Comprehensive Organometallic Chemistry IV. Volume 5: Groups 5 to 7 - Part 1 9780128202067

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COMPREHENSIVE ORGANOMETALLIC CHEMISTRY IV

COMPREHENSIVE ORGANOMETALLIC CHEMISTRY IV EDITORS-IN-CHIEF

GERARD PARKIN Department of Chemistry, Columbia University, New York, NY, United States

KARSTEN MEYER Department of Chemistry and Pharmacy, Friedrich-Alexander-Universität, Erlangen, Germany

DERMOT O’HARE Department of Chemistry, University of Oxford, Oxford, United Kingdom

VOLUME 5

GROUPS 5 TO 7 - PART 1 VOLUME EDITOR

SCOTT R. DALY Department of Chemistry, The University of Iowa, Iowa City, IA, United States

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2022 Elsevier Ltd. All rights reserved No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers may always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein.

Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN 978-0-12-820206-7 For information on all publications visit our website at http://store.elsevier.com

Publisher: Oliver Walter Acquisition Editor: Blerina Osmanaj Content Project Manager: Claire Byrne Associate Content Project Manager: Fahmida Sultana Designer: Christian Bilbow

CONTENTS OF VOLUME 5 Editor Biographies

vii

Contributors to Volume 5

xiii

Preface 5.01

xv

Overview and Introduction

1

Scott R Daly

5.02

Cyclic and Non-Cyclic Pi Complexes of Vanadium

2

Jan Honzí cek

5.03

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

49

Grant E Forsythe and Louis Messerle

5.04

Cyclic and Non-cyclic Pi Complexes of Chromium

81

Tingting Song and Ying Mu

5.05

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

174

Wenguang Wang, Xiaofang Zhai, and Shu-Fen Hou

5.06

Cyclic and Non-Cyclic Pi Complexes of Tungsten

257

Timothy P Curran

5.07

Cyclic and Non-Cyclic Pi Complexes of Manganese

378

Jeffrey S Price and David JH Emslie

5.08

Organometallic Complexes of Technetium

547

Henrik Braband

5.09

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

587

Kotohiro Nomura

5.10

Group 6 Complexes With Metal-Carbon Sigma Bonds

651

Brian J Bellott and Matthew A Klyman

5.11

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

671

Richard R Schrock, Michael R Buchmeiser, Jonas Groos, and Mathis J Benedikter

v

EDITOR BIOGRAPHIES Editors in Chief Karsten Meyer studied chemistry at the Ruhr University Bochum and performed his Ph.D. thesis work on the molecular and electronic structure of first-row transition metal complexes under the direction of Professor Karl Wieghardt at the Max Planck Institute in Mülheim/Ruhr (Germany). He then proceeded to gain research experience in the laboratory of Professor Christopher Cummins at the Massachusetts Institute of Technology (USA), where he appreciated the art of synthesis and developed his passion for the coordination chemistry and reactivity of uranium complexes. In 2001, he was appointed to the University of California, San Diego, as an assistant professor and was named an Alfred P. Sloan Fellow in 2004. In 2006, he accepted an offer (C4/W3) to be the chair of the Institute of Inorganic & General Chemistry at the Friedrich-Alexander-University ErlangenNürnberg (FAU), Germany. Among his awards and honors, he was elected a lifetime honorary member of the Israel Chemical Society and a fellow of the Royal Society of Chemistry (UK). Karsten received the Elhuyar-Goldschmidt Award from the Royal Society of Chemistry of Spain, the Ludwig Mond Award from the RSC (UK), and the Chugaev Commemorative Medal from the Russian Academy of Sciences. He has also enjoyed visiting professorship positions at the universities of Manchester (UK) and Toulouse (F) as well as the Nagoya Institute of Technology (JP) and ETH Zürich (CH). The Meyer lab research focuses on the synthesis of custom-tailored ligand environments and their transition and actinide metal coordination complexes. These complexes often exhibit unprecedented coordination modes, unusual electronic structures, and, consequently, enhanced reactivities toward small molecules of biological and industrial importance. Interestingly, Karsten’s favorite molecule is one that exhibits little reactivity: the Th symmetric U(dbabh)6. Dermot O’Hare was born in Newry, Co Down. He studied at Balliol College, Oxford University, where he obtained his B.A., M.A., and D.Phil. degrees under the direction of Professor M.L.H. Green. In 1985, he was awarded a Royal Commission of 1851 Research Fellowship, during this Fellowship he was a visiting research fellow at the DuPont Central Research Department, Wilmington, Delaware in 1986–87 in the group led by Prof. J.S. Miller working on molecular-based magnetic materials. In 1987 he returned to Oxford to a short-term university lectureship and in 1990 he was appointed to a permanent university position and a Septcentenary Tutorial Fellowship at Balliol College. He has previously been honored by the Institüt de France, Académie des Sciences as a leading scientist in Europe under 40 years. He is currently professor of organometallic and materials chemistry in the Department of Chemistry at the University of Oxford. In addition, he is currently the director of the SCG-Oxford Centre of Excellence for chemistry and associate head for business & innovation in the Mathematics, Physical and Life Sciences Division. He leads a multidisciplinary research team that works across broad areas of catalysis and nanomaterials. His research is specifically targeted at finding solutions to global issues relating to energy, zero carbon, and the circular economy. He has been awarded numerous awards and prizes for his creative and

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Editor Biographies

ground-breaking work in inorganic chemistry, including the Royal Society Chemistry’s Sir Edward Frankland Fellowship, Ludwig Mond Prize, Tilden Medal, and Academia–Industry Prize and the Exxon European Chemical and Engineering Prize. Gerard Parkin received his B.A., M.A., and D.Phil. degrees from the Queen’s College, Oxford University, where he carried out research under the guidance of Professor Malcolm L.H. Green. In 1985, he moved to the California Institute of Technology as a NATO postdoctoral fellow to work with Professor John E. Bercaw. He joined the Faculty of Columbia University as assistant professor in 1988 and was promoted to associate professor in 1991 and to professor in 1994. He served as chairman of the Department from 1999 to 2002. He has also served as chair of the New York Section of the American Chemical Society, chair of the Inorganic Chemistry and Catalytic Science Section of the New York Academy of Sciences, chair of the Organometallic Subdivision of the American Chemical Society Division of Inorganic Chemistry, and chair of the Gordon Research Conference in Organometallic Chemistry. He is an elected fellow of the American Chemical Society, the Royal Society of Chemistry, and the American Association for the Advancement of Science, and is the recipient of a variety of international awards, including the ACS Award in pure chemistry, the ACS Award in organometallic chemistry, the RSC Corday Morgan Medal, the RSC Award in organometallic chemistry, the RSC Ludwig Mond Award, and the RSC Chem Soc Rev Lecture Award. He is also the recipient of the United States Presidential Award for Excellence in Science, Mathematics and Engineering Mentoring, the United States Presidential Faculty Fellowship Award, the James Flack Norris Award for Outstanding Achievement in the Teaching of Chemistry, the Columbia University Presidential Award for Outstanding Teaching, and the Lenfest Distinguished Columbia Faculty Award. His principal research interests are in the areas of synthetic, structural, and mechanistic inorganic chemistry.

Volume Editors Simon Aldridge is professor of chemistry at the University of Oxford and director of the UKRI Centre for Doctoral Training in inorganic chemistry for Future Manufacturing. Originally from Shrewsbury, England, he received both his B.A. and D.Phil. degrees from the University of Oxford, the latter in 1996 for work on hydride chemistry under the supervision of Tony Downs. After post-doctoral work as a Fulbright Scholar at Notre Dame with Tom Fehlner, and at Imperial College London (with Mike Mingos), he took up his first academic position at Cardiff University in 1998. He returned to Oxford in 2007, being promoted to full professor in 2010. Prof. Aldridge has published more than 230 papers to date and is a past winner of the Dalton Transactions European Lectureship (2009), the Royal Society of Chemistry’s Main Group Chemistry (2010) and Frankland Awards (2018), and the Forschungspreis of the Alexander von Humboldt Foundation (2021). Prof. Aldridge’s research interests are primarily focused on main group organometallic chemistry, and in particular the development of compounds with unusual electronic structure, and their applications in small molecule activation and catalysis (website: http:// aldridge.web.ox.ac.uk). (Picture credit: John Cairns)

Editor Biographies

ix

Eszter Boros is associate professor of chemistry at Stony Brook University with courtesy appointments in radiology and pharmacology at Stony Brook Medicine. Eszter obtained her M.Sc. (2007) at the University of Zurich, Switzerland and her Ph.D. (2011) in chemistry from the University of British Columbia, Canada. She was a postdoc (2011–15) and later instructor (2015–17) in radiology at Massachusetts General Hospital and Harvard Medical School. In 2017, Eszter was appointed as assistant professor of chemistry at Stony Brook University, where her research group develops new approaches to metal-based diagnostics and therapeutics at the interfaces of radiochemistry, inorganic chemistry and medicine. Her lab’s work has been extensively recognized; Eszter holds various major federal grants (NSF CAREER Award, NIH NIBIB R21 Trailblazer, NIH NIGMS R35 MIRA) and has been named a 2020 Moore Inventor Fellow, the 2020 Jonathan L. Sessler Fellow (American Chemical Society, Inorganic Division), recipient of a 2021 ACS Infectious Diseases/ACS Division of Biological Chemistry Young Investigator Award (American Chemical Society), and was also named a 2022 Alfred P. Sloan Research Fellow in chemistry. Scott R. Daly is associate professor of chemistry at the University of Iowa in the United States. After spending 3 years in the U.S. Army, he obtained his B.S. degree in chemistry in 2006 from North Central College, a small liberal arts college in Naperville, Illinois. He then went on to receive his Ph.D. at the University of Illinois at Urbana-Champaign in 2010 under the guidance of Professor Gregory S. Girolami. His thesis research focused on the synthesis and characterization of chelating borohydride ligands and their use in the preparation of volatile metal complexes for chemical vapor deposition applications. In 2010, he began working as a Seaborg postdoctoral fellow with Drs. Stosh A. Kozimor and David L. Clark at Los Alamos National Laboratory in Los Alamos, New Mexico. His research there concentrated on the development of ligand K-edge X-ray absorption spectroscopy (XAS) to investigate covalent metal–ligand bonding and electronic structure variations in actinide, lanthanide, and transition metal complexes with metal extractants. He started his independent career in 2012 at George Washington University in Washington, DC, and moved to the University of Iowa shortly thereafter in 2014. His current research interests focus on synthetic coordination chemistry and ligand design with emphasis on the development of chemical and redox noninnocent ligands, mechanochemical synthesis and separation methods, and ligand K-edge XAS. His research and outreach efforts have been recognized with an Outstanding Faculty/Staff Advocate Award from the University of Iowa Veterans Association (2016), a National Science Foundation CAREER Award (2017), and a Hawkeye Distinguished Veterans Award (2018). He was promoted to associate professor with distinction as a College of Liberal Arts and Sciences Deans Scholar in 2020. Lena J. Daumann is currently professor of bioinorganic and coordination chemistry at the Ludwig Maximilian Universität in Munich. She studied chemistry at the University of Heidelberg working with Prof. Peter Comba and subsequently conducted her Ph.D. at the University of Queensland (Australia) from 2010 to 2013 holding IPRS and UQ Centennial fellowships. In 2013 she was part of the Australian Delegation for the 63rd Lindau Nobel Laureate meeting in chemistry. Following postdoctoral stays at UC Berkeley with Prof. Ken Raymond (2013–15) and in Heidelberg, funded by the Alexander von Humboldt Foundation, she started her independent career at the LMU Munich in 2016. Her bioinorganic research group works on elucidating the role of lanthanides for bacteria as well as on iron enzymes and small biomimetic complexes that play a role in epigenetics and DNA repair. Daumann’s teaching and research have been recognized with numerous awards and grants. Among them are the national Ars Legendi Prize for chemistry and the Therese von Bayern Prize in 2019 and the Dozentenpreis of the “Fonds der Chemischen Industrie“ in 2021. In 2018 she was selected as fellow for the Klaus Tschira Boost Fund by the German Scholars Organisation and in 2020 she received a Starting grant of the European Research Council to study the uptake of lanthanides by bacteria.

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Editor Biographies

Derek P. Gates hails from Halifax, Nova Scotia (Canada) where he completed his B.Sc. (Honours Chemistry) degree at Dalhousie University in 1993. He completed his Ph.D. degree under the supervision of Professor Ian Manners at the University of Toronto in 1997. He then joined the group of Professor Maurice Brookhart as an NSERC postdoctoral fellow at the University of North Carolina at Chapel Hill (USA). He began his independent research career in 1999 as an assistant professor at the University of British Columbia in Vancouver (Canada). He has been promoted through the ranks and has held the position of professor of chemistry since 2011. At UBC, he has received the Science Undergraduate Society—Teaching Excellence Award, the Canadian National Committee for IUPAC Award, and the Chemical Society of Canada—Strem Chemicals Award for pure or applied inorganic chemistry. His research interests bridge the traditional fields of inorganic and polymer chemistry with particular focus on phosphorus chemistry. Key topics include the discovery of novel structures, unusual bonding, new reactivity, along with applications in catalysis and materials science. Patrick Holland performed his Ph.D. research in organometallic chemistry at UC Berkeley with Richard Andersen and Robert Bergman. He then learned about bioinorganic chemistry through postdoctoral research on copper-O2 and copper-thiolate chemistry with William Tolman at the University of Minnesota. His independent research at the University of Rochester initially focused on systematic development of the properties and reactions of three-coordinate complexes of iron and cobalt, which can engage in a range of bond activation reactions and organometallic transformations. Since then, his research group has broadened its studies to iron-N2 chemistry, reactive metal–ligand multiple bonds, iron–sulfur clusters, engineered metalloproteins, redox-active ligands, and solar fuel production. In 2013, Prof. Holland moved to Yale University, where he is now Conkey P. Whitehead Professor of Chemistry. His research has been recognized with an NSF CAREER Award, a Sloan Research Award, Fulbright and Humboldt Fellowships, a Blavatnik Award for Young Scientists, and was elected as fellow of the American Association for the Advancement of Science. In the area of N2 reduction, his group has established molecular principles to weaken and break the strong N–N bond, in order to use this abundant resource for energy and synthesis. His group has made a particular effort to gain an insight into iron chemistry relevant to nitrogenase, the enzyme that reduces N2 in nature. His group also maintains an active program in the use of inexpensive metals for transformations of alkenes. Mechanistic details are a central motivation to Prof. Holland and the wonderful group of over 80 students with whom he has worked. Steve Liddle was born in Sunderland in the North East of England and gained his B.Sc. (Hons) and Ph.D. from Newcastle University. After postdoctoral fellowships at Edinburgh, Newcastle, and Nottingham Universities he began his independent career at Nottingham University in 2007 with a Royal Society University Research Fellowship. This was held in conjunction with a proleptic Lectureship and he was promoted through the ranks to associate professor and reader in 2010 and professor of inorganic chemistry in 2013. He remained at Nottingham until 2015 when he was appointed professor and head of inorganic chemistry and co-director of the Centre for Radiochemistry Research at The University of Manchester. He has been a recipient of an EPSRC Established Career Fellowship and ERC Starter and Consolidator grants. He is an elected fellow of The Royal Society of Edinburgh and fellow of the Royal Society of Chemistry and he is vice president to the Executive Committee of the European Rare Earth and Actinide Society. His principal research interests are focused on f-element chemistry, involving exploratory synthetic chemistry coupled to detailed electronic structure and reactivity studies to elucidate structure-bonding-property relationships. He is the recipient of a variety of prizes, including the IChemE Petronas Team Award for Excellence in Education and Training, the RSC Sir Edward Frankland Fellowship, the RSC Radiochemistry

Editor Biographies

xi

Group Bill Newton Award, a 41st ICCC Rising Star Award, the RSC Corday-Morgan Prize, an Alexander von Humboldt Foundation Friedrich Wilhelm Bessel Research Award, the RSC Tilden Prize, and an RSC Dalton Division Horizon Team Prize. He has published over 220 research articles, reviews, and book chapters to date. David Liptrot received his MChem (Hons) in chemistry with Industrial Training from the University of Bath in 2011 and remained there to undertake a Ph.D. on group 2 catalysis in the laboratory of Professor Mike Hill. After completing this in 2015 he took up a Lindemann Postdoctoral Fellowship with Professor Philip Power FRS (University of California, Davis, USA). In 2017 he began his independent career returning to the University of Bath and in 2019 was awarded a Royal Society University Research Fellowship. His interests concern new synthetic methodologies to introduce main group elements into functional molecules and materials.

David P. Mills hails from Llanbradach and Caerphilly in the South Wales Valleys. He completed his MChem (2004) and Ph.D. (2008) degrees at Cardiff University, with his doctorate in low oxidation state gallium chemistry supervised by Professor Cameron Jones. He moved to the University of Nottingham in 2008 to work with Professor Stephen Liddle for postdoctoral studies in lanthanide and actinide methanediide chemistry. In 2012 he moved to the University of Manchester to start his independent career as a lecturer, where he has since been promoted to full professor of inorganic chemistry in 2021. Although he is interested in all aspects of nonaqueous synthetic chemistry his research interests are currently focused on the synthesis and characterization of f-block complexes with unusual geometries and bonding regimes, with the aim of enhancing physicochemical properties. He has been recognized for his contributions to both research and teaching with prizes and awards, including a Harrison-Meldola Memorial Prize (2018), the Radiochemistry Group Bill Newton Award (2019), and a Team Member of the Molecular Magnetism Group for the Dalton Division Horizon Prize (2021) from the Royal Society of Chemistry. He was a Blavatnik Awards for Young Scientists in the United Kingdom Finalist in Chemistry in 2021 and he currently holds a European Research Council Consolidator Grant. Ian Tonks is the Lloyd H. Reyerson professor at the University of MinnesotaTwin Cities, and associate editor for the ACS journal Organometallics. He received his B.A. in chemistry from Columbia University in 2006 and performed undergraduate research with Prof. Ged Parkin. He earned his Ph.D. in 2012 from the California Institute of Technology, where he worked with Prof. John Bercaw on olefin polymerization catalysis and early transition metal-ligand multiply bonded complexes. After postdoctoral research with Prof. Clark Landis at the University of Wisconsin, Madison, he began his independent career at the University of Minnesota in 2013 and earned tenure in 2019. His current research interests are focused on the development of earth abundant, sustainable catalytic methods using early transition metals, and also on catalytic strategies for incorporation of CO2 into polymers. Prof. Tonks’ work has recently been recognized with an Outstanding New Investigator Award from the National Institutes of Health, an Alfred P. Sloan Fellowship, a Department of Energy CAREER award, and the ACS Organometallics Distinguished Author Award, among others. Additionally, Prof. Tonks’ service toward improving academic safety culture was recently recognized with the 2021 ACS Division of Chemical Health and Safety Graduate Faculty Safety Award.

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Editor Biographies

Timothy H. Warren is the Rosenberg professor and chairperson in the Department of Chemistry at Michigan State University. He obtained his B.S. from the University of Illinois at Urbana-Champaign in 1992 and Ph.D. from the Massachusetts Institute of Technology in 1997. After 2 years of postdoctoral research at the Organic Chemistry Institute of the University of Münster, Germany with Prof. Dr. Gerhart Erker, Dr. Warren started his independent career at Georgetown University in 1999 where he was named the Richard D. Vorisek professor of chemistry in 2014. He moved to Michigan State University in 2021. Prof. Warren’s research interests span synthetic and mechanistic inorganic, organometallic, and bioinorganic chemistry with a focus on catalysis. His research group develops environmentally friendly methods for organic synthesis via C–H functionalization, explores the interconversion of nitrogen and ammonia as carbon-free fuels, and decodes ways that biology communicates using nitric oxide as a molecular messenger. Mechanistic studies on these chemical reactions catalyzed by metal ions such as iron, nickel, copper, and zinc enable new insights for the development of useful catalysts for synthesis and energy applications as well as lay the mechanistic groundwork to understand biochemical nitric oxide misregulation. Dr. Warren received the NSF CAREER Award, chaired the 2019 Inorganic Reaction Mechanisms Gordon Research Conference, and has served on the ACS Division of Inorganic Chemistry executive board and on the editorial boards of Inorganic Synthesis, Inorganic Chemistry, and Chemical Society Reviews.

CONTRIBUTORS TO VOLUME 5 Brian J Bellott Western Illinois University, Macomb, IL, United States

Matthew A Klyman Western Illinois University, Macomb, IL, United States

Mathis J Benedikter Institute of Polymer Chemistry, University of Stuttgart, Stuttgart, Germany

Louis Messerle Department of Chemistry, The University of Iowa, Iowa City, IA, United States

Henrik Braband Department of Chemistry, University of Zurich, Winterthurerstrasse, Zürich, Switzerland

Ying Mu The State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, People's Republic of China

Michael R Buchmeiser Institute of Polymer Chemistry, University of Stuttgart, Stuttgart, Germany Timothy P Curran Department of Chemistry, Trinity College, Hartford, CT, United States Scott R Daly Department of Chemistry, The University of Iowa, Iowa City, IA, United States David JH Emslie Department of Chemistry and Chemical Biology, McMaster University, Hamilton, ONT, Canada Grant E Forsythe Department of Chemistry, The University of Iowa, Iowa City, IA, United States

Kotohiro Nomura Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, Hachioji, Tokyo, Japan Jeffrey S Price Department of Chemistry and Chemical Biology, McMaster University, Hamilton, ONT, Canada Richard R Schrock Department of Chemistry, Pierce Hall, University of California at Riverside, Riverside, CA, United States Tingting Song The State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, People's Republic of China

Jonas Groos Institute of Polymer Chemistry, University of Stuttgart, Stuttgart, Germany

Wenguang Wang College of Chemistry, Beijing Normal University, Beijing, China; School of Chemistry and Chemical Engineering, Shandong University, Jinan, China

Jan Honzícek Institute of Chemistry and Technology of Macromolecular Materials, Faculty of Chemical Technology, University of Pardubice, Pardubice, Czech Republic

Xiaofang Zhai School of Chemistry and Chemical Engineering, Shandong University, Jinan, China

Shu-Fen Hou School of Chemistry and Chemical Engineering, Shandong University, Jinan, China

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PREFACE Published 40 years ago in 1982, the first edition of Comprehensive Organometallic Chemistry (COMC) provided an invaluable resource that enabled chemists to become efficiently informed of the properties and reactions of organometallic compounds of both the main group and transition metals. This area of chemistry continued to develop at a rapid pace such that it necessitated the publication of subsequent editions, namely Comprehensive Organometallic Chemistry II (COMC2) in 1995 and Comprehensive Organometallic Chemistry III (COMC3) in 2007. Organometallic chemistry has continued to be vibrant in the 15 years following the publication of COMC3, not only by affording compounds with novel structures and reactivity but also by having important applications in organic syntheses and industrial processes, as illustrated by the awarding of the 2010 Nobel prize to Heck, Negishi, and Suzuki for the development of palladium-catalyzed cross couplings in organic syntheses. Comprehensive Organometallic Chemistry IV (COMC4) thus serves the same important role as its predecessors by providing an indispensable means for researchers and educators to obtain efficiently an up-to-date analysis of a particular aspect of organometallic chemistry. COMC4 comprises 15 volumes, of which the first provides a review of topics concerned with techniques and concepts that feature prominently in current organometallic chemistry, while 5 volumes are devoted to applications that include organic synthesis, materials science, bio-organometallics, metallo-therapy, metallodiagnostics, medicine, and environmental chemistry. In this regard, we are very grateful to the volume editors for their diligent efforts, and the authors for producing high-quality chapters, all of which were written during the COVID-19 pandemic. Finally, we wish to thank the many staff at Elsevier for their efforts to ensure that the project, initiated in the winter of 2018, remained on schedule. Karsten Meyer, Erlangen, March 2022 Dermot O’Hare, Oxford, March 2022 Gerard Parkin, New York, March 2022

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5.01

Overview and Introduction

Scott R Daly, Department of Chemistry, The University of Iowa, Iowa City, IA, United States © 2022 Elsevier Ltd. All rights reserved.

Volume 5 and Volume 6 of Comprehensive Organometallic Chemistry IV continues the survey of organometallic complexes across the periodic table by reviewing the synthesis, structures, and reactivity of complexes containing transition metals in Groups 5 – 7. Highlighting the diversity of topics and international expertise in this area, these two volumes contain twenty-three chapters by authors from nine countries. The chapters in Volumes 5 and 6 are organized by metal identity and ligand class, starting with complexes containing cyclic and non-cyclic p ligands such as cyclopentadienyls, arenes, and olefins. These include p complexes with vanadium (Honzícek, Chapter 5.02), niobium and tantalum (Forsythe and Messerle, Chapter 5.03), chromium (Mu, Chapter 5.04), molybdenum (Wang and coworkers, Chapter 5.05), tungsten (Curran, Chapter 5.06), and manganese (Price and Emslie, Chapter 5.07). Due to the lack of non-radioactive technetium isotopes, organometallic technetium complexes have received less attention compared to other metals in this volume. Thus, new examples of organometallic technetium complexes reported since Comprehensive Organometallic Chemistry III, including those with p ligands, have been compiled in Chapter 5.08 by Braband and Alberto. The next four chapters focus on metal complexes containing metal-carbon bonds with anionic organometallic ligands, such as alkyls, aryls, alkylidenes, and alkylidynes. Chapter 5.09 (Nomura) and Chapter 6.01 (Jia) describe Group 5 and Group 7 complexes containing metal-carbon sigma and multiple bonds. Group 6 metals are divided into two chapters according to metal-carbon bond order. Alkylidene and alkylidyne complexes containing metal-carbon multiple bonds are covered by Buchmeiser and 2005 Nobel Laureate Richard Schrock (Chapter 5.11), whereas chapter 5.10 (Klyman and Bellott) reviews Group 6 complexes containing metal-carbon single bonds. The six chapters that follow focus on metal complexes containing certain carbenes and pi acidic ligands, such as isocyanides and the ubiquitous carbonyl. N-Heterocyclic and mesoionic carbene complexes of Group 5 and 6 metals are described by Buchmeiser and coworkers (Chapter 6.02) and Royo reviews those that contain Group 7 metals (Chapter 6.03). Marchetti and Pampaloni then provide a survey of organometallic complexes of Group 5 with pi-acidic ligands (Chapter 6.04), followed by a review by Fischer of Group 6 complexes containing carbonyl and isocyanides (Chapter 6.05). Carbonyl and isocyanide complexes of manganese and rhenium are divided according to metal into two chapters by Lacy (Chapter 6.06) and Ko and coworkers (Chapter 6.07). To round out Volume 6, several new chapters not included in previous editions of Comprehensive Organometallic Chemistry highlight areas of special and contemporary interest. Organometallic complexes containing pincer and some non-innocent ligands are compiled for the first time in three chapters, with Hollis and coworkers covering Group 5 (Chapter 6.08), Grzybowski and Daly covering Group 6 (Chapter 6.09), and Luca and coworkers covering Group 7 (Chapter 6.10). Earl and Messerle survey examples of tri- and polynuclear organometallic clusters of Groups 5 – 7 (Chapter 6.11), and the volume is closed out with a review from Mock and coworkers on Group 5 – 7 complexes containing N2, NO, and related small molecules (Chapter 6.12). It is important for readers, especially those who come to this volume years after its production, to understand that the information found here was compiled during the COVID-19 pandemic. Several authors became very ill during production, and a few were unable to continue due to lingering effects. Several planned chapters were left unfinished, including one on Re complexes with cyclic and non-cyclic p ligands. While unfortunate, these chapters’ absence is trivial when considering the health of our authors and the incomprehensible toll that the pandemic has taken on humanity. This edition of Comprehensive Organometallic Chemistry is testament to the resilience and determination of the many contributors from around the world who worked under extraordinary conditions to deliver this work to you, our reader and colleague.

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00167-0

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5.02

Cyclic and Non-Cyclic Pi Complexes of Vanadium

Jan Honzícek, Institute of Chemistry and Technology of Macromolecular Materials, Faculty of Chemical Technology, University of Pardubice, Pardubice, Czech Republic © 2022 Elsevier Ltd. All rights reserved.

5.02.1 5.02.2 5.02.2.1 5.02.2.1.1 5.02.2.1.2 5.02.2.2 5.02.2.2.1 5.02.2.2.2 5.02.2.2.3 5.02.2.2.4 5.02.2.2.5 5.02.2.2.6 5.02.2.3 5.02.2.3.1 5.02.2.3.2 5.02.2.3.3 5.02.2.3.4 5.02.2.3.5 5.02.2.3.6 5.02.3 5.02.3.1 5.02.3.1.1 5.02.3.1.2 5.02.3.1.3 5.02.3.1.4 5.02.3.2 5.02.3.3 5.02.3.3.1 5.02.3.3.2 5.02.3.4 5.02.3.5 5.02.3.5.1 5.02.3.5.2 5.02.3.5.3 5.02.3.5.4 5.02.3.6 5.02.3.7 5.02.3.7.1 5.02.3.7.2 5.02.4 5.02.4.1 5.02.4.2 5.02.5 5.02.5.1 5.02.5.2 5.02.6 5.02.7 5.02.8 5.02.8.1 5.02.8.2 5.02.8.3 5.02.9 References

2

Introduction Mono(h5-cyclopentadienyl) complexes Mono(Z5-cyclopentadienyl)vanadium(I) complexes Tetracarbonyl(Z5-cyclopentadienyl)vanadium Polynuclear carbonyl complexes Mono(Z5-cyclopentadienyl) complexes of vanadium(II) and vanadium(III) Halogenido complexes Carboxylato and phosphato complexes Complexes with S-, Se- and Te-donors Complexes bearing anionic N-donor ligands Hydrido, alkyl, alkynyl, alkylidene and silyl derivatives Boron-containing complexes Mono(Z5-cyclopentadienyl) complexes of vanadium(IV) and vanadium(V) Halogenido complexes Oxido complexes Imido complexes Chalcogenido complexes Azido and nitrido complexes Applications Bis(h5-cyclopentadienyl) complexes Bis(Z5-cyclopentadienyl)vanadium Synthesis of vanadocene and ring-substituted derivatives Sandwich complexes related to vanadocene Reactivity of vanadocene with unsaturated molecules Reactivity of vanadocene with digallanes and silylene Carbonyl and isonitrile complexes Halogenido and pseudohalogenido complexes Halides Pseudohalides Complexes with weakly nucleophilic ligands Complexes of group 15 and 16 donor ligands Vanadocene complexes bearing bio-ligands Other vanadocene complexes with O-donor ligands Chalcogenides and other S- and Se-donor ligands Compounds with N- and P-donor ligands Hydrido, alkyl, aryl and alkynyl derivatives Applications Biological activity Applications in catalysis and material science h6-Arene complexes Bis(Z6-benzene)vanadium and its derivatives Half-sandwich, mixed-ring sandwich and multi-decker complexes h7-Cycloheptatrienyl complexes (Z7-Cycloheptatrienyl)(Z5-cyclopentadienyl)vanadium and its derivatives Half-sandwich and bis(cycloheptatrienyl) complexes h8-Cyclooctatetraene complexes Miscellaneous cyclic p-complexes Non-cyclic p-complexes Z2-Alkene, alkyne and Z4-butadiene complexes Z3-Allyl complexes Z5-Pentadienyl and Z7-heptadienyl complexes Concluding remarks

Comprehensive Organometallic Chemistry IV

3 3 3 3 5 6 6 7 7 8 9 10 10 10 11 11 12 13 13 13 13 13 15 16 18 18 19 19 21 21 22 22 23 25 25 26 27 27 27 28 28 30 32 32 36 36 36 37 37 38 38 39 39

https://doi.org/10.1016/B978-0-12-820206-7.00006-8

Cyclic and Non-Cyclic Pi Complexes of Vanadium

5.02.1

3

Introduction

During the last 15 years, the number of organometallic vanadium complexes bearing p-bonded ligands expanded rapidly, especially in the area of vanadocene, trovacene and bis(arene)vanadium complexes. The rapid development of vanadocene complexes is due to their potential application in medicine. The number of studies dealing with bis(arene)vanadium and trovacene compounds can be ascribed to the development of efficient synthetic procedures for preparing ring-substituted and ansa-bridged complexes. Undoubtedly, improved development and access to X-ray diffraction (XRD) instrumentation, now a routine characterization tool, has strongly contributed to progress observed in organovanadium chemistry. It is noteworthy that the rather limited number of studies of organovanadium complexes is related to their paramagnetism, which often precludes routine NMR characterization. This chapter surveys the literature from 2005 up to early 2020 including some relevant review articles. The organization of this chapter broadly follows the previous editions; COMC I (1982),1 COMC II (1995),2 COMC III (2007).3 A survey of older XRD crystallographic studies is included in order to improve clarity of the presentation and to provide a more comprehensive overview about the topic of p-complexes of vanadium.

5.02.2

Mono(h5-cyclopentadienyl) complexes

5.02.2.1

Mono(h5-cyclopentadienyl)vanadium(I) complexes

5.02.2.1.1

Tetracarbonyl(
5-cyclopentadienyl)vanadium

5.02.2.1.1.1 Synthesis, structure and ring-substituted derivatives [(Z5-Cp)V(CO)4] is a diamagnetic 18-electron compound (d4-configuration) appearing as an orange crystalline solid. Its mononuclear four-legged piano stool structure is evidenced by gas-phase electron diffraction4 and XRD analysis.5 Unusually strong delocalization of d-electrons to the carbonyl ligands has been evidenced by gas-phase photoelectron spectroscopy and well reproduced by density functional theory (DFT) and ab initio calculations.6 Due to its diamagnetic character, [(Z5-Cp)V(CO)4] and its carbonyl containing derivatives are routinely characterized by NMR spectroscopy including 51V NMR technique.7 [(Z5-Cp)V(CO)4] is conveniently prepared by a reduction of [(Z5-Cp2)V] with potassium at low temperature followed by carbonylation upon warming to room temperature (Scheme 1).8 This synthetic protocol has replaced an earlier route involving a high-pressure carbonylation of [(Z5-Cp2)V].9,10

Scheme 1 Convenient syntheses of [(Z5-Cp)V(CO)4] and [(Z5-Cp0 )V(CO)4].

[(Z5-Cp)V(CO)4] and its ring-substituted derivatives are accessible by a reaction of [Cp0 HgCl] with [V(CO)6]−.9,11 A more convenient pathway to the ring-substituted derivatives involves a reaction of corresponding cyclopentadiene (Cp0 H) with [V(CO)6] (Scheme 1).5,12,13 Low reactivity of anellated cyclopentadienes (e.g., indene and fluorene) can be overcome when starting from halogenated cyclopentadienes.5 This modified route has been used for derivatives bearing halogenated cyclopentadienyl rings Z5-C5Cl5 and Z5-C5Br5.14 The electron-rich metal center in [(Z5-Cp)V(CO)4] enables derivatization through Friedel-Crafts acylation.5,15 Reductive coupling of acenaphthylene, induced by [V(CO)6], gives a rather unusual dinuclear compound with coordinated five-membered rings.16 A more detailed overview of the synthetic procedures affording [(Z5-Cp0 )V(CO)4] is given in the previous editions of COMC; COMC I and COMC II. Structures of the ring-substituted derivatives of [(Z5-Cp)V(CO)4], confirmed by single-crystal XRD analysis, are shown in Scheme 2.

Scheme 2 Structures of [(Z5-Cp0 )V(CO)4] determined by XRD analysis.

4

Cyclic and Non-Cyclic Pi Complexes of Vanadium

5.02.2.1.1.2 Carbonyl substitution [(Z5-Cp)V(CO)4] and its ring-substituted analogues serve as precursors for a wide variety of mono(cyclopentadienyl)vanadium compounds. Under photochemical conditions, one or two carbonyls can be substituted by Lewis bases, as comprehensively overviewed in COMC II. Briefly, a convenient procedure involves low-temperature irradiation of [(Z5-Cp0 )V(CO)4] in tetrahydrofuran generating labile green [(Z5-Cp0 )V(CO)3(thf )], which reacts with a given Lewis base (e.g., amines, phosphines, arsanes, stibanes) to afford the ligand-exchange product.17,18 A promising modification of the reaction protocol involves a reaction of [(Z5-Cp0 )V(CO)4] with organic sulfide (e.g., tetrahydrothiophene and dimethyl sulfide) to produce a stable intermediate, [(Z5-Cp0 )V(CO)3(SR2)], which can be easily isolated but stays reactive enough under mild thermal conditions.19,20 Formation of [(Z5-Cp)V(CO)(k3-triphos)] by a photochemical reaction of [(Z5-Cp)V(CO)4] with the tridentate phosphine ligand represents a rare example of the substitution reaction involving replacement of three CO ligands.21 An alternative route to phosphine complexes involves exposition of the chlorido complex [{(Z5-Cp)V(PR3)}2(m-Cl)2] with CO, as exemplified with the synthesis of [(Z5-Cp)V(CO)3(PEt3)].22 The structurally characterized phosphine complex [(Z5-Cp)V(CO)2(k2-dmpe)] has been synthesized by a reaction of [(Z5-Cp)V(k2-dmpe)(SiHPh2)] with CO.23 Photochemical reaction of the niobocene(V) compound [(Z5-Cp)2NbH3] with [(Z5-Cp)V(CO)4] gives [{(Z5-Cp)V(CO)3} (m-H){(Z5-Cp)2Nb(CO)}] with two organometallic moieties connected with a hydride bridge. From a mechanistic point of view, the transfer of CO ligand is accompanied by reductive elimination of H2. The long vanadium-to-niobium distance (3.71 A˚ ), elucidated by XRD analysis, suggests formation of a three-center two-electron VdHdNb bond.24 The anionic vanadium(I) complex [Ph4P][(Z5-Cp)V(CO)3GeH3] is accessible by a reaction of [(Z5-Cp)V(CO)4] with potassium germanide followed by cation exchange. XRD analysis of the complex confirmed its half-sandwich structure with a rare VdGe bond.25 Photochemical activation of [(Z5-Cp)V(CO)4] has been successfully used for intermetallic transfer of borylene from [(OC)5Cr] BN(SiMe3)2] to give the complex [(Z5-Cp)V(CO)3{]BN(SiMe3)2}] (Scheme 3).26 A follow-up theoretical study at the DFT level of theory revealed a significantly higher value of Wiberg bond indices (0.79) for the VdB bond than for VdX bonds of the virtual alylene (0.60) and gallylene analogues (0.65).27

Scheme 3 Synthesis of borylene, gallylene and silylene vanadium compounds.

The half-sandwich vanadium complex bearing anionic gallium(I) heterocycle with sterically demanding 2,6-diisopropylphenyl substituent [(Z5-Cp)V(CO)3(Ga{N(DiPP)CH}2)]− has been synthesized under mild thermal conditions from [(Z5-Cp)V(CO)4] and [K(tmeda)][Ga{N(DiPP)CH}2] (Scheme 3). A theoretical study on a model compound suggests the VdGa bond involves only negligible back-bonding, thereby resembling an analogue bearing a neutral N-heterocyclic carbene ligand.28 As previously reported in COMC III, unusual zirconoxy- and hafnoxycarbene complexes are accessible by photochemically induced coupling of butadiene, coordinated in [(Z5-Cp)2M(Z4-diene)] (M ¼ Zr, Hf ), with a CO ligand in [(Z5-Cp)V(CO)4]. XRD analysis has confirmed appearance of the zirconium species with a distorted Z3-allylic moiety in the solid state.29 The insertion of aldehydes, ketones and nitriles into a ZrdC bond gives nine-membered zircona- and hafnacycles with an untouched vanadium coordination sphere, as documented in several examples.30–32 The compounds lack the typical electrophilic properties of carbene complexes but can be easily transformed to ordinary Fischer-type carbenes through hydrolysis and subsequent alkylation with [Et3O][BF4].30 The first structurally characterized vanadium silylene complex has been synthesized by a reaction of [(Z5-Cp)V(CO)4] with dichlorosilylene stabilized with an N-heterocyclic carbene. In contrast to the reactions with typical Lewis bases (e.g., phosphines), the vanadium precursor does not need photochemical activation and proceeds under mild thermal conditions (Scheme 3).33 A similar synthetic strategy has been used for assembly of a half-sandwich compound bearing a stable N-heterocyclic chlorosilylene. It is formed at room temperature from [(Z5-Cp)V(CO)4] and the corresponding chlorosilylene (Scheme 3).34 The thermally stable vanadium phosphanylidene complexes [(Z5-Cp)V(CO)3(]PNR2)] (R ¼ iPr, Cy) are accessible by a reaction of [Na]2[(Z5-Cp)V(CO)3] with Cl2PNR2 (Scheme 4).35 It is noteworthy that the starting [Na]2[(Z5-Cp)V(CO)3] is

Cyclic and Non-Cyclic Pi Complexes of Vanadium

5

generated by a titration of sodium dispersion with a solution of [(Z5-Cp)V(CO)4] or through a reduction of [(Z5-Cp)V(CO)4] with sodium amalgam.36 Interestingly, a reaction with Cl2PMes gives the vanadium diphosphene complex [{(Z5-Cp)V(CO)3}2{m(PMes)2}]37 while more sterically demanding Cl2PC6Ht2Bu3 affords the dinuclear phosphanylidene complex [{(Z5-Cp)V (CO)2}2(m-PC6Ht2Bu3)].38 The electrophilic nature of the [(Z5-Cp)V(CO)3(]PNiPr2)] is demonstrated in reactions with unsaturated substrates (Scheme 4). Indeed, treatment with diphenylethyne gives the phosphirene. Stabilization of the parent phosphanylidene complex [(Z5-Cp)V(CO)3(]PNiPr2)] can be achieved by an N-heterocyclic carbene. Complexes bearing Z2-coordinated phosphaimines, serving as four electron donors, are formed by reactions with Ph2C]N]N and PhN3. It should be noted that all crucial structure types, given in Scheme 4, have been documented by XRD analysis.35 The bonding situation in phosphanylidene complexes has been clarified by DFT calculations.39 More details about reactivity of [(Z5-Cp0 )V(CO)4] at metal center is given in COMC I, COMC II and COMC III.

Scheme 4 Synthesis and reactivity of vanadium phosphanylidene complexes.

5.02.2.1.1.3 Photochemical activation of SidH and SndH bond Oxidative addition of triethylsilane and tributylstannane to [(Z5-Cp)V(CO)3], photochemically generated from [(Z5-Cp)V(CO)4], has been investigated by ultrafast infrared spectroscopy.40,41 Quantum-chemical calculations suggested that [(Z5-Cp)V(CO)3] stays either in the singlet or the triplet state. The triplet intermediate plays a key role in oxidative addition of silanes to produce [(Z5-Cp)V(CO)3H(SiMe3)] while the singlet counterpart gives solely weak adducts with the alkyl substituent of the silane or with hydrocarbon solvent.40,42 It should be noted that this type of adduct is experimentally suggested by photoacoustic calorimetry43 and femtosecond infrared spectroscopy.40 The triplet species, 3[(Z5-Cp)V(CO)3], is also primarily responsible for activation of the stannane SndH bond in the picosecond time-scale.41 5.02.2.1.1.4 Applications Application of [(Z5-Cp)V(CO)4] in organic synthesis has been scrutinized in COMC III. Recently, it was reported that radical cyclization of various alkyl iodides is catalyzed by [(Z5-Cp)V(CO)3H]− under H2 and in the presence of a sacrificial base.44 The catalytically active species is generated by reduction of [(Z5-Cp)V(CO)4] followed by hydrolysis.36 Its molecular structure has been determined by XRD analysis.45 Free energies of H+/H/H−/e− transfer from [(Z5-Cp)V(CO)3H]−, necessary for understanding the driving force of the H transfer, have been estimated by experimental solution-phase techniques and quantum chemical calculations.45 [(Z5-Cp)V(CO)4] is also used as a catalyst for photochemical dehydro-coupling of amine-borane adducts. Variation of amine-borane substrates yields monomeric, dimeric or polymeric products.46

5.02.2.1.2

Polynuclear carbonyl complexes

Photochemical decarbonylation of [(Z5-Cp)V(CO)4] gives dinuclear [(Z5-Cp)2V2(CO)5] as the main isolable product.47 X-ray crystallographic studies have revealed its molecular structure has two semibridging carbonyl ligands and a short vanadiumto-vanadium bond distance (2.46 A˚ ).48,49 Theoretical DFT studies indicate the presence of formal V^V triple bond with each vanadium atom in an 18-electron configuration (Scheme 5). These studies ruled out the suggested Z2-m bridging mode of virtually 4-electron donor CO ligands, sometimes appearing in literature, as both CO ligands serve as the usual 2-electron donors.50 This

6

Cyclic and Non-Cyclic Pi Complexes of Vanadium

formulation is consistent with the absence of CO stretching modes at very low frequencies.48,50 Photochemical reaction of [(Z5-Cp)2V2(CO)5] with PPh3 leads to substitution of one terminal carbonyl ligand as confirmed by XRD analysis (Scheme 5). NMR and infrared spectroscopic investigations do not indicate appearance of any other isomer of [(Z5-Cp)2V2(CO)4(PPh3)].49

Scheme 5 Reaction of [(Z5-Cp)2V2(CO)5] with PPh3.

A theoretical investigation rationalized the absence of expected byproducts ([(Z5-Cp)2V2(CO)n] (n ¼ 1–4),50 [(Z5-Cp)3V3(CO)9] and [(Z5-Cp)4V4(CO)4])51 from photochemical cleavage of [(Z5-Cp)V(CO)4] and pyrolysis of [(Z5-Cp)2V2(CO)5].52 Thermodynamic studies have also clarified the low stability of suggested intermediates upon [(Z5-Cp)2V2(CO)5] formation, namely complexes with the formula [(Z5-Cp)2V2(CO)n] (n ¼ 6–7).53 Computational study of thermodynamic stability on mixed carbonyl/thiocarbonyl complexes with different formulas predicts formation of a stable [(Z5-Cp)2V2(CS)2(CO)2] bearing two four-electron donor bridging CS groups.54 More details about polynuclear carbonyl complexes is given in COMC I and COMC II.

5.02.2.2 5.02.2.2.1

Mono(h5-cyclopentadienyl) complexes of vanadium(II) and vanadium(III) Halogenido complexes

Cyclopentadienyl halide complexes serve as precursors for a wide variety of vanadium(II) and vanadium(III) compounds. A detailed overview of the synthetic procedures for cyclopentadienyl halides of vanadium(II) and vanadium(III), including ring-substituted complexes, is given in the previous editions of COMC. Briefly, monomeric vanadium(III) dihalides of the type [(Z5-Cp)VX2(PR3)2] (X ¼ Cl, Br; R ¼ Me, Et) and their analogues bearing the methylcyclopentadienyl ligand Z5-C5H4Me are accessible from in situ prepared VX3 ∙2PR3 and NaCp0 or other reactive main group metal cyclopentadienyl compounds (e.g., TlCp, Cp2Mg, CpSnBu3); see Scheme 6.55,56 The X-ray crystallographic studies revealed their four-legged piano stool structure and suggest formation of the VX2P2 moiety solely in the trans-configuration.55–57 The only reported analogue appearing in cis-configuration is the iodido complex supported by a bidentate phosphine ligand in [(Z5-Cp)VI2{k2-(Ph2PCH2)2NMe}]. This complex has been synthesized by a different pathway starting from [(Z5-Cp)VI2] and the given phosphine ligand.58

Scheme 6 Convenient syntheses of chlorido complexes.

It is noteworthy that the general protocol giving [(Z5-Cp0 )VX2(PR3)2] is not suitable for synthesis of pentamethylcyclopentadienyl compounds (Cp ) and for complexes bearing more sterically demanding or bidentate phosphines (e.g., PPh3, PMePh2, PMe2Ph, dppe, dmpe).55 Complexes bearing cyclopentadienyl ligands with nitrogen donor atom in the side chain [(Z5-C5H4R) VCl2(PMe3)2] (R ¼ (CH2)2NMe2, (CH2)3NMe2, (CH2)2N(CH2)5, CH(CH2CH2)2NMe) are accessible from VCl3 ∙ 2PMe3 and NaC5H4R.57 Closely related compounds with hemilabile intramolecular N–V coordination [(Z5:kN-C5H4R)VCl2(PMe3)] (R ¼ (CH2)2NMe2, (CH2)2NHiPr) were synthesized by a very similar procedure. It is noteworthy that both structure types have been structurally characterized by XRD analysis.59,60 In particular cases, the use of a phosphine stabilizing ligand is not necessary as documented on species bearing an indenyl ligand with intramolecularly coordinated N-heterocyclic carbene [{Z5:kC-4,6-Me2-2-(CH2CH2NHC)C9H4}VCl2] (NHC ¼ C3H2N2DiPP); see Scheme 7. This compound has been synthesized by a reaction of functionalized potassium indenide with VCl3 ∙3thf.61 Related [(Z5:kC-C9H6CH2CH2NHC)VBr(NMe2)] is accessible by aminolysis of V(NMe2)4 by indene functionalized with a side arm derived from a sterically demanding imidazolium salt. In this case, the reaction is accompanied with reduction of VIV to VIII.62 According to XRD analysis, both compounds adopt a three-legged piano stool structure protected by 2,6-diisopropylphenyl substituent (DiPP).61,62

Cyclic and Non-Cyclic Pi Complexes of Vanadium

7

Scheme 7 Synthesis of indenyl compounds stabilized by intramolecular coordination.

A similar three-legged piano stool structure has been implied for the cyclopentadienyl vanadium(III) compound [(Z5-Cp) VCl2{C3H2N2(Mes)2}] bearing a coordinated N-heterocyclic carbene decorated with two bulky mesityl groups. This compound, accessible by a reaction of [(Z5-Cp)2VCl] with {C3H2N2(Mes)2}∙HCl, is highly sensitive to both air and moisture. Addition of stoichiometric amounts of HCl ∙dioxane gives the anionic trichlorido species [(Z5-Cp)VCl3]− compensated with imidazolium.63 A stable iodido complex [(Z5-Cp)VI2(thf )] is reported as the oxidation product formed upon treatment of the triple-decker compound [{(Z5-Cp)V}2(m-Z6:Z6-C6H3R3)] (R ¼ H, Me) with iodine. Its monomeric structure has been elucidated from cryoscopic measurements.64 Oligomeric complexes [{(Z5-C5Me4R)V}n(m-X)2n] (R ¼ Me, Et; X ¼ Cl, Br) bearing a pentaalkylcyclopentadienyl ring are prepared by a reduction of [(Z5-C5Me4R)VX4] with sodium amalgam or more conveniently by a reaction of (C5Me4R)SnBu3 with VX3 ∙ 3L (X ¼ Cl, Br; L ¼ thf, tetrahydrothiophene).65,66 X-ray crystallographic studies indicate appearance of trimeric chlorides [{(Z5-C5Me4R)V}3(m-Cl)6] (R ¼ Me, Et)65–67 and dimeric bromide [{(Z5-C5Me4Et)V}2(m-Br)4],65 which is consistent with magnetic behavior of the compounds.65,67 The tetrameric fluorido complex [{(Z5-Cp )V}4(m-F)8] has been prepared by a reaction of aforementioned trimeric chloride with a convenient fluorination agent Me3SnF and structurally characterized by XRD analysis.68 A series of dimeric vanadium(II) halides [{(Z5-Cp0 )V(PR3)}2(m-X)2] (Cp0 ¼ Cp, C5H4Me; R ¼ Me, Et; X ¼ Cl, Br) has been prepared by one-electron reduction of dihalides [(Z5-Cp0 )VX2(PR3)2] with zinc or preferably with aluminum powder.22 Structure of one representative, [{(Z5-Cp)V(PEt3)}2(m-Cl)2], has been elucidated by X-ray diffraction analysis.22,69 This approach has been used successfully for the synthesis of the vanadium(II) species [({Z5:kN-C5H4(CH2)2NMe2}V)2(m-Cl)2] containing an intramolecularly coordinated amine arm. It is given by a reduction of [{Z5:kN-C5H4(CH2)2NMe2}VCl2(PMe3)] with sodium amalgam. The appearance of phosphine-free compound has been determined by NMR spectroscopy. XRD analysis has revealed a puckered V2Cl2 core with the Cp0 ligands in a cis-arrangement70 resembling aforementioned [{(Z5-Cp)V(PEt3)}2(m-Cl)2].22 Monomeric species, [(Z5-Cp)VX(PR3)2] (X ¼ Cl, Br; R ¼ Me, Et) and [(Z5-Cp)VCl(k2-P,PL)] (P,PL ¼ dppe, dmpe), have been obtained from dimers [{(Z5-Cp0 )V(PR3)}2(m-X)2] after addition of monodentate or bidentate phosphine ligands, respectively (Scheme 6).22 XRD structure data are available for monomeric iodides [(Z5-Cp)VI{k2-(R2PCH2)2NR0 }] (R ¼ Ph, R0 ¼ Me; R ¼ Et, R0 ¼ Ph; R ¼ Et, R0 ¼ Et) and [(Z5-Cp)VI{k2-(PhP)2(CH2NPhCH2)2}]. They have been synthesized by a reduction of the corresponding diiodides by zinc powder. These species adopt the expected three-legged piano stool structure with the pendant amino groups in the second coordination sphere.58 Vanadium(II) chlorido complex [{(Z5-Cp)V(thf )}2(m-Cl)2] has been prepared by oxidative cleavage of mixed-ring complex [(Z5-Cp)(Z6-C10H8)] with 1,2-dichloroethane in the presence of thf (Scheme 6).71 This compound has great synthetic potential due to a relatively low affinity of the low-valent vanadium for thf. It is known as an effective CpV transfer reagent and its structure has been recently elucidated by XRD analysis. Unlike the PEt3 analogue, it contains a planar V2Cl2 core with the Cp ligands in a transarrangement.72

5.02.2.2.2

Carboxylato and phosphato complexes

Dimeric vanadium(III) carboxylates [{(Z5-Cp0 )V}2(m-OOCR)4] are conveniently prepared by treatment of [(Z5-Cp0 )V(CO)4] or [(Z5-Cp0 )2V] with the corresponding carboxylic acid.73,74 Isostructural N,N-dialkylcarbamate derivatives [{(Z5-Cp0 ) V}2(m-OOCNEt2)4] are obtained by a reaction of [(Z5-Cp)2V] with [Ti(OOCNR2)4].75 The following species were structurally characterized by XRD analysis: [{(Z5-Cp)V}2(m-OOCR)4] (R ¼ CF3,76 Ph,74 2-furyl,77 NEt2,75 cymantrenyl),78 [{(Z5-Cp ) V}2(m-OOCPh)4],74 [{(Z5-C5HPh4)V}2(m-OOCCF3)4].79 Structurally related species bearing four diphenylphosphinate bridges [{(Z5-Cp)V}2(m-OOPPh2)4] is formed upon a reaction of [(Z5-Cp)2VMe] with 2 equiv. of Ph2P(O)OH. Mononuclear dianionic compound [(Z5-Cp)V(k4-P4O12)]2− appears upon a treatment of [(Z5-Cp)2VMe] with dihydrogen tetrametaphosphate acid.80 More details about carboxylato complexes is given in COMC I, COMC II and COMC III.

5.02.2.2.3

Complexes with S-, Se- and Te-donors

Photochemical decarbonylation of [(Z5-Cp0 )V(CO)4] (Cp0 ¼ Cp, Cp ) in the presence of diorganodichalcogenide REER (E ¼ S, Se, Te; R ¼ Me, Ph, Fc) leads to diamagnetic dinuclear species [{(Z5-Cp0 )V(CO)2}2(m-ER)2] with antiferromagnetically coupled vanadium(II) centers.81,82 XRD structures of [{(Z5-Cp )V(CO)2}2(m-SR)2] (R ¼ Me, Ph)81 and [{(Z5-Cp0 )V(CO)2}2(m-TeR)2] (Cp0 ¼ Cp, R ¼ Me; Cp0 ¼ Cp , R ¼ Ph)82 show a strongly puckered V2E2 core with Cp0 ligands in cis-arrangement. Long

8

Cyclic and Non-Cyclic Pi Complexes of Vanadium

vanadium-to-vanadium distances imply absence of a direct VdV single bond in these compounds. An excess of REER induces a full decarbonylation to afford paramagnetic vanadium(III) compounds [{(Z5-Cp0 )V}2(m-ER)4].81,82 Structurally related species [{(Z5-Cp)V}2{m-k2:k2-(SCH2)2}2]83 and [{(Z5-Cp)V}2{m-k2:k2-S2(C6H4)}2]84 are formed at ordinary thermal conditions by a reaction of [(Z5-Cp)V(CO)4] with ethane-1,2-dithiol and benzene-1,2-dithiol, respectively. Vanadium(III) compound [{(Z5-Cp) V}2(m-Se)(m-SePh)2] appears as a side product upon a reduction of [(Z5-Cp)2VCl2] with LiBH4
thf in the presence of PhSeSePh.85 More details about chalcogenido complexes is given in COMC I, COMC II and COMC III.

5.02.2.2.4

Complexes bearing anionic N-donor ligands

Amidinate complex [(Z5-Cp )V{k2-(NiPr)2CMe}Cl] has been obtained by a reaction of chlorido precursor [{(Z5-Cp )V}3(m-Cl)6] with corresponding lithium amidinate. One-electron reduction with sodium amalgam, done under nitrogen atmosphere, has produced a paramagnetic dinuclear d3-d3 vanadium dinitrogen-bridged compound [((Z5-Cp )V{k2-(NiPr)2CMe})2(m-kN,kN0 -N2)] that is unusually thermally robust in solution up to 90 C (Scheme 8).86 Structural data, obtained from XRD analysis, suggest appearance of a ground-state electronic configuration incompatible to achieving N^N triple bond cleavage under mild thermal conditions.86,87 Reaction of the dinitrogen complex with 2,6-Me2C6H3NC led to structurally characterized isocyanide [(Z5-Cp )V {k2-(NiPr)2CMe}(CNC6H3Me2)2].86

Scheme 8 Synthesis and reactivity of dinuclear dinitrogen-bridged complex.

Reaction of vanadocene with 1,2-phenylene diimide (N,NL) gives the unusual anionic dinuclear vanadium(III) complex [Li(thf )4][{(Z5-Cp)V}{V(N,NL)}(m-N,NL)2] (Scheme 9). XRD analysis of this diamagnetic compound revealed a short vanadiumto-vanadium bond distance (2.3742(9) A˚ ) suggesting the presence of V]V double bond between d2-vanadium(III) centers. Nevertheless, quantum-chemical calculations indicate the presence of a direct VdV single bond rather than the double bond. The proximity of metal centers seems to be a result of geometrical constrains induced by the rigid bridging ligands.88

Scheme 9 Synthesis of dinuclear complex supported by 1,2-phenylene diimido ligands.

A convenient route to imidovanadium(III) compounds [(Z5-Cp)V(]NR)(PMe3)2] (R ¼ DiPP, tBu) uses a reduction of chlorides [(Z5-Cp)V(]NR)(Cl)2] by magnesium in the presence PMe3.89,90 An alternative method, used for synthesis of [(Z5-Cp)V(]NR) (PMe3)2] and [(Z5-Cp)V(]NR)(k2-dmpe)] (R ¼ p-Tol, tBu), involves a thermolysis of vanadium(V) precursors [(Z5-Cp)V(]NR) (Me)(NiPr2)] in the presence of the corresponding phosphine.91 A structurally related imido complex bearing a 1-isoindolenine substituent was obtained from benzyne compound [(Z5-Cp)V(PMe3)2(Z2-C6H4)] by a double insertion of tBuCN.92 Imidovanadium(III) complex [(Z5-Cp)V{]NC(tBu)]CHtBu}(k2-dmpe)] is formed by a reaction of alkylidene complex [(Z5-Cp)V(]CHtBu)(k2-dmpe)] with tBuCN.93 The short vanadium-to-nitrogen bond distance (1.70 A˚ ) in these vanadium(III) imido compounds is indicative for a V]N multiple bond.89,91–93 Cyclopentadienyl vanadium(II) compound [(Z5-Cp)V(k3-Tp)] bearing tripodal tridentate hydridotris(pyrazolyl)borate ligand, often presented as an analogue of Cp, is formed upon a reduction of [(Z5-Cp)2VCl] by the action of KTp. It is noteworthy that Tp seems to be a sufficiently strong ligand to replace both cyclopentadienyl rings as evidenced on the reaction with [(Z5-Cp)2V]. [(Z5-Cp)V(k3-Tp)] is a high-spin d3-compound that can be oxidized to [(Z5-Cp)V(k3-Tp)]+ by the action of ferrocenium. The vanadium(III) product is paramagnetic and forms adducts with common Lewis bases as documented on the reactions with MeCN,

Cyclic and Non-Cyclic Pi Complexes of Vanadium

9

PMe3 and CNtBu. Formulation of the species [(Z5-Cp)V(k3-Tp)][BAr4], [(Z5-Cp)V(PMe3)(k3-Tp)][BAr4] (Ar ¼ C6H3(CF3)2-3,5) and [(Z5-Cp)V(MeCN)(k3-Tp)][PF6] is confirmed by XRD analysis.94 The electronic structure of the Tp complexes has been elucidated from magnetic susceptibility measurements, spectroscopic data and quantum-chemical calculations.95 Vanadium(III) complex of dianionic tetradentate ligand dibenzotetramethyltetraaza[14]annulene, also known as Goedken’s macrocycle, [(Z5-Cp)V(k4-tmtaa)] is given by a reaction of [(Z5-Cp)2V] with H2tmtaa under rigorously anaerobic conditions. XRD analysis confirmed its molecular structure with vanadium atom displaced 0.875(1) A˚ out of the N4-plane of the saddle-shaped tmtaa.96 More details about complexes bearing anionic N-donors is given in COMC I and COMC III.

5.02.2.2.5

Hydrido, alkyl, alkynyl, alkylidene and silyl derivatives

Dinuclear hydrido compounds [{(Z5-Cp)V(k2-dmpe)}2(m-H)2] and [{(Z5-Cp)VH(k2-dmpe)}2(m-dmpe)2] are given by a reaction of the alkyl vanadium(II) complex [(Z5-Cp)V(nPr)(k2-dmpe)] with molecular hydrogen. The hydrido-bridged species, [{(Z5-Cp)V(k2-dmpe)}2(m-H)2], is considered to consist of two low-spin d3-centers connected by VdV single bond. Such interpretation is consistent with observed diamagnetic behavior and a long vanadium-to-vanadium distance elucidated by XRD analysis (2.701 (1) A˚ ). The species with terminal hydrido ligands [{(Z5-Cp)VH(k2-dmpe)}2(m-dmpe)2] is EPR active as the vanadium atoms achieve a low-spin d3-configuration.97 Cyclopentadienyl vanadium(II) complexes bearing s-bonded alkyls, [(Z5-Cp)VR(k2-dmpe)] (R ¼ Me, nPr, Ph), and their vanadium(III) congeners, [(Z5-Cp)VR2(PMe3)2] (R ¼ Me, Ph), have been prepared by alkylation of halogenido analogues by common alkylation reagents (e.g., LiR, RMgBr).98 A similar route has been used for synthesis of alkynyl complexes.92 Formulation of [(Z5-Cp)VMe(k2-dmpe)],98 [(Z5-Cp)VCl(kC-C^CPh)(PMe3)2] and [(Z5-Cp)V(kC-C^CPh)2(PMe3)2]92 was confirmed by XRD analysis. The increasing size of alkyl substituents stabilizes strongly electron-deficient vanadium(III) compounds such as 14-electron species [(Z5-Cp)V(CH2CMe2R)2(PMe3)] (R ¼ Me, Ph) and phosphine-free 12-electron species [(Z5-Cp)V {CH(SiMe3)2}2].93 Vanadium(III) species with a pendant amine arm, [{Z5-C5H4(CH2)2NMe2}VMe2(PMe3)2], has been obtained from chlorido precursor having intramolecularly coordinated amine arm [{Z5:kN-C5H4(CH2)2NMe2}VCl2(PMe3)] by methylation, preferably with added PMe3. Reaction of [{Z5-C5H4(CH2)2NMe2}VMe2(PMe3)2] with 1 equiv. of Brønsted acid [PhNMe2H][BPh4] produces unexpected cationic species [{Z5:Z2-C5H4(CH2)2N(Me)CH2}V(PMe3)2][BPh4] with intramolecularly coordinated Z2-NCH2, which is formed from the NMe2 moiety by deprotonation of one methyl group. The reaction is accompanied with evolution of 2 equiv. of CH4 as confirmed by experiments using a Toepler pump.59 The neopentyl vanadium(III) compound [(Z5-Cp)VNp2(PMe3)] was found to be a suitable precursor for the Schrock-type alkylidene complex [(Z5-Cp)V]CHtBu(dmpe)]. It is formed by thermolysis in the presence of dmpe.93 A short vanadium-tocarbon bond distance (1.809(3) A˚ ), elucidated from XRD analysis, reveals the appearance of a V]C multiple bond indicative of Schrock-type carbenes. The vanadium(III) compound bearing neophyl ligand [(Z5-Cp)V(CH2CMe2Ph)2(PMe3)] decomposes at room temperature to give a dinuclear species [{(Z5-Cp)V}2(m-k2:k2-C6H4CMe2CH2)2], in which four carbon atoms of the two ortho-metallated neophyl ligands serve as bridges between vanadium atoms. The vanadium-to-vanadium distance 2.313(2) A˚ indicate the presence of VdV single bond.93 Structurally related species bearing butane-1,4-diyl bridging ligands [{(Z5-Cp) V}2{m-k2:k2-(CH2)4}2] has been synthesized by a reaction of [(Z5-Cp)V(Z6-C10H8)] with ethylene.71 Mononuclear vanadium species [(Z5-Cp)V(k2-C6H4CMe2CH2)(PMe3)2] appears as the ortho-metallation product of [(Z5-Cp)V(CH2CMe2Ph)2(PMe3)] when treated with PMe3.93 Vanadium(II) complexes bearing silyl ligands [(Z5-Cp)V(k2-dmpe)(SiH2Mes)] and [(Z5-Cp)V(k2-dmpe)(SiHPh2)] are formed upon a reaction of [(Z5-Cp)V(k2-dmpe)Me] with the corresponding primary and secondary silane, respectively. Both compounds are paramagnetic due to high-spin d3-configuration. Tertiary silanes (e.g., Et3SiH and Ph3SiH) are not reactive enough to undergo the reaction under mild conditions while chlorosilane Ph2SiHCl gives trivial [(Z5-Cp)V(k2-dmpe)Cl]. Nevertheless, the silyl complex [(Z5-Cp)V(k2-dmpe)(SiHPh2)] can be easily converted to the chlorosilyl analogue [(Z5-Cp)V(k2-dmpe)(SiClPh2)] upon silyl group exchange using Ph2SiHCl (Scheme 10). The one-electron oxidation of this product with Ph3CCl gives chlorido complex [(Z5-Cp)V(k2-dmpe)Cl(SiClPh2)] that is a suggested precursor to vanadium silylene species. It is noteworthy that formulation of crucial species, [(Z5-Cp)V(k2-dmpe)(SiH2Mes)], [(Z5-Cp)V(k2-dmpe)(SiHPh2)] and [(Z5-Cp)V(k2-dmpe)Cl(SiClPh2)], is supported by XRD data.23 More details about this group of complexes is given in COMC I, COMC II and COMC III.

Scheme 10 Synthesis of stabilized vanadium(II) silyl complexes.

10

Cyclic and Non-Cyclic Pi Complexes of Vanadium

5.02.2.2.6

Boron-containing complexes

Structurally characterized borohydrido vanadium(II) compound [(Z5-Cp)V(k2-dmpe)(k2-BH4)] has been prepared by a reaction of [(Z5-Cp)VCl(k2-dmpe)] with LiBH4. The compound is a low-spin d3-complex as elucidated from measurements of magnetic susceptibility and electron paramagnetic resonance spectroscopy.98 A similar molecular structure motif is suggested for [(Z5-Cp)V(kC-C^CPh)(PMe3)(k2-BH4)] based on spectroscopic evidence.92 A reaction of [{(Z5-Cp )V}3(m-Cl)6] with excess NaB3H8 gives monomeric vanadium(III) compound bearing two borohydrido ligands [(Z5-Cp )V(k2-B3H8)2]. As revealed by XRD analysis, both B3H8 ligands are coordinated to the vanadium atom in the same way via two vicinal hydrogen atoms.99 Air-stable tricarbadecaboranyl vanadium(III) compound [(Z5-Cp)VBr(Z6-C3B7H9Ph)] is formed upon a reduction of [(Z5-Cp)VBr3] with Li[(6-Ph-nido-5,6,9-C3B7H9)].100 Reaction of [(Z5-Cp)2VCl2] with LiBH4
thf followed by thermolysis in the presence of monoborane reagent BH3
thf gives the symmetrical divanadaborane species bearing two B2 H6 2 − units [{(Z5-Cp)V}2(m-B2H6)2] along with its congener [{(Z5-Cp) V}2(m-B5H11)] and the cluster [{(Z5-Cp)V}2(m-B2H5)(m-BH3OEt)] with an oxygen atom incorporated in the cage (Scheme 11).101,102 According to XRD analysis, the vanadium-to-vanadium bond distance in the parent cluster [{(Z5-Cp)V}2(m-B2H6)2] is 2.787(2) A˚ .101 The modified protocol involving thermolysis in the presence of 2-mercaptobenzothiazole and PhTeTePh has been used for controlled synthesis of derivatives with incorporated sulfur atoms [{(Z5-Cp)V}2(m-BH3S)2] and tellurium atoms [{(Z5-Cp) V}2(m-BHTe)(m-TePh)2]∙thf, respectively.102 Chalcogen-stabilized divanadaboranes [{(Z5-Cp)V}2(m-BH3E)(m-EPh)2] (E ¼ S, Se) and [{(Z5-Cp)V}2(m-BHSe)(m-SePh)2]∙thf are accessible by an alternative route starting from [(Z5-Cp)2VCl2] and Li[BH3EPh] (E ¼ S, Se).103 Decoration of the divanadaborane cage in [{(Z5-Cp)V}2(m-B2H6)2] has been achieved by a treatment with PhSSPh and BzSeSeBz. Monosubstituted product, [{(Z5-Cp)V}2(m-B2H6)(m-B2H5SeBz)], has been isolated when starting from diselenide while di- and trisubstituted derivatives, [{(Z5-Cp)V}2(m-B2H5SPh)2] and [{(Z5-Cp)V}2{m-B2H4(SPh)2}(m-B2H5SPh)], are isolated from the reaction with disulfide.104 A reaction of [{(Z5-Cp )V}3(m-Cl)6] with LiBH4
thf followed by heating with BH3
thf gives the ring-substitute derivative [{(Z5-Cp )V}2(m-B2H6)2]. Cage-substituted derivatives [{(Z5-Cp )V}2(m-B2H4Cl2)2] and [{(Z5-Cp ) V}2{m-B2H4(SePh)2}2] are formed upon treatment with CCl4 and PhSeSePh, respectively. In both cases, the tetrasubstituted compounds have been characterized by XRD analysis together with products of a partial substitution (Scheme 11).105 More details about the borane and carbaborane complexes is given in COMC III.

Scheme 11 Schematic view of divanadaboranes characterized by XRD analysis (hydrogens on boron atoms are omitted for clarity).

5.02.2.3 5.02.2.3.1

Mono(h5-cyclopentadienyl) complexes of vanadium(IV) and vanadium(V) Halogenido complexes

Cyclopentadienyl vanadium(IV) trihalides are 13-electron complexes serving as strong oxidizing reagents. Detailed overview of the synthetic procedures for this type of compounds is given in COMC I, COMC II and COMC III. Briefly, the trihalides [(Z5-Cp0 )VX3] (Cp0 ¼ Cp, C5H4Me, Cp , C5Me4Et; X ¼ Cl, Br, I) are formed upon oxidation of [(Z5-Cp0 )V(CO)4] with halogens.106,107 More conveniently, chlorides [(Z5-Cp0 )VCl3] (Cp0 ¼ Cp, C5H4Me) are synthesized by reaction of [(Z5-Cp0 )2VCl2] with SOCl2.108,109 Conversion of [(Z5-Cp)VCl3] to the corresponding bromide is conveniently provided by the action of BBr3.110 Pentaalkyl-substituted derivatives [(Z5-C5Me4R)VX3] (Cp0 ¼ Me, Et; X ¼ Cl, Br) are prepared by halogenation of [{(Z5-C5Me4R)

Cyclic and Non-Cyclic Pi Complexes of Vanadium

11

V}3(m-Cl)6] and [{(Z5-C5Me4Et)V}2(m-Br)4].65 XRD structural data are currently available for [(Z5-C5H4Me)VCl3],109 [(Z5-C5Me4Et)VCl3]106 and [(Z5-Cp)VI3].107 Strong oxidizing properties of [(Z5-Cp0 )VX3] have been documented by electrochemical studies.107,109,111 Bromido complex [(Z5-Cp)VBr3] is capable of oxidizing ferrocene to thermally sensitive [(Z5-Cp)2Fe][(Z5-Cp)VBr3].107 Oxidation of tetramethyltetrathiafulvalene (TmTTF) with [(Z5-C5Me4Et)VCl3] gives the charge-transfer salt [TmTTF][(Z5-C5Me4Et)VCl3].111 Vanadium(IV) species [{Z5:kN-C5H4(CH2)2NiPr}VCl2] with intramolecularly bonded amide is accessible by two-electron oxidation of the Z4-diene compound [{Z5:kN-C5H4(CH2)2NiPr}V(Z4-CH]CMeCMe]CH)] with one equivalent of PhI ∙Cl2. The complex is considered a 15-electron species due to sp2 hybridization of the N atom with a lone pair giving a N(pp) ! V(dp) donation as elucidated from planar geometry on the N atom and a short vanadium-to-nitrogen bond distance (1.831(1) A˚ ).60

5.02.2.3.2

Oxido complexes

A variety of synthetic protocols have been developed for mononuclear vanadium(V) oxido compounds [(Z5-Cp0 )VOCl2] (Cp0 ¼ Cp, C5H4Me, Cp ), obvious precursors of monocyclopentadienyl vanadium(V) species, as comprehensively overviewed in the previous editions of COMC; COMC II and COMC III. The procedures are usually based on oxidation or oxidation/chlorination reactions starting from [(Z5-Cp0 )V(CO)4], [(Z5-Cp0 )VCl3], [(Z5-Cp0 )2V] or [(Z5-Cp0 )2VCl2]. Nevertheless, the protocols often suffer from low yields, appearance of polynuclear side products or poor reproducibility. The most convenient route to [(Z5-Cp)VOCl2] utilizes an oxidative decarbonylation of [(Z5-Cp)V(CO)4] by SOCl2112 while the pentamethyl analogue is readily available by oxidation of trimeric vanadium(III) chloride [{(Z5-Cp )V}3(m-Cl)6] by the action of O2.113 XRD data documents molecular structures of various species containing oxido bridges. Structurally characterized dimers involve vanadium(IV) iodide [{(Z5-Cp )VI2}2(m-O)],114 vanadium(V) oxides [{(Z5-Cp )VOX}2(m-O)] (X ¼ Cl, N3),115,116 alcoholate [{(Z5-Cp )VO}2(m-O){m-(OCH2)2}]117 and the charge-transfer salt [TmTTF][{(Z5-C5H4Me)VOCl2}2(m-O)].111 Tetranuclear species appear as the adamantane-like structure [{(Z5-Cp)V}4(m-O)6]118 or as cyclic tetramers evidenced in the case of compounds bearing terminal chlorides [{(Z5-Cp )VCl}4(m-O)4],114 oxides [{(Z5-Cp )VO}4(m-O)4]119 or nitrosyl ligands [{(Z5-Cp)V (NO)}2{(Z5-Cp)VI}2(m-O)4].120 Pentanuclear compound [{(Z5-Cp)V}5(m3-O)6]121 and its hexanuclear derivative bearing one terminal oxido ligand [{(Z5-Cp)V}5(VO)(m3-O)8] have a polyhedral structure.122 Several congeners of the later compound have been described. They consist of two [{(Z5-Cp)V}5(V)(m3-O)8] clusters linked by an O, (Z5-Cp)VO2(NMe3)2 or {(Z5-Cp) V}4(m-O)8 bridge.122,123 More details about oxido complexes is given in COMC I, COMC II and COMC III.

5.02.2.3.3

Imido complexes

Imidovanadium(V) complexes bearing halido ligands [(Z5-Cp)V(]NR)Cl2] (R ¼ tBu, p-Tol, DiPP) and their congeners with alkyl substituted Cp ring are prepared by a reaction of [RN]VCl3] with LiCp0 or Cp0 SiMe3.89,124–127 Indenyl derivatives [(Z5-Ind)V(] NtBu)X2] are accessible by a similar route starting from LiInd and [RN]VX3] (X ¼ Cl, Br).128 Compounds bearing aromatic imines are also accessible from [(Z5-Cp)V(]NtBu)Cl2] by imine exchange.129,130 This pathway has been utilized for attachment of the imidovanadium(V) species on a polystyrene-based support.131 The tert-butyl- or arylimido ligands are able to stabilize monomeric cyclopentadienyl vanadium(V) species more effectively than the aforementioned terminal oxido ligand, which is documented on a variety of mixed chlorido/alkoxido and chlorido/amido compounds synthesized by simple chloride exchange from [(Z5-Cp0 )V(] NR)Cl2].124,132,133 An alternative and more convenient route to these mixed-ligand species involves a reaction of [RN]VXCl2] with Cp0 Li.128,134–136 The amido/chlorido derivatives were found to be suitable precursors for mixed amido/alkyl, amido/silyl, amido/ germyl, amido/stannyl and amido/plumbyl compounds.134,137 Thermolysis of the mixed amido/methyl complexes [(Z5-Cp)V(] NR)(NiPr2)Me] (R ¼ tBu, p-Tol) has been scrutinized in the presence or absence of ancillary stabilizing ligands.91 Treatment of [(Z5-Cp0 )V(]NR)Cl2] with common alkylation reagents does not give expected dialkyl derivatives but leads to reduction or decomposition. Dimeric species [{(Z5-Cp)VMe}2{m-N(p-Tol)}2] bearing bridging imides has been prepared by treating the chlorido precursor with AlMe3. This compound has been characterized by XRD analysis together with its chlorido analogue obtained upon treatment with HCl/CHCl3.125 Structurally related alkoxide [{(Z5-Cp)VOtBu}2(m-NtBu)2] is formed upon a reduction of mixed chlorido/alkoxido precursor [(Z5-Cp)V(]NtBu)Cl(OtBu)] by tBuLi. Treatment of [(Z5-Cp)V(]NtBu)Cl2] with 1 equiv. of tBuLi gives the chloride-bridged dimer [{(Z5-Cp)V(]NtBu)}2(m-Cl)2].138 Reduction of the vanadium(V) complex bearing sterically-demanding imine [(Z5-Cp)V{]N(DiPP)}Cl2] with MeMgCl gives a mixed-valent VIII/VIV dimer [{(Z5-Cp)V}2{mN(DiPP)}2(m-Me)].129 Reaction of [(Z5-Cp)V{]N(p-Tol)}(NiPr2)Me] with Lewis acids (e.g., B(C6F5)3 and [Ph3C][B(C6F5)4]) or Brønsted acids (e.g., [PhNMe2H][BAr4] (Ar ¼ Ph, C6F5)) gives electron-deficient cationic species [(Z5-Cp)V{]N(p-Tol)}(NiPr2)]+ stabilized by interaction with solvents, counterion [B(C6F5)4]− or amine. The structure bearing coordinated thf has been determined by XRD analysis.134 Treatment of ring-substituted vanadium(V) compounds [{Z5-C5H4CMe2(CH2)nC6H3Me2}V{]N(p-Tol)}(NiPr2)Me] (n ¼ 0, 1), accessible from chlorido precursors, with Lewis acidic [Ph3C][B(C6F5)4] gives ionic complexes with intramolecularly coordinated arene [{Z5:kC-C5H4CMe2(CH2)nC6H3Me2}V{]N(p-Tol)}(NiPr2)][B(C6F5)4] (Scheme 12). XRD analysis implies that one carbon in the ortho-position is coordinated to the electron-deficient vanadium center (V–C ¼ 2.411(2) A˚ ). The strength of the arene interaction seems to be almost insensitive to the length of the interannular bridge as revealed by 2D EXSY NMR spectroscopy at different temperatures. Quantum-chemical calculations on a simplified model compound have proven that the hemilabile coordination of the pendant arene arm is not enforced by the steric demand of the NiPr2 ligand but is forced by electron-rich vanadium atom.135

12

Cyclic and Non-Cyclic Pi Complexes of Vanadium

Scheme 12 Synthesis of the complex with intramolecularly coordinated arene.

Vanadium(V) compounds with intramolecularly coordinated amide arm [{Z5:kN-C5H4(CH2)2NR}V(]NtBu)Cl] (R ¼ Me, iPr) are prepared from amine-functionalized cyclopentadiene and imido complex [tBuN]V(NMe2)2Cl] by diethylamine elimination.134,139 Methyl derivatives, available through alkylation with MeLi, react with Brønsted acidic [PhNMe2H][B(C6F5)4] in thf-d8 to produce cationic compounds [{Z5:Z2-C5H4(CH2)2NCR0 2}V(NHtBu)(thf-d8)][B(C6F5)4] (R0 ¼ H, Me) with an intramolecularly coordinated Z2-NCH2 moiety, as evidenced by multinuclear NMR spectroscopy.134 It strongly contrasts reactions with Lewis acids B(C6F5)3 and [Ph3C][B(C6F5)4] that allow ionic species [{Z5:kN-C5H4(CH2)2NR}V(]NtBu)]+ to be isolated as contact or solvent-separated ion-pairs. Such electron-deficient compounds readily form adducts with ethylene and propylene. In the case of 2-butyne, products of mono- and bis-insertion into the VdN single bond have been observed instead of simple p-adducts, as documented by multinuclear NMR spectroscopy (Scheme 13).134,139

Scheme 13 Reactivity imidovanadium(V) complexes with intramolecularly bonded amide.

Vanadium(V) alkylidene compound [(Z5-Cp)V{]N(DiPP)}(]CHPh)(PMe3)] has been prepared from a low-valent vanadium precursor [(Z5-Cp)V{]N(DiPP)}(PMe3)2] by alkylidene transfer from l5-phosphane Ph3P]CHPh. The reaction proceeds with loss of PMe3 and PPh3. The product has a three-legged piano stool structure. The vanadium-to-carbon bond distance (1.922(6) A˚ ), determined by XRD analysis, is significantly longer than in case of cyclopentadienyl vanadium(III) alkylidenes.89 More details about imidovanadium complexes is given in COMC II and COMC III.

5.02.2.3.4

Chalcogenido complexes

Oxidative decarbonylation of [(Z5-Cp )V(CO)4] by the action of sulfur or selenium gives tetrachalcogenido [{(Z5-Cp ) V}2(m-E)2(m-E2)] (E ¼ S, Se) and pentachalcogenido species [{(Z5-Cp )V}2(m-E)(m-E2)(m-Z2:Z2-E2)].140 Synthetic procedures to mixed sulfido/selenido and chalcogenido/oxido analogues have been developed as well.140,141 Although the oxidative decarbonylation protocol is compatible with species bearing unsubstituted Cp ligand,142 pentachalcogenido compounds [{(Z5-Cp)V}2(m-E) (m-E2)(m-Z2:Z2-E2)] (E ¼ S, Se) and their congeners with monoalkyl-substituted Cp rings are more conveniently synthesized by thermolysis of [(Z5-Cp0 )2V(k2-E5)].143,144 Unusual hexasulfide [{(Z5-Cp)V}2(m-S2)(m-Z4:Z4-S4)] is formed upon pyrolysis of [{(Z5-Cp)V}2(m-B2H6)2] in the presence of sulfur powder.104 Treatment of pentasulfides [{(Z5-Cp0 )V}2(m-S)(m-S2)(m-Z2:Z2-S2)] with 1 equiv. of Bu3P leads to elimination of one sulfur atom to give [{(Z5-Cp0 )V}2(m-S)2(m-S2)].145 Electron-deficient heterocubane [{(Z5-Cp0 )V}4(m3-S)4] and pentanuclear species with polyhedral structure [{(Z5-Cp0 )V}5(m3-S)6] are formed when an excess of the phosphine is used.146,147 An alternative route for these clusters involves a reaction of [(Z5-Cp0 )2V] with dithioacetic acid or tBuSH.148,149 Cationic species [{(Z5-C5H4Me)V}4(m3-S)4][BF4] is obtained by oxidation of the aforementioned heterocubane by [Ph3C][BF4].146 Treatment of the pentanuclear cluster with 7,7,8,8-tetracyano-p-quinodimethane was used for preparation of the charge-transfer complex [{(Z5-C5H4Me)V}5(m3-S)6] [TCNQ].147

Cyclic and Non-Cyclic Pi Complexes of Vanadium

13

The tetrasulfide [{(Z5-C5H4Me)V}2(m-S)2(m-S2)] readily reacts with an electrophilic alkyne to produce the adduct [{(Z5-C5H4Me)V}2(m-Z2:Z2-S2){m-k2:k2-SC(CF3)]C(CF3)S}].145 Furthermore, the tetrasulfide seems to be a very suitable precursor for a variety of mixed-metal clusters.150–154 Mononuclear compound [(Z5-Cp )VO(k2-S5)] has been prepared by a metathesis reaction of chloride [(Z5-Cp )VOCl2] with polysulfide.155 A similar synthetic strategy, starting from convenient chlorido precursors and thiolates or selenolates, has been used for assembly of [(Z5-Cp)VO(SPh)2]156 and the ferrocene-bridged derivative [Fe(m-Z5:kSe-C5H4Se)2{(Z5-Cp )VO}].157 Dinuclear species [{(Z5-Cp )VCl}2(m-S)2] appears as a product of [(Z5-Cp )2VCl] oxidation when treated by an excess of sulfur.158 Oxidation of imidovanadium(III) compound [(Z5-Cp)V(]NtBu)(PMe3)2] by elemental selenium and tellurium gives the dinuclear complex with puckered V2N2 core and bridging dichalcogenides [{(Z5-Cp)V}2(m-NtBu)2(m-E2)] (E ¼ Se, Te).90,159 It should be noted that all structure types reported in this section have been determined with XRD crystallography. More details about chalcogenido complexes is given in COMC II.

5.02.2.3.5

Azido and nitrido complexes

The dimeric vanadium(IV) complex bearing two azido bridges, [{(Z5-Cp )V(N3)2}2(m-N3)2], has been prepared by reaction of [(Z5-Cp )V(CO)4] with Me3SiN3. The decarboxylation involves oxidation of the central metal by the action of air-oxygen. An alternative route to the azido-bridged compound uses [(Z5-Cp)VCl3] as the starting compound and enables isolation of the chlorido/azido intermediate [{(Z5-Cp )VCl(N3)}2(m-N3)2]. Both compounds have a planar V2N2 core with Cp ligands in trans-configuration, as proved by XRD analysis.160 Dimeric nitridovanadium(V) compound [{(Z5-Cp )VCl}2(m-N)2] has been originally synthesized by a reaction of [{(Z5-Cp ) V}3(m-Cl)6] with Me3SiN3161 but a more convenient protocol uses sodium azide as the source of nitrogen.66 XRD analysis of [{(Z5-Cp )VX}2(m-N)2] (X ¼ Cl, N3) has documented an almost square V2N2 core with delocalized p-electrons.116,161 The chloride [{(Z5-Cp )VCl}2(m-N)2] is reduced by sodium amalgam to afford tetrameric species [{(Z5-Cp )V}4(m3-N)4] with a heterocubane structure.162 The bonding situation in the heterocubane and possibility of V–V interaction have been examined by quantum-chemical calculations.163 Monomeric vanadium(V) complex [(Z5-Cp)VCl(^N){C3H2N2(Mes)2}] with terminal nitrido ligand has been prepared by a reaction of the dichlorido precursor with sodium azide (Scheme 14). In this case, formation of oligomeric species is prevented by use of the sterically demanding and strongly electron-releasing N-heterocyclic carbene decorated by mesityl groups. Treatment of the nitride with HCl ∙dioxane leads to the anionic species [(Z3+2-Cp)VCl2(^N)]−. In both nitride compounds, infrared V^N stretching frequencies have been assigned based on a 15N isotopic labelling study. XRD structural data, reported for the anionic compound, suggest appearance of an unusual Z3+2-coordination mode of Cp ligands induced by trans-effect of the strong p-electron-releasing nitrido ligand (Scheme 14).63 More details about azido and nitrido complexes is given in COMC III.

Scheme 14 Synthesis of cyclopentadienyl vanadium complexes with terminal nitrido ligand.

5.02.2.3.6

Applications

Catalytic activity of vanadium(IV) species bearing intramolecularly bonded amide [{Z5:kN-C5H4(CH2)2NiPr}VCl2] and imidovanadium(V) compounds [(Z5-Cp0 )V(]NR)Cl2] (Cp0 ¼ Cp, C5Ht4Bu; R ¼ tBu, p-Tol) toward ethylene polymerization has been investigated. Both structure types show moderate catalytic activity when activated with methylaluminoxane (MAO).60,127 Oxidovanadium species, synthesized from [(Z5-Cp )VOCl2] and incompletely condensed silsesquioxanes, are moderately active for the ethylene polymerization under atmospheric pressure when EtAlCl2 or polymethylaluminoxane (PMAO) are used as activators.164 Imido complexes [(Z5-Cp)V(]NR)(NiPr2)Me] (R ¼ tBu, p-Tol) were found to be active catalysts in cyclotrimerization of alkynes.91

5.02.3

Bis(h5-cyclopentadienyl) complexes

5.02.3.1

Bis(h5-cyclopentadienyl)vanadium

5.02.3.1.1

Synthesis of vanadocene and ring-substituted derivatives

[(Z5-Cp)2V], known as vanadocene, is a paramagnetic 15-electron compound in a high-spin d3-configuration appearing as a purple crystalline solid. Its sandwich structure has been determined by gas-phase electron diffraction.165 XRD analysis on a single crystal

14

Cyclic and Non-Cyclic Pi Complexes of Vanadium

reveals staggered conformation of Cp rings at 108 K. At temperature around 170 K, the Cp ring starts to disorder due to appearance of the second orientation of the ring.166 Electronic structure of [(Z5-Cp)2V] has been reevaluated by conventional EPR in X-band, high-frequency and -field EPR (HFEPR), electronic absorption spectroscopy, variable-temperature magnetic circular dichroism (VTMCD), two-dimensional penning ionization electron spectroscopy and quantum-chemical calculations on various level of theory.167–170 Hyperfine coupling tensor, estimated from the X-band EPR spectra, well correlates with data published previously.171 The HFEPR spectra established a precise value for the axial zero-field splitting and confirmed the absence of rhombic zero-field splitting, which is in line with presence of fivefold axis.167,168 Assignment of the electronic absorption bands, consistent with previously reported data,172 has been done based on experimental VT-MCD data and results of computations by multireference ab initio methods. It is noteworthy that commonly used time-dependent DFT approach167,168,173 as well as coupled-perturbed DFT give misleading results with erroneous ordering of excited states.167,168 The hyperfine coupling tensor, g-tensor and zero-field splitting tensor of [(Z5-Cp)2V] have been calculated using approximate relativistic methods in order to construct the paramagnetic nuclear magnetic resonance shielding tensor by applying an implementation of the classic Kurland–McGarvey theory.174 The heterolytic dissociation enthalpy of [(Z5-Cp)2V] has been estimated by DFT methods and multiconfigurational perturbation theory.175 Coordination modes of indenyl ligands in bis(indenyl)vanadium has been investigated by DFT methods. As expected, high-spin species with Z5-coordinated ligands [(Z5-Ind)2V] is the most stable structure176 consistent with experimental observations.177 Convenient synthesis of [(Z5-Cp)2V] involves a metathesis reaction between NaCp and [V2Cl3(thf )6]2[Zn2Cl6] (Scheme 15). This procedure has been optimized to reach [(Z5-Cp)2V] in a multigram scale.178,179 Modifications of this synthetic protocol have been successfully used for a variety of vanadocenes symmetrically substituted on both Cp ligands including species with benzannulated Cp rings [(Z5-Ind)2V],177 derivatives bearing bulky substituents (e.g., [(Z5-Cp )2V],180 [(Z5-C5Hi2Pr3)2V] and [(Z5-C5HiPr4)2V]),181 species with nitrogen donor atom in the side chains [(Z5-C5H4R)2V] (R ¼ (CH2)2NMe2, (CH2)2N(CH2)5, (CH2)3NH2, (CH2)3NMe2, CH(CH2CH2)2NMe, CH2C5H4N)57 or decorated with metallocene moieties [{(Z5-Cp)M}2{mZ5:Z5-C5H3(SiMe2)2C5H3}2V] (M ¼ Fe, Ni).182 It is noteworthy that the starting vanadium(II) precursor is air-sensitive but is readily accessible by reduction of VCl3 ∙3thf with Zn.183 Alternative pathway to ring-substituted vanadocenes involves reduction of [(Z5-Cp0 )2VCl] with potassium amalgam as exemplified on [{Z5-C5H3(SiMe3)2}2V].184 Mixed-ring vanadocene compounds have been prepared by several procedures.185,186 The most convenient protocol, recently utilized for synthesis of the dinuclear naphthalene-bridged compound [{(Z5-Cp)2V}2{m-Z5:Z5-(C5H4)2C10H6}],72 involves a metathesis reaction between [{(Z5-Cp)V (thf )}2(m-Cl)2] and alkali-metal cyclopentadienide as originally exemplified on 1,2,3,4,5-pentamethylvanadocene (Scheme 15).71

Scheme 15 Convenient syntheses of vanadocenes.

The unusual functionalized vanadocene Li2[{Z5-C5Ph2Me2(NMes)}2V]∙tmeda has been synthesized by two-electron reduction of the corresponding anilydenecyclopentadiene with the 2 equiv. of lithium metal followed by treatment with VCl2 ∙ 2tmeda (Scheme 16). The capability of the amide donors to coordinate transition metal has been exemplified in a reaction with FeCl2. The isolated heterobimetallic complex [V{m-Z5:kN-C5Ph2Me2(NMes)}2Fe] is thermally stable. It is reduced by KC8 under nitrogen atmosphere to afford a dinitrogen complex with significantly elongated nitrogen-to-nitrogen bond (1.208(5) A˚ ) compared to free N2 (1.098 A˚ ) (Scheme 16).187

Scheme 16 Synthesis of heterobimetallic vanadocene complexes.

Computational methods have been used to investigate bonding situation in hypothetical helical vanadocene188 and predicted species from vanadium binding inside a super[5]phane cage.189 Inclusion compound of the composition [(Z5-Cp )2Yb]/[(Z5-Cp)2V]/[(Z5-Cp )2Yb] has been isolated from toluene solution and its magnetic and related physical properties were reported.190

Cyclic and Non-Cyclic Pi Complexes of Vanadium

15

More detailed overview of the synthetic procedures affording vanadocene and its ring-substituted derivatives were given in the previous editions of COMC. Schemes 16 and 17 summarize structures of sandwich vanadocene compounds determined by single-crystal XRD analysis.57,72,181,182,184,191–194

Scheme 17 Ring-substituted vanadocenes characterized by XRD analysis.

5.02.3.1.2

Sandwich complexes related to vanadocene

5.02.3.1.2.1 Pentalene complexes Pentalene (Pn, C8H6) is the dianionic double ring-fused relative of the Cp ligand bearing 10 p-electrons. Dinuclear d3-d3 compound bearing bridging pentalene ligand, [{(Z5-Cp)V}2(m-Z5:Z5-Pn)], is afforded by a reaction of [{(Z5-Cp)V(thf )}2(m-Cl)2] with Li2Pn∙ 2dme. The compound shows antiferromagnetic behavior and poor stability in solution, decomposing to [(Z5-Cp)V(Z8-Pn)] mentioned below (Scheme 18). X-ray structural data show a small folding of Pn ligand apart of vanadium atoms (folding angle ¼ 13 ) and a rather short vanadium-to-vanadium bond distance (2.5380(5) A˚ ). Such observations together with initial theoretical calculations suggested formation of a V^V triple bond in [{(Z5-Cp)V}2(m-Z5:Z5-Pn)].195 Nevertheless, later in-depth quantum-chemical study describes its ground state as an open-shell singlet with a VdV single bond and two unpaired electrons on each vanadium atom.196 The electronic structure of the hypothetical congener bearing bridging Z5:Z5-coordinated acepentalene ligand has been investigated by DFT methods.197

Scheme 18 Pentalene vanadium complexes.

The homoleptic dinuclear vanadium(II) compound bearing hexamethylpentalene ligands [(m-Z5:Z5-Pn )2V2] has been synthesized by reaction of Li2Pn with VCl2 ∙ dme (Scheme 18). Polynuclear structure of the vanadium precursor with bridging chlorides seems to be crucial for the assembly as other common vanadium(II) precursors have failed to give [(m-Z5:Z5-Pn )2V2]. XRD analysis confirmed the presence of flat pentalene ligands and a very short vanadium-to-vanadium bond distance (2.1689(5) A˚ ) compatible with appearance of a V^V triple bond,198 which has been supported by theoretical DFT calculations.198,199 XRD structures of mononuclear vanadium(III) complexes [(Z5-Cp)V(Z8-Pn)], [(Z5-Cp )V(Z8-Pn)] and [(Z5-Cp)V 8 (Z -2-MeC8H5)], accessible by reaction of Li2Pn0 with the corresponding vanadocene monochloride, exhibit a strong folding of the Pn0 ligand toward the vanadium center along the bond between the bridging carbon atoms.200 Thermodynamic stability of the compounds and observed folding angle (43 ) have been rationalized in theoretical DFT studies.201,202 A hypothetical derivative bearing two pentalene ligands has been investigated by computational methods as well.203 Considerable theoretical effort has been given to the one-dimensional sandwich nanowires based on [(Pn)V2]n204 and hypothetical tetranuclear vanadium species bearing two dicyclopenta[a,e]pentalene ligands.205 More details about pentalene compounds is given in COMC III.

16

Cyclic and Non-Cyclic Pi Complexes of Vanadium

5.02.3.1.2.2 Complexes bearing heterocyclic Cp-like ligands The vanadocene analogue bearing two different phosphorus-containing Cp-like ligands [(Z5-P2Ct3Bu3)(Z5-P3Ct2Bu2)V] has been synthesized by co-condensation of electron-beam generated vanadium atoms with tBuC^P at a low temperature.206 The metal vapor route has also been used for synthesis of the compound with 1,2-azaborolinyl ligands [{Z5-C3H3B(Me)N(SiMe3)}2V] (Scheme 19).207 As confirmed by XRD analysis, both complexes have the typical sandwich structure with parallel five-membered rings.206,207

Scheme 19 Synthesis and reactivity of vanadocene analogues.

Formation of the thermally robust sandwich compound [(Z5-C4H2Me2NdAlClMe2)2V], upon a reaction of VCl3 ∙ 3thf with 2,5-dimethylpyrrole and AlMe3 (Scheme 19), documents that Z5-coordination mode of the pyrrolide anion can by enforced by a strong Lewis acid (e.g., AlClMe2)208 even through this ligand usually forms s-complexes with vanadium.209 Structure of [(Z5-C4H2Me2NdAlClMe2)2V], together with a product of azobenzene oxidative addition [{(Z5-C4H2Me2NdAlClMe2) VMe}2(m-NPh)2], are documented by XRD analysis (Scheme 19). In both species, AlClMe2 groups lock the nitrogen atoms through s-bonding giving species with hemilabile p-bonded Cp-like ligands. In principle, the p-bonding interaction could be tuned through strength of the Lewis acid coordination and thus considerably modify reactivity of the species. Polymerization tests have revealed that these compounds can serve as a homogenous single-component and -site catalysts in ethylene polymerization without necessity of further activation.208 More details about sandwich complexes bearing Z5-coordinated heterocycles is given in COMC III.

5.02.3.1.3

Reactivity of vanadocene with unsaturated molecules

Vanadocene is a coordinatively and electronically unsaturated molecule that readily reacts with various unsaturated X]Y or X^Y molecules to give 1:1 adducts summarized in this section. The reactions are usually taken as a formal oxidative addition of a carbene-like species to the multiple bond giving three-membered metallacycles with vanadium in the formal oxidation state IV. This interpretation is supported by a decrease in unpaired electron number (from 3 to 1), considerable lowering of XY stretching frequencies and prolongation of the XY bond. 5.02.3.1.3.1 Reactions with alkenes and alkynes Reaction between [(Z5-Cp)2V] and activated alkenes gives vanadacyclopropanes as evidenced by the diethyl fumarate adduct [(Z5-Cp)2V{Z2-(CHCOOEt)2}] characterized by XRD analysis.210,211 The structural data revealed significant prolongation of the C]C bond upon coordination to the value expected for a single bond (1.468(11) A˚ ).211 Exchange of coordinated fumarate by carbon monoxide has been studied by gas volumetry. This equilibrium reaction was found to be only slightly exothermic.212 Substituted alkynes give vanadacyclopropenes upon reaction with vanadocene as documented by the structurally characterized mononuclear species [(Z5-Cp)2V(Z2-RC]CR)] (R ¼ COOMe, C6F5, Py).211,213,214 Appearance of the [(Z5-Cp)2V(Z2-PyC]CPy)] as the sole product of the reaction between [(Z5-Cp)2V] and PyC^CPy can be taken as an example of the low affinity of [(Z5-Cp)2V] to aromatic N-heterocycles. Interestingly, coordination of [(Z5-Cp )2VIII]+ species on nitrogen binding sites of [(Z5-Cp)2V (Z2-PyC]CPy)] leads to dissociation of the metallacycle and liberation of the alkyne, which is ascribed to deactivation of acetylene bridge.214 Protonation of vanadacyclopropenes by anhydrous HCl leads to liberation of the corresponding alkene as a mixture of Eand Z-isomers, which has been observed with [(Z5-Cp)2V(Z2-PhC]CPh)].215 Structurally characterized mononuclear [(Z5-Cp)2V{Z2-PhC]CP(C6Ht2Bu3)C^CPh}],216 [(Z5-Cp)2V(Z2-tBuC^CC] and dinuclear complexes [{(Z5-Cp)2V}2(m-Z2:Z2-RC]CC]CR)] [R ¼ SiMe3, PPh2],218 CC^CC^CtBu)]217 5 2 2 t 217 5 [{(Z -Cp)2V}2(m-Z :Z -RC]CC^CC^CC]CR)] (R ¼ Bu, Ph) have been prepared by reaction of [(Z -Cp)2V] with the corresponding diyne or tetrayne. The unusual dinuclear d2-d2 vanadium compound [{(Z5-Cp)2V}2(m-kC,kC-Me3SiC^CC]

Cyclic and Non-Cyclic Pi Complexes of Vanadium

17

CC^CSiMe3)] with two vanadocene(III) moieties s-bonded to neighboring carbon atoms has been reported as a product of triyne coordination.219 Reaction of [(Z5-Cp)2V] with group IV metallocenes bearing s-bonded alkynyl ligands [(Z5-C5H4R)2M(kC-C^CPh)2] (R ¼ H, Me, tBu, SiMe3; M ¼ Ti, Zr) leads to the heterobimetallic species [(Z5-C5H4R)2M(m-Z4:Z2-PhC]CC]CPh)V(Z5-Cp)2] bearing a bridging butadiene moiety.220,221 More details about the reactivity of [(Z5-Cp0 )2V] with alkenes, alkynes and polyalkynes is given in the overview study published previously,222 and in COMC I, COMC II and COMC III.

5.02.3.1.3.2 Reactions with keteneimines, carbodiimides, activated nitriles and arylphosphines Oxidative addition of [(Z5-Cp)2V] to a C]N double bond is evidenced on series of keteneimines and carbodiimides.223,224 XRD structures reported for [(Z5-Cp)2V(Z2-PhNdCCMe2)]223 and [(Z5-Cp)2V{Z2-(p-Tol)NdCN(p-Tol)}]224 document formation of vanadaaziridine cycles. The latter compound undergoes methylation on the uncoordinated nitrogen atom to give [(Z5-Cp)2V {Z2-(p-Tol)NdCNMe(p-Tol)}]+ when MeI is used as the alkylating reagent.224 The vanadaazirine cycle is accessible by a reaction of vanadocene with nitrilium cations, isoelectronic species to acetylenes, as evidenced by structurally characterized cationic species [(Z5-Cp)2V(Z2-PhC]NMe)][BF4].225 Low affinity of nitriles to vanadocene has been overcome by activation with Lewis acids (e.g., triaryl boranes and BCl3), which enabled synthesis and structural characterization of [(Z5-Cp)2V(Z2-F3CC6H4C]NBX3)] (X ¼ Cl, Ph, C6H3F2, C6H2F3, C6F5) and dinitrile adducts [(Z5-Cp)2V {Z2-(C6F5)3BN]CCH2C^NB(C6F5)3}] and [{(Z5-Cp)2V}2{m-Z2:Z2-(C6F5)3BN]C(CH2)4C]NB(C6F5)3}].226,227 Unexpected CdF⋯ V interaction between vanadium and an ortho-fluorine atom of the borane phenyl ring were detected by fluid-solution EPR experiments.226 It is noteworthy that isonitriles are not suitable precursors for the assembly of vanadacycles owing to RdNC bond cleavage as documented in the reaction of [(Z5-Cp )2V] with CyN^C to produce the vanadium(III) complex [(Z5-Cp )2V(kC-CN)(kC-CNCy)] with s-bonded ligands.191 Due to their pronounced oxidation properties, tetracyano ligands TCNX (X ¼ E: tetracyanoethylene; X ¼ Q: 7,7,8,8-tetracyanop-quinodimethane) activated with B(C6F5)3 do not produce metallacycles but instead yield dinuclear complexes bearing s-bonded dianionic TCNX bridges. The vanadium(III) species [{(Z5-Cp)2V}2{m-TCNX ∙2B(C6F5)3}] is formed when TCNX∙ 2B(C6F5)3 is treated with 2 equiv. of [(Z5-Cp)2V] while the use equimolar amount of the reagents affords vanadium(IV) compounds [{(Z5-Cp)2V}2{m-TCNX∙2B(C6F5)3}2] (Scheme 20). XRD analysis revealed twisted TCNE ligands in the complexes while TCNQ remains planar upon coordination.228 The related mononuclear vanadocene(IV) complex [(Z5-Cp)2VBr(kN-TCNE)] bearing the N-coordinated monoanionic form of TCNE is afforded by oxidation of [(Z5-Cp)2VBr] by TCNE.229

Scheme 20 Synthesis of vanadocene complexes of tetracyanoligands.

Vanadocene(IV) species containing diphosphirane cycles [(Z5-Cp)2V(Z2-ArPdPAr)] (Ar ¼ Ph, Mes) are reported as products of a reaction between [(Z5-Cp)2V] and corresponding arylphosphines (ArPH2). Alternatively, the phenyl derivative is accessible from [(Z5-Cp)2VClMe] when treated by Li[PHPh].230 More details about oxidative addition of CN multiple bonds can be found in a previously published review article,222 and in COMC II and COMC III.

5.02.3.1.3.3 Reactions with compounds containing C]O and C]S double bonds The vanadocene compound containing a vanadaoxirane cycle [(Z5-Cp)2V(Z2-OCH2)] has been synthesized by a reaction of vanadocene with paraformaldehyde and characterized by XRD analysis. Reactivity of this compound with Lewis acids, alkylation and acylation reagents has been investigated. Stable species containing a five-membered chelate ring [(Z5-Cp)2V{kO,kC-O]C(R) OCH2}]+ (R ¼ Me, Ph) are accessible by acylation with RCOCl.231 Oxidative addition of vanadocene to the C]O double bond of ketenes has been documented with XRD structures of [(Z5-Cp)2V(Z2-OdC]CRR0 )] (R ¼ R0 ¼ Ph, CF3; R ¼ Me, R0 ¼ Ph).232–234 Investigation of these species has been motivated by

18

Cyclic and Non-Cyclic Pi Complexes of Vanadium

problems associated with carbon dioxide activation since given ketene adducts can serve as models for Z2-coordination mode of CO2.232 Carbon disulfide reacts with vanadocene to afford [(Z5-Cp)2V(Z2-SdC]S)]. This adduct has been characterized by XRD analysis together with the alkylation product [(Z5-Cp)2V{Z2-SdC(SMe)}]+.235 Furthermore, vanadathiirane cycles are formed by reactions of [(Z5-Cp0 )2V] (Cp0 ¼ Cp, Cp ) with thioketones, thioketenes and isothiocyanates as observed in XRD structures of [(Z5-Cp)2V(Z2-SdCPh2)],236 [(Z5-Cp)2V(Z2-SdC]CR2)] (R ¼ tBu, CF3, SiMe3)237,238 and [(Z5-Cp )2V(Z2-SdC]NPh)],239 respectively. More details about adducts on C]O and C]S double bonds can be found in COMC I, COMC II and COMC III. 5.02.3.1.3.4 Reactions with miscellaneous unsaturated reagents Oxidative addition of vanadocene on the W^C triple bond of the alkylidyne complex [(Z5-Cp)W^C(p-Tol)(CO)2] gives the bimetallic species [{(Z5-Cp)2V}{m-C(p-Tol)}(m-CO){(Z5-Cp)W(CO)}] with a bridging C(p-Tol) group and one semibridging carbonyl ligand. The product has been characterized by infrared spectroscopy and EPR. XRD analysis confirmed the presence of the vanadium-tungsten single bond with a VdW bond distance of 2.994(5) A˚ .240

5.02.3.1.4

Reactivity of vanadocene with digallanes and silylene

Although no stable adducts of vanadocene with N-heterocyclic carbenes (NHCs) are known, several compounds bearing heavier low-valent group 13 and group 14 carbene analogues have been reported.28,241,242 Reaction of digallane (Ga{N(DiPP)CH}2)2 with 2 equiv. of [(Z5-C5H4Me)2V] gives the monomeric species [(Z5-C5H4Me)2V(Ga {N(DiPP)CH}2)], which is ascribed to oxidative insertion of the vanadocene into the GadGa single bond followed by comproportionation with the second vanadocene molecule. Subsequent reaction with the carbene-like species [K(tmeda)][Ga{N(DiPP) CH}2] leads to the anionic complex [(Z5-C5H4Me)2V(Ga{N(DiPP)CH}2)2]−.28 Similar reactivity is observed for digallane stabilized by bis(imino)acenaphthene (Scheme 21).241

Scheme 21 Synthesis of gallylene and silylene complexes.

The vanadocene adduct with the sterically demanding silylene [(Z5-Cp)2V(Si{N(DiPP)CH}2)] is formed upon a treatment of [(Z5-Cp)V] with the N-heterocyclic silylene Si{N(DiPP)CH}2. XRD analysis of the product confirmed coordination of the silylene ligand with a vanadium-to-silicon bond length of 2.358 A˚ (Scheme 21). EPR and magnetic susceptibility studies revealed a low-spin configuration of the complex, which was supported by computational DFT study. In contrast to adducts of unsaturated molecules, both vanadium and silylene remain in the formal oxidation state II as elucidated from observed VdSi and SidN bond distances and the calculated electronic structure. Absence of an NHC analogue is ascribed to low stabilization owing to a combination of electron-rich vanadium with a poor NHC p-acceptor. Calculated Wiberg bond indices for [(Z5-Cp)2V(Si {N(DiPP)CH}2)] (0.90) and the virtual carbene analogue [(Z5-Cp)2V(C{N(DiPP)CH}2)] (0.63) support this hypothesis.242

5.02.3.2

Carbonyl and isonitrile complexes

Vanadocene compounds [(Z5-Cp0 )2V] (Cp0 ¼ Cp, Cp ) react with carbon monoxide under mild conditions to give monoadducts [(Z5-Cp0 )2V(CO)].191,243 In both cases, the appearance of monocarbonyl species was confirmed by EPR spectroscopy on samples labeled by 13CO.244 The structure of [(Z5-Cp )2V(CO)] has been determined by XRD analysis.191 Under similar conditions, the dicarbonyl adduct with one slipped indenyl ring [(Z3-Ind)(Z5-Ind)V(CO)2] is formed from [(Z5-Ind)2V] without evidence of the monocarbonyl intermediate.245 Such a preference is probably not only due to lower activation energy of the Z5-to-Z3 haptotropic

Cyclic and Non-Cyclic Pi Complexes of Vanadium

19

rearrangement but also a result of lower stability of the Z5-InddM and Z3-CpdM bonds when compared to Z5-CpdM and Z3-InddM, respectively.246 [(Z5-Cp)2V(CO)] reacts with Lewis acidic B(C6F5)3 to give a mixture of zwitterionic ring-borylated vanadium(III) complex [{Z5-C5H4B(C6F5)3}(Z5-Cp)V(CO)2], its carbonyl-free derivative with intramolecularly coordinated ortho-fluorine of one C6F5 group [{Z5:kF-C5H4B(C6F5)3}(Z5-Cp)V], cationic [(Z5-Cp)2V(CO)2][HB(C6F5)3] and the hydrido complex [(Z5-Cp)2V {kH-HB(C6F5)3}].247,248 The product distribution is ascribed to the proposed reaction pathway involving an oxidative addition of the borane followed by redox and disproportionation processes.247 The hydrido complex is also formed in mixture with [(Z5-Cp)2V{kO-HOB(C6F5)3}] by a reaction of [(Z5-Cp)2V] with H2O∙ B(C6F5)3, which can be taken as vanadocene-mediated water ionization.248 Vanadium(III) dicarbonyl complexes [(Z5-Cp0 )2V(CO)2][BPh4] (Cp0 ¼ Cp, Ind) are conventionally synthesized by a reaction of monohalides [(Z5-Cp0 )2VX] (Cp0 ¼ Cp, X ¼ Cl; Cp0 ¼ Ind, X ¼ I) with carbon monoxide in the presence of a large counterion.177,210,249 Reaction of [(Z5-Cp )2V] with isonitriles leads to labile monoadducts [(Z5-Cp )2V(CNR)] (R ¼ tBu, Cy) that decompose to dealkylation products [(Z5-Cp )2V(CN)(CNR)] upon heating, as confirmed by XRD analysis of the cyclohexyl derivative.191 The vanadium(III) isonitrile species [(Z5-C5H4Me)2V(CNtBu)2][VCl4(thf )2] is formed upon reaction of VCl3 ∙ 3thf with Na(C5H4Me) in the presence of tBuNC together with a monocyclopentadienyl side product [(Z5-C5H4Me)VCl2(CNtBu)2].56 A more convenient pathway to the vanadocene(III) isonitrile complexes involves a reaction of tBuNC with the corresponding vanadocene(III) monochloride as demonstrated with the structurally characterized ansa-vanadocene species [{Z5:Z5-(C5H4)2(CMe2)2}V(CNtBu)2]+.250 More details about carbonyl and isonitrile complexes can be found in COMC I, COMC II and COMC III.

5.02.3.3 5.02.3.3.1

Halogenido and pseudohalogenido complexes Halides

Bis(cyclopentadienyl) monochloride [(Z5-Cp0 )2VCl] and dichloride compounds [(Z5-Cp0 )2VCl2] have been comprehensively investigated as precursors for a wide variety of vanadocene compounds. Stability of vanadocene dichloride in air predetermines it for application even in areas not directly related to organometallic chemistry such as materials science or medicinal chemistry. This section deals with synthesis and properties of vanadocene monohalides and dihalides, including ring-substituted and ansa-bridged species. More details can be found in previous editions of COMC. 5.02.3.3.1.1 Vanadocene monohalides Vanadocene monochloride, [(Z5-Cp)2VCl], is a paramagnetic 16-electron compound (d2-configuration) that is reported as a deep-blue crystalline solid highly sensitive to moisture and dioxygen. Unlike its titanium(III) analogue, it has a monomeric structure with non-parallel Cp rings and chlorido ligand situated at the C2 axis, as confirmed by XRD analysis.251 A convenient synthesis of [(Z5-Cp)2VCl] involves treatment of VCl3 ∙3thf with thallium cyclopentadienide.252 Attention should be given to purity of starting VCl3 as a contamination with VCl2 reduces the reaction yield considerably due to vanadocene formation.251 The use of NaCp as the source of Cp is not recommended as simultaneous formation of [(Z5-Cp)2VCln] (n ¼ 0, 1, 2) is observed probably due to monochloride disproportionation, as documented with the methylcyclopentadienyl derivative.56 An alternative route to [(Z5-Cp)2VCl], involving a reaction of V(acac)2Cl ∙thf with CpMgCl, utilizes a strong affinity of magnesium(II) to pentane2,4-dionato ligands and mild reducing properties of Grignard reagents. This synthetic strategy has been successfully used for the assembly of vanadocene species with interannularly bridged Cp rings [{Z5:Z5-(C5H4)2(CMe2)2}VCl] (Scheme 22).250 Vanadium(III) complex bearing Cp-like 1,2-azaborolinyl ligands [{Z5-C3H3B(Me)N(SiMe3)}2VCl] are formed upon a reaction of VCl3 ∙3thf with 2 equiv. of corresponding lithium salt.253

Scheme 22 Vanadocene monohalides characterized by XRD analysis.

Conversion of vanadocene to vanadocene monochloride can be realized by various chlorination reagents but the reaction with 1 equiv. of PCl3 seems to be the most convenient as shown with [(Z5-C5H4R)2VCl] (R ¼ (CH2)2N(CH2)5, CH(CH2CH2)2NMe)

20

Cyclic and Non-Cyclic Pi Complexes of Vanadium

bearing amine functional groups in the side chain.57 Vanadium(III) compounds [(Z5-Cp)2VF] and [(Z5-Ind)2VI] are accessible by oxidation of corresponding [(Z5-Cp0 )2V] by the action of a small excess of SF6 or 0.5 equiv. of I2, respectively (Scheme 22).177,254 The XRD structure has been reported for the indenyl species [(Z5-C9H5Me2)2VCl] but without synthetic details.194 5.02.3.3.1.2 Vanadocene dihalides Vanadocene dichloride, [(Z5-Cp)2VCl2], is a paramagnetic 17-electron compound (d1-configuration) appearing as a grass green crystalline solid. Although air-stable in the solid state, its solutions in organic solvents slowly decompose in air to give unidentified oxidovanadium(IV) species. The molecule of [(Z5-Cp)2VCl2] has a typical bent-metallocene structure with non-parallel Cp rings and chlorides symmetrically disposed in the plane bisecting the molecule, as documented by XRD analysis.255 The electronic structure of [(Z5-Cp)2VCl2] has been investigated by photoelectron spectroscopy,256 K-edge X-ray absorption and Kb X-ray emission spectroscopies257 and DFT computational methods.256–258 Electrochemical behavior,259,260 magnetic anisotropy of a polycrystalline sample261 and weak interactions in the crystal lattice have been described.262 EPR studies on magnetically diluted single-crystals of [(Z5-Cp)2VCl2],263 [(Z5-C5H4Me)2VCl2]264 and [(Z5-Cp)2VS5]265 have revealed that the unpaired electron in these vanadocene(IV) compounds is localized in the orbital antibonding with respect to the VdX bonds in agreement with more recent theoretical studies.258 This bonding situation allowed chloride substitution to be followed by conventional fluid-solution EPR spectroscopy in X-band.266 Although Cp ring substitution has a considerably lower effect on the giso and Aiso values, distortions caused by connection of the Cp rings with a short ansa-bridge results in observable giso increase and Aiso decrease.267 Dichloride complexes [(Z5-C5H4R)2VCl2] (R ¼ H, Me) are accessible in a multigram scale by a reaction between alkali-metal cyclopentadienide and vanadium tetrachloride.264,268 This protocol has been used for assembly of a variety of species bearing modified Cp rings as exemplified by the following structurally characterized species [(Z5-C5H4R)2VCl2] (R ¼ SiMe3, CH2C6H4OMe, CH2C6H4OCF3, CH2C6H2(OMe)3, CH2C6H3F(OMe), CH2C6H4CH2OMe, N-methyl-5-methoxyindol-3-yl); see Scheme 23.269–273

Scheme 23 Vanadocene dihalides characterized by XRD analysis.

Unfortunately, this synthetic strategy fails in particular cases or gives considerably reduced reaction yields, which is ascribed to the pronounced reducing properties of MCp0 .274 Indeed, [(Z5-Cp )2VCl2] is commonly prepared by a reaction of [(Z5-Cp )2V] with an excess of PCl3.275 Furthermore, the chlorination of vanadocene precursor by PCl3 has been used for synthesis of vanadocene(IV) compounds decorated with alkyl substituents [(Z5-C5H4R)2VCl2] (R ¼ Me, iPr, tBu)276 and with tertiary amines [(Z5-C5H4R)2VCl2] (R ¼ (CH2)2N(CH2)5, CH(CH2CH2)2NMe).57 A more convenient alternative, suitable for ring-substituted and ansa-bridged derivatives, involves chlorination of monochlorides [(Z5-Cp0 2VCl] with excess of PCl3.250 Such monochlorides, accessible from Cp0 MgCl and VCl3 ∙3thf (or preferably V(acac)2Cl ∙thf or V(acac)3), are usually not isolated, which minimalizes the risk of decomposition upon manipulation. The following dichlorides, characterized by XRD analysis, have been synthesized according to this protocol: [(Z5-C5H4R)2VCl2] (R ¼ (CH2)2OMe, CH2C6H4OMe, COOPh),277,278 [{Z5:Z5-(C5H4)2A}VCl2] (A ¼ CMe2, (CMe2)2),267,274 [{Z5:Z5-(C5H2Me2)2(C6H4)2}VCl2]279 and [{Z5:Z5-(C5H3)2(CH2CH2)2}VCl2] (Scheme 23).280 Bromide and iodide complexes [(Z5-Cp0 )2VX2] (X ¼ Br, I) are accessible from dichlorides by equilibrium ligand-exchange reaction using a large excess of alkali-metal halides in acetone281 or upon treatment of [(Z5-Cp0 )2V] with corresponding phosphorus trihalide.275,282 Alternatively, the bromides are readily formed at mild conditions upon treatment of the chlorides with BBr3 in CH2Cl2.267,278 Mixed-halide complexes [(Z5-Cp0 )2VICl] and [(Z5-Cp0 )2VIBr] have been prepared by oxidative addition of IBr and ICl, respectively, to [(Z5-Cp0 )2V].275,282

Cyclic and Non-Cyclic Pi Complexes of Vanadium

21

The high-valent vanadocene(V) species [(Z5-Cp)2VCl2][AsF6] is reported as a product of [(Z5-Cp)2VCl2] oxidation by the action of AsF5 containing traces of HF or in the presence of NaF.283,284 Formation of host/guest inclusion compounds of vanadocene dichlorides [(Z5-Cp0 )2VCl2] (Cp0 ¼ Cp, C5H4Me) with b- and g-cyclodextrins is evidenced by a typical EPR pattern of magnetically diluted samples while a-cyclodextrin does not form inclusion compounds due to a smaller size of the cyclodextrin cavity.285 A more recent study reports a more detailed thermal and spectroscopic characterization of the inclusion compounds.286 Nevertheless, very high Aiso values, reported in the later study (up to 115.1  10−4 T) suggest, in particular cases, unnoticed Cp-ring loss that is most likely due to exposure to air-oxygen.

5.02.3.3.2

Pseudohalides

Cyanido complex [(Z5-Cp )2V(CN)] is a rare example of structurally characterized vanadocene(III) pseudohalide. It appears upon isonitrile dealkylation induced by [(Z5-Cp )2V(CO)].191 Vanadocene(IV) cyanides [(Z5-Cp0 )2V(CN)2] (Cp0 ¼ Cp, C5H4Me) are readily accessible from the corresponding dichloride by ligand-exchange reaction using an excess of alkali-metal cyanide in acetone. EPR study on samples of 13C labeled cyanides revealed strong super-hyperfine coupling in fluid- and frozen-solution spectra, which correlates well with kC-coordination observed in XRD analysis of [(Z5-C5H4Me)2V(CN)2].287 This synthetic strategy is suitable for complexes with other linear (NCO, NCS, NCSe, N3)281,288–290 and non-linear pseudohalides (N(CN)2, C(CN)3, ONC(CN)2).255,291 These pseudohalides are usually coordinated to vanadium through nitrogen donor atoms with the exception of O-coordinate ONC(CN)2 as observed in the XRD structures of [(Z5-C5H4Me)2V(NCO)2],288 [(Z5-Cp)2VX2] (X ¼ SCN, N3),289,290 [(Z5-C5H4CH2C6H4OMe)2V(NCSe)2],292 [(Z5-Cp0 )2V(NCNCN)2] (Cp0 ¼ Cp, C5H4Me),255,291 [(Z5-Cp)2VCl {NCC(CN)2}]255 and [(Z5-C5H4Me)2V{ONC(CN)2}2].255 Coordination through nitrogen donor atom is also evidenced in the case of dicyanomethanidobenzoate, (PhCOC(CN)2), which contrasts with reported O-bonding in the titanocene analogue and demonstrates a lower “Pierson hardness” of vanadocene(IV) dication.293 It should be noted that an alternative pathway to pseudohalide complexes [(Z5-Cp)2VX2] (CN, NCSe, N3) involves the reaction of vanadocene with (CN)2, (SeCN)2 and HN3, respectively.294–296 Mixed halide/pseudohalide and pseudohalide/pseudohalide species are formed by oxidation with XCN (X ¼ Cl, Br, I, SCN, SeCN, N3).294–296 More details about pseudohalide complexes can be found in COMC I, COMC II and COMC III.

5.02.3.4

Complexes with weakly nucleophilic ligands

Unsolvated cationic species [(Z5-Cp)2V][BPh4] with 14-electron configuration has been synthesized by one-electron oxidation of vanadocene with [(Z5-Cp)2Fe][BPh4] but its structure has not been confirmed by XRD analysis.297 CpdV bond dissociation energy (BDE) in the “naked” cation [(Z5-Cp)2V]+ has been reexamined using threshold collision-induced dissociation on a guided ion-beam tandem mass spectrometer and compared with available first-row metallocene ions.298 The average ionic BDE of 4.02  0.14 eV points to a strong overestimation in the previously reported data obtained by electron-ionization mass spectrometry (5.32  0.15 eV).299 Nevertheless, the latest advanced computational investigation by means of multiconfigurational perturbation theory and restricted coupled cluster theory are in better agreement with the original electron-ionization experiments.300 Cationic decamethylvanadocene(III) complexes free of a Lewis base, [(Z5:kH-Cp )2V][BAr4] (Ar ¼ Ph, C6F5), are prepared by reaction of [(Z5-Cp )2VMe] with the corresponding Brønsted acid [PhNMe2H][BAr4] in toluene. XRD analysis of these species reveals that one of the Cp rings is distorted due to the agostic interaction of one methyl group with vanadium (CdH⋯ V). In contrast to the titanium(III) analogue, [(Z5:kH-Cp )2V][BPh4] does not form adducts with mono- and 1,2-difluorobenzene, which is ascribed to a lower electropositivity of the metal center; the interaction with fluorinated benzenes (CdF⋯ V) is suggested to be predominantly electrostatic.301 Interestingly, the CdF ⋯ V interaction is evidenced in the case of another Lewis base-free vanadium(III) species; namely the zwitterionic borylated compound [{Z5:kF-C5H4B(C6F5)3}(Z5-Cp)V].247 Tetrahydrofuran adduct [(Z5-Cp )2V(thf )] is formed upon treatment of [(Z5:kH-Cp )2V][BPh4] with thf.301 This synthetic strategy has been further utilized for the synthesis of ketone complexes [(Z5-Cp )2V(kO-OCR2)][BPh4] (R ¼ Me, Ph).302 The analogue with unsubstituted Cp rings [(Z5-Cp)2V(kO-OCMe2)][BPh4] has been obtained in attempts to isolate “naked” cationic species [(Z5-Cp)2V][BPh4] through a reaction between an aqueous solution of [(Z5-Cp)2VCl] and Na[BPh4] upon recrystallization from acetone.210 Acetone adducts [(Z5-Cp0 )2V(kO-OCMe2)][BPh4] (Cp0 ¼ Cp, Cp ) show a bent coordination mode of the ketone with V–O–C angles of 136.8(4) and 141.66(13) , respectively,232,302 that is due to donation of one pair of electrons to a free valance orbital of vanadium. A nearly linear arrangement (V–O–C ¼ 176.9(3) ), observed in case of benzophenone adduct, is ascribed to steric effects of the larger ligand.302 Vanadocene(III) species with coordinated acetonitrile [(Z5-Cp )2V(kN-NCMe)][PF6] is formed by oxidation of [(Z5-Cp )2V] by ferrocenium hexafluorophosphate in acetonitrile.180 The dinuclear congener with bridging fulvalene ligands [{mZ5:Z5-(C5H4)2}2{V(kN-NCMe)}2][PF6]2, prepared accordingly, has been characterized by XRD analysis.303 Vanadocene(IV) complexes bearing acetonitrile [(Z5-Cp)2VCl(kN-NCMe)][FeCl4]304 and [(Z5-Cp)2VMe(kN-NCMe)][BPh4]305 are formed by a reaction of [(Z5-Cp)2VCl2] with the Lewis acid FeCl3 and [(Z5-Cp)2VMe2] with the Brønsted acid [PhNMe2H][BAr4], respectively. XRD analysis indicates nearly linear coordination of the MeCN ligands in both complexes.304,306 Structurally characterized complexes [(Z5-Cp)2V(kF-MF6)] (M ¼ As, Sb) are formed upon a reaction of [(Z5-Cp)2VCl2] with 3 equiv. of AsF5 and Ag[SbF6], respectively.284,307 The hexafluoroarsenate complex [(Z5-Cp)2V(kF-AsF6)] can be oxidized to the high-valent species [(Z5-Cp)2V(kF-AsF6)][AsF6] by an excess of AsF5 in the presence of HF.283

22

Cyclic and Non-Cyclic Pi Complexes of Vanadium

Chlorido ligand exchange in [(Z5-Cp0 )2VCl2] (Cp0 ¼ Cp, C5H4Me) is often carried out in an aqueous solution to facilitate VdCl bond cleavage. Chlorides can be easily precipitated from the aqueous solution by AgClO4 before a treatment with the corresponding ligand.308,309 A more convenient approach, suitable even for non-aqueous media and avoiding explosive perchlorate, involves the use of vanadocene(IV) species [(Z5-Cp0 )2V(OTf )2] bearing weak-donating triflato ligands.310 Isolation of the triflate species is first reported with the ansa-bridged derivative [{Z5:Z5-(C5H4)2(CMe2)2}V(OTf )2]250 but the use of in situ generated species for synthesis of cationic vanadocene(IV) complexes bearing chelating ligands appears much earlier.308 The XRD structure of [(Z5-Cp)2V(OTf )2], obtained by a reaction of [(Z5-Cp0 )2VCl2] with 2 equiv. of AgOTf in tetrahydrofuran, confirms the formulation with monodentate coordination of the triflato ligands.310 Perfluorobutanesulfonate analogue [(Z5-Cp)2V(O3SC4F9)2] has been prepared accordingly starting from Ag(O3SC4F9).311 More details about complexes bearing weak-donating ligands can be found in COMC II.

5.02.3.5 5.02.3.5.1

Complexes of group 15 and 16 donor ligands Vanadocene complexes bearing bio-ligands

5.02.3.5.1.1 Hydrolysis of vanadocene dichloride Hydrolysis of vanadocene dichloride has been studied by several research groups but some doubts still remain about its behavior in neutral and basic solutions.312–315 Dissolution of [(Z5-Cp)2VCl2] in deoxygenated water leads to immediate VdCl bond cleavage that is accompanied with considerable pH decrease.266,312 Formation of the dicationic aqua complex [(Z5-Cp)2V(OH2)2]2+ (Scheme 24) has been verified by XRD structures of [(Z5-Cp)2V(OH2)2]X2 (X ¼ O2(OPh)2, OTf ).316,317 Under inert atmosphere, the aqua complex seems to be stable for days.313 Although, earlier studies suggested formation of a stable dihydroxido species [(Z5-Cp)2V(OH)2] upon neutralization,312,313 recent re-examination through EPR studies concluded that the appearing signal (Aiso ¼ 62.98  10−4 T, giso ¼ 1.9861), incorrectly attributed to [(Z5-Cp)2V(OH)2], originates from the carbonate complex [(Z5-Cp)2V(k2-CO3)]314,318 as confirmed on samples bearing 13C labeled carbonate.318 Titration with carbonate-free hydroxide leads to EPR signal disappearance at pH value 6.4, which is ascribed to formation of EPR-silent oligomeric species accompanied with Cp-ring loss. Attempts to clarify this observation led to isolation of the trinuclear species [{(Z5-Cp)2V}2(m-OH)4(VO)] [PF6]2 (Scheme 24), which has been structurally characterized by XRD analysis. The behavior of this complex in more basic solutions stays unclear.314 It should be noted that the incorrect attribution probably originates from sodium hydroxide contamination and the narrow lines of the eight resonances, which enables detection of traces of the carbonate complex in the mixture with EPR silent species. Although recent DFT study suggests virtually the same Aiso values for [(Z5-Cp)2V(OH)2] and [(Z5-Cp)2V(k2-CO3)],315 the stability of the unusual VIV(OH)2 is questionable due strong nucleophilicity of the non-bridging hydroxyl groups. It is noteworthy that EPR spectra of [(Z5-Cp)2V(OH2)2]2+, presented by the same authors,315 show signal loss upon neutralization, which is hardly compatible with suggested quantitative [(Z5-Cp)2V(OH)2] formation. Attempts to examine the [(Z5-Cp)2VCl2] hydrolysis by electrospray ionization mass spectrometry (ESI-MS) showed [(Z5-Cp)2VCl]+ and [(Z5-Cp)2VOH]+ fragments, but they were assigned to the fragmentation-recombination products in the ESI phase.315

Scheme 24 Reactivity of vanadocene dichloride with low molecular mass bio-ligands.

Cyclic and Non-Cyclic Pi Complexes of Vanadium

23

5.02.3.5.1.2 Interaction with low molecular mass bio-ligands Formation of the chelate complexes [(Z5-Cp)2V(k2-CO3)] and [(Z5-Cp)2V(k2-PO4H)] has been documented by EPR spectroscopy in aqueous solution of the corresponding oxoanion, as well as in therapeutic and physiologic media (e.g., phosphate buffered saline and Krebs-Ringer solution); see Scheme 24.318,319 The phosphate complex is easily recognized due to a strong super-hyperfine coupling of the EPR signal (aiso(31P)) ¼ 29.3  10−4 T).319 Complexes of monocarboxylic [(Z5-Cp0 )2V(kO-OCOR)2] (Cp0 ¼ Cp, C5H4Me; R ¼ H, Ph, CCl3, CF3) and dicarboxylic acids [(Z5-Cp0 )2V{k2-(OCO)2A}] (Cp0 ¼ Cp, C5H4Me; A ¼ d, CH2) have been synthesized from [(Z5-Cp0 )2VCl2] by ligand-exchange reactions.320,321 Structures of [(Z5-Cp)2V(kO-OCOCCl3)2],320 [(Z5-Cp)2V {k2-(OCO)2}]320 and [(Z5-C5H4Me)2V(kO-OCOCF3)2]321 have been determined by XRD analysis. Particular attention has been given to synthesis and isolation of amino acid complexes including those with donor atoms in the side-chains. Formation of N, O-chelate complexes [(Z5-Cp0 )2V(k2-aa)]+ (Cp0 ¼ Cp, C5H4Me; aa ¼ gly, ala, val, leu, ile, phe, his, trp, met, ser, thr, asp, glu, asn, gln, lys, arg) was observed by EPR spectroscopy.322–325 The following compounds have been characterized by XRD analysis: [(Z5-Cp)2V(kO,N-aa)][PF6] (aa ¼ ala, val, ile, thr, asn), [(Z5-C5H4Me)2V(kN,O-aa)][X] (aa ¼ gly, ala, leu, ile; X ¼ PF6 or BPh4) and [(Z5-C5Hi4Pr)2V(kN,O-gly)][PF6].323,325,326 Interestingly, the coordination mode of cysteine is pH dependent. Formation of O, S-chelate [(Z5-Cp)2V(kO,S-cys)]+ has been observed in acidic solutions by EPR spectroscopy while the N,S-chelate [(Z5-Cp)2V(kN,S-cys)] appears in neutral media (Scheme 24). The EPR signal assignment was checked by comparison to simplified model complexes bearing chelating 3-mercaptopropionate [(Z5-Cp)2V(kO,S-mpa)] and cysteamine [(Z5-Cp)2V(kO,S-csam)] [BPh4]. The structure of the later species has been determined by XRD analysis.324 Amino acids bearing a secondary amino group (e.g., proline, N-methylglycine and N-phenylglycine) afford N,O-chelate complexes [(Z5-Cp0 )2V(kN,O-aa)]+, complexes with amino acids bonded via a single oxygen donor atom [(Z5-Cp0 )2V(kO-aa)2]2+ or complexes bearing and O,O-coordinated amino acid [(Z5-Cp0 )2V(kO,O-aa)]+. These three structure types have been documented in solution by EPR spectroscopy and in the solid state by XRD analysis.327 The O,O-coordination mode is also reported for adducts of simple oligopeptides gly-gly, gly-ala, gly-leu, gly-ile and gly-gly-gly (Scheme 24).266 The behavior of [(Z5-Cp)2VCl2] with important bio-ligands in blood plasma has been investigated. ESI-MS, tandem mass spectrometry (MS-MS) and EPR measurements were used for detection of adducts with oxalate, carbonate, lactate and hydrogen phosphate but no interaction with blood plasma proteins was indicated. The stability of the complexes decreases in the order [(Z5-Cp)2V{k2-(OCO)2}]  [(Z5-Cp)2V(k2-CO3)] > [(Z5-Cp)2V(k2-{OCOCH(Me)O})] > [(Z5-Cp)2V(k2-PO4H)] (Scheme 24). The study on blood plasma models have indicated that vanadocene dichloride is transported in the bloodstream as the oxalate complex [(Z5-Cp)2V{k2-(OCO)2}] at low concentration (10 mmol L−1). At higher dosage, 9.2 mmol L−1 of vanadocene(IV) binds as the oxalate adduct and remaining part forms carbonate and lactate complexes.315 Among the other scrutinized low molecular mass bio-ligands of the cytosol, only diphosphate, ATP and ADP form adducts with vanadocene(IV) moiety at physiological pH.328 More details about the reactivity of [(Z5-Cp)2VCl2] with low molecular mass bio-ligands is given in the overview study published previously.266

5.02.3.5.1.3 Interaction with high molecular mass bio-ligands Nucleic acids, particularly DNA, are a probable primary target of vanadocene-based drugs as implied by accumulation of vanadium in the nuclear heterochromatin and cytoplasmic ribosomes.329 Although some studies rule out covalent interaction of vanadocene(IV) species with DNA,316,328,330 its intercalation into DNA328 or nuclease activity,328 recent investigation using high-resolution tandem mass spectrometry identified adducts with deoxydinucleoside monophosphates.331 It has been demonstrated that vanadocene(IV) complexes undergo extensive substitution of both cyclopentadienyl ligands upon coordination to the deprotonated phosphate group and one of the nucleobases. In the absence of a second phosphate group, coordination to a nucleobase occurs.331 It is noteworthy that systemic tumor inhibiting properties of cyclopentadiene and its dimer, released upon hydrolytic cleavage of the CpdV bonds, has been ruled out in the early stage of investigation.332 It has been further demonstrated that vanadocene dichloride inhibits DNA-processing enzymes topoisomerase II333 and protein kinase C.334 Inhibition of Aurora B kinase has been evidenced by immunoblotting assays on human cervix adenocarcinoma HeLa cells.335 The interaction of vanadocene dichloride with apo-transferrin and albumin has been investigated by several research groups through various spectroscopic methods336–338 and by computational methods.339 Recent reexamination by EPR spectroscopy pointed out the absence of an anisotropic EPR signal at room temperate that rules out interaction with such high molecular mass ligands.315

5.02.3.5.2

Other vanadocene complexes with O-donor ligands

Pentane-2,4-dionato complex [(Z5-Cp)2V(k2-acac)][OTf] and its congeners bearing Me, Et, Cl and NO2 groups in 3-position of the dionate are conveniently prepared from aqueous solution of [(Z5-Cp)2V(OH2)2][OTf]2 and the appropriate pentane-2,4-dione.308,340 Electrochemical studies reveal a reversible VIV/VV couple in MeCN with E1/2 values dependent on the nature of the substituent in 3-position. The E1/2 values increase in the order Me < Et < H < Cl while the nitro derivative gives an irreversible VIV/VV couple.340 It is noteworthy that the pentane-2,4-dionato complexes are also accessible from [(Z5-Cp)2VCl2] when treated with aqueous solution of pentane-2,4-dione. The cationic species [(Z5-Cp)2V(k2-acac)]+ is then isolated by precipitation with a large counter ion (e.g., BF4, BPh4, PF6).308 XRD analysis has been used for structure determination of [(Z5-Cp)2V(k2-acac)] [OTf]341 and [{Z5:Z5-(C5H2R2)2(CMe2)2}V(acac)][BPh4] (R ¼ H, tBu).250

24

Cyclic and Non-Cyclic Pi Complexes of Vanadium

The dinuclear complex [{(Z5-Cp)2V}2{m-O2C6H2(OMe)2}] bearing bridging 2,5-dimethoxybenzene-1,4-dialcoholato ligand is formed upon treatment of vanadocene with 2,5-dimethoxy-1,4-benzoquinone. Oxidation with 1 and 2 equiv. of [(Z5-Cp)2Fe][BF4] gives mono- and dicationic analogues, respectively.342 One-electron oxidation of vanadocene by the action of flavanthrone or chloranil affords dinuclear zwitterionic complexes with paramagnetic vanadocene motifs separated by diamagnetic dianionic quinonoid spacers, as documented by XRD analysis.343 Vanadocene(III) diphenylphosphinate [(Z5-Cp)2V(kO-O2PPh2)], prepared by protonation of [(Z5-Cp)2VMe] with 1 equiv. of Ph2P(O)OH, undergoes dimerization upon oxidation by ferrocenium hexafluorophosphate to give [{(Z5-Cp)2V}2(m-O2PPh2)2] [PF6]2 (Scheme 25). Treatment of [(Z5-Cp)2VMe] with 2 equiv. of Ph2P(O)OH leads to dinuclear species bearing four bridging diphenylphosphinato ligands [{(Z5-Cp)V}2(m-OOPPh2)4], which is formed by concomitant elimination of CH4 and CpH. The Cp-ring loss has also been observed in a reaction with dihydrogen tetrametaphosphate acid that produces dianionic [(Z5-Cp)V (k4-P4O12)]2−.80 An alternative route to [(Z5-Cp)2V(kO-O2PPh2)] involves Me3SiF elimination from [(Z5-Cp)VF] and Ph2P(O) OSiMe3. Treatment of the phosphinate complex with Lewis acidic BPh3 or B(C6F5)3 leads to coordination to the P]O oxygen atom to give adducts [(Z5-Cp)2V{kO-OP(Ph2)OBAr3}] (R ¼ Ph, C6F5). A similar procedure has been used for assembly of dinuclear [{(Z5-Cp)2V}2{m-O2P(R)OB(C6F5)3}] (R ¼ Ph, OSiMe3) and trinuclear complexes [{(Z5-Cp)2V}3{m3-O3POB(C6F5)3}]. Chemical oxidation of [(Z5-Cp)2V{kO-OP(Ph2)OB(C6F5)3}] by Ag[B(C6F5)4] leads to the vanadium(IV) complex [(Z5-Cp)2V{kO-OP(Ph2)OB(C6F5)3}2]. Dinuclear mixed-valent VIII/VIV compound [{(Z5-Cp)2V}2{m-k1:k2-O3POB(C6F5)3}] has been reported as an oxidation product of [{(Z5-Cp)2V}3{m3-O3POB(C6F5)3}] (Scheme 25). In both cases the naked cationic vanadocene(III) species [(Z5-Cp)2V][B(C6F5)4] is formed as a side product.344 It is noteworthy that the phosphate and phosphinate complexes mentioned here serve as models for industrially relevant vanadium phosphorus oxide catalyst used for the conversion of butane to maleic anhydride.80,344

Scheme 25 Synthesis of phosphate and phosphinate complexes.

Me3SiF elimination is a suitable synthetic strategy for triphenylphosphine oxide substitution on the periphery by vanadocene(III) units as demonstrated with [(Z5-Cp)2V(kO-OOCC6H4POPh2)] and [{(Z5-Cp)2V}3{m3-kO:kO:k O-(OOCC6H4)3PO}].345 The monodentate coordination mode through the carboxylate is consistent with the XRD structure of the salicilato compound [(Z5-Cp)2V(kO-OOCC6H4OH)] reported earlier.346 Oxidation of [(Z5-Cp)2V(kO-OOCC6H4POPh2)] by the action of [Ph3C][B(C6F5)4] leads to the cationic vanadium(IV) species [(Z5-Cp)2V(k2-OOCC6H4POPh2)][B(C6F5)4] with a bidentate coordinated carboxylate group345 that is structurally related with the amino acid derivative [(Z5-C5H4Me)2V(k2OOCCH2NHPh)][BPh4].327

Cyclic and Non-Cyclic Pi Complexes of Vanadium

25

Heterobimetallic Y–V and Gd–V complexes with bridging 1,10-phenathroline-5,6-dionate [{(Z5-Cp0 )2V}(m-kO,O:kN,N-pd) {Ln(k2-hfac)3}] (Cp0 ¼ Cp, C5HMe4; Ln ¼ Y, Gd) are formed by oxidative addition of vanadocene on coordination compounds [Ln(k2-hfac)3(kN,N-pd)] as evidenced by XRD analysis on representative complexes. Spectroscopic data and results of theoretical DFT calculations are consistent with two-electron oxidation of vanadium(II) and two-electron reduction of pd ligand.347 A similar synthetic strategy has been used to prepare the heterobimetallic Fe–V complex [{(Z5-Cp)2V}(m-kO,O:kN,N-pd)(FeCl2)].348 Arsenate complex [(Z5-Cp)2V(k2-AsO4H)] is formed by treating an aqueous solution of [(Z5-Cp)2VCl2] with sodium arsenate as evidenced by strong super-hyperfine coupling of the EPR signal (aiso(75As)) ¼ 46.6  10−4 T). Variation of the EPR parameters upon connection of the cyclopentadienyl rings by an interannular bridge has been reported.349 More details about complexes bearing O-donor ligands can be found in COMC I, COMC II and COMC III.

5.02.3.5.3

Chalcogenides and other S- and Se-donor ligands

Reaction of vanadocene dichloride with (NH4)2S5 gives the stable pentasulfide vanadium(IV) complex [(Z5-Cp)2V(k2-S5)].350 This compound is isostructural with the diamagnetic titanium(IV) analogue,351 which allowed a detailed EPR investigation on a magnetically diluted single-crystal.265 Under similar conditions, [(Z5-Cp )2VCl2] gives the three-membered metallacycle [(Z5-Cp )2V(Z2-S2)],352 which could be ascribed to the larger cyclopentadienyl ligands. Alternative pathways to the disulfide complex involves a reaction of [(Z5-Cp )2VCl2] with Na2S2352 or oxidation of [(Z5-Cp )2V] by elemental sulfur.158 Vanadocene(III) thiolates [(Z5-Cp)2V(SR)] (R ¼ Ph, C6H2(tBu)2OH) are formed by one-electron oxidation of vanadocene by the corresponding benzenethiol, as evidenced by XRD crystallography.353 This synthetic strategy enables formation of complexes with bridging dithiolates as documented with the dinuclear species [{(Z5-Cp)2V}2(m-1,3-S2C6H4)] prepared from [(Z5-Cp)2V] and benzene-1,3-dithiol.84 Vanadocene(IV) methanethiolato complex [(Z5-Cp)2V(SMe)2] has been synthesized by a reaction of vanadocene dichloride with NaSMe.354 The benzenethiolato analogue is formed upon oxidative addition of [(Z5-Cp)2V] to disulfide PhSSPh.355 Complexes bearing chelating thiolato ligands [(Z5-Cp)2V(k2-S2A)] (A ¼ (CH2)3; CH]CH, 1,2-C6H4) are accessible by a reaction of [(Z5-Cp)2V] with corresponding dithiol84 or through a more convenient procedure starting from [(Z5-Cp)2VCl2] and corresponding sodium thiolate.356,357 The latter synthetic strategy has been used for complexation of porphyrazines functionalized in the periphery by thiolate substituents. As evidenced by EPR spectroscopy, this macrocycle is capable of mediating magnetic exchange interactions between coordinated vanadium(IV) atoms separated by 14.5 A˚ .358 Reaction of vanadocene dichloride with LiSetBu gives the unstable selenolato complex [(Z5-Cp)2V(SetBu)2].359 Electronic structures of the 1,2-ethenedithiolate and 1,2-benzenedithiolate complexes have been comprehensively investigated by photoelectron spectroscopy and quantum-chemical methods as they serve as a part of minimum molecular models of the active sites of pyranopterin Mo/W enzymes.356,357 Cationic vanadocene(IV) complexes bearing chelating dithiocarbamato [(Z5-Cp0 )2V(k2-S2CNR2)]+ (Cp0 ¼ Cp, C5H4Me), xantato [(Z5-Cp)2V(k2-S2COR)]+, dithiophosphato [(Z5-Cp)2V{k2-S2P(OR)2}]+ and dithioarsenato ligands [(Z5-Cp)2V {k2-S2As(OR)2}]+ are conveniently prepared from dichloride [(Z5-Cp0 )2VCl2] and the corresponding alkali-metal salt.309,360–362 An alternative synthetic strategy to dithiophosphinates and dithiophosphates [(Z5-Cp)2V(k2-S2PR2)][S2PR2] (R ¼ Et, OEt, OiPr) involves oxidative addition of vanadocene to the SdS bond of (S2PR2)2.363 Complexes bearing nuclei of non-zero nuclear spin (13C in 13C-enriched species; 31P; 75As) at the twofold axis of the molecule exhibit strong super-hyperfine coupling of the EPR signal.309,362–364 XRD structures have been reported for [(Z5-Cp)2V(k2-S2CNEt2)][BF4]365 and [(Z5-C5H4Me)2V{k2-S2CN (CH2CH2)2O}]Cl.361 More details about thiolato and sulfido complexes can be found in COMC I, COMC II and COMC III.

5.02.3.5.4

Compounds with N- and P-donor ligands

Vanadocene reacts with 2,20 -bipyridine to give [(Z3-Cp)(Z5-Cp)V(k2-bpy)] with one Cp ring Z3-slipped. The Cp ring rearrangement, confirmed by XRD analysis, is induced by a strong chelating effect of N,N-chelating ligand to avoid the unfavorable 19-electron configuration.366 Electronic structure of [(Z3-Cp)(Z5-Cp)V(k2-bpy)] has been examined by theoretical DFT calculations. This complex contains a high-spin vanadium(III) center antiferromagnetically coupled with bipyridine radical-anion.367 Formal one-electron oxidation flips the Cp ring back to common Z5-coordination mode as evidenced by [(Z5-Cp)2V(k2-bpy)][BPh4] prepared by a reaction between [(Z5-Cp)2V][BPh4] and 2,20 -bipyridine (Scheme 26).366

26

Cyclic and Non-Cyclic Pi Complexes of Vanadium

Scheme 26 Vanadocene complexes bearing pyridine-based ligands.

A reaction of decamethylvanadocene, [(Z5-Cp )2V], with azobis(4-pyridine) gives the dinuclear complex [{(Z5-Cp )2V}2(m-Py2N2)] with the N-heterocyclic bridging ligand. XRD analysis has confirmed bonding through pyridine nitrogen donors while the central azo moiety stays out of the coordination sphere of vanadium atoms (Scheme 26). Cationic vanadocene(III) complex [(Z5-Cp)2V(py)][BPh4] is accessible by a reaction of vanadocene monochloride with pyridine in the presence of a large anion, as reported in the preliminary investigation210 and more recently confirmed by XRD analysis.214 Structurally related pyrazine complex [(Z5-Cp)2V(pz)][BPh4] is given by a reaction of [(Z5-Cp)2V][BPh4] with pyrazine. A similar synthetic strategy has been used for decamethylvanadocene analogues [(Z5-Cp )2V(NL)][BPh4] (NL ¼ py, pz) and for the assembly of dinuclear [{(Z5-Cp )2V}2(m-N,NL)][BPh4]2 (N,NL ¼ 4,40 -bpy, Py2C2) and trinuclear vanadocene(III) species [{(Z5-Cp )2V}3(m3-Py2C3N3)][BPh4]3 (Scheme 26).214 Vanadocene(IV) complexes bearing N,N-chelating ligands [(Z5-Cp0 )2V(N,NL)][OTf]2 (Cp0 ¼ Cp, C5H4Me) are conveniently synthesized from triflate complex [(Z5-Cp0 )2V(OTf )2], prepared in situ, and the corresponding chelating ligand (Scheme 26).310,368 XRD analyses are reported for complexes bearing benzene-1,2-diamine,369 (E)-N-[(pyridin-2-yl)methylene] benzenamine,369 2,20 -bipyridine,368,370 4,40 -dimethyl-2,20 -bipyridine,370 4,40 -dimethoxy-2,20 -bipyridine,370 1,10-phenanthroline,310,368 1,10-phenanthroline-5-amino,310 4,5-diazafluoren-9-one,370 dipyrido[3,2-a:20 ,30 -c]phenazine369 and 1,2-bis(4-methoxyphenylimino)acenaphthene.369 Nitrene complexes [(Z5-Cp)2V(NR)] (R ¼ SiMe3, SiPh3) and [(Z5-Cp )2V(NR)] (R ¼ Ph, C6H3Me2, C6H4Ph) have been prepared by a reaction of organic azides RN3 with the corresponding vanadocene. XRD structures of three representative complexes have revealed a linear V–N–C(Si) arrangement, typical for the R–N group donating 4-electrons to the metal, suggesting appearance of formally 19-electron vanadocene species with V^N triple bond.371–373 Structurally related [(Z5-Cp)2V{NN(SiMe3)2}] is formed by a reaction of [(Z5-Cp)2V] with bis(trimethylsilyl)diazene, (Me3Si)2N]N(SiMe3)2.374 A reaction of [(Z5-Cp)2VCl] with phenyl phosphine (PhPH2) in the presence of a large anion leads to cationic vanadocene(III) compound [(Z5-Cp)2V(PH2Ph)2][BPh4] as confirmed by XRD analysis.230 More details about complexes bearing N- and P-donor ligands can be found in COMC I, COMC II and COMC III.

5.02.3.6

Hydrido, alkyl, aryl and alkynyl derivatives

Vanadocene(III) borohydrido complex [(Z5-Cp)2V(k2-BH4)] is formed by a reaction of [(Z5-Cp)2VCl2] with an excess of MBH4 (M ¼ Li, Na). This compound has been characterized by spectroscopic methods.375 Structure of the ansa-bridged congener

Cyclic and Non-Cyclic Pi Complexes of Vanadium

27

[{Z5:Z5-(C5H4)2(CMe2)2}V(k2-BH4)], prepared by a similar protocol, has been determined by XRD analysis.376 Structurally characterized hydrido complex [(Z5-Cp)2V{kH-HB(C6F5)3}] appears as a co-product of vanadocene-mediated water ionization.248 The interaction between hydride complexes [(Z5-Cp)2VH(L)] (L ¼ CO, CH2]CH2, PMe3) and Lewis acidic BF3 and AlF3 has been investigated by computational methods.377 Synthetically important methyl complex [(Z5-Cp )2VMe] is formed upon alkylation of [(Z5-Cp )2VCl] by methyllithium.378 Recently, [(Z5-Cp )2VMe] has been recognized as a promising methyl transfer reagent. It is capable of oxidizing decamethylytterbocene to dinuclear species [{(Z5-Cp )2Yb(Me)}(m-Me){(Z5-Cp )2Yb}] and [{(Z5-Cp )2Yb}2(m-Me)].379 Dimethyl complex [(Z5-Cp)2VMe2] is conveniently prepared by methylation of vanadocene dichloride in toluene.380 Cationic derivative [(Z5-Cp)2VMe(kN-NCMe)][BPh4], appearing upon protonation with [PhNMe2H][BAr4], has been structurally characterized by XRD analysis.305,306 Dinuclear d2-d2 vanadium complexes [{(Z5-Cp0 )2V}2(m-C6H4)] (Cp0 ¼ Cp, C5H4Me) with 1,4-phenylene spacers are formed upon treatment of [(Z5-Cp0 )2VCl] with 1,4-dilithiobenzene. The XRD structure was reported for the methylcyclopentadienyl species.381 A structurally related complex bearing a ferrocene bridge has been synthesized accordingly starting from 1,10 dilithioferrocene.382 Alkynyl complexes of the formula [(Z5-Cp0 )2V(kC-C^CR)] are accessible from corresponding vanadocene halides and alkali-metal alkynides as documented on XRD structures of [(Z5-Cp)2V(kC-C^CtBu)]383 and [(Z5-C5Me4Et)2V(kC-C^CMes)].384 Dinuclear complex with vanadium carbon s-bonds [{(Z5-Cp)2V}2(m-kC,kC-Me3SiC^CC]CC^CSiMe3)] is formed upon addition of vanadocene to corresponding triyne.219 More details about this group of complexes can be found in COMC I, COMC II and COMC III.

5.02.3.7 5.02.3.7.1

Applications Biological activity

Antitumor activity of vanadocene(IV) halides and pseudohalides has been reported in COMC II and reviewed by several authors.266,385–388 Therefore, only a brief overview is given bellow. Biological activity of vanadocene dichloride has been under extensive scrutiny since 1979 when antitumor properties against Ehrlich ascites tumor in CF1 mice was reported.389 In early stages of the investigation, it underwent preclinical assays against various animal and human cell lines in parallel with structurally related titanocene(IV) dichloride.386,390–393 Vanadocene dichloride and series of its derivatives exhibit induced apoptosis on human testicular cancer cell lines Tera-2 and Ntera-2,281 breast cancer cell line BT-20394 and leukemia cell line MOLT-4.395 The apoptotic signal is not triggered by primary DNA damage and induction of protein p53 is not a necessity.395,396 Recent investigation on human T-leukemic cells of a different p53 status exhibit an induced apoptotic process when treated with [(Z5-Cp)2V(k2-5-NH2-phen)][OTf]2. The apoptosis is related with caspase 8 and caspase 9 activation. The expression of p53 and its phosphorylated form increases in the case of the p53 wild-type cells.397 Spin-trapping experiments ruled out a mechanism based on the capability of generating reactive oxygen species; vanadocene dichloride does not produce appreciable amounts of the hydroxyl radical in Fenton-like reactions at physiological pH.328 Recent studies are often focused on modified vanadocene(IV) complexes with improved pharmacological properties. Substitution of the cyclopentadienyl rings can enhance cytotoxicity as demonstrated on in vitro assays toward human leukemia cell line MOLT-4,277 human renal cell line Caki-1,271-273,292 porcine kidney cell line LLC-PK270,271 and human colon cancer cells HCT-8.398 Anti-angiogenesis assay against human umbilical vein endothelial cells (HUVEC) is reported on vanadocenes [(Z5-C5H4CH2C6H4OMe-4)2VX2] (X ¼ Cl, SeCN). These derivatives exhibit in vivo antitumor activity against Caki-1 in xenograft mice.292 Substitution of chloride ligands is another synthetic strategy that can improve cytotoxicity of vanadocene(IV) complexes as demonstrated with a series of derivatives bearing N,N-chelating ligands.310,370 Promising in vitro cytotoxicity against A549 lung adenocarcinoma cell line was reported for complexes with 1,2-bis(phenylimino)acenaphthene ligands [(Z5-C5H4Me)2V(N,NL)] [OTf]2 (bian, 4-MeO-bian).369 Pentane-2,4-dionate complex [(Z5-Cp)2V(k2-acac)][OTf] performs as a dual-function antitumor agent with anti-angiogenic and anti-mitotic properties as demonstrated with classical in vitro cytotoxicity assays and experiments with chick embryo and zebrafish.341 Cytotoxicity against cell line MOLT-4 has been investigated on a series of dithiocarbamate compounds. Improved activity is reported for species bearing cyclic dithiocarbamates [(Z5-Cp0 )2V{k2-S2CN(CH2)5}][OTf] (Cp0 ¼ Cp, C5H4Me).361 It has been demonstrated that the activity of vanadocene dichloride against A549 cells is enhanced in the presence of human serum transferrin and apo-transferrin.337 Vanadocene dichloride and its derivatives have been extensively investigated for their spermicidal properties and potential application as a vaginal contraception as mentioned in COMC III. Such topic has been reviewed recently and no new studies have been reported afterwards.266

5.02.3.7.2

Applications in catalysis and material science

Pinacol-type coupling reactions of aldehydes, aldimines and arylidene malononitriles by a system [(Z5-Cp)2VCl2]/Me3SiCl/Zn (or Al) has been overviewed in COMC III. A short addition to the vanadium-catalyzed synthesis of dl-1,2-dicyclohexylethanediol has been reported recently.399

28

Cyclic and Non-Cyclic Pi Complexes of Vanadium

[(Z5-Cp)2VCl2] performs as an efficient catalyst in opening of the epoxide ring by aromatic amines under solvent-free conditions. Produced vicinal aminoalcohols have been tested for fungistatic activity.400 It has been demonstrated that [(Z5-Cp)2VCl2] selectively catalyzes benzylic C–H oxidation when treated with an aqueous solution of tBuOOH.401 Insertion of dimethylacetylene dicarboxylate into the vanadium-hydrogen bond of the hydrido complexes [(Z5-Cp)2M(L)H] (L ¼ CO, P(OMe)3) has been investigated by computational methods.402 Catalytic systems based on vanadocene dichloride, activated by AlEt2Cl or AlEtCl2 and heterogenized on a supported ionic-liquid system, are capable of producing high molecular-weight polyethylene of fluffy or fibrous morphology.403,404 A similar system has been used for ethylene/norbornene and ethylene/oct-1-ene copolymerization. Relatively narrow molecular-weight distribution suggests the action of a single-site catalytic system.405 Aqueous solution of vanadocene dichloride is capable of catalyzing emulsion polymerization of styrene and methyl methacrylate at elevated temperature.406,407 Roles of vanadocene and co-catalysts in immobilized single-site olefin polymerization have been scrutinized with a combined experimental/theoretical investigation.408 [(Z5-Cp)2VCl2] acts as a source of vanadium for MCM-41 type silica tubes functionalized on the surface with vanadium oxide.409 Catalytic properties of various mesoporous silicates functionalized by [(Z5-Cp)2VCl2] have been overviewed in COMC III. A coating process involving vanadocene enabled deposition of a uniform layer of vanadium carbide, which could serve as a barrier against lanthanide fission products.410 Furthermore, vanadocene has been utilized as a dopant source for semi-insulating SiC411 or for hydrogen storage materials based on vanadium-doped magnesium.412 Reducing properties of [(Z5-Cp)2V] have been utilized for deposition of Cu metal from copper(II) acetylacetonate.413 Carbon-encapsulated vanadium oxide nanocrystals have been formed using a low-temperature procedure based on oxidation of [(Z5-Cp)2VCl2] by ammonium persulfate.414 Pyrolysis of [(Z5-Cp)2VCl2] with functionalized phosphazene gives mixtures of V2O5 and VO(PO4)n of different morphology including laminar and xerogelous V2O5.415 Single-walled carbon nanotubes/silicon photovoltaic devices have been doped with [(Z5-Cp)2V(O3SC4F9)2] using a room-temperature spin-coating process to modify the performance of solar cells.311 More details about catalytic activity of vanadocene compounds and their application in material science can be found in COMC II and COMC III.

5.02.4

h6-Arene complexes

5.02.4.1

Bis(h6-benzene)vanadium and its derivatives

Bis(arene)vanadium compounds have attracted considerable attention since the discovery of bis(Z6-benzene)chromium, an unprecedented species with two formally neutral benzene molecules p-bonded to a formally neutral, zero-valent metal atom.416 Bis(Z6-benzene)vanadium is a paramagnetic 17-electron species in a low-spin configuration with a positive charge on vanadium atom (NBO charge ¼ 0.216 a.u.).417 It exhibits a typical sandwich structure with two parallel benzene rings Z6-coordinated to the vanadium atom as confirmed by XRD analysis.418 Interpretation of electronic absorption spectra has been recently reevaluated by time-dependent DFT.419 Electronic states of [(Z6-C6H6)2V] and [(Z6-C6H6)2V]+ have been elucidated by pulsed field ionization electron spectroscopy.420 Rotational disorder of [(Z6-C6H3Me3)2V] in channels of 2,4,6-tris(4-Br-phenoxy)-1,3,5-triazine has been investigated by EPR spectroscopy on oriented single crystals. The analysis of temperature-dependent spectra revealed two types of paramagnetic guest entities capable a reorientational motion.421 The adsorption of [(Z6-C6H6)2V] on a self-assembled monolayer of n-alkanethiols and stability of the composites have been investigated by infrared reflection absorption spectroscopy and temperature-programmed desorption.422 Mesoporous silica doped with [(Z6-C6H6)2V] is a potential hydrogen storage material with high enthalpy of hydrogen adsorption.423 [(Z6-C6H6)2V] is conveniently prepared by a reductive arene complexation, known as Fischer-Hafner synthesis. Reaction of VCl4 with a mixture of aluminum powder and aluminum trichloride in benzene gives cationic species [(Z6-C6H6)2V]+ that can be converted to the neutral [(Z6-C6H6)2V] by the action of aqueous solution of Na2S2O4 (Scheme 27).424 Studies on toluene and mesitylene congeners have confirmed appearance of the cationic species as the primary reaction product. This conclusion is supported by XRD structures of following species: [(Z6-C6H3Me3)2V][AlCl4],425 [(Z6-C6H5Me)2V]2[Al4O2Cl10] and [(Z6-C6H5Me)2V][catena-Al4O2Cl9].426 It is noteworthy that the conversion of [(Z6-C6H6)2V]+ to [(Z6-C6H6)2V] in aqueous solutions does not proceed as usual one-electron reduction but preferentially through disproportionation to vanadium(0) and vanadium(III) or vanadium(II) complexes depending on pH of the solution.427 The use of the Fischer-Hafner route for alkyl-substituted derivatives of [(Z6-C6H6)2V] is very limited owing to transalkylation and migration of the substituent under Friedel-Crafts conditions.428 Modified protocols starting from VCl3 are conveniently used only for preparation of [(Z6-C6H5Me)2V]425 and [(Z6-C6H3Me3)2V].429,430

Cyclic and Non-Cyclic Pi Complexes of Vanadium

29

Scheme 27 Synthesis and reactivity of ansa-bridged bis(arene)vanadium complexes.

Complexes bearing polynuclear arenes are accessible by reduction of VCl3 ∙3thf by alkali-metal arene radical-anions. The reaction with 4 equiv. of sodium naphthalide gives the anionic species Na[(Z6-C10H8)2V] that has been oxidized to [(Z6-C10H8)2V] by alumina.431 Pyrene derivative [(Z6-C16H10)2V] has been synthesized in one step by reduction of VCl3 ∙3thf with 3 equiv. of Na[C16H10].432 XRD structures of these compounds show eclipsed conformation of the polynuclear arenes with vanadium atom disordered over a crystallographic inversion center.431,432 An alternative pathway to bis(arene) compounds involves co-condensation of aromatic hydrocarbons with electron-beam generated vanadium atoms. This strategy enables the synthesis of complexes not compatible with Friedel-Crafts conditions. The following examples represent structurally characterized complexes prepared by the metal vapor pathway. They involve complexes bearing substituted benzenes [(Z6-C6H4F2)2V],433,434 [(Z6-C6Ht3Bu3)2V],429 2,6-dimethylpyridine [(Z6-C5H3Me2N)2V],435 tetramethylpyrazine [(Z6-C4Me4N2)2V]436,437 and phosphinine [(Z6-C5H5P)2V].438 It is noteworthy that bis(Z6-benzene)vanadium has been reported as a side product from reactions of cyclopentadienyl neodymium(II) and dysprosium(II) iodides with vanadocene.439 Ring-substituted and ansa-bridged derivatives of [(Z6-C6H6)2V] are conveniently synthesized by a selective double deprotonation of [(Z6-C6H6)2V] with 2.5 equiv. of nBuLi and tmeda. The 1,10 -dilithiated sandwich complex [(Z6-C6H5Li)2V]∙tmeda has been characterized by elemental analysis and the structure of thf solvate has been confirmed by XRD analysis.440 Reaction of [(Z6-C6H5Li)2V]∙tmeda with chloride reagents leads to salt elimination as exemplified with the recently prepared and structurally characterized ansa-bridged compounds [{Z6:Z6-(C6H5)2SntBu2}V],441 [{Z6:Z6-(C6H5)2BNR2}V] (R ¼ iPr, SiMe3),440,442 [{Z6:Z6-(C6H5)2(SiMe2)2}V],440 [{Z6:Z6-(C6H5)2(BNMe2)2}V],440 [(Z6:Z6-(C6H5)2{MNMe2SiMe2C(SiMe3)2})V] (M ¼ Al, Ga)443; see Scheme 27. A similar synthetic strategy has been used for ansa-bridged compounds [V(m-Z6:kC-C6H5)2{Zr(Z5-C5Ht4Bu)2}]444 and [{Z6:Z6-(C6H5)2Si(CH2)3}V].445 Structurally characterized dinuclear species with two ansa-moieties connected by a silicon-silicon single bond [(m-Z6:Z6:Z6:Z6-{(C6H5)2Si(C6H4Pr)}2)V2] is formed upon reduction of chloride precursors [{Z6:Z6-(C6H5)2Si(C6H4Pr)Cl}V] by lithium naphthalide.446 A dinuclear species with a (PMe2)2Co2(CO)4(Me2P)2 spacer has been prepared from the phosphine functionalized arene complex [(Z6-C6H5PMe2)2V] and the norbornadiene complex [{(Z2:Z2-nbd)Co(CO)}2(m-CO)2].447 A similar strategy has been used for synthesis of compounds with (PMe2)2CoH(Me2P)2 and (PMe2)2Ni(Me2P)2 spacers.448

30

Cyclic and Non-Cyclic Pi Complexes of Vanadium

The boron-bridged compound [{Z6:Z6-(C6H5)2BN(SiMe3)2}V] reacts with C6D6 in the presence of a catalytic amount of [Pt(PEt3)4] to afford the ring opened product [(Z6-C6H5B{]N(SiMe3)2}Ph)(Z6-C6D6)V] as elucidated by XRD analysis.442 Under similar conditions the diboron-bridged compound [{Z6:Z6-(C6H5)2(BNMe2)2}V] reacts with 1 equiv. of [Pt(PEt3)4] to give [V(m-Z6:kB-C6H5BNMe2)2{Pt(PEt3)2}] from oxidative addition into the BdB single bond. The reaction is driven by a reduction of the molecular ring strain and by the formation of two stable PtdB bonds.440 The activation of the BdB single bond by transition metal catalysts has been further documented on the insertion of but-2-yne (Scheme 27).440 The silicon-bridged compound [{Z6:Z6-(C6H5)2SiMeiPr}V] undergoes a ring-opening reaction in the presence of Karstedt’s catalyst to afford a polymer containing spin-active metal centers in the main chain. Weight-averaged molecular weight of the polymeric material (28 kg mol−1) has been determined by a small-angle X-ray scattering.442 Although attempts to polymerize the tin-bridged analogue [{Z6:Z6-(C6H5)2SntBu2}V] were not successful, activation of the CipsodSn bond has been evidenced by reaction with the low-valent platinum complex [Pt(PEt3)4] to afford [V(m-Z6:kC-C6H5)(m-Z6:kSn-C6H5SntBu2){Pt(PEt3)2}] (Scheme 27).441 One-electron oxidation of [(Z6-C6Ht3Bu3)2V] with Ag[BPh4] gives the stable cationic compound [(Z6-C6Ht3Bu3)2V][BPh4]. The structurally related charge-transfer compound [(Z6-C6Ht3Bu3)2VI][TCNE] is formed upon treatment of the neutral arene complex with 1 equiv. of TCNE, as revealed in an investigation of molecular magnets with the empirical formula V(TCNE)n ∙ xCH2Cl2 (n  2; x  0.5).429,449 The interaction between [(Z6-C6H6)2V] and TCNE in acetonitrile has been utilized for templated assembly of the macrocyclic oxidovanadium(IV) octacyanoporphyrazine complex.450 Lewis acid adduct of malononitrile CH2{CNB(C6F5)3}2 reacts with bis(Z6-benzene)vanadium to give the cationic vanadium(I) species [(Z6-C6H5Me)2V][CH{CNB(C6F5)3}2]. Exchange of the arene ligand, oxidation of the vanadium center and deprotonation of the malononitrile reagent occur during the synthesis. [(Z6-C6H6)2V] can be oxidized by the action of water adduct H2O∙ B(C6F5)3.451 Heat capacity of the crystalline fulleride [(Z6-C6H3Me3)2V][C60] has been established by calorimetric measurements in the temperature range 7–345 K.452 A reduction of bis(arene)vanadium compounds by the action of potassium metal affords anionic vanadium(−I) complexes as exemplified with the structurally characterized mesitylene complex K[(Z6-C6H3Me3)2V].430,453 In accordance with the 18-electron rule, anionic [(Z6-C6H6)2V]− is thermodynamicaly more stable than neutral [(Z6-C6H6)2V] as confirmed by ab initio calculations.417 Hypothetical bis(coronene)vanadium454 and dinuclear species of the type [(Z6-L)2V2] (L ¼ naphthalene, anthracene, triphenylene, phenazine)455–458 have been studied by computational methods. Binding of vanadium metal and vanadium dication inside a hypothetical super[6]phane cage has been investigated as well.189 Sandwich complex [(Z6-C5H5BMe)2V] bearing the Z6-boranuidabenzene ligands is a high spin vanadium(II) compound isoelectronic to vanadocene, as documented by theoretical459 and experimental studies.460 More details about bis(arene) compounds can be found in overview articles428,461 and previous editions of COMC.

5.02.4.2

Half-sandwich, mixed-ring sandwich and multi-decker complexes

Cationic carbonyl vanadium(I) complexes bearing arene ligands [(Z6-C6H6-nMen)V(CO)4][V(CO)6] are formed upon reaction of [V(CO)6] with alkylated arenes. The formulation of the reaction products has been elucidated from spectroscopic data and supported by an XRD structure of [(Z6-C6H2Me4)V(CO)4][V(CO)6].462 Dinuclear vanadium(0) complex [{(Z6-C6H5Me)V (CO)2}2] bearing toluene ligands appears under similar reaction conditions. A short vanadium-to-vanadium bond distance (2.388(2) A˚ ), elucidated from XRD analysis, suggests a presence of V^V triple bond.463 The half-sandwich complex [(Z6-C5H5BMe)V(CO)4] bearing Z6-boranuidabenzene ligand has been prepared by a multistep procedure starting from the alkali metal precursor M[C5H5BMe]. The Z6-coordination mode of the ligand has been confirmed by XRD analysis.460 More details about carbonyl complexes bearing p-bonded arene ligands can be found in COMC I and COMC II. Half-sandwich vanadium(I) complexes supported by ketiminato ligands are formed upon a two-electron reduction of [{k2-(ArNCMe)2CH}VCl2] (Ar ¼ 2,6-Me2C6H3, 2,6-Et2C6H3, 9-anthracenyl) with KC8. XRD analysis of the reaction products revealed their dimeric structure [{m-Z6:k2-(ArNCMe)2CH}2V2] with ketiminate ligands serving as bridges between the vanadium atoms through Z6-coordination of the flanking benzene rings (Scheme 28).464 Under the same reaction conditions, the ketiminate complex decorated by more sterically hindered DiPP substituents [(k2-{(DiPP)NCPh}2CH)VCl2] undergoes a reductive CdN bond cleavage on the ketiminate backbone to afford mononuclear imidovanadium(III) species with Z4-coordinated azabutadienyl ligand (Scheme 28).464 Inverted sandwich dinuclear complex [{(k2-{(DiPP)NCMe}2CH)V}2(m-Z6:Z6-C6H5Me)] is formed upon a two-electron reduction of [(k2-{(DiPP)NCMe}2CH)VCl2] by the action of KC8 in toluene (Scheme 28).465 Its electronic structure has been clarified by DFT and ab initio calculations.466 The compound can serve as a source for the dicoordinate ketiminatovanadium fragment or as a multi-electron reductant for NdN bond cleavage.465,467 Monomeric vanadium(I) complex bearing Z6-coordinated 1,3,5-triphenylbenzene [(k2-{(DiPP)NCMe}2CH)V(Z6-C6H3Ph3)] appears as a stable intermediate upon cyclotrimerization of phenylethyne catalyzed by ketiminatovanadium compounds (Scheme 28).464

Cyclic and Non-Cyclic Pi Complexes of Vanadium

31

Scheme 28 Half-sandwich arene complexes supported by ketiminate ligands.

Another example of a ketiminatovanadium(I) species stabilized by arene coordination involves the zwitterionic vanadium(II) complex [(k2-{(DiPP)NCMe}2CH)V(Z6-C6H5BPh3)] formed upon thermally induced reductive elimination of neopentane from the cationic alkylidene compound [(k2-{(DiPP)NCMe}2CH)V(]CHtBu)(thf )][BPh4]. The related iodido complex [(k2-{(DiPP) NCMe}2CH)V(]CHtBu)(I)] exhibits, under similar conditions, Wittig-like reactivity to give an imidovanadium(IV) species bearing an anilido ligand and an Z2-coordinated diene pendant arm, as documented by XRD analysis (Scheme 29).468

Scheme 29 Thermolysis of alkylidene vanadium complexes.

The dimeric vanadium(II) compound [{m-Z6:kS,S-(DiPP)2C6H3S}2(VI)2] bearing sterically demanding terphenyl thiolate ligands has been synthesized by a reaction of VI2 ∙4thf with 1 equiv. of corresponding lithium thiolate. XRD analysis revealed that vanadium atoms and thiolate bridges form a flat V2S2 core. Each low-valent vanadium center is stabilized by the intramolecular coordination of the flanking aryl ring from the terphenyl substituent.469 Treatment of VI2 ∙4thf with 2 equiv. of the thiolate leads to disproportionation. Isolated monomeric vanadium(III) species [{Z6:kS-(DiPP)2C6H3S}{kS-(DiPP)2C6H3S}(VI)] is stabilized by Z6-coordination of one benzene ring.469 Vanadium(III) complex bearing Z6-coordinated aniline-based ligand is formed upon oxidation of [{kN-(DiPP)NSiiPr3}2V] by ferrocenium.470 Mixed-ring sandwich compound [(Z5-Cp)(Z6-C6H6)V] is conveniently prepared by alcoholysis of the lithium-vanadium complex [(Z5-Cp)V(m-Z6:Z6-C6H6)Li(thf )x], which is accessible by a reaction between vanadocene, cyclohexa-1,3-diene and n BuLi.471,472 Synthetically important naphthalene analogue [(Z5-Cp)(Z6-C10H8)V] is given by oxidation of anionic complex K [(Z5-Cp)(Z6-C10H8)V] formed upon a treatment of vanadocene by potassium naphthalide.71 The bonding in [(Z5-Cp)(Z6-L)V] (L ¼ C6H6, C10H8) has been evaluated by computational methods.473,474

32

Cyclic and Non-Cyclic Pi Complexes of Vanadium

Structurally characterized dinuclear bis(indenyl) complex [(m-Z5:Z6-Ind)2V2] is formed upon a reduction of [(Z5-Ind)2V] by potassium metal.475 Under similar conditions, [(Z5-Cp)2V] is reduced to K[(Z5-Cp)2V], which then undergoes oxidative addition of cyclohexa-1,3-diene to afford the unusual dinuclear hydrido complex [{(Z5-Cp)V}2(m-Z4:Z4-C6H6)(m-H)2] that is stabilized by a puckered Z4:Z4-coordinated benzene ligand.476 DFT calculations suggest a partial lack of benzene ring aromaticity allowing fluxional isomerism.477 A hypothetical mixed cyclopentadienyl/[60]fullerene complex has been investigated by DFT computational methods. Conformational analysis has predicted formation of [(Z5-Cp)V(Z6-C60] coordinated to a hexagonal site of fullerene.478 Similarly, the Z6-coordination mode of fullerene is preferred for virtual sandwich-like structures of [(C60)n+1Vn] (n ¼ 1–3).479 Triple-decker sandwich complex [{(Z5-Cp)V}2(m-Z6:Z6-C6H6)] with benzene ring as the middle deck is conveniently prepared by a reaction of [(Z5-Cp)V(Z3-C3H5)2] with cyclohexa-1,3-diene.64,472 It is also formed upon hydrogenation of the benzyne complex [(Z5-Cp)V(PMe3)2(Z2-C6H4)]92 or phosphine complex [(Z5-Cp)VMe2(PMe3)2] in benzene.98 The electronic structure of the triple-decker compound [{(Z5-Cp)V}2(m-Z6:Z6-C6H6)] has been investigated theoretically using DFT.480 The benzene middle deck undergoes arene exchange reactions with retention of the triple-decker sandwich structure as documented by XRD analysis on the mesitylene analogue.64 Electronic structure of hypothetical hexafluorobenzene derivative [{(Z5-Cp)V}2(m-Z6:Z6-C6F6)] has been examined by DFT methods.481 Complex [{(Z5-Cp)V}2(m-Z6:Z6-C10H8)] with bridged naphthalene is formed by reaction of vanadocene with [(C10H8)Yb(thf )3].482 The structurally characterized tetra-decker complex [{(Z5-Cp)V}2(m-Z6:Z6-C10H8)2{Eu(thf )(dme)}] appears upon treatment of EuI2 ∙3dme with a mixture of KCp and K[CpV(C10H8)].483 Triple-decker complexes bearing hexaphosphabenzene as the middle deck [{(Z5-Cp0 )V}2(m-Z6:Z6-P6)] are formed by thermolysis or photolysis of [(Z5-Cp0 )V(CO)4] (Cp0 ¼ Cp, Cp , C5Me4Et) with white phosphorus.484,485 Several intermediates of the reaction and side products have been isolated and structurally characterized by XRD analysis.485,486 The electronic structure of [{(Z5-Cp)V}2(m-Z6:Z6-P6)], calculated at the DFT level of theory, rationalizes asymmetry of the planar P6 ring.487 The middle-deck of the vanadium complex [{(Z5-Cp )V}2(m-Z6:Z6-P6)] can be contracted by phosphorus cation abstraction induced by the action of an N-heterocyclic carbene. The product of the ring-contraction [{(Z5-Cp )V}2(m-Z5:Z5-P5)]− is accompanied with two other unprecedented triple-decker complexes (Scheme 30).488

Scheme 30 Synthesis and reactivity of the hexaphosphabenzene complex.

Neutral half sandwich complex [(Z6-C6Me6)V] is generated in a supersonic metal cluster beam source. Time-of-flight mass spectrometry and pulsed field ionization zero electron kinetic energy spectroscopy have been used for characterization of this species.489 Gas-phase condensation of benzene with laser-vaporized vanadium atoms has been used for assembly of multi-decker vanadium(0) complexes [(Z6-C6H6)2Vn(m-Z6:Z6-C6H6)n–1] (n ¼ 1–4). Magnetic properties of the formed species have been investigated experimentally and theoretically.490–492 The soft-landing of [(Z6-C6H6)3V2] on monolayer of n-octadecanethiol has been documented by infrared spectroscopy and thermal desorption studies.493 Effects of the multidecker-structure on nonlinear optical properties of metallocene-like chromophores has been evaluated theoretically using DFT and ab initio methods.494,495 Structural, energetic, electronic and magnetic properties of infinite one-dimensional sandwich cyclopentadienylvanadium,496,497 benzene-vanadium,496,498–501 naphthalene-vanadium,502,503 anthracene-vanadium,497,504,505 cyclopentadienylvanadium-benzene-vanadium497 and cyclopentadienyl-vanadium-cyclopentadienyl-iron497,506–509 nanowires have been calculated by quantum-chemical computational methods. Theoretical efforts have also been reported for several related sandwich nanowires containing vanadium.510–514 More details about mixed-ring sandwich and multi-decker complexes can be found in COMC II and COMC III.

5.02.5

h7-Cycloheptatrienyl complexes

5.02.5.1

(h7-Cycloheptatrienyl)(h5-cyclopentadienyl)vanadium and its derivatives

Mixed-ring sandwich compound [(Z5-Cp)(Z7-C7H7)V], known as trovacene, is a paramagnetic d1-compound appearing as a purple crystalline solid. Its sandwich structure has been documented by XRD analysis reported in early stages of investigation.515 More

Cyclic and Non-Cyclic Pi Complexes of Vanadium

33

recent low-temperature XRD data describe electron density distribution and atom displacements.516 The bonding in [(Z5-Cp) (Z7-C7H7)V] has been clarified by ab initio and DFT calculations.517,518 Time-dependent DFT has been utilized for description of low-lying electronic excited states.519 Localization of valence electrons in [(Z5-Cp)(Z7-C7H7)V] suggests that the cycloheptatrienyl ring acts more as a trianionic ligand than monocationic but neither of these formal extremes gives a satisfactory description of the cycloheptatrienyl–vanadium bond owing to its significant covalent character.518 Nevertheless, the trianionic formalism is still less commonly used than the monocationic.520 Trovacene has been synthesized by thermolysis of the carbonyl complex [(Z5-Cp)V(CO)4] in the presence of cycloheptatriene521 or by reaction of the cycloheptatrienyl complex [(Z7-C7H7)V(CO)3] with cyclopentadiene.522 The limitation of these routes for alkyl [5]trovacenes and Z5-indenyl congeners has been discussed.522 Trovacene is also formed upon a reaction of chlorido complex [(Z5-Cp)VCl3] with cycloheptatriene and the Grignard reagent iPrMgBr523 but a more convenient procedure, developed recently, involves a readily available vanadium(III) precursor [(Z5-Cp)VCl2(PEt3)2]. At room temperature, it reacts with cycloheptatriene and magnesium turnings to afford trovacene in 55% yield. It should be noted that the triethylphosphine could be easily recovered from the reaction mixture.524 Alternatively, [(Z5-Cp)(Z7-C7H7)V] is accessible by a reaction between [(Z5-Cp)2V], nBuLi and cycloheptatriene followed by alcoholysis (Scheme 31).524 This procedure utilizes the synthetic strategy developed for the mixed-ring sandwich compound [(Z5-Cp)(Z6-C6H6)V].

Scheme 31 Synthesis of trovacene and its derivatives.

Ring-substituted and ansa-bridged derivatives of trovacene are commonly prepared from lithiated trovacenes (Scheme 31). Monolithiation proceeds preferentially at the five-membered ring,525,526 which has been utilized for assembly of a variety of [5] trovacenyl compounds including bi-, tri- and tetra[5]trovacenes. Dilithiated compound [(Z5-C5H4Li)(Z7-C7H6Li)V] is formed upon treatment of trovacene with 2 equiv. of tmeda527 or preferably with 2.5 equiv. of nBuLi in the presence of N,N,N0 ,N00 , N00 -pentamethyldiethylenetriamine. Structure of the solvate [(Z5-C5H4Li)(Z7-C7H6Li)V]∙pmdta has been determined by XRD analysis.528 Synthetically important formyl[5]trovacene is formed upon treatment of lithiated trovacene with dimethyl formamide.529,530 The acetyl derivative is conveniently prepared from [(Z5-C5H4COMe)V(CO)4] by heating with cycloheptatriene.530 A series of [5-5] bitrovacenes bearing saturated and unsaturated carbon spacers [{(Z7-C7H7)V}2{m-Z5:Z5-(C5H4)2(A)}] (A ¼ CH2, CH2CH2, >C] CH2, E-(CH]CH) and Z-(CH]CH)) have been synthesized from these derivatives and structurally characterized. Their investigation has been motivated by electronic and magnetic communication between vanadium centers. The extent of magneto-communication has been analyzed be EPR spectroscopy and quantified by the exchange coupling constant J.530 This approach has been applied to the majority of the bi-, tri- and tetranuclear trovacene species given below. Structurally characterized aryl-substituted trovacenes [(Z5-C5H4Ar)(Z7-C7H7)V] (Ar ¼ Ph, 1-C10H7) have been prepared by the Negishi route. In this protocol, monolithiated trovacene is transmetalated by the action of ZnCl2 followed by cross-coupling with

34

Cyclic and Non-Cyclic Pi Complexes of Vanadium

ArI catalyzed by Pd(dppf )Cl2.531,532 This synthetic strategy has been used for the assembly of [5-5]bitrovacenes bearing phenylene and naphthylene spacers,532,533 1,3,5-tri([5]trovacenyl)benzene533 and 1,2,4,5-tetra([5]trovacenyl)benzene.534 Parent [5-5] bitrovacene has been synthesized by a modified protocol using a homocoupling of [5]trovacenyllithium by the action of iodocyclohexane.531 XRD structural data are available for [{(Z7-C7H7)V}2{m-Z5:Z5-(C5H4)2}],531 [{(Z7-C7H7)V}2{mZ5:Z5-(C5H4)2C6H4}], [{(Z7-C7H7)V}2{m-Z5:Z5-(C5H4)2C10H6}]532 and [{(Z7-C7H7)V}4{m4-Z5:Z5:Z5:Z5-(C5H4)4C6H2}].534 [5-5]Bitrovacene species with a C^C spacer has been prepared by a Stille coupling reaction with iodo[5]trovacene and Me3SnC^CSnMe3. Attempts to synthesize this compound by the Sonogashira route from iodo[5]trovacene and [5]trovacenylethyne led to isolation of a species with a longer alkyne spacer (C^CC^C) owing to initial H/I exchange on the ethyne substituent. Mixed phenylene/alkyne spacer (C^CC6H4C^C) has been obtained by Negishi coupling starting from lithiated [5]trovacenylethyne and 1,4-diiodobenzene.529 [5]Trovacenylethyne has been further utilized for assembly of dinuclear complexes with C^C(M2R2)C^C bridges (M ¼ Al, Ga, R ¼ Et, tBu). They appear upon treatment with the corresponding dialkylmetal hydride (R2MH).535 Derivatives of trovacene [(Z5-C5H4R)(Z7-C7H7)V] (R ¼ OH, COOH, B(OH)2) are bearing Cp ligands modified with fundamental functional groups not available for other cyclopentadienyl vanadium compounds. [5]Trovacenol is given by a reaction of the lithiated trovacene with bis(trimethylsilyl)peroxide followed by subsequent desilylation.536 [5]Trovacenecarboxylic acid, prepared by a treatment of [5]trovacenyllithium with carbon dioxide, is a suitable precursor for a variety of functional derivatives as exemplified with the anhydride and acid chloride.526 [5]Trovacenylboronic acid is accessible by a reaction of lithiated trovacene with tributylborate and subsequent hydrolysis. The corresponding cyclic anhydride is formed upon heating under vacuum.537 Mono-, di- and trinuclear [5]trovacenylboranes [{(Z7-C7H7)V}n{(C5H4)nB(Mes)(3-n)}] (n ¼ 1–3) are given by lithiation of trovacene and subsequent reaction with (Mes)2BF, (Mes)BF2 and BF3, respectively.538 Di([5]trovacenyl)ketone is formed upon a reaction of [5]trovacenyllithium with dimethylcarbamoyl chloride. The procedure is sensitive to reaction conditions as their slight variation lead to unusual trinuclear species with a reacted cycloheptatrienyl ring (Scheme 31).539 Dinuclear trovacene compounds bearing a single atom bridge of group 14 elements [{(Z7-C7H7)V}2{m-Z5:Z5-(C5H4)2ER2}] (E ¼ Si, Ge, Sn, Pb; R ¼ Me or Ph) have been synthesized by a convenient procedure starting from [5]trovacenyllithium and the corresponding dichloride R2ECl2.540,541 A similar synthetic strategy has been used for synthesis of siloxane-bridged compounds [{(Z7-C7H7)V}2{m-Z5:Z5-(C5H4SiR2)2O}] (R ¼ Me, Ph), species with SiMe2CH2SiMe2 spacer542 and tetranuclear tin-bridged compound [{(Z7-C7H7)V}4{m4-Z5:Z5:Z5:Z5-(C5H4)4Sn}].540 The trinuclear dichloridostannane(IV)-bridged complex is formed upon treatment of [(Z5-C5H4Li)(Z7-C7H7)V] with SnCl2 (Scheme 31).543 Phosphorus bridged compound [{(Z7-C7H7)V}2{m-Z5:Z5-(C5H4)2PPh}], prepared conveniently from [(Z5-C5H4Li)(Z7-C7H7) V] and PhPCl2, was found to be a suitable precursor of heterobimetallic complexes as documented in reactions with palladium(II) and chromium(0) complexes.544 Di([5]trovacenyl)arsinous acid has been prepared by a reaction of [5]trovacenyllithium with AsCl3 and consequent hydrolysis (Scheme 32).545

Scheme 32 Synthesis and reactivity of bi- and tetra[5]trovacenes.

Cyclic and Non-Cyclic Pi Complexes of Vanadium

35

A series of trovacene species bearing disulfide, diselenide and ditelluride spacers is formed upon treatment of [5]trovacenyllithium with 2 equiv. of corresponding elemental chalcogen546 or more conveniently by oxidation of [(Z5-C5H4ELi)(Z7-C7H7)V] (E ¼ S, Se, Te) intermediates by elemental sulfur.547 The thiolate complex [(Z5-C5H4SLi)(Z7-C7H7)V] has been used for assembly of the dinuclear species with S3, S4 and SCH2S spacers. They are formed upon the reaction with SCl2, S2Cl2 and CH2Cl2, respectively.548 Under slightly different conditions, the treatment of [(Z5-C5H4SLi)(Z7-C7H7)V] with S2Cl2 leads to trisulfide-bridged ansa-compound [{Z5:Z7-(C5H4)S3(C7H6)}V]. Mononuclear species bearing thioethers [(Z5-C5H4SR)(Z7-C7H7)V] (R ¼ CH2Ph, SiMe3) are accessible by a metathesis reaction between the [5]trovacenylthiolate and corresponding chloride. [(Z5-C5H4SMe) (Z7-C7H7)V] is formed upon treatment of [(Z5-C5H4Li)(Z7-C7H7)V] with S2Me2.549 Dinuclear species with a single sulfur atom bridge [{(Z7-C7H7)V}2{m-Z5:Z5-(C5H4)2S}] is given by a metathesis reaction between [5]trovacenyllithium and SCl2 (Scheme 32).548 Ferrocene-bridged bi[5]trovacene has been synthesized from a tetramethylcyclopentadiene-containing precursor by deprotonation and consequent reaction with Fe2Cl4 ∙3thf (Scheme 32).550 Complexes selectively substituted on the seven-membered ring are not accessible through the lithiation procedure. Therefore, the [7-7]bitrovacene [{(Z5-Cp)V}2{m-Z7:Z7-(C7H6)2}] has been synthesized by a reaction of thermally rearranged dicycloheptatriene with [(Z5-Cp)V(CO)4].551 No other [7]trovacenyl compounds have been reported till now probably due to the lack of a more convenient synthetic procedure. Dilithiated trovacene [(Z5-C5H4Li)(Z7-C7H6Li)V], prepared in situ, has been used for assembly of boron-bridged ansa-trovacenes [{Z5:Z7-(C5H4){BN(SiMe3)2}(C7H6)}V] and [{Z5:Z7-(C5H4)(BNMe2)2(C7H6)}V]. They are given by a reaction with the corresponding chloride reagent. The diboron-bridged compound undergoes oxidative addition with the low-valent platinum complex [Pt(PEt3)4] to afford bimetallic [V(m-Z5:kB-C5H4BNMe2)(m-Z7:kB-C7H6BNMe2){Pt(PEt3)2}] (Scheme 33).524

Scheme 33 Synthesis and reactivity of ansa-trovacenes.

A similar synthetic strategy has been used for series of ansa-trovacene compounds bridged by group 14 elements. Structures of the following complexes have been documented by XRD analysis: [{Z5:Z7-(C5H4)A(C7H6)}V] (A ¼ SiPh2,552 SiMeiPr,528 Si2Me4,552 GeMe2,528 SntBu2,441 Snt2Bu4).528 Species with single atom bridge undergo oxidative addition and regioselective insertion of [Pt(PEt3)4] into the Si(Sn)dC single bond in the cycloheptatrienyl ring. Structures of the resulting bimetallic complexes [V(m-Z5:kE-C5H4ER2)(m-Z6:kC-C7H6){Pt(PEt3)2}] (E ¼ Si, R ¼ Me; E ¼ Sn, R ¼ tBu) have been confirmed by XRD analysis (Scheme 33).441,527

36

Cyclic and Non-Cyclic Pi Complexes of Vanadium

Ring opening polymerization of [{Z5:Z7-(C5H4)SiR2(C7H6)}V] (SiR2 ¼ SiMe2, SiMeiPr) proceeds in the presence of the Karstedt’s catalyst with comparable values of number-averaged molecular weights, estimated by gel permeation chromatography (A ¼ SiMe2: 5 kg mol−1; A ¼ SiMeiPr: 10 kg mol−1).528 Significantly higher molecular weight (89 kg mol−1) is reported for macromolecules formed upon optimized ring-opening polymerization of the compounds with the SntBu2 bridge. In this case, the scaffold with repeating paramagnetic centers has been evidenced by magnetic susceptibility measurements (Scheme 33).441 Hypothetical dinuclear azulene compound [(C10H8)2V2], investigated by computational DFT methods, prefers opposed orientation of bicyclic ligands with each ring Z5-coordinated. The short vanadium-to-vanadium bond distance suggests appearance of a formal V]V double bond.553 Some other p-complexes of azulene have been theoretically examined as well.554 The Z7-coordination mode has been predicted for azepine ligand in [(Z5-Cp)(Z7-C6NH7)V].555 More details about trovacene and its derivatives can be found in COMC I and COMC III.

5.02.5.2

Half-sandwich and bis(cycloheptatrienyl) complexes

Synthesis, reactivity and properties of the Z7-cycloheptatrienyl complex [(Z7-C7H7)V(CO)3] and its ring-substituted derivatives have been overviewed in COMC I and COMC III. No new experimental studies have been reported since then. Computational studies on [(C7H7)V(CO)n] (n ¼ 3–5) confirm the tricarbonyl complex [(Z7-C7H7)V(CO)3] as the preferred structure and suggest dinuclear species [(Z7-C7H7)2V2(CO)3] as a thermodynamically stable synthetic target.556 Hypothetical bis(cycloheptatrienyl) species [(Z7-C7H7)2V] has found to be unstable as revealed by computational studies. It lays 15 kcal mol−1 above the global minimum [(Z5-C7H7)(Z7-C7H7)V], which is isoelectronic to trovacene.557 Dinuclear bis(heptalene) compound without direct vanadium-vanadium bond [(m-Z7:Z7-C12H10)2V2] has been predicted based on theoretical DFT calculations.558

5.02.6

h8-Cyclooctatetraene complexes

The bis(cyclooctatetraene)vanadium [(Z4-C8H8)(Z8-C8H8)V] is conveniently prepared by a reaction between VCl3 ∙ 3thf, K2[C8H8] and cyclooctatetraene in the molar ratio 2:3:1. XRD analysis revealed different coordination modes of the cyclooctatetraene ligands that is rationalized by a strong preference of the stable 17-electron configuration.559 Electronic structure of [(Z4-C8H8)(Z8-C8H8)V] has been elucidated by DFT calculations.560 Another example of the Z8-coordination mode of the cyclooctatetraene ligand involves the mixed-ring sandwich complex bearing dianionic nido-carbaborane [(Z8-C8H8)(Z5-C2B4H4Et2)V].561 Structurally characterized dinuclear complex bearing puckered Z5:Z5-bridging cyclooctatetraene [{(Z5-Cp)V}2(m-Z5:Z5-C8H8)] is formed upon a reaction of [V2Cl3(thf )6]2[Zn2Cl6] with a mixture of K2[C8H8] and Na[Cp].562 Formation of a vanadium-vanadium multiple bond has been verified by DFT calculations.563 Congeners with a short bridge between cyclopentadienyl ligands [{m-Z5:Z5-(C5H4)2A}(m-Z5:Z5-C8H8)V2] (A ¼ CH2, SiMe2, GeMe2) are accessible by a modified route starting from Li2[(C5H4)2A]. Structure of the species bearing the SiMe2 bridge has been documented by XRD analysis.564 Hypothetical trinuclear cyclooctatetraene complex [(m-C8H8)3V3], investigated by computational methods, has the vanadium atoms in the usual 17-electron configuration. Due to hypoelectronic character, relative to the known iron analogue, the central V3 moiety forms a scalene triangle with one vanadium-vanadium single bond, one double bond and one triple bond.565 Conformational studies have been performed on virtual carbonyl complexes of the general formula [(C8H8)V(CO)n] (n ¼ 1–4)566 and their octafluorocyclooctatetraene analogues.567 More details about the cyclooctatetraene compounds is given in the in COMC I and COMC II.

5.02.7

Miscellaneous cyclic p-complexes

The cyclopropenyl complex [(Z3-Ct3Bu3)V(CO)4], accessible by photolysis of a mixture of [Ct3Bu3][BF4] and Na[V(CO)6], exhibits unsymmetrical coordination of the p-ligand. The Z3-cyclopropenyl vanadium moiety is best described as an intermediate between a regular p-complex and vanadacyclobutadiene as revealed by variable temperature NMR spectra and XRD analysis.568 The p-coordination of tetraphenylbutadiene is documented in the cyclopentadienyl vanadium compound [(Z5-Cp)V(CO)2(Z4-C4Ph4)], which is formed upon heating of the Z2-alkyne complex [(Z5-Cp)V(CO)2(Z2-C2Ph2)] with diphenylethyne. XRD analysis revealed a regular Z4-coordination of the symmetrical ligand.569 A vanadium(II) compound bearing Z5-coordinated pyrrole ring has been synthesized according to the route given in Scheme 34. Deprotonation of terminal pyrroles on the tripyrrole proligand followed by treatment with VCl3 ∙3thf led to the vanadium(III) species with the N-methylated ring s-coordinated to vanadium atom via quaternized nitrogen atom. The kN-to-Z5 slippage of the central pyrrole ring is enforced by a reduction with sodium metal as confirmed by XRD analysis. Extraction of the coordinated thf molecule by Lewis acidic AlMe3 under the N2 atmosphere leads to dinuclear dinitrogen VII/VII complex that can be reduced to mixed-valent VIII/VIV nitrido species by the action of KC8 (Scheme 34). The formulation of both species has been verified by magnetic susceptibility measurements and XRD analysis.570

Cyclic and Non-Cyclic Pi Complexes of Vanadium

37

Scheme 34 Vanadium complexes bearing Z5-coordinated pyrrole ring.

The unusual dinuclear sandwich compound K2[{m-Z3:Z5:kN,N-C6H4(CMe2C4H3N)}2V2]∙4thf, in which a bridging interaction between vanadium atoms is realized via the benzene ring of the pincer-like ligand, is formed by a metathesis reaction from the pyrrolide-based precursor and VCl3 ∙3thf (or VCl2 ∙ 2dmeda) followed by reduction with KH. Theoretical DFT calculations have been used to clarify the bonding situation and strength of the vanadium-vanadium bond.571

5.02.8

Non-cyclic p-complexes

5.02.8.1

h2-Alkene, alkyne and h4-butadiene complexes

A brief overview of simple Z2-alkene complexes, supported by chloride and carbonyl ligands, is given in COMC I. The Z2-coordination of alkene is documented in the XRD structure of the ethylene vanadium(I) complex [(Z5-Cp)V(PMe3)2(Z2-H2C] CH2)], which is stabilized by phosphine ligands. It is formed upon reaction of [(Z5-Cp)VCl(PMe3)2] with 0.5 equiv. of BrMg(CH2)4MgBr probably via a dinuclear butylene intermediate.572 Another example of structurally characterized alkene coordination involves the imidovanadium species formed upon thermolysis of [(k2-{(DiPP)NCMe}2CH)V(CHtBu)(I)] (Scheme 29).468 Structurally characterized Z2-alkyne vanadium(I) complex [(Z5-Cp)V(PMe3)2(Z2-PhC^CPh)] is formed upon reduction of [(Z5-Cp)VCl2(PMe3)2] in the presence of diphenylethyne or by a ligand exchange reaction from the aforementioned ethylene complex [(Z5-Cp)V(PMe3)2(Z2-H2C]CH2)].572 The benzyne analogue [(Z5-Cp)V(PMe3)2(Z2-C6H4)] is accessible by thermal decomposition of the diphenyl complex [(Z5-Cp)VPh2(PMe3)2].92 Its reactivity is described in detail in COMC III. Another approach to cyclopentadienyl vanadium(I) compounds bearing an Z2-coordinated alkyne ligand involves photochemically induced substitution of carbonyl ligands in [(Z5-Cp0 )V(CO)4] as documented with the XRD structure of [(Z5-Ind)V(CO)2(Z2-HC^CPh)].573 The dinuclear vanadium(I) dinitrogen-bridged complex [({Z5-C5H4(CH2)2NMe2}V(Z2-PhC^CPh)(PMe3))2(m-kN,kN0 -N2)] is afforded by two-electron reduction of [{Z5:kN-C5H4(CH2)2NMe2}VCl2(PMe3)] with an excess of magnesium in the presence of diphenylethyne (Scheme 35). The product is diamagnetic, which enabled characterization with multinuclear NMR spectroscopy.574 The relatively high frequency of the C^C stretching vibration (1717 cm−1) indicates a weaker alkyne p-back-donation compared to the aforementioned cyclopentadienyl compound [(Z5-Cp)V(PMe3)2(Z2-PhC^CPh)] (nC^C ¼ 1600 cm−1).572,574 The vanadium(II) compound stabilized by an intramolecularly coordinated amine-functionalized cyclopentadienyl ligand [{Z5:k N-C5H4(CH2)2NMe2}VCl(Z2-PhC^CPh)] has been synthesized by a reaction of dimeric [({Z5:kN-C5H4(CH2)2NMe2} V)2(m-Cl)2] with diphenylethyne in thf (Scheme 35).70

Scheme 35 Synthesis of Z2-alkyne complexes.

The structurally characterized Z4-diene compound [(Z5-Cp)V(PMe3)(Z4-H2C]CHCPh]CPhEt)] is formed upon linear 1,2-cotrimerization of the diphenylethyne coordinated in [(Z5-Cp)V(PMe3)2(Z2-PhC^CPh)], with 2 equiv. of ethylene.572 A reduction of the chlorido complex [{Z5:kN-C5H4(CH2)2NiPr}VCl(PMe3)] by the action of sodium amalgam in the presence of 2,3-dimethylbuta-1,3-diene gives Z4-diene compound [{Z5:kN-C5H4(CH2)2NiPr}V(Z4-CH]CMeCMe]CH)].60 Conformations of the hypothetical homoleptic butadiene complexes [(Z5-C4H6)nV] (n ¼ 2, 3) have been investigated by DFT methods.575,576

38

Cyclic and Non-Cyclic Pi Complexes of Vanadium

More details about alkene, alkyne and diene complexes supported by cyclopentadienyl ligands can be found in COMC II and COMC III. It is noteworthy that species with high metallacycle character are omitted in this overview due to negligible p-character of the metal-carbon bonds.

5.02.8.2

h3-Allyl complexes

An overview of Z3-allyl complexes, supported by carbonyl ligands, is given in COMC I. The formulation of [(Z3-C3H5)V(CO)4PPh3],577 [(Z3-C3H5)V(CO)3(k2-dppe)]578 and [(Z3-C3H4Me)V(CO)3{k2-(Me2As)2C6H4}]579 has been documented by XRD analysis. Conformations of hypothetical homoleptic allyl vanadium complexes [(Z3-C3H5)nV] (n ¼ 2, 3) have been analyzed by DFT methods.580 Vanadium(III) complexes supported by amidinate ligands are formed upon treatment of the corresponding chloride precursor with an allyl Grignard reagent as exemplified with structurally characterized compounds [(Z3-C3H5)V{k2-(Me3SiN)2CPh}2] and [(Z3-C3H5)2V{k2-(iPrN)2CtBu}].581 Stabilization of an allyl complex by an intramolecularly coordinated amine-functionalized cyclopentadienyl ligand is documented by the XRD structure of the cationic vanadium(III) compound [{Z5:kN-C5H4(CH2)2NMe2}V(PMe3)(Z3-C3H5)][BPh4].59 Allylvanadium hydride [(Z3-C3H5)VH] has been observed by infrared spectroscopy as a stable intermediate of the reaction between vanadium atoms and propene. It appears upon isomerization of the primary product [(kC-CH2CH]CH2)VH] in the absence of collision with the second propene molecule. The role of the Z3-allyl complex in catalyzed propene hydrogenation has been discussed.582 The Z3-1-azaallylvanadium moiety is observed in the XRD structure of imidovanadium complex [V(]NSiMe3)(NHSiMe3){Z3:k C-CH2CMe2Si(tBu)2NC(Et)CH(Et)}].583 The coordination of the azaallyl fragment is highly asymmetric and somewhat reminiscent of anilide coordination in [{Z3-AdN(C6H3Me2)}V{kN-AdN(C6H3Me2)}2]584 and [{Z3-N(p-Tol)2}V{k2-(DiPPNCMe)2CH}].585

5.02.8.3

h5-Pentadienyl and h7-heptadienyl complexes

The half-sandwich pentadienyl compound [(Z5-C5H7)V(CO)4] is formed upon oxidative addition of 1-chloropenta-2,4-diene on Na[V(CO)6] under UV-irradiation. XRD analysis of [(Z5-C5H7)V(CO)3(PMe2Ph)] documents spontaneous carbonyl ligand exchange under mild thermal conditions.586 Derivatives of bis(Z5-pentadienyl)vanadium, known as open-vanadocenes, are synthesized by a reaction of VCl3 ∙ 3thf with an excess of corresponding potassium pentadienide587–589 or more conveniently from vanadium(II) precursors [V2Cl3(thf )6]2[Zn2Cl6]587,590 or [VCl2(Py)4].587 Open vanadocenes are paramagnetic 15-electron compounds in a low-spin d3-configuration, which contrasts with high-spin [(Z5-Cp)2V].587,588,590 XRD structures have been reported for [(Z5-2,4-R2C5H5)2V] (R ¼ Me, tBu)587,591 and species bearing enantiomerically pure pentadienyl ligands synthesized from the natural product (1R)-(−)-myrtenal.588,589 The larger size of the pentadienyl ligand and low nuclear charge at vanadium seem to be responsible for stronger metal-ligand bonding compared to vanadocene, as implied by shorter vanadium-carbon bonds.588,591 Results of computational conformation analysis for [(Z5-2,4-Me2C5H5)2V] are in agreement with experimental observations.592 Species with two terminal dienyl carbon atoms connected via a bridging unit, known as edge-bridged open vanadocenes, form an intermediate between vanadocenes and open vanadocenes. Compounds bearing cyclohexadienyl and cyclooctadienyl rings [(Z5-1,5-EC5H5)2V] (E ¼ CMe2, (CH2)3) have been synthesized according to the aforementioned protocol starting from VCl3 ∙3thf.593,594 Magnetic susceptibility measurements and XRD structural data indicate that both edge-bridged compounds closely resemble classical open vanadocenes.593–595 The unusual edge-bridged open vanadocene [{Z5-(DiPP)NSitBu2Me}2V], formed upon a reaction of [V2Cl3(thf )6]I with the corresponding anilide, represents the first example of open vanadocene species in high-spin configuration. Its reduction by KC8 in the presence of 18-crown-6 affords an anionic vanadium(I) species with similar sandwich structure as documented by XRD-analysis (Scheme 36). It is noteworthy that the less sterically demanding SiiPr3-functionalized anilide affords s-complex with N-coordinated ligands but the sandwich analogue has been obtained upon its reduction (Scheme 36).470

Scheme 36 New type of edge-bridged open vanadocene complexes.

Cyclic and Non-Cyclic Pi Complexes of Vanadium

39

The open vanadocene species bearing ethylene inter-ligand bridge [{Z5:Z5-(C5H5Me)2(CH2)2}V] has been prepared from [V2Cl3(thf )6]2[Zn2Cl6] and the corresponding bridging pentadienide. XRD data confirm a large tilt of the bridging and bridgehead atoms to vanadium that is typical for ansa-bridged metallocenes.596 Monoadducts are formed upon treatment of open vanadocenes (including edge-bridged derivatives) with phosphines or carbon monoxide.597–599 The coordination stabilizes the open metallocene framework due to the 17-electron configuration and enables isolation of otherwise unisolable open vanadocene species, as exemplified with XRD structures of [(Z5-1-MeC5H6)2V(CO)] and [(Z5-3-MeC5H6)2V(CO)].597 XRD structural data are also available for the following derivatives: [(Z5-2,4-Me2C5H5)2VL] (L ¼ CO, PMe3)597,599 and [{Z5-1,5-(CH2)3C5H5}2VL] (L ¼ CO, P(OCH2)3CEt).598 It is noteworthy that sterically demanding pentadienyl ligands can prevent the formation of stable adducts as documented by EPR spectroscopic measurements.587,589 Mixed pentadienyl/cyclopentadienyl compounds, so-called half-open vanadocenes, are conveniently prepared by a metathesis reaction from the chloride precursor [{(Z5-Cp)V(PR3)}2(m-Cl)2] (R ¼ Me, Et) and corresponding pentadienide.600 These phosphine adducts have a stable 17-electron configuration. The synthetic protocol is suitable for dinuclear compounds bearing bridged pentadienyl ligands as exemplified with the structurally characterized compound [{(Z5-Cp)V(PMe3)}2(m-Z5:Z5-3,8Me2C10H10)].601 Phosphine ligands in half-open vanadocenes are easily exchanged by PF3 and CO.600,602 XRD structural data are reported for the following half-open vanadocenes: [(Z5-C5H7)(Z5-Cp)VL] (L ¼ PEt3, CO)600 and [(Z5-Me2C5H5) (Z5-Cp)V(CO)].602 A complex bearing Z5-coordinated cycloheptadienyl ligand and isoelectronic to trovacene [(Z7-C7H7)(Z5-C7H9)V] is formed upon treatment of VCl3 ∙ 3thf with 3 equiv. of potassium cycloheptadienide. This compound can be taken as an example of edge-bridged half-open trovacene.603 Evidence of an edge-bridged Z7-heptatrienyl system with dinuclear half-open trovacenes formed by vanadium-induced coupling of cyclooctatetraene has been obtained during attempts to synthesize [(Z8-C8H8)(Z5-Cp) V].604 Instability of the hypothetical species [(Z8-C8H8)(Z5-Cp)V] is discussed elsewhere.605 A hypothetical open vanadocene species with a split p-system is exemplified with bis(bicyclo[3.2.1]octa-2,6-dien-4-yl)vanadium. In this complex, both ligands prefer an Z3:Z2-coordination mode and the vanadium center stays in a high-spin d3-configuration.606 More details about Z5-pentadienyl vanadium compounds can be found in COMC II and COMC III.

5.02.9

Concluding remarks

Although considerable progress has been made in the investigation of vanadium p-complexes during the last 15 years, the field remains underdeveloped compared to p-complexes with neighboring group 4 and 6 transition metals. This is well documented by the considerably smaller number of structurally characterized p-complexes in the Cambridge Structure Database for V compared to Ti and Cr (V: 717; Ti: 4646; Cr: 3360). The smaller number of Group 5 p-complexes cannot be ascribed solely to experimental difficulties associated with their synthesis, characterization or handling. Also playing a significant role is the limited number of identified applications, which is often a potent driving force for fundamental investigations and curiosity driven research. This situation is further worsened by limited accessibility to organovanadium complexes; only three compounds ([(Z5-Cp)V(CO)4], [(Z5-Cp)2V] and [(Z5-Cp)2VCl2]) are currently available commercially. However, recent studies on bis(benzene)vanadium and trovacene have shown that modification of p-ligands presents new opportunities that could improve the situation. In principle, this approach can be used for fine-tuning physical, chemical and biological properties of mono- and bis(cyclopentadienyl)vanadium in order to better fit desirable applications in catalysis, materials science or medicine. It is noteworthy that these derivatives should be accessible by general synthetic procedures described in this overview.

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236. 237. 238. 239. 240. 241. 242. 243. 244. 245. 246. 247. 248. 249. 250. 251. 252. 253. 254. 255. 256. 257. 258. 259. 260. 261. 262. 263. 264. 265. 266. 267. 268. 269. 270. 271. 272. 273. 274. 275. 276. 277. 278. 279. 280. 281. 282. 283. 284. 285. 286. 287. 288. 289. 290. 291. 292. 293. 294. 295. 296. 297. 298. 299. 300. 301. 302. 303. 304. 305. 306.

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307. 308. 309. 310. 311. 312. 313. 314. 315. 316. 317. 318. 319. 320. 321. 322. 323. 324. 325. 326. 327. 328. 329. 330. 331. 332. 333. 334. 335. 336. 337. 338. 339. 340. 341. 342. 343. 344. 345. 346. 347. 348. 349. 350. 351. 352. 353. 354. 355. 356. 357. 358. 359. 360. 361. 362. 363. 364. 365. 366. 367. 368. 369. 370. 371. 372. 373. 374. 375. 376.

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521. 522. 523. 524. 525. 526. 527. 528. 529. 530. 531. 532. 533. 534. 535. 536. 537. 538. 539. 540. 541. 542. 543. 544. 545. 546. 547. 548. 549. 550. 551. 552. 553. 554. 555. 556. 557. 558. 559. 560. 561. 562. 563. 564. 565. 566. 567. 568. 569. 570. 571. 572. 573. 574. 575. 576. 577. 578. 579. 580. 581. 582. 583. 584. 585. 586. 587. 588. 589. 590. 591. 592. 593.

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B.; Schaefer, H. F., III J. Organomet. Chem. 2010, 695, 2461–2468. Swisher, R. G.; Sinn, E.; Grimes, R. N. Organometallics 1984, 3, 599–605. Elschenbroich, C.; Heck, J.; Massa, W.; Nun, E.; Schmidt, R. J. Am. Chem. Soc. 1983, 105, 2905–2907. Zhai, X.; Li, G.; Li, Q. S.; Xie, Y.; King, R. B.; Schaefer, H. F., III J. Phys. Chem. A 2011, 115, 3133–3143. Bachmann, B.; Baum, G.; Heck, J.; Massa, W.; Ziegler, B. Z. Naturforsch. B 1990, 45, 221–238. Wang, H.; Sun, Z.; Xie, Y.; King, R. B.; Schaefer, H. F., III Inorg. Chem. 2011, 50, 9256–9265. Wang, H.; Du, Q.; Xie, Y.; King, R. B.; Schaefer, H. F., III J. Organomet. Chem. 2010, 695, 215–225. Wang, H.; Wang, H.; Die, D.; King, R. B. Transition Met. Chem. 2014, 39, 95–109. Blunden, R. B.; Cloke, F. G. N.; Hitchcock, P. B.; Scott, P. Organometallics 1994, 13, 2917–2919. Gusev, A. I.; Aleksandrov, G. G.; Struchkov, Y. T. J. Struct. Chem. 1969, 10, 562–655. Vidyaratne, I.; Crewdson, P.; Lefebvre, E.; Gambarotta, S. Inorg. Chem. 2007, 46, 8836–8842. Ilango, S.; Vidjayacoumar, B.; Gambarotta, S.; Gorelsky, S. I. Inorg. Chem. 2008, 47, 3265–3273. Hessen, B.; Meetsma, A.; Bolhuis, F.; Teuben, J. H. Organometallics 1990, 9, 1925–1936. Alt, H. G.; Engelhardt, H. E.; Razavi, A.; Rausch, M. D.; Rogers, R. D. Z. Naturforsch. B 1988, 43, 438–444. Liu, G.; Liang, X.; Meetsma, A.; Hessen, B. Dalton Trans. 2010, 39, 7891–7893. Fan, Q.; Fu, J.; Li, H.; Feng, H.; Sun, W.; Xie, Y.; King, R. B.; Schaefer, H. F. Phys. Chem. Chem. Phys. 2018, 20, 5683–5691. Fan, Q.; Li, H.; Fu, J.; Fan, Z.; Xu, Y.; Feng, H.; Xie, Y.; King, R. B.; Schaefer, H. F., III J. Phys. Chem. A 2019, 123, 5542–5554. Schneider, M.; Weiss, E. J. Organomet. Chem. 1976, 121, 189–198. Franke, U.; Weiss, E. J. Organomet. Chem. 1977, 139, 305–313. Franke, U.; Weiss, E. J. Organomet. Chem. 1979, 168, 311–319. Pu, M. P.; Li, Q. S.; Xie, Y.; King, R. B.; Schaefer, H. F., III J. Phys. Chem. A 2011, 115, 4491–4504. Brussee, E. A. C.; Meetsma, A.; Hessen, B.; Teuben, J. H. Organometallics 1998, 17, 4090–4095. Thompson, M. G. K.; Walker, S. W. C.; Parnis, J. M. Inorg. Chem. 2011, 50, 7317–7323. de With, J.; Horton, A. D. Organometallics 1993, 12, 1493–1496. Ruppa, K. B. P.; Desmangles, N.; Gambarotta, S.; Yap, G.; Rheingold, A. L. Inorg. Chem. 1997, 36, 1194–1197. Tran, B. L.; Singhal, M.; Park, H.; Lam, O. P.; Pink, M.; Krzystek, J.; Ozarowski, A.; Telser, J.; Meyer, K.; Mindiola, D. J. Angew. Chem. Int. Ed. 2010, 49, 9871–9875. Lin, W. J.; Lee, G. H.; Peng, S. M.; Liu, R. S. Organometallics 1991, 10, 2519–2521. Reiners, M.; Baabe, D.; Schweyen, P.; Freytag, M.; Jones, P. G.; Walter, M. D. Inorg. Chim. Acta 2014, 422, 167–180. Fecker, A. C.; Glöckner, A.; Daniliuc, C. G.; Freytag, M.; Jones, P. G.; Walter, M. D. Organometallics 2013, 32, 874–884. Fecker, A. C.; Craciun, B. F.; Schweyen, P.; Freytag, M.; Jones, P. G.; Walter, M. D. Organometallics 2015, 34, 146–158. Wilson, D. R.; Liu, J. Z.; Ernst, R. D. J. Am. Chem. Soc. 1982, 104, 1120–1122. Campana, C. F.; Ernst, R. D.; Wilson, D. R.; Liu, J. Z. Inorg. Chem. 1984, 23, 2732–2734. Fan, Q.; Feng, H.; Sun, W.; Li, H.; Xie, Y.; King, R. B.; Schaefer, H. F., III New J. Chem. 2016, 40, 8511–8521. DiMauro, P. T.; Wolczanski, P. T. Organometallics 1987, 6, 1947–1954.

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594. 595. 596. 597. 598. 599. 600. 601. 602. 603. 604. 605. 606.

Cyclic and Non-Cyclic Pi Complexes of Vanadium

Kulsomphob, V.; Tomaszewski, R.; Yap, G. P. A.; Liable-Sands, L. M.; Rheingold, A. L.; Ernst, R. D. J. Chem. Soc. Dalton Trans. 1999, 3995–4001. Ernst, R. D.; Basta, R.; Arif, A. M. Z. Kristallogr. NCS 2006, 221, 289–290. Weng, W.; Arif, A. M.; Ernst, R. D. Organometallics 1993, 12, 1537–1542. Newbound, T. D.; Rheingold, A. L.; Ernst, R. D. Organometallics 1992, 11, 1693–1700. Kulsomphob, V.; Tomaszewski, R.; Rheingold, A. L.; Arif, A. M.; Ernst, R. D. J. Organomet. Chem. 2002, 655, 158–166. Ernst, R. D.; Freeman, J. W.; Stahl, L.; Wilson, D. R.; Arif, A. M.; Nuber, B.; Ziegler, M. L. J. Am. Chem. Soc. 1995, 117, 5075–5081. Gedridge, R. W.; Hutchinson, J. P.; Rheingold, A. L.; Ernst, R. D. Organometallics 1993, 12, 1553–1558. Weng, W.; Arif, A. M.; Ernst, R. D. Organometallics 1998, 17, 4240–4248. Wang, Y.; Liang, Y.; Liu, J.; Jin, S.; Lin, Y. J. Organomet. Chem. 1992, 435, 311–318. Basta, R.; Arif, A. M.; Ernst, R. D. Organometallics 2003, 22, 812–817. Bachmann, B.; Heck, J.; Meyer, G.; Pebler, J.; Schleid, T. Inorg. Chem. 1992, 31, 607–614. Wang, H.; Chen, X.; Xie, Y.; King, R. B.; Schaefer, H. F., III Organometallics 2010, 29, 1934–1941. Li, H.; Wan, D.; Wu, X.; Fu, J.; Fan, Z.; Fan, Q.; King, R. B. New J. Chem. 2020, 44, 6902–6915.

5.03

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

Grant E Forsythe and Louis Messerle, Department of Chemistry, The University of Iowa, Iowa City, IA, United States © 2022 Elsevier Ltd. All rights reserved.

5.03.1 5.03.2 5.03.2.1 5.03.2.2 5.03.3 5.03.4 5.03.5 5.03.6 5.03.7 5.03.8 5.03.8.1 5.03.8.2 5.03.9 5.03.10 5.03.10.1 5.03.10.2 5.03.10.3 5.03.10.4 5.03.11 5.03.11.1 5.03.11.2 5.03.11.3 5.03.11.4 5.03.12 References

5.03.1

Introduction h2-Complexes of tantalum Alkene complexes Alkyne complexes h3-Complexes of tantalum h4-Complexes of tantalum h6-Complexes of tantalum h7-Complexes of tantalum h8-Complexes of tantalum h2-Complexes of niobium Alkene complexes Alkyne complexes h4-Complexes h5-Complexes of tantalum Mono(cyclopentadienyl) complexes Tantalum mono(cyclopentadienyl) heterodimetallic compounds Bis(cyclopentadienyl) complexes of tantalum Linked cyclopentadienyl complexes of tantalum h5-Complexes of niobium Mono(cyclopentadienyl) complexes of niobium Bis(cyclopentadienyl) complexes on niobium Linked cyclopentadienyl complexes of niobium Indenyl complexes of niobium Conclusions

49 49 49 50 53 54 55 56 56 56 56 57 59 59 59 70 71 72 72 72 75 77 77 78 78

Introduction

This chapter is a review of niobium and tantalum cyclic and non-cyclic p-complexes, their preparations, and their reactions published from 2005 to August 2020. The organization is generally congruous with those found in the previous editions of COMC, with the exclusion of non-p-donor ligands. In general, compounds containing tantalum will be considered first unless both tantalum and niobium complexes appear in the original publication. Otherwise, the chapter will be organized by increasing hapticity of the p-donor ligands with the exception of Z-cyclopentadienyl derivatives. Where Z-cyclopentadienyl or a derivative thereof is the only p-donor in the complex, those compounds will be treated last in their own sections. The chemistries of niobium and tantalum primarily involve the three highest formal oxidation states, all of which are featured below. Significant advances have been made in CdH activation, dinitrogen activation, synthesis and reactivity of metal-ligand multiple bonds with small molecules, polymerization catalysis, homo- and hetero-multinuclear compounds, and metallocene cytotoxicity. While much of the chemistry described here relates to Z-cyclopentadienyl compounds, there is still a range of chemistry involving new and interesting compounds bearing other p-donor ligands.

5.03.2

h2-Complexes of tantalum

5.03.2.1

Alkene complexes

Wolczanski, Cundari, Lobkovsky et al. synthesized a large series of tantalum and niobium tris(siloxane) alkene complexes with C2H4, MeC2H3, EtC2H3, p-X-C6H4C2H3 (X ¼ OMe, H, CF3), tBuC2H3, c-C5H8, c-C6H10, and C7H10 (norbornene) ligands via reduction of the corresponding (silox)3MCl2 (silox ¼ tBu3SiO) complexes with sodium in the presence of excess alkene.1 In the case of Nb, (silox)3Nb(Z-N,C-pic) (pic ¼ 1,4-picoline or 4-pic) was treated with excess alkene. Tantalum homologues were obtained simply by the addition of excess alkene to Ta(silox)3. The mechanism, kinetics, and thermodynamics of their subsequent rearrangements to alkylidenes was then studied. Later, further alkene substitution was explored for a range of silox Ta or Nb complexes. The reported complexes were (silox)3M (alkene) (silox ¼ tBu3SiO; M ¼ Nb, Ta; alkene ¼ 13C2H4, C2D4, MeC2H3, EtC2H3, cis-2-C4H8, iBu, PhC2H3, c-C5H8, c-C6H10, and C7H10 (norbornene)), and their binding energetics were calculated.2 Niobium olefin complexes formed at temperatures lower than

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00107-4

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Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

those observed for conventional rearrangements. The authors concluded that the faster kinetics observed for Nb alkene association and substitution originated from the greater density of states associated with second-row transition metals that gave a flatter transition state. The complex Cp TaIIICl2(Z2-C2H4), used in tandem with an iridium pincer hydride compound, was found to be an alkene coupling catalyst.3 The Ir pincer [POCOP] hydride complex performed transfer hydrogenation of alkanes to terminal alkenes, and the tantalum component coupled these alkenes, allowing the use of alkanes as a feedstock. In a later publication, Cp TaIIICl2 (Z2-C2H4) was used as one of two kinetically-independent catalysts (along with [PCP]IrH2), for the coupling of alkanes to alkenes, with alkenes serving as hydrogen acceptors.4 Bonitatebus, Schrock, and Lopez found that diamidoamine- and triamidoamine-ligated tantalum complexes formed Z2-ethylene compounds when treated with CH3CH2MgBr.5 For example, CH3CH2MgBr (3.3 equiv.) reacted with [(3,4,5-F3C6H2 NCH2CH2)2NMe]TaCl3 to form [(3,4,5-F3C6H2NCH2CH2)2NMe]Ta(Z2-CH2CH2)(CH2CH3). The triamidoamine complex [(3,5-Cl2C6H3NCH2CH2)3N]TaCl2 reacted with 2 equiv. of EtMgBr to generate [(3,5-Cl2C6H3NCH2CH2)3N]Ta(Z2-CH2CH2). As part of a larger project aimed at utilizing hydrogenolysis of methyl ligands as a clean, simple way to access TaIV and TaIII compounds, Arnold et al. synthesized the dinuclear TaIII complex {[H2B(MesIm)2]TaIII}2(m-H)2(m-Z4:Z4-C6H5R) (H2B(MesIm)2 ¼ bis(N0 -mesitylimidazol-1-yl)-dihydroborate); R ¼ H, Me) via two alternative pathways from mononuclear or dinuclear precursors.6 [H2B(MesIm)2]TaVMe3Cl was reduced under H2 to {[H2B(MesIm)2]TaIVCl}2(m-H)4. Further reduction with 1 equiv. of KC8 in benzene or toluene furnished {[H2B(MesIm)2]Ta}2(m-H)2(m-Z4:Z4-C6H5R). [H2B(MesIm)2]TaVMe3Cl could be first methylated to [H2B(MesIm)2]TaVMe4 and then reduced with dihydrogen to form the m-Z4:Z4-C6H5R alkene complex. Tantalum benzylidene complexes bearing 1,4-disubstituted-1,4-diaza-1,3-butadiene (DAD) ligands were synthesized by thermolysis of Cp Ta(CH2Ph)2(Z4-prone-Ar-DAD) (Ar ¼ p-MeO, p-Me, p-F, p-Cl), inducing a-elimination and loss of toluene (Scheme 1).7 The complex Cp Ta(]CHPh)(Z4-prone-p-MeOC6H4-DAD) reacted with protic reagents neopentanol and pyrrole via protonolysis to yield alkoxide Cp Ta(OCH2CMe3)(CH2Ph)(Z2-N,N0 -prone-p-MeOC6H4-DAD) and amide Cp Ta(Z1-NC4H4) (CH2Ph)(Z4-supine-p-MeOC6H4-DAD) products. Reaction of Cp Ta(]CHPh)(Z4-prone-p-MeOC6H4-DAD) with carbon monoxide cleanly generated the Z2-C,O-ketene complexes. Treatment with tBuCN gave the imido complex Cp Ta[]NC(tBu)]CHPh](Z2-N, N0 -prone-p-MeOC6H4-DAD), while the carbodiimides (iPrN]C]NiPr, CyN]C]NCy) yielded azametallacyclobutanes. Reaction with acetophenone generated exclusively the E-isomer of 1-methyl-1,2-diphenylethene and a dinuclear m-O complex. The reactant ethyl benzoate generated Cp Ta(OMe)[OC(Ph)]CHPh](Z2-N,N0 -prone-p-MeOC6H4-DAD).

Scheme 1

5.03.2.2

Alkyne complexes

In 2007 the Takai group synthesized compounds of the general formula TaCl3(R1C^CR2)(DME) (R1¼ SiMe3, Me; R2¼ SiMe3, Me) directly from tantalum pentachloride via zinc metal reduction and subsequent addition of the alkyne and dimethoxyethane (DME) in toluene.8 When TaCl3(R1C^CR2)(DME) was added to other internal alkynes, alkyne exchange was observed. Novel compounds bearing the activated alkynes PhC^CR (R ¼ COOMe, CONMe2)] were synthesized by alkyne exchange with TaCl3(Me3SiC^CSiMe3)DME. Fujita et al. synthesized the complex TaCl3(Z2-3-hexyne)(DME) reacted with KTp (Tp − ¼ HB(3,5-dimethylpyrazolyl)−3) to give the corresponding Tp complex Tp TaCl2(Z2-3-hexyne)(DME).9 Addition of 2 equiv. of PhCH2MgCl gave Tp TaCl(Z2-3-hexyne)

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

51

(CH2Ph), which did not polymerize ethylene. The bulkier TpMs∗TaCl2(Z2-3-hexyne) (TpMs∗(−) ¼ HB(3-mesitylpyrazolyl)2 (5-mesitylpyrazolyl)−) complex, synthesized in a similar manner, displayed greater activity in ethylene polymerization. Mashima et al. in 2015 reported that AlCl3 reacted with TaCl3(Z2-EtC^CEt)(DME) to form the dinuclear complex Ta2Cl6(m2-C4Et4) (Scheme 2).10 Lewis bases THF, pyridine, and THT (THT ¼ tetrahydrothiophene) formed mono-adducts bearing a tantallacyclopentadienyl species coordinated to the other tantalum in Z4 fashion. The parent ditantalum compound as well as the Lewis base adducts catalyzed cyclotrimerization reactions of alkynes, with varying yields and the adducts generally being less effective. The THF adduct reacted with 10 equiv. of 3-hexyne to produce Ta2Cl4(m-Z4:Z4-C6Et6)(m-Z2:Z2-EtC^CEt). Reduction of Ta2Cl6(m-C4Et4) with Me4-BTDP (Me4-BTDP ¼ 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene) in the presence of 3-hexyne gave similar compounds, including the analogue bearing a m-Z2:Z2-Me3SiC^CSiMe3 ligand. The latter compound was shown to undergo the first example of direct [4 +2] cycloaddition between a metallacyclopentadiene and an external alkyne.

Scheme 2

Similar work from the Mashima group demonstrated that Cl2Ta(m,Z4-C4Et4)(m-Cl)TaCl3 reacted with the nitrogen donor ligand benzo[c]cinnoline, forming a tantallacyclopentadienyl complex coordinated to the second Ta. Pyridazine associated as a bridging m-k(N),k(N) ligand between both Ta atoms.11 Cl3Ta(m,Z4-C4Et4)(m-Cl)TaCl2(phen) was produced with 1,10-phenanthroline, analogous to the benzo[c]cinnoline complex. Contrasting reactivity was observed when the starting material was reduced by two electrons prior to ligation. Structurally uncharacterized [Ta2Cl4(C4Et4)]n (n unknown) reacted with 1,10-phenanthroline (phen) to form a doubly Z2-alkyne-bridged compound Ta2Cl4(C2Et2)2(phen)2 as the result of a retro-oxidative cyclization. Treatment of TaCl3(Z2-RC^CR)(DME) with NaOMe in THF gave the dinuclear compound [(Z2-RC^CR)TaCl2]2(m-OMe)OMe (m-THF) (R ¼ Et, nPr).12 Following addition of catalytic amounts of 3-hexyne, the bridged tantallacyclopentadiene compound Ta2Cl4(OMe)2(m-C4Et4) was formed, with release of RCCR. 4-Octyne gave the n-propyl substituted derivative. A mono(methoxy) derivative was prepared by addition of SiCl4, replacing the terminal methoxide with chloride. When excess 3-hexyne was added, both the dimethoxy and monomethoxy compounds produced hexaethylbenzene in nearly quantitative yields, showing that these complexes effect [2+2+2] cyclotrimerization. The fully-chlorinated compound bearing a bridging 2,20 ,5,50 -tetraethylenetantallacyclopentadiene also cyclotrimerized 3-hexyne. The dinuclear complex ([PhP(CH2SiMe2NPh)2]Ta)2(m-H)4 was shown by Fryzuk and coworkers to activate both dinitrogen and terminal acetylenes to give unsymmetrical bridging dialkylidenes in the latter case (Scheme 3).13 In order to further probe the reactivity of the dinuclear ([PhP(CH2SiMe2NPh)2]Ta)2(m-H)4, a reaction with diphenylacetylene was conducted in an effort to form the symmetrically-bridged product. However, the slow reaction gave a radically different product, an asymmetric dinuclear complex, wherein one Ta was bound to an Z2-diphenylacetylene ligand, and the other Ta-supported ancillary ligand had undergone rearrangement to a new imido ligand and a s-phenyl bound to the Ta, with losses of 1 equiv. of H2 and benzene observed.

Scheme 3

52

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

In 2014 Fryzuk et al. published the results of the reaction of TaCl3(Z2-RC]CR)(DME) (R ¼ SiMe3, Et) with an o-phenylene-linkeddiamidophosphine that resulted in the facially-coordinated [Ta(Z2-alkyne)(PhNPN )]Cl (PhNPN ¼ 2,20 -phenylphosphinidenebis (4-methyl-N-mesityl-aniline)).14 Reaction of the product with KBEt3H gave the corresponding hydride (Scheme 4). It was then discovered that reaction with 2,6-dimethylisocyanide or phenylacetylene led to insertion into the TadH bond, resulting in the formation of iminoformyl (proposed, undetected) and phenylvinyl (well-characterized) complexes. Reductive elimination and intramolecular reduction led to CdC bond formation and cyclometallation, yielding azadiene and phenyl-vinyl derivatives, respectively.

Scheme 4

The Z2-alkyne benzyl compounds Ta(RC]CR)(CH2Ph)[NPN ] (R ¼ SiMe3, Et; NPN ¼ PhP(2-(N-mesityl)-5-Me-C6H3)2) were prepared from the corresponding alkyne chloride [Ta(2-alkyne)(PhNPN )]Cl, synthesized above, and KCH2Ph (Scheme 5).15 Addition of H2 to these compounds resulted in reduction of the alkyne ligands with loss of toluene and formation of a TadH bond. In the case of the 3-hexyne analogue, alkyne hydrogenation was accompanied by isomerization of 3-hexene to 1-hexene.

