Comprehensive Organometallic Chemistry IV. Volume 6: Groups 5 to 7 - Part 2 [6] 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 6

GROUPS 5 TO 7 - PART 2 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 6 Editor Biographies

vii

Contributors to Volume 6

xiii

Preface 6.01

xv

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

1

Guochen Jia

6.02

N-Heterocyclic and Mesoionic Carbene Complexes of Group 5 and Group 6 Metals

208

Philipp M Hauser, Felix Ziegler, Janis V Musso, Pradeep KR Panyam, Mohasin Momin, Jonas Groos, and Michael R Buchmeiser

6.03

N-Heterocyclic and Mesoionic Carbene Complexes of Group 7 Metals

264

Beatriz Royo, Sara Realista, and Sofia Friães

6.04

Organometallic Complexes of Group 5 With p-Acidic Ligands

299

Fabio Marchetti and Guido Pampaloni

6.05

Group VI Metal Complexes of Carbon Monoxide and Isocyanides

352

Paul J Fischer

6.06

Carbonyl and Isocyanide Complexes of Manganese

449

David C Lacy, Sanchita Paul, Vipulan Vigneswaran, and Preshit C Abhyankar

6.07

Carbonyl and Isocyanide Complexes of Rhenium

553

Chi-On Ng, Shun-Cheung Cheng, and Chi-Chiu Ko

6.08

Organometallic Complexes of Group 5 Metals With Pincer and Noninnocent Ligands

623

Samuel D Juárez-Escamilla, Maitreyee Rawat, and T Keith Hollis

6.09

Organometallic Pincer Complexes With Group 6 Metals

648

Scott Grzybowski and Scott R Daly

6.10

Organometallic Complexes of Group 7 Metals With Pincer and Noninnocent Ligands

695

Oana R Luca, Tessa HT Myren, Haley A Petersen, and Shea J O’Sullivan

6.11

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

720

James C Earl and Louis Messerle

6.12

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

772

Olivia L Duletski, Roark D O’Neill, Charles Beasley, Molly O’Hagan, and Michael T Mock

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

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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 6 Preshit C Abhyankar University at Buffalo, State University of New York, Buffalo, NY, United States Charles Beasley Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT, United States Michael R Buchmeiser Chair of Macromolecular Compounds and Fiber Chemistry, Institute of Polymer Chemistry, University of Stuttgart, Stuttgart, Germany

Philipp M Hauser Chair of Macromolecular Compounds and Fiber Chemistry, Institute of Polymer Chemistry, University of Stuttgart, Stuttgart, Germany T Keith Hollis Department of Chemistry, Mississippi State University, Starkville, MS, United States Guochen Jia Hong Kong University of Science and Technology, Hong Kong, China

Shun-Cheung Cheng Department of Chemistry, City University of Hong Kong, Kowloon, Hong Kong, China

Samuel D Juárez-Escamilla Department of Chemistry, Mississippi State University, Starkville, MS, United States

Scott R Daly Department of Chemistry, The University of Iowa, Iowa City, IA, United States

Chi-Chiu Ko Department of Chemistry, City University of Hong Kong, Kowloon, Hong Kong, China

Olivia L Duletski Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT, United States

David C Lacy University at Buffalo, State University of New York, Buffalo, NY, United States

James C Earl Department of Chemistry, The University of Iowa, Iowa City, IA, United States Paul J Fischer Chemistry Department, Macalester College, Saint Paul, MN, United States Sofia Friães ITQB NOVA, Instituto de Tecnologia Quí mica e Biológica António Xavier, Universidade Nova de Lisboa, Oeiras, Portugal Jonas Groos Chair of Macromolecular Compounds and Fiber Chemistry, Institute of Polymer Chemistry, University of Stuttgart, Stuttgart, Germany Scott Grzybowski Department of Chemistry, The University of Iowa, Iowa City, IA, United States

Oana R Luca Department of Chemistry, University of Colorado Boulder, Boulder, CO, United States Fabio Marchetti Department of Chemistry and Industrial Chemistry, University of Pisa, Pisa, Italy Louis Messerle Department of Chemistry, The University of Iowa, Iowa City, IA, United States Michael T Mock Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT, United States Mohasin Momin Chair of Macromolecular Compounds and Fiber Chemistry, Institute of Polymer Chemistry, University of Stuttgart, Stuttgart, Germany

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Contributors to Volume 6

Janis V Musso Chair of Macromolecular Compounds and Fiber Chemistry, Institute of Polymer Chemistry, University of Stuttgart, Stuttgart, Germany Tessa HT Myren Department of Chemistry, University of Colorado Boulder, Boulder, CO, United States Chi-On Ng Department of Chemistry, City University of Hong Kong, Kowloon, Hong Kong, China Molly O’Hagan Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT, United States Roark D O’Neill Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT, United States Shea J O’Sullivan Department of Chemistry, University of Colorado Boulder, Boulder, CO, United States Guido Pampaloni Department of Chemistry and Industrial Chemistry, University of Pisa, Pisa, Italy Pradeep KR Panyam Chair of Macromolecular Compounds and Fiber Chemistry, Institute of Polymer Chemistry, University of Stuttgart, Stuttgart, Germany

Sanchita Paul University at Buffalo, State University of New York, Buffalo, NY, United States Haley A Petersen Department of Chemistry, University of Colorado Boulder, Boulder, CO, United States Maitreyee Rawat Department of Chemistry, Mississippi State University, Starkville, MS, United States Sara Realista ITQB NOVA, Instituto de Tecnologia Quí mica e Biológica António Xavier, Universidade Nova de Lisboa, Oeiras, Portugal Beatriz Royo ITQB NOVA, Instituto de Tecnologia Quí mica e Biológica António Xavier, Universidade Nova de Lisboa, Oeiras, Portugal Vipulan Vigneswaran University at Buffalo, State University of New York, Buffalo, NY, United States Felix Ziegler Chair of Macromolecular Compounds and Fiber Chemistry, Institute of Polymer Chemistry, University of Stuttgart, Stuttgart, Germany

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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6.01

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Guochen Jia, Hong Kong University of Science and Technology, Hong Kong, China © 2022 Elsevier Ltd. All rights reserved.

6.01.1 6.01.2 6.01.2.1 6.01.2.1.1 6.01.2.1.2 6.01.2.2 6.01.2.2.1 6.01.2.2.2 6.01.2.3 6.01.2.4 6.01.2.4.1 6.01.2.4.2 6.01.2.5 6.01.3 6.01.3.1 6.01.3.1.1 6.01.3.1.2 6.01.3.1.3 6.01.3.2 6.01.3.2.1 6.01.3.2.2 6.01.3.3 6.01.3.3.1 6.01.3.3.2 6.01.3.4 6.01.3.5 6.01.3.6 6.01.4 6.01.4.1 6.01.4.1.1 6.01.4.1.2 6.01.4.1.3 6.01.4.2 6.01.4.2.1 6.01.4.2.2 6.01.4.2.3 6.01.5 6.01.5.1 6.01.5.1.1 6.01.5.1.2 6.01.5.1.3 6.01.5.1.4 6.01.5.2 6.01.6 6.01.6.1 6.01.6.1.1 6.01.6.1.2 6.01.6.1.3 6.01.6.1.4 6.01.6.1.5 6.01.6.2 6.01.7 6.01.7.1 6.01.7.1.1

Introduction Alkyl complexes M(I) complexes Synthesis Reactivity M(II) alkyl complexes Synthesis Reactivity M(III) and M(IV) complexes M(V) complexes Synthesis Reactivity M(VI) and M(VII) complexes Aryl complexes M(I) complexes Complexes with terminal Z1-aryl ligands Complexes with bridging aryl ligands Cyclometalated complexes M(II) complexes Synthesis Reactivity M(III) complexes Mn(III) complexes Re(III) complexes M(IV) complexes M(V) complexes M(VII) complexes Vinyl complexes Synthesis Complexes with Z1-vinyl or Z2-vinyl ligands Cyclometalated vinyl complexes Complexes with bridging vinyl ligands Reactivity Protonation reactions Reductive elimination Reactions in which vinyl complexes are implicated Alkynyl complexes Synthesis M(I) complexes M(II) and M(III) complexes M(V) complexes Cluster complexes Reactivity Acyl and related complexes Synthesis Acyl and formyl complexes LnMCOR (R]H, alkyl, aryl) Alkoxycarbonyl (metallaester) complexes LnMC(O)OR Carbamoyl complexes LnMC(O)NR2 Bora-acyl complexes Iminoacyl, imidate and related complexes LnMdC(]NR)R0 Reactivity Carbene complexes Carbene complexes without a hetero substituent at the carbene carbon Synthesis

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00073-1

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Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

6.01.7.1.2 6.01.7.2 6.01.7.2.1 6.01.7.2.2 6.01.7.3 6.01.7.4 6.01.7.5 6.01.7.6 6.01.7.6.1 6.01.7.6.2 6.01.7.7 6.01.8 6.01.8.1 6.01.8.1.1 6.01.8.1.2 6.01.8.1.3 6.01.8.1.4 6.01.8.2 6.01.8.3 6.01.8.3.1 6.01.8.3.2 6.01.8.3.3 6.01.8.3.4 6.01.8.3.5 6.01.9 6.01.9.1 6.01.9.1.1 6.01.9.1.2 6.01.9.1.3 6.01.9.1.4 6.01.9.1.5 6.01.9.1.6 6.01.9.2 6.01.9.2.1 6.01.9.2.2 6.01.9.2.3 6.01.9.2.4 6.01.9.2.5 6.01.9.2.6 6.01.9.2.7 6.01.10 6.01.10.1 6.01.10.2 6.01.10.3 6.01.11 References

Reactivity Carbene complexes with one oxygen substituent at the carbene carbon Synthesis Reactivity Complexes with one SR or SeR substituent at the carbene carbon Carbene complexes with a PR2 substituent at the carbene carbon Carbene complexes with one nitrogen substituent at the carbene carbon Complexes with N,N-hetero carbenes Complexes with acyclic diaminocarbenes A brief comment on complexes with N-heterocyclic carbenes (NHCs) Complexes with N,O-, N,S- and N,B-hetero carbenes Vinylidene and allenylidene complexes Synthesis of vinylidene complexes and related complexes By protonation of alkynyl complexes By isomerization of alkynes By CdC bond activation of alkynols Preparation of M]C]M0 complexes Synthesis of allenylidene complexes Reactivity of vinylidene and allenylidene complexes Electrophilic addition, abstraction and oxidative coupling reactions Deprotonation reactions Nucleophilic addition reactions Formation of vinylidene bridged dinuclear complexes and clusters Reactions in which vinylidene complexes are implicated Carbyne complexes Synthesis By electrophilic abstraction of Fischer carbenes By oxidation of vinylidene complexes By electrophilic addition of vinylidene or allenylidene complexes By oxidation of dinuclear complexes with a C]CHdCH]C or C^CdC^C bridge By activation of hydrocarbons and halocarbons By electrophilic addition to carbonyl and reactions of phosphorus ylides Reactivity Migratory insertion reactions Alkyne metathesis and formation of metallacyclobutadienes Reactions of phosphoniocarbyne complexes Redox reactions Deprotonation reactions Nucleophilic addition reactions Reactions with anionic metal-carbonyl complexes Metallacarbocycles Four-membered metallacycles Five-membered metallacycles Six-membered metallacycles Conclusion

Nomenclature  C acac Ar BArF4 BDI bpy COD Cp Cp

Temperature in degree centigrade Acetylacetonate Aryl B{3,5-(CF3)2C6H3}4 N,N0 -Bis(2,6-diisopropylphenyl)-2,4-dimethyl-b-diketiminate 2,20 -Bipyridine derivatives Cyclooctadiene Z5-Cyclopentadienyl Z5-Pentamethylcyclopentadienyl

137 143 143 152 154 155 156 161 161 162 162 165 165 165 166 167 167 168 169 169 169 169 172 175 176 176 176 177 178 182 183 183 184 184 185 186 186 187 188 188 189 189 192 194 196 197

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Cp0 Cy DAAm DAPdipp DAPMes depe Dipp DMAD dmap DMF dmpe dmpm DMSO dppe dppf dppm Et fac Fc Hppy HRPz IMes IPr iPr L M Me MeCN mer Mes nBu NHC NMR nPr OAc Ph Phen Por PPN+ PTA py ROMP salen tacn tBu THF TMEDA TMS Tp Tp tpy triphos X Xyl

Z5-Cyclopentadienyl and Z5-indenyl derivatives Cyclohexyl (C6F5NHCH2CH2)2NMe 2,6-Bis(((2,6-diisopropylphenyl)amino)methyl)pyridine 2,6-Bis((mesitylamino)methyl)pyridine 1,2-Bis(diethylphosphino)ethane 2,6-Diisopropylphenyl Dimethyl acetylenedicarboxylate (Dimethylamino)pyridine N,N0 -Dimethylformamide 1,2-Bis(diethylphosphino)ethane 1,2-Bis(diethylphosphino)methane Dimethylsulfoxide 1,2-Bis(diphenylphosphino)ethane 1,10-Bis(diphenylphosphino)ferrocene 1,2-Bis(diphenylphosphino)methane Ethyl Facial (isomerism) Ferrocenyl 2-Phenylpyridine Pyrazole 1,3-Di-mesitylimidazol-2-ylidene 1,3-Bis(2,6-diisopropylphenyl)imidazol-2-ylidene Isopropyl A neutral two-electron donor Metal Methyl Acetonitrile Meridional (isomerism) 2,4,6-Me3C6H2 Normal butyl N-Heterocyclic carbene Nuclear magnetic resonance Normal propyl Acetate Phenyl 1,10-Phenanthroline Porphyrin based ligand Bis(triphenylphosphine)iminium cation 1,3,5-Triaza-7-phosphaadamantane Pyridine Ring-Opening Metathesis Polymerization N,N0 -Ethylene-1,2-salicylidiniminate 1,4,7-Triazacyclononane tert-Butyl Tetrahydrofuran N,N,N,N0 -Tetramethylethylenediamine Trimethylsilyl Hydrotrispyrazol-1-ylborate Hydrotris(3,5-dimethylpyrazol-1-yl)borate 2,20 :60 ,200 -Terpyridine 1,1,1-Tris(diphenylphosphino-methyl)ethane An anionic monodentate ligand 2,6-Me2C6H3

3

4

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

6.01.1

Introduction

Group 7 metals, especially manganese and rhenium, form a large number of organometallic compounds with metal-carbon s-bonds and/or metal-carbon multiple bonds, including alkyl, aryl, alkenyl, alkynyl, acyl, carbene (alkylidene) and carbyne (alkylidyne) complexes. These organometallic compounds can display rich chemical reactivities and play an important role in stoichiometric synthesis, catalysis and materials sciences. This chapter focusses on the synthesis, structure, and reactivity of organometallic compounds of group 7 metals with at least one metal-carbon s-bond or metal-carbon multiple bond reported in the literature in the period of 2005–19. The contents are arranged in sections according to the type of the hydrocarbon ligands and presented in the order of alkyl, aryl, alkenyl, alkynyl, acyl, carbene (alkylidene) and carbyne (alkylidyne). In each section, the materials are grouped according to oxidation states of metals and types of chemical reactions. The reader is referred to the corresponding chapters in COMC (1982), COMC (1995) and COMC (2007) for a thorough introduction to this field and for a complete overview of organometallic complexes of group 7 metals reported prior to 2005. Several relevant review articles have appeared over the years. The organomanganese and organorhenium chemistry has been highlighted in 20151 and 2011,2 respectively. A review summarizing the uses of manganese compounds as catalysts appeared in 2012.3 A comprehensive overview of catalytic processes mediated by manganese intermediates bearing MndC and/or MndH bonds appeared in 2016.4 More specific review articles are mentioned in the corresponding sections. For the sake of convenient presentation, a carbene (or alkylidene) ligand without a hetero substituent is treated as a dianionic ligand, while vinylidene, allenylidene and a carbene ligand with one or two p-donating hetero substituents (e.g. OR, NR2) is treated as a neutral two-electron donor ligand. A carbyne (or alkylidyne) ligand is treated as a trianionic ligand. Some abbreviations are used in the presentations and are defined in the Nomenclature section. The most common ones include Cp ¼ Z5-C5H5, Cp ¼ Z5-C5Me5, Tp ¼ hydrotrispyrazol-1-ylborate, Tp ¼ hydrotris(3,5-dimethylpyrazol-1-yl)borate, L ¼ neutral two electron donor, X ¼ monoanionic ligand and NHC ¼ N-heterocyclic carbene.

6.01.2

Alkyl complexes

In the review period, well-characterized group 7 metal alkyl complexes have been described for those containing a Mn(I), Mn(II), Mn(III), Mn(IV), Re(I), Re(III), Re(V), Re(VI), or Re(VII) center. Most of the rhenium alkyl complexes are diamagnetic 18-electron complexes. Manganese alkyl complexes are more diverse and can be diamagnetic or paramagnetic with the coordination numbers ranging from two to six.

6.01.2.1

M(I) complexes

Well-characterized group 7 M(I) alkyl complexes have been described for both Mn(I) and Re(I) systems. Most of the complexes are diamagnetic 18-electron complexes containing strongly p-accepting ligands such as CO, NO, CNR and pyridine derivatives, while a few open-shell Mn(I) alkyls are also known.

6.01.2.1.1

Synthesis

6.01.2.1.1.1 By transmetalation reactions with main group metal alkyls A common route to group 7 M(I) alkyl complexes is to use transmetalation reactions of appropriate M(I) precursors (often halide complexes) with alkylating reagents such as LiR, RMgX, MgR2 and ZnR2. For example, the rhenium methyl complex [Re(Me)2(MeCN)(NO)(PiPr3)2] (2) could be obtained by the reaction of [ReBr2(MeCN)(NO)(PiPr3)2] (1) with LiMe (Scheme 1).5 The manganese(I) methyl complex [Mn(Me)(CO)5] could be synthesized by the reaction of [MnBr(CO)5] with ZnMe2.6 Treatment of [ReCl(CO)3(P((CH2)14)3P)] (3) with LiMe produced the methyl complex [Re(Me)(CO)3(P((CH2)14)3P)] (4).7

Scheme 1

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

5

The rhenium alkyl complexes [Re(R)(CO)3(bpy)] (5, R ¼ Me, Et, nPr, bipy ¼ 2,20 -bipyridine) have been prepared by the reactions of [ReBr(CO)3(bpy)] with the Grignard reagents MeMgI, EtMgBr and nPrMgBr (Scheme 2).8 A similar strategy has been used to prepare analogous complexes such as [Re(R)(CO)3(4,40 -dimethyl-2,20 -bipyridine)] (R ¼ Me (6), Et (7))9 and [Re(Me) (CO)3(phen)] (8, phen ¼ 1,10-phenanthroline).10

Scheme 2

Treatment of the cationic complex 9 bearing a macrocycle moiety with ZnMe2 in CH2Cl2 resulted in both methylation at Re and insertion of Zn into the macrocycle to give the methyl complex 10 (Scheme 3). Passing a THF solution of 10 over alumina gave the zinc-free methyl complex 11.11

Scheme 3

The reaction of the indenide [C9H6CH2CH(Me)(OMe)]Li (12) with [ReBr(CO)5] yielded a nearly equimolar mixture of the indenyl complexes [{Z1-C9H6CH2CH(Me)OMe}Re(CO)5] (13) and [{Z3-C9H6CH2CH(Me)OMe}Re(CO)4] (14) (Scheme 4).12 At room temperature, the Z1-indenyl complex 13 evolved quantitatively to the Z3-indenyl complex 14 in polar solvents such as CH2Cl2 and MeCN.

OMe OMe

[ReBr(CO)5] OC

CO Re

OC

12

CO

13

OMe + Re

CO

OC OC

CO CO

14

Scheme 4

M(I) alkyl complexes can also be prepared from reactions of [MX(CO)5] with other alkylating reagents. For example, treatment of the metallo nitrile ylide Li[W(CO)5CNCHCO2Et] (15) with [MBr(CO)5] (M ¼ Mn, Re) produced the bimetallic alkyl complexes [(OC)5W{m-CNCH(CO2Et)}M(CO)5] (16, M ¼ Mn (a), Re (b)) (Scheme 5).13

Scheme 5

6

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

6.01.2.1.1.2 By reactions of [LnM(I)]− complexes with electrophilic RX reagents Alkylation of anionic [LnM(I)]− complexes with electrophilic RX reagents represents another effective route to group 7 M(I) alkyl complexes. For example, the 1-carbomethoxyethyl manganese(I) complex [Mn{CH(Me)COOMe}(CO)5] (17) has been synthesized by the reaction of K[Mn(CO)5] with methyl 2-bromopropionate (Scheme 6).14 The methyl complex [Mn(Me)(CO)5] can be synthesized by treating the complexes M[Mn(CO)5] (M ¼ Na, K) with MeI in diethyl ether.15,16 A similar reaction has been used to obtain the benzyl complex [Mn(CH2Ph)(CO)5].17–19 As an additional example, the alkyl complex [Mn(CH2CO2Me)(PPh3)(CO)4] (18) has been synthesized in good yield by the reaction of TsOCH2CO2Me with Na[Mn(PPh3)(CO)4].20 O OMe

O OMe

K[Mn(CO)5]

CO

OC Mn

Br

CO

OC CO 17 O

OMe

TsO

CO

OC

O

Na[Mn(PPh3)(CO)4] OMe

Mn CO

Ph3P CO 18

Scheme 6

The Mn(I) alkyl complexes fac-[Mn(R)(dpre)(CO)3] (20, dpre ¼ 1,2-bis(di-npropylphosphino)ethane, R ¼ CH3 (a), CH2CH3 (b), CH2CH2CH3 (c)) have been obtained by treatment of fac-[MnBr(dpre)(CO)3] (19a) with NaK (three equivalents) followed by addition of the organohalides CH3I, CH3CH2Br and CH3CH2CH2Br, respectively (Scheme 7).21,22 The analogous Mn(I) alkyl complex fac-[Mn(CH2CH2CH3)(dippe)(CO)3] (21, dippe ¼ 1,2-bis(diisopropylphosphino)ethane) has been obtained similarly from the reaction of fac-[MnBr(dippe)(CO)3] (19b) with NadK and CH3CH2CH2Br. Treatment of [MBr(CO)3(bpy)] (22, M ¼ Mn, Re) with Na(Hg) followed by ClCH2OCH3 afforded the (methoxy)methyl complexes [M(CH2OCH3)(CO)3(bpy)] (23, M ¼ Mn, 24, M ¼ Re).23

Scheme 7

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

7

The alkyl complexes cis-[Re(CH2Ph)(k1-(P)-PNN)(CO)4] (26) and cis-[Re(CH2OCH3)(k1-(P)-PNN)(CO)4] (27) have been prepared by reduction of 25 with Na/Hg, followed by treatment with PhCH2Br and ClCH2OCH3, respectively (Scheme 8). The analogous methyl complex cis-[Re(Me)(k1-(P)-PNN)(CO)4] (29) was obtained by heating [Re(Me)(CO)5] with the PNN ligand 28 at 110  C for 24 h.24

Scheme 8

Triethanolamine reacted with [Mn(SiCl3)(CO)5] in CH2Cl2 in the presence of excess Et3N to give the alkyl complex [Mn(CH2Cl) (CO)5]. The product is likely formed by the reaction of the solvent CH2Cl2 with [Mn(CO)5]− generated in situ from cleavage of the SidMn bond by the amine.25 M(I) alkyl complexes could also be obtained by the reactions of RX with other [LnM]− complexes. For example, treatment of Na [Mn(CO)3(CNArDipp2)2] (30, Dipp ¼ 2,6-diisopropylphenyl) with methyl iodide (MeI) generated the methyl complex mer, trans[MnMe(CO)3(CNArDipp2)2] (31) (Scheme 9).26

Scheme 9

6.01.2.1.1.3 By alkene insertion reactions Insertion reactions of M(I) hydride complexes with alkenes have been occasionally used to prepare M(I) alkyl complexes. For example, the ethyl complex [ReH(Et)(THF)(NO)(PTA)2] (33, PTA ¼ 1,3,5-triaza-7-phosphadamantane) was produced in the reaction of ethylene with [ReH2(THF)(NO)(PTA)2] (32) (Scheme 10).27 The cis-hydrido-ethyl complex [ReH(Et)(Z2-C2H4) (NO)(PPh3)2] (34) can be synthesized by the reaction of [ReH2(Z2-C2H4)(NO)(PPh3)2] with excess ethylene.28 Analogous ethyl complexes were produced from the reactions of ethylene with [ReBr(H)(Z2-C2H4)(NO)(L2)] [L2 ¼ 1, 10 -bisdiphenylphosphinoferrocene, 1,10 -bisdiisopropylphosphinopherrocene].29

8

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 10

Treatment of the hydrido-alkene complex [MnH(CH2]CH2)(dmpe)2] (35) with CNR (R ¼ tBu, Xyl (Xyl ¼ 2,6-C6H3Me2)) afforded the manganese(I) ethyl complexes [MnEt(CNR)(dmpe)2] (36, R ¼ tBu (a), Xyl (b)) (Scheme 11).30 The manganese hydride [MnH(CO)5] normally does not undergo insertion reactions with alkenes and alkynes unless the alkenes and alkynes contain a strongly electron-withdrawing group. Interestingly, the hydride complex [MnH(CO)5] in C6D6 reacted with ethylene, ethyl acrylate and acrylonitrile under the catalysis of [Pd(PPh3)4] at 20  C to give the corresponding alkyl complexes [Mn {CH(R)Me}(CO)5] (37, R ¼ H (a), CO2Et (b), CN (c)).31,32

Scheme 11

6.01.2.1.1.4 By decarbonylation of acyl complexes Another route to M(I) alkyl complexes is to perform decarbonylation of acyl complexes. For example, treatment of K[Mn(CO)5] with anhydrides (RCO)2O (e.g. R ¼ CF3, CHF2) or acyl chlorides RCOCl (e.g. R ¼ CH2CF3, CF2CH3) at room temperature yielded the acyl complexes 38, which can undergo decarbonylation reactions to give the alkyl derivatives [Mn(R)(CO)5] (39, R ¼ CF3 (a), CHF2 (b), CH2CF3 (c), CF2CH3 (d)) upon heating (Scheme 12).33

Scheme 12

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

9

6.01.2.1.1.5 By reduction of carbonyl complexes Reduction of M(I)dCO complexes by NaBH4 or LiAlH4 has often been used to prepare methyl or oxymethyl complexes. This strategy has been used to prepare complexes of the type [Cp0 Re(Me)(NO)(L)] (Cp0 ¼ cyclopentadienyl derivatives). For example, the reaction of NaBH4 with [CpRe(CO)2(NO)]PF6 produced the methyl complex [CpRe(Me)(CO)(NO)].34 Treatment of the carbonyl complex 40 with NaBH4 produced the methyl complex 41 (Scheme 13).35 The reaction of the complex [CpRe(CO) (NO){P(Me)(Ph)(C9H6N)}]BF4 (42) with sodium borohydride gave the methyl complex [CpRe(Me)(NO){P(Me)(Ph)(C9H6N)}] (43).36 Analogous complexes of the type [CpRe(Me)(NO){P(Me)(Ph)(2-C6H4NMe2)}] have been prepared similarly.37

Scheme 13

Hydride reduction has also been reported for neutral carbonyl complexes. For example, the tetranuclear yttrium polyhydride complex {[Cp0 Y(m-H)2]4(THF)} (44, Cp0 ¼ Z5-C5Me4SiMe3) can reduce coordinated CO in [Cp∗Re(CO)3] to give the unusual heterometallic oxymethyl dicarbonyl complex 45 (Scheme 14).38

Scheme 14

6.01.2.1.1.6 By other routes Mn(I) alkyl complexes can also be obtained by other routes. For example, visible light induced reductive elimination of the heterodinuclear triorganoplatinum-manganese complex [(tBu2bpy)Me2PhPtdMn(CO)5] (46) to give the methylmanganese complex [Mn(Me)(CO)5] and the methylphenylplatinum complex [Pt(Me)(Ph)(tBu2bpy)] (47) (Scheme 15).39

Scheme 15

10

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Although rare, a few group 7 metal complexes supported by a chelating alkyl ligand have been obtained by cyclometalation reactions. For example, the rhenium complex 49 bearing a [C,C,C]-chelating ligand was produced in the process of recrystallization of the complex 48 from pentene/hexafluorobenzene (Scheme 16).40 Treatment of [MnBr(CO)5] with the pyridyl-(benzamide)functionalized imidazolium salt 50 afforded the Mn(I) complex 51 bearing a [C,N,O]-chelating ligand.41

Scheme 16

Most of the well-characterized group 7 M(I) alkyl complexes are diamagnetic 18-electron complexes. Two-coordinated Mn(I) open-shell complexes have been reported recently. Treatment of a pale yellow Et2O solution of [Mn{C(SiMe3)3}2] (52) with one equivalent of KC8 in the presence of two equivalents of 15-crown-5 produced the anionic two-coordinate Mn(I) salt [K(15crown-5)2][Mn{C(SiMe3)3}2] (52) (Scheme 17). In the presence of 18-crown-6, the reaction produced [K2(18-crown-6)3]+[Mn {C(SiMe3)3}2]−2.42

Scheme 17

6.01.2.1.2

Reactivity

6.01.2.1.2.1 Homolytic cleavage of MdC bonds Group 7 metal alkyl complexes can undergo homolytic cleavage reactions to give alkyl radicals. A computational study of the fluorinated ethyl derivatives [Mn(CH2-nFnCH3-mFm)(CO)5] (n ¼ 0–2; m ¼ 0–3) predicted that the MndC bond dissociation enthalpies (BDEs) are in the range of 42–53.5 kcal/mol, depending on fluoro substitution on the a-C and b-C atoms.43 A kinetic study reveals that the homolytic MndRF bond cleavage reactions of [Mn(R)(CO)5] (R ¼ CF3, CHF2, CH2CF3, and CF2CH3) have the activation parameters DH# in the range of 46–53 kcal/mol.33 The activation enthalpy DH{ for homolytic MdC bond cleavage of [Mn(CHMeCO2Me)(CO)5] was experimentally determined to be 35.3  2.8 kcal/mol, which is close to the bond dissociation enthalpy (36.9 kcal/mol) estimated by DFT calculations.14

6.01.2.1.2.2 Protic cleavage of MdC bonds The metal-alkyl bonds in group 7 metal alkyl complexes can be cleaved by acids. For example, the methyl complex [Re(Me)(CO)5] can react with HBF4 to give [Re(CO)5BF4].44,45 The methyl complex [CpRe(Me)(NO)(PPh3)] rapidly reacted with CF3SO3H to give the triflate complex [CpRe(OSO2CF3)(NO)(PPh3)].46 Treatment of the methyl complex 43 with HBF4 gave the complex [CpRe(NO){P(Me)(Ph)(C9H6N)}]BF4 (54) (Scheme 18).36 A similar reaction occurred between MeSO3H and [CpRe(Me) (NO){P(Me)(Ph)(2-C6H4NMe2)}].37

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

11

Scheme 18

Protonation of the alkyl complexes 23 and 24 with HOTf produced the corresponding triflate complexes [M(OTf )(CO)3(bpy)] (55, M ¼ Mn, 56, M ¼ Re) (Scheme 19).23 Protonation of the methyl complex [Mn(Me)(CO)5] with HBF4 in the presence of phenols produced Z6-phenol complexes (e.g. 57).47 Protonation of the methyl complex [Re(Me)(CO)5] with HBF4 produced [Re(CO)5(FBF3)].48 Chemical vapor deposition of [Mn(Me)(CO)5] on partially dehydroxylated MgO powder generated MgO-supported manganese carbonyl complexes [Mn(CO)4(Os)2] (Os ¼ MgO support).49

Scheme 19

The tryptophan-derived manganese complex 59 has been obtained by treating the benzyl complex [Mn(CH2Ph)(CO)5] with 1-tryptophan 58 in MeOH followed by MeCN (Scheme 20). The complex 59 can release 1.4 mol of CO at upon irradiation at 465 nm and 2 mol of CO at 400 nm.17

Scheme 20

The MndMe bond in [Mn(Me)(CO)5] could also be cleaved by weak acids like phenols, amines and 2-alkylated pyridines. For example, treatment of the methyl complex [Mn(Me)(CO)5] with one equivalent of H-POP (60, 2,6-bis[(diphenylphosphino) methyl]-4-methylphenol) produced [Mn(CO)3(POP)] (61) and methane (Scheme 21). Heating a mixture of PNP (62, 2,6-bis (di-tert-butylphosphinomethyl)pyridine) and [Mn(Me)(CO)5] in toluene resulted in elimination of CH4 to give the complex

12

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 21

[Mn(CO)3(PNP0 )] (63) bearing a deprotonated PNP ligand.16 Treatment of [Mn(Me)(CO)5] with H-PNP2 (64, bis[(2-diisopropylphosphino)ethyl]amine) in toluene at 120  C led to the formation of the complex [Mn(CO)2(PNP)] (65) and methane.16 6.01.2.1.2.3 Insertion reactions with CO and CNR M(I) alkyl complexes can undergo CO insertion reactions. For example, the reaction between H-POP (60, 2,6-bis[(diphenylphosphino)methyl]-4-methylphenol) and 2.5 equivalents of [Mn(Me)(CO)5] at room temperature produced the bimetallic acyl complex 66 (Scheme 22).16

Ph2 P

PPh2 [Mn(Me)(CO)5] OH

OC OH

CO Mn

66

CO

CO O CO

OC

PPh2 60

O

P Ph2

Mn CO

CO

Scheme 22

Carbonyl migratory insertion reaction can be assisted by a Lewis acid. For example, ZnCl2 reacted with the methyl complex [Re(Me)(k1-(P)-PNN)(CO)4] (29) bearing a phosphine-functionalized bipyridine ligand to afford the adduct 67 (Scheme 23), which underwent facile CO migratory insertion to yield the complex [Re(k1-(P)-PNN
ZnCl)(m2-Cl)(CO)3{m2-C(O)Me}] (68) featuring a bridging acyl ligand.24 Zn-promoted migratory insertion was also observed for the benzyl complex [Re(CH2Ph) (k1-(P)-PNN)(CO)4] but not for [Re(CH2OCH3)(k1-(P)-PNN)(CO)4], probably because coordination of the methoxy oxygen to Zn may block coordination of Zn to the carbonyl oxygen which facilitates migratory insertion. The rhenium carbonyl-alkyl complex 11 bearing a phosphine-functionalized aza crown ether ligand reacted with Group 2 Lewis acids such as [CaI2(THF)4] and [SrI2(THF)5] to form the complexes 69, which underwent slow insertion of CO into the RedMe bond to give the Lewis acid-stabilized acyl complexes 70 (M ¼ Ca (a), Sr (b)).24

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

13

Scheme 23

Mn(I) alkyl complexes such as fac-[Mn(R)(dpre)(CO)3] (20, dpre ¼ 1,2-bis(di-npropylphosphino)ethane, R ¼ CH3 (a), CH2CH3 (b), CH2CH2CH3 (c)) and fac-[Mn(CH2CH2CH3)(dippe)(CO)3] (21, dippe ¼ 1,2-bis(diisopropylphosphino)ethane) are efficient additive-free manganese catalysts for hydrogenation of olefins and nitriles RCN.21,22 Migratory insertion of CO to form acyl complexes has been suggested as one of the key steps in the activation of the precatalysts. For example, a computational study reveals that the alkyl complex [Mn(CH2CH2CH3)(dippe)(CO)3] (21) could react with H2 and CH2]CHMe to give hydrido-alkene complex [MnH(CH2]CHMe)(dippe)(CO)3] (73) via the acyl complex 71 and the aldehyde complex 72 (Scheme 24).

Scheme 24

Insertion reactions of CNR have also been reported. For example, the ethyl complex 36b reacted with excess the isocyanide CN-2,6-C6H3Me2 to form the iminoacyl complex [Mn{C(]N-Xyl)CEt(]N-Xyl)}(CN-Xyl)3(dmpe)] (74, Xyl ¼ 2,6-C6H3Me2) (Scheme 25).30

Scheme 25

14

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

6.01.2.1.2.4 Reductive elimination and s-bond metathesis reactions M-alkyl bonds in group 7 metal alkyl complexes could be cleaved by silanes and H2. For example, photoirradiation of a 1:1 mixture of [Mn(Me)(CO)5] and the bis(silane) xantsilH2 (75, xantsil ¼ 9,9-dimethylxanthene-4,5-diyl)-bis(dimethylsilyl) in pentane for 0.5 h gave the silane-silyl complex [Mn(xantsilH)(CO)4] (76) (Scheme 26). The complex 76 could be formed by photoinduced dissociation of a carbonyl ligand in [Mn(Me)(CO)5], oxidative addition of an SiH bond of xantsilH2, methane elimination, and intramolecular coordination of the other SiH bond in the resulting xantsilH ligand to Mn.50 It has also been reported that photolytic reactions of the para-methyl benzyl manganese complex [Mn(CH2-p-C6H4OCH3)(CO)5] with R3SiH in benzene at 5  C can yield the silyl manganese complexes [Mn(SiR3)(CO)5] (SiR3 ¼ SiMe2Ph, SiMePh2, SiPh3, SiHPh2, SiEt3, SiMe2-tBu, and SiMe2Et).51,52

Scheme 26

The Wilkinson’s manganese(I) ethylene hydride complex trans-[MnH(CH2]CH2)(dmpe)2] (35, dmpe ¼ Me2PCH2CH2PMe2) can react as a source of the low coordinate manganese(I) ethyl complex [Mn(CH2CH3)(dmpe)2] (79) (Scheme 27). For example, the complex 35 reacted with H2 at 60  C to afford ethane and the dihydrogen hydride complex [MnH(H2)(dmpe)2] (77), presumably through intermediates 79 and 80. The complex 35 reacted with primary silanes RSiH3 (R ¼ Ph, nBu) at 60  C to afford ethane and the disilyl hydride manganese complexes [MnH(SiH2R)2(dmpe)2] (78, R ¼ Ph, R ¼ nBu), presumably through intermediates 79 and 81.30 In the reaction of the ethyl complex 79 with H2 and silanes, the ethane could be formed by either a s-bond metathesis reaction or oxidative addition of XdH (X ¼ H, silyl) followed by reductive elimination.

Scheme 27

Treatment of trans-[MnH(CH2]CH2)(dmpe)2] (35) with the secondary silane Et2SiH2 afforded the silylene hydride complex [(dmpe)2MnH(]SiEt2)] (82) via ethane elimination (Scheme 28). The reaction of 35 with Ph2SiH2 afforded ethane and a mixture of complexes trans-[MnH(]SiPh2)(dmpe)2] (83a), cis-[MnH(]SiPh2)(dmpe)2] (83b, featuring a MndHdSi bridging interaction) and [MnH2(SiHPh2)(dmpe)2] (84) along with Ph2EtSiH.53

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

15

Scheme 28

6.01.2.1.2.5 Oxidation and a-H abstraction Alkyl complexes [Re(Me)(CO)5] and [Re(R)(CO)3(bpy)] (R ¼ Me, Et, Pr) can react with SeO2 in CD3CN/H2O to give RSeO2H along with [Re(O2SeOH)(CO)5] and [Re(O2SeOH)(CO)3(bpy)] (Scheme 29).8 The reactions were proposed to proceed by electrophilic attack on the alkyl by SeO2 to form Re-O2SeR species, as illustrated by the reaction of [Re(Me)(CO)5] with SeO2 to give MeSO2H and [Re(O2SeOH)(CO)5] (86) (Scheme 29).

Scheme 29

Alkyl complexes with an a-hydrogen can undergo electrophilic abstraction reactions to give carbene complexes. For example, methyl complexes of the type [Cp0 Re(Me)(NO)(PPh3)] (Cp0 ¼ cyclopentadienyl derivatives) can react with Ph3CBF4 to give carbene complexes of the type [Cp0 Re(]CH2)(NO)(PPh3)]BF4.35,54 These reactions will be described in detail in the section dealing with carbene chemistry.

6.01.2.2

M(II) alkyl complexes

Well-characterized group 7 M(II) alkyl complexes are mainly those of Mn(II) systems. The synthesis and applications (in organic synthesis) of manganese (II) organometallic compounds in the +2 oxidation state have been reviewed in 200855,56 and 2009.57

6.01.2.2.1

Synthesis

6.01.2.2.1.1 MR2 and related complexes The simplest Mn(II) alkyl complexes are homoleptic complexes with the empirical formula MnR2. These complexes can be prepared by reactions of MnCl2 with suitable alkyl lithium or Grignard reagents. The solid structures of these homoleptic Mn(II) complexes are dependent on the steric bulkiness of the alkyl groups. The reaction between MnCl2 and tris(trimethylsi1yl)methyl lithium in 1:2 M ratio produced the monomeric two-coordinate complex [Mn{C(SiMe3)3}2] (52) (Scheme 30).58 The reaction of MnCl2 with the neophyl Grignard reagent Mg(CH2CMe2Ph)Cl produced [Mn(CH2CMe2Ph)2]2 (or [(PhCMe2CH2)Mn(m-CH2CMe2Ph)2Mn(CH2CMe2Ph)]) (87), which adopts a dimeric structure containing two bridging and two terminal alkyls.59

16

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 30

The reaction of MnCl2 with Mg(CH2CMe3)2(1,4-dioxane) produced the alkyl complex [Mn(CH2CMe3)2]4 or ([(Me3CCH2) Mn(m-CH2CMe3)2{Mn(m-CH2CMe3)2}2Mn(CH2CMe3)]) (88), which has a tetrameric structure as confirmed by X-ray diffraction study (Scheme 31).60 It should be noted that an electron diffraction study suggests that the complex 88 has a monometallic structure in the vapor phase.61 The reaction of MnCl2 with the trimethylsilylmethyl Grignard reagent Mg(CH2CMe3)Cl produced the alkyl compound [Mn(CH2SiMe3)2]n (89) which has a polymeric structure containing distorted tetrahedral spiro Mn atoms linked together via m2-bonding alkyl ligands.62 The reaction of MnCl2 with Mg(CH2Ph)2 produced the alkyl compound [Mn(CH2Ph)2]n which also adopts a similar polymeric structure.59

Scheme 31

Mn(II) alkyl complexes of the formulae [Mn(H)(R)] and [MnR(Ar)] have also been described. The gas phase reactions of excited Mn atoms with CH4, C2H6 and C2H5Cl were reported to give CH3dMnH, C2H5dMnH and C2H5dMnCl, respectively.63 The reaction of the sterically crowded Mn(I) iodo-aryl complex {Li(THF)Ar∗MnI2}2 (Ar∗ ¼ C6H3-2,6-(C6H2-2,4,6-iPr3)2) with LitBu gave the mononuclear complex [Mn(Ar )(tBu)] (90), which has a linear geometry (Scheme 32).64 The iodo-aryl complex {Li(THF) Ar∗MnI2}2 (Ar∗ ¼ C6H3-2,6-(C6H2-2,4,6-iPr3)2) reacted with sterically less demanding LiMe to produce the dinuclear methyl complex [{Ar Mn(m-Me)}2] (91), in which the Mn centers have a distorted trigonal-planar geometry and are linked by two bridging methyl groups.65

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

17

Scheme 32

Homoleptic manganese(II) dialkyls with sterically less demanding alkyls are often highly sensitive and tend to have a polymeric structure with bridging alkyls. They usually have low solubility in common organic solvents owing to their polymeric structures. In contrast, adducts with the empirical formulae MnR2(L) and MnR2(L2) (L ¼ neutral two-electron donors) can be much more soluble in hydrocarbon solvents and be more readily isolated and stored compared with the corresponding homoleptic alkyls compounds. Several complexes with the empirical formula MnR2(L) have been reported. These complexes can adopt either a monomeric structure or a dinuclear structure, depending on the steric property of R and L. The polymeric dialkyl complex [Mn(CH2SiMe3)2]n can be de-aggregated by the potent two-electron s-donor 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) to yield the monomeric three-coordinate alkyl complex [Mn(CH2SiMe3)2(IPr)] (93a) (Scheme 33). Similarly, the monomeric three-coordinate alkyl complex [Mn{CH(SiMe3)2}2(IPr)] (93b) was obtained by displacement of the THF from the disilyl-substituted alkyl-THF complex [Mn{CH(SiMe3)2}2(THF)] with IPr. The complexes 93 represent rare examples of dialkylmanganese(II) species stabilized by N-heterocyclic-carbenes.66 Treatment of [Mn(CH2SiMe3)2]n with 94, a gallium(I) analog of an NHC ligand, afforded the interesting ion-contacted alkyl complex 95 with a GadMn bond.67

[Mn(CH2SiMe3)2]n

Dipp +

or

N

N

Dipp

Mn Dipp

[Mn{CH(SiMe3)2}2(THF)] 92

N

Ph

Ph

K N

N

[K(tmeda)] 94 Scheme 33

Dipp

SiMe3

Ga N

N

N

Ph N

+ Ph

N

93: x = 2 (a), 1 (b)

[Mn(CH2SiMe3)2]n Ga

CHx(SiMe3)3-x

(SiMe3)3-xCHx

95

Mn SiMe3

18

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Three-coordinated Mn(II) alkyl complexes can also be obtained by other routes. For example, the formally Mn(0)-alkene complex 96 undergoes oxidative coupling reactions with alkenes, allenes, and alkynes to form metallacyclic complexes (e.g. 97, 98) (Scheme 34). The complex 96 can also react with H2 at room temperature to give the Mn(II) alkyl complex 99.68

Scheme 34

Dinuclear complexes with the formula [MnR2(L)]2 have been reported for a few of those with either a bulky alkyl R group or a bulky ligand L. In these complexes, the two Mn centers are usually bridged by two alkyl groups and thus can be better represented by the formula [MnR(m-R)(L)]2 or [R(L)Mn(m-R)2MnR(L)]. For example, treatment of MnCl2 with the alkylmagnesium reagents RMgCl (R ¼ CH2SiMe3, CH2CMe2Ph, CH2Ph) in THF produced the dinuclear THF adducts of bis-(alkyl)manganese complexes [MnR(m-R) (THF)]2 (R ¼ CH2SiMe3 (100), CH2CMe2Ph (101), CH2Ph (102)), featuring two symmetrical Mn(m-R)(R)(THF) units related by a crystallographic inversion center (Scheme 35). The Mn centers have a distorted tetrahedral environment, configured by the two bridging and the terminal alkyls and completed by the THF ligand. The analogous dinuclear alkyl complex [Mn(CH2SiMe3) (m-CH2SiMe3)(py)]2 has been obtained by the reaction of pyridine with [Mn(CH2SiMe3)(m-CH2SiMe3)(THF)]2 (100).59

Scheme 35

Reactions of the manganese(II) dichloro complexes [MnCl(m-Cl)(L)]2 (103) (L ¼ IiPr, 1,3-diisopropyl-4,5-dimethylimidazole2-ylidene (a); IMes, 1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene (b); IPr, 1,3-bis(2,6-diisopropylphenyl)imidazole2-ylidene (c)) with LiMe afforded the corresponding dinuclear dimethylmanganese(II) complexes [MnMe(m-Me)(L)]2 (104) (L ¼ IiPr (a), IMes (b), IPr (c)) (Scheme 36).69,70

Scheme 36

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

19

Dinuclear methyl-bridged complexes have also been isolated for manganese(II) complexes supported by a cyclopentadienyl appended with a phosphine donor. For example, treatment of the complex 105 with MeMgCl produced the manganese(II) methyl-bridged complex 106 (Scheme 37).71

Scheme 37

Four coordinate Mn(II) alkyl complexes of the type [MnR2(L)2] or [MnR2(L2)] are quite common. These complexes were often obtained by reactions of MnR2 or their THF adducts with L or L2. For example, the alkyl complexes [Mn(CH2SiMe3)2(pyridine)2] (107), [Mn(CH2SiMe3)2(TMEDA)] (108, TMEDA ¼ Me2NCH2CH2NMe2) and [{(dioxane)[Mn(CH2SiMe3)2]}n] (109) have been obtained by the reactions of [Mn(CH2SiMe3)2]n with pyridine, TMEDA and dioxane, respectively (Scheme 38).62

Scheme 38

Addition of pyridine to the neophyl complex [Mn(CH2CMe2Ph)2(THF)]2 produced the four-coordinate complex [Mn(CH2CMe2Ph)2(pyridine)2] (110) (Scheme 39). The reactions of [Mn(CH2CMe2Ph)2]2 and [Mn(CH2SiMe3)(THF)]2 with 2,20 -bipyridine were reported to give the bipyridine-supported alkyl complexes 112 and 111, respectively.59

Scheme 39

20

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Reactions of bisphosphines PP (PP ¼ dmpm (Me2PCH2PMe2), dmpe (Me2PCH2CH2PMe2)) with [{Mn(m-CH2SiMe3)2}n] and [{Mn(CH2CMe3)2}4] were reported to give three distinct types of complexes: (a) monometallic complexes [MnR2(PP)] (e.g. 113), (b) dimetallic complexes [R2Mn(m-PP)2MnR2] (e.g. 115), and (c) dimetallic complexes [{RMn(m-R)}2(m-PP)] (e.g. 114 and 116) (Scheme 40). The selectivity of the products are dependent on the phosphines, the alkyl groups as well as reaction conditions.60 SiMe3 SiMe3

P

dmpe

Mn P

dmpm

CH2 SiMe3

P Mn

[{Mn(CH2SiMe3)2}n] P

SiMe3

Mn

CH2 SiMe3 SiMe3

113

114 CMe3

Me3C

Mn

Mn Me3C

CMe3

P

P

P

P

CMe3

dmpe

[{Mn(CH2CMe3)2}4]

dmpe

P Mn P

Mn

CH2 CMe3 CH2 CMe3

CMe3 115

116

Scheme 40

Mn(II) complexes with a MndC(sp3) bond have also been described for a few of those containing ligands such as allyls, pentadienyls and cyclopentadienyls. Treatment of MnCl2 with two equivalents of K[TMSCH2CH]CHTMS] (117) in THF produced the allyl complex [Mn(Z1-TMSCH2CH]CHTMS)2(THF)2] (118), bearing two s-bonded allyl ligands (Scheme 41). Treatment of MnCl2 with K [TMSCH2CH]C(TMS)2] (119) in THF/TMEDA produced the analogous s-bonded allyl complex [Mn(Z1-TMSCH2CH] C(TMS)2)2(TMEDA)] (120).72 Geometry optimizations of the manganese(II) tris-allyl anions [Mn(CH2]CHdCH2)3]3− and [Mn(Me3SiCH]CHdCHSiMe3)3]3− in the high-spin (or S ¼ 5/2) state at the BP86/AE1 level of theory indicate that these complexes adopt the mixed hapticity structures [Mn(Z3-CH2]CHCH2)(Z1-CH2]CHCH2)2]3− and [Mn(Z3-Me3SiCH] CHCHSiMe3)(Z1-Me3SiCH]CHCHSiMe3)2]3−, respectively.73

Scheme 41

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

21

Treatment of [MnI2(THF)2] with the potassium pentadienylide K[CH2C(tBu)]CHC(tBu)]CH2] (121) resulted in the formation of the complex [Mn{Z3-(CH2C(tBu)]CHC(tBu)]CH2)}2] (122) (Scheme 42).74 Treatment of MnCl2 with two equivalents of K[indenyl] in THF followed by recrystallization from THF afforded the complex [(Z1-indenyl)(Z3-indenyl)Mn(THF)2] (123) bearing two differently bonded indenyl ligands.75 DFT calculations suggest that the complex [Mn(indenyl)2] would have two Z5-bound ligands.75 Indeed, single-crystal x-ray studies of indenyl complexes [{Z5-2-(SiMe3)C9H6}2Mn], [{Z5-1, 3-(SiMe3)2C9H5}2Mn] and [{Z5-1,3-(iPr)2C9H5}2Mn] reveal that they adopt classic Z5-bound sandwich structures. Interestingly the complex {(4,7-Me2C9H5)2Mn}8 with a dimethyl substituted indenyl ligand is a cyclic octamer containing both bridging and terminal indenyl ligands in the solid-state. The cyclopentadienyl ligands in Mn(II) complexes of the type [Cp2MnL2] can also be in Z1-coordination mode.76

Scheme 42

6.01.2.2.1.2 M(R)(X) (X ¼ halide, amide) and related complexes Mn(II) alkyls with the empirical formula Mn(R)(X), Mn(R)(X)(L) and Mn(R)(LX) have also been documented. For applications in organic synthesis, benzylic manganese chlorides of the empirical formula Mn(CH2Ar)Cl have often been generated in situ by reactions of benzyl chlorides with magnesium turnings and MnCl2
2LiCl (e.g. preparations of 124a–i, Scheme 43)77 or reactions of benzyl chlorides with manganese powder in the presence of a catalytic amount of InCl3 and PbCl2.78

Scheme 43

Treatment of the chloro-bridged dinuclear complex [MnCl(m-Cl)(IPr)]2 (103c) with PhCH2MgCl produced the dimeric alkyl Mn(II) complex [Mn(CH2Ph)(m-Cl)(IPr)]2 (125) (Scheme 44). Stoichiometric reactions of these complexes with bromocyclohexane demonstrate that they are not chemically competent in CdC coupling reactions with alkyl electrophiles.69 Heating the hydride complex 126 in the presence of two equivalents of 4-tert-butylstyrene at 130  C for 16 h yielded the monomeric three-coordinate alkyl complex [Mn{CH(CH3)C6H4-tBu-4}(2,6-iPr2PhBDI)] (127), as a result of an alkene insertion reaction.79

22

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 44

6.01.2.2.1.3 Alkyl complexes supported by polydentate ligands There are several reports on the chemistry of mononuclear Mn(II) alkyl complexes supported by a tridentate or tetradentate ligand. The complexes [TpPh,MeMn(CH2R)] (129, R ¼ TMS, Ph) bearing a hydrotris(pyrazolyl)borate (Tp) ligand have been obtained by the reactions of [TpPh,MeMnCl(CH3CN)] (128) with the corresponding RCH2MgCl reagents (Scheme 45).80 The tetrahedron-like methyl complex [Mn(Me)(PPN)] (131) bearing a [P,P,N]-tridentate ligand has been obtained by treating the metal-halide complex [MnCl(PPN)] (130) with methyl lithium in Et2O at −40  C.81

Scheme 45

Treatment of the complex 132 bearing an anionic tris(phosphino)borate ligand with alkyl Grignard RMgBr (R ¼ Me, CH2Ph) at −90  C in THF produced the pseudo tetrahedral alkyl complexes [Mn(R){PhB(CH2PiPr2)3}] (133, R ¼ Me (a), CH2Ph (b)) (Scheme 46).82 The [PhB(CH2PiPr2)3]-supported manganese alkyls 133 are thermally stable up to 65  C even under an atmosphere of dihydrogen or CO. The anionic alkyl complex 135 bearing a [P,N,P]-pincer ligand has been obtained by treatment of the chloro complex 134 with two equivalents of LiMe in THF.83

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

23

Scheme 46

The manganese dichloride complex 136a bearing a 2,20 :60 ,200 -terpyridine (tpy) ligand reacted with two equivalents of LiCH2SiMe3 to give the bisalkyl complex 137a (Scheme 47). Analogous complexes 137b,c bearing a phenyl or 4-chlorophenyl substituent on the 40 -position of the tpy ligand can be prepared by a similar procedure.84 These well-defined manganese complexes supported by a 2,20 :60 ,200 -terpyridine ligand and their derivatives are catalytically active for selective hydroboration of alkenes, ketones and aldehydes. Treatment of the homoleptic alkyl [{Mn(CH2SiMe3)2}n] or its THF adduct [{Mn(CH2SiMe3)2(THF)}2] with 2,6-[(2,6-iPr2C6H3)N]C(Me)]2C5H3N (138, iPrBIP) in toluene produced the dialkyl complex 139.85

Scheme 47

The manganese complex [Mn(CH2SiMe3)(boxmi)] (141) bearing a chiral bis(oxazolinyl-methylidene)isoindoline (“boxmi”) pincer ligand can be obtained from either the reaction of the boxmi-H protio ligand 140 with [Mn(CH2SiMe3)2]n, or alkylation of the manganese chloride complex [MnCl(boxmi)] with LiCH2SiMe3 (Scheme 48).86 The complex 141 can add a pyridine ligand to form a five-coordinated complex. The complex 141 is a precatalyst for enantioselective hydroboration of ketones.

Scheme 48

24

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Treatment of the dichloro manganese(II) complexes 142 with PhCH2MgCl provided the monobenzyl manganese complexes 143 bearing a [N,N,N,N]-tetradentate ligand (Scheme 49).87 The Mn complexes 143 are catalytically active for ethene polymerizations when activated by 30% MAO.

Scheme 49

6.01.2.2.1.4 Anionic alkyl complexes Anionic Mn(II) alkyl complexes of the type MnR−3 can be synthesized by the reactions of MR2 with alkali metal alkyls. For example, treatment of the manganese alkyl [Mn(CH2SiMe3)2]n with alkali metal alkyls M(CH2SiMe3) (M ¼ Na, K) in hexane/benzene produced the alkali metal triorganomanganates [{NaMn(CH2SiMe3)3}]n (144) and [KMn(CH2SiMe3)3
C6H6]2 (145), respectively (Scheme 50).88 In the solid-state, the unsolvated sodium manganate 144 displays an infinitely aggregated structure in which the Mn and Na centers are bridged by their methylene groups. The benzene-solvated potassium manganate 145 has a dimeric structure with the benzene molecule coordinated to a K center.

Scheme 50

These alkali metal triorganomanganates can react with bidentate ligands to give dinuclear or polynuclear complexes. For example, the sodium complex 144 reacted with TMEDA to give the monomanganese complex [(TMEDA)2NaMn(CH2SiMe3)3] (146), with 1,4-dioxane to give the dimanganese complex [{NaMn(CH2SiMe3)3}2(dioxane)7] (147), and with 1,4-diazabicyclo [2,2,2]octane (DABCO) to give the polymeric complex [Na2Mn2(CH2SiMe3)6(DABCO)2]n (148) (Scheme 51).88

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

SiMe3 O

H2C

O

[{NaMn(CH2SiMe3)3}]n (144)

Na

O O

N Me2N

N

NMe2

NMe2

Mn

CH2

O

SiMe3

O

O

SiMe3 CH2 O O H2C Na O Me3Si O O 147 O O

Mn

SiMe3 CH2

Me2N

Me3Si

Me3Si

O O

25

Na

SiMe3

N N

Mn

Me2N

Na

CH2 NMe2 146

SiMe3

N N

SiMe3

SiMe3

SiMe3

CH2

CH2

CH2

Mn

Mn

Na

CH2

CH2

CH2

SiMe3

SiMe3

SiMe3

148

n

Scheme 51

Anionic Mn(II) complexes with halide co-ligands are also known. For example, treatment of anhydrous MnCl2 with LiC(SiMe3)3 give the anionic alkyl complex [Li(THF)4][(Mn{C(SiMe3)3})3(m-Cl)4(THF)] (149). The complex 149 reacted with K [TMSCH2CH]CHTMS] (117) to give the alkyl-allyl mixed complex [Mn{C(SiMe3)3}(Z3-TMSCH2CH]CHTMS){ClLi(THF)3}] (150) (Scheme 52). The LiCl moiety in the complex 150 can be displaced by ligands L such as PMe3, THF, quinuclidine, and dmap (dmap ¼ 4-(dimethylamino)pyridine) to give the neutral alkyl complexes [Mn{C(SiMe3)3}(Z3-TMSCH2CH]CHTMS)(L)] (151, L ¼ PMe3 (a), THF (b), quinuclidine (c), dmap (d)). Treatment of the PMe3 complex 151a with BPh3 yielded the complex [Mn {C(SiMe3)3}(Z3-TMSCH2CH]CHTMS)] (152) accompanied by Ph3B(PMe3).89

Scheme 52

6.01.2.2.2

Reactivity

MndC bonds in Mn(II) alkyl complexes have significant ionic character. The low covalency of the MndC bonds results in their unique chemical reactivities and applications. 6.01.2.2.2.1 Acid-base reactions MndC bonds in organomanganese(II) can be protolytically cleaved by AdH acidic substrates. For example, the dialkyl complex 139 reacted with the pyridinium salt [H(Py)2]BArF4 (ArF ¼ 3,5-C6H3(CF3)2) to produce the monoalkyl complex 153 (Scheme 53).85 The alkyl complex 112 reacted with [H(OEt2)]BArF4 (ArF ¼ 3,5-C6H3(CF3)2) to give the cationic trisbipyridine complex [Mn(bpy)3](BArF4)2, presumably through the intermediate 154.59

26

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

BArF4

N N iPr

iPr N

Mn

N

N

N

Mn

iPr

iPr Me3Si

iPr

iPr Me3Si

iPr

iPr [HPy2]BArF4

N

SiMe3 139

153 CMe2Ph

N Mn N

CMe2Ph 112

[H(OEt)2]BArF4

N

+ Mn

CMe2

[Mn(bipy)3](BArF4)2

N 154

Scheme 53

Even weak acids such as alcohol and amine can protonate Mn-alkyl bonds in organomanganese(II) compounds. For example, the manganese alkyl complex [Mn(CH2SiMe3)(boxmi)] (141) bearing a bis(oxazolinyl-methylidene)isoindoline (“boxmi”) pincer ligand reacted with alcohols to give the corresponding alkoxide complexes (e.g. it reacted with PhCH(OH)Me to give the alkoxide complex 155, Scheme 54).90

Scheme 54

The reactivity has been used to prepare Mn-containing polymers. For example, the alkyl complex [Mn(CH2tBu)2(TMEDA)] (156, TMEDA ¼ tetramethylethylenediamine) reacted with partially dehydroxylated silica at 700 (SiO2/700) to yield the silica-supported alkyl complexes [(^SiO)Mn(CH2tBu)(TMEDA)] (157a) as the major surface species and [(^SiO)2Mn(TMEDA)] (157b) as the minor surface species (Scheme 55).91 The alkyl complex [Mn(CH2SiMe3)2(THF)]2 reacted with a catechol porous organic polymer to give a material which is catalytically active for alkyne semihydrogenation.92 Treatment of bis(trimethylsilylmethyl) manganese [Mn(CH2SiMe3)2]n with anhydrous hydrazine produced a manganese(II) hydrazide gel (158), which shows hydrogen storage capability.93

Scheme 55

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

27

Combination of Mn(II) alkyl complexes with alkaline alkyls can generate highly basic Mn species. For example, mixing [Mn(CH2SiMe3)2]n, LiTMP (TMP ¼ 2,2,6,6-tetramethylpiperidide) and TMEDA produced the heteroleptic amido-alkyl manganite compound [(TMEDA)Li(TMP)Mn(CH2SiMe3)2] (159) (Scheme 56). A freshly prepared solution of 159 in hexane can deprotonate ferrocene to give [(TMEDA)2Li2Mn2{Fe(C5H4)2}3] (160),94 in which the ferrocenediide ligands bridge the two tetrahedral manganese(II) centers and two tetrahedral lithium cations.

Scheme 56

Treatment of bis(trimethylsilylmethyl) manganese [Mn(CH2SiMe3)2]n with NaTMP and TMEDA produced the monoalkyl-bisamido complex [(TMEDA)Na(TMP)(m-CH2SiMe3)Mn(TMP)] (161) (Scheme 57).95 The resulting Mn(II)

N N

N N

SiMe3 Mn

Na

SiMe3

N

Mn

Na

N

Mn

Na

N

N

O

N

N 162

164

166 OMe

[Mn(CH2SiMe3)2]n + BuNa N +

N N

O SiMe3

N

H

N

N

+

N

Mn

Na

NiPr2

161 O

N N

Na

Na

N

N Mn

Mn N

N

Na

Na

N N

Mn

Na N

SiMe3

O

N N

O iPr2N

N 163 Scheme 57

165

SiMe3 Mn

Na

167

28

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

amido-alkyl complex 161 can perform direct metalations (manganations) of aromatic compounds including benzene (to give 162),95 toluene (to give 163),96 anisole (to give 166), N,N-diisopropylbenzamide (to give 167),97 naphthalene (to give 164), 1-methoxynaphthalene and 2-methoxynaphthalene (to give 165).98 Combination of BuNa, [Mn(CH2SiMe3)2]n and DAH (diisopropylamide, HNiPr2) in a 1:1:3 M ratio produced the hydrido inverse crown [Na2Mn2(m-H)2{NiPr2}4(toluene)2] (168) (Scheme 58). The complex can affect the manganation of ferrocene to generate the trimanganese-triferrocenophane [{Fe (C5H4)2}3{Mn3Na2(NiPr2)2(HNiPr2)2}].99

Scheme 58

6.01.2.2.2.2 CdC bond activation The sodium-manganate [(TMEDA)Na(TMP)(m-CH2SiMe3)Mn(TMP)] (161) can promote cleavage of at least six bonds in THF to give O and CH]CHdCH]CH fragments that are captured in complexes 169 and 170 (Scheme 59). The oxide fragment occupies the guest position in the bimetallic inverse crown ether 169 while the C4 fragment appears in the bimetalated butadiene complex 170.100

Scheme 59

6.01.2.2.2.3 s-bond metathesis reactions Manganese(II) alkyls can undergo s-bond metathesis reactions with boranes and silanes. For example, the manganese alkyl complex [Mn(CH2SiMe3)(boxmi)] (141) bearing a bis(oxazolinyl-methylidene)isoindoline (“boxmi”) pincer ligand reacted with pinacol borane to yield Me3SiCH2BPin, the s-bond metathesis product of the reaction of the alkyl species, and the transient manganese hydride complex 171 (Scheme 60).90 The alkyl complex 127 reacted with PhSiH3 to give the bimetallic hydride complex 173 and the silane product 172, probably also through an s-bond metathesis reaction. A similar reaction may occurred in hydrosilylation of a-olefin and styrene with PhSiH3 catalyzed by the complex 127.79

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

29

Scheme 60

6.01.2.2.2.4 Addition to unsaturated substrates Mn(II) alkyl complexes can undergo addition reactions with unsaturated substrates. For example, the alkyl complex 139, which can be obtained by the reaction of [Mn(CH2SiMe3)2]n with 2,6-[(2,6-iPr2C6H3)N]C(Me)]2C5H3N (iPrBIP) in toluene, evolved to 174 on standing at room temperature for 72 h, as a result of migration of one of the alkyl groups to the 4-position of the BIP pyridine ring (Scheme 61). Similar reactions have been described for the alkyl complexes [MnR2(THF)n] (R ¼ CH2CMe2Ph, CH2Ph, CH2CH]CH2).85,101,102

Scheme 61

6.01.2.2.2.5 Reactions with oxygen A few reactions of Mn(II) alkyl complexes with oxygen have been reported. Treatment of the alkyl complex 111 with oxygen produced the complex 175, a tetranuclear oxo-bridged alkyl complex containing two Mn(R)(bpy) and two MnR2 units bridged by two m3-oxo units (Scheme 62).59 The reaction of NaHMDS, [Mn(CH2SiMe3)2]n and HMDSH in a 1:1:1 M ratio in the absence of

Scheme 62

30

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

O2 gave the oxygen-free monoalkyl-bisamido manganate [{Na(HMDS)2Mn(R)}n] (176). In the presence of O2, the reaction produced the sodium–manganese inverse crown ether [Na2Mn2(HMDS)4(O)] (177).103 6.01.2.2.2.6 Nucleophilic substitution and addition reactions Metal-carbon bonds in Mn(II) alkyls have significant ionic character, which enable Mn(II) alkyls to be useful nucleophilic alkylation reagents for organic synthesis. For example, treatment of ICH2CH2CH2CO2Et with zinc dust followed by trimethylmanganate Me3MnMgCl led to quantitative formation of MeZnCH2CH2CH2CO2Et (178) (Scheme 63).104 The methyl complexes 104 can undergo nucleophilic substitution reactions with Si(OEt)4 to selectively form MeSi(OEt)3.70

Scheme 63

Benzylic manganese chlorides can undergo coupling reactions with acid chlorides and allyl bromides; as well as addition reactions with aldehydes, enones (e.g. formation of 179 from 124e, Scheme 64) and nitro olefins without any additional transition metals.105,106 Benzylic manganese reagents can also undergo cross-couplings with functionalized alkenyl iodides, bromides and triflates (e.g. formation of 180 from 124e) in the presence of 10 mol% FeCl2 probably via reactive iron-alkyl intermediates.77,107 Benzylic manganese reagents have also been used in Pd-catalyzed cross-coupling and copper-catalyzed conjugate addition reactions.78

Scheme 64

Manganese(II) halo complexes such as MnCl2(TpmiPr,iPr) (Tpm ¼ tris(pyrazolyl)methane), [Mn(TpmiPr,iPr)Br(HOMe)2]Br and [MnCl(TptBu,iPr)] (Tp ¼ hydrotris(pyrazolyl)borate), when activated by triisobutylaluminum/triphenylcarbenium tetrakis(pentafluorophenyl)borate or MAO, are catalytically active for ethylene polymerization.108 MnI2 catalyzed intramolecular iodoamination of olefins.109 These catalytic reactions may also involve Mn(II) alkyl species.

6.01.2.3

M(III) and M(IV) complexes

Well-characterized group 7 M(III) alkyl complexes are less common, but have been reported for a few Mn(III) and Re(III) systems. M(IV) complexes are rare and have only been described for a few Mn(IV) systems. The Mn complexes are usually paramagnetic open-shell complexes while the Re complexes are usually diamagnetic 18-electron complexes. The trimethylmanganese(III) complex [MnMe3(TMEDA)] (181, TMEDA ¼ N,N,N,N0 -tetramethylethylenediamine) has been synthesized by the reaction of [Mn(acetylacetonate)3] with methyllithium in the presence of TMEDA at −78  C in diethyl ether (Scheme 65). The trimethyl complex 181 adopts a highly distorted trigonal bipyramidal structure in the solid state.110 In solution, it decomposes to give the Mn(II) complex [(TMEDA)MnMe2] (182) and the Mn(IV) complex [(TMEDA)MnMe4] (183), presumably through a bimolecular disproportionation process.

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

31

Scheme 65

Oxidation of the triflate complex 184 bearing a bis(imidazolin-2-imino)pyridine pincer ligand with o-(tBuSO2)C6H4IO in MeCN at −30  C gave the cyclometalated Mn(III) alkyl complex 186, probably via the oxo intermediate 185 which undergoes intramolecular CdH bond activation of one of the methyl groups of the ligand by a putative MnIV]O moiety (Scheme 66).111 It has also been reported that the reaction of methyl radical with the Mn(II) complex [Mn(ntp)(H2O)2] (H3ntp ¼ nitrilotris(methylenephosphonic-acid)) produced the unstable Mn(III) methyl complex [Mn(Me)(ntp)(H2O)].112

2+

N

N

Mn

2OTf

OTf

N

O

N

OTf

S O

+

N N

N

tBu N N

N

184

N

Mn

N N

IO

2+

O

N

185

2OTf N N

N

Mn

N N

N CH2

OH

N

186 Scheme 66

The Mn(III) complex [Mn(dpm)3] (188, dpm ¼ dipivaloylmethanato) catalyzed hydrohydrazination of unactivated alkenes by di(tert-butyl)azodicarboxylate (BocN]NBoc) and PhSiH3 in iPrOH (e.g. formation of 187) (Scheme 67).113,114 The catalytic reaction was proposed to involve [MnH(dpm)2] as the active species, which undergoes an insertion reaction of an alkene (into the MndH bond) to form the Mn(III) s-alkyl intermediate 189. The latter reacts with BocN]NBoc to give the Mn(III) amide complex 190, which reacts with iPrOH and PhSiH3 to give the hydrohydrazination product 187 and regenerates the hydride complex [MnH(dpm)2].

32

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 67

Several Re(III) alkyl complexes of the types [Cp0 Re(R)(X)L2] and [Cp0 Re(R)L3]+ have been described. Complexes of the type [Cp0 Re(R)L3]+ can be obtained by oxidative addition reactions of RX with Re(I) complexes [CpReL3]. For example, the complex [(Z5:Z1-C5Me4(CH2)2NMe2)Re(CO)2] (191a) reacted with MeOTf to form the cationic methyl complex trans[(Z5:Z1-C5Me4(CH2)2NMe2)Re(Me)(CO)2]OTf (192a) (Scheme 68).115 Similarly, the complex [(Z5:Z1-C5Me4CH2PPh2)Re(CO)2] (191b) reacted with MeOTf to yield the cationic methyl complex trans-[(Z5:Z1-C5Me4CH2PPh2)Re(Me)(CO)2]OTf (192b).116 The reaction of the cationic complex 192b with KI and Me3NO
2H2O yielded the neutral alkyl complex cis-[(Z5:Z1-C5Me4CH2PPh2)Re(I) (Me)(CO)] (193).

Scheme 68

Treatment of Na[CpRe(BDI)] (194, BDI ¼ N,N0 -bis(2,6-diisopropylphenyl)-3,5-dimethyl-b-diketiminate) with tetramethylammonium fluoride (Me4NF) gave solely the rhenium(III) methyl complex [CpRe(Me)(BDI)] (195) (Scheme 69).117 The analogous ethyl complex [CpRe(Et)(BDI)] was similarly produced in the reaction of Na[CpRe(BDI)] with tetraethylammonium chloride (Et4NCl).

Scheme 69

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

33

Re(III) alkyl complexes bearing an [N,N,N]-tridentate ligand are also known. For example, treatment of the acetate complex 196 with MeMgBr produced the methyl complex 197 (Scheme 70).118 The methyl complex 196 can undergo insertion reactions with RNC to give iminoacyl complexes.

Scheme 70

6.01.2.4

M(V) complexes

No Mn(V) and Tc(V) alkyl complexes have been described in the review period. On the other hand, a series of Re(V) alkyl complexes have been isolated especially for those containing an oxo, nitrido or carbyne ligand.

6.01.2.4.1

Synthesis

6.01.2.4.1.1 By transmetalation reactions of main group alkyls The most common route to Re(V) alkyl complexes is to use transmetalation reactions of rhenium halo complexes with alkylating reagents such as LiR and RMgBr. For example, treatment of the carbyne complexes 198 with MeMgCl produced the monomethyl complexes [Re(^CCH]CRC^CTMS)(Me)Cl(PMe2Ph)3] (199, R ¼ tBu (a), 1-adamantyl (b)) (Scheme 71).119 Similarly, treatment of the chloro complex [ReCl(N)(PNP)] (200, PNP ¼ (2-PiPr2-4-MeC6H3)2N) with MeMgBr gave the methyl complex [Re(Me) (N)(PNP)] (201).120 PMe2Ph Cl H Cl

Re

MeMgCl R

PMe2Ph Me H Cl

Re

R

PhMe2P PhMe2P

PhMe2P PhMe2P

Si

Si 198a: R = CMe3 198b: R = 1-adamantyl

PiPr2 N N

Re PiPr2 200

Cl

199a: R = CMe3 199b: R = 1-adamantyl

MeMgCl

PiPr2 N N

Re

Me

PiPr2 201

Scheme 71

A series of complexes of the type [Re(O)(R)(NNN)] bearing a dianionic tridentate ligand have been synthesized by the same strategy. For example, oxorhenium alkyl and aryl complexes of the formula [Re(O)(R)(DAPMes)] (203, R ¼ Me (a), Ph (b), 4-MeOC6H4 (c), 4-ClC6H4 (d); H2DAPmes ¼ 2,6-bis((mesitylamino)methyl)pyridine) have been obtained by the reactions of [Re(O)Cl(DAPMes)] (202) with the corresponding transmetalating reagents RMgBr (Scheme 72).121,122 Additional examples of complexes of the type [Re(O)(R)(NNN)] (NNN ¼ dianionic [N,N,N]-tridentate ligand) prepared by this strategy include [Re(O) (Me)(DAPdipp)] (205, H2DAPdipp ¼ 2,6-bis((2,6-diisopropylphenylamino)methyl)pyridine) 123 and [Re(O)(CH2Ar)(DAAm)] (207, Ar ¼ Ph (a), 4-MeC6H4 (b), 4-FC6H4 (c), 4-MeOC6H4 (d); DAAm ¼ (C6F5NHCH2CH2)2NMe).124

34

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Mes

Mes

N

N O

N

Re N

O

RMgBr

Cl

Mes

202

N

Re N

R = Me (a), Ph (b), 4-MeOC6H4 (c), 4-ClC6H4 (d)

R

Mes

203

dipp

dipp

N

N O

N

Cl

Re N

O

MeMgBr N

N

dipp

C6 F 5

Re

dipp

C6F5

N O

N

Me

205

204

N

Re

Cl

O

RCH2MgBr N

Re R

N 206

C6F5

N

C6F5

207

R = Ph (a), 4-MeC6H4 (b), 4-FC6H4 (c), 4-MeOC6H4 (d) Scheme 72

A series of oxorhenium alkyl and aryl complexes [Re(O)R(SSS)] (e.g. 208a–c, 209) bearing a dianionic tridentate SSS tridentate ligand (SSS ¼ 2-mercaptoethylsulfide) have also been successfully synthesized via transmetalation of the bromo complex [Re(O)Br(SSS)] with the corresponding transmetalating reagents (ZnR2 or RMgBr) (Scheme 73).125

Scheme 73

6.01.2.4.1.2 By insertion reaction of alkenes Insertion of an alkene into a RedH bond represents another route to Re(V) alkyl complexes. For example, the hydride complex [Re {OB(C6F5)3}H(DAPMes)] (210) reacted with CH2]CH2 to afford the alkyl complex [Re{OOB(C6F5)3}(CH2CH3)(DAPMes)] (211)121 (Scheme 74). Similarly, the hydride complex [Re{OB(C6F5)3}H(DAPdipp)] (212) reacted with styrene derivatives CH2]CHC6H4-4-R (R ¼ H, Me, CF3) to afford the alkyl complexes [Re{OOB(C6F5)3}(CH2CH2C6H4R)(DAPdipp)] (213).126 It is noted that the parent complexes without coordinated Lewis acids do not undergo insertion reactions under similar conditions.