Scheme 5

Fryzuk and Parker have published a comprehensive review of Ta and Nb alkyne complexes.16 It was found that TaIII and NbIII dinuclear halide complexes containing thio- and selenoether ligands of the form III (M X2L)2(m-X)2(m-L) (M ¼ Nb: X ¼ Cl, Br; L ¼ R2S, R2Se;M ¼ Ta: X ¼ Cl, Br; L ¼ R2S) were effective regioselective catalysts for

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

53

alkyne cyclotrimerization, giving in some cases >99:1 selectivity of head-to-tail cyclotrimerization of phenylacetylene, in up to 96% yields under mild conditions.17 The choice of ligand and solvent were shown to be critical, with Me2S and THT in dichloromethane being optimal for the transformation to take place selectively and efficiently. It was presumed that the active catalyst bears two Z2-phenylacetylenes at either transition metal center. Smith and Hill found that the reaction of Ta(Z2-RC]CR)Cl3(DME) with Na[HB(methimazolyl)3] (R ¼ Et, Ph) led to the corresponding tantalum tris(methimazolyl)borato complexes, abbreviated in the scheme for clarity (Scheme 6).18

Scheme 6

5.03.3

h3-Complexes of tantalum

The compound TaCl3(NtBu)py2 was shown to react with either 2-methallyl or 2-butenyl magnesium chloride to yield tris(Z3-2methallyl)tBu-imidotantalum, or the 2-butenyl compound, respectively, in good yields (Scheme 7).19 Treatment of these compounds with 2 equiv. of 2,6-dimethylphenylisocyanide gave the bis(iminoacyl). A mono(iminoacyl) intermediate was isolated by simple addition of 1 equiv. of isocyanide. The tris(allyl) compounds also reacted with ketones, as illustrated by the reaction of benzophenone with tris(Z3-2-methallyl)tBu-imido tantalum, yielding the trialkoxy compound Ta(OCPh2(CH2CMeCH2))3(NtBu).

Scheme 7

Further research showed that [MCl2(m-Cl)(NtBu)py]2 (M ¼ Nb, Ta), when treated with allyl magnesium chloride (4 equiv.), was converted to M2(m-C3H5)(Z3-C3H5)(m-Cl)2(NtBu)2py2 (Scheme 8).20 The Nb and Ta products were found to be diamagnetic by NMR spectroscopy, and single-crystal X-ray diffractometry gave a Nb–Nb distance of 3.214(1) A˚ , interpreted as a Nb(III) to Nb(V) dative bond.

54

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

Scheme 8

The first example of a mono(allyl) tantalum tetrachloride complex was prepared by Bochmann et al. via metathesis of TaCl5 with (Me3Si)CH]CHCH(SiMe3)(SnMe3), yielding [Z3-1,3-(SiMe3)2C3H3]TaCl4 and Me3SnCl (Scheme 9).21 A crystal structure was unobtainable, so the compound was treated with 2 equiv. of TMEDA, resulting in formation of the tantalum alkylidene Me3SiCHCH(SiMe3)C]TaCl3(TMEDA) via loss of TMEDA•HCl, the first known allyl-to-alkylidene transformation. The solidstate structure was obtained for a derivative.

Scheme 9

In 2007 the ylid complex Cp TaCl4(CH2]PPh3) was reacted with NaN(SiMe3)2, resulting in double CdH activation at two adjacent cyclopentadienyl methyl groups, yielding the Z3:Z4-tuck-in compound (Z3:Z4-C5Me3(CH2)2TaCl2(CH2PPh3).22 In an attempt to obtain the ylid carbyne, Sundermeyer et al. found that Cp TaCl4 reacted with CH2]PPh3 and LiHMDS in a 1:3:3 ratio, giving the alkylidene-alkylidyne compound (Scheme 10).

Scheme 10

5.03.4

h4-Complexes of tantalum

The first tantalum Z4-butadiene complex [Ta(Z4-C4H6)3]− (Fig. 1) was synthesized by Ellis and co-workers in the reaction of [Ta(Z4-C10H8)3]− (C10H8 ¼ naphthalene) with 1,3-butadiene, and shown to be a useful source of Ta-23 The analogous niobate [Nb(Z4-C4H6)3]− was prepared from the anionic [Nb(Z4-C10H8)2(PMe3)2]−. These derivative compounds represented the first isolable homoleptic butadienemetallates of the 4d and 5d metals. In a later publication, the Ellis group used tris(Z4-naphthalene) tantalate to perform an unprecedented double-orthometallation of the bis(triphenylphosphano)iminium [PPN] cation, on two

Fig. 1

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

55

separate phenyl groups of the triphenylphosphine moiety (Scheme 11).24 The third phenyl group was reduced to 1,3cyclohexadiene, bound Z4 to tantalum. This was the first example of an irreversible reaction of the PPN cation, proving that it is not always an ancillary cation.

Scheme 11

5.03.5

h6-Complexes of tantalum

A variety of neutral, monocationic, and dicationic ansa-(Z5-cyclopentadienyl,Z6-arene) tantalum compounds were obtained from the cationic complex [(Z6-ArCMe2-Z5-C5H4)TaPr]+[B(C6F5)4]− (Ar ¼ 3,5-Me2C6H3) (Scheme 12).25,26 Hydrogenolysis of [(Z6-ArCMe2-Z5-C5H4)TaPr]+[B(C6F5)4]− formed the cationic Ta hydride [(Z6-ArCMe2-Z5-C5H4)TaH]+[B(C6F5)4]−, which inserted into the di- and trisubstituted alkenes cyclopentene and 2-methyl-2-pentene to give Ta alkyl compounds. By contrast, styrene yielded a s3-benzyl compound. The neutral compound (Z6-ArCMe2-Z5-C5H4)Ta(nPr)Br was synthesized by addition of [Bu4N]+Br− to [(Z6-ArCMe2-Z5-C5H4)TaPr]+[B(C6F5)4]−. Hydride abstraction from [(Z6-ArCMe2-Z5-C5H4)TaH]+[B(C6F5)4]− by [Ph3C]+[B (C6F5)4]− in C6D5Br generated the dication [(Z6-Ar-CMe2-Z5-C5H4)Ta(C6D5Br)]2+{[B(C6F5)4]−2 2. Displacement of the by perdeuterated THF proceeded in near-quantitative yield, giving the bis(d8-THF) adduct. All compounds retained the ansa-configuration with the same general donor-acceptor interaction, though structural and computational studies suggested that increasing positive charge leads to a greater degree of electrostatic interaction between the tantalum and the arene ring.

Scheme 12

The tantalum complex Bn3Ta]NCMe3 (Bn¼ benzyl) reacted with B(C6F5)3 to afford the zwitterionic Z6-arene-coordinated compound [Bn2Ta]NCMe3]+[Z6-BnB(C6F5)3]− (Scheme 13).27 The benzylborate anion was displaced when 3 equiv. of pyridine were added.

Scheme 13

56

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

5.03.6

h7-Complexes of tantalum

The scope of cycloheptatriene-tantalum chemistry was greatly expanded by the work of Girolami and co-workers, who synthesized the synthetically-useful Ta(Z6-C7H8)Cl2(PMe3)2 (Scheme 14).28 The compound was prepared from TaCl5, 3 equiv. of BuLi, and PMe3, after which a brown solution formed to which cycloheptatriene was then added. Addition of MeLi prompted the loss of HCl, yielding the cycloheptatrienyl complex (Z-C7H7)TaCl(PMe3)2. Alternatively, sodium cyclopentadienide formed the mixedsandwich structure. The Cp analogue was synthesized by treating TaCl5 with Cp SiMe3 to yield the known piano-stool complex Cp TaCl4, followed by reduction by Mg in the presence of cycloheptatriene. (Z-C7H7)Cp Ta had been prepared previously by the same researchers and reported earlier, as well as the methylcyclopentadienyl derivative.29 (Z-C7H7)Cp Ta, when treated with allyl bromide, afforded (Z-C7H7)Cp TaBr. Treatment of (Z-C7H7)Cp TaBr with LiAlH4 gave the tantalum hydride (Z-C7H7)Cp TaH, while with alkyllithium, Grignard, or di(Grignard) reagents the alkylated (C7H7)Cp TaR were formed. While yields were generally modest, the utility of the starting complex was demonstrated clearly.

Scheme 14

5.03.7

h8-Complexes of tantalum

The compound Ta(Z4-C8H8)(Z8-C8H8)I was synthesized from napthalenate complexes by cation-exchange metathesis via the following sequence. Reacting [Na(THF)6]+[Ta(Z4-C10H8)3]− with 3 equiv. of COT (COT ¼1,3,5,7-cyclooctatetraene) generated [Na(THF)x]+[Ta(Z4-COT)3]−.30 Addition of bis(triphenylphosphine)iminium chloride formed [PPN]+[Ta(Z4-COT)3]−. Finally, addition of elemental iodine gave Ta(Z4-C8H8)(Z8-C8H8)I.

5.03.8

h2-Complexes of niobium

5.03.8.1

Alkene complexes

Carbon tetrachloride was used as a chlorine source in the radical chlorination of a number of alkenes, catalyzed by NbVCl3(enediamido) complexes.31 The non-innocent enediamido ligand both accepted and released a single electron as CdCl bond homolysis and formation occurred. The reaction was facilitated by the different coordination modes of the enediamido ligand, either as Z4(s2,p) or as planar k2(N,N0 ). The NbVCl3(enediamido) complex hydrodehalogenated alkyl chlorides and bromides in the

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

57

presence of PhSiH3.32 At low catalyst loading, moderate to excellent yields were obtained, with the best results in cases where bromide was removed. NbVCl3(enediamido) catalytically converted benzyl ethers to the corresponding benzyl chlorides in the presence of stoichiometric SiCl4, with production of SiO2 as by-product.33 rac-(Monoazabutadiene)Nb(NtBu)(NPh)(OEt2) reacted with diphenylacetylene or 3-hexyne, forming eight-membered metallacycles from the alkyne, monoazabutadiene and imido ligands (Scheme 15).34 Single-crystal X-ray diffractometry showed that the newly-formed alkene alpha to nitrogen interacts with niobium in an Z2 fashion. Similar reactivity was observed with norbornene.

Scheme 15

5.03.8.2

Alkyne complexes

Alkyl- and aryl-substituted niobium Tp’ (Tp’− ¼ hydrotris(3-methylimidazolyl)borate) alkyne complexes have been reported. Tp’Nb(alkyne) could be synthesized by either metathesis of NbIIICl3(PhC^CMe)(DME) and KTp’ or reduction of Tp0 NbCl3 with Zn in the presence of the alkyne.35,36 Tp’NbMe(c-C3H5)(MeC^CMe) generated a transient intermediate, Tp’Nb(Z2-c-C3H4) (MeC^CMe), by b-H abstraction from the cyclopropyl ligand to give methane as by-product (Scheme 16).32,37,38 The intermediate activated stereospecifically CdH bonds of benzene, leading to phenyl and 2-benzyne compounds when PMe3 or CO were added.37,39 Benzylic protons could also be abstracted across the Nb(Z2-cyclopropene) bond.40 The CdH bonds of the compounds furan, thiophene, phenylacetylene, ferrocene, pentaflurobenzene, and cyclopentene were activated to form metal-carbon s-bonded derivatives.32 Interestingly, the complex Tp’Nb(MeC^CMe)Me2(C6F5)(c-C3H5) underwent cyclopropyl ring-opening followed by intramolecular alkyne coupling to two isomers at 318 K. This pattern of reactivity was established in an earlier publication.39,41

Scheme 16

Tetrapodal amine bis(phenolato) and tripodal amine phenolato compounds of niobium were synthesized by addition of a mixture of triethylamine and the corresponding phenol or bis(phenol) to NbCl3(MeC^CMe)(DME). The new compounds Nb(MeC^CMe)Cl[ONNO]R ([ONNO]R ¼ N(CH2CH2NMe2)2(CH2C6H2-3,5-R2-2-O); R ¼ H, Me), Nb(MeC^CMe)Cl2[ONN]Me ([ONN]Me ¼ NMe(CH2CH2NMe2)(CH2C6H2-3,5-Me2-2-O), and Nb(MeC^CMe)Cl2[ONOO]R ([ONOO]R ¼ N(CH2CH2OMe)2 (CH2C6H2-3,5-R2-2-O); R ¼ tBu, Me) were formed (Scheme 17).42

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

Scheme 17

58

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

59

Bergman, Arnold, and coworkers synthesized Nb(Z2-PhC^CPh)(BDI)(NtBu)(CO) (BDI ¼ N,N0 -bis(2,6-diisopropylphenyl)-bdiketiminate) by reaction of Nb(BDI)(NtBu)(CO)2 with diphenylacetylene (Scheme 18).43 The analogous isocyanide compound Nb(Z2-PhC^CMe)(BDI)(NtBu)(CNtBu) was synthesized by the reaction of 5 equiv. of tBuNC and 1-phenyl-1-propyne with Nb(BDI)(NtBu)(CO)2. These studies were conducted in order to obtain information on the structure of the thermally-unstable Nb(Z2-MeC^CPh)(BDI)(NtBu)(CO), which produced a 2:1 ratio of b-methylstyrene and 1-phenyl-1-propyne.44 The unstable carbonyl compound was proposed as an intermediate in the partial hydrogenation of alkynes to cis-alkenes under a CO/H2 atmosphere. Further studies of Nb(BDI)(NtBu)(CO)2 with terminal alkynes45 showed that it reacted with 2 equiv. of t-butylacetylene, resulting in coupling of the alkyne and carbonyl ligand to an “yne-one-ene” niobium complex.

Scheme 18

Mindiola and co-authors found that Nb(Z2-AdCCH)(PNPP)(ArO) (Ad¼ 1-adamantyl; PNP− ¼ N(2-PiPr2-4-methylphenyl)−2, Ar ¼ 2,6-iPr2C6H3) is produced by the addition of phosphaalkyne adamantylidyne phosphorus (PCAd) to (PNP)(ArO)Nb^CH through PdP and C^C bond formation.46

5.03.9

h4-Complexes

The first niobium naphthalene complex [Na(THF)]+[Nb(Z4-C10H8)2(PMe3)2]− was synthesized by Ellis and co-workers by addition of NaC10H8 to NbCl4(THF)2, followed by treatment with excess trimethylphosphine, and shown to be a reliable source of Nb− as it reacted with 1,3-butadiene to give the homoleptic [Nb(Z4-C4H6)3]−.23 This compound and the Ta analog represent the first isolable homoleptic butadienemetallates of the Group five 4d and 5d metals.

5.03.10

h5-Complexes of tantalum

The chemistry of Z5-cyclopentadienyl-ligated complexes of tantalum is varied and myriad. As the cyclopentadienyl moiety and related derivative ligands typically function as ancillary ligands in transition metal chemistry, the chemistry of note featured in the following sections will generally occur at other ligands.

5.03.10.1 Mono(cyclopentadienyl) complexes An assortment of variably-substituted tantalum amidate complexes Cp Ta(RNC(O)R0 )X3 (R ¼ Me2C6H3, iPr, R0 ¼ tBu, Ph, X ¼ Cl, Me) were synthesized by metathesis of Cp TaCl4 and the appropriate sodium amidate.47 The reduction of the tantalum chloro amidates with 2.2 equivalents of KC8 under dinitrogen atmosphere resulted in the extrusion of the amidate oxygen, forming a terminal oxo-compound of the form Cp TaO(Z2-RNCR0 )Cl. The same results were observed when Cp2Co was used as the reducing agent. (Cp TaCl2)2(m-N2) was used to form dinuclear amidate complexes similarly by salt metathesis. Smith et al. synthesized novel chlorobis(methimazolyl)borate compounds containing niobium or tantalum from Ph3Sn[HB(mt)3] (mt ¼ methimazolyl) and CpMCl4 (M ¼ Ta, Nb).48 Products could not be isolated from simple metathesis with TaCl5 or NbCl5. Sodium dihydrobis(methimazolyl)borate reacted with Cp TaCl4 to give the reduced TaIV compound Cp TaCl2[H2B(mt)2].49 The d1 paramagnetic compound, expectedly, gave little information by way of NMR spectroscopy. Single-crystal X-ray diffractometry showed a nearly linear BdHdTa interaction.

60

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

In 2006, 1,4-(SiMe3)2N)2C6Me4 (N,N,N0 ,N0 -tetrakis(trimethylsilyl)-2,3,5,6,-tetramethyl-1,4-phenylene-diamine) was used to form dinuclear and mononuclear imido complexes of the general formulas [M(Z5-C5H4SiMe3)Cl2]2(m-1,4-NC6Me4N) and M(Z5-C5H4SiMe3)(]NC6Me4-4-N(SiMe3)2)Cl2 (M ¼ Ta, Nb) (Scheme 19).50

Scheme 19

In 2011 Howard et al. published two studies, the first where Li+[Cp Ta(C10H15)(2-tBuC6H4)2Cl(Et2O)]− was synthesized by reacting Cp TaCl4 with 4 equivalents of LiNH(2-tBuC6H4) in ether.51 In the second study, a series of Cp Ta imido compounds Cp Ta(NtBu)(CH2R)2 (R ¼ Ph, CMe2Ph, CMe3) were synthesized by reaction of alkyl Grignard reagents with Cp Ta(NtBu)Cl2. Reaction of the dibenzyl compounds with 2 equiv. of C6F5OH yielded the dinuclear [Cp Ta(CH2Ph)(OC6F5)(m-O)]2, via loss of t-butylamine, and an assumed loss of bis(pentafluorophenyl) ether, the m-oxygen ligands balancing the charge. The homobenzyl analogue yielded a tetraphenoxide derivative, when reacted with C6Cl5OH, as well as a m-O dinuclear compound of similar structure to that above.52 TaRCl3[C6H4(2-CH2NMe2)-k2C,N] (R ¼ Cl, Z5-C5H5, Z5-C5H4SiMe3, Z5-C5Me5) were synthesized from the corresponding TaRCl4 and ortho-lithiated dimethylbenzylamine.53 Cp TaCl4 was reacted with polyhedral ibutyloligosilsesquioxane, eliminating HCl and forming a polyhedral tantalum oligosilsesquioxane chloride (Scheme 20).54 Chloride is readily displaced with methyllithium, silver triflate, or LiB(C6F5)4 to give the corresponding metathesis products.

Scheme 20

In 2007 Sundermeyer et al. reported that tantalum phosphinomethylidyne complexes were obtained by treatment of CpTaCl4 with either 5 or 7 equivalents of the ylide Ph3PCH2, yielding [CpTa(^CPPh3)(]CHPPh3)Cl] and [CpTa(^CPPh3) (]CHPPh3)2].55 Cp Ta(^CPh)(PMe3)2Cl treated with 3-hexyne, with and without Na(HCB11Cl11), led to the formation of cationic and neutral tantallacyclic complexes, respectively.56 Treatment of Cp Ta(^CPh)(PMe3)2Cl with alkyne in perdeuterotoluene resulted in a tantallacyclobutadiene. When the starting alkylidyne was first treated with a chloride abstracting agent, AgBArF4 for example, followed by addition of 3-hexyne, the cationic tantallacyclobutadiene was obtained.

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

61

The Li group reported that reactions of CpRMCl4 (CpR ¼ Cp , Cp; M ¼ Ta, Nb) with one equivalent of the phosphorus ylid (Et2N)3P]CH2 gave the pseudo-octahedral ylid adducts of general formula CpRMCl4[CH2]P(NEt2)3].57 When Cp TaCl4 was reacted with 3 equiv. of this ylid, a salt was obtained having the formula [H3CP(NEt2)3]+[Cp TaCl5]−. Its formation was proposed to proceed through an ylid adduct, alkylidene formation (not isolated, from elimination of HCl), yielding [H3CP(NEt2)3]+Cl−. This phosphonium salt then reacted with an additional equivalent of Cp TaCl4 to give the ionic product. Later in 2013, the same researchers reported that reactions of (C5Me4H)TaCl4 with 5 equiv. of Ph3P]CH2 gave the phosphoniomethylidyne complex [(C5Me4H)Ta(^CPPh3)(]CHPPh3)Cl], alternatively synthesized from the same starting materials along with 3 equiv. of LiN(SiMe3)2 and 3 equiv. of Ph3P]CH2, in quantitative yield.58 Many tantallaboranes bearing Cp ligands have been synthesized by Ghosh et al. over the past fifteen years since an earlier report from the Messerle group of Cp 2Ta2(B2H6)Cl2 and Cp 2Ta2(B2H6)2 syntheses from the reactions of Cp 2Ta2(m-Cl)4 with LiBH4.59 The complex (Cp Ta)2B5H11 reacted at the terminal B-H site with arenes via CdH activation and formation of a CdB bond (Scheme 21).60 When the arene in question has a substituent, para is favored over meta-activation, with no observed ortho-coupling. It is also of note that halide-substituted arenes were not reduced. Thiophene and pyrrole derivatives were also studied. In 2012 Ghosh et al. reacted (Cp Ta)2B5H11 with halide sources, yielding (Cp TaX)2B5H11 (X ¼ Cl, Br, I).61 (Cp Ta)2B4H9(m-BH4) reacted with iodine to give (Cp Ta)2B4H8I(m-BH4).

Scheme 21

The reaction of Cp TaCl4 or CpNbCl4 with LiBH4, then subsequent addition of Ph2Se2, gave (Cp∗ Ta)2B4H11(SePh), the known (Cp∗ Ta)2B5H11, and, in the case of Nb, (CpNb)2B4H9(m-SePh) and the known (CpNb)2(B2H6)2.62 (Cp Ta)2B5H11 was also obtained from the reaction of (Cp Ta)2B4H8(m-BH4) and excess Fe2(CO)9 that gave (m3-BH)(Cp TaCO)2(m-CO)[Fe(CO)3] (Fig. 2).63,64 Addition of 6 equiv. of LiBH4
THF, followed by BH3
THF, to Cp TaCl4 formed (Cp Ta)2B4H10, (Cp Ta)2B5H11, (Cp Ta)2B5H10(C6H4CH3), and (Cp TaCl)2B5H11.65 The treatment of Cp TaCl4 with LiBH4
THF, followed by addition of BH3
THF, generated an oxatantallaborane, (Cp Ta)2B4H10O, in low yield.66 The authors speculated that the source of oxygen was water or air bound to silica gel used to purify the reaction mixture. Further reaction of the oxatantallaborane with Fe2(CO)9 yielded the hetero-tetrametallic compound (Cp Ta)2B2H4O [H2Fe2(CO)6BH]. (Cp Ta)2B4H9(m-BH4) reacted with Fe2(CO)9 to generate three products, (Cp Ta)2B5H7[Fe(CO)3]2, (displaying bicapped pentagonal bipyramidal geometry), the fused (Cp Ta)2B5H9[Fe(CO)3]4, and (m3-BH)(Cp TaCO)2(m-CO)[Fe(CO)3] (Fig. 3).67 (Cp Ta)2B4H8(m-BH4) reacted with Fe2(CO)9, forming (m3-BH)(Cp TaCO)2(m-CO)[Fe(CO)3].64 Oxametallaboranes (Cp M)2(B4H10O) (M ¼ Ta, Nb) were synthesized by treating (Cp M)2(B2H6)2 with dioxygen.68 Asymmetric homodimetallic (Cp Ta)(Cp TaCl)B9H16 (Fig. 4) was obtained from the reaction of Cp TaCl4 and LiBH4
THF, followed by addition of BH3
THF, and displayed a 1-vertex nido-cage geometry.69 LiBH4
THF reacted with Cp TaCl4 and BH3
THF to form the m-Z2 acyl compound (Cp Ta)2B4H8(m-Z2-COCH3).70 It was determined that the source of the organic ligand was the borane-THF adduct, as shown by deuterium labeling studies. (Cp Ta)2B4H9(m-BH4) was isolated from the reaction of BH3
THF and (Cp Ta)2(B2H6)2.64,71

Fig. 2

62

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

Fig. 3

Fig. 4

Cp TaCl4 reacted with Li(BH2S3) to yield three structurally-related trinuclear clusters, [(Cp Ta)3(m-S)3(m3-S)3B(R)] (R ¼ H, SH, Cl) and the tetranuclear (Cp Ta)4(m-S)6(m3-S)(m4-O), the oxo source resulting from the chromatographic separation on silica gel. Similar reactions were carried out with Li(BH2Se3) and Li(BH2Te3) and gave (Cp Ta)3(m-Se)3(m3-Se)3B(H) and (Cp Ta)2(m-Te)2, respectively.72 Thermolysis of Cp TaCl4 in the presence of Li(BH2Se3) yielded (Cp Ta)3(m-Se)3(m3-Se)3B(OBuCl) and (Cp∗Ta)3 (m-Se)6.73 Cp TaCl4 reacted with LiBH4 and S2CPPh3 to give (Cp Ta)2(m,k2-B2H5)(m-H)(m,k2-S2CH2)2.74 Additionally in this study, [CpNb(m-EPh)2(m-Z2:Z2-B2H4E), (E ¼ S or Se) was reported and structurally characterized. Dinuclear and trinuclear cyclopentadienyl sulfide tantalum complexes were synthesized by addition of bis(trimethylsilyl) sulfide to Cp TaCl4. When a 4:3 ratio of (Me3Si)2S to Ta was employed, Cp 3Ta3Cl3(m3-Cl)(m-S)3(m3-S) was generated.75 Cp 2Ta3Cl4(m3-Cl) (m-S)3(m3-S) was synthesized when 1 equiv. of Cp TaCl4 was replaced with TaCl5. The former trinuclear complex could be reduced by thermal treatment with phenylsilane, yielding dinuclear Ta2(Z5-C5Me5)2Cl2(m-S)2, which was also formed directly by reaction of Cp TaCl4, bis(trimethylsilyl) sulfide, and phenylsilane. Treatment of the dinuclear compound with alkyl nucleophiles resulted in substitution of the chloride ligands and generation of the peralkylated complexes Cp 2Ta2R2(m-S)2 (R ¼ Me, Et, CH2SiMe3, C3H5, Ph). Cp MCl4 (M ¼ Nb or Ta) reacted with Na5[B(SCH2S)4] to give dimetallic thiolate complexes [(Cp M)2(m-S){m-C(H) S3-k2S:k2-S0 ,S00 }{m-SC(H)S-k2C:k2S000 ,S0000 }].76 Notable is the inclusion of a square pyramidal carbon in both thiolate complexes. The compounds [(Cp Nb)2(m-S)(m-SCH2S-k2S,S0 )(m,Z2:Z2-BH3S)] and [(Cp Ta)2(m-O)(m-SCH2S-k2S,S0 )(m-H){m-S2C(H)SCH2Sk2S00 :k2S000 ,S0000 }] were also isolated. [(Cp Ta)2(m-S){m-(SBS)S(CH2S)2(BH2S)-k2B:k2S:k4S0 ,S00 ,S000 ,S0000 }] was prepared by addition of Na5[B(SCH2S)4] to Cp TaCl4 and found to contain a boron with square-pyramidal geometry (Fig. 5). When (Cp Ta)2(B2H6)Cl2 reacted with Ru3(CO)12, Cp TaCl(m-Cl)B2H4Ru3(CO)8 and (Cp Ta)2(B2H6)(B2H4Cl2) were formed.77 Trimetallic tantalum boronate clusters were readily synthesized from Cp TaMe4 and the boronic acids PhB(OH)2 or iBuB (OH)2 and water, generally of the form (Cp Ta)3(m-Z2-RBO2)3(m-O)2(m-OH)(m3-OH).78 For R ¼ Ph, a Lewis-acidic cavity was accessible to Lewis bases such as acetone. Poly-mono(cyclopentadienyl)tantalum and poly-niobocene complexes with arylimido-based simple or dendritic carbosilanes were obtained by reaction of N,N-bis(trimethylsilyl) arylamine dendrimers with Cp TaCl4 or Cp0 NbCl4 (Cp0 ¼ C5H4SiMe3).79 In order to form the niobocene, the dendrimer-ligated mono(cyclopentadienyl)niobium complex was reacted with LiCp0 . Hydrolysis of the dendritic niobocene yielded Cp0 2NbCl(O).

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

63

Fig. 5

Thiobis(phenolato) complexes, Cp TaCl2(k3-tbop), Cp TaCl2(k3-tbcp), Cp TaMe2(k3-tbop), Cp TaMe2(k3-tbcp), Cp TaMe (k -tbop)(OTf ), Cp TaMe(k3-tbcp)(OTf ), Cp Ta(k3-tbop)(OTf )2, and Cp Ta(k3-tbcp)(OTf )2 (tbop ¼ 2,20 -thiobis(6-tert-octylphenolato), tbcp ¼ 2,20 -thiobis(4,6-dichlorophenolato); OTf ¼ triflate), were synthesized by treatment of Cp TaCl4 with the appropriate thiobis(phenol) and triethylamine or Cp TaMe4 and thiobis(phenol) alone.80 The triflates were prepared by protonolysis of methyl ligands with triflic acid. Cp TaMe2(k3-tbcp) surprisingly did not react with carbon monoxide nor isocyanides, but the triflate derivatives did react, albeit slowly, with one equivalent of 2,6-dimethylphenylisocyanide or tbutyl isocyanide, yielding Cp Ta(k2-Me2CNxylyl)(k3-tbcp) (xylyl ¼ 2,6-dimethylphenyl) and Cp TaMe(k2-MeCNtBu)(k3-tbcp). In 2010, the cationic tantalum aminocarbene complexes [Cp Ta{C(Me)N(H)Ar-k1-C}(OH)(k3-tbop)]OTf and [Cp Ta{C(Me)N(H)Ar-k1-C}(OH)(k3-tbcp)] OTf were synthesized via insertion of the isocyanides into the TadMe bond of Cp TaMe(k3-tbop)OTf and Cp TaMe(k3-tbcp)OTf, reported earlier,80 in the presence of 1 equiv. of water.81 Salt metathesis with NaBPh4 released NaOTf and yielded the tetraphenylborate salts. The chloride is easily formed by addition of HCl in dichloromethane. It was found that the triflate and chloride were air-stable, while the tetraphenylborate salts were not. The source of the stability was thought to be H-bonding interactions between the anion and N-H and O-H moieties in the cation. Cp TaCl2{(N-C6H4)2S-k3-N,S,N} and Cp TaMe2{(N-C6H4)2S-k3-N,S,N} were synthesized from Cp TaCl4 or Cp TaMe2Cl2 and 2,20 -diaminophenyl sulfide, using piperidine or nBuLi respectively as base (Scheme 22).82 Cp TaCl2{(N-C6H4)2S-k3-N,S,N} was then monomethylated by addition of 1 equiv. of trimethylaluminum or converted to the dihydride by addition of 2 equiv. of sodium triethylborohydride. The latter reacted with tbutylisocyanide via double-insertion, yielding a tantallaziridine. The dimethyl derivative reacted with B(C6F5)3 to give the unstable cationic complex [Cp TaMe{(N-C6H4)2S-k3-N,S,N}][MeB(C6F5)3]. 3

Scheme 22

Cp TaX2{[O(CH2)2]2S-k3O,S,O} (X ¼ Cl, Me), as well as the 2,20 -thiobis(6-tert-octylphenolate) ligand analogue, were synthesized respectively by treatment of Cp TaX4 with triethylamine and the appropriate ligand or by protonolysis of a methyl ligand.83 The chloride compounds were further derivatized to the dihydrides by reaction with NaBHEt3 and mono or bistriflate compounds by treatment of the dimethyl derivative with one or two equivilents of triflic acid, respectively. The reactivities of the dimethyl, dihydride, and monomethyl(monotriflate) compounds were examined with the isocyanides tbutyl isocyanide and 2,6-dimethylphenyl isocyanide. Treatment of Cp TaH2{[O(CH2)2]2S-k3O,S,O} with either isocyanide produced the azatantallacyclopropanes in quantitative yields. Also interesting was the reaction of Cp TaMe(OTf ){[O(CH2)2]2S-k3O,S,O} with 2,6-dimethylphenyl isocyanide in water, as it yielded a cationic aminocarbene (Scheme 23).

Scheme 23

64

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

Cp TaMe[(OCH2)2py]OTf reacted with LiN(SiMe3)2 in acid-base fashion, deprotonating the methylene position alpha to one alkoxy-oxygen.84 Further reaction with phenylacetylene or (trimethylsilyl)acetylene gave vinylic alkenyl tantalum products regioselectively via attack at the internal position of the alkyne. Similar nucleophilicity was observed in reaction with dipyridylketone, giving rise to a dipyridylalkoxy ligand. Further treatment of the alkenyl tantalum complexes with 2 equiv. of triflic acid generated, through protonation of the alkyl and vinylic MdC bonds, bis(triflate) compounds with a non-coordinating alkene moiety.85 However when the vinylic substituent bore a primary amino group, three products of protonolysis were formed. When the amine was tertiary, only one product formed from loss of methane and protonation of the amine. Other examples of dearomatization and derivatization of tantalum-containing O,N,O pincer ligands were reported by Rodriguez et al.86 The water-soluble cationic tantalum complexes [Cp TaMe(OCH2)2py(H2O)]OTf, [Cp Ta(OH)(OCH2)2py(H2O)]OTf, and [Cp Ta(OCH2)2py(H2O)2](OTf )2 were obtained by the addition of water to [Cp TaMe(OCH2)2py]OTf, yielding [Cp TaMe (OCH2)2py(H2O)]OTf that hydrolysized slowly to [Cp Ta(OH)(OCH2)2py(H2O)]OTf (Scheme 24).87 Protonation of the hydroxo complex by treatment with triflic acid led to the diaqua species without destruction of the dialkoxy pincer ligand. These compounds marked the first examples of water-soluble organotantalum complexes with alkyl ligands. Further studies were reported of ONO ligand reactivity displacement with fluoride and additional reactivity studies.88,89

Scheme 24

Cp TaMe(OTf )[(OOC)2py-k3O,N,O] reacted with methanol to eliminate methane and introduce a methoxide ligand, or with triflic acid to give the bis(triflate).90 When reacted with tBuCN, Cp TaMe(OTf )[(OOC)2py-k3O,N,O] generated the iminoacyl derivative as a triflate salt. Remarkably, this compound was water-soluble when converted to the aqua-adduct. In 2012, it was reported that Cp TaMe4 reacted with 2-(salicylideneamino)benzoic acid to yield Cp TaMe2(OOCPhN]CHPhO-k3O,N,O).91 The compound was then reacted with Bronsted and Lewis acids. Reaction with triflic acid generated methane and formed a coordinated triflate. Addition of B(C6F5)3 formed an adduct between boron and a carboxylate oxygen. The adduct was split by addition of t butylisocyanide. Addition of HBF4 resulted in loss of methane and formation of an adduct between BF3 and a carboxylate oxygen. Addition of tbutylisocyanide or 2,6-dimethylphenylisocyanide to the triflate derivatives formed tantalum iminoacyl compounds via insertion into the TadMe bond. In order to further their studies of 2,6-dicarboxylic acids as ligands en route to water-soluble organotantalum compounds, Ruiz, Fandos, Hernández, Otero and Rodríguez reacted chelidamic acid with Cp TaMe3(OTf ), releasing two equivalents of methane and yielding the pincer complex Cp TaMe(OTf )((OOC)2py(OH)-k3O,N,O).92 Hydrolysis gave rise to the dinuclear complex [{Cp Ta((OOC)2py(OH)-k3O,N,O)}2(m-OH)3]OTf, which was only partially soluble in water. Similar experiments with citric and diglycolic acids gave the expected bis(carboxylate) complexes, citrate also bearing a tantalum alkoxide bond. Cp TaCl4 reacted with 2,6-pyridinedicarboxylic acid with loss of 2 equivalents of HCl to yield the air- and moisture-stable pincer complex [Cp TaCl2{2,6-(O2C)2py-k2O,O}].93 Reaction with silver triflate gave the expected [Cp TaCl(OTf ){2,6-(O2C)2py-k2O,O}]. [Cp TaCl2{2,6-(O2C)2py-k2O,O}] gave rise to the dinuclear derivative [Cp Ta(m-O){2,6-(O2C)2py-k2O,O}]2. When reacted with water, the dinuclear oxo compound was found to be in equilibrium with [Cp TaCl2(m-OH)]2(m-O), with a slight preference for the latter. Treatment of either of these dimeric compounds with triflic acid yielded [{Cp Ta{2,6-(O2C)2py-k2O,O}}2(m-OH)3](OTf ). Facial and meridional derivatives of tantalum-digol (digol ¼ diethylene glycolate) complexes were synthesized from either treatment of Cp TaMe4 with 2,20 -oxybis(ethanol) or Cp TaCl4 with the dilithio salt of 2,20 -oxybis(ethanol) (Fig. 6).94 Facial isomers were observed when the starting material was Cp TaMe4. Treatment of fac-Cp TaMe2(k3-digol) with H2O
B(C6F5)3 yielded the tantalum-oxo borane TaCp {OB(C6F5)3}(k3-digol). Triflic acid and fac-Cp TaMe2(k3-digol) generated the triflate complex, which was found to react slowly with 2,6-dimethylphenylisocyanide in water to generate a dicationic aminocarbene complex, [TaCp {C(Me)NH(xylyl)}(OH2)(k3-digol)][OTf]2.

Fig. 6

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

65

In a series of studies that investigated the ligation of Group 4 Cp complexes with di(2-pyridyl)amide (dpa), the tantalum analogue was also synthesized.95 A solution of Cp TaCl4 was treated with dpaLiin toluene. 1H NMR spectroscopic and single-crystal X-ray crystallographic studies showed two distinct pyridine groups, with the amido group situated in the equatorial plane, and pyridine axial in the pseudo-octahedron. One pyridine was not complexed in the solid-state structure, and no evidence of pyridine exchange was observed in the 1H NMR spectrum. The tantalum siloxide complex Cp TaMe3(OSiPh3) was synthesized by simple addition of triphenylsilyl alcohol to Cp TaMe4, with loss of methane (Scheme 25).96 The siloxide complex was then reacted with isocyanides to produce azatantallacyclopropanes Cp Ta(Z2-Me2CNtBu)Me(OSiPh3) and Cp Ta(Z2-Me2CN(2,6-Me2Ph)Me(OSiPh3). The latter complex reacted with an additional equivalent of 2,6-dimethylphenylisocyanide to give the alkenylamido complex Cp Ta(NAr)[N(xylyl)CMe]CMe2](OSiPh3) (xylyl ¼ 2,6-dimethylphenyl). This alkenyl-amido complex was synthesized directly by addition of two equivalents of isocyanide to the parent siloxide. Also probed was the reactivity of Cp TaMe3(OSiPh3) with excess carbon monoxide, which resulted in formation of the alkenylalkoxide Cp Ta(O)(OCMe]CMe2)(OSiPh3) with all methyl ligands incorporated into the alkenyloxide ligand. Infrared spectroscopy however showed no strong Ta]O stretching absorption, and it was therefore concluded that the molecule existed in oligomeric forms with m-oxo ligands.

Scheme 25

It was shown with a variety of cyclopentadienyl ligands that TaCpxCl2Me2 (Cpx ¼ Cp, Cp , C5H4SiMe3) reacted with 2,6-dimethylphenylisocyanide to yield TaCpxCl2(CMe2NAr-k2C,N) (Ar ¼ 2,6-Me2C6H3).97 Alkylation with Mg(CH2SiMe3)Cl yielded Cp TaCl(CH2SiMe3)(CMe2NAr-k2C,N) which, in turn, decomposed to CH3SiMe3 and the propenyl imido compound Cp TaCl{C(Me)]CH2}(NAr). Similar reactivity was observed when other alkyllithium or alkyl Grignard reagents were added; however, the intermediate alkyl species was not detected in 1H NMR spectroscopic studies. TaCpxCl2(CMe2NAr-k2C,N) reacted with ethylene to form the corresponding azatantallacyclopentane that decomposed in solution to give dichloro(imido) tantalum compounds. 1H NMR spectroscopy showed the simultaneous formation of 2-methyl-2-butene and 3-methyl-1-butene. The reactivity was tested with acetylene, and azatantallacyclopentene products were formed. The azatantallacyclopentene was alkylated giving derivatives that were found to rearrange to the alkenyl-enamido(imido)(methyl) derivatives TaCp Me(NAr)(Z1-N(Ar)C(Me)]CHdCH]CMe2).98 Gomez et al. prepared trialkylchloro and dialkyldimethyl Cp Ta complexes of the form Cp TaClxMeyRz (x ¼ 1, y ¼ 0, z ¼ 3, R ¼ CH2SiMe3; x ¼ 0, y ¼ z ¼ 2, R ¼ CH2Ph, CH2SiMe3, CH2CMe3), by reaction of Cp TaCl4 or Cp TaCl2Me2 with the alkylating agent.99 When Cp TaCl2Me2 was reacted with 2 equiv. of LiCH2CMe2Ph, three products were formed: [Cp TaMe2(CH2CMe2Ph)]2 (m-O), Cp TaMe2 (CH2CMe2-o-C6H4-k2C,C), and Cp TaMe(CH2CMe2Ph)(CH2). Under rigorously anhydrous conditions, the same reaction did not yield the dimethyl(neophyl)(m-oxo) complex. Heating Cp TaMe2R2 (R ¼ CH2SiMe3, CH2CMe3) led to formation of the alkylidenes Cp TaMeR(CHSiMe3) (R ¼ CH2SiMe3, Me in 3:2 ratio) and Cp TaMeR(CHCMe3) (R ¼ Me CH2CMe3 in 4:1 ratio), respectively. Photochemical studies gave similar products, albeit in different ratios. MCl5 (M ¼ Ta, Nb) reacted with 5-(SiClMe2)-2,5-(SiMe3), affording [M{Z5-C5H3(SiClMe2)(SiMe3)}Cl4].100 In the solid-state the niobium homologue was found to be dinuclear with two m-Cl ligands. Reaction of either homologue with 2,6dimethylphenylisocyanide produced the pseudo-octaheadral adducts M{Z5-C5H3(SiMe3)(SiClMe2)}Cl4(CNAr) (Ar ¼ 2,6-Me2C6H3). Imido compounds were synthesized by a variety of approaches, for example by reaction of the starting material with 1.5 equiv. LiNHtBu. Finally, the SidCl bond was cleaved by addition of 4 equiv. of tBuNH2, forming Nb[Z5-C5H3(SiMe3)(SiMe2NtBu-kN)]Cl(NtBu), or by

66

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

addition of ethylenediamine and triethylamine which gave M{Z5-C5H3(SiMe3)[SiMe2N(CH2)2NH2-k2N,N]}Cl3 (M ¼ Ta, Nb). Further methylation studies on these compounds were also reported.101 Cp (Z-C5H4SiMe3)TaIVCl2 and Cp CpTaIVCl2, when exposed to air, were converted in moderate yields to the corresponding cationic species [Cp (Z-C5H4SiMe3)TaCl(OH)]+ Cl− and [Cp CpTaCl(OH)]+ Cl−.102 The hydroxo ligand was easily deprotonated with either KOH or Et3N in toluene to afford the tantalum-oxo compounds in good yield. The oxo compounds were converted back to cations by treatment with either acid or trimethylsilyl triflate. When treated with the alcohols ROH (R ¼ SiiPr3, 2,6-Me2C6H3, 2,6-iPr2C6H3), Cp TaMe4 formed the corresponding alkoxide/ aryloxide with loss of methane.103 Reaction of these complexes with methyl methacrylate in the presence of Al(C6F5)3 yielded highmolecular-weight syndiotactic poly(methylmethacrylate), albeit with low catalytic efficiency. [Cp TaMe2(OR)]+[MeAl(C6F5)3]− cationic intermediates were isolated, which reacted with a further equivalent of Al(C6F5)3 to yield [Cp TaMe2(OR)]+[Me {Al(C6F5)3}2]−. The boron analogue B(C6F5)3 was not effective in the polymerization of methyl methacrylate. Whereas typical hydrolyses of Cp TaCl4 formed oxo-bridged dinuclear complexes, a Lewis-acid-stabilized terminal oxo-complex was synthesized by the controlled hydrolysis of Cp TaCl4.104 When reacted with Lewis acid-base complex H2O
B(C6H5)3, Cp TaCl4 formed the borane-stabilized oxo complex Cp TaCl2O
B(C6F5)3. Analogous compounds were prepared when Cp Ta(CHPh)(CH2Ph)2 and Cp TaMe4 were used as starting materials, forming Cp TaR2O
B(C6F5)3 (R ¼ CH2Ph, Me). Cp TaCl2O
B(C6F5)3 also reacted with pyridine to afford the known oxo-bridged dinuclear compound [Cp TaCl2(m-O)]2. When Cp TaR2O
B(C6F5)3 (R ¼ CH2Ph, Me) were reacted with CO or aryl isocyanide, insertion into the metal alkyl bond was observed, with Z2 side-on bonding of the acyl or iminoacyl groups.105 Interestingly, and unlike the alkylated precursor materials, the oxo-borane bond could be broken using pyridine without the formation of dinuclear m-O complexes. The cationic complex [Cp TaMe3]+[MeB(C6F5)3]− was prepared by the abstraction of methyl anion from Cp TaMe4 by B(C6F5)3 (Scheme 26).106 Reaction with pyridine cleanly gave the Lewis acid-base adduct. Hydrolysis with water afforded the dicationic dinuclear complex [(Cp TaMe2)2(m-O)][MeB(C6F5)3]2, whereas hydrolysis with H2O
B(C6F5)3 resulted in the formation of, with loss of methyl groups as methane, the previously-reported compound Cp TaCl2O
B(C6F5)3. Reactions of [Cp TaMe3]+[MeB(C6F5)3]− with simple organic substrates were also studied. When this cationic Ta complex was generated in situ in the presence of C2H4, polymerization produced HDPE but with poor activity at 5 atm of pressure. When reacted with benzaldehyde, [Cp TaMe3]+[MeB(C6F5)3]− attacked at the carbonyl by its methyl group to give the expected tantalum alkoxide in 1 h at room temperature. This compound was prepared by first reacting Cp TaMe4 with benzaldehyde and then treating with B(C6F5)3, albeit at 70  C over 24 h. The alkoxide complex reacted further with benzaldehyde, forming the corresponding bis(alkoxide). Reactivity with phenyl isocyanate formed the cationic dimethyl amidate complex [Cp TaMe2{Z2-OC(Me)NPh}] [MeB(C6F5)3]. Addition of a further equivalent of isocyanate gave rise to cationic [Cp TaMe{Z2-OC(Me)NPh}2][MeB(C6F5)3]. Contrasting reactivity was observed when phenyl isocyanate was reacted with Cp TaMe4, returning only [(Cp TaMe3)2(m-O)].

Scheme 26

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

67

 00 The d2-d2 dinuclear compounds (Z5-C5Me4R)2TaIII 2 (m-Cl)4 (R ¼ Me, Cp ; R ¼ Et, Cp ) were found to react under 100 psi of dinitrogen in toluene to form the disproportionation product of four-electron reduction of dinitrogen, [(C5Me4R)TaVCl2]2(m-Z1,Z1-N2), along with the cationic organotritantalum cluster [(C5Me4R)3Ta3(m-Cl)6]+[(C5Me4R)TaCl4]− (Scheme 27).107 The m-dinitrogen complex was easily synthesized directly from mononuclear Cp TaCl4 under dinitrogen pressure with Na/Hg reduction, or stepwise in high yield by the reaction of Cp TaCl4 with N2H4 to form the m-hydrazine adduct [(C5Me4R)TaCl4]2(m-Z1, Z1-N2H4), followed by deprotonation with triethylamine. The (C5Me4R)2Ta2(m-Cl)4 compounds were also shown to react with allene, giving a novel dinuclear allene coordination, [(C5Me4R)2Ta2(m-Cl)Cl3(m-Z1,Z3-C3H4), which was described as a four-electron reduced allene ligand with the central carbon doubly-bonded in alkylidene-type fashion to one Ta and the sp3-hybridized CH2 groups bonded as alkyls to the other Ta with one b-agostic CdH...Ta interaction.108 It was shown earlier that this allene complex, upon reduction with Na/Hg, underwent a double intramolecular CdH activation to give the novel m-propynylidene complex, Cp 2Ta2Cl2 (m-Z1,Z1-HCCCH)(m-H)2 with a planar tetracoordinate carbon and, based on theory, a three-center, two-electron bond involving the central planar carbon and both tantalums.109

Scheme 27

(Z5-C5Me4R)Ta(hpp)Cl3 (R ¼ Me, Et; hpp ¼ anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine, hppH) was formed readily by addition of (hpp)SiMe3 to (Z5-C5Me4R)TaCl4, with loss of volatile Me3SiCl.110 Reduction of the former with sodium amalgam produced the first lower-valent Ta hpp complex, (C5Me4R)TaIV(hpp)Cl2 (R ¼ Me, Et). Interestingly, it was found that the same compound was synthesized when two equivalents of (hpp)SiMe3 were added to the dinuclear (Z5-C5Me4R)2Ta2(m-Cl)4. The amidinate complex Cp Ta[N(iPr)C(Me)N(iPr)]Cl3 was reduced by 2.5 equiv. of KC8 under dinitrogen atmosphere to give the amidinate m-dinitrogen complex [Cp Ta{N(iPr)C(Me)N(iPr)}Cl]2(m-N2) (Scheme 28).111 Further reduction removed the chlorides, yielding [Cp Ta{N(iPr)C(Me)N(iPr)}]2(m-N2), which was also prepared directly by employing 4 equiv. of KC8. Upon warming, the structure isomerizes to Cp Ta{N(iPr)C(Me)N(iPr)}(m-N)2, splitting the N2 ligand to two m-nitrides. Further treatment with PhSiH3 yielded an asymmetric dinuclear complex stereospecifically, bearing only one m-nitrido ligand, with one tantalum bearing a hydride and the other the imidophenylsilane. Further attempts to reduce the compound, even at temperatures up to 100  C, were unsuccessful and gave only epimerization.

68

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

Scheme 28

A class of TaIV neutral and cationic alkyl compounds have been synthesized that exhibited little to no b-hydride and b-methyl elimination.112 Alkylation of Cp TaV[N(iPr)C(Me)N(iPr)]Cl3 with 3 equiv. nBuLi gave the dialkylated Cp TaV[N(iPr)C(Me)N (iPr)](Bu)2. Three equivalents of EtLi yielded the ethyl ethylidene (Z5-C5Me5)TaEt(CHCH3)[N(iPr)C(Me)N(iPr)]. The diethyl derivative was obtained also by first reducing the starting material to TaIV, followed by addition of 2 equiv. of EtLi. iBuLi gave results similar to that of n-butyl lithium (Scheme 29). Different results were observed when NpLi (Np ¼ CH2CMe3) was employed, as addition of 3 equiv. of NpLi produced the monoalkylated compound, wherein the acetamidinate ligand had been converted to

Scheme 29

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

69

the eneamidinate. The compound was protonated with (PhNHMe2)+[B(C6F5)4]− to return the acetamidinate cation complex. All compounds were found to be stable in solution at room temperature over extended periods. Tantalum amidinate and guanidinate imido complexes were synthesized as mononuclear models of end-on-bridged dinuclear m-dinitrogen compounds.113 Lithium tbutylamide reacted with Cp Ta[N(iPr)C(Me)N(iPr)]Cl2 to afford Cp Ta[N(iPr)C(Me)N(iPr)] [NH(tBu)]Cl. Addition of a second equivalent of amide gave ill-defined oils. However, treatment with LiNEt2 deprotonated the amidinate ligand’s methyl position, yielding the enamido(amido) compound Cp Ta{N(iPr)C(CH2)N(iPr)}(NHtBu). Tautomerization yielded the tbutylimido complex Cp Ta[N(iPr)C(CH3)N(iPr)][N(tBu)]. Synthesis of the guanidinate analogue required a decidedly different approach. Cp Ta[N(iPr)C(NMe2)N(iPr)]Cl2 reacted with LiNHtBu, yielding Cp Ta[N(iPr)C(NMe2)N(iPr)] [NH(tBu)]Cl with no methyl group available for deprotonation. However, it was found that the formation of the imido moiety could be easily achieved by oxidative means, namely the employment of TEMPO radical (TEMPO ¼ 2,2,6,6-tetramethylpiperidin-1yloxyl) to yield the TaV imido Cp Ta[N(iPr)C(NMe2)N(iPr)][NtBu)]Cl. Reduction with KC8 formed the desired TaIV guanidinate. Both complexes were inert to H2 after 72 h at 100  C. Reaction of the guanidinate compound with MeI gave two products, the methyl imido and iodo imido Cp Ta guanidinates. When a symmetrical coupling partner was used in the reaction, only one product formed. Diphenyl disulfide yielded only a sulfido imido compound. Similar reactivity was observed when the dinuclear [Cp Ta {N(iPr)C(Me)N(iPr)}]2(m-N2) was reacted with (PhS)2, which formed the symmetrical {Cp Ta[N(iPr)C(Me)N(iPr)](SPh)}2 (m-Z1:Z1-N2). [Cp M(N(iPr)C(R)N(iPr))]2(m-N2) (M ¼ Ta, Nb; R ¼ Me, Ph, NMe2) rearranged upon thermolysis to form dinuclear bis (m-nitrido) complexes.114 Detailed kinetic analyses found that an intermolecular isomerization via side-on m-N2 was the likely pathway. The R groups had a pronounced effect on the rate constant of the reaction, namely that more sterically-demanding R groups increased the kinetic stability of diniobium N2 compounds, thereby making them isolable. In order to evaluate a Schrock-type dinitrogen fixation cycle with tantalum, Cp Ta[N(iPr)C(NMe2)2N(iPr)](NtBu) was reacted with lithium 1,1-dimethylhydrazide to provide the TaV hydrazido complex Cp Ta[N(iPr)C(NMe2)N(iPr)](Z-NNMe2)Cl.115 Subsequent reduction provided Cp TaIV[N(iPr)C(NMe2)N(iPr)](Z-NNMe2), shown to be paramagnetic and thermally unstable. Both TaV and TaIV complexes could be methylated at the N-dimethylamino nitrogen in high yields by addition of methyl triflate. Reduction of the TaIV compound with sodium amalgam in THF resulted in the TaVTaV bis(m-nitrido) complex [Cp Ta(N(iPr)C (NMe2)N(iPr))(m-N)]2 in both cis and trans configurations. When a similar reduction was conducted in toluene, however, the mononuclear Cp Ta[N(iPr)C(NMe2)N(iPr)](NH)(OTf ) was formed. The different reduction results were postulated to arise from the lower concentration in toluene of a presumed terminal nitride intermediate, allowing competition of other reactions to predominate. Under dinitrogen atmosphere, reaction of Cp TaCl4 with di-p-tolyl disulfide and 5 equiv. of KC8 yielded the dinuclear, 4-electron-reduced m-dinitrogen complex [Cp Ta(SC6H4Me)2]2(m-Z1:Z1-N2).116 Addition of Mo(CO)6 or Cr(CO)6 gave sulfur bridged tri- and tetranuclear complexes. Tantalum homoenolates Cp TaCl3(CH2CR12C(]O)OR2) were synthesized from Cp TaCl4 and Zn(CH2CR12CO2R2)2 (R1 ¼ H, Me; R2 ¼ Me, Et, p-tolyl).117 Treatment with AgOTf led to the substitution of the chlorides by triflates. While the triflate compound was inert to CO, methyl acrylate, and PhCCPh, 2,6-dimethylphenylisocyanide led to multiple isocyanide insertions (Scheme 30) starting at the TadC bond of the homoenolate. Treatment of the homoenolate bearing CdH bonds a to the carbonyl with KN(SiMe3)2 yielded Cp TaCl2(CH2CH]C(O)OEt) with an enolate alkene-metal dative bond. In the absence of acidic a-hydrogens, addition of the dilithium salt of 1,4-bis(p-methoxyphenyl)-1,4-diaza-1,3-butadiene (p-MeOC6H4-DAD) resulted in Cp (Z4-p-MeOC6H4-DAD)Ta(CH2CMe2(C]O)O).

Scheme 30

A diethylsilyl-bridged tetramethylcyclopentadienyl-phenoxy ligand was synthesized by addition of nBuLi to (2-allyloxy-3-tbutyl5-methylphenyl)(2,3,4,5-tetramethylcyclopenta-2,4-dien-1-yl)diethylsilane, forming the dilithio species, followed by ligation to TaCl5, generating the piano-stool complex (Z-C5Me4SiEt2-3-tBu-5-Me-2-C6H2-Z1-O)TaCl3 in moderate yield.118 Treatment with MeLi gave the trimethyl derivative (Z5-C5Me4)SiEt2(3-tBu-5-Me-2-C6H2O)TaMe3. Reaction with [Ph3C]+[B(C6F5)4]− resulted in

70

Cyclic and Non-Cyclic p-Complexes of Tantalum and Niobium

methyl abstraction and formation of the B(C6F5)−4 salt of (Z5-C5Me4SiEt2(3-tBu-5-Me-2-C6H2-Z1-O)TaMe2, while treatment with B(C6F5)3 led to the formation of the MeB(C6F5)−3 salt of (Z5-C5Me4SiEt2(3-tBu-5-Me-2-C6H2-Z1-O)TaMe2. Cp-carboranyl groups were ligated to TaCl2Me3 employing the dilithio species Li2[C5H4CMe2(C2B10H10)](OEt2)2, yielding Ta(Cp-carboranyl)Me3.119 Subsequent reaction with carbodiimides resulted in the formation of azametallacyclic compounds. Addition of B(C6F5)3(THF) formed the salt [Ta{Z5-C5H4CMe2(C2B10H10)-s-C}Me2(THF)][B(C6F5)3Me].119 The new salt-free reducing agents 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene and 1,10 -bis(trimethylsilyl)-1,10 -dihydro-4,40 -bipyridine were shown to cleanly reduce Cp TaCl4 by 2 electrons per equivalent, and the organotantalum product was trapped with N,N0 -bis(4-methoxyphenyl)-1,4-diaza-1,3-butadiene (a-diimine), yielding Cp TaCl2(a-diimine).120 When no trapping ligand was employed, one electron reduction gave the known dinuclear [Cp TaCl2]2(m-Cl)2. CpTa(PMe3)(NAr)(H)(SiR3) (Ar ¼ 2,6-di-isopropylphenyl) and CpTa(PMe3)(NAr0 )(H)(SiR3) (Ar0 ¼ 2,6-dimethylphenyl) were discovered as intermediate compounds in the reactions of CpTa(NAr0 )(PMe3)2 with hydrosilanes.121 Chlorosilanes showed a negative coupling constant for J(Si-H) in 1H NMR spectroscopy, consistent with Ta-H agostic interactions, in contrast to classical alkylsilyl metal (hydrides), which display positive coupling constants. In the presence of excess NaBH4 and PMe3, CpRTa(NAr)Cl2 (CpR ¼ Cp, Cp ; M ¼ Ta, Nb; Ar ¼ 2,6-C6Hi3Pr2) were converted to their dihydride derivatives CpRM(NAr)H2(PMe3) (Scheme 31).122 Without excess phosphine, these reactions inevitably led to the formation of the dinuclear products [CpRM(m-NAr)(k2-BH4)]2 (M ¼ Ta, Nb), bearing a single MdM bond.

Scheme 31

CpTa(NAr)Cl2 were readily converted to CpTa(NAr)[NtBu(SiHMe2)]Cl by addition of LiN(SiHMe2)tBu
THF.123 It should be noted that Ar was not defined in the report, nor in supporting information for the tantalum compounds. Chloride abstraction was not achievable despite the steric crowding of the Cp, arylimido, and amido ligands. In order to work around the issue, CpTa(NAr) (H)(Cl)(PMe3) was used as an alternative starting material. With one equivalent triphenylborane and LiN(SiHMe2)tBu
THF, CpTa(NAr){Z3-NtBu(SiMe2H)}(H) was formed, exhibiting spectral evidence for an agostic tantalum H-Si interaction. The terminal phosphinidene complex Cp Ta(c-P6Ph5)(Ph) was synthesized in low yields by reaction of Cp TaCl4 with Na2(P4Ph4) in a 1:2 molar ratio.124 The structure was determined by single-crystal X-ray diffractometry and 1H and 31P NMR spectroscopies. Terminal alkyl(phosphanylidene)tantalum complexes with TaIV and TaV formal oxidation states were formed from addition of 2 equiv. of LiPHR (R ¼ Cy, tBu, Ph, Mes) to Cp TaCl4.125 One product was the paramagnetic trans-[{Cp TaIVCl(m-PR)}2] complex. In an effort to preclude reductive dimerization, bis(amido) compound [Cp Ta{1,2-(NSiMe3)2C6H4}Cl2] was employed as reactant. Under the same reaction conditions, [Cp TaV{1,2-(NSiMe3)2C6H4}(PR)] [R ¼ Cy, iPr] were obtained. Attempts to synthesize the adamantyl and tbutyl phosphanylidene derivatives by the latter method were unsuccessful, resulting instead in reduction to [Cp TaIV{1,2-(NSiMe3)2C6H4}Cl]. Later, it was found that [Cp Ta{1,2-(NSiMe3)2C6H4}(PR)] (R ¼ Cy, iPr) reacted with CO or CyNC to give the first examples of tantalum phosphaketene and phosphaazaallene complexes.125 The latter formed both E- and Z-isomers, exhibiting different electronic structures as evidenced by large differences in the 31P NMR chemical shifts. Cp TaCl4 and Cp0 TaCl4 (Cp0 ¼ C5Me4H) were reacted with primary and secondary ferrocenylphosphines to form the octahedral phosphine adducts CpRTaCl4(PH2Fc) (CpR ¼ Cp , Cp0 ), CpRTaCl4(PH2CH2Fc), and CpRTaCl4[PH(CH2Fc)2].111 With a bis(phosphine) ligand, wherein one phosphino group was on each of the two ferrocenyl rings, a bridged complex was formed.

5.03.10.2 Tantalum mono(cyclopentadienyl) heterodimetallic compounds Reaction of the tantalum alkylidene [Cp Ta(CHSiMe3)(CH2SiMe3)Cl], prepared from Cp TaCl4 and LiCH2SiMe3, and Li[Cp IrH3] afforded the heterobimetallic Cp (Me3SiCH2)2TaIrCp (H)2.126 The short intranuclear distance between the metals, 2.4457(3) A˚ , was judged to be consistent with an unbridged Ta]Ir double bond. Hydrolysis yielded Cp (O)(Me3SiCH2)Ta(m-H)3IrCp . Addition of primary amines such as iPrNH2 to Cp (Me3SiCH2)2TaIrCp (H)2 gave the heterodinuclear m-trihydride imido complex [Cp (iPrN)(Me3SiCH2)Ta(m-H)3IrCp ] with liberation of 1 equiv. of SiMe4. The reaction was accelerated under an atmosphere of dihydrogen. Ammonia, on the other hand, afforded both the heterodinuclear-bridged hydride imido Cp (HN)(Me3SiCH2)Ta (m-H)3IrCp and the tantalum amide Cp (H2N)(Me3SiCH2)TaIrCp (H)2, with the Ta–Ir interaction still intact as shown by 1H NMR spin-saturation transfer experiments. Cp (Me3SiCH2)2TaIrCp (H)2 combined with 2 equiv. of substituted phenols, ArOH, gave the tantalum(bis)aryloxide, which was further reacted with an additional equivalent of the substituted phenol to give Cp (ArO)3Ta(m-H)3IrCp .

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Fig. 7

Tethered heterobimetallics of Ta or Nb and Ti have been synthesized. [TiCl2{Z5-C5H4SiMe2-m-N(CH2)2-kNH2}MCpRCl4] (Fig. 7) (M ¼ Ta, Nb; CpR ¼ C5H4SiMe3, Cp ) were prepared by reaction of CpRMCl4 (M ¼ Nb, Ta; CpR ¼ C5H4SiMe3, Cp ) with Ti(Z5-C5H4SiMe2N(CH2)2-s-NH2)Cl2 in benzene-d6 and CDCl3.127 Cp∗TaIVCl3PMe3, when reacted with Li[C5H4(CH2)2PR2], yielded Cp∗[Z5-C5H4(CH2)2PR2]TaIVCl2 (R ¼ Ph, Cy), with the phosphine moiety pendant to the cyclopentadienyl ring not coordinated to tantalum.128 The phosphine was then ligated to the complex [Ru(p-cymene)Cl2]2 (p-cymene ¼ 1-iPr-4-MeC6H4) in order to generate the Ta-Ru heterodimetallic Cp∗TaCl2(Z5-C5H4(CH2)2PR2Ru (Z6-p-cymene)Cl2. This species was exposed to air to generate a monohydroxo-tantalum chloride which was deprotonated readily with Et3N to form the terminal tantalum-oxo derivative. When tested as a catalyst in cyclopropanation, the Ta-Ru heterodimetallic complexes Cp∗TaCl2(Z5-C5H4(CH2)2PR2Ru(Z6-p-cymene)Cl2 (R ¼ Ph, Cy) activated ethyl diazoacetate for addition to phenylethylene, with a slight preference for the trans cyclopropane. Tantalum c-P4 Cp compounds were used as c-P4 transfer reagents to copper halides, yielding non-classical, fullerene-like supramolecules composed of c-P4 units, and varying stoichiometries of copper halide.129 2-{(E)-[(2-Hydroxyphenyl)imino]methyl}phenol (H2salaph) was used to produce [TaCp Cl2(salaph)] by reaction with  Cp TaCl4 under microwave conditions.130 It was found to exhibit strong cytotoxicity to cancer cells while exhibiting low in vitro toxicity to healthy MRC-5 and human hepatocyte cells. 2-Ethoxy-6-{(E)-[(2-hydroxyphenyl)imino]methyl}phenol was used to generate a similar Ta compound shown to be cytotoxic in vitro against the A2780 human ovarian carcinoma cell line.131

5.03.10.3 Bis(cyclopentadienyl) complexes of tantalum The tantalum alkylidene Cp2Ta(CH2)(CH3) reacted with 1 equiv. of Al(C6F5)3 to give the acyclic Cp2Ta+(CH3)[m-CH2Al−(C6F5)3].132 Excess Al(C6F5)3 yielded the four-membered heterometallacyclic [Cp2Ta(m-CH3)(m-CH2)Al(C6F5)2)]+[Al(C6F5)4]−. Cp2Ta(CH2)(CH3) and 2 equiv. of Al(C6F5)3 catalyzed the polymerization of methyl methacrylate and of N,N-dimethylacrylamide. The hydrazido complex (dme)TaCl3(NNPh2) was found to be a reactive synthon for a variety of derivatives.133 In particular, reaction with 2 equiv. of NaCp gave Cp2TaCl(NNPh2) in good yield. The monocyclopentadienyl complex was not synthesized; attempts to do so gave 0.5 equiv. starting material and 0.5 equiv. of the Cp2TaCl(NNPh2) complex. The anionic dihydride [K(crypt]+[Cp2TaH2]− was synthesized by deprotonation of Cp2TaH3 with KH in the presence of [2.2.2] cryptand (crypt) (Scheme 32).134 Reaction of [K(crypt]+[Cp2TaH2]− with CO2 resulted in the neutral compound Cp2TaH(CO) as well as KOH and KHCO3, with the K+ cations bound by crypt. [Rh(m-Cl)(dippp)]2 (dippp ¼ 1,3-bis(di-ipropylphosphino)propane) reacted with [K(crypt]+[Cp2TaH2]−, eliminating KCl and forming the m-dihydride Cp2Ta(m-H)2Rh(dippp). Similarly, RuHCl(CO) (PiPr3)2 and RuHCl(N2)(PiPr3)2 reacted to form the corresponding Cp2Ta(m-H)2RuH(L)(PiPr3)2 (L ¼ CO, N2).

Scheme 32

Cp2TaH3 was shown to act as a halogen-bond acceptor towards the iodine in iodopentafluorobenzene.135

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5.03.10.4 Linked cyclopentadienyl complexes of tantalum Methylene-bridged bis(cyclopentadienyl) groups were shown to form dinuclear Ta and Nb carbonyl complexes of the form (OC)4M(Z-C5H4CH2-Z-C5H4)M(CO)4 when Na[M(CO)6] and Na2[CH2(C5H4)2] were combined in a 2:1 molar ratio in the presence of I2.136 Single-crystal X-ray diffractometry showed the adoption of an “anti” configuration of NbI centers in the diniobium species, whereas the ditantalum analogue adopted a “syn” orientation of TaI centers.

5.03.11

h5-Complexes of niobium

The following section encompasses the literature reports that feature a cyclopentadienyl ligand coordinated to a niobium center. Many papers, while reporting CpNb complexes, also focused on Ta analogs and are discussed more thoroughly in the previous Ta cyclopentadienyl section. Generally, the syntheses of CpNb starting materials were conducted by the addition of CpSiR3 (R ¼ Me) or CpSnR3 (R ¼ Bu) to the relevant niobium pentahalide, almost exclusively (NbCl5)2. Previous editions of COMC have highlighted much of that relevant literature.

5.03.11.1 Mono(cyclopentadienyl) complexes of niobium Publications from the Royo and Mosquera groups focused in part on the reactivity of the piano-stool complexes CpRNbCl4 (CpR ¼ C5Me4H, C5H4SiMe2Cl, or C5H4SiMe3).137,138 In 2005 they reported that [Mg(NAr)(THF)]6 (Ar ¼ 2,6-Me2C6H3, Ph) reacted with CpClNbCl4 (CpCl ¼ C5H4(SiMe2Cl)) to form CpClNbCl2(NAr) (Ar ¼ 2,6-Me2C6H3, Ph) (Scheme 33).137 Subsequently, the niobium imido compounds were reacted with LiNHAr (Ar ¼ 2,6-Me2C6H3) to yield the mononuclear compound (Z-C5H4SiMe2-s-NAr)NbCl(NAr) compound, and with H2O to give imido piano-stool Nb2 complexes linked by Si-O-Si-linked cyclopentadienyl groups, Cl2(ArN)Nb(Z-C5H4SiMe2OSiMe2-Z-C5H4)Nb(NAr)Cl2. In 2008 it was reported that the a,b-unsaturated carbonyl compounds (denoted LL) methyl acrylate, methyl methacrylate, and 4-methylpent-3-en-2-one reacted with NbCpRCl4 (CpR ¼ C5Me4H, C5H4SiMe2Cl, C5H4SiMe3) and sodium amalgam to form enolate oxametallacylic NbIII compounds of the general form NbCpRCl2(LL).2

Scheme 33

Royo and Mosquera reported the formation of terminal M]O bonds in 2010. Hydrolysis of Cp0 NbCl4 (Cp0 ¼ Z-C5H4SiMe3) with the adduct H2O
B(C6F5)3 formed the oxo-borane complex Cp0 NbCl2[OB(C6F5)3]. Subsequent reaction with Mg(CH2Ph)2 (THF)2 resulted in the expected alkylation, yielding Cp0 Nb(CH2Ph)2[OB(C6F5)3].139 The corresponding methyl derivative was synthesized directly from Cp0 NbMe4 and H2O
B(C6F5)3. These compounds were converted to the oxo-alane through substitution by Al(C6F5)3, with loss of the borane.

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Royo et al. showed that mono- and bis-cyclopentadienyl complexes of niobium (and tantalum) bearing a silylallyl substituent were synthesized readily from 1,1-[SiMe2(CH2CH]CH2)]2C5H4 and NbCl5.140 Reduction of Nb{Z5-C5H4SiMe2(CH2CH]CH2)} Cl4 with Na/Hg in the presence of LiCp resulted in the formation of the NbIV compound CpNb(Z5-C5H4SiMe2CH2CH]CH2)Cl2. The tantalum homologue was synthesized in reverse fashion by addition of LiCp followed by reduction. Alternatively, reaction of Nb(Z5-C5H4SiMe2(CH2CH]CH2))Cl4 with LiNHtBu afforded the imido derivative Nb{Z5-C5H4SiMe2(CH2CH]CH2)}(NtBu) Cl2. The latter was alkylated with Grignard reagents or an additional LiCp. The dicyclopentadienyl tantalum homologue was also synthesized, albeit from the lithium salt of the allylsilylcyclopentadienyl proligand. The above authors went on to explore imido niobium complexes with the (dichloromethylsilyl)Cp ligand. CpCl2NbCl4 (CpCl2 ¼ 5 Z -C5H4(SiCl2Me)) was synthesized from NbCl5 and C5H4(SiCl2Me)(SiMe3) (Scheme 34).103 Further ligand substitution with 2 equiv. of LiNHtBu gave exclusively CpClNNbCl2(NtBu) (CpClN ¼ Z5-C5H4[SiClMe(NHtBu)]) by displacement of both NbdCl and SidCl bonds. No singly-substituted product could be isolated from these experiments. However, treatment of CpClNNbCl2(NtBu) with MeSiCl3 resulted in breaking the SidN bond, followed by formation of CpClNbCl2(NtBu). CpNNbCl2(NtBu) reacted further with LiNHtBu to yield the amido-imido complex CpNNbCl(NHtBu)(NtBu). Upon heating, chloro and amido ligands migrated, generating [Nb{Z5-C5H4[SiMe(NHtBu)2]}Cl2(NtBu)], which formed an amido-imido derivative upon treatment with LiNHtBu. Contrasting results were observed when excess LiNHtBu was added to CpNNbCl2(NtBu), giving rise to the constrained-geometry complexes [Nb{Z5-C5H4[SiMe(NHtBu)(NtBu)]}(NHtBu)(NtBu)] and [Nb{Z5-C5H4[SiClMe(NtBu)]}(NHtBu)(NtBu)].

Scheme 34

A niobium b-diketiminate (BDI) complex containing cyclopentadienyl and imido ligands was synthesized via the reaction of NaCp with (BDI)Nb(NtBu)Cl2py (Scheme 35).141 The initial result was formation of a (k1-N-BDI)CpNb(NtBu)Cl complex, the increased steric congestion having forced monodentate coordination of the BDI ligand. To encourage bonding in a bidentate fashion, and to synthesize potentially more reactive species, the chloride ligand was abstracted or substituted. Treatment of (k1-N-BDI)CpNb(NtBu)Cl with NaBPh4 resulted in chloride abstraction and formation of the corresponding cation, wherein the BDI ligand returned to the more-commonly observed chelation. Simple reduction of (k1-N-BDI)CpNb(NtBu)Cl with KC8 formed KCl and the neutral Nb(IV) compound, (k2-N,N0 -BDI)CpNb(NtBu). The latter compound was also formed when cobaltocene was employed as the reducing agent. Finally, substitution by hydride and deuteride reagents was studied. (k1-N-BDI)CpNb(NtBu)Cl was reacted with NaAlH2(OCH2CH2OCH3)2 (Red-AlTM) to yield (k2-N,N0 -BDI)CpNb(NtBu)H or, when LiAlD4 was employed, (k2-N, N0 -BDI)CpNb(NtBu)D formed, both complexes displaying a chelating b-diketiminate ligand.

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Scheme 35

Cp2Nb(NtBu)(PPh2) was synthesized from the lithium phosphide LiPPh2
Et2O and Cp2Nb(NtBu)Cl by the Nikonov group (Scheme 36).142 Reacting Cp2Nb(NtBu)(PPh2) with [Rh(C2H4)2(m-Cl)]2 resulted in Cp for chloride exchange and formation of the heterodinuclear compound Cp(Cl)Nb(m-NtBu)(m-PPh2)RhCp. However, when [Rh(COD)(m-Cl)]2 was employed as the Rh source, Cp2Nb(NtBu)(m-PPh2)Rh(Cl)(COD) was produced. The latter reacted with AgBF4 to cyclize into a heterodinuclear cation [Cp2Nb (m-NtBu)(m-PPh2)Rh(COD)]+(BF4)−. The parent compound Cp2Nb(NtBu)(PPh2) and Cp(Cl)Nb(m-NtBu)(m-PPh2)RhCp were each found to be precatalysts for the hydrosilylation of acetophenone and benzaldehyde.

Scheme 36

A phosphinomethylidyne complex, CpNb(CPPh3)Cl2, was synthesized by transylidation of CpNbCl4 with 3 equiv. of Ph3P] CH2, likely through a six-coordinate ylid complex that was deprotonated twice in sequence by the additional two equivalents of Ph3P]CH2. Sundermeyer et al. further reported on the formation mechanisms and the single-crystal X-ray diffraction results, with a three-legged piano-stool conformation observed in the solid-state.143 In 2009 Hernandez et al. synthesized Nb{Z5-C5H3(SiMe3)2}RR0 (NtBu) (R ¼ Cl, R0 ¼ Me; R ¼ R0 ¼ Me, CH2SiMe3) by addition of aminosilsesquioxane Si8O12R7(NH2) to [Z5-C5H3(SiMe3)2]NbCl4.144 In 2019 Santamaria and coworkers found that the trinuclear compound [Cp 3Nb3Cl3(m3-Cl)(m-S)3(m3-S)] could be prepared by the reaction of (Me3Si)2S with Cp NbCl4, via Me3SiCl elimination.145 Reduction with various agents, phenylsilane in particular, resulted in the formation of the symmetric, diamagnetic, dinuclear NbIV compound Cp 2Nb2Cl2(m-S)2. This dinuclear complex was also prepared directly via a one-step procedure from Cp NbCl4. A series of alkylated derivatives were then synthesized via simple metathesis by alkyllithiums or Grignard reagents.

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{Me3SiNP(t-Bu)2CH2SiMe2}2O reacted with CpNbCl4 to produce organodiniobium phosphinimide compounds.146 A heterodimetallic compound containing titanium was synthesized by reaction of a pre-ligated titanium phosphinimide compound with CpNbCl4. In both cases the niobium- and titanium-chloride bonds could be replaced completely with methyl groups through the action of MeMgCl. Similar compounds were reported in 2005 by Cuecna and Jimenez et al.127 Tethered heterodinuclear niobiumtitanium compounds could be formed by the reaction of Nb(Z5-C5H4SiMe2Cl)Cl4 with Ti[Z5-C5H4SiMe2-Z-N(CH2)2NRR0 ] Cl2 (NRR0 ¼ NH2, NHMe) produced TiCl2[Z5-C5H4SiMe2-Z-N(CH2)2-s-NRR0 ]Nb(Z5-C5H4SiMe2Cl)Cl4, with the NRR’ group bound datively to Nb, as observed by 1H NMR spectroscopy. Jimenez and Cuecna et al. reacted Nb(Z5-C5H4SiMe2Cl)Cl4 with the amines 2-methoxyethylamine, 3-methoxypropylamine, and allylamine to form amine adducts and, in the presence of Et3N, imido compounds.147 In the case of 2-methoxyethylamine, a constrained-geometry product resulted from nucleophilic attack at Si to form a SidN bond. On the other hand, 1,2phenylenediamine reacted by aminolysis of a NbdCl bond and formed an amido-amine k2-ligand. In the presence of base, the SidCl bond was broken and an aminosilane formed.148 A new route to niobium alkylidene complexes was devised by Beckhaus et al. Reaction of NbCl4(THF)2 with 2 equiv. of di-p-tolylpentafulvene and 1.5 equiv. Mg formed the niobocene compound bis(Z5:Z1-(di-p-tolyl)pentafulvene)niobium chloride (Z5:Z1-(4-Me-C6H4)2CH2C5H4)2NbCl, featuring a dianionic p-Z5:s-Z1 bonding arrangement (Scheme 37).149 Reaction of this compound with H2C]CHMgCl gave a cyclic niobium alkylidene, via displacement of the chloride to form a vinylic NbdC bond followed by rearrangement to the cyclic niobium alkylidene. It was found that primary amines would react with loss of both hydrogens, forming an imido compound, incorporating hydrogens onto each ligand.

Scheme 37

5.03.11.2 Bis(cyclopentadienyl) complexes on niobium The Antinolo and Otero groups published a significant number of papers between 2005 and 2020. A large percentage involved the structure and behavior of a Nb trihydride complex, Nb(Z5-C5H4SiMe3)2H3. The structure of Nb(Z5-C5H4SiMe3)2H3 in the solid state was determined by single-crystal X-ray diffractometry. Finding differences between the distances from the two outer hydrides to the central hydride, the compound was classified by the authors as a compressed hydride, where the two hydrides in closer proximity to one another resemble coordinated dihydrogen, consistent with its facile loss of dihydrogen upon heating.150 The compound catalyzed ring-opening-polymerization of the lactones e-caprolactone and d-valerolactone. Little polymerization was noted when rac-lactide and b-butyrolactone were employed. A proposed mechanism involved the loss of dihydrogen, coordination of lactone, and insertion of the hydride to generate a terminal aldehyde and metal alkoxide. Further propagation by lactone coordination and alkoxide insertion generated high-molecular-weight polyesters.151 The trihydride also reacted readily with carboxylic acids to form the corresponding carboxylates.152 The carbonyl derivative Nb(Z5-C5H4SiMe3)2(H)(CO) reacted with a,b-unsaturated acids, e.g. fumaric acid, to yield the Z1-carboxylate carbonyl via hydride insertion into the double bond followed by proton transfer from the acid. Heating induced loss of CO2, yielding the k2-carboxylate. It was later found that oxidation in air led to the formation of Z1-carboxylato peroxides.153 Other niobaheterocycles were formed by addition of substituted benzothiazole heterocycles.154 Treatment of Nb(Z5-C5H4SiMe3)2H3 with isocyanide and heat gave products isostructural to that of the carbonyl compound, generally of the form Nb(Z5-C5H4SiMe3)2H(CNR) (Scheme 38).155 When Nb(Z5-C5H4SiMe3)2H(CNR) was reacted with chlorodiphenylphosphine, PPh2Cl added into the NbdH bond producing an ionic complex, [Nb(Z5-C5H4SiMe3)2(PHPh2) (CNR)]+ Cl−. Deprotonation of the phosphine by NaOH yielded a series of niobium phosphide derivatives, all of which functioned as starting materials for the formation of cationic phosphinoniobocene compounds from the addition of MeI, PhCH2Br, or BrCH2CH2Ph. Nucleophilic additions to 2-chloroacetophenone or other acyl chlorides gave the expected phosphine and acylated compounds as chloride salts.13 Alternatively, use of ICH2CH2I or I2 produced Nb(Z5-C5H4SiMe3)2(P(I)Ph2)(CNR)I3 rather than a dicationic species. Addition of CS2 to [Nb(Z5-C5H4SiMe3)2(PHPh2)(CNR)]+ Cl− produced the phosphinodithioformate insertion

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Scheme 38

product. Similar reactivity was observed when CO replaced the isocyanide in the ionic complex, [Nb(Z5-C5H4SiMe3)2(PHPh2) (CNR)]+ Cl−, or with replacement by isothiocyanates.156,157 The phosphidoniobium complex added to alkynes, both in terminal and internal fashion, forming phospha-aza-niobabicyclopentene compounds incorporating the isocyanide.158 Nb(Z5-C5H4SiMe3)2H3 reacted with P(OMe)3 to give Nb(Z5-C5H4SiMe3)2H[P(OMe)3]. The addition of electron-poor alkynyl esters to Nb(Z5-C5H4SiMe3)2H[P(OMe)3] led to nucleophilic addition by niobium into the triple bond, followed by deprotonation by the remaining hydride ligand to form a metalla-alkenyl complex.159 Interestingly, similar reactivity was found with niobocene phosphidocarbonyl and niobocene phosphidoisocyano compounds. When reacted with alkynes, diphenylphosphinoalkenyl niobocene complexes [Nb(Z5-C5H4SiMe3)2(Z1-CdC(]Nxylyl)C(R1)]C(R2)PPh2-Z1-P)] (R1 ¼ H, Me; R2 ¼ CO2Me) were formed (Scheme 39).160

Scheme 39

Tercero-Morales et al. reacted (Z5-C5H4SiMe3)2NbH3 with Sb(C6H5)3 to form the niobium-stibine (Z5-C5H4SiMe3)2NbH [Sb(C6H5)3] with loss of dihydrogen.161 The hydride was examined for its basicity, and upon addition of trifluoroacetic acid, the cationic Z2-dihydrogen-niobium complex (Z5-C5H4SiMe3)2Nb(Z2-H2)(Sb(C6H5)3)+ was formed with a trifluoroacetate counteranion. The monodeuterated analogue (Z2-HD) was synthesized for spectroscopic studies in order to confirm the nature of the Z2-H2 ligand. Addition of cis- or trans-stilbene to (Z5-C5H4SiMe3)2NbH[Sb(C6H5)3)], with heating, generated the single adduct (Z5-C5H4SiMe3)2NbH(trans-Z2-PhCH]CHPh) with loss of stibine; the same result was observed when the trihydride was used as the niobocene source. The reaction of (Z5-C5H4SiMe3)2NbL(PPh2) (L ¼ CO, CNxylyl) with MClx (M ¼ Nb, Ta, x ¼ 5; M ¼ Ti, Zr, x ¼ 4) gave the monobridged homo- and heterodimetallic compounds.162 Similar compounds were synthesized from CpTiCl3. Hexachlorometallate salts were generated by addition of HCl, resulting in protonation of the phosphanido P and chlorination of the transition metal.

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The triflate Cp2Nb(OTf )2 was prepared cleanly by addition of AgOTf to Cp2NbCl2 in THF. An in-depth IR characterization of all compounds was included.163 In 2011, Lledos, Braunschweig, and Pandey164 synthesized (Me2Si)2(Z5-C5H4)2NbCl2 by addition of the lithium dianion of 1,2-bis(cyclopentadienyl)-1,1,2,2-tetramethyldisilane, Li2(C5H4)2(Me2Si)2, to NbCl4(thf )2. The trihydride was synthesized by addition of LiAlH4 and then degassed water. While the trihydride is thermally-unstable, it can be converted by heating or photolysis to a diamagnetic, dinuclear compound [(Me2Si)2{m-(Z1:Z5-C5H3)}(Z5-C5H4)NbH]2, albeit in poor yield. Hey-Hawkins and coworkers studied the reaction of Nb(NtBu)Cl3(py)2 with 2 equiv. of the phosphine-containing cyclopentadienyl reagent Li[(C5H4)CMe2PHPh] to initially produce a non-phosphine-coordinated Nb(NtBu)Cl2[(Z5-C5H4)CMe2PHR]2 complex and the HCl elimination product [Nb(NtBu){(Z5-C5H4)CMe2PPh-Z1-P}{(Z5-C5H4)CMe2PHPh}], both of which exhibit 3 chiral centers.165 Interestingly, only two of the possible diastereomers were detected by 31P{1H} NMR spectroscopy. An electrochemical study of bis(Cp) dichloride compounds of Group 4 and Nb metals, and not including Group 5 element Ta, was published in 2016.166 The niobocene compound Cp2NbCl2 showed a current enhancement when experiments were conducted under CO2 rather than argon, but the reduction processes were not observed with added water present. Zabel published two studies that explored niobocene and tellurium chemistry. In 2007 a novel niobium-palladium-tellurium cluster, (Cp 2Nb)2PdTe4, was formed from a reaction between Cp 2Nb(Te2H), Pd(dibenzylideneacetone)2, and Ph2PCH2PPh2.167 The second publication reported that Cp 2NbIIIBH4 was a useful reagent for the synthesis of cobalt and ruthenium carbonyl compounds, albeit with low selectivity.168 The niobocene [Cp 2NbIII(CO)2]+ served as a charge-balancing cation. Varghese, Ghosh, and coworkers reported the formation of Nb-containing clusters characterized by mass spectrometry, NMR spectroscopy, and SCXRD. CpNbCl4 reacted with LiBH4 in the presence of PhTeTePh or 2-mercaptobenzothiazole to form three products, (CpNb)2B4H11TePh, (CpNb)2B4H10S, and (CpNb)2B4H11S(tBu)2C6H2OH (the sulfide is the source of the tBu and phenol groups).169 When Ph2Se2 was combined with CpNbCl4, a variety of compounds were formed, (CpNb)2BHSe4 in particular, but also the known [CpNb(B2H6)]2 and (CpNb)2B4H9(m-SePh).170 Niobium mono(cyclopentadienyl) and niobocene complexes have been shown to exhibit cytotoxic properties. Sebestova et al. studied a wide array of methoxy-substituted, phenylmethylene decorated niobocene(IV) complexes for their cytotoxic properties with human T-lymphocytic MOLT-4 leukemia cells by WST-1 viability assays, with [2,4-(MeO)2C6H3CH2C5H4]2NbCl2 found to be particularly active and comparable to the activity of cisplatin.171 Electron-withdrawing cyclopentadienyl ligands were successfully ligated to niobium(IV) centers and then the Nb complexes investigated for their cytotoxic effects on the human leukemia MOLT-4 cell line by Klepalova et al.172 Niobocene complexes with an ester group on the cyclopentadienyl ring, (Z-C5H4COOR)2NbCl2 (R ¼ Me, Et, Ph, CH2CH2OMe), were synthesized from their sodium salts and NbCl4(THF)2. BBr3 was used to derivatize (Z-C5H4COOR)2NbCl2 (R ¼ Me, Et, Ph, CH2CH2OMe) to the dibromide analogs (Z-C5H4COOR)2NbBr2.

5.03.11.3 Linked cyclopentadienyl complexes of niobium Kelland, Leach, and Green found in 2006 that the silyl-linked cyclopentadienyl niobocene complex NbIV(Z-C5H3SiMe3) SiMe2(Z-C5H3SiMe3)Cl2 was formed by the reaction of NbCl4(THF)2 and Li2[(C5H3SiMe3)SiMe2(C5H3SiMe3)].173 Treatment with LiBH4 gave the NbIII(Z-C5H3SiMe3)SiMe2(Z-C5H3SiMe3)(k2-BH4) complex.

5.03.11.4 Indenyl complexes of niobium A series of bis(indenyl) niobium peroxo complexes bearing a variety of functional groups decorating the indenyl ligands were synthesized by Baleeva et al. (Scheme 40).174 These were then tested for their catalytic ability to oxidize sulfides to the corresponding sulfoxide and sulfonyl complexes in the presence of H2O2. When the indenyl ligands were linked via an ethylene bridge, high yield and selectivity for conversion of sulfides to the sulfonyl was observed. When the indenyl ligands were not coupled, the peroxo complexes produced the sulfoxide selectively.

Scheme 40

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5.03.12

Conclusions

The chemistry displayed in this chapter shows the rich and varied reactivity of organometallic complexes of tantalum and niobium. Continued research into the activation of small molecules, spurred by the investigations of metal-ligand bonding (in particular formation of amido and imido complexes) should be pursued. The very encouraging results from reactions of tantalum organometallics with dinitrogen should lead to the further development of small-molecule activation chemistry. A deeper foray into similar chemistry with niobium may well yield promising results. The dearth of reactions of the various Ta and Nb compounds with carbon dioxide also offers a possible new direction. New catalytic applications of tantalum and niobium p-complexes, besides heavily-studied and now mature alkene polymerization, may be discovered and provide new insights into regio- and stereoselectivity. Other than C2-symmetric, covalently-linked cyclopentadienyl ligands, the chemistry with other chiral ligands for these elements is in its infancy. Bulky basic trialkylphosphine, non-redox-innocent, scorpionate (other than Tp derivatives), and pincer-type ligands in mid-valent, non-cyclopentadienyl Nb and Ta complexes, chemistries far-less explored compared to that of the late transition metals, will lead to fundamentally new classes of complexes, small-molecule reactions, new catalytic chemistries, and bonding insights. The presumed oxophilicity and air-sensitivity of early transition metal complexes has certainly inhibited Nb and Ta chemistry research in the past. There is also considerable reaction space to be explored for synthesis and reactivity of organodimetallic complexes with metal-metal bonding. There remains a wide gulf in the small number of publications reporting molecules without cyclopentadienyl ligands compared to that for cyclopentadienyl ligand complexes. The chemistry of these non-cyclopentadienyl compounds is of some note and very much worth pursuing. New, highly-donating ancillary ligands with otherwise disparate characteristics to Cp-type ligands, or further studies of soft donor ligand, such as N-heterocyclic carbene, Nb and Ta complexes will generate new classes of compounds with new insights into bonding and altered reactivity. It will also be interesting to see what develops from the early, promising in vitro studies into the use of niobocenes, tantalocenes, and forthcoming piano-stool Group 5 complexes as cytotoxic agents in human cancer cell lines, especially those similar in selective cytotoxicity to that of cisplatin and other platinum chemotherapeutics. There has been considerable clinical success in titanocene and arene ruthenium compounds in this regard, so there is ample reason to anticipate further advances for p-complexes of Nb and Ta.

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Organometallics 2012, 31 (13), 4658–4661. Maestre, M. C.; Paniagua, C.; Herdtweck, E.; Mosquera, M. E. G.; Jimenez, G.; Cuenca, T. Organometallics 2007, 26 (17), 4243–4251. Goux, J.; Le Gendre, P.; Richard, P.; Moise, C. J. Organomet. Chem. 2006, 691 (15), 3239–3244. Dielmann, F.; Peresypkina, E. V.; Kraemer, B.; Hastreiter, F.; Johnson, B. P.; Zabel, M.; Heindl, C.; Scheer, M. Angew. Chem. Int. Ed. 2016, 55 (47), 14833–14837. Starha, P.; Travnicek, Z.; Dvorak, Z. Chem. Commun. (Cambridge, UK) 2018, 54 (68), 9533–9536. Travnicek, Z.; Starha, P.; Cajan, M.; Dvorak, Z. Acta Crystallogr., Sect. C: Struct. Chem. 2019, 75 (3), 248–254. Mariott, W. R.; Gustafson, L. O.; Chen, E. Y. X. Organometallics 2006, 25 (15), 3721–3729. Tonks, I. A.; Bercaw, J. E. Inorg. Chem. 2010, 49 (10), 4648–4656. Ostapowicz, T. G.; Fryzuk, M. D. Inorg. Chem. 2015, 54 (5), 2357–2366. Smith, D. A.; Brammer, L.; Hunter, C. A.; Perutz, R. N. J. Am. Chem. Soc. 2014, 136 (4), 1288–1291. Bitterwolf, T. E.; Gallagher, S.; Rheingold, A. L.; Guzei, A. I.; Liable-Sands, L. Inorg. Chim. Acta 2005, 358 (2), 449–451. Arteaga-Mueller, R.; Sanchez-Nieves, J.; Royo, P.; Mosquera, M. E. G. Polyhedron 2005, 24 (11), 1274–1279. Arteaga-Muller, R. A.; Sanchez-Nieves, J.; Royo, P.; Mosquera, M. E. G. Eur. J. Inorg. Chem. 2008, 14, 2313–2320. Sanchez-Nieves, J.; Frutos, L. M.; Royo, P.; Castano, O.; Herdtweck, E.; Mosquera, M. E. G. Inorg. Chem. 2010, 49 (22), 10642–10648. Nicolas, P.; Royo, P. Inorg. Chim. Acta 2005, 358 (5), 1494–1500. Ziegler, J. A.; Bergman, R. G.; Arnold, J. Dalton Trans. 2016, 45 (32), 12661–12668. Leelasubcharoen, S.; Zhizhko, P. A.; Kuzmina, L. G.; Churakov, A. V.; Howard, J. A. K.; Nikonov, G. I. Organometallics 2009, 28 (15), 4500–4506. Li, X.; Sun, H.; Harms, K.; Sundermeyer, J. Organometallics 2005, 24 (19), 4699–4701. Garcia, C.; Gomez, M.; Gomez-Sal, P.; Hernandez, J. M. Eur. J. Inorg. Chem. 2009, 29-30, 4401–4415. Gomez, M.; Hernandez-Prieto, C.; Martin, A.; Mena, M.; Santamaria, C. J. Organomet. Chem. 2019, 897, 148–154. Martinez, G.; Stephan, D. W. Can. J. Chem. 2006, 84 (9), 1180–1187. Maestre, M. C.; Gratal, P. B.; Mosquera, M. E. G.; Cuenca, T.; Jimenez, G. Eur. J. Inorg. Chem. 2017, 2017 (7), 1060–1066. Maestre, M. C.; Mosquera, M. E. G.; Jacobsen, H.; Jimenez, G.; Cuenca, T. Organometallics 2008, 27 (5), 839–849. Manssen, M.; Dierks, A.; de Graaff, S.; Schmidtmann, M.; Beckhaus, R. Angew. Chem., Int. Ed. 2018, 57 (37), 12062–12066. Alonso-Moreno, C.; Antinolo, A.; Garcia-Martinez, J. C.; Garcia-Yuste, S.; Lopez-Solera, I.; Otero, A.; Perez-Flores, J. C.; Tercero-Morales, M. T. Eur. J. Inorg. Chem. 2012, 2012 (7), 1139–1144. Antinolo, A.; Dorani, K.; Garcia-Yuste, S.; Lopez-Solera, I.; Otero, A.; Tercero-Morales, M. T.; Kovacs, G.; Ujaque, G.; Lledos, A. Organometallics 2012, 31 (14), 5177–5184. Antinolo, A.; Garcia-Yuste, S.; Lopez-Solera, I.; Otero, A.; Perez-Flores, J. C.; del Hierro, I.; Salvi, L.; Cattey, H.; Mugnier, Y. J. Organomet. Chem. 2005, 690 (13), 3134–3141. Antinolo, A.; Garcia-Yuste, S.; Lopez-Solera, I.; Otero, A.; Tercero-Morales, M. T.; Carrillo-Hermosilla, F. J. Organomet. Chem. 2019, 897, 120–129. Antinolo, A.; Garcia-Yuste, S.; Otero, A.; Perez-Flores, J. C.; Lopez-Solera, I.; Rodriguez, A. M. J. Organomet. Chem. 2007, 692 (16), 3328–3339. Antinolo, A.; Evrard, D.; Garcia-Yuste, S.; Otero, A.; Perez-Flores, J. C.; Reguillo-Carmona, R.; Rodriguez, A. M.; Villasenor, E. Organometallics 2006, 25 (15), 3670–3677. Antinolo, A.; Garcia-Yuste, S.; Otero, A.; Perez-Flores, J. C.; Reguillo-Carmona, R.; Rodriguez, A. M.; Villasenor, E. Organometallics 2006, 25 (5), 1310–1316. Antinolo, A.; Garcia-Yuste, S.; Otero, A.; Reguillo-Carmona, R. Eur. J. Inorg. Chem. 2009, 4, 539–544. Antinolo, A.; Garcia-Yuste, S.; Lopez-Solera, M. I.; Otero, A.; Perez-Flores, J. C.; Reguillo-Carmona, R.; Villasenor, E. Dalton Trans. 2006, (12), 1495–1496. Antinolo, A.; Garcia-Yuste, S.; Otero, A.; Espinosa, A. Organometallics 2015, 34 (11), 2695–2698. Antinolo, A.; Garcia-Yuste, S.; Solera, I. L.; Otero, A.; Perez-Flores, J. C.; Reguillo-Carmona, R.; Villasenor, E.; Santos, E.; Zuidema, E.; Bo, C. Dalton Trans. 2010, 39 (8), 1962–1971. Antinolo, A.; Perez-Flores, J.-C.; Hervas, M. J.; Garcia-Yuste, S.; Lopez-Solera, M. I.; Otero, A.; Fernandez-Pacheco, A. R.; Tercero-Morales, M. T. Organometallics 2013, 32 (3), 862–868. Reguillo-Carmona, R.; Antinolo, A.; Garcia-Yuste, S.; Lopez-Solera, I.; Otero, A. Dalton Trans. 2011, 40 (11), 2622–2630. Angus-Dunne, S. J.; Lee Chin, L. E. P.; Burns, R. C.; Lawrance, G. A. Trans. Met. Chem. (Dordrecht, Neth.) 2006, 31 (2), 268–275. Pandey, K. K.; Braunschweig, H.; Lledos, A. Inorg. Chem. 2011, 50 (4), 1402–1410. Gomez-Ruiz, S.; Hoecher, T.; Prashar, S.; Hey-Hawkins, E. Organometallics 2005, 24 (9), 2061–2064. Grice, K. A.; Saucedo, C.; Sovereign, M. A.; Cho, A. P. Electrochim. Acta 2016, 218, 110–118. Kubicki, M. M.; Schwarz, P.; Meier, W.; Wachter, J.; Zabel, M. J. Organomet. Chem. 2007, 692 (18), 3931–3935. Lange, A.; Meier, W.; Wachter, J.; Zabel, M. Inorg. Chim. Acta 2006, 359 (3), 1006–1011. Roy, D. K.; Bose, S. K.; Geetharani, K.; Chakrahari, K. K. V.; Mobin, S. M.; Ghosh, S. Chem. Eur. J. 2012, 18 (32), 9983–9991, S9983/1-S9983/3. Shankhari, P.; Roy, D. K.; Geetharani, K.; Anju, R. S.; Varghese, B.; Ghosh, S. J. Organomet. Chem. 2013, 747, 249–253. Honzickova, I.; Honzicek, J.; Vinklarek, J.; Padelkova, Z.; Rezacova, M.; Sebestova, L. Appl. Organomet. Chem. 2014, 28 (4), 252–258. Klepalova, I.; Honzicek, J.; Vinklarek, J.; Padelkova, Z.; Sebestova, L.; Rezacova, M. Inorg. Chim. Acta 2013, 402, 109–115. Green, M. L. H.; Leach, J. B.; Kelland, M. A. J. Organomet. Chem. 2006, 691 (6), 1295–1297. Rakhmanov, E. V.; Zhong, S.; Tarakanova, A. V.; Anisimov, A. V.; Akopyan, A. V.; Baleeva, N. S. Russ. J. Gen. Chem. 2012, 82 (6), 1118–1121.

5.04

Cyclic and Non-cyclic Pi Complexes of Chromium

Tingting Song and Ying Mu, The State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, People’s Republic of China © 2022 Elsevier Ltd. All rights reserved.

5.04.1 5.04.2 5.04.2.1 5.04.2.1.1 5.04.2.1.2 5.04.2.1.3 5.04.2.1.4 5.04.2.1.5 5.04.2.1.6 5.04.2.1.7 5.04.2.1.8 5.04.2.1.9 5.04.2.1.10 5.04.2.1.11 5.04.2.1.12 5.04.2.1.13 5.04.2.1.14 5.04.2.1.15 5.04.2.1.16 5.04.2.1.17 5.04.2.1.18 5.04.2.1.19 5.04.2.1.20 5.04.2.2 5.04.2.2.1 5.04.2.2.2 5.04.2.2.3 5.04.2.2.4 5.04.2.2.5 5.04.2.2.6 5.04.2.3 5.04.3 5.04.4 5.04.5 5.04.5.1 5.04.5.2 5.04.5.3 5.04.5.4 5.04.6 5.04.7 5.04.8 5.04.9 5.04.10 5.04.11 5.04.12 References

5.04.1

Introduction h6-Arene chromium complexes
6-Arene chromium carbonyls Aromatic nucleophilic substitution or addition reactions Arene lithiations and reactions with electrophiles Palladium catalyzed coupling of arene chromium carbonyls Palladium catalyzed CdH activation of arene chromium carbonyls Desymmetrization of difunctionalized arene chromium carbonyls Chiral arene chromium complexes as ligands for asymmetric catalysis Cycloaddition reactions of arene chromium carbonyls Ring-closing metathesis reaction of arene chromium carbonyls Bimetallic and multimetallic arene chromium carbonyls Gold(I)-catalyzed cyclization of arene chromium carbonyls Chromium-stabilized cations and related complexes Chromium carbonyls of polycyclic hydrocarbons Haptotropic migration of chromium carbonyl group Benzannulations of Fischer-type carbene complexes Polymer-bound arene chromium carbonyls Miscellaneous arene chromium carbonyls Arene chromium carbonyls with heteroatoms on the periphery Molecular modeling and other computational approaches Spectroscopic studies on arene chromium carbonyls Applications of arene chromium carbonyls Bis(
6-arene) complexes Compounds with hydrocarbon-substituted arenes Compounds with heteroatom-substituted arenes Ion-radical salts with bis(arene)chromium complexes Bimetallic and multimetallic bis(arene)chromium complexes Theoretical considerations and spectroscopic studies Applications of bis(arene)chromium complexes Mono(
6-arene), half-sandwich, mixed sandwich, multidecker
6-arene chromium complexes h6-Heteroarene chromium complexes Other h6-pi-ligand chromium complexes h5-Cyclopentadienyl chromium complexes Sandwich chromium complexes Half-sandwich chromium complexes as catalysts or activators Other half-sandwich chromium complexes Theoretical studies on the electronic structures h5-Heterocyclopentadienyl chromium complexes Other h5-pi-ligand chromium complexes h2-Pi-ligand chromium complexes h3-Pi-ligand chromium complexes h4-Pi-ligand chromium complexes h7-Pi-ligand chromium complexes Summaries and suggestions

81 82 82 82 85 87 88 89 90 91 92 93 99 101 102 103 106 110 113 123 125 126 126 126 126 127 128 129 129 130 130 131 134 135 135 138 143 154 155 156 158 161 163 166 166 166

Introduction

The chemistry involving the syntheses, structures, reactivity, and applications of cyclic and non-cyclic p complexes of chromium has been extensively investigated in the past decades. The previous editions, COMC1 (1982), COMC2 (1995) and COMC3 (2007), covered the important developments up to 2005. This chapter attempts to highlight the major progresses on the synthesis, characterization, reactions, and applications of the organochromium compounds of this type since 2006 up to early 2020.

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00056-1

81

82

Cyclic and Non-cyclic Pi Complexes of Chromium

During the past 15 years, studies on the
6-arene chromium complexes have been the major works of the scientists in this area, which include the
6-arene carbonylchromium complexes, bis(
6-arene) and mono(
6-arene) sandwich chromium complexes, and half-sandwich and multidecker
6-arene chromium complexes. In addition, as a small family of
6-coordinated p complexes of chromium, works on the
6-heteroarene chromium complexes were also reported. The
5-cyclopentadienyl chromium complexes have also been intensively studied during the past 15 years. The major works focused on
5-cyclopentadienyl carbonylchromium complexes, chromocene and mixed sandwich chromium complexes, half-sandwich cyclopentadienyl and functionalized cyclopentadienyl chromium complexes, as well as a number of
5heterocyclopentadienyl chromium complexes. Many of the functionalized cyclopentadienyl chromium complexes have been developed as catalysts or activators for olefin polymerization, N2 activation and other interesting organic transformations. Some chromium complexes bearing other cyclic and non-cyclic
n-(n ¼ 2–7)-p-ligands have also been exploited and summarized in this chapter with the above-mentioned work.

5.04.2


6-Arene chromium complexes

A large number of studies on the chemistry of arene chromium complexes, especially complexes with supporting carbonyl ligands, have been reported since 2006. Here we review major progress on the synthesis, characterization, reactions, and applications of this important type of complexes.

5.04.2.1 5.04.2.1.1


6-Arene chromium carbonyls Aromatic nucleophilic substitution or addition reactions

The electron-withdrawing effect of the organometallic tripod of tricarbonylchromium renders the ring susceptible to be attacked by a wide variety of nucleophiles to give cyclohexadienyl–chromium complexes. When there is a suitable leaving group in the ring, the nucleophile can be directly incorporated; in other cases, cyclohexadiene will be formed. The ways of forming intermediate cyclohexadiene complexes are diverse. A highly modular strategy for synthesis of the novel chiral 1,3-diamines has been developed. This can be achieved through nucleophilic aromatic substitution of fluorine in [(R,R)-1-fluoro-2-{(1-dimethylamino)ethyl}benzene]tricarbonylchromium with amines (Scheme 1).1

Scheme 1

The
6-1,2-bis-alkylsulfanylbenzene tricarbonyl chromium(0) complexes have been obtained by electrophilic addition of an alkylthio group to C-2 of the benzene ring and a subsequent SNAr substitution reaction. This reaction provides an effective way to introduce two adjacent sulfur substituents into the aromatic ring.2 An illustrative example of halogenophilic attack in nucleophilic aromatic substitution has been given via the reactions of the iodobenzene Cr(CO)3 p-complex with carbonylmetallates.3 The first strategy for nucleophilic CdH perfluoroalkylation of arenes as ligands of stable (
6-arene)tricarbonylchromium complexes has been developed, as shown in Scheme 2.4

Scheme 2

Cyclic and Non-cyclic Pi Complexes of Chromium

83

The binaphthyl-modified chiral phase transfer catalyst has effectively promoted the SNAr reaction of a-amino acid derivatives and electron-rich fluoroaromatic hydrocarbon-derived chromium complexes, resulting in the corresponding a-a-disubstituted a-amino acids, which contain various aromatic substituents with highly enantioselectivities, as depicted in Scheme 3.5 Another successful example of using an intramolecular SNAr reaction of an (
6-arene)chromium complex to construct of a macrocycle with an aryl ether linkage has been reported. Through further crystallographic and computational studies, the intramolecular p-p interaction between the chromium complexed aromatic ring and the enol ether moiety of the cyclization precursor has been investigated.6

Scheme 3

Efficient methods for stereoselective synthesis of N-arylindoles and related compounds with NdC axial chirality, using arene chromium complexes as chiral scaffolds or platforms has been developed,7–9 and a comprehensive review has been made.10 These complexes are obtained in good to excellent yields with high diastereo- or enantioselectivities. Furthermore, upon air-oxidation chromium tricarbonyl fragments in these complexes are easily removed with the retention of axial chirality. As shown in Scheme 4, both enantiomers of N-aryl indoles have been synthesized using the same planar chiral arene chromium complex. N-Aryl indoles with axially chiral NdC bonds have been prepared by stereoselective nucleophilic aromatic substitution reactions of planar chiral tricarbonyl(2,6-disubstituted-1-fluorobenzene)chromium complexes. It is found that the stereochemistry of the products is highly dependent on the position of the substituent in the indole. The N-aryl indole chromium complexes having anti orientation with respect to the tricarbonylchromium fragment are obtained diastereoselectively when using indoles devoid of a substituent at the 2-position. While a totally different result can be achieved when 2-substituted indoles are used, which result in the N-aryl indoles with syn orientation between the chromium tricarbonyl fragment and the benzene ring of the indole.7,8

Scheme 4

A synthetic route to the tricarbonylchromium complex of N-methylisatin and some related complexes has been reported. Complexation of the acetal 1 with tricarbonyl(naphthalene)chromium affords complex rac-2. Treatment of rac-2 with formic acid at 25  C for 1 h gives tricarbonyl(N-methylisatin)chromium(0) (rac-3). Tricarbonyl(isatin)chromium(0) (rac-3) undergoes nucleophilic addition with organolithium and Grignard reagents. Treatment with methyllithium at −78  C affords the methylated complex rac-4 as a single diastereomer. The addition of vinylmagnesium bromide at rac-3 results in a regioisomeric mixture of monoadducts rac-5 and rac-6, respectively, along with divinyl adduct rac-7 with the product distribution being highly dependent on the reaction time, as shown in Scheme 5.11

84

Cyclic and Non-cyclic Pi Complexes of Chromium

Scheme 5

SNAr reactions of fluoroarene complex 8 and the corresponding potassium alkoxide afford chiral (alkoxyarene)chromium tricarbonyl complexes 9 in good yields (Scheme 6).12 And the structure of the complex bearing a simple alkyl group instead of trimethylsilyl, tricarbonyl(
6-1-methyl-4-{spiro-[(1R,2S)-1,7,7-trimethylbicyclo[2.2.1]-heptane-3,20 -1,3-dio-xolan]-2-yloxy}-benzene)chromium, has been determined.13

Scheme 6

As shown in Scheme 7, reaction of aromatic chromium tricarbonyl complex 10 with LDA affords
5-chromium tricarbonyl anionic intermediates 11 and 12. The intermediates 11 and 12 are oxidized to give a mixture of spirocyclohexanedione 13 and tetrahydronaphthalene 14. The product distribution depends on the nature of the substituents R1 and R2 and the temperature of the reaction.14

Scheme 7

Cyclic and Non-cyclic Pi Complexes of Chromium

85

Treatment of (styrene)Cr(CO)3 complexes 15 by adding various heteronucleophiles such as alcohols, amines, and thiols, in the presence of KOH, gives b-addition products 16 in moderate to good yields (Scheme 8).15

Scheme 8

The corresponding secondary alcohols can be obtained in high diastereoisomeric purity from the diastereoselective addition of Grignard reagents or Super-Hydride® to (aryl aldehyde)- and (aryl ketone)-chromium tricarbonyl complexes ortho-substituted with the chiral auxiliary O-methyl-N-(a-methylbenzyl)hydroxylamine, which can be easily decomplexed and deprotected to yield the corresponding enantiopure amino alcohols.16

5.04.2.1.2

Arene lithiations and reactions with electrophiles

The method of synthesizing an enantioenriched C2-symmetric molecule by a chiral-base-mediated kinetic resolution of an (arene)tricarbonylchromium(0) complex has been developed. The tricarbonylchromium(0) complex 17 has been used to control the conformational preferences of a 1,2-disubstituted (arene)tricarbonylchromium(0) complex. Furthermore, the enantiomerically enriched C2-symmetric bis-ether (+)-19 can be obtained by the kinetic resolution of the mono-methyl derivative ()-18 using a chiral base/methyl iodide quench sequence, as shown in Scheme 9.17

Scheme 9

The lithiation/electrophile trapping reactions have been developed using highly enantiomerically enriched complex [Cr(5bromonaphthalene)(CO)3] (20). Electrophile quenching with ClPPh2, PhCHO, and (Me3SiO)2 gives the enantiomerically enriched planar chiral 5-substituted naphthalene complexes 21 and 22 with PPh2, CH(Ph)OH, and OH substituents, respectively. By deprotonating with NaH in dichloromethane and then treating with Meerwein’s salt, the methylation of 22 can be achieved, resulting in 23 in a high yield (Scheme 10). The planar chiral aryl-, heteroaryl-, alkynyl-, and alkenylnaphthalene chromium

Scheme 10

86

Cyclic and Non-cyclic Pi Complexes of Chromium

complexes 24 and 25 have been obtained with high enantiomeric purity from the Pd-catalyzed Suzuki–Miyaura cross-coupling reactions using highly labile [Cr(5-bromonaphthalene)(CO)3] (20) as starting material. Etherification of 20 has been achieved with potassium tert-butoxide by using a [Pd(dba)2]/Q-Phos (Q-Phos ¼ 1,2,3,4,5-pentaphenyl-10 -di-tert-butylphosphinoferrocene) catalyst system to give complex 26 in a moderate yield (Scheme 11).18

Scheme 11

Another enantioselective reaction of a chiral bisamide and an electrophile with a benzyl ether complex of arene tricarbonyl chromium(0) has been used to synthesize novel enantiopure planar chiral ligands. Treatment of benzyl ether complexes of arene tricarbonyl chromium [
6-4-t-BuC6H4CHE1(OR)]Cr(CO)3 [E1 ¼ Me, Bn, SPh, PPh2] via a diastereoselective ortholithiation/chlorophosphane quench protocol leads to the synthesis of planar chiral monophosphane complexes with high enantiomeric purity (typically 96–98% ee).19 The syntheses of benzoyl-substituted
6-arene tricarbonylchromium complexes by lithiation/electrophilic quench sequence using a Weinreb amide as the electrophile have been reported. The reactions of the anions of complexes 27 obtained by treatment with nBuLi at low temperature, with the Weinreb amide lead to the formation of the complexes 28 substituted by a benzoyl group (Scheme 12). The reactions are highly regioselective and only ortho-substituted complexes have been isolated.20

Scheme 12

A novel diastereoselective electrophilic fluorination using N-fluorobenzenesulfonimide (NFSI) results in [(R,R)-1-fluoro-2{(1-dimethylamino)ethyl}benzene]tricarbonylchromium, which has been examined as precursor for the highly modular synthesis of a rare class of chiral 1,3-diamines, as shown in Scheme 13.1

Scheme 13

Cyclic and Non-cyclic Pi Complexes of Chromium

87

Allyl ethers of (
6-benzylic alcohol)tricarbonylchromium(0) complexes have been shown to undergo highly stereoselective functionalization at their benzylic positions using a chiral base/electrophilic quench reaction sequence. Using catalytic amounts of tetrakis(triphenylphosphane)palladium(0), the obtained chiral ethers are readily deprotected without loss of enantiopurity.21 Directed lithiation of 1,3-diheteroatom substituted arene chromium complexes gives regioselectively 2- or 4-functionalized arene chromium complexes depending on the nature of quenching electrophiles (Scheme 14).22 Regioselective introduction of functional groups at different position of arene ring would be a significantly useful tool for organic synthesis.

Scheme 14

The use of O-methyl-N-(a-methylbenzyl)hydroxylamine as a novel chiral auxiliary in asymmetric ortho-deprotonation of the (
6-arene) chromium tricarbonyl complexes has been developed. The quench reaction of the resultant ortho-lithiated complex with an electrophile gives 1,2-disubstituted (
6-arene) chromium tricarbonyl complexes in good yields with excellent levels of diastereoselectivity.23

5.04.2.1.3

Palladium catalyzed coupling of arene chromium carbonyls

The palladium-catalyzed reactions of alkynes 30 with halogeno derivatives 29 give the
6-(arene)tricarbonylchromium complexes 31 linked to 20 -deoxyuridine via a triple bond (Scheme 15), and their antiviral activity in cell-based assays have been studied.24

Scheme 15

Palladium-catalyzed coupling reactions of 6,60 -dihydroxyboron-2,20 -dimethoxy-1,10 -binaphthyl 32 with chloroarenetricarbonylchromium complexes 33 give complexes 34, in which the binaphthyl residue is directly linked to the (
6-arene) chromium tricarbonyl entity. Coupling reactions of 2,20 -dimethoxy, 3,30 -diodo, and 6,60 -diodo-1,10 -binaphthyl 35 and 38 with ethynylarenetricarbonylchromium derivatives 36 afford binaphthyl compounds linked to arenetricarbonylchromium derivatives 37 and 39 through a triple bond, respectively. The reactions of methyltriphenylphosphonium bromide of arenetricarbonylchromium 41 and the dialdehyde 40 in the presence of tBuOK give three products 42, 43, and 44, respectively, as shown in Scheme 16.25

88

Cyclic and Non-cyclic Pi Complexes of Chromium

Scheme 16

5.04.2.1.4

Palladium catalyzed CdH activation of arene chromium carbonyls

Palladium-catalyzed CdH functionalization through the complexation of
6-arene-chromium has made great progress.26 The CdH activation of weakly acidic aryl sp2 and benzylic sp3 bonds has been achieved through these reactions. These methodologies greatly extend the traditional methods of functionalizing
6-arene–chromium complexes via stoichiometric lithiation and nucleophilic addition. A general, high-yielding cross-coupling method for preparing a wide range of di- and triarylmethanes between {Cr(CO)3}activated toluene derivatives and aryl bromides has been developed, which is a supplement to the F-C method and can synthesize polyarylmethanes that are not accessible through electrophilic aromatic substitution reactions alone.27 The palladium/triphenylphosphine-catalyzed cross-coupling of aryl bromides and (
6-C6H5CH2Z)Cr(CO)3 complexes (45) in the presence of LiN(SiMe3)2 yields a range of di- and tri-arylmethanes 46 and 47 (Scheme 17).28

Scheme 17

Cyclic and Non-cyclic Pi Complexes of Chromium

89

Another new dynamic kinetic resolution involving palladium-catalyzed cross-coupling of aryl triflates with [(
6-benzylamine) Cr(CO)3] has been developed. In this process diastereoselective transmetalation of one enantiomer of a rapidly equilibrating planar-chiral secondary benzyllithium species may be the enantioselectivity-determining step.29 Cr(CO)3-complexed aryl pyridines have been shown to readily undergo stoichiometric palladation and subsequent arylation using boronic acid nucleophiles as shown in Scheme 18.30 The positioning of the aryl substituents was found to be key in controlling the reactivity of the substrate, whereby sterically crowded compounds prevent the geometry required for cyclometalation.

Scheme 18

The approach for aromatic CdH activation of arenes via p-complexation to a Cr(CO)3 unit, which can greatly enhance the reactivity of the aromatic CdH bonds without altering the connectivity of the arenes, has been applied to CdH arylation of highly unreactive monofluorobenzenes. Upon complexation to the Cr(CO)3 unit, the monofluorobenzenes become nearly as reactive as pentafluorobenzene itself in their couplings with iodoarenes. DFT calculations indicate that CdH activation via a concerted metalation–deprotonation transition state is facilitated by the predisposition of CdH bonds in (ArdH)Cr(CO)3 to bend out of the aromatic plane.31 A mild and straightforward method of producing biaryls is also described.32 The direct arylation of the CdH bond of (arene)Cr(CO)3 48 upon deprotonation with Mg(2,2,6,6-tetramethylpiperazide)2
2LiCl is followed by transmetalation to CuI and cross-coupling with an aryl iodide to give the biaryls 49 (Scheme 19).

Scheme 19

A more efficient Pd-catalyzed direct arylation process for anisole-type arenes via p-complexation to a Cr(CO)3 unit has also been demonstrated. This mild methodology can be applied to the late stage functionalization of bioactive compounds containing the anisole motif, allowing the construction of novel organic scaffolds with few synthetic steps.33 In a catalytic asymmetric direct CdH arylation of (
6-arene)chromium complexes to obtain planar-chiral compounds by a Pd-catalyzed/Ag-promoted catalytic system, it was found that the use of a hemilabile ligand H8-BINAP(O) is key to providing high enantioselectivity in this transformation. This methodology provides a new approach for the synthesis of a variety of novel (arene) chromium-based chiral phosphine ligands.34

5.04.2.1.5

Desymmetrization of difunctionalized arene chromium carbonyls

A series of methods to desymmetrical appropriate difunctional (aromatic) chromium complexes have been carried out, such as palladium-catalyzed hydrogenolysis,35,36 gold-catalyzed intramolecular nucleophilic additions37,38 and asymmetric Suzuki–Miyaura coupling.39 The palladium-catalyzed hydrogenolysis of one of the two CdBr bonds in complex 50, using LiBH4 as the reducing agent and [Pd(dba)2] (dba ¼ dibenzylideneacetone) as the catalyst, gives the [Cr(5-bromonaphthalene)(CO)3] (51) with high enantioselectivity (Scheme 20).35 The asymmetric catalytic hydrogenolysis of 50 has been investigated in details with different chiral ligands.36 It

Scheme 20

90

Cyclic and Non-cyclic Pi Complexes of Chromium

was found that addition of DABCO as a borane-trapping reagent to the reaction system is necessary, which can prevent the formation of the BH3-ligand adduct and hence enhance the stereochemical outcome of the reaction.40 The planar chiral isochromene chromium complexes 54 have been obtained with high enantioselectivity by gold(I)-catalyzed asymmetric cyclization of 1,3-dihydroxymethyl-2-alkynylbenzene chromium complexes 5237 or 1,3-bis(carbamate)-2-alkynylbenzene tricarbonylchromium complexes 5338 with axially chiral diphosphine ligand (Scheme 21).

Scheme 21

A highly efficient desymmetrization reaction of a tricarbonylchromium 1,4-dibromonaphthalene complex by Pd-catalyzed asymmetric Suzuki–Miyaura coupling39 has been developed. In this reaction, high selectivities of up to 98% ee have been achieved using a bulky chiral phosphoramidite ligand (Scheme 22).

Scheme 22

5.04.2.1.6

Chiral arene chromium complexes as ligands for asymmetric catalysis

New “Daniphos” type chiral P\ P- and P \ N-ligands 57 with an [(
6-arene)Cr(CO)3] core structure have been prepared through a stereoselective synthetic strategy from optically pure benzylamines 56 bearing a second substituent on the arene other than the benzyldimethylamino group (Scheme 23).41

Scheme 23

Cyclic and Non-cyclic Pi Complexes of Chromium

91

The highly diastereoselective induction of planar chirality in N-substituted
6-arene chromium tricarbonyl complexes has been achieved using the isopropoxy-modified pyrroloimidazolones as viable directing groups (Scheme 24).42

Scheme 24

5.04.2.1.7

Cycloaddition reactions of arene chromium carbonyls

The introduction of the (
6-phenyl)tricarbonylchromium group in both dipolar and dipolarophilic molecules enhances the regioand diastereoselectivity of 1,3-dipolar cycloaddition process compared with the reactions taking place between similar substances.43–46 Among them, a number of bimetallic isoxazolidines have been obtained through the 1,3-dipolar cycloaddition reactions (Scheme 25). It has been shown that the formation of these compounds occurred with high regio- and diastereoselectivity.45,46 A number of (
6-arene)tricarbonylchromium-containing amino alcohols have been prepared and applied to the synthesis of new classes of (
6-arene)tricarbonylchromium derivatives of 1,3-oxazolidines and 1,3-oxazinanes (Scheme 26).47

Scheme 25

Scheme 26

92

Cyclic and Non-cyclic Pi Complexes of Chromium

5.04.2.1.8

Ring-closing metathesis reaction of arene chromium carbonyls

Performing metathesis reactions on organometallic complexes have offered special methods to study interesting structures that are difficult to study by other methods. The synthesis of phosphinechelate (
6-arene)chromium complexes by ruthenium-catalyzed ring closing metathesis has been studied.48 The ring-closing metathesis reaction of [
6-(o-alkenyl)benzene][(o-alkenyl)phosphine] chromium(0) dicarbonyl complexes 58 catalyzed by the Grubbs’ ruthenium-carbene complexes gives the phosphine-chelate (piarene)chromium complexes 59 with good to excellent yields, as shown in Scheme 27.

Scheme 27

Development of new enantioselective reactions has always been one of the most attractive research topics in organic chemistry. On the one hand, most of asymmetric reactions have been reported in their respective product configuration of the stereogenic carbon atoms. On the other hand, compounds with axial or planar chirality have also made important contributions to asymmetric synthesis. An effective kinetic resolution method for planar-chiral (
6-1,2-disubstituted benzene)chromium complexes with excellent selectivity by the asymmetric ring closing metathesis (ARCM) strategy has been developed. After appropriate derivatization, the obtained optically active (diphenylphosphinoarene)chromium complexes have been utilized as chiral ligands in Rh-catalyzed asymmetric reactions (Scheme 28).49

Scheme 28

A planar-chiral alkenylene-bridged (phosphino-p-arene)(phosphine)chromium species 60 has been reported to be capable of coordinating to a rhodium(I) cation in bidentate fashion with the phosphorus atom and the olefin moiety. Some of rhodium complexes bearing ligand 60 show high enantioselectivity in the asymmetric 1,4- and 1,2-addition reactions of the arylboron nucleophiles. The new synthetic method opens the door to the synthesis of various (arene)Cr-based phosphine–olefin ligand, resulting in the discovery of phosphine-olefin ligands with higher enantioselective variants.50 A series of planar-chiral (pi-arene)chromium complexes have been obtained in up to 99% ee upon the molybdenum-catalyzed ARCM of the Cs-symmetric substrates. When there are indolyl or naphthyl substituents in the p-arene moiety, the induction of axial and planar chirality can be achieved in ARCM products. Even after removing the chromium dicarbonyl fragment, the induced axial chirality is still maintained, and the resolution of the axial chiral biaryl/heterobiaryl compounds is as high as 99% ee (Scheme 29).51

Cyclic and Non-cyclic Pi Complexes of Chromium

93

Scheme 29

In addition, the enantioselectivity of the phosphine-olefin ligands with a planar chiral (
6-arene)chromium(0) framework has been compared with that of corresponding manganese(I) complexes in various asymmetric addition reactions.52 The origin of enantioselectivity in Rh-catalyzed asymmetric 1,4-additon to enones using these phosphine-olefin ligands has been studied by DFT calculations.53 The synthesis of planar-chiral (arene)chromium complexes has been briefly reviewed.54

5.04.2.1.9

Bimetallic and multimetallic arene chromium carbonyls

The reaction of complex 61 with photochemically generated Cr(CO)5(THF) gives a novel homobimetallic d-borane complex (OC)3Cr(
6-C6H5CH2NMe2BH2dHdCr(CO)5) (62), wherein one of the BH moieties is bound to the chromium center in an
1-fashion, as shown in Scheme 30. The d-borane complex 62 has been isolated in moderate to good yield, and has been characterized by X-ray crystallography.55

Scheme 30

A planar-chiral bimetallic tetranuclear chloro-bridged ruthenium–chromium complex has been obtained as a racemic mixture by cyclometalation of a planar pro-chiral 2-[tricarbonyl(
6-phenyl)chromium]pyridine complex with [Ru(CO)2Cl2]n (Scheme 31).56

Scheme 31

Complexes 64 and 65 have been prepared efficiently by the cyclometalation of 63 with both [Cp RhCl2]2 and [Cp IrCl2]2 at room temperature. The reaction of 65 carried out overnight with a large excess of NaBH4 and 2-propanol in dry refluxing THF affords an air-sensitive admixture of the starting complex with a single new hydrido complex 66, as depicted in Scheme 32.57

94

Cyclic and Non-cyclic Pi Complexes of Chromium

Scheme 32

Two phenyl and methyl iridium(III) complexes 68 have been prepared by reaction of (S p,R Ir)-chlorido{2-[(tricarbonyl) 0 (
6-phenylene-C1 )chromium(0)]pyridine-N}(pentamethyl-cyclopenta-dienyl)iridium(III) (anti-67) with PhMgBr or MeMgBr, as shown in Scheme 33.58

Scheme 33

A coupling reaction of the iodo complex 69 and chromium derivative 70 in the presence of triethylamine using Pd(PPh3)2Cl2 and copper(I) iodide as the catalysts affords the bimetallic iron–chromium complex 71 (Scheme 34).59

Scheme 34

Treatment of (
6-C6H5C^CH)Cr(CO)3 (70) with the gold(I) chloride complex 72 gives the heterotrimetallic FedAudCr complex FcPPh2AuC^C(
6-C6H5)Cr(CO)3 (73), in which three different transition metal atoms are linked by carbon-rich bridging units, as shown in Scheme 35.60

Scheme 35

A number of heterobimetallic
6-[(ferrocenyl)indene]-Cr(CO)3 complexes 74–78 which differ in the position of the ferrocenyl group, 1-(ferrocenyl)indene and 2-(ferrocenyl)indene, and the number of indene methylation (tetramethyl- and hexamethyl-) have been obtained and investigated by DFT analysis.61

Cyclic and Non-cyclic Pi Complexes of Chromium

95

As shown in Scheme 36, [Cr(CO)3(
6-C6H5)C^C-{(
5-C5H4)Fe(
5-C5H5)}] (80) is prepared using complex 79 as starting material. The reaction of [Cr(CO)3(
6-C6H5)C^C-{(
5-C5H4)Fe(
5-C5H5)}](80) with Co2(CO)8 or Cp2Mo2(CO)4 affords the heterotetrametallic complexes [Cr(CO)3(
6-C6H5){Co2(CO)6-m2-
2-C^C–}{(
5-C5H4)Fe(
5-C5H5)}] (81), and [Cr(CO)3 (
6-C6H5){Mo2Cp2(CO)4-m2-
2-C^C–}{(
5-C5H4)Fe(
5-C5H5)}] (82), respectively.62 Both complexes have been characterized by single-crystal X-ray diffraction analysis.

 Scheme 36

Trimetallic Fe(II)-Rh(I)-Cr(0) complexes 83 and 84 have been prepared by the stepwise synthesis depicted in Scheme 37. The influence of different stereochemical dispositions among three metal units on electronic interactions has been studied by a variety of methods including IR and near-IR data, electrochemical behavior and density functional theory (DFT) calculations.63

96

Cyclic and Non-cyclic Pi Complexes of Chromium

Scheme 37

A convergent growth method for the preparation of novel heterometallic dendritic molecules derived from carbosilane frameworks and functionalized with silicon-bridged ferrocenyl and (
6-aryl)tricarbonylchromium moieties has been developed.64 Bimetallic complexes 85–87, a new type of Grubbs-Hoveyda-Blechert alkene metathesis catalysts, in which the aromatic ring of the benzylidene ligand coordinates to a highly electron-withdrawing tricarbonylchromium moiety in a hexadentate fashion, have been prepared.65

The complexes bearing trans-benzoate chromium tricarbonyl moieties trans-M2(TiPB)2[O2CC6H5-
6-Cr(CO)3]2, where M ¼ Mo (88) or W (89), and TiPB ¼ 2,4,6-triisopropylbenzoate, have been prepared from the reactions between M2(TiPB)4 and HO2CC6H5-
6-Cr(CO)3 (2 equiv.), as depicted in Scheme 38. The electronic structure and excited-state dynamics of complexes 88 and 89 have been studied by density functional theory calculations.66

Scheme 38

A series of planar chiral heterobimetallic pincer complexes 90 have been facilely synthesized by p coordination of the aromatic ring of nonsymmetric POCOP pincer derivatives of nickel, palladium, and platinum to the [Cr(CO)3] fragments (Scheme 39).67 And only coordination of Cr(CO)3 group and noncyclometalated ring can be observed in these reactions.

Cyclic and Non-cyclic Pi Complexes of Chromium

97

Scheme 39

Two series of novel derivatives of lanthanides and (
6-benzoic acid)tricarbonylchromium [benchrotrenecarboxylic acid, BcrCOOH (Bcr ¼ (
6-C6H5)Cr(CO)3)], namely, the mononuclear complexes [Ln(BcrCOO)(acac)2(H2O)2] and the 1D-coordination polymers [Ln(BcrCOO)(acac)2(H2O)]n, have been synthesized (Scheme 40).68

Scheme 40

Reactions of the planar chiral arene chromium complexes 91 with allyl alkoxide, followed by photo-oxidative demetalation, give a-allyl b-arylpropionates up to 97% ee. Similar reactions of the chiral heterobinuclear tungsten carbene complexes 92 afford anti a-allyl b-hydroxy b-arylpropionates as major products in up to 92/8 dr (Scheme 41). High diastereoselectivity in these reactions is

Scheme 41

98

Cyclic and Non-cyclic Pi Complexes of Chromium

attributed to the planar chirality of the arene chromium complexes. The reaction products, a-allyl b-arylpropionates, are derived by 1,3-M(CO)5 shift and subsequent [3,3]-sigmatropic rearrangement.69 As shown in Scheme 42, deprotonation of the benzo[b]thiophene (BT) chromium complex (
6-benzo[b]thiophene)tricarbonylchromium(0) Cr(CO)3 with nBuLi, followed by reaction with chromium hexacarbonyl Cr(CO)6, and finally treatment with triethyloxonium tetrafluoroborate or titanocene dichloride yields the binuclear carbene complex [Cr(CO)5{C(OEt)((
6-2-BT)Cr(CO)3)}] (93) and the trinuclear carbene complex [Cr(CO)5{C(OTiCp2Cl)((
6-2-BT)Cr(CO)3)}] (94), respectively.70

Scheme 42

Similar complexes 95 and 96 with a tungsten Fischer carbene substituent have also been synthesized. Their structural features and relevance to the analogous chromium complexes have been investigated. The effect of metal-containing substituents on the carbene ligands has been studied by DFT calculations.71

Neutral d,p-coordinated bimetallic carbene complexes 97–103 have been obtained by treatment of {
6-Me2NC6H5}Cr(CO)3 or {
-MeOC6H5}Cr(CO)3 with nBuLi and subsequent reactions with W(CO)6 and [Et3O][BF4]. Molecular structures of these complexes have been confirmed by single crystal X-ray diffraction analysis. Spectroscopic data, electrochemistry studies and DFT calculations on these complexes have been conducted.72 6

Reactions of in situ formed tricarbonyl(
6-2-methylindenyl)chromium anion with metallacycles such as [(
2-2-Me2NCH2C6H4)M(m-Cl)]2, [(
2-2-Me2NCHMe-C6H4)M(m-Cl)]2, [(
2-2-Me2NCH2-4-F-C6H4)M(m-Cl)]2, [(
2-2-Me2NCH2-Fc)M(m-Cl)]2 and {[
2-2-(20 -C5H4N)(
6-C6H4)Cr(CO)3]M(m-Cl)}2 [M ¼ Pd, Pt] afford synfacial heterobimetallic complexes 104–109, in which the indenyl fragment binds the metal Pd or Pt in an
1 fashion, leaving the fourth coordination site of the metal atom virtually vacant and making a T-shaped coordination environment around the metal atom.73 The preferential forming of these synfacial

Cyclic and Non-cyclic Pi Complexes of Chromium

99

heterobimetallic complexes is attributed to the weak non-covalent attractive interactions between the Pd or Pt metal atom and the Cr(CO)3 moiety. This kind of coordination model around the Pd, Pt or Rh metal atom by the (
6-lindenyl)Cr(CO)3 anion has been called “hemichelation”.73–75 The concept of “hemichelation”, as a genuine coordination mode of polytopic ligands to metal centers, combines covalent and non-covalent binding interactions, which outlines the potential of non-covalent attractive interactions in ensuring structural cohesion and opens new perspectives in the design of catalysts and materials with new architectures.

Antarafacial isomers of these T-shaped complexes have also been observed and the trans-cis isomerization of the T-shaped complexes has been studied.76 In subsequent investigations, more hemichelated complexes have been synthesized and characterized in the solid state. So far, hemichelated complexes of Pd(II),73,74,77 Pd(I),78 Mn(I),79,80 Re(I),81 Pt(II)73 and Rh(I)75 have been reported. An investigation on the molecular dynamics of a Pd(II) hemichelated complex further confirms the non-covalent nature of the interaction between the Pd(II) atom and the Cr(CO)3 fragment, which exhibits fluxionality, the Pd atom binding alternatively the two proximal Cr-bound CO ligands and the rotation of the Cr(CO)3 moiety being largely slower than the rapid coordinative exchange equilibrium.82 Another study has demonstrated that elusive trans-CdPddC hemichelated complexes can be formed when a fluorene-based tricarbonylchromium anionic ligand is used. Structural characterization and spectroscopic investigations reveals their ability to isomerize into the thermodynamically stable cis-CdPddC isomers within hours.76

5.04.2.1.10

Gold(I)-catalyzed cyclization of arene chromium carbonyls

Activation of acetylene groups by metal complexes has become an attractive strategy for catalyzing carbon-carbon bond reactions. In particular, gold has been studied as a homogeneous catalyst for the electrophilic activation reaction of alkynes and the enyne cyclization reaction. There are also some reports on gold catalyzed benzannulation reactions for the substituted aromatic compounds. Gold(I)-catalyzed cycloisomerization of planar chiral arene chromium complexes with enyne bonds affords stereoselectively axially chiral biaryl–chromium complexes in good yields (Schemes 43 and 44).83,84

Scheme 43

100

Cyclic and Non-cyclic Pi Complexes of Chromium

 Scheme 44

Gold(I)-catalyzed cyclization of o-alkynyl benzaldehyde and benzaldimine chromium complexes with nucleophiles gives stereoselectively 1-anti- and 1-syn-functionalized heterocycle chromium complexes depending on the nature of nucleophiles (Scheme 45).85

Scheme 45

Gold(I)-catalyzed asymmetric cyclization of 1,3-dihydroxymethyl-2-alkynylbenzene chromium complexes forms planar chiral isochromene chromium complexes with high enantioselectivity (Scheme 46).37

Scheme 46

In addition, gold(I)-catalyzed asymmetric intramolecular cyclization of prochiral 1,3-bis(carbamate)-2-alkynylbenzene tricarbonylchromium complexes with axially chiral diphosphine ligand generates planar chiral tricarbonylchromium complexes of 1H-isochromene or 1,2-dihydroisoquinoline with high enantioselectivity (Scheme 47).38

Scheme 47

Cyclic and Non-cyclic Pi Complexes of Chromium

5.04.2.1.11

101

Chromium-stabilized cations and related complexes

Treatment of complex 110 with one equiv. of Ag[Al(OR)4] (OR ¼ OC(CF3)3 or OC(CF3)2Me) in CH2Cl2 gives a highly air-sensitive seventeen-electron half-sandwich radical, [(C6Me6)Cr(CO)3]+ (110+), which has been characterized by single-crystal X-ray analysis, EPR spectroscopy, and theoretical calculations. A more stable cation, [(C6Me6)Cr(CO)2(PPh3)]+ (111+) can be formed through a substitution reaction with PPh3 (Scheme 48).86

Scheme 48

Chemical oxidations of piano-stool chromium/cobalt carbonyl complexes Cr(CO)3(
6,
5-C6H5C5H4)Co(CO)2 (112) and Cr(CO)3(
6,
6-C6H5C6H5)Cr(CO)3 (113) have been investigated. Upon one-electron oxidation, 112 is converted to a heterometalloradical species, 112%+. However, either one- or two-electron oxidation of 113 affords a decomposition product, 114. The formation of 115 can be observed via a crystal-to-crystal transformation with the removal of solvent molecules when dipping 114 into pentane (Scheme 49).87

Scheme 49

The synthesis, characterization, crystal structures, and theoretical studies on the first complexed diphosphene radical cation (
6-arene)chromium complex 116 and the first 17e Cr radical cation (
6-arene)chromium complex with a side-on p-bonded ligand

102

Cyclic and Non-cyclic Pi Complexes of Chromium

117 have been reported.88 Both radical cation species are stable in the solid state under anaerobic conditions at room temperature, but readily decompose in air. The X-ray crystallographic analysis reveals that the PdP bonds are coordinated to the Cr metal in an end-on (in 116) or side-on (in 117) fashion.

5.04.2.1.12

Chromium carbonyls of polycyclic hydrocarbons

Reactions of 1,4-dihydronaphthalene with (NH3)3Cr(CO)3 or Py3Cr(CO)3 in the presence of BF3
Et2O afford a mixture of isomeric
6-chromium tricarbonyl complexes of 1,4- and 1,2-dihydronaphthalene 118 and 119 in the ratio 63:37 and 55:45, respectively (Scheme 50).89

Scheme 50

An (
6-arene)Cr(CO)3 complex connected with a platensimycin derivative 120 has been prepared in a five-step synthesis starting from 2-methyl-1-tetralone.90

Optimized synthetic procedures to [7]heliphene have been developed, which allows the synthesis of (
6-[7]heliphene)tricarbonylchromium (121) and a preliminary exploration of its chemical behavior. DFT calculations provide structural and energetic information on this complex and its haptomeric isomers.91 Treatment of pyrido[2,1-]isoindole with (CH3CN)3Cr(CO)3 affords p complex (
6-iczH)Cr(CO)3 (122), as shown in Scheme 51.92

Scheme 51

Cyclic and Non-cyclic Pi Complexes of Chromium

103

A combined experimental and theoretical study on the structure anomaly in (benzocyclobutenedione)tricarbonylchromium complexes has been reported, in which the syntheses of methoxy- and trifluoromethyl-substituted benzocyclobutenone and benzocyclobutenedione tricarbonylchromium complexes were described.93 An aromatic molecule 2,3-diethyl-1,4-dimethoxytriphenylene and its Cr(CO)3 complex [
6-(1,2,3,4,4a,12b)-(2,3-diethyl1,4-dimethoxytriphenylene)]tricarbonylchromium (123) were deposited on a Ag(111) surface by vapor deposition. X-ray photoelectron spectroscopy and scanning tunneling microscopy studies on their monolayers indicate that both substances adsorb with the extended p-system parallel to the surface and form long-range ordered monolayers.94 Phenyl-, anthracenyl- and triptycyl-substituted indene tricarbonylchromium complexes [
6-2-phenylindene]Cr(CO)3 (124), 6 [
-2-(9-anthracenyl)indene]Cr(CO)3 (125) and [
6-2-(9-triptycyl)indene]Cr(CO)3 (126) have been synthesized by reactions of the corresponding indene derivatives with Cr(CO)6. Treatment of these complexes with NaOtBu in DMSO leads to their conversion into anionic complexes Na[
5-2-phenylindenyl]Cr(CO)3 (127), Na[
5-2-(9-anthracenyl)indenyl]Cr(CO)3 (128), Na[
5-2-(9triptycyl)indenyl]Cr(CO)3 (129). The dynamic behavior of these complexes has been studied by experiments and DFT calculations.95,96

5.04.2.1.13

Haptotropic migration of chromium carbonyl group

Haptotropic metal migration of transition metal p-complexes has been observed, where p-bound ligands offer various coordination possibilities. In some aromatic chromium complexes, metal fragments can be considered as mobile functional groups moving between the different benzene rings. Stereoselective tricarbonylchromium migration to another arene ring and its application to the stereoselective synthesis of axially chiral biaryl tricarbonylchromium complexes have been demonstrated. Incorporation of a directing group such as a hydroxyl group plays an important role in the migration of the tricarbonylchromium group. In the tricarbonylchromium migration process, both the axial chirality and the chirality at the side chain from a single chiral source can been controled.97 The sterically hindered N-aryl indole chromium complexes 130 reflux in toluene to give complexes 131 through stereoselective chromium tricarbonyl migration, as shown in Scheme 52.7

Scheme 52

The thermodynamic isomers 134 and 135 have been obtained by a haptotropic migration of the chromium fragment of the kinetic diastereoisomeric anti- and syn-heterobimetallic CrdMn polyarene complexes 132 and 133, respectively, as shown in Scheme 53.98

104

Cyclic and Non-cyclic Pi Complexes of Chromium

Scheme 53

Similarly, to study the influence of cyclomanganation on the haptotropic migration of the Cr(CO)3 fragment, complexes 136 and 137 were heated to 105  C in di-n-butyl ether to give complexes 138 and 139, respectively, as shown in Scheme 54.99

Scheme 54

Kinetic study on the ethene-bridged complex 140 reveals a haptotropic metal shift onto the adjacent naphthalene ring to give mainly complex 141 with minor amounts of complex 142 as well as binuclear complexes 143 and 144. The formation of 143 and 144 involves competing intermolecular pathways (Scheme 55).100

Cyclic and Non-cyclic Pi Complexes of Chromium

105

Scheme 55

Complexes [(
6-C6H5)(C6H4-4-NH2)]Cr(CO)3 (145) and [(
6-C6H5)(C6H4-2-NH2)]Cr(CO)3 (147) have been synthesized as shown in Schemes 56 and 57. Complexes [(
6-C6H4-4-NH2)(C6H5)]Cr(CO)3 (146) and [(
6-C6H4-2-NH2)(C6H5)]Cr(CO)3 (148) can be obtained by heating 145 and 147 or reactions of 4-aminobiphenyl or 2-aminobiphenyl with Cr(CO)6 in dibutyl ether/ THF (9:1).101

Scheme 56

106

Cyclic and Non-cyclic Pi Complexes of Chromium

Scheme 57

5.04.2.1.14

Benzannulations of Fischer-type carbene complexes

The stable silyl vinylketenes 150 have been prepared via the thermal reactions of Fischer carbene complexes 149 with triisopropylsilyl- or tert-butyldimethylsilyl-substituted alkynes, as shown in Scheme 58. The corresponding cyclopentenone products 151 were obtained by [4 +1] annulation reactions of silyl vinylketenes 150 with carbenoid reagents.102

Scheme 58

The photochemical [2 +2] reactions of the chromium Fischer carbene complexes with triisopropylsilyl-substituted alkynes afford the TIPS-vinylketene complexes 152–155 or the 2-TIPS-substituted cyclobutanone complex 156 with the Cr(CO)3 moiety attached to the phenyl ring.103 Moreover, reactions of Fischer carbene complexes with conjugated enediynes that feature a pendant alkene group have also been examined.104

Cyclic and Non-cyclic Pi Complexes of Chromium

107

As shown in Schemes 59–61, densely substituted hydroquinoid phenanthrene, acephenanthrene and triphenylene chromium tricarbonyl complexes 160–166 have been prepared via benzannulation of naphthalenyl, acenaphthenyl and phenanthrenyl carbene complexes 157–159, respectively.105

Scheme 59

Scheme 60

108

Cyclic and Non-cyclic Pi Complexes of Chromium

Scheme 61

The benzannulation of pentacarbonyl[(methoxy)-1-naphthylcarbene]chromium with 3-hexyne gives hydroquinoid phenanthrene Cr(CO)3 complex 167 as the kinetically favored product. The complex 167 and its thermodynamically stable isomer 168 undergo a photoinduced substitution with phosphites PR3 (R ¼ Me, OMe, Ph, OPh) to give (
6-phenanthrene)Cr(CO)2PR3 complexes 169 and 170, respectively, as depicted in Scheme 62.106

Scheme 62

Cyclic and Non-cyclic Pi Complexes of Chromium

109

The cymanthrene-type Fischer carbene complex 171 undergoes a clean benzannulation with 3-hexyne upon warming in refluxing tert-butyl methyl ether. The formed diastereomers anti-132 and syn-133 could be isolated after in situ O-protection with tert-butyldimethylsilyl triflate, as depicted in Scheme 63.98

Scheme 63

Methoxy(benzo[h]quinolinyl)carbene chromium 172 undergoes a chromium-templated benzannulation with 3-hexyne to give (dibenzo[f,h]-quinoline)tricarbonylchromium complex 136, in which the hydroquinoid ring is selectively labeled by a Cr(CO)3 fragment. Cyclomanganation of (benzo[h]quinolinyl)carbene complex 172 gives bimetallic chromium carbene 173. Reaction of 173 with 3-hexyne forms the hydroquinoid dibenzo[f,h]quinoline s-manganese-p-chromium complex 137 as shown in Scheme 64.99

Scheme 64

The naphthohydroquinoid tricarbonyl chromium complexes 140 and 176, bearing a styryl or phenylazo moiety, have been synthesized in similar way from benzannulation of 174 or 175, respectively (Scheme 65).100

Scheme 65

110

Cyclic and Non-cyclic Pi Complexes of Chromium

Heterobimetallic (di)benzoindenyl RedCr complexes have been prepared through a sequence of reactions, as shown in Scheme 66. Reaction of (8-bromobenzo[e]-indenyl)potassium with pentacarbonylrhenium bromide affords tricarbonylrhenium complex 177, which is then converted to rhenium–chromium carbene complex 178. The latter undergoes a chromium-templated [3 +2 +1] benzannulation with 3-hexyne to give the anti-(Cr(CO)3dRe(CO)3) dibenzoindenyl complex 179 as the major product along with the syn diastereoisomer 180.107

Scheme 66

The reaction of pentacarbonyl{methoxy-[1-(
5-4-methoxy-6-phenylcyclohexadienyl)tricarbonylmanganese]carbene}chromium (181) with 3-hexyne via a chromium templated benzannulation gives the formation of complex 182 (Scheme 67). A syn position of the phenyl group with respect to the Cr tripod examined in the structure of complex 182 confirms participation of the Mn moiety in the benzannulation process.108

Scheme 67

Another synthetic route to the chromene ring system is via the reaction of an a,b-unsaturated Fischer carbene complex of chromium with a propargyl ether bearing an alkenyl group on the propargylic carbon. The transformation procedure involves a cascade of reactions that begins with a benzannulation reaction, then forms an o-quinone methide, and finally generates a chromene complex through an electrocyclization reaction.109

5.04.2.1.15

Polymer-bound arene chromium carbonyls

Due to potential applications in various fields including catalysis, multielectron reservoirs, electron-transfer mediators and ion sensors, polymers containing organometallic motifs have attracted great research interest. Among the organometallic motifs used for such purposes, the polymers containing arenetricarbonylchromium complexes [(
6-arene)Cr(CO)3] have been attracting significant attention in recent years.

Cyclic and Non-cyclic Pi Complexes of Chromium

111

As shown in Scheme 68, reaction of N-vinylcarbazole with (NH3)3Cr(CO)3 affords (N-vinylcarbazole)Cr(CO)3 (183) and (N-vinylcarbazole)[Cr(CO)3]2 (184), wherein 183 can be conveniently converted into (N-vinylcarbazole)Cr(CO)2(PPh3) (185) and (N-vinylcarbazole)Cr(CO)2(C3H5)+ BF−4 (186) by reactions with PPh3 or C3H5OH/HBF4 under appropriate conditions. Under radical initiation conditions, the vinyl-type monomers 183 and 185 with organometallic push–pull chromophores undergo a controlled organometallic radical polymerization and produce the corresponding well-defined polymers 187 and 188.110

Scheme 68

Reactions of (benzoic acid) chromium tricarbonyl with zinc acetate in the presence of various organic dipyridyl linkers produce different coordination polymers depending on the nature of the linker. Reactions of (benzoic acid) chromium tricarbonyl with zinc acetate and bidentate pyridine-based ligands 4,40 -bipyridine (4,40 -bipy), 1,2-bis(4-pyridyl)ethane (bpe), and 1,3-bis(4-pyridyl) propane (tmdp) afford the coordination polymers [Zn[{
6-C6H5COO}Cr(CO)3]2(4,40 -bipy)]n (189), [Zn[{
6-C6H5-COO}Cr (CO)3]2(bpe)]n (190), [Zn[{
6-C6H5COO}Cr(CO)3]2(tmdp)]n (191), as shown in Scheme 69.111

Scheme 69

112

Cyclic and Non-cyclic Pi Complexes of Chromium

Similarly, the reaction of 4,4-bipyridine with (benzoic acid)tricarbonylchromium and cadmium acetate leads to the onedimensional coordination polymer [Cd2{(
6-C6H5COO)Cr(CO)3}4(4,40 -bipy)2
3DMF]n (192) (Scheme 70).112

Scheme 70

Treatment of (
6-benzoic acid)-tricarbonylchromium with lead(II) acetate in the presence of trans-1,2-bis(4-pyridyl)ethene (bpe) gives the one-dimensional coordination polymer [Pb{(
6-C6H5COO)Cr-(CO)3}2(bpe)]n (193) (Scheme 71).113

Scheme 71

Organostannoxanes have been utilized as platforms for supporting organometallic [(
6-arene)Cr(CO)3] complexes by forming coordination assemblies.114 As shown in Schemes 72 and 73, reactions between Me3SnCl and [
6-C6H4(COOH)2-1,3]Cr(CO)3

Scheme 72

Cyclic and Non-cyclic Pi Complexes of Chromium

113

Scheme 73

(L1H2) or [
6-C6H4(COOH)2-1,4]Cr(CO)3 (L2H2) in a 2:1 ratio afford planar two-dimensional coordination polymers [(Me3Sn)2(m4-L1)]n (194) and [(Me3Sn)2(m4-L2)]n (195), respectively, while the reaction of L2H2 with Me3SnCl in the presence of 4,40 -bipyridine forms a 1D-coordination polymer [(Me3Sn)2(m-L2)(m-4,40 -bipy)]n (196). In addition, arenetricarbonyl metal complexes ([dphCr(CO)3d] and [dbiphCr(CO)3d]; ph ¼ phenylene, biph ¼ biphenylene) constructed within porous coordination polymers Zr-based MOFs play as highly active and selective catalysts in the system of epoxidation of cyclooctene. Catalytic activities of these complexes increase with increasing pore sizes of the Zr-based MOFs.115

5.04.2.1.16

Miscellaneous arene chromium carbonyls

Dihydrotestosterone (DHT) derivatives bearing a (
6-C6H5)Cr(CO)3 unit at the C-17 position of the steroid skeleton have been prepared as shown in Scheme 74. The chromium complex 198 could be obtained from the reaction of 197 with Cr(CO)3(NH3)3 in dioxane. Deprotection of 198 with PTSA produces the chromium DHT derivative 199 in a moderate yield.116

Scheme 74

114

Cyclic and Non-cyclic Pi Complexes of Chromium

Reaction of 200 with Cr(CO)6 in Bu2O/THF at 120  C in a glass autoclave forms an isomeric mixture of complexes 201 and 202, which could not be separated by recrystallization or chromatography, but crystals could been separated by hand because of the different crystal shapes (Scheme 75).117

Scheme 75

Reaction of 1,2-dihydro-2-phenyl-1,2-benzazaborine (203) with Cr(CO)3(CH3CN)3 in THF at 140  C for 40 h gives complex 204, in which the phenyl is
6-bound to chromium. The same reaction under milder conditions gives a mixture of 206 and an isomer tentatively identified as 204. Pure 206 can’t be isolated from the reaction mixture, but can be converted to 204 by heating the reaction mixture to 140  C. Complex 204 can be quantitatively deprotonated by LiTMP in THF to form a solution of 205 (Scheme 76).118

Scheme 76

The synthesis of Cr(CO)3-complexed aromatic nitrones has been reported. Reactions of Cr(CO)3-complexed aromatic aldehydes with N-benzyl-hydroxylamine afford the corresponding nitrone derivatives 207 and 208. Treatment of 207a and 207b with 2 equiv. of SmI2 in THF at −78  C results in the homocoupling of Cr(CO)3-complexed nitrones to form the binuclear complexes 209.119

Treatment of N,N0 -diphenylurea or N-methyl-N0 -phenylurea with (NH3)3Cr(CO)3 by refluxing in dioxane gives the complexes [(N,N0 -diphenylurea)Cr(CO)3] (210a) or [(N-methyl-N0 -phenylurea)Cr(CO)3] (210b), respectively, in which each urea-bound Cr(CO)3-benzene fragment can act as a good hydrogen bond donor and acceptor.120

Cyclic and Non-cyclic Pi Complexes of Chromium

115

In addition, a fulleropyrrolidine-tricarbonylchromium complex, 1-methyl-2-(4-methoxyphenyl)-3,4-[60]fulleropyrrolidinetricarbonylchromium has been synthesized and its electrochemical properties were investigated (Scheme 77).121

Scheme 77

New (
6-arene-quinoline)Cr(CO)3 complexes [
6-N-(7-chloroquinolin-4-yl)-N0 -(2-dimethyl-aminomethyl-benzyl)ethane-1, 2-diamine]tricarbonylchromium (213) and [
6-N-(7-chloro-quinolin-4-yl)-N0 -(2-dimethylamino-benzyl)ethane-1,2-diamine] tricarbonylchromium (218) have been synthesized from (
6-benzyldimethylamine)tricarbonylchromium (211) and (
6phenyldimethylamine)tricarbonylchromium (214) as depicted in Scheme 78.122 Lithiation of 211 and 214, followed by reaction with DMF, yields [
6-2-(dimethylaminomethyl)-benzaldehyde]tricarbonylchromium (212) and [
6-2-(dimethylamino) benzaldehyde]-tricarbonylchromium (215), [
6-3-(dimethylamino)benzaldehyde]tricarbonylchromium (216), [
6-4-(dimethylamino)benzaldehyde]tricarbonylchromium (217), respectively. Reductive amination of 212 and 215 with N1-(7-chloroquinolin-4-yl)ethane-1,2-diamine gives complexes 213 and 218, respectively.

Scheme 78

116

Cyclic and Non-cyclic Pi Complexes of Chromium

A number of dCOOH and dNH3 containing (
6-arene)chromium tricarbonyl complexes (
6-C6H5CH2COOH)Cr(CO)3 (219), [
6-1,2-(NH2)(COOEt)C6H4]Cr(CO)3 (220), [
6-2,6-(iPr)2C6H4NH2]Cr(CO)3 (221) and [
6-1,4-(NH2)(CH3)C6H4] Cr(CO)3 (222) have been synthesized. Heating a mixture of Cr(CO)6 and the corresponding arenes 1,2-(NH2)(COOEt)C6H4, 2,6-(iPr)2C6H4NH2 or 1,4-(NH2)(CH3)C6H4 in di-n-butyl ether/THF directly affords the complexes 220, 221 and 222, respectively. Hydrolysis of an ester-containing complex (
6-C6H5CH2COOEt)Cr(CO)3 gives the complex 219. These complexes form hydrogen bonds in the solid state, leading to dimeric or polymeric networks. The complex 219 just forms a dimer via hydrogen bonding of two carboxylic acid functions. The aniline derivatives form hydrogen bonds between the NH2 groups and one CO group of the Cr(CO)3 moiety. Moreover, the connectivity is also influenced by the steric demand of the ligand.123

Complexes 223 and 224 have been synthesized and used as ligands for synthesis of the Cr(CO)3-containing Ir and Cr complexes.124 A tricarbonylchromium complex 225 bearing the ligand 1,8-bis(dimethylamino)-2-(4-methoxyphenyl)naphthalene has also been synthesized and characterized by ESI-MS analysis.125 A series of azines derivatives and reaction intermediates bearing a (
6-C6H5)Cr(CO)3 unit have been obtained from metal-mediated CdC coupling reactions of azines with lithium benzenechromiumtricarbonyl and characterized by NMR and X-ray analysis.126

Cyclic and Non-cyclic Pi Complexes of Chromium

117

A series of glycoside-derived tricarbonyl(
6-arene)chromium complexes have been prepared from phenyl and benzyl O-, N-, Sand C-glycosides, and have been fully characterized.127 (
6-naphthalene)Cr(CO)3 has been used in the study of tricarbonylchromium-mediated cycloaromatization of enediynes. NMR spectroscopic analysis suggests that enediyne alkene coordination to the tricarbonylchromium species occurs in preference to alkyne coordination, forming a THF-stabilized olefin intermediate; subsequent alkyne coordination leads to cyclization.128 Heating the mixture of dibenzo[a,e]cyclooctatetraene and hexacarbonylchromium in di-n-butyl ether/THF to reflux for an extended period, both the syn,anti and anti,anti isomers of the bis-tricarbonylchromium complex anti,anti-DBCOT[Cr(CO)3]2 (226) and syn,antiDBCOT[Cr(CO)3]2 (227) are formed. DFT calculations on both isomers confirm that the syn,anti isomer is favored.129

The reaction of pentaphenylbenzene with chromium hexacarbonyl yields (
6-C6Ph5H)Cr(CO)3 complexes 228 and 229 in which the metal tripod is attached either to an ortho peripheral phenyl ring or to the central phenyl ring (Scheme 79).130

Scheme 79

Tricarbonyl(
6-arene)chromium complexes with tethered pyridinyl groups [
6-C6H5(CH2)n(2-Py)]Cr(CO)3 (230) (2-Py ¼ 2-pyridinyl, n ¼ 1–3) have been synthesized. Irradiation of 230 with 300 nm light affords the corresponding dicarbonyl chelates [
6-C6H5(CH2)n(2-Py)-kN]Cr(CO)2 (231) (Scheme 80).131 The effect of chain length has been examined for the parent tricarbonyl chromium complexes as well as the dicarbonyl chromium pyridinyl chelates. Variable-temperature NMR spectra for two dicarbonyl chromium pyridyl chelates (n ¼ 2 and 3) indicate chelate ring inversions happening in solution, which is supported by DFT calculations.132

118

Cyclic and Non-cyclic Pi Complexes of Chromium

Scheme 80

Reactions of cycloparaphenylenes ([n]CPPs, n ¼ 9 and 12) with Cr(CO)6 afford the corresponding
6-[n](CPP)Cr(CO)3 complexes. 1H NMR spectra of these complexes show characteristic upfield-shifted singlet signals corresponding to the four hydrogen atoms attached to the coordinated C6H4 ring of the CPPs at 5.4–5.9 ppm. X-ray crystallographic analysis on [9]CPPCr (CO)3 reveals that chromium-CPP coordination occurs at the convex surface of [9]CPP. By using CPPCr(CO)3 complexes, CPPs have been functionalized to give highly monoselective silyl-, boryl-, and methoxycarbonyl-CPPs complexes.133 A phenylethynyl-substituted (arene)Cr(CO)3 complex (
6-PhC^CC6H5)Cr(CO)3 (232) has been synthesized by treatment of (
6-benzene)Cr(CO)3 with tert-butyllithium first and reaction then with phenylethynyl tolylsulfonate as shown in Scheme 81.134 A couple of (
6-arene)Cr(CO)3 complexes wherein the arene is chlorobenzene, phenyl trimethyl silane and acenaphthene have also been synthesized and characterized by NMR, IR and UV-visible spectroscopy and DFT calculations.135

Scheme 81

Solid-state structures of 2-fluoro-biphenyl and 4-fluoro-biphenyl tricarbonylchromium complexes [(
6-C6H5)(20 -F-C6H4)] Cr(CO)3 (233), and [(
6-C6H5)(40 -F-C6H4)]Cr(CO)3 (234) have been investigated, to explore how the steric and electronic effects of a fluorine substituent on the biphenyl ligand would influence ring selectivity of chromium coordination.136

Reactions of Cr(CO)6 with fluorous benzenes C6H6-x[(CH2)3Rf8]x [x ¼ 1–3, Rf8 ¼ (CF2)7CF3] and p-Rf8(CH2)3C6H4CH[S(CH2)3Rf8](CH2)2Rf8 give the p complexes {
6-C6H6-x[(CH2)3Rf8]x}-Cr(CO)3 [x ¼ 1 (235), 2 (236), 3 (237)] and {
6-p-Rf8(CH2)3 C6H4CH[S(CH2)3Rf8](CH2)2Rf8}-Cr(CO)3 (238) (Scheme 82).137 The CF3C6F11/toluene partition coefficients of 235–238 have been determined by HPLC and found to increase with the number of ponytails (three, 99:1 to 97:3; two, 91:9; one, 49:51).

Cyclic and Non-cyclic Pi Complexes of Chromium

119

Scheme 82

Treatment of (
6-5,8-dihydroxynaphthalene)Cr(CO)3 with CF3COOH in degassed dry benzene yields its tautomer (
-1,2,3,4-tetrahydronaphthalene-5,8-dione)Cr(CO)3 (239) together with a small amount of the less stable starting material in a ratio of 92:8 (Scheme 83). Pure 239 is obtained by successive recrystallizations from iPr2O.138 6

Scheme 83

The reaction of flavone and Cr(CO)6 in Bu2O/THF under an oxygen free atmosphere gives the compound [Cr(C15H10O2)(CO)3] ((C15H10O2) ¼ flavone) (240), in which the Cr(CO)3 unit exhibits a three-legged piano-stool conformation.139 An (arene) tricarbonylchromium complex (241) with a steroid-like arene ligand has been synthesized in THF by a microwave irradiation assisted reaction of hexacarbonylchromium with the ligand in a 78% isolated yield.140 It has been demonstrated experimentally that Cr(CO)3 complexation has a significant activation effect on the homolytic cleavage of aromatic CdCl bonds.141 Treatment of (chloroarene)Cr(CO)3 complexes 242 with SmI2/HMPA at room temperature

120

Cyclic and Non-cyclic Pi Complexes of Chromium

results in the complete dechlorination of 242 to form the corresponding products 243 in almost quantitative yields. Similar reactions of complexes 244 and 246 with SmI2/HMPA generate the corresponding dechlorinated cyclization products 245 and 247 in good to excellent yields, as shown in Scheme 84. DFT calculations indicate that the spin delocalization effect of the metal center plays an important role in reducing the CdCl bond dissociation energy.

Scheme 84

It has been known that tricarbonylchromium complexes of aryl triflates undergo a base-mediated anionic thia-Fries rearrangement to generate push–pull substituted [o-(trifluoromethylsulfonyl)phenol]tricarbonylchromium complexes.142 As shown in Scheme 85, reactions of the ligands with Cr(CO)6 in dibutyl ether/THF afford the corresponding phenol tricarbonylchromium complexes 248. Subsequent treatment with triflic anhydride affords the tricarbonylchromium complexes of aryl triflates 249. Finally, Treatment of complexes 249 with LDA at −78  C cause the regioselective rearrangement to generate [o-(trifluoromethylsulfonyl)phenol]tricarbonylchromium complexes 250 under mild reaction conditions.

Scheme 85

Heterogeneous hydrogenation reactions of the styrene and stilbene chromium tricarbonyl complexes RCH]CHPhCr(CO)3 [R ¼ H, Ph] by molecular hydrogen on skeletal Ni/C and Pd/C catalysts have been studied. The reactions were found to be considerably slow in comparison to the hydrogenation reactions of styrene and stilbene.143 Radical aromatic substitution of tert-butyl groups for hydrogen on simple arene tricarbonylchromium complexes has also been investigated. It was found that pre-existing tert-butyl and methoxy groups direct incoming radicals primarily to the meta-position.144 The reaction of a benzaldiminetricarbonylchromium complex 251 with benzylzinc bromide in the presence of scandium triflate selectively gives single diastereomer 252 exclusively (Scheme 86). The o-TMS group on the benzaldiminetricarbonylchromium complex was found necessary for affording the diastereoselectivity.145

Cyclic and Non-cyclic Pi Complexes of Chromium

121

Scheme 86

A number of highly selective chiral auxiliaries containing arene chromium complexes have been prepared using biogenic precursors. The systems derived from isomannide, prolinol, and xylofuranose were applied to the asymmetric Diels–Alder reaction of the derived acrylate esters, and good stereoselectivity with >95% ee and 98:2 exo:endo ratio was achieved in the cycloaddition with cyclopentadiene.146 Photolysis of (
6-C6H6)Cr(CO)3 in the presence of 2,3-dihydrofuran (DHF) gives two linkage isomers (
6-C6H6)Cr(CO)2 1 [
-(O)-2,3-DHF] and (
6-C6H6)Cr(CO)2[
2-(C,C)-2,3-DHF]. The rearrangement from the oxygen bound isomer to the thermodynamically favored p bound isomer has been followed on the millisecond to microsecond time scale using step-scan FTIR, which suggests that the rearrangement proceeds intramolecularly with the metal migrating from one functional group to another.147 Studies on the displacement of
2-L ligands in the photolytically generated (
6-C6H6)Cr(CO)2(
2-L) [L ¼ Furan and 2,3-DHF] complexes by pyridine indicate that the displacement reaction spans a wide range of time scales from microseconds to hours, and the interaction between the metal and the DHF ligand is stronger ( 6–10 kcal mol−1) than with furan due to resonance in the aromatic furan molecule.148 Photolysis of (1,3,5-iPr3-C6H3)Cr(CO)3 (253) in fluorobenzene-d5 or cyclopentane solution at 230 K in the presence of H2 leads to a dihydrogen complex (1,3,5-iPr3-C6H3)Cr(CO)2(H2) (254) (Scheme 87). The resulting complex 254 has moderate thermal stability and is identified as a sigma complex of H2 by NMR spectroscopy.149 Sigma borane complexes of the type (
6-arene) Cr(CO)2(
1-HBH2
NMe3) (arene ¼ fluorobenzene, benzene and mesitylene) (255) can also be prepared via photolysis of (
6-arene)Cr(CO)3 and Me3N
BH3 (Scheme 88). These derivatives are unstable towards decomposition, and they could only be characterized in solution using multinuclear NMR spectroscopy.150 Photolysis of the complex 61 results in an intramolecular d-borane complex [
1-(
6-C6H5CH2NMe2
BH2dH)]Cr(CO)2 (256) via elimination of one CO ligand,55 as shown in Scheme 89.

Scheme 87

Scheme 88

Scheme 89

122

Cyclic and Non-cyclic Pi Complexes of Chromium

Two chromium dithiolate complexes 257 and 258 have been synthesized by salt metathesis and characterized by electronic spectroscopy, X-ray crystallography, and magnetic measurements using Evans’ method and SQUID magnetometry.151

The (phenyl triflate)chromium complexes, even electron-rich ones, have been shown to have a highly dominant tendency to occur anionic thia-Fries rearrangements over triflate elimination, resulting in the rearrangement products with high yields.152 Treatment of a phenyl triflate tricarbonylchromium complex 259 with LDA in THF forms quantitatively the phenolate complex 260, instead of the desired benzyne tricarbonylchromium complex from triflate elimination. An enantiospecific C(sp2)dC(sp3) cross-coupling reaction with easily accessible and electronically diverse chromium arene complexes as efficient mediators to produce the desired products in high yields, with excellent ortho-selectivity, complete enantiospecificity, and excellent functional group tolerance, has been developed (Scheme 90).153

Scheme 90

Hexahapto graphene tricarbonylchromium complexes have been synthesized using a facile and simple photochemical route, as shown in Scheme 91. The
6-graphene tricarbonylchromium complexes show significantly enhanced conductivity that ranges from 4-fold increase [prepared from reaction with Cr(CO)6] to about 2 times [prepared from reaction with (
6-benzene)Cr(CO)3 reagent].154

Scheme 91

In addition, solution molar volumes and structures of a series of chromium tricarbonyl complexes of arenes have been analyzed. The results indicate that the coordination bond polarity increases with growing p-donor ability of the ligand, and the molar volume increment of Cr(CO)3 increases as well, due to transfer of p-electron density from the ligand.155

Cyclic and Non-cyclic Pi Complexes of Chromium

5.04.2.1.17

123

Arene chromium carbonyls with heteroatoms on the periphery

(
6-arene)chromium tricarbonyl complexes with heteroatoms on the periphery of the arene ring have been extensively studied. The reaction of 1,2-dihydro-2-phenyl-1,2-oxaborine with Cr(CO)3(CH3CN)3 in THF at 70  C gives the complex 261 (Scheme 92). Structural characterization of the complex 261 indicates that the 1,2-dihydro-1,2-oxaborine ring has a p-delocalized structure.156

Scheme 92

Reactions of a B-phenylated 1,2,3-triphenyl-o-carborane with 2 or 3 equiv of Cr(CO)6 in a mixture of di-n-butyl ether and THF afford the tricarbonylchromium complexes 1,2-bis(
6-phenyl-chromiumtricarbonyl)-3-phenyl-o-carborane (262) and 1,2,3-tris (
6-phenyl-chromiumtricarbonyl)-o-carborane (263).157

As shown in Scheme 93, reactions of Cr(CO)6 with PhMe2SiCH2GeMe3, PhMe2SiCH-(GeMe3)(SnMe3) or PhMe2SiCH(GeMe3) (PbMe3) lead to the corresponding (arene)Cr(CO)3 complexes, {(
6-C6H5)Cr(CO)3}Me2SiCH2(GeMe3) (264) and {(
6-C6H5) Cr(CO)3}Me2SiCH-(GeMe3)(EMe3), E ¼ Sn (265), Pb (266). The thermal reaction of PhMe2SiCH(GeMe3)-(SnMe3) with Cr(CO)6 in a wet THF/di-n-butyl ether mixture results in the formation of an arene tricarbonylchromium silanol {(
6-C6H5) Cr(CO)3}Me2SiOH (267) as well as the complex 264 in an approximately 1:2 ratio.158

Scheme 93

124

Cyclic and Non-cyclic Pi Complexes of Chromium

The alkynylsilylarene tricarbonylchromium complexes PhC^CMe2SiPhCr(CO)3 (268), n-PrC^CMe2SiPhCr(CO)3 (269), n-PrC^CMe2SidSiMe2-SiMe2PhCr(CO)3 (270), PhC^CMe2SidSiMe2PhCr(CO)3 (271) and the trinuclear tricarbonylchromium complex (OC)3CrPhSiMe2PhC^C
Ni2Cp2 (272) have been synthesized by direct complexation of the corresponding arenes or intermolecular complexation.159 Similar chromium tricarbonyl complexes {
6-[(Me3Si)3SiC6H4R]Cr(CO)3(R ¼ H, p-Me, p-OMe)} (273) have also been prepared and characterized. Density functional theory calculations have been conducted to predict the SidSi bond activation.160

Interaction of organolithium reagent (
6-C6H5Li)Cr(CO)3 with ClSiMe2SiMe2Cl results in [(OC)3Cr(
6-C6H5)]SiMe2SiMe2 [(
-C6H5)Cr(CO)3] (274), Me2Si[(
6-C6H5)Cr(CO)3]2 (275) and the tricarbonylchromium complex with molecular aryl polysilanes, [(OC)3Cr(
6-C6H5)] [SiMe2]4[(
6-C6H5)Cr(CO)3] (276) (Scheme 94). These complexes have been fully characterized by NMR, IR, UV/vis spectroscopy and mass spectrometry, and the molecular structure of 274 has been studied by single crystal XRD analysis The thermodynamics of the interaction has been studied by DFT calculations, which indicates several possible ways of forming the unexpected 275 and 276.161 6

Scheme 94

A series of tricarbonyl chromium complexes with oligogermane substituents R3GedGeAr2(R0 C6H4-
6)Cr(CO)3 (277–281) have been prepared using several different synthetic approaches. The physicochemical properties of these derivatives have been investigated by IR, UV/Vis, NMR spectroscopy, electrochemistry, and DFT calculations.162

Cyclic and Non-cyclic Pi Complexes of Chromium

125

A number of bimetallic arene tricarbonylcromium complexes with germane-linkages have been synthesized by different methods. Thermal reaction of Cr(CO)6 with Me3GeGePh3 leads to the formation of Me3GeGePh[(
6-C6H5)Cr(CO)3]2 (282). Lithiation of [(
6-C6H5)Cr(CO)3] with nBuLi, followed by reaction with Me2GeCl2 or ClGeMe2GeMe2Cl affords Me2Ge[(
6-C6H5) Cr(CO)3]2 (283) and [(OC)3Cr(
6-C6H5)]GeMe2GeMe2[(
6-C6H5)Cr(CO)3] (284), respectively. The molecular structures of 282 and 283 have been determined by X-ray diffraction analysis.163

5.04.2.1.18

Molecular modeling and other computational approaches

Theoretical investigations of haptotropic rearrangements of tricarbonyl chromium complexes on naphthalene,164–166 phenanthrene106,164–167 and other polycyclic aromatic systems165,168 using DFT calculations have been carried out. The haptotropic rearrangement of a Cr(CO)3 unit in naphthalene and phenanthrene and their derivatives has been investigated by means of gradient-corrected density functional theory.164 The haptotropic rearrangements of the tricarbonylchromium complexes between two six-membered rings arranged in polycyclic aromatic hydrocarbons (PAHs) with three and four six-membered rings have been investigated through DFT calculations.165,168 The mechanisms of thermally and photochemically induced haptotropic rearrangements of Cr(CO)3 and Cr(CO)2(cyclooctene) complexes on naphthalene systems have been studied by means of quantum mechanical methods.166 The quantum theory of atoms in molecules (QTAIM) has been used to analyze experimental charge density for (
6-C6H6)Cr(CO)3. And the electronic structure of (
6-C6H6)Cr(CO)3 has been investigated by ab initio density functional theory calculations, and the result gave a good agreement between theory and experiment in terms to the topological parameters.169 The electronic excitation and the first static hyperpolarizability of (benzene)chromium tricarbonyl have been calculated by a series of density functional approaches.170 The energies of filled and unfilled orbitals in (benzene)chromium tricarbonyl have been studied by various methods.171 A variety of ab initio methods have been employed to determine intermolecular interactions in (
6-C6H6)Cr(CO)3.172 The haptotropic rearrangement in tricarbonyl chromium complexes with carbo- and heterocyclic polyaromatic ligands have been studied by using DFT calculations.173 The electronic structure and IR spectroscopic properties of a series of arene tricarbonyl chromium complexes (
6-C6H5Y) Cr(CO)3 (Y ¼ NH2, OCH3, H, CHO, or CO2CH3) have been investigated using DFT methods and time dependent DFT calculations.174 The geometry and electronic structures of Cr(CO)3C21H12R6 (R ¼ H, F, Cl, Br and CN) have been explored by the density functional theory in the PBE approximation.175 The energy local minimum and structural parameter of CrPhC(C3H5)3, in which the Cr atom is coordinated by three double bonds of the corresponding triene, have been studied by DFT calculations. The result indicates that the PhC(
2-C3H5)3 Cr(CO)3 isomer with the coordination of double bonds is less stable than the
6-PhC(C3H5)3Cr(CO)3 isomer with the coordination of the benzene ring.176 The molecular structures and relative stabilities of bis(tricarbonylchromium) complexes of two- to four-fused benzenoid rings have been theoretically calculated by means of the B3LYP method.177 The metal-to-ligand charge transfer in (
6-naphthalene)Cr(CO)3 and (
6-phenanthrene)Cr(CO)3 has been studied by resonance Raman spectroscopy and DFT calculations.178 The molecular geometries, electronic properties and second-order nonlinearities of a variety of mono- and bi-nuclear chromium carbazole complexes have been studied by DFT and TDDFT calculations.179 Structural and electronic properties for a series of half-sandwich complexes of the type
6-(CnSimH6)Cr(CO)3 (where n ¼ 0–6, m ¼ 0–6, and n + m ¼ 6) have been studied through DFT calculations.180 The TDDFT calculations have been used to explore the potential energy profiles of the optically accessible excited states of (
6-arene)Cr(CO)3 systems. Two processes in photochemical reactions, CO-loss and the haptotropic or ring-slip of the arene ligand, have been investigated.181 The hybrid density functional theory method (B3LYP/LANL2DZ) have been used to study the tricarbonylchromium-complexed benzyl cation, radical, and anion, focusing on the role of substituents.182 The coordination of graphene to (C6H6)Cr and (CO)3Cr fragments has been studied by density functional theory method, M11-L.183 The effect of fluorine substitution on the conformation and aromaticity of fluorine substituted
6-benzenetricarbonylchromium has been investigated using DFT calculations at B3LYP/LANL2DZ level.184 The effect of substituents in manipulating the cation-p interaction in substituted
6-C6H6Cr(CO)3 complexes has been studied in terms of interaction energy, charge transfer, aromaticity, hardness, and UV-VIS spectra.185

126

Cyclic and Non-cyclic Pi Complexes of Chromium

The isomerism and mechanisms of inter-ring haptotropic rearrangements in Cr(CO)3 complexes with potential organometallic ligand 1,6-methyl[10]cyclopentene have been investigated by DFT calculations.186 The effect of the arene ligand on the structure and properties in (arene)Cr(CO)3 complexes (arene ¼ benzene, biphenyl, triphenyl, tetraphenyl) has been studied through MPW1PW91 quantum chemical computations.187 Based on the geometric properties of structures determined by X-ray diffraction analysis, the electronic structure and structural features of a series of 34-electrons syn-facial CrdMn benzylic complexes have been investigated by DFT methods.79 The high-level ab initio electronic structure methods have been performed to explore the properties of the “heterodox bond” in bimetallic tricarbonyl(
6-indene)chromium complexes, with particular attention to the contribution of London dispersion.188 The [2 +2 +2] cycloaddition reaction of acetylene and benzene mediated by the bimetal system [Cr(CO)3IndRh] has been studied theoretically. The potential energy surfaces (PESs) have been studied in detail using relativistic density functional theory methods to determine intermediates and transition states.189

5.04.2.1.19

Spectroscopic studies on arene chromium carbonyls

In order to better study the properties of compounds, some spectroscopic studies have been carried out, and the relevant works are summarized below. Vibrational IR and Raman spectroscopy of (
6-C6H6)Cr(CO)3 have been studied. The vibrational properties between free and coordinated ligands, as well as between solution and solid-state spectra of the complexes have been compared.190 The 13C NMR parameters of a series of sandwich chromium complexes of (
6-C6H5)Cr with polycyclic aromatic hydrocarbons including naphthalene, phenanthrene, pyrene, fluoranthene, and biphenylene, have been analyzed. And the 13C NMR chemical shifts of these complexes have been calculated through the gauge-independent atomic orbital method.191 A series of half-sandwich complexes (
6-arene)Cr(CO)3 (arene ¼ benzene, methylbenzoate, naphthalene, phenanthrene, benzophenone, styrene, allylbenzene) systems have been studied on the picosecond time scale by time-resolved infrared spectroscopy.192–195 An efficient photoinduced CO-loss process has occurred in these systems. An efficient photoinduced cis-trans isomerization of cis-(
6-1,2-diphenylethene)Cr(CO)3 has been confirmed using NMR, UV-vis spectroscopy, and matrix isolation studies.196 A series of arene tricarbonylchromium complexes (C6H5YMe)Cr(CO)3 [Y ¼ O, CH(OH), N(Pr), CH]CH] have been characterized by three carbonyl bands using the IR spectrum. Intramolecular CO ⋯ H interaction between hydrogen and carbonyl groups can be measured by the position of the third carbonyl stretching vibration.197 The improved spectral characterization of radical cation Cr(CO)3(
6-arene)+ has been reported alongside synthetic-level substitution reactions that are possible when performing the electrochemistry in a relatively benign medium that includes weakly-coordinating supporting electrolyte.198 High resolution neutron vibrational spectroscopy has been used to study the vibrations of benzene chromium tricarbonyl and its mesityl analogue, for analysis of arene-to-metal bonding.199 Raman spectra of two methylbenzoate complexes, (
6-C6H5CO2CH3)Cr(CO)2(CX) (X ¼ O, S), have been examined under different pressures.200

5.04.2.1.20

Applications of arene chromium carbonyls

Photoinitiation of acrylate polymerization with (
6-arene)Cr(CO)3 complexes and organic chloro compounds as initiators has been studied. In the bimolecular systems, the polymerization activities show a good correlation with the strength of the metal-aromatic and metal-carbonyl bonds.201 A chromiumtricarbonyl-containing iridacycle displays catalytic activities for the hydroamination of terminal aromatic alkynes and hydrosilation/protodesilylation reactions under mild conditions.202 In the past ten years, significant progress has been made in the (
6-arene)Cr(CO)3 chemistry with carbon nanotubes and graphene as ligands.203–205 In addition, organometallic sidewall complexes of SWNTs, (
6-SWNT)Cr(CO)3, have also been studied.113,204,206 Composite films of graphene oxide/poly ortho aminophenol/Ionic liquid/(
6-C6H5)Cr complex have been obtained through electrochemical deposition method and studied as active electrodes for electrochemical supercapacitors.207

5.04.2.2

Bis(
6-arene) complexes

In 2010, a review was published that summarized advances in the chemistry of bis(
6-arene) complexes.208 In the present section, we will focus on the synthesis and reactivity of bis(arene)chromium derivatives. The use of bis(
6-arene) complexes in material applications is also summarized.

5.04.2.2.1

Compounds with hydrocarbon-substituted arenes

Cr(I) derivatives containing the [Cr(
6-arene)2]+ cations, with arene ¼ 1,3,5-Me3C6H3 (Mes) have been synthesized and characterized.209 The reaction of Cr(
6-arene)2 with fulvenes derivatives results in evolution of dihydrogen with the formation of ionic derivatives [Cr(
6-arene)2][X] where arene ¼ toluene, Mes; X ¼ pentakis(methoxycarbonyl)-cyclopentadienyl, [pcmcp]−, 1,2dibenzoylcyclopentadienyl, [dbcp]−, 1,2-dibenzoyl-4-nitro-cyclopentadienyl, [dbncp]−.210

Cyclic and Non-cyclic Pi Complexes of Chromium

5.04.2.2.2

127

Compounds with heteroatom-substituted arenes

5.04.2.2.2.1 Derivatives with heteroatoms in the aromatic ring Treatment of the lithium salt of 1-mesityl-2-(trimethylsilyl)boratabenzene with 1/2 equiv. of CrCl2 in THF affords bis(1-mesityl2-(trimethylsilyl)boratabenzene)chromium(II) (285) (Scheme 95).211

Scheme 95

5.04.2.2.2.2 Derivatives with heteroatoms on the periphery The selective diboration has been achieved from the metal-mediated insertion of alkynes into the BdB bridge of bis(benzene) chromium.212 As shown in Scheme 96, treatment of complex 286 with a 10-fold excess of propyne or 2-butyne gives the corresponding ansa-bis(boryl)alkene 287. Alternatively, complex 287 can be obtained from reactions of [2]borachromoarenophane [Cr{(
6-C6H5)2(BNMe2)2}] with propyne and 2-butyne respectively using catalytic amounts of [Pt(PEt3)3] or Pd/C. An extension of this reactivity to the diboration of the N]N double bond of azobenzene by [n]metalloarenophanes of chromium has also been achieved by reaction of [3]diboraplatinachromoarenophane 286 with azobenzene giving ansa-bis(boryl)hydrazine 288.213 The molecular structure of 288 has been determined by X-ray diffraction.

Scheme 96

The [2]silachromoarenophane [Cr(
6-C6H5)2Si2Me4] (289) has been obtained through salt elimination reaction of the dilithiated precursor [Cr(
6-C6H5Li)2]
tmeda and Cl2Si2Me4 in aliphatic solvent. In similar way for the reactions of [2]borachromoarenophanes with alkynes, the reaction of [2]silachromoarenophane 289 with propyne in the presence of a catalytic amount of Pd(PPh3)4 gives the ansa-bis(silyl)alkene [Cr(
6-C6H5)SiMe2C(Me)]C(H)SiMe2(
6-C6H5)] (290) (Scheme 97).214

Scheme 97

128

Cyclic and Non-cyclic Pi Complexes of Chromium

Bis(
6-arene) complex (PhC^CMe2SiPh)2Cr (291) has been synthesized by modification of precomplexed arenes, while the dinuclear cyclobutadiene bis(
6-arene) complex CpCo(PhCCSiMe2Ph)2Cr (292) can be obtained from intramolecular [2 + 2] cycloaddition of two alkynyl groups of 291, followed by reaction with cyclopentadienyl lithium.159 Aluminum- and gallium-bridged chromarenophanes 293 and 294 have been prepared from Cr(LiC6H5)2 with element dichlorides (Me2Ntsi)ECl2 [E ¼ Al, Ga; Me2Ntsi ¼ C(SiMe3)2(SiMe2NMe2)] in moderate to high yields (Scheme 98).215

Scheme 98

Addition of 10 equiv. of TEA to a dichloromethane solution of [Cr(CO)4Ph2PN(iPr)PPh2][Al(OC(CF3)3)4] (295) gives the formation of a bis(
6-arene) complex, [Cr(1-bis-
6-arene)]+ (296) along with complete loss of CO (Scheme 99).216

Scheme 99

5.04.2.2.3

Ion-radical salts with bis(arene)chromium complexes

Reactions of bis(arene)chromium complexes with [60]fullerene derivatives lead to ion-radical bis(arene)chromium [60]fullerides. Bis(1,2,4,5-tetramethylbenzene)chromium [70]fulleride has also been synthesized in similar way. The obtained compounds have been characterized by spectroscopic methods, their thermal decomposition has been studied and thermal stability of their dimers has also been evaluated.217 Indolizidinofullerenes containing trimethoxyarene substituents in the indolizidine cycle, 1-amidoalkylfullerenes, and their ion-radical salts with bis(arene)chromium complexes have been synthesized and characterized by a variety of spectral methods.218

Cyclic and Non-cyclic Pi Complexes of Chromium

5.04.2.2.4

129

Bimetallic and multimetallic bis(arene)chromium complexes

As shown in Scheme 100, the regioselective para monodeprotonation of bis(toluene)chromium manifested in the mixed-metal sodium-magnesium amido complex 297 has been reported as the first example of a metallo derivative of bis(toluene)chromium. The molecular structure of 297 has been determined by X-ray crystallography.219

Scheme 100

Tungsten- and molybdenum-dinitrogen complexes bearing bis(dialkylphosphinobenzene)chromiums have been prepared and characterized by X-ray crystallography.220 Treatment of a suspension of phenazine and CrCl2(THF)2 in toluene with excess Et3Al affords heterometallic aluminum–chromium complex [AlEt2]2(m-
1:
1-
6:
6-C12H8N2Cr)2 (298) (Scheme 101). The molecular structure of 298 has been determined by X-ray crystallography.221

Scheme 101

Moreover, a bis(benzene)chromium polymer [{K((C6H6)2Cr)1.5(Mes)}+{Mg(HMDS)3}−]1 has been synthesized.222

5.04.2.2.5

Theoretical considerations and spectroscopic studies

A theoretical study on interactions between the bis(arene)chromium complex (
6-C6H6)(
6-C6F6)Cr and different cations/anions has been carried out by high-level ab initio and DFT calculations.223 In addition, the structure, reactivity, spectroscopic properties of (C6F6)Cr(C6H6) have also been explored with DFT calculations.224 The possible existence of derivatives, (
6–p–C60R6)Cr(C6H6), (C6H6)Cr(
6–p–R6C60R6)Cr(C6H6), and (
6–p–C60R6) Cr(
6–p–R6C60) (R ¼ –, H, F, Cl, Br, CN) has been analyzed using the DFT method within the Perdew–Burke–Ernzerhof approximation.225 The geometries and electronic structures of (
6-C21H12R6)Cr(C6H6) (R ¼ H, Hal, and CN) have been modeled by molecular mechanics and studied by MNDO/PM3 calculations.175 Chromium sandwich complexes with alkylbenzenes, (
6-RPh)2Cr (R ¼ Me, Et, iPr, and tBu) have been investigated by massanalyzed threshold ionization (MATI) spectra and DFT methods.226 The electronic properties of a number of bis(benzene)chromium complexes with hexaarylbenzene molecules have been studied by DFT calculations.227 Some theoretical studies on bis(arene)chromium complexes with heteroatoms in the aromatic ring have been performed. DFT calculations have been carried out to investigate bonding situation and stability of the sandwich complexes Cr(
6-EC5H5)2 (E ¼ N, P, As, Sb, Bi). The
6-complexes Cr(
6-EC5H5)2 has been compared with Cr(
6-C6H6)2.228 Structures and electronic properties for a variety of sandwich complexes (
6-CnSimH6)2Cr (where n ¼ 0–6, m ¼ 0–6, and n + m ¼ 6) have been investigated by DFT methods.180 The geometries and electronic structures, binding energies and aromaticities of (
6-1,3,5-C3P3H3)Cr and (
6-1,3,5-C3P3H3)2Cr have been studied using DFT calculations.229 The properties of the sandwich complexes Cr(aniline)2 on self-assembled monolayers have been studied by means of different methods including density functional theory.230

130

Cyclic and Non-cyclic Pi Complexes of Chromium

In addition, DFT calculations have also been used to investigate the interaction of Cr(
6-C6H6)2 intercalated in a series of semiconducting single-walled carbon nanotubes.231 Two-color multiphoton mass-analyzed threshold ionization (MATI) spectra of two deuterated derivatives of bis(
6-benzene) chromium, (
6-C6D6)2Cr and (
6-C6D5H)(
6-C6D6)Cr have been measured via vibronic components of the 3d2z !R4px,y Rydberg transition.232

5.04.2.2.6

Applications of bis(arene)chromium complexes

A series of bis(benzene)chromium-reduced microporous titanium oxide composite materials were synthesized and characterized by nitrogen adsorption, XRD, XPS, and elemental analysis.233 The graphitic materials (
6-arene)2Cr [arene ¼ single-walled carbon nanotubes (SWNT), exfoliated graphene (XG), epitaxial graphene (EG) and highly-oriented pyrolytic graphite (HOPG)] have been explored. The use of graphene as a ligand is expected to expand the scope of graphene chemistry in connection with the application of this material in organometallic catalysis, where graphene can act as an electronically conjugated catalyst support.206

5.04.2.3

Mono(
6-arene), half-sandwich, mixed sandwich, multidecker
6-arene chromium complexes

The addition of iso-butylaluminoxane (IBAO) to complex 299 results in reduction of the metal center and partial transmetalation, forming a new
6-arene chromium complex [
4-{2,6-[2,6-(iPr)2PhN]C(CH3)]2(C5-H3N)}Al2(iBu)3(m-Cl)]Cr-(
6-C7H8) (300) (Scheme 102). Complex 300 is catalytically inactive for ethylene polymerization reaction.234

Scheme 102

A systematic theoretical study of cationic chromium–benzene sandwich and half-sandwich complexes of CrnBzn+1 (n ¼ 1, 2) has been described. The size- and element-dependent properties, such as binding energies, ionization potentials, and magnetic moments of the cationic CrdBz complexes have been discussed and analyzed and compared to their neutral counterparts.235 Aromatic p–p interaction in the presence of a metal atom has been investigated experimentally and theoretically with the model system of bis(
6-benzene)chromium–benzene cluster (Cr(Bz)2dBz) in which a free solvating benzene is non-covalently attached to the benzene moiety of Cr(Bz)2.236 On the basis of density functional theory calculations, three classes of multidecker bis(benzene)chromium molecular wires with -(arene-chromium(0)-arene)-sandwich complexes as monomer units have been designed. The arene fragments of the wires are either [2.2]paracyclophane, biphenylene or biphenyl compounds with two strongly coupled benzene rings.237 An efficient procedure for the selective dilithiation of paramagnetic sandwich complex [Cr(
5-C5H5)(
6-C6H6)] affording [Cr(
5-C5H4Li)(
6-C6H5Li)]
pmdta has been reported. This dilithiated complex was used to prepare a series of heteroleptic [n] chromoarenophanes (n ¼ 1, 2, 3) 301–303 that feature silicon, germanium, and tin atoms at the bridging positions (Scheme 103).

Scheme 103

Cyclic and Non-cyclic Pi Complexes of Chromium

131

Treatment of [Cr(
5-C5H4)(
6-C6H5)SntBu2] with [Pt(PEt3)3] gives dinuclear chromium-platinum complex, cis-[Cr(
5-C5H4) (
6-C6H5)SntBu2Pt(PEt3)2] (304) (Scheme 104). The electronic and structural properties of these complexes have been probed by X-ray diffraction analysis, cyclic voltammetry, and by UV/Vis and EPR spectroscopy.238

Scheme 104

As shown in Scheme 105, two inverted arene sandwich dichromium complexes (m-
6:
6-1,3,5-(Me3Si)3C6H3)[Cr{k2-HC(N2,6-iPr2C6H3)(N-2,6-R2C6H3)}]2 (R ¼ iPr (307), Me (308)) have been formed by using the quintuply bonded dichromium amidinates [Cr{k2-HC(N-2,6-iPr2C6H3)(N-2,6-R2C6H3)}]2 (R ¼ iPr (305), Me (306)) as catalysts to mediate the [2 +2 +2] cyclotrimerization of terminal alkynes giving 1,3,5-trisubstituted benzenes. During the catalysis, reversible cleavage/formation of the ultrashort CrdCr quintuple bond has been corroborated.239 In the presence of d donors, such as THF and 2,4,6-Me3C6H2CN, the bridging arene 1,3,5-(Me3Si)3C6H3 in 307 and 308 are extruded with 305 and 306 being regenerated. Theoretical calculations have been employed to disclose the reaction pathways of these highly regioselective [2 +2+ 2] cyclotrimerization reactions of terminal alkynes.

Scheme 105

5.04.3


6-Heteroarene chromium complexes

Some chromium carbonyl complexes with numerous heteroarene-type rings, such as pyridines, phosphabenzenes, arsabenzenes, thiabenzene oxides, borazoles, or borathiins, have been explored. Reaction of 1,2-dihydro-2-phenyl-1,2-azaborine, 309 with [Cr(CH3CN)3(CO)3] in THF at 50  C gives the Cr(CO)3 complex 310 in which the Cr atom is
6-bound to the heterocyclic ring. Heating 310 causes it to isomerize to complex 311, in which the Cr atom is
6-bound to the phenyl ring. The Cr(CO)3 group of 311 may be switched back to the heterocyclic ring by base conversion to the anion 313, followed by thermal isomerization to anion 312. Protonation of 312 reforms 310. It is worth noting that X-ray structure of 311 shows that the uncoordinated 1,2-dihydro-1,2 azaborine ring is aromatic, as shown in Scheme 106.240

132

Cyclic and Non-cyclic Pi Complexes of Chromium

Scheme 106

Treatment of 3a,7a-azaborindene (314) with Cr(CO)3(CH3CN)3 forms a tricarbonylchromium complex [
6-3a,7aazaborindene]Cr(CO)3 (315) which can be quantitatively deprotonated at −60  C with potassium bis(trimethylsilylamide) to afford a potassium salt of
6-3a,7a-azaborindenylCr(CO)3 (316). Quenching 316 with excess trimethylsilyl chloride results in the 3-trimethylsilyl derivative 317. Allowing the reaction solution of the potassium salt of 316 to warm to −40  C causes a
6 !
5 haptotropic rearrangement to its
5-regioisomer 318 which can be trapped by reaction with trimethylstannyl chloride to form adduct 319 (Scheme 107).241

Scheme 107

Complexation of 1,2-azaborine 320 to {Cr(CO)3} affords the piano-stool adduct 321.240 Subsequent removal of the N-protecting group gives 322 (Scheme 108).242

Scheme 108

Cyclic and Non-cyclic Pi Complexes of Chromium

133

A series of constrained-geometry Cr(III) complexes 323–326 bearing the amidino-boratabenzene ligands have been prepared and found to show excellent catalytic activity for ethylene polymerization upon activation.243

As shown in Scheme 109, reaction of the heteroarene 1,4-bis(CAAC)-1,4-diborabenzene [327, CAAC ¼ cyclic (alkyl)(amino) carbine] with [(MeCN)3Cr(CO)3] yields a half-sandwich complex [(
6-diborabenzene)Cr(CO)3] (328). Investigation of 328 with a combination of X-ray diffraction, spectroscopic methods and DFT calculations shows that ligand 327 is a remarkably strong electron donor. In particular, complex 328 displays the lowest CO stretching frequencies yet observed for this class of (
6-arene) Cr(CO)3 complexes.244

Scheme 109

Reaction of 329 with tris(acetonitrile)tricarbonylchromium at room temperature in THF gives the piano-stool complex 330 (Scheme 110). Coordination of dibenzo-18-crown-6 to the potassium ion allows for the isolation and crystallographic characterization of the
6-boratabenzene complex 330.245

Scheme 110

A Cr(CO)3 complex 331 with a 1,2-thiaborine coordinated to chromium in an
6 fashion has been obtained by the reaction of excess Cr(CO)3(CH3CN)3 with 1,2-thiaborine (Scheme 111).246

Scheme 111

134

Cyclic and Non-cyclic Pi Complexes of Chromium

The ligand exchange reaction of 2-stannanaphthalene 332 with [Cr(CH3CN)3(CO)3] at room temperature in THF results in the regioselective formation of the first
6-2-stannanaphthalene chromium complex 333 in 89% yield, as shown in Scheme 112.247 In a related reaction, the
6-stannabenzene complex 334 with two steric protection groups, the Bbt {2,6-bis[bis(trimethylsilyl)methyl]4-[tris(trimethylsilyl)methyl] phenyl} group on the tin atom and the t-butyl group on the 2-position to the stannabenzene skeleton, has been prepared. The isolation of complex 334 was found to be difficult and only a small amount of single crystals of it have been obtained and proven by X-ray structure determination.248

Scheme 112

A stable
6-germabenzene complex Cr(CO)3(
6-C5H5GeTbt) 336 [Tbt ¼ 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl] has been synthesized by ligand-exchange reaction between [Cr(CO)3(CH3CN)3] and the kinetically stabilized germabenzene 335 (Scheme 113), which has been characterized by 1H and 13C NMR, IR, UV/Vis spectroscopy and X-ray crystallographic analysis.249

Scheme 113

Moreover, two mono- and bis-
6-phosphinine chromium complexes bearing bis(phosphinine) [bis{3-methyl-6-(trimethylsilyl)-phosphinine-2-yl}dimethylsilane] ligand, 337 and 338, have been prepared, and the molecular structure of 338 has been determined by X-ray crystallography. Unfortunately, both complexes 337 and 338 could not be isolated as pure products.250

5.04.4

Other
6-pi-ligand chromium complexes

The electrochemical reduction of two types of Cr(
6-triolefin)(CO)3 complexes 339 and 340 has been studied by voltammetry and electrolysis. In contrast to Cr(
6-arene)(CO)3 compounds, the bicyclic ligand in Cr(
4:
2-C9H8Ph2)(CO)3 (339) and the fulvene-type ligand in Cr(
6-C5H4CPh2)(CO)3 (340) are nonaromatic and display two separate one-electron reductions to the nominally 19e and 20e complexes.251

Cyclic and Non-cyclic Pi Complexes of Chromium

135

Static first hyperpolarizabilities of vinylogue bimetallic sesquifulvalene chromophores [(
5-C5H5)M{m-(
5-C5H4)CH] CH(
6-C7H7)}Cr(CO)3]+ (M ¼ Fe, Ru, and Os) have been studied with DFT calculations.252 The cyclooctatetraene chromium carbonyl (
6-C8H8)Cr(CO)3 has been synthesized by the reaction of cyclooctatetraene with fac(CH3CN)3Cr(CO)3. The complex has been investigated by density functional theory.253

The synthesis and structural characters of a series of chromium tricarbonyl complexes bearing substituted pentafulvene ligands, tricarbonyl(1,3,6-triphenylfulvene)chromium(0) benzene hemisolvate, [Cr(C24H18)(CO)3]0.5C6H6 (341), tricarbonyl[1, 3-diphenyl-6-(3-vinylphenyl)fulvene]chromium(0), [Cr(C26H20)(CO)3] (342), and tricarbonyl[1,3-diphenyl-6-(pyren-1-yl)fulvene]chromium(0), [Cr(C34H22)(CO)3] (343), have also been studied.254 Molecular and electronic structures of the known mononuclear heptalene chromium tricarbonyl complex (
6-C12H10) Cr(CO)3 (344) and binuclear derivatives C12H10Cr2(CO)n (n ¼ 3, 4, 5, 6) of the known tetramethylheptalene dichromium hexacarbonyl complexes 345 and 346 have been studied by DFT methods.255

5.04.5


5-Cyclopentadienyl chromium complexes

5.04.5.1

Sandwich chromium complexes

Methyl group substituted bis(indenyl)chromium(II) complexes (
5-1-MeC9H6)2Cr (347) and (
5-2-MeC9H6)2Cr (348) as well as a binuclear complex [(
5-4-MeC9H6)Cr]2(m,
3-4-MeC9H6)(m-Cl) (349) have been prepared from reactions of 2 equiv. of corresponding alkali metal indenide with CrCl2. X-ray diffraction results indicate that 348 exists in a staggered conformation and is a high-spin species in the solid state while 347 is eclipsed in the solid state and exhibits spin-crossover behavior over a wide temperature range.256 A number of polymethylated indenyl complexes containing two, three, and five methyl groups on the indenyl ligand have also been synthesized and their structures, and magnetic properties have been studied. Methyl groups on the benzo portion appear to be critical in supporting a low-spin ground state, and the order of the preference to exist as a low-spin complex is approximately (2,4,5,6,7-Me5C9H2)2Cr (350) > (2,4,7-Me3C9H4)2Cr (351)  (4,7-Me2C9H5)2Cr (352) > (1,2,3-Me3C9H4)2Cr (353) > (1-MeC9H6)2Cr (347)  (2-MeC9H6)2Cr (348).257 Chromocenes with hexa(trimethylsilyl), hexa(tert-butyl) or hexa (cyclopentyl) substituents, (1,2,4-R3C5H2)2Cr [R ¼ Me3Si (354), tBu (355), cyclopentyl (356)], have been synthesized from the reaction of chromium(II) acetate with the corresponding cyclopentadienylsodium.258

136

Cyclic and Non-cyclic Pi Complexes of Chromium

Reactions of chromocene Cp2Cr with cyclopentadienyltitanium trichlorides Cp0 TiCl3 in toluene result in the formation of ion pairs of [Cp2Cr(III)]+[Cp0 Ti(III)Cl3]− [Cp0 ¼ C5H5 (357), C5H4Me (358), C5H3Me2 (359), C5H2Me3 (360), C5HMe4 (361), C5Me5 (362), C5H4Et (363)]. Heating their toluene solutions to 100  C yields chloride-bridged titanocene chloride– cyclopentadienylchromium dichloride complexes [CpCp0 Ti(m-Cl)2Cr(Cp)Cl] [Cp0 ¼ C5H5 (364), C5H4Me (365), C5H3Me2 (366), C5H2Me3 (367), C5HMe4 (368), C5Me5 (369), C5H4Et (370)]. Complexes 368–370 are unstable in toluene at 100  C and decompose to CpCp0 TiCl2 and polymeric (CpCrCl)n.259 Reaction of dimethylchromocene (C5H4Me)2Cr with a gallium heterocyclic compound, the digallane(4) {Ga[(2,6-iPrC6H3) NCH]2}2, gives a neutral heterobinuclear complex (C5H4Me)2CrGa[(2,6-iPrC6H3)NCH]2 (371) (Scheme 114).260

Scheme 114

Interaction of decamethylchromocene, Cp 2Cr, with metal-free phthalocyanine (H2Pc) in o-dichlorobenzene produces a deep blue charge transfer complex [Cp 2Cr]+[H2Pc]%−
4C6H4Cl2 (372) which contains a metal-free phthalocyanine (Pc) radical anion and a decamethylmetallocenium cation.261 An unstable single-carbon-bridged constrained ansa-chromocene complex Me2C(C5H4)2CrCO has been obtained from the reaction of Me2C(C5H4)2Mg and CrCl2 in THF under an atmosphere of carbon monoxide.262

A number of ansa-chromocene isocyanide complexes (R1R2C)2(C5H4)2CrCNR3 [R1 ¼ R2 ¼ Me, R3 ¼ tBu (373), p-tolSO2CH2 (374), xyl (375); R1 ¼ H, R2 ¼ Ph, R3 ¼ tBu (376), Fc (377)] and [(R1R2C)2(C5H4)2CrCN]2X [R1 ¼ R2 ¼ Me, X ¼ C6H4 (378), Fc (379); R1 ¼ H, R2 ¼ Ph, X ¼ Fc (380)] have been obtained via ligand substitution on the corresponding carbonyl complexes (R1R2C)2(C5H4)2CrCO with isocyanide ligands. DFT calculations have been performed on model complexes to elucidate the nature of the coordination bonding between the chromium and the isocyanide ligand.263 In contrast to earlier claims,264 by slow addition of CrCl2(THF)x to the reaction mixture, the ansa-chromocene Me2Si(C5Me4)2Cr (381) is formed directly from reaction with Me2Si(C5Me4)2Li2 without the participation of an auxiliary ligand such as xylyl isocyanide. Similar reaction of CrCl2(THF)x with C2H4(Ind)2Li2 yields a small amount of dimeric complex [C2H4(
5-Ind) (m-,
3-Ind)Cr]2 (382).265 Treatment of a solution of trochrocene [Cr(
5-C5H5)(
7-C7H7)] and tmeda with nBuLi affords [Cr(
5-C5H4Li)(
7-C7H6Li)]
tmeda (383), which reacts with RR0 SiCl2 and Me4Si2Cl2 to produce corresponding silatrochrocenophane complexes [Cr(
5-C5H4) (
7-C7H6)SiRR0 ] [R ¼ R0 ¼ Me (384); R ¼ Me, R0 ¼ iPr (385); R ¼ R0 ¼ iPr (386); R + R0 ¼ (CH2)3 (387)] and [Cr(
5-C5H4) (
7-C7H6)Si2Me4] (388). Treatment of the complex 384 with Pt(PEt3)4 forms a binuclear chromium-platinum complex [Cr(
5-C5H4)SiMe2Pt(PEt3)2(
7-C7H6)] (389) in a high yield (Scheme 115).266

Cyclic and Non-cyclic Pi Complexes of Chromium

137

Scheme 115

Reactions of 383 with monochlorophosphines R2PCl form the corresponding 1,10 -bis(phosphanyl)trochrocene derivatives, [Cr(
5-C5H4PR2)(
7-C7H6PR2)] [R ¼ Me (390), Cy (391), Ph (392)]. Treatment of the complexes 391 and 392 with M(CO)6 results in the formation of the heterobimetallic trochrocenophane species [Cr(
5-C5H4PR2)(
7-C7H6PR2)M(CO)4] [R ¼ Cy, M ¼ Cr (393), Mo (394), W (395); R ¼ Ph, M ¼ Cr (396), Mo (397), W (398)] (Scheme 116).267

Scheme 116

In a similar way, treatment of a solution of [Cr(
5-C5H5)(
6-C6H6)] and pmdta (3 equiv) with three equiv. of nBuLi affords [Cr(
5-C5H4Li)(
6-C6H5Li)]
pmdta (399), which reacts with RR0 ECl2, R4E2Cl2 and (ClSiMe2)2CH2 to form ansa complexes [Cr(
5-C5H4)(
6-C6H5)ERR0 ] [E ¼ Si, R ¼ R0 ¼ Me (400); E ¼ Si, R ¼ Me, R0 ¼ iPr (401); E ¼ Si, R ¼ R0 ¼ iPr (402); E ¼ Ge, R ¼ R0 ¼ Me (403); E ¼ Sn, R ¼ R0 ¼ tBu (404)], [Cr(
5-C5H4)(
6-C6H5)E2R4] [E ¼ Si, R ¼ Me (405); E ¼ Sn, R ¼ tBu (406)] and [Cr(
5-C5H4SiMe2)(
6-C6H5SiMe2)CH2] (407), respectively. Treatment of the complex 404 with Pt(PEt3)3 at elevated temperature results in the formation of binuclear chromium-platinum complexes [Cr(
5-C5H4)Pt(PEt3)2Sn(tBu)2(
6-C6H5)] (408) and [Cr(
5-C5H4)Sn(tBu)2Pt(PEt3)2(
6-C6H5)] (409) (Scheme 117).238

Scheme 117

138

Cyclic and Non-cyclic Pi Complexes of Chromium

Reaction of 2 equiv. of 1,4-bis(triisopropylsilyl)pentalenyl potassium, 1,4-(iPr3Si)2C8H4K2, with Cr2(OAc)4 affords the corresponding binuclear bispentalenyl complex [
5:
5-(iPr3Si)2C8H4]2Cr2 (410). Structural data, solid-state magnetic studies and density functional theory calculations indicate a strong metal-metal interaction which can be described by a double bond with each metal having an 18-electron count and a pair of antiferromagnetically interacting S ¼ ½ centers which reach spin equilibrium between a singlet ground state and a triplet excited state.268

5.04.5.2

Half-sandwich chromium complexes as catalysts or activators

Cyclopentadienyl chromium complexes with a coordinating pyridine side arm 411 and 412 have been synthesized and studied as ethylene269 and methyl methacrylate270 polymerization catalysts. Complexes 411 and 412 both show moderate catalytic activity for ethylene polymerization. Similar cyclopentadienyl and indenyl chromium complexes 413–418 have been studied as catalysts for propylene polymerization and found that the obtained polymers have structures resembling those of ethylene/propylene copolymers with regioinverted propylene units due to chain-walking.271

Quinoline,272–275 1,5-naphthyridine276 and 7-bromo-2,1,3-benzothiadiazole277 functionalized cyclopentadienyl chromium complexes 419–439 have been synthesized and investigated as ethylene polymerization catalysts. Some of these complexes show high catalytic activity and 1-hexene incorporation ability for ethylene/1-hexene copolymerization. For the quinoline-based cyclopentadienyl chromium complexes (C9H6N)(Me2RSi)Me3C5CrCl2 [R ¼ 3,5-(CF3)2C6H3 (424), C6F5 (425), (CH2)2CF3 (426), CH] CH2 (427), Allyl (428), Bz (429), (CH2)3CN (430)], the additional side arm R has been found to minimize chain-transfer processes, leading to an increase in molecular weights of the resulting polymers. The catalytic performance of complex 423 for the carboalumination of unactivated terminal olefins has also been examined.278 EPR and NMR spectroscopies of some of these complexes have been studied.272,275

Cyclic and Non-cyclic Pi Complexes of Chromium

139

Neutral O- and S-donor functionalized cyclopentadienyl chromium complexes 440–446 and related dimeric complexes 447 and 448 have been synthesized and investigated as ethylene polymerization catalysts.279,280

Half-sandwich chromium complexes 449–452 with (2-((arylimino)methyl)phenyl)-tetramethylcyclopentadienyl ligands have been synthesized and found to show good catalytic activity for ethylene polymerization upon activation with trialkylaluminum.281 A similar imine-coordinated cyclopentadienyl chromium complex, 2,6-Me2C6H3N]C(Ph)CH2CMe2CpCrCl2 (453), was also prepared.282 By reduction of the imine C]N bond in the ligands of this type of complexes, a number of half-sandwich secondary amine-coordinated chromium complexes chelated by 2-(tetramethylcyclopentadienyl)benzylamine ligands, 2-(Me4Cp)C6H4CH2NH(R)CrCl2 [R ¼ iPr (454), Cy (455), Ph (456), 4-MeC6H4 (457), 2,6-Me2C6H3 (458), 2,6-Et2C6H3 (459)], have also been synthesized and investigated as catalysts for ethylene polymerization and ethylene/1-hexene copolymerization.283

Imidazolin-2-imino-functionalized tetramethylcyclopentadienyl and indenyl chromium complexes 460–463 have also been synthesized and investigated as catalysts for ethylene polymerization reaction.284

140

Cyclic and Non-cyclic Pi Complexes of Chromium

Phosphine-functionalized tetraethylcyclopentadienyl chromium chloride complexes, 2-(Et4Cp)C6H4PR2CrCl [R ¼ Ph (464), Cy (465), iPr (466)], have been synthesized and studied for N2 activation. Reduction of the complex 464 or 465 with 1 equiv. of KHBEt3 under N2 gives a trinuclear Cr(I)dN2 complex [2-(Et4Cp)C6H4PPh2Cr]3(m-N2)2(N2)2 (467) or a dinuclear Cr(I) dN2 complex [2-(Et4Cp)C6H4PCy2Cr]2(m-N2)(N2)2 (468), respectively. Reduction of 465 with excess K, Rb, or Cs in THF under N2 atmosphere, with the aid of 2.2.2-cryptand, results in the formation of complexes [2-(Et4Cp)C6H4PCy2Cr(N2)2]-[M(crypt-222)] [M ¼ K (469), Rb (470), Cs (471)]. Reaction of 469 with 1 equiv. of Me3SiCl produces a hydrazido complex 472, along with regeneration of 465.285

Similar phosphine-functionalized tetraethylcyclopentadienyl chromium chloride complexes, 2-(Et4CpCH2)C6H4PR2CrCl [R ¼ Ph (473), Cy (474), iPr (475)], have also been synthesized and studied for N2 activation. Reduction of the complex 473 or 475 with 1 equiv. of KC8 under N2 gives a trinuclear Cr(I)dN2 complex [2-(Et4CpCH2)C6H4PPh2Cr]3(m-N2)2(N2) (476) or a dinuclear Cr(I)dN2 complex [2-(Et4CpCH2)C6H4PiPr2Cr]2(m-N2)(N2) (477), respectively. Reaction of 473 with 3.0 equiv. of phenylsilane at room temperature results in the formation of a mixed-valence dinuclear Cr dinitrogen complex [2-(Et4CpCH2) C6H4PPh2Cr](m-N2)[2-(Et4CpCH2)C6H4PPh2CrH(SiH2Ph)] (478). Similar oxidative addition reactions of 473 with 3.0 equiv. of azobenzene or benzylideneaniline, or excess 2-butyne generate mononuclear Cr(III) complexes [2-(Et4CpCH2)C6H4PPh2Cr (PhNNPh)] (479), [2-(Et4CpCH2)C6H4PPh2Cr(PhNCHPh)] (480) and [2-(Et4CpCH2)C6H4PPh2Cr(CMe)4] (481), respectively.286 Hydrogenolysis of the C5Me4SiMe3-ligated chromium(II) methyl complex [(C5Me4SiMe3)Cr(m-Me)]2 (482) with H2 under N2-free conditions in a dilute hexane solution affords a dinuclear Cr(II) dihydride complex [(C5Me4SiMe3)Cr(m-H)]2 (483), while the same reaction in a concentrated solution provides a trinuclear chromium tetrahydride complex [(C5Me4SiMe3)Cr]3(m3-H) (m-H)3 (484). When the hydrogenolysis reaction is carried out in the presence of N2 in a dilute solution, a tetranuclear diimide/ dihydride complex [(C5Me4SiMe3)Cr]4(m3-NH)2(m3-H)2 (485) is formed. Reaction of 483 with N2 forms a dinitride intermediate [(C5Me4SiMe3)Cr]4(m3-N)2(m-H)2 (486) at −30  C and gives a tetranuclear imide/nitride/dihydride complex [(C5Me4SiMe3)Cr]3 [(C5Me3SiMe3CH2)Cr](m3-NH)(m3-N)(m-H)2 (487) at room temperature. Reaction of 484 with N2 at 100  C affords a trinuclear diimide complex [(C5Me4SiMe3)Cr]3(m3-NH)2 (488).287

Cyclic and Non-cyclic Pi Complexes of Chromium

141

A number of (phosphanylethyl)cyclopentadienyl chromium chelate complexes [
1:
5-(tBu)(Ph)PCH2CH2Cp]Cr(III)Cl2 (489), [
:
5-Ph2PCH2CH2Cp]Cr(II)H(CO)2 (490), [
1:
5-Ph2PCH2CH2Cp]Cr(II)I(CO)2 (491), and K[18-crown-6]+{[
1:
5-Ph2 PCH2CH2Cp]Cr(II)(CO)2}− (492) have been synthesized and structurally characterized.288,289 1

N-heterocyclic carbene-functionalized indenyl chromium complexes L1CrXY [L1 ¼ 1-(2,6-diisopropylphenyl)-3-[b-(4,7dimethylindenyl)ethyl]-imidazol-2-ylidene, X ¼ Y ¼ Cl (493); X ¼ Cl, Y ¼ Me (494), Ph (495); X ¼ Y ¼ benzyl (496)] and L2CrXY [L2 ¼ 1-(2,6-diisopropylphenyl)-3-[g-(4,7-dimethylindenyl)propyl]-imidazol-2-ylidene, X ¼ Cl, Y ¼ Cl (497), Me (498)] have been prepared and found to show moderate to high catalytic activity for ethylene polymerization upon activation with MMAO.290 A series of half-metallocene chromium complexes bearing salicylaldiminato ligands, Cp0 Cr[2-R1-4-R2-6-(CH]NR3)C6H2O]Cl [Cp0 ¼ C5H5, R1 ¼ iPr, R2 ¼ H, R3 ¼ tBu (499); Cp0 ¼ C5H5, R1 ¼ tBu, R2 ¼ H, R3 ¼ iPr (500), tBu (501); Cp0 ¼ C5H5, R1 ¼ Ph, R2 ¼ H, R3 ¼ iPr (502); Cp0 ¼ C5H5, R1 ¼ Ph, R2 ¼ NO2, R3 ¼ iPr (503); Cp0 ¼ C5Me5, R1 ¼ iPr, R2 ¼ H, R3 ¼ iPr (504);

142

Cyclic and Non-cyclic Pi Complexes of Chromium

Cp0 ¼ C5Me5, R1 ¼ iPr, R2 ¼ R3 ¼ tBu (505); Cp0 ¼ C5Me5, R1 ¼ tBu, R2 ¼ H, R3 ¼ iPr (506), tBu (507); Cp0 ¼ C5Me5, R1 ¼ R2 ¼ tBu, R3 ¼ iPr (508), tBu (509), Ph (510), 2,6-iPr2C6H3 (511); Cp0 ¼ C5Me5, R1 ¼ Ph, R2 ¼ H, R3 ¼ iPr (512), tBu (513); Cp0 ¼ C5Me5, R1 ¼ Ad, R2 ¼ tBu, R3 ¼ iPr (514); Cp0 ¼ C5Me5, R1 ¼ Ph, R2 ¼ NO2, R3 ¼ iPr (515), tBu (516); Cp0 ¼ C5Me5, R1 ¼ tBu, R2 ¼ NO2, R3 ¼ iPr (517); Cp0 ¼ C5Me5, R1 ¼ iPr, R2 ¼ Br, R3 ¼ tBu (518); R1 ¼ tBu, R2 ¼ H, R3 ¼ iPr, Cp0 ¼ C5H2Ph3 (519), C5Me4Ph (520)],291–294 have been synthesized and investigated as ethylene polymerization catalysts. Some of them show high catalytic activity upon activation with a small amount of trialkylaluminum. Related mono- and binuclear half-metallocene chromium complexes with binaphthyl-based salicylaldiminato ligands, Cp Cr[2-O-20 -R1-3-(CH]NR)C20H11]Cl [Cp ¼ C5Me5, R1 ¼ nBuO, R ¼ iPr (521), Ph (522), 2,6-iPr2C6H3 (523); R1 ¼ H, R ¼ iPr, (524)] and {Cp Cr[2-O-3-(CH]NR) C10H5]Cl}2 [R ¼ iPr (525), Ph (526), 2,6-iPr2C6H3 (527)],295 as well as half-metallocene chromium complexes with hydroxyindanimine ligands, Cp Cr[2-R1-4-R2-5,6-C2H3Me(CH]NR3)C6HO]Cl [R1 ¼ H, R2 ¼ Me, R3 ¼ Ph (528), 2,6-iPr2C6H3 (529); R1 ¼ H, R2 ¼ Cl, R3 ¼ Ph (530), 2,6-iPr2C6H3 (531); R1 ¼ tBu, R2 ¼ Me, R3 ¼ Ph (532)],296 have also been synthesized and studied as ethylene polymerization catalysts.

Chloride-bridged binuclear half-metallocene chromium(III) aryloxide complexes [Cp0 Cr(OAr)(m-Cl)]2 [Cp0 ¼ C5H5, Ar ¼ 2, 6- Pr2C6H3 (533), 2,6-tBu2C6H3 (534); Cp0 ¼ C5Me5, Ar ¼ 2,6-iPr2C6H3 (535), 2,6-tBu2C6H3 (536)] have been synthesized and found to show moderate catalytic activity for ethylene polymerization upon activation with a small amount of trialkylaluminum.297 Related chromium complexes bearing aryloxy-phosphine ligands and aryloxy-phosphine oxide ligands, Cp[2-R1-4-R2-6-(Ph2P) C6H2O]CrCl [R1 ¼ R2 ¼ H (537); R1 ¼ Ph, R2 ¼ H (538); R1 ¼ tBu, R2 ¼ H (539); R1 ¼ R2 ¼ tBu (540); R1 ¼ F, R2 ¼ H (541); R1 ¼ C6F5, R2 ¼ H (542)] and [2-R1-4-R2-6-(Ph2P]O)C6H2O]CrCl [R1 ¼ Ph, R2 ¼ H (543); R1 ¼ tBu, R2 ¼ H (544); R1 ¼ R2 ¼ tBu (545); R1 ¼ F, R2 ¼ H (546)] have been reported to show moderate catalytic activity for ethylene polymerization and ethylene/ norbornene copolymerization in the presence of modified methylaluminoxane (MMAO).298 A number of half-sandwich Cr(III) complexes bearing bidentate bis(imino)pyrrole ligands, Cp[
2-2,5-C4H2N(CH]NAr)2]CrCl [Ar ¼ C6H5 (547), 2,6-Me2C6H3 (548), 2,6-iPr2C6H3 (549), C6F5 (550)] and Cp [
2-2,5-C4H2N(CH]NAr)2]CrCl [Ar ¼ C6H5 (551), 2,6-iPr2C6H3 (552)] have been synthesized and studied as catalysts for ethylene polymerization and ethylene/norbornene copolymerization. These complexes were found to show moderate to good catalytic performances in the presence of methylaluminoxane.299 Chromium complexes with a b-diketiminate ligand CpCr(ArNCMeCHCMeNAr0 ) [Ar ¼ Ar0 ¼ 2,6-Me2C6H3 (553), 2,4,6-Me3C6H2 (554), 2,6-Et2C6H3 (555), 2,6-iPr2C6H3 (556); Ar ¼ 2,6-iPr2C6H3, Ar0 ¼ Ph (557), 2,6-Me2C6H3 (558), 4-MeC6H4 (559), 4-MeOC6H4 (560), 4-CF3C6H4 (561)]300–302 and CpCr(ArNCMeCHCMeNAr0 )X [Ar ¼ Ar0 ¼ 2,6-Me2C6H3, X ¼ Cl (562), I (563), OTs (564), Me (565), Et (566), CH2CMe3, (567), CH2SiMe3, (568), CH2CN (569), CH2Ph (570), CH2C6H4Me (571), Ph (572), C^CH (573); Ar ¼ Ar0 ¼ 2,4,6-Me3C6H2, X ¼ Cl (574), I (575), Me (576); Ar ¼ Ar0 ¼ 2,6-Et2C6H3, X ¼ Cl (577), I (578), Me (579); Ar ¼ Ar0 ¼ 2,6-iPr2C6H3, X ¼ CH2CHMe2 (580), CH2Ph (581), CH2SiMe3 (582); Ar ¼ 2,6-iPr2C6H3, Ar0 ¼ Ph, X ¼ Cl (583), I (584), Me (585); Ar ¼ 2,6-iPr2C6H3, Ar0 ¼ 4-MeC6H4, X ¼ Cl (586), I (587), Me (588); Ar ¼ 2,6-iPr2C6H3, Ar0 ¼ 4-MeOC6H4, X ¼ Cl (589), I (590), Me (591); Ar ¼ 2,6-iPr2C6H3, Ar0 ¼ 4-CF3C6H4, X ¼ Cl (592), I (593), Me (594)]301–304 have been synthesized and some of them have been tested as catalysts for polymerization of ethylene, radical polymerization of vinyl acetate,305 or radical cyclization of bromo and chloroacetals.306 The CrdR bond homolysis of some CpCr[(ArNCMe)2CH]R complexes has been investigated.304 Complex 559 has been used to catalyze the reaction of Ph2PY (Y ¼ Cl, PPh2, H) and CyX (X ¼ Br, Cl) to form Ph2PCy.307 Oxidation of CpCr[(XylNCMe)2CH] [Xyl ¼ 2,6-Me2C6H3 (553)] with pyridine N-oxide or air generates the m-oxo dimeric Cr(III) complex {CpCr[(XylNCMe)2CH]}2(m-O) (595) which can be converted to complexes CpCr[(XylNCMe)2CH]X [X ¼ Cl (596), OH (597), OTs (598), OCMe3 (599), OCMe2Ph (600), O2CPh (601)] via protonolysis or salt metathesis reactions.308 i

Cyclic and Non-cyclic Pi Complexes of Chromium

143

Half-sandwich chromium(III) complexes bearing bidentate and tridentate b-ketoimines chelating ligands of the types, Cp[O(R)CC(H)C(Me)NAr]CrCl [R ¼ Me, Ar ¼ Ph (602), 2,4,6-Me3C6H2 (603), 2,6-iPr2C6H3 (604); R ¼ CF3, Ar ¼ Ph (605), 2,6-iPr2C6H3 (606)],309 Cp[O(Ar)CC(H)C(CF3)NPh]CrCl [Ar ¼ Ph (607), 2-MeC6H4 (608), 2-FC6H4 (609), 2-CF3C6H4 (610), 2-biphenyl (611), Naphthyl (612)],310 and Cp[O(R)CC(H)C(CF3)NAr]CrCl [R ¼ tBu, Ar ¼ 2-MeOC6H4 (613), 2-MeSC6H4 (614); R ¼ Ph, Ar ¼ 2-MeSC6H4 (615)]311 have been synthesized and investigated as catalysts for ethylene polymerization or copolymerization.

5.04.5.3

Other half-sandwich chromium complexes

The half-sandwich complex (616) with the 1,2,4-(Me3C)3C5H2 ligand has been prepared and studied by using a variety of spectroscopic techniques, X-ray diffraction and magnetic susceptibility measurements.312

In the past decade, very few borido chromium complexes have been reported. Fortunately, a half-sandwich borido chromium complex Cp(CO)2Cr(m-H)[BN(SiMe3)2] (617) has been obtained from the reaction of CpCr(CO)3H with a borylene transfer reagent (OC)5Cr]BN(SiMe3)2.313

Single-electron oxidation of a known Cr(II) complex CpCr[2-(Me2NCH2)C6H4] with I2, AgOTs, AgO2CPh, (OCMe2Ph)2 and (SPh)2 affords corresponding Cr(III) complexes CpCr[2-(Me2NCH2)C6H4]X [X ¼ I (618), OTs (619), O2CPh (620), OCMe2Ph

144

Cyclic and Non-cyclic Pi Complexes of Chromium

(621), SPh (622)]. Reactions of 619 with PhCH2MgCl or Mg(4-MeC6H4)2 reagents form the Cr(III) benzyl (623) and p-tolyl (624) complexes, respectively. Similarly, oxidation of CpCr[2-(Me2NCH2)C6H4(Ph)C]C(Ph)] with I2 and (SPh)2 gives Cr(III) complexes CpCr[2-(Me2NCH2)C6H4(Ph)C]C(Ph)]X [X ¼ I (625), SPh (626)].314 Reaction of chromocene (Cp2Cr) with 1,3-diisopropylimidazolium chloride forms CpCr(iPr-NHC)Cl (627) (iPr-NHC ¼ 1,3diisopropylimidazol-2-ylidene). 627 reacts with MesMgBr to give CpCr(iPr-NHC)Mes (628) (Mes ¼ 2,4,6-trimethylphenyl). Oxidation of 627 with PbCl2 leads to the Cr(III) dichloro complex CpCr(iPr-NHC)Cl2 (629), and oxidation of 628 with iodine gives the Cr(III) iodo complex CpCr(iPr-NHC)(Mes)I (630). Reaction of the chromocenium iodide [Cp2Cr]+ I− with 1,3-dimethylimidazolium iodide provides the Cr(III) diiodo complex CpCr(Me-NHC)I2 (631).315 Similarly, reactions of Cp2Cr with 1-R-3-R1-imidazolium bromides, followed by oxidation in CCl4, afford Cr(III) complexes CpCr(1-R-3-R1-NHC)ClBr [R ¼ Me, R1 ¼ Bn (632), C2H4Ph (633); R ¼ R1 ¼ Bn (634); R ¼ Bn, R1 ¼ C2H4Ph (635); R ¼ R1 ¼ C2H4Ph (636)]. Complexes 632–636 have been found to be active catalysts for the dehydration of glucose to form 5-hydroxymethylfurfural.316 In addition, reactions of [Cp2Cr]+ X− [X− ¼ I− or (O3SCF3)−] with neutral b-diketones, salicylaldimines and b-ketoimines form corresponding complexes [CpCrL]X, in which the structures of CpCr[OC(Ph)CHC(Me)O]I (637), CpCr[OC6H4CHN(2,6-iPr2C6H3)]I (638), and CpCr[OC (Ph)CHC(Me)N(3,5-Me2C6H3)](O3SCF3) (639) have been determined. In a similar way, reaction of chromocene with DBU
HCl (DBU ¼ 1,8-diazabicyclo[5.4.0]undec-7-ene) generates CpCr(DBU)Cl (640), which reacts with MesMgBr to form CpCr(DBU)Mes (641). Treatment of 641 with iodine affords CpCr(DBU)(Mes)I (642). Dissociation of DBU from 641 in Et2O affords the dimeric [CpCr(m-Mes)]2 (643), which can be directly prepared by sequential reactions of CrCl2 with NaCp and MesMgBr.315 Reactions of the known anionic radical a-diimine chromium complexes (RCNAr)2Cr(III)Cl2(THF)2 with CpNa afford the corresponding half-sandwich complexes CpCr[(ArNCR)2]Cl [R ¼ H, Ar ¼ 2,6-iPr2C6H3 (Dpp) (644); R ¼ Me, Ar ¼ 2,6Me2C6H3 (Xyl) (645), 2,4,6-Me3C6H2 (Mes) (646)]. Reduction of 644 with Zn gives a Cr(II) complex CpCr[(DppNCH)2] (647). Protonolysis of Cp2Cr with HOCR2R0 in the presence of a neutral a-diimine and a catalytic amount of base [such as NaN(SiMe3)2] forms alkoxide complexes CpCr[(DppNCH)2](OCR2R0 ) [R ¼ Me, R0 ¼ Ph (648); R ¼ iPr, R0 ¼ H (649)]. Similar reactions using a pyridine-imine instead of the a-diimine produce the corresponding anionic radical pyridine-imine complexes CpCr(PyCHNMes)Cl (650), CpCr(PyCHNMes) (651), and CpCr(PyCHNMes)(OCMe2Ph) (652). Oxidation of 651 with iodine gives CpCr(PyCHNMes)I (653).317

Oxidation of the dianionic phenylenediamido chromium complexes CpCr(III)[1,2-(RN)2C6H4] [R ¼ SiMe3 (654), CH2CMe3 (655), Ph (656)] with PbCl2 generates the corresponding monoanionic radical diiminosemiquinonate complexes Cp [1,2-(RN)2C6H4]Cr(III)Cl [R ¼ SiMe3 (657), CH2CMe3 (658), Ph (659)]. Reactions of complexes 654 or 655 with N3Ad or N3Mes azides afford dianionic phenylenediamido Cr(V) complexes Cp[1,2-(RN)2C6H4]Cr(V)NR1 [R ¼ SiMe3, R1 ¼ Ad (660), Mes (661); R ¼ CH2CMe3, R1 ¼ Ad (662)], while the reactions of 654 or 655 with N3SO2Ar azides give monoanionic radical diiminosemiquinonate Cr(III) complexes Cp[1,2-(RN)2C6H4]Cr(III)NHSO2Ar [R ¼ SiMe3, Ar ¼ 4-MeC6H4 (663), 2,4,6-iPr3C6H2 (664); R ¼ CH2CMe3, Ar ¼ 4-MeC6H4 (665)].318 Reaction of CpCr(NO)2Cl with benzil-bis(trimethylsilyl)diimine in 1:2 molar ratios in the presence of excess AgOTf leads to the formation of complex 666 (Scheme 118).319

Scheme 118

Cyclic and Non-cyclic Pi Complexes of Chromium

145

Reactions of [CpCr(CO)3]2 with tricyclohexylphosphine (PCy3) or N-heterocyclic carbene 1,3-dimethylimidazol-2-ylidene (IMe) form 17-electron radical complexes CpCr(CO)2(PCy3)% (667) and CpCr(CO)2(IMe)% (668). Reduction of the radical complexes with KC8 and 18-crown-6 produces 18-electron complexes K+(18-crown-6)[CpCr(CO)2(PCy3)]− (669) and K+(18crown-6)[CpCr(CO)2(IMe)]− (670). Protonation of 669 and 670 with NH4PF6 gives the hydrides CpCr(CO)2(PCy3)H (671) and CpCr(CO)2(IMe)H (672).320 Reaction of [CpCr(CO)3]2 with 1-adamantyl azide (N3Ad) results in the formation of 1-adamantyl isocyanate (OCNAd) and [CpCr(CO)2]2. Under a CO atmosphere, [CpCr(CO)3]2 can be regenerated and approximately five turnovers can be achieved.321 Substitution of one cyclopentadienide ligand of chromocene with the pnictogen-centered nucleophiles LiE(SiMe3)2 (E ¼ N, P, or As) gives the heterobimetallic coordination polymer {(m:
2:
5-Cp)Cr[m-N(SiMe3)2]2Li}n (673) and the homometallic dimeric complexes {(
5-Cp)Cr[m-E(SiMe3)2]}2 [E ¼ P (674) or E ¼ As (675)].322

Reaction of the complex [CpCr(CO)2]2(m,
2-P2) (676) with copper(I) halides leads to the formation of the one-dimensional linear coordination polymers {[CpCr(CO)2]2(m,
2:
1:
1-P2)CuX}n [X ¼ Cl (677), Br (678), I (679)]. The solid-state structures of 677–679 were determined by X-ray crystallography.323

Reaction of [CpCr(CO)3]2 with the symmetrical 3,5-diphenylthiatriazinyl radical or the asymmetrical 3-trifluoromethyl-5phenylthiatriazinyl radical in toluene affords a mixture containing a thiatriazinyl complex 680 or 681 and a known complex [CpCr(CO)2]2S (Scheme 119).324,325 In complex 680 the bonding is via the perpendicular ps orbital of the sulfur atom, while in complex 681 the bonding is via pp orbitals on both sulfur and nitrogen.