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Mes

Mes

N

N O

N

Re N

B(C6F5)3

O

H

N

N

Mes

Dipp

Re N

H

N

R B(C6F5)3

212

Dipp O

N

Re

B(C6F5)3 R

R = H (a), Me (b), CF3 (c)

N Dipp

Mes 211

N O

B(C6F5)3

Re

210

N

35

Dipp 213

Scheme 74

6.01.2.4.1.3 By cyclometalation and CdS bond activation reactions Cyclometalation reactions have been occasionally used to prepare alkyl complexes. For example, the cyclometalated complex 215 bearing a [N,N,N,C]-tetradentate ligand was produced in the reaction of the hydride complex 214 with B(C6F5)3, as a result of cyclometalation of one of the methyl groups (Scheme 75).121

Scheme 75

RedC bonds could also be generated by CdS bond cleavage reactions of thiolate complexes. For example, treatment of the thiolate complexes [Re(O)(SR)(mtp)(PPh3)] (216, R ¼ Ph (a), Et (b), H2mtp ¼ 2-(mercaptomethyl)thiophenol) with excess PPh3 led to the formation of the metallacyclic Re(V) complexes 217 (Scheme 76). Similar CdS bond cleavage reactions have been reported for [Re(O)(S-p-C6H4X)(mtp)(PPh3)] (X ¼ OMe, Me, F, Cl).127 Treatment of the thiolate complex 218 with PPh3 yielded the cyclometalated alkyl complex 219.

Scheme 76

6.01.2.4.1.4 By modification of preformed alkyl complexes Rhenium(V) alkyl complexes have been routinely obtained by ligand substitution reactions of preformed alkyl complexes. For example, treatment of the methyl complex [Re2(Me2)(O)2(SPh)4] with PPh3 and 1,3-propanedithiol (H2pdt) produced [Re(O) (Me)(PPh3)(pdt)] (220), which can be oxidized by air to give the bimetallic methyl complex 221 (Scheme 77). Similarly, the

36

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 77

reaction of (EtO)3SiCH2CH2CH2OC(O)CH2CH2CH2CH2CH(SH)CH2CH2SH (222, RTAH2) with the methyl complex [Re(O) (SPh)2(Me)(PPh3)] produced the methyl complex [Re(O)(Me)(PPh3)(PTA)] (223). The reaction has been used to prepare a silica-tethered oxorhenium(V) dithiolate alkyl complex, which can catalyze selective oxidation of methyl(p-tolyl)sulfide, dibenzothiophene and 4,6-dimethyldibenzothiophene to their corresponding sulfoxides and sulfones by tert-butylhydroperoxide.128 As another example, treatment of the methyl complex {Re(O)(Me)(edt)}2 (224, edt ¼ 1,2-ethanedithiolate) with two equivalents of 2,20 -bipyrimidine (bpym) produced the methyl complex [Re(O)(Me)(edt)(bpym)] (225), which is catalytically active for oxygen atom transfer from picoline N-oxide to triarylphosphines.129 Re(V) alkyl complexes could also be generated by reduction of preformed Re(VII) alkyl complexes. For example, the methyl complexes [ReO2(Me)(PR3)2] (226, R ¼ Ph (a), Cy (b)) have been obtained by the reactions of MeReO3 (MTO) with PPh3 and PCy3 (Scheme 78).130 The methyl complexes [Re(O)(Me){(RNCH2CH2)2N(Me)}] (R ¼ C6F5 (207e), mesityl (228)) bearing a diamidoamine ancillary ligand have been obtained by treating a dichloromethane solution of MeReO3 with one equivalent of PPh3 and an equivalent of the triamines 227.131 The methyl complex [Re(O)(Me)(DAPMes)] (203a) can be obtained by the reaction of MeReO3 with PPh3 and bis((mesitylamino)methyl)pyridine (229, H2DAPMes).132

Scheme 78

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

37

The strategy has been used to prepare rhenium alkyl-alkyne complexes. For example, treatment of MeReO3 with 3-hexyne and 3-pentanol (a reductant) at 155  C produced the organorhenium(V) alkyl compound 230 along with 3-pentanone (Scheme 79).133 The alkyne complex 230 could also be obtained by heating a mixture of indoline, 3-hexyne and MTO (5:2:1) at 100  C for 19 h.134 Treatment of MeReO3 with one equivalent of the diyne 231 in the presence of triphenylphosphine produced the alkyl complex 232.135

Scheme 79

Re(V) alkyl complexes could also be obtained from Re(VII) diolate-alkyl complexes. For example, treatment of the Re(VII) glycolate alkyl complex 233 with PPh3 or the sulfite salt Na2SO3 gave the dinuclear Re(V) glycolate alkyl complex 2340 derived from the coordinatively unsaturated Re(V) glycolate alkyl complex 234 formed by deoxygenation of the complex 233 (Scheme 80).136

Scheme 80

The Re(VII) diolate alkyl complex 235 underwent photo-decomposition at room temperature or thermal decomposition at 140  C to produce a mixture of MTO and the Re(V) alkyl complex 236 (Scheme 81).130 The Re(V) diolate methyl complex 235 reacted with [ReO2(Me)(PCy3)2] (226b) to give a mixture of MTO and the Re(V) alkyl complex 237. The Re(V) alkyl complex 236 can react with 3-octanol to give MeReO2, trans-stilbene and 3-octanone. Ph

Me Me

O

Ph

O O

Re

h or heating O

O

Re

+ O

O O

O

PhCHO

Ph

Me

236

235

PCy3 Re

O

PCy3 226b Scheme 81

Me

Me

O Me

Re

O Ph

O

Re O O

O

+

Re

PCy3 O

Ph

O Ph

237

38

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

6.01.2.4.2

Reactivity

Oxorhenium alkyl complexes can undergo migratory insertion reactions with CO. For example, the alkyl and aryl complexes [Re(O) (R)(SSS)] (R ¼ Me (208a), Et (208b), Ph (209)) bearing a dianionic tridentate SSS ligand (SSS ¼ 2-mercaptoethylsulfide) reacted with CO (400 psi) in toluene at 50  C to give the corresponding acyl complexes [Re(O)(COR)(SSS)] (239, R ¼ Me (a), Et (b), Ph (c)) (Scheme 82).125 A computational study suggests that the insertion reaction most likely proceeds through a stepwise pathway involving the unstable CO adducts 238. Interestingly, the analogous complex [Re(O)(CH2Ph)(SSS)] (208c) does not undergo CO insertion reaction under the same condition.

O S Re S

O

O R S

CO

S

R S

R

Re

Re S

O

S S

S

CO 208a: R = Me 208b: R = Et 209: R = Ph

238

239a: R = Me 239b: R = Et 239c: R = Ph

Scheme 82

The alkyl and aryl complexes of the type [Re(O)(R)(DAPMes)] (203, R ¼ Me (a), Ph (b), 4-MeOC6H4 (c), 4-ClC6H4 (d); H2DAPmes ¼ 2,6-bis((mesitylamino)methyl)pyridine) reacted with CO at 80  C to give the acyl complexes [Re(O)(COR)(DAPMes)] (241) (Scheme 83).122 DFT studies suggest that the carbon–carbon bond formation process most likely involves direct insertion of CO into the RedR bond via transition state 240 without prior formation of a CO-complex intermediate. It is noted that the methyl complex 203a reacted with CO at a significantly faster rate than the aryl complexes 203b–d for kinetic reason.

Scheme 83

The methyl complex [Re(O)(CH2R)(DAAm)] (207, R ¼ Ph (a), 4-MeC6H4 (b), 4-FC6H4 (c), 4-MeOC6H4 (d), H (e), DAAm ¼ (C6F5NCH2CH2)2NMe) underwent insertion reaction with CO (60 psi) at RT to form the acyl complex 242 (Scheme 82). The acyl complex 242e can react further with CO at 80  C to give the rhenium(III) acetate complex [Re(O2CCH3)(CO)(DAAm)] (243) (Scheme 84).124,137 Mechanistic, kinetic, and computational studies reveal that the insertion of CO involves a direct insertion mechanism and not the typical two-step intramolecular mechanism.138 It is noted that the related aryl complex [Re(O)(Ph) (DAAm)] does not undergo analogous CO insertion reaction even after 6 days under harsh reaction conditions (800 psi CO, 80  C). A computational study reveals that the barrier for CO insertion into the RedPh bond is higher than that into the RedMe bond, which may be attributed to the stronger RedPh bond compared with the RedMe bond. It has also been reported that the analogous hydride complex [Re(O)H(DAAm)] reacted with CO to give [Re(OH)(CO)(DAAm)].139

Scheme 84

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

39

The methyl complex [Re(N)(Me)(PNP)] (201, PNP ¼ [2-PiPr2-4-MeC6H3]2N) reacted with CO at 80  C to give the Re(V) acyl complex 244 in 1 h, and the carbonyl complex 245 and MeC(O)NCO in 24 h (Scheme 85). In contrast the complex 201 reacted with the isocyanide CNdC6H4Me2-2,6 to give the isocyanide adduct 246.120 DFT calculations suggest that the difference might be of kinetic origin.

Scheme 85

Oxorhenium alkyl complexes [Re(O)(R)(NNN)] (NNN ¼ dianionic tridentate ligand) could also undergo other reactions. For example, the methyl complex 207e reacted with Me2PhSiH under N2 for 24 h to generate the dinuclear dinitrogen complex 247 (Scheme 86).140 Complexes of the type [Re(O)(R)(DAPMes)] (DAPMes ¼ 2,6-bis((mesitylamino)methyl)-pyridine) and [Re(O)(R) (DAAm)] (DAAm ¼ (C6F5NCH2CH2)2NMe) can serve as a Lewis base via lone pairs of the oxo ligand to form frustrated Lewis pairs (FLPs) with Lewis acids such as B(C6F5)3 and Al(C6F5)3 (e.g. formation of 248 from 203a).123,141 The resulting frustrated Lewis pairs are capable of hydrogenation of unactivated olefins such as ethylene, propylene, 1-hexene and tert-butylethylene through an ionic mechanism. Alkyl complexes of the type [Re(O)(R)(DAAm)] (DAAm ¼ (C6F5NCH2CH2)2NMe) were also reported to be catalytically active for hydrosilylation reactions.140

Scheme 86

40

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

As will be discussed in the section dealing with carbyne chemistry, treatment of complexes of type [Re(^CR)Cl2(PMe2Ph)3] with tert-butylmagnesium chloride can lead to the formation of hydridochloridocarbyne complexes [Re(^CR)HCl(PMe2Ph)3] and CH2]CMe2, presumably through b-H elimination of the alkyl intermediate [Re(^CR)(tBu)Cl(PMe2Ph)3].119

6.01.2.5

M(VI) and M(VII) complexes

Group 7 M(VI) alkyl complexes are rare, but have been described for a few Re(VI) complexes. Treatment of Re2O7 with three equivalents of ZnMe2 produced the bimetallic Re(VI) tetramethyl compound [Me2Re(m-O)2ReMe2] (249) as the major product. The reaction of Re2O7 with four equivalents of ZnMe2 above 0  C produced the bimetallic Re(VI) hexamethyl compound [Me3Re(m-O) ReMe3] (250) as the major product (Scheme 87). The tetramethyl compound 249 reacted with H2O2 to give the peroxy alkyl complexes 252 and 253. The hexamethyl compound 250 can react with H2O2 to give the trimethyldioxorhenium complex 251 as the major product. The tetramethyl compound [Me2Re(m-O)2ReMe2] (249) can be applied as an oxidation catalyst precursor.142

Scheme 87

Re(VII) alkyl complexes are more common. The most important member of Re(VII) alkyl complexes is MeReO3 (methyltrioxorhenium, MTO). A classic route to this compound is to react Re2O7 with tetramethyltin. Efforts have been made to improve the synthesis by performing reactions of rhenium(VII) precursors Re2O7, acetyl perrhenate, trifluoroacetyl perrhenate, chlorotrioxorhenium and trimethylsilyl perrhenate with various tin free methylating agents (e.g. LiMe, MeMgCl, ZnMe2, AlMe3 and MeAlCl2).143 It has been demonstrated that MeReO3 can be efficiently synthesized by the reaction of perrhenylacetate (MeCO2ReO3) (254) with methylzinc acetate (MeZnO2CMe) (Scheme 88).144–146 This new route is highly efficient and eliminates the uses of sensitive dirhenium heptoxide as well as the expensive and toxic methyltin reagent.

Scheme 88

MeReO3 can catalyze a variety of organic transformations, especially deoxydehydration and oxidation. Examples of the catalytic deoxydehydration reactions include deoxydehydration of vicinal diols and epoxides to give alkenes with reducing agents such as hydrogen,147 sulfite136,148 alcohols130,133,149–154 and hydroaromatics.134 Examples of the catalytic oxidation reactions include oxidation of purine to give purine N-oxide,155,156 oxidation of pyridines to give pyridine N-oxides,157–169 oxidation of imines (and amines) to give nitrones,170–174 oxidation of thiols and thioethers,175,176 oxidation of diaryl- and dialkyldisulfides to sulfonic acids,177 epoxidation of olefins,178–186 oxidation of porphyrins to give N-oxides,187,188 oxidation of ketones to give bishydroperoxides,189,190 oxidation of phenolic and non-phenolic lignin model compounds191 and allylic CdH oxidation.192 Other reactions catalyzed by MeReO3 include Diels-Alder reactions of 1,2,3,4,5-pentamethylcyclopentadiene with benzoquinones,193 coupling reactions of N-sulfonylpiperidones with ketones to give N-sulfonylpiperidine tetraoxanes194 and mono-trifluoromethylation of corannulene with 1-trifluoromethyl-1,2-benziodoxol-3-(1H)-one (Togni’s reagent).195–197

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

41

MeReO3 can react with monodentate ligands L to form five-coordinate trigonal bipyramidal complexes of the type [MeReO3
L] (255), and with bidentate ligands L2 to form six-coordinate octahedral complexes of the type [MeReO3(L2)] (256) (Scheme 89). These complexes are often catalytically active for olefin epoxidation.

Scheme 89

The complexes 257 are examples of complexes bearing a Schiff-base ligand derived from salicylaldehydes and aniline derivatives (Scheme 90).198–204 The complexes 258 are examples of complexes bearing a pyridine ligand.205,206 The complex 259 is an example of complexes bearing an aniline ligand.134 The complex 260 is an example of complexes bearing an indoline ligand.134

Scheme 90

The complexes 261 are examples of complexes bearing a Schiff-base ligand derived from pyridinecarboxaldehyde and primary amines (Scheme 91).207–210 The complexes 262–263 are examples of complexes bearing a bipyridine ligand.211,212 The complex 264 is an example of complexes bearing a 1,2-diamine bidentate ligand.142,213 The complexes 265 are examples of complexes bearing a benzimidazole-pyridine bidentate ligand.214

Scheme 91

MTO can also form inclusion complexes with b-cyclodextrin which are catalytically active for oxidation of indigo blue dyes with H2O2.215 Lewis base adducts of methyltrioxorhenium (MTO) with 4-methoxyaniline and 2-pyridinemethanamine can be encapsulated in polystyrene to generate recyclable catalysts for epoxidation of alkenes using hydrogen peroxide as the oxidant.216 Efforts have been made towards heterogenization of MeReO3 with supports such as mesoporous silica MCM-41 functionalized with a pyrazolylpyridine ligand,217 graphene oxide functionalized with Schiff bases,218 carbon nanofibers219 and nitrogen-containing polymers such as poly(4-vinylpyridine), polybenzylamine and poly(melamine-formaldehyde).220 The graphene oxide-supported system was found to be catalytically active for oxidation of amines to the corresponding N-oxides using hydrogen peroxide as the oxidant. The carbon nanofiber-supported system was found to be an efficient catalyst for oxidation of secondary as well as tertiary amines to corresponding nitrones or N-oxides, respectively with hydrogen peroxide. The polymer-supported systems are catalytically active for deoxydehydration of tartaric acid.

42

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Grafting MeReO3 on Al2O3 generated a heterogeneous system that can catalyze the metathesis of ethylene with ethyl oleate, an alkene containing an ester functional group.221–225 Detailed NMR and computational studies of MeReO3/Al2O3 reveals that two major surface Re-species are present in this system. The major species (85–90%) is 266a which is formed by coordination of MeReO3 to surface Al atoms by Re]O moieties (Scheme 92). The minor species (10–15%) (266b) contains a m-methylene bridging ligand resulting from activation of a CdH bond in MeReO3 by an AldO bond. The minor surface species can evolve to the carbene species 267, which is responsible for the olefin metathesis activity.226,227 Solid-state NMR studies reveal that the reaction of Re2O7/ Al2O3 with Me4Sn can generate m-methylene AldCH2ReO3 species, similar to the species observed in the MeReO3/Al2O3 system. The m-methylene species was probably formed through in situ generated MeReO3.228 Me

Me O

+ Al2O3

Re O O

O

Re O Als

Als

266a (major)

Re

Re

+ O

O Als

CH2

O

O

O

CH2 Als O

266b (minor)

H

O

O

Als

Als O

H

267

Scheme 92

Methyltrioxorhenium can react with other Lewis acids. For example, it reacts with B(C6F5)3 to give [MeReO2{OB(C6F5)3}] (268) which exhibits catalytic activity in the ROMP reaction of norbornene and the metathesis reaction of 1-hexene (at 150  C) (Scheme 93).229

Scheme 93

Reactions of methyltrioxorhenium (MTO) with 1,2-diols and epoxide derivatives often lead to the formation of diolate-methyl complexes. For example, treatment of MTO with phenylethanediol (269) or styrene oxide produced the glycolate complex 233 (Scheme 94).136 Treatment of MTO with hydrobenzoin (270) in the presence of molecular sieves at room temperature gave the diolate complex 235.130 Similar reactions have been described for 2-mercaptoethanol derivatives. For example, mixing MTO and enantiopure (−)-2-mercaptocyclohexanol (271) produced the enantiopure dioxorhenium complex 272.230 Analogous complexes bearing a propane-2-olato-3-thiolato ligand have been made similarly.231 Theses complexes have been used as candidates for observation of parity violation (PV) effects.

Scheme 94

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

43

MTO can undergo methyl transfer reactions with transition metal complexes. For example, the iridium pincer complex 273 in acetonitrile reacted with methyltrioxorhenium (MTO) to give the IrdRe bimetallic complex 274 with a net result of methyl transfer to Ir (Scheme 95).232,233 A similar reaction occurred between methyltrioxorhenium (MTO) and [PtMe2(bpy)] (bpy ¼ 2,20 -bipyridine) (275) to give the bimetallic complex [(bpy)Me3PtReO3] (276).234

Scheme 95

Reactions of MTO with oxidizing agents have also been described. Treatment of MTO with O-17 labeled (trimethylsilyl)peroxide (277) generated the O-17 labeled bis(peroxo) rhenium methyl complex [MeRe(]O)(Z2-O 2)2(OH2)] (278) (Scheme 96).235

Scheme 96

Methyltrioxorhenium can react with O-atom donors such as IO−4, H2O2 and PhIO in aqueous solutions at room temperature to cleanly generate methanol and ReO−4. Oxidation of MTO by H2O2 can be catalyzed by flavins and related molecules.236 DFT studies suggest that these reactions most likely proceed through a Baeyer-Villiger (BV) type mechanism involving non-redox electrophilic O-atom insertion.237 For example, the reaction of MTO with H2O2 under basic condition most likely proceeds through intermediates 279 and 281 via the transition state 280 (Scheme 97).238 Methyltrioxorhenium also readily reacted with osmium tetroxide in basic aqueous medium to give MeOH probably via a low energy (2 + 3) transition state.239 Methyltrioxorhenium (MTO) could also be oxidized electrochemically to methanol in an acidic aqueous solution on tin-doped indium oxide mesoporous nanoparticle film electrodes modified with the complex [Ru(Mebimpy)(4,40 -((HO)2OPCH2)2bpy)(OH2)]2+ (Mebimpy ¼ 2,6(1-methylbenzimidazol-2-yl)).240

Scheme 97

As described in previous section, oxo-Re(VII) methyl complexes can be reduced to Re(V) methyl complexes. Ethyltrioxorhenium (282, ETO), the closest congener of MTO, can be synthesized via the reaction of diethylzinc with rhenium heptoxide (Scheme 98). ETO can also serve as a catalyst for the epoxidation of olefins using either tert-butylhydroperoxide (TBHP) or hydrogen peroxide as the oxidants. The active species was assumed to be an alkyl peroxo species.241

44

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 98

ETO is not a thermally stable compound. A sample of colorless liquid ETO could turn black within a few minutes at room temperature to give [ReOx] particles, ethane and ethylene. Experimental and theoretical studies on decomposition of ETO in diluted polar solvents reveal that the degradation can proceed via b-hydrogen elimination (path B, Scheme 96) and/or a radical process induced by homolytic cleavage of the RedC bond (path A, Scheme 96).241 The barriers for b-H elimination pathway and the radical pathway in THF were calculated to be 54.0 and 51.9 kcal/mol, respectively. Isolation of ethyltrioxorhenium (282) in a solid argon matrix at ca. 16 K and exposure to UV radiation with the wavelengths of 200–400 nm results in b-hydrogen elimination with the formation of the hydrogen-bonded ethene complex [C2H⋯ 4 HOReO2] (286) (Scheme 99). Visible radiation with wavelengths of 400–800 nm caused 286 to isomerize to the Z2-ethene complex [Re(OH) O2(Z2-C2H4)] (287).242

Scheme 99

Other well-characterized Re(VII) alkyl complexes include those contain a carbene or carbyne ligand, for example, [ReO2(CH2CMe3)(]CHCMe3)],229 [Re(^CCMe3)(]CHCMe3)(CH2CMe3)2] and [Re(^CCMe3)(]CHCMe3)(OSiR3) (CH2CMe3)].243–247 These complexes will be described in detail in sections dealing with carbene and carbyne complexes.

6.01.3

Aryl complexes

Group 7 metal aryl and heteroaryl complexes have been described for those containing a Mn(I), Mn(II), Mn(III), Mn(IV), Tc(I), Tc(V), Re(I), Re(III), Re(IV), Re(V), or Re(VII) center. In these complexes, the aryl groups can serve as either terminal or a bridging ligand. The chemistry of group 7 metal complexes bearing a cyclometalated aryl ligand has also been actively explored.

6.01.3.1 6.01.3.1.1

M(I) complexes Complexes with terminal
1-aryl ligands

Most of the well characterized M(I) aryl complexes are 18-electron complexes with p-accepting ligands such as CO and phosphines. A few open-shell Mn(I) aryl complexes have also been isolated with bulky aryl groups. 6.01.3.1.1.1 Synthesis Treatment of the dinuclear Mn(II) chloroaryl complex [(3,5-iPr2-1,2-Ar2-C6H)Mn(m-Cl)]2 (288, Ar ¼ 2,4,6-iPr3-C6H2) with K [CpFe(CO)2] yielded the Mn(I) complex [(3,5-iPr2-1,2-Ar2-C6H)MnFe(CO)2Cp] (289) containing a MndFe metal bond (Scheme 100).248 DFT calculations reveal a dative bonding interaction between the metals (Mn(I) to Fe(I)) in the complex 289.

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

iPr

iPr

iPr iPr

iPr

iPr

iPr iPr

Cl iPr

Mn

Mn

iPr

iPr

iPr iPr K[CpFe(CO)2]

Cl iPr

iPr

iPr

iPr Fe

Mn iPr iPr

iPr iPr

45

CO CO

iPr

iPr

iPr 288

289

Scheme 100

The Mn(I) inverted sandwich aryl complex [(Z6:Z6-C6H5Me){Mn(3,5-iPr2-1,2-Ar2-C6H)}2] (291) has been prepared by the reduction of the in situ generated iodo-alkyl [(3,5-iPr2-1,2-Ar2-C6H)MnI]2 (290) with KC8 in THF, followed by extraction with toluene (Scheme 101).249 The complex 291 represents a rare example of open-shell one-coordinate metal moieties stabilized by coordination to an arene. iPr iPr iPr

iPr iPr

iPr

iPr iPr

iPr

I

Mn iPr

iPr Mn

I

iPr

iPr

iPr 290

KC8

iPr

THF/toluene iPr iPr

iPr

iPr

iPr

iPr iPr

iPr iPr

iPr Mn

Mn iPr

iPr

iPr

iPr iPr

iPr

iPr 291

Scheme 101

Saturated 18-electron group 7 M(I) aryl complexes are more common. Several routes have been used to install the M(I)-aryl linkages of these complexes. The most common strategy is to use transmetalation reactions of transmetalating reagents such as LiAr and ZnAr2 with M(I) precursors. For examples, the aryl complexes [Re(Ar)(CO)4{PPh2(OEt)}] (293, Ar ¼ C6H5 (a), Ar ¼ 4-CH3C6H4 (b)) have been prepared by the reactions of the triflate complex [Re(OTf )(CO)4{PPh2(OEt)}] (292) with the corresponding aryllithium reagents (Scheme 102).250 Analogous aryl complexes such as [Re(Ph)(CO)3(P)2] (294, P ¼ P(OEt)3 (a), PPh(OEt)2 (b)), [Re(Ph)(CO)2{PPh(OEt)2}3] (295), [Re(Ph)(CO)3{Ph2PO(CH2)3OPPh2}] (296) and the Mn(I) aryl complex [Mn(Ph)(CO)3{POEt3}2] (297) have been prepared similarly.

Scheme 102

Treatment of the bromo complex [ReBr(CO)3(4,40 -dimethoxy-2,20 -bipyridine)] (298) with ArMgBr (Ar ¼ Ph, p-C6H4Me) produced the aryl complexes [Re(Ar)(CO)3(4,40 -dimethoxy-2,20 -bipyridine)] (299, Ar ¼ Ph (a), p-C6H4Me (b)) (Scheme 103).9 Treatment of the chloro complex [ReCl(CO)3(P((CH2)14)3P)] (3) with ZnPh2 produced the aryl complex [Re(Ph)(CO)3 (P((CH2)14)3P)] (300).7,251

46

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 103

Treatment of the trinuclear chloro complex Li[Re2(AuPPh3)(m-PCy2)(CO)7Cl] (301) with LiPh gave the trinuclear aryl complex anion [Re2(AuPPh3)(m-PCy2)(CO)7Ph]−, which was isolated as its PPh4-salt 302 (Scheme 104).252 The Tc(I) aryl complex [CpTc(Ph)(NO)(PPh3)] (304) has been obtained from the reaction of [CpTcBr(NO)(PPh3)] (303) with PhMgBr.253

Cy2 P

CO Li

CO Re

(CO)4Re PPh3

CO PPh4

(CO)4Re

CO

Au

CO

Cy2 P

1) LIPh 2) PPh4Cl

Re CO

Au

Cl

PPh3

301

Ph

302

PhMgCl

Tc

Tc

PPh3

ON

PPh3

ON

Cl

Ph

303

304

Scheme 104

Apart from typical alkaline or alkaline-earth transmetalating agents, bismuth and antimony aryl compounds can also serve as sources of aryls for group 7 M(I) aryl complexes. The carbonyl complex [Re2(CO)10] reacted with SbPh3 under UV–Vis irradiation to give a mixture of species including [Re2(CO)8(SbPh3)(Ph)(m-SbPh2)] (305), [Re(Ph)(CO)4(SbPh3)] (306), fac-[Re(Ph) (CO)3(SbPh3)2] (307) and [ReH(CO)4(SbPh3)] (308) (Scheme 105).254 The aryl complex [Re(Ph)(CO)4(SbPh3)] (306) was also produced by heating the bimetallic complex [PtRe(CO)4(Ph)(PtBu3)(m-SbPh2)(m-H)] at 100  C.255 Clusters containing a RedPh moiety have also been generated in the reaction of [Re2(CO)8(m-SbPh2)(m-H)] with [Pt(PtBu3)2].256

CO [Re2(CO)10] + SbPh3

h

Ph3Sb

Ph2 Sb

Re OC

Ph Re CO

305

Ph3Sb +

CO

CO OC CO

Scheme 105

CO

CO

OC

SbPh3

CO

Ph

Ph Ph3Sb H Ph3Sb Re Re Re + + CO CO CO OC OC CO CO CO 306

307

308

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

47

Heating the rhenium-bismuth dinuclear carbonyl complex [Re2(CO)8(m-BiPh2)2] (309) at 110  C for 45 min produced a mixture of the six-membered metallacycle 310, the mononuclear aryl complex [Re(Ph)(CO)5] and the bimetallic complex [Re2(CO)8(m-BiPh)2] (311) (Scheme 106).257 Clusters containing a RedPh moiety have also been generated in the reaction of [Pt(PtBu3)2] with the bismuthtrirhenium complex [Re3(CO)12(m-BiPh2)(m-H)2].258

Scheme 106

Even arylphosphines can be a source of aryls for group 7 metal aryl complexes. For example, the reaction between [Mn2(CO)10] and tri(2-furyl)phosphine (PFu3) in refluxing toluene afforded [Mn2(CO)8(PFu3)2] (312) together with the furanyl complex mer-[Mn(Z1-C4H3O)(CO)3(PFu3)2] (313) as a result of cleavage of a phosphorus-carbon bond of the phosphine (Scheme 107).259

CO [Mn2(CO)10] + PFu3

110 oC

CO CO

Fu3P OC

Mn

Mn OC

O CO PFu3 + Fu P Mn 3 OC

CO

CO 312

CO PFu3

CO 313

Scheme 107

Treatment of [Mn2(CO)10] with PTh3 (Th ¼ 2-thienyl) in refluxing toluene afforded the phosphine-substituted product [Mn2(CO)9(PTh3)] and the two carbon-phosphorus bond cleavage products [Mn2(CO)6(m-PTh2)(m-Z1:Z5-C4H3S)] (314) and [Mn2(CO)5(PTh3)(m-PTh2)(m-Z1:Z5-C4H3S)] (315) containing a 2-thienyl moiety (Scheme 108).260 Thermolysis of the rhenium dinuclear PTh3-complex 316 in refluxing xylene afforded the thienyl complex [Re2(CO)8(m-PTh2)(m-Z1:k1-C4H3S)] (317) (Th ¼ 2thienyl), resulting from cleavage of a carbon-phosphorus bond of a coordinated PTh3 ligand. Similar PdC bond cleavage reactions have been described for the reactions of phenyldi(2-thienyl)phosphine (PPhTh2) with M2(CO)10 (M ¼ Re, Mn)261 and the reactions of tri(2-furyl)phosphine (PFu3) with [Re2(CO)10-n(NCMe)n] (n ¼ 1, 2).262

Scheme 108

Group 7 M(I) aryl complexes have been occasionally obtained by oxidative addition of arenes to low valent precursors. For example, the ortho-pyridyl-ditechnetium hydrido complex [Tc2(m-H)(m-NC5H4)(NC5H5)2(CO)6] (319) was produced upon refluxing a pyridine solution of [Tc2(CO)8(py)2] (318) (Scheme 109).263

48

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 109

Another route to M(I) aryl complexes is to use decarbonylation reactions of acyl complexes. For example, the complex [(Z6-iodobenzene)Cr(CO)3] (320) reacted with Na[Re(CO)5] to give the iodo(acyl)rhenate Na[{Z6-C6H5C(O)ReI(CO)4} Cr(CO)3] (321), which evolved to the rhenium aryl complex Na[{Z6-C6H5ReI(CO)4}Cr(CO)3] (322) on standing at 20  C for months (Scheme 110).264 The analogous reaction of [(Z6-iodobenzene)Cr(CO)3] with K[Mn(CO)5] gave a mixture of s-aryl complexes [{Z6-C6H5Mn(CO)5}Cr(CO)3] (324) and K[{Z6-C6H5Mn(CO)4I}Cr(CO)3] (323).264

I OC OC

O

I

Na

Na[Re(CO)5]

Re(CO)4

Cr CO

OC OC

320

Cr

I Re(CO)4

OC OC

CO 321

Na

Cr CO 322

I

Na

Mn(CO)5

Mn(CO)4 K[Mn(CO)5] OC OC

Cr CO 323

+

OC OC

Cr CO 324

Scheme 110

Re(I) aryl complexes of the type [CpRe(Ar)L2]− can be made by nucleophilic addition reactions of aryl-fulvene complexes. For example, the fulvene complex [(Z6-C5Me4CH2)Re(C6F5)(CO)2] (325) reacted at the exocyclic methylene carbon with KPPh2 and vinylmagnesium bromide to yield the anionic aryl complexes K[(Z5-C5Me4CH2PPh2)Re(C6F5)(CO)2] (326) and [(Z5-C5Me4CH2CH]CH2)Re(C6F5)(CO)2]− (327), respectively (Scheme 111).116,265

Scheme 111

A complex structurally related to group 7 M(I) aryl complexes is the C82 derivative [Re(CO)5-Z1-Y@C2v(9)-C82], which was obtained by photochemical reaction of the rhenium complex [Re2(CO)10] with the paramagnetic compound [Y@C2v(9)-C82]. An X-ray diffraction study reveals that the rhenium moiety is bound to a [5,6,6]-carbon atom of the cage via a single RedC s bond.266

6.01.3.1.1.2 Reactivity M-aryl bonds in group 7 metal aryl complexes can be cleaved by H2. For example, treatment of the phenyl complex 305 with hydrogen led to the cleavage of the RedPh bond to give benzene and a mixture of the dirhenium complexes [Re2(CO)7(SbPh3) (m-SbPh2)2] (328), [Re2(CO)7(SbPh3)(m-SbPh2)(m-H)] (329) and [Re2(CO)8(m-SbPh2)(m-H)] (330) (Scheme 112).254

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

49

Scheme 112

Cleavage of M-aryl bonds in group 7 metal aryl complexes by acids is a common reaction. For example, protonation of the aryl complexes [Re(Ph)(CO)3(P)2] (294, P ¼ P(OEt)3, PPh(OEt)2) with HBF4 gave C6H6 and [Re(CO)3(P)2(BF4)] (331) (Scheme 113). Similar reactions have been reported for [Re(Ph)(CO)2{PPh(OEt)2}3], [Re(Ph)(CO)3{Ph2PO(CH2)3OPPh2}] and the Mn(I) aryl complex [Mn(Ph)(CO)3((POEt)3)2].250 Group 7 M-aryl bonds could even be cleaved by [AuCl(PPh3)]. For example, the aryl complex 302 reacted with one equivalent of [AuCl(PPh3)] to give [AuPh(PPh3)] and PPh4[Re2(AuPPh3)(m-PCy2) (CO)7Cl] (301).252

Scheme 113

Group 7 metal aryl complexes may undergo insertion reactions with alkynes and alkenes. The manganese complex [Mn2(CO)8Br2] catalyzed hydroarylation of internal alkynes with ArB(OH)2 (e.g. formation of 334 from 332) (Scheme 114).267,268 Alkyne insertion involving metal-aryl complexes (e.g. 333) has been suggested as one of the elementary step in the catalytic cycle. Rhenium aryl complexes have been suggested as one of the key intermediates in the catalytic cycle of [Re2(CO)10]-catalyzed alkylation of phenols with terminal alkenes (e.g. formation of 335).269,270

CO OH

[Mn2(CO)8Br2] + ArB(OH)2

Ar

Mn OC

NaOAc

Ph

Ar

OC

OH CO

332

333 Ph OH

OH + nC6H13

OMe

Scheme 114

nC6H13

[Re2(CO)10] OMe 335

Ph

OH 334

50

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

The aryl complexes [Re(Ar)(CO)3(4,40 -dimethoxy-2,20 -bipyridine)] (299, Ar ¼ Ph, p-C6H4Me) can react with SeO2 in CD3CN/ H2O to give ArSeO2H (Scheme 115).8 The reactions were proposed to proceed by initial electrophilic attack of the aryls with SeO2 to form RedO2SeAr species (e.g. 336).

Scheme 115

6.01.3.1.2

Complexes with bridging aryl ligands

Apart from the simple Z1 coordination mode, an aryl group could also function as a m2-bridging ligand or a m2-Z2-bridging ligand. These complexes have been described for a series of Re(I) systems. The dirhenium aryl complex [Re2(CO)8{m-Au(IPr)}(m-Ph)] (338a, IPr ¼ 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) containing a bridging aryl group and a bridging gold-NHC moiety has been obtained from the reaction of [Re2(CO)8(m,Z2-CH] CHnBu)(m-H)] (337) with [PhAu(IPr)] (Scheme 116).271 The analogous complex [Re2(CO)8(m-AuPPh3)(m-Ph)] (338b) containing bridging phenyl and gold phosphine moieties has been similarly obtained from the reaction of [Re2(CO)8(m,Z2-CH]CHnBu) (m-H)] (337) with [PhAu(PPh3)].272 Treatment of the phenyl-bridged complex [Re2(CO)8(m-AuPPh3)(m-Ph)] (338b) with HgI2 produced the rhenium-mercury complex [Re2(CO)8(m-HgI)(m-Ph)]2 (339), the dimer of [Re2(CO)8(m-HgI)(m-Ph)] held together by iodide ligands bridging the two mercury atoms.273

CO

OC OC

Re

Re H

CO

PhAu(L) OC OC

337

CO Re

Re Au

CO

CO

CO

CO

CO

CO

CO

CO

CO

CO

Re

Re

CO

OC

tBu

CO

OC

CO

L = PPh3

CO OC

338

339

CO

CO I

L L = IPr (a),PPh3 (b)

Hg

HgI2

I CO Hg

Re

Re

CO

CO

OC

CO CO

Scheme 116

Treatment of the phenyl-bridged complex [Re2(CO)8(m-AuPPh3)(m-Ph)] (338b) with HSnPh3 produced the dirhenium complex [Re2(CO)8(m-Ph)(m-H)] (340) containing a bridging Z1-phenyl ligand and a bridging hydrido ligand across the RedRe bond (Scheme 117).272 The complex 340 could also be obtained by the reaction of [Re2(CO)8{m-Au(IPr)}(m-Ph)] (338a, IPr ¼ 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with HCl which selectively removes the bridging Au(IPr) group in the form of [AuCl(IPr)].271

Scheme 117

The dirhenium phenyl-bridged complex [Re2(CO)8(m-Ph)(m-H)] (340) is remarkably reactive. It can be easily hydrolyzed to give [Re2(CO)8(m-OH)(m-H)].274 More interestingly, it can reductively eliminate benzene and readily react by CH activation with other arenes. For example, it reacted with C6D6, N,N-diethylaniline and naphthalene to yield the complexes [Re2(CO)8(m-C6D5)(m-D)] (341), [Re2(CO)8(m-H)(m,Z1-NEt2C6H4)] (342)275 and [Re2(CO)8(m-H)(m,Z2-1,2-C10H7)] (343), respectively (Scheme 118).275

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

51

Scheme 118

The dirhenium phenyl-bridged carbonyl complex [Re2(CO)8(m-Ph)(m-H)] (340) can undergo multiple CH activation reactions with aromatic compounds.276,277 The simplest example of these reactions is the self-condensation of 340. Heating a solution of 340 in CH2Cl2 at 40  C for 24 h produced benzene and the tetrarhenium complex [Re2(CO)8(m-H)(m,Z1-1-m,Z1-3-C6H4) Re2(CO)8(m-H)2] (344) (Scheme 119). The reaction of 340 with naphthalene yielded the two doubly CH activated isomeric complexes [Re2(CO)8(m-H)(m,Z2-1,2-m,Z2-3,4-C10H6)Re2(CO)8(m-H)] (345) and [Re2(CO)8(m-H)(m,Z2-1,2-m,Z2-5,6-C10H6) Re2(CO)8(m-H)] (346) via the mono CH activated complex [Re2(CO)8(m,Z2-C10H7)(m-H)] (343).

Scheme 119

The dirhenium phenyl-bridged complex [Re2(CO)8(m-Ph)(m-H)] (340) reacted with anthracene to yield a mixture of the mono-CH activated complex 347, two doubly CH activated complexes 348 and 349, and a small amount (5% yield) of the tetra-substituted anthracene product 350 (Scheme 120). Multiple CdH bond activations of bowl-shaped corannulene (C20H10) with the complex 340 have also been achieved.278

52

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 120

The phenyl-bridged dinuclear complex [Re2(CO)8(m-Ph)(m-H)] (340) is also effective in activation of CdH bonds in heteroaromatic compounds. For example, it reacted with furan in CH2Cl2 at 40  C to give the two isomeric dirhenium compounds 351 and 352, which contain a bridging (s + p)-coordinated furyl ligand formed by activation of the CdH bond at the 2 and 3 positions of furan, respectively (Scheme 121).279 Double-CdH activation of furan with 340 can also be achieved. Similar compounds have been obtained from the reactions of 340 with 2,5-dimethylfuran and thiophene.280

Scheme 121

Activation of alkenes by the dinuclear phenyl-bridged complex [Re2(CO)8(m-Ph)(m-H)] (340) has also been reported. The reaction of 340 with vinyl acetate in CH2Cl2 at 40  C produced three products: [Re2(CO)8(m-Z2-O2CCH3)(m-H)] (353) (9% yield), [Re3(CO)12(m,Z2-C2H3)] (354) (24% yield), and [Re2(CO)8(m-Z2-CHCHO2CCH3)(m-H)] (355) (13% yield) (Scheme 122). The complex 354 can also be obtained from the reaction of [Re2(CO)8(m-H)(m-Z2-C(H)]CHBu)] with vinyl acetate.281

Scheme 122

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

53

Clusters with an aryl ligand bridging Re and another metal center are also known. For example, treatment of [Pd(PtBu3)2] with [Re2(CO)8(m-SbPh2)(m-H)] (330) produced a small amount of the cluster [Pd2Re4(CO)16(m4-SbPh)(m3-SbPh2)(m-Ph)(m-H)2] (356) which contains two mutually bonded palladium atoms and a phenyl ring serving as a bridging ligand across the RedPd bond (Scheme 123).282,283

Scheme 123

6.01.3.1.3

Cyclometalated complexes

Group 7 M(I) cyclometalated aryl complexes have attracted considerable attention in the review period. They have been described for both Mn(I) and Re(I) systems. Most of the complexes are those bearing a bidentate cyclometalated aryl ligand, while a few complexes with a polydentate cyclometalated aryl ligand have also be documented. Many of these complexes were obtained by cyclometalation reactions.284 These complexes can play an important role in catalysis.285–290 6.01.3.1.3.1 Synthesis 6.01.3.1.3.1.1 Mn(I) complexes with bidentate cyclometalated aryl ligands Mn(I) cyclometalated aryl complexes could be prepared via CdH activation of arenes and heteroarenes bearing a directing group. Many of the cyclometalation reactions were carried out by using alkyl or aryl complexes [Mn(R)(CO)5] (e.g. R ¼ Me, CH2Ph and Ph) as the starting material. For example, treatment of the benzyl complex [Mn(CH2Ph)(CO)5] with 2-phenylpyridine produced the metallacycle [Mn(ppy)(CO)4] (357) (Scheme 124).18

Scheme 124

The complexes 358,18 359,291 360, 361, 362, 36319 and 364292 are additional examples of metallacycles derived from cyclometalation reactions of [Mn(CH2Ph)(CO)5] with substrates bearing a 2-pyridyl moiety as the directing group (Scheme 125). It is noted that the metallacycles 358d, 359b, 360, 361, 363 and 364 are carbon-monoxide-releasing molecules (CO-RMs).

54

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 125

Cyclometalated Mn(I) aryl complexes have also been obtained from arenes bearing other nitrogen-based directing groups. For example, treatment of 2,3-diphenylbenzo[g]quinoxaline (365) with three equivalents of [Mn(CH2Ph)(CO)5] in refluxing n-hexane and benzene for 8 h produced the mono-manganated compound 366 (Scheme 126). The reaction of 2,3-diphenylbenzo[g] quinoxaline (365) with [Mn(CH2Ph)(CO)5] in 1:2 M ratio in boiling toluene for 14 h afforded the cyclometalated complex 367. The reaction of 365 with [Mn(CH2Ph)(CO)5] in boiling n-heptane for 18 h produced mainly the bis-cyclomanganated 2,3-diphenylbenzo[g]quinoxaline complex 368. The bis-benzylation product 368 was likely formed via trapping of benzyl radicals formed upon homolytic cleavage of the MndC bond in [Mn(CH2Ph)(CO)5] by the metallated 2,3-diphenylbenzo[g]quinoxaline 367.293

Scheme 126

The cyclometalation reaction of 2,5-diphenyl-1,3,4-oxadiazole (369) with [Mn(CH2Ph)(CO)5] afforded a mixture of the mononuclear complex 370 and the dinuclear complex 371 (Scheme 127).294 5-Bromobenzo[h]quinolone (372a) underwent a cyclometalation reaction with [Mn(CH2Ph)(CO)5] to give the cyclometalated complex 373a. A similar reaction between [Mn(CH2Ph)(CO)5] and the (benzo[h]quinolinyl)carbene complex 372b occurred to give the MndCr bimetallic complex 373b.295

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

55

N N CO N N

O 369

CO

OC Mn CO

Mn(CO)4

N N

+

O

CH2Ph

OC

(OC)4Mn

Mn(CO)4

O

370

371

OMe OMe Cr(CO)5

CO

Br CO

OC Mn

372b N (OC)4Mn

Br

Cr(CO)5

CH2Ph

OC

N

372a N (OC)4Mn

CO

373b

N

373a

Scheme 127

Carbonyl is another common directing group for cyclometalation reactions of [Mn(CH2Ph)(CO)5]. For example, N-methyl1,8-naphthalimide (374) reacted with [Mn(CH2Ph)(CO)5] in refluxing heptane to give a mixture of the monocyclometalated complex 375 and the biscyclometalated complex 376 (Scheme 128).296 The complexes 377,297 378,298,299 379300 and 380301 are additional examples of metallacycles derived from cyclometalation reactions of [Mn(CH2Ph)(CO)5] with substrates bearing a carbonyl directing group.

CO O

CO

OC Mn

N

O

O

N

O

(OC)4Mn

+

O +

CH2Ph

OC CO

374

375

377

Mn(CO)4

O

O

Mn(CO)4

Mn(CO)4

S

CO)4 Mn N

O Mn(CO)4

O

376

N=PR3 Fe

N

(OC)4Mn

N

O

Mn(CO)4

378 PR3 = PPh3 (a),PTA (b)

379

380

Scheme 128

Cyclometalation reactions involving halide complexes [MnX(CO)5] (X ¼ Cl, Br) have been frequently used in recent years for preparing cyclometalated Mn(I) aryl complexes. The reactions were in general carried out in the presence of an external base. For example, the five-membered manganacycle [Mn(ppy)(CO)4] (357) can be obtained by the reaction of 2-phenylpyridine with [MnBr(CO)5] in the presence of Cy2NH302–304 or ZnMe2 (Scheme 129).6

Scheme 129

With a similar strategy, Mn(I) metallacycles have been obtained from the reactions of [MnBr(CO)5] with substrates bearing other directional groups (Scheme 130). Examples of the substrates include N-2-pyridylindoles (e.g. to give 381305), N-2-pyrimidylindoles

56

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 130

(e.g. to give 382a,306 382b307–309), thiazolyl-indoles (e.g. to give 383),310 acetophenone (e.g. to give 384 (R ¼ Me),311 385 (R ¼ tBu)312), bis(4-methoxyphenyl)methanimine (e.g. to give 386313) and (E)-4-methoxy-N-(1-phenylethylidene)aniline (e.g. to give 387 and 388314,315). Additional examples of cyclometalation reactions will be discussed in the section dealing with vinyl complexes. There are other routes to Mn(I) cyclometalated aryl complexes. For example, (2-(2-pyridyl)phenyl)boronic acid (389) reacted with [Mn2Br2(CO)8] in the presence of sodium acetate at 90  C to give the metallacyclic complex 357 (Scheme 131).267,268 The complex [MnBr(CO)5] reacted with the alcohol 390 in benzene at 70  C for 18 h to produce the metallacycle 391, presumably via an alkoxide complex intermediate which undergoes acetone extrusion.316

[Mn2Br2(CO)8]

CO CO

+ Mn

NaOAc N

N (HO)2B

CO CO

389

357 N

N N

[MnBr(CO)5]

CO Mn

OH O

390

CO

N

CO

, HBr

CO

391

Scheme 131

6.01.3.1.3.1.2 Re(I) complexes with bidentate cyclometalated aryl ligand Most of the reported Re(I) cyclometalated aryl complexes were also obtained via CdH activation of arenes bearing a directing group. A few of the cyclometalation reactions were achieved with alkyl complexes [Re(R)(CO)5]. For example, treatment of [Re(CH2Ph) (CO)5] with 2,3-diphenylquinoxaline (392) produced the cyclometalated complex 393 (Scheme 132).317 The Cr(CO)3-containing 2-phenyl,2-oxazoline derivative 394 reacted with [Re(CH2Ph)(CO)5] in refluxing heptane to give the CrdRe bimetallic cyclometalated complex 395.318

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

57

Scheme 132

The dinuclear cyclometalated Re(I) complex 397 has been obtained by the reaction of [Re(Me)(CO)5] with the imino compound 396 (Scheme 133).319

O

[Re(Me)(CO)5] N

O

O

O

N

CO

CO

Re

Re

CO

OC OC

nC7H15

nC7H15 396

CO

N

N

CO

CO

nC7H15

nC7H15 397

Scheme 133

Cyclometalation reactions could also be achieved with Re(0) complexes such as [Re2(CO)10] and [Re2(CO)8(CH3CN)2]. For example, heating a mixture of azobenzene (398) and [Re2(CO)10] in toluene produced the cyclometalated complex 399 (Scheme 134).320,321 Heating a mixture of [Re2(CO)8(CH3CN)2] and the polypyridine derivative 400 in a 1:1 M ratio in chlorobenzene in a sealed tube generated a mixture of species. One of the products is the metallacycle 401.322 The complex 401 and [Re(ppy)(CO)4] are catalytically active for coupling reactions of terminal alkynes with acetoacetates to form 2-pyranones.

Scheme 134

58

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Reactions of phosphines or phosphinites with [Re2(CO)10] in chlorobenzene could give rhenium(I) complexes bearing a [C,P]-bidentate ligand (e.g. 402–404) (Scheme 135).323

Scheme 135

Re(0) complexes could also promote PdC bond activation to give cyclometalated products. For example, treatment of [Re2(CO)8(MeCN)2] with two equivalents of dppn (405) in refluxing toluene led to the formation of a mixture of species, including the cyclometalated complexes fac-[Re(CO)3(k1:Z1-PPh2C10H6)(PPh2H)] (406), fac-[Re(CO)3{k1:k1:Z1-PPh2C10H6P(O)Ph(C6H4)}] (407) and fac-[ReCl(CO)3(PPh2C10H6PPh2)] (Scheme 136).324

CO OC

PPh2PPh2 [Re2(CO)8(MeCN)2]

Ph2 P

Re

+

Ph2P +

OC

Ph

Re OC

PPh2

CO 407

406

405

P

O

OC

Scheme 136

Cyclometalation reactions involving Re(-I) and Re(I)-halo complexes, as well as cluster complexes have been occasionally observed. For example, treatment of Na[Re(CO)5] with [Cp RuCl(dppm)] (408) afforded the heterodinuclear complex [Cp Ru (m-CO)2(m-dppm)Re(CO)3] (409) and the orthometalated complex [Cp (CO)Ru(m-H){m-C6H4P(Ph)CH2PPh2}Re(CO)3] (410) containing a cyclometalated dppm ligand (Scheme 137).325

O Na[Re(CO)5] Ru Cl

Ph2P PPh2 408

Ru

Re

CO CO CO

Ph2P

O PPh2

H +

Ru OC PhP

Re

CO PPh2

409 410

Scheme 137

CO CO

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

59

Treatment of [ReCl(CO)5] with 2-phenylbenzothiazole (411, HPBT) afforded the complex [Re(CO)4(PBT)] (412) bearing a [C,S]-bidentate ligand (Scheme 138).326 The complex [Re(CO)4(PBT)] (412) shows intense luminescence originating from the p-p intraligand electronic transition and was considered as a potential “triplet emitter” for OLED devices. A cyclometalation reaction of a C60-functionalized phosphine with the cluster complex [H3Re3(CO)11(NCMe)] has also been reported.327

Scheme 138

6.01.3.1.3.1.3 Complexes with polydentate cyclometalated aryl ligands There are a few reports on the synthesis of Mn(I) and Re(I) complexes bearing a tridentate cyclometalated aryl ligand. The Mn(I)-PCP pincer complex [Mn(PCPNEt-iPr)(CO)3] (414) has been obtained by treating [Mn2(CO)10] with 2-chloro-N1,N3-bis(diisopropylphosphanyl)-N1,N3-diethylbenzene-1,3-diamine (413, P(CdCl)PNEt-iPr) in MeCN under solvothermal conditions in a sealed microwave glass tube for 8 h at 140  C (Scheme 139). The complex 414 can be protonated to give the cationic Mn(I) complex [Mn{k3-(P,CH,P)-P(CH)PNEt-iPr}(CO)3]BF4 (415) featuring an Z2-Caryl-H agostic bond. The agostic CdH moiety in 415 is acidic and can be deprotonated by NEt3 to re-form [Mn(PCPNEt-iPr)(CO)3] (414). Treatment of 414 with NOBF4 led to the substitution of one CO ligand by NO+ to afford the cationic complex [Mn(PCPNEt-iPr)(CO)2(NO)] BF4 (416).328

Scheme 139

The complex 418 bearing a [P,P,C]-pincer ligand has been obtained by the reaction of the diphosphine (4-MeC6H4CH2)2PCH2P(CH2-4-C6H4Me)2 (417) with [Mn(Me)(CO)5] (Scheme 140).329 Treatment of [ReBr2(MeCN)(NO)(Sixantphos)] (419, Sixantphos ¼ 4,6-bis(diphenylphosphino)-10,100 -dimethylphenoxasilin) with ethylene in the presence of HSiEt3 produced the complex [ReBr(Z2-C2H4)(NO)(Z3-o-C6H4–Sixantphos)] (420) which bears a [C,P,P]-pincer ligand generated by cyclometalation of the diphosphine ligand. The complex is highly active for catalytic hydrogenation of monosubstituted olefins and nitriles.29,330

60

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

CO CO

Mn

P

[Me(Me)(CO)5]

P P

OC P

418

417 Br NO Re NCMe Ph2P Br PPh2 O

Ph

Et3Si-H/CH2=CH2

NO P

Re

Br O PPh2

Si

Si 419

420

Scheme 140

Treatment of the triply bonded dirhenium(II) complex [Re2Cl4(m-Ph2PCH2PPh2)2] (421) with triphenylguanidine (422) in refluxing ethanol produced the complex 423 bearing a cyclometalated ligand derived from orthometalation of one of the phenyl rings of the dppm ligand (Scheme 141). Similar reactions were observed for triarylguanidines with aryl groups functionalized with Me and OMe at the para position.331

Ph

Ph Ph2P

PPh2 Cl Cl

Re Cl

Re

Cl

Ph2P

PPh2 421

HN Ph

Ph N 422

N H

H N

N PhP

Cl

Re

Ph

N Ph Ph P 2

PPh2

Re Cl

Cl PPh2

423

Scheme 141

6.01.3.1.3.2 Reactivity of M(I) cyclometalated aryl complexes 6.01.3.1.3.2.1 Ligand dissociation and substitution reactions The complex [Mn(CO)4(ppy)] (357) and its derivatives derived from modified 2-phenylpyridine can act as CO-releasing molecules (CO-RMs).18,19,291 Time-resolved multiple-probe spectroscopy (TRMPS) with IR detection has demonstrated that the Mn(I) 2-phenylpyridyl (ppy) complex [Mn(CO)4(ppy)] (357) can undergo photochemical induced loss of a carbonyl ligand on a sub-picosecond time scale to give solvent (S) complexes fac-[Mn(S)(CO)3(ppy)] (424, S ¼ nC7H16, CH2Cl2, NCMe, C6H5CH3, THF, 1,4-dioxane, Bu2O, and DMSO) (Scheme 142).332

Scheme 142

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

61

6.01.3.1.3.2.2 Cleavage of MdC bonds by electrophiles MndC bonds in Mn(I) cyclometalated aryl complexes can be cleaved by electrophilic reagents such as HgCl2 and ICl. For example, treatment of the cyclometalated iminophosphorane manganese complexes 378 with HgCl2 produced the organomercury derivatives [HgCl(C6H4-2-COdN]PR3)] (425, PR3 ¼ PPh3, PTA) (Scheme 143).298,299 The cyclometalated complex 375 reacted with HgCl2 and ICl to give HgCl (426) and iodo (427) derivatives, respectively.296 Similarly, refluxing a mixture of [Mn{C6H4-2-P(]S) Ph2}(CO)4] and a slight excess of HgCl2 in methanol produced the organomercury compound [Hg{C6H4-2-P(]S)Ph2}], an analog of compounds 425 and 426.333

Scheme 143

The RedC bond in the cyclometalated complexes 402 can also be cleaved by CF3CO2H and CHCl3 upon irradiation with UV lamp to give complexes 428 and 429, respectively (Scheme 144).323 The complexes 402 were reported to be moderately active for catalytic direct arylation of arenes with aryl halides under UV irradiation. Photolysis of 402 with five equivalents of bromobenzene in benzene under N2 for 1–4.5 h produced the bromo complexes 430 together with biphenyl.

Scheme 144

6.01.3.1.3.2.3 Carbene insertion reactions Group 7 M(I) metallacycles can undergo carbene insertion reactions. For example, 1,1-diphenyldiazomethane reacted with the monometallated 2,5-diphenyl-1,3,4-oxadiazole Mn(I) complex 370 to give the trihaptobenzylic complex 431 (Scheme 145).294 The bis-cyclometalated quinoxaline derivatives 432 (M ¼ Mn) and 393 (M ¼ Re) reacted with Ph2CN2 to give the benzyl complexes 433.317,334 The complex 432 also reacted with (tBu)(Ph)CN2 to give the analogous benzyl complex 434.

62

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 145

Similar products were formed in reactions of diazoalkanes with biscyclomanganated pyrazine (e.g. formation of 436 from 435), pyrimidine (e.g. formation of 438 from 437) and benzo[g]quinoxaline (e.g. formation of 439 from 367) derivatives (Scheme 146).334 The resulting organomanganese complexes can react with metallic potassium to afford paramagnetic anionic radical species.317,335

Scheme 146

The cyclometalated complex 440 reacted with diazoalkanes RR0 CN2 (Me3Si)(H)CN2, (Ph)(Me)CN2 and (Ph)(tBu)CN2) to produce syn-facial CrdMn-bimetallic benzyl complexes 441 (Scheme 147).336,337

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

63

Scheme 147

Reactions of group 7 M(I) metallacycles with diazoalkanes can give other products. For example, treatment of the cyclometalated complex 395 with excess diphenyldiazomethane produced the complex 442a as the major product resulting from a mono-insertion of the Ph2C: carbene into the RedAr bond, and the side product 442b resulting from a reaction of 442a with dissolved CO from decomposition (Scheme 148). The reaction of the cyclometalated complex 395 with tbutylphenyldiazomethane produced the main product 443a along with two minor products 443b and 443c, formed by attacking on the aromatic ring with carbenes.318

Scheme 148

The cyclometalated complex 444 reacted with (tBu)(Ph)CN2 to give a mixture of species, including the expected carbene insertion product 445 and the unexpected complexes 446a–c, due to side reactions of the diazo reagent with the aromatic ring (Scheme 149).334

Scheme 149

64

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

The complex [MnBr(CO)5] catalyzed C2-alkylation of N-(2-pyrimidinyl)indole (447) with diazoalkanes (e.g. to give 448) (Scheme 150). The reaction was proposed to proceed through cyclometalation (to give 382a), carbenoid insertion (to give 450), followed by protolysis.338

Scheme 150

6.01.3.1.3.2.4 Insertion reactions of unsaturated substrates and related reactions Another common reactivity of Group 7 M(I) cyclometalated complexes is their insertion reactions with unsaturated substrates such as alkynes, alkenes and carbonyl compounds. Alkyne insertion reactions of group 7 M(I) metallacycles derived from orthometalation of aryl ketone, amide, ester and aldehyde derivatives can produce a range of products including indenols, indenones and cyclohexadieny p-complexes.339 For example, treatment of the mono-cyclomanganated N-methyl naphthalimide derivative 375 with phenylacetylene gave the Z5-cyclohexadienyl complex 452, presumably via the intermediate 451 (Scheme 151).296

O

N

Mn(CO)4 Ph

O

O

Ph

Mn(CO)4

O

O

N

N

OPh Ph

Ph 375

Mn(CO)3

451

452

Scheme 151

The cyclometalated complex 453 reacted with RC^CR (R ¼ CO2Et, Ph) to give indenols 456 after protic work-up, as a result of monoinsertion of the alkyne to give the seven-membered metallacycle 454, followed by addition across the C]O bond to give the alkoxide complex 455 and protolysis (Scheme 152).300 Reactions of the orthomanganated acetophenone complex [Mn(Z2-2-acetylphenyl)(CO)4] with alkynes have also been documented.339

R

N

Mn(CO)4 R = Ph, CO Me 2 O 453

R

R

R

N O 454

Mn(CO)4

N

R

R O Mn(CO) 4 455

H+

R

N

OH

R

456

Scheme 152

Time-resolved infrared spectroscopy reveals that the photochemical reaction of the complex 357 with phenylacetylene produced the short-lived 7-membered metallacyclic complex 457,340,341 which can evolve to complex 458 as a result of formal reductive elimination (Scheme 153). The complex 457 can react with phenylacetylene to give the alkenes 459 and 460, presumably by protolysis of the MndC bond of vinyl species by PhC^CH. The reaction of 357 with excess PhC^CPh produced a complex analogous to 458. The reaction of 357 with excess HC^C-p-tolyl produced an alkene analogous to 459.302

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

65

Scheme 153

The complex [MnBr(CO)5] catalyzed selective addition of CdH bonds of 2-phenylpyridines to alkynes (e.g., to give the alkene derivatives 461)302,342 and C2-alkenylation of N-(2-pyrimidinyl)indole (447) with terminal alkynes (e.g. to give 462) (Scheme 154).306,343–346 These catalytic reactions were also proposed to involve cyclometalation, insertion of alkyne and protolysis by terminal alkyne as in the reactions of 357 with phenylacetylene to give the vinyl derivative 459 (Scheme 153).

Scheme 154

The complex 386 reacted with PhC^CPh to give the pyridine derivative 464 (Scheme 155).313,347 The transformation presumably proceeds through an initial insertion reaction of the alkyne to give the seven-membered metallacycle 463, which OMe

MeO MeO

H

CO

N

Ph

CO Mn

H CO N

+ CO

CO

Ph

MeO

CO

Br CO CO

CO 382a Scheme 155

TIPS

Ph Ph

"[MnH(CO)4]" 464

CO

N N

+

Mn N

Ph CO 463

386

N

N MeO

CO

Ph

OMe

N

CO Mn

N

CO

N

Mn CO

Br TIPS CO 465

"[MnBr(CO)4]"

N

N 466

TIPS

66

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

undergoes cyclization with the elimination of [MnH(CO)4]. The complex 382a reacted with BrC^CTIPS to give the alkyne derivative 466.307 The transformation may proceed through initial formation of the seven-membered metallacycle 465, which undergoes b-bromine elimination to give [MnBr(CO)4] and 466. Catalytic versions of the reactions have also been reported. The rhenium complex [ReBr(CO)3(THF)2] catalyzes reactions of aromatic aldimines with alkynes to give indene derivatives (e.g. formation of 468 from 467, Scheme 156). The catalytic reactions were proposed to proceed via CdH bond activation to form a metallacycle (e.g. 469), insertion of an alkyne (to give species like 470), intramolecular nucleophilic cyclization to generate an amido-rhenium species (e.g. 471), and reductive elimination.348,349 Similar processes have also been suggested for [MnBr(CO)5]-catalyzed ortho-alkenylation of aromatic amidines with alkynes350 and [Mn2(CO)10]-catalyzed CdH alkenylation of aromatic imidates with alkynes.351

Scheme 156

MnBr(CO)5 catalyzed coupling of 3-alkenyl- and 3-allylindoles with propargylic carbonates to afford fused eight- and four-membered carbocycles (e.g. formation of 473 from 472, Scheme 157).352 The transformation was proposed to involve cyclometalation of indoles, alkyne insertion (to give species like 474), elimination of CO2 and Mn (I) species to give the allene 475, and a subsequent pericyclic reaction. It is noted that [MnBr(CO)5] catalyzed CdH allenylation of N-(2-pyridyl)indoles with propargyl carbonate, presumably via a similar process.353

Scheme 157

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

67

Treatment of the metallacycle 357 with methyl acrylate followed by protic work-up gave the alkylated organic product 477, which could also be obtained by a [MnBr(CO)5]-catalyzed selective addition of a CdH bond of 2-phenylpyridine to methyl acrylate (Scheme 158).303 The compound 477 can be formed by insertion of the olefin moiety into the MndCaryl bond of 357 to give the metallacycle 476 followed by protolysis.

Scheme 158

Treatment of the metallacycle 381 with N-phenyl maleimide (478) followed by protic work-up gave 3-(indol-2-yl)succinimide (480), which could also be obtained by [Mn2(CO)10]-catalyzed CdH addition reaction of indole 481 to N-phenyl maleimide (Scheme 159).354 The transformation was proposed to involve insertion of the olefin moiety into the MndCaryl bond and protolysis. A similar process has been suggested for [MnBr(CO)5]/AgBF4-catalyzed coupling reactions of N-(2-pyridyl)indole with a-diazoketones.355

Scheme 159

Treatment of the metallacycle 384 with styrene followed by protic work-up gave the benzylic alcohol 482 (Scheme 160).311 The transformation may proceed through insertion of styrene into the MndCaryl bond to give the seven-membered metallacycle 483, isomerization of 483 to the alkoxide complex 485 via the hydride intermediate 484, and protolytic cleavage of the MndO bond in 485. MnBr(CO)5 catalyzed redox-neutral CdH olefination of aryl ketones with unactivated alkenes (e.g. formation of 482 from styrene and acetophenone). The catalytic reactions presumably proceed through a similar sequence.

68

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 160

Treatment of the metallacycle 387 with methyl acrylate followed by protic work-up gave the cis-b-amino acid ester 488,315 which could also be obtained by a [Mn2(CO)10]-catalyzed reaction of (E)-4-methoxy-N-(1-phenylethylidene)aniline 489 with methyl acrylate (Scheme 161).356 The compound 488 might be formed via the sequence of olefin insertion, intramolecular nucleophilic attack to form the five membered ring, and protolytic cleavage. Interestingly, the reaction catalyzed by [MnBr(CO)5]/Zn produced a b-lactam derivative formed by intramolecular nucleophilic substitution of OEt group in 488 by the amide group.

Scheme 161

The metallacycle 381 reacted with the allyl carbonate CH2]CHCH2OCO2Me to give the allyl compound 491 (Scheme 162).305 The compound 491 might be formed via insertion of the allyl substrate to the MndC bond of 381 to give the intermediate 490 which undergoes b-oxygen elimination. A similar process has been suggested for the formation of the allyl compound 494 in the Mn(I)-catalyzed coupling reaction of ketamine 492 with the allyl carbonate CH2]CHCH2OCO2Me.

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

69

Scheme 162

The metallacycle 382a reacted with the allylbromide CH2]C(CH2Br)(CO2Et) (495) to give the allyl compound 497, which might be formed via insertion of the allyl substrate into the MndC bond of 382a to give the seven-membered intermediate 496 and subsequent b-bromine elimination and demetallation (Scheme 163).307 The metallacycle 382a similarly reacted with the gem-difluoroalkene 498 to give the vinyl compound 500, presumably via alkene insertion (to give the intermediate 499) and b-fluorine elimination.357 Similar sequences have been suggested in the catalytic cycles of Mn(I)-catalyzed C2-allylation of indoles with allylbromides and C2-fluoroalkenylation of indoles with gem-difluoroalkenes.358

Scheme 163

Treatment of the metallacycle 382a with the allene 501 followed by protic work-up gave the vinyl compound 503, which might be formed by the sequence of allene insertion to give the seven-membered intermediate 502 and protolysis (Scheme 164).310 Interestingly, the metallacycle 383 reacted with the allene 504 to give the heterocyclic compound 506. The compound 506 might be formed from intermediate 505 by N-to-C 1,4-migration of the directing group followed by an intramolecular displacement of the ethoxy group of the ester to furnish the cyclized product. Related catalytic reactions have also been reported.359

70

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 164

Treatment of the metallacycle 388 with the methylenecyclopropane 507 followed by protic work-up gave the polycyclic anilines 508 (Scheme 165). The transformation might procced by the sequence of alkene insertion to give the seven-membered metallacycle 509, cleavage of a CdC bond of the cyclopropane ring to give 510, cyclization to give 511, proto-demetallation to give 512 and intramolecular hydroarylation. A catalytic version of the transformation has been developed.314

Scheme 165

Treatment of the metallacycle 381 with the vinyl cyclopropane 513 followed by water gave the indole derivative 514 (Scheme 166). The transformation might procced by the sequence of alkene insertion to give the seven-membered metallacycle 515, cleavage of a CdC bond of the cyclopropane ring to give 516, and proto-demetallation. A similar process has been proposed for C2-functionalization of N-(2-pyridinyl)indoles with vinylcyclopropanes catalyzed by [MnBr(CO)5].360,361

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

71

Scheme 166

Treatment of the cyclometalated complex 357 with PhCHO and ZnBr2 followed hydrolysis produced the alcohol 518, which could be formed by an insertion reaction of PhCHO with 357 to give the seven-membered metallacycle intermediate 517 followed by hydrolysis (Scheme 167).6 Similarly, treatment of 381 with CF3C(O)CO2Et (519) followed by hydrolysis produced the alcohol 521.362 In line with the results of the stoichiometric reactions, [MnBr(CO)5]/ZnMe2/ZnBr2 catalyzed selective addition of CdH of 2-phenylpyridines to aldehydes and nitriles, while [Mn2(CO)10] catalyzed addition of the C(2)dH of N-(2-pyridinyl)indoles to ketones.362 [MnBr(CO)5] also catalyzed CdH functionalization of arenes bearing an imidazole or imidazoline moiety with aldehydes and tertiary silanes to give silyl ethers (e.g. formation of 523 from 522).363,364

Scheme 167

Treatment of the metallacycle 524 with PhCHO produced the isobenzofuranone 527 (Scheme 168). The transformation might proceed by initial insertion of PhCHO into the MndC bond of 524 to give the seven-membered metallacycle 525, followed by intramolecular nucleophilic addition to give the alkoxide complex 526, protolysis of the MndO bond in 526 and elimination of methanol (Scheme 168). A similar process has been suggested for [Mn2(CO)10]/BPh3-mediated synthesis of isobenzofuranones (e.g. formation of 528) from esters and oxiranes (a source of terminal aldehydes).365

72

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 168

Treatment of the metallacycle 399 with PhCHO followed by protic work-up gave 2H-indazole 531, which could also be obtained by a [Re2(CO)10]-catalyzed [4 + 1] annulation reaction of azobenzene with PhCHO (Scheme 169).321 The transformation presumably involves insertion of the olefin moiety into the MndCaryl bond of 399 to give the intermediate 529 followed by protolysis to give the alcohol 530, which undergoes cyclization and dehydration.

Scheme 169

Insertion of imines and isocyanates into MdC bonds of cyclometalated group 7 metal complexes has also been reported. Treatment of the metallacycle 381 with the imine 532 followed by protic work-up gave the amine 534, resulting from imine insertion and protolysis (Scheme 170).312 Similarly, the amine derivative 536 was produced in the reaction of the metallacycle 385 with the imine 535.366 Treatment of the metallacycle 399 with the isocyanate 537 followed by protic work-up gave the amine 539 which might be formed by protolysis of the intermediate 538 formed by insertion of the isocyanate.320 Analogous insertion reactions have been suggested as one of the elementary steps in the catalytic cycles of several catalytic reactions. These catalytic reactions include [MnBr(CO)5]-catalyzed aromatic C(sp2)dH addition of ketones to imines,312 [Mn2(CO)10]-catalyzed addition of C(2)dH of indoles to imines,366 [Re2(CO)10]-catalyzed aminocarbonylation of azobenzenes with isocyanates to give o-azobenzamides,320 [MnBr(CO)5]-catalyzed CdH aminocarbonylation reactions of heteroarenes with isocyanates,367 and [ReBr(CO)3(THF)]2-catalyzed reaction of aromatic aldimine with isocyanates to give phthalimidines.348,349

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

73

Scheme 170

The metallacycle 357 reacted with N-cyano-N-(4-methoxy)phenyl-p-toluenesulfonamide (540) to give the cyano compound 542 (Scheme 171).304 The transformation may involve insertion of the nitrile into the MndCaryl bond of 357 to give the seven-membered metallacycle 541, which undergoes elimination of [Mn{NTs(p-C6H4OMe)}(CO)4] (543).304 A catalytic version of the reaction has also been described.

Scheme 171

6.01.3.2

M(II) complexes

Well-defined group 7 M(II) aryl complexes are mainly those of Mn(II) systems, including MAr2 and Mn(Ar)X (X ¼ halide, amide) as well as their coordination compounds.

6.01.3.2.1

Synthesis

Mixed Mn(II) arylhalide complexes can be prepared by reactions of MnX2 with aryl Grignard or lithium reagents. For example, treatment of manganese(II) chloride with 2,4,6-trimethylphenylmagnesium bromide (MesMgBr) with manganese(II) chloride in an equimolar ratio in THF produced the dinuclear arylhalide complex [(THF)4Mg(m-Cl)2MnBrMes] (544) which contains a four coordinate tetrahedral Mn(II) center (Scheme 172).368 Treatment of MnCl2 with one equivalent of Ar0 Li (545, Ar0 ¼ C6H3-2,6-(C6H3-2,6-iPr2)2) in THF produced the Mn(II) arylhalide complex [Li2(THF)3Ar0 MnCl2]2 (546).64 The complex has a pseudo cuboidal structure featuring three different bridging chlorides and four-coordinated Mn and Li centers.

74

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 172

Treatment of MnI2 with one equivalent of Ar0 Li (546, Ar0 ¼ C6H3-2,6-(C6H3-2,6-iPr2)2) in Et2O produced the Mn(II) arylhalide complex [Li(OEt2)Ar0 MnI2]2 (547a) (Scheme 173). The complex possesses a distorted cubane Li2Mn2I4 core, in which the lithiums are bound to an ether and the manganese metals are bound to a terphenyl group.369 The analogous iodo-aryl complex [Li(THF) Ar0 MnI2]2 (547b) could be obtained by carrying out the reaction in THF.64 Treatment of [Li(THF)Ar0 MnI2]2 (547b) with (Trip)MgBr(THF)2 (548) in THF produced the iodo-aryl complex [{Li(THF)}Ar0 Mn(m-I)(C6H2-2,4,6-iPr3)] (549), in which the Mn center adopts an unusual distorted T-shaped geometry.64

Scheme 173

Treatment of [Mn2Cl2(m-Cl)2(IPr)2] (103c) with ArMgCl produced the dimeric Mn(II) aryl complexes [Mn(Ar)(m-Cl) (IPr)]2 (550, Ar ¼ Ph (a), o-tolyl (b)) (Scheme 174).69 For the purpose of organic synthesis, organomanganese reagents of the empirical formula ArMnX have often been prepared in situ by reactions of magnesium with aryl halides in the presence of MnCl2
2LiCl (e.g. formation of 551, Scheme 174).78,370

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

75

Scheme 174

Several reports have appeared on the syntheses and structures of manganese(II) amido-aryl complexes. The four-coordinate dinuclear amido-aryl complex {Ar0 Mn(m-NH2)(NH3)}2 (553, Ar0 ¼ C6H3-2,6-(C6H3-2,6-iPr2)2) has been obtained by the reaction of the two-coordinate diaryls MnAr0 2 (552) with excess NH3 (Scheme 175).371

Scheme 175

The antiferromagnetic dinuclear amido-aryl complex {Ar0 Mn(m-NMe2)}2 (554, Ar0 ¼ C6H3-2,6-(C6H3-2,6-iPr2)2) has been obtained by the reaction of the arylhalide complex {Li(THF)Ar0 MnI2}2 (547b) with LiNMe2 in a 1:2 M ratio (Scheme 175).372 In complex 554, each metal center is three coordinated with a distorted trigonal planar geometry. Three-coordinated aryl-amidomanganese(II) complexes have also been obtained by direct metalations (manganations) of aromatic compounds with alkylamido-Mn(II) complexes, as described in the section dealing with reactivity of Mn(II) alkyl complexes. It is possible to isolate two-coordinated ArMnNHR complexes provided that both Ar and NHR groups are sterically bulky. For example, treatment of [Li(THF)Ar0 MnI2]2 (547b, Ar0 ¼ C6H3-2,6-(C6H3-2,6-iPr2)2) with the bulky amido agent LiN(H)Ar (555, Ar ¼ C6H3-2,6-(C6H3-2,4,6-Me3)2) at room temperature gave the monomeric two-coordinate amido-aryl complex [Ar0 MnNHAr] (556), an rare example of heteroleptic quasi-two coordinate open shell transition metal complexes (Scheme 176).373

Me2 N Mn

Mn LiNMe2

N Me2

I 554

Mn

Mn I THF Li

I

I Li

Mn THF

547b

NH Li 555

Scheme 176

556

H N

76

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Additional examples of two-coordinated amido-aryl complexes [ArMnNHR] have been obtained from the reactions of the Mn(I) inverted sandwich complex [(Z6-C7H8){MnAr -3,5-iPr2}2] (291, Ar ¼ C6H-2,6-(C6H2-2,4,6-iPr3)2-3,5-iPr2) with bulky terphenyl azides (Scheme 177). Treatment of the complex 291 with the azide 557 afforded the dimeric Mn(II) amido-aryl complex 558, arising from methyl hydrogen abstraction by the nitrogen atom and dimerization via radical coupling with CdC bond formation. Treatment of the complex 291 with the azide 559 afforded the dinuclear Mn(II) amido-aryl complex 560, formed by phenyl hydrogen abstraction by the imido nitrogen atom, followed by incorporation of a further MnAr -3,5-iPr2 unit. The complex 560 has two different manganese environments.374

Scheme 177

Homoleptic Mn(II) aryls with the empirical formula MnAr2 can often be obtained by transmetalation reactions of Mn(II) halide precursors with aryl Grignard or lithiating reagents. These compounds can be monomeric, dimeric, trimeric and even polymeric depending on the steric properties of the aryl groups.375,376 For example, treatment of MnI2 with phenyl lithium in diethyl ether produced [MnPh2]n (561) which has a chain-like structure with the metal centers bridged by phenyl groups (Scheme 178).368 The reaction of 2,4,6-trimethylphenylmagnesium bromide (MesMgBr) with manganese(II) chloride in 2:1 M ratio in THF yielded the dimeric aryl complex [Mes(THF)Mn(m-Mes)2Mn(Mes)(THF)]2 (562) with two bridging mesityl groups, which evolved to the THF-free trimetallic complex [MnMes2]3 (563) upon crystallization from toluene.

Scheme 178

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

77

The iodo-aryl complex {Li(THF)Ar0 MnI2}2 (547b, Ar0 ¼ C6H3-2,6-(C6H3-2,6-iPr2)2) reacted with four equivalents of (C6H2-2,4,6-iPr3)MgBr(THF)2 to afford the homoleptic dimer [Mn(C6H2-2,4,6-iPr3)(m-C6H2-2,4,6-iPr3)]2 (or [Mn(Trip) (m-Trip)]2) (564), as a result of displacement of the bulkier Ar0 ligand as well as the halogen (Scheme 179). The metal center in the dimeric complex 564 has a trigonal planar geometry.64

Scheme 179

Two-coordinate linear complexes Mn(2,6-Xyl2C6H3)2 (565), Mn(2,6-Mes2C6H3)2 (566) and MnAr0 2 (567, Ar0 ¼ C6H3-2,6(C6H3-2,6-iPr2)2) have been prepared by the reaction of MnBr2 with (2,6-Xyl2C6H3Li)2 (2,6-Xyl ¼ 2,6-Me2C6H3),377 the reaction of MnCl2 with (2,6-Mes2C6H3Li)2378 and the reaction of {Li(THF)Ar0 MnI2}2 with LiAr0 ,64 respectively (Scheme 180).

iPr

iPr iPr

Mn

Mn

iPr

Mn iPr iPr iPr

565

566

iPr 567

Scheme 180

Transmetalation reactions of MX2 with alkylating reagents conducted in a coordinating solvent may give solvated complexes instead of simple homoleptic diaryls. As mentioned earlier, the reaction of 2,4,6-trimethylphenylmagnesium bromide (MesMgBr) with manganese(II) chloride in 2:1 M ratio in THF yielded the dimeric complex [Mes(THF)Mn(m-Mes)2Mn(Mes)(THF)] (562). As an additional example, treatment of MnBr2 with one equivalent of (2,6-Tmp2C6H3Li)2 (568, Tmp ¼ 2,4,6-Me3C6H2) in a mixture of toluene and THF at room temperature yielded the three-coordinate diaryl complex [Mn(2,6-Tmp2C6H3)2(THF)] (569) (Scheme 181).379

Scheme 181

78

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

In the area of organic synthesis, diaryl manganese reagents have often been generated380 in situ by transmetalation of ArMgBr with MnCl2 (e.g. formation of 570) (Scheme 182),381 or directed manganation using TMP2Mn
2MgCl2
4LiCl (e.g. formation of 573 from 571).382

Scheme 182

6.01.3.2.2

Reactivity

MnAr2 compounds are valuable starting materials for preparation of other Mn aryl complexes. For example, treatment of [Mn3(mes)6] (563) with 2,20 -bipyridine produced the four-coordinated complex [Mn(mes)2(bpy)] (574), which reacted with K metal and 18-crown-6 to give the anionic complex [K(18-crown-6)][Mn(mes)2(bpy)] (575) (Scheme 183). Single-crystal X-ray diffraction data, DFT calculations, spectroscopic and magnetic measurements indicate that the anionic complex 575 can be best formulated as an Mn(II) complex of the 2,20 -bipyridyl radical anion.383

Scheme 183

The aryl complex [Mn3(mes)6] (563) reacted with IPr (1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene) to give the three-coordinated mononuclear aryl complex [Mn(mes)2(IPr)] (576) (Scheme 184).384a Treatment of 576 with KC8 produced the monoanionic pseudo-tetrahedral bis(dicarbene) complex K[{[:CN(2,6-iPr2C6H3)]2(CH)C}2Mn(mes)(THF)] (577). The complex 577 reacted with two molar equivalents of triethylaluminium in the presence of the cation sequestering agent 2,2,2-cryptant to give the complex [K(2,2,2-crypt)][{(Et)3Al:C[N(2,6-iPr2C6H3)]2(CH)C}2Mn(mes)] (578). In complex 578, the manganese(II) metal center is bonded to a mesityl group, a THF molecule and two deprotonated IPr ligands in an abnormal fashion through the C4 atoms.

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

N N Mn

Mn

79

N

N

Mn

Mn

563: [MnMes2]3

576 KC8 K K(2,2,2-crypt)

AlEt3

N

N

N

N AlEt3/2,2,2-crypt THF

Mn

Mn

THF

N

N N

N

AlEt3

578

577

Scheme 184

Mn(II) aryls of the type MnAr2 have also been used as starting materials for preparation of mixed Mn-main group metal complexes. For example, the halide-free diphenylmanganese ([MnPh2]n) reacted with activated calcium powder to give the aryl-rich heterobimetallic ion pair [(THF)3Ca(m-Ph)3Ca(THF)3]+[(THF)2PhCa(m-Ph)3MnPh]− (579) (Scheme 185). The anion of this complex contains a calcium(II) as well as a manganese(I) center which are bridged by three phenyl groups.385 The reaction of an ethylenediamine solution of K4Pb9 and 2,2,2-crypt (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) with a THF solution of [Mn3(Mes)6] (563, Mes ¼ 2,4,6-trimethylphenyl) yielded the anionic cluster [K(2,2,2-crypt)]3[Mn@Pb12] (580) along with mesitylene (1,3,5-trimethylbenzene). This reaction presumably proceeds through reductive cleavage of the MdC bonds in [Mn3(Mes)6] by the solvated electrons present in the K4Pb9 solution.384b

+ THF

THF Ca

4/n [MnPh2]n+ 3 Ca THF THF

Ca

Mn

Ca

THF THF

THF THF 579 3 K(2,2,2-crypt) Pb

3

Pb Pb Mn

Mn

Pb Pb

Pb

K4Pb9 /2,2,2-crypt Mn

Mn Pb

Pb

Pb

Pb Pb

563: [MnMes2]3 Scheme 185

580

Pb

80

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Mn(II) aryls are nucleophilic and can react with a variety of electrophiles. Thus, Mn(II)-aryl bonds can be easily cleaved by acids. For example, treatment of [Mn3(Mes)6] (563) with one equivalent of 8-hydroxyquinoline (581, q-H) resulted in the formation of the tetranuclear organomanganese complex [Mn4(Mes)2(q)6] (582) (Scheme 186). The complex 582 in THF is emissive with the maxima at 580 nm.386 The compound [Mn3(Mes)6] (563) reacted with the polyamine tbsLH6 (583, tbsLH6 ¼ 1,3,5-C6H9(NHC6H4-o-NHSiMe2tBu)3) in THF to give the trinuclear complex [Mn3(THF)(tbsL)] (584).

Scheme 186

Mn(II)-aryl bonds can be cleaved by I2. For example, treatment the aryl compounds 16396 and 16597 with I2 lead to the formation of 3,5-diiodo-toluene and 3-iodo-2-methoxynaphthalene, respectively (Scheme 187).

Scheme 187

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

81

The Mn(II) aryl compound [MnPh2]n reacted with Si(OR)4 (R ¼ Me, Et) to give a mixture of PhSi(OR)3 and Ph2Si (OR)2 (Scheme 188). Similar reactions have been reported for [Mn(Xyl)2]3 (Xyl ¼ 2,6-Me2C6H3) and [MnPh2(PCy3)].70 R = Me, Et 1/n [MnPh2]n + Si(OR)4

1/3 [MnXyl2]3 + Si(OEt)4

[MnPh2(PCy3)]+ Si(OEt)4

PhSi(OR)3 + Ph2Si(OR)2

Xyl = 2,6-Me2C6H3

XylSi(OEt)3

PhSi(OEt)3 + Ph2Si(OEt)2

Scheme 188

Organomanganese reagents of the type ArMnX generated in situ by reactions of magnesium with aryl halides in the presence of MnCl2
2LiCl can smoothly undergo 1,2-addition, acylation (e.g. preparation of 585, Scheme 189), allylation with allyl bromides (e.g. formation of 586), Pd-catalyzed cross-coupling with electrophiles and oxidative homocoupling,78,370 and Ni-catalyzed cross-coupling with arylhalides and carbonitriles.387,388 Mixed amido-aryl Mn(II) complexes (e.g. 166) can also be used as a partner for Pd-catalyzed cross-coupling reactions with iodobenzenes.97

Scheme 189

Diaryl manganese reagents generated in situ by transmetalation of ArMgBr with MnCl2 have been used to perform cross-coupling reactions with primary and secondary alkyl halides, as well as vinyl halides through catalysis with species such as FeCl2,389 Fe(acac)3390 and CoCl2 (e.g. formation of 586 and 587) (Scheme 190).381

Scheme 190

Oxidation of diaryl and diheteroaryl manganese(II) reagents can lead to homocoupling of the aryl ligands. For example, the compound 573 reacted with chloranil to give the homocoupling product 589 (Scheme 191).382

82

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 191

Mn(II) aryls have often been suggested as reactive intermediates in transformations catalyzed by the manganese halide MnCl2. For example, MnCl2 catalyzed homocoupling of aryl, alkenyl and alkynyl Grignard reagents using oxygen as the oxidant (Scheme 192).391,392 The coupling reaction was proposed to involve the neutral Mn(II) intermediate MnR2 (590). The species 590 may react with O2 to give the Mn(IV) peroxo complex 591, which undergoes reductive elimination to give the coupled product RdR and the peroxo Mn(II) intermediate 592. The latter can react with a Grignard reagent to regenerate the organomanganese species 590 and closes the catalytic cycle.

Scheme 192

MnCl2 also can catalyze homocoupling of ArMgCl reagents using DCE as an oxidant (e.g. formation of 593) (Scheme 193).393 The reaction was proposed to proceed through an MnAr2 intermediate 594. The MnAr2 species can react with ArMgCl to give the anionic triaryl manganate(II) species 595, which could be alkylated with DCE to form the Mn(IV) intermediate 596. The species 596 could undergo a reductive elimination reaction to form a biaryl and the Mn(II) complex 597, which could be transformed into the chloroaryl species 598 by elimination of ethylene. Alkylation of 598 with ArMgCl would regenerate the active species 594.

Scheme 193

MnCl2 catalyzed dimerization of aryl bromides (ArBr) in the presence of Mg (e.g. formation of biphenyl (599) from PhBr, Scheme 194). The reaction was proposed to involve MnBr2 as the active species. Alkylation of MnBr2 with in-situ generated ArMgBr gives ArMnBr. Oxidative addition of the aryl bromide ArBr onto the ArMnBr species gives the Mn(IV) species Ar2MnBr, which can undergo reductive elimination of Ar2 to generate MnBr2 to start another catalytic cycle.394 MnCl2 also catalyzed coupling reactions of aryl Grignard reagents with arylhalides (e.g. formation of 601 from 600).395,396 These reactions presumably proceed through Mn(II)-aryl intermediates via a mechanism similar to that proposed for the dimerization of arylbromides in the presence of Mg.

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

83

Scheme 194

Mn(II) aryls have also been suggested as reactive intermediates in other Mn-catalyzed reactions. Examples of the reactions include three-component coupling of PhMgCl, aromatic imines and THF (e.g. formation of 602, Scheme 195) catalyzed by MnCl2, Mn(acac)2, Mn(acac)3, [MnBr(CO)5] or [Mn(tmhd)3] (tmhd ¼ 2,2,6,6-tetramethyl-3,5-heptanedionate),397 amination of aryl and heteroaryl halides with nitrogen nucleophiles (e.g. formation of 603, Scheme 195) catalyzed by MnCl2, MnF2 or MnF2/CuI in combination of a diamine ligand.398–402

Scheme 195

The aryl complexes [Mn(2,6-Mes2C6H3)2] and [Mn(2,6-Tmp2C6H3)2(THF)] are efficient catalysts for selective cyclotrimerization of primary aliphatic isocyanates to give isocyanurates (604, Scheme 196).379 In these catalytic reactions, Mn(II) aryls were proposed to act as a Lewis acid catalyst. Two- and three-coordinate manganese m-terphenyl complexes [Mn(2,6-Ar2C6H3)2] (Ar ¼ 2,6-Me2C6H3; 2,4,6-Me3C6H2) and [Mn(2,6-Tmp2C6H3)2(THF)] (Tmp ¼ 2,4,5-Me3C6H2) are precatalysts for dehydrogenation of dimethylamine-borane Me2NH
BH3 to afford one equivalent of hydrogen and half an equivalent of [Me2N-BH2]2 (605).377

Scheme 196

6.01.3.3

M(III) complexes

Well-characterized M(III) aryl complexes have been isolated for both Mn(III) and Re(III) systems. The Mn(III) systems are usually paramagnetic open-shell complexes while the Re(III) systems are usually diamagnetic 18-electron complexes.

6.01.3.3.1

Mn(III) complexes

The homoleptic Mn(III) aryl complex [Mg(THF)6][Mn(C6F5)4]2 (606) has been obtained by treatment of MnBr2 with C6F5MgBr in THF/Et2O. The anion [Mn(C6F5)4]− of 606 has an approximately square-planar geometry around the metal center (Scheme 197).403

84

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

C6F5 MnBr2 + 4 C6F5MgBr

THF

[Mg(THF)6]

2+

2 C6F5

- MgBr2

Mn

C6F5

C6F5 606

Scheme 197

The Mn(III) complex [(tBuN3CBr)MnBr2] (608) bearing a tetradentate cyclometalated aryl ligand has been obtained by the oxidative addition reaction of [MnBr(CO)5] with tBuN3CBr (607) at room temperature under Hg lamp irradiation (Scheme 198).404 The complex 608 can react with AgBF4 to give the BF4-complex 609, which can further react with NaI to give the iodo complex 610. These Mn(III)–aryl complexes can undergo 2e-reductive elimination of ArdX (X ¼ Br, I, and CN) induced by 1e-oxidation, presumably via transient reactive Mn(IV) species. For example, the iodo complex 610 reacted with 1.5 equivalent of NOBF4 to give tBuN3CI (611) and a Mn(II) species. Treatment of the complex 608 with AgBF4 followed by NaCN and NOBF4 produced the CN-containing compound tBuN3CCN (612).

Scheme 198

6.01.3.3.2

Re(III) complexes

Re(III) aryl complexes have been described for [(Z5-C5R5)Re(Ar)(X)L2] and [(Z5-C5R5)Re(Ar)(LX)]. A common route to [(Z5-C5R5)Re(Ar)(X)L2] is to perform oxidative addition reactions of ArX with Re(I) complexes [(Z5-C5R5)Re(CO)3]. For example, photochemical reactions of the d6 Re(I) complex [Cp∗Re(CO)3] with dichloroarenes 1-R-2,4-dichlorobenzene (R ¼ H, Me, OMe, CF3 and F) yielded the Re(III) chloro-aryl complexes trans-[Cp Re(C6H3ClR)Cl(CO)2] (613, 614) via CdCl bond activation (Scheme 199).405,406 The analogous bromo-aryl complex trans-[Cp Re(2,5-C6H3Cl2)Br(CO)2] has been obtained similarly from the reaction of [Cp Re(CO)3] with 2-bromo-1,4-dichlorobenzene at room temperature.407

Scheme 199

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

85

Similarly, the photolytic reaction of [(Z5-C5H3)2(SiMe2)2][Re(CO)3]2 (615) with C6F6 produced the pentafluorophenyl Re(III) fluoro complex trans-[(Z5-C5H3)2(SiMe2)2][Re(CO)3][Re(C6F5)F(CO)2] (616), as a result of activation of a CdF bond in C6F6 (Scheme 200).408

Scheme 200

Complexes of type [(Z5-C5R5)Re(Ar)(X)L2] could also be synthesized by oxidative addition reactions of pre-formed Re(I) aryl complexes [(Z5-C5R5)Re(Ar)(CO)2]−, or ligand substitution reactions of pre-formed Re(III) aryl complexes. For example, protonation of the anionic aryl complex K[(Z5-C5Me4CH2PPh2)Re(C6F5)(CO)2] (326) with HCl at 0  C produced the hydrido-aryl complex trans-[(Z5-C5Me4CH2PPh2)Re(C6F5)H(CO)2] (617) (Scheme 201).116 Protonation of the anionic aryl complex [(Z5-C5Me4CH2CH]CH2)Re(CO)2(C6F5)]− (327) with HCl at 0  C produced the hydrido-aryl complexes trans[(Z5-C5Me4CH2CH]CH2)Re(C6F5)H(CO)2] (618).265 Analogous hydrido-aryl complexes trans-[Cp∗Re(Ar)H(CO)2] have been obtained by treatment of trans-[Cp Re(Ar)X(CO)2] (X ¼ Cl, Br; Ar ¼ 1,2,3,4-C6HCl4, 1,2,4-C6H2Cl3, 1,4-C6H3Cl2 and 1,3-C6H3Cl2) with LiBHEt3.407

Scheme 201

Re(III) Aryl complexes may also be produced by other routes. For example, the rhenium(II) complex [CpRe(BDI)] (619, BDI ¼ N,N0 -bis(2,6-diisopropylphenyl)-2,4-dimethyl-diketiminate) reacted with Gomberg’s dimer (a source of the trityl radical, Ph3C) to produce the cyclometalated aryl complex 620 as a result of radical coupling between trityl radical and the Cp ligand of 619 and cyclometalation of an aryl group (Scheme 202).409

Scheme 202

A common reactivity of Re(III) aryl complexes of the type [(Z5-C5R5)Re(Ar)(X)L2] is the reductive elimination of ArX.407 For example, thermolysis of a hexane solution of the aryl complex 617 produced the chelated complex [(Z5:Z1-C5Me4CH2PPh2)Re(CO)2] (622). The thermolysis of 617 under a CO atmosphere gave the Re(I) complex [(Z5-C5Me4CH2PPh2)Re(CO)3] (621) (Scheme 203).116 Similar reactions have been described for the complex [(Z5-C5Me4CH2CH]CH2)Re(C6F5)H(CO)2].265

86

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 203

A thermal reaction of the chloroaryl complex trans-[Cp Re(Ar)Cl(CO)2] (614b, Ar ¼ 3-ClC6H3(4-Me)) in acetonitrile produced the complex [ReCl(CO)2(NCMe)3] (623a) and the 5-ArCl-1,2,3,4,5-pentamethylcyclopentadiene 624 (Scheme 204). Similar products were obtained when the rhenium complex 614b was treated with trimethylphosphite.406

Scheme 204

6.01.3.4

M(IV) complexes

Group 7 M(IV) aryl complexes are rare and have only been described for a few Mn(IV) and Re(IV) systems. The Mn(IV) aryl complexes are those with a manganese corrole core (e.g. 625,410,411 626412) (Scheme 205). These complexes can be obtained by transmetalation reactions of aryl Grignard or lithiating reagents with the corresponding halide complexes.

Scheme 205

A Re(IV) aryl complex has been obtained by the cyclometalation reaction of a rhenium complex bearing an aryl-functionalized tren-based ligand. The reaction of samarium diiodide with the complex [ReOCl(HN3N)] (627) containing the unsymmetric tren-based ligand HN3N (({C6F5NCH2CH2}2NCH2CH2CH2NHC6F5)2−) produced a mixture of the iodide complex [ReI(N3N)] (628) and the unusual rhenium(IV) complex [ReF(HN3N∗)] (629, HN3N∗ ¼ ({C6F5NCH2CH2}2NCH2CH2CH2NHC6F4)3−) as a result of intramolecular CdF bond activation (Scheme 206).413

Scheme 206

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

6.01.3.5

87

M(V) complexes

Well-characterized group 7 M(V) aryl complexes have been described for a few Tc(V) and Re(V) complexes containing an oxo, nitride, imido or hydride ligand. These complexes were usually obtained by transmetalation of the M(V)dX (X ¼ Cl or Br) precursors. Treatment of [Re(O)Cl2(BDI)] (630, BDI ¼ N,N0 -bis(2,6-diisopropylphenyl)-2,4-dimethyl-bdiketiminate) with diphenylmagnesium (MgPh2) gave the diphenyl complex [Re(O)Ph2(BDI)] (631) (Scheme 207).414 A similar strategy has been used to prepare Re(V) oxo aryl complexes such as [Re(O)Ph(SSS)] (209, SSS ¼ SCH2CH2SCH2CH2CH2S, 2-mercaptoethylsulfide),125 [Re(O)Ar(DAPMes)] (e.g. 203, Ar ¼ Ph (b), 4-MeC6H4 (c), 4-ClC6H4 (d); DAPMes ¼ 2,6-bis((mesitylamino)methyl)-pyridine)121,122 as detailed in the section dealing with alkyl complexes. Additional examples of aryl complexes prepared by this route include [Re(O)Ph(DAPdipp)] (632, DAPdipp ¼ 2,6-bis(((2,6-diisopropylphenyl)amino)methyl)-pyridine) 123 and [Re(O)Ph(DAAm)] (633, DAAm ¼ (C6F5NCH2CH2)2NMe).124

Scheme 207

Oxo aryl complexes such as [Re(O)Ph(SSS)] (209c, SSS ¼ SCH2CH2SCH2CH2CH2S, 2-mercaptoethylsulfide)125 and [Re(O) (Ar)(DAPMes)] (Ar ¼ Ph (203b), 4-MeOC6H4 (203c), 4-ClC6H4 (203d); H2DAPmes ¼ 2,6-bis((mesitylamino)methyl)pyridine) can undergo insertion reactions with CO at 80  C to give the acyl complexes [Re(O)(COAr)(DAPMes)].122 The complex [Re(O)(Ph) (DAPMes)] (203b, DAPMes ¼ 2,6-bis((mesitylamino)methyl)-pyridine) can react with Lewis acids B(C6F5)3 and Al(C6F5)3 to form frustrated Lewis pairs (FLPs) [Re{OX(C6F5)3}(Ph)(DAPMes)] (X ¼ B, Al).121,123 The frustrated Lewis pairs are capable of hydrogenation of unactivated olefins such as ethylene, propylene, 1-hexene and tert-butylethylene through an ionic mechanism. It is also noted that the phenyl complex 203a can slowly react with B(C6F5)3 to give the C6F5-complex 203e as a result of C6F5/Ph exchange (Scheme 208).

Scheme 208

The PCP-pincer complexes [Re(O)Cl2(PCP)] (635, PCP ¼ 2,6-(R2PCH2)2C6H3, R ¼ tBu (a), iPr (b)) have been synthesized by the reactions of the ligands PCHP (634, PCHP ¼ C6H4(R2PCH2)2-1,3, R ¼ tBu, iPr) with [Re(O)Cl3(SMe2)2] or [ReOCl2(OEt) (PPh3)2] in the presence of NEt3 (Scheme 209).415,416 These complexes can be derivertized to give complexes with ligands such as hydride (e.g. the Re(VII) complex 636), acetate (e.g. 637) and oxo (e.g. 638). The complex 635b could undergo a deprotonation reaction upon treatment with LiN(SiMe3)2 to give the pseudo-carbenoid complex 639, which can react with CO2 to give the carboxylate complex 640.417

88

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 209

Treatment of [MNCl2(PPh3)2] (641, M ¼ Re (a), Tc (b)) with LiPh produced the air- and H2O-stable phenyl complexes [MNPh2(PPh3)2] (642, M ¼ Re (a), Tc (b)) with a nitrido M(V) core (Scheme 210).418 The complex [TcNPh2(PPh3)2] (642b) is of special interest as it represents a rare example of well-characterized Tc(V) aryl complexes.

Scheme 210

Treatment of the nitridorhenium complex [Re^NCl2(PPh3)2] or [ReNCl2(PMe2Ph)3] with the methoxo-protected form (643) of 1,3,4-triphenyl-1,2,4-triazol-5-ylidene (HLPh) gave the aryl complex [ReNCl(HLPh)(LPh)] (644) as a result of cyclometalation of one of the aromatic rings (Scheme 211).419 The complex 644 reacted with KX (X ¼ CN, SCN) and RSH (RSH ¼ 2-mercaptopyridine N H OMe Ph Ph N N N

Cl

Re

ReNCl2(PPh3)2

N

N N

N

Ph

Ph

Ph 643

N

Ph

Ph

N N

Ph

KSCN or KCN

Re

Ph N

N N

N N

N

Ph Ph Ph 645, X = CN, SCN

644 N

X

N

RSH PhLi

N N

N

N

Ph Ph

Ph Re N N Ph

N N

N Ph

N

N Ph

Ph

N

N

N

Ph

Ph 647

Scheme 211

Re

N

N N

Re

Ph

Ph N

N N Ph

648

SR

N Ph

Ph

N N

646, R- = pyS-, Ph2PS2-

Ph

Ph

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

89

and Ph2PS2H) to give the aryl complexes 645 and 646, respectively, and with PhLi to give the aryl complex 648. The complex 644 also reacted with 1,3,4,5-tetramethylimidazol-2-ylidene in benzene to give complex 647 containing an imidazolylidene ligand and two cyclometalated aryl ligands.418 The imido complexes [Re(O)(BDI)(]NDipp)] (649a) and [Re(O)(BDI)(]NtBu)] (649b) reacted with B(C6F5)3 to yield the rhenium(V) imido arylborinate complexes 650 (Scheme 212). The compound BPh3 reacted similarly at room temperature to give the analogous aryl complex 651, which undergoes a cyclometalation reaction to give the cyclometalated complex 652.420

Scheme 212

The cyclometalated rhenium (V) polyhydride complex [ReH4(ppy)(PPh3)2] (653) has been prepared through the reaction of [ReH7(PPh3)2] with 2-phenylpyridine (Scheme 213).421

Scheme 213

6.01.3.6

M(VII) complexes

Group 7 M(VII) aryl complexes have been described for a few Re(VII) complexes containing an oxo or hydride ligand. The complexes [ReH6(PCP)] (636, PCP ¼ 2,6-(R2PCH2)2C6H3, R ¼ tBu (a), iPr (b)) are examples of Re(VII) hydrido-aryl complexes.416 Dioxidodiphenylrhenium propionate ([ReO2(Ph2)(O2CEt)] (654)) has been obtained by treatment of propylperrhenate ([ReO3(O2CEt)]) with one equivalent of phenylzinc acetate (Scheme 214).422 Aryl compounds of the type ArReO3 are more common and can be synthesized by the reactions of ClReO3 or Re2O7 with ZnAr2.229,423–425 For example, treatment of Re2O7 with Zn(2,6-Me2C6H3)2 produced (2,6-dimethylphenyl)trioxorhenium (655). The aryl complexes 656–660 are additional examples of complexes prepared from this route.

90

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 214

Re(VII) aryl complexes of the type ArReO3 can be catalytically active for epoxidation of cyclooctene and for olefin metathesis reactions. It has also been reported that aryltrioxorhenium of the type ArReO3 can react with O-atom donors, such as H2O2 and NaIO4 to selectively generate the corresponding phenols. For example, MesReO3 (661) reacted with H2O2 to give 2,4,6-trimethylphenol (663) (Scheme 215). 18O-Labeling experiments as well as DFT studies reveal that the reaction most likely proceeds through a Baeyer-Villiger type mechanism involving intermediates like 662 via nucleophilic attack of the aryl group on an electrophilic oxygen of peroxide coordinated to rhenium.426

Scheme 215

6.01.4

Vinyl complexes

Group 7 metal vinyl complexes have been described for those containing an Mn(I), Mn(II), Re(I), Re(III) or Re(V) center. In most the well-characterized vinyl complexes, the vinyl moiety is present as either an Z1-vinyl or Z2-vinyl ligand. A vinyl moiety could also serve as a bridging ligand to form dinuclear or cluster complexes. In addition, cyclometalated vinyl complexes or complexes with a vinyl moiety bearing a pendant donor have also attracted attention.

6.01.4.1 6.01.4.1.1

Synthesis Complexes with
1-vinyl or
2-vinyl ligands

6.01.4.1.1.1 By nucleophilic substitution reactions of vinylhalides A series of Mn(I) and Re(I) vinyl complexes of the type [M(vinyl)(CO)5] (M ¼ Mn, Re) have been synthesized by nucleophilic substitution reactions of [M(CO)5]− with vinylhalides.427 For example, the fluorinated alkene (CF3)3CCF]CF2 (664) reacted with Na[Re(CO)5] and K[Mn(CO)5] to give the vinyl complex [M{CF]CH(CF3)2}(CO)5] (665, M ¼ Mn (a), Re (b)) (Scheme 216).428 The fluorovinyl-imidazolium salt 666 reacted with Na[Mn(CO)5] in THF to give the fluorovinyl-imidazolium Mn(I) complex 667.429 A similar vinyl complex was produced in the reaction of 1-chlorononafluorocyclohexene with Na[Re(CO)5].430 The transformation presumably proceeds through an addition–elimination process. In these nucleophilic substitution reactions, [Re(CO)5]− is in general much more reactive than [Mn(CO)5]−.

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

91

Scheme 216

Common vinylhalides for the nucleophilic substitution reactions include styryl derivatives with an electron-withdrawing group428,430 and vinyl esters or ketones. For example, [M(CO)5]− (M ¼ Mn, Re) reacted with the chlorostyrene derivative PhCCl]C(CN)2 (668) to give the vinyl complexes 669 (Scheme 217). Na[Re(CO)5] reacted with (E)-b-chloroacrylate (670)430 and b-chlorovinyl phenyl ketone (672)431 to produce the vinyl complexes 671 and 673, respectively.

Scheme 217

Interestingly, (E)-a-bromostilbene (674) reacted with Na[Re(CO)5] at 22  C in THF to give a mixture of the vinyl complexes Na [Re{(Z)-C(Ph)]CHPh}Br(CO)4] (675) and Na[Re2(CO)9{(Z)-C(Ph)]CHPh}] (676) as the main products (Scheme 218), probably via a radical mechanism.432 The complex Na[Re{C(Ph)]CHPh}Br(CO)4] (675) reacted with CO (1 bar) to give the vinyl complex [Re{C(Ph)]CHPh}(CO)5].

Ph

Ph

H

Br

Ph +

674

Scheme 218

Na[Re(CO)5]

Ph

Ph Na +

H

Re(CO)4 Br 675

Ph Na

H

Re(CO)4

(OC)5Re 676

92

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

6.01.4.1.1.2 By insertion reactions of alkynes and allenes Insertion of alkynes into MdH bonds represents another common route to group 7 metal vinyl complexes. For example, the five-coordinate rhenium(I) hydride complexes of the type [ReBr(H)(NO)(PR3)2] (R ¼ Cy (a), iPr (b)) reacted with terminal alkynes PhC^CH and 2-methyl-1-buten-3-yne to give the rhenium(I) Z1-vinyl complexes [Re{CH]CHPh}Br(NO)(PR3)2] (677, R ¼ Cy (a), iPr (b)) and [Re{CH]CHC(Me)]CH2}Br(NO)(PR3)2] (678, R ¼ Cy (a), iPr (b)), respectively (Scheme 219).433 Similar reactions also occurred for HC^CH, HC^CSiMe3 and 1,7-octadiyne. The rates of the insertion reactions of HC^CR were found to be dependent on the substituent of the alkynes and are in the order SiEt3 > Ph > H. Treatment of the dihydrido complex [ReH2(bpy)P3]BPh4 (679, P ¼ PPh(OEt)2; bpy ¼ 2,20 -bipyridine) with excess terminal alkynes HC^CR (R ¼ Ph, p-tolyl, t-butyl) in 1,2-dichloroethane gave the Re(III) bisvinyl complexes [Re(CH]CHR)2(bpy)P3]BPh4 (680, R ¼ Ph (a), p-tolyl (b), t-butyl (c)).434

Scheme 219

Treatment of the polyhydride complex [ReH5(PMe2Ph)3] with HC^CR (R ¼ Ph (a), SiMe3 (b) (CH2)4Me (c), p-C6H4CF3 (d)) in the presence of 2.2 equivalents of HCl produced a mixture of the carbyne complexes [Re(^CCH2R)Cl2(PMe2Ph)3] (681) and the Z2-vinyl complexes [Re(Z2-CH2]CR)Cl2(PMe2Ph)3] (682) (Scheme 220).435,436 The Z2-vinyl complexes 682 were proposed to be formed by the insertion reaction of HC^CR with the intermediate [ReH2Cl(PMe2Ph)3]. Similar products have been obtained from the reaction of HC^CC(OH)Ph2 with [ReH5(PMe2Ph)3] in the presence of HCl.

Scheme 220

Insertion of allenes into MdH bonds could also lead to vinyl complexes. For example, treatment of the monohydride complex [MnH(CO)5] with excess tetrafluoroallene at room temperature led to the vinyl complex [Mn{C(CF2H)]CF2}(CO)5] (683), which can rearrange to the vinyl complex [Mn{C(CF3)]CHF2}(CO)5] (684) upon heating due to shifting of a fluorine atom.437 The reaction of [MnH(CO)5] with excess 1,1-difluoroallene at room temperature yielded a mixture of the insertion products 685, 686 and 687 (Scheme 221).

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

93

Scheme 221

6.01.4.1.1.3 By oxidative addition of vinyl halides Vinyl complexes could also be produced by oxidative addition reactions of halogenated olefins. For example, storage of a CH2Cl2 solution of the alkene complex [Cp Re(CO)2(Z2-Cl2C]CCl2)] (688) for 24 h produced the CdCl bond activation product cis-[Cp Re(CCl]CCl2)Cl(CO)2] (689) (Scheme 222).438 Reactions of alkenyl halides with Mn(0) have been used to generate alkenylmangnese(II) reagents in situ for organic synthesis. For example, the alkenylmangnese reagent “p-NCC6H4-CH]CHMnI” has been generated from the reaction of Mn-graphite with p-NCC6H4dCH]CHI.439

Scheme 222

6.01.4.1.1.4 By transmetalation reactions with anionic vinyl reagents Transmetalation from organolithium or organomagnesium vinyl reagents represents another effective route to vinyl complexes. For example, the oxorhenium vinyl complex [Re(O)(CH]CMe2)(DAPMes)] (690, DAPMes ¼ 2,6-bis((mesitylamino)-methyl)pyridine) has been synthesized by the transmetalation reaction of [Re(O)Cl(DAPMes)] (202) with the Grignard reagent Me2C]CHMgBr (Scheme 223).122 Transmetalation reactions have often been used to generate alkenylmangnese(II) reagents in situ for organic synthesis. For example, treatment of 5-chloropentenyl iodide (ClCH2CH2CH2CH]CHI) with BuLi followed by MnI2 at low temperature gave 5-chloropentenylmanganese iodide (“ClCH2CH2CH2CH]CHMnI”), although the detailed structure of the compound was not described.440

Mes

Mes

N

N O

N

Re N

202

Cl + Me2C=CHMgBr

Mes

O N

Re N

690

Mes

Scheme 223

6.01.4.1.1.5 By nucleophilic addition to alkyne, vinylidene, allenylidene and carbonyl complexes Several Re(V) phosphorus-substituted vinyl complexes have been obtained from reactions of phosphine-containing rhenium oxo complexes with alkynes. For example, the complex [ReOCl3{C(Ph)]CH(PPh3)}(PPh3)] (691) was generated in the reaction of PhC^CH with [ReOCl3(PPh3)2], while the complexes 692 and 693 have been obtained from the reaction of PhC^CH with [ReOBr3(PPh3)2] (Scheme 224).441 These reactions presumably proceed through nucleophilic addition of PPh3 to coordinated PhC^CH.

94

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 224

Addition of phosphines (PR3) and phosphites to Mn and Re vinylidenes and allenylidene complexes may also lead to vinyl complexes. For example, Mn(I) and Re(I) phosphoniostyryl complexes of type [CpM{C(PR3)]CHPh}(CO)2] (696, M ¼ Mn, 697, M ¼ Re) can be obtained by the reactions of Mn and Re vinylidene complexes [CpM(]C]CHPh)(CO)2] (694, M ¼ Mn, 695, M ¼ Re) with phosphines (PR3) (Scheme 225). These reactions will be described in detail in the section dealing with the chemistry of vinylidene complexes.

Scheme 225

Vinyl complexes have also been obtained from 1,2,3-diheterocyclization reactions of the allenylidene complex [Re{C]C] CPh2}(CO)2(triphos)]OTf (698; triphos ¼ MeC(CH2PPh2)3) with amine-functionalized heterocycles. For example, the complex 698 reacted with 2-aminopyridine to give the vinyl complex 699 (Scheme 226).442 The NH proton of 699 can be deprotonated to give a neutral vinyl complex. Vinyl complexes have been occasionally obtained by nucleophilic addition of nitrile ylides to carbonyl complexes. For example, the metallo nitrile ylide Li[W{CNCHCO2Me}(CO)5] reacted with [Re(CO)6]PF6 to give the Re(I) vinyl complex 700 (Scheme 226).443

Scheme 226

6.01.4.1.2

Cyclometalated vinyl complexes

Cyclometalated vinyl complexes have been described for Mn(I), Mn(III), Re(I) and Re(V) systems. The cyclometalated vinyl ligands can be either bidentate or polydentate.

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

95

6.01.4.1.2.1 Complexes with bidentate cyclometalated vinyl ligands 6.01.4.1.2.1.1 By cyclometalation reactions Cyclomelation involving CdH bond activation represents one of the commonest routes to prepare cyclometalated vinyl complexes. For example, the benzyl complex [Re(CH2Ph)(CO)5] reacted with 1,4-diphenyl-1-azabutadiene PhCH]CHdCH]NPh to give the cyclometalated vinyl complex [Re{k2-(C,N)-PhC]CHdCH]NPh}(CO)4] (701) together with the substituted complex [Re(k1-(N)-PhCH]CHdCH]NPh)(k2-(C,N)-PhC]CHdCH]NPh)(CO)3] (702) (Scheme 227).444 The Mn(I) benzyl complex [Mn(CH2Ph)(CO)5] reacted with the indene derivative 703292 and the 2-pyrone derivatives 704445 to give the metallacycles 705 and 706, respectively. The N-pyridyl-2-pyridones 707 reacted with [MnBr(CO)5] to give the metallacycles 708 (R ¼ H (a),446 Cl (b)447).

Ph Ph CO CO N Re CO Ph CO

CO CO

OC

N

Ph

Re

Ph

CH2Ph

OC CO

N

Ph N

+

Re CO Ph CO

701

CO Mn N

702

CO

Mn

703

CH2Ph

OC

CO

CO

O

CO

OC N

O

O

CO CO

Ph CO

CO

N R

704

Mn

O

N R

R = H (a), OMe (b)

705

CO

O

R

X

CO

OC

+

Mn Br

OC CO

CO CO

CO

O

N N 707

NHCy2 X = H (a), Cl (b)

706

CO CO

N

Mn N

CO CO

708

Scheme 227

6.01.4.1.2.1.2 By insertion or nucleophilic addition reactions of alkynes Insertion or nucleophilic addition reactions of alkynes may also give cyclometalated vinyl complexes. For example, UV irradiation (Hg/Xe Arc lamp, 200–2500 nm) of a mixture of the five-membered metallacycle 706a and PhC^CH in THF at 240 K produced the seven-membered manganacycle 709 as a result of alkyne insertion (Scheme 228).445 A similar transformation has been observed by time-resolved infrared spectroscopy for the photochemical reaction of the complex [Mn(ppy)(CO)4] with phenylacetylene.340,341

Scheme 228

The alkylideneamido complex [Re(N]CPh2)(CO)3(bpy)] (710) reacted with an equimolar amount of dimethyl acetylenedicarboxylate (DMAD) to yield the metallacyclic complex 713 (Scheme 229).448 The reaction presumably proceeds through initial intermolecular nucleophilic addition of the amido nitrogen to DMAD to give the intermediate 711, followed by a series of intramolecular nucleophilic addition reactions starting from the intermediate 712.

96

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 229

Treatment of the dinitrosyl rhenium complex [Re(NO)2(PR3)2]BArF4 (R ¼ Cy, iPr; BAr−F4 ¼ tetrakis((3,5-bis(trifluoromethyl) phenyl)borate)) with acetylene yielded the alkynyl-vinyl nitrosyl complexes [Re(CH]C(H)ONH)(C^CH)(NO)(PR3)2] BArF4 (715, R ¼ Cy (a), iPr(b)) (Scheme 230).449 The complexes 715 could be formed via the nitroxyl acetylide intermediate 714, which undergoes 1,3-dipolar addition of acetylene with the ReN(]O)H moiety.

Scheme 230

Several rhenium(V) cyclometalated vinyl complexes have been obtained by reactions of alkynols with [ReOCl2(OEt)(PPh3)2] (638) through phosphine addition to coordinated alkynes. Both internal and terminal alkynols can be used for the reactions. In the reactions of terminal alkynols, the phosphine usually attacks on the terminal carbon. For example, treatment of [ReOCl2(OEt) (PPh3)2] (716) with HC^CCH2CH(OH)Me, HC^CCH2CH2CH2OH and HC^CC6H4-2-CH2OH gave the five-membered ylide complexes [ReOCl2(C(]CH(PPh3))CH2CH2CHMeO)(PPh3)] (717), the six-membered ylide complexes [ReOCl2(C(]CH(PPh3)) CH2CH2CH2O)(PPh3)] (718) and [ReOCl2(C(]CH(PPh3))dC6H4dCH2O)(PPh3)] (719), respectively (Scheme 231).450 Treatment of [ReOCl2(OEt)(PPh3)2] (716) with the internal alkynols RC^CCH2CH2OH (R ¼ Me, Et, Ph) produced the six-membered ylide complexes [ReOCl2(CR]C(PPh3)CH2CH2O)(PPh3)] (720) as a result of attacking of phosphine on the alkyne carbon closer to the OH group. Computational studies reveal that the different regioselectivities in the reactions of [ReOCl2(OEt)(PPh3)2] with alkynols is related to the effect of chelating ring sizes and steric interactions.

Scheme 231

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

97

6.01.4.1.2.1.3 By miscellaneous routes Cyclometalated vinyl complexes could also be generated by other routes. For example, the complex 721 reacted with five equivalents of PEt3 in toluene at 120  C to yield the cyclometalated vinyl complex [(ArN)ReCl2(MAD)(PEt3)] (722, Ar ¼ 2,6-diisopropylphenyl, MAD ¼ 4-((2,6-diisopropylphenyl)imino)pent-2-en-2-ide), as a result of cleavage of one of the CdN bonds in 721 (Scheme 232).451 The chlorovinyl compound 723 reacted with Na[Re(CO)5] and K[Mn(CO)5] to give the cyclometalated vinyl complexes 724.431 Treatment of [Re(CO)3(N-MeIm)2(PR3)]BArF4 (725, PR3 ¼ PPh3, PMePh2) with an equimolar amount of KN(SiMe3)2 in THF at 78  C followed by addition of MeOTf produced the dinuclear cyclometalated vinyl complexes 726.452

Scheme 232

6.01.4.1.2.2 Complexes with polydentate cyclometalated vinyl ligands Treatment of fac-[Re(CO)3(THF){Fc-C(O)CHC(O)Me}] (727) with the terminal alkynes HC^CR (R ¼ Ph, CO2Me) gave the Z2-alkyne complexes fac-[Re(CO)3(Z2-HC^CR){FcC(O)CHC(O)Me}] (728, R ¼ Ph (a), CO2Me (b)) which are not stable and underwent a 1,4-addition reaction with regiospecific alkyne attack on the g-methine site of the Fc-acac moiety to afford the cyclometalated vinyl complex fac-[Re(CO)3{k3-(C,O,O)-FcC(O)CH((E)-RC]CH)C(O)Me}] (729, R ¼ Ph (a), CO2Me (b)) (Scheme 233).453 In contrast, the complex fac-[Re(CO)3(THF){Fc-C(O)CHC(O)Me}] (727) reacted with the internal alkyne dimethyl acetylenedicarboxylate (DMAD) to furnish the dimeric complex [fac-Re(CO)3{FcC(O)CH2C(CO2Me)C(CO2Me)}]2 (731).453 The reaction was proposed to proceed through the initial formation of the tricyclic species fac-[Re(CO)3{k3-C,O, O-FcC(O)CH(MeO2CC^CCO2Me)C(O)Me}] (730), which undergoes hydrolysis and dimerization.

98

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 233

Treatment of the complex 732 with PhC^CH in the presence of CuSO4 produced the trimetallic complex 733 bearing a tridentate cyclometalated vinyl ligand (Scheme 234). The complex 733 reacted with NH3 to give the mononuclear cyclometalated vinyl complex 734.454 CO OC

CO

Ph CuSO4 Na ascorbate

N

OC Re OC

N

N Re

OC

N3

N

N N N

CO

Ph OC

CO Ph Cu CuBr2-

N CO

Re

N N N

N

OC Re

NH3

OC

N

Ph

N

N

Br 733

732

N N

734

Scheme 234

The Mn(III) complex [Mn(III)(NCTPP)(py)2] (736) bearing an N-confused porphyrin ligand has been prepared by treating [Mn2(CO)10] with the precursor ligand 735 followed by air oxidation in the presence of pyridine (Scheme 235).455

Ph

Ph

Ph N H N H N

Ph

Ph 735

Py

1) [Mn2(CO)10] N

Ph N N Mn Py N

2), air, pyridine (Py) Ph

N Ph

736

Scheme 235

Manganese complexes of N-confused porphyrin (NCP) can be more conveniently obtained by reactions of N-confused porphyrins with Mn(II) salts. For example, treatment of the N-methyl N-confused 5,10,15,20-tetraarylporphyrin 737 with Mn(OAc)2 in CH2Cl2/MeOH under aerobic conditions produced the manganese(III) complex 738 (Scheme 236).456 The analogous Mn(III) complex 739 can be prepared by heating a solution of the N-confused porphyrin 737 and MnBr2 in CH2Cl2/MeOH under aerobic conditions.457 The analogous complexes 740,458 741,459 742459 and 743460 have been prepared similarly from MnBr2. Manganese (III) complexes of N-confused porphyrins can display interesting photophysical properties,461 be active for photo cleavage of supercoiled plasmid DNA,462 and serve as catalysts for alkene oxidation using PhIO as the terminal oxidant.456

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Ph

Ph

Ph

N

N Cl N Mn

Ph N

N

N

Br N Mn

N

Mn(OAc)2 Ph CH Cl / 2 2 MeOH

Ph

Ph N

MnBr2

Ph

CH2Cl2/ MeOH

HN

N

Ph

N

Ph

Ph

Ph

738

737

739

CH2CO2Et Ph

CH2CO2CH2Ph Ph

CH2-p-C6H4CO2Me Ph

CH2Ph Ph

N

N

N

N

Br N Mn

Ph N

N

Ph

Br N Mn

Ph N

N

99

Ph

Br N Mn

Ph N

N

Ph

Br N Mn

Ph N

Ph

N

Ph

Ph

Ph

Ph

740

741

742

743

Scheme 236

Rhenium complexes of N-confused porphyrins have also been described. For example, the N-methyl-NCP 737 reacted with [ReBr(CO)5] in the presence of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and K2CO3 to give the oxorhenium(V) complex 744, which can react with pyridine to form the hexacoordinate rhenium(V) complex 745 (Scheme 237).463 Interestingly, heating a mixture of the NCP 737 and [ReBr(CO)5] (2 equivalents) in the presence of 2,6-lutidine (5 equivalents) at 140  C for 23 h in air produced the rhenium(V)-NCP complex 746.464

Scheme 237

100

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

6.01.4.1.3

Complexes with bridging vinyl ligands

Vinyls can serve as a bridging ligand to form dinuclear or cluster complexes. Several such complexes have been obtained by insertion reactions of alkynes with hydride-bridged dinuclear rhenium complexes. The heterodinuclear hydride complex [MoReCp(m-H)(m-PCy2)(CO)5(NCMe)] (747) reacted with a slight excess of dimethyl acetylenedicarboxylate to give a mixture of the dinuclear alkenyl complexes 748 and 749 (Scheme 238).465 A similar complex has been obtained by protonation of the alkyne complex Na[MoReCp(m-PCy2)(CO)5{Z2-HC^C(p-tolyl)}].466 The dinuclear hydride complex [Re2(CO)8{m-Au(IPr)}(m-H)] (750, IPr ¼ 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) reacted with HC^CH in refluxing benzene to give the vinyl-bridged complex [Re2(CO)8{m-Au(IPr)}(m-CH]CH2)] (751).271

Cp Mo OC OC

Cy2 CO P CO R R Re CO H R = CO2Me NCMe

CO

Cy2 P

Cp Mo

Re

Mo

+

OC

CO

R

H

747

748 CO OC OC

H

Re

CO CO

OC

Re

CO Au

CO

OC

CO

CO

CO

Re

Re

CO CO Re

Cp

R CO R

Cy2 P

OC

CO

H

C

749

OMe

CO O

CO CO Au

IPr

CO CO

IPr 751

750

Scheme 238

The dinuclear vinyl-bridged complex [Re2(CO)8(m,Z2-CH]CHnBu)(m-H)] (752), which can be prepared from the photolytic reaction of [Re2(CO)10] with HC^CnBu, reacted with HgPh2 to give the mercury-containing vinyl-bridged complex [{Re2(CO)8(m,Z2-CH]CHnBu)}2(m4-Hg)] (753) (Scheme 239).273 nBu CO nBu CO OC OC

OC

CO CO

Re

Re

CO H 752

HgPh2

CO Re

Re

CO

CO

OC

CO Hg

CO CO

CO

OC OC Re OC CO

CO

753 CO

Re CO CO nBu

Scheme 239

6.01.4.2 6.01.4.2.1

Reactivity Protonation reactions

Vinyl complexes can be attacked by an electrophile to give carbene complexes. For example, the vinyl complexes 677 and 678 reacted with HBF4
OEt2 to afford the carbene complexes [Re(F)(Br)(]CHCH2Ph)(NO)(PR3)2] (754, R ¼ Cy (a), iPr (b)) and [Re(F)(Br)(]CHCH]CMe2)(NO)(PR3)2] (755, R ¼ Cy (a), iPr (b)) via protonation at the b- and d-carbons of the vinyl groups, respectively (Scheme 240). Similarly the dinuclear vinyl complexes [(PR3)2(NO)(Br)(F)Re{m-CH]CH-(CH2)4-CH]CH}Re(F) (Br)(NO)(PR3)2] (R ¼ Cy (a), iPr (b)) reacted with HBF4
OEt2 to afford the dinuclear carbene complexes [(PR3)2(NO)(Br)(F)Re{] CHd(CH2)6dCH]}Re(F)(Br)(NO)(PR3)2] (756).433 Additional examples of carbene formation via protonation of vinyl complexes are given in the section dealing with carbene chemistry.

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Ph

Ph

R3P ON

Br PR3

Re

F PR3

HBF4.OEt2 Re

R3P ON

677

Br 754

R = Cy (a), iPr (b)

R3P ON

101

HBF4.OEt2

Br PR3

Re

F R3P ON

Re

PR3

Br 755

678 Scheme 240

Protonation of vinyl complexes may lead to the cleavage of the MdC bonds to give olefin or olefin complexes. For example, treatment of the vinyl complex 734 with HBr and HOTf produced the complexes 757 and 758, respectively (Scheme 241).454 CO

OC

N

OC

Br Ph N

HOTf

Re

HBr

Re OC

N

Ph

N N

N

757

N

OC

N

OC

Re

+

CO

CO N

OC

N N

N N

OTf-

N

N

Ph

734

758

Scheme 241

Treatment of the phosphine-substituted vinyl complexes [CpM{HC(PR3)]CHPh}(CO)2] (696, M ¼ Mn, 697, M ¼ Re; PR3 ¼ PMe3 (a), PMePh2 (b)) with HBF4 gave the Z2-phosponioalkene complexes [CpM{Z2-(E)-HC(PR3)]CHPh}(CO)2] BF4 (759, M ¼ Mn, 760, M ¼ Re) (Scheme 242).467 Low temperature experiments show that the protonation reactions most likely proceed through the hydride intermediates [CpMH{HC(PR3)]CHPh}(CO)2]+, which undergo reductive elimination.467 The phosphite-substituted vinyl complexes [CpM{C(P(OR)3)]CHPh}(CO)2] (761, M ¼ Mn; 762, M ¼ Re; P(OR)3 ¼ P(OMe)3 (a), P(OEt)3 (b), P(OiPr)3 (c)) can even be protonated (and hydrolyzed) by water to afford the corresponding Z2-phosphorylalkene complexes [CpM{Z2-(E)-HC(P(O)(OR)2)]CHPh}(CO)2] (763, M ¼ Mn, 764, M ¼ Re).468 Ph C M

H

Ph H HBF4

BF4-

M

C OC

OC

PR3

OC

H

OC

PR3

PR3 = PMe3 (a), PMePh2 (b) 696, M = Mn 697, M = Re

759: M = Mn 760: M = Re

Ph

Ph

C M OC OC

H

H2O

C P(OR)3

H C

M OC OC

C P

H O

R = Me (a), Et (b), iPr (c) 761: M = Mn 762: M = Re

Scheme 242

763: M = Mn 764: M = Re

OR OR

102

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

The partially deuterated metallacyclic complex 765 reacted with acetic acid (MeCO2H) to afford the deuterated compound 766 as a result of protolysis of the Mn-vinyl bond (Scheme 243).340,341 The metallacyclic complex 706a reacted with neat PhC^CH at 100  C to give a mixture of species, including the alkene derivative 767.445 The reaction might proceed through alkyne insertion into the MndCaryl bond of 706a to give the intermediate 709, which is formally protonated by phenylacetylene.

Scheme 243

6.01.4.2.2

Reductive elimination

Vinyl complexes can undergo reductive elimination reactions. For example, warming of a THF solution of the complex 709 to room temperature led to the formation of the reductive elimination product 768 (Scheme 244).445 The complex 706b reacted with phenylacetylene to give the vinyl complex 769 (an analog of 709) which evolved to the complex 770.

Scheme 244

Treatment of the cyclometalated vinyl complex 702 with phenyl acetylene gave [(Z5-(1,2,4-triphenyl-1-aza-cyclohexadienyl)) Re(CO)3] (772),444 which might be formed by insertion of the alkyne to give the seven-membered metallacycle 771 followed by reductive elimination (Scheme 245). Treatment of a benzene solution of the vinyl complex 677a with one equivalent of Et3SiH at 75  C led to the formation of the rhenium(I) hydride compound [ReH(Br)(NO)(PCy3)2] and (E)-PhCH]CH(SiEt3).433 The reaction presumably proceeds through SidH oxidative addition to give the intermediate 773 followed by reductive elimination of SiEt3 and the vinyl group.

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Ph Ph N

Ph CO

Ph

CO Re

Ph

Ph

CO

CO

Re

Ph

Ph CO

Ph

CO

OC CO

771

702

Re

N CO

N Ph

Re

Cy3P ON

Ph

Ph N

103

772

Ph Et3SiH

Br PCy3

H SiEt3 ON Cy3P Re PCy3

Ph

Re

Br PCy3

H

Br 677a

Cy3P ON SiEt3

773

Scheme 245

6.01.4.2.3

Reactions in which vinyl complexes are implicated

Vinyl complexes have often been suggested as reactive intermediates in organometallic synthesis and catalysis. Some of the reactions have been described in the section dealing with the chemistry of cyclometalated group 7 aryl metal complexes. Additional examples are described below. Treatment of the metallacycle 708a with the propargylic carbonate PhC^CCMe2dOCO2Me gave the fused indolone 774 (Scheme 246).446 The compound 774 might be formed by initial insertion of the alkyne moiety into the MndCaryl bond of 708a to give the seven-membered metallacycle 775, which can evolve to the allene 776 by b-O elimination. [4 + 2] Diels–Alder reaction of 776 would give 777 which can eliminate HCN through a retro-Diels–Alder reaction. A similar process has been proposed for [MnBr(CO)5]-catalyzed Alder domino annulation of pyridones bearing a pyridine directing group with propargylic carbonates.446,447 OCO2Me CO O

Ph

Ph

CO

N

Mn

CO

N

CO

+

Mn N

CO CO

708a

N

O

OCO2Me

CO CO 775 -O elimination

- "[Mn(OCO2Me)(CO)4]"

N retro [4+2] Ph

N

O

N

HCN

N

[4+2] O

Ph O

N

•

Ph 774

777

776

Scheme 246

The Re(I) complex [ReBr(CO)3(THF)2] catalyzed CdH functionalization of olefins bearing a directing group with a,b-unsaturated carbonyl compounds, alkynes and aldehydes to give g,d-unsaturated carbonyl compounds (e.g. 778), dienes (e.g. 779), and allyl silyl ethers (e.g. 780), respectively (Scheme 247).364 These catalytic reactions were proposed to proceed via CdH bond activation, insertion of unsaturated molecules into the formed rhenium-carbon bond, and reductive elimination (or transmetalation in the cases of aldehydes).364

104

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

N

N

[ReBr(CO)3(thf)2] O

N

[ReIII]-H

+

O OEt

OEt 778

Ph O

N

[ReBr(CO)3(thf)2]

+

O

O

N

N Ph

III

[Re ]-H

Ph

Ph 779

N

N + PhCHO + SiHEt3

[ReBr(CO)3(thf)2]

N

N

N [ReIII]-H

N Ph OSiEt3 780

Scheme 247

The manganese(I) complex [MnBr(CO)5] catalyzed coupling reactions of terminal alkynes with isocyanates to give hydantoin derivatives (e.g. formation of 781) (Scheme 248).469 The reaction was proposed to proceed by initial oxidative addition of terminal alkyne to give the alkynyl-hydride complex intermediate 782, which undergoes insertion of two molecules of the isocyanate to give the amido-hydride complex intermediate 783. The latter undergoes a reductive elimination reaction to give the alkyne complex 784. An intramolecular nucleophilic attack of the amine group on the Z2-alkyne ligand in the complex 784 gives the vinyl complex 785. The product 781 is formed upon protonation of the MndCvinyl bond with the terminal alkyne.

Scheme 248

The manganese(III) tetraphenylporphyrin complex 786 catalyzed cycloisomerization of enynes 787 to give cyclic compounds 788 and 789 (Scheme 249). The catalytic reaction was proposed to proceed via initial intramolecular nucleophilic addition of the internal olefin moiety to the activated Z2-alkyne ligand in intermediate 790. Attacking on the internal carbon of 790 would give the carbocationic intermediate 791, which can undergo a direct proton shift to give the five-membered diene 788 with concomitant regeneration of starting porphyrin complex. Attacking the terminal carbon atom of 790 would give the isomeric carbocationic intermediate 792, which can evolve to the carbene complex 793 by addition of the carbocation to the C]C double bond. The six-membered product 789 is liberated upon subsequent carbene-alkene rearrangement.470

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

105

Scheme 249

The manganese(I) complex MnBr(CO)5 catalyzed intermolecular dehydrative [2 + 2 + 2] coupling of one mole of a 1,3-dicarbonyl compound with two moles of a terminal acetylene to give a benzene derivative (e.g. formation of 794) (Scheme 250).471,472 DFT calculations suggest that473 the catalytic reaction most likely involves an acac-Mn(I) species (795). This species can undergo stepwise alkyne insertion reactions to give the vinyl complexes 796 and 797. The intermediate 797 could then undergo a stereoselective intramolecular nucleophilic attack of the vinyl moiety on one of the carbonyl groups to give the alkoxide complex 798, which can liberate the cyclohexadienol 799 and regenerate active Mn(I) species upon reaction with acetylacetone and two alkyne molecules. The final arene product 794 is generated via aromatization of cyclohexadienol 799 by elimination of a water molecule. O O

O

[MnBr(CO)5]

Ph

+2

Ph

Ph 794

H2 O Ph O [Mn]

O HO

O

Ph 799

795 Ph Ph

Ph Ph Ph

[Mn]

O

[Mn]

O

O

O

796

797

Ph

carbonyl addition

O [Mn]

O Ph 798

Scheme 250

Mn(II)-vinyl species have been suggested as key intermediates for MnCl2 catalyzed coupling reactions of aryl Grignard reagents with alkenyl halides474 and cross-coupling of Grignard reagents with vinylic organo tellurides.475

6.01.5

Alkynyl complexes

Group 7 metal alkynyl complexes have received continuous interest in the review period. They have been described for Mn(I), Mn(II), Mn(III), Re(I), Re(II), and Re(V) systems. In most of the complexes, the alkynyl ligands serve as a terminal ligand, while complexes with a bridging alkynyl ligand are also known.

106

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

6.01.5.1

Synthesis

6.01.5.1.1

M(I) complexes

Group 7 M(I) alkynyl complexes are common and have been isolated with metal fragments such as [M(CO)5]+ (M ¼ Mn, Re), [Re(CO)3(diamine)]+, [M(CO)3(diimine)]+, [Re(CO)3(CNR)2]+, [Re(CO)2(PR3)3]+, [Re(NO)2(PR3)2]+ and [Cp0 Re(PR3)2] (Cp0 ¼ cyclopentadienyl derivatives). The most common route to construct the MdC^CR linkage of M(I) alkynyl complexes is to use transmetalation reactions of M(I) precursors with alkynyl reagents RC^CM (e.g., M ¼ Li, MgX, Ag, and Au). For example, the manganese(I) alkynyl complex [Mn(C^C-p-tolyl)(CO)5] (800) has been synthesized by the reaction of LiC^C-p-tolyl with [MnBr(CO)5] (Scheme 251).302,341

Scheme 251

Several rhenium complexes of the type [Re(C^CR)(CO)3(diamine)] (e.g. 802a–d) have been prepared by the reactions of the bromo complex 801 with the corresponding silver acetylides (Scheme 252).476 These complexes can be derivertized by modification of the acetylide ligand (e.g. to give 802e,f) or substitution of the diamine ligand with a stronger ligand such as isocyanides (e.g. to give 803).

Scheme 252

Treatment of the acyl complex Li[Re2(m-H)(m-PCy2)(CO)7{C(O)Ph}] (804) with [Au(C^CPh)(PPh3)] gave benzaldehyde and the trinuclear alkynyl complex anion [Re2(AuPPh3)(m-PCy2)(CO)7(C^CPh)]−, which was isolated as its PPh+4 salt 805 (Scheme 253). The latter reacted with coinage metal complexes [MCl(PPh3)] (M ¼ Cu, Ag, Au) to give chiral heterometallatetrahedranes of the general formula [Re2(AuPPh3)(MPPh3)(m-PCy2)(CO)7(C^CPh)] (806, M ¼ Cu (a), Ag (b), Au (c)). The analogous complex [Re2(AgPPh3)2(m-PCy2)(CO)7(C^CPh)] has been obtained from the reaction of [Re2(AgPPh3)2(m-PCy2)(CO)7Cl] with LiC^CPh. The complex [Re2(AgPPh3)2(m-PCy2)(CO)7(C^CPh)] can undergo a metathesis reaction with [CuCl(PPh3)] to give [Re2(AgPPh3)(CuPPh3)(m-PCy2)(CO)7(C^CPh)] and [AgCl(PPh3)].252

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

CO

Cy2 P (CO)4Re

+ Li CO

1) + Au(CCPh)(PPh3) - PhCHO (CO)4Re

Re O

CO CO

2) + PPh4Cl - LiCl

Ph

PPh4+

Re CO

Au

CO

H

Cy2 P

107

PPh3 Ph

804

805 MCl(PPh3) M = Cu, Ag, Au PPh3

PPh4Cl

Au CO

Cy2 P (CO)4Re

CO Re CO

M

806 M = Cu (a), Ag (b), Au (c)

Ph3P Ph Scheme 253

Group 7 M(I) alkynyl complexes have often been prepared by reactions of coordinatively unsaturated complexes with alkynes in the presence of a base or deprotonation reactions of preformed terminal alkyne or vinylidene complexes. For example, treatment of the cationic dinitrosyl bisphosphine rhenium complexes [Re(NO)2(PR3)2]BArF4 (R ¼ Cy, iPr; BAr−F4 ¼ tetrakis[(3,5-bis (trifluoromethyl)phenyl)borate]) with acetylene in the presence of 2,6-di(tert-butyl)pyridine gave the neutral alkynyl complexes [Re(C^CH)(NO)2(PR3)2] (807) (Scheme 254).449 The reaction in the absence of a base yielded the alkynyl complexes [Re{CH] C(H)ONH}(C^CH)(NO)(PR3)2]BArF4 (715), as mentioned in the section dealing with vinyl complexes.

PR3

+ BArF4-

PR3 H

ON

H [Re(NO)2(PR3)2]BArF4

Re ON

tBu

N

H

H

O

H N

NO Re

tBu

RP3

RP3

R = Cy (a), iPr (b)

807

715

Scheme 254

Complexes of the type [Cp Re(C^CR)(NO)(PR3)] can be obtained by deprotonation of [Cp Re(Z2-HC^CR)(NO)(PR3)]+. For example, treatment of the alkyne complexes [Cp Re(Z2-HC^CR)(NO)(PR3)]BF4 (808, R ¼ p-C6H4Me, p-C6H4Ph, p-C6H4-tBu, c-C6H11) with KOtBu produced the corresponding alkynyl complexes [Cp Re(C^CC^CSiMe3)(NO)(PR3)] (809) (Scheme 255).477 Deprotonation of vinylidene complexes [LnM]C]CHR]+ can also lead to alkynyl complexes, as will be described in the section dealing with vinylidene complexes.

+ BF4-

tBuOK Re

Re

SiMe3 808 Scheme 255

SiMe3

ON R3P

ON R3P R = p-C6H4Me, p-C6H4Ph p-C6H4-tBu, c-C6H11

809

108

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Complexes of the type [Cp Re(C^C(C^C)nC^CSiEt3)(NO)(PPh3)]478 have been used to make RedFe heterobimetallic complexes.479 For example, treatment of [Cp Re(C^C(C^C)nC^CSiEt3)(NO)(PPh3)] (810, n ¼ 0 (a), 1 (b)) with Bu4NF produced the desilyated alkynyl complexes [Cp Re(C^C(C^C)nC^CH)(NO)(PPh3)] (811) which reacted with [Cp FeCl(dppe)] in the presence of alcoholic KPF6 and KOtBu to give the bimetallic complexes 812, presumably via vinylidene intermediates (Scheme 256). The bimetallic complex [Cp Re(NO)(PPh3)(m-C^CC^CC^C)Fe(dppe)Cp ] (812b) reacted with [Co2(CO)8] to give the Co-containing complex 813, and with ferricinium hexafluorophosphate in THF to give the cationic radical complex [Cp Re(NO)(PPh3)(m-C^CC^CC^C)Fe(dppe)Cp ]PF6.

wet nBu4NF Re

SiEt3

n

ON Ph3P

810: n = 0 (a), 1 (b)

Re H n ON Ph3P 811: n = 0 (a), 1 (b) [Cp*FeCl/dppe)]/KPF6

Co(CO)3

CO)3Co

[Co2(CO)8] Fe

Re ON PPh 3

Ph2P

n =1 PPh2

Re ON Ph3P

Fe n

PPh2 Ph2P

812: n = 0 (a), 1 (b)

813

Scheme 256

Alkynyl complexes could also be obtained by direct reactions of terminal alkynes with complexes bearing a basic site. For example, treatment of the diphosphinomethanide complex [Mn(CO)4{(PPh2)2CH}] (814) with MeO2CC^CH produced a mixture of the alkynyl complex fac-[Mn(C^CCO2Me)(CO)3(dppm)] (815) and the vinyl-substituted diphosphinomethanide complex [Mn(CO)4{(PPh2)2CCH]CH(CO2Me)}] (816) (Scheme 257). The complex 816 is likely formed by insertion of the alkyne into the CdH bond, while the alkynyl complex 815 might be formed by oxidative addition of the alkyne followed by protonation of the methanide carbon.480

Scheme 257

Re(I) alkynyl complexes have been occasionally obtained by nucleophilic addition reactions of allenylidene complexes. For example, treatment of the allenylidene complex 817 with 1.2 equivalent of the phosphine PMePh2 at −40  C produced the g-phosphonioalkynyl complex [Re{C^CCPh(p-C6H4NO2)(PMePh2)}(CO)2(triphos)]OTf (818, triphos ¼ MeC(CH2PPh2)3, OTf ¼ OSO2CF3) (Scheme 258).481 + OTf-

PPh2 CO Ph2P Ph2P

Re

C

C Ph

CO Scheme 258

C

NO2

817

+ OTf-

PPh2 CO

PMePh2 Ph2P Ph2P

Re

C

C

NO2

C PMePh2

CO

Ph 818

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

109

The allenylidene complex [Re(]C]C]CPh2)(CO)2(triphos)]OTf (698, triphos ¼ MeC(CH2PPh2)3, OTf ¼ OSO2CF3) reacted with pyrazole to give heterocyclic vinyl complex 819 which can react with NaOMe to give the alkynyl complex 820 via ring opening of the triazaindenyl moiety (Scheme 259).442 The alkynyl complex 821 has been similarly obtained from 1H-benzotriazole and the complex 698.

Scheme 259

Reaction of hydride complexes with excess of terminal alkynes may also lead to the formation of alkynyl complexes. For example, the hydride complexes [ReH(Rpz)(HRpz)(NO)(PPh3)2] (822, HRpz ¼ pyrazoles, R ¼ H (a), 5-Me (b)) reacted with excess terminal arylalkynes HC^CAr (Ar ¼ Ph, p-tolyl) in refluxing 1,2-dichloroethane to give the alkynyl-pyrazolato complexes [Re(C^CAr)(Rpz)(HRpz)(NO)(PPh3)2] (824, R ¼ H (a), 5-Me (b); Ar ¼ Ph, p-tolyl) (Scheme 260).482 These reactions presumably proceed through initial insertion of the alkyne into the RedH bond to give the vinyl intermediates 823, followed by a metathesis reaction of the intermediate with alkyne.

PPh3 H

N N Re N

ON PPh3 822

R

PPh3

N

R H

Ar

N N

Ar

Re N

ON

R Ar = Ph, p-tolyl R = H (a), Me (b)

PPh3 823

N

R H

Ar

NH N

PPh3 Re N

ON

R CH2=CHAr

Ar N R

PPh3 824

Scheme 260

The most intensively studied Group 7 M(I) alkynyl complexes are complexes of the type [M(C^CR)(CO)3(diimine)].483 These complexes usually contain three facially bound carbonyl ligands. Upon photolysis, the facial isomer can rearrange to meridional isomers. For example, photolysis of the facial isomer of the alkynyl complex fac-[Mn(C^CPh)(CO)3(bpy)] (825) in MeCN produced the meridional isomer mer-[Mn(C^CPh)(CO)3(bpy)] (826) (Scheme 261).484

Scheme 261

110

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Complexes of the type [M(C^CR)(CO)3(diimine)] (M ¼ Re, Mn) could be prepared by modification of preformed M(I) alkynyl complexes, or alkynylation of [MX(CO)3(diimine)]. For example, the Re(I) alkynyl complex 828 can be prepared by the reaction of the complex 802a with the 827 (Scheme 262).476 Br N

Hexn

nHex

N OC

Br

Br N

nHex

nHex N

N 827

Re

Br

Re

OC

OC

N

CO

OC CO

802a

828

Scheme 262

A more common route to construct the metal-alkynyl linkage of complexes of the type [Re(C^CR)(CO)3(diimine)] is to perform one-pot reactions of [ReX(CO)3(diimine)] (X ¼ Cl, Br) with TlPF6/HC^CR/NEt3. For example, treatment of [ReBr(CO)3(bpy)] (829) with PhC^CH/TlBF4/NEt3 produced the alkynyl complex 830 (Scheme 263).485 Analogous complexes have been obtained for those with bipyridine derivatives such as 4,40 -dimethyl-2,20 -bipyridine, 6,60 -dimethyl-2,20 -bipyridine and 5,50 -dimethyl-2,20 -bipyridine. In these reactions, TlBF4 serves as a halogen abstraction agent. As a halogen abstraction agent, thallium(I) salt has advantages over Ag(I) salts as Tl(I) is electrochemically inert while Ag(I) is electrochemically active.