Scheme 119

Similarly, reactions of [CpCr(CO)3]2 with 1,2,3,5-dithiadiazolyls (ArCN2S2)2 yield mainly complexes CpCr(CO)2(
2-S2N2CAr) [Ar ¼ 4-MeC6H4 (682), 4-ClC6H4 (683), 4-MeOC6H4 (684), 4-CF3C6H4 (685), 3-CN-5-tBuC6H3 (686)] and [CpCr(CO)2]2S, together with a trace amount of the tetranuclear complex 687.326,327

146

Cyclic and Non-cyclic Pi Complexes of Chromium

Reaction of [CpCr(CO)3]2 with 2,4-bis(phenyl)-1,3-diselenadiphosphetane-2,4-diselenide (Woollins’ reagent) gives a mixtures of CpCr(CO)2(SeP(H)Ph) (688), trans-[CpCr(CO)2(SePPh)]2 (689) and a known complex [CpCr(CO)2]2Se.328 In a similar reaction of [CpCr(CO)3]2 with Bz2S2, complexes [CpCr(CO)2(SBz)]2 (690), [CpCr(SBz)]2S (691) and [CpCr(CO)2]2S (692) are formed.329 An analogue of 691, [CpCr(StBu)]2S, reacts with Cb Co(CO)2I (Cb ¼ tetramethylcyclobutadienyl) to give a heterometallic cluster Cb Co(m3-S)2(CpCr)2(m-StBu) (693).330 Reactions of Na[CpCr(CO)3] with N-heterocyclic carbene copper complexes (IPr)CuCl and (IMes)CuCl [where IPr ¼ N, N0 -bis(2,6-diisopropylphenyl)imidazol-2-ylidene, IMes ¼ N,N0 -dimesitylimidazol-2-ylidene] afford heterobimetallic complexes Cp(CO)(m-CO)2CrdCu(IPr) (694) and Cp(CO)(m-CO)2CrdCu(IMes) (695). The complex 695 exists in solution equilibrium with the mononuclear copper complex (IMes)2Cu and a heterotrimetallic complex [Cp(CO)(m-CO)2Cr]2Cu (696).331

Investigation on the kinetics of the hydrogenation of 2CpCr(CO)3%/[CpCr(CO)3]2 to CpCr(CO)3H indicates that the reaction is second-order in Cr and first-order in H2. DFT calculations rule out a side-on H2 complex as an intermediate and suggest homolytic cleavage via a collinear CrdHdHdCr transition state to CpCr(CO)3H as the most possible reaction route.332 The single-crystal X-ray structure of CpCr(CO)3H is close to a distorted three-legged piano stool geometry, whereas a four-legged piano stool arrangement should be preferred in the gas phase based on DFT calculations.333 Anti-Markovnikov alcohols are formed from the ring-opening hydrogenation of epoxides in a Cp2TiX2/[CpCr(CO)3]2 cooperative catalytic system, in which the titanocene catalyzes the epoxide ring-opening to form the more highly substituted radical and the chromium species catalyzes the hydrogen activation and sequentially transfers the hydrogen atom, proton, and electron as shown in Scheme 120.334

Scheme 120

Cyclic and Non-cyclic Pi Complexes of Chromium

147

Reactions of the tris(3,5-dimethylpyrazolyl)methanide amido complexes [C(3,5-Me2pz)3]MN(SiMe3)2 [M ¼ Mg, Zn, Cd; 3,5-Me2pz ¼ 3,5-dimethylpyrazolyl] with two equivalents of CpCr(CO)3H afford different types of heterobimetallic complexes, [HC(3,5-Me2pz)3]Mg(thf )[CpCr(m-CO)(CO)2]2 (697), {[HC(3,5-Me2pz)3]ZndCrCp(m-CO)2(CO)}+[CpCr(CO)3]− (698) and {[HC(3,5-Me2pz)3](thf )CddCrCp(CO)3}+{Cd[CpCr(CO)3]3}− (699). In these reactions, the tris(3,5-dimethylpyrazolyl)methanide C(3,5-Me2pz)−3 ligand is converted to the neutral tris(3,5-dimethylpyrazolyl)methane HC(3,5-Me2pz)3 ligand and the valence of the chromium changes from Cr(II) to Cr(0).335 The cyclopentadienylchromium(II) hydride CpCr(CO)3H has also been studied as a catalyst or H% transfer agent for the radical cyclization of a non-conjugated diene compound (Scheme 121).336

Scheme 121

Photochemical reactions of complexes 700 and 701 with (CpCr)2(m-S)(m-SCMe3)2 give heterometallic heterochalcogenide methylarsine clusters 702 and 703 (Scheme 122).337

Scheme 122

Reaction of Cp2Cr2(CO)4[m-SC5H4Mn(CO)3]2 with sulfur gives a new bichromium complex Cp2Cr2[m-SC5H4Mn(CO)3]2(m-S) (704) and a triangular cluster Cp3Cr3[m-SC5H4Mn(CO)3](m-S)2(m3-S) (705). Reaction of 704 with Co2(CO)8 forms a mixed-metal cluster Cp2Cr2[m-SC5H4Mn(CO)3](m-S)2Co(CO)2 (706) (Scheme 123).338

Scheme 123

148

Cyclic and Non-cyclic Pi Complexes of Chromium

Ligand substitution of the heterometallic cluster PhCCo2CrCp(CO)8 with the diphosphine ligand 2,3-bis(diphenylphosphino) maleic anhydride (bma) leads to the formation of a phosphido-bridged cluster 707 (Scheme 124).339

Scheme 124

The reaction of [MeC5H4Cr(CO)3]2 with dibutyldisulfide (BuS2Bu) results in the dimeric complex [MeC5H4Cr(CO)2(SBu)]2 (708) which interacts with a Pt(0) complex, (PPh3)2Pt(PhC^CPh) to give a heterobinuclear complex MeC5H4Cr(CO)(m-CO) (m-SBu)Pt(PPh3)2 (709).340

Cyclic and Non-cyclic Pi Complexes of Chromium

149

Reactions of (
6-C6Me6)Ru(II)[
3-S(CH2CH2S)2] with [CpCr(CO)3]2, [CpCrCl2(SPyH)], and [Cp Cr(CH3CN)4](PF6)2 produce corresponding thiolate-bridged RudCr heterobimetallic complexes, [(
6-C6Me6)Ru(II){m:
3:
2-S(CH2CH2S)2}{CpCr(II)(CO)2}] [CpCr(0)(CO)3] (710), [(
6-C6Me6)Ru(II){m:
3:
2-S(CH2CH2S)2}{CpCr(III)Cl}](PF6) (711), and [(
6-C6Me6)Ru(II){m:
3:
2S(CH2CH2S)2}{Cp Cr(III)(MeCN)}](PF6)2 (712) as shown in Scheme 125. The single-crystal X-ray structures of 710, 711 and 712 have been determined.341

Scheme 125

Reaction of a b-diketiminato yttrium complex [(2,6-iPr2C6H3NCMe)2CH]YI2(THF) with two equiv. of Na[CpCr(CO)3] affords a hexanuclear complex {[(2,6-iPr2C6H3NCMe)2CH]Y(THF)[Cr(Cp)(CO)3]2}2 (713).342 Reaction of a silicon(II) dibromide–carbene adduct SiBr2L1 (L1 ¼ 1,3-bis[2,6-bis(isopropyl)-phenyl]imidazolidin-2-ylidene) with Li[CpCr(CO)3] affords complex 714, which features an NHC-stabilized bromosilylidyne ligand. Treatment of 714 with two equivalents of the NHC 1,3-dihydro-4,5-dimethyl-1,3-bis(isopropyl)-2H-imidazol-2-ylidene (L2) forms complex 715. Addition of one equivalent of LiB(C6F5)4 to a fluorobenzene solution of 715 leads to the formation of a dimeric complex 716.343 Similarly, reaction of SiBr2L1 with Li[Cp Cr(CO)3] gives complex 717, which can be selectively converted to a chromium silylidyne complex salt [Cp (CO)2Cr^SiL1][B(ArF)4] (718) by bromide abstraction with Na[B(ArF)4] (ArF ¼ 3,5-(CF3)2C6H3) in fluorobenzene. Exposure of the fluorobenzene solution of 718 to CO results in selective formation of a tricarbonyl chromiosilylene complex salt [Cp (CO)3CrSiL1][B(ArF)4] (719). Furthermore, exposure of 719 to an atmosphere of N2O leads to a metallosilanone complex 720.344

150

Cyclic and Non-cyclic Pi Complexes of Chromium

Reactions of [(C5R5)Cr(CO)3]2 (R ¼ H, Me) with (trimethylsilyl)diazomethane (N]N]CHSiMe3) under an Ar or N2 atmosphere stoichiometrically produce the Cr^Cr triply bonded complex [(C5R5)Cr(CO)2]2 (721), while the same reactions under a CO atmosphere catalytically convert N]N]CHSiMe3 to (trimethylsilyl)ketene (O]C]CHSiMe3). Two key intermediates in the reaction, %Cr(CO)2(ketene)(C5R5) (722) and (C5R5)2Cr2(CO)5 (723) have been detected spectroscopically.345 Reactions of [Cp Cr(CO)3]2 with Ph3SnH and Cy3SnH lead to complexes Cp Cr(CO)3SnPh3 (724) and Cp Cr(CO)3 SnCy3 (725).346 Reactions of Cr(CO)6 with Cp Li or CpNa, followed by treatment with (CF3CO)2O, give the perfluoroacyl Cr(II) complexes  Cp Cr(CO)3(COCF3) (726) and CpCr(CO)3(COCF3) (727). Complexes 726 and 727 can be readily converted to their corresponding perfluoroalkyl complexes Cp Cr(CO)3CF3 (728) and CpCr(CO)3CF3 (729) by heating under N2 in the solid state.347 Exposure of a hexane solution of complex 728 to air in the presence of light affords a diamagnetic oxo-bridged chromium(V) complex, [Cp Cr(O)CF3]2O (730).348

Reactions of [Cp Cr(CO)3]2 with P4 and As4 in toluene afford butterfly complexes [Cp Cr(CO)3]2(m,
1:1-P4) (731) and [Cp Cr(CO)3]2(m,
1:1-As4) (732) in good yields. Dimerization of 732 takes place upon crystallization to form a As8 cuneane complex [Cp Cr(CO)3]4(m4,
1:1:1:1-As8) (733).349

Reaction of decamethylchromocene Cp 2Cr with 3,6-di-tert-butyl-o-benzoquinone gives a mixture containing complexes [Cp 2Cr(III)][Cp Cr3(m3-O)(3,6-tBu2C6H2O2)4] (734), [Cp Cr(IV)(3,6-tBu2C6H2O2)]2(m-O) (735), and Cp Cr(III)(3,6-tBu2 C6H2O2)(thf ) (736). Compound 734 is a complex salt with the anionic part either in the form [Cp Cr(III)3(m3-O) t t (3,6-tBu2C6H2Osq 2 )(3,6- Bu2C6H2O2)3] in which one of the four 3,6- Bu2C6H2O2 ligands exists in the anion radical t  semi-quinolate state or in the form [Cp Cr(IV)Cr(III)2(m3-O)(3,6- Bu2C6H2O2)4] in which all four 3,6-tBu2C6H2O2 ligands in the dianionic catecholate state.350

Cyclic and Non-cyclic Pi Complexes of Chromium

151

Reaction of Cp 2Cr with thioindigo (TI) affords a thioindigo-bridged dimeric complex [Cp Cr(m,
2-TI)]2 (737) with a CrdCr distance of 3.12 A˚ . According to magnetic data, two diamagnetic TI2− dianions and two Cr3+ atoms with a high S ¼ 3/2 spin state are present in 737.351 Reactions of half-sandwich chromium(III) complex [Cp CrBr2]2 with the scorpionate salts K[HB(mt)3], Na[H2B(mt)2], and Li [HB(mt)2(pz)] (mt ¼ N-methyl-2-mercaptoimidazol-1-yl, pz ¼ pyrazolyl) give the Cr(III) complexes [Cp Cr{k3-S,S0 ,S00 -HB(mt)3}] Br (738), [Cp Cr{k2-S,S0 -H2B(mt)2}Br] (739), and [Cp Cr{k2-S,S0 -HB(mt)2(pz)}Br] (740). By treatment with AgPF6 in acetonitrile or toluene to remove the bromide ligand, 741 is converted to complexes [Cp Cr{k2-S,S0 -H2B(mt)2}(NCMe)]PF6 (741) or [Cp Cr {k3-H,S,S0 -H2B(mt)2}]PF6 (742), while 740 is converted to the pyrazolyl coordinating complex [Cp Cr{k3-N,S,S0 -HB(mt)2(pz)}] PF6 (743).352

Treatment of the half-sandwich chromium(III) complex [Cp CrCl2]2 with 2 eq. of NaB3H8 in diethyl ether affords the octahydrotriborate complex Cp Cr(B3H8)2 (744).353

152

Cyclic and Non-cyclic Pi Complexes of Chromium

Reaction of [Cp CrCl]2 with Li[BH3(SePh)] affords a Se inserted binuclear chromium complex, [(Cp Cr)2(m-Se2SePh)2] (745).354 Reaction of Cp 2Cr with SF6 in toluene forms a multinuclear complex (Cp CrF2)6(CrF3)2 (746). Introduction of p-dimethylaminopyridine (dmap) to the reaction mixture leads to a mononuclear complex Cp CrF2(dmap) (747). The same reaction in hexane produces the ionic complex, [Cp 2Cr(III)]+[(Cp CrF2)2(m-F)]− (748).355

Treatment of [(
5-C5Ph5)CrCl(m-Cl)]2 with TlCl with in CH2Cl2 yields a dimeric heterobimetallic complex [(
5-C5Ph5)CrCl (m-Cl)2Tl]2
2CH2Cl2 (749).356

Reactions of Cr(CO)6 with methyl iodide and sodium 1,2,4-tritertbutylcyclopentadienide (tBu3CpNa) forms the tricarbonyl methyl chromium(II) complex
5-tBu3CpCr(CO)3CH3 (750). Oxidation of complex 750 with PCl5 in dichloromethane affords the dimeric dichloride [tBu3CpCrCl(m-Cl)]2 (751) which can be converted to a mononuclear THF adduct tBu3CpCrCl2(THF) (752) when the solution is exposed to THF vapor. Treatment of complex 751 or 752 with 2 equiv. of sodium 2,6-diisopropylphenolate gives the bis(diisopropylphenolate) derivative tBu3CpCr(OC6Hi3Pr2)2 (753). Reactions of tBu3CpNa with CrBr2 or CrI2 produce the corresponding dimeric bromo and iodo complexes [tBu3CpCr(m-Br)]2 (754) and [tBu3CpCr(m-I)]2 (755). Complex 754 reacts with 2 equiv. of sodium phenolate and sodium 2,6-ditertbutylphenolate to form complexes [tBu3CpCr(m-OC6H5)]2 (756) and t Bu3CpCr(OC6Ht3Bu2)(THF) (757), respectively. Oxidation of 755 with iodine in THF results in the dimeric chromium(III) complex [tBu3CpCrI(m-I)]2 (758).357 Bromination/nitrosylation of (
5-C5H4R)Cr(CO)2(NO) with hydrogen bromide/isoamylnitrite gives bromodinitrosylchromium complexes (
5-C5H4R)Cr(NO)2Br [R ¼ COOH (759), COOMe (760), COPh (761)]. Reactions of the known complex (
5-C5H4COOMe)Cr(NO)2Cl with excess potassium thiocyanate and selenocyanate produce isothiocyanate and isoselenocyanate complexes (
5-C5H4R)Cr(NO)2NCS (762) and (
5-C5H4R)Cr(NO)2NCSe (763).358 Copper(I) iodide catalyzed reactions of (
5-C5H4R)Cr(NO)2Cl and (
5-C5H4COOMe)Cr(CO)3Cl with phenylacetylene form corresponding d-alkynyl complexes (
5-C5H4R)Cr(NO)2(C^CdPh) [R ¼ H (764), COOMe (765)] and (
5-C5H4COOMe)Cr(CO)3(C^CdPh) (766).359 Similarly, chlorination/nitrosylation of (
5-C5H4NHCO2CH2C6H5)Cr(CO)2(NO) with hydrogen chloride/isoamylnitrite forms chlorodinitrosylchromium complexes (
5-C5H4R)Cr(NO)2Cl [R ¼ COPh (767), COFc (768), NHCO2CH2Ph (769)]. Reactions of the chlorodinitrosylchromium complexes with potassium iodide give the corresponding iodo derivatives (
5-C5H4R)Cr(NO)2I [R ¼ COPh (770), COFc (771), NHCO2CH2Ph (772)].360,361 Methylation of 772 with the Grignard reagent CH3MgI affords the methylated complex (
5-C5H4NHCO2CH2Ph)Cr(NO)2Me (773). Mono-demethylation of Cp2TiMe2 with 1 equiv. of (
5-C5H4COOH)Cr(CO)2NO, (
5-C5H4COOH)Cr(NO)2Cl and 5 (
-C5H4COOH)Cr(NO)2I forms heterobinuclear complexes Cp2TiMe(OOCC5H4)Cr(CO)2NO (774), Cp2TiMe(OOCC5H4) Cr(NO)2Cl (775) and Cp2TiMe(OOCC5H4)Cr(NO)2I (776),362 while complete demethylation of Cp2TiMe2 with 2 equiv. of

Cyclic and Non-cyclic Pi Complexes of Chromium

153

corresponding chromium complexes affords heterotrinuclear complexes Cp2Ti[(OOCC5H4)Cr(CO)2NO]2 (777), Cp2Ti[(OOCC5H4)Cr(NO)2Cl]2 (778) and Cp2Ti[(OOCC5H4)Cr(NO)2I]2 (779),363 respectively. A heterobimetallic anionic complex [
5-(2-ferrocenyl)indenyl]Cr(CO)−3 (780) has been obtained by metalation of the neutral complex [
6-(2-ferrocenyl)indene]Cr(CO)3 with potassium hydride, followed by a
6 !
5 haptotropic rearrangement. Treatment of 780 with Diazald gives a neutral complex [
5-(2-ferrocenyl)indenyl]Cr(CO)2NO (781).364

Reactions of Cr(CO)6 with methyldiphenylphosphonium cyclopentadienylide, Ph2MePC5H4, and methyldiphenylphosphonium 1-indenylide, 1-Ph2MePC9H6, give the zwitterionic complexes (
5-Ph2MePC5H4)Cr(CO)3 (782) and (
5-1-Ph2 MePC9H6)Cr(CO)3 (783).365,366 The molecular structures of the ligands and complexes 782 and 783 have been characterized spectroscopically and crystallographically. Oxidation of 782 with [Cp2Fe][B(C6F5)4] gives a dimeric product [(
5-Ph2MePC5H4) Cr(CO)3]2[B(C6F5)4]2 (784).367 Reaction of in situ formed Na[(
5-Me2NCH2CH2C5H4)Cr(CO)3] with iodine, followed by methylation with CH3I, provides [(
5-Me3NCH2CH2C5H4)CrI(CO)3]I (785). Reduction of 785 with sodium naphthalenide affords a zwitterionic complex [(
5-Me3NCH2CH2C5H4)Cr(CO)3] (786).368 Similar zwitterionic tricarbonyl chromium complexes with electron-poor mono(imidazolium)- and bis(imidazolium)-substituted cyclopentadienylide ligands 787, 788 and 789 are formed from the reactions of Cr(CO)3(MeCN)3 with corresponding ligands.369 In these zwitterionic complexes, due to electrostatic interaction between the positively charged atom(s) and the negatively charged metal center, the donor ability of the cyclopentadienyl or indenyl ligand toward Cr(CO)3 fragment is less than that of classical cyclopentadienyl or indenyl ligand. Binuclear complexes 790 and 791 are formed from condensation reactions of hydrazine with each corresponding carbonyl derivatives (
5-C5H4CHO)Cr(CO)2(NO) and (
5-C5H4COCH3)Cr(CO)2(NO).370



154

Cyclic and Non-cyclic Pi Complexes of Chromium

Reactions of Cr(CO)3(MeCN)3 with NaC5H4CH2CH2PPh2, followed by treatment with CuCl and PR3 form heterobimetallic complexes (m:
5:
1-CpCH2CH2PPh2)(CO)3CrdCuPR3 [R ¼ Me (792), Cy (793), Ph (794)]. A related complex (m:
5:
1-CpCH2 CH2NMe2)(CO)3CrdCuPPh3 (795) with a bridging
5:
1-CpCH2CH2NMe2 ligand can be obtained in the same way.371 Similar reactions of Cr(CO)3(MeCN)3 with NaC5H4CH2CH2PPh2, followed by salt elimination with [M(
3-L)(m-X)]2 [M ¼ Ni, L ¼ allyl, 2-methylallyl, cyclohexenyl, X ¼ Cl, Br; M ¼ Pd, L ¼ allyl, X ¼ Cl] afford heterobimetallic complexes (m:
5:
1-CpCH2CH2PPh2) (CO)3CrdM(
3-L) [M ¼ Ni, L ¼ allyl (796), 2-methylallyl (797), cyclohexenyl (798); M ¼ Pd, L ¼ allyl (799)]. The competitive reactions of allyl complexes 792 and 795 with phenyl and trityl radicals provide 4,4,4-triphenyl-1-butene as the sole allyl ligand coupling product.372 Reaction of Cr(CO)3(MeCN)3 with a potassium salt of pentamethyl[60]fullerene, KC60Me5, prepared by deprotonation of C60Me5H with KH, forms a known anionic chromium complex, K+[(
5-C60Me5)Cr(CO)3]− (800). Treatment of 800 with Diazald (N-methyl-N-nitroso-p-toluenesulfonamide) causes evolution of carbon monoxide gas and affords a chromium(II) complex (
5-C60Me5)Cr(CO)2(NO) (801), which reacts with ethanol to afford an ethoxy chromium(III) complex (
5-C60Me5)Cr(NO) (OEt) (802). Treatment of a solution of 801 in carbon disulfide with a few drops of ethanol results in the insertion of carbon disulfide into the chromium-oxygen bond to form a dithiocarbonate complex, (
5-C60Me5)Cr(
2-S2COEt)(NO) (803).373

Reactions of alkoxy alkynyl Fischer carbene complexes (CO)5Cr]C(OMe)(C^CR) with symmetrical internal alkynes R1C^CR1 at or above 80  C afford new cyclopentadienyl chromium organometallic species [
1,
5-(R1)C]C(R1)CO-2-(OMe)3,4-R12-5-RC5Cr(CO)3] [R ¼ Ph, R1 ¼ Et (804), Pr (805); R ¼ 4-ClC6H4, R1 ¼ Et (806); R ¼ 4-(MeO)C6H4, R1 ¼ Et (807); R ¼ 3,4,5-(MeO)3C6H2, R1 ¼ Et (808); R ¼ styryl, R1 ¼ Et (809); R ¼ ferrocenyl, R1 ¼ Et (810)] and [
3,
5-1,3,4,5-R14-2-O-C5 CO-2-(OMe)-3,4-R12-5-RC5Cr(CO)2] [R ¼ Ph, R1 ¼ Et (811), Pr (812); R ¼ 4-ClC6H4, R1 ¼ Et (813); R ¼ 4-(MeO)C6H4, R1 ¼ Et (814); R ¼ 3,4,5-(MeO)3C6H2, R1 ¼ Et (815); R ¼ styryl, R1 ¼ Et (816); R ¼ ferrocenyl, R1 ¼ Et (817)] as shown in Scheme 126. Complexes 804–810 and 811–817 should be formed from consecutive insertions of several alkyne units and carbonyl groups into the metal–carbon bond.374

Scheme 126

5.04.5.4

Theoretical studies on the electronic structures

Density functional theory (DFT) calculations on all intermediates and relevant transition states in the ethylene trimerization reaction catalyzed by a
5-CpCr complex, starting from the species
5-CpCrMe2, have been carried out for establishing the rate-determining reaction barrier and facilitating the development of improved ethylene trimerization catalysts of this type.375 The catalytic cycle and possible termination reactions for ethylene polymerization with cationic chromium complexes of the type [CpCr(L)R]+ (L ¼ PR3 or NR3) have been studied by DFT. The calculations indicate that termination of the polymerization reaction by b-hydrogen elimination and subsequent dissociation of the resulting olefin are thermodynamically less feasible than the alternative termination process by b-hydrogen transfer to a monomer, and the ethylene insertion into the chromium–alkyl bond is the rate-determining step in the catalytic cycle.376

Cyclic and Non-cyclic Pi Complexes of Chromium

155

Electronic structures and electron affinities (EAs) of the 17-electron radical chromium(I) complex CpCr(CO)3% and its corresponding 18-electron anionic complex Na+[CpCr(CO)3]− have been investigated by photodetachment photoelectron spectroscopy (PES) and DFT calculations. The calculated EAs and band gaps are in good agreement with the experimental data.377 Molecular and electronic structures of cyclopentadienylchromium carbonyls Cp2Cr2(CO)n and Cp 2Cr2(CO)n (n ¼ 3, 2) and cyclopentadienylchromium carbonyl thiocarbonyls Cp2Cr2(CS)2(CO)n (n ¼ 4, 3, 2, 1) have also been studied by DFT calculations.378,379 Geometry and bond energy analysis of the coordinated hydrosilyl groups in chromium complexes, [CpCr(
2-HdSiMe2) (dmpe)] (818), and [CpCr(CO)2(
2-HdSiMe2)] (819) have been investigated using DFT calculations. The optimized geometry of model complexes are in agreement with the experimental values.380 Electronic and molecular structures of [(
5-C5H5) (CO)2Cr^EMe] (820) and [(
5-C5H5)(CO)3CrdEMe] [E ¼ Si (821), Sn (822), Pb (823)] have also been calculated at the DFT level. The theoretically predicted bond lengths and angles of the model compounds are in agreement with experimental values.381 Similar DFT calculations have been performed for germylene382 and phosphinidene383 complexes of [(
5-C5H5)(CO)3CrdGeN (SiMe3)R] [R ¼ Ph (824), Mes (825)], [(
5-C5H5)(CO)3CrdGeN(Ph)R] [R ¼ Ph (826), Mes (827)], [(
5-C5H5)(CO)2Cr^PMe]+ (828) and [(
5-C5H5)(CO)3Cr]PMe]+ (829). The effective exchange integrals of [Cp 2Cr]+[TCNE]− [Cp 2Cr]+ have been calculated using a broken-symmetry hybrid-DFT method. The calculated results show that effective exchange integrals are positive and the signs of spin densities on the cyclopentadienyl rings are negative, and indicate that orbital orthogonality is an important key factor for explaining the ferromagnetism of the system.384 The molecular and electronic structures of complex (
5-C5H5)Cr(
6-C8H8) were investigated by DFT calculations.385 Structural and optical properties of (
5-C5H5)Cr(
6-C60) were also studied by DFT calculations.386 Molecular structures of a series of binuclear azulene chromium carbonyl complexes (
5,
5-C10H8)Cr2(CO)n [n ¼ 6, 5, 4, 3, 2, 1], especially the chromium–chromium bonding in these complexes, have also been studied by DFT calculations.387 The hydrogen storage capacity of the sandwich-type dichromocene CpCr2Cp has been studied by DFT calculations, and the results demonstrate that each CpCr2Cp complex could adsorb up to six hydrogen molecules.388 DFT calculations on a triple-decker sandwich chromium compound (CpCr)2(m-C6H6) indicate that the global minimum of this derivative is a singlet spin state cis-(CpCr)2(
4:
4-m-C6H6) structure with a very short CrdCr distance of 2.06 A˚ , suggesting a formal quadruple bond.389 DFT calculations on a series of polyhedral chromadicarbaborane complexes show that the lowest-energy CpCrC2Bn−3Hn−1 (8  n  11) structures all have both carbon atoms at degree 4 vertices, while the lowest-energy CpCrC2B9H11 structures all have central CrC2B9 icosahedra and thus lack degree 4 vertices for the carbon atoms (the degree of a vertex is the number of edges incident to it or the number of vertices adjacent to it). For all of the CpCrC2Bn−3Hn−1 (8  n  12) systems the lowest-energy isomers are those with the maximum number of CrdC edges.390

5.04.6


5-Heterocyclopentadienyl chromium complexes

Combination of tetramethylpyrrole, CrCl2(thf )4 and AlMe3 affords a binuclear chromium complex {[
5-2,3,4,5-Me4C4N (AlClMe2)]Cr}2(m-Me)2 (830), which shows good catalytic activity for selective ethylene oligomerization. Similar reaction of tetramethylpyrrole, CrCl3(thf )3 and AlMe3 forms a chromocene-type complex [
5-2,3,4,5-Me4C4N(AlClMe2)]2Cr(II) (831). Treatment of 831 with azobenzene gives a pentavalent chromium species [
5-2,3,4,5-Me4C4N(AlClMe2)]Cr(V)Me(m-NPh)2AlMe2 (832). Both 831 and 832 can catalyze ethylene polymerization to produce high molecular weight polyethylene.391

156

Cyclic and Non-cyclic Pi Complexes of Chromium

Reaction of deprotonated 2,5-di-tert-butylpyrrole with CrCl3(thf )3 affords a p-bonded pyrrolyl chromium(III) complex [
5-(tBu)2C4H2N]CrCl2(thf ) (833) which reacts with 0.5 equivalent of AlEt3 to give a binuclear complex {[
5-(tBu)2C4H2N] Cr}2(m-Cl)2(m-CHCH3) (834) and reacts with 1 equivalent of AlEt3 to form a dimeric complex {[
5-(tBu)2C4H2N]CrEt}2 (m-Cl)2 (835). Treatment of 835 with AlEt3 in THF results in a binuclear d-bonded pyrrolyl complex {[d-(tBu)2C4H2N]Cr(II) (thf )}2(m-Cl)2 (836). Complexes 834 and 835 show high catalytic activity and selectivity for ethylene trimerization when activated with AlEt3.392 A theoretical study on the pyrrolyl chromium catalyzed ethylene trimerization system has been carried out by DFT methods, which supports that the selective ethylene oligomerization goes through the metallacycle mechanism involving the Cr(I/III) redox cycle.393 Reactions of Cr(CO)3(MeCN)3 with the appropriate ferrocenyl-substituted thiophenes afford ferrocenyl-functionalized half-sandwich
5-thiophene Cr(CO)3 complexes (
5-2,5-Fc2C4H2S)Cr(CO)3 (837), (
5-3,4-Fc2C4H2S)Cr(CO)3 (838), (
5-2,3-Fc2 C4H2S)Cr(CO)3 (839), and (
5-2-FcC4Me3S)Cr(CO)3 (840).394

DFT calculations on the structures and energetics of the binuclear phospholyl chromium carbonyl derivatives (C4H4P)2 Cr2(CO)n (n ¼ 6, 5, 4, 3) indicate that their lowest energy structures (C4H4P)2Cr2(CO)n (n ¼ 6, 4, 3) are analogous to their Cp2Cr2(CO)n analogues with two terminal pentahapto phospholyl rings, while the lowest energy structure of the pentacarbonyl complex (C4H4P)2Cr2(CO)5 has a bridging seven-electron donor
5,
1-C4H4P ligand using the phosphorus lone pair and the p-system of the phospholyl ligand.395 The singlet and triplet surfaces for the interaction of thiophene or selenophene with a chromium tricarbonyl unit in complexes (
5-C4H4E)Cr(CO)3 [E ¼ S or Se] have been calculated using DFT.396 The structures and energetics of a number of neutral binuclear nickelacyclopentadienyl chromium carbonyls (CpNiC4H4)2 Cr2(CO)n (n ¼ 6, 5, 4, 3) have been investigated by DFT calculations. The obtained lowest energy (CpNiC4H4)2Cr2(CO)n (n ¼ 6, 4) structures are similar to the corresponding experimentally characterized Cp2Cr2(CO)n structures.397

5.04.7

Other
5-pi-ligand chromium complexes

As shown in Scheme 127, the complex (
5-silacyclohexadienyl)Cr(H)(CO)3 (842) has been isolated as an intermediate from the hydration reaction of Cr(
6-silabenzene)(CO)3 (841).398 Moreover, complex 842 can also be obtained from reaction of silanol 843 and Cr(CH3CN)3(CO)3 through formal insertion of Cr(0) into CdH or SidH bond.399 A similar reaction of chlorosilane 844 gives the formation of corresponding complex 845 (Scheme 128).

Scheme 127

Scheme 128

Cyclic and Non-cyclic Pi Complexes of Chromium

157

In addition, the reaction of hydrosilane 846 with Cr(CH3CN)3(CO)3 gives (
5-1-silacyclohexa-1,3-dienyl)Cr(H)(CO)3 (847) (Scheme 129).

Scheme 129

The applicability of the nucleoside alkylation with 8-dioxane-(3,30 -chromium-1,2,10 ,20 -dicarbollide) adducts for incorporation of chromium unit into nucleoside molecule has been developed and preliminary electrochemical characteristics of the chromium containing conjugate has been studied.400 Furthermore, the electrochemical detection of DNA hybridization using a metallacarborane unit covalently linked to oligonucleotide strand has been investigated.401 A method for the synthesis of cholesterol–metallacarborane conjugates bearing chromium has also been developed.402 The coordination chemistry of the enantiomerically pure dimethylnopadienyl ligand (Pdl ) with chromium has been studied. As shown in Scheme 130, an open chromocene [Cr(
5-Pdl )2] (849) has been readily obtained from the reaction of CrCl2 and 2 equiv. of 848.403

Scheme 130

In addition, the coordination chemistry of the 2,4-di(tert-butyl)pentadienyl (Pdl0 ) ligand with chromium has also been investigated. The open chromocene [(
5-Pdl0 )2Cr] (850) has been synthesized and characterized by a series of spectroscopic techniques and X-ray diffraction.404

Reaction of dicarbollylamino aluminum compounds [(
5-RC2B9H9)(
1-CH2NMe2)AlMe] with CrCl3
THF3 in toluene gives dicarbollyl chromium complexes [(
5-RC2B9H9 CH2NMe2)Cr(m-Cl)Cl]2 (R ¼ H and Me). Structural analysis by X-ray diffraction reveals a three-legged piano stool structure with a planar dichromium Cr unit containing bridged chloride ions for both complexes.405 A radical cation salt type of molecular conductor based on the bis(1,3-propylenedithio)tetrathiafulvalene (BPDT-TTF) radical cation and chromium bis(dicarbollides) anion (BPDT-TTF)[3,30 -Сr(1,2-C2B9H11)2] has been synthesized and characterized by X-ray diffraction.406 The geometry and energy of the bis(bicyclo[3.2.1]octa-2,6-dien-4-yl) chromium complex, (bcod)2Cr, have been studied by density functional theory.407

158

5.04.8

Cyclic and Non-cyclic Pi Complexes of Chromium


2-Pi-ligand chromium complexes

Binuclear
2-alkenylarene chromium complexes have been obtained via photolytic decomposition of styrene, stilbene, and 1,4-diphenylbuta-1,3-diene tricarbonyl chromium complexes in solution together with elimination of the carbonyl ligands.408 In addition, photolysis of (C5H5)CrNO(CO)2 in the presence of butadiene gives
2-diene complexes 851 and 852. Photolysis of dicarbonyl complex (C5Me5)CrNO(CO)2 in the presence of a large excess of propene or allyltrimethylsilane affords a mixture of
2-alkene complexes 853 and 854. Similar photolysis of dicarbonyl complex (C5Me5)CrNO(CO)2 in the presence of excess 2-butyne gives the
2-2-butyne complex 855 with a modest isolated yield.409

A number of arenechromium dicarbonyl complexes 856–858 with a
2-allene ligand have been synthesized, as shown in Scheme 131. A mixture of all three possible isomers can be obtained with stereochemical nonrigidity. Similar results have been observed for methylallene chromium complexes.410

Scheme 131

A study on the origin of restricted rotation of an aryl ring in transition-metal aminocarbene complexes has been reported. The influence of electronic effects, steric factors, and aromatic ortho-substituent on chelated tetracarbonyl[(
2-N-allyl-N-allylamino)(aryl)carbene]chromium complexes 859, N-alkyl derivatives 860, and NdH derivatives 861 have been investigated.411 A series of
2-pi-ligand chromium complexes have been speculated as intermediates in the coupling reaction of cyclopropylcarbene–chromium complex with ferrocenyl alkynes, although these
2-pi-ligand chromium complexes have not been isolated.412 The first selective insertion of the borylene moiety into a CdH bond has been reported. Reaction of [(OC)5Cr]BN(SiMe3)2] with 3,3-dimethyl-1-butene gives the complex (OC)4Cr[Me3CHC]CHBHN(SiMe3)2] (862).413

Cyclic and Non-cyclic Pi Complexes of Chromium

159

Metal-olefin bond dissociation enthalpies for a series of complexes Cr(CO)5(C2H4-nCln) (n ¼ 0–4) have been calculated via density functional theory.414 A bistable photochromic organometallic transformation based on the linkage isomerization between arene chromium complexes with tethered alkene and pyridine groups Cr{
6-C6H5CH(2-Py)CH2-
2-CH]CH2}(CO)2 (863) and Cr{
6-C6H5CH(2-Py-kN) CH2CH)CH2}(CO)2 (864) has been studied (Scheme 132).415

Scheme 132

The reaction of 1,2-dipiperidinoacetylene with two equivalents of the Cr(CO)5 transfer reagent [(
2-COE)Cr(CO)5] (865) (COE ¼ cis-cyclooctene) affords the chromium complex 866, which can be isolated via redissolution and filtration through alumina (Scheme 133).416

Scheme 133

The p and CdH insertion complexes [(
2-C2H2)Cr and HCrC^CH] have been confirmed via matrix infrared spectra from reactions of laser-ablated Cr atoms with acetylene. The p complex is produced first, and then converted to the insertion product during photolysis. No vinylidene product has been observed.417 The mononuclear alkyne complex (iPr2Ph)2nacnacCr[
2-C2(SiMe3)2] (867) has been isolated by KC8 reduction of [(iPr2Ph)2 nacnacCr(m-Cl)]2 in the presence of bis(trimethylsilyl)-acetylene.418 In addition, the reaction of complex 867 with dioxygen and the mechanism of oxygen cleavage have been studied experimentally and computationally. Reaction of [(iPr2Ph)2nacnacCr]2(m-N2) with ethylene leads to the formation of a binuclear complex [(iPr2Ph)2nacnacCr]2 (m-
2:
2-C2H4) bearing a single ethylene ligand symmetrically coordinated to two metal centers.419

The side-on alkyne complex 869 resulting from the reaction of quasi-linear chromium(I) silylamide, K(18c6){Cr[N(SiMe3)2]2} (868) (18c6 ¼ 18-crown-6), has been isolated and characterized by X-ray diffraction analysis (Scheme 134).420

Scheme 134

160

Cyclic and Non-cyclic Pi Complexes of Chromium

Irradiation of the complex 870 leads to the formation of a complex with the tethered C^C bond coordinating to the Cr atom, which is demonstrated by a low-field shift of the 29Si NMR signal of the silicon atom. However, the anticipated main product (
2:
6-nPrC^CSiMe2-SiMe2SiMe2Ph)Cr(CO)2 could not be isolated (Scheme 135).159

Scheme 135

A simple three-step mechanism for the dimerization of ethylene by gas-phase chromium hydroxide (CrOH+) has been proposed based on the formation of a series of chromium-aqua cation intermediate with the coordination of two ethylene molecules.421 The exchange reaction of a 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (dpp-bian) radical anion chromium complex 871 with PhC^CNa happens upon heating to afford the corresponding derivative [(dpp-bian)Cr(m-C^CPh)]2 (872) (Scheme 136).422

Scheme 136

Reactions of 873 with various p-acceptors have been examined (Scheme 137). A binuclear product, k2-TptBu,MeCr(CO)2(m
1:
1-CO)(Et2O)CrTptBu,Me (874) is formed as a mixed-valent (Cr0CrII) isocarbonyl complex upon treatment with CO. An ethylene bridged binuclear complex, [k2-TptBu,MeCr]2-(m-
2:
2-C2H4) (875) with the ethylene p-bonded to two metals, is produced upon reaction with ethylene. The reactions of 873 with less hindered alkynes give the pseudotetrahedral alkyne complexes 876 and 877. In addition, the reaction of 873 with N2 (1 atm) affords a N2 bridged binuclear complex 878.423

Scheme 137

Cyclic and Non-cyclic Pi Complexes of Chromium

161

UV-promoted four-component reactions with tricarbonyl(arene)chromium complexes, propargyl alcohol and its derivatives, PPh3, and HBF4, afford the isomeric terminal and internal phosphonioallene complexes 879–884 shown in Scheme 138.424

Scheme 138

5.04.9


3-Pi-ligand chromium complexes

An allyl complex, Cr[(Me3SiN)2CPh]2(
3-C3H5) (885) has been prepared and characterized by X-ray crystallography. The thermal stability of 885 in the solid state and in solution may be attributed to the increased barriers to bimolecular decomposition due to the spatial protection of
3-allyl provided by the bulky N,N0 -bis-(trimethylsilyl)benzamide ligands.425

Cationic
3-butadienyl chromium complexes 886, 887 and 888 have been synthesized from reactions of (arene)Cr(CO)3 with enynes or allenylcarbinol as shown in Scheme 139. Reaction of complex 887 with NaBH4 gives a 1,1-dimethylallene complex 889. Complex 888 is easily converted to complex 890 by treatment with PPh3. The allene ligands in both complexes 889 and 890 coordinate to the metal atoms with their terminal double bonds.426

162

Cyclic and Non-cyclic Pi Complexes of Chromium

Scheme 139

The allyl chromium complexes bis(
3-allyl)chromium and tri(
3-allyl)chromium have been studied theoretically by DFT calculations.427 Complex 891 has been prepared by photolytic transfer of an arylborylene ligand to diphenylacetylene from the Cr(CO)5 precursor as shown in Scheme 140. This is the first example of a complex containing an
3-coordinated borirene ligand bound to a single metal atom. The bonding between the chromium center and the borirene ring in 891 has been studied by DFT calculations.428

Scheme 140

One of the two NiPr2 ligands in complex NCr(NiPr2)2(
1-Cp) (892) is replaced with a weaker donor to give
3-Cp coordinated chromium complexes NCr(NiPr2)(X0 )(Cp) (893–895) as shown in Scheme 141. The complex 892 reacts with benzoic acid in

Scheme 141

Cyclic and Non-cyclic Pi Complexes of Chromium

163

toluene to result in the formation of NCr(NiPr2)Cp(O2Ph) (893), while it reacts with HCl to afford the chloride complex 894. Treatment of 894 in acetonitrile with silver hexafluoroantimonate leads to a cationic acetonitrile complex [NCr(NPri2)Cp(NCMe)] [SbF6] (895).429

The trinuclear complex [Cr{Cr(tBuP NatBuP )Cl}2] (896) has been isolated and characterized by X-ray crystallography. The structure of 896 features one internal 12 electron Cr connecting the two subunits via two
3-allylic-type interactions.430 Complex 896 has been found to be an active catalyst for ethylene oligomerization.431 The UV-promoted decarbonylation of arene tricarbonyl chromium complexes in the presence of propargyl alcohol affords the cooresponding dicarbonyl chromium complexes 898 with a
2-coordinated propargyl alcohol ligand. Deprotonation of complexes 898 gives highly reactive
3-propargyl chromium species 899 which react with water to form complexes 897 (Scheme 142).424

Scheme 142

Ene-amides have been studied as ligands and substrates for oxidative coupling reactions. Treatment of CrCl2 with 2 equiv. of [(2,6-iPr2C6H3)(1-cyclohexenyl)N]Li affords a pseudosquare planar enamide complex {
3-C,C,N-[(2,6-iPr2C6H3)(1-cyclohexenyl) N]}2Cr (900).432

5.04.10


4-Pi-ligand chromium complexes

Photolysis of complex (C5H5)CrNO(CO)2 in the presence of butadiene or 2-methylbutadiene affords
4-s-trans-diene complexes 901 in low yields. In a similar way, the
4-diene complexes 902 can be prepared by photolysis of (C5Me5)CrNO(CO)2 in the presence of butadiene, 2-methylbutadiene or 2,3-dimethylbutadiene, respectively.409

164

Cyclic and Non-cyclic Pi Complexes of Chromium

As shown in Scheme 143, reaction of the divalent [(t-Bu)NP(Ph)2N(t-Bu)]CrCl2Li(THF)2 (903) with 1 equiv. of vinyl Grignard (CH2]CH)MgCl gives triangulo {p-[(t-Bu)NdP(Ph)2-N(t-Bu)]Cr}2(m,m0 ,
4,
40 -C4H4){s-[(t-Bu)NdP(Ph)2-N(t-Bu)]Cr} (904) containing a s-/p-bonded butadiene-diyl unit. The structure of complex 904 has been determined by X-ray crystallography and studied by DFT calculations.433

Scheme 143

Deprotonation of cis-{(t-Bu)N(H)P[m-N-(t-Bu)]2PN(H)(t-Bu)} with n-BuLi, followed by reaction with CrCl3(THF)3 affords the trivalent chromium complex (cis-{(t-Bu)NP-[m-N(t-Bu)]2PN(t-Bu)}[Li(THF)])CrCl2 (905). Subsequent reaction with 2 equiv. of vinyl Grignard CH2]CHMgCl gives the
4-butadiene chromium complex (cis-{(t-Bu)NP[m-N-(t-Bu)]2PN(t-Bu)}[Li(THF)])Cr (cis-
4-butadiene) (906) as shown in Scheme 144. DFT calculations on the electronic structure of 906 support the presence of

Scheme 144

Cyclic and Non-cyclic Pi Complexes of Chromium

165

Cr(I) in the complex. Treatment of 906 with PMe3 leads to a new
4-butadiene chromium complex [(t-Bu)NPN(t-Bu)] Cr(cis-
4-butadiene)PMe3 (907) which contains a regular monoanionic NPN ligand. Complexes 906 and 907 have been tested as single-component self-activating catalysts for selective ethylene dimerization and trimerization, respectively.434 Reactions of complex 908 with 1,3-butadiene or isoprene give complexes 909 and 910, respectively in good yields (Scheme 145). Structures of 909 and 910 have been characterized by X-ray crystallography.435

Scheme 145

The crystal structure of complex (
4-s-cis-1,3-butadiene)tetracarbonylchromium(0) (911) has been determined by X-ray crystallography. The complex displays a distorted octahedral coordination environment around the Cr0 atom from four carbonyl ligands and the two p-bonds of the s-cis-1,3-butadiene ligand.436 An intriguing butadiene/butadiene-diyl cluster, {[(
4-butadiene)Cr(m,
4-butadienediyl)-(m-NP)Mg]2(m-Cl)4Mg(THF)2} {[(THF)3Mg]2(m-Cl)3}2 (912) has been isolated and used as a highly selective self-activating ethylene trimerization catalyst.437

Reaction of K2COT (COT ¼ 1,3,5,7-cyclooctatetraene, C8H8) with the aryl chromium(II) halide [AriPr4Cr(m-Cl)]2 (AriPr4 ¼ 2,6(2 ,60 -iPr2C6H3)2C6H3) gives a COT bridged complex (CrAriPr4)2(m2-
3:
4-COT) (913) with a configuration previously unknown for chromium complexes of COT. The nonplanar COT ring is complexed between two CrAriPr4 moieties, as shown in Scheme 146.438 0

Scheme 146

166

Cyclic and Non-cyclic Pi Complexes of Chromium

The (
4-1,3-butadiene)tetracarbonylchromium complex (
4-C4H6)Cr(CO)4 has been synthesized and structurally characterized using X-ray crystallography.436 The structures and energetics of the butadiene chromium carbonyl complexes (C4H6)Cr (CO)m (m ¼ 2, 3, 4, 5) and (C4H6)2Cr2(CO)n (n ¼ 5, 6, 7) have been investigated via DFT calculations.439 A related carbonyl chromium complex [
4-(CH2)3C]Cr(CO)4 containing the umbrella-shaped trimethylenemethane ligand has been synthesized. The prospect of using this ligand in the complex [(CH2)3C]2Cr has been investigated by density functional theory.440 The effect of the umbrella-shaped trimethylmethane ligand on the structure and stability of dicarbonyl chromium complexes has also been studied by DFT calculations.441 Compared with cyclic p-conjugated hydrocarbons, the coordination chemistry of inorganic heterocycles has been less developed. Cyclic dicarbondiphosphides stabilized by N-heterocyclic carbenes containing a four-membered C2P2 ring with an aromatic 6p-electron configuration have been developed and employed as ligands for preparing Cr(CO)3 complexes (
4-L2C2P2)Cr(CO)3 914 and 915.442

5.04.11


7-Pi-ligand chromium complexes

A number of silatrochrocenophane complexes [Cr(
5-C5H4)(
7-C7H6)SiRR0 ] [R ¼ R0 ¼ Me (384); R ¼ Me, R0 ¼ iPr (385); R ¼ R0 ¼ iPr (386); R + R0 ¼ (CH2)3 (387)] and [Cr(
5-C5H4)(
7-C7H6)Si2Me4] (388), as well as a binuclear chromium-platinum complex [Cr(
5-C5H4)SiMe2Pt(PEt3)2(
7-C7H6)] (389) have been discussed in above sections.266 A number of 1,10 -bis (phosphanyl)trochrocene complexes, [Cr(
5-C5H4PR2)(
7-C7H6PR2)] [R ¼ Me (390), Cy (391), Ph (392)], and their heterobimetallic derivatives [Cr(
5-C5H4PR2)(
7-C7H6PR2)M(CO)4] [R ¼ Cy, M ¼ Cr (393), Mo (394), W (395); R ¼ Ph, M ¼ Cr (396), Mo (397), W (398)] have also been discussed in previous sections.267 Microwave spectra of the complex (
7-cycloheptatriene)Cr(
5-cyclopentadienyl) have been determined and the molecular structure of this complex has been studied by DFT calculations.443

5.04.12

Summaries and suggestions

In the past 15 years, the synthesis, characterization, reactions, and applications of the
6-arene tricarbonylchromium(0) complexes have been the major works of scientists in the area of
6-arene chromium chemistry due to the accessibility of complexes of this type, although some bis(
6-arene) and mono(
6-arene) sandwich chromium complexes, half-sandwich and multidecker
6-arene chromium complexes have also been studied. In the near future, the development tendency of the
6-arene chromium chemistry may not show any significant change. However, it would be good to try something new. Considering that the chemistry of chromium(II) and chromium (III)
6-arene complexes has not been extensively studied, investigations on the synthesis, properties and applications of new chromium(II) and chromium (III)
6-arene complexes, especially complexes bearing
6-arene ligands with chelating side arms, should be worthwhile. The
5-cyclopentadienyl carbonylchromium complexes, chromocene and mixed sandwich chromium complexes, half-sandwich cyclopentadienyl and functionalized cyclopentadienyl chromium complexes have also been intensively investigated during the past 15 years. Some of the functionalized cyclopentadienyl chromium complexes have been studied as catalysts or activators for olefin polymerization, N2 activation and other interesting organic transformations. In the next decade, the synthesis, properties and applications of new functionalized cyclopentadienyl chromium complexes would still be a major area for scientists working in the field of cyclopentadienyl chromium chemistry.

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Cyclic and Non-cyclic Pi Complexes of Chromium 148. 149. 150. 151. 152. 153. 154. 155. 156. 157. 158. 159. 160. 161. 162. 163. 164. 165. 166. 167. 168. 169. 170. 171. 172. 173. 174. 175. 176. 177. 178. 179. 180. 181. 182. 183. 184. 185. 186. 187. 188. 189. 190. 191. 192. 193. 194. 195. 196. 197. 198. 199. 200. 201. 202. 203. 204. 205. 206. 207. 208. 209. 210. 211. 212. 213. 214. 215. 216. 217.

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5.05

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

Wenguang Wanga,b, Xiaofang Zhaib, and Shu-Fen Houb, aCollege of Chemistry, Beijing Normal University, Beijing, China; bSchool of Chemistry and Chemical Engineering, Shandong University, Jinan, China © 2022 Elsevier Ltd. All rights reserved.

5.05.1 5.05.1.1 5.05.1.1.1 5.05.1.1.2 5.05.1.1.3 5.05.1.1.4 5.05.1.1.5 5.05.1.1.6 5.05.1.2 5.05.1.2.1 5.05.1.2.2 5.05.1.2.3 5.05.1.2.4 5.05.2 5.05.2.1 5.05.2.2 5.05.2.3 5.05.2.4 5.05.2.4.1 5.05.2.4.2 5.05.2.4.3 5.05.2.5 5.05.3 References

Cyclic p complexes of molybdenum Cyclopentadienyl molybdenum compounds Cyclopentadienyl molybdenum tricarbonyls Complexes containing group 13 ligands Complexes containing group 14 ligands Complexes containing group 15 ligands Complexes containing group 16 ligands Bis(cyclopentadienyl) molybdenum complexes Arene-containing complexes of molybdenum Molybdenum
2-arene complexes Molybdenum
4-arene complexes Molybdenum
6-arene complexes Molybdenum
7-arene complexes Non-cyclic p complexes of molybdenum Alkene complexes Alkyne complexes Allyl complexes Heteroatom-substituted p complexes Carboxylate complexes Amidinate complexes Thiocarboxylate complexes Trispyrazolylborate-based molybdenum p complexes Summary and outlook

174 174 175 177 180 191 209 216 223 223 224 225 228 229 230 234 235 238 238 240 243 244 246 246

As a sequel to COMC III (2007), this review of molybdenum p complexes is divided into two subchapters, cyclic p complexes and non-cyclic p complexes. Such material covers the literature from mid-2005 to the end of 2020. It is organized broadly based on the type of ligands. Cyclic p ligands are classified into cyclopentadienyl and modified cyclopentadienyl ligands, and arene ligands. Non-cyclic p ligands mainly include alkenyl, alkynyl, allylic ligands, and other
2 and
3 ligands such as carboxylate, amidinate, and thiocarboxylate.

5.05.1

Cyclic p complexes of molybdenum

5.05.1.1

Cyclopentadienyl molybdenum compounds

The chemistry of cyclopentadienyl molybdenum complexes has continued to thrive during the last decade. If any trend can be discerned, it is perhaps the increased emphasis on novel CpRMo complexes and their applications in small molecule activations. Corresponding to Chapter 7 in COMC II (1995) and Section 5.06.3.1.9, 5.07.8 in COMC III (2007), this subchapter outlines the molybdenum complexes featuring cyclopentadienyl (Cp) and modified cyclopentadienyl ligands, as well as indenyl ligands. The molybdenum cluster compounds are not included. Overall, the organization of this subchapter is based on the dominant ligands following groups 13–16. Throughout the chapter, the abbreviations Cp, CpMe, and Cp refer to
5-C5H5,
5-C5H4Me, and
5-C5Me5, respectively. CpR generally represents various modified cyclopentadienyl ligands, including substituted cyclopentadienyl rings, tethered cyclopentadienyl groups, bis(cyclopentadienyl) ligands, indenyl ligands, and other functionalized cyclopentadienyl ligands as shown below.

174

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00031-7

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

5.05.1.1.1

175

Cyclopentadienyl molybdenum tricarbonyls

Cyclopentadienyl ligands are among the most essential ligands in organometallic chemistry, and especially varying the substituents at the Cp ring can tailor the electronic properties of this CpR family, leading to diverse reactivity of such transition-metal complexes.1–3 This section mainly focuses on the synthesis and reactivity of CpRMo tricarbonyl compounds in the anionic [CpRMo(CO)3]− type, and molybdenum(II) complex in the form of CpRMo(CO)3X (X ¼ H, Cl, Br or I). Molybdenum complexes with tethered cyclopentadienyl ligands are separately discussed in Section 5.05.1.1.3. 5.05.1.1.1.1 [CpRMo(CO)3]n Synthesis and Properties: Mo(CO)6 is a well-known precursor to react with CpRNa salts leading to the formation of anionic compound [Mo(CO)3(
5-Cp)]−. The synthesis of this kind has been thoroughly discussed in Chapter 5.06 of COMC III. It should be noted that Mo(CO)3(CH3CN)3 derived from Mo(CO)6 is a potent precursor for neutral molybdenum(0) tricarbonyl compounds. By the reactions of Mo(CO)3(CH3CN)3 with phosphonium-substituted cyclopentadienyl ligands C5HPh3PR3,4–7 for example, a series of new hitherto molybdenum complexes (5-C5HPh3PR3)Mo(CO)3 1 (R3 ¼ Et3, Me2Ph, MePh2) have been synthesized (Eq. 1).8

ð1Þ

Source: Ning, G.-l.; Gong, W.-T.; Na, D.; Mehdi, H.; Ye, J.-W. J. Organomet. Chem. 2014, 772–773, 314–319. Due to the well-established chemistry of [Mo(CO)3(
5-Cp)]−, the zwitterionic species containing this fragment are continually reported (Scheme 1). Reaction of Mo(CO)6 with NaCpN followed by metathesis with PPNCl (PPNCl ¼ Bis (triphenylphosphoranylidene)ammonium chloride) affords complex [PPN][Mo(CO)3(
5-CpN)] (CpN ¼ (2-(dimethylamino) ethyl)cyclopentadiene) 2.9 Protonation of [Mo(CO)3(
5-CpN)]− with acetic acid proceeds at the N atom to provide the zwitterionic complex Mo(CO)3(
5-CpNH) 3. The intramolecular N ⋯ Mo separations in complex 3 are consistent with threecenter-four-electron NdH ⋯ Mo hydrogen bonding.10 Oxidation of in situ-generated Na[Mo(CO)3(
5-CpN)] by iodine, followed by methylation with CH3I provides [MoI(CO)3(
5-CpNMe)]I. Reducing the resultant iodide salt with sodium naphthalenide affords a zwitterion Mo(CO)3(
5-CpNMe) 4.11 Related to 3, increased steric hinders at the tethered counterion of 4 reduces the

Scheme 1 (A) Fischer, P. J.; Krohn, K. M.; Mwenda, E. T.; Young, V. G. Organometallics 2005, 24, 1776–1779. (B) Fischer, P. J.; Krohn, K. M.; Mwenda, E. T.; Young, V. G. Organometallics 2005, 24, 5116–5126. (C) Fischer, P. J.; Herm, Z. R.; Kucera, B. E. Organometallics 2007, 26, 4680–4683.

176

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

metal-based reactivity. For example, complex 3 reacts with diverse electrophiles such as HCl, Ph3PAuCl, CH3I, and I2 to provide the corresponding robust Mo(II) complexes. However, isolable products for 4 can only be obtained from the reactions with I2 and CH3I.

Mo(CO)6 and Mo(CO)3(CH3CN)3 are also applied as precursors in the preparation of dinuclear molybdenum carbonyl compounds, and the synthetic methods have been discussed in COMC II (1995) and COMC III (2007). In the last decade, new binuclear complexes 5–7 are derived from the reaction of Mo(CO)6 with aryl-substituted cyclopentadienes, and complex 8 is obtained from Mo(CO)3(CH3CN)3 with the corresponding cyclopentadienyl ligands.12–20 For unbridged bis(cyclopentadienyl) metal carbonyl dimers, the electronic properties of Cp substituents have significant influence on the bimetallic structures. The electron-withdrawing substituent (X ¼ Cl) at the para position of the phenyl ring is favorable to give ModMo triple bonded complexes 6, with the ModMo distance in the range of 2.5254–2.5376 A˚ .21 In contrast, the electron-donating substituent such as Me, and OMe favors to form a ModMo single bond. Wang’s group explored the reactivity of doubly bridged bis(cyclopentadienyl) complex 8 toward a series of classic substrates, such as nitrile, diazoalkane, Ph2E2 (E ¼ S, Se), carbon disulfide, carboxylate-substituted allenes and others.22–25 Upon oxidation by I2, the ModMo bond of complex 8 undergoes cleavage to give [(
5-C5H3)2(CMe2)(SiMe2)]Mo2(CO)6I2 9. In contrast, oxidative addition of the EdE bond of Ph2E2 to complex 8 under refluxing or photochemical condition leads to the triple-bonded compound 10, featuring a short Mo^Mo bond distance of 2.483(1) A˚ .26 (Eq. 2).

ð2Þ

Source: Zhu, B.-L.; Xu, S.-S.; Zhou, X.-Z.; Wang, B.-Q. Polyhedron 2012, 42, 57–65.

5.05.1.1.1.2 CpRMo(CO)3H Synthesis: The synthetic details of HMo(CO)3(5-C5H5) has been thoroughly reviewed in COMC II (1995)27 and COMC III (2007).28 Further research in this class of molybdenum hydride is concentrated on the Cp derivatives. Reaction of LiCpBz with Mo(CO)6 followed by quenching with HOAc gives the hydride complex 11 (Eq. 3).14,29,30 Oxidative addition of the substituted cyclopentadienes C5HR4SiMe2CH2CH]CH2 (R ¼ H, CH3) to precursor Mo(CO)3(NCMe)3 affords the hydride compound 12a (Eq. 4).31,32

ð3Þ

Source: Namorado, S.; Cui, J.; de Azevedo, C. G.; Lemos, M. A.; Duarte, M. T.; Ascenso, J. R.; Dias, A. R.; Martins, A. M. Eur. J. Inorg. Chem. 2007, 1103–1113.

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

177

ð4Þ

Source: Royo, E.; Acebron, S.; Gonzalez Mosquera, M. E.; Royo, P. Organometallics 2007, 26, 3831–3839. Properties and reactivity: Since the hydride ligand of CpRMo(CO)3H can be easily abstracted by trityl cation,33 reaction of 12a with [Ph3C][B(C6F5)4] gives the cationic species 13, in which coordination of the olefin pendant unit to molybdenum is rationalized through 1H and 13C NMR spectroscopic analysis. Besides, treatment of compound 12a with equimolar amount of trimethylamine N-oxide produces the bimetallic Mo(I) species 14, with the formation of H2O and elimination of NMe3 (Eq. 5).31

ð5Þ

Source: Royo, E.; Acebron, S.; Gonzalez Mosquera, M. E.; Royo, P. Organometallics 2007, 26, 3831–3839. CpMo(CO)3H can undergo hydrogen atom transfer (HAT) to a proper transition-metal acceptor, forming new metal hydrides. Hoff and coworkers reported that the reaction of CpMo(CO)3H with Mo(N[tBu]Ar)3 (Ar ¼ 3,5-C6H3Me2) produces HMo(N[tBu] Ar)3.34 Thermodynamic and kinetic studies on the HAT process were conducted. They later reported HAT reactions from CpMo(CO)3H to a radical
Sn(Si(tBu)2Me)3 to give HSn(Si(tBu)2Me)3.35 Tack’s group reported reaction of a silylene compound with CpMo(CO)3H leads to a cationic five-coordinate silicon complex.36 Breher’s group found CpMo(CO)3H reacts with [M0 {C(3,5-Me2pz)3}{N(SiMe3)2}] (M0 ¼ Mg, Zn, Cd) affording a series of trinuclear complexes.37 5.05.1.1.1.3 CpRMo(CO)3X The synthetic strategies of tricarbonyl halides CpRMo(CO)3X (X ¼ Cl, Br, I) through treatment of MoH(CO)3CpR with CX4 or the cleavage of [Mo(CO)3CpR]2 with halogens have been covered in COMC II (1995)27 and COMC III (2007).28 The new synthetic route to indenyl molybdenum(II) complex 15 is based on oxidative addition of 1-bromo-1H-indene to Mo(CO)3(CH3CN)3 (Eq. 6).38 Similarly, the reaction of the chiral menthyl-derived hydride complex [(–)-menthylCp]Mo(CO)3H with CCl4 gives the chiral chloride derivative,39 which was tested as catalyst for the epoxidation of alkenes in the presence of tert-butyl hydroperoxide (TBHP).40 Substituted tetramethylcyclopentadienyl dimolybdenum carbonyl complexes [(
5-C5Me4R)Mo(CO)3]2 (R ¼ allyl, nBu, t Bu, Ph, Bz) react with I2 to give the corresponding mononuclear substituted molybdenum carbonyl compounds 16 (Eq. 7). All of these mononuclear molybdenum carbonyl complexes 16 are catalytically active in Friedel-Crafts alkylation reactions.41,42

ð6Þ

Source: Honzícek, J.; Mukhopadhyay, A.; Romão, C. C. Inorg. Chim. Acta 2010, 363, 1601–1603.

ð7Þ

Source: Ma, Z.-H.; Lv, L.-Q.; Wang, H.; Han, Z.-G.; Zheng, X.-Z.; Lin, J. Transit. Metal Chem. 2016, 41, 225–233.

5.05.1.1.2

Complexes containing group 13 ligands

5.05.1.1.2.1 Boron-based complexes Over the past decade, transition-metal complexes of boron have been established as a novel class of compounds involving direct metal-boron interactions. According to metal-boron coordination modes, the boron-based molybdenum compounds described here are classified into borane, boryl and borylene complexes.43,44

178

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

Borane complexes can be considered as Lewis acid-base adducts of acidic boranes, BR3, with basic metal centers resulting in a fourfold coordination of the boron atom. Ghosh’s group has thoroughly studied molybdenum diborane compounds. Treatment of molybdenumborane complex (Cp Mo)2(m-Cl)2B2H645 with CO at room temperature leads to the formation of the highly fluxional species [Cp Mo(CO)2]2(m-
2:
2-B2H4) 17, which is the first example of a bimetallic diborane(4) conforming to a singly bridged Cs structure.46 When replacing two hydrogen atoms of the diborane(4) in 17 with W(CO)5
THF, diborene(2) species [Cp Mo (CO)2]2B2H2W(CO)4 18 is generated (Eq. 8). Complex 17 is the first structurally characterized B2H4 complex. The solid structure shows that the BdB bond distance of 1.700 (12) A˚ is significantly shorter than that in some base-stabilized diborane(4) complexes.47–51 The 1H NMR spectrum of 17 at −40  C shows a peak at −11.3 ppm that may be due to a ModHdB hydrogen atom. Furthermore, two broad resonances appear at −0.86 and 3.28 ppm that may be due to the presence of BdHdB and BHt hydrogen atoms, respectively.

ð8Þ

Source: Mondal, B.; Bag, R.; Ghorai, S.; Bakthavachalam, K.; Jemmis, E. D.; Ghosh, S. Angew. Chem. Int. Ed. Engl. 2018, 57(27), 8079–8083. Boryl complexes with a three-coordinate boron is obtained by linking a -BR2 group to a metal center. Among transition metal complexes of boron, the group of boryl compounds LxM-BR2 (II) is the largest one and has been well reviewed.43 The development of molybdenum boryl complexes in recent years is discussed here. Photoreaction of diaminosubstituted-phosphiteborane, BH3P(NMeCH2)2(OMe) with a methyl molybdenum complex, CpRMo(CO)3Me (R ¼ Me5, Me4H, H5) yields a phosphiteboryl molybdenum complex 19, CpRMo(CO)3BH2[P(NMeCH2)2 (OMe)].52,53 The ModB bond in 19 can be activated by the reaction with MeI to give Cp Mo(CO)3Me and BH2I
P (NMeCH2)2(OMe). In the reaction with an equimolar amount of PMe3, the BdP bond of 19 is cleaved affording phosphiteboryl molybdenum complex Cp Mo(CO)3(BH2PMe3) 20 and free P(NMeCH2)2(OMe). In solution, the pentamethylcyclopentadienyl molybdenum complex 19 (R ¼ Me5) can gradually convert to a hydrido phosphite complex Cp MoH(CO)3[P(NMeCH2)2(OMe)] 21 (Scheme 2). The reaction mechanism is not clear, but the BdH bond in 19 is cleaved and this type of reaction has not been reported for the corresponding trimethylphosphineboryl complex.

Scheme 2 (A) Nakazawa, H.; Ohba, M.; Itazaki, M., Organometallics 2006, 25(12), 2903–2905. (B) Nakazawa, H.; Itazaki, M.; Ohba, M. J. Organomet. Chem. 2007, 692(1–3), 201–207.

The solid structures show that the BdP bond distances in 19 (1.903(4) A˚ : R ¼ Me5, 1.908(3) A˚ : R ¼ Me4H) are obviously shorter than that in 20 (1.949(5) A˚ ). Complex 21 takes a typical four-legged piano-stool geometry. The hydride is located at the cis position to the phosphite ligand. In 1H NMR spectrum, the hydride resonance appears at −5.81 ppm with 2JHP ¼ 62.7 Hz, which is in the range of the reported 2JHP values for cis HdMo(Cp)dP complexes.54

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

179

Borylene transfer from the Group VI borylene complexes M(BX)(CO)5 (M ¼ Cr, Mo, W) to metal-carbonyl complexes has been proved to be a very practical strategy for the synthesis of both borylene and metalloborylene (borido) complexes.55–57 Braunschweig’s group reported the borylene ligand transfer from (OC)5Mo[BN(SiMe3)2] to molybdenum hydrido species CpMo(CO)3H gives the hydridoborylene complex [CpMo(CO)2](m-H)[BN(SiMe3)2] 22 (Eq. 9).58 Analogous reaction of molybdenum hydrido complex CpMo(CO)3H with [Cp (OC)2Fe](m-B)[Cr(CO)5] affords the dinuclear species [CpMo(CO)2](m-H)[BFeCp (OC)2] 23 (Eq. 10). In the 1H NMR spectra, the broad signals in the hydride region indicate the presence of hydrido ligand in complex 22 (−10.3 ppm) and 23 (−7.34 ppm). The solid-state structures show that they are definitely molybdenum borylene complexes. The ModBdN axes remain effectively linear and the ModB bond distances are 2.020(3) A˚ and 2.028(4) A˚ , respectively.

ð9Þ

Source: Bauer, J.; Bertsch, S.; Braunschweig, H.; Dewhurst, R. D.; Ferkinghoff, K.; Horl, C.; Kraft, K.; Radacki, K. Chemistry 2013, 19(51), 17608–17612.

ð10Þ

Source: Bauer, J.; Bertsch, S.; Braunschweig, H.; Dewhurst, R. D.; Ferkinghoff, K.; Horl, C.; Kraft, K.; Radacki, K. Chemistry 10013, 19(51), 17608–17610. The synthetic method of complex arachno-2-[CpMo(
5:
1-C5H4)B4H7] 24 from Cp2MoH2 has been well established in the previous volumes of COMC II (1995).59 The reactivity of arachno-2-[CpMo(
5:
1-C5H4)B4H7] 24 has been studied by Kelland’s group in recent years (Scheme 3). The molybdaborane 24 reacts with W(PMe3)3H6, giving the novel tungstaborane

Scheme 3 (A) Green, M. L. H.; Leach, J. B.; Kelland, M. A. J. Organomet. Chem. 2005, 690(18), 4203–4205. (B) Green, M. L. H.; Leach, J. B.; Kelland, M. A. Inorg. Chim. Acta 2008, 361(2), 495–501. (C) Green, M. L. H.; Leach, J. B.; Kelland, M. A. J. Organomet. Chem. 2006, 691(6), 1295–1297.

180

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

nido-2-W(PMe3)3H2B4H7{CpMo(
5:
1-C5H4)H2} 25.60 This is a rare example of metal fragment exchange within a metalla-borane cage. The molybdenum atom is retained in the molecule via a s-bond between the substituted cyclopentadienyl ring and a basal boron atom in the metallaborane cluster. The reaction between complex 24 and PEt3 produces a mixture of two products Cp(
5:
1-C5H4)-arachno-2-MoHB4H4PEt3 26 and Cp(
5:
1-C5H4)-arachno-1-MoHB3H3PEt3 27.61 Treatment of complex 24 with NEt3 gives the molybdacarbaborane nido-1-[CpMo(
3:
2-C3H3)C2B3H5] 28. The 1H{11B} NMR spectrum shows two resonances at −2.59 ppm and 3.70 ppm, which are assigned to the two equivalent BdHdB bridging hydrogen atoms and the terminal BdH hydrogen atoms, respectively.62 5.05.1.1.2.2 Gallium-based complexes The coordination chemistry of gallium(I) heterocycles, GaR (R ¼ bulky aryl, alkyl, C5Me5−, etc.), has been extensively developed in recent years.63 These compounds have been employed as metal donor Lewis bases in the formation of complexes with transition metal elements including molybdenum. For example, Jones and coworkers reported that the treatment of molybdenum carbonyl complex [CpMo(CO)2]2 with four-membered gallium(I) heterocycle [:Ga(Giso)] (Giso ¼ [{N(Ar)}2CN(C6H11)2]−) affords a hydrolysis product CpMo(CO)2[{(Giso)Ga}2OH] (29), which arises from the presence of adventitious water in the reaction mixture.64 Afterward, Fedushkin and coworkers developed a new redox-active ligand-heterocyclic gallium carbenoid 1,2-bis [(2,6-diisopropylphenyl)imino]acenaphthene (dpp-bian) and it reacts with [CpMo(CO)3]2 to produce the diamagnetic complex (dpp-bian)GadMo(CO)3Cp 30 (Eq. 11). The complex 30 adopts a four-leg piano stool geometry and can be viewed as a molybdenum gally with a GadMo covalent bond.65

ð11Þ

Source: (A) Jones, C.; Stasch, A.; Moxey, G. J.; Junk, P. C.; Deacon, G. B. Eur. J. Inorg. Chem. 2009, 2009(24), 3593–3599. (B) Fedushkin, I. L.; Sokolov, V. G.; Piskunov, A. V.; Makarov, V. M.; Baranov, E. V.; Abakumov, G. A. Chem. Commun. 2014, 50(70), 10108–10111.

5.05.1.1.3

Complexes containing group 14 ligands

5.05.1.1.3.1 Carbon-based complexes 5.05.1.1.3.1.1 Alkyl-containing complexes In day to day organic synthesis, particularly from the application point of view, the transition-metal alkyls are often perceived as a source of stabilized carbanions for reactions with various electrophiles. The behavior of alkyl ligands usually depends on the metal center and the ligand’s substituents. In recent years, new molybdenum complexes of the type CpMo(CO)3X containing ligands where X ¼ CH2COOH (31), CH2-p-C6H4-COOMe (32), CH2COR1 (33, R1 ¼ OEt, menthyl, bornyl), CHR2COOEt (34, R2 ¼ Me, pH) have been synthesized by treatment of [CpMo(CO)3]+ with different alkyl halides (31 −33) and alkylmesonic acid 34,66–69 according to a general method for the synthesis of molybdenum alkyl species (COMC III 2007). 1 H NMR spectra of compounds 31–34 usually show the ModCH proton signals in the range of 1.80–2.90 ppm, but for CpMo(CO)3{CH(Ph)CO2Et} the ModCH resonance is highly deshielded appearing downfield at 4.15 ppm. The ModC bond distances lie in the range of 2.325(2)–2.377(2) A˚ . These compounds are active catalyst for achiral and chiral epoxidation of unfunctionalized olefins.

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

181

The above strategy was also applied to the synthesis of modified cyclopentadiene molybdenum methyl complexes CpRMo(CO)3(CH3). Reactions of various alkali salts LiCpR (CpR ¼ 1,2,4-triphenylcyclopenta-1,3-diene, 2,5-dimethyl-4H-cyclopenta [b]thiophene, (N-benzyloxycarbonylprolyl) cyclopentadienyl) or NaCpR (R ¼ CHO, COCH3, CO2CH3) with Mo(CO)6 followed by treatment of MeI gives complexes 35–37.70–73 By condensation with corresponding primary amine in the presence of the Lewis acid BF3, complexes 37a and 37b convert to Schiff’s bases compounds 38 and 39.74–76

The reaction of [CpMo(CO)3]+ with Br(CH2)5CH3 in the presence of PPh3 produces CpMo(CO)2(PPh3)CO(CH2)5CH3 40 (Eq. 12).77 Molybdenum alkyl complexes can also be obtained through the insertion of alkenes into the ModH bond of CpMo(CO)3H complex.78 For example, in the presence of 5 mol% of the zerovalent palladium complex Pd(PPh3)4, the instant insertion of an olefin into CpMo(CO)3H affords alkylmolybdenum complexes 41 in excellent yields at room temperature (Eq. 13).79

ð12Þ

Source: Murshid, N.; Rahman, M. A.; Wang, X. J. Organomet. Chem. 2016, 819, 109–114.

ð13Þ

Source: Kuramoto, A.; Nakanishi, K.; Kawabata, T.; Komine, N.; Hirano, M.; Komiya, S. Organometallics 2006, 25(2), 311–314. Legzdin’s group has thoroughly studied molybdenum compounds of Cp Mo(NO)(CH2CMe3)2 and Cp Mo(NO) (]CHCMe3).80,81 The synthetic method has been reviewed in detail in COMC III (2007).82 These compounds react with cycloalkane through intramolecular CdH activation.83 For instance, dissolution of Cp Mo(NO)(CH2CMe3)2 in cyclopentene at 20  C for 18 h results in the formation of complex 42 (n ¼ 1). When cyclooctene is used, metallacycle complex 43 (n ¼ 4) is obtained. Heating 43 (n ¼ 4) in cyclohexane at 50  C gives complex 44 (n ¼ 4) which can be isolated by column chromatography (Eq. 14).

ð14Þ

Source: Graham, P. M.; Buschhaus, M. S. A.; Legzdins, P. J. Am. Chem. Soc. 2006, 128(28), 9038–9039.

182

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

The chemistry of
5,
1-coordinated cyclopentadienyl molybdenum complexes featuring s-metal–carbon ansa bridges has been well reviewed by Kühn and coworkers.84–87 An improved synthetic strategy to the spiro-bicycles ansa complexes 45 is oxidative addition of spiro cyclopentadienes to Mo(CO)3(tach) precursor (tach ¼ 1,3,5-trimethylhexahydro-1,3,5-triazine) (Eq. 15).88,89

ð15Þ

Source: Kühn, F. E.; Capapé, A.; Raith, A.; Herdtweck, E.; Cokoja, M. Adv. Synth. Catal. 2010, 352, 547–556. 5.05.1.1.3.1.2 N-Heterocyclic carbene-containing complexes N-Heterocyclic carbenes (NHCs) have been widely used as phosphine alternatives, and they usually serve as spectator in transition metal-based catalysis. One of the advantages of NHC ligands over phosphines is their higher oxidation resistance. The chemistry of molybdenum-NHCs complexes have been well-reviewed in COMC III (2007), and the progress made in last decade is summarized in this section. Through a transmetalation route,90 a series of molybdenum-NHC complexes CpMo(CO)2(NHC)X 46 have been synthesized by heating CpMo(CO)3Br with silver carbene in toluene (Eq. 16). These compounds show good stability in both the solid state and solution at room temperature. The structure of CpMo(CO)2(NHC)X adopts characteristic four-legged piano stool geometry, and the two CO groups are cis to each other. The ModCcarbene bond lengths in 46 and 47 are in the range of 2.22–2.25 A˚ , which is longer than those reported with similar structures,91,92 such as CpMo(CO)2(IMes)H (2.187(8) A˚ ) and (Cp -NHC)Mo(CO)2I (Cp -NHC ¼
5-C4Me4-CH2-CHPh-NHCMe) (2.207(3) A˚ ). Metathesis of CpMo(CO)2(MesNHC)Br with AgBF4 in MeCN provides a highly efficient catalyst 47 (Eq. 17) for selective olefin epoxidation (TOFs  3400 h−1).93

ð16Þ

Source: Li, S.; Kee, C. W.; Huang, K.-W.; Hor, T. S. A.; Zhao, J. Organometallics 2010, 29(8), 1924–1933.

ð17Þ

Source: Li, S.; Kee, C. W.; Huang, K. W.; Hor, T. A.; Zhao, J. Organometallics 2010, 29, 1924–1933. Andy Hor and coworkers reported that transmetalation of Ag(I)-NHC complexes with CpMo(CO)3Br in toluene and subsequent anion exchange by AgX (X ¼ BF4−, PF6−, OTf −) afforded the CpMo(II) dicarbonyl cationic complex [CpMo(CO)2(k2-N,C-NHC)][X] 48 (Eq. 18).94 Due to the chelation effect of the hybrid NHC ligand, and ModCcarbene bond lengths are in the range of 2.143 (2)–2.166(3) A˚ , which are slightly shorter than the normal molybdenum(II) NHC complexes CpMo(CO)2(NHC)Br (NHC ¼ IMe or IBz).

ð18Þ

Source: Wang, Z.; Ng, S. W. B.; Jiang, L.; Leong, W. J.; Zhao, J.; Hor, T. S. A. Organometallics 2014, 33(10), 2457–2466. Due to the unique 1,2,4-N-substitution pattern, 1,2,3-triazolylidenes are stronger sigma donors than the common NHC ligands.95 Molybdenum complex 49 is synthesized in good yield by using 1,2,3-triazolylidene silver compound as carbene transfer agent to react with CpMo(CO)3Cl (Eq. 19). The ModCcarbene bond length of 2.221(4) A˚ in complex 49 is very close to that of 46 (2.22–2.25 A˚ ).

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

183

ð19Þ

Source: Schaper, L.-A.; Graser, L.; Wei, X.; Zhong, R.; Oefele, K.; Poethig, A.; Cokoja, M.; Bechlars, B.; Herrmann, W. A.; Kuehn, F. E. Inorg. Chem. 2013, 52(10), 6142–6152. 5.05.1.1.3.1.3 Alkenyl-containing complexes Half-sandwich molybdenum complexes containing
3-ligands have been reviewed in COMC I (1982)96, COMC II (1995)97 and COMC III (2007).98 This section focuses on the synthesis and properties of CpR molybdenum complexes with alkenyl ligands. 5.05.1.1.3.1.3.1 [CpRMo(CO)2(h3-allyl)] complexes Synthesis: The synthesis, properties and reactivities of complexes [CpRMo (CO)2(
3-allyl)] have been investigated extensively. Herein, some new [CpRMo(CO)2(
3-allyl)] structures will be emphasized. A series of
3-allyl [CpRMo(CO)2(
3-C3H5)] complexes 50–53 are prepared from the reaction of [MoX(CO)2(NCMe)2(
3-C3H5)] (X ¼ Br, Cl) with LiCpR or NaCpR through the replacement of acetonitrile and chlorido ligands by the cyclopentadienyl ligand (Eq. 20).99–114

ð20Þ

Source: (A) Honzicek, J.; Mukhopadhyay, A.; Bonifacio, C.; Romao, C. C. J. Organomet. Chem. 2010, 695(5), 680–686. (B) Honzícková, I.; Vinklárek, J.; Romão, C. C.; Ru˚ žicková, Z.; Honzícek, J. New J. Chem. 2016, 40(1), 245–256. (C) Pammer, F.; Sun, Y.; Thiel, W. R. Organometallics 2008, 27(5), 1015–1018. (D) Ryan, D. E.; Cardin, D. J.; Hartl, F. Coord. Chem. Rev. 2017, 335, 103–149.

The allylic compound 54 is prepared via the reaction of allylic benzoates CH3CH(OBz)CH]CHR (R ¼ iPr, COOEt) and Mo(CO)4(THF)2 or Mo(CO)6 followed by salt metathesis with LiCp. Treatment of 54 with [NO][BF4] affords a cationic complex 55 (Eq. 21).115,116

ð21Þ

Source: (A) Kocienski, P. J.; Christopher, J. A.; Bell,R.; Otto,B. Synthesis 2005, 75–84 (B) Cooksey, J. P. Org. Biomol. Chem. 2013, 11, 5117–5126. Properties: [CpRMo(CO)2(
3-allyl)] compounds have been applied in many aspects, such as in alcoholysis of styrene oxide with ethanol.117–122 Furthermore, the common feature of
3-allyl in these complexes, exo-endo isomerization123, has been described in detail in COMC I (1982)96, COMC II (1995)97 and COMC III (2007)98. In most [CpRMo(CO)2(
3-allyl)] complexes, the Mo–Cg (allyl) distances (Cg: the centroid of the allyl moiety) are in the range of 1.99–2.06 A˚ . The allyl ligands in these Mo(II)-allyl compounds allow for protonation to generate the propene moiety, which is labile and readily replaced by coordinating solvent

184

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

molecules. For example, protonating CpMo(CO)2(
3-C3H5) with HBF4 in the mixture of CH2Cl2 and MeCN gives CpRMo (CO)2(NCMe)2 (Eq. 22).124–127

ð22Þ

Source: Pereira, C. C. L.; Braga, S. S.; Paz, F. A. A.; Pillinger, M.; Klinowski, J.; Gonçalves, I. S. Eur. J. Inorg. Chem. 2006, 21, 4278–4288. 5.05.1.1.3.1.3.2 h3-Indenyl complexes Synthetic routes to the complexes of [CpRMo(CO)2(
3-Ind)] type have been mentioned in COMC III (2007).98 Honzícek’s group reported compounds {(
5-Ind0 )Mo(CO)2(m-Cl)}2 56 are convenient starting materials to prepare (
5-Cp0 )Mo(CO)2(
3-Ind0 ) 57 through the reaction with appropriate cyclopentadienides. The incorporation of the Cp0 M causes a haptotropic shift of the
5-indenyl ligand to a
3-indenyl complex (Eq. 23). Compounds 57 (except 57b) exhibit the proton resonance in the 2-position of the indenyl at very low field (d  6.6) in 1H NMR spectrum, which is typically observed for
3-indenyl compounds.128,129

ð23Þ

Source: Honzicek, J.; Vinklarek, J.; Erben, M.; Lodinsky, J.; Dostal, L.; Padelkova, Z. Organometallics 2013, 32(12), 3502–3511. 5.05.1.1.3.1.3.3 h3-Benzyl complexes Several synthetic methods for
3-benzyl complexes have been reviewed in COMC I (1982)96 and COMC II (1995)97. This section introduces a new method of preparing
3-benzyl molybdenum complex Cp Mo (CO)2(
3-CH2C6H5) 58 by treatment of Cp Mo(CO)3Cl with NaBt2 (Bt2 ¼ dihydrobis(2-mercapto-benzothiazolyl)borate) in toluene. The reaction involves a selective binding of toluene through CdH activation followed by orthometalation (Eq. 24).130 In the solid-state structure of 58, the distances of three ModCtoluene bonds are 2.262(4), 2.367(4) and 2.449(4) A˚ . In addition, the bond lengths of C1dC2 and C2dC3 are 1.424(6), 1.400(7) A˚ which are almost identical, indicating that a benzylic group is coordinated to molybdenum in a
3-fashion. The benzylic group acts as a formal allyl-like three-electron donor allowing the metal center to satisfy the 18-electron rule.

ð24Þ

Source: Ramalakshmi, R.; Maheswari, K.; Sharmila, D.; Paul, A.; Roisnel, T.; Halet, J. F.; Ghosh, S. Dalton Trans. 2016, 45 (41), 16317–16324. 5.05.1.1.3.1.3.4 Heteroatom-substituted h3-ligands Novel
3-silapropargyl/alkynylsilyl 59131,132 and
3-silaallyl/alkenylsilyl Mo complexes 60133 are reported by Sakaba and co-workers via the reactions of Cp Mo(CO)2MeL (L ¼ MeCN, py) with HPh2SiC^CR and HPh2SiCH]CMe2, respectively (Eq. 25). According to the crystallographic analysis, the SidC bond length in 60 (1.817(3) A˚ ) lies between SidC single (1.86–1.91 A˚ ) and double bond (1.70–1.76 A˚ ) lengths, close to those of 59a (1.806 (3) A˚ ) and 59b (1.795(3) A˚ ). The C − C bond in 60 (1.414(3) A˚ ) is intermediate between CdC single and double bonds but longer than that of 59a (1.251(4) A˚ ) and 59b (1.254(4) A˚ ), which is intermediate between CdC double and triple bonds. The aforementioned SidC and CdC lengths in 59 are very close to the calculated SidC and CdC lengths in Cp(CO)2Mo(
3-H2SiCCtBu) (1.814 and 1.263 A˚ ).134

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

185

ð25Þ

Source: (A) Yabe-Yoshida, M.; Kabuto, C.; Kabuto, K.; Kwon, E.; Sakaba, H. J. Am. Chem. Soc. 2009, 131(26), 9138–9139. (B) Sakaba, H.; Tonosaki, H.; Isozaki, K.; Kwon, E. Organometallics 2015, 34(6), 1029–1037. Complexes 59 shows intriguing reactivity toward methanol to give the four-membered cyclic complex Cp Mo(CO)2{k2-C, OdC(]CHtBu)SiPh2OMe} 61 or the silyl-substituted
3-allyl complex Cp Mo(CO)2{
3-(MeOPh2Si)HCCHCMe2} 62 depending on the substituent R (Eq. 26). The reaction of 60 with primary amines RNH2 (R ¼ tBu, iPr, Et) is examined in detail, revealing unique transformation to ModNdSi three-membered cyclic complexes Cp Mo(CO)2(k2-N,Si-RHNSiPh2) 64 with elimination of isobutene via ModNdSidC four-membered metallacycles Cp Mo(CO)2(k2-N,C-RHNSiPh2CHiPr) 63 intermediates (Eq. 27).

ð26Þ

Source: Yabe-Yoshida, M.; Kabuto, C.; Kabuto, K.; Kwon, E.; Sakaba, H. J. Am. Chem. Soc. 2009, 131(26), 9138–9139.

ð27Þ

Source: Sakaba, H.; Tonosaki, H.; Isozaki, K.; Kwon, E. Organometallics 2015, 34(6), 1029–1037. 5.05.1.1.3.1.3.5 h4-diene ligands The synthesis and properties of CpRMo complexes bearing
4-diene ligands have been discussed in COMC III (2007).135 Reaction of [CpMo(CO)2(CH3CN)2]+ with spiro[2.4]hepta-4,6-diene or spiro[4.4]nona1,3-diene gives the
4-diene cyclopentadiene complexes 65 or 66, respectively (Eq. 28).136,137 This strategy can be also applied to the synthesis of a
4-diene indenyl complex [Mo{
4-C5H4(CH2)4}(CO)2(Ind)]+ by reaction of [Mo(CO)2(CH3CN)2(Ind)]+ with spiro[4.4]nona-1,3-diene.

ð28Þ

Source: Honzícek, J.; Almeida Paz, F. A.; Romão, C. C. Eur. J. Inorg. Chem. 2007, 18, 2827–2838.

186

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

5.05.1.1.3.1.4 Alkynyl-containing complexes 5.05.1.1.3.1.4.1 Carbon-based h1-alkyne ligands Synthesis: Transition metal alkynyl complexes (MdC^CR) are versatile and robust synthons for many applications.138–140 The synthetic methods for CpRMo complexes containing alkynyl ligands have been reviewed in the previous volume of COMC II (1995).141 Many reported metal-alkynyl complexes are obtained from the reaction of CpMo(CO)3Cl with HC^CR in NEt3 in the presence of a catalytic amount of CuI. Such a method is also applicable for 1-alkynes containing propargyl alcohol or ether substituents (HC^CCH2OR) to synthesize the corresponding molybdenum alkynyl complexes of CpMo(CO)3(C^CCR1R2OH) 67–71 and CpMo(CO)3(C^CCH2OR) 72 (R ¼ OAc, CH2Ph, salicylate, aspirin, fructopyranose).142 A variety of substituted alkyne ligands have been incorporated into the dinuclear molybdenum carbonyl compounds {CpMo (CO)2}2(m-
2:
2-RC^CR1) by a general reaction of [CpMo(CO)3]2 with various alkynes. In particular, refluxing the mixture of N3P3F5(C^CR) or P3N3F5(C^CdC^CdR) (R ¼ Ph, Fe(C5H5)2) with [CpMo(CO)3]2 in toluene yields the first examples of phosphazene-derived molybdenum carbonyl clusters 73 and 74.143 The simplest possible fluorocarbon molecule, CF, unlike its isoelectronic analog NO, is not an isolable monomeric species. Trifluoromethylmolybdenum complex Cp Mo(CO)3(CF3) is well-known since reported in 1991.144 When it is reduced by potassium graphite or magnesium graphite, the terminal fluoromethylidyne complex Cp Mo(CO)3(^CF) 75 is produced in 80–90% yield.145 Complex 53 is the first reported molybdenum complex containing a terminally bound CF ligand (Eq. 29). The crystallographic analysis provides the ModC bond length of 1.838 A˚ and CdF bond length of 1.290 A˚ . This compound can react with Co2(CO)8 leading to the formation of the heteronuclear ModCodCo carbonyl cluster 76.146

ð29Þ

Source: Huang, H.; Hughes, R. P.; Rheingold, A. L. Organometallics 2010, 29(8), 1948–1955. Properties: In the IR spectra of complexes 67–72, the nC^C absorption bands are usually displayed in the range of 2010–2114 cm−1. The 13C{1H} NMR spectrum displays signals for the a-carbon of MdC^CR in the field of d 121–143, while the b-carbon signals usually fall in d 75.9–84.1. The alkynyl complex CpMo(CO)3{C^CCH2(OH)} 67 reacts fast with the secondary amine in the presence of CuI to give substituted allyl molybdenum complex (Eq. 30).

ð30Þ

Source: Zhang, W. Q.; Atkin, A. J.; Fairlamb, I. J.; Whitwood, A. C.; Lynam, J. M. Organometallics 2011, 30, 4643–4654. In the dinuclear molybdenum carbonyl compounds 73 and 74, two CO groups are semi-bridging the Mo(II)dMo(II) centers, as evidenced by IR spectra. The 31P NMR spectra give rise to doublet multiplet signals in the range of 51.36–53.35 ppm for the PF(C) moiety and triplet multiplet peaks in the range of 7.67–8.26 ppm for the PF2 moiety. Complex 75 shows two strong CO absorbance (2002, 1930 cm−1) in the IR spectrum, and the 13C NMR spectrum displays a doublet signal for the CF at d 219.12 with a very large coupling constant to fluorine (1JCF ¼ 556 Hz).

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

187

5.05.1.1.3.1.4.2 Carbon-based h2-alkyne ligands Biscyclopentadienone molybdenum(0) compound Mo(CO)2(
4-C4Ph4CO)2 77147 is a suitable precursor for preparing CpRMo(
2-alkyne) complexes. Heating a toluene solution of 77 with excess MeO2CC^CCO2Me provides molybdenum(II) complex [Mo(CO)(MeO2CC^CCO2Me)(
5,s-C4Ph4COMeO2CC]CCO2Me)] 78, in which two alkyne molecules have been incorporated: one is linked to the carbonyl group of the tetracyclone ligand, whereas the other is p-bound to the metal as a four-electron donor. Oxidation of this compound affords Mo(IV)-oxo compound Mo(O)(MeO2CC^CCO2Me)(
5,sC4Ph4COMeO2CC]CCO2Me) 79.148 When 77 is treated with aryl terminal alkynes, Mo(0) compound Mo(CO)(
4-C4Ph4CO) (
6-2,3,6-C4H2R2C]CHR) (R ¼ Ph, p-tol) 80 is produced, in which [2 + 2 + 1] cyclotrimerization of three alkyne molecules forms a fulvene ligand (Eq. 31).149,150

ð31Þ

Source: (A) Adams, H.; Booth, Y. K.; Cook, E. S.; Riley, S.; Morris, M. J. Organometallics 2017, 36(11), 2254–2261. (B) Adams, H.; Brown, P.; Cook, E. S.; Hanson, R. J.; Morris, M. J. Organometallics 2012, 31(21), 7622–7624. It is well established that in Mo(II) complexes the typical 13C chemical shifts of alkyne ligands are approximately at 200 ppm for 4-electron-donor alkynes and 115 ppm for 2-electron donors. In structure 78, both the C^C bond length of 1.312(6) A˚ and the ModC bond lengths (2.031(4) and 2.059(4) A˚ ) are commensurate with this ligand acting as a 4-electron donor, as required by electron-counting considerations. The length of Mo]O bond (1.693(5) A˚ ) confirms that complex 79 is a typical Mo(IV) oxo complex. The
6-fulvene fragment in complex 80 shows the five-membered ring being essentially planar. The bonds within the five-membered ring do not show a clear division between single and double bonds (CdCavg 1.432 A˚ ) and the exocyclic C]C bond has lengthened to 1.432(7) A˚ from a typical value of 1.345 A˚ in free fulvenes. The tetracyclone ligand is
4 bonded to the metal, with ModC bond lengths between 2.276(5) and 2.376(5) A˚ . 5.05.1.1.3.2 Silicon-based complexes 5.05.1.1.3.2.1 Silylidyne complexes Compared to metal alkylidyne complexes, silylidyne complexes are much less explored due to their synthetic difficulty. Schnakenburg and coworkers found N-heterocyclic carbene can stabilize aryl silicon(II) chlorides,151 which is demonstrated by the isolation of the first molybdenum complex featuring a Mo^Si triple bond 82.152,153

ð32Þ

Source: Filippou, A. C.; Chernov, O.; Stumpf, K. W.; Schnakenburg, G. Angew. Chem. Int. Ed. 2010, 49(19), 3296–3300. The reaction of SiRCl(Im-Me4) (R ¼ Me4C6H3-2,6-Trip2, Im-Me4 ¼ tetramethylimidazol-2-ylidene) with Li[CpMo(CO)3] causes an instant color change from yellow to brown, resulting in the formation of CpMo(CO)2{]Si(Me4C6H3-2,6-Trip2)Cl(Im-Me4)} 81 (Eq. 32). Molybdenum silylidene complex 81 is fully characterized and its molecular structure has been established. Reflux of 81 with B(p-Tol)3 in o-xylen gives molybdenum silylidyne complex 82 in good yield. The triarylborane B(C6H4-4-Me)3 can serve as a Lewis acid to capture Im-Me4 and form the carbeneborane adduct Im-Me4
B(C6H4-4-Me)3. The ModSi double bond in 81 is 2.345 A˚ , which falls in the range of 2.288(2)–2.3872(7) A˚ reported for molybdenum arylsilylidene complexes.154–156 The IR spectrum shows that the two nCO bands of 81 appear at much lower wavenumbers (1859 and 1785 cm−1) compared to 1937 and 1875 cm−1 for 82, which indicates that the silylidene ligand is a much weaker p-acceptor ligand than the silylidyne ligand. 5.05.1.1.3.2.2 Silylidene complexes The molybdenum silylidyne complexes 82 reacts smoothly with various anionic nucleophiles to produce unprecedented anionic silylidene complexes on account of the electrophilic silicon center (Scheme 4).157,158 For example, the reaction with (NMe4)Cl affords the bright orange chlorosilylidene salt 83. With (NMe4)N3, the reaction gives azidosilylidene complex 84 almost in quantitative yield. The reaction of 82 with one equivalent LiMe at low temperature produces methylsilylidene complex 85, whereas the silyl complex 86 is produced when two equivalent LiMe is used. Notably, an extremely rare dianionic silyl complex 87 is obtained through the reaction with potassium graphite.

188

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

Scheme 4

Filippou, A. C.; Chernov, O.; Schnakenburg, G. Angew. Chem. Int. Ed. 2011, 50(5), 1122–1126.

The ModSi bond distances of 2.300(1) A˚ in 83 and 2.287(1) A˚ 84 consists with the values reported for molybdenum arylsilylidene complexes (in the range of 2.288(2)–2.3872(7) A˚ ). The ModCO bond lengths (1.971(3) A˚ , 82; 1.915(15) A˚ , 83; 1.926(4) A˚ , 84) are slightly shorter than 2.063(3) A˚ reported for Mo(CO)6, indicating a stronger metal-carbonyl back bonding. The strong ModCO interactions are also reflected by the low nCO wavenumbers (83, 1840 and 1762 cm−1; 84, 1826 and 1756 cm−1; 85, 1824 and 1695 cm−1). Two very strong nCO absorption bands are found at low wavenumbers for complex 86 (1677 and 1589 cm−1) and 87 (1685 and 1593 cm−1), suggesting that the s-donor/p-acceptor ratio increases in the ligand series SiR+ < SiR(Im-Me4)+ < Si(R)Cl < Si(R)N3 < Si(R)Me. The 29Si{1H} NMR spectra of complex 86 and 87 display signals at 27.4 ppm and 50.8 ppm, respectively, typical for transition metal silyl complexes (50–70 ppm).159,160 The ModSi single bond length in 87 is 2.480(2) A˚ , which is slightly longer than the ModSi double bonds of 83 (2.300(1) A˚ ) and 84 (2.287(1) A˚ ) but shorter than the ModSi single bonds of silyl complexes (2.487–2.669 A˚ ) (Table 1). The first donor-free (silyl)(silylene)molybdenum complex 88, Cp Mo(CO)2(]SiMes2)(SiMe3) is synthesized through photolysis of Cp Mo(CO)3Me with HSiMe2SiMeMes2 in hexane (Eq. 33).161 The insertion of the isocyanide tBuCH2CMe2NC into the SidC bond of the silylene ligand gives complex 89.

ð33Þ

Source: Modified from Hirotsu, M.; Nunokawa, T.; Ueno, K. Organometallics 2006, 25(7), 1554–1556. The molybdenum-silylene bond (ModSi ¼ 2.3872(7) A˚ ) in 88 is much shorter than the molybdenum-silyl bond (ModSi ¼ 2.6391(7) A˚ ).162 The 29Si NMR spectrum of 88 shows two signals at 35.0 and 414.1 ppm, which is assigned to the silyl and silylene ligands, respectively. The 29Si NMR signal at 58.6 ppm for the silylene ligand of 89 shows significant high-field shift compared to 88, indicating that 89 is a base-stabilized silyl silylene complex. The iminoacyl group in 89 is coordinated to the silylene silicon atom Table 1

Selected spectroscopic and structure data for complexes 83–87.

Complex

29

ModSi (A˚ )

ModCO (avg. A˚ )

vCO (cm−1)

83 84 85 86 87

228.2 228.5 138.0 27.4 50.8

2.300(1) 2.287(1)

1.915(15) 1.926(4)

2.480(2)

1.9005(10)

1840, 1762 1826, 1756 1824, 1695 1677, 1589 1685, 1593

Si{1H} NMR (ppm)

Filippou, A. C.; Chernov, O.; Schnakenburg, G. Angew. Chem. Int. Ed. 2011, 50(5), 1122–1126.

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

189

through C and N to form a SidCdN three-membered ring. The SidN bond length (1.939(4) A˚ ) is significantly longer than the formal SidN single bond (1.70–1.76 A˚ ).163–165 The 13C NMR spectrum shows two signals at 89 ppm and 177 ppm corresponding to the Mo-(
2-S]C) and ester carbonyl carbons, respectively. The reaction of 88 with excess thiirane leads to Mo-(
2-S]C) cyclic complex 90 Cp (S)Mo[{
2-S]C(SiMe3)}C(]O)OSi (Mes)2S] in 52% yield.166 The Mo]Si bond in complex 88 can also react with 1 equiv. of PNO in the presence of DMAP in toluene to produce silanone molybdenum complexes Cp (OC)2Mo(SiMe3){O]SiMes2(DMAP)} 91, or with 2 equiv. of PNO to afford Cp (OC)2Mo(SiMe3){O]SiMes2(PNO)} 92 (Scheme 5). The silanone ligand is coordinated through
1-coordination mode via the oxygen atom.167

Scheme 5 (A) Muraoka, T.; Nakamura, T.; Nakamura, A.; Ueno, K. Organometallics 2010, 29(24), 6624–6626; (B) Muraoka, T.; Abe, K.; Kimura, H.; Haga, Y.; Ueno, K.; Sunada, Y. Dalton Trans. 2014, 43(44), 16610–16613.

5.05.1.1.3.2.3 Silyl complexes Transition-metal silyl complexes play important roles in the synthesis of organosilicon compounds, which are presumed to be key intermediates in metal-mediated transformations of organosilanes such as hydrosilylation of unsaturated organic molecules. The photochemical reaction of Cp Mo(CO)3Me with DMAP produces Cp Mo(CO)2(DMAP)Me 93, which undergoes the metathesis reaction with HSi(p-Tol)3 to form the molybdenum(II) silyl compound Cp Mo(CO)2(DMAP){Si(p-Tol)3} 94 (Eq. 34).168 Direct irradiation of Cp Mo(CO)3Me with xantsilH2 (xantsil ¼ (9,9-dimethylxanthene-4,5-diyl)bis(dimethylsilyl)) gives bis(silyl) hydrido Mo(IV) complex Cp Mo(k2-Si,Si-xantsil)(CO)(H) 97 (Eq. 35).169 The trisila-bridged ansa half-sandwich complex 98 is synthesized in one step from the reaction of a dilithiated precursor Li[Mo(
5-C5H4Li)(CO)3] with stoichiometric amount of 1,3-dichlorohexamethyltrisilane under dilute conditions (Eq. 36).170

ð34Þ

Source: Modified from Komuro, T.; Kanno, Y.; Tobita, H. Organometallics 2013, 32(9), 2795–2803. The structure of 94 shows a relatively long ModSi bond distance (2.6218(10) A˚ ) compared to the normal ModSi single bond distance ((2.480(2)–2.6815(13) A˚ )). The 29Si{1H} NMR spectrum shows a signal at 34.7 ppm, which is shifted more downfield than that of HSi(p-Tol)3 (−17.9 ppm) and is comparable to those of typical (silyl) molybdenum complexes with a SiR3 (R3 ¼ Me3,

190

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

Et3, MePh2) ligand (27.0–35.2 ppm).171 The reaction of 94 with BPh3 leads to the formation of the
3-a-silabenzyl complex Cp Mo(CO)2{
3(Si,C,C)-Si(p-Tol)3} 95, in which the
3-a-silabenzyl ligand is in an exo geometry. Complex 95 converts to the
2-N-silyliminoacyl complex Cp Mo(CO)2{
2(C,N)-C(Me)]NSi(p-Tol)3} 96 in the presence of acetonitrile. The resonance at 236.4 ppm in the 13C{1H} NMR spectrum is assigned to the iminoacyl carbon.

ð35Þ

Source: Komuro, T.; Begum, R.; Ono, R.; Tobita, H. Dalton Trans. 2011, 40(10), 2348–2357.

ð36Þ

Source: Braunschweig, H.; Doerfler, R.; Hammond, K.; Mies, J.; Radacki, K. Eur. J. Inorg. Chem. 2010, 34, 5383–5385. Further decarbonylation of 97 by photolysis affords Cp Mo(k3-Si,Si,O-xantsil)(CO)(H) 99 in which the oxygen in the xanthene backbone coordinates to the molybdenum with a k3-Si,Si,O-xantsil coordination mode.172 The conversions between 97 and 99 is reversible, i.e. purging the solution of 99 with CO gas leads to the recovery of compound 97. Complex 99 is reactive toward tBuCN gives N-silyliminoacyl complex 100 by the migratory insertion of nitrile into a molybdenum–silyl bond, which is similar to the formation of complex 96 (Eq. 37).

ð37Þ

Source: Komuro, T.; Begum, R.; Ono, R.; Tobita, H. Dalton Trans. 2011, 40(10), 2348–2357.

5.05.1.1.3.3 Ge, Sn-based complexes The syntheses of bimetallic ModGe and ModSn complexes are well-described in COMC II (1995), and COMC III (2007).173,174 Two major methods have been developed, and they both require the use of thermally stable organoelement(II) halides of Ge and Sn or arylchlorogermylidenes stabilized by N-heterocyclic carbene. For example, treatment of Li[CpMo(CO)3] with one equivalent of 1,2-dichlorotetratbutyldistannane tBu2(Cl)Sn-Sn(Cl)tBu2 affords the new molybdenum distannane complex CpMoSntBu2-Sn (Cl)tBu2(CO)3.175 A new synthetic protocol has been established by treating Li[CpMo(CO)3] with germylenoid Li(THF)3GeCl2 C(SiMe3)3 to synthesize the germylidyne complex Cp(CO)2Mo^GedC(SiMe3)3 101 (Eq. 38).176 Complex 101 bears an electrophilic germanium center, which is susceptible to addition of nucleophiles. The reaction of 101 with Im-Me4(1,3,4,5-tetramethylimidazol-2-ylidene) at ambient condition produces germylidene complex Cp(CO)2Mo]Ge(Im-Me4){C(SiMe3)3} 102. The ModGe bond length in 102 is 2.4004(5) A˚ which is considerably shorter than most ModGe double bond. The IR spectrum of 102 in toluene displays two very strong nCO absorption bands with almost equal intensity at wavenumbers (1861 and 1785 cm−1) lower than those of the germylidyne complexes 101 (1930 and 1868 cm−1). This evinces the considerably higher s-donor/ p-acceptor ratio of the germylidene ligand Ge(Im-Me4)C(SiMe3)3 as compared to that of the germylidyne ligand GeC(SiMe3)3, leading to a stronger metal-carbonyl back-bonding in 102 (Eq. 38). Addition of Na[CpMo(CO)3] to [LPhGeCl] results in the formation of metalla-germylene complex 103. Under reflux or irradiation, the amino-germylyne complex 104, can be obtained in quantitative yield (Eq. 39). The ModGe bond lengths of complex 103 and 104 are 2.7377(3) and 2.2811(4) A˚ , respectively, which are consistent with the ModGe single bond and triple bond.177–179

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

191

ð38Þ

Source: Modified from Filippou, A. C.; Stumpf, K. W.; Chernov, O.; Schnakenburg, G. Organometallics 2012, 31(2), 748–755.

ð39Þ

Source: Hicks, J.; Hadlington, T. J.; Schenk, C.; Li, J.; Jones, C. Organometallics 2013, 32(1), 323–329. Treatment of complexes 105, prepared from molybdenum-carbonylate salts [CpMo(CO)3]− Na+ and ClCH2GeR2Cl, with 1 equiv. of lithium diisopropylamide, LDA, results in the formation of the metallacycles (
5-C5H4)Mo(CO)3CH2GeR2 (R ¼ Me, nBu) 106. The thermal reaction of 106 with Ph3P in THF resulted in the formation of the simple phosphine-substituted products (
5-C5H4)Mo(CO)2(Ph3P)CH2GeMe2, and the release of CO (Eq. 40).180

ð40Þ

Source: Apodaca, P.; Kumar, M.; Cervantes-Lee, F.; Sharma, H. K.; Pannell, K. H. Organometallics 2008, 27, 3136–3141. Addition of Ni(CNtBu)4 to the molybdenum distanna ansa half-sandwich complex {k1-SntBu2SntBu2(
5-C5H4)}Mo(CO)3 107 leads to carbonyl-isocyanide exchange, resulting in enhanced reactivity toward chalcogen insertion in the case of the isocyanide compound 108. The 1,3-distanna-2-chalcogena ansa half-sandwich complexes 109 were then obtained (Eq. 41).181–183

ð41Þ

Source: (A) Bera, H.; Braunschweig, H.; Dorfler, R.; Hammond, K.; Oechsner, A.; Radacki, K.; Uttinger, K. Chem. A Eur. J. 2009, 15, 12092–12098. (B) Braunschweig, H.; Dorfler, R.; Gruss, K.; Kohler, J.; Radacki, K. Organometallics 2011, 30, 305–312.

5.05.1.1.4

Complexes containing group 15 ligands

5.05.1.1.4.1 Nitrogen-based complexes Monodentate ligands: This section mainly includes the CpRMo complexes containing amino, imido, and nitride ligands. Spectroscopic and reactivity studies on the nitrosyl-containing dinuclear complexes are discussed separately with phosphorus examples in Section 5.05.1.1.4.2.2. Photolysis of CpMo(CO)3I with succinimide or maleimide in the presence of dipa (diisopropanolamine) affords CpMo(CO)3 (
1-N-succinimidato) or CpMo(CO)3(
1-N-maleimidato), respectively (Eq. 42).184,185 The C]C bond in CpMo(CO)3 (
1-N-maleimidato) is reactive towards cyclopentadiene186, HP(]O)(OR)2 (R ¼ Me, Ph)187 or cysteine derivatives,188 leading to the formation of Diels-Alder adducts or Michael addition adducts.

ð42Þ

Source: Rudolf, B.; Palusiak, M.; Zakrzewski, J.; Salmain, M.; Jaouen, G. Bioconjug. Chem. 2005, 16, 1218–1224.

192

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

CpMoCl4 is a useful precursor for half-sandwich molybdenum imido complexes, which is well documented in COMC III (2007).189 Treatment of CpMoCl4 with NH2C6H4CH2CH]CHCH3 affords the imido complex CpCl2Mo(]N-C6H4-CH2CH] CHCH3) with tethered olefins.190 The cyclopentadiene imido complexes can also be derived from other type of imido species. Reaction of (Me3P)3(Cl)(ArN])MoH (Ar ¼ 2,6-diisopropylphenyl) with CpNa gives the imido complex CpMo(H)(PMe3) (]NAr), which is capable of catalyzing the hydrosilylation reactions.191,192 An unusual method to synthesize the imido complex CpMo(CO)2(N]N-CH2SiMe3) via reaction of CpMo(CO)3H with (trimethylsilyl)diazomethane N]N]CHSiMe3 is reported by Hoff’s group (Eq. 43).193,194 The thermodynamic, kinetic, and DFT studies disclose that a reversible 1,2-addition occurs to form an intermediate [Me3SiCH2N]N][MoCp(CO)3] which then goes through heterolytic or homolytic cleavage. The heterolytic process leads to the formation of imido complex CpMo(CO)2(N] N-CH2SiMe3).

ð43Þ

Source: Fortman, G. C.; Isrow, D.; McDonough, J. E.; Schleyer, P. R.; Schaefer III, H. F.; Scott, B.; Kubas, G. J.; Kégl, T.; Ungvάry, F.; Hoff, C. D. Organometallics 2008, 27, 4873–4884. The example of the nitride-bridged heterometallic molybdenum compounds 110 is synthesized by the reaction of [CpMo (CO)2L]− (L ¼ CO, PPh3, P(OMe)3) with [CpMeRe(NO)(CO)2]+ (Eq. 44).195 Experimental and DFT studies indicate that the reaction involves elimination of CO2. The typical nMo^N stretch of such Mo^NdRe compounds in IR spectra are observed in the range of 908–921 cm−1.

ð44Þ

Source: Garcίa, M. E.; Melón, S.; Ruiz, M. A.; López, R.; Sordo, T.; Marchiò, L.; Tiripicchio, A. Inorg. Chem. 2008, 47, 10644–10655. Bidentate ligands: During the last decade, half-sandwich molybdenum complexes containing NdN chelating ligands are mainly reported by the groups of Sita, Gonçalves, Vinklárek, and Honzícek. Starting from Cp MoCl4 and lithium amidinate salt, a series of molybdenum complexes with anionic NdN chelating ligands have been reported.196–198 For example, treating Cp MoCl4 with Li[N(iPr)C(R)N(iPr)] (R ¼ Me or NMe2) gives trichlorides Cp Mo [N(iPr)C(R)N(iPr)]Cl3 111, which can be further reduced to the dichloride complex Cp Mo[N(iPr)C(R)N(iPr)]Cl2 112 (Scheme 6).

Scheme 6 (A) Fontaine, P. P.; Yonke, B. L.; Zavalij, P. Y.; Sita, L. R. J. Am. Chem. Soc. 2010, 132(35), 12273–12285. (B) Yonke, B. L.; Reeds, J. P.; Zavalij, P. Y.; Sita, L. R. Angew. Chem. Int. Ed. Engl. 2011, 50(51), 12342–12346. (C) Yonke, B. L.; Reeds, J. P.; Fontaine, P. P.; Zavalij, P. Y.; Sita, L. R. Organometallics 2014, 33(13), 3239–3242.

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193

Direct chemical reduction of complex 111 under N2 atmosphere with 3 equiv. Na/Hg produces a dinitrogen-bridged complex {Cp Mo[N(iPr)C(R)N(iPr)]}2(m-N2) 113. The ModNdNdMo framework of 113 adopts a slight transoid “zigzag” conformation and the NdN bond length of 1.267(2) A˚ indicates a moderate degree of N^N bond activation. The bridging dinitrogen can be substituted by CO, CNAr, and MeCN to give Cp Mo[N(iPr)C(Me)N(iPr)](L)2 (L ¼ CO, CNAr, MeCN) 114. In addition, such dinitrogen-bridged complex 113 is reactive towards CO2 or N2O, producing the molybdenum (IV)-oxo complex Cp Mo(O)[N(iPr)C(Me)N(iPr)] 115 through facile oxygen atom transfer (OAT). The ModO bond length of ˚ 199 Based 1.7033(19) A˚ in complex 115 is shorter than the reported bis(
5-cyclopentadienyl) complex CpMe 2 Mo(O) (1.721(2) A ). on the bond length comparison, the ModO bond order of complex 115 lies somewhere between two and three, which manifests a lower nucleophilicity of the terminal oxo group. Complexes in a Cp Mo[N(iPr)C(Me)N(iPr)](L)2 type 114 readily react with propylene oxide, Me3SiN3, and N2O (Scheme 7).200–202 In the reaction with propylene oxide, a ring-opened product 116 is formed through oxidative CdO bond activation of the coordinated oxide to give a transient metallaoxetane, followed by 1,1-migratory insertion of CO into the new ModC bond. Analogous reaction of 114 with excess Me3SiN3 affords the terminal imido complex Cp Mo[N(iPr)C(Me)N(iPr)](NSiMe3) 117 and OCNSiMe3. The facile NdN bond cleavage of N2O occurs in the reaction with Cp Mo[N(iPr)C(Me)N(iPr)](CO)2 under CO, producing nitrosyl, isocyanate complex 118.

Scheme 7 (A) Farrell, W. S.; Zavalij, P. Y.; Sita, L. R. Angew. Chem. Int. Ed. Engl. 2015, 54 (14), 4269–4273. (B) Farrell, W. S.; Yonke, B. L.; Reeds, J. P.; Zavalij, P. Y.; Sita, L. R. Organometallics 2016, 35(8), 1132–1140. (C) Reeds, J. P.; Yonke, B. L.; Zavalij, P. Y.; Sita, L. R. J. Am. Chem. Soc. 2011, 133(46), 18602–18605.

Removal of the allyl ligand in CpRMo(
3-allyl)(CO)2 (CpR ¼ Ind-CH2C4H3S, Cp-C5F4N, Ind-C2H4NMe2, Cp-C2H4NMe2, Cp-(CO)Me, Cp-(CO)Ph, Cp-COOH, Cp-(CO)C6H2(OMe)3, Cp-CH2C6H4X-4 (X ¼ F, Cl, Br)) can be achieved by protonation of the parent complexes in MeCN,203 which formally produces cationic complexes [CpRMo(NCMe)2(CO)2][BF4]. The MeCN ligands in such molybdenum complexes are easily replaced by neutral NdN chelating ligands (NNL) such as bipyridine (bpy) and phenanthroline (phen) to provide [CpRMo(CO)2(NNL)][BF4] 119 (Scheme 8).100–102,107,109,124,125,129 Notably, Poli and coworkers reported a straightforward method of synthesizing CpMoCl2(diazadienes) 120 by treating [CpMoCl2]2 dimer with asymmetric diazadiene ligands.204 Some of these [CpRMo(CO)2(NNL)][BF4] complexes exhibit high activity against the human leukemia cell lines MOLT-4 and HL-60. The cytotoxicity strongly depends on nature of the coordinated NdN chelating ligands, and these complexes may have great potential in the design of new highly cytotoxic active species.

194

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

Scheme 8 (A) Pereira, C. C. L.; Costa, P. J.; Calhorda, M. J.; Freire, C.; Rodrigues, S. S.; Herdtweck, E.; Romão, C. C. Organometallics 2006, 25, 5223–5234.  cová, M.; Eisner, A.; Ru˚ žicková, Z.; Honzícek, J. Eur. J. Inorg. Chem. 2016, 519–529. (C) Schejbal, J.; Honzícˇek, J.; (B) Mrózek, O.; Šebestová, L.; Vinklárek, J.; Rezá  cová, Vinklárek, J.; Erben, M.; Ru˚ žicková, Z. Eur. J. Inorg. Chem. 2014, 5895–5907. (D) Honzícˇek, J.; Vinkla´rek, J.; Erben, M., Padelková, Z.; Šebestová, L.; Rezá  cová, M.; Ru˚ žicková, Z.; Císarˇová, I.; Honzícek, J. Inorg. Chim. M. J. Organomet. Chem. 2014, 749, 387–393. (E) Schejbal, J.; Melounková, L.; Vinklárek, J.; Rezá Acta 2018, 479, 66–73. (F) Mrózek, O.; Melounková, L.; Dostál, L.; Císarˇová, I.; Eisner, A.; Havelek, R.; Peterová, E.; Honzícek, J.; Vinklárek, J. Dalton Trans. 2019, 48, 11361–11373. (G) Pereira, C. C. L.; Braga, S. S.; Paz, F. A. A.; Pillinger, M.; Klinowski, J.; Gonçalves, I. S. Eur. J. Inorg. Chem. 2006, 4278–4288.  cová, M.; Honzícek, J. Appl. Organometal. Chem. 2017, 31, e3759. (I) Lodinsky´, J.; Vinkla´rek, J.; Dostál, L.; (H) Honzícková, I.; Vinklárek, J.; Ru˚ žicková, Z.; Rezá Ru˚ žicková, Z.; Honzícˇek, J. RSC Adv. 2015, 5, 27140–27153. (J) Stoffelbach, F.; Richard, P.; Poli, R.; Jenny, T.; Savaryc, C. Inorg. Chim. Acta 2006, 359, 4447–4453.

Multidentate ligands: Multidentate ligands play a significant role in coordination chemistry. They cover number of shapes and constrain geometries that strongly affect their coordination ability. The restrictions in the coordination sphere of the central metal, possessed by a multi-dentate ligand, are often used for a tuning of the physical and chemical properties of transition metal complexes that are widely applied in catalysis205,206 and they also serve as synthetic models of the active site of metalloenzymes.207,208 Complexes [CpRMo(CO)2(k2-N,N,NL)][BF4] 121 are conveniently synthesized by the reactions of [CpRMo(CO)2(NCMe)2][BF4] (CpR ¼ Cp, Ind, or 1,3-Ph2C9H5) with a series of planar N,N,N-chelating ligands (Eq. 45). These complexes spontaneously release CO under certain conditions to form [CpRMo(CO)(k3-N,N,NL)][BF4] 122 depending on the basicity of the tridentate ligands as well as the nature of the starting compounds.209

ð45Þ

Source: Honzícek, J.; Honzícková, I.; Vinklárek, J.; Ru˚ žicková, Z. J. Organomet. Chem. 2014, 772–773, 299–306. Using excess SiCl4, the terminal oxo ligand of metal corroles 123 can be replaced by two chloride ligands, yielding the dichloride complexes 124 (Eq. 46). Further reaction of CpNa with complexes 124 results in the formation of robust molybdenum corrolocene

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

195

complexes 125,210 which can be reversibly transformed into the corresponding corrolocenium ions 1250 . The high stability of the molybdenum compounds in both redox states makes them suitable for potential applications in catalysis as redox switch.

ð46Þ

Source: Schweyen, P.; Brandhorst, K.; Hoffmann, M.; Wolfram, B.; Zaretzke, M. K.; Broring, M. Chem. Eur. J. 2017, 23(56), 13897–13900.

5.05.1.1.4.2 Phosphorus-based complexes Following the fundamental aspects of phosphorus bonding, the discussion herein is divided into three sections of phosphine (PY3), phosphido (PY2), and phosphene (PY) ligands, where Y represents carbon, hydrogen and heteroatom (O, S, N, P, halogen) substituents.

5.05.1.1.4.2.1 Phosphine ligands (PY3) Monodentate ligands: The synthetic strategies of cyclopentadienyl molybdenum phosphine complexes CpRMoLn(PY3)4-n include ligand substitution, salt metathesis, and oxidation addition of CpH to Mo(PMe3)6. Many novel molybdenum phosphine compounds have been prepared with these strategies. For example, the g- and b-agostomers of 16-electron complexes 126 can be directly obtained by treatment of CpMo(CO)2(H)(PiPr3) with [Ph3C]+. The short Mo ⋯ Cag (ca. 2.7 A˚ ) and Mo ⋯ Hag (ca. 2.0 A˚ ) distances in 126 suggest that the molecules exhibit an unusual agostic interaction between the methyl groups of PiPr3 and the molybdenum center.211 In the analogous complex 127, however, coordination of the phenyl C]C bond of PPh3 to the molybdenum center instead by an agostic CdH interaction is observed.212

The reaction of Mo(CO)3(solv)3 (solv ¼ diglyme, EtCN) with [C5H4(CH2)2PR2]− (R ¼ Ph, Cy, and tBu) followed by protonation with acid affords a phosphine ansa complexes 128 (Eq. 47).213,214 The halide derivatives Mo(CO)2X[
5-C5H4(CH2)2PPh2] (X ¼ Cl, Br, I) are prepared by a hydrogen-halogen exchange with CHX3.

ð47Þ

Source: (A) Fischer, P. J.; Neary, M. C.; Avena, L.; Sullivan, K. P.; Hackbarth, K. C. Organometallics 2012, 31, 2437–2444. (B) Kimmich, B. F. M.; Fagan, P. J.; Hauptman, E.; Marshall, W. J.; Bullock, R. M. Organometallics 2005, 24, 6220–6229. The preparation of dimolybdenum complexes bearing mix-donor polydentate ligands via addition of bidentate phosphorus ligand to precursor Mo2Cp2(CO)6 is described in COMC II (1995).215 Mo2Cp2(CO)6 is also applied to react with (chloro)palladium(I) dimer [PdCl(m-MeN{P(OR)2}2)]2 (R ¼ CH2CF3, Ph) to synthesize the heterometallic complexes CpMo(CO) (m-MeN{P(OR)2}2)2PdCl.216 The complexes with bridging R2PdXdPR2 ligands can be derived from triple bonded dimolybdenum complexes.217–220 For example, addition of R2PdXdPR2 to Cp2Mo2(m-PCy2)(m-H)(CO)2 provides the diphosphine-bridged derivatives 129 (Eq. 48).

196

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

ð48Þ

Source: Alvarez, M. A.; García, M. E.; Ramos, A.; Ruiz, M. A. Organometallics 2007, 26, 1461–1472. The molybdenum(II) hydride Cp Mo(PMe3)3H is extremely reactive. It can abstract CO from CO2, (CH2O)n, MeOH, HCO2H, and etc. to produce Cp Mo(CO)(PMe3)2H (Scheme 9).221–223 Protonation of Cp Mo(CO)(PMe3)2H by HBF4
(Et2O)2 in THF affords dihydrogen complex Cp Mo(CO)(PMe3)2(
-H2) at 200 K, which is evidenced by exhibition of a broad signal at −5.14 ppm for the hydride resonance. In CH2Cl2, however, the appearance of the hydride resonance at −3.92 ppm suggests the selective formation of a dihydride complex Cp Mo(CO)(PMe3)2(H)2. Above 230 K, both cationic complexes lose H2 to generate Cp Mo (CO)(PMe3)2(FBF3), which slowly degenerate to a HF adduct Cp Mo(CO)(PMe3)2(FH ⋯ FBF3).224

Scheme 9 (A) Shin, J. H.; Churchill, D. G.; Parkin, G. J. Organomet. Chem. 2002, 642, 9–15. (B) Dub, P. A.; Belkova, N. V.; Filippov, O. A.; Daran, J.-C.; Epstein, L. M.; Lledos, A.; Shubina, E. S.; Poli, R. Chem. A Eur. J. 2010, 16(1), 189–201.

The 33-electron radicals [Mo2Cp2(m-CO)2(CO)2(m-L2)]+ (130, L2 ¼ Ph2PCH2PPh2 or Me2PCH2PMe2) can be trapped by NO to give stable binuclear nitrosyl derivatives in the type of Mo2Cp2(CO)4(NO)(m-L2). Such diphosphine-bridged dimolybdenum radicals are quite reactive toward EdH (E ¼ O, S, N) bonds (Scheme 10). Two main reaction pathways seem to be operative in the reactions with H2O or thiols, one of them leading to unsaturated derivatives 131 (E ¼ OH, SPh) and the second one leading to the tricarbonyl derivatives 132.225 The reaction of 130 with 1-alkynes HC^CR0 (R0 ¼ p-tol, tBu, CO2Me) evolves along three different reaction pathways.226,227 The first one implies additional decarbonylation to give the paramagnetic alkyne-bridged complexes 133 (R0 ¼ p-tol). The second one requires H-abstraction by the O atom of a carbonyl ligand and CdC coupling of the

Scheme 10 (A) Alvarez, M. A.; Anaya, Y.; García, M. E.; Ruiz, M. A. Organometallics 2004, 23, 3950–3962. (B) Alvarez, M. A.; Anaya, Y.; García, M. E.; Ruiz, M. A.; Vaissermann, J. Organometallics 2005, 24, 2452–2465. (C) Alvarez, M. A.; Anaya, Y.; García, M. E.; Ruiz, M. A. J. Organomet. Chem. 2007, 692, 983–990.

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

197

resulting hydroxycarbyne group with the internal carbon atom of the alkyne to give the diamagnetic derivatives 134 (R0 ¼ tBu). The third pathway involves the PdC coupling of the external carbon atom of the alkyne with one of the P atoms of the diphosphine ligand and H-dissociation to give the phosphonioalkyne-bridged derivatives 135. Bidentate ligands: CpMo complexes containing CdP chelating ligands are sparse. Xi and coworkers reported the synthesis of molybdenum(II) complexes CpMo(CO)2[(CO)CR]CRPPh2] (R ¼ Ph, 136 and R ¼ Tolyl, 137) through transmetalation-CO insertion by Cp2ZrCl(CR]CRPPh2) with CpMo(CO)3Cl in the presence of a catalytic amount of Pb(NCMe)2Cl2 (Eq. 49).228 The IR spectrum of 136 shows three nCO bands, 1960 cm−1 and 1884 cm−1 for ModCOterminal and 1670 cm−1 for ModCOacyl. The bond length of alkene C]C is 1.342(4) A˚ , indicative of a typical C]C bond.

ð49Þ

Source: Yan, X.; Yu, B.; Wang, L.; Tang, N.; Xi, C. Organometallics 2009, 28(23), 6827–6830. By the reaction of [Cp Mo(NO)Cl2]2 with iPr2PN ligand followed by reduction with 2 equiv. of Cp2Co, Legzdins and coworkers reported a new molybdenum(0) complex Cp Mo(NO)(k2-P,N-iPr2PN) 138 (Eq. 50).229 The synthesis of [Cp Mo(NO)Cl2]2 from Cp Mo(CO)2(NO) with PCl5 or Cl2 has been reported in 1991.230

ð50Þ

Source: Handford, R. C.; Patrick, B. O.; Legzdins, P. Inorg. Chem. 2017, 56(20), 12641–12651. Synthetic routes of installing diphosphine chelating ligands such as dppe (dppe ¼ Ph2PC2H4PPh2) and dmpe (dmpe ¼ Me2PC2H4PMe2) to CpRMo fragments are summarized in Scheme 11. Starting from Cp MoCl4, Tilley and Poli independently

Scheme 11 (A) Mork, B. V.; Tilley, T. D.; Schultz, A. J.; Cowan, J. A. J. Am. Chem. Soc. 2004, 126, 10428–10440. (B) Pleune, B.; Poli, R.; Fettinger, J. C. Organometallics 1997, 16, 1581–1594. (C) Handford, R. C.; Wakeham, R. J.; Patrick, B. O.; Legzdins, P. Inorg. Chem. 2017, 56(6), 3612–3622. (D) Holmes, A. S.; Patrick, B. O.; Levesque, T. M.; Legzdins, P. Inorg. Chem. 2017, 56(18), 11299–11309. (E) Cheng, T.-Y.; Szalda, D. J.; Zhang, J.; Bullock, R. M. Inorg. Chem. 2006, 45(12), 4712–4720. (F) Roberts, H. N.; Brown, N. J.; Edge, R.; Lewin, R.; Collison, D.; Low, P. J.; Whiteley, M. W. Organometallics 2011, 30(14), 3763–3778.

198

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

reported the synthesis of complexes Cp Mo(dmpe)X 139 and Cp Mo(dppe)X (X ¼ Bn, H3) 140, which had been well reviewed in previous volumes of COMC III (2007).155,231 Further studies on complex 139 show that it facilitates the EdH (E ¼ Ge, Si) bond activation to access silylene- and germylene-hydride complexes.232 The synthetic precursor of Cp Mo(NO)Cl2 reacts with 2 equiv. AgSbF6 in the presence of PhCN and dppe, or directly with dmpe affording [Cp Mo(dppe)(NCPh)NO][SbF6]2 or [Cp Mo(dpme)(Cl)NO][Cl], respectively. Further reduction of the two complexes by Cp2Co produces the corresponding 18e− neutral compounds Cp Mo(dppe)NO 141 and Cp Mo(dmpe)NO 142 (Scheme 11).233,234 Cp0 Mo(dppe)COCl complexes (Cp0 ¼ Cp or Cp ) which are usually prepared from the reaction of Cp0 Mo(CO)3Cl with dppe, have shown high reactivity towards small molecules to furnish a series of CpMo diphosphine complexes. Bullock and coworkers reported that the reaction of CpMo(dppe)COCl with Na+[AlH2(OCH2CH2OCH3)2]− gave molybdenum hydride complex CpMo(CO)(dppe)H 143.235 The hydride resonance of 143 in the 1H NMR spectrum at −87  C is observed at −6.44 ppm, with 2 JPH ¼ 16 Hz for the coupling of the hydride to the trans phosphine and 2JPH ¼ 69 Hz for the coupling of the hydride to the cis phosphine. A series of Cp0 Mo(dppe) alkynyl complexes 144 have been prepared by Whiteley and Low through the reaction of Cp0 Mo(dppe)COCl with alkyne, followed by treatment with CO and NaOMe.236 The preparation of hydrosulfido complexes Cp0 Mo(SH)(dppe)(CO) 145 from the precursor of Cp0 Mo(H)CO3 (Cp0 ¼ Cp or Cp ) is discussed in Section 5.05.1.1.5.2. Beside dppe- and dmpe-based complexes, there are some novel CpRMo complexes containing functionalized diphosphine chelating ligands. For example, Nishibayashi and coworkers reported a series of Cp Mo complexes containing two electrondonating ferrocenyldiphosphine ligands.237–239 Treatment of the tetrachloride dimer [Cp MoCl4]2 with depf (depf ¼ 1,1bis(diethylphosphino)ferrocene) or group IV metallocenyldiphosphines (
5-C5H4PEt2)2MCl2 (M ¼ Zr, Hf ) affords the dimolybdenum(V) complexes (Cp MoCl4)2(m-depf ) 146 and MCl2{(m-
5:
1-C5H4PEt2)(Cp MoCl4)}2 147, respectively (Scheme 12). Further reduction of 146 with excess Na/Hg under dinitrogen atmosphere gives a mixture of Cp Mo(H)(N2)(depf ) 148 and [Na(THF)3][Cp Mo(m-N2)(depf )] 149 (isolated as [Na(15-crown-5)][Cp Mo(m-N2)(depf )] 1490 ). Reducing 147 with KC8 affords the chloride-bridged complexes [MCl(m-
5:
1-C5H4PEt2)(m-Cl)Cp MoCl] 150. Compared to the previously reported dinitrogenbridged sodium anionic molybdenum complexes,240–243 the NdN bond length of 1.168(7) A˚ , as well as the strong IR nN]N band at 1791 cm−1 reflect the diazenido character (N]N) of the terminal dinitrogen ligand in 1490 . By contrast, the dinitrogen in complex 148 is unactivated (nN^N ¼ 1967 cm−1).

Scheme 12 (A) Miyazaki, T.; Tanabe, Y.; Yuki, M.; Miyake, Y.; Nakajima, K.; Nishibayashi, Y. Chem. Eur. J. 2013, 19(36), 11874–11877. (B) Miyazaki, T.; Tanabe, Y.; Yuki, M.; Miyake, Y.; Nishibayashi, Y. Organometallics 2013, 32(6), 2007–2013. (C) Miyazaki, T.; Tanaka, H.; Tanabe, Y.; Yuki, M.; Nakajima, K.; Yoshizawa, K.; Nishibayashi, Y. Angew. Chem. Int. Ed. Engl. 2014, 53(43), 11488–11492.

Bullock and coworkers synthesized a series of CpMo complexes bearing the diphosphine ligand functionalized a pendant amine. 0 These complexes are able to activate small molecules such as H2 and NH3.244–246 Complexes CpMo(H)(CO)(PRNR PR) 151 or R R0 0 CpMo(H)(CO)(P2 N2 ) 152 (R ¼ Ph or Et, R ¼ Ph, Bn or Me) are synthesized from CpMo(H)(CO)3 and the diphosphine ligand 0 0 0 PR2 N2R or PRNR PR (Eq. 51). The reaction of CpMo(CO)Cl with PR2 N2R followed by the addition of Me3NO affords the chloride 0 complexes CpMo(Cl)(CO)(PR2 N2R ). Subsequent treatment with LiBHEt3 also produces the corresponding hydride complexes 0 CpMo(H)(CO)(PR2 N2R ).

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

199

ð51Þ

Source: Zhang, S.; Bullock, R. M. Inorg. Chem. 2015, 54(13), 6397–6409. Zhang, S.; Appel, A. M.; Bullock, R. M. J. Am. Chem. Soc. 2017, 139(21), 7376–7387. Bhattacharya, P.; Heiden, Z. M.; Wiedner, E. S.; Raugei, S.; Piro, N. A.; Kassel, W. S.; Bullock, R. M.; Mock, M. T. J. Am. Chem. Soc. 2017, 139(8), 2916–2919. 0 The reactivities of CpMo(X)(CO)(PR2 N2R ) (X ¼ H 152, Cl 153) are shown in Scheme 13. Protonation of the molybdenum(II) F hydride 152 by [H(OEt2)2][BAr4] results in formation of a “proton-hydride” species 155. Alternatively, abstraction of chloride in 153 allows for heterolytic cleavage of H2 cooperative ModN reactivity to form 155. Variable-temperature NMR spectra show rapid exchange between the proton and hydride presumably through a molybdenum dihydride or dihydrogen intermediate, indicating that the HdH bond is reversibly formed and cleaved at the metal center.

Scheme 13 (A) Zhang, S.; Bullock, R. M. Inorg. Chem. 2015, 54(13), 6397–6409. (B) Zhang, S.; Appel, A. M.; Bullock, R. M. J. Am. Chem. Soc. 2017, 139 (21), 7376–7387. (C) Bhattacharya, P.; Heiden, Z. M.; Wiedner, E. S.; Raugei, S.; Piro, N. A.; Kassel, W. S.; Bullock, R. M.; Mock, M. T. J. Am. Chem. Soc. 2017, 139(8), 2916–2919.

In the presence of NaBArF4, complex CpMo(CO)(PPh2NtBu2)Cl 153 also reacts with NH3 gas to give a cationic ammonia complex [CpMo(CO)(PPh2NtBu2)NH3][BArF4]. Homolytic cleavage of the three NdH bonds is realized by the addition of tBu3ArO, affording a Mo-alkylimido complex [CpMo(PPh2NtBu2)(N2,4,6-tBuC6H2O)][BArF4] 156. Multi-dentate ligands: The benzene solution of trans-[Mo(N2)2(dppe)2] is heated at reflux in the presence of dppe, resulting in CdH and PdPh bond cleavage and the following coupling of the two dppe moieties take place to give a molybdenum(0) complexes 157.247,248 Treatment of 157 with cyclopentadiene, oxidative addition occurs to produce the hydride complex CpMoH (k3-P4) (P4 ¼ meso-o-C6H4(PhPCH2CH2PPh2)2) 158 (Eq. 52).249 Protonation of the molybdenum(II) hydride 158 with a strong acid produces k4-coordination cationic complex 159, and the reaction involves reductive elimination of H2 from a dihydride complex [CpMoH2(k3-P4)]+.250

ð52Þ

Source: Yasuda, R.; Iwasa, K.; Niikura, F.; Seino, H.; Mizobe, Y. Dalton Trans. 2014, 43(24), 9344–9355.

200

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

5.05.1.1.4.2.2 Phosphido ligands (PY2) Mononuclear molybdenum complexes: The mononuclear molybdenum PY2 species include phosphenium (M]PR2), phosphido and phosphaalkenes complexes. Synthetic methods for this class of compounds have been documented in COMC III (2007).251 In 2006, Vega et al. reported a new method of synthesizing phosphenium complexes CpMo(CO)2(PHMes ) (Mes ¼ 2,4,6-C6Ht2Bu3) by irradiation of Cp2Mo2(CO)6 in the presence of PH2Mes (Scheme 14).252 The 1H spectrum for PdH displays a down-field resonance at 10.20 ppm with JPH ¼ 341 Hz. The characteristic deshielded 31P resonance at 272.7 ppm suggests the P atom is in a planar environment. Cleavage of the PdH bond in the PHR ligand can be realized by photochemical activation, deprotonation or one-electron reduction. Consequently, CpMo(CO)2(PHMes ) is a versatile precursor for heterometallic phosphinidene-bridged ModM (M ¼ Fe, Re, Au) complexes. For example, irradiation of CpMo(CO)2(PHMes ) with Re2(CO)10 in solution produces a mixture of the ModRe complexes 160 and 161. An improved method for the synthesis of 160 is the salt metathesis of CpMo(CO)2(PClMes ) with Na[Re(CO)5]. Heating the solution of 160 at 363 K leads to the formation of 161 in high yields.

Scheme 14 (A) Alvarez, M. A.; Burgos, M.; Garcia, M. E.; Garcia-Vivo, D.; Ruiz, M. A.; Vega, P. Dalton Trans. 2019, 48 (39), 14585–14589. (B) Alvarez, M. A.; Garcia, M. E.; Garcia-Vivo, D.; Ruiz, M. A.; Vega, P. Inorg. Chem. 2020, 59(14), 9481–9485.

The p-bonding interaction between the ModPdRe backbone in complex 161 is localized between the phosphorus and molybdenum atoms.253 Such special structure endows it to exhibit both nucleophilic (Mo]P double bond) and electrophilic (dative single P ! Re bond) characters. Nucleophilic-like behavior is found in reactions with alkynes and isocyanides, which formally involves [2 + 2] and [2 + 1] cycloadditions, respectively. Electrophilic-like behavior can be traced in the thermal or photochemical rearrangement of 161 into its hydride isomer MoReCp(m-H)[m-P(CH2CMe2)C6Ht2Bu2](CO)6, because this likely involves a CdH bond cleavage step at the phosphorus site. The phosphinidene oxide compound [MoCp{P(O)(Mes )}(CO)2]− exhibits typical double Mo]P and P]O bond character. Three reactive sites at O, P and Mo atoms endow it with unique redox and acid-base properties.254–257 For example, electrophilic attack at the oxygen atom of [MoCp{P(O)(Mes )}(CO)2]− affords the neutral phosphenium complexes 162 and 163 (Eq. 53).

ð53Þ

Source: Alonso, M.; Alvarez, M. A.; García, M. E.; Ruiz, M. A.; Hamidov, H.; Jeffery, J. C. Inorg. Chem. 2010, 49, 11595–11605. Dinuclear dimolybdenum complexes: The synthesis of PRR0 -bridged complexes Cp2Mo2(CO)4(m-H)(m-PRR0 ) have been introduced in COMC III (2007).258 Oxidative addition of ClPR2 (R ¼ Cy, pH, OEt, tBu) to Cp2Mo2(CO)6 followed by two-electron reduction gives the anionic triple-bonded Mo^Mo complex [Cp2Mo2(CO)2(m-PR2)]− (Eq. 54).259,260 In the past decade, most studies on this subject are concentrated on the compounds derived from Cp2Mo2(CO)4(m-H)(m-PRR0 ) and [Cp2Mo2(CO)2(m-PR2)]−. A review involving synthesis and reactivity of triple bonded Mo^Mo complexes bridged by PY2 ligands has been published.261 Herein, only examples not involved in the above review and display unusual structure, bonding or reactivity is discussed in detail.

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

201

ð54Þ

Source: (A) García, M. E.; Melón, S.; Ramos, A.; Riera, V.; Ruiz, M. A.; Belletti, D.; Graiff, C.; Tiripicchio, A. Organometallics 2003, 22, 1983–1985. (B) Alvarez, M. A.; Casado-Ruano, M.; García, M. E.; García-Vivó, D.; Ruiz, M. A. Inorg. Chem. 2017, 56, 11336–11351. A set of di- or trinitrosyl bimetallic ModMo compounds have been reported.262,263 Reaction of Cp2Mo2(CO)4(m-H)(m-PCy2) with [NO][BF4] and following decarbonylation by NaNO2 gives the targeted product 164 in a total yield of 44% (Scheme 15). The bridging nitrosyl ligand exhibits pyramidalization at the N atom (average ModModNdO ca. 163.5 ), which might increase the electron density of the bridged N site and suggest weakening of the NdO bond. In the reaction with P(OR)3 or Na/Hg, the NdO bond of 164 undergoes cleavage and complexes 165 and 166 are formed. Protonation or methylation of the bridging nitrosyl ligand leads to a novel coordination mode in an alkenyl-like fashion in complex 167.

Scheme 15 (A) Alvarez, M. A.; Garcia, M. E.; Garcia-Vivo, D.; Ruiz, M. A.; Toyos, A. Inorg. Chem. 2015, 54(22), 10536–10538. (B) Alvarez, M. A.; Garcia, M. E.; Garcia-Vivo, D.; Ramos, A.; Ruiz, M. A.; Toyos, A. Inorg. Chem. 2018, 57(24), 15314–15329.

The molybdenum site of PRR0 -bridged radicals [Cp2Mo2(CO)4(m-PRR0 )(m-H)]− (R ¼ R0 ¼ Cy, Ph) reacts readily with p-benzoquinone to afford the hemiquinone complexes 168, which exhibit facile homolytic cleavage of the corresponding ModO bonds (Scheme 16).264 In contrast, an insertion of p-benzoquinone into the PdH bond of Cp2Mo2(CO)4(m-PRR0 )(m-H) (R ¼ Cy; R0 ¼ H) provides the anionic complex 169. The latter can release the hemiquinone group via heterolytic cleavage of the PdO bond by reaction with ER0 (E ¼ O, S; R ¼ H, alkyl, aryl) to give derivatives 170 in which the molybdenum atoms are bridged by novel PR(ER) phosphide ligands.

Scheme 16

Alvarez, C. M.; Alvarez, M. A.; Alonso, M.; Garcia, M. E.; Rueda, M. T.; Ruiz, M. A.; Herson, P. Inorg. Chem. 2006, 45(23), 9593–9606.

202

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

The ModP bond and molybdenum center can be potentially protonated in the phosphide-bridged complexes Mo2Cp2(m-PR2) (m-PR0 2)(CO)2 (R, R0 ¼ Ph, Cy, Et) 171 depending on the substituted groups on phosphorus (Scheme 17).265 When R ¼ R0 ¼ Ph, the terminal-hydride complex 172 is afforded at 253 K, and 172 further isomerizes to a more stable hydride-bridged complex 174d at 293 K. In comparison, protonation of 171 supported by electron-rich phosphide ligands gives the agostic type complexes 173 which experiences an intramolecular exchange of the agostic H atom to reach an equilibrium with hydride-bridged isomers 174a–c. The solid-state structure of agostic-like complex 173a reveals that the agostic H atom is disordered in two equivalent positions with half occupancy.

O

Scheme 17

Alvarez, M. A.; Garcia, M. E.; Martinez, M. E.; Ramos, A.; Ruiz, M. A.; Saez, D.; Vaissermann, J. Inorg. Chem. 2006, 45(17), 6965–6978.

Reaction Cp2Mo2(m-PPh2)2(CO)(O) with HBArF4
(OEt2)2 also gives a hydride-bridged bimetallic complex 175, which is intermediated by hydroxo-derivative [Cp2Mo2(OH)(m-PPh2)2(CO)]+ and hydride-hydroxo complex [Cp2Mo2(m-H)(OH) (m-PPh2)2(CO)]2+ (Eq. 55).266 However, treating Mo2Cp2(m-PPh2)2(CO)(O) with HBF4
(OEt2)2 provides the tetrafluoroborate complex 176. The k2-F2BF2 ligand of 176 is easily replaced by mono- and bidentate ligands having EdH bonds (E ¼ O, S, Se, N, P), with an elimination of HBF4.267

ð55Þ

Source: Cimadevilla, F.; Garcίa, M. E.; Garcίa-Vivó, D.; Ruiz, M. A.; Rueda, M. T.; Halut, S. J. Organomet. Chem. 2012, 699, 67–74. The unsaturated benzylidyne complex Mo2Cp2(m-CPh)(m-PCy2)(CO)2 displays a multisite reactivity of its central Mo2PC core, involving the addition of electron donors (N2CPh2) at the electron-deficient metal site or the addition of ambiphilic reagents (HC^CCO2Me) to the Mo2C face (Scheme 18).268,269 Protonation reaction of Mo2Cp2(m-CPh)(m-PCy2)(CO)2 occurs at the

Scheme 18 (A) Alvarez, M. A.; García, M. E.; Menéndez, S.; Ruiz, M. A. Organometallics 2011, 30(14), 3694–3697. (B) Esther Garcia, M.; Garcia-Vivo, D.; Menendez, S.; Ruiz, M. A. Organometallics 2016, 35(20), 3498–3506.

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

203

carbyne atom or at the ModP bonds to generate the agostic-like complex 177 (major product) and carbene-bridged complex 178 (minor product), respectively. The presence of the agostic-type ModHdP interaction in 177 is displayed by the appearance of relatively shielded 1H and 31P resonances (−4.30 and 131.4 ppm, respectively). As for compound 178, the 1H resonance of m-CH is located at 9.18 ppm, and the 13C resonance of m-CHPh is at 180 ppm, indicating the transformation of the bridging carbyne ligand into a carbene ligand. Dicarbyne-bridged dimolybdenum complexes usually react with CO to afford CdC coupling products.261,270,271 Although the reaction of hydroxycarbyne complex 179 with CNR also gives the CdC coupling product 180 (Eq. 56),272 proton transfer from m-C(OH) to CNR (R ¼ tBu, Xyl) ligand leads to the bridging aminoalkyne ligand m-C(NHR).

ð56Þ

Source: Alvarez, M. A.; Garcίa, M. E.; Menéndez, S.; Ruiz, M. A. Organometallics 2015, 34, 1681–1691. Photolysis of benzylidyne complex 181 in the presence of Ph2SiH2 yields silylene-bridged complex 182, which can be further oxidized by O2 to form diphenylsilanone-bridged complex 183 (Scheme 19).273 One-electron oxidation by [Cp2Fe]+ greatly increases the reactivity of the relatively inert 30-electron complex 181, thus enabling it to rapidly react with many different molecules under mild conditions.274 For example, the cationic radical [181]+ is reactive towards PhOH, selectively concerting to the phenoxo complex [Mo2Cp2(m-CPh)(OPh)(m-PCy2)(CO)]+. The reactions of 181 with H2SiPh2 or H3BNHt2Bu in the presence of [FeCp2]+, however, result in the selective H transfer to the O atom of the carbonyl ligand, giving the hydroxycarbyne complex [Mo2Cp2(m-COH)(m-PCy2)(m-CPh)]+.

Scheme 19 (A) Angeles Alvarez, M.; Esther Garcia, M.; Garcia-Vivo, D.; Menendez, S.; Ruiz, M. A. Chem. A Eur. J. 2016, 22(26), 8763–8767. (B) Angeles Alvarez, M.; Esther Garcia, M.; Garcia-Vivo, D.; Menendez, S.; Ruiz, M. A. Organometallics 2013, 32(1), 218–231.

Treatment of complex [Cp2Mo2(CO)2(m-PtBu2)]− with MeI followed by photolysis leads to the formation of the alkyl-bridged complex 184 with an agostic ModHdC interactions.260 The bridging methyl ligand is actively involved in the reactions of 184 with donor molecules such as Ph2SiH2, Ph2PH, BH3, and P4 (Scheme 20).275 Methane elimination occurs in the reaction with SiPh2H2 followed by SidH bond oxidative addition to give the hydride silylene derivative 185. Dehydrogenation, however, is the dominant process in the reaction with Fe2(CO)9, affording the unsaturated methylidyne cluster 187. By contrast, PMe elimination takes place in the reaction with P4, to give the unsaturated triphosphorus complex 186.

204

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

Scheme 20 (A) Alvarez, M. A.; Casado-Ruano, M.; Garcia, M. E.; Garcia-Vivo, D.; Ruiz, M. A. Inorg. Chem. 2017, 56(18), 11336–11351. (B) Alvarez, M. A.; Casado-Ruano, M.; Garcia, M. E.; Garcia-Vivo, D.; Ruiz, M. A. Chem. Eur. J. 2018, 24(38), 9504–9507.

Chalcogenophosphinidene-bridged complexes 188–190 in a m-k2P,E:k1P-bound (E ¼ S, O, Se, Te) mode are usually synthesized via [2 + 1] addition of phosphinidene-bridged dimolybdenum complexes to chalcogen atoms, such as S8, Se and Te (Eq. 57). Specifically, reaction of Me2CO2 instead of oxygen with the corresponding precursor affords oxophosphinidene-bridged compound.276 A significant p bonding interaction between the chalcogen atom with phosphorus and perhaps with the metal atom in the EPMo ring (E ¼ S, O, Se, Te), is suggested by DFT calculation, structural and spectroscopic analysis.

ð57Þ

Source: Alvarez, B.; Alvarez, M. A.; Amor, I.; García, M. E.; García-Vivó, D.; Suárez, J.; Ruiz, M. A. Inorg. Chem. 2012, 51, 7810–7824. These chalcogenophosphinidene-bridged compounds are reactive species towards electrophiles, such as H+, Me+, and AuP (p-tol)3+ (Scheme 21).277 For instance, Me+ and AuP(p-tol)3+ add to the chalcogen atom of complex 188 to give the corresponding

Scheme 21 (A) Alvarez, B.; Angeles Alvarez, M.; Amor, I.; Esther Garcia, M.; Garcia-Vivo, D.; Suarez, J.; Ruiz, M. A. Eur. J. Inorg. Chem. 2014, (10), 1706–1718. (B) Alvarez, M. A.; Alvarez, B.; Garcia, M. E.; Garcia-Vivo, D.; Ruiz, M. A. Dalton Trans. 2013, 42(31), 11039–11042.

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

205

derivatives 191. Protonation of 188 results in an endo addition of proton to the carbon atom of the Mes ring to eventually afford complex 192, in which the coordination mode of the aromatic ring shift from
4 to
5. The bridging S atom in the chalcogenophosphinidene ligand is susceptible to proton transfer in the initial protonation. The cationic complex 192 further reacts with strong base K[BHR3] (R ¼ sBu) to give hydrocarbation product 193 and hydronitration product 194 in a ratio of 2:5.278 This reaction is supposed to be mediated by
4-cyclopentadiene complex and ModH species. Bearing a bulky Xyl group, complex 194 undergoes the aldehyde/aldimine transformations in the presence of NHn2Bu or water.279 The triply-bonded complex Mo2Cp2(m-CPh)(m-PCy2)(m-CO) and the thiophosphinidene complexes 188–190 can transform to di- and multi-nuclear complexes by reacting with metal fragments, such as Mn2(CO)10, Ru3(CO)12, Co2(CO)8, and Fe2(CO)9.280–283 For example, treatment of complex 189 with Mn2(CO)10 in toluene gives dinuclear product 195. However, the reaction produces the trinuclear ModMndMo carbonyl compound 196 in THF (Eq. 58). The formation of 196 is thought to involve abstraction of H atom from the solvent or trace of water by the aromatic phenyl ring.

ð58Þ

Source: Alvarez, B.; Alvarez, M. A.; García, M. E.; García-Vivó, D.; Ruiz, M. A. Inorg. Chem. 2018, 57, 1901–1911. Heterobimetallic molybdenum complexes: Heterobimetallic complexes can be synthesized from phosphinidene- or chalcogenophosphinidene-bridged complexes, which was covered earlier in this section. The phosphenium complex CpMo(CO)2 (PClMes ) is used as precursors to give many heterobimetallic complexes. Dinuclear MoIVdFeII complexes with bridging hydrides 197 are synthesized via deprotonation of CpRMo(PMe3)H5 (CpR ¼ Cp, Me Cp ) by Cp FeN(SiMe3)2 (Eq. 59).284 Removal of the terminal ModH hydride in 197 by [Ph3C]+ leads to the formation of cationic THF adducts 198. Further reaction of 198 with LiPPh2 gives rise to a phosphido-bridged complex 199. These multi-hydride species are capable of catalyzing the silylation of nitrogen. Another ModFe hydride complex 201 is provided when the cluster 200 is treated with excess CNtBu. Dinuclear ModMn complex 202 is prepared in analogous method (Eq. 60).285

ð59Þ

Source: Ishihara, K.; Araki, Y.; Tada, M.; Takayama, T.; Sakai, Y.; Sameera, W. M. C.; Ohki, Y. Chem. Eur. J. 2020, 26, 9537–9546.

ð60Þ

Source: Ohki, Y.; Araki, Y.; Tada, M.; Sakai, Y. Chem. Eur. J. 2017, 23, 13240–13248. Reaction of Cp(CO)3ModM(CO)5 (M ¼ Re, Mn) which is given by the photolysis of Cp2Mo2(CO)6 with M2(CO)10, with R2PH (R ¼ Ph, Cy) affords the phosphide- bridged heterometallic complex 203 (Eq. 61).286,287 Deprotonation of 203 with DBU followed by treatment of I2 produces iodide-bridged complex 204. The latter is finally reduced by Na/Hg to provide the unsaturated anionic product 205.288

ð61Þ

Source: Alvarez, M. A.; García, M. E.; García-Vivó, D.; Huergo, E.; Ruiz, M. A. Eur. J. Inorg. Chem. 2017, 1280–1283. The reactivities of the acetonitrile adduct 206, which is prepared by photolysis of the acetonitrile solution of complex 203 (M ¼ Re; R ¼ Cy), were examined.289,290 The acetonitrile ligand of 206 is easily replaced by electron donor molecules (such as

206

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

PhSH and Ph2PH) or displaced upon two-electron reduction. The substitution step is followed by additional processes such as EdH bond cleavage, MdH bond insertion, H2 elimination, etc. For example, addition of complex 206 to dppm
BH3 (dppm ¼ Ph2PCH2PPh2) under reflux conditions gives the corresponding diphosphine-borane compound 207, which is followed by decarbonylation and dehydrogenation to form the agostic boryl-bridged complex 208 (Eq. 62).

ð62Þ

Source: Alvarez, M. A.; García, M. E.; García-Vivó, D.; Huergo, E.; Ruiz, M. A. Inorg. Chem. 2019, 58, 16134–16143. Complex 205b exhibits both nucleophilic and electrophilic character. The electrophilic property of 205 is illustrated by the reaction of rhenium center with nucleophiles such as ClSnPh3, HSPh, PPh2H and HC^C(p-tol).288,291 In particular, protonation of compound 205 with NH+4 leads to the ammonia derivative 209 instead of forming the corresponding hydride-bridged complex observed for its homometallic analog [Mo2Cp2(m-PCy2)(m-CO)2]− (Eq. 63).

ð63Þ

Source: Alvarez, M. A.; García, M. E.; García-Vivó, D.; Huergo, E.; Ruiz, M. A. Eur. J. Inorg. Chem. 2017, 1280–1283. 5.05.1.1.4.2.3 Phosphene ligands (PY) A review on the chemistry of bridging phosphinidene dimolybdenum complexes has been reported.292 The audience is referred to this review for the synthesis, structural studies and reactivity of the PY-bridged dimolybdenum complexes. Molybdenum complexes with terminal phosphinidene ligands are relatively rare. When AlCl3 is used as a halide abstraction agent, Cp Mo(CO)3(PClR) can convert to terminal phosphinidene molybdenum complexes. In particular, the aminophosphinidene complex [Cp Mo(CO)3(PNiPr2)][AlCl4] has been isolated and well-characterized.293,294 This complex is stabilized by the p back donation from metal, as well as the p donation from N atom to the empty pz orbital of P atom. Though longer than typical ModP double bonds, the ModP bond distance of 2.4506 A˚ in [Cp Mo(CO)3(PNiPr2)][AlCl4] is closer to the bond distance of phosphine ligand with strong donating effect such as ModPMe3 complexes (ModP ¼ 2.462(46) A˚ ). Thus, it is clarified as phosphinidene complex for its s-donor/p-acceptor abilities, as reflected by the extremely down-field 31P resonance at d 1007. The reaction of K[CpMo(CO)3] with the chlorophosphaalkene species ClP]C(SiMe3)2 produces the phosphavinylidene complex Cp(CO)2Mo]P]C(SiMe3)2 in high yield.295,296 The ModP bond length (ModP ¼ 2.174(1) A˚ ) is significantly shorter than those found in phosphine-Mo complexes (2.40–2.57 A˚ ), in which ModP multiple bonding is implied. The PdC bond length (1.649(4) A˚ ) is consistent with a P]C double bond. In addition, the ligand configuration is such that the angle at phosphorus (ModPdC ¼ 178.3(2) ) is almost linear and the methylene carbon is trigonal-planar. These terminal phosphinidene molybdenum complexes can be easily attacked by nucleophilic reagents, such as alkynes,294 diazoalkanes297 and phosphorous compounds.298,299 Besides, [Cp Mo(CO)3(]PiPr)][AlCl4] generated in situ from the reaction of Cp Mo(CO)3[P(Cl)iPr] with AlCl3, is able to activate the CdH of ferrocene and SidH bond of H2SiPh2 (Eq. 64).293

ð64Þ

Source: Rajagopalan, R. A.; Sterenberg, B. T. Organometallics 2011, 30, 2933–2938. Using phosphaalkene RP]C(NMe2)2 (R ¼ tBu, Cy) as phosphinidene transfer reagent, the
3-1,2-diphosphaallyl complexes 210a and 210b are afforded from the phosphavinylidene complexes Cp(CO)2Mo]P]C(SiMe3)2.300–306 A similar strategy has been applied for synthesis of
3-arsaphosphaallyl complex 211.307–310 In particular, treatment of Cp(CO)2Mo]P]C(SiMe3)2 with 10 equivalent tBuC(O)As]C(NMe2)2 yields the
3-2-phospha-1,3-diarsaallyl complexes 212 and
3-1,2,3-triarsaallyl complexes 213 in a ratio of 38:62 (Scheme 22). A further study of the complex 214 with alkynes has been reported (Eq. 65).311 The outcome of the reactions is found to be very dependent on the steric hindrance of terminal alkynes, and two different types of products (215 and 216) are isolated. The bridging

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

207

Scheme 22 (A) Weber, L.; Noveski, G.; Lassahn, U.; Stammler, H.-G.; Neumann, B. Eur. J. Inorg. Chem. 2005, 2005(10), 1940–1946. (B) Weber, L.; Bayer, P.; Braun, T.; Stammler, H.-G.; Neumann, B. Organometallics 2006, 25(7), 1786–1794. (C) Weber, L.; Bayer, P.; Stammler, H. G.; Neumann, B. Chem. Commun. 2005, (12), 1595–1597.

formylalkenyl derivative 216 has cis and trans configurations, and the ratio of the two isomers is significantly affected by the radiation wavelength.

ð65Þ

Source: García, M. E.; García-Vivó, D.; Ruiz, M. A.; Sάez, D. Organometallics 2017, 36, 1756–1764. The Mo]P double bond of phosphinidene-bridged complex 217 can go through cycloaddition reaction with unsaturated organic molecules.312 Reaction of 217 with CS2 results in [2 + 2] cycloaddition of C]S bond to the Mo]P bond and a further insertion of S into the remaining ModP bond, giving the phosphanyl derivative 218 (Eq. 66). In the case of diazoalkane, the phosphaalkene-bridged derivatives 219 are afforded. The products 219 are shown to proceed through a [3 + 2] cycloaddition of the diazoalkane molecule, followed by N2 elimination.

ð66Þ

Source: Albuerne, I. G.; Alvarez, M. A.; Amor, I.; García, M. E.; García-Vivó, D.; Ruiz, M. A. Inorg. Chem. 2016, 55, 10680–10691. The phosphorus atom in pyramidal phosphinidene complex 220 allows for attack by the terminal N atom of diazoalkanes to generate the P:P-bridged phosphadiazadiene derivatives, which are stabilized through protonation or methylation at the P-bound N atom to give complexes 221 (Scheme 23).313 In a similar way, nucleophilic attack of the phosphinidene ligand in 220 to the terminal N atom of the azide affords the corresponding P:P-bridged phosphatriazadiene derivatives. These complexes, however, evolve rapidly at low temperature to give products depending on the azide used. For aryl azides 4-C6H4X (X ¼ Me, F), fast denitrogenation takes place to give the phosphamine-bridged complexes, which are isolated as in the protonated or methylated forms 222. For benzyl azide, a decarbonylation process leads to the coordination of the remote NR nitrogen to the MoCp fragment, and the unprecedented phosphatriazametallacyclic complex is initially formed. Methylation at the P-bound N atom of the latter provides the isolated complex 223.

208

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

Scheme 23

Albuerne, I. G.; Alvarez, M. A.; Garcia, M. E.; Garcia-Vivo, D.; Ruiz, M. A.; Vega, P. Inorg. Chem. 2020, 59(11), 7869–7883.

5.05.1.1.4.2.4 “Naked” phosphorus ligands (Pn) The chemistry of complexes bearing a “naked” phosphorus (Pn) ligand has been subjected to much study and documented in reviews.314,315 An improved method to synthesize complex (Cp Mo)2(m,
6:
6-P6) is via the reaction of [Cp Mo(CO)2]2 with white phosphorus P4 (Eq. 67).316,317 Treatment of (Cp Mo)2(m,
6:
6-P6) with MeNHC (MeNHC ¼ 1,3,4,5-tetramethylimidazol-2-ylidene) leads through NHC-induced phosphorus cation abstraction to the ring contraction product [(MeNHC)2P][(Cp Mo)2(m,
3:
3-P3) (m,
2:
2-P2)].318 Complexes of [CpMo(CO)2]2(m,
2:2-P2), Cp Mo(CO)2(
3-P3), and (Cp Mo)2(m,
6:
6-P6), are versatile precursors for preparation of clusters.316,319–321

ð67Þ

Source: (A) Fleischmann, M.; Dielmann, F.; Gregoriades, L. J.; Peresypkina, E. V.; Virovets, A. V.; Huber, S.; Timoshikin, A. Y.; Balάzs, G.; Scheer, M. Angew. Chem. Int. Ed. 2015, 54, 13110–13115. (B) Piesch, M.; Reichl, S.; Seidl, M.; Balάzs, G.; Scheer, M. Angew. Chem. Int. Ed. 2019, 58, 16563–16568. By reaction of the triple bonded bimetallic complex [Mo2Cp2(m-PCy2)(m-CO)2]− with P4, the bridging diphosphorus dimolybdenum complex 224 is afforded and along with the cleavage of PdP bond (Scheme 24).322,323 The bridging P atoms of P2 ligand in

Scheme 24 (A) Alvarez, M. A.; Garcia, M. E.; Garcia-Vivo, D.; Ramos, A.; Ruiz, M. A. Inorg. Chem. 2011, 50(6), 2064–2066. (B) Alvarez, M. A.; Garcia, M. E.; Garcia-Vivo, D.; Ramos, A.; Ruiz, M. A. Inorg. Chem. 2012, 51(20), 11061–11075. (C) Angeles Alvarez, M.; Esther Garcia, M.; Garcia-Vivo, D.; Lozano, R.; Ramos, A.; Ruiz, M. A. Inorg. Chem. 2014, 53(20), 11261–11273.

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

209

224 are chemically inequivalent as suggested by VT-NMR studies. The unique donor behavior of the P2 ligand enables it to be attacked by electrophiles ER3X (E ¼ C to Pb). These electrophiles bind to the P2 ligand in a terminal way to afford the diphosphenyl derivatives. For example, its methylation derivative 225 converts into trinuclear clusters in the presence of metal fragments such as Fe2(CO)9 and W(CO)4(THF)2. Treating 225 with MLn(THF) (MLn ¼ MnCp0 (CO)2, Mo(CO)5, W(CO)5) gives trigonal-planar phosphide derivatives 226, with the spontaneously release of methylphosphinidene. The strong p bonding interaction of the two molybdenum centers in complex 226 (MLn ¼ W(CO)5) leads to a short ModMo bond distance of 2.749(1) A˚ .324 5.05.1.1.4.3 Arsenic, antimony, and bismuth-based complexes This section reviews CpMo complexes bearing arsenic-, antimony-, and bismuth-donor ligands. Recently reviewed areas include complexes containing transition metal-bismuth bond325,326 and complexes with group 15 element ligands.314 Generally, the synthesis, structure, and reactivity of these complexes share many similarities with their phosphorus analogs. A synthetic strategy for the preparation of compounds 227 via the reaction of [Cp2Mo2(CO)6][SbCl] with S8 or Se8 has been reported (Eq. 68).327 Using GaCl3 as chloride ion acceptor, the corresponding cationic complex [Cp2Mo2(CO)4ESb]+ (E ¼ S, Se) is produced.328 Salt metathesis reaction of [Cp(CO)3Mo]Na with (C2F5)2BiCl provides complex Cp(CO)3ModBi(C2F5)2 (Eq. 69).329

ð68Þ

Source: Wagener, C.; Kerzweiler, K. Z. Anorg. Allg. Chem. 2011, 637, 651–654. ð69Þ Source: Hoge, B.; Solyntjes, S.; Bader, J.; Neumann, B.; Stammler, H.-G.; lgnat’ev, N. Chem. Eur. J. 2017, 23, 1557–1567. The complexes Cp2Mo2(CO)4(m,
2:
2-PE) (E ¼ As, Sb) and Cp2Mo2(CO)4(m,
2:
2-Sb2) are reported to react with CuI, affording a set of supramolecular assemblies.330,331 The oxidation of complexes Cp2Mo2(CO)4(m,
2:
2-E2) (E ¼ P, As, Sb, Bi) with [C12H8S2]+ selectively produces the dicationic E4 complexes [Cp2Mo2(CO)4(m4,
2:
2:
2:
2-E4)]2+.332 Treatment of complex Cp Mo(CO)2 (
3-As3) with AgI leads to the formation of a novel oligomeric complex.333

5.05.1.1.5

Complexes containing group 16 ligands

5.05.1.1.5.1 Oxygen-based complexes A number of CpRMoO2Cl (CpR ¼ Cp, Cp , CpBz,) compounds have been covered in COMC II (1995) and COMC III (2007). The synthetic routes are summarized in Scheme 25.334–336 Treatment of
2-acyl complex Mo(CpBz)Cl3(COCH3)337 with one equiv. of

Scheme 25 (A) Morris, M. J. In Comprehensive Organometallic Chemistry II, Abel, E. W., Stone, F. G. A., Wilkinson, G. W., Eds.; Elsevier: Oxford, UK, 1995; Vol. 5, Chapter 7, pp. 431–433. (B) Flower, K. R. In Comprehensive Organometallic Chemistry III; Abel, E. W., Stone, F. G. A., Wilkinson, G. W., Eds.; Elsevier: Oxford, UK, 2007; Vol. 5, Chapter 5, pp. 562–564. (C) Poli, R. Coord. Chem. Rev. 2008, 252(15), 1592–1612. (D) Martins, A. M.; Romão, C. C.; Abrantes, M.; Azevedo, M. C.; Cui, J.; Dias, A. R.; Duarte, M. T.; Lemos, M. A.; Lourenço, T.; Poli, R. Organometallics 2005, 24(11), 2582–2589.

210

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

t

BuOOH leads to Mo(CpBz)Cl2(O) in 86% yield, whereas the same reaction in the presence of excessive tBuOOH gives Mo(CpBz)Cl(O)2 in 73% yield. The Mo]O distance of 1.735(4) A˚ in Mo(CpBz)Cl2(O) is close to 1.740(6) A˚ found for CpMo(Cl2)O but slightly longer than that in Cp Mo(Cl2)O (1.683(2) A˚ ).338 High-valent molybdenum complexes CpRMoO2Cl (CpR ¼ Cp, Cp , CpBz, CpiPr) are efficient catalysts or catalytic intermediates for olefin epoxidation and sulfide and sulfoxide oxidation.339–349 The synthesis of Cp 2Mo2O5 has been achieved by aerial oxidation of Cp Mo(CO)2 derivatives such as Cp Mo(CO)2(NO), [Cp Mo(CO)2]2, and others.336,350–352 Poli and co-workers have developed synthetic procedures to access Cp 2Mo2O5 from commercially available Mo(CO)6, Cp Na, tBuOOH (or H2O2), and CH3COOH reagents.353 Cp2Mo2O5 can catalyze epoxidation of cyclooctene or thiophene derivatives even in the presence of water.354,355 The reaction of Cp2Mo2O5 with ROOH produces the polyoxometallate complex.356,357 The ionic NHC complex [CpMo(CO)2(NHC)(CH3CN)][BF4] (NHC ¼ IBz ¼ 1,3-dibenzylimidazol-2-ylidene or NHC ¼ IMes ¼ 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) is oxidized to a thermally stable molybdenum(VI) dioxo complex [CpMoO2(NHC)][BF4].358 The Mo]O bonds of 1.711(3) and 1.694(3) A˚ in [CpMoVIO2(IBz)]2[Mo6O19] are comparable with the related dioxo compounds such as Mo(C5Ph4R)O2(OR0 ) and Cp2Mo2O5 (1.677(3)–1.720(2) A˚ ).352,359,360 Both [CpMoO2 (NHC)][BF4] and its carbonyl precursor show high activity towards olefin epoxidation. [Cp MoO3]− 228, which is synthesized from the reaction of [Cp MoO2]2(m-O) with NBu4OH, is a practical synthetic precursor for high valent heterometallic compounds containing molybdenum.361,362 A stop-flow study on the protonation of [Cp MoO3]− at low pH (down to zero) in a mixed H2O-MeOH (80:20) solvent at 25  C allowed the simultaneous determination of the first acid dissociation constant of the oxo-dihydroxo complex, [Cp MoO(OH)2]+ (pKa1 ¼ − 0.56), and the rate constant of its isomerization to the more stable dioxo-aqua complex, [Cp MoO2(H2O)]+ (k−2 ¼ 28 s−1).363 [Cp2Mo(OH)(OH2)]+ catalyzes the hydrolysis of ethyl vinyl, divinyl, and diethyl ethers. Addition of Ph3BiBr2 to two equivalents of 228 leads to the formation of MoVIBiVMoVI compound 229. The average ModO single bond length of 1.828 A˚ in 229 is much shorter than that in (Cp MoO2)2O (1.855(6)–1.894(4) A˚ )364. By contrast, treatment of 228 with [(o-tolyl)2Bi(hmpa)2][SO3CF3] (o-tolyl ¼ 2-(Me)C6H4; hmpa ¼ hexamethylphosphoric acid triamide) affords the MoVIBiIIIMoVI compound 230.365,366 The tetranuclear ModM (M ¼ Fe, Co, Zn) complexes [(TPA)M(m-Cp MoO3)]2(OTf )2 231 (TPA ¼ tris(2-pyridylmethyl)-amine, OTf ¼ trifluoromethanesulfonate) can be synthesized from [Cp MoO3]− and [(TPA)M (MeCN)2](OTf )2 (Scheme 26).367

Scheme 26 (A) Sundermeyer, J.; Radius, U.; Burschka, C. Chem. Ber. 1992, 125(11), 2379–2384. (B) Schoettel, G.; Kress, J.; Osborn, J. A. J. Chem. Soc., Chem. Commun. 1989, (15), 1062–1063. (C) Roggan, S.; Limberg, C.; Brandt, M.; Ziemer, B., J. Organomet. Chem. 2005, 690(23), 5282–5289. Doggan, S.; Limberg, C.; Ziemer, B. Angew. Chem. Int. Ed. Engl. 2005, 44(33), 5259–5262. (D) Falkenhagen, J. P.; Limberg, C.; Demeshko, S.; Horn, S.; Haumann, M.; Braun, B.; Mebs, S. Dalton Trans. 2014, 43(2), 806–816.

5.05.1.1.5.2 Sulfur-based complexes Transition metal sulfur compounds have exhibited intriguing properties in bonding, structure, and reactivity, and they have attracted significant attention in the last decades due to the unique ModS moieties widely found in the active sites of biological and industrial catalysts.368,369

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

211

Mono(hydrosulfido) complexes: CpRMo(SH)(CO)3 232 (CpR ¼ Cp, Cp ), which are generated in situ from hydrido-carbonyl complexes CpRMoH(CO)3 are practical synthons due to the unique SH bond and the labile CO (Scheme 27). Monothiocarboxylate complexes of molybdenum CpMo(CO)3SCOR (R ¼ Me, CH2Cl, Ph, 4-C6H4NO2, 3,5-C6H3(NO2)2) 233 have been obtained by the reaction of the hydrosulfido complex CpMo(CO)3SH 232, with acid chlorides.370 Compound 232 is treated with dppe under UV irradiation in THF, new mono(hydrosulfido) complexes 145 are obtained by replacing two CO ligands with dppe. Compound 145 reacts with RhCl(PPh3)3 or [IrCl(coe)2]2/PPh3 (coe ¼ cyclooctene) in the presence of base to afford sulfide-bridged dinuclear heterometallic complexes 234. The dinuclear ModRu compound is prone to form the tetranuclear cluster (CpMo)2{Rh(dppe)}2 (m2-CO)2(m3-S)2 in solution.371

Scheme 27 (A) El-khateeb, M.; Rueffer, T.; Lang, H. Polyhedron 2006, 25(17), 3413–3416. (B) Iwasa, K.; Seino, H.; Mizobe, Y. J. Organomet. Chem. 2009, 694(23), 3775–3780.

The IR spectra of 232 display three absorption bands in the range of 2048–2042, 1980–1971, and 1936–1942 cm−1 for the terminal carbonyl ligands. These bands are lower than those observed for CpMo(CO)3SH372,373 (2039, 1963 cm−1), but similar to those observed for CpMo(CO)3S2CSR (2050–2046, 1976, and 1951–1952 cm−1)374. An additional distinctive absorption appears between 1644 and 1736 cm−1, corresponding to the ketonic carbonyl group of the thiocarboxylate ligand. The nCO value of 1829 cm−1 observed for 145a, is somewhat lower than those of the Cl and H analogs, CpMoCl(CO)(dppe) (1845 cm−1)375 and CpMoH(CO)(dppe) (1840 cm−1)235. PdS chelating complexes: Wang and coworkers reported a Cp Mo complex supported by PdS chelating ligand (Scheme 28).376 The reaction of Cp Mo(CO)3Cl with 1,2-Ph2PC6H4SNa affords a dicarbonyl complex Cp Mo(1,2-Ph2PC6H4S)(CO)2 235.

Scheme 28

Hou, S. F.; Chen, J. Y.; Xue, M.; Jia, M.; Zhai, X.; Liao, R. Z.; Tung, C. H.; Wang, W. ACS Catal. 2020, 10, 380–390.

212

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

Complex 235 is then treated with 2 equiv. FcBF4, and the IR spectrum of the reaction mixture exhibits one nCO band at 2005 cm−1. The reaction is stirred at 50  C overnight to promote decarbonylation and produce the molybdenum(IV) complex [Cp Mo (1,2-Ph2PC6H4S)(NCMe)3][BF4]2 236. Further reduction of 236 with 2 equiv. Cp2Co (bis(cyclopentadienyl)cobalt) produces a
2-nitrile molybdenum(II) complex [Cp Mo(1,2-Ph2PC6H4S)(
2-NCMe)] 237. Complex 237 is able to catalyze transfer hydrogenation of nitriles to primary amines with ammonia borane. Further mechanistic study reveals that the reaction of 237 with H3B
THF or H3N
BH3 readily affords a molybdenum(II) hydride complex Cp MoH(1,2-Ph2PC6H4SBH2) 238 by molybdenum-thiolate cooperation (Eq. 70). It demonstrates a new method of activating BH3 and stabilizing the parent H2B fragment in the form of HM-L(BH2). The hydride ligands of 238 exhibit a terminal hydride ModH, a bridging hydride ModHdB and a terminal BdH hydride resulted from the cleavage of one BdH bond of BH3 with ModS cooperation. The 1H NMR spectrum of compound 238 exhibits three resonances at d −4.49, −4.65 and −7.15, which are assigned to ModHdBdH, ModHdBdH and ModH, respectively. Besides, VT NMR experiment suggests a fast exchange between the bridging hydride ModHdB and the terminal BdH hydride.

ð70Þ

Source: Hou, S. F.; Chen, J. Y.; Xue, M.; Jia, M.; Zhai, X.; Liao, R. Z.; Tung, C. H.; Wang, W. ACS Catal. 2020, 10, 380–390. SdS chelating complexes: The SdS chelating ligand transfer reaction between Ni(S2C2Ph2)2 and CpMo(CO)3Cl affords bis(dithiolene) complex CpMo(S2C2Ph2)2 239 (Eq. 71).377 Ghosh, Halet and coworkers reported a series of CpMo-CS2complexes.378,379 Treatment of CpMoCl4 with LiBH4
THF at low temperature followed by addition of CS2 produces complex Cp Mo(
2-S2CH2)(
3-S2CH) 240 (Eq. 72). In the solid-state structure, the CdS bonds in the
3-CHS2 group (1.720(2) A˚ and 1.719 (2) A˚ ) are significantly shorter than those in the
2-CH2S2 fragment (1.810(2) A˚ and 1.815(2) A˚ ), suggesting the partial CdS double bond character. Using Cp Mo(CO)3Me as the synthetic precursor, the reaction provides a mixture of Cp Mo (CO)2(
2-S2CSMe) 241a, Cp Mo(CO)2(
2-S2CMe) 241b and Cp Mo(CO)2(m-(
2-S2CS))Mo(CO)3Cp 241c (Eq. 73).

ð71Þ

Source: Adams, H.; Gardner, H. C.; McRoy, R. A.; Morris, M. J.; Motley, J. C.; Torker, S. Inorg. Chem. 2006, 45(26), 10967–10975.

ð72Þ

Source: Rao, C.; Barik, S.; Yuvaraj, K.; K, B.; Roisnel, T.; Dorcet, V.; Halet, J. F.; Ghosh, S. Eur. J. Inorg. Chem. 2016, 2016.

ð73Þ

Source: Modified from Ramalakshmi, R.; Roisnel, T.; Dorcet, V.; Halet, J.-F.; Ghosh, S. J. Organomet. Chem. 2017, 849, 256–260. The macrocycle 1,4,7-trithiacyclononane (ttcn) reacts with [
5-CpRMo(CO)2(NCMe)2][BF4] (CpR ¼ Ind) to give [(
3-CpR)Mo (CO)2(k3-ttcn)][BF4] 242. In contrast, the reaction with (
5-CpR)Mo(CO)2(C3H6)(FBF3) affords the CdS bond cleaved product [(
5-CpR)Mo(CO)(k3-1,4,7-trithiaheptanate)][BF4] 243 (Eq. 74).380

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

213

ð74Þ

Source: Gamelas, C. A.; Bandeira, N. A. G.; Pereira, C. C. L.; Calhorda, M. J.; Herdtweck, E.; Machuqueiro, M.; Romão, C. C.; Veiros, L. F. Dalton Trans. 2011, 40(40), 10513–10525. Organometallic trisulfido complexes Cp MS3 (M ¼ Mo, W) are valuable synthetic precursors for various heterometallic sulfido clusters.381,382 Tatsumi et al. have reported the synthesis of Cp MS3 (M ¼ Mo, W) via the reaction of KCp with trithio-chloro molybdate [MoClS3]− in DMF (Eq. 75).383 It can be alternatively synthesized by the reaction of Cp MoCl4 with LiStBu through CdS bond cleavage.384

ð75Þ

Source: Ito, J.-I.; Ohki, Y.; Iwata, M.; Tatsumi, K. Inorg. Chem. 2008, 47(9), 3763–3771. Thiolate-bridged homonuclear complexes are synthesized from the reaction of molybdenum carbonyl compounds with organic sulfide or element sulfur, which has been reviewed in COMC II (1995) and COMC III (2007).385,386 Transition metal carbonyl compounds undergo decarbonylation with organic sulfide to yield the thiolato-bridged complexes. For instance, treatment of [CpMo(CO)3]2 with Bz2S3 produced [CpMo(CO)2(SBz)]2, [CpMo(CO)(SBz)]2S, and [CpMo(SBz)S]2 stepwise.387 The dithiolene compounds are also accessible in other methods. Cp Mo(NO)(CO)2 reacts with element sulfur and dimethyl acetylenedicarboxylate (C2Z2 (Z ¼ COOMe)) in one pot to give a mixture of mononuclear [2 + 2] cycloaddition products.388 Dimolybdenum dithiolates compounds: Dimolybdenum dithiolates are reactive towards alkynes to produce alkenedithiolate compounds. The bridging sulfide ligands in (MenCpMom-S)2S2CH2 (n ¼ 0 or 1) react with alkynes, including diynes to produce the alkenedithiolate compounds.389 Besides, Cp2Mo(S4) reacts with alkynes in the presence of radical initiator 2,2-azobisbutyronitrile (AIBN) to afford Cp2Mo{S2C2(R)(CMe2CN)}.390 Additionally, the bis(dithiolene) compound 244 with an excess of alkyne refluxing in dichloromethane affords 245. In contrast, with one equivalent of alkyne at room temperature, the mixed dithiolene-dithiolate species Cp2Mo2(m-SCR1]CR2S)(m-SCH2CH2S) 246 are formed (Scheme 29). The remaining dithiolate ligand in 246 can convert to a different dithiolene compound by further reaction with additional alkyne. This methodology was applied to prepare a bis(diphenylphosphino)acetylene compound containing phosphine-substituted dithiolene ligands.391

Scheme 29

Adams, H.; Morris, M. J.; Riddiough, A. E.; Yellowlees, L. J.; Lever, A. B. P. Inorg. Chem. 2007, 46(23), 9790–9807.

214

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

The SdH bond reactivity in thiolate-bridged homonuclear complex 247 was thoroughly studied by hydrogen atom transfer kinetics including hydrogen addition, alkyl group addition and transfer, and disulfide formation and cleavage.392 Complex 248 and its derivatives are efficient electrocatalysts for proton reduction, even with 100% current efficiencies. The catalysis is initiated by protonation of the sulfur site of the catalyst, and the rate-determining step is the elimination of H2.393,394

Reactivity of [Mo2Cp2(m-SR)n]: Recently, the reactivity of tris(thiolato) bridged complexes with [Mo2Cp2(m-SR)n] core has been reviewed.395 Their reactivities towards various reagents such as alkynes, isonitriles, nitriles, propargylic alcohols, azides, hydrazines, and carbon disulfide, demonstrate their ability to activate small unsaturated molecules through new CdX (X ¼ C, N, O, S. . .) bonds formation or cleavage. Attempts to methylate cis-[Mo2Cp2(m-SMe)3L2]2+ (L ¼ CO, CH3CN, tBuCN, XylNC) with methyl triflate give the corresponding thioether-bridged cations [Mo2Cp2(m-SMe)2(m-SMe2)L2]2+, except in the case of L ¼ CO which does not react at room temperature. The electronic properties of the ancillary ligands L thus have a crucial influence on the course of this reaction.396 Anionic reagents such as NO−3, BH−4, OH−, H−, N−3, and SH− react with [Mo2Cp2(m-SMe)nL2] species to produce a series of products depending on various L ligands.397–401 For instance, in the reaction of [Mo2Cp2(m-SMe)3(CH3CN)2][BF4] cation with NaBH4, the hydride transfer process ends in the ligand replacement stage and tetrahydroborate-bridged compound 249 is obtained with the concomitant formation of a m-azavinylidene product 250.402 [Mo2Cp2(m-SMe)3(CO)2][BF4] or [Mo2Cp2(m-SMe)3 (XylNC)2][BF4] reacts with NaBH4, resulting in hydride addition to the Cp ligand to produce compound 251 as major products. Treatment of [Mo2Cp2(m-SMe)3(tBuNC)2][BF4] with NaBH4, formimidoyl-bridged compound 252 is formed (Scheme 30). The alkylidene complex [Mo2Cp2(m-SMe)3(m-
1:
2-CHCH2Tol)][BF4] reacts with NaBH4 to afford the semibridging alkyl species [Mo2Cp2(m-SMe)3(m-CH2CH2Tol)].403

Scheme 30 (A) Cabon, N.; Le Goff, A.; Le Roy, C.; Pétillon, F. Y.; Schollhammer, P.; Talarmin, J.; McGrady, J. E.; Muir, K. W. Organometallics 2005, 24(25), 6268–6278. (B) Schollhammer, P.; Cabon, N.; Pétillon, F. Y.; Talarmin, J.; Muir, K. W. Chem. Commun. 2000, (21), 2137–2138.

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

215

Organolithium and organomagnesium reagents LiR and RMgX react with [Mo2Cp2(m-SMe)3L2][BF4] at three sites: (i) at a Cp ring, (ii) at the metal with formation of a transient metal-nucleophile bond, (iii) at a terminal ligand with possible displacement of a second ligand, depending on the relative Lewis acidities and steric constraints of the metal and ligand sites.404 Synthesis of {[Mo2Cp2(L)2(m-SMe)3]X} (L ¼ Cl, Br, I) compounds: Thermal transformation of the 34-electron dicarbonyl precursor {[Mo2Cp2(CO)2(m-SMe)3]Cl} 253 is conducted at 160  C in toluene, giving the quadruply bridged species 254 as the major product in 65% yield.405 Treatment of 254 with HPPh2 results in the substitution of one SMe3 group by PPh2 affording 255 (Eq. 76). Mo2Cp2(m-SMe)3(m-X) (X ¼ Br or I) 257 are synthesized through the reaction of 256 with 1.2 equiv. of [Bu4N]Br or [Bu4N] I at room temperature in the presence of zinc dust. The [Mo2Cp2(m-SMe)3(m-X)]+ cation 258 is obtained by addition of one equivalent HBF4 to the neutral complex Mo2Cp2(m-SMe)3(m-X) (X ¼ Br or I) 257 (Eq. 77).406

ð76Þ

Source: Ojo, W.-S.; McGrady, J. E.; Petillon, F. Y.; Schollhammer, P.; Talarmin, J. Organometallics 2011, 30(3), 649–652.

ð77Þ

Source: Le Roy, C.; Petillon, F. Y.; Muir, K. W.; Schollhammer, P.; Talarmin, J. J. Organomet. Chem. 2006, 691(5), 898–906. Multimetallic molybdenum complexes: Mixed-metal clusters having two metals with different chemical properties in close proximity have potential catalytic applications.407,408 Heating Mn2(CO)6(pyS)2 with (CpMo(CO)3)2 in toluene affords a mixture of the heterobimetallic MndMo complexes 259, a 46-electron trimolybdenum cluster 260, and a mononuclear molybdenum complex 261 (Eq. 78). The trimolybdenum cluster 260 is an electron-deficient compound, but it is stable toward CO and PR3. Compound 261 can also be obtained from the reaction of [CpMo(CO)3]2 with pyridine-2-thiol at 110  C.409

ð78Þ

Source: Begum, N.; Kabir, S. E.; Hossain, G. M. G.; Rahman, A. F. M. M.; Rosenberg, E. Organometallics 2005, 24(2), 266–271. Multimetallic complexes composed of electron-deficient early-transition metals and electron-rich late-transition metals have shown excellent reactivity due to their unique electronic properties.410–415 Mononuclear Cp IrH(SH)(PPh3) reacts with Cp MCl2(NO) (M ¼ Mo, W) in the presence of NEt3 giving an early-late heterobimetallic (ELHB) complex 262, in which both metals are chiral centers. The hydride on 262 is abstracted by [Ph3C][BF4] affording the cationic acetonitrile complex 263 (Eq. 79). The labile acetonitrile ligand is easily substituted by CO or isocyanide to form the heterodinuclear carbonyl complex or isocyanide complex, respectively.416

ð79Þ

Source: Modified from:Arashiba, K.; Matsukawa, S.; Tanabe, Y.; Kuwata, S.; Ishii, Y. Inorg. Chem. Commun. 2008, 11(5), 587–590.

216

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

5.05.1.1.5.3 Selenium and tellurium-based complexes The chalcogen chemistry of transition metals has received significant attention due to their potential applications in photovoltaic materials, industrial materials and magnetic resonance imaging.417,418 Transition metal-selenide and telluride complexes represent a wide range of structural varieties and recent investigations focus on their cluster growth reactions.419,420 The reaction of Cp2Mo2(m-PR) with E ligands (E ¼ S, Se, Te) has been reviewed in detail in Section 5.05.1.1.4.2.2. Ghosh’s group reported that chalcogen elements like Se, Te can stabilize dimolybdaboranes. Reaction of Cp MoCl4 with LiBH4
THF and followed by pyrolysis with excess dichalcogenide RE-ER (R ¼ Ph, CH2Ph, 2,6-(tBu)2-C6H2OH; E ¼ S, Se) yields a new class of hybrid clusters, such as (Cp Mo)2B5H8(SePh), (Cp Mo)2B2H5(BSR)2(m-
1-SR), (R ¼ 2,6-(tBu)2-C6H2OH), and (Cp Mo)2B2H5(BSePh)2(m-
1-SePh).421–423 Synthesis of hybrid clusters with chalcogen powders: Reaction of Cp MoCl4 with six-fold excess of LiBH4
THF followed by thermolysis with excess chalcogen powder (S, Se and Te) affords dichalcomolybda, (Cp Mo)2B4H4E2 (E ¼ S, Se, Te) in modest yield.424 The reaction of dimolybdaborane cluster which is produced from the reaction of Cp MoCl4 with LiBH4
THF with various metal carbonyls such as Fe2(CO)9, Co2(CO)8, and Ru3(CO)12 to produce triple-decker mixed metal clusters 264–266 by replacement of one of the open face boron vertices by chalcogen atoms.425–430

Cp Mo(CO)3Me reacts with various chalcogenide salts, such as Li[BH2E3] (E ¼ S, Se or Te) to afford metal chalcogenide complexes Cp Mo(CO)2(
2-S2CCH3) 267 and Cp Mo(CO)2(
1-SeC2H5) 268. Treatment of Cp Mo(CO)3Me with Li[BH3(EFc)] or Li(BH3EPh) (E ¼ S, Se or Te) gives borate complexes Cp Mo(CO)2(m-H)(m-EFc)BH2 269 (Scheme 31).431,432

Scheme 31 (A) Ramalakshmi, R.; Saha, K.; Paul, A.; Ghosh, S. J. Chem. Sci. 2016, 128(7), 1025–1032. (B) Ramalakshmi, R.; Saha, K.; Roy, D. K.; Varghese, B.; Phukan, A. K.; Ghosh, S. Chem. Eur. J. 2015, 21(48), 17191–17195.

5.05.1.1.6

Bis(cyclopentadienyl) molybdenum complexes

5.05.1.1.6.1 [CpR2MoCl2] and their derivatives The preparation of compound Cp 2MoCl2 has been reported in COMC II (1995).433 Cp 2MoCl2 is a versatile precursor for a variety of permethylmolybdenocene derivatives. The reactivity of Cp 2MoCl2 and its derivatives is shown in Scheme 32. The reaction of Cp 2MoCl2 with LiAlH4 or MeLi affords the Mo dihydride complex Cp 2MoH2 270 or the dimethyl complex Cp 2Mo(Me)2 271, respectively.434 When treated with LiOH,

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

217

Scheme 32 (A) Shin, J. H.; Churchill, D. G.; Bridgewater, B. M.; Pang, K.; Parkin, G. Inorg. Chim. Acta 2006, 359(9), 2942–2955. (B) Kruczynski, T.; Grubba, R.; Baranowska, K.; Pikies, J. Polyhedron 2012, 39(1), 25–30. (C) Lindsell, W. E.; Rosair, G. M.; Rothmann, M. M. J. Organomet. Chem. 2005, 690(1), 126–133. (D) Takanashi, K.; Lee, V. Y.; Yokoyama, T.; Sekiguchi, A. J. Am. Chem. Soc. 2009, 131(3), 916–917.

the Mo-oxo complex Cp 2Mo(O) 272 is produced. The reduction of Cp 2MoCl2 with Na(Hg) in the presence of CO affords the Mo carbonyl complex Cp 2Mo(CO) 273. Cp2MoCl2 reacts with R2PdP(SiMe3)Li yielding phosphanylphosphido complexes by formal insertion of the phosphinidene P-atom into the CdH bond of a cyclopentadiene ring and migration of a hydrogen atom in the case of Cp(C5H4PdPtBu2)MoH 274 or a SiMe3 group in the case of Cp(C5H4PdPtBu2)Mo(SiMe3) 275 and Cp[C5H4PdP(NiPr2)2] Mo(SiMe3) 276 to the Mo atom.435 Reaction of bis(cyclopentadienyl)molybdenum dichloride in ethanol with 1,3-propanedithiol either in the presence of base or its thallium derivative produces Cp2Mo(SCH2CH2CH2S) 277, which exhibits a six-membered ring with a chair conformation.436 In contrast, reactions of Cp2MoCl2 with 2,20 -thiodiethanethiol or 2,20 -oxydiethanethiol under the same conditions lead to SdC or OdC bond cleavage, to yield Cp2Mo(SCH2CH2E) (E ¼ O or S) 278 containing 5-membered rings. Crystal structures of 277 reveal envelope conformations. The reaction of Ca[(tBu2MeSi)4Si4] with Cp2MoCl2 results in the clean formation of bicyclic silylene complexes Cp2Mo(
1-Si4R4) (R ¼ SiMetBu2) 279.437 A series of bis(cyclopentadienyl) dithiolene molybdenum complexes have been synthesized by hydrolysis of the dithiolene complexes with base such as MeONa, CsOH or CsCO3 to release the corresponding dithiolene ligands in metathesis with Cp2MoCl2 (Eq. 80).438–443

ð80Þ

218

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

Source: (A) Whalley, A. L.; Blake, A. J.; Collison, D.; Davies, E. S.; Disley, H. J.; Helliwell, M.; Mabbs, F. E.; McMaster, J.; Wilson, C.; Garner, C. D. Dalton Trans. 2011, 40(40), 10457–10472. (B) Le Gal, Y.; Roisnel, T.; Dorcet, V.; Guizouarn, T.; Sady, L. P.; Lorcy, D. J. Organomet. Chem. 2015, 794, 323–329. (C) Bsaibess, T.; Guerro, M.; Le Gal, Y.; Sarraf, D.; Bellec, N.; Fourmigue, M.; Barriere, F.; Dorcet, V.; Guizouarn, T.; Roisnel, T.; Lorcy, D. Inorg. Chem. 2013, 52(4), 2162–2173. (D) Vacher, A.; Le Gal, Y.; Roisnel, T.; Dorcet, V.; Devic, T.; Barrière, F.; Lorcy, D. Organometallics 2019, 38(22), 4399–4408. (E) Taylor, A. J.; Davies, E. S.; Weinstein, J. A.; Sazanovich, I. V.; Bouganov, O. V.; Tikhomirov, S. A.; Towrie, M.; McMaster, J.; Garner, C. D. Inorg. Chem. 2012, 51(24), 13181–13194. (F) Bellec, N.; Vacher, A.; Barrière, F.; Xu, Z.; Roisnel, T.; Lorcy, D. Inorg. Chem. 2015, 54(10), 5013–5020. Bioorganometallic chemistry: The aqueous chemistry of CpR2MoCl2 has been explored.444–448 Kuo’s group reported the first aqueous investigation of the Mo(V) metallocene [Cp2MoCl2][BF4] which showed a novel and unprecedented autocatalytic reduction to give Cp2MoCl2 that is mediated by water.449 Key results of Cp2MoCl2 and derivatives that have appeared over the last 15 years include studies on peptides,450,451 ribonucleosides,452 nitrile hydration to amides,453 hydrolysis of ester,454 hydrolysis studies,455 cytotoxic,456–460 the cleavage of thiophosphinate,461–465 hydrolysis of ethers.466 Kuo and co-workers have done significant research on the degradation phosphonothioates of neurotoxin also in recent years; they reported the first case of thiophosphinate hydrolysis involving PdS bond scission by an organometallic compound Cp2MoCl2, which may open up a new class of reagents for the degradation of organophosphorus neurotoxins.461–465 A short review on bioorganometallic chemistry of Cp2MoCl2 is available.467

5.05.1.1.6.2 [CpR2MoH2] and their derivatives The synthetic methods of CpR2MoH2 (CpR ¼ Cp, Cp ) have been summarized in the previous volumes of COMC II (1995).59 The synthesis of a series of phenylchalcogenolate-hydride complexes and chalcogenido complexes from CpR2MoH2 (CpR ¼ Cp, Cp ) has been reported (Scheme 33).434 Treatment of dihydride complex Cp 2MoH2 with Ph2E2 (E ¼ S, Se or Te) affords Cp 2Mo(H) (EPh) 280. The reaction of Cp 2MoH2 with 4 equiv. of elemental chalcogen (E ¼ S, Se) gives Cp 2Mo(
2-E4) 281. Besides, Cp 2MoH2 reacts with MeI to generate the hydride-iodide complex Cp 2Mo(H)I 282, and single-crystal X-ray diffraction shows the bond length of ModI is 2.8100 A˚ .

Scheme 33

Shin, J. H.; Churchill, D. G.; Bridgewater, B. M.; Pang, K.; Parkin, G. Inorg. Chim. Acta 2006, 359(9), 2942–2955.

5.05.1.1.6.3 Ansa-bridges complexes The incorporation of an ansa bridge may have a profound influence on the chemistry of a metallocene system. Gerard Parkin and coworkers synthesized [Me2Si(C5Me4)2]MoH2 283 via reaction of MoCl5 with a mixture of [Me2Si(C5Me4)2]Li2 and NaBH4 followed by addition of CHCl3 and LiAlH4.468 A summary of the reactivity of [Me2Si(C5Me4)2]MoH2, and its derivatives is shown in Scheme 34. Upon photolysis, complex 283 is capable of inducing CdS bond cleavage of thiophene to give 284 [Me2Si(C5Me4)2]Mo(
2-C,S-SC4H4).469 In addition, selenophene reacts with [Me2Si(C5Me4)2]MoH2 in a manner analogous to that of thiophene and give the complex [Me2Si(C5Me4)2]Mo(
2-C,Se-SeC4H4) 285.470 However, the reactivity of [Me2Si(C5Me4)2] MoH2 towards furan gives the CdH bond cleaved products 286 and 287. Besides, it should be noted that the oxidation of CdH bond of furan is reversible and treatment of complexes 286 with thiophene affords complex 284. DFT calculations indicate that CdE (E ¼ O, S, Se) bond cleavage is thermodynamically favored for all of the chalcogens.

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

Scheme 34

219

Churchill, D. G.; Bridgewater, B. M.; Zhu, G.; Pang, K. L.; Parkin, G. Polyhedron 2006, 25(2), 499–512.

Metallocenophanes have attracted great attention in recent years due to their applications in catalysis and material science.471–480 Holger Braunschweig’s group synthesized complex 1,1,2,2-tetramethyl[2]disilamolybdenocenophanedihydride 288 by dilithiation of 1,2-bis(cyclopentadienyl)tetramethyldisilane and subsequent reaction with MoCl5 in the presence of NaBH4. Under photolysis, complex 288 undergoes reductive elimination of H2, and a facile oxidative addition of the SidSi bond to the molybdenum center gives complex (1, 1)disilamolybdenocenophane 289 (Eq. 81).481

ð81Þ

Source: Braunschweig, H.; Gross, M.; Radacki, K.; Rothgaengel, C. Angew. Chem. Int. Ed. 2008, 47(51), 9979–9981. DFT calculations predict that the relatively high charge of the metal center and the highly deformed geometry determine the high reactivity of compound 289 towards unsaturated organic substrates (Scheme 35). Treatment of 289 with the unsaturated substrates 2-butyne and trans-azobenzene results in the clean formation of
2-coordinated alkyne complex 290.482 The 13C NMR spectrum of complex 290 shows the alkyne carbon at 114.6 ppm, which is comparable to the 13C NMR shift at 114.8 ppm for the non-bridged derivative Cp2Mo(
2-MeCCMe).483 The reaction of compound 289 with trans-azobenzene gives the azo complex 291. The NdN bond length of 1.4061(19) A˚ is typical for
2-bound azobenzene. Complex 289 reacts with the polar substrate tBuNC through 1,2-addition of the silyl groups to the C^N bond, and coordination of the carbenoid carbon atom to the molybdenum center to form compound 292. The ModC bond length of 2.0653(12) A˚ in complex 292 is obviously shorter than the normal Fischer-type Mo-carbene complexes Mo(CO)5[CMeNHP (NiPr2)2] (2.204(3) A˚ )484 and is close to the ModC distance of the related molybdenum isonitrile complex Cp2Mo(CNtBu) (1.997(4) A˚ )485. The complex 293 has been synthesized by the reaction of 289 with P4.486 The PdP bond length of 2.1721(5) A˚ lies between PdP single bond (2.21 A˚ in P4) and double bond (2.031(3) A˚ in Cp P] PCp ,487 2.034(2) A˚ in (tBu)3C6H2dP]PdC6H2(tBu)3488), and is elongated compared to that of Cp2Mo(P2H2).489 The reaction of 289 with IMe (IMe ¼ N,N0 -dimethylimidazol-2-ylidene) produced ansa-metallocene-carbene complex 294.490 Treatment of 289

220

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

Scheme 35 Arnold, T.; Braunschweig, H.; Jimenez-Halla, J. O. C.; Radacki, K.; Sen, S. S. Chem. A Eur. J. 2013, 19(28), 9114–9117. Arnold, T.; Braunschweig, H.; Hupp, F.; Radacki, K.; Sen, S. S. Eur. J. Inorg. Chem. 2013, 5027–5032. Arnold, T.; Braunschweig, H.; Gross, M.; Kaupp, M.; Mueller, R.; Radacki, K. Chem. A Eur. J. 2010, 16(10), 3014–3020.

with dimethyl(phenyl)phosphane sulfide results in the cleavage of the P]S bond and gives complex 295. Complex 289 reacts with one of the C]N bonds in N,N0 -dicyclohexylcarbodiimide to give the
2-coordinated complex 296. The side-on coordinated CdN bond (1.303(3) A˚ ) is slightly elongated. The reaction of 289 with 2-adamantyl-1-phosphaalkyne produced complex 297. The PdC bond distance of 1.625(3) A˚ is longer than that in 2-tert-butyl-1-phosphaethyne (1.536(2) A˚ )491 but shorter than PdC double bond (1.693(2) A˚ 492 and 1.655(3) A˚ 493). Holger Braunschweig’s group also studied the reactivity of 289 towards zerovalent platinum complexes (Scheme 36).494 Treatment of complex 289 with 2 equiv. of Pt(PEt3)3 results in the formation of complex 298. The long ModPt bond length  (3.8869(14) A˚ ) and the ModSidPt angle (103.35(5) ) are indicative of a bismetalated silane complex, consistent with the 29Si 162,495 NMR resonance at −27.2 ppm. Reaction of 289 with 2 equiv. of Pt(PCy3)2 gives complex 299. The most striking feature of the molecular structure of 299 is the MoPt2 triangle, with one platinum atom incorporated into the bridging PtdSi moiety, which connects both cyclopentadienyl rings of the molybdenocene unit. To gain insight into the mechanistic aspects of the formation of

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

Scheme 36

221

Braunschweig, H.; Brenner, P.; Gross, M.; Radacki, K. J. Am. Chem. Soc. 2010, 132(32), 11343–11349.

the trinuclear cluster 299, the two fold-bridged molybdenocene 289 was treated with equimolar amount of Pt(PCy3)2. The constitutional isomers 300 and 301, both representing adducts of 289 with one Pt(PCy3) fragment, were identified by X-ray diffraction. Complex 300 presumably depicts an intermediate in the formation of the trinuclear cluster 299. However, the predominant isomer in solution, 301, was identified as the product of CdH oxidative addition to the platinum phosphine fragment.

5.05.1.1.6.4 Multimetallic complexes The reactivity of CpR2Mo(H)2 (R ¼ H, Me) towards Bi(OtBu)3 has been documented in COMC III (2007).496 Further research shows that the products of the reaction are strongly affected by the stoichiometry of substrates and the substituted groups of Bismuth.497,498 The Bi-substituted monohydride complexes 302 CpR2Mo(H)(Bi(o-tolyl)2) (R ¼ H, Me) which are difficult to isolate, were prepared via the reaction of molybdocene dihydrides with the polymer [(o-tolyl)2BiOMe]n (Eq. 82).499

ð82Þ

Source: Roggan, S.; Limberg, C.; Ziemer, B.; Siemons, M.; Simon, U. Inorg. Chem. 2006, 45(22), 9020–9031. Treatment of LBiNMe2 (L ¼ [1,8-C10H6-(NSiMe3)2]2−) with CpMe2Mo]O yields a heterobimetallic molybdenum/bismuth compound 303, wherein the two metal centers are linked by an oxide bridge as well as a s,p-binding [m-
5:
1-CH2(C5H4)]2− ligand (Eq. 83).500

222

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

ð83Þ

Source: Knispel, C.; Limberg, C. Organometallics 2011, 30(14), 3701–3703. t Inspired by reactivity studies with CpR2MoH2 (R ¼ Me) and Bi(OtBu)3, analogous reactions of CpMe 2 MoH2 with Sb(O Bu)3, 501 SbCl3, SnCl2, and SnCl4 were also examined (Scheme 37).

Scheme 37

Knispel, C.; Limberg, C.; Zimmer, L.; Ziemer, B. Z. Anorg. Allg. Chem. 2007, 633(13–14), 2278–2284.

Oxidation of the dicyclopentadienyl imido complex Cp2Mo(]NtBu) with PhIO produces the m-oxodimolybdenum dimer 308 (Eq. 84).502 DFT calculations suggest that the oxygen-atom transfer reaction is intermediated by a Mo(IV)-oxo complex. The 1H NMR and X-ray crystallography of 308 indicates a slipping of one of the
5-Cp ring to
1-Cp.

ð84Þ

Source: Askari, M. S.; Ottenwaelder, X. Dalton Trans. 2010, 39(10), 2644–2650.

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

5.05.1.2

223

Arene-containing complexes of molybdenum

Aromaticity is a fundamental concept with implications spanning the chemical sciences. Hückel’s (4n + 2) p-electron rule is the standard criterion to determine aromaticity, and it applies well to neutral arenes as well as to charged species such as the cyclopentadienyl anion and the cycloheptatrienyl cation (tropylium).503 From the structural point of view, aromatic compounds have a planar or nearly planar unsaturated cyclic structure with exceptional stability. Heterocycles, as long as they comply with Huckel’s rule, are also considered aromatic compounds.504,505 This chapter covers molybdenum arene complexes reported from 2005 to 2020, comprising four sections based on the coordination mode of the arene rings, i.e.,
2-arene,
4-arene,
6-arene,
7-arene complexes.

5.05.1.2.1

Molybdenum h2-arene complexes

Aromatic rings can coordinate to molybdenum center through carbon-carbon double bond or carbon-heteroatom double bond in
2-fashion. In molybdenum
2-arene complexes, Mo-arene bonds are stabilized primarily by the interaction of a filled metal dp orbital with a p orbital of the aromatic ligand. The shift of electron density from the metal to the arene (backbonding) prompts
2-bound aromatic systems tending to electrophilic rather than nucleophilic addition reactions. Thus, the metal acts as a protecting group for the coordinated double bond and activates the uncoordinated portion of the aromatic through p-donation.506 The synthesis of
2-coordinate arene molybdenum complexes based on p-basic metal fragments have been reviewed.507 These reviews have covered the organic reactions of molybdenum complexes supported by
2-aromatic ligands and their applications in the synthesis of novel organic substances.508–515 Additionally, Song and coworkers reported the fullerene complexes emerged as an essential category in molybdenum
2-arene complexes, and they are typically prepared from Mo(CO)3(CH3CN)3 with fullerene.516–518 Synthesis of carbon-based molybdenum
2-arene complexes: The reported carbon-based molybdenum
2-arene complexes in the last decade are largely based on TpMo(L)(NO) (Tp ¼ Hydridotris(pyrazolyl)borate) fragment (Section 5.05.2.5) and they are generally derived from MoTp(NO)(L)(I) 309 (Eq. 85). The
2-arene complexes TpMo(L)(NO)(3,4-
2-arene) 310 (L ¼ MeIm, DMAP, PMe3; MeIm ¼ 1-methylimidazole; DMAP ¼ 4-(dimethylamino)pyridine) are readily formed by reduction of iodide complex 309 with Na in the presence of arene. When a solution of the molybdenum complex 310 is treated with Brønsted acid and nucleophiles, substituted
2-1,3-cyclohexadiene complexes 311 are cleanly formed. Subsequently, iodine effectively oxidizes molybdenum
2-arene complexes 311 to release diene and regenerate the molybdenum(I) precursor 309.519

ð85Þ

Source: Liebov, B. K.; Harman, W. D. Chem. Rev. 2017, 117, 13721–13755. The structural features of molybdenum
2-arene complexes are exemplified by the Tp(MeIm)Mo(NO)(3,4-
2-naphthalene) 310a. NMR spectral analysis indicates that the protons at the two bound carbons appear at d 3.62 (d, J ¼ 8 Hz) and 3.06 (dd, J ¼ 6, 8 Hz), and the corresponding 13C signals are displayed at d 75.4 and 69.3, both showing an upfield shift compared to the free naphthalene. Single crystal X-ray diffraction analysis shows elongation of the C1dC2 bond to 1.437(5) A˚ and C2dC3 to 1.449 (5) A˚ and a contracting C3dC4 bond of 1.319(5) A˚ as shown in Eq. (85). The 1H resonances of
2-1,3-cyclohexadiene in complex 311a are shown at 3.52 (dd, J ¼ 16.7, 8.9 Hz, H4a), 2.55 (d, J ¼ 16.7 Hz, H4b), 3.23 (d, J ¼ 8.1 Hz, H3) ppm, which confirms the occurrence of dearomatization and nucleophilic addition.520 Synthesis and properties of heteroatom-substituted molybdenum
2-arene complexes: Arene-Mo complexes containing PNP (PNP ¼ 2,6-bis(2-(diisopropylphosphinoyl)-phenyl)pyridine) ligands are found to undergo interconversion of the s-bound and the p-bound pyridine.521,522 The dicationic complex 312 reacts with two equivalent tetrabutylammonium azide (TBAN3) in THF

224

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

affording nitride azide complex 313 via spontaneous azide thermolysis (Eq. 86). Salt metathesis of 313 with TMSCl affords a nitride chloride molybdenum(IV) complex 314.

ð86Þ

Source: Wan, R.; Buss, J. A.; Horak, K. T.; Agapie, T. Polyhedron 2020, 187, 114631–114638. To satisfy the electronic and geometric requirements engendered by these Mo centers, the ligand PNP in 312–314 can adaptively switch its coordination mode between pyridine nitrogen s-bonding and C]N p-bonding. The pyridine N]C distance is lengthened to 1.402(9) A˚ in 313 compare to the N]C bond observed in 312 with s-binding (1.359–1.371 A˚ ), which is consistent with electron back donation from Mo to N]C p orbital.

5.05.1.2.2

Molybdenum h4-arene complexes

Molybdenum
4-bound complexes have been reported in COMC III (2007).523 Remarkably, cycloC4 or cycloP4C4 complexes are isolated from cyclodimerization reactions of alkynes or phosphaalkynes on a variety of molybdenum templates. Herein, we review the synthesis and properties of
4-arene molybdenum complexes, with a particular focus on mononuclear Mo complexes containing neutral (NHCs)2C2P2 ligands and dianionic [cycloP4]2− ligands.524 (NHCs)2C2P2 ligands: Li and Grützmacher disclosed zero-valent molybdenum complexes [Mo(CO)3(
4-(NHCs)2C2P2)] (315, 316). The (NHCs)2C2P2 ligands are planar C2P2 rings with a delocalized, moderately aromatic, 6p-electron configuration that interacts with two N-heterocyclic carbenes (NHCs).525 When L2C2P2 reacts with Mo(CO)6 in THF at 100  C for several days, the neutral molybdenum(0) complexes 315 and 316 were isolated in high yields (86.4% and 89.6%). The 31P NMR spectra of 315 and 316 show singlet resonances at d ¼ 60.1 and d ¼ 73.4, respectively, which are largely low-frequency shifted compared to free ligand L2C2P2 (31P NMR ¼ 196.3, 227.8 ppm). The trend is observed for the 13C nuclei within the
4-bound C2P2 rings (315: 67.8 ppm; 316: 70.4 ppm), which are shifted downfield by about 70 ppm compared to ligands (−4.7 and −3.9 ppm). Combining with NMR spectroscopic studies and crystallographic analysis, DFT calculations reveal that the planar C2P2 rings are strong 6p-electron donors to the molybdenum centers.

Cyclo-P4 ligands: Herein, a new approach to white phosphorus storage and release is described. The approach involves photolytic reductive elimination of the tetrahedral P4 molecule from a mononuclear cyclo-P4 molybdenum complex. The mononuclear cycloP4 molybdenum complexes are direct electronic analogs of the classic
6-benzene molybdenum complex (
6-C6H6)Mo(CO)3, and allow for a direct assessment of the effects of metal coordination on the ostensible aromatic framework of the [cyclo-P4]2− dianion.526 Figueroa’s group has reported the interaction between molybdenum and phosphorus (P4) (Scheme 38). The molybdenum iodido complex MoI2(CO)2(CNArDipp2)2 317 (ArDipp2 ¼ 2,6-(2,6-(iPr)2C6H3)2C6H3) is a 16e− species.527 By reaction with white phosphorus, it converts to [cyclo-P4]2− complex (
4-P4)MoI2(CO)(CNArDipp2)2 318. In the presence of HSnBu3, HNiPr2, and P4, zero-valent neutral molybdenum complex (
4-P4)Mo(CO)2(CNArDipp2)2 319 is obtained.528

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

225

Scheme 38 (A) Mandla, K. A.; Moore, C. E.; Rheingold, A. L.; Figueroa, J. S. Angew. Chem. Int. Ed. 2019, 58, 1779–1783. (B) Ditri, T. B.; Moore, C. E.; Rheingold, A. L.; Figueroa, J. S. Inorg. Chem. 2011, 50, 10448–10459. (C) Figueroa, J. S.; Mandla, K. A.; Moore, C. E.; Rheingold, A. L. Angew. Chem. Int. Ed. 2018, 57, 6853–6857. (D) Mandla, K. A.; Neville, M. L.; Moore, C. E.; Rheingold, A. L.; Figueroa, J. S. Angew. Chem. Int. Ed. 2019, 58, 15329–15333.

Complexes 318 and 319 are candidates for a storage/photolytic-release system of white phosphorus. DFT calculations on the model complex 319 reveals the presence of two occupied non-bonding d-orbitals on the Mo center, consistent with a d4 electronic configuration. The interaction between the cyclo-P4 unit and the Mo center is best formulated as featuring the coordination of an ostensible six p-electron [cyclo-P4]2− dianion. Also, the calculations of 318 show that the highest-lying, filled molecular orbital manifold consists predominantly of orbitals with both iodide lone pair and PdP s-bonding character. Correspondingly, the lowest-lying unoccupied orbitals for 318 possess significant
4-P4/Mo antibonding character, which, when coupled with its formally d2 electronic configuration, signifies that lower-energy P4-releasing LMCTs (ligand-to-metal charge-transfer) relative to complex 319 may be accessible. Therefore, after the photolysis of P4 from complex 318, heating the mixture of white phosphorus and complex 320 to 80  C for 30 min, results in the complete reversion to obtain
4-P4 complex 318, thereby establishing that white phosphorus can be easily and reversibly reincorporated onto the Mo center in this system.529

5.05.1.2.3

Molybdenum h6-arene complexes

Ubiquitous
6-arene complexes display classic electrophilic reactivity at the arene and have been broadly utilized in medicinal, organometallic and synthetic organic chemistry for the functionalization and substitution of arenes.530,531 The detailed preparation of
6-arene molybdenum complexes has been well reviewed in COMC III (2007).532 This section is focused on novel structures and reactivities of (
6-arene)Mo and (
6-arene)2Mo complexes.

5.05.1.2.3.1 Carbon-substituted molybdenum
6-arene complexes Mono-arene complexes: A series of (
6-arene)Mo(CO)3 complexes have been obtained by refluxing Mo(CO)6 in the presence of substituted arene. For instance, complexes 321 containing one or multiple sterically expanded substituents on the aromatic ring are prepared with similar synthetic strategy.533–535 Mild reduction of MoCl5 with Mg metal in the presence of PMePh2 and multi-dentate phosphine ligands readily affords zero-valent Mo complex (
6-C6H5PMePh)Mo(triphos) (triphos ¼ 1,1,1-tris-(diphenylphosphinomethyl)ethane) 322. Multi-dentate phosphine ligands play a significant role in steric restriction of the geometry.536,537

226

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

Mo(PMe3)6 is also a potent precursor for Mo
6-arene complexes. The
6-arene compounds (
6-PhH)Mo(PMe3)3 323, (
-NpH)Mo-(PMe3)3 324, and the anthracene complex (
6-AnH)Mo(PMe3)3 325 can be conveniently obtained via reaction of Mo(PMe3)6 with PhH (benzene), NpH (naphthalene), and AnH (anthracene), respectively (Scheme 39). Oxidative addition of H2 to anthracene complex 325 affords the
4-anthracene compound (
4-AnH)Mo(PMe3)3H2 326, which is accompanied by an
6-to-
4 reversible haptotropic shift of the anthracene ligand. Reductive elimination of H2 results in regeneration of 325.538 6

Scheme 39

Zhu, G.; Janak, K. E.; Figueroa, J. S.; Parkin, G. J. Am. Chem. Soc. 2006, 128, 5452–5461.

Heating para-terphenyl diphosphine in the presence of Mo(CO)3(MeCN)3 cleanly affords the
2-arene molybdenum complex 327 (Eq. 87). Decarbonylation of this molybdenum(0) compound 327 is achieved through sequential oxidation-photolysischemical reduction under an N2 atmosphere, which eventually affords zero-valent molybdenum dinitrogen complex 330. Compound 327 reveals the
2-binding mode with partial disruption of aromaticity in the central ring. The solid-state structures and spectroscopic data of 328–330 are consistent with the
6-binding fashion.539

ð87Þ

Source: Buss, J. A.; Edouard, G. A.; Cheng, C.; Shi, J.; Agapie, T. J. Am. Chem. Soc. 2014, 136, 11272–11275.

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

227

The aromatic ring ligand plays a significant role in stabilizing the metal center in different oxidation state by switching its coordination mode between
2,
4, and
6. Herein, the role of the pendant arene acted as a hemilabile ligand to stabilize metal in various oxidation states is demonstrated by complex 328 in carbon monoxide deoxygenation and coupling (Eq. 88). Reduction of MoII-arene carbonyl compound 328 with KC8 results in the formation of a new
6-arene molybdenum(0) 16e− complex 331 and the polyanionic dicarbonyl complex 332. Trianion 332 adopts a polynuclear structure, with an
4 metal-arene interaction, a deplanarization of the central arene ring of 43.7 . Treatment of compound 332 with iPr3SiCl affords a silyl carbyne complex 333 via CdO bond cleavage and CdSi formation. Subsequent CO deoxygenation and CdC coupling occur through addition of KC8 to 333, giving C2O1 fragments. Molybdenum complexes 331–333 coordinated by para-terphenyl diphosphine ligands have shown versatile reactivities which not only play a significant role in the activation of carbon monoxide540–542 but also promote the reaction of proton induced carbon dioxide cleavage,543 the dehydrogenation reaction of ammonia borane,539 etc.544,545

ð88Þ

Source: Buss, J. A.; Agapie, T. J. Am. Chem. Soc. 2016, 138, 16466–16477. Bis-arene complexes: The preparation of bis-arene molybdenum complexes Mo(
6-arene)2 has been reviewed in COMC III (2007).546 The homoleptic sandwich complex bis(benzene)molybdenum Mo(
6dC6H6)2 can be dilithiated by BuLi at slightly elevated temperature to produce the highly reactive, ring metalated species [Mo-(
6-C6H5Li)2]
tmeda 334 (tmeda ¼ N,N,N0 , N0 -tetramethylethylenediamine). Addition of the dilithio precursor to dihalide X2BNR2 (X ¼ Cl, Br; R ¼ iPr, SiMe3) or i Pr2SiCl2 facilitates the isolation of unstrained 1,1-disubstituted derivatives Mo{
6-C6H5(BN(SiMe3)2X)}2 (X ¼ Cl, Br) 335 or Mo{
6-C6H5(SiiPr2Cl)}2 336, respectively.547,548

5.05.1.2.3.2 Heteroatom-substituted molybdenum
6-arene complexes Mono-arene complexes: The
6-silabenzene549 or
6-germabenzene550 molybdenum complexes are given by ligand-exchange reactions between Mo(CO)3(CH3CN)3 and silabenzene or germabenzene. This synthetic method is covered in COMC III (2007).551 Herein, we mainly discuss heteroatom-substituted
6-arene complexes with the formulation of (
6-NHetH)Mo(PMe3)3 (NHetH ¼ pyridine, pyrimidine, pyrazine, triazine, quinolone, isoquinoline, quinoxaline, phenazine, etc.). Mo(PMe3)6 reacts reversibly with N-heterocyclic arene to give (
2-C,N-arene)MoH(PMe3)4 337, which involves oxidative addition of a CdH bond adjacent to the nitrogen atom. At higher temperature, the
6-heterocycle coordinated complexes (
6-C5N)Mo(PMe3)3 338 are obtained from complexes 337. Both
2-arene complexes 337 and 338 react with H2 at elevated temperature to give Mo(PMe3)4H4 339 with the release of pyridine or 1,2,3,4-tetrahydroquinoline, respectively (Scheme 40).552 Notably, Mo(PMe3)4H4 can catalyze the hydrogenation of quinolone and its derivatives to 1,2,3,4-tetrahydroquinoline and its derivatives.553,554

Scheme 40 (A) Zhu, G.; Pang, K.; Parkin, G. J. Am. Chem. Soc. 2008, 130, 1564–1565. (B) Zhu, G.; Pang, K.; Parkin, G. Inorg. Chim. Acta. 2008, 361, 3221–3229. (C) Sattler, A.; Zhu, G.; Parkin, G. Multiple J. Am. Chem. Soc. 2009, 131, 7828–7838.

228

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

Bis-arene complexes: Heteroarenes of the group 15 elements EC5H5 (E ¼ NdBi) in Mo(EC5H5)2 complexes 340 are ambient ligands which can either bind in
1 fashion via the s-lone electron pair of atom E or in
6 fashion through the six p electrons of the aromatic ring.555 The coordination chemistry of this class of ligands has been systematically investigated with the focus on the preparation of neutral compounds. For instance, with the combination of Mo/C5H5As, the primary product is the sandwich complex bis(
6-arsenine)molybdenum which is synthesized by means of metal-atom ligand-vapor co-condensation (CC). The bis(
6-arsenine)molybdenum can be exploited for the synthesis of hexa(
1-arsenine)molybdenum in excess of C5H5As.556

5.05.1.2.4

Molybdenum h7-arene complexes

Previous work on
7-cycloheptatrienyl (cht) molybdenum complexes has been reviewed in COMC II (1995)557 and COMC III (2007).558 This section covers recent synthetic and reactivity studies about Mo
7-cycloheptatrienyl alkynyl complexes, metallacumulenylidene complexes, and others. Molybdenum
7-arene complexes with highly unsaturated carbon-chain ligands have attracted extensive attention due to their applications as molecular materials in electronic, magnetic and optical devices.559 The monometallic alkynyl complexes 341 are commonly synthesized from the reaction of MoX(L-L)(
7-C7H7) (L-L ¼ bipy, dppe; X ¼ Br, I) with Li(C^C)nR in THF,560,561 which have been reviewed in COMC III (2007).558 Alternatively, 341 can be obtained from [MoX(L-L)(
7-C7H7)] with Me3Si(C^C)nR in the presence of KF.562 Complexes 341 are potent precursors for a series of carbon chain bridged bimetallic systems as shown in Scheme 41. A methanol solution of 341 is treated with MoBr(dppe)(
7-C7H7) and KOtBu, and the reaction mixture is refluxed for 6 h to give the diyndiyl-bridged bimetallic complex 342. Further reaction of 342 with 1 equiv. of [FeCp2][PF6] affords the mixed-valence complex 344 in good yield (78%).563,564 The infrared data of the alkyne carbons at 1900–2100 cm−1 confirms the diyndiyl-bridged structure of 342 and 344. Additionally, complex 341 undergoes oxidative dimerization in the reaction with [FeCp2][PF6] to give the bis(vinylidene) complexes [{Mo(L-L)(
7-C7H7)}2(m-C^CRCR^C)][PF6]2 (R ¼ H, C^CSiMe3) 343. Deprotonation of 343 (R ¼ H) with KOtBu followed by aerial oxidation gives [{Mo(dppe)(
7-C7H7)}2(m-C4)][PF6] 344 as the major product.565,566

Scheme 41 (A) Roberts, H. N.; Brown, N. J.; Edge, R.; Fitzgerald, E. C.; Ta, Y. T.; Collison, D.; Low, P. J.; Whiteley, M. W. Organometallics 2012, 31, 6322–6335. (B) Brown, N. J.; Collison, D.; Edge, R.; Fitzgerald, E. C.; Helliwell, M.; Howard, J. A. K.; Lancashire, H. N.; Low, P. J.; McDouall, J. J. W.; Raftery, J.; Smith, C. A.; Yufit, D. S.; Whiteley, M. W. Organometallics 2010, 29, 1261–1276. (C) Brown, N. J.; Lancashire, H. N.; Fox, M. A.; Collison, D.; Edge, R.; Yufit, D. S.; Howard, J. A. K.; Whiteley, M. W.; Low, P. J. Organometallics 2011, 30, 884–894. (D) Fitzgerald, E. C.; Ladjarafi, A.; Brown, N. J.; Collison, D.; Costuas, K.; Edge, R.; Halet, J.-F.; Justaud, F.; Low, P. J.; Meghezzi, H.; Roisnel, T.; Whiteley, M. W.; Lapinte, C. Organometallics 2011, 30, 4180–4195. (E) Brown, N. J.; Collison, D.; Edge, R.; Fitzgerald, E. C.; Low, P. J.; Helliwell, M.; Ta, Y. T.; Whiteley, M. W. Chem. Commun. 2010, 46, 2253–2255. (F) Fitzgerald, E. C.; Brown, N. J.; Edge, R.; Helliwell, M.; Roberts, H. N.; Tuna, F.; Beeby, A.; Collison, D.; Low, P. J.; Whiteley, M. W. Organometallics 2012, 31, 157−169.

A series of vinylidene complexes (345, 346) and oxacyclocarbene complexes 347 with the cycloheptatrienyl molybdenum auxiliary have been synthesized from the reaction of MoX(L-L)(
7-C7H7) (X ¼ Br, I; L-L ¼ dppe, bipy, dppm) with the terminal alkynes HC^CR in the presence of KPF6, or reaction of [Mo(
6-C6H5CH3)(
7-C7H7)] with the terminal alkynes HC^CR in refluxing condition (Scheme 42).567–570

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

229

Scheme 42 (A) Grime, R. W.; Helliwell, M.; Hussain, Z. I.; Lancashire, H. N.; Mason, C. R.; McDouall, J. J. W.; Mydlowski, C. M.; Whiteley, M. W. Organometallics 2008, 27, 857–871. (B) Tamm, M.; Dreßel, B.; Lügger, T. J. Organomet. Chem. 2003, 684, 322–328. (C) Carter, E.; Collison, D.; Edge, R.; Fitzgerald, E. C.; Lancashire, H. N.; Murphy, D. M.; McDouall, J. J. W.; Sharples, J.; Whiteley, M. W. Dalton Trans. 2010, 39, 11424–11431. (D) Kuwabara, T.; Sakajiri, K.; Oyama, Y.; Kodama, S.; Ishii, Y. Organometallics 2019, 38, 1560–1566.

Recently, Tamm’s group has reported the isolation of stable cationic 16-electron molybdenum complexes of the type [(
7-C7H7)Mo(BLR)]+ (R ¼ iPr, Me) 348 containing the neutral 1,2-bis(imidazolin-2-imino)ethane ligands (BLiPr ¼ 1,2-bis(1,3-diisopropyl-4, 5-dimethylimidazolin-2-imino)ethane; LMe ¼ 1,2-bis(1,3,4,5-tetramethylimidazolin-2-imino)ethane). The reaction of the ligands 1,2-bis(imidazolin-2-imino)ethane with the cycloheptatrienyl-molybdenum complexes [(
7-C7H7)Mo(CH3CN)3]X (X ¼ BF4, PF6) leads to the formation of stable 16-electron half-sandwich complexes [(
7-C7H7)Mo(BLR)]X (X ¼ BF4, PF6) 348, which are crystallographically characterized to reveal undistorted two-legged piano stool geometries. Complexes of the sterically crowded, bis(triphenylphosphine) auxiliary [Mo(CO)(PPh3)2(
7-C7H7)][PF6] 349 have been synthesized from low-temperature reaction of MoMe(CO)(PPh3)(
7-C7H7) with [Ph3C][PF6] in the presence of PPh3.571–573

5.05.2

Non-cyclic p complexes of molybdenum

This subchapter covers molybdenum complexes with non-cyclic p ligands. It is organized based on the nature of the ligands and is divided into:
2-alkene/alkyne complexes,
3-allyl complexes,
3-carboxylate/amidinate/thiocarboxylate complexes, and complexes containing other heteroatom-based p ligands, such as ketone, amide, and nitrile.

230

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

5.05.2.1

Alkene complexes

In this section, non-cyclic Mo(0), Mo(I), Mo(II), Mo(III), and Mo(IV) complexes with alkenyl ligands are reviewed in sequence. Mo(0) complexes containing alkenes: A typical method of synthesizing Mo(0)-alkene complexes is the reduction of molybdenum chloride precursors in the presence of the alkene ligand. For example, coordination of phosphine-based tridentate ligand such as triphos (triphos ¼ (Ph2PCH2CH2)2PPh) and pincer PNP (PNP ¼ 2,6-bis(diphenylphosphinomethyl)pyridine) to MoCl3(THF)3 provides trichloro molybdenum complexes in L3MoCl3 type.574,575 These Mo(III) complexes are then reduced by Na/Hg in the presence of ethylene to afford the molybdenum-ethylene compound (triphos)Mo(N2)2(C2H4) 350576,577 and (PNP)Mo(C2H4)3 351578, respectively. Alternatively, complex 350 can be synthesized by the reaction of (triphos)MoCl3 with NaEt3BH followed by exposing the reaction mixture to ethylene.579

Other methods of synthesizing the Mo(0)-alkene complexes involve using Mo(0) complexes as precursors.580–584 Notably, the diphosphine chelating ligand, dppe, in Mo(dppe)(k4-P4) (P4 ¼ meso-o-C6H4(PPhCH2CH2PPh2)2)248 can be directly substituted by 1,5-COD to produce the alkene complex 352.581 Photolyzing the solution of the allylaminocarbene complex (OC)5Mo] C(NHCH2CH]CH2)Fc results in dissociation of the CO ligand and coordination of the alkene in the side-chain to the Mo center, affording an alkene complexes 353.582,583

Bernskoetter and co-workers reported that the coordination of C2H4 in (triphos)Mo(N2)2(C2H4) 350 is labile, as reflected by the reversible conversions with (triphos)Mo(N2)2(PPh3) 354 (Scheme 43).585 Both 350 and 354 are reactive towards CO2 with ethylene, affording an acrylate Mo(II)dH compound.576,577 Reaction of 350 with CO2 gives a dimeric hydride compound [(triphos)Mo(H)(CO2CH]CH2)]2 355, and the subsequent addition of PPh3 affords (triphos)Mo(H)(PPh3)(CO2CH]CH2) 356. The 1H NMR spectrum of 356 displays the 1H resonance at 4.68, 5.15, and 5.48 ppm for the acrylic olefin, while the ModH signal is observed at −4.71 ppm. Alternatively, the monomeric hydride 356 can be synthesized by the reaction of 354 or (Triphos)MoH4PPh3579 with CO2 and C2H4.

Scheme 43

Zhang, Y.; Hanna, B. S.; Dineen, A.; Williard, P. G.; Bernskoetter, W. H. Organometallics 2013, 32(14), 3969–3979.

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

231

In contrast, the pincer-type molybdenum(0) ethylene complex 357 is unable to activate CO2 (Eq. 89). Instead, one of the C2H4 ligand in 357 can be replaced by CO2 to give Mo(C2H4)2(CO2)(PNP) 358, which exhibits both
2-C2H4 and k2-CO2 ligands. In the 13C NMR spectrum of 358, a triplet at d 200.8 (2JC–P ¼ 14 Hz) is displayed for the CO2, while the signals for the
2-C2H4 is shown at d 43.8 as a broad peak and d 50.6 as a triplet (2JC–P ¼ 4 Hz). Crystallographic analysis indicates that binding CO2 at the Mo(0) in k2-fashion increased the C]O bond length by 0.05 A˚ that is 1.261(4) A˚ vs. 1.213(4) A˚ .578

ð89Þ

Source: Álvarez, M.; Galindo, A.; Perez, P. J.; Carmona, E. Chem. Sci. 2019, 10(37), 8541–8546. Mo(I) complexes containing alkenes: A representative example of molybdenum(I) alkene complex supported by terpyridine ligand is shown in Eq. (90). After chloride abstraction from (PhTpy)(PPh2Me)2MoCl (PhTpy ¼ 40 -Ph-2,20 ,60 ,200 -terpyridine)586 with NaBArF4, the Mo(I) center can bind an ethylene molecule to form [(PhTpy)(PPh2Me)2Mo(C2H4)]+ 359.587 The C]C bond distance in 359 is 1.425(6) A˚ , 0.08 A˚ longer than that of a free C2H4 molecule. Notably, complex 359 reacts with a Mo(I) ammonia complex [(PhTpy)(PPh2Me)2Mo(NH3)][BArF4]588, resulting in formations of Mo(II) ethyl complex [(PhTpy)(PPh2Me)2Mo(CH2CH3)][BArF4] 360 and amido complex [(PhTpy)(PPh2Me)2Mo(NH2)][BArF4]. Addition of 2,4,6-tritert-butylphenoxyl radical to 360 regenerates 359. Crystallographic analysis of 360 reveals a b-agostic interaction between the ethyl moiety and the metal center. The shortened ethyl CdC contact (1.452(4) A˚ ) is consistent with the ethyl carbons increased sp2 character.

ð90Þ

Source: Bezdek, M. J.; Chirik, P. J. J. Am. Chem. Soc. 2018, 140(42), 13817–13826. Mo(II) complexes containing alkenes: Reduction of molybdenum(III) chlorides (iPrPDI)MoCl3 (iPrPDI ¼ 2,6-(2,6-iPr2C6H3N] CMe)2C5H3N)589 in benzene gives molybdenum(II) benzene complex (iPrPDI)Mo(C6H6) 361. The benzene ligand can then be replaced by NH3 and ethylene to afford complex 362 containing both NH3 and C2H4 ligands (Eq. 91).531 When the reduction reaction is conducted in the presence of ethylene, diethylene complex (iPrPDI)Mo(C2H4)2 363 is produced. Interestingly, changing the 2,6-diisopropyl substituent on the aromatic ring to 2,4,6-trimethyl (MesPDI) leads to complex (MesPDI)Mo(
4-butadiene) (
2-ethylene) 364, which exhibits a 1,3-butadiene ligand and an ethene ligand (Eq. 92).590 The reaction pathway involves the coupling of ethylene to 1-butene and followed by allylic dehydrogenation to produce 1,3-butadiene.

ð91Þ

Source: Margulieux, G. W.; Bezdek, M. J.; Turner, Z. R.; Chirik, P. J. J. Am. Chem. Soc. 2017, 139(17), 6110–6113.

232

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

ð92Þ

Source: Joannou, M. V.; Bezdek, M. J.; Al-Bahily, K.; Korobkov, I.; Chirik, P. J. Organometallics 2017, 36(21), 4215–4223. Under ethylene atmosphere, reducing (PNMeP)MoCl3 (PNMeP ¼ MeN(CH2CH2PPh2)2) with excess Na/Hg affords (k4-PNCH2P)Mo(H)-(C2H4)2 366 (Eq. 93).591 Especially, the formation of 366 involves the intermediate of (PNMeP)Mo (C2H4)2 365 featuring b-agostic C − H interaction between the N-methyl group and the metal center. Crystallographic analysis reveals that the coordination sphere is saturated by a b-agostic C − H interaction from the N-methyl group. Dissolving the crystals of 365 in benzene-d6 at ambient temperature leads to the conversion to the cyclometalated product 366, which is supported by a characteristic triplet ModH resonance at −2.06 ppm (2JP−H ¼ 19 Hz) and a complex multiplet signal from 1.42 to 1.48 ppm assigned to the cyclometalated NdCH2 in 1H NMR spectrum.

ð93Þ

Source: Zhang, Y.; Williard, P. G.; Bernskoetter, W. H. Organometallics 2016, 35(6), 860–865. The abstraction of hydrogen atoms from NH3 in 362 by 3 equiv. of tBu3ArO radical regent affords molybdenum(IV) nitride complex (iPrPDI)Mo(N)(C2H4) 367 (Eq. 94).531 The coordination of ethylene (C]C 1.410(4) A˚ ) and the terminal nitride ligand (Mo^N 1.653(2) A˚ ) in 367 is determined by X-ray diffraction. The ModH in complex 366 is proved to be sufficiently nucleophilic for CO2 insertion, generating molybdenum(II) formate complex (k4-PNMeP)dMo(C2H4)(k2-O2CH) 368, which is different from the acrylate product 356 (Eq. 95). The C − O bond lengths of 1.247(6) and 1.257(7) A˚ in 368 indicate strong resonance delocalization of the C]O bond in the formate ligand. Importantly, complex 368 is capable of catalyzing hydrogenation of CO2 to formate under basic conditions.591

ð94Þ

Source: Margulieux, G. W.; Bezdek, M. J.; Turner, Z. R.; Chirik, P. J. J. Am. Chem. Soc. 2017, 139(17), 6110–6113.

ð95Þ

Source: Zhang, Y.; Williard, P. G.; Bernskoetter, W. H. Organometallics 2016, 35(6), 860–865.

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

233

Mo(III) complexes containing alkenes: Synthesis of Mo(III) alkene complex is challenging due to their highly reactive nature. [{3,5(2,4,6-iPr3-C6H2)2C6H3NCH2CH2}3N]3− ([HIPTN3N]3−) is found to be a suitable scaffold to stabilize Mo(III) complexes. The synthesis of [HIPTN3N]MoCl complex 369 (Eq. 96) is reported, and it is applied to study the conversion of dinitrogen to ammonia.592 Reducing 369 with Na/Hg under an ethylene atmosphere affords [HIPTN3N]Mo(C2H4) 370, a low-spin Mo(III) ethylene complex.593 Oxidation of 370 by [Cp2Fe][BArF4] gives cationic Mo(IV) ethylene complex [{HIPTN3N}Mo(C2H4)][BArF4]. The oxidation causes the framework of [HIPTN3N]Mo(C2H4) to become more compact.

ð96Þ

Source: Byrnes, M. J.; Dai, X.; Schrock, R. R.; Hock, A. S.; Müller, P. Organometallics 2005, 24, 4437–4450. Mo(IV) complexes containing alkenes: Schrock-type alkylidene complexes Mo(NAr)(CHCMe2Ph)(OTf )2(dme) 371 have been reviewed in COMC III (2007)594, and they are versatile precursors for molybdenum(IV) alkene complexes.595–602 For instance, alkylidene complexes 371 slowly reacts with ethylene (60 psi) to give ethylene complex Mo(NAr)(CH2CH2)(OTf )2(dme) 372 (Scheme 44). Addition of lithium 2,5-dimethylpyrrolide (Me2PyrLi) to 372 in the presence of ethylene produces Mo(NAr) (CH2CH2)(Me2Pyr)2 373. The solid-state structure of 373 confirms that the pyrrolide ligands are bound in an
1 and
5 manner.597 Otherwise, addition of 2 equiv. of Me2PyrLi to a diethyl ether solution of 371 produces Mo(NAr)(CHCMe2Ph)(Me2Pyr)2 and subsequent addition of Ph3SiOH to this complex yields Mo(NAr)(CHCMe2Ph)(OSiPh3)(
1-Me2Pyr) species 374.598,599 Exposure of heptane solutions of 374 to ethylene atmosphere leads to formation of the ethylene complex Mo(NAr)(CH2CH2)(
5-Me2Pyr) (OSiPh3) 375. Furthermore, protonolysis of the pyrrolide ligand in 375 with Ph3SiOH gives dialkoxyl molybdenum complex 376.597

Scheme 44 (A) Marinescu, S. C.; King, A. J.; Schrock, R. R.; Singh, R.; Muller, P.; Takase, M. K. Organometallics 2010, 29(24), 6816–6828. (B) Singh, R.; Czekelius, C.; Schrock, R. R.; Müller, P.; Hoveyda, A. H. Organometallics 2007, 26, 2528–2539. (C) Singh, R.; Schrock, R. R.; Müller, P.; Hoveyda, A. H. J. Am. Chem. Soc. 2007, 129, 12654–12655.

As an analog of Schrock-type alkylidene complex, Mo(]NtBu)(OSitBu3)2(PMe3) reacts with ethylene by replacing the PMe3 ligand to give molybdenum(IV) ethylene complex 377 (Eq. 97).603 Nikonov and co-workers reported the reaction of (ArN])2Mo(PMe3)604 with PhSiH3 produces the b-agostic NSi-H. . .Mo silylamido (
3-ArN-SiHPh-H) complex 378. Treatment of 378 with ethylene cleanly affords the ethyl vinylsilyl derivative 379a. Similarly, reaction of 378 with styrene prepares the hydrido vinylsilyl complex 379b, which probably emerges from styrene coupling with the silanimine intermediate, followed by CdH activation in the CH2CHPh fragment (Eq. 98).605,606

ð97Þ

Source: Rosenfeld, D. C.; Wolczanski, P. T.; Barakat, K. A.; Buda, C.; Cundari, T. R.; Schroeder, F. C.; Lobkovsky, E. B. Inorg. Chem. 2007, 46(23), 9715–9735.

234

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

ð98Þ

Source: Khalimon, A. Y.; Simionescu, R.; Kuzmina, L. G.; Howard, J. A.; Nikonov, G. I. Angew. Chem. Int. Ed. Engl. 2008, 47(40), 7701–7704.

5.05.2.2

Alkyne complexes

The synthesis and properties of non-cyclic molybdenum alkyne complexes in various valence states are reviewed in this section.593,603,607–615 Molybdenum alkyne complexes can be obtained through ligand substitution. For example, the five-coordinate alkyne complex Mo(
2-RC^CR0 )(k4-P4) 380 is synthesized by treatment of the Mo(0) tetraphosphine complex Mo(dppe)(k4-P4) with internal alkynes RC^CR0 .611 Reaction of [HIPTN3N]Mo(N2) with 2–10 equiv. of acetylene affords paramagnetic [HIPTN3N]Mo(C2H2) 381.593 Schrock-type alkylidene complex, Mo(]NtBu)(OSitBu3)2(PMe3) reacts with 2-butyne by replacing of the PMe3 ligand to give the mono-2-butyne adduct 382.603

The monocarbonyl molybdenum(II) alkyne complexes (trop)2Mo(CO)(RC^CR0 ) 383 and (acac)Mo(CO)(RC^CR0 ) 384 (trop ¼ tropolonate; acac ¼ acetylacetonate) can be directly synthesized through the reaction of [Mo(CO)4I3]− with tropolone or acetylacetone in the presence of NEt3, followed by coordination of alkyne (Eq. 99).612 Alkyne complexes 383, 384 are oxidized in air to give the d2 species (trop)2Mo(O)(RC^CR0 ) 385 and (acac)2Mo(O)(RC^CR0 ) 386 in which the alkyne has been rotated perpendicular to the ModO bond and locked in position.

ð99Þ

Source: Becica, J.; Jackson, A. B.; Koronkiewicz, B. M.; Piro, N. A.; Kassel, W. S.; West, N. M. Polyhedron 2014, 84, 51–58. The reaction of [Ti](C^CSiMe3)2 toward Mo(CO)5(thf ), Mo(CO)4(nbd), and Mo(CO)3(MeC^N)3 results in the formation of the heterobimetallic complexes [[Ti](m-
1:
2-C^CSiMe3)][Mo(m-
1:
2-C^CSiMe3)(CO)4] 387. The molecular structure indicates that in doubly alkynyl-bridged 387, the alkynides are bridging the metals Ti and Mo as a s-donor to one metal and as a p-donor to the other with the [Ti](C^CSiMe3)2Mo core being planar. The bimetallic alkyne compound 387 reacts with O2 and undergoes decarbonylation, leading to molybdenum(IV) bis(alkyne)MoO2 complex 388 (Eq. 100).614,615

ð100Þ

Source: Hildebrandt, A.; Mansilla, N.; Rheinwald, G.; Rueffer, T.; Lang, H. J. Organomet. Chem. 2011, 696(20), 3231–3237.

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

5.05.2.3

235

Allyl complexes

Mononuclear molybdenum allyl complexes: Molybdenum(II) complexes containing Mo(allyl)(CO)2 fragment have been widely investigated. The synthetic strategy of molybdenum allyl complexes is generally derived from [MoX(
3-allyl)(CO)2(MeCN)2] (X ¼ Br, Cl)616 via ligand substitutions, which has been reviewed in COMC I (1982)96 and COMC II (1995)617. MoX(
3-allyl) (CO)2(MeCN)2 can react with different amount of pyrazoles618, imidazoles619, NaNCBH3620, ferrocenoyl imidazoles621, thiadiazoles622 to provide mono-, di-, and tri-substituted Mo-allyl complexes, accordingly. Reactions of MoX(
3-allyl)(CO)2(MeCN)2 with stoichiometric amount bidentate126,623–637 and tridentate619,633,638,639 ligands give complexes of MoX(
3-allyl)(CO)2(L-L) or [Mo(
3-allyl)(CO)2(L-L-L)]n+ (n ¼ 0 or 1) (Scheme 45). MoX(
3-allyl)(CO)2L2 can further convert to a series of novel molybdenum allyl derivatives.640–649 For instance, The azide complexes Mo(N3)(
3-allyl)(bpy)(CO)2 (bpy ¼ 2,2-bipyridine) are prepared from the corresponding halides MoCl(
3-allyl)(bpy)(CO)2 with NaN3.645

Scheme 45 (A) Martinho, P. N.; Quintal, S.; Costa, P. J.; Losi, S.; Felix, V.; Gimeno, M. C.; Laguna, A.; Drew, M. G. B.; Zanello, P.; Calhorda, M. J. Eur. J. Inorg. Chem. 2006, 4096–4103. (B) Alonso, J. C.; Neves, P.; Pires da Silva, M. J.; Quintal, S.; Vaz, P. D.; Silva, C.; Valente, A. A.; Ferreira, P.; Calhorda, M. J.; Felix, V.; Drew, M. G. B. Organometallics 2007, 26, 5548–5556. (C) Saraiva, M. S.; Nunes, C. D.; Nunes, T. G.; Calhorda, M. J. J. Mole. Catal. A: Chem. 2010, 321, 92–100. (D) Honzícková, I.; Honzícek, J.; Vinklárek, J.; Padelková, Z. Polyhedron 2014, 81, 364–369. (E) Quintal, S.; Fedi, S.; Barbetti, J.; Pinto, P.; Felix, V.; Drew, M. G. B.; Zanello, P.; Calhorda, M. J. J. Organomet. Chem. 2011, 696, 2142–2152. (F) Abernethy, R. J.; Foreman, M. R. S. J.; Hill, A. F.; Tshabang, N.; Willis, A. C.; Young, R. D. Organometallics 2008, 27, 4455–4463.

Besides MoX(
3-allyl)(CO)2(MeCN)2, [Mo(
3-methallyl)(CO)3(THF)3]+ is also a suitable synthetic precursor in the synthesis of Mo-allyl complexes (Eq. 101). The in situ reaction of [Mo(
3-methallyl)(CO)3(THF)3]+ with the a-diimine ligand such as bipyridine at room temperature offers the cationic
3-methallyl molybdenum complexes Mo(
3-methallyl)(CO)3(bpy).650

ð101Þ

Source: Turki, T.; Guerfel, T.; Bouachir, F. Polyhedron 2009, 28, 569–573.

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Cyclic and Non-Cyclic Pi Complexes of Molybdenum

Exo-endo isomerization is a common feature of all these complexes in solution, and this structural fluxionality has been covered by relevant review651 and COMC III (2007)98.

The average bond length of Mo-Callyl is about 1.95–2.05 A˚ . Absorption bands of the carbonyl groups appear at 1850 cm−1 and 1950 cm−1. Representative complexes and their 13C NMR and IR spectra are given in Table 2. One of the most relevant catalytic applications of these Mo-allyl complexes has been as precursors in the epoxidation of olefins and other molecules in the presence of t-butyl hydroperoxide or other oxidant.624,652,653 The reaction is initiated by oxidation of Mo(II) to Mo(VI), losing the allyl and the carbonyls, gaining oxides, and retaining the L-L ligand(s). Their versatility makes functionalization to graft or intercalate in various materials very easy.627,654–663 Binuclear molybdenum allyl complex. MoX(
3-allyl)(CO)2L2 complexes can be suitable precursors to access binuclear molybdenum allyl complexes. Synthetic methods to prepare these binuclear complexes generally follow one of the following pathways: (a) [MoX(
3-allyl)(CO)2(NCMe)2] + L. (b) [MoX(
3-allyl)(CO)2L2] + NaX. (c) [MoL0 (
3-allyl)(CO)2L2] + base. Treatment of MoCl(
3-allyl)(CO)2(NCMe)2 with tBu2pzK (pz ¼ pyrazole) leads to the dimeric complex 393.641 Reactions of cis[Mo(
3-methallyl)Cl(CO)2(NCMe)2] with 0.5 equiv. of pyrazolate (pz) NnBu4 salts give bimetallic anionic complexes [NnBu4] [{Mo(
3-methallyl)(CO)2(m-Cl)}2(m-pz )] 394 (method (a)).664 By the reaction with NaCN, complex [Mo(
3-allyl)CO2(en)2] (en ¼ ethylenediamine) is assembled to cyanide-bridged dinuclear molybdenum compound 395, which adopts the asymmetric endo-conformation. The reactions of cis-[MoCl(
3-methallyl)(CO)2(NCMe)2] with Na(NCNCN) and pz H (pyrazole, 3,5-dimethylpyrazole) lead to complex cis-[Mo(
3-methallyl)(CO)2(pz H)-(m-NCNCN-k2-N,N)]2 396 as yellow solids.665 The terminal carbon atoms of the methallyl group are oriented over the carbonyl groups, as has been demonstrated to be the most energetically favorable arrangement (method (b)).666 Dimolybdenum complexes [cis-{Mo(
3-allyl)CO2(m-pypz)}]2 and [cis-{Mo (
3-methallyl)CO2(m-pypz)}]2 397 are synthesized by treating the corresponding [Mo(Br)(
3-allyl)CO2(pypzH)] (pypzH ¼ 3-(2-pyridyl)pyrazole) precursor with an equimolar amount of NaOMe (method (c)).667

Table 2

Selected Mo-allyl complexes and their spectroscpoic data.

Complex

13

vCO (cm−1)

References

389 390 391 392

59.9, 74.0 59.1, 70.8 57.6, 73.7 53.5, 77.7

1945, 1853 1940, 1846 1931, 1833 1934, 1850

619 616 627 649

C NMR of allyl group (CH2, CH ppm)

(A) Perez, J.; Morales, D.; Nieto, S.; Riera, L.; Riera, V.; Miguel, D. Dalton Trans. 2005, (5), 884–888. (B) Liu, F.-C.; Chen, J.-H.; She, J.-J.; Lee, G.-H.; Peng, S.-M. J. Organomet. Chem. 2006, 691(17), 3574–3580. (C) Saraiva, M. S.; Quintal, S.; Portugal, F. C. M.; Lopes, T. A.; Felix, V.; Nogueira, J. M. F.; Meireles, M.; Drew, M. G. B.; Calhorda, M. J. J. Organomet. Chem. 2008, 693(21–22), 3411–3418. (D) Taylor, J. O.; Veenstra, F. L. P.; Chippindale, A. M.; Calhorda, M. J.; Hartl, F. Organometallics 2019, 38(6), 1372–1390.

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Treatment of [Mo(
3-methallyl)(CO)2(Metz)(bipy)][OTf] (bipy ¼ 2,2-bipyridine, Metz ¼ 1-Methyl-1,2,3-triazole) with strong base KN(SiMe3)2 results in the deprotonation of the C − H of the triazole moiety, which is followed by nucleophilic attack to the ortho CdH group of a bipy or phen ligand to afford a cyclic, bimetallic complex 398 (method (c)).668 The complex is bridged by triazole, and one of the bipyridine ligands is dearomatized during CdC coupling (Eq. 102). The reaction of the neutral bipy derivative with an acid leads to the formation of dihydropyridyl units by protonation of a CdH group of the dearomatized rings, the dimeric nature of complexes being maintained upon protonation. Similarly, treatment of [Mo(
3-methallyl)(CO)2(Mespz) (bipy)][OTf] (Mespz ¼ 1-mes-pyrazole) with stoichiometric amount of KN(SiMe3)2 generates the corresponding mononuclear complex (right side in Eq. 102) via intramolecular CdC bond coupling.669

ð102Þ

Source: Fombona, S.; Espinal-Viguri, M.; Huertos, M. A.; Díaz, J.; López, R.; Menéndez, M. I.; Pérez, J.; Riera, L. Chem. Eur. J. 2016, 22, 17160–17164. Prolonged refluxing of Mo(
3-allyl)(OAc)(CO)2(pz )2 (pz ¼ 2,4-dimethylpyrazole) in toluene in the presence of O2 and H2O formed the tetranuclear oxo complex [Mo2O2(O2CCH3)2{(m3-O)(m-pz )Mo(
3-allyl)(CO)2(pz H)}2] 399.670

238

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

The variable temperature 1H NMR spectrum of complex 393 is the most distinct at 213 K. It is possible that the allyl group on the Mo atom has cis and trans configurations, so NMR signals of the mixture are observed. The IR spectrum of complex 394 shows two nCO bands at 1924 and 1830 cm−1 as expected for cis-dicarbonyl complexes. The binuclear complex 395 shows sharper 1H NMR signals at 323 K. Two sets of allylic signals are found corresponding to a fast-trigonal twist rotation around each Mo center. Cyclic voltammograms of 397 exhibit two reversible oxidation waves at 0.47 V and 0.83 V (vs. SCE). A controlled-potential electrolysis at each oxidation events confirms the occurrence of a single-electron transfer process. The 1H NMR spectrum of 398 features distinct signals for every hydrogen of the bipy moiety, and for each H of the methallyl ligand, indicating the asymmetry of 398. In addition, the signals between 5.85 and 4.54 ppm are clearly indicative of a dearomatized 2-pyridyl ring.

5.05.2.4

Heteroatom-substituted p complexes

Besides the carbon-based non-cyclic p ligands, heteroatom-based p ligands are also reported for molybdenum complexes. This section covers the chemistry of Mo complexes stabilized by heteroatom-substituted
3-ligands, such as carboxylate, amidinate, and thiocarboxylate ligands. These ligands are widely employed in bimetallic molybdenum complexes containing multiple metal-metal bonds. A couple of Mo complexes with other heteroatom-based p ligands are also discussed in this section.

5.05.2.4.1

Carboxylate complexes

Carboxylate ligands are analogous to
3-allylic ligands. The majority of ModMo carboxylate complexes are quadruply bonded,671 featuring a five-coordinate (considering the metal-metal bond) paddle-wheel geometry. Mo2(O2CR)4 complexes and their derivatives have shown interesting electrochemical and photochemical properties and are potential materials for LED devices.672–678 Especially, they can be used to prepare low-coordinate dinuclear ModMo multiply bonded complexes since the bidentate carboxylate ligands can be replaced. The synthetic methods for Mo2(O2CR)4 compounds were established by Wilkinson and coworkers in the 1960s and were improved by Cotton.679,680 Reaction of Mo(CO)6 with carboxylic acids at high temperature for a prolonged time affords Mo2(O2CR)4 which are usually air-stable (Eq. 103).

ð103Þ

Source: Stephenson, T. A.; Bannister, E.; Wilkinson, G. J. Chem. Soc. 1964, 2538–2541. Mo2(O2CR)4 is a practical precursor to synthesize alkyl or aryl dimolybdenum complexes, which are sparse. Power and Carmona have reported that bulky terphenyl ligands can stabilize such ModMo multiple bond system.681,682 The reaction of Mo2(O2CR)4 (R ¼ Me, H) with LiAr0 (Ar0 ¼ C6H3-2,6-(C6H3-2,6-Me2)2) at low temperature directly provides the mono-quadruply bonded dimolybdenum complexes Mo2(Ar0 )(O2CMe)3 400 as shown in Scheme 46. However, further substitution of the acetate group in 400 is difficult, even with excess LiAr0 . Using a better leaving group such as HCO−2 or CF3CO−2, bis-terphenyl complex Mo2(Ar0 )2(O2CH)2 401, and Mo2(Ar0 )2(O2CF3)2 403 are successfully prepared via salt metathesis of the corresponding precursor. The substitution of the bidentate carboxylate ligand gives rise to the formation of Mo-Caryl s-bond and a coordination unsaturation at the metal center. Complexes 400–403 all have a four-coordinate geometry at the metal center and a formal 14-electron count, which makes them highly unusual. The acetate ligand in the aryl dimolybdenum complex Mo2(Ar0 )(O2R)3 (R ¼ CH3 or CF3) can be replaced by an iodide anion, resulting in the formation of Mo2(Ar0 )I(OEt2)(O2R)3 404 (Scheme 47). Such an ether adduct can further react with phosphine ligands to provide bimetallic Mo complex in the Mo2(Ar0 )I(PR3)(O2R)3 form 405, leading to the two metal centers entirely asymmetrically substituted. The secondary interaction between Mo and the arene ring is thought to have effects on stabilizing bimetallic Mo complexes.683 The Mo-parene interaction in complexes 404 and 405 is analyzed based on crystallographic data and computational studies (Scheme 47).

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

239

Scheme 46 (A) Carrasco, M.; Faust, M.; Peloso, R.; Rodriguez, A.; Lopez-Serrano, J.; Alvarez, E.; Maya, C.; Power, P. P.; Carmona, E. Chem. Commun. 2012, 48, 3954–3956. (B) Carrasco, M.; Mendoza, I.; Faust, M.; Lopez-Serrano, J.; Peloso, R.; Rodriguez, A.; Alvarez, E.; Maya, C.; Power, P. P.; Carmona, E. J. Am. Chem. Soc. 2014, 136, 9173–9180.

Scheme 47 Carrasco, M.; Mendoza, I.; Álvarez, E.; Grirrane, A.; Maya, C.; Peloso, R.; Rodríguez, A.; Falceto, A.; Álvarez, S.; Carmona, E. Chem. A Eur. J. 2015, 21, 410–421.

The Mo-Carene distances are in the range of 2.57–2.78 A˚ . Comparison of these distances with the sums of the covalent and van der Waals radii (2.17 and 4.22 A˚ , respectively) suggest the existence of a Mo-Carene bonding interaction. Also, the bonding character is computed between Mo and the arene ring. These effects vary with different phosphorus ligands to fulfill electronic and steric requirements for stabilizing such unsaturated quadruple ModMo complexes.

240

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

5.05.2.4.2

Amidinate complexes

Since the recognition of the first stable quintuply bonded Cr(I)dCr(I) complex in 2005, the studies on quintuply bonded Mo(I) dMo(I) complexes have gained extensive attention.684–686 The first isolated quintuple ModMo complex was reported by Tsai et al., in which the Mo(I) centers are supported by two bulky amidinate ligands.687 Soon afterwards, many ModMo quintuple bonded complexes stabilized by sterically hindered N-based bidentate ligands emerged. In addition to the intriguing electronic and photochemical properties,688 these complexes have shown great potential in small molecule activation and organic transformation. The synthesis, bonding and reactivity of these quintuple bonded complexes have been thoroughly reviewed689–695 and this section only covers ModMo complexes containing
3-amidinate ligands. Quintuply bonded ModMo complexes are mostly prepared through the reduction of the bimetallic precursors having quadruple bonds. Tsai’s group reported the first two ModMo quintuple bonded complexes with a new synthetic protocol as shown in Scheme 48.687 Treatment of the quadruply bonded precursor K4Mo2Cl8 with 2 equiv. of lithium amidinates in THF leads to the ModMo quadruply “ate” complexes 406 and 407, which are then reduced by KC8 at low temperature to afford the desired ModMo quintuple complexes 408 and 409. The same group later reported a new lantern type of ModMo quintuple bonded complexes Mo2(m-Li){m-CH(N-2,6-Et2C6H3)2}3 412 in which the ModMo quintuple bond is spanned by one lithium atom in addition to three amidinato ligands.696 The synthetic pathway is different from that of 408 and 409. An unusual intermediate [Mo2{m-HC-(N-2,6-Et2C6H3)}2(m-Cl){(m-Cl)2-Mo(m-Cl)2Li(THF)(OEt2)}] 410 which is prepared through the reaction of the lithiated amidinate Li[HC(N-2,6-Et2-C6H3)] with a mixture of Mo2Cl6(THF)3 and Zn powder in THF at −35  C is isolated. Subsequent reduction of 410 with 4 equiv. of KC8 gives 412 in an 11% yield. Alternatively, 412 is obtained in a 17% yield through KC8 reduction of the quadruply bonded [Mo2{m-HC(N-2,6-Et2C6H3)}2(m-Cl){(m-Cl)2Li(THF)2}] 411, which is structurally similar to the previously reported molybdenum complexes 406 and 407. They emphasized the formation of the quintuply bonded dimolybdenum complex 412 in low yields is mainly due to a complicated ligand redistribution process.

Scheme 48 (A) Tsai, Y.-C.; Chen, H.-Z.; Chang, C.-C.; Yu, J.-S. K.; Lee, G.-H.; Wang, Y.; Kuo, T.-S. J. Am. Chem. Soc. 2009, 131, 12534–12535. (B) Liu, S.-C.; Ke, W.-L.; Yu, J.-S. K.; Kuo, T.-S.; Tsai, Y.-C. Angew. Chem. Int. Ed. 2012, 51, 6394–6397.

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

241

Alvarez and Carmona have reported an alternative synthetic route to such dinuclear quadruply “ate” complexes.697 The reaction of Mo2(O2CCH3)2{HC(N-2,6-iPr2C6H3)2}2 with MeLi under H2 atmosphere produces the dihydrido dimolybdenum complex 413, which undergoes H2 elimination to give the quintuply bonded complex 408 (Scheme 49). Notably, upon photolysis, complex 408 reversibly converts to 413 by reacting with H2. The IR spectrum of complex 413 features a ModH stretching band at 1525 cm−1 and 1 H NMR spectrum shows a hydride resonance at d 5.67.

Scheme 49

Carrasco, M.; Curado, N.; Maya, C.; Peloso, R.; Rodriguez, A.; Ruiz, E.; Alvarez, S.; Carmona, E. Angew. Chem. Int. Ed. 2013, 52, 3227–3231.

With a similar synthetic method, Carmona and coworkers later reported a series of quadruply bonded dimolybdenum “ate” complexes in the form of Mo2{m-Me2Li(S)}(m-X)(m-N^N)2 (S ¼ THF or Et2O, N^N ¼ aminopyridine or amidinate ligand, X ¼ MeCO2, Me), 414 and 415,698 which are derived from the corresponding Mo2(O2CCH3)2(m-N^N)2 precursor with LiMe. In these complexes, the Mo2 core is spanned by a –CH3Li ⋯ CH3–fragment and bridged by aminopyridinate or amidinate ligands. The two ModCH3 units are bonded to a solvated lithium cation by 3c-2e agostic interactions. Computational studies on the dimethyl heterotrinuclear Mo2Me2Li framework revealed that the CH3 ⋯ Li interactions are mainly ionic with a non-negligible covalent character, which is consistent with NMR observations. Heating solution of 415 in toluene at 100  C for several hours results in the elimination of LiMe and formation of a four-coordinate, 14-electron dimethyl complex Mo2Me2{m-HC(NDipp)2}2 (Dipp ¼ 2, 6-iPr2C6H3), 416.699

A series of mixed-ligated tris(amidinate)dimolybdenum complex Mo2(DAniF)3(L) (DAniF ¼ N,N0 -di(p-anisyl)formamidinate; L ¼ acetate) 417a-e are synthesized by Mashima et al.700 These dimolybdenum complexes are obtained in good yield by treating an acetate precursor Mo2(DAniF)3(OAc) with the corresponding carboxylic acids, such as m-diphenylphosphino benzoic acid (a), nicotinic acid (b), benzoic acid (c), furan-3-carboxylic acid (d), and isonicotinic acid (e) in the presence of sodium methoxide (Eq. 104). Complexes 386a–e are active catalysts for radical addition of CCl4 to 1-hexene to give 1,1,1,3-tetrachloroheptane. Mechanistic studies suggest that the leaving nature of the L ligands is a crucial factor for initiating the catalytic reaction.

ð104Þ

Source: Rej, S.; Majumdar, M.; Kando, S.; Sugino, Y.; Tsurugi, H.; Mashima, K. Inorg. Chem. 2017, 56, 634–644.

242

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

The ModMo quintuple bond lengths in 408 and 409 are 2.01879(9) A˚ and 2.0157(4) A˚ , respectively. They are much shorter than that in the quadruply complex, i.e., 2.08750 A˚ for 406 and 2.0756 A˚ for 407. It’s noteworthy that the ModMo quintuple bond is substantially shorter than the computed bond lengths (2.03–2.10 A˚ ).687 The amidinato ligands in 408 and 409 remain intact, as reflected by the CdN bond lengths of 1.32–1.35 A˚ . The average ModN bond length of 2.14 A˚ in 406 and 407 is comparable to 2.12 A˚ observed for 408 and 409. The d-bond character of the metal-metal multiple bonds are analogous to the CdC p bond, thus the reactivity of multiple-bonded dinuclear complexes has been highly expected. These quadruply or quintuply bonded ModMo complexes exhibit versatile reactivity towards a variety of small molecules (Scheme 50). DFT calculations disclosed that the HdH s-bond cleavage by complex 408 occurs with nearly no barrier to afford the cis-dihydride intermediate and undergoes cis-trans isomerization to form the trans-dihydride product 413. Computational studies also suggest that the ModMo quintuple bond can be applied to various s-bond cleavage, such as OdH and CdH bond cleavages due to the easy polarization of the dd-type MOs of the ModMo quintuple bond.701

Scheme 50 (A) Chen, H.-Z.; Liu, S.-C.; Yen, C.-H.; Yu, J.-S. K.; Shieh, Y.-J.; Kuo, T.-S.; Tsai, Y.-C. Angew. Chem. Int. Ed. 2012, 51, 10342–10346. (B) Carrasco, M.; Curado, N.; Alvarez, E.; Maya, C.; Peloso, R.; Poveda, M. L.; Rodriguez, A.; Ruiz, E.; Alvarez, S.; Carmona, E. Chem. A Eur. J. 2014, 20, 6092–6102. (C) Wu, P.-F.; Liu, S.-C.; Shieh, Y.-J.; Kuo, T.-S.; Lee, G.-H.; Wang, Y.; Tsai, Y.-C. Chem. Commun. 2013, 49, 4391–4393. (D) Chen, H.-G.; Hsueh, H.-W.; Kuo, T.-S.; Tsai, Y.-C. Angew. Chem. Int. Ed. 2013, 52, 10256–10260.

Complex 408 reacts with 1-alkynes through [2 + 2 + 2] cycloaddition, accomplishing a dimolybdenum-based benzylic structure. With internal alkynes, in contrast, they undergo [2 + 2] cycloaddition to give the corresponding metallacycloalkylidene products.702 Notably, complex 378 catalyzes the tricyclomerization of alkynes to benzene.702,703 Complex 408 forms arene adducts 418 with benzene or toluene when it is dissolved in the corresponding aromatic solvent.697 Photolysis of the bis(hydride) compound 413 in benzene or toluene also gives the same arene complexes, in which the aromatic ring acts as a
4-bridging ligand.704 Tsai and coworkers reported complex 408 can react with NO gas at low temperature to produce a

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

243

quadruple-bonded ModMo complex 419,705 which displays a paddlewheel structure, and the Mo2 center is bridged by two amidinato and two bidentate nitrito ligands. It is interesting that the two nitrito groups are in cis configuration, despite the presence of two bulky amidinates. The quintuple metal-metal bond undergoes haloacylation, an alkyne-like addition to yield quadruple-bonded dimolybdenum acyl complexes, and subsequently react with aroyl halides to afford three-membered dimolybdenum arylidyne heterocyclic compounds via acyl disproportionation, 420.706 DFT studies suggest that complex 420 equilibrates with the oxo alkylidyne.707 The reactivities of bis(hydride) complex 413 towards alkenes and alkynes are also explored.708 Complex 413 experiences facile bimetallic migratory insertion of C2H4, affording the bis(ethyl) complex Mo2Et2{m-HC(NDipp)2}2 421. This bimetallic system is found to undergo reversible ethylene migration insertion and b-H elimination, and the intermediate readily experiences reductive elimination of C2H6. When 413 is exposed to acetylene (1 bar), a black precipitate of polyacetylene is formed, while the alkynyl complex Mo2(C^CPh)2{m-HC(NDipp)2}2(THF)2 is produced by reacting with phenylacetylene. The dimethyl (Mo2)4+ complex 416 reacts towards Lewis bases, such as THF, PMe3 and 4-dimethylaminopyridine to yield a series of adducts. Facile hydrogenolysis of 416 leads to the bis(hydride) complex 413 with elimination of CH4.699

5.05.2.4.3

Thiocarboxylate complexes

Early examples of Mo complexes containing the thiocarboxylate ligand [S2CR]− have been reviewed in COMC III (2005).709 In the past decade, complexes of this class are slightly reported. Brown et al. reported a heterometallic system supported by trithiocarboxy ligands to achieve a higher degree of activation of metal-bound N2.710 The reaction of [Mo2(S2CNEt2)6][OTf]2 gives the ModRe bimetallic dithiocarbamato complex 422, which is stable with respect to N2 cleavage (Eq. 105). According to the crystallographic analysis of 422, the NdN distance of 1.167(6) A˚ is about 0.06 A˚ longer than a free N2 molecule (dN–N ¼ 1.0977 A˚ ). In the IR spectrum, the vN^N stretch of 1818 cm−1 is shifted to higher energy relative to 1925 cm−1 observed for the monomeric molybdenum precursor. The ModS bond lengths lie in the range of 2.49–2.52 A˚ . The coordination of N2 between ModRe centers is weak, and it can be replaced by phosphine ligand. For example, in the presence of PPh3, [422]OTf degenerates to (PhMe2P)4Re(N2)Cl and [Mo(PPh3)(S2CNEt2)2][OTf].

ð105Þ

Source: Seymore, S. B.; Brown, S. N. Inorg. Chem. 2006, 45, 9540–9550. Mo complexes with other heteroatom-based p ligands: A couple of Mo complexes with heteroatom-based p ligands, such as ketone and nitrile, have been reported in last decade.606,711–715 A ketone Mo complex 423 containing a
2-OCMe2 ligand was synthesized by the reaction of 378 with acetone (Scheme 51). By the reaction with PhSiH3, it can be reversibly converted to the silylamido molybdenum(IV) complex 378 featuring agostic Si − H interaction.605,606 Such silylamido complexes are active towards nitriles to form the
2-NCMe adducts 424. These nitrile adducts are unstable in solution and undergo a slow rearrangement through the insertion of the C^N moiety into the Mo − Si bond to form silanimine complexes 425.

Scheme 51 (A) Khalimon, A. Y.; Ignatov, S. K.; Okhapkin, A. I.; Simionescu, R.; Kuzmina, L. G.; Howard, J. A. K.; Nikonov, G. I. Chem. A Eur. J. 2013, 19, 8573–8590. (B) Khalimon, A. Y.; Farha, P. M.; Nikonov, G. I. Dalton Trans. 2015, 44, 18945–18956.

244

5.05.2.5

Cyclic and Non-Cyclic Pi Complexes of Molybdenum

Trispyrazolylborate-based molybdenum p complexes

Trispyrazolylborates,716 Tp−, is one of the most broadly used anionic tridentate and tripodal ligands in coordination chemistry. In the last decade, a large number of molybdenum complexes supported by Tp ligands have appeared. This section includes Tp-based molybdenum p complexes, typically in the form of TpMo(NO)(L)(
2-Lp) or TpMo(CO)2(
3-Lp). The p ligands can be alkene, alkyne, ketone, formate, amide, or aromatic heterocycle. Especially, considerable progress made on TpMo chemistry includes: (i) TpMo(CO)2(
3-Lp) (
3-Lp is typically pyridinyl or pyranyl ligands) complexes as organometallic enantiomeric scaffolds for the asymmetric synthesis of heterocyclics;717–720 (ii) dearomatization of arene compounds based on [TpMo(NO)(L)] fragments (Section 5.05.1.2.1). [TpMo(CO)3]− is a potent precursor for molybdenum p complexes of the type TpMo(CO)2(
3-Lp). The preparation of [TpMo(CO)3]− by Tp salt with Mo(CO)6 has been described in COMC III (2005).721,722 In 2008, Liebeskind reported an efficient method for large-scale synthesis of TpMo(CO)2(
3-pyridinyl) and TpMo(CO)2(
3-pyranyl) complexes using the oxa- and aza-Achmatowicz reaction as shown in Scheme 52.723 For example, the racemic complex 426 is conveniently prepared in one-pot, which involves oxidative addition of hydroxypyranone to Mo(DMF)3(CO)3 followed by treating the reaction mixture with KTp (Scheme 52).

Scheme 52 (A) Coombs, T. C.; Lee, M. D.; Wong, H.; Armstrong, M.; Cheng, B.; Chen, W.; Moretto, A. F.; Liebeskind, L. S. J. Org. Chem. 2008, 73, 882–888. (B) Mocella, C. J.; Delafuente, D. A.; Keane, J. M.; Warner, G. R.; Friedman, L. A.; Sabat, M.; Harman, W. D. Organometallics 2004, 23, 3772–3779. (C) Graham, P. M.; Mocella, C. J.; Sabat, M.; Harman, W. D. Organometallics 2005, 24, 911–919.

As a strong p-acceptor, NO ligand is often used to stabilize low-valence molybdenum complexes. A series of TpMo(NO)(MeIm) (
2-Lp) (MeIm ¼ 1-methylimidazole) complexes 427 are synthesized by the reaction of TpMo(NO)Br2 with 1-methylimidazole followed by an addition of a suitable p-ligand in the presence of sodium dispersion.724 Similar synthetic procedures have been applied to prepare dihapto-coordinated aldehyde, ketone, and amide complexes 428–430.725 The most striking feature of complexes 427–430 is the strong p-donating ability of the TpMo(NO)(L) fragment, as reflected by their robust thermal stability. The bond length of the alkyne in the hexyne complex 427 is 1.23 A˚ , only slightly shorter than that of free 1-hexyne (1.21 A˚ ). The complex TpMo(NO)(MeIm)(
2-1,3-cyclohexadiene) undergoes protonation to afford a
3-1Hcyclohexadienium complex which is best described as an
3-allyl cation. The studies of protonation and isomerization reactions indicated that the TpMo(NO)(MeIm) system is a more effective p-base than [Os(NH3)5]2+, very similar to TpRe(CO)(py) and TpRe(CO)(PMe3), but not as effective as TpRe(CO)(MeIm).

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The aromatic ring in TpMo(NO)(MeIm)(
2-furan) complex undergo dissociation upon heating. When the reaction is performed under slightly elevated pressures of CO2, the corresponding CO2 adduct TpMo(NO)(MeIm)(
2-CO2) 431 is afforded (Eq. 106).726 The IR spectrum of 431 displays a strong absorbance for
2-CO2 at 1755 cm−1, and the 13C NMR spectrum features resonance near 200 ppm for the CO2 carbon. Complex 431 is air-stable and resist decomposition in solution.

ð106Þ

Source: Carden, R. G., Jr.; Ohane, J. J.; Pike, R. D.; Graham, P. M. Organometallics 2013, 32, 2505–2508. The nitrosyl stretching frequency for the formate complex 428, ketone complex 429 and amide complex 430 are 1574 cm−1, 1571 cm−1 and 1558 cm−1, respectively. The formate complex 428 undergoes decarbonylation to form TpMo(NO)(MeIm)(CO) and methanol. Complex 430, TpMo(NO)(MeIm)(
2-DMF), is an rare example of
2-amide complex and stable towards decarbonylation. Interestingly, 430 undergoes
2 to
1 isomerization at room temperature as suggested by 1H NMR spectroscopic analysis. Due to the p-basic property of TpMo(NO) fragment, these complexes are widely used in the activation of aromatic molecules, which is separately discussed in Section 5.05.1.2.1. TpMo(CO)2(
3-Lp) complexes, such as TpMo(CO)2(
3-pyridinyl) and TpMo(CO)2(
3-pyranyl) have been employed as “organometallic chiron” to synthesize a variety of enantiopure heterocyclic compounds. TpMo(CO)2 moiety functions as a non-traditional protecting and auxiliary group in organic transformations. The enantioselective synthesis of bioactive molecules and natural products using these complexes as enantiomeric scaffolds is promising, which is summarized in Scheme 53.727–731

Scheme 53 (A) Coombs, T. C.; Zhang, Y.; Garnier-Amblard, E. C.; Liebeskind, L. S. J. Am. Chem. Soc. 2009, 131, 876–877. (B) Coombs, T. C.; Huang, W.; Garnier-Amblard, E. C.; Liebeskind, L. S. Organometallics 2010, 29, 5083–5097. (C) Wong, H.; Garnier-Amblard, E. C.; Liebeskind, L. S. J. Am. Chem. Soc. 2011, 133, 7517–7527. (D) Chen, W.; Liebeskind, L. S. J. Am. Chem. Soc. 2009, 131(35), 12546–12547. (E) Arrayás, R. G.; Yin, J.; Liebeskind, L. S. J. Am. Chem. Soc. 2007, 129, 1816–1825. (F) Coombs, T. C.; Lee; Wong, H.; Armstrong, M.; Cheng, B.; Chen, W.; Moretto, A. F.; Liebeskind, L. S. J. Org. Chem. 2008, 73, 882–888.

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5.05.3

Summary and outlook

We have outlined the synthesis and characterization of molybdenum p complexes reported in the last 15 years. From a structural point of view, molybdenum complexes bearing Cp and CpR ligands dominant the molybdenum p complexes. Most of these complexes are derived from Mo carbonyl precursors, and the synthetic protocols often involve decarbonylation of CO ligands from the Mo center, somewhat challenging under most circumstances. Although various novel cyclic molybdenum p complexes have emerged, the reactivity relevant to organic catalytic synthesis and sustainable transformations is largely unexplored. However, the broader reactivity studies are critically dependent on the development of new synthetic strategies to access unsaturated molybdenum complexes, which will be of particular interest in Mo-based catalysis research. Recall that molybdenum is an essential component in the FeMo cofactor of nitrogenase (FeMoco). A significant development in non-cyclic molybdenum p complexes is synthesizing bimetallic molybdenum complexes supported by heteroatom-substituted
3-ligands. Such bimetallic systems feature multiple metal-metal bonds and exhibit versatile reactivity towards small molecules. Notably, a few molybdenum p complexes have shown great potential in processing N2 reduction. However, homogenous catalysis of nitrogen reduction to ammonia is exceptionally challenging because it must proceed through multiple and consecutive electron/ proton transfer steps. To understand the enzymatic nitrogen fixation mechanism for exploiting efficient synthetic systems, studies on the heteronuclear FeMo complexes and FeMo-based clusters will continue to thrive in the future.

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Cyclic and Non-Cyclic Pi Complexes of Molybdenum 619. 620. 621. 622. 623. 624. 625. 626. 627. 628. 629. 630. 631. 632. 633. 634. 635. 636. 637. 638. 639. 640. 641. 642. 643. 644. 645. 646. 647. 648. 649. 650. 651. 652. 653. 654. 655. 656. 657. 658. 659. 660. 661. 662. 663. 664. 665. 666. 667. 668. 669. 670. 671. 672. 673. 674. 675. 676. 677. 678. 679. 680. 681. 682. 683. 684. 685. 686.

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Cyclic and Non-Cyclic Pi Complexes of Molybdenum Tsai, Y.-C.; Chen, H.-Z.; Chang, C.-C.; Yu, J.-S. K.; Lee, G.-H.; Wang, Y.; Kuo, T.-S. J. Am. Chem. Soc. 2009, 131, 12534–12535. Jiang, C. C.; Young, P. J.; Durr, C. B.; Spilker, T. F.; Chisholm, M. H. Inorg. Chem. 2016, 55, 5836–5844. Harisomayajula, N. V. S.; Nair, A. K.; Tsai, Y.-C. Chem. Commun. 2014, 50, 3391–3412. Kempe, R.; Noor, A. Inorg. Chim. Acta 2015, 424, 75–82. Mendoza, I.; Curada, N.; Carrasco, M.; Álvarez, E.; Peloso, R.; Rodríguez, A.; Carmona, E. Inorg. Chim. Acta 2015, 424, 120–128. Nair, A. K.; Harisomayajula, N. V. S.; Tsai, Y.-C. Inorg. Chim. Acta 2015, 424, 51–62. Hua, S.-A.; Tsai, Y.-C.; Peng, S.-M. J. Chin. Chem. Soc. 2014, 61, 9–26. Kang, L.; Sun, Z.; Meng, L. Chem. Phys. Lett. 2019, 731, 13660–13665. Nair, A. K.; Harisomayajula, N. V. S.; Tsai, Y.-C. Dalton Trans. 2014, 43, 5618–5638. Liu, S.-C.; Ke, W.-L.; Yu, J.-S.; Kuo, T.-S.; Tsai, Y.-C. Angew. Chem. Int. Ed. 2012, 51, 6394–6397. Carrasco, M.; Curado, N.; Maya, C.; Peloso, R.; Rodríguez, A.; Ruiz, E.; Alvarez, S.; Carmona, E. Angew. Chem. Int. Ed. 2013, 52, 3227–3231. Curado, N.; Carrasco, M.; Álvarez, E.; Maya, C.; Peloso, R.; Rodríguez, A.; López-Serrano, J.; Carmona, E. J. Am. Chem. Soc. 2015, 137, 12378–12387. Curado, N.; Carrasco, M.; Campos, J.; Maya, C.; Rodríguez, A.; Ruiz, E.; Álvarez, S.; Carmona, E. Chem. A Eur. J. 2017, 23, 194–205. Rej, S.; Majumdar, M.; Kando, S.; Sugino, Y.; Tsurugi, H.; Mashima, K. Inorg. Chem. 2017, 56, 634–644. Chen, Y.; Sakaki, S. Inorg. Chem. 2017, 56, 4011–4020. Chen, H.-Z.; Liu, S.-C.; Yen, C.-H.; Yu, J.-S. K.; Shieh, Y.-J.; Kuo, T.-S.; Tsai, Y.-C. Angew. Chem. Int. Ed. 2012, 51, 10342–10346. Chen, Y.; Sakaki, S. Dalton Trans. 2014, 43, 11478–11492. Carrasco, M.; Curado, N.; Álvarez, E.; Maya, C.; Peloso, R.; Poveda, M. L.; Rodríguez, A.; Ruiz, E.; Álvarez, S.; Carmona, E. Chem. A Eur. J. 2014, 20, 6092–6102. Wu, P.-F.; Liu, S.-C.; Shieh, Y.-J.; Kuo, T.-S.; Lee, G.-H.; Wang, Y.; Tsai, Y.-C. Chem. Commun. 2013, 49, 4391–4393. Chen, H.-G.; Hsueh, H.-W.; Kuo, T.-S.; Tsai, Y.-C. Angew. Chem. Int. Ed. 2013, 52, 10256–10260. Harisomayajula, N. V. S.; Chen, H.-G.; Kuo, T.-S.; Tsai, Y.-C. Organometallics 2016, 35, 1534–1546. Pérez-Jiménez, M.; Campos, J.; López-Serrano, J.; Carmona, E. Chem. Commun. 2018, 54, 9186–9189. Tamm, M.; Baker, R. J. In Comprehensive Organometallic Chemistry III; Mingos, D. M. P., Crabtree, R. H., Eds.; Elsevier: Oxford, UK, 2007; vol. 5; pp 447–448. Seymore, S. B.; Brown, S. N. Inorg. Chem. 2006, 45, 9540–9550. Khalimon, A. Y.; Ignatov, S. K.; Simionescu, R.; Kuzmina, L. G.; Howard, J. A. K.; Nikonov, G. I. Inorg. Chem. 2012, 51, 754–756. Khalimon, A. Y.; Ignatov, S. K.; Okhapkin, A. I.; Simionescu, R.; Kuzmina, L. G.; Howard, J. A. K.; Nikonov, G. I. Chem. A Eur. J. 2013, 19, 8573–8590. Khalimon, A. Y.; Farha, P. M.; Nikonov, G. I. Dalton Trans. 2015, 44, 18945–18956. Kuramshin, A. I.; Kuramshina, E. A.; Cherkasov, R. A. Russ. J. Org. Chem. 2005, 41, 649–655. Kuramshin, A. I.; Vatsadze, S. Z.; Galkin, V. I.; Cherkasov, R. A. Russ. J. Gen. Chem. 2016, 86, 645–655. Trofimenko, S. J. Am. Chem. Soc. 1966, 88, 1842–1844. Shu, C.; Liebeskind, L. S. J. Am. Chem. Soc. 2003, 125, 2878–2879. Zhang, Y.; Liebeskind, L. S. J. Am. Chem. Soc. 2006, 128, 465–472. Cheng, B.; Liebeskind, L. S. Org. Lett. 2009, 11, 3682–3685. Chen, W. Y.; Sana, K.; Jang, Y.; Meyer, E. V. S.; Lapp, S.; Galinski, M. R.; Liebeskind, L. S. Organometallics 2013, 32, 7594–7611. Tamm, M.; Baker, R. J. In Comprehensive Organometallic Chemistry III; Mingos, D. M. P., Crabtree, R. H., Eds.; Elsevier: Oxford, UK, 2007; vol. 5; pp 412–415. Kisała, J.; Białonska, A.; Ciunik, Z.; Kurek, S.; Wołowiec, S. Polyhedron 2006, 25, 3222–3230. Coombs, T. C.; Lee, M. D.; Wong, H.; Armstrong, M.; Cheng, B.; Chen, W.; Moretto, A. F.; Liebeskind, L. S. J. Org. Chem. 2008, 73, 882–888. Mocella, C. J.; Delafuente, D. A.; Keane, J. M.; Warner, G. R.; Friedman, L. A.; Sabat, M.; Harman, W. D. Organometallics 2004, 23, 3772–3779. Mocella, C. J.; Sabat, M.; Harman, W. D.; Graham, P. M. Organometallics 2005, 24, 911–919. Carden, R. G.; Ohane, J. J.; Pike, R. D.; Graham, P. M. Organometallics 2013, 32, 2505–2508. Arrayás, R. G.; Yin, J.; Liebeskind, L. S. J. Am. Chem. Soc. 2007, 129, 1816–1825. Chen, W.; Liebeskind, L. S. J. Am. Chem. Soc. 2009, 131, 12546–12547. Cheng, B.; Coombs, T. C.; Zhang, Y.; Garnier-Amblard, E. C.; Liebeskind, L. S. J. Am. Chem. Soc. 2009, 131, 876–877. Coombs, T. C.; Huang, W. W.; Garnier-Amblard, E. C.; Liebeskind, L. S. Organometallics 2010, 29, 5083–5097. Wong, H.; Garnier-Amblard, E. C.; Liebeskind, L. S. J. Am. Chem. Soc. 2011, 133, 7517–7527.

5.06

Cyclic and Non-Cyclic Pi Complexes of Tungsten

Timothy P Curran, Department of Chemistry, Trinity College, Hartford, CT, United States © 2022 Elsevier Ltd. All rights reserved.

5.06.1 5.06.1.1 5.06.1.2 5.06.1.2.1 5.06.1.2.2 5.06.1.2.3 5.06.1.2.4 5.06.1.2.5 5.06.1.2.6 5.06.1.2.7 5.06.1.2.8 5.06.1.2.9 5.06.1.2.10 5.06.1.2.11 5.06.1.2.12 5.06.1.2.13 5.06.2 5.06.3 5.06.3.1 5.06.3.2 5.06.3.3 5.06.4 5.06.4.1 5.06.4.2 5.06.4.3 5.06.4.4 5.06.4.5 5.06.5 5.06.6 5.06.7 5.06.8 5.06.9 5.06.10 5.06.10.1 5.06.10.1.1 5.06.10.1.2 5.06.10.1.3 5.06.10.2 5.06.10.2.1 5.06.10.2.2 5.06.10.2.3 5.06.10.2.4 5.06.10.2.5 5.06.10.2.6 5.06.10.2.7 5.06.10.2.8 5.06.10.2.9 5.06.10.2.10 5.06.10.2.11 5.06.10.2.12 5.06.10.2.13 5.06.10.2.14 5.06.10.2.15 5.06.10.3

Monoalkene complexes Complexes involving simple alkenes Complexes derived from dearomatization reactions Reactions of benzene Reactions of naphthalene and anthracene Reactions of anisole Reactions of phenol Reactions of anilines Reactions of indolines and quinolines Reactions of benzenes with electron withdrawing groups Deuteration reactions Stereospecific reactions Reactions of pyridines and pyrimidine Reactions of furan, thiophene and pyrrole Cycloaddition reactions Other monoalkene complexes Bis-alkene complexes Allyl complexes Carbon allyl complexes Allyl and Cp complexes Heteroallyl complexes Monoalkyne complexes Alkyl-substituted alkynes Hetero-substituted alkynes from Seidel Alkyl-substituted alkyne complexes from Templeton Additional complexes Alkynylpeptide complexes Bis-alkyne complexes Tris-alkyne complexes Nitriles and heteroalkyne complexes Carbonyl complexes Imine and iminium complexes Cyclopentadienyl complexes Cp complexes Tungsten-Cp complexes Cp metal cluster complexes Ruiz tungsten-Cp complexes Cp complexes Tungsten-Cp oxide and sulfide complexes Tungsten-Cp carbonyl complexes Tungsten-Cp hydride complexes Tungsten-Cp complexes with silicon ligands Tungsten Cp monoalkene complexes Tungsten Cp bis-alkene complexes Tungsten Cp monoalkyne complexes Tungsten Cp allyl complexes Tungsten Cp imine and iminium complexes Tungsten Cp and ketone complexes Tungsten Cp complexes with NO ligands Tungsten-Cp complexes with nitrogen ligands Tungsten-germanium complexes Tungsten-Cp complexes with CO ligands Cp -W clusters Modified Cp and Cp complexes

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00108-6

258 258 259 260 262 262 263 265 267 269 270 272 273 278 279 281 281 281 282 284 289 289 289 291 296 298 300 302 304 304 306 307 307 307 307 310 314 326 327 327 328 330 338 340 340 340 341 341 342 346 348 350 351 358

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5.06.10.3.1 5.06.10.3.2 5.06.11 5.06.12 5.06.13 5.06.13.1 5.06.13.2 5.06.14 5.06.15 References

Modified Cp complexes Modified Cp complexes Indenyl-tungsten complexes Cyclobutadiene complexes Arene complexes Tungsten complexes to benzene rings Tungsten 2,5-dimethylpyrrolide complexes Cycloheptatrienyl complexes Summary

359 362 365 365 365 366 367 368 368 368

This review covers the literature on tungsten pi complexes prepared and examined by researchers from 2006 to 2020. The chapter is organized around the various pi ligands (alkenes, alkynes, allyls, cyclobutadienes, cyclopentadienes, pyrrolides, arenes cycloheptatrienyls and C60). Within each section, the different complexes described are organized by the types of other ligands attached to the metal center. It does not cover complexes examined solely by computational methods.

5.06.1

Monoalkene complexes

A carbon-carbon double bond can form Z2 complexes with tungsten. This can occur with simple alkenes, or with molecules having multiple pi bonds, like dienes or arenes. Most of the work in this area since 2006 has centered on the latter complexes.

5.06.1.1

Complexes involving simple alkenes

The simplest tungsten pi complexes those containing a bond between tungsten and a single carbon-carbon double bond. There has been considerable research on these complexes over the last 15 years. This section describes most of these complexes. In 2014, Berke and co-workers were investigating the development of new hydrogenation catalysts, and reported the preparation and characterization of the ethylene complexes 1a-b. Catalytic hydrogenations employing 1b were examined.1

Cedeno and co-workers examined the chloroethylene complexes 2–4 using experimental and DFT calculations. They discovered that electron-withdrawing groups on the alkene did not increase the strength of the pi bond.2

Other monoalkene complexes were prepared and examined by the Szyma nska-Buzar group. They devised a new route for the synthesis of 5, and obtained an X-ray structure of this molecule.3 Both 6 and 7 were prepared via displacement of an Z2-silane ligand on tungsten. The resulting monoalkene complexes 6 and 7 then underwent ring-opening metathesis reactions.4

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259

In the course of their extensive investigations into metathesis catalysts, Schrock and co-workers prepared and characterized the monoalkene complex 8.5

Harman and co-workers have also examined monoalkene complexes with the tungsten center utilized in their dearomatization studies (see Section 5.06.1.2), and have described complexes with trans-3-hexene (9),6 cyclopentene (10),7 and a cyclopentenyl cation (11).7 A complex bearing a methylnitrosyl ligand and cyclopentene (12) was examined.7 Another Z2 complex to cyclohexene (13)8 was prepared. Finally Harman and co-workers complexed tungsten to several naturally occurring alkenes, including R-pinene (14),9 S-pinene (15),9 limonene (16)6 and humulene (17).6

5.06.1.2

Complexes derived from dearomatization reactions

By far, the most extensive number of monoalkene Z2 complexes were prepared and studied by Harman and co-workers, who detailed in a large number of papers their successful efforts to dearomatize and functionalize aromatic organic molecules, including benzene, naphthalene, anthracene, phenol, anisole, aniline, furan, pyrrole, thiophene, pyridine and pyrimidine. A timely review of their work was published in 2017.10 The process of breaking the aromaticity of these molecules begins with the coordination of the aromatic species to tungsten as Z2 complexes. Protonation of these species, followed by the judicious selection of further reactants lead to a large number of non-aromatic species linked to tungsten. Harman and co-workers have also developed mild methods for separating the resulting organic molecules from the tungsten center. This process is outlined in Scheme 1 for the generic conversion of benzene into a functionalized cyclohexadiene. The process starts with hexacoordinate complex 18,11 where benzene serves as an Z2 ligand to tungsten, which is also linked to the tridentate Tp ligand, NO and PMe3. Treatment of 18 with an electrophile, E+, generates the cation 19, which can be isolated and studied. Subsequent treatment of 19 with nucleophiles (Nu-) generates complexes having the general structure 20. To isolate the organic molecule 21 from the complex can then be readily accomplished using a variety of oxidative reagents.

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Scheme 1 The steps involved in the dearomatization of benzene using a tungsten complex.

5.06.1.2.1

Reactions of benzene

The complex 18 has been prepared and characterized,11 as has its structural relative TpW(NO)(PBu3)(Z2-benzene).8 Of these two species, 18 has been the one used in most studies. It has been converted to the Z2-cyclohexadiene complexes 22a-h and 23a-h.11 The reaction of 18 leading to these products yields two isomeric products, with 22a-h being the major isomer, and 23a-h being the minor isomer.

The cyclohexadienes 22a-h can also undergo further reactions to yield cyclohexene derivatives. Complex 22f can be converted into 24a-e and 25a-e, complex 22g can be converted into 26a-d and 27a-d, and complex 22d can be made into 28a-d and 29a-d.11

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An isomer of cyclohexadiene complex 22d is 30, which could be converted into 31a-e and 32a-e.11

Substituted benzenes can also be coordinated to tungsten in this system. Reaction of 9 with a substituted benzene was used to make complexes 33a-g. The substituent on the benzene leads to formation of different isomers of 33a-g.12 The hydrogens on benzene can also be replaced with fluorines, as complex 34 also has been prepared.13

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5.06.1.2.2

Reactions of naphthalene and anthracene

Beyond benzene, both naphthalene and anthracene will also coordinate to tungsten and undergo reactions. The naphthalene complexes 35a-f, 36a-j and 37a-d have been prepared and studied.14–16

The parent naphthalene complex 35a will react like benzene in dearomatization reactions, yielding the substituted dihydronaphthalene complexes 38a-c.17

Anthracene will also complex with the tungsten to yield 39.10

5.06.1.2.3

Reactions of anisole

Anisole is another aromatic ligand that has been extensively studied in the dearomatization reactions. It coordinates to tungsten to yield 40. Protonation of 40 with a mild acid can then yield the cation 41, which can then undergo a number of reactions.14

Cyclic and Non-Cyclic Pi Complexes of Tungsten

263

If 40 is presented with an electrophile like acrolein or methyl vinyl ketone, then cations 42a and 42b are obtained. Harman and his co-workers then found that 42a could react with an enolate ion to produce the tungsten complex 43. In another route, nucleophilic attack on the carbonyl groups in 42a-b triggered an intramolecular Michael addition, producing the oxahydrodecalin ligands 44a-d, 45 and 46. Even more elaborate reactions were used to prepare the novel, bicyclic ligand shown in 47.18

5.06.1.2.4

Reactions of phenol

Further elaborations of phenol were also discovered. The cyclohexadienone complexes 48, 49a-b, 50a-b and 51a-b were prepared and examined.19

Phenol and m-cresol were also used to prepare a different set of cyclohexadienone complexes 52a-c, 53 and 54.20

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Cyclic and Non-Cyclic Pi Complexes of Tungsten

Phenol and m-cresol were used as ligands to prepare cyclohexendione complexes 55, 56a-i, 57a-b, 58 and 59. The organic ligands here have a wide range of multiple functional groups.20,21

Cyclic and Non-Cyclic Pi Complexes of Tungsten

265

Phenol was also used to prepare the 4-substituted cyclohexendione complexes 60, 61 and 62a-d.17,22

5.06.1.2.5

Reactions of anilines

Harman and co-workers have also extensively studied the coordination and reactions of N,N-dimethylaniline. Initial complexation of N,N-dimethylaniline generated complexes 63 and 64, which differ where the dienes are located in the ring.23 Complex 63 is the predominant isomer.

Both 63 and 64 can then react to yield derivatives with various substituents around the ring. One set of such derivatives are 65a-e where a number of heterocyclic aromatics were appended to the cyclohexene ring via Friedel-Crafts reactions.17

Another set of complexes made from aniline were 66a-e, where two substituents were added to the ring.17

Simmons-Smith cyclopropanation of 63 was used to create 67a, which could then be converted to 68. Simmons-Smith cyclopropanation of 68 then yielded 69b. Subsequent reactions of 67a-b with a variety of nucleophiles were used to generate complexes 69a-b, 70a-c and 71a-f.23

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Cyclic and Non-Cyclic Pi Complexes of Tungsten

Another set of aniline derivatives prepared by Harman and co-workers were 72a-c and 73a-e, which were produced by either single or double additions to the coordinated aniline.24

Cyclic and Non-Cyclic Pi Complexes of Tungsten

5.06.1.2.6

267

Reactions of indolines and quinolines

Related to the aniline work were complexes derived from N-alkylindolines. Coordination of these indoline derivatives generated 74a-c and 75a-c.25

Reaction of 74b with a variety of nucleophiles generated the dihydroindoline complexes 76a-m. It was also found that 74b can undergo Simmons-Smith cyclopropanation to give complex 77.25

Dual additions to indoline were also discovered. Complexes bearing fluorines (78a-e), hydroxyls (79a-d), cyclic iminoesters 80 and 81a-b) and cyclicimide (82) were prepared and characterized. In a similar manner, the sulfonylchloride 83 was obtained.25

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Cyclic and Non-Cyclic Pi Complexes of Tungsten

Complexes 76a-m and 77 and 78a-d possess an iminium bond. Harman and co-workers discovered that the iminium in some of these complexes could be reduced, yielding the tetrahydroindoline complexes 84a-e, 85a-c and 86. It was also possible to convert 84a to the dihydroindoline complex 87.26

Tetrahydroquinolines behave like indolines, and coordinate to tungsten as well. The complexes 79 and 80 have been prepared and evaluated.26

Cyclic and Non-Cyclic Pi Complexes of Tungsten

5.06.1.2.7

269

Reactions of benzenes with electron withdrawing groups

Anisole, phenol, aniline and indoline are aromatic systems possessing electron donating groups. As noted earlier, aromatics with electron withdrawing groups (33a-g and 34) will also bond with tungsten.15 Harman and co-workers have discovered that these molecules too can undergo reactions leading to more complex systems. Among the molecules they have described are 90a-c, 91a-c, 92 and 93, all of which feature at least one trifluoromethyl substituent on the six-membered ring.13,27

Protonation of the trifluorotoluene complex 33c resulted in formation of the cation 94. Reactions of 94 with nucleophiles then generated cyclohexadiene complexes 95a-c.27

The Harman group then found that 95b could be protonated to yield a mixture of two different cations, 96 and 97.27

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Cyclic and Non-Cyclic Pi Complexes of Tungsten

Deprotonation of the mixture of 96 and 97 generated a new cyclohexadiene complex, 98. Reaction of the mixture of 96 and 97 with nucleophiles produced the complexes 99a-c and 100a-b.27

Likewise, protonation of 95c followed by addition of hydride generated the cyclohexadiene complexes 101 and 102. Protonation of 95c followed by addition of an amine led to the formation of 103a-b.27

5.06.1.2.8

Deuteration reactions

In a recent paper, Harman and his group have demonstrated how to prepare selectively deuterated cyclohexadienes, cyclohexenes and cyclohexanes utilizing tungsten complexes.28 Thus, they prepared and characterized 104–128, which comprise six-ring complexes that range from being monodeuterated to octadeuterated. Within this group of complexes, the placement of the deuterium label can be controlled. Release of the organic ligand from the complex can then be used to isolate the selectively deuterated cyclohexene or cyclohexadiene.

Cyclic and Non-Cyclic Pi Complexes of Tungsten

271

In addition to deuterations starting from benzene complexes, it has also been shown that deuteration can be controlled by dearomatization of benzonitrile. Harman and co-workers have described the preparation of complexes 129–134.26

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Cyclic and Non-Cyclic Pi Complexes of Tungsten

Similarly, this same group has accomplished selective deuteration of trifluorotoluene to yield complexes 126–130.28

5.06.1.2.9

Stereospecific reactions

One thing to note about these complexes is that they are typically formed as a pair of enantiomers. For example, coordination of benzene results in the formation of the R and S isomers of R-18 and S-18. Harman and co-workers have described a method for resolving complexes made from 1,3-dimethoxybenzene. The method involves the protonation of the complex with the L and D isomers of dibenzoyltartaric acid (DBTH2) to yield salts of L-DBTH− (140) and D-DBTH− (141).9

As outlined in Scheme 2, coordination of 1,3-dimethoxybenzene to tungsten produces a mixture of the coordination diastereomers 142 and 143. Protonation of this mixture with L-DBTH2 produces two salts, 144 and 145. The salt of the R isomer, 144, is insoluble in the reaction solution and precipitates out, while the salt of the S isomer, 145, remains soluble. Thus, by filtration methods the two enantiomers can be separated. As expected, using D-DBTH2 for the protonation step results in another set of two salts, 146 and 147. Here the R isomer 146 is soluble in the reaction solution, while the S isomer 147 is insoluble. Separation of the enantiomers is then accomplished using filtration methods.9

Cyclic and Non-Cyclic Pi Complexes of Tungsten

273

Scheme 2 Separation of chiral isomers of a tungsten-benzene complexes using dibenzoyltartaric acid.

5.06.1.2.10

Reactions of pyridines and pyrimidine

As stated at the start of this section, Harman and co-workers have investigated coordination of various aromatics to tungsten in this system. So far the complexes with benzene and benzene derivatives have been delineated. But there are also aromatic molecules containing heteroatoms, and the Harman group explored their behavior too. Pyridine and several of its substituted derivatives were coordinated to tungsten to yield the coordination diastereomers 148a-h and 149a-h. A competing reaction here is for the N to directly coordinate to tungsten, rather than forming the Z2 complex with the aromatic pi bond. Substituents at the 2 or 2 and 6 positions help favor Z2 coordination. Protonation of 148a-h and 149a-h occurred at nitrogen, producing another pair of coordination diastereomers, the cations 150a-h and 151a-h.29

274

Cyclic and Non-Cyclic Pi Complexes of Tungsten

Another set of coordination diastereomers produced were the acetals 152 and 153, which could be converted to the ketone 154 when subjected to acidic hydrolysis.29

The problem of N-coordination to tungsten here can be solved by acylation of the pyridine nitrogen. Thus, acetylpyridine was coordinated to tungsten to yield 155.22,30 Subsequent reaction of this complex with a wide array of nucleophiles produced the N-acetyldihydropyridine complexes 156a-m.31,32

The complexes 156a-m can then undergo further elaboration to produce the tetrahydropyridine complexes 157a-d and 158a-d.30,31

Cyclic and Non-Cyclic Pi Complexes of Tungsten

275

Other elaborations with these complexes yielded 159, 160 and 161.31

Another set of N-acetylpyridine complexes were elaborated into the N-acetyldihydropyridine complexes 162a-c, 163a-d and 164.33

Complexes 162a-c underwent further reactions to yield the tetrahydropyridines 165a-c.33

Harman and co-workers also prepared cation 166, and converted it to the tetrahydropyridine 167. Similarly, cation 168 was converted to 169.30

Complex 155 could also undergo oxidation reactions, producing the dimethoxy complexes 170 and 171.34

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Cyclic and Non-Cyclic Pi Complexes of Tungsten

In this same study, complex 172 was prepared, and then subjected to similar oxidations, which yielded 173a-d, 174 and 175.34

N-acetylpyridine derivatives were not the only N-substituted pyridines examined. A number of different moieties can be appended to the pyridine nitrogen that then allow coordination to the tungsten. These complexes are 176a-f.32,35

A particularly interesting pyridine examined was the 2-dimethylaminoderivative, 177a-b. It could be functionalized to produce the tungsten complexes 178a-b and 179.36

Cyclic and Non-Cyclic Pi Complexes of Tungsten

277

One interesting feature of complex 155 is that it can undergo additions followed by ring opening. Thus, complexes 180–182 were obtained by reactions of 155.37

Ring opening reactions were also noted starting from the dihydropyridine complex 183. These reactions produced complexes 184–187.37

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Cyclic and Non-Cyclic Pi Complexes of Tungsten

Beyond pyridine, Harman and co-workers have also prepared and examined complexes of pyrimidines 179a-b.38

5.06.1.2.11

Reactions of furan, thiophene and pyrrole

Other aromatic rings bearing heteroatoms have also been examined. These include the two furans 189a-b14 and 190a-b,39 the thiophene 1917 and the pyrroles 19240 and 193.41

More heavily substituted pyrroles have also been coordinated to tungsten here, including the monocyclic complexes 194a-g, the diastereomers 195–196, and the bicyclic complexes 197a-d and 198.41

Cyclic and Non-Cyclic Pi Complexes of Tungsten

5.06.1.2.12

279

Cycloaddition reactions

One reaction that these aromatic ligands bonded to tungsten readily undergo are Diels-Alder reactions. Benzenes and substituted benzenes react readily with dienophiles to produce the complexes 199a-e, 200a-d, and 201a-b. Acidic hydrolysis of 201a-b then generates the ketone products 202a-b.42

Naphthalene complexes also undergo Diels-Alder reactions. Thus, complexes 203a-f have been described.16

Another aromatic ring that undergoes Diels-Alder additions when linked to tungsten is 2,6-dimethoxybenzene. Its complex produced the bicyclic products 204–207, 208a-b, 209a-d and 210a-b.43

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Cyclic and Non-Cyclic Pi Complexes of Tungsten

N-acetyldihydropyridine complexes also undergo Diels-Alder additions to produce 211a-b, 212a-c and 213a-c. Both 212a-c and 213a-c react with an N]O bond in the cycloaddition reaction.33

One other pyridine derivative that undergoes a Diels-Alder 2-dimethylaminopyridine. It reacted with acrylonitrile to produce 214.42

cycloaddition

is

the

tungsten

complex

with

Pyrimidine, when coordinated to tungsten, also participate in Diels-Alder reactions. Reactions with methylacrylate and cyanoacrylate yielded tungsten complexes 215a-b.38

Furan complexes also functioned as dienes in the Diels-Alder reaction, producing complexes 216a-b and 217.39

Likewise, pyrrole complexes would undergo Diels-Alder reactions, yielding complexes 218a-b, 219a-b, 220 and 221.40

Cyclic and Non-Cyclic Pi Complexes of Tungsten

281

Harman and co-workers also explored 2 + 2 cycloadditions to a tungsten cyclohexadienone complex to produce complexes 222a-c and 223. Treatment of 222c with a reducing agent produced the alcohol 224, while 222a underwent a Favorskii rearrangement to yield the cyclopropane 225.44

5.06.1.2.13

Other monoalkene complexes

There are other monoalkene complexes that are discussed in this review. These complexes possess other pi ligands and are described later in the review. They include 455–456, 470, 726–751, and 905.

5.06.2

Bis-alkene complexes

Since 2006, only a small number of bis-alkene complexes have been prepared and described; four of them from the group of Szyma nska-Buzar. They are the bis-ethylene complex 226, the bis-norbornene complex 227, the bis complex derived from 1,4-cyclooctodiene (228) and the bis complex derived from norbornadiene (229).45–47

Several other bis-alkene complexes had other pi ligands, 752–754, and they are described later in this review.

5.06.3

Allyl complexes

There have been a number of reports about tungsten-allyl complexes since 2006. These complexes are Z3 species, and typically are comprised of an allyl group consisting of the three carbon atoms. However, there are also complexes where the allyl group contains heteroatoms. Lastly, there are a large number of allyl complexes that also feature another set of pi bonds, this time to Cp, Cp and related ring systems. These categories of allyl complexes are outlined in the sections that follow.

282

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Cyclic and Non-Cyclic Pi Complexes of Tungsten

Carbon allyl complexes

Bitterwolf published a study where the properties of tungsten-allyl complexes 230a-b were studied.48

Hill recently described two allyl complexes, 231 and 232, supported by multidentate borate ligands.49

Liu and co-workers prepared a number of tungsten-allyl complexes, including 233, 234, 235a-c and 236a-c. The common feature of all these complexes is the use of cyanoborohydride as another ligand around the tungsten center.50,51

The Liu group also prepared the allyl complexes 237a-c and 238a-c, which differ in how the bridging ligand is arranged relative to the allyl group.52 Finally, Liu has described 239, which features two tungsten allyl complexes tied together by a cyanide ion.52

Cyclic and Non-Cyclic Pi Complexes of Tungsten

283

Schatzschneider and his group also prepared similar allyl complexes, 240a-d, using bipy as the bridging ligand.53

Vinklárek and co-workers made similar complexes, 241a-b, 242 and 243.54

Winter’s group also contributed new allyl complexes of this variety, describing 244–247. This group also found that pyridine could be eliminated from 246 to generate 248.55

284

Cyclic and Non-Cyclic Pi Complexes of Tungsten

The Yamaguchi group prepared complexes 249 and 250, which share similarity to 246. Notable about 250 is the methylenephosphine ligand.56

Another set of allyl complexes were prepared during the Harman group’s work on complexes that induce dearomatization of arenes. They reported complexes 251–253, 254a-b and 255.22

There are additional allyl complexes that have other pi ligands (647, 658, 756–759, 914–915, 931, 934 and 940) that are described later in this review.

5.06.3.2

Allyl and Cp complexes

Since 2006 there have been other allyl complexes described in the literature, but these complexes also contain another pi ligand, typically a Cp or Cp group (or a close structural relative). This section summarizes these compounds. Tobita and his co-workers have described complexes 256a-b and 257, which have both an allyl and Cp ligand.57 The allyl ligand here is linked to a germanium moiety. In contrast, this same group has described 258a-f, which have an allyl ligand bonded to a silicon moiety58; they also have Cp or Cp derivative ligands.

Cyclic and Non-Cyclic Pi Complexes of Tungsten

285

Goncalves and co-workers have examined the three tungsten allyl complexes 248–250, which are linked respectively to Cp (259),59 Cp (260)59 and indenyl (261)60 ligands.

Vinklárek has also made a substituted indenyl allyl complex with tungsten, 262. This complex undergoes a reaction to yield the cation 263, where the allyl ligand has become a monoalkene complex.61

The majority of Cp and allyl complexes since 2006 came from studies focused on studying CdH bond activation, which was spearheaded by the Legzdins group. Their work was reviewed in 201462 and in 2016.63 As shown in general in Scheme 3, CdH bond activation begins with a neopentyl complex (264a,64 264b,64–69 264c,64–67,69–74 265d64–67,75,76 or 265e67,77,78), which will lose 2,2-dimethylpropane when allowed to react with a hydrocarbon, RdH, to generate a new allyl complex (265). When left to react further, complexes 265 become the hydride complexes 266. Similar chemistry occurs with neopentyl complexes 267a-b.64,79

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Cyclic and Non-Cyclic Pi Complexes of Tungsten

Scheme 3 CdH bond activation triggered by a tungsten-Cp complex.

A large number of complexes having the general structure of 265 have been studied by the Legzdins group (265a-ai), and they are shown below.65,67–69,71,73,75–77 As is evident from this list of compounds, a large number of hydrocarbons will react with complexes 264b, 264c, 264d, and 264e.

Cyclic and Non-Cyclic Pi Complexes of Tungsten

287

288

Cyclic and Non-Cyclic Pi Complexes of Tungsten

The resulting hydride products from these reactions, 266a-n and 268–274,65–69,75,78–80 have been isolated and characterized.

In exploring the reactivity of this system, Legzdins and co-workers prepared derivatives of some of the above complexes using modified Cp ligands. Variants of 264 and 266 using isopropyl Cp (275 and 276) and isopropyl Cp (277 and 278) have been described.74 Similarly, a variant of hydride 266 employing a Cp ligand where one of the methyls is replaced with a proton (279) was also examined.72

Following up on the work from the Legzdins group, Huang has performed DFT calculations on 264c and 265c.81 Etienne and co-workers have recently prepared and examined the allyl complexes 264a along with 280a-b.82,83

In closing out this section on allyl complexes also having a Cp ligand, complexes 281a-b, which also have a Cl ligand, have been prepared by the Legzdins group.67

Cyclic and Non-Cyclic Pi Complexes of Tungsten

5.06.3.3

289

Heteroallyl complexes

Allyl complexes are typically thought to be made from a linear chain of 3 carbon atoms. However, one or more of these carbon atoms can be replaced with another atom, generating a heteroallyl ligand. This section details most of the tungsten-heteroallyl complexes that have been described since 2006. A small number of heteroallyl complexes have other pi ligands, 713–715 and 933), and are described in later sections of this review. Tobita’s group has described complexes 282a-d, 283 and 284a-b. All three complexes possess a Cp ligand, along with a heteroallyl ligand. In 282a-d, the heteroallyl ligand is composed of two carbon atoms and a boron.84 In 283, the heteroallyl ligand is composed of two carbons and a nitrogen,85 while complexes 272a-b are comprised of two carbons and a silicon atom.86

Weber’s group has published data on another set of heteroallyl complexes, 285–287. These complexes only have one carbon atom along with two heteroatoms. In complexes 285 and 286 these two heteroatoms are P and As,87–89 while in complex 287 they are both P atoms.90

5.06.4

Monoalkyne complexes

Tungsten is well-known for making complexes with alkynes, and since 2006 there have been a number of monoalkyne complexes prepared and studied. All but one of these complexes are described in this section. The last one, 755, can be found in a later section.

5.06.4.1

Alkyl-substituted alkynes

Two groups studied the reactivity of pentacarbonyl complexes. Bengali and co-workers did experimental and DFT studies on 288,91 while the Szyma nska-Buzar group examined 289a-b.3,4

290

Cyclic and Non-Cyclic Pi Complexes of Tungsten

The most common ligand for monoalkyne complexes was Tp’. Connelly and co-workers used this ligand to study the redox behavior of complexes 290a-c,92 291,92 292a-i,93 293a-e,94 294a-b,94 295a-b,95 296a-b95 and 297a-d.95 The monoalkyne complexes 285a-d feature a Cp group.

Connelly and his group also examined tungsten monoalkyne complexes that were linked to a manganese center via a bridging CN ligand between tungsten and manganese. These include complexes 298a-c, 299a-d, 300a-b, and 301a-b.96

Cyclic and Non-Cyclic Pi Complexes of Tungsten

5.06.4.2

291

Hetero-substituted alkynes from Seidel

The Seidel group has also made extensive use of the Tp ligand to form tungsten monoalkyne complexes. In 2006 they described complexes 302–305, which utilize alkyne-1,2-dithiolato ligands.97

Seidel and co-workers utilized this chemistry to prepare the bimetallic complexes 306a-b, which include Ni and Pd centers.97

Another set of bimetallic complexes were examined by the Seidel group. The starting complex was the dithiolate dianion 307, which was used to make the tungsten-ruthenium complexes 308a-b.98 In this same study trimetallic complexes featuring two tungsten monoalkynes coordinated to nickel, palladium and platinum were also examined (309a-c).98,99

292

Cyclic and Non-Cyclic Pi Complexes of Tungsten

In a subsequent paper, Seidel and co-workers prepared 310a-b and then converted them to the thiolates 311-b.100

They also prepared the tungsten monoalkyne-ruthenium complexes 312–315.99

Cyclic and Non-Cyclic Pi Complexes of Tungsten

293

They also prepared the tungsten monoalkyne-palladium complexes 316–317.100

One final study on 1,2-dithioalkynes led to studies on 318a-c which feature a tungsten oxide coordinated to monoalkynes.101

Seidel and co-workers also examined S,N-substituted alkynyl complexes 319c, 320, 321, 322a-b, 323 and 324. They were all accessed starting from 319a-b.102 Lastly, the trimetallic complex 325 which features two monoalkynes linked to a palladium complex, was prepared and characterized.102

294

Cyclic and Non-Cyclic Pi Complexes of Tungsten

These same researchers also made other bimetallic complexes using S,N-alkynyl ligands. These included a ruthenium complex (326) and an iridium complex (327).103

Seidel’s group has also extended their studies to include S,P-alkynyl ligands. In recent work they described the use preparation of 328 and 329a-b, which feature an S-alkynyl ligand, and then 330a-b, which has the S,P-alkynyl ligand. The complexes 330a-b could then be used to coordinate a second metal center to the tungsten monoalkyne complex, generating the tungsten-ruthenium complexes 331–333.104

Also examined by the Seidel group have been P,P-alkynyl ligands. In their first report on this topic the acetylene complex 334a was described, along with the P-alkynyl complexes 334b-c.105 Complexes having P,P-alkynyl ligands were 335a-c and the tungstenplatinum complex 336.105

Cyclic and Non-Cyclic Pi Complexes of Tungsten

295

In another study by Seidel, P,P-alkynyl ligands using oxidized phosphine generated the monoalkyne complexes 337a-b and their companion complexes 338a-b.106 Also prepared were the novel cyclic complexes 339, 340 and 341, which also include a palladium center.106

Seidel also reported on some monophosphinealkynyl ligands that would coordinate to tungsten, and described 342, 343, 344, 345a-b, 346, 347 and 348.107

One final set of heterosubstituted alkynyl ligand complexes reported by Seidel were 349a-d. Three of these complexes (349a-c) have iodine bonded to the alkyne, while the last one (349d) has two amines linked to the alkynes.108

296

5.06.4.3

Cyclic and Non-Cyclic Pi Complexes of Tungsten

Alkyl-substituted alkyne complexes from Templeton

Another researcher who made great use of the Tp ligand with tungsten monoalkyne complexes was Templeton. In 2006 he described complexes 350–357.109 Both 338 and 339 were the starting points for making the amine tungsten-alkyne complexes 352–357, which feature pyrrolidine.

Another study on amine tungsten-alkyne complexes followed in 2008, where 358–365 were prepared and characterized.110 In this study, Templeton and co-workers analyzed other cyclic amines as ligands for tungsten-monoalkyne complexes.

The use of acyclic amines in this chemistry was explored when this group prepared 366 and 367 using aniline.111

Cyclic and Non-Cyclic Pi Complexes of Tungsten

297

Following the exploration of amine ligands, Templeton and co-workers next examined imines as ligands, and prepared and characterized 368–371.112 The alkyne in all of these complexes is ethyne. In 368, 369a-b and 370, the imine is in various protonation or alkylation states. Complexes 371a-b were obtained by addition of nucleophilic carbon to the imine.

The final report from the Templeton group on these tungsten-monoalkyne complexes utilizing the Tp ligand and N-ligands came in 2017. Three new complexes, 372–374, were described.113 Complex 372 has acetonitrile as a ligand around tungsten, while 373 is a variant of 370, differing only in the nature of the counterion. Finally, complex 374 features two tungsten-monoalkyne complexes that are linked via a diaminoethane bridge.

In addition to these monoalkyne complexes using Tp ligands, Templeton and co-workers also examined tungsten monoalkyne complexes using acac ligands.114,115 Monoalkyne complexes 375a-b have two acac ligands plus a CO. Monoalkyne complexes 376a-b also have two acac ligands, but they have an oxide in place of the CO.

Another set of monoalkyne complexes prepared by Templeton and co-workers are 377–380.116 All these complexes contain two acac ligands, an alkyne. The other ligand are pi ligands – iminium or iminoacyl groups.

298

5.06.4.4

Cyclic and Non-Cyclic Pi Complexes of Tungsten

Additional complexes

Hill and co-workers reported the monoalkyne complexes 381a-b and 382 in 2007.117 These feature borate-supported methimazolyl ligands.

Similar complexes were also studied in the Mösch-Zanetti group, who prepared and examined 383–388 as possible mimics for the active site of the enzyme acetylene hydratase.118

Cyclic and Non-Cyclic Pi Complexes of Tungsten

299

Other monoalkyne complexes prepared by the Mösch-Zanetti group intended to mimic the active site of acetylene hydratase include 389a-c, 390a-c, 391a-c and 392a-c.119,120

This same group also reported the monoalkyne complexes 393–396.121 Complex 393 was the starting complex used to prepare 394–396.

West and co-workers described tungsten monoalkyne complexes 397a-b and 398a-d. These species feature two supporting tropolonate ligands.122

300

Cyclic and Non-Cyclic Pi Complexes of Tungsten

The Yeh group also described two very different tungsten mono-alkyne complexes, 399 and 400. Complex 399 has several organometallic bonds besides the tungsten-pi bonding.123 Complex 400 has the monoalkyne, and another pi ligand, a cyclobutadiene, and all of this is linked via another pi bond to the C60 center.124

Leung and co-workers prepared a novel tungsten monoalkyne complex, 401, which has four tungsten-selenium bonds.125 Several variations of a tungsten-acetylene complex were examined in theory by the Bayse group. They examined 402a-d and 403a-c as models for the active site of acetylene hydratase.126

5.06.4.5

Alkynylpeptide complexes

In the area of bioorganometallic chemistry, two groups have examined tungsten monoalkyne complexes. The Metzler-Nolte group described complexes 404a-b, which feature an alkynyl acid or alkynyl amino acid linked to tungsten.127 But more importantly, this group coordinated alkynylpentapeptides to tungsten by using the coordination found in 404a-b. This work produced peptide complexes 405–407.127

Cyclic and Non-Cyclic Pi Complexes of Tungsten

301

The other bioorganometallic complexes were prepared by this author and his co-workers. In one study mono and bis-alkynyl esters were coordinated to tungsten to produce 408a-c and 409a-e. The conformational behavior of the peptides were probed after coordination to the metal center.128

302

Cyclic and Non-Cyclic Pi Complexes of Tungsten

In another study, peptide derivatives containing a diphenylacetylene moiety were coordinated to tungsten to yield 410 and 411.129 In the absence of tungsten these peptides adopt a b-sheet conformation. It was found that the peptides in these complexes retained their b-sheet conformation after coordination to tungsten.129

5.06.5

Bis-alkyne complexes

Tungsten can form bonds to more than one alkyne. Several research groups reported on tungsten bis-alkyne complexes during the last 15 years. The Mösch-Zanetti group, in their studies aimed at modeling the active site of acetylene hydratase, prepared complexes 412a-c, 413 and 414.130

Another set of bis-alkyne complexes came from the Sheridan group, which described 415a-b and 416a-e.131 The tungsten center here coordinates both alkynes, along with another pi ligand.

The author’s own group has prepared the cyclic peptidyl tungsten bis-alkyne complexes 417–421.132,133 In 417 and 418 the tungsten links the two ends of the peptide,132 while in 419–421 the tungsten links the two amino acid side chains together.133

Cyclic and Non-Cyclic Pi Complexes of Tungsten

303

The final set of tungsten bis-alkyne complexes are derived by coordination of two alkynes emanating from a 1,10 -disubstituted ferrocene. Complexes 422–425 were prepared and their conformational behavior examined.134 Both 422 and 423 formed a conformationally rigid ring system where the two alkynes are held to only the syn orientation.134 In the course of this work, the novel dimeric complex, 426, was also prepared.134 This species features two tungsten bis-alkyne complexes.

304

5.06.6

Cyclic and Non-Cyclic Pi Complexes of Tungsten

Tris-alkyne complexes

There were only two reports of tungsten tris-alkyne complexes in the period between 2006 and 2020. The Seidel group reported 427,135 which has three dithioalkyne ligands, and Yeh reported on 428,124,125,136,137 which was used as a starting material for the preparation of other tungsten complexes.

5.06.7

Nitriles and heteroalkyne complexes

Tungsten can also make pi bonds to other groups bearing triple bonds, and one of these is the nitrile. Templeton and co-workers prepared a number of such complexes. The neutral complexes 429a-c138,139 and 431115 have two acac ligands and either a CO or oxide ligand. The cations 430,116 432a-c,115 433,115 434a-c116 and 425a-c116 again have two acac ligands and the nitrile N-alkylated with various alkyl groups. There is some variety too with the axial ligands here. Complex 430 also has another pi ligand, a phosphinylalkene.

Cyclic and Non-Cyclic Pi Complexes of Tungsten

305

306

Cyclic and Non-Cyclic Pi Complexes of Tungsten

There were two other research groups that also described N-alkylated nitrile complexes. Tobita and his co-workers reported 436a-c140 and 437a-h.85,86 In these complexes the nitrile is N-alkylated with silyl groups. They also possess another pi ligand, the Cp group.

The Legzdins group, in the course of their studies on tungsten allyl complexes, also described 438a-b, 439a-b and 440.141 The N-alkyl groups here are bulky, sterically hindered moieties.

Interesting tungsten tris-heteroalkyne complexes, 424a-c, were described in 2019 by Goicoechea and co-workers.142 These complexes utilize three phosphaalkynes as pi ligands to tungsten.

5.06.8

Carbonyl complexes

There were 2 reports of tungsten-carbonyl pi complexes from 2006 to 2020, both coming from the Templeton group. They prepared and characterized 442a-c138 and 443a-b.115 Coordination of both aldehydes (442a, 442c and 443a) and ketones (442b and 443b) was observed.

Cyclic and Non-Cyclic Pi Complexes of Tungsten

5.06.9

307

Imine and iminium complexes

A small number of tungsten pi complexes to imines have also been reported. In 2007 the Templeton group reported complex 444,138 and in 2012 they reported on the related complex 433.115 These two differ only with regards to the axial CO or oxide ligand. In 2012 they also reported on complexes 434a-b and 435a-b, which are similar to 432.115

5.06.10

Cyclopentadienyl complexes

The most prevalent pi ligand for tungsten is the cyclopentadienyl group. There are a large number of such complexes that have been prepared in the last 15 years. These complexes can be divided into three groups: those containing Cp (C5H5) ligands, those containing Cp (C5Me5) ligands, and those containing structural derivatives of the two.

5.06.10.1 Cp complexes 5.06.10.1.1

Tungsten-Cp complexes

There were a number of tungsten cyclopentadienyl (Cp]C5H5) complexes described from 2006 to 2020. Harris and co-workers examined the photochemistry of the dimeric tungsten complex 448 and reported on the products 449a-c, 450, 451a-c, 452a-c and the cation 453.143–145

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Cyclic and Non-Cyclic Pi Complexes of Tungsten

Another study from the Harris group reported on the Cp complexes 454–456.146 Complexes 455–456 are also monoalkene pi complexes, while 454 has a conventional WdC s bond with an ethyl ligand.

Another researcher who examined the reactivity of 448 was O’Connor, who reported on its conversion to the hydride 457.147 The hydride 457 was also investigated by Shubina and co-workers,148,149 who reported on its ability to form novel hydrogen bonded species with nickel complexes to make 458 and 459.150

Cationic tungsten-Cp complexes 460–462 were also reported by the Honzícek group.151 The two acetonitrile ligands in 460 were readily replaced with bipy (461) and phen (462) ligands. This group measured the cytotoxicity of 461 and 462.151

Cyclic and Non-Cyclic Pi Complexes of Tungsten

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The Filippou group described tungsten-Cp complexes 463 and 464, which also feature tungsten-germanium bonds.152

Other tungsten-Cp complexes that also involve other metal centers were reported by Chatterjee and co-workers. Complexes 465a and 456a possess a ferrocene moiety, while complex 467 includes two cobalt centers.153 The other complexes reported here, 465b and 466b, feature an eneyne group bonded to tungsten.153

Weber and his co-workers prepared several new tungsten-Cp complexes in the course of their work making heteroallyl complexes. One set of these complexes have the tungsten bonded to carbon chains as in 468a-c, 469 and 470a-c.89,90 This last set of complexes can also be considered as tungsten-monoalkene complexes to allene.90

310

Cyclic and Non-Cyclic Pi Complexes of Tungsten

The Weber group has also prepared tungsten-Cp complexes that are also bonded to phosphorus, as shown below for 471–472, 473a-d, 474 and 475a-b.87,88,90,154

The Weber group also reported arsenic complexes 476a-d, 477 and 478.87–90 One set of complexes, 476a-c, are the arsenic analogs of the phosphorus complexes 473a-d.89,90 Two other complexes, 477 and 478, have both arsenic and phosphorus bonded to the tungsten center.87–89

5.06.10.1.2

Cp metal cluster complexes

With regards to tungsten-Cp complexes and arsenic, Scheer and co-workers reported 479, which has tungsten linked to two Cp rings and then an arsenic-sulfur cluster.155

Cyclic and Non-Cyclic Pi Complexes of Tungsten

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The Scheer group has also described supramolecular clusters that involve tungsten-Cp complexes along with phosphine ligands that link the tungstens to other metal atoms. The starting compound for this work is the ditungsten complex 480, which was used to prepare the supramolecular clusters 481a-b and 482a-c, which involve coordination of silver (481a-b) and copper (482a-c) to phosphorus.156

The group led by Humphrey has also employed tungsten-Cp complexes in their studies of iridium metal clusters. In a 2015 paper they first described clusters 483a-b, 484, 485a-b, 486 and 487a-d.157

312

Cyclic and Non-Cyclic Pi Complexes of Tungsten

Cyclic and Non-Cyclic Pi Complexes of Tungsten

313

This group went on to prepare two larger tungsten-iridium clusters, 488 and 489, where disubstituted benzene rings join two individual clusters.157

314

Cyclic and Non-Cyclic Pi Complexes of Tungsten

The group led by Ghosh has also explored metal clusters that utilize tungsten-Cp complexes. In two papers they described the tungsten-boron clusters 490158,159 and 491,158 and the tungsten-boron-tellurium cluster 492.159

5.06.10.1.3

Ruiz tungsten-Cp complexes

5.06.10.1.3.1 Complexes with a single tungsten The final set of tungsten-Cp complexes described from 2006 to 2020 came from the Ruiz group. One set of these complexes involved only one tungsten-Cp interaction. In 2008 they reported on 493a-c, which were prepared starting from 450 or 451a.160 These complexes connect a tungsten center to a manganese or rhenium center, where these other metals are coordinated to a monomethylcyclopentadienyl group.

Another set of complexes they explored are 494, 495, 496a-d and 497.161 These all feature a tungsten-phosphorus bond, along with the Cp ligand. The phosphorus atoms here are in a variety of oxidation states. These species were then used to prepare the bimetallic complexes 498a-d and 499, which have tin, lead, gold and zirconium adjoining the tungsten center.162

Cyclic and Non-Cyclic Pi Complexes of Tungsten

315

5.06.10.1.3.2 Complexes having a tungsten-tungsten single bond The majority of the complexes explored by Ruiz and co-workers have two tungstens bonded directly to each other. These species come with either a single, double or triple bond between the two tungstens. This review will begin with the complexes that have a single tungsten-tungsten bond. From a 2010 paper the Ruiz group reported on the bridging hydride 500,161 and its companion anion without a bridging hydride, 501.161 Both complexes also possess a bridging phosphine hydride ligand.

316

Cyclic and Non-Cyclic Pi Complexes of Tungsten

Another, similar bridging hydride complex, 502, was described first in 2017.163–165 In 502 the other bridging group is sampling a phosphine.

In 2018 the bridging hydride complexes 503 and 504 were reported.166 These complexes differ from 500 and 502 because of the other ligands around tungsten. In 503 each tungsten has its own hydride and NO ligands, while in 504 one tungsten has hydride and NO, while the other center has NO and a tin group. Another paper from 2018 reported the cation 505 having a bridging hydride.164 Another cation having a bridging hydride is 506, which also has two bridging phophines.167 Hydrides are not the only bridging ligands in these tungsten-Cp complexes. The Ruiz group reported on 507a-b,163 which have a bridging CO ligand. Bridging oxide are present in complexes 508166 and 509,168 with the latter species also possessing a rhenium center.

Bridging oxide and sulfur ligands were reported in cations 510a-b,166 which also have two bridging phosphine ligands. Another complex with a bridging sulfur is 511.169

Complexes having bridging carbons were also reported by the Ruiz group. In 512 the bridge is made by a CdH bond,166 while in 513–516 the carbon bridge is more direct. In 513–516 there are additional bridges involving nitrogens (513,166 515170 and 516170) and oxygen (514171).

Cyclic and Non-Cyclic Pi Complexes of Tungsten

317

Other nitrogen bridged complexes of this type have been reported by the Ruiz group. Both 517 and 518 are derived from diazo groups,166 while 519 and 520a-c are derived from NO ligands.164 Complex 521 is derived from ammonia164 while complex 522 has both a nitrogen and carbon bridge.171

318

Cyclic and Non-Cyclic Pi Complexes of Tungsten

Similar complexes which feature a bridging carbon have also been described by the Ruiz group. These complexes are the cations 523a-b, 524, 525, 526a-e and 527a-b.172

Although bridging ligands in these systems overwhelmingly involve only one atom and a 3-membered ring, complexes with larger bridges have been prepared, as seen in 528–531.171,173 The bridges here are extended carbon chains. Complexes 528,171 530173 and 531173 feature coordination between carbonyl oxygens and one of the tungsten centers.

A constant in most of the complexes described above is that they have had a particular bridging ligand that is often paired with a bridging phosphine ligand. The bridging phosphine ligands are ubiquitous in these complexes that possess tungsten-Cp bonds. Neutral complexes 532–535 possess two bridging phosphine ligands, in addition to the other ligands around the two tungstens.174 Complex 534 has both a bridging phosphine and a bridging phosphine oxide.

Cyclic and Non-Cyclic Pi Complexes of Tungsten

319

Cationic complexes 536a-c169 and 525174 also possess two bridging phosphine ligands.

Two bridging phosphine ligands are present in the nitride complexes 538167 and 539a-b.174

Two bridging phosphines were part of the isomeric tungsten oxide cations 540–541.174

Lastly, there were complexes prepared by Ruiz and co-workers that only had one bridging phosphine ligand. These are the oxide, 542,171 and complexes 543, 544a-b and 545a-b.163

320

Cyclic and Non-Cyclic Pi Complexes of Tungsten

The final set of complexes having a tungsten-tungsten single bond along with tungsten-Cp bonds all incorporate other metals beyond the two tungstens. One such complex that contains rhenium, 509, has been discussed already. Two other complexes that have rhenium are 546 and 547168; notably, 547 has four tungstens to go along with the rhenium. Other metals that are found in the complexes described by Ruiz and co-workers are manganese (548),175 cobalt (549),175 a third tungsten (550a),175 and molybdenum (550b).175 Two complexes contain iron; 551 contains one iron center,175 while 552 has two.176 Two other complexes possess two copper centers, 553 and 554.176

Cyclic and Non-Cyclic Pi Complexes of Tungsten

321

5.06.10.1.3.3 Complexes having a tungsten-tungsten multiple bonds Ruiz and co-workers have also reported on complexes that have tungsten-tungsten double bonds, and where the two tungstens are coordinated to Cp ligands. One set of these complexes have a bridging hydride ligand. Complexes 555177 and 556166,175 are neutral, while 557a-c178 and 558a-b169 are cations.

Complexes with bridging carbon atoms have also been reported. The neutral complexes 559,170 560170 and 561171 all have the bridging carbon bonded to a nitrogen. The same is true for the cations 562 and 563.167

Complexes of this type that have two bridging carbons have also been reported. These are derived from reactions of a starting ditungsten complex with alkynes. In this manner complexes 564,171 565a-b,173 566a-c,173 567a-b,173 568a-b,173 569a-b173 and 570173 were constructed. Interestingly, 567a-b173 are also tungsten monoalkyne complexes.

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Other complexes having a tungsten-tungsten double bond are simpler in their structure. Complexes 571,177 572a-c178 and 573ac just have phosphine and CO ligands to go along with the two Cp ligands. Complexes 572a-c and 573a-c are isomers of each other. The cations 574a-c178 feature a hydride ligand, while cation 575167 has a PH2Cy ligand. 178

Cyclic and Non-Cyclic Pi Complexes of Tungsten

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Ruiz and co-workers also examined the neutral complexes 576a-b and 577,169 which have multiple bonds between tungsten and oxygen, sulfur and nitrogen. The same is true for cations 578169 and 579a-c,167,169 which have multiple bonds between tungsten and oxygen, sulfur and bromine.

In the above complexes one of the tungstens forms a multiple bond with a heteroatom. In another set of complexes prepared by the Ruiz group, one of the tungstens is bonded to two heteroatoms. This can be two sulfurs (580), two oxygens (581) or a nitrogen and a sulfur (582–585).169 Complexes 580–582 are neutral, while 583–585 are cations.

In another set of complexes, Ruiz and co-workers bridged the two tungstens with either two oxygens (586a-b),179 carbon and nitrogen (587),170 nitrogen (588)167 and two phosphines (589).174

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One last set of complexes in this group are 590 and 591, which have a bridging iodine (590) and a bridging trialkyltin (591).177

Attachment of other metal centers to these tungsten-tungsten complexes is also possible. Ruiz and co-workers prepared the diiron complex 592, the tritungsten complex 593 and the ruthenium complex 594.176

The final set of complexes examined by Ruiz and co-workers all are reported with a tungsten-tungsten triple bond. It is reactions of these species that ultimately generated the compounds described above that have tungsten-tungsten double and single bonds. Hydride complex 595170,171,173,179 was used to prepare cations 596a-b,179 as well as the bridged aryl complex 597.179

Complexes with bridging CO ligands (598167,174,180 and 599170) were also obtained. These could be converted into 600a-b,177 anion 601168 and cations 602a-b.167,174,180

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Like all the other tungsten-tungsten complexes explored by Ruiz, this particular set could also be coupled to other metal centers. Accordingly, the tin adduct 603 and the gold adducts 604a-b and 605a-b.179

5.06.10.1.3.4 Complexes having a tungsten-molybdenum bond Although the bulk of work from the Ruiz group centered on ditungsten complexes, they did explore some mixed metal complexes, with molybdenum complexes being the most notable. Complexes 606–611165,179 all possess a tungsten-molybdenum triple bond. The isomeric hydrides 607 and 608 are found in equilibrium with each other. Bridging CO ligands are present in 606, 609 and 610a-c, while 611 has a bridging iodine.

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Tungsten-molybdenum double bonds are present in complexes 612 and 613a-b,165 while 614 has a tungsten-molybdenum single bond.165

Ruiz and co-workers also prepared complexes with tungsten-cobalt single bonds (605)175 and tungsten-iron single bonds (606).176 As with all these complexes, the tungsten has a Cp ligand.

5.06.10.2 Cp complexes Tungsten-Cp (C5Me5) complexes were more numerous than tungsten-Cp complexes in the period 2006–2020. Given the large number of complexes to discuss, this section is divided into sections that focus on the ligands besides Cp that are also around the tungsten center.

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5.06.10.2.1

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Tungsten-Cp oxide and sulfide complexes

Tungsten-Cp (C5Me5) complexes were more numerous than tungsten-Cp complexes in the period 2006–2020. The simplest such complexes involved tungsten oxides and sulfides. In a 2013 paper Gunnoe and co-workers reported the tungsten oxides 617–620.181

Erker and co-workers prepared the sulfide 621, then converted it to 622, which would interconvert to 623 via a Claisen-type rearrangement.182

Poli and his co-workers also examined tungsten oxides, and reported work with 624183,184 and the cations 625 and 626.185

Poli and his group also investigated complexes that had both oxides and sulfides, describing 627 and 628.186

5.06.10.2.2

Tungsten-Cp carbonyl complexes

Tungsten-Cp carbonyl complexes also received attention from researchers. Harris and co-workers prepared 629 and examined its photochemical reaction with pentane to yield the tungsten hydride complex 630 and the boronic acid ester 631 (see Scheme 4).187 Sterenberg and co-workers examined novel phosphine ligands bonded to tungsten, and generated the cations 632–636, and the dimeric dications 637–638.188

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Scheme 4 Photochemical oxidation of pentane by a tungsten-Cp complex.

Low and co-workers recently reported tungsten-Cp complexes that were linked to other metal centers via a dialkynylbenzene. Starting with the tungsten complex 639 they were able to prepare the tungsten-ruthenium complexes 640a and 641a, along with the tungsten-iron complexes 640b and 641b.189

5.06.10.2.3

Tungsten-Cp hydride complexes

Tungsten hydride complexes that have Cp ligands have also been looked at. Both the Poli and Shubina groups prepared and examined the trihydride 632.190,191

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Earlier in this chapter the work of Legzdins and co-workers on allyl complexes was discussed. In the course of this work they prepared a number of tungsten-Cp hydride complexes. One set of these are 633–636,192 which have in common a phosphine ligand.

The Legzdins group also reported the hydride complex 647, which also has an allyl ligand.193 In another paper the same group described hydrides 648–649.66

Further studies from this group generated additional hydrides. Complex 650, which is a structural isomer of 645, was reported, along with the phosphite hydrides 651a-b, 652 and 653.74 Additionally, a dihydride, 654, was described.74

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The final set of hydrides reported by the Legzdins group were 655–658.194,195 They all possess unusual phosphine ligands, and in the case of 656 and 657,195 they have Lewis acids coordinating to the NO ligand.

Another set of tungsten-Cp hydrides came from the Sita research group, which is exploring complexes capable of fixing nitrogen. In their work they described two hydrides, 658196 and 659.197 Complex 658 is also an allyl complex.

5.06.10.2.4

Tungsten-Cp complexes with silicon ligands

A number of research groups have explored tungsten-Cp complexes that also possess silicon ligands, which could include silanes, silylenes and silylynes. These complexes are detailed in the sections that follow.

5.06.10.2.4.1 Tungsten-Cp complexes with silane ligands Tungsten hydride complexes that also have Cp and hydride ligands have been explored by Sakaba and co-workers, who focused on tungsten hydrides 660, 661a-b, 662, 663, 664a-b and 665a-b.198

Tobita and co-workers have also prepared tungsten hydrides which also possess tungsten-silane bonds. Complexes 666–669 have two tungsten-silicon bonds, along with the hydride.199–201

Cyclic and Non-Cyclic Pi Complexes of Tungsten

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Another set of tungsten hydride complexes are 670–677. These species have only one silicon, and are either a monohydride (670) or dihydrides (671–677). The alkenyl groups attached to silicon are all derived from epoxides.202

In two following publications the Tobita group discussed two other dihydrides, 678203 and 679,204 that are structurally related to 671–677.

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Other researchers have probed silanes that do not possess hydrides. Sakaba and co-workers have prepared and examined silyltungsten complexes 680a-c205,206 and 681.206

The Tobita group has examined several classes of silyltungsten complexes. One class involves the coordination of a disilyldimethylxanthene to tungsten as found in complexes 682a-b.201

Another set of tungsten-silyl complexes prepared by this group are 683a-f.86,207,208 A range of silyl groups were employed in these species.

Two unique tungsten silyl complexes studied by the Tobita group are 684a-b209 and the salt 685.210 These are silicon hydrides where the silicon is bonded also to tungsten and either nitrogen (684a-b) or oxygen (685), which is also joined to tungsten.

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One last set of tungsten silyl complexes examined by the Tobita group were 686a-b and 687.211 Here there are two silyl groups attached to tungsten, along with the Cp group. There are bridging ligands here too, H in 686a-b and methoxy in 687.

Another group that has investigated tungsten-Cp complexes with silyl groups is headed by Muraoka. In recent publications they have described 688a-b212–215 and 689.215 These species employ silanone ligands.

5.06.10.2.4.2 Tungsten-Cp complexes with silylene ligands Tungsten-Cp complexes that also possess tungsten-silylene bonds have been investigated. Sakaba and co-workers have examined 690a-b,205 691a-c216 and 692a-c.216 Complexes 690a-b are dimeric with both tungstens linked via an alkyne, while 691a-c and 692a-c are monomeric and differ only in the spatial arrangement of the ligands around tungsten.

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Tobita and co-workers prepared similar complexes in which 4-dimethylaminopyridine was coordinated to silicon. Complexes 693a-d207 have methyl and tolyl groups bonded to silicon, while the isomeric complexes 694 and 695208 have methoxy groups bonded to silicon.

The Tobita group also prepared the silylene tungsten hydrides 807,217 808,218 809204 and 810.209

The hydrides 696 and 697 were converted to the salts 700,210,217 701217 and 702.218 Complex 812 has tris(pentafluorophenyl) borane coordinated to one of the CO ligands.

Cyclic and Non-Cyclic Pi Complexes of Tungsten

335

The Tobita group also prepared tungsten silylenes where the silylene was part of a 4-membered ring involving tungsten, silicon, carbon and either oxygen (703a-b,218 704219 and 705219) or nitrogen (706218).

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The final tungsten silylene reported was 707, prepared by the Muraoka group.212,215,220,221 This complex has both a silylene and silane bond to tungsten.

5.06.10.2.4.3 Tungsten-Cp complexes with silylene ligands Between 2006 and 2020 there were two reports of tungsten-Cp silylynes, both from the Tobita group. They are the monomer 708218 and its dimer, 709,218,219 and another monomer, 710.217

5.06.10.2.4.4 Tungsten-Cp complexes with oxysilanes These complexes have a tungsten-Cp moiety, along with an OSiR3 moiety linked to the tungsten. The Tobita group reported two complexes of this type, 711204 and 712a-b.218

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Muraoka and co-workers have examined oxysilanes 713–716.215

5.06.10.2.4.5 Tungsten-Cp complexes with thiosilanes The Muraoka group has also examined tungsten-Cp thiosilane complexes 717a-c,214 718220,221 and salt 719.214

5.06.10.2.4.6 Other silicon-related Cp complexes Sakaba and co-workers examined tungsten-Cp complexes 720a-b217 and 721,206 while the Tobita group explored complexes 722ac219 and 723.84 All of these species have a triple bond between carbon and tungsten. Complexes 720a-b, 721 and 723 also contain silicon, whereas the carbonyl component in complexes 722a-c is strictly carbon based.

Tungsten-Cp complexes that have an isocyanosilane ligand were examined by the Tobita group. They prepared and studied the cis and trans isomers 724a-b and 725a-b.85

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5.06.10.2.5

Tungsten Cp monoalkene complexes

Tungsten-Cp complexes that are also monoalkene complexes have been examined. Legzdins and co-workers, in their investigations of CdH bond activation, prepared complexes 726a-b,192 727a-b192 and 728.78

This work was followed by reports on the monoalkene complexes 729–735. All of these complexes have an NO ligand. Complexes 685–686222 pair the NO ligand with a CO, while complexes 687–691193 pair the NO with a PMe3 ligand.

In other reports from the Legzdins group, monoalkene complexes of alkenyl ketones, 736–73868 and 739a-d,76 were reported. All of these complexes were obtained by CO insertion reactions.

Cyclic and Non-Cyclic Pi Complexes of Tungsten

339

Monoalkene complexes involving allenes, 74075 and 74177 were also reported by this group, as was a monoalkene complex, 742,70,71 derived from 1,3-butadiene.

Finally, the Legzdins group reported intramolecular monoalkene complexes 743223 and 744–746.76 In all four of these molecules the tungsten forms a pi bond to an alkene that has another carbon directly bonded to the metal center, forming a ring.

Sita and co-workers also reported tungsten-Cp complexes with a pi bond to a monoalkene. The alkene ligands were ethylene (747), styrene (748), cyclopentene (749) and norbornene (750).196

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Cyclic and Non-Cyclic Pi Complexes of Tungsten

Tobita’s group, in their studies on tungsten-silicon chemistry, have described the boroalkenyl complex 751.84

5.06.10.2.6

Tungsten Cp bis-alkene complexes

There was only one report of tungsten-Cp bis-alkene complexes. Complex 752 and the isomeric complexes 753–754 were prepared and examined by the Legzdins group.193

5.06.10.2.7

Tungsten Cp monoalkyne complexes

The Legzdins group also described the only tungsten complex having both a monoalkyne and Cp ligand, compound 755.193

5.06.10.2.8

Tungsten Cp allyl complexes

Besides the Cp allyl complexes noted earlier in this chapter, Tobita and co-workers prepared the allyl complexes 756a-c,58 and the heteroallyl complexes 757,85 758a-b86 and 759a-d,84 in the course of their work on tungsten silylenes.

Cyclic and Non-Cyclic Pi Complexes of Tungsten

5.06.10.2.9

341

Tungsten Cp imine and iminium complexes

Legzdins and co-workers prepared the tungsten-Cp imine complex 760,64 as well as the iminium complexes 761–762.223

Tobita and co-workers have also examined iminium complexes of this type. With 763a-b and 764a-h,85 the nitrogen is bonded to silicon.

5.06.10.2.10

Tungsten Cp and ketone complexes

There was one report of a tungsten-Cp complex also forming a pi bond to ketones. Legzdins and co-workers reported complexes 765a-b.192

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5.06.10.2.11

Tungsten Cp complexes with NO ligands

A number of tungsten-Cp complexes also include the NO ligand. Legzdins and co-workers employed complexes 766,222 767,64,68,71 768,77 769,224 770a-c224 and the dimeric species 771194 as their starting materials for their CdH insertion studies.

These molecules were used to make tungsten-Cp complexes that have a single phosphine ligand. These include 772, which has a bidentate N,P ligand, the salt 773, which has two tungsten-Cp components, and the dimeric species 774.194

Use of 766–771 allowed for the preparation of 775a-b,192,193 776,74 777a-b,193,194 778a-b,194,195 779a-b195 and 780.195 All of these complexes coordinate two phosphine ligands. These ligands can be two individual phosphines (775a-b and 776), or two bridging phosphines (777a-b, 778a-b, 779a-b and 780).

Cyclic and Non-Cyclic Pi Complexes of Tungsten

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There were a number of tungsten-Cp complexes that also had NO and a carbon chain as ligands to the tungsten. Complexes 737a-c were prepared by the Legzdins64,68,71,77 and Etienne groups.83 In addition to the Cp and carbon chains bonded to tungsten, they also have NO and Cl ligands.

Tungsten complexes with oxide and alkoxide ligands were also described by the Legzdins group. These complexes include dioxides 782a-b,77,225 the alkoxide 783192 and 784,225 which features a 3-membered ring involving tungsten, oxygen and nitrogen.

Tungsten-Cp complexes having pyrrolidine or piperidine ligands include the pyrrolidine complexes 785,79 786a-c79 and 787,64 and the piperidine complexes 788,79 789,64 790226and 791.226 Complexes 790 and 791 are isomers of each other that differ only in the relationship of the carbon-carbon double bond and the other ligands on the metal.

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Cyclic and Non-Cyclic Pi Complexes of Tungsten

One last structural variant in this category is 792, a dimeric structure with two tungsten centers linked via phosphates.74 Both tungstens in 792 have a methyl group bonded to tungsten.

Related to the above tungsten-Cp complexes are ones that have two carbon chains appended to the tungsten, along with an NO ligand. The simplest of these are 793a-b, which were prepared by Legzdins and co-workers.193,225,227,228 The cyclopropyl complexes 794a-c were prepared by Etienne and co-workers.82,83

The Legzdins group, in their studies on CdH bond activation, prepared a large number of complexes bearing substituted phenyls. The first set of these complexes involved monosubstituted phenyl and generated the ortho complexes 795a-e, the meta complexes 796a-e and the para complexes 797a-e.227

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This group also prepared complexes with disubstituted phenyls. These included phenyls where both substituents are identical (798a-b, 799a-b, 800a-b, 801a-b and 802a-b), and phenyls with two different substituents (803a-c and 804a-c).227

Another variation with these tungsten-Cp complexes that have two alkyl ligands is where the two alkyl groups form a ring involving the tungsten center. Complexes made by the Legzdins group include 805,226 806,193 807,193 808193 and 809.193

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Cyclic and Non-Cyclic Pi Complexes of Tungsten

Finally, Legzdins and co-workers prepared 810a-c, which has a 3-membered ring involving tungsten, carbon and nitrogen.67

5.06.10.2.12

Tungsten-Cp complexes with nitrogen ligands

The group led by Sita has been searching for transition metal complexes that could be used to fix nitrogen, and have explored tungsten-Cp complexes in their studies. The starting compounds for this work are 811,229 812,229 813,229,230 814197,231 and 815.196,197,229,230 They were used to prepare the complexes described below.

These starting materials were then used to make complexes 816,231 817,196,230 818,196 819,229 820,197,229,232 821231 and 822,230 where N2 has been coordinated to two tungstens (816–818), and then undergo initial reduction steps (819–822).

Cyclic and Non-Cyclic Pi Complexes of Tungsten

347

The Sita group also prepared 823,196 which came from a reaction with acetonitrile.196

Sita and co-workers also reported on 824,230 825,232 826,229 827a-b,197,230,232 828231 and 829,232 which represent complexes where N2 has been cleaved. Another complex, 830,230,232 a tungsten oxide, was also generated.

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Cyclic and Non-Cyclic Pi Complexes of Tungsten

Other products from the reactions studied by Sita and co-workers included 3-membered ring species 831a-b, 832, 833 and 834.196,197

5.06.10.2.13

Tungsten-germanium complexes

Just as tungsten-Cp complexes including silicon could be prepared, tungsten-Cp complexes involving germanium have been probed.

5.06.10.2.13.1 Tungsten-Cp germyl complexes Sakaba and co-workers detailed their work on 835a-b and 836, which feature a tungsten-germanium single bond, along with a tungsten-carbon double bond.233

Cyclic and Non-Cyclic Pi Complexes of Tungsten

349

Tobita and co-workers described 837,234,235 838a-b234 and 839a-b,234 which are tungsten-Cp complexes with a tungstengermanium single bond. With 837, 838a-b and 839a-b the Ge forms a ring that includes the tungsten center.

5.06.10.2.13.2 Tungsten-Cp germylene complexes Researchers who explored tungsten-Cp silylene complexes also explored their germylene counterparts. Sakaba and co-workers prepared germylenes 840a-b, and the isomers 841a-b and 842a-b, which are in equilibrium with each other.233

The Tobita group extended their studies to include germanium, and they have described the hydrides 843234,236 and 844a-d.234,237

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The Tobita group also prepared germylene complexes 845a-b236 and 846a-c.237 In these complexes a neighboring nitrogen (835a-b) or oxygen (836a-c) coordinates to the germanium.

5.06.10.2.13.3 Tungsten-Cp germylyne complexes From 2006 to 2020, there was one report of a tungsten-Cp germylyne complex, 847, which was prepared by the Tobita group.57,235

5.06.10.2.13.4 Other related tungsten-Cp germanium complexes There was also only one report of a carbonyl complex that also contained germanium, which was examined by the Sakaba group.233

5.06.10.2.14

Tungsten-Cp complexes with CO ligands

The Cp -W CO complexes 849,205,233,236 850,211 851a-b207,208,218 and 852213 were used as starting materials for the preparation of Cp -W complexes with silicon and germanium. Sakaba and co-workers employed 839 in their work, while Tobita and co-workers employed 840 and 841a-b in their work. The dimeric species 842 was employed by the Muraoka group.

Cyclic and Non-Cyclic Pi Complexes of Tungsten

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In the course of their work on silicon and germanium adducts, the Tobita group also produced 853a-b,84 which are hydrides having a WdB bond, and 854a-b,140 which has an unique N,C bridging ligand.

5.06.10.2.15

Cp -W clusters

From 2006 to 2020 there were several reports of metal clusters that utilized Cp -W moieties. In 2006 Suzuki and co-workers reported on two sets of tungsten-ruthenium clusters, 845a-b and 846a-b.238 These are metal hydride clusters. 845a-b has one tungsten and two ruthenium centers, while 846a-b has two tungsten and one ruthenium center.

The group led by Scheer examined the clusters 857 and 858, which contain multiple tungsten centers.239 The hydride 857 has one Cp -W moiety, while 858 has two.

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Cyclic and Non-Cyclic Pi Complexes of Tungsten

This same group prepared other clusters (859–862) which contain both tungsten and molybdenum, along with phosphorus.239 Cluster 859 contains 3 tungstens and 1 molybdenum, while 860 contains 4 tungstens and 1 molybdenum. Cluster 861 contains 2 tungstens and 2 molybdenums, while 862 contains 1 tungsten and 2 molybdenums.

Ghosh and co-workers have explored Cp -W clusters, most notably with boron. They prepared complexes 863,240–245 864,246 865,240 866240 and 867.247 These metalloboranes only contain Cp , W, B and H.

Cyclic and Non-Cyclic Pi Complexes of Tungsten

353

Some other metalloborane clusters this group examined, 868,243 869,243 870243 and 871248 also included CO ligands. Of these, 859 and 860 also include an additional tungsten center that does not have a Cp ligand.

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Cyclic and Non-Cyclic Pi Complexes of Tungsten

Some even larger complexes examined by the Ghosh group are 872246 and 873.240 These are essentially two clusters joined together by a central tungsten (872) or by a BdB bond (873).

Cyclic and Non-Cyclic Pi Complexes of Tungsten

355

The Ghosh group also included other atoms in these clusters. In 874249 sulfur is included, while in 875242 selenium is included.

356

Cyclic and Non-Cyclic Pi Complexes of Tungsten

Complexes involving only sulfur and selenium were prepared by Ghosh and co-workers. Complexes 876249 and 877249 link two Cp -W centers via 4 sulfurs, while 878250 has only one Cp -W center linked to 4 sulfurs. Complex 879245 links 4 seleniums to two Cp -W centers. 

Besides adding sulfur and selenium to the boron clusters, the Ghosh group has also incorporated other metals in these systems. Ruthenium clusters were one target of this group. They prepared 880,247 which has one Cp -W center, and 871, 872 and 873,247 which have in common 3 Cp -W centers, but differ in the number and bonding of the ruthenium centers. Still one other ruthenium complex is 884a.251

Cyclic and Non-Cyclic Pi Complexes of Tungsten

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Another metal that was incorporated into these clusters was iron. Complex 884b is identical to 884a, except iron has replaced ruthenium.251 Complex 885251 also shares a similar structure with 884b. Clusters 886245 and 887240 incorporate the tungsten and irons in different arrangements.

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Cyclic and Non-Cyclic Pi Complexes of Tungsten

One last set of metals incorporated into this type of cluster by Ghosh and co-workers were chromium, molybdenum and tungsten. Clusters 888a-c incorporate chromium, molybdenum or an additional tungsten into the framework,245 while clusters 889a-b incorporate molybdenum or an additional tungsten.245

5.06.10.3 Modified Cp and Cp complexes In this section complexes having tungsten coordinated to a structural variant of Cp or Cp are delineated.

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5.06.10.3.1

359

Modified Cp complexes

The Cp ring system can be modified by adding substituents to the cyclopentadienyl ring. Some complexes that fit in this category, 275–279, were described earlier. The Legzdins group, in their studies of CdH bond activation, prepared complexes 890–883,74,228 which have an isopropyl group on the Cp ring. Their preparation of 891–893 began with 890a-c, which could undergo substitution reactions. In another study these researchers also prepared 894.80

A tert-butyl variant of the Cp ring was used by the Scheer group to prepare the polymeric cluster 895.156

The Wang research group has examined a large number of carbonyl- (896b-i)252–258 and imine- (896j-k)256,257 substituted derivatives of Cp-W complex 896a. Two interesting members of this set of complexes are 896k and 896l,257 both of which have a second metal center, chromium.

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Cyclic and Non-Cyclic Pi Complexes of Tungsten

Two other complexes reported by Wang and co-workers are 897252,254 and 898,254 which contain a second metal center, titanium.

The Wang group extended their work to also include amine-substituted complexes, 899a-b.256,258

Wang and co-workers also examined derivatives of ester 896g, where the methyl group on tungsten is replaced by either Cl (900a) or an alkyne (900b).253,255

The Royo group has examined tungsten complexes made to Cp rings functionalized with silylalkenyl groups. They prepared the isomeric hydrides 901 and 902, as well as the dimeric complex 903.259

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This group also looked at the conversion of 901 to the anion 904, and its subsequent conversion to the tungsten-monoalkene complex 905.259

Mizuta and co-workers, who have examined species with multiple metal centers, prepared 906 and 907.260 In these species, two Cp rings are linked via a thiophosphate linker.

Fischer and his students prepared the cation 908 and the zwitterion 909.261 These two complexes have Cp rings that are substituted with an alkylamine chain.

Cuenca and co-workers reported 910a-b, which feature a disubstituted Cp ring bonded to tungsten.262 The two substituents are in a 1,3-arrangement, and both groups are silanes, with one of the silanes having an alkene group.

Two research groups detailed complexes where a substituent on the Cp ring possessed a ligand that could bond back to the tungsten. Fischer and his students reported on 901a-b and 902, which have an alkylphosphine chain emanating from the Cp ring.263 This same phosphine arrangement was used to prepare the bimetallic complexes 903a-b,264 which have a CudW bond.

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Cyclic and Non-Cyclic Pi Complexes of Tungsten

The Fischer group also prepared and examined 914a-c and 915,265 which are similar to 913a-b, except that the Cu has been replaced by either Ni or Pd, and that the Ni and Pd form an allyl complex.

The Wang group reported on 916, which has an alkyl group bonded to tungsten and the Cp ring.255,256,258

5.06.10.3.2

Modified Cp complexes

There have been reports on tungsten complexes made to structural variants of the Cp ligand. With complexes to tungsten made from 2006 to 2020, in general, only one of the methyls on the Cp ring was replaced with another group. Some complexes that fit this category, 278–279, were described earlier. Cuenca and co-workers reported the hydride 917, which has a Cp ring where a hydrogen takes the place of one of the methyls.266

In their studies on CdH bond activation, Legzdins and co-workers prepared 918,80 where the methyl on Cp ring has been swapped for an ethyl group.

Sakaba and co-workers, during their investigations of tungsten-silicon complexes, prepared 909 and 910,205 which are in equilibrium with each other. Again, these species have an ethyl group in place of one of the Cp methyl groups.

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The Muraoka group also explored this same variant of the Cp ring, making complexes 921 and 922.212

The Tobita group has made extensive use of this ethyl variant of Cp in their studies on tungsten-silicon complexes. They prepared and examined 923210 and 924a-b,209 which have a single bond between tungsten and silicon.

Tobita and co-workers also examined 925,58,204,209 926,204 927,204 928209 and 929,210 which are silylene complexes having the ethyl variant of the Cp ligand.

Two other complexes of this type reported by the Tobita group are the oxysilyl complex 930204 and the allyl complexes 931a-c.58

364

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The Scheer group reported complex 932, where a methyl on the Cp ring was replaced with a methylenephosphine, with the phosphorus bonding back to two tungsten centers, including the one bonded to the modified Cp ring.239

Yeh and co-workers have explored the Cp variant where all the methyls have been replaced by phenyl groups. They prepared the two allyl complexes 933136 and 934.123 In 934 one of the phenyls on the cyclopentadienyl ring has reacted via CdH bond activation.

The Yeh group has also prepared complexes of this type that are also pi bonded to C60. Complexes 935a-b link the pentaphenylcyclopentadiene and C60 to tungsten via pi bonds.123,136 Two other adducts are 936a-b, which link C60 the tungsten via sigmabonds, and one of the phenyls on the cyclopentadienyl ring has been linked to the tungsten center.123,136 Complexes 935ab can also be considered monoalkene complexes because of the bonding between tungsten and the C60 unit.

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5.06.11

365

Indenyl-tungsten complexes

From 2006 there were four complexes reported that exhibited pi bonding between tungsten and an indenyl ligand, which is simply another variation of the Cp ligand. Three complexes reported earlier in this review that use and indenyl ligand are 261–263. In addition to these complexes, Honzícek and co-workers reported on tricarbonyl complex 937.267 Vinklárek and co-workers reported on the cations 938 and 939,61 where derivatization of the indenyl group allowed for addition of another ligand for the tungsten.

Thiel and co-workers described the allyl complex 940, which has a highly elaborated indenyl ligand bonded to the tungsten center.268

5.06.12

Cyclobutadiene complexes

There was one report of a tungsten complex with cyclobutadiene, complex 400, which was described with the monoalkyne complexes.

5.06.13

Arene complexes

Tungsten can also form pi bonds to arene rings. This section delineates the complexes that have been made and studied from 2006 to 2020.

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Cyclic and Non-Cyclic Pi Complexes of Tungsten

5.06.13.1 Tungsten complexes to benzene rings Fischer and his students have recently prepared the bimetallic complexes 941a-c,269 942,269 943a-b,270 944271 and 945.271 At one end of these complexes tungsten is pi bonded to a boranylphenyl (941a-c, 942 and 943a-b) or a dimethylboranylphenyl (944 and 945). The boron is linked to three phosphines, which are in turn coordinated to a variety of other metal centers. Some of the complexes are anions (941a-c, 943a-b and 944), while others are zwitterions (942 and 945).

In 2010 the Heinekey group reported the preparation of the dihydride 946, which features a pi bond between tungsten and a 1,3,5-triisopropylbenzene.272

In recent work, Zaitsev and co-workers described the preparation of 947, which has a pi bond between tungsten and a germanium-substituted phenyl ring.273

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Lastly, Yeh and his co-workers made complex 948, which has tungsten coordinated to hexaphenylbenzene.274 The tungsten is also pi bonded to a double bond on the C60 moiety.

5.06.13.2 Tungsten 2,5-dimethylpyrrolide complexes Besides benzene, 2,5-dimethylpyrrolide has been utilized as a pi ligand for tungsten. The first reports of this came from the Schrock group, which made 949, 950a-b and 951a-b, along with the cation 952.275

In recent work, the Buchmeiser group has prepared and examined 949 and its structural relatives 953a-c.276

368

5.06.14

Cyclic and Non-Cyclic Pi Complexes of Tungsten

Cycloheptatrienyl complexes

The cycloheptatrienyl ligand, C7H7, has also been utilized as a pi ligand for tungsten. Whiteley and co-workers reported complexes 954a-c in 2006.277

They followed this with reports about 953, which could be converted to the dppe complex 954. Subsequent elaboration of 954 generated the cations 955 and 956a-c, along with neutral complexes 957a-b.278

5.06.15

Summary

In the last 15 years over a thousand new tungsten pi complexes have been detailed in the literature. This review tried to best summarize all the developments in this area, and anticipates even further growth in the coming years.

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Wang, Y.-P.; Tang, W.-D.; Lui, X.-H.; Cheng, H.-Y.; Lin, T.-S. Crystal Structures of Azine Derivatives of (Z5-acylcyclopentadienyl)dicarbonylnitrosylchromium and Unequivocal Assignments of C(2,5) and C(3,4) on the Cyclopentadienyl Ring of Azine Derivatives of (Z5-acylcyclopentadienyl)dicarbonylnitrosylchromium, (Z5-acylcyclopentadienyl) tricarbonylmethylmolybdenum, and (Z5-acylcyclopentadienyl)tricarbonylmethyltungsten in 13C NMR Spectra. Polyhedron 2015, 100, 231–242. https://doi.org/10.1016/j. poly.2015.07.082. 258. Wang, Y.-P.; Yang, H.-H.; Wu, J.-C.; Cheng, H.-Y.; Lin, T.-S. Unequivocal Assignments of C(2,5) and C(3,4) on the Cyclopentadienyl Ring of Cp2Fe, CpCr(CO)2(NO), and CpW(CO)3(CH3) Bearing an Electron-Donating or Electron-Withdrawing Substituent via Resonance in 13C NMR Spectra. J. Organomet. Chem. 2015, 794, 168–180. https:// doi.org/10.1016/j.jorganchem.2015.07.008. 259. Royo, E.; Acebrón, S.; Mosquera, M. E. G.; Royo, P. Allyl Isomerization Mediated by Cyclopentadienyl Group 6 Metal Compounds. Organometallics 2007, 26, 3831–3839. https://doi.org/10.1021/om700293a. 260. Imamura, Y.; Kubo, K.; Mizuta, T.; Miyoshi, K. Reactions of Ring-Slipped Iron Complexes Derived from P(S)Ph-Bridged [1]Ferrocenophane: Synthesis of Bis(half-sandwich) Heterodinuclear Complexes. Organometallics 2006, 25, 2301–2307. https://doi.org/10.1021/om060020v. 261. Fischer, P. J.; Herm, Z. R.; Kucera, B. E. [(2-(Trimethylammonium)ethyl)cyclopentadienyl]tricarbonylmetalates: Group VI Metal Zwitterions. Organometallics 2007, 26, 4680–4683. https://doi.org/10.1021/om700497z. 262. Petrisor, C. E.; Chahboun, G.; El Amrani, M. A.; Royo, E.; Cuenca, T. Mixed Disilyl-Substituted Cyclopentadiene Derivatives and Corresponding Zirconium, Molybdenum and Tungsten Compounds. Eur. J. Inorg. Chem. 2010, 2010, 3666–3674. https://doi.org/10.1002/ejic.201000394. 263. Fischer, P. J.; Neary, M. C.; Avena, L.; Sullivan, K. P.; Hackbarth, K. C. Dicarbonyl{[2-(diphenylphosphino)ethyl]cyclopentadienyl} Group VI Metal Hydrides, Halides, and Anions: Precursors for Olefin Epoxidation Catalysts. Organometallics 2012, 31, 2437–2444. https://doi.org/10.1021/om300057n. 264. Fischer, P. J.; Heerboth, A. P.; Herm, Z. R.; Kucera, B. E. [(2-(Diphenylphosphino)ethyl)cyclopentadienyl]tricarbonylmetalates: Supporting Ligands for Reactions at Group VI Metal − Copper Bonds. Organometallics 2007, 26, 6669–6673. https://doi.org/10.1021/om700861r. 265. Fischer, P. J.; Neary, M. C.; Heerboth, A. P.; Sullivan, K. P. Allylnickel(II) and Allylpalladium(II) Derivatives of [(2-(Diphenylphosphino)ethyl)cyclopentadienyl]tricarbonylmetalates: Reactions with Free Radicals. Organometallics 2010, 29, 4562–4568. https://doi.org/10.1021/om1006825. 266. Chahboun, G.; Petrisor, C. E.; Gómez-Bengoa, E.; Royo, E.; Cuenca, T. Insight into cis-to-trans Olefin Isomerisation Catalysed by Group 4 and 6 Cyclopentadienyl Compounds. Eur. J. Inorg. Chem. 2009, 2009, 1514–1520. https://doi.org/10.1002/ejic.200900069. 267. Honzícek, J.; Mukhopadhyay, A.; Romão, C. C. Improved Preparation of Indenyl Molybdenum(II) and Tungsten(II) Compounds. Inorg. Chim. Acta 2010, 363, 1601–1603. https://doi.org/10.1016/j.ica.2010.01.022. 268. Pammer, F.; Sun, Y.; Thiel, W. R. Group VI Allyl Complexes of Dibenzo[c,g]fluorenide. Organometallics 2008, 27, 1015–1018. https://doi.org/10.1021/om701022q. 269. Fischer, P. J.; Weberg, A. B.; Bohrmann, T. D.; Xu, H.; Young, V. G. Group VI Metal Complexes of tris(diphenylphosphinomethyl)phenylborate: Modulation of Ligand Donation via Coordination of M(CO)3 Units at the Borate Phenyl Substituent. Dalton Trans. 2015, 44, 3737–3744. https://doi.org/10.1039/C4DT03857F. 270. Fischer, P. J.; Senthil, S.; Stephan, J. T.; Swift, M. L.; Storlie, M. D.; Chan, E. T.; Vollmer, M. V.; Young, V. G. Inductive Modulation of tris(phosphinomethyl)phenylborate Donation at Group VI Metals via Borate Phenyl Substituent Modification. Dalton Trans. 2018, 47, 6166–6176. https://doi.org/10.1039/C8DT00703A. 271. Fischer, P. J.; Senthil, S.; Stephan, J. T.; Swift, M. L.; Young, V. G. Seventeen-Electron Chromium(I)tricarbonyltris(phosphine) Complexes Supported by tris(phosphinomethyl) phenylborates. Dalton Trans. 2019, 48, 16705–16712. https://doi.org/10.1039/C9DT03562A. 272. Heinekey Egbert, J. D.; Heinekey, D. M. Dihydrogen Complexes of the Chromium Group: Synthesis and Characterization of (Arene)M(CO)2(H2) Complexes. Organometallics 2010, 29, 3387–3391. https://doi.org/10.1021/om100416w. 273. Zaitsev Zaitsev, K. V.; Lam, K.; Tafeenko, V. A.; Korlyukov, A. A.; Poleshchuk, O. K. Aryl Oligogermanes as Ligands for Transition Metal Complexes. Eur. J. Inorg. Chem. 2018, 2018, 4911–4924. https://doi.org/10.1002/ejic.201801095. 274. Chen, C.-H.; Chen, C.-S.; Dai, H.-F.; Yeh, W.-Y. Synthesis of the Phosphino–fullerene PPh2(o-C6H4)(CH2NMeCH)C60 and Its Function as an Z1-P or Z3-P,C2 Ligand. Dalton Trans. 2012, 41, 3030–3037. https://doi.org/10.1039/C1DT11769F. 275. Kreickmann, T.; Arndt, S.; Schrock, R. R.; Müller, P. Imido Alkylidene Bispyrrolyl Complexes of Tungsten. Organometallics 2007, 26, 5702–5711. https://doi.org/10.1021/ om7006985. 276. Musso, J. V.; Benedikter, M. J.; Gebel, P.; Elser, I.; Frey, W.; Buchmeiser, M. R. Synthesis of Tungsten(VI) Imido Alkylidene Bispyrrolide Complexes via the Isocyanate Route. Organometallics 2020, 39, 3072–3076. https://doi.org/10.1021/acs.organomet.0c00435.

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277. Fitzgerald, E. C.; Grime, R. W.; Knight, H. C.; Helliwell, M.; Raftery, J.; Whiteley, M. W. Synthesis of the Sterically Crowded Cycloheptatrienyl Complexes [M(CO)(PPh3)2 (Z-C7H7)]+ (M ¼Mo or W): X-Ray Crystal Structures of [W(CO)(PPh3)2(Z-C7H7)][BF4] and [W(CO)2(PPh3)(Z-C7H7)][BF4]
CH2Cl2. J. Organomet. Chem. 2006, 691, 1879–1886. https://doi.org/10.1016/j.jorganchem.2006.01.013. 278. Lancashire, H. N.; Brown, N. J.; Carthy, L.; Collison, D.; Fitzgerald, E. C.; Edge, R.; Helliwell, M.; Holden, M.; Low, P. J.; McDouall, J. J. W.; Whiteley, M. W. Synthesis, Spectroscopy and Electronic Structure of the Vinylidene and Alkynyl Complexes [W(CCHR)(dppe)(Z-C7H7)]+ and [W(CCR)(dppe)(Z-C7H7)]n+ (n ¼ 0 or 1). Dalton Trans. 2011, 40, 1267–1278. https://doi.org/10.1039/C0DT01150A.

5.07

Cyclic and Non-Cyclic Pi Complexes of Manganese

Jeffrey S Price and David JH Emslie, Department of Chemistry and Chemical Biology, McMaster University, Hamilton, ONT, Canada © 2022 Elsevier Ltd. All rights reserved.

5.07.1 5.07.1.1 5.07.1.2 5.07.2 5.07.2.1 5.07.2.1.1 5.07.2.1.2 5.07.2.1.3 5.07.2.1.4 5.07.2.2 5.07.2.2.1 5.07.2.2.2 5.07.2.2.3 5.07.2.3 5.07.2.4 5.07.2.4.1 5.07.2.4.2 5.07.2.4.3 5.07.2.4.4 5.07.2.4.5 5.07.2.4.6 5.07.2.5 5.07.2.5.1 5.07.2.5.2 5.07.2.5.3 5.07.2.6 5.07.2.6.1 5.07.2.6.2 5.07.2.6.3 5.07.2.6.4 5.07.2.7 5.07.2.7.1 5.07.2.7.2 5.07.2.8 5.07.3 5.07.3.1 5.07.3.1.1 5.07.3.1.2 5.07.3.1.3 5.07.3.1.4 5.07.3.1.5 5.07.3.1.6

5.07.3.1.7 5.07.3.1.8 5.07.3.1.9 5.07.3.1.10 5.07.3.2 5.07.3.2.1 5.07.3.2.2 5.07.3.2.3

378

Introduction and organization Introduction Coverage and organization Acyclic p ligands Alkene complexes Mn(I) alkene complexes Mn(0) alkene complexes Reactions of previously reported alkene complexes Computational reports regarding alkene complexes Cumulene and ketene complexes Neutral cumulene complexes Cationic allene complexes Ketene complexes Alkyne complexes Complexes featuring
2-coordinated heteroatom-containing p Ligands
2-Silene complexes
2-Imine complexes
2-Methylenephosphonium complexes
2-Aldehyde complexes
2-Alkylideneborane complexes Computational reports on heteroatom-containing
2-coordinated p systems Allyl, benzyl, propargyl, and trimethylenemethane complexes Mn(I) allyl and benzyl complexes Mn(II) allyl complexes Propargyl and trimethylenemethane complexes Polyalkene complexes
4-Vinylketene complexes
6-Cycloheptatriene complexes
4-Quinone complexes
4-Butadiene complexes Polyalkenyl complexes
5-Pentadienyl complexes (not including cyclohexadienyl complexes)
5-Cyclohexadienyl complexes (and related species) Complexes containing
n-coordinated (n > 2) heteroatom-containing p ligands Cyclic p ligands Cyclopentadienyl complexes Derivatives of cymantrene (I) synthesis and reactivity of complexes containing the “(C5H5-xMex)Mn(CO)” fragment (nitrosyl-free) Derivatives of cymantrene (II) synthesis and reactivity of complexes containing the “(C5H5-xMex)Mn(NO)” fragment Derivatives of cymantrene (III) [(C5H4R)Mn(CO)3] (R 6¼ H, CyH2y + 1 alkyl) Derivatives of cymantrene (IV) [(C5H5-xRx)Mn(CO)3] (x ¼ 2-5, R 6¼ H or CyH2y + 1 alkyl); not including fused polycyclic cyclopentadienyl derivatives Derivatives of cymantrene (V) [(C5H5-xRx)Mn(CO)3] (C5H5-xRx ¼ fused polycyclic cyclopentadienyl ligand) Derivatives of cymantrene (VI) derivatives of cymantrene with both (a) one or more non-alkyl substituent on the cyclopentadienyl ligand, and (b) one or more non-carbonyl co-ligand; [(C5H5-xRx)Mn(CO)3-nLn] (x ¼ 1–5, n ¼ 1-2, R 6¼ H or CyH2y +1 alkyl, L 6¼ CO) Derivatives of cymantrene (VII) manganese(I) dicarbonyl complexes with a chelating Lewis base-appended cyclopentadienyl ligand Carbonyl-free complexes with one cyclopentadienyl ligand on Mn Complexes with two cyclopentadienyl ligands on Mn Miscellaneous cyclopentadienyl chemistry Arene complexes Synthesis and reactivity of cationic Mn(I)
6-arene complexes; [(
6-arene)Mn(CO)3)]+ Anionic manganese
6-arene complexes Neutral manganese
6-arene complexes

Comprehensive Organometallic Chemistry IV

381 381 381 382 382 382 384 385 386 386 386 387 388 388 390 390 390 391 391 391 392 392 392 395 395 396 396 396 396 398 400 400 402 414 416 416 416 443 445 482 489

494 499 505 512 519 520 520 527 527

https://doi.org/10.1016/B978-0-12-820206-7.00077-9

Cyclic and Non-Cyclic Pi Complexes of Manganese 5.07.3.2.4 Neutral manganese
x (x < 6) arene complexes 5.07.3.2.5 Miscellaneous arene chemistry 5.07.3.3 Complexes with cyclic heteroatom-substituted p ligands 5.07.3.4 Miscellaneous (cyclic) 5.07.4 Concluding remarks Acknowledgments References

Abbreviations Ac acac ALD Ar, Ar0 ArCl Arx bcod bian bipy bmim Bn Boc Bt i Bu n Bu s Bu t Bu CAAC CVD COD Cp Et Cp Me Cp H,Me Cp R Cp Cp Cy Cym DABSO dba DBU DCC DCI DDQ depe DFT DIAD Dipp DMAP dme DMF dmpe DMPU Dmpz DMSO DOSY dppe dppf dpph dppm DSC

Acetyl Acetylacetonate Atomic Layer Deposition Aryl substituent (variable) 3,5-C6H3Cl2 Halide-substituted aryl substituent Bicyclo[3.2.1]octa-2,6-dien-4-yl Bis(imino)acenaphthene Bipyridine 1-Butyl-3-methylimidazolium Benzyl tert-Butoxycarbonyl Benzotriazolyl iso-Butyl n-Butyl sec-Butyl tert-Butyl Cyclic(alkyl)(amino)carbene Chemical Vapour Deposition 1,5-Cyclooctadiene C5H5 C5H4Et C5H4Me Either Cp or MeCp (variable) C5H4R (variable) C5Me5 Cyclohexyl Cymantrenyl ([(C5H4)Mn(CO)3]) 1,4-Diazabicyclo[2.2.2]octane bis(sulfur dioxide) Dibenzylideneacetone 1,8-Diazabicyclo[5.4.0]undec-7-ene N,N0 -dicyclohexylcarbodiimide 4,5-Dicyanoimidazole 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone 1,2-Bis(diethylphosphino)ethane Density Functional Theory Diisopropyl azodicarboxylate 2,6-Diisopropylphenyl 4-(N,N-dimethylamino)pyridine 1,2-Dimethoxyethane Dimethylformamide 1,2-Bis(dimethylphosphino)ethane 1,3-Dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone Dimethylpyrazolyl Dimethyl sulfoxide Diffusion-Ordered NMR Spectroscopy 1,2-Bis(diphenylphosphino)ethane 1,10 -Bis(diphenylphosphino)ferrocene 2,2-Diphenyl-1-picrylhydrazyl 1,1-Bis(diphenylphosphino)methane Differential Scanning Calorimetry

379

528 528 529 533 533 534 534

380

Cyclic and Non-Cyclic Pi Complexes of Manganese

dvtms EHMO ELF Et Fc FTIR HATU HOMO IMe IMes Ind IPr IR ItBu ITol L LDA Ln LUMO M mCPBA Me Me-Phos Mes Mes MLD MMT MOCVD NADH NFSI NHC NMR Nu PGSE Ph pmdta i Pr n Pr PTC PTSA py pyr pz QTAIM RCM SOMO TBAF TBDPS TBME TBS TCNQ TEMP-H TEMPO Tf TFA TGA THF TMEDA TMP TMS Tol

Divinyltetramethyldisiloxane Extended Hückel Molecular Orbital Electron Localization Function Ethyl Ferrocenyl Fourier-transform Infrared 1-[Bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium 3-oxide hexafluorophosphate Highest Occupied Molecular Orbital 1,3-Dimethylimidazol-2-ylidene 1,3-Dimesitylimidazol-2-ylidene Indenyl 1,3-(2,6-Diisopropylphenyl)imidazol-2-ylidene Infrared 1,3-Di-tert-butylimidazol-2-ylidene 1,3-Ditolylimidazol-2-ylidene Ligand (variable) Lithium diisopropylamide Lanthanide (variable) Lowest Unoccupied Molecular Orbital Metal (variable) Meta-chloroperoxybenzoic acid Methyl 2-Dicyclohexylphosphino-20 -methylbiphenyl Mesityl 2,4,6-Tri-tert-butylphenyl Molecular Layer Deposition Methylcyclopentadienyl manganese tricarbonyl ([MeCpMn(CO)3]), alternatively referred to in the literature as tricarbonyl (methylcyclopentadienyl)manganese (TCMn) Metal Organic Chemical Vapour Deposition Nicotinamide adenine dinucleotide N-fluorobenzenesulfonimide N-heterocylic carbene Nuclear Magnetic Resonance Nucleophile Pulsed Gradient Spin-Echo Phenyl N,N,N0 ,N00 ,N00 -pentamethyldiethylenetriamine iso-Propyl n-Propyl Phase-transfer catalyst p-Toluenesulfonic acid Pyridine Pyridinyl Pyrazolyl Quantum Theory of Atoms in Molecules Ring Closing Metathesis Singly Occupied Molecular Orbital Tetrabutylammonium fluoride tert-Butyldiphenylsilyl tert-Butyl methyl ether tert-Butyldimethylsilyl Tetracyanoquinodimethane 2,2,6,6-Tetramethylpiperidine 2,2,6,6-Tetramethyl-piperidin-1-oxyl Triflyl Trifluoroacetic acid Thermogravimetric analysis Tetrahydrofuran N,N,N0 ,N0 -Tetramethyl-1,2-ethylenediamine Tetramethylpiperidide Trimethylsilyl Tolyl

Cyclic and Non-Cyclic Pi Complexes of Manganese

Tp Tp0 Trip TRIR Ts UV VE WBI Xyl

381

Tris(pyrazolyl)borate Tris(3,5-dimethylpyrazolyl)borate 2,4,6-Triisopropylphenyl Time-resolved infrared Tosyl Ultraviolet Valence electron(s) Wiberg bond index Xylyl

5.07.1

Introduction and organization

5.07.1.1

Introduction

Manganese has played a key role in the development of organometallic chemistry, given that it is a relatively low cost metal with high abundance, minimal toxicity, and access to a variety of oxidation states. The evolution of organomanganese chemistry can be traced back to 1954, which saw seminal publications on dimanganese decacarbonyl ([Mn2(CO)10]),1 cymantrene ([CpMn(CO)3]),2 and manganocene ([Cp2Mn]),3 each of which has become a commonly used precursor to access novel manganese-containing species in oxidation states 0, +1, and +2, respectively (Fig. 1). The latter two, which were independently reported by Ernst Fischer and Geoffrey Wilkinson (who would later share the 1973 Nobel Prize in Chemistry “for their pioneering work. . .on the chemistry of organometallic, so called sandwich compounds”), are the first reported p complexes of manganese. The chemistry of organomanganese p complexes is incredibly broad, ranging from extremely robust monovalent species {which are of interest in fields ranging from catalysis to medicine (e.g. anticancer drugs or imaging) and transportation (as additives to gasoline)} to extremely air sensitive divalent species which often display unusual magnetic properties. Mn(II) has been referred to as “the black sheep of the organometallic family” due to the uncommonly varied chemistry and structures found in organomanganese(II) chemistry, which is due in part to a particularly significant ionic contribution to MndC bonds in high spin manganese(II) compounds.4 The reader is referred to previous editions of COMC for historical overviews of organomanganese chemistry. The first edition (1982), volume 4 Ch. 29 (written by P. M. Treichel) provides a general overview of organomanganese chemistry. The large scope of this field is reflected in the second edition (1995), which divides organomanganese chemistry into multiple chapters (volume 6, chapters 1-7), one of which (Ch. 5; P. M. Treichel) is devoted to manganese cyclopentadienyl complexes. The third edition (2007) divides organomanganese chemistry into carbonyl-containing (5.10; D. A. Sweigart, J. A. Reingold, and S. U. Son) and carbonyl/isocyanide-free (5.11; J. B. Sheridan) species.

5.07.1.2

Coverage and organization

Herein, we aim to provide an overview of organomanganese chemistry incorporating cyclic or acyclic p ligands reported in the peer-reviewed literature over the period 2005–2020. This chapter will highlight synthetic routes reported for novel organometallic p complexes, while applications, characterization, and computational analyses will be only briefly mentioned. For the purposes of this chapter, p complexes are considered only when at least one of the metal-coordinated atoms is carbon. Metallacarborane complexes, and associated systems such as metalladicarbollide complexes, are beyond the scope of this chapter. Unless otherwise indicated, ligands with an even number of p electrons (e.g. alkenes, alkynes, and butadienes) will be described using the neutral canonical form. Complexes are provided with identification numbers only if they correspond to species first reported in 2005–2020; complexes without identification numbers include previously reported p complexes for which new reactivity or analysis has recently been published, hypothetical complexes investigated computationally, and non-p complexes. This chapter is organized by the nature of the p-ligand, starting with complexes featuring acyclic p ligands (e.g. alkenes, alkynes, allyls, polyalkenes), and finishing with complexes containing cyclic p ligands (primarily cyclopentadienyl and arene ligands). Where complexes contain multiple different p-ligands, one of which is a cyclopentadienyl anion, synthesis and reactivity is typically discussed in the section on cyclopentadienyl complexes (Section 5.07.3.1), and mentioned only briefly elsewhere. The primary purpose of this chapter is to introduce the reader to the scope of manganese p complexes first reported from 2005 to 2020, with a focus on synthesis and reactivity. Given the large scope of this chapter, it is necessary to omit many details, and for further information the reader should consult individual references.

Fig. 1 Early organomanganese complexes.

382

Cyclic and Non-Cyclic Pi Complexes of Manganese

5.07.2

Acyclic p ligands

5.07.2.1

Alkene complexes

5.07.2.1.1

Mn(I) alkene complexes

Many of the
2-alkene complexes reported in the 2005–2020 period feature both cyclopentadienyl and carbonyl co-ligands, and are of the form [H,MeCpMn(CO)2(
2-alkene)]. These complexes (e.g. 1–3 in Fig. 2) have often been prepared from [H,MeCpMn(CO)3] by photochemical carbonyl substitution reactions with free alkenes (see Scheme 43 for synthetic details). Examples of alkenes installed this way are styrene or methyl methacrylate, and the resulting [CpMn(CO)2(
2-alkene)] complexes (1 and 2) were investigated for catalytic alkene polymerization, although characterization of 1 and 2 was limited.5,6 Also, carbonyl substitution from either cymantrene or its methylcyclopentadienyl analogue permitted isolation of
2-trifluorovinylferrocene complexes 3.7

Fig. 2 Examples of [H,MeCpMn(CO)2(
2-alkene)] complexes.

Photochemical carbonyl ligand substitution from tricarbonyl manganese(I) pyrrolyl or scorpionate precursors ([(
5dimethylpyrrolyl)Mn(CO)3] or [TpMn(CO)3], respectively) by a free alkene was used to prepare [LMn(CO)2(
2-cyclooctene)] complexes 4 and 5 (Scheme 1), and the mechanism of alkene substitution (by 2-picoline or THF, respectively) in these complexes was investigated.8,9 Irradiation of cymantrene in the presence of dihydrofuran was also investigated (Scheme 1), and initially yielded a mixture of an
1-oxygen bound complex and the alkene isomer (6). However, the former complex rapidly converted to thermodynamically favored 6, and this process was studied by step-scan FTIR spectroscopy and DFT calculations.10 In addition, irradiation of cymantrene in wet hexane (Scheme 1) yielded a variety of short-lived decomposition products; monometallic (7) and dimetallic (8)
2-cyclopentadiene complexes were proposed on the basis of FTIR spectroscopy.11

Scheme 1

Cyclic and Non-Cyclic Pi Complexes of Manganese

383

A series of alkene dicarbonyl complexes (Fig. 3) have also been reported where the alkene ligand is tethered to a cyclopentadienyl anion; 9,12 10,12 11,12,13 12,14 13,15 14,12 15a,16 15b,15 16,12 and 1717 (see Section 5.07.3.1.7 for more information on the synthesis and isomerization of these species). Some of these complexes were characterized by X-ray crystallography, and the C]C distances range from 1.395(4)-1.43(4) A˚ , while the MndC distances range from 2.139(3) to 2.171(1) A˚ . Stemming from a vinylidene precursor, a series of Cp/dicarbonyl
2-phosphonioalkene cations [CpMn(CO)2{
2-E-H(PR3)C] C(H)Ph}]+ (18)18 and neutral
2-E-phosphorylalkene complexes [CpMn(CO)2(
2-Ph{H}C]C{H}{PR2(]E)})] (E ¼ O or S) (19)19,20 have been prepared; Fig. 4 (see Schemes 58 and 67, respectively, for synthetic details). 13C NMR spectra could not be obtained for 18 due to trace paramagnetic impurities which limited full spectroscopic characterization,18 and 13C NMR data was only obtained for a single derivative of 19 (where E ¼ O and R ¼ OEt).19,20 Unfortunately, an X-ray crystal structure was not obtained for any derivative of 18 or 19. A tricarbonyl Mn(I) complex with a cyclic ligand containing
3-allyl and
2-alkene donors (20; Fig. 4) was also formed as one of several products from the reaction of [(
5-2,4-dimethyl-2,4-pentadienyl)Mn(CO)3] with acetylene (see Scheme 10 for synthetic details).21

Fig. 3 Manganese alkene dicarbonyl complexes where the alkene is tethered to a cyclopentadienyl anion.

Fig. 4 Manganese carbonyl complexes with
2-phosphonioalkene,
2-E-phosphorylalkene, or
2:
3-1,3-dimethylbicyclo[3.3.1]nona-3,6-dien-2-yl co-ligands.

A Mn(I) tetracarbonyl complex featuring a cyclometallated 6-methyl-4-(2-pyridinyl)-2H-pyran-2-one ligand was shown to undergo photochemically-induced insertion of phenylacetylene, and the product (21) was determined by DFT calculations to feature a bond between Mn and the pyrone olefinic bond; Scheme 2.22 This reactivity is related to a key step in [BrMn(CO)5]-catalyzed CdH alkenylation of 2-arylpyridine derivatives with alkynes in the presence of an amine.23–25 In the presence of excess phenylacetylene, the cyclometallated ligand in 21 was removed from the metal center by protonation. Furthermore, 21 could only be observed at low temperature; upon warming to room temperature, 21 underwent reductive elimination of the pyridine and alkenyl donors to yield a complex in which manganese is
4-coordinated to the butadiene unit within a pyridinium ring (22).22

384

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 2

5.07.2.1.2

Mn(0) alkene complexes

Though most reported alkene complexes feature Mn(I) centers (Section 5.07.2.1.1), a handful of zero-valent Mn alkene complexes have also been reported (Scheme 3). One of these (23) is a heterobimetallic complex where a vinyl ligand on an adjacent Pt atom is
2-coordinated to manganese. Compound 23 was prepared by deprotonation of a benzyl substituent on a carbene ligand attached to platinum (by NEt3 or the pentacarbonyl manganate ion).26 Monometallic NHC-supported Mn(0) alkene complexes 24 and 25

Scheme 3

Cyclic and Non-Cyclic Pi Complexes of Manganese

385

were prepared by reduction of MnCl2 in the presence of free alkenes and an NHC. Compound 24 features two monodentate
2-styrene ligands,27 whereas 25 features a single divinyltetramethyldisiloxane (dvtms) ligand.28 The MndCalkene and CdC distances in 24 and 25 range from 2.081(3) to 2.132(1) A˚ and 1.382(8) to 1.456(4) A˚ , respectively. The reactivity of 25 with alkene, allene, or alkyne reagents was explored, which yielded Mn(II) dialkyl complexes via oxidative coupling followed by 1,2-insertion. In addition, 25 reacted with H2 or water to afford either a dialkyl manganese(II) complex or a free disiloxane {Me2EtSiOSiMe2(C2H3)}, while the alkene was displaced from the metal center upon exposure to I2 or CO.28

5.07.2.1.3

Reactions of previously reported alkene complexes

One of the earliest known manganese alkene complexes, trans-[(dmpe)2MnH(
2-ethylene)] (an unusual example of a 1st row transition metal complex containing ethylene and hydride co-ligands), was first reported in 1983.29 Despite being known for decades, its reactivity remained largely unexplored. However, it was recently demonstrated that this complex can be a useful precursor to new manganese complexes (Scheme 4), with reactivity proceeding via either (i) ethylene substitution by a Lewis base, or (ii) initial isomerization to a Mn(I) ethyl intermediate. Via the ethylene substitution route, new pentaphosphino hydride and k2-BH4 borohydride complexes were prepared via reactions with free phosphines or BH3(NMe3), respectively. Reactions with aryl or alkyl hydroboranes yielded isostructural k2-borohydride complexes, though this reactivity was demonstrated to proceed via initial isomerization of the ethylene hydride precursor to [(dmpe)2MnEt].30 Additionally, reactions of [(dmpe)2MnH(
2-ethylene)] with H2 to afford the previously reported dihydrogen hydride complex [(dmpe)2MnH(H2)] or isonitriles (yielding a “trapped” 6-coordinate ethyl species) also proceeded with initial isomerization to [(dmpe)2MnEt].31 However, perhaps the richest chemistry of [(dmpe)2MnH(
2-ethylene)] was derived from reactions with hydrosilanes, which also proceeded via [(dmpe)2MnEt]. Exposure to primary hydrosilanes yielded disilyl hydride complexes [(dmpe)2MnH(SiH2R)2] (R ¼ Ph, nBu), which feature significant SidH interligand interactions,31 while reactions with secondary hydrosilanes afforded silylene hydride complexes [(dmpe)2MnH(]SiR2)] (R ¼ Et, Ph); the first group 7 complexes with sterically and electronically unstabilized silylene ligands (the phenyl derivative is also the first example of a silylene hydride complex which exists in solution as an equilibrium between two isomers with- and without- an interligand SidH interaction).32 In addition to being interesting in their own right, the disilyl hydride and silylene hydride complexes prepared from [(dmpe)2MnH(
2-ethylene)] could be used to obtain formate,33 amidinylsilyl,33 silyl dihydride,32,34 and silene32,35 complexes of manganese (see Section 5.07.2.4.1 for a more detailed discussion of the syntheses and structures of the silene complexes).

Scheme 4

386

Cyclic and Non-Cyclic Pi Complexes of Manganese

5.07.2.1.4

Computational reports regarding alkene complexes

Hypothetical alkene complexes have been the subject of multiple computational reports. This includes Mn(V) ethylene complexes,36 a variety of tetracarbonyl alkene anions [Mn(CO)4(
2-alkene)]−,37 an inverse sandwich-type complex [{(b-diketiminate)Mn}2(m-ethylene)],38 and Mn(0) bis-1,5-cyclooctadiene species.39 The bonding between cyclooctene and a CpMn(CO)2 fragment has been probed computationally, and compared to similar complexes involving other transition metals.40 Structures of alkene complexes were also computed as part of a combined theoretical/experimental study on the reactions of ethane or chloroethane with laser-ablated Mn atoms.41 Calculations have also been conducted on the electronic structure of hypothetical [Mn(bcod)2] (bcod ¼ bicyclo[3.2.1]octa-2,6-dien-4-yl), where the cyclic ligand features both
3-allylic and
2-alkene ligands.42 Alkene complexes have also been proposed (on the basis of DFT calculations) as intermediates in the catalytic CdH alkenylation of 2-arylpyridine derivatives with terminal alkynes in the presence of [BrMn(CO)5] and a base.23,24 Additionally, transient alkene complexes were proposed to be formed (on the basis of DFT calculations and time-resolved IR spectroscopy) upon irradiation of [(2-phenylpyridinyl)Mn(CO)4] in the presence of H2C]CHCOn2Bu.25 Furthermore, calculations on a previously reported (though not structurally characterized) dimetallic ethylene dithiolate Mn carbonyl complex, and various analogues, indicated the presence of an
2 alkene interaction.43 Lastly, DFT calculations have recently been conducted on the various possible isomers of previously reported trans-[(dmpe)2MnH(
2-ethylene)], which demonstrated that the observed ethylene hydride structure is lower in energy than the ethyl isomer (with a b-agostic interaction) due to the steric environment enforced by the chelating dmpe ligands, and maintenance of this structure (previously only verified in solution) in the solid state was demonstrated crystallographically.31

5.07.2.2

Cumulene and ketene complexes

Manganese cumulene (e.g. allene; R2C]C]CR2) and ketene (O]C]CR2) complexes are structurally related to alkene complexes, and in all examples in this section the ligands bind to the metal in an
2-fashion involving a single C]C bond. In cumulene complexes, this results in elongation of the bound C]C bond relative to the unbound one(s). All allene complexes of manganese reported in the period 2005-2020 feature a monovalent metal center, along with cyclopentadienyl and carbonyl co-ligands. Neutral and cationic ketene complexes of manganese(I) in which the “ketene” moiety is part of a multi-dentate system containing a phosphine donor have also been reported. Complexes with
4-vinylketene ligands are discussed in Section 5.07.2.6.

5.07.2.2.1

Neutral cumulene complexes

Photoinduced carbonyl substitution reactions between [CpMn(CO)3] and a free allene {H2C]C]C(H)dCH2OH} provided access to neutral [CpMn(CO)2{
2-H2C]C]C(H)dCH2OH}] (26); Scheme 5. Three isomers of 26 were observed in solution (by 1 H NMR and IR spectroscopy): a dominant isomer with the metal bound to the internal C]C bond, and two minor isomers with the metal bound to the terminal C]C bond.44 Furthermore, various MeCp analogues, ([MeCpMn(CO)2(
2-allene)]; 27, 28, and 29), have been reported as intermediates in the catalytic conversion of alkynes to allenes (see Scheme 57 for details), some of which were isolated and/or spectroscopically characterized.45,46

Scheme 5

Additional [MeCpMn(CO)2(
2-allene)] complexes (Fig. 5) have been prepared from carbene precursors as described in Scheme 66. In these complexes, the metal center is bound to an internal ArRC]C bond where the R substituent is an H (30a, 31, 32a, 33a-b),47 SR0 (30b),47 or PR0 2 (32b)48 group. The 13C NMR chemical shifts arising from the terminal metal-bound sp2 carbon atoms (27.4–39.1 ppm) are far lower in frequency than those from the internal allene sp2 carbons (164.4-189.2 ppm) or the un-bound sp2 carbons (123.3–134.0 ppm). For 32a, both syn and anti stereoisomers were observed (and individually crystallography characterized).

Cyclic and Non-Cyclic Pi Complexes of Manganese

387

Fig. 5 Neutral [MeCpMn(CO)2(
2-allene)] complexes synthesized from carbene-containing precursors.

Cross coupling of alkynyl ligands in a Pt(II) precursor upon exposure to [Mn2(CO)9(CH3CN)] yielded a trimetallic Pt/Mn/Mn species (34) where a 1,4-butatriendiyl ligand bridges between the three metal atoms (Scheme 6). MndC distances to the two terminal 1,4-butatriendiyl carbon atoms (2.00(3)–2.06(2) A˚ ) are consistent with s bonds, and those involving the internal 1,4-butatriendiyl carbons range from 2.20(2) to 2.28(2) A˚ , which is consistent with an interaction with a p ligand. The three 1,4-butatriendiyl C]C distances of 1.37(3)–1.45(3) A˚ are statistically indistinguishable due to large standard deviations, and are all consistent with multiple bonding character.49

Scheme 6

5.07.2.2.2

Cationic allene complexes

The neutral allene species 26 has been used to prepare the cationic allene complex [CpMn(CO)2{
2-H2C]C]C(H)CH2PPh3}]+ (35; Fig. 6), though characterization was limited (the synthesis of 35 is described in Scheme 58).44 In addition, a series of cationic or

Fig. 6 Cationic [RCpMn(CO)2(
2-allene)]+ complexes.

388

Cyclic and Non-Cyclic Pi Complexes of Manganese

dicationic
2-phosphonioallene complexes [RCpMn(CO)2{
2-H(R3P)C]C]CR0 R00 }]+ or [{CpMn(CO)2}2{m-
2:
2-Ph2C]C] CH(Ph2PCH2CH2PPh2)HC]C]CPh2}]2+ (Fig. 6: 36a-b, 37, and 38; RCp ¼ Cp or Cp ) have been prepared by protonation of a-phosphonioallenyl complexes50 or coordination of PPh3 to a cationic propargyl complex (see Scheme 58 for synthetic details).51 For derivatives where 13C NMR data was reported, the chemical shifts for the three allene sp2 carbon atoms (coordinated terminal: 1.0–11.8 ppm, internal: 120.8–145.0 ppm, non-coordinated terminal: 94.3–141.4 ppm) are lower in frequency than the analogous values for neutral allene species (see Section 5.07.2.2.1). Structural parameters in the X-ray crystal structures are consistent with previously reported
2-allene complexes of manganese.

5.07.2.2.3

Ketene complexes

A series of neutral Mn(I) cyclopentadienyl-carbonyl complexes [CpMn(CO){
3-P,C,C-R0 2PC(R)]C]O}] (39; Fig. 7) have been reported, where the ketene moiety and an adjacent phosphine donor are
3-P,C,C-coordinated (see Schemes 63 and 64 for synthesis).52,53 These complexes spontaneously isomerized to form other species (see Scheme 64 for details), though many derivatives were sufficiently stable for storage over months at low temperature. The C]C bond in crystallographically characterized examples (1.456(3)–1.473(6) A˚ ) is significantly elongated relative to a double CdC bond, and the C]C]O angles of 135.0 (4)–135.9(3) deviate substantially from linearity. For examples where 13C NMR signals were located for the sp2 carbon atoms, the atom adjacent to oxygen yields an extremely high frequency signal (238.6–245.2 ppm) and the carbon adjacent to phosphorus yields a very low frequency signal (−34.6 to −24.7 ppm).52,53 Reactions of 39 are covered in Schemes 64 and 65. In addition, a computational analysis of bonding and isomerization processes for the previously reported ketene complex [MeCpMn(CO)2(
2-O]C]CPh{(m-
2:
2-C^CPh)Co2(CO)6})] was recently published.54 A pair of cationic phosphonioketene complexes [CpMn(CO){
2-C,C-
1-P-Ph2PCH2PPh2C(R)]C]O}]+ (40; Fig. 7) have also been reported (synthesis is described in Scheme 63), where the ketene moiety and a phosphine donor form a chelating ligand system. The structural data for the ketene fragment in the crystallographically characterized benzyl derivative {R ¼ Bn; d(C]C) ¼ 1.453(2) A˚ , angle(C]C]O) ¼ 139.66(18) } is similar to that in neutral 39, as are the 13C NMR chemical shifts (of all reported derivatives of 40) for the sp2 carbon adjacent to oxygen (245.9–246.6 ppm). However, the 13C NMR chemical shifts for the other sp2 carbon atom in 40 (−17.9 and −9.7 ppm) are significant higher in frequency relative to those in 39.55

5.07.2.3

Alkyne complexes

Given the ubiquitous nature of complexes of the type [RCpMn(CO)2L] (see Section 5.07.3.1.1 for an overview of this class of complex), it is unsurprising that examples of such species have been reported where L is an
2-coordinated alkyne (Fig. 8). [CpMn(CO)2(
2-HC^CPh)] (41) and [RCpMn(CO)2(HC^CCR0 2OH)] (42; RCp ¼ Cp, Cp and R0 ¼ H, Me) have been prepared by photochemically-induced carbonyl substitution from cymantrene (see Scheme 43 for synthesis)19,51 or [Cp Mn(CO)3].51 Compound 41 slowly isomerized to the known vinylidene complex [CpMn(CO)2(]C]CHPh)], and also reacted with (C6F5)2POH to afford (C6F5)2P(O)CH2CHPhP(O)(C6F5)2 (presumably via similar reactivity to that illustrated in Scheme 67, which describes chemistry stemming from [CpMn(CO)2(]C]CHPh)] and secondary phosphine oxides).19 Derivatives of 42 have been used as precursors to access propargyl complexes, as described in Scheme 58.51 In addition, high resolution X-ray diffraction

Fig. 7 Manganese cyclopentadienyl carbonyl
2-ketene complexes.

Fig. 8 Manganese cyclopentadienyl carbonyl complexes with an alkyne co-ligand.

Cyclic and Non-Cyclic Pi Complexes of Manganese

389

has been utilized in an analysis of the topology of the electron density of previously reported [Cp(CO)2Mn(
2-PhC^CPh)] to probe the nature of the p-interaction.56 Furthermore, various analogues of [MeCpMn(CO)2{
2-RR0 HCdC^CdC(O)R00 }] (43) have been reported (and often observed spectroscopically) as intermediates in [MeCpMn(CO)3]-mediated isomerization of alkynes to allenes (see Scheme 57 for more details on this isomerization). In addition, a complex with two MeCpMn(CO)2 fragments bridged by a bis(alkyne) ligand (44) was observed as a minor byproduct in the synthesis of alkynyl carbene complexes (see Scheme 63 for synthetic details).48 An alkyne complex (45) has also been prepared where the alkyne moiety is tethered to a cyclopentadienyl moiety (see Scheme 154 for synthesis).17 Furthermore, alkyne complexes were proposed as intermediates in the syntheses of several vinylidene complexes described in Scheme 157.57,58 A series of anionic Mn(I) alkyne complexes has been prepared by exposure of bis(amido) manganese anions to free alkynes (Scheme 7). The reactions of [Mn{N(Dipp)(SiMe3)}2]− with diphenylacetylene or 3-hexyne yielded 3-coordinate products of alkyne coordination; [(
2-RC^CR)Mn{N(Dipp)(SiMe3)}2]− (46; R ¼ Ph, Et). Exposure of the same precursor to bis(trimethylsilyl)acetylene resulted in a solution colour change which suggested formation of a similar complex, but crystallization exclusively yielded the alkyne-free precursor, suggesting that this reaction is reversible. By contrast, reactions of alkynes with [Mn {N(SiMe3)2}2]− yielded complex mixtures of products. For example, the reaction with diphenylacetylene yielded an anion isostructural to 46 (compound 47), an anion with
2-alkyne and
6-arene ligands ([(
2-PhC^CPh)Mn(
6-C6Ph6)]; 48), homoleptic [Mn{N(SiMe3)2}3]−, and a dimetallic dianion where the metal atoms are bridged by an alkenediyl moiety. Alternatively, [Mn {N(SiMe3)2}2]− reacted with bis(trimethylsilyl)acetylene to yield a mixture which includes 49 (a dimetallic anion with a single bridging amido ligand, one terminal amido anion per Mn center, and a syn-m-
2:
2-coordinated alkyne). The MndMn distance of 2.6699(4) A˚ in 49 suggests some metal-metal bonding. However, 49 could not be isolated on a preparative scale. Unlike many of the Mn(I) complexes covered in this chapter, 46, 47, and 49 are paramagnetic, though the magnetic susceptibilities are lower than expected for high-spin d6 complexes, indicative of a more complicated electronic situation. MndC distances for 46 and 47 ranged from 2.022(5) to 2.081(2) A˚ ; slightly shorter than to the bridging alkyne in 49 (2.081(2)-2.138(2) A˚ ), and significantly longer than in 48 (1.893(4)-1.905(5) A˚ ). C^C distances in these complexes range from relatively long in arene-bound 48 (1.346(7) A˚ ) and bridging 49 (1.353(3) A˚ ), to relatively short in 46 and 47 (1.271(4)–1.305(7) A˚ ).59

Scheme 7

The nature of the
2-alkynedMn bond has been probed computationally in hypothetical model complexes, including zero-valent acetylene complexes with porphyrin,60 H2,61 or no62 co-ligands, as well as carbonyl complexes of difluoroacetylene.63 Additionally, DFT calculations have been conducted on an acetylene-coordinated Mn atom, in the context of the formation of ethynyl manganese hydride species from the reaction of acetylene with laser-ablated Mn atoms.64 Furthermore, manganese(I) alkyne complexes have been proposed as intermediates in the catalytic cycle for aromatic CdH alkenylation with terminal alkynes (supported by DFT calculations and time-resolved IR spectroscopy),23–25 in the synthesis of 21 (described in Scheme 2),22 and in catalytic vinylation of arenes (with computational and experimental corroboration).24 DFT calculations have also been conducted on [MnC2]n (n ¼ 0, −1), some isomers of which can be considered to contain
2-alkynediyl ligands; the anionic derivative was detected by photoelectron spectroscopy.65 Calculations have also been conducted on MnC2O2 (formed from exposure of [MnC2] to O2), which also features
2-CC coordination to Mn.66

390

Cyclic and Non-Cyclic Pi Complexes of Manganese

5.07.2.4 5.07.2.4.1

Complexes featuring
2-coordinated heteroatom-containing p Ligands h2-Silene complexes

Silenes (R2Si]CR2) are far more unstable than alkenes, and are normally only isolable with very bulky substituents. However, simple silenes have been stabilized by metal coordination. Reactions of silylene hydride complexes [(dmpe)2MnH(]SiR2)] (R ¼ Et, Ph) with ethylene were found to yield room-temperature stable Mn(I)
2-silene complexes ([(dmpe)2MnH(
2-R2Si]CHMe)]; 50), which are the first examples of isolable or crystallographically characterized first row transition metal complexes bearing a sterically and electronically unstabilized silene ligand (Scheme 8). The diphenyl derivative (which was crystallographically charactered) features a short C]Si bond (1.781(5) A˚ ), and MndC and MndSi distances of 2.270(4) and 2.409(2) A˚ , respectively.32 Similar complexes with an SiH substituent ([(dmpe)2MnH(
2-RHSi]CHMe)]; 51) have been prepared via the reactions of disilyl hydride complexes [(dmpe)2MnH(SiH2R)2] (R ¼ nBu, Ph) with ethylene (Scheme 8). These are the first silene complexes with an SiH substituent to be spectroscopically characterized, and they reacted with a second equivalent of ethylene to form new silene complexes [(dmpe)2MnH(
2-REtSi]CHMe)] (52) via apparent ethylene insertion into the SidH bond. Compounds 52 then reacted with a third equivalent of ethylene to form previously reported [(dmpe)2MnH(
2-C2H4)]. In the absence of excess ethylene, 51 was found to slowly isomerize in solution to the silylene hydride isomer.35 These silene complexes (50, 51, and 52) all display significant interligand SidH interactions (demonstrated by NMR spectroscopy and DFT calculations), and have shown activity towards catalytic hydrosilylation of ethylene. 13C NMR chemical shifts of the sp2 carbon (−19.3 to −22.9 ppm) and 29Si chemical shifts of −17.4 to 0.7 ppm are consistent with a silene bonding description, as are Si]CHMe 1JC,H coupling constants of 136–139 Hz. Both the silylene hydride to silene hydride and the silene hydride to silylene hydride transformations are unprecedented.

Scheme 8

5.07.2.4.2

h2-Imine complexes

Imine ligands are ubiquitous in transition metal chemistry, and often bind to metals in an
1-fashion via the N donor. However, imines can also engage in
2-coordination. A manganese complex (53) where imine ligands bridge between two metal centers, and are
1-N-coordinated to one metal and
2-C,N-coordinated to the other, was synthesized by reduction of a Mn(II) dichloride complex containing a bis(imine)-pyridine pincer ligand (Scheme 9). 53 contains Mn centers which are formally zero-valent, though magnetic susceptibility and EPR analysis indicated the presence of intermediate spin Mn(I) centers which are antiferromagnetically coupled to singly reduced diimine ligands. In addition, an elongated C]N distance of 1.395(6) A˚ for the bridging imine groups (c.f. 1.339(7) A˚ for the C]N bonds in 53 which are only coordinated in an
1-fashion) is indicative of significant p-backdonation. Complex 53 proved to be active towards hydrosilylation of aldehydes.67

Cyclic and Non-Cyclic Pi Complexes of Manganese

391

Scheme 9

5.07.2.4.3

h2-Methylenephosphonium complexes

A pair of unusual air-stable cationic
2-methylenephosphonium complexes [Cp(CO)2Mn{
2-R0 2P]C(H)Ph}]+ (R0 ¼ Ph, Cy; 54) have been isolated (Fig. 9), and display unprecedented (for these types of ligands) reactivity with nucleophiles (synthesis and reactivity are described in Scheme 65).68 A crystal structure was obtained for one derivative (where R0 ¼ Ph), and the C]P bond length of 1.735(2) A˚ is intermediate between that which would be expected for a single or a double bond. Surprisingly, the MndP distance of 2.2405(7) A˚ is not elongated relative to CpMn(CO)2L complexes with PR3 ligands, and the MndCsp2 distance of 2.257 (2) A˚ is much longer than that commonly observed for
2 alkene complexes. Combined with reactivity data, this suggests that the structure of 54 is dominated by a resonance structure with the cationic site at the sp2 carbon atom, in contrast to un-coordinated methylenephosphonium salts.68

5.07.2.4.4

h2-Aldehyde complexes

To our knowledge, no
2-ketone or aldehyde complexes of manganese were isolated in the period 2005-2020. However, one example, [MeCpMn(CO)2(
2-O]CHPh)], was proposed as a transient intermediate formed initially upon reaction of [MeCpMn(CO)2(^CPh)][BCl4] with water, followed by reaction/decomposition to yield the observed products, which include [MeCpMn(CO)3] and benzaldehyde.48 Complexes with
2-aldehyde ligands have also been proposed as intermediates in carbonyl or carboxylate hydrosilylation.67

5.07.2.4.5

h2-Alkylideneborane complexes

A series of
2-alkylideneborane complexes [CpMn(CO)2{
2-(L)(tBu)BC(]NR)}] {L ¼ CNtBu (55) or IMe (56); Fig. 10} were prepared from base-free and NHC-stabilized borylene complexes (see Scheme 75 for details on synthesis and reactivity). Notably, the NHC stabilized example 56 exists in solution as an equilibrium mixture with an alternative isomer {[CpMn(CO)2{CN(R)B (tBu)(IMe)}] (322); discussed in Scheme 75}, and only one of the prepared derivatives crystallized as the
2-alkylideneborane isomer.69 All of these complexes were characterized spectroscopically, but 13C NMR signals were not located for the BC]N carbon atoms. In the solid state structures of both 55 and 56 (R ¼ Cy), significant B]C double bond character is apparent from the short B–C distances of 1.516(4)–1.521(3) A˚ (c.f. 1.622(4)–1.638(3) A˚ to the tBu substituent). Furthermore, short Ccarbonyl–B distances (2.441(4)–2.496(4) A˚ ) in the X-ray crystal structures suggest a significant interligand C–B interaction involving one of the adjacent

Fig. 9 Manganese
2-methylenephosphonium complexes.

Fig. 10 Manganese
2-alkylideneborane complexes.

392

Cyclic and Non-Cyclic Pi Complexes of Manganese

carbonyl ligands.69 An
2-alkylideneborane complex isostructural to 56 was also suggested as an intermediate in the synthesis of (2,4,6-Ct6Bu3)N]C]B(IMe)iBu {from the manganese borylene complex [CpMn(CO)2{]BiBu(IMe)}] (319); Scheme 75}.69

5.07.2.4.6

Computational reports on heteroatom-containing h2-coordinated p systems

Unsymmetrically bridged carbonyl ligands are often
1-coordinated to one metal and
2-coordinated to another (Fig. 11). This has been observed computationally for a variety of dimanganese complexes63,70–73 or manganese-containing heterometallic complexes.74 Computational reports have also described dimanganese complexes containing both CO and CS ligands, including examples with a m-
1:
2-CS ligand.71,75 Carbon dioxide is known to bind to transition metals in various ways, including side-on via the C]O bond (Fig. 11). For [CpMn(CO)2(CO2)], the
2-bound isomer was examined by DFT calculations, though determined to be higher in energy than the isomer with end-on
1-O CO2 coordination.76 Calculations have been used to support the formation of transient
2-CO2 complexes in a number of situations, including a b-diketiminate-ligated complex proposed to be an intermediate in a manganese-catalyzed reverse water-gas shift reaction,77 [Mn(CO2)n]− anions (detected by photoelectron spectroscopy),78 and reactions of the [ClMn]− anion with carbon dioxide (products were detected by mass spectrometry).79 Infrared spectra of complexes formed from the reaction of acetonitrile with laser-ablated Mn atoms in excess argon have been obtained, and DFT calculations suggest that one of these species may feature an
2-bound nitrile ligand.80 Calculations on various isomers of [(PCO)2Mn2(CO)6] located energy minima corresponding to structures featuring a bridging phosphaketenyl ligand which binds to one Mn center through the phosphorus atom and to the other via
2-PC coordination, though they were  27 kcal mol−1 higher in energy than an isomer with a m-P2 ligand (by contrast, calculations on monometallic [(PCO)Mn(CO)3] determined that the global minimum featured
3-coordination of the phosphaketenyl ligand to manganese).81 In addition, a manganese complex with an
2-bound (via N]C) isocyanate ligand has been proposed as an intermediate in catalytic isocyanate cyclodimerization.82

Fig. 11 Calculated bonding motifs involving
2-coordination of heteroatom-containing p systems (CO, CS, CO2, RCN, PCO−, RNCO) to manganese.

5.07.2.5

Allyl, benzyl, propargyl, and trimethylenemethane complexes

Allyl and propargyl ligands can be
1- or
3-coordinated, while benzyl complexes can adopt a range of hapticities; only
n (n > 1) complexes are discussed in this section.

5.07.2.5.1

Mn(I) allyl and benzyl complexes

A neutral Mn(I) allyl complex, [(
3-C3H5)Mn(CO)3(PEt3)] (57), has been prepared by allyl transfer from [{(
3-C3H5)Pd(m-Cl)}2] to an anionic [Mn(CO)4(PEt3)]− precursor (Scheme 10; top).83 Other neutral Mn(I) allyl complexes were prepared via reactivity involving 1,2-insertion (Scheme 10; middle and bottom). This includes a pair of a Mn(I) carbonyl complexes (20 and 58) synthesized by the reaction of [(
5-2,4-dimethylpentadienyl)Mn(CO)3] with acetylene, each of which contains an
3-allyl anion and either an
2-alkene (20) or an
4-butadiene (58) moiety.21 In addition, 59 (with a chelating allyl-ketone ligand) was synthesized by successive insertion of three equivalents of acetylene into a MndC bond, followed by intramolecular cyclization and metal-mediated hydride migration.84 In addition to the aforementioned experimental studies, the fluxionality of [(
3-cyclohexenyl)Mn(CO)3] (stabilized by an agostic interaction) was probed computationally.85

Cyclic and Non-Cyclic Pi Complexes of Manganese

393

Scheme 10

An
3-benzyl species (60) has been synthesized by thermolytic coupling of 1,1-diphenyldiazomethane with a monometallated 2,5-diphenyl-1,3,4-oxadiazole ligand (Scheme 11).86 In addition, thermolytic coupling of two equivalents of a diazomethane derivative (PhRCN2) with a dimetallic Mn(I) complex featuring a doubly cyclomanganated diaryl diazine core afforded a series of Mn(I)
3-benzyl complexes (61, 62, 63, 64, 65, and 66); Scheme 11. These reactions resulted in “CRPh” insertion into both Mn— Caryl bonds, in most cases leading to complexes (61, 62, 63, 64, and 65) in which both manganese centers are
3-coordinated to a benzyl fragment, in addition to a nitrogen atom in the diazine core. However, in 66, only one manganese center is
3-coordinated to a benzyl unit, while the other is
5-coordinated to a cyclohexadienyl fragment and is no longer coordinated to nitrogen. In two cases, a mixture of products was observed, resulting from reactivity at only one of the two Mn–C linkages (67 and 68) or decomposition of the target species, possibly during work-up (69, 70, and 71).87 The electrochemistry, and in particular the reduction chemistry, of these compounds has been extensively explored, and in one case (63 when R ¼ tBu), a crystal structure was obtained of the radical anion.88 In addition, the radical anion of a previously reported derivative of 61 (where tBu groups are replaced with Ph groups) has recently been synthesized and characterized.89 In both radical anions, the structural environment around the Mn centers remained qualitatively unchanged upon reduction.

394

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 11

Additionally, a cationic
3-butadienyl complex containing cyclopentadienyl and carbonyl co-ligands (72; Fig. 12) has been prepared from an allene-containing precursor (see Scheme 58 for synthetic details). In 72, the butadienyl anion binds to the metal center via only three carbon atoms, in an
3-allyl fashion. This complex was observed to exist in solution as a mixture of (dominant) endo and (minor) exo isomers, though X-ray crystal structures were not obtained.44

Fig. 12 [CpMn(CO)2(
3-butadienyl)][BF4] (72).

Cyclic and Non-Cyclic Pi Complexes of Manganese

5.07.2.5.2

395

Mn(II) allyl complexes

The first structurally authenticated Mn(II) complex with allyl ligands, Li[Mn{
3-(1,3-SiMe3)2C3H3}{
1-(1,3-SiMe3)2C3H3}2], was reported in 2004 (shortly before the 2005-2020 period covered by this chapter).90 More recently, the potassium analogue of this anion (73) was isolated during an attempt to prepare the neutral bis(allyl) species from the reaction of MnCl2 with two equivalents of the potassium allyl salt in diethyl ether (Scheme 12); this allyl complex co-crystallized with a single equivalent of the potassium allyl reagent. In the solid state, two of the allyl ligands are
1-coordinated to manganese (in addition to being
3-coordinated to potassium), while one allyl ligand is
3-coordinated to manganese. When this reaction was conducted in THF or in the presence of TMEDA, two equivalents of potassium allyl salt were consumed to yield neutral base-coordinated Mn(II) allyl complexes, though both allyl ligands on Mn were
1-coordinated (not shown in Scheme 12).91 The same potassium allyl salt utilized in the synthesis of 73 has also been used to prepare a series of manganese(II) mixed alkyl-allyl complexes upon reaction with a trimetallic manganese alkyl chloride precursor, where the alkyl group is an extremely bulky C(SiMe3)3 ligand (Scheme 12). The complex initially prepared in this reaction (74) was isolated as a LiCl adduct stabilized by three equivalents of THF (bound to Li). Exposure of 74 to PMe3, quinuclidine, or dmap resulted in adduct formation, displacing the “LiCl” ligand and maintaining the
3-coordination mode of the allyl ligand (75). Alternatively, attempted sublimation of 74 afforded a derivative of 75 where L ¼ THF. A base-free mixed allyl/alkyl complex [{(Me3Si)3C}Mn{
3-(1,3-SiMe3)2C3H3}] (76) could be isolated by phosphine abstraction from the PMe3 derivative of 75, and was the first room-temperature-stable example of a mononuclear transition metal complex bearing only alkyl and allyl ligands. Complexes 74, 75, and 76 are the first high spin d5 mixed alkyl/allyl complexes, and in each case the allyl group is
3-coordinated.92

Scheme 12

In addition, divalent manganese allyl complexes have been the subject of computational investigation. This includes DFT calculations on the structure of a bis(allyl) intermediate in the formation of a 1,5,9-cyclododecatriene species from tris(butadiene) manganese via oxidative coupling followed by 1,2-insertion and reductive elimination,93 the electronic structure of hypothetical [Mn(bcod)2] (bcod ¼ bicyclo[3.2.1]octa-2,6-dien-4-yl) where the cyclic ligand features both
3-allylic and
2-alkene fragments,42 and [(
3-C3H5)2Mn] (which was compared to a hypothetical homoleptic Mn(III) triallyl species with
3- and
1-coordinated ligands).94 A computational investigation has also been conducted on the previously reported [Mn{
3-(1,3-SiMe3)2C3H3} {
1-(1,3-SiMe3)2C3H3}2]− anion, along with the silyl-free derivative, to determine the relative energies of high-, intermediate-, and low-spin structures, as well as the nature of the manganese-allyl bonding in each case.95

5.07.2.5.3

Propargyl and trimethylenemethane complexes

In the conversion of neutral
2-alkyne complexes [RCpMn(CO)2(
2-HC^CCR2OH)] (42) to cationic
2-allene complexes [RCpMn(CO)2{
2-R0 2C]C]C(H)(PPh3)}]+ (37 for R0 ¼ H, 38 for R0 ¼ Me), cationic
3-propargyl complexes [RCpMn(CO)2 (
3-HC^CdCR0 2)]+ (RCp ¼ Cp, Cp and R0 ¼ H, Me; 77 in Fig. 13) were proposed as intermediates (see Scheme 58 for details of this reaction). The derivative of 77 where R ¼ Me and R0 ¼ H was isolated and spectroscopically characterized.51

Fig. 13 Manganese
3-propargyl complexes.

396

Cyclic and Non-Cyclic Pi Complexes of Manganese

A Computational report has also been provided on the hypothetical trimethylenemethane complex [Mn{C(CH2)3}2], where the lowest energy structure was found to involve
4-coordination of the p ligands.96

5.07.2.6 5.07.2.6.1

Polyalkene complexes h4-Vinylketene complexes

Reactions of carbene complexes [MeCpMn(CO)2{]CR(CCR’)}] with secondary phosphines yielded a mixture of products, including
4-vinylketene complexes [MeCpMn(CO){
4-(R2P)(Ar)C]C(H)C(Ar0 )]C]O}] (78; Fig. 14); synthetic details can be found in Scheme 66. For derivatives of 78 where Ar 6¼ Ar0 , two regioisomers were observed (which differ from each other by switching of the Ar and Ar0 groups) which could not be separated. For derivatives of 78 which were crystallographically characterized, the C]C bond adjacent to the oxygen atom was significantly longer than the other two carbon-carbon bonds in the
4-coordinated fragment (1.448(2)–1.453(3) A˚ vs. 1.394(4)–1.429(4) A˚ ), and the Mn–C distances increased with distance from the oxygen atom (with bonds to the two internal carbon atoms roughly similar).47,48

Fig. 14 Manganese
4-vinylketene complexes.

5.07.2.6.2

h6-Cycloheptatriene complexes

6

An
-cycloheptatriene Mn(I) complex with a bulky cyclopentadienyl co-ligand (79) was prepared by photochemically-induced carbonyl substitution (Scheme 13). Complex 79 was used to access new manganese complexes with unusual phosphorus-based ligands; see Scheme 157 for details.97 Reactivity stemming from previously reported [H,MeCpMn(
6-cycloheptatriene)] is also covered in Scheme 157.

Scheme 13

5.07.2.6.3

h4-Quinone complexes

Multimetallic complexes containing a tricarbonyl manganese fragment with an
4-ortho-quinone ligand, where the two oxygen atoms chelate to another transition metal center {Cu (80), Pd (81), or Rh (82)}, have been prepared by the reaction of [(
6-catechol)Mn(CO)3]+ with transition metal precursors under conditions in which deprotonation of the hydroxyl moieties occurs (Scheme 14). While the quinone ligand in each of these complexes remains planar in the solid state, the two oxygen-coordinated carbon atoms exhibit considerably longer Mn–C distances than the other four carbon atoms in the ring. These data are most consistent with
4-coordination to Mn, although contributions from
5- or
6-resonance forms were suggested to be pertinent in these structures; indeed, the average n(CO) for these complexes are most consistent with a semiquinone ligand.98

Cyclic and Non-Cyclic Pi Complexes of Manganese

397

Scheme 14

In addition, [(
4-para-benzoquinone)Mn(CO)3]− anions have been used to bridge between metal centers via the oxygen atoms (Scheme 14). For example, the reaction of [(
4-para-benzoquinone)Mn(CO)3]− with Eu(NO3)3 in DMSO yielded a polymeric structure where Eu3+ ions connect a framework formed from [(
4-para-benzoquinone)Mn(CO)3]− fragments (83). When this reaction was performed in the presence of 1,10-phenanthroline, a tetrametallic (Eu2Mn2) complex was formed where two [(
4-para-benzoquinone)Mn(CO)3]− anions bridge between two Eu3+ atoms, which are themselves bound to 1,10-phenanthroline, DMSO, and two nitrate ligands (84).99 In addition, Fe3O4 nanoparticles have been functionalized with coordination polymers based on [(
4-para-benzoquinone)Mn(CO)3]− linkers and Mn2+ or Cd2+ ions (85); these were synthesized by reaction of a surface-modified nanoparticle (86, which features oleic acid and [(
5-semiquinone)Mn(CO)3] surface-bound

398

Cyclic and Non-Cyclic Pi Complexes of Manganese

species; see Scheme 38 for synthesis of 86) with excess [(
5-semiquinone)Mn(CO)3] and a transition metal acetate salt.100 In addition, a Mn-quinonoid nanoparticle core (87) composed of [(
4-para-benzoquinone)Mn(CO)3]− and Ti-containing fragments has been prepared (using [(
6-hydroquinone)Mn(CO)3]+ and Ti(OiPr)4); not shown in Scheme 14. After addition of a Rh-quinonoid shell to 87, the resulting organometallic nanosphere was investigated for utility as a semiheterogeneous catalyst in carbene transfer reactions.101

5.07.2.6.4

h4-Butadiene complexes

Butadiene ligands can coordinate to transition metals as neutral bis(alkene) ligands or as dianionic ligands (forming a metallacyclopentene). Calculations have been conducted on various
4-butadiene complexes of manganese to probe the relative energies of different energy minima, including homoleptic bis(butadiene)102 and tris(butadiene)93 complexes. Calculations have also been conducted on dimanganese carbonyl complexes with bridging hexafluorocyclopentadiene ligands (though energy minima were only located when the butadiene ligand was
1–3-coordinated).103 In addition, a mixture of products was formed from the reaction of [(
5-2,4-dimethyl-2,4-pentadienyl)Mn(CO)3] with acetylene (see Scheme 10), including a tricarbonyl Mn(I) complex with a ligand which features
3-allyl and
4-butadiene moieties (58).21 A series of Mn(I) tricarbonyl complexes have been reported with
4-phosphoryl ligands composed of a P atom (which does not interact with the metal center) in a cycle with an
4-coordinated butadiene fragment (Scheme 15). Anionic analogues 88 and 89 were prepared by the reactions of 2,5-diphenylphosphacymantrene with a base (e.g. solid potassium hydroxide, the combination of an amine with water, or the combination of diethylamine with tert-butyl hydroperoxide); these complexes could be crystallized, sometimes with the assistance of crown ethers.104–107 An experimental charge density distribution study and QTAIM analysis of the [NH2Et2]+ derivative of 88 was later conducted.108 Alkoxypalladation of the same phosphacymantrene precursor with sodium tetrachloropalladate in the presence of sodium acetate yielded a family of dimeric Mn(I) complexes with a phosphole ligand which is
4-coordinated to Mn, with a Pd center bridging between manganese and the phosphorus donor (90). Structurally characterized examples of 90 feature a nearly planar 5-membered C4P ring (similar to that in the
5-phospholyl precursor).109–111 Compound 90 (where R ¼ CHi2Bu) reacted with PPh3 to yield monometallic 91, demonstrating that the dimer can be disrupted using a Lewis base.111

Scheme 15

Reduction of a Mn(I) bis-isonitrile complex with two equivalents of KC8 yielded an anionic manganese dicarbonyl mono-isonitrile complex with a dianionic
4-azabenz[b]azulene ligand (92; Scheme 16). The
4-hapticity in 92 is supported by the X-ray crystal structure, which shows that the p ligand is not planar; four carbon atoms of the C7 ring interact with Mn, and the other three atoms are bent out of the plane. Compound 92 was proposed to form from a reduced (Mn−1) intermediate by spontaneous aza-Büchner ring expansion of a flanking 2,6-diisopropylphenyl substituent on an isocyanide ligand. Notably, 92 forms a one-dimensional coordination polymer with the potassium counterion in the solid state.112

Cyclic and Non-Cyclic Pi Complexes of Manganese

399

Scheme 16

A neutral manganese tricarbonyl complex with an
4-NC5R6 ligand (93; the authors propose that the butadiene moiety in 93 is best considered to have been reduced to afford a but-2-ene-1,4-diyl ligand) has been prepared via reactivity proposed to involve initial 1,2-insertion of two equivalents of a terminal alkyne into the MndC bond of a formamidinyl precursor (Scheme 17). The
4-coordination mode of the ligand is clearly observed from the X-ray crystal structure, which shows that one carbon atom and the nitrogen atom are raised above the plane formed by the four Mn-bound carbon atoms.113

Scheme 17

A related manganese complex (22; Fig. 15), in which manganese is
4-coordinated to the butadiene unit within a pyridinium ring, was also reported via reactivity described in Scheme 2.22 In addition, [(2-phenylpyridinyl)Mn(CO)4] reacted with phenylacetylene or diphenylacetylene to afford similar
4-butadiene complexes 94, presumably via initial alkyne insertion into the MndC bond, followed by CdN bond-forming reductive elimination (Scheme 18).24 X-ray crystal structures of 22 and 94 (where R ¼ Ph) were obtained.

Fig. 15 A manganese tricarbonyl complex with an
4-coordinated pyridinium ring.

Scheme 18

400

Cyclic and Non-Cyclic Pi Complexes of Manganese

5.07.2.7 5.07.2.7.1

Polyalkenyl complexes h5-Pentadienyl complexes (not including cyclohexadienyl complexes)

Though far less common than cyclopentadienyl complexes, several manganese complexes have been reported featuring
5pentadienyl ligands. DFT calculations have been conducted on the possible structures of homoleptic bis(2,4-dimethylpentadienyl)manganese(II), and indicate that doublet, quartet, and sextet spin state structures are closely spaced in energy.114 However, in the 2005–2020 period, all experimentally reported manganese complexes with
5-pentadienyl ligands involve
5-bonding to a Mn(I) center. For example (Scheme 19), addition of a bis(phosphine) to [(
5-pentadienyl)Mn(CO)2(THF)] or [(
5-2,4dimethylpentadienyl)Mn(CO)2(THF)] yielded, upon THF substitution, a pair of dimetallic Mn(I)
5-pentadienyl carbonyl complexes with a bridging bis(phosphine) ligand (95).115

Scheme 19

An
5-pentadienyl bis(trimethylphosphine) carbonyl complex (96a) has been prepared from the tricarbonyl precursor [(
-2,4-dimethylpentadienyl)Mn(CO)3] (Scheme 20), and the reaction was shown to proceed via stepwise associative substitution. Initial exposure of the precursor to PMe3 at room temperature resulted in phosphine coordination to the metal center, with concomitant slippage of the
5-pentadienyl to afford an
3 species (96b). Heating this complex led to carbonyl dissociation to yield a dicarbonyl phosphine species where the pentadienyl ligand has resumed
5-coordination (96c). Compound 96a was formed via a second substitution under photochemical conditions. It is notable that although the tricarbonyl precursor was previously reported, an X-ray crystal structure has only recently been obtained.116 5

Scheme 20

A pair of carbonyl-free monometallic manganese(II) complexes, [Mn(2,4-tert-butylpentadienyl)2] (97) and [(
5-1,3,5-C5Ht2Bu3)Mn(2,4-tert-butylpentadienyl)] (98), were synthesized by reactions of a potassium pentadienyl salt with (for 97) a half equivalent of MnI2(THF)2 or (for 98) a half equivalent of [{(
5-1,3,5-C5Ht2Bu3)Mn(THF)}2(m-I)2] (753; see Scheme 160 for the synthesis of 753); Scheme 21. In both 97 and 98, the acyclic pentadienyl ligand coordinates to manganese in a bidentate
1,
1-fashion. The bidentate coordination of the acyclic pentadienyl-based ligands in 97 and 98 to Mn(II) is asymmetric, where one set of Mn–C distances (2.1665(16)–2.1943(16) A˚ ) is far shorter than the other (2.3181(16)–2.5132(16) A˚ ). Compound 98 is also notable in that it is the first stable half-open manganocene derivative. In addition, a trimetallic mixed valence Mn(I)/Mn(I)/Mn(II) complex (99) was prepared by the reaction of the same potassium pentadienyl salt with manganocene (with 2,4,7,9-tetra-tertbutyl-1,3,7,9-decatetraene and KCp elimination); Scheme 21. In 99, the two Mn(I) atoms are each bound to an
5-cyclopentadienyl ligand, while the pentadienyl ligand binds to Mn(I) as a p ligand (with
5-coordination) and to Mn(II) in a bidentate
1,
1-fashion (analogous to what was observed in 97 and 98). Notably, bidentate coordination of the acyclic pentadienyl ligand to the central Mn(II) center in 99 is not clearly asymmetric (as it is in 97 and 98), with Mn(II)–C distances ranging from 2.3437(18) to 2.394(2) A˚ .117

Cyclic and Non-Cyclic Pi Complexes of Manganese

401

Scheme 21

In addition to chemistry which yielded novel p complexes of manganese, reports have also been provided on reactions stemming from [(
5-pentadienyl)Mn(CO)3] to afford products lacking a p ligand (Scheme 22). This pentadienyl complex reacted with thiols118 or selenothiols119 in the presence of phosphines or phosphites to afford a family of dimetallic complexes, [{Mn(CO)2(PR3)}2(m-CO)(m-EAr)2] (E ¼ S, Se), with the metal centers bridged by a single carbonyl and two thiolate or selenolate moieties. When analogous chemistry was carried out in the presence of a bis(phosphine) ligand, a monometallic Mn(I) thiolate120 or selenolate119 complex was obtained; [(RE)Mn(CO)3(dppe)] (E ¼ S, Se). For sulfur analogues, equivalent chemistry was observed using a bis(amine) ligand in place of the bis(phosphine) ligand (affording [(ArS)Mn(CO)3{k2-H2N(CH2)2NH2}]).120 In the absence of a thiol or selenothiol, the reaction with ethylenediamine yielded a bimetallic complex with bridging 2-aminoethylamido ligands; [{Mn(CO)3}2{m-1k1:2k2-HN(CH2)2NH2}2].120 Furthermore, in the absence of any additional Lewis base, reactivity with PhSeH yielded a heterocubane tetramanganese structure with Mn(CO)3 and SePh vertices; [({Mn(CO)3}{m3-SePh})4].119 Reactivity with alkanedithiols yielded mixed-valence thiolate complexes where a central Mn(IV) atom is bound to 6 thiolate donors, and the terminal manganese tricarbonyl moieties feature monovalent metal centers; [Mn{m-S(CH2)nS}3{Mn(CO)3}2] (n ¼ 2, 3, 4).121 In addition, tricarbonyl(
5-2,4-dimethyl-2,4-pentadienyl)manganese(I) underwent insertion reactivity with acetylene to form p-complexes containing an allyl-like fragment as well as an
2-coordinated alkene (20) or
4-coordinated butadiene (58) moiety (see Scheme 10).21 Furthermore, [(
5-pentadienyl)Mn(CO)3] has been investigated as a precursor for deposition of thin manganese-containing films by MOCVD.122

402

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 22

5.07.2.7.2

h5-Cyclohexadienyl complexes (and related species)

Many examples of tricarbonyl manganese(I) complexes with an
5-cyclohexadienyl ligand have been reported in the period 2005–2020. [(
5-cyclohexadienyl)Mn(CO)3)] complexes are isoelectronic with cymantrene and its derivatives, which feature cyclopentadienyl ligands (see Sections 5.07.3.1.1–5.07.3.1.7 for an overview of cymantrene derivatives). Several general routes have been used to access these complexes. One common route to access [(
5-cyclohexadienyl)Mn(CO)3)] complexes is nucleophilic attack on the arene ligand in a cationic 6 [(
-arene)Mn(CO)3]+ complex; Scheme 23 (for synthesis of new arene precursors 802, 800, 801, 804, and 811, see Schemes 171 and 172). Using LiAlH4, organolithium reagents, or Grignard reagents, hydride, alkyl or aryl groups have been attached to the

Scheme 23

Cyclic and Non-Cyclic Pi Complexes of Manganese

403

precursor, forming an sp3 carbon atom and converting the arene to a cyclohexadienyl ligand; 100, 101, 102, 103a-b, 104, 105, and 106.123–132 In all cases, nucleophilic attack (by hydride, organolithium, or Grignard reagents) occurred exclusively at a C–H carbon atom. This reactivity was found to proceed even in the presence of chloro or bromo arene substituents. For methoxy substituted arenes, the addition occurred regioselectively meta to the OMe substituent. By contrast, reactivity stemming from a derivative of 801 with a silyl in place of a methoxy substituent yielded a mix of regioisomers, where substitution occurs meta (80%) or para (20%) to the silyl group; 103a–b.125 Conversion of cationic arene precursors to [(
5-cyclohexadienyl)Mn(CO)3)] complexes has also been accomplished via a variety of additional routes (Scheme 24). One is electrochemical reduction, followed by apparent abstraction of a hydrogen radical from

Scheme 24

404

Cyclic and Non-Cyclic Pi Complexes of Manganese

solvent (this method was used in the synthesis of an analogue containing a fluorene-based ligand; 107).133 On a preparative scale, indium-mediated alkyl radical addition and reduction allowed various analogues of [(
5-cyclohexadienyl)Mn(CO)3)] complexes (108 and 109) to be prepared from cationic arene complexes using alkyl iodide precursors.134 Using this method with cationic arene precursors containing methyl, isopropyl, or methoxy substituents, the alkyl group was found to add primarily to the meta position (for methoxy, 100%); 109.134 Also, the reaction of a terminal alkene with [(
6-benzene)Mn(CO)3]+ under hydrogen atom transfer conditions (in the presence of [Co(acac)2], HSiEt3, and tBuOOH) resulted in the
5-cyclohexadienyl complex 110.135,136 Alternatively, a radical process for alkylation and reduction using alkyl-substituted NADH models was used to prepare [(
5-cyclohexadienyl)Mn(CO)3)] complexes with tBu or iPr groups on the sp3 carbon of the cyclohexadienyl ring (most of the complexes prepared in this way had previously been synthesized via alternative methods).137 Another route to the formation of [(
5-cyclohexadienyl)Mn(CO)3)] complexes from similar cationic arene precursors involves deprotonation of an atom directly attached to the manganese-bound arene (Scheme 25; syntheses of several of the arene precursor complexes are discussed in Section 5.07.3.2.1). For example, [(
6-octamethylfluorene)Mn(CO)3]+ (809) formed an
5-cyclohexadienyl complex (111) upon deprotonation at one of the methyl groups (this reactivity contrasts that of a previously reported non-methylated fluorene cation138 which underwent deprotonation on the 5-membered ring to afford a fluorenyl complex).139 While all 6 carbon atoms of the Mn-bound cycle of 111 are sp2 hybridized, one of them participates in a p bond to a terminal methylene unit, so it does not participate in bonding to Mn; this is clear from the X-ray crystal structure in which this atom lies out of the plane formed by the other 5 sp2 carbon atoms in the ring.139 Similarly, [(
6-dibenzosuberane)Mn(CO)3]+ (806) underwent deprotonation at a benzylic position within the 7-membered ring to afford
5-cyclohexadienyl complex 112.140 In addition, an
5-cyclohexadienyl complex where the uncoordinated ring carbon atom is part of an exocyclic alkene moiety (66) was formed as a minor component in the reaction of 1-phenyl-10 -tert-butyldiazomethane with a dimetallic Mn(I) aryl complex (see Scheme 11).87 In related chemistry, deprotonation of [{
6-o-C6H4(OH)(OMe)}Mn(CO)3]+ (812) afforded a neutral manganese(I) tricarbonyl
5-cyclohexadienyl complex with an exocyclic C]O double bond (113); in the solid state, the ketone carbon is raised above the plane formed by the other five cyclic carbons, and the ketone is angled away from the Mn center.141 Lastly, the reaction of a tetralin-bound arene cation with tBuOK unexpectedly yielded a neutral
5-cyclohexadienyl complex with a second tetralin moiety attached to the newly-formed sp3 carbon (114); this reactivity was proposed to proceed via initial deprotonation of the tetralin ligand, resulting in a carbanion which acts as a nucleophile towards the arene ligand in a second equivalent of starting material, followed by loss of “Mn(CO)+3”.140

Scheme 25

While [(
5-cyclohexadienyl)Mn(CO)3)] complexes have generally been formed from cationic arene precursors (see Schemes 23-25) or manipulation of other [(
5-cyclohexadienyl)Mn(CO)3)] complexes (Schemes 29–36), some have alternatively been prepared (Scheme 26) via reactions involving double or triple alkyne-insertion into an MndC bond in [Mn(k2-L)(CO)4] precursors where the k2-L ligand consists of an aryl donor and a neutral ketone or pyridine, forming a 5-membered metallacycle; the products of these reactions are 115,22 116,142 117,142 118 (as a minor product in the preparation of 1H-inden-1-ols),142 119,143 and 120.142

Cyclic and Non-Cyclic Pi Complexes of Manganese

405

Scheme 26

In contrast to reactions described in Scheme 26, single insertion of ethynyl ferrocene into the MndC bond of a cyclomanganated chalcone yielded a Mn(I)
5-pyranyl complex, where the oxygen atom is located out of the plane formed by the five sp2 carbon atoms in the 6-membered ring, and is not involved in bonding to Mn (much like an
5-cyclohexadienyl species); 121 and 122 (Scheme 27). This reactivity proceeded in benzene or acetonitrile, while a Mn-free product was observed when the reaction was carried out in CCl4. Protonation or oxidation of 121 and 122 afforded Mn-free ferrocene compounds; not shown in Scheme 27.144 In addition, the mechanisms of related (previously reported) reactions between alkynes and cyclomanganated 1,5-diaryl-penta1,4-dien-3-ones, which yielded
5-pyranyl and/or
5-6-oxocycloheptadienyl tricarbonyl manganese complexes (Scheme 27), were recently investigated. This report focussed on understanding why different analogues of the starting materials yielded one or both of these products, and included a new X-ray crystal structure for (
5-2,4-diphenyl-6-(2-phenylethenyl)pyranyl) tricarbonylmanganese(I).145

406

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 27

A rather unusual tetrametallic complex (123) was prepared (in a mixture with a dimetallic complex lacking a p-ligand) by the reaction of dimanganese decacarbonyl with 2,6-dimethyl-4-phenylphosphabenzene in the presence of PdO (Scheme 28). This complex can be considered to contain two doubly-reduced phosphabenzene ligands, each of which features (a) a pentadienyl anion which is
5-coordinated to a Mn(CO)3 fragment, and (b) a PR−2 anion which bridges between the two manganese centers of a (CO)4Mn–Mn(CO)3 core.146

Scheme 28

[(
5-cyclohexadienyl)Mn(CO)3)] complexes can be manipulated to install various new substituents on the p system, and a significant amount of effort has been devoted to converting these species to new [(
5-cyclohexadienyl)Mn(CO)3)] derivatives in a regioselective fashion (Schemes 29–36). One common method for installing new substituents directly on one of the sp2 carbon atoms is lithiation followed by addition of an appropriate electrophile (Scheme 29). Substituents installed on the cyclohexadienyl ligand include formyl, alkoxy, ester, alkyl (including C(OH)RR0 groups), silyl, stannyl, boryl, thiolate, phosphide, and iodo groups. In most cases, this method leads to functionalization para to the sp3 carbon (124, 125, 126, 127, 128, 129, 130, 131, 132, 133, or 134), though it could be directed to the meta position (135), and in one case a mixture of the meta and para substituted products (136 and 137) was obtained (for the synthesis of newly reported 145, the precursor in the synthesis of 134, see Scheme 32).126,128,147–152 Stepwise repetition of lithiation/electrophilic quenching has also been used for multiple functionalization of a cyclohexadienyl ligand. For example, lithiation followed by addition of electrophiles to 131 or 132 afforded complexes

Cyclic and Non-Cyclic Pi Complexes of Manganese

407

Scheme 29

138 and 139 where the second substituent has added ortho to the initial electrophile (E ¼ CH2NMe2 or P(O)Ph2).150,152 The regioselective nature of cyclohexadienyl ligand lithiation has been investigated computationally,148 as have the consequences of the regiochemistry on the electronic properties of formyl-149,153 or acyl-153 substituted derivatives. Studies directed towards resolving a single enantiomer of a cyclohexadienyl complex have also been reported.147,149,150,152 For cyclohexadienyl tricarbonyl manganese(I) complexes with four or five methyl substituents (104), lithiation followed by electrophilic quenching occurred at a methyl group (meta to the sp3 ring carbon), forming 140; Scheme 30.127 When R0 ¼ R ¼ H in 104, the aforementioned reactivity was not regioselective, occurring at either of the inequivalent methyl positions (forming a mixture of 140 and 141).129 In some cases, a second cycle of lithiation/electrophilic quenching was carried out, installing a PPh2 or formyl group on the other methyl group meta to the sp3 ring carbon to afford 142.127 This process has been investigated computationally.129

408

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 30

An alternative route to install acyl groups on [(
5-cyclohexadienyl)Mn(CO)3)] complexes is by organomanganese transmetallation catalyzed by [Fe(acac)3]; exposure to n-butyl lithium, followed by Li2MnCl4, [Fe(acac)3], and then an acyl chloride yielded Mn(I) tricarbonyl
5-cyclohexadienyl complexes with acyl substituents (Scheme 31). In reactions stemming from precursors with a methoxy or chloride substituent meta to the sp3 carbon, the acyl group was installed para to the sp3 carbon (143). In the absence of a directing group, a mixture of para (143) and meta (144) regioisomers was produced, where the former was the major product.153

Scheme 31

In contrast to lithiation/electrophilic quenching reactions of chloro-substituted manganese(I)
5-cyclohexadienyl complexes, which installed new substituents without loss of chloride substituents (Scheme 29), similar reactivity involving bromo-substituted derivatives 100, 102, 103a, and 103b proceeded via initial halogen-metal exchange (Scheme 32). In most cases, functionalization occurred at the carbon which was originally bound to bromine (145, 146, 147, 148, 149, and 150), though in some cases a mixture of products was observed; 126 and 151, or 152 and 153 (the components of the latter mixture could be isolated chromatographically).125,128,132,152 Furthermore, removal of a silyl substituent (in 150) was achieved using tetra-n-butylammonium fluoride (to form 154).125 Quenching of the lithiated species derived from 100 (where R ¼ Ph) with [Cr(CO)6], followed by reaction of the resulting acyl species with methyl triflate, allowed installation of a chromium Fischer carbene fragment on the cyclohexadienyl ligand; 155. Compound 155 was subsequently reacted with 3-hexyne (followed by addition of NEt3 and tBuMe2SiOTf ) to afford, via benzannulation, a manganese-free chromium arene complex.154

Cyclic and Non-Cyclic Pi Complexes of Manganese

409

Scheme 32

Replacement of chloride substituents in [(
5-cyclohexadienyl)Mn(CO)3)] complexes (achieved for bromo-substituents by lithiation followed by electrophilic quenching; Scheme 32) was achieved in a variety of manners (Scheme 33). For example, Pd-catalyzed Suzuki-Miyaura coupling reactions have been used to replace chloro substituents with aryl moieties, forming 156, 157, and 158 (in the syntheses of 158, a byproduct was observed with a second methoxy group installed in place of the aryl substituent).155 In addition, conversion of a Cl to an H substituent has been demonstrated by sequential addition of LiAlH4 followed by an acid to form 159.124 Furthermore, Pd-catalyzed Stille coupling reactions with 2-thienyl tributyl tin have been used to replace the chloride substituents in some analogues of 101 or 136 with a thienyl substituent to form 160.151,152 Similar chemistry in the presence of carbon monoxide resulted in
5-cyclohexadienyl manganese complexes with a carbonyl group inserted between the installed thienyl group and the p system; 161.123,126

410

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 33

Aldehyde and ketone substituents appended to the p ligand in [(
5-cyclohexadienyl)Mn(CO)3)] complexes 136, 125, and 161 have been reduced to alcohols using NaBH4 or a Grignard reagent (with associated CdC bond formation in the latter case); Scheme 34. Products in these reactions were 162,126 163, 164,149 165 (a metal free cyclohexadiene compound was also observed as a minor byproduct, and could be isolated),123,126 and 166.126,156 Utilizing an excess of NaBH4 (with subsequent exposure to trifluoroacetic acid), 161 could be further reduced to form complex 167 (where the thienyl fragment is tethered to the cyclohexadienyl ring by a CH2 linker).123 Reactions of 165 (R ¼ Ph, 2-thienyl) or 166 (R ¼ H, R0 ¼ Ph), which contain a CR(thienyl)(OH) group ortho to the sp3 ring carbon, with (CF3CO)2O in the presence of triethylamine unexpectedly yielded metal-free trienones.126 By contrast, similar reactivity involving 130 {with a CH(thienyl)(OH) substituent para to the sp3 carbon} yielded metal-free arenes (the exact composition was dependent on the substituent on the sp3 ring carbon in the precursor),126 and related reactivity involving 162 and 136 {with a CHR(OH) substituent meta to the sp3 carbon} yielded [(
5-cyclohexadienyl)Mn(CO)3)] complexes with a CHR(O2CCF3) substituent (168).126 CRR0 (OH)-appended [(
5-cyclohexadienyl)Mn(CO)3)] complexes were found to react with acids to yield vinyl (169; via reactivity from 166)156 or carbenium (170, 171, and 172 via reactivity from 151, 126 and 148, respectively; some of these complexes showed significant catalytic activity for the Diels-Alder reaction of cyclopentadiene and methacrolein)128 moieties attached to the p system, depending on the nature of the R and R0 groups. In the synthesis of 169, a metal-free dienone byproduct was also observed, potentially due to the reaction of 169 with a second equivalent of acid, or decomposition of an intermediate enroute to compound 169.156

Cyclic and Non-Cyclic Pi Complexes of Manganese

411

Scheme 34

Sonogashira coupling involving an alkynyl-substituted [(
5-cyclohexadienyl)Mn(CO)3)] precursor provided access to a variety of new [(
5-cyclohexadienyl)Mn(CO)3)] derivatives where the alkyne (which is ortho to the sp3 ring carbon) links to a pendent benzodithiophene (173),157 tetrathia[7]helicene (174; depending on whether the organic reactant had one or two iodo substituents, the resulting complex contained either one or two manganese fragments),158 or 20 -deoxyuridine (175; derivatives where R ¼ H were evaluated for antiviral activity against a series of DNA and RNA viruses, though no significant activity was observed)159 fragment; Scheme 35.

412

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 35

A final class of [(
5-cyclohexadienyl)Mn(CO)3)] complexes reported between 2005 and 2020 involve coordination of a pendent Lewis base on the Mn-bound p system to a palladium center (Scheme 36). For example, phosphine-appended 129, 149 (where R ¼ Ph and R0 ¼ PPh2), and 146 (when R ¼ Ph) reacted with enantiopure (S)-(+)-bis(m-chloro)bis[2-{(dimethylamino) ethyl}phenyl-C2,N]dipalladium(II) to afford heterometallic complexes 176, 177, and 178, respectively. In each case, two diastereomers were formed; for reactions stemming from 129, and 149, they could be separated and the Pd fragment removed (by exposure to TMEDA), allowing enantiopure samples of the initial monometallic Mn-containing species to be isolated.152 Reaction of phosphine-containing 129 with [{ClPd(C3H5)}2] resulted in formation of another heterometallic complex, 179.150,152 In addition, an [(
5-cyclohexadienyl)Mn(CO)3)] complex with pendent phosphine and amine substituents (138 where E0 ¼ PPh2) reacted with [{ClPd(C3H5)}2] to afford 180 upon coordination of the CH2NMe2 and PPh2 substituents on the Mn-bound cyclohexadienyl ligand to Pd; subsequent chloride abstraction yielded a cationic complex with an
3-bound allyl ligand on Pd (181).152 Heterometallic complexes 138 and 180, along with others formed in situ from [{ClPd(C3H5)}2] and various manganese(I)
5-cyclohexadienyl complexes, displayed catalytic activity towards allylic alkylation of 1,3-diphenyl-3-acetoxy-1-propene by dimethyl malonate.152

Cyclic and Non-Cyclic Pi Complexes of Manganese

413

Scheme 36

Photoirradiation of [(
5-cyclohexadienyl)Mn(CO)3)] complexes has also been investigated (Scheme 37) and can yield organic products with aromatic or non-aromatic 6-membered rings depending on the solvent or presence/absence of acetic acid.160 In addition, computational work on previously reported [(
5-cyclohexadienyl)Mn(CO)3] complexes has included an analysis of isomerization via manganese-mediated 1,4-hydride migration.161

Scheme 37

Nanoparticles or surfaces functionalized with manganese complexes incorporating an
5-coordinated cyclohexadienyl moiety have also been reported. Surface modification of Fe3O4 or FePt nanoparticles with a single layer of [(
5-semiquinone)Mn(CO)3] fragments has been carried out (to form 86) by exposure of an oleylamine-functionalized nanoparticle to [(
6-hydroquinone)Mn(CO)3]+ in DMSO (Scheme 38; top).162 For the Fe3O4 derivative, further functionalization with an H-bonding polymer formed from [(
5-semiquinone)Mn(CO)3] units (to form 182) was achieved by exposure of 86 to an excess of free [(
5-semiquinone)Mn(CO)3].100 In addition, a derivative of [(
5-semiquinone)Mn(CO)3] functionalized by dodecoxy substituents in the 2- and 5- positions (183) has been prepared (Scheme 38; bottom), and it was demonstrated that it adsorbs on highly ordered pyrolytic graphite surfaces to form a 2D structure. Compound 183 was synthesized by the reaction of [(
6-acenaphthene)Mn(CO)3] with the free arene 2,5-didodecoxy-1,4-dihydroxybenzene, which presumably proceeds by initial formation of a cationic arene intermediate followed by proton loss (arene substitution from the same manganese-containing precursor has been demonstrated to yield a series of [(
6-arene)Mn(CO)3]+ complexes; see Scheme 172 for details).163

414

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 38

5.07.2.8

Complexes containing
n-coordinated (n > 2) heteroatom-containing p ligands

Stoichiometric hydroboration of terminal alkynes by a Mn(I) bis(s)borate complex yielded
4-E-boratabutadiene complexes (each equivalent of the manganese-containing precursor reacted with two equivalents of the alkyne); 184 (Scheme 39). Notably, the internal BdC bond appears to display a lower bond order than the terminal BC and CC bonds in the CBCC p system {the terminal BdC bond distance of 1.482(4) A˚ in the derivative for which an X-ray crystal structure was obtained (R ¼ p-tolyl) is far shorter than that of the internal BdC bond distance of 1.541(4) A˚ }. Interaction of all four atoms in the p system with manganese is also supported by X-ray crystallography and DFT calculations, with MndB and MndC distances (and Wiberg bond indices) of 2.246 (3) A˚ (WBI: 0.26) and 2.199(2)–2.572(2) A˚ (WBI: 0.31–0.37), respectively.164

Scheme 39

A Mn(I) tricarbonyl complex with an
5-1-oxopentadienyl ligand (185) was prepared by an aldol-like condensation of two equivalents of acetylferrocene, promoted by benzyl pentacarbonylmanganese(I) in the presence of 9-diazofluorene (Scheme 40). The X-ray crystal structure and DFT calculations highlight the delocalized and polarized nature of the
5-coordinated ligand.165

Cyclic and Non-Cyclic Pi Complexes of Manganese

415

Scheme 40

Complexes with carboxylate, amidinate, guanidinate, or related ligands are for the most part not covered in this chapter because, despite the fact that the metal-coordinated ligand fragment involves a delocalized p-system, the bonding environment with manganese is normally k2 where the central carbon atom in the 3-atom p-system is not coordinated to Mn. However, a series of bimetallic dithiocarboxylate complexes (186) have been reported where a dithiocarboxylate ligand bridges between two Mn atoms; one metal is k2-coordinated while the other engages in allyl-like
3-coordination (Scheme 41). These complexes were prepared by carbonyl substitution from dimanganese decacarbonyl by an imidazole(in)ium-2-dithiocarboxylate zwitterion, or alternatively from the comproportionation-decarbonylation reaction of Na[Mn(CO)5] with a monometallic Mn(I) thiocarboxylate carbonyl bromide complex. For structurally characterized derivatives of 186, the “allylic” MndC distances ranged from 2.03 to 2.08 A˚ , comparable to true allylic systems (see Section 5.07.2.5 for an overview of Mn allyl complexes). Also, the MndS distances involving the metal center interacting with the p-system (2.33 A˚ ) were only slightly longer than those involved in the k2-interaction (2.28 A˚ ).166

Scheme 41

In addition, the previously reported heterometallic Re/Mn complex [{Mn(CO)3}{Re(CO)3}{m-S2C(PCy3)}], which contains a bridging phosphine-carbon disulfide ligand (where, analogous to 186, the SCS moiety interacts in an
3-allyl fashion with the Mn atom and a k2 fashion with the Re atom), was reacted with nBuLi, followed by protonation and Lewis base coordination to yield dimetallic Mn/Re species where one of the bridging ligands is a trialkylphosphoniothiolateylide {SC(H)PR3} which is
2-C,S-coordinated to manganese (Scheme 42). The behavior of these Mn/Re complexes as electrocatalysts was explored.167

Scheme 42

416

Cyclic and Non-Cyclic Pi Complexes of Manganese

5.07.3

Cyclic p ligands

5.07.3.1

Cyclopentadienyl complexes

By far the most common class of p ligand utilized in manganese chemistry is the cyclopentadienyl anion. The vast majority of cyclopentadienyl-containing Mn complexes reported in the 2005–2020 period can be considered to be derivatives of the first isolated monovalent organomanganese complex, [CpMn(CO)3] (cymantrene), with either one or more carbonyl ligands replaced by an alternative co-ligand, one or more non-hydrogen substituents on the cyclopentadienyl ligand, or both of these. These Mn(I) complexes generally feature a piano-stool structure, and are sometimes referred to as half-sandwich complexes. Other cyclopentadienyl-containing manganese complexes described below include carbonyl-free complexes with one or two cyclopentadienyl ligands on manganese (e.g. metallocene derivatives).

5.07.3.1.1 Derivatives of cymantrene (I) synthesis and reactivity of complexes containing the “(C5H5-xMex)Mn(CO)” fragment (nitrosyl-free) A wide variety of [(C5H5-xMex)Mn(CO)2L] and [(C5H5-xMex)Mn(CO)L2] complexes have been reported. Neutral ligands bound to the (C5H5-xMex)Mn(CO)n fragment include both stable species (e.g. phosphines) and moieties unstable in the absence of the metal fragment (e.g. borylenes). 5.07.3.1.1.1 Synthesis of cymantrene derivatives from [(C5H5-xMex)Mn(CO)2L] (L ¼ neutral ligand, x ¼ 0-5) via substitution or ligand dissociation One common route to access [H,MeCpMn(CO)3-nLn] (n ¼ 1 or 2) complexes is photochemically-induced carbonyl substitution from cymantrene or MMT in the presence of a free ligand, as illustrated in Scheme 43 (new complexes of this type are numbered only

Scheme 43

Cyclic and Non-Cyclic Pi Complexes of Manganese

417

when they appear in other figures or schemes). In some cases, it was shown that a solvent-coordinated species is initially generated, and the solvent subsequently undergoes thermal substitution. In fact [H,MeCpMn(CO)2(THF)], which can be generated by UV irradiation of cymantrene or MMT in THF, is often used as an isolable precursor to prepare [H,MeCpMn(CO)2L] complexes by substitution of the THF ligand without the need for UV irradiation. Neutral pnictogen ligands reported to have been incorporated into [H,MeCpMn(CO)3-nLn] (n ¼ 1 or 2) complexes via CO or THF substitution (Scheme 43) include amines,168 phosphines or phosphites,169–173 arsines,174 stibines,174 and bismuthines,175 as well as cyanamides,176 pyrazole,177,178 pyridines,178 imidazole,178 or a lithium phosphanide salt (in this example, the P donor in the resulting complex bridges between two CpMn(CO)2 fragments).179 Polymers containing either pyridine or phosphine donors have also been used to attach the MeCpMn(CO)2 fragment to a polymer.180–183 In addition, various chelating neutral ligands have been incorporated by substitution of two carbonyl ligands, including a Schiff base184 and thiosemicarbazide185 (both of which coordinate to Mn via nitrogen). Furthermore, THF substitution from [Cp Mn(CO)2(THF)] has been used to prepare a complex in which two cymantrene fragments are tethered together by a bis(diphenylphosphino)alkyne ligand; the electronic structures of this complex and a hypothetical Cp derivative were probed computationally.115 Group 14-based ligands (aside form CO and H,MeCp) incorporated into [H,MeCpMn(CO)3-nLn] (n ¼ 1 or 2) complexes by carbonyl or THF substitution (Scheme 43) include alkenes (see Section 5.07.2.1.1 for structures of these complexes),5–7 allenes (see Section 5.07.2.2.1 for structures of these complexes),44 alkynes (see Section 5.07.2.3 for structures of these complexes),45,51 NHCs,186,187 germylenes,188 a germavinylidene,189 and stannylenes,190 while group 16 based ligands include alcohols,19,191 an imidazolinethione (see Scheme 50 for a discussion of dimanganese complexes formed via reversible imidazolinethione dissociation),192 and dodecanethiol.191 Furthermore, a polymer containing GedH bonds was functionalized with CpMn(CO)2 fragments by exposure to cymantrene under photochemical conditions; the report suggested GedH bond oxidative addition to form manganese(III) germyl hydride species.193 E–H co-ligands (where E is a group 12 or 13 element) which have been incorporated into the cymantrene or MMT framework via carbonyl or THF substitution (Scheme 43) include hydroborane-Lewis base adducts,194–196 hydroalanes,197 a dihydrogallane (upon further irradiation, this complex underwent spontaneous H2 elimination to form a gallylene complex),198 and a zinc hydride.199 With the exception of the hydroborane-Lewis base adducts (which coordinate to the metal center in a k1-fashion via a BdH substituent), these group 12 and 13 based ligands interact with manganese via an HdM bond (M ¼ Al, Ga, or Zn) to form s complexes. Furthermore, the reaction of an anionic 5-membered Ga-containing heterocycle with MMT led to the anionic gallyl complex [MeCpMn(CO)2{Ga({N(Ar)C(H)}2)}]−.200 Displacement of THF in [H,MeCpMn(CO)2(THF)] by a series of P (or, in one case, Te) donors coordinated to other transition metal centers afforded a family of heterometallic complexes (Scheme 44). For example, [MeCpMn(CO)2(THF)] reacted with the molybdenum-containing precursor [{CpMo(CO)}2(m-PCy2)(m-
2:
2-P2Me)] to afford a trimetallic species [{MeCpMn(CO)2} {CpMo(CO)}2(m-Mo,Mo-PCy2)(m3-
2-Mo-
2-Mo-
1-Mn-P2Me)] (187) where a lone pair on the P2Me ligand coordinates to the manganese center. Complex 187 was unstable, spontaneously undergoing apparent methylphosphinidene loss to form phosphide complex [{MeCpMn(CO)2}{CpMo(CO)}2(m-Mo,Mo-PCy2)(m3-P)] (188).201 Exposure of 188 to carbon monoxide yielded [{MeCpMn(CO)2}{CpMo(CO)}{CpMo(CO)2}(m-Mo,Mo-PCy2)(m3-P)] (189), in which an additional CO ligand has coordinated to one of the two molybdenum centers.201 However, when excess [MeCpMn(CO)2(THF)] reacted with the same dimetallic molybdenum precursor ([{CpMo(CO)}2(m-PCy2)(m-
2:
2-P2Me)]), the unstable tetrametallic cluster [{MeCpMn(CO)2}2{CpMo(CO)}2(m-Mo,Mo-PCy2)(m-
2-Mo-
1-Mo-
1-Mn-
1-Mn-P2Me)] (190) was observed (presumably via initial formation of 187), which decomposed to form 188 upon attempts to conduct further chemistry.202

418

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 44

Cyclic and Non-Cyclic Pi Complexes of Manganese

419

An anionic cluster has also been prepared utilizing similar chemistry to that used in the synthesis of the neutral cluster 187, but involving an anionic dimolybdenum precursor (Scheme 44). The bis(molybdenum) anion [{CpMo(CO)}2(m-PCy2)(m-
2:
2-P2)]− displaced THF from [MeCpMn(CO)2(THF)] to yield [{MeCpMn(CO)2}{CpMo(CO)}2(m-Mo,Mo-PCy2)(m3-
2-Mo-
2-Mo-
1-MnP2)]− (191), which is the result of coordination of a lone pair on the P2 ligand to Mn. The presence of a second equivalent of manganese precursor led to the tetrametallic cluster [{MeCpMn(CO)2}2{CpMo(CO)}2(m-Mo,Mo-PCy2)(m4-
2-Mo-
2-Mo-
1-Mn
1-Mn-P2)]− (192) in which both phosphorus atoms in the P2 ligand are coordinated to manganese. Both 191 and 192 can be protonated to yield neutral [{MeCpMn(CO)2}{CpMo(CO)}2(m-Mo,Mo-PCy2)(m3-HP2)] (193) or [{MeCpMn(CO)2}2{CpMo(CO)} (CpMoH)(m-Mo,Mo-PCy2)(m4-
2-Mo-
2-Mo-
1-Mn-
1-Mn-P2)] (194), respectively. In 193, the hydride bridges between Mo and P atoms, while in 194, a carbonyl ligand on one Mo center has been replaced by a terminal hydride ligand.203 Mixtures of heterometallic complexes were obtained by reactions between [CpMn(CO)2(THF)] and [(1,3-C5Ht2Bu2)2Zr(P4)], the latter of which features a k2-coordinated P4 ligand (with a tetraphosphabicyclo[1.1.0]butane structure) with four Lewis basic sites (Scheme 44). Utilizing two equivalents of the manganese precursor, a mixture of complexes was obtained resulting from coordination of one or two of the P atoms to CpMn(CO)2 fragments; [{CpMn(CO)2}(m2-P4){(1,3-C5Ht3Bu2)2Zr}] (195a-b) and [{CpMn(CO)2}2(m3-P4){(1,3-C5Ht3Bu2)2Zr}] (195c), respectively. The ratio of 195a:195b:195c was 10:1:0.5, though only 195a and 195c were isolated by column chromatography. The structures of the monosubstituted products 195a and 195b differ by which P site coordinates to Mn, and the major product (195a) involves a bond between manganese and one of the bridgehead phosphorus atoms (consistent with DFT calculations on these isomers, which indicated that 195a is 9 kJ mol−1 more stable than 195b). In 195c, both P atoms bound to Mn are in the bridgehead position. When an excess of [CpMn(CO)2(THF)] was employed, the mixture of products obtained was composed of 195c and a species tentatively identified as the trisubstituted product [{CpMn(CO)2}3(m4-P4){(1,3-C5Ht3Bu2)2Zr}] (195d), the latter of which could not be isolated.204 In addition, mixtures of homometallic complexes were obtained by reactions between [CpMn(CO)2(THF)] and the (CpMnP)4 cage 735; Scheme 44 (see Scheme 157 for the synthesis of 735). Mono- and di-substituted species 196a-b were formed when the reaction was conducted with one equivalent of each reagent, and the tri- and tetra-substituted complexes 196c-d were obtained when four equivalents of [CpMn(CO)2(THF)] were used. While X-ray crystal structures were obtained for all four compounds (196a-d), only 196b-d were isolated (chromatographically).205 Substitution chemistry stemming from cymantrene or [MeCpM(CO)2(THF)] has also been used to install the H,MeCpMn(CO)2 fragment onto iron complexes (Scheme 44). For example, irradiation of [CpMn(CO)3] in the presence of [CpFe(CO)(PPh3) (TePh)] afforded the dimetallic complex [{CpMn(CO)2}(m-TePh){CpFe(CO)(PPh3)}] (197) with a bridging tellurolate ligand.206 In addition, THF substitution from [MeCpMn(CO)2(THF)] by the phosphinidene fragment in [Fe2Cp2(m-PCy)(m-CO)(CO)2] afforded the cluster [{MeCpMn(CO)2}{CpFe(CO)}2(m-Fe,Fe-CO)(m3-PCy)] (198), containing a triply-bridging phosphinidene ligand. Exposure of the MnFe2 complex 198 (see Scheme 44 for the synthesis of 198) to either visible or ultraviolet light at 288 K resulted in different clusters with carbonyl ligands bridging between manganese and iron; [(MeCpMn)(CpFe)2(m-Fe,Fe-CO)(m-Mn, Fe-CO)2(m3-PCy)] (199), [(MeCpMn){CpFe(CO)}2(m-Fe,Mn-CO)2(m3-PCy)] (200a), and [{MeCpMn(CO)}(CpFe){CpFe(CO)} (m-Fe,Mn-CO)(m-Fe,Fe-CO)(m3-PCy)] (200b) in Scheme 45. These reactions were reversible in the presence of CO at room temperature.207

Scheme 45

420

Cyclic and Non-Cyclic Pi Complexes of Manganese

Clusters with trimetallic cores composed of Mn and two group 6 metals have been prepared by substitution of CO or THF from manganese cyclopentadienyl precursors (Scheme 46). For example, [MeCpMn(CO)3] reacted under photochemical conditions with an unsaturated methyl-bridged dimolybdenum precursor to form, upon H2 and CO elimination, the trimetallic complex [{MeCpMn(CO)2}{CpMo(CO)}2(m-Mo,Mo-PCy2)(m3-CH)] (201). The structure of 201 features a bridging methylidyne ligand, and the two manganese-bound carbonyl ligands engage in weak bridging interactions with the Mo centers.208 Furthermore, displacement of THF from [H,MeCpMn(CO)2(THF)] by dimetallic molybdenum methoxycarbyne or hydride complexes yielded trimetallic clusters with a m3-methoxycarbyne ligand in [{H,MeCpMn(CO)2}{CpMo(CO)}2(m-Mo, Mo-PCy2)(m3-COMe)] (202)209,210 or a m3-hydride ligand in [{MeCpMn(CO)2}{CpMo(CO)}2(m-Mo,Mo-PCy2)(m3-H)] (203),211,212 respectively. In addition, the bis(tungsten) hydride manganese cluster [{MeCpMn(CO)2}{CpW(NO)}2(mW,W-PPh2)(m3-H)] (204) was accessed by coordination of the MeCpMn(CO)2 unit to the core of the W2 dimer [{CpW(NO)}2(m-PPh2)(m-H)].213

Scheme 46

In addition to the examples provided in Scheme 43, irradiation of cymantrene or MMT in the presence of a free ligand allowed transient alkane,76,214,215 carbon dioxide,76 noble gas,76
2-cyclopentadiene,11 thiol,216 and water11 derivatives of [H,MeCpMn(CO)2L] (205) to be spectroscopically detected (top of Scheme 47). Furthermore, DH and DV for the formation of [CpMn(CO)2(alkane)] via photochemical irradiation of cymantrene in alkane solvents were measured using photoacoustic calorimetry.217 In addition, photochemical carbonyl dissociation from [MeCpMn(CO)2(L)] (L ¼ NHC or PPh3) yielded low-coordinate species with agostic interactions between the neutral non-carbonyl co-ligand and the metal center, which were detected using time-resolved IR spectroscopy (bottom of Scheme 47).186,218

Cyclic and Non-Cyclic Pi Complexes of Manganese

421

Scheme 47

The coordinatively unsaturated MeCpMn(CO)2 fragment (presumed to be an intermediate in photochemical carbonyl substitution stemming from MMT) has been crystallographically observed upon low-temperature photoirradiation of [MeCpMn(CO)3] contained within a self-assembled coordination cage (attempts to extend this chemistry to the CpMn(CO)2 fragment were unsuccessful, presumably due to suppressed carbonyl dissociation or rapid carbonyl re-coordination); Scheme 48.219,220 In addition, the cymantrene analogue of this low-coordinate species, and its indenyl, C5H4Me, and C5Me5 derivatives, have been observed in the singlet and triplet states in Ar, CH4, and Xe matrices at 10 K; irradiation initially forms the triplet species which thermally decays to the singlet species (Scheme 48).221

Scheme 48

Upon irradiation of tetra- or penta-methylated cymantrene derivatives in benzene (even in the presence of water), carbonyl dissociation was instead followed by coordination of an intact [nMeCpMn(CO)3] fragment to yield the carbonyl-bridged species [{(C5H5-xMex)Mn(CO)2}2(m-CO)] (x ¼ 4, 5; 206); Scheme 49. While these complexes were spectroscopically characterized, they could not be isolated.222

Scheme 49

In solution, the monometallic imidazolinethione complex [CpMn(CO)2(SC3N2Me4)] (207; prepared as illustrated in Scheme 43) was shown to be in equilibrium with the dimetallic species [{CpMn(CO)2}2(m-SC3N2Me4)] (208); Scheme 50. While monometallic 207 is the dominant species in solution, dimetallic 208 was isolated in the solid state and crystallographically characterized. The MndMn distance in the X-ray crystal structure of 208 is 4.1878(18) A˚ , ruling out the presence of a MndMn bond, and the S atom is significantly pyramidalized. Combined, this indicates that the bonding situation in 208 would best be described by contributions from the resonance structures shown in Scheme 50, two of which are zwitterionic and place a lone pair on the S atom.192

422

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 50

Linkage isomerization has also been investigated for cymantrene or MMT derivatives where a bifunctional neutral ligand can bind to the metal via different sites; e.g. O-bound and alkene isomers (see Scheme 1),10 or pyridine- and nitrile-bound isomers (Scheme 51).223 In the latter case, a solvent-coordinated (alkane) intermediate was observed using time-resolved IR spectroscopy, which suggests a mechanism involving initial ligand dissociation.223 As well, the strength of bonding between Mn and very weak neutral ligands in [CpMn(CO)2L] has been investigated by a combination of computational studies and measurements of the rate of substitution by pyridine ligands and other small molecules. Examples of weak neutral ligands in these systems are haloalkanes,224
2-dihydrofuran,225 arenes (
2-coordinated or bound via an arene CdH bond),226 and carbon dioxide.76

Scheme 51

5.07.3.1.1.2 Synthesis and reactivity of new cymantrene derivatives prepared via other methods While [(C5H5-xMex)Mn(CO)2L] and [(C5H5-xMex)Mn(CO)L2] complexes have often been prepared by substitution of a neutral ligand (typically CO or THF; Section 5.07.3.1.1.1), reactions which convert one [(C5H5-xMex)Mn(CO)2L] or [(C5H5-xMex)Mn(CO) L2] derivative into another have also been extensively researched. These reactions are the focus of this section. Precursors in the chemistry described below were often synthesized via the chemistry described in Section 5.07.3.1.1.1.