Scheme 263

The strategy has been used to prepare rhenium(I) diimine alkynyl complexes with a variety of functionalities as illustrated in Scheme 264. They include complexes containing a carboxaldehyde moiety (e.g. 831) which are emissive at 630–672 nm in the

R OC

R

R'

N

N R Re

CHO

R OC

R

R'

N

N R

S

R OC

S

Re

R

R'

N

N

CN R

S

CO2H

Re

S OC

CO

OC

831 R = R' = H; R = Me, R'= H; R = H, R' = Br tBu

CO

OC

832 R = R' = H; R = Me, R'= H; R = H, R' = Br C18H37O

tBu

CO

833 R = R' = H; R = Me, R'= H; R = H, R' = Br O

O

O

OC18H37

O

C18H37O R OC

N

N

OC18H37

OC

OC CO

Mes

B OC

Mes

834

R OC

R

R'

N

N

OC

N B

CO

835 R = R' = H; R = Me, R'= H; R = H, R' = Br Scheme 264

OC18H37

Re

R CO

836

R Re

N

N

C18H37O

R Re

OC OC

N

CF3

N

N N

Re

OC CO

837a

CF3

Re

OC

CO

837b

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

111

solution state at room temperature486; complexes with a fused thiophene (e.g. 832) or cyanoacrylic acid moiety (e.g. 833) which show photosensitizing property,487 complexes functionalized with a B(ppy)Mes2 moiety (e.g. 834),488 complexes with a triarylboron moiety (e.g. 835) which are capable of binding F− ions with spectral changes in the UV–vis absorption and emission spectra,489 complexes that show thermotropic gelation behavior upon CuI or AgI coordination (e.g. 836),490 and complexes bearing CF3 (e.g. 837a) or pyridine (e.g. 837b) moiety which are photosensitizers for H2 production.491 Complexes of the type [Re(C^CR)(CO)3(diimine)] have also been made by one-pot reactions of [ReX(CO)3(bpy)] (X ¼ Cl, Br) with AgOTf/HC^CR/NEt3. For example, a series of luminescent 8-hydroxyquinoline-containing alkynylrhenium(I) tricarbonyl bipyridyl complexes (e.g. 840) have been synthesized by one-pot reactions of [ReCl(CO)3(bipyridine)] (839) with the 8-tertbutyloxycarbonyloxy-containing alkyne 838, AgOTf and NEt3 (Scheme 265).492 R

R

N

R

Br N

N

CO Re

N

OtBu

CO

OC

Re

OC

AgOTf/NEt3

CO

R

O

838

OtBu O

CO 840

R = H, Me, tBu

839

N

N

Scheme 265

A series of functional rhenium(I) diimine alkynyl complexes have been synthesized by the same strategy (Scheme 266). They include alkynyl complexes with electron- and hole-transporting moieties such as carbazole (e.g. 841), oxadiazole (e.g. 842),493 and triarylamine (e.g. 843), many of which could act as emitters in OLED devices and show electroluminescence494; complexes displaying thermotropic gelation behavior (e.g. 844)495; and the complex 845 bearing a chromone moiety which is a potentially luminophore for use in cell imaging.496

R N OC

N

N Re

OC

OC

NnBu

841

N

O

Re

OC

CO

N

CO

R

OC

N N

N Re

OC

842

N CO

843: R = H, Me, tBu

OC18H37 N OC OC

O

N Re

NH CO

844

OC18H37 OC18H37

N OC OC

N

H N

O

O

Br

Re CO

845

Scheme 266

Another route to construct the metal-alkynyl linkage of complexes of the type [Re(C^CR)(CO)3(diimine)] is to react [ReX(CO)3(diimine)] (X ¼ Cl, Br) with LiC^CR. For example, treatment of triethynylbenzene (846) with LiMe followed by [ReCl(CO)3(tBu2bpy)] produced the alkynyl complex 847 (Scheme 267).497 The alkynyl complexes 848498 and 849499 have been obtained similarly by treating the corresponding alkynes with LiN(SiMe3)2 followed by [ReCl(CO)3(tBu2bpy)]. It is noted that the complex 849 could not be prepared by the reaction of the alkyne with [ReCl(CO)3(tBu2bpy)] in presence of NEt3 and AgOTf.

112

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

tBu

tBu N

1) LiMe OC

2) [ReCl(CO)3(tBu2bpy)]

N Re

OC

CO

846

847

tBu

tBu

tBu N

OC

PPh2

N

N OC

Re

OC

tBu N Re

OC

CO

Fe

CO

848

849

Scheme 267

The mononuclear alkynyl complexes [Re(C^Cd(C6H4)ndC^CH)(CO)3(diimine)] (diimine ¼ bpy, tBu2bpy, (CF3)2bpy, NO2phen) and [Re(C^CdC^CH)(CO)3(tBu2bpy)] have been used as starting materials to prepare heterobimetallic complexes (Scheme 268). For example, treatment of [Re(C^CdC6H4dC^CH)(CO)3(R2bpy)] (850, R ¼ H (a), tBu (b), CF3 (c)) with [PtCl(tBu3tpy)]OTf (851) in the presence of CuI and Et3N produced the luminescent heterobimetallic rhenium(I)-platinum(II) polypyridine alkynyl complexes [(R2bpy)(CO)3Re{C^Cd(C6H4)n-C^C}Pt(tBu3tpy)]OTf (852).500 Analogous complexes have also been made from [Re(C^CdC^CH)(CO)3(tBu2bpy)].

tBu

tBu R

R N

OC

Re

+ Cl

Pt

CO

OC

+ OTf-

N

N

R

tBu

N

N

CuI/NEt3

Pt

N

tBu

N

CO

OC

852

851

850: R = H (a), tBu (b), CF3 (c)

N

N Re

OC

N

+ OTf-

R

tBu

tBu

Scheme 268

The alkynyl complex [Re(C^CC^CH)(CO)3(tBu2bpy)] (853) can serve as a starting material for the preparation of coinage metal clusters (Scheme 269). For example, it reacted with [Cu2(dppm)2(NCMe)2](PF6)2 in acetone containing an excess of KOH to give the cluster [Cu3(m-dppm)3{m3-C^CC^CdRe(CO)3(tBu2bpy)}2]PF6 (854), and with [Ag2(dppm)2(NCMe)2](PF6)2 in CH2Cl2/MeOH to produce the cluster [Ag3(m-dppm)3(m-Cl){m3-C^CC^CdRe(CO)3(tBu2bpy)}]PF6 (855).501 tBu

tBu N

OC OC

tBu

N

N

[Ag2(dppm)2(NCMe)2](PF6)2

Re

tBu

OC CO

Ph2P

N Re

OC

C C C C

855 KOH/MeCN

N OC OC

Ph2P

N Re

C C C C Ph2P

CO 854

Scheme 269

But

tBu

tBu

PPh2 Ph2 Cu P

N

C C C C

Cu

Cu PPh2 PPh2

OC

N Re

Ag PPh2 PPh2

[Cu2(dppm)2(NCMe)2](PF6)2 tBu

Cl

Ag

Ph2P

CO

853

PPh2 Ph2 Ag P

CO CO

+ PF6-

+ PF6-

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

113

The complex 849 has been used as a starting material to prepare a series of complexes with different organometallic building blocks unsymmetrically arranged around the periphery of a 1,3,5-triethynylbenzene core.499 The complexes 856–858 are examples of such complexes (Scheme 270). The trimetallic alkynyl complex 857 can be further manipulated to give a series of heteromultinuclear transition metal complexes with up to seven different metal atoms (e.g. Ti, Fe, Ru, Os, Re, Pt, and Cu).

tBu

tBu N

OC

tBu

Fe

N

N

1) [CpMCl(PPh3)2]/NH4PF6

OC

Re

OC

tBu N Re

OC

2) KOtBu

CO

Fe

CO 856

849

M = Ru, Os

M Ph3P Ph3P

CuCl, O2

cis-[PtCl2(PPh3)2]

Fe

Fe tBu

tBu N

OC OC

Fe

N tBu

Re

tBu

CO

N PPh3

tBu

Pt Ph3P

857

N OC

Cl

N

Re

CO

OC 858

CO

Re

OC

N

tBu

CO

Scheme 270

A series of polymetallic complexes have also been made from the alkynyl complexes 847497 and 848498,502 bearing a C^CH moiety by manipulating the C^CH and/or the PPh2 groups. The complexes 859–860 are examples of the complexes derived from these studies (Scheme 271). Ph2 P

Ph2 P P Ph2

Pt

OC

Re

tBu 859

CO

Cl

N

CO

Re

N OC

CO

Ph2 P Ru

Ru

P Ph2

Pt P Ph2

OC Scheme 271

Re

P Ph2

tBu

PPh3

N N

Fe P Ph2

PPh3

But

tBu

860

CO

Ph2 P Fe

CO CO

N OC

861

Rh

PPh2

But

Cl

PPh3

N N

P Ph2

PPh3

But

tBu

Ru

Fe

Ru

Fe

OC

Re

N CO

tBu

Cl

114

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

6.01.5.1.2

M(II) and M(III) complexes

There are a few reports on the chemistry of group 7 M(II) and M(III) alkynyl complexes. The M(II) ethynyl metal hydride molecules [MH(C^CH)] (862, M ¼ Mn (a), Re (b)) can be generated in the gas phase by reactions of laser-ablated Mn and Re atoms with acetylene. Matrix infrared spectroscopy and DFT calculations suggest that the ethynyl Mn hydride [MnH(C^CH)] (862a) has a linear structure while the Re analog 862b has a planar Cs structure (Scheme 272).503 H H

156o 174.2o Re C C

Mn C C H

H

179.5o 862a

862b

Scheme 272

The four coordinated homoleptic Mn(II) alkynyl complex [Li(THF)]2[Mn(C^C-{2,6-(Me3Si)2C6H3}4)] (864) has been made by treatment of the alkyne 863 with LDA followed by MnI2 in THF (Scheme 273). The complex is a high spin d5 complex with a tetrahedral geometry.504

Scheme 273

A few heteroleptic Mn(II) alkynyl-aryl complexes have been obtained by reactions of the iodo-aryl complex {Li(THF) Ar0 MnI2}2 (547b, Ar0 ¼ C6H3-2,6-(C6H2-2,6-iPr3)2) with lithium alkynyl reagents. The reaction of {Li(THF)Ar0 MnI2}2 with two equivalents of LiC^CtBu and LiC^CPh in hexane or hexane/THF–Et2O produced [Ar0 Mn(C^CtBu)4{Li(THF)}3] (865) and [Ar0 Mn(C^CPh)3Li3(THF)(Et2O)2(m-I)] (866), respectively (Scheme 274).64 The complex 865 has an MnLi3C4 cubane core. In this

I Mn

Mn I THF

Li I

I

547b

Li THF tBu C

Ph C

CLi

CLi

hexane/Et2O tBu C C

Mn

THF tBu THF Li C C Li tBu C C Li C THF

Ph Li

C

I

C Li Ph C C

Li

Mn C

Scheme 274

tBu

OEt2 C

C 865

THF OEt2

Ph 866

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

115

compound, the manganese has a distorted tetrahedral geometry and the manganese atom is s-bonded to three C^CtBu groups through the b-carbons and to one Ar0 ligand. The complex 866 is structurally similar to 865 and has an MnLi3C3I cubane structure. The manganese atom is also coordinated to three C^CPh groups through the b-carbons and an Ar0 ligand. Treatment of the divalent manganese complex [Mn{N(SiMe3)2}2] (867) with phenylacetylene in n-hexane afforded the manganese(II) alkynyl complex [{Mn(m3-C^CPh)(N(SiMe3)2)}4] (868), which adopts an Mn4C4 cubane structure (Scheme 275). The complex 868 represents a rare example of complexes with a m3-alkynyl Mn3 motif. The cluster is moderately active for catalytic cyclotrimerization of phenylacetylene. It also catalyzed hydrogenation of alkenes.505

Scheme 275

Manganese bis(alkynyl) complexes with bis(phosphino)ethane ligands have been extensively studied by Berke and co-workers. Early examples of such complexes include mononuclear complexes trans-[Mn(C^CR)2(dmpe)2]n+ (n ¼ 0, 1; R ¼ Ph, SiMe3, SiEt3, SiiPr3, Si(Me)2tBu, SiPh3) and dinuclear complexes [{Mn(C^CR)(dmpe)2}2(m-C4)]n+ (n ¼ 0, 1, 2; R ¼ H, SiMe3, SiEt3, R ¼ SiiPr3, Si(tBu)Me2).506,507 Several new derivatives with the metal fragments [MnI(dmpe)2] and [Mn(C^CAr)(diphosphine)2] have been reported by the same group in 2009. The symmetrical Mn(II) d5 bis-alkynyl complexes [Mn(C^CC6H4R)2(dmpe)2] (870, R ¼ C^C-TIPS (a); F (b), dmpe ¼ Me2P (CH2)2PMe2) have been obtained by the reactions of [MnI2(dmpe)2] (869) with two equivalents of the corresponding acetylides LiC^CC6H4R (Scheme 276). Treatment of the alkynyl complexes 870 with Cp2FePF6 yielded the corresponding d4 alkynyl complexes [Mn(C^CC6H4R)2(dmpe)2]PF6 (871) which can be desilyated to give 872. Similar complexes have been described for those with the diphosphine Me2P(CH2)3PMe2.508 Me2 I P

Me2 P

Mn P Me2 I

Me2P

Li

R

R

PMe2 Mn

P Me2

Me2P 870:

869

R=

R

PMe2 TIPS (b), F (b)

Cp2FePF6 Me2P

PMe2 Mn

R= Me2P

872

Me2P

NBu4F/H2O TIPS

R

PMe2 Mn

PMe2

Me2P 871:

R=

+ PF6R

PMe2 TIPS (b), F (b)

Scheme 276

The unsymmetrically substituted iodo-alkynyl complex trans-[MnI(C^CC6H4dC^C-TIPS)(dmpe)2] (873) has been obtained by treating [(Z5-MeC5H4)MnI(dmpe)] with one equivalent of HC^CC6H4dC^C-TIPS in the presence of dmpe (Scheme 277). Deprotection of the TIPS group in 873 with TBAF gave the alkynyl complex [MnI(C^CC6H4C^CH)(dmpe)2]. The dinuclear MnII/ MnII complexes {[Mn(C^CC6H4R)(dmpe)2]2(m-C4)} (875, R ¼ H, Me, n-pentyl, F) can be prepared by reactions of {[MnI (dmpe)2]2(m-C4)} with the corresponding alkynyl lithium reagents LiC^CC6H4R (R ¼ H, Me, n-pentyl, F). The MnII/MnII dinuclear complexes 875 can be oxidized to give the corresponding mixed-valent complexes 876 and the dicationic complexes 877 using [Cp2Fe]PF6.508 The complexes 877 can be reduced by Cp Co to go back to complexes 876 and 875.

116

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Me2P

TIPS I

PMe2

Me2P I Me2P

dmpe

TIPS

Mn

I

Mn

Me2P

PMe2

Me2P

PMe2 873

PMe2 Me2P Mn

PMe2 Mn

PMe2

Me2P

R

Me2P

Li

I

R

PMe2

PMe2 Me2P

Me2P

874

Mn

Mn

R

PMe2 Me2P

Mn

Mn Me2P

PMe2

PMe2 877

Me2P

PMe2

+ 2PF6- Cp2FePF6 R

Me2P

R Cp*2Co

R

PMe2

875 Cp2FePF6

Me2P

Me2P

PMe2

R = H, Me, n-pentyl, F

PMe2

Cp*2Co

PMe2 Me2P

Mn

Mn Me2P

PMe2

PMe2

Me2P

+ PF6R

PMe2

876

Scheme 277

6.01.5.1.3

M(V) complexes

Group 7 M(V) alkynyl complexes have been reported for a few of those with a carbyne ligand. For example, the mononuclear Re alkynyl-carbyne complex trans-[Re(C^CSiMe3)(^CdMe)(PMe3)4]PF6 (878) has been obtained by heating a mixture of the dinitrogen complex trans-[ReCl(N2)(PMe3)4], TlPF6, and excess HC^CSiMe3 (Scheme 278).509 The carbyne complex 878 can be deprotonated to give the vinylidene-carbyne complex 879.509 The chemistry of these complexes will be described in more detailed in sections dealing with carbyne and vinylidene complexes.

Scheme 278

The 12-membered-ring metallacycles [mer-Re(^CCH]CRC^Cd)Cl(PMe2Ph)3]2 (880, R ¼ tBu (a), 1-adamantyl (b)), which are organometallic analogs of antiaromatic octadehydro[12]annulene, have been prepared by heating the methyl-carbyne complexes mer-[Re(^CCH]CRC^CH)(Me)Cl(PMe2Ph)3] (881) (Scheme 279). The complexes are likely formed by an intermolecular s-bond metathesis between the RedMe bond and the acetylenic CdH bond.119 The complexes 880 could also been prepared by treatment of the SiMe3-protected dichloro-rhenium carbyne complexes [Re{^CCH]C(R)C^CSiMe3}Cl2(PMe2Ph)3] (882, R ¼ tBu (a), 1-adamantyl (b)) with CsF in methanol.510

Scheme 279

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

6.01.5.1.4

117

Cluster complexes

Alkynyls can serve as a bridging ligand to form rhenium dinuclear and polynuclear complexes. For example, treatment of [Re2(CO)8(NCMe)2] with the gold-alkynyl complex [Au(C^CC^CFc)(PPh3)] in toluene at 90  C for 1 h produced the trinuclear gold-dirhenium complex [Re2(AuPPh3)(m-C^CC^CFc)(CO)8] (883) in which the C^CC^CFc unit acts as an Z1:Z2 ligand.511 If the reaction was carried out for 5 h, the cluster complex [Re4(AuPPh3)(m4-C^C)(m3-C^CFc)(NCMe)(CO)13] (884) was formed as a result of cleavage of the CdC bond of the butadiynyl ligand (Scheme 280).

Scheme 280

Refluxing the cluster complex [Re2(AuPPh3)(CO)8(m-C^CFc)] (885) in benzene for 2.5 h generated the digold-tetrarhenium cluster [Re4(AuPPh3)2(CO)12(m3-C^CFc)2] (886), which contains two m3-Z1:Z2:Z2-alkynyls symmetrically bridging the open M. . .M edge (Scheme 281).512

Fc C

Fc CO

CO

CO

OC Re

Re Au

OC CO 885

CO PPh3

CO

benzene

Fc C C

(CO)3Re

Re(CO)3

C Re(CO)3

reflux

Au Ph3P

Re(CO)3Au 886

PPh3

Scheme 281

6.01.5.2

Reactivity

Group 7 metal alkynyl complexes can undergo ligand substitution reactions, redox reactions and other chemical transformations without affecting the M-alkynyl linkage, as described in previous discussion. Group 7 metal alkynyl complexes could also undergo chemical reactions involving change in the M-alkynyl functionality as described below. MdC bonds of group 7 metal alkynyl complexes can be cleaved by acid. For example, treatment of the metallacyclic complex {ReCl(PMe2Ph)3}2{^CCH]C(CMe3)C^Cd}2 (880a) with HCl produced the dichloro-rhenium carbyne complex [Re {^CCH]C(CMe3)C^CH}Cl2(PMe2Ph)3] (887) (Scheme 282).510 The alkynyl complex [Mn(C^C-p-tolyl)(CO)5] (800) reacted with 2-phenylpyridine to give [Mn(CO)4(ppy)] (357).302

Scheme 282

118

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

M(I) alkynyl complexes could also be protonated at the b-carbon to give vinylidene complexes,449,480 as will be described in the section dealing with preparation of vinylidene complexes. Re(I) g-phosphonioalkynyl complexes could isomerize to a-phosphonioallenyl complexes. For example, the g-phosphonioalkynyl complex [Re{C^CCPh(p-C6H4NO2)(PMePh2)}(CO)2(triphos)]OTf (818) is thermally unstable and isomerized at −20  C to the a-phosphonioallenyl complex [Re{C(PMePh2)]C]CPh(p-C6H4NO2)}(CO)2(triphos)]OTf (888) (Scheme 283).481 + OTf-

PPh2 CO Ph2P Ph2P

C

Re

C

NO2

PPh2 CO

> 20 oC

C PMePh2

Ph2P Ph2P

Ph

CO

Ph

+ OTf-

C C

Re

C

CO

PMePh2

NO2

888

818 Scheme 283

The Mn(II) alkynyl complex 868 was reported to react with PhC^CH to give the Mn(II) (alkynyl)manganese complex [Mn(C^CPh)2]n (889), which decomposed at 60  C to give 1,4-diphenylbutadiyne (890) and heterogeneous Mn(0) species (Scheme 284).505

(Me3Si)2N Ph

N(SiMe3)2 Ph C C

Mn

C C

Ph C

CH Mn C

Mn Ph C C C

(Me3Si)2N 868

2

Mn(0) species Mn

Mn

C Ph

889

Ph C N(SiMe3)2

C

C

C Ph

890

C Ph

Scheme 284

Mn(II)-alkynyl complexes have been proposed as reactive intermediates in catalytic reactions. For example, the manganese halide MnCl2 catalyzed homocoupling of alkynyl Grignard reagents (e.g. formation of 892 from 891) and cross-coupling of aryland alkynyl Grignard reagents (e.g. formation of 893) using oxygen as the oxidant (Scheme 285).392,513 As illustrated by the formation of 893, the catalytic reaction may proceed through the formation of the organomanganese(II) compound 894, which can react with O2 to give the manganese(IV) species 895, followed by reductive elimination.

Scheme 285

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

6.01.6

119

Acyl and related complexes

The chemistry of group 7 metal acyl and formyl complexes has attracted attention due to its relevance to organometallic synthesis and catalysis. There has also been interest in the chemistry of alkoxycarbonyl, carbamoyl, bora-acyl and iminoacyl complexes.

6.01.6.1

Synthesis

6.01.6.1.1

Acyl and formyl complexes LnMCOR (R]H, alkyl, aryl)

Well-characterized Group 7 metal acyl and formyl complexes are mainly those of Mn(I) and Re(I) systems. The most common route to group 7 metal acyl complexes is to use nucleophilic addition reactions of carbonyl complexes. For example, treatment of the carbonyl complex [(Z5-C5H4Et)Mn(CO)3] with phenyllithium in Et2O produced the acyl complex Li[(Z5-C5H4Et)Mn{C(O)Ph} (CO)2](Et2O) (896) which adopts a dimeric structure in the solid state and can also be viewed as a carbene complex (Scheme 286).514 Et

Et

OEt2

LiPh Mn

OC

CO

OC

OC

OEt2

OC

Et

Et

Li

Mn C

O

Ph

Mn O

Li

C

OC

CO

OC

CO

Ph

OEt2

896

OEt2

Et

Li

Mn C

O

Mn O

Ph

Li

896A

OEt2

C

CO

CO

Ph

Scheme 286

The carbonyl complex [Re(k1-(P)-PNN)(CO)5]OTf (897) reacted with dialkylzinc reagents ZnR2 (R ¼ Me, Et, Bn) to form the acyl complex [Re(k1(P)-PNN
ZnR)(CO)4{m2-C(O)R}]OTf (898, R ¼ Me, Et, Bn), in which an alkyl group has been transferred to a carbonyl carbon and the resulting monoalkyl Zn is bound both to the bipyridine nitrogens and the acyl oxygen. The zinc ion in 898 can be removed by treatment with water to give the Zn-free acyl complex 899 (Scheme 287).24

Scheme 287

The a-fluorovinyl complex [Re(CF]CF2)(CO)5] (900a) reacted with PhC^CNa to give the anionic acyl complexes Li[Re(CF] CF2){]C(O)C^CPh}(CO)4] (901a) which exhibits F ⋯ Na interaction (Scheme 288).515 A similar complex (901b) was produced in the reaction of [Re(CF]CFPh)(CO)5] with PhC^CNa.

Ph

Ph CO OC

Na

Re

OC

F

R

OC

O Re

F

OC

CO R = F (a), Ph (b) 900

CO

CO

CO F

CO F 901

R

Na

OC

O Re

F

OC CO F

Na

R

901A

Scheme 288

Acyl complexes have also been obtained by reactions of vinylhalides with [M(CO)5]− (M ¼ Mn, Re). For example, the iodide compound PhIC]C(CN)2 reacted with K[Mn(CO)5] and Na[Re(CO)5] to produce the halo(acyl)metalates 903 (Scheme 289).431

120

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 289

The reaction might proceed through the intermediates [MI(CO)5] (902, M ¼ Mn (a), Re(b)) and [PhC]C(CN)2]− formed by a halogen exchange reaction. Similar acyl complexes were formed in the reactions of Na[Re(CO)5] with Ph(CN)C]CHI,431 a-bromostilbene (PhCH]CHBr)432 and CF2]CFCl.516 The 2-alkylidene-4-oxothiazolidine vinyl bromide 904 reacted with Na [Re(CO)5] to give the metallacyclic acyl complex 906, presumably via the intermediate 905.517 Group 7 metal formyl complexes can be similarly obtained by reactions of carbonyl complexes with hydride donors. For example, treatment of the carbonyl complex 907 with two equivalents of LiBHEt3 produced the formyl complex 908 (Scheme 290).518

Scheme 290

Even transition metal hydride complexes can serve as the hydride source for formyl complexes. For example, treatment of the carbonyl complexes 909 bearing a functionalized phosphine ligand with PPN[WH(CO)4{P(OMe)3}] (PPN ¼ bis-(triphenylphosphine)iminium) produced the formyl complexes 910 (Scheme 291).518 The hydride complex PPN[cis-WH{P(OMe)3}(CO)4] similarly reacted with the cationic cyclopentadienyl rhenium carbonyl compound [(Z5-C5H4DMEG)Re(CO)2(NO)]BF4 (911, DMEG ¼ dimethylethyleneguanidine) to give the neutral formyl complex [(Z5-C5H4DMEG)Re(CO)(NO)(CHO)] (912).519 The cationic hydride complex [PtH(dmpe)2]PF6 can also react with [Re(k1-(P)-PNN)(CO)5]OTf (913) to give the rhenium(I) formyl complex 914.24

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

121

Scheme 291

Migratory insertion reactions of CO represent another common route to group 7 metal acyl complexes. Several group 7 M(I) acyl complexes of the type LnMCOR (R ¼ alkyl) have been obtained by migratory insertion reactions of alkyl complexes with CO as described in sections dealing with chemistry of alkyl, vinyl and aryl complexes. Acyl complexes have been occasionally obtained by carbonyl insertion reactions of in situ generated alkyl complexes. For example, the tosylate PhCH2CH2C(OTs)Me reacted with one equivalent of Na[Mn(CO)5] under a CO atmosphere to give the acylmanganese complex 916, presumably through the alkyl intermediate 915 (Scheme 292).520 Treatment of the PtdMn bimetallic complex 917 with cis and trans-2,3-dimethylthiirane at room temperature produced the PtdMn bimetallic acyl complex 919.521 The reaction presumably proceeded through initial formation of the alkyl complex 918, followed by CO insertion and coordination of the sulfur donor.

Scheme 292

122

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Acyl complexes of the type [M(acyl)(CO)5] can be conveniently prepared by reactions of [M(CO)5]− with carboxylic acid chlorides or anhydrides. The generation of acyl derivatives [Mn{C(O)R}(CO)5] (R ¼ CF3, CHF2, CH2CF3, CF2CH3) by this route has been described in the section dealing with alkyl complexes.33 As additional examples, the acyl complexes 921 and 923 have been obtained by reactions of [Mn(CO)5]− and [Re(CO)5]− with pyridine carboxylic acid chlorides (Scheme 293).522

Scheme 293

6.01.6.1.2

Alkoxycarbonyl (metallaester) complexes LnMC(O)OR

Alkoxycarbonyl or metallaester complexes have been described for a few Re(0), Re(I) and Re(III) systems. They were usually obtained by nucleophilic addition of alkoxides to coordinated carbonyls. The neutral metallaester complexes [(Z5-C5H4DMEG)Re{C(O)OCH3}(CO)(NO)] (924, DMEG ¼ dimethylethyleneguanidine), [CpRe{C(O)OCH3}(CO)(NO)] (925a) and [Cp Re{C(O)OCH3}(CO)(NO)] (925b) have been obtained by treating the respective cationic carbonyl precursors with one equivalent of methanolic KOMe in thawing acetonitrile (Scheme 294).519

Scheme 294

The cationic carbonyl Re(III) complex [Cp∗ReBr(CO)3]SbF6 reacted with methanol to give the Re(III) methoxycarbonyl complex trans-[Cp∗ReBr(COOCH3)(CO)2]. Similarly, the complex [Cp∗ReBr(CO)3]Br reacted with 3-fluorobenzyl alcohol (3-FBA) to give the alkoxycarbonyl complex trans-[Cp∗ReBr(COOCH2-C6H4F)(CO)2Br] (926) (Scheme 295).523

Scheme 295

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

123

The a-fluorovinyl complex [Re(CF]CFR)(CO)5] (900, R ¼ F (a), Ph (b)) reacted with tBuONa to give the respective anionic alkoxycarbonyl complexes Na[Re{C(O)(OtBu)}(CF]CFR)(CO)4] (927) which exhibit F ⋯ Na interaction (Scheme 296).515 Similar complexes were produced in reactions with tBuOM (M ¼ Li, K). Related alkoxycarbonyl complexes have been obtained by the reaction of CF2]CFCl (I) with Na[Re(CO)5] in the presence of tBuOH,516 and the reaction of Na[Re(CO)5] with PhIC]C(CN)2 in the presence of tBuOH.431

CO OC

CO Re

OC

F

F

tBuONa R

OC

CO OtBu O Re

F

OC

CO R = F (a), Ph (b) 900

CO F 927

R

Na

OC

CO OtBu O Re

OC CO F

Na

F R

927A

Scheme 296

The complexes [ReCl(CO)5] and [Re2(CO)10] reacted with RONa (R ¼ Me3Si, Me) to give the anionic Re(I) alkoxycarbonyl complexes cis-[ReCl(COOR)(CO)4]− (928) and the Re(0) alkoxycarbonyl complexes [Re2(CO)9(COOR)]− (929) respectively (Scheme 297). The Re(I) alkoxycarbonyl complexes of the type cis-[ReCl(OCOR)(CO)4]− are in general stable only at low temperature and can further react with NaOR to give dimeric species [Re2(CO)6(m-OR)3].524

Scheme 297

6.01.6.1.3

Carbamoyl complexes LnMC(O)NR2

Carbamoyl complexes have been described for a few Mn(I), Re(I) and Re(III) systems. They were usually obtained by nucleophilic addition of amides to coordinated carbonyls. Treatment of a dichloromethane solution of the cationic complexes fac-[Mn(CNR)(CO)3(bpy)]+ (930, R ¼ Ph (a), Xyl (b), Me (c), CH2Ph (d), tBu (e)) at −30  C with NH2Me produced the corresponding neutral carbamoyl complexes cis-[Mn{C(O)NHMe} (CNR)(CO)2(bpy)] (931) (Scheme 298).525 Treatment of the N,N0 -diarylformamidine complex fac-[Mn(ArN]CHNHAr)(bpy) (CO)3]+ (932, Ar ¼ 4-dimethylaminophenyl) with KOH produced the carbamoyl complex 933 featuring a cyclic carbamoyl moiety as a result of nucleophilic attack of the uncoordinated N-aryl moiety on a vicinal carbonyl ligand. Similar reactions occurred for fac-[Mn(RN]C(H)NHR)(bpy)(CO)3]+ (R ¼ phenyl, 2-naphthyl, 4-methoxyphenyl)526 and fac-[Mn(RN]C(H)NHMe)(bpy) (CO)3]+ (R ¼ phenyl, Me).527,528

124

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

+

CNR N

CO Mn

N

CO CO

CNR N

CO

2 NH2Me

Mn

+ NH3Me+

N

- 30 oC

CO C NHMe

O

930

931

R = Ph (a), xyl (b), Me (c), CH2Ph (d), tBu (e)

NMe2 Me2N

+ Me2N

HN

NMe2

N N

N CO

Mn N

KOH

N C O

N Mn

CO

CO

N

CO

CO

932

933

Scheme 298

The Mn(0) complex [Mn2(CO)9(NCMe)] reacted with RC(S)NH2 (R ¼ Me, thioacetamide; R ¼ Ph, thiobenzamide) to produce the chelating carbamoyl complexes [Mn{k1:Z1-RC(S)NHCO}(CO)4] (934, R ¼ Me (a), R ¼ Ph (b)) (Scheme 299).529 A similar reaction occurred between the complex [Mn2(CO)9(NCMe)] and thpymSH (thpymS ¼ tetrahydropyrimidine-2-thionato) at 25  C to afford a mixture of the dinuclear complex [Mn2(CO)6(m-thpymS)2] (936) and the mononuclear carbamoyl complex [Mn(k1:Z1-SCNHC3H6NCO)(CO)4] (935).530

Scheme 299

Reactions of the cationic carbonyl complex [Cp ReI(CO)3]+ with aliphatic and aromatic primary amines produced the chelated carbamoyl complexes trans-[(Z5:Z1-C5Me4CH2NRC(]O))ReI(CO)2] (937, R ¼ Me, Pr, Ph, p-C6H4OMe) (Scheme 300).531 The reaction was proposed to proceed by initial nucleophilic addition of the amine to one of the CO ligands of the cation [Cp ReI (CO)3]+ to form the carbamoyl complex [CpReI(CONHR)(CO)2], followed by intramolecular cyclization (involving intramolecular nucleophilic attack of the N-carbamoyl to a carbon atom of a methyl group with simultaneous elimination of H2).

Scheme 300

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

125

Treatment of [MnBr(CO)5] with iPrPNNNP (938, iPrPNNNP ¼ N,N0 -bis(diisopropylphosphino)-2,6-diaminopyridine) yielded a mixture of the carbamoyl complex [Mn(CO)3(iPrPNNNCO)] (939) and [Mn(CO)3(iPrPNNNP)]Br (940) (Scheme 301). The carbamoyl complex 939 is likely formed by a formal loss of iPr2PBr and amide formation from the remaining pyridine “NH” fragment and CO.532 The reaction of the Re(I) carbonyl complex 941 with Br2 at room temperature afforded the Re(III) carbamoyl complex 942, which represents a rare example of rhenium complexes containing a three-membered ring acyl amide bond.533

Scheme 301

N-carbamoyl complexes could also be obtained by CdH bond activation processes. For example, the complex [Re2(CO)8{m,Z2-C(H)]C(H)nBu}(m-H)] (752) reacted with N,N-dimethylformamide (DMF) at 70  C for 6 h to give a mixture of the carbamoyl complexes [Re2(CO)8(m,Z2-O]CNMe2)(m-H)] (943), [Re2(CO)7(NHMe2)(m,Z2-O]CNMe2)(m-H)] (944) and [Re2(CO)9(NHMe2)] (Scheme 302). The compounds 943 and 944 are likely formed by elimination of hexene from 752 and oxidative addition of the formyl CdH bond of DMF to the dirhenium complex.534

nBu CO OC OC

H

CO

Re

Re

CO H

CO Me N 2

752

O

OC

NMe2 O CO

Re

hexene

CO CO +

Re

OC

CO CO

CO

CO CO

H 943

CO

OC

NMe2 O CO

Re

CO

Re

OC CO

H

NHMe2 CO

944

Scheme 302

6.01.6.1.4

Bora-acyl complexes

Re(I) bora-acyl complexes have been obtained by nucleophilic substitution reactions of metallaesters with lithiated boryl reagents. For example, treatment of the rhenium ester [Re{C(O)OMe}(CO)5] with boryllithium 945 generated the bora-acyl complex 946, which is thermally unstable and slowly evolves to the boryl complex 947 by decarbonylation (Scheme 303).535 The reaction of [Mn {C(O)OEt}(CO)5] with boryllithium 945 only led to the isolation of a boryl complex analogous to 947.

Scheme 303

126

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

6.01.6.1.5

Iminoacyl, imidate and related complexes LnMdC(]NR)R0

Iminoacyl complexes can be obtained by insertion reactions of isocyanide with metal alkyl complexes. For example, the ethyl complex [Mn(Et)(CN-Xyl)3(dmpe)] (36b) reacted with excess the isocyanide CN-2,6-C6H3Me2 to form the iminoacyl complex [Mn {C(]N-Xyl)CEt(]N-Xyl)}(CN-Xyl)3(dmpe)] (74, Xyl ¼ 2,6-C6H3Me2) (Scheme 25).30 Treatment of the Re(III) methyl complex [Re(Me)(DAAm)(CO)] (197, DAAm ¼ (C6F5NCH2CH2)2NMe) with the isocyanides RN^C (R ¼ Xyl (a), tBu (b)) resulted in insertion of the isocyanides into the RedMe bond to form the iminoacyl complexes 948 (Scheme 304).118 As a related synthesis, the Mn(I) boryl complex [Mn(BCl2)(PCy3)(CO)4] (949) reacted with tert-butylisocyanide at room temperature to give the iminoacyl complex 950, which has a relatively short MndC distance (2.013(4) A˚ ), implying the contribution of the cationic alkylidene/borate resonance structure 9500 to its structure.

Scheme 304

Iminoacyl complexes could also be obtained by nucleophilic addition to coordinated isocyanide ligands. For example, the isocyanide complexes [Re(CO)3(N-RIm)2(C^NtBu)]BArF4 (951, R ¼ Me, Mes; ArF ¼ 3,5-bis(trifluoromethyl)phenyl) reacted with KN(SiMe3)2 to afford the iminoacyl complexes 952 derived from the nucleophilic attack of the deprotonated imidazole moiety on the isocyanide sp-carbon (Scheme 305).536

Scheme 305

The imidate complex 953 has been obtained by the reaction of the isocyanide complex 930a with NaOMe (Scheme 306).537 A series of amidinate complexes have been obtained by deprotonations reactions of acyclic diaminocarbenes (e.g. formation of the amidinate complexes 955 from the diaminocarbenes 954).528,537,538

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

127

Scheme 306

There are other routes to obtain amidinate complexes. For example, the borylene complex 956 reacted with CyN]C]NCy to give the amidinate complex 957 (Scheme 307).539,540 The oxo complex trans-[ReOCl3(PPh3)2] reacted with 1-(1,3-benzothiazol-2yl)-3-benzoylthiourea (958, Hbbt) in methanol to give the cyclic amidinate complex [ReOCl2(bbt)(PPh3)] (959), which contains the monoanionic bidentate chelate N-((benzothiazol-2-ylamino)methylene)benzamide.541 The reaction of the bimetallic manganese(0) compound 960 with iPrN]C]NiPr produced the magnesium manganesio-amidinate complex 961 as a result of two-electron reduction of the substrate and its insertion into the MndMg bond of 960.542

NCy CyN

tBu

Mn B

C

C

NCy

956

Cl

PPh3 Cl Re Cl

S Cl HN C N N Re O Ph PPh3 959

S HN N NH

+ S O

O Cl S=PPh3

Ph

958 Mes SiiPr3

N Mn

N

N

CHPh2

iPrN

C

NiPr

Mes

iPr

N

N Mg

N Mes

Ph2HC 960

tBu

957

PPh3

Mg

B

OC OC

OC OC

O

NCy

Mn

SiiPr3 Mn

N

CHPh2

N Mes

Ph2HC

iPr 961

Scheme 307

Complexes closely related to iminoacyl complexes are complexes bearing an Z1-(C)-azolyl containing a C]N moiety within a five-membered ring. These complexes can often be obtained by deprotonation of coordinated azoles. For example, the reaction of fac-[Mn(N-phenylimidazole)(CO)3(dppe)]ClO4 (962) with KOtBu produced the imidazolyl complex 963 (Scheme 308).543 It should be noted that the imidazolyl ligand in the complex 963 can also be viewed as an NHC ligand containing a nonsubstituted nitrogen. A number of other imidazolyl544 (e.g. 964a,545 964b,452 964c546–548), and related oxazolyl (e.g. 965a), thiazolyl (e.g. 965b),549 benzimidazolyl (e.g. 966a), benzoxazolyl (e.g. 966b)550 and benzothiazolyl (e.g. 966c)543 complexes have been similarly obtained.

128

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 308

Azolyl complexes have been occasionally obtained by other routes. For example, benzoxazole-2-thiol (967) reacted with [ReOCl3(PPh3)2] to give the complex 968 as a result of CdS bond activation (Scheme 309).551 The Re(0) dinuclear complex [Re2(CO)8(MeCN)2] reacted with thiazoles to give the dirhenium compounds 969 bearing a bridging thiazolyl ligand.552 The complex [Re(CO)6]+ underwent a [2 + 3] cycloaddition reaction with the chromium nitrile imine [Cr{]C]NNH}(CO)5]− to give the 1,3,4-oxdiazolinyl complex 970.443 Deprotonation of benzoxazole and 1-methyl-benzimidazole by TMP2Mn
2MgCl2
4LiCl (TMP ¼ 2,2,6,6-tetramethylpiperidyl) to give the Mn(II) species Mn(azolyl)2 has also been described.382

O Cl

PPh3

PPh3 Cl Re + Cl PPh3

N

Cl N

air

SH

Cl Cl

O

Re O PPh3

967

S CO

R

S

N

OC N

[Re2(CO)8(MeCN)2]

Re

R CO CO Re

H

OC

N

CO

R

969

N C N NH

+ + [Re(CO)6]

S

CO

R = H (a), Me (b)

[(CO)5Cr

968

(OC)5Re O

NH Cr(CO)5

970 Scheme 309

6.01.6.2

Reactivity

The acyl oxygen is basic and can interact with Lewis or protic acids. For example, treatment of the acyl complex 899a with ZnCl2 produced the Zn-acyl complex 971 (Scheme 310).24 Protonation of the formyl complex 912 in thawing acetonitrile with pyridinium tetrafluoroborate generated the complex [Re(Z5-C5H4DMEGH)Re(CHO)(CO)(NO)]BF4 (972) (DMEG ¼ dimethylethyleneguanidine) in which the formyl oxygen is hydrogen bonded to a NdH proton.519 Similar reactions occurred between protonated pyridine derivatives and the formyl complexes 910 supported by a nitrogen functionalized phosphine ligands (e.g. protonation of 910a gives 973).518

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

129

Scheme 310

Addition of 5–20 equivalents of water to a CD2Cl2 solution of the acyl complex 70a produced acetaldehyde and the Re(I) complex 976 (Scheme 311). The reaction most likely proceeds through protonation of the acyl oxygen of 70a to afford the hydroxycarbene intermediate 974, followed by a 1,2-hydride shift and nucleophilic attack by iodide on Re.11

Scheme 311

Acyl complexes can undergo CO deinsertion reactions. Several examples of such reactions have been described in the section dealing with alkyl complexes. As an additional example, the acyl complex 70a evolved to the methyl complex 11 upon treatment with [12]crown-4 which sequestrates CaI2 (Scheme 312).11

130

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

O O Ph2P I

OC

O

N

Re C

OC

O

Ca O

CO

OC I

O

N

Ph2P

[12]crown-4

O

O I I O

+

Ca

Re

OC

O

Me

CO

O

CO 70a

977

11

Scheme 312

A metal-acyl bond could be cleaved by an electrophile, nucleophile, oxidizing agent or reducing agent. For example, the acyl complex 978 in THF reacted with trifluoroacetic acid to give initially [Cp∗ReBr(CO)2(THF)]CF3CO2 (979) and HCO2Me and eventually [Cp∗Re(CO)3] as a result of reduction of the complex 979 by formic acid derived from hydrolysis of HCO2Me (Scheme 313).523

Me

Re OC OC

C

Br

O

+ CF3CO2CF3CO2H

Re

THF

OC OC

978

O

+ HCO2Me

Br 979

Scheme 313

Under basic conditions (1 M NaOH), the N-carbamoyl complex 942 underwent reductive decarbonylation to give the bis-carbonyl complex [(tacn)ReBr(CO)2(tacn)] (980) (Scheme 314). In acidic media (1 M HBr), the complex 942 was partially converted to the monocarbonyl m-oxo bridged dinuclear complex {[ReBr(CO)(tacn)]2O}2+. The complex 942 reacted with H2O2 to give the trioxo complex [ReO3(tacn)]ReO4 (981).553

Scheme 314

The acyl complex [ReO{C(O)Me}(DAAm)] (242e, DAAm ¼ (C6F5NCH2CH2)2NMe) in benzene reacted with CO (60 psi, 80  C) to give the acetate complex [Re(O2CMe)(CO)(DAAm)] (243) (Scheme 315). A computational study reveals that the complex 243 is likely generated by migration of the acyl ligand to the terminal oxo.137

C 5F 5

C 5F 5

Re CO 243

Scheme 315

OAc

N

Re

N CO

O Me2PhSiH

N

Re

o

Re

N

OC

80 C, 8 h

N C6F5

O

CO (60 psi)

N

N

N

N

C5F5

C5F5

N

N 242e

C 6F 5

N C6F5

C6F5 982

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

131

Treatment of the acyl complex 242e with dimethylphenylsilane resulted in the formation of the dirhenium(II) complex 982 (although the fate of the acyl group was not described) (Scheme 315).140 The acyl complex [Mn(COMe)(CO)5] was reported to react with Me2PhSiH in C6D6 to give initially the alkyl complex [Mn{C(OSiMe2Ph)Me}(CO)5] and eventually a mixture of species include [MnH(CO)5] and [Mn(SiMe2Ph)(CO)5].554 The acyl complex [Mn(COMe)(CO)5] reacted with [Cu(tol)(IPr)] under an atmosphere of CO (3 atm) to give the ketone product 983 (Scheme 316).555 The acyl complex 916 reacted with NaBH4 to give the alcohol 984.520

Scheme 316

Amidinate complexes can undergo coupling reactions with alkynes. For example, the amidinate complex 955a reacted with alkynes RC^CH (R ¼ Ph, CO2Me) to give the coupled products 987 (Scheme 317). The complexes 987 might be formed by two successive insertion reactions of the alkynes into the carbon-metal bond of the amidinyl ligand in a head-to-tail fashion to give rise to the alkenyl intermediates 985 which undergo electrocyclic rearrangement to give the intermediates 986 followed by loss of the bipyridine ligand.538

R

N Ph

MeHN

NHMe

R

C N

R

CO Mn

N

CO

R = Ph (a), CO2Me (b)

CO

C

C N Mn N

Ph CO

CO 985

955a NHMe

R

Ph N R Mn OC

N

CO

R C

Ph

CO CO 987

N N

N

C NHMe N Ph CO

Mn N

CO CO

986

Scheme 317

Acyl, iminoacyl and amidinate complexes can also undergo other reactions. For example, they can react with electrophiles to give carbene complexes, as will be discussed in the section dealing with chemistry of carbene complexes. Acyl complexes have been suggested as reactive intermediate in catalytic reactions. For example, the complex [Mn2(CO)10] is catalytically active for stereospecific hydroxymethylation of alkyl tosylates with CO and NaBH4 (e.g. formation of 989 from 988) (Scheme 318).520 The active species of the system was proposed to be Na[Mn(CO)5] which can be generated by reduction of [Mn2(CO)10] with NaBH4. Na[Mn(CO)5] can react with an alkyl tosylate to form an alkylmanganese intermediate (e.g. 990), which undergoes migratory insertion of CO to give an acyl manganese complex (e.g. 991). The latter can react with NaBH4 to regenerate the active catalyst and to form an aldehyde. The aldehyde formed in this step can be further reduced to give the hydroxymethylation product.

132

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

[Mn2(CO)10] Ph

OTs + CO + NaBH4

Ph

OH 989

988

NaBH4 O Ph

Mn(CO)5

CO

Ph

Mn(CO)5 991

990

Scheme 318

The system (IPr)CuCl/Na[Mn(CO)5] catalyzed carbonylative CdC coupling of arylboronic esters with alkyl halides to give ketones (e.g. formation of 993 from 992, Scheme 319). The catalytic process was proposed to involve a Heck–Breslow cycle for alkyl halide carbonylation combined with a Cu cycle that generates catalytic quantities of an arylcopper species from an arylboronic ester nucleophile. The Mn-carbonyl co-catalyst activates the alkylhalide by formation of an alkyl intermediate (e.g. 994) which undergoes reversible carbonylation to generate an acylmanganese electrophile (e.g. 995).555 The final product was formed by the reaction of the Mn-acyl complex with arylcopper species (e.g. 996).

O

Na[Mn(CO)5]/(IPr)CuCl

O +

Ar B

R

I + CO

Ar KOMe

O

R 993

992 IPr-Cu Ar 996

O R Mn(CO)5

R

994

Mn(CO)5 995

Scheme 319

The PPN+ salts of [Mn(CO)4(L)]− (L ¼ CO, PPh3) are catalytically active for selective formation of cyclic carbonates (e.g. 998) from epoxides (e.g. 997) and CO2 under neat conditions (Scheme 320). The catalytic reaction was proposed to proceed through initial nucleophilic addition of the metal complex to CO2 to give the carboxylate intermediate 999, which reacts with an epoxide to give the intermediate 1000 bearing an alkoxide functionality. The latter undergoes an intramolecular SN2 nucleophilic substitution reaction to form the final carbonate product with concomitant regeneration of the starting anionic species.556 Carboxylate complexes like 1000 have also been suggested as intermediates in formation of cyclic carbonates from epoxides and CO2 catalyzed by the heterobimetallic complexes [CpRu(CO)(m-dppm)Mn(CO)4] and [Cp Ru(m-dppm)(m-CO)2Mn(CO)3] (dppm ¼ bis(diphenylphosphino)methane).557

PPN[Mn(CO)4(L)] L = CO, PPh3

O + CO2 R

997

998

Mn L 999

CO

O C O O

CO

OC

CO

OC

Scheme 320

O O

CO OC

O

R

R

Mn OC L 1000

C O O O

R

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

133

Re(I) and Mn(I) complexes of the type fac-[MX(CO)3(R2bpy)] (M ¼ Mn, Re; R2bpy ¼ 4,40 -disubstituted-2,20 -bipyridine; X ¼ halogen or solvent molecule with counter anion) are catalytically active for electrochemical reduction of CO2 to CO (Scheme 321).558 The catalytic reaction was suggested to involve [M(CO)3(R2bpy)]− species (1001) which binds CO2 and H+ to form the hydroxycarbonyl complex [M(CO2H)(CO)3(R2bpy)] (1002). This 18-electron M(I)dCO2H species can be reduced to give the formally 19-electron species 1003. In the presence of H+, the species 1003 can evolves to the neutral species 1004 which can be reduced and dissociates CO to regenerate [M(CO)3(R2bpy-R)]− species (1001). The complex [Re(COOH)(CO)3(Me2bpy)] (Me2bpy ¼ 4,40 -dimethyl-2,20 -bipyridine) has been successfully detected by means of cold-spray ionization spectrometry (CSI-MS).559,560

[MX(CO)3(R2bpy)] CO2 + 2H+ + 2e-

M = Mn, Re CO + H2O

R

N

CO M

N

e-

CO

R

CO

CO2, H+

CO 1001

R

O

R N

CO N

CO

N

M

CO CO

R

CO CO

R

OH

M

CO

N

C

1002 O

R

1004 H 2O

N

C

OH

-

e-

CO M

H+

N

CO 1003 CO

R Scheme 321

6.01.7

Carbene complexes

6.01.7.1

Carbene complexes without a hetero substituent at the carbene carbon

Well-characterized manganese carbene complexes without a hetero substituent at the carbene carbon are relatively rare and have only been isolated for a few low valent systems with metal fragments such as [Mn(CO)5]+ and [Cp0 Mn(CO)2] (Cp0 ¼ cyclopentadienyl and it derivatives). Rhenium carbene complexes are more common and have been isolated for both low and high valent systems. No technetium carbene complexes have been isolated in the review period.

6.01.7.1.1

Synthesis

6.01.7.1.1.1 By nucleophilic addition to carbyne complexes Manganese alkynylcarbene complexes of the type [Cp0 Mn{]C(R)C^CR0 }(CO)2] can be prepared by reactions of carbyne complexes [Cp0 Mn(^CR)(CO)2]+ with alkynyl lithium reagents LiC^CR0 . For example, treatment of the carbyne complex [(Z5-MeC5H4)Mn(^CPh)(CO)2]BPh4 (1005a) with alkynyl lithium reagents LiC^CR (R ¼ Ph (a), p-tolyl (b)) produced the carbene complexes [(Z5-MeC5H4)Mn{]C(Ph)C^CAr}(CO)2] (1006, Ar ¼ Ph (a), p-tolyl (b)) (Scheme 322). The carbene complex [(Z5-MeC5H4)Mn{]C(p-tolyl)C^CPh}(CO)2] (1006c) has been similarly prepared from the reaction of [(Z5-MeC5H4)Mn(^C-p-tolyl)(CO)2]BPh4 (1005b)with LiC^CPh.561 + BPh4-

R Mn OC OC

Li

C Ph

1006: R = Ph (a), p-tolyl (b)

Scheme 322

Li

R

Ar = Ph

Ph

Mn OC OC

Ph

Mn

C Ar Ar = p-tolyl

1005: Ar = Ph (a), p-tolyl (b)

C

OC OC

p-tolyl

1006c

134

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

6.01.7.1.1.2 By metathesis reactions of borylene complexes with carbonyl compounds Several manganese complexes of the type [CpMn(]CRR0 )(CO)2] have been obtained by metathesis reactions of the terminal alkylborylene complex [CpMn{]B(tBu)}(CO)2] (956) with ketones via a sequential cycloaddition and cycloreversion process (Scheme 323).562,563 For example, the complex [CpMn{]B(tBu)}(CO)2] (956) reacted with benzophenone to give the cycloadded complex [CpMn{k2-(B,C)-B(tBu)OCPh2}(CO)2] (1007), which loses (tBuBO)3 to give the carbene complex [CpMn(]CPh2) (CO)2] (1008a). The complexes 1008b–f are additional examples of carbene complexes made through this strategy. O

Mn B

Ph

tBu

Ph

Ph

C

Ph

C

Ph

OC OC

B

OC OC

956

OC OC

(tBuBO)3

tBu

CF3

Ph

Mn C

1008b: R = Me 1008c: R = NMe2

1008d

OC OC

CF3

OC OC

R

Mn C

CF3

Mn C

OC OC

Ph

1008a

1007 R

C

Mn

O

Mn

CF3

Mn C OC OC

Ph

1008e

1008f

Scheme 323

6.01.7.1.1.3 By reactions of quinolizinium salts The manganese benzo[c]quinolin-6-ylidene complex 1010 has been synthesized by the reaction of Na[Mn(CO)5] with 6-trifluoromethanesulfonylbenzo[c]quinolizinium trifluoromethanesulfonate (1009) at low temperature (−78  C) (Scheme 324).564

OTf-

N N+

+

Na[Mn(CO)5] OC

OTf

CO Mn

OC CO

1009

OTf-

CO 1010

Scheme 324

6.01.7.1.1.4 By activation of halocarbons Several rhenium nitrosyl carbene complexes of the type [ReX2(]CHR)(NO)(PR0 3)2] have been obtained by reactions of halocarbons with Re(I) hydride complexes [Re(Br)(H)(NO)(PR0 3)2]. For example, treatment of the Re(I) hydride complexes [Re(Br)(H) (NO)(PR3)2] (R ¼ Cy (a), iPr (b)) with dibromomethane produced a mixture of the 17-electron mononuclear Re(II) hydride complexes [Re(Br)2(H)(NO)(PR3)2] and the carbene complexes [Re(]CH2)(Br)(H)(NO)(PR3)2] (1012) in about 2:1 M ratio, as a result of activation of the CdBr bonds by single-electron transfer (Scheme 325). The carbene complexes 1012 are likely formed via H

Br

H

H CHBr3

Br R3 P ON

Re

H

PR3

R3 P ON

PR3 + R3P ON

Re

CH2Br2

1011 H

R3P

CH2Br H Re PR3 Br 1013

R3P ON

Scheme 325

Re

Re Br 1012

H Br

R3P ON

H

H H

ON

PR3

Br PR3

Br

Br R = Cy (a), iPr (b)

Re

PR3 + R3P ON

Re Br

Br PR3

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

135

the intermediates [Re(CH2Br)(Br)(H)(NO)(PR3)2] (1013) which undergo bromide abstraction by [Re(Br)(H)(NO)(PR3)2]. Similarly, bromoform reacted with [Re(Br)(H)(NO)(PR3)2] (R ¼ Cy, iPr) to give a mixture of the Re(II) hydride complexes [Re(Br)2(H) (NO)(PR3)2] and the carbene complexes [Re(]CHBr)(Br)(H)(NO)(PR3)2] (1011).565 6.01.7.1.1.5 By protonation of vinyl complexes Protonation of vinyl complexes could lead to carbene complexes. Formation of carbene complexes by protonation of the rhenium(I) vinyl complexes [ReBr(CH]CHR0 )(NO)(PR3)2] (R ¼ Cy (a), iPr (b)) has been described in the section dealing with the reactivity of vinyl complexes.433 As an additional example, the cationic carbene complex [Cp(NO)(PPh3)Re(2,5-dimethyl-2thiophene-3-ylidene)]+ (1015) has been made by protonation of the thiophenyl complex [Cp(NO)(PPh3)Re(2,5-Me2C4HS-3)] (1014, C4HS-3 ¼ 3-thienyl) (Scheme 326).566 + BF4HBF4

Re Ph3P

Re Ph3P

C

C ON

ON S

H

1015

1014

S

Scheme 326

6.01.7.1.1.6 By electrophilic abstraction of alkyl complexes Electrophilic abstraction of alkyl complexes represents another route to carbene complexes. For example, methyl complexes of the type [Cp0 Re(Me)(NO)(PPh3)] (Cp0 ¼ cyclopentadienyl derivatives) can undergo electrophilic abstraction reactions with Ph3CBF4 to give carbene complexes [Cp0 Re(]CH2)(NO)(PPh3)]BF4.35,54

6.01.7.1.1.7 By reactions of diazoalkanes Reactions of diazoalkanes with coordinatively unsaturated complexes represent one of the most convenient routes to carbene complexes. For example, the rhenium-benzylidene bis(nitrosyl) complexes [Re(]CHPh)(NO)2(PR3)2]BArF4 (1016, R ¼ Cy (a), iPr (b); ArF ¼ 3,5-bis(trifluoromethyl)phenyl) can be made by the reactions of phenyldiazomethane with [Re(NO)2(PR3)2] BArF4 (Scheme 327).567 Treatment of the carbene complex 1016b with dioxygen resulted in the oxidation of one of the nitrosyl ligands into an Z2-nitrito ligand to give the carbene complex [Re(]CH2)(NO2)(NO)(PiPr3)2]BArF4 (1017), which can be further oxidized to give the nitrato complex [Re(]CH2)(NO3)(NO)(PiPr3)2]BArF4 (1018).568 Interestingly, the tricyclohexylphosphine derivative [Re(]CHPh)(NO)2(PCy3)2]BArF4 (1016a) does not react with oxygen under a similar condition.

+ PR3 ON

Ph

ON PhCHN2

Re

BArF4-

Re ON

ON R = Cy (a), iPr (b)

PR3

+

PR3

BArF4-

H PR3 1016

PR3 = PiPr3 O2 PiPr3 NO O N O O

Ph

Re H PiPr3 1018

Scheme 327

+

PiPr3 NO

BArF4-

Re

O

O2 N

+ Ph H

O PiPr3 1017

BArF4-

136

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

6.01.7.1.1.8 By activation of alkenes Carbene complexes can also been obtained by activation of alkenes. For example, the reaction of [TpRe(CO)(MeIm)(Z2-benzene)] (1019, MeIm ¼ N-methylimidazole) with 2-methoxyfuran produced the carbene complex 1022, presumably through intermediates 1020 and 1021 (Scheme 328).569

Scheme 328

Carbene complexes of the type [ReH2(cycloalkylidene)(PNPR)] bearing a pincer PNP-tridentate ligand have been obtained by reactions of the operationally unsaturated complexes [ReH4(PNPR)] (1023; PNPCy ¼ (Cy2PCH2SiMe2)2N−, PNPPr ¼ (iPr2PCH2SiMe2)2N−) at 22  C with cyclic olefins (Scheme 329). For example, the complexes 1023 reacted with cyclohexene to give the carbene complexes 1024, which possesses a b-agostic cyclohexylidene ligand. Analogous complexes (e.g. 1025–1027) have been obtained by the reactions of the hydride complexes 1023 with cyclopentene, cyclooctene, norbornene and cyclododecene.570

Scheme 329

Rhenium is known to form well-defined Re(VII) d0 carbene complexes as documented in previous COMCs, although few studies have been carried out with these complexes in the review period. Re(VII) d0 carbene complexes have been suggested as reactive intermediates in catalytic olefin metathesis reactions mediated by Re(VII) oxo complexes such as [MeReO3], [ReO3(Ph)], [ReO3(C6F5)(THF)] and [ReO3{3,5-(CF3)2C6H3}]. Calculations show that the active carbene species is likely formed via a pseudo-Wittig mechanism, which involves [2 + 2] cycloaddition to form a metallaoxetane that undergoes retro-cycloaddition to give a rhenium carbene (e.g. 1030) and an aldehyde as illustrated in Scheme 330 for the formation of 1030.229,571 The heterogeneous system Re2O7/Al2O3 catalyzes the self-metathesis of (Z)-stilbene and its cross-metathesis reaction with ethane. The necessary carbene species was also proposed to be formed via a pseudo-Wittig reaction.572

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

X O +

Re O

X

X O

Re

O

O

O

O

O

+

Re

CH2

O

1028

137

CH2

1030

1029

Scheme 330

6.01.7.1.1.9 Preparation of supported carbene complexes In the review period, efforts have been made to prepare heterogeneous systems by anchoring Re(VII) carbene species to solid supports with surface organometallic chemistry. Grafting of the carbene complex 1031 on silica (SiO2) (1032) yielded a dark red material containing [(^SiO)Re(O)(]CHdCH]CPh2)(OtBuF6)2(THF)] (1033a, tBuF6 ¼ CMe(CF3)2) as the main surface species and [(^SiO)2Re(O)(]CHdCH]CPh2)(OtBuF6)(THF)] (1033b) as the minor species (Scheme 331).573 These species do not show significant olefin metathesis activity. Grafting of the carbene complex 1031 on Al@SiO2 (1034) yielded a dark purple solid containing [(^SiO)Re(O)(]CHdCH]CPh2)(OtBuF6)2(THF)/Al] (1035) as the surface species, which shows good olefin metathesis activity. O

OH Si 1032 O O (SiO2)

O THF

Ph +

Re

O

Si O

Ph

THF O

tBuF6O O

OtBuF6 Ph

Re

tBuF6O

THF

O

O

OtBuF6 Ph

Re

O

Ph

O

O Si

Si

O O

O

OtBuF6 Ph

O

O 1033b

1033a

OtBu OtBuF6 OtBuF6 Ph

1031

O

O Si

O

O

O

O

Si O

Si

THF

O

O

Al

O O

O

Re

O

Si O

Al O

O

O Si O

Si 1034 (Al@SiO2)

Ph

O

1035

Scheme 331

6.01.7.1.2

Reactivity

6.01.7.1.2.1 Migratory insertion reactions Carbene complexes of the type [LnM(R)(carbene)] (R ¼ H, alkyl) can undergo reductive elimination or migratory insertion reactions. For example, the hydridocarbene complex 1024a reacted with cyclohexene to give 1036 and cyclohexane (Scheme 332).570

PiPr2 Si N Si H

H Re H PiPr2 1024a

Scheme 332

PiPr2 Si N

Re

Si H

H PiPr2 1036

138

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

The cyclomanganated complex 1037 reacted with the diazoalkane (Ph)(FcCH2CH2)CN2, which is a precursor of electrophilic alkylidene, to afford the syn-facial heterobimetallic benzyl complex 1039, presumably via migratory insertion reaction of the carbene intermediate 1038 (Scheme 333).337 Additional examples of carbene insertion reactions can be found in the section dealing with reactivity of cyclometalated aryl complexes. (CO)3 Cr

Cr(CO)3

Cr(CO)3 FcH2CH2C C

Ph

Mn(CO)4

CH2CH2Fc Ph

CH2CH2Fc

N2

Mn(CO)3 N

N

Ph

N

1039

1038

1037

Mn(CO)3

Scheme 333

6.01.7.1.2.2 Deprotonation reactions b-Hydrogen of alkyl carbene complexes can be deprotonated. For example, the cationic carbene complex [Cp(NO)(PPh3)Re(2, 5-dimethyl-2-thiophene-3-ylidene)]BF4 (1015) can react with KOH to give the thienyl complex [Cp(NO)(PPh3)Re(2,5-Me2C4HS-3)] (1014) (Scheme 334).566 It has been estimated that the carbene complex 1015 has a pKaCH value of 2.64 in CH3CN.

+ BF4Re Ph3P

Re

KOH

C

Ph3P

1015

H

C ON

ON

S

S 1014

Scheme 334

6.01.7.1.2.3 Nucleophilic addition reactions Carbene complexes can undergo nucleophilic addition reactions. The carbene complexes [(Z5-MeC5H4)Mn{]C(Ar)C^CAr0 } (CO)2] (Ar, Ar0 ¼ Ph or Tol) were reported to react with phosphines via two apparently competing pathways. For example, the carbene complex [(Z5-MeC5H4)Mn{]C(Ph)C^CR}(CO)2] (1006, R ¼ Ph (a), p-tolyl (b)) reacted with PPh3 to afford the Z1-allenylphosphonium complexes [(Z5-MeC5H4)Mn{C(Ph)]C]C(PPh3)R}(CO)2] (1040) as a result of nucleophilic attack on the remote alkynylcarbon atom (Scheme 335). In contrast, the more basic PMe3 attacks on the carbene carbon to afford the s-propargylphosphonium complexes [(Z5-MeC5H4)Mn{C(Ph)(PMe3)C^CR}(CO)2] (1041), which are extremely unstable in solution, evolving within minutes at 25  C to give the s-allenylphosphonium complexes (1042). Heating 1042 in refluxing CH2Cl2 for 2 h produced the s-dihydrophospholium complexes [(Z5-MeC5H4)Mn{C]C(Ph)PMe2CH2CH(R)}(CO)2] (1043).561 Similar reactions have been described for the carbene complexes [(Z5-MeC5H4)Mn{]C(p-tolyl)C^CPh}(CO)2]. R R Mn

C

OC OC

C PPh3

Ph

PPh3

C Mn OC OC

1006

C Ph

1040 R = Ph (a), p-tolyl (b)

PMe3 Ph R

C

R

PMe3

C Mn

C PMe3

OC OC

Ph 1041

Scheme 335

Mn C OC R OC 1042

Mn OC OC

P Ph

1043

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

139

Reactions of alkynylcarbene complexes [(Z5-MeC5H4)Mn{]C(R)C^CR0 }(CO)2] with secondary phosphines in general give a mixture of Z4-vinylketene and Z2-allene complexes.561 For example, the alkynylcarbene complex [(Z5-MeC5H4)Mn{]C(p-tolyl) C^CPh}(CO)2] (1006c) reacted with HPPh2 to produce a mixture of the Z4-vinylketene complexes [(Z5-MeC5H4)Mn {Z4-Ph2P(Ph)C]CHC(p-tolyl)]C]O}(CO)] (1046, 4%) and [(Z5-MeC5H4)Mn{Z4-Ph2P(p-tolyl)C]CHC(Ph)]C]O}(CO)] (1050, 4%), and the Z2-allene complex [(Z5-MeC5H4)Mn{Z2-Ph2P(p-tolyl)C]C]C(Ph)H}(CO)2] (1051, 53%) (Scheme 336). The complex 1046 is likely formed via initial nucleophilic attack of the secondary phosphine on the remote alkynylcarbon atom Cg, whereas the complexes 1050 and 1051 were from a nucleophilic attack on the carbene carbon. HPCy2 behaved similarly except that the nucleophilic attack by the bulkier phosphine takes place preferentially at the less hindered remote alkynylcarbon atom. Treatment of the carbene complex [(Z5-MeC5H4)Mn{]C(tolyl)C^CPh}(CO)2] with LiPR2 (R ¼ Ph, Cy) followed by protonation gave the same products.

Ph C

PHPh2

C

C C

Mn OC OC

Ph

OC OC

p-tolyl

C

C

Mn

PPh2 Ph

PPh2

H

Mn OC

p-tolyl

C

1045

1044

O

C p-tolyl 1046 Ph

Ph

Ph

C Mn

+ PHPh2

C

OC OC

Mn

C

p-tolyl

Mn C OC p-tolyl Ph P 2 OC

PHPh2 p-tolyl

OC OC 1047

1006c

1051

p-tolyl C

PHPh2

C

C

OC OC

C

Mn OC OC

Ph 1048

PPh2 p-tolyl

PPh2

H

C Mn

H

C

C

Mn

p-tolyl

OC

Ph

C O

1049

C Ph 1050

Scheme 336

Alkynylcarbene complexes of the type [Cp0 Mn{]C(R)C^CR0 }(CO)2] can also undergo nucleophilic addition reactions with enolates and thiolates. For example, treatment of the alkynylcarbene complex 1006c with cyclohexanone lithium enolate followed by protonation afforded the Z2-allene complex [(Z5-MeC5H4)Mn{Z2-H(p-tolyl)C]C]C(Ph)R}(CO)2] (1052, R ¼ 2-oxocyclohexyl), resulting from a nucleophilic attack on the Cg carbon in 1006c (Scheme 337). Treatment of [(Z5-MeC5H4)Mn{]C(p-tolyl) Ph OLi

Ph

C

1)

C

OC OC

2) NH4Cl

p-tolyl

C

OC OC

1052 SLi

1) 2) NH4Cl

Ph Ph C

C

C H

C S

Mn OC OC

C H

1054 1053

S

C +

Mn OC OC p-tolyl

p-tolyl

H

1006c

Scheme 337

O

Mn

C

Mn

p-tolyl

140

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

C^CPh}(CO)2] (1006c) with lithium p-toluenethiolate at −80  C followed by protonation with NH4Cl gave the Z2-allene complexes 1053 and 1054 in a 1:2.3 M ratio.574 Carbene complexes of the type [Cp0 Re(]CH2)(NO)(PPh3)]+ (Cp0 ¼ cyclopentadienyl derivatives) readily undergo nucleophilic addition reactions with amines and phosphines. The chemistry has been used to prepare rhenium-containing phosphine and amine ligands from [Cp0 Re(]CH2)(NO)(PPh3)]+ usually generated in situ from the reactions of [Cp0 Re(Me)(NO)(PPh3)] with Ph3CBF4 or Ph3CPF6.35,54 For example, treatment of [CpRe(]CH2)(NO)(PPh3)]PF6 (generated by the reaction of [CpRe(Me) (NO)(PPh3)] with Ph3CPF6) with both primary and secondary amines gave [CpRe(NO)(PPh3)(CH2NHRR0 )]PF6 (1055, e.g. R/R0 ¼ H/CH2Ph, H/CH2(CH2)2CH3, Et/Et, nBu/nBu, CH2Ph/CH2Ph;Me/CH2CH2OH). Deprotonations of 1055 with tBuOK afforded the corresponding amines [CpRe(NO)(PPh3)(CH2NRR0 )] (1056) (Scheme 338).46 + PF6-

+ PF6- NHRR' Re Ph3P

Ph3P

CH2

ON

Re

tBuOK Re Ph3P

CH2 NHRR'

ON

CH2 NRR'

ON

1056

1055

R/R' = H/CH2Ph, H/CH2(CH2)2CH3, Et/Et, nBu/nBu, CH2Ph/CH2Ph;Me/CH2CH2OH Scheme 338

Treatment of the carbene complex [CpRe(]CH2)(NO)(PPh3)]X (X ¼ PF6 or BF4) with R2PH (R ¼ Ph, tBu) gave the cationic complexes [CpRe(NO)(PPh3)(CH2PHR2)]X (1057), which can be deprotonated by tBuOK to yield the neutral phosphine derivatives 1058 (Scheme 339).575 Analogous compounds supported by a Z5-C5H4Br or Z5-C5H4I ligand have been made similarly from the corresponding methyl complexes.35 These species are effective ligands for Pd-catalyzed Suzuki and Heck coupling reactions.576–579

+ XRe Ph3P ON

CH2

X = PF6 or BF4

+ XPHR2

Re Ph3P ON

R = Ph (a), tBu (b)

1057

CH2 PHR2

tBuOK

Re Ph3P ON

CH2 PR2

1058

Scheme 339

Reactions of the enantiopure complex (SM)-[CpRe(]CH2)(NO)(PPh3)]PF6 with PH2(CH2)xPH2 (1059, x ¼ 2, 3) followed by deprotonation gave the metal-chiral tetrarhenium diphosphines 1060 (Scheme 340). Complexation of 1060 with [Rh(NBD)2] PF6 gave the rhodium complexes [Rh(NBD)(L2)]PF6 (1061, L2 ¼ 1060). The complexes 1061 can catalyze hydrogenations of protected dehydroamino acids and hydrosilylations of propiophenone with modest enantioselectivities. The ligands 1060 have also been applied in rhodium-catalyzed conjugate addition reactions of aryl boronic acids and palladium-catalyzed allylic alkylation reactions.54

Scheme 340

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

141

The carbene complexes [Re(]CHPh)(NO)2(PR3)2]BArF4 (1016, R ¼ Cy (a), iPr (b)) in chlorobenzene solution rearranged at 60 C to give the ylide complexes 1062 (Scheme 341).567 The reaction can be viewed as phosphine migration onto a coordinated carbene carbon or intramolecular attack of phosphine on the carbene carbon. The cationic benzylidene complexes [Re{CHPh}(NO)2(PR3)2]BArF4 (1016, R ¼ Cy (a), iPr (b)) undergo unusual coupling 

+

PR3 Ph

ON

H

BArF4-

ON 60 oC

Re ON

PR3

Re ON

H PR3

PR3

R = Cy (a), iPr (b)

1016

1062

Scheme 341

reactions with MeCN to give consecutively the complexes [Re(NCCH3)(NCPh)(NO)(OCMe]NH)(PR3)2]BArF4 (1065) and [Re(NCCH3)(OCMe]NdCPh]NH)(NO)(PR3)2]BArF4 (1067) (Scheme 342). The reaction was proposed to proceed through initial coupling of the nitrosyl and the benzylidene ligand (an intramolecular nucleophilic addition reaction) to give the oximate species 1064, followed by a heteroene reaction of the oximate with MeCN to yield 1065, and cyclization (to give 1066) and proton shift to give 1067.580

PR3 Ph

ON

+ BArF4-

NO CH3CN

Re ON

PR3 H3CCN

H PR3

H

ON Re H3CCN

H

ON

1016

Ph

Re

+

ON Re H3CCN

H N O

PR3 1067

Ph N CH3

BArF4-

BArF4-

N O

PR3

PR3

1063

1064

R = Cy (a), iPr (b)

PR3

+

PR3 Ph

+ BArF4-

CH3CN +

PR3 Ph

ON Re H3CCN

N O

PR3 1066

NH CH3

+

PR3

BArF4-

Ph

ON Re H3CCN

N O

PR3

BArF4-

NH CH3

1065

Scheme 342

Treatment of the (methoxy)methyl complexes [M(CH2OCH3)(CO)3(bpy)] (M ¼ Mn (23), Re (24)) with methyl triflate in the presence of SMePh produced the alkylidenesulfonium complexes [M(CH2SPhMe)(CO)3(bpy)]OTf (1068) (Scheme 343). In the presence of styrene, the reaction produced phenylcyclopropane and the corresponding triflate complexes [M(OTf )(CO)3(bpy)] (M ¼ Mn (55), Re (56)).23 These reactions could proceed through the electrophilic carbene intermediates [M(]CH2)(CO)3(bpy)]+ or cationic alkylideneoxonium complexes [M{CH2OMe2}(CO)3(bpy)]+.23

Scheme 343

142

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Mn(ClO4)2 catalyzed the coupling reactions of secondary amides, tosylamine with a-diazo esters to give N-tosyl amidates (e.g. formation of 1069, Scheme 344).581 The transformation was proposed to involve an electrophilic manganese carbene species LnMn]CHCO2Et that can be attacked by amide intermediates.581 O O

O R1

+

2 NH R

R

3

S

O NH2

+

N2

Mn(ClO4)2

R

3

OEt

R2

S

N

O

N

OEt

O

O

R1 1069

Scheme 344

6.01.7.1.2.4 Metathesis and cycloaddition reactions with alkenes Carbene complexes can undergo metathesis reactions with olefins. The reactions have been demonstrated for both low valent and high valent rhenium carbene complexes. The cationic complexes [Re(NO)2(PR3)2]BArF4 (R ¼ Cy (a), iPr (b)) are catalytically active for ring-opening metathesis polymerization (ROMP) of strained cyclic olefins such as norbornene to give polymers (e.g. 1070, Scheme 345) with relatively high polydispersity indices. DFT analysis suggests that bis(nitrosyl) rhenium carbene complexes 1074 are the likely species responsible for the catalytic reactions. In the reaction of norbornene, the carbene complexes 1074 can be generated by initial formation of the olefin complexes [Re(NO)2(alkene)(PR3)2]BArF4 (1071) followed by phosphine addition to the C]C double bond and CdC bond cleavage as illustrated by the formation of the complexes 1074 from the olefin complexes 1071.567 The carbene complex 1074 can react further with norbornene to give polymers through typical metathesis reactions. It is noted that the carbene complexes [Re(]CHPh)(NO)2(PR3)2]BArF4 (R ¼ Cy (a), iPr (b)) are almost inactive in ROMP catalysis with norbornene and in olefin metathesis.567

[Re(NO)2(PR3)2]BArF4n 1070 + ON

ON

+

+

PR3

PR3

Re

Re

Re

R = Cy (a), iPr (b) ON

ON PR3

R3P

ON ON PR3

PR3

1071

1072

+

+

R3P

R3P ON

ON Re n

1070

ON

Re ON

PR3

PR3

1074

1073

Scheme 345

Olefin metathesis reactions of high valent d0 Re(VII) carbene complexes are well-documented reactions, and have attracted continuing interest in the review period. The complex [ReO2(CH2tBu)(]CHtBu)] (1075) is catalytically active for the ROMP reaction of norbornene, but has a low activity for typical olefins such as 1-hexene. Interestingly, the complex [ReO2(CH2tBu)(] CHtBu)] (1075) reacted with B(C6F5)3 to afford the adduct [(C6F5)3BdO]Re(O)(CH2tBu)(]CHtBu)] (1076), which can undergo an metathesis reaction with 1-hexene readily to give the carbene complexes 1077 and 1078, and display enhanced catalytic activity (Scheme 346).229

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

143

Scheme 346

The silica supported Re-based catalyst, [(^SiO)Re(^CtBu)(]CHtBu)(CH2tBu)], displays high activity in propene metathesis at low temperatures without the need of co-catalysts. The system also displays alkane metathesis activities, albeit with a poor performance.582 The structure, spectroscopic and electronic properties of this highly active system have been studied by DFT calculations and NMR.245,246,583,584 The reaction pathways accounting for metathesis reactions mediated by the heterogeneous catalyst [(^SiO)Re(^CtBu)(]CHtBu)(CH2tBu)] have also been investigated by DFT calculations.244,246,585 It was found that the alkene metathesis is a four-elementary step reaction involving coordination of the alkene, [2 + 2]-cycloaddition to generate metallacyclobutanes and the corresponding reverse steps. The heterogeneous catalyst [(^SiO)Re(^CtBu)(]CHtBu)(CH2tBu)] showed a relatively fast deactivation and produced 1-butene as a primary side product in the metathesis reaction of propene. DFT calculations suggest that the byproduct formation and deactivation can occur via b-H elimination of metallacyclobutane intermediates.243 Computations study shows that the carbene complex [MeReO2(]CH2)] can undergo a [2 + 2] cycloaddition reaction with CH2]CH2 to yield the rhenacyclobutane [MeReO2(dCH2CH2CH2d)] in an exothermic process.586 There are catalytic reactions that were proposed to involve rhenium vinylcarbene intermediates generated from vinylidene species. These materials will be described in detail in the section dealing with the chemistry of vinylidene complexes.