5.07.3.1.1.2.1 Derivatives with Group 16-based Ligands A family of trimetallic complexes have been prepared which feature a m3-bridging dichalcogen (S, Se, or Te) moiety bound
1 to two “CpMn(CO)2” fragments and
2 to a group 10 fragment; [{CpMn(CO)2}2{m-E2M(PR3)2}] (209; Scheme 52). S and Se derivatives of 209 {M ¼ Pt or (for E ¼ S only) Ni} were prepared by reaction of [(
2-PhC^CPh)M(PR3)2] with the bridging S2 or Se2 ligand in [{CpMn(CO)2}2(m-E2)] (E ¼ S, Se). Relative to the precursors, the EdE and MndE bonds in 209 are significantly elongated.227,228 Related [{(
5-C5H4Et)Mn(CO)2}2{m-S2Pt(PPh3)2}] was also prepared via similar chemistry (not shown in Scheme 52).228 A tellurium/platinum derivative of 209 was also prepared by reaction of [(
2-PhC^CPh)Pt(PPh3)2] with [{CpMn(CO)2}2}3 (m3-Te2)] (210), in which two Te atoms bridge between three CpMn(CO)2 fragments (complex 210 can be considered an analogue of the sulfur and selenium precursors with another CpMn(CO)2 fragment bound side-on to the EdE bond).228 Compound 210 was prepared in two steps from Li[CpMn(CO)2{]C(Ph)(O)}] by initial exposure to elemental tellurium followed by oxygen. The intermediate was tentatively identified as Li2[{CpMn(CO)2}2(m-Te)] (211), and reacted with PhCH2Br to form [{CpMn(CO)2}2{m-Te(CH2Ph)2}] (212), or with [M(CO)5(THF)] (M ¼ Cr, W) to form the trimetallic Te-bridged compound [{CpMn(CO)2}2{M(CO)5}(m3-Te)] (213).229

Cyclic and Non-Cyclic Pi Complexes of Manganese

423

Scheme 52

[CpMn(CO)2(TePhI)] (214) was synthesized by I2 oxidation of a m-diphenyl ditelluride precursor (Scheme 53). Compound 214 was the first example of a transition metal complex with an unsupported PhTeI ligand. Analysis of the X-ray crystal structure of 214 showed that, unlike in many charge transfer compounds where the PhTeI fragment is bound to a Lewis base, the manganese center is not located trans to the iodo substituent across the Te atom (the MndTedI angle is 105.90(4) ).230

Scheme 53

5.07.3.1.1.2.2 Derivatives with Group 15-based Ligands Secondary phosphine-containing [MeCpMn(CO)2(PR2H)] (215; prepared as described in Scheme 43) reacted with nBuLi followed by either TEMPO or [TEMPO][BF4] to yield an anionic manganese complex with a k1-P-PR2O− ligand (216) or a phosphinite derivative with a 2,2,6,6-tetramethyl-piperidin-1-oxyl (OTEMP) substituent on phosphorus (217), respectively (Scheme 54). Both complexes were unstable, decomposing to [MeCpMn(CO)2{PR2(OH)}] (218) at room temperature (for 217, this was rapid; the source of the H atom in these cases was proposed to be the solvent). For R ¼ Cy, 216 could be regenerated from 218 by addition of the amine base TEMP-H. The hydroxyl group on 218 could alternatively be stannylated with HSnPh3 to yield [MeCpMn(CO)2{PCy2(OSnPh3)}] (219).171

424

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 54

[CpMn(CO)2(PH2Ph)] (220; prepared as described in Scheme 43), which features a primary phosphine ligand, was used to prepare a series of [CpMn(CO)2L] derivatives with unusual phosphorus-based co-ligands (Scheme 55); 220 reacted with 1,2-diethynylbenzene in the presence of KOH to yield the stable benzophosphepine complex [CpMn(CO)2(3-phenyl-benzophosphepine)] (221).231 At elevated temperatures, 221 underwent cycloheptatriene-norcaradiene valence isomerization to form a phosphanorcaradiene complex (222; unobserved), followed by naphthalene extrusion to afford the phosphinidene complex [CpMn(CO)2(]PPh)] (223; unobserved), which itself reacted with unsaturated hydrocarbons to form isolable (in the case of PhCCH) phosphirene (224) or (in the case of 1-hexene) phosphirane (225) complexes.232

Scheme 55

A cationic manganese cyclopentadienyl complex (226) was prepared by abstraction of a methoxy substituent from a neutral P {(NMeCH2)2}(OMe) donor in [CpMn(CO)(P{(NMeCH2)2}{OMe})2] (227; prepared as illustrated in Scheme 43); Scheme 56. The resulting P{(NMeCH2)2} ligand was described as a phosphenium cation, isoelectronic with an NHC.173

Scheme 56

Cyclic and Non-Cyclic Pi Complexes of Manganese

425

5.07.3.1.1.2.3 Derivatives with Group 14-based Ligands [MeCpMn(CO)3] (MMT) has been used for alkyne {RR0 HCdC^CdC(O)R00 } to allene transformations. This involved initial photochemical formation of an alkyne-bound species [MeCpMn(CO)2{
2-RR0 HCdC^CdC(O)R00 }] (43; see Scheme 43), followed by manipulation of the newly installed p-ligand on Mn and removal of the resulting allene ligand from the metal’s coordination sphere (Scheme 57). For example, alkyne-coordinated 43 underwent isomerization into the allene-coordinated complex [MeCpMn(CO)2{
2-RR0 C]C]CHC(O)R00 })] (27) in the presence of a base.45 This reaction was also carried out using various chiral phase-transfer catalysts, affording derivatives of 27 with enantiomeric excesses of 54%–83% under the optimized conditions (for brevity, only the optimized reaction conditions involving a geranyl-substituted cinchonine phase transfer catalyst are shown in Scheme 57).46 In addition, aldehyde-containing derivatives of 27 were converted to secondary alcohol-containing allene complexes (28) using a Grignard reagent.233,234 Furthermore, aldol reactivity involving alkyne complex 43, leading to allene complex 29, has been reported. Under some reaction conditions used in the synthesis of 29, compound 27 was also formed.233,234 In most cases, the allene ligand could be liberated from complexes 27, 28, or 29 by mild oxidation.

i. ii. H+

Scheme 57

A series of cationic dicarbonyl cyclopentadienyl manganese complexes containing butadienyl, propargyl, phosphonium-substituted alkene, or phosphonium-substituted allene ligands have been prepared by protonation of a neutral ligand in a [CpMn(CO)2L] precursor, in some cases followed by reaction with PPh3 (Scheme 58). For example, protonation of
2-allene complex 26 (prepared as described in Scheme 5), which contains an H2C]C]CH(CH2OH) ligand, resulted in (upon H2O elimination) a cationic
3-butadienyl complex (72: see Section 5.07.2.5.1 for structures of isomers of 72 observed in solution). This complex proved susceptible to nucleophilic attack; exposure of 72 to PPh3 resulted in a cationic
2-allene complex (35).44 Related chemistry has been reported stemming from alcohol-containing alkyne complexes [RCpMn(CO)2(
2-HC^CCR0 2OH)]+ (42; prepared as described in Scheme 43), which upon protonation (and water elimination) yielded cationic
3-propargyl complexes [RCpMn(CO)2{
3-HC^CdCR0 2}] (RCp ¼ Cp, Cp and R0 ¼ H, Me; 77). In chemistry similar to that of
3-butadienyl complex 72, exposure of 77 to PPh3 yielded cationic
2-allene complexes [RCpMn(CO)2{
2-R0 2C]C]C(H)(PPh3)}]+ (37 for R0 ¼ H, 38 for R0 ¼ Me).51 Protonation of zwitterionic s-phosphoniostyryl complexes [CpMn(CO)2{C(PR3)(]CHPh)}] (278; prepared as described in Scheme 67) yielded
2-phosphonioalkene cations [CpMn(CO)2{
2-E-H(PR3)C]C(H)Ph}]+ (18).18 Various mechanisms for this reaction were investigated computationally, with the lowest energy pathway involving initial formation of a hydride intermediate.18 By contrast, protonation of zwitterionic a-phosphonioallenyl complexes [CpMn(CO)2{C(PR3)(]C] CR0 Ph)}] and [{CpMn(CO)2}{m-C(]C]CPh2)(Ph2PCH2CH2PPh2)C(]C]CPh2)}] (282a-b; prepared as described in Scheme 69) generated different structures depending on the identity of the PR3 substituents; in most cases, the product was one of the expected
2-phosphonioallene cations [CpMn(CO)2{
2-HC(PR3)]C]CR0 Ph}]+ (36a) or [{CpMn(CO)2}2{m-
2:
2-Ph2C] C]CH(Ph2PCH2CH2PPh2)HC]C]CPh2}]2+ (36b). However, a derivative of 282a where PR3 ¼ dppm yielded an isomer featuring a manganese-phosphine linkage (228), presumably by isomerization of an unobserved intermediate analogous to 36a.50

426

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 58

When treated with n-butyl lithium, the NHC-coordinated species [CpMn(CO)2(NHC)] (229; prepared as described in Scheme 43) underwent lithiation on the NHC backbone; this organolithium species (230) could be isolated, and subsequent reactivity allowed further modification of the NHC ligand (Scheme 59).235,236 Reaction of 230 with CO2 yielded the carboxylic acid-appended 231, which could be used to incorporate the CpMn(CO)2 moiety into heterometallic complexes upon exposure to a Cu(II) or Zn(II) precursor (affording 232).235 Alternatively, two equivalents of 230 underwent CuCl2-catalyzed oxidative coupling to form dimetallic 4,40 -bis(2H-imidazol-2-ylidene) complexes 233.236 A derivative with a fluorinated backbone (234) was also prepared from 230 upon exposure to N-fluorobenzenesulfonimide (NFSI), and a difluorinated derivative (235) was synthesized from 234 via subsequent lithiation followed by reaction with NFSI.237 These monometallic or dimetallic species (233, 234, and 235) reacted with TfOH to form imidazolium salts, which could be used to access the free NHCs.236,237

Cyclic and Non-Cyclic Pi Complexes of Manganese

427

Scheme 59

A common route to install Fischer-type carbene ligands on manganese is stepwise nucleophilic attack at a carbonyl C atom in [H,MeCpMn(CO)3] (using an organolithium reagent), followed by quenching by addition of an electrophile to the resulting anionic oxygen center (Scheme 60). In the examples provided in Scheme 60, quenching was most often done using [Et3O] [BF4], which installs an ethyl group, though [Cp2TiCl2] has also been used (which installs a terminal Cp2TiCl or a bridging Cp2Ti moiety). Substituents installed on the sp2 carbene carbon in these various examples include ferrocenyl (236, 237, 238, and 239),238 (
5-C5H4)M(CO)3 (240),239 thienothiophenyl (241),240 thiophenyl (242),241 furanyl (243 and 244),241 and bithiophenyl (245, 246, 247, and 248)240,242 moieties. Complexes with bis(carbene) ligands bridging between two H, Me CpMn(CO)2 fragments have been prepared either using a dilithiated reagent (where the initial nucleophilic attack occurs on two equivalents of [H,MeCpMn(CO)3]; 237 and 239 were prepared via this route)238 or in a stepwise fashion by reaction of [H,MeCpMn(CO)3] with a monolithiated reagent, followed by lithiation of the product, treatment with a 2nd equivalent of [H, Me CpMn(CO)3], and quenching (244, 246, and 247 were prepared via this route).241,242 Alternatively, heterometallic bis(carbene) complexes have been prepared by reaction of [H,MeCpMn(CO)3] with an organolithium reagent which contains a pendent metal-carbene unit, followed by quenching with [Et3O][BF4] (e.g. 248).242 The electrochemical behavior of some of these complexes has also been investigated; notably, ferrocenyl-substituted derivatives undergo oxidation of Mn(I) in preference to oxidation at iron.241

428

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 60

In some cases, the carbene-coordinated manganese center or a metal-carbene linkage in the carbene complexes [H,MeCpMn (CO)2{]C(OEt)R}] described in Scheme 60 was subjected to further reactivity (Scheme 61). For example, heating a homometallic derivative of 240 (i.e. where M ¼ Mn and R ¼ H) in the presence of free triphenylphosphine resulted in substitution of one carbonyl ligand on the carbene-coordinated metal atom to afford 249 (this reactivity did not require irradiation).243 In addition, the Mn/Cr derivative of complex 248 reacted (at the carbene attached to chromium) with 3-hexyne to yield monometallic carbene complex 250 via a regioselective Dötz reaction.242

Cyclic and Non-Cyclic Pi Complexes of Manganese

429

Scheme 61

Heterometallic Ir2Mn2 complexes [Mn2Ir2Cp2(m-CPh)(m3-CPh)(m-CO)3(CO)3] (251) and [Mn2Ir2Cp2{m-C(CO)Ph}(m3-CPh) (m-CO)3(CO)4] (252) have been prepared from the cationic carbyne [CpMn(CO)2(^CPh)]+, and feature bridging m2-carbonyl and m2- or m3-carbyne ligands (252 also contains a m2-carbene ligand) between Mn and Ir atoms (Scheme 62).244 The same manganese-containing precursor was found to react with Na2[Ru(CO)4] to yield the previously reported (via an alternative synthetic route) heterometallic complex [MnRu2Cp(m3-CPh)(m-H)(m-CO)2(CO)6]. The authors proposed that a possible source of the hydride ligand in this complex is a Na[HRu(CO)4] contaminant in the Na2[Ru(CO)4] reagent. It is also notable that the carbene complex [CpMn(CO)2{]C(NH2)Ph}] (253) was observed in significant amounts in this reaction mixture due to the presence of another contaminant (NaNH2) in the batch of Na2[Ru(CO)4] used (see Scheme 63 for the independent synthesis of 253).244

Scheme 62

430

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 63

Cationic carbyne complexes [H,MeCpMn(CO)2(^CR)]+ are also commonly used to access neutral monometallic manganese complexes with carbene ligands via nucleophilic attack at the sp carbon atom (Scheme 63). Reactions with NaNH2 or an alkynyl lithium reagent yielded the aminocarbene complex [CpMn(CO)2{]C(NH2)Ph}] (253)244 or the alkynyl-substituted carbene [MeCpMn(CO)2{]CR(C^CR0 )}] (254; reactions of this complex are illustrated in Scheme 66),48 respectively. In the synthesis of 254, a dimetallic byproduct where a bis(alkyne) ligand bridges between two MeCpMn(CO)2 fragments (44) was observed (presumably formed by dimerization of 254 triggered by an excess of alkynyl lithium reagent present in the reaction mixture, since 254 was shown to undergo electrocatalytic dimerization upon reduction by controlled potential electrolysis).48 Reactions of the same cationic carbyne precursors [CpMn(CO)2(^CR)]+ with alcohols and primary or secondary phosphines also proceeded via initial coordination of the nucleophile to the sp carbon atom, forming cationic carbene species, though these complexes were generally unstable and underwent spontaneous proton loss to afford neutral aryloxy-substituted carbene complexes [CpMn(CO)2{]CPh(OAr)}] (255; the IR spectrum of 255 is surprisingly complex due to interligand interactions)245 or phosphinocarbene complexes [CpMn(CO)2{]CR(PR0 2)}] (256),52,53 respectively (Scheme 63). For reactions of the primary or secondary phosphines, the cationic carbene intermediate [CpMn(CO)2{]CR(PR0 2H)}]+ (257) was observed spectroscopically, and isolated for PR0 2 ¼ P(NiPr2)2.52,53,246 In addition, the carbene ligand in 256 underwent spontaneous insertion into a metal-carbonyl bond to form phosphinoketene complexes [CpMn(CO){
3-P,C,C-R0 2PC(R)]C]O}] (39).52,53 In most cases (PR0 2 ¼ PPh2, P(NiPr2)2, and PMeMes), this decomposition occurred immediately upon synthesis of the carbene. However, derivatives of 256 with a PHMes substituent could be isolated (isomerization only occurred rapidly in THF).52,53 Reactions of both 256 and 39 have been explored to access new phosphorus-containing manganese complexes (see Schemes 64 and 65). In some cases, it was found that in reactions of [CpMn(CO)2(^CR)]+ with an excess of secondary phosphine, the cationic intermediate 257 underwent further reactivity to yield [CpMn(CO)2{k1-PR0 2C(H)(Me)(PR00 2H)}]+ (258), containing a k1-coordinated mono-protonated bis(phosphine) ligand, which itself underwent deprotonation by alumina or Et3N to form isolable neutral k1-bis(phosphine) complexes [CpMn(CO)2{k1-PR0 2C(H)(Me)(PR00 2)}] (259). Free bis(phosphines) could be isolated upon UV irradiation of 259.246

Cyclic and Non-Cyclic Pi Complexes of Manganese

431

Unlike reactions of [CpMn(CO)2(^CR)]+ with primary or secondary phosphines, for which the nature of the carbyne substituent did not significantly affect the nature of the products, varied products were observed upon exposure of different [CpMn(CO)2(^CR)]+ derivatives to tertiary bis(phosphines); Scheme 63. Reactions of aryl-substituted carbyne cations with dppm yielded metal-free zwitterionic cyclic semi-ylide cations,55,246 while reactions involving alkyl-substituted [CpMn(CO)2(^CR)]+ yielded cationic
3-a-phosphonioketene complexes [CpMn(CO){
2-C,C-
1-P-Ph2PCH2PPh2C(R)]C]O}]+ (40).55 In both cases, reactivity presumably proceeds via initial formation of an unobserved cationic carbene intermediate 260, which does not contain a PH substituent susceptible to proton loss (preventing formation of a neutral carbene species similar to 256).55 For chemistry yielding semi-ylide species, intramolecular coordination of the pendent phosphine moiety to the sp2 carbene carbon in 260 would afford intermediate 261 (observed for R ¼ Ph), followed by dissociation of the semi-ylide cation from the metal center.55,246 It is notable that reactions of [CpMn(CO)2(^CPh)]+ with asymmetric bis(phosphine)s yielded asymmetric semi-ylide cations.246 By contrast, for reactivity which yields phosphonioketene complexes 40, the putative carbene intermediate 260 would undergo insertion of the carbene ligand into a metal-carbonyl bond.55 Phosphinocarbene complexes [CpMn(CO)2{]CR(PHMes)}] (isolable derivatives of 256, which were synthesized as outlined in Scheme 63) have been shown to be useful precursors to access new manganese complexes with phosphorus-containing ligands (Scheme 64). For example, 256 was found to react with borane-dimethylsulfide via coordination of BH3 to the phosphine to afford [CpMn(CO)2(]CR{PHMes(BH3)})] (262), confirming the availability of the lone pair on phosphorus.53 Deprotonation of the same precursor by exposure to nBuLi yielded the anion [CpMn(CO)2{]CR(PMes)}]− (263). In the presence of even very weak acids, 263 underwent acid catalyzed rearrangement to form a chelating phosphaalkene complex with a single carbonyl co-ligand (264). This reactivity was proposed to proceed via undetected hydride-containing neutral k1-C(R)]PMes and k2-C(O)C(R)]PMes intermediates. The same complex (264) was formed by deprotonation of [CpMn(CO){
3-P,C,C-HMesPC(R)]C]O}] (39). Compound 264 was also formed directly from 256 by deprotonation with LiOtBu in THF; this reactivity probably proceeds via either 263 or 39 (256 isomerizes to 39 in THF; Scheme 63) as an intermediate. Compound 264 reacted with MeI or I2 to afford new
3-phosphinoketene complexes [CpMn(CO){
3-P,C,C-R0 2PC(Ph)]C]O}] (derivatives of 39 when R0 2P ¼ PMeMes or PIMes), though the iodo-containing derivative (which was observed spectroscopically) was unstable and isomerized rapidly to form the
1-phosphaalkene complex [CpMn(CO)2{
1-P(Mes)]C(I)Ph}] (265).52,53 Protonation of 264 yielded a neutral
1-phosphaalkene complex [CpMn(CO)2{P(Mes)(]CHR)}] (266). However, under some conditions, a small concentration of 39 was observed during this reaction, consistent with some reversibility of the aforementioned deprotonation of 39. Complex 266 was alternatively formed by isomerization of phosphinoketene complexes 39, which occurred faster in THF than in noncoordinating solvents, or via DBU-catalyzed isomerization of carbene complex 256.52,53

Scheme 64

432

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 65

Protonation of phosphinoketene complexes [CpMn(CO){
3-P,C,C-R0 2PC(Ph)]C]O}] (39 where R0 ¼ Ph or Cy; see Scheme 63 for synthesis) yielded
2-methylenephosphonium cations [CpMn(CO)2{
2-R0 2P]C(H)Ph}]+ (54; see Section 5.07.2.4.3 for structural details); Scheme 65.68 Compound 54 reacted cleanly with a variety of nucleophiles at the phosphaalkene carbon to form k1-phosphine complexes [CpMn(CO)2(PPh2{CHPh(OMe)})] (267, prepared using methanol), [CpMn(CO)2{PPh2(CHPhCl)}] (268, prepared using benzyltrimethylammonium chloride), [CpMn(CO)2(PPh2{CHPh(NHMes)})] (269, prepared using 2,4,6-trimethylaniline), [CpMn(CO)2(PPh2{CHPh(L)})]+ {270, prepared using PPh3 or SC(NMe2)2}, [CpMn(CO)2(PPh2{CHPh (N2C3H3R)})]+ (271, prepared using derivatives of imidazole) and [CpMn(CO)2(PPh2{CHPh(N2C3H3)})] (272, prepared using un-substituted imidazole); Scheme 65. This reactivity contrasts that of free methylenephosphonium salts, for which nucleophilic attack occurs at phosphorus. For reactions with imidazole, 272 reacted with a second equivalent of 54 to afford dimetallic [{CpMn(CO)2}2{m(PPh2CHPh)2C2N2H3}][BF4] (273), which features a cationic bis(phosphine) ligand bridging between two CpMn(CO)2 fragments.68 The cationic imidazolium-containing phosphine ligands in 271 and 273 could be liberated by photolysis under acidic conditions, and are of interest as precursors to bidentate or tridentate ligands containing an NHC and one or two adjacent phosphine donors.68,247 Alkynyl-substituted carbene complexes [MeCpMn(CO)2{]CR(C^CR0 )}] (254; prepared as shown in Scheme 63) were found to be useful precursors to access a variety of new manganese-containing species (Scheme 66). Reactions of 254 with either p-toluenethiol (in the presence of catalytic NEt3) or lithium p-toluenethiolate (followed by protonation) yielded
2-allene complexes [MeCpMn(CO)2(
2-{H(Tol)C]C]C(Ph)STol}] (30a) and [MeCpMn(CO)2(
2-{TolS(Tol)C]C]C(Ph)H}] (30b), which differ by allene regiochemistry and which double bond coordinates to the metal center.47 Similarly, reaction of cyclohexanone lithium enolate with 254 followed by protonation afforded the
2-allene complex [MeCpMn(CO)2{
2-(H{Tol}C]C]C{Ph} {CH(CH2)4C(O)})}] (31) as a single regioisomer.47 Furthermore, when exposed to water, 254 decomposed to a variety of

Cyclic and Non-Cyclic Pi Complexes of Manganese

433

Scheme 66

species including benzaldehyde and [MeCpMn(CO)3], potentially via [MeCpMn(CO)2{]CPh(OH)}] and/or [MeCpMn(CO)2 (
2-O]CHPh)] intermediates.48 Room temperature reactions of 254 with tertiary phosphines ultimately yielded two isomers of the zwitterionic s-allenylphosphonium complex [MeCpMn(CO)2{C(R)]C]C(PR00 3)(R0 )}] (274a,b) which differ in the position of the R and R0 substituents (Scheme 63). While both isomers were observed for reactions involving PPh2Me, reactions with PPh3 yielded 274a, the product of phosphine reactivity at the g position of the carbene ligand in 254, and reactions with PMe3 exclusively afforded 274b. Formation of 274b was proposed to proceed via initial phosphine coordination to the a carbene carbon to generate the unstable s-propargylphosphonium complex [MeCpMn(CO)2{C(R)(PMe3)C^CR0 }] (275; an intermediate was spectroscopically observed in the synthesis of 274b, though instability prevented definitive identification as 275), which could form 274b by a 1,3-[Mn] shift. Upon mild heating, derivatives of 274a-b with PPh2Me or PMe3 substituents isomerized to form one of two s-dihydrophospholium isomers [MeCpMn(CO)2{C]C(R0 )PR00 2CH2CH(R)}] (276a) or [MeCpMn(CO)2{C]C(R)PR00 2CH2CH (R0 )}] (276b).47,48 In contrast to reactions with tertiary phosphines, 254 reacted with secondary phosphines or LiPR2 (with subsequent protonation) to yield a mixture of
2-allene regioisomers [MeCpMn(CO)2{
2-(R)(H)C]C]C(R0 )(PR00 2)}] (32a) and [MeCpMn (CO)2{
2-(R00 2P)(R)C]C]C(R0 )(H)}] (32b), along with
4-vinylketene regioisomers [MeCpMn(CO){
4-(R00 2P)(R)C] C(H)C(R0 )]C]O}] (78a) and [MeCpMn(CO){
4-(R00 2P)(R0 )C]C(H)C(R)]C]O}] (78b); Scheme 66. Reactions involving HPR2 yielded both
4-vinylketene regioisomers (78a-b) and a single allene regioisomer (32b), while the opposite was observed for reactions involving LiPR2; i.e. 32a-b and 78b (apart from when NH+4 was used for acid treatment, when only one regioisomer of each was observed). Syntheses of 32b and 78a presumably proceeded by initial formation of [MeCpMn(CO)2{C(R)(PHR00 2) C^CR0 }] (analogous to 275). By contrast, 32a and 78b presumably results from 1,3-hydrogen migration from initially formed [MeCpMn(CO)2{C(R)]C]C(PHR00 2)(R0 )}] (analogous to 274a-b) to form a transient carbene complex, followed by CO insertion

434

Cyclic and Non-Cyclic Pi Complexes of Manganese

reactivity.47 Upon heating
2-allene isomers 32a-b in THF, rearrangement to the respective
1-phosphinoallene regioisomer [MeCpMn(CO)2(
1-{R00 2P(R0 )C]C]C(R)H}] (277a) or [MeCpMn(CO)2(
1-{R00 2P(R)C]C]C(R0 )H}] (277b) was observed (via intermediates, one of which was identified as a free allene; the same free allene was produced quantitatively upon photolysis of 32a,b). In addition, both 32b and 277a (when R ¼ Tol and R0 ¼ R00 ¼ Ph) reacted with [MeCpMn(CO)2(THF)] to form dimetallic [{MeCpMn(CO)2}2(m-
1-P-
2-C,C-{Ph2P(Tol)C]C]C(Ph)H}] (33a), where the allene ligand is
2-coordinated to one manganese center and the phosphine donor coordinates to the second manganese atom.47 Complex 33b (an isomer of 33a where the phenyl and tolyl groups on the allene moiety are switched) was also observed spectroscopically during the aforementioned rearrangement of 32b (where R00 ¼ Ph) to 277b.47 The vinylidene complex [CpMn(CO)2(]C]CHPh)] has also been investigated with respect to reactions with tertiary phosphines18 and phosphites,20 reacting via initial PR3 coordination to the a-vinylidene carbon atom to yield zwitterionic s-phosphoniostyryl complexes [CpMn(CO)2{C(PR3)(]CHPh)}] (278); Scheme 67.18,20 The derivatives of 278 derived from phosphites proved very susceptible to reactions with trace water, forming
2-E-phosphorylalkene complexes [Cp(CO)2Mn {
2-Ph(H)C]C(H)P(]O)(OR)2}] (19) overnight in organic solvents.20 By contrast, reactions of [CpMn(CO)2(]C]CHPh)] with secondary phosphine oxides (which exist in equilibrium with a tertiary phosphine containing a hydroxyl group, which is the reactive species) directly yielded
2-E-phosphorylalkene complexes 19. This reactivity was presumed to proceed via initial Lewis base coordination to the a carbene carbon to form unobserved 279, followed by g-proton abstraction from the POH moiety to form a manganese hydride and subsequent reductive elimination. When HP(O)Ph2 was used, 19 proved susceptible to decomposition in the presence of excess secondary phosphine oxide to afford, among other products, a diphosphine dioxide {Ph2P(O)CHPhCH2P (O)Ph2}. Compound 19 could alternatively be prepared in a stepwise fashion from the same vinylidene precursor by reaction with LiOPR2 to yield anionic intermediate 280, which afforded 19 upon acid treatment.19 Using LiSP(OEt)2, sulfur-containing derivatives of 19 and 280 were also prepared.19

Scheme 67

Exposure of the phosphine-substituted vinylidene precursor [CpMn(CO)(PPh3)(]C]CHPh)] to [Fe2(CO)9] allowed isolation of the benzylideneketene [(
4-C{CpMn(CO)(PPh3)}{CO}CHPh)Fe(CO)3] (281), in which iron can be considered to be
4-coordinated to a heterotrimethylenemethane ligand (Scheme 68).248 This reaction presumably involves carbonyl transfer from the Fe fragment to the alpha carbon on the vinylidene ligand on Mn.249

Scheme 68

Cyclic and Non-Cyclic Pi Complexes of Manganese

435

Reactions of tertiary phosphines with allenylidene precursors mirror those of vinylidene precursors; [CpMn(CO)2(]C]C] CRPh)] reacted with free phosphines (Scheme 69) to form zwitterionic a-phosphonioallenyl complexes [CpMn(CO)2{C(PR0 3) (]C]CRPh)}] (282a) and (when bis(diphenylphosphino)ethane was used as the free phosphine) [{CpMn(CO)2}2{m-C(]C] CPh2)(Ph2PCH2CH2PPh2)C(]C]CPh2)}] (282b).50 Reactions of 282a–b were discussed in Scheme 58.

Scheme 69

Recently, a newly observed tetrametallic byproduct, [(dppe)PdMn(m3-C]CHPh)PdMn(m-C]CHPh)(CO)4Cp2] (283), was isolated from the previously reported reaction of [CpMn(CO)2Pd(m-C]CHPh)(PPh3)2] with free dppe (the major product was that of simple phosphine substitution); Scheme 70. Compound 283 is related to the major product by loss of one equivalent of dppe, and features carbonyl ligands bridging between Mn and Pd centers, as well as a m-vinylidene ligand that is
2-coordinated to an adjacent Pd atom.250

Scheme 70

5.07.3.1.1.2.4 Derivatives with Group 13-based Ligands Manganese(I) cyclopentadienyl dicarbonyl hydride anions [H,MeCpMnH(CO)2]− were found to react with boron or aluminium dihalides to yield the s-hydroborane complexes [CpMn(CO)2{s-HB(X)R0 }] (284; X ¼ Br and R0 ¼ Fc, 285; X ¼ Cl and R0 ¼ tBu or Mes)251 or the s-hydroalane complex [MeCp(CO)2(s-HAl{Cl}{(NiPr)2CPh})] (286),197,252 respectively (Scheme 71). The hydridic nature of the bridging hydrogen atoms in 284, 285, and 286 are reflected by the low frequency 1H NMR signals ranging from −13.9 to −16.8 ppm. The M–E (E ¼ B, Al) distances of 2.11(1) A˚ (for the crystallographically characterized R0 ¼ tBu analogue of 285) and 2.446(1) A˚ (for 286) are also consistent with s-hydroborane and s-hydroalane complexes, respectively.

Scheme 71

436

Cyclic and Non-Cyclic Pi Complexes of Manganese

Borylene (and metallaborylene) moieties, which are highly unstable in the absence of transition metal coordination, have been stabilized in a series of manganese complexes. In particular, the “CpMn(CO)2” fragment has shown utility in stabilizing these ligands. Two classes of dimetallic borylene complexes have been prepared from the anionic hydride complex [CpMn(CO)2H]−. First, reactions with transition metal complexes containing a dihaloboryl ligand yielded the metallaborylene complexes [CpMn(CO)2(]B–[M])] (287; [M] ¼ Mn(CO)5, Mn(CO)4(PCy3), Fe(CO)2Cp ); the derivative containing a Mn(CO)5 fragment was unstable and could not be isolated (Scheme 72). This reactivity proceeded by initial formation of a spectroscopically observed intermediate [CpMn(CO)2{H–B(X)([M])}] with a s-metallahydroborane ligand (288; [M] ¼ Mn(CO)5, Mn(CO)4(PCy3), Fe(CO)2Cp and X ¼ Cl, Br); conversion to 287 occurred via HX (X ¼ Cl, Br) elimination over several hours at room temperature.253 Second, exposure of the hydride precursor [CpMn(CO)2H]− to Bt2Bu2Cl2 yielded the borylene bridged complex [{CpMn(CO)2}2(mBtBu)] (289) as shown in Scheme 72 (reactivity of this complex is discussed in Scheme 75). While the synthesis of 289 reflects that of the previously reported MeCp derivative, 289 was the first alkylborylene complex to be crystallographically characterized.254 Subsequently, the nature of the Mndborylene bonding in 289 was investigated by computational analysis of the electron density and Electron Localization Function, and compared to bonding in dimetallic complexes with bridging BNMe2, methylene, or vinylidene ligands.255,256

Scheme 72

Reduction of dimetallic borylene complexes [{H,MeCpMn(CO)2)}2(m-BCl)], which contain a terminal BCl substituent, permitted isolation of C2-symmetric [{H,MeCpMn(CO)2}2(m-B)]− (290); Scheme 73.257,258 The MeCp derivative of 290 was crystallographically characterized; the Mn]B]Mn angle is nearly linear (176.1 ) and the MndB distances are rather short (1.8809(14)–1.8812 (14) A˚ ), indicative of two “full” Mn]B borylene bonds to the same boron atom.257 Compound 290 proved to be a useful access point to a wide array of multimetallic borylene-containing species. For example, 290 (R ¼ Me) reacted with MeI to yield

Scheme 73

Cyclic and Non-Cyclic Pi Complexes of Manganese

437

borylene-bridged [{MeCpMn(CO)2}2(m-BMe)] (291), the methyl derivative of tBu-terminated 289. This reactivity highlights the nucleophilicity of the bridging boron atom in 290.257 By contrast, complex 290 reacted with germyl, stannyl, or lead(IV) chlorides to yield neutral [{CpMn(CO)2}{CpMn(CO)2(ER3)}(m-B)] (292; ER3 ¼ GeMe3, SnMe3, SnPh3, PbPh3), formed from nucleophilic attack of manganese on the group 14 element. Relative to 290, the bond between boron and the ER3-bound manganese atom is significantly weakened (notably, the borylene Mn]B bond to the other metal center is not elongated).258 Reactions of 290 with coinage metal NHC or phosphine chlorides proceeded via nucleophilic attack by the boron center, yielding symmetric (293) or asymmetric (294) neutral trimetalloboride complexes [{H,MeCpMn(CO)2}2B(ML)] (M ¼ Cu, Ag, Au and L ¼ ITol, CAAC, PPh3, PCy3); Scheme 73.258–260 The symmetric or asymmetric nature of these structures was attributed to a competition between the preferred coordination number for the coinage metal and the ability of the cationic “ML+” fragment to be involved in backbonding.260 Computational models of the copper-containing derivatives of 293 where L ¼ PCy3 or a simplified NHC have been utilized to probe the nature of the B–Cu interaction by ELF.261 Furthermore, 290 reacted with [PhHgCl] to yield an asymmetric trimetallic structure [{CpMn(CO)2}2B(HgPh)] (295) analogous to 294.262 [Pt(PCy3)2] also reacted with 290 to yield a trimetallic boride species, [{CpMn(CO)(m-Mn,Pt-CO)}2B{Pt(PCy3)}]− (296; Scheme 73). Compound 296 was also described as a transition metal-base-stabilized metalloborylene.263 Structurally, 296 differs from 293 by the bridging nature of two carbonyl ligands. Exposure of this T-shaped complex to a single equivalent of [(ITol)MCl] {ITol ¼ N,N0 -bis(4-methylphenyl)imidazole-2-ylidene, M ¼ Cu or Au} yielded borido complexes [{CpMn(CO)2}2B{Pt(PCy3)} {M(ITol)}] (297; M ¼ Cu, Au), where boron and the four transition metals lie in a nearly perfect plane and a significant (group 11 metal)–Mn interaction was observed.264,265 Complex 297 was described by the authors as a “B(Mn)2(Au or Cu)” trimetalloborane with the Pt(PCy3) fragment acting as a Lewis base; in the dominant resonance structure, both Mn atoms were proposed to be divalent, with some contribution from a mixed valence Mn(III)/Mn(I) resonance structure.264 Interestingly, exposure of 296 to the isoelectronic Cu precursor [(Cy3P)CuCl] yielded a tetrametallic cluster with a Cu–Pt interaction [{CpMn(CO)2}2B{Pt(PCy3)} {Cu(PCy3)}] (298).265 Complex 298 was also prepared from 290 via an alternative route, involving initial reactivity with [ClCu(SMe2)] to form the hexametallic diboride cluster [{(CpMn{CO}2)2B}2Cu2] (299; each Mn atom bridges between Cu and B), which afforded 298 upon exposure to [Pt(PCy3)2].265 The gold-containing derivative of diboride cluster 299 could also be isolated from the reaction of 290 with [ClAu(SMe2)].265 Synthesis of hexametallic 299 presumably proceeded via initial formation of an (unobserved) trimetallic complex analogous to 294, followed by SMe2 dissociation and dimerization.265 The bridging borylene complexes [{MeCpMn(CO)2}2(m-BR)] (with tBu254 or Cl266 substituents on boron) were found to react with [Pd(PCy3)2] to yield the first reported heterometallic m3-borylene trimetallic complexes, [{MeCpMn(m2-CO)2} {Pd(PCy3)}2{m3-BR}] (300; Scheme 74). This reactivity (which also yielded previously reported [MeCpMn(CO)2(PCy3)] as a byproduct) was proposed to proceed via an unobserved terminal borylene intermediate [CpMn(CO)2(]BR)], presumably formed by spontaneous MndMn bond cleavage.254,266

Scheme 74

In the presence of excess phosphine, the dimetallic bridging borylene complex [{CpMn(CO)2}2(m-BtBu)] (289; prepared as illustrated in Scheme 72) underwent MndMn bond cleavage to yield the terminal borylene complex [CpMn(CO)2(]BtBu)] (301) and [CpMn(CO)2(PCy3)]; Scheme 75.267 Complex 301 was the first terminal alkylborylene complex to be reported, and was sufficiently stable to allow for extensive characterization and investigation of its reactivity. DFT calculations indicated that the M]B bond involves strong metal-to-boron p backbonding. The MndB distance of 1.809(9) A˚ in the crystal structure of 301 is the shortest yet observed (and is 22 pm shorter than that in dimetallic 289), and the MndBdC angle is nearly linear (174.3(7) ).267 Complex 301 has been utilized to access a rich tapestry of new multi- and mono-metallic manganese-containing species, as well as metal-free compounds.

438

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 75

Cyclic and Non-Cyclic Pi Complexes of Manganese

439

Monometallic [CpMn(CO)2(]BtBu)] (301) has been used to access multimetallic complexes via reactions with various transition metal precursors (Scheme 75). For example, 301 reacted with cymantrene to re-form the dimanganese borylene complex 289 (used as a precursor in the synthesis of 301; vide supra).268 Compound 301 also reacted with other mid-transition metal cyclopentadienyl carbonyl complexes to yield heterobimetallic alkylborylene complexes [{CpMn(CO)2}{m-B(tBu)}{H,MeCpCo (CO)}] (302) and [{CpMn(CO)2}{m-B(tBu)}{CpCr(CO)2}] (303), in which all carbonyl ligands are terminal.268 The same monometallic precursor reacted with electron rich zero-valent group 10 precursors [M(PCy3)2] (M ¼ Pd, Pt) to form dimetallic structures with bridging borylene and carbonyl ligands (304).267 This reactivity differs from the reaction of [Pd(PCy3)2] with the Me Cp derivative of 301 (generated spontaneously from the MeCp derivative of 289), which afforded trimetallic 300 (see Scheme 74). Compound 301 reacted with Au(I) precursors [LAuCl] (L ¼ PR3, NHC) to yield complexes which could be described as involving s-coordination of a MndB bond to Au(I); [CpMn(CO)2{m-B(tBu)Cl}Au(L)] (305: L ¼ PPh3, PCy3, ITol).269 The derivative of 305 where L ¼ PPh3 could be further modified to replace the BCl substituent with Ph, C^CPh, or SCN moieties {affording [CpMn(CO)2{m-B(tBu)R}Au(PPh3)] (306); R ¼ Ph, C^CPh, SCN}.269,270 Compound 306 (where R ¼ C^CPh) was alternatively prepared directly from 301 by transition metal-mediated BdC coupling with a gold acetylide complex.270 Notably, exposure of the derivative of 305 where L ¼ PCy3 to [Pt(PCy3)2] afforded the previously discussed dimetallic MnPt complex 304.270 Furthermore, 301 and 305 (where L ¼ PCy3) each reacted with the extremely bulky supermesityl (Mes ¼ 2,4,6-tri-tert-butylphenyl) isonitrile to afford a complex with two carbonyl ligands bridging between the metal and borylene fragments; [{CpMn(CNMes )} (m-CO)2{B(tBu)(CNMes )}] (307). Conversion of 301 to 307 could be reversed in the presence of B(C6F5)3, which abstracts both isonitrile ligands.270,271 Compound 305 reacted with Na[BArX4] (ArX ¼ 3,5-C6H3(CF3)2, 3,5-C6H3Cl2) to yield [{CpMn(CO)2}2Au{m-B(tBu)}2][BArX4] (308), which can be described as two borylene complexes side-on coordinated to a monovalent gold cation (a boron analogue of Stone’s alkylidyne-bridged multinuclear complexes); Scheme 75.270 This reactivity presumably proceeded via halide abstraction to yield [{CpMn(CO)2}{m-B(tBu)}{Au(PPh3)}][BArX4] (309; not observed), from which the observed bis(borylene) product 308 was formed via [Au(PPh3)2][BArX4] elimination. The silver-containing derivative of 308 was prepared by addition of a half equivalent of Ag[BArCl 4 ] to the monometallic borylene precursor 301, presumably via simple coordination of the silver ion to the Mn]B bonds. The same product was also formed, though in low yield, by the reaction of 301 with [(Ph3P)AgCl] and Na[BArCl 4 ] (this reaction presumably proceeded via unobserved intermediates similar to 305 and 309). However, a similar reaction with [(Ph3P)CuCl] in place of the silver precursor instead yielded (in low yield) the pentametallic complex [{CpMnH(CO)2}(Cu{m-BtBu}{CpMn (CO)2})2][BArCl 4 ] (310), where two copper atoms each interact with a Mn]B borylene bond on a separate Mn fragment and a [CpMn(CO)2H] fragment bridges between the two copper centers (the source of the MnH atom is unclear). Interestingly, when this reaction was carried out in the presence of cymantrene, [{CpMn(CO)2}2{m-B(tBu)}2Cu][BArX4] (the Cu-containing derivative of 308) was obtained. Unfortunately, both Cu complexes (308 where M ¼ Cu and 310) were highly unstable at room temperature so could not be isolated, and spectroscopic characterization was limited. Complexes 309 and 310 were the first structurally characterized examples of bis(metal-borylene)-coordinated coinage metal complexes.270 Furthermore, interesting reactivity has been reported involving the terminal borylene complex 301 and unsaturated organic reagents (Scheme 75). For example, the Mn]B bond in 301 underwent 2 +2 cycloaddition with ketones or dicyclohexylcarbodiimide to yield boryl complexes containing a 4-membered MndBdEdC (E ¼ O or N) metallacycle {[CpMn(CO)2{k2-C,B-R2COB(tBu)}] (311) or [CpMn(CO)2{k2-C,B-(CyN])CN(Cy)B(tBu)}] (312)}. The oxygen-containing products (311) decomposed at room temperature to form tri-tert-butylboroxine and the carbene complexes [CpMn(CO)2 (]CR2)] (313; some derivatives had previously been synthesized by other routes prior to 2005); the overall 301 !311! 313 transformation represents the first example of concerted metathesis facilitated by a borylene complex. For reactions involving more electron-releasing ketones {O]CR2 where CR2 ¼ adamantylidene, cyclo-[C3Ph2], or C(C6H5-4-NEt2)2}, intermediate 311 was not observed (311 was observed for R ¼ Ph, 3,5-C6H3(CF3)2, or p-Tol).272,273 While the product of dicyclohexylcarbodiimide reactivity (312) was stable at room temperature, heating to 65  C resulted in formation of a carbene isomer [CpMn(CO)2{]C(m-NCy)2 B(tBu)}] (314). In addition, 301 reacted with S]PPh3 to afford the short-lived complex [CpMn(CO)2(
2-SBtBu)] (315; observed spectroscopically but not isolated), which rapidly decomposed (in the presence of PPh3 formed as a byproduct in the synthesis of 315) to (tBuBS)3 and [CpMn(CO)2(PPh3)].273 Furthermore, upon exposure to P-chloro(supermesityl)iminophosphane, 301 underwent an unprecedented borylene-to-phosphinidene transformation, yielding [CpMn(CO)2{]PN(Mes )]B(Cl)(tBu)}] (316). Phosphinidene complex 316 is isolobal to a terminal borylene complex, and was the first structurally characterized phosphinidene complex with carbonyl co-ligands.274 Finally, reactions of borylene complex 301 with a variety of Lewis bases were investigated (Scheme 75). Compound 301 reacted with two equivalents of tert-butyl isonitrile to form the
2-alkylideneborane complex [CpMn(CO)2{
2-(CNtBu)(tBu)BC(]NtBu)}] (55), the product of apparent insertion reactivity followed by coordination of the second isonitrile equivalent to boron.69 This is in contrast to the reaction of 301 with the extremely bulky supermesityl isonitrile, which resulted in coordination of one equivalent of CNMes to boron, and one equivalent to the metal center (307; vide supra). By comparison, pyridine derivatives or 1,3-dimethylimidazol-2-ylidene (IMe) coordinated to the boron center of 301, yielding [CpMn(CO)2{]BtBu(L)}] (317) where the Mn]B borylene double bond has been significantly elongated relative to 301, and the Mn]BdtBu linkage is no longer linear.69,275 When 301 was exposed to a sterically demanding NHC (1,3-di-tert-butylimidazol-2-ylidene; ItBu), a mixture of [CpMn(CO)2(ItBu)] and an NHC-stabilized borane was formed; this reactivity was proposed to proceed via initial formation of an unobserved intermediate analogous to 307 ([CpMn(ItBu)(m-CO)2{BtBu(ItBu)}]; 318), followed by CdH activation by the boron center.276

440

Cyclic and Non-Cyclic Pi Complexes of Manganese

Lewis base stabilized borylene complexes 317 themselves underwent unusual reactivity (Scheme 75). For example, in the DMAP and IMe derivatives of 317, the butyl substituent on boron underwent a thermally-induced tert-butyl to iso-butyl rearrangement, forming a new set of Lewis base-coordinated borylene adducts; [CpMn(CO)2{]BiBu(L)}] (319). However, under an atmosphere of carbon monoxide, 317 instead reacted to form [CpMn(CO)(m-CO)2{BtBu(L)}] (320).69,275 A PMe3 derivative of 320 could be formed directly from 301 by exposure to PMe3 under an atmosphere of carbon monoxide (notably, in the absence of CO, no reaction was observed between 301 and PMe3).277 In addition, the DMAP adduct of 320 isomerized via insertion of the borylene moiety into a cyclopentadienyl CdH bond, forming [RCpMn(CO)3] (321: RCp ¼ C5H4{B(H)(tBu)(DMAP)}).277 The NHC-stabilized derivative of 317 underwent insertion reactivity with isonitriles, forming an equilibrium mixture of [CpMn(CO)2{
1-CN(R)B(tBu)(IMe)}] (322) and [CpMn(CO)2{
2-(IMe)(tBu)BC(]NR)}] (56) isomers (for tert-butyl isonitrile, 56 was not observed).69,275 It is notable that 56 is isostructural to 55, the product of isonitrile insertion into the base-free borylene 301 (vide supra). The other NHC-stabilized borylene adducts 319 also reacted with isonitriles; sequential exposure to supermesityl isonitrile and free NHC afforded the manganese-free compound Mes N]C]B(IMe)(iBu), potentially via an intermediate analogous to 56.69 Exposure of Lewis base stabilized borylene complexes 317 to a single equivalent of elemental chalcogen yielded [CpMn(CO)2{k1-E]B(tBu)(IMe)}] (E ¼ S, Se, Te; 323), the product of insertion chemistry involving the Mn]B bond; Scheme 75.278 The Te-containing derivative of 323 was the first complex in which a B]Te double bond has been observed. Exposure of 323 to a second equivalent of Te (or the reaction of 317 with an excess of Te) yielded the first reported example of a boraditellurirane complex, [CpMn(CO)2{k1-TeTeB(tBu)(IMe)}] (324). Interestingly, addition of PMe3 to 324 abstracted one of the Te atoms, regenerating 323 with concomitant formation of Te]PMe3. By contrast, the S or Se derivatives of 323 formed Mn-free chalcogen analogues of dioxiranes upon addition of a second equivalent of chalcogen (the Se derivative was the first of its kind to be reported), presumably via decomposition of initially formed 324.279 Furthermore, a series of unsymmetrical B2E2 (E ¼ S, Se, Te) heterocycles have been prepared by i) the reaction of the boraditellurirane complex 324 with the chromium borylene complex [(OC)5Cr {]B(C6H3Mes2)}], or ii) the reactions of free boradichalcogeniranes (E ¼ S, Se) with the manganese borylene complex 301.280 5.07.3.1.1.3 Oxidation of manganese(I) cyclopentadienyl/CO complexes Oxidation of methanol-containing [CpMn(CO)2(CH3OH)] (325) or 2-aminoanthracene-containing [Cp Mn(CO)2(H2NC14H9)] (326) afforded radical complexes 327191 and 328,168 respectively, by H atom abstraction using O2, H2O2, or (for the synthesis of 327 only) dpph; Scheme 76. Both precursors 325 and 326 were prepared by UV irradiation of the parent tricarbonyl species in the presence of free alcohol or amine (as described in Scheme 43 for reactions stemming from [CpMn(CO)3]). Radicals 327 and 328 (low spin Mn(II)) were persistent, with lifetimes of 10–20 min (for 327) or a week (for 328). In both complexes, the unpaired electron is primarily localized on the metal (similar to the situation in the previously reported281 NH(p-tolyl) analogue).

Scheme 76

Despite the fact that cationic derivatives of cymantrene-based species have been known for decades, only recently (in 2008) has the 17 electron singly oxidized “parent” cymantrene cation [CpMn(CO)3]+ been prepared in a manner that allowed it to persist long enough to be spectroscopically characterized (by electrochemical generation using [NBu4][B(C6F5)4] as a particularly unreactive and weakly-coordinating electrolyte); Scheme 77. While attempts to isolate [CpMn(CO)3]+ were unsuccessful, derivatives with C5Me5 or C5H4(NH2) cyclopentadienyl ligands (329 or 330, respectively) were generated chemically using [CpRe(CO)3][B(C6F5)4] as the oxidant; these complexes were characterized spectroscopically and crystallographically (Scheme 77). Investigation of [CpMn(CO)3]+, 329, and 330 suggested that the SOMOs of these radical cations have 40%–50% Mn d-orbital character.282 In addition, oxidation of [MeCpMn(CO)3] confined within carbon nanotubes has allowed direct observation of the one-electron oxidized cation without reaction with the solvent (MeCN), which occurred in the absence of this confinement; the nanotubes were considered to act as a nano-electrode and a nano-reactor in this reaction.283 17 electron [CpMn(CO)2{C(PR3)(]CHPh)}]+ cations (331) have also been prepared by one-electron oxidation of neutral precursors using, for example, ferrocenium hexafluorophosphate,284,285 and have been investigated as catalysts for radical polymerization of methyl methacrylate and styrene (Scheme 77; bottom left).284 The same oxidizing agent has been used to prepare a 17 electron cationic cymantrene derivative with an NHC ligand; 332 (Scheme 77; bottom right).286

Cyclic and Non-Cyclic Pi Complexes of Manganese

441

Scheme 77

5.07.3.1.1.4 Miscellaneous chemistry of cymantrene and its derivatives Cymantrene itself, despite being one of the first isolated organomanganese complexes, continues to be the subject of significant experimental and computational investigation. Of note, an Inorganic Syntheses publication was recently reported for the preparation of cymantrene, as well as the pentamethylcyclopentadienyl derivative [Cp Mn(CO)3], using [BrMn(CO)3(py)2] and an alkali metal cyclopentadienyl salt.287 Furthermore, it has been investigated as a catalyst for photoconversion of thiols into disulfides and dihydrogen,216,288 dehydrocoupling of secondary and primary aminedborane adducts,289 silane reduction of DMF,290 photochemical water splitting,222 and photochemical arene CdH arylation.291 Cymantrene has also been used to incorporate Mn into heterogenous heterometallic Rh/Fe/Mn catalysts which displayed activity towards synthesis gas conversion,292 and into MnO2 in supercritical carbon dioxide.293 Recently reported experimental analyses of cymantrene have included ultrahigh-field solid state 55 Mn NMR spectroscopy,294 analysis of radicals formed upon pyrolysis,295 analysis of the sublimation energy (in combination with molecular dynamics simulations),296 determination of 2p orbital population in the carbonyl ligands by measuring NMR relaxation times,297 hydrothermal Raman microscopy,298 determination of rotational diffusion using narrow-band IR pump broad-band IR probe spectroscopy,299 2D IR spectroscopy (including an investigation into the influence of nanoconfinement in the cavity of b-cyclodextrin on the spectral diffusion dynamics),300,301 and adsorption on silica gel.302 Given its widespread use, some effort has been placed towards the development of analytical methods for the accurate and sensitive detection of cymantrene.303–306 Many computational studies have also been conducted on cymantrene, either to gain further understanding of its chemical properties, or to demonstrate the utility of new computational methods. These include calculations related to the quantum chemistry of carbonyl group photodissociation,307,308 the application of optimal control theory to ultrafast non-resonant multiphoton transitions,309 assigning photoelectron spectra on the basis of Kohn-Sham orbital energies,310 ionization without carbonyl dissociation (including analysis of temporary anion states),311,312 applications of the theory of ultrafast non-resonant multiphoton transitions,313 QTAIM analysis,314,315 determination of electric field gradient,316 determination of AM1 parameters,317 investigation of host-guest interactions with carbon nanotubes,318 and an analysis of core-electron binding energies (using new basis sets).319 [MeCpMn(CO)3] (MMT) continues to be investigated for catalytic activity towards various reactions including conversion of isocyanates to carbodiimides.320 Furthermore, MMT has been used in the fabrication of polyimide films in combination with nanoTiO2, N,N-dimethylacetamide, 4,40 -oxydianiline, and pyromellitic dianhydride.321 In addition to its synthetic uses, MMT is a commonly used precursor for chemical vapour deposition (CVD) of manganese-containing films such as MnOx,322 Zn1323–325 Cd1-xMnxTe,326 MnAs,327–329 MnP,330,331 In1-xMnxSb,332 and elemental manganese (though in the latter case, the xMnOx, deposited film often underwent further reactivity with the substrate to form a new manganese-containing material).333–338 CVD using MMT has also been used to introduce manganese as a dopant into other films such as InAs,339 ZnO,340 and ZnSe341 (the pentamethylated derivative [Cp Mn(CO)3] has also been investigated for use in the CVD of manganese-containing films).122 MMT has also been used as the manganese-containing precursor in deposition of manganese-containing materials by other methods, such as X-ray beam induced deposition.342 In addition, adsorption of this complex on magnetic composite sorbents based on hyper-crosslinked polystyrene impregnated with magnetic nanoparticles has been investigated.343

442

Cyclic and Non-Cyclic Pi Complexes of Manganese

MMT has also been, and in some locations still is, used as an additive in automotive fuels. Because of this, reports continue to be made regarding the effect of MMT on soot suppression in fuels,344 electrochemistry in kerosine,345 volatility of gasoline,346 vehicle emissions and combustion,347–350 octane number of gasoline,351 and anti-knock efficiency when present in high-octane fuels,352 as well as the environmental fate of manganese (or in some cases intact MMT)306,353–362 and health effects (such as increased Mn concentration in blood, and associated toxicity studies)356,359,363–373 caused by MMT use in gasoline. Furthermore, effort has been placed to develop sensitive and accurate analytical techniques for the detection of MMT in a variety of media (often, MMT is one of a number of analytes used to show the effectiveness of a specific analytical technique).303,305,306,374–381 In addition, the poisoning of a TiO2 based lambda oxygen sensor by MMT has been investigated.382 Recently, an X-ray crystal structure of MMT (which is liquid at room temperature) has been obtained, and the bonding in the structure was analyzed using QTAIM.383 Derivatives of [H,MeCpMn(CO)2L] (often hypothetical ones) have been studied computationally to probe the nature of manganese–ligand interactions. Examples include complexes where the neutral L ligand is an alkane,384,385 alkene,40 s-hydrosilane,386–390 acetonitrile,187 amine,191 thiol,191 s-H–PR2,391 PF3,392 s-hydroborane,393 borane–Lewis base adduct,394 borylene,395 B2H4,396 and tetracyanoethylene (in the latter complex, metal-to-ligand charge transfer was investigated).397 [CpMn(CO)(NHC)] complexes featuring agostic interactions with a CdH bond of the NHC ligand have also been computationally investigated.186 Calculations have also been used to study the relative energies and electronic structures of different rotamers for the carbene complex [CpMn(CO)2{]CMe(OMe)}], demonstrating that intramolecular interactions between the carbene and carbonyl ligands affect the relative energies of the rotamers.398 Previously reported cymantrene derivatives with a non-carbonyl co-ligand have also been the subject of new experimental reports (in some cases with a computational component) regarding the nature of these complexes. These include determining manganesedchalcocarbonyl bond strengths in [CpMn(CO)2(CE)] (E ¼ S, Se) using threshold photoelectron photoion coincidence spectroscopy,399 analysis of bonding in [MeCp(CO)2(s-hydrosilane)] complexes using a combination of calculations, X-ray crystallography, charge density analysis, and T1 NMR measurements,400,401 as well as DSC and TGA analysis of [CpMn(CO)2{SnCl2(THF)}].402 Interaction of halides with [CpMn(CO)2L] complexes where L contains a terminal OH or NH substituent (within a 3- or 4-hydroxypyridine, imidazole, or 3,5-dimethylpyrazole ligand) has been shown to have a marked effect on the carbonyl stretching frequencies (a shift of up to 12 cm−1); Scheme 78.178 In addition, new X-ray crystal structures for various manganese cyclopentadienyl carbonyl complexes with unsaturated ligands have recently been reported (Fig. 16).398,403,404

Scheme 78

Fig. 16 Previously reported manganese cyclopentadienyl carbonyl complexes for which an X-ray crystal structure has recently been published.

Various reactions of previously reported cymantrene derivatives with a non-carbonyl co-ligand have also been reported. For example, Mn2P2O7-forming pyrolysis of a dimetallic species with two methylcyclopentadienyl dicarbonyl manganese(I) moieties bridged by an unusual bis(phosphine) ligand was studied (Scheme 79).405 In addition, previously reported phosphine or NHC

Cyclic and Non-Cyclic Pi Complexes of Manganese

443

substituted derivatives [CpMn(CO)2L] have been investigated as catalysts for hydrosilylation of aldehydes and ketones,406 and previously reported cymantrene derivatives with vinylidene or alkene co-ligands have been investigated as catalysts for the polymerization of methyl methacrylate and styrene.407 Derivatives with vinylidene or allenylidene co-ligands have also shown activity for catalytic proton reduction.408 A pair of cationic triple-decker [{(
5-C5Me4R)Fe}2(m-
5:
5-C5Me4H)] derivatives (where R ¼ H or Me) have been prepared via the reactions of [(
5-C5Me4R)(
5-C5HMe4)Fe] with [{(
5-C5Me4R)Fe}(m
5:
5-C5HMe4){Mn(CO)3}]+ (R ¼ H or Me), in which a Mn(CO)3 fragment is bound to the outer face of a ferrocene derivative (Scheme 79). These reactions were proposed to proceed via initial decomposition of the manganese-containing precursor to form previously reported [(
5-C5HMe4)Mn(CO)3] (observed by IR spectroscopy) and [(
5-C5Me4R)Fe]+ (R ¼ H, Me), the latter of which coordinates to the outer face of the tetramethylcyclopentadienyl ring in [(
5-C5Me4R)(
5-C5HMe4)Fe] (R ¼ H or Me).409,410

Scheme 79

5.07.3.1.2

Derivatives of cymantrene (II) synthesis and reactivity of complexes containing the “(C5H5-xMex)Mn(NO)” fragment

Cationic precursors [H,MeCpMn(CO)2(NO)]+ have been used to access a series of manganese cyclopentadienyl complexes with nitrosyl (and usually carbonyl) co-ligands; Scheme 80. For example, carbonyl substitution from [CpMn(CO)2(NO)]+ by a tellurophenyl ligand on a neutral iron cyclopentadienyl complex yielded cationic heterometallic Fe/Mn complexes 333, where the tellurophenyl ligand bridges between the two transition metal centers.411 In addition, [MeCpMn(CO)2(NO)]+ has been used to access the heterometallic carbonyl-bridged complex [{MeCpMn(NO)}(m-CO)2{CpRu(CO)}] (334) via reaction with [CpRu(CO)2]−.412

444

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 80

Manganese nitrosyl complexes with stannyl or stannylene co-ligands were accessed from [CpMn(CO)2(NO)][SnCl3] (335), which was formed from the reaction of [CpMn(CO)2(NO)]Cl with SnCl2; Scheme 80.413 Some of this reactivity proceeded via initial formation of previously reported [{CpMn(CO)}{CpMn(NO)}(m-CO)(m-NO)] by an alternative route involving NaBH4 reduction of [CpMn(CO)2(NO)][SnCl3].414 This dimanganese complex reacted with one equivalent of SnCl2 to yield [{CpMn(CO)(NO)}2(m-SnCl2)] (336) or with excess SnCl2 to afford [{CpMn(SnCl3)}{CpMn(NO)}(m-NO)2] (337).414 In addition, [CpMn(CO)2(NO)][SnCl3] (335) decomposed upon mild heating to form neutral [CpMn(CO)(NO)(SnCl3)] (338), the product of SnCl−3 substitution of a carbonyl ligand.415 The chloride substituents on the stannyl group in 338 were replaced using salt metathesis with thiolate,416 selenolate,416 aryl,417 or alkynyl415 moieties to afford 339, 340, 341, or 342, respectively. The thiolate derivative [CpMn(CO)(NO){Sn(SPh)3}] (339) was alternatively prepared directly from exposure of the [CpMn(CO)2(NO)] [SnCl3] salt (335) to NaSPh, which may proceed by initial formation of a [Sn(SPh)3]− anion which then displaces a carbonyl ligand.413 The thiolate and alkyne fragments in 339 and 342 can coordinate to other transition metal centers; the dimetallic MnW complex 343 was formed (from 339) by “thioether” coordination to tungsten,416 while the trimetallic MnMo2 complex 344 was formed (from 342) by coordination of an alkyne to two molybdenum centers.418 Exposure of 338 to an anionic transition metal complex yielded a family of dimetallic species with a stannylene group bridging between two transition metal centers

Cyclic and Non-Cyclic Pi Complexes of Manganese

445

([{CpMn(CO)(NO)}(m-SnCl2){M}]; {M} ¼ Co(CO)4, Mn(CO)5, MeCpMo(CO)3, {
5-C5H3(tBu)2}Mo(CO)3: 345). In the reaction with [{
5-C5H3(tBu)2}Mo(CO)3]−, the monometallic molybdenum stannyl complex [{
5-C5H3(tBu)2}Mo(CO)3(SnCl3)] was observed as a byproduct.419 In addition to the chemistry described in Scheme 80 for reactions of [H,MeCpMn(CO)2(NO)]+, the methylcyclopentadienylcontaining derivative has also been utilized in the preparation of heterometallic complexes with bridging nitride groups (Scheme 81). For example, the reaction with the cyclopentadienyl tricarbonyl tungsten anion [CpW(CO)3]− afforded (as the major product) the nitride-bridged structure [{MeCpMn(CO)2}(m-N){CpW(CO)2}] (346), in which NdO bond cleavage has occurred (this unprecedented, low-temperature, transformation was proposed to proceed via initial nucleophilic attack of [CpW(CO)3]− on the nitrosyl nitrogen atom).412 NdO bond cleavage was also observed in the reaction of [CpMn(CO)2(NO)]+ with the dimolybdenum anion [(CpMo)2(m-CO)2(m-PPh2)]−; the initially formed trimetallic cluster [{MeCpMn(CO)2}(m-N)(CpMo) (m-O)(m-PPh2){CpMo(CO)}] (347), the result of NdO bond cleavage and CO dissociation, was extremely air sensitive and upon exposure to O2 eliminated CO2 to produce a terminal oxo ligand in the isolated trimetallic product [{MeCpMn(CO)2}(m-N)(CpMo) (m-O)(m-PPh2){CpMo(O)}] (348).420

Scheme 81

5.07.3.1.3

Derivatives of cymantrene (III) [(C5H4R)Mn(CO)3] (R 6¼ H, CyH2y+1 alkyl)

A large number of cymantrene derivatives have been reported where one or more CH substituent on the cyclopentadienyl ligand has been replaced with an alternative substituent (not including CyH2y+1 alkyl substituents; derivatives with CyH2y+1 alkyl substituents on the cyclopentadienyl ring were covered in Section 5.07.3.1.1). In examples with a single cyclopentadienyl substituent, the metal and its immediate coordinate sphere are often considered as a substituent on a larger molecule, including complexes and clusters. In these cases, the [(
5-C5H4)Mn(CO)3] fragment is often referred to as a formally anionic cymantrenyl substituent (Cym; Fig. 17), and this nomenclature will often be used in this section for compounds with a single elaborate substituent on the cyclopentadienyl ring. Herein we provide an overview of new cymantrenyl-containing species (CymR) reported between 2005 and 2020, arranged by the synthetic method used to attach the cymantrenyl group. Cymantrene derivatives with multiple non-alkyl (alkyl ¼ CyH2y+1) substituents on the cyclopentadienyl ring are covered in Section 5.07.3.1.4.

Fig. 17 The cymantrenyl substituent.

446

Cyclic and Non-Cyclic Pi Complexes of Manganese

One common access point to cymantrene derivatives CymR is reactivity of an alkali metal or thallium cyclopentadienyl salt with a Mn(I) halide precursor (usually [BrMn(CO)5]) via salt metathesis with concomitant dissociation of one or more neutral ligands (e.g. two equivalents of CO); Scheme 82 (analogues of CymR prepared by this method have been numbered 349). Examples of the cyclopentadienyl substituent in complexes prepared in this manner include an alkyl group with a pendent trimethylstannyl moiety,421 a perfluorinated tolyl group,422 a perfluorinated pyridine group,423 an SiMe2(C^CPh) group,424 an alkyl group with a pendent acetylsalicylic acid moiety (which was investigated for anticancer properties),425 and an iron(II) cyclopentadienyl phospholide compound attached via a CH2 linker.426 Related chemistry was used to prepare the tetracymantrenylcyclobutadiene iron complex 350 (this complex was also prepared by an alternative route: see Scheme 90).427

Scheme 82

As an alternative to salt metathesis, Friedel-Crafts acylation of unsubstituted cymantrene has been used to attach various acyl substituents onto the cyclopentadienyl ring (Scheme 83). In the presence of AlCl3, reactions involving cyclic anhydrides have been used to prepare a series of carboxylic acid-terminated acyl-substituted cymantrene analogues with different alkyl (351)428,429 or aryl (352)430 spacers. Both complexes were used as precursors to prepare other complexes; 351 proved useful for the preparation of peptide-linked cymantrene complexes (see Scheme 121), while the terminal hydroxyl group in 352 could be methylated to form a derivative with a pendent ester moiety (353).430 Additional carboxylic acid appended species were prepared by sequential Friedel-Crafts acylation of cymantrene with acyl chlorides, followed by hydrolysis of the terminal ester using sodium hydroxide in methanol (affording 354 and 355).429 Another example of Friedel-Crafts reactivity involves exposure of cymantrene to 3,4,4-trichlorobut-3-enoyl chloride, which afforded acyl complex 356.431 The carbonyl group in 356 could be reduced, and the resulting alcohol (357) was used to attach a 1,2-azole fragment (via an amide linkage) using 4,5-dichloroisothiazole-3-carbonyl chloride, affording 358.432

Cyclic and Non-Cyclic Pi Complexes of Manganese

447

Scheme 83

Deprotonation of the methyl group in [MeCpMn(CO)3], followed by acylation by reaction with an ester, was used to prepare complexes 359 and 360, which contain a pendent ketone moiety (Scheme 84). An equivalent procedure was also reported as a higher yielding route to a previously reported derivative of 360 containing a 2-pyridyl group (not shown in Scheme 84). The pendent Lewis bases in these complexes could be used to prepare cymantrene derivatives with a chelating cyclopentadienyl/ketone ligand (see Scheme 149 for the synthesis and structures of the resulting products).433

Scheme 84

448

Cyclic and Non-Cyclic Pi Complexes of Manganese

The lithiated derivative of cymantrene, CymLi, is another precursor commonly used to install new substituents on the cyclopentadienyl ring (see Schemes 85–88). In general, this reagent is generated in situ from cymantrene and nBuLi. An example of reactivity stemming from CymLi is the preparation of vinyl-substituted cymantrene derivatives (361) upon exposure to fluoroalkenes; Scheme 85.434,435 For the reaction with pentacarbonyl(trifluoroethenyl isonitrile)chromium, 361 was obtained as both E and Z isomers.434 By contrast, the reaction of CymLi with trifluorovinylferrocene yielded only the E isomer, along with small amounts of a trimetallic cymantrene derivative with two vinylferrocenyl substituents on the Mn-coordinated cyclopentadienyl ligand (362).435 An analogue of cymantrene with a silyl substituent on the cyclopentadienyl ring (363) has also been obtained via the reaction of CymLi with ClSiMe2(NEt2); Scheme 85. Compound 363 was further modified to produce a species with a terminal SiCl substituent (364), which reacted with an alkynyl Grignard reagent to install an alkynyl substituent on silicon (affording 365; this complex was also prepared by an alternative route outlined in Scheme 82).424 Furthermore, coupling of CymLi with (1R,2S,5R)-(–)-methyl-(S)-p-toluenesulfinate installed a sulfoxide moiety on cymantrene (to afford 366); Scheme 85.436 By contrast, the reaction of CymLi with phenylethynylsulfone was shown to provide a new high yielding route to a previously reported CymC^CPh (Scheme 85).437 Lastly, CymLi reacted with 2-formylpyridine to afford, upon CdC bond formation and subsequent quenching, a cymantrene derivative bearing a CH(OH)(2-pyridyl) substituent (367); Scheme 85. Deprotonation of 367, followed by exposure to allyl bromide, permitted installation of an allyl group on the oxygen atom to afford 368. The pendent pyridine and alkene units in 368 are both capable of coordinating to Mn upon photochemically-induced carbonyl co-ligand dissociation (see Scheme 154 for the structures of chelating products 712 and 14).12

Scheme 85

CymLi has also been used in the syntheses of transition metal carbene complexes by nucleophilic attack on a CO ligand of a transition metal carbonyl precursor, followed by quenching with an electrophile such as [Et3O][BF4] (Scheme 86). One example of this reactivity involves the reaction of CymLi with [CpRe(CO)3] to afford (upon quenching with [Et3O][BF4]) heterobimetallic 369 (Scheme 86).239 Analogous reactions between CymLi and [H,MeCpMn(CO)3], followed by quenching with [Et3O][BF4], afforded the dimanganese complex 240 (as described in Scheme 62). Reactions of CymLi with bimetallic Mn(0) or Re(0) carbonyl precursors followed by quenching with [Et3O][BF4] yielded carbene complexes 370 where the M2 unit remained intact.239,438 Group 6 cymantrenyl-substituted carbene complexes [M(CO)5{]C(OR)(Cym)}] (371: R ¼ Me, Et and M ¼ Cr, W, Mo) were prepared in

Cyclic and Non-Cyclic Pi Complexes of Manganese

449

Scheme 86

a similar manner; reaction of CymLi and a hexacarbonyl transition metal precursor to afford intermediate [M(CO)5{]C(O) (Cym)}]− (372), which upon quenching with [Me3O][BF4] or [Et3O][BF4] afforded various derivatives of 371.438,439 Like 240 (see Scheme 61), 371 is susceptible to carbonyl substitution by PPh3, forming 373 where substitution has occurred exclusively on the group 6 metal center cis to the carbene ligand.243 Compound 371 also displays unusual photochemical properties; irradiation of the Cr and W derivatives (with a methoxy substituent on the carbene) in the presence of methanol led to metal-free fulvene-based products.439 Furthermore, in the presence of catalytic amounts of Pd(OAc)2 and NEt3, 371 (Cr and W derivatives with a methoxy carbene substituent) underwent dimerization of the carbene residues to form dimetallic chromium- or tungsten-free E and Z alkene isomers (374).440 Another set of reactions stemming from CymLi involved the preparation of azine-substituted cymantrene derivatives 375 and 376; Scheme 87 (compound 375 has also been prepared by an alternative route as shown in Scheme 90). Upon exposure of CymLi to free azines and quenching with water, a cymantrene derivative with a non-aromatic N-containing ring substituent (377) was formed (this was only isolated for the reaction involving 3,6-diphenyl-1,2,4-triazine), which underwent dehydrogenation/aromatization to form 375 or 376 in the presence of 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ). In all cases, the ring carbon adjacent to N was bound to the cymantrene cyclopentadienyl ring, unless if it was occupied by another substituent. The authors were unable to obtain an X-ray crystal structure of intermediate 377. Therefore, the aforementioned isolated derivative of 377 was derivatized by installation of an ethyl group at the NH site, and the resulting complex (378) was crystallographically characterized.441,442 Reactions of CymLi with azine N-oxides and quenching with water were also described (Scheme 87; left), and presumably proceed via a similar pathway to the reactions of azines to form 377. However, two different structures were isolated, depending on the exact identity of the azine N-oxide used in the reaction; some yielded azine N-oxide compounds after oxidation with DDQ (379), while others underwent spontaneous water elimination from the initially formed product, generating azine compounds (380) which are directly analogous to 375 and 376.443

450

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 87

Reactions of CymLi with N-alkoxyamides have also been used as the first step in the multistep synthesis of 4-(2-(cymantrenyl)-2oxo-ethoxy)phenyl-1,2-di(p-hydroxyphenyl)but-1-ene (381; Scheme 88), a complex with potential anticancer activity. Initial formation of a protected derivative of 381 was accomplished from CymLi in one step using an N-alkoxyamide appended alkene (which yielded exclusively the E isomer, although isomerization to an E/Z mixture was observed upon dissolution in acetone) or in two-steps via initial formation of a bromide-terminated cymantrene derivative, followed by coupling with an alkene precursor via a hydroxyl group (this route yielded a mixture of E and Z isomers). Deprotection of the intermediate by HCl afforded the isolated product 381.444

Scheme 88

Cyclic and Non-Cyclic Pi Complexes of Manganese

451

Cymantrene derivatives where a substituent on the cyclopentadienyl ligand contains a pendent lithiated moiety have also been used to prepare complexes with more complex cyclopentadienyl substituents (Scheme 89). For example, a cymantrene derivative with a lithiated alkynyl substituent reacted with pentacarbonyl(trifluoroethenyl isocyanide)chromium to form the heterometallic complex 382 (both E and Z isomers were observed).434 Another example involves lithiation of N-(tert-butoxycarbonyl)-N-methyl-1-cymantrenylethylamine at the a-carbon; subsequent quenching with alkyl halides installed an alkyl group (383), while exposure to benzaldehyde afforded oxazolidinone complex 384. Similarly, lithiation of N-(1-cymantrenylethyl)-N,N0 dimethyl-N0 -phenylurea or N-Boc-N-methyl-1-cymantrenylethylamine, followed by quenching with allyl bromide, lead to allylation of the a-carbon to afford 385445 or 386,12 respectively (the pendent alkene and carbonyl groups in 386 are capable of coordinating to the metal center upon CO dissociation under photochemical conditions; see Scheme 153 for the structures of chelating products 705 and 16).

Scheme 89

An alternative route used to install cymantrenyl groups on various halide-containing reagents involves the use of cymantrenyl zinc precursors (Scheme 90). For example, in situ generated Cym2Zn (formed by exposure of CymLi to half an equivalent of ZnBr2) reacted with half an equivalent of tetraiodocyclobutadieneiron tricarbonyl to form the pentametallic radial complex 350 via palladium-catalyzed coupling (350 was also prepared via an alternative route; see Scheme 82). In the synthesis of 350, a tetrametallic derivative (387) was also observed as a byproduct.427 In addition, palladium-catalyzed Negishi coupling of 2-bromopyridine with in situ generated cymantrenyl zinc chloride (CymZnCl) allowed installation of a 2-pyridyl substituent onto the cymantrene cyclopentadienyl ring (affording 375; this complex was also synthesized by an alternate route, as described in Scheme 87).446

452

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 90

Another common class of precursor used to access new cymantrenyl species are halide-substituted derivatives CymX. New substituents can be installed on the cyclopentadienyl ligand from CymX through several different routes. First, bromo-substituted cymantrene (CymBr) was used as a precursor to prepare derivatives with a fluorinated alkyl substituent (388 and 389) by Pd(II) catalyzed alkylation with a zinc alkyl halide (Scheme 91).447

Scheme 91

Iodo-substituted cymantrene (CymI) was used to prepare the Pd complex [(Cym)IPd(PPh3)2] (390) by oxidative addition to Pd(0); Scheme 92. 390 proved unstable to ligand scrambling, forming [I2Pd(PPh3)2] and [Cym2Pd(PPh3)2] (391); further decomposition of 391 via CdC bond-forming reductive elimination afforded previously reported dimetallic Cym2 (cymantrene was also observed as a byproduct, but the source of the H atom is unclear).448 Alternatively, Stille coupling of CymI with Bu3Sn(C2F3) allowed attachment of a trifluorovinyl substituent onto the cymantrene cyclopentadienyl ring, yielding 392 (Scheme 92). Upon heating, 392 underwent [2 + 2] cycloaddition to form 393, which contains a fluorinated cyclobutane ring (2 isomers were observed).435 Pd-catalyzed Stille coupling has also been used to attach an alkynyl steroid to a cymantrenyl group, and the resulting complex (394) was investigated for anti-cancer properties (Scheme 92).449

Cyclic and Non-Cyclic Pi Complexes of Manganese

453

Scheme 92

Cymantrene derivatives containing a halide substituent that is not directly attached to the cyclopentadienyl ring have also been used to prepare new cymantrenyl-containing species (Schemes 93–95). For example, reactions of CymCH2Br or CymCHMeBr with derivatives of 4-quinazolinone in the presence of a base allowed the preparation of various new quinazoline and 4-quinazolinone derivatives with cymantrenyl substituents in various locations (Scheme 93). Both brominated precursors reacted with 2-methylquinazolin-4-one in the presence of NaH to form products where the nitrogen atom adjacent to the carbonyl group has coupled to the cymantrenylalkyl moiety (395). However, when tBuOK was used in place of NaH (for the reaction involving CymCH2Br), 4-(cymantrenylmethoxy)-2-methylquinazoline (396) was observed as a substantial byproduct. Interestingly, CymCH2Br proved unreactive under these conditions towards the vinyl-substituted reagent 2-styryl-3H-quinazolin-4-one, though a derivative of 395 was produced when AcOH/NaOAc was used in place of NaH, and a mixture of 395 and 396 derivatives were produced when K2CO3 was used as the base. When alkylation of 6,7-difluoroquinazolin-4-one was attempted (using K2CO3 in combination with CymCH2Br), a mixture of 395 and 396, as well as a minor byproduct (which is a derivative of 395 where one of the fluoro groups has been converted to a hydroxyl moiety) was observed. The fluorescent properties of these various complexes were investigated.450

Scheme 93

A chloromethyl-substituted cymantrene complex, CymCH2Cl, reacted with metallated 2-cyanomethylpyridine to yield cymantrene derivatives with pendent pyridine donors (397 and 398),451,452 and further manipulation of 397 allowed installation of a pendent thioamide moiety (399); Scheme 94. The pendent Lewis bases in 397 and 399 are capable of coordinating to the metal center upon carbonyl co-ligand dissociation (see Scheme 149 for the structures of the resulting complexes 692 and 682).452 In addition, 3-chloropropionylcymantrene reacted with nucleobases in the presence of NEt3 to afford a series of cymantrene

454

Cyclic and Non-Cyclic Pi Complexes of Manganese

derivatives containing acyl cyclopentadienyl substituents with appended fluorouracil, uracil, thymine, or adenine fragments (400, 401); Scheme 94. The ketone groups in these complexes were susceptible to reduction by NaBH4 (most derivatives) or LiAlH4 (adenine-containing derivative) to form new alcohol-appended species 402 or 403. For the fluorouracil derivative of 402 or adenine derivative 403, subsequent alcohol deprotonation followed by treatment with MeI allowed isolation of methoxy-substituted derivatives 404 and 405, respectively. The electrochemical properties of these various complexes were probed, and the anticancer, antitrypanosomal, and/or antimicrobial activities were investigated.453,454 Analogous chemistry involving ciprofloxacin afforded a compound where the cymantrenyl group is attached to the antibiotic (406), which showed some antimicrobial and cytotoxic potential.455

Scheme 94

Additionally, halide appended cymantrene derivatives have been used to prepare complexes with pendent triazole moieties (Scheme 95). For example, 1-benzyl-5-phenyl-1,2,3-triazole reversibly reacted with cymantrenylmethyl iodide (CymCH2I) to yield the triazolium iodide salt 407. As illustrated in Scheme 100, compound 407, and various derivatives of it, were alternatively prepared by alkylation of a neutral triazole precursor (the reverse reaction was favored at high temperature).456 Furthermore, CymCH2Br reacted with benzotriazole in the presence of sodium hydroxide to afford a mixture of 1- and 2-cymantrenylmethylbenzotriazole complexes 408 and 409, both of which could be isolated chromatographically.457 Bromomethylcymantrene also reacted with 1,3-thiazolidine-2-thione in the presence of K2CO3 to yield a species with a pendent thiazole moiety (410).458 The pendent donors in these complexes can coordinate to metal centers upon dissociation of carbonyl co-ligands; see Schemes 149 and 153 for the structures of chelating products 688, 690, 706, and 707.

Cyclic and Non-Cyclic Pi Complexes of Manganese

455

Scheme 95

Cymantrene derivatives with SbX2 or Sb(OH)2 substituents have been prepared from previously reported tricymantrenyl antimony (SbCym3); reactions with Cl2 or Br2 yielded antimony dihalide species 411, which could be converted to hydroxyl species 412 by exposure to potassium hydroxide (Scheme 96).459

Scheme 96

A cobalt complex (413) with the same tetracymantrene-substituted cyclobutadiene ligand as in 350 (Schemes 82 and 90) was prepared by cobalt-mediated 2+ 2 cycloaddition of dicymantrenylacetylene (C2Cym2); Scheme 97.460 A new X-ray crystal structure of C2Cym2 has also recently been obtained.461

Scheme 97

A series of complexes containing cymantrenylthiolate fragments (CymS−) have been reported (Scheme 98). For example, a Pt(II) complex with two cymantrenylthiolate ligands (414) was prepared both from Pt(II) or Pt(0) precursors using thiolate or disulfide derivatives of cymantrene, respectively.462 In addition, the reaction of thiol-substituted cymantrene (CymSH) with a Cr dimer containing bridging thiolate and sulfide ligands yielded a new tetrametallic complex (415) containing two bridging cymantrenylthiolate ligands. This complex could alternatively be prepared from the reaction of S8 with a previously reported tetrametallic Cr2Mn2 species containing m-cymantrenylthiolate and terminal carbonyl ligands, although this reaction also produced the tetrametallic Cr3Mn cluster 416 as a byproduct. A tetrametallic Cr2CoMn cluster (417) was prepared by exposure of 415 to [Co2(CO)8].463

456

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 98

Alkynyl- and vinyl-substituted cymantrenyl derivatives have also been used to incorporate organic substituents onto the manganese-coordinated cyclopentadienyl ring (Scheme 99). For example, alkynylcymantrene (CymCCH) can be used to generate triazole substituents through copper-catalyzed azide-alkyne cycloaddition (to afford 418).464 Additionally, a Kinugasa reaction (with diphenylnitrone in the presence of CuCl, Cy2NMe, and indaBOX) yielded azetidinone complex 419.465 By contrast, the exposure of vinylcymantrene (CymC2H3) to nitrones in the presence of catalytic amounts of catechol resulted in cycloaddition to afford cymantrene derivatives with isoxazolidinyl substituents where the cymantrenyl group is ortho to the oxygen atom (420). When the NR substituent of the nitrone reagent was a methyl group (for Ar ¼ Ph or cymantrenyl), a mixture of cis and trans isomers of 420 was observed, though in the other derivatives the cis isomer was formed exclusively.466,467 Reactions of styrene or [{
6-C6H5(CH]CH2)}Cr(CO)3] with cymantrene derivatives featuring a nitrone substituent (for reactions involving styrene, in the presence of catalytic catechol) also yielded isoxazolidnyl-substituted derivatives (421), but in this case the positions of N and O within the isoxazolidinyl ring are reversed relative to 420. These reactions exclusively afforded the cis isomers, with the exception of the chromium-containing derivative where R ¼ Me, for which the trans isomer was observed as a minor product.466,467

Cyclic and Non-Cyclic Pi Complexes of Manganese

457

Scheme 99

The copper-catalyzed azide-alkyne cycloaddition chemistry used in the synthesis of 418 (Scheme 99) has been mirrored in chemistry stemming from a cymantrene-based precursor with a pendent terminal alkyne group separated from the metal-coordinated cyclopentadienyl fragment by a CMe(OH) unit (Scheme 100). The resulting triazole-containing complex (422) could be further manipulated at the hydroxyl site to install pendent propargyl or allyl moieties (affording 423 or 424, respectively).17 Copper-catalyzed azide-alkyne cycloaddition chemistry has also been used with azide-substituted cymantrene derivatives and free alkynes, yielding triazole-containing 425. These species (425) could be alkylated to yield triazolium cations 407 (one derivative of 407 was also prepared from cymantrenylmethyl iodide; Scheme 95). In some cases, the halide anion in 407 was replaced with a weakly-coordinating BF−4 ion, to facilitate subsequent reactivity.17,456 In the syntheses of some iodide salts of 407, a metal-free dialkylated 1,3-dialkyl-4-phenyl-1,2,3-trazolium iodide byproduct was observed; this byproduct became the major product when the initial reaction was carried out with excess alkyl halide at high temperature. This process was proposed to proceed via decomposition of 407 to form a halide-appended cymantrene derivative and a 1,5-disubsituted 1,2,3-triazole, which would react with the alkyl halide.456 Furthermore, in the synthesis of the derivative of 407 where R0 ¼ R ¼ Me, three byproducts were observed: i. dimethyltriazolium iodide, presumably formed in a manner similar to that described for the reaction where R0 ¼ H, ii. 2-(1-cymantrenylethyl)-4-phenyl-1,2,3-triazole (an isomer of 425), potentially formed from MeI elimination from 407, and iii. previously reported vinylcymantrene (CymC2H3), presumably formed from HI elimination from the expected 1-cymantrenylethyl iodide byproduct.456,457 Notably, various pendent substituents in these complexes have been shown to be able to coordinate to Mn upon carbonyl ligand dissociation; see Scheme 149.

458

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 100

Formylcymantrene (CymCHO), also referred to as cymantrene-carbaldehyde, has been used to access new cyclopentadienyl-substituted cymantrene derivatives using standard methods for aldehyde functionalization (Schemes 101–105). Many of these reactions involve CdN bond formation using amine-based reagents (Scheme 101). For example, condensation with a variety of amines yielded imine-substituted cymantrene complexes 426, where the R groups on the imine include alkyl groups with a pendent alcohol (this was used in the synthesis of complex phosphites),468 as well N-phenylmaleimidetriazole or quinonetriazole derivatives.469 When the R group of 426 was a daunorubicin moiety, the imine functionality was reduced in situ to form an amine-containing species (which was isolated).470 The reaction of formylcymantrene with 2-aminobenzylalcohol afforded a cymantrene derivative with an imine substituent (427), which in solution exists in equilibrium with the ring-closed amino-ether tautomer 428 (in the solid state, 428 preferentially crystallizes); this mixture displayed anti-trypanosoma cruizi activity.471 When anthranilamide was used as the amine reagent, copper-catalyzed cyclization of the amine and (in situ generated) imine moieties yielded the 4-quinazolinone compound 429, which could be further modified to form a 2-cymantrenyl-4-aryl-quinazoline (430; the luminescence properties of 430 were investigated).472 In addition, a chloroquine-containing cymantrene derivative (431) was prepared from the reaction of CymCHO with N-(7-chloroquinolin-4-yl)ethane-1,2-diamine in the presence of N-methylmorpholine, isobutyl chloroformate, and NEt3. Compound 431 was examined for anti-malaria, -leishmaniasis, and -trypanosomiasis effects.473 Furthermore, similar reactivity allowed the cymantrenyl group to be attached to dendrimers of various sizes (432); these cymantrene-containing dendrimers were also investigated for cytotoxic activity. Derivatives of 432 were prepared for three generations of dendrimer (only G1 is shown in Scheme 101).474 As well, a cymantrene derivative with a pendent 1,3-thiazolidine-2-thione moiety (433) was prepared in a stepwise fashion by reductive amination involving an amine with pendent alcohol (forming intermediate 434), which yielded 433 by condensation with CS2 in the presence of potassium hydroxide (433 was used as a precursor to prepare 681, the product of thione donor coordination to the metal center upon carbonyl ligand dissociation; see Scheme 149).458

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 101

459

460

Cyclic and Non-Cyclic Pi Complexes of Manganese

In the presence of a fullerene, CymCHO and sarcosine (MeHNCH2CO2H) reacted via the Prato reaction to attach a fullerene to the cyclopentadienyl ring, affording 435; Scheme 102 (this complex, along with other fullerene-appended species, is of interest due to unusual electrophysical and optical properties). An analogue of 435 with a vinyl group between the cyclopentadienyl ring and the fullerene-containing moiety (436) was synthesized via initial preparation of 3-cymantrenylprop-2-enal (437), followed by similar chemistry to that used to prepare 435.475 Notably, the amine group in 435 could be oxidized with mCPBA to afford an N-oxide (438).476

Scheme 102

Reactions of hydrazines with formylcymantrene provided an alternative route to generate new substituents on the cyclopentadienyl ring of cymantrene derivatives via CdN bond formation (Scheme 103). For example, the microwave-assisted reaction of CymCHO with FcC(O)NHNH2 yielded a ferrocenyl/cymantrenyl hydrazone species 439, where the cymantrenyl and ferrocenyl cyclopentadienyl rings are tethered together by a acylhydrazone linker.477 Additionally, thermally-induced reactions of CymCHO with acylhydrazides or p-toluenesulfonyl hydrazide yielded a family of cymantrene derivatives with acyl- (440)478 or tosyl- (441)479 hydrazone substituents on the cyclopentadienyl ring. Derivatives of 440 were investigated as inhibitors of human carbonic anhydrases,478 and 441 was evaluated for antiproliferative and antitubercular activity, though initial results were not promising.479 The reaction of CymCHO with S-methyldithiocarbazate afforded CymCH]NNHC(S)SMe (442), which reacted with K2[PdCl4] in the presence of PPh3 to yield a Pd(II) complex linked to the cymantrene fragment via imine and thiolate donors (443).480 In addition, CdN bond-forming reactivity stemming from CymCHO has been achieved using a modified Strecker synthesis to prepare a cymantrene derivative of an amino acid (444), which was studied in order to model a 99mTc analogue which may have applications in cancer imaging.481

Cyclic and Non-Cyclic Pi Complexes of Manganese

461

Scheme 103

In the presence of NEt3 and TMSCl, formylcymantrene (CymCHO) reacted with the chromium carbene complex [Cr(CO)5{] CMe(OEt)}] to yield the heterobimetallic cymantrene derivative 445, where the sp2 carbon of the chromium carbene is tethered to the cymantrenyl group via a vinyl linker (Scheme 104).439 Like 371 (see Scheme 86), compound 445 underwent dimerization of the carbene moieties in the presence of catalytic Pd(OAc)2 and NEt3, forming a polyalkene (446; the central alkene forms as a mixture of E and Z isomers).440

Scheme 104

462

Cyclic and Non-Cyclic Pi Complexes of Manganese

Additional CdC bond-forming reactivity stemming from formylcymantrene (Scheme 105) include a reaction with phenyl Grignard followed by a magnesium-Oppenauer oxidation using benzaldehyde to prepare previously reported cymantrenyl phenyl ketone (CymCOPh).482 CymCHO also reacted with (2-pyridinyl)methyllithium to yield a cymantrene complex with pendent hydroxy and pyridine groups, the former of which could be converted to an allyl-oxy group by deprotonation followed by exposure to allyl bromide (affording 447, where the pendent alkene and pyridine moieties can both bind to metal centers upon carbonyl co-ligand dissociation as illustrated in Scheme 153).12 Alternatively, CymCHO reacted with acetone in the presence of a catalytic amount of the ionic liquid 1-butyl-3-methylimidazolium (bmim) tetrafluoroborate and water to yield the aldol 448.483,484 In addition, the reaction of CymCHO with quinazolinone derivates in the presence of sodium acetate (when R ¼ Ph, X ¼ H) or zinc chloride (when R ¼ H, X ¼ F) afforded derivatives of cymantrene with a vinyl substituent linked to a quinazolinone moiety (449), which display interesting fluorescent properties.450

Scheme 105

Mirroring the reactions of formyl cymantrene with hydrazines (see Scheme 103), acetyl cymantrene (CymCOMe) also reacted with hydrazine reagents to produce complexes with hydrazone substituents on the manganese-coordinated cyclopentadienyl ring (Scheme 106). This process has been used to prepare cymantrene derivatives with Rhodamine-6G or Rhodamine-B substituents (450), which are fluorescent and exhibit distinctive sensing and switching properties.485 In addition, CymCOMe reacted with acylhydrazides or p-toluenesulfonyl hydrazide to yield a family of cymantrene derivatives with acyl- (451)486 or tosyl- (452)479 hydrazone substituents on the cyclopentadienyl ring. Derivatives of 451 were investigated for antibacterial activity,486 and 452 was evaluated for antiproliferative and antitubercular activity, though initial results were not promising.479 Furthermore, reactions of acetyl cymantrene with aldehydes in the presence of potassium hydroxide afforded a series of a,b-unsaturated ketones with heterocycle-containing substituents (453). Reactions of some of these derivatives (453a-c) with semicarbazide or thiosemicarbazide afforded (in the presence of KOH) 4,5-dihydro-1H-pyrazole-1-carboxamides or -1-carbothioamides 454, while reactions with hydroxylamine (in the presence of KOH) yielded dihydroisoxazole derivatives of cymantrene (455). Furthermore, reactions of 453a-b with thiourea or guanidine (both in the presence of KOH) yielded dihydropyrimidine-2(1H)-thione (456) or 2-aminopyrimidine (457)487 complexes, respectively.

Cyclic and Non-Cyclic Pi Complexes of Manganese

463

Scheme 106

Acetyl cymantrene was also used as a precursor to access ferrocenyl-cymantrenyl chalcones (458 and 459) upon reaction with formylferrocene or 1,10 -diformylferrocene in the presence of NaOH (Scheme 107). These complexes were investigated for antimalarial and antibacterial activity.488 A monometallic chalcone species with a pendent crown ether (460) was prepared via similar chemistry (Scheme 107), and functioned as a radical trap upon solvated electron reduction.489,490 The photoinduced isomerization of E-460 to Z-460 was the subject of subsequent steady state and flash photolysis experiments.491 In addition, the reaction of acetylcymantrene with acetylferrocene catalyzed by SiCl4 yielded a complex mixture of species (Scheme 107), including previously reported (E)-1,3-dicymantrenyl-2-buten-1-one, (E)-1,3-diferrocenyl-2-buten-1-one, 1,3,5-tricymantrenylbenzene, and 1,3,5-triferrocenylbenzene, as well as novel complexes (E)-3-cymantrenyl-1-ferrocenyl-2-buten-1-one (461), 1-cymantrenyl-3ferrocenyl-2-buten-1-one (462; this complex is a close relative of 458 and 459, which displayed antimalarial and antibacterial activity), 1,3-dicymantrenyl-5-ferrocenylbenzene (463), and 1-cymantrenyl-3,5-diferrocenylbenzene (464). The ratio of products was rationalized by the different abilities of ferrocenyl or cymantrenyl groups to stabilize cationic intermediates.492

464

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 107

In addition to reactions stemming from CymCOMe and 453 (see Schemes 106 and 107), other ketone-appended cymantrene derivatives have been investigated as precursors to new cymantrenyl-containing complexes (Scheme 108). For example, cymantrenyl phenyl ketone (CymCOPh) has been used to access the oxime complex 465 by reaction with NH2OH(HCl), which upon reduction by zinc in the presence of acetic acid yielded the expected amine product 466, in addition to three minor byproducts (467, 468, and 469) which could be chromatographically separated on a preparative scale. The pendent amide group in 469 is capable of

Cyclic and Non-Cyclic Pi Complexes of Manganese

465

coordinating to the metal center upon CO dissociation under photochemical conditions (see Scheme 149).16 In addition, propionyl cymantrene (CymCOEt) underwent McMurry cross-coupling with benzophenone493 or 4,40 -dihydroxybenzophenone494 to yield vinyl-substituted cymantrene species 470. The phenol derivative of 470 (R ¼ OH) was investigated for potential cytotoxic effects,494,495 and the electrochemistry of the hydroxyl-free derivative (R ¼ H) was probed.493 Furthermore, oxidation of the hydroxyl-free derivative of 470, as well as a previously reported derivative where R ¼ OMe, was investigated.496

Scheme 108

Claisen condensation of the methyl ester of cymantrene carboxylic acid with acetylferrocene yielded a b-diketone complex with cymantrenyl and ferrocenyl substituents (471); Scheme 109. Compound 471 reacted further with hydrazine to afford a heterobimetallic pyrazole compound (472), or with diido(p-cymene)ruthenium(II) in the presence of Cs2CO3 to afford a heterotrimetallic acetylacetonate complex (473). The structures and electrochemical properties of these complexes were investigated.497

466

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 109

An a,b-unsaturated ketone-substituted cymantrene derivative has underwent an asymmetric Michael reaction with 4-hydroxy-2H-chromen-2-one to form 474, catalyzed by a combination of AcOH and a primary amine-derived organocatalyst modified with a pendent N-(4-carboxy-n-butyl)imidazolium cation (Scheme 110).498 This chemistry bears a distinct similarity to Michael reactions of an a-nitroolefin cymantrene precursor with cyclohexanone (catalyzed by a C2-symmetric pyrrolidinium salt and NEt3, yielding 475),499 acetylacetone (catalyzed by an asymmetric ionic-liquid supported catalyst, yielding 476),500 dimethylmalonate (catalyzed by a C2-symmetric tertiary amine-squaramide, yielding 477),501 and ArHC]CHC(O)Ph where Ar ¼ 2-(tosylamino)phenyl (catalyzed by the same species as used in the formation of 476, and yielding the tetrahydroquinoline derivative 478); Scheme 110.502

Cyclic and Non-Cyclic Pi Complexes of Manganese

467

Scheme 110

Derivatives of cymantrene with appended alcohol moieties have also been used to form new cymantrene derivatives upon standard organic synthetic manipulation of the hydroxyl group (Scheme 111). For example, phosphorus donors have been installed on the cymantrene cyclopentadienyl ring by reaction of cymantrenylcarbinol (CymCH2OH)503 or dicymantrenylmethanol (Cym2CHOH)504 with ClP(OR)2 or ClP(NR)2 in the presence of NEt3, yielding 479 or 480, respectively. Upon combination with a Rh cyclooctadiene complex, 479 afforded a species (presumably via coordination of the P donor in 479 to Rh) which was catalytically active towards enantioselective hydrogenation of (Z)-methyl-2-acetamido-3-phenylacrylate and dimethyl itaconate.503

468

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 111

In addition, allyl substituents were installed on the oxygen atom in 1-cymantrenylethanol (CymCHMeOH) by sequential deprotonation and exposure to an allyl halide (affording 481, which contains an alkene fragment that can, as described in Scheme 149, act as a donor to the metal center upon carbonyl co-ligand dissociation).16 Furthermore, 1-cymantrenylethanol could be acetylated using an acyl chloride in the presence of NEt3, permitting the synthesis of 1,2-thiazole- and 1,2-oxazole-containing esters (482).487 A cymantrene derivative with an NH2 cyclopentadienyl substituent (CymNH2) reacted with suberic acid monomethyl ester (in the presence of N-methylmorpholine and isobutyl chloroformate) to yield an ester-terminated species (483; Scheme 112) which was used to model an intermediate in the preparation of a 99mTc-containing analogue.505

Scheme 112

Cymantrene derivatives with pendent amine groups (i.e. where the nitrogen atom is not directly bonded to the cyclopentadienyl ring) have also been used to attach various moieties to the manganese-coordinated cyclopentadienyl ligand (Scheme 113). For example, coupling of aminomethylcymantrene (CymCH2NH2) with 7-chloro-4-fluoroquinoline allowed formation of 484.506 Interestingly, oxidation of 484 was found to be electrochemically irreversible, consistent with oxidation of the secondary amine rather than the metal center.506,507 Additionally, methylation of (R)-1-cymantrenylethylamine (CymCH(NH2)CH3) afforded the secondary amine 485, to which a P(OR)2 group could be attached yielding 486. Upon reaction with [Rh(COD)2]+, these phosphoramidite species afforded active catalysts for asymmetric hydrogenation of dimethyl itaconate in supercritical CO2; the attachment of the cymantrenyl group is useful for conducting this reactivity in supercritical CO2, as it has beneficial solubility effects.508 Both CymCH2(NH2) and CymCH(NH2)CH3 reacted with (boc)2O to yield carbamate complexes 487, and the methyl derivative was then methylated at the nitrogen atom to afford 488.509 Both primary amine precursors also reacted with acyl chlorides to afford cymantrene complexes with a pendent amide group (489), which could undergo subsequent reactivity to install an allyl group on nitrogen (yielding 490 or 491).12,13,510,511 The amide groups on 487, 488, 489, 490, and 491 (as well as the pendent alkenes on the latter two compounds) can coordinate to the metal center upon carbonyl ligand dissociation (see Schemes 149 and 153). An alternative route to access derivatives of 491 with extremely bulky aryl substituents adjacent to the carbonyl group was also developed, involving the reaction of a secondary amine-containing cymantrene species (already containing an allyl substituent) with an acyl chloride.511 Utilizing equivalent chemistry to that used to convert 489 into 490 or 491, allyl groups

Cyclic and Non-Cyclic Pi Complexes of Manganese

469

have been installed at the NH site of previously reported carbamate-appended derivatives of cymantrene to afford 492 or 493.12 In addition, the NH substituent in secondary amines was shown to react with phenylisothiocyanate to yield cymantrene derivatives with a pendent thiourea moiety (494), which are capable of donating to the metal center upon carbonyl co-ligand dissociation (see Scheme 149 for the structure of the chelating product 683).15 Notably, a new synthetic route (involving the reductive amination of acetylcymantrene with NaBH4 and NH3 on alumina) was recently reported for the synthesis of 1-cymantrenylethylamine (CymCH(NH2)CH3), which was used to access several of the new complexes in Scheme 113.512

Scheme 113

Cymantrene with a carboxylate substituent on the cyclopentadienyl ring (CymCO−2) has commonly been incorporated into polymetallic systems as a terminal or bridging carboxylate ligand on various metal centers. These complexes are generally prepared from a cymantrenecarboxylate salt or the parent carboxylic acid (CymCO2H). For reactivity stemming from CymCO−2, both the sodium and potassium salts (often prepared in situ from CymCO2H) are commonly employed. Crystal structures containing both

470

Cyclic and Non-Cyclic Pi Complexes of Manganese

CymCO−2 and CymCO2H have been obtained by mixing CymCO2H with heterocyclic nitrogen-bases or ethanolamine followed by crystallization.513 Carboxylic acids CymCO2H and CymC(O)CH2CH2CO2H have also been used to prepare a series of salts with alkali metal or ammonium counter ions, and in the solid state these species form supramolecular structures, in some cases including interactions between the oxygen atom of a manganese-bound carbonyl ligand and an alkali metal cation.514 The syntheses of various lanthanide-containing cymantrenecarboxylate compounds are discussed in Schemes 114 (from CymCO−2) and 115 (CymCO2H), and are primarily of interest due to the magnetic, physical, and chemical properties of the varied solid state structures obtained. Properties including magnetic behavior and luminescence were investigated. In addition, lanthanide-containing structures were analyzed as potential precursors to LnMnO3 or LnMn2O5 (formed via thermal decomposition in air). Cymantrenecarboxylate (also referred to as cymantrenate) complexes of lanthanides were reviewed in 2016.515 Cymantrenecarboxylate (CymCO−2) salts were found to react with Ln(NO3)3 to form various structures depending on the specific metal and solvent combination used (Scheme 114). When reactions were conducted in THF (Ln ¼ Nd, Gd, Eu), the resulting structures feature both terminally-bound and bridging cymantrenecarboxylate moieties (495); the bridging CymCO−2 anions are k1-coordinated to each metal.516 However, in the presence of DMSO517 (Ln ¼ Ce, Nd, Gd, Eu) or pyridine518 (Ln ¼ Pr, Sm. Eu. Gd), structures were obtained which feature, in addition to the bonding motifs present in 495, bridging CymCO−2 anions which are k1-coordinated to one metal and k2-coordinated to the other (496). Analysis of the magnetic exchange interaction between the Gd atoms in the Gd/DMSO derivative of 496 was later used, along with similar analyses involving other Gd-containing species, to provide a magnetostructural correlation with the GddGd distance.519 Dy and Tb DMSO derivatives of 496 were prepared using LnCl3 in place of Ln(NO3)3; when nitrate precursors were used, a structure was obtained (497) with four bridging m-cymantrenecarboxylate moieties, as well as two DMSO ligands and one terminal nitrate ligand per lanthanide center (analogous to 495, but with nitrate groups in place of terminal cymantrenecarboxylate groups).520

Scheme 114

Cyclic and Non-Cyclic Pi Complexes of Manganese

471

Reactions of CymCO−2 with a half equivalent of Ln(NO3)3 (Ln ¼ Tb, Dy, Ho, Er, Yb, Pr, Eu, Gd) in the presence of dme yielded one of two structures (in some cases, both); 498 and 499 (Scheme 114). In compounds 498 and 499, the lanthanides are bridged by four cymantrenecarboxylate moieties, and each lanthanide is coordinated to one dme and one NO−3 ligand. In 498, each carboxylate O atom is bound to one lanthanide, while in 499, two of the bridging carboxylate ligands are k1-coordinated to one metal and k2-coordinated to the other (i.e. they are m-1k1:2k2-coordinated). In some cases, interconversion was observed between 498 and 499. By contrast, analogous reactivity involving Yb(NO)3 afforded a tri-ytterbium complex where each pair of lanthanide atoms is bridged by three cymantrenecarboxylate groups (500).521 Interestingly, the reaction of CymCO−2 with Er(NO3)3(H2O)5 in the presence of OP(OMe)3 yielded a complex with an Er2Mn core featuring bridging and terminal cymantrenecarboxylate moieties (501), where the Mn atom in the core has been oxidized to Mn(II); Scheme 114. Another species containing divalent manganese (within an MnCl2− 4 counterion) was prepared via reaction of the CymCO−2 anion with TbCl3(H2O)6 in methanol, and a subsequent workup involving THF; four Tb atoms formed a dication with bridging hydroxyl and cymantrenecarboxylate ligands, as well as terminal water and THF donors (502; Scheme 114).520 FurtherII more, tetranuclear LnIII 2 Mn2 cymantrenecarboxylate complexes (503) have been prepared by partial destructive photolysis of the cymantrenecarboxylate anion CymCO−2 in the presence of LnCl3 (Ln ¼ Dy, Ho), O2, and H2O.522 LnCl3 (Ln ¼ Nd, Gd, Dy, Ho, Er) reacted with CymCO−2 in the presence of water and organic solvent to afford a series of polymeric structures, which contained terminal and bridging cymantrenecarboxylate ligands (504); Scheme 114. These structures feature two metal coordination environments in each repeating unit (one with two terminal cymantrenecarboxylate moieties and the other with four alcohol or water ligands).523 Various cymantrenecarboxylate complexes of lanthanides have alternatively been prepared using cymantrene carboxylic acid (CymCO2H) as the manganese-containing reagent (Scheme 115), rather than a cymantrenecarboxylate salt (see Scheme 114). When CymCO2H was exposed to lanthanide acetylacetonate precursors (Ln ¼ Eu, Gd, Tb, Dy, Ho, Er), polymeric structures were obtained where a single cymantrenecarboxylate moiety bridges between two lanthanide centers, each of which is coordinated to two chelating acac ligands and a single water ligand (505).524 Polymeric species were also obtained from the reaction of lanthanide acetate precursors with CymCO2H, though the resulting structures differed depending on the metal and crystallization conditions. Reactions involving Nd, Gd, or Dy precursors yielded, in the presence of methanol (as well as THF and water), 506 which features acetate and cymantrenecarboxylate moieties bridging between multiple lanthanide atoms (as well as terminal cymantrenecarboxylate

Scheme 115

472

Cyclic and Non-Cyclic Pi Complexes of Manganese

and methanol ligands on each lanthanide). Alcohol-free derivatives of these species were obtained when isopropanol was used in place of methanol; these structure feature one or two water ligands coordinated to each lanthanide (507 and 508). By contrast, reactions involving Ho, Er, or Tm acetate yielded a non-polymeric structure with two lanthanide atoms bridged by two cymantrenecarboxylate moieties, and each lanthanide featuring terminal cymantrenecarboxylate, acetate, and (two) water ligands (509). Attempted recrystallization of the Er derivative of 509 from pure methanol afforded an Er derivative of polymeric 506.525 Multimetallic complexes with cymantrenecarboxylate moieties coordinated to transition metal centers have also been reported (Schemes 116 and 117). For example, CymCO−2 reacted with Cu(II) salts to yield “Chinese lantern” or “paddle-wheel” structures where four cymantrenecarboxylate moieties bridge between two copper centers, and one equivalent of ethereal solvent is bound to each Cu (510; Scheme 116).526 Upon exposure of 510 to CymCO2H, the cymantrene carboxylic acid displaced the ether ligands, binding via the carbonyl oxygen atom to afford 511.526 Alternatively, displacement of the neutral ligands from the THF derivative of 510 by 2,6-lutidine yielded a mono-copper complex (512) with two k1-coordinated cymantrenecarboxylate anions.527 However, exposure of 510 to 2,6-lutidine in the presence of Mn(O2CCym)2 (which was formed in situ from two equivalents of CymCO−2 and MnCl2) formed a structure with a trimetallic Cu/Mn(II)/Cu core, three bridging cymantrenecarboxylate ligands between manganese and each copper center, and terminal lutidine ligands on copper (513).527 Cymantrene carboxylic acid (CymCO2H) has also been utilized to prepare a series of multimetallic complexes with cymantrenecarboxylate substituents (Scheme 117). For example, the reaction of CymCO2H with titanocene-based precursors [MeCp2TiX2] (X ¼ BH4 or Cl) yielded a titanocene complex with two k1-cymantrenecarboxylate ligands (514).527 A bis-vanadium(III) species with a “Chinese lantern” core and terminal Cp ligands (515) was reported to be formed via the reaction of CymCO2H and vanadocene.527 Also, using CymCO2H, PPh3-terminated nickel(II) and cobalt(II) analogues of 515 were prepared from metal pivalate precursors in the presence of free triphenyl phosphine (516). However, when the reaction of CymCO2H with [Co(pivalate)2] was conducted in the presence of more sterically demanding 2,6-lutidine, the resulting complex (517) featured three Co centers, each bridged by three cymantrenecarboxylate ligands.528

Scheme 116

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 117

473

474

Cyclic and Non-Cyclic Pi Complexes of Manganese

Reactions of CymCO2H with transition metal acetate complexes afforded a variety of different structures depending on the metal and solvent combination (Scheme 117). Reactions involving Cu(II) acetate yielded, in acetonitrile, an acetonitrile-terminated copper analogue of 516; 518.529 By contrast, in methanol this reaction yielded a structurally related complex with terminal methanol ligands, two bridging acetate groups, and two bridging CymCO−2 groups (519).529 CymCO2H reacted with nickel(II), cobalt(II), and manganese(II) acetate in methanol to afford octahedral M(II) species with four equatorial methanol ligands and two axial k1-coordinated cymantrenecarboxylate ligands (520).529–531 Upon dissolution of the Ni or Co derivatives of 520 in acetonitrile (as well as the Co species in THF), new tri-nickel or tri-cobalt complexes were formed with three cymantrenecarboxylate ligands bridging between each pair of late transition metals centers, and terminal acetonitrile or THF ligands (521).529,530 The methanol ligands in the manganese(II)-containing derivative of 520 were also susceptible to substitution by pyrazole, forming 522 which is isostructural to 520. By contrast, exposure of 520 to 3,5-dimethylpyrazole yielded a structure with two octahedral Mn(II) centers bridged by a pair of cymantrenecarboxylate moieties, each with three pyrazole and one k1-cymantrenecarboxylate terminal ligands (523).531 Furthermore, recrystallization of the manganese(II)-containing derivative of 520 from hot acetonitrile in the presence of water afforded a polymeric methanol-free structure with Mn atoms bridged by cymantrenecarboxylate moieties and containing terminal water and acetonitrile ligands (not shown in Scheme 117).532 A complex isostructural to 523 with one of the neutral ligands replaced with water was prepared by the reaction of CymCO2H and manganese(II) acetate in the presence of 3,5-dimethylpyrazole and water; 524.533 By contrast, the same reaction in THF instead yielded a 1D polymer with divalent manganese centers bridged by cymantrenecarboxylate and acetate moieties (each acetate ligand bridges between 3 Mn(II) centers), with one THF ligand per Mn(II); [{Mn2(m-O2CCym)3(m3-O2CMe)(THF)2}1] (525). Exposure of 525 to 3,5-dimethylpyrazole (Hdmpz) led to substitution of every other THF ligand to form an essentially isostructural polymer; [{Mn2(mO2CCym)3(m3-O2CMe)(THF)(Hdmpz)}1] (526).534 A pair of zinc cymantrenecarboxylate complexes have been synthesized from CymCO2H and zinc acetate under different solvent conditions (Scheme 118). For example, the reaction of CymCO2H with hydrous zinc acetate in acetonitrile yielded a polymeric structure with Zn centers bridged by a single acetate moiety, and featuring water and k1-coordinated cymantrenecarboxylate ligands (527). Exposure of 527 to 3,5-dimethylpyrazole resulted in a water- and acetate-free bis(zinc) complex with each zinc center bridged by two pyrazolate moieties, and Zn atoms bound to terminal k1-cymantrenecarboxylate and k1-pyrazole ligands (528).535

Scheme 118

Cyclic and Non-Cyclic Pi Complexes of Manganese

475

Alternatively, when the reaction of CymCO2H and zinc acetate was conducted in the presence of methanol, an octahedral mono-zinc complex with 4 methanol ligands and 2 axial k1-cymantrenecarboxylate moieties (529) was isolated. Recrystallization of 529 from THF yielded a tri-zinc complex (530), where the central zinc atom is connected to the two terminal zinc atoms by (in each case) three cymantrenecarboxylate ligands (the terminal zinc atoms are also bound by two THF ligands each). Exposure of 530 to 1,10-phenanthroline led to substitution of the THF ligands on the terminal zinc atoms to afford 531.536 Complexes containing tin with cymantrenecarboxylate substituents have been prepared both from CymCO−2 and CymCO2H (Scheme 119). In the former case, CymCO−2 reacted with an iron(II) SnCl3 complex to form 532, an Fe complex featuring a Sn(k1-O2CCym)3 ligand.537 In addition, CymCO2H reacted with Me2SnCl2 in the presence of NEt3 in methanol to form a tetratin oxo cluster with two terminal (chelating) and two bridging cymantrenecarboxylate ligands (533).538

Scheme 119

CymCO2H has also been used as a precursor to prepare monometallic cymantrene derivatives by installation of new organic moieties at the carboxylic acid site (Scheme 120). For example, CymCO2H reacted with pentafluorophenyltrifluoroacetate, followed by benzylamine, to yield an amide derivative (534) for comparison with a 99mTc analogue.539 Reactions of the same precursor with amine-terminated peptides has allowed peptides to be attached with microwave assistance (535).540 Derivatives of 535 have been investigated for use in IR labeling using scattering scanning near-field infrared microscopy541 and to study their cytotoxicity.429,542 CymCO2H also reacted with ciprofloxacin to prepare an organometallic analogue of this species (536), which showed some antimicrobial and cytotoxic potential.455 A cymantrenyl-containing derivative of the Nematocidal drug Monopantel (537), formed via Steglich esterification using CymCO2H, has shown substantial activity against D. immitis microfilariae, though it was not active against nematodes.543

476

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 120

Protein-containing cymantrene derivatives (4 isomers) with an additional methyl substituent have also been prepared starting from derivates of MMT with a carboxylic acid substituent on the cyclopentadienyl ring, and the effect of different isomers on the structures of the proteins was investigated (538: Fig. 18).544

Fig. 18 Isomers of the protein-containing MMT derivative 538.

Additionally, cymantrene derivatives with pendent carboxylic acid functionalities have been used to prepare a range of complexes using reactions similar to those used to functionalize CymCO2H (Scheme 121). For example, the reaction of CymC(O)CH2CH2CO2H (351a; prepared as shown in Scheme 83) with Me2SnCl2 in the presence of NEt3 in methanol afforded the tetratin structure 539 featuring two CymC(CO)CH2CH2CO−2 anions (both of which are k1-coordinated to different tin centers) and two methoxide ligands. Structurally, 539 differs significantly from 533 (the product of analogous reactivity involving CymCO2H; Scheme 119).538 Multi-step procedures have been used to convert the same carboxylic acid-appended precursor into a series of cymantrene-containing platensimycin derivatives (540, 541, 542, 543, 544, and 545), which were investigated for antibacterial and antitumor activities.545 Another example is a chloroquine derivative featuring a cymantrenyl group (546), prepared from the reaction of N-(7-chloroquinolin-4-yl)ethane-1,2-diamine with Cym(CH2)3CO2H (in the presence of NaBH4), which was examined for anti-malaria, -leishmaniasis, and -trypanosomiasis effects.473

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 121

477

478

Cyclic and Non-Cyclic Pi Complexes of Manganese

Furthermore, various carboxylic acid-appended cymantrene precursors have been used to attach proteins to the manganese-bound cyclopentadienyl ring via an amide linker (Scheme 121). In general, the amide connection is made using one of a variety of standard organic synthetic techniques involving a protein tethered to a solid surface, from which it can be cleaved following connection to the cymantrenyl unit. Derivatives of these species have been prepared with various different spacers between the manganese-coordinated cyclopentadienyl fragment and the amide bond (547, 548, 549, 550, 551, and 552; 548-550 were prepared from derivatives of 351, which were synthesized as illustrated in Scheme 83).429,542,546–551 Most of these bioconjugates were investigated for cytotoxic effects. In addition, 547 was investigated with respect to altered intracellular distribution of the peptide in MCF-7 cells relative to the cymantrenyl-free protein,546 and 548 was examined for potential use in IR labeling.428 Among the derivatives of 548 prepared, some have other metal fragments incorporated into the protein at various points, with potential applications in multi-modal bioimaging and antibiotics.547–550 An acyl-chloride substituted cymantrene derivative (CymCOCl) has additionally been used to install new substituents to the Mn-coordinated cyclopentadienyl ligand, via CdN bond formation upon exposure to amines (Scheme 122). This method, followed by a deprotection step (conversion of ArOSitBuMe2 to ArOH groups within the organic portion of the molecule), was used to prepare the E isomer of a cymantrenyl-appended hydroxytamoxifen derivative (553), which was investigated for anticancer activity. The 99mTc analogue of 553 was also prepared, and in vivo distribution studies (of the 99mTc analogue) were undertaken in rats to model the potential biodistribution of Mn species 553.552

Scheme 122

Cymantrenyldibromoborane (CymBBr2) has been used to prepare a variety of boron-based species containing cymantrenyl fragments by reactivity which supplants one or both of the halide substituents on boron (Scheme 123). For example, exposure of CymBBr2 to highly nucleophilic cyclopentadienyl or pentamethylcyclopentadienyl iron dicarbonyl anions allowed for the isolation of [RCpFe(CO)2{BBr(Cym)}] (RCp ¼ Cp, Cp ; 554). These complexes can be considered to be iron complexes with a cymantrenyl-substituted boryl ligand.553 In addition, a wide variety of new complexes have been prepared using lithiated or sodiated reagents, installing various substituents on boron including amido (555 and 556), k1-cyclopentadienyl (557), or k1-indenyl (558 and 559; initially, the indenyl carbon atom bound to boron is sp3 hybridized, but NEt3 was shown to catalyze isomerization to the 3-indenyl isomer in which the carbon atom bound to boron is now sp2-hybridized) groups.554 Exposure of CymBBr2 to water yielded the tri-cymantrenyl-substituted boroxine [({OB(
5-C5H4)}Mn(CO)3)3] (560), in which all three cyclopentadienyl rings lie in the same plane as the boroxine unit in the solid state.554 CymBBr2 reacted with CymB(NMe2)2 to afford CymBBr(NMe2) (561) via substituent scrambling, and subsequent alkylation with MeMgCl yielded CymBMe(NMe2) (562).177 Sequential exposure of 562 to pyrazole and potassium pyrazolate afforded cymantrenylbis(pyrazol-1-yl)methylborate 563 (this complex contains two pyrazole rings capable of coordinating to manganese upon CO ligand dissociation; see Scheme 149 for the structure of the chelating product 695).177 Another cymantrenyl-containing borate (564) was generated by exposure of CymBBr2 to three equivalents of PhLi.177

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 123

479

480

Cyclic and Non-Cyclic Pi Complexes of Manganese

Complexes with boron-containing moieties bridging between two cymantrenyl fragments were prepared by exposure of CymBBr2 to HSiEt3; using 1.5 equivalents of HSiEt3 at room temperature, (Cym)2BBr (565) was formed with loss of B2H6, and this complex could be converted to Cym2BNMe2 (566) by reaction with Me3SiNMe2 (Scheme 123). By contrast, the low temperature reaction of CymBBr2 with excess HSiEt3 provided (CymBH2)2 (567) featuring an HB(m-H)2BH core. The BH substituents in compound 567 underwent insertion chemistry with HC^CtBu. When this chemistry was conducted in THF, a mixture of dimetallic 568 and monometallic 569 was formed, and preferential crystallization allowed for isolation of dimetallic 568. By contrast, when this reaction was performed in neat tert-butyl acetylene, monometallic 569 was formed exclusively. In addition, dimer 567 reacted with NMe2Et to afford monomeric CymBH2(NMe2Et) (570).555 As previously discussed (see Scheme 75), another Lewis base adduct of a cymantrenyl borane fragment (321) was prepared serendipitously by intramolecular insertion of a borylene fragment into a cyclopentadienyl CdH bond in [CpMn(CO)2(m-CO)2BtBu(DMAP)] (320).277 The reaction of CymBBr2 with 1,1-dimethyl-2,3,4,5-tetraphenylstannole afforded borole complex 571 upon tin-boron exchange. Compound 571 displays a weak MndB interaction, apparent from XRD and UV/Vis data, along with localized double bonds on the boron-containing ring and a planar environment at boron.556 The antiaromatic nature of 571 has been computationally probed.557 Furthermore, exposure of 571 to 4-subsituted pyridines allowed adducts to be synthesized in which boron is pyramidalized (572).556 Me CymBBr2, the methylated derivative of CymBBr2, has also been used to prepare new cymantrene-based complexes with boron-based substituents on the cyclopentadienyl ring (Scheme 124). For example, a diazaborolidine-substituted cymantrene derivative (573) was prepared upon reaction with tBuNH(CH2)2NHtBu in the presence of NEt3 (2 equiv.), and a diazaborole-substituted derivative of cymantrene (574) was prepared by reduction of a diazaborolium salt formed upon reaction of MeCymBBr2 with an a-diimine.558 It is notable that although the precursor used for this chemistry (MeCymBBr2) has been known for decades, its crystal structure has only recently been reported.553

Scheme 124

A pair of borate anions with cymantrenyl and pyrazolyl substituents on boron have been prepared from neutral manganese-containing starting materials (Scheme 125). For example, CymBMe2 reacted with potassium pyrazolate to yield the cymantrenylmono(pyrazol-1-yl)dimethylborate anion 575.177 Furthermore, a new synthetic route was reported for the previously reported cymantrenyl tris(pyrazol-1-yl)borate anion, in this case as the potassium salt.177 The pyrazolyl rings in these complexes are capable of coordinating to Mn upon CO ligand dissociation (see Scheme 149 for the structures of chelating products 695).

Scheme 125

A series of heterobimetallic complexes where a substituent on the cyclopentadienyl ring in a cymantrene fragment is coordinated to another transition metal center have been reported (Scheme 126). For example, the reaction of the (2-pyridyl)-substituted cymantrene derivative CymPyr with cis-[Cl2Pt(DMSO)2] yielded the cycloplatinated complex 576.559 Additionally, deprotonation of a pendent indene group followed by reaction with metal halides or a PF−6 salt afforded cymantrene-containing indenyl complexes of Rh, Ir, or Fe (577; as illustrated in Scheme 136, a derivative of 577 with a manganese indenyl fragment linked to the Mn-coordinated cyclopentadienyl ring was generated via analogous reactivity with [BrMn(CO)5]).560

Cyclic and Non-Cyclic Pi Complexes of Manganese

481

Scheme 126

Cymantrene-containing ladder complexes with a “CpM(CO)2” substituent on the cyclopentadienyl ring are of interest given their unusual electrochemical properties. The previously reported example [({
5-CpFe(CO)2}C5H4)Mn(CO)3], along with a number of other Fe and W derivatives, were recently the subject of electrochemical and spectral studies of intramolecular charge transfer.561 In addition, [({
5-CpFe(CO)2}C5H4)Mn(CO)3] was found to undergo lithiation selectively at the iron-coordinated cyclopentadienyl group, allowing a PPh2 group to be attached to that ring (affording 578); Scheme 127.562

Scheme 127

Lastly, a cymantrene derivative with a 1-methyl-3-acetyl-2-indolylmethyl cyclopentadienyl substituent (579) was serendipitously formed via thermal decomposition of the
3-allyl complex 59 by manganese-mediated hydride migration (Scheme 128).84

Scheme 128

DFT calculations have also been reported in the 2005–2020 time period on a previously reported cymantrene derivative with a cationic tropylium substituent on the cyclopentadienyl ring, to investigate its unusual electronic structure (Fig. 19; left).563 In addition, DFT calculations have been conducted on a hypothetical hydrazone-substituted cymantrene derivative (Fig. 19; right) to investigate molecular properties such as the HOMO-LUMO gap, charge distribution, nucleophilicity, polarizability, and hyperpolarizability.564

482

Cyclic and Non-Cyclic Pi Complexes of Manganese

Fig. 19 Previously reported and hypothetical derivatives of cymantrene, with a non-alkyl substituent on the cyclopentadienyl ligand, which have recently been the subjects of computational studies.

The electrochemical properties of many of the newly reported cymantrene-based species have been investigated. Furthermore, electrochemical analysis has been performed on previously reported tamoxifen derivatives of cymantrene (in which the radical cations were found to be charge-delocalized between the “cymantrenyl” and diphenylethene moieties).565 Electrochemical analysis has been carried out on cymantrene complexes which have been tethered to a glassy carbon electrode via dinitrogen liberation from a cyclopentadienyldiazonium manganese tricarbonyl cation,566 or covalently attached to electrode surfaces via an ethynyl linkage by anodic oxidation of CymCH2C^CLi.567 It is notable that upon oxidation, cymantrene derivatives undergo more facile carbonyl substitution in the presence of Lewis bases than the neutral species (i.e. under ambient conditions in the absence of irradiation).565–567 Previously reported substituted cymantrene derivatives have been investigated for various applications. For example, the effect of a series of azinyl-substituted cymantrene species on the radical polymerization of MMA was investigated.568 In addition, trimethylsilyl or tert-butyl substituted cymantrene derivatives were investigated as potential precursors for CVD of manganese-containing films (primarily oxides),122 and the ability of fluorine atoms in [{
5-C5H4(p-C6F4CF3)}Mn(CO)3] (Fig. 20) to act as a halogen bond donor has been discussed.569 Furthermore, cymantrenyl groups covalently bound to bovine serum albumin proteins via amidine linkers have been used as redox markers in an electrochemical microbead-based immunoassay (Fig. 20).570 Lastly, new single crystal X-ray structures have been obtained for previously reported iodo or formyl substituted, and alkyne bridged, cymantrene derivatives, as well as a BBr2/methyl disubstituted derivative of cymantrene (Fig. 20).461,571,553

Fig. 20 Previously reported derivatives of cymantrene, with a non-alkyl substituent on the cyclopentadienyl ligand, which have recently been the subjects of publications involving new characterization or applications.

5.07.3.1.4 Derivatives of cymantrene (IV) [(C5H5-xRx)Mn(CO)3] (x ¼ 2-5, R 6¼ H or CyH2y+1 alkyl); not including fused polycyclic cyclopentadienyl derivatives This section focuses on the synthesis of complexes of the type [(C5H5-xRx)Mn(CO)3] (x ¼ 2-5, R 6¼ H or CyH2y+1 alkyl); derivatives of cymantrene with multiple non-alkyl (alkyl ¼ CyH2y+1) substituents on the cyclopentadienyl ligand. For the most part, synthetic routes used to access these species mirror some of those discussed in Section 5.07.3.1.3 for cymantrene derivatives with a single non-alkyl (alkyl ¼ CyH2y+1) substituent on the cyclopentadienyl ring, and will be discussed in a similar order.

Cyclic and Non-Cyclic Pi Complexes of Manganese

483

A common route to access polysubstituted cymantrene derivatives is reaction of a Mn(I) bromide precursor with an alkali metal or thallium(I) cyclopentadienyl salt (with 2, 3, or 5 non-hydrogen substituents on the ring); Scheme 129. Examples of the substituents on cyclopentadienyl ligands that have been installed in this way are fluorinated tolyl groups,422 other aryl groups,572,573 and acyl groups with furan574 or bromofuran575 substituents. New complexes of this type are numbered only when they appear in other schemes.

Scheme 129

Very bulky pentaphenyl or pentatolyl-substituted cymantrene derivatives (580) were formed by coupling between a Mn(I) halide precursor and a mono-brominated cyclopentadiene promoted by elemental zinc; these reactions presumably proceeded via initial formation of a cyclopentadienyl zinc compound (Scheme 130).572 Compound 580 was also prepared using the cyclopentadienyl lithium salt (Scheme 129).

Scheme 130

Utilizing n-butyl lithium, deprotonation of cymantrene derivatives containing a substituent on the cyclopentadienyl ring can occur either on the previously installed substituent (as in Scheme 89) or on the Mn-coordinated cyclopentadienyl ring itself (Scheme 131). In the latter case, quenching with an appropriate electrophile can install a second substituent on the cyclopentadienyl ring, often regioselectively (Scheme 131). Using an excess of nBuLi, deprotonation of the cyclopentadienyl ring in a carbamate- or urea-substituted cymantrene derivative {CymCHMeNHC(O)R} followed by quenching with dimethylformamide (DMF) or allyl bromide selectively installed a formyl or allyl group at the position ortho to the carbamate or urea substituent, forming 581 and 582 (this is in contrast to analogous reactivity with a similar cymantrene complex containing an NMe group in place of an NH group, where deprotonation occurred at the aliphatic carbon bound to the cyclopentadienyl ring; see syntheses of 383–386 in Scheme 89). By contrast, lithiation of an allyl-substituted cymantrene derivative occurred stereoselectively at the meta position, whereas lithiation of an allyloxymethyl-substituted cymantrene species proceeded non-stereoselectively; these deprotonated species were quenched with DMF, yielding formyl-substituted species (583, 584, and 585).445

484

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 131

Cyclic and Non-Cyclic Pi Complexes of Manganese

485

Deprotonation of the cyclopentadienyl ring in various derivatives of cymantrene was also accomplished using other reagents (Scheme 131). For example, a silyl-substituted cymantrene precursor was stereoselectively lithiated using tBuLi at the meta position, and subsequent alkylation yielded 1-methyl-3-trimethylsilylcyclopentadienyltricarbonylmanganese(I) (586; this derivative of cymantrene was investigated as a potential CVD precursor to prepare manganese-containing thin films).122 Using LDA, the sulfoxide-substituted cymantrene species 366 (prepared as illustrated in Scheme 85) was selectively deprotonated ortho to the sulfoxide substituent, yielding 587. A quinolinyl substituent was installed at the lithiated site of 587 to afford 588, either via SNH reactivity using quinoline and DDQ, or by initial reaction with zinc bromide followed by Negishi cross-coupling with 2-bromoquinoline in the presence of [Pd(dba)2] and P(o-furyl)3.436 Lastly, lithium tetramethylpiperidide (LiTMP) could be used to deprotonate bromo-substituted cymantrene (CymBr) selectively at the ortho position; successive quenching with acetone and ammonium chloride afforded a cymantrene analogue with ortho-disposed bromo and CMe2(OH) substituents (589), which underwent dehydration to form a 2-propenyl-containing cymantrene derivative (590) upon exposure to acid.576 In addition, the bromo substituent on 590 was replaced with a CH2CMe]CH2 group by successive lithiation and coupling with 3-bromo-2-methylpropene in the presence of CuCl and 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU), affording a vinyl-allyl disubstituted cymantrene derivative (591).576 Cymantrene derivatives with a polyhalogenated cyclopentadienyl ligand have often been utilized as precursors to access new polysubstituted cymantrene complexes (Schemes 132-133). For example, treatment of [(C5Cl4Br)Mn(CO)3] with nBuLi resulted in lithium-bromine exchange, and subsequent reaction with divalent group 10 halides afforded heterometallic complexes with phosphine (592) or alkene (593) co-ligands on the group 10 metal; Scheme 132. In the latter case, an alkene-free dimer, 594, was also observed. Furthermore, reactions of halide-substituted cymantrene derivatives (including those with SiMe3 or SMe substituents) with in situ generated [(ethylene)Ni(PMe3)2] yielded products (Scheme 132) where a carbon-halogen bond has undergone oxidative addition at the Ni(0) center (595 and 596a-b; in some cases an equilibrium was observed between the starting materials and the products).577

Scheme 132

486

Cyclic and Non-Cyclic Pi Complexes of Manganese

Polybromide-substituted derivatives of cymantrene [(
5-CH5-xBrx)Mn(CO)3] (x ¼ 3-5) have been used to access cymantrene complexes with multiple fluorinated alkyl substituents on the cyclopentadienyl ring (597a–f) via Pd(II) catalyzed alkylation with a zinc alkyl halide (Scheme 133). While reactions involving tri- and tetra- substituted precursors primarily afforded a single product where each bromide substituent was replaced with a fluorinated alkyl group (597c–e), reactivity involving [(
5-C5Br5)Mn(CO)3] afforded a mixture of di-, tri-, tetra- and pentasubstituted products [(
5-CH5-xRx)Mn(CO)3] (x ¼ 2-5, R ¼ (CH2)n(CF2)7CF3, n ¼ 2 or

Scheme 133

Cyclic and Non-Cyclic Pi Complexes of Manganese

487

3; 597a–f). Protonation of these complexes allowed isolation of the free cyclopentadienes, which could then be deprotonated and attached to other transition metals.447 A family of “starburst” or “radial” oligocyclopentadienyl metal complexes based on the [5]radialene core (598) have been prepared by palladium-catalyzed coupling of pentaiodo cymantrene [(C5I5)Mn(CO)3)] (CymI5) with [(C5H4ZnBr)Re(CO)3], [(CpFe)2{m-(C5H4)2Zn}], or [{Mn(CO)3}2{m-(C5H4)2Zn}] (Scheme 133). Photochemical decarbonylation of the hexamanganese derivative of 598 yielded a structure where two adjacent Mn centers are linked via a MndMn bond and a bridging carbonyl ligand (599); the stability of this complex is remarkable, give the instability of the parent [Cp2Mn2(CO)5]. Furthermore, irradiation of the iron-containing derivative of 598 in the presence of 1-heptyne yielded the free substituted cyclopentadiene, whereas a phenoxy-substituted cyclopentadiene was formed when this reaction was performed in the presence of phenol.578 Other reactions of 598, which resulted in annulation of adjacent substituents on the central cyclopentadienyl ring, are covered in Scheme 141. Various new disubstituted derivatives of cymantrene have been prepared by manipulating previously reported disubstituted derivatives with a formyl group on the cyclopentadienyl ligand (Scheme 134). For example, (Rp)-(hydroxymethyl)

Scheme 134

488

Cyclic and Non-Cyclic Pi Complexes of Manganese

cymantrenecarbaldehyde and ((2S,4S,5R)-3,4-dimethyl-5-phenyloxazolidin-2-yl)cymantrene reacted with amines to generate imine functional groups (600579 or 601,580 respectively). The hydroxyl moiety in 600 was further functionalized by reaction with ClP(NR2)2 to form a phosphorodiamidite group, and the resulting chiral bidentate ligands (602) were shown to afford high enantioselectivities in Pd-catalyzed allylic substitution of 1,3-diphenylallyl acetate with various organic reagents.579 In addition, the hydroxyl group in 601 could be phosphorylated (by reaction with ClP(OAr)2) to yield 603, which contains both phosphite and imine donors, as well as multiple chiral elements. The pendent imine and phosphite donors in 603 were coordinated to Pd(II) (via reactions with [Cl2Pd(COD)] or [{ClPd(allyl)}2], in the latter case with concomitant chloride abstraction using AgBF4) to afford neutral (604) or cationic (605) heterometallic complexes, the latter of which was active towards stereoselective allylic alkylation of 1,3-diphenylallyl acetate with dimethyl malonate.580 Furthermore, the Prato reaction involving sarcosine and a dimetallic cymantrene derivative (containing both a formyl and cymantrenyl group) was used to prepare dimetallic fullerene-substituted complexes (606), in a similar manner to the synthesis of monometallic 435 (Scheme 102), which are of interest due to their unusual electrophysical and optical properties.581–583 In addition, manipulation of the formyl substituent in 1-formyl-3-(N,N-dimethylaminomethyl)cymantrene has been used to prepare cymantrene derivatives with a pendent quinoline moiety tethered to the Mn-bound cyclopentadienyl ring by a CH2NHCH2CH2NH (607) or CH2NH (608) linker (Scheme 134). Compound 607 was prepared by reductive amination using a quinoline derivative with a tethered primary amine. By contrast, the synthesis of 608 (a direct analogue of the potent antimalarial drug ferroquinone) was achieved via a multi-step process involving initial reduction of the aldehyde to an alcohol, which could be converted to a phthalimide using the Mitsunobu reaction, followed by transformation into a primary amine by a Gabriel-type reaction, and finally coupling with 7-chloro-4-fluoroquinoline.506 Like monosubstituted 484 (an analogue of 608 without the CH2NMe2 substituent; see Section 5.07.3.1.3), oxidation of 607 and 608 was found to electrochemically irreversible, consistent with oxidation of the secondary amine rather than the metal center.506,507 Upon exposure to excess BBr3, cymantrene reacted to afford [{1,3-C5H3(BBr2)2}Mn(CO)3] (609), a cymantrene derivative with two BBr2 substituents (Scheme 135).584 Compound 609 reacted with Me3SiNMe2 to afford a species with B(NMe2)2 substituents ([(1,3-C5H3{B(NMe2)2}2)Mn(CO)3]; 610), which provided a useful starting point to attach more complex substituents to the boron atoms on the cyclopentadienyl ring. For example, 610 reacted with four equivalents of pyrazole and two equivalents of potassium pyrazolate to afford a derivative of cymantrene with two scorpionate substituents on the Mn-coordinated cyclopentadienyl ring (611). Reaction of 611 with 2 equivalents of [BrMn(CO)5] afforded a trimanganese species with a central cymantrene

Scheme 135

Cyclic and Non-Cyclic Pi Complexes of Manganese

489

fragment (612). In addition, scorpionate coordination to the central manganese center occurred upon carbonyl co-ligand dissociation from 611 (see Scheme 149 for the synthesis of 696).585 Compound 609 has also been used to access a cymantrene derivative with two borole substituents (613), which reacted with pyridines to form the Lewis base adducts 614 (Scheme 135). This chemistry is closely analogous to the syntheses of mono-substituted derivatives 571 and 572 (see Scheme 123), and the complexes bear significant structural similarities. Electrochemical reduction of bis(borole) 613 proceeded in three steps; initial single electron reduction (to a monoanion with the radical centerd on one of the borole rings), a second single electron reduction (forming a dianion with a radical centered on each borole ring), and a two-electron reduction to a tetraanion; this reduced species (615) could be isolated upon chemical reduction using Mg-anthracenide, and features a rare interaction between one of the Mg centers and a carbonyl ligand on Mn.586 Both neutral 613 and anionic 615 were probed computationally to analyse the antiaromatic nature of neutral borole moieties, and their increasing aromaticity upon reduction.557

5.07.3.1.5

Derivatives of cymantrene (V) [(C5H5-xRx)Mn(CO)3] (C5H5-xRx ¼ fused polycyclic cyclopentadienyl ligand)

Various cymantrene-like complexes ([(C5H5-xRx)Mn(CO)3]) have been reported where the cyclopentadienyl ring is incorporated into a fused ring system. In this section, complexes are discussed where the adjacent fused ring is either aromatic or aliphatic, although in each case the part of the polycyclic ligand which coordinates to Mn is a cyclopentadienyl-like 5-membered ring. The majority of these complexes reported in the 2005–2020 period feature monoanionic fused polycylic rings, and were prepared by one of two synthetic routes. One of these is salt metathesis involving a Mn(I) halide precursor and a polycyclic anion (where at least one of the incorporated rings is a C5 cyclopentadienyl-like moiety), generally introduced as an alkali metal, thallium, or tin reagent (left in Scheme 136). Complexes synthesized by this route, which mirrors approaches discussed in Schemes 82 and 129 to install substituted cyclopentadienyl ligands on a tricarbonyl manganese(I) fragment, are discussed in section Section 5.07.3.1.5.1. The other commonly employed route to prepare fused ring cymantrene derivatives involves manipulation of ortho-disposed substituents within a [nRCpMn(CO)3] complex (n ¼ 2–5) to tether them together (right in Scheme 136); complexes prepared in this manner are discussed in section Section 5.07.3.1.5.2. Manganese tricarbonyl complexes containing formally dianionic pentalene ligands are covered in section Section 5.07.3.1.5.3.

Scheme 136

5.07.3.1.5.1 Analogues of cymantrene featuring a fused cyclopentadienyl ligand prepared via salt metathesis Examples of cymantrene analogues containing a fused cyclopentadienyl ligand which were prepared via the salt metathesis route (left in Scheme 136) are provided in Fig. 21. These include many manganese(I) tricarbonyl complexes with singly-substituted
5-indenyl ligands (inset in Fig. 21), with dimethylamino (616),587 2-pyridyl (617),588 triptycenyl (618),589 and transition metal-containing (619 and 620)560,590 substituents. In 618, it was found that rotation of the CdC bond to the triptycenyl substituent was slowed by a factor of 108 relative to metal-free triptycenyl indene.589 Oxidation of the various dimetallic derivatives of 619 yielded complexes in which intermetallic interactions were proposed, and oxidation of ferrocenyl-substituted 620 yielded a rare example of a complex with straightforward correlation between metal-to-metal electronic coupling and the redox asymmetry of the donor and acceptor sites predicted by Hush theory.560,590 Additionally, the salt metathesis route was used to access a pair of cymantrene derivatives with extremely bulky polycyclic octamethyloctahydrodibenzofluorenyl (621)591 or dibenzo[c,g]fluorenyl (622)592 ligands. Crystals of 621 gave rise to unusual diffuse X-ray diffraction,591 and the high electron donating ability of the polycyclic ligand was demonstrated by IR spectroscopy and DFT analysis.593 Heteroatom-containing cymantrene derivatives with thiophene (623)594 or pyridazine (624)595 rings fused to the cyclopentadienyl fragment were also prepared by the salt metathesis route.

490

Cyclic and Non-Cyclic Pi Complexes of Manganese

Fig. 21 Analogues of cymantrene which feature a fused cyclopentadienyl ligand and have been prepared via salt metathesis; the inset highlights examples where the fused cyclopentadienyl ligand is an
5-indenyl anion.

A pair of manganese(I) tricarbonyl complexes featuring a fused tricyclic ligand bound to the metal via a C5 ring (
5-benzo[e] indenyl-coordinated 625596 and
5-hydroindacenyl-coordinated 626,597 as shown in Fig. 21) were also prepared by the salt metathesis route (left in Scheme 136). Manipulation of complexes 625 and 626 was used to access heterometallic complexes where another metal fragment is bound to one of the other (i.e. not coordinated to Mn) hydrocarbyl rings in the polycyclic system (Scheme 137). Starting from bromo-substituted 625, lithiation followed by exposure to [Cr(CO)6] and quenching with MeOSO2CF3 installed a chromium carbene moiety onto the ring system (627). Chromium-templated benzannulation of the carbene moiety yielded two isomers of a tetra-fused ring system, where the 5-membered ring remains coordinated to Mn and the newly generated C6 arene ring is coordinated to a Cr(CO)3 fragment; 628 (the two isomers differ in whether the two metals are located on the same or opposite faces of the ring system). Upon heating, both isomers of 628 underwent a rearrangement in which the Cr(CO)3 fragment migrated from one arene ring to another, while remaining coordinated to the same face of the ring system (yielding 629).596 Stemming from the polyalkyl-s-hydroindacenyl derivative 626, a second transition metal could be coordinated to the manganese-free C5 ring by deprotonation followed by exposure to [Cp Ni(acac)] or [{ClRh(COD)}2], affording 630597 or 631,598 respectively. In 630 and 631, the two metal fragments bind to opposite faces of the fused dianionic tricyclic ligand, and the COD ligand in the Rh-containing species 631 could be supplanted by two equivalents of CO to form 632. Furthermore, reduction of 630 with 1,10 ,3,30 -tetramethyl-2,20 -biimidazolidine yielded an organometallic radical anion, which afforded EPR spectra with very well resolved hyperfine coupling (indicative of spin distribution displaced towards the nickel fragment).599

Cyclic and Non-Cyclic Pi Complexes of Manganese

491

Scheme 137

5.07.3.1.5.2 Analogues of cymantrene featuring a fused cyclopentadienyl ligand prepared via annulation The
5-cyclopenta[c]thienyl- and pyridazine-fused cyclopentadienyl tricarbonyl manganese(I) complexes (623 and 624) described in Section 5.07.3.1.5.1 were alternatively prepared (with a broader range of R-groups) by annealing together two ortho-substituted acyl substituents on a derivative of cymantrene (Scheme 138). Synthesis of 623 was accomplished using P4S10 in the presence of sodium bicarbonate and CS2,594 while exposure of the bis(acyl)-substituted precursor to hydrazine afforded 624.574,600 Structurally, 623 features a planar bicyclic ligand with the Mn atom coordinating to the C5 ring, displaced to the opposite side of the centroid from the ring-fusion bond in the bicyclic ligand.594 Notably, 624 was only observed spectroscopically, and spontaneously decomposed to yield various Mn-containing species and a free pyridazine.574,600 Direct Analysis in Real Time (DART) Mass-spectrometric analysis of 624 (where R ¼ p-tolyl) was later reported.601

Scheme 138

Reaction of chlorosilyl/halide polysubstituted manganese(I) tricarbonyl complexes with water formed oxadisilole rings fused to the Mn-coordinated cyclopentadienyl fragments. Depending on the reaction conditions and nature of the precursor, various oxadisilole-containing derivatives (633, 634, 635, 636, and 637) were obtained; Scheme 139.602

492

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 139

Coupling of two ortho-situated substituents in a cymantrene derivative has been utilized to prepare new cymantrene analogues with a fused polycyclic cyclopentadienyl ligand stemming from a chiral manganese(I) tricarbonyl precursor where the cyclopentadienyl ligand contains an acetal group (Scheme 140). An enantiopure cymantrene-fused phosphole complex (638) was prepared from this precursor in a multistep process; top of Scheme 140. First, conversion of the acetyl precursor to a formyl/halide disubstituted analogue of cymantrene (639) was accomplished by diastereoselective lithiation ortho to the acetal, halogenation (RLi !RBr), and then hydrolysis of the acetal moiety. After this, the formyl substituent on the cyclopentadienyl ring was transformed into a dibromovinyl substituent (CH]CBr2) by Wittig dibromoolefination (to afford intermediate 640), followed by regioselective reduction (CH]CBr2 ! Z-CH]CHBr) using HSn(nBu)3 in the presence of catalytic [Pd(PPh3)4] to afford intermediate 641. The phosphole fragment in 638 was then formed by lithiation of the adjacent halide and halogen-terminated vinyl substituents in 641 followed by reaction with PhPBr2.603 The same acetal-substituted precursor was used to access a pair of cymantrene derivatives with a cyclopentadienyl-fused 4-dialkylaminopyridine ring (642; bottom of Scheme 140). This was accomplished by initial ortho-amination (by lithiation, followed by reaction with TsN3 and reduction by NaBH4) to form NH2-substituted species 643, followed by tosylation of the amine and acetal hydrolysis (RCH(OR)2 !RCHO) to afford 644. After this, cyclization with ethynyl magnesium bromide (using MnO2 and [NBu4][I]) afforded a pyridone-fused cymantrene moiety in 645. Finally, exposure of 645 to N-trimethylsilyamine reagents in the presence of TiCl4 yielded enantiopure (S)-642, which demonstrated activity for catalytic enantioselective acetylation of rac-1-phenyl-1-hydroxy-2,2-propane with acetylacetone.604

Scheme 140

Cyclic and Non-Cyclic Pi Complexes of Manganese

493

The pentaferrocenyl derivative of the “starburst” complex 598 (prepared as outlined in Scheme 133) reacted with TsOH or [Cu(OTf )2], in both cases resulting in Fe extrusion and coupling of ortho-situated cyclopentadienyl substituents to generate 646 or 647, respectively (Scheme 141).578

Scheme 141

5.07.3.1.5.3 Derivatives of cymantrene with pentalene-based ligands In addition to the formally monoanionic fused ring systems described above, dimetallic complexes with a bridging dianionic pentalene ligand (Pn2−) have also been reported (Scheme 142). The p ligand in these complexes can be considered to consist of two fused cyclopentadienyl rings, and results in a 10 p-electron system with some similarities to a cyclooctatetraenide dianion. Alkyl-substituted derivatives of [(anti-m-
5:
5-pentalene){Mn(CO)3}2] (648) have been prepared by exposing LiR2 Pn to two equivalents of a manganese(I) bromide reagent.605 These structures are isostructural to the previously reported unsubstituted derivative (i.e. 648 where R ¼ H), though the CdC bond between the substituted carbon atom and the adjacent quaternary carbon atom is slightly elongated relative to the analogous CdC bonds in the unsubstituted rings.605 DFT analysis of the electronic structure of the previously reported unsubstituted pentalene derivative has also recently been reported.606,607 Furthermore, calculations have been conducted on the electronic structures of derivatives of syn and anti 648 with 3-5 or 7-8 carbonyl ligands.608 Related to this, EHMO and DFT calculations have been conducted on the potential energy hypersurface for the haptotropic rearrangement of a single Mn(CO)+3 fragment from one 5-membered ring to the neighbouring 5-membered ring in an asymmetrically substituted dianionic syn-dibenzpentalene moiety.609

Scheme 142

5.07.3.1.5.4 Analogues of cymantrene featuring a fused cyclopentadienyl ligand: Miscellaneous chemistry Several cymantrene-like complexes with a fused cyclopentadienyl ligand (540, 541, and 542) were included in Scheme 121, which covers chemistry stemming from carboxylic acid-appended cymantrene complexes (including CymC(O)CH2CH2CO2H, which was the precursor in the syntheses of these complexes).545 In addition, compound 649 (which contains a tetrasubstituted indenyl moiety; Fig. 22) was observed as a by-product in the synthesis of P(OMe)3-substituted analogue 675, and this reactivity is included in Scheme 175. Notably, 649 could be isolated from this reaction mixture chromatographically.140

Fig. 22 An analogue of cymantrene featuring a tetrasubstituted
5-indenyl Ligand.

494

Cyclic and Non-Cyclic Pi Complexes of Manganese

Reactivity of a previously reported tricarbonyl indenyl manganese(I) complex with ortho-situated NMe2 and P(S)iPr2 substituents on the C5 ring was also reported in the 2005-2020 period (Scheme 143). Exposure of this complex to Cl3SiSiCl3 resulted in reduction of the phosphine sulfide group to a phosphine, affording iPr2P/Me2N-disubstituted 650.587 Compound 650 reacted with [{ClRh(COD)}2] and AgBF4 to afford a cationic heterobimetallic complex where the pendent phosphine and amine donors on the manganese-coordinated cyclopentadienyl ring coordinate to Ru (651). This complex was active towards catalytic hydroboration of styrene.610

Scheme 143

5.07.3.1.6 Derivatives of cymantrene (VI) derivatives of cymantrene with both (a) one or more non-alkyl substituent on the cyclopentadienyl ligand, and (b) one or more non-carbonyl co-ligand; [(C5H5-xRx)Mn(CO)3-nLn] (x ¼ 1–5, n ¼ 1-2, R 6¼ H or CyH2y+1 alkyl, L 6¼ CO) Sections 5.07.3.1.1, 5.07.3.1.3, and 5.07.3.1.4 reviewed derivatives of cymantrene where a carbonyl group has been substituted by another neutral ligand, and cymantrene derivatives with one or more non-alkyl (alkyl ¼ CyH2y+1) substituents on the cyclopentadienyl ligand, respectively. However, a small number of cymantrene analogues which incorporate both a substituted cyclopentadienyl ligand and one or more non-carbonyl co-ligand were reported in the 2005-2020 period; [(C5H5-xRx)Mn(CO)3-nLn] (x ¼ 1-5, n ¼ 1–2, R 6¼ H or CyH2y+1 alkyl, L 6¼ CO; note that CyH2y+1 alkyl-substituted cyclopentadienyl derivatives where L 6¼ CO were covered in Section 5.07.3.1.1). The most common non-carbonyl neutral ligands employed in [(C5H5-xRx)Mn(CO)3-nLn] complexes are phosphines (e.g. Schemes 144 and 145). Many examples of manganese dicarbonyl phosphine complexes with a monosubstituted cyclopentadienyl ligand ([(C5H4R)M(CO)2(PR0 3)], R 6¼ H; 652) have been prepared by photochemically-induced carbonyl substitution from tricarbonyl precursors (Scheme 144).16,453,454,488,503,509,576,611 Related chemistry with multiple cyclopentadienyl substituents has also been described {when PR0 3 ¼ PR2(CH2CR0 ]CH2); not shown in Scheme 144}.576,611 Similar chemistry has also been used to displace two carbonyl ligands (one on each metal) from a dimetallic precursor where two [RCpMn(CO)3] fragments are tethered together by a ferrocene-containing linker (458; yielding 653).488 Carbonyl substitution has also been conducted under non-photochemical conditions, where installation of phosphine ligands was promoted by (a) employing the carbonyl-abstraction agent trimethylamine N-oxide {used in the synthesis of [(C5Ph5)Mn(CO)2(PMe2Ph)] (654) from 580}572 or (b) by electrochemical oxidation of the initial cymantrene derivative to a 17 electron radical cation, which can undergo associative rather than dissociative substitution; Scheme 144. Oxidation-induced ligand substitution has been used to prepare 17 electron mono- (655) and di- (656) substituted P(OMe3) species where the cyclopentadienyl ligand is covalently attached to an electrode via an alkynyl linker,567 as well as P(OPh)3-containing species with an NH2, iodo, formyl, or vinyl substituent on the cyclopentadienyl ligand (657).612 The derivative of 17 electron complex 657 with a vinyl substituent was subsequently reduced, to isolate the 18 electron phosphite-substituted cymantrene derivative 658 (the overall reaction sequence of oxidation, substitution, and reduction has been referred to as the “electrochemical switch method”).493 Phosphine-substituted cymantrene derivatives with various substituents on the cyclopentadienyl ring have alternatively been prepared via thermally-promoted substitution of a labile neutral ligand in a dicarbonyl precursor (Scheme 145). For example, displacement of a weakly-bound carbamate donor (within a chelating cyclopentadienyl-carbamate ligand) in 680 (prepared as outlined in Scheme 149) afforded phosphine-coordinated 659.509 In addition, a phosphite-substituted cymantrene derivative with a substituent on the cyclopentadienyl ring (660) was formed upon heating allyl complex 59 in the presence of P(OMe)3 (in the absence of free phosphite, an isostructural tricarbonyl analogue, 579, was formed; see Scheme 128 for synthesis of 579).84

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 144

495

496

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 145

Irradiation of some cymantrenylquinazolinone derivatives 395 or 449 (prepared as outlined in Schemes 93 and 105, respectively) resulted in carbonyl substitution by solvent (THF, cis-3-hexene, or acetonitrile; 661 and 662); Scheme 146. The weakly-bound solvent ligands in 662 proved susceptible to thermal substitution by cis-3-hexene, PPh3, or phenylacetylene (to afford 663). This reactivity contrasts that described for other derivatives of 395 (with a styryl group in place of the methyl substituent on the quinazolinone ring) which, upon irradiation and carbonyl dissociation, yielded complexes resulting from intramolecular alkene coordination (see preparation of 12 and 711 in Scheme 153).14 In addition, reaction of pyridine or THF with various cymantrene derivatives featuring intramolecular coordination of a pendent ketone or amide donor (677 or 678; prepared as illustrated in Scheme 149) resulted in displacement of the weakly-bound oxygen donor to afford complexes 664433 and 665,14 as shown in Scheme 146.

Scheme 146

Cyclic and Non-Cyclic Pi Complexes of Manganese

497

A derivative of cymantrene with an extremely bulky pentasubstituted cyclopentadienyl ligand, [{C5(p-C6Hn4Bu)5}Mn(CO)3] (666: prepared as described in Scheme 129), has been used to access a series of complexes with unusual phosphorus-based co-ligands in place of one of the carbonyl ligands (Scheme 147). Initial photochemically-induced CO substitution installed a weakly bound THF ligand (forming 667), which could be thermally displaced by P4 to form complexes with bridging (668) or terminal (669) P4 ligands.573 These complexes are unusual for transition metal P4 complexes, in that they are both soluble in most organic solvents and stable at room temperature (in solution and solid states).573 Upon exposure of 668 to Co or Fe arene complexes, opening of the P4 ligand and coordination to the Co or Fe atoms was observed to generate 670 and 671 (where the co-ligand on the non-manganese transition metal is a cyclopentadienyl or b-diketiminate derivative, respectively).613 The planar 1 cyclo-P4 ligand in these complexes (which is best described as a p-delocalized P2− 4 system) is k coordinated to the two Mn atoms 4 613 and
-coordinated to Fe or Co.

Scheme 147

Reactions of phosphine-substituted derivatives of cymantrene containing various substituents on the Mn-coordinated cyclopentadienyl ring have also been reported, generally involving manipulation of the ring substituent (Scheme 148). For example, a PPh3-substituted derivative of cymantrene with a pendent phosphite donor (672)503 was prepared by reaction of [{
5-C5H4(CH2OH)}Mn(CO)2(PPh3)] with ClPR2 (in the presence of NEt3); this chemistry is equivalent to that described in Scheme 111 for the synthesis of tricarbonyl complex 479. Additionally, a PPh3-substituted derivative of cymantrene with a chromium carbene substituent on the cyclopentadienyl ring (673)439 was prepared by lithiation of the cyclopentadienyl ring in [CpMn(CO)2(PPh3)], followed by reaction with [Cr(CO)6] and quenching with [Me3O][BF4]; this chemistry is analogous to that described in Scheme 86 for the synthesis of tricarbonyl complex 371. Also, lithiation of a ladder complex where a CpFe(CO)2 moiety is a substituent on a Mn-bound cyclopentadienyl ring, followed by exposure to Cl3SiMe, yielded a hexametallic species (674) formed via attachment of three iron-coordinated cyclopentadienyl rings to a central silicon atom.562 A derivative of cymantrene with a tetrasubstituted indenyl ligand and a phosphine or phosphite co-ligand (675; Fig. 23) has been prepared from a cationic trindane precursor via a reaction with KOtBu which was proposed to occur via elimination of two equivalents of H2 from initially formed [(
6-trindane)Mn(CO)2H] (see Scheme 175).140

498

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 148

Fig. 23 A derivative of cymantrene featuring both a tetrasubstituted
5-indenyl ligand and a phosphine or phosphite co-ligand.

Lastly, a family of hypothetical dimetallic complexes in which an alkane bridges between the two manganese centers in [C2{(
5-C5H4)Mn(CO)2}2] (Fig. 24) were investigated computationally, and binding energies to the alkane were calculated to range from 8.0-12.7 kcal mol−1 depending on the alkane investigated.384

Fig. 24 Hypothetical dimanganese complexes with a bridging alkane ligand which have been investigated computationally.

Cyclic and Non-Cyclic Pi Complexes of Manganese

499

5.07.3.1.7 Derivatives of cymantrene (VII) manganese(I) dicarbonyl complexes with a chelating Lewis base-appended cyclopentadienyl ligand The final class of cymantrene derivatives discussed in this chapter involves those where the Mn(I) center has two carbonyl ligands and a chelating Lewis base-appended cyclopentadienyl ligand; these species will be referred to herein as intramolecular Lewis base-coordinated cymantrene derivatives. In most cases, these complexes were prepared from a tri-carbonyl precursor containing a Lewis base tethered to the cyclopentadienyl ring, which coordinates to the metal center upon photochemically-induced carbonyl dissociation (Scheme 149). For the syntheses of (tricarbonyl) derivatives of cymantrene with pendent Lewis bases first prepared in the 2005-2020 period, see Sections 5.07.3.1.3 and 5.07.3.1.4. In some cases, carbonyl dissociation was effectively irreversible (potentially due to removal of carbon monoxide from the reaction atmosphere) so intramolecular Lewis base-coordinated cymantrene derivatives could be isolated. In other cases, intramolecular Lewis base-coordinated cymantrene derivatives were not isolated, either due to reaction reversibility, or because they were used as precursors for further reactivity in situ.

Scheme 149

500

Cyclic and Non-Cyclic Pi Complexes of Manganese

A handful of the intramolecular Lewis base-coordinated cymantrene derivatives reported in the 2005–2020 period feature coordination of a neutral carbon donor to manganese upon photochemically induced carbonyl co-ligand dissociation (Scheme 149). For example, formation of a family of intramolecular alkene-bound cymantrene derivatives 15 were reported with an ether or amine linker between the alkene and cyclopentadienyl donors.15,16 In addition, a pair of examples have been reported involving coordination of an NHC to manganese (676).614 Many of the intramolecular Lewis base-coordinated cymantrene derivatives prepared from tricarbonyl precursors in the manner illustrated in Scheme 149 involve coordination of a group 16 atom to manganese. Examples where the pendent Lewis base which coordinates to Mn is an oxygen atom in a carbonyl group include ketone (677),433 amide (678 and 679),14,16,510,511 and carbamate (680) complexes.509 Analogues of these complexes with a sulfur donor have also been reported, involving thione (681),458 thioamide (682),452 and thiourea (683)15 moieties. In addition, two intramolecular Lewis base-coordinated cymantrene derivatives with pendent metal-coordinated thiolate moieties (684 or 685) were isolated from [(CymS)2Pt(PPh3)2] depending on reaction conditions, and their potential as catalysts for oxygen reduction was investigated.462,615 Furthermore, investigations have been carried out on the formation and ring-inversion of previously reported intramolecular Lewis base-coordinated cymantrene derivatives with thioether moieties under various conditions (not included in Scheme 149).616–619 A wide array of intramolecular Lewis base-coordinated cymantrene derivatives with group 15 donors have also been reported (Scheme 149). The nitrogen donors include 1,2,3-triazole (686, 687, 688, 689, and 690),52,492 triazolium (691),52 pyridine (692),452 imine (693)458 and pyrazole (694; the tricarbonyl precursor to the derivative of 694 with a methylene linker between the amine and pyridyl moieties has been previously reported, while the derivative with an ethyl linker was recently reported using an equivalent synthetic method)481 moieties. In addition, anionic (695)177 and dianionic (696)585 intramolecular Lewis base-coordinated cymantrene derivatives which feature pendent pyrazolyl borate anions have been reported. Notably, 696 could not be isolated because it readily decomposed to form a [C5H4(Bpz3)2]2− dianion (along with small amounts of a trimetallic Mn(I)/ Mn(I)/Mn(II) species).585 Lastly, an intramolecular phospholyl-coordinated cymantrene derivative (697; Scheme 149) was prepared from a tricarbonyl precursor with a pendent iron(II) cyclopentadienyl phospholide substituent (a derivative of 349 in Scheme 82).426 Reactivity of various intramolecular Lewis base-coordinated cymantrene derivatives has also been investigated. For example, the oxygen atoms in ketone, amide, and carbamate-based intramolecular Lewis base-coordinated cymantrene derivatives are weakly coordinated, and susceptible to displacement by various Lewis bases yielding cymantrene derivatives without intramolecular Lewis base coordination; [RCpMn(CO)2L] (see Schemes 145 and 146 for details). By contrast, the bonds between manganese and the NHC ligand in 676 or the phospholyl ligand in 697 were sufficiently strong that photochemically induced carbonyl substitution with hydrosilanes or phosphines, respectively, afforded new complexes where the chelating ligand remains intact (698614 and 699,426 respectively); Scheme 150. Upon photochemical removal of the remaining carbonyl ligand, 698 generated a species which was active towards catalytic hydrosilylation of 2-acetonaphthone.614

Scheme 150

Other examples of the reactivity of intramolecular Lewis base-coordinated cymantrene derivatives involve manipulation of a portion of the chelating ligand outside of the metal coordination sphere (Scheme 151). These reactions include conversion of the pendent cyano moiety in pyridine-bound 692 to a thioamide, forming 700,452 insertion of phenyl isothiocyanate into the NdH bond in alkene-coordinated 15b to yield a structure containing a pendent thiourea moiety (13),15 and reversible reactions of 683 or 701 (prepared as outlined in Schemes 149 and 152) with air to form stable Mn-based radicals (702) which converted back to the thiourea-coordinated precursors upon irradiation.15

Cyclic and Non-Cyclic Pi Complexes of Manganese

501

Scheme 151

Many intramolecular Lewis base-coordinated cymantrene derivatives feature substituents on the cyclopentadienyl ring which can bind to the metal by more than one possible pendent neutral substituent, and in some cases application of different conditions could lead to interconversion between these regioisomers. For example, pyridine-coordinated 700 isomerized to thioamide-coordinated 682 upon heating (top of Scheme 152).452 In addition, a photochemical/thermal equilibrium was observed involving linkage isomers 13 (in which the alkene substituent coordinates to Mn) and 701 (where the S atom is bonded to manganese); bottom of Scheme 152.15

Scheme 152

In addition to the examples highlighted in Scheme 152, many pairs of Lewis base-coordinated cymantrene derivatives capable of interconverting with each other were produced by irradiation of tricarbonyl manganese(I) precursors containing a substituent on the cyclopentadienyl ring with two pendent donors (Scheme 153). Irradiation of cymantrene derivatives containing an alkene and either a pyridine, ester, or ketone donor (447, 490, 491, 492, and 386) formed mixtures of the two possible linkage isomers (703 and 9, 704 and 11, or 705 and 16).12,13 Analogous reactivity was observed stemming from a thiazole-appended derivative of cymantrene (410) to yield a mixture of Lewis base-coordinated cymantrene derivatives with Mn coordination to the nitrogen (706) or sulfur (707) donor in the thiazole ring.458 Similarly, irradiation of the ketone- and pyridine-appended

502

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 153

Cyclic and Non-Cyclic Pi Complexes of Manganese

503

manganese(I) tricarbonyl species 708 (produced as a byproduct in Siegrist alkene synthesis using [MeCpMn(CO)3], tBuOK, DMF, and 2-pyridinecarboxaldeyde) afforded a mixture of intramolecular Lewis base-coordinated cymantrene derivatives with oxygen (709) or nitrogen (710) coordination to manganese.452 In many cases, formation of the intramolecular Lewis base-coordinated cymantrene derivatives was reported to be reversible if the reaction was carried out in a closed system (except for the reactions of 490, 492, and 708). Additionally, the resulting linkage isomers generally could be interconverted thermally or photochemically (with the exception of products formed from 386, where thermal conversion of 705 to 16 was observed but the reverse photochemical isomerization was not). By contrast, irradiation of 395 (where R ¼ R00 ¼ H and R0 ¼ styryl) initially formed alkene-coordinated isomer 12 (which could be isolated), and further irradiation was required to access the alternative linkage isomer 711 (and heating 711 reformed 12).14 In contrast to the reactivity described in Scheme 153, irradiation of 493 (which contains pendent alkene and ester moieties) exclusively formed the alkene-coordinated isomer (10); Scheme 154.12 In addition, irradiation of 368, 423, and 424 (which contain a pendent alkene or alkyne moiety, along with a pendent pyridine or 1,2,3-triazole donor; see Schemes 85 and 100 for synthesis of these precursors) formed in each case a mixture of both linkage isomers (712 and 14, 713 and 45, or 714 and 17); Scheme 154. However, unlike the examples illustrated in Scheme 153, no thermal or photochemical interconversion between these linkage isomers was reported.12,17

Scheme 154

Deprotonation of the pyrazole group in 694 (which also contains pendent pyridine and tertiary amine donors; see Scheme 149 for synthesis), followed by exposure to various metal salts, allowed the preparation of dimetallic species with a bridging pyrazolate moiety (Scheme 155). Reactions of the derivative of 694 with a methylene spacer between the amine and pyridine functionalities (i.e. where n ¼ 1) with various manganese(II) salts yielded hybrid organometallic/Werner-type systems (715).620 Similar reactions of 694 involving zinc precursors allowed isolation of dimetallic Mn(I)/Zn(II) species (716, 717, and 718), with different coordination numbers at the zinc center depending on the nature of the Zn reagent and the spacer between the amine and pyridine substituents in the 694 precursor.621 By contrast, analogous chemistry involving nickel(II) or cobalt(II) precursors yielded isostructural products regardless of the linker size; 694 (where n ¼ 2) reacted sequentially with KOtBu and the appropriate acetate

504

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 155

precursor to yield products (717 where M ¼ Ni, Co) directly analogous to that observed for zinc acetate (717 where M ¼ Zn), and related reactions involving 694 (where n ¼ 1 or 2) with nitrate precursors afforded similar products with an NO−3 ligand in place of the acetate ligand (719).622,623 An alternative route to prepare intramolecular Lewis base-coordinated cymantrene derivatives involves connection of a previously installed neutral ligand on the Mn(I) center with a substituent on the cyclopentadienyl ring (Scheme 156). This has been achieved by ring-closing metathesis (RCM) of pendent alkene moieties on the cyclopentadienyl ring and an allylphosphine co-ligand, affording 720 (in some cases, kinetic resolution was achieved using a chiral olefin metathesis catalyst).576,611,624 When a precursor containing a cyclopentadienyl ring bearing two different alkene-containing substituents was employed, a mixture of two products was observed (721 and 722), resulting from metathesis of the allyl group on the phosphine with either of the two pendent alkenes on the cyclopentadienyl ring (an alternative structure resulting from RCM involving the two cyclopentadienyl-bound alkene substituents was not observed).576 Furthermore, for derivatives of 720 with a bromo substituent ortho to a vinyl group (i.e. 720 when R ¼ Br and n ¼ 0), the bromo substituent could be converted to a C(O)OEt (723), PAr2 (724), CHO (725), or vinyl (726) group.611,624 In the preparation of 725, small amounts of an impurity where the alkene is in a different position (727) was formed, though the two products could be separated chromatographically.625 In combination with [{ClRh(C2H4)2}2], enantiopure samples of phosphine-appended 724611 or vinyl-appended 726624 formed species which were catalytically active for asymmetric addition reactivity between ArB(OH)2 {Ar ¼ Ph, o-Tol, p-C6H4R (R ¼ Me, OMe, F, CF3)} and aldehydes or enones (for 726, enones only). The combination of enantiopure 724 and [{ClRh(C2H4)2}2] was also active for asymmetric addition reactivity involving phenylboroxine and arylaldehyde N-tosylimines.611 Subsequently, the mechanism of 724/[Rh]-catalyzed asymmetric 1,4-addition reactions involving PhB(OH)2 and enones was probed computationally.626

Cyclic and Non-Cyclic Pi Complexes of Manganese

505

Scheme 156

5.07.3.1.8

Carbonyl-free complexes with one cyclopentadienyl ligand on Mn

The vast majority of cyclopentadienyl manganese complexes either feature carbonyl co-ligands (see Sections 5.07.3.1.1–5.07.3.1.7) or feature two cyclopentadienyl ligands bound to Mn (e.g. sandwich complexes; see Section 5.07.3.1.9). This section covers manganese cyclopentadienyl complexes which do not fall within these two categories. Note that this section considers a complex carbonyl-free if there are no carbonyl ligands on the cyclopentadienyl-coordinated Mn atom, and includes dimetallic complexes with CO ligands on the other metal. Derivatives of [CpMn(
6-cycloheptatriene)] have proven useful to access carbonyl-free non-sandwich Mn(I) cyclopentadienyl complexes (Scheme 157). A family of Mn(I) vinylidene complexes with bis(phosphine) and cyclopentadienyl co-ligands (728) was prepared by exposure of this cycloheptatriene precursor to RdC^CdSnMe3 in the presence of the free bis(phosphine) (when R ¼ Ph and depe was used as the bis(phosphine) ligand, the divalent manganese byproduct [(depe)2Mn(C^CdPh)2] was also observed). Abstraction of the terminal stannyl moiety in 728 by TBAF in the presence of water allowed formation of the “parent” vinylidene complexes 729.57 Upon reaction with [Cp2Fe][PF6], these neutral Mn(I) vinylidene complexes (729) underwent oxidative coupling to form dimetallic dications with a bis(carbyne) ligand bridging between the metal centers (730). Interestingly, 730 was converted back to the neutral monometallic precursor 729 upon reduction by [Cp 2Co]. Furthermore, in the oxidative coupling of some 729 derivatives, a pair of byproducts were observed; a dimetallic dication similar to 730 though with an unsaturated bridging linker (731), and a monometallic cation with a terminal carbyne ligand (732).57 Calculations have also been carried out to study the dimerization process using a model of 729 with PH3 ligands (and R0 ¼ H).627 Employing stannyl-terminated bis(alkyne) reagents, similar chemistry to the synthesis of 729 instead afforded dimanganese vinylidene complexes where a bis(vinylidene) moiety bridges between two Mn(I) centers (733). The stannyl groups in 733 could be replaced with a proton from methanol, yielding “parent” bis(vinylidene) dimetallic complexes (734).58

506

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 157

[CpMn(
6-cycloheptatriene)] also reacted with P4 to yield the heterocubane cluster [{CpMn}4(m3-P)4] (735); Scheme 157. The utility of 735 as a building block in coordination chemistry is reflected by the formation of 1D coordination polymers upon reaction with copper(I) halides; regardless of the reactant stoichiometry, the 1:1 complex 736 was obtained (reactions of 735 with [CpMn(CO)2(THF)] were discussed in Scheme 44).205 By contrast, the extremely sterically hindered [{C5(p-C6Hn4Bu)5}

Cyclic and Non-Cyclic Pi Complexes of Manganese

507

Mn(
6-cycloheptatriene)] (79; for synthesis see Scheme 13) reacted with excess P4 to yield a pair of triple-decker Mn complexes with two terminal bulky cyclopentadienyl ligands and a m-
5:
5-P5 ligand (737) or two m-
2:
2-P2 ligands (738); these complexes, which could not be separated from each other, were the most phosphorus-rich manganese complexes to have been characterized by X-ray crystallography (bottom of Scheme 157).97 A series of homo- and hetero-bimetallic complexes where the metal centers are bridged by a cyanide ligand have been synthesized from manganese nitrosyl cyclopentadienyl precursors (Scheme 158). [RCpMn(NO)(L)(CN)] (RCp ¼ MeCp, Cp ) was used to prepare the dimanganese complexes cis- or trans-[{RCpMn(NO){P(OR0 )3}(m-CN){MnL(CO)2(dppm)}] (RCp ¼ MeCp, Cp ;

Scheme 158

508

Cyclic and Non-Cyclic Pi Complexes of Manganese

739a-b),628 as well as heterobimetallic complexes [{MeCpMn(NO)(PR3)}(m-CN){Ru(NH3)5}] (740; R ¼ Ph, OPh),629 [{Cp Mn (NO)(CNR)}(m-CN){(k2-O2C6Cl4)Ru(CO)2(PPh3)}] (741a; R ¼ tBu, xylyl),630 and [{Cp Mn(NO)(CNR)}(m-CN){(k3-Tp0 )M (CO)(
2-R0 C^CR0 )}]+ (742a; M ¼ Mo or W, R ¼ tBu or xylyl, R0 ¼ Me or Ph),631 by coordination of the N atom in the cyanide ligand of the cyclopentadienyl manganese precursor to the other transition metal (a vacant coordination site was formed by halide abstraction, dimer cleavage, or dissociation of a carbonyl or water ligand). In addition, isomers of 739a-b, 741a, and 742a were prepared where the cyano N atom is coordinated to the cyclopentadienyl-ligated manganese center (743a-b, 741b, and 742b, respectively) from reactions of [RCpMn(NO)(L)(I)] (RCp ¼ MeCp, Cp ) with cyanide-containing transition metal complexes (usually involving the employment of a halide abstraction agent).628,630,631 One-electron oxidation of the dimanganese complexes 739a,b and 743a,b afforded mixed-valence dicationic species; it was found that the nature of the supporting co-ligands had a significant effect on the order of oxidation for the two metal centers, and therefore the direction and energy of metal-metal charge transfer through the cyanide bridge.628 In addition, one-electron oxidation of the Mn/Ru dimetallic dications 740 (which occurred at the Ru center) yielded trications (744; see Scheme 158), and cyclic voltammetry and electronic spectroscopy indicated strong solvatochromism with higher energy metal-to-metal charge transfer in solvents with greater hydrogen-bond acceptor ability.629 Cyclic voltammetry of neutral Mn/Ru dimetallic complexes 741a,b indicated that both complexes undergo three sequential one-electron oxidation processes (the first two at the Ru center, and the third at Mn).630 Lastly, cyclic voltammetry of cationic Mn/M (M ¼ W or Mo) complexes 742a,b indicated that oxidation these species proceeded in a stepwise fashion via two one-electron processes, the first at the metal coordinated to the N atom of the m-cyanide ligand, and the second at the other metal center.631 Manganocene has been used to access various new divalent carbonyl-free complexes of manganese with a single Cp ligand on the metal center (Scheme 159). For example, the reaction of [Cp2Mn] with a single equivalent of Li(hpp) (hpp ¼ hexahydropyrimidinopyrimidine) yielded a dimetallic structure with terminal cyclopentadienyl ligands and m-1k1:2k2-hpp ligands (745). Magnetic measurements indicated weak antiferromagnetic coupling between the two Mn(II) centers in 745. Exposure of 745 to a second equivalent of Li(hpp) afforded a cyclopentadienyl-free manganate cage [{LiMn(hpp)3}2].632 In addition, manganocene reacted with LiP(SiMe3)2 or LiAs(SiMe3)2 to afford [(CpMn)2{m-E(SiMe3)2}2] (746; E ¼ P, As), which are dimetallic manganese(II) complexes with terminal Cp and bridging phosphido or arsenido ligands. Magnetic studies on 746 indicated temperature dependence of the magnetic susceptibility due to antiferromagnetic exchange and spin-crossover; the arsenic-containing derivative features a two-step spin-crossover with hysteresis, and was assigned to the conversion of one, and then both manganese(II) centers from high-spin to intermediate-spin.633 DFT calculations on 746 were later conducted, and indicated that the undecaplet state is lowest in energy.634 Another complex with terminal Cp and bridging phosphido ligands ([{CpMn(PHt2Bu)}2(m-PHtBu)2]; 747) was prepared via exposure of [Cp2Mn] to H2PtBu, and differs from 746 by the inclusion of terminal phosphine ligands on each Mn center. Despite involving a pair of Mn(II) centers, 747 is diamagnetic due to MndMn bonding, which provides an 18-electron count at Mn.635

Cyclic and Non-Cyclic Pi Complexes of Manganese

509

Scheme 159

Interestingly, reactions of manganocene with different iminophosphoranes Me3SiN]P{NHR}3 yielded high-spin Mn(II) complexes which contain bis(imino)bis(amino)phosphate ligands in three different binding motifs with Mn(II), resulting in formal VE counts ranging from 13 to 17 for Mn (Schemes 159 and 170). Reactivity involving the least bulky iminophosphorane (where R ¼ nPr) afforded a dimetallic complex with terminal Cp ligands, where one of the N atoms on each ligand bridges between the two metal centers (748; Scheme 159). By contrast, reactions involving the bulkiest iminophosphorane (where R ¼ tBu) produced a monometallic structure with one k2-coordinated bis(imino)bis(amino)phosphate ligand and one Cp ligand (749; Scheme 159). However, reactivity involving the iminophosphorane where R ] Cy resulted in a cyclopentadienyl-free species, and is instead discussed in Scheme 170.636

510

Cyclic and Non-Cyclic Pi Complexes of Manganese

Manganocene reacted with either Ph3SiOH or Ph3SiOLi to afford the same trimetallic complex, [(CpMn)2Mn(m-OSiPh3)4] (750), which features Cp ligands on the two terminal Mn atoms, a central tetrahedral Mn atom, and two siloxy groups bridging between each pair of neighbouring metal centers; Scheme 159. Analogous sulfur chemistry afforded the heterocubane complex [{CpMn(m3-SSiPh3)}4] (751); Scheme 159. In both 750 and 751, the Mn(II) centers are antiferromagnetically coupled.637 Furthermore, reactivity of manganocene with a digallane yielded a gallium-free Mn(II) product where one of the bidentate bian ligands from the gallium-containing precursor has been installed on Mn, which is also k1-coordinated to a molecule of dme solvent (752); Scheme 159. This reactivity contrasts that of the same digallane with other metallocenes, which resulted in products which feature gallyl ligand coordination to the transition metal.638 As discussed in Section 5.07.2.7.1, manganocene also reacted with a potassium pentadienyl salt to yield a mixed valence Mn(I)/Mn(I)/Mn(II) species (99) with cyclopentadienyl coordination to the Mn(I) centers (see Scheme 21 for the synthesis of 99). As discussed in Section 5.07.3.1.9 (Scheme 163), manganocene derivatives have often been prepared from the reactions of manganese(II) halide precursors with two equivalents of an alkali metal cyclopentadienyl salt.639 However, when extremely bulky cyclopentadienyl ligands are involved, careful control of stoichiometry and reaction conditions permitted isolation of dimetallic intermediates, where two Mn(II) centers (each bound to a terminal cyclopentadienyl ligand and neutral donor such as THF or a phosphine) are bridged by two halide ligands (Scheme 160). THF-coordinated examples, [{(1,2,4-C5Ht2Bu3)Mn(THF)}2(m-X)2] (753; X ] Cl, Br, I), were prepared using a sodium cyclopentadienyl salt in THF.640 A related complex with a chelating

Scheme 160

Cyclic and Non-Cyclic Pi Complexes of Manganese

511

cyclopentadienyl-phosphine ligand on each metal center (754) was synthesized from the reaction of MnX2 (X ] Cl, Br) with a potassium cyclopentadienyl salt where a phosphine group is tethered (via a phenylene linker) to the C5 ring.641 When this reaction was conducted in the presence of a crown ether, a monomanganese structure (with a similar core to 754) was obtained, but in this case with the two halide ligands bridging between Mn and K (755).641 Complexes 753 and 754 proved to be useful precursors to access a variety of new Mn(II) complexes (see Scheme 160; reactions yielding manganocene derivatives can be found in Scheme 163). Stemming from 753, installation of an amido ligand in place of a halide yielded the low-coordinate monometallic “pogo-stick” Mn(II) complex [(1,2,4-C5Ht2Bu3)Mn{N(SiMe3)2}] (756). In addition, the iodo derivative of 753 reacted with KHBEt3 to form a 1:1 mixture of previously reported [(1,2,4-C5Ht2Bu3)2Mn] and the pentametallic (all Mn(II)) polyhydride complex [{(1,2,4-C5Ht2Bu3)Mn}4{MnH6}] (757), consisting of a 17 VE [MnH6]4− core with face capping [(1,2,4-C5Ht2Bu3)CpMn]+ units (these complexes could be separated by preferential recrystallization).640 The iodo derivative of 753 was also used as a precursor to access the half-open manganocene complex [(
5-1,3,5-C5Ht2Bu3)Mn(2,4-tertbutylpentadienyl)] (98; see Scheme 21 for the synthesis and structure of 98).117 Stemming from the chloro derivative of 754, isostructural complexes with bridging NH2 or CH3 ligands in place of the halides (758 and 759, respectively), were prepared by exposure to NaNH2 or MeMgCl. These reactions contrast that of 754 with EtMgCl (or other reagents) which yielded the metallocene 776 (see Scheme 163). The MndMn distance in 759 is 0.2 A˚ shorter than that in 758, perhaps indicative of some interaction between the two metal centers. Like the halide bridged precursor 754, compounds 758 and 759 both display antiferromagnetic coupling between neighbouring Mn(II) centers.641 A series of Mn(I) complexes with both a neutral arene and an anionic cyclopentadienyl ligand have been reported (Scheme 161). One example is dilithiated [(
5-C5H4Li)Mn(
6-C6H5Li)]∙pmdta (760), which is a rare example of a structurally characterized dimetallated transition metal sandwich complex (760 crystallized as a dimer).642 This complex could be isolated, and used as a

Scheme 161

512

Cyclic and Non-Cyclic Pi Complexes of Manganese

precursor in the formation of strained ansa-type heteroleptic sandwich complexes, including [1]manganoarenophanes with Si (761, 762, and 763),642,643 Ge (764)643 Sn (765),643 B (766),643 or Zr (767)644 linkers, and [2]manganoarenophanes with B2 (768)642 or Sn2 (769)643 linkers. Insertion of sulfur into the SndSn bond in 769 allowed isolation of the [3]manganoarenophane 770.643 Similarly, the reaction of 765 with [Pt(PEt3)3] yielded the two possible isomers formed from Pt insertion into the SndCCp or SndCarene bond (771 and 772).643 By contrast, the reaction of 760 with Cl2BN(tBu)(SiMe3) resulted in the 1,10 -disubstituted sandwich complex 773.643 DFT calculations have also been conducted on the electronic structure of the parent heteroleptic sandwich complex [CpMn(benzene)].645 In addition, the divalent methylcyclopentadienyl-containing precursor [MeCpIMn(dmpe)] has been used to prepare a cyclopentadienyl-free manganese alkynyl iodide complex (Scheme 162).646

Scheme 162

5.07.3.1.9

Complexes with two cyclopentadienyl ligands on Mn

[Cp2Mn] (manganocene) and its derivatives have been the focus of extensive investigation due in part to the effect that different substituents on the cyclopentadienyl ligand have on the magnetic properties of the complex, including whether manganese(II) has a low spin or high spin configuration. Furthermore, bis(cyclopentadienyl) manganese complexes show a diverse range of hapticity in the cyclopentadienyldMn interaction (deviation from
5-coordination is often referred to as ring slippage); qualitative geometric parameters are traditionally used to identify cyclopentadienyl ring hapticity, though some attempts have been made to quantify deviations from ideal
5-hapticity in Cp2MnLx complexes.647 Syntheses of a variety of new manganocene derivatives with various substituents on the cyclopentadienyl rings have recently been reported (Scheme 163). For example, [(1,3-C5Ht3Bu2)2Mn] (774a), [{1,3-C5H3(SiMe3)2}2Mn] (774b), and

Scheme 163

Cyclic and Non-Cyclic Pi Complexes of Manganese

513

[{1,3-C5Ht3Bu(SiMe3)}2Mn] (774c) have been prepared via the reactions of MnI2(THF)2 with the appropriate magnesocene reagent ([RCp2Mg]), though 774c formed as a mixture of diastereomers which could not be separated.639 In addition, [{1,2,4-C5H2(SiMe3)3}2Mn] (775), in which each cyclopentadienyl ligand has three silyl substituents, was prepared from the reaction between MnI2(THF)2 and two equivalents of the cyclopentadienyl sodium salt.639 The structures and spin states of 774 and 775 were investigated by X-ray crystallography, as well as variable temperature extended X-ray absorption fine-structure measurements, UV-Visible spectroscopy, and magnetic susceptibility studies.639 Salt metathesis from MnCl2 and two equivalents of a potassium cyclopentadienyl precursor has also been used to access a highly substituted manganocene derivative with pendent phosphine donors (776). Alternatively, 776 was also prepared from the dimetallic cyclopentadienyl/halide complex 754 (see Scheme 160 for synthesis of 754) upon exposure to any one of a variety of reagents; EtMgCl, PhMgBr, KHBEt3, nBuLi, or KC8.641 Additionally, the halide-bridged dimetallic cyclopentadienyl complex [{(1,2,4-C5Ht2Bu3)Mn(THF)}2(m-I)2] (753; prepared as outlined in Scheme 160) reacted with Na[1,3-C5Ht3Bu2] to afford the asymmetric manganocene derivative [(1,2,4-C5Ht2Bu3) (1,3-C5Ht3Bu2)Mn] (777).117 The Mn(II) center in 777 adopts a high-spin configuration,117 similar to [(1,2,4-C5Ht2Bu3)2Mn], but in contrast to [(1,3-C5Ht3Bu2)2Mn] (774a), which behaves as a spin-crossover molecule.639 Another asymmetric manganocene derivative, [CpCp Mn] (778), has been prepared by ligand exchange between manganocene and decamethylytterbocene (isolation of 778 was facilitated by the insolubility of the Yb-containing byproduct in hydrocarbon solvents). Compound 778 displays various properties which are not intermediate between manganocene and decamethylmanganocene; the sublimation temperature is similar to that of [Cp2Mn], the magnetic characteristics (a low spin divalent Mn center) match [Cp 2Mn], and the melting point is lower than both of the homoleptic analogues (though closer to [Cp2M]); the low melting point is explained by less efficient crystal packing for 778 relative to the two homoleptic structures.648 Manganocene derivatives have also been used to access tris(cyclopentadienyl)manganese(II) “paddle-wheel” anions (Scheme 164). For example, [Cp2Mn] reacted with the methylcyclopentadienyl potassium salt in THF to afford K[CpMe 2 CpMn] (779), which forms an extended structure in the solid state due to contacts between the outer face of each cyclopentadienyl ring with a potassium ion. Like previously reported and isostructural K[Cp3Mn], each cyclopentadienyl ligand is
2-coordinated to Mn (and
5-coordinated to K+). By contrast, Mg2+ salts of [CpMeCp2Mn]− and [MeCp3Mn]− {780; prepared via the reactions of manganocene or its methylated derivative with bis(methylcyclopentadienyl)magnesium in THF} feature anionic units which do not interact with the [Mg(THF)6]2+ cation, though the cyclopentadienyl ligands in 780 also bind to Mn in an
2-fashion. Magnetic analyses of 779 and 780 indicated, in each case, that the Mn(II) centers are in an intermediate S ¼ 3/2 spin state; this was the first time this spin state had been identified for a Mn(II) complex. By contrast, magnetic analyses for previously communicated K[Cp3Mn] and [Mg(THF)6] [Cp3Mn]2 were more complex, and these structures may have a contribution from the thermal population of an S ¼ 5/2 excited state. The lithium salt [Li(12-crown-4)][Cp3Mn] (781) was also prepared reproducibly, though in extremely low yield, by the reaction of manganocene with cyclopentadienyl lithium in THF in the presence of 12-crown-4. An X-ray crystal structure was obtained, and contained several independent structures in the unit cell, in which two Cp rings are
2-coordinated whereas the third Cp ring is either
2-,
3-, or
5-coordinated ([(
2-Cp)3Mn]−, [(
2-Cp)2(
3-Cp)Mn]−, and [(
2-Cp)2(
5-Cp)Mn]−). Unlike 779 and 780, 781 has magnetic properties consistent with a high spin (S ¼ 5/2) configuration.649

Scheme 164

514

Cyclic and Non-Cyclic Pi Complexes of Manganese

Bis(fulvalene)dimetallic complexes of most 1st row transition metals have been known for decades, but the manganese analogue was noticeable by its absence from the literature. Recently, the substituted derivative [Mn2(2,20 ,4,40 -(tBu)4C10H4)2] (782), which resembles two connected manganocene moieties, has been prepared from the reaction of the bis(fulvalene)dimagnesium precursor with manganese(II) triflate (Scheme 165). In the solid state, the two metallocene units are twisted relative to each other, and magnetic analysis indicated that both divalent manganese centers are high spin and weakly antiferromagnetically coupled.650

Scheme 165

Despite the decades-long history of bis(indenyl) sandwich complexes, Mn-containing analogues have only recently been reported. A series of bis(
5-indenyl) metallocene complexes of Mn have now been reported (Scheme 166), including those with a single SiMe3 substituent in the 2-position of each indenyl ligand (783), prepared by halide metathesis from MnCl2 and the potassium indenyl salt.651 Derivatives with two substituents per indenyl ring have also been prepared, both from MnCl2 (784; with SiMe3 or iPr substituents)651 or MnI2(THF)3 (785; with tBu or cyclohexyl substituents).652 While DFT calculations suggest that the unsubstituted derivative would be isostructural to these species, attempts to prepare it in THF instead yielded the bis(THF) adduct, which features
3- and
1-coordinated indenyl ligands (786). Unexpected structures were also obtained for the 4,7-dimethyl substituted derivative, which was isolated (in the absence of 1,4-dioxane) as an octametallic ring species where each Mn center is
5-coordinated to a terminal indenyl ligand and is
1-coordinated to two bridging indenyl ligands (787). When this reaction was carried out in the presence of 1,4-dioxane, a salt was isolated where each Mn atom is
2-coordinated to three indenyl ligands; [K(1,4-dioxane)1.5][Mn(
2-4,7-indenyl)3] (788).651 For all of the bis(indenyl) derivatives included in Scheme 166, magnetic analyses indicated a high spin configuration.

Cyclic and Non-Cyclic Pi Complexes of Manganese

515

Scheme 166

Addition of Lewis bases to metallocenes has long been used to access adducts of bis(cyclopentadienyl) complexes. In the chemistry of manganese, recently reported examples (Scheme 167) include the addition of free NHCs to manganocene to yield a family of 1:1 adducts [Cp2Mn(NHC)] (789). Magnetic analysis of these complexes, as well as a previously reported 1,3-dimesitylimidazolin-2-ylidene derivative (for which an X-ray crystal structure has only recently been obtained) indicated that the Mn centers are high spin. It is notable that recently obtained magnetic data on the previously reported derivative contradicts the originally proposed spin state.653 A series of adducts with two or three pyridine donors on Mn (either two pyridine ligands, one bipyridine-type ligand, or terpyridine) have been prepared similarly (790, 791, and 792, respectively); it was noted that these adducts feature high spin Mn(II) centers and are yellow or red, likely due to charge transfer. Cyclopentadienyl ring coordination in complexes 790, 791, and 792 was described using a “non-idealized hapticity parameter” (0 indicates an ideal
5-bound Cp ligand and 1 indicates an
1-bound Cp ligand), which ranged from 0.364 to 0.786 (for simplicity, all ligands in Scheme 167 are shown as
5-coordinated).647

516

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 167

Dimetallic bis(pentalene) sandwich complexes have also been reported (Scheme 168). One derivative, with iPr3Si-substituted pentalene ligands (793), was synthesized from the reaction of MnCl2 with the potassium salt of the pentalene. Interestingly, 793 is asymmetric, with the two Mn atoms [which are 2.609(2) A˚ from each other] coordinating to the two halves of the fused ligand in different ways; one is
5-coordinated to both pentalene ligands, and the other is
1-bound (via the silyl-substituted carbon atom) to both pentalene ligands. Computational and magnetic analyses indicate that this feature is due to the presence of both low spin (
5-cyclopentadienyl coordinated) and high spin (
1-cyclopentadienyl coordinated) Mn centers.654 By contrast, the bulkier permethylpentalene derivative 794 (prepared in a similar fashion) is symmetric, with a short MndMn distance of 2.277(5) A˚ consistent with some degree of manganese–manganese bonding. Structurally, the local environment at Mn bears significant similarities to low-spin Cp 2Mn, with
5 coordination and similar average MndC distances (the low-spin nature of 794 is supported by magnetic susceptibility, which is indicative of a triplet state in the solid and in solution).655 The electrochemistry of 794 was investigated using cyclic voltammetry,655 and the nature of bonding in 794 was probed computationally.656 DFT calculations have also been conducted on nanowires formed from repeating (pentalene)Mn2 units; these structures were predicted to be ferromagnetic.657 In addition, the structure of hypothetical monometallic [(pentalene)2Mn] has been investigated computationally.658

Scheme 168

Cyclic and Non-Cyclic Pi Complexes of Manganese

517

Charge-transfer salts which contain the decamethylmanganocenium cation [Cp 2Mn]+ (Scheme 169) are of interest due primarily to their unusual magnetic properties. Examples of such species have been prepared via two general routes. First, oxidation of decamethylmanganocene with dialkyl dicyanofumarates or ethyl tricyanoethylenecarboxylate allowed formation of charge-transfer salt magnets which include the [Cp 2Mn]+ cation; 795659 or 796,660 respectively. Compound 796 exhibits complex magnetic behavior consisting of two frequency-dependent peaks in the ac susceptibility data.660 The second general synthetic route is anion-exchange from [Cp 2Mn][PF6] to incorporate an anion which contains a group 10 metal (797, 798, and 799). Compound 797, with the [Ni(1,3-dithiol-2-one-4,5-dithiolato)2]− anion, exhibits different crystal structure arrangements and magnetic properties depending on which solvent co-crystallized with the target species (e.g. ferromagnetic with acetone, or antiferromagnetic with benzonitrile).661 Compound 798, with the [Ni(a-2,3-thiophenedithiolate)2]− anion, shows magnetic behavior typical for a frustrated magnet and has a blocking temperature of 4 K. Additionally, an unusual inverted hysteresis loop was detected in the magnetic field induced transition.662,663 Lastly, examples with bis[bis(trifluoromethyl)ethylene diselenato]metalate(III) anions (M ¼ Ni, Pt) show metamagnetic behavior where the phase diagrams contain paramagnetic and antiferromagnetic phases (799).664 Theoretical studies have also been conducted on the ferromagnetic behavior of the charge-transfer salt [Cp 2Mn][TCNQ] (TCNQ ¼ tetracyanoquinodimethane).665 Furthermore, the MndCp bond energies in the [Cp2Mn]+ cation have been determined experimentally (using threshold collision-induced dissociation mass spectrometry)666 and computationally.667

Scheme 169

Though the anion of decamethylmanganocene has been known for decades, its solid-state structure has only recently been reported. Reduction of neutral [Cp 2Mn] by elemental potassium yielded an isolable salt, with potassium bridging between the outer faces of the Cp ligands from adjacent [Cp 2Mn]− anions, leading to an extended polymeric structure. When this reaction was performed in the presence of a crown ether, the potassium ion could be abstracted, leading to a structure containing isolated [Cp 2Mn]− anions. In both cases, the angle between the centroids of the two cyclopentadienyl rings is approximately 180 , like in the neutral and cationic analogues, though the MndC distances are significantly shorter than those in the neutral or cationic structures.668 In addition, Cs[Cp 2Mn] has been used as a strong reducing agent for the preparation of Cs3C60.669

518

Cyclic and Non-Cyclic Pi Complexes of Manganese

Reports have also been published on reactions of manganocene which afforded non-p complexes (Scheme 170). For example, [Cp2Mn] reacted with the 1,2-(NH)2C6H4 dianion in THF (and, presumably, traces of air) to yield a MnII4MnIII 2 oxo cage [Mn6{1,2(NH)2C6H4}6(m6-O)(THF)4].670 In addition, [Cp2Mn] reacted with [Li(THF)3][Ru(m-H)3{P(p-tolyl)3}3] to form the heterometallic polyhydride complex [Mn(Ru{m-H}3{P(p-tolyl)3}3)2].671 Manganocene also reacted with lithium N,N0 -dimethylformamidinate in the presence of trace oxygen to yield the tetrametallic interstitial oxide [Mn4O(MeNCHNMe)6], while reaction of [Cp2Mn] with the guanidine salt LiNC(NMe2)2 afforded [Mn{m-NC(NMe2)2}4{Li(THF)}{Li(THF)2}].672 Manganocene also reacted with indium(I) triflate to form an unusual indium complex with dimetallic cationic and anionic units, [In(m,
5-Cp)In][Cp3In(m,
1-Cp) InCp3], though the fate of manganese in this reaction is unclear.673 [Cp2Mn] reacted with the iminophosphorane Me3SiN]P {NHCy}3 to afford a high-spin Mn(II) complex with two bis(imino)bis(amino)phosphate ligands (in contrast to reactions described in Scheme 159 between manganocene and other iminophosphorane derivatives, which yielded Cp-containing manganese complexes).636 Reactions of manganocene which resulted in loss of one or both cyclopentadienyl ligands but yielded complexes with p ligands on manganese are covered elsewhere in this chapter.

Scheme 170

In addition to solution-based chemical reactivity, CVD of GaMnN thin films has been reported using [Cp2Mn] as the manganese source.674 Derivatives of manganocene have also been used as precursors for CVD of manganese-containing thin films, including [MeCp2Mn] (for MnAs,675 InMnAs,676 and Ga1-xMnxN677) and [EtCp2Mn] (for MnO678,679), while CVD stemming from [MeCp2Mn] has also been used to dope AlN,680,681 and ZnO682,683 films with manganese. Furthermore, methods for Atomic Layer Deposition (ALD), or the related technique Molecular Layer Deposition (MLD), have been reported where the manganese-containing precursor is [MeCp2Mn] (for MnAs684 or GaMnAs685 thin films) or [EtCp2Mn] (for manganese oxide,686–701 MnF2,702 MnS,703 manganese alkoxide,704 or (Mn0.8Co0.2)Ox705 films). In addition, manganocene has been used for manganese doping of Mg crystals in a solution-based method,706 formation of Mn3N2 by ammonolysis at 700  C,707 co-pyrolysis of manganocene and sulfur (to prepare branched carbon-encapsulated MnS core/shell nanochains),708 and creation of carbon microspheres with unique micro/nanostructure by in situ pyrolysis.709 Lastly, decamethylmanganocene has been used as the Mn precursor in the deposition of CuMn alloys films from supercritical carbon dioxide.710

Cyclic and Non-Cyclic Pi Complexes of Manganese

519

Despite being known for over a half century, calculations continue to be done on manganocene, including those focused on the nature of the metal–cyclopentadienyl bonding,711 the effects of Jahn-Teller distortion on electronic, spin, and/or magnetic characteristics,712–715 calculations regarding the NMR shielding tensor716 or chemical shift,717,718 charge transfer and spin alignment when deposited on graphene,719 heterolytic dissociation enthalpy,720 electronic configurations and magnetic anisotropy,721 shielding anisotropy,718 and the influence of adsorption configurations on the spin-state energetics when deposited on a Cu(111) surface and subjected to an electron field.722,723 Also, calculations have been done on manganocene and related species encapsulated in carbon nanotubes.724 Given the importance of manganocene as a precursor to a wide range of chemistry (e.g. see Schemes 159, 164, 167, and 170), reports continue to be published on its chemical and physical properties, including solubility in supercritical CO2,725 and determination of the spin equilibria and thermodynamic constants in solid solutions of diamagnetic diluents (for both [Cp2Mn] and the closely related [MeCp2Mn]).726 In addition, the fire-suppression properties of manganocene have been studied.727 Many computational investigations have focused on hypothetical species with two cyclopentadienyl donors on manganese. These include investigations into the structures of sandwich complexes with
5-coordinated indenyl ligands (or one indenyl and one cyclopentadienyl ligand),606,728 one of which features two metals bridging a pair of indenyl ligands ([Mn2(indenyl)2]).729 DFT calculations have also been conducted on the structures of hypothetical bis(indenyl) dimers Mn2(indenyl)4 (and compared to hypothetical manganocene dimers Mn2Cp4).730 In addition, the electronic and structural properties have been investigated for manganocene derivatives where both cyclopentadienyl rings are tethered together by incorporation (as the terminal rings) in a [7] helicene731 or as part of a super[5]phane cage.732 Furthermore, incorporation of one or two manganocene moieties as head groups in thiol-based molecular junctions has been computationally investigated, with a focus on the effects of these changes on the rectifying and spin filtering properties of the junctions.733,734 Other calculations have investigated the electronic and geometric structures of [(C5HtBu4)2Mn].735 Calculations have also been done on the effects that cyclopentadienyl substituents on neutral and cationic manganocene derivatives have on the formation of magnetic superalkali or superhalogen systems (where the complex features a lower ionization energy than alkali metals or a higher electron affinity than halogens, respectively),736 on one- and two-electron reduction of [(C5R5)2Mn] where R ¼ CN or BO (compared to where R ¼ H),737 on the geometries of [(azulene)2Mn2] and [(azulene)Cp2Mn2],738,739 on multidecker complexes containing a “CpMnCp” fragment,740 on the potential structures formed upon chemisorption of [EtCp2Mn] on a “Mn(OH)” surface (to model nucleation during Atomic Layer Deposition of MnO),698 on the electronic structures of polymers formed from repeated manganocene units (bound via a CdC bond between cyclopentadienyl rings of different monomers),741 and on the effect that changing the substituent on [RCp2Mn] (RCp ¼ MeCp, iPrCp, or tBuCp) has on the spin-crossover temperatures.742 Finally, calculations were also carried out to probe the electronic structure of an s-indacene complex with each 5-membered ring bound to a “MnCp” fragment.743 New structural and magnetic data have been reported for a series of previously reported alkyl- or silyl-substituted derivatives of manganocene (Fig. 25). For example, new X-ray crystal structures for [MeCp2Mn] (the structure of which is a chain polymer, qualitatively similar to the parent manganocene),726 [(C5Ht4Bu)2Mn],639 and [(1,2,4-C5Ht2Bu3)2Mn]639 have been reported. For [(C5Ht4Bu)2Mn], a new synthetic route was also reported involving the reaction of MnBr2 with [(C5Ht4Bu)2Mg]. In addition, new magnetic characterization was obtained for [{C5H4(SiMe3)}2Mn].639

Fig. 25 Previously reported derivatives of manganocene for which new X-ray crystal structures or magnetic analysis has recently been reported.

5.07.3.1.10

Miscellaneous cyclopentadienyl chemistry

In addition to reports pertaining to the synthesis of new complexes, publications have been devoted to the reactivity and analysis of previously reported systems. For example, decomposition of [{CpFe(CO)}{MeCpMn(CO)}(m-CO)(m-COCH3)] in the absence or presence of PPh3 has been analyzed, given the unusual methyl migration from oxygen to iron described in the original report.744 Cyclopentadienyl co-ligands have often been incorporated into computational models where the purpose of the calculation is to investigate the bonding between Mn and another ligand. In addition to examples discussed in Sections 5.07.3.1.1–5.07.3.1.9, these calculation have involved investigations into the bonding between Mn and benzene,645 naphthalene,745 triphenylene,746 indacene,743 fulvene,747 [3]- or [4]radialene,748 cyclooctatetraene,749 fullerene,750,751 azepine,752 dibenzazepine,753 benzoquinoline,754 phenanthridine,755 phenazine,756,757 dicarbaborane,758 B40,751 and phosphinidene759 ligands. In addition, calculations have been carried out on dimetallic Cp-containing manganese species with carbonyl,70,760,761 thiocarbonyl,71 or nitrosyl762 co-ligands. Other computational reports have focused on the reaction pathways of [CpMnO3] with ethylene,763 and

520

Cyclic and Non-Cyclic Pi Complexes of Manganese

the ability of a series of cyclopentadienone species with carbonyl, hydride, and nitrosyl co-ligands to act as catalysts for CO2 hydrogenation.764 A cyclopentadienyl co-ligand was also incorporated into a computational model of a hypothetical manganese metallacycle, which was investigated for “Möbius” aromaticity.765 Calculations have also been performed on the transformation of a metallabenzene tetracarbonyl complex into cymantrene (with carbonyl ligand dissociation), which was shown to be very energetically favorable compared to other transition metal analogues.766 In addition, calculations have been used to gain insight into the electronic structure of a hypothetical inverse-sandwich bipyramidal cation [(m-
5:
5-Cp){Mn (CO)3}2]+, along with heterometallic and multidecker derivatives.767 Similar calculations have involved homo- and hetero-metallic multidecker structures where the bridging ligands are carbonyl, cyclopentadienyl, or cyclobutadiene ligands.761,768 Calculations have also been used to probe the fluxionality of [(
5-Cp)(
1-Cp)Mn(CO)2]−.769 Furthermore, computational investigations have also been performed on a series of nanowires; [{(C5H4dC5H4)Mn}1] (which contain fulvalene dianions bridging between neighbouring manganese centers),770 [{CpMn}1],771 [{CpMnCpM}1],772,773 [{MnCpFeCpFeCp}1], [{VCpMnCp}1], [{NiCpMnCp}1], [{VCpMnCpMnCp}1], or [{VCpMnCpMnCpMnCp}1],774 often focusing on the predicted magnetic properties of these structures. Similarly, calculations have been conducted on infinite molecular wires with alternating Cp and heteroatom-substituted 5-membered rings [{CpMn(m-
5:
5-L)Mn}1] (L ¼ C4BH5, C3B2H5, C4NH4, or C3N2H3),775 molecular wires of [{CpMn}1] on the surface of, or encapsulated within, single walled carbon nanotubes,776 and finite [CoCpFeCpMnCpFeCpCo] wires acting as junctions between gold electrodes.777

5.07.3.2

Arene complexes

While not quite as commonly utilized as cyclopentadienyl ligands, neutral arene ligands are also prevalent in manganese chemistry. Various cationic, neutral, and anionic arene complexes (usually, though not exclusively, of Mn(I)) have been reported. Newly reported arene complexes with a cyclopentadienyl co-ligand were covered in Scheme 161.

5.07.3.2.1

Synthesis and reactivity of cationic Mn(I) h6-arene complexes; [(h6-arene)Mn(CO)3)]+

Cationic Mn(I) tricarbonyl
6-arene complexes [(
6-arene)Mn(CO)3)]+ are by-far the most explored class of arene-coordinated manganese complex. One common route to prepare [(
6-arene)Mn(CO)3)]+ complexes involves addition of free arenes to manganese carbonyl cations, which are generally prepared in situ via the “silver(I) method”; addition of AgBF4 to [BrMn(CO)5] (Scheme 171). In the 2005–2020 period, the silver(I) method has been used to synthesize [(
6-arene)Mn(CO)3)]+ complexes with various substituents on the arene, including a halogen (800, 801, 802)125 or an OH group (803; this complex, as well as a previously reported NH2-substituted arene derivative, showed activity towards the catalytic transfer hydrogenation of ketones).778 Additionally, hydrocarbyl-substituted arene ligands have been installed in a similar manner, including durene (804 where R ¼ H), pentamethylbenzene (804 where R ¼ Me),127 hexaethylbenzene (805),779 and dibenzosuberane (806).140 When this reaction was carried out with 8-methyl-1-(p-tolyl)naphthalene, the initially formed (kinetic) product was proposed to involve
6-coordination to the methyl-substituted naphthalene ring (807). However, the thermodynamic product (808), involving
6-coordination to the p-tolyl group, was formed upon heating with an excess of the arene ligand.780 In addition, detailed procedures using the silver(I) method to prepare previously reported acenaphthene and naphthalene derivatives (useful precursors to access other [(
6-arene)Mn(CO)3)]+ derivatives; Scheme 172) have recently been reported in Inorganic Syntheses.781 Furthermore, the combination of AlCl3 and [NH4][PF6] has been employed in place of AgBF4 to provide a silver-free route to an [(
6-arene)Mn(CO)3)]+ complex with an octamethylfluorene ligand (809); Scheme 171.139 Additionally, another silver-free route (reaction of [MeMn(CO)5] with HBF4 in the presence of the free arene) was reported to prepare the
6-phenol derivatives 803 (Scheme 171).778

Cyclic and Non-Cyclic Pi Complexes of Manganese

521

Scheme 171

Another commonly employed method to prepare [(
6-arene)Mn(CO)3)]+ complexes is substitution of the weakly-bound polycyclic arene in [(
6-acenaphthene)Mn(CO)3]+ or [(
6-naphthalene)Mn(CO)3]+ by a free arene (Scheme 172); the precursors are sometimes referred to as “tricarbonyl transfer reagents” or “Mn(CO)+3 transfer reagents”. Recently, this method has been used to install arenes with OnOct substituents (810; this complex formed a monolayered 2D structure on a highly ordered pyrolytic graphite surface)163 or methyl and methoxy (811)130 substituents, as well as guaiacol (812; R ¼ H), 2-methylguaiacol (812; R ¼ Me)141 or hexaethylbenzene (805)782 ligands. In addition, an Inorganic Syntheses publication has been provided for the preparation of previously reported N,N-dimethylaniline derivatives from a naphthalene-containing precursor via Mn(CO)+3 transfer reactivity.781

522

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 172

Another route often used to form cationic
6-arene Mn(I) tricarbonyl complexes is hydride abstraction (generally using [CPh3] [BF4]) from a neutral manganese(I) tricarbonyl
5-cyclohexadienyl precursor which contains two hydrogen atoms on the sp3 ring carbon (syntheses of these precursors are discussed in Section 5.07.2.7.2; in Scheme 173, the number in italics represents the code for the [(
5-cyclohexadienyl)Mn(CO)3)] precursor used to access each arene complex). Various examples of [(
6-arene)Mn(CO)3)]+ complexes prepared by this route in the 2005-2020 period are shown in Scheme 173 (813,155 814,132,148,149 815,126 816,156 817,132

Scheme 173

Cyclic and Non-Cyclic Pi Complexes of Manganese

523

818,159 and 819157), and include derivatives with unusual arene substituents such as an alkynyl-connected 20 -deoxyuridine (818),159 or a benzodithiophene (819, synthesized from 173 where R ¼ H)157 moiety. PPh2-substituted 814 (E ¼ PPh2)132 and 817132 were synthesized in high enantiopurity when prepared using enantiopure cyclohexadienyl precursors. Given that tricarbonyl Mn(I) arene cations and neutral manganese(I) tricarbonyl
5-cyclohexadienyl complexes can be interconverted by addition or abstraction of a nucleophile (see Schemes 23–25 and 173), this process has been used to resolve racemic mixtures of chiral [(
6-arene)Mn(CO)3)]+ cations. For example (Scheme 174), initial addition of a camphor enolate (selected as the chiral carbanion due to the ease with which it can subsequently be removed from the complex) formed from nBuLi and D-camphor produced a mixture of
5-cyclohexadienyl diastereomers which could be chromatographically separated (105). Once separate, abstraction of the camphor moiety (using AgBF4 and Me3SiCl) allowed isolation of enantiopure arene cations.131,132 Similar reactivity (not shown in Scheme 174) permitted chiral resolution of the o-trimethylsilylanisole-coordinated derivative [{
6-o-C6H4(OMe)(SiMe3)}Mn(CO)3]+.131,132 Furthermore, enantiopure arene complexes were used to access enantiopure 3,4-disubstituted cyclohexanones, which may have utility as organic synthons (Scheme 174). This was achieved via i. reaction with LiAlH4 to form an
5-cyclohexadienyl complex, ii. addition of a second nucleophile to afford an anionic intermediate containing an
4-coordinated 1,3-cyclohexadiene ligand, and iii. oxidative demetallation using FeCl3 followed by acidic hydrolysis. A halide-free enone was also prepared by exposure of the halide-containing
5-cyclohexadienyl intermediate to nBuLi followed by water prior to subsequent reactivity.131,132

Scheme 174

In addition to reactions of [(
6-arene)Mn(CO)3)]+ cations with strong (typically anionic) nucleophiles, which results in nucleophilic attack on the arene ligand (yielding [(
5-cyclohexadienyl)Mn(CO)3)] complexes; discussed in Scheme 23), reactivity resulting in carbonyl ligand substitution can also be accessed (Scheme 175). For example, carbonyl substitution (promoted photochemically or using the carbonyl abstraction agent trimethylamine N-oxide) by P(OMe)3 or Br− afforded cationic 820 and 821,140 or neutral 822,779 respectively. In addition to substitution reactivity (which afforded 821), [(
6-trindane)Mn(CO)3)]+ also reacted with methyl iodide or allyl bromide in the presence of tBuOK to yield a neutral arene dicarbonyl halide complex (823). The same precursor reacted with free phosphine or phosphite ligands (again in the presence of tBuOK) to afford neutral complexes featuring a tetrasubstituted
5-indenyl ligand (675 and 649). Syntheses of both 823 and 675 were proposed to proceed by initial nucleophilic attack of tert-butoxide at a carbonyl ligand, forming a neutral hydride intermediate (824; unobserved) via elimination of isobutene and CO2.140 While formation of halide-containing 823 from 824 (via reactivity with allyl bromide or methyl iodide) is straightforward, mirroring previously reported reactivity of [(C6Me6)Mn(CO)2H] with CHCl3 or ClCl4,783 conversion of 824 to 675 requires loss of two equivalents of dihydrogen from 824.140 In the synthesis of 675 using P(OMe)3, an isostructural PR3-free byproduct was also obtained (649).140 Reflecting previously reported reactivity in which tBuOK was utilized to deprotonate [(
6-fluorene)Mn(CO)2L]+ (L ¼ CO, PnBu3) to afford the neutral fluorenyl complexes [(
5-fluorenyl)Mn(CO)2L],138,784 the same transformation has been accomplished by one-electron reduction (with concomitant hydrogen radical elimination), either electrochemically or using Na/K alloy. In addition, cyclic voltammetry suggested that an isostructural product would be obtained upon reduction of [(
6-9-methylfluorene)Mn(CO)3]+.133

524

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 175

In reactivity which mirrors previously discussed reactions with free arenes, the weakly bound p ligands in [(
6-polycyclic arene)Mn(CO)3]+ (polycyclic arene ¼ naphthalene or acenaphthene) cations can be displaced by non-p ligands (Scheme 176). For example, [(
6-naphthalene)Mn(CO)3]+ has been used as a precursor to prepare neutral tricarbadecaboranyl785 or cationic trispyridine786 complexes via reactions with the tricarbadecaboranyl anion or pyridine ligands, respectively. Additionally, [(
6-acenaphthene)Mn(CO)3]+ has been reported to afford an FeMn carbonyl dithiolate species via a reaction with an iron(II) dithiolate precursor.787

Cyclic and Non-Cyclic Pi Complexes of Manganese

525

Scheme 176

Electrochemical studies have been conducted on previously reported [(
6-arene)Mn(CO)3)]+ complexes. These include an investigation of the kinetics and nature of the products formed upon reduction of [(
6-C6Et6)Mn(CO)3]+, as compared to a previously reported electrochemical study on the mesitylene analogue788 and analogous reduction of rhenium analogues.782 In addition, an in-depth electrochemical study has been reported on the reduction of [(
6-arene)Mn(CO)3)]+ complexes with polycyclic arenes. The previously reported789 2-electron reduction of these complexes, which afforded an anionic product with
4-arene coordination, has been further investigated; it was found that the second single-electron reduction event was thermodynamically more favorable but kinetically less favored relative to the first. In addition, this reduction chemistry was extended to a phenanthrene-containing cationic precursor, affording anionic 825 (Scheme 177). Interestingly, reduction of [(
6-naphthalene)Mn(CO)3)]+ in the presence of a single equivalent of [CpFe(
6-naphthalene)]+ afforded a zwitterionic anti-facial bimetallic species (826) with
4-coordination of naphthalene to the Mn center (where the naphthalene ring in the product originates from the Fe precursor); Scheme 177.790

526

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 177

New X-ray crystal structures have also been reported for previously reported [(
4:
6-polycyclic arene)Mn2(CO)5] derivatives (arene ¼ 1,4-dimethylnaphthalene or hexahydropyrene), the products of reduction of the monometallic cationic precursors [(
6-arene)Mn(CO)3)]+ by a single equivalent of cobaltocene (per Mn). Furthermore, the chemistry of [(
4:
6-1,4-dimethylnaphthalene)Mn2(CO)5] was further investigated; in the presence of catalytic oxidant, this complex reacted with free carbon monoxide or P(OEt)3 to afford a zwitterionic syn-facial bimetallic complex [(
4,
6-1,4-dimethylnaphthalene) Mn2(CO)5(L)] (827: L ¼ CO, 828: L ¼ P(OEt)3). X-ray crystal structures of 827 and 828 showed that the diene plane is angled at 45 relative to the rest of the naphthalene ligand. In addition, CO abstraction from 827 by Me3NO permitted re-formation of the pentacarbonyl precursor.790 A variety of reports have been published recently which provide further analysis of previously reported [(
6-arene)Mn(CO)3)]+ complexes. Spectroscopic investigations of such complexes include a detailed study of the minor changes in the IR spectra of the [(
6-benzene)Mn(CO)3]+ cation with different counterions791 and PGSE NMR diffusion studies to investigate ion pairing (strong or weak ion pairing appears to be dependent on the solvent, anion, and arene substituents).792 In addition, DFT calculations have been published on previously reported [(
6-C6H5Cl)Mn(CO)3]+ to compare the CdCl bond strength with that in free chlorobenzene.793 X-ray crystal structures have also recently been published for previously reported [(
6-arene)Mn(CO)3)]+ complexes with 1,3,5-trimethoxybenzene,792 acenaphthylene, acenaphthene, and pyrene ligands (for the pyrene derivative, the Mn fragment coordinated to the less-substituted arene ring); Fig. 26.790

Fig. 26 Previously reported [(
6-arene)Mn(CO)3)]+ complexes for which new X-ray crystal structures have recently been reported.

Cyclic and Non-Cyclic Pi Complexes of Manganese

5.07.3.2.2

527

Anionic manganese h6-arene complexes

To our knowledge, only a single anionic manganese complex with an
6-arene ligand has been reported in the 2005–2020 period: [(
6-C6Ph6)Mn(
2-PhC^CPh)]− (48). This anion (along with a variety of byproducts) was prepared via the reaction of excess diphenylacetylene with an anionic bis(amido)manganese(I) precursor (see Scheme 7 for synthetic details).59

5.07.3.2.3

Neutral manganese h6-arene complexes

A variety of neutral manganese complexes with
6-arene ligands have been reported over the 2005–2020 period. Heteroleptic Mn(I) sandwich complexes with cyclopentadienyl and arene ligands are covered in Scheme 161,642–644 while the syntheses two additional recently reported examples of neutral manganese arene complexes with
6-toluene ligands are illustrated in Scheme 178. The inverse sandwich carbonyl-free manganese(I) complex [(Mn{C6H-2,6-(C6H2-2,4,6-iPr3)2-3,5-iPr2})2(m-
6:
6-toluene)] (829) was synthesized by reduction of the Mn(II) aryl iodide precursor in THF, followed by exposure to toluene. Presumably, the appreciable thermal stability of 829 (which was characterized spectroscopically and crystallographically) is imparted by the very bulky aryl ligands. Notably, the Mn–centroid (arene) distance in 829 is longer than is commonly found in complexes featuring a terminal
6-arene ligand, and magnetic studies indicated that the complex is paramagnetic with two high-spin d6 Mn(I) centers lacking any detectable magnetic exchange.794 Compound 829 was found to react with bulky terphenyl azides to yield different dimetallic aryl/amido manganese(II) complexes. This reactivity was proposed to proceed via an unobserved Mn(III) imido

Scheme 178

528

Cyclic and Non-Cyclic Pi Complexes of Manganese

intermediate, which could form the observed species by a) hydrogen abstraction from a methyl group by the nitrogen atom in the imido ligand, followed by CdC bond-forming radical coupling, or b) hydrogen abstraction from an aryl ring by the nitrogen atom in the imido ligand, followed by addition of a second manganese aryl unit.795 In addition, a neutral trivalent manganese complex containing an
6-toluene ligand has been reported (830), featuring a carbonyl, a hydride, and two silyl co-ligands (the silyl ligands are incorporated into the 4- and 5-positions of a xanthene backbone). Compound 830 was formed by photochemically-induced carbonyl substitution from tri- or tetracarbonyl precursors containing manganese-coordinated hydrosilane fragments.796

5.07.3.2.4

Neutral manganese hx (x < 6) arene complexes

In most cases, arene ligands are
6-coordinated manganese (see Sections 5.07.3.2.1–5.07.3.2.3), though some exceptions have been reported. In addition to the
4-arene coordination to manganese observed in cationic, zwitterionic, and anionic species described in Scheme 177, a pair of neutral manganese complexes with planar
3-coordinated arene fragments have been synthesized from a manganese(II) amidinato bromide precursor (Scheme 179). This precursor reacted with [({(MesNCMe)2CH}Mg)2] or K[HBEt3] to yield Mn(I) complex 831797 or Mn(II) complex 832,798 respectively, both of which contain amidinato ligands (one per Mn) bound to the metal center by the anionic nitrogen donor as well as one of the flanking arene groups. Compound 831 is a rare example of a carbonyl-free complex with an unsupported MndMn single bond, while 832 (which features two hydride ligands bridging between the two metal centers) was the first structurally-authenticated example of an amidinato-manganese(II) hydride complex. Interestingly, both complexes reacted with small molecules to afford identical Mn(I) (via reactions with CO) or Mn(III) (via reactions with O2 or N2O) products, indicating that the Mn(II) hydride species (832) reacts as a “masked” source of an amidinato-manganese(I) fragment.798

Scheme 179

5.07.3.2.5

Miscellaneous arene chemistry

In addition to the synthetic chemistry described in Sections 5.07.3.2.1–5.07.3.2.4, the structures of many manganese complexes with arene ligands have been investigated computationally. This includes various sandwich complexes such as [Mn(
6-benzene)2] (to investigate electronic structure and stability,799 as well as to model the intercalation of Mn into bilayered graphene800), [Mn2(naphthalene)2],801 [Mn2(anthracene)2],802 [{Mn(
6-phenazine)}2],803 [CpMn(
6-benzene)],645,799,804 [CpMn(
6-benzene)]+/−,645 [(
5-C60)Mn(
6-benzene)],805 [(fulvene)Mn(
6-benzene)],747 [CpMn(
6-naphthalene)] (including haptotropic rearrangement),745 [CpMn(
6-triphenylene)],746 [CpMn(
6-benzoquinoline)] (including structures where the polycyclic arene binds to Mn via the C6 or C5N rings),754 [CpMn(
6-dibenzazepine)],753 [CpMn(
6-phenanthridine)],755 [(CpMn)2(mphenazine)],756 [Mn2(indenyl)2],729 [Mn4(pyrene)2],806 [Mn2(benzene)2],807 sandwich808–810 and half-sandwich808,810 [Mn(
6-benzene)n]+ (n ¼ 1, 2) cations, the one-dimensional [{Mn(
6-benzene)}1] polymer (predicted to display half-metallic ferromagnetism and, if elongated, the properties of an antiferromagnetic insulator),811 BN graphene functionalized with Mn(
6-benzene) fragments,812 and a sandwich-like structure with a single bis(arene) super[6]phane ligand.732 The electronic and magnetic properties were also calculated for sandwich-like nanowires composed of Mn2(naphthalene),813 Mn2(anthracene),814 or Mn(benzene)771 repeating units. Similarly, calculations have been conducted on the structures of hypothetical triple decker complexes (as well as rice-ball isomers) with terminal
6-arene ligands {where the bridging ligand is B6X (X ¼ B, C, N) or {(CH)x(BH)y(N)z}− (x + y + z ¼ 5) },775,815 or terminal cyclopentadienyl ligands and a bridging arene ligand,816,817 as well as tris(benzene)dimanganese.818 In addition, the structure of [C6{(C6H5)Mn(C5H4)}3] was investigated.819 The nature of bonding to arene ligands has been investigated for various complexes, including inverse-sandwich complexes of Mn(I) (with terminal

Cyclic and Non-Cyclic Pi Complexes of Manganese

529

b-diketiminate ligands)820,821 and of Mn(III) (with terminal imido and b-diketiminate ligands).822 Also, calculations have been conducted on Mn2+ ions binding to phenylalanine, tyrosine, or tryptophan (with no additional ligands; in each case the lowest energy structure involves
2-coordination to the arene).823 Furthermore, calculations have been reported on the possible structures of arene complexes with carbonyl co-ligands, including [(
6-benzene)2Mn2(CO)n] (n ¼ 4, 3, 2, 1)824 and [{Mn(CO)3}2(m-phenazine)] (notably, the syn and anti
6 isomers were nearly identical in energy).757 In addition, calculations have been conducted on the interactions of H2 with various manganese arene complexes, including [Mn(
6-benzene)2] (focused on the inhomogeneity of spin density distribution and its relevance to ortho-para H2 conversion),825 or with cationic Mn(I) arene complexes with and without additional carbonyl and/or cyano co-ligands (focused on analysing potential H2 binding by metal-organic frameworks incorporating open transition metal sites).826 Other reactions involving arene complexes which have been probed computationally include the gas-phase oxidation of benzene by N2O catalyzed by Mn+,827 and the formation of [CpMn(
6-benzene)] from cymantrene {for comparison to formation of a hypothetical [CpMn(
n-B40)] (n ¼ 6 or 7) sandwich complex}.751 A series of multidecker and ring sandwich clusters {[{Mn(C6H6)}n]− (n ¼ 1–5, 18)} were prepared in the gas phase using a laser vaporization and supersonic expansion method, and detected by anion photoelectron spectroscopy; structures of the observed species were probed by DFT calculations.828 The binding and reactivity of methane with [Mn(benzene)2]2+ has also been investigated through computational and experimental means; the UV photofragmentation mass spectrum of the dication was obtained, and cationic products from methane activation, which were proposed to incorporate methane, methyl, water, and/or carbon dioxide ligands, were detected.829 Furthermore, neutral and cationic Mn(I) arene complexes were proposed as intermediates in cymantrene-catalyzed photocatalytic CdH arylation of arenes,291 and transient tricarbonyl Mn(I) arene complexes were observed by time-resolved IR spectroscopy upon exposure of [(2-phenylpyridyl)Mn(CO)4] to toluene,830 phenylacetylene,25 or benzaldehyde25 under photochemical conditions.

5.07.3.3

Complexes with cyclic heteroatom-substituted p ligands

Several novel organomanganese complexes have been reported in the 2005–2020 period which bear cyclic p ligands containing carbon and one or more heteroatom within the manganese-coordinated ring. Most of these complexes feature anionic pyrrolyl or phospholyl ligands (Fig. 27), which are related to cyclopentadienyl ligands by substitution of a CdH group for nitrogen or phosphorus, respectively. Mn(I) tricarbonyl complexes with these ligands are often referred to as azacymantrene or phosphacymantrene complexes (Fig. 27). Using reactivity analogous to that commonly employed for the syntheses of new cymantrene derivatives (see Schemes 82 and

Fig. 27 Structures of heteroatom-containing analogues of the cyclopentadienyl ligand and cymantrene.