6.01.7.2

Carbene complexes with one oxygen substituent at the carbene carbon

Well-characterized group 7 oxygen substituted carbene complexes have been described for those with a Mn(0), Mn(I), Re(0), Re(I) or Re(V) center. The oxygen substituents can be OR (R ¼ alkyl), OH, OTiClCp and OBR−3. These complexes can be made by several routes as will be described below.

6.01.7.2.1

Synthesis

6.01.7.2.1.1 By Fischer synthesis Fischer synthesis represents the most common route to low valent group 7 metal alkoxycarbene complexes. A number of Mn(0) and Re(0) alkoxycarbene complexes have been obtained from [M2(CO)10] by Fischer synthesis. For example, treatment of the bimetallic decacarbonyl complex [Mn2(CO)10] with lithiated thiophene or furan in THF at low temperature followed by Et3OBF4 produced the bimetallic carbene complexes 1079a and 1079b, respectively (Scheme 347).587 Analogous reactions with [Re2(CO)10] produced the carbene complexes 1080. It is interesting to note that the carbene ligands are at an axial position in the dimanganese complexes 1079, but are at an equatorial position in the dirhenium complexes 1080. It has been suggested that, for dinuclear complexes of the type [(OC)5MdM(CO)4(carbene)], the equatorial position is the electronically favored position for the carbene ligands. Steric effects can force a carbene ligand to occupy an axial position. The steric constraint is much more pronounced for manganese complexes compared to rhenium complexes because a MndMn bond is in general shorter compared to a RedRe bond.

Scheme 347

144

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

As illustrated in Scheme 348, dinuclear Fischer carbenes with moieties such as bithienyl (e.g. 1081, 1082),588 thieno[2,3-b] thiophenyl (2,3-b-TTH), thieno[3,2-b]thiophenyl (e.g. 1083),589 [(Z5-C5H4-)Mn(CO)3] (e.g. 1084, 1085)590 and ferrocenyl (e.g. 1086)591 have been prepared similarly. The methodology can also be applied for synthesis of tetranuclear complexes (e.g. 1087,589 1088,592 and 1089) from reactions of the corresponding dilithiated reagents.

S CO CO OEt Mn Mn OC S OC OC CO CO CO

EtO

CO

S

Re

Re

CO

CO Mn

OC

OC

OC CO

OC OC

Re

Re

CO

CO

CO

EtO

Re

CO

Mn(CO)3 Re

CO CO

CO Re

OC

OC

CO

CO 1085

1084

CO CO OEt Re Fe

OC

OC

CO

CO 1083

CO Mn(CO)3 OC OC

CO CO

OC CO

1082

CO CO OEt Mn

CO

CO

CO CO

OC

1081

EtO

CO

CO OC OC

S

S

S

CO

CO 1086

CO OC OC Re2(CO)9

Re

OC OC

OEt

S OEt

CO

S

S

EtO

CO

n 1087:n = 1 (a), 2 (b) (OC)5Re OC

Re CO

CO CO

Re2(CO)9

EtO

CO Re(CO)5

EtO

OC OC

Re

CO CO

OEt

CO

CO CO

OC CO

1088

Re

Re OC

Fe

CO

CO CO

Re

CO

1089

Scheme 348

M(0) alkynyl-alkoxycarbene can be prepared similarly. For example, treatment of [Re2(CO)10] with LiC^CTMS followed by Me3OBF4 resulted in the Fischer type alkynylcarbene complex [(CO)5Re(CO)4Re(]C(OMe)C^CTMS)] (1090) (Scheme 349). The complex 1090 reacted with HNMe2 at −100  C to give the vinylcarbene complex [(CO)5Re(CO)4Re(]C(OMe)CH]C(NMe2) TMS)] (1091), which could be desilylated to form the carbene complex [(CO)5Re(CO)4Re(]C(OMe)CH]C(NMe2)H)].593

TMS

TMS 1) TMS

MeO

MeO Li

[Re2(CO)10] 2) Me3OBF4

(OC)5Re OC

- 78 oC 1090

CO Re CO CO

NHMe2 - 100 oC

(OC)5Re OC 1091

NMe3 CO Re CO CO

Scheme 349

Electrophilic reagents such as HBF4 and titanocene dichloride instead of typical electrophilic alkylating reagents have been used in Fischer synthesis to give related carbene complexes. For example, treatment [Re2(CO)10] with mono lithiated thiophene and bithiophene followed by HBF4 produced the carbene complexes 1092 featuring a hydroxycarbene fragment connected to an acyl fragment via an OdH⋯ O hydrogen bond and a RedHdRe bond linking the two Re centers (Scheme 350).592

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

R

R S

R

1)

S

S CO

Li

[Re2(CO)10]

CO

OC 2) HBF4

145

OH

Re

CO

O

Re CO

OC R = H (a), 2-thienyl (b)

CO

H

CO

1092 Scheme 350

The reaction of FcLi (Fc ¼ ferrocenyl) with [Re2(CO)10] in THF followed by titanocene dichloride produced a mixtures of species, including the Re(0) monocarbene complex ax-[Re2(CO)9{C(OTiCp2Cl)Fc}] (1093), the Re(I) carbene complex fac-[(Μ-Cl)2(Re(CO)3{C(OEt)Fc})2] (1095), and the unique hydroxycarbene-acyl complex [(m-H)(Re(CO)4{]C(OH)Fc}) {C(O)Fc}] (1094) (Scheme 351). The reaction of FcLi2 with [Re2(CO)10] in THF followed by titanocene dichloride also produced a mixtures of species, including ax,ax-[{m-TiCp2O2}{m-Fe(C5H4)2}{CRe2(CO)9}2] (1097) and eq-[Re2(CO)9{C(OTiCp2Cl) (Fc-CHO)}] (1096).591

OH Fe OC

O

Fe

CO OC

CO

Re

OEt EtO

Fe OC

+

H

Re

OC

CO

CO

CO

Re

Re

OC

Fe

Cl Cl

CO

CO 1095

CO 1094

CO

+ CO CO

1) FcLi

[Re2(CO)10]

OC 2) Cp2TiCl2, H2O

Re OC

OC CO

TiClCp2 CO CO O Re Fe CO

1) FcLi2

1093

2) Cp2TiCl2, H2O CO

Cp2ClTi CO OC

O Fe CO

Re

OC

CO

CO Re CO

+ OC

CO CO CO O Re Re OC CO CO

OC CHO

CO OC OC O OC Re Re

CO

CO CO CO CO

Fe

OC CO

Cp2 Ti

1097

1096

Scheme 351

The Fischer methodology has also been successfully used to prepare carbene complexes of Mn and Re using cymantrene and cyclopentadienyl rhenium tricarbonyl as the starting synthons. For example, the Fischer carbene complexes [CpMn{]C(OEt)Ar} (CO)2] (1098, Ar ¼ thienyl (a), furyl (b)) and biscarbene complexes [Cp(CO)2Mn]C(OEt)-Ar0 -(OEt)C]Mn(CO)2Cp] (Ar0 ¼ 2,5-thienylene (1099), 2,5-furylene, 1,10 -ferrocenediyl) have been synthesized by treating [CpMn(CO)3] with the corresponding lithiated reagents followed by alkylation with Meerwein’s reagent Et3OBF4 (Scheme 352).594

S OEt Mn

Li

C

OC OC

1) Li

S

Li

2) Et3OBF4

OC OC

Mn CO

X = S (a), O (b)

2) Et3OBF4

EtO

OEt

Mn X

1098

Scheme 352

1)

C

S

C

Mn CO CO

OC OC 1099

146

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 353 shows additional examples of complexes prepared through this route. They include mononuclear complexes [CpMn{]C(OEt)(TT)}(CO)2], (1100, TT ¼ 3,6-dimethylthieno[3,2-b]thiophenyl),595 the ferrocene-containing carbene complexes [CpMn{]C(OEt)Fc}(CO)2] (1101, Fc ¼ ferrocenyl) and [Cp(CO)2Mn]C(OEt)-Fc0 -(OEt)C]Mn(CO)2Cp] (1102, Fc0 ] 1,10 -ferrocenediyl),594 the dinuclear monocarbene complexes 1103–1104,590,596 the homodinuclear biscarbene complex [(Z5-MeC5H4)(CO)2Mn]C(OEt)C4H2S-C4H2SC(OEt)]Mn(CO)2(Z5-MeC5H4)] (1105a) and the heterodinuclear biscarbene complexes [(Z5-MeC5H4)(CO)2Mn]C(OEt)C4H2S-C4H2SC(OEt)]MLn] (1105, [MLn] ¼ Cr(CO)5 (b), Mo(CO)5 (c), and W(CO)5 (d)).597 Conformations of complexes of the type [CpMn{]C(OR)R0 }(CO)2] have been studied computationally.598

OEt

OEt Mn

C

Mn OC OC

S

Fe

OEt Mn

Re

OC OC

Fe

Mn OC OC

(OC)3M

1103

EtO S

C

C

S

MLn

1105

MLn = (

1104

M = Mn (a), Re (b)

CO CO

OEt

C

OC OC (OC)3M

Mn

1102

OEt

C

C

OC OC

1101

1100

EtO

C

Mn

OC OC

S

OEt

C

5

-C5H4Me)Mn(CO)2 (a),

Cr(CO)5 (b), Mo(CO)5 (c), W(CO)5 (d)

M = Mn (a), Re (b)

Scheme 353

Treatment of [CpMn(CO)3] with FcLi and FcLi2 followed by titanocene dichloride yielded the trimetallic monocarbene complex [MnCp(CO)2{C(OTiCp2-Cl)Fc}] (1106) and the double-bridged bisoxy titanocene ferrocen-1,10-diyl complex [{m-TiCp2O2} {m-Fe(C5H4)2}{CMnCp(CO)2}2] (1107), respectively (Scheme 354).591

O TiClCp2 Mn OC OC 1106

C

O

1) FcLi Mn

Fe

2) Cp2TiCl2 OC OC

Mn

1) FcLi2 CO

2) Cp2TiCl2

OC OC

Cp2 Ti

O C

C Fe

Mn CO CO

1107

Scheme 354

Fischer synthesis could also be carried out with other precursors. Deprotonation of [Re(CO)3(N-MeIm)2(PPh3)]BArF4 (725b, ArF ¼ 3,5-bis(trifluoromethyl)phenyl) with KN(SiMe3)2 followed by methylation with MeOTf produced a mixture of the dinuclear carbene complex 726b and the complex 1108b (Scheme 355).452The complex 726b features an abnormal NHC ligand generated by deprotonation of a coordinated imidazole and a Fischer carbene generated by nucleophilic attack of CO by another deprotonated imidazole followed by methylation. The complex [Re(CO)3(N-MeIm)2(PPh2Me)]BArF4 (725a) reacted similarly to give a mixture of complexes 726a and 1108a.

Scheme 355

Closely related to Fischer synthesis are reactions of carbonyl complexes with transition metal hydride complexes. For example, treatment of [Re2(CO)10] with trans-[WH(NO)(PMe3)4] resulted in the formation of the dinuclear carbene-bridged complex trans[W(NO)(PMe3)4(m-OCH])Re2(CO)9] (1109) (Scheme 356).599 The analogous complex trans-[W(NO)(PMe3)4(m-OCH])

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

NO W

Me3P Me3P

PMe3

H

CO

PMe3 [Re2(CO)10]

CO OC OC

H

Re

Et2 P

W P Et2 H 1110

P Et2

PMe3 PMe3 NO W PMe3

OC CO

BEt3 N Et2 P

O Me3P CO Re CO

147

CO

1109

Et2P H [Re2(CO)10]

CO CO OC OC

Re

O

W

Et2P CO Re CO

PEt2 N BEt3

PEt2

OC CO

CO

1111

Scheme 356

Mn2(CO)9] has been similarly obtained from the reaction of [Mn2(CO)10] with trans-[WH(NO)(PMe3)4]. In addition, the hydride complex [WH(NBEt3)(depe)2] (1100) also reacted with [Re2(CO)10] to afford the carbene complex trans-[Mo[(m-OCH]) Re2(CO)9](NBEt3)(depe)2] (1111).600 Alkoxycarbene complexes made from Fischer synthesis can be used as starting materials for other carbene complexes. The MdM bonds in dinuclear Fischer carbene complexes [(CO)5MdM(CO)4(carbene)] can be oxidatively cleaved by halogens. The reaction provides an effective method to prepare mononuclear group 7 halo-carbene complexes. For example, treatment of the dinuclear Fischer carbene complexes [Mn2(CO)9{]C(OEt)R}] (1079a, R ¼ 2-thienyl) with I2 afforded the mononuclear Fischer carbene complexes cis-[MnI(CO)4{]C(OEt)R}] (1112) (Scheme 357), treatment of the dinuclear carbene complex [Re2(CO)9{]C(OEt)R}] (1080a, R ¼ 2-thienyl) with Br2 afforded the mononuclear carbene complex cis-[ReBr(CO)4{]C(OEt)R}] (1113, R ¼ 2-thienyl).588

Scheme 357

148

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

The reaction of the tetrarhenium biscarbene complex 1088 containing a thieno[3,2-b]thiophene moiety with Br2 afforded the dirhenium biscarbene dibromide complex [Re(CO)4Br-m-{]C(OEt)-3,2-b-TT-C(OEt)]}Re(CO)4Br] (1114) bearing a thienothiophene spacer, as a result of cleavage of both metal-metal bonds in 1088.589 The phosphine-substituted Fischer carbene complex [Cp(CO)(PPh3){]C(OEt)(C5H4Mn(CO)3)}] (1115) has been synthesized through thermal substitution of a carbonyl ligand in 1103a with triphenylphosphine (Scheme 358).601 The Mn(I) carbene complex 1101 can be oxidized by AgPF6 to yield the radical species 1116.594

OEt OEt PPh3

C

Mn OC OC

C

Mn Ph3P OC

Mn OC OC CO 1115

(OC)3Mn 1103a

Mn OC OC

+ PF6-

OEt

OEt C

Fe

1101

C

Mn

AgPF6 OC OC

Fe

1116

Scheme 358

6.01.7.2.1.2 By electrophilic addition to acyl complexes Electrophilic addition to acyl complexes represents another common route to group 7 metal alkoxycarbene complexes. For example, the acyl complex [CpRe(COMe)(NO)(PPh3)] can be reversibly protonated by o-toluidinium (1117) to give the hydroxycarbene complex [CpRe{]CMe(OH)}(PPh3)(NO)]+ (1118) (Scheme 359). The reaction has an equilibrium constant of 0.8  0.3 and the hydroxycarbene [CpRe{]CMe(OH)}(PPh3)(NO)]+ (1118) was estimated to have a pKa value of 10.6  0.4 in acetonitrile.602

+ Me

Me Re Ph3P

+ C

ON

Me

+ NH3

O 1117

+

Re Ph3P

C ON

OH

NH2

Me

1118

Scheme 359

The carbene complexes [Mn(]CHOMe)(CO)5-x(PPh3)x]+ (x ¼ 1 or 2) can be obtained by methylation of the formyl complexes [Mn(CHO)(CO)5-x(PPh3)x] with methyl triflate. For example, treatment of the formyl complexes [M(CHO)(CO)3(PPh3)2] (M ¼ Mn, Re) with MeOTf produced the carbene complexes [M(]CHOMe)(CO)3(PPh3)2]OTf (1119, M ¼ Mn (a), Re (b)) (Scheme 360).20 The reaction of the boroxycarbene complex [Re(]CHOBF3)(CO)3(PPh3)2] with CH3OTf can also give the methoxycarbene complex [Re(]CHOMe)(CO)3(PPh3)2]OTf (1119b). The siloxycarbene complex [Re(]CHOSiMe3) (PPh3)2(CO)3]BPh4 (1120) has been obtained similarly by the reaction of [Re(CHO)(PPh3)2(CO)3] with SiMe3OTf/LiBPh4.20 Similarly, treatment of the acyl complex [Mn{C(O)CH2OCH3}(CO)4(PPh3)] (1121) with triethyloxonium tetrafluoroborate resulted in alkylation of the acyl oxygen to give the cationic carbene complex [Mn{]C(OEt)CH2OMe}(PPh3)(CO)4]BF4 (1122). The acyl complex 1121 reacted with HOTf to give the hydroxycarbene complex [Mn{]C(OH)CH2OMe}(PPh3)(CO)4]BF4 (1123).

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

149

Scheme 360

The strategy has also been used to prepare Re(V) alkoxycarbenes. For example, treatment of the acyl complex [Re(O){C(O)Me} (DAPMes)] (241a, DAPmes ¼ 2,6-bis-((mesitylamino)methyl)pyridine) with methyl triflate resulted in the formation of the cationic rhenium(V) carbene complex 1124 (Scheme 361).118

Mes O N

N Me

O

MeOTf

Re

N

O N

+ OTf-

Mes

N

Me

Re OMe N

Mes

241a

Mes

1124

Scheme 361

6.01.7.2.1.3 By electrophilic abstraction of alkyl complexes Group 7 metal alkoxycarbene complexes have been occasionally obtained by hydride abstraction of MdCH2OR complexes. For example, the reaction of [CpRe(CH2OMe)(PPh3)(NO)] in CD3CN with [Cp Re(NO)(CO)2]+ (Cp ¼ C5Me5) produced equal amounts of the carbene complex [CpRe(]CHOMe)(PPh3)(NO)]+ (1125), [CpRe(Me)(PPh3)(NO)] and [Cp Re(COOMe)(NO) (CO)] (Scheme 362). The reaction was proposed to proceed by initial transfer of MeO− from [CpRe(CH2OMe)(PPh3)(NO)] to [Cp Re(NO)(CO)2]+ to give [CpRe(]CH2)(PPh3)(NO)]+ and [Cp Re(COOMe)(NO)(CO)]. Irreversible H-transfer from [CpRe(CH2OMe) (PPh3)(NO)] to transient [CpRe(]CH2)(PPh3)(NO)]+ would give [CpRe(]CHOMe)(PPh3)(NO)]+ (1125) and [CpRe(Me)(PPh3)(NO)].602

+ Re Ph3P

C ON

Scheme 362

OMe

H H

+

Re

Re OC ON

+

CO

Ph3P

C ON 1125

H

OMe

+ Ph3P

+

Re ON

CH3

Re OC C ON MeO

O

150

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

6.01.7.2.1.4 By nucleophilic addition to carbyne or vinylidene complexes Group 7 metal Fisher carbene complexes could also be obtained by addition of alkoxides to carbyne or vinylidene complexes. For example, the carbene complexes [CpMn{]C(Ph)OC6H2R3-2,4,6}(CO)2] (1127, R ¼ F, Cl, Br, I, H, Me) can be readily obtained by treating the cationic carbyne complex [CpMn(^CPh)(CO)2]BCl4 (1126aBCl4) with the corresponding phenol derivatives in the presence of Et3N (Scheme 363).603 The complex 1128 reacted with 3-butyn-1-ol in refluxing THF to give the Re(I) oxocyclocarbene complex [Re{]C(CH2)3O}{k2-(P,S)-Ph2PCH2P]NP(]S)(OPh)2}Ph2(CO)3]SbF6 (1129), presumably via an intramolecular OH addition reaction of a vinylidene intermediate.604

Scheme 363

6.01.7.2.1.5 By reactions of boranes with carbonyl complexes Acyl complexes can form adducts with boranes or boroxycarbene complexes, an interesting class of oxygen substituted carbene complexes. Well-characterized group 7 metal boroxycarbene complexes have been isolated with Mn(I), Re(I) and Re(V) centers. They were in general prepared by reactions of acyl complexes with BR3. For example, BF3 reacted with the Re(V) oxo-acyl complexes 241a and 242e to give the boroxycarbene complexes 1130 and 1131, respectively (Scheme 364)141 Treatment of the M(I) formyl complexes [M(CHO)(PPh3)2(CO)3] (M ¼ Mn, Re) with BX3 (X ¼ F, C6F5) gave the carbene complexes [M(]CHOBX3) (PPh3)2(CO)3] (1132, M ¼ Mn; 1133, M ¼ Re; X ¼ F (a), C6F5 (b)).20,605 The acyl complex [Mn(COMe)(CO)5] reacted with B(C6F5)3 to form the carbene complex [Mn{]CMe(OB(C6F5)3)}(CO)5].606

Scheme 364

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

151

Boroxycarbene complexes could often be obtained by trapping in-situ generated acyl complexes with BR3. For example, treatment of [Re(CO)4(PPh3)2]BF4 with [PtH(dmpe)2]PF6 (dmpe ¼ 1,2-bis(dimethylphosphino)ethane) in the presence of BEt3 produced the boroxycarbene complex [Re(]CHOBEt3)(CO)3(PPh3)2] (1134), presumably via an in-situ generated formyl complex intermediate (Scheme 365).607

PPh3 CO OC

Re

OC PPh3

CO

+ BF4-

BEt3 [PtH((dmpe)2]+

PPh3 CO H CO Re O BEt3 OC PPh3 1134

Scheme 365

The reaction of the carbonyl complex [Re(CO)4{Ph2PCH2B(C8H14)}2]B(C6F5)4 (1135) with [PtH(dmpe)2]PF6 produced the boroxycarbene complex 1136, which reacted with pyridine to give the carbene complex 1137 (Scheme 366).607,608 The carbonyl complexes [M(CO)4{Ph2P(CH2)2B(C8H14)}2]B(C6F5)4 (1138, M ¼ Mn (a), Re(b)) reacted with NaBHEt3 to give the dinuclear boroxycarbene complexes 1139, which can further react with NaBHEt3 to give the complex 1140 bearing a chelating boroxycarbene ligand.

Scheme 366

The carbonyl complexes [M(CO)4(BP2)]BF4 (1141, M ¼ Mn (a), Re (b); BP2 ¼ Ph2PCH2CH{B(C8H14)}PPh2) bearing a diphosphine tethered with the Lewis acidic group B(C8H14) reacted with NaBHEt3 to give the boroxycarbene complexes [M{] CHOdBP2}(CO)3] (1142, M ¼ Mn (a), Re (b)) (Scheme 367).609 The bimetallic complex 1143 in THF reacted with excess trialkylborane BR3 (R ¼ Et, nBu) and triphenylphosphine to give the complexes 1144 bearing a chelating boroxycarbene ligand.610

152

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 367

6.01.7.2.2

Reactivity

A common reactivity of alkoxycarbene complexes is that they can undergo nucleophilic substitution reactions with amines to give aminocarbene complexes. This topic will be described in the section dealing with aminocarbenes. Fischer carbene can undergo nucleophilic addition reactions to give alkyl complexes. For example, the formyl complex [CpRe(CHO)(PPh3)(NO)], a hydride donor, reacted with the carbene complex [CpRe(]CHOMe)(PPh3)(NO)]+ (1125) to give the carbonyl complex [CpRe(PPh3)(NO)(CO)]+ and the alkyl complex [CpRe(CH2OMe)(PPh3)(NO)] via a hydride transfer process (Scheme 368).602

+

+ +

Re Ph3P

OMe

C ON

Re

Re Ph3P

C ON

H

O

Ph3P

C ON

H

OMe

Re

+

Ph3P ON

H H

CO

1125 Scheme 368

A similar reaction was observed in the protonation reaction of [Mn(CHO)(PPh3)(CO)4] in methanol. Treatment of the formyl complex [Mn(CHO)(PPh3)(CO)4] with p-toluenesulfonic acid in methanol resulted in the formation of equal molar amounts of the alkyl complex [Mn(CH2OMe)(PPh3)(CO)4] and [Mn(PPh3)(CO)5]+ (Scheme 369). The reaction presumably proceeded through initial formation of the hydroxycarbene intermediate 1145 which exchanges with methanol to give the methoxycarbene intermediate 1146, followed by hydride transfer.20 CO

CO CO H OC

OC

Mn MeOH

O

OC

+ CO

Mn

OC

OH H

PPh3 1145 Scheme 369

+

OC

Mn

CO

OC PPh3

+

CO OC

O PPh3

PPh3 H

CO

Mn

OC

+

CO

CO

TsOH

CO MeOH H 2O

OC

CO

+ CO

Mn

OC

OMe H

PPh3 1146

CO H OC

Mn O

OC PPh3

OTs-

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

153

Alkoxycarbene complexes can undergo 1,2-hydride shift and deprotonation reactions. For example, the cationic carbene complex [Mn{]C(OEt)CH2OMe}(PPh3)(CO)4]BF4 (1122) decomposed over time to liberate trans-1-ethoxy-2-methoxyethene, via the 1,2-dialkoxyethylene complex intermediate 1148 formed by 1,2-hydride shift (Scheme 370). The carbene complex 1122 can be deprotonated by NEt3 to give the vinyl complex [Mn{C(OEt)]CH(OMe)}(PPh3)(CO)4] (1147).20

Scheme 370

The hydroxycarbene [Mn{]C(OH)CH2OMe}(PPh3)(CO)4]OTf (1123) decomposes over time to yield the manganese triflate complex [Mn(OTf )(PPh3)(CO)4] and methyl acetate (Scheme 371). The transformation may involve protolysis of the methoxy group to yield MeOH and the ketene intermediate [Mn(PPh3)(CO)4(Z2-CH2]C]O)]OTf, which undergoes nucleophilic attack by MeOH at the central carbon of the ketene to form the carbomethoxymethyl complex [Mn(CH2CO2Me)(PPh3)(CO)4] followed by protolysis.

CO OC

CO

+ CO

OTf-

OH

OC

Mn

OC

CO

CH2OMe

Mn

OTf + CH3CO2Me

OC PPh3

PPh3 1123 Scheme 371

Fischer carbene complexes can be hydrolyzed. For example, treatment of the dinuclear carbene complex 1087 with wet THF produced the mononuclear carbene complex 1149 as a result of hydrolysis of one of the metal-carbene moieties in 1087 (Scheme 372).592

Re2(CO)9

Re2(CO)9

Re2(CO)9

S OEt

EtO n

1087: n = 1 (a), 2 (b)

H2O

S

CHO

EtO n 1149: n = 1 (a), 2 (b)

Scheme 372

Fischer carbenes [LnMR{]C(OR)R0 }] can also undergo migratory insertion reactions. For example, treatment of the cyclometalated complex 440 sequentially with nBuLi followed MeOTf gave the complex 1150 (Scheme 373).337 Treatment of the complex 440 sequentially with PhLi, nBuLi at −50  C, nBuLi at −20  C followed by MeOTf produced the complex 1151. The complexes 1150 and 1151 are likely formed via alkoxycarbene intermediates.

154

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

(CO)3 Cr

Cr(CO)3 OMe nBu

N

1) PhLi, - 50 oC 2) nBuLi, -20 oC 3) MeOTf

1) nBuLi 2) MeOTf Mn(CO)4

Mn(CO)3

N

(CO)3 Cr OMe Ph N

nBu

Mn(CO)3 nBu

440

1150

1151

Scheme 373

Boroxycarbene complexes can decompose via formation of Lewis acid-base adducts of boranes. For example, the boroxycarbene complex 1132b evolved at room temperature to the borohydride salt of the cationic carbonyl complex [Mn(CO)4(PPh3)2] [(C6F5)3BH] (Scheme 374). On exposure to light, the complex 1132b is converted to the manganese hydride [MnH(CO)4(PPh3)] and the phosphine-borane adduct Ph3P-B(C6F5)3.20 The boroxycarbene complex 1139b can serve as a hydride source to react with CO2 to give the complex 1152.611

Scheme 374

6.01.7.3

Complexes with one SR or SeR substituent at the carbene carbon

Group 7 metal carbene complexes with a SR or SeR group are still rare and limited to a few Re(I) systems with the metal fragment [Re(NO)2(PR3)2]+. These complexes can be obtained by addition of RXH (X ¼ S, Se) to vinylidene ligands. For example, thiophenol and benzeneselenol reacted with the vinylidene complexes [Re(]C]CH2)(NO)2(PR3)2][BArF4] (1153, R ¼ Cy (a), iPr (b)) to yield the cationic Fischer-type carbene complexes [Re{]C(XPh)Me}(NO)2(PR3)2]BArF4 (1154S, X ¼ S, 1154Se, X ¼ Se) (Scheme 375).449

+ BArF4-

PR3 ON Re

C

XPh Re

ON PR3

Scheme 375

ON

CH2

ON

1153

+

PR3 PhXH

Me PR3

R = Cy (a), iPr (b)

1154S: X = S 1154Se: X = Se

BArF4-

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

6.01.7.4

155

Carbene complexes with a PR2 substituent at the carbene carbon

Group 7 metal carbene complexes with an PR2 substituent at the carbene carbon have been reported for a few manganese(I) complexes. The most studied manganese carbene complexes with one phosphorus substituent are complexes of the type [CpMn{]C(PR3) R0 }(CO)2]+ or [CpMn{]C(PR2)R0 }(CO)2], which were usually obtained by nucleophilic addition reactions of phosphines with carbyne complexes [CpMn(^CR0 )(CO)2]+. For example, the carbyne complex [CpMn(^CPh)(CO)2]BCl4 (1126aBCl4) reacted with the primary phosphine H2PMes to give the phosphoniocarbene complex [CpMn{]C(Ph)PH2Mes}(CO)2]BCl4 (1155), which can be deprotonated by NEt3, or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to give the Z1-phosphinocarbene complex [CpMn{] C(Ph)PHMes}(CO)2] (1156) (Scheme 376).612,613 Secondary phosphines HPPh2, HPCyPh, HP(N(iPr)2)2 and HPMeMes reacted similarly to give the corresponding phosphoniocarbene complexes [CpMn{]C(Ph)PHRR0 }(CO)2]BCl4 (1157), which can be deprotonated to give the Z1-phosphinocarbene complexes [CpMn{]C(Ph)PRR0 }(CO)2] (1158). The complexes 1158 are highly thermolabile and undergo intramolecular CO insertion to give the Z3-phosphinoketene complexes [CpMn{Z3-C(O)C(Ph)PRR0 } (CO)] (1159). The reactions have been explored for syntheses of organophosphorous derivatives, including chiral pincer-type phosphine-NHC-phosphine ligands.614

+ BCl4C Ph

Mn

H2 P

PH2Mes Mn

OC OC 1126aBCl4

OC OC

C

+ BCl4Mes

H P

NEt3

Mn

C

OC OC

Ph

Mes

Ph 1156

1155

PHRR'

Mn

+ PHRR' BCl4- NEt 3

C

OC OC

PRR' Mn OC OC

Ph

OC

Ph

C

C

1158

1157

PRR'

Mn

C

Ph

O 1159

R/R' = Ph/Ph (a), Cy/Cy (b), N(iPr)2/N(iPr)2 (c), Me/Mes (d) Scheme 376

Analogous phosphinocarbene complexes have also been obtained from the carbyne complex [CpMn(^CMe)(CO)2] BCl4 (1160a). For example, treatment of [CpMn(^CMe)(CO)2]BCl4 (1160a) with the primary phosphine H2PMes produced the phosphoniocarbene complex [CpMn{]C(Me)PH2Mes}(CO)2]BCl4 (1161), which can be deprotonated to give the Z1-phosphinocarbene complex [CpMn{]C(Me)PHMes}(CO)2] (1162) (Scheme 377).615 The carbene carbon of phosphinocarbene complexes [CpMn{]C(R)PHR2}(CO)2]+ can be further attacked by secondary phosphines. The chemistry has been used to prepare backbone-substituted diphosphinomethanes. For example, addition of PHPh2 to the methylcarbyne complex [CpMn(^CMe)(CO)2]BCl4 (1160a) at low temperature produced the Mn(I) phosphoniocarbene complex [CpMn{]C(Me) PPh2H}(CO)2]BCl4 (1163a) which can uptake a second molecule of the phosphine to give the cationic alkyl complex [CpMn {C(Me)(PPh2H)2}(CO)2]BCl4 (1164a) featuring a pendant PdCdP structural motif resulting from formal coupling of the carbyne fragment with two secondary phosphines. Deprotonation of 1164a with alumina afforded the neutral diphosphine complex [CpMn(Z1-Ph2PCHMePPh2)(CO)2] (1165a). Treatment of 1165a with borane or acid gave the free ligand Ph2PCHMePPh2. The unsymmetrical ligand Ph2PCHMePCy2 could be obtained similarly from reactions involving HPPh2 and HPCy2. + BCl4Mn

C Me

H2 P

PH2Mes Mn

OC OC 1160a

OC OC

C

+ BCl4Mes

H P

base

Mn

C

OC OC

Me

Me 1162

1161

PHR2

Mn

C

OC OC

PHR2 PHR2

Me 1163

Scheme 377

+ BCl 4 PHR2

R = Ph (a), Cy (b)

Mn OC OC

C Me

1164

+ BCl4-

PHR2

base

Mn OC OC 1165

PR2 Me

PR2 H

Mes

156

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Tertiary phosphine can also react with [CpMn(^CR)(CO)2]BX4 to give reactive phosphorus-substituted carbene complexes.615,616 For example, the cationic Mn(I) carbyne complexes [CpMn(^CAr)(CO)2]BCl4 (1126aBCl4, Ar ¼ Ph, 1126bBCl4, Ar ¼ p-tolyl) reacted with bis(diphenylphosphino)methane to afford selectively the cyclic 1,3-diphosphetium semiylides [ArC(dPPh2CH2PPh2d)]BCl4 (1169) (Scheme 378). The related diphosphine Ph2PCHMePPh2 behaved similarly. Interestingly, the alkyl carbyne complex [CpMn(^CR)(CO)2]BCl4 (1160, R ¼ Me (a), CH2Ph (b)) reacted with bis(diphenylphosphino) methane to afford selectively the Mn(I) complexes [CpMn(Z3-(P,C,C)dPh2PCH2PPh2C(R)]C]O)(CO)]BX4 (1167) featuring a phosphonioketene ligand. All the reactions presumably proceed through the carbene intermediates 1166.

Scheme 378

Manganese(II) carbene complexes with two phosphorus atoms at the carbene carbon have also been reported. For example, bis(2-pyridyl)tetraphenylcarbodiphosphorane (CDPPy2, 1170) reacted with MnCl2 in THF to give the Mn(II) complex [MnCl2(CDPPy2)] (1171) (Scheme 379).617

Ph2P

MnCl2

C Ph2P 1170

Ph2P

N

N

N Mn

Ph2P

Cl Cl

N

1171

Scheme 379

6.01.7.5

Carbene complexes with one nitrogen substituent at the carbene carbon

Group 7 metal carbene complexes with one nitrogen substituent at the carbene carbon (aminocarbene complexes) have been reported for Mn(0), Mn(I), Mn(II), Re(0), Re(I), Re(III) and Re(V) systems. Group 7 metal aminocarbene complexes can often be obtained by reactions of alkoxycarbenes with primary or secondary amines. For example, the manganese alkoxycarbene complexes 1079 reacted with NH3 and n-propylamine to give the aminocarbene complexes 1172 and 1173, respectively (Scheme 380).587 It is noted that the location of the carbene ligand in 1172 (at the equatorial position) is different from that in the precursor alkoxycarbene complexes 1079, presumably due to steric effect. The rhenium alkoxycarbene complexes 1080 reacted with NH3 and n-propylamine to give the aminocarbene complexes 1174 and 1175, respectively.587

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

X CO

CO

CO CO OEt OC Mn Mn X OC OC CO CO

H2 N NH3

CO CO Mn CO OC Mn OC OC CO CO

X = S (a), O (b)

1172

CO

CO

nPrNH2

OC OC

X

EtO CO CO

Re

CO CO Re

OC CO

X

1173

X CO

CO CO H N Mn

CO

OC Mn OC OC CO CO

1079

157

RNH2 R = H, nPr

RHN CO

OC OC

Re

Re

CO

CO CO

OC CO

CO X = S (a), O (b)

1080

1174: R = H 1175: R = nPr

Scheme 380

Interestingly, treatment of the ethoxycarbene complex 1176 with dimethylamine hydrochloride and sodium hydroxide produced a mixture of the tetracarbonyl chlorido dimethylaminocarbene complex cis-[Re{]C(NMe2)-2,3-b-TTH}Cl(CO)4] (1177) and pentacarbonyl rhenium chloride [ReCl(CO)5], as a result of cleavage of the RedRe bond in 1176 (Scheme 381).589

S S CO

CO NMe 2

CO OC OC

CO

EtO

Re

Re OC

CO 1176

CO CO

1) Me2NH.HCl 2) NaOH

OC Re OC Cl

CO

+

S

[ReCl(CO)5]

S

1177

Scheme 381

The alkynylcarbene complex [(CO)5Re(CO)4Re{]C(OMe)C^CTMS}] (1090) reacted with HNMe2 at −130  C to give the aminocarbene complex [(CO)5Re(CO)4Re{]C(NMe2)C^CTMS}] (1178) which could be desilylated to form the carbene complex [(CO)5Re(CO)4Re{]C(NMe2)C^CH}] (1179) bearing a terminal alkynyl moiety (Scheme 382).593

Scheme 382

Nucleophilic addition of amines to carbyne complexes represents another common route to aminocarbene complexes. For example, NaNH2 reacted with the cationic carbyne complexes [CpMn(^CPh)(CO)2]BBr4 (1126aBBr4) and [CpRe(^CPh)(CO)2] BBr4 (1180aBBr4) to give the aminocarbenes [CpM{]C(NH2)Ph}(CO)2] (1181, M ¼ Mn (a), Re (b)) (Scheme 383).618

158

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

+ BBr4C Ph

M

NH2

NaNH2

OC OC

M

C

OC OC

Ph 1181

1126aBBr4: M = Mn 1180aBBr4: M = Re

M = Mn (a), Re (b)

Scheme 383

Electrophilic addition of iminoacyl complexes have been occasionally employed to prepare aminocarbene complexes. For example, the iminoacyl complexes 952 reacted with HOTf at room temperature to give the cationic Re(I) aminocarbene complexes 1182 (Scheme 384).536 CO

CO

N R

N R OC

N Re

OC tBu N

HOTf

N R = Me (a), Mes (b)

N

C

R 952

+ OTf-

N

OC

Re N

OC tBu N H

N

C

R 1182

Scheme 384

The strategy can be applied for preparation of Re(III) and Re(V) aminocarbene complexes. For example, the iminoacyl complexes [Re{C](NR)Me}(CO)(DAAm0 )] (948, R ¼ Mes (a) Xyl (b), DAAm0 ¼ (MesNHCH2CH2)2NMe) reacted with methyl triflate and H2O to give the Re(III) carbene complexes 1183 (Scheme 385).118 Treatment of the hydride complex 214 with CN-Xyl followed by methyl triflate/H2O resulted in the formation of the oxorhenium(V) carbene complex 1184, presumable via a Re(III) iminoacyl intermediate.

Scheme 385

Group 7 metal complexes supported by an acyclic aminoalkyl carbene (cAAC) ligand can be prepared by addition or substitution reactions of preformed carbenes. For example, the reaction of the free carbene cAAC (1185) with MnCl2 in a 1:1 ratio in THF at room temperature produced the three coordinated Mn(II) carbene complex [MnCl2(cAAC)] (1186) (Scheme 386). Treatment of 1186 with KC8 and cAAC (1074) in a 1:2.1:1 ratio afforded the linear two-coordinate formally Mn(0) complex [Mn(cAAC)2] (1187).619 The carbene complex 1187 reacted with H2 to give the bisalkyl complex 1188.

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

N

MnCl2

MnCl2

N

cAAC, KC8 1185 (cAAC)

H

N

Mn

N

H2

N

N H

1188

1187

1186

Mn

159

Scheme 386

The one-pot reaction of MnCl2 with Et2-cAAC (1189), dvtms (1190, dvtms ¼ divinyltetramethyldisiloxane) and KC8 produced the three-coordinate formal zero-valent manganese complex [Mn(Z2:Z2-dvtms)(Et2-cAAC)] (1191) (Scheme 387). Spectroscopic studies and theoretical calculations reveal an S ¼ 3/2 ground-spin state for these formal Mn(0) complexes that feature highly covalent metal-ligand bonding arising from pronounced metal-to-alkene back-donation.68 Dipp

MnCl2

+

Et Et

N

+

O

N

Si

Si

Dipp

+ KC8

Si

O

Mn

Si Et Et

1189 (Et-cAAc)

1191

1190 (dvtms)

Scheme 387

1,2,3-Triazolylidenes can be regarded as a special class of carbenes with one nitrogen substituent at the carbene carbon. Group 7 metal complexes with such ligands have been reported for a few of those with a Mn(0), Mn(I) or Re(I) center. Treatment of [MnBr(CO)5] with the ditriazolium triflate salt 1192a bearing an Mes substituent in the presence of two equivalents of KOtBu produced the monometallic manganese(I) triazolylidene complex [MnBr(CO)3(ditrz)] (1193) containing a bidentate chelating di(triazolylidene) ligand (ditrz) (Scheme 388). Interestingly, the ditriazolium triflate salt 1192b bearing an ethyl group reacted under a similar condition to give the bimetallic manganese(0) triazolylidene complex [Mn(CO)8(m-ditrz)] (1194) with the two metal centers bridged by a di-triazolylidene ligand. The bimetallic complex 1194 was found to be catalytically active for oxidation of secondary alcohols and benzyl alcohol with tert-butyl hydroperoxide under mild conditions.620

N

N N N Mes CO R = Mes Mn

CO

N N Br CO Mes

N N N R

N N N

2+ 2 OTf-

Et

N N N

N N N Et CO

R = Et

R

CO

1192: R = Mes (a), Et (b)

OC OC

+ Bu4NBr, KOtBu, [MnBr(CO)5]

CO

1193

Mn

Mn OC

CO

CO

1194

Scheme 388

Several rhenium complexes bearing a bidentate ligand containing a triazolylidene moiety combining with a pyridine, triazole or triazolylidene group have been prepared by the reactions of [ReCl(CO)5] with the corresponding 1,2,3-triazol-3-ium salts in the presence of NEt3.621,622 The complexes 1195–1197 shown in Scheme 389 are examples of such complexes.

N N N R CO N Cl

Re

CO

CO

1195 R = CH2Ph, CH2C6H5, Dipp Scheme 389

N N N dipp CO N dipp N N Cl

Re CO 1196

CO

N

N N N dipp CO

Re N N Cl CO dipp 1197

CO

160

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Treatment of the triazolyl complex 734 with Me3OBF4 produced the complex 1198 bearing a [C,N,N]-tridentate ligand with a triazolylidene mesoionic carbene moiety (Scheme 390).454 The complex 1200 bearing a tris(carbene)-borate ligand that incorporates 1,2,3-triazol-5-ylidene donors has been obtained by treating the ligand precursor 1199 with LDA followed by [MnBr(CO)3(tBuNC)2]2.623

Scheme 390

Re(I) complexes bearing N-confused porphyrin imbedded with an monosubstituted N-heterocyclic carbene donor have been serendipitously synthesized by reactions of inner-methylated N-confused porphyrins with [Re2(CO)10]. For example, the N-confused porphyrin 1201 reacted with two equivalents of [Re2(CO)10] in 1,2-dichlorobenzene at 140  C for 16 h to give the complex 1202 in 18% yield together with the isomeric complex 1203 in 6.1% yield (Scheme 391).624 Upon heating a mixture of the N-confused porphyrin 1204 and [ReBr(CO)5] (four equivalents) in the presence of 2,6-lutidine at 140  C for 24 h, the rhenium(I) tricarbonyl carbene complex 1205 was obtained.464

Ph

Ph

Ph N

N N OC

NH Ph

Ph N

Re2(CO)10

Ph

CO

Ph +

Ph

N

N

Ph [ReBrCO)5] N Ph

1203

N

1204

OC

CO Re

HN

Ph

Ph

R N

N

N Ph

N

Ph CO N

Ph

1202

N

CO

OC Re

Ph

Ph 1201

Scheme 391

N CO Re

N

HN

N

N

1205

CO Ph

N

Ph

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

6.01.7.6

161

Complexes with N,N-hetero carbenes

Group 7 metals form a variety of complexes with carbene ligands bearing two nitrogen substituents. These carbene ligands can be in either acyclic or cyclic form, which are referred as diaminocarbenes and N-heterocyclic carbenes in the following discussion.

6.01.7.6.1

Complexes with acyclic diaminocarbenes

Several Mn(I) and Re(I) diaminocarbene complexes have been obtained by reactions of isocyanide complexes with primary or secondary amines. For example, treatment of the isocyanide complex fac-[Mn(CNR)(bpy)(CO)3]+ (930, R ¼ Ph (a), Xyl (b), Me (c), CH2Ph (d)) with NH2Me produced the diaminocarbene complexes fac-[Mn{CNHR(NHMe)}(bpy)(CO)3]+ (954) (Scheme 392).538 The cationic isocyanide complexes fac-[Mn(CNR)(CO)3(bpy)]+ (930, R ¼ Ph (a), Xyl (b), Me (c), CH2Ph (d)) reacted with 3-bromopropylamine to give the diaminocarbene complexes 1206 containing an azetidine group, instead of the expected N,N-heterocyclic carbenes.525

Scheme 392

Treatment of the isocyanide complexes fac-[ReBr(CNR)2(CO)3] (1207, e.g. RNC ¼ 4-ClC6H4NC, 2,6-(CH3)2C6H3NC) with amino-substituted N-donor ligands such as 2-aminopyridine, 2-aminothiazole and 2-aminobenzothiazole produced luminescent rhenium(I) complexes (e.g. 1208–1210) bearing a [C,N]-bidentate diaminocarbene ligand (Scheme 393). Similar complexes were also obtained from the reactions of these amino-substituted N-donor ligands with [ReI(CNR)5] and fac[Re(CNR)3(CO)3](CF3SO4).625 R

R CO OC

C N R

Re

OC

Br

OC N

NH2

OC

N H

N

1208 AgOTf

NH2

N

AgOTf

NH2

S R C

OC

S

C OC

N H 1209

N

CO

NHR

Re N

R

N

CO OC

NHR

Re

1207

N S

C

AgOTf

C

N

CO

N

OC

NHR

Re N H

N S

1210

Scheme 393

The diaminocarbene ligands in acyclic diaminocarbene complexes fac-[Mn{CNHR(NHMe)}(bpy)(CO)3]+ (954) can be deprotonated to give amidinate complexes537 as mentioned in the section dealing with acyl complexes. They reacted with Ag2O to give carbodiimide complexes (e.g. 1211, Scheme 394).537 Acyclic protic diaminocarbene (ADC) complexes can also react with [AuCl(PPh3)] to give Au-N-heterocyclic carbene complexes (e.g. 1212) as a result of translocation of the carbene carbon atom from Mn(I) to Au(I).527,528,626

162

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 394

6.01.7.6.2

A brief comment on complexes with N-heterocyclic carbenes (NHCs)

Group 7 metal complexes with N-heterocyclic carbenes (NHCs) are among the most studied group 7 metal carbene complexes. These complexes have been reported for those containing an Mn(0), Mn(I), Mn(II), Mn(III), Mn(IV), Mn(V), Tc(V), Re(0), Re(I), Re(III) or Re(V) center. These complexes are interesting as they are potentially useful in areas such as catalysis, light-emitting materials, molecular sensoring, biological markers and radiopharmaceuticals. A comprehensive review on NHC complexes of group 7 transition metals appeared in 2013.627 A review of the coordination chemistry of technetium and rhenium complexes with N-heterocyclic carbenes appeared in 2005.628 The chemistry of manganese(IV, V)-NHC complexes has been reviewed 2016,629 and that of manganese(0)-NHC complexes in 2020.630 Cationic rhenium NHC complexes631 and rhenium(I) tricarbonyl N-heterocyclic carbene complexes632 have been summarized in 2016 and 2018, respectively.631 The chemistry of group 7 metal NHC complexes has also been reviewed in reports dealing with photoluminescence,633 complexes bearing “protic” N-heterocyclic carbene (NHC) ligands,634 high oxidation state organometallic complexes635 and late transition metal catalysis.636 For further information on the topic, readers are directed to the chapter dealing with N-heterocyclic and mesoionic carbene complexes of Group 7 metals in this series.

6.01.7.7

Complexes with N,O-, N,S- and N,B-hetero carbenes

Several low valent group 7 metal complexes with N,O-, N,S- and N,B-hetero carbenes have been described. Complexes with N,O-hetero carbenes could be generated by alkylation of carbamoyl complexes. For example, treatment of the carbamoyl complexes 1212 and 1213 with methyl triflate yielded the N,O-heterocarbene complexes 1214 and 1215, respectively (Scheme 395).527,528

Scheme 395

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

163

The carbamoyl complexes 1216 can be similarly alkylated at the carbamoyl oxygen atom by methyl triflate to give the carbene complexes 1217 (Scheme 396). The complex 1217a reacted with [AuCl(PPh3)], CuCl and [Rh(cod)Cl]2 (cod ¼ 1,5-cyclooctadiene) to produce the carbene complexes 1215, 1218 and 1219, respectively. PPh3 R

R

N Me

N N Mn

MeOTf

N

R = Ph (a), Me (b)

R = Ph

Cl Cu N N Mn N

N

R = Ph

+ OTf-

N Me

N

C

Mn

CO

1216

Ph

[AuCl(PPh3)]

CO

CO

Ph

OMe

N

CO

Au LiHMDS

C

Mn

O

N

N Me

N

C

+ OTf-

OMe

N

CO

1217

CO

1215

LiHMDS R = Ph [RhCl(COD)] 2 LiHMDS CuCl

RhCl(CO)2

RhCl(COD)

N Me

Ph

C

N

OMe

CO

CO

N

OMe

N

CO

CO CO

CO

1218

C

Mn

OMe

N

N Me

N

C

Mn

CO

Ph

N Me

N

1220

1219

Scheme 396

Another approach to complexes bearing a N,O-hetero carbene ligand is to react imidate complexes with electrophiles. For example, the neutral imidate complexes 953 can be protonated with HBF4 to give the alkoxyaminocarbene complexes 1221 (Scheme 397). It is noted that treatment of 1221 with Ag2O produced the formimidate complex 1222.537

MeO

N Ph

MeO

C N

C CO

HBF4

N

Mn N

H

H + N Ph BF4CO

Ph N

Ag2O

CO Mn

Mn N

CO CO 953

OMe N

N

CO

CO

CO

CO

1221

1222

Scheme 397

Intramolecular addition of alcohols to isocyanides can also lead to the formation of Group 7 metal N,O-heterocyclic carbene complexes. For example, treatment of the acetonitrile complexes [Re(L)(MeCN)(CO)2(phen)](CF3SO3) (1223, L ¼ CO (a), PPh3 (b) PPh2Me (c), P(OEt)3 (d)) with 2-trimethylsiloxyphenyl isocyanide (1224, 2-(Me3SiO)C6H4NC) followed by NH4PF6 produced the rhenium(I) phenanthroline carbene complexes 1225 (Scheme 398). The complex 1225a can be methylated to give the carbene complex 1226 (Scheme 398).550

NCMe

N

N

OC Re

N

OC L

1223

OSiMe3 1224 NH4PF6

L = CO (a), PPh3 (b), PMePh2 (c), P(OEt)3 (d) Scheme 398

C HN

N

O

OC

N

Me2SO4 K2CO3

N

L = CO

Re OC

L 1225 L = CO (a), PPh3 (b), PMePh2 (c), P(OEt)3 (d)

O N

OC Re

N

OC CO 1226

164

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

The Re(I) complex [ReBr(CO)5] reacted with two equivalents of 2-(Me3SiO)C6H4NC (1224) to give the bis(carbene) complex fac-[Re{CN(H)C6H4-2-O}2(CO)4] (1227) (Scheme 399). The bis(carbene) complex 1227 can undergo substitution reactions with diimine ligands to form cis-dicarbonyl Re(I) diimine complexes 1228 bearing one carbene ligand and one 2-hydroxyphenyl isocyanide ligand.637 These rhenium(I) diimine complexes containing a benzoxazol-2-ylidene ligand are luminescent.

N

N HN C

N

HN

[ReBr(CO)5]

OC

OSiMe3

OC

1224

O CO H N Re O CO

N

N

N

OH

O N

N

C Re

OC

R

N N

CO 1228

1227

N

R N

R = H, Me, CMe3

Scheme 399

Group 7 metal N,O-heterocyclic carbene complexes have also been obtained by reactions of isocyanide complexes with Br(CH2)nOH. For example, the isocyanide complex 1229 reacted with 2-bromoethanol to give the N,O-heterocyclic carbene complex 1230 (Scheme 400).637 The related N,S-heterocyclic carbene complex 1231 has been obtained by the reaction of the complex 1229 with thiirane. These N,O- and N,S-heterocyclic carbene complexes are catalytically active for electrochemical and photochemical reduction of CO2.638 Cl

O N PF6-

N Br

Cl OH

N

C Re

OC CO 1230

N

Cl

Cl

Cl

Cl

NH4PF6

C C Re

OC

N

S

N N

S N

N NH4PF6 N

CO 1229

PF6-

N

C Re

OC

N

CO 1231

Scheme 400

Complexes with a N,O-heterocyclic carbene ligand have been occasionally obtained by reactions of nitrogen-containing reagents with a carbonyl complexes. For example, the complex fac-[Mn(CN-Xyl)(CO)3(bpy)]+ (930b) reacted with NH2CH2CH2CH2Br to give the complex 1232 bearing a six-membered N,O-heterocyclic carbene (Scheme 401).525 The hydrazinediido complex 1233 reacted with the dinuclear metal carbonyl [Mn2(CO)10] to give the trinuclear complex 1234 bearing a N,O-carbene ligand.639

Scheme 401

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

165

Although rare, group 7 metal complexes bearing a N,B-heterocyclic carbene ligand have also been isolated. For example, treatment of the manganese borylene complex [CpMn{]BtBu(IMe)}(CO)2] (1235) with tBuNC produced the carbene complex 1236a as the only product (Scheme 402). Reactions of 1235 with other isocyanides RNC (R ¼ Me, Mes, and Cy) produced a mixture of the three-membered metallacycles 1237b–d and the carbene complexes 1236b–d.539,540 A crystallographic study reveals that the MndC1 bond in 1236a (1.975(2) A˚ ) is slightly shorter than typical MndCNHC bonds in manganese NHC complexes of the type [CpMn(NHC)(CO)2] and longer than M]C bonds in typical Fischer-type Mn(I) carbene complexes. IMe

IMe Mn B OC OC

B

CNR

Mn

C

OC OC

tBu

CO

tBu + OC

NR

C

tBu

NR

R = tBu (a), Me (b), Mes (c), Cy (d) 1235

IMe B

Mn

1237

1236 Scheme 402

6.01.8

Vinylidene and allenylidene complexes

In the review period, well-characterized group 7 metal vinylidene complexes have been isolated for those with d6 metal fragments such as [Mn(CO)3(dppm)]+, [Mn(C^CR)(dmpe)2], [CpMn(diphosphine)], [CpRe(CO)2], [Re(NO)2(PR3)2]+, [Re(C^CSiMe3) (PMe3)4] and [ReI(dppm)2]. Allenylidene complexes have been isolated for those with d6 metal fragments such as [CpMn(CO)2], [Re(triphos)(CO)2]+ and [ReI(dppm)2]. While rhenium is also known to form stable high valent d0 Re(VII) allenylidene complexes (with the metal fragment Re(]NCMe3)2(S-adamantyl)640), no additional examples of high valent vinylidene or allenylidene complexes of group 7 metals have been reported in the review period. Allenylidene and higher cumulenylidene complexes have been reviewed in 2009.641 A review summarizing the syntheses, structures, physicochemical properties and reactivity of complexes containing Z1-, m-, m3- and m4-vinylidene ligands derived from [CpMn(]C]CRR0 )(CO)2] appeared in 2007.642

6.01.8.1

Synthesis of vinylidene complexes and related complexes

6.01.8.1.1

By protonation of alkynyl complexes

A common route to group 7 metal vinylidene complexes is to protonate alkynyl complexes. For example, treatment of the alkynyl complexes [Re(C^CH)(NO)2(PR3)2] (807, R ¼ Cy (a), iPr (b)) with equimolar amounts of [H(Et2O)2]BArF4 yielded the cationic vinylidene complexes [Re(]C]CH2)(NO)2(PR3)2]BArF4 (1153, R ¼ Cy (a), iPr (b)) (Scheme 403).449 Protonation of the alkynyl complex fac-[Mn(C^CCO2Me)(CO)3(dppm)] (815) with HBF4 at 200 K afforded the vinylidene complex fac-[Mn(]C] CHCO2Me)(CO)3(dppm)]BF4 (1238).480 PR3

PR3 ON

[H(OEt2)2]BArF4

Re ON

R = Cy (a), iPr (b)

Re PR3 1153

807 CO2Me Ph2 P

MeO2C

H

Mn OC CO 815 Scheme 403

C CH2

ON

PR3

OC

+ BArF4-

ON

HBF4

OC

H •

H

+ BF4H

Mn OC

P Ph2

Ph2 P

CO 1238

P Ph2

H

166

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

The anionic alkynyl complexes Na[Mn(C^CR)2(dmpe)2] (1239, R ¼ SiEt3 (a), R ¼ SiiPr3 (b), Si(tBu)Me2 (c)) can be protonated by methanol to give the Mn(I) alkynyl-vinylidene complexes trans-[Mn(C^CR)(]C]CHR)(dmpe)2] (1240, R ¼ SiEt3 (a), SiiPr3 (b), Si(tBu)Me2 (c)), which can be converted to the parent Mn(I) alkynyl-vinylidene complexes trans-[Mn(C^CR)(]C] CH2)(dmpe)2] (1241, R ¼ SiEt3 (a), R ¼ SiiPr3 (b), Si(tBu)Me2 (c)) upon treatment with MeOH (Scheme 404).506 R PMe2 Mn

Me2P Me2P

C

Me2P

one eq. MeOH

Na C R

PMe2 Mn

R Me2P

PMe2

C

H

Me2P

excess MeOH

C

Mn

R

R

PMe2

PMe2 C

H C H

PMe2

Me2P

R = SiEt3 (a), R =SiiPr3 (b), Si(tBu)Me2 (c) 1241

1240

1239 Scheme 404

6.01.8.1.2

By isomerization of alkynes

The most convenient route to vinylidene complexes is the isomerization of alkynes on d6 metal fragments. For example, mononuclear vinylidene complexes of the type [(Z5-MeC5H4)Mn{]C]CR(SnMe3)}(dmpe)] (1243, R ¼ SnMe3 (a), Ph (b), 3-thienyl (c), 4-MeC6H4 (d), SitBuMe2 (e)) have been obtained by the reactions of [(Z5-MeC5H4)Mn(Z6-cycloheptatriene)] (1242) with one equivalent of RC^CSnMe3 and the diphosphine Me2PCH2CH2PMe2 (dmpe) in toluene at 50  C for 3 h (Scheme 405). Treatment of these tin-substituted complexes with one equivalent of 1.0 M TBAF yielded the corresponding parent vinylidene complexes [(Z5-MeC5H4)Mn(]C]CHR)(dmpe)] (1244).507 Similar complexes have been obtained with the metal fragments [CpMn(depe)], [CpMn(dmpe)] and [(Z5-MeC5H4)(depe)].

Mn

Me3Sn

R

Mn

C

C

C R

Me2P PMe2

PMe2

R = H (a), Ph (b), 3-thienyl (c), 4-MeC6H4 (d), SitBuMe2 (e)

R = SnMe3 (a), Ph (b), 3-thienyl (c), 4-MeC6H4 (d), SitBuMe2 (e) 1242

C

Mn TBAF

R

Me2P

dmpe

H

SnMe3

1244

1243

Scheme 405

Analogous half-sandwich dinuclear bis-vinylidene-bridged manganese complexes have been prepared similarly. For example, the d6 low-spin Mn(I) dinuclear complexes of the type [(Z5-MeC5H4)(dmpe)Mn]C]C(SnMe3)XC(SnMe3)]C]Mn (Z5-MeC5H4)(dmpe)] (1245, X ¼ 1,4-C6H4 (a), 1,3-C6H4 (b), 4,40 -C6H4C6H4 (c), and 2,5-C4H2S (d)) were obtained by treatment of [(Z5-MeC5H4)Mn(Z6-cycloheptatriene)] (1242) with 0.5 equivalent of the corresponding trimethylstannylated diacetylenes Me3SnC^CXC^CSnMe3 and dmpe at 50  C for 12 h (Scheme 406). These dinuclear tin-substituted vinylidene complexes reacted

+

Me3Sn

+

SnMe3

X

Me2P

Mn

PMe2

X : 1242

Mn

C

C

Me2P

S (a)

(b)

H

SnMe3 MeOH

Me2P

X PMe2

PMe2 C

(d)

(c)

C

Mn

C

Mn Me2P

Scheme 406

Me2P

X PMe2

1246

PMe2 C

H

Me3Sn 1245

C C

Mn

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

167

with excess of MeOH to give the corresponding tin-free dinuclear vinylidene complexes 1246.643,644 Similar complexes have been obtained with the metal fragment [(Z5-MeC5H4)(depe)]. Re(I) vinylidene complexes of the type [ReI(]C]CHR)(dppm)2] (dppm ¼ PPh2CH2PPh2) can be prepared by reactions of [Re(dppm)3]I with terminal alkynes. For example, treatment of [Re(dppm)3]I with HC^CPh produced the rhenium vinylidene complex [ReI(]C]CHPh)(dppm)2] (1247) (Scheme 407).645 The closely related complexes [Re(]C]CHR)(C^CTMS)(PMe3)4]509 have also been prepared by a similar route as will be described in the section dealing with the chemistry of carbyne complexes. + I-

Ph2P Ph2 P Re

PPh2

Ph2P H

Ph

H C

C

Ph

Ph2P

PPh2

P Ph2 Ph2P

PPh2

Re

I

PPh2

dppm

1247

Scheme 407

6.01.8.1.3

By CdC bond activation of alkynols

Vinylidene complexes of the type [ReI(]C]CHR)(dppm)2] can also be obtained by CdC bond cleavage reactions of [Re(dppm)3] I with internal propargyl alcohols. For example, the vinylidene complex [ReI(]C]CHPh)(dppm)2] (1247) can be obtained by treating [Re(dppm)3]I with PhC^CC(OH)R0 R00 (C(OH)R0 R00 ¼ C(OH)Ph2, C(OH)Me2, C(OH)HPh and C(OH)H2) in the presence of DBU. The vinylidene complexes 1249 and 1251 were similarly obtained from alkynols 1248 and 1250, respectively (Scheme 408). Computational studies suggest that the reactions most likely proceed via b-alkynyl elimination involving the alkoxide complex intermediates [Re{OC(R0 )(R00 )C^CR}(dppm)2].646

Ph2P HO

OH

I

H Ph

Ar Ar 1248

PPh2

[Re(dppm)3]I (1) - dppm, PhCH=O

C

PPh2

1249

Ph2P

Ar = Ph (a), p-tolyl (b)

[Re(dppm)3]I

HO

- dppm, Me2C=O 1250

H C

Re

Ph2P

PPh2

I Re C Ph2P PPh2

Ar

C OH Ar

H C

1251

Scheme 408

6.01.8.1.4

Preparation of M]C]M0 complexes

Complexes closely related to vinylidene complexes are m-carbido dinuclear complexes. The m-carbido complex [{Cp (CO)2Re}2(m-C)] (1252) was produced along with complexes [CpRe(CO)2(CS)] and [CpRe(CO)2(PPh3)] in the reaction of [CpRe(THF)(CO)2] with CS2 and PPh3 (Scheme 409).647 The trimetallic complexes [(porphyrin)Fe]C]M(CO)4-M(CO)5] (1254a, M ¼ Mn, porphyrin ¼ tetrakis(p-cyanophenyl)porphyrinato; 1254b, M ¼ Re, porphyrin ¼ tetrakis(phenyl)porphyrinato) were generated in the reactions of the porphyrinato-dichlorocarbene iron complexes 1253 with the carbonyl metalates Na[M(CO)5] (M ¼ Mn, Re).648

168

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

CS2/PPh3

Re OC OC

O

Re OC OC

Ph3P=S

OC OC

CS

+

Re

+

CO

PPh3

M

OC

1253

CO

1252

CO M

CO M = Mn, Re

CO OC

C

Fe(porphyrin)

OC

OC

+ [M(CO)5]-

Re

CO CO

Cl2C Fe(porphyrin)

C

Re OC

CO

1254a:M = Mn, porphyrin = TCNP 1254b: M =Re, porphyrin = TPP

Scheme 409

6.01.8.2

Synthesis of allenylidene complexes

Electrophilic abstraction of alkoxy vinylcarbene complexes represents a common route to allenylidene complexes. For example, the allenylidene complex [CpMn{]C]C]CHPh}(CO)2] (1256) can be generated by the reaction of the alkoxycarbene complex [CpMn{]C(OEt)dCH]CHPh}(CO)2] (1255) with BCl3 (Scheme 410).649 Another route to allenylidene complexes is to deprotonate vinylcarbyne complexes. For example, the allenylidene complex [ReI(]C]C]CPh2)(dppm)2] (1258) can be obtained by treating the vinylcarbyne complex [ReI(^CCH]CPh2)(dppm)2]I (1257a) with LiBHEt3.646

H Mn

Ph

OC

Ph

BCl3/Et3N

C

H

OC

H OC

1255

1256

Ph2P

H

PPh2 I

Re

C

1257a

+ I-

Ph2P LiBHEt3

C C

PPh2

C C

OC

OEt

Ph2P

C

Mn

Ph

Ph

Ph

PPh2 I

Re

C

C

C Ph

Ph2P PPh2 1258

Scheme 410

The most convenient route to allenylidene complexes is to use reactions of terminal alkynols with coordinatively unsaturated d6 metal complexes. For example, treatment of the Re(I) complex [Re(OTf )(CO)2(triphos)] (1259) with 1-(phenyl)-1(p-nitrophenyl)-2-propyn-1-ol afforded the cationic allenylidene complex [Re{]C]C]CPh(p-C6H4NO2)}(CO)2(triphos)]OTf (817) (Scheme 411).481 Analogous mononuclear and dinuclear allenylidene rhenium complexes (e.g. 1260–1263) have been prepared similarly from the corresponding bisalkynols.650

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

169

Scheme 411

6.01.8.3

Reactivity of vinylidene and allenylidene complexes

6.01.8.3.1

Electrophilic addition, abstraction and oxidative coupling reactions

Vinylidene complexes [LnM]C]CRR0 ] can be protonated at the b-carbon to give carbyne complexes [LnM^CCHRR0 ]+. Hydroxyvinylidene complexes [LnM]C]CHC(OH)RR0 ] can undergo electrophilic abstraction of the OH group to give vinylcarbyne complexes [LnM^CCH]CRR0 ]+. Vinylidene complexes [LnM]C]CRR0 ] may also undergo oxidative coupling reactions to give dinuclear carbyne complexes [LnM^C-CRR0 -CRR0 C^MLn0 ]2+ These materials will be presented in the section dealing with . preparation of carbyne complexes.

6.01.8.3.2

Deprotonation reactions

Vinylidene complexes with a b-hydrogen atom can be deprotonated to give alkynyl complexes. For example, treatment of the vinylidene complexes [Mn(C^CR)(]C]CH2)(dmpe)2] (1241, R ¼ SiEt3 (a), R ¼ SiiPr3 (b), Si(tBu)Me2 (c)) with two equivalents of [Cp2Fe]PF6 and one equivalent of quinuclidine produced the asymmetrically substituted trans-bis(alkynyl) complexes [Mn(C^CR)(C^CH)(dmpe)2] (1264) (Scheme 412).506

Me2P R

PMe2 Mn

C

PMe2

Me2P

H C

quinuclidine H

Me2P

[Cp2Fe]PF6 R

Mn Me2P

+ PF6-

PMe2 C

C

H

PMe2

R = SiEt3 (a), R =SiiPr3 (b), Si(tBu)Me2 (c) 1241

1264

Scheme 412

6.01.8.3.3

Nucleophilic addition reactions

Both vinylidene and allenylidene complexes can undergo nucleophilic addition reactions. Tertiary phosphines were reported to add stereoselectively to the vinylidene complexes [CpM(]C]CHPh)(CO)2] (694, M ¼ Mn, 695, M ¼ Re) to give the corresponding zwitterionic (Z)-phosphoniostyryl complexes [CpM{C(PR3)]C(H)Ph}(CO)2] (e.g. 696, M ¼ Mn; 697, M ¼ Re; PR3 ¼ PPh2Me (a), PMe3 (b)) (Scheme 413).467 Phosphites similarly reacted with [CpM(]C] CHPh)(CO)2] (M ¼ Mn, Re) to afford analogous zwitterionic complexes (e.g. complexes 761, 762, 1265, 1266). These complexes are sensitive to H2O and can be readily hydrolyzed to afford the corresponding Z2-phosphorylalkene complexes [CpM {Z2-(RR0 P(O))CH]CHPh}(CO)2].468

170

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Ph C

Ph C

M

+ PR3

C

OC

M OC

H

OC

OC PR3 = PMePh2 (a), PMe3 (b) 694: M = Mn 695: M = Re

H

C PR3

696: M = Mn 697: M = Re Ph

Ph C M OC

H

C

M

C

C

OC

P(OR)3

OC

Ph

P(OR)3

C

P(OEt)2Ph

M OC

H

P(OEt)2Ph

OC

OC

R = Me (a), Et (b), iPr (c)

694: M = Mn

1265: M = Mn

761: M = Mn;

695: M = Re

1266: M = Re

762: M = Re

H

C

Scheme 413

Hydrophosphoryl compounds HP(O)R2 (R ¼ Ph, C6F5) and HP(S)Ph2 reacted with the vinylidene complexes [CpM(]C] CHPh)(CO)2] (694, M ¼ Mn, 695, M ¼ Re) to give Z2-(E)-phosphorylalkene complexes [CpM{Z2-(R2(O)P)CH]CHPh}(CO)2] (1267, M ¼ Mn; 1268, M ¼ Re; R ¼ Ph (a), C6F5 (b)) and [CpM{Z2-(Ph2(S)P)CH]CHPh}(CO)2] (1269, M ¼ Mn, 1270, M ¼ Re), respectively (Scheme 414). DFT/B3LYP(6-31G ) analysis shows that the reactions most likely proceed via initial nucleophilic attack followed by protonation.651

Ph C C OC

H P

O

Ph

R2P(O)H

M OC

Ph

H

R R

R = Ph (a), C6F5 (b) 1267: M = Mn; 1268: M = Re

M

C

C

OC

H OC

C

Ph2P(S)H

H

M OC

C OC

H P

S 694: M = Mn

1269: M = Mn

695: M = Re

1270: M = Re

Ph Ph

Scheme 414

The vinylidene complex fac-[Mn(]C]CHCO2Me)(CO)3(dppm)]BF4 (1238) generated at 200 K by protonation of the corresponding alkynyl complex underwent a spontaneous insertion of the vinylidene ligand into a MndP bond to form the coupled product 1271 above 243 K (Scheme 415).480 The reaction can be viewed as intramolecular nucleophilic addition of a phosphine to a vinylidene ligand. Vinylidene complexes can also undergo nucleophilic addition reactions with RXH. For example, thiophenol and benzeneselenol reacted with the vinylidene complexes [Re(]C]CH2)(NO)2(PR3)2][BArF 4] (1153, R ¼ Cy (a), iPr (b)) to yield the cationic Fischer-type carbene complexes [Re{]C(XPh)Me}(NO)2(PR3)2]BArF4 (1154S, X ¼ S, 1154Se, X ¼ Se) (Scheme 375).

Scheme 415

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

171

Allenylidene complexes can be attacked by nucleophiles at the a or g carbons. For example, the manganese allenylidene complex [CpMn(]C]C]CPh2)(CO)2] (1272) reacted selectively with tertiary phosphines PR0 3 (e.g. PPh2Me (a), PPhMe2 (b), and PMe3 (c)) at the Ca atom to give the (Z)-a-phosphonioallenyl complexes [CpMn{C(PR0 3)]C]C(Ph)R}(CO)2] (1273) (Scheme 416). The reaction of the allenylidene complex 1272 with the diphosphine dppm produced the analogous mononuclear adduct 1273d. In contrast, the reaction of the allenylidene complex 1272 with dppe afforded the dinuclear adduct 1274 even if the reagent ratio is 1:1.649 Similarly, treatment of the monophenylallenylidene complex [CpMn(]C]C]CHPh)(CO)2] (1256) with methyldiphenylphosphine afforded the adduct 1275, which can react with HBF4 to give the phosphonioallene complex 1276.