129), the reaction of a potassium pyrrolyl salt with [BrMn(CO)5] was used to prepare new azacymantrene derivatives [{
5-C4N(2,5-tBu2)(3-PR2)H}Mn(CO)3] (833: R ¼ Ph, 834: R ¼ iPr), which feature phosphino and tert-butyl substituents on the pyrrolyl ligand; top of Scheme 180. The substituents on the phosphine were found to have a significant effect on the energy of the HOMO, and as a consequence the MndCO bond strength for these complexes; the phenyl-substituted derivative 833 featured a stronger MndCO bond, leading to a lower energy CO stretching feature, than in alkyl-substituted 834.831 Another azacymantrene derivative (835) was prepared via the reaction of a 2-(pyrrolylmethyl)phosphaferrocene precursor with [Mn2(CO)10] (bottom of Scheme 180). Compound 835 was then used as an exotic bidentate ligand to coordinate to another transition metal center; reaction with [Mo(CO)6] yielded a complex (836) where the phosphorus and nitrogen atoms of the phospholyl and pyrrolyl ligands in 835 act as s-donors to Mo. While an X-ray crystal structure was not obtained for 835, the crystal structure of 836 supports
5-pyrrolyl coordination to Mn.832 In addition, the rates of substitution of weakly bound Lewis bases in [(
5-2,5-dimethylpyrrolyl)Mn(CO)2L] (L ¼ cyclohexane, benzene, 1-bromohexane, THF, and cyclooctene; the synthesis and structure of the derivative where L ¼ cyclooctene, 3, is mentioned in Section 5.07.2.1.1) with other Lewis bases has been investigated computationally and experimentally; the data suggests that this process proceeds by a dissociative or dissociative interchange (Id) mechanism, as opposed to the associative mechanism associated with carbonyl substitution reactions of [(
5-2,5-dimethylpyrrolyl)Mn(CO)3].8 Unlike the aforementioned pyrrolyl complexes of Mn(I), in which
5-coordination to Mn was exclusively observed, pyrrolyl complexes of manganese(II) feature a variety of different hapticities (Scheme 181). For example, reaction of 2 equivalents of

530

Cyclic and Non-Cyclic Pi Complexes of Manganese

Scheme 180

2,5-di-tert-butylpyrrolyl sodium with MnI2(THF)3 afforded [(2,5-di-tert-butylpyrrolyl)2Mn], in which the pyrrolyl donors are 1-N-coordinated to manganese. A structurally analogous complex was formed via the reaction of MnCl2 with 2 equivalents of 2,5-di-tert-butyl-3,4-dimethylpyrrolyl potassium in toluene. By contrast, reaction of the latter potassium salt with 2 equivalents of MnI2(THF)3 in THF afforded a mixture of the aforementioned base-free species and THF-coordinated 837, in which one of the pyrrolyl ligands is 1-N-coordinated and the other is 2-coordinated via a CN bond. Both [(1-N-2,5-di-tert-butyl-3,4-dimethylpyrrolyl)2Mn] and THF-bound 837 sublimed to yield [(3-C,N,C-2,5-di-tert-butyl-3,4-dimethylpyrrolyl)2Mn] (838) with 3-C,N,C coordination of the pyrrolyl ligands. In addition, MnI2(THF)3 reacted with a single equivalent of 2,5-di-tert-butyl3,4-dimethylpyrrolyl potassium in THF to afford, in extremely low yield, a dimetallic Mn(II) complex featuring bridging halide and terminal 3-pyrrolyl and THF ligands (839). The X-ray crystal structures of 838 and 839 feature planar pyrrolyl ligands where the Mn center lies far closer to the N atom than the centroid of the ring. DFT calculations indicated that 838 (the thermodynamic product) is only 4.5 kcal mol−1 lower in energy than the kinetically-formed 1-isomer. All of these Mn(II) pyrrolyl complexes are high-spin.833 Reflecting the chemistry of formylcymantrene (see Schemes 101–105), a formyl-substituted phosphacymantrene complex (840) has been used to access phosphacymantrene derivatives with various other substituents on the phospholyl ligand (Scheme 182). Chiral resolution of 840 was achieved by formation of acetal diastereomers (841) via reaction with (S,S)-1,2-diphenylethane-

Scheme 181

Cyclic and Non-Cyclic Pi Complexes of Manganese

531

1,2-diol, followed by chromatographic separation and re-formation of enantiopure 840 by acidic workup on silica gel. Installation of various substituents at the formyl moiety of enantiopure 840 has been demonstrated, including reduction with LiAlH4 to yield alcohol-substituted 842, where the hydroxyl group could be converted into a pendent phosphine by protonation in the presence of HPPh2 and acetic anhydride to afford 843. Alternatively, exposure of enantiopure 840 to aminodiphenylmethane yielded a phosphacymantrene derivative with a pendent imine substituent (844). Palladium complexes formed in-situ by reaction of [Cl2Pd(COD)] with enantiopure samples of 843 or 844 were found to be active towards catalytic condensation of sodium malonate and 1,3-diphenylallyl acetate, though the enantioselectivity of this process was poor.834 A series of phosphacymantrene complexes with allyl or vinyl substituents on 5-phospholyl ligands (845 or 846, respectively) have been prepared by the reaction of an alkali metal salt of the appropriate phospholide with [BrMn(CO)5] (Scheme 182). Substitution of the carbonyl ligands in 845 or 846 by phosphines proceeded under photochemical conditions, and this method was

Scheme 182

532

Cyclic and Non-Cyclic Pi Complexes of Manganese

used to install Ph2P(allyl) ligands on manganese, generating 847 or 848 (respectively). In a similar manner to chemistry reported for cyclopentadienyl analogues of 847 and 848 (e.g. formation of 720 in Scheme 156), ring-closing metathesis provided access to derivatives of phosphacymantrene where the 5-coordinated ring and the neutral phosphine co-ligand are tethered together (849 or 850). This chemistry was catalyzed by chiral molybdenum catalysts, and depending on the R groups of the catalyst and manganese-based precursor, different enantiopurity (up to a maximum of 99% enantiomeric excess) was observed for 849 or 850 due to kinetic resolution. Furthermore, one of the enantiopure derivatives of 850 was used as a ligand for palladium-catalyzed asymmetric allylic alkylation of rac-1,3-diphenyl-2-propenyl acetate with dimethylmalonate, yielding up to a 74% enantiomeric excess of the product when the reaction was carried out at 0  C.835 Reactions of previously reported 2,5-diphenylphosphacymantrene with various bases, which yielded 4-phosphoryl complexes 88, 89, 90, and 91 (which are bound to Mn centers exclusively by the carbon atoms), are described in Scheme 15. DFT calculations have also been conducted on various 2,5-disubstituted phosphacymantrenes to investigate their electronic structures and the relative energies of different confomers,108,836 and an X-ray crystal structure of previously reported 2,5diphenylphosphacymantrene has recently been obtained.108 Additional computational reports have focused on the electronic structures of unsaturated dimetallic phosphacymantrene derivatives, which include complexes containing a m-5:1-phospholyl ligand.837 Several manganese 1,2-diphospholide complexes have also been reported. [BrMn(CO)5] reacted with a 3,4,5-triaryl1,2-diphospholide tin species {P2C3Ar3(SnMe3)} to yield, at elevated temperature, a family of low-spin manganese(I) diphosphacymantrene derivatives (851); Scheme 183. Alternatively, 851 could be formed by decomposition (at elevated temperature or upon UV irradiation) of the dimetallic m-1:1-diphospholide-bridged species which were formed (and isolated) when the reaction between [BrMn(CO)5] and P2C3Ar3(SnMe3) was performed at lower temperature.838 A triple decker heterobinuclear Co/Mn complex with a bridging 5:5-diborolyl ligand (852) has been prepared by substitution of labile ligands in cationic Mn(I) precursors with a [CpCo(5-diborolyl)]− anion (Scheme 184). Though close relatives of 852 have previously been reported, this represents a new reaction pathway to access this type of complex.839

Scheme 183

Lastly, dimanganese decacarbonyl reacted with excess tris(2-thienyl)phosphine or bis(2-thienyl)phenylphosphine to generate a mixture of three products (Scheme 185); [Mn2(CO)9{P(2-thienyl)2R)] (the product of phosphine substitution of one carbonyl group) and two dimetallic complexes formed via PdC bond oxidative addition (853 and 854), with and without substitution of a

Scheme 184

Cyclic and Non-Cyclic Pi Complexes of Manganese

533

carbonyl ligand by a second equivalent of the phosphine. Compounds 853 and 854 both contain bridging phosphido and thienyl ligands, the latter of which is 1-C-coordinated to one Mn center and 5-coordinated to the other. These complexes are diamagnetic, and the crystal structures indicate that the bridging phosphido ligand lies closer to the Mn center to which the thienyl group is 5-coordinated.840,841 In addition to experimentally observed species, various calculations have been conducted on complexes featuring heteroatom-containing cyclic p-ligands. These include complexes with fused polycyclic arenes which could bind via a C6 or C5N fragment; the lowest energy calculated structure of [CpMn(6-benzoquinoline)] involved Mn binding to the C5N ring.754 DFT

Scheme 185

calculations have also been conducted to probe the electronic structures of manganese complexes with azepine ligands.752,842 Other calculations have involved determining the electronic structures of hypothetical sandwich complexes with carbon boronyl (CBO)n ligands ([Mn{5-(CBO)5}2]− or [Mn{6-(CBO)6}2]+)843 or methylboratabenzene ligands ([Mn(6-C5H5BMe)2]),844 as well as neutral and anionic [Mn{6-BC5(CN)6}2]0/−.845 Calculations have also been reported on the electronic structures of manganese(I) structures containing one B6C2− or two B6N− ligands (with no additional co-ligands),846 as well as triple decker complexes with terminal arene rings and a bridging C3B3H6 ring.740 Additionally, calculations have been reported on dimetallic carbonyl complexes with pyrrolyl, phospholyl, or arsolyl ligands,847 triple-decker complexes with m-5:5-1,2-C3X2H5 (X ¼ B or N) ligands,775 dimetallic manganese carbonyl species with 1,3-diphosphacyclobutadiene ligands,848 an analogue of manganocene with two 5-coordinated 1,2,3,5-tetramethyl-1,2-diaza-3,5-diborolyl ligands,735 and [Mn(5-C4H4BMe)2].849

5.07.3.4

Miscellaneous (cyclic)

DFT calculations on a hypothetical Mn(I) tetracarbonyl hydride C60 complex have been performed to compare the relative energies of different coordination modes, and predicted that 2-coordination would be lowest in energy.850 Calculations have also been conducted on sandwich clusters where Mn is bound to a fullerene and either a benzene805 or a cyclopentadienyl750 co-ligand. Additionally, DFT calculations have been conducted on BN graphene functionalized with Mn(6-benzene).812 Computational investigations have also been carried out on a wide variety of other manganese complexes with cyclic p ligands, including cyclooctatetraene,749,851–855 fluorinated cyclooctatetraene,856,857 fulvene,747 [3]- and [4]radialene,748 cycloheptatrienyl,858,859 azulene,738,860 and heptalene.861 Furthermore, while novel 4-cyclobutadiene complexes of manganese have not been reported during the 2005-2020 period, [(4-C4H4)2Mn(PMe3)] has been used as a precursor to access a variety of manganese telluride clusters.862 Calculations have also been conducted of a series of manganese-containing complexes with cyclobutadiene ligands, including examples where the cyclobutadiene ligand bridges between two metal centers in an inverted-sandwich m-4:4-coordination mode.761,767,768,863

5.07.4

Concluding remarks

The period from 2005 to 2020 has seen a remarkably broad array of publications on organometallic manganese p complexes. Mono-cyclopentadienyl complexes, principally of manganese(I), are particularly prevalent and will undoubtedly see further development, given that they provide a robust platform for the study/development of unique bonding situations and reactivity, as well as complexes for use in medicine and materials chemistry. Manganese(II) p complexes are also likely to see additional development with a focus on unusual magnetic properties, as are manganese(I) arene and cyclohexadienyl complexes in the context of arene functionalization (via nucleophilic attack on the arene ligand in cationic manganese(I) arene complexes). By contrast, the manganese chemistry of cyclopentadienyl-, arene-, and cyclohexadienyl-free p complexes, such as those bearing reactive acyclic or heteroatom-containing p ligands, is significantly less developed and represents a fertile area for future development.

534

Cyclic and Non-Cyclic Pi Complexes of Manganese

Acknowledgments D. J. H. E thanks NSERC of Canada for a Discovery Grant.

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Cyclic and Non-Cyclic Pi Complexes of Manganese 619. 620. 621. 622. 623. 624. 625. 626. 627. 628. 629. 630. 631. 632. 633. 634. 635. 636. 637. 638. 639. 640. 641. 642. 643. 644. 645. 646. 647. 648. 649. 650. 651. 652. 653. 654. 655. 656. 657. 658. 659. 660. 661. 662. 663. 664. 665. 666. 667. 668. 669. 670. 671. 672. 673. 674. 675. 676. 677. 678. 679. 680. 681. 682. 683. 684. 685. 686. 687. 688. 689. 690.

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Cyclic and Non-Cyclic Pi Complexes of Manganese 761. 762. 763. 764. 765. 766. 767. 768. 769. 770. 771. 772. 773. 774. 775. 776. 777. 778. 779. 780. 781. 782. 783. 784. 785. 786. 787. 788. 789. 790. 791. 792. 793. 794. 795. 796. 797. 798. 799. 800. 801. 802. 803. 804. 805. 806. 807. 808. 809. 810. 811. 812. 813. 814. 815. 816. 817. 818. 819. 820. 821. 822. 823. 824. 825. 826. 827. 828. 829. 830.

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5.08

Organometallic Complexes of Technetium

Henrik Braband, Department of Chemistry, University of Zurich, Winterthurerstrasse, Zürich, Switzerland © 2022 Elsevier Ltd. All rights reserved.

5.08.1 Introduction 5.08.2 Technetium carbonyls and their halide and hydride derivatives 5.08.2.1 Binary and mixed metal carbonyls 5.08.2.2 Halo and hydrido technetium carbonyls 5.08.3 Other technetium carbonyl derivatives 5.08.3.1 Oxygen and sulfur 5.08.3.2 Nitrogen and phosphorus 5.08.4 Technetium isocyanides and their derivatives 5.08.4.1 Binary technetium isocyanides 5.08.4.2 Technetium isocyanide derivatives 5.08.5 Technetium cyclopentadienyl complexes and other p-complexes 5.08.5.1 Cyclopentadienyl complexes 5.08.5.2 Arene complexes 5.08.5.3 Other p-complexes 5.08.6 Derivatives containing single- or multiple-bonded h1-carbon groups 5.08.6.1 Alkyl/aryl complexes 5.08.6.2 Carbene complexes 5.08.6.3 Carbyne complexes 5.08.7 Structural data and 99Tc NMR studies 5.08.7.1 Structural data 99 Tc NMR 5.08.7.2 5.08.8 Conclusion and perspective Acknowledgment References

5.08.1

547 548 548 550 552 552 556 559 559 561 566 566 570 573 574 574 575 579 579 579 581 582 583 583

Introduction

The element technetium (Tc) is located in the middle of the periodic table (Z ¼ 43). As its position in the periodic table suggests, technetium can be characterized as a typical second row transition metal. Its chemical properties are more similar to those of its heavier homolog rhenium than those of manganese. There is one property that dramatically distinguishes it from the rest of the second row transition metals: Technetium is a radioactive element and does not have any stable isotopes. Technetium is often referred to as the first artificial element, produced by mankind. After a long search it was discovered and characterized by C. Perrier and the Nobel laureate E. Segrè in 1937.1 The question why technetium does not have stable isotopes puzzled and fascinated researchers from the beginning of its discovery. A simple explanation is that the position of technetium in the periodic table can be described as very unfavorable. None of the proton/neutron configuration leads to a nucleus which is energetically favored. This simple fact is the reason why Tc isotopes either convert by b− decay to a stable ruthenium isotope or by b+ decay and electron capture (e) to a stable molybdenum isotope (see Fig. 1). A much more precise and detailed explanation can be found in an excellent publication by Johnstone and coworkers.2 Out of the 37 known Tc isotopes, only one has a significant impact on society. Technetium-99 (99Tc) and its nuclear isomer 99mTc are important for different reasons. 99Tc is a fission product in nuclear reactors (fission yield 6%).3 It is a weak b− emitter (Emax ¼ 294 keV) with a half-life time of 2.111  105 years. Whereas the first technetium samples were isolated in trace amounts from cyclotron-radiated molybdenum foils, 99Tc became available in macroscopic amounts by the workup of spent nuclear fuels.4 Due to its long half-life, 99Tc became an important isotope for nuclear waste management and protocols to separate long-lived 99Tc species from high-level nuclear waste are still under development.5 99m Tc became one of the most important nuclear isomers for radiodiagnostics. Its radiation characteristics (g-emitter, radiation energy ¼ 142.68 keV, half-life time ¼ 6.01 h) make it a very practical nuclear isomer in the development of probes for radiodiagnostic applications, utilizing single-photon emission computed tomography (SPECT). Since the early years of radiodiagnostics several 99mTc drugs have been developed for imaging the functional efficiency of all organs and bones.6–13 A comprehensive overview about recent developments in the field of 99mTc radiodrugs is published by R. Alberto in the chapter ‘Organometallic chemistry of drugs based on technetium and rhenium’ of this series. Nuclear waste management, including environmental studies, and nuclear medical applications are the main driving forces for technetium research. The close relationship between technetium and questions that significantly influence our society (health, energy) leads to a strong focus on practical applications in fundamental technetium research.

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00019-6

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Fig. 1 Excerpt from the chart of nuclides, showing the unfavorable situation of the element technetium. Technetium either converts by b− decay (blue isotopes) or b+ decay and electron capture (red isotopes) into energetically more favored (stable) ruthenium of molybdenum nuclei (black isotopes).

Considering the fact that technetium has been discovered in the late 1930s it is fair to say that organometallic chemistry has a long tradition. First reports about Tc bisarene chemistry ([Tc(Z6-C6R6)2]+) were published in the beginning of the 1960s. Driven by pure fundamental interest in organometallic chemistry, these studies focused on the synthesis of 99Tc bisarene, carbonyl and cyclopentadienyl complexes. Technetium bisarene complexes, the synthesis of water-stable hexakis(isocyanide)technetium(I) complexes ([Tc(CNR)6]+) and later the tris(aquo)tris(carbonyl)technetium(I) compound ([Tc(CO)3(H2O)3]+) contradicted the general assumption, that metal-carbon bonds are generally highly reactive and thus paved the way for organometallic technetium compounds in life sciences (Scheme 1).

Scheme 1 General Lewis structures of hexakis(isocyanide)technetium(I) complexes ([Tc(CNR)6]+) and of the complex tris(aquo)tris(carbonyl)technetium(I) ([Tc(CO)3(H2O)3]+).

For application in life sciences, research focuses strongly on the nuclear isomer 99mTc. Since 99mTc is only available at tracer level (generator nuclide), 99Tc is often used to synthesize structural models which can be analyzed and characterized at the macroscopic level. Furthermore, 99Tc is used to develop novel reactions or classes of compounds, which generate new opportunities and tools for the application of 99mTc. Low valent 99Tc carbonyl species have been found in high-level nuclear waste tanks at the Hanford Reservation.14 It has been stated that the presence of soluble, lower-valent technetium carbonyl species in this nuclear waste complicates the immobilization of this high-level nuclear waste.14 Consequently, organometallic 99Tc chemistry gains momentum as well in the field of nuclear waste management, in recent years. This article focuses on organometallic chemistry of the isotope 99Tc, summarizing the developments from the beginning until the year 2020. 99Tc carbonyl, isocyanate, cyclopentadienyl, arene, alkyl, aryl, carbene and carbyne complexes are discussed. Where available, structural information is highlighted, and links are made to research fields that go beyond pure fundamental organometallic 99Tc chemistry.

5.08.2

Technetium carbonyls and their halide and hydride derivatives

5.08.2.1

Binary and mixed metal carbonyls

Various technetium starting compounds, such as TcO2, [Tc2O7], (NH4)[TcO4] and Na[TcO4] have been used to synthesize homoleptic ditechnetium decacarbonyl [Tc2CO10], which has been an extremely important precursor for organometallic technetium chemistry. However, all these reactions have been performed at elevated temperature and CO pressure in stainless steel

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549

autoclaves.15–18 The risk assessment of reactions that combine radioactive material, high pressure and a highly toxic gas usually leads to the recognition of a very high risk potential. Therefore, the feasibility of these reactions strongly depend on national laws and local safety regulations/guidelines. The highest yields of [Tc2CO10] in multigram scale reactions have been achieved by using a variant of the [Re2(CO)10] synthesis developed by Heinekey and coworkers.17,19 It has been stated that a small amount of copper powder is crucial for the success of this carbonylation reaction (Scheme 2).20

Scheme 2 High-pressure synthesis of [Tc2CO10].17,20

An interesting low-pressure synthesis for [Tc2(CO)10] has been described in a French patent in 1997.21 It has been claimed that the reduction of (NH4)[MO4] (M ¼ 99Tc, Re) suspended in toluene, saturated with carbon monoxide, at ca. 75  C with diisobutyl aluminum hydride (DIBAL) yields [M2(CO)10] (M ¼ 99Tc, Re) (Scheme 3).

Scheme 3 Low-pressure synthesis of [Tc2(CO)10].21

The examples given in this patent exclusively contain Re as metal. Until today, no reports have been published, using this synthetic pathway for the synthesis of [Tc2(CO)10]. Nevertheless, it would be a remarkably interesting reaction, which should be studied in more detail. [Tc2(CO)10] is moderately air stable and can be purified by vacuum sublimation. Handling [Tc2(CO)10] should be done carefully. Even though this compound is not volatile at 1 atm, it has been shown that it slowly migrates through plastic.22 After some time, this property leads to contaminations on the outside of the plastic. In the solid state the dimeric [Tc2(CO)10] is isomorphous to [Mn2(CO)10] and [Re2(CO)10].23 It has a D4d symmetry with two metal centers coordinated octahedrally by five carbonyl groups (Fig. 2). The equatorial carbonyl groups are arranged in a staggered configuration. The metal-metal bond distance in [Tc2(CO)10] is 3.036(6) A˚ and the average TcdC bond distances are 2.00(1) A˚ (equatorial) and 1.90(1) A˚ (axial). Besides this dimeric 99Tc complex the mixed metal MndTc and TcdRe compounds are also known.24 The infrared spectrum of [Tc2(CO)10] measured in cyclohexane solution shows three strong absorptions for the carbonyl stretching vibrations at 2064, 2017 and 1984 cm−1.25

Fig. 2 Ball and stick representation of the crystal structure of [Tc2(CO)10].23

Infrared spectroscopy and the comparison to the corresponding Mn complex showed, that the reduction of [Tc2(CO)10] by sodium amalgam leads to the formation of the 99Tc−I complex [Tc(CO)5]−.25 The 99Tc+I hexacarbonyl cation ([Tc(CO)6]+, 1) can be prepared, following two different synthetic strategies. The reaction of [TcX(CO)5] (X ¼ Cl, I; 2) with CO gas in an autoclave leads to the formation of [Tc(CO)6]+ with yields up to 60% (Scheme 4).26–28 Alternatively, the trisaquo complex [Tc(CO)3(H2O)3]+ (3) and the tetra-nuclear cluster [Tc(OH)(CO)3]4 (4) can be used as starting compounds (Scheme 4).29,30

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Organometallic Complexes of Technetium

Scheme 4 Synthetic strategies for the synthesis of [Tc(CO)6]+ (1), using [TcX(CO)5] (X ¼ Cl, I; 2),26–28 or [Tc(CO)3(H2O)3]+ (3) and [Tc(OH)(CO)3]4 (4) as starting compounds.29,30

In its crystalline form [Tc(CO)6]+ has a nearly ideal octahedral coordination geometry.27 The TcdC bond lengths are in the range of 2.025(3)–2.029(3) A˚ and the bond angles of the cis-CO groups are very close to 90 . In solution (H2O, MeOH, MeCN), [Tc(CO)6]+ is stable at room temperature for several days.28 However, in MeCN a slow ligand exchange has been observed, leading exclusively to the fac-tricarbonyl 99Tc complex [Tc(CO)3(MeCN)3]+. The rate constant (7.68  0.16  10−6 s−1 at 321.6 K) and the activation energy (118  4 kJ/mol) of this decarbonylation reaction have been determined and it has been found that the rate constant is lower by more than on order of magnitude than that of [TcBr(CO)5].28 Furthermore, the CO replacement in higher technetium carbonyls, such as [Tc(CO)6]+, has been analyzed by quantum-chemical calculations.31 The study revealed that the CO ligands are weaker bound to the Tc center in the hexacarbonyl cation as compared to pentacarbonyl halides. However, [Tc(CO)6]+ shows the higher resistance to CO replacement. This trend is attributable to considerable enhancement of bonding between the technetium atom and the halide ligand in the transition state of the decarbonylation reaction of technetium pentacarbonyl halides, which reduces the barrier for CO elimination. [Tc(CO)6]+ is an interesting starting compound for the synthesis of 99Tc hydrido carbonyl clusters (see Section 5.08.2.2).

5.08.2.2

Halo and hydrido technetium carbonyls

99

Tc pentacarbonyl halides, [TcX(CO)5] (X ¼ Cl, Br, I) can be synthesized by two different synthetic strategies. The direct oxidation of [Tc2(CO)10] with X2 in CCl425 and the high pressure carbonylation of [TcX6]2− (X ¼ Cl, Br, I),26 or [TcO4]−.32 Recently, an alternative procedure has been developed, which does neither rely on [Tc2(CO)10] as starting compound nor high CO gas pressure. By reacting K2[TcX6] (X ¼ Cl, Br, I) with a mixture of HCOOH and H2SO4, 99Tc pentacarbonyl halides can be synthesized at ambient-pressure (Scheme 5).33

Scheme 5 Low pressure synthesis of [TcX(CO)5] (X ¼ Cl, Br, I).33

[TcF(CO)5] was synthesized for the first time by substitution of the iodo ligand in [TcI(CO)5] with fluoride ions.34 [TcF(CO)5] is unstable and rapidly undergoes decarbonylation at room temperature in dichloromethane. Low temperature crystallization of [TcF(CO)5] led to the isolation of the mixed tricarbonyl hydroxyfluoride–pentacarbonyl tetrafluoroborate technetium complex [Tc(OH)0.49F0.51(CO)3]4
[Tc(BF4)(CO)5], which has been analyzed by X-ray diffraction analysis. In addition, the structures of the other pentacarbonyl halides, [TcX(CO)5] (X ¼ Cl, Br, I) have been elucidated by X-ray diffraction analysis.35,36 In these compounds the metal is coordinated in a slightly distorted octahedral fashion. Whereas [TcCl(CO)5] and [TcBr(CO)5] are isostructural, [TcI(CO)5] is not isostructural to the chloro- and the bromo-complex. An interesting structural feature of these [TcX(CO)5] compounds is the elongated TcdC bond length in cis-position to the respective halogen atom (mean bond length X ¼ Cl: 2.016 A˚ , X ¼ Br: 2.021 A˚ , X ¼ I 2.015 A˚ ) in comparison to the TcdC bond lengths in trans position (X ¼ Cl: 1.915 A˚ , X ¼ Br: 1.937 A˚ , X ¼ I: 1.938 A˚ ). These findings are underlined by the reactivity of this class of 99Tc carbonyl complexes, which is characterized by the lability of the CO groups in cis-position to the halogen atom. The pentacarbonyl halides decarbonylate in a stepwise manner by heating in non-coordinating solvents or by vacuum sublimation.37 Decarbonylation initially results in the formation of halide-bridged dimers and, ultimately, tetrameric 99Tc+I complexes (Scheme 6). The decarbonylation rate increases in the order I < Br < Cl.38

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Scheme 6 Stepwise decarbonylation of [TcX(CO)5] complexes.37

Fig. 3 Ball and stick representation of the X-ray structures of (A) [TcI(CO)4]2 and (B) the anion of the structure of (NBu4)[Tc2Cl3(CO)6].39,42

The decarbonylation of 99Tc pentacarbonyl complexes has been studied in detail by kinetic investigations as well as theoretical calculations.31,38 The X-ray structures of the dimeric [TcI(CO)4]2 and the tetranuclear clusters [TcCl(CO)3]4 and [TcI(CO)3]4 have been reported.39–41 The structure of [TcI(CO)4]2 can be described as an edge-shared bioctahedral (Fig. 3). The structures of [TcCl(CO)3]4 and [TcI(CO)3]4 are of cubane-type, as it is also known for the hydroxide cluster [Tc(OH)(CO)3]4.43 [TcX(CO)5] as well as [TcX(CO)3]4 (X ¼ Cl, Br, I) can be reacted with bidentate N- and O-donor ligands (L), such as 1,10-phenanthroline and 2,20 -bipyridine to give compounds of the type [TcX(CO)3L].44 The reaction of [TcBr(CO)5], [TcBr(CO)4]2 or [TcBr(CO)3]4 with ethylenediamine (en) yields [TcBr(CO)3(en)].45 In the X-ray structure of [TcBr(CO)3(en)] the TcdBr bond distance is 2.640(1) A˚ and the TcdN distances are 2.211(7) and 2.233(6) A˚ . The reduction of [TcO4]− in THF under a CO gas atmosphere (1 atm) with BH3
THF in the presence of chloride yields the dinuclear 99Tc complex [Tc2(m-Cl)3(CO)6]− of which the tetrabutylammonium salt has been analyzed by X-ray diffraction analysis (Scheme 7).42 A representation of this structure is shown in Fig. 3. The TcdCl bond lengths in [Tc2(m-Cl)3(CO)6]− are between 2.503(2) and 2.566(2) A˚ . Upon the addition of (NEt4)Cl to a solution of [Tc2(m-Cl)3(CO)6]− in EtOH, the Cl− bridges can be cleaved and (NEt4)2[TcCl3(CO)3] precipitated from the reaction mixture.

Scheme 7 Low pressure synthesis of (NBu4)[Tc2(m-Cl)3(CO)6] and its transformation into (NEt4)2[TcCl3(CO)3].42

These reactions and the obtained knowledge about the products formed by the reduction of [TcO4]− in THF under an CO gas atmosphere (1 atm) with BH3
THF led to the development of a direct low pressure synthesis of (NEt4)2[TcCl3(CO)3].46 Besides (NEt4)2[ReBr3(CO)3], (NEt4)2[TcCl3(CO)3] has become one of the most important precursors for the synthesis of fac-{M(CO)3}+ (M ¼ 99Tc, Re) type complexes, which are often used to elucidate the structure of novel fac-{99mTc(CO)3}+ based radiodiagnostics. If the reduction of [TcO4]− in THF under a CO gas atmosphere (1 atm) is exclusively performed with BH3
THF (no addition of (NBu4)Cl) the 99Tc hydrido carbonyl cluster [Tc3(m-H)3(CO)12] is formed.47 The structure of [Tc3(m-H)3(CO)12] has been elucidated by X-ray diffraction analysis and is shown in Fig. 4. Since this triangular cluster is electronically unsaturated, the formation of TcdTc contacts can be expected. The TcdTc separation in this cluster average 3.28 A˚ . Besides [Tc2(CO)10], the pale yellow [Tc3(m-H)3(CO)12] has been obtained after hydrolysis of [Tc(CO)6]+ in aqueous solution at pH  7 and workup at elevated temperature and under vacuum.49 Recently, it has been found, that the formation of [Tc2(CO)10] and [Tc3(m-H)3(CO)12] is caused by the decomposition of the primary yellow product [Tc3(m-H)(CO)14] (Fig. 4).48 The molecular structure of [Tc3(m-H)(CO)14] is composed of a {Tc2(CO)9} fragment with a weak metal −metal bond (TcdTc bond distance ¼ 3.0441(5) A˚ ) bound to a {Tc(CO)5} fragment via a hydrogen bridge. A number of attempts have been made to synthesize and isolate the simple, and volatile carbonyl hydride complex [HTc(CO)5].25 Although the corresponding Mn and Re hydrido carbonyls are reasonably stable,25,50 [HTc(CO)5] as well as the anion [Tc(CO)5]− are unstable in solution.49 Most likely, [HTc(CO)5] oligomerizes to [Tc3(m-H)3(CO)12] or [HTc(CO)4]3 in solution. A dimeric Tc carbonyl hydrido complex is obtained by refluxing [Tc2(CO)10] or [Tc2(m-CO)2(CO)6(py)2] in pyridine

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Organometallic Complexes of Technetium

Fig. 4 Ball and stick representation of the X-ray structure of (A) [Tc3(m-H)3(CO)12] and (B) [Tc3(m-H)(CO)14].47,48

(py) (Scheme 8).51 [Tc2(m-H)(CO)6(m-py)(py)2] (5) was crystallized from a 1:1 mixture of acetone and n-hexane and analyzed by X-ray diffraction analysis. In the solid state one pyridine ring acts as a bridge between the metal centers forming an additional TcdC bond with the ortho-carbon atom. A hydrogen atom serves as a second bridge, forming a planar five membered TcdHdTcdCdN ring. The TcdTc and TcdH distances are 3.23 and 1.82 A˚ , respectively.

Scheme 8 Synthesis of [Tc2(m-H)(CO)6(m-py)(py)2] (py ¼ pyridine; 5), starting from [Tc2(CO)10] or [Tc(m-CO)2(CO)6(py)2].51

5.08.3

Other technetium carbonyl derivatives

5.08.3.1

Oxygen and sulfur

Upon reacting [TcI(CO)5] with Ag(ClO4) in CH2Cl2, the carbonyl perchlorate complex [Tc(ClO4)(CO)5] is formed.52 The yellow crystals have been analyzed by X-ray diffraction. In this crystal structure the (ClO4)− anion is coordinated to the Tc center in a monodentate fashion (TcdO bond length ¼ 2.1996(15) A˚ ). The perchlorate ligand is only weakly bound and can readily be substituted by coordinating solvent molecules such as MeCN. When the carbonylation of sodium pertechnetate is performed with lower CO gas pressure and temperature (135 atm CO, 150  C, 48 h) than for the synthesis of [Tc2(CO)10] (see Section 5.08.2.1), the trinuclear Tc+I cluster Na[Tc3(OMe)4(CO)9] (6) can be isolated (Scheme 9).17 The cubane type geometry of this cluster has been proven by X-ray diffraction analysis. Under harsher

Scheme 9 Synthesis of Na[Tc3(OMe)4(CO)9] (6).17

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conditions (190 atm CO, 230  C, 48 h) this cluster can be converted into [Tc2(CO)10]. Na[Tc3(OMe)4(CO)9] has proven to be an interesting starting compound for the synthesis of new organometallic Tc+I complexes, containing the fac-{Tc(CO)3}+ moiety. The cluster is already broken up by weak proton sources such as HC5H5 and HC5Me5 (Section 5.08.5.1).17 As mentioned before (Section 5.08.2.2), the development of a low pressure synthesis for the precursor complexes (NEt4)2 [TcX3(CO)3] (X ¼ Cl, Br), starting form (NBu4)[TcO4] in the 1990s had an enormous impact onto the field of technetium carbonyl chemistry.42,43,53 With these compounds in hand, laboratories, which were not equipped for high pressure syntheses, could enter the field of technetium carbonyl chemistry. Analogous to the low-pressure synthesis of (NBu4)[Tc2(m-Cl)3(CO)6] (Section 5.08.2.2, Scheme 7), (NEt4)2[TcX3(CO)3] (X ¼ Cl, Br) is synthesized by the reduction of [TcO4]− in THF under 1 atm CO gas pressure with BH3
THF. Evaporation of the solvent followed by treatment of the residue with ethanolic (NEt4)X (X ¼ Cl, Br) yields (NEt4)2[TcX3(CO)3] (X ¼ Cl, Br).54 Even after years of extensive research with this compound the reaction mechanism of this six-electron reduction is still unclear. As described in Section 5.08.2.2, [Tc2(m-Cl)3(CO)6]− was the only intermediate compound, which had been trapped from this reaction.42 The halide-metal bonds in [TcX3(CO)3] (X ¼ Cl, Br) are weakened due to the trans-effect of the carbonyl groups. The halides can be replaced by a wide range of electron donating ligand systems or coordinating solvent molecules, such as water, methanol or acetonitrile. The substitution of the halide ligands with water yields [Tc(CO)3(H2O)3]+. Aqueous solutions of [Tc(CO)3(H2O)3]+ are stable at neutral and lower pH. At higher pH, reversible hydrolytic oligomerization takes place, leading to the hydroxyl carbonyl cluster [Tc(m3-OH)(CO)3]4 (Scheme 10).43,55 The structure of [Tc(m3-OH)(CO)3]4 has been elucidated by X-ray diffraction analysis and is depicted in Fig. 5.43

Scheme 10 Reversible hydrolytical oligomerization, leading to the hydroxyl carbonyl cluster [Tc(m3-OH)(CO)3]4.43,55

Due to the chemical properties of [Tc(CO)3(H2O)3]+, such as its water stable core structure and the ability to coordinate soft as well as rather hard ligand systems, it quickly became clear that this compound had the potential to be a key compound for future radioprobe developments with the nuclear isomer 99mTc. Thus, the desire for a synthesis of [99mTc(CO)3(H2O)3]+ quickly arose. For the first synthetic protocol of [99mTc(CO)3(H2O)3]+ the reaction conditions of the low pressure synthesis of [99TcCl3(CO)3]2− were translated into the 99mTc regime.56 However, a direct and elegant synthetic protocol for the synthesis of [99mTc(CO)3(H2O)3]+ uses potassium boranocarbonate (K[H3BCO2H]) as reagent.57 In this reaction K[H3BCO2H] acts as reducing agent as well as the source of CO. The development of this reaction is a beautiful piece of research and led to the commercialization of the so called ‘IsoLink’ kit for nuclear medical application. The low concentrations of CO in these reaction solutions exclude any stepwise mechanism of reduction followed by CO coordination.57 The impact of this synthesis on the field of radioprobe developments was enormous. Over almost two decades, research with the fac-{99mTc(CO)3}+ moiety dominated the search for novel radiodiagnostics with 99m Tc.6–13 ‘Kläui’ type ligands Na[CpCo[PO(OR)2]3] (NaLOR: R ¼ Me, Et) have been used to synthesize structural models for the [Tc(CO)3 (H2O)3]+ cation.58 (NEt4)2[TcCl3(CO)3] rapidly reacts with NaLOR (R ¼ Me, Et) in water, yielding [TcLOR(CO)3] (R ¼ Me, Et). Both

Fig. 5 Ball and stick representation of the crystal structure of [Tc(OH)(CO)3]4
H2O.43 TcdO bond length are between 2.180(3) and 2.191(3) A˚ , TcdC bond length are between 1.886(4) and 1.905(4) A˚ .

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Organometallic Complexes of Technetium

Fig. 6 Ball and stick representation of the crystal structure of the 99Tc complex [TcLOEt(CO)3].58 TcdO bond length are between 2.168(7) and 2.178(6) A˚ , TcdC bond length are between 1.84(2) and 1.877(14) A˚ .58

Fig. 7 Ball and stick representation of the crystal structure of (A) the [Tc2(m-SCH2CH2OH)3(CO)6]− anion (TcdS bond length ¼ 2.5099(9)–2.5157(9) A˚ ) and (B) the [Tc(9-ane-S3)(CO)3]+ cation (TcdS bond length ¼ 2.447(2)–2.465(2) A˚ ).53,59

ligands coordinate the fac-{Tc(CO)3}+ core in a tridentate fashion utilizing an {O,O,O} donor set. Fig. 6 shows the crystal structure of [TcLOEt(CO)3]. As mentioned before, [TcCl3(CO)3]2− and [Tc(CO)3(H2O)3]+ are excellent precursors to a wide variety of compounds containing the fac-{Tc(CO)3}+ moiety. Substitution reactions are possible with mono-, bi-, and tridentate ligand systems. One of the first reported examples was the reaction of (NEt4)2[TcCl3(CO)3] with 2-mercaptoethanol (HSCH2CH2OH).53 In this reaction, two intermediates were observed with IR spectroscopical methods. The final product was the dimeric complex (NEt4)[Tc2(m-SCH2CH2OH)3(CO)6] (Fig. 7), the structure of which was determined by X-ray diffraction analysis (TcdS bond length ¼ 2.5099(9)–2.5157(9) A˚ ).53 The cyclic thio-crown ether 1,4,7-trithiacyclononane (9-ane-S3), hexadentate 1,4,7,10,13,16-hexathiacyclooctadecane (18-ane-S6), and 3,6,9,13,16,19-hexathiacycloicosanol (20-ane-S6-OH) react with [Tc(CO)3(H2O)3]+ in methanol to provide the corresponding complexes [Tc(9-ane-S3)(CO)3]+ (Fig. 7), [Tc2(18-ane-S6) (CO)6(tosylate)2], and [Tc2(20-ane-S6-OH)(CO)6]2+.59 All compounds have been structurally characterized. In the case of (18-ane-S6) and (20-ane-S6-OH) the formation of the 1:2 ([Tc(ane)2(CO)3]+, ane ¼ 18-ane-S6, 20-ane-S6-OH) as well as the 1:1 complex ([Tc(ane)(CO)3(tosylate)], ane ¼ 18-ane-S6, 20-ane-S6-OH) could be observed in 99Tc NMR experiments, depending on the ligand to metal ratio. Furthermore, (NEt4)2[TcCl3(CO)3] reacts with a series of thiourea derivatives, such as N,N-diethylthiocarbamoylbenzamidine (L1) and morpholinylthiocarbamoylbenzamidine (L2), under formation of stable sulfur bridged dimeric complexes (7, 8; Scheme 11).60 In a reaction with a thiosemicarbazone derived form 2,20 -dipyridyl ketone (L3), the ligand surprisingly coordinates only via the pyridine nitrogen atoms (9).61 The same reaction with a thiosemicarbazone ligand derived from 4-acetylpyridine (L4) yielded a dinuclear complex (10, Scheme 11). This compound represents the first technetium complex with a coordinated thiosemicarbazone functionality.61 A series of tridentate thiocarbazone ligands of the N,N-dialkylamino(thiocarbonyl)-N0 -picolylbenzamidine type

Organometallic Complexes of Technetium

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Scheme 11 Reactions of (NEt4)2[TcCl3(CO)3] with N,N-diethylthiocarbamoylbenzamidine (L1) morpholinylthiocarbamoylbenzamidine (L2) and thiosemicarbazones derived from 2,20 -dipyridyl ketone (L3) and 4-acetylpyridine (L4).60,61

has been reacted with (NEt4)2[TcCl3(CO)3], as well.62 These reactions demonstrated, that N,N-dialkylamino(thiocarbonyl)N0 -picolylbenzamidines are excellent ligands for the fac-{Tc(CO)3}+ core and the reactions have already been translated into a 99m Tc labeling protocol. Besides these interesting tridentate thiocarbazones also simple bidentate 1,2-ethanedithiol derivatives have been used to coordinate biomolecules to the fac-{Tc(CO)3}+ core. This approach has been used to synthesize a 99Tc structure model of the receptor binding 99mTc imaging probe [TcCl(CO)3 ((1,4.dithiapent-1-yl)progesterone)].63 Tetracarbonyl complexes of the type [Tc(CO)4L] can be synthesized by reacting [TcCl(CO)5] with dithiocarbamates and xanthates (L ¼ S2CNEt2, S2COMe).64 [Tc(CO)4(S2CNEt2)] is prone to decarbonylation, leading to the dimeric complex [Tc(CO)3(S2CNEt2)]2. Furthermore, both compounds decarbonylate in coordinating solvents, forming tricarbonyl solvates [Tc(CO)3L(sol)]. All these complexes have been analyzed by X-ray diffraction analysis. The reactions of dihydrobis(2-mercapto-1-methylimidazolyl)borate ([H2B(timMe)2]−) or trihydro(2-mercapto-1-methylimidazolyl)borate ([H3B(timMe)]−) with (NEt4)2[TcCl3(CO)3] led to a facial coordination of the ligands and the formation of one or two three-center, two-electron bonds (B-H ⋯ Tc), respectively.65,66 The compounds [Tc{k3-H(m-H)B(timMe)2}(CO)3] and [Tc{k3-H (m-H)2B(timMe)}(CO)3] represent unique examples of structurally characterized Tc carbonyl complexes exhibiting three-center, two-electron bonds (Fig. 8). The bidentate ligand sodium bis(diphenylthiophosphoryl)amide (Na((Ph2PS)2N)) reacts with (NEt4)2[TcCl3(CO)3] in acetonitrile to provide colorless crystals of the neutral complex [Tc ((Ph2PS)2N)(CO)3(CH3CN)].67 The crystal structure of this complex reveals TcdS distances of 2.546(3) and 2.526(3) A˚ and a TcdN distance of 2.155(6) A˚ .

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Organometallic Complexes of Technetium

Fig. 8 Ball and stick representations of the crystal structure of (A) [Tc{k3-H(m-H)B(timMe)2}(CO)3] (TcdS bond length ¼ 2.4931(12), 2.5190(11) A˚ ; TcdH bond length ¼ 1.65(6) A˚ ) and (B) [Tc{k3-H(m-H)2B(timMe)}(CO)3] (TcdS bond length ¼ 2.4739(6) A˚ ; TcdH bond length ¼ 1.89(3), 1.940(3) A˚ ).65,66

A rather unique reaction is the reduction of (NH4)[TcO4] with formamidinesulphinic acid in the presence of sodium diethyldithiocarbamate and aqueous base. This reaction provides the seven coordinated 99Tc+III complex [Tc(S2CNEt2)3(CO)] (11; Scheme 12).68

Scheme 12 Synthesis of [Tc(S2CNEt2)3(CO)] (11). It is suggested that the carbonyl ligand is formed by a metal mediated decomposition of coordinated formamidinesulphinic acid.68

The coordinated CO ligand does not originate from gaseous CO. It is suggested that the carbonyl ligand is formed by a metal mediated decomposition of coordinated formamidinesulphinic acid. Alberto has commented that the same mechanistic picture is likely to be valid for the formation of the fac-{Tc(CO)3}+ moiety as well.9 In the solid state [Tc(S2CNEt2)3(CO)] adopts a distorted pentagonal bipyramidal coordination geometry with TcdS bond distances between 2.440(3) and 2.520(3) A˚ . A series of other 99 +III Tc carbonyl complexes have been synthesized by reacting the precursor complexes [Tc(SAr)3(MeCN)2] (SAr ¼ 2,3,5,6tetramethylbenzenethiolate, 2,4,6-triisopropylbenzenethiolate) with CO gas.69 The final products are of the [Tc(SAr)3(CO)2] type. It has been shown, that one of the CO ligands is labile and can be replaced by monodentate ligands, such as pyridine or MeCN, leading to monocarbonyl complexes. An interesting report about sulfur containing 99mTc carbonyl complexes has been published by Alberto and coworkers, very recently.70 The formation of di- or polynuclear complexes at nanomolar concentrations is generally too slow to be observed with 99m Tc due to the low 99mTc concentration (10−6–10−9 M). However, appropriate thiols as bridging ligands can accelerate the dimerization reaction to an extent that dinuclear complexes of the type [99mTc2(m2-SR)3(CO)6]− are formed at very high dilution.

5.08.3.2

Nitrogen and phosphorus

The development of a low pressure synthesis for the 99Tc precursor complex [TcX3(CO)3] (X ¼ Cl, Br) enabled the work with the fac{Tc(CO)3}+ core also for less specialized laboratories (Section 5.08.3.1). The combination of a kit development for the direct synthesis of [99mTc(CO)3(H2O)3]+ and the highly interesting coordination properties of the fac-{Tc(CO)3}+ core for radioprobe developments led to increased research activities with this precursor. Especially, the coordination of molecules, considered as ‘biological active’ or ‘biocompatible’ to the core was studied intensively. This rationalizes the rather high number of structural characterized fac-{99Tc(CO)3}+ complexes with N-donor ligands compared to other ligand systems. As mentioned before, the fac{Tc(CO)3}+ core can be coordinated to a wide range of mono-, bi-, or tridentate Lewis bases. The products are in general kinetically stable. However, if a reaction with a monodentate ligand is performed in saline solution, as it has to be done for the radioprobe synthesis with [99mTc(CO)3(H2O)3]+, it is often found that the reaction is fast for the first two incoming ligands and much slower for the third. This is rationalized by the fact that the excess chloride ions compete favorably for the sixth coordination site. Until today, structurally characterized complexes, containing the fac-{99Tc(CO)3}+ core, are known with monodentate nitrogen ligands,28,51,71,72 bidentate ligands with NN-donor40,73–78 and NO-donor sets,79–82 as well as tridentate ligands with NNN-83–86 and NNO-donor sets.56,87 Furthermore, Tc carbonyl complexes with nitrogen donor ligands based on the {Tc(CO)2(PR3)}+ core,88–91 and the {Tc(NO)(CO)2} core92 are known. With very strong tripodal N donor ligand systems, such as trispyrazolyborate and 1,4,7-triazacyclononane, complexes, containing a {Tc(CO)2(PR3)}+ core have been realized.93 Very interesting is the reaction of the trispyrazolyborate containing complex [Tc(HB(3,5-Me2C3N2)3)(CO)3] with elemental nitrogen under UV radiation in THF.94,95 This reaction yields the air-stable N2-bridged binuclear complex [Tc(HB(3,5-Me2C3N2)3)(CO)2]2(m-N2) (Fig. 9), which is one of the very few 99 Tc complexes, binding elemental nitrogen. A photoreaction based procedure was also applied for the synthesis of [Tc(HB(C3N2)3) (CO)(PPh2Me)2].95

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Fig. 9 Ball and stick representation of the crystal structure of [Tc(HB(3,5-Me2C3N2)3) (CO)2]2(m-N2).94 In this complex the TcdNPyrazolyl bond lengths are between 2.13(1) and 2.21(1) A˚ and the TcdNN2 bond length 1.94(2) A˚ . The NN2dNN2 distance is 1.160(3) A˚ .

Although [Tc2(CO)10] is challenging to prepare and to handle, it provides access to an enormous range of organometallic technetium complexes. This also applies for reactions with phosphine ligands. Numerous mixed carbonyl-phosphine Tc0 and Tc+I complexes are known. Photolytic or thermal substitution reactions of [Tc2(CO)10] with PF3 led to twenty-four different derivatives.96 The compounds of the type [Tc2(CO)10-x(PF3)x] (x ¼ 1–8) were observed by GC-MS or GC-IR spectroscopy. Under thermal conditions, substitution at the axial sites is preferred. Substitution of both axial and equatorial sites can be observed during photolysis. In this study it has also been observed that the substitution rate of CO by PF3 is faster for the Tc complex than for the corresponding Mn and Re compounds. The homoleptic trifluorophosphine complex [Tc2(PF3)10] is not formed via ligand substitution reactions, starting form [Tc2(CO)10]. It has been synthesized by condensing technetium metal vapor with PF3 at 77 K.97 The axially substituted bis-triphenylphosphine derivative, [Tc2(CO)8(PPh3)2] has been prepared by the reaction of [Tc2(CO)10] with PPh3 in decaline at 100–150  C.98 [Tc2(CO)8(PPh3)2] as well as the monosubstituted intermediate [Tc2(CO)9(PPh3)] have only been analyzed by IR spectroscopy due to the small amount of starting compound available. The established starting compounds [TcX(CO)5] (X ¼ Br, I) have been used for a series of mixed Tc pentacarbonyl phosphine complexes (Scheme 13). The reaction of [TcI(CO)5] with AgOTf (OTf ¼ trifluoromethanesulfonate) and PPh3 in dry CH2Cl2 yields

Scheme 13 Reactions of [TcX(CO)5] (X ¼ Cl, Br, I) with phosphine ligands, such as PPh3, 1,4,7-triaza-9-phosphatricyclo[5.3.2.14,9]tridecane (CAP) and 1,3,5-triaza-7-phosphaadamantane (PTA) in CH2Cl2.32,99

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Organometallic Complexes of Technetium

Fig. 10 Ball and stick representation of the crystal structure of [TcCl3(CO)(PPhMe2)3]
EtOH (solvent molecule omitted for clarity).102 TcdP bond lengths are in the range of 2.44(1)–2.45(1) A˚ , TcdCl bond lengths are between 2.47(1) and 2.49(1) A˚ , and the TcdC bond distance is 1.86(2) A˚ .

[Tc(CO)5(PPh3)]OTf (12).32 The crystal structure of this compound shows a TcdP bond length of 2.49(2) A˚ . Water stable, mixed Tc pentacarbonyl phosphine complexes have been prepared by the reaction of [TcX(CO)5] (X ¼ Br, I) with 1,4,7-triaza-9-phosphatricyclo[5.3.2.14,9]tridecane (CAP) and 1,3,5-triaza-7-phosphaadamantane (PTA).99 The compounds [Tc(CO)5(CAP)]+ (13) and [Tc(CO)5(PTA)]+ (14) have been synthesized in CH2Cl2 at room temperature. At elevated temperature decarbonylation can be observed, which led to the isolation of the mixed 99Tc tricarbonyl phosphine complex [TcCl(CO)3(CAP)2] (15). [Tc(CO)5(CAP)]+ as well as [Tc(CO)3(CAP)2]+ have been analyzed by X-ray diffraction and the TcdP bond length are 2.4179(4) and 2.4575(9) A˚ , respectively. Other precursor complexes for the synthesis of mixed Tc carbonyl phosphine complexes are trans-[TcCl(CO)3(PPh3)2] and cis[TcCl(CO)2(PMe2Ph)3]. trans-[TcCl(CO)3(PPh3)2] is accessible in very good yield by treatment of (NBu4)[TcO4] with PPh3 in an CO gas atmosphere (1 atm).93 cis-[TcCl(CO)2(PMe2Ph)3] is synthesized in good yield by the reaction of mer-[TcCl3(PMe2Ph)3] with CO gas in the presence of an excess PMe2Ph in toluene.88 cis-[TcCl(CO)2(PMe2Ph)3] crystalizes as colorless crystals from the reaction solution, when concentrated and treated with pentane. trans-[TcCl(CO)3(PPh3)2] and cis-[TcCl(CO)2(PMe2Ph)3] have been used as precursors for the synthesis of a series of mixed Tc carbonyl phosphine complexes.88–91,93,95 A variety of 99Tc+III precursor complexes for the synthesis of mixed Tc carbonyl phosphine complexes have been developed. The reaction of [TcX2(PPh (OEt)2)4]ClO4 (X ¼ Cl, Br, I) with CO gas (1 atm) in EtOH and an excess of PPh(OEt)2 at 50  C leads to the formation of a mixture of trans- and cis-[Tc(CO)2(PPh(OEt)2)4](ClO4).100 The two isomers were separated by the workup procedure. Cooling the reaction mixture led to precipitation of the trans-isomer of [Tc(CO)2(PPh(OEt)2)4](ClO4). The cis-isomer was isolated by treating the mother liquor with pentane. [TcCl3(PPh3)2(MeCN)] reacts with CO gas (1 atm) in toluene at 60  C by substitution of the MeCN molecule.101 The first known seven-fold coordinated 99Tc complex, [TcCl3(CO)(PPhMe2)3] has been prepared by the reaction of the 99Tc+III complex mer-[TcCl3(PPhMe2)3] with CO gas (1 atm) in boiling ethanol (Fig. 10).102 99Tc+IV complexes of the type [TcCl4(PR3)2] (R ¼ alkyl, aryl) are common starting materials for 99Tc organometallic chemistry.103 [TcCl4(PPh3)2] has been used as starting compound for the synthesis of the 99Tc hydride complex trans-[HTc(CO)(dppe)2] (dppe ¼ 1,2-bis(diphenylphosphino) ethane), via the corresponding N2 derivative, [HTc(N2)(dppe)2].104 Similar to [Tc(HB(3,5-Me2C3N2)3)(CO)2]2(m-N2), [HTc(N2) (dppe)2] is one of the very few known 99Tc dinitrogen compounds. It can be synthesized by the reaction of [TcCl4(PPh3)2] with dppe and sodium amalgam in THF under a N2 gas atmosphere.105,106 [TcCl4(PPh3)2] has also been used to synthesize the bistriphenylphosphine derivative of [HTc(CO)5], [HTc(CO)3(PPh3)2].107 The reaction of [TcCl4(PPh3)2] with NaBH4 in the presence of an excess of PPh3 in EtOH yields the yellow 99Tc trihydrido complex [H3Tc(PPh3)4]. This highly reactive compound has been transformed into [HTc(CO)3(PPh3)2], by stirring in benzene under a CO gas atmosphere (1 atm). The insertion chemistry of [HTc(CO)3(PPh3)2] (16) has been studied in detail.108 It reacts with a variety of unsaturated substrates, including heterocumulenes (L1), electron-deficient acetylenes (L2), and diazonium salts (L3, Scheme 14). Reacting [HTc(CO)3(PPh3)2] (16) in benzene under a CO gas atmosphere, followed by treatment with HBF4/ether, yields trans[Tc(CO)4(PPh3)2](BF4) (17).107 It has been shown that this complex is prone to nucleophilic attacks.109 It reacts with lithium triethylborohydride in toluene to afford the yellow 99Tc+I formyl complex [Tc(C(O)H)(CO)3(PPh3)2] (18) in good yield (Scheme 15). The latter complex is unstable in solution and slowly decomposes to [HTc(CO)3(PPh3)2]. However, by methylation of the formyl complex [Tc(C(O)H)(CO)3(PPh3)2] with CH3SO3CF3 one of the first 99Tc ‘Fischer’ type carbenes was synthesized (see Section 5.08.6.2). Aqueous sodium hydroxide reacts with trans-[Tc(CO)4(PPh3)2]+ (17) in MeCN to give the neutral 99Tc hydroxy– carbonyl complex [Tc (C(O)OH)(CO)3(PPh3)2] (19). If this reaction is carried out in the presence of MeOH or EtOH, the corresponding alkoxycarbonyl complexes [Tc(C(O)OR)(CO)3(PPh3)2] (R ¼ Me, Et) (20, 21) are obtained (Scheme 15). Finally, the treatment of trans-[Tc(CO)4(PPh3)2]+ with NaN3 gives the isocyanate complex [Tc(NCO)(CO)3(PPh3)2]. This reaction is believed to occur via an acyl-azide intermediate which quickly loses N2.109

Organometallic Complexes of Technetium

559

Scheme 14 Insertion reactions of [HTc(CO)3(PPh3)2] (16) with unsaturated substrates, including heterocumulenes (L1), electron-deficient acetylenes (L2), and diazonium salts (L3).108

Scheme 15 Reaction of trans-[Tc(CO)4(PPh3)2]+ (17) with lithium triethylborohydride and NaOH in different organic solvents, yielding the unstable formyl complex [Tc(C(O)H)(CO)3(PPh3)2] (18) and 99Tc hydroxyl-, and alkoxy-carbonyl complexes of the type [Tc(C(O)OR)(CO)3(PPh3)2] (R ¼ OH, Me, Et) (19–21).109

Besides all described mixed 99Tc carbonyl phosphine complexes, the mixed [(Z5-C5H5)Tc(NO)(CO)(PPh3)]+ is known and discussed in Section 5.08.5.1.

5.08.4

Technetium isocyanides and their derivatives

5.08.4.1

Binary technetium isocyanides

99

Tc carbonyl phosphine nitrosyl complex

The synthesis of hexakis(isocyanide)technetium(I) ([Tc(CNR)6]+, 22, Scheme 16) complexes had an enormous impact on the development of organometallic technetium compounds. This type of compounds contradicts the general assumption, that

560

Organometallic Complexes of Technetium

metal-carbon bonds are highly reactive in general and thus expanded the focus of radioprobe developments onto organometallic 99m Tc compounds. The first time hexakis(isocyanide)technetium(I) complexes have been mentioned was in a review article by Jones and Davison in 1982.114 Later, initial studies in humans disclosed excellent heart uptakes of these type of compounds.115 This discovery initiated intensive investigations, pushing 99mTc-hexakis(isocyanide) complexes into clinical applications.6–9,11 The myocardial perfusion radiotracers 99mTc-Sestamibi (Cardiolite®) is still one of the most frequently used radiodiagnostic agents.116 In this 99mTc-hexakis(isocyanide) complex the metal center is coordinated by six methoxy-2-methylpropylisonitrile (MIBI) ligands (Scheme 16). The compound is stable under physiological conditions and represents the first organometallic 99mTc compound for routine nuclear medical applications. Until today, two major synthetic pathways for the synthesis of [99Tc(CNR)6]+ type complexes are known (Scheme 16): (A) reduction of [TcO4]− by dithionite in the presence of an isocyanide,110–112 and (B) reductive substitution of a hexakis(thiourea)technetium(III) complex ([Tc(tu-S)6]3+, 23) by isocyanides.111,113

Scheme 16 General synthetic pathways for the synthesis of 99Tc complexes of the [Tc(CNR6)]+ type (22). (A) via reduction of [TcO4]− by dithionite in the presence of a isocyanide,110–112 and (B) reductive substitution of a hexakis(thiourea)technetium(III) complex ([Tc(tu-S)6]3+; 23) by isocyanides.111,113

Alternative to the synthetic route (A), bulk electrolysis of [TcO4]− in the presence of acetate or formate has been applied to form reactive ‘Tc-carboxylate precursors’. These precursors react with isocyanides to form [Tc(CNR6)]+ type complexes.117 The formed complexes are stable over a wide pH and temperature range. Furthermore, they are inert toward substitution by other isocyanides. Surprisingly, only one crystal structure of a 99Tc-hexakis(isocyanide) complex can be found in the Cambridge Crystallographic Database (Fig. 11).63 Reacting the precursor complex [Tc(tu-S)6]3+ with a mixture of isocyanides, leads to a statistical distribution of the different isocyanides coordinated to the Tc center.118

Fig. 11 Ball and stick representation of the cation of the crystal structure of [Tc(CNCCOOMe)6][TcO4].63 Selected bond length and angles: TcdC 2.032(3)–2.044 (3) A˚ , CdNdC 167.7(3)–176.4(3) .

Organometallic Complexes of Technetium

5.08.4.2

561

Technetium isocyanide derivatives

As mentioned before, binary technetium isocyanide complexes of the [Tc(CNR)6]+ type are inert toward substitution. These complexes can only be reacted further under harsh conditions. For example, the reaction of [Tc(CNCMe3)6]+ (24) with NOPF6 or nitric acid in glacial acetic acid leads to the formation of [Tc(NO)(CNCMe3)5]+ (25; Scheme 17).119 Exploration of alternative synthetic pathways for the synthesis of this nitrosyl complex showed that the reaction of [Tc(NO)Br4]− (26) with tert-butyl isocyanide leads to [Tc(NO)Br2(CNCMe3)3] (27) instead of [Tc(NO)(CNCMe3)5]+. The crystal structure of [Tc(NO)Br2(CNCMe3)3] showed the metal center being coordinated by the six ligands in an octahedral geometry. The three isocyanide ligands are meridionally coordinated and one isocyanide is trans to the almost linear NO+ group. TcdC bond length of the isocyanide ligand in trans-position to the NO+ group is elongated (2.137(22) A˚ ) compared to the two others (2.080(15) and 2.082(14) A˚ ). The related complex [Tc(NO)Cl2(CNCMe3)3] can be isolated from the reaction of the precursor complex [Tc(NO)Cl2(PPh3)2(MeCN)] (28) with excess tert-butylisocyanide in refluxing benzene (Scheme 17).120 Similar reactions in refluxing CH2Cl2 or at room temperature lead to the formation of [Tc(NO)Cl2(PPh3)2(CNCMe3)] and [Tc(NO)Cl2(PPh3)(CNCMe3)2], respectively. Another example, in which [Tc(CNR)6]+ type complexes have been employed as starting compounds, is the synthesis of seven coordinated Tc+III isocyanide complexes (Scheme 17).121 [TcX(CNCMe3)6]+ (29; X ¼ Cl, Br) is synthesized by the oxidation of the homoleptic [Tc(CNCMe3)6]+ (24) with elemental chlorine or bromine. These light yellow complexes readily dealkylate upon heating in the presence of 2,2´-bipyridine in acetonitrile, forming the cyanide complexes [Tc(CN)X(CNCMe3)5]+ (30; X ¼ Cl, Br).

Scheme 17 Synthesis of 99Tc+I isocyanide complexes, starting from binary technetium isocyanide complexe [Tc(CNCMe3)6]+ (24) and alternative synthetic pathways for the synthesis of isocyanide containing 99Tc nitrosyl complexes.119–121

[Tc(CNR6)]+ complexes are rather unsuitable for substitution reactions with neutral phosphine or nitrogen donor ligands. Photolysis of [Tc(CNR)6]+ starting compounds in the presence of bidentate aromatic amine ligands did not lead to yields higher than 15%.112 Better results have been achieved by using 99Tc+III complexes as starting material and reduction in the presence of neutral phosphine or nitrogen donor ligands and isocyanides.112,122–124 Following this strategy, a series of 99Tc-isocyanide complexes, containing aromatic amine ligands have been prepared.112 The reduction of [TcO4]− with dithionite in the presence of an excess of amines, such as 2,20 -bipyridine (bpy), 4,40 -dimethyl-2,20 -dipyridyl (Me2bpy), 1,10-phenanthroline (phen), 4-methyl-1,10-phenanthroline (Me-phen) and isocyanide leads to the formation of compounds of the type [Tc(CNR)4(NN)]+ (NN ¼ bidentate aromatic amine) (31; Scheme 18). Out of this series of compounds the structure of [Tc(CNCMe3)4(bpy)]PF6 has

Scheme 18 Synthesis of Tc+I isocyanide complexes, containing neutral phosphine and amine.112,122–124

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Organometallic Complexes of Technetium

Fig. 12 Ball and stick representation of the cation of the [Tc(CNCMe3)4(bpy)]PF6 X-ray structure, depicting the bent isocyanide ligand.112

been elucidated by X-ray diffraction analysis. The structure reveals a bent isocyanide ligand (CdNdC ¼ 148(2) ) with a shortened TcdC bond length of 1.90(2) A˚ (Fig. 12). This structural feature, along with additional spectroscopic data, suggests that this compound may have undergone an internal oxidation from Tc+I to Tc+III, leading to the bent isocyanide ligand. Replacing dithionite by phosphines as reducing agents enables the synthesis of mixed phosphine-isocyanide complexes.122 The reaction of [TcO4]− with triphenylphosphine leads to the formation of [Tc(CNCMe3)4(PPh3)2] (32) or [Tc(CNCMe3)5(PPh3)] (33; Scheme 18). The product formation is controlled by the addition of the stoichometrically correct amount of isocyanide. By using mer-[TcCl3(PMe2Ph)3] as starting compound and controlling the reaction conditions, the two related mixed phosphine-isocyanide complexes trans-[Tc(CNCMe3)4(PMe2Ph)2] and [Tc(CNCMe3)5(PMe2Ph)] have been isolated.123 Both complexes have been crystalized and studied by X-ray diffraction analysis. The TcdC bond lengths in trans-[Tc(CNCMe3)4(PMe2Ph)2] are 2.033(13) and 2.046(11) A˚ and the TcdP bond lengths are 2.392(3) A˚ . The CdNdC angles of the isocyanide ligands are 169.1(12) and 173.4(11) . In the structure of [Tc(CNCMe3)5(PMe2Ph)], the TcdC bond length of the isocyanide ligand in trans-position to the phosphine ligand is significantly shorter (1.966(19) A˚ ) compared to others (2.002(18)–2.064(18) A˚ ). The CdNdC angles of the isocyanide ligands are between 164.9(2) and 178.7(2) . As mentioned before, an alternative approach for the synthesis of Tc+I isocyanide complexes is the reductive substitution of the hexakis(thiourea)technetium(III) complex ([Tc(tu-S)6]3+, 23, Scheme 16). This strategy has been used for the synthesis of trans-[Tc(CNCMe3)2(dppe)2]+ (dppe ¼ bis(diphenylphosphino)ethane).124 X-ray diffraction analysis of crystals of trans[Tc(CNCMe3)2(dppe)2](PF6) disclosed TcdC and TcdP bond lengths of 2.034(4) and 2.432(2), 2.421(2) A˚ , respectively. The CdNdC angles of the isocyanide ligands are 178.0(5) . trans-[Tc(CNCMe3)2(dppe)2]+ is one of the rather few 99Tc complexes which has been studied by cyclic voltammetry and shows a reversible one-electron oxidation at 0.91 V vs. SCE. However, the 99Tc+II species, observed in the cyclic voltammetry experiment has not been isolated. For the first synthesis of mixed carbonyl-isocyanide 99Tc complexes, [TcBr(CO)5] has been used as precursor. Lorenz and coworkers reported the mixed carbonyl-isocyanide 99Tc complexes of the type [TcBr(CO)3(CNCR)2] (34, R ¼ Me3, C6H11) in 1988 (Scheme 19).125 Refluxing [TcBr(CO)5] in the presence of the isocyanides leads to the formation of the corresponding mixed carbonyl-isocyanide 99Tc complexes. These complexes have been studied only spectroscopically. By using [TcI(CO)5] as starting material and precipitating the halide atom from the reaction solution by the addition of Ag+, the reaction with tert-butyl isocyanide led to the formation of [Tc(CO)5(CNCMe3)]+ (35) at room temperature (Scheme 19).32 The structure of [Tc(CO)5(CNCMe3)]+ has been analyzed by X-ray diffraction analysis. In this structure, the TcdCCO bond length are between 1.999(6) and 2.022(5) A˚ and the TcdCCNR bond length is 2.095(5) A˚ . The reaction of (NEt4)2[TcCl3(CO)3] with excess CNCMe3 in THF at room temperature leads to the formation of [TcCl(CO)3(CNCMe3)2] (36, Scheme 19).46 By treating (NEt4)2[TcCl3(CO)3] with AgNO3 in H2O the halide atoms can be removed as AgCl. The resulting [Tc(CO)3(H2O)3]+ reacts with an excess CNCMe3 in ethanol at room temperature, yielding the trisubstituted cation [Tc(CO)3(CNCMe3)3]+ (37, Scheme 19).53 Alternatively, [Tc(CO)3(CNCMe3)3]+ can also be prepared without previous precipitation of the halide ions, performing the reaction in methanol as solvent. This prevents the precipitation of [TcCl(CO)3(CNCMe3)2] from the reaction solution. The structures of [TcCl(CO)3(CNCMe3)2] and [Tc(CO)3(CNCMe3)3](NO3) have been elucidated by X-ray diffraction analysis. Structural features of both complexes are very similar. The TcdCCNR bond lengths are 2.097(2) and 2.102(6) A˚ in [TcCl(CNCMe3)2(CO)3] and 2.097(3), 2.082(3) A˚ in [Tc(CNCMe3)3(CO)3](NO3).46,53 These TcdCCNR bond lengths are significantly longer than in [Tc(CNCCOOMe)6][TcO4] (2.032(3)–2.044(3) A˚ ). This can be explained by the stronger p-backbonding of the CO ligands over isocyanides. In the context

Organometallic Complexes of Technetium

Scheme 19

563

Synthesis of mixed carbonyl-isocyanide 99Tc complexes, starting from [TcX(CO)5] (X ¼ Br, I) and [TcCl3(CO)3]2−precursor complexes.32,46,53,125,126

of radioprobe development, Alberto and coworkers introduced the [2+ 1] mixed ligand approach in 2004.79 The [2 + 1] approach allows the labeling of bioactive molecules containing a monodentate or a bidentate donor site with [99mTcCl3(H2O)3]+. In a systematic 99Tc NMR study Suglobov and coworkers showed that mixed ligand complexes of the [Tc(CO)3(DTC)L] type (38; DTC ¼ dithiocarbamate, L ¼ isocyanide, imidazole, or phosphine) are the most stable complexes with respect to a histidine challenge experiment.126 The labeling of biomolecules with 99mTc by a mixed ligand approach has been suggested already before the introduction of the [2 + 1] approach. In 1996 Pietsch and coworkers presented the [4 + 1] approach, based on the coordination of Tc+III center by a tetradentate NS3 ligand system and one monodentate ligand.127 99Tc complexes of this type have been known since 1991 and have been reported by Davison and coworkers.128 Reducing [TcO4]− in a MeCN/H2O mixture with sodium dithionite in the presence of tris(o-mercaptophenyl)phosphinate (P(SH)3) as ‘umbrella’ ligand and the addition of an isocyanide ligand led to the formation of [Tc(PS3)(CNMe)] and [Tc(PS3)(CNCMe2)] (39, Scheme 20). Furthermore, it was shown that in the presence of a large excess of isocyanide, these electron-deficient complexes (14 electron complexes) binds an additional ligand, forming [Tc(PS3)(CNMe)2] or [Tc(PS3)(CNCMe2)2] (40). [Tc(PS3)(CNCMe2)] as well as [Tc(PS3)(CNCMe2)2] have been studied by X-ray diffraction analysis, showing the typical trigonal-bipyramidal coordination geometry of this type of complexes. The TcdC bond lengths are 2.06(8) ([Tc(PS3)(CNCMe2)]) and 2.081(7), 2.058(8) A˚ ([Tc(PS3)(CNCMe2)2]), respectively. By changing from the tetradentate phosphine ligand to a tetradentate nitrogen ligand namely 2,20 ,200 -nitrilotris(ethanethiol) (N(SH)3), three 99Tc+III isocyanide complexes of the

Scheme 20 Synthesis of ‘umbrella’ type 99Tc+III, containing the tetradentate ligand systems tris(o-mercaptophenyl)phosphinate (PS3) and 2,20 ,200 nitrilotris(ethanethiol) (N(SH)3).128,129

564

Organometallic Complexes of Technetium

Fig. 13 Ball and stick representation of the crystal structures of (A) [Tc(NS3)(CNC6H11)], (B) [Tc(NS3)(CNCH2C6H5)] and (C) [Tc(NS3)(CN CH2C(O)OC2H5)].129

type [Tc(NS3)(CNR)] (42; R ¼ CH2C6H5, C6H11, CH2C(O)OC2H5) have been synthesized.129 These complexes can be obtained by a two-step reduction/substitution procedure starting from [TcO4]− via the phosphine-containing precursor complex [Tc(NS3) (PMe2Ph)] (41, Scheme 20). The structures of all three complexes have been elucidated by X-ray diffraction analysis (Fig. 13), showing again a trigonal-bipyramidal coordination geometry. The central nitrogen atom of the chelate ligand and the monodendate isocyanides occupy the apical positions. The TcdC bond lengths in these [Tc(NS3)(CNR)] type of complexes are significantly shorter (R ¼ CH2C6H5: 1.932(10) A˚ , R ¼ C6H11: 1.942(6) A˚ , R ¼ CH2C(O)OC2H5: 1.945(3) A˚ ) as in the complexes, containing the tetradentate phosphine ligand PS3. Very recently, the coordination chemistry of 99Tc at different oxidation states (+I, +III, +V) with two sterically demanding isocyanide ligands has been studied (Schemes 21 and 22).19

Scheme 21 Reactions of the m-terphenyl isocyanide CNArDipp2 (Dipp ¼ 2,6-diisopropylphenyl) with 99Tc starting compounds, such as (NBu4)[Tc2Cl3(CO)6], [Tc(NO)Cl2(PPh3)2(MeCN)], [Tc(NPh)Cl3(PPh3)2], and [TcCl3(PMe2Ph)3].130

Organometallic Complexes of Technetium

565

Scheme 22 Reactions of the m-terphenyl isocyanide CNArMes2 (Mes ¼ 2,4,6-trimethylphenyl) with 99Tc starting compounds, such as [TcCl3(PMe2Ph)3], (NBu4) [TcNX4] (X ¼ Cl, Br), and [TcNCl2(PPh3)2].130

The reaction of the m-terphenyl isocyanide CNArDipp2 (Dipp ¼ 2,6-diisopropylphenyl) with 99Tc starting compounds, which contain the metal at the oxidation states +I ((NBu4)[Tc2Cl3(CO)6], [Tc(NO)Cl2(PPh3)2(MeCN)]) and +V ((NBu4)[TcNX4], [Tc(NPh)Cl3(PPh3)2]) led to the formation of octahedral coordinated products, which contain two isocyanide ligands in transposition. Consequently, all other ligands occupy the equatorial plane (Scheme 21). In trans,mer-[Tc+I(CO)3Cl(CNArDipp2)2] (43), the meridional coordination of the three carbonyl ligands renders this compound rather unique. Only a few other 99Tc+I complexes with meridional coordinated CO ligands are known, and these contain two axial bound PPh3 groups.93,108 It has been shown, that the exclusive trans-coordination of the CNArDipp2 ligand is caused by the high steric stress that this ligand type exerts onto the metal center. No evidence for the formation of ligand-exchange intermediates or the formation of solvent-coordinated species has been observed. The reaction of [Tc+IIICl3(PMe2Ph)3], containing the metal in the oxidation +III yielded [Tc+IIICl3(PMe2Ph)2(CNArDipp2)] (44). In contrast to the complexes at the oxidation states +I and +V, only one CNArDipp2 ligand is coordinated in this compound. To probe if this structural feature is a consequence of the high steric demand of the ligand or due to electronic reasons a series of reactions has been performed using the sterically less encumbered ligand CNArMes2 (Mes ¼ 2,4,6-trimethylphenyl) (Scheme 22). From this series, reactions with (NBu4)[TcNX4] (X ¼ Cl, Br) and [TcNCl2(PPh3)2] as starting compounds always yielded products with two CNArMes2 isocyanide ligands coordinated in a cis-conformation in contrast to the reaction of [TcCl3(PMe2Ph)3] with CNArMes2. As observed before in the reaction with CNArDipp2, the reaction of [TcCl3(PMe2Ph)3] with CNArMes2 exclusively yields the 99Tc+III complex with only one coordinated CNArMes2 ligand. This finding did support the hypothesis that the coordination of only one CNArDipp2 ligand in [Tc+IIICl3(PMe2Ph)2(CNArDipp2)] (44) is not caused by steric factors. All isolated products with the ligands CNArDipp2 and CNArMes2 are air-stable and the compounds with two trans-coordinated CNArDipp2 ligands possess highly shielded metal centers. The structures of trans,mer-[TcCl(CO)3(CNArDipp2)2], trans-[Tc(NO)Cl2(PPh3)(CNArDipp2)2], [TcCl3(PPhMe2)2(CNArDipp2)], trans-[TcNCl2(CNArDipp2)2], trans-[Tc(NPh)Cl3(CNArDipp2)2], trans-[Tc(NPh)Br3(CNArDipp2)2], [TcCl3(PPhMe2)2(CNArMes2)], cis-[TcNCl2(CNArMes2)(MeOH)], cis-[TcNCl2(CNArMes2)2(H2O)] have been elucidated by X-ray diffraction analysis. The TcdCisocyanides bond lengths are summarized in Table 1, reflecting the steric repulsion of the isocyanide ligands at the metal center.

Table 1 TcdCisocyanides bond lengths in the complexes containing the isocyanide ligands CNArDipp2 (Dipp ¼ 2,6-diisopropylphenyl) and CNArMes2 (Mes ¼ 2,4,6-trimethylphenyl).130 Compound

˚) TcdCisocyanides bond length (A

trans,mer-[TcCl(CO)3(CNArDipp2)2] trans-[Tc(NO)Cl2(PPh3)(CNArDipp2)2] [TcCl3(PPhMe2)2(CNArDipp2)] trans-[TcNCl2(CNArDipp2)2] trans-[Tc(NPh)Cl3(CNArDipp2)2] trans-[Tc(NPh)Br3(CNArDipp2)2] [TcCl3(PPhMe2)2(CNArMes2)] cis-[TcNCl2(CNArMes2)(MeOH)] cis-[TcNCl2(CNArMes2)2(H2O)]

2.049(2)–2.057(2) 2.064(2) 2.034(5) 2.098(4), 2.116(4) 2.090(1), 2.081(1) 2.074(3), 2.079(3) 2.003(3) 2.039(1) 2.062(1), 2.51(2)

566

Organometallic Complexes of Technetium

Fig. 14 Ball and stick representation of the [Tc2(m, Z1, Z2-MeCN)(MeCN)10]3+ cation of the [Tc2(m, Z1, Z2-MeCN)(MeCN)10][BF4]3
0.83 MeCN structure.131

A unique 99Tc compound has been isolated by Cotton, Sattelberger and coworkers. The reduction of the solvated dinuclear 99Tc complex [Tc2(MeCN)10]4+ by either zinc or cobaltocene in acetonitrile leads to the formation of the red-brown product [Tc2(m, Z1, Z2-MeCN)(MeCN)10]3+.131 Even if this compound cannot be considered as a isocyanide complex it does contain a TcdC bond. The crystal structure of [Tc2(m, Z1, Z2-MeCN)(MeCN)10]3+ showed two independent molecules per asymmetric unit. In these dinuclear complexes the Tc atoms are bridged by an acetonitrile molecule that is bound to one Tc center via the lone pair on the nitrogen atom (TcdN bond length: 2.169(8), 2.019(8) and 2.150(7), 2.040(7) A˚ ) and bound to the second Tc atom in a Z2 fashion, using the filled p-orbitals of the CdN triple bond (TcdC bond length: 2.09(1) and 2.067(9) A˚ , Fig. 14). In contrast to the isocyanide complexes, [Tc2(m, Z1, Z2-MeCN)(MeCN)10]3+ is air sensitive in solution and in the solid state.

5.08.5

Technetium cyclopentadienyl complexes and other p-complexes

5.08.5.1

Cyclopentadienyl complexes

From the beginnings of organometallic chemistry, cyclopentadiene and its anion cyclopentadienyl (Cp) have been the focus of research. However, the need for air and water free reaction conditions as well as the low vapor pressure of some reagents and products limited the synthesis of technetium cyclopentadienyl complexes to pure and fundamental research. The field gained momentum in the 1980s after the nuclear isomer 99mTc came into the focus of radioprobe development. Cyclopentadienyl is an attractive ligand system to develop small neutral 99mTc probes. Today, several synthetic procedures are known for the synthesis of water stable 99mTc complexes, based on the fac-{Tc+I(CO)3}+ core. The first technetium cyclopentadienyl complexes, [(Z5-C5H5)Tc(CO)3] and [(Z5-C5H5)2TcH] were reported in the early 1960s.132–134 An important starting compound for many early cyclopentadienyl complexes of technetium is anhydrous technetium tetrachloride ([TcCl4]). For the synthesis of the first structural characterized technetium cyclopentadienyl complex, [TcCl4] was reacted with K(C5H5) in THF, leading to the diamagnetic, air-stable red-brown bent-sandwich complex [(Z5-C5H5)2TcCl] (45).135 With an excess of K(C5H5), the dark red complex [(Z5-C5H5)2Tc(Z1-C5H5)] is formed (46, Scheme 23).

Scheme 23 Synthesis of [(Z5-C5H5)2TcCl] (45) and [(Z5-C5H5)2Tc(Z1-C5H5)] (46).135

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567

Besides these two complexes the dimeric complex, [(Z5-C5H5)4Tc2] (47) is formed by the reduction of the [(Z5-C5H5)2TcCl] with potassium naphthalenide, or thermolysis of [(Z5-C5H5)2Tc(Z1-C5H5)]. Single crystal X-ray diffraction analysis of [(Z5-C5H5)2TcCl] reveals a short ring-metal distance of 1.877(5) A˚ and a TcdCl bond distance of 2.450(3) A˚ . In [(Z5-C5H5)2Tc (Z1-C5H5)] the ring-metal distance has been found to be 1.8833(2) and 1.7811(3) A˚ and the TcdC(Z1-C5H5) distance is 2.30(1) A˚ .136 Besides the dimeric complex [(Z5-C5H5)4Tc2] the thermolysis of [(Z5-C5H5)2Tc(Z1-C5H5)] leads to the formation of the hydride complex [(Z5-C5H5)2TcH] as a side product. A direct synthesis of this air-sensitive but thermally stable Tc-hydride complexes is possible by the reaction of [(Z5-C5H5)2TcCl] with sodium borohydride in THF.134 In accordance to the synthesis of the first Tc-arene complexes by Fischer and coworkers via element transmutation,137 Matsu et al. studied the formation of [(Z5-C5H5)2Tc] (Tc ¼ 95Tc, 99mTc) complexes by recoil atom reaction induced by high energy g-irradiation of ruthenocene [(Z5-C5H5)2Ru].138 Element transmutation via (n, g) reactions has also been used to synthesize the first cyclopentadienyl containing 99mTc carbonyl complex [(Z5-C5H5)99mTc(CO)3].139 The vapor pressure of [(Z5-C5H5)Tc(CO)3] has been determined to be about 1 Pa.140 Synthesis and reactions with this compound can be described as challenging, from a radiation protection point of view. Therefore, [(Z5-C5Me5)Tc(CO)3] and derivatives came into focus of interest. These compounds were proven to be key compounds for this research and numerous substituted Z5-cyclopentadienyl and one Z5-idenyl technetium carbonyl complexes have been reported.17,18,43,95,141–147 Furthermore, [(Z5-C5Me5)Tc(CO)3] was an important starting material for the synthesis of the first technetium carbene and carbyne complexes (see Sections 5.08.6.2 and 5.08.6.3). Multiple synthetic procedures were developed for the synthesis of fac-{99Tc(CO)3}+ complexes, containing cyclopentadienyl derivatives (48, Scheme 24).

(A)

(B)

(C)

Scheme 24 Synthesis of [(Z5-C5Me5)Tc(CO)3] and derivatives (48) via (A) the starting material [Tc2(CO)10],17,18,95,141,142 (B) [Tc(CO)3(H2O)3]+ or Tc-tricarbonyl clusters,43,143–146 and (C) double ligand transfer.147

Out of the depicted synthetic pathways two involve the synthesis of [Tc2(CO)10].17,18,95,141,142 The synthesis of this compound depends on a high pressure carbonylation reaction. Today, whenever possible, the use of [Tc2(CO)10] as a precursor has to be circumvented and substituted by different starting compounds that do not have such high preparative demands and are in agreement with radiation protection standards. An elegant way to circumvent [Tc2(CO)10] as starting complex for the synthesis of 99Tc-cyclopentadienyl complexes is by using fac-[Tc(CO)3(H2O)3]+ and corresponding fac-{Tc(CO)3}+ clusters such as [{Tc(m3-OH)(CO)3}]4, which is synthesized via the [Tc(CO)3(H2O)3]+ precursor.43,143–146 The direct use of fac-[Tc(CO)3(H2O)3]+ as a precursor strongly depends on the pKa of the cyclopentadienyl derivatives.144,148 Electron withdrawing substituents allow the Cp ring to be deprotonated to an extent at physical pH and enable the formation of complexes of the type [(Z5-C5H4COR)Tc (CO)3]. These reports strongly influenced the field of radioprobe development because they enable the linking of the small robust, highly lipophilic [(Z5-C5H4COR)Tc(CO)3] unit onto a wide variety of targeting functions. Furthermore, they initiated a series of developments, focusing on the synthesis of 99mTc containing [(Z5-C5H4R)Tc(CO)3] complexes directly from water or saline.149–157 Alternatively, double ligand transfer (DLT) reactions with a ferrocene precursor and [Cr(CO)6] as CO source gave access to compounds of the [(Z5-C5H4R)Tc(CO)3] type.147 Especially for the synthesis of 99mTc labeled proteins and peptides the DLT reactions proved to be convenient, provided that the substituents of the cyclopentadienyl can withstand the rather harsh reaction conditions (T > 100  C). Following this synthetic strategy but using [Tc(CO)3(H2O)3]+ as starting compound instead of [TcO4]−, [(Z5-C5H4COPhOMe)Tc(CO)3] has been isolated and characterized.145 It has been shown, that this type of building block can efficiently be linked to ethylestradiol derivatives, such as a 17a-ethynyl estradiol derivative. Radiolabeled estrogen derivatives are in the focus of interest for imaging and therapy of breast tumors.

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Organometallic Complexes of Technetium

Fig. 15 Ball and stick representation of the crystal structure of [(Z5-C5Me5)Tc(CO)3], [(Z5-C5Me4Et)Tc(CO)3], [(Z5-C9H7)Tc(CO)3], [(Z5-C5H4COCH2Ph)Tc(CO)3] and [(Z5-C5H4COPhOMe)Tc(CO)3], including selected bond lengths.141,144,145

The structure of [(Z5-C5Me5)Tc(CO)3] has been elucidated by single crystal X-ray diffraction analysis.141 The structure can be described as three-legged piano stool, and the TcdCO distances average at 1.91 A˚ and the ring-metal distance is 1.944(3) A˚ (see Fig. 15). Two closely related complexes [(Z5-C5Me4Et)Tc(CO)3] and [(Z5-C9H7)Tc(CO)3] have also been prepared and structurally characterized. Furthermore, the structure of the pale yellow phenyl acetyl derivative [(Z5-C5H4COCH2Ph)Tc(CO)3] and [(Z5-C5H4COPhOMe)Tc(CO)3] have been determined by X-ray diffraction analysis.144,145 All structures are summarized in Fig. 15, including relevant bond lengths. Photochemical reactions of [(Z5-C5H5)Tc(CO)3] and [(Z5-C5Me5)Tc(CO)3] with the neutral phosphorous donor ligand PPh3 yield the complexes [(Z5-C5H5)Tc(CO)2(PPh3)] and [(Z5-C5Me5)Tc(CO)2(PPh3)] respectively (49, Scheme 25).95 Both compounds have been isolated in crystalline form and studied by X-ray diffraction analysis. In these structures the TcdP distance are 2.341(1) ([(Z5-C5H5)Tc(CO)2(PPh3)]) and 2.340(1) A˚ ([(Z5-C5Me5)Tc(CO)2(PPh3)]). In the absence of donor ligands, two dinuclear compounds are formed that can be isolated by column chromatography (Scheme 25). The deep red triply CO-bridged compound, [(Z5-C5Me5)Tc(m-CO)3Tc(Z5-C5Me5)] (50) contains a metal-metal bond (2.413(3) A˚ ).142 The structure of the yellow [(Z5-C5Me5)Tc(CO)2(m-CO)Tc(CO)2(Z5-C5Me5)] (51) is inferred from spectroscopic data and analogy to the known rhenium analog. By reacting [(Z5-C5Me5)Tc(CO)3] with NO(PF)6 in acetonitrile the first 99Tc-dicarbonyl-nitrosyl complex [(Z5-C5Me5)Tc(NO) (CO)2]+ (52) has been formed in high yield (Scheme 25).18 Attempts to oxidize the [(Z5-C5Me5)Tc(CO)3] by bromine showed a strong solvent dependence. Whereas the reaction of [(Z5-C5Me5)Tc(CO)3] with Br2 in trifluoroacetic acid leads to the formation of the neutral 99Tc+III complex [(Z5-C5Me5)Tc(CO)2Br2] as a mixture of cis- (cis-53) and trans-isomers (trans-53),18 oxidation in methylene chloride yields the complex salt [(Z5-C5Me5)TcBr(CO)3][(CO)3Tc(m-Br)3Tc(CO)3] (54).43 To avoid the formation of the dinuclear anion, which reduces the maximum yield of this reaction to 30%, toluene can be used as solvent.146 Following this procedure [(Z5-C5Me5)TcBr(CO)3]Br can be isolated in very good yields of 85%. By sonicating a aqueous suspension of [(Z5-C5Me5)TcBr(CO)3]Br for 30 min at room temperature this complex can be transferred into the neutral [(Z5-C5Me5) TcBr2(CO)2] (cis-, trans-isomers).146 X-ray diffraction analysis of [(Z5-C5Me5)TcBr(CO)3][(CO)3Tc(m-Br)3Tc(CO)3] and trans-

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Scheme 25 Reactivity of [(Z5-C5R5)Tc(CO)3] type complexes.18,43,95,142,146

[(Z5-C5Me5)TcBr2(CO)2] showed TcdCCO bond length of 1.974(10)–2.076(9) A˚ (cation) and 1.948(8)–1.982(7) A˚ respectively. The TcdBr bond length are 2.5893(11) A˚ in [(Z5-C5Me5)TcBr(CO)3][(CO)3Tc(m-Br)3Tc(CO)3] (cation) and 2.5852(2) and 2.5874 (7) in trans-[(Z5-C5Me5)TcBr2(CO)2].146 The 99Tc+III complex [(Z5-C5Me5)TcBr(CO)3]+ (54) is of high interest. It was hoped that the synthesis and characterization of the high-valent 99Tc+VII-trioxo complex [(Z5-C5Me5)TcO3] could be achieved by oxidation of [(Z5-C5Me5)TcBr(CO)3]+.43 [(Z5-C5Me5)TcO3] is in the focus of organometallic technetium chemistry since the late 1980s and many attempts have been made to synthesize this complex without success.19,43,158 In one case the formation of the polynuclear compound [(Z5-C5Me5)Tc2O3]n has been claimed.158 However, the extremely short TcdTc bond length in the crystal structure of [(Z5-C5Me5)Tc2O3]n (1.867(4) A˚ ) led to controversial discussions about the existence of this compound. In 1995 Cotton and coworkers stated, that [(Z5-C5Me5)Tc2O3]n is most likely [(Z5-C5Me5)ReO3], whose complicated and distorted structure was misinterpreted as that of [(Z5-C5Me5)Tc2O3]n.159 Besides all these setbacks [(Z5-C5Me5)TcO3] is still a highly interesting target compound for fundamental organometallic 99Tc chemistry. New approaches should be initiated to finally synthesize, isolate and characterize this mysterious compound. Ab initio SCF studies predict that [(Z5-C5Me5)TcO3] and derivatives should be stable, although the MdCp p-bonding is supposed to be relatively weak.160 Recently, Abram and coworker have presented the novel robust 99Tc metal core {(Z5-C5H5)Tc(NO)(PPh3)}+.161,162 [Tc(NO) X2(PPh3)2(CH3CN)] (X ¼ Cl, Br) reacts with an excess of potassium cyclopentadienyl in boiling toluene, forming the red pseudotetrahedral organotechnetium compounds [(Z5-C5H5)Tc(NO)X(PPh3)] (X ¼ Cl, Br). The halide ligand in [(Z5-C5H5)Tc (NO)Cl(PPh3)] (55) has been replaced by a series of monodentate ligands, demonstrating the potential of this compound as precursor (Scheme 26.). All isolated compounds are stable as solids. However, oxidation of the metal center leads to loss of the PPh3 and/or the cyclopentadienyl ligand. Solutions of [(Z5-C5H5)Tc(NO)(I3)(PPh3)] (56) undergo an internal oxidation to form the Tc+II complex [(Z5-C5H5)Tc(NO)(I)2(PPh3)].162 The red [(Z5-C5H5)Tc(NO)(SCN)(PPh3)] (57) is the first technetium complex in which the pseudohalide SCN− is coordinated via its sulfur atom. In solution a slow isomerization of the thiocyanato complex to the isothiocyanato species has been observed. 99Tc NMR studies and DFT calculations showed that the thiocyanato complex can be considered as the kinetic product which can be transferred into the thermodynamic product.

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Organometallic Complexes of Technetium

Scheme 26 Ligand replacement reactions of [(Z5-C5H5)Tc(NO)Cl(PPh3)] (55) with a series of monodentate ligands.161,162

Table 2 Bond length (A˚ ) in compounds of the type [(Z5-C5H5)Tc(NO)X(PPh3)], analyzed by single crystal X-ray analysis. X¼

Tcd(5-C5H5)centroid

TcdN

TcdP

TcdX

Br CO Ph SO3CF3 OOCCF3 SCN I3

1.937(2) 1.944(1) 1.972(1) 1.9261(10) 1.9339(1) 1.9330(2) 1.9373(2)

1.869(9) 1.679(8) 1.758(2) 1.766(2) 1.763(3) 1.760(6) 1.819(6)

2.369(8) 2.369(8) 2.357(1) 2.346(5) 2.3702(1) 2.371(2) 2.375(2)

2.471(9) 1.832(2) 2.149(2) 2.162(1) 2.108(2) 2.469(2) 2.7037(9)

Besides [(Z5-C5H5)Tc(NO)Br(PPh3)] several synthesized derivatives have been crystalized and studied by single crystal diffraction analysis. Table 2 summarizes the important bond lengths, disclosing the flexibility of the {(Z5-C5H5)Tc(NO)(PPh3)}+ core to react to electronic and steric influences.

5.08.5.2

Arene complexes

The coordination chemistry of technetium with arenes has been comprehensively reviewed in the past by Imamoto and coworkers.163 The first 99Tc bisarene complex is known since 1961.137 [Tc(C6H6)2]+ (58) has been synthesized by the rather uncommon synthetic strategy of element transmutation. Trace amounts of [Tc(C6H6)2]+, sufficient for spectroscopic characterization, have been isolated after the b− decay of the starting compound [99Mo(C6H6)2]+ (99Mo ! 99mTc ! 99Tc) (Scheme 27). Shortly after the first spectroscopic characterization of [Tc(C6H6)2]+, weighable amounts of this compound have been synthesized with low yields by the reaction of [TcCl4] with Al0 and AlCl3 in benzene.164 Starting from the 99Tc carbonyl [Tc3(OCH3)4(CO)9]− Herrmann and coworkers synthesized the 99Tc tricarbonyl complex fac-[Tc(CO)3(Ph)]+.17 Based on the reactions under Fischer-Hafner conditions Wester et al. synthesized a series of 99mTc bisarene complexes.165 Some members of

Organometallic Complexes of Technetium

(A)

571

(B)

Scheme 27 Synthesis of [Tc(C6H6)2]+ (58) by (A) element transmutation and (B) applying Fischer–Hafner conditions.137,164

this class of compounds showed extraordinary water-stability in in vivo studies. However, in this publication the author stated that: ‘the total yield of species containing 99mTc that was isolated by this procedure varied widely from day to day and from arene to arene . . .’. This fact and the complex reaction procedures suppressed further developments with this type of compounds. More than 50 years later Kudinov and coworkers reported an improved synthesis of [Re(C6H6)2]+, starting directly from [ReO4]−.166 This report re-initiated the interest for technetium bisarene complexes. Many pharmaceutical lead structures contain phenyl groups. The replacement of a phenyl groups in a lead structure of interest by cyclopentadienyl and subsequent labeling with 99mTc is a valid strategy to implement the ‘building blocks’ concept into modern radioprobe development. However, it is even more attractive to address the phenyl groups of lead structures directly as potential ligand systems. Believing in the high potential of technetium arene complexes for the development of radioprobes for nuclear medical applications, the Alberto group initiated a research project aiming at the synthesis, characterization and utilization of technetium and rhenium bisarene complexes, in 2014. Until today, this research project led to a constantly increasing number of (structural) characterized 99(m)Tc bisarene complexes. Translating the synthetic strategy of Kudinov and coworkers into technetium chemistry, Alberto and coworkers reported a series of water stable 99(m)Tc-bisarene complexes.167–169 [TcO4]− reacts with an excess of AlCl3 in a liquid arene to the corresponding 99Tc-bisarene complex (Scheme 28). In contrast to the rhenium chemistry most of these reactions do not need Al0 or Zn0 as additional reducing agent. The high Cl− concentration in these reaction mixtures is supposed to be sufficient to reduce the metal from its highest oxidation state +VII to the oxidation state +I.167 The isolated complexes show an extraordinarily stability, where oxidation is found to occur at potentials higher than +1.3 V and reduction at potentials below −2 V vs. Fc/Fc+.

Scheme 28 Synthesis of [Tc(arene)2]+ complexes with AlCl3.167

A disadvantage of this reaction procedure is that Fischer-Hafner reaction conditions are incompatible with most functional groups. Since post-synthetic modifications of 99mTc complexes are less favored for routine applications in nuclear medicine many efforts have been taken to develop procedures for the direct synthesis of functionalized 99(m)Tc bisarene complexes. By using hexamethyl-benzene (Ph(CH3)6) as co-ligand, the direct synthesis of the 99Tc bis-hexamethyl-benzene complex [Tc(Ph(CH3)6)2]+ (59) and the functional group containing mixed-arene sandwich complexes [Tc(Ph(CH3)6)(PhR)]+ (R ¼ N(SiMe3)2 (60), NH2 (61), Br (62), OH (63)) have been achieved (Scheme 29).168,169 In addition the 99Tc bisarene complex [Tc(PhN(CH3)2)2]+ (64) has been synthesized by the reaction of [TcO4]− with Zn and N,N-dimethylaniline and analyzed spectroscopically.170 A drawback of this procedure is the fact, that under the given Fischer-Hafner conditions hexamethyl-benzene tend to trans- or demetallate. Therefore, it is not surprising that the isolated 99Tc hexamethyl-benzene complex are always accompanied by the corresponding mono-demethylated 99Tc side product. Purification of the 99Tc hexamethyl-benzene complexes from unwanted side products has been proven to be challenging.

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Organometallic Complexes of Technetium

Scheme 29 Synthesis of 99Tc bis-hexamethyl-benzene (59), mixed-arene sandwich complexes containing functional groups (60–63) and [Tc(PhN(CH3)2)2]+ (64).168–170 The isolated 99Tc hexamethyl-benzene complexes are always accompanied by the corresponding mono-demethylated 99Tc side product.

Besides the developed synthetic procedures for 99Tc bisarene complexes a strategy for the synthesis of the corresponding 99mTc has been successfully established.167 However, the 99mTc reaction also had to be performed under Fischer–Hafner conditions. This drastically limits the scope of this reaction type for nuclear medical applications. A real breakthrough has been published by Nadeem et al., very recently. The new approach makes 99mTc bisarene complexes, containing arenes incompatible with Fischer– Hafner conditions directly accessible from water and [99mTcO4]−.171 By using reducing agents such as Zn0, H3N
BH3, or NaBH4 and small amounts of sodium dodecylsulfate (SDS) as a surfactant to increase the solubility of the arenes in water a series of functionalized 99mTc bisarene complexes has been synthesized (Scheme 30).

Scheme 30 Direct synthesis of 99mTc bisarene complexes from [99mTcO4]− in water.171 The coordinated arenes are incompatible with Fischer–Hafner conditions.

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This new synthetic route for the synthesis of 99mTc bisarene complexes paves the way for the labeling of many organic molecules comprising phenyl groups without the need of chelators. The exact conditions need to be optimized for the individual derivatives, but the given reaction schemes can be applied to a wide variety of phenyl groups.

5.08.5.3

Other p-complexes

The heterocyclic half-sandwich complex [Tc(Me4C4N)(CO)3] (65) has been synthesized by the reaction of K(Me4C4N) with precursor complex [TcBr(CO)5] (Scheme 31).172

99

Tc

Scheme 31 Synthesis of [Tc(Me4C4N)(CO)3] (65).172

In the crystal structure of [Tc(Me4C4N)(CO)3]
(HNC4Me4), the heterocycle is Z5-coordinated to the fac-{Tc(CO)3}+ metal core. Furthermore, an uncoordinated tetramethylpyrrole is bonded to the N atom of the Z5-coordinated pyrrolyl ring by a hydrogenbridged. The carborane [nido-7,8-(C2H9H11)]2− (dicarbollide dianion) and its derivatives are formal isolobal to the cyclopentadienyl anion (Cp). To circumvent the synthetic challenges encountered when attempting to prepare [CpM(CO)3] (M ¼ Tc and Re) based radiopharmaceuticals, Valliant and coworkers studied the coordination chemistry of this carboranes with the fac-{M(CO)3}+ (M ¼ 99Tc, Re) core.173 The 99Tc precursor complex [TcBr3(CO)3]2− readily reacts with [nido-7,8-(C2H9H11)]2− in THF in the presence of TlOEt to yield beige [Tc(Z5-C2H9H10)(CO)3]−. This complex has been isolated and crystalized as an ammonium salt, which has been investigated by X-ray diffraction analysis (Fig. 16). (NEt4)[Tc(Z5-C2H9H10)(CO)3] is the first, and until today only, structural characterized Tc carborane complex. The structure of (NEt4)[Tc(Z5-C2H9H10)(CO)3] is nearly isostructural to the Re complex.174 First attempts showed that this type of compound can be prepared in water. The authors claim that, because of the (water) stability, bifunctional derivatives of this type of Tc carborane complexes can be considered as novel synthons for the preparation of organometallic 99mTc and 186/188Re radiopharmaceuticals.

Fig. 16 Ball and stick representation of the anion of the crystal structure of (NEt4)[Tc(Z5-C2H9H10)(CO)3].174 The bond distance from the Tc center to an atom of the bonding face of the carborane is in the range of 2.270(13)–2.38(2) A˚ .

574

Organometallic Complexes of Technetium

5.08.6

Derivatives containing single- or multiple-bonded h1-carbon groups

5.08.6.1

Alkyl/aryl complexes

Alkyl- and aryl complexes of technetium are extremely rare. This lack of knowledge is surprising, given the fact that alkyl and aryl complexes of all other transition metals are well-known. The first 99Tc-alkyl complex was isolated in 1990 by the group of W. A. Herrmann.143,175 Treatment of technetiumoxid [Tc2O7] with tetramethyltin yields [TcCH3O3] (66) along with the stannyl ester of pertechnetium acid [TcO3(OSn(CH3)3)] (68) and the paramagnetic reduced side product [(TcO(CH3)2(m-O))2] (67, Scheme 32).

Scheme 32 Synthesis of the 99Tc-alkyl complexes [TcCH3O3] and [(TcO(CH3)2(m-O))2].175

Both 99Tc-alkyl complexes are air and moisture sensitive. [TcCH3O3] is stable up to 0  C under an inert atmosphere. The experimental setup for this synthesis can be characterized as challenging since the starting compound [Tc2O7] is highly moisture sensitive and volatile. X-ray diffraction analysis of the side product [(TcO(CH3)2(m-O))2] showed the metal in a distorted square-pyramidal coordination with the oxo ligand in the apical position. The TcdC bond lengths are in the range of 2.086 (3)–2.133(2) A˚ in this dinuclear 99Tc complex. Searching for synthetic pathways for the synthesis of Z5-Cp (Cp ¼ cyclopentadienyl) coordinated 99Tc complexes, the group of J. C. Bryan synthesized the 99Tc-alkyl complex [(Z1-Cp)Tc(NAr)3] (69, Ar ¼ 2,6-diisopropylphenyl) in 1992.176 [(Z1-Cp)Tc(NAr)3] was isolated from a reaction solution of [TcI(NAr)3] and 1 equiv of KCp (Scheme 33) in yields of 94%.

Scheme 33 Synthesis of the 99Tc-alkyl complexes [(Z1-Cp)Tc(NAr)3] (69).176

The green complex is air and water stable and can be purified by column chromatography. X-ray diffraction analysis of [(Z1-Cp)Tc(NAr)3] showed the metal center coordinated by four ligands in a tetrahedral geometry (TcdCCp bond length ¼ 2.156(3) A˚ ). The same reaction with the Cp derivative pentamethylcyclopentadienyl (Cp ) does not lead to a similar compound. The product appeared to be a result of electron transfer rather than metathesis. Reducing [TcI(NAr)3] with 1 equiv. of sodium yields the homoleptic dinuclear imido complex [Tc2(NAr)4(m-NAr)2] (70).177 Treatment of [Tc2(NAr)4(m-NAr)2] (70) with 2 equiv. of the Grignard reagent MeMgCl in THF results in the formation of the deep red compound [TcMe2(NAr)(m-NAr)2Tc (NAr)2] (71, Scheme 34).177 This compound is stable in air for a short period. Treatment of [TcMe2(NAr)(m-NAr)2Tc(NAr)2] (71)

Scheme 34 The reaction of [Tc2(NAr)4(m-NAr)2] (70) with the Grignard reagent MeMgCl leads to the formation of [TcMe2(NAr)(m-NAr)2Tc(NAr)2] (71) and [Tc2(NAr)2(m-NAr)2Me4] (72, Ar ¼ 2,6-diisopropylphenyl).177

Organometallic Complexes of Technetium

575

with additional 2 equiv. of MeMgCl results in the formation of [Tc2(NAr)2(m-NAr)2Me4] (72). [Tc2(NAr)2(m-NAr)2Me4] (72) is air stable and can be purified by column chromatography. It crystallizes as a red/orange solid. X-ray diffraction analysis of [TcMe2(NAr)(m-NAr)2Tc(NAr)2] showed TcdC bond lengths of 2.119(7) and 2.144(17) A˚ and a TcdTc distance of 2.673(2) A˚ . In the crystal structure of [Tc2(NAr)2(m-NAr)2Me4] TcdC bond lengths are 2.134(2) and 2.149(2) A˚ and the TcdTc distance is 2.733(1) A˚ .177 The reaction of [TcO4]− with the strong Lewis acid Me3SiCl, yields [TcO3(OSiMe3)].178 The 99Tc complexes [Tc(OSiMe3)(NAr)3] (Ar ¼ 2,6-diisopropylphenylisocyanate) has been isolated by reacting 2,6-diisopropylphenylisocyanate with [TcO3(OSiMe3)] in methylsiloxane at elevated temperature.179 The trimethylsiloxy ligand in [Tc(OSiMe3)(NAr)3] is readily substituted by Grignard reagents, such as MeMgCl, EtMgCl and (C3H5)MgCl, leading to the corresponding 99Tc alkyl complexes of the type [TcR(NAr)3] (R ¼ Me, Et, C3H5).179 These complexes are resistant to reduction and moderately air-sensitive. The structure of [TcMe(NAr)3] has been elucidated by X-ray diffraction analysis, showing a TcdC bond length of 2.136(17) A˚ .180 A rational synthesis for the first 99Tc-aryl complex has been presented by U. Abram and coworkers.181 Treatment of [TcNCl2(PPh3)2] with 3 equiv of PhLi in dry THF afforded orange-red crystals of air- and water-stable complex [TcNPh2(PPh3)2] (73, Scheme 35).

Scheme 35 Synthesis of the first 99Tc-aryl complex [TcNPh2(PPh3)2] (73).181

The TcdCPh bond lengths in the crystal structure of [TcNPh2(PPh3)2] are 2.138(4) and 2.127(4) A˚ , which is shorter than those in the 99Tc-alkyl complex [(Z1-Cp)Tc(NAr)3] (2.156 (3) A˚ ) and in the same range as those found in the dinuclear 99Tc-alkyl complex [(TcO(CH3)2(m-O))2] (2.086(3)–2.133(2) A˚ ). [TcN(Ph)2(PPh3)2] is air and water stable and not prone to substitution reactions. Following the same synthetic strategy but using the 99Tc-NHC complex [TcN(TPh2HPh)Cl2] as starting compound led to the formation of the first mixed aryl/carbene complex [TcN(TPh2HPh)Ph2] (85, Scheme 40). Recently a new 99Tc metal core has been introduced.161 The coordination sphere of the new 99Tc core structure {Tc(NO)(Cp)(PPh3)}+ has one vacant coordination side which can be occupied by monodentate s-donors, such as halides, CO, and Ph−. The Tc–phenyl bond length in [Tc(NO)(Cp)(PPh3) (Ph)] of 2.149(2) A˚ is similar in the magnitude to other isolated Tc–aryl bonds. By demonstrating which technetium metal core is suited for the synthesis of 99Tc-aryl and alkyl complexes more complexes of this type will be isolated hopefully in the future.

5.08.6.2

Carbene complexes

The chemistry of technetium complexes containing carbene ligands has been reviewed before by the groups of Abram (2005) and Kühn (2013).182–184 After the first synthesis and structural characterization of a technetium complex containing N-heterocyclic carbenes (NHCs) in 2003, technetium carbene complexes came into focus of recent organometallic technetium chemistry.185 Since then the number of new 99Tc-NHC complexes is slowly increasing. However, the first technetium carbene complexes were already synthesized in 1972 by E. O. Fischer et al.22 These complexes can be described as classical ‘Fischer-type’ carbene complexes, prepared by the addition of LiCH3 or LiC6H5 to [Tc2(CO)10] and subsequent methylation (74, Scheme 36).

Scheme 36 Synthesis of 99Tc ‘Fischer-type’ carbene complexes.22,109,186

576

Organometallic Complexes of Technetium

This classical synthetic pathway, nucleophilic attack onto a carbonyl carbon atom followed by methylation of the intermediate, has been also successfully applied for the synthesis of 99Tc complexes containing pentamethylcyclopentadienyl (Cp ) and triphenylphosphine (PPh3) (75 and 76, Scheme 36).109,186 [(Cp )(CO)2Tc]C(OEt)Ph] (75) is the only 99Tc ‘Fischer-type’ complex, which has been analyzed by X-ray diffraction analysis. The TcdCcarbene bond length has been found to be 1.97(2) A˚ , which is significantly shorter than TcdC bonds found in 99Tc alkyl and aryl complexes (see Section 5.08.6.1). The TcdCp centroid distance is 1.966(1) A˚ .186 Like most classical ‘Fischer-type’ carbene complexes these compounds are highly sensitive to moisture and air, which suppressed any development of potential applications. This changed after the introduction of NHCs as ligand systems into technetium chemistry. Already, the first 99Tc-NHC complex disclosed a surprising stability for dry air. Until today, most 99 Tc-NHC complexes contain the metal in its formal oxidation state +V, which can be attributed to the high stability of the technetium(V) oxo and nitride core. By reacting (NBu4)[TcOCl4] with 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (IiPr2Me2) the yellow compound [TcO2(IiPr2Me2)4][TcO4] (77) was isolated in 25% yield (Scheme 37).185

Scheme 37 Synthesis of 99Tc-NHC complexes, containing the {Tc+VO2}+ core.185,187

1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene is one of the few NHCs that can be prepared in advance and applied as free carbene for the reaction with a 99Tc precursor complex.188 Most NHCs have to be prepared by deprotonation of the imidazolium salts with a strong base in situ. Under basic conditions (NBu4)[TcOCl4] does hydrolyze very fast and forms the thermodynamically stable TcO2 polymer. This reactivity renders most deprotonation procedures for NHC precursors incompatible with (NBu4)[TcOCl4] as starting compound. (NBu4)[TcO(glyc)2] (78) was found to be an excellent alternative precursor for the synthesis of 99Tc-NHC complexes containing 3-dimethylimidazoline-2-ylidene (IMe2H2) as ligand (Scheme 37).187 The yellow compound [TcO2(IMe2H2)4]+ (79) has been isolated in yields of 55%. [TcO2(IMe2H2)4]+ is stable as a solid under inert atmosphere, but decomposes in the presence of H2O and oxygen. In both compounds the trans-{99TcO2} core is coordinated by four NHC ligands in a paddlewheel-like arrangement, which is a typical structural feature for all 99Tc and Re compounds at the oxidation state +V, containing four monodentate NHC ligands. The 99Tc−C bond lengths in [TcO2(IMe2H2)4](PF6) (2.188(3)–2.191(3) A˚ ) are slightly shorter than those in [TcO2(IiPr2Me2)4][TcO4] (2.220(3)–2.232(3) A˚ ) because of the reduced steric repulsion of the ligand IMe2H2 in comparison to IiPr2Me2. Water stable 99Tc-NHC complexes have been synthesized after extending the ligand systems from mono- to bidentate NHC ligands.187 Following the same synthetic strategy as for the synthesis of [TcO2(IMe2H2)4]+, complexes of the type [TcO2(IXH2CH2H2XI)2]+ (80, X ¼ Me, Et) have been isolated in yields up to 50% ([TcO2(IMeH2CH2H2MeI)2]+) and 33%

Organometallic Complexes of Technetium

577

([TcO2(IEtH2CH2H2EtI)2]+). At low pH these water stable 99Tc-NHC complexes show a unique reactivity. At pH 1 a reversible metal-core transformation is initiated (monooxo−dioxo interconversion) without the involvement of the equatorial ligands (Scheme 38).187

Scheme 38 pH-controlled metal-core transformation (monooxo− dioxo interconversion).187

The first 99Tc-nitrido-NHC complexes, containing 1,3-dialkyl-4,5-dimethylimidazol-2-ylidene type ligands have been synthesized by the reaction of [TcNCl2(PPhR2)2] (R ¼ Me, Ph) with 1,3-diethyl-4,5-dimethylimidazol-2-ylidene (IEt2Me2) (Scheme 39).189 [TcN(IEt2Me2)4]2+ (81) is stable as a solid in dry air, but slowly decomposes in solution. The crystal structure of [TcN(IEt2Me2)4]2+ disclosed the metal coordinated in a distorted square pyramidal geometry with the strong p-donating nitrido ligand in the apical position. Surprising results have been observed when the reaction solution was contaminated with traces of silicon grease. The highly nucleophilic carbene activates silicon grease which led to the formation of the 99Tc-NHC complexes [TcN(IEt2Me2)2(OSiMe2OSiMe2O)] (82) and [TcN(IEt2Me2)(PPhMe2)(OSiMe2OSiMe2O)] (83), containing silicon grease fragments as bidentate ligands (Scheme 39).189

Scheme 39 Synthesis of 99Tc-NHC complexes, containing the {Tc+VN}2+ core. The silicon-containing ligands (1,1,3,3-tetramethyldisiloxane-1,3-diolato) are the result of activation of silicon grease by the highly nucleophilic carbene.189

Besides the 1,3-diethyl-4,5-dimethylimidazol-2-ylidene, 1,3,4-triphenyl-1,2,4-triazol-5-ylidene (TPh2HPh) has been reacted with the precursor complex [TcNCl4]−.181 TPh2HPh is a weaker nucleophile compared to the 1,3-dialkyl-4,5-dimethylimidazol-2ylidenes. The reaction of [TcNCl4]− with TPh2HPh led to the formation of the air-stable neutral 99Tc-NHC complex [TcN(TPh2HPh) Cl2] (84, Scheme 40). In contrast, the reaction with TPh2HPh with various nitridorhenium precursors always led to the monochloro

578

Organometallic Complexes of Technetium

Scheme 40 Synthesis and reactivity of 99Tc-triazolylidenes complexes.181

complex [ReN(TPhPh-HHPh)(TPh2HPh)Cl], which contains one orthometalated NHC ligand.190,191 The chloro ligands of [TcN(TPh2HPh)Cl2] can be substituted by phenyl ligands, resulting in the mixed aryl/carbene complex [TcN(TPh2HPh)Ph2] (85). The crystal structure analysis of [TcN(TPh2HPh)Cl2] disclosed a five-coordinate 99Tc metal center with square-pyramidal geometry. In this mixed aryl/carbene complex the technetium-carbene carbon bond length (2.129(3) A˚ ) is in a similar range as in [TcN(IEt2Me2)2(OSiMe2OSiMe2O)] (2.127(2), 2.134(2) A˚ ) and slightly shorter than the bonds to the phenyl carbon atoms (2.168 (3) A˚ ). Comparing the crystal structures of 99Tc+V-NHC complexes demonstrates the fact, that the metal-carbene bond length is more influenced by steric constraints than nucleophilicity (see Section 5.08.7.1). Challenging experiments with [TcN(TPh2HPh) Cl2] (84) and the stronger nucleophile 1,3,4,5-tetramethylimidazol-2-ylidene (IMe2Me2) resulted in the replacement of all equatorial ligands and the formation of [TcN(IMe2Me2)4Cl]+ (86, Scheme 40).181 Until today, the only characterized 99Tc-NHC complex which does not contain the metal at the oxidation state +V is [99(m)Tc+I(CH3CN)(CO)3(IPyMeH2)]+ (87).192 The 99mTc complex represents the first successful labeling of an NHC with 99mTc. To confirm the nature of this 99mTc-NHC complex, a carrier added (99Tc) reaction solution was employed and the formed [99Tc+I(CH3CN)(CO)3(IPyMeH2)]+ (87) identified by mass spectrometry (Scheme 41).

Scheme 41 Synthesis of 99(m)Tc-NHC complexes, containing the fac-{99(m)Tc+I(CO)3}+ core.192

By introducing NHCs into technetium chemistry and the great potential of this type of ligand systems for drug development this field of organometallic chemistry picked up new momentum. With the first water-stable 99(m)Tc-NHC complexes in hand, the development of the first NHC-based radiopharmaceuticals for preclinical trials is only a matter of time.

Organometallic Complexes of Technetium

5.08.6.3

579

Carbyne complexes

The first isolated 99Tc-carbyne complexes were reported by A. K. Burrell and coworkers in 1994.193 By protonating vinylidene complexes of the type [Tc(]C]CHR)Cl(dppe)2] (88, R ¼ Ph, tBu; dppe ¼ 1,2-bis(diphenylphosphino)ethane) the respective terminal carbine complexes [Tc(^CCH2R)Cl(dppe)2]+ (89, R ¼ Ph, tBu) were synthesized (Scheme 42). An alternative approach for the synthesis of a 99Tc-carbyne complex was developed by E. O. Fisher.186 By reacting the 99Tc ‘Fischer-type’ carbene complex [Cp (CO)2Tc(]C(OCH3)Ph)] (90) with BCl3, [Cp (CO)2Tc(^CPh)](BCl4) (91) was isolated and characterized.

(A)

(B)

Scheme 42 Synthesis of 99Tc-carbyne complexes by (A) protonation of corresponding 99Tc-vinyliden complexes and (B) the reaction of a 99Tc ‘Fischer-type’ carbene complex with BCl3.186,193 A nucleophilic attack onto the carbyne carbon atom regenerates a 99Tc ‘Fischer-type’ carbene complex.

[Tc(^CCH2 tBu)Cl(dppe)2]+ was analyzed by single crystal X-ray diffraction analysis. The Tc^C bond length is 1.724(7) A˚ , which is significantly shorter than in the structures of 99Tc ‘Fischer-type’ carbene complexes (1.97(2), 2.01(2) A˚ ) and 99Tc-NHC complexes (2.071(9)–2.232(3) A˚ ). The yellow [Cp (CO)2Tc(^CPh)](BCl4) is surprisingly stable as a solid, which enabled further studies, characterizing the reactivity of this rare type of compounds. The nucleophilic attack of NaOCy (Cy ¼ cyclohexyl) on the carbyne carbon atom of compound [Cp (CO)2Tc(^CPh)]+ led to the formation of the 99Tc ‘Fischer-type’ carbene complex [Cp (CO)2Tc(]C(OCy)Ph)] (92, Scheme 42).

5.08.7

Structural data and 99Tc NMR studies

5.08.7.1

Structural data

The number of 99Tc complexes analyzed by single crystal X-ray diffraction analysis is lagging behind its heavier homolog Re. The reasons for this are the limited number of laboratories licensed to handle the long-lived radioisotope 99Tc and the strong focus on only three field of research, namely fundamental research, nuclear medical applications and nuclear waste management. Nevertheless, the structural elucidation of 99Tc compounds is of utmost importance, due to the limitations in other analytical methods restricted by the radioactivity of this element. There are some reviews published, focusing on 99Tc complexes whose structure has been studied by X-ray diffraction analysis.10,194–196 The reports cover both coordination and organometallic compounds, and include compounds containing the fac-{Tc(CO)3}+ core.

580

Organometallic Complexes of Technetium

Most technetium–carbon bond length from this review (except TcdCO bond length) are summarized in Tables 3 and 4. The summary gives a good overview about the bond distances of the individual classes of organometallic 99Tc complexes. TcdC bond lengths in 99Tc isocyanide complexes strongly depend on the steric demand of the isocyanide ligand. Whereas for smaller ligands, bond distances in the range of 1.932(10)–2.102(6) A˚ were found, sterically very demanding ligands can lead to rather long bond lengths of up to 2.51(2) A˚ (cis-[TcNCl2(CNArMes2)2(H2O)]). TcdC bond lengths for 99Tc alkyl and aryl complexes are known in the range of 2.086(3)–2.159(2) A˚ . This range is in good agreement with the range of 2.00–2.13 A˚ which has been predicted by ab initio

Table 3

Overview about TcdC bond length in 99Tc isocyanide and alkyl/aryl complexes.

Isocyanide complexes [Tc(CNCCOOMe)6][TcO4] [Tc(CNCMe3)4(bpy)](PF6) [Tc(CNCMe3)4(PMe2Ph)2] [Tc(CNCMe3)2(dppe)2](PF6) [Tc(CO)5(CNCMe3)]+ [TcCl(CNCMe3)2(CO)3] [Tc(CNCMe3)3(CO)3](NO3) [Tc(PS3)(CNCMe2)] [Tc(PS3)(CNCMe2)2] [Tc(NS3)(CNCH2C6H5)] [Tc(NS3)(CN C6H11)] [Tc(NS3)(CN CH2C(O)OC2H5)] trans-mer-[TcCl(CO)3(CNArDipp2)2] trans-[Tc(NO)Cl2(PPh3)(CNArDipp2)2], [TcCl3(PPhMe2)2(CNArDipp2)] trans-[TcNCl2(CNArDipp2)2], trans-[Tc(NPh)Cl3(CNArDipp2)2], trans-[Tc(NPh)Br3(CNArDipp2)2], [TcCl3(PPhMe2)2(CNArMes2)], cis-[TcNCl2(CNArMes2)(MeOH)] cis-[TcNCl2(CNArMes2)2(H2O)] Alkyl/aryl complexes [(TcO(CH3)2(m-O))2] [Tc(NAr)3Me] [TcMe2(NAr)(m-NAr)2Tc(NAr)2] [Tc2(NAr)2(m-NAr)2Me4] [(Z1-Cp)Tc(NAr)3] [TcNPh2(PPh3)2] [Tc(NO)(Cp)(PPh3)(Ph)]

Table 4

TcdCx

Tc]Cx

x ¼ Isocyanide 2.032(3)–2.044(3) 1.96(2)–2.04(2) 2.033(13)–2.046(11) 2.034(4) 2.095(5) 2.097(2), 2.102(6) 2.097(3), 2.082(3) 2.06(8) 2.081(7), 2.058(8) 1.932(10) 1.942(6) 1.945(3) 2.049(2)–2.057(2) 2.064(2) 2.034(5) 2.098(4), 2.116(4) 2.090(1), 2.081(1) 2.074(3), 2.079(3) 2.003(3) 2.039(1) 2.062(1), 2.51(2) x ¼ Alkyl/aryl 2.086(3)–2.133(2) 2.136(17) 2.119(17), 2.144(17) 2.134(2), 2.149(2) 2.156(3) 2.138(4), 2.127(4) 2.149(2)

x ¼ Isocyanide

Tc^C

References

63 112 123 124 32 46 53 128 128 129 129 129 130 130 130 130 130 130 130 130 130

1.90(2)

175 180 177 177 176 181 161

Overview about TcdC bond length in 99Tc carbene and carbyne complexes. TcdCx

Carbene complexes [(Cp )(CO)2TcC(OEt)Ph] [Tc(]C]CHR)Cl(dppe)2] [TcO2(IMe2H2)4](PF6) [TcO2(IiPr2Me2)][TcO4] [TcO2(IMeH2CH2H2MeI)]+ ([TcO2(IEtH2CH2H2EtI)]+). [99TcN(IEt2Me2)4]2+ [99TcN(IEt2Me2)2 (OSiMe2OSiMe2O)] [99TcN(IEt2Me2)(PPhMe2)(OSiMe2OSiMe2O)] [TcN(TPh2HPh)Cl2] [TcN(TPh2HPh)Ph2] [TcN(IMe2Me2)4Cl]+ Carbyne complexes [Tc(^CCH2 tBu)Cl(dppe)2]+

Tc]Cx

Tc^C

x ¼ Carbene 1.97(2) 1.861(9) 2.188(3), 2.191(3) 2.220(3)–2.232(3) 2.166(4)–2.170(4) 2.159(3)–2.181(3) 2.165(5)–2.192(5) 2.127(2), 2.134(2) 2.053(6) 2.129(3) 2.129(3) 2.207(3)

References

186 193 187 185 187 187 189 189 189 181 181 181 1.724(7)

193

Organometallic Complexes of Technetium

581

SCF calculations.197 The Tc]C bond lengths listed in Tables 3 and 4 (except TcdCNHC bond length) are in the range of 1.861 (9)–1.97(2) A˚ , which is significant shorter than the TcdC single bonds. Technetium-NHC complexes are a big exception. Formal the technetium–carbon bonds in Tc-NHC complexes are described as double bonds. However, the observed bond lengths in the known complexes are in the range of 2.053(6)–2.232(3) A˚ , which are better described as TcdC single bonds. The only structural characterized 99Tc carbyne complexes shows a Tc^C bond distance of 1.724(7) A˚ . Besides single crystal X-ray diffraction analysis, extended X-ray absorption fine structure (EXAFS) spectroscopy is an established analytical method to gain deeper insight into binding modes and bond distances of 99Tc complexes in solution. In recent years, this analytical method has been applied more for studies with inorganic 99Tc compounds, but a few studies of organometallic 99Tc compounds are known. [Tc2(CO)10] was one of the first organometallic 99Tc compounds studied by EXAFS.198 Fitting the experimental spectrum was possible with theoretically determined phase and amplitude functions for multiple scattering along the TcdCdO vector. The structural parameters were in reasonable agreement with the single X-ray data (TcdTc distance ¼ 0.09 A˚ more than obtained by X-ray diffraction analysis).23 Extended X-ray absorption fine structure spectroscopy has also been used to study the structure of 99Tc thioether complexes such as [TcCl(CO)3(SdS)] (SdS ¼ bidentate dithioether) and [TcCl(CO)3 (SdSdO)] (SdSdO) (SdSdO ¼ tidentate caboxylato dithioether) in aqueous solution and their reactivity toward histidine.199,200 It was shown that these fac-{99Tc(CO)3}+ complexes undergo ligand exchange reactions with histidine, yielding [Tc(his)(CO)3]. Structural parameters of the thioether complexes and [Tc(his)(CO)3] are given. The determined TcdS distances are in the range of 2.50  0.02 A˚ in the thioether complexes and the TcdN distance is 2.20  0.02 A˚ in the histidine complex. The authors conclude that similar ligand exchange reactions can lead to the strong protein binding of thioether complexes, containing the fac-{Tc(CO)3}+ core in vitro as well as in vivo. Besides other analytical methods EXAFS spectroscopy was used to identify 99Tc+I carbonyl species in high-level nuclear waste at the Hanford Reservation.14 The presence of soluble, lower-valent technetium species in this nuclear waste complicates the immobilization of this high-level nuclear waste. By analyzing XANES, EXAFS and 99Tc NMR spectra and comparison with spectra of model compounds the authors stated that the lower-valent technetium species is most likely those of fac-[Tc(glyconate)(CO)3]2−. Very recently, further studies at the Hanford Reservation demonstrated the presence of several low-valent Tc species in nuclear waste tanks.5 In this report a methodology for accurate quantification of the multiple 99Tc species from a multicomponent mixture has been developed by correlating XAS and NMR with quantitative techniques such as ICP and radiochemistry.

5.08.7.2 99

99

Tc NMR

The Tc NMR nucleus is extremely convenient for NMR. The sensitivity of the Tc resonance relative to the proton resonance is 0.275 which allows for rapid data acquisition.201 However, the large nuclear spin of I ¼ 9/2 in combination with the large quadrupole moment of Q ¼ -0.19(5)  10−28 m2 can lead to significant line broadening in solution.202 This effect can inhibit the analysis of 99 Tc complexes with a rather low symmetry in the first coordination sphere of the metal center. On the other hand, the line broadening of the 99Tc signal can be used as a first qualitative information to estimate the symmetry of a 99Tc compound. Due to the large nuclear spin and the large quadrupole moment line width (Dn1/2) of 99Tc signals are known in the large range of 2.7 Hz ([TcO4]− in H2O)–20,000 Hz ([TcOCl4]− in CH3OD).202 Another problem caused by the large nuclear spin of the 99Tc nucleus is, especially in organometallic complexes, due to the coupling with the nuclear spin 9/2 it is almost impossible to measure 13C NMR signals from carbon atoms coordinated directly at the 99Tc metal center. 99Tc NMR spectroscopy exhibits a broad chemical shift range of over 9000 ppm. Correlating the formal oxidation state of diamagnetic 99Tc complexes with chemical shifts (vs. [TcO4]− in H2O) shows a regular increase in shielding with decreasing oxidation state.202 Only 99TcVII complexes do not follow this trend. Most likely due to the absence of d-electrons and varying degrees of ligand-to-metal charge transfer. The use of 99Tc NMR is extremely efficient in studying the kinetics of complex formation.202,203 Aqueous solutions of [Tc(CO)3(H2O)3]+ are stable at neutral and lower pH. At higher pH, reversible hydrolytically oligomerization takes place, leading to the hydroxyl carbonyl cluster [Tc(m3-OH) (CO)3]4.43 It has been shown by 99Tc NMR and potentiometry that at a ratio of OH/Tc < 1 the [Tc(OH)(CO)3(H2O)] monomer is formed, which polymerizes to give the dimer [Tc(OH)(CO)3(H2O)]2 and the tetramer [Tc(m3-OH)(CO)3]4.204 Water exchange on fac-[Tc(CO)3(H2O)3]+ has been studied, applying different 17O NMR techniques. From isotopic enrichment experiments an exchange rate constant (kex) has been determined of 0.49  0.05 s−1 (298 K) and its activation parameters DH{ (78.3  1 kJ mol−1) and DS{ (+11.7  3 JK−1 mol−1).203 A slow CO exchange in fac-[Tc(CO)3(H2O)3]+ with free CO can be observed in solution. The exchange has been studied by 99Tc and 13C NMR and CO exchange rate constants (kCO) of kCO ¼ (10.0  0.2)  10−4 s−1 M−1 (T ¼ 310 K) and kCO ¼ (0.82  0.01)  10−4 s−1 M−1 (T ¼ 277 K) have been measured.29 The slow CO self-exchange (compared to the water exchange) confirms the kinetic inertness of the fac-{Tc(CO)3}+ moiety. The exchange of water by CO in fac-[Tc(CO)3(H2O)3]+ has be studied by 99Tc and 13C NMR and a kinetic model has been developed to describe the exchange.29 In this study the first 99Tcd13C coupling constant has been determined (354 Hz) and the intermediates fac[Tc(CO)4(H2O)2]+, fac-[Tc(CO)5(H2O)]+ and fac-[Tc(CO)6]+ have been found. Interestingly, [Tc(CO)6]+ has been found to be relatively stable in aqueous solution at lower pH. The 99Tc NMR signals of fac- [Tc(CO)4(H2O)2]+ and fac-[Tc(CO)5(H2O)]+ disappeared within hours, after the CO gas pressure was released. In contrast, the 99Tc NMR signal of [Tc(CO)6]+ disappeared slowly within 2 days at room temperature. A mechanistic model, which explains this observation has been developed by Alberto and coworkers.205 They suggest that once one CO molecule is replaced by a water molecule the two other CO molecules are cleaved rapidly (see Scheme 43).

582

Organometallic Complexes of Technetium

Scheme 43 Decarbonylation of [Tc(CO)6]+ in aqueous solution, including observed relative rates of CO release.205

Recent self-exchange experiments of CO in [M(CO)3X3]2− (M ¼ Re, X ¼ Br; M ¼ 99Tc, X ¼ Cl) in organic solvents disclosed a significant different between these two homologs.206 Whereas [ReBr3(CO)3]2− showed a high affinity for CO, forming [ReBr2(CO)4]−, no evidence for the formation of [TcCl2(CO)4]− has been obtained by 99Tc NMR. As XANES and EXAFS, 99Tc NMR is a promising tool to characterize and identify unknown 99Tc carbonyl complexes in nuclear waste (see Section 5.08.7.1). However, identification in nuclear waste is challenging due to the lack of reference data especially for Tc compounds. Levitskaia and coworker reported efforts to establish a spectroscopic library corresponding to the relevant conditions of extremely high ionic strength typical for the legacy nuclear waste. This study focuses on 99Tc compounds with the general formula of fac-[Tc(OH)n(CO)3(OH2)3−n]1−n (where n ¼ 0− 3) and includes spectroscopic techniques such as 99Tc NMR, 13C NMR, IR, XPS, and XAS.207 Furthermore, some theoretical studies have been made to develop a profound theoretical background for the calculation of 99 Tc NMR shifts.208,209 Finally, theoretical studies showed that 99Tc NMR, due to its high sensitivity, can be a powerful tool for investigation of the chemical structure of biomolecules at the molecular level in different environments.210

5.08.8

Conclusion and perspective

From the beginning of technetium chemistry, organometallic chemistry has played an important role. Today, the 99mTc probe Cardiolite can be considered as early example of an organometallic compound in the rapidly developing field of metals in medicine and bioorganometallic chemistry. Very intense 99Tc fundamental research was the cornerstone for this successful development in the past. However, in many cases fundamental organometallic research is demanding regarding the infrastructure (toxic gas, inert atmosphere, etc.). This characteristic of organometallic chemistry in combination with the constantly decreasing number of licensed laboratories dedicated to fundamental 99Tc chemistry led to an unsatisfactory situation. Analyzing the literature quotations of the last fifteen years show that all reports, dealing with synthetic 99Tc organometallic chemistry, were published by less than five laboratories. This picture does not change much if we look at technetium chemistry in general. These 99Tc laboratories ensure that technetium chemistry continues to advance. Furthermore, they work against a foreseeable lag of trained people. The basis for new developments is strong and rigorous fundamental chemistry. To meet today’s safety requirements low-pressure syntheses of organometallic 99Tc precursor complexes are crucial. The established syntheses of [TcCl3(CO)3]2− and [Tc(CO)5X] (X ¼ Cl, Br, I) are prototypical examples. However, more precursors, such as [Tc2(CO)10], are needed to generate new opportunities and to diversify organometallic 99Tc chemistry. Furthermore, new approaches and synthetic strategies have to be developed or adapted to give access to highly interesting target compounds, currently not considered as an option for molecular imaging. The successful synthesis and characterization of the controversially discussed [(Z5-C5Me5)TcO3] for example would demonstrate the synthetic possibilities of modern organometallic 99Tc chemistry without radioactivity of technetium being a limitation. Such findings clearly motivate the search for other ‘non-existing’ 99Tc compounds. As described in the introduction, fundamental technetium chemistry is strongly influenced by its potential application in life sciences. For many years, research with the nuclear isomer 99mTc focused on the development of a ‘magic bullet’ for radiodiagnostic. However, modern radioprobe development is not focused exclusively on one radiometal anymore, and the influence of 99mTc research as a driving force for basic 99Tc research is clearly waning. For this reason, 99mTc research must be embedded in a broader strategy, focusing on the unique chemical properties of this radiometal. The matched-pair concept, the combination of 99mTc and 186/188Re for radiodiagnostic and radiotherapy was formulated a long time before the term ‘theranostics’ was created. However, very few examples exist demonstrating the high potential of this strategy. The developed technetium bis-arene chemistry aims to reactivate this concept. Recently, a conceptual extension of the ‘theranostic pair’ Tc/Re has been described by the combination of a 99mTc-based bio-mimetic with its organometallic but non-radioactive rhenium homologue.168 The rhenium complex should be therapeutically active through its structural characteristics while the 99mTc homologue allows for visualization of the pharmacology. In this context bis-arene complexes of technetium and rhenium represent a novel organometallic theranostic platform for (nuclear)medical applications. This platform is certainly not the only one and more such are needed to demonstrate the unique potential of technetium. The success of 99mTc for routine applications in radiodiagnostics is based on so called de novo complexes (small biological active molecules). Research, focusing on this class of Tc compounds has been neglected for many years. Thus, new classes of (organometallic) technetium compounds, such as Tc-NHC complexes with interesting bioprofiles and properties, deserve intense development to revive the importance and the potential of this type of compounds for nuclear medical applications. In summary, the few active 99Tc laboratories are core for setting the base of a strong future for Tc chemistry. However, fundamental 99Tc chemistry hardly fits into any popular grant schemes, which leaves the funding of this research primarily to universities, research institutes and national laboratories. This fact in combination with the low number of laboratories leads to the current global situation, which has to be described as ‘metastable’.

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Acknowledgment The author would like to thank Roger Alberto, Robin Bolliger, Manuel Besmer, Joshua Csucker and Raphael Lengacher for support and valuable discussions. Furthermore, H. Braband acknowledges financial support from the Swiss National Science Foundation (200021_140665 and 200020_159800) and the University of Zurich.

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Ed. 1989, 28, 1055. Burrell, A. K.; Cotton, F. A.; Daniels, L. M.; Petricek, V. Inorg. Chem. 1995, 34, 4253–4255. Szyperski, T.; Schwerdtfeger, P. Angew. Chem. Int. Ed. 1989, 28, 1228–1231. Ackermann, J.; Hagenbach, A.; Abram, U. Chem. Commun. 2016, 52, 10285–10288. Ackermann, J.; Abdulkader, A.; Scholtysik, C.; Jungfer, M. R.; Hagenbach, A.; Abram, U. Organometallics 2019, 38, 4471–4478. Gridnev, I. D.; Imamoto, T. Science of Synthesis; Thieme Chemistry, 2003; vol. 2. Palm, C.; Fischer, E. O.; Baumgärtner, F. Tetrahedron Lett. 1962, 3, 253–254. Wester, D. W.; Coveney, J. R.; Nosco, D. L.; Robbins, M. S.; Dean, R. T. J. Med. Chem. 1991, 34, 3284–3290. Trifonova, E. A.; Perekalin, D. S.; Lyssenko, K. A.; Kudinov, A. R. J. Organomet. Chem. 2013, 727, 60–63. Benz, M.; Braband, H.; Schmutz, P.; Halter, J.; Alberto, R. Chem. Sci. 2015, 6, 165–169. Meola, G.; Braband, H.; Jordi, S.; Fox, T.; Blacque, O.; Spingler, B.; Alberto, R. Dalton Trans. 2017, 46, 14631–14637. 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K.; Miller, M. M.; Smith, W. H.; Burns, C. J.; Sattelberger, A. P. Polyhedron 1993, 12, 1769–1777. Benson, M. T.; Bryan, J. C.; Burrell, A. K.; Cundari, T. R. Inorg. Chem. 1995, 34, 2348–2355. Oehlke, E.; Kong, S.; Arciszewski, P.; Wiebalck, S.; Abram, U. J. Am. Chem. Soc. 2012, 134, 9118–9121. Braband, H.; Kueckmann, T. I.; Abram, U. J. Organomet. Chem. 2005, 690, 5421–5429. Jain, K. R.; Herrmann, W. A.; Kühn, F. E. Current Organic Chemistry; Bentham Science Publishers Ltd., 2008; vol. 12; pp 1468–1478 Hock, S. J.; Schaper, L.-A.; Herrmann, W. A.; Kühn, F. E. Chem. Soc. Rev. 2013, 42, 5073–5089. Braband, H.; Zahn, T. I.; Abram, U. Inorg. Chem. 2003, 42, 6160–6162. Fischer, E. O.; Apostolidis, C.; Dornberger, E.; Filippou, A. C.; Kanellakopulos, B.; Lungwitz, B.; Müller, J.; Powietzka, B.; Rebizant, J.; Roth, W. Z. Naturforsch. 1995, 50 (b), 1382–1396. Benz, M.; Spingler, B.; Alberto, R.; Braband, H. J. Am. Chem. Soc. 2013, 135, 17566–17572. Kuhn, N.; Kratz, T. 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5.09 Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds Kotohiro Nomura, Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, Hachioji, Tokyo, Japan © 2022 Elsevier Ltd. All rights reserved.

5.09.1 5.09.2 5.09.2.1 5.09.2.2 5.09.2.3 5.09.2.4 5.09.3 5.09.3.1 5.09.3.2 5.09.3.3 5.09.3.4 5.09.4 5.09.4.1 5.09.4.2 5.09.4.3 5.09.4.4 5.09.5 5.09.5.1

Introduction: Group 5 organometallics as promising catalysts for efficient carbon-carbon bond formations Organovanadium complexes and related chemistry Vanadium complexes containing cyclopentadienyl ligands Vanadium complexes containing monodentate or bidentate ligands Vanadium complexes containing tridentate, tetradentate ligands (Imido)vanadium complexes and some reaction chemistry Organoniobium complexes and related chemistry Niobium complexes containing cyclopentadienyl, hydridotris(pyrazolyl)borate ligands Niobium complexes containing monodentate anionic donor ligands Niobium complexes containing bidentate or tridentate anionic donor ligands (Imido)niobium complexes and some reaction chemistry Organotantalum complexes and related chemistry Tantalum complexes containing cyclopentadienyl ligands Tantalum complexes containing monodentate anionic donor ligands Tantalum complexes containing bidentate or tridentate anionic donor ligands (Imido)tantalum complexes and some reaction chemistry Selected topics Vanadium(V)-, niobium(V)-alkylidene complexes as catalysts for ring-opening metathesis polymerization (ROMP) of cyclic olefins and living polymerization of internal alkynes 5.09.5.1.1 Introduction: Synthesis of vanadium-, niobium-alkylidenes 5.09.5.1.2 Vanadium(V)-, niobium(V)-alkylidene complexes as catalysts for ring-opening metathesis polymerization (ROMP) of cyclic olefins and living polymerization of internal alkynes 5.09.5.2 Solution XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure) analysis for exploring homogeneous catalytically active species 5.09.5.2.1 Introduction 5.09.5.2.2 Solution XAS analysis of active species of vanadium complex catalysts in ethylene polymerization/dimerization 5.09.6 Concluding remarks Acknowledgments References

5.09.1

587 589 589 593 595 598 607 607 612 614 616 619 619 624 626 630 633 633 634 637 641 641 641 644 644 645

Introduction: Group 5 organometallics as promising catalysts for efficient carbon-carbon bond formations

Although this chapter will describe a summary of recent organometallic complexes and their related chemistry in group 5 transition metals (vanadium, niobium, and tantalum), several unique characteristics as well as important points displayed/demonstrated by group 5 organometallics in early days (from 1960s through 1980s), which have been recognized as very important contributions not only in organometallics, but also in catalysis and materials chemistry, should be described in the introduction. Important reports are also cited as reference. Classical Ziegler type vanadium catalyst systems [consisting of VOCl3, VCl4, VCl3 – AlBr3, AlCl3 – AlPh3, AliBu3, SnPh4 or V (acac)3 (acac ¼ acetylacetonato) – Et2AlCl] developed in 1960s display unique characteristics in olefin polymerization (Scheme 1); the remarkable propagation afforded ultrahigh molecular weight polymers with low polydispersity indexes (PDIs, narrow molecular weight distribution).1–8,11–14 The unique characteristics were applied for industrial production of synthetic rubber (called EPDM, ethylene/propylene/diene copolymers),8 and then ethylene/cyclic olefin copolymers (copolymers of ethylene with tetracyclododecene). Synthesis of syndiotactic polypropylene with narrow molecular weight distribution was demonstrated under deep cooling conditions (below −65  C),9 and the method was applied to synthesis of diblock copolymer of propene and methyl methacrylate (MMA),10 recognized as the first clear demonstration of synthesis of olefin/polar monomer copolymer using transition metal catalysts (Scheme 1). Although the very high initial catalytic activities were exhibited, the rapid catalyst deactivation [probably associated with conversion to the inactive species by reduction, from vanadium(III) to vanadium(II)] causes poor overall productivities.1–10 Vanadium(III) complexes were proposed to be the catalytically active species,6,7 directly observed very recently by solution V K-edge XANES (X-ray Absorption Near Edge Structure) and EXAFS (Extended X-ray Absorption Fine Structure) analysis.15,16 Design of vanadium complex catalysts for olefin coordination/insertion polymerization still has been an important subject.14,17–20

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00042-1

587

588

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

Scheme 1 Olefin polymerization by classical Ziegler type vanadium catalyst systems.1–10

Group 5 complexes, especially Ta-, Nb-alkylidene complexes, play a pioneering contribution in organometallic chemistry containing multiple metal-carbon bonds,21–25 and these complexes play roles as catalysts and/or important intermediate for organic transformations such as olefin metathesis and Wittig-type coupling. The first metal-alkylidene complex, Ta(CHtBu) (CHt2Bu)3, was prepared by treating Ta(CHt2Bu)3Cl2 with 2 equiv. of LiCHt2Bu in pentane (Scheme 2). The possibility of a-hydrogen abstraction (elimination) was proposed from isotopic distribution of the product in reaction of Ta(CDt2Bu)3Cl2 with 2 equiv. of LiCHt2Bu.26 Reaction of Ta(CHt2Bu)2Cl3 with 1 equiv. of CpTl gave CpTa(CHtBu)Cl2 and further treatment with 1 equiv. of CpTl afforded Cp2Ta(CHtBu)Cl;27 the structure was later determined by X-ray crystallography.28 Cp2Nb(CHtBu)Cl was also prepared by treatment of Nb(CHt2Bu)2Cl3 with CpTl (2 equiv.).29 The synthesis of Nb(CHtBu)(CHt2Bu)3 was also demonstrated by treatment of Nb(CHt2Bu)3Cl2 with LiCHt2Bu but the complex was unstable and decomposed extensively in C6D6 at 25  C.30 In contrast, reaction of Ta(CH3)3Cl2 with 2 equiv. of CpTi gave Cp2Ta(CH3)3, and the subsequent reaction with [Ph3C][BF4] followed by treatment with Me3P]CH2 gave the methylene complex, Cp2Ta(CH2)(CH3),27 confirmed by X-ray crystallography [Ta-CH3 2.246(12) A˚ ; Ta-CH2 2.026(10) A˚ ]. To M(CHt2Bu)2Cl3 (M ¼ Nb, Ta) was added PMe3 (2 equiv.) to afford M(CHtBu)2Cl3(PMe3)2 by a-hydrogen elimination.31 Reaction of the Ta tribenzyl complex, Ta(CH2Ph)3Cl2, with 2 equiv. of CpTl gave Cp2Ta(CHPh)(CH2Ph)3;29 the similar reaction with 1 equiv. of Cp Li (Cp ¼ C5Me5) gave Cp Ta(CHPh)(CH2Ph)Cl.32 These are nowadays known as the common method for synthesis of high-oxidation state metal alkylidenes by a-hydrogen abstraction, which is induced by steric crowding, thermally or photochemically, or by treatment with a base.21–25

Scheme 2 Synthesis of high oxidation state metal-alkylidene and metal-alkylidyne complexes.21

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

589

As also shown in Scheme 2, the first Ta-carbyne complex, [Ta(^CtBu)(CHt2Bu)3][Li(dmp)] (dmp ¼ N,N-dimethylpiperazine), was isolated from Ta(CHtBu)(CHt2Bu)3 by treatment with nBuLi in the presence of dmp (confirmed by X-ray crystallography).33 Treating CpTa(CHtBu)Cl2 with PMe3 in THF followed by addition of Ph3P]CH2 or with LiCHt2Bu followed by addition of PMe3 gave the alkylidyne complex, Ta(^CtBu)Cl(PMe3)2.32 Ta(^CPh)Cl(PMe3)2 was also formed by treatment of CpTa(CH2Ph)3Cl with PMe3.32 Addition of PMe3 to Ta(CHtBu)(CHt2Bu)3 gave the bis(alkylidene) complex, Ta(CHtBu)2(CHt2Bu) (PMe3)2,34 and reaction of Ta(^CtBu)Cl(PMe3)2 with LiCHt2Bu also gave another bis(alkylidene) complex, Cp Ta(CHtBu)2 (PMe3).34 Although many efforts were reported for synthesis and reaction chemistry especially in Ta(V)-alkylidene and Ta(V)-alkylidyne complexes, reports for their application as highly active olefin metathesis catalysts were limited; as described below, one example for living metathesis polymerization of 1-butyne.35 Selective olefin dimerization through metallacycle intermediate was first demonstrated by Ta catalysts.36–39 First, reaction of CpTa(CHtBu)Cl2 with propylene in pentane at 0  C afforded 2,4,4-trimethyl-1-pentene and relatively stable metallacycle complex, CpTaCl2(C6H12), which decomposed to give 2,3-dimethyl-1-butene (Scheme 3).36 Various Ta(III) olefin complexes, Cp TaCl2 (olefin) (olefin ¼ styrene, tert-butylethylene, cyclooctene, 1-pentene, and cis-2-pentene), were prepared from the propylene analog by treating with the corresponding olefins,37 and subsequent reactions with olefin gave the corresponding metallacyclopentanes.37 Selective catalytic propylene dimerization to afford 2,3-dimethyl-1-butene was demonstrated by Cp TaCl2(propylene) in toluene under propylene pressure (40 psi, 0.28 MPa).39 The results clearly demonstrate a different mechanism proposed in ordinary olefin polymerization by Ziegler-Natta catalysts and nickel catalyzed ethylene oligomerization (called Shell Higher Olefin Process, SHOP)40,41 through insertion pathway via metal-hydride or -alkyl intermediate. Formation of metallacycle by oxidative coupling and b-hydride elimination pathway, which has been proposed for selective ethylene trimerization using Cr catalysts, was demonstrated at this stage.42–45

Scheme 3 Selective olefin dimerization through metallacyclopentene intermediate.36–39

In this chapter, recent reports for synthesis of group 5 (V, Nb, Ta) complexes containing metal-alkyl (aryl) and multiple metal-carbon bonds and related reaction chemistry have been summarized. Additional explanations of background have also been placed for uninitiated readers in this field for better understanding. Since we recognize the progress especially of organometallic chemistry and/or molecular catalysis in vanadium and niobium in recent 15–20 years, the manuscript could be helpful for researchers who have interests. Moreover, two topics concerning metal-alkylidene catalysts for olefin metathesis polymerization and solution XAS (X-ray absorption spectroscopy) studies for in situ analysis of catalytically active species have also been introduced on this occasion.

5.09.2

Organovanadium complexes and related chemistry

5.09.2.1

Vanadium complexes containing cyclopentadienyl ligands

Cp2V and Cp 2V46,47 are known as stable and easily accessible (electronically and coordinatively unsaturated) metallocenes. As demonstrated mostly by Gambarotta and Florani in 1980s, the unsaturated nature of [Cp2V] accepts various organic compounds (ketene, thioketene, ketene imine etc.) to generate complexes with
2-C,X (X ¼ C, O, S, N) bonding (Scheme 4), which have been considered as model compounds for basic organic transformations.48 The reaction with 2 or 3 equiv. of Me3SiC^CdC^CdC^CSiMe3 gave dimeric vanadium(III) complex, (Cp2V)2(3
:4
-Me3SiC^CdC]CdC^CSiMe3) (1),

590

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

Scheme 4

and the reaction conducted with 1 equiv. in pentane yielded the monomeric vanadium(IV) complex, Cp2V(3–4
2-Me3SiC^CdCdCdC^CSiMe3) (2).49 The reaction with 0.5 equiv. of RC^C–C^CR (R ¼ SiMe3, PPh2) afforded homodimeric vanadacyclopropanes (3,4),50 and the reaction with 1 equiv. of RC^CdC^CdC^CdC^CR afforded monomeric complex 5 [R ¼ tBu (a), Ph (b)]. Further addition of [Cp2V] to 5a gave the homodimeric complex (6).51 Cp 2VMe52 was treated with [PhN(H)Me2][BPh4] (1 equiv.) in toluene to afford a base free cationic [Cp 2V][BPh4],53 and an agostic C-H interaction of a methyl group on Cp with V was demonstrated through the X-ray crystallographic analysis. The reaction with [PhN(H)Me2][B(C6F5)4] in toluene or C6H5F afforded [Cp 2V][B(C6F5)4].53 Cp2VMe2 was treated with [PhN(H)Me2][BPh4] in acetonitrile to yield the cationic methyl complex, [Cp2VMe(CH3CN)]+[BPh4]− (7),54,55 and the similar reaction of Cp2VMe2 with B(C6F5)3 in THF initially gave [Cp2VMe(THF)]+ (by EPR spectrum), which eventually converted by a disproportionation to give cationic vanadium(III) species, [Cp2V(THF)][MeB(C6F5)3], (Scheme 5).55 Treatment of Cp2V with RC^N
B(C6F5)3 (R ¼ CH3, 4-CF3C6H4), prepared by mixing RC^N and B(C6F5)3 in toluene, afforded Cp2V[
2-RC]N
B(C6F5)3] (8), and the similar reaction with RC^N
BAr3 (R ¼ 4-CF3C6H4, Ar ¼ 2,6-F2C6H3, 3,4,5-F3C6H2) gave Cp2V[
2-RC]N
BAr3] (9).56 The reaction of Cp2V with 1,4-dicyanobenzene upon presence of B(C6F5)3 (2 equiv.) gave [Cp2V]2[
2:C,N-(C6F5)3B
N]C(C6H4)C]N
B(C6F5)3] (10) and the similar reactions with N^CCH2C^N, N^C(CH2)4C^N yielded Cp2V[
2-{(C6F5)3B
N]CCH2}C^N
B(C6F5)3] (11), [Cp2V]2[
2:C,N-(C6F5)3B
N]C(CH2)4C]N
B(C6F5)3] (12), respectively.57

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

591

Scheme 5

CpVX(dmpe) [dmpe ¼ 1,2-bis(dimethylphosphino)ethane] or CpVCl2(PMe3)2 were known examples of (coordinatively saturated) low-valent vanadium species. Reduction of CpVCl2(PMe3)2 with Zn or Al in THF gave a dimeric [CpVCl(PMe3)2]2, and subsequent addition of dmpe afforded a monomeric CpVCl(dmpe) (13).58 The corresponding vanadium(II)-methyl, -isopropyl, and -phenyl complexes, CpVR(dmpe) (R ¼ Me, iPr, Ph), were prepared by treating with MeLi, iPrMgCl, PhMgBr, respectively,59 and their structures were confirmed by X-ray crystallography (Scheme 6).60 CpV(iPr)(dmpe) was treated under hydrogen to give a dimeric [CpV(m-H)(dmpe)]2.61 CpVCl2(PMe3)2 was also reduced by Na/Hg in THF to afford CpVCl(PMe3)2, and subsequent treatment with 0.5 equiv. of BrMg(CH2)4MgBr gave vanadium(I) complex, CpV(
2-ethylene)(PMe3)2;62 the availability of CpVC12(PMe3)2 thus demonstrated synthesis of a wide range of low valent and V(III) organometallics, and some reactions of 14-electron and 12-electron vanadium(III) dialkyl species, such as CpV(CHt2Bu)2(PMe3), CpV(CH2CMe2Ph)2(PMe3), and CpV(CH2SiMe3)2 were studied.63 As shown in Scheme 6, a-hydrogen abstraction of CpV(CHt2Bu)2(PMe3) in the presence of dmpe afforded the first vanadium(III)-alkylidene, CpV(CHtBu)(dmpe) (14), and subsequent reaction with tBuCN afforded the insertion product (15). In contrast, reaction of CpV(CH2CMe2Ph)2(PMe3) in the presence of excess PMe3 gave the metallacycle CpV(s2-CH2CMe2C6H4)(PMe3)2 (16) via orthometalation, whereas the reaction in the absence of PMe3 gave the bridged dimer, [CpV(m2-CH2CMe2C6H4)]2 (17).63 Moreover, CpV(C6H5)2(PMe3)2 in C6D6 gave the benzyne complex (18) upon heating (50  C), and insertion of PhC^CPh, ethylene, propylene and isobutene gave the corresponding inserted complexes (exemplified as 16). Insertion reactions of the Cp-arylimido analog, CpV(NAr0 )(PMe3)2 (19, Ar0 ¼ 2,6-iPr2C6H3), prepared from CpV(NAr0 )Cl2 by treatment with Mg in the presence of PMe3, with ethylene, CO and PhC^CPh were studied,64 and the reaction with Ph3P]CHPh gave the vanadium(V)-benzylidene complex, CpV(CHPh)(N-2,6-iPr2C6H3)(PMe3) (20), via benzylidene transfer.65 Complexes 15 and 20 are known as the early examples of isolated alkylidenes, but showed poor capabilities as olefin metathesis catalysts.

592

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

Scheme 6

As shown in Scheme 6, reaction of VCl3(PMe3)2 with lithium salt, prepared from C5H5(CH2)2NH(iPr) by treating with MeLi, afforded [
5,
1-C5H4(CH2)2N(H)iPr]VCl2(PMe3) (21), and the further reaction of 21 with MeLi gave vanadium(III) amide complex, [
5,
1-C5H4(CH2)2NiPr]VCl2(PMe3) (22).66 Reduction of 22 with Na/Hg in 2,3-dimethyl butadiene (dmb) yielded vanadium(II) complex, [
5,
1-C5H4(CH2)2NiPr]V(dmb) (23), and the reaction of 23 with PhICl2 gave the vanadium(IV) dichloride complex, [
5,
1-C5H4(CH2)2NiPr]VCl2 (24).66 [
5,
1-C5H4(CH2)2NMe2]VCl2(PMe3) (25) in place of 21 was reacted with MeLi in

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

593

the presence of PMe3 afforded [
5-C5H4(CH2)2NMe2]VMe2(PMe3)2 (26), and a cationic complex, [{
5,
2-C5H4(CH2)2N(Me)CH2} V(PMe3)2][BPh4] (27) was obtained by subsequent treatment with [PhN(H)Me2][BPh4].67 Treatment of 25 with 1 equiv. of allylmagnesium chloride in Et2O and followed by addition of MeLi gave [
5-C5H4(CH2)2NMe2]VMe(allyl)(PMe3)2 (28); treatment of 28 with [PhN(H)Me2][BPh4] gave a cationic p-allyl complex (29).67

5.09.2.2

Vanadium complexes containing monodentate or bidentate ligands

Vanadium(II) complexes, V[N(SiiPr3)Ar0 ]2 (30, Ar0 ¼ 2,6-iPr2C6H3) or V[N(SitBu2Me)Ar0 ]2 (31), were prepared by reaction of V2Cl3(THF)6 with K[N(SiiPr3)Ar0 ] or K[N(SitBu2Me)Ar0 ].68 On the basis of X-ray crystallographic analysis, complex 31 affords a sandwich structure in which both silyl(aryl)amido ligands are bound to the vanadium center through the aryl rings (Scheme 7),68 whereas complex 30 affords a linear structure [V −N 1.9824(5) A˚ ]; spectroscopic data supported the structural change of 30 between linear and sandwich in the solution by isomerization.68 Another vanadium(II) complex [
5-{2,5-Me2C4H2N(AlClMe2)}]2V (32), prepared by treatment of VCl3(THF)3 with 2,5-dimethylpyrrole and AlMe3 (6 equiv.), showed catalytic activity for ethylene polymerization without any additional cocatalysts to afford linear ultrahigh molecular weight polyethylene with unimodal molecular weight distributions and the activity increased at 75  C.69 Reaction of 32 with PhNNPh gave the bridged phenylimido complex (33).69

Scheme 7

Vanadium(IV) bis(amide) complexes, (R0 2N)2VCl2 [34, R0 ¼ iPr, cyclohexyl (Cy)]70 and [Me3Si(Ar)N]2VCl2 (35, Ar ¼ 2,6-Me2C6H3),71 prepared from VCl3(THF)3 by treatment with the corresponding lithium amide, were used as catalyst precursors for ethylene (co)polymerization in the presence of Et2AlCl
EtAlCl2 cocatalyst (Scheme 7).70,71 The dialkyl (diphenyl) complexes were prepared by treatment of (Cy2N)2VCl2 with 2 equiv. of LiCHt2Bu, LiPh, PhCH2MgCl.70 Vanadium(IV) tetraaryl complexes, V(ArCl)4 (36, ArCl ¼ C6Cl5, 2,4,6-Cl3C6H2, 2,6-Cl2C6H3), were prepared from [Li(THF)4][V(ArCl)4], which was prepared by reaction VCl3(THF)3 with LiArCl, upon oxidation (with Cl2 etc.).72 Analogous approach for synthesis of V(Mes)4 (Mes ¼ 2,4,6-Me3C6H2) was known previously,73 whereas V(CH2SiMe3)4 was obtained from VCl4 by treating with LiCH2SiMe3 in petroleum ether.74

594

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

b-Diketiminate ligands are widely used as supporting ligands in organometallic chemistry due to the capability of stabilizing various oxidation states and coordination environments.75 As shown in Scheme 8, the vanadium(III) dichloride complex, (nacnac) VCl2 (37, nacnac ¼ HC(C(Me)NAr0 )2, Ar0 ¼ 2,6-iPr2C6H3),76 was treated with KC8 in toluene to afford a toluene-bridged vanadium(I) complex, (m-
6:
6-C7H8)[V(nacnac)]2 (38).77 The dinuclear vanadium(I) complexes [V{m-(
6-ArN)C(Me)CHC(Me)C(NAr)}]2 (39) was obtained as dark brown crystals by two-electron reduction of [HC(C(Me)NAr)2]VCl2 (Ar ¼ 2,6-Me2C6H3, 2,6-Et2C6H3, 9-anthracenyl) and KC8 in THF.78 Treatment of 15 with LiNAr(p-tol) [Ar ¼ 2,4,6-Me3C6H2, p-tol; p-tol ¼ p-tollyl] yielded (nacnac)VCl[N(p-tol)2] (40),78 and vanadium(II) complex, (nacnac)V[N(p-tol)2] (41) was obtained by subsequent reaction of 40 with KC8 or Na/Hg.79 Treatments of 41 with PhC^CPh gave (nacnac)V(
2-C2Ph2)[N(p-tol)2].80

Scheme 8

Vanadium(IV)-alkylidene complex, [(nacnac)V(CHtBu)(THF)]+[BPh4]− (42), was prepared via oxidatively induced a-hydrogen abstraction of (nacnac)V(CHt2Bu)2 (41), prepared from (nacnac)VCl2 (37), by treating with AgBPh4 in THF (Scheme 9).81 Treatment of 42 with 0.5 equiv. of I2 or MgI2 gave (nacnac)V(CHtBu)(I) (43),81 and subsequent reaction with LiCH2SiMe3 gave the alkyl, alkylidene complex, (nacnac)V(CHtBu)(CH2SiMe3) (44).23,82,83 Reaction of 42 with LiPHR in Et2O gave the alkylidene, phosphinidene complex, (nacnac)V(CHtBu)(PR) [45, R ¼ 2,4,6-iPr3C6H2, 2,4,6-tBu3C6H2).23,84 Treatment of 44 with AgOTf (Tf ¼ CF3SO2) or AgBPh4 gave (nacnac)V(CHtBu)(L) [46, 46-OTf or 46-THF-BPh4, Scheme 9],82 which were stable in solid but were converted in solution to give the (arylimido)vanadium(IV) complexes containing amide-vinyl ligand, [tBuC]C(Me)CHC(Me)N (Ar0 )]V(NAr0 )(OTf ) or [{tBuC]C(Me)CHC(Me)N(Ar0 )}V(NAr0 )(THF)]+[BPh4]− (47).82 Ligand deprotonation of the neutral alkylidyne, (nacnac)V^CtBu(OTf ) (46-OTf), with KCH2Ph or LitBu in THF or Et2O, resulted in formation of another alkylidene, (nacnac)V^CtBu(L) (48, L¼ THF, Et2O), and subsequent reaction with B(C6F5)3 gave zwitterionic alkylidyne (49) containing weakly coordinating borate moiety (WCA-nacnac).84 Reaction of 46-OTf with Li[B(C6F5)4](Et2O) gave the cationic alkylidyne, [(nacnac)V^CtBu(L)][B(C6F5)4] (50), which was also obtained by reaction of 48 with [PhN(H)Me2][B(C6F5)4].85

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

595

Scheme 9

5.09.2.3

Vanadium complexes containing tridentate, tetradentate ligands

As described in the introduction, classical Ziegler type vanadium catalyst systems display unique characteristics in olefin polymerization,14,17,18 yielding ultrahigh molecular weight polymers.1–7 Bis(amidinate)vanadium(III) methyl complexes, [PhC(NSiMe3)]2VMe (51a), which was prepared from [PhC(NSiMe3)2]VCl2(THF)286 by treatment with MeLi, showed catalytic activity for ethylene oligomerization without any additional cocatalysts, affording linear a-olefins with Schultz-Flory distribution.87 Introduction of C6F5 in place of Ph led to increase in the activity and the molecular weight of the resultant oligomers.87 Vanadium(III) trichloride complex containing bis(imino)pyridine ligand (52, Scheme 10) showed high catalytic activity for ethylene polymerization in the presence of MAO.88 Vanadium(II) (53) and vanadium(III) (54) complexes containing bis(pyrrolide)phenyl ligand were prepared by treating VCl3(THF)3 or VCl2(tmeda)2 with the corresponding K salt, and subsequent reaction with KH gave a dinuclear, diamagnetic vanadium(I), [1,3-bis-{[(10 -pyrrol-2-yl)-1,10 -dimethyl]methyl}benzene]V}2] (55), the bridging interaction between the two metal centers through sharing coordination of the benzene ring.89 Vanadium(III) complex containing dianionic tridentate bis(pyrrolide) pyrrole (MeTP) ligand, [(MeTP)VCl(THF)]
THF (56), was reduced by Na to yield the divalent complex (57) where the central N-methylated ring adopted a more regular p-orientation. Upon treatment of 57 with a strong Lewis acid (AlMe3), THF was extracted from the vanadium coordination sphere, resulted in formation of the dinuclear dinitrogen complex (58). Reduction of 58 with potassium graphite gave cleavage of dinitrogen, affording the mixed-valent nitride-bridged complex.90 Dinuclear vanadium(III) chloride complex containing bis(amido)amine ligand, (Me3SiNCH2CH2)2NSiMe3 (59), exhibited remarkable catalytic activity for ethylene polymerization in the presence of MAO, Me2AlCl at 50  C;91 the activities decreased at higher temperature. The catalyst system was short-lived, remaining active for no more than 20-30 min, probably due to the reduction of the vanadium(III) center to an inactive divalent species. The reaction of 59 with AlMe3, Me2AlCl and MAO in hexane initially gave a red solution, which after a few days afforded another vanadium(II) complex identified as 60b, suggesting that no ligand dissociation occurred in the present catalyst system but aggregation with the cocatalyst. The reaction of 59 with AlCl3 afforded disproportionated compounds, (tetravalent) 60c and (one divalent and two trivalent) 60d, explaining that disproportionation was the basis of a reduction of the vanadium center. These results suggested the possibility of the reduction of 59 affording 60b and that the addition intermediate 60a, which is likely to be the catalytically-active species, has the intrinsic instability of a vanadium-carbon bond.91

596

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

Scheme 10

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

597

Reaction of L0 VCl2 [61, L0 ¼ HC{Ph2P]N(SiMe3)}2] with 2 equiv. of MeLi in Et2O afforded the dimethyl complex (62) and subsequent treatment with H2 gave the dimeric vanadium(II) complex [L0 V]2(m-H)2 (63).92 Oxidative addition to the styrene gave dinuclear vanadium(III) complex, [L0 V]2(H)(m-H)2(m,
1:
2-CHCHPh), and the room-temperature hydrogenolysis re-formed 63 and ethylbenzene catalytically.93 The subsequent reaction with ethylene afforded the mixed-valence [L0 V]2(m-H)3 and a mixture of 2-butene and 3-methyl-1,4-pentadiene.94 Reaction of 61 with either 1.0 or 1.5 equiv. of MeLi followed by hydrogenolysis of the resulting solutions afforded the dinuclear and mixed-valence V(II)/V(III) species [L0 V]2(m-Cl)2(m-H) and [L0 V]2(m-Cl)(m-H)2, respectively.92 Vanadium(III) complexes containing [ONNO] tetradentate amine bis(phenolate) ligand, VCl[(O-2,4-Me2C6H2-6-CH2)2 (Me2NCH2CH2)N](THF) (64) was prepared by reaction of VCl3(THF)3 with [(KO-2,4-Me2C6H2-6-CH2)2(Me2NCH2CH2)N] in THF.95 Treatment of 64 with PhCH2MgCl gave the corresponding benzyl complex (65).96 Complex 64 showed notable catalytic activity for ethylene polymerization in the presence of EtAlCl2, but showed negligible activity in the presence of MAO.96 Treatment of methylene bridged anilide(bisphenolate) complex (66) with 2-butyne gave the metallacyclopropene complex (67).97 Vanadium(III) dichloride complexes containing another monoanionic tridentate ligand [PNP0 ¼ 2,5-bis(dialkylphosphinomethyl) pyrrolide], VCl2(PNP0 ) (R ¼ tBu, iPr) was treated with 1 equiv. of lithium phenoxide or LiAr (Ar ¼ 2,6-Me2C6H3) gave VCl(Y)(PNP0 ) [68, R,Y ¼ tBu,OAr (a); tBu,OC6H5; tBu,O-2,6-iPr2C6H3; iPr,OAr; tBu,Ar].98 Vanadium diiodide complexes containing PCCP ligand, VI2(ArC]CAr) (Ar ¼ 2-iPr2P-C6H4), which was prepared by reaction of VI2(THF)4 with ArC^CAr, was treated with Mg(CH2SiMe3)2 gave the dialkyl complex, V(CH2SiMe3)2(ArC]CAr) (Scheme 10).99 Vanadium(III) dialkyl complex containing monoanionic tridentate ligand, V(CHt2Bu)2(PNP) (69), prepared from VCl3(THF)3 by treating with Li(PNP) [PNP ¼ N{4-Me-2-(PiPr2)C6H3}-2] followed by addition of LiCHt2Bu,88 has been used for synthesis of vanadium(V)-alkylidenes upon oxidative additions.100–102 Addition of chalcogens to vanadium(III) dialkyl complex containing tridentate PNP ligand, V(CHt2Bu)2(PNP) (69), gave the corresponding vanadium(V)-alkylidene chalcogenide species, V(X)(CHtBu)(PNP) (70, X ¼ O, S, Se, Te, Scheme 11) upon oxidation.100–102 Similarly, the reaction with N2CPh2 in Et2O gave the alkylidene-hydrazido complex, V(CHtBu)(]NdNCPh2)(PNP);100 the alkylidene, phosphinidene complex (71) was also obtained by reaction with Me3P]PAr00 (Ar00 ¼ 2,4,6-tBu3C6H2).102 The bridging end-on dinitrogen–alkylidene, [(PNP)V(CHtBu)]2

Scheme 11

598

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

(m2,
1:
1-N2) (72) was generated if a toluene solution of complex (69) was placed under nitrogen atmosphere over the course of several days or treating with 1 equiv. of PhHC]PPh3 (or Ph3P]NPh).100 The related vanadium(III)-alkylidene complex (73) was also prepared upon addition of 2,20 -bipyridine by a-hydrogen abstraction.100 Reaction of 69 with 2 equiv. of tBuCN under N2 afforded the bridged (imido)vanadium dinitrogen complex, whereas the reaction of 72 with excess tBuCN did not occur even at 40  C after 48 h.101 These results thus suggest a presence of a transient vanadium(III)-alkylidene intermediates.101 Breaking of the carbon–hydrogen bond of benzene and pyridine is observed with the dialkyl complex (69), and in the case of benzene, the formation of a benzyne intermediate is proposed.103 Reaction of (PNP)V(CHt2Bu)2 with C6D6 affording (PNP)V(C6D5)2 [74, PNP ¼ N{2-PiPr2-4-Me-C6H3}-2] postulated via a vanadium(III)-alkylidene intermediate.103 Further reaction of 74 or 74-d6 in C6D6 or C6H6 upon N2O yielded (PNP)V(O) (
2-C6D4) or (PNP)V(O)(
2-C6H4) (75 or 75-d10). The same reaction of 69 in the presence of N2O afforded, the benzyne complex (75), the proposed intermediate. It thus seems likely that the second C-H (C-D) activation would proceed via the benzyne intermediate.104 Thermolysis of the dialkyl complex, (PNP)V(CHt2Bu)2 (69), in C6D6 afforded the assumed intermediate, (PNP)V[CH(D)tBu](C6D5), with monitoring formation of CHt3Bu and CD2HtBu (by 1H NMR and UV-vis spectroscopy and GC-MS), and subsequent oxidation with N2O yielded a benzyne complex, (PNP)V]O(
2-C6D4) (75), which was also prepared from thermolysis of 69 in C6D6 in the presence of N2O.103 The result thus suggests a-hydrogen abstraction to form [(PNP)N] CHtBu], prior to the C–H bond activation, and (PNP)V[CH(D)tBu](C6D5) could be an assumed intermediate generated by C–H bond activation. As considered in Scheme 11, a stepwise mechanism via a s-bond metathesis of benzene (path 1) or a concerted b-hydrogen abstraction step (path 2), were proposed to yield the vanadium(III) benzyne intermediate, (PNP)V(
2-C6D4) (76).103

5.09.2.4

(Imido)vanadium complexes and some reaction chemistry

Vanadium(V) complexes containing aryl- and/or alkyl-imido ligands, which can be prepared by treating VOCl3 with various arylisocyanates (ArNCO) in octane under reflux conditions, are useful precursors due to their thermal stability as well as better stability toward organometallic reagents.65,105,106 For example, reactions of V(N-2,6-iPr2C6H3)(CH2Ph)3 (77) prepared by treatment of V(N-2,6-iPr2C6H3)Cl3
(THF) with PhCH2MgCl in n-hexane,107 with Ar0 OH or (CF3)3COH gave V(NAr0 )(CH2Ph)2(OAr0 ) or V(NAr0 )(CH2Ph)2[OC(CF3)3], respectively (Scheme 12),107 and the approach is useful for synthesis of a series of vanadium(V)alkyls.15,24,108,109 Placement of appropriate alkyl (aryl) substituent on the phenyl group is in fact important, because the reaction

Scheme 12

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

599

of V(N-p-tol)Cl3 (78, p-tol ¼ 4-MeC6H4) with PhCH2MgCl or MesMgCl (Mes ¼ 2,4,6-Me3C6H2) gave (arylimido)vanadium(IV) dimers, [V(m2-N-p-tol)(CH2Ph)2]2 or [V(m2-N-p-tol)(Mes)2]2, accompanied by reduction.110 Reaction of analogous (dimethylphenylimido)vanadium(V) tribenzyl was treated with 2,6-iPr2C6H3OH to afford the dibenzyl complex (79), which initiated ring-opening metathesis polymerization (ROMP) of norbornene (NBE) in toluene at 25  C.111 A series of (imido)vanadium(V) dichloride complexes containing anionic ancillary donor ligand [phenoxide (80),111–113 ketimide (81),113,114 imidazolidin-2-iminato (82),115 imidazolin-2-iminato (83),104 and phosphinimide (84)116 ligands, Scheme 13] were prepared by treating the (imido)vanadium(V) trichloride complexes, V(NR0 )Cl3, with corresponding lithium phenoxide (LiOAr) or phenol (ArOH), lithium ketimide [LiN]C(tBu)R, R ¼ tBu, CH2SiMe3], lithium iminoimidazolidide 1,3-R2(CH2N)2C]NLi (R ¼ tBu, 2,6-Me2C6H3, 2,6-iPr2C6H3, Ph), 1,3-R2(CHN)2C]N-SiMe3 [R ¼ tBu, 2,6-Me2C6H3, 2,6-iPr2C6H3, 2,6-(Ph2CH)24-MeC6H2] or Me3Si-N]PR3 (R ¼ Ph, iPr, tBu). As described below, these are used as precursors for synthesis of various alkyl complexes as well as alkylidene complexes. The phenoxide complexes (80a-c) as well as the imidazolin2-iminato and the imidazolidin-2-iminato complexes (82, 83) showed high catalytic activities for not only ethylene polymerization, but also ethylene copolymerization with norbornene (NBE) in the presence of MAO or Et2AlCl cocatalysts.104,111,117–119

Scheme 13

600

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

These ketimide complexes also showed catalytic activities for ethylene polymerization in the presence of Al cocatalyst;113,120 These arylimido-phenoxide (80a-c) or -ketimide (81) complexes also catalyzed ROMP of NBE in the presence of MeMgBr and PMe3.121 Similarly, the dichloride complexes containing anionic N-heterocyclic carbene–borate (WCA-NHC) ligands, V(NR0 ) Cl2(WCA-NHC) (85, R0 ¼ Ad, C6H5, 2,6-Me2C6H3), were prepared by treating V(NR0 )Cl3 with lithium salt in toluene. V(N-2,6-Me2C6H3)Cl2(WCA-NHC-Ar0 ) (85e, Ar0 ¼ 2,6-iPr2C6H3) exhibited the highest catalytic activities for ethylene polymerization among the reported (imido)vanadium(V) dichloride complexes containing anionic ancillary donor ligands in the presence of Al cocatalyst.111–115,117–120,122–125 In particular, 85e exhibited the highest activity in the presence of AliBu3 cocatalyst,126 which is ineffective as Al cocatalyst in ordinary olefin polymerization catalysts. As shown in Scheme 14, treatment of the (imido)vanadium(V) dichloride complexes with organolithium or Grignard reagents gave the corresponding dialkyl complexes; reactions of V(NAr)Cl2(N]PtBu3) (84) with LiMe or PhMgBr gave the corresponding dimethyl, diphenyl complexes (86), respectively.116 Reactions of V(NAr)Cl2(N]CtBu2) (81a), V(NAr0 )Cl2[1,3-Ar0 2(CHN)2C]N] (83c), or V(NR0 )Cl2[1,3-Ar0 2(CH2N)2C]N] (82) with 2 equiv. of LiCH2SiMe3 gave the corresponding dialkyl complexes, V(NAr) (CH2SiMe3)2(N]CtBu2) (87),114 V(NAr0 )(CH2SiMe3)2[1,3-Ar0 2(CHN)2C]N] (88),104 V(NR0 )(CH2SiMe3)2[1,3-Ar0 2(CH2N)2C] N] (89),127 respectively. The corresponding vanadium(V)-alkylidenes containing ketimide, imidazolin-2-iminato, or imidazolidin2-iminato ligands (90, 91, 92), which catalyze ROMP of cyclic olefins, could be obtained by a-hydrogen abstraction from the dialkyl complexes upon addition of excess PMe3 (Scheme 14).104,109,114,127,128 Degree (rate) of the a-hydrogen abstraction was dependent upon the imido substituent as well as nature of the anionic donor ligand; the phenylimido-iminoimidazolidide analog (92c) required 9–10 days at 70  C for completion, whereas the reaction of the 2,6-dimethylphenyl analog (92a) completed after 24 h at 60  C probably due to steric crowding to induce the a-hydrogen abstraction.127

Scheme 14

Reactions of V(NR)(CH2SiMe3)3 (R ¼ aryl, adamantyl) with 1.0 equiv. of phenols and alcohols in n-hexane took place cleanly at 25  C to afford the corresponding dialkyl complexes in high yields (Scheme 15).129–136 The resultant dialkyl complexes were converted to the corresponding alkylidene complexes (93,94) through a-hydrogen abstraction (elimination) upon presence of PMe3 in excess amount,129–133,136 and degree (rate) of a-hydrogen elimination was dependent upon both imido and phenoxy or alkoxy substituents employed.129–133,136 Since cleanly isolation of the dialkyl complexes in the reaction of the dichloride complexes with LiCH2SiMe3 is sometimes difficult,129 an approach using the trialkyl complexes has been used for the synthesis of a series of phenoxide-modified (imido)vanadium(V)-alkylidene complexes.

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

601

Scheme 15

The alkyl-alkylidene complex, V(CHSiMe3)(NAd)(CH2SiMe3)(PMe3)2 (95) was formed from a n-hexane solution containing V(NAd)(CH2SiMe3)3 upon addition of PMe3 (12 equiv.).137 Reactions of the trialkyl complexes in the presence of 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (NHC) gave the corresponding alkyl-alkylidene complexes, V(CHSiMe3)(NR) (CH2SiMe3)(NHC) (96, 97).138 Reaction of the alkyl-alkylidene (95) with 1.0 equiv. of 2,6-Me2C6H3OH gave V(CHSiMe3)(NAd) (O-2,6-Me2C6H3)(PMe3)2 (98) in high yield, whereas the reaction with C6F5OH in C6D6 gave a mixture of the phenoxy-alkylidene (99) and the dialkyl analog, V(NAd)(CH2SiMe3)2(OC6F5), and the PMe3 adduct. The conversion rate of 95 to form 99 was retarded by addition of PMe3 or use of C6F5OD in place of C6F5OH, suggesting that the reaction proceeds via coordination of C6F5OH after dissociation of one PMe3 (Scheme 15).137 A series of (imido)vanadium(V) dichloride complexes containing (2-anilidomethyl)pyridine ligands of the type, V(NR0 ) Cl2[2-(2,6-R2C6H3)NCH2(C5H4N)] [R0 ¼ 2,6-Me2C6H3 (Ar), C6H5, adamantyl, cyclohexyl; R ¼ Me, iPr], were prepared by treating V(NR0 )Cl3 with Li[2-(2,6-R2C6H3)NCH2(C5H4N)] in Et2O.139,140 V(NR0 )Cl2[2-Ar0 NCH2(C5H4N)] [R0 ¼ Ar (100), adamantyl (101); Ar0 ¼ 2,6-iPr2C6H3] was reacted with LiCH2SiMe3 (2.0 equiv.) to yield V(NR0 )(CH2SiMe3)2[2-Ar0 NCH2(C5H4N)] (102, 103b), and the subsequent reaction of 102(iPr) with (CF3)2CHOH (2.0 equiv.) in C6D6 at 25  C to yield V(NAr)(CH2SiMe3)[OCH(CF3)2]

602

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

[2-Ar0 NCH2(C5H4N)], and synthesis of the bis(alkoxo) analog, V(NAr)[OCH(CF3)2]2[2-Ar0 NCH2(C5H4N)] (104), required 3 days for completion even upon addition of 3.0 equiv. of (CF3)2CHOH.139 Similarly, the reactions of V(NAd)Cl2[2-ArNCH2(C5H4N)] (101a) with 2.0 equiv. of LiMe in toluene gave V(NAd)Me2[2-ArNCH2(C5H4N)] (103a).141 Reactions of the dimethyl complex with [Ph3C] [B(C6F5)4] in Et2O gave the corresponding cationic complex, [V(NAd)Me{2-ArNCH2(C5H4N)}(Et2O)]+[B(C6F5)4]− (105), and the reaction with R0 OH (1.0 equiv.) gave the mono alkoxide complexes, V(NAd)Me(OR0 )[2-ArNCH2(C5H4N)] [106, OR0 ¼ OC(CF3)3, OC(CH3)(CF3)2, OC(CH3)3]; further reaction with B(C6F5)3 afforded the corresponding cationic complex (107).141 The (adamantylimido) complexes, V(NAd)Cl2[2-(2,6-R2C6H3)NCH2(C5H4N)] (101, R ¼ Me, iPr), exhibit remarkable catalytic activities for selective ethylene dimerization in the presence of MAO [ex. by 101a TOF 2730000 h-1 (758 sec-1), selectivity ¼ 90.4  99%], whereas the dimethylphenyl analogs (100) exhibited catalytic activities for ethylene polymerization (Scheme 16).139 The cyclohexylimido, and the phenylimido analogs also yielded 1-butene with high selectivity, suggesting that a steric bulk in the imido ligand plays a role for the selectivity (polymerization vs dimerization).140 The electronic nature directly affects the catalytic activity (activity: R ¼ Ad > Cy > Ph).140 Moreover, the dichloride complex containing the 8-(2,6-dimethyl-anilide)5,6,7-trihydroquinoline ligand, V(NAd)Cl2[8-(2,6-Me2C6H3)N(C9H10N)] (108) exhibited the remarkable activities, which are higher than those by 101a, affording 1-butene as the major product [ex. TOF: 9600000 h-1 (2670 sec-1), selectivity ¼ 95.0–99.4%).142

Scheme 16

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

603

The dimethyl complex (103a) showed similar both the activity and the selectivity as the dichloride complex (101a) in the ethylene dimerization.141 On the basis of ethylene pressure dependence,53,143 V NMR spectra (in the presence of Al cocatalysts) and attempted ESR spectra,141,144 it was demonstrated that the anionic chelate ligand plays a role in stabilizing the metal oxidation state in this catalysis. These results thus suggest a possibility that the cationic alkyl complex, [V(NAd)Me{2-ArNCH2(C5H4N)}]+, plays an important role in this catalysis. As described below, the hypothesis was further confirmed by solution-phase V K-edge XAS (X-ray absorption spectroscopy) analysis from the XANES (X-ray Absorption Near Edge Structure) and FT-EXAFS (Extended X-ray Absorption Fine Structure) spectra of 101a and 103a in the presence of Al cocatalysts.141 Metal alkyls in early transition metal complexes are in general more nucleophilic than those in the late transition metal complexes and are thus highly reactive toward Brönsted/Lewis acids. However, some alkyl complexes with high oxidation state, such as tetrakis(alkyl)-chromium(IV)145,146 and zirconium(IV)147 complexes [exemplified as CrtBu4 and Zr(CHt2Bu)4], and the (arylimido)vanadium(V) trialkyl complexes129,134 showed unique stability toward alcohols due to steric crowding. For example, reaction of V(NAr)(CH2SiMe3)3 (Ar ¼ 2,6-Me2C6H3) with phenols and alcohols (1.0 equiv.) generally proceeded at 25  C (Scheme 17), except that the reaction with 2,6-tBu2-4-MeC6H2OH (in C2D2Cl4 at 50  C) did not take place even with excess

Scheme 17

604

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

addition (2.6 equiv.).129 Moreover, synthesis of the bis(phenoxide), V(NAr)(CH2SiMe3)(OAr)2, by treatment of V(NAr) (CH2SiMe3)2(OAr) with ArOH required 50 h at 60  C for completion (no reaction at 25  C).148 Further studies revealed that substitution rate in the reactions of V(NAr1)(CH2SiMe3)3 (Ar1 ¼ C6H5, 2-MeC6H4, 2,6-Me2C6H3, 2,6-Cl2C6H3) with various phenols (1.0 equiv., Ar2OH, Ar2 ¼ 2,6-F2C6H3, 2,6-Cl2C6H3, 2,6-Me2C6H3, 2,6-iPr2C6H3, 2-tBuC6H4, 2,6-tBu2C6H3) was affected by a steric bulk in the arylimido ligand in the order: Ar1 ¼ 2,6-Me2C6H3 < 2,6-Cl2C6H3 < 2MeC6H4 < C6H5.134 The reaction rate of V(NAr1)(CH2SiMe3)3 with various disubstituted phenols increased in the order, irrespective of the kind of the aryl imido ligands: 2,6-iPr2C6H3OH < 2,6-Me2C6H3OH < 2,6-Cl2C6H3OH < 2,6-F2C6H3OH. As described above, the reactions of V(NAr1)(CH2SiMe3)3 with 1.0 (and 3.0) equiv. of 2,6-tBu2C6H3OH did not take place even at 60  C. These results indicate that the reactions proceed via coordination of ArOH to vanadium and steric crowding around vanadium play a role toward the reactivity. The hypothesis was also supported by an immediate phenoxide ligand exchange in V(NAr) (CH2SiMe3)2(OAr) with 1.0 equiv. of C6F5OH at 25  C (Scheme 17).148 Reaction of V(NAr)Me(N]CtBu)(OAr) (108) or V(NAr)Me(N]CtBu)(O-2,6-iPr2-4-tBuC6H2) (109) with 2,6-iPr2-4-tBuC6H2OH or 2,6-Me2C6H3OH (1.0 equiv.) in CDCl3 at 25  C gave a 1:1 mixture of 108 and 109 with a rapid phenoxy ligand scrambling (Scheme 17).149 The solution then gave three species as the bis(phenoxide)s (110-112) upon heating at 60  C for 12 h. Both phenol-scrambling and the phenol/ketimide exchange reaction should be preferred if the phenol approaches the electrophilic vanadium metal center trans to the Me group (NNO face in 108,109, and not the NNC faces), to give pentacoordinated trigonal bipyramidal intermediates (shown in brackets in Scheme 17).149 Reaction of a (1-adamantylimido)vanadium(V)-dialkyl complex containing a chelating alkoxo(imino)pyridine ligand, V(NAd) (CH2SiMe3)2(L0 ) [113, L0 ¼ 6-OC(Me)2-2-{(2,6-iPr2C6H3)N]CMe}dC5H3N] with 1.0 equiv. of (CF3)2CHOH first generated an intermediate, V(NAd)(CH2SiMe3)2(L0 )[(CF3)2CHOH] (confirmed by NMR spectra), formed by coordination of oxygen in (CF3)2CHOH to the vanadium accompanied with dissociation of the two imino group; the formed species were converted to the original (113) by placing the mixed solution in vacuo (Scheme 18). The reaction completed upon heating to give the monoalkyl-alkoxo species, V(NAd)(CH2SiMe3)(L0 )[OCH(CF3)2] (114a), which folds a distorted tetrahedral geometry around vanadium, and two neutral nitrogen donors in L0 were dissociated.150 These results strongly demonstrate that reactions of the dialkyl complex (113) with ROH [R ¼ CH(CF3)2, C(CH3)(CF3)2] proceeded via five coordinate intermediates (Scheme 18) formed by coordination of the oxygen in ROH (not via addition of H+) accompanied by dissociation of two neutral nitrogen donors (imino groups) in the chelate tridentate ligand.

Scheme 18

Reactions of a (arylimido)vanadium(V)-methyl complex, V(NAr)Me(N]CtBu2)2 (115)151 with phenols (1.0 equiv.) gave other methyl complexes, V(NAr)Me(N]CtBu2)(OAr0 ) [Ar0 ¼ 2,6-Me2C6H3 (108), 4-tBu-2,6-iPr2C6H2 (109), Ph, C6F5] exclusively in all cases by substitution with the ketimide ligand and without reaction with the methyl group (Scheme 19).149,151 The reactions with i PrOH, 3-buten-1-ol or 5-hexen-1-ol also gave V(NAr)Me(N]CtBu2)(OiPr) (116), VMe(NAr)(N]CtBu2)-[OCH2(CH2)nCH] CH2] [n ¼ 1, 3 (117)], respectively.149,151 The reaction with 2-Me-6-{(2,6-iPr2C6H3)N]CH}C6H3OH (1.1 equiv.) to afford V(NAr)Me(N]CtBu2)-[O-2-Me-6-{(2,6-iPr2C6H3)N]CH}C6H3] required 3 days for the completion (in C6D6 at 25  C).152 In contrast, the reaction with 2,6-Me2C6H3SH (1.0 equiv.) gave V(NAr)(N]CtBu2)2(S-2,6-Me2C6H3) (118) via cleavage of the V-Me bond,149 and the reaction with n-C6H13SH in n-hexane afforded V(NAr)(N]CtBu2)2(S-n-C6H13).149 Moreover, the catoinic complexes, [V(NAr)(N]CtBu)2(THF)2][MeB(C6F5)3] (120) and [V(NAr)(N]CtBu)2(THF)2][B(C6F5)4] (119), were formed exclusively in the reaction of 115 with B(C6F5)3, [PhN(H)Me2][B(C6F5)4] or [Ph3C][B(C6F5)4] (in THF).149 The crystallographic analysis indicates that 120 adopts a pseudo-tetrahedral geometry around the vanadium with the coordination of one THF molecule.149 The unique contrast in the reactivity between phenol (alcohol) and thiols (and borate) suggests a presence of different mechanism (reaction pathway).

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

605

Scheme 19

Reported methods for synthesis of vanadium-alkylidene complexes are summarized in Scheme 20. a-Hydrogen elimination from the dialkyl complexes lacking b-hydrogen is the common method for generation of early transition metal alkylidenes with high-oxidation state.21–25 Certain assistances such as (i) addition of neutral donor ligand (phosphine etc.) to increase the steric crowding, (ii) addition of a base, and (iii) photochemical stimulation, are often required to induce the reaction.21–25

Scheme 20

606

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

Vanadium(III)-alkylidene, CpV(CHtBu)(dmpe) [14, dmpe ¼ bis(dimethylphosphino)ethane], was prepared from the dialkyl analog by a-hydrogen abstraction with addition of dmpe,63,153 and vanadium(V)-alkylidene, CpV(CHPh)(NAr0 )(PMe3) (20, Ar0 ¼ 2,6-iPr2C6H3), was prepared from CpV(NAr0 )(PMe3)2 by benzylidene transfer with Ph3P]CHPh (upon oxidation).65 A bicyclic vanadium(V)-alkylidene (121) was formed by treating a vanadium(III) borohydride complex with PhC^CPh (excess).154 Vanadium(IV)-alkylidenes, [(nacnac)V(CHtBu)(THF)]+[BPh4]− (42) was prepapred by a-hydrogen elimination induced by oxidation of vanadium(III) dialkyl complex, (nacnac)V(CHt2Bu)2 [nacnac- ¼ {Ar0 N-C(Me)}2CH-] with AgBPh4.81 Several vanadium(V)-alkylidenes (70) were also prepared from the vanadium(III) dialkyl complexes containing monoanionic tridentate ligand, V(CHt2Bu)2(PNP) [PNP ¼ N{4-Me-2-(PiPr2)C6H3}-2], by addition of p-acids or two electron oxidants.100,101 Treating CpV(CHCMe3)(dmpe) (14) with tBuC^N at 60  C afforded CpV[]NC(CMe3)]CHCMe3](dmpe) (122),153 and similar reactions of the ketimide-modified vanadium(V)-alkylidene (90) with RC^N (R ¼ Me, tBu, Ph) also gave ring-opened bis(imido) complexes (123, a mixture of E-, Z- isomers, Scheme 21).155 Reaction of 90 with PhC^CPh (1.0 equiv.) gave the metallacyclobutene, (ArN)V[C(Ph)]C(Ph)CHSiMe3)](NCtBu2)(PMe3) (124),155 and the reaction with styrene (1.0 equiv.) to afford the metallacyclopropane analog, V(NAr)(CH2CHPh)(NCtBu2) (125). Two reaction pathways, (i) formation of the metallacylobutane and subsequent b-hydride elimination156 or (ii) dimeric pathway by-producing Me3SiHC]CHSiMe3, could be considered; the latter pathway via vanadium(III) was considered more likely because the reaction with 1.0 equiv. of styrene yielded 125 exclusively.155 In contrast, reaction products of halogenated alkylidene complexes (126, X ¼ F, Cl) were propylene, allyl trimethylsilane, and paramagnetic vanadium(IV) complexes (127, 128) containing Cl or OC6F5 ligand; formation of propylene suggested a possibility of b-hydrogen elimination pathway.136,157

Scheme 21

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

607

Thermolysis of (arylimido)vanadium(V) dialkyl complexes containing an imidazolin-2-iminato ligand in C6D6 (at 60 or 70  C) gave V(NAr)(CHDSiMe3)(C6D5)[1,3-Ar0 2(CHN)2C]N] (129-d6, Ar0 ¼ 2,6-iPr2C6H3) quantitatively through 1,2-C-H (or C-D) bond activation (Scheme 22). Kinetic studies (conversion to 129-d6) at 70  C showed first-order kinetics; these results excluded a mechanism through a bimetallic intermediate or s-bond metathesis between the dialkyl complex and C6D6.104 The corresponding alkylidene complex (91) was isolated by a-hydrogen abstractions in n-hexane in place of benzene in the presence of PMe3. The fact that D atom in CHDSiMe3 incorporated from C6D6 was confirmed in 129-d6 by a 2H NMR spectrum, whereas no resonances corresponding to D atoms in CD2SiMe3 (in the 2H NMR spectrum) nor protons in CH2SiMe3 (in the 1H NMR spectrum) were observed. Further thermolysis of 129 or gave the diphenyl complex (130), and the reaction in the presence of PMe3 gave the benzyne complex, V(NAr)(
2-C6H4)[1,3-Ar0 2(CHN)2C]N] (131),104 proposed as intermediate for formation of the diphenyl complex. The results also support a mechanism as postulated in the reaction of (PNP)V(CHt2Bu)2 (69) with C6D6 affording (PNP)V(C6D5)2 [74, PNP ¼ N{2-PiPr2-4-Me-C6H3}-2] via a vanadium(III)-alkylidene intermediate, [(PNP)N]CHtBu] (Scheme 11).103 The same reaction of 69 in the presence of N2O afforded (PNP)V(O)(
2-C6H4) (75 or 75-d10); the benzyne complex (76), the proposed intermediate in the second C-H (C-D) activation.104

Scheme 22

5.09.3

Organoniobium complexes and related chemistry

5.09.3.1

Niobium complexes containing cyclopentadienyl, hydridotris(pyrazolyl)borate ligands

As described in the introduction, niobium-alkylidene complexes, as demonstrated for synthesis of Cp2Nb(CHtBu)Cl,29 Nb(CHtBu) (CHt2Bu)3,30 play an important role in organometallic chemistry containing multiple metal-carbon bonds.21–25 Reaction of substituted ansa-niobocene dichlorides, [Me2Si(Cp0 )2]NbCl2 with EtMgBr in Et2O gave [Me2Si(C5H4)(3-RC5H3)]NbH(ethylene) [R ¼ iPr (132a), tBu (132b)] or [Me2Si(3-tBuC5H3)2]NbH(ethylene) (132c, 50:50 mixture of meso- and rac- forms, Scheme 23), and the structures (the ethylene is coordinated in the more open portion of the metallocene wedge, away from the isopropyl substituent,

608

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

Scheme 23

the distal isomer) were determined by X-ray crystallography. The reaction with PhCH2CH2MgBr gave the styrene bridged [Me2Si (C5H4)(3-tBuC5H3)]NbH(ethylene) (132d).158 Reaction of (
5-C5H5)[
5-C5H4-SiMe2(CH2CH]CH2)]NbCl2 with 2 equiv. of RMgCl (R ¼ Me, PhCH2) or LiCH2SiMe3 gave olefin coordinated (
5-C5H5)[
5-C5H4-SiMe2(CH2-
2-CH]CH2)]NbR (135, R ¼ Me, CH2Ph, CH2SiMe3).159 Bimetallic cationic vinylidene [(Me3SiC5H4)2Nb{]C]C(Me)]}(CO)]2[BPh4]2 (135) was prepared from (Me3SiC5H4)2Nb(RC^CR0 ) (134) by treating with [Cp2Fe][BPh4] (1 equiv.),160 whereas reactions of 134 conducted under the similar conditions afforded cationic acetylene complexes coordinated CH3CN (136, Scheme 23).160 Treatment of (tBuC5H4)2Nb(CH2Ph)2 (137) with AgBPh4 afforded a cationic benzylidene, [(tBuC5H4)2Nb(CHPh)][BPh4] (138), which is kinetically unstable and was converted to the cyclometallated product (139) via C-H bond activation.161 There were also reports concerning synthesis and reaction chemistry of half-metallocene type niobium-alkylidenes including in situ formation. Reactions of Cp NbCl4 with LiCH2SiMe3 afforded the alkyl, alkylidene complex, Cp M(CHSiMe3) (CH2SiMe3)2.162 Cp NbMe2(
4-C4H6) (140), which was thermally unstable and generated the methylidene species in situ, was added norbornene or acenaphthylene to afford the metallacyclobutane derivatives (141, 142), respectively (Scheme 24); complex 141 was found to be a catalyst for the ROMP of norbornene.163. Synthesis of dibenzyl complex containing 1,4-diaza-1,3-butadiene ligand (144) was prepared from the corresponding dichloride complex (143) with Mg(CH2Ph)2; reaction of 143 with [slight excess of 1,3-butadiene-magnesium adduct, [Mg(diene)(THF)]n, gave mixed-ligand complexes, Cp Nb(
2-N,N0 -p-MeOC6H4-dad) (
4-s-cis-1,3-butadiene) (145).164 Cp analog (146) was also prepared in the same manner.164

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

609

Scheme 24

Synthesis of half-sandwich imido complexes bearing alkyne, benzyne and benzylidene ligands were reported by Gibson et al.165 Reactions of Cp Nb(NAr0 )Cl2 (147, Ar0 ¼ 2,6-iPr2C6H3) with 2 equiv. of KCH2Ph gave the dibenzyl complex (148), and treatment of 148 with PMe3 afforded the benzylidene complex, Cp Nb(CHPh)(NAr0 )(PMe3) (149), by a hydrogen elimination (Scheme 24).165 CpNb(NAr0 )Cl2 (150) reacted with PhMgCl in the presence of PMe3 to give CpNb(NAr)(C6H5)2(PMe3) (151), which converted to the
2-benzyne complex (152) upon heating at 70  C in heptane.165 Reduction of the dichloride complexes (147,153) with Mg in the presence of PMe3 (excess) afforded Nb(III) complex (154,155), that easily reacted with appropriate alkyne to give the adduct (156,157), respectively.165 Insertion of arylisocyanide (ArNC, Ar ¼ 2,6-Me2C6H3) to various chloro alkyl or dialkyl imido complexes (
5-C5H4SiMe3)Nb(NAr)(R)X (158, R,X ¼ Me,Cl; Me,NMe2; Me,Me; CH2SiMe3,CH2SiMe3; CH2Ph, CH2Ph; CHt2Bu,CHt2Bu) yielded
2-iminoacyl derivatives, Cp0 Nb(NAr)(X)[
2-ArN]CR] (159).166 Bimetallic half-niobocene complexes bridged by 1,4- or 1,3-phenyldiimido ligands, [Cp0 NbCl2]2(1,3-NC6H4N) or [Cp0 NbCl2]2(1,4-NC6H4N) (Cp0 ¼ Cp, Me3SiC5H4, C5Me5), were prepared by reaction of Cp0 NbCl4 with 1,3- or 1,4-(Me2N)2C6H4 in CDCl3, and treatment of the Me3SiCp analogs with MeLi, Me3SiCH2MgCl, Mg(CH2Ph)2(THF)2 gave the corresponding dialkyl complexes, [(Me3SiC5H4)NbR2]2(1,3-NC6H4N) (160) or [(Me3SiC5H4)NbR2]2(1,4-NC6H4N) (161), respectively (Scheme 24).167 Similar to complexes 159, insertion of arylisocyanide (ArNC)

610

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

gave the dimeric, [(Me3SiC5H4)NbR(
2-ArN]CR)]2(1,3-NC6H4N) (162) or [(Me3SiC5H4)NbR(
2-ArN]CR)]2(1,4-NC6H4N) (163), respectively.168 A series of [
5-1-(Me3Si)-3-(Me2ClSi)-C5H3]Nb(CH3)n(Cl)4-n [n ¼ 1 (164), 2 (165), 4 (166)] or [
5-1,3(Me3Si)2C5H3]NbMe4 (167) were prepared from the trichloride by treatment with ZnMe2 or MeMgCl.169 Reaction of Tp NbCl2(PhC]CR) (168, Tp ¼ hydridotris(3,5-dimethylpyrazolyl)borate) with allyl Grignard, CH2] CHCH2MgCl (1 equiv.), gave five-membered niobacycles (169) through an allyl-alkyne coupling reaction accompanied by a 1,3-hydrogen shift in the allyl moiety (Scheme 25).170 Treatment of the dichloride (168) with NaOCH3 afforded Tp NbCl(OCH3) (PhC]CR0 ) (170), and the complex was reacted with MeLi to give the methyl, methoxy complex (171). Subsequent CO insertion afforded alkyne coupled products (172).171 Reactions of the dichloride complex (168) with 2 equiv. of MeLi or PhCH2MgCl gave the corresponding dialkyl complexes (173,174), respectively.172 The reaction (168, R0 ¼ Me) with EtMgCl eventually gave the five-membered niobacycle Tp Nb(CH2CH3)[C(Ph)C(CH3)CHCH2] containing Nb-carbon double bond (resonance at 233.5 ppm in 13C NMR spectrum in C6D6), probably formed by dehydrogenation of one ethyl group and subsequent coupled to the coordinated phenylpropyne.172 Moreover, the 1H NMR spectrum strongly suggested the presence of an a-agostic interaction.172

Scheme 25

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

611

The dimethyl complex (173, R0 ¼ Me) showed catalytic activity for ethylene polymerization in the presence of B(C6F5)3 (100 kg-PE/ mol-Nb
h, ethylene 1 atm, toluene), and introduction of Cl into the Tp ligand (Tp0 ¼ hydridotris(4-chloro-3,5-dimethylpyrazolyl)borate) improved the activity (130 kg-PE/mol-Nb
h).173 Reaction of the dimethyl complex (176) with borate, [H(OEt2)2][BArF4] [ArF ¼ 3,5-(CF3)2C6H3], gave the Et2O coordinated cationic complex (177), and Et2O was replaced with PMe2Ph or PEt3.108 Reaction of Tp NbCl3 with terminal alkyne (RC^CH) in the presence of Zn gave Tp NbCl2(RC]CH) (178), and, as shown in Scheme 25, the stable rotational isomer was different from that in Tp NbCl2(PhC]CR) (168).174 Reaction of Tp NbCl2(PhC]CHR0 ) (168, R0 Me, Et, nPr, Ph) with RCH2MgCl in toluene/Et2O gave the a-agostic n-alkyl complexes, Tp NbCl(CH2R)(PhC]CMe) (179, R ¼ Me, Et, SiMe3), and the thermolysis in toluene (>338 K) gave the methyl complexes Tp NbCl(Me)(PhC]CCH2R) (180) via alkyl exchange, through a rate determining reversible migratory insertion of the alkyl group onto the alkyne.175 It seems likely that the reaction proceeds via Tp NbCl(Me)[]C(Ph)C(Me)CH2R]. In contrast, a mixture of alkyl exchange products were formed on the thermolysis and rotation of Tp NbCl(CH2CH3)(PhC]CCH2CH2CH3) and Tp NbCl(CH2CH2CH3)(PhC]CCH2CH3) (181).175 Moreover, compounds arising from the migratory insertion into the alkyne are not observed (through 1H NMR spectra) for the secondary alkyl compounds. Instead, isomerization of the secondary alkyl group to a primary one occurs; for example, as shown in Scheme 25 (bottom right), thermolysis of Tp NbCl[CH(CH3)CH2R](R0 C]CCH3) (182, R ¼ H, Me; R0 ¼ Me, Ph) at 343 K gave more stable linear a-agostic n-alkyl complexes, Tp NbCl[CH2CH2CH2R](R0 C]CCH3) (183) via rearrangement through b-hydrogen elimination.176 Reaction of Tp NbCl(Et)(PhC]CEt) with PhC^CCH3 gave Tp NbCl[C(Ph)]CEt2](PhC]CMe) (184).175 As shown in Scheme 26, facile C–H bond activation of benzene under mild conditions occurred in benzene solution containing the cyclopropyl-methyl complex (185) to afford the phenyl-cyclopropyl complex (186), and thermolysis of 185 or 186 in benzene

Scheme 26

612

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

at 323 K gave the diphenyl complex, Tp Nb(C6H5)2(MeC]CMe) (187).176 Reaction of 185 in C6D6 gave a mixture of isomers, indicating that formation of an unsaturated
2-cyclopropene complex (niobiabicyclobutane intermediate) from 185 by a b-H or 1,3-abstraction of CH4 and subsequent addition of CH/CD bond of C6H6/C6D6 across a Nb-C bond in a stereospecific 1,3-fashion.177 Formation of the diphenyl complex (124) was proposed via benzyne intermediate (188), since the reaction of 186 in the presence of excess CO or PMe3 in cyclohexane afforded Tp Nb(
2-C6H4)(MeCCMe)(L) (189, L ¼ PMe3, CO).178 Similarly, treatment of 185 with pyridine in n-pentane gave Tp Nb(cyclo-C3H4)(MeCCMe)(pyridine) (190).178 Reactivity of the cyclopropyl-methyl complex (185) with alkylbenzenes were explored.177,179 The reaction with toluene gave the bis(p-tolyl) complex, Tp Nb(p-MeC6H4)2(MeC]CMe),177 the reaction with mesitylene in cyclohexane at 308 K gave the 3,5-dimethylbenzyl complex (191); the reaction with o-xylene gave 3,4-dimethylphenyl complex (192).179 The reaction of p-xylene gave the 4-methylbenzyl complex (193) and the reaction with m-xylene gave a mixture of the 3,5-dimethylphenyl complex (194) and the 2-methylbenzyl complex (195) with 3:1 molar ratio (Scheme 26).179 On the basis of computational studies, benzylic activation dominates in cases where all aromatic groups lie ortho to a methyl group, whereas the aromatic activation pathway is preferred, both kinetically and thermodynamically.179 As described above, the niobiabicyclobutane intermediate can be considered as the important intermediate. The reaction of 185 with CD4 gave a mixture of deutrated methyl-cyclopropyl complexes (185-d4) and rapid methane exchange was also confirmed by the reaction with 13CH4.180 The reaction of 3,5-dimethylbenzyl complex (191), formed by treatment of 185 with mesitylene, with methane (40 bar) in C6F6 gave the methyl complex (185).180 Various substrates such as C6F5H, furane or thiophene, cyclopentene, phenyl acetylene, and ferrocene, were reacted with 185 to afford the corresponding products via C–H activation181; the ferrocenyl analog (197) was further treated with benzene to afford the phenyl complex (186).180

5.09.3.2

Niobium complexes containing monodentate anionic donor ligands

Treatment of NbCl5 with LiC6Cl5 (5 equiv.) in Et2O followed by addition of 2 equiv. of [NBu4]Br (after extraction with CH2Cl2) gave Nb(III) tetraphenyl complex, [NBu4][Nb(C6Cl5)4], and corresponding Nb(IV) complex, Nb(C6Cl5)4, was obtained upon oxidation with Br2 in CCl4.182 Their four coordinate tetrahedral structure was determined by X-ray crystallography.182 Reaction of NbOCl3 with LiC6F5 (5 equiv.) in Et2O followed by addition of 1 equiv. of [NBu4]Br gave (oxo)niobium(V) pentaphenyl complex, [NBu4]2[Nb(O)(C6F5)5]. The crystallographic analysis revealed its pentagonal-pyramidal structure with coordination of the oxo ligand in the apical position that is tightly bound to the niobium center.183 Treatment of PhLi or MeLi to a diethyl ether solution containing NbCl[N(Ad)Ar]3 (Ad ¼ 1-adamantyl; Ar ¼ 3,5-Me2C6H3), prepared by reaction of NbCl4(THF)4 with LiN(Ad)Ar gave the corresponding methyl (phenyl) complex (198, Scheme 27); their monomeric tetrahedral geometry around niobium was determined by crystallographic analysis.184 NbCl2[N(Ar)iPr]3 (199) was treated with Na/Hg in THF to yield the niobium(IV) mono chloride complex, NbCl[N(Ar)iPr]3 (200), and the subsequent reaction with Na/Ph2C]O yielded the oxacyclopropane, Nb(
2-OCPh2)[N(Ar)iPr]3 (201).185 The reaction of 199 with AgOTf followed by addition of LiBH4 gave
3-borohydride metallaaziridine complex (202) and subsequent treatment with quinuclidine (to remove BH3) gave the hydride complex (203),185 as a dimeric form confirmed later by X-ray crystallographic analysis.186 The reaction of 203 with benzophenone gave the diphenylmethoxide complex (204).185 Reaction of dimeric [(Me3SiCH2)2Nb (
2-CSiMe3)2Nb(CH2SiMe3)2] (205)187 with excess of carbazole (cb) gave another dimeric [(cb)2Nb(
2-CSiMe3)2Nb(cb)2] (206), and addition of 2,6-dimethylphenyl isocyanide gave the inserted product (207).188 Reactions of NbCl3(OAr0 )2 (208, Ar0 ¼ 2,3,5,6-Ph4C6H) with Me3SiCH2MgCl, PhCH2MgCl were explored, and the reaction with 1 or 2 equiv. of the Grignard reagents gave the corresponding mono- (209) and dialkyl complexes (210,211); heating a C6D6 solution containing 211 gave the chloro-alkylidene complex, Nb(CHSiMe3)(Cl)(OAr0 )2 (212) observed by NMR spectroscopy.189 Attempts to synthesize trialkyl complex, Nb(CH2SiMe3)3(OAr0 )2 (213) accompanied formation of the alkyl-alkylidene complex, Nb(CHSiMe3)(CH2SiMe3)(OAr0 )2 (214), together with niobium(IV) dialkyl complex, Nb(CH2SiMe3)2(OAr0 )2 (215). Complexes 214 and 215 were cocrystallized (their structures were determined as a 1:1 mixture) and could be separated first from the trialkyl complex (150), and alkylidene complex (214) was also separated from 215 (Scheme 27).189

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

613

Scheme 27

Treatment of NbCl(CH3)2Cl(OAr00 )2 [216, Ar00 ¼ 2,6-(Ph2C)2-4-tBuC6H2], which was prepared from reaction of [Nb(CH3)2Cl3]2 with 2 equiv. of NaOAr00 , with 2 equiv. of H2C]PPh3 gave the terminal methylidene complex, Nb(CH2)(CH3) (CH2PPh3)(OAr00 )2 (217) (Scheme 27).190 In contrast, the bridged methylidene [(Ar00 O)2Nb]2(m2-Cl)2(m2-CH2)] (218) was obtained exclusively (90% isolated yield) by thermolysis of 216 at 80  C for 5 days.191 The methylidene complex 218 was also obtained via photoirradiation (Xe lamp) of 216 in benzene (18 h); the complex 218 was also obtained from NbCl(CH3)2(OAr00 )2(DMAP) in benzene at 80  C for 5 days. Reduction of 218 with KC8 (2 equiv.) gave methylidyne complex, [K][{(ArO)2Nb}2(m2-CH)(m2-H)(m2-Cl)] (219), via a-hydrogen elimination. Oxidation of 219 with 2 equiv. of ClCPh3 gave the starting 218.191 Further reactions of Nb(CH2)(CH3)(CH2PPh3)(OAr00 )2 (217) with AdNH2 (Ad ¼ 1-adamantyl), PhPH2 gave the corresponding phosphinidene (220), imide (221) complexes, respectively.192 NbCl2(OSitBu3)3 was treated with Na/Hg in pyridine to give Nb(
2-pyridine)(OSitBu3)3 (222) and thermolysis of 222 in benzene led to a ring cleave to afford the imido-alkylidene complex, (OSitBu3)3Nb]CH-CH]CH-CH]CH-N]Nb (OSitBu3)3 (223, Scheme 28).193 Similarly, NbCl2(OSitBu3)3 was also treated with Na/Hg in olefin (excess) to afford olefin coordinated Nb(OSitBu3)3(olefn) [224, olefin ¼ cyclooctatetraene (COT), cyclohexene, 1-butene], and the reaction with

614

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

Scheme 28

4-picoline gave Nb(
2-picoline)(OSitBu3)3 (225).194 Complex 225 was shaken with Ta(OSitBu3)3 in cyclohexane followed by addition of Nb(OSitBu3)3(COT) gave a dimeric [Nb(OSitBu3)3]2(COT) (227), and thermolysis of 226 or 225 in benzene afforded a dimeric alkylidene-olefin complex (228).194 Similar alkene-alkylidene rearrangement reactions were observed in the cyclohexene, 1-butene complexes to afford the corresponding alkylidene complexes (229,230), respectively (Scheme 28).194 Detailed kinetic studies concerning substitution rate (with ethylene) and rearrangement in niobium and tantalum complexes were also reported (also described below, Scheme 38).195

5.09.3.3

Niobium complexes containing bidentate or tridentate anionic donor ligands

As described above, b-diketiminate ligands are widely used as supporting ligands in organometallic chemistry due to their capability of stabilizing various oxidation states and coordination environments.75 Cationic niobium(V)-methyl complexes, [(nacnac)Nb(NtBu)Me]+ A- [A- ¼ Me(B(C6F5)3 (232), B(C6F5)4 (233)], were prepared from the dimethyl complex (231)196 by treating with B(C6F5)3 or Ph3CB(C6F5)4 in chlorobenezene [or {(Et2O)2H}B(C6F5)4 in Et2O, Scheme 29].197 The dimethyl complex was prepared from (nacnac)Nb(NtBu)Cl2(pyridine) by treating with MeMgBr; the reaction with Mg(4-MeC6H4)2 gave (nacnac)Nb(NtBu)(4-MeC6H4)2.196 The cationic methyl complex (233) was treated with CO in the presence of PR3 [R ¼ Et, cyclohexyl (Cy)] to yield [(nacnac)Nb(NtBu)(R3PC(O)Me)]+ (234), and the reaction with NCXy (Xy ¼ 2,6-Me2C6H3) afforded [(nacnac)Nb(NtBu)

Scheme 29

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

615

(XyN]CMe)]+ (235) as a mixture of isomers. The reaction with PhCOMe, Ph3SiOH gave [(nacnac)Nb(NtBu)(OCMe2Ph)]+ (236), [(nacnac)Nb(NtBu)(OSiPh3)]+ (239), respectively; the reaction with CyNCNCy yielded the amidinate complex (237); the cationic iminoacyl complex (235) was treated with MeMgBr to afford neutral (nacnac)Nb(NtBu)(XyN-CMe2) (240).197 As also demonstrated in the vanadium complexes,88,98–103 monoanionic tridentate bis(phosphino)amide ligand, [PNP ¼ N {4-Me-2-(PiPr2)C6H3}-2], was employed as the supporting ligand to stabilize the oxidation states. Reactions of (PNP)NbCl3 (241) with 3 equiv. of LiCH2SiMe3, Mg(CHt2Bu)2 or Mg(CHt2Bu)Cl, conducted under argon atmosphere gave the corresponding Nb(V) bis-alkylidene complexes, (PNP)Nb(CHSiMe3)2 (242) or (PNP)Nb(CHtBu)2 (243), respectively (Scheme 30).198 The reaction with 2 equiv. of LiCH2SiMe3 gave another niobium(V) chloro-alkylidene complex, (PNP)Nb(CHSiMe3)Cl (244).198 Reaction of (PNP)NbCl2(OAr0 ) (245, Ar0 ¼ 2,6-iPr2C6H3), prepared from 241 by treating with NaOAr0 , with CH3MgCl (2 equiv.) gave the dimethyl complex, (PNP)Nb(CH3)2(OAr0 ) (246), and the subsequent oxidation with [FeCp2][OTf] gave (PNP)Nb(CH3)2(OAr0 )(OTf ) (247). As shown in Scheme 30, photolysis of 247 in benzene at room temperature gave the methylidene complex, (PNP)Nb(CH2)(OAr0 )(OTf ) (248), and the reaction of 247 with H2C]PPh3 afforded the mononuclear methylidyne complex, (PNP)Nb(CH)(OAr0 ) (249). Treatment of the methylidene complex (248) with LiN(SiMe3)2 also gave the methylidyne complex (259); structures of 248 and 249 were confirmed by the X-ray crystallographic analysis.199 Nitride cross-metathesis of the methylidyne complex (249) with RCN (R ¼ tBu or 1-adamantyl) afforded neutral Nb(V)-nitride, (PNP)Nb(^N)(OAr0 ) (250).199 Reaction chemistry of the methylidyne complex (249) was explored.200,201 The reaction with t BuNCO gave an oxo complex, (PNP)Nb(O)(OAr0 )(CH]C]NtBu) (251) and P^C bond cleavage occurred in the reaction

Scheme 30

616

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

with P^CAd to afford (PNP)Nb(OAr0 )(
2-AdCCH) (252).200 The reaction of 249 with ethylene first gave the propenyl-ethylene complex (PNP)Nb(HC]CHCH3)(
2-H2C]CH2)(OAr0 ), which was converted upon gentle heating to give a low-spin Nb(III) allyl complex, (PNP)Nb(
3-H2CCHCH2)(OAr0 ) (253);201 the reaction with norbornene gave the analogous species (254).201 NbCl3(dme)(MeC^CMe) (dme ¼ 1,2-dimethoxyethane) with N(CH2CH2NMe2)(CH2C6H2-3,5-R2-2-OH)2 (R ¼ H, Me), NMe(CH2CH2NMe2)(CH2C6H2-3,5-Me2-2-OH), N(CH2CH2OMe)2(CH2C6H2-3,5-R2-2-OH) (R ¼ tBu, Me) gave the tetradentate, tridentate complexes (255,256,257), respectively.202 Reactions of Nb(V) dichloro complex containing trianionic ONO pincer ligand, [N{2-(CF3)2CO-4-MeC6H3}2]NbCl2(OEt2), wth RMgCl in Et2O gave corresponding dialkyl complexes, [N{2-(CF3)2CO4-MeC6H3}2]NbR2 [258, R ¼ CH2Ph (a), CH2SiMe3 (b), CH2CMe2Ph (c), Scheme 30].202 The dialkyl complex 258c was used as precatalysts for ring opening metathesis polymerization (ROMP) of norbornene and the resultant polymers possessed highly cis (75–85%) olefinic double bonds with syndiorich tacticity; analogous Ta dialkyl complex exhibited low catalytic activities compared to the Nb analog.203 Niobium(V) trichloride complexes containing PCCP ligand, NbCl3(ArC]CAr) (259, Ar ¼ 2-R3P-C6H4; R ¼ i Pr, Ph), which was prepared by treatment of NbCl3(Me3SiC]CSiMe3)(dme) with ArC^CAr in a mixed solution of toluene and CH2Cl2, with Mg(CH2SiMe3)2 gave the alkyl-alkylidene complex, Nb(CHSiMe3)(CH2SiMe3)(ArC]CAr) (260, R ¼ iPr) that initiated ROMP of norbornene.99

5.09.3.4

(Imido)niobium complexes and some reaction chemistry

Imido (M]NR) ligand, as seen in many reports for synthesis and reaction chemistry of the niobium complexes containing both tertbutyl-imido ligands (also shown in Scheme 29),75,196,197,204–209 as well as of half-sandwich imido complexes derived from Cp Nb(NAr0 )Cl2 (147, Ar0 ¼ 2,6-iPr2C6H3)165 has been known as an effective ligand for stabilization of the high oxidation state in early transition metals.210,211 Synthesis of (arylimido)niobium(V) complexes were reported,165,212–224 however, synthesis of the low coordinative (4 coordinate) unsaturated organo-niobium(V) complexes containing an anionic donor (monodentate) ligands still have been limited until recently.220,223,224 Nb(NAr0 )(NMe2)3 (261, Ar0 ¼ 2,6-iPr2C6H3) was used as precursors for synthesis of solvent free unsaturated complexes. Reaction of 261 with cyclopentadiene in toluene gave CpNb(NAr0 )(NMe2)2 (262)212 and the similar reaction with Me2C(indenyl)(cyclopentadiene) gave Me2C(Cp)(indenyl)Nb(NAr0 )(NMe2) (263).214 As shown in Scheme 31, reaction of diimido complex, [NbCl3(CH3CN)2]2(m-1,4-NC6H4N) with Me3SiCH2MgCl or Mg(CHt2Bu)2(THF)2 gave the corresponding alkyl complexes, [Nb(CH2SiMe3)3(CH3CN)]2(m-1,4-NC6H4N) or [Nb(CHt2Bu)3(THF)]2(m-1,4-NC6H4N) (264),215 and the similar reaction of 1,3-diimido complex gave [NbR3(CH3CN)]2(m-1,4-NC6H4N) (265, R ¼ CH2SiMe3, CHt2Bu);216 acetonitrile in 265 was replaced with THF by dissolving. In contrast, reaction of [NbCl3(CH3CN)2]2(m-1,4-NC6H4N) with Ph(CH3)2CCH2MgCl gave solvent free [Nb(CH2CMe2Ph)3]2(m-1,4-NC6H4N) (266), whereas it seemed difficult to remove CH3CN or THF from the trialkyl complex (264).216 As described above, (nacnac)Nb(NtBu)Me2 (231) was prepared from the dichloride complex by treating with MeMgBr, and the similar reaction with Mg(4-MeC6H4)2 gave (nacnac)Nb(NtBu)(4-MeC6H4)2.196 The cationic methyl complexes, [(nacnac)Nb(NtBu)Me]+ A- [A- ¼ Me(B(C6F5)3 (232), B(C6F5)4 (233)], were prepared by treating 231 with B(C6F5)3 or Ph3CB (C6F5)4 in chlorobenzene; reaction chemistry of 233 with various organic compounds were thus explored (Scheme 29).75,197 Reaction of Nb(NtBu)Cl3(pyridine)2 with 3 equiv. of PhCH2MgCl gave the tribenzyl complex, Nb(NtBu)(CH2Ph)3 (267),205 and the subsequent reaction with B(C6F5)3 gave the cationic dibenzyl complex, [Nb(NtBu)(CH2Ph)2][(
6-C6H5CH2)B(C6F5)3] (268), coordinated with benzene in
6 fashion (Scheme 31).205 The dibenzyl complexes with phenoxide ligand, Nb(NtBu) (CH2Ph)2(OAr0 )(THF) (269, Ar0 ¼ 2,6-iPr2C6H3), was also prepared by treatment of the trichloride with LiOAr0 followed by addition of PhCH2MgCl.205 A series of dibenzyl complexes containing guanidine ligands (270,271) were prepared by treating the tribenzyl complex (267) with guanidine proligands [2-(4-(tert-butyl)phenyl)-1,3-diisopropylguanidine etc.].206 Niobium chloride complex containing an imido-diamido-pyridine ligand, Nb(NtBu)Cl[MeC(CH2NSiMe3)2(CH2-2-C5H4N](pyrdine) (273), was treated with PhCH2MgCl to afford the benzyl complex (272), whereas the reaction with LiCH2SiMe3 or LiCH(SiMe3)2 gave the complexes with dissociated pyridine in the chelate bis(amide)pyridine ligand (274);207 the reaction with Li(MeC5H4) also gave the complex with dissociated pyridine in the chelate ligand (275) (Scheme 31).207 Insertion of arylisocyanide into a series of monoalkyl complexes, Tp Nb(NtBu)(R)Cl [276, Tp ¼ hydridotris(3,5-dimethylpyrazolyl)borate, R ¼ Et, CH2Ph, CHt2Bu, CH2SiMe3, CH2CMe2Ph],208 afforded the corresponding iminoacyl complexes (277), and their irreversible conversions of endo-form to the exo form were studied.209

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

Scheme 31

617

618

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

As described above, Nb(NAr)Cl3(dme), Nb(NAr0 )Cl3(dme) (Ar ¼ 2,6-Me2C6H3, Ar0 ¼ 2,6-iPr2C6H3, dme ¼ 1,2-dimethoxyethane) or their THF, pyridine analogs are useful starting complexes for synthesis of various (imido)niobium(V) complexes, because the complexes with various arylimido or alkylimido ligands could be prepared from NbCl5 by treating with various amines (RNH2) in the presence of ZnCl2 (and pyridine).225 However, it was difficult to prepare, low coordinate, dme-free complexes, as exemplified in the reaction with lithium phenoxide.205,220,221 As shown in Scheme 31, reactions of Nb(NR0 )Cl3(dme) [R0 ¼ Ar, Ar0 or 1-adamantyl (Ad)] with LiCH2SiMe3 in n-hexane to afford the corresponding trialkyl complexes, Nb(NR0 )(CH2SiMe3)3 (278).220 The similar reaction with Me3SiCH2MgCl in place of LiCH2SiMe3 gave Nb(N-2,6-Cl2C6H3)(CH2SiMe3)3 and Nb(N-2-MeC6H4) (CH2SiMe3)3.223 Treatment of the trialkyl complex with HOC(CF3)3 gave the fluorinated alkoxo dialkyl complexes, Nb(NR0 ) (CH2SiMe3)2[OC(CF3)3] (279), and heating the n-hexane solution containing 279 in the presence of excess PMe3 gave the corresponding niobium(V)-alkylidene complexes, Nb(CHSiMe3)(NR0 )[OC(CF3)3](PMe3)2 (280,281) (Scheme 32).220,223

Scheme 32

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

619

As described below, these complexes are effective catalysts for ring opening metathesis polymerization of cyclic olefins and living metathesis polymerization of internal alkynes.223 Nb(NR0 )(NMe2)3 (R0 ¼ Ar, Ar0 , 2-MeC6H4) with 2-aminoethylpyridine, 2-Ar(H)NCH2-C5H4N, gave the corresponding bis(amide) complexes, Nb(NR0 )(NMe2)2[2-ArNCH2(C5H4N)] (282), the complexes were treated with HOCH(CF3)2 to yield the bis(alkoxy) complexes, Nb(NR0 )[OCH(CF3)2]2[2-ArNCH2(C5H4N)] (283).222 Further reaction of 283 with Me3SiCH2MgCl gave the dialkyl complex, (284) and the reaction with MeMgBr afforded the dimethyl complexes, Nb(NR0 )Me2[2-ArNCH2(C5H4N)] (285), which showed capabilities as the catalyst precursors for selective ethylene dimerization in the presence of MAO (methylalumioxane) cocatalyst (especially 2,6-diisopropylphenyl imido analog).222 Reaction of Nb(NR0 )Me2[2-ArNCH2(C5H4N)] (285) with [Ph3C][B(C6F5)4] in Et2O gave the cationic [Nb(NR0 )Me{2-ArNCH2(C5H4N)}]+[B(C6F5)4]− (286), which catalyze ethylene dimerization without MAO cocatalyst (Scheme 32).222 Similarly, reaction of Nb(NR0 )(NMe2)3 (R0 ¼ Ar, Ar0 , 2-MeC6H4) with 2,6-tBu2C6H3OH yielded the corresponding bis(amide) complexes, Nb(NR0 )(NMe2)2(O-2,6-tBu2C6H3) (287) and the dimethyl complexes, Nb(NR0 )Me2(O-2,6-tBu2C6H3) (288), were obtained by treating 287 with AlMe3. The dimethyl complexes (especially the dimethylphenylimido analog) showed catalytic activity for ethylene polymerization in n-octane in the presence of MAO; the activity increased at high temperature (80  C).223 The reaction of 288 with [Ph3C][B(C6F5)4] in THF gave the cationic [Nb(NAr0 )Me (O-2,6-tBu2C6H3)]+[B(C6F5)4]− (289), which catalyze ethylene polymerization without MAO cocatalyst (Scheme 32).224

5.09.4

Organotantalum complexes and related chemistry

5.09.4.1

Tantalum complexes containing cyclopentadienyl ligands

Reaction of substituted ansa-tantalocene dichlorides, [Me2Si(Cp0 )2]TaCl2 with EtMgBr in Et2O gave [Me2Si(C5H4)(3-RC5H3)] TaH(ethylene) [R ¼ iPr (290a), tBu (290b)], and the structures (the ethylene is coordinated in the more open portion of the metallocene wedge, away from the isopropyl substituent, the distal isomer) were determined by X-ray crystallography. In contrast, the reaction of [Me2Si(C5H4)(3,5-iPr2C5H2)]TaCl2 gave a mixture of isomers (290c, Scheme 33).158 The reaction of [Me2Si(C5H4) (3-RC5H3)]TaCl2 with nPrMgCl gave [Me2Si(C5H4)(3-RC5H3)]TaH(propylene) [R ¼ iPr (290d), tBu (290e)] observed as a distal isomer (anti-, endo- form). In contrast, the reaction with PhCH2CH2MgBr gave a mixture of isomers in [Me2Si(3-iPrC5H3)2] TaH(styrene) (290f) and [Me2Si(C5H4)(3,5-iPr2C5H2)]TaH(styrene) (290h), whereas the resultant complex possessed an anti-, endo- form in [Me2Si(3-tBuC5H3)2]TaH(styrene) (290g), suggesting steric bulk plays a role.158 Doubly bridged Ta-methylidene complex (291) was isolated in the reaction with TaCl2Me3 and the thermolysis in C6D6 afforded ethylene hydride complex (292) via a-migratory insertion of the methyl group into the carbene ligand, followed by b-H elimination.158 Reaction of (
5-RC5H4)2Ta(]CH2)(CH3) (293, R ¼ H, CH3) with B(C6F5)3 gave a zwitterionic complex (294), and the subsequent insertion of tBuNC proceeded in THF with excess to give the N-out iminoacyl zwitterions (295a) as metastable kinetic isomers. Thermolysis of 295a converted to more thermodynamically stable N-in isomers (295b).226 Reaction of (
5-C5H5)[
5-C5H4-SiMe2(CH2CH]CH2)] TaCl2 or (
5-C5Me5)[
5-C5H4-SiMe2(CH2CH]CH2)]TaCl2 with 2 equiv. of RMgCl (R ¼ Me, PhCH2) or LiCH2SiMe3 gave olefin coordinated (
5-C5H5)[
5-C5H4-SiMe2(CH2-
2-CH]CH2)]NbR (296, R ¼ Me, CH2Ph) or (
5-C5Me5)[
5-C5H4-SiMe2

620

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

Scheme 33

(CH2-
2-CH]CH2)]NbR (297, R ¼ Me, CH2Ph, CH2SiMe3).159 The coordinated olefin in 296, 297 was dissociated by insertion of CO, tBuNC and (2,6-Me2C6H3)NC (Scheme 33).159 (
6-cycloheptatriene)TaCl2(PMe3)2 (299), prepared by the reduction of TaCl5 with n-butyllithium in the presence of PMe3 and cycloheptatriene and holds a four-legged piano stool structure, was reacted with MeLi afforded (
7-C7H7)TaCl(PMe3)2 (300) with loss of HCl.227 Treatment of the previously described sandwich compound Cp Ta(
7-C7H7) (301) with allyl bromide afforded Cp Ta(
7-C7H7)Br (302),228 which reacted with LiAlH4 to give Cp Ta(
7-C7H7)H (303). Reactions of 4 with MeLi, Li(CH2CH] CH2), or with Mg(cyclo-C3H5)2 gave Cp Ta(
7-C7H7)R (304, R ¼ Me,
1-CH2CH]CH2, cyclopropyl), respectively.227 As shown in Scheme 34, (
5-C5Me4R)2Ta2(m-X)4 (305; R ¼ Me, Et; X ¼ Cl, Br) with 2 equiv. of 3,3-diphenylcyclopropene afforded a ring-opened mononuclear (diphenylalkenyl)alkylidenes, (C5Me4R)Ta(CHCHCPh2)X2 (306), and the subsequent reaction with K/Hg in toluene afforded the tethered s2-arene complex (C5Me4R)Ta(CHCHPhC6H5) (307).229 The reaction of [Cp TaCl4] with the potassium salt of carbazole (cbK) led to formation of (C5Me4CH2)Ta(cb)2Cl as a major product, and the

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

621

Scheme 34

following reaction with LiCH2SiMe3 or PhCH2MgCl gave the monoalkyl complexes, (C5Me4CH2)Ta(cb)2(R) (308, R ¼ CH2Ph, CH2SiMe3).230 Reaction of Cp0 TaCl2(
4-C4H6) (Cp0 ¼ Cp, Cp ) with PhCH2MgCl in THF gave the dibenzyl complexes, Cp0 Ta (CH2Ph)2(
4-C4H6) (309), and thermolysis of the dibenzyl complex (309, Cp0 ¼ Cp ) in the presence of PMe3 afforded the benzylidene complex Cp Ta(CHPh)(
4-C4H6)(PMe3) (311). Reaction of Cp TaCl2(
4-C4H6) with Me3SiCH2MgCl or tBuCH2MgCl followed by addition of MeMgI gave the other dialkyl complexes, Cp0 TaMe(R)(
4-C4H6) (310, R ¼ CH2SiMe3, CHt2Bu).231 Similarly, thermolysis of another dibenzyl complex, Cp Ta(CH2Ph)2(
4-o-(CH2)2C6H4) (312), gave the benzylidene complex, Cp Ta(CHPh)(
4-o-(CH2)2C6H4) (313); the benzylidene complex (306) was treated with acenaphthylene to afford the metallacyclobutane complex (314).231 Complexes 309,310, and 312,313 exhibited catalytic activites for ring-opening metathesis polymerization of norbornene, affording ring-opened polymers containing highly cis (by 309, 310) or highly trans (312, 313) olefinic double bonds (Scheme 34).231 Cp TaCl3(N]PR3) (315, R ¼ iPr, tBu), prepared from Cp TaCl4 with Me3SiN]PR3, was reacted with MeLi to afford the trimethyl complex Cp TaMe3(N]PR3) (316), and the reaction with 1 equiv. of PhCH2MgCl gave the monobenzyl complex (317) and subsequent reaction with MeLi to give the dimethyl-benzyl complex Cp TaMe2(CH2Ph)(N]PR3) (318).232 The reaction with EtMgCl to give the metallacyclopropane (319) and subsequent reaction with EtMgCl gave the ethyl complex (320). In contrast, reaction of the trichloride (315) with 3 equiv. of PhMgCl gave the benzyl-benzylidene complex, Cp Ta(CHPh)(CH2Ph)(N]PR3) (321). The benzylidene complex (321) was reacted with MeI to afford the metallacyclopropane (322).232 Reaction of the trimethyl

622

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

complex (316) with B(C6F5)3 or [Ph3C][B(C6F5)4] gave the corresponding cationic dimethyl complexes (323) and the reaction of metallacyclopropane complex (319) afforded the zwitterionic ring-opened products (324, 325).232 A series of [
5-C5H3(SiMe3){Si(X)Me2}]TaCl3Me (326), [
5-C5H3(SiMe3){Si(X)Me2}]TaCl2Me2 (327), and [
5-C5H3(SiMe3) {Si(X)Me2}]TaMe4 (328) were prepared from the tetrachloride complexes by modification of the reaction conditions (equivalence of ZnMe2 or MeMgCl, Scheme 35).169 Cp Ta(PMe3)2Cl(^CPh) (330) was prepared from the benzyl-benzylidene complex, Cp Ta(CHPh)Cl(CH2Ph) (329),32 upon addition of PMe3, and subsequent addition of 3-hexyne gave the metallacyclobutane

Scheme 35

(331) with loss of two PMe3.233 Treatment of the benzylidyne complex (330) with Na(HCB11Cl11) or Na[B(3,5-(CF3)2C6H3)4] followed by addition of 3-hexyne gave the cationic tantallacyclobutene complex (332).233 Reaction of Cp TaCl4[(iPr2N)2CMe] (333) containing acetamidinate ligand with EtLi (3 equiv.) in Et2O gave the alkyl-alkylidene complex, Cp Ta(CHMe)(Et)[(iPr2N)2CMe] (333),whereas the reactions with iBuLi or nBuLi (3 equiv.) gave tantalum(IV) dialkyl complexes, Cp Ta(i-C4H9)2[(iPr2N)2CMe] (335), Cp Ta(n-C4H9)2[(iPr2N)2CMe] (336), respectively.234 The reaction with neopentyllithium gave the mononeopentyl complex, Cp Ta(CHt2Bu)[(iPr2N)2C]CH2] (337), and subsequent addition of 1 equiv. of [PhN(H)Me2][B(C6F5)4] yielded the cationic neopentyl complex [Cp Ta(CHt2Bu){(iPr2N)2CMe}]+[B(C6F5)4]− (338).234 The trialkyl complex (333) was reduced to the dichloride complex (339) upon treatment with Na/Hg, and the reaction with EtLi or n-BuLi gave the corresponding dialkyl complexes, Cp TaEt2[(iPr2N)2CMe] (340), Cp Ta(n-C4H9)2[(iPr2N)2CMe] (336), respectively.234 The reaction of 339 with 1 equiv. of RCMe2CH2MgCl (R ¼ H, Me) gave the monoalkyl complex, Cp TaCl(RCMe2CH2) [(iPr2N)2CMe] (341), and the subsequent reaction with MeLi gave the dialkyl complex Cp TaMe(RCMe2CH2)[(iPr2N)2CMe] (342).234

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

623

Insertion of arylisocyanide (ArNC, Ar ¼ 2,6-Me2C6H3) to various chloro alkyl or dialkyl imido complexes Cp Ta(NAr)(R)X (343, X ¼ Cl, R ¼ Me, CH2SiMe3, CH2Ph, CHt2Bu, CH2CMe2Ph, 2-(CH2NMe2)C6H4, NMe2; R ¼ X¼ CH2SiMe3, CH2Ph, CHt2Bu, CH2CMe2Ph, Ph)235 yielded
2-iminoacyl derivatives, Cp Ta(NAr)(X)[
2-ArN]CR] (344).166 The reaction Cp Ta(NAr)(Me)R gave a mixture of different (R or Me) insertion products.166 Bimetallic half-tantalocene complexes bridged by 1,4-phenyldiimido ligands, [CpTaCl2]2(1,4-NC6H4N), were treated with MeLi, Me3SiCH2MgCl, Mg(CH2Ph)2(THF)2 to give the corresponding dialkyl complexes, [CpTaNbR2]2(1,4-NC6H4N) (345) (Scheme 35).167 Reaction of ArNC to the oxo complex, Cp Ta(O)(R)X [R,X ¼ Me,Cl; CH2Ph,CH2Ph or R¼ X ¼ Me, CH2Ph] pretreated with B(C6F5)3 (346) gave Cp Ta{O B(C6F5)3}(X)[
2-ArN]CR] (347); subsequent addition of pyridine to afford Cp Ta(O)(X)[
2-ArN]CR] (348).236

Scheme 36

624

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

As shown in Scheme 36, Tp TaCl2(
2-3-hexyne) [349; Tp ¼ HB(3,5-dimethylpyrazolyl)-3] and TpMs TaCl2(
2-3-hexyne) [350; Tp ¼HB(3-mesitylpyrazolyl)2(5-mesitylpyrazolyl)-] were prepared from TaCl3(
2-3-hexyne)(dme) (dme ¼ 1,2-dimethoxyethane) by treating with Tp K or TpMs Tl in toluene.237 Similar reactions of Ta(NAr0 )Cl3(dme) or Ta(NtBu)Cl3(dme) to afford the imido dichloride complexes (352a, 352b), respectively. Reaction of 349 with PhCH2MgCl gave the monobenzyl complex, Tp TaCl(CH2Ph) (
2-3-hexyne) (351).237 These complexes (350-352) showed catalytic activities for ethylene polymerization, and the complex 352a showed the highest activity at 40  C or 60  C in the presence of AliBu3 – [Ph3C][B(C6F5)4] cocatalyst.237 Tp Ta(NtBu)(R)Cl2 was treated with MeMgCl to give the dimethyl complex (353).208 Insertion of arylisocyanide into a series of monoalkyl complexes, Tp Ta(NtBu)(R)Cl [354, Tp ¼ hydridotris(3,5-dimethylpyrazolyl)borate, R ¼ Et, CH2Ph, CHt2Bu, CH2SiMe3, CH2CMe2Ph],208 afforded the corresponding iminoacyl complexes (355), and their irreversible conversions of endo-form to the exo form were studied.209 The dichloro tantalacarborane, CpTaCl2(Et2C2B4H4) (356), was treated with LiAlH4 in THF to afford a Cl bridged hydridotantalum dimer, [CpTaH(Et2C2B4H4)]2(m2-Cl)2 (357, Scheme 36).238 Reaction of 357 with 2 equiv. of diphenyl acetylene or methyl phenyl acetylene gave the hydridotantalum–alkyne complexes (358) with alkyne p-coordination, and the insertion of p-tolyl acetylene gave trans-CpTaCl[CH]CH-p-MeC6H4](Et2C2B4H4) (359) exclusively.238 Reaction of 2 equiv. of styrene to 357 gave CpTaCl[CH2CH2PhMeC6H4](Et2C2B4H4) (360).238 Insertion of various alkynes and alkenes to the dimethyl complex, CpTaMe2(Et2C2B4H4) (361),239 was studied.240 Insertion of disubstituted alkynes (R-C^C-R, R ¼ Me, Et, Ph) gave CpTaMe[CR] C(R)Me](Et2C2B4H4) (362); the reactions with 3-hexyne, diphenylacetylene gave only one isomers through g-agostic Ta-H3C interactions which are sufficiently strong to stabilize the structure in two different regioisomers (Scheme 36).240 Insertion of alkynes [R1-C^C-R2: R1,R2 ¼ Ph,H; H,Ph; Ph,CH3; CH3,Ph; Ph,Ph; Et,Et; SiMe3,H] to the diphenyl complex (363) afford the five-membered metallacycles (364).240 The reaction of 363 with Ph-C^C-C^CPh first gave the five-membered metallacycles (365), and subsequent reaction with 343 gave the dimer (366) confirmed by X-ray crystallography.240 Insertion of styrene to the diphenyl complex 363 first gave the five-membered metallacycles (367), subsequent styrene insertions upon heating afforded another five-membered metallacycles (368) with repeated 3 styrene insertions; the reaction proceeded via two benzyne intermediates.240 CpTaCl2(Et2C2B4H3Br) (369) was treated with 2 equiv. of MeMgBr to afford the dimethyl complex, CpTaMe2(Et2C2B4H3Br) (370), and subsequent reaction with MeMgBr or nBuMgBr gave the corresponding CpTaCl2(Et2C2B4H3R) (371).241 Similar reaction with PhMgBr also gave CpTaPh2(Et2C2B4H3Br) (372), CpTaPh2(Et2C2B4H3Ph) (373), respectively (Scheme 36).241 As shown in Scheme 36, reaction of linked amide-tantalacarborane methyl complex, [
1:
5-(Me2NCH2CH2)C2B9H10] TaMe3 (374) with Me3SiCH2NC gave the corresponding iminoacyl complex, [
1:
5-(Me2NCH2CH2)C2B9H10]TaMe[
2-C, N-C(Me2)NCH2SiMe3] (375) and the reactions with iPrNC or CyNC (Cy ¼ cyclohexyl) gave [s:
1:
5-{MeN(CH2)CH2CH2} C2B9H10]TaMe[N(iPr)R] (376, R ¼ iPr, Cy).242 The reaction of 374 with 4 equiv. of Me3SiCH2NC gave multiply inserted 377 and with iPrNC gave another complex (378).242 Insertion of [
1:
5-(MeOCH2CH2)C2B9H10]TaMe3 (379), prepared in an equimolar reaction of [(MeOCH2CH2)C2B9H10]Na2 with Me3TaCl2 in THF, with CyNC gave the multiply inserted product (380).242 Ms

5.09.4.2

Tantalum complexes containing monodentate anionic donor ligands

TaCl2(OMe)3 was treated with EtLi (8 equiv.) in Et2O followed by addition of Me2NCH2CH2NMe2 (tmeda) to afford the tantalum(-I) complex, [Li(tmeda)]3[TaHEt(
2-C2H4)3]
1/2tmeda (small amount) and the structure was determined by X-ray crystallography (and 1H NMR spectroscopy at −80  C).243 Reaction of Ta(CHCMe2R)Cl3(L) (R ¼ Me, Ph; L ¼ THF or 1,2-dimethoxyethane)244 with [Pd(2-C6H4CH2NMe2)(m-Cl)]2 gave a Cl bridged Ta-Pd heterbimetallic dimer, Pd(2-C6H4CH2NMe2) (m-Cl){m-CH(CMe2R)}TaCl3(L) with
2-interaction of Ta]C with Pd.245 The reaction with Ta(CHtBu)(OAr0 )3 (Ar0 ¼ 2,6-iPr2C6H3)246 gave stable Pd(2-C6H4CH2NMe2)(m-Cl)(m-CHtBu)Ta(OAr0 )3.245 The structure was different from that reported previously in [(dmpe)Pd(m-CH2)2TaCp2]Cl (a planar geometry of the four-membered rings including Ta]C and Pd).247 In the reaction of alkyl-alkylidene complexes with silanes, the silane elimination could be preferred over alkane elimination; in the reactions of silanes with alkylidene, the M¼ CH p bonds attack Si in silanes.248,249 In the reactions of (Me3ECH2)3TaCl2 (E ¼ C, Si) with LiSi(SiMe3)3(THF)3 (2 equiv.) preferential silane elimination led to formation of (Me3ECH2)2Ta(CHEMe3)[Si(SiMe3)3] (381, Scheme 37).250 Reaction of 381 with HCl gave the trialkyl complex, (R0 3Si)TaCl(CH2R0 3)3 (382, R0 ¼ SiMe3), which reverted to 381 by addition of LiSiR0 3.250 Treatment of 382 with LiCH2SiR0 3 gave the alkyl-alklylidene, Ta(CHR0 )(CH2R0 )3 (383), and subsequent SiMe3 elimination to afford a dimer, [Ta(m-CR0 )(CH2R0 )2]2 (384). Elimination of SiMe4 from 382 gave [Ta(CHR0 )Cl(CH2R0 )2]2 (385), which was converted to the bridged 384 by treatment with LiCH2R0 ; further SiMe4 elimination yielded the alkylidene-alkylidyne, Ta2(CR0 )(CHR0 )Cl2(CH2R0 )3 (386).250 Formation of bis(alkylidene) complexes (Me3ECH2)Ta(PMe3)2(CHEMe3)2 (387) from (Me3ECH2)2Ta(CHEMe3)(SiPht2Bu) (E ¼ C, Si) was achieved via preferential silane elimination upon addition of PMe3, which was gradually decomposed thermally to convert the bis(alkylidene) complex to [Ta(m-CHEMe3)(CH2SiMe3)2]2.251 Reaction of Ta(CHSiMe3)2(CH2SiMe3)(PMe3)2 (387) with SiH2(R)Ph (R ¼ Me, Ph) gave metallasilacyclobutadiene complex (388), and addition of SiH2(R)Ph to Ta(CHR0 )(CH2R0 )3(PMe3) (389) afforded Ta[CR0 [Si (H)RPh]](CH2R0 )3 (390).252,253 Reaction of [Ta(CHR0 )Cl(CH2R0 )2]2 (385) with excess PMe3 gave [TaCl(PMe3)2](m-CHR0 )2 [TaCl(CH2R0 )2]2 (391) and subsequent reaction of 391 with LiCH2SiMe3 yielded [Ta(CH2R0 )(PMe3)2](m-CHR0 )2[TaCl

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

625

Scheme 37

(CH2R0 )2]2 (392), which was converted to a dimer, [Ta(m-CR0 )(CH2R0 )2]2 (384) by reaction with LiCH2SiMe3; The reaction of 392 with LiSiR0 3 gave [Ta(CH2R0 )(SiR0 3)](m-CHR0 )2[TaCl(CH2R0 )2]2 (393).254 As described in the introduction (Scheme 2), Ta(CHtBu)(CHt2Bu)3, the first metal-alkylidene complex, was prepared by treating Ta(CHt2Bu)3Cl2 with 2 equiv. of LiCHt2Bu in pentane. A possibility of a-hydrogen abstraction (elimination) was proposed from isotopic distribution of the product in reaction of Ta(CDt2Bu)3Cl2 with 2 equiv. of LiCHt2Bu.26 Kinetic isotope effect (KIE) of 14.1 at 273 K was observed in conversion of Ta(CDt2Bu)5, which was prepared in situ from Ta(CDt2Bu)3Cl2 or Ta(CDt2Bu)4Cl by treatment with LiCDt2Bu, to yield Ta(CDtBu)(CDt2Bu)3, because Ta(CDt2Bu)5 showed much longer lifetime compared to Ta(CHt2Bu)5.255 Dimeric [Ta(cb)2]2(m-CSiMe3)2 (394, cb ¼ carbazole, Scheme 37) was reacted with Ar00 OH (Ar00 ¼ 2,6-Ph2-3,5-tBu2C6H) to afford [Ta(cb)(OAr00 )]2(m-CSiMe3)2[Ta(cb)2] (395), and insertion of Me3SiC^CH to complexes 394, 395 gave the corresponding 6 membered complexes, [(cb)2Ta](m-CSiMe3){m-C(SiMe3)CHC(SiMe3)}[Ta(cb)2] (396) [(Ar00 O)(cb)Ta](m-CSiMe3){m-C(SiMe3) CHC(SiMe3)}[Ta(cb)2] (397), respectively; insertion of EtC^CEt to complexes 394 gave [(cb)2Ta](m-CSiMe3){m-C(Et)C(Et)C (SiMe3)}[Ta(cb)2] (398).256 Reactions of TaCl3(O-2,6-Ph2C6H3) with Me3SiCH2MgCl or TaCl3(O-2,3,5,6-Ph4C6H) with PhCH2MgCl gave the corresponding mono- and bis- complexes (399-402), respectively.189 Reaction of [(Me2N)3Ta(
2-CH2SiMe2-NSiMe3)] (403)

626

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

with H3SiPh with underwent C–H activation to afford a Ta/m-alkylidene/hydride complex, [(Me2N)2{(Me3Si)2N}Ta] (m-H)2(m-C-
2-CHSiMe2NSiMe3)[Ta(NMe2)2] (404); deuterium-labeling studies with [D3]SiPh showed H–D exchange between the Ta–D–Ta unit and all methyl groups.257 b-Hydrogen abstraction between a hydride and the NMe2 ligand gave a new complex, [(Me2N)2{(Me3Si)2N}Ta](m-H)(m-N-
2-C,N-CH2NMe)(m-C-
2-C,N-CHSiMe2-NSiMe3)[Ta(NMe2)2] (405).257 Ta(OSitBu3)3, prepared from TaCl2(OSitBu3)3 by treating with Na/Hg in THF,258 was added with olefins (e.g., 1.1 equiv. for styrene to 30 equiv. for cyclohexene) to yield olefin coordinated Ta(OSitBu3)3(olefn) [403, olefin: ethylene, propylene, 1-butene, cyclopentene, cyclohexene, H2C]HC-4-XC6H4 (X ¼ OMe, H, CF3), norbornene],195,259 and the crystallographic analysis of Ta(
-C2 H3Et)(OSitBu3)3] was also reported. Alkyl-alkylidene rearrangements, as observed in the niobium analogs,194 were explored by thermolysis of 403 in C6D6, and two products called “tuck-in” alkyls, Ta-(k2-O,C-OSitBu2CMe2CH2)(R)(OSitBu3)2 (404), and the alkylidene (405) were observed in most cases, except that only alkylidene (norbornylidene) complex was observed.195 Two pathways (i) double abstractions or (ii) hydrogen migration were considered for formation of the alkylidene from the olefin complex, and the observed “tuck-in” alkyl (404) supports the double abstractions. Further labeling studies, kinetic studies and computational studies revealed the difference between Ta and Nb species; once formed, tuck-in alkyl niobium species converted

Scheme 38

to the alkylidene species and the reaction between olefin and the alkyl complex was reversible (irreversible in Ta-cyclohexyl, Scheme 38).195 Reaction of TaCl2Me3 with Ar00 OH [Ar00 ¼ 2,6-(Ph2CH)2-4-tBuC6H2] gave the bis(phenoxy) methyl complex, TaCl(CH3) (OAr00 )2 (406, a square pyramidal structure), and treatment of 406 with excess Ph3P]CH2 gave the anionic methylidene, Ta(CH2)Cl(OAr00 )2(CH2PPh3) (407).260 In contrast, the reaction of TaCl2Me3 with Ar00 ONa afforded the trimetyl complex, TaMe3(OAr00 ) (408, trigonal bipyramidal), and further reaction with PPh3]CH2 did not take place (Scheme 38).260

5.09.4.3

Tantalum complexes containing bidentate or tridentate anionic donor ligands

As shown in Scheme 39, tantalum trialkyl complexes containing 2,20 -ethylenebis(6-isopropylphenoxide) (409) were prepared by treating the trichloride, prepared by reaction of TaCl5 with the bis(phenol), with RMgCl (R ¼ Me, PhCH2); the ethylene bridge constrains the aryloxide portions of the bis(phenoxide) ligand to reside in a trigonal bipyramidal equatorial plane.261 Tantalum(V) alkyl complexes containing two resolved 3,30 -disubstituted-1,10 -bi-2,20 -naphthoxide ligands, (R,R)-[H2NEt2][Ta (O2C20H10{R}2-3,30 2Cl2] [R ¼ SiMe3, SiMe2Ph, SiMePh2, SiPh3], were prepared by treatment of dimeric (Et2N)2Cl2Ta]2(m-Cl)2 with

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

627

Scheme 39

the bis(phenol), and the further reaction with RMgCl gave the corresponding mono alkyl complex, (R,R)-Ta(O2C20H10(SiMe3)2-3,30 2R0 (410, R0 ¼ CH2SiMe3, CH2Ph, cyclo-C5H9).262 Reactions of tantalum methylidene-methyl complex containing bis(amidinate)ligand, [p-MeC6H4C(NSiMe3)2]2Ta(CH2) CH3 (411)263 with RCN (R ¼ p-MeC6H4, Me), MeI, and MeCHO gave [p-MeC6H4C(NSiMe3)2]2Ta(]NC(]CH2)R)CH3 (412), [p-MeC6H4C(NSiMe3)2]2Ta(CH3)(CH2CH3)I (413) and [p-MeC6H4C(NSiMe3)2]2Ta(]O)CH3 (414), respectively. The reaction with CNAr (Ar ¼ 2,6-Me2C6H3) gave the isocyanide inserted [p-MeC6H4C(NSiMe3)2]2Ta[
2-N(Ar)C(]CH2)]CH3, and the reaction with propylene sulfide gave the sulfur transferred
2-thioformaldehyde complex, [p-MeC6H4C(NSiMe3)2]2Ta(SCH2)CH3.264 Tantalum trimethyl complexes containing
2-N,O-chelated phosphoamidinate ligand, TaMe3[
2-N(Ar)-P(OEt)2-O] (415), prepared by reaction of TaCl2(CH3)3 with [P(O)(OEt)2{NaN(Ar)} in hexanes, was employed as efficient catalyst for hydroaminoalkylation of unactivated alkenes with N-methyl-p-methoxyaniline under mild conditions (at room temperature).265 The complex possessed trigonal bipyramidal geometry with the N,O-chelated phosphoramidate ligand trans to the axial chloride ligand. A series of tantalum complexes containing bis(phenoxide) linked with pyridine, phenyl, furan and thiophene were prepared.266 Reaction of TaCl2R3 (R ¼ Me, PhCH2) with [2,6-KOC6H2-2,4-tBu2]2(C5H3N) gave the corresponding trialkyl complexes, TaR3[{2,6-OC6H2-2,4-tBu2}2(C5H3N)] (416), and the reaction with TaCl2Me3 with [2,6-HOC6H2-2,4-tBu2]2(C5H3N) gave the mono methyl complex (417); the similar reaction of TaCl3Me2 gave the trichloride complex, TaCl3[{2,6-OC6H2-2,4-tBu2}2 (C5H3N)]. Thermolysis of the tribenzyl complex (416) in toluene in the presence of PMe2Ph gave the benzyl-benzylidene complex, Ta(CHPh)(CH2Ph)[{2,6-OC6H2-2,4-tBu2}2(C5H3N)](PMe2Ph) (419). Similarly, reactions of TaCl2Me3 with [2,6-KOC6H2-2, 4-tBu2]2(C4H2O), [2,6-KOC6H2-2,4-tBu2]2(C4H2S) afforded the corresponding trimethyl complexes (420,421), respectively (Scheme 39).266 The reaction of TaCl2Me3 with [2,6-KOC6H2-2,4-tBu2]2(C6H4) gave the dimethyl complex containing bis(phenoxy)phenyl ligand, TaMe2[{2,6-OC6H2-2,4-tBu2}2(C5H4)] (422), and the reaction with [2,6-HOC6H2-2,4-tBu2]2(C6H4)

628

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

gave the dichloride complex. Reaction of 422 with PhCN, Ph2CO, or with tBuNC gave the corresponding inserted product (into Ta-phenyl bond), 423-425, respectively; their structures were determined by X-ray crystallography.266 Tantalum(V) dibenzyl complexes containing amine tris(phenolate) ligands, Ta(CH2Ph)2[(O-2-R1-4-R2C6H2-6-CH2)3N] (426, R1,R2 ¼ Me,Me; Cl,Cl; Ph,tBu), were prepared by treating Ta(CH2Ph)5 with [(HO-2-R1-4-R2C6H2-6-CH2)3N], and thermolysis of the dimethylphenyl complex afforded bridged bis(benzylidene) complex, [Ta{O-2,4-Me2C6H2-6-CH2)3N}]2(m2-CHPh) (427), and the structure (two forms) was confirmed by X-ray crystallographic analysis (Scheme 40).267 Ta dichloride complex containing SiEt3 substituted tris(amide)amine ligand, TaCl2[(Et3SiNCH2CH2)3N] (428) was treated with 2 equiv. of MeMgCl to give the dimethyl complex, TaMe2[(Et3SiNCH2CH2)3N] (429), and the similar reaction with EtMgCl afforded ethylene complex, Ta(ethylene)[(Et3SiNCH2CH2)3N], contaminated with the ethylidene.268 Thermolysis of the dimethyl complex (429) led to evolution of methane to afford TaMe[(Et3SiNCH2CH2)2N](Et3SiNCH]CH2)]. Reactions of the dichloride complex (428) with LiCH2SiMe3, RCH2CH2MgX (X ¼ Cl, Mg; R ¼ Me, Et, iPr) gave the corresponding alkylidene complexes, Ta(CHSiMe3)[(Et3SiNCH2CH2)3N] (430), Ta(CHCH2R)[(Et3SiNCH2CH2)3N] (431), respectively.268 The reaction with CH2]CHMgCl gave the dimeric [Ta{(Et3SiNCH2CH2)3N}]2(m2-CHCH2CH2CH) (432) and the structure was determined (Scheme 40).268 The

Scheme 40

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

629

trimethyl complexes containing bis(amide)amine ligand, TaMe3[{(3,5-Cl2C6H3)NCH2CH2}2NMe] (433), or TaMe3[(3,5-Cl2C6H3 NCH2)2C(2-C5H4N)Me] (434) were also prepared by treating TaCl2Me3 with the corresponding lithium amide.269 Reaction of TaCl2Me3 with lithium amide, [PhP(CH2SiMe2NSiMe2CH2)2PPh]Li2(dioxane), gave the corresponding trimethyl complex containing macrocyclic (tetradentate) bis(amide-phosphine) ligand, TaMe3[PhP(CH2SiMe2NSiMe2CH2)2PPh] (435), that exhibited fluxional behavior of capped-trigonal-prismatic behavior in solution.270 Photolysis of 435 induced a-hydrogen elimination to afford the methylidene-methyl complex, Ta(CH2)Me[PhP(CH2SiMe2NSiMe2CH2)2PPh] (436), which afforded trans phosphines with a P-Ta-P angle of 165.11(7) and cis amides with a N-Ta-N angle of 114.1(5) . Reaction of 435 with [Ph3C]BF4, [PhN(H)Me2][B(C6F5)4] gave the cationic dimethyl complexes (437,438), respectively, and the further treatment of 437 with base [LiNiPr2, NaN(SiMe3)2
THF] gave Ta(F)Me2[PhP(CH2SiMe2NSiMe2CH2)2PPh] (439) as the major product; the similar reaction of 438 gave another product (440) with formation of TadC bond.270 As also shown in Scheme 40, Ta(V) trimethyl complex, TaMe3[PhP(CH2SiMe2NPh)2] (441), was reacted with H2 (excess) to generate the dinuclear ditantalum tetrahydride complex [{PhP(CH2SiMe2NPh)2}Ta]2(m2-H)4 (442), which reacted spontaneously with molecular nitrogen to afford the side-on end-on dinitrogen complex, [{PhP(CH2SiMe2NPh)2}Ta]2(m-
2:
1-N2)(m-H)2 (443)271–273 Reaction of 443 with propylene afforded propyl complex bridged with two imido ligand, [{PhP(CH2SiMe2NPh)2}Ta(CH2CH2CH3)]2(m2-N2) (444),272 and the reaction with n BuSiH3 once gave [{PhP(CH2SiMe2NPh)2}Ta]2{m-
2:
1-N-NSi(nBu)H2}(m-H)2, and converted to a bridged imido-amido complex, [{PhP(CH2SiMe2NPh)2}Ta]2(m2-N){m2-NSi(nBu)H2}.273 As reported for 435, photolysis of the trimethyl complex (441) gave the methylidene-methyl complex, Ta(CH2)Me[PhP(CH2SiMe2NPh)2] (445).274 Thermolysis of 441 in toluene at 70  C (48 h) afforded the cyclometallated TaMe2[PhP(CH2SiMe2NPh)(CH2SiMe2N-o-C6H4)] (446), and the similar thermolysis with the mesityl analog, TaMe3[PhP{CH2SiMe2N(2,4,6-Me3C6H2)}2], gave a complex mixture.274 Complexes 445 and 446 also gave the tetrahydride complex (442) by treatment of H2, and rates of hydrogenolysis of the trimethyl (441), the methylidene-methyl (445) and the cyclometallated (446) to give 442 were thus compared; the complex 446 showed faster than 441 and 445 required longer reaction hours, suggesting some of the steps in the reaction to generate 442.274 Ta(V) chloro complexes containing alkyne (RC^CR, R ¼ Et, SiMe3) and diamidophosphine ligands, TaCl(CR]CR)[PhP{2-(N2,4,6-Me3C6H2)-5-Me-C6H3}2] (447), prepared from Ta(RC]CR)Cl3(dme) (dme ¼ 1,2-dimethoxyethane) by treating with [PhP {2-(KN-2,4,6-Me3C6H2)-5-Me-C6H3}2](THF), was reacted with KBEt3H to afford the hydride, TaH(CR]CR)[PhP{2-(N2,4,6-Me3C6H2)-5-Me-C6H3}2] (448), and the subsequent reaction of 448 with arylisocyanide (ArNC) afforded the inserted azatantalacyclopentene, Ta[C(R)C(R)CHN(Ar)][PhP{2-(N-2,4,6-Me3C6H2)-5-Me-C6H3}2] (449), as shown in Scheme 41.275 The reaction of the hydride (448) with phenylacetylene once afforded alkyne-vinylphenyl complex (450) and converted to the metallacyclopentene (alkyl-alkylidene) Ta[C(R)C(R)CHCH(Ph)][PhP{2-(N-2,4,6-Me3C6H2)-5-Me-C6H3}2] (451), after rearrangement of the phenyl group275; it was proposed that the sequence of reductive elimination followed by internal reduction of the organic fragment by the electron-rich Ta(III) species leads to the observed product.275 Reactions of 447 with KCH2Ph gave the benzyl complex, Ta(CH2Ph)(CR]CR)[PhP{2-(N-2,4,6-Me3C6H2)-5-Me-C6H3}2] (452), and the geometry was dependent upon the alkyne substituent (R: Et vs SiMe3), and subsequent reaction with hydrogen gave the hydride alkene (metallacyclopropane) complexes, TaH{C(H)SiMe3-C(H)SiMe3}[PhP{2-(N-2,4,6-Me3C6H2)-5-Me-C6H3}2] or TaH{CH2-C(H)nBu}[PhP{2-(N2,4,6-Me3C6H2)-5-Me-C6H3}2] (453), as shown in Scheme 41.276 These complexes (453) were eventually converted to the dimeric tetrahydride complex (454) after long reaction times (10 days, 4 atm H2, R ¼ SiMe3).276 The trimethyl complex, TaMe3[PhP{2-(N2,4,6-Me3C6H2)-5-Me-C6H3}2] (455), prepared by treating of TaCl2Me3 with [PhP{2-(KN-2,4,6-Me3C6H2)-5-Me-C6H3}2](THF), was also treated with H2 to afford the dimeric tetrahydride (454, 4 atm H2, 24 h).276 Another trimethyl complex, TaMe3[PhP (CH2C6H4-o-NPh)2] (456), which folded a distorted trigonalprismatic coordination environment, was prepared by reaction of TaCl2Me3 with the corresponding K salt, and the reaction with Ta(EtC]CEt)Cl3(dme) instead of TaCl2Me3 followed by addition of LiCH2SiMe3 gave the alkyl-alkyne complex (457).277 Reaction of Ta(NMe2)5 with PhP{CH2C6H4-o-N(H)Ar}2 (Ar ¼ 2,6-Me2C6H3) gave the bis(amide) complex, Ta(NMe2)2[PhP(CH2C6H4-o-NAr)(C6H4-o-CHNAr)] (458), and the reaction of 458 with Me3SiI gave the diiodo complex and the subsequent reaction with LiCH2SiMe3 gave the dialkyl complex (459, Scheme 41).277 Moreover, thermolysis of 459 in the presence of PMe3 or dmpe (dimethylphosphinoethane) gave the bis-cyclometalated complexes, Ta(CH2SiMe3)[PhP(C6H4-o-CHNAr)2](PMe3) (460) or Ta(CH2SiMe3)[PhP(C6H4-o-CHNAr)2](dmpe) (461), respectively; hydrogenolysis of 461 gave the mono hydride, TaH[PhP(C6H4-o-CHNAr)2](dmpe) (462).277 Reaction of TaF5 with LiN{4-Me-2-(PiPr2)C6H3}2 gave the tetrafluoride complex, TaF4[N{4-Me-2-(PiPr2)C6H3}2], and subsequent treatment with MeMgBr gave the tetramethyl complex, TaMe4[N{4-Me-2-(PiPr2)C6H3}2] (463); thermolysis of 463 (in toluene, 74  C, 24 h) or photolysis (22 h, room temperature) afforded the bis(methylidene), Ta(]CH2)2[N{4-Me-2-(PiPr2) C6H3}2] (464).278 As observed in the niobium analog (259, 260, Scheme 30), tantalum(V) trichloride complexes containing PCCP ligand, NbCl3(ArC]CAr) (465, Ar ¼ 2-R3P-C6H4; R ¼ iPr, Ph), which was prepared by treatment of TaCl3(Me3SiC]CSiMe3) (dme) with ArC^CAr in a mixed solution of toluene and CH2Cl2, with Mg(CH2SiMe3)2 gave the alkyl-alkylidene complex,

630

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

Scheme 41

Ta(CHSiMe3)(CH2SiMe3)(ArC]CAr) (466, R ¼ iPr) that initiated ROMP of norbornene.99 Hydrogenolysis of the alkyl-alkylidene gave the dimeric hydride complex, (ArC]CAr)Ta(H)2(m-H)3Ta(H)(ArC]CAr) (467).99

5.09.4.4

(Imido)tantalum complexes and some reaction chemistry

As shown in Scheme 42, reactions of diimido complex, [TaCl3(CH3CN)2]2(m-1,3-NC6H4N) with R0 MgCl (R0 ¼ CH2SiMe3, CH2CMe2Ph) or Mg(CHt2Bu)2(THF)2 gave the corresponding alkyl complexes, [TaR3(CH3CN)]2(m-1,3-NC6H4N) (468, R ¼ CH2SiMe3, CH2CMe2Ph, CHt2Bu),216 and the similar reaction of the 1,3-diimido complex gave [Ta(CH2SiMe3)3(CH3CN)]2(m-1,4-NC6H4N) (469, R ¼ CH2SiMe3, CHt2Bu), as observed in the niobium analogs (Scheme 30, 264, 265).216

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

631

Scheme 42

Ta(NAr )(NHAr )Me2 [470, Ar ¼ 2,6-(2,4,6-Me3C6H2)2-C6H3; Mes ¼ 2,4,6-Me2C6H2] was prepared from TaCl2Me3 with 2 equiv. of LiNHAr .279 The similar reaction with Ar NH2 gave the dichloride, Ta(NAr )(NHAr )Cl2; the dichloride complex was treated with LiCHt2Bu to afford Ta(NAr )(NHAr )(CHt2Bu)2 (471). The dimethyl complex (470) was reacted with B(C6F5)3 to afford the cationic, [Ta(NAr )(NHAr )Me]+[MeB(C6F5)3]− (472), and the reaction with AgOTf (OTf ¼ CF3SO3) to give Ta(NAr )(NHAr )Me(OTf ) (473). The triflate complex (473) was reacted with hydrogen to afford Ta(NAr )(OTf )[NH{2-(
5-2,4,6-Me3C6H3)-6(2,4,6-Me3C6H2)-C6H3}] (474) confirmed by X-ray crystallography (Scheme 42).279 Reaction of 473 with 1-hexene (10 equiv.) under hydrogen gave the n-hexyl complex, suggesting the formation of hydride (476) as the intermediate.279 The similar reaction of Cp TaCl4 with 1 equiv. of LiNHAr in Et2O in the presence of NEt3 gave the dichloride complex, Cp Ta(NAr )Cl2 (477), which was converted to the dimethyl complex, Cp Ta(NAr )Me2 (478), by treating with MeLi (2 equiv.).280 The dichloride complex (477) was reacted with KSi(SiMe3)3 to afford Cp Ta(NAr )Cl[Si(SiMe3)3], which was converted immediately to the chloro-hydride complex Cp Ta(NAr )(H)Cl (479) upon exposure of hydrogen.280 The dimethyl complex was treated to AgOTf to afford Cp Ta(NAr )Me (OTf ) (480), which was also converted cleanly (in bromobenzene at 100  C, 3 days under H2 atm) to the hydride-triflate complex, Cp Ta(NAr )H(OTf ) (481). Complex 481 was reacted with LiCHt2Bu to afford Cp Ta(NAr )(CHt2Bu)(OTf ) (482), which was converted to bridged hydride complex, [Cp Ta(NAr )(H)]2(m2-H)2 (483), under atmospheric pressure of H2 (or upon heating at 65  C).280 Cp Ta(NAr0 )Cl2 (Ar0 ¼ 2,6-iPr2C6H3), known as useful starting complex for synthesis of a series of alkyl complexes,281,282 was added LiSiPh3(THF)3 or LiSiHMes2(THF)2 afforded the corresponding chloro-silyl complex, which was converted to a dimeric hydride, [Cp Ta(NAr0 )Cl]2(m2-H)2.283 The resultant complex was further treated with LiSi(SiMe3)3 to afford hydrido-silyl complex,

632

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

Cp Ta(NAr0 )H[Si(SiMe3)3] (484).283 As summarized in Scheme 42, insertion of CH3CN, ArNC (Ar ¼ 2,6-Me2C6H3) gave Cp Ta (NAr0 )H[N]CMeSi(SiMe3)3], Cp Ta(NAr0 )[
2-N(Ar)]C{Si(SiMe3)3}], respectively, and reactions with ethylene, diphenyl acetylene afforded the metallacycle, Cp Ta(NAr)(CH2CH2CH2CH2) (485), Cp Ta(NAr0 )[C(Ph)]C(Ph)dC(Ph)]C(Ph)] (486); reaction with H2 gave the dimeric hydride, [Cp Ta(NAr0 )H]2(m2-H)2 (487).283 Ta(NtBu)R3 (488, R ¼ Me, CH2CMe2Ph, CH2SiMe3) were prepared from Ta(NtBu)Cl3(pyridine)2 by treating with RMgCl,284 as conducted in the reaction with PhMgCl for synthesis of Ta(NtBu)(CH2Ph)3.205 The reaction of Ta(NtBu)Cl3(pyridine)2 or Ta(NAr0 ) Cl3(pyridine)2 with LiOAr0 (Ar0 ¼ 2,6-iPr2C6H3) followed by treatment with PhCH2MgCl gave Ta(NtBu)(CH2Ph)2(OAr0 )(THF) (489) or Ta(NAr0 )(CH2Ph)2(OAr0 )(THF), respectively (Scheme 43).205 As observed in the niobium analogs (Scheme 31, 267-269),

Scheme 43

reaction of Ta(NtBu)(CH2Ph)3 with B(C6F5)3 gave [Ta(NtBu)(CH2Ph)2]+[(
6-C6H5CH2)B(C6F5)3]− (490). THF-free Ta(NtBu) (CH2Ph)2(OAr0 ) (491) was obtained by treatment of 489 with Al(C6F5)3, and subsequent reaction with B(C6F5)3 gave the cationic benzyl complex [Ta(NtBu)(CH2Ph)(OAr0 )]+[(
6-C6H5CH2)B(C6F5)3]− (492); reaction of 490 with B(C6F5)3 followed by addition of Al(C6F5)3 gave the same complex (492).205 Ta(NSiMe3)(CHt2Bu)2[NSi(SiMe3)2] (493) was prepared from [Ta(NSiMe3)Cl{N(SiMe3)2}]2(m2-Cl)2 with LiCHt2Bu; reaction with oxygen gave the bis(alkoxy) complex, Ta(NSiMe3)[NSi(SiMe3)2]-(OCHt2Bu)2 (494).285 The similar reaction with MeLi, PhCH2MgCl gave the corresponding dialkyl complexes, Ta(NSiMe3)Me2[NSi(SiMe3)2] (495), Ta(NSiMe3)(CH2Ph)2[NSi(SiMe3)2], respectively.286 The dimethyl complex (495) was treated with 0.5 equiv. of O2 to afford a dimeric alkoxo complex, [Ta(NSiMe3)Me {N(SiMe3)2}]2(m2-OMe)2 (496), as a mixture of cis-/trans- isomers (but obtained as trans-form after recrystallization).286 As observed in the niobium complexes with b-diketiminate ligands (Scheme 29), (nacnac)Ta(NtBu)Me2 (497, nacnac ¼ HC(C(Me)NAr0 )2, Ar0 ¼ 2,6-iPr2C6H3) was obtained from (nacnac)Ta(NtBu)Cl2(pyridine) by treating with MeMgBr, which was converted to the cationic methyl complex (498) by reaction with [Ph3C][B(C6F5)4] or [(Et2O)2H][ B(C6F5)4] in Et2O.197 Hydrogenolysis of the dimethyl complex afforded cyclometallated [Ar0 NC(Me)CHC(Me)N{2-(CHMeCH2)-6-iPrC6H3}]Ta(NtBu)H (499) and thermolysis of 499 in toluene gave double cyclometallated [HC{(C(Me)N(2-Me2C-6-iPrC6H3}2]Ta(NtBu) (500); introduction of CO to an Et2O solution of 499 resulted in formation of Ta(III) complex, (nacnac)Ta(NtBu)(CO)2 (501).287 Insertion of ArNC (1 equiv., Ar ¼ 2,6-Me2C6H3) to 499 gave [Ar0 NC(Me)CHC(Me)N{2-(CHMeCH2)-6-iPrC6H3}]Ta(NtBu){
2-N(Ar)]CH} (502),

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

633

and further ArNC insertion afforded Ta(V) amidinylidene complex, [Ar0 NC(Me)CHC(Me)N{2-(CHMeCH2CN(Ar)C(H)N(Ar)) -6-iPr-C6H3}]Ta(NtBu) (503).288 Reaction of 503 with AgB(C6F5)4 (1 equiv.) gave a paramagnetic cationic complex (504), by removal of one electron from the redox-noninnocent amidinylidene ligand, whereas the reaction with 2 equiv. of AgB(C6F5)4 in Et2O gave another diamagnetic cationic complex (505), as shown in Scheme 43.288 Reaction of Ta(NAr0 )(iPr2-tacn)Cl2 (506, iPr2-tacn ¼ diisopropyl-1,4,7-triazacyclononane, Ar0 ¼ 2,6-iPr2C6H3) with 3 equiv. of LiCH2SiMe3 in toluene gave the trialkyl complex containing Li ion, Li[Ta(NAr0 )(CH2SiMe3)2(m-CH2SiMe3)(m-
1:
3-iPr2-tacn)] (507), which proceeded a-H abstraction under gentle heating to form the alkylidene Li[Ta(NAr0 )(CH2SiMe3)(m-CHSiMe3) (m-
1:
3-iPr2-tacn)] (508), as shown in Scheme 44.289 Reaction of 508 with Ph2CO gave a dimeric oxo bridged complex (510), and the reaction with FeCl2(tmeda) (tmeda ¼ N,N,N0 ,N0 -tetramethyl ethylene diamine) led to replacement of Li with Fe (511), and

Scheme 44

CO insertion gave the oxatantallacyclopropane complex (512), respectively; structures of these complexes were determined by X-ray crystallography.290 Reaction of 509 with pyridine hydrochloride gave the amide-dialkyl complex, Ta(NAr0 )(iPr2-tacn) (CH2SiMe3)2 (513), whereas one nitrogen ligand in tacn coordinated to Ta.291 The reaction with hydrogen afforded dialkyl-hydride complex (514); the reaction with 2,4,6-Me2C6H2OH gave the dialkyl-phenoxy complex (515).291 Hydroxyalkyl-functionalized NHC was treated with Ta(NtBu)(CHt2Bu)3 or Ta(CHtBu)(CHt2Bu)3 to afford the corresponding dialkyl complex (516) and alkyl-alkylidene complex (517), respectively (Scheme 44).292 The alkyl-alkylidene complex was decomposed at room temperature for several hours; and thermolysis of 517 afforded the cyclometallated complex (518).292 Reaction of imido-dialkyl complex (516) with [RhCl(COD)]2 (COD ¼ 1,5-cyclooctadiene) gave the binuclear Rh-Ta complex with dissocation of NHC from Ta (519).292 Reaction of the alkyl-alkylidene complex (517) with HOSi(OtBu)3 gave the trialkyl complex with dissociation of NHC (Scheme 44).293

5.09.5

Selected topics

5.09.5.1 Vanadium(V)-, niobium(V)-alkylidene complexes as catalysts for ring-opening metathesis polymerization (ROMP) of cyclic olefins and living polymerization of internal alkynes Metal alkylidene complexes with group 5 transition metals, especially Ta- and Nb-alkylidene complexes reported by Schrock, contributed as a pioneering role in organometallic chemistry of multiple metal-carbon bonds.21–25 As demonstrated especially

634

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

by high oxidation state metal-alkylidenes with group 6 transition metals such as Mo and W,21,22,294,295 these complexes play roles as catalysts and/or important intermediate for organic transformations such as olefin metathesis and Wittig-type coupling.21–25,109,128,294–311 As described in the introductory (Scheme 2), the first metal-alkylidene complex, Ta(CHtBu) (CHt2Bu)3, was prepared by a-hydrogen abstraction (elimination).26 As introduced below, certain assistances such as (i) addition of neutral donor ligand (phosphine etc.) to increase the steric crowding, (ii) addition of a base, and (iii) photochemical stimulation, are often required to facilitate the reaction.21–25 However, as described below, reports concerning design of (highly) active vanadium-, niobium-alkylidenes for olefin metathesis were limited until recently. Since these alkylidenes display unique characteristics, here summarizes the synthesis of high oxidation state alkylidene (alkylidyne) complexes with vanadium and niobium, and application as catalysts for the metathesis polymerization.24,109,128,303,308

5.09.5.1.1

Introduction: Synthesis of vanadium-, niobium-alkylidenes

Scheme 45 summarizes reports for synthesis of vanadium-alkylidene and the alkylidyne complexes. As described above, a-hydrogen elimination from the dialkyl complexes lacking b-hydrogen is the common method,21–25 as first demonstrated for synthesis of vanadium(III)-alkylidene, CpV(CHtBu)(dmpe) (14) from the dialkyl analog upon addition of dmpe.63,153 Various (imido)vanadium(V)-alkylidenes containing both arylimido (or adamantylimido) and phenoxide (alkoxide) ligands (93,94) were pre-

Scheme 45

pared from the dialkyl complexes upon addition of PMe3;129–133,136 various imido-alkylidenes containing ketimide (90),114 iminoimidazolide (91),104 iminoimidazolidide (92)127 were then reported. These complexes play a role as catalysts for ring-opening metathesis polymerization of cyclic olefins. Similarly, the (imido)vanadium(V)-alkyl, alkylidene complexes (95-97) were prepared from the trialkyl complexes by addition of NHC or PMe3.137,138

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

635

Benzylidene transfer to CpV(NAr0 )(PMe3)2 with Ph3P]CHPh (upon oxidation) gave the vanadium(V)-alkylidene, CpV(CHPh) (NAr0 )(PMe3) (20, Ar0 ¼ 2,6-iPr2C6H3).65 Oxidation of vanadium(III) dialkyl complex, (nacnac)V(CHt2Bu)2 [nacnac- ¼ {Ar0 N-C (Me)}2CH-] with AgBPh4 gave the vanadium(IV)-alkylidenes, [(nacnac)V(CHtBu)(THF)]+[BPh4]− (42) by a-hydrogen elimination.81 Similarly, several vanadium(V)-alkylidene complexes (70) containing monoanionic tridentate ligand, V(CHt2Bu)2(PNP) [PNP ¼ N{4-Me-2-(PiPr2)C6H3}-2], were prepared by addition of p-acids or two electron oxidants.100,101 Moreover, the resultant alkyl-alkylidene complex (44) was oxidized upon addition of AgOTf (Tf ¼ CF3SO2) or AgBPh4 to afford neutral or cationic vanadium(V)-alkylidynes (46).82 A bicyclic vanadium(V)-alkylidene (121) was formed through acetylene (PhC^CPh) insertion from a vanadium(III) borohydride complex.154

Scheme 46

636

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

Scheme 46 summarizes reports for synthesis of niobium-alkylidene complexes. As described in the introduction, reaction of Nb(CHt2Bu)2Cl3 with CpTl gave Cp2Nb(CHtBu)Cl,29 and treatment of Nb(CHt2Bu)3Cl2 with LiCHt2Bu gave Nb(CHtBu)(CHt2Bu)3;30 moreover, addition of PMe3 to Nb(CHt2Bu)2Cl3 induced a-hydrogen elimination to form Nb(CHtBu)2Cl3(PMe3)2.31 Similarly, treatment of Cp Nb(NAr0 )(CH2Ph)2 (148, Ar0 ¼ 2,6-iPr2C6H3) with PMe3 afforded the benzylidene complex, Cp Nb(CHPh)(NAr0 ) (PMe3) (149).165 Reactions of NbCl3(OAr0 )2 (208, Ar0 ¼ 2,3,5,6-Ph4C6H) with 3 equiv. of Me3SiCH2MgCl gave a mixture of alkyl-alkylidene complex, Nb(CHSiMe3)(CH2SiMe3)(OAr0 )2 (214) and the dialkyl/trialkyl complexes; the alkylidene complex (214) was separated by repetitive recrystallization.189 Treatment of (tBuC5H4)2Nb(CH2Ph)2 (137) with AgBPh4 afforded the cationic niobium(V) benzylidene, [(tBuC5H4)2 Nb(CHPh)][BPh4] (138), which is kinetically unstable and was converted to the cyclometallated product (139) via C-H bond activation.161 Reaction of (Me3SiC5H4)2Nb(RC^CR0 ) (134) by treating with [Cp2Fe][BPh4] (1 equiv.) gave a bimetallic cationic vinylidene [(Me3SiC5H4)2Nb{]C]C(Me)]}(CO)]2[BPh4]2 (135).160 Reaction of Tp NbCl2(PhC]CR) (168, Tp ¼ hydridotris(3,5-dimethylpyrazolyl)borate) with CH2]CHCH2MgCl (1 equiv.), gave five-membered niobacycles (169).170 The similar reaction of 168 (R0 ¼ Me) with EtMgCl gave Tp Nb(CH2CH3)[C(Ph)C(CH3)CHCH2] containing a Nb-carbon double bond, probably formed by dehydrogenation of one ethyl group and subsequent coupled to the coordinated phenylpropyne; a presence of a-agostic interaction was suggested by the 1H NMR spectrum.172 Insertion of 2,6-dimethylphenyl isocyanide to a dimeric [(cb)2Nb(
2-CSiMe3)2Nb(cb)2] (206) gave the dimeric complex containing niobium-carbon double bond (207).188 Alkene-alkylidene rearrangement reactions were observed in the cyclohexene, 1-butene complexes, Nb(OSitBu3)3(olefin), to afford the corresponding alkylidene complexes (229,230), respectively.194 Thermolysis of dimeric [Nb(OSitBu3)3]2(COT) (227) in benzene afforded a dimeric alkylidene-olefin complex (228).194 Detailed investigation of the mechanism was explored in the following studies.195 Scheme 47 summarizes more recent reports for synthesis of niobium-alkylidene and the alkylidyne complexes. Treatment of NbCl(CH3)2Cl(OAr00 )2 [216, Ar00 ¼ 2,6-(Ph2C)2-4-tBuC6H2] with 2 equiv. of H2C]PPh3 gave the terminal methylidene complex, Nb(CH2)(CH3)(CH2PPh3)(OAr00 )2 (217).190 Thermolysis of 216 or NbCl(CH3)2(OAr00 )2(DMAP) in benzene (80  C for 5 days) gave the bridged methylidene [(Ar00 O)2Nb]2(m2-Cl)2(m2-CH2)] (218) exclusively; the photoirradiation (Xe lamp) of 216 in benzene (18 h) also gave the bridged methylidene (218).191 Reduction of 218 with KC8 (2 equiv.) gave methylidyne complex, [K] [{(ArO)2Nb}2(m2-CH)(m2-H)(m2-Cl)] (219), via a-hydrogen elimination. Oxidation of 219 with 2 equiv. of ClCPh3 gave the starting 218.191 Reactions of (PNP)NbCl3 (241) with 3 equiv. of LiCH2SiMe3 or Mg(CHt2Bu)2 gave the corresponding Nb(V) bis-alkylidene complexes, (PNP)Nb(CHSiMe3)2 (242) or (PNP)Nb(CHtBu)2 (243), respectively.198 The reaction with 2 equiv. of LiCH2SiMe3 gave another niobium(V) chloro-alkylidene complex, (PNP)Nb(CHSiMe3)Cl (244).198 Photolysis of (PNP)Nb(CH3)2(OAr0 )(OTf ) (247), prepared by oxidation of(PNP)Nb(CH3)2(OAr0 ) (246) with [FeCp2][OTf], in benzene gave the methylidene complex, (PNP)Nb(CH2)(OAr0 )(OTf ) (248), and the reaction of 247 with H2C]PPh3 afforded the mononuclear

Scheme 47

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

637

methylidyne complex, (PNP)Nb(CH)(OAr0 ) (249). The methylidene complex (248) was treated with LiN(SiMe3)2 to afford the methylidyne complex (259).199 Niobium(V) trichloride complexes containing PCCP ligand, NbCl3(ArC]CAr) (259, Ar ¼ 2-R3P-C6H4; R ¼ iPr, Ph) with Mg(CH2SiMe3)2 gave the alkyl-alkylidene complex, Nb(CHSiMe3)(CH2SiMe3)(ArC]CAr) (260, R ¼ iPr).99 Heating the (imido)niobium(V)-dialkyl complexes, Nb(NR0 )(CH2SiMe3)2[OC(CF3)3] (279), prepared by treatment of Nb(NR0 ) (CH2SiMe3)3 (278) with HOC(CF3)3, in the presence of excess PMe3 gave the corresponding niobium(V)-alkylidenes, Nb(CHSiMe3)(NR0 )[OC(CF3)3](PMe3)2 (280,281).220,223 As described below, these complexes (280,281) are effective catalysts for ring opening metathesis polymerization of cyclic olefins220 and living metathesis polymerization of internal alkynes.223

5.09.5.1.2 Vanadium(V)-, niobium(V)-alkylidene complexes as catalysts for ring-opening metathesis polymerization (ROMP) of cyclic olefins and living polymerization of internal alkynes As described above, high oxidation state metal-alkylidenes play an important role as catalysts in olefin metathesis and Wittig-type coupling. Olefin metathesis has been the useful method applied for synthesis of various fine chemicals, polymers, and advanced materials.22,24,109,128,294–311 Ring-opening metathesis (ROM, reaction with cyclic olefins), ring closing metathesis (RCM, intramolecular reaction with acyclic diene), and acyclic diene metathesis (ADMET, intermolecular reaction with acyclic diene), cross metathesis (CM, reaction with acyclic olefins)] proceed via metallacycle intermediate formed from the metal-carbene (alkylidene) species (Scheme 48).298–300 Ring-opening metathesis polymerization (ROMP) and ADMET polymerization are widely used in

Scheme 48

synthesis of advanced polymeric materials.304–311 Catalyst development has been considered an important subject because there are still concerns such as thermal stability and, catalyst deactivation over time (and subsequent isomerization and/or generating radicals etc. especially by Ru catalysts). However, reports for synthesis of the alkylidene complexes with vanadium and niobium,10,15 which should be (highly) active for olefin metathesis were limited until recently.23,24,128,308,312,313 Reports for olefin metathesis by metal-alkylidene complexes with group 5 transition metals were limited before 2000.35,63,163,231,246,314,315 Schrock et al. reported (in 1981) reactions of ethylene, propylene, 1-butene, cis-2-pentene, and with styrene using niobium- and tantalum-alkylidenes, M(CHPh)L2X3 or M(CHtBu)L2X3 (M ¼ Nb, Ta; X ¼ Cl, Br; L ¼ phosphine);314 only metathesis products were obtained in the reactions with terminal and internal olefins in the presence of Nb(CHtBu) (OtBu)2Cl(PMe3).314 The first olefin metathesis active vanadium(III)-alkylidene CpV(CHtBu)(dmpe) (14) was reported by Hessen and Teuben in 1993, which was prepared by a-hydrogen abstraction from the dialkyl analog, CpV(CHt2Bu)2(PMe3)2, upon addition of dmpe (Scheme 49).11b Reaction of norbornene (NBE) by the alkylidene (14) was attempted but showed extremely low (catalytic) activity for ROMP (0.92 turnovers after 96 h at 20  C); the dialkyl complex showed the higher activity (8 turnovers after 96 h at 20  C).63 Attempted reaction of NBE with the vanadium(V)-alkylidene, CpV(CHPh)(N-2,6-iPr2C6H3)(PMe3) (20), gave negligible product.65 Mashima et al. reported in 1997 that metallacycle derived from the dimethyl analog, Cp NbMe2(
4-C4H6) (140), by treatment with NBE initiated ROMP of NBE (12 turnovers after 30 h at 60  C) affording low molecular weight oligomers (Mn ¼ 2900, Mw/Mn ¼ 4.78).163 Cp Ta(CHPh)[
4-o-(CH2)2C6H4] (313) and the dibenzyl analogs (312) also initiated ROMP of NBE (8 and 94 turnovers after 30 h at 60  C).231 Tantalum(V)-alkylidene complexes containing phenoxide ligands showed catalytic activity for ROMP of NBE,246,315 and a living polymerization behavior was demonstrated by adopting the metallacyclobutane analogs (521, Scheme 49).315 Ta[]C(Me)C(Me)]CHtBu](O-2,6-iPr2C6H3)3(pyridine) (522), prepared by addition of 1-butyne and pyridine to Ta(CHtBu)(O-2,6-iPr2C6H3)3(THF) polymerized not only NBE, but also 2-butyne, 1-pentyne.35 In 2001, our group

638

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

Scheme 49

reported ROMP of NBE initiated by V(N-2,6-Me2C6H3)(CH2Ph)2(O-2,6-iPr2C6H3) (79, 37 turnovers after 11 h at 25  C in toluene);111 synthesis of ultrahigh molecular weight ring-opened polymers with unimodal molecular weight distribution has been demonstrated (Mn ¼ 2.43  106, Mw/Mn ¼ 1.93), strongly suggesting a possibility of vanadium(V)-alkylidene generation in situ.111 We reported in 2005 that V(CHSiMe3)(NAr)(N]CtBu)(PMe3) (90) showed catalytic activity for ROMP of NBE at 80  C (1580 turnovers after 1 h) affording ultrahigh molecular weight polymer with unimodal molecular weight distribution (Mn ¼ 1.33  106, Mw/Mn ¼ 1.4).114 The activity by 90 increased at 80  C (267 turnovers after 3 h at 25  C), whereas the activities by ordinary molybdenum-alkylidene and ruthenium-carbene catalysts decreased under these conditions due to the catalyst decomposition. Unique characteristics (thermal resistance affording ultrahigh molecular weight polymers) by using vanadium catalysts in addition with much improvement in the activity compared to those previously reported [CpV(CHtBu)(dmpe) (14) and CpV(CHt2Bu)2(PMe3)2,63 V(N-2,6-Me2C6H3)(CH2Ph)2(O-2,6-iPr2C6H3)80] were demonstrated.114 Later, a series of (imido)vanadium(V)-alkylidene complexes of type, V(CHSiMe3)(NR)(Y)(PMe3)n [R ¼ aryl or 1-adamantyl; Y ¼ iminoimidazolide

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

639

Scheme 50

(91),104 iminoimidazolidide (92),127 phenoxide (93),129–133 alkoxide (94);131,132,136 n ¼ 1 or 2], shown in Scheme 45 were reported.128,312,313 These complexes showed moderate to remarkable catalytic activities for ROMP of NBE (Scheme 50); living polymerization systems were also demonstrated by using some catalysts even at high temperature.131,132 It turned out that, as summarized in Scheme 50, the activity by the phenoxide analogs (93) increased in the order (at 25  C in benzene): V(CHSiMe3)(N-2,6-Me2C6H3)(O-2,6-iPr2C6H3)(PMe3) (TOF 150 h−1)129  V(CHSiMe3)(N-2,6-Me2C6H3) (O-2,6-F2C6H3)(PMe3)2 (TOF 1270 min−1) < V(CHSiMe3)(NAd)(OC6F5)(PMe3)2 (1650 min−1) < V(CHSiMe3)(N-2,6-Me2C6H3) (OC6F5)(PMe3)2 [3830 min−1 (64 sec−1)]  V(CHSiMe3)(N-2,6-Cl2C6H3)(OC6F5)(PMe3)2 [523, 7070 min−1 (118 sec−1), >99% conv.].131,132 The results clearly indicate that an electronic factor plays a role in the high catalytic activity; both the imido and the phenoxide ligands containing electron-withdrawing groups, generating electron deficient alkylidene species, led to the higher activity. The activity was higher than that by Mo(CHCMe2Ph)(N-2,6-Me2C6H3)[OCMe(CF3)2], which was known as the most active catalyst in the early transition metal alkylidene.131 The ROMP by V(CHSiMe3)(N-2,6-Me2C6H3)(OC6F5)(PMe3)2 proceeded in a living manner to yield ultrahigh molecular weight polymers with rather low PDI (Mw/Mn) values.131 Linear relationships between the polymer yields and the Mn values were consistent with low PDIs, and the first order kinetics without catalyst deactivation were demonstrated in this catalysis.131 Although the resultant polymers prepared by the OC6F5 analog (93) possess a mixture of cis/trans olefinic double bonds, polymers prepared by the fluorinated alkoxide analogs, V(CHSiMe3)(N-2,6-X2C6H3)[OC(CF3)3](PMe3)2 (94, X ¼ H, Cl), possessed high cis selectivity (98%). The activities increased by addition of PMe3 even at 80  C probably due to improvement of stability of the active species by excess PMe3, and the high cis percentages (97%, 98%) were maintained even at 50  C and 80  C in the presence of

640

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

PMe3. Although cis specific (Z selective) olefin metathesis reactions303,305 including ROMP using molybdenum316–328 and ruthenium329–340 complex catalysts were known, these reactions were generally conducted at room temperature; these are rare examples as thermally robust, highly efficient cis specific ROMP catalysts. It was assumed that coordination of NBE for subsequent metathesis would be controlled in this catalysis; NBE would coordinate to V trans (opposite) to PMe3 and high cis selectivity would be achieved due to a proposed intermediate consisting of a steric bulk of small arylimido and large alkoxo ligands (Scheme 50).128,131,132,303,308 Efficient synthesis of end-functionalized ring-opened polymers was reported by combined cis-specific ROMP of NBE with terminal olefins (1-hexene, allyltrimethylsilane, vinylcyclohexane, 4-vinyl-1-cyclohexene etc.) as the chain transfer (cross metathesis) agent in the presence of 94 (X ¼ Cl).341 ROMPs of low strained cis-cyclooctene, cycloheptene were demonstrated by V(CHSiMe3)(N-2,6-Cl2C6H3)(OC6F5) (PMe3)2 (523) and V(CHSiMe3)(N-2,6-Cl2C6H3)(OC6Cl5)(PMe3)2 (524) and the OC6Cl5 showed the higher activity. The driving force in the ROMP has been invoked to be release of the ring strain, and ROMP of COE by metal-alkylidene with early transition metals were very limited. The ROMP of cycloheptene, cis-cyclooctene proceeded in a living manner even at 80  C.133 Cis-specific ROMP of low strain cycloheptene was also demonstrated by using V(CHSiMe3)(NC6F5)[OC(CF3)3](PMe3)2 (525).136 After the first report in ROMP of NBE by half-niobocene (141) in 1997,163 as far as we know, there had been no reports concerning synthesis of niobium(V)-alkylidene complexes that initiate ROMP.24 In 2016, as shown in Scheme 51, two groups reported ROMP of NBE at the same time by (imido)alkylidene catalysts, Nb(CHSiMe3)(NR)[OC(CF3)3] (280, R ¼ 1-adamantyl, 2,6-Me2C6H3, 2,6-iPr2C6H3),220 and dialkyl complex containing chelate trianionic ligand, Nb(CH2CMe2Ph)2[N{2-OC (CF3)2-4-MeC6H3}2}] (258).203 Much improvement in the catalyst efficiencies (1020–6530 turnovers after 2–10 min, at 25–50  C; TOF 211–3180 min−1) compared to 141 (12 turnovers after 30 h; TOF 0.4 h−1)163 was demonstrated by 280,220 whereas the resultant polymer by 280 possessed cis-, syndiotactic (85%) stereoregularity (62 turnovers after 15 h; TOF 4.1 h−1) although the activity seemed rather low.203 Later ROMP of NBE by 260 was also reported (85 turnovers after 120 h).99 The activity in the ROMP by Nb(CHSiMe3)(NR)[OC(CF3)3] (280) conducted at 25  C increased in the order: R ¼ 1-adamantyl (203 min−1) < 2,6-iPr2C6H3 (305 min−1) < 2,6-Me2C6H3 (928 min−1). The activity further increased at 50  C (3180 min−1 by the

Scheme 51

2,6-Me2C6H3 analog).220 Although the related vanadium complexes (94)132 gave the ring-opened polymers with high cis-selectivity, the resultant polymers by 280 did not have regularity; these polymerizations by 280 proceed in a living manner, affording ultrahigh molecular weight polymers with rather low PDIs.220 Nb(CHSiMe3)(N-2,6-Me2C6H3)[OC(CF3)3] (280a) polymerized 2-hexyne at 25  C, although the polymerization by V(CHSiMe3)(N-2,6-Cl2C6H3)(OC6F5)(PMe3)2 (523) did not take place.220 Later, living polymerizations of internal alkynes (2-hexyne, 3-hexyne, 4-methyl-2-pentyne, and 1-phenyl-1-propyne) were demonstrated at 50  C by Nb(CHSiMe3)(NAr)[OC (CF3)3](PMe3)2 [Ar ¼ 2,6-Me2C6H3 (280a), 2-MeC6H4 (280b), 2,6-Cl2C6H3 (280c)], in the presence of PMe3 (Scheme 51).223 The living nature by 280a was preserved even at 80  C in the 2-hexyne polymerization upon presence of excess PMe3; the activity by 280b and 280c showed higher than that by 280a. Effect of the internal alkynes toward the activity by 280b (at 50  C in the presence of PMe3) is in the order: 2-hexyne > 4-methy-2-pentyne > 3-hexyne > 4-octyne  5-decyne (negligible).223 These are only one

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

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example for living metathesis polymerization of internal alkynes by Nb(V)-alkylidenes, followed by one report by Schrock using Ta []C(Me)C(Me)]CHtBu](O-2,6-iPr2C6H3)3(pyridine) (522), shown in Scheme 49.35

5.09.5.2 Solution XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure) analysis for exploring homogeneous catalytically active species 5.09.5.2.1

Introduction

Analysis of catalytically active species has been one of the most important subject in terms of not only of clear understanding the catalysis mechanism, but also of catalyst design on the basis of information of the structural/electronic nature. NMR is the principal technique for identification of (diamagnetic) inorganic/organometallic compounds in (partially) deuterated solvent, and single crystal X-ray crystallography is the solid method for obtainment of the structural information of the proposed intermediate(s), which is (are) generally isolated by certain stabilization. Some reaction chemistry using model complexes (and using the isolated species into the catalytic reaction) and computational analysis are helpful to draw a proposed catalysis mechanism. ESR spectroscopy has also been used for the paramagnetic compounds that show negligible or broad resonances in the NMR spectra.342–346 It seems however difficult to obtain clear structural information (image) in solution by these (NMR, ESR) spectroscopies. Moreover, ESR generally lacks the quantitative analysis,342,343,347 and cannot exclude a the possibility of formation of “ESR silent” species; generally considered, this is exemplified in vanadium(III) with 3d2 electron configuration [S ¼ 1, triplet, S ¼ spin quantum number, due to an interaction between the two unpaired electrons by spin-spin coupling (SSC)] or antiferromagnetically coupled vanadium(IV) dimer due to spin-orbit coupling (SOC).347,348 Analysis by XAS (X-ray absorption spectroscopy) at synchrotron facilities provides information of the oxidation state and the geometry by XANES (XANES ¼ X-ray Absorption Near Edge Structure) and of their coordinated atoms to the metal center by EXAFS (EXAFS ¼ Extended X-ray Absorption Fine Structure) spectra. The methods are popular in heterogeneous catalysis349–353 but the reports applied to homogeneous catalysis have been still limited.15,16,93,126,141,144,222,224,354–357 It has been recognized that the methods are useful for analysis of catalysis species especially with early transition metals, as exemplified in analysis of ESR silent paramagnetic vanadium(III) species that cannot be observed by NMR and ESR spectroscopies.126,144,356,357

5.09.5.2.2

Solution XAS analysis of active species of vanadium complex catalysts in ethylene polymerization/dimerization

It has been known that pre-edge peak intensity and edge absorption (exemplified in Fig. 1) in XANES spectra are influenced by the oxidation state and the basic geometry around the centered metal.15,16,93,349–353,358–364 V(NAd)Cl2[2-(2,6-Me2C6H3) NCH2(C5H4N)] (101a) shows two pre-edge peaks (absorptions) at 5465.2 and 5467.3 eV (ascribed to a transition from 1s ! 3d + 4p),358–364 which are similar to those by the dimethyl analog, V(NAd)Me2[2-(2,6-Me2C6H3)NCH2(C5H4N)] (103a, 5465.0 and 5467.1 eV), except that 101a showed a shoulder-edge at 5477.8 eV corresponded to an absorption in the V–Cl bond141 [known as a shakedown peak assigned to the metal 1s to 4p coupled with the ligand to metal charge transfer (LMCT)].364–366 As shown in Scheme 52, both 101a and 103a showed remarkable catalytic activities for selective ethylene dimerization in the presence of methylaluminoxane (MAO) cocatalyst5,141 whereas 101a gave ultrahigh molecular weight polyethylene in the presence of chlorinated Al cocatalysts (Me2AlCl, Et2AlCl).143 The pre-edge peak positions and intensities did not change by treatment of 101a with 10 equiv. of MAO, whereas decrease in the shoulder edge intensity was observed due to cleavage of VdCl bonds by MAO, which was also confirmed by the FT-EXAFS spectrum (as the coordination number).141 Moreover, no significant changes in the EXAFS oscillation and the FT-EXAFS spectra, except that apparent decrease in the intensity ascribed to V–Cl bond, were observed when toluene solution of 101a was added MAO.141 In contrast, treatment of 101a with 10 equiv. of Me2AlCl led to a slight low energy

Fig. 1 V K-edge XANES spectra (in toluene at 25  C) for V(NAd)Cl2[2-(2,6-Me2C6H3)NCH2(C5H4N)] (101a) and V(NAd)Me2[2-(2,6-Me2C6H3)NCH2(C5H4N)] (103a) in the presence of Al cocatalyst [methylaluminoxane (MAO) or Me2AlCl, 10.0 equiv.].141

642

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

Scheme 52

shift with appearance of the shoulder-edge absorption (suggesting formation of VdCl bond) without changes in the pre-edge peaks (5476.4 eV). The similar spectral change was observed by treatment of the dimethyl complex (103a) with 10 equiv. of Me2AlCl, strongly suggesting formation of the same species in situ (as also observed in the 51V NMR spectra).141 On the basis of (i) 51V NMR and ESR spectra (diamagnetic species observed),141,143 (ii) isolation of the dimethyl (and cationic) complexes and their reaction chemistry,141,143 and (iii) XANES and EXAFS analysis,141 it is concluded that cationic vanadium(V) alkyl species play a role as the active species in the ethylene dimerization as well as polymerization.141 The observed difference as a function of Al cocatalyst could be explained as catalyst/cocatalyst nucelarity effect367–370 (isolated cationic alkyl species with MAO or associated cationic alkyl species which are generated by dissociation from a Cl bridged alkyl species present as a dormant coupled with halogenated Al alkyl).17,141,143 The reaction of the trichloride, V(NAd)Cl3, with Me2AlCl gave reduction to form a vanadium(III) species (observed in the XANES spectrum),141 strongly suggesting that the chelate ligand plays a role to stabilize the oxidation state in reaction with Al alkyls. The phenoxide modified (arylimido)vanadium(V) dichloride complex, V(N-2,6-Me2C6H3)Cl2(O-2,6-Me2C6H3) (80a, Schemes 13 and 53), showed high activities for ethylene polymerization in the presence of Al cocatalysts;111,117–119 The activity was highly affected by the Al cocatalyst employed; the activity in the presence of halogenated Al alkyls (Me2AlCl, Et2AlCl, iBu2AlCl etc.) was higher than that in the presence of MAO.117,119 (Arylimido)vanadium dichloride complexes containing anionic NHCs with a weakly coordinating B(C6F5)3 moiety (WCA-NHC), expressed as 85 in Scheme 13,125,126 showed high activities for ethylene polymerization in the presence of Al cocatalyst, especially the diisopropyl-phenyl analog V(N-2,6-Me2C6H3)Cl2(WCA-NHC) (85e).126 The activity by 85e in the presence of MAO (19,500 kg-PE/mol-V h) or AliBu3 (66000 kg-PE/mol-V h) is higher than

Scheme 53 Effect of Al cocatalyst and anionic donor ligand in ethylene polymerization.115,117,125,126,144

those by the other (imido)vanadium(V) dichlorides containing phenoxide,111,117–119 iminoimidazolide,115 iminoimidazolidide.115 No or negligible resonances in the 51V NMR and ESR spectra were observed when a toluene solution of 85c was treated with AliBu3.356 Fig. 2 shows V K-edge XANES spectra for complexes 80a and 85e in the presence of Al cocatalyst (V 50 mmol/mL in toluene at 25  C).126 Complex 85e shows sharp pre-edge peak(s), which are typically observed in the spectra of vanadium(V) complexes with tetrahedral geometry [5466.9 eV (and 5465.1 eV)] (ascribed to a 1s ! 3d + 4p transition),358–364 and a shoulder-edge peak at

Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

643

Fig. 2 V K-edge XANES spectra (in toluene at 25  C) for V(N-2,6-Me2C6H3)Cl2(O-2,6-Me2C6H3) (80a) and V(N-2,6-Me2C6H3)Cl2(WCA-NHC) (85e) in the presence of Al cocatalyst (MAO, Me2AlCl or AliBu3, 10.0 equiv.].126,356,357

5478 eV ascribed to an absorption of the VdCl bond,126 assigned to the metal 1s ! 4p coupled with the ligand to metal charge transfer.364–366 Similarly, the V K-edge XANES spectra of 80a showed a pre-edge (and a shoulder) peak [5466.8 (and 5465.3) eV] and a shoulder-edge absorption (5478.2 eV).126,357 No significant spectral changes in the pre-edge peaks (position and the intensity) and the edge absorption were observed when MAO (10 equiv.) was added to a toluene solution of 85e, whereas the pre-edge peaks in 85e were shifted slightly at 5464.5 eV with decreasing in intensity of the shoulder-edge absorption.126 The similar spectra (and the spectral changes) were observed in the dimethylphenyl analog (85c),356 and the results clearly suggest that the oxidation state and the basic framework were preserved from 85c,e by treating with MAO. Similarly, the pre-edge peak positions in 80a did not change (5466.8 eV) by addition of MAO (10 equiv.) with increasing in the intensity accompanied with slight shift in edge absorptions (Fig. 2).126,357 The result also suggest that the oxidation state and the basic framework were preserved even upon addition of MAO. In contrast, a low energy shift in the edge absorption with decreasing in the pre-edge intensity was observed by treatment of 85e with AliBu3, indicating reduction of the vanadium. The results are consistent with those observed in the 51V NMR and the ESR spectra, which showed negligible resonances in the spectra by addition of AliBu3.356 The apparent decrease in the pre-edge intensity upon addition of AliBu3 clearly suggest a structural change, and the large low energy shift in the edge absorption [even compared with VIV(NAd)Cl2(NMe2H)2] strongly suggests a formation of vanadium(III) species by reduction;126,356,357 the structural changes were also suggested by the EXAFS spectra (described below). The results are an interesting contrast to those observed in the reaction of V(NAd)X2[2-ArNCH2C5H4N] [X ¼ Cl (101a), Me (103a)] with MAO and Me2AlCl (Fig. 1), indicating that anionic chelate ligand plays a role to stabilize the oxidation state in reaction with Al alkyls.126,141 Moreover, low energy shifts (shoulder-edge at 5475.7 eV) in the edge absorption were observed, when 80a was treated with Me2AlCl or Et2AlCl; the pre-edge peak in 80a [5466.8 (and 5465.3) eV] shifted to low energy or became one absorption by adding Me2AlCl (5465.7 eV) or Et2AlCl (5465.9 eV).126 The results strongly suggest that 80a was reduced by Me2AlCl or Et2AlCl to afford a vanadium(III) species.126 Note that, as summarized in Fig. 2, the observed XANES spectrum of 80a with addition of Me2AlCl are apparently different from that of 85e with addition of AliBu3. Also note that the spectra (80a–Me2AlCl),126 especially in the pre-edge intensities, are different from that for the other imido analog containing 2-(20 -benzimidazolyl)-6-methylpyridine ligand (526, Scheme 53) by treatment with Me2AlCl, which exhibited remarkable catalytic activity in ethylene polymerization144 The shoulder-edge intensity (at 5475.7 eV) decreased when Cl3CCO2Et (ETA, 50 equiv.), known as the reactivator (re-oxidant) commonly used in Ziegler type vanadium catalysis,11–14 was added into a toluene solution containing 80a and Me2AlCl (and NBE),126 whereas the intensity increased with decreasing the pre-edge intensity (at 5465.5 eV) when a toluene solution of 526 and Me2AlCl was added with ETA.144 These observations also correspond to the results that the activity in ethylene polymerization by 80a decreased upon addition of ETA,119 whereas the activity by 526 increased upon addition of ETA,144 as seen in most vanadium complex and classical Ziegler type vanadium catalyst systems (as described in the introduction). These also suggest that different catalytically active vanadium(III) species would play roles in these two reactions. EXAFS analysis revealed that addition of AliBu3 to a toluene solution containing 85e led to a formation of an (arylimido)vanadium(III) species containing one V–Cl bond (2.34  0.04 A˚ ), which is longer than those in the reported (imido)vanadium (V) dichloride complexes containing monodentate anionic ligands [2.1901(8)–2.2462(8) A˚ , by X-ray crystallography] was suggested. The analysis of 80a (in toluene solution after treatment with Me2AlCl) suggests a formation of another (arylimido) vanadium species containing two VdCl bonds (2.45  0.03 A˚ ). The EXAFS analysis of toluene solution containing 526 and

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Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

Me2AlCl also suggests a presence of vanadium-imido species containing more than two (three) VdCl bonds [2.455(7) A˚ ] with dissociation of 2-(20 -benzimidazolyl)-6-methylpyridine ligand (Scheme 53).126 However, in all cases, presence of the VdCalkyl bond, which should be present as catalysis, could not be defined. On the basis of these EXAFS analyses, it should be considered that three different vanadium(III) species (with different number of Cl ligands as a neutral donor) play roles in the polymerization depending upon the anionic donor ligand employed. These vanadium(III) species could be stabilized by coordination of neutral donor ligands exemplified with the cheoride complexes,126 although more precise analysis (on the basis of more clear spectra or advanced analysis) should be required. For comparison, it should be noted that toluene solutions containing classical Zielger type vanadium catalysts (such as VOCl3 and halogenated Al alkyls) underwent rapid decomposition observed by V K-Edge XANES spectra under these conditions (at 25  C).371

5.09.6

Concluding remarks

The synthesis and reactivity of organometallic group 5 transition metal complexes (vanadium, niobium, and tantalum) containing metal-carbon single and multiple bonds reported over the past 20 years were reviewed alongside historically important results that contextualize these more recent developments. Reports concerning certain types of catalysis (organic transformations using organometallic catalysts formed in situ, e.g., oxidation including CdC cleavage by vanadium catalysis), chemistry with (oxo) vanadium complexes,372–380 and recent advances in nitrogen fixation chemistry381 were omitted from this chapter, but some of these topics are covered elsewhere in COMC4. As described in the introduction, classic Ziegler-type vanadium catalyst systems (consisting of vanadium complexes and organometallic reagents) display remarkable reactivity toward olefins to afford ultrahigh molecular weight polymers.1–14 Many reports concerning the synthesis of new vanadium complexes and their use as catalyst precursors for olefin polymerization were introduced.11–14 However, in most cases, the oxidation states of the active catalytic species in solution in the presence of Al cocatalysts were uncertain, and the catalyst systems often required excess addition of a so-called re-oxidant [e.g., Cl3CCO2Et (ETA)] to avoid rapid deactivation. Certain vanadium(III) species have been assumed to play a role as the active species, but conventional spectral methods have not been sufficient for quantitative analysis and cannot be applied to “ESR silent” vanadium(III) species.342–348 As described in Section 5.09.5.2, XAS can be more useful for analysis of vanadium catalyst solutions and oxidation states, especially those containing ESR silent paramagnetic species.15,16,93,355 Existence of vanadium(III) species was clearly showed based on different edge absorptions in the XANES spectra [low energy shift compared to vanadium(IV) and vanadium(V)], whereas no or negligible resonances were observed in the corresponding 51V NMR and the ESR spectra.15,126,144 No significant changes in oxidation state was also confirmed in the catalyst solution containing (imido)vanadium(V) complexes containing chelate anionic donor ligand in the presence of Al cocatalyst; these results clearly indicated that the ligand structure plays an essential role in controlling the desired catalyst performance.15,16,93,141 Moreover, EXAFS provides information concerning the number and kind of coordinated atoms,382 and helped to provide a clear structure of the active species. Collectively, these result suggest that XAS should be used by more researchers in the near future as an in-situ means to interrogate catalytically-relevant vanadium systems. In the last 15 years, significant progress has been made in synthesis and reaction chemistry of vanadium- and niobiumalkylidene complexes, especially those containing bidentate nacnac [nacnac ¼ HC(C(Me)NAr0 )2, Ar0 ¼ 2,6-iPr2C6H3), tridentate PNP ligand [PNP ¼ N{4-Me-2-(PiPr2)C6H3}−2], and imido-alkylidene complexes containing anionic ancillary donor ligands. As described in the introduction, group 5 complexes, especially Ta- and Nb-alkylidene complexes, have played a pioneering contribution in the chemistry of organometallic complexes containing multiple metal-carbon bonds.21–25 As an extension, careful studies in alkylidene chemistry should lead to the design of new and more efficient catalysts. In particular, (arylimido)vanadium (V)-alkylidene and (arylimido)niobium(V)-alkylidene complexes introduced in Section 5.09.5.1 exhibited unique characteristics in olefin metathesis polymerization [ring-opening metathesis polymerization (ROMP) and living polymerization of internal alkynes by Nb(V)-alkylidenes220,223],24,128,308,312,313 including some reactivity not accessible using known ruthenium and molybdenum catalysts.294–311 Vanadium and niobium complexes containing both imido and (chelate) anionic donor ligands also exhibited high activities (and selectivity) in ethylene polymerization and dimerization reactions. This progress in V and Nb organometallic chemistry is paving new roads in homogeneous catalysis chemistry with group 5 transition metals. These results suggest there is a brilliant future in organometallic chemistry with group 5 transition metals, especially at their intersection with molecular catalysis, bioinorganic chemistry, and the development of alternative methods to analyze ESR and NMR silent species, as exemplified with XAS.

Acknowledgments The author would like to express his heartfelt thanks to some of his students in the laboratory of organic chemistry, Tokyo Metropolitan University (TMU) who helped review the chapter, and to his student assistants, Ms. Yuri Sugiyama (TMU), who helped prepare this extensive manuscript. The synchrotron XAFS analysis (described in 5-2) were performed at the SPring-8 beam lines of BL01B1 with the approval of Japan Synchrotron Radiation Research Institute (JASRI, 2015B1308, 2016A1455, 2016B1509, 2017A1512, 2018A1245, 2018B1335).

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Wu, Z.; Xian, D. C.; Natoli, C. R.; Marceli, A.; Paris, E.; Mottana, A. Appl. Phys. Lett. 2001, 79, 1918–1920. Rehr, J. J.; Ankudinov, A. L. Coord. Chem. Rev. 2005, 249, 131–140. Yamamoto, T. X-Ray Spectrom. 2008, 37, 572–584. Glatzel, P.; Smolentsev, G.; Bunker, G. J. Phys.: Conf. Ser. 2009, 190, 012046. Yi, J.; Nakatani, N.; Nomura, K.; Hada, M. Phys. Chem. Chem. Phys. 2020, 22, 674–682. Yokoyama, T.; Kosugi, N.; Kuroda, H. Chem. Phys. 1986, 103, 101–109. Bair, R. A.; Goddard, W. A., III Phys. Rev. B 1980, 22, 2767–2776. Macchioni, A. Chem. Rev. 2005, 105, 2039. Li, H.; Marks, T. J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15295. Bochmann, M. Organometallics 2010, 29, 4711. McInnis, J. P.; Delferro, M.; Marks, T. J. Acc. Chem. Res. 2014, 47, 2545. Nomura, K. Unpublished results. Langeslay, R. R.; Kaphan, D. M.; Marshall, C. L.; Stair, P. C.; Sattelberger, A. P.; Delferro, M. Chem. Rev. 2019, 119, 2128–2191. Hanson, S. K.; Baker, R. T. Acc. Chem. Res. 2015, 48, 2037–2048. da Silva, J. A. L.; Fraústo da Silva, J. J. R.; Pombeiro, A. J. L. Coord. Chem. Rev. 2013, 257, 2388–2400. Sutradhar, M.; Pombeiro, A. J. L. Coord. Chem. Rev. 2014, 265, 89–124. Amadio, E.; Di Lorenzo, R.; Zonta, C.; Licini, G. Coord. Chem. Rev. 2015, 301-302, 147–162. Sutradhar, M.; Martins, L. M. D. R. S.; Guedes da Silva, M. F. C.; Pombeiro, A. J. L. Coord. Chem. Rev. 2015, 301-302, 200–239. Schwendt, P.; Tatiersky, J.; Krivosudský, L.; Šimuneková, M. Coord. Chem. Rev. 2016, 318, 135–157. Maurya, M. R. Top. Catal. 2018, 61, 1500–1513. Maurya, M. R. Coord. Chem. Rev. 2019, 383, 43–81. Tanabe, Y.; Nishibayashi, Y. Coord. Chem. Rev. 2019, 381, 135–150. Linehan, J. C.; Balasubramanian, M.; Fulton, J. L. In XAFS Techniques for Catalysts, Nanomaterials, and Surfaces; Iwasawa, Y., Asakura, K., Tada, M., Eds.; Springer: Switzerland, 2017; pp 431–450.

5.10

Group 6 Complexes With Metal-Carbon Sigma Bonds

Brian J Bellott and Matthew A Klyman, Western Illinois University, Macomb, IL, United States © 2022 Elsevier Ltd. All rights reserved.

5.10.1 5.10.2 5.10.2.1 5.10.2.2 5.10.2.3 5.10.2.4 5.10.2.5 5.10.3 5.10.3.1 5.10.3.2 5.10.3.3 5.10.3.4 5.10.3.5 5.10.3.6 5.10.4 References

Introduction Cr complexes Alkyl and aryl complexes without Cp-type ligands Alkyl and aryl complexes with Cp-type ligands Alkyl and aryl complexes with metal-element multiple bonds Alkynyl complexes Multimetallic Cr complexes and clusters Molybdenum and tungsten complexes Alkyl and aryl complexes without Cp-type ligands Alkyl and aryl complexes with Cp ligands and without NO ligands Alkyl and aryl complexes with Cp and NO ligands Complexes with metal-element multiple bonds Alkynyl complexes Multimetallic Mo and W complexes and clusters Conclusion

651 652 652 656 658 658 659 659 659 660 660 666 668 668 668 669

Abbreviations dmpe dppe depe DIPP BARF TMEDA TMCDA bipy mes dpp xyl dep HLiPr THF THP MTBE

5.10.1

1,2-Bis(dimethylphosphino)ethane 1,2-Bis(diphenylphosphino)ethane 1,2-Bis(diethylphosphino)ethane 2,6-Diisopropylphenyl B(3,5-(CF3)2C6H3)4− N,N,N0 ,N0 -Tetramethylethylenediamine ((R,R)-N,N,N0 ,N0 -Tetramethylcyclohexanediamine) 2,20 -Bipyridine 2,4,6-Me3C6H2 2,6-(Me2CH)2C6H3 xylyl, 2,6-Me2C6H3 2,6-Diethylphenyl, 2,6-(CH3CH2)2C6H3 N,N0 -Bis(2,6-diisopropylphenyl-1,4-diazadiene) Tetrahydrofuran Tetrahydropyran (Tert-butylmethyl ether)

Introduction

This chapter deals with group 6 complexes containing metal-carbon sigma bonds with anionic organometallic ligands such as alkyls, aryls, alkenyls, and alkynyls. Complexes that contain metal-carbon sigma bonds with neutral ligands such CO, isocyanide, N-heterocyclic carbenes, or pincer ligands are covered in other sections of Comprehensive Organometallic Chemistry IV. This chapter has been broken into two major sections with one discussing chromium complexes and the other section discussing molybdenum and tungsten complexes together. The chromium section contains five subsections and the molybdenum/tungsten section contains the same subsections but there is a break in complexes Cp complexes which either do or do not contain an NO ligand. The complexes are numbered in the order they appear in the text and according to reference number. So, for example, if reference 17 of this section had four complexes they would be numbered 17-1, 17-2, 17-3, and 17-4. Sometimes the authors carried over abbreviations from the original manuscript for continuity. Where appropriate the MdC sigma bond length(s) is/are reported using data from the manuscript, supporting information, or from the .cif file obtained from the Cambridge Crystallographic Data Centre (CCDC).

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00163-3

651

652

Group 6 Complexes With Metal-Carbon Sigma Bonds

5.10.2

Cr complexes

5.10.2.1

Alkyl and aryl complexes without Cp-type ligands

Treatment of {(Me3Si)2NC(NCy)2}2Cr (1-1) in toluene with trimethylaluminum gives [{(Me3Si)2NC(NCy)2CrMe}2]∙(C7H8)0.5 (1-2) as green blocks. The complex was characterized by 1H NMR and IR spectroscopy, single crystal X-ray crystallography, and elemental analysis. The CrdC bond lengths were found to be 2.192(7), 2.521(8), and 2.187(7) A˚ .1 The ligands Ap-1 and Ap-2 are shown in Chart 1. [(Ap-1)Cr-m-CH2]2 (2-1) can be synthesized as a mixture with [(Ap-1) Cr-CH3]2 (2-2) or as a pure complex depending on the amount of MeLi used in the synthesis. The synthesis of only 2-1 involves adding excess MeLi to [(Ap-1)Cr(THF)2(Cl)2] (2-3) in hexanes. After filtration and low temperature crystallization, (2-1) was isolated as red crystals. The complex was characterized by 1H NMR spectroscopy, single crystal X-ray diffraction, and elemental analysis. The CrdC bond lengths in 2-1 were found to be 2.143(5) and 2.223(5) A˚ . Adding only 2 equivalents of MeLi to 2-3 instead of excess affords a mixture of 2-1 and 2-2. Complex 2-2 was only characterized by single-crystal X-ray diffraction, and the CrdC bond length was found to be 2.097(8) A˚ . Adding methyl lithium to [Ap-2-Cr(THF)2(Cl)2] (2-4) in hexanes affords green crystals of[(Ap-2)Cr-CH3]2 (2-5) after workup and isolation. The CrdC bond length in 2-5 was reported to be 2.057(5) A˚ . Adding methyl iodide to [(Ap-2)Cr]2 (2-6) yields brown-red crystals of [(Ap-2)Cr-m-CH3-m-I]2 (2-7) after filtration and crystallization. The CrdC bond lengths in 2-7 were determined to be 2.308(16) and 2.329(16) A˚ .2 {Cr(2,20 -bipy)(mes)2}
1.5C6H6 (4-1.1.5C6H6) is prepared by mixing [Cr(PMe3)2(mes)2] with 2,20 bipyridine in THF. Diffusion into benzene yielded X-ray quality crystals of (4-1.1.5C6H6). The complex was characterized by IR, Raman, solid-state EPR (very weak resonance with g-factor of 1.986), single-crystal X-ray crystallography, and elemental analysis. (4-1.1.5C6H6) has been reported as a different solvate (THF) with CrdC bond lengths of 2.130(5) and 2.099(5) A˚ at (T ¼ 130 K) 3 vs. 2.1166(17) and 2.1194(17) A˚ at (T ¼ 150 K).4 Dissolving a mixture of Cr(2,20 -bipy)(mes)2, potassium on graphite, and dibenzo-18-crown-6 in THF followed by filtration and layering with hexane yields brown crystals of [K(dibenzo-18-crown-6)
THF][Cr(2,20 -bipy)(mes)2] (4-2). The sample was characterized by IR and Raman spectroscopy, solid-state EPR spectroscopy (sharp resonance with a broad resonance alongside that has a g-factor of 1.986), and single crystal X-ray crystallography. The CrdC bond lengths in 4-2 were reported as 2.1166(17) and 2.1194(17) A˚ .4 Adding solid LiCH2SiMe3 to [(HLiPr)Cr(m-Cl)2] (5-1) in THF at −30  C gives [Li(THF)4][(HLiPr)Cr{CH2Si(CH3)3}2] (5-2) after workup. The CrdC bond lengths in 5-2 are 2.131(5) and 2.136(5) A˚ . Adding 4 equivalents of PhLi to 5-1 in THF yields [Li(THF)2] [(HLiPr)CrPh2] (5-3a). A single crystal of [PhMg(THF)][(HLiPr)CrPh2] (5-3b) was obtained by using PhMgCl instead of PhLi and its CrdC bond lengths were reported as 2.164(2) and 2.203(2) A˚ . Single crystals of 5-3a could not be obtained, but the reported synthesis using PhLi gave higher yields than the magnesium counterpart 5-3b prepared using PhMgCl. Adding 5-1 to THF followed by the addition of MeLi gave [Li(THF)4][(HLiPr)Cr2(m-Me)3] (5-4) as blue crystals after isolation. The CrdC bond lengths for 5-4 were reported as 2.192(4), 2.235(5), and 2.202(5) A˚ . Adding MeLi to a THF solution of 5-1 produced red needles of [Li2(THF)3] [(HLiPr)CrMe3] (5-5) after workup. The CrdC bond lengths for 5-5 were reported as 2.159(5), 2.150(5), and 2.107(4) A˚ . Combining 5-1 and 5-2 in pentane produced (HLiPr)Cr(CH2SiMe3)(THF) (5-6) after concentration and crystallization. The CrdC bond length for 5-6 was reported as 2.102(5) A˚ . In a similar manner, the combination of 5-5 and 5-1 produced [(HLiPr)Cr(m-Me)]2 (5-7). The CrdC bond lengths for 5-7 were 2.192(3) and 2.197(3) A˚ . Treating 5-6 in Et2O with [Cp2Fe] [BARF] in Et2O gives [(H,TMSML )Cr(THF)(Et2O)][BARF] (5-8) after isolation. The CrdC bond length in 5-8 was reported as 2.053(6) A˚ . Complexes 5-2-8, were also characterized by melting point, IR spectroscopy, magnetic susceptibility measurements, single-crystal X-ray crystallography, and UV-Vis spectroscopy. Complexes 5-3a, 5-4, 5-6, 5-7, and 5-8 were also characterized by elemental analysis.5 Fisher and co-workers synthesized a series of Cr-Ph complexes. [{Li(THF)}2.3{Li(OEt2)}0.7][CrPh6] (6-1-THF-OEt2) was prepared by adding Ph3Cr(THF)3 in small portions to a stirred solution of PhLi in Et2O at −30  C. Taking 6-1-THF-OEt2 suspended in Et2O and adding HCl ∙ OEt2 solution yielded turquoise crystals of [{(Et2O)Li}{(THF)2Li}][CrPh5] (6-2-THF-OEt2), which were characterized by 1H NMR spectroscopy, single-crystal X-ray crystallography, and Cr elemental analysis. Fisher et al. reported two methods for the synthesis of [(12-crown-4)Li(THF)]2[CrPh5] (6-2-THF-12C4). The first method involved the addition of

Chart 1

Group 6 Complexes With Metal-Carbon Sigma Bonds

653

12-crown-5 to 6-2-THF-OEt2 in Et2O (Scheme 1) while the second involved adding 12-crown-4 to 6-1-THF-OEt2 in THF. Complex [{(Et2O)Li}2OCrPh3]2 (6-3-Li2O) was also prepared in two different methods: either by dissolving 6-2-THF-OEt2 in Et2O or stirring 6-1-THF-OEt2 in Et2O at room temperature for 3 days. Treating 6-2-THF-OEt2 with HCl ∙OEt2 in Et2O produced [{(THF)Li} CrPh3]2 (6-3-CrII-THF). Taking (6-3-CrII-THF) and stirring in THF produced [(THF)4Li]2[Cr2Ph6] (6-3-CrII-THF4). These Cr complexes were characterized by a combination of 1H NMR spectroscopy, single-crystal X-ray diffraction, and/or Cr elemental analysis.6

Scheme 1

The synthesis of fac-(C6H5)3Cr(THF)3
0.25 dioxane (7-1) proceeded by first suspending CrCl3(THF)3 in THF and dioxane. Adding Ph2Mg(dioxane) in THF to the suspension yielded red crystals of 7-1 after workup. Increasing the molar ratio of Ph2Mg (dioxane) to CrCl3(THF)3 (1.96:1 vs. 0.60:1 CrCl3(THF)3:Ph2Mg(dioxane)) yielded trans-Cl2CrPh(THF)3 (7-2-Cl). Taking complex 7-1 in THF and adding molecular iodine yielded I2CrPh(THF)3 (7-2-I) upon heating to 40  C. Crystals suitable for X-ray diffraction (Cr-C]2.065(9) A˚ ) were obtained by cooling the 7-2-I reaction mixture to −40  C. [(THF)5MgCl][(Ph2Cr(THF))2(m-Cl)3]− (7-3 (-Cl)2-MgCl2) was prepared by suspending CrCl3(THF)3 in a THF dioxane mixture and adding 0.98:1 equivalents of Ph2Mg (dioxane). 7-3(-Cl)2-MgCl2 was only characterized by single-crystal X-ray diffraction (Cr-C ¼ 2.076(10), 2.078(11), 2.104(11), and 2.100(10) A˚ ). [Li(DME)3]+[(C6H5)4Cr(DME)]− (7-4-Li) was prepared by suspending (7-1) in DME and adding PhLi in Et2O (Scheme 2). After workup, red crystals of 7-4-Li were isolated. Taking complex 7-1 and suspending it in DME followed by rapid addition of Ph2Mg(dioxane) produced a red precipitate. Adding THF and heating to 60  C yields red crystals of [(C6H5)Mg(DME)2(THF)]+[(C6H5)4Cr(DME)]− (7-4-Mg) upon cooling. 7-4-Mg was only characterized by single-crystal X-ray crystallography (Cr-C ¼ 2.0901(17), 2.1750(16), 2.0875(16), and 2.1870(17) A˚ ). Taking 7-1 and suspending it in DME, followed by the addition of PhLi in ether, yields green crystals of [Li(DME)]2[Cr(C6H5)5] (7-5-Li) after workup. Adding THF dropwise to 7-5Li produces [Li(DME)3]+ [Li(DME)Cr(C6H5)5}−] (7-5-Li0 ) after workup. Adding diphenylcalcium in THF to 7-1 in DME produces green crystals of [Ca(DME)(THF)]2[Cr(C6H5)6]+[(THF)Cr(C6H5)4]− (7-5-Ca) after workup. 7-5-Ca was only characterized by single-crystal X-ray diffraction (Cranion-C ¼ 2.124(2), 2.1298(18), 2.034(2), and 2.134(2) A˚ ; Crcation-C ¼ 2.251(2), 2.228(3), and 2.246(3) A˚ ). Suspending 7-1 in THF followed by dropwise addition of PhLi in Et2O at 0  C yielded [Li(THF)4]+[(Li(THF))2Cr(C6H5)6]− (7-6-Li) as a yellow solid. Suspending 7-6-Li in THF at −40  C for 5 weeks produced [Ph3Cr(m-O)]2[(THF)Li2(m-THF)]2 (7-7) in near quantitative yield. Dissolving 7-1 in THP and adding PhLi in Et2O dropwise gave [Li(THP)4]+ [{(THP)Li}2Cr(C6H5)6]− (7-6-Li0 ) as yellow solid. Complexes 7-1, 7-2-Cl, 7-4-Li, 7-4-Mg, 7-5-Li, 7-5-Li0 , 7-6-Li, 7-6-Li0 , and 7-7 were characterized by both Cr-only elemental analysis and single-crystal X-ray diffraction. The CrdC bond lengths are 2.086(3) A˚ for all three CrdC bonds in 7-1. The CrdC bond length in 7-2-Cl is 2.0597(15) A˚ . The CrdC bond lengths for 7-4-Li are 2.177(2), 2.074(2), 2.081(2), and 2.167(2) A˚ . There are two molecules of 7-5-Li in the asymmetric unit with CrdC bond lengths ranging from 2.020(5) to 2.168(5) A˚ . For 7-7 the CrdC bond lengths are 2.1530(19), 2.142(2), 2.0592(19) A˚ . Complexes 7-2-I, 7-3(-Cl)2-MgCl2, 7-5-Ca were only characterized by single-crystal X-ray diffraction. The CrdC bond lengths for 7-2-I are 2.083(4) and 2.139(4) A˚ . The authors report that crystals of 7-5-Li0 , 7-6-Li, and 7-6-Li0 were extremely thin or of low quality. Structures obtained for those three complexes were only used to confirm identity and were not deposited in the CCDC.7

Scheme 2

Complex {N2P2}CrMe (8-1) [HN2P2 ¼ tBuN(H)SiMe2N(CH2CH2PiPr2)2; Chart 2] was prepared by reacting {N2P2}CrCl (8-2) with MeLi in THF or Et2O at −78  C (Scheme 3). This complex can also be prepared by adding a suspension of KC8 in THF to a solution of 8-2 in THF at −78  C followed by slow addition of AlMe3 at −78  C. After workup, 8-1 was characterized by X-ray diffraction 1H NMR, IR, and UV-Vis spectroscopy, mass spectrometry, elemental analysis, melting point analysis, and magnetic susceptibility measurements (Evans method). The CrdC bond length was determined to be 2.127(3) A˚ . If instead of adding AlMe3, the reaction between KC8 and 8-2 occurs under a 1 atm atmosphere of ethylene, {N2P2}Cr(C4H8) (8-3) is produced. 8-3 was

654

Group 6 Complexes With Metal-Carbon Sigma Bonds

Chart 2

Scheme 3

characterized by single-crystal X-ray diffraction (Cr-C ¼ 2.081(4) A˚ ), 1H NMR and IR spectroscopy, mass spectrometry, elemental analysis, melting point analysis, and the Evans method for magnetic susceptibility. Adding LiCH2SiMe3 in toluene to 8-2 in toluene gives purple {N2P2}CrCH2SiMe3 (8-4) after workup. 8-4 was characterized using the same methods as 8-2 and 8-3. The CrdC bond length in 8-4 was determined to be 2.122(3) A˚ . Treating 8-4 in THF with AgBF4 in THF gave {(N2P2)CrCH2SiMe3}BF4 (8-5). Adding LiCH2SiMe3 in toluene to 8-1 in toluene instead yields (N2P2)Cr{Cl}CH2SiMe3 (8-6). The CrdC bond length in 8-6 was determined to be 2.066(6) A˚ .8 If methyl lithium in Et2O is added to 8-1 in THF at −78  C, (N2P2)Cr{Cl}Me (8-7) is produced. Complexes 8-5-8-7 were characterized by single-crystal X-ray diffraction, 1H NMR and IR spectroscopy, elemental analysis, melting point analysis, and the Evans method for magnetic susceptibility. All complexes CpCr[DppNC(Me)CHC(Me)NC6H4(OMe)](CH3) (9-1a), CpCr[DppNC(Me)CHC(Me)NC6H4Me](CH3) (9-1b), CpCr[DppNC(Me)CHC(Me)NC6H5](CH3) (9-1c), CpCr[DppNC(Me)CHC(Me)NC6H4CF3](CH3)] (9-1d) were prepared and characterized in the same manner (Scheme 4). Briefly, the corresponding complexes CpCr[DppNC(Me)CHC(Me)NC6H4(OMe)] (9-2a), CpCr[DppNC(Me)CHC(Me)NC6H4CH3] (9-2b), CpCr[DppNC(Me)CHC(Me)NC6H5] (9-2c), CpCr[DppNC(Me)CHC (Me)NC6H4CF3] (9-2d) were dissolved in Et2O and methyl iodide was added in MTBE (tert-butylmethyl ether). After stirring, MeMgI was added and the mixture was allowed to react overnight. Excess 1,4-dioxane was added to the reaction mixture and 9-1a-d were isolated after workup and characterized by UV-Vis spectroscopy, single-crystal X-ray diffraction, and elemental analysis. The reported CrdC bond distances were 2.0563(15) A˚ (9-1a), 2.0608(17) A˚ (9-1b), 2.0799(18) A˚ (9-1c), and 2.052(2) A˚ (9-1d). Complexes 9-1b and 9-1d were also prepared by a different method which mixed the corresponding Cr complex (9-2b or 9-1d) in THF with methyl iodide in MTBE and then added SmI2 in THF. After workup the corresponding complexes 9-1b and 9-1d were isolated.9

Scheme 4

Complex 10-1 was prepared by adding silver triflate to an ether solution of [MeLiPrCr(m-Me)]2 (where MeLiPr ¼ 2,4-pentane N,N-bis(2,6-diisopropylphenyl)diketiminate). Slow cooling the filtrate gives MeLiPrCrCH3(OTf ) (10-1a) and recrystallizing 10-1a from THF gives [MeLiPrCrCH3(OTf )(THF)] (10-1b). Complex 10-1b was characterized by single-crystal X-ray diffraction only whereas 10-1a was characterized by 1H NMR, IR, and UV-Vis spectroscopy, magnetic susceptibility (Johnson-Matthey balance), mass spectrometry, melting point analysis, single-crystal X-ray crystallography, and elemental analysis. The CrdC bonds reported

Group 6 Complexes With Metal-Carbon Sigma Bonds

655

are 2.049(3) A˚ (10-1b) and 2.025(6) A˚ (10-1b). Methyl lithium was added dropwise to a −30  C ether solution of MeLiPrCr(OTf )2. After stirring at room temperature for 2 h, the mixture was worked up to yield brown crystals of MeLiPrCr(CH3)2(THF) (10-2). This complex was also characterized by 1H NMR, IR, UV-Vis, magnetic susceptibility (Johnson-Matthey balance), melting point, single crystal X-ray crystallography, and elemental analysis. The CrdC bond length was determined to be 2.045(4) A˚ .10 Complex [(11-L1)Cr(THF)(Cl)2] 11-1 was prepared by mixing N-phenyl-2-(4-(trifluoromethyl)phenyl)quinolin-8-amine (11-L1H; Chart 3) and CH3CrCl2(THF)3 in closed vessel at 100  C for 2 h. After cooling 11-1 was isolated as a brown solid. Complex 11-1 was analyzed by 1H and 19F NMR spectroscopy, single-crystal X-ray diffraction, and elemental analysis. The CrdC bond length was determined to be 2.0416(19) A˚ . Complex [(11-L2)Cr(p-toly)(THF)(Cl)2] 11-2 was prepared by mixing 8-(piperidin-1-yl)quinoline 11-L2 in THF (Chart 3). After workup 11-2 was collected as a brown solid. The single-crystal X-ray diffraction revealed the CrdC bond length in 11-2 as 2.083(3) A˚ . For the synthesis of [(11-L2)Cr(p-toly)3] 11-3, p-tolyMgCl in THF was added dropwise to a suspension of [(11-L2)Cr(THF)(Cl)3] 11-4 in THF at 0  C. After filtering the product 11-3 was isolated as a green product. Complexes [(11-L3a-d)Cr(THF)(Cl)2] 11-5-9 and [(11-L2)Cr(THF)(CH3)(Cl)2] (11-16) were all synthesized in a similar manner. Briefly, the corresponding ligand (2-phenyl-8-(piperidin-1-yl)quinoline (11-L3a), 8-(piperidin-1-yl)-2p-tolylquinoline (11-L3b), 8-(piperidin-1-yl)-2-(4-(trifluoromethyl)phenyl)quinoline (11-L3c), 2-(3,5-dimethylphenyl)-8-(piperidin-1-yl)quinoline (11-L3d), 8-(pyrrolidin-1-yl)-2-p-tolylquinoline (11-L3b), 8-(piperidin-1-yl)quinoline (11-L2) were mixed with CH3CrCl2(THF)3 in toluene to produce the desired complexes 11-5-11-10 after workup and isolation. The CrdC bond lengths for complexes where single crystal X-ray data were reported are 2.369(14) A˚ (11-5), 2.0416(19) A˚ (11-7), 2.308(16) A˚ (11-8) (11-L3), and 2.329(16) A˚ (11-9) (11-L4). All complexes were analyzed by elemental analysis and the structures for all complexes except 11-6 and 11-10 were determined by single-crystal X-ray diffraction.11 For the synthesis of (nacnac)Cr(CHt2Bu) (12-1) and (nacnac)Cr(CH2CH3)] (12-2), a solution of either LiCHt2Bu (12-1) or ClMg(CH2CH3) in Et2O was added to a suspension of [{(nacnac)Cr(m2-Cl)}2] (12-3) in Et2O. After isolation and crystallization, 12-1 and 12-2 were characterized by 1H NMR, IR, and UV-Vis-spectroscopy, magnetic susceptibility measurements (Evans method), X-ray crystallography, and elemental analysis. In a similar manner (nacnac)Cr(CH3) (12-4) was prepared by adding a THF solution of ClMgCH3 in a suspension of 12-3 in ether. After crystallization 12-4 was characterized by the same methods used for 12-1 and 12-4. The CrdC bond lengths report are as follows 2.1385(19) A˚ (12-1), 2.118(5) A˚ (12-2), and 2.097(3) A˚ (12-4).12 Cold methyl lithium in Et2O was added to a suspension of {2,6-[2,6-(i-Pr)2PhN]C(CH3)]2(C5H3N)}CrCl2 ∙ 0.75THF (13-1) in Et2O at −35  C. After isolation {2,6-[2,6-(i-Pr)2PhN]C(CH3)]2(C5H3N)}CrMe(m-Me)Li(THF)3 (13-2) was characterized by FTIR spectroscopy, magnetic susceptibility measurements (Johnson Matthey), X-ray diffraction, and elemental analysis. The CrdC bond length in 13-2 are 2.062(8) and 2.135(9) A˚ . To a −35  C suspension of {2,6-[2,6-(i-Pr)2PhN]C(CH3)]2(C5H3N)}CrCl (13-3) in Et2O was added cold methyl lithium in Et2O, which was stirred over 4 h while warming to room temperature. After crystallization {2-[2,6-(i-Pr)2PhN]C(CH3)]-6-[2,6-(i-Pr)2PhNC]CH2](C5H3N)}Cr(m-Me)Li(THF)3. 0.66(toluene) (13-4) was characterized by FTIR spectroscopy, magnetic susceptibility measurements (Johnson-Matthey), single crystal X-ray diffraction, and elemental analysis. The reported CrdC bond length for 13-4 was 2.129(4) A˚ . Adding Me3Al in toluene to a solution of 13-3 in toluene at −35  C gave a green-brown mixture. Crystallization at room temperature from toluene gave{2,6-[2,6-(i-Pr)2PhN] C(CH3)]2(C5H3N)}CrCH3 ∙ 0.5(hexane) (13-5). The complex was characterized by IR spectroscopy, magnetic susceptibility measures, X-ray diffraction, and elemental analysis. The CrdC bond length was found to be 1.971(13) A˚ .13 A solution of cyclohexyl isocyanide in toluene was added to a (m-Z1:Z1-HLiPr)2Cr2 (HLiPr ¼ N,N0 -bis(2,6-diisopropylphenyl1,4-diazadiene)) solution in toluene at 0  C for 2 h, then left at room temperature overnight. After workup and crystallization, H i L PrCr(CNCy)4 (14-1) was characterized by 1H NMR, FTIR, UV-Vis spectroscopy and X-ray diffraction. The reported CrdC bond lengths for 14-1 are 1.929(3) and 1.939(3) A˚ . The synthesis of 14-2 ([HLiPrCr]2(m-CyNC)4) involves adding cyclohexyl isocyanide (4 equivalents) in toluene to (m-Z1:Z1-HLiPr)2Cr2. After crystallization 14-2 was characterized using similar methods as described for 14-1. The three reported CrdC bond lengths for 14-2 are 1.954(3), 2.386(3), and 2.404(3) A˚ .14 Dissolving [(2,6-Me2Ph)2nacnac]CrCl2(THF)2 (15-1) in Et2O followed by addition of MeLi in hexanes afforded [(2,6-Me2Ph)2nacnac]Cr(THF)Me2 (15-2) as green crystals. 15-2 was characterized via 1H NMR, IR, and UV-Vis spectroscopy, melting point analysis, mass spectrometry, and elemental analysis. Adding solid LiCH2Si(CH3)3 instead of MeLi to 15-1 yielded [(2,6-Me2Ph)2nacnac]Cr(CH2SiMe3)2 (15-3) as orange-brown crystals after workup. The complex was characterized by 1H NMR, IR, and UV-Vis spectroscopy, melting point, mass spectrometry, and elemental analysis. In the same manner adding BnMgCl in Et2O instead of methyl lithium to 15-1 produced [(2,6-Me2Ph)2nacnac]Cr(Bn)2 (15-4) as red crystals. 15-4 was characterized by 1H NMR and IR spectroscopy and melting point analysis. Dissolving 15-2 in THF followed by the addition of [HNEt3]BPh4 yields {[(2,6-Me2Ph)2nacnac]Cr(THF)2Me}BPh4 (15-5) as green crystals. The complex was characterized by the methods described

Chart 3

656

Group 6 Complexes With Metal-Carbon Sigma Bonds

above along with magnetic susceptibility measurements (Johnson Matthey balance). The synthesis of {[(2,6-Me2Ph)2nacnac] Cr(OEt2)CH2SiMe3}BARF (15-6) involved slowly adding [H(Et2O)2]BARF to 15-3. Complex 15-6 was characterized as described above, and the CrdC bond length in 15-6 was determined to be 1.930(3) A˚ .15 Zhou and Leznoff synthesized a series of phtalocyanine-Cr (PcCr) complexes. In general, the majority of the reported complexes where synthesized by reacting PcCr (16-1) with the appropriate reagent(s) PcCr(CH2PPh3)2 (CH2PPh3) (16-2), [Li(DME)3] PcCrCCPh(DME) (LiCCPh) (16-3), {Li2(DME)4}PcCr(CH2CH3) (LiCH2CH2) (16-4), Li(DME)5PcCr(CH2CH3) (LiCH2CH3 followed by Cp2FeBF4) (16-5), PcCrPh (16-6) (LiPh, then Cp2FeBF4), PcCrCH2SiMe3 (16-7) (LiCH2SiMe3, then Cp2FeBF4), PcCrMe2 (16-8) (MgMe2, then MeI; Scheme 5), and PcCrPhMe (16-9) (LiPh, then MeI). For K(DME)PcCr(CH2CH3) (16-10), K2(DME)PcCr was used as the starting material and LiCH2CH3 was added followed by Cp2FeBF4 to yield blue K(DME)PcCr (CH2CH3) (16-10). Complex PcCrEt (16-11) was synthesized by treating a solution of 16-4 in DME with Cp2FeBF4.16

Scheme 5

Table 1

Reported CrdC bond lengths for complexes in reference 16.

Complex

˚) Reported Cr-C (A

Complex

˚) Reported Cr-C (A

16-2 16-3 16-5 16-6

2.231(5) and 2.261(5) 2.003(2) 2.070(6) 2.072(6)

16-7 16-8 16-9 16-10

2.073(4) 2.303(2) 2.060(4) 2.222(18)

Complexes 16-2, 16-3, 16-5, 16-6, 16-7, 16-8, 16-9, and 16-10 were characterized by X-ray diffraction, and Cr-C distances are compiled in Table 1. All complexes 16-2 through 16-11 were characterized by elemental analysis. Complexes 16-6, 16-7, 16-8, 16-9, and 16-11 were characterized by mass spectrometry and magnetic susceptibility data, determined by the Evans methods, was collected for complexes 16-2, 16-7, 16-8, and 16-9.16

5.10.2.2

Alkyl and aryl complexes with Cp-type ligands

Several complexes were synthesized by MacLeod et al. using one general method.17 Briefly, the appropriate chromium starting material (see Table 2) was added to a flask containing Et2O. Then the appropriate alkylation reagent was added dropwise. The mixture was stirred at room temperature after which the solvent was removed and the residue extracted with hexanes. Cooling the filtrate yielded the desired complex as crystals. The synthesized complexes via this method were CpCr[(2,6-Et2C6H3NCMe)2CH] (CH2CMe3) (17-1b), CpCr[(2,4,6-Me3C6H2NCMe)2CH](CH2CMe3) (17-1c), CpCr[(2,6-Me2C6H3NCMe)2CH](CH2SiMe3) (17-2), CpCr[(2,6-iPr2C6H3NCMe)2CH](CH2SiMe3) (17-2a), CpCr[(2,6-Et2C6H3NCMe)2CH](CH2SiMe3) (17-2b), CpCr[(2,4,6-Me3C6H2NCMe)2CH](CH2SiMe3) (17-2c), CpCr[(2,6-Me2C6H3NCMe)2CH](CH2CH3) (17-3), CpCr[(2,6-Me2C6H3NCMe)2CH](CH2CHMe2) (17-4), CpCr[(2,6-iPr2C6HNCMe)CH](CH2CHMe2) (17-4a), CpCr[(2,6-Me2C6H3NCMe)2CH](CH2CH2Ph) (17-5), CpCr[(2,6-Me2C6H3NCMe)2CH](CH2Ph) (17-6), CpCr[(2,6-iPr2C6H3NCMe)2CH](CH2Ph) (17-6a), CpCr[(2,6-Et2C6H3NCMe)2CH](CH2Ph) (17-6b), CpCr[(2,4,6-Me3C6H2NCMe)2CH](CH2Ph) (17-6c), CpCr[(2,6-Me2C6H3NCMe)2CH](Ph) (17-7), CpCr [(2,6-Me2C6H3NCMe)2CH](CHCMe2) (17-8), and CpCr[(2,6-Me2C6H3NCMe)2CH](CCH) (17-9). Two other complexes were synthesized via different methods. CpCr[(2,6-Me2C6H3NCMe)2CH](CH2CN) (17-10) was synthesized by adding KN[(SiMe)3]2 to CpCr[(2,6-Me2C6H3NCMe)2CH](Cl) in Et2O. The mixture was stirred for 3 days and the solvent removed in vacuo. The resulting residue was extracted and recrystallized to give 17-10. Adding 17-1 to a solution of p-xylene in benzene at room temperature afforded CpCr[(2,6-Me2C6H3NCMe)2CH](CH2C6H4Me) (17-11) after purification and crystallization. The preceding complexes were characterized by UV-Vis spectroscopy, X-ray crystallography, and elemental analysis except

Group 6 Complexes With Metal-Carbon Sigma Bonds

Table 2

657

Listing of Cr reagents and alkylation reagents for complexes synthesized in reference 17.

Product

Cr reagent

Alkylation reagent

17-1b 17-1c 17-2 17-2a 17-2b 17-2c 17-3 17-4 17-4a 17-5 17-6 17-6a 17-6b 17-6c 17-7 17-8 17-9

CpCr[(2,6-Et2C6H3NCMe)2CH](OTs) CpCr[(2,4,6-Me3C6H2NCMe)2CH](OTs) CpCr[(2,6-Me2C6H3NCMe)2CH](OTs) CpCr[(2,6-iPr2C6H3NCMe)2CH](OTf ) CpCr[(2,6-Et2C6H3NCMe)2CH](OTs) CpCr[(2,4,6-Me3C6H2NCMe)2CH](OTs) CpCr[(2,6-Me2C6H3NCMe)2CH](Cl) CpCr[(2,6-Me2C6H3NCMe)2CH](Cl) CpCr[(2,6-iPr2C6HNCMe)CH](OTf ) CpCr[(2,6-Me2C6H3NCMe)2CH](Cl) CpCr[(2,6-Me2C6H3NCMe)2CH](Cl) CpCr[(2,6-iPr2C6H3NCMe)2CH](OTf ) CpCr[(2,6-Et2C6H3NCMe)2CH](Cl) CpCr[(2,4,6-Me3C6H2NCMe)2CH](Cl) CpCr[(2,6-Me2C6H3NCMe)2CH](OTs) CpCr[(2,6-Me2C6H3NCMe)2CH](OTs) CpCr[(2,6-Me2C6H3NCMe)2CH](Cl)

Mg(CH2CMe3)2 ∙1.05(1,4dioxane) Mg(CH2CMe3)2 ∙1.26(1,4-dioxane) Mg(CH2SiMe3)2 ∙1.05(1,4-dioxane) Mg(CH2SiMe3)2 ∙1.05(1,4dioxane) Mg(CH2SiMe3)2 ∙1.05(1,4-dioxane) Mg(CH2SiMe3)2 ∙1.05(1,4-dioxane) ClMgCH2CH3 ClMgCH2CHMe2 Mg(CH2CHMe2)2 ∙ 0.618(1,4dioxane) ClMgCH2CH2Ph ClMgCH2Ph ClMgCH2Ph ClMgCH2Ph ClMgCH2Ph Mg(Ph)2 ∙ 2.74(1,4-dioxane) BrMgCHCMe2 BrMgCCH

17-1b and 17-2 which were not analyzed crystallographically. The reported CrdC bond lengths for the complexes are 2.128(2) A˚ (17-1c), 2.0987(14) A˚ (17-2), 2.110(2) and 2.118(12) A˚ (17-2a), 2.1098(15) A˚ (17-2b), 2.1031(17) A˚ (17-2c), 2.090(5) and 2.104(5) A˚ (17-3), 2.121(5) and 2.125(5) A˚ (17-4), 2.1269(15) and 2.1199(15) A˚ (17-4a), 2.200(7) and 2.107(7) A˚ (17-5), 2.1526(15) A˚ (17-6), 2.1239(13) A˚ (17-6a), 2.144(2) A˚ (17-6b), 2.1384(19) A˚ (17-6c), 2.098(3) A˚ (17-7), 2.050(4) and 2.038 (4) A˚ (17-8), 2.009(5) A˚ (17-9), 2.126(2) A˚ (17-10), and 2.131(4) A˚ (17-11). MacLeod, Patrick, and Smith reported an updated synthesis and crystal structure of CpCr(C6H4CH2NMe2) (18-1) by the treatment of CrCl2 with NaCp and LiC6H4CH2NMe2 (Scheme 6). Treatment of 18-1 with diphenylacetylene in toluene produces CpCr[C(Ph)C(Ph)C6H4CH2NMe2)] (18-2). 18-2 was characterized by single crystal X-ray crystallography (Cr-C 2.088(3) A˚ ), EA, and UV-Vis spectroscopy. To access different alkyl complexes, McLeod and co-workers synthesized a tosyl complex which was treated with Grignard reagents. The synthesis of the tosyl complex involved treating 18-1 in Et2O with AgOTs in THF. After workup, the complex CpCr(C6H4CH2NMe2)(OTs) (18-3) was characterized by EA and UV-Vis spectroscopy. Treatment of 18-3 with PhCH2MgCl in Et2O gives CpCr(C6H4CH2NMe2)(CH2Ph) (18-4). EA data collected on 18-4 was consistently low in C, which the authors attributed to the known loss of a benzyl radical in similar CpCr(III) benzyl complexes.17 Complex CpCr(C6H4CH2NMe2)(C6H4Me) (18-5) was synthesized by the treatment of 18-3 with Mg(C6H4Me).21.89(1,4-dioxane) in Et2O. 18-5 was analyzed by single-crystal X-ray crystallography (CrdC bond lengths ¼ 2.040(1) and 2.075(1) A˚ ), UV-Vis spectroscopy, and showed consistently low results for both C and H in the elemental analysis data. The authors did not identify the impurity. Complex CpCr(C6H4CH2NMe2)(CH2CMe3) (18-6) was prepared in low yield by the treatment of CpCr(C6H4CH2NMe2) (OTs) (18-3) with Mg(CH2CMe3).21.05-(1,4-dioxane) in Et2O. The complex was only identified by single-crystal diffraction with CrdC bond lengths ranging from 2.028(2) to 2.206(2) A˚ .18

Scheme 6

658

Group 6 Complexes With Metal-Carbon Sigma Bonds

5.10.2.3

Alkyl and aryl complexes with metal-element multiple bonds

Treatment of (ArN)2CrCl2 (Ar ¼ 2,6-iPr2C6H3) with Me3SiCH2MgCl in ether yielded (ArN)2Cr(CH2SiMe3)2 (19-1) after workup (Scheme 7). It was characterized by single-crystal X-ray crystallography (Cr-C ¼ 2.0182(18) A˚ and Cr-C ¼ 2.00113(19) A˚ ), 1H and 13 C{1H} NMR spectroscopy, elemental analysis, mass spectrometry, IR spectroscopy, UV-Vis spectroscopy, and melting point analysis. The metallacycle (ArN)2Cr(C10H18)] (19-2) was prepared by stirring (ArN)2Cr(CHt2Bu)2 (19-3) in cyclopentane. After workup, 19-3 complex was characterized via similar methods as described for 19-1.19

Scheme 7

Complex 20-1 was synthesized with two different counterions: K{(ArN)2Cr(]CHSiMe3)-(CH2SiMe3)} (20-1K) and Et4N {(ArN)2Cr(]CHSiMe3)(CH2SiMe3)} (20-1Et4N). The synthesis of 20-1K starts with (ArN)2Cr(CH2SiMe3)2 dissolved in THF. KN(SiMe3)2 was added and, after workup, 20-1K was isolated and characterized by 1H and 13C{1H} NMR, IR, and UV-Vis spectroscopy, elemental analysis, and melting point analysis. Treating 20-1K with Et4NCl in THF yielded 20-1Et4N, which was characterized by similar methods. Complex (Et4N){(ArN)2Cr(]CHPh)(CH2Ph)} (20-2) was synthesized in a manner similar to 20-1, but only the Et4N salt was isolated and characterized. Briefly, KN(SiMe3) was added to (ArN)2Cr(CH2Ph)2 dissolved in THF. The potassium counterion is exchanged with Et4N using Et4Cl. 20-2 was analyzed by similar methods to those described above.20 In 2016, Tilley et al. reported the synthesis of five alkyl complexes supported by the bulky silylamido ligand [N(SiMe3)DIPP] (DIPP ¼ 2,6 diisopropylphenyl). Complex Cr[k2-N(SiCH2Me2)DIPP][N(SiMe3)DIPP] (21-1) was prepared by treating CrCl3(THF)3 in toluene with Li[N(SiMe3)DIPP] in toluene. After workup, 21-1 was characterized by single-crystal X-ray diffraction (Cr-C ¼ 2.062(3) A˚ ), elemental analysis, and magnetic susceptibility (Evans method). Taking 21-1 and heating it to 80  C for 24 h under 10−5 mbar vacuum yielded an orange solid. Purification of the crude solid revealed Cr[k2-N(SiMe3)(2-CH(Me)CH2,6-isopropylphenyl)][N(SiMe3)DIPP] (21-2), which was characterized by single-crystal X-ray diffraction (Cr-C ¼ 2.053(5) A˚ ), elemental analysis, and magnetic susceptibility (Evans method) as well. Complexes Cr[N(SiMe3)DIPP]2(Vinyl) (21-3), Cr [N(SiMe3)DIPP]2(Bn) (21-4), and Cr[N(SiMe3)DIPP]2(Me) (21-5) were all prepared in similar manner. Cr[N(SiMe3)DIPP]2Cl (21-6) in Et2O was treated with either vinyl lithium (21-3), benzyl potassium (21-4), or methyl magnesium chloride (21-5). After workup each of the complexes were characterized by single-crystal X-ray diffraction (21-3 Cr-C ¼ 2.003(8) A˚ , 21-4 2.094(2) A˚ , and 21-5 2.036(6) A˚ ) elemental analysis, and magnetic susceptibility (Evans method).21

5.10.2.4

Alkynyl complexes

To a solution of trans-(dmpe)2CrCl2 and Me3SiCCH in THF, n-BuLi in hexanes was added (Scheme 8). After workup, trans[(Me3SiCC)(dmpe)2Cr]2(m-N2)
hexane (22-1) was isolated and characterized by UV-Vis and IR spectroscopy, mass spectrometry, single-crystal X-ray crystallography, and elemental analysis. The CrdC bond length was determined to be 2.007(10) A˚ . For the synthesis of trans-(dmpe)2Cr(CCSiMe3)2 (22-2), n-BuLi in hexanes was added to Me3SiCCH in THF, stirring for 1 h before trans(dmpe)2CrCl2 was added dropwise to produce a red solution. After workup, trans-(dmpe)2Cr(CCSiMe3)2 (22-2) was isolated and characterized as described above for 22-1. Complex 22-2 was also prepared by adding Me3SiCCH to trans-(dmpe)2Cr(N2)2 in toluene. The CrdC bond length in trans-(dmpe)2Cr(CCSiMe3)2 (22-2) was determined to be 2.043(3) A˚ .22

Scheme 8

Group 6 Complexes With Metal-Carbon Sigma Bonds

659

In the absence of light, (Z5-CpCr(NO)2Cl) (23-1) was dissolved in Et2O and diethylamine (Scheme 9). Phenylacetylene was added while stirring followed by CuI. After isolation, (CpCr(NO)2(C^C-Ph) (23-2) was characterized by 1H and 13C{1H} NMR and IR spectroscopy, mass spectrometry, X-ray diffraction, and elemental analysis. The NO absorptions for 23-2 were reported at 1805 and 1689 cm−1. The CrdC bond length in 23-2 was determined to be 2.013(3) A˚ .23

Scheme 9

5.10.2.5

Multimetallic Cr complexes and clusters

Treating [Et2O)Na]Cr2Me8 in Et2O with TMEDA produced red crystals of {(TMEDA)Na}3Cr2Me7 (24-1). The complex was characterized by 1H and 13C{1H} NMR spectroscopy, single-crystal X-ray diffraction, and magnetic susceptibility (Gouy balance). The CrdC bond lengths in 24-1 vary between 2.153(5) and 2.302(7) A˚ . Substituting TMCDA ((R,R)-N,N,N0 , N0 -tetramethylcyclohexanediamine) for TMEDA in the previous synthesis yielded {(TMCDA)Na}3Cr2Me7 (24-2) as yellow-green crystals that were characterized by similar methods. The CrdC bond lengths in 24-2 varied between 2.142(2) and 2.302(2) A˚ .24 Me8Cr2[Na(OEt2)]4 (25-1) was prepared by Campbell et al. in a two-step process. First, methyl lithium in Et2O was added to a slurry of CrCl2 in Et2O at −30  C. After removal of the LiCl by-product, NaOtBu was added to yield a green solution. After workup, 25-1 was isolated as emerald-green crystals that are highly pyrophoric when exposed to air. The complex was characterized by magnetic susceptibility measurements and single-crystal X-ray diffraction. The CrdC bond lengths were all identical at 2.195(3) A˚ .25

5.10.3

Molybdenum and tungsten complexes

5.10.3.1

Alkyl and aryl complexes without Cp-type ligands

Benzene was added to a mixture of Mo(PMe3)6 and benzothiphene. The mixture was stirred until all solids dissolved and then the reaction was frozen. After workup Mo(k2-CHCHC6H4S)(PMe3)4 (26-1) was isolated and characterized by 1H NMR and single-crystal X-ray crystallography (Mo-C ¼ 2.154(5) A˚ ). Mo(k1,Z2-CH2CC6H4S)(PMe3)4 (26-2) was prepared by mixing Mo(PMe3)6 and benzothiphene in benzene and refluxing for 2.5 h (Chart 4). After workup, 26-2 was characterized by 1H, 31P {1H}, and 13C{1H} NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction (ModC]1.9532(12) and 2.2936 (12) A˚ ). Dissolving a mixture of Mo(PMe3)6 and benzothiphene in C6D6 and cooling to 10  C for days yielded [Mo(k1,Z2-CH2CC6H4S)(Z2-CH2PMe2)(PMe3)3] (26-3) that was characterized as described for 26-1 and 26-2 (ModC]2.2883 (16) A˚ ). {(k2-C4H4)Mo(PMe3)3(Se)}2{Mo(PMe3)4} (26-4) was prepared by suspending Mo(PMe3)6 in toluene and adding selenophene. The ModC bond lengths range from 2.161(3) to 2.208(3) A˚ in 26-4. 26 Methyl lithium in Et2O was added dropwise to a THF solution of trans-{Mo(N2)2(dppe)2}. After workup, trans-{MeMo (dppe)2N2}Li(THF)3 (27-1Mo) was isolated as a red powder. To a solution of trans-{Mo(depe)2(N2)2} in THF was added MeLi in Et2O. After workup trans-[MeMo(depe)2N2]Li(THF)3 (27-2Mo) was obtained as a mixture with MeLi. Adding an Et2O solution of methyl lithium to a THF solution of cis/trans-{Mo(PMe2Ph)4(N2)2} yielded a mixture of complexes trans-[MeMo(PMe2Ph)4N2] Li(THF)3 (27-3Mo) and fac-[(PMe2Ph)3MoMe(N2)2]Li(THF)3 (27-4Mo) in a 76:24 mixture. Adding MeLi in Et2O to trans{W(N2)2(dppe)2} in THF gave trans-[MeW(dppe)2N2][Li(THF)3] (27-1W) after being irradiated for 3 h at 460 nm. Treating trans-{W(depe)2(N2)2} in THF with MeLi in Et2O and irradiating for 3 h at 460 nm gives trans-[MeW(depe)2N2]Li(THF)3 (27-2W) as a mixture with MeLi after workup. Adding an Et2O solution of MeLi to a THF solution of cis/trans-{W(PMe2Ph)4(N2)2} followed by irradiation at 460 nm for 3 h gives trans-[MeW(PMe2Ph)4N2]Li(THF)3 (27-3W) as a major component in the reaction mixture. Complex [W(N2)2(PMe2Ph)4] was dissolved in THF and methyl lithium was added as an Et2O solution. After stirring for 1 day at 60  C [k2-(PMe2C6H4)W(PMe2Ph)3N2]Li(THF)3 (27-4W) was obtained as a single crystal in low yield. Complexes 27-1Mo,

Chart 4

660

Group 6 Complexes With Metal-Carbon Sigma Bonds

27-2Mo, 27-3Mo, 27-4Mo, 27-1W, 27-2W, and 27-3W, were characterized by 1H, 13C{1H}, and 31P{1H} NMR spectroscopy. Complexes 27-1Mo, 27-2Mo, 27-3Mo, 27-1W, and 27-2W were characterized by FTIR spectroscopy. Elemental analysis data was obtained for 27-1Mo and 27-1W. 27-2Mo was further characterized by 1H{31P} NMR spectroscopy and 27-4W was characterized by single-crystal X-ray diffraction (WdC]2.230(3) A˚ ) and 31P{1H} NMR spectroscopy.27

5.10.3.2

Alkyl and aryl complexes with Cp ligands and without NO ligands

The complex Mo(C5H5)2(C3H5)PF6 was treated with ClMgCH2C(Me)]CH2 in Et2O to yield Mo(C5H5)2(m-CH2)2C(H)[CH2C(Me) CH2] (28-1) after workup (Scheme 10). The complex was characterized by single-crystal X-ray diffraction (Mo-C ¼ 2.258(2) A˚ ), 1H and 13C{1H} NMR spectroscopy, and elemental analysis. The author notes the Grignard must be THF free as the reaction will not occur in the presence of THF.28

Scheme 10

W{(Z-C5H4)CMe2(Z-C5H4)}{(CH2)2CH3}2 (29-1) was synthesized by treatment of [W{Z-(C5H4)CMe2(Z-C5H4)}Cl2] with [Zn{(CH2)2CH3}2]. Complexes W{(Z-C5H4)CMe2(Z-C5H4)}{(CH2)3CH3}2 (29-2) and W{(Z-C5H4)CMe2(Z-C5H4)} {(CH2)3CH3}Cl (29-3) were synthesized from the same reaction mixture. Briefly, to [W{(Z-C5H4)CMe2(Z-C5H4)}Cl2], [Zn {CH2)3CH3}2] was added. The resulting crude mixture was extracted either in pentane to yield 29-2 or a 1:1 mixture of toluene/ pentane to afford 29-3. Complexes W{(Z-C5H4)CMe2(Z-C5H4)}{(CH2)4CH3}Cl (29-4) and [W{(Z-C5H4)CMe2(Z-C5H4)} {(CH2)4CH3}2] (29-5) were synthesized by treatment of [W{(Z-C5H4)CMe2(Z-C5H4)}Cl2] with [Zn{CH2)4CH3}2]. The complexes were purified by column chromatography with 29-4 eluting with Et2O and 29-5 eluting with toluene. W{(Z-C5H4) CMe2(Z-C5H4)}{(CH2)5CH3}2 (29-6) was prepared by treating [W{Z-C5H4)CMe2(Z-C5H4)}Cl2] with [Zn{CH2)5CH3}2] in toluene. Treating 29-1 with NH4I in THF produced [W{(Z-C5H4)CMe2(Z-C5H4)}{(CH2)2CH3}I] (29-7) as a pale red solid after workup (Scheme 11). Treatment of 29-7 with [Na{AlH2(OCH2CH2OCH3)2}] in toluene yields W{(Z-C5H4)CMe2(Z-C5H4)} {(CH2)2CH3}H (29-8). Under similar reaction conditions as those for 29-8; W{(Z-C5H4)CMe2(Z-C5H4)}{(CH2)3CH3}H (29-9) and [W{(Z-C5H4)CMe2(Z-C5H4)}{(CH2)3CH3}D] (29-10) were synthesized from 29-3 and 29-4. All synthesized complexes were characterized via elemental analysis and 1H and 13C{1H} NMR spectroscopy. Complexes 29-1-7,10 were also characterized via mass spectrometry.29

Scheme 11

5.10.3.3

Alkyl and aryl complexes with Cp and NO ligands

30-1-30-4 were prepared by dissolving [Cp W(NO)(CH2CMe3)(Z3-CH2CHCHMe)] in 1-chloropropane [Cp W(NO) (CH2CH2CH2Cl)(Z3-CH2CHCHMe)] (30-1), 1-chlorobutane [Cp W(NO)(CH2(CH2)2CH2Cl)(Z3-CH2CHCHMe)] (30-2), 1-chloropentane [Cp W(NO)(CH2(CH2)2CH2Br)(Z3-CH2CHCHMe)] (30-3), ethylcyclohexane [Cp W(NO)(CH2CH2C6H11) (Z3-CH2CHCHMe)] (30-4) and leaving the samples for 24 h at room temperature. The solvent was removed, and the residue was purified by column chromatography to obtain the product after recrystallization. The complexes were characterized by IR, 1H and 13C{1H} NMR spectroscopy, and elemental analysis. The n(NO) absorption bands are 1599 (30-1), 1599 (30-2), 1597 (30-3), and 1598 (30-4) cm−1. In a similar manner, 30-5 and 30-6 were prepared by dissolving [Cp W(NO)(CH2CMe3) (Z3-CH2CHCHMe)] in di-n-butyl ether for the synthesis of [Cp W(NO)((CH2)4O(CH2)3CH3)(Z3-CH2CHCHMe)] (30-5) or THF for [Cp W(NO)(C4H7O)(Z3-MeCHCHCH2)] (30-6). After isolation and purification after stirring for 24 h, the complexes were characterized as described above for 30-1. The reported n(NO) are 1598 (30-5) and 1598 (30-6) cm−1. Complex [Cp Mo(NO) (CH2CMe3)(Z3-C3H5)] (30-7) was synthesized by adding a suspension of Cp Mo(NO)Cl2 to Mg(CH2CMe3)2 ∙ x(dioxane) in THF, after which the volatiles were removed to yield a brown residue. The residue was taken up in Et2O and added dropwise to Mg(C3H5)2 ∙ x(dioxane) in Et2O. After workup and crystallization 30-7 was characterized by IR, 1H and 13C{1H} NMR spectroscopy, mass spectrometry, and elemental analysis. The NO absorption in the IR spectrum was reported as 1613 cm−1. Mixing Cp Mo(NO) Cl2 with Mg(CH2SiMe3)2 ∙ x(dioxane) and CH2SiMe3 in THF at −196  C followed by warming first to −70  C, then room temperature gave a dark indigo residue after removal of the volatiles. Dissolving the residue in Et2O and adding it dropwise to

Group 6 Complexes With Metal-Carbon Sigma Bonds

661

Mg(CH2CHCHMe)2 ∙ x(dioxane) in Et2O gave [Cp Mo(NO)(CH2SiMe3)(Z3-CH2CHCHMe)] (30-8) after workup. The complex was characterized by IR, 1H NMR spectroscopy, and mass spectrometry. The NO band was reported at 1609 cm−1 in the IR spectrum. X-ray structures were also reported for complexes 30-2,5,6, and 7. The M-alkyl carbon bond lengths are 2.228(3) A˚ (30-2), 2.230 (2) A˚ (30-5), 2.214(3) A˚ (30-6), and 2.269(2) A˚ (30-7).30 To a frozen Et2O solution of Cp W(NO)(CH2CMe3)Cl was added a hexanes solution of Li[CH2CHCHSiMe3]. After purification and crystallization, [Cp W(NO)(CH2CMe3)(Z3-CH2CHCHSiMe3)] (31-1; Chart 5) was characterized by IR (nNO ¼ 1589 cm−1), 1H and NMR spectroscopy, X-ray diffraction (W-C(alkyl) ¼ 2.2643(17) A˚ ), and elemental analysis. Thermolyzing 31-1 in pentane at 55  C for 2 days yielded [Cp W(NO)((CH2)4CH3)(Z3-CH2CHCHSiMe3)] (31-2) after workup. The complex was characterized by IR (nNO ¼ 1596 cm−1), 1H and 13C{1H} NMR spectroscopy, and mass spectrometry. The following complexes were characterized using similar methods, and NO stretches are provided. Thermolyzing 31-1 in methylcyclohexane at 55  C for 2 days yielded [Cp W(NO)(Z3-CH2CHCHSiMe3)(CH2Cy)] (31-3) after workup (nNO ¼ 1568 cm−1). In a bomb, 31-1 was dissolved in cyclohexane and placed under 1000 psig of methane. The resulting mixture was heated to 55  C for 2 days and gave [Cp W(NO)(Me) (Z3-CH2CHCHSiMe3)] (31-4) after isolation (nNO ¼ 1589 cm−1). Complex [Cp W(NO)(Et)(Z3-CH2CHCHSiMe3)] (31-5) was synthesized in the same manner as 31-4 except ethane was used instead of methane (nNO ¼ 1596 cm−1). Thermolyzing 31-1 in mesitylene at 55  C for 2 days gave [Cp W(NO)(CH2-3,5-Me2C6H3)(Z3-CH2CHCHSiMe3)] (31-6) after crystallization (nNO ¼ 1568 cm−1). Heating 31-1 in tetramethylsilane at 55  C for 1 day gives [Cp W(NO)(CH2SiMe3)(Z3-CH2CHCHSiMe3)] (31-7) after filtering through silica and solvent removal (nNO ¼ 1561 cm−1). Heating a Et2O solution of 31-1 at 65  C for 1 day and removing the solvent gave [Cp W(NO)(CH2CH2OCH2CH3)(Z3-CH2CHCHSiMe3)] (31-8) as an orange oil (nNO ¼ 1592 cm−1). A cyclohexane solution of 31-1 and 500 psig of CO was stirred for 2 h at room temperature in a pressure reactor. Solvent removal and crystallization yielded [Cp W(NO)(Z1-C(]O)CH2CMe3)(Z3-CH2CHCHSiMe3)] (31-9), which was characterized by IR spectroscopy (nNO ¼ 1588 cm−1) and X-ray crystallography (W-C ¼ 2.2099(17) A˚ ), and the other usual methods. The synthesis for 31-1 was followed using Cp Mo(NO)(CH2CMe3)Cl instead of the tungsten reagent to give [Cp Mo(NO)(CH2CMe3)(Z3-CH2CHCHSiMe3)] (31-10) (nNO ¼ 1604 cm−1 and Mo-C(alkyl) ¼ 2.275(3) A˚ .31 A glass bomb was loaded with Cp Mo(NO)(CH2SiMe3)2, hexanes, PMe3, and hydrogen gas. The mixture was stirred for 5 h after which the volatiles were removed and the residue crystallized from pentane to give [Cp Mo(NO)(CH2SiMe3)(PMe3)H] (33-1). The complex was characterized by IR (nNO ¼ 1592 cm−1), 1H, 13C{1H}, and 1P{1H} NMR spectroscopy and elemental analysis. The already reported32 Cp W(NO)(Ph)(H)PPh3 was prepared by dissolving [Cp W(NO)[Z2-(C6H4)PPh2]H] (33-2) in benzene. This solution was heated at 50  C for 24 h, and the product isolated by removal of the solvent and crystallization from toluene/ hexanes. The complex was characterized by IR (nNO ¼ 1572 cm−1) and NMR spectroscopy.33 To a cyclohexane solution of [Cp Mo(NO)(CH2CMe3)2] was added cumene hydroperoxide in cyclohexane. After purification and crystallization [Cp Mo(O)(Z2-ONCH2CMe3)(CH2CMe3)] (34-1; Chart 6) was characterized by IR, 1H and 13C{1H} NMR spectroscopy, mass spectrometry, X-ray crystallography, and elemental analysis. The ModC bond length in 34-1 is 2.204(4) A˚ . [Cp W(O)(Z2-ONCH2SiMe3)(CH2SiMe3)] (34-2) was prepared in an analogous manner using [Cp W(NO)(CH2SiMe3)2] as the starting material and characterized by similar methods (W-C ¼ 2.1555(19) A˚ ).34 A Et2O solution of Mg(CH2CMe3)2 ∙ x(dioxane) was added dropwise to a flask containing CpEtW(NO)Cl2 (CpEt ¼ C5Me4Et) in Et2O at −78  C. After warming to room temperature for 1 h the reaction was cooled to −78  C again and additional Mg(CH2CMe3)2 ∙ x(dioxane) was added dropwise. The mixture was allowed to warm to room temperature and [CpEtW(NO) (CH2CMe3)2] (35-1) was isolated after workup. The complex was characterized by IR spectroscopy (nNO ¼ 1579 cm−1), 1H and 13 C{1H} NMR spectroscopy, mass spectrometry, X-ray crystallography, and elemental analysis. The WdC bond lengths are 2.148 (3) and 2.139(3) A˚ .35 Complex [Cp W(NO)(CH2CMe3)(Z3-CH2CHCHPh)] (37-1) was prepared by modifying a previous synthesis.36 Briefly, solid Cp W(NO)Cl2 and solid Mg(CH2CMe3)2 ∙ x(dioxane) were cooled to −196  C and THF was added. The mixture was warmed

Chart 5

Chart 6

662

Group 6 Complexes With Metal-Carbon Sigma Bonds

and stirred for 1 h after which the solvent was removed and dissolved in Et2O. A Et2O solution of Mg(CH2CHCHPh)2 ∙ x(dioxane) was added dropwise to the above mixture at −78  C. After workup, 37-1 was isolated and characterization methods were consistent with those previously reported. Heating 37-1 with n-heptane [Cp W(NO)(n-C7H15)(Z3-CH2CHCHPh)] (37-2), n-octane [Cp W (NO)(n-C8H17)(Z3-CH2CHCHPh)] (37-3), n-pentane [Cp W(NO)(n-C5H11)(Z3-CH2CHCHPh)] (37-4), or p-xylene [Cp W(NO) (CH2C6H4-4-Me)(Z3-CH2CHCHPh)] (37-5) at 55  C for 3 days yielded the corresponding complexes. These complexes were analyzed by IR spectroscopy (37-2 nNO ¼ 1598, 37-3 nNO ¼ 1599, and 37-4 nNO ¼ 1598 cm−1), 1H NMR spectroscopy (all complexes), 13C APT NMR spectroscopy (all complexes), mass spectrometry (37-2, 37-3, 37-4, and 37-5), and elemental analysis (37-3 and 37-4). Complex 37-2 was also characterized by X-ray crystallography, which revealed a WdC bond length of 2.2408 (18) A˚ the bound n-heptyl group. In the same manner, treating 37-1 with either methylcyclohexane [Cp W(NO)(CH2C6H11) (Z3-CH2CHCHPh)] (37-7) or ethylcyclohexane [Cp W(NO)(CH2CH2C6H11)(Z3-CH2CHCHPh)] (37-8) at 55  C for 2 days yielded 37-7 or 37-8, respectively. Complex 37-7 was characterized by IR (nNO ¼ 1562 cm−1), 1H NMR spectroscopy, 13C APT NMR spectroscopy, mass spectrometry, and X-ray crystallography. The W-methylcyclohexane bond length in 37-7 was reported at 2.226(5) A˚ . 37-8 (nNO ¼ 1562 cm−1) was characterized by similar methods as 37-7, but the structure was not determined. Heating 37-1 at 75  C for 18 h in mesitylene gave Cp W(NO)(CH2C6H3-3,5-Me2)(Z3-CH2CHCHPh) (37-9) after isolation. This complex, as well as those that follow, were characterized using similar methods as described above, and only select IR and XRD data is reported (nNO ¼ 1604 cm−1 and W-mesitylene ¼ 2.254(3) A˚ for 37-9). Complex [Cp Mo(NO)(CH2CMe3)(Z3-CH2CHCHPh)] (37-10) was prepared by dissolving Cp Mo(NO)Cl2 in THF and adding it dropwise to a frozen Mg(CH2CMe3)2 ∙ x(dioxane) solution in THF. After warming to room temperature and stirring for an hour the resulting solution was added to a frozen solution of Mg(CH2CHCHPh)∙x(dioxane) in Et2O (nNO ¼ 1611 cm−1). Heating 37-10 with mesitylene at 35  C for 24 h gave [Cp Mo(NO)(Z3-CH2CHCHPh)(CH2-3,5-(CH3)2C6H3)] (37-11) after workup and isolation (nNO ¼ 1616 cm−1; Mo-mesitylene ¼ 2.275(2) A˚ ).37 In a pressure reaction, [Cp W(NO)(CH2CMe3)(Z3-CH2CHCHPh)] was dissolved in cyclohexane and charged with 1250 psig of methane. After stirring and heating at 88  C for 4 h the solvent was removed and the resulting residue was purified to give [Cp W(NO)(CH3)(Z3-CH2CHCHPh)] (38-1) (nNO ¼ 1559 cm−1; W-C ¼ 2.209(7) A˚ ). Heating a pressure reactor charged with [Cp W(NO)(CH2CMe3)(Z3-CH2CHCHPh)], cyclohexane, and 300 psig of ethane to 55  C for 3 days give [Cp W(NO) (CH2CH3)(Z3-CH2CHCHPh)] (38-2) after workup (nNO ¼ 1553 cm−1;W-ethyl ¼ 2.221(13) A˚ ). Following the same procedure except using 250 psig of propane or butane gave [Cp W(NO)(CH2CH2CH3)(Z3-CH2CHCHPh)] (38-3) or [Cp W(NO) (CH2CH2CH2CH3)(Z3-CH2CHCHPh)] (38-4), respectively after isolation (nNO ¼ 1558 cm−1 for 38-3 and 1598 cm−1 for 38-4). Both complex were also analyzed by X-ray diffraction to reveal a W-propane bond length of 2.230(5) A˚ (38-3) and a W-butane bond length of 2.253(7) A˚ (38-4). Heating a solution of [Cp W(NO)(CH2CMe3)(Z3-CH2CHCHPh)] in n-butyl ether at 55  C for 3 days gave [Cp W(NO)((CH2)4O(CH2)3CH3)(Z3-CH2CHCHPh)] (38-5) after purification and isolation (nNO ¼ 1589 cm−1). A glass bomb containing [Cp W(NO)(CH2CMe3)(Z3-CH2CHCHPh)] and 1-chloropropane was heated to 55  C for 3 days. After purification of the crude reaction mixture complex [Cp W(NO)(CH2CH2CH2Cl)(Z3-CH2CHCHPh)] (38-6) was isolated (nNO ¼ 1602 cm−1; W-C(alkyl) ¼ 2.227(8) A˚ ). Heating [Cp W(NO)(CH2CMe3)(Z3-CH2CHCHPh)] and SiMe4 in a reaction bomb at 85  C for 9 h gave [Cp W(NO)(CH2SiMe3)(Z3-CH2CHCHPh)] (38-7) after removal of the solvent and crystallization from Et2O (nNO ¼ 1574 cm−1). A pressure reaction was loaded with 38-2, n-pentane, and 450 psig of CO. It was then heated for 2 h at 55  C after which the solvent was removed and the residue was dissolved in Et2O. Crystallization was induced using pentane to give [Cp W(NO)(C(]O)CH2CH3)(Z3-CH2CHCHPh)] (38-8) (nNO ¼ 1599 cm−1; W-C(alkyl) ¼ 2.220(5) A˚ ). Stirring a pressure reactor containing 38-5, Et2O, and 450 psig of CO for 4 days at room temperature gave [Cp W(NO)(C(]O)(CH2)4O(CH2)3CH3) (Z3-CH2CHCHPh)] (38-9) after removal of the solvent. The complex was characterized by 1H NMR and 13C APT NMR spectroscopy. A pressure reactor was charged with 38-7, pentane, and 250 psig of CO. The reactor was the sealed and heated to 55  C for 2 h to give [Cp W(NO)(C(]O)CH3)(Z3-CH2CHCHPh)] (38-10) after workup (nNO ¼ 1598 cm−1). The orientation of the allyl ligand of each isomer was determined by 1H and 13C NMR spectra for complexes 38-1-38-7.38 Loading a pressure reactor with [Cp W(NO)(CH2CMe3)(Z3-CH2CHCMe2)], cyclohexane, and 1250 psig of CH4, followed by heating the sealed mixture for 0.5 h at 88  C gave [Cp W(NO)(CH3)(Z3-CH2CHCMe2)] (39-1) after purification and crystallization (nNO ¼ 1591 cm−1; W-CH3 ¼ 2.216(5) A˚ ). The 1H NMR signals for both the exo and endo orientations of the allyl ligand were reported. Charging a pressure reactor with [Cp W(NO)(CH2CMe3)(Z3-CH2CHCMe2)], Et2O, and 650 psig of CO then heating to 65  C for 1 hour affords [Cp W(NO)(C(O)CH2CMe3)(Z3-CH2CHCMe2)] (39-2) after purification and crystallization (nNO ¼ 1567 cm−1; W-C(O)CH2CMe3 ¼ 2.226(5) A˚ ). Heating a pressure reactor loaded with 39-1, Et2O, and 500 psig of CO to 75  C for 3 h followed by purification and crystallization yielded [Cp W(NO)(C(O)CH3)(Z3-CH2CHCMe2)] (39-3) (nNO ¼ 1592 cm−1; W-C(O)CH3 ¼ 2.191(8) A˚ ).39 Complex [Cp W(NO)(CH2CMe3)(Z3-CH2CHCHMe)] (40-1) was prepared by loading a Schlenk vessel with Cp W(NO) (CH2CMe3)Cl and Et2O. In a separate vessel Mg(CH2CHCHMe)2 ∙ x(dioxane) and Et2O were mixed and then frozen using a liquid N2 bath. The tungsten solution was added dropwise to the frozen tube at a rate slow enough to ensure the tungsten solution would freeze upon addition. After the addition was complete the mixture was warmed to −78  C and stirred for 45 min at that temperature. Complex 40-1 is thermally unstable so all workup and isolation was carried out using solvents at −30  C. The complex was characterized by IR (nNO ¼ 1594 cm−1), 1H and 13C{1H} NMR spectroscopy, mass spectrometry, X-ray diffraction (W-CH2CMe3 bond length ¼ 2.257(3) A˚ ), and elemental analysis. Complexes 40-2-40-12 were prepared by mixing 40-1 with the corresponding reactants for 24 h at room temperature: pentane [Cp W(NO)(n-C5H11)(Z3-CH2CHCHMe)] (40-2), n-heptane

Group 6 Complexes With Metal-Carbon Sigma Bonds

663

[Cp W(NO)(n-C7H15)(Z3-CH2CHCHMe)] (40-3), methylcyclohexane [Cp W(NO)(CH2(cyclohexyl))(Z3-CH2CHCHMe)] (40-4), tetramethylsilane [Cp W(NO)(CH2SiMe3)(Z3-CH2CHCHMe)] (40-5), 1-chloropetane [Cp W(NO)((CH2)5Cl)(Z3CH2CHCHMe)] (40-6), Et2O [Cp W(NO)(CH2CH2OCH2CH3)(Z3-CH2CHCHMe)] (40-7), triethyl amine [Cp W(NO) (CH2CH2N(CH2CH3)2)(Z3-CH2CHCHMe)] (40-8), cyclohexane [Cp W(NO)(Z3,Z1-CH2CHCHCH2CbH(C4H8)CaH)] (40-9), acetone [Cp W(NO)(Z3,Z1-CH2CHCHCH2C(CH3)2O)] (40-10), 2-butyne [Cp W(NO)(Z3,Z1-CH2CHCHCH2CCH3]CCH3)] (40-11), or 2,3-dimethyl-2-butene [Cp W(NO)(Z1-CH2CMe]CMe2)(Z3-CH2CHCHMe)] (40-12). The samples were purified by column chromatography and recrystallized to yield the respective complexes. All complexes were characterized by IR (nNO ¼ 1597 (40-2), 1593 (40-3), 1591 (40-4), 1593 (40-5), 1598 (40-6), 1595 (40-7), 1596 (40-8), 1588 (40-9), 1594 (40-10), 1600 (40-11), and 1597 (40-12) cm−1), 1H NMR, 13C{1H} NMR, and mass spectrometry. Additionally, 40-2-7, and 9-12 were characterized by elemental analysis. Complexes 40-2 and 40-11-12 were analyzed by X-ray diffraction to show the WdC(alkyl) bond lengths of 2.242(3), 2.191(3), and 2.261(5) A˚ for 40-2, 40-11, and 40-12, respectively. In a reaction bomb, 40-1, C6F6, and propane were combined and stirred for 20 h at room temperature. After removal of the volatiles, [Cp W(NO)(CH2CH2CH3)(Z3-CH2CHCHMe)] (40-13) was isolated in the same manner as 40-2. 40-13 was characterized by IR (nNO ¼ 1598 cm−1), 1H and 13C{1H} NMR spectroscopy, mass spectrometry, and elemental analysis. The following complexes were characterized using similar methods, and only select IR and XRD data are reported when provided. Stirring a mixture of 40-1, C6F6, and 400 psig of C2H6 for 20 h at room temperature affords [Cp W(NO)(CH2CH3)(Z3-CH2CHCHMe)] (40-14) after purification and crystallization (nNO ¼ 1600 cm−1; W-CH2CH3 ¼ 2.332(11) A˚ ). Mixing 40-1, hexane, and 1025 psig of methane in a pressure reactor for 20 h at room temperature yielded [Cp W(NO)(CH3)(Z3-CH2CHCHMe)] (40-15) after purification and crystallization (nNO ¼ 1600 cm−1). Dissolving 40-1 in toluene gave an orange solution, which was left for 20 h at room temperature and then heated to 50  C for 20 h to give [Cp W(NO)(CH2C6H5)(Z3-CH2CHCHMe)] (40-16) after purification and crystallization (nNO ¼ 1603 cm−1; WdCH2C6H5 bond length ¼ 2.216(7) A˚ ).40 Vigorously shaking a mixture of Mg(c-C3H4)2(1,4-C4H8O2)x and the corresponding complexes [Cp W(NO)(CH2SiMe3)Cl], [Cp W(NO)(CH2Ph)Cl], or [Cp W(NO)(CH2-t-Bu)Cl] gave the following complexes and NO stretches: 1572 cm−1 [Cp W(NO) (c-C3H5)(CH2SiMe3)] (41-1-SiMe3), 1572 cm−1 [Cp W(NO)(c-C3H5)(CH2Ph)] (41-1-Ph), and 1564 cm−1 [Cp W(NO)(c-C3H5) (CH2-t-Bu)] (41-2-t-Bu). Treating [Cp W(NO)(CH2SiMe3)Cl], [Cp W(NO)(CH2Ph)Cl], or [Cp W(NO)(CH2-t-Bu)Cl] with Mg(c-C3H5)2 ∙ x(1,4-C4H8O2) in THF for 1 h gave the corresponding complexes [Cp W(NO)(Z3-C3H5)(CH2SiMe3)] (41-2SiMe3), [Cp W(NO)(Z3-C3H5)(CH2Ph)] (41-2-Ph), and [Cp W(NO)(Z3-C3H5)(CH2-t-Bu)] (41-2-t-Bu) after purification and crystallization. These complexes were characterized by FTIR (nNO ¼ 1584 (41-2-SiMe3), 1589 (41-2-Ph), or 1585 (41-2-t-Bu) cm−1) and 1H and 13C{1H} NMR spectroscopy. Additionally complex 41-2-Ph was analyzed by X-ray diffraction to show a WdPh bond length of 2.252(3) A˚ .41 Warming a −35  C solution of Cp W(NO)(CH2SiMe3)Cl with a half equivalent of Mg(c-C3H5)2 ∙ x(1,4-C4H8O2) in THF to room temperature over the course of an hour (Scheme 12), followed by removal of the solvent gave a dark yellow-brown residue. The mixture was purified by layering pentane on dichloromethane and cooling to −40  C. The complex Cp W(CO)(CH2SiMe3)(allyl) (42-1TMS) was characterized by FTIR (nNO ¼ 1584 cm−1), 1H and 13C{1H} NMR spectroscopy, elemental analysis, and X-ray diffraction (W-C ¼ 2.238(4) A˚ ).42

Scheme 12

Cp W(NO)(H)(Z1-CH2CH]CMe2)(PMe3) (43-1) was prepared by heating a mixture of Cp W(NO)(H)(Z3-CH2CH]CMe2) (PMe3) with PMe3 in a glass bomb to 60  C for 18 h. The complex 43-1 was isolated from the mixture by first crystallizing from pentane then by column chromatography. The complex was characterized by FTIR (nNO ¼ 1563 cm−1), 1H and 31P NMR and 13C APT NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray diffraction (W-C 2.259(2) A˚ ). Heating a sealed mixture of Cp W(NO)(H)(Z3-CH2CH]CHPh) and PMe3 to 60  C for 18 h gave Cp W(NO)(H)(Z1-CH2CH]CHPh)(PMe3) (43-2) after crystallization from Et2O (characterized by similar methods; nNO ¼ 1536 cm−1). Two asymmetric units of 43-2 are located in the unit cell with WdC bond lengths of 2.284(4) and 2.302(4) A˚ .43 A mixture of Cp0 W(NO)Cl2 and Mg(CH2CMe3)2 ∙ x(dioxane) was cooled to −196  C and THF was added to the mixture. After warming to room temperature, the solvent was removed and the residue extracted with Et2O. The combined extracts were cooled to −60  C and a separate flask with Mg(CH2CHCHMe)2 ∙ x(dioxane) was cooled to −196  C. The extract solution was added dropwise at a rate slow enough to freeze upon addition. After addition was complete, the reaction was moved to a dry ice/acetone bath and stirred for 45 min. The solvent was removed and the residue was purified to give Cp0 W(NO)(CH2CMe3)(Z3-CH2CHCHMe) (44-10 ). This complex (as well as the others that follow) were characterized by FTIR (nNO ¼ 1595 cm−1), 1H NMR and 13C APT NMR spectroscopy, mass spectrometry, and elemental analysis. Two isomers of Cp0 W(NO)(Ph)(Z3-MeCHCHCH2) (44-2a0 and 44-2b0 ) were prepared by dissolving 44-10 in benzene and leaving the solution sit undisturbed for 48 h. The solvent was removed and

664

Group 6 Complexes With Metal-Carbon Sigma Bonds

residue purified to give a mixture of isomers. The FTIR spectrum revealed a single nNO ¼ 1590 cm−1 and X-ray diffraction revealed W-C ¼ 2.214(3) A˚ for isomer 44-2a0 and W-C ¼ 2.210(3) A˚ for isomer 44-2b0 . In an analogous reaction, Cp W(NO)(CH2CMe3) (Z3-CH2CHCHMe) (44-1 ) was dissolved in benzene and left undisturbed for 24 h. The solvent was then removed and recrystallization from pentane at −20  C yielded Cp W(NO)(Ph)(Z3-CH2CHCHMe) (44-2a ) (nNO ¼ 1604 cm−1; W-C ¼ 2.216(5) A˚ ). If 44-10 is dissolved in pentane, instead of benzene, and left undistributed for 24 h, the complex Cp0 W(NO)(CH2(CH2)3CH3) (Z3-CH2CHCHMe) (44-30 ) is instead isolated after removal of the solvent and recrystallization from a 2:1 pentane:Et2O mixture (nNO ¼ 1600 cm−1). Substituting 1-chlorobutane for pentane in the reaction above yields Cp0 W(NO)(CH2(CH2)2CH2Cl) (Z3-CH2CHCHMe) (44-40 ) after recrystallization from pentane (nNO ¼ 1601 cm−1). To synthesize Cp0 W(NO)(CH2CMe3) (Z3-CH2CHCHMe) (44-50 ), THF was added to a reaction vessel containing Cp0 W(NO)Cl2 and (CH2CMe3)2Mg ∙x(dioxane) at −196  C. The reaction was then warmed to −78  C and the solvent removed to yield a dark red oily residue that was taken up in Et2O and added to a separate flask containing (CH2CHCHPh)2Mg ∙ x(dioxane). Complex 44-50 was isolated by removal of the solvent (nNO ¼ 1604 cm−1; W-C ¼ 2.249(6) A˚ ). Thermolyzing 44-50 in benzene at 55  C for 22 h gave Cp0 W(NO)(C6H5) (Z3-CH2CHCHMe) 44-60 after removal of the solvent and purification (nNO ¼ 1578 cm−1).44 To a frozen mixture of Mg(CH2CHCHMe)2 ∙ x(dioxane) in Et2O at −196  C was added a Et2O solution of Cp W(NO) (CH2SiMe3)Cl. The resulting mixture was allowed to warm to room temperature over the course of 1 h. The solvent was removed and the residue purified to yield Cp W(NO)(CH2SiMe3)(Z3-CH2CHCHMe) (45-1; nNO ¼ 1593 cm−1). Following the same process as above, but instead using Cp W(NO)(CH2C6H5)Cl and Mg(CH2CHCHMe)2 ∙ x(dioxane), gave Cp W(NO)(CH2C6H5) (Z3-CH2CHCHMe) (45-2; nNO ¼ 1603 cm−1). Treating an Et2O solution of 45-1 with CNC6H3Me2 for 2 h at room temperature gave Cp W(NO)(CH2SiMe3)(Z2-CH3CHCHCH2C]NC6H3Me2) (45-3a) after removal of the solvent and crystallization from Et2O (nNO ¼ 1553 cm−1; W-C ¼ 2.197(7) A˚ ). A CNC6H3Me2 solution in acetonitrile was added to an acetonitrile solution of 45-1 and then stored in the freezer for 1 h. The resulting mixture was transferred to an alumina column and the mixture purified to yield Cp W(NO)(CH2SiMe3)(Z2-CH3CH2CHCHC ¼ NC6H3Me2) (45-3b) as red crystals (nNO ¼ 1554 cm−1); W-C ¼ 2.211(2) A˚ ). A mixture prepared by adding a THF solution of 45-2 dropwise to a THF solution of CNC6H3Me2 was left at room temperature for 20 h. The solvent was removed and the residue recrystallized from Et2O at −5  C to give Cp W(NO)(CH2C6H5) (Z2-CH3CHCHCH2C]NC6H3Me2) (45-4a) (nNO ¼ 1560 cm−1). In a similar manner, a Et2O solution of 45-2 was added dropwise to a Et2O solution of CNC6H3Me2 and left at room temperature for 20 h. The solution was then transferred to an alumina column where Cp W(NO)(CH2C6H5)(Z2-CH3CH2CHCHC]NC6H3Me2) (45-4b) was separated and recrystallized from Et2O (nNO ¼ 1560 cm−1). When THF was used instead of Et2O for the reaction, Cp W(NO)(CH2C6H5)(Z2-CH3CH2CHCHC]NC4H9) (45-5b) was isolated as a viscous red oil (nNO ¼ 1548 cm−1). A benzene solution of 45-1 was placed in a steel pressure vessel and pressured to 1000 psig with CO. The mixture was stirred for 72 h after which the solvent was removed and the residue purified to yield Cp (W)(NO)(C(O)Me)(Z3-CH2CHCHMe) (45-6) (nNO ¼ 1594 cm−1). Stirring a pentane solution of Cp (W)(NO)(CH2CMe3)-(Z3-CH2CHCHMe) under 850 psig of CO for 20 h yielded Cp (W)(NO)(C(O)CH2CMe3)(Z3-CH2CHCHMe) (45-7) after workup and recrystallization from pentane (nNO ¼ 1548 cm−1; W-C ¼ 2.203(3) A˚ ). In a similar experiment, pressurizing a solution of Cp (W)(NO)(CH2CMe3) (Z3-CH2CHCHPh) in Et2O with 850 psig of CO gas for 20 h yielded Cp (W)(NO)(C(O)CH2CMe3)(Z3-CH2CHCHPh) (45-8) after isolation and crystallization from Et2O (nNO ¼ 1591 cm−1; W-C ¼ 2.240(5) A˚ ). The complex Cp (W)(NO)(C(O)CH2CMe3) (Z3-CH2CMe-CH2) (45-9) was prepared in the same manner using Cp (W)(NO)(CH2CMe3)(Z3-CH2CMeCH2) instead of Cp (W) (NO)(CH2CMe3)-(Z3-CH2CHCHPh). Complex 45-9 was characterized by the usual means: FTIR (nNO ¼ 1596 cm−1), 1H and 13C {1H} NMR, mass spectrometry, elemental analysis, and X-ray diffraction (W-C ¼ 2.220(5) A˚ ). Cp (W)(NO)(C(O)CH2CMe3) (Z3-CH2CHCH2) (45-10) was also prepared by this method using Cp (W)(NO)(CH2CMe3)(Z3-CH2CHCH2) as the starting material (nNO ¼ 1599 cm−1).45 To a Et2O solution containing dimethyl allyl frozen at −196  C was added a −60  C Et2O solution containing Cp W(NO) (CH2CMe3)Cl dropwise at a slow enough rate to ensure it froze upon addition. After the addition was complete, the solution was warmed to −60  C with stirring for a total of 45 min. The solvent was reduced and the mixture separated using an alumina column to yield Cp W(NO)(CH2CMe3)(Z3-CH2CHCMe2) (36-1). The product was characterized by FTIR (nNO ¼ 1546 cm−1), 1H and 13C {1H} NMR spectroscopy, mass spectrometry, and elemental analysis. As described in the previous paragraphs, all the complexes that follow were characterized by similar methods as 36-1, but only NO stretches and metal-carbon distances are reported here. Using similar reaction conditions and isolation methods, Cp W(NO)(CHCMe3)(Z3-CH2CMeCH2) (36-2) was prepared by mixing Cp W(NO)(CH2CMe3)Cl and (CH2-CMeCH2)2Mg ∙x(dioxane) (nNO ¼ 1563 cm−1). The thermally unstable complex Cp W(NO) (CH2CMe3)(Z3-CH2CHCHMe) (36-3) was prepared by treating Cp W(NO)(CH2CMe3)Cl with (CH2-CHCHMe)2Mg ∙ x(dioxane) being sure to use solvents at −30  C (nNO ¼ 1594 cm−1; W-C ¼ 2.257(3) A˚ ). Cp W(NO)(CH2CMe3)(Z3-CH2CHCHPh) (36-4) was prepared by reacting Cp W(NO)(CH2CMe3)Cl and (CH2CHCHPh)2Mg ∙ x(dioxane) in a manner analogous to the synthesis and isolation of 36-1 (nNO ¼ 1588 cm−1; W-C ¼ 2.265(3) A˚ ). Also following the procedure for the synthesis and isolation of 36-1, Cp W(NO)(CH2SiMe3)(Z3-CH2CHCHMe) (36-5) was prepared by the reaction between Cp W(NO)(CH2SiMe3)Cl and (CH2CHCHMe)2Mg ∙ x(dioxane) (nNO ¼ 1593 cm−1; W-C ¼ 2.212(6) A˚ ). Cp W(NO)(CH2CMe3)(Z3-C3H5) (36-6) was also prepared in a similar manner using Cp W(NO)(CH2CMe3)Cl and (C3H5)2Mg ∙x(dioxane) (nNO ¼ 1597 cm−1; W-C ¼ 2.267(6) A˚ ). Stirring a solution of 36-1 in pyrrolidine for 17 h at room temperature affords Cp W(NO)(CH2CMe3)(NC4H7-2-Me2-CCH]CH2) (36-7) after separation and crystallization (nNO ¼ 1561 cm−1; W-C ¼ 2.192(3) A˚ ). Stirring 36-1 in piperidine of pyrrolidine instead yielded Cp W(NO)(CH2CMe3)(NC5H9-2-Me2CCH]CH2) (36-8) after separation and crystallization (nNO ¼ 1564 cm−1; W-C ¼ 2.190(3) A˚ ). Stirring a pyrrolidine solution of 36-2 for 64 h under an N2 flow yielded Cp W(NO)(CH2CMe3)

Group 6 Complexes With Metal-Carbon Sigma Bonds

665

(NC4H7-2-CH2CMe]CH2) (36-9) after separation and crystallization (nNO ¼ 1563 cm−1; W-C ¼ 2.184(3) A˚ ). Stirring 36-3 with pyrrolidine for 17 h at room temperature yielded Cp W(NO)(CH2CMe3)(NC4H7-2-CHMe-CH]CH2) (36-10) after separation and crystallization (nNO ¼ 1563 cm−1). If 36-4 is used instead in the previous reaction, Cp W(NO)(CH2CMe3)(NC4H7-2-CHPh-CH] CH2) (36-11) is produced (nNO ¼ 1562 cm−1). Under identical conditions, using 36-5 yielded Cp W(NO)(CH2SiMe3) (NC4H7-2-CHMeCH]CH2) (36-12) after separation and crystallization (nNO ¼ 1561 cm−1). Leaving 36-2 in excess pyrrolidine for 5 h gave Cp W(NO)(NC4H8)(CHMeCH2NC4H8) (36-13) (nNO ¼ 1553 cm−1; W-C ¼ 2.198(6) A˚ ). When the previous reaction described for 36-13 was left for 2 days instead of 5 h, Cp W(NO)(NC5H10)(CHMeCH2NC5H10) (36-14) was prepared (nNO ¼ 1561 cm−1).36 For the following set of complexes, the convention used is a:b:c ¼ ortho:meta:para is used to be consistent with the presentation of materials in the original manuscript. Unless otherwise stated, the only complex isolated was the “a” isomer. The other two isomers were only identified by 1H NMR spectroscopy. The isolated complexes were characterized by various combinations of FTIR and NMR spectroscopy, mass spectrometry, elemental analysis and X-ray diffraction, but only NO stretches and WdC bond distances are described here. Cp W(NO)(CH2CMe3)(C6H4F) (46-1a-c) was prepared in a 97:2:1 ratio by heating Cp W(NO)(CH2CMe3)2 in fluorobenzene (ortho isomer; nNO ¼ 1592 cm−1; W-C ¼ 2.162(3) and 2.101(3) A˚ ). If chlorobenzene is used instead, the distribution of products is 75:18:8 for Cp W(NO)(CH2CMe3)(C6H4Cl) (46-2a; nNO ¼ 1589 cm−1). Using bromobenzene in the reaction yielded a distribution of 63:25:12 of Cp W(NO)(CH2CMe3)(C6H4Br) (46-3a-; nNO ¼ 1596 cm−1). Using anisole for the solvent in the thermolysis of Cp W(NO)(CH2CMe3)2 gave a distribution of 87:7:6 for Cp W(NO)(CH2CMe3)(C6H4OMe) (46-4a; nNO ¼ 1564 cm−1, W-C ¼ 2.149(3) A˚ and 2.122(4) A˚ ). Heating a mixture of Cp W(NO)(CH2CMe3)2 with diphenylacetylene for 40 h at 70  C gave 51:33:16 as the distribution, but also yielded new complex Cp W(NO)(Z3,Z1-(Me3C)HC-CPh]CPh-CPh]CPh) (46-5d) in low yield. X-ray diffraction data collected for 46-5d was not high enough quality to be included in the publication, but this complexes was also characterized by 1H NMR and FTIR spectroscopy (nNO ¼ 1563 cm−1). The 1H NMR data for 46-5b and 46-5c were not reported, but 46-5a was characterized by FTIR (nNO ¼ 1568 cm−1), 1H and 13C{1H} NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray diffraction (W-C(phenyl) ¼ 2.156(4) and 2.118(4) A˚ ). The only product identified from the thermolysis of Cp W(NO)(CH2CMe3)2 in 3-hexyne was Cp W(NO)(Z3,Z1-(Me3C)HC-CEt]CEt-CEt]CEt) (46-6; nNO ¼ 1583 cm−1; W-C(alkyl) ¼ 2.192(4) A˚ ). When thermolyzing Cp W(NO)(CH2CMe3)2 with ortho-difluorobenzene or ortho-dichlorobenzene, only ortho or meta products were formed. In the case of ortho-difluorobenzene the ratio of a:b was 98:2 and the complex Cp W(NO)(CH2CMe3)(C6H3F2) (46-7a) was isolated and characterized (nNO ¼ 1596 cm−1). For ortho-dichlorobenzene, the distribution of ortho:meta products was 82:18 and Cp W(NO)(CH2CMe3)(C6H3Cl2) (46-8a) was isolated and characterized (nNO ¼ 1580 cm−1). The thermolysis of Cp W(NO)(CH2CMe3)2 in either meta-difluorobenzene or meta-dichlorobenzene gave only Cp W(NO)(CH2CMe3)(C6H3F2) (46-9 a:c) (84:16 ratio) products for the meta-difluorobenzene and only Cp W(NO) (CH2CMe3)(C6H3Cl2) (46-10 a:b) (89:11) products for the meta-dichlorobenzene. 46-9a was characterized by FTIR (nNO ¼ 1595 cm−1), 1H and 19F{1H} NMR, mass spectrometry, and elemental analysis. 46-9c was also characterized by FTIR (nNO ¼ 1609 cm−1), 1H NMR spectroscopy, mass spectrometry, and X-ray diffraction (W-C ¼ 2.1657(17) and 2.1002(17) A˚ ). 46-10a was characterized by similar means (nNO ¼ 1580 cm−1). The thermolysis of Cp W(NO)(CH2CMe3)2 in para-chlorofluorobenzene gave an 85:15 a:b distribution of Cp W(NO)(CH2CMe3)(C6H3ClF) (46-11a and 46-11b) where a is when the fluorine is ortho with respect to tungsten and b is when the chlorine is ortho. 46-11a was characterized by FTIR (nNO ¼ 1599 cm−1), 1H and 19F{1H} NMR spectroscopy, mass spectrometry, and elemental analysis. The thermolysis of Cp W (NO)(CH2CMe3)2 in para-fluoroanisole yielded only a and b isomers of Cp W(NO)(CH2CMe3)(C6H3FOMe) (46-12a and 46-12b) in a ratio of 71:21 where isomer a is when fluorine is ortho with respect to tungsten and isomer b is when methoxy is ortho. Both isomers were isolated and characterized by FTIR (nNO ¼ 1597 cm−1 (46-12a) and nNO ¼ 1561 cm−1 (46-12b)), 1H and 19 F{1H} NMR spectroscopy, mass spectrometry, and elemental analysis. Cp W(NO)(CH2CMe3)2 was thermolyzed in para-chloroanisole and both a and b isomers of Cp W(NO)(CH2CMe3)(C6H3ClOMe) (46-13a and 46-13b) were isolated and characterized in a ratio of a:b 77:23 with a having the methoxy group ortho with respect to tungsten and b have the chlorine group ortho. Both complexes were characterized by FTIR (nNO ¼ 1584 cm−1 (46-13a) and nNO ¼ 1567 cm−1 (46-13b)), 1H NMR spectroscopy, and mass spectrometry. 46-13a was further characterized by 13C{1H} NMR and elemental analysis. Addition of Et4NCN in MeNO2 to Cp W(NO)(CH2CMe3)(C6H4F) (46-1a) at room temperature for 5 min yielded [Et4N][Cp W(NO)(CH2CMe3)(CN) (o-C6H4F)] (46-14) after purification and crystallization (nNO ¼ 1530 cm−1; nCN ¼ 2103 cm−1). Stirring a hexanes solution of Cp W(NO)(CH2CMe3)2 while bubbling CO through the solution yielded Cp W(NO)(Z2-C(]O)CH2CMe3)(o-C6H4F) after crystallization (n ¼ 1584 and 1566 cm−1; W-C ¼ 2.211(9) A˚ ).46 Stirring a benzene solution of Cp W(NO)(Z3-CH2CHCMe2)(Ph) in benzene with excess PPh3 for 8 days at 70  C yielded trans Cp W(NO)(H)(k2-PPh2C6H4) (47-1) after purification and precipitation from a 1:1 Et2O/n-pentane mixture (nNO ¼ 1552 cm−1). To prepare (Z5-C5Hi4Pr)W(NO)(CH2CMe3)2 (47-2), a Et2O solution of Mg(CH2CMe3)2 at −78  C was added to a Et2O solution of (Z5-C5Hi4Pr)W(NO)I2 also at −78  C. After warming the mixture, the volume was reduced and the product was separated by column chromatography (nNO ¼ 1594 cm−1). Bubbling H2 through a solution containing PPh3 and (Z5-C5Hi4Pr)W(NO)(CH2CMe3)2 in n-pentane gave trans-(Z5-C5Hi4Pr)W(NO)(H)(k2-PPh2C6H4) (47-3) that was crystallized from a 50:50 mixture of CH2Cl2 and hexanes (nNO ¼ 1579 cm−1; W-C ¼ 2.2134(18) A˚ ). Adding P(OMe)3 instead of PPh3 to the above reaction yielded transCp W(NO)(CH2CMe3)(H)(P(OMe)3) (47-4) after separation and crystallization from a/pentane solution (nNO ¼ 1586 cm−1; W-C ¼ 2.250(11) A˚ ). Heating 47-4 in SiMe4 at 50  C for 16 h yielded the dimeric [Cp W(NO)(Me)(PO(OMe)2)]2 (47-5) with the W2P2O2 adapting either a boat or chair conformation. Conducting the thermolysis reaction in n-pentane instead of

666

Group 6 Complexes With Metal-Carbon Sigma Bonds

SiMe4 yielded only the boat conformation after crystallization from 50:50 Et2O/THF. The boat form of the complex was characterized by FTIR (nNO ¼ 1571 cm−1), 1H and 31P NMR and 13C APT NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray diffraction (W-C ¼ 2.051(4) and 2.060(6) A˚ ). The chair form was characterized by 1H NMR and 13C APT NMR spectroscopy. Sealing Cp W(NO)(CH2CMe3)2 and P(OCH2)3CMe under a hydrogen atmosphere and stirring at room temperature overnight gave trans-Cp W(NO)(CH2CMe3)(H)(P(OCH2)3CMe) (47-6) upon isolation and crystallization from Et2O (nNO ¼ 1583 cm−1; W-C ¼ 2.2654(18) A˚ ). Heating a sealed flask of benzene and trans-Cp W(NO)(CH2CMe3)(H)(P(OCH2)3CMe) at 50  C for 14 h yielded trans-Cp W(NO)(H)(C6H5)(P(OCH2)3CMe) (47-7) after column purification. The complex was characterized by 1H and 31P{1H} NMR and 13C APT NMR spectroscopy and mass spectrometry. Stirring a mixture containing H2, Cp W(NO)-(CH2CMe3)2, and P(OPh)3 at room temperature for 1 h affords trans-Cp W(NO)(CH2CMe3)(H)(P(OPh)3) (47-8) after crystallization from Et2O (nNO ¼ 1590 cm−1; W-C ¼ 2.274(4) A˚ ).47 Adding a benzene solution of Cp W(NO)(C5H11)(Z3-CH2CHCHMe) dropwise to a benzene solution of CNC6H3Me2 yielded Cp W(NO)(Z2-C5H11C]NC6H3Me2)(Z1-CH2CHCHMe) (48-1) after separation and crystallization from pentane (nNO ¼ 1566 cm−1 and nCN ¼ 1652 cm−1; W-CH2CHCHMe ¼ 2.214(3) A˚ and W-C2,6xylylisonitrile ¼ 2.101(3) A˚ ). Using the supernatant from the above purification process and reducing the volume yielded Cp W(NO)(Z2,Z1-CH2CHCHMe)N (C6H3Me2)]C(C5H11) (48-2). The complex was characterized by FTIR (nNO ¼ 1560 cm−1 and nCN ¼ 1596 cm−1), 1H and 13C {1H} NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray diffraction (W-C2,6xylylisonitrile ¼ 2.134(3) A˚ ). The complexes Cp W(NO)(Z2-C7H15C]NC6H3Me2)(Z1-CH2CHCHMe) (48-10 ), Cp W(NO)(Z2,Z1-CH2CHCHMe)N(C6H3Me2)] C(C7H15) (48-20 ), Cp W(NO)(Z2-C8H17C]NC6H3Me2)(Z1-CH2CHCHMe) (48-100 ), and Cp W(NO)(Z2,Z1-CH2CHCHMe)N (C6H3Me2)]C(C8H17) (48-200 ) were prepared in an analogous manner by substituting the appropriate tungsten starting material. All four complexes where characterized by 1H and 13C{1H} NMR spectroscopy, mass spectrometry, and elemental analysis in addition to FTIR spectroscopy: 48-10 (nNO ¼ 1560 cm−1 and nCN ¼ 1653 cm−1), 48-20 (nNO ¼ 1560 cm−1 and nCN ¼ 1653 cm−1)], 48-100 ((nNO ¼ 1559 cm−1 and nCN ¼ 1653 cm−1)], and 48-200 (nNO ¼ 1560 cm−1 and nCN ¼ 1653 cm−1)].48 In a glass bomb, Cp W(NO)(H)(Z3-CH2CHCMe2) and N-methylmorpholine were stirred at 80  C for 18 h. The solvent was removed and the residue recrystallized from 1:1 Et2O/hexanes to yield Cp W(NO)(Z2-CH2NC4H8O)(Z1-CH2CH2CHMe2) (49-1) (n ¼ 1522 cm−1; W-CH2CH2CHMe2 ¼ 2.222(5) A˚ and W-CH2NC4H8O ¼ 2.137(5) A˚ ). Heating a mixture of Cp W(NO)(H) (Z3-CH2CHCHPh) and N-methylmorpholine to 80  C for 72 h yielded Cp W(NO)(Z2-CH2NC4H8O)(Z1-CH2CH2CH2Ph) (49-2; Chart 7) after removal of the volatiles and purification by flash chromatography. The complex was characterized by 1H NMR and 13C NMR spectroscopy. Heating a mixture of Cp W(NO)(Z2-CH2NC4H8O)(Z1-CH2CH2CH2Ph) and N-methylmorpholine at 75  C for 3 days yielded two isomers of Cp W(NO)(Z2-CH2NC4H8O)(Z1-CH2CH2CH2Me) (49-3a and 49-3b) in an 80:20 ratio. The minor isomer was characterized by 1H and 13C NMR spectroscopy. The major isomer was characterized by NMR spectroscopy as well as FTIR spectroscopy (nNO ¼ 1564 cm−1), mass spectrometry, and X-ray diffraction. There are two asymmetric units in the unit cell with W-C distances of W1-CH2CH2CH2Me ¼ 2.218(5) A˚ , W1-CH2NC4H8O ¼ 2.143(5) A˚ , W2-CH2CH2CH2Me ¼ 2.222(5) A˚ , and W2-CH2NC4H8O ¼ 2.141(6) A˚ .49 Heating Cp W(NO)(CH2CMe3)(Z3-CH2CHCHSiMe3) in pentafluorobenzene for 48 h at 55  C gave Cp W(NO)(C6F5) (Z3-CH2CHCHSiMe3) (50-1) after removal of the solvent and crystallization by slow diffusion of pentane into an Et2O extract of the residue (nNO ¼ 1581 cm−1; W-C ¼ 2.216(3) A˚ ). Heating Cp W(NO)(CH2CMe3)(Z3-CH2CHCHSiMe3) and paradifluorobenzene at 55  C for 48 h yielded Cp W(NO)(2,5-F2C6H3)(Z3-CH2CHCHSiMe3) (50-2) after removal of the solvent and crystallization of the resulting material from pentane (nNO ¼ 1592 cm−1; W-C ¼ 2.207(3) A˚ ). Heating Cp W(NO)(CH2CMe3) (Z3-CH2CHCHSiMe3) in ortho-difluorobenzene for 2 days at 55  C yielded a minor product Cp W(NO)(Z3-CH2CHCHSiMe3) (2,3-F2C6H3) (50-3) which was only identified by 1H NMR spectroscopy.50

5.10.3.4

Complexes with metal-element multiple bonds

The complex Mo(O)(RC^CR)(Z5,s-C4Ph4COCR]CR) (51-1) was prepared via two different routes. First, Mo(CO)(RC^CR) (Z5,s-C4Ph4COCR]CR) was dissolved in THF known to contain peroxides, which, after workup, yielded Mo(O)(RC^CR) (Z5,s-C4Ph4COCR]CR) (51-1) in near quantitative yield. The second synthesis was completed by stirring a dichloromethane solution of Mo(CO)(RC^CR)(Z5,s-C4Ph4COCR]CR) in air. After workup, Mo(O)(RC^CR)(Z5,s-C4Ph4COCR]CR) (51-1) was characterized by melting point analysis, IR spectroscopy, 1H and 13C{1H} NMR spectroscopy, mass spectrometry, and elemental analysis. The similar complex Mo(O)(RC^CH)(Z5,s-C4Ph4COCH]CR) (51-2) was prepared by treating a mixture of

Chart 7

Group 6 Complexes With Metal-Carbon Sigma Bonds

667

Mo(CO)(RC^CH)(Z5,s-C4Ph4COCH]CR) with THF containing peroxides in CH2Cl2 or exposing Mo(CO)(RC^CH) (Z5,s-C4Ph4COCH]CR) briefly to air. After workup, the products, both major and minor, were characterized by IR and NMR spectroscopies, mass spectrometry, and elemental analysis. The reaction produced a ratio of 1.3:1 for Mo(O)(RC^CH) (Z5,s-C4Ph4COCH]CR) (51-2a:51-2b) with 51-2b having the methyl propiolate ligand rotated 180  C compared to 51-2a.51 Mo(N-t-Bu)2(CH2-t-Bu)2 (52-1) was prepared by treatment of Mo(N-t-Bu)2Cl2(DME) with neopentyl magnesium chloride in THF. After workup, Mo(N-t-Bu)2(CH2-t-Bu)2 (52-1) was isolated as a crude brown oil, which was only characterized by 1H NMR spectroscopy.52 Complex Mo(NC6F5)(CHCMe2Ph)(ONO)] was dissolved in pentane and placed under 1 atm of ethylene. After workup, the solid Mo(NC6F5)(CH2CH2CH2)(ONO) (53-1) was characterized by single crystal X-ray diffraction (Mo-C ¼ 2.2037(14) and 2.1948(14) A˚ ), 1H, 19F, and 13C{1H} NMR spectroscopy, and elemental analysis. Alternatively, 53-1 can be synthesized by dissolving H2ONO in benzene and adding it to Mo(NArF)2(CH2CMe2Ph)2 and heating to 60  C for 12 h. After removing the solvent, the residue was dissolved in pentane and exposed to 1 atm of ethylene for 4 h. The tungsten analog of Mo(NC6F5) (CH2CH2CH2)(ONO) (53-1) was synthesized by storing W(O)(CHCMe2Ph)(ONO)(PPh2Me) (53-1(PPh2Me)) in pentane under 1 atm of ethylene. After workup, W(O)(CH2CH2CH2)(ONO) (53-3) was characterized by 1H and 13C{1H} NMR spectroscopy and elemental analysis.53 W(NAr)(C3H6)(OR )(Me2Pyr) (54-1) (Ar ¼ 2,6 diisopropylphenyl and Me2Pyr ¼ 2,5-dimethylpyrrolide) was prepared by the addition of 1 atm of ethylene to W(NAr)(CH-CMe2Ph)(OR )(ME2Pyr) (OR ¼ (R)-3,30 -dibromo-20 -(tert-butyldimethylsilyloxy)5,50 ,6,60 ,7,70 ,8,80 -octahydro-1,10 binaphtyl-2-olate) in pentane (Scheme 13). This complex can also be synthesized by generating W(NAr)(CH2)(OR )(Me2Pyr) and exposing it to 1 atm of ethylene in toluene. 54-1 was characterized by 1H and 13C{1H} NMR, 2D 1 H-13C HSQC NMR, 2D 1H-1H NOESY NMR, and 2D 1H-1H EXSY spectroscopy, single-crystal X-ray diffraction, and elemental analysis. The crystal structure showed non-equivalent carbon atoms on the -CH2-CH2-CH2 ring with one WdC bond being 2.040 (4) A˚ and the other being 2.057(4) A˚ .54

Scheme 13

Complex {WO(ONOtBu)Me2} (55-1) was prepared using two different starting materials using identical procedures. Treating either trans- or cis-{WO(ONOtBu)Cl2} in Et2O with MeMgI affords a yellow air stable solid after workup (Scheme 14). The complex was characterized via IR and 1H NMR spectroscopy, melting point analysis, single-crystal X-ray diffraction, and elemental analysis. There were two 55-1 molecules in the asymmetric unit with identical WdCH3 bond lengths within experimental error. One complex has bond lengths of 2.182(5) and 2.176(6) A˚ while the other has bond lengths of 2.178(5) and 2.185(6) A˚ .55

Scheme 14

W(NArCl)(CH2CH2CH2)[OC(CF3)2Me]2 (56-1) was prepared by exposing W(NArCl)(CHCMe3)(ORF6)2 to 1 atm of ethylene in benzene. After workup, the complex was characterized by 1H, 13C{1H}, and 19F NMR spectroscopy, single-crystal X-ray diffraction, and elemental analysis. The carbon atoms bound to tungsten in the -CH2-CH2-CH2- ligand had W-C distances of 2.078(4) and 2.016(4) A˚ . In a similar manner, W(NArCl)(C3H6)(Biphen) (56-2) was prepared by treating W(NArCl)(CHCMe3) (Biphen)(THP) in benzene with 1 atm of ethylene. After workup, the product was characterized by 1H and 13C{1H} NMR spectroscopy and elemental analysis. In the same manuscript Arndt et al. also prepared W(NArCl)(C4H8)(Biphen) (56-3) by placing W(NArCl)(Biphen)(C2H4)(THF) under 1 atm of ethylene in benzene. After workup, the complex was characterized via 1H and 13C{1H} NMR spectroscopy, single-crystal X-ray crystallography, and elemental analysis. The MdC bond lengths were 2.129 (8) and 2.112(8) A˚ .56

668

Group 6 Complexes With Metal-Carbon Sigma Bonds

In a glass bomb, a toluene solution of [(CF3N2NMe)Mo(CH2SiMe3)2] was added to a 2-butyne in toluene. The bomb was sealed and heated for 5 h at 88  C, which after workup yielded {(CF3N2NMe)Mo(CHSiMe3)(Z2-MeC^CMe)} (57-1) as yellow blocks. The sample was characterized by 1H, 13C{1H}, and 19F NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction. The ModC bond length was reported as 1.959(8) A˚ .57 Addition of 1 equivalent of the corresponding ligands in THF to [MoO2Cl2(THF)2] in THF, followed by slow addition of CH3MgBr at −20  C, yielded the following complexes: (4,40 -bismethoxycarbonyl-2,20 -bipyridine ¼ [MoO2(CH3)2{4,40 bis-methoxycarbonyl-2,20 -bipyridine}] (58-1); 4,40 -bisethoxycarbonyl-2,20 -bipyridine ¼ [MoO2(CH3)2{4,40 -bis-ethoxycarbonyl2,20 -bipyridine}] (58-2); or 5,50 -bisethoxycarbonyl-2,20 -bipyridine ¼ [MoO2(CH3)2{5,50 -bis-ethoxycarbonyl-2,20 -bipyridine}] (58-3)). After workup and isolation, 58-1-51-3 were characterized by FTIR, 1H, 13C, and 95Mo NMR spectroscopy, and elemental analysis. The Mo-CH3 1H NMR resonances of the complexes were observed as d 0.57 (58-1), 0.57 (58-2), and 0.61 (58-3).58 Adding 15 psi of ethylene gas to a pentane solution of [(NPh)Mo(C4H8)(o-(Me3SiN)2C6H4)] gave [(NPh)Mo(C(Ph)C(H) CH2CH2)(o-(Me3SiN)2C6H4)] (59-1) after crystallization. Complex 59-1 was characterized by 1H and 13C{1H} NMR spectroscopy and elemental analysis. A Et2O solution of [(NPh)Mo(Cl)2(THF)(o-(Me3SiN)2C6H4)] was treated with EtMgCl2 at −78  C followed by phenylacetylene and stirred overnight. After isolation, [(NPh)Mo(C(H)C(Ph)CH2CH2)(o-(Me3SiN)2C6H4)] (59-2) was characterized by 1H and 13C{1H} NMR spectroscopy, X-ray crystallography, and elemental analysis. The reported ModC bond lengths for 59-2 are 2.1520(19) A˚ and 2.175(2) A˚ . Adding phenylacetylene to a stirring solution of [(NPh)Mo(C(H)-C(Me)2) (o-(Me3SiN)2C6H4)] in pentane resulted in a color change from green to yellow. Workup and crystallization revealed [(NPh)Mo(C(Ph)CHC(Ph))(o-(Me3SiN)2C6H4)] (59-3), which was characterized by 1H and 13C{1H} NMR spectroscopy, X-ray crystallography, and elemental analysis. The ModC bond lengths for 59-3 are 2.188(3) and 2.197(3) A˚ .59

5.10.3.5

Alkynyl complexes

Mo(C^CC6H4-4-C^CH)(dppe)(Z-C7H7) (60-1) was prepared by refluxing a mixture of [MoBr(dppe)(Z-C7H7)]∙0.5 CH2Cl2, 1,4-HC  CC6H4C  CH, and KOBut in methanol. After workup, 60-1 was isolated and characterized via 1H, 13C{1H}, and 31P {1H} NMR spectroscopy, IR spectroscopy, mass spectrometry, and elemental analysis. Methanol was added to a mixture of 60-1, [FeCl(dppe)Cp ], and NaBPh4. After workup, [{Fe(dppe)Cp }{-C^CC6H4(H)C]C-}{Mo(dppe)(Z-C7H7)}]BPh4 (60-2) was isolated and characterized by 31P{1H} NMR and IR spectroscopy, mass spectrometry, and elemental analysis. Treatment of 60-2 with KOBut in THF yielded {Fe(dppe)Cp }(-C^CC6H4C^C-){Mo(dppe)(Z-C7H7)} (60-3), which was characterized by 1H, 13C {1H}, and 31P{1H} NMR spectroscopy, IR spectroscopy, mass spectrometry, and elemental analysis. Adding [FeCp2]PF6 to 60-3 produced [{Fe(dppe)Cp }(-C^CC6H4C^C-){Mo(dppe)(Z-C7H7)}](PF6)2 (60-3[PF6]2) which was characterized by IR spectroscopy and elemental analysis. Mixing 60-3 and 60-3[PF6]2 in CH2Cl2 yielded [{Fe(dppe)Cp }{-C^CC6H4(H)C]C-}{Mo(dppe) (Z-C7H7)}]PF6 (60-3[PF6]) after workup. 60-3[PF6] was analyzed by similar methods used to characterize 60-3[PF6]2.60

5.10.3.6

Multimetallic Mo and W complexes and clusters

Pineda and co-workers reported two molybdenum clusters that co-crystallized from the same reaction mixture. The clusters were synthesized by refluxing Mo(CO)6 in a mixture of propionic acid and propionic anhydride for 8 h. During workup, the sample was loaded on a Dowex 50WX2 ion exchange column to yield an orange band that yielded a nearly 50:50 mixture of {Mo3(m3-CCH3) (m3-O)(m-O2CCH2CH3)6(H2O)3}Br
4H2O (61-1) and {Mo3(m3-CCH2CH3)(m3-O)(m-O2CCH2CH3)6(H2O)3}Br
4H2O (61-2) upon workup. The clusters were characterized by 1H and 13C{1H} NMR spectroscopy, UV-Vis spectroscopy, single-crystal X-ray crystallography and molybdenum-only elemental analysis. The average ModC bond length for the bridging C-group was determined to be 2.060(2) A˚ .61 To synthesize {(TMEDA)Na}4Mo2Me8 (24-1), [Mo(O2CCH3)2]2 was suspended in Et2O followed by dropwise addition of MeLi in Et2O at 0  C for 18 h. The suspension was then filtered and the resulting solution was treated with NaOtBu and TMEDA (N,N,N0 , N0 -tetramethylethylenediamine). After workup, red crystals of {(TMEDA)Na}4Mo2Me8 (24-1) were isolated and characterized by 1H and 13C{1H} NMR spectroscopy and single-crystal X-ray diffraction. The ModC bond lengths in {(TMEDA) Na}4Mo2Me8 (24-1) vary between 2.305(2) and 2.324(2) A˚ .24 Mg(CH2CMe3)2 ∙ 1.05(1,4-dioxane) in Et2O was added dropwise to CpCr(nacnacxyl,xyl)OTs (nacnacxyl.xyl ¼ xyl-N-C(Me)CHC(Me)-N-xyl, xyl ¼ 2,6-C6H3Me3) in Et2O at room temperature. After workup the complex was crystallized from hexanes and characterized by UV-Vis spectroscopy, X-ray crystallography, and elemental analysis. The CrdC bond length for CpCr(nacnacxyl,xyl) CH2CMe3 62-1 is 2.136(3) A˚ .62

5.10.4

Conclusion

There has been significant activity in the area of group 6 metal complexes containing metal-carbon sigma bonds since the last edition of this text. Focusing in on the less common supporting ligands shows there is room for exploration. The majority of the work conducted in this field has been with chromium complexes which leaves a large space for analogous molybdenum and tungsten complexes to be explored. The authors note the particular potential of NO serving as good ligand for complexes which contain metal-carbon sigma bonds as a gateway for catalytic reactions. Numerous examples of group 6 metal complexes which contain a ligand that is sigma bound via carbon and a NO were discussed, but these were focused on a just a few supporting ligands. Exploring other ligand scaffolding with sigma bound carbon atoms and NO ligands appears to be an area with great potential.

Group 6 Complexes With Metal-Carbon Sigma Bonds

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M.; Tsang, J. Y. K.; Thibault, M. E.; Patrick, B. O.; Legzdins, P. Factors Influencing the Outcomes of Intermolecular C-H Activations of Hydrocarbons Initiated by CpW(NO)(CH2CMe3)(Z3-Allyl) Complexes (Cp ¼ Z5-C5Me5 (Cp ), Z5-C5Me4H (Cp’)). Organometallics 2012, 31 (3), 1055–1067. 45. Semproni, S. P.; Graham, P. M.; Buschhaus, M. S. A.; Patrick, B. O.; Legzdins, P. Toward Alkane Functionalization Effected With Cp W(NO)(alkyl)(Z3-allyl) Complexes. Organometallics 2009, 28 (15), 4480–4490. 46. Tsang, J. Y. K.; Buschhaus, M. S. A.; Legzdins, P.; Patrick, B. O. Ortho-Selective C-H Activation of Substituted Benzenes Effected by a Tungsten Alkylidene Complex Without Substituent Coordination. Organometallics 2006, 25 (17), 4215–4225. 47. Fabulyak, D.; Handford, R. C.; Holmes, A. S.; Levesque, T. M.; Wakeham, R. J.; Patrick, B. O.; Legzdins, P.; Rosenfeld, D. C. Thermal Chemistry of Cp W(NO)(CH2CMe3)(H) (L) Complexes (L ¼ Lewis Base). Inorg. Chem. 2017, 56 (1), 573–582. 48. Semproni, S. P.; Legzdins, P. Unprecedented Reactivity Initiated by Insertion of 2,6-Xylylisonitrile into the W-Alkyl Linkages of Cp W(NO)(n-alkyl)(Z3-CH2CHCHMe) Complexes. Organometallics 2009, 28 (21), 6139–6141. 49. Baillie, R. A.; Wakeham, R. J.; Lefevre, G. P.; Bethegnies, A.; Patrick, B. O.; Legzdins, P.; Rosenfeld, D. C. Thermal Chemistry of Cp W(NO)(H)(Z3-allyl) Complexes. Organometallics 2015, 34 (13), 3428–3441. 50. Chow, C.; Patrick, B. O.; Legzdins, P. Thermal Chemistry of a Tungsten Trimethylsilylallyl Complex in Benzene and Fluorobenzenes. Organometallics 2012, 31 (23), 8159–8171. 51. Adams, H.; Booth, Y. K.; Cook, E. S.; Riley, S.; Morris, M. J. Reactions of Tetracyclone Molybdenum Complexes with Electrophilic Alkynes: Cyclopentadienone-Alkyne Coupling and Alkyne Coordination. Organometallics 2017, 36 (11), 2254–2261. 52. Bukhryakov, K. V.; VenkatRamani, S.; Tsay, C.; Hoveyda, A.; Schrock, R. R. Syntheses of Molybdenum Adamantylimido and t-Butylimido Alkylidene Chloride Complexes Using HCl and Diphenylmethylphosphine. Organometallics 2017, 36 (21), 4208–4214. 53. Sues, P. E.; John, J. M.; Schrock, R. R.; Muller, P. Molybdenum and Tungsten Alkylidene and Metallacyclobutane Complexes That Contain a Dianionic Biphenolate Pincer Ligand. Organometallics 2016, 35 (5), 758–761. 54. Jiang, A. J.; Simpson, J. H.; Muller, P.; Schrock, R. R. Fundamental Studies of Tungsten Alkylidene Imido Monoalkoxidepyrrolide Complexes. J. Am. Chem. Soc. 2009, 131 (22), 7770–7780. 55. Lehtonen, A.; Sillanpaeae, R. Synthesis and Structure of Stable Cis-Dimethyl Complex of Oxotungsten(VI). J. Organomet. Chem. 2007, 692 (12), 2361–2364. 56. Arndt, S.; Schrock, R. R.; Mueller, P. Synthesis and Reactions of Tungsten Alkylidene Complexes That Contain the 2,6-Dichlorophenylimido Ligand. Organometallics 2007, 26 (5), 1279–1290. 57. Hock, A. S.; Schrock, R. R. Oxidative Reactions of the MoIV Dialkyl Complex [{(3-CF3C6H4NCH2CH2)2NMe}Mo(CH2SiMe3)2]. Chem. Asian J. 2007, 2 (7), 867–874. 58. Guenyar, A.; Betz, D.; Drees, M.; Herdtweck, E.; Kuehn, F. E. Highly Soluble Dichloro, Dibromo and Dimethyl Dioxomolybdenum(VI)-Bipyridine Complexes as Catalysts for the Epoxidation of Olefins. J. Mol. Catal. A Chem. 2010, 331 (1-2), 117–124. 59. Ison, E. A.; Abboud, K. A.; Boncella, J. M. Synthesis and Reactivity of Molybdenum Imido Diamido Metallacyclopentenes and Metallacyclopentadienes and the Mechanism of Ethylene Exchange With Metallacyclopentane Complexes. Organometallics 2006, 25 (7), 1557–1564. 60. Fitzgerald, E. C.; Ladjarafi, A.; Brown, N. J.; Collison, D.; Costuas, K.; Edge, R.; Halet, J.-F.; Justaud, F.; Low, P. J.; Meghezzi, H.; Roisnel, T.; Whiteley, M. W.; Lapinte, C. Spectroscopic Evidence for Redox Isomerism in the 1,4-Diethynylbenzene-Bridged Heterobimetallic Cation [{Fe(Dppe)Cp }(m-C  CC6H4C  C){Mo(Dppe)(Z-C7H7)}]PF6. Organometallics 2011, 30 (15), 4180–4195. 61. Pineda, K. A.; Fettinger, J. C.; Houston, J. R. A New Aqueous Molybdenum Cluster With a Mo3(m3-CCH3)(m3-O) Trinuclear Core. Inorg. Chim. Acta 2012, 392, 485–489. 62. Champouret, Y.; MacLeod, K. C.; Baisch, U.; Patrick, B. O.; Smith, K. M.; Poli, R. Cyclopentadienyl Chromium b-Diketiminate Complexes: Initiators, Ligand Steric Effects, and Deactivation Processes in the Controlled Radical Polymerization of Vinyl Acetate. Organometallics 2010, 29 (1), 167–176.

5.11

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Richard R Schrocka, Michael R Buchmeiserb, Jonas Groosb, and Mathis J Benedikterb, aDepartment of Chemistry, Pierce Hall, University of California at Riverside, Riverside, CA, United States; bInstitute of Polymer Chemistry, University of Stuttgart, Stuttgart, Germany © 2022 Elsevier Ltd. All rights reserved.

5.11.1 5.11.2 5.11.2.1 5.11.2.2 5.11.2.2.1 5.11.2.2.2 5.11.2.2.3 5.11.2.2.4 5.11.2.2.5 5.11.2.2.6 5.11.2.2.7 5.11.2.2.8 5.11.2.2.9 5.11.2.2.10 5.11.2.3 5.11.3 5.11.3.1 5.11.3.2 5.11.3.3 5.11.3.4 5.11.4 5.11.5 5.11.5.1 5.11.5.1.1 5.11.5.1.2 5.11.5.1.3 5.11.5.1.4 5.11.5.1.5 5.11.5.1.6 5.11.5.1.7 5.11.5.1.8 5.11.5.1.9 5.11.5.1.10 5.11.5.1.11 5.11.5.2 5.11.5.3 5.11.5.4 5.11.5.5 5.11.5.6 5.11.5.6.1 5.11.5.6.2 5.11.5.7 5.11.5.8 5.11.6 References

5.11.1

Introduction Alkylidene complexes Early tantalum chemistry Tungsten and molybdenum alkylidenes Discovery Imido alkylidenes Metallacyclobutanes Decomposition and olefin complexes Metallacyclopentanes Oxo alkylidenes Cationic alkylidenes M]CHX complexes Disubstituted alkylidenes Oxo- and imido-free alkylidenes Chromium alkylidenes Alkylidyne complexes Discovery Development of catalysts for alkyne metathesis Alkyl/alkylidyne vs bisalkylidene Other alkylidyne chemistry Pincer alkylidenes and alkylidynes NHC alkylidene and alkylidyne complexes Molybdenum imido alkylidene complexes that contain a monodentate NHC Bistriflates Cationic monotriflates Monotriflate monoalkoxides Bisalkoxides Pyrrolides Halides Cationic monoalkoxides Carboxylates Complexes that contain a chelating alkylidene Ionically tagged complexes Silica-supported complexes Molybdenum imido alkylidene complexes that contain a chelating NHC Neutral molybdenum alkylidyne complexes Cationic molybdenum alkylidyne complexes Supported molybdenum alkylidyne complexes Tungsten alkylidene complexes Oxo complexes Imido alkylidene complexes Alkoxide-based tungsten alkylidyne complexes Halide-based tungsten alkylidyne and tungsten oxo alkylidene complexes derived therefrom Conclusions and perspective

671 682 682 691 691 691 702 704 708 708 712 713 715 717 719 719 719 722 726 727 731 735 735 735 737 738 738 739 740 741 742 743 745 746 747 748 752 753 753 753 754 761 763 765 766

Introduction

Transition metal complexes that contain a carbene ligand, CRR0 (R and R0 are C-based or H), bound to a metal in groups 4–7 is best described as an alkylidene, [CRR0 ]2−, when that description results in a d0 electron count at the metal. From this viewpoint, alkylidene ligands are analogous to oxo (O2−) or imido (NR2−) ligands, which are also common ligands in the high oxidation state chemistry of metals in groups 4–7. Oxo and imido ligands are bound to d0 metals through triple M]O or M]N bonds as a

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00062-7

671

672

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

consequence of an electron pair being available on O or N to form a triple bond. In contrast, the M]C bond in an alkylidene complex has comparatively limited options for increasing the electron count at the metal. An alkylidyne ligand, [CR]3−, is an analog of the nitrido (N3−) ligand. If R or R0 in [CRR0 ]2− is not C or H, then the character of [CHX]2− or [CXY]2− (X and/or Y being a heteroatom), can be dramatically altered, the extreme being where the CRR0 ligand is strictly a two electron s donor, a circumstance found with N heterocyclic carbenes (NHC’s). It should also be noted that alkylidenes are isomers of olefins (e.g., ethylidene vs ethylene or CMePh vs styrene) and bind similarly to a metal in otherwise identical circumstances. Olefins are usually viewed as neutral ligands in much of organo transition metal chemistry, although viewing them as dianions if bound to d0 metals is not absurd. Olefins are central to “alkene metathesis” chemistry and alkynes to “alkyne metathesis” chemistry, both driving forces for much research in organometallic chemistry starting around 1970. The mechanisms of the two metathesis reactions also are closely related. Therefore, both high oxidation state (d0) alkylidene complexes and high oxidation state alkylidyne complexes are covered in this article.

ð1Þ

ð2Þ

- H+

- H+

M

M=CHR

M-CH2R

+ H+

CR

ð3Þ

+ H+

The first isolable compounds that contain a M]C bond were reported by E. O. Fischer in 19641 (e.g., (CO)5W]CPh(OMe)) and were called “carbene” complexes. The first “carbyne” (M^CR) complexes (e.g., Br(CO)4Mo^CPh) were prepared in 1973.2 The first complex that qualifies as a d0 alkylidene complex was prepared in 19743 in the process of exploring the synthesis and chemistry of pentaalkyl tantalum complexes. (“Peralkyl” or “homoleptic” alkyl complexes such as hexamethyltungsten, WMe6, prepared in 1973,4 were of fundamental interest in organometallic chemistry at the time.) TaMe5 was prepared from TaMe3Cl25 and methyllithium and found to be unstable above 0  C.6 Ta(CH2Ph)5 was found to be isolable and stable up to 50  C.7 An attempt to prepare Ta(CH2-t-Bu)5 yielded thermally stable Ta(CH-t-Bu)(CH2-t-Bu)3 and neopentane in what is best described as an internal deprotonation (movement of H from C to C) of one neopentyl group in Ta(CH2-t-Bu)5 by another (Eq. 1).3 (Highly unstable Ta(CH2-t-Bu)5 and Ta(CD2-t-Bu)5 have since been observed and studied.8–10) A 13C NMR resonance for the alkylidene a carbon atom in Ta(CH-t-Bu)(CH2-t-Bu)3 is found at 250.1 ppm and the JCH value is only 90 Hz, consistent with a substantial reduction of the s character in the alkylidene CHa bond and therefore an increased protonic character for Ha. The neopentylidene a proton in Ta(CH-t-Bu)(CH2-t-Bu)3 is removed by butyllithium in the presence of dimethylpiperazine (DMP) to yield the first d0 alkylidyne, a [Li(DMP)]+ salt of [Ta(C-t-Bu)(CH2-t-Bu)3]− (Eq. 2), in which lithium is weakly bound more or less to the electron density in the triple bond between Ta and the alkylidyne a carbon atom.11 A unifying view is that high oxidation state MdC, M]C, and M^C bonds in d0 complexes are polarized M(d +) and C(d −) and can be interconverted through loss or gain of a proton, as shown in Eq. (3) (without balancing charges). The vast majority of alkylidene and alkylidyne research has concerned d0 compounds of tantalum, tungsten, molybdenum, and rhenium, largely those in which the alkylidene or alkylidyne ligand contains no heteroatom bound to the a carbon atom. An attempt to place all “carbene” complexes into classes on the basis of whether the carbene ligand is viewed as a neutral (carbene) or a dianionic (alkylidene) entity in the end is futile. The truth lies somewhere in between. Nevertheless, we will still refer the alkylidene and alkylidyne complexes described here as having a d0 electron configuration.

R1CH=CHR2

R1CH=CHR1 + R2CH=CHR2

ð4Þ ð5Þ

The driving force for organometallic research in high oxidation state alkylidene and alkylidyne chemistry of (initially) tantalum compounds, and later tungsten, molybdenum, and rhenium compounds, were the discoveries that heterogeneous catalysts (initially metal oxides on silica) that contain Mo, W, or Re could catalyze what came to be known as the olefin or alkene metathesis reaction,12 in which C]C bonds overall are cleaved and reformed to give a new alkene (Eq. 4), and the acetylene or alkyne

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

673

metathesis reaction,13 in which C^C bonds overall are cleaved and reformed to give a new alkyne (Eq. 5). These reactions cannot be explained through classical organic chemistry.

ð6Þ

ð7Þ

The challenge beginning in  1975 was first to determine the mechanism of the reactions in Eqs. (4) and (5), then to synthesize and characterize homogeneous compounds that demonstrably carry out these reactions, and finally to add these reactions to the chemistry toolbox as new ways to manipulate C]C and C^C bonds. The mechanisms for alkene and alkyne metathesis have been proven over a period of several decades to consist of reversible reactions between pseudotetrahedral d0 M]C compounds (M ] Mo, W, Re) with (usually) a 14 electron count and olefins to give intermediate metallacyclobutane complexes (Eq. 6)14 and between pseudotetrahedral d0 compounds (M ] Mo, W) with (usually) a 12 electron count and acetylenes to give intermediate metallacyclobutadiene complexes (Eq. 7).15 All 14e alkylidene complexes are subject to a variety of decomposition reactions that destroy the alkylidene, although evidence in both homogeneous and heterogeneous systems suggest that the alkylidene can arise from the olefin itself.16 A “classical” heterogeneous catalyst consisting of WO3 on (MgO-doped) silica is probably the largest scale industrial metathesis reaction today, one in which ethylene is dimerized to butenes, and 2-butenes and ethylene are metathesized to form propylene at high temperatures (300–400  C).17 Overall ethylene is turned into propylene (3 CH2]CH2 ! 2 CH3CH]CH2), the demand for which greatly exceeds that for the much more abundant ethylene today. The metathesis portion of this conversion (ethylene + 2-butene ! 2 propylenes), like all olefin and acetylene metathesis reactions, in theory can be run in either direction because metathesis reactions are close to thermoneutral if none of the olefins is highly strained. Catalytically manipulating olefins to make new olefins and acetylenes to make new acetylenes is an inherently attractive “atom economic” feature of alkene and alkyne metathesis reactions. In the 1990s well-characterized olefin metathesis catalysts of ruthenium (e.g., Ru(CHCH]CPh2)(PCy3)2Cl2) were developed by R. H. Grubbs,18,19 and ultimately many others. It turns out that a four-coordinate carbene complex with a 14e count at Ru also is required to form a relatively unstable and (rarely observable) ruthenacyclobutane intermediate in an olefin metathesis reaction. Therefore, a donor ligand must be lost in a typical 16e complex to yield the 14e catalytically active four-coordinate species. The carbene ligands in Ru complexes are not produced through a proton abstraction reactions and the Ru]C bond is not as strongly polarized as in d0 complexes, all of which cause Ru catalysts generally to be less sensitive than 14e d0 complexes to O2, moisture, and to relatively acidic protons. In this article we will focus on the synthesis and properties of alkylidene and alkylidyne complexes that contain molybdenum or tungsten in its highest oxidation state, especially those that initiate olefin and acetylene metathesis reactions. (Cr(VI) organometallic chemistry is rare and no catalytic metathesis activity for alkene or alkyne metatheses has been reported; see Section 5.11.2.3) Many reviews, focus articles, etc., have been published in the last 50 years that describe catalyst syntheses and mechanisms and applications of metathesis in organic and polymer chemistry, and research in the area continues today. We will take care to point out recent findings and state their relevance to the field from today’s perspective. All compounds that have been prepared since the last comprehensive review on high oxidation state multiple metal-carbon bonds in 200220 will be listed in tables which, as much as possible, allow the reader to find relatively quickly what is known in this increasingly large field and also generate questions that lead to future work. The text will be largely a description of history and highlights since 2001, leaving the reader to consult Tables 1–3 for all such complexes in a given category of interest. Abbreviations used within this text can also be found in the tables, if not defined in the text. It is impractical to list all reviews and books on the subjects of alkene and alkyne metathesis, and there has been much overlap and repetition over the years. Previous reviews should be consulted as necessary.20,217–233 We will not discuss comprehensively the theoretical studies surrounding d0 alkylidene complexes, metallacyclobutanes, and other issues relevant to olefin metathesis,234–242 many of which are ongoing. We also will not discuss in detail the many important advances in surface organometallic chemistry243–246 that consist of attaching well-defined complexes to a surface, usually via a surface siloxide (OSisurf) bond that results from protonation and elimination of a ligand, often an alkoxide; this work too is ongoing. Applications of olefin and acetylene metathesis in organic or polymer chemistry will be limited to selected examples. Surveys of the chemistry of all carbon-transition metal double and triple bonds are published annually; they are listed here as references for the years 2000–19 for convenience.247–265

674

Table 1

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes Isolated Mo(VI) and W(VI) alkylidene and related complexes ( ) X-ray; R1 ¼t-Bu; R2 ¼CMe2Ph).