Ph

Ph C

C H

C

Ph

PR3

Mn C OC

1273

C

dppe

OC

C C Mn

Mn C

Mn C C C

PR3

OC

Ph H H C

OC

Ph

OC

OC

Ph2P

PPh2

1272

PR3 = PMePh2 (a), PMe2Ph (b), PMe3 (c), dppm (d)

1274 Ph

Ph C Ph Mn

C

C C

OC

H

OC

Mn

OC

C

OC OC

PMePh2

1256

H

C HBF4

Mn C

OC

C

H

C

PMePh2

CO CO

1275

BF4H

PMePh2

1276

Scheme 416

The allenylidene complex [Re{C]C]CPh(p-C6H4NO2)}(CO)2(triphos)]OTf (817) reacted with 1.2 equivalents of PMePh2 in CD2Cl2 at −85  C to give the g-phosphonioalkynyl complex [Re{C^CCPh(p-C6H4NO2)(PMePh2)}(CO)2(triphos)]OTf (818) (Scheme 417).481 In contrast, the allenylidene 817 reacted with the sterically less demanding phosphine PMe3 to produce the a-phosphonioallenyl derivative 1277. A theoretical study on reactions of [Re(]C]C]CPh2)(CO)2(triphos)]+ with tertiary phosphines PMe3-xPhx (x ¼ 0, 1, 2, and 3) indicates that the products resulting from the attack on Cg are kinetically favored, while the products from the attack on Ca are thermodynamically favored.652

PMePh2 Ph2P Ph2P

Re

PPh2 CO Ph2P Ph2P

Re

C

C

C

C Ph

818

NO2 + OTf-

C Ph

CO

PPh2 CO PMe

817 PMe3

Ph2P Ph2P

Re

3

C C

CO 1277 Scheme 417

C

NO2

PMePh2 CO

+ OTf-

+ OTf-

PPh2 CO

C Ph

NO2

172

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

The allenylidene ligand in the complex [(triphos)(CO)2Re{C]C]CPh2}]OTf (698; triphos ¼ MeC(CH2PPh2)3, OTf ¼ OSO2CF3) can undergo 1,2,3-diheterocyclization reactions with N,N-heterocycles. One example of such reactions involving reaction of 2-aminopyridine has been described in the section dealing with vinyl complexes. As additional examples (Scheme 418), the complex 698 reacted with pyrazole to afford the heterobicyclic product [Re{C]C(H)C(Ph)2N(CH)3N}(CO)2(triphos)]OTf (1278), with 1H-benzotriazole to give [Re{C]C(H)C(Ph)2N(C6H4)N]N}(CO)2(triphos)]OTf (1279), and with 2-aminothiazole to give [Re{C]C(H)C(Ph)2N(H)CS(CH)2N}(CO)2(triphos)]OTf (1280).442 + OTf-

PPh2 CO

Ph

C

Re

Ph2P Ph2P

N

C

C

NH2 S

Ph2P Ph2P

Ph CO

698

N N

+

Ph

PPh2 H CO C Re C N CO

Ph

OTf-

C N H S

1280

NH

N NH

Ph2P Ph2P

+ H Ph PPh2 OTfCO C Ph C Re C N N CO

Ph2P Ph2P

+ H Ph OTfPPh2 CO C Ph C Re C N N CO N 1279

1278 Scheme 418

6.01.8.3.4

Formation of vinylidene bridged dinuclear complexes and clusters

Complexes of the type [CpM(]C]CHR)(CO)(L)] (M ¼ Mn, Re; L ¼ CO, phosphine) have often been used as starting materials for preparation of vinylidene-bridged dinuclear complexes and clusters. Reactions of the vinylidene complex [CpRe(]C]CHPh)(CO)2] (695) with [M(PR3)4] (M ¼ Pd, Pt) can give m-vinylidene complexes [Cp(CO)2ReM(m-C]CHPh)(PR3)2]. For example, the reactions of [CpRe(]C]CHPh)(CO)2] (695) with [M(PPh3)4] (M ¼ Pd, Pt) gave the m-vinylidene complexes [Cp(CO)2ReM(m-C]CHPh)(PPh3)2] (1281, M ¼ Pt, 1282, M ¼ Pd) (Scheme 419).653–656 Dinuclear RePt complexes of the type [Cp(CO)2RePt(m-C]CHPh)(L)2] (1283, L ¼ P(OPh)3 (a), P(OEt)3 (b), P(OiPr)3 (c)) were similarly prepared from reactions of the vinylidene complex [CpRe(]C]CHPh)(CO)2] (695) with [Pt{P(OR)3}4] (R ¼ iPr, Et, Ph).657 Treatment of the vinylidene complex [CpRe(]C]CHPh)(CO)2] (695) with CuCl resulted in the formation of the heterometallic m-vinylidene complex [Cp(CO)2ReCu(m2-C]CHPh)(m2-Cl)]2 (1284).658,659

Ph

H C

Ph C

Re

C

OC

[Pt(PPh3)4] or [Pd(PPh3)4]

C

OC

M

Re

H

CO

PPh3 PPh3

OC 695

1281: M = Pt; 1282: M = Pd

[Pt(P(OR)3)] Ph

CuCl H

C

Ph Pt

Re CO

H C

C

OC

P(OR)3 P(OR)3

C

OC Re

OC

Cl Cu

Scheme 419

CO

C

R = Ph (a), Et (b), iPr (c) 1283

Re

Cu Cl

CO

C 1284

H

Ph

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

173

The reaction of the vinylidene complex [CpMn(]C]CHPh)(CO)2] (694) with [Fe2(CO)9] gave the heterometallic trimethylenemethane complex 1286 (Scheme 420), presumably via the intermediate 1285 that undergoes intramolecular coupling between the carbonyl and the vinylidene ligands to generate the benzylideneketene [PhHC]C]C]O] fragment. The complex [CpMn(] C]CHPh)(CO)(PPh3)] (1287) reacted similarly to give the metalla-trimethylenemethane complex 1288.660–662

Ph

H C

Ph Mn C

[Fe2(CO)9]

C

OC

Fe CO

OC 694

Mn

C

OC Mn

H

OC

CO CO

C O

CO CO

OC Mn C OC Ph3P 1287

Mn

[Fe2(CO)9]

C H

Ph H Fe(CO)3

1286

1285

Ph

CO

C O

PPh3 Ph H Fe(CO)3

1288

Scheme 420

The PPh3 ligand in the dinuclear m-vinylidene complexes [Cp(CO)2ReM(m-C]CHPh)(PPh3)2] can be replaced by ligands such as phosphites, carbon monoxide and chelate diphosphines (Scheme 421). For example, the dinuclear complex [Cp(CO)2Re(m-C]CHPh)Pt(PPh3)2] (1281) reacted with P(OR)3 to give [Cp(CO)2RePt(m-C]CHPh)(PPh3){P(OR)3}] (1289, P(OR)3 ¼ P(OPh)3 (a), P(OEt)3 (b), P(OiPr)3 (c)),657 with [Co2(CO)8] or [Rh(acac)(CO)2] to produce the heterodinuclear m-vinylidene complex [Cp(CO)2Re(m-C]CHPh)Pt(PPh3)(CO)] (1290),663 and with diphosphines Ph2P(CH2)nPPh2 (n ¼ 1, 2, 3) to afford complexes [Cp(CO)2RePt(m-C]CHPh)(Ph2P(CH2)nPPh2)] (1291, n ¼ 1 (a), 2 (b) and 3 (c)).656

Scheme 421

Similarly, the RedPd dinuclear m-vinylidene complex [Cp(CO)2RePd(m-C]CHPh)(PPh3)2] (1282) reacted with the diphosphines dppe and dppp to give [Cp(CO)2RePd(m-C]CHPh)(dppe)] (1292) and [Cp(CO)2RePd(m-C]CHPh)(dppp)] (1293), respectively (Scheme 422).655 Interestingly, the reaction between the MndPd dinuclear m-vinylidene complex [Cp(CO)2MnPd(m-C]CHPh)(PPh3)2] (1294) and dppe produced the expected dinuclear complex [Cp(CO)2MnPd(m-C]CHPh) (dppe)] (1295) and the unexpected tetranuclear complex 1296.664

174

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Ph

H

Ph

C C

OC

dppe

CO

Ph

H dppe

C

OC

Pd

Mn

CO Ph P 2

PPh3

1293

H

H C

C

C

CO

PPh2

C

OC Mn

PPh3

C

PPh3

Pd

PPh2

Ph H

C

C O

CO

C Pd

Mn

Ph2P

Ph C

Pd

+ Ph2P

O 1295

1294

PPh2

Pd

Re

dppp

PPh3

1282

1292

Ph

C

OC Pd

Re

CO Ph P 2

H C

C

OC PPh2

Pd

Re

Ph

H C

1296

Mn C

C

O

O

Scheme 422

Treatment of the RedPd dinuclear m-vinylidene complex [Cp(CO)2RePd(m-C]CHPh)(PPh3)2] (1282) with [Cp(CO)2RePd(m-C]CHPh)(dppe)] (1292) in dichloromethane produced the tetranuclear di-vinylidene complex [Cp2(CO)4RePd(m-C]CHPh)RePd(m3-C]CHPh)(dppe)] (1297) (Scheme 423). This cluster consists of two RePd fragments linked to each other through the bonding of the palladium atom in the [Cp(CO)2RePd(m-C]CHPh)] fragment to the carbonyl and vinylidene ligands in the Cp(CO)2RePd(m-C]CHPh)(dppe) fragment.665

Ph

Ph

H C

OC

CO

PPh3

PPh2

CO

1292

1282

C

C

Pd

PPh2

Re CO

Pd

Re

CO Ph P 2

PPh3

Ph

C

OC Pd

Re

H C

C

OC

+ Pd

Re

Ph

C

C

Ph2P

H

H

C

1297

O

Scheme 423

The dinuclear complexes [Cp(CO)2RePt(m-C]CHPh)(PR3)2] can be used as building block for the synthesis of trimetallic MM0 M00 m3-vinylidene clusters. For example, treatment of the RedPt dinuclear m-vinylidene complex [Cp(CO)2RePt(m-C]CHPh) (PPh3)2] (1281) with [Fe2(CO)9] afforded the heterometallic m3-vinylidene cluster [CpReFePt(m3-C]CHPh)(CO)6(PPh3)] (1298), in which the m3-C]CHPh ligand is Z1-bound to the Re and Pt atoms and Z2-coordinated to the Fe atom (Scheme 424).666,667 The analogous cluster [CpReFePt(m3-C]CHPh)(CO)5{P(OiPr)3}2] has been synthesized similarly by the reaction of [Cp(CO)2RePt(m-C]CHPh){P(OiPr)3}2] with [Fe2(CO)9].668 Clusters complexes have also been obtained from the reactions of [Fe2(CO)9] with [Cp(CO)2RePd(m-C]CHPh)(dppe)] (1292) and [Cp(CO)2RePd(m-C]CHPh)(dppp)] (1293).655,669

Ph Ph

H C [Fe2(CO)9]

C

OC

Pt

Re CO

C

OC PPh3

Scheme 424

CO

C

OC

Fe

Mn

PPh3 1298

CO

CO CO

Pt Ph3P

1281

H

CO

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

6.01.8.3.5

175

Reactions in which vinylidene complexes are implicated

Vinylidene complexes have been suggested as reactive intermediates in several catalytic reactions. The rhenium hydridocarbonyl complex [ReH(CO)4]n catalyzed the anti-Markovnikov addition of stabilized carbanions (e.g. anions derived from esters R0 CH (CO2Et)2) to terminal alkynes to give (E)-olefin derivatives (e.g. 1299, Scheme 425).670 The transformation was proposed to proceed through generation of a rhenium vinylidene intermediate 1300, followed by nucleophilic addition of an ester to afford an hydrido-vinyl intermediate 1301, which undergoes reductive elimination to give the anti-Markovnikov adduct 1299 with the regeneration of the active rhenium species. R' R

+

[ReH(CO)4]n EtO2C

CO2Et CO2Et R'

R

CO2Et R' = CO2Et, Ar

1299

R R

H

[Re] C C

H

C

[Re] C

H

R'

1300 EtO2C

CO2Et

CO2Et 1301 R' CO2Et

Scheme 425

The rhenium complex [ReBr(CO)5] catalyzed reactions of terminal alkynes with imines to give N-alkylideneallylamine derivatives (e.g. formation of 1302) (Scheme 426).671,672 The transformation was proposed to proceed through a vinylidene rhenium intermediate 1303 generated by nucleophilic attack of an alkynyl intermediate on an imine carbon. An 1,5-shift of the hydrogen atom adjacent to the nitrogen atom to the a-carbon atom of the vinylidene moiety in 1303 and elimination of HBr would give the rhenium vinyl species 1304, which undergoes protolysis to afford the product 1302. R C6H13

+ H2 C N

H R'

C6H13

[ReBr(CO)5]

N 1302

[Re]

C6H13 [Re] C C Br

H H R

R' 1303

R

NH HBr

R' R

C6H13

C C N

R' 1304

Scheme 426

The complex [ReI(CO)5] catalyzed formal [3 + 4] cycloaddition of propargylic benzyl ethers and dienes to give seven-membered ring products (e.g. formation of 1307 from the alkyne 1305 and the diene 1306) (Scheme 427).673 The transformation was proposed to proceed through the initial formation of the rhenium vinylidene complex 1308, which undergoes hydride transfer (to give 1309) and elimination of PhCHO to afford the carbene complex 1310. The reaction of 1310 with the diene CH2]C(OTIPS)CH]CHAr would afford the divinylcyclopropane 1311, which can rearrange to the seven-membered ring product 1307.

176

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Ph TIPSO

O +

H

Ar 1305

PhCHO

1306

Ph

Ph

OTIPS C

H C

C

C

[Re]

C

H

C

TIPSO Ar

H

H

[Re]

H 1308

Ar 1307

PhCHO

O

O [Re]

OTIPS

[ReI(CO)5]

1309

1310

Ar 1311

[Re]

Scheme 427

As shown in Scheme 428, Re(I) complexes such as [ReBr(CO)5], [ReI(CO)5] and [ReBr(CO)3(THF)2] can catalyze allylation of indoles with propargylic ethers (e.g. formation of 1313 from 1305 and 1312),674 propargylation of silyl enol ethers and propargylic ethers to give alkyne-functionalized silyl ethers (e.g. formation of 1316 from 1314 and 1315),675 and isomerization of 3-iodopropargyl ethers to give 2-iodo-1H-indene derivatives (e.g. formation of 1318 from 1317).676 These catalytic reactions were suggested to involve vinylidene complexes analogous to 1308 and carbene complexes analogous to 1310.

Ph [ReI(CO)5]

O +

H 1305

N

N

1312

1313

Ph Ph O H

+

iPr OTIPS 1315

1314

iPr

[ReX(CO)5] (X = Br, I)

OTIPS H 1316

Ph O

[ReBr(CO)3(thf)]2

I

I Ph 1317

1318

Scheme 428

6.01.9

Carbyne complexes

In the review period, only a few mononuclear manganese carbyne complexes of the type [Cp0 Mn(^CR)L2]+ have been described, and no new technetium carbyne complexes have been isolated. In contrast, the chemistry of rhenium carbyne complexes has received much interest. Well characterized rhenium carbyne complexes have been described for those with a Re(V) or Re(VII) center, with the former ones being dominant. A review on the chemistry of rhenium carbyne complexes appeared in 2013.677 The chemistry of rhenium carbyne complexes has also been reviewed in articles dealing photophysical properties of carbyne complexes.678

6.01.9.1 6.01.9.1.1

Synthesis By electrophilic abstraction of Fischer carbenes

Complexes of the type [Cp0 M(^CR)(CO)2]+ can be prepared by electrophilic abstraction of the alkoxy group OR0 of Fischer carbenes [Cp0 M{]C(OR0 )R}(CO)2]. For example, treatment of [(Z5-MeC5H4)Mn{]C(OEt)Ar}(CO)2] (1319, Ar ¼ Ph (a), p-tolyl (b)) with BCl3 produced the carbyne complexes [(Z5-MeC5H4)Mn{^CAr}(CO)2]BCl4 (1005BCl4), which underwent metathesis

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

177

reactions with NaBPh4 to give the BPh4 salts [(Z5-MeC5H4)Mn{]CAr}(CO)2]BPh4 (1005) (Scheme 429). The BPh4 salts 1005 are much more stable than the corresponding BCl4 and BBr4 salts in the solid state, and could be washed with water.561 The carbyne complex [Cp Re(^CPh)(CO)2]BCl4 has been prepared in a manner similar to that for carbyne complexes 1005BCl4 from [Cp Re {]C(OR0 )Ph}(CO)2].618

+ BCl4-

Ar

BCl3

C

Mn OC OC

NaBPh4

C Ar

Mn

Mn

OC OC

OEt Ar = Ph (a), p-tolyl (b)

1319

+ BPh4C Ar

OC OC 1005

1005BCl4

Scheme 429

6.01.9.1.2

By oxidation of vinylidene complexes

Carbyne complexes of the type [Cp0 Mn(^CR0 )(R2PCH2CH2PR2)]+ can be synthesized by oxidation of vinylidene complexes [CpMn(]C]CHR0 )(R2PCH2CH2PR2)]. For example, treatment of vinylidene complexes of the type [CpMn(depe)(]C]CHR)] (1320, R ¼ H (a), Ph (b), 3-thienyl (c)) with one equivalent of [Cp2Fe]PF6 led to the dinuclear biscarbyne complexes [Cp(depe) Mn^CCHRCHRC^Mn(depe)Cp](PF6)2 (1321) (Scheme 430).507,643 Similar reactions occurred for [(Z5-MeC5H4)Mn(dmpe) (]C]CHR)] (1244, R ¼ H (a), Ph (b), 3-thienyl (c)) to give the dinuclear biscarbyne complexes [(Z5-MeC5H4)(dmpe) Mn^CCHRCHRC^Mn(dmpe)(Z5-MeC5H4)](PF6)2 (1322). The oxidative coupling reactions presumably proceed through the radical cations [CpMn(depe)(]C]CHR)]+. The thermodynamic feasibility of dimerization of the radical cations [CpMn(]C] CHR)(PH3)2]+ (R ¼ H, Me, Ph) has been studied computationally.679

H Mn

C

[Cp2Fe]PF6

C

Mn R

Et2P

C

Et2P

PEt2

H

H

C

C

R

R

2+ 2PF6C

PEt2

PEt2

H Mn

Et2P 1321

R = H(a), Ph (b), 3-thienyl (c)

1320

C

Me2P

C

PMe2 1244

[Cp2Fe]PF6 Mn

R

Mn

Me2P

C

H

H

C

C

R

R

PMe2 R = H(a), Ph (b), 3-thienyl (c)

2+ 2PF6C

Mn PMe2 Me2P

1322

Scheme 430

Oxidation of vinylidene complexes may give other products. For example, treatment of the vinylidene complexes [CpMn(]C] CHR)(dmpe)] (1323, R ¼ H (a), Ph (b), 3-thienyl (c), 4-tolyl (d)) with one equivalent of [Cp2Fe]PF6 led to a mixture of species, including the oxidative coupling products [Cp(dmpe)Mn^CCHRCHRC^Mn(dmpe)Cp](PF6)2 (1324), the dinuclear biscarbyne complexes of the type [Cp(dmpe)Mn^CCR]CRC^Mn(dmpe)Cp](PF6)2 (1325) and the cationic mononuclear carbyne complexes [CpMn(^CCH2R)(dmpe)]PF6 (1326) (Scheme 431).507 Similar reactions have also been described for [(Z5-MeC5H4)Mn(depe)(]C]CHR)].

178

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

H Mn

Mn R

Me2P

C

C

R

R

PMe2

1323

C

C

Me2P

C PMe2

Mn

C

PMe2 Me2P

1324

2+ 2PF6-

R Mn

C

Me2P

PMe2

+

H

H

[Cp2Fe]PF6

C

C

2+ 2PF6-

C

+

Mn

Mn Me2P

PMe2

R

H C

R

H PMe2 1326

Me2P

1325

C

+ PF6-

R = H(a), Ph (b), 3-thienyl (c), p-tolyl (d) Scheme 431

Oxidation of the vinylidene complex trans-[(Me3SiC^C)Re(]C]CH2)(PMe3)4] (879) with 1.2 equivalent of [Cp2Fe]PF6 at −78  C gave the CbdC0 b coupled dinuclear Re biscarbyne complex trans-[(Me3SiC^C)(PMe3)4Re^CdCH2dCH2dC^Re (PMe3)4(C^CSiMe3)](PF6)2 (1327) (Scheme 432).509 Me3P

PMe3

Me3Si

Re

C CH2

+

[Cp2Fe]PF6

PMe3

Me3P 879

Me3P Me3Si

Re Me3P

C CH2 CH2 C

PMe3

+2

PMe3

Me3P

PMe3

SiMe3

Re

Me3P

2PF6-

PMe3

1327 Scheme 432

6.01.9.1.3

By electrophilic addition of vinylidene or allenylidene complexes

Protonation of vinylidene or allenylidene complexes represent a common route to Re(V) carbyne complexes. For example, protonation of the vinylidene complex trans-[(Me3SiC^C)Re(]C]CH2)(PMe3)4] (879) with HCl produced the carbyne complex trans-[Re(C^CSiMe3)(^CMe)(PMe3)4]Cl (878Cl) (Scheme 433).509 The PF6-salt of the mononuclear rhenium carbyne complex trans-[Re(C^CSiMe3)(^CdMe)(PMe3)4]PF6 (878) can be obtained by heating a mixture of the dinitrogen complex trans[ReCl(N2)(PMe3)4], TlPF6, and an excess of HC^CSiMe3.

Me3P Me3Si

PMe3 Re C CH2

Me3P

PMe3 879

Scheme 433

HCl

Me3P Me3Si

PMe3 Re

Me3P

Me

PMe3

878Cl

+ Cl-

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

179

The strategy has been used to prepare Re(V) carbyne complexes of the type [ReI(^CR)(dppm)2]+. For example, protonation of the vinylidene complexes 1247645 and 1251646 with HI produced the carbyne complexes 1328a and 1328b, respectively (Scheme 434). Protonation of the hydroxyvinylidene complexes 1249 with HI produced the vinylcarbyne complexes 1329.646

Scheme 434

Re(V) carbyne complexes of the type [ReI(^CR)(dppm)2]+ can also been prepared by protonation of vinylidene intermediates generated in situ from the reaction of [Re(dppm)3]I with terminal alkynes. For example, one-pot reactions of [Re(dppm)3]I/HI with HC^CAr (Ar ¼ Ph, p-tolyl, C6H4-o-CHO) and HC^C(CH2)4CH3/NaBPh4 give the rhenium carbyne complexes [ReI(^CCH2Ar) (dppm)2]I (1328) and [ReI(^C(CH2)5Me)(dppm)2]BPh4 (1330b), respectively (Scheme 435).645 The analogous reaction with HC^CSiMe3 gives [ReI(^CMe)(dppm)2]I (1330a). One-pot reactions of [Re(dppm)3]I, alkynols HC^CC(OH)RR0 (RR0 ¼ Ph2, Me2, PhMe) and HI produced the vinylcarbyne complexes [ReI(^CCH]CRR0 )(dppm)2]I (1329).

Ph2P I Ph2P

PPh2 H H Re C C

ITMS

H

PPh2

HI/H2O

(CH2)4Me

PPh2 H

Re C I Ph2P PPh2

H

(CH2)4Me

PPh2

HI

+ BPh4-

C

HI

Ph2P I Ph2P

+

PPh2 C

C C

PPh2

H

I-

C(OH)(Me)Ph

R

Re

PPh2 H H Ar Re C C I Ar Ph2P PPh2 1328a, Ar = Ph 1328c, Ar = p-C6H4OMe, 1328d , Ar =p-C6H4CHO

C(OH)R2

HI, NaBPh4 Ph2P

Ph2P

PPh2

P Ph2 Ph2P

1330a

+

+ I-

Ph2P Ph2 P Re

+

I-

HI

+ Ph2P

R I

Ph2P

Me PPh2

Re PPh2

C

C

I-

Ph

C H

R = Ph (a), Me (c) 1330b

1329

1329b

Scheme 435

A series of Re(V) carbyne complexes have been obtained by treatment of [ReH5(PR3)3] (PR3 ¼ PMe2Ph, PMePh2) with terminal alkynes in the presence of HCl. For example, treatment of the PMePh2-complex [ReH5(PMePh2)3] with the terminal alkyne HC^CPh in the presence of two equivalents of HCl produced the carbyne complex [Re(^CCH2Ph)Cl2(PMePh2)3] (1331) (Scheme 436).680 The reactions of the PMe2Ph-complex [ReH5(PMe2Ph)3] with terminal alkynes HC^CR (R ¼ Ph, SiMe3, nC5H11, p-C6H4CF3) in the presence of two equivalents of HCl produced a mixture of the carbyne complexes [Re(^CCH2R) Cl2(PMe2Ph)3] (681) and the Z2-vinyl complexes [Re(Z2-CH2]CR)Cl2(PMe2Ph)3] (682), as described in the section dealing with vinyl complexes.435,436 In contrast, treatment of [ReH5(PMe2Ph)3] with HC^CAr (Ar ¼ o-C6H4CF3 (a), o-C6H4Me (b), o-C6H4Br (c)) bearing an ortho-substituted aryl group in the presence of two equivalents of HCl produced only the carbyne complexes [Re(^CCH2Ar)Cl2(PMe2Ph)3] (1332).436

180

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

PMePh2 ReH5(PMePh2)3 +

Cl

2 HCl

Ph

CH2Ph

Cl Re Ph2MeP

toluene

PMePh2 1331 PMe2Ph R ReH5(PMe2Ph)3 +

R

Cl

2 HCl

CH2

Cl Re PhMe2P

toluene

PMe2Ph R = CF3 (a),Me (b), Br (c)

1332

Scheme 436

A computational study reveals that the carbyne complexes of the type [Re(^ CCH2R0 )Cl2(PR3)3] formed in the one-pot reactions are likely produced via the hydrido-alkyne intermediates [Re(Z2-HC^CR0 )Cl(H)2(PR)3] as illustrated in Scheme 437.436 Protonation of [ReH5(PR3)3] with HCl may initially give the cationic hexahydride 1333 which can lose two molecules of H2 to give the neutral unsaturated hydride complex 1334. The hydride complex 1334 can then react with a molecule of HC^CR0 to give the Z2-alkyne complex 1335. The complex 1335 can undergo an insertion reaction by transferring the hydride to the internal carbon of the alkyne to give the vinyl complex 1336, which can evolve to the vinylidene complex 1337 via a-hydrogen elimination.681,682 Protonation of 1337 with HCl at the b-carbon with elimination of H2 would give the carbyne complexes 1338.

- 2 H2

HCl ReH5(PR3)3

[ReH6(PR3)3]Cl

1334

1333

H

PR3 H

R3P

+ H

Re

R 3P Cl

R'

H

Cl

R 3P

PR3 1334 H

1338

H Re H PR3

R'

R'

C H

Cl

PR3

R'

1335

PR3 Cl Re R3P

ReH2Cl(PR3)3

R'

HCl

R3P Cl

H2

R3P

C

H

Cl

H

R3P

Re

C

H

C

PR3 Re H

PR3

PR3

1337

1336

Scheme 437

The chemistry can be extended to prepare rhenium vinylcarbyne complexes using alkynols. For example, the reaction of [ReH5(PMe2Ph)3] with the alkynol HC^CC(OH)Ph2 gave the carbyne complexes [Re{^CCH2C(OH)Ph2}Cl2(PMe2Ph)3] (1339) and [Re(^CCH]CPh2)Cl2(PMe2Ph)3] (1340) along with the Z2-vinyl complex Re{Z2-CH2]CC(OH)Ph2} Cl2(PMe2Ph)3 (1341) (Scheme 438).435 The complex [ReH5(PMe2Ph)3] reacted with the alkynol HC^CC(tBu)(OH) C^CSiMe3 in the presence of HCl to give a mixture of the vinylcarbyne complex [Re{^CCH]C(tBu)C^CSiMe3}Cl2(PMe2Ph)3] (198a) and the hydroxycarbyne complex [Re{^CCH2C(OH)(tBu)C^CSiMe3}Cl2(PMe2Ph)3] (1343) in a molar ratio of 0.95:1.683,684 Under similar condition, the hydride complex [ReH5(PMe2Ph)3] reacted with HC^CC(OH)(tBu)C^C(CH2)4Me and HC^CC(OH)(R)C^CSiMe3 (R ¼ isopropyl, and adamantyl) to give the carbyne complex [Re{^CCHC(tBu) C^C(CH2)4Me}Cl2(PMe2Ph)3] (1342)685 and [Re{^CCHC(R)C^CSiMe3}Cl2(PMe2Ph)3] (198, R ¼ adamantly (b), isopropyl (c)),683,684 respectively. These carbyne complexes are presumably formed by protonation of hydroxyvinylidene intermediates.

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

PMe2Ph Cl

PMe2Ph Cl Cl

Re

+

C CH2

PhMe2P

Cl

OH

Re

PMe2Ph

PMe2Ph

Cl

+ Ph

PhMe2P

Ph Ph

PMe2Ph OH Cl Ph C Re Ph

H

C

PhMe2P

Ph

1340

1339

181

C H2 PMe2Ph 1341

Ph Ph HCl OH PMe2Ph Cl H Cl

Re

Cl

OH

OH

Re

PhMe2P

Si PhMe2P

PMe2Ph Cl H

R

ReH5(PMe2Ph)3

PMe2Ph

HCl

R

PMe2Ph TMS

HCl

198b, R= admantyl 198c, R= iPr

1342 HCl

OH Si PMe2Ph Cl H Cl

PMe2Ph Cl

Re

PhMe2P

Cl

+

PhMe2P

PMe2Ph Me3Si

Re PMe2Ph

OH

Me3Si 1343

198a

Scheme 438

Complexes of the type [Re(^CR0 )Cl2(PR3)3] could be used as starting materials to make Re(V) carbyne complexes with other ligand environments. For example, treatment of [Re(^CCH2Ph)Cl2(PMePh2)3] (1331) with PMe3, NaI and Na2(mnt) produced the carbyne complexes [Re(^CCH2Ph)Cl2(PMe3)3] (1344), [Re(^CCH2Ph)I2(PMePh2)3] and [Re(^CCH2Ph)(mnt)(PMePh2)2] (1345), respectively (Scheme 439).680 The complex [Re(^CPh)Cl2(PMePh2)3] (1346) reacted with NaS2CNEt2 and NaCp to give the carbyne complexes 1347 and 1348, respectively. PMe3

PMePh2

Cl Cl Re Me3P

CH2Ph

PMe3

Cl Re Ph2MeP

PMe3

Na2(mnt) CH2Ph

NC

PMePh2

S

Ph

NaS2CNEt2

Cl

PMePh2

S 1345

PMePh2 Cl

S Re

PMePh2

S

1331

Et2N S

NC

Re

PMePh2

1344

Et2N

Ph

Cl

Re

Ph

ClNaCp

+ Re

Ph2MeP

S

PMePh2

1347

1346

PMePh2

PMePh2

Ph 1348

Scheme 439

The chloride ligands in complexes of the type [Re(^CR0 )Cl2(PR3)3] could also be replaced by hydride, alkyls, and alkynyls. For example, treatment of the SiMe3-protected dichloro carbyne complexes [Re{^CCH]C(R)C^CSiMe3}Cl2(PMe2Ph)3] (198, R ¼ tBu (a), 1-adamenyl (b)) with CH3MgCl in THF gave the methyl-carbyne complexes [Re{^CCH]C(R)C^CSiMe3}(Me)Cl (PMe2Ph)3] (199), which can be desilylated with nBu4NF to give the complexes [Re{^CCH]C(R)C^CH}(Me)Cl(PMe2Ph)3] (1349) (Scheme 440).119 Treatment of the complexes [Re{^CCH]C(R)C^CSiMe3}Cl2(PMe2Ph)3] (198, R ¼ tBu (a), 1-adamenyl (b), isopropyl (c)) with tert-butylmagnesium chloride produced the hydridochloridocarbyne complexes [Re {^CCH]C(R)C^CSiMe3}HCl(PMe2Ph)3] (1351) and CH2]CMe2, presumably through the intermediate [Re{^CCH]C(R) C^CSiMe3}Cl(tBu)(PMe2Ph)3] (1350). The Me3Si group in 1351 could be easily removed to give carbyne complexes 1352 by treatment with nBu4NF.683,684

182

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

PMe2Ph Cl Cl

CH3 CH3MgCl

Re

Cl

R

Re

TMS

Cl

H

Re R

PhMe2P PhMe2P TMS

199

1349

H

R = tBu (a), adamantyl (b), isopropyl (c) PMe2Ph

PMe2Ph

PMe2Ph

H

H Cl

Re

H

H nBu4NF

Re R

R

Cl

PhMe2P PhMe2P TMS

TMS

H

Re R

PhMe2P PhMe2P

PhMe2P PhMe2P 1350

nBu4NF

PhMe2P PhMe2P

tBuMgCl

Cl

CH3

H R

PhMe2P PhMe2P 198

PMe2Ph

PMe2Ph H

H

1351

1352

Scheme 440

6.01.9.1.4

By oxidation of dinuclear complexes with a C]CHdCH]C or C^CdC^C bridge

Dinuclear carbyne Re(V) complexes with a CdCH]CHdC or CdC^CdC bridge and the metal fragment [Re(PMe3)4(C^CSiMe3)] have been prepared by oxidation of dinuclear complexes with a C]CHdCH]C or C^CdC^C bridge.509 Deprotonation of the dinuclear complex 1327 with excess KOtBu gave the diamagnetic bisvinylidene complex [(Me3SiC^C)(PMe3)4Re]C]CHdCH]C]Re(PMe3)4(C^CSiMe3)] (1353) with an (E)-butadienediylidene bridge (Scheme 441). Oxidation of 1353 with two equivalents of [Cp2Fe]PF6 resulted in the formation of the diamagnetic dicationic complex [(Me3SiC^C)(PMe3)4Re^CdCH]CHdC^Re(PMe3)4(C^CSiMe3)](PF6)2 (1354) with an ethylenylidene dicarbyne

Me3P Me3Si

Re C CH2 CH2 C Me3P

PMe3

Me3Si

PMe3 Re C

Me3P

C

C

Me3Si

Re C Me3P

C

C

Re

Me3Si Me3P

Me3P C

C

PMe3

C Me3P

1355 Scheme 441

SiMe3

1354

PMe3 Re C

2PF6-

PMe3

Me3P

KOtBu [Cp2Fe]PF6 Me3P

+2

PMe3

Me3P

C

H

PMe3

1353

H

PMe3

SiMe3

Re

Me3P

[Cp2Fe]PF6 PMe3

PMe3

PMe3

Me3P

C H

Me3P

SiMe3

1327

H

PMe3

2PF6-

Re

Me3P

KOtBu Me3P

+2

PMe3

Me3P

PMe3

+2

PMe3 Re PMe3

2PF6SiMe3

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

183

bridge. The dicationic complex [(Me3SiC^C)(PMe3)4Re^CdC^CdC^Re(PMe3)4(C^CSiMe3)](PF6)2 (1355) with a butynedi(triyl) bridge could be obtained by deprotonation of the complex 1354 with KOtBu followed by oxidation with two equivalents of [Cp2Fe]PF6.

6.01.9.1.5

By activation of hydrocarbons and halocarbons

Rhenium(V) carbyne complexes could also be obtained by activation of alkanes. For example, thermolysis (24 h, 110  C) of a mixture of [ReH4(PNPiPr)] (1023a, PNPiPr ¼ (iPr2PCH2SiMe2)2N) or [ReH2(cyclooctyne)(PNPiPr)] and three equivalents of cyclooctene in hydrocarbons RCH3 containing a methyl group produced a series of hydrido carbyne complexes of the type [ReH(^CR)(PNPiPr)] (1356) along with cyclooctane (Scheme 442).570 Similar carbyne complexes could also be produced in the reactions of the carbene complex [ReH2{]C(CH2)5}(PNPiPr)] with heptane and neohexane, the reactions of [ReH2(Z4-cyclohexadiene)(PNPiPr)] with heptane and ethylbenzene, and the reaction of the complex [ReH4(PNPiPr)] (1023a) with norbornene. P(iPr)2

P(iPr)2 Si N Si

H Re

H

110 oC, 24 h

+ R-CH3

+ 3

Si N

H

Re

R

Si H

H

P(iPr)2

P(iPr)2 1023a

1356 CH3-(CH2)nCH3 ( n = 3, 4, 5. 6)

CH3 R-CH3:

CH3-CH2R ( R = CHMeEt, CMe3, Ph)

CH3

Scheme 442

Four-coordinate halo-carbyne species HC^ReX3 and XC^ReX3 (X ¼ F, Cl) can be generated from reactions of laser-ablated Re atoms with CHX3 and CX4.686 Analogous carbyne complexes HC^ReH3, HC^ReH2F, HC^ReH2Cl, HC^ReH2Br, HC^ReHF2, HC^ReHFCl, HC^ReHCl2 and CH3C^ReH3 were formed in the reactions of laser-ablated Re atoms with methane, methylhalides, methylenehalides and ethane.687,688 The cyano substituted four-coordinate carbyne complexes HC^ReH2(NC) was produced in the reaction of laser-ablated Re atoms with MeCN.689,690 These carbyne complexes were characterized by matrix-isolation infrared spectra. DFT calculations suggest that these complexes have a trigonal pyramidal structure.

6.01.9.1.6

By electrophilic addition to carbonyl and reactions of phosphorus ylides

Rhenium carbyne complexes have been occasionally obtained by other routes. Treatment of the Re(I) carbonyl complex [Na(OEt2)] [CpRe(BDI)(CO)] (1357, BDI ¼ N,N0 -bis(2,6-diisopropylphenyl)-3,5-dimethyl-b-diketiminate) with excess Me3SiCl or 0.5 equivalent of [Cp2ZrCl2] at room temperature afforded the complexes 1358 or 1359, respectively (Scheme 443).117 The RedC(CO) distances in 1358 (1.776(4) A˚ ) and 1539 (1.785(3) A˚ ) are relatively short and are on the order observed for rhenium carbyne complexes. Thus these complexes can be viewed as siloxycarbyne complexes, which are rare for rhenium.

O

Ar C

N Re

C

N Re

TMSCl

Na(OEt2)+

N Ar

N Ar

OTMS

Ar

-

1358

Ar = 2,6-diisopropylphenyl

1357 Cp2 Zr Cp2ZrCl2

O

Ar

O

C

N Re

N Re

N

N

Ar

Ar 1359

Scheme 443

Ar C

184

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

The reaction of [Ph3PMe]StBu with [ReCl3(]NtBu)2] (1360) and Ph3P]CH2 was reported to give the rhenium phosphonio-methylidyne complex [(tBuN])2(tBuS)Re^CPPh3] (1361) (Scheme 444).691 The complex 1361 represents a rare example of well-characterized Re(VII) carbyne complexes isolated in the review period.

Ph3P=CH2

Cl N Cl

N

[Ph3PCH3]StBu

Re

Re

N

N

PPh3

S Cl 1360

1361

Scheme 444

6.01.9.2

Reactivity

6.01.9.2.1

Migratory insertion reactions

Carbyne complexes of the type LnM(R0 )^CR (R0 ¼ hydride, alkyl) can undergo migratory insertion reactions to give carbene complexes LnM]CR0 R. The transformation has often been observed for carbyne complexes of ruthenium, osmium and tungsten. A few such reactions have also been demonstrated with rhenium carbyne complexes. For example, heating the hydridocarbyne complexes [Re{^CCHC(R)C^CH}HCl(PMe2Ph)3] (1352, R ¼ CMe3 (a), adamantyl (b), isopropyl (c)) in benzene at 50  C produced the complexes [Re{]CHCH]C(R)C^CH}Cl(PMe2Ph)3] (1362) (Scheme 445). The complexes 1362 can be viewed as either bicyclic rhenabicyclohexatriene complexes (1362) or alkyne-carbene complexes (13620 ). The structural data indicate that these complexes can be best described as bicyclic rhenabicyclohexatriene complexes (1362).

PR3 Cl R3P

H

Re

50 C

R3P

R3P H R3 P Re

PR3 = PMe2Ph

Re

1342

Re

R'

H

R3P H

H

R = CMe3 (a) adamantyl (b), iPr (c) 1362

H

1362'

R3P 50 oC

Cl

+

R3 P

R3P Re

Re PR3 = PMe2Ph

PR3

R3P H

Cl R3P H

H

R'

Cl

PR3

PR3

R3P

o

R'

H 1352

Cl

H

Cl

R3P R3P H 1363

H

R3P H

H

1363A

Scheme 445

A similar reaction occurred for the carbyne complex 1342 to give an isomeric mixture of [Re{]CHCH]C(CMe3) C^C(CH2)4Me}Cl(PMe2Ph)3] (1363 and 1363A) (Scheme 445).683,684 In contrast, the analogous complex [Re(^CCH] C(CMe3)C^CSiMe3)HCl(PMe2Ph)3], which bears a SiMe3 group on the C^C moiety, does not undergo a similar transformation under the same condition. A computational study suggests that the difference in the reactivity of the hydrido carbyne complexes is related to steric effects in the corresponding hydride-shift products.685 The migratory insertion reactions of the model rhenium vinylcarbyne complexes [Re{^CCH]C(tBu)C^CH}(R)Cl(PMe3)3] (1364, R ¼ H, 1366, R ¼ Me) have been studied computationally (Scheme 446). Calculations at the B3LYP/6-31G(d) + lanl2dz level suggest that the migratory insertion reaction of the hydrido-rhenium vinylcarbyne complex 1364 to give 1365 is kinetically more favorable, but thermodynamically less favorable than the reaction of the methyl-carbyne complex 1366 to give 1367.119

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

PMe3 PMe3 Cl

Me3P H Me3P

G = -7.9 kcal/mol

H

G# = 23.5 kcal/mol

Re

Re

H

Cl

PMe3

Me3P H 1365

H

1364

PMe3 PMe3 Cl

185

Me3P H Me3P

G = -12.6 kcal/mol

H

G# = 30.7 kcal/mol

Re

Re

Me

Cl

PMe3

Me3P Me H

1366

H

H

1367

Scheme 446

6.01.9.2.2

Alkyne metathesis and formation of metallacyclobutadienes

Alkyne metathesis reactions are well-documented for d0 carbyne complexes. However, such reactions have rarely been observed for non-d0 systems. Recent work demonstrates that d2 Re(V) carbyne complexes [Re(^CR)X2(PR0 3)3] can undergo stoichiometric alkyne metathesis reactions. For example, the complex [Re{^CCH2(o-C6H4Br)}Cl2(PMe2Ph)3] (1332c) reacted with TMSC^CR (R ¼ CO2Et, CH2Ph) and PhC^CPh to produce the metathesis products [Re(^CR)Cl2(PMe2Ph)3] (R ¼ CO2Et) (1368), CH2Ph (681a) and [Re(^CPh)Cl2(PMe2Ph)3] (1346), respectively (Scheme 447). The complex 1332c undergoes metathesis with PhC^C(CH2)5Me to give a mixture of the carbyne complexes 681c and 1346. The carbyne complex 1346 undergoes metathesis with TMSC^CCOEt to give the complex 1368. The complex 1368 reacted with PhC^CPh to give the expected alkyne metathesis product 1346.692

Cl

PMe2Ph Cl Re

O OEt

PhMe2P PMe2Ph TMS

1368

CO2Et

TMS

Cl

PMe2Ph Cl Re

Br Ph

Ph

PhMe2P

Ph

Ph

CO2Et

Cl

PMe2Ph Cl Re

PhMe2P PMe2Ph

PMe2Ph

1346

1332c Ph

CH2Ph

(CH2)5CH3 TMS

Cl

PMe2Ph Cl Re

PhMe2P PMe2Ph 681a Scheme 447

Cl Ph

PMe2Ph Cl (CH2)5CH3 + Cl Re

PhMe2P

PMe2Ph Cl Re

PhMe2P PMe2Ph

PMe2Ph

681c

1346

186

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

The carbyne complexes [Re(^CCH2Ar)Cl2(PR3)3] (PR3 ¼ PMe3 and PMePh2) as well as [Re(^CCH2Ph)I2(PMePh2)3] were also found to undergo alkyne metathesis reactions with PhC^CPh. The reactivity of [Re(^CCH2Ar)Cl2(PR3)3] towards alkyne metathesis reactions with PhC^CPh were found to be in the order of PMe3 < PMe2Ph < PMePh2. The iodo-carbyne complex [Re(^CCH2Ph)I2(PMePh2)3] is slightly less reactive than its chloride analog. In contrast, carbyne complexes such as [Re(^CPh) (S2CNEt2)2(PMePh2)], [CpRe(^CPh)(PMePh2)2]Cl and Re(^CCH2Ph)(mnt)(PMePh2)2 are inactive for alkyne metathesis reactions with PhC^CPh.680

6.01.9.2.3

Reactions of phosphoniocarbyne complexes

The rhenium(VII) phosphoniocarbyne complex [Re(^CPPh3)(]NtBu)2(SCMe3)] (1361) undergoes an unusual C,C-coupling reaction with CO to give the Z2-ketene rhenium(VII) complex [Re(]NtBu)2{Z2-(C,C)-O]C]CPPh3}(StBu)] (1369) (Scheme 448).691

Scheme 448

6.01.9.2.4

Redox reactions

Reduction of the dinuclear carbyne complexes [Cp0 (R0 2PCH2CH2PR0 2)Mn^CCHRCHRC^Mn(R0 2PCH2CH2PR0 2) Cp0 ](PF6)2 (1321, Cp0 ¼ Cp, R0 ¼ Et; 1322, Cp0 ¼ MeC5H4, R0 ¼ Me, R ¼ H (a), Ph (b), 3-thienyl (c)) with Cp 2Co produced the corresponding mononuclear vinylidene complexes [Cp0 (R0 2PCH2CH2PR0 2)Mn]C]CHR] involving a reductive decoupling process (Scheme 449).507

Mn

C

Et2P

H

H

C

C

R

2+ 2PF6C

Mn

R

Mn

Mn

C

R

1320

1321

Me2P

C

PEt2

Et2P

H

C

Et2P

PEt2

PEt2

H

[Cp*2Co]

2+ 2PF6-

H

C

C

R

R

PMe2

C

Mn PMe2

H

[Cp*2Co]

Mn

C R

Me2P

Me2P 1322

C

PMe2 1244

R = H(a), Ph (b), 3-thienyl (c)

Scheme 449

The dicationic dinuclear carbyne complex trans-[(Me3SiC^C)(PMe3)4Re^CdC^CdC^Re(PMe3)4(C^CSiMe3)] (PF6)2 (1355) with a butynedi(triyl) bridge can be reduced by KH in the presence of 18-crown-6 to give the neutral dinuclear complex 1370 (Scheme 450). DFT calculations indicate that the complex 1370 could possess two equilibrating electronic states: a diamagnetic structure (1370) with a butatrienediylidene bridge and a diradical structure with a but-2-ynediylidene bridge.509

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

187

Scheme 450

The manganese phenylvinylidene [CpMn(]C]CHPh)(CO)(PPh3)] (1287) and the allenylidene complexes [CpMn(]C]C] CPh2)(CO)2] (1272) are catalytically active for electrochemical reduction of protons from HBF4 into dihydrogen in CH2Cl2 or CH3CN (Scheme 451). The reaction was proposed to involve carbyne complexes formed by protonation reactions. For example, protonation of the vinylidene complex [CpMn(]C]CHPh)(CO)(PPh3)] (1287) would give the cationic carbyne complex [CpMn(^CCH2Ph)(CO)(PPh3)]BF4 (1371), which could be reduced to the 19-electron carbyne radical [CpMn(^CCH2Ph) (CO)(PPh3)] (1372). The carbyne radical can undergo a homolytic cleavage of a CdH bond to generate an H-radical that combines to produce molecular hydrogen with concomitant recovery of the neutral vinylidene complex 1287.693 A similar process has been suggested for the reaction catalyzed by the allenylidene complex [CpMn(]C]C]CPh2)(CO)2] (1272).

HBF4

Ph 2HBF4 + 2e-

Mn C OC

Ph

C

Mn C C OC

H

Ph3P

+ BF4-

Ph3P

1287

H

H

1371

1287 e-

H. Ph H2 +

Mn C C

0.5H2

2BF4-

OC Ph3P

BF4-

H

H 1372

Scheme 451

6.01.9.2.5

Deprotonation reactions

Carbyne (1373) and vinylcarbyne (1374) complexes, especially positively charged ones, with a proton at the b-carbon can be deprotonated to give vinylidene and allenylidene complexes, respectively as shown in Scheme 452. For example, carbyne complexes of the type [Re(C^CSiMe3)(^CdCHR)(PMe3)4]+ can be deprotonated by KOtBu to give the vinylidene complexes [Re(C^CSiMe3)(]C]CHR)(PMe3)4].509 The vinylcarbyne complex [ReI(^CCH]CPh2)(dppm)2]I can be deprotonated by LiBHEt3 to give the allenylidene complex [ReI(]C]C]CPh2)(dppm)2].646 +

H LnM C C R R 1373 H

LnM BH+

+

B:

LnM C R R 1374 Scheme 452

R

B: C

C R

1375

R LnM

C

C

C R

BH+ 1376

188

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

6.01.9.2.6

Nucleophilic addition reactions

Complexes of the type [(Z5-C5H4R)M(^CR0 )(CO)2]+ (M ¼ Mn, Re) are electrophilic and can undergo nucleophilic addition reactions with nucleophiles such as NaNH2,618 alkoxides,603 alkynyl lithium reagents561 and phosphines612–616 to give carbene complexes as detailed in the section dealing with chemistry of carbene complexes. As an additional example of such reactivity, the carbyne complex [(Z5-MeC5H4)Mn(^CPh)(CO)2]BBr4 (1005aBBr4) reacted with water to give the hydroxycarbene complex [CpMn{]C(OH)Ph)}(CO)2]BBr4 (1377) which can be detected spectroscopically (Scheme 453). The hydroxycarbene complex is unstable and decomposes into [(Z5-MeC5H4)Mn(CO)3] and benzaldehyde through an acid–base process via the intermediate 1378.561

+ BBr4Mn

Ph

H2O

C

Mn

C Ph OC OC

OC OC

HC

OH

Mn

OC

O

OC OC

PhCHO

CO

OC

1378

1377

1005aBBr4

Ph

Mn

Scheme 453

6.01.9.2.7

Reactions with anionic metal-carbonyl complexes

Low valent carbyne complexes [Cp0 M(^CR)(CO)2]+ can react with anionic metal-carbonyl complexes to give bimetallic or polymetallic complexes with a bridging carbyne ligand. For example, the cationic manganese carbyne complex [CpMn(^CPh) (CO)2]BBr4 (1126aBBr4) reacted with [(Ph3P)2N][Ir(CO)4] to give a mixture of the Mn2dIr2 mixed tetrametal complex 1379 with both an m2 and an m3 bridging carbyne ligand, and the Mn2dIr2 mixed-tetranuclear cluster 1380 with an m2-bridging carbene ligand and a m3 bridging carbyne ligand (Scheme 454). Under similar condition, the analogous rhenium carbyne complex [CpRe(^CPh) (CO)2]BBr4 (1180aBBr4) reacted with [(Ph3P)2N][Ir(CO)4] to afford a mixture of the corresponding Re2dIr2 mixed-tetrametal cluster 1382 and the RedIr2 mixed-trimetallic complex 1381 with an m3 bridging carbyne ligand and an allyl ligand. The allyl group (C3H5) in 1381 is presumably derived from the cyclopentadienyl ligand (C5H5) of the cationic carbyne complex 1180aBBr4. The reaction of [Cp Re(^CPh)(CO)2]BBr4 with [(Ph3P)2N][Ir(CO)4] gave only a Re2dIr2 mixed-tetranuclear complex analogous to 1379.618

Cp Cp + BBr4Mn

C Ph

O PPN[Ir(CO)4]

OC OC

Mn

C

O

O Ph

Ir

Mn

Ir

OC

1126aBBr4

+ OC

Re

C Ph

OC OC 1180aBBr4

O PPN[Ir(CO)4] OC

O C

C Ph

O 1379

Re

Ir

Cp

C Ph 1381

Mn CO

C

C Ph

O 1380

Cp O

O

C Ir

OC

Ir

OC OC

CO

C

C

C

Ir

Cp + BBr4-

O Ph

Mn

C Cp

OC

C

+

OC

C

Re

Ir

OC OC

O Ph C

O C

Ir

Cp

Re

C

C

O

O

CO

1382

Scheme 454

The cationic manganese carbyne complex [CpMn(^CPh)(CO)2]BBr4 (1126aBBr4) reacted with the anionic compound Na2[Ru(CO)4] to give the heterotrimetallic complex 1383 (Scheme 455). The reaction of the analogous rhenium complex [CpRe(^CPh)(CO)2]BBr4 (1180aBBr4) with Na2[Ru(CO)4] produced the heterotrimetallic complex 1384 along with a Re2dRu2 mixed-metal cluster.618

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

189

Cp + BBr4Mn

O

C

C

H

Na2[Ru(CO)4]

C Ph

O

Mn

Ru(CO)3

(OC)3Ru

OC OC

C

1126aBBr4

1383

Ph Cp + BBr4Re OC OC

C Ph

O Na2[Ru(CO)4]

O

Re

C

C

H Ru(CO)3

(OC)3Ru C

1180aBBr4

Ph

1384

Scheme 455

6.01.10

Metallacarbocycles

Group 7 metals form a number of metallacarbocycles. In this section, the synthesis and reactivity of four-, five- and six-membered metallacarbocycles are described.

6.01.10.1 Four-membered metallacycles Well characterized four-membered metallacarbocycles have been described for rhenacyclobutenes and rhenacyclobutadienes. Several rhenacyclobutene complexes have been obtained by nucleophilic addition reactions of Z3-propargyl complexes. For example, PMe3 adds selectively to the central carbon of the Z3-propargyl ligand in the complex [Cp Re(Z3-CH2CCCMe3)(CO)2] BF4 (1385) to form the metallacyclobutene [Cp Re(Z3-CH2C(PMe3)]CCMe3)(CO)2]BF4 (1386a) (Scheme 456). Analogous metallacyclobutenes can be obtained by the reactions of [Cp Re(Z3-CH2C]CCMe3)(CO)2]BF4 (1385) with other phosphines such as PPh2Me, PPh3 and P(p-C6H4F)3.694

OC

Re

CMe3

+ BF4PR3

BF4-

OC OC

OC 1385

CMe3

Re

PR3 = PMe3 (a), PMePh2 (b), PPh3 (c), P(p-C6H4F) (d)

PR3 1386

Scheme 456

The Z2-alkyne complex [CpReBr2(BnSC^CSBn)] (1387, Bn ¼ PhCH2) reacted with an excess of the dithioalkyne BnSC^CSBn to give the complex [CpReBr2{Z3-(BnS)2C]CC(SBn)dC(SBn)}] (1388), which features a ReC3 ring with an exocyclic double bond and can be regarded as a rhenacyclobutene complex (Scheme 457).695 Calculations suggest that the reaction most likely proceeds through the mixed alkyne/vinylidene intermediate [CpReBr2(BnSC^CSBn){]C]C(SBn)2}] formed by 1,2-SBn shift, which undergoes a migratory insertion reaction.695

Re Br Br

SCH2Ph

Re

SCH2Ph

Br

1387

PhCH2S

SCH2Ph 1388

Re

Br SCH2Ph

PhCH2S SCH2Ph

Scheme 457

PhCH2S

Br

PhCH2S

Br SCH2Ph

PhCH2S

SCH2Ph 1388'

190

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

A series of rhenacyclobutadienes of the type [Re{dC(R)]C(CO2R0 )C(OR00 )¼}(CO)4] (1390) have been obtained by alkylating rhenacyclobutenone complexes [Re{dC(R)]C(CO2R0 )dC(O)]}(CO)4] (1389) generated from the reactions of [Re(CO)5]− with electron-deficient alkynes as illustrated in Scheme 458.696–698 The alkylation reactions can be achieved by treatment of rhenacyclobutenone complexes 1389 with R00 3OPF6 or MeCOCl followed by alcohols R00 OH. Complexes 1390a–f are examples of rhenacyclobutadienes prepared by this method.

OC

CO

CO R

R

OC

Re

CO2R'

OC CO O

CO2R'

CO

CO R

R"3OPF6 or OC

OC

Na Re CO +

Re

1) MeCOCl

OC

CO OR"

2) R"OH

Na

1390

1389

CO Re OC

OC

OC

CO2Et

Re OC

CO OEt

Re

CO2Et

OC

OMe 1390c

1390b

CO

CO

OC Re

CO

OC

CO2Me

OC Re

OC

CO OnPr

CO2Me

CO

CO OEt

1390a

OC

CO

CO

OC

CO2R'

CO2Et

Re OC

CO OEt

CO OEt

1390e

1390d

CO2Et

1390f

Scheme 458

Rhenacyclobutadienes of the type [Re{dC(R)]C(CO2R0 )C(OR00 )]}(CO)4] (1390) can react with HC^COEt to give rhenabenzene complexes (1391) and vinyl-substituted alkoxy-rhenacyclobutadienes (1392) (Scheme 459). The formation of rhenabenzene complexes will be discussed in the section dealing with chemistry of rhenabenzenes. The complexes 1392a–d are examples of vinyl-substituted alkoxy-rhenacyclobutadienes generated from these reactions. In complexes 1392, the ethyl of the ester group is from the alkyne HC^COEt, the alkyl of alkoxy group OR0 at the a-carbon is from the ester group of the starting rhenacyclobutadienes, and the alkyl group of OR00 of the vinyl substituents is from the [Re]C(OR00 )] group of starting rhenacyclobutadienes.

CO R

R

CO OEt

OC Re

OC Re

CO2R'

OR"

OC

OC

OEt

OC Re

OC H

OC CO OEt 1392a

1392

CO2Et

Re

Re H

OC

OC

OEt 1392b

CO2Et

OMe 1392c

OnPr

OC H

CO

CO

CO

OMe

OC

OEt

CO2Et

OR'

CO

CO

H

OC

1391

1390

OR"

Re

OEt

OR"

CO

CO R OC

CO

CO

CO

Scheme 459

CO2R'

CO2Et

Re

H

OC CO OMe 1392d

CO2Et

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

191

A plausible mechanism for the formation of vinyl-substituted rhenacyclobutadienes 1392 is shown in Scheme 460. Direct nucleophilic attack of the terminal carbon of HC^COEt on the carbene carbon of the rhenacyclobutadienes 1390 produces complexes 1393. The zwitterionic complexes 1393 can undergo a cyclization reaction to give the bicyclic metallacycles 1394 by nucleophilic addition of the carbonyl oxygen to the [C]OEt]+ carbon. Complexes 1394 then evolve into the zwitterionic carbene complexes 1395 (three resonance forms are shown) by detaching the C(OR00 ) carbon from the metal center. Re-coordination of the C(OR0 ) carbon would produce the bicyclic metallacycles 1396. Ring opening of the complexes 1396 would eventually produce the vinyl-substituted rhenacyclobutadiene complexes 1392.697

R

R

OR'

OR' H C C OEt [Re] O R"O HC

[Re]

OR' [Re]

1393

[Re]

OR'

R"O

O C

OR" 1390

R

R

O

O

R"O OEt

1394 OEt

1395A

H

OEt

[Re] = Re(CO)4 R

R R

OR"

OR'

R"O

H

H

OR'

R"O O

R'O O O

[Re]

[Re]

[Re] OR'

R

[Re]

OR"

OEt

OEt

H 1395C

1396

1392

O H

OEt

OEt

1395B

Scheme 460

Analogous low valent amino-rhenacyclobutadienes can be prepared by nucleophilic substitution reactions of alkoxy-rhenacyclobutadienes with amines. For example, treatment of the alkoxy-rhenacyclobutadiene 1390a with Et2NH produced the amino-rhenacyclobutadiene 1397 (Scheme 461), treatment of the alkoxy-rhenacyclobutadiene 1392d with 4-methoxyanilline produced the vinyl-substituted amino-rhenacyclobutadiene complex 1398.

CO

CO

OC

OC CO2Et + Et2NH

Re OC

- EtOH

CO

CO OEt

NEt2

1390a

CO

1397

CO

OnPr

OC Re

H

OC CO OMe 1392d

CO2Et

Re OC

CO2Et

H2N

OMe

OnPr

OC Re

H

OC CO NH 1398

CO2Et

OMe

Scheme 461

Rhenacyclobutanes have been suggested as key intermediates in olefin metathesis reactions promoted by complexes such as [Re(NO)2(PR3)2]BArF4 (R ¼ Cy, iPr, BAr−F4¼tetrakis{3,5-bis(trifluoromethyl)phenyl}borate)567,699 and [(^SiO)Re(^CtBu) (]CHtBu)(CH2tBu)]243,572,585 as described in the section dealing with carbene chemistry.244

192

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

6.01.10.2 Five-membered metallacycles Well-defined five-membered metallacarbocycles have been described for manganacyclopentadienes, rhenacyclopentatriene, rhenacyclopentadiene and rhenacyclopentanes. The one-pot reaction of the dilithio reagents 1399 with 0.5 equivalents of MnCl2 and 0.5 equivalents of lithium produced the trilithio spiro manganacycles 1400, which can further react with an excess amount of lithium in THF to give the tetralithio spiro manganacycles (or manganacyclopentadienes) 1401 (Scheme 462). The complex 1401 can also be obtained by the reactions of the dilithio reagents 1399 with 0.5 equivalents of MnCl2 and an excess amount of lithium in the mixed solvents of hexane, diethyl ether (Et2O) and tetrahydrofuran (THF) at room temperature. The complexes 1401 represents an interesting examples of metalla-aromatics with two spirofused independent and perpendicular aromatic rings.700

Scheme 462

Treatment of the alkyne complex [Cp ReBr2(BnSC^CSBn)] (1387, Bn ¼ benzyl) with BnSC^CSBn and AgBF4 produced the rhenacyclopentatriene 1402 (Scheme 463).701 The rhenacyclopentatriene complex 1402 can react with a stoichiometric amount of NBu4NBr to give the neutral Z4-cyclobutadiene complex 1403.702

Re Br Br

SCH2Ph

PhCH2S

SCH2Ph

SCH2Ph

Re Br

AgBF4 CH2Ph

1387

Br PhCH2S

Bu4NBr SCH2Ph

PhCH2S PhCH2S

+ BF4-

Re

SCH2Ph

PhCH2S

1402

Br SCH2Ph

1403

Scheme 463

Treatment of [Cp Sm(THF)2] (1404) with [Re2(CO)10] in toluene at room temperature give the complex [{(Cp )2Sm}3{(m-O4C4)(m-CO)3(CO)3Re2}Sm(Cp )2(THF)] (1405), consisting of one [(m-O4C4)(m-Z2-CO)2(m-Z1-CO) (CO)3Re2]4− (a rhenacyclopentatriene), one [(Cp )2Sm(THF)]+, and three [Cp 2Sm]+ moieties (Scheme 464).703 The reaction of [(Cp )2Sm(THF)2] with half an equivalent of [Mn2(CO)10] in toluene does not give analogous metallacyclopentatriene complex, but give the polymeric complex [{Cp 2Sm(THF)}(m-CO)2{Mn(CO)3}]n. Cp*2 Sm O O O Sm O

O + [Re2(CO)10]

Cp*2Sm

C

O

1405

CO

O

CO

SmCp*2

Re

Re

O 1404

OC

C

OC

O Sm Cp*2

Scheme 464

Thermal treatment of [Re2(CO)8(CH3CN)2] with 1,8-bis(aryl)-1,7-octadiyne derivatives (e.g. 1406, aryl ¼ 2-thienyl (a), 2-pyridyl (b), 2-quinolyl (c) and 9-phenanthrenyl) produced symmetrical 2,5-bis(aryl)rhenacyclopentadienes 1407 and 1408, along with alkyne complexes 1409 and/or 1410 (Scheme 465).704

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Ar

Ar

Re(CO)3

+

Ar

Ar

1406

+

1409 1407

[Re2(CO)8(NCMe)2] Ar =

Re(CO)3(NCMe)

(NCMe)(CO)3Re

Re(CO)3

193

Ar

Re(CO)3

S

+ N

Re(CO)2

N 1408

Re(CO)3

+

1410

Ar

NCMe

Re(CO)3(NCMe)2

Scheme 465

Treatment of cis-[Pt(C^CPh)2(dppe)] (1411) with [Mn2(CO)8(CH3CN)2] in refluxing toluene afforded the mixed transition metal complex [Mn2Pt(m3-Z1:Z1:Z2:Z4-PhCCCCPh)(CO)6(dppe)] (1412), as a result of cross coupling of the alkynyl ligands (Scheme 466).705

Ph2 P Pt P Ph2

Ph [Mn2(CO)8(NCMe)2] Ph

Mn(CO)3 Ph

Ph2 P Pt P Ph2

1411

Mn(CO)3 Ph 1412

Scheme 466

Rhenacyclopentanes have been suggested as reactive intermediates in organometallic reactions. Reduction of [ReBr2(NO) (MeCN)(PR3)2] (1413, R ¼ Cy (a), iPr (b)) with sodium amalgam in THF at room temperature under one bar of ethylene produced the butadiene-ethylene species [Re(Z4-C4H6)(Z2-H2C]CH2)(NO)(PR3)] (1417) (Scheme 467). The transformation was proposed to proceed through first generation of a transient Re(ethylene)2 complex 1414, which undergoes oxidative coupling of the two ethylene ligands to give the metallacyclopentane intermediate 1415, followed by double b-hydrogen elimination, H2 reductive elimination, and the ethylene replacement by one phosphine ligand.5

PR3

PR3 Br

Br Re

MeCN

NO

CH2=CH2

ON

Re

PR3

H

PR3 H2C

ON

Re

Na

PR3

PR3

PR3

1413

1414

1415

C

CH

Re CH2 R3P H ON H 1416

CH2=CH2 H2

R3P ON

Re

1417

R = Cy (a), iPr (b) Scheme 467

Group 7 metallacyclopentenes and metallacyclopentadienes have also been suggested as reactive intermediates in catalytic reactions. Rhenium and manganese complexes [ReBr(CO)3(THF)2] and [MnBr(CO)5] catalyzed coupling reactions between b-ketoesters and aryl- and alkenyl-substituted terminal alkynes (followed by treatment with tetrabutylammonium fluoride) to give 2-pyranone derivatives (e.g. formation of 1419 from 1418 and alkynes) (Scheme 468).706,707 The catalytic cycle was proposed to involve initial formation of a metallacyclopentene intermediate like 1420, which undergoes a reductive elimination reaction to liberate the cyclobutenol 1421. The resulting cyclobutenol 1421 undergoes a ring opening rearrangement into the b-ketoester 1422, which can evolve to isomeric b-ketoester 1423. The latter undergoes intramolecular cyclization to afford the final 2-pyranone 1419.

194

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

O O

OH

O

[MnBr(CO)5]

O

OEt

O

R

+

OEt

R 1419

1418

HOEt

H OH O

O

O

H

OEt

O

OEt

O

OEt

O

OH

OEt

[Mn] R

R

[Mn]

1420

1421

1422

R

1423

R

Scheme 468

Rhenium and manganese complexes [ReBr(CO)3(THF)2] and [MnBr(CO)5] catalyzed the reactions of cyclohexanone2-carboxylic acid ethyl esters with alkynes to give eight-membered ring products (e.g. formation of 1425 from 1424) (Scheme 469).708,709 The reaction was also proposed to proceed through formation of metallacyclopentene intermediates as illustrated by the reaction mediated by the rhenium complex [ReBr(CO)3(THF)2] shown in Scheme 469. The rhenacyclopentene intermediate 1426 could be formed by oxidative coupling of the alkyne and the enol form of a cyclohexanone-2-carboxylic acid ethyl ester. The rhenacyclopentene intermediate 1426 could undergo reductive elimination to give the cyclobutene 1427, which can evolve to the eight-membered ring product 1425 through ring opening by a retro-aldol reaction (to give 1428) and isomerization. Alternatively, the rhenacyclopentene intermediate 1426 can evolve to 1425 through ring opening by a retro-aldol reaction, followed by isomerization and reductive elimination. O O

OH

O

O OEt

OEt

R

+

R

[ReBr(CO)3(thf)2]

OEt 1425

1424

HO

HO

O

[Re] R OEt

O OEt

[Re]

R 1427

1426

HO

O

O OEt R 1428

Scheme 469

[MnBr(CO)5] catalyzed intermolecular [2 + 2 + 2] coupling of HC^C-o-C6H4OMe (1429) to give the benzene derivative 1431 (Scheme 470).473 The reaction was proposed to involve a manganacyclopentadiene intermediate like 1430. o-C6H4OMe

o-C6H4OMe o-C6H4OMe

[MnBr(CO)5] NMO/MgSO4

1429

[Mn] o-C6H4OMe 1430

o-MeO-C6H4

o-C6H4OMe 1431

Scheme 470

6.01.10.3 Six-membered metallacycles Six-membered rhenacarbocycles have been described for rhenabenzenes, metallabicyclo[3.1.0]hexatrienes and rhenabenzynes. A series of rhenabenzenes have been isolated from reactions of low valent rhenacyclobutadienes [Re{dC(R)]C(CO2R0 )C (OR00 )]}(CO)4] (1390) with HC^COEt (Scheme 471).696–698 As mentioned in the section dealing with metallacyclobutadiene complexes, the reactions in general produced a mixture of species, including the rhenabenzenes 1391 and the vinyl-substituted rhenacyclobutadienes 1392. The complexes 1391a–f are the examples of rhenabenzenes synthesized through this route.

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

195

Scheme 471

DFT calculations suggest that the rhenabenzenes are most likely formed through the mechanism shown in Scheme 472. HC^COEt first attacks on the C(OR00 ) carbene carbon of a rhenacyclobutadiene 1390 to produce the zwitterionic complex 1393, which rearranges to the five-membered metallacycle 1342 containing a free-carbene group stabilized by a OEt group. The complex 1342 then evolves to a metallabenzene complex 1391 by coordination of the carbene carbon.

Scheme 472

The NMR spectroscopic and structural data as well as the aromatic stabilization energy (ASE) and nucleus-independent chemical-shift (NICS) values suggest that these rhenabenzenes 1391 have aromatic character. Rearrangement of metallabenzenes to cyclopentadienyl complexes is one of the common reactivities of metallabenzenes. A computational study indicates that the substituent on the metallacycle and the ligands around the metal center can influence the thermodynamics of the rearrangement of low valent group 7 metallabenzenes to give Z5-cyclopentadienyl complexes.710–712 As shown in Scheme 473, with respect to the formation of cyclopentadienyl complexes, the rhenabenzene [Re(]CHCH]CHCH] CH)(CO)4] (1433Re) was calculated to be thermodynamically more stable than the analogous Mn and Tc metallabenzene complexes [M{]CHCH]CHCH]CH}(CO)4] (1433Mn, M ¼ Mn, 1333Tc, M ¼ Tc).

196

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

Scheme 473

Calculations show that an alkoxy group may significantly increase the stability of the metallabenzenes with respect to the formation of cyclopentadienyl complex if it is at the ortho or para position on the metallacycle, and decrease slightly the stability of the metallabenzenes if it is at the meta position. For example, the unsubstituted rhenabenzene 1433Re and analogous rhenabenzene with one OMe substituent are less stable than the corresponding Z5-cyclopentadienyl complexes, while rhenabenzenes with two or three OMe groups at ortho and para positions (e.g. 1434) are more stable than the corresponding Z5-cyclopentadienyl complexes (Scheme 473). The p-donating groups NMe2 and SMe have an effect similar to OMe. The stabilizing effect of the three substituents is in the order of OMe > NMe2 > SMe when they are at the ortho position, and in the order of NMe2 > OMe > SMe when they are at the para position. Computational studies also reveal that the ligand effect is much smaller than the substituent effect on the stability of metallabenzenes. For example, the PMe3-supported rhenabenzene 1436 has a stability similar to the CO-supported rhenabenzene 1433Re. A series of rhenabicyclohexatrienes (e.g. 1362, 1363) have been obtained by rearrangement reactions of hydrido vinylcarbyne complexes as described in the section dealing with carbyne complexes.119,683–685 The rhenabicyclohexatrienes 1632 in benzene can slowly rearrange to form the metallabenzyne complexes 1438 (Scheme 474). The NMR spectroscopic and structural data as well as the aromatic stabilization energy (ASE) and nucleus-independent chemical-shift (NICS) values suggest that these rhenabenzynes have aromatic characters.683,684 PMe2Ph

Me2PhP H Me2PhP

benzene R. T., 7 days

Re

Ph2MeP Re

R PR3 = PMe2Ph

Cl Me2PhP H 1362

Cl

R = CMe3 (a) adamantyl (b), iPr (c)

R

Ph2MeP Re

R

Cl PMe2Ph

H

PMe2Ph

1438

PMe2Ph 1438'

Scheme 474

6.01.11

Conclusion

The chemistry of Group 7 metal (especially manganese and rhenium) organometallic compounds with metal-carbon s-bonds and metal-carbon multiple bonds has been expanded significantly in the review period. There are still ample opportunities for further development in this field, especially in the design and synthesis of organometallic compounds for applications in areas such as

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds

197

catalysis, materials science and biomedicine. For example, exciting new findings are expected in the chemistry and applications of organometallic compounds of manganese, the third most abundant transition metal; further investigation of group 7 metal carbene and carbyne complexes may yield metathesis catalysts that are complimentary to group 6/8 ones.

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Chem. 2015, 56, 774–776. Burmakina, G. V.; Verpekin, V. V.; Maksimov, N. G.; Zimonin, D. V.; Piryazev, D. A.; Chudin, O. S.; Rubaylo, A. I. Inorg. Chim. Acta 2017, 463, 70–79. Hori, S.; Murai, M.; Takai, K. J. Am. Chem. Soc. 2015, 137, 1452–1457. Fukumoto, Y.; Daijo, M.; Chatani, N. J. Am. Chem. Soc. 2012, 134, 8762–8765. Fukumoto, Y.; Daijo, M.; Chatani, N. Pure Appl. Chem. 2014, 86, 283–289. Sogo, H.; Iwasawa, N. Angew. Chem. Int. Ed. 2016, 55, 10057–10060. Chen, J.; Wu, J. Chem. Sci. 2018, 9, 2489–2492. Iwasawa, N.; Watanabe, S.; Ario, A.; Sogo, H. J. Am. Chem. Soc. 2018, 140, 7769–7772. Murai, M.; Takai, K. Org. Lett. 2019, 21, 6756–6760. Shi, C.; Jia, G. Coord. Chem. Rev. 2013, 257, 666–701. Da Re, R. E.; Hopkins, M. D. Coord. Chem. Rev. 2005, 249, 1396–1409. Satpati, P. Organometallics 2007, 26, 4771–4775. Bai, W.; Wei, W.; Sung, H. H.; Williams, I. D.; Lin, Z.; Jia, G. Organometallics 2018, 37, 559–569. Oliván, M.; Clot, E.; Eisenstein, O.; Caulton, K. G. Organometallics 1998, 17, 3091–3100. Lee, J.-H.; Pink, M.; Smurnyy, Y. D.; Caulton, K. G. J. Organomet. Chem. 2008, 693, 1426–1438. Chen, J.; Shi, C.; Sung, H. H.; Williams, I. D.; Lin, Z.; Jia, G. Chem. A Eur. J. 2012, 18, 14128–14139. Chen, J.; Sung, H. H. Y.; Williams, I. D.; Lin, Z.; Jia, G. Angew. Chem. Int. Ed. 2011, 50, 10675–10678. Chen, J.; Shi, C.; Sung, H. H.-Y.; Williams, I. D.; Lin, Z.; Jia, G. Organometallics 2012, 31, 1817–1824. Lyon, J. T.; Cho, H.-G.; Andrews, L.; Hu, H.-S.; Li, J. Inorg. Chem. 2007, 46, 8728–8738. Cho, H.-G.; Andrews, L. Inorg. Chem. 2008, 47, 1653–1662. Cho, H.-G.; Andrews, L. Organometallics 2007, 26, 4098–4101. Xiao, Y.; Ji, W.-X.; Chen, X.-Y.; Wang, S.-G. Dalton Trans. 2014, 43, 9508–9517. Cho, H.-G.; Andrews, L. Organometallics 2012, 31, 6095–6105. Li, X.; Schopf, M.; Stephan, J.; Kipke, J.; Harms, K.; Sundermeyer, J. Organometallics 2006, 25, 528–530. Bai, W.; Lee, K.-H.; Sung, H. H.; Williams, I. D.; Lin, Z.; Jia, G. Organometallics 2016, 35, 3808–3815. Valyaev, D. A.; Peterleitner, M. G.; Semeikin, O. V.; Utegenov, K. I.; Ustynyuk, N. A.; Sournia-Saquet, A.; Lugan, N.; Lavigne, G. J. Organomet. Chem. 2007, 692, 3207–3211. Casey, C. P.; Boller, T. M.; Samec, J. S.; Reinert-Nash, J. R. Organometallics 2009, 28, 123–131. Seidel, W. W.; Meel, M. J.; Schallenberg, D.; Pape, T.; Villinger, A.; Michalik, D. Eur. J. Inorg. Chem. 2010, 2010, 5523–5528.

Complexes of Group 7 Metals with Metal-Carbon Sigma and Multiple Bonds 696. 697. 698. 699. 700. 701. 702. 703. 704. 705. 706. 707. 708. 709. 710. 711. 712.