M(NR)(CHR0 )X2 a. X ¼ halide or triflate Mo(NAr)(CHR2)(bipy)Cl2 Mo(NAd)(CHR2)(PPh2Me)Cl2 Mo(N-t-Bu)(CHR1)(PPh2Me)Cl2 Mo(NC6F5)(CHR2)(PPhMe2)2Cl2 Mo(NArTrip2)(CHR1)(py)Cl2 Mo(NArTrip2)(CHR1)Cl2 Mo(NArMes2)(CHR2)Cl2(L) Mo(NR)(CHR2)(OTf )2(bipy) Mo(NC6F5)(CHR2)(DME)(OTf )2 Mo(NFedipp)(CHR2)(DME)(OTf )2 Mo(NArCl2)(CHR1)(OTf )2(DME)x(THF)y Mo(NArCl2)(CHR2)(OTf )2(THF) Mo(NArF2)(CHR2)(OTf )2(DME) Mo(NR)(CHR2)(OTf )2(DME) Mo(NR)(CHR2)(OTf )2(DME) Mo(NArTrip)(CHR1)(OTf )2(DME) Mo(NC6H4-2-CH2CH2CMe2CH)(DME)(OTf )2 Mo(N-2,4-i-PR2C6H2-6-CH2CH2CMe2CH)(DME)(OTf )2 Mo(NR)(CHR2)(OTf )2(DME) Mo(NR)(CHR2)(OTf )2(DME) W(NR)(CHR1)(OTf )2(DME) W(NArTrip2)(CHR1)Cl2 W(NArTrip2)(CHR1)Cl2(py) W(N-t-Bu)(CH-o-MeOC6H4)(Cl)2(py) W(N-t-Bu)(CHR2)(py)2Cl2 W(NR)(CHR1)Cl2(py)2 W(NR)(CHR2)Cl2(bipy) W(O)(CHR2)(PMe3)2Cl2 W(NArMes2)(CHR2)Cl2(L) W(NArMe2)(CHR2)Cl2(DME) [W(NC6F5)(CHR1)(m-DME)(OTf )2]x W(NArMe2)(CHR1)(OTf )2(DME) W(NArMe2)(CHR2)(OTf )2(DME) W(NArCl2)(CHR1)(OTf )2(DME) W(NR)(CHR2)(DME)Cl2 W(NArtBu)(CHR2)(OTf )2(DME) W(NR)(CHR2)(DME)Br2 W(Nt-Bu)(CHR2)(DME)Cl2 b. X ¼ pyrrolide and other N-based ligands Mo(NAr)(CHR)(Pyr)2 Mo(NR)(CHR2)(Pyr)2 Mo(NAr)(CHR2)X2 Mo(NAr)(CHR1)(Ph2Pyr)2 Mo(NAr)(CHR2)(indolide)2(THF) Mo(NAr)(CHR2)(R2Pz)2 Mo(NAr)(CHR1)(Me2Pyr)2 Mo(NAr)(CHR2)(MesPyr)2 Mo(NR)(CHR2)(Me2Pyr)2 Mo(N-t-Bu)(CHR1)(Pyr)2(bipy) Mo(NAd)(CHR2)(Pyr)2 Mo(NAd)(CHR2)(Pyr)2(PMe3) Mo(NAd)(CHR)(Me2Pyr)2 Mo(NAd)(CHR2)(indolide)2 Mo(NAd)(CHR2)(MesPyr)2 Mo(NAd)(CHR2)(CNPyr)2 Mo(N-2,6-Br2-4-MeC6H2)(CHR1)(Pyr)2 Mo(NR)(CHR2)(Pyr)2(bipy) Mo(NC6F5)(CHR2)(Me2Pyr)2 Mo(NArX)(CHR2)(Me2Pyr)2 Mo(NArX)(CHR1)(Me2Pyr)2 Mo(NArMes2)(CHR2)(Me2pyr)2 Mo(NArMes2)(CHR2)(Pyr)2(py) Mo(NArTrip2)(CHR1)(Pyr)2 Mo(NAr)(CHR)(NPh2)2

L ¼ pyridine or 3,5-lutidine R ¼ Ar , Ad, ArMe2, ArCl, AriPr, ArtBu, ArMes

NR ¼ NPhCy or NPhAd R ¼ ArCl or ArMes (imido tethered to alkylidene) (imido tethered to alkylidene) NR ¼ 2,6-R0 2-4-((CH2)6Br)C6H2, R0 ¼ Me, i-Pr NR ¼ 2,6-i-Pr2-4-((CH2)6Cl)C6H2 R ¼ 2,4,6-Cl3C6H2, 2,4,6-Br3C6H2, 2,6-Cl2-4-CF3C6H2, 3,5-(CF3)2C6H3

R ¼ t-Bu or Ad R ¼ ArMe2, Ar, ArCl2, AriPr L ¼ py or bipy

R ¼ Ar, ArMe2, ArCl2, Ar3,5Me2 R ¼ Ar, ArMe2, ArCF3

R ¼ R1 or R2 R ¼ Ar, ArMe2, Art-Bu X ¼ Me4Pyr , i-Pr2Pyr , Ph2Pyr , Indolide

R2Pz ¼ 3,5-diphenylpyrazolide or 3,5-di-t-butylpyrazolide

R ¼ Ad, ArMe2, ArCF3

R ¼ t-Bu or CMe2Ph

R ¼ Ar , Ad, ArMe2, ArCl, AriPr, ArtBu, ArMes X ¼ Cl, i-Pr, Mes X ¼ CF3 , t-Bu, Trip

R ¼ t-Bu or CMe2Ph

21 22 22 21 23 23 24 25 26 27 28 29 30 31 32 32 33 34 35 35 36 23 23 37 21 38 38 39 40 41 26 42 43 44 45 46 47 47 48 49 50 50 50 51 52 53 54 55 48 50 50 50 56 56 4 25 26 32 32 24 40 23 57

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Table 1

675

(Continued)

Mo(NArMe2)(CHR2)(NPh2)2 Mo(NAr)(CHR1)[N(i-Pr)(3,5-C6H3Me2)]2 Mo(NAr)(CHR1)[N(t-Bu)(3,5-C6H3Me2)]2 Mo(NArt-Bu)(CHR2)(MesPyr)2 W(NR)(CHR2)(DME)(Pyr)2 W(NAr)(CHR2)(DME)(Pyr)2 W(NArCl2)(CHR1)(Pyr)2(DME) W(NArMe2)(CHR2)(Pyr)2(DME) W(NAr3,5Me2)(CHR2)(Pyr)2(DME) W(NC6F5)(CHR1)(Pyr)2(DME) W(N-t-Bu)(CHR1)(Pyr)2(bipy) W(N-t-Bu)(CH-o-MeOC6H4)(Pyr)2(bipy) W(NArMes2)(CHR2)(Pyr)2(py) W(NR)(CHR2)(Me2Pyr)2 W(NAr)(CHR2)(Me2Pyr)3(DME)Li W(NAr)(CHR0 )(Me2Pyr)2 W(NR)(CH2)(Me2Pyr)2 W(NArCl2)(CHR1)(Me2Pyr)2 W(NArMe2)(CHR2)(Me2Pyr)2 W(NArtBu)(CHR2)(Me2Pyr)2 W(NAr3,5Me2)(CHR2)(Me2Pyr)2 W(NC6F5)(CHR1)(Me2Pyr)2 W(N-t-Bu)(CHR1)(Me2Pyr)2 W(NArMes2)(CHR2)(Me2Pyr)2(py) W(NArMes2)(CHR2)(Me2Pyr)2 W(NArCl3)(CHCMe3)(Me2pyr)2 W(NArCl2)(CHR2)(Me2Pyr)2 W(NAr3,5Me2)(CHR2)(MesPyr)2 c. X ¼ OR Mo(NAr)(CHR2)(bipy)(OC6F5)2 Mo(NAr)(CHR2)(OTPP)2 Mo(NR)(CHR2)(ODFT)2 Mo(N-t-Bu)(CHR1)(OR)2(NH2-t-Bu) Mo(N-t-Bu)(CH-o-MeOC6H4)(OR)2(t-BuNH2) Mo(NC6F5)(CHR2)(OR)2 Mo(NArMes)(CHR2)(OTPP)2 Mo(NAr)(CHR2)(OR)2 Mo(NAr)(CHR2)(OCHMe2)2 Mo(NArMe2)(CHR2)(O-t-Bu2)2 Mo(NArMe2)(CHR2)(OR)2(quin) Mo(NArMe2)(CHR2)(ORF9)2 Mo(NAd)(CHR2)(OR)2 [(DME)(RF6O)2(ArN)Mo]CH]2(1,4-C6H4) [(THF)(RF6O)2(ArN)Mo]CH]2(1,4-C6H4) [(t-BuO)2(NAr)Mo]CH]2(1,4-C6H4) [(RO)2(ArN)Mo]CH(C5H4)]2Fe Mo(NFedipp)(CHR2)(O-t-Bu)2 Mo(NAr)(CHR2)(Br2Bitet-OTBS)2 Mo(N-2,4-i-Pr2C6H2-6-CH2CH2CMe2CH)(quin)(ORF6)2 Mo(N-2,4-i-Pr2-6-MeC6H2)(CHR1)(quin)(ORF6)2 Mo(NAr)(CHR2)(ORF6)2(L) Mo(NAr)(CHCMe2R)(OSiPh3)2(phen) Mo(NAr)(CHR2)(Biphen)(bipy) Mo(NAr)(CHR2)(ORF6)2(4,40 -dibromobipy) Mo(NR)(CHR2)(ORF6)2 Mo(NR)(CHR2)(ORF6)2 W(NR)(CHR1)(ODBMP)2 W(NAr3,5-Me2)(CHR2)(ODBMP)2 W(NAr3,5-(CF3)2)(CHR1)(ODBMP)2 W(N-t-Bu)(CHR1)(OR)2 W(NC6F5)(CHR1)[OC(CF3)3]2(DME) W(NC6F5)(CHR1)[OC(C6F5)3]2 W(NC6F5)(CHR1)(ODFT)2

R ¼ Ar, ArCl2, ArMe2, Ar3,5Me2

R ¼ Ar, ArCl2, ArMe2, Ar3,5Me2 R0 ¼ CMe2Ph , t-Bu R ¼ Ar or ArCl2

R ¼ Ar, Ad, ArMe2 OR ¼ OC6F5 or OCH(CF3)2 OR ¼ OC6F5 or ORF9 OR ¼ ORF9, OC(C6F5)3, ODFT OR ¼ OCMe(C6F5)2, OBINAP-TBS OR ¼ OSiMe2(t-Bu), OSiPh3

R ¼ OCH(CF3)2 or OCH(CH3)2 OR ¼ OSi(t-Bu)3, OSi(TMS)3, OSi(O-t-Bu)3

RO ¼ RF6O or t-BuO

tethered imido/alkylidene L ¼ phen, bipy R ¼ Me, Ph

NR ¼ 2,6-Me2-4-((CH2)6Br)C6H2 NR ¼ 2,6-i-Pr-4-((CH2)6Cl)C6H2 R ¼ 2,4,6-Cl3C6H2, 2,4,6-Br3C6H2, 2,6-Cl2-4-CF3C6H2, 2,6-Cl2C6H3, C6F5, ArMe2

OR ¼ OHMT or ODFT

57 57 57 58 45 59 59 60 43 26 38 37 40 45 45 59 59 59 60 46 43 61 38 40 40 62 63 43 21 64 65 55 37 26 32 66 67 68 68 68 69 67 70 70 70 70 27 71 34 34 72 72 72 73 35 35 36 36 36 37 26 26 26 (Continued )

676

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Table 1

(Continued) 2

W(NAr)(CHR )(OR)2 W(NArMe2)(CHR1)(OCMe2CF3)2 W(NArCl)(CHR1)[OC(CF3)2Me]2 W(NAr)(CHR2)(OSiPh3)2(L) M(NR)(CHR0 )(Biphenoxide) (S)-Mo(NR)(CHR2)[Biphen] (R)-Mo(NR)(CHR2)[Biphen] (R)-Mo(NR)(CHR2)[Trip2BINAP](THF) (R)-Mo(NAr)(CHR2)[(t-Bu)2Bitet] (R)-Mo(NArCl2)(CHR1)(Benz2Bitet)(THF) (R)-Mo(NAr)(CHR2)(L)(THF) (S)-Mo(NArCl2)(CHR1)(Biphen)(THF) (R)-Mo(NArCl2)(CHR1)(L)(THF) Mo(NAr)(CHR2)[Me2SiBiphen] Mo(NAr)(CHR2)[TMS2Biphen] Mo(NAr)(CHR2)[Mes2BINAP](THF) (R)-Mo(NAr)(CHR2)[Ph2BINAP](THF) Mo(NAd)(CHR2)(Biphen) Mo(NR)(CHR2)(BinaphAnth)(THF) Mo(NR)(CHR2)(BinaphC6F5)(THF) Mo(NAr)(CHR2)(BinaphCF3)(THF) Mo(NR)(CHR2)(BiphenCF3)(THF) W(N-t-Bu)(CHR1)(Biphenoxide)(py) W(NR)(CHR2)(Biphen)(THF)x W(NArCl2)(CHR1)(Biphen)(Tetrahydropyran) [W(NArCl2)(Biphen)(m-CH2)]2 M(NR)(CHR0 )(X)(Y); Y ¼ OR a. X ¼ pyrrolide (MAP) Mo(NAr)(CHR2)(X)(ORF6) Mo(NAr)(CHR2)(X)(ORF6)(PMe3) Mo(NAr)(CHR2)(Me2Pyr)(OR) Mo(NAr)(CHR2)(Me2Pyr)(OR) Mo(NAr)(CHR2)(Me2Pyr)(OSiPh3) (R)-Mo(NAr)(CHR2)(Me2Pyr)(OBr2Bitet) (S)-Mo(NAr)(CHR2)(Me2Pyr)(OBr2Bitet) (R)-Mo(NAr)(CHR2)(Me2Pyr)(OBr2Bitet)(PMe3) Mo(NAr)(CH2)(Pyr)(OHIPT) Mo(NAr)(CHR2)(Pyr)(OTPP) Mo(NAr)(CHR2)(Pyr)[OSi(t-Bu)3] Mo(NAr)(CHR2)(Me2Pyr)(OCPh3) Mo(NR)(CHR2)(Me2Pyr)(OHMT) Mo(NAr)(CHR1)(Me2Pyr)(OHMT) Mo(NAr)(CHR2)(Pyr)(OR) Mo(NAr)(CHR2)(Me2Pyr)(ODPPPh) Mo(NAr)(CH2)(Me2Pyr)(OTPP) Mo(N-t-Bu)(CHR1)(Pyr)(OHMT) Mo(NAd)(CHR2)(Me2Pyr)(ODPPPh) Mo(NAd)(CHR2)(Me2Pyr)(OTPP) Mo(NAd)(CHR2)(Pyr)(OHIPT) Mo(NAd)(CHR1)(Pyr)(OHIPT) Mo(NAd)(CHR2)(Pyr)(OR) Mo(NAd)(CHR2)(MesPyr)(OR) Mo(NAd)(CHR2)(CNPyr)(OHIPT) Mo(NAd)(CHR2)(Pyr)(OHMT) Mo(NAd)(CHR2)(Pyr)(OR) Mo(NAd)(CHR2)(Pyr)(OHMT) Mo(NAd)(CH2)(OHIPT)(Pyr) Mo(NArMe2)(CHR2)(Pyr)(OHIPT) Mo(NR)(CHR2)(Pyr)(OHMT) Mo(NArX)(CHR2)(Me2Pyr)(OHMT) Mo(NArX)(CHR1)(Me2Pyr)(OHMT)

OR ¼ OCMe(C6F5)2, OAr, OBINAP-TBS, OCMe2(CF3), OC(CF3)3, OSiPh3

L ¼ phen, bipy NR ¼ NPhCy or NPhAd NR ¼ 2,6-R0 2-4-((CH2)6Br)C6H2, R0 ¼ Me, i-Pr NR ¼ NPhCy or NPhAd

L ¼ Mes2Bitet or Benz2Bitet L ¼ Trip2BINAP or Mes2Bitet rac and S rac and S rac and R

R ¼ Ar, Ad, ArMe2 R ¼ Ar, Ad, ArMe2, ArCF3 R ¼ Ar, Ad, ArMe2 Biphenoxide ¼ Biphen or BiphenCF3 R ¼ Ar or ArMe2, rac and (S), x ¼ 0 or 1

X ¼ Me4Pyr, i-PR2Pyr, Ph2Pyr X ¼ Me2Pyr, Me4Pyr, i-PR2Pyr, Ph2Pyr OR ¼ OTPP , OArPh2 , ORF6 OR ¼ O-1-PhC6H10, OSi(O-t-Bu)3, OSiPh3

R ¼ Ar or ArMe2 OR ¼ ODPPPh or ODPPiPr

OR ¼ O-(3,5-R0 2C6H3)2C6H3 (R0 ¼ Me or t-Bu), OCPh3, OSiTMS3 OR ¼ OTPP , OBr2Bitet , OHIPT

OR ¼ ODPPPh or ODPPiPr

R ¼ Ar, Ad, ArMe2, ArCl, AriPr , ArtBu, ArMes X ¼ Cl, i-Pr, Mes X ¼ CF3, t-Bu, TRIP

66 42,74 44 73 31 35 31 75 28 28 28 28 76 76 77 77 78 54 54 54 54 37 41 44 44

50 50 79 66 80 81 82 83 46 84 67 85 52 52 86 86 87 55 86 88 88 60 85 56 56 89 86 90 88 52 25 32 32

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Table 1

677

(Continued)

Mo(NArt-Bu)(CHR2)(MesPyr)(OHMT) Mo(NArMes)(CHR2)(Me2Pyr)(OHIPT) Mo(NArTrip)(CHR1)(Me2Pyr)(OTPP) Mo(NArMes2)(CHR2)(Me2pyr)(O-t-Bu) Mo(NArMes2)(CHR2)(Me2pyr)(OR) Mo(NArMes2)(CHR2)(OR)(Pyr)(py) Mo(NArMe2)(CHR2)(Me2Pyr)(ODFT) Mo(NC6F5)(CHR2)(Me2Pyr)(OR) Mo(NC6F5)(CHR2)(Me2Pyr)(ODFT)(MeCN) Mo(NC6F5)(CHR2)(3,4-(CF3)2-Pyrrolide)(TPPO)(PPhMe2) Mo(NArMes2)(CHR2)(Me2Pyr)(OR) Mo(NArMes2)(CHR2)(Pyr)(OR)(py) Mo(NArMes2)(CHR1)(Pyr)(OC6F5)(MeCN) Mo(NAr)(CHR2)(Me2Pyr)(OR) Mo(NAr)(CHR2)(Me2Pyr)(OBMes2) Mo(NR)(CHR2)(Pyr)(OHMT)(bipy) W(NAr)(CHR2)(Pyr)(OMes2Bitet) W(NAr)(CHR)(Me2Pyr)(OBr2Bitet) W(NAr)(CHR)(Me2Pyr)(OTPP) W(NAr)(CH2)(Me2Pyr)(OTPP) W(NAr)(CH2)(Me2Pyr)(OR)(PMe3) W(NAr)(CHR2)(Me2Pyr)(OR) W(NAr)(CHR2)(Pyr)(OHIPTNMe2) W(NAr)(CH2)(Me2Pyr)(OTPP)(L) W(NAr)(CHR1)(Me2Pyr)(OHMT) W(NAr)(CHR1)(Pyr)(OR) W(NAr)(CHR2)(Me2Pyr)[OCPh(CF3)2](L) W(N-t-Bu)(CHR1)(Me2Pyr)(OHMT) W(N-t-Bu)(CHR1)(Pyr)(OHIPT) W(N-t-Bu)(CHR1)(Pyr)(OHMT) W(N-t-Bu)(CHR1)(Pyr)(ODFT)(py) W(N-t-Bu)(CH-o-MeOC6H4)(Pyr)(OHMT) W(NArCl)(CHR1)(Pyr)(OHIPT) W(NArtBu)(CHR2)(Me2Pyr)(OTPP) W(NAr3,5-Me2)(CHR2)(Me2Pyr)(OHIPT) W(NAr3,5-Me2)(CHR2)(Me2Pyr)(OTPP) W(NAr3,5Me2)(CH2)(Me2Pyr)(OHIPT) W(NR)(CHR2)(Pyr)(OHMT) W(NArMes2)(CHR2)(Me2pyr)(OR) W(NArMes2)(CHR2)(Me2Pyr)(OR) W(NC6F5)(CHR1)(Me2Pyr)(OR) W(NArMes2)(CH2)(Me2Pyr)(OR) W(N-2,4,6-Cl3C6H2)(CHR1)(Me2pyr)(OR) b. M(NR)(CHR0 )(X)(Y) complexes (X ¼ halide or triflate) Mo(NAr)(CHR2)(OHMT)(Cl)(L) Mo(NAr)(CHR2)(TripON)(OTf ) Mo(N-t-Bu)(CHR1)(OR)(Cl)(PPh2Me) Mo(N-t-Bu)(CHR2)(OTPP)(Cl)(PPh2Me) Mo(N-t-Bu)(CHR2)(L)Cl Mo(N-t-Bu)(CHR1)(OTTBT)(Cl)(L) Mo(N-t-Bu)(CHR1)(OHIPT)(3-Brpy)(Cl) Mo(NAd)(CHR2)(OR)(Cl)(PPh2Me) Mo(NAd)(CHR2)(OHIPT)(Cl)(PPh2Me) Mo(NAd)(CHR2)[O-2,6-(2,4,6-t-Bu3C6H2)2C6H3](Cl)(PMe3) Mo(NAd)(CHR2)(OHMT)(Cl)(py) [Mo(NAd)(CHR2)(Cl)(PMe3)]2(m-O) Mo(NAd)(CHR2)(Br)(OHMT)(Py) Mo(NC6F5)(CHR2)(ONO)(OTf )(DME)(Li) Mo(NC6F5)(CHR2)(L)Cl Mo(NC6F5)(CHR2)(OR)Cl(PPhMe2)

R ¼ CMe(CF3)2, OSiPh3, OArMe2 R ¼ OCMe(CF3)2, OCHMe2, OCH(CF3)2, OArMe2, OSi(i-Pr)3, OSiPh3, OSi(SiMe3)3 OR ¼ OHMT, ODFT

OR ¼ ORF6, OSiPh3, O-2,6-Me2C6H3 OR ¼ ORF6, OCHMe2, OCH(CF3)2, OArMe2, OSi(i-Pr)3, OSiPh3, OSi(SiMe3)3 OR ¼ O-t-Bu, OCHMe2, OAr , OCH(CF3)2, OCMe(CF3)2), ORCamph R ¼ Ar , Ad, ArMe2, ArCl, AriPr, ArtBu, ArMes R ¼ H, CMe2Ph R ¼ H, CMe2Ph OR ¼ OBr2Bitet or OTPP OR ¼ OAr, ORF6, OSiPh3 L ¼ THF , PMe3 OR ¼ ODPPPh, ODPPi-pr L ¼ phen, bipy , 5,50 -dimethylbipy

R ¼ Ar, ArMe2 R ¼ CMe(CF3)2, OSiPh3, OArMe2 OR ¼ O-t-Bu , ORF6, OSiPh3, ODMP OR ¼ OHMT, OHIPT OR ¼ O-t-Bu, OSiPh3, OArMe2 OR ¼ OHMT or ODFT L ¼ t-BuCN , 3-Brpy, PMe2Ph OR ¼ OHIPT or OHMT L ¼ MesON or TRIPON L ¼ MeCN or 3-Brpy OR ¼ OTPP or OHMT

L ¼ MesON or TRIPON OR ¼ OHIPT or OHMT

58 32 32 24 40 40 65 65 65 91 40 40 23 92 93 25 60 94 94 46 94 66 95 94 61 86 73 38 38 38 37 37 60 46 43 43 43 52 40 40 61 96 62 21 97 22 22 97 21 90 22 22 22 21 22 90 39 97 21 (Continued )

678

Table 1

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

(Continued)

Mo(NC6F5)(CHR2)(ODFT)(Cl)(3-Brpy) Mo(NC6F5)(CHR2)(OR)(Cl)(PPhMe2) Mo(NArMe2)(CHR2)(L)Cl W(N-t-Bu)(CHR1)(OHMT)(Cl)(py) W(N-t-Bu)(CHR2)(OHMT)(Cl)(L) W(NAr)(CHR2)(TripON)(OTf ) c. Other M(NR)(CHR0 )(X)(Y) complexes Mo(NAd)(CHR2)(OHIPT)(OR) Mo(NAd)(CHR2)(OHIPT)(OTf )(PMe3) Mo(NR)(CHR2)(OHMT)(ORF6) Mo(NR)(CHR2)[N(H)HMT](ORF6) Mo(NAd)(CHR2)(HMT)(ORF6) Mo(NArMes2)(CHR2)Cl(OR)(py) Mo(NAr)(CHR2)(Pyr)(OTf )(DME) Mo(NAr)(CHR2)[OSi(t-Bu)3](OTf ) Mo(NAr)(CHR2)(TMHD)(OTf )(THF) Mo(NAr)(CHR2)(ArMe-nacnac)(OTf ) Mo(NAr)(CHR2)(ArCl-nacnac)(OTf ) Mo(NR)(CHR2)(ArCl-nacnac)(OTf ) Mo(NArCl2)(CHR1)(ArCl-nacnac)(OTf ) Mo(NAr)(CHR2)(Ar0 -nacnac)(OTf ) Mo(NR)(CHR1)(Ar0 -nacnac)(OTf ) Mo(NAr)(CHR2)(Ar00 -nacnac)(OTf ) Mo(NAr)(CHR2)(ArF-nacnac)(OTf ) Mo(NAr)(CHR2)(O2CCF3)2(quin) [Mo(NAr)(CHR2)(O2CCF3)(Et2O)]2(m-O2CCF3)2 M]CHX complexes Mo(NAr)(CH[6])[OCMe(CF3)3]2 Mo(NAr)(CH[6])(O-t-Bu)2[LiOCMe(CF3)2]2 Mo(NAr)(trans-CHCH]CHMe)(O-t-Bu)2(quin) Mo(NAr)(CH[5]CH]CMe2)(OCMe(CF3)2)2 Mo(NAr)(CH[5]CH]CMe2)(O-t-Bu)2 Mo(CH[5](CO2-i-Pr)2)(NAr)(ORF6)2 [[Mo(NAr)(ORF6)2]CH]2[5](CO2Et)2 [[Mo(NArMe2)(ORF6)2]CH]2[5](CO2-i-Pr)2 [[Mo(NAr)(ORF6)2]CH[5](CO2Et)2CH]2 [Mo(NArt-Bu)(ORF6)2]2(CH[6](CO2Et)2C) Mo(NAr)(CHCH]CRMe)(Me2Pyr)(OHMT) Mo(NAr)(CHX)(Me2Pyr)(OTPP) [Mo(N-t-Bu)(CHPPh2Me)(OHMT)(Cl)(Bipy)]Cl Mo(N-t-Bu)(CHX)(OHIPT)(Cl)(Bipy) Mo(N-t-Bu)(CHX)(OHIPT)(Cl)(PPh2Me) Mo(NAr)(CHCO2-t-Bu)(Me2Pyr)(OTPP) Mo(CHSiMe2Ph)(NAr)(ORF3)2 [Mo(NAr)(ORF3)2(CH)]2(SiR2) [Mo(NAr)(ORF3)2(CH)]2(SiPhVinyl) Mo(CHE)(NAr)(ORF3)2 Mo(NAr)(CHSiPh3)(ORF3)2 Mo(NAr)(CHGeR3)(ORF3)2 W(NAr)(CHE)(ORF3)2 W(NAr)(CHE)(ORF3)2 [(t-BuO)2Cl2W]CH]2SiPh2 W(NAr)(CHGeMe3)(ORF3)2 [(RF3O)2(ArN)W]CH]2SiR2 [(RF3O)2(ArN)W]CHSiMe2]2 Oxo alkylidenes Mo(O)(CHAro)Cl2(PMe3) Mo(O)(CHAro)(ORF6)2(L) Mo(O)(CHAro)(OHIPT)(PMe3)Cl Mo(O)(CHArx)(OHMT)2 Mo(O)(CHArp)(ORF9)2(L) Mo(O)(CHArp)(ORF9)2(THF)2

OR ¼ OTPP or OTTBT L ¼ MesON or TRIPON L ¼ MeCN, py, 3-Brpy, t-BuCN

OR ¼ OTf, O-t-Bu R ¼ Ar, ArMe2, AriPr, Ad R ¼ ArMe2, AriPr OR ¼ ORF6, O-t-Bu, OArMe2, OHMT

NR ¼ NAr, NArMe2, NAd

NR ¼ ArCl2, ArtBu

R ¼ H or Me X ¼ B(pin) , SiMe3 , Carbazole, Pyrrolidinone , PPh2 , OPr , or SPh X ¼ Cl, CF3, CN X ¼ Cl, CF3, CN

R ¼ Me or Ph (E ¼ SiEt3, GeEt3 ) R ¼ Me or Ph (E ¼ SiMe3, SiMe2Ph ) (E ¼ SiEt3, GeEt3 )

(R ¼ Me, Ph)

Aro ¼ o-OMeC6H4 L ¼ PPhMe2 or PMe3 Arx ¼ Aro or p-OMeC6H4 L ¼ PMePh2, PMe2Ph, PEt3, P(i-Pr3)

21 91 97 37 21 97 56 56 56 56 56 24 60 67 98 98 98 99 99 99 99 99 100 100 101 101 101 101 101 102 102 102 102 102 52 87 103 103 103 104 105 105 106 107 108 109 110 107 111 112 113 113 114 114 114 115 115 116

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Table 1

679

(Continued)

Mo(O)(CHR)(ORF9)2(TMEDA) Mo(O)(CHR1)(ORF9)2(bipy) Mo(O)(CHArp)(OR)2 Mo(O)(CHR1)Cl2(bipy) Mo(O)(CHR1)Cl(OR)(3-Brpy) W(O)(CHR)Cl2(PMe2Ph)2 W(O)(CHR1)(Me2Pyr)(OHIPT) W(O)(CHR1)(Me2Pyr)(OHMT)(PMe2Ph) W(O)(CHR1)(Me2Pyr)(OHMT)[B(C6F5)3] W(O)(CHR1)(Cl)(OR)(PMe2Ph) W(O)(CHR1)(Ph2Pyr)(OHMT) W(O)(CHR1)[N(C6F5)2](OHMT)(PMe2Ph) W(O)(CH2)(OHMT)2 W(O)(CHR1)(OHMT)2 W(O)(CHR1)(dAdPO)2 W(O)(CHR2)(Cl)2(L)2 W(O)(CHR2)(OR)2 W(O)(CHR2)(OR)2(PPh2Me) W(O)(CHR2)(ODBMP)2(MeCN) W(O)(CHR2)(ORF9)2(PPh2Me) W(O)(CHR2)(OR)(Cl)(PPh2Me) W(O)(CHR2)(Me2Pyr)(OHMT)(L) W(O)(CHR2)(Me2Pyr)(OR) W(O)(CHR2)(Me2Pyr)(OR)(PPh2Me) W(O)(CHR)(Me2Pyr)2(PMe2Ph) W(O)(CHR2)(Me2Pyr)[(R)-OBr2Bitet](PMe2Ph) W(O)(CHR2)(ONO)(L) W(O)(CHR2)(Me2Pyr)(L) rac-W(O)(CHR2)(L)(PPhMe2) (S)-W(O)(CHR2)(Biphen)(PPhMe2) (R)-W(O)(CHR2)[(t-Bu)2Bitet](PPhMe2) (R,S)-[W(m-O)(CHR2)(L)]2 W(O)[C(t-Bu)(Si-t-BuPh2](CH2-t-Bu)2 Alkylidene/Alkyl and Alkylidene/Alkylidyne Mo(NAr)(CHR1)R2 Mo(NAr)(CHR2)(CH2-t-Bu)2 Mo(NCPh3)(CHR1)(CH2-t-Bu)2 Mo(NAr)(CHR1)(CH2-t-Bu)(OR) Mo(NAr)(CHR1)(CH2-t-Bu)(OC6F5)(PMe3) Mo(NAr)(CHR1)(CH2-t-Bu)(OArMe2) Mo(NCPh3)(CHR1)(CH2-t-Bu)(OR) Mo(NAr)(CHR1)(CH2-t-Bu)[OSi(O-t-Bu)3]) Mo(C-t-Bu)(CHR1)(X)(PMe2Ph)2 W(NAr)(CHR1)(CH2-t-Bu)2 W(NArMe2)(CHR1)(CH2-t-Bu)2 W(NAr)(CHR1)(CH2-t-Bu)(OR) W(NAr)(CHR1)(CH2-t-Bu)(OR) W(NArMe2)(CHR1)(CH2-t-Bu)(OC6F5) W(NAr)(CHR1)(CH2-t-Bu)(Cl) W(NAr)(CHR1)(CH2-t-Bu)[OSi(O-t-Bu)3] [W(NAr)(CHR1)(CH2-t-Bu)(OC6F5)]2 W(NAr)(CHR1)(CH2-t-Bu)(OC6F5)(PMe3) W(CHSiMe3)2(CH2SiMe3)2(PMe3)a W(CHSiMe3)2(CH2SiMe3)2(PMe2Ph)b Thiolates Mo(NAr)(CHR2)(STPP)2 Mo(NAr)(CHR2)(Me2Pyr)(STPP) Mo(NAr)(CHR2)(Pyr)(SHMT) [Mo(NAr)(CHR2)(Cl2S2)]2

R ¼ Arp, t-Bu, Mes OR ¼ OHMT or OTPP OR ¼ OHMT or OHIPT R ¼ t-Bu or CMe2Ph

OR ¼ OHMT or OHIPT

L ¼ PPh2Me or PPhMe2 OR ¼ OHMT, OdAdP OR ¼ ODFT, OTPP

OR ¼ OHMT, ODFT, OTPP, OdAdP, ODBMP L ¼ PPh2Me, MeCN OR ¼ OHMT, OdAdP OR ¼ ODFT, OTPP, ODBMP R ¼ t-Bu, CMe2Ph L ¼ PPh2Me, PPhMe2, PMe3 L ¼ TripON or MesON L ¼ Biphen, (t-Bu)2Bitet, (Benzyl)2Bitet, Trip2BINAP

L ¼ Biphen or (t-Bu)2Bitet

R ¼ CH2-t-Bu or CH2CMe2Ph

OR ¼ OCH(CF3)2, OAd, O-t-Bu, OAr, OC6F5, OC(CF3)3

OR ¼ O-3,5-Trip2C6H3 , OArMe2, OAr X ¼ Cl, Me2Pyr, OSiPh3

OR ¼ OC6F5, OCMe3, ORF3, ORF6, OCH(CF3)2, OAr OR ¼ OC(CF3)3, OAd, OC6F5

116 116 116 116 116 117 118 118 118 118 117 117 117 117 119 120 120 120 120 120 120 120 120 120 64 121 39 97 122 122 122 122 123 124 125 126 124,125 125 125 126 125,127 128 42 74 125 42 42 125 74 74 74 129 130 64 64 64 131 (Continued )

680

Table 1

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

(Continued) 2

[Mo(NAr)(CHR )(Cl4S2)]2 Mo(NAr)(CHR2)(Cl4S2)(py) Mo(NAr)(CHR2)(Br4S2)(py) Mo(NAr)(CHR2)(Bu4S2)(PMe3) Mo(NAd)(CHR2)(Cl2S2) Mo(NAd)(CHR2)(Cl2S2)(PPh2Me) Mo(NR)(CHR2)(t-Bu4OS)(PMe3) W(NAr)(CHR2)(Pyr)(SHMT) W(O)(CHR1)(Me2Pyr)(SHMT)(PMe2Ph) W(O)(CHR1)(SR)2(PMe2Ph) W(NAr)(CHR2)(t-Bu4OS)(PMe3) Disubstituted Alkylidenes Mo(NAr)(CMePh)(OMes)2 [Mo(NAr)(CMePh)(OC6F5)2]2 Mo(NAr)(CMePh)(OC6F5)2(THF)x Mo(NAr)(CMePh)(OC6F5)2(L) Mo(NAr)(CMePh)(Cl)2(bipy) Mo(NAr)(CMePh)(Cl)(OHMT) Mo(NAr)(CMePh)(Cl)(OHMT)(MeCN) Mo(NAr)(CMePh)(OHMT)(Pyr) Mo(NAr)(Adene)(OTf )2(DME) Mo(NAr)(Adene)(OR)2 Mo(NAr)(Adene)(OC6F5)2(MeCN) Mo(NAr)(Adene)(Cl)2(bipy) Mo(NAr)(Adene)(Cl)(OHMT) Mo(NAr)(Adene)(Cl)(OHMT)(t-BuCN) Mo(NAr)(Adene)(OHMT)(Pyr) Mo(N-t-Bu)(Adene)(Cl)2(bipy) Mo(N-t-Bu)(Adene)(Cl)(OHMT)(L) Mo(N-t-Bu)(Adene)(OHMT)(Pyr)(THF) Mo(NArF)(Adene)(Me2Pyr)2 Mo(NArF)(Adene)Cl2(bipy) Mo(NArF)(Adene)(OTf )2(DME) Mo(NArF)(Adene)(ORF9)2 Mo(NArF)(Adene)(ORF9)2(L) Mo(NArF)(Adene)(OTf )(OHMT)(L) Mo(NArF)(Adene)(OHMT)(Pyr)(Piv) M(O)(Adene)(ORF9)2(PPh2Me) M(O)(Adene)(ORF9)2 W(O)(Adene)(Me2Pyr)2 M(Adene)(ORF9)2Cl2 W(O)(Adene)(Me2Pyr)(OHMT) Alkylidene Cations [Mo(NAr)(CHR2)(NC4H4)(THF)3][BArF4] [Mo(NAr)(CHR2)(Me2NC4H2)(THF)2][BArF4] [Mo(NAr)(CHR2)(Me2Pyr)(2,4-Lutidine)][BArF4] [Mo(NAr)(CHR2)[OC(CF3)2Me](THF)x][BArF4] [Mo(NAr)(CHR2)(OAr)(THF)x][BArF4] [Mo(NAr)(CHR2)(OAd)(THF)2][BArF4] [Mo(NAr)(CHR2)(ODFT)(THF)2][BArF4] [Mo(NAr)(CH2CMe2Ph)(rac-BIPHEN)][BArF4] [Mo(NAr)(CHR2)(TMHD)(THF)][BArF4] [Mo(NAr)(CHR2)(ArCl-nacnac)][BArF4] [Mo(NR)(CHR1)(ArCl-nacnac)][BArF4] [Mo(NR)(CHR2)(ArCl-nacnac)][BArF4] [Mo(NAr)(CHR2)(Ar0 -nacnac)(THF)][BArF4] [Mo(NArCl2)(CHR1)(Ar0 -nacnac)(THF)][BArF4] [Mo(NAr)(CHR2)(ArF-nacnac)(THF)][BArF4] [Mo(NAr)(CHR2)(Me2Pyr)(1,3,5-trimethylpyrazole)][BArF4] [Mo(NAr)(CHR2)(ODFT)(THF)][BArF4] [W(NArMes2)(CHR2)Cl(bipy)][Zn2Cl6]0.5  [W(NR)(CHCMe3)(Z5-Me2Pyr)(NC4H3-2,5-Me2)][BArF4] [(OSSO)W(C]CHMe)(X)][B(C6F5)4] [(OSSO)W(CHEt)(O-t-Bu)][B(C6F5)4]

R ¼ Ar or Ad

SR ¼ SHMT or STPP

x¼1 or 2 L ¼ MeCN or bipy

Ad ¼ 2-Adamantylidene OR ¼ OMes, OC6F5

L ¼ THF or t-BuCN

L¼THF, Piv L ¼ THF, Piv M ¼ Mo or W M ¼ Mo or W M ¼ Mo or W

x¼2 or 3 x¼1 or 2

NR ¼ NAr, NArCl2 NR ¼ NAd, NArMe2 NR ¼ NArCl2, NArtBu

R ¼ ArCl, Ar X ¼ O-t-Bu or CH2SiMe3

131 131 131 131 131 131 132 64 64 64 132 133 133 133 133 133 133 133 133 134 134 134 134 134 134 134 134 134 134 135 135 135 135 135 135 135 136 136 134 136 136 53 53 53 53 53 53 30 53 98 98 99 99 99 99 99 30 30 40 59 137 137

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Table 1

681

(Continued)

Complexes that contain cationic ligands [Mo(NR)(CHR2)(O-2,6-Ph2-4-(2,4,6-Ph3)NC5H2)2][OTf]2 Mo(NArCl2)(CHR1)(OTf )2(O-2,6-Ph2-4-(2,4,6-Ph3)NC5H2) [Mo(NAr)(CHR1)(Me2Pyr)(O-2,6-Ph2-4-(2,4,6-Ph3)NC5H2)][X] [Mo(NAr)(CHR2)(Me2Pyr)(O-2,6-Ph2-4-(2,4,6-Ph3)NC5H2)][X] [Mo(NAr)(CHR2)(Me2Pyr)(O-2,6-Ph2-4-(2,4,6-Me3)NC5H2)][X] [Mo(NR)(CHR2)(Me2Pyr)(O-2,6-t-Bu2-4-(PPh3))][BArF4] [Mo(NR)(CHR2)(Pyr)(O-2,6-Mes2-4-(PPh3))][BArF4] Olefin complexes Mo(NAr)(Alkene)(OSiPh3)2 Mo(NAr)(C2H4)(OSiPh3)2(Et2O) Mo(NAr)(C2H4)(Pyrrolide)2 Mo(NAr)(C2H4)(Me2Pyr)(OR) Mo(NR)(C2H4)(OTf )2(DME) Mo(NAr)(C2H4)[OCH(CF3)2)2](Et2O) Mo(NAr)(CH2CH2)(TripON)(OTf )(ether) Mo(NAr)(C2H4)(t-Bu4OS) Mo(NC6F5)(C2H4)(ODFT)2 Mo(NC6F5)(C2H4)(DCMNBD)(ODFT)2 Mo(NC6F5)(C2H4)(ONO) Mo(NArCl2)(C2H4)(Biphen)(Et2O) [Mo(NAr)(C2H4)(ORF6)(THF)3][BArF4] [Mo(NAr)(C2H4)[OC(CF3)2Me](THF)3][BArF4] [Mo(NAr)(C2H4)(Ar0 -nacnac)(THF)][BArF4] Mo(NAr)(CH2]CHGeEt3)(ORF3)2 W(NArCl2)(C2H4)(Biphen)(THF) W(O)(trans-4,4-dimethylpent-2-ene)(dAdPO)2 W(NAr)(CH2]CHGeMe3)(ORF3)2 W(NPh)(olefin)[1,2-(TMSN)2C6H4](PMe3)2 W(O)(ORF6)2(olefin)py2 Metallacyclobutanes Mo(NAr)(C3H6)(Me2Pyr)(OBr2Bitet) Mo(NAr)(C3H6)(Pyr)(OHIPT) Mo(NAr)(C3H6)(OSiPh3)2 Mo(NC6F5)(C3H6)(ONO) W(NAr)(C3H6)(Pyr)(OHIPT) W(NAr)(C3H6)(Me2Pyr)(OBr2Bitet) W(NAr)(C3H6)(Me2Pyr)(OTPP) W(NArMe2)(C3H6)(Pyr)(OHIPT) W(NArtBu)(C3H6)(Me2Pyr)(OTPP) W(NAr3,5Me2)(C3H6)(Pyr)(OHIPT) W(NAr3,5Me2)(C3H6)(MesPyr)(OTPP) W(NC6F5)(C3H6)[OC(CF3)3]2 W(NAr)(C3H6)[OC(CF3)3]2 W(NAr)(C3H6)(Pyr)(OHIPTNMe2) W(NAr)[CH2CH(CMe2Ph)CH2](Pyr)(OHIPTNMe2) W(O)(C3H6)(OHMT)[OSi(t-Bu)3] W(O)(C3H6)(OHMT)2 W(NAr)(C3H6)(Pyr)(ODPPPh) W(NArMes2)(C3H6)(Me2Pyr)(OR) W(N-t-Bu)(C3H6)(Pyr)(ODFT) W(N-t-Bu)(C3H6)(ODFT)2 W(O)(C3H6)(ONO) W(NArCl2)(C3H6)(Biphen) W(NArCl2)(C3H6)[OC(CF3)2Me]2 W(NAr)[CH2C(Me)(R)CH2)[(S)-Biphen] W(NAr)[CH2CH(CMe2Ph)CH2)[(S)-Biphen] W(NAr)[trans-CH(GeMe3)CH(GeMe3)CH2](ORF3)2 Metallacyclopentanes Mo(NAr)(C4H8)(OSiPh3)2 W(NArCl)(C4H8)(Biphen) W(O)(C4H8)(OHMT)2

R ¼ Ar, ArCF3, ArMe2, ArCl2, Ad [X]− ¼ [BArF4]−  or [Al(OC(CF3)3)4]− [X]− ¼ [BArF4]−, [Al(OC(CF3)3)4]−, [B(C6F5)4]−, [OTf]− [X]− ¼ [OTf]− or [BF4]− R ¼ Ar or Ad R ¼ Ar or t-Bu Alkene ¼ C2H4, styrene , trans-3-hexene Pyrrolide ¼ Me2Pyr, MesPyr OR ¼ OSiPh3, OAr, ORF6 NR ¼ NAr, NAd

olefin ¼ ethylene, propylene, styrene olefin ¼ ethylene, propylene

OR ¼ ORF6, OSiPh3 , OArMe2

(See Eq. 17 for the identity of R)

138 138 138 138 138 138 138 139 139 139 139 139 139 97 132 65 65 39 140 53 53 99 107 44 119 112 141 142 80 46 139 39 88 94 94 60 46 43 43 26 66 95 95 117 117 86 96 37 37 39 44 44 41 41 112 139 44 119 (Continued )

682

Table 1

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

(Continued)

W(ArMe2)(C4H8)(Me2Pyr)(OHIPT) Oxo- and Imido-free alkylidenes W(CHR1)[OC6H3(Ph)C6H4](O-2,6-Ph2C6H3)Cl W(CHR1)(OAr)4 W(CH2)(OAr)4 W(CHEPh3)(O-t-Bu)2Cl2 Pincers [t-BuOCHO]Mo(NAr)(CHR2) [Ph3PMe][t-BuOCO]Mo(NAr)(CHR2) [(t-BuOCO)(NAr)Mo(CHR2)][PPh3Me] Mo(NC6F5)(CHR2)(ONO) [CBZ-ONO](t-BuO)W]CHR1 [pyr-ONO](t-BuO)W]CHR1 [CF3-ONO](RO)W]CHEt [CF3-ONO](t-BuO)W]C(Me)(Et) [t-Bu-OCO](OAr)W]CHR1 [[t-Bu-OCO]W]CHR1]2[m-t-Bu-OCHO] [ONCH2O](t-BuO)W]CHR1 [t-BuOCO]W(CHR1)(OAr) [[t-BuOCO]W(CHR1)]2(t-BuOCHO) [CF3ONO]W(CHEt)(O-t-Bu) [CF3ONO]W[C(Me)(Et)](O-t-Bu) [ONO]W(CHR1)(O-t-Bu) [CBZ-ONO]W(CHR1)(O-t-Bu)

43

E ¼ Si or Ge

R ¼ t-Bu or Me3Si

143 144 145 146 147 147 147 39 148 149 150 150 151 151 152 151 151 150 150 149 148

Abbreviations: CHR1 ¼ CH-t-Bu; CHR2 ¼ CHCMe2Ph; Bipy ¼ 2,20 -bipyridyl, DME ¼ 1,2-dimethoxyethane, Mes ¼ 2,4,6-Me3C6H2, Trip ¼ 2,4,6-(i-Pr)3C6H2, TMEDA ¼ dMe2NCH2CH2NMe2, Pyr ¼ pyrrolide, MesPyr ¼ 2-mesitylpyrrolide, Me2Pyr ¼ 2,5-dimethylpyrrolide, CNPyr ¼ 2-cyanopyrrolide, Me4Pyr ¼ 2,3,4,5-tetramethylpyrrolide, R2Pyr ¼ 2,5-R2pyrrolide, TBS ¼ dimethyl-t-butylsilyl, Adene ¼ 2-Adamantylidene, HIPT ¼ 2,6-(2,4,6-i-Pr3C6H2)2C6H3, HMT ¼ 2,6-Mes2C6H3, 3-Brpy ¼ 3-bromopyridine, NAd ¼ N-1-Adamantyl, NAr ¼ N-2,6-i-Pr2C6H3, NArR2 ¼ N-2,6-R2C6H3, NArR ¼ N-2-RC6H4; NAr3,5Me2 ¼ N-3,5-Me2C6H3; OHIPT ¼ O-2,6-(2,4,6-i-Pr3C6H2)2C6H2, OHMT ¼ O-2,6-Mes2C6H3, ODFT ¼ O-2,6-(C6F5)2C6H3; OArPh2 ¼ 2,6-Ph2C6H3, ODIPP ¼ O-2,6-i-Pr2C6H3, ODBMP ¼ O2,6-Benzyl2-4-MeC6H2, ORF3 ¼ OCMe2(CF3), ORF6 ¼ OCMe(CF3)2, ORF9 ¼ OC(CF3)3, OTPP ¼ O-2,3,5,6-Ph4C6H, ODPPR ¼ 2,6-(2,5-R2pyrrolyl)2Phenoxide (R ¼ i-Pr or Ph); OTTBT ¼ (O-2,6-(3,5-(t-Bu)2C6H3)2C6H3, ODMP ¼ O-2,6-Me2C6H3, MesON ¼ 20 ,40 ,60 -trimethyl-5-methyl-3-(pyridin-2-yl)-[1,10 -biphenyl]-2-olate, TRIPON ¼ 20 ,40 ,60 -triisopropyl-5-methyl-3-(pyridin-2-yl)-[1,10 -biphenyl]-2-olate, ONO ¼ 2,6-[3,5-(t-Bu)2-2-olate]2pyridine, [BArF4]− ¼ B[3,5-(CF3)2C6H3]−4 . For Biphen, Bitet, and BINAP variations see Fig. 4. a In equilibrium with W(CSiMe3)2(CH2SiMe3)3(PMe3). b In equilibrium with W(CSiMe3)(CH2SiMe3)3(PMe2Ph).

5.11.2

Alkylidene complexes

5.11.2.1

Early tantalum chemistry

The vast majority of d0 alkylidene complexes are terminal and are prepared through a hydrogen abstractions (essentially deprotonations) in a complex that contains at least two alkyls, with each having no beta proton (e.g., Eq. 1). If an alkylidene is to be isolable the alkylidene complex must be relatively stable toward decomposition reactions, which in the absence of an olefin are largely bimolecular coupling to give an olefin or further proton abstraction (either intramolecularly or intermolecularly) to yield an alkylidyne. Neopentyl (CH2-t-Bu) and neophyl (CH2CMe2Ph) are almost exclusively the alkyl ligands of choice for preparing M] CHR complexes. Other possibilities are CH2SiMe3, CH2Ph, and CH3, although the ease of forming the alkylidene through a hydrogen abstraction decreases in that order and the instability of the resulting alkylidene (if formed) toward bimolecular coupling increases dramatically in the order M]CHSiMe3 < M]CHPh  M]CH2. In short, neophyl and neopentyl form alkylidenes most easily and they are the most stable M]CHR complexes toward bimolecular alkylidene coupling. However, they are also the most likely themselves to be deprotonated to yield an alkylidyne. Disubstituted alkylidenes cannot form alkylidynes and are only now beginning to be explored in some detail (see Section 5.11.2.2.9). In highly unstable 10e Ta(CH2-t-Bu)5,8,9 described above, steric repulsion between the neopentyl ligands increases the Ta-Ca-Cb angle (Eq. 1), which in turn pushes an a hydrogen toward the metal, creates a more significant “agostic” interaction235,236,241,266 of the CHa electrons with the metal, and thereby activates an a hydrogen to move as a proton from one carbon atom to another. (The a agostic interaction also can be described as a consequence of the metal attracting electron density through the CdC bonding system and attempting to form a multiple M]C bond.) Stability of Ta(CH-t-Bu)(CH2-t-Bu)3 toward bimolecular decomposition is

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Table 2

683

Isolated Mo(VI) and W(VI) alkylidyne and related complexes ( ) X-ray).

Monodentate alkoxides, phenoxides, or siloxides Mo(CAro)(ORF6)3(DME) Mo(CArp)(OR)3(DME) Mo(CArp)(OR)3(THF)2 Mo(C-t-Bu)(ORF9)3 Mo(C-3-CF3-4-NO2C6H3)(ORF6)3(DME) Mo(C-4-NO2C6H4)(ORF6)3 Mo(CEt)(OR)3 Mo(CEt)(POSS)2(NRAr)(HNRAr) Mo(CPh)[OSi(O-t-Bu)3]3 Mo(CR)(OSiPh3)3(L) [Mo(CPh)(OSiPh3)4][K(DME)] Mo(CEt)(ORF6)2(NImt-Bu) Mo(C-p-C6H4R)(ORF6)3 Mo(CAryl)(OSiPh3)3(L) [Ph3PMe][Mo(NAr)(CR)(ORF6)2] Mo(C-t-Bu)[N(R)Ar][ORF6]2 [Ph3PMe][Mo(C-t-Bu)(CO2NAr)(ORF6)] [Mo(CMe2Ph)(CH2-t-Bu)2(NArMe2)MgCl(Et2O)]2 M(CMes)(OR)3 M(CMes)(ORF6)3 M(CMes)[OC(CF3)2Ph]3 M(CMes)(ORF6)2(NImt-Bu) M(CMes)[OSi(O-t-Bu)3]3 M(CMes)(ORF9)3(THF) M(CPh)(ORF6)3(DME) M(CPh)(ORF6)2(NImt-Bu) W(CMes)(ORF3)3 W(C-t-Bu)(OR0 )2(NImR) W(CPh)(ORF3)3 W(C-t-Bu)(ORF6)2[NRAr] W(C-t-Bu)[OC(Ph)(CF3)2]3 W(CEt)(ORF6)2(NImt-Bu) W(C-t-Bu)(ORF9)2(Cl)(DME) W(C-t-Bu)(ORF6)3(DME) (ORF9)(NImt-Bu)(t-BuC)W[N(t-Bu)CH]CHN(t-Bu)C N]W(C-t-Bu) (ORF9)3 W(C-t-Bu)(O-2,6-Ph2C6H3)3 W(CPh)(L)(ORF6)2 [(t-BuO)3SiO]3W^CdC^CdC^W[OSi(O-t-Bu)3]3 W(C-t-Bu)(ORF6)2(NPR3) [W(CPh)(OSiPh3)4][K(phen)] [Ph3PCH3][W(CH)(OAr)4] [(t-BuO)3W C]2EPh2 (t-BuO)3WCEPh3 W(CPh)[OSi(O-t-Bu)3]3 W(C-t-Bu)(CH2-t-Bu)2[OSiPh2(t-Bu)] W(CSiMe3)(CH2SiMe3)3(PMe3)a W(CSiMe3)(CH2SiMe3)3(PMe2Ph)b W(C-t-Bu)(O-2,6-Ph2C6H3)3 Bidentate Ligands Mo(C-t-Bu)(NH-2,6-Cl2C6H3)[Biphen] Mo(CCH2SiMe3)[Biphen](OAd) Tridentate C3 Symmetric Ligandsc [(OC6H4-o-CH2)3N]Mo^CR [[(OC6H4-o-CH2)3NMe]Mo^CR]+ [(OC6H4-o-CH2)3SiMe]Mo^CR [(OC6H4-o-CH2)3CH]Mo^CR (Ph-Podand)Mo^CAryl (R-Podand)Mo^CAryl

Aro ¼ o-(OMe)C6H4 Arp ¼ p-(OMe)C6H4; OR ¼ ORF6 or ORF9 Arp ¼ p-(OMe)C6H4; OR ¼ ORF6 or ORF9

OR ¼ OC(CF3)2Ph or ORF6 POSS ¼ polyhedral oligomeric silsesquioxane R ¼ t-Bu, L ¼ MeCN ; R ¼ Ph, L ¼ Et2O or phen

R ¼ NMe2, OCH3, CH3, H, OAc, CF3, NO2 L ¼ phen, bipy R ¼ t-Bu, CMe2Ph R ¼ Me, SiMe3

M ¼ Mo; OR ¼ O-t-Bu, ORF3, ORF9 M ¼ Mo or W M ¼ Mo or W M ¼ Mo or W M ¼ Mo or W M ¼ Mo or W M ¼ Mo or W M ¼ Mo or W R ¼ t-Bu, OR0 ¼ O-t-Bu, ORF6 R ¼ Ar, OR0 ¼ O-t-Bu, ORF6

L ¼ 1,3-di-t-butylimidazolidin-2-iminato or 4,5-dimethyl-1,3-di-t-butylimidazolin2-iminato R ¼ Cy or i-Pr

E ¼ Si, Ge, or Sn E ¼ Si, Ge, or Sn

rac or (S)-Biphen

Aryl ¼ Mesityl or p-tolyl R ¼ Me or i-Pr

114 115 115 116 153 153 154 155 156 157 157 158 159 160 161 161 161 162 163 164 154 156 156 156 158 158 163 165,166 167 166 166 166 166 166 166 144 168 169 170 157 145 171,172 171,172 173 123 129 130 144 29 29 174 175 176 177 178 179 (Continued )

684

Table 2

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

(Continued)

(MeCN)(Ar-Podand)Mo^C-p-OMeC6H4 Pincer2 − and Pincer3 − Complexes [K(O-i-Pr2)][(RF9O)2(3,4-t-Bu2C6H2O)2C5H3N)]Mo^C-p-tolyl] [t-BuOCHO](OR)Mo^CMes [PPh3Me][[t-BuOCO](OR)Mo^CMes] [t-BuOCO](THF)2Mo^CMes [OCH2NHCH2O](t-BuO)W^C-t-Bu [OCH2NHCH2O](t-BuO)(Et2O)W^CEt [PPh3Me][[OCH2NCH2O](t-BuO)W^C-t-Bu] [CF3ONO](Et2O)W^C-t-Bu [PPh3Me][CBZ-ONO](t-BuO)W^C-t-Bu] [PPh3Me][pyr-ONO](t-BuO)W^C-t-Bu] [[CF3ONO](RO)W^CEt][PPh3Me] [t-Bu-OCHO](t-BuO)(THF)W^C-t-Bu [t-Bu-OCHO](t-BuO)(Et2O)W^C-t-Bu [t-Bu-OCHO](t-BuO)(THF)2W^C-t-Bu [Ph3PMe][t-BuOCO]W(CCMe2Ph)(O-t-Bu) [t-BuOCO]W(C-t-Bu)(Et2O) [t-BuOCO]W(C-t-Bu)(THF)2 [ONHCH2O](t-BuO)W^C-t-Bu [PPh3Me][ONCH2O](t-BuO)W^C-t-Bu] [Ph3PMe][CF3ONO]W(CCH2CH3)(O-t-Bu) [CF3ONO]W(CEt)(OSiMe3) [Ph3PMe][CF3ONO]W(C-t-Bu)(OTf ) [CF3ONO]W(C-t-Bu)(Et2O) [Ph3PMe][[ONO]W(C-t-Bu)(O-t-Bu)] [Ph3PMe][(CBZ-ONO)W(C-t-Bu)(O-t-Bu)] [(t-BuOCO)(OCC-t-Bu)]W(NR)(THF)] [(t-BuOCO)(t-BuC)]W(2-(N,C)-RN]C]O)(THF)] [CF3ONO]W[N-t-Bu][t-BuC]C]O] [t-BuOCO]W[N-t-Bu][t-BuC]C]O] Other Motifs Mo(C-t-Bu)(CHR1)(Cl)(PMe2Ph)2X [ArCl2NNMe]Mo(C-t-Bu)(CH2-t-Bu) Mo(NR)(CCMe2Ph)(Me2Pyr)(bipy) Mo(NR)(C-t-Bu)(Me2Pyr)(bipy) [F3NMe]M(C-t-Bu)(CH2-t-Bu) [F3NMe]W(CSiMe3)(CH2SiMe3) W(C-t-Bu)(CH2-t-Bu)(X)2 (OSSO)W(CEt)(X) [(OSSO)W(CEt)(PhMe2N)][B(C6F5)4] W(CX)(Cy2PCH2CH2PCy2)Br3 W(CCH2-t-Bu)(Cy2PC2H4PCy2)Cl3 W[C-o-(OMe)C6H4](ORF6)2(OTf )(L) W[C-o-(OMe)C6H4](ORF3)2(OTf )(L) Metallacyclobutadienes and related complexes Mo(C3Et3)(ORF6)3 Mo(C3Et3)(ORF9)3 Mo[C(Mes)CC(Ph)](ORF9)2 Mo(C3Et3-hedrane)(Ph-Podand) W(C3Et3)[OC(CF3)2Ph]3 W[C(Ph)C(Me)C(Me)][OC(CF3)2Ph]3 W[C(C^CMe)C(Me)C(C^CMe)][OC(CF3)2Ph]3 W[C(Me)C(Me)C(C^CMe)](ORF9)3 W(C3Et3)(ORF6)2(NImt-Bu) [CF3ONO]W[C(t-Bu)C(Me)C(R)] [CF3ONO]W[C(t-Bu)C(CH2)6C]

Ar ¼ Ph or p-OMeC6H4

OR ¼ O-t-Bu or ORF6 OR ¼ O-t-Bu or ORF6

R ¼ t-Bu, Cy, Ph R ¼ t-Bu, Cy, Ph

X ¼ Cl, Me2Pyr, OSiPh3 R ¼ Ar, Ad, ArMe2, AriPr, and ArMes R ¼ ArCF3 and ArTrip M ¼ Mo or W X ¼ OAr, NPh2, or OAd X ¼ O-t-Bu, Cl, CH2SiMe3 X ¼ SiPh3 or Br; Cy ¼ cyclohexyl Cy ¼ cyclohexyl L ¼ DME or THF L ¼ DME or THF

(Deprotiometallacycle) (Metallatetrahedrane)

R ¼ Ph or t-Bu C(CH2)6C ¼ cyclooctyne

Abbreviations: [(3,4,5-C6F3H2NCH2CH2)2NMe]2− ¼ [F3NMe]2−, [(ArCl2NCH2CH2)2NMe]2− ¼ [ArCl2NNMe]2− In equilibrium with W(CHSiMe3)2(CH2SiMe3)2(PMe3). b In equilibrium with W(CHSiMe3)2(CH2SiMe3)2(PMe2Ph). c Only the parent ligand type is listed, not those with substituents on the aromatic rings.

a

180 181 182 182 182 183 183 183 184 148 149 150 185 185 185 185 185 185 152 152 150 150 184 184 149 148 186 186 186 186 128 187 25 25 188 188 189 137 137 190 191 192 192 154 163 154 178 154 154 154 154 165 184 184

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Table 3

685

Isolated Mo(VI) and W(VI) NHC alkylidene and alkylidyne complexes ( ) X-ray; R1 ¼t-Bu; R2 ¼CMe2Ph).

Molybdenum Alkylidene a. Mo(NR)(CHR0 )(OTf )2(NHC) Mo001—Mo(NAr2,6Me2)(CHR1)(OTf )2(IMesH2) Mo002—Mo(NArMe2)(CHR1)(OTf )2(ItBu) Mo003—Mo(NArCl2)(CHR1)(OTf )2(IMes) Mo004—Mo(NArCl2)(CHR1)(OTf )2(IMesH2) Mo005—Mo(NArCl2)(CHR1)(OTf )2(IMesCl2) Mo006—Mo(NArMe2)(CHR2)(OTf )2(IMes) Mo007—Mo(NArMe2)(CHR2)(OTf )2(IMesH2) Mo008—Mo(NArMe2)(CHR2)(OTf )2(IMesCl2) Mo009—Mo(NArMe2)(CHR2)(OTf )2(IMesMe2) Mo010—Mo(NArMe2)(CHR2)(OTf )2(ItBu) Mo011—Mo(NArMe2)(CHR2)(OTf )2(TPT) Mo012—Mo(NArMe2)(CHR2)(OTf )2(IMesMePhH2) Mo013—Mo(NArCl2)(CHR2)(OTf )2(IMes) Mo014—Mo(NArCl2)(CHR2)(OTf )2(IMesH2) Mo015—Mo(NArCl2)(CHR2)(OTf )2(IMesCl2) Mo016—Mo(NAd)(CHR2)(OTf )2(IMes) Mo017—Mo(NAd)(CHR2)(OTf )2(IMesH2) Mo018—Mo(NAd)(CHR2)(OTf )2(ICy) Mo019—Mo(NArCF3)(CHR2)(OTf )2(IMes) Mo020—Mo(NArF2)(CHR2)(OTf )2(IMes) Mo021—Mo(NArtBu)(CHR2)(OTf )2(IMes) Mo022—Mo(N-t-Bu)(CHR2)(OTf )2(IMes) Mo023—Mo(N-t-Bu)(CHR2)(OTf )2(IMeCl2) Mo024—Mo(NAr3,5Me2)(CHR2)(OTf )2(IMes) Mo025—Mo(NAr3,5Me2)(CHR2)(OTf )2(IMesH2) Mo026—Mo(N-t-Bu)(CHR2)(OTf )2(MIC) Mo027—Mo(NArMe2)(CHFc)(OTf )2(IMes) Mo028—Mo(NArMe2)(CHFc)(OTf )2(IMesH2) b. [Mo(NR)(CHR0 )(OTf )(NHC)][A] Mo029—[Mo(NArCl2)(CHR1)(OTf )(IMes)][BArF4] Mo030—[Mo(NArCl2)(CHR1)(OTf )(IMes)(MeCN)][BArF4] Mo031—[Mo(NArCl2)(CHR2)(OTf )(IMes)][BArF4] Mo032—[Mo(NArCl2)(CHR1)(OTf )(IMesH2)][BArF4] Mo033—[Mo(NArCl2)(CHR1)(OTf )(IMesCl2)][BArF4] Mo034—[Mo(NArMe2)(CHR2)(OTf )(IMes)][BArF4] Mo035—[Mo(NArMe2)(CHR2)(OTf )(IMesH2)][BArF4] Mo036—[Mo(NArMe2)(CHR2)(OTf )(IMesCl2)][BArF4] Mo037—[Mo(NArMe2)(CHR2)(OTf )(IMesMe2)][BArF4] Mo038—[Mo(NArMe2)(CHR2)(OTf )(IMes)(MeCN)][BArF4] Mo039—[Mo(NArMe2)(CHR2)(OTf )(IMesH2)(MeCN)][BArF4] Mo040—[Mo(NArMe2)(CHR2)(OTf )(IMesH2)(MeCN)][BPh4] Mo041—[Mo(NAr3,5Me2)(CHR2)(OTf )(IMes)(MeCN)][BArF4] Mo042—[Mo(NAr3,5Me2)(CHR2)(OTf )(IMesH2)(MeCN)][BArF4] Mo043—[Mo(NArtBu)(CHR2)(OTf )(IMes)][BArF4] Mo044—[Mo(NArtBu)(CHR2)(OTf )(IMes)][B(Ph)4] Mo045—[Mo(NArtBu)(CHR2)(OTf )(IMes)(MeCN)][BArF4] Mo046—[Mo(NArCF3)(CHR2)(OTf )(IMes)(MeCN)][BArF4] Mo047—[Mo(NArF2)(CHR2)(OTf )(IMes)(MeCN)][BArF4] Mo048—[Mo(N-t-Bu)(CHR2)(OTf )(IMes)(MeCN)][BArF4] Mo049—[Mo(NArMe2)(CHFc)(OTf )(IMes)] BArF4] Fc ¼ ferrocenyl Mo050—[Mo(NArMe2)(CHFc)(OTf )(IMesH2)][BArF4] Fc ¼ ferrocenyl c. Mo(NR)(CHR0 )(OTf )(Alkoxide)(NHC) Mo051—Mo(NArMe2)(CHR2)(OTf )(OC6F5)(IMesH2) Mo052—Mo(NArMe2)(CHR2)(OTf )(OCH(CF3)2)(IMesH2) Mo053—Mo(NArMe2)(CHR2)(OTf )(ORF9)(IMesH2) Mo054—Mo(NAr3,5Me2)(CHR2)(OTf )(ORF9)(IMesH2) Mo055—Mo(NAr3,5Me2)(CHR2)(OTf )(OArPh2)(IMesH2) Mo056—Mo(NArMe2)(CHR2)(OTf )(OC6F5)(IMes) Mo057—Mo(NArMe2)(CHR2)(OTf )(OC6H5)(IMes)

MIC ¼ 1-(2,6-diisopropylphenyl)-3isopropyl-4-phenyl-1H-1,2,3-triazol-5-ylidene Fc ¼ ferrocenyl Fc ¼ ferrocenyl

193 193 194 194 30 194 194 69 69 195 196 195 197 197 30 198 195 195 198 30 198 199 200 195 195 196 69 69 30 201 30 30 30 30 69 69 69 201 194 202 201 203 201 202 201 201 30,201 30 69 69 202 194 195 195 195 202 204 (Continued )

686

Table 3

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

(Continued)

Mo058—Mo(NArMe2)(CHR2)(OTf )(O-2-Cl-C6H4)(IMes) Mo059—Mo(NArMe2)(CHR2)(OTf )(ORF9)(IMes) Mo060—Mo(NAd)(CHR2)(OTf )(OC6F5)(IMes) Mo061—Mo(N-t-Bu)(CHR2)(OTf )(O-t-Bu)(MIC) d. Mo(NR)(CHR0 )(Alkoxide)2(NHC) Mo062—Mo(N-t-Bu)(CHR1)(OC6F5)2(IMeCl2) Mo063—Mo(N-t-Bu)(CHR1)(OC6F5)(OHIPT)(IMeCl2) Mo064—Mo(NArCF3)(CHR2)(ORF6)2(IMe) Mo065—Mo(NArCF3)(CHR2)(ORF6)2(IiPr) Mo066—Mo(NArMe2)(CHR2)(ORF6)2(IMe) Mo069—Mo(NAd)(CHR2)(OCH(CF3)2)2(TPT) Mo070—Mo(NArMe2)(CHR2)(OCH(CF3)2)2(IiPr) Mo071—Mo(NArMe2)(CHR2)(O-2,6-Br2-C6H3)2(IiPr) e. Mo(NR)(CHR0 )(Pyrrolide)2(NHC) and [Mo(NR)(CHR0 )(Pyrrolide)(NHC)][A] Mo072—Mo(NArtBu)(CHR2)(Pyr)2(IiPr) Mo073—Mo(NArtBu)(CHR2)(Pyr)2(TPT) Mo074—Mo(NArtBu)(CHR2)(Pyr)2(IMeCl2) Mo075—Mo(NArMe2)(CHR2)(Pyr)2(IiPr) Mo076—Mo(NAr)(CHR2)(Pyr)2(IiPr) Mo077—Mo(N-t-Bu)(CHR2)(Pyr)2(IiPr) Mo078—Mo(NAd)(CHR2)(Pyr)2(IMeCl2) Mo079—[Mo(NArtBu)(CHR2)(Pyr)(IiPr)][BArF4] Mo080—[Mo(NArtBu)(CHR2)(Pyr)(TPT)][BArF4] Mo081—[Mo(NArtBu)(CHR2)(Pyr)(IMeCl2)][BArF4] Mo082—[Mo(NArMe2)(CHR2)(Pyr)(IiPr)][BArF4] Mo083—[Mo(NAr)(CHR2)(Pyr)(IiPr)(MeCN)][BArF4] Mo084—[Mo(N-t-Bu)(CHR2)(Pyr)(IiPr)][BArF4] Mo085—[Mo(NAd)(CHR2)(Pyr)(IMeCl2)][BArF4] f. Mo(NR)(CHR0 )(Halide)2(NHC) Mo086—Mo(NC6F5)(CHR2)(Br)2(6Mes) Mo087—Mo(NArCF3)(CHR2)(Br)2(6Mes) Mo088—Mo(NArMe2)(CHR2)(Br)2(6Mes) Mo089—Mo(NArCF3)(CHR2)(Br)(6Mes)(MeCN)2][BArF4] Mo090—Mo(NAd)(CHR2)(Cl)2(IMes) Mo091—Mo(NAd)(CHR2)(Cl)2(IMesH2) Mo092—Mo(NAd)(CHR2)(Cl)2(IiPr) Mo094—Mo(NArCF3)(CHR2)(Cl)(OHIPT)(IMeCl2) Mo095—Mo(NArCF3)(CHR2)(Cl)(OHMT)(IMeCl2) Mo096—Mo(NArMe2)(CHR2)(Cl)(OHIPT)(IMeCl2) g. [Mo(NR)(CHR0 )(Alkoxide)(NHC)][A] Mo097—[Mo(NArMe2)(CHR2)(OC6F5)(IMesH2)(MeCN)][BArF4] Mo098—[Mo(NArMe2)(CHR2)(OC6F5)(IMes)][BArF4] Mo099—[Mo(NAd)(CHR2)(OC6F5)(IMes)(MeCN)][BArF4] Mo101—[Mo(NArMe2)(CHR2)(OC6F5)(IMes)(MeCN)][BArF4] Mo102—[Mo(NArtBu)(CHR2)(OCH(CF3)2)(IMes)][BArF4] Mo103—[Mo(NArCF3)(CHR2)(OCH(CF3)2)(IMes)(MeCN)][BArF4] Mo104—[Mo(NArCF3)(CHR2)(ORF6)(IiPr)(MeCN)2][Al(OC(CF3)3)4] Mo105—[Mo(NArCF3)(CHR2)(OC6F5)(6Mes)(MeCN)][BArF4] Mo106—[Mo(NArMe2)(CHR2)(OC6F5)(6Mes)(MeCN)][BArF4] Mo107—[Mo(NC6F5)(CHR2)(ORF6)(6Mes)(MeCN)][BArF4] Mo108—[Mo(NArMe2)(CHR2)(OHIPT)(IMeCl2)][BArF4] Mo109—[Mo(NArCF3)(CHR2)(OHMT)(IMeCl2)][BArF4] Mo110—[Mo(NArCF3)(CHR2)(OHIPT)(IMeCl2)][BArF4] Mo111—[Mo(NAr3,5Me2)(CHR2)(OCPh(CF3)2)(IMesH2)][BArF4] Mo112—[Mo(NArMe2)(CHR2)(ORF9)(IMes)][BArF4] Mo113—[Mo(NArCl2)(CHR2)(ORF9)(IMes)(MeCN)][BArF4] Mo114—[Mo(NArCl2)(CHR1)(O-t-Bu)(IMes)][BArF4] Mo115—[Mo(NArMe2)(CHR2)(ORF9)(IMesCl2)(MeCN)][BArF4] Mo116—[Mo(NArMe2)(CHR2)(ORF9)(IMesMe2)(MeCN)][BArF4] Mo117—[Mo(NPh)(CHR2)(ORF9)(IMesH2)(MeCN)][BArF4] Mo118—[Mo(NArtBu)(CHR2)(OHMT)(IiPr)][BArF4] Mo119—[Mo(NArMe2)(CHR2)(OHMT)(IiPr)][BArF4]

MIC ¼ 1-(2,6-diisopropylphenyl)-3isopropyl-4-phenyl-1H-1,2,3-triazol-5-ylidene

100 202 198 196

205 205 205 205 205 205 205 205 49 49 49 49 49 49 49 49 49 49 49 49 49 49 205 205 205 205 205 205 205 205 205 205 195 30,47 198 198 198 198 205 205 205 205 205 205 205 203 30 30 47 69 69 69 49 49

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Table 3

687

(Continued)

Mo120—[Mo(NAd)(CHR2)(OHMT)(IMeCl2)][BArF4] Mo121—[Mo(NArtBu)(CHR1)(OHIPT)(IMeCl2)][BArF4] h. [Mo(NR)(CHR0 )(Carboxylate)(NHC)][A] Mo122—Mo(N-t-Bu)(CHR2)(OTf )(O2C-C6F5)(IMes) Mo123—[Mo(N-t-Bu)(CHR2)(O2C-C6F5)(IMes)][BArF4] Mo124—[Mo(NArMe2)(CHR2)(O2C-C6F5)(IMes)][Al(OC(CF3)3)4] Mo125—[Mo(NAr3,5Me2)(CHR2)(O2C-C6F5)(IMes)][Al(OC(CF3)3)4] Mo126—[Mo(NArCl2)(CHR1)(O2C-C6F5)(IMes)][BArF4] Mo127—[Mo(NArCl2)(CHR1)(O2C-2,6-CF3-C6H3)(IMes)][Al(OC(CF3)3)4] Mo128—[Mo(NArCl2)(CHR1)(O2C-2,6-CF3-C6H3)(IMes)][BArF4] Mo129—[Mo(NAd)(CHR2)(O2C-2,6-CF3-C6H3)(IMes)][Al(OC(CF3)3)4] Mo130—[Mo(NAd)(CHR2)(O2C-2,6-CF3-C6H3)(IMesH2)][Al(OC(CF3)3)4] i. Complexes with Chelating Alkylidene Ligands Mo131—Mo(NAr3,5Me2)(CHR2)(OTf )(OC6H5)(IMes) Mo132—Mo(NAr3,5Me2)(CH-2-OMe-C6H4)(OTf )2(IMes) Mo133—Mo(NAr3,5Me2)(CH-2-OMe-C6H4)(OTf )2(IMes) Mo134—Mo(NArMe2)(CH-2-OMe-C6H4)(OTf )2(IMes) Mo135—Mo(NArMe2)(CH-2-OMe-C6H4)(OTf )2(IMesH2) Mo136—Mo(NArMe2)(CH-2-OMe-C6H4)(OTf )(OC6F5)(IMesH2) Mo137—Mo(NAr3,5Me2)(CHOCOC3H7)(OTf )2(IMes) Mo138—Mo(NAr3,5Me2)(CHOCOC3H7)(OTf )(OC6F5)(IMes) Mo139—Mo(NArMe2)(CH-2-O-NC4H6)(OTf )2(IMes) Mo140—Mo(NAr3,5Me2)(CH-2-O-NC4H6)(OTf )2(IMes) Mo141—Mo(NAr3,5Me2)(CH-2-OMe-5-Cl-C6H3)(OTf )2(IMes) Mo142—Mo(NAr3,5Me2)(CH-2-OCF3-C6H4)(OTf )2(IMes) Mo143—Mo(NAr3,5Me2)(CH-2-OMe-5-F-C6H3)(OTf )2(IMes) Mo144—Mo(NAr3,5Me2)(CH-2,4,5-(OMe)3-C6H3)(OTf )2(IMes) Mo145—Mo(NAr3,5Me2)(CH-2-NMe2-C6H4)(OTf )2(IMes) Mo146—Mo(NArMe2)(CH-C5H4N)(OTf )2(IMes) Mo147—Mo(NAr3,5Me2)(CH-C5H4N)(OTf )2(IMes) Mo148—Mo(NAr3,5F2)(CH-C5H4N)(OTf )2(IMes) Mo149—Mo(NArCF3)(CH-C5H4N)(OTf )2(IMes) Mo150—Mo(NArMe2)(CH-2-i-Pr-C6H4)(OTf )(OPh)(IMes) Mo151—Mo(NArMe2)(CH-C5H4N)(OTf )(O-2-Cl-C4H4)(IMes) Mo152—[Mo(NArMe2)(CH-CH2-O-(CH2)2-OH)(OTf )(IMes)][BArF4] j. Ionically Tagged Alkylidenes Mo153—[Mo(NArMe2)(CHR2)(OTf )(OSO2-(CH2)3-NC5H5)(IMesH2)][OTf] Mo154—[Mo(NArMe2)(CHR2)(OC6F5)(OSO2-(CH2)3-NC5H5)(IMesH2)][BArF4] Mo155—[Mo(N-t-Bu)(CHR2)(OTf )(O-2,6-Ph2-4-(2,4,6-Ph3)NC5H2-C6H2) (IMeCl2)][OTf] Mo156—[Mo(N-t-Bu)(CHR2)(O-2,6-Ph2-4-(2,4,6-Ph3)NC5H2-C6H2)(IMeCl2)2] [OTf]2 Mo157—[Mo(N-t-Bu)(CHR2)(O-2,6-Ph2-4-(2,4,6-Ph3)NC5H2-C6H2)(IMeCl2)] [BArF4]2 k. Alkylidenes with Chelating NHCs Mo164—Mo(NArMe2)(CHR2)(OTf )(N-mesityl-N0 -O-C6H4-imidazolidine-2ylidene) Mo165—Mo(NAr)(CHR2)(OTf )(N-2,6-(i-Pr)2-C6H3-N0 -O-C6H4-imidazolidine-2ylidene) Mo166—Mo(NAr)(CHR2)(OC6F5)(N-2,6-(i-Pr)2-C6H3-N0 -O-C6H4-imidazolidine2-ylidene) Mo167—[Mo(NAr)(CHR2)(N-2,6-(i-Pr)2-C6H3-N0 -O-C6H4-imidazolidine-2ylidene)][BArF4] Mo168—Mo(NArMe2)(CHR2)(OTf )2(1-(2-pyridyl)methylene-3mesitylimidazol-2-ylidene) Mo169—Mo(NArMe2)(CHR2)(OTf )(OC6F5)(1-(2-pyridyl)methylene-3mesitylimidazol-2-ylidene) Mo170—Mo(NArMe2)(CHR2)(OTf )(O-t-Bu)(1-(2-pyridyl)methylene-3mesitylimidazol-2-ylidene) Mo171—[Mo(NArMe2)(CHR2)(OC6F5)(1-(2-pyridyl)methylene-3mesitylimidazol-2-ylidene)][BArF4] Mo172—[Mo(NArMe2)(CHR2)(O-t-Bu)(1-(2-pyridyl)methylene-3mesitylimidazol-2-ylidene)][BArF4]

49 205 30 30 30 30 30 30 30 30 30 199 199 199 199 202 202 199 199 204 204 204 204 204 204 204 204 204 204 204 204 204 30 200 200 200 200 200

206 206 206 206 207 200 200 200 200 (Continued )

688

Table 3

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

(Continued)

Mo173—[Mo(NArMe2)(CHR2)(OTf )(1-(2-pyridyl)methylene-3mesitylimidazol-2-ylidene)][BArF4] Mo174—Mo(NArMe2)(CHR2)(OTf )(1-mesityl-3-(C9H11)-imidazol-2-ylidene) Molybdenum Alkylidynes Mo067—Mo(NHC6F5)(CR2)(ORF6)2(IMes) Mo068—Mo(NH-2-CF3-C6H5)(CR2)(ORF6)2(6Mes) Mo093—Mo(NAr)(CR2)(Cl)(IPr)2 Mo177—Mo(CArp)(ORF6)3(IMe) Mo178—Mo(CArp)(ORF6)3(IMeCl2) Mo179—Mo(CArp)(ORF6)3(IMeCN2) Mo180—Mo(CArp)(ORF6)3(IiPr) Mo181—Mo(CArp)(ORF6)3(ICy) Mo182—Mo(CArp)(ORF6)3(ItBu) Mo183—Mo(CArp)(ORF6)3(TMTh) Mo184—Mo(CArp)(OC6F5)3(IMes) Mo185—Mo(CArp)(OC6F5)3(IiPr) Mo186—Mo(CArp)(ORF6)2(N-2,6-Me2-C6H3-N0 -O-C6H4-imidazolidine-2ylidene) Mo187—Mo(CArp)(ORF6)2(N-mesityl-N0 -O-C6H4-imidazolidine-2-ylidene) Mo188—Mo(CArp)(ORF6)2(N-2-Ph-C6H4-N0 -O-C6H4-imidazolidine-2-ylidene) Mo189—Mo(CArp)(ORF6)(N-(CH2)(Me)2C-N0 -O-C6H4-imidazolidine-2-ylidene) Mo190—Mo(CArp)(ORF6)(N,N0 -(CH2-C(CF3)2O)2-imidazol-2-ylidene) Mo191—Mo(CArp)(ORF6)2(OTf )(DME) Mo192—Mo(CArp)(ORF6)2(OTf )(THF)2 Mo193—Mo(CArp)(ORF6)2(OTf )(IMes) Mo194 – [Mo(CArp)(ORF6)2(IMes)] [BArF4] Mo195 – [Mo(CArp)(ORF6)2(IMes)] [BArF4] Tungsten Alkylidenes W002—W(O)(CHR2)(Cl)2(IMes)(PMe2Ph) W003—W(O)(CHR2)(Cl)(OArPh2)(IMes) W004—W(O)(CHR2)(Cl)(ORF6)(IMes) W005—W(O)(CHR2)(Cl)(OTf )(IMes)(PMe2Ph) W006—[W(O)(CHR2)(OArPh2)(IMes)(MeCN)2][BArF4] W007—[W(O)(CHR2)(OArPh2)(IMes)][BArF4] W008—[W(O)(CHR2)(ORF6)(IMes)(MeCN)2][BArF4] W009—[W(O)(CHR2)(ORF6)(IMes)][BArF4] W010—[W(O)(CHR2)(OTf )(IMes)(MeCN)2][BArF4] W012—[W(NAr)(CHR2)(Cl)(O-(CH2)2-OMe)(IMesH2)][OTf] W013—[W(NAr)(CHR2)(OTf )(CH2IMesH2)][OTf] W017—W(NAr)(CHR2)(OSiPh3)2(IiPr) W018—W(NAr)(CHR2)(O-t-Bu)2(IiPr) W019—W(NAr)(CHR2)(ORF6)2(IiPr) W020—[W(NAr)(CHR2)(OSiPh3)(IiPr)(MeCN)][BArF4] W021—[W(NAr)(CHR2)(O-t-Bu)(IiPr)][BArF4] W023—W(NAr)(CHR2)(Me2Pyr)2(IiPr) W024—[W(NAr)(CHR2)(Me2Pyr)(IiPr)][BArF4] W025—[W(NAr)(CHR2)(OC6F5)(IiPr)][BArF4] W026—[W(NAr)(CHR2)(ORF9)(IiPr)][BArF4] W027—[W(NAr)(CHR2)(OPh)(IiPr)][BArF4] W028—[W(NAr)(CHR2)(Me2Pyr)(O-2,6-(t-Bu)2-4-PPh3-C6H2)(IiPr)][BArF4] W029—W(NAr)(CHR2)(Me2Pyr)(OC6F5)(IiPr) W031—W(NAr)(CHR2)(Br)2(IMes) W032—W(NAr)(CHR2)(Br)2(IiPr) W033—W(NAr)(CHR2)(Br)2(IMeCl2) W034—W(NAr)(CHR2)(Br)2(IMe) W035—W(NAr)(CHR2)(Br)2(IPr) W036—W(NAr)(CHR2)(Br)2(6Mes) W037—[W(NAr)(CHR2)(Br)(IMes)][BArF4] W038—[W(NAr)(CHR2)(Br)(IMeCl2)(t-BuCN)2][BArF4] W039—[W(NAr)(CHR2)(Br)(IMe)(t-BuCN)][BArF4] W040—[W(NAr)(CHR2)(Br)(IiPr)(t-BuCN)][BArF4] W041—[W(NAr)(CHR2)(Br)(IPr)][BArF4] W042—[W(NAr)(NCMe]CH-CMe2Ph)(Br)(6Mes)][BArF4] W043—[W(NAr)(CHR2)(Br)(6Mes)(t-BuCN)][BArF4]

200 200 205 205 205 208 208 208 209 209 209 209 209 209 209 209 209 209 209 210 210 210 210 210 211 211 211 211 211 211 211 211 211 212 212 213 213 213 213 213 213 213 213 201 201 200 214 47 47 47 47 47 47 47 47 47 47 47 47 47

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Table 3

689

(Continued)

W049—W(NArMe2)(CHR2)(Br)2(IMes) W050—W(NArCF3)(CHR2)(Br)2(IMes) W051—W(NArCl2)(CHR2)(Cl)2(IMes) W052—W(NAr3,5Me2)(CHR2)(Cl)2(IMes) W053—W(NArtBu)(CHR2)(Cl)2(IMes) W054—[W(NArMe2)(CHR2)(Br)(IMes)(t-BuCN)][BArF4] W055—[W(NArCF3)(CHR2)(Br)(IMes)(t-BuCN)][BArF4] W056—[W(NArCl2)(CHR2)(Cl)(IMes)(t-BuCN)][BArF4] W057—[W(NAr3,5Me2)(CHR2)(Cl)(IMes)(t-BuCN)][BArF4] W058—[W(NArtBu)(CHR2)(Cl)(IMes)(t-BuCN)][BArF4] W059—[W(NAr)(CHR2)(OC6F5)(IMes)(t-BuCN)][BArF4] W060—[W(NArMe2)(CHR2)(OC6F5)(IMes)(t-BuCN)][BArF4] W061—[W(NArCF3)(CHR2)(OC6F5)(IMes)(t-BuCN)][BArF4] W062—[W(NArCl2)(CHR2)(OC6F5)(IMes)(t-BuCN)][BArF4] W063—[W(NAr3,5Me2)(CHR2)(OC6F5)(IMes)(t-BuCN)][BArF4] W064—[W(NArtBu)(CHR2)(OC6F5)(IMes)(t-BuCN)][BArF4] W065—[W(NAr)(CHR2)(OTf )(IMes)(t-BuCN)2][BArF4] W066—[W(NArMe2)(CHR2)(OTf )(IMes)(t-BuCN)][BArF4] W067—[W(NArCF3)(CHR2)(OTf )(IMes)(t-BuCN)][BArF4] W068—[W(NArCl2)(CHR2)(OTf )(IMes)(t-BuCN)][BArF4] W069—[W(NAr3,5Me2)(CHR2)(OTf )(IMes)(t-BuCN)][BArF4] W070—[W(NArtBu)(CHR2)(OTf )(IMes)(t-BuCN)][BArF4] W071—W(NArMe2)(CHPh)(O-t-Bu)(CH2Ph)(IMes) W072—W(NArMe2)(CHPh)(O-t-Bu)(CH2Ph)(ICy) W073—W(NArMe2)(CHPh)(O-t-Bu)(CH2Ph)(IiPr) W074—W(NArCl2)(CHPh)(O-t-Bu)(CH2Ph)(IMes) W075—W(NArCl2)(CHPh)(O-t-Bu)(CH2Ph)(ICy) W076—W(NArCl2)(CHPh)(O-t-Bu)(CH2Ph)(IiPr) W077—W(NAr)(CHPh)(O-t-Bu)(CH2Ph)(ICy) W078—W(NAr)(CHPh)(O-t-Bu)(CH2Ph)(IiPr) Tungsten Alkylidyne W081—W(CArp)(ORF6)3(IiPr) W082—W(CArp)(ORF6)3(ICy) W083—W(CArp)(ORF3)3(IiPr) W084—W(CArp)(ORF3)3(ICy) W085—[W(CArp)(ORF6)2(IiPr)(t-BuCN)][BArF4] W086—[W(CArp)(ORF3)2(IiPr)(t-BuCN)][BArF4] W089—W(CArp)(ORF6)2(OTf )(IiPr)2 W090—W(CArp)(ORF3)2(OTf )(IMes) W091—[W(CArp)(ORF3)2(IMes)][BArF4] W092—[W(C3(C6H4OMe)(Me)(Ph))(ORF6)2(IiPr)(t-BuCN)][BArF4] W093—W(CArp)(ORF6)2(N-2,6-Me2-C6H3-N0 -O-C6H4-imidazolidine-2-ylidene) W094—W(CArp)(ORF6)2(N-mesityl-N0 -O-C6H4-imidazolidine-2-ylidene) W095—W(CArp)(ORF6)3(N-mesityl-N’-O-C6H4-imidazolinium) W096—W(CArp)(ORF3)2(N-2,6-Me2-C6H3-N0 -O-C6H4-imidazolidine-2-ylidene) W097—W(CArp)(ORF3)2(N-mesityl-N0 -O-C6H4-imidazolidine-2-ylidene) W098—W(CArp)(ORF6)3(N-t-Bu-N0 -O-C6H4-imidazolinium) W099—W(CArp)(ORF6)(Cl)(N-mesityl-N0 -O-C6H4-imidazolidine-2-ylidene) W100—W(CArp)(ORF6)(N-mesityl-N0 -O-C6H4-imidazolidine-2-ylidene)(O-t-Bu)] [Al(ORF9)4] W101—W(CArp)(ORF6)(N,N0 -(CH2-C(CF3)2O)2-imidazol-2-yliden) W102—W(CArp)(Cl)(N,N0 -(CH2-C(CF3)2O)2-imidazol-2-yliden) W103—[W2(m-(Arp)CC(Arp))(Cl)(N,N0 -(CH2-C(CF3)2O)2-imidazol-2-yliden)2] [BArF4] W104—W(CArp)(Br)3(IMes) W105—W(CArp)(Br)3(IPr) W106—[W(CArp)(Br)2(IMes)(t-BuCN)2][BArF4] W107—[W(CArp)(Br)2(IPr)(t-BuCN)2][BArF4] W108—W(CArp)(OTf )3(IMes) W109—[W(CArp)(OTf )2(IMes)(t-BuCN)2][BArF4] W110—W(C-t-Bu)(Cl)3(IMes) W111—W(C-t-Bu)(Cl)3(IPr) W112—W(C-t-Bu)(Cl)3(IMesCl2)

47 47 47 47 47 47 47 47 47 47 47 47 47 47 47 47 47 47 47 47 47 47 215 215 215 215 215 215 215 215 192 192 192 192 192 192 192 192 192 192 209 209 209 209 209 209 192 192 192 192 192 216 216 216 216 216 216 216 216 216 (Continued )

690

Table 3

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

(Continued)

W113—W(C-t-Bu)(Cl)(OTf )2(IMes) W114—W(C-t-Bu)(Cl)(OTf )2(IPr) W115—W(C-t-Bu)(Cl)(OTf )2(IMesCl2) W116—[W(C-t-Bu)(OTf )2(IMes)(t-BuCN)2][BArF4] W117—[W(C-t-Bu)(OTf )2(IMesCl2)(t-BuCN)2][BArF4] W118—[W(C-t-Bu)(Cl)2(IMes)(t-BuCN)][BArF4] W119—[W(C-t-Bu)(Cl)2(IPr)(t-BuCN)2][BArF4] W120—[W(C-t-Bu)(Cl)2(IMesCl2)(t-BuCN)][BArF4] W121—[W(O)(CHR1)(OTf )(IMes)(t-BuCN)2][BArF4] W122—[W(O)(CHR1)(OTf )(IMesCl2)(t-BuCN)2][BArF4] W123—[W(O)(CHR1)(Cl)(IMes)(t-BuCN)][BArF4] W124—[W(O)(CHR1)(Cl)(IMesCl2)(t-BuCN)][BArF4] W125—[W(O)(CHR1)(OTf )(IPr)(t-BuCN)2][BArF4] W126—[W(O)(CHR1)(OC6F5)(IMes)(t-BuCN)][BArF4] W127—[W(O)(CHR1)(OC6F5)(IMesCl2)(t-BuCN)][BArF4] W128—[W(O)(CHR1)(OC6F5)(IPr)(t-BuCN)2][BArF4]

216 216 216 216 216 216 216 216 216 216 216 216 216 216 216 216

provided by the neopentyl and neopentylidene ligands themselves; intramolecular pressure toward losing neopentane to yield three-coordinate Ta(C-t-Bu)(CH2-t-Bu)2, an unusually low coordination number, is essentially absent. In contrast, attempts to prepare Ta(CH2SiMe3)5 or Nb(CH2SiMe3)5 leads to formation of [M(CSiMe3)(CH2SiMe3)2]2 through a double, sequential proton abstraction to give alkylidynes which bridge the two metal centers (symmetrically).267 [Ta(CSiMe3)(CH2SiMe3)2]2 is the product of intermolecular proton abstraction of the alkylidene proton in Ta(CHSiMe3)(CH2SiMe3)3 by an alkyl ligand in a second molecule, a reaction that is not sterically blocked as it is in Ta(CH-t-Bu)(CH2-t-Bu)3. Cp2Ta(CH2)(CH3) is an 18e methylene complex that decomposes slowly in a bimolecular fashion at room temperature to give a 50% yield of the ethylene complex, Cp2Ta(CH2]CH2) (CH3) and uncharacterizeable other decomposition products; under ethylene Cp2Ta(CH2]CH2)(CH3) is the only product in essentially 100% yield.268 Steric crowding that results from binding one or two electron donor ligands (which could include a coordinating solvent such as THF) to the metal often dramatically accelerates alkylidene formation. For example, Ta(CH2-t-Bu)2Cl3 is a yellow five-coordinate complex in pentane, but when dissolved in THF, neopentane and purple Ta(CH-t-Bu)(THF)2Cl3 are formed.269 Other electron donors (e.g., small and relatively basic phosphines) can also induce a hydrogen abstraction and give Ta(CH-t-Bu) (L)2Cl3 complexes, but Ta(CH-t-Bu)(L)2Cl3 readily loses one L (phosphine) per Ta to give chloride-bridged [Ta(CH-t-Bu)(L) Cl3]2 complexes. A neutron diffraction study of [Ta(CH-t-Bu)(PMe3)Cl3]2 showed that the neopentylidene ligand is T-shaped with a large Ta]CdC bond angle (161.2 ), a Ta]CdH angle of less than 90 (84.8 ), a slightly shortened M]Ca bond (1.898 A˚ ), and a relatively long CHa bond (1.131 A˚ );270 all are characteristic of an agostic interaction between the electron poor metal and the electrons in the CHa bond of the alkylidene. Ta(CH-t-Bu)(CH2-t-Bu)3 can be induced to lose neopentane through addition of PMe3 to yield bisneopentylidene complexes, Ta(CH-t-Bu)2(CH2-t-Bu)(PMe3)2, in which the inequivalent neopentylidene ligands (one being considerably more distorted than the other) are found in equatorial positions in a trigonal bipyramid (Eq. 8).271,272 Bisalkylidenes are rare for Mo and W because the alternative M(C-t-Bu)(CH2-t-Bu) complexes almost always are lower in energy (see later).129,273,274 Conversely, Ta(C-t-Bu)(CH2-t-Bu)2(PMe3)2 has never been observed, although it could still be the first species formed when PMe3 is added to Ta(CH-t-Bu)(CH2-t-Bu)3.

ð8Þ

Tantalum alkylidenes were the first to be studied in depth and led to clues as to what type of Mo or W complex would be active in metathesis reactions. Some initial studies involved monocyclopentadienyl complexes such as CpTa(CH-t-Bu)Cl2.275,276 If the Cp (Z5-C5H−5) ligand is viewed as a 6e donor ligand then (Z5-C5H5)Ta(CH-t-Bu)Cl2 has a 14e count and a pseudotetrahedral geometry. CpTa(CH-t-Bu)Cl2 reacts readily with olefins, but the unobservable metallacyclobutane intermediates rearrange through b hydrogen elimination reactions to give CpTa(olefin)Cl2 complexes277,278 and metallacyclopentane complexes (see later) through addition of a second olefin to the olefin complex. A lot was learned, but had it been noticed that a hypothetical 14e tetrahedral W(NR)(CHR0 )X2 complex, where (NR)2− is a 6e ligand and X is a monodentate monoanionic ligand, is a next door 14e cousin of CpTa(CH-t-Bu)Cl2, the route to Mo- and W-based metathesis catalysts might have been much shorter than it turned out to be. In some brief studies279 it was found that tantalum could be coaxed into metathesizing olefins when alkoxides are coordinated to the metal instead of chlorides, and the alkoxides are big enough to discourage bimolecular decomposition reactions. Many alkoxides are more electron-donating than chlorides in both a s and p sense, although both are variable in an alkoxide and can

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

691

change significantly on the basis of the nature of the R group in an OR ligand. (It is worth noting that the pKa of t-butanol is 19 in water, while the pKa of (CF3)3COH is 5.4.280) The p donating ability of alkoxides depends upon the MdOdC angle at the a oxygen, and that angle depends upon interligand steric interactions within the complex, the nature of the alkoxide’s R group, and the symmetry of orbitals on the metal that are used to form an orbital that can accept nonbonding electrons on oxygen. p Donation can be attenuated for ligand conformational reasons, e.g., in bidentate bisalkoxides, tridentate trisalkoxides, siloxides, pincer ligands (see Section 5.11.4), etc. In early studies of tantalum chemistry it was postulated that an alkylidene could rearrange to an alkene, e.g., ethylidene to ethylene, through abstraction of a b proton and delivery or migration of it to the a carbon. However, only recently has an alkylidene been shown to rearrange to an olefin in Mo(VI) or W(VI) chemistry that is relevant to olefin metathesis (see Section 5.11.2.2.9). Rearrangement of an olefin to an alkylidene, was demonstrated for Nb and Ta281,282 some time ago.

5.11.2.2 5.11.2.2.1

Tungsten and molybdenum alkylidenes Discovery

The discovery of Ta(CH-t-Bu)(CH2-t-Bu)3 begged the question as to what an attempted “peralkylation” of W(VI) starting materials with neopentyl reagents would produce, if reduction of the metal is either avoided or if disproportionation of lower oxidation state complexes allows some of the metal to be reoxidized? The answer is W(C-t-Bu)(CH2-t-Bu)3,283 a volatile, yellow, thermally stable compound whose steric environment and physical properties are essentially the same as for Ta(CH-t-Bu)(CH2-t-Bu)3. W(C-t-Bu)(CH2-t-Bu)3 and derivatives of it provided invaluable information, including a route to imido alkylidenes (and recently oxo alkylidenes; see Section 5.11.2.2.6). A full discussion of alkylidyne complexes and alkyne metathesis catalysts can be found in Section 5.11.3.

ð9Þ

With knowledge in hand concerning the apparent value of alkoxides for olefin metathesis by tantalum compounds,279 a Wittig-like reaction between W(O)(O-t-Bu)4 and Ta(CH-t-Bu)Cl3(PMe3)2 to give (still unknown) W(CH-t-Bu)(O-t-Bu)4 was attempted. After a series of successive intermetallic ligand transfers that were not elucidated, the end result was formation of Ta(O-t-Bu)4Cl and W(O) (CH-t-Bu)(PMe3)2Cl2 in high yield (Eq. 9).284 W(O)(CH-t-Bu)(PMe3)2Cl2 is an 18e complex in which the t-Bu group of the neopentylidene points toward the oxo ligand (the syn isomer) as opposed to away from the oxo ligand (the anti isomer). The two possible isomers result from the fact that dxy, dxz, and dyz orbitals are used to make a total of three p bonds (two to O and one to C). In the presence of 5% AlCl3, W(O)(CH-t-Bu)(PMe3)2Cl2 catalyzes metathesis reactions. New W(O)(CHR)(PMe3)2Cl2 complexes were isolated in the presence of RCH]CH2; even W(O)(CH2)(PMe3)2Cl2 was formed in the presence of ethylene and could be isolated. These experiments were the first to demonstrate that an oxo bound through a triple bond to the metal was compatible with metathesis reactions and perhaps required in order to prevent ready formation of a W^C bond. The role of AlCl3 in these reactions was never fully elucidated. The original intent was for AlCl3 to scavenge any dissociated phosphine and produce a < 18e complex, e.g., W(O)(CHR)(PMe3)Cl2.285 However, AlCl3 could also remove a chloride, or bind to the oxo ligand, or even, as Osborn showed later,286,287 remove the oxo ligand and replace it with two halides to yield “oxo-free” alkylidene complexes such as W(cyclopentylidene)(OCH2-t-Bu)2Br2. (Alkylidenes of this type will be revisited in Section 5.11.2.2.10). But oxo alkylidene chemistry was not pursued because, it was feared, oxo M]CHR complexes would decompose bimolecularly too readily because of a lack of steric hindrance. An oxo ligand also cannot be varied sterically or electronically and can bridge readily between metals. For these reasons attention was directed toward synthesizing imido alkylidene complexes.

5.11.2.2.2

Imido alkylidenes

In 1982 it was found that tungsten oxo neopentylidene complexes could be prepared through addition of water to a neopentylidyne complex in the presence of triethylamine (Eq. 10).288 2,6-Diisopropylphenylamido (NHAr) neopentylidyne complexes could also be prepared and the a proton on the amido N could be moved to the alkylidyne C with a catalytic amount of NEt3 to give an imido neopentylidene complex. Because Et3N is not a strong 2e donor, this is not likely to be a reaction induced by coordination of triethylamine. Instead, triethylamine most likely deprotonates the amido ligand to give an anionic imido alkylidyne intermediate and [Et3NH]+, which delivers the proton to the alkylidyne a carbon atom. With W(NAr)(CH-t-Bu)(DME)Cl2 complexes from the catalytic NEt3 method in hand, W(CH-t-Bu)(NAr)(OR)2 complexes could be prepared in which OR ¼ O-t-Bu, OCMe2(CF3), OCMe(CF3)2, and OC(CF3)2(CF2CF2CF3).289,290

ð10Þ

692

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

In view of the low and unpredictable yield of Mo(C-t-Bu)(CH2-t-Bu)3283 the “triflic acid” method of preparing Mo(NAr)(CH-t-Bu) (OR)2 complexes was developed (Eq. 11). The triflic acid method relies on conversion of molybdates to Mo(NAr)2(DME)Cl2 and subsequently to Mo(NAr)2(CH2-t-Bu)2,291 and then conversion of Mo(NAr)2(CH2-t-Bu)2 into Mo(NAr)(CH-t-Bu)(OTf )2(DME) through addition of 3 equiv. of triflic acid; one amido nitrogen is protonated three times to give the proposed five-coordinate Mo(NAr)(CH2-t-Bu)2(OTf )2 followed by a abstraction and coordination of DME or vice versa. The triflic acid route was shown to be successful for a variety of imido variations292 (initially 1-adamantyl, 2,6-Me2C6H3, 3,5-Me2C6H3, 2,6-i-Pr2C6H3, 4-Br-2,6i-Pr2C6H2, 2-i-PrC6H4, 2-CF3C6H4, 2-t-BuC6H4, 2-i-PrC6H4, or 2-PhC6H4) and for making neophylidene (M]CHCMe2Ph) complexes. Neophyl halides are cheaper than neopentyl halides, but one disadvantage of neophyl is that the t-butylbenzene product formed through proton abstraction is not as easily removed as is neopentane. The triflic acid method also worked well to make tungsten imido alkylidene complexes. A pernicious problem with the triflic acid route is that it can be difficult to remove all traces of the ammonium triflate product, but high yields of complexes prepared from a bistriflate can be ensured through adequate purification of the bistriflate through recrystallization. Some of the imido ligands that have been used are shown in Fig. 1. Substitution of HCl or pyridinium chloride for triflic acid is possible in some variations of the synthesis shown in Eq. (11). For example, routes to tungsten alkylidene complexes that contain t-butylimido or adamantylimido ligands were devised that begin with a reaction between WCl6 and 4 equiv. of HNR(TMS) to give [W(NR)2Cl(m-Cl)(RNH2)]2 (R ] t-Bu or 1-adamantyl) complexes.38 Alkylation of [W(NR)2Cl(m-Cl)(RNH2)]2 leads to W(NR)2(CH2R0 )2 (R0 ] t-Bu or CMe2Ph), which upon treatment with pyridinium chloride yields W(NR)(CHR0 )Cl2py2 complexes (Eq. 12). HCl is considerably cheaper than triflic acid, and also provides more direct access to monophenoxide monochloride (MAC) complexes (see later).

ð11Þ

ð12Þ

The a abstraction reaction for making alkylidenes has not been explored when one or more b protons is/are present in the alkyls, because of a presumed preference for abstracting a b proton. Whether the assumption that b abstraction to give olefins (and reduction of the metal) is faster than a abstraction to give terminal alkylidenes is true or not has not been explored in depth from today’s perspective, nor have isomers of known terminal alkylidene complexes, namely disubstituted alkylidenes (see Section 5.11.2.2.9). (Rare examples of reactions in which b abstraction or elimination reactions (formation of olefin hydrides) are faster than a reactions

Fig. 1 Imido ligands appearing in imido alkylidene complexes.

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

693

are known, but not for Mo and W in circumstances that are relevant to metathesis chemistry.293,294) An exception to the first statement above is when the alkyl is 2-adamantyl. In this case b abstraction would give a high energy C]C bond within an adamantyl cage, which is not observed in adamantyl chemistry, so a abstraction takes place cleanly and gives high yields of 2-adamantylidene complexes.134 2-Adamantylidene and other disubstituted alkylidenes will be discussed later in this article. One obvious and important difference between internal (M]CRR0 ) and terminal (M]CHR) complexes is that an alkylidyne cannot be formed from M]CRR0 through a proton abstraction if neither R nor R0 is H. Disubstituted alkylidenes are required for rare metathesis reactions that give tetrasubstituted olefins. The triflic acid route replaced a route that had been developed for making imido neopentylidene complexes of tungsten that involved addition of PCl5 to W(NAr)(O-t-Bu)2(CH2-t-Bu)2. W(NAr)(O-t-Bu)2(CH2-t-Bu)2 is a rare example of an imido bisalkoxide dineopentyl complex that can be made readily in good yield (Scheme 1) and that does not undergo a abstraction. As shown in Eq. (13), it is proposed that PCl5 attacks a t-butoxide ligand in W(NAr)(O-t-Bu)2(CH2-t-Bu)2 to give W(NAr)(O-t-Bu)(Cl) (CH2-t-Bu)2 and t-BuOPCl4, which decomposes to isobutene, HCl, and P(O)Cl3; HCl protonates the second t-butoxide to yield the (proposed) intermediate W(NAr)(CH2-t-Bu)2Cl2,295 which does undergo a hydrogen abstraction to generate the alkylidene in the presence of DME. This “PCl5 method” has been shown to provide access to W(NR)(CHCMe2Ph)(DME)Cl2 complexes (where R ¼ ArMe2, ArCl2, or Ar3,5Me2);45 the bistriflate analogs of these dichloride complexes are available via the triflic acid route, but dichlorides are required for making useful monochloride monoaryloxide (MAC) complexes (see later). No similar PCl5 route for molybdenum has been reported.

ð13Þ

Scheme 1 Synthesis of a five-coordinate dineopentyl tungsten imido complex.

Mo(NCPh3)(CH-t-Bu)(CH2-t-Bu)2 was prepared via an “azide route” in which the imido group is installed in a reaction between MoCl4(THF)2 and Ph3CN3 to give Mo(NCPh3)Cl4(THF).126 Mo(NCPh3)(CH-t-Bu)(CH2-t-Bu)(OR) complexes were prepared where OR ¼ O-3,5-Trip2C6H3, OArMe2, and OHIPT, but there seemed to be little promise that catalysts based on the Mo] NCPh3 scaffold could compete with other imido-containing catalysts. Other imido groups that were explored include 1-phenylcyclohexylimido (NPhCy) and 2-phenyl-2-adamantylimido;31 the few catalysts that contain them also did not reveal any obvious advantages over other imido possibilities. Some of the most challenging phenylimido alkylidene complexes that have been made are those in which the phenyl ring is substituted in the 2 and 6 positions with sterically demanding 2,4,6-R3C6H2 groups, where R is methyl (ArMes2) or isopropyl (ArTrip2). The required bisimido dichloride complex, Mo(NArMes2)2(DME)Cl2, could not be prepared, probably for steric reasons. The solution was to make an alkylidene precursor that contains one NArMes2 group and one N-t-Bu group, namely Mo(NArMes2) (N-t-Bu)(CH2CMe2Ph)2, as shown in Scheme 2a.24 The t-BuN group is then protonated selectively to give the desired imido alkylidene. Tungsten analogs were synthesized through similar approaches.40 As a consequence of the steric demand of the NArMes2 ligand approximately a 1:1 mixture of syn and anti isomers was observed instead of the usual 100% syn isomer. The imido ligand that contains the NArMes2 group is significantly more sterically demanding near the metal than the hexamethyl terphenoxide ligand (OHMT) to be described later because what is essentially a triple M]N bond (1.75 A˚ ) is significantly shorter than the single MdO bond (2.0 A˚ ). Some variation of the azide route just discussed above may be an option to prepare complexes of these sterically demanding monoimido ligands. Several tungsten methylene and unsubstituted metallacyclobutane complexes were prepared that contain the NArMes2 ligand including W(NArMes2)(CH2)(Me2Pyr)(OSiPh3) and W(NArMes2)(C3H6)(Me2Pyr)(OSiPh3) (a metallacyclobutane complex). W(NArMes2)(CH2)(Me2Pyr)(OSiPh3) reacted with 5,6-dicarbomethoxynorbornadiene (DCMNBD) to give the monoinsertion product, which rearranged to the form in which the cyclopentene double bond is conjugated with the double bond in the CH] CH2 group in the presence of triethylamine, as was proven through an X-ray study of the acetonitrile adduct (Eq. 14).96 This type of rearrangement has not been shown to compete with polymerization in reported polymerizations of DCMNBD so far.226

694

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

ð14Þ

Mo(N-t-Bu)2Cl2(dme)

Mo - LiCl

NEt3 cat

t-BuN

N-t-Bu

+ LiNHArMes2 t-BuN Cl

NHArMes2

Mo Cl

+ 2 RCH2MgCl

Mo Cl

NArMes2

t-BuN

CH2R Mo

- 2 MgCl2 - t-BuNH2

NH2-t-Bu

NH-t-Bu + 2,6-LutHCl t-BuN

ArMes2N

+ 3 pyHCl

Cl NArMes2

Mo(NArMes2)(CHR)Cl2(py)

CH2R - t-BuNH3Cl - MeR

(R = CMe2Ph)

Scheme 2a Syntheses of MoNArMes2 complexes.

Mo and W analogs that contain the imido analog, NArTrip2, were prepared in a manner similar to that just described for NArMes2.23 Largely anti isomers were observed. In the end the impressive steric demands of NArMes2 and NArTrip2, the predominance of the normally disfavored anti isomers, and a solution to the Z selectivity problem with bulky terphenoxides (see later) led to a loss of interest in exploring them further as metathesis initiators. An X-ray structure of W(NHIPT)(CH-t-Bu)Cl2 (Fig. 2) showed it to contain a disordered alkylidene that is 86% in the anti configuration and 14% in the syn configuration. No other monomeric four-coordinate imido alkylidene dichloride has been reported to our knowledge. Imido alkylidene compounds where CH-t-Bu or CHCMe2Ph is the alkylidene constitute a majority of those known. Early studies of Mo and W alkylidenes showed that syn isomers are usually lowest in energy because of a CHa agostic interaction, which manifests itself in a lower value for JCH in the syn vs the anti isomer.241,296,297 The agostic interaction lowers the energy for the more sterically demanding situation in which the substituent points toward the imido ligand in 14e complexes. Anti isomers can be favored in monoadducts or bisadducts or when the imido group is exceedingly sterically demanding, as just discussed (see Fig. 2). The rate of interconversion of syn and anti isomers can vary by six or seven orders of magnitude, being the fastest when X and/or Y are relatively good electron donors (e.g., fast for O-t-Bu but slow for OCMe(CF3)2). Interconversion of the alkylidene isomers appears to be possible only in 14e complexes, not 16e (monoadducts) or 18e (bisadducts). When X and Y are good s and especially good p donors, then the rate of M]C rotation may be fast, but the rate of reaction with olefins suffers to a significant degree. An example is

Fig. 2 The X-ray structure of W(NHIPT)(CH-t-Bu)Cl2.

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

695

Fig. 3 An example of a tethered imido alkylidene complex.

Mo(NAr)(CH-t-Bu)(NPh2)2 in which the diphenyl amido group is sterically demanding, a good p donor, and orbitals on the metal are available for both NPh2 ligands to form M]N double bonds and a planar amido nitrogen; Mo(NAr)(CH-t-Bu) (NPh2)2 therefore can be regarded as an 18e complex if all p interactions are included.57 Thiolato analogs of alkoxide complexes have also been found to be poor metathesis catalysts.64,298 There has been some interest in tethering an alkylidene to the imido ligand, especially for applications in ROMP chemistry. This has been achieved, although it is challenging synthetically. The phenylimido complex, Mo(NC6H4-2-CH2CH2CMe2CH)(DME) (OTf )2 is shown in Fig. 3 (R ¼ H).33 A related, sterically more protected version in which R ¼ i-Pr and the triflates are cis to one another, Mo(N-2,4-i-Pr2C6H2-2-CH2CH2CMe2CH)(DME)(OTf )2, was also prepared.34 Mo(N-2,4-i-Pr2C6H2-2-CH2CH2CMe2CH) (quin)(ORF6)2 (quin ¼ quinuclidine) and Mo(N-2,4-i-Pr2-6-MeC6H2)(Me3CCH)(quin)(ORF6)2 (in which the tether was replaced with a methyl group) were also prepared and structurally characterized. Structures and NMR characteristics were not changed to any significant degree in complexes that contain a tether between the imido group and the alkylidene compared to complexes that do not contain a tether.

5.11.2.2.2.1 Bisalkoxides M(NR)(CH-t-Bu)(OR0 )2 and M(NR)(CHCMe2Ph)(OR0 )2 complexes, where OR0 is relatively sterically demanding and can be varied widely electronically and sterically, have been the preferred metathesis initiators if Z-selectivity is not required. They are usually prepared through nucleophilic attack on a bistriflate or dichloride derivative with a Li+, Na+, or K+ salt of the alkoxide.20 The step-wise replacement of chlorides or triflates with alkoxide salts is usually relatively unselective, with a monoalkoxide not being formed in high yield unless it is too sterically demanding to form the bisalkoxide readily. Attempts to make bisalkoxides through addition of lithium salts of smaller alkoxides (e.g., LiOCH(CF3)2) can lead to retention of 1 equiv. of the lithium salt in the product, i.e., formation of an “ate” complex.299 Much has been learned through varying both the sterics and electronics of the alkoxide, aryloxide, or siloxide, with the t-butoxide series (O-t-Bu, OCMe2(CF3), OCMe(CF3)2, and OC(CF3)3) standing out as one in which primarily the electronic factors can be addressed. Another alkylidene derivative usually can be prepared through addition of either ethylene (in order to prepare a methylene or a metallacylobutane complex) or a terminal olefin (in order to prepare a some other CHR complex) to a neopentylidene or neophylidene complex. A neopentylidene complex is the better choice as a starting material for this type of reaction, because relatively volatile t-BuCH]CH2 can be removed (sometimes in several cycles) to drive the reaction to completion and prevent back reaction to give the starting alkylidene. A potential complication in the synthesis of some alkoxide derivatives is that the nucleophile can remove the a proton in the M]CHR complex to yield imido alkylidyne complexes (see later section) instead of substituting a chloride or triflate on the metal. M(NR)(CHR0 )(OR)2 complexes have been prepared that do not react readily with olefins. One example is Mo(NAd) (CHCMe2Ph)[OSi(t-Bu)3]2 (Ad ¼ 1-adamantyl),67 which does not react with 5 atm of ethylene at 125  C. Oxo complexes will be discussed later, but it can be noted now that the small size of an oxo vs an imido ligand allow W(O)(CH-t-Bu)(OHMT)2 and W(O)(CH-t-Bu)(OdAd)2 (dAd ¼ 2,6-(1-adamantyl)2-4-MeC6H2) to be prepared, but they are poor initiators in general for steric reasons. The more sterically demanding the alkoxide the less likely the required TBP metallacyclobutane complex can form in which the two alkoxides are 90 to one another in axial and equatorial positions. (See the metallacyclobutane section below.) In the mid 1990s the synthesis of biphenoxide complexes drew attention with the discovery that chiral biphenoxides could be used as initiators for the polymerization of norbornenes and norbornadienes in a cis,isotactic manner (see below).300 This result raised questions as to what is necessary for making cis products selectively in the metathesis of ordinary olefins (the “cis problem”),

696

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

what asymmetric metathesis reactions can be carried out with an enantiomerically pure biphenoxide complex, and what other polymers can be prepared through ring-opening metathesis polymerization (ROMP) reactions that have a single structure. (Studies of ROMP reactions with classical catalysts301 dominated the early literature, although stereoselective polymerizations were virtually unknown at that time.) Some biphenoxides were known in the literature and those that could be made more sterically demanding through substitution in the 3 and 30 positions became attractive ligands to explore. Their bidentate nature perhaps also limits their ability to donate p electron density to the metal efficiently. It has been shown that 2,20 -bipyridyl or 1,10-phenanthroline adducts of imido alkylidene complexes yield 18e complexes that are relatively resistant to air and moisture.72 They can be activated under dinitrogen in solution through addition of some Lewis acid, e.g., MnCl2 or ZnCl2, in order to remove bipy or phen from solution.72 The bidentate bipy or phen ligand must be somewhat labile in order to be scavenged by the Lewis acid. More recent studies have shown that some bipy adducts spontaneously lose bipy, especially in dilute solution and/or at elevated temperatures, to yield the catalytically active 14e initiators.73 Therefore easily isolated bipy adducts that can be isolated in high yield and stored in air become activators once dissolved. 5.11.2.2.2.2 Biphenoxides A number of biphenoxides (Fig. 4) have been used to make rac and enantiomerically pure imido alkylidene complexes. The biphenoxide should have large substituents in the 3 and 30 positions (e.g., t-butyls) in order to prevent bimolecular alkylidene coupling, formation of ate complexes, etc. Biphenoxide derivatives are usually prepared from a lithium, sodium, or potassium salt of the biphenoxide in question or through addition of the biphenol to a bispyrrolide (see Section 5.11.2.2.2.3). Many biphenoxide complexes are five-coordinate as a consequence of binding a solvent (e.g., THF), but the solvent must dissociate readily in solution to yield an active 14e metathesis catalyst. Many of the ligands shown in Fig. 4 are chiral and C2-symmetric. Rac complexes were usually explored as initiators for ROMP reactions, because the same polymer structure is generated from each enantiomer of the initiator. Much was learned about mechanisms of metathesis from studies of biphenoxide complexes, among them the finding that unsubstituted metallacyclobutanes and metallacyclopentanes are fluxional on the NMR time scale (see later sections). The ROMP studies are summarized in a review in 2014,226 but some of the details of the mechanism of ROMP are covered here because of their importance in understanding metathesis in general. Complexes that contain the TMS2Biphen and Me2SiBiphen ligands76 were prepared in order to connect them to polystyrene.302 Preliminary studies suggest that activities and efficiencies in metathesis reactions of the polystyrene-bound initiators are comparable to the homogeneous versions of these initiators.

Fig. 4 Some biphenoxides used in metathesis catalysts.

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

697

Enantiomerically pure biphenoxide complexes are required for asymmetric organic transformations. A review appeared in 2009,228 but many papers concerned with asymmetric transformations were published after this review.31,302–312 A typical catalytic desymmetrization initiated by (S)-W(NAr)(CHCMe2Ph)(Biphen) is shown in Eq. (15). Similar desymmetrizations have been carried out with molybdenum catalysts and planar-chiral ferrocenylphosphines and molecules related thereby synthesized.313

ð15Þ

ð16Þ

Substituents in the 3 and 30 positions of a biphenoxide allow efficient discrimination of the two faces of the M]C bond. In the right circumstance one of the faces of the M]C bond is sterically blocked in the initial and subsequently formed alkylidenes in a ROMP reaction with a chiral biphenoxide complex. In 1993 it was shown300 that the substrate approaches the syn form of the initial and any subsequent alkylidene to give an all cis metallacycle (Eq. 16 for norbornene itself ). Formation of this metallacycle is essentially irreversible for a highly strained norbornene. The metallacycle then opens to yield the insertion product and a new syn alkylidene with a cis C]C bond. Repetition of the process leads to a polymer that contains cis,isotactic dyads (repeating units in the polymer).

+

O

W(NAr)(CHCMe2Ph)(Biphen)

ð17Þ - CH2=CH2

- CH2=CHCMe2Ph

Rac- and (S)-W(NAr)(CHCMe2Ph)(Biphen) and rac- and (S)-W(NArMe2)(CHCMe2Ph)(Biphen) were prepared and mechanistic studies of the asymmetric desymmetrization of the substrate shown in Eq. (15) were carried out.41 A stoichiometric reaction between (S)-W(NArMe2)(CHCMe2Ph)(Biphen) and one equiv. of the substrate led to isolation and structural characterization of a b-substituted metallacylobutane complex formed from (S)-W(NArMe2)(CH2)(Biphen) and CH2]CHCMe2Ph and a b,b-disubstituted metallacyclobutane product derived from the substrate in which the furan oxygen is coordinated to the metal (Eq. 17). Studies of reactions between W(NR)(CHCMe2Ph)(Biphen) (R ¼ Ar or ArMe2) complexes and 13CH2]13CH2 allowed observation of 13C-labeled complexes (no 13C labels shown) that include W(NR)[CH(CMe2Ph)CH2CH2](Biphen), W(NR) [CH2CH(CMe2Ph)CH2](Biphen), W(NR)(CH2CH2CH2)(Biphen), W(NR)(CH2]CH2)(Biphen), and W(NR)(CH2CH2CH2CH2) (Biphen) (a metallacyclopentane). Most importantly, and unexpectedly, from a rac starting material a highly unusual dimeric methylene complex, heterochiral [W(NR)(m-CH2)(Biphen)]2, was observed in NMR studies. The chemical shift of the methylene (185.9 ppm) in [W(NAr)(m-13CH2)(R-Biphen)][W(NAr)(m-13CH2)(S-Biphen)] is less than a typical terminal methylene (>200 ppm) and the inequivalent methylene protons have JCH values of 131 and 148 Hz. The methylenes must be bridging unsymmetrically because two sets of tungsten satellites were observed for the m-13C resonance with JCW ¼ 78.9 and 36.7 Hz. Effectively, one methylene is acting as a donor (exact details unknown) toward the other tungsten center. (An example of a structurally-characterized dimeric methylene complex can be found in Section 5.11.2.2.4.) Methylenes must be able to couple in some variation of this dimeric complex to give ethylene. (S)-W(NR)(CH2CH2CH2)(Biphen) has a TBP structure at low temperature and the six metallacycle proton resonances merge at higher temperatures to give three resonances as a consequence of the metallacycle “flipping over” through a pseudorotation process, but loss and gain of ethylene is not part of the process. This result

698

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

clearly establishes that an unsubstituted metallacyclobutane complex can be fluxional at the metal center on the NMR scale. W(NR) (CH2CH2CH2CH2)(Biphen) was observed and found to slowly produce 1-butene catalytically under ethylene through a b hydride elimination reaction. Reactions between molybdenum imido alkylidene biphenoxide complexes and 13CH2]13CH2 were explored further for a variety of Mo biphenoxide catalysts.140 In NMR studies a-substituted molybdacyclobutanes Mo(NR)(∗CH∗ 2 CH2CHCMe2Ph) ∗ (Biphenoxide) (∗C ] 13C), unsubstituted molybdacyclobutanes Mo(NR)(∗CH∗ 2 CH2 CH2)(Biphenoxide), olefin complexes ∗ ∗ Mo(NR)(∗CH2]CHR)(Biphenoxide) (R ¼ H or CMe2Ph), molybdacyclopentane complexes Mo(NR)(∗CH∗ 2 CH2 CH2 CH2) (Biphenoxide), and base-free methylene complexes Mo(NR)(∗CH2)(Biphenoxide), were observed. The methylene complexes Mo(NAr)( CH2)(Biphen) (dC ¼ 253.5 ppm, JCH ¼ 134 and 163 Hz), Mo(NAd)( CH2)(Biphen) (d∗C ¼ 253.7 ppm, JCH ¼ 137 and 157 Hz), and Mo(NAr)( CH2)(Benz2Bitet)(THF) (d∗C ¼ 275.3 ppm, JCH ¼ 140 and 161 Hz) have chemical shifts for the methylene carbon atoms that are more downfield than found for [W(NAr)(m-∗CH2)(R-Biphen)][W(NAr)(m-13CH2)(S-Biphen)] (d∗C ¼ 185.9 ppm; JCH ¼ 131 and 148 Hz), consistent with these methylene ligands not bridging between two tungsten centers; the methylene carbon in each is therefore coupled only to one tungsten atom. These studies provided invaluable mechanistic information that supports the accepted mechanism of reactions of neophylidene complexes with ethylene, but the only compound that was isolated and crystallographically characterized in these particular studies was the ethylene complex, Mo(NArCl)(CH2] CH2)(rac-Biphen)(Et2O). 5.11.2.2.2.3 Pyrrolides Applications of asymmetric metathesis to organic chemistry stimulated a search for complexes that could be turned into bisalkoxides or biphenoxides in situ through addition of an alcohol in order to relatively quickly form and evaluate metathesis initiators in various organic reactions of interest. Imido dineopentyl neopentylidene complexes at one point were explored as precursors to mono or bisalkoxide complexes through protonation of one or both neopentyl groups.42,74,124–127 However, protonation of the neopentylidene ligand competed with protonation of a neopentyl carbon atom in many situations and led to complexes that then depended on a hydrogen abstraction again in order to generate an alkylidene. The additional possible complication of forming an alkylidyne complex through loss of the alkylidene a proton led to a loss of interest in dineopentyl complexes as potential metathesis catalyst precursors. Diphenylamido and related amido ligands were also explored as the more logical ligands to protonate an alcohol to give an alkoxide. Diphenylamido ligands are relatively sterically demanding and their p electrons are donated to the metal to give pseudo 18e complexes, but they nevertheless can be protonated with several alcohols according to initial results.57 (Other substituted amido ligands also have proven to be valuable precursors in the synthesis of trialkoxide alkylidyne complexes, as discussed later in this article.) The search for precursors that could be protonated by alcohols to give bisalkoxides culminated with the synthesis of bispyrrolide complexes, first of Mo,48,50 and then of W.59 At the time, pyrrolide complexes of either metal were virtually unknown. The pyrrolide ligand ([NC4H4]− ] Pyr), is the aza analog of the cyclopentadienyl ligand ([C5H5]− ] Cp), and therefore a 6e donor when it binds to a metal to give a [Z5-NC4H4]− complex. Unlike a Cp ligand, it also binds readily in an Z1 fashion (through N) and the Z1 and Z5 forms have been found to interconvert readily often on the NMR time scale at 22  C in bispyrrolide complexes. A pyrrolide is a relatively poor donor of p electrons to the metal. The most readily available pyrroles are pyrrole itself (to give Pyr complexes) and 2,5-dimethyl pyrrole (to give Me2Pyr complexes).50 An X-ray structural study of Mo(NAd)(CHCMe2Ph)(Me2Pyr)2 (Fig. 5), for example, reveals that it contains one

Fig. 5 The structure of Mo(NAd)(CHCMe2Ph)(Me2Pyr)2.

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

699

Z1-Me2Pyr and one Z5-Me2Pyr; NMR studies show that the two interconvert on the NMR time scale at room temperature. The structures of Mo(NAr)(CHR)(Pyrrolide)2 complexes that contain two 2,3,4,5-tetramethylpyrrolides, 2,5-diisopropylpyrrolides, or 2,5-diphenylpyrrolides are analogous to the structure shown in Fig. 5. In contrast, Mo(NAr)(CH2CMe2Ph)(indolide)2 contains two Z1-bound indolides, and 2-mesitylpyrrolide binds only in the Z1 form in the situations documented so far, presumably because binding in an Z5 manner is blocked by the mesityl group.53 In general, the range of pyrrolides in MAP chemistry has been underexplored, with pyrrolide or 2,6-dimethylpyrrolide being the two of choice so far. In 2006 it was shown that monoalcohols and biphenols would cleanly protonate bispyrrolides to yield the bisalkoxide or biphenoxide.48 Therefore, bisalkoxides or biphenoxides could be prepared in situ for evaluation as metathesis initiators, or the initiators could be isolated, if desired. For example, in the process of evaluating many Mo and W compounds for the alkane metathesis reaction,66 bis-2,5-dimethylpyrrolide complexes were used as precursors. In situ methods that consist of protonation of bispyrrolides are now used routinely to rapidly evaluate the efficacy of catalysts in organic applications.21,314–319 The reaction between W(NArCl2)(CH-t-Bu)(Me2Pyr)2 and [PhMe2NH][BArF4] in dichloromethane resulted in protonation of one dimethylpyrrolide to give the 18e complex, [W(NArCl2)(CH-t-Bu)(Z5-Me2Pyr)(NC4(2,3,4-H3)(2,5-Me2)][BArF4], in good yield.59 An X-ray structural study (Fig. 6) confirmed that this cationic complex contains a pyrrolenine ligand formed through protonation of one of the 2,5-dimethylpyrrolide ligands on a carbon next to the pyrrolide nitrogen (C22 or C25 in Fig. 6); the remaining dimethylpyrrolide ligand is bound in an Z5 manner. (Protonation of C23 or C24 in Mo(2,5-Dimethylpyrrolide)3 has also been observed.320) The proton NMR resonance for H25 is found at 4.48 ppm. Upon dissolution of [W(NArCl2)(CH-t-Bu)(Z5-Me2Pyr) (NC4(2,3,4-H3)(2,5-Me2))][BArF4] in THF-d8 the pyrrolenine is replaced to give [W(NArCl2)(CH-t-Bu)(Z5-Me2Pyr)(THF)][BArF4]. The unobserved free pyrrolenine then rearranges rapidly to the much more stable (by 20 kcal for pyrrole itself in one study321) 2,5-dimethylpyrrole. Two key features of the method are that a pyrrole is relatively inconsequential in any further chemistry and the loss of ligand through deprotonation therefore is irreversible. The reaction between W(NAr)(CH-t-Bu)(Me2Pyr)2 and [PhMe2NH] [BArF4] in dichloromethane yielded analogous results. The reaction between W(NArCl2)(CH-t-Bu)(Me2Pyr)2 and ethylene is rapid and produced the 18e complex, W(NArCl2)(CH2)(Me2Pyr)(Z5-Me2Pyr), while the reaction between W(NArCl2)(CH-t-Bu)(Pyr)2 and ethylene resulted in decomposition of intermediate W(NArCl)(CH2)(Pyr)2 to give ethylene and [W(m-NArCl)(Pyr)2]2.

ð18Þ

Addition of 2,20 -bipyridyl (bipy) to Mo(NAr)(CHCMe2Ph)(Pyr)2 leads to formation of the relatively insoluble adduct, Mo(NAr) (CHCMe2Ph)(Pyr)2(bipy). However, an attempt to prepare the analogous bipy adduct of Mo(NAr)(CHCMe2Ph)(Me2Pyr)2 led to formation of the imido alkylidyne complex, Mo(NAr)(CCMe2Ph)(Z1-Me2Pyr)(bipy) (Eq. 18) and dimethylpyrrole.25 In Mo(NAr) (CCMe2Ph)(Me2Pyr)(bipy) Mo^C ] 1.764(3) A˚ , Mo^CdC ] 161.5(2) , Mo^ N ¼1.804(3) A˚ , and Mo^NdCipso ¼ 159.6 (2) ; these distances and angles are characteristic of slightly bent Mo^CdR and Mo^NdR ligands. Reactions of this type are proposed to result from a ligand-induced migration of an alkylidene a proton from the alkylidene to a dimethylpyrrolide ligand.

Fig. 6 The structure of [W(NArCl2)(CH-t-Bu)(Z5-Me2Pyr)(NC4(2,3,4-H3)(2,5-Me2)]+.

700

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

The mechanism could be more complex with bipyridyl removing the alkylidene proton and delivering it to the pyrrolide. The main message is that deprotonation of a M]CHR complex is a potential complication when any substitution at the metal is attempted with a nucleophile that can remove a proton from the alkylidene. Therefore, deprotonation of the alkylidene could play a more significant role in bimolecular decomposition of M]CHR complexes than generally believed, especially when the proton ends up on a ligand, possibly on another metal, that is then lost from the coordination sphere. Proton migrations or transfers from a M]CHR ligand to N- or O-based ligands may be a more significant problem than generally acknowledged. 5.11.2.2.2.4 Monoaryloxide (or monoalkoxide) pyrrolide (MAP) complexes Bispyrrolide complexes are protonated readily by a wide variety of alcohols to give bisalkoxide derivatives or monoalkoxide pyrrolide (MAP) complexes in which OR is O-t-Bu, O-i-Pr, OAr, OCH(CF3)2, or OCMe(CF3)2 (Eq. 19).92 When the OR group in ROH is sterically demanding (e.g., HMTO or HIPTO) protonation of the second pyrrolide is relatively slow. The pyrrolide is bound in an Z1 fashion in Mo(NAr)(CHCMe2Ph)(Me2Pyr)(OR) derivatives if OR is relatively sterically demanding. The reaction between Mo(NAr)(CHCMe2Ph)(Me2Pyr) and HOBMes2 to give Mo(NAr)(CHCMe2Ph)(Me2Pyr)(OBMes2) has also been reported.93

ð19Þ

An important feature of MAP complexes is that the metal is a stereogenic center. Upon addition of an enantiomerically pure alcohol derived from camphor (HOCamph) to Mo(NAr)(CHCMe2Ph)(Me2Pyr)2, Mo(NAr)(CHCMe2Ph)(OCamph)(Me2Pyr) was isolated and shown to be a 3:1 mixture of two diastereomers. As expected, the diastereomers do not interconvert readily on the NMR time scale.92 In 2009 a monoTBS-protected biphenol (enantiomerically pure) was shown to react with Mo(NAr)(CHCMe2Ph) (Me2Pyr)2 to give a 7:1 mixture of diastereomers of Mo(NAr)(CHRCMe2Ph)(OR )(Me2Pyr) (OR ¼ Br2Bitet-OTBS).83 R OH is too large to protonate the second dimethylpyrrolide, which remains bound in an Z1 fashion to the metal. Both diastereomers were isolated through partial recrystallizations and structurally characterized. A PMe3 adduct of one diastereomer was shown in an X-ray study to be closest to a square pyramid with the syn alkylidene in the apical position and the phosphine trans to the pyrrolide. It was subsequently shown that PMe3 catalyzes the interconversion of the two diastereomers of Mo(NAr)(CHCMe2Ph)(OR )(Me2Pyr) through formation and rearrangement of five-coordinate phosphine adducts (Eq. 20). Interconversion of diasteromers catalyzed by pyridine was found to be 600 times slower than interconversion of diasteromers catalyzed by PMe3, presumably because PMe3 binds more strongly than pyridine; the PMe3 adduct might also rearrange more rapidly than the pyridine adduct.

ð20Þ

ð21Þ

Ethylene reacts rapidly with Mo(NAr)(CHCMe2Ph)(OR )(Me2Pyr) to yield two diastereomers of the methylidene complex, Mo(NAr)(CH2)(OR )(Me2Pyr) in equilibrium with the molybdacyclobutane complex, Mo(NAr)(CH2CH2CH2)(OR )(Me2Pyr) where OR is Br2Bitet-OTBS.83 It was proposed that an olefin enters the coordination sphere trans to the pyrrolide and the metathesis product olefin leaves trans to the pyrrolide to yield an alkylidene with the opposite chirality at the metal (Eq. 21). Facile five-coordinate rearrangement of the metallacycle allows an olefin reactant to enter and an olefin product to leave in the same manner. This proposal has been supported through theoretical studies.240 It was concluded that “rearrangement of the five-coordinate metalacyclobutane itself might compete with loss of olefin in circumstances where the metalacyclobutane lifetime is relatively long, i.e., in tungstacyclobutane species. The speed of metathesis/inversion at M is expected to vary widely and exceptionally finely as steric interactions generated in reactions between a given diastereomer and a given olefin become more significant and unavoidable.”83

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

O-

O-

OPh

Ph

Ph

Ph

Mes Trip

Mes

TPPO

H MTO

Trip

H IPTO t-Bu

t-Bu t-Bu O-

t-Bu

701

t-Bu

t-Bu HTBTO Fig. 7 Terphenoxides found in Mo and W alkylidene complexes.

Biphen-OTMS and Br2Bitet-OTBS have played significant roles in the chemistry and applications of MAP catalysts in organic chemistry, not necessarily because the ligands and therefore their complexes can be enantiomerically pure, but because of their steric demands in a metal complex with a stereogenic metal center. They stimulated an interest in using other bulky aryloxides to prepare MAP complexes. In that category are phenoxides that are disubstituted in the 2 and 6 positions with aryl groups, i.e., 2,6-terphenoxides such as 2,3,5,6-terphenylphenoxide (TPPO), 2,6-dimesitylterphenoxide (hexamethylterphenoxide or HMTO), and 2,6-Trip2terphenoxide (hexaisopropylterphenoxide or HIPTO; see Fig. 7). Several terphenoxide and related ligands of this type were synthesized and popularized by Power322,323 who used them to synthesize transition metal complexes that have an unusually low coordination number. In all terphenoxides the planes of the rings in the 2 and 6 positions are perpendicular to the plane of the central phenyl ring, thus creating a relatively rigid terphenoxide that sweeps out a substantial volume of space around the metal. The largest in this series of electronically similar but sterically significantly different 2,6-terphenoxides is hexa-t-butylterphenoxide (HTBTO). Preliminary results showed that in at least one case a single HTBTO ligand creates a complex that is too crowded to react readily with olefins,22 while TPPO is sometimes not sterically demanding enough for the most efficient stereoselective reactions, leaving HMTO and HIPTO as the sterically demanding terphenoxides that have been used in much of MAP chemistry explored so far.

ð22Þ

The exploration of MAP complexes, especially those in which the aryloxide is a 2,6-terphenoxide, led to the discovery of how the chirality at the metal controls the structure of the polymer in ROMP and how crucial is the terphenoxide for formation of solely cis internal olefins from terminal olefins.88 Fundamental studies of tungsten94 and molybdenum83 MAP complexes led to the conclusions that (i) metallacyclobutanes that contain axial imido and aryloxide groups are intermediates in metathesis, (ii) the metal inverts with each forward metathesis step, and (iii) a “sufficiently large” aryloxide in one axial position of the TBP metallacycle intermediate and a relatively small X group (imido or oxo) in the other axial position could direct formation of all-cis substituted metallacycles from syn alkylidenes, as shown in Eq. (22), and therefore cis products. These findings suggest that cis selectively could be generalized to acyclic olefins, which has been found to be the case in numerous applications in organic chemistry since that time.21,43,81,82,90,104,118,312,314,318,319,324–336

ð23Þ

702

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

R

R

H H

R enantiomer A

H

R

+

R H

enantiomer B

R R

ð24Þ R

cis,syndiotactic,alt

ð25Þ

The impact of MAP complexes as initiators in ROMP also has been substantial. For example, Mo adamantylimido HIPTO complexes have been shown to yield >99% cis and >99% syndiotactic polyDCMNBD (DCMNBD is 5,6-dicarbomethoxynorbornadiene) and related norbornenes and norbornadienes through “stereogenic metal control” of the insertion process. As shown in Eq. (23) the olefin approaches the syn alkylidene selectively trans to the pyrrolide with the norbornene ring pointing up, followed by formation of the insertion product with the opposite configuration at the metal; the resulting dyads therefore have a cis,syndiotactic structure. It was later shown that the enantiomers in a racemic mixture are also recognized by a chiral metal center and incorporated selectively and alternatingly to give a cis,syndiotactic,alt polymer (Eq. 24).89 Finally, it was shown that a mixture of two different, but similar, pseudoenantiomers could be polymerized to yield an even more elaborate structure, a cis,syndiotactic,alt0 polymer (Eq. 25).337,338 Polymers with this degree of structural elaboration are extraordinarily rare. Other complexes in which only one alkoxide or aryloxide is present have been prepared, with the most important so far being monoaryloxide chloride (MAC) complexes. An example is Mo(NAd)(CHCMe2Ph)(Cl)(CH3CN)(OHMT).21 MAC complexes have been shown to be significantly more reactive than MAP complexes, so much so that they are initiators of stereoselective cross metathesis reactions that involve ClCH]CHCl or (CF3)CH]CH(CF3) as one of the cross metathesis partners.21,90

5.11.2.2.3

Metallacyclobutanes

Theoretical studies suggest that a TBP metallacyclobutane forms in a smooth 2 + 2 reaction between the olefin and the M]C bond.238 An olefin is lost in the same manner, possibly with inversion at the metal center if the metal is a stereogenic center. A metal-olefin interaction appears to be required before the metallacylobutane forms, and as it breaks up, but the interaction must be relatively weak (99% cis, 98% syndiotactic polymer,118 as observed for 14e imido alkylidene MAP complexes. Addition of 2 equiv. of B(C6F5)3 to W(O) (CH-t-Bu)(OHMT)(Me2Pyr)(PMe2Ph) led to scavenging of PMe2Ph and addition of B(C6F5)3 to the oxo ligand (Fig. 16). The WdO distance in W(O)[B(C6F5)3](CH-t-Bu)(OHMT)(Me2Pyr) is slightly longer (1.759(2) A˚ ) than what it is in W(O)(CH-t-Bu)(OHMT) (Me2Pyr)(PMe2Ph) (1.717(2) A˚ ) or in WO(CH-t-Bu)Cl(OHMT)(PMe2Ph) (1.695(3) A˚ ). The long BdO bond (1.571(3) A˚ ) suggests that B(C6F5)3 is relatively weakly coordinated in W(O)[B(C6F5)3](CH-t-Bu)(OHMT)(Me2Pyr), and indeed B(C6F5)3 dissociates readily in solution. Coordination of B(C6F5)3 to the oxo is proposed to lead to a more electrophilic metal center and accelerated metathesis but probably also decompositions. Data in support of those assertions are difficult to come by because B(C6F5)3 dissociates in solution and slow decomposition ensues. The discussion of the decomposition of the metallacyclobutane made from W(O)(CH-t-Bu)(dAdPO)2 and ethylene in Section 5.11.2.2.4 (Eq. 27 and Fig. 15) is an example of how important activation of the oxo ligand by a Lewis acid could be in some metathesis systems. In 2012 it was shown that W(O)2(CH2-t-Bu)2(bipy) reacts with a mixture of ZnCl2(dioxane), PMe2Ph, and Me3SiCl in toluene at 100  C to yield W(O)(CH-t-Bu)Cl2(PMe2Ph)2.117 Although details of the mechanism have not been elucidated, a plausible scenario is that TMSCl replaces one oxo with two chlorides (thereby stimulating a abstraction), zinc chloride removes bipy from solution, and the phosphines add to the metal to give the final product. Forming the neopentylidene directly on tungsten is a far more practical method of making W(O)(CH-t-Bu)Cl2L2 complexes than relying on transfer of an alkylidene from tantalum to tungsten.284 Off-white W(O)(CH-t-Bu)Cl(OR)(PMe2Ph) complexes (OR ¼ OHMT or OHIPT) were prepared and served as starting materials for the synthesis of W(O)(CH-t-Bu)(OHMT)(2,6-diphenylpyrrolide), W(O)(CH-t-Bu)[N(C6F5)2](OHMT)(PMe2Ph), W(O)(CH-t-Bu)[OSi(t-Bu)3](OHMT), and W(O)(CH-t-Bu)(OHMT)2. W(O)(CH-t-Bu)(OHMT)2 reacts with ethylene to yield the square pyramidal metallacyclobutane complex, W(O)(C3H6)(OHMT)2, which rearranges to a TBP and loses ethylene to yield isolable W(O)(CH2)(OHMT)2. The structure of W(O)(CH2)(OHMT)2 (Fig. 17) reveals the extraordinary steric protection of the methylene and oxo toward bimolecular reactions by the two OHMT ligands. Steric protection of this magnitude by two OHMT ligands understandably slows reactions of bis-OHMT complexes with olefins in general. Further studies showed that B(C6F5)3 can bind reversibly to the oxo ligand in W(O)(CH2)(OHMT)2 but not as well to the oxo ligand in W(O)(CH-t-Bu)(OHMT)2, presumably because of the greater steric demand of the neopentylidene ligand.119 Addition of ethylene to a mixture of B(C6F5)3 and W(O)(CH-t-Bu)(OHMT)2 in C6D6 led to rapid formation of propylene and the tungstenacyclopentane complex, W(O)(C4H8)(OHMT)2, over a period of a few minutes (Eq. 30). The unsubstituted metallacyclobutane intermediate rearranges to a propylene complex. Ethylene then displaces propylene to form an ethylene complex and ultimately W(O)(C4H8)(OHMT)2. Addition of 0.2 equiv. of B(C6F5)3 to W(O)(C3H6)(OHMT)2 in the absence of ethylene leads to formation of two isomers of W(O)(propylene)(OHMT)2, according to 13C NMR studies.

Fig. 16 The structure of W(O)[B(C6F5)3](CH-t-Bu)(OHMT)(Me2Pyr).

710

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Fig. 17 The structure of W(O)(CH2)(OHMT)2.

ð30Þ

In spite of much effort, Mo(O)(CH-t-Bu)Cl2(phosphine)2 complexes could not be prepared from MoO2(CH2-t-Bu)2(bipy) under conditions that were successful for making W(O)(CH-t-Bu)Cl2(phosphine)2 complexes. Formation of significant quantities of known diamagnetic Mo(O)(phosphine)3Cl2 complexes suggests that reduction of Mo(VI) to Mo(IV) at some stage is perhaps the primary cause of the failure to prepare Mo oxo alkylidene complexes in this manner. The first Mo oxo alkylidene, Mo(O)(CHSiMe3)[NP(t-Bu)3]2, was synthesized in 2015 by a classical synthesis that involves a abstraction in unobservable Mo(O)(CH2SiMe3)2[NP(t-Bu)3]2.354 The synthesis is successful in part because the [NP(t-Bu)3]− is a powerful s and p electron donor that allows alkylation of an oxo complex to proceed without metal reduction. Unfortunately, for the same reason Mo(O)(CHSiMe3)[NP(t-Bu)3]2 was found to be essentially unreactive in metathesis reactions, even toward norbornene. The [NP(t-Bu)3]− ligand is related sterically to the [OSi(t-Bu)3]− (Silox) ligands and bisSilox imido alkylidenes were found to be essentially unreactive toward olefins.67 The two large [NP(t-Bu)3]− ligands would have to be at 90 to each other (one axial, one equatorial) in a TBP metallacyclobutane intermediate, which is an untenable steric problem that likely plays a role in preventing formation of the required metallacyclobutane. In 2018 it was shown that addition of one equiv. of water to Mo(CAro)(ORF6)3(1,2-dimethoxyethane) (Aro ¼ o-(OMe)C6H4) produces hexafluoro-t-butanol and the dimeric alkylidyne hydroxide complex, [Mo(CAro)[OCMe(CF3)2]2(m-OH)]2(DME), in which each bridging hydroxide proton points toward an oxygen atom in an arylmethoxy group (Fig. 18).114 Addition of PMe3 to [Mo(CAro)(ORF6)2(m-OH)]2(DME) gives the alkylidene oxo complex, Mo(O)(CHAro)(ORF6)2(PMe3). Removal of the ORF6 ligands from Mo(O)(CHAro)(ORF6)2(PMe3) with HCl followed by addition of one LiOHIPT gave 18e Mo(O)(CHAro)(OHIPT)(Cl)(PMe3) (Fig. 19). Only in the presence of B(C6F5)3 (to scavenge PMe3) will Mo(O)(CHAro)(OHIPT)Cl(PMe3) initiate metathesis reactions, among them the polymerization of 5,6-dicarbomethoxynorbornadiene (DCMNBD) to give cis,syndiotactic-poly(DCMNBD), as found for analogous four-coordinate imido alkylidene initiators. Addition of 1 equiv. of water (in THF) to the benzylidyne complex, Mo(CArp)(OR)3(THF)2 (Arp ¼ para-methoxyphenyl, OR ¼ ORF6 or ORF9) leads to formation of [Mo(CArp)(OR)2(m-OH)(THF)]2(m-THF) complexes that are analogous to that in Fig. 19,115 which confirms that an ortho methoxide in the benzylidyne ligand is not necessary to stabilize the hydroxy alkylidyne complex. Addition of 1 equiv. of a phosphine (L) and water to Mo(CArp)(ORF9)3(THF)2 gave Mo(O)(CHArp) (ORF9)2(L) complexes (L ¼ PMe2Ph, PMePh2, PEt3, or P(i-Pr)3) in good yields. The reaction between Mo(O)(CHArp)(ORF9)2(PEt3) and 2 equiv. of LiOHMT proceeds smoothly at 90  C in toluene to give Mo(O)(CHArp)(OHMT)2. Mo(O)(CHArp)(OHMT)2 reacts with 2,3-dicarbomethoxynorbornadiene to yield syn and anti isomers of the first-insertion product that contains a cis C]C bond (Eq. 31), a result that suggests that the first-insertion product in a bis-OHMT complex is much less reactive than Mo(O)(CHArp) (OHMT)2, presumably for steric reasons, as noted earlier.

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

711

Fig. 18 The structure of [Mo(CAro)[OCMe(CF3)2]2(m-OH)]2(DME).

Fig. 19 The structure of Mo(O)(CHAro)(OHIPT)(Cl)(PMe3).

ð31Þ

Ultimately it was shown that phosphines are not necessary for the synthesis of oxo alkylidenes from alkylidynes.116 Addition of 1 equiv. of water to Mo(CArp)(ORF9)3 (Arp ¼ p-methoxyphenyl; ORF9 ¼ OC(CF3)3) in the presence of 5% NEt3 in THF led to formation of Mo(O)(CHArp)(ORF9)2(THF)2 in good yield. This compound provides access to Mo(O)(CHArp)(OHMT)2 and Mo(O)(CHArp)(OTPP)2. In the presence of TMEDA (2.5 equiv.) the addition of 1 equiv. of water to Mo(CR)(ORF9)3 (R ¼ Arp, Mesityl, or t-Bu) yields Mo(O)(CHR)(ORF9)2(TMEDA) complexes. Other complexes that were prepared include Mo(O)(CHt-Bu)Cl(OHIPT)(3-Brpy) (Fig. 20), which is an active metathesis initiator as a consequence of dissociation of 3-Brpy to expose the 14e Mo(O)(CH-t-Bu)Cl(OHMT) core. A full study and comparisons of various imido MAC complexes in metathesis reactions remain to be carried out. Although the syntheses of Mo oxo alkylidenes are currently relatively lengthy, shorter ones seem likely to be developed in due course.

712

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Fig. 20 Structure of Mo(O)(CH-t-Bu)Cl(OHIPT)(3-Brpy).

5.11.2.2.7

Cationic alkylidenes

A required feature of all Mo and W olefin metathesis catalysts is an electrophilic metal and a TBP metallacyclobutane intermediate. Therefore a cationic metal complex should be considerably more reactive than a neutral metal complex, all else being equal. However, decomposition reactions may also be faster as the metal becomes more electrophilic and especially if it is cationic overall. An obvious requirement for making cations is that the anion be relatively weakly coordinating. Two anions in this category that have been used to form ion pairs with relatively reactive cationic metal complexes are [B(3,5-(CF3)2C6H3)4]− ([BArF4]−) and [B(C6F5)4]−. One concerning issue with cationic M]CHR complexes is the likely increased protonic character of the a hydrogen in the alkylidene and loss of it to a ligand or external base to give an alkylidyne. Osborn was one of the first to attempt to determine experimentally the nature of the catalysts in a homogeneous tungsten-based metathesis system in which aluminum alkyls are the source of the alkylidene. In 1980 he published a paper in which he reported the synthesis of Mo and W complexes of the type M(O)(CH2-t-Bu)3Cl and M(O)(CH2-t-Bu)4.287 The M(O)(CH2-t-Bu)4 complexes later were found to be M(O)(CH2-t-Bu)3(OCH2-t-Bu) complexes. The neopentoxide apparently arises through attack on the oxo by neopentyl alkylating agents. In 1981 he reported that aluminum halide adducts of oxo alkyl complexes could be prepared that were active for metathesis, although few experimental details were provided. In 1982 he reported that the metathesis-active catalysts are not oxo complexes, but M(CH-t-Bu)(OCH2-t-Bu)2Cl2(AlX3) complexes,363 and in 1983 that the active form of M(CH-t-Bu) (OCH2-t-Bu)2Cl2(AlX3) complexes were actually ion pairs that contain a four-coordinate cation, namely [W(CH-t-Bu) (OCH2-t-Bu)2Cl](AlX4). (Cationic tungsten oxo alkylidene complexes formed through addition of AlCl3 were also proposed in 1982, but they were not isolated or characterized.364) In 1987 he reported NMR studies for the cationic alkylidene complex and metallacyclobutane intermediates prepared from norbornene.365 In the next few years he reported studies concerned primarily with polymerization of cyclic olefins.339,366,367 In 1988 the X-ray structures of the [W(cyclopentylidene)(OCH2-t-Bu)2Br2]2 (pseudooctahedral with bridging bromides and trans alkoxides) and W(cyclopentylidene)(OCH2-t-Bu)2Br(GaBr4) (pseudooctahedral with trans alkoxides and a bidentate GaBr−4 ligand) were published.368 Dissociation of the GaBr−4 ligand leaves [W(cyclopentylidene)(OCH2-tBu)2Br]+ as the 10e four-coordinate metathesis-active complex. W(cyclopentylidene)(OCH2-t-Bu)2Br2 will metathesize linear and cyclic olefins in chlorobenzene or dichloromethane, but addition of 1 equiv. of GaBr3 yields a “vastly more efficient” catalyst.368 No Mo analogs of the reported W complexes were reported. By this point 14e four-coordinate W oxo and imido alkylidene and Mo imido alkylidene complexes had been prepared and established as metathesis catalysts.289,290,364,369–374 Several cationic alkylidene complexes were published during the period covered by this article. Mo(NAr)(CHCMe2Ph) (OTf )2(DME) reacts with the lithium salt of various b-diketonate and b-diketiminates to give complexes of the type Mo(NAr) (CHCMe2Ph)(L)(OTf ) (L ¼ b-diketonate or b-diketiminate).98,99 Treatment of these compounds with NaBArF4 in dichloromethane affords cationic imido alkylidene complexes.286,287,363 An example is [Mo(NAr)(CHCMe2R)(Ar-nacnac)(THF)n]BArF4 (n ¼ 0 or 1). The cationic species were found to have short lifetimes as metathesis catalysts and decomposition modes often involved the nacnac ligands. Addition of 1 equiv. of [HNMe2Ph]BArF4 to Mo(NAr)(CHCMe2Ph)(Pyrrolide)2 (Pyrrolide ¼ Pyr or Me2Pyr) complexes in THF produced [Mo(NAr)(CHCMe2Ph)(Pyrrolide)(THF)x]BArF4 complexes, from which a number of cationic complexes were prepared such as [Mo(NAr)(CHCMe2Ph)(Me2Pyr)(2,4-lutidine)]BArF4, [Mo(NAr)(CHCMe2Ph)[OC(CF3)2Me](THF)3]BArF4, and [Mo(NAr) (CHCMe2Ph)(O-2,6-i-Pr2C6H3)(THF)]BArF4.53 (See Table 1 for a complete list.) The addition of ArOH or BiphenH2 to [Mo(NAr) (CHCMe2Ph)(Pyrrolide)(THF)x]BArF4 led to protonation of the alkylidene to give the neophyl complexes, [Mo(NAr)(CH2CMe2Ph) (OAr)2]BArF4 and [Mo(NAr)(CH2CMe2Ph)(rac-Biphen)]BArF4, which are rare examples of cationic Mo(VI) alkyl complexes. Reactivity of the cationic alkylidenes in metathesis reactions is attenuated by strongly bound ligands (or coordinating solvents such as THF) or an Z5 binding mode of the pyrrolide ligand. In order to remove bipy from W(NArMes2)(CHCMe2Ph)Cl2(bipy), ZnCl2(dioxane) was added to W(NArMes2)(CHCMe2Ph) Cl2(bipy) suspended in CD2Cl2. Instead of forming W(NArMes2)(CHCMe2Ph)Cl2, a chloride was abstracted by ZnCl2 to give [W(NArMes2)(CHCMe2Ph)Cl(bipy)][Zn2Cl6]0.5, as proven through an X-ray structural study.40 No further studies of this cationic tungsten complex have been reported.

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

713

The reactions of [PhMe2NH]BArF4 with W(NR)(CH-t-Bu)(Z5-Me2Pyr)2 (R ¼ ArCl2 or Ar) to give the pyrrolenine complexes, [W(NR)(CH-t-Bu)(Z5-Me2Pyr)[NC4(2,3,4-H3)(2,5-Me2)]]BArF4, were mentioned in the pyrrolide section. Numerous cationic alkylidene complexes that contain an NHC ligand have been prepared and will be discussed in Section 5.11.5. Cationic complexes have been prepared in which the positive charge(s) is(are) located on the ligands. The purpose is to carry out olefin metathesis reactions in two phases, one of which being an ionic liquid.138 Examples are anilinium complexes, [Mo(NAr)(CHt-Bu)(Me2Pyr)(O-2,6-Ph2-4-(2,4,6-Ph3)NC5H2)][X], in which [X]− ¼ [BArF4]−, [Al(OC(CF3)3)4]−, [B(C6F5)4]−, or [OTf]−.138

5.11.2.2.8

M]CHX complexes

The vast majority of alkylidene complexes are M]CHX complexes, where X is either a proton, a saturated carbon center, silicon (usually SiMe387), or an aryl group. Both Mo]NAr and W]NAr compounds that contain ]CHX groups in which X is Si or Ge have been prepared through reactions of a neopentylidene or neophylidene complex with CH2]CHX.105–113,146,171,375 Many have been crystallographically characterized and found to be competent initiators in routine metathesis reactions. (See Table 1.) M]CHX complexes in which X is a vinyl group are rare, but they are key intermediates in the cyclopolymerization of diynes such as diethyldipropargyl malonate to give polyenes that contain five- or six-membered rings or in polymerization of terminal alkynes,101,102 but simple vinyl-substituted alkylidene complexes are relatively uncommon and their chemistry unexplored. Recent examples include M¼CHCH¼CRMe complexes in which R ¼ H or Me.87 When X contains a donor group that can bind to the metal, metathesis activity is usually attenuated. An example is the Mo(NAr)[CH(N-pyrrolidinonyl)](Me2Pyr)(OTPP) complex shown in Fig. 21, which is one of a series of Mo]CHX complexes prepared through reactions between Mo(NAr)(CHCMe2Ph)(Me2Pyr)(OTPP) and CH2]CHX;87 the complex is relatively unreactive toward olefins as a consequence of coordination of the pyrrolidinonyl oxygen to the metal. Other Mo(NAr)(CHX)(Me2Pyr)(OTPP) examples are those in which X ¼ B(pin) (Fig. 22) or SPh (Fig. 23).87 The 2,5-dimethylpyrrolide is bound in an Z1-fashion in all cases. These Mo(NAr)(CHX)(Me2Pyr)(OTPP) complexes have characteristics that are typical for imido alkylidene complexes (e.g., syn and anti isomers, etc.) and any adverse reactions that are a consequence of X being bound to the alkylidene carbon do not seem

Fig. 21 The structure of Mo(NAr)[CH(N-pyrrolidinonyl)](Me2Pyr)(OTPP).

Fig. 22 The structure of Mo(NAr)[CHB(pin)](Me2Pyr)(OTPP).

714

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Fig. 23 The structure of Mo(NAr)(CHSPh)(Me2Pyr)(OTPP).

to be significant. However, metathesis activities of these M]CHX complexes have not been explored in depth. The X groups do not seem to change the nature of the alkylidene in a structural sense. Side reactions that promote catalyst decomposition also do not appear to be a serious limitation, although again limited experiments were carried out with these compounds. Unusual stereoselective cross-metathesis reactions that involve ClCH]CHCl,21,315 CF3CH]CHCF3,317 and NCCH]CHCN316 led to a search for examples of M]CHX complexes that must be intermediates in these reactions.103 Reactions between XCH]CHX where X ¼ Cl, CF3, or CN and Mo(N-t-Bu)(CH-t-Bu)(OHIPT)Cl(PPh2Me) produce Mo(N-t-Bu)(CHX)(OHIPT)Cl(PPh2Me) complexes. Addition of 2,20 -bipyridyl led to isolation of Mo(N-t-Bu)(CHX)(OHIPT)Cl(Bipy) complexes. The reaction between Mo(N-t-Bu)(CH-t-Bu)(OHMT)Cl(PPh2Me) and ZdClCH]CHCl in the presence of Bipy produces a mixture that contains both Mo(N-t-Bu)(CHCl)(OHMT)Cl(PPh2Me) and Mo(N-t-Bu)(CHCl)(OHMT)Cl(Bipy); the product that crystallizes from toluene-d8 is the phosphoniomethylidene complex, [Mo(N-t-Bu)(CHPPh2Me)(OHMT)(Cl)(Bipy)]Cl. The Mo(N-t-Bu)(CHX)(OHIPT)Cl(PPh2Me) complexes (X ¼ Cl or CF3) were confirmed to initiate the stereoselective cross-metathesis between Z-5-decene and ZdXCH]CHX. Studies showed that the structures of all Mo(N-t-Bu)(CHX)(OHIPT)Cl(Bipy) complexes are analogous; the Mo]CH(CF3) structure is shown in Fig. 24. It should be noted that the reaction between W(NAr)(CHCMe2Ph)(ORF6)2 and 1 equiv. of acrylonitrile had been found to give [W(NAr)(CHCN)(ORF6)2]4 in which the cyano group in the alkylidene nitrogen binds as a donor to another tungsten center and effectively blocks olefin metathesis activity.376 The cyano methylidene reacts with 2 equiv. of acetonitrile to give a diazacyclohexatriene ring.

Fig. 24 The structure of Mo(N-t-Bu)[CH(CF3)](OHIPT)Cl(Bipy).

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

5.11.2.2.9

715

Disubstituted alkylidenes

Until relatively recently, the only examples in the literature of simple disubstituted high oxidation state Mo or W alkylidene complexes were Mo(NAd)(CMePh)[OCH(CF3)2]2(2,4-Lutidine),377 Mo(NAd)(CPh2)[OCH(CF3)2]2(2,4-Lutidine),377 and Mo(NAr)(CMePh)[OCMe(CF3)2]2.378 All were prepared through reactions between neopentylidene or neophylidene complexes and CH2]CMePh or CH2]CPh2, but they were not structurally characterized nor studied in any detail. Polymerization of certain acetylenes or cyclopolymerization of diynes proceeds via reaction of the acetylene to give a metallacyclobutene intermediate and then M]C(vinyl)(R) complexes, where R is found in the terminal alkyne such as o-TMS phenylacetylene.377 (Examples of M] C(vinyl)(R) complexes are listed in the M]CHX section in Table 1.) The first insertion product reacts further with the acetylene to give polyacetylenes. It was proposed that any disubstituted alkylidene would be inherently less reactive than neopentylidene initiators toward ordinary olefins, but no quantitative data were offered. A key feature of disubstituted alkylidene complexes that distinguishes them from M]CHR complexes is that alkylidyne complexes cannot form. Therefore, some multiple metal-carbon bond chemistry will likely turn out to be unique to disubstituted alkylidenes. A potential benefit of working with disubstituted alkylidenes in terms of synthesis is that bimolecular decomposition pathways will no longer include a proton abstractions that are possible in M]CHR complexes and that may be contributing to bimolecular decompositions. The synthesis of Mo]CMePh complexes from CH2]CMePh378 has been revisited and expanded.133 The reaction between Mo(NAr)(CH-t-Bu)[OCMe(CF3)2]2 and CH2]CMePh led to several Mo]CMePh complexes, many of which were structurally characterized. These include Mo(NAr)(CMePh)(OMesityl)2 (Fig. 25), [Mo(NAr)(CMePh)(OC6F5)2]2, Mo(NAr)(CMePh) (OC6F5)2(MeCN), Mo(NAr)(CMePh)(OC6F5)2(bipy), Mo(NAr)(CMePh)(Cl)2(bipy), Mo(NAr)(CMePh)(Cl)(OHMT)(MeCN), and Mo(NAr)(CMePh)(Pyrrolide)(OHMT). The dominant Mo]CMePh isomer in the solid state is usually that in which the phenyl group points toward the imido nitrogen. The Mo]CdC angles in the two 14e complexes examined differ by 29.7 and 36.4 , consistent with a Mo. . .(CdHb) agostic interaction (Fig. 25) in which the a hydrogen is only 2.51 A˚ from the metal. In the two 16e monoadducts examined the Mo]CdC angles differed by 12.3 and 18.4 , and in the two 18e bipy complexes the Mo]CdC angles differed by only 0.4 and 2.6 ; these data imply that the extent of the agostic interaction is inversely proportional to the electron count at the metal, as one might expect. These CHb agostic interactions are the first to be observed in Mo high oxidation state alkylidenes. No data are yet available as to whether a CHa agostic interaction is favored over a CHb agostic interaction in a Mo]CHCH3 complex, for example. Until recently (see below) there has been no example of rearrangement of an alkylidene to an olefin among any d0 alkylidene complexes going back as far as Cp2Ta(CHMe)(Me), which is converted into Cp2Ta(propylene)(H), not (known) Cp2Ta(CH2] CH2)(Me), through methyl migration to the alkylidene carbon followed by b hydride elimination.379 However, there is substantial evidence in niobium and tantalum chemistry281,282 that olefins can rearrange to alkylidenes if a source of H is available, as described earlier. Preliminary olefin metathesis studies with cyclooctene and 1-decene confirm that the Mo]CMePh complexes described above are competent metathesis initiators. A realization that b proton abstraction cannot take place in a 2-adamantyl ligand led to the synthesis of 2-adamantylidene complexes through techniques that are standard for synthesizing neopentylidene and neophylidene complexes,134 starting with the reaction between Mo(NAr)2(2-adamantyl)2 and 3 equiv. of triflic acid to give Mo(NAr)(Adene)(OTf )2(DME) (Ad ¼ 2-adamantylidene). t-Butylimido complexes were prepared readily in a similar fashion. Examples of characterized complexes include Mo(NAr) (Adene)(Cl)(OHMT), Mo(NAr)(Adene)(OHMT)(Pyr), Mo(N-t-Bu)(Adene)(Cl)(OHMT)(t-BuCN), and Mo(N-t-Bu)(Adene)(Pyr) (OHMT)(THF) (Fig. 26). X-ray structural studies suggest that one of the Adene b protons is engaged in a CHb agostic interaction with the metal when the electron count is less than 18e, although the differences in the Mo]CdC angles (26.2 , 20 , 23.2 , 18.1 , 24.8 , and 18.6 ) are significant less than the differences found in the M]CMePh complexes just described (29.7 and 36.4 in two 14e M]CMePh complexes). Two adamantylidene b proton resonances between 3.5 and 4.5 ppm are observed in all admantylidene complexes prepared so far, consistent with no “alkylidene rotation” on the NMR time scale; they are reliable markers in proton NMR studies of 2-adamantylidene complexes. Preliminary olefin metathesis studies with cyclooctene and 1-decene confirmed that 2-adamantylidene complexes are competent metathesis initiators. Synthesis of 2-adamantylidene complexes through a abstraction has been extended to Mo pentafluorophenylimido (NArF) complexes.135 Addition of HCl and bipy to Mo(NArF)2(Ad)2 gave Mo(NArF)(Adene)Cl2(bipy), while addition of triflic acid to Mo(NArF)2(Ad)2 in the presence of DME gave Mo(NArF)(Adene)(OTf )2(DME). Mo(NArF)(Adene)(OTf )2(DME) served as a starting point to prepare Mo(NArF)(Adene)(ORF9)2, Mo(NArF)(Adene)(ORF9)2(THF), Mo(NArF)(Adene)(ORF9)2(Piv) (Piv ¼ pivalonitrile), Mo(NArF)(Adene)(OTf )(OHMT)(THF) (OHMT ¼ O-2,6-Mesityl2C6H3), Mo(NArF)(Adene)(OTf )(OHMT) (Piv), Mo(NArF)(Adene)(OHMT)(Pyr)(Piv), and Mo(NArF)(Adene)(Me2Pyr)2. X-ray structural studies showed that the Adene ligand is distorted by a CdHa agostic interaction most significantly in 14e Mo(NArF)(Adene)(ORF9)2. Preliminary studies show

i-Pr

i-Pr N MesitylO

MesitylO Fig. 25 The structure of Mo(NAr)(CMePh)(OMesityl)2.

138.7°

Ph C 118.9° 102.4° CH2 2.51Å H Mo

716

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Fig. 26 Structure of Mo(N-t-Bu)(Adene)(OHMT)(Pyr)(THF).

that 14e or 16e 2-adamantylidene complexes are initiators for homocoupling of 1-decene and polymerization of cyclooctene and 5,6-dicarbomethoxynorbornadiene. Tungsten oxo adamantylidene complexes have now been prepared through addition of 2-methyleneadamantane to a neopentylidene or neophylidene complex.136 Addition of 2-methyleneadamantane to W(O)(CHCMe2Ph)(PPh2Me)2Cl2 in the presence of 2 equiv. of NaORF9 gave the 16e complex, W(O)(Adene)(ORF9)2(PPh2Me). The phosphine dissociates from W(O)(Adene) (ORF9)2(PPh2Me) and if it is scavenged by B(C6F5)3, the dimer, [W(O)(Adene)(ORF9)2]2, precipitates from solution. An X-ray structural study of the dimer (Fig. 27) shows two unsymmetrically bridging oxo ligands to be present, with each oxo being triply bound to one metal (W]O ¼ 1.778 A˚ ) and behaving as a donor to the other (WdO ¼ 2.168 A˚ ). Formation of a dimer through bridging oxo ligands and dissociation thereof to give monomers was also found in a tungsten biphenolate complex that readily polymerizes norbornenes and norbornadienes to give cis,isotactic polymers.122 [W(O)(Adene)(ORF9)2]2 dissolves readily in a donor solvent such as THF, presumably to give one or more monomeric THF adducts. Similar approaches led to the isolation of Mo(O) (Adene)(ORF9)2, which is a monomer as a consequence of Mo-ligand bonds being weaker than W-ligand bonds.380 Mo(O)(Adene) (ORF9)2 is remarkably thermally stable toward bimolecular decomposition (85 in toluene-d8 for 8 h). W(O)(Adene)(Me2Pyr)2 was prepared readily through addition of LiMe2Py to [W(O)(Adene)(ORF9)2]2 and W(O)(Adene)(Me2Pyr)(OHMT) was prepared through addition of HMTOH to W(O)(Adene)(Me2Pyr)2. The suggested reason for the thermal stability of the M(O)(Adene) (ORF9)2 complexes is a resistance of 2-adamantylidene ligands to couple bimolecularly or to the fact that no a hydrogen is present that encourages bimolecular decompositions. Both may be true.

Fig. 27 The structure of [W(O)(Adene)(ORF9)2]2.

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

717

The existence of Mo(NAr)(styrene)(OSiPh3)2 (a mixture of two isomers)139 and the possibility of making its alkylidene isomer, Mo(NAr)(CMePh)(OSiPh3)2,133 created an opportunity of determining when and how the two might interconvert.381 Mo(NAr) (CMePh)(OSiPh3)2 was prepared and its structure shown to be analogous to that of Mo(NAr)(CMePh)(OMesityl)2 (Fig. 25,133). Upon adding 5% [PhMe2NH][BArF4] (BArF4 ¼ B(3,5-trifluoromethylphenyl)4) or [PhMe2NH][B(C6F5)4] to Mo(NAr)(styrene) (OSiPh3)2 in C6D6, carbon and proton NMR resonances characteristic of Mo(NAr)(CMePh)(OSiPh3)2 grew in slowly over 2 days at 22  C to reach an equilibrium in which Keq was found to be 0.32 in favor of the styrene complex. The equilibrium can be reached from either direction, but is complicated by the fact that [PhMe2NH]+ is slowly consumed during the process through some unknown and irreversible reaction or reactions. A van’t Hoff plot of nine determinations between 22 and 55  C showed DH ¼ 1.0 (3) kcal mol−1 and DS ¼ 1.2(9) cal mol−1 K−1 with the styrene complex being the slightly more stable form. The ratio of BH+ to B, and other concentrations, could be monitored during the reaction and the proposed mechanism shown in Scheme 2b could be modeled. The most reliable and readily determined rate constant is k1 (0.158(4) L/mol ∙ min). Other values obtained through modeling are k−1 ¼ 117(11) L/(mol∙ min), k2 ¼ 1460(240) L/(mol∙ min), and k−2 ¼ 7.7(1.6) L/(mol ∙ min). There is no evidence for formation of a terminal alkylidene (Mo]CHCH2Ph), although that alkylidene is likely to be subject to conversion to the secondary alkylidene through a protonation/deprotonation sequence if the secondary alkylidene is lower in energy. Protonation of W(NAr) (CMePh)(OSiPh3)2 gives an equilibrium mixture of W(NAr)(CMePh)(OSiPh3)2 and Mo(NAr)(styrene)(OSiPh3)2 and is remarkably similar overall to the Mo results both in rate and equilibrium constant.

Ar

Ar + BH+ - B

N

Ar

N

+

k1 Ph3SiO Ph3SiO

Ph3SiO

M CHPh CH2

+ B - BH

Ph

Mo C

k-1 +

Ph3SiO

H3C

H

+ B - BH

+

N

k2 k-2 + BH+ - B

Ph3SiO Ph3SiO

Mo

Ph C CH3

Scheme 2b

An X-ray study of a “double-salt” that contains the Mo 1-phenethyl cation finally confirmed its structure. Treatment of the Mo 1-phenethyl cation with dimethylaniline allowed a determination of k2 to k−1, i.e., the rate of deprotonation in the a position to give the Mo]CMePh complex vs deprotonation in the b position (which is three times more likely) to give the styrene complex. The ratio of k2 to k−1 was found to be 10 at 22  C, i.e., the a proton is removed 30 times more rapidly than a b proton. If more than 1 equiv. of triethylamine is added to the cationic 1-phenethyl complex the deprotonation is less selective for removing the a proton and the resulting mixture of styrene and Mo]CMePh complexes remains unchanged, i.e., the interconversion of the styrene and 1-phenethylidene complexes is not catalyzed readily by the triethylammonium ion, presumably because it is significantly less acidic (>9) than dimethylanilinium (pKa  5.1 in water). The above studies are the first to confirm and clarify an H-assisted mechanism for interconversion of an olefin and what turn out to be internal metathesis-active Mo and W complexes, provide a detailed mechanism for that interconversion, and establish the relative energies of the olefin and alkylidene complexes. Among the many variations of the protonation/deprotonation mechanism of interconverting olefin and alkylidene complexes that can now be studied, an immediate and pressing question is whether olefins that contain one or more allylic protons can still yield metallacyclobutane intermediates and therefore alkylidenes via some allylic intermediate, and if so, what are the relative rates of the protonation/deprotonation and allyl/metallacyclobutane pathways. Future studies are expected to further delineate the mechanisms of interconverting molybdenum and tungsten alkylidene and olefin complexes within the context of forming active olefin metathesis catalysts and should lead to a greater understanding of the fundamentals of proton movement within the primary coordination sphere of high oxidation state Mo and W complexes relevant to olefin metathesis.

5.11.2.2.10

Oxo- and imido-free alkylidenes

A prototypical complex in Osborn’s studies in the 1980s was [W(cyclopentylidene)(OCH2-t-Bu)2Br2]2, which has a pseudooctahedral arrangement about each W with two bridging bromides and trans alkoxides.368 In 1985 it was shown that W(C-t-Bu) (O-t-Bu)3 reacts with 2 equiv. of HX (X ¼ C1, Br, MeCO2, PhCO2, OPh, OC6F5, and O-p-C6H4Cl) to give the trigonal-bipyramidal neopentylidene complexes, W(CH-t-Bu)(O-t-Bu)2 X2 or octahedral complexes of the type W(CH-t-Bu)(O-t-Bu)2 X2(py) (X ¼ Cl, Br, I, O2CCF3).382 Addition of only one equiv. of R0 CO2H to W(CR)(O-t-Bu)3 (R ¼ Me, Et), yields relatively stable alkylidene complexes of the type W(CHR)(O-t-Bu)3(O2CR0 ). Complexes that contain Si or Ge (E) in the alkylidene, (t-BuO)2Cl2W]CHEPh3, are formed through addition of HCl to (t-BuO)3W^CEPh3 complexes.146 It should be noted that all of these complexes contain relatively electron-donating t-butoxide ligands, which may be the reason why the alkylidene complex does not lose HX (where X is an anionic ligand) to an external base or internal base/ligand.

718

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

H t-Bu

Cl W

Ph

O

O-2,6-Ph2C6H3

Fig. 28 The structure of W(CH-t-Bu)(O-2,6-Ph2C6H3)(O-2-Ph-6-C6H4-C6H3)Cl.

The reaction between W(C-t-Bu)(DME)Cl3 and 2 equiv. of Li(O-2,6-Ph2C6H3) produces the complex shown in Fig. 28; the presumed intermediate is W(C-t-Bu)(O-2,6-Ph2C6H3)2Cl, which undergoes CH metalation and transfer of H to the neopentylidyne to give the neopentylidene.143 This complex is active for the stereoretentive metathesis of an internal olefin (e.g., of cis-2-pentene) in chlorobenzene, but activity ceases at 30% conversion and the trans/cis ratio of the products increases sharply. It is possible that the active initiator may actually be the cation, W(CH-t-Bu)(OC6H3(Ph)C6H4)(O-2,6-Ph2C6H3)]+ (in chlorobenzene, at least), and that olefin metathesis activity ceases because an alkylidyne, W(CR)[OC6H3(Ph)C6H4](O-2,6-Ph2C6H3) and HCl are formed and are the tungsten alkylidyne complex is inactive for metathesis of olefins. The reaction between an anthracene-based methylidene transfer agent and square planar W(OAr)4 (OAr ¼ 2,6-diisopropylphenoxide) led to formation of W(CH2)(OAr)4 (Eq. 32).145 The methylene is sterically protected in a dramatic fashion and shows no metathesis-like reactions with ethylene or 1-hexene. It is deprotonated by Ph3P]CH2 to give the methylidyne complex, [Ph3PCH3] [W(CH)(OAr)4], in which the W^C distance was found to be 1.749(1) A˚ and the geometry to be close to a square pyramid (t ¼ 0.21) in an X-ray study.

ð32Þ

A neopentylidene analog of the complex shown in Eq. (32) has been prepared by adding 4 equiv. of ArOH to W(C-t-Bu) (CH2-t-Bu)3;144 intermediate W(C-t-Bu)(CH2-t-Bu)x(OAr)3−x complexes where x ¼ 2, 1, or 0 were observed in NMR spectra. As proposed in the reaction of W(C-t-Bu)(CH2-t-Bu)3 with HCl to give W(C-t-Bu)(dme)Cl3 (see Section 5.11.3) the reaction probably proceeds step-wise through protonation of the alkylidyne to give a five-coordinate alkylidene intermediate followed by a hydrogen abstraction to reform another neopentylidyne complex. The robustness of the 2-adamantylidene complexes described in Section 5.11.2.2.9 inspired reactions in which the oxo ligand is removed and replaced by two chlorides. Addition of PCl5 to [W(O)(Adene)(ORF9)2]2 and Mo(O)(Adene)(ORF9)2 gave the pale blue complexes, W(Adene)(ORF9)2Cl2 and Mo(Adene)(ORF9)2Cl2 (Fig. 29), respectively, in high yields, along with P(O)Cl3. W(Adene)(ORF9)2Cl2 is a relative of the [W(cyclopentylidene)(OCH2-t-Bu)2Br2]2 complexes prepared by Osborn, but Mo(Adene)(ORF9)2Cl2 appears to be the first example of an analogous five-coordinate Mo complex. Not surprisingly, W(Adene) (ORF9)2Cl2 and Mo(Adene)(ORF9)2Cl2 are poor metathesis catalysts in the absence of a Lewis acid; the possibility that [Mo(Adene) (ORF9)2Cl][Mo(Adene)(ORF9)2Cl3] and the analogous propagating complexes are the active catalysts has to be considered. The promising recent results in 2-adamantylidene chemistry suggests that much remains to be learned about disubstituted alkylidene complexes in general.

Fig. 29 The structure of Mo(Adene)(ORF9)2Cl2.

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

5.11.2.3

719

Chromium alkylidenes

Cr(VI) alkyl and alkylidene complexes have been pursued for decades. The first stable chromium(VI) alkylidene complexes to be prepared in (1996) were Cr(NAr)2(CH-t-Bu)(L) complexes (L ¼ THF or PMe3), which were formed through the loss of neopentane from Cr(NAr)2(CH2-t-Bu)2 in the presence of L;383 they were not isolated. These findings were extended in 2018 with the isolation of Cr(NAr)2(CHR)(PPh3) (R ¼ t-Bu or SiMe3).384 Three-coordinate Cr(NAr)2(CHR) is proposed to form from Cr(NAr)2(CHR)2(PPh3) and be trapped by PPh3. A determination of the structure of Cr(NAr)2(CH-t-Bu)(PPh3), including location of the a proton, along with NMR studies, showed it to be a bona fide neopentylidene complex with no evidence of any significant CHa agostic266 interaction. It is possible that PPh3 induces a hydrogen abstraction through binding to the metal, a phenomenon well-known to give rise to Mo(VI) and W(VI) alkylidene complexes, although Cr(NAr)2(CH2-t-Bu)2 was shown to react with SiMe4 to form (sequentially) Cr(NAr)2(CH2-t-Bu)(CH2SiMe3) and Cr(NAr)2(CH2SiMe3)2, presumably through formation of, and CH activation by, intermediate Cr(NAr)2(CHR) complexes. Cr(NAr)2(CH2-t-Bu)2 also was shown to react with cyclopentene slowly to yield the structurally-characterized metallacyclobutane complex, Cr(NAr)2[CH(CH2CH2CH2)CHCH(t-Bu)] upon treatment of Cr(NAr)2(CH-t-Bu) with cyclopentene.384 Further studies published in 2018385 showed that Cr(NAr)2(CH2R)2 (R ¼ t-Bu, SiMe3, or Ph) complexes could be deprotonated to give [cat][Cr(NAr)2(CHR)(CH2R)]− complexes in which [cat]+ is Li+, K+, or Et4N+. The authors also showed that Cr(NAr)2(CH2-t-Bu)2 would react with cyclohexane to yield neopentane, cyclohexene, and diamagnetic Cr(IV) complexes derived from intermediate “Cr(NAr)2.” The mechanism is proposed involve CH activation of cyclohexane by intermediate Cr(NAr)2(CHt-Bu) to give Cr(NAr)2(CH2-t-Bu)(cyclohexyl). The reaction between Ag(1,3-dimethylimidazol-2-ylidene)I and Cr(NAr)2(CH2-t-Bu)2 in THF at 22  C has been found to yield Cr(NAr)2(CH-t-Bu)[AgI(1,3-dimethylimidazol-2-ylidene)], along with the tetrahedral chromium(V) complex Cr(NAr)2(1,3dimethylimidazol-2-ylidene)I.386 It is not known whether formation of the neopentylidene complex is induced through coordination of the NHC to the metal or not. To our knowledge no other chromium (VI) alkylidene complexes have been isolated or observed in solution. It should be noted the Cr(NAr)2(CHR)(L) complexes are structurally distinct from all other Mo(VI) and W(VI) complexes described here, and Mo or W M(NAr)2(CHR)(L) complexes are not known. (Molybdenum and tungsten dineopentyl or dineophyl M(NAr)2(CH2R)2 complexes are well-known, but no report of their reaction with PMe3, for example, to give M(NAr)2(CHR)(PMe3), has appeared.) On the other hand, M(N-t-Bu)2(propylene)(PMe3) is known,387 as is Cr(NAr)2(olefin)(L). A 14e count at the metal in an alkylidene complex is required for rapid reaction between an alkylidene and an olefin to form a metallacyclobutane complex (and the reverse), an electron count that is prevented by the bisimido construct if both imido ligands form MdN triple bonds. A bisimido complex is related to a biscyclopentadienyl or “metallocene” framework in which 12 electrons are provided by two cyclopentadienyl anions if they both remain p bound to the metal.387 Cr(NAr)2(CH-t-Bu) therefore is analogous to unobservable intermediate Cp2Ti(CHR) complexes, which can be trapped with norbornene to form metallacyclobutane complexes analogous to Cr(NAr)2[CH(CH2CH2CH2)CHCH (t-Bu)] described above388–391 and to biscyclopentadienyl titanacyclobutane complexes of this type that are catalysts for the polymerization of norbornene.392

5.11.3

Alkylidyne complexes

5.11.3.1

Discovery

A driving force in alkylidyne chemistry was the discovery of homogeneous alkyne metathesis in 1974 by Mortreux,393 who found that Mo(CO)6 in phenol at 160  C would metathesize C6H5C^CC6H4-p-CH3 to give an equilibrium mixture of the three possible alkynes. In 1975 in an article concerned with the mechanism of the olefin metathesis reaction, which was first proposed by Chauvin in 1971,14 Katz suggested that the mechanism of the alkyne metathesis reaction might be analogous to that of olefin metathesis.15 In view of the discovery of Ta(CH-t-Bu)(CH2-t-Bu)3 and [Ta(C-t-Bu)(CH2-t-Bu)3]− and the questions surrounding what type of Mo and W are catalysts for metathesis of alkenes and what type for alkynes, an exploration of the peralkylation of tungsten and molybdenum with neopentyl reagents was undertaken concurrent with the development of tantalum alkylidene chemistry. In 1978 it was shown that addition of 6 equiv. of LiCH2-t-Bu to WCl6 in diethyl ether produced yellow sublimable (t-BuCH2)3W^C-t-Bu in 25% yield.283 Tungsten was reduced at least twice in this reaction, but some reduced tungsten complexes evidently disproportionate to form (t-BuCH2)3W^C-t-Bu in low yield. Soon thereafter it was shown that W(C-t-Bu)(Et3P]O)Cl3 could be prepared in a reaction between W(O)(CH-t-Bu)(Et3P)2Cl2 and hexachloroethane (or R3PCl2), thereby providing another synthetic path to neopentylidyne complexes.394 Ultimately, it was shown that the reaction between HCl and (t-BuCH2)3W^C-t-Bu in the presence of [Et4N]Cl gave [Et4N][W(C-t-Bu)Cl4], from which off-white and sublimable (t-BuO)3W^C-t-Bu was prepared readily. In the absence of [Et4N]Cl and the presence of DME the synthetically even more valuable blue W(C-t-Bu)(DME)Cl3 was formed. It was shown by that tungsten neopentylidyne complexes could be turned into oxo or imido neopentylidene complexes, as discussed earlier in this article, via transfer of a proton to the alkylidyne carbon atom from a hydroxo or amido group.288,395 These and other observations established that a proton on N

720

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

or O could move, or be moved, to an a carbon in alkylidyne complexes and imido, amido, alkoxo, and oxo ligands were likely to be compatible with MdCH2R, M]CHR, and M^CR groups in d0 complexes, and perhaps involved in proton movement within the primary coordination sphere.

ð33Þ

ð34Þ

W(C-t-Bu)(DME)Cl3 was found to react with 3-hexyne to yield the first observed metallacylobutadiene complex, trigonal bipyramidal W[(C(t-Bu)C(Me)C(Me)]Cl3 (Eq. 33) in which the metallacylobutadiene ring lies in the equatorial plane.396 The WdCa bond lengths in the metallacycle are 1.86 A˚ , less that most W]C bond lengths and  0.2 A˚ shorter than WdCa bond lengths in metallacyclobutanes. W[(C-t-Bu)(CMe)(CMe]Cl3 does not lose an alkyne; it reacts with more alkyne to give W(IV) cyclopentadienyl intermediates,397 presumably through formation of a metallacyclohexatriene intermediate and subsequent CdC coupling (Eq. 34), followed by disproportionation of W(IV) to W(V) and W(III) through a chloride radical transfer. It was shown in 1981 that (t-BuO)3W^C-t-Bu is a highly efficient catalyst for metathesis of 3-heptyne, 1600 equiv. of which (neat) were metathesized to equilibrium in less than one minute at 25  C.398,399 Other trialkoxide complexes were soon prepared, along with TBP metallacylobutadiene complexes, and both were shown to be active metathesis catalysts.400,401 Loss of alkyne from a metallacycle to give a four-coordinate (RO)3W^CR0 complex was found to be the rate-limiting step when OR ¼ OAr. However, [(CF3)2HCO]3W(C3Et3) was found to metathesize olefins through an associative process as a consequence of the relatively small size of (CF3)2HCO; a tungstacyclohexatriene complex was proposed as the intermediate.401 A persistent problem with acetylenes as substrates in alkyne metathesis reactions was their often rapid polymerization via insertion into MdC or M]C bonds in a wide variety of circumstances, including a “runaway” expansion of the metallacyclohexatriene ring shown in Eq. (34),402 formation of metallacyclopentadiene rings from lower oxidation state complexes,403 etc. The interest in alkoxides and aryloxides raised a question concerning arylthiolates as compatible ligands, but aryl thiolate alkylidyne complexes were found to be relatively unreactive toward acetylenes, presumably because thiolates are so much more electron-donating than alkoxides.298 Molybdenum trialkoxy alkylidyne complexes were prepared from (t-BuCH2)3Mo^C-t-Bu in the same way as the tungsten complexes just described.404,405 The Mo(C-t-Bu)(OR)3 compounds where OR ¼ O-t-Bu, OCMe2(CF3), OCMe(CF3)2, OC(CF3)3, and OAr (O-2,6-Me2C6H3) were readily prepared and all but Mo(C-t-Bu)(O-t-Bu)3 were found to react with internal alkynes to give metathesis products and the expected (RO)3Mo^CR products. Mo(C-t-Bu)(OAr)3 was found to react with excess 3-hexyne to give the molybdacyclobutadiene complex, Mo(C3Et3)(OAr)3, but the metallacycle readily loses 3-hexyne when redissolved in benzene to yield (ArO)3Mo^CEt in equilibrium with Mo(C3Et3)(OAr)3. Further studies firmly established that the principles of alkyne metathesis by molybdenum alkylidynes were the same as for tungsten alkylidynes,405 but Mo alkylidynes could not be fully explored because their synthesis relied on the capricious synthesis of (t-BuCH2)3Mo^C-t-Bu.405 Attention was also being drawn toward the development of alkene metathesis catalysts of Mo and W at the cost of alkyne metathesis catalyst development. The product of the reaction between CpCl2W^C-t-Bu and diphenylacetylene was found to be 16e WCp[C(Ph)C(t-Bu)C(Ph)] Cl2, a metallacyclobutadiene complex in which the ring is dramatically nonplanar (Fig. 30) and has rearranged to give the a,a-diphenyl disubstitution pattern, possibly via formation of a metallatetrahedrane intermediate.406 A rare early example of an isolable metallatetrahedrane was formed through addition of TMEDA to the metallacyclobutadiene complex, W[(C(t-Bu)C(Me)C(Me)]Cl3 (Eq. 35).407 Triscarboxylate variations (carboxylate ¼ CO2Me, CO2Et, CO2Ph) were also prepared.408 One theoretical study of Mo(C3Me3)Cl3 found that the metallatetrahedrane could only form from a metallacyclobutadiene;409 approach of an alkyne to a Mo^C bond with its axis perpendicular to the Mo^C bond axis was predicted to be forbidden. (Other theoretical

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

721

Fig. 30 The structure of WCp[C(Ph)C(t-Bu)C(Ph)]Cl2.

studies examined a possible interaction between the metal and Cb in a metallacyclobutadiene complex.410,411) Although a metallatetrahedrane is a viable alternative to a planar TBP metallacyclobutadiene intermediate, it still today does not seem likely to be a required intermediate.

ð35Þ

The discovery of (t-BuO)3W^C-t-Bu raised questions as to what type and size groups could be present in the alkylidyne and whether one decomposition pathway might be a bimolecular decomposition to give the acetylene and (t-BuO)3W^W(O-t-Bu)3, a type of compound studied extensively by Chisholm,412,413 or a variation in which the acetylene remains bonded between the two metals as part of a W2C2 tetrahedral core, a “dimetallatetrahedrane.”413 Another question was whether the reverse reaction of (t-BuO)3W^W(O-t-Bu)3 with an internal acetylene might yield (t-BuO)3W^CR complexes (Eq. 36). This stoichiometric metathesis-like reaction between an alkyne and (t-BuO)3W^W(O-t-Bu)3 was found to be smooth for symmetric or unsymmetric internal alkynes (RC^CX; R ¼ Me or Et; X ¼ Me, Et, Pr, CMe3, Ph, CH]CH2, CH2NR2, CH2OMe, CH2OSiMe3, CH(OEt)2, CO2Me, CH2CO2Me, C(O)Me, and S-t-Bu). About half of the compounds could be isolated only when quinuclidine was used to form an adduct, (quin)(t-BuO)3W^CX, which prevented formation of some bimetallic complex.414 It was also found that the C^N bonds in acetonitrile and benzonitrile would react with (t-BuO)3W^W(O-t-Bu)3 to give a 1:1 mixture of (t-BuO)3W^CR and (t-BuO)3W^N (Eq. 37).415 Although compounds were known that contained a W2C2 tetrahedral core, an alternative planar and unsymmetrical 1,3-dimetallacyclobutadiene ring was proposed to be formed and to dissociate to yield two monomeric alkylidyne complexes (Eq. 38).416 The compound shown in Eq. (38) could be viewed as a dimeric version of (t-BuO)3W^CR in which the alkylidyne bridges unsymmetrically. Reactions between acetylenes and (t-BuO)3Mo^Mo(O-t-Bu)3 were explored as a route to (t-BuO)3Mo^CR complexes, but such reactions were found to be successful only for terminal acetylenes and only when the methylidyne was trapped with quinuclidine.417 Alkyne polymerization, especially of terminal acetylenes, continued to be a significant problem. ð36Þ

ð37Þ

ð38Þ

One of the most unusual findings in the chemistry of metallacyclobutadiene complexes is that a b proton in a metallacycle can be lost to a ligand to yield a “deprotiometallacyclobutadiene” complex (Eq. 39). 405,418,419 This reaction, along with alkyne

722

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

polymerization and the bimolecular combination of M^CH complexes to give dimetallatetrahedrane dimeric complexes, hindered facile catalytic metathesis homocoupling of terminal acetylenes to give acetylene itself, an analog of the homocoupling of terminal olefins in olefin metathesis chemistry to give ethylene. However, if ROH is relatively acidic ((CF3)3COH has a pKa of 5.4 in water),280 and a deprotiocycle does not become involved in some other irreversible decomposition, there is no obvious reason why deprotiocycles could not reenter the metathesis cycle. Early studies showed that (t-BuO)3W^C-t-Bu is susceptible to protonation in water420–422 to give oxo neopentyl complexes such as [(t-BuCH2)WO3]− and [O]W(CH2-t-Bu)3]2(O). The neopentyl group itself is relatively resistant to being protonated in water to give neopentane and [WO4]2−. The nucleophilic character of alkylidynes was also evident in Wittig-like reactions, an example of which is shown in Eq. (40).423,424 (t-BuO)3W^C-t-Bu also reacts with HX (HX ¼ HCl, HBr, RCO2H, ArylOH) to give trigonal-bipyramidal neopentylidene complexes of the type W(CH-t-Bu)(O-t-Bu)2  2382 (Eq. 41), which are analogs of the tungsten cyclopentylidene complexes reported by Osborn.

ð39Þ

ð40Þ

ð41Þ

5.11.3.2

Development of catalysts for alkyne metathesis

Alkylidyne chemistry played a major role in the development of alkene metathesis catalysts, but research in alkylidyne chemistry and alkyne metathesis was delayed in the process. The growing success of alkene metathesis as a synthetic method in organic chemistry toward the end of the century led to a realization by organic chemists that alkyne metathesis could also prove to be of great value in organic synthesis. The movement began in 1998 with a paper concerning the synthesis of cyclic alkynes using (t-BuO)3W^C-t-Bu or (t-BuO)3W^W(O-t-Bu)3, the most readily available initiators at the time.425 Several groups then aimed to develop alkyne metathesis catalysts that are highly active, easy to prepare, tolerant of functional groups, successful in metathesizing terminal alkynes, etc.223,426–429 We will be concerned here largely with fundamental catalyst developments rather than efficiency and scope in alkyne metathesis chemistry in organic chemistry, although it is clear that efficiency in catalysis and catalyst/ligand design are intimately linked. Isolated and characterized alkylidyne complexes are listed in Table 2. It should be noted that several important versions of alkyne polymerization by well-defined alkylidene or alkylidyne initiators were eventually tamed with well-defined catalyst initiators. The three most prominent, which had been explored briefly in the early days of alkyne metathesis chemistry, are polymerization of terminal alkynes such as substituted phenylacetylenes429–431 by alkylidene initiators,377 cyclopolymerization of diynes, especially 1,6-heptadiynes,432 by alkylidene initiators, and ring-opening alkyne polymerization (ROAMP) by alkylidyne initiators (see below). The search for more highly active, accessible, and proton and functional group tolerant alkyne metathesis catalysts led to many discoveries and ultimately many improved catalysts. As with alkene metathesis, there was a need to evaluate ligands and efficiency of catalysts in organic reactions without the need to make and isolate each catalyst separately. The catalyst could then be isolated at some point if desired. A synergistic relationship between fundamental advancements and efficiency of catalysts proved mutually beneficial to both organic and inorganic/organometallic chemists and to the fields of both alkene and alkyne metatheses. A potentially attractive method of synthesizing tungsten alkylidyne complexes was the addition of alkynes to (t-BuO)3W^W(O-t-Bu)3 to give (t-BuO)3W^CX complexes, as noted above.414,415 The reaction between (RF3O)3W^W(ORF3)3 and acetylenes was reported in

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

723

1985433 and was recently used to make (RF3O)3W^CPh from 2-phenyl-1-propyne.167 However, the cleavage of W^W and Mo^Mo bonds by internal or terminal acetylenes has been increasingly unsuccessful as the electron-withdrawing ability of the alkoxides increases402,417 because the 1,3-dimetallacyclobutadiene (Eq. 38) becomes less prone to split spontaneously into monomers. ð42Þ

ð43Þ ð44Þ 15

14

Chisholm was the first to note that (t-BuO)3W^N would catalyze the scrambling of N and N between organonitriles, presumably via diazametallacylobutadiene complexes (Eq. 42),434 and that (t-BuO)3Mo^N was not an efficient alkyne metathesis catalyst. Johnson later found that (RF6O)3Mo^N and (RF9O)3Mo^N complexes were competent catalysts for a similar degenerate nitrogen atom scrambling, in contrast to (t-BuO)3Mo^N.435 He therefore turned to possible reactions between W or Mo nitrido trialkoxide complexes and internal alkynes and discovered the organonitrile alkyne cross-metathesis (NACM) reaction shown in Eq. (43), in which a sacrificial alkyne such as 3-hexyne is used to form RC^CR from RCN, where R is an aryl group; the initiators for this reaction are (RF3O)3W^N and (RF6O)3W^N.436 These reactions can be driven to completion through loss of relatively volatile propionitrile. He found that (RF6O)3Mo^N and (RF9O)3Mo^N complexes would react with 3-hexyne437 to give (RF6O)3Mo^CEt and (RF9O)3Mo^CEt complexes (Eq. 44). Furthermore, it was found that Lewis acids (e.g., MgCl2) will speed up more sluggish conversions (presumably through reversibly binding to the nitride) so that even (RF3O)3Mo^CEt and (t-BuO)3Mo^CEt complexes could be prepared from nitrides.438 Moore also discovered this “Lewis acid effect.”439 (RF6O)3W^N also will react with ArylC^CEt (Aryl ¼ p-OMeC6H4 or p-CF3C6H4) to give (RF6O)3W^CAryl derivatives or DME adducts thereof.440 These investigations suggest that both Mo and W alkylidyne trialkoxide complexes can be made from the analogous nitrides, although it is not necessarily a reliable synthesis for all combinations and alkyne polymerization can still hinder isolation of a pure metal-containing product. Molybdenum came to be preferred as an alkyne metathesis catalyst vs tungsten because molybdacyclobutadiene complexes lose alkyne more readily405 and therefore yield a higher alkyne metathesis rate. In 2000 Cummins showed that molybdenum alkylidyne complexes could be accessed through rearrangement of Z2-vinyl complexes, as shown in Eq. (45).441 Three years later442 a related strategy was published in which cationic acetylene complexes were deprotonated and coupled to give the ene bisalkylidyne shown in Eq. (46) for R ¼ Ph, Me, n-Bu, t-Bu, TMS, (CH2)3CN, 4-CF3C6H4, 4-MeOC6H4, and CH2NPh2 (RArN ¼ (t-Bu)(Ar3,5Me2)N]−). Addition of 2-phenylphenol (PPOH) gave the analogous triphenoxide derivatives, from which the enediynes could be isolated upon metathesis cleavage with diphenylacetylene to give trans-PhC^CCH(R)]CH(R)C^CPh and 2 equiv. of (PPO)3Mo^CPh. Analogous reactions with (HC^CCH2)2X (X ¼ CH2, (CH2)2, (CH2)3, or O) led to formation of the bisalkylidynes shown in Eq. (47) and corresponding enediynes. The apparent ease of replacing amido ligands of the type [N(t-Bu)(Ar3,5Me2)]− simply through addition of alcohols or phenols ligand drew attention to [NArR]3Mo itself, a rare trigonally symmetric platform pioneered and developed by Cummins.443–445

ð45Þ

ð46Þ

ð47Þ

Fürstner discovered that [NArR]3Mo was a modestly active alkyne metathesis catalyst for ring-closing alkyne metathesis (at 10% loading) and proposed it to be a “precatalyst.”446 But he also noted that it reacts immediately with dichloromethane to give [NArR]3MoCl, and, among the other products, [NArR]3Mo^CH,447 a compound previously prepared by Cummins.448 However,

724

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

[NArR]3Mo^CH was found to be a poor alkyne metathesis catalyst, if at all, which is not surprising in view of the s and p electron-donating ability of amides and the lack of metathesis activity of imido alkylidene complexes that contain two disubstituted amido groups such as Mo(NAr)(CH-t-Bu)(NPh2)2.57 However, Cummins also showed that [NArR]− ligands could be replaced by adamantoxides through addition of adamantanol to give highly active analogs of known alkyne metathesis catalysts.441 Moore then showed that the reaction between [NArR]3Mo and RCHCl2 led to formation of [NArR]3MoCl and [NArR]3Mo^CR complexes, and moreover, that [NArR]3MoCl could be reduced with magnesium back to [NArR]3Mo in situ, therefore providing an efficient synthesis of [NArR]3Mo^CR complexes in good yield. Finally, [NArR]3Mo^CEt was shown to react with a variety of alcohols and phenols, silanols, etc., to yield active alkyne metathesis catalysts.155,449–452 [NArR]3Mo^CEt therefore became the pre-catalyst of choice for evaluating new ligands for alkyne metathesis reactions of interest to organic chemists, and isolating those catalysts if so desired. Given that dichloromethane can be a ready source of a chloride radical, a plausible explanation of the formation of [NArR]3Mo^CR complexes (often R ¼ Et) is shown in Eqs. (48) and (49). The proposed loss of HCl from [RArN]3Mo(CHClR0 ) (Eq. 48) is related to the loss of molecular hydrogen from trigonal bipyramidal Mo(IV) and W(IV) triamidoamine alkyl complexes, [N(CH2CH2NR00 )3]MCH2R.293,294 ð48Þ

ð49Þ The alkyne metathesis catalyst field was further advanced through syntheses of aryl-substituted alkylidyne tribromide complexes of both Mo and W, a route based on the work by Mayr in the 1980s.453,454 This method is now called the low oxidation-state route because the alkylidyne tribromide is prepared through oxidative bromination of a monobromide. The full series of M(CMes) (OR)3 complexes (Mes ¼ mesityl) where OR ¼ O-t-Bu, ORF3, ORF6, and ORF9 has now been prepared, along with W(CMes) (ORF3)3, W(CMes)(ORF9)3, and related complexes.156,163,164 Coordinating solvents such as THF and DME are best avoided when the metal is highly electrophilic, as they can significantly slow the rate of reaction with an alkyne. Mo(CMes)(ORF9)3 was found to react with excess 3-hexyne to give a purple triethylmetallacyclobutadiene complex, and although it could be isolated and characterized structurally at low temperature, the crystal suffered from disorder problems and lost alkyne at ambient temperatures. Several molybdacyclobutadiene complexes were observed in solution in the initial phase of molybdenum alkylidyne chemistry,405 but structural studies are still challenging. Be that as it may, there is no doubt that Mo and W form analogous TBP metallacyclobutadiene intermediates in alkyne metathesis. Trisiloxide complexes of molybdenum have been explored because the three siloxide ligands resemble what might be found on a silica surface where alkyne metathesis activity was first observed, namely three MdOdSisurf bonds.13 They also make the metal relatively electrophilic (due in part to reduced p donation from O to the metal) and can provide significant bulk and molecular weight that aid isolation of catalysts. The silicon must of course be relatively resistant to attack and resulting Si-O bond cleavage. Moore has made Mo^CEt complexes that contain two polyhedral oligomeric silsesquioxanes (POSS ligands), but only two of these sterically demanding ligands would add to [NArR]3Mo^CEt to give [NArR][HNArR][POSS]2Mo^CEt.155 A related tungsten compound, W(CPh)[OSi(O-t-Bu)3]3, was also reported and found to be active for the metathesis of terminal alkynes in the presence of 5 A˚ molecular sieves to absorb acetylene.173 W(CPh)[OSi(O-t-Bu)3]3 was found to be a relatively efficient catalyst for the metathesis coupling of terminal alkynes. It also was found to initiate the formation of diynes from the coupling of terminal alkynes and then cross-metathesize the symmetric diynes to give unsymmetric diynes, the mechanism of which was found to be consistent with an alkyne metathesis mechanism.169 The only compound that could be isolated was [(t-BuO)3SiO]3W^CdC^CdC^W[OSi(t-BuO)3]3, which was also crystallographically characterized. Fürstner found that triphenylsiloxides are valuable ligands in alkylidyne chemistry and that (Ph3SiO)3Mo^CR complexes can be prepared in a variety of ways and made relatively stable to air in the solid state as bipy or 1,10-phenanthroline adducts.160 The triphenylsiloxide complex is formed when a Lewis acid is added (e.g., ZnCl2 or MnCl2) to bind bipy or phen.157 It is also possible to use the nitrido tristriphenylsiloxide complex as the catalyst precursor.455 The 18e octahedral bipy adduct is relatively stable to air, but can be activated in solution. (Ph3SiO)3Mo^CR complexes have also been found to be effective initiators for metathesis of terminal alkynes.456 Complexes of the type shown in Fig. 31 contain a ligand in the “podand” class,178–180 also called “canopy catalysts,”457 in which R is phenyl, methyl, or isopropyl.179 These structurally relatively flexible ligands resist being readily removed from the metal through protonolysis, yet maintain an electrophilic Mo center and the flexibility to access the intermediates required for efficient alkyne metathesis. They also appear to block access by an alkyne to the metallacyclobutadiene intermediate and subsequent polymerization of the alkyne. Catalysts in the Podand class may supersede catalysts that contain monodentate siloxides. Other tripodal constructs are known (see below). The challenges encountered in arriving at the final design have been described.179 Pseudorotation of metallacyclobutadiene complexes have been found to be crucial features of catalysts that otherwise seem to be locked into some relatively non-productive form.457 As mentioned earlier in this section the metallatetrahedrane complex, Mo(C3Et3-hedrane)(Ph-Podand) (where the Ph-Podand is the one in which R in Fig. 31 is phenyl), is formed upon treatment of the initiator with excess 2-hexyne and was isolated and

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

725

MeO R

R

C

R R

O Si R Mo O O Si R Si

Fig. 31 An example of an alkylidyne Podand complex.

crystallographically characterized. The prevailing opinion is that a TBP metallacyclobutadiene complex is the required intermediate in alkyne metathesis and the corresponding metallatetrahedrane may or may not form to a significant extent (reversibly) before an alkyne is lost from the TBP metallacyclobutadiene complex.

ð50Þ

ð51Þ

ð52Þ

Although the majority of alkyne metathesis catalysts are trisalkoxides or siloxides, other types have been prepared. Tamm synthesized a series of variations in which one of the alkoxides in a trisalkoxide alkylidyne tungsten complex is replaced with a bulky amido ligand ([NArR]−) or an imidazolin-2-iminato derivative ([NImR]−; see Eq. 50). Some complications reported in these initial studies included a CH bond cleavage in a OC(Ph)(CF3)2 ligand (Eq. 50) or opening of an [NImt-Bu]− ligand to give the

726

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

bimetallic complex shown in Eq. (51). (An aryl CH cleavage had also been observed when the synthesis of a bis[OC(CF3)2(p-tolyl)]− imido neopentylidene derivative was attempted.295) M(CPh)(ORF6)3(DME) and M(CPh)(ORF6)2(NImt-Bu) (M ¼ Mo or W) formed the propylidyne complexes upon reaction with 3-hexyne.158 This direction has been expanded to include phosphoraneiminato complexes, W(C-t-Bu)(NPR3)(ORF6)2, where R ¼ Cy or i-Pr170, and W(CPh)(L)(ORF6)2, where L is 1,3-di-t-butylimidazolidin2-iminato or 4,5-dimethyl-1,3-di-t-butylimidazolin-2-iminato (Eq. 52).168 The “push/pull” combination of the ligands was proposed to lead to high metathesis rates. Recent studies have confirmed that metathesis intermediates (metallacyclobutadiene and a deprotio version) can be prepared starting with molybdenum or tungsten complexes that contain fluoroalkoxy ligands, often OC(CF3)2Ph, ORF6, or ORF9.154 Examples of stable, but catalytically active, tungstenacyclobutadiene complexes are W[C(Ph)C(Me)C(Me)][OC(CF3)2Ph]3, W(C3Et3)[OC(CF3)2Ph]3, and W[C(C^CMe)C(Me)C(C^CMe)][OC(CF3)2Ph]3. Metallacyclobutadienes tend to have some alternation of double and single bond lengths around the MC3 ring. Mo(C3Et3)[OC(CF3)2Me]3 could be isolated at low temperature and its connectivity confirmed in an X-ray study; it should be compared with the isolated, but not crystallographically characterized, Mo(C3Et3)(OAr)3 analog, which also loses 3-hexyne at room temperature in solution.404,405 The reaction between Mo(CMes) (ORF9)3 and phenylacetylene yields a deprotiocyclobutadiene complex (see Eq. 39) in which Ph and Mes are on the a carbon atoms.

ð53Þ

Ring-opening alkyne metathesis polymerization (ROAMP) was first demonstrated with cyclooctyne and tri-t-butoxide alkylidyne complexes of Mo and W, or (t-BuO)3W^W(O-t-Bu)3, as initiators.458,459 Much more interesting dibenzocyclooctynes were prepared and shown to be polymerized immediately by (t-BuO)3W^C-t-Bu and by molybenum p-nitrophenoxide Mo catalysts prepared in situ from [ArRN]3Mo^CEt after addition of a phenol.460 A triphenol precursor to a tridentate podand ligand, N(CH2C6H4OH)3, was used to prepare a molybdenum alkylidyne, [N(CH2C6H4O)3]Mo^CEt, which was shown to be a dimer of the idealized structure shown in Eq. (53) with two asymmetrically bridging phenoxide oxygen atoms (2.00 and 2.34 A˚ ) and with a somewhat longer than expected donor ModN bond (2.28 A˚ ).461 The dimer was a poor initiator for ROAMP of dibenzocyclooctynes, but methanol was found to cleave the dimer to produce a proposed more reactive adduct of monomeric [N(CH2C6H4O)3] Mo^CEt, perhaps without the nitrogen donor still being coordinated. The podand ligand was resistant to being removed totally, but eventually it was fully protonated and removed, and metathesis activity ceased. The relatively flexible nature of the multidentate podand ligand is an asset in that it allows access to all important intermediates in the metathesis reaction if the nitrogen donor can dissociate. This first-generation podand ligand led to many other tripodal ligands designed to resist protonolysis, block alkyne polymerization, and provide increased metathesis reactivity and longevity all at the same time. As far as is known, the principles of any alkyne metathesis reaction remain the same, i.e., a five-coordinate metallacyclobutadiene intermediate is required for facile metathesis. A para nitro version of the ligand in Eq. (53) was introduced by Zhang in 2011.174 Versions in which the nitrogen is methylated led to much more active catalysts because nitrogen could no longer bind to the metal.175 For similar reasons a central SiMe group was a successful component of various multidentate triphenol-based ligands176 as was a tertiary carbon center (CH).177 In the last decade the ROAMP reaction has become a unique way to synthesize polymers in a living manner from dibenzocyclooctynes that have low polydispersities, controllable molecular weights, and valuable optical properties. The topologies of the polymers also depend dramatically upon the nature of the initial alkylidyne and on the ligands present on the metal. Typical initiators have been (RF6O)3(DME)Mo^C(p-C6F4X) (X ¼ NMe2, OCH3, CH3, H, OAc, CF3, NO2),159 K[(ONO)(RF6O)2Mo^Cp-tolyl],181 or (XC6H4O)3Mo^CEt where X is o-NO2, o-CH]NPh, o-Me, p-CH]NPh, or p-NO2.462 The last can be prepared in situ from [ArRN]3Mo^CEt] and the phenol. Addition of 3 equiv. of HO-2,6-Ph2C6H3 to W(C-t-Bu)(CH2-t-Bu)3 gave W(C-t-Bu)(O-2,6-Ph2C6H3)3,144 but the use of W(C-t-Bu)(CH2-t-Bu)3 as a pre-catalyst has not taken hold relative to alternative approaches.

5.11.3.3

Alkyl/alkylidyne vs bisalkylidene

Xue has studied the isomerization of several types of tungsten alkyl/alkylidyne complexes to give bisalkylidene complexes.10 For example, (t-BuPh2Si)(t-BuCH2)2W^C-t-Bu was found to be in equilibrium with (t-BuPh2Si)(t-BuCH2)W(CH-t-Bu)2 and a mixture of the two was found to react with oxygen to give the oxo alkylidene complex, (t-BuCH2)2W(O)[C(t-Bu)(t-BuPh2Si)].123 It was proposed that the silyl group in (t-BuPh2Si)(t-BuCH2)2W^C-t-Bu first migrates to the neopentylidyne carbon atom to give (t-BuPh2Si)(t-BuCH2)2W]C(t-Bu)(t-BuPh2Si), possibly as oxygen attacks the metal to give an oxygen complex, (t-BuPh2Si) (t-BuCH2)2W(O2)[C(t-Bu)(t-BuPh2Si)], which then loses an oxygen atom to (t-BuPh2Si)(t-BuCH2)2W]C(t-Bu)(t-BuPh2Si) to give two molecules of (t-BuCH2)2W(O)[C(t-Bu)(t-BuPh2Si)].

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

727

ð54Þ

The alkylidyne complex, (Me3SiCH2)3W^CSiMe3, reacts reversibly with PMe3 to give the monophosphine complex, (PMe3) (Me3SiCH2)3W^CSiMe3, which is then converted into (Me3SiCH2)2W(CHSiMe3)2(PMe3) through migration of a proton on a neopentyl a carbon atom to the alkylidyne a carbon atom, as shown in Eq. (54).129,130 An X-ray structure of the bisalkylidene was disordered around the pseudo-three-fold axis of the TBP structure, so the W]C values could not be determined accurately (avg ¼ 1.963(12) A˚ ); the axial WdC distance was 2.042(18) A˚ , which suggests that the alkylidenes are in the equatorial positions with JCHa values of 123.5 Hz and 102.6 Hz; one has a CHa agostic interaction larger than the other, as was found in Ta(CHt-Bu)2(PMe3)2Cl (98 and 86 Hz).271,272 An analogous PMe2Ph derivative, (Me3SiCH2)2W(CHSiMe3)2(PMe2Ph), was prepared, but was relatively unstable. In the presence of PMe3 (Me3SiCH2)2W(CHSiMe3)2(PMe3) was converted into W(CSiMe3)(CHSiMe3) (CH2SiMe3)(PMe3)2 and neopentane evolved.463 Similar results were observed when dmpe (Me2PCH2CH2PMe2) was added to (Me3SiCH2)3W^CSiMe3 to yield first (Me3SiCH2)2W(CHSiMe3)2(dmpe), in which only one P is bound to W, and then W(CSiMe3)(CHSiMe3)(CH2SiMe3)(dmpe), in which dmpe is bidentate. A review of equilibria between bisalkylidene and alkyl/ alkylidyne complexes appeared in 2011.10

5.11.3.4

Other alkylidyne chemistry

ð55Þ

As noted in the first part of this article, attempts to prepare bipy adducts of bisdimethylpyrrolide complexes led to formation of imido alkylidyne complexes of the type Mo(NR)(CR0 )(Me2Pyr)(bipy) (R0 ¼ t-Bu or CMe2Ph) through movement of an alkylidene a proton to a dimethylpyrrolide ligand.25 It has also been shown that Ph3P]CH2 deprotonates the alkylidene in Mo(NAr)(CHt-Bu)[OCMe(CF3)2]2 to give the imido alkylidyne complex, [Ph3PMe][Mo(NAr)(C-t-Bu)[OCMe(CF3)2]2].161 The reaction between Mo(NArMe2)(CHCMe2Ph)(OTf )2(DME) and excess Me3CCH2MgCl produces the neophylidyne complex (Mo^C ¼ (1.7500(16) A˚ ) shown in Eq. (55);162 the ModN distance (1.8930(13) A˚ ) is more consistent with a metal-amido bond, as that nitrogen is also bound to Mg. There are also many examples of deprotonation of an alkylidene ligand in Pincer complexes (see Section 5.11.4). What is not known is the extent to which an a proton in a M]CHR complex can be lost to a ligand or external base in alkylidene complexes other than neopentylidene or neophylidene under relatively mild conditions, whether cationic complexes or those that contain highly electron-withdrawing ligands might be especially susceptible to loss of an a proton, and, ultimately, whether loss of an a proton is a more common mode of decomposition of alkylidene complexes than previously assumed. Unless “decomposition” yields an identifiable product it is difficult to answer these questions. However, studies concerned with the chemistry of disubstituted alkylidene complexes (see earlier section) are beginning to shed some light on the topic.

ð56Þ

728

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Mo(C-t-Bu)(CH-t-Bu)(Cl)(PMe2Ph)2 was prepared as off-white crystals in 26% yield through addition of 2.5 equiv. of Mg(CH2-t-Bu)2 to Mo(O)[OC(CF3)3]4 in diethyl ether followed by 3 equiv. of PMe2Ph and a workup that includes dichloromethane (the source of a chloride radical; Eq. 56).128 Mo(C-t-Bu)(CH-t-Bu)(Cl)(PMe2Ph)2 is obtained as largely a syn isomer that equilibrates to give approximately a 1:1 mixture of syn and anti isomers within 1–2 h in solution. Mo(C-t-Bu)(CH-t-Bu)(Cl) (PMe2Ph)2 reacts with Li(3,5-dimethylpyrrolide) to give Mo(C-t-Bu)(CH-t-Bu)(Z1-Me2Pyr)(PMe2Ph)2 as a pale yellow solid, while subsequent addition of Ph3SiOH to the pyrrolide complex gave a mixture of syn and anti Mo(C-t-Bu)(CH-t-Bu)(OSiPh3)(PMe2Ph)2. All three compounds tend to lose PMe2Ph to give 14e monophosphine complexes with the formulas Mo(C-t-Bu)(CH-t-Bu)(X) (PMe2Ph) (X ¼ Cl, Me2Pyr, or OSiPh3), none of which could be isolated. This work underscores the ease with which oxo ligands can be removed by a strong alkylating agent and the likehood that reduced Mo complexes are produced and reoxidized through abstraction of a chloride radical from dichloromethane to give the observed Mo(VI) product. The structures of the isolated TBP complexes (axial phosphines) are analogous to tantalum bisalkylidene complexes such as Ta(CH-t-Bu)2(PMe3)2Cl.271,272 The 14e initiator in a hypothetical metathesis reaction would be Mo(C-t-Bu)(CH-t-Bu)(X)(PMe2Ph), but no significant studies were carried out.

ð57Þ

ð58Þ

The reaction between WCl3(OAr)3 and 4 equiv. of t-BuCH2MgCl in diethyl ether yields yellow crystalline W(C-t-Bu)(CH2-t-Bu) (OAr)2 (Eq. 57).189 This complex reacts with 2-butyne and 3-hexyne to generate symmetric metallacyclobutadiene species, W(C3R3) (CH2-t-Bu)(OAr)2 (R ¼ Me and Et, respectively), which could be isolated, but which decomposed thereafter readily and therefore could not be fully characterized. W(C-t-Bu)(CH2-t-Bu)(OAr)2 reacts with LiNPh2 to yield W(C-t-Bu)(CH2-t-Bu)(NPh2)2, which in turn gives W(C-t-Bu)(CH2-t-Bu)(OAd)2 upon treatment with 1-adamantanol. Preliminary studies concerning the catalytic metathesis efficiency of W(C-t-Bu)(CH2-t-Bu)(OAr)2, W(C-t-Bu)(CH2-t-Bu)(NPh2)2, and W(C-t-Bu)(CH2-t-Bu)(OAd)2 for 3-heptyne revealed that only W(C-t-Bu)(CH2-t-Bu)(OAd)2 is a relatively efficient catalyst that generates no polymer in the process. There is no evidence yet that this route to alkylidynes will prove to be competitive with other routes to tungsten alkylidynes. Addition of one equiv. of water to Mo(CAro)(ORF6)2(DME) (Aro ¼ o-(OMe)C6H4) leads to protonation of one ORF6 and formation of a dimeric hydroxy alkylidyne complex in which each bridging hydroxide proton points toward an oxygen atom in an arylmethoxy group (Eq. 58).114 In spite of the hydrogen bonding of the OH group to the methoxy oxygen in this dimer, an orthomethoxy group is not required; addition of 1 equiv. of water (in THF) to the benzylidyne complexes Mo(CArp) (OR)3(THF)2 (Arp ¼ para-methoxyphenyl, OR ¼ ORF6 or ORF9) leads to formation of similar complexes in which THF is hydrogen-bonded to the bridging hydroxo groups.115 These dimeric hydroxyalkylidyne complexes will react with phosphines to give oxo alkylidene complexes, as noted in an earlier section, but the crucial proton movement from the OH to the benzylidyne carbon atom is proposed to take place in a monomeric complex. Nonafluoro-t-butoxide complexes seem to give consistently higher yields of monomeric oxo alkylidene products. Mo(CArp)(OR)3(DME) and Mo(CArp)(OR)3(THF)2 complexes (OR is ORF6 or ORF9) were prepared and characterized as part this study. Later studies showed that it appears best to avoid formation of the dimer shown in Eq. (58) and that phosphines are not necessary for preparing oxo alkylidenes from alkylidynes.116

ð59Þ

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

729

ð60Þ

ð61Þ

Addition of the diphenol H(OSSO)H (Eq. 59) to (t-BuO)3W^CEt gave (OSSO)W(CEt)X (X ¼ O-t-Bu), from which X ¼ Cl and CH2SiMe3 derivatives were prepared.137 The (OSSO)W(CEt)(X) derivatives where X ¼ O-t-Bu or CH2SiMe3 yielded the rare cationic vinylidene complexes, [(OSSO)W(CC]CHMe)(X)][B(C6F5)4], upon abstraction of a beta hydride in the alkylidene through addition of [Ph3C][B(C6F5)4]. The (OSSO)W(CEt)(CH2SiMe3) complex could be protonated with [PhNMe2H][B(C6F5)4] to give [(OSSO)W(CHEt)(CH2SiMe3)][B(C6F5)4], which could be isolated or heated in situ to give a cationic propylidyne complex, [(OSSO)W(CEt)(NMe2Ph)][B(C6F5)4], through a hydrogen abstraction (Eq. 60). A vinylidene complex could be prepared from a neutral propylidyne complex through the beta hydride abstraction shown in Eq. (61). The vast majority of alkylidynes that have been prepared have a carbon atom or a proton (rarely) attached to the alkylidyne carbon atom. Exceptions are (t-BuO)3W^CS(t-Bu) and (quin)(t-BuO)3W^CNEt2 (quin ¼ quinuclidine),414 and complexes of the type (t-BuO)3W^CEPh3 and [(t-BuO)3W^C]2EPh2 where E is Si, Ge, or Sn.171,172 All were made from alkynes through metathesis reactions with W^C or W^W bonds. With the view that we have a lot to learn about M^CX complexes where X is not C or H, and about reactions that involve alkynes that contain an X group directly bound to the alkyne carbon, reactions between R2NC^CNR2 (R ¼ Et or piperidin-1-yl) and Mo(CMes)(ORF6)3 were explored.464 The resulting compounds (Eq. 62) are paramagnetic. The authors suggest that these are Mo(IV) complexes that contain an anionic diaminodicarbene ligand, [(R2N)CC(Mes)C(NR2)]−. Calculations suggest that the initial metallacycle, (RF6O)3Mo[(R2N)CC(NR2)C(Mes)], rearranges to the final form without forming a metallatetrahedrane intermediate. In spite of the exotic nature of this result, it could be argued that the area of heteroatom-containing M^CX complexes and substrates is underexplored. It remains to be seen whether alkyne metathesis with heteroatom (X) groups in the alkyne could take part in actual catalytic metathesis reactions.

ð62Þ

ð63Þ

730

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

ð64Þ

In this vein, it should be noted that the first high oxidation state bromomethylidyne complex, W(CBr)Br3(Cy2PCH2CH2PCy2), has been prepared through addition of bromine to W(CSiPh3)Br3(Cy2PCH2CH2PCy2) (Cy ¼ cyclohexyl).190 The authors propose that a cationic W]C(SiPh3)(Br) complex is formed (Eq. 63) through attack on the alkylidyne a carbon atom by Br+ and that a bromide anion then attacks Si in the cationic complex to give Ph3SiBr and the observed product. There is a vast chemistry of M^CX complexes of polypyrazolylborate alkylidynes in which the metal is in an oxidation state lower than M(VI),465 so analogous high oxidation state alkylidyne chemistry could be accessible. Treatment of the triethylammonium salt of [[F3NMe]MoCl3]− (where [F3NMe]2− is [(3,4,5-C6F3H2NCH2CH2)2NMe]2−) with 3 equiv. of t-BuCH2MgCl produced the five-coordinate alkylidyne/alkyl complex, [F3NMe]Mo(C-t-Bu)(CH2-t-Bu) (Eq. 64), while analogous reactions between [Et3NH][[F3NMe]WCl3] and Me3CCH2MgCl or Me3SiCH2MgCl lead to five-coordinate complexes of the type [F3NMe]W(CR)(CH2R) where R ¼ CMe3 or SiMe3, respectively.188 Similarly, treatment of [Et3NH][[ArClNNMe]MoCl3] (where [(ArCl2NCH2CH2)2NMe]2− ¼ [ArCl2NNMe]2−) with 3 equiv. of t-BuCH2MgCl gave [ArCl2NNMe]Mo(C-t-Bu)(CH2-t-Bu).187 In all cases it was proposed that [diamidoamine]M(CH2R)2 complexes are intermediates that undergo first an a elimination to yield [diamidoamine]M(CHR)(H)(CH2R) intermediates of M(VI), and that these then lose molecular hydrogen to give the observed alkylidyne/alkyl products (Eq. 64). a,a-Dehydrogenation of neopentyl and often other alkyls in [triamidoamine]Mo(CH2R) and especially [triamidoamine]W(CH2R) complexes to give [triamidoamine]Mo(CR) complexes was observed in the late 1990s.293,294,466 The reaction between K2[Biphen] and Mo(NArCl2)(CH-t-Bu)(OTf )2(DME) (in the presence of 10 equiv. of triethylamine gave Mo(C-t-Bu)(NHArCl2)[Biphen], while addition of K2[S-Biphen] to Mo(NArCl2)(CHCMe2Ph)(OTf )2(THF) in THF led to the isolation of Mo(NHArCl)(CCMe2Ph)[S-Biphen].29 Addition of 1 equiv. of H2[Biphen] to Mo(CCH2SiMe3)[N(i-Pr) Ar00 )]3 (Ar00 ¼ 3,5-dimethylphenyl) produced Mo(CCH2SiMe3)[Biphen][N(i-Pr)Ar00 )] in situ, which when treated with one equiv of 1-adamantanol gave a mixture of Mo(CCH2SiMe3)[Biphen](OAd) and 3 equiv. HN(i-Pr)Ar00 , from which Mo(CCH2SiMe3)[Biphen](OAd) could be isolated. Addition of 10 equiv. of 2-butyne or 3-hexyne to a pale yellow solution of Mo(CCH2SiMe3)[Biphen](OAd) produced the molybdacyclobutadiene complexes Mo(C3R3)[Biphen](OAd), but both decomposed slowly in solution even in the presence of added alkyne. These results are consistent with facile relocation of protons on C and N atoms in the coordination sphere in some circumstances.

ð65Þ

ð66Þ

A characteristic feature of all high oxidation state alkylidyne complexes that do not contain a rigid multidentate ligand is that they do not react with olefins (an YneEne reaction), perhaps in part because the only facile further step is to revert to form the alkylidyne. In contrast, alkynes usually react readily with alkylidenes (an EneYne reaction) to give a vinylalkylidene. However, Veige has recently proposed that an YneEne reaction of a Pincer complex (see Section 5.11.4) initiates polymerization of norbornene to give a new alkylidene bound to the metal at one end and through a vinyl MdC bond at the other (Eq. 65). After many alkylidene-catalyzed norbornene insertion steps the reverse of the initiation reforms the alkylidyne and a cyclic polymer.403,467 He also could observe (by NMR) reversible formation of a metallacyclobutene made through addition of ethylene to the alkylidyne. This behavior might be ascribed to the aggressive reactivity of the alkylidyne in the [CF3ONO]3− ligand system (see Section 5.11.4).

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

731

Further evidence that YneEne reactions are possible outside a system that contains the [CF3ONO]3− ligand is a recent publication by Bukhryakov.153 He reports ring-closing of olefins, homocoupling of olefins, and polymerization of cyclooctene by a Mo(CAryl) (ORF6)3(DME) complex in which the R1 is nitro and R2 is CF3 (e.g., Eq. 66; >99% yield); the yield is 500,000 for 1-octene and 1-nonene.186 This is in line with the mechanistic investigations regarding the neutral bistriflate NHC complexes, from which a triflate dissociates to form the active cationic species (vide supra).194,195 Another noteworthy feature of cationic monotriflate NHC complexes is their tolerance toward alcohols, e.g., Mo029 catalyzes the cross-metathesis of 4-penten-1-ol, 5-hexen-1-ol, 7-octen-1-ol and 2-allylphenol in iPrOH with TONs of up to 1100.30

5.11.5.1.3

Monotriflate monoalkoxides

Monotriflate monoalkoxide complexes were synthesized by treating a bistriflate complex with a lithium or potassium alkoxide in 1,2-dichloroethane (Eq. 85, Fig. 36). No ligand scrambling was observed upon substitution of one of the two triflate ligands. Mo051, Mo052, Mo054, and Mo055 have geometries intermediate between SP and TBP, while Mo054 adopts a distorted SP geometry at the metal center with the alkylidene ligand in the apical position and Mo058 a TBP geometry with the IMes and triflate ligands in axial positions. Monotriflate monoalkoxide complexes demonstrate productivities comparable to the bistriflate complexes in selected metathesis reactions. A mesoionic version has also been prepared (Eq. 86).196

ð85Þ

ð86Þ

5.11.5.1.4

Bisalkoxides

Bisalkoxide complexes were synthesized by reacting a bisalkoxide precursor with less sterically demanding IMeCl2. t-Butylamine was replaced by the NHC to give Mo062 (Eq. 87).195 Addition of lithium hexaisopropylterphenolate (LiOHIPT) to Mo062 gave Mo063.

Fig. 36 Monotriflate monoalkoxide complexes.

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

739

ð87Þ

Bis(hexafluoro-tert-butoxide) complexes Mo064–Mo065 were synthesized by treating of the respective bisalkoxide precursor with either the free carbene or the silver NHC adduct in methylene chloride or n-pentane (Eq. 88). When sterically more demanding NHCs such as IMes (Mo067) or 6Mes (Mo068) were employed, amido alkylidyne complexes were formed. Complexes Mo069– Mo071 were prepared as shown in Eq. (89). Single-crystal X-ray analysis of Mo062 and Mo065 revealed a distorted SP geometry at the metal center with the syn alkylidene in the apical position. Bisalkoxide complexes are virtually inactive in olefin metathesis reactions because alkoxides do not dissociate readily.

ð88Þ

ð89Þ

5.11.5.1.5

Pyrrolides

Bispyrrolide complexes were prepared from bispyrrolide precursors through addition of the free carbene in diethyl ether or the silver carbene complex in methylene chloride (Eq. 90).49,201 Only sterically less demanding NHCs such as IiPr, TPT, and IMeCl2 can coordinate to these precursors. Molybdenum imido alkylidene NHC bispyrrolide complexes Mo072–Mo078 were transformed into cationic species through protonation of one of the pyrrolide ligands using N,N-dimethylanilinium BAr−F4. Except for Mo082 the complexes do not contain a donor solvent because the pyrrolide is bound in an 5-fashion to give 18 electron complexes. The cationic monopyrrolide complexes showed moderate to good productivities in the ring-closing metathesis (RCM) and cross-metathesis (CM) of simple substrates. Aldehydes, alcohol, and secondary amine moieties were not tolerated, possibly due to further protonation of a pyrrolide and formation of inactive complexes.

740

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

ð90Þ

5.11.5.1.6

Halides

The dibromo complexes Mo086–Mo088 were synthesized by treating the corresponding molybdenum imido alkylidene DME dibromide complex with the highly basic 6Mes (pKa  28)477 in toluene (Eq. 91). Dihalide complex Mo086 can be converted into Mo089 via treatment with Ag(MeCN)2[BArF4] in methylene chloride. The molybdenum center in Mo089 adopts an intermediate geometry between SP and TBP (t5 ¼ 0.50) and the imido ligand was found to be somewhat bent (Mo]NdC ¼ 162 ).

ð91Þ

Addition of 3 equiv. of HCl to Mo(NAd)2(CH2CMe2Ph)2 followed by 2 equiv. of the NHC complexes yields the dichloride complexes Mo090–Mo092 (Eq. 92). Upon reaction with LiOHIPT in toluene, the alkylidyne complex that was formed (Mo093) contains two NHC ligands. Another pathway to prepare complexes that contain bulky alkoxides and small NHC ligands uses MAC complexes as precursors, as shown in Eq. (93). MAC complexes are treated with IMeCl.2AgI in the presence of B(C6F5)3 as a phosphine scavenger to give complexes Mo094 and Mo095. An X-ray study of Mo094 shows it to be a square pyramid (t5 ¼ 0.05) with the alkylidene ligand in the apical position. The related complex Mo096 was prepared as shown in Eq. (94).

ð92Þ

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

741

ð93Þ

ð94Þ

5.11.5.1.7

Cationic monoalkoxides

Seven routes have been used to synthesize cationic NHC monoalkoxide complexes Mo097–Mo120 with the formulas [Mo(NR) (CHCMe2R0 )(NHC)(OR00 )(L)]+; they are summarized in Scheme 3, described below, and listed also in Table 3.30,49,194,198,201–203 (Four-coordinate Mo121 also was obtained as shown in Eq. 95.) The methods are the following:

Scheme 3 Seven routes to cationic monoalkoxide NHC complexes.

742

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

1. Treatment of a monotriflate monoalkoxide complex with Na[BArF4] or Ag[BArF4] in methylene chloride in the presence of acetonitrile. 2. Reaction of a bistriflate complex with Na[BArF4] in the presence of MeCN in methylene chloride to generate the cationic monotriflate species in situ, to which a lithium alkoxide is then added. 3. Treatment of a bisalkoxide complex with LiAl(OCCF3)3)4 in methylene chloride in the presence of MeCN. 4. Addition of Na[BArF4] to a dibromide complex in methylene chloride in the presence of MeCN to generate the cationic monobromide species in situ, followed by addition of a potassium alkoxide. 5. Reaction of a monoalkoxide monochloride complex with Na[BArF4] in methylene chloride. 6. Substitution of the triflate in a monotriflate complex through addition of a lithium alkoxide in methylene chloride. 7. Addition of a sterically demanding terphenol (HOHIPT/HOHMT) to a pyrrolide complex.

ð95Þ

Cationic monoalkoxide NHC complexes are highly active olefin metathesis catalysts, with TONs up to 80,000 in some cases. They also tolerate several functional groups and those that contain a nitrile donor ligand are often stable in air in the solid state. Mo101 is stable in air in the solid state for at least 2 weeks with no sign of decomposition.47

5.11.5.1.8

Carboxylates

The stability of cationic molybdenum imido alkylidene NHC monotriflate species can be further enhanced by introducing a carboxylate. A molybdenum imido alkylidene NHC bistriflate was treated with Ag[O2CC6F5] in methylene chloride to yield the neutral monotriflate carboxylate complex Mo122 which was then treated with Na[BArF4] in methylene chloride to provide the cationic carboxylate complex Mo123 (Scheme 4).30

Scheme 4 Synthesis of Mo122 and cationic Mo123–Mo128.

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

743

The synthesis of the cationic carboxylate species can be accomplished in a more straightforward manner in a one-pot reaction of the bistriflate or dichloride precursor with Na[BArF4] or Li[Al(OC(CF3)3)4], followed by salt metathesis of the triflate/chloride. Complexes Mo123–Mo128 were synthesized from the bistriflate NHC complex while complexes Mo129 and Mo130 were prepared from a dichloride precursor. A single-crystal X-ray study of Mo128 revealed a monomeric structure, in contrast to neutral carboxylate complexes that contain relatively small ligands.100 Mo128 adopts a slightly distorted SP (t5 ¼ 0.1) geometry at the metal center with the alkylidene ligand in the apical position and one of the carboxylate oxygens is positioned roughly trans to the NHC. Synthesis of the cationic carboxylate complexes Mo129 and Mo130 derived from a dichloride precursor is shown in Eq. (96).30 Upon exposure to air for 5 days, the carboxylate complexes Mo123, Mo125, Mo127, and Mo129 in the solid state showed no signs of decomposition. They showed moderate to good activity in the olefin metathesis of substrates containing an alcohol OH group.

ð96Þ

5.11.5.1.9

Complexes that contain a chelating alkylidene

A series of six-coordinate O- and N-chelating alkylidene complexes were synthesized with the intent of preparing latent and air-stable pre-catalysts (Scheme 5). Mo132 was obtained by treating the monoalkoxide monotriflate complex Mo131 with

Scheme 5 Synthesis of O-chelating alkylidene complexes Mo132–Mo138.199,202

744

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

2-methoxystyrene in methylene chloride. Compounds Mo133–Mo136 were obtained by reaction of the corresponding precursor with 2-methoxystyrene in methylene chloride. The O-chelating alkylidene bistriflate complex Mo137 was synthesized by treatment of the precursor with butyric acid vinyl ester in methylene chloride. The product was further reacted with lithium pentafluorophenolate in methylene chloride to provide Mo138. According to the single-crystal X-ray analysis, Mo138 adopts a distorted octahedral geometry in the solid-state with the triflate trans to the NHC. Compounds Mo133–Mo136 are thermally latent initiators for the ROMP of dicyclopentadiene (DCPD) without being activated by an additional Lewis acid. In the solid state Mo133 and Mo134 are relatively stable in air in the solid state. Syntheses of the O-chelated complexes Mo139–Mo144 and N-chelated Mo-imido complexes Mo145–Mo149 are shown in Scheme 6. Compounds Mo146–Mo149 yield alkylidenes stabilized through formation of a four-membered ring, in contrast to Mo132–Mo145, which all contain alkylidenes as part of a five-membered ring.204

Scheme 6 Synthesis of Mo139–Mo144 and Mo145–Mo149.204

The monotriflate monoalkoxide complex shown in Eq. (97) was treated with 2-isopropoxystyrene in methylene chloride to furnish Mo150. The complex adopts a distorted SP geometry at the metal with the alkylidene ligand occupying the apical position. Coordination of the isopropoxy group to give an octahedral complex is apparently discouraged, in part for steric reasons.204 Addition of 2-vinylpyridine to the monotriflate in methylene chloride yielded Mo151. These complexes allowed for tailoring the temperature of the onset of polymerization in the ROMP of DCPD between 52 and 142  C.

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

745

ð97Þ

Reaction of the cationic complex Mo029 with 2-allyloxyethanol produces Mo152 (Eq. 98). In the chelating alkylidene, the allyl ether oxygen forms a four-membered ring through coordination trans to the imido ligand. The hydroxyl group is located trans to the NHC, therefore forming an octahedral geometry, and the alkylidene is in an anti-configuration, both in solution and as a solid.

ð98Þ

5.11.5.1.10

Ionically tagged complexes

Complexes that contain a cationic ligand create the possibility of biphasic reactions in which one phase contains the catalyst and the other the substrates and products, thereby facilitating work-up and reuse of the catalyst.478–483 The molybdenum imido alkylidene complex Mo153 was synthesized through reaction of Mo007 with the betaine-type ligand 3-(1-pyridinium)-1-propanesulfonate in methylene chloride (Eq. 99). The molybdenum center adopts a distorted SP geometry (t5 ¼ 0.11) with the alkylidene ligand occupying the apical position. The triflate ligand is situated trans to the NHC ligand and the ionic sulfonate is located trans to the imido ligand. Addition of 1 equiv. of lithium pentafluorophenoxide resulted in the exchange of the betaine-type sulfonate ligand. A similar reaction could still be performed with the monotriflate monoalkoxide complex Mo051. However, addition of 1 equiv. of Na [BArF4] is required to afford the monoalkoxide complex Mo154 with [BArF4]− as the anion.

ð99Þ Complex Mo155 (and byproduct Mo156) were prepared from Mo023 and 2,6-Ph-4-(2,4,6-Ph3-pyridinium)phenolate (Scheme 7). According to the distinct resonances in the 19F NMR spectrum (s ¼ − 77.78 and −78.69 ppm), Mo155 contains two triflate ligands, one bound to the molybdenum center and one acting as counterion to the ionically tagged alkoxide ligand. The molybdenum imido alkylidene bis-NHC monophenoxide complex Mo156 formed as byproduct (11%) through NHC transfer and was isolated as THF adduct. Both triflate ligands are dissociated in CDCl3 according to the 19F NMR spectrum. Dicationic Mo157 was synthesized through addition of 2 equiv. Na[BArF4] to Mo155, with one cationic charge at the molybdenum center and one at the aryloxide ligand.

746

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Scheme 7 Synthesis of Mo155–Mo157.200

5.11.5.1.11

Silica-supported complexes

Two different methods have been implemented for immobilizing molybdenum imido alkylidene NHC catalysts on silica. In the first approach the catalyst was bound to an NHC which itself had been immobilized on silica (Scheme 8) by treating silica G60 with 2-hydroxymethyl-1,3-dimesitylimidazolium chloride in the presence of a catalytic amount of sulfuric acid. The free carbene was obtained after subsequent deprotonation of the imidazolium salt in THF at room temperature with lithium hexamethyldisilazane (LiHMDS) and was then reacted with the molybdenum imido alkylidene DME bistriflate precursor in benzene. The reactivities of Mo158@SiO2 and Mo159@SiO2 were comparable to the corresponding homogeneous bistriflate species. Immobilization via the NHC moiety preserved the reactivity of the homogeneous catalysts and allowed for the preparation of virtually metal-free products with metal contents 1,200,000 were obtained in the self-metathesis of propylene. During the immobilization process, the 1,1,1,3,3,3-hexafluoro-tert-butoxide ligand is proposed to be protonated by surface silanols.

ð103Þ

5.11.5.6.2

Imido alkylidene complexes

Although the most common precursors for the preparation of molybdenum imido alkylidene NHC complexes are molybdenum imido alkylidene bistriflate DME complexes,193–195,232 the analogous tungsten bistriflate complexes have not been accessible in this manner so far. When W011 is reacted with 1,3-dimesitylimidazolin-2-ylidene (IMesH2), a reaction cascade is initiated, which entails transmethylation of one of the DME’s methyl groups and ultimately leading to the formation of the NHC complex W012 and the N-heterocyclic olefin (NHO) complex W013 (Eq. 104).212 All compounds were characterized by X-ray crystallography and the methyl group transfer was unambiguously proved by 13C-labeling. This unexpected behavior was attributed to the ability of the triflate ligand to dissociate. The focus was therefore shifted to complexes containing other anionic ligands, such as alkoxides or 2,5-dimethylpyrrolide.213

ð104Þ

The bisalkoxide complexes W014–W016 reacted with 1 equiv. of 1,3-diisopropylimidazol-2-ylidene (IiPr) to form the corresponding NHC containing bisalkoxide complexes W017–W019 (Eq. 105). W017 and W018 could be transformed into cationic-at-metal alkoxide complexes W020 and W021 by protonation with N,N-dimethylanilinium BArF4. In the case of W019, the attempted protonation resulted in decomposition.

ð105Þ

Bispyrrolide W022 also reacts with IiPr to form the bispyrrolide NHC complex W023 (Scheme 16), which turned out to be more versatile than W017 to W019. It can be transformed into its cationic counterpart through addition of N,N-dimethylanilinium BArF4 and then further modified by subsequent protonation of the remaining pyrrolide ligand with various alcohols to produce complexes W025–W027.201,213 The bispyrrolide complex W023 can also be reacted with alcohols to replace one of the pyrrolide ligands with an alkoxide, leading to the formation of the mixed complexes W028 and W029. Surprisingly, complexes W028 and W029 decomposed upon reaction with N,N-dimethylanilinium BArF4. Compound W025 can be prepared from W024, but not from W029.

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

755

Scheme 16 Reaction of IiPr with W022 and modifications of the resulting NHC complex.

While the alkoxide complexes W017–W019 and the bispyrrolide complex W022 offer access to a limited number of tungsten imido alkylidene NHC complexes, all complexes contain the same imido ligand as well as the same NHC. Complexes that contain more sterically demanding NHCs such as IMes could not be prepared. To enable the coordination of sterically more demanding NHCs as well as to explore the influence of different imido ligands, a new synthetic approach was required that begins with tungsten imido alkylidene dihalide DME complexes (Scheme 17).47 The analogous molybdenum compounds had previously been shown to be precursors for the coordination of NHCs.205 Tungsten dibromide W030 was prepared through the reaction of bistriflate W011 with potassium bromide. Various different NHCs could be coordinated to W030 to yield W031–W036. Even highly basic NHCs such as 3,4,5,6-tetrahydro-1,3-dimesitylpyrimidin-2-ylidene (6-Mes) or the sterically demanding 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene (IiPr) could be coordinated to the metal center. The NHCs were either introduced through a reaction with the free carbene or with its silver salt.

756 Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Scheme 17

Synthesis of the tungsten imido alkylidene dibromide DME complex W030 and subsequent coordination of various NHCs.

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

757

Complexes W031 – W035 could then be transformed into cationic complexes by reaction with NaBArF4 or, in the case of W032, Ag(pivCN)3BArF4. However, the addition of a nitrile as an additional donor ligand was necessary to obtain stable, crystalline complexes (Eq. 106). W036 decomposed upon reaction with NaBArF4. If Ag(MeCN)3BArF4 was used instead, the reaction proceeded cleanly, but the insertion of acetonitrile into the alkylidene was observed, leading to the formation of the bisimido complex W042 (Eq. 107). If the pivalonitrile-containing compound Ag(pivCN)3BArF4 is used, the desired cationic complex W043 is formed.

ð106Þ

ð107Þ

Tungsten imido alkylidene dihalide DME complexes that contain a variety of different imido ligands were also used as precursors for the coordination of IMes (Eq. 108). In all cases, the reaction proceeded cleanly and in high yield.

ð108Þ

The IMes complexes W031 and W049–W053 were reacted with NaBArF4, typically in the presence of pivalonitrile, to create the cationic monohalide complexes W037 and W054–W058 (Scheme 18). The remaining halide ligand could then be exchanged for pentafluorophenoxide by reaction with LiOC6F5, to yield the alkoxide complexes W059–W064. Alternatively, the halide ligand can also be replaced by a triflate ligand through reaction with AgOTf, yielding complexes W065–W070. The cationic complexes are moderately active in olefin metathesis and several complexes are stable in air in the solid state for at least 2 weeks.

758 Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Scheme 18

Syntheses of W031 and W049–W053 and subsequently pentafluorophenoxide or triflate complexes W059–W070.

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

759

Compound W029, immobilized on silica (Eq. 109), was found to be a less efficient catalyst for metathesis of internal olefins than W029 in the homogeneous phase. However, the activity of the complexes toward the metathesis of a-olefins increased significantly compared to immobilized NHC free tungsten-based olefin metathesis initiators.214

ð109Þ

Another approach to tungsten imido alkylidene NHC complexes is based on NHC-induced a-H-abstraction. If tungsten imido dibenzyl bis-tert-butoxide complexes are reacted with NHCs, tert-butanol is eliminated and a benzylidene ligand is formed upon coordination of the NHC (Scheme 19).215 This reaction was successful with different NHCs and imido ligands. Interestingly, the addition of one equivalent of an NHC leads to a mixture of starting material and product since the process is an equilibrium. Only upon removal of the solvent as well as the tert-butanol by-product under reduced pressure does the equilibrium shift toward the product.

760 Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Scheme 19

Reaction of tungsten imido dibenzyl bis-tert-butoxide complexes with IMes, ICy, and IiPr.

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

5.11.5.7

761

Alkoxide-based tungsten alkylidyne complexes

The reaction of tris(alkoxide) complexes W079-DME and W080-THF with IiPr and ICy led to the formation of the corresponding NHC complexes W081–W084 (Scheme 20).192 W081 and W083 were then transformed into the corresponding cationic complexes via reaction with N,N-dimethylanilinium BArF4.

Scheme 20 Coordination of monodentate NHCs ICy and IiPr to tris(alkoxide) alkylidyne complexes W079-DME and W080-THF.

The approach shown in Scheme 20 was limited to sterically less demanding NHCs. Therefore, one of the alkoxide ligands was protonated with triflic acid in the presence of DME to yield the resulting bis(alkoxide) triflate alkylidyne complexes W087-DME and W088-DME (Scheme 21). Attempted coordination of IiPr to W088-DME in THF led to the formation of the bis(NHC)-complex W089 and isolation of the starting material, albeit coordinated by THF instead of DME. However, if a larger NHC IMes was reacted with W088-THF, the expected NHC complex W090 was obtained and can then undergo salt metathesis with NaBArF4 to produce cationic complex W091.

Scheme 21 Synthesis of bis(alkoxide) triflate alkylidyne NHC complexes.

762

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Complex W085 was examined as a catalyst for the metathesis of phenyl propyne. Surprisingly, at room temperature, no activity was observed. 1H NMR studies indicated the formation of a stable tungstacyclobutadiene complex W092 (Eq. 110), as confirmed in an X-ray study.192

ð110Þ

Bidentate NHC ligands were introduced to the metal alkylidyne as shown in Scheme 22.209 The chelating NHCs consisted of an imidazolin-2-ylidene having a phenolic substituent on one of the nitrogen atoms. Both the imidazolinium ring as well as the phenol were deprotonated either by LiHMDS or KH and then reacted with tris(alkoxide) precursors W079-DME and W080-THF without isolation of the carbene. However, in the case of 1-tert-butyl-3-(2-hydroxyphenyl)-4,5-dihydroimidazolium tetrafluoroborate (ligand 3), only the phenol could be deprotonated even in the presence of two equivalents of base, thus leading to the formation of ionically tagged complex W098.

Scheme 22 Synthesis of tungsten alkylidyne NHC complexes containing bidentate NHC ligands.

Protonation of complex W094 with N,N-dimethylanilinium BArF4 did not lead to the cationic compound, therefore N, N-dimethylanilinium chloride was employed instead, leading to the formation of W099 (Eq. 111). The latter could then be transformed into W100 through reaction with the silver salt of tetrakis-(nonafluoro-tert-butoxy)-aluminate in the presence of pivalonitrile.192

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

763

ð111Þ A tridentate NHC was also added to W079-DME to yield complex W101 (Scheme 23). Protonation with N,N-dimethylanilinium chloride yielded the chloride complex W102. When this compound was reacted with Ag(MeCN)3BArF4, the expected cationic complex did not form. Instead, the ditungstatetrahedrane complex W103 was formed, as shown in an X-ray study.192

Scheme 23 Formation of a ditungstatetrahedrane.

5.11.5.8

Halide-based tungsten alkylidyne and tungsten oxo alkylidene complexes derived therefrom

Reaction of a tris(bromide) precursor with bulky NHCs such as IMes and IPr in THF affords the tris(bromide) NHC complexes W104 and W105 (Scheme 24).216 The neutral complexes W104 and W105 can then be transformed into the respective cationic species W106 and W107 by reaction with Ag(pivCN)3BArF4 or into the neutral tris(triflate) NHC complex W108 by reaction with AgOTf. The tris(triflate) NHC complex W108 can subsequently be transformed into the cationic complex W109. Reaction of a tris(chloride) precursor bearing a tert-butyl alkylidyne substituent with NHCs in toluene leads to the respective complexes W110, W111 and W112 (Scheme 25).216 The neutral complexes can be converted into the cationic counterparts W118, W119 and W120 by reaction with Ag(pivCN)3BArF4 or into the respective bis(triflate) mono(chloride) NHC complexes W113, W114 and W115 by addition of AgOTf in CH2Cl2. Complexes W113 and W115 were then reacted with AgBArF4 in the presence of pivalonitrile to form the cationic species W116 and W117.

764

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Scheme 24 Synthesis of neutral and cationic alkylidyne NHC complexes bearing a para-methoxy phenyl alkylidyne substituent.216

Scheme 25 Synthesis of neutral and cationic tungsten alkylidyne NHC complexes bearing a tert-butyl alkylidyne substituent.216

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

765

The addition of one equivalent water in acetonitrile to the bis(triflate) complexes W116 and W117 or the respective bis(chloride) complexes W118 and W120 in diethyl ether and pivalonitrile leads to the formation of the respective tungsten oxo NHC complexes W121, W122, W123 and W124 (Scheme 26).216 In case of complex W114, the neutral species was reacted with Ag(pivCN)3BArF4 to form the cationic species in-situ followed by reaction with one equivalent of water in acetonitrile to provide the cationic tungsten oxo complex W125. For the synthesis of the alkoxide complexes W126, W127 and W128, the respective mono(chloride) complexes W123 and W124 and the mono(triflate) complex W125 were mixed with LiOC6F5 in CH2Cl2.

Scheme 26 Synthesis of cationic tungsten oxo NHC complexes derived from tungsten alkylidyne NHC complexes.216

The tungsten alkylidyne complexes W104–W115 did not show any activity in alkyne metathesis reactions at all. However, the derived tungsten oxo NHC complexes showed decent activity and tolerance against functional groups in olefin metathesis reactions with substrates such as diallyl diphenyl silane, allyl benzene, allyl sulfide and methyl oleate. For cyclooctadiene, the alkoxide complexes W123–W125 reached TONs of up to 10,000.

5.11.6

Conclusions and perspective

The progress in high oxidation state alkylidene and alkylidyne chemistry and applications that involve alkene and alkyne metathesis has been enormous in the last two decades. We know now that in many cases a M]CHR alkylidene carries out a balancing act between an alkyl and an alkylidyne, but it can be a robust “natural product” that is likely to be reformed in some circumstances continuously without our knowledge. Metathesis catalysts can be highly active as well as regio- and stereospecific, characteristics that are of significant interest to the chemical and pharmaceutical industry, as well as basic research. Both Mo and W are much less expensive than Ru, possess low toxicity to life, and can be removed readily. Functional group tolerance and stability toward air and moisture have been improved dramatically. Perhaps more importantly, Mo and W metathesis catalysts are demonstrating that they have the power to metathesize olefins stereospecifically that contain Cl, F, CF3, or CN groups directly attached to an olefinic carbon atom, and to make olefins that are rare or even new molecules. Well-defined high-oxidation state Mo- and W-alkylidenes offer access to silica- and polymer-supported systems that can be used under continuous flow.486 In spite of the progress, fundamental questions continue to challenge the researcher. For example, now that we have experimental confirmation that the energy of at least one type of olefin complex and an (internal) alkylidene isomer are approximately the

766

Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

same, we have to ask “How, then, exactly, should an alkylidene and alkene in high oxidation state complexes, and the oxidation state of the metal, therefore be viewed?” Management of protons on atoms in the primary coordination sphere always has been and continues to be a key feature of the chemistry of metathesis-relevant organometallic Mo and W chemistry. But how much proton mobility is not visible to the observer? How can alkylidenes be reformed routinely and how can catalyst lifetimes therefore be extended? To what extent will the metathesis reaction continue with heteroatoms directly bound to the alkylidene carbon atom? What unites the principles of alkene and alkyne metathesis? Are internal alkylidenes much more stable than terminal alkylidenes and how do they fit into the metathesis picture? Are apparent a or b CH agostic interactions actually the result of some other fundamental characteristic of multiple metal-carbon bonds? Will polymer- or silica-supported catalysts ever be developed that rival homogeneous catalysts in selectivity? It has taken more than 50 years to get here. The rate of our understanding is increasing rapidly, but the picture may not be complete for some time. The only surprise would be if there were no other surprises.

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