Lin, R.; Lee, K. H.; Poon, K. C.; Sung, H. H.; Williams, I. D.; Lin, Z.; Jia, G. Chem. A Eur. J. 2014, 20, 14885–14899. Lin, R.; Lee, K.-H.; Sung, H. H.; Williams, I. D.; Lin, Z.; Jia, G. Organometallics 2015, 34, 167–176. Poon, K. C.; Liu, L.; Guo, T.; Li, J.; Sung, H. H. Y.; Williams, I. D.; Lin, Z.; Jia, G. Angew. Chem. Int. Ed. 2010, 49, 2759–2762. Frech, C. M.; Blacque, O.; Berke, H. Pure Appl. Chem. 2006, 78, 1877–1887. Zhang, Y.; Wei, J.; Zhu, M.; Chi, Y.; Zhang, W.-X.; Ye, S.; Xi, Z. Angew. Chem. Int. Ed. 2019, 58, 9625–9631. Kleinschmidt, S.; Schallenberg, D.; Helmdach, K.; Hinz, A.; Villinger, A.; Seidel, W. W. Organometallics 2015, 34, 1091–1097. Kleinschmidt, S.; Hinz, A.; Villinger, A.; Seidel, W. W. Z. Anorg. Allg. Chem. 2018, 644, 1268–1273. Yadav, R.; Simler, T.; Gamer, M. T.; Köppe, R.; Roesky, P. W. Chem. Commun. 2019, 55, 5765–5768. Cardozo, C.; Mendoza, A.; Farías, G.; Formiga, A. L. B.; Peña, D.; Fuentes, F.; Arce, A.; Otero, Y. Organomet. Chem. 2019, 881, 34–44. Yamazaki, S.; Taira, Z.; Yonemura, T.; Deeming, A. J. Organometallics 2005, 24, 20–27. Kuninobu, Y.; Kawata, A.; Nishi, M.; Takata, H.; Takai, K. Chem. Commun. 2008, 6360–6362. Kuninobu, Y.; Nishi, M.; Kawata, A.; Takata, H.; Hanatani, Y.; Salprima, Y. S.; Iwai, A.; Takai, K. J. Org. Chem. 2010, 75, 334–341. Kuninobu, Y.; Kawata, A.; Nishi, M.; Yudha, S.; Chen, J.; Takai, K. Chem. Asian J. 2009, 4, 1424–1433. Kuninobu, Y.; Kawata, A.; Takai, K. J. Am. Chem. Soc. 2006, 128, 11368–11369. Iron, M. A.; Martin, J. M.; Van der Boom, M. E. J. Am. Chem. Soc. 2003, 125, 13020–13021. Iron, M. A.; Lucassen, A. C.; Cohen, H.; van der Boom, M. E.; Martin, J. M. J. Am. Chem. Soc. 2004, 126, 11699–11710. Shi, C.; Guo, T.; Poon, K. C.; Lin, Z.; Jia, G. Dalton Trans. 2011, 40, 11315–11320.

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6.02 N-Heterocyclic and Mesoionic Carbene Complexes of Group 5 and Group 6 Metals Philipp M Hauser, Felix Ziegler, Janis V Musso, Pradeep KR Panyam, Mohasin Momin, Jonas Groos, and Michael R Buchmeiser, Chair of Macromolecular Compounds and Fiber Chemistry, Institute of Polymer Chemistry, University of Stuttgart, Stuttgart, Germany © 2022 Elsevier Ltd. All rights reserved.

6.02.1 6.02.2 6.02.2.1 6.02.2.2 6.02.2.3 6.02.2.4 6.02.3 6.02.3.1 6.02.3.2 6.02.4 6.02.4.1 6.02.4.2 6.02.4.3 6.02.5 6.02.5.1 6.02.5.2 6.02.5.2.1 6.02.5.2.2 6.02.5.2.3 6.02.5.2.4 6.02.5.2.5 6.02.5.2.6 6.02.5.2.7 6.02.5.2.8 6.02.5.2.9 6.02.5.2.10 6.02.5.3 6.02.5.4 6.02.6 6.02.7 6.02.7.1 6.02.7.1.1 6.02.7.1.2 6.02.7.1.3 6.02.7.1.4 6.02.7.1.5 6.02.7.2 6.02.8 References

6.02.1

Introduction Vanadium NHC complexes Synthesis from various precursors Complexes bearing arene-appended NHCs Complexes bearing tridentate NHCs NHC-induced reactions Niobium NHC complexes Complexes bearing monodentate NHCs Complexes bearing multidentate NHCs Tantalum NHC complexes Complexes bearing monodentate NHCs Complexes bearing bidentate NHCs Complexes bearing tridentate NHCs Chromium NHC complexes Chromium (0) NHC complexes Chromium (I), (II), (III) NHC complexes Chromium NHC complexes bearing 5-bound cyclopentadienyl rings Chromium indenyl NHC complexes Chromium fluorenyl NHC complexes Chromium Di-NHC complexes Chromium tetra-NHC complexes Chromium NHC complexes bearing pincer ligands Chromium NHC complexes bearing amidine ligands Chromium NHC complexes bearing chiral ligands Chromium NHC complexes bearing N-phosphanyl substituents Chromium complexes bearing imino-functionalized NHCs Chromium (VI) NHC complexes Chromium (0) mesoionic tetrazolylidene complexes Molybdenum NHC complexes Tungsten NHC complexes Tungsten (0) NHC carbonyl complexes Structure and synthesis of monodentate tungsten (0) NHC carbonyl complexes Structure and synthesis bidentate tungsten(0) NHC carbonyl complexes Template-controlled synthesis of NHC ligands via the use of isocyanides Tungsten carbonyl complexes bearing an abnormal NHC (aNHC) Tungsten allyl and cyclopentadienyl carbonyl complexes Tungsten (VI) NHC complexes without alkylidene or alkylidyne ligands Summary

208 209 209 212 213 214 215 215 216 217 217 218 220 222 222 223 223 224 225 226 228 229 230 231 231 232 233 234 234 247 247 247 250 251 254 255 260 261 261

Introduction

After the first investigation on the structure and reactivity of N-heterocyclic carbenes (NHCs) in 19621 and the first realization of metal NHC complexes in 1968 by Wanzlick,2 the further use of NHCs in metal complexes remained comparably dormant until the first isolation of a stable NHC in 1991 by Arduengo.3 At that time, the tremendous potential of NHCs as well as of abnormal (mesoionic or remote) NHC complexes4,5 as strong s-donating ligands for (transition) metals was realized and boosted the area of both homogeneous6 and heterogeneous7,8 (late) transition metal catalysis. In parallel, NHCs also entered the field of organocatalysis.9–11 The special reactivity and peculiar properties of metal-NHC complexes are a result of the unique nature of the metal-NHC bond, which has been studied in detail.12,13 While some reports on group 5 and 6 metal complexes date back to the

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1970s, the majority of the complexes prepared so far have been reported over the last 30 years. This article provides a comprehensive survey over all relevant group 5 and 6 metal complexes prepared so far, thereby excluding group 6 metal alkylidene and alkylidyne NHC complexes, which will be outlined in a separate chapter.

6.02.2

Vanadium NHC complexes

6.02.2.1

Synthesis from various precursors

Vanadium (imido) alkyl alkylidene NHC complexes were developed by Nomura et al. in 2008. These complexes were prepared from the vanadium imido trialkyl progenitors via a-hydrogen elimination in presence of an NHC in C6D6. In case 1 equiv. of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) was added to a vanadium imido trialkyl complex, no coordination of the NHC was observed. However, the target complex V001 became accessible when 2/3 equiv. of IPr were used (Scheme 1). In case of V001, an aliphatic imido complex, the isolated yield was only 22% while for V002, an aromatic imido complex, isolated yields reached 75%. The alkylidene protons in V001 and V002 were observed at d ¼ 13.4 ppm as a broad resonance due to the coupling with vanadium. Further, reaction of the vanadium imido trialkyl complex V(N]Ad)(CH2SiMe3), with 1,3-di-tert-butylimidazol-2ylidene did not give the target complex and no a-hydrogen abstraction was observed. Crystallographic data of V001 revealed that the vanadium complex adopts a distorted tetrahedral geometry. The VdC bond distances for the alkyl, alkylidene and the NHC

Scheme 1 Synthesis of vanadium (imido) alkyl alkylidene NHC complexes. Modified from Zhang, W.; Nomura, K. Facile Synthesis of (Imido)vanadium(V)−Alkyl, Alkylidene Complexes Containing an N-Heterocyclic Carbene Ligand From Their Trialkyl Analogues. Organometallics 2008, 27, 6400–6402.

ligand were found to be 206.9(3), 182.9(3) and 217.2(2) pm, respectively.14 Furthermore, Lorber and Vendier expanded the family of vanadium NHC complexes by treating several vanadium precursors with 1,3-dimesitylimidazol-2-ylidene (IMes) or its imidazolium salt. Upon treatment of V(NMe2)4 with 2 equiv. of IMes ∙ HCl in THF at 70  C, the vanadium (III) NHC complex VCl2(NMe2)(IMes)2 V003 and the vanadium (IV) NHC complex VCl2(NMe2)2(IMes) V004 were obtained in 65% and 20% isolated yield, respectively (Scheme 2).15 Both complexes were found to be paramagnetic, V003 was EPR and NMR silent. Single crystal X-ray analysis of V003 confirmed that the pentacoordinate vanadium adopts a distorted square pyramidal geometry with two carbene carbons trans to each other and the two remaining chlorine atoms eventually trans to each other, two NHCs and two chlorine atoms form the base of the pyramid. The short distances between the carbene-carbon and the chlorine atoms supported the presence of intramolecular Ccarbene-Cl interactions. The synthesis of a vanadium oxo NHC complex was reported via a two-step reaction sequence starting from a vanadium amide precursor (Scheme 2). In the first step, V(]O)Cl2(NHMe2)2 was obtained by reacting V(NMe2)4 with an amide RC(O)NH2 (R ¼ tBu, Ph) in the presence of excess Me3SiCl in toluene. It was verified that the oxo complex formed through the intermediate acetamido complex V(]NC(O)R)Cl2(NHMe2)2. Next, the oxo complex was reacted with 2 equiv. of IMes in toluene, which resulted in the formation of the vanadium oxo bis-NHC dichloride complex V(]O)Cl2(IMes)2 V005 in 73% yield. Alternatively, crystals of V005 were obtained by exposure of a THF solution of complex V006 to air. Single-crystal X-ray analysis of V005 revealed a distorted square pyramidal geometry around vanadium with the oxo ligand in the apical position.15

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Scheme 2 Synthesis of vanadium oxo NHC complexes from an amide precursor. Modified from Lorber, C.; Vendier, L. Synthesis and Structure of Early Transition Metal NHC Complexes. Dalton Trans. 2009, 6903–6914.

VCl3(THF)3 was also treated with 2 equiv. of IMes in toluene, furnishing VCl3(IMes)2, V006, as an air-sensitive, orange compound in 70% yield. As V006 was NMR and EPR silent, formation of V006 was confirmed by single-crystal X-ray analysis, revealing a pentacoordinate metal center with distorted square pyramidal geometry.15 However, Wu et al. also reported that the reaction of VCl3(THF)3 with NHCs in equimolar quantities in toluene yielded the vanadium (III) mono-NHC complexes V007 and V008 (Scheme 3). Both complexes were characterized by elemental analysis and the vanadium content was confirmed by titration.16

Scheme 3 Synthesis of vanadium NHC complexes from vanadium chloride precursors. Modified from Zhang, S.; Zhang, W.-C.; Shang, D.-D.; Wu, Y.-X. Synthesis of Ultra-High-Molecular-Weight Ethylene-Propylene Copolymer Via Quasi-Living Copolymerization With N-Heterocyclic Carbene Ligated Vanadium Complexes. J. Polym. Sci. Pol. Chem. 2019, 57, 553–561, Zhang, S.; Zhang, W.-C.; Shang, D.-D.; Zhang, Z.-Q.; Wu, Y.-X. Ethylene/Propylene Copolymerization Catalyzed by Vanadium Complexes Containing N-Heterocyclic Carbenes. Dalton Trans. 2015, 44, 15264–15270, and Lorber, C.; Vendier, L. Synthesis and Structure of Early Transition Metal NHC Complexes Dalton Trans. 2009, 6903–6914.

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Wu et al. also described vanadium (V) oxo NHC complexes V009–V012 (Scheme 3)17 obtained by the reaction of VOCl3 with different NHCs in equimolar quantities in toluene. All complexes were characterized by 1H, 13C and 51V NMR. Lorber and Vendier reported a V (II) NHC complex by exchanging one pyridine by IMes from the VCl2(Py)4 precursor complex and obtained VCl2(IMes)(Py)3 V013 as dark-red compound in THF. The geometry around the V (II) center was found to be distorted octahedral, with mutually trans-located chlorine atoms (Cl-V-Cl ¼ 176.78(3) ). By contrast, the synthesis of V (IV) NHC chloride complexes was hampered by their very poor solubility, which significantly impeded their full characterization. Lorber and Vendier also reported on a vanadium imido NHC complex by reacting equimolar amounts of V(¼N-2,6-iPr2-C6H3) Cl2(NHMe2)2 and IMes in toluene to yield V(¼N-2,6-iPr2-C6H3)Cl2(NHMe2)(IMes) V014 in 84% isolated yield. The single-crystal X-ray analysis revealed V014 as distorted square-pyramid with the arylimido ligand in the apical position and the NHC ligand trans to the dimethylamino ligand, which forms the base of the pyramid along with two chlorine atoms. The imido linkage was found to be almost linear with a V–N1–C1 angle of 172.6(3) . In order to obtain a vanadium bis-NHC imido complex, the dimethylamine-free oligomeric complex [V(¼N-2,6-iPr2-C6H3)Cl2]n was reacted with 2 equiv. of IMes (Scheme 3). However, unexpectedly, the new ionic complex [V(¼N-2,6-iPr2-C6H3)Cl3(IMes)]−[HIMes]+ V015 formed, whose structure was unambiguously confirmed by single-crystal X-ray analysis.15 In 2016 Nomura et al. reported on the reaction of a weakly coordinating anionic-NHC (WCA-NHC) i.e., a frustrated carbene-borane Lewis pair, Li(R’NHC-B(C6F5)3)(toluene) (R0 ¼ Mes, Dipp) with vanadium imido complexes in toluene to obtain 0 complexes of the general formula [V(] NR)Cl2(WCA-R NHC)] (R ¼ 1-adamantyl, C6H5, 2,6-Me2-C6H3) V016–V020 (Scheme 4).18,19 Single-crystal X-ray analysis of V016–V018 revealed considerably shorter VdCcarbene bond distances 203.9(3)–204.9(2) pm compared to the previously reported vanadium oxo complex20 or to vanadium alkylidene complexes14 (approx. 213–217 pm), which suggests a strong s-donor nature of the WCA-NHC ligand. V019 and V020 showed slightly longer VdCcarbene bond distances of 206.0(2) and 207.6(3) pm, respectively, although these were comparably shorter than those in the previously reported vanadium oxo or vanadium alkylidene complexes.

Scheme 4 Synthesis of vanadium WCA-NHC complexes from a vanadium imido trichloride precursor. Modified from Igarashi, A.; Kolychev, E.L.; Tamm, M.; Nomura, K. Synthesis of (Imido)Vanadium(V) Dichloride Complexes Containing Anionic N-Heterocyclic Carbenes That Contain a Weakly Coordinating Borate Moiety: New MAO-Free Ethylene Polymerization Catalysts. Organometallics 2016, 35, 1778–1784 and Nomura, K.; Nagai, G.; Izawa, I.; Mitsudome, T.; Tamm, M.; Yamazoe, S. XAS Analysis of Reactions of (Arylimido)vanadium(V) Dichloride Complexes Containing Anionic NHC That Contains a Weakly Coordinating B(C6F5)3 Moiety (WCA-NHC) or Phenoxide Ligands with Al Alkyls: A Potential Ethylene Polymerization Catalyst with WCA-NHC Ligands ACS Omega 2019, 4, 18833–18845.

Recently, Tavcar et al. successfully isolated [(LDipp)H][VOF4] V021, (LDipp ¼ 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro2H-imidazol-2-ylidene) and the neutral complex [(LDipp)VOF3] V022 (Scheme 5). They reacted VOF3 with a “naked” fluoride reagent i.e., imidazolium fluoride [(LDipp)H][F] in acetonitrile to obtain complex V021; reaction with the free carbene (LDipp) in an ethereal solvent (THF, diethyl ether) resulted in the formation of complex V022. Complex V021 crystallized from acetonitrile and was found to contain two acetonitrile molecules, notably both as solvate molecules not coordinated to the vanadium(V) center. A square pyramidal arrangement with apical oxygen and basal fluorine atoms at the vanadium center were observed, while V022 exhibits a distorted square pyramidal geometry with oxygen in the apical position. The VdF bond distance (180.4(2) pm) trans to the VdCcarbene bond was found to be elongated compared to the remaining two VdFcis bond distances (177.1(1) and 178.1(1) pm). It was speculated that this elongated VdFtrans bond was the result of crystal packing as supported by computational structure studies. Further, the ionic complex [(LDipp)H][VO2F2] V023 was obtained from V022 upon controlled exposure of the solution to air.21

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Scheme 5 Synthesis of vanadium NHC complexes from a vanadium oxo trifluoride precursor. Modified from Županek, Z.; Tramšek, M.; Kokalj, A.; Tavcar, G. Reactivity of VOF3 With N-Heterocyclic Carbene and Imidazolium Fluoride: Analysis of Ligand–VOF3 Bonding with Evidence of a Minute p Back-Donation of Fluoride. Inorg. Chem. 2018, 57, 13866–13879.

6.02.2.2

Complexes bearing arene-appended NHCs

In 2006, Danopoulos et al. reported on a novel arene-appended NHC bearing a pendant indenyl moiety, e.g., N-(inden-1-ylethyl)N0 -di(2,6-isopropylphen-1-yl) imidazolium bromide (IndH-NHC-H)Br, which was reacted with V(NMe2)4 in toluene to obtain the red-brown crystalline (Ind-NHC)V(NMe2)Br complex, V024 (Scheme 6).22 Crystallographic data for V024 showed that the bridgehead C atoms of the indenyl ring were planar, indicating the absence of strain in the chelate. The VdCcarbene bond distance was 218.5 pm, the VdN(amido) bond distance was 188.5 pm and thus in the range of related literature-known compounds. Further, in 2007, Danopoulos et al. reported on a 4,7-dimethylindenyl-functionalized NHC complex of vanadium (III) V025, prepared by reacting the potassium salt of the indenylcarbene with VCl3(THF)3 (Scheme 6).23 Crystallographic data of V025 revealed that the VdCcarbene bond length was about 6 pm shorter than the one in V024. Interestingly, the VdCcarbene bond length was found to be dependent on the nature of the co-ligands. Both, complex V024 and V025, exhibit distorted tetrahedral geometries.

Scheme 6 Synthesis of vanadium complexes bearing a bidentate NHC. Modified from Downing, S.P.; Danopoulos, A.A. Indenyl- and Fluorenyl-Functionalized N-Heterocyclic Carbene Complexes of Titanium and Vanadium. Organometallics 2006, 25, 1337–1340 and Downing, S.P.; Guadano, S.C.; Pugh, D.; Danopoulos, A.A.; Bellabarba, R.M.; Hanton, M.; Smith, D.; Tooze, R.P.; Indenyl- and Fluorenyl-Functionalized N-Heterocyclic Carbene Complexes of Titanium, Zirconium, Vanadium, Chromium, and Yttrium. Organometallics 2007, 26, 3762–3770.

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Complexes bearing tridentate NHCs

Dagorne et al. reported on an air-stable vanadium(V) oxo complex bearing a tridentate chelating NHC ligand (Scheme 7). The complex V(]O)Cl[Z3-O,C,O-(3,5-di-tert-butyl-C6H2O)2N2C3H4] V026 was synthesized by reacting the imidazolium salt with 1 equiv. of V(]O)(iPrO)3 in THF under concomitant elimination of isopropanol.24 X-ray crystallographic studies showed that V026 exhibits a slightly distorted square-pyramidal geometry with the oxo moiety occupying the apical position. The VdCcarbene bond distance was 209.5(3) pm and thus somewhat shorter than in other reported V (V)-NHC complexes, probably reflecting the geometrical constraints in V026.

Scheme 7 Synthesis of vanadium complexes bearing tridentate NHCs. Modified from Bellemin-Laponnaz, S.; Welter, R.; Brelot, L.; Dagorne, S. Synthesis and Structure of V (V) and Mn(III) NHC Complexes Supported by a Tridentate Bis-Aryloxide-N-Heterocyclic Carbene Ligand. J. Organomet. Chem. 2009, 694, 604–606 and Pugh, D.; Wright, J.A.; Freeman, S.; Danopoulos, A.A. ‘Pincer’ Dicarbene Complexes of Some Early Transition Metals and Uranium. Dalton Trans. 2006, 775–782.

A vanadium (IV) complex bearing another type of pincer ligand, bis(NHC)-pyridine, was reported by Danopoulos et al.25 The vanadium (II) complex, VCl2(tmeda)2 (tmeda ¼ tetramethylethyenediamine) was reacted with [2,6-bis(3-(2,6-diisopropylphenyl) imidazol-2-ylidene)]pyridine (DippCNC) in THF to obtain (DippCNC)VCl2(THF) V027, in 98% yield (Scheme 7). Crystallographic studies revealed that the two VdCcarbene bond distances were slightly different, (220.9(9) and 216.2(9) pm) and that the Cl-V-Cl angle was 173.73(10) with the chlorine atoms pointing away from the pyridine ring. Oxidation of V027 and (DippCNC)VCl3 with 4-methylmorpholine N-oxide (NMO) in THF resulted in the formation of the vanadium (IV)-oxo complex V028 in variable yield.

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Single-crystal X-ray analysis revealed that the V (IV)dCcarbene bond lengths were in the range of 216.9(10)–218.2(11) pm, comparable to the corresponding VdCcarbene bond length in V027. Further, the chloride ligand was easily abstracted from V028 using AgBF4 in acetonitrile to obtain the ionic complex V030. Single-crystal X-ray analysis showed an octahedral geometry in V030 with the acetonitrile ligands occupying the trans positions. Finally, reaction of V028 with p-tolyl azide gave the brown paramagnetic complex V029; its structure was confirmed by the V]N(tolyl) bond stretching observed at 970 cm−1. No single-crystal X-ray analysis of V029 could be accomplished due to facile solvent loss from the crystal.25

6.02.2.4

NHC-induced reactions

Vanadium complexes were also assessed in the activation of N2O. Severin et al. reported on the reaction of equimolar amounts of the IMes-N2O adduct with V(Mes)3(THF) in THF, resulting in a blue to red-purple solution.26 Further precipitation with n-hexane allowed the isolation of V031 in 53% yield. X-ray analysis confirmed the cleavage of the NdO bond and transfer of oxygen to vanadium resulting in an oxo-vanadium complex. Also, the insertion of the IMesN2 fragment into the VdMes bond was observed. Further, upon heating V031 to 80  C in toluene, a color change from red-purple to orange-red was observed and V032 was obtained in good yield (75%). Crystallographic analysis of V032 confirmed the cleavage of the NdN bond and migration of the formerly vanadium-bound mesityl group to the N atom26 (Scheme 8).

Scheme 8 NHC-induced reaction of vanadium complex and N2O. Modified from Tskhovrebov, A.G.; Solari, E.; Wodrich, M.D.; Scopelliti, R.; Severin, K.; Sequential N–O and N–N Bond Cleavage of N-Heterocyclic Carbene-Activated Nitrous Oxide With a Vanadium Complex. J. Am. Chem. Soc. 2012, 134, 1471–1473.

NHC-induced ring contraction was reported by Scheer et al. through pnictogen abstraction. Here, reaction of the vanadium complex [(Cp V)(m,6:6-P6)] with 2 equiv. of 1,3,4,5-tetramethylimidazol-2-ylidene (MeNHC) resulted in the expected ring contraction product [(MeNHC)2P][(Cp V)2(m,5:5-P5)] V034 along with two additional complexes [(MeNHC)2P][(Cp V)2(m,6:6-P6)] V033 and [(Cp V)2(m,3:3-P3)(m-PMeNHC)] V035. Complexes V033 and V034 co-crystallized while V035 crystallized separately; all structures were confirmed by single-crystal X-ray analysis27 (Scheme 9).

Scheme 9 Synthesis of vanadium complexes V033–V035. Modified from Piesch, M.; Reichl, S.; Seidl, M.; Balazs, G.; Scheer, M. Ring Contraction by NHC-Induced Pnictogen Abstraction. Angew. Chem. Int. Ed. 2019, 58, 16563–16568.

N-Heterocyclic and Mesoionic Carbene Complexes of Group 5 and Group 6 Metals

6.02.3

Niobium NHC complexes

6.02.3.1

Complexes bearing monodentate NHCs

215

Niobium pentahalide complexes bearing monodentate NHCs were first synthesized by Marchetti et al. via the reaction of a niobium pentahalide and a free carbene, e.g., 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, IPr or 1,3-dimesitylimidazol-2-ylidene, IMes, in toluene to obtain complexes Nb001–Nb004.28,29 DFT calculations suggest that the formation of the pentahalide NHC complexes Nb002 and Nb003 takes place via the cationic intermediate [Nb(NHC)X4]+ with [NbX6]− as the counterion. In fact, [NbCl6]− and [NbBr6]− were observed as intermediates in the 93Nb NMR spectra during the reaction. Interestingly, if moisture is present during the reaction to Nb001, the binuclear niobium oxo fluoride complex Nb005 bearing two monodentate NHCs can be isolated.30 Slight changes in the reaction conditions for the synthesis of Nb004 by Wei and Cai et al. allowed for the isolation of the cationic complex Nb006 with two NHCs trans to each other and NbCl−6 as an anion (Scheme 10, (I)).31 Petrov et al. showed that the reaction of Nb(NMe2)5 with the imidazolium salt IPr ∙HBF4 leads to a mixture of the binuclear complex Nb007, IPr, and other unknown byproducts, while from the reaction with IMes ∙ HBF4 and Nb(NMe2)5 Nb008 can be isolated. Nb008 consists of an NbF−6 anion and an imidazolium cation, which is hydrogen-bonded to the free carbene IMes (Scheme 10, II).32

Scheme 10 Synthesis of niobium complexes bearing monodentate NHCs. Modified from Bortoluzzi, M.; Ferretti, E.; Marchetti, F.; Pampaloni, G.; Zacchini, S. Coordination Complexes of Niobium and Tantalum Pentahalides With a Bulky NHC ligand. Dalton Trans. 2016, 45, 6939–6948, Bortoluzzi, M.; Ferretti, E.; Marchetti, F.; Pampaloni, G.; Zacchini, S. A Structurally-Characterized NbCl5–NHC Adduct. Chem. Commun. 2014, 50, 4472–4474, Bortoluzzi, M.; Ferretti, E.; Marchetti, F.; Pampaloni, G.; Zacchini, S. A Crystallographic and DFT Study on a NHC Complex of Niobium Oxide Trifluoride. J. Coord. Chem. 2016, 69, 2766–2774 and Wei, Z.; Zhang, W.; Luo, G.; Xu, F.; Mei, Y.; Cai, H.; Mono- and Bis-N-Heterocyclic Carbene Complexes of Tantalum and Niobium With High Oxidation States. New J. Chem. 2016, 40, 6270–6275, Petrov, P.A.; Golubitskaya, E.A.; Kompankov, N.B.; Eltsov, I.V.; Sukhikh, T.S.; Sokolov, M.N. Binuclear Niobium Complex With Coordinated N-Heterocyclic Carbene. J. Struct. Chem. 2020, 60, 1989–1994 and Bortoluzzi, M.; Ferretti, E.; Marchetti, F.; Pampaloni, G.; Pinzino, C.; Zacchini, S. Coordination Compounds of Niobium (IV) Oxide Dihalides Including the Synthesis and the Crystallographic Characterization of NHC Complexes. Inorg. Chem. 2016, 55, 4173–4182 and Zupanek, Ž.; Tramšek, M.; Kokalj, A.; Tavcar, G. The Peculiar Case of Conformations in Coordination Compounds of Group V Pentahalides With N-Heterocyclic Carbene and Synthesis of Their Imidazolium Salts J. Fluor. Chem. 2019, 227, 109373.

Niobium oxo halide complexes bearing two NHCs were synthesized by Marchetti et al. For the synthesis of Nb009, NbOBr3 was reacted with DME and SnBu3H to obtain NbOBr2(DME), which was then reacted with the free carbene IXyl (IXyl ¼ 1,3-bis(2,6-dimethylphenyl)imidazol-2-ylidene).33 NbOCl3 was reacted with acetonitrile and SnBu3H to produce NbOCl2(MeCN)2, which reacts with IMes to complex Nb010 (Scheme 10, III).33 The crystal structures of Nb001, Nb002, Nb005, Nb006 and Nb007 show octahedral geometries at the metal center, while the pentacoordinate complexes Nb009 and Nb010 show

216

Table 1

N-Heterocyclic and Mesoionic Carbene Complexes of Group 5 and Group 6 Metals NbdNHC bond lengths [pm] of niobium complexes bearing monodentate NHCs.28–34

Complex

Nb001

Nb002

Nb005

Nb006

Nb007

Nb009

Nb010

NbdNHC bond [pm] 2nd NbdNHC bond [pm]

231.9(2) –

239.6(12) –

233.9(2) 232.2(2)

235.2(4) 235.6(4)

235.0(6) –

233.7(7) 230.5(8)

232.2(6) 233.2(5)

slightly distorted trigonal bipyramidal geometries. Nb002 shows the longest NbdNHC bond length with 239.6(12) pm, while the measured bond length in the other complexes ranges from 231–236 pm (Table 1).

6.02.3.2

Complexes bearing multidentate NHCs

The groups of Bergman and Arnold developed Niobium complexes supported by bis-NHC borate ligands.35 The imidazolium salt H2B(MesImH)2Cl was deprotonated with 2 equiv. lithium diisopropylamide and further reacted with the metal precursor Nb(NtBu) Cl3py2 to obtain the Nb dichloro complex Nb011 (Scheme 11). Reaction of a slurry of Nb011 in diethyl ether with 2 equiv. of MeMgCl led to the formation of the analogous dimethyl complex Nb012. Both pentacoordinate complexes adopt a distorted square pyramidal geometry with the imido ligand in the apex.

Scheme 11 Synthesis of Nb011 and subsequent reaction with MeMgCl to Nb012. Modified from Ziegler, J.A.; Prange, C.; Lohrey, T.D.; Bergman, R.G.; Arnold, J. Hydroboration Reactivity of Niobium Bis(N-heterocyclic carbene)borate Complexes. Inorg. Chem. 2018, 57, 5213–5224.

The NbdNHC bonds in Nb012 (231.6(2) and 231.1(2) pm) are slightly longer than the ones in Nb011 (228.4(3) and 227.4 (3) pm). The reaction of Nb012 with a large excess of carbon monoxide leads to the formation of the niobium imido dicarbonyl isopropoxyboryl bis-NHC complex Nb013, which exhibits a distorted octahedral geometry with the imido trans to the oxygen. The absence of a large excess of CO leads to the formation of the dimeric acetone adduct Nb014, which is bridged by the oxygen atoms of the acetone. In the reaction of Nb012 with carbonyl-containing substrates such as paraformaldehyde and benzophenone, hydroboration is observed and leads to the formation of double-insertion products Nb015 and Nb016. Nb016 spontaneously releases methane by C-H activation of the o-hydrogen atom of a phenyl group to form the cyclometalated complex Nb017. The addition of H2 to Nb016 results in the formation of the arene complex Nb018. Nb017 can also be reacted with H2 to obtain complex Nb018 (Scheme 12).

Scheme 12 Reactions of Nb012 with CO, paraformaldehyde, benzophenone, and subsequent reactions. Modified from Ziegler, J.A.; Prange, C.; Lohrey, T.D.; Bergman, R.G.; Arnold, J. Hydroboration Reactivity of Niobium Bis(N-heterocyclic carbene)borate Complexes. Inorg. Chem. 2018, 57, 5213–5224.

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217

With Nb011, the hydroboration of carbonyl-containing substrates is observed, too. Reaction with benzophenone results in the formation of Nb019, while the reaction with paraformaldehyde results in the formation of Nb020. Even excess of paraformaldehyde and elevated temperatures do not result in the formation of the double-insertion product, while Nb011 and benzaldehyde react to form the double insertion product Nb021. The hexacoordinate insertion products show distorted octahedral geometries. Complex Nb011 also shows reactivity in the hydroboration of isocyanates. The reaction of Nb011 with (p-C6H4Me)NCO resulted in a mixture of the O-borylated product Nb022 and the N-borylated complex Nb024. Heating the reaction mixture to 100  C for several days resulted in the accumulation of Nb024. Similarly, reaction of DippNCO (2,6-diisopropylphenyl isocyanate) at elevated temperature results in the formation of the O-borylated product Nb023, while additional heating for several days resulted in the formation of the N-borylated product Nb025 (Scheme 13).

Scheme 13 Reactions of complex Nb011 with aldehydes, ketones, and isocyanates. Modified from Ziegler, J.A.; Prange, C.; Lohrey, T.D.; Bergman, R.G.; Arnold, J. Hydroboration Reactivity of Niobium Bis(N-heterocyclic carbene)borate Complexes. Inorg. Chem. 2018, 57, 5213–5224 and Baltrun, M.; Watt, F.A.; Schoch, R.; Wölper, C.; Neuba, A.G.; Hohloch, S. A New Bis-Phenolate Mesoionic Carbene Ligand for Early Transition Metal Chemistry. Dalton Trans. 2019, 48, 14611–14625.

The niobium complex Nb026 bearing a mesoionic carbene was isolated by Hohloch et al. in 2019 by deprotonation of the triazolium precursor with 3 equiv. of lithium diisopropylamide and subsequent reaction with Nb(NtBu)Cl3py2 in a mixture of THF and diethyl ether (Scheme 13, II).36 The niobium atom is coordinated in a distorted octahedral fashion with the tridentate ligand and the chloride in one plane and the imido ligand and the pyridine trans to each other.

6.02.4

Tantalum NHC complexes

6.02.4.1

Complexes bearing monodentate NHCs

The first tantalum pentahalide complexes bearing monodentate NHCs were synthesized by Marchetti et al. by reacting the free carbene IPr with TaX5 (X ¼ F, Cl, Br) at 80  C in toluene to obtain complexes Ta001–Ta003 (Scheme 14, I). Similar to the analogous niobium complexes, the synthesis of Ta002 and Ta003 proceeds via formation of an intermediate product. DFT calculations support the ionic intermediates [TaX4(NHC)]+[TaX6]− in analogy to the proposed intermediates for niobium pentahalide NHC complexes.28

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Scheme 14 Synthesis of tantalum complexes bearing monodentate NHCs. Modified from Bortoluzzi, M.; Ferretti, E.; Marchetti, F.; Pampaloni, G.; Zacchini, S. Coordination Complexes of Niobium and Tantalum Pentahalides With a Bulky NHC Ligand. Dalton Trans. 2016, 45, 6939–6948 and Wei, Z.; Zhang, W.; Luo, G.; Xu, F.; Mei, Y.; Cai, H. Mono- and Bis-N-Heterocyclic Carbene Complexes of Tantalum and Niobium With High Oxidation States. New J. Chem. 2016, 40, 6270–6275 and Petrov, P.A.; Sukhikh, T.S.; Sokolov, M.N. NHC Adducts of Tantalum Amidohalides: The First Example of NHC Abnormally Coordinated to An Early Transition Metal. Dalton Trans. 2017, 46, 4902–4906.

Table 2

TadNHC bond lengths [pm] in tantalum complexes bearing monodentate NHCs.28,31,34,37

Complexes

Ta001

Ta002

Ta003

Ta004

Ta005

Ta006

Ta008

TadNHC bond [pm] 2nd TadNHC bond [pm]

230.9(3) –

237.3(5) –

238.3(10) –

234.6(6) 234.6(6)

238.9(5) –

243.2(2) –

233.9(2) –

During efforts to expand the scope to different NHCs (IMes, IXyl) the degradation product [IXylH]+[TaF6]− was isolated.28 The isolation of Ta004, a cationic bis-NHC complex with a [TaCl6]− anion obtained via the reaction of TaCl5 with IMes at lower temperatures, was published by Wei and Cai et al. (Scheme 14, II).31 Reaction of Ta(NMe2)5 with imidazolium salts such as IMes ∙ HBF4 and SIMes ∙HBF4 at 60  C by Petrov et al. resulted in the formation of the mixed amidofluoride complexes Ta005 and Ta006, with the fluorides derived from the BF−4 anion.37 Complexes Ta005 and Ta006 display octahedral geometries with one amide trans to the carbene and the fluorides in one plane along with the second amide (Scheme 14, III). The reaction of Ta(NMe2)5 with IMes ∙HCl resulted in a mixture of the free carbene IMes, Ta007, which consists of a [TaCl3(NMe2)3]− anion and the IMes imidazolium cation hydrogen-bonded to IMes, and the first isolated example of abnormal neutral NHC bound to tantalum in Ta008. Deliberate synthesis of Ta008 was achieved by the reaction of the dimer [Ta(NMe2)3Cl2]2 with the free carbene IMes.37 In the pentahalide NHC complexes, the TadNHC bond is shortest for the fluoride complex Ta001 (230.9(3) pm) and longest for the bromide complex Ta003 (238.3(10) pm) (Table 2). The TadNHC bonds in the cationic complexes Ta004 (234.6(6) pm) and Ta008 with the abnormally bound carbene (233.9(2) pm) are rather short, while the amidofluoride complexes Ta005 (238.9(5) pm) and Ta006 (243.2(2) pm) exhibit the longest TadNHC bonds in tantalum complexes with monodentate NHCs.

6.02.4.2

Complexes bearing bidentate NHCs

To obtain tantalum complexes bearing bidentate NHCs, Arnold et al. deprotonated H2B(MesImH)2Cl with 2 equiv. of lithium diisopropylamide and then reacted the free carbene with TaMe3Cl2 to obtain Ta009, which can be reacted with MeLi in diethyl ether to replace the remaining chloride in Ta009 by a methyl group in complex Ta010.38 Even though Ta009 and Ta010 only differ by one ligand, they significantly differ in reactivity upon the addition of H2. Thus, the hydrogenolysis of Ta009 in benzene results in the formation of Ta011, a dimer in which the tantalum atoms are bridged by four hydrogen atoms (Scheme 15). In an atmosphere of D2, Ta009 reacts to form the analogous deuterium-bridged complex. In contrast, reaction of Ta010 with H2 in benzene yields complex Ta012, in which two tantalum atoms are bridged by two hydrogen atoms and a former benzene molecule, which was reduced by two electrons. If the reaction is conducted in toluene, complex Ta013 results, in which the tantalum atoms are bridged by a former toluene molecule, which was reduced by two electrons. Ta011 can be reduced in benzene with 2 equiv. of KC8 to obtain Ta012. Ta013 can be obtained via the analogous reaction in toluene. Hydrogenolysis of Ta010 in hexane leads to the formation of the trimeric complex Ta014 in which three tantalum atoms are bridged by a total of four hydrogens. Two of the NHC ligands undergo cyclometalation via the methyl group of the mesityl ligand.

N-Heterocyclic and Mesoionic Carbene Complexes of Group 5 and Group 6 Metals

219

Scheme 15 Synthesis of complexes Ta009 and Ta010 and their reactions with H2. Modified from Fostvedt, J.I.; Lohrey, T.D.; Bergman, R.G.; Arnold, J. Structural Diversity in Multinuclear Tantalum Polyhydrides Formed Via Reductive Hydrogenolysis of Metal–Carbon Bonds. Chem. Commun. 2019, 55, 13263–13266.

Tantalum complexes bearing a bidentate NHC, which binds via the carbene moiety and an alkoxide moiety in the N-substituent of the NHC, were developed by Camp et al.39 Alcoholysis of Ta(NtBu)(CHt2Bu)3 with the NHC-alcohol 1-mesityl-3-(2-hydroxyisobutyl)imidazol-2-ylidene led to the formation of complex Ta015 with concurrent release of neopentane. Analogously, the reaction of the NHC-alcohol with the alkylidene Ta(CHtBu)(CHt2Bu)3 led to the formation of Ta016 (Scheme 16).

Scheme 16 Synthesis of complexes Ta015 and Ta016 and subsequent reactions with [Rh(COD)Cl]2 and (tBuO)3SiOH. Modified from Srivastava, R.; Moneuse, R.; Petit, J.; Pavard, P.A.; Dardun, V.; Rivat, M.; Schiltz, P.; Solari, M.; Jeanneau, E.; Veyre, L.; Thieuleux, C.; Quadrelli, E.A.; Camp, C. Early/Late Heterobimetallic Tantalum/Rhodium Species Assembled Through a Novel Bifunctional NHC-OH Ligand. Chem. Eur. J. 2018, 24, 4361–4370 and Srivastava, R.; Quadrelli, E.A.; Camp, C. Lability of Ta–NHC Adducts as a Synthetic Route Towards Heterobimetallic Ta/Rh Complexes. Dalton Trans. 2020, 49, 3120–3128.

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The alkylidene complex Ta016 was found to be stable at −40  C in the solid state but decomposed readily in a toluene solution at room temperature accompanied by the release of 1 equiv. of neopentane. CdH bond activation of one of the methyl groups in the mesityl-substituent resulted in the formation of the cyclometalated compound Ta017. Ta015 reacts with half an equivalent of [Rh(COD)Cl]2 (COD ¼ cycloocta-1,4-diene) to obtain the heterobimetallic compound Ta018, in which the carbene moiety is coordinated to the rhodium center, while the alkoxide binds to tantalum. Alternatively, Ta018 can be synthesized via the reaction of Rh(COD)Cl(NHC) and Ta(NtBu)(CHt2Bu)3. The reaction of Ta016 with [Rh(COD)Cl]2 in toluene resulted in a mixture of the heteropolymetallic complex Ta019, the cyclometalated complex Ta017, and the heterobimetallic complex Ta020, whereas in case the reaction was carried out in THF, only Ta019 was isolated. Two equivalents (tBuO)3SiOH react with Ta016 to Ta021, in which the carbene moiety is no longer coordinated to tantalum, but to the hydroxyl group of one (tBuO)3SiOH, while the other silanol deprotonates the alkylidene and forms a siloxide bond with the tantalum. In case only 1 equiv. of silanol is added, Ta022 can be isolated, in which also a siloxide bond is formed with tantalum while the alkylidene is protonated and the carbene does not coordinate to the tantalum. Ta022 can also be isolated by heating Ta021 to 120  C under vacuum. If Ta022 is reacted with [Rh(COD)Cl]2, the heterobimetallic complex Ta023 is obtained.40

6.02.4.3

Complexes bearing tridentate NHCs

The tridentate NHC ligand precursors with p-tolyl- and mesityl-groups as N-substituents of the NHC were coordinated to Ta(NMe2)5 and Ta(CH2Ph)5, respectively, to obtain complexes Ta024 und Ta025 in which, next to the carbene, only one of the amides in the N-substituents is coordinated to the metal center, while a proton remains on the other amine. Generation of Ta024 is accompanied by the protonation and dissociation of one dimethylamide, while the reaction of the NHC with Ta(CH2Ph)5 results in the release of 2 equiv. of toluene, with the formation of a benzylidene moiety (Scheme 17, I). To assure coordination of both chelating ligands in the NHCs, the amines were deprotonated with 2 equiv. of nBuLi to obtain the dilithium salts of the NHCs. The di-p-tolyl substituted NHC dilithium salt was reacted with TaCl2(NMe2)3 to obtain Ta026. If the NHC was reacted with TaCl3(NMe2)2(THF), complex Ta027, in which one chloride remains at the complex, was obtained. Reaction of the di-p-tolyl substituted NHC dilithium salt with TaCl4(NEt2)(OEt2) resulted in a mixture of complexes Ta028 with two remaining chlorides trans to each other and the diethylamide trans to the NHC, and Ta029, in which one chloride is trans to the NHC while the other is trans to the diethylamide. If the mesityl substituted NHC dilithium salt was reacted with tantalum precursors, CdH activation of the ethylene linker of the NHC occurred.

∙ Scheme 17 Synthesis of tantalum complexes with tridentate NHC ligands. Modified from Spencer, L.P.; Beddie, C.; Hall, M.B.; Fryzuk, M.D. Synthesis, Reactivity, and DFT Studies of Tantalum Complexes Incorporating Diamido-N-heterocyclic Carbene Ligands. Facile Endocyclic C −H Bond Activation. J. Am. Chem. Soc. 2006, 128, 12531–12543.

N-Heterocyclic and Mesoionic Carbene Complexes of Group 5 and Group 6 Metals

221

If the NHC is reacted with the alkylidene complex TaCl3(CHtBu)(THF)2, the cyclometalated complex Ta030, in which the alkylidene is protonated, was isolated. Reaction of the di-mesityl-substituted NHC dilithium salt with the mixed alkyl-chloro complexes TaCl2R3 (R ¼ CHt2Bu, CH3, CH2Ph) resulted in the formation of the tantalum bis-alkyl NHC complexes Ta031–Ta033 (Scheme 17, II).41 Tantalum complexes with CCC-NHC pincer ligands were developed by Hollis and Webster and coworkers.42 The reaction of Ta(NtBu)(NMe2)3 with stoichiometric amounts of the bis(imidazolium) iodide precursor resulted in the formation of a mixture of complexes Ta034 and Ta035, in which the bis(NHC) ligand is coordinated to the metal center as a tridentate ligand via triple CdH bond activation. While one dimethylamide is exchanged with one iodide in Ta034, both amides are exchanged in Ta035 (Scheme 18). If the bis(imidazolium) iodide was reacted with 3 equiv. of Ta(NtBu)(NMe2)3 under the same conditions, complex Ta034 with one iodide and one amide was isolated. In case the reaction of Ta(NtBu)(NMe2)3 was carried out with bis(triazolium) iodide, the bis(triazol-3-ylidene) tantalum complex Ta036 was obtained. Complexes Ta034–Ta036 were found to possess distorted octahedral symmetry.42

Scheme 18 Synthesis of tantalum complexes bearing bis(NHC) CCC-pincer ligands. Modified from Helgert, T.R.; Hollis, T.K.; Oliver, A.G.; Valle, H.U.; Wu, Y.; Webster, C.E.; Synthesis, Characterization, and X-Ray Molecular Structure of Tantalum CCC-N-Heterocyclic Carbene (CCC-NHC) Pincer Complexes With Imidazoleand Triazole-Based Ligands. Organometallics 2014, 33, 952–958 and Liang, G.; Hollis, T.K.; Webster, C.E. Computational Analysis of the Intramolecular Oxidative Amination of an Alkene Catalyzed by the Extreme p-Loading N-Heterocyclic Carbene Pincer Tantalum(V) Bis(imido) Complex. Organometallics 2018, 37, 1671–1681 and Helgert, T.R.; Zhang, X.; Box, H.K.; Denny, J.A.; Valle, H.U.; Oliver, A.G.; Akurathi, G.; Webster, C.E.; Hollis, T.K. Extreme p-Loading as a Design Element for Accessing Imido Ligand Reactivity. A CCC-NHC Pincer Tantalum Bis(Imido) Complex: Synthesis, Characterization, and Catalytic Oxidative Amination of Alkenes. Organometallics 2016, 35, 3452–3460 and Helgert, T.R.; Webster, C.E.; Hollis, T.K.; Valle, H.U.; Hillesheim, P.; Oliver, A.G. Free Methylidyne? CCC-NHC Tantalum Bis(Imido) Reactivity: Protonation, Rearrangement to a Mixed Unsymmetrical CCC-N-Heterocyclic Carbene/N-Heterocyclic Dicarbene (CCC-NHC/NHDC) Pincer Tantalum Bis(Imido) Complex. Inorg. Chim. Acta 2018, 469, 164–172.

Addition of LiNHtBu to Ta034 led to the formation of the dimer Ta037, in which two bisimido complexes are bridged by lithium iodide, which is coordinated to the imido ligand. Complex Ta037 was employed in the catalytic oxidative amination reactions of alkenes.43,44 Out of the hydrocarbon washing phases of Ta037, Hollis and Webster et al. were able to isolate the dimeric complex Ta038, in which one former NHC moiety contains two carbenes as an anionic N-heterocyclic dicarbene (NHDC). While the C5carbon binds to tantalum, the C2-carbon is coordinated to lithium, which bridges the dimers via coordination to one imido ligand. Heating Ta037 in toluene to 120  C led to the formation of Ta039, in which the iodide recoordinated to the tantalum and concomitantly one imido ligand was transformed into a tert-butylamido ligand.45

222

6.02.5

N-Heterocyclic and Mesoionic Carbene Complexes of Group 5 and Group 6 Metals

Chromium NHC complexes

One of the first metal NHC complexes isolated was the chromium complex Cr(IMe)(CO)5, which was synthesized by Öfele in 1968.46 Since then, various chromium NHC compounds have been synthesized including mono-NHC complexes,47–49 di-NHC complexes50–54 as well as tetra-NHC complexes of chromium55–57 in various oxidation states.58–60

6.02.5.1

Chromium (0) NHC complexes

In 2007, Chung et al. reported on the synthesis of Cr (0) NHC complexes by utilizing the Fischer chromium carbene complex [Cr¼C(OMe)(Ph)(CO)5] as a source for chromium carbonyls.47 The imidazolium salt was deprotonated with KOtBu and subsequently reacted with the Fischer carbene to afford complexes Cr001–Cr011 (Scheme 19). Single-crystal X-ray analysis of Cr001 unveiled that the NHC ligand is eclipsed to the equatorial chromium tetracarbonyl and the eclipsed carbonyls are highly bent (169.4(2) ) compared to the other carbonyls (176.4 ). The benzimidazolin-2-ylidene complexes Cr012–Cr014 were also synthesized; however, product yields were on the lower side.

Scheme 19 Chromium (0) carbonyl NHC complexes Cr001–Cr014. Modified from Kim, S.; Choi, S.Y.; Lee, Y.T.; Park, K.H.; Sitzmann, H.; Chung, Y.K. Synthesis of Chromium N-Heterocyclic Carbene Complexes Using chromium Fischer Carbenes as a Source of Chromium Carbonyls J. Organomet. Chem. 2007, 692, 5390–5394.

Later, Bielawski et al. reported the synthesis of complex Cr015 containing 1,3-diferrocenyl imidazol-2-ylidene and of Cr016 based on 1,3-dimesitylnaphthoquinoimidazol-2-ylidene by reacting the free carbenes with [Cr(CO)5(THF)] (Scheme 20).49 Notably, the CrdCcarbene distances in Cr015 and Cr016 were 215.8(3) and 212.5(3) pm, respectively.

Scheme 20 Synthesis of the chromium (0) carbonyl NHC complexes Cr015–Cr016. Modified from Rosen, E.L.; Varnado, C.D.; Tennyson, A.G.; Khramov, D.M.; Kamplain, J.W.; Sung, D.H.; Cresswell, P.T.; Lynch, V.M.; Bielawski, C.W. Redox-Active N-Heterocyclic Carbenes: Design, Synthesis, and Evaluation of Their Electronic Properties. Organometallics 2009, 28, 6695–6706.

In 2012, Arduengo and Streubel et al. reported the synthesis of phosphoryl-functionalized chromium pentacarbonyl imidazol2-ylidene complexes Cr017–018 and the bis(phosphoryl)-functionalized chromium pentacarbonyl 1-methyl-2-isopropylimidazol2-ylidene complex Cr019 (Scheme 21).48

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Scheme 21 Synthesis of the chromium (0) carbonyl NHC complexes Cr017–Cr019. Modified from Majhi, P.K.; Sauerbrey, S.; Schnakenburg, G.; Arduengo, A.J.; Streubel, R. Synthesis of Backbone P-Functionalized Imidazol-2-ylidene Complexes: En Route to Novel Functional Ionic Liquids. Inorg. Chem. 2012, 51, 10408–10416.

6.02.5.2 6.02.5.2.1

Chromium (I), (II), (III) NHC complexes Chromium NHC complexes bearing h5-bound cyclopentadienyl rings

In 2010, Smith and co-workers reported on a series of cyclopentadienyl mesityl complexes of chromium (II) and chromium (III) (Cr020–Cr024).58 The chromium (II) chloro complex Cr020 was synthesized by the reaction of chromocene with diisopropyl imidazolium chloride. Reaction of the Cr020 with MesMgBr yielded the Cr (II) mesityl complex Cr021, which upon subsequent oxidation with 0.5 equiv. of iodine yielded the Cr (III) iodo complex CpCr(IPr)(Mes)I (Cr022), whereas oxidation with excess of PbCl2 led to the Cr (III) dichloro complex CpCr(IPr)Cl2 (Cr023). Single-crystal X-ray analysis of Cr021 confirmed the monomeric and two-legged geometry. Notably, the direct synthesis of the Cr (III) dihalo complex Cr024 from a chromium (III) precursor has also been demonstrated as shown in Scheme 22.

Scheme 22 Synthesis of the [CpCr(Mes)]X complexes Cr020–Cr024. Modified from Zhou, W.; Therrien, J.A.; Wence, D.L.K.; Yallits, E.N.; Conway, J.L.; Patrick, B.O.; Smith, K.M. Cyclopentadienyl Mesityl Complexes of Chromium(II) and Chromium(III). Dalton Trans. 2011, 40, 337–339.

In 2013, Bullock et al. reported the synthesis of the 17-electron radical complex CpCr(CO)2(IMe)% (Cr025) via the reaction of IMe with [CpCr(CO)3]2 (Scheme 23).59 Complex Cr025 adopts a piano stool geometry in which the three legs are aligned irregularly around the chromium center and the CCO-Cr-CCO angle is significantly smaller than the Ccarbene-Cr-CCO angles. The CrdCcarbene bond length was found to be 205.3(6) pm. Subsequent reduction of Cr025 employing KC8 yielded the 18-electron Cr (II) complex K[CpCr(CO)2(IMe)] (Cr026). Addition of 18-crown-6 to Cr026 yielded K(18-crown-6)[CpCr(CO)2(IMe)] (Cr027), which was structurally characterized by single-crystal X-ray analysis. Protonation of K(18-crown-6)[CpCr(CO)2(IMe)] was proposed to the hydride complex Cr028 by employing [H-DBU]BF4. However, the complex was not isolated.

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Scheme 23 Synthesis of the chromium cyclopentadienyl carbonyl NHC complexes Cr025–Cr028. Modified from van der Eide, E.F.; Helm, M.L.; Walter, E.D.; Bullock, R.M.; Structural and Spectroscopic Characterization of 17- and 18-Electron Piano-Stool Complexes of Chromium. Thermochemical Analyses of Weak Cr–H Bonds. Inorg. Chem. 2013, 52, 1591–1603.

In recent years, Conradie and Landman et al. synthesized the air-stable heterohalo Cr (III) NHC complexes Cr029–Cr033 by the reaction of chromocene with the corresponding imidazolium salts (Scheme 24).61 Halide exchange took place slowly at room temperature in the presence of chlorinated solvents (CHCl3/CCl4) to yield the corresponding dichloride complex Cr034. Complexes Cr029–Cr033 possess a monometallic, three-legged piano stool structure with the centroid of the cyclopentadienyl ring, the chromium center, as well as the carbenic carbon atom all located in the same plane.

Scheme 24 Synthesis of the chromium cyclopentadienyl NHC halo complexes Cr029–Cr034. Modified from Malan, F.P.; Singleton, E.; van Rooyen, P.H.; Conradie, J.; Landman, M.; Base-Free Glucose Dehydration Catalysed by NHC-Stabilised Heterohalo Cyclopentadienyl Cr(III) Complexes. New J. Chem 2018, 42, 19193–19204.

6.02.5.2.2

Chromium indenyl NHC complexes

In 2012, Danopoulos and Hanton et al. reported the synthesis of chromium indenyl NHC compounds Cr035–Cr045 (Scheme 25).62 Complexes Cr035–Cr036, Cr037–Cr038 and Cr044 were synthesized by reacting the potassium salt of the corresponding indenyl-functionalized NHC ligand with CrCl3(THF)3, CrMeCl2(THF)3 and CrCl2, respectively. Complexes Cr035–Cr036 were found to be monometallic and to adopt distorted tetrahedral geometries with the indenyl centroid occupying one coordination site and a CrdCcarbene bond length of 207.5(4) pm. The CrdCindenyl bond lengths are comparable to those in chromium cyclopentadienyl complexes and in support of a Z5-indenyl coordination. Reactions of Cr035 with dibenzylmagnesium and diphenylmagnesium resulted in complexes Cr039 and Cr041, which are isostructural with Cr035 with a CrdCcarbene bond length of 211.7(5) pm for Cr039 and 209.9(5) pm for Cr041. Complexes Cr035 and Cr036 react with Na[B(ArF)4] resulting in the ionic complexes Cr042 and Cr043, respectively. Complex Cr043 also adopts a distorted tetrahedral geometry with a CrdCcarbene bond length of 206.2(10) pm. The thermally unstable complex Cr040 forms upon reaction of Cr039 with [H(Et2O)+2][B(ArF)−4] at −78  C and also shows a distorted tetrahedral geometry with a CrdCcarbene bond length of 213.0(5) pm. A binuclear complex Cr045 was crystallized by slow vapor diffusion of petroleum ether into a concentrated THF solution of the Cr (II) complex Cr044 at room temperature. Cr044 adopts a distorted three-coordinate geometry with a CrdCcarbene bond length of 208.5(4) pm. Complex Cr045 was reported to be a binuclear centrosymmetric dimer in which each Cr center adopts a distorted tetrahedral geometry with a CrdCcarbene bond length of 210.8(8) pm.

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Scheme 25 Synthesis of the chromium indenyl NHC complexes Cr035–Cr045. Modified from Conde-Guadano, S.; Danopoulos, A.A.; Pattacini, R.; Hanton, M.; Tooze, R.P. Indenyl Functionalized N-Heterocyclic Carbene Complexes of Chromium: Syntheses, Structures, and Reactivity Studies Relevant to Ethylene Oligomerization and Polymerization. Organometallics 2012, 31, 1643–1652.

6.02.5.2.3

Chromium fluorenyl NHC complexes

Danopoulos et al. synthesized the chromium (II) NHC complex Cr046 (Scheme 26) via the aminolysis of Cr[N(SiMe3)2]2(THF)2 with fluorenyl-substituted imidazolium salts. In the resulting complex Cr046 the ligand adopts a monodentate binding mode with a dangling fluorenyl group.23 The geometry around the metal center is square planar and the CrdCcarbene bond length was within the expected range (207.5–215.5 pm).

Scheme 26 Synthesis of the chromium fluorenyl NHC complex Cr046. Modified from Downing, S.P.; Guadaño, S.C.; Pugh, D.; Danopoulos, A.A.; Bellabarba, R.M.; Hanton, M.; Smith, D.; Tooze, R.P. Indenyl- and Fluorenyl-Functionalized N-Heterocyclic Carbene Complexes of Titanium, Zirconium, Vanadium, Chromium, and Yttrium. Organometallics 2007, 26, 3762–3770.

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6.02.5.2.4

Chromium Di-NHC complexes

In 2012, Satsch et al. synthesized two N-heterocyclic carbene adducts of CrCl2 (Cr047 and Cr048).50 Both, Cr047 and Cr048, adopt a distorted square planar geometry with the NHCs trans to each other. The CrdCcarbene bond distances are 214.7 and 217.6 pm, respectively. At the same time, Yang et al. demonstrated the synthesis of NHC-stabilized chromium (II) alkyl, aryl and alkynyl complexes.51 Metathesis reaction of the complex Cr048 with 2 equiv. of CH3Li, PhLi and PhC^CLi yielded the air-sensitive complexes Cr049–Cr051 (Scheme 27). The structural analysis of complexes Cr049–Cr051 revealed a near square planar arrangement of the four coordinated chromium with the two carbene ligands and the two organo-groups in the trans orientation. The average CrdCcarbene bond lengths were 215.2(8) pm in Cr049, 216.8(5) pm in Cr050, and 215.2(2) pm in Cr051. Furthermore, reaction of Cr050 with an organic azide, i.e., 1-azidoadamantane, yielded the organonitridochromium (V) compound (IPM)2Ph2Cr^N (Cr052) (IPM ¼ 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene). The structural analysis of Cr052 revealed that the chromium atom was five-coordinated and adopted a square pyramidal arrangement with the nitrogen occupying the apex. The CrdCcarbene bond lengths were 211.1(2) and 211.0(2) pm.

Scheme 27 Synthesis of the dinuclear chromium NHC complexes Cr047–Cr052. Modified from Jones, C.; Dange, D.; Stasch, A. Synthesis and Crystal Structures of bulky Guanidinato Zirconium (IV) and Hafnium (IV) Chloride Complexes. J. Chem. Crystallogr. 2012, 42, 866–870 and Wang, J.; Tan, G.; An, D.; Zhu, H.; Yang, Y.; Organochromium(II) and Organonitridochromium(V) N-Heterocyclic Carbene Complexes: Synthesis and Structural Characterization. Z. Anorg. Allg. Chem. 2011, 637, 1597–1601.

Recently, Danopoulos et al. reported the open-shell Cr(II) benzyl NHC complexes Cr053 and Cr054 (Scheme 28).52 Reaction of Cr(benzyl)3(THF)3 with N,N0 -diisopropylimidazol-2-ylidene resulted in the formation of complex Cr053, which adopts a distorted square-planar geometry with an unusual cis-arrangement of the benzyl ligands. Complex Cr054 was synthesized by the reaction of N,N0 -bis(2,6-diisopropylphenyl)imidazol-2-ylidene with CrCl2(THF)2/Mg(benzyl)2. It exhibits a distorted trigonal planar configuration with the chromium center coordinated by one NHC and two benzyl groups.

Scheme 28 Synthesis of the dinuclear chromium (II) alkyl NHC complexes Cr053 and Cr054. Modified from Danopoulos, A.A.; Monakhov, K.Y.; Robert, V.; Braunstein, P.; Pattacini, R.; Conde-Guadaño, S.; Hanton, M.; Tooze, R.P. Angular Distortions at Benzylic Carbons Due to Intramolecular Polarization-Induced Metal–Arene Interactions: A Case Study With Open-Shell Chromium(II) NHC Complexes. Organometallics 2013, 32, 1842–1850.

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Theopold et al. reported on the synthesis of the chromium (III) NHC complex Cr055 by reacting 1,10 -bis(2,6-diisopropylphenyl)-3,30 -methylenediimidazolin-2,20 -diylidene with CrCl3(THF)3 in THF (Scheme 29).53 Cr055 exhibits an octahedral geometry with CrdCcarbene bond lengths of 213.4(5) and 210.9(5) pm, respectively. Reaction of Cr055 with Na[B(ArF)4] resulted in chloride abstraction and yielded the dinuclear chloro-bridged complex Cr060 with a CrdCr distance of 311.16(12) pm. One dicarbene ligand, one terminal chloride and three bridging chlorides occupy the coordination sphere around the metal atom. Another dinuclear chloride-bridged complex (Cr061) was obtained as a result of alkylation of Cr055 with AlMe3 and turned out to be isostructural with Cr060. Interestingly, in case the alkylation of Cr055 was performed with LiMe, it yielded the bimetallic chromium (II) complex Cr062 with a shorter CrdCr bond distance (189.7 pm) compared to Cr060 and Cr061. The two chromium centers are different with one bridging dicarbene ligand, one bridging methyl and three terminal methyl ligands. The methylene bridge of the dicarbene ligand is deprotonated and is coordinated to one of the chromium centers, which is balanced by the accompanying THF solvated Li+ ion.

Scheme 29 Synthesis of the chromium di-NHC complexes Cr055–Cr063. Modified from Kreisel, K.A.; Yap, G.P.A.; Theopold, K.H.A. Chelating N-Heterocyclic Carbene Ligand in Organochromium Chemistry. Organometallics 2006, 25, 4670–4679 and Kreisel, K.A.; Yap, G.P.A.; Theopold, K.H. A Bimetallic N-Heterocyclic Carbene Complex Featuring a Short Cr–Cr Distance. Chem. Commun. 2007, 1510–1511.

Complexes Cr056–Cr058 were synthesized employing CrCl2, CrPh3(THF)3 and CrRCl2(THF)3, respectively; the corresponding chromium precursors are depicted in Scheme 29. Cr057 exhibits a square pyramidal geometry with the phenyl at the apex. Addition of methylenebis(imidazolium bromide) to a THF solution of Cr[N(SiMe3)2]2 yielded complex Cr059. Its structure is square pyramidal with CrdCcarbene lengths of 216.2(7) and 217.3(6) pm, respectively. Methylation of Cr056 with MeMgCl yielded Cr063, a homobimetallic complex with a bridging di-NHC ligand across two chromium (II) atoms with a short CrdCr distance.54 In 2012, Yamaguchi and Ito et al. synthesized the chromium complex [Cr(CO)4(bis-NHC)] (Cr064) by treatment of the BEt3-adducts of a di-NHC with Cr(CO)4(Z4-norbornadiene) (Scheme 30).63

Scheme 30 Synthesis of the chromium di-NHC complex Cr064. Modified from Ogata, K.; Yamaguchi, Y.; Kurihara, Y.; Ueda, K.; Nagao, H.; Ito, T. Twisted Coordination Mode of Bis(N-Heterocyclic Carbene) Ligands in Octahedral Geometry of Group 6 Transition Metal Complexes: Synthesis, Structure, and Reactivity. Inorg. Chim. Acta 2012, 390, 199–209.

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6.02.5.2.5

Chromium tetra-NHC complexes

The first examples of a tetracarbene Cr-complex Cr065 was reported by Jenkins et al. in 2012 (Scheme 31). The preparation involved transmetalation of a dinuclear silver complex with a macrocyclic tetra-NHC.57 Complex Cr065 exhibits an octahedral geometry at the metal center; the CrdCcarbene bond lengths of 209 and 214 pm, respectively, were found to be in good agreement with those in other chromium dicarbene complexes.

Scheme 31 Synthesis of the chromium tetra-NHC complex Cr065. Modified from Lu, Z.; Cramer, S.A.; Jenkins, D.M. Exploiting a Dimeric Silver Transmetallating Reagent to Synthesize Macrocyclic Tetracarbene Complexes. Chem. Sci. 2012, 3, 3081–3087.

Recently, Jenkins et al. reported six new chromium complexes Cr067–Cr072 (Scheme 32)56 synthesized from an electronically highly unsaturated, square planar chromium (II) complex (Cr066) supported by a macrocyclic tetracarbene ligand. Reaction of Cr066 with Me3NO resulted in the highly stable chromium (IV)-oxo complex Cr067, which adopts a square pyramidal geometry with the chromium atom raised 52.5 pm above the plane of the carbene carbons. The CrdCcarbene bond distances range from 202.6 to 209.4 pm. Reaction of Cr066 with less bulky organic azides yielded the respective metallotetrazenes (Cr068 for n-octylazides and Cr069 for p-tolylazides). A [2 + 3] cycloaddition of an additional equivalent of organic azide with a transient metal imido was proposed as the reason for the formation of tetrazenes. The CrdC bond distances ranged from 205.6 to 212.0 pm for Cr069 and from 207.0 to 215.9 pm for Cr068. In contrast, reaction of Cr066 with a relatively bulky adamantyl azide and mesityl azide resulted in the formation of the chromium (IV) imido complexes Cr071 and Cr072, respectively. Cr072 adopts a bent square pyramidal geometry with the central chromium atom sitting 54.2 pm out of the tetracarbene plane. The CrdCcarbene bond distances ranges from 206.7 to 210.2 pm. The first linearly bridging nitrido chromium species (Cr070) was obtained by reacting Cr066 with trimethylsilyl azide. The molecular structure of Cr070 comprises an asymmetric bridging nitrido ligand (Cr1–N1 ¼ 166.7 pm versus Cr2–N1 ¼ 189.9 pm).

Scheme 32 Synthesis of the chromium tetra-NHC complexes Cr066–Cr072. Modified from Elpitiya, G.R.; Malbrecht, B.J.; Jenkins, D.M. A Chromium(II) Tetracarbene Complex Allows Unprecedented Oxidative Group Transfer. Inorg. Chem. 2017, 56, 14101–14110.

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In 2019, Jenkins et al. reported a series of chromium (II)—chromium (V) complexes bearing a 16-atom membered dianionic tetra-NHC macrocycle (Cr073–Cr079, Scheme 33).55 Cr073 was synthesized by the direct deprotonation of the imidazolium salt with nBuLi followed by the addition of CrCl2. Its structure turned out to be dimeric with a CrdCr quadruple bond. The CrdCr bond distance of 193.3(4) pm is among the shortest ever reported. When the same ligand is subjected to deprotonation followed by the addition of CrCl3(THF)3, Cr074 was formed, which contained one molecule of acetonitrile bound trans to the bromide. Complex Cr074 possessed an octahedral coordination and an average CrdCcarbene bond distance of 205.2(2) pm. Reaction of Cr074 with TlPF6 yielded Cr075 with two acetonitrile molecules coordinated and PF−6 as the counter anion. Cr073 was reported to undergo oxidative cleavage of the CrdCr bond resulting in the formation of a highly stable diamagnetic chromium (IV) oxo complex (Cr079). Cr079 can be oxidized to form the cationic chromium (V) oxo complex Cr076. Reactions of Cr073 with organic azides such as 2,6-diisopropylphenyl azide and tert-butyl azide results in the formation of the corresponding paramagnetic chromium (IV) imide complexes Cr077 and Cr078.

Scheme 33 Synthesis of the chromium tetra-NHC complexes Cr073–Cr079. Modified from Anneser, M.R.; Powers, X.B.; Peck, K.M.; Jensen, I.M.; Jenkins, D.M. One Macrocyclic Ligand, Four Oxidation States: A 16-Atom Ringed Dianionic Tetra-NHC Macrocycle and Its Cr(II) Through Cr(V) Complexes. Organometallics 2019, 38, 3369–3376.

6.02.5.2.6

Chromium NHC complexes bearing pincer ligands

In 2006, Danopoulos et al. reported the synthesis of pincer dicarbene complexes (Cr080–Cr081, Scheme 34).25 Cr080 was synthesized by the reaction of CrCl2(THF)2 with 2,6-bis(imidazolylidene)pyridine. Similarly, Cr081 was formed from the aminolysis of bisimidazolium salt (CH-N-CH)Br2 with Cr[N(SiMe3)2]2(THF)2. Cr081 exhibited a distorted square pyramidal geometry with bromide at the apex. The CrdCcarbene bond lengths are 212.2(10) and 212.5(10) pm.

Scheme 34 Synthesis of the chromium pincer dicarbene complexes Cr80 and Cr81. Modified from Pugh, D.; Wright, J.A.; Freeman, S.; Danopoulos, A.A. ‘Pincer’ Dicarbene Complexes of Some Early Transition Metals and Uranium. Dalton Trans. 2006, 775–782.

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In 2008, McGuinness and Davies et al. reported the synthesis of a series of Cr-NHC complexes containing bidentate carbene-pyridine and carbene-thiophene ligands Cr082 and Cr086 (Scheme 35).64

Scheme 35 Synthesis of the pyridine- and thiophene-based chromium-NHC complexes Cr082–Cr086. Modified from McGuinness, D.S.; Suttil, J.A.; Gardiner, M.G.; Davies, N.W. Ethylene Oligomerization With Cr− NHC Catalysts: Further Insights Into the Extended Metallacycle Mechanism of Chain Growth. Organometallics 2008, 27, 4238–4247.

Treatment of 1,6-bis(3-methylimidazol-2-yliden-1-yl)pyridine with CrCl3(THF)3 at −78  C yielded complex Cr082. Cr082 displayed a distorted octahedral geometry with a meridional arrangement of tridentate ligand. The CrdCcarbene bond distance was 209.8(3) pm. The N-pyridyl-substituted NHC complex Cr083 was synthesized by reacting CrCl3(THF)3 with 1-(2,6-diisopropylphenyl)-3-pyrid-1-ylimiazol-2-ylidene in THF. Attempts to recrystallize Cr083 invariably resulted in the formation of Cr084. Similarly, the thiophene-based pincer chromium complexes Cr085 and Cr086 were synthesized from 2,5-bis(1isopropylimidazol-2-yliden-1-yl)thiophene and 1-isopropyl-3-thienylimidazol-2-ylidene, respectively.

6.02.5.2.7

Chromium NHC complexes bearing amidine ligands

In 2012, Danopoulos et al. reported the synthesis of chromium NHC complexes functionalized with the neutral amidine and anionic amidinato ligands.65 Ligands were bidentate through the NHC and amidine nitrogen donation. 3-(2,6-Diisopropylphenyl)-1-[2-N, N0 bis(2,6-diisopropylphenylamidinato)ethyl]imidazol-2-ylidene potassium was reacted with CrMeCl2(THF)3, CrCl2(THF)2 and CrCl3(THF)3 to yield the respective Cr-NHC complexes Cr087, Cr088 and Cr089 (Scheme 36). Cr087 adopts a distorted square-pyramidal geometry with the methyl group occupying the apex. The CrdCcarbene bond distance in Cr087 was 210.7(3) pm. Cr088 turned out to be a centrosymmetric dimer with each metal center adopting a distorted square planar geometry and bridging chlorides. The CrdCcarbene bond distance in Cr088 was 216.3(2) pm. Cr089 was binuclear, too, comprising two different metal centers of which one metal center is sixfold coordinated in a distorted octahedral geometry while the other is fivefold coordinated with a distorted trigonal bipyramidal geometry. The CrdCcarbene bond distances were 209.3(4) and 212.0(4) pm, respectively.

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Scheme 36 Synthesis of the amidine-bearing chromium NHC complexes Cr087–Cr089. Modified from Conde-Guadano, S.; Hanton, M.; Tooze, R.P.; Danopoulos, A.A.; Braunstein, P.; Amidine- and Amidinate-Functionalised N-Heterocyclic Carbene Complexes of Silver and Chromium. Dalton Trans. 2012, 41, 12558–12567.

6.02.5.2.8

Chromium NHC complexes bearing chiral ligands

In 2014, Nakada et al. reported on the synthesis of the chiral, C2-symmetrcial tridentate NHC-coordinated chromium (III) complex Cr090 (Scheme 37).66 At that time, this was the first chromium complex bearing a chiral NHC ligand. Cr090 adopts a distorted octahedral geometry with both oxazoline rings not located in the plane of the benzimidazol-2-ylidene. The NHC is ligated in a tridentate fashion to the chromium center, which is also bound to two chlorines and one bromine atoms. However, the complex Cr090 is not C2-symmetric due to bromine atom, which is located above the plane of NHC ligand.

Scheme 37 Synthesis of the oxazoline-based chromium NHC complex Cr090. Modified from Uetake, Y.; Niwa, T.; Nakada, M.; Synthesis and Characterization of a New C2-Symmetrical Chiral Tridentate N-Heterocyclic Carbene Ligand Coordinated Cr(III) Complex. Tetrahedron: Asymmetry 2015, 26, 158–162.

6.02.5.2.9

Chromium NHC complexes bearing N-phosphanyl substituents

In 2015, Danopoulos and Braunstein et al. reported on the synthesis of chromium complexes Cr091–Cr096 bearing N-phosphanyl-substituted NHC ligands (Scheme 38).67 (1-(Di-tert-butylphosphino)-3-tert-butylimidazol-2-ylidene), (1-(di-tertbutylphosphino)-3-mesitylimidazol-2-ylidene), (1-(di-tert-butylphosphino)-3-(2,6-diisopropylphenyl)imidazol-2-ylidene), and (1,3-bis(di-tert-butylphosphino)imidazol-2-ylidene) were reacted with [CrCl2(THF)2] or [CrCl3(THF)3] or [Cr(Me)Cl2(THF)3] to yield Cr091–Cr094, respectively. All complexes adopt a perfect square-planar geometry with the two NHCs and the two chlorides trans to each other. The CrdCcarbene bond distances were in the range of 215.2(6)–220.5(2) pm. Alkylation of complexes Cr093 and Cr094 with Mg(benzyl)2(THF)2 led to the formation of the [Cr(benzyl)3NHC] complexes Cr095 and Cr096, respectively. Both complexes Cr095 and Cr096 were found to be five-coordinated and slightly distorted square pyramidal.

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Scheme 38 Synthesis of the chromium NHC complexes Cr091–Cr096 bearing N-phosphanyl substituents. Modified from Ai, P.; Danopoulos, A.A.; Braunstein, P. N-Phosphanyl- and N,N0 -Diphosphanyl-Substituted N-Heterocyclic Carbene Chromium Complexes: Synthesis, Structures, and Catalytic Ethylene Oligomerization. Organometallics 2015, 34, 4109–4116.

Danopoulos and Braunstein et al. also synthesized the tridentate Cr-NHC complex Cr097 (Scheme 39) based on a dearomatized pyridine backbone.68 The metal center in complex Cr097 adopts a distorted square planar geometry and the anionic planar pincer ligand occupies three coordination sites.

Scheme 39 Synthesis of the Cr-NHC complex Cr097 bearing a dearomatized pincer. Modified from Simler, T.; Danopoulos, A. A.; Braunstein, P. N-Heterocyclic Carbene–Phosphino-Picolines as Precursors of anionic ‘Pincer’ Ligands With Dearomatised Pyridine Backbones; Transmetallation From Potassium to Chromium. Chem. Commun. 2015, 51, 10699–10702.

6.02.5.2.10

Chromium complexes bearing imino-functionalized NHCs

In 2011, Lavoie et al. reported on the synthesis of chromium NHC complexes Cr098 and Cr099 containing an aryl-substituted acyclic imino NHC ligand (Scheme 40)69 via reaction of CrCl3(THF)3 and CrCl2(THF)2 with 1-(1-(2,6-dimethylphenylimino)-2,2-dimethylpropyl)-3-(2,4,6-trimethylphenyl)-imidazol-2-ylidene. Cr098 was reported to possess a highly distorted octahedral geometry with a CrdCcarbene bond distance of 204.1(4) pm. In continuation, the same authors also explored the synthesis of chromium (III) bis(imino) NHC complexes (Scheme 41).70,71 The copper bis(imino)-pyrimidin-2-ylidene adduct was reacted with CrCl3(THF)3 to yield Cr100 (Scheme 40), which adopts a distorted octahedral geometry and the bis(imino) carbene ligand coordinating to the metal in a tridentate fashion. The CrdCcarbene bond distance is 198.9(3) pm. Complexes Cr101–Cr103 were synthesized via transmetalation of the silver or copper NHC adducts with CrCl3(THF)3. Cr102 adopts a distorted octahedral geometry with the ligand chelated to the metal to form a five-membered metallacycle. The CrdCcarbene bond distance was found to be 204.8(3) pm.

Scheme 40 Synthesis of the chromium complexes Cr098 and Cr099 containing acyclic imino ligands. Modified from Larocque, T.G.; Badaj, A.C.; Dastgir, S.; Lavoie, G.G. New Stable Aryl-Substituted Acyclic Imino-N-Heterocyclic Carbene: Synthesis, Characterisation and Coordination to Early Transition Metals Dalton Trans. 2011, 40, 12705–12712.

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Scheme 41 Synthesis of the chromium bisimino NHC complexes Cr100–Cr103. Modified from Thagfi, J.A.; Lavoie, G.G. Preparation and Reactivity Study of Chromium(III), Iron(II), and Cobalt(II) Complexes of 1,3-Bis(imino)benzimidazol-2-ylidene and 1,3-Bis(imino)pyrimidin-2-ylidene. Organometallics 2012, 31, 7351–7358 and Al Thagfi, J.; Lavoie, G.G. Synthesis, Characterization, and Ethylene Polymerization Studies of Chromium, Iron, and Cobalt Complexes Containing 1,3-Bis(imino)-N-Heterocyclic Carbene Ligands. Organometallics 2012, 31, 2463–2469.

In 2019, Braunstein et al. synthesized the chromium NHC complexes Cr104 and Cr105 comprising N-amine and N-imine substituents (Scheme 42).72 Transmetalation of the NHC-silver adducts with either CrCl3(THF)3 or CrCl2(THF)2 yielded Cr104 and Cr105, respectively. Both complexes exhibit slightly distorted octahedral geometries and are sixfold coordinated.

Scheme 42 Chromium NHC complexes Cr104 and Cr105 with mixed amine-imine ligands. Modified from Ren, X.; Wesolek, M.; Braunstein, P. Cu(I), Ag(I), Ni(II), Cr(III) and Ir(I) Complexes With Tritopic NimineCnhcNamine Pincer Ligands and Catalytic Ethylene Oligomerization. Dalton Trans. 2019, 48, 12895–12909.

6.02.5.3

Chromium (VI) NHC complexes

Recently, Buchmeiser et al. synthesized chromium (VI) bisimido dichloro and chromium (V) bisimido iodo NHC complexes. Treatment of the chromium bisimido dichloro complexes with the free carbenes yields monodentate and bidentate chromium (VI) NHC complexes Cr106–Cr115 (Scheme 43).60

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Scheme 43 Synthesis of chromium (VI) NHC complexes, Cr106–Cr115. Modified from Panyam, P.K.R.; Stöhr, L.; Wang, D.; Frey, W.; Buchmeiser, M.R. Chromium (VI) Bisimido Dichloro, Bisimido Alkylidene and Chromium (V) Bisimido Iodo N-Heterocyclic Carbene Complexes. Eur. J. Inorg. Chem. 2020, 3673–3681.

6.02.5.4

Chromium (0) mesoionic tetrazolylidene complexes

Kühn et al. reported on the synthesis of mesoionic 1,3-dimethyltetrazol-5-ylidene NHC complexes of chromium in 2013 (Scheme 44).73 The precursor tetrafluoroborate salt was treated with Na2[Cr2(CO)10] to yield the respective chromium pentacarbonyl complex, Cr116.73

Scheme 44 Synthesis of the mesoionic chromium tetrazol-5-ylidene complex Cr116. Modified from Schaper, L.-A.; Wei, X.; Altmann, P.J.; Öfele, K.; Pöthig, A.; Drees, M.; Mink, J.; Herdtweck, E.; Bechlars, B.; Herrmann, W.A.; Kühn, F.E. Synthesis and Comparison of Transition Metal Complexes of Abnormal and Normal Tetrazolylidenes: A Neglected Ligand Species. Inorg. Chem. 2013, 52, 7031–7044.

6.02.6

Molybdenum NHC complexes

Early work on molybdenum carbonyl mono- and di-NHC complexes was already reported by Öfele et al.74–77 and Lappert et al.78–80 in the 1970s. The first high oxidation state molybdenum complexes, however, have been synthesized by Herrmann et al. in 1996.81 In the following chapter, progress on molybdenum NHC complexes made during the last years is presented. Recently Buchmeiser et al. developed various molybdenum alkylidene and alkylidyne NHC complexes for olefin and alkyne metathesis82–86; however, these will be discussed in a separate chapter. For the preparation of molybdenum dioxo NHC complexes, either 1 or 2 equiv. of the free NHC such as 1,3-dimethyl4,5-dimethylimidazol-2-ylidene (IMeMe), 1,3-dimethyl-4,5-diisopropylimidazol-2-ylidene (IMeiPr) and 1,3-dibutyl-4,5-dichloroimidazol-2-ylidene (IBuCl) were reacted with MoO2Cl2(THF)2 to obtain the corresponding purple- or yellow-colored complexes Mo001–Mo00587 (Scheme 45).

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Scheme 45 Preparation of the molybdenum (VI) dioxo NHC complexes Mo001–Mo005. Modified from Mas-Marzá, E.; Reis, P.M.; Peris, E.; Royo, B. Dioxomolybdenum(VI) Complexes Containing N-Heterocyclic Carbenes. J. Organomet. Chem. 2006, 691, 2708–2712.

Mo001–Mo005 are moisture- and air-sensitive and the solids show thermal decomposition at room temperature over several hours. Attempts to crystallize the compounds by slow diffusions of diethyl ether in CH2Cl2 at low temperatures led to decomposition. The decomposition pathway involves the protonation of the NHC with water and yields [Mo2(m-O)(m-Cl)2Cl2O4] [HIMeiPr]2 for Mo002. As already shown in 2003,88 it is possible to synthesize CpMo(CO)2(IMes)H (Mo006) by the reaction of 1,3-dimesitylimidazol2-ylidene (IMes) with CpMo(CO)2(PPh3)H.89 The weakly coordinating phosphine ligand is slowly replaced by the NHC. The competing deprotonation of the metal hydride by the NHC leads to [CpMo(CO)2(PR3)][HIMes] and is substantially faster. (Scheme 46).

Scheme 46 Preparation of CpMo(CO)2(IMes)H (Mo006) and the competing protonation of the NHC. Modified from Yoshitaka, Y.; Ryoji, O.; Katsunori, S.; Kimiko, K.; Makoto, M.; Takashi, I. Synthesis and Crystal Structure of N-Heterocyclic Carbene Complexes of Bis(Z5-cyclopentadienyl)molybdenum. Bull. Chem. Soc. Jpn. 2003, 76, 991–997 and Wu, F.; Dioumaev, V.K.; Szalda, D.J.; Hanson, J.; Bullock, R.M. A Tungsten Complex With a Bidentate, Hemilabile N-Heterocyclic Carbene Ligand, Facile Displacement of the Weakly Bound W −(CC) Bond, and the Vulnerability of the NHC Ligand Toward Catalyst Deactivation During Ketone Hydrogenation. Organometallics 2007, 26, 5079–5090.

The rate of reaction to the ionic [CpMo(CO)2(PR3)][HIMes] byproduct was found to strongly depend on the solvent. For example, the reaction can be performed at 95  C in toluene to yield 86% of the target compound Mo006. In THF or acetonitrile, conversion is reduced to 60% and 5%, respectively. The proton transfer reaction is an unproductive equilibrium and Mo006 can still be generated. The other possibility of a productive cycle involves the oxidative addition of the CdH bond, which is already known for Ni, Pd and Pt complexes.90–92 Mo006 can be separated from PPh3 by crystallization. In case CpMo(CO)2(PMe3)H is used as starting material, the reaction can be carried out under dynamic vacuum to remove the volatile PMe3 from the reaction mixture to obtain Mo006. The carbonyl ligands in Mo006 are cis to each other and the ligands are arranged in a square pyramidal configuration around the molybdenum. The ModNHC bond length is 218.7(8) pm. The preparation of cyclopentadienyl-NHC molybdenum(II) ((Cpx-NHC)Mo(CO)2I) Mo007–Mo010 complexes were accomplished by the in situ deprotonation of the imidazolium salt with 2 equiv. of nBuLi in THF and the reaction with MoCl(3-C3H5) (CO)2(NCMe)2.93 (Scheme 47).

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Scheme 47 Preparation of cyclopentadienyl-NHC molybdenum (II) complexes Mo007–Mo010. Modified from Kandepi, V.V.K.M.; da Costa, A.P.; Peris, E.; Royo, B. Molybdenum(II) Complexes Containing Cyclopentadienyl-Functionalized N-Heterocyclic Carbenes: Synthesis, Structure, and Application in Olefin Epoxidation. Organometallics 2009, 28, 4544–4549.

Isolation of the free NHC was not necessary and Mo007–Mo010 were obtained in moderate yields. The complexes show good stability in the solid-state and can be easily handled under air. Although two diastereomers can be expected for Mo007–Mo010 due to the backbone and the chirality at the molybdenum center, only in the case of Mo010 both could be detected. In terms of the carbonyl ligands, compounds Mo007 and Mo010 were obtained as cis isomers, while Mo008 and Mo009 were reported to be a mixture of the corresponding cis and trans isomers. For Mo008 it was possible to convert the trans isomer in boiling CH2Cl2 to the cis isomer. The structure of Mo007 can be described as a distorted four-legged piano stool. The ModNHC bond length is 220.7(3) pm. Mo011–Mo015 were obtained by the reaction of different imidazolium bromide salts with Ag2O and a subsequent transmetalation reaction with CpMo(CO)3Br (Scheme 48).94

Scheme 48 Preparation of molybdenum (II) cyclopentadienyl NHC complexes. Modified from Li, S.; Kee, C. W.; Huang, K.-W.; Hor, T. S. A.; Zhao, J. Cyclopentadienyl Molybdenum(II/VI) N-Heterocyclic Carbene Complexes: Synthesis, Structure, and Reactivity Under Oxidative Conditions. Organometallics 2010, 29, 1924–1933.

Due to the use of the corresponding Ag ∙NHC adduct, reactions were performed under the exclusion of light. Products were obtained as air-stable violet crystals in moderate yields. Single crystals of the complexes Mo011–Mo015 were obtained by crystallization from a mixture of n-hexane and ethyl acetate. Single-crystal X-ray analyses revealed a typical four-legged piano structure for all compounds. Based on IR and NMR data it was assumed that the carbonyl ligands are positioned cis to each other. The ModNHC bond lengths were found to be in the range of 224.4(3)–222.1(3) pm. To increase the activity during the epoxidation of cyclooctene, Mo013 was further reacted with AgBF4 in acetonitrile to yield the ionic complex Mo01694 (Scheme 49).

Scheme 49 Preparation of an ionic cyclopentadienyl molybdenum (II) NHC complex. Modified from Li, S.; Kee, C. W.; Huang, K.-W.; Hor, T. S. A.; Zhao, J. Cyclopentadienyl Molybdenum(II/VI) N-Heterocyclic Carbene Complexes: Synthesis, Structure, and Reactivity Under Oxidative Conditions. Organometallics 2010, 29, 1924–1933.

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Similar to Mo011–M015, the carbonyl ligands in Mo016 and Mo017 are cis to each other. However, Mo016 decomposes in air within 1 day. Due to the coordinating acetonitrile, the crystal structures of Mo016 and Mo017 also exhibit a four-legged piano structure with a ModNHC bond length of 224.9 and 222.8 pm, respectively. By oxidation of Mo016 or Mo017 with tert-butylhydroperoxide in acetonitrile, Mo018 and Mo019 can finally be obtained in moderate yield in form of yellow crystals. In the epoxidation of olefins, Mo018 and Mo019 show a similar reactivity as Mo016. Mo020 was prepared analogously to Mo007–Mo010. The imidazolium iodide was deprotonated in situ with potassium tert-butoxide and then reacted with MoCl(Z3-C3H5)(CO)2(NCMe)2 to yield Mo020 in the form of red crystals95 (Scheme 50). Based on the IR data it was assumed that the carbonyl ligands are cis to each other.

Scheme 50 Preparation of the molybdenum bis-NHC complex Mo020. Modified from Krishna Mohan Kandepi, V.V.; Cardoso, J.M.S.; Royo, B. N-Heterocyclic Carbene-Based Molybdenum and Tungsten Complexes as Efficient Epoxidation Catalysts With H2O2 and Tert-Butyl Hydroperoxide. Catal. Lett. 2010, 136, 222–227.

In the one-pot synthesis of Mo021 and Mo022, the imidazolium iodide, Mo(CO)6, and potassium tert-butoxide were reacted in refluxing DME.96 The products were obtained as yellow solids in moderate yields (Scheme 51).

Scheme 51 Preparation of the C2-symmetric chiral bidentate molybdenum (0) bis-(NHC) complexes Mo021 and Mo022. Modified from Satoshi, A.; Yoshitaka, Y.; Masatoshi, A. Syntheses of Novel C2-Symmetric Chiral Bidentate Bis(N-heterocyclic Carbene) Ligands and Their Molybdenum Complexes. Chem. Lett. 2010, 39, 398–399.

In (R,R)-Mo021, (S,S)-Mo021 and (R,R)-Mo022 the ligands are arranged in an octahedral configuration around the molybdenum. For the synthesis of Mo024, the imidazolium bromide was deprotonated with LiBEt3H, which resulted in the formation of a BEt3-protected NHC. Afterward, lithiation of the indenyl residue was accomplished. MoCl(3-C3H5)(CO)2(NCMe)2 was then reacted with the lithiated indenyl-substituted NHC to obtain Mo023. Refluxing Mo023 in pyridine led to the coordination of the NHC to the metal center to yield Mo02497 (Scheme 52) while refluxing in toluene resulted only in low conversion. The authors concluded that the Lewis base adduct of BEt3 with pyridine played an essential role in the preparation of Mo024.

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Scheme 52 Preparation of the indenyl-functionalized NHC molybdenum (II) complexes Mo023–Mo025. Modified from Takaki, D.; Okayama, T.; Shuto, H.; Matsumoto, S.; Yamaguchi, Y.; Matsumoto, S. Indenyl-Functionalised Triethylborane Adduct of N-Heterocyclic Carbene: Stepwise Coordination of Indenyl and NHC Ligands Toward Molybdenum Fragment. Dalton Trans. 2011, 40, 1445–1447.

The ionic liquid BMIMCl was deprotonated with nBuLi in THF. The resulting 1-butyl-3-methyl-imidazol-2-ylidene (IBuMe) was purified by distillation.98 Longer storage was not recommended because IBuMe turned out to be sensitive to air and water. Mo026 was obtained in moderate yield by the reaction of IBuMe with molybdenum hexacarbonyl in THF (Scheme 53). After recrystallization from n-pentane, Mo026 was obtained in form of yellow crystals. The ModNHC bond length was found to be 228.8(9) pm.

Scheme 53 Preparation of a molybdenum (0) NHC complex from an anionic liquid-derived carbene. Modified from Cole, M.L.; Gyton, M.R.; Harper, J.B. Metal Complexes of an Ionic Liquid-Derived Carbene. Aust. J. Chem. 2011, 64, 1133–1140.

For the preparation of the molybdenum bis-NHC complexes Mo027–Mo030, the bis-imidazolium bromide or iodide had to be fist reacted with LiBEt3H to obtain the corresponding bis-BEt3-protected bis(NHCs). The bis-NHCs were then reacted in n-heptane with molybdenum hexacarbonyl to obtain Mo027–Mo03099 (Scheme 54). In case toluene was used instead of n-heptane, Mo(CO)3(Z6-toluene) was formed as a by-product. The ligands in Mo027 and Mo028 are arranged in a pseudo-octahedral configuration around the molybdenum and show C2-symmetric structures whereby the bis-NHC ligand is coordinated in a twisted conformation. The ModNHC bond lengths were 230.6(2) and 229.4(2) pm in Mo027 and 229.0(3) pm in Mo027. Mo027, Mo028, and Mo030 were further modified with P(OMe)3. Complexes Mo031–Mo033 were obtained via carbonyl exchange99 (Scheme 54). Mo032 and Mo033 exhibit an octahedral ligand sphere around the metal and a twisted configuration of the bis-NHC ligand. The ModNHC bond length are 231.2(6) and 230.4(6) pm for Mo032 and 233.1(5) and 232.1(5) pm for Mo033.

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Scheme 54 Preparation of molybdenum (0) bis-NHC complexes Mo027–Mo033. Modified from Li, S.; Wang, Z.; Hor, T. S. A.; Zhao, J. First Crystallographic Elucidation of a High-Valent Molybdenum oxo N-Heterocyclic Carbene Complex [CpMoVIO2(IBz)]2[Mo6O19]. Dalton Trans. 2012, 41, 1454–1456.

Complex Mo034 was prepared via transmetalation of the AgCl∙ 1-methyl-2-phenyl-3-tolyltriazol-5-ylidene complex with Mo(CO)3CpCl100 (Scheme 55). The reaction was performed under the exclusion of light. Mo034 was reported to be stable in air as well as in water. The ModNHC bond length in Mo034 is 222.1(4) pm and thus same as for Mo014 based on IBz.

Scheme 55 Preparation of the molybdenum (II) triazol-5-ylidene complex Mo034. Modified from Schaper, L.-A.; Graser, L.; Wei, X.; Zhong, R.; Öfele, K.; Pöthig, A.; Cokoja, M.; Bechlars, B.; Herrmann, W.A.; Kühn, F.E.; Exploring the Scope of a Novel Ligand Class: Synthesis and Catalytic Examination of Metal Complexes With ‘Normal’ 1,2,3-Triazolylidene Ligands. Inorg. Chem. 2013, 52, 6142–6152.

For the synthesis of Mo035–Mo037, the imidazolium chloride salts Benz-CC, Benz-PC and DMBenz-PCP were deprotonated with KOtBu and reacted with cis-[Mo(CO)4(pip)2]101 (Scheme 56). Hereby only DMBenz-PCP was able to displace a carbonyl ligand in addition to the piperidine ligands. The ligands in Mo035 are arranged in a distorted octahedral configuration around the molybdenum. The ModNHC bond lengths in Mo035 and Mo036 are 224.9(2) and 225.7(2) pm. The ligands in Mo037 are arranged in a distorted octahedral configuration around the molybdenum. The ModNHC bond length is 223.9(3) pm.

Scheme 56 Preparation of molybdenum (0) carbonyl complexes Mo035–Mo037 supported by mixed benzimidazol-2-ylidene/phosphine ligands. Modified from Gradert, C.; Krahmer, J.; Sönnichsen, F.D.; Näther, C.; Tuczek, F.; Molybdenum(0)–Carbonyl Complexes Supported by Mixed Benzimidazol-2-ylidene/Phosphine Ligands: Influence of Benzannulation on the Donor Properties of the NHC Groups. J. Organomet. Chem. 2014, 770, 61–68.

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Fig. 1 Molybdenum (0) carbonyl complexes containing mono-, bi- and tridentate NHCs Mo038–Mo043. Modified from Wang, Z.; Li, S.; Teo, W.J.; Poh, Y.T.; Zhao, J.; Hor, T.S.A. Molybdenum (0) and Tungsten (0) Carbonyl N-Heterocyclic Carbene Complexes as Catalyst for Olefin Epoxidation. J. Organomet. Chem. 2015, 775, 188–194.

The general procedure for the synthesis of Mo038 and Mo042–Mo043 comprised the in situ deprotonation of the imidazolium salt in THF and subsequent reaction with Mo(CO)3(CH3CN)3102 (Fig. 1). A Mo(CO)3(NHC)3 complex could not be synthesized via this approach. Two sets of carbonyl signals were identified for Mo042 and Mo043. It was therefore assumed that the NHC ligands are cis to each other. For the synthesis of Mo039 – Mo041, first the corresponding Ag ∙NHC complexes were prepared and used for the transmetalation with Mo(CO)3(CH3CN)3 to yield the desired complexes. The complexes are both air and moisture stable. The ModNHC bond length in Mo041 is 223.9(6) pm. Reaction of 1,3-bis(2-(diphenylphosphanyl)ethyl)imidazol-2-ylidene (PCP) with trans-[Mo(N2)2(PPh2Me)4] in THF yielded Mo044.103 Mo044 displayed the lowest N-N stretching vibration in a molybdenum mono(dinitrogen) complex. It was therefore assumed that the N2 moiety is trans to the NHC. By reacting PCP with trans-[Mo(N2)2(dmpm)(PPh2Me)2] (dmpm ¼ bis(dimethylphosphino)methane), complex Mo045 was obtained.103 However, Mo045 was thermally unstable and therefore lacked full characterization. Complexes Mo046–Mo048 were prepared by the reaction of the free NHC PCP with MoX3(THF)3 (X ¼ Cl, Br, I); Mo049 was accessible by a sodium amalgam reduction of Mo046–Mo048 in the presence of 2 equiv. of trimethyl phosphite.103 As a side reaction, Mo050 was isolated and characterized. To receive Mo050 in the mentioned reaction, an additional demethylation/demethoxylation reaction took place. The ModNHC bond length in Mo050 is 226.1(4) pm (Scheme 57).

Scheme 57 Preparation of molybdenum (0) complexes Mo044–Mo048 supported by mixed NHC/phosphine ligands. Modified from Gradert, C.; Stucke, N.; Krahmer, J.; Näther, C.; Tuczek, F.; Molybdenum Complexes Supported by Mixed NHC/Phosphine Ligands: Activation of N2 and Reaction With P(OMe)3 to the First Meta-Phosphite Complex. Chem. Eur. J. 2015, 21, 1130–1137.

Treatment of N-[2-(pyrazol-1-yl)phenyl]-N0 -benzylimidazolium chloride (PBCl), N-[2-(3,5-dimethylpyrazol-1-yl)phenyl]N0 -benzylimidazolium chloride (DPBCl) or N-[2-(3,5-dimethylpyrazol-1-yl)phenyl]-N0 -2-pyridylmethylimidazolium chloride (DPPCl) with Ag2O followed by the addition of Mo(CO)5THF yielded Mo061, Mo062, and Mo063, respectively104 (Scheme 58). The ModNHC bond length in Mo061 and Mo062 are 224.9(2) and 254.4(3) pm, respectively. The free NHCs in PBCl and DPBCl act as a bidentate ligand and form a seven-membered ring. The ModNHC bond length in Mo063 is 224.2(3) pm. Surprisingly, N-[2-(3,5-dimethylpyrazol-1-yl)phenyl]-N0 -2-pyridylmethylimidazol-2-ylidene does acts as a bidentate (not tridentate) ligand, whereby the pyrazolyl nitrogen does not coordinate to the metal center.

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Scheme 58 Preparation of molybdenum (0) carbonyl NHC complexes based on 1-[2(pyrazol-1-yl)phenyl]imidazole. Modified from Cheng, C.-H.; Guo, R.-Y.; Cui, Q.; Song, H.-B.; Tang, L.-F. Synthesis and Catalytic Activity of N-Heterocyclic Carbene Metal Carbonyl Complexes based On 1-[2-(Pyrazol-1-yl)phenyl]imidazole. Transit. Met. Chem. 2015, 40, 297–304.

Mo064–Mo066 have been prepared by the reaction of the free NHCs IMeMe, 1,3-diethyl-4,5-dimethylimidazol-2-ylidene (IMeEt) and IMeiPr and KC8 with MoCl4(THF)2 under 1 atm of nitrogen105 (Scheme 59). The nitrogen atoms in complex Mo064 have a meridional arrangement. The two nitrogen ligands in Mo065–Mo066 were found to be trans to each other. The ligands in the complexes Mo064–Mo066 adopt an octahedral position around the metal atom. The NHCs are all located in the equatorial plane.

Scheme 59 Preparation of NHC-supported di- and tri-nitrogen molybdenum (0) complexes Mo064–Mo066. Modified from Ohki, Y.; Aoyagi, K.; Seino, H.; Synthesis and Protonation of N-Heterocyclic-Carbene-Supported Dinitrogen Complexes of Molybdenum(0). Organometallics 2015, 34, 3414–3420.

The addition of IPr to [Mo2(m-OTFA)4] resulted in the yellow complex Mo067.106 (Scheme 60) Mo067 was reported to be highly air sensitive. With [Mo2(m-OAc)4] and [Mo2(m-OPiv)4] it was impossible to isolate the corresponding IPr complex. The two IPr ligands in Mo067 are perpendicular to each other. In analogy to Mo067, complexes Mo068–Mo070 can be prepared by adding IMes to [Mo2(m-OTFA)4], [Mo2(m-OAc)4] or [Mo2(m-OPiv)4]106 (Scheme 60). Although an excess of NHC was used, only a 1:1 ratio of NHC and (Mo2) complex could be detected by NMR spectroscopy. Single-crystal X-ray analysis confirmed this finding. Complexes Mo068–Mo070 all have a lantern-like structure, in which the Mo atoms are surrounded by acetate ligands and the NHC is linearly coordinated to one of the Mo atoms (Mo-ModC angle 180 ). The extended p-interaction between a mesityl ring of the coordinating NHC and another (Mo2) unit leads to the formation of a pseudopolymer. Similar to Mo067–Mo070, complexes Mo071–Mo076 can be synthesized through the addition of IMeEt or IMeiPr to [Mo2(m-OTFA)4], [Mo2(m-OAc)4] or [Mo2(mOPiv)4]106 (Scheme 60). Elemental analysis for Mo072 and Mo073 revealed a complex with an NHC:Mo ratio of 1:1. Complexes Mo071, Mo074, and Mo075 were found to be isostructural. In contrast to Mo067–Mo070, they do not show mono- and axial coordination; instead, the NHC ligands are in equatorial position with a transodial configuration. The equatorial position causes two of the carboxylate ligands to be pushed out. Consequently, there are both bridging and monodentate ligands. In case Me3SiCl was added during the synthesis of Mo075, Mo077 was obtained.

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Scheme 60 Preparation of the molybdenum (II) NHC tetracarboxylate complexes Mo067–Mo077. Modified from Robinson, T.P.; Johnson, A.L.; Raithby, P.R.; Kociok-Kohn, G.; N-Heterocyclic Carbene Adducts of Molybdenum Tetracarboxylate Complexes. Organometallics 2016, 35, 2494–2506.

Mo078 was obtained by reacting CNC-Mes with [Mo(CO)6] or [Mo(CO)3(THF)3] in benzene107 (Scheme 61). 1H NMR data indicated a C2v symmetry in solution, whereas single-crystal X-ray analysis revealed C2 symmetry in the solid state. Mo078 adopts a significantly distorted octahedral geometry with the NHC in a meridional arrangement. In contrast to CNC-Mes, a,a0 -(diimidazol-2ylidenedodecamethylene)lutidine (C^N^C-12) had to be deprotonated in situ for further reaction with [Mo(CO)3(PhMe)] to Mo079107 (Scheme 61). The NHC in Mo079 was found to be coordinated in a facial instead of a meridional manner allowing the complex to adopt an almost perfect octahedral geometry.

Scheme 61 Preparation of molybdenum (0) carbonyl complexes containing NHC-based pincer ligands Mo078–Mo079. Modified from Apps, S.L.; Alflatt, R.E.; Leforestier, B.; Storey, C.M.; Chaplin, A.B.; Divergent Stereoisomers of Molybdenum Carbonyl Complexes of NHC-Based Pincer Ligands. Polyhedron 2018, 143, 57–61.

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Treatment of benzothiazol-2-yl-imidazolium salts with Ag2O followed by the transmetalation with Mo(CO)3(CH3CN)3 yielded Mo080, Mo081, and Mo082, respectively.108 Ligand scrambling was observed, which is why 1.3 equiv. of the precursor were used in the synthesis. Mo080 exists in two polymorphic configurations. The binding parameters of the two complexes are identical, but the orientation of the allyl group is different. The conformers have a small energy difference. Single crystals of Mo081 were obtained from an acetonitrile solution. Mo080 and Mo081 both have a distorted octahedral structure. Mo083–Mo085 can be obtained by the oxidative addition of Mo080–Mo082 with I2. Mo083–Mo085 are stable and can be used in air under ambient laboratory conditions. Mo083–Mo085 are rare examples of sevenfold coordinated molybdenum complexes (Scheme 62).

Scheme 62 Preparation of the molybdenum (II) diiodo complexes Mo083–Mo085 containing benzothiazol-based NHC ligands and their molybdenum (0) precursor Mo080–Mo082. Modified from Wang, Z.; Song, X.; Jiang, L.; Lin, T.T.; Schreyer, M.K.; Zhao, J.; Hor, T.S.A. Seven-Coordinate MoII − Diiodo Complexes With Benzothiazole−N-Heterocyclic-Carbene Ligands and Their Mo0 Precursors: Synthesis, Structures, and Catalytic Application in the Epoxidation of CisCyclooctene. Asian J. Org. Chem. 2018, 7, 395–403.

Addition of 6,60 (1H-1,2,3-triazol-1,4-diyl)bis(2,4-di-tert-butylphenol) (TBBP) to LDA and (DME)Mo(NtBu)2Cl2 in Et2O/THF resulted in the formation of Mo086 (Scheme 63).36 The ModNHC bond length in this compound is 202.4(1) pm.

Scheme 63 Preparation of the mesoionic carbene-containing molybdenum (VI) bis-imido bis-phenolate complex Mo086. Modified from Baltrun, M.; Watt, F.A.; Schoch, R.; Wölper, C.; Neuba, A.G.; Hohloch, S. A New Bis-Phenolate Mesoionic Carbene Ligand for Early Transition Metal Chemistry. Dalton Trans. 2019, 48, 14611–14625.

For the preparation of Mo087, H4(BMe2,MeTCH)Br2 was deprotonated in situ with nBuLi and further reacted with (Mo2(OAc)4)55 (Scheme 64).

Scheme 64 Preparation of the molybdenum (II) complex Mo087 containing a 16-membered imidazolium precursor. Modified from Anneser, M.R.; Powers, X.B.; Peck, K.M.; Jensen, I.M.; Jenkins, D.M.; One Macrocyclic Ligand, Four Oxidation States: A 16-Atom Ringed Dianionic Tetra-NHC Macrocycle and Its Cr(II) Through Cr(V) Complexes. Organometallics 2019, 38, 3369–3376.

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By adding KHMDS and ROTf to MoCl(3-C4H7)(CO)2(4,40 -tBu2bipy)(M-RIm)OTf, complexes Mo088–Mo091 were obtained.109 Single-crystal X-ray analysis confirmed the proposed structure of Mo088 and Mo089. In both complexes, the NHC is trans to the Z3-methallyl ligand. The bipyridyl and the two carbonyl ligands are in the meridional plane of the pseudooctahedron. The ModNHC bond lengths are 224.7(4) and 222.8(4) pm, respectively (Scheme 65).

Scheme 65 Deprotonation of the molybdenum (II) N-alkylimidazole complexes to the corresponding molybdenum (II) NHC complexes Mo088–Mo091. Modified from Espinal-Viguri, M.; Fombona, S.; Álvarez, D.; Díaz, J.; Menéndez, M.I.; López, R.; Pérez, J.; Riera, L.; Regiochemistry Control by Bipyridine Substituents in the Deprotonation of ReI and MoIIN-Alkylimidazole Complexes. Chem. Eur. J. 2019, 25, 9253–9265.

Complexes Mo092–Mo096 were synthesized via treatment of MoO2Cl2(DME), Mo(O)(NtBu)Cl2(DME) or Mo(NtBu)2Cl2(DME) with bis(2-hydroxy-3,5-di-tert-butylphenyl)benzimidazolium chloride deprotonated in situ with triethylamine or LDA110 (Scheme 66). Furthermore, it was possible to convert complex Mo094 into Mo095 by adding LiHMDS or LDA. During the thermal treatment of Mo095 in benzene, it was also possible to obtain Mo093. The absence of the benzimidazolium-2H proton in the 1H NMR clearly showed the formation of the free NHC. Based on 1H NMR data, a C2v symmetry was concluded for Mo092; final proof for the structure was obtained by the single crystal X-ray analysis. Mo092 crystallizes in two different molecules, a Mo(VI) dimer with two six-coordinated Mo atoms with a strongly distorted octahedral environment and a Mo(VI) monomer with a five-coordinated Mo. In solution, only one set of signals can be identified. The Mo in Mo093 turned out to be six-coordinated, with one of the ligands being a weakly coordinating THF. Only one of the three possible isomers of Mo094 was obtained, i.e., the one in which the NHC is trans to the imido ligand. The Mo in Mo095 is five-coordinated with a slightly distorted square pyramidal geometry, whereby the Mo in Mo096 is also five-coordinated but with a distorted trigonal bipyramidal geometry.

Scheme 66 Preparation of molybdenum(VI) dioxo-, oxo-, imido-, and bis-imido complexes containing a bis(hydroxyphenolate)-based NHC ligand. Modified from Baltrun, M.; Watt, F.A.; Schoch, R.; Hohloch, S. Dioxo-, Oxo-imido-, and Bis-Imido-Molybdenum(VI) Complexes With a Bis-phenolate-NHC Ligand. Organometallics 2019, 38, 3719–3729.

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Karthik et al. described the syntheses of Mo097 prepared in the reaction of 1,3-dimethyl-4,5-bis(phenylthio)imidazolium iodide with KOtBu and Mo(CO)6 in THF111 (Scheme 67). Mo097 has an octahedral structure and a ModNHC bond length of 227.3(4) pm.

Scheme 67 Preparation of the molybdenum pentacarbonyl bis(4,5-diphenylthio)-1,3-dimethylimidazol-2-ylidene complex Mo097. Modified from Karthik, V.; Bhat, I.A.; Anantharaman, G.; Backbone Thio-Functionalized Imidazol-2-ylidene–Metal Complexes: Synthesis, Structure, Electronic Properties, and Catalytic Activity. Organometallics 2013, 32, 7006–7013.

By exchanging the THF ligand in Mo(CO)5THF by the imidazolium salt of 1,3-dimesitylnaphthoquinoimidazol-2-ylidene (NqMes) and 1,3-diferrocenylbenzimidazolium chloride deprotonated in situ with NaHMDS, complexes Mo098 and Mo099 were obtained49 (Scheme 68). The ModNHC bond lengths in Mo098 and Mo099 are 232.8(2) and 225.7(5) pm, respectively. Both compounds were reported to be bench stable and could even be purified by column chromatography.

Scheme 68 Preparation of the molybdenum pentacarbonyl naphthoquinonoimidazol-2-ylidene and 1,3-di ferrocenylbenzimidazol-2-yliden complexes Mo089–Mo099. Modified from Rosen, E.L.; Varnado, C.D.; Tennyson, A.G.; Khramov, D.M.; Kamplain, J. W.; Sung, D.H.; Cresswell, P.T.; Lynch, V.M.; Bielawski, C.W. Redox-Active N-Heterocyclic Carbenes: Design, Synthesis, and Evaluation of Their Electronic Properties. Organometallics 2009, 28, 6695–6706.

Mo100–Mo102 were prepared via the reaction of Mo(CO)5(CHCN) with 1,3-dialkyl-4-diphenylphosphinoimidazolium hydrogensulfate and 1,3-dialkyl-4,5-diphenylphosphinoimidazolium hydrogensulfate, respectively, using KOtBu in THF48 (Scheme 69). Mo100 was reported to have a slightly distorted octahedral geometry.

Scheme 69 Synthesis of nono- and bis(diphenylphosphino)imidazol-2-ylidene complexes Mo100–Mo102. Modified from Majhi, P.K.; Sauerbrey, S.; Schnakenburg, G.; Arduengo, A.J.; Streubel, R. Synthesis of Backbone P-Functionalized Imidazol-2-ylidene Complexes: En Route to Novel Functional Ionic Liquids. Inorg. Chem. 2012, 51, 10408–10416.

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Mo103–Mo105 were prepared in analogy to Mo035–Mo037. The imidazolium salts were deprotonated in situ with KOtBu in THF and reacted with cis-[Mo(CO)4(pip)2]112 (Scheme 70). For Mo106, no deprotonation reagent was used to prevent dimerization of di(1-ethylimidazol-2-ylidene)methane (CC). Similar to Mo035 and Mo036, only the piperidine ligands of the precursor cis[Mo(CO)4(pip)2] in Mo103 and Mo106 were replaced by 3-(2-diphenylphosphanylethyl)-1-ethylimidazol-2-ylidene (CP) and CC In Mo104 and Mo105, one additional carbonyl ligand was replaced. Mo104 and Mo105 could only be isolated in their facial form. The ModNHC bond length in Mo104 is 226.3(2) pm. It should be noted that the ModCO bond length of the carbonyl trans to NHC is longer than that of the carbonyl trans to the phosphine. Like Mo104, Mo106 shows a strongly distorted octahedral structure. The distances of the ModNHC bonds are 225.9(4) and 226.1(4) pm.

Scheme 70 Synthesis of molybdenum (0) carbonyl complexes supported by mixed imidazol-2-ylidene/phosphanyl hybrid ligands Mo103–Mo106. Modified from Gradert, C.; Krahmer, J.; Sönnichsen, F.D.; Näther, C.; Tuczek, F. Small-Molecule Activation With Molybdenum(0) Complexes Supported by Mixed Imidazol-2Ylidene/Phosphanyl Hybrid Ligands—Electronic and Structural Consequences of Substituting a Phosphane by a Carbene Group. Eur. J. Inorg. Chem. 2013, 2013, 3943–3955.

Willms et al. reported the synthesis of a series of molybdenum (0) NHC phosphaferrocene complexes Mo107–Mo110. These complexes can be prepared by the reaction of [Mo(CO)6] with the corresponding free NHC113 (Fig. 2). Mo110 has a distorted octahedral geometry with the NHC in a meridional position. The ligands in Mo108 and Mo109 are arranged in a distorted octahedral configuration around the molybdenum. For Mo108 the ModNHC bond length of 230.0(4) pm is in the upper range compared to other ModNHC complexes. Mo109 exhibits a ModNHC bond length of 226.8(5) pm.

Fig. 2 Preparation of molybdenum (0) NHC phosphaferrocene complexes Mo107–Mo110. Modified from Willms, H.; Frank, W.; Ganter, C. Hybrid Ligands With NHeterocyclic Carbene and Chiral Phosphaferrocene Components. Chem. Eur. J. 2008, 14, 2719–2729.

In 2014, Wang et al. described the syntheses of Mo111–Mo119 by reacting Mo(CO)3Br with different benzothiazol2-ylimidazolium salts.114 Here, the benzothiazol-2-ylimidazolium salts were reacted with Ag2O under the exclusion of light to get the corresponding silver salts. Mo114–Mo119 were prepared by exchanging the complex silver anions in complexes Mo111–Mo113 with AgX (Scheme 71).

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Scheme 71 Preparation of molybdenum (II) cyclopentadienyl dicarbonyl complexes Mo111–Mo119 containing N, C-chelating benzothiazol-2ylimidazol-2-ylidenes. Modified from Wang, Z.; Ng, S.W.B.; Jiang, L.; Leong, W.J.; Zhao, J.; Hor, T.S.A. Cyclopentadienyl Molybdenum(II) N,C-Chelating Benzothiazole-Carbene Complexes: Synthesis, Structure, and Application in Cyclooctene Epoxidation Catalysis. Organometallics 2014, 33, 2457–2466.

Complexes Mo111, Mo112, Mo114, Mo115, and Mo119 all adopt a 4-legged piano chair structure. The NHC acts as a bidentate C, N-ligand, and forms a five-membered ring with the Mo center. The ModNHC bond lengths are summarized in Table 3. Table 3

ModNHC bond lengths in complexes Mo111, Mo112, Mo114, Mo115, and Mo119.

Complexes

Mo111

Mo112

Mo114

Mo115

Mo119

Mo-NHC [pm]

216.1(9)

214.3(5)

216.6(3)

216.2(3)

214.3(2)

The ModNHC bond lengths are significantly shorter than those of other ModNHC complexes, which can be explained by the chelating effect of the hybrid NHC.

6.02.7

Tungsten NHC complexes

Several examples of tungsten NHC complexes date back to the 1970ies. Here, pioneering work by Öfele et al. has to be mentioned.74,77 Later, more examples of tungsten NHC complexes were provided by Herrmann et al.81—also synthesizing the first high oxidation state tungsten NHC complexes—and Hahn et al. working on the template-controlled synthesis of tungsten NHC complexes.115–117 Furthermore, Buchmeiser et al. published on a series of tungsten alkylidene and alkylidyne NHC complexes.82,83,85,118–122 Several applications of tungsten NHC complexes are reported such as catalysts for epoxidation of olefins,102,123 redox active complexes49,124 and highly active metathesis catalysts.82,83,85,118–122 Possible applications of the following metal complexes will be mentioned but not reviewed in detail. The focus of the following discussion is on the synthesis and structural properties of tungsten NHC complexes with an emphasis on the ones that have been published since 2006 as well as the template-assisted synthesis of tungsten NHC complexes since it features a unique strategy for the introduction of NHC ligands in group 6 metals.

6.02.7.1

Tungsten (0) NHC carbonyl complexes

Tungsten (0) NHC carbonyl complexes represent one of the largest and most studied classes of tungsten NHC complexes. Not only have tungsten NHC carbonyl complexes been the first tungsten NHC complexes synthesized but they often have been used as model complexes since the electronic influence of the NHC on the carbonyl resonances can easily be studied by IR spectroscopy.74

6.02.7.1.1

Structure and synthesis of monodentate tungsten (0) NHC carbonyl complexes

First examples of monodentate tungsten NHC carbonyl complexes were provided by Öfele in the 1970ies alongside with chromium and molybdenum NHC carbonyl complexes, which were all synthesized by addition of the free carbene to the corresponding metal (0) carbonyl complexes and analyzed with various techniques such as 1H NMR, IR spectroscopy and cyclic voltammetry. Early examples of tungsten (0) NHC carbonyl complexes are depicted in Fig. 3.74,77,125 Öfeles bis-NHC complex W(IMe)2(CO)4 remained one of a few rare examples of tungsten NHC carbonyl complexes until the research on tungsten NHC complexes slowly gathered pace in the early 1990ies. A variety of complexes was synthesized by Öfele and Herrmann such as an oxy-functionalized heterobimetallic complex and a tungsten mono-NHC carbonyl complex bearing a chiral NHC.125,126 More elaborate synthetic strategies were researched by Herrmann et al. and Hahn et al., which yielded a number of different tungsten NHC carbonyl complexes.127,128129 Also, the synthesis of ferrocenyl substituted NHCs and their tungsten carbonyl complexes was researched as well as elucidation of structural and electrochemical properties.130

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Fig. 3 Early examples of tungsten (0) NHC carbonyl complexes.125,126 Modified from Kreiter, C.G.; Öfele, K.; Wieser, G.W. Gehinderte Ligandenbewegungen in Übergangsmetallkomplexen, VIII. Untersuchungen an Übergangsmetallkomplexen mit cyclischen Carbenliganden, II. Chrom-, Molybdän- und Wolframkomplexe mit 1,3-Dimethyl-4-imidazolin-2-yliden-Liganden. Chem. Ber. 1976, 109, 1749–1758 and Rieke, R.D.; Kojima, H.; Oefele, K. Studies on Transition Metal Complexes With Cyclic Carbene Ligands. 4. Electrochemical Oxidation of Dicarbene Metal Carbonyl Complexes. Isomerization Via an Electrochemical Reaction With no net Current Flow. J. Am. Chem. Soc. 1976, 98, 6735–6737.

Benzimidazolin-2-ylidene ligands can easily be introduced by the addition of the free carbene to a metal complex. This procedure is applicable to the synthesis of benzimidazol-2-ylidene complexes of tungsten(0) as well (Scheme 72A).129

(A)

(B)

Scheme 72 (A) Synthesis of a benzimidazol-2-ylidene carbonyl complex of tungsten(0); (B) synthesis of a benzothiazol-2-ylidene carbonyl complex of tungsten(0). Modified from Nonnenmacher, M.; Kunz, D.; Rominger, F.; Oeser, T. First Examples of Dipyrido[1,2-c:20 ,10 -e]Imidazolin-7-Ylidenes Serving as NHC-Ligands: Synthesis, Properties and Structural Features of Their Chromium and Tungsten Pentacarbonyl Complexes. J. Organomet. Chem. 2005, 690, 5647–5653.

Compared to the synthesis of benzimidazolin-2-ylidene metal complexes, the introduction of the analogous benzothiazol2-ylidene complexes was not possible from the free carbenes since rapid dimerization upon deprotonation of the corresponding azolium salts lead to a dimer that undergoes N-allyl cleavage to give benzothiazole which eventually coordinates to the metal.131–134 An alternative route for the introduction of the N-allyl benzothiazole-2-ylidene ligand is presented in Scheme 72B. In situ deprotonation of the C2 carbon of benzoxazole with nBuLi gave 2-lithobenzothiazole that was reacted with photochemically produced W(CO)5THF. Subsequently, treatment with 1 equiv. of ally bromide resulted in the formation of the N-allyl benzimidazolin-2-ylidene W(0) complex in a reasonable yield of 62%. The complex was subjected to single crystal X-ray analysis which revealed a slightly distorted octahedral structure with the NHC being in the axial position. The WdCOtrans, the WdCOcis as

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well as the WdC(NHC) bond distance (223.1(3) pm) are comparable to those in other tungsten N,N0 -diallyl-benzimidazol2-ylidene complexes.129,135,136 Metal complexes containing redox-active ligands have been used in various areas including catalysis.137–139 Previous studies of tungsten (0) ferrocenyl-NHC carbonyl complexes solely focused on the steric properties of the ferrocenyl rather than on the electrochemical properties.130 Redox-active tungsten (0) NHC complexes that were also studied for their electrochemical properties are depicted in Fig. 4A.25

(A)

(B)

Fig. 4 (A) Redox-active tungsten(0) NHC carbonyl complexes; (B) Tungsten(0) NHC carbonyl complexes for olefin epoxidation; Tungsten(0) NHC carbonyl complexes for catalytic reduction of imines. (C) depicts tungsten(0) NHC carbonyl complexes that have been used for catalytic olefin epoxidation. Notably, in this case W(CO)3(MeCN)3 was used instead of W(CO)5THF as starting material in combination with the in situ generated free carbenes or silver(I) NHC adducts leading to only low to moderate yields after column chromatography. The presence of additional carbonyl groups compared to the starting material is attributed to “CO ligand scrambling”140 and features a reasonable explanation for low isolated yields. Single crystal X-ray analysis was performed for the tungsten mono- and bis-NHC complexes for R ¼ Bz showing a slightly distorted octahedral structure. In both cases, unsuspicious WdC(NHC) bond distances between 226.5(4) and 227.9(6) pm and the previously discussed trans effect of the NHC on the carbonyl groups (vide supra) were observed. Modified from Reshi, N.; Kathuria, L.; Samuelson, A. Reduction of Imines Catalysed by NHC Substituted Group 6 Metal Carbonyls. Inorg. Chim. Acta 2018, 486, 119–128 and Rosen, E.L.; Varnado Jr., C.D.; Tennyson, A.G.; Khramov, D.M.; Kamplain, J.W.; Sung, D.H.; Cresswell, P.T.; Lynch, V.M.; Bielawski, C.W. Redox-Active N-Heterocyclic Carbenes: Design, Synthesis, and Evaluation of Their Electronic Properties. Organometallics 2009, 28, 6695–6706, and Wang Z.; Li, S.; Teo, W.J.; Poh, Y.T.; Zhao, J.; Hor, T.S.A.; Molybdenum (0) and Tungsten (0) Carbonyl N-Heterocyclic Carbene Complexes as Catalyst for Olefin Epoxidation. J. Organomet. Chem. 2015, 775, 188–194.

The synthetic route to those complexes was accomplished via the corresponding free carbenes that can be synthesized from aminoferrocene in a convenient 3- to 4-step reaction sequence in combination with W(CO)5THF. Single crystal X-ray analysis of W(N,N´-ferrocenylbenzimidazol-2-ylidene)(CO)5 revealed bond distances for the WdC(NHC), the WdCOtrans and the WdCOcis bond of 229.9(4), 197.8(4) and 203.2 pm, respectively, which indicates a weaker carbonyl p-back bonding of the trans CO due strong s-donor properties. The WdC(NHC) bond distance of the naphthoquinone substituted WNHC(CO)5 complex, in contrast, is slightly shorter (225.9(5) pm).

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Lastly, Fig. 4C shows a series of tungsten(0) NHC carbonyl complexes that are active in the catalytic reduction of imines using phenyl silane as reductant. All depicted complexes were synthesized from W(CO)5THF and silver(I) NHC adducts. The nCO IR stretching vibrations for R ¼ H and R ¼ Bz were measured and compared to find only minor differences (R ¼ H: nCO ¼ 2062, 1924 cm−1; R ¼ Bz: nCO ¼ 2058, 1961, 1884 cm−1).141

6.02.7.1.2

Structure and synthesis bidentate tungsten(0) NHC carbonyl complexes

One of the first examples for a bidentate tungsten (0) NHC carbonyl complex was provided by Öfele and Herrmann et al. in 1993 (Fig. 5).142

N N

CO CO W CO CO

N N

Fig. 5 One of the first examples of a bidentate tungsten (0) NHC carbonyl complex. Modified from Öfele, K.; Herrmann, W.A.; Mihalios, D.; Elison, M.; Herdtweck, E.; Scherer, W.; Mink, J. Mehrfachbindungen zwischen hauptgruppenelementen und übergangsmetallen: CXXVI. Heterocyclen-carbene als phosphananaloge liganden in metallkomplexen. J. Organomet. Chem. 1993, 459, 177–184.

The complex was synthesized by twofold in situ deprotonation of the imidazolium salt and subjected to single crystal x-ray analysis, which revealed that the complex has an almost perfect octahedral geometry. The WdC(NHC) bond distances are 221.1 and 223.1 pm, which is not unusual for tungsten(0) NHC carbonyl complexes of this type. Both carbene ligands are twisted in regard to each other (interplanar angle ¼ 71.9 ) and form a coplanar structure with one carbonyl group.142 Other examples of bidentate tungsten (0) NHC carbonyl complexes feature an N-chelation, which can be seen in Scheme 73. Treatment of an azaaryl-functionalized imidazolium salt (R ¼ H) with Ag2O followed by addition of W(CO)5THF leads to formation of a monodentate complex (Scheme 73A). In contrast to the analogous complex for R ¼ Me, which in the reaction with the imidazolium salt and Ag2O directly forms a bidentate complex, the monodentate complex for R ¼ H requires heating to reflux in dioxane for 9 h to induce monodecarboxylation and, thereby, formation of a bidentate complex. The bidentate complex for R ¼ Me was analyzed by single crystal X-ray crystallography to shows a regular WdC(NHC) bond distance of 223.5 pm. Both complexes show activity in the catalytic oxidation of styrene using H2O2 or tBuOOH as an oxidant at a catalyst loading of 1 mol%.104 (A)

(B)

Scheme 73 (A) Synthesis of a pyrazole-chelating bidentate tungsten (0) NHC carbonyl complex; (B) synthesis of a pyridine-chelating bidentate tungsten (0) NHC carbonyl complex. Modified from Cheng, C.-H.; Guo, R.-Y.; Cui, Q.; Song, H.-B.; Tang, L.-F. Synthesis and Catalytic Activity of N-Heterocyclic Carbene Metal Carbonyl Complexes Based on 1-[2-(pyrazol-1-yl)phenyl]imidazole. Transit. Met. Chem. 2015, 40, 297–304 and Huckaba, A.J.; Shirley, H.; Lamb, R.W.; Guertin, S.; Autry, S.; Cheema, H.; Talukdar, K.; Jones, T.; Jurss, J.W.; Dass, A.; Hammer, N.I.; Schmehl, R.H.; Webster, C.E.; Delcamp, J.H. A Mononuclear Tungsten Photocatalyst for H2 Production. ACS Catal. 2018, 8, 4838–4847.

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Scheme 73B shows the synthesis of a pyridine-chelated tungsten NHC complex, which can be used for photocatalytic H2 production. In contrast to previously discussed synthesis routes for the introduction of NHCs, neither a silver salt nor a free carbene is used. Instead, the imidazolium salt is heated with W(CO)6 and a weak base, namely triethylamine, to yield the complex in 28% yield. The WdCCO bond trans to the NHC was found to be shorter than the carbonyl trans to the pyridyl moiety. The WdC(NHC) bond distance, on the other hand, is in the usual range (218.5(4) pm) and shorter than the WdN bond (225.9(3) pm).143

6.02.7.1.3

Template-controlled synthesis of NHC ligands via the use of isocyanides

The use of coordinated isocyanides can be utilized for the synthesis of a variety of group VI metal carbene complexes.144–148 This method reaches back to 1915, when the Russian chemists Tschugajeff and Skanawy-Grigorjewa treated platinum (II) tetrakis(methyl isocyanide) with hydrazine.149,150 However, it took another 50 years until it was realized that a metal carbene complex had formed.151–153 More recently, it was shown that this strategy can also be exploited for the template-controlled synthesis of tungsten NHC complexes.115,116,154 By combining the isocyanide ligand with a nucleophile—for the synthesis of NHC complexes the nucleophile has to be an amine or an hydroxy group—it is possible to furnish cyclic carbenes. One of the first examples comprise the synthesis of benzannulated NHC carbene ligands and the corresponding chromium and tungsten complexes by Hahn et al.116,117,155 The general strategy is depicted in Scheme 74.

Nu N

H HN

Nu

RN

Nu

C MLX

MLX

MLX

M = Cr, W Nu = OH, NH2

Scheme 74 General approach for the template-controlled synthesis of N,O- and N,N0 stabilized benzannulated chromium and tungsten NHC complexes. Modified from Hahn, F.E.; Langenhahn, V.; Pape, T. Template Synthesis of tungsten Complexes With Saturated N-Heterocyclic Carbene Ligands. Chem. Commun. 2005, 5390–5392, Hahn, F.E.; Langenhahn, V.; Meier, N.; Lügger, T.; Fehlhammer, W.P. Template Synthesis of Benzannulated N-Heterocyclic Carbene Ligands. Chem. Eur. J. 2003, 9, 704–712, and Hahn, F.E.; Tamm, M. Synthesis and Molecular Structure of a Complex Containing All Three Intermediates of the Carbene Synthesis From Coordinated Isocyanides. J. Chem. Soc. 1995, 569–570.

Since free 2-aminophenyl isocyanides and 2-hydroxyphenyl isocyanides spontaneously undergo cyclization,156 a synthon for the in situ generation of amino or hydroxy group is required that, once coordinated to the metal, can be deprotected. The deprotected isocyanide ligand then readily undergoes cyclization to an NH,NH-carbene or NH,O-carbene. Subsequent double alkylation of the N-termini or single alkylation in case of the NH,O-carbene, respectively, then give access to the corresponding NHC complexes. It was shown by the group of Langenhahn that 2-azidophenyl isocyanide reacts with tungsten pentacarbonyl THF to yield the moisture- and air-sensitive tungsten pentacarbonyl monoisocyanide complex in good yield (Scheme 75).116 Single crystal X-ray analysis of the complex revealed that bond lengths and angles do not substantially differ from other published tungsten carbonyl complexes bearing an isocyanide ligand trans to the CO.117,157 The weak back bonding of the WdC bond is indicated by a linear geometry of the C^NdC group and a nearly unchanged wavenumber of the C^N stretching vibration compared to the free isocyanide (2142 cm−1 vs. 2141 cm−1 for M ¼ W). Consequently, the isocyanide carbon atom has an almost unchanged electrophilicity and, hence, can still be attacked by a nucleophile.117 The azido moiety can be transformed to an iminophosphorane in a Staudinger-type reaction with triphenylphosphine.158 Hydrolysis of the formed 2-phosphiniminophenyl isocyanide complex with a mixture of water and methanol containing catalytic amounts of HBr furnishes the corresponding 2-aminophenyl isocyanide complex which spontaneously cyclizes via a nucleophilic attack of the amino group of the isocyanide carbon to the NH,NH-carbene complex.

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Scheme 75 Synthesis of benzannulated N,N0 stabilized NHC complexes. Modified from Hahn, F.E.; Langenhahn, V.; Meier, N.; Lügger, T.; Fehlhammer, W.P.; Template Synthesis of Benzannulated N-Heterocyclic Carbene Ligands. Chem. Eur. J. 2003, 9, 704–712.

Double alkylation of the N-termini leads to an NHC complex. In the whole reaction sequence the geometry remains slightly distorted octahedral. Bond lengths, on the other hand, differ from the isocyanide complexes compared to the NH,NH-carbene and NHC complexes due to strong s-donor properties of the carbene ligands. A shorter MdCOtrans bond (188.7(3) pm) and a longer MdCOcis bond (202.2(4)–202.9(4) pm) were observed in the tungsten NH, NH-carbene complex. The trans and cis MdCO bond lengths, in contrast, are virtually the same in all isocyanide complexes. In a similar way, 6-membered benzannulated tungsten NHC complexes can be synthesized from 2-(azidomethyl)-, 2-(chloromethyl)- and 2-(iodomethyl)phenyl isocyanides in combination with tungsten hexacarbonyl (Scheme 76).154

Scheme 76 Synthesis of quinazolin-2-ylidene complexes. Modified from Basato, M.; Facchin, G.; Michelin, R.A.; Mozzon, M.; Pugliese, S.; Sgarbossa, P.; Tassan, A.; Transition Metal Coordination and Reactivity of 2-(Azidomethyl)-, 2-(Chloromethyl)- and 2-(Iodomethyl)Phenyl Isocyanides. Inorg. Chim. Acta 2003, 356, 349–356.

After the synthesis of the metal isocyanate pentacarbonyl complex, which in this case can be accomplished via reaction of [W(CO)5I][NEt4] with 1 equiv. of AgBF4 and the corresponding isocyanide, a Staudinger-type reaction can also performed for Y ¼ N3 to afford iminophosphorane derivatives, which are then hydrolyzed to the free amine to undergo cyclization to the NH,NHcarbene. The alkyl halide isocyanide complexes (Y ¼ I), however, can react with a primary amine (NH2R) which is a well-known reaction in organic synthesis.159 Benzylic halides in this regard have proved to be outstandingly reactive. So much so, that the reaction of the iodo-derivative with NH2Me can be conducted in 1,2-dichloroethane at room temperature to first yield the secondary amine which then cyclizes to the quinazolin-2-ylidene complex (R ¼ Me) in an isolated yield of 29%. Employing this synthetic strategy, the synthesis of saturated 5-membered N-heterocyclic carbenes was also achieved by Hahn et al. (Scheme 77).115

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Scheme 77 Synthesis of saturated N,N0 stabilized NHC complexes. Modified from Hahn, F. E.; Langenhahn, V.; Pape, T. Template Synthesis of Tungsten Complexes With Saturated N-Heterocyclic Carbene Ligands. Chem. Commun. 2005, 5390–5392.

2-Azidoethyl isocyanide readily coordinates to photochemically produced W(CO)5THF and undergoes the same reaction sequence as for the synthesis of benzannulated NHCs. The iminophosphorane complex was not isolated but treated in situ with a catalytic amount of HCl to first obtain the NH,NH-carbene complex and, after double alkylation, the unsaturated N,N´-diethylimidazolin-2-ylidene complex. Another example for the template-controlled synthesis of saturated tungsten NHC carbonyl complexes can be found in the work of Liu (Scheme 78).160

Scheme 78 Synthesis of saturated N,N0 stabilized NHC complexes from iminophsophoranes. Modified from Liu, C.-Y.; Chen, D.-Y.; Lee, G.-H.; Peng, S.-M.; Liu, S.-T. Synthesis of Cyclic Diamino-Substituted Metal Carbene Complexes. Organometallics 1996, 15, 1055–1061.

It was shown that the synthesis of the iminophosphorane can be conducted by a Staudinger-type reaction from triphenylphosphine and the corresponding azido compound before the coordination of the template ligand takes place. The resulting amino-phosphinimines react with tungsten hexacarbonyl under extrusion of O]PPh3 and deoxygenation of one carbonyl ligand by the iminophosphorane moiety to form the isocyanide complex, which cannot be isolated but is subsequently attacked by the amino group to furnish five- and six-membered carbene pentacarbonyl complexes. In an additional step, the second N-terminus can be alkylated by deprotonation with sodium hydride and treatment with alkylation reagent. Even before the first examples for the template-assisted synthesis of N,N´-stabilized NHCs existed, a similar synthetic pathway was exploited for the template-controlled synthesis of tungsten benzoxazol-2-ylidene carbonyl complexes. It is known that free 2-hydroxyalkylisocanide readily cyclizes to the aromatic heterocycle benzoxazole156 and that 2-hydroxyalkylisocanide complexes of M(CO)5 (M ¼ Cr, Mo, W) on the contrary are stabilized and less prone to cyclization due to (d ! p)p back bonding, which results in a lower electrophilicity.161 Addition of 2-(trimethylsiloxyl)phenyl isocyanide to W(CO)5THF results in the formation of an isocyanide complex (Scheme 79).115,117,148

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Scheme 79 Synthesis of saturated N,O stabilized NHC complexes by Hahn and Tamm et al.148 Modified from Hahn, F.E.; Langenhahn, V.; Pape, T.; Template Synthesis of Tungsten Complexes With Saturated N-Heterocyclic Carbene Ligands. Chem. Commun. 2005, 5390–5392, Hahn, F.E.; Tamm, M.; Synthesis and Molecular Structure of a Complex Containing All Three Intermediates of the Carbene Synthesis From Coordinated Isocyanides. J. Chem. Soc. 1995, 569–570, and Hahn, F.E.; Tamm, M.; Carbenkomplexe aus koordinierten 2-Siloxyphenylisocyaniden. J. Organomet. Chem. 1993, 456, C11–C14.

Deprotection of the trimethylsilyl-group with a catalytic amount KF over 2 days furnishes the free hydroxy moiety, which can then undergo cyclization. As mentioned above, the bonded ligand has a lower tendency to cyclize than the free 2-hydroxyphenyl isocyanide. This, indeed, results in the ligand being in an equilibrium between the 2-hydroxyphenyl isocyanide and the benzoxazol2-ylidene form. The driving force of the cyclization is the high stability of the formed ylidene ligand with an aromatic five-membered heterocycle. The backreaction can be prevented by trapping the amine by deprotonation with a strong base and subsequent alkylation.115,117,148

6.02.7.1.4

Tungsten carbonyl complexes bearing an abnormal NHC (aNHC)

Only few examples of tungsten (0) aNHC (abnormal NHC) carbonyl complexes exist. Early examples can be found in the work of Raubenheimer and Zippel et al. (Fig. 6).5,162

Fig. 6 Early examples of tungsten (0) aNHC carbonyl complexes. Modified from Schuster, O.; Yang, L.; Raubenheimer, H.G.; Albrecht, M. Beyond Conventional N-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands With Reduced Heteroatom Stabilization. Chem. Rev. 2009, 109, 3445–3478 and Aumann, R.; Yu, Z.; Fröhlich, R.; Zippel, F. Mesoionic Pyrrolium Complexes and Dihydropyrroles by Cycloaddition of (Non-Enolizable) Imines to an [(1-Alkynyl) carbene]tungsten Complex. Eur. J. Inorg. Chem. 1998, 1998, 1623–1629.

Their discoveries can rather be attributed to serendipitous discoveries than to a rational approach. Raubenheimers tungsten aNHC complex was obtained by alkylation of an anionic tungsten isothiazole complex whereas Zippel was investigating cycloadditions of imines to an [(1-alkynyl)carbene] tungsten complex.5,162 A more recent but, nonetheless, also indirect approach to the introduction of an aNHC ligand to tungsten carbonyl complexes was found by the rearrangement of an NHC with a silylene tungsten complex. The silylene complex [(IPr)SiCl2]W(CO)5 can be obtained by treatment of a freshly prepared solution of W(CO)5THF in THF with SiCl2(IPr). The addition of 1 equiv. of CsOH in the presence of IPr as an HCl scavenger then facilitates the formation of W(aIPrH)(CO)5. Alternatively, it was found that the use of Robinson’s anionic dicarbene (dc)NHC163 to W(CO)5THF and subsequent addition of a proton source features a different synthetic approach for the formation of W(aIPrH)(CO)5. Its single crystal X-ray structure was compared to the analogous NHC complex W(IPr)(CO)5 and revealed a WdC(NHC) bond length of 226.0(2) pm, comparable to the one in its aNHC counterpart (224.80 (19) pm)164 (Scheme 80).

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Scheme 80 Synthesis of a tungsten aNHC complex from [(IPr)SiCl2]W(CO)5 or direct addition of Robinson’s dcNHC to W(CO)5THF. Modified from Ghadwal, R.S.; Rottschäfer, D.; Andrada, D.M.; Frenking, G.; Schürmann, C.J.; Stammler, H.-G.; Normal-to-Abnormal Rearrangement of an N-Heterocyclic Carbene With a Silylene Transition Metal Complex. Dalton Trans. 2017, 46, 7791–7799 and Wang, Y.; Xie, Y.; Abraham, M.Y.; Wei, P.; Schaefer, H.F.; Schleyer, P.V.R.; Robinson, G.H. A Viable Anionic N-Heterocyclic Dicarbene J. Am. Chem. Soc. 2010, 132, 14370–14372.

6.02.7.1.5

Tungsten allyl and cyclopentadienyl carbonyl complexes

Grubbs,165,166 Herrmann,167 and Lin et al.168 showed that NHC-chloroform adducts or Ag(I) adducts of NHCs, respectively, can be used for NHC transfer onto metal complexes. More recently, it was shown that borane adducts of the type NHC
BEt3 can be likewise utilized for this purpose (Scheme 81).92

Scheme 81 NHC borane adducts for the transfer of NHCs onto tungsten allyl amidinato(pyridine) complexes. Modified from Ogata, K.; Yamaguchi, Y.; Kashiwabara, T.; Ito, T. The reaction of Amidinato(pyridine) Complexes of Molybdenum and Tungsten With Triethylborane Adduct of N-Heterocyclic Carbene (NHC
BEt3): Investigation on the Reactivity of NHC
BEt3 as Carbene Precursor Toward Transition Metal Complexes. J. Organomet. Chem. 2005, 690, 5701–5709.

The labile nature of the pyridine ligand in [W(Z3-allyl)92(CO)2(NC6H5)] was earlier exploited for an exchange with phosphine or phosphite ligands.169 Under thermal conditions a ligand exchange can also be achieved using NHC
BEt3 (NHC ¼ 1,3-dimesitylimidazol-2-ylidene; 1,3-diisopropylimidazol-2-ylidene) obtaining the corresponding tungsten allyl amidinato NHC complexes in moderate to good yields of 71–92%. The nCO IR stretching vibrations in both tungsten NHC complexes (1904 and 1818, 1897 and 1797 cm−1) are lower than that of the previously synthesized phosphine complexes, which is accordance with the strong s-donor properties of the NHCs and thereby a stronger p-back bonding on the carbonyl ligands. Single crystal X-ray diffraction revealed a pseudo-octahedral geometry in which the amidinato ligand is located coplanar with the Z3-allyl ligand whose open face points towards the carbonyls. WdC(NHC) bond distances are in the usual range of 222.(2) pm (R ¼ iPr) and 228.(1) pm (R ¼ Mes).92 Tungsten Z3-allyl carbonyl bis-NHC complexes can be synthesized from the free carbenes in a similar manner as seen in the reaction of the NHC borane adduct (Scheme 82).95

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Scheme 82 Synthesis of bidentate [W(Z3-allyl)Cl(CO)2(bis-NHCBz)] (bis-NHC ¼ 1,10 -dibenzyl-3,30 -methylenediimidazolin-2,2-diylidene). Modified from Krishna Mohan Kandepi, V.V.; Cardoso, J.M.S.; Royo, B. N-Heterocyclic Carbene-Based Molybdenum and Tungsten Complexes as Efficient Epoxidation Catalysts With H2O2 and tert-Butyl Hydroperoxide. Catal. Lett. 2010, 136, 222–227.

Treatment of [W(Z3-allyl)Cl(CO)2(NCMe)2] with bis-NHCBz (bis-NHC ¼ 1.10 -dibenzyl-3,30 -methylenediimidazolin-2,2-diylidene) leads to the rapid formation of [W(Z3-allyl)Cl(CO)2(bis-NHCBz)] in THF at room temperature in a moderate yield of 47%. The nCO IR stretching vibrations are with 1915 and 1806 cm−1 comparable to those previously discussed and lower compared to the starting complex (1933 and 1839 cm−1) due to back bonding. Although no crystallographic data is available for this compound, the authors proposed a pseudo-octahedral structure with the orientation of the open allyl face towards the carbonyl groups. Catalytic activity was found for the epoxidation of olefins using tert-butyl hydroperoxide as oxidant.95 A rich variety of tungsten cyclopentadienyl NHC complexes for applications such as catalytic ketone hydrogenation89 or SidH bond activation170 exists. Scheme 83 displays the synthesis of the neutral tungsten cyclopentadienyl complex WCpHI(NHC) (CO)2 (NHC ¼ IMes, IMe; IMes ¼ 1,3-bismesitylimidazol-2-ylidene, IMe ¼ 1,3-dimethylimidazol-2-ylidene) by Bullock et al.89,171

Scheme 83 Synthesis of WCp(H)(IMes)(CO)2 and underlying equilibrium between an NHC, WCp(H)(PPh3)(CO)2, an imidazolium salt and WCp(PPh3)(CO)−2 . Modified from Wu, F.; Dioumaev, V.K.; Szalda, D.J.; Hanson, J.; Bullock, R.M.A Tungsten Complex With a Bidentate, Hemilabile N-Heterocyclic Carbene Ligand, Facile Displacement of the Weakly Bound W −(CC) Bond, and the Vulnerability of the NHC Ligand Toward Catalyst Deactivation During Ketone Hydrogenation. Organometallics 2007, 26, 5079–5090.

The synthesis of WCpH(NHC)(CO)2 was conducted by the addition of 1 equiv. of the free NHC to WCpHI(PPh3)(CO)2, which tentatively results in an equilibrium of starting materials and an anionic complex and imidazolium salt. In case the reaction was carried out at ambient temperature the ionic species could be isolated and fully characterized whereas a reaction at elevated temperatures of 100  C in toluene led to the formation of WCpH(NHC)(CO)2. Polar solvents stabilize the ionic form and therefore conversion is much slower in THF compared to toluene. Although an oxidative pathway of the ionic species to WCpH(NHC) (CO)2 cannot be ruled out completely (oxidative addition of imidazolium salts on transition metal complexes has been reported for Ni, Pd and Pt90,172,173 as well as in Ir and Rh studies174,175) it seems rather unlikely since tungsten is less prone to oxidative addition. X-ray single crystal analysis was used to determine a four-legged piano stool structure with the carbonyls in cis-position as indicated by both NMR and IR. The relatively short WdC(NHC) bond distances are 193.6(6) pm (R ¼ Mes) and 219.6(2) pm (R ¼ Me), respectively. In addition, tritylium reagents can be used as a hydride abstraction reagent to either yield the bidentate, hemilabile complex CpW(CO)2(IMes)+ or, in presence of a donor solvent, the monodentate CpW(CO)2(IMes)L+ (Scheme 84).89,176

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257

Scheme 84 Hydride transfer from WCpH(IMes)(CO)2 for the formation of CpW(CO)2(IMes)+ and CpW(CO)2(IMes)L+. Modified from Wu, F.; Dioumaev, V.K.; Szalda, D.J.; Hanson, J.; Bullock, R.M.A Tungsten Complex With a Bidentate, Hemilabile N-Heterocyclic Carbene Ligand, Facile Displacement of the Weakly Bound W −(CC) Bond, and the Vulnerability of the NHC Ligand Toward Catalyst Deactivation During Ketone Hydrogenation. Organometallics 2007, 26, 5079–5090, and Roberts, J.A.S.; Franz, J.A.; van der Eide, E.F.; Walter, E.D.; Petersen, J.L.; DuBois, D.L.; Bullock, R.M. Comproportionation of Cationic and Anionic Tungsten Complexes Having an N-Heterocyclic Carbene Ligand to Give the Isolable 17-Electron Tungsten Radical CpW(CO)2(IMes)%. J. Am. Chem. Soc. 2011, 133, 14593–14603.

CpW(CO)2(IMes)+ is virtually insoluble in apolar solvents such as toluene and hexane and reacts with more polar solvents to form monodentate adducts. For this reason, limited analytical methods could be used for its characterization. The nCO IR stretching vibrations (Nujol) are 1980 and 1890 cm−1. Single crystal X-ray analysis was performed on a high-intensity synchrotron and revealed that, indeed, the mesityl group binds to tungsten in an Z2-fashion with bond distances of the two adjacent carbons to tungsten being 290.1(13) and 307.2(13) pm. This is a significantly longer bond distance than in other non-chelating Z2-tungsten arene complexes (typically 210–220 pm)177–180 and more in the range of constrained Z2-arenes of tungsten and molybdenum (typically 250–280 pm).181–187 The WdC(NHC) bond distance, on the other hand, is with 192.8(15) pm in the usual range. If the reaction is conducted in coordinating solvents such as THF and MeCN, direct formation of the solvent adduct CpW(CO)2(IMes)L+ can be observed. In contrast to the hydride abstraction, a deprotonation of WCpH(IMes)(CO)2 was also reported by Bullock and co-workers (Scheme 85). WCpH(IMes)(CO)2 can readily be deprotonated by KH in THF under the release of H2. 18-Crown-6 can be added to the reaction mixture to facilitate crystallization and indeed seems to be required to grow single crystals for further analysis.171,176 Single crystal X-ray diffraction shows that [WCp(IMes)(CO)2]− [K(18-crown-6)]+ adopts a three-legged piano-stool configuration with a 213.1 pm WdC(NHC) bond distance. The pKa of WCpH(IMes)(CO)2 in MeCN was determined using Verkades superbase (VSB). The equilibrium constant was found Keq ¼ 0.10 and, thereby, a pKa of 31.9 was calculated for WCpH(IMes)(CO)2. A similar pKa of 31–5(3) was determined in the same way for WCpH(IMe)(CO)2.188,189

Scheme 85 Proton abstraction from WCp(H)(IMes)(CO)2 for the formation CpW(CO)2(IMes)−. Modified from Roberts, J.A.S.; Franz, J.A.; van der Eide, E.F.; Walter, E.D.; Petersen, J.L.; DuBois, D.L.; Bullock, R.M.; Comproportionation of Cationic and Anionic Tungsten Complexes Having an N-Heterocyclic Carbene Ligand to Give the Isolable 17-Electron Tungsten Radical CpW(CO)2(IMes)%. J. Am. Chem. Soc. 2011, 133, 14593–14603, van der Eide, E.F.; Liu, T.; Camaioni, D.M.; Walter, E.D.; Bullock, R.M.; Facile Thermal W–W Bond Homolysis in the N-Heterocyclic Carbene Containing Tungsten Dimer [CpW(CO)2(IMe)]2. Organometallics 2012, 31, 1775–1789, and Roberts, J.A.S.; Appel, A.M.; DuBois, D.L.; Bullock, R.M.; Comprehensive Thermochemistry of W–H Bonding in the Metal Hydrides CpW(CO)2(IMes)H, [CpW(CO)2(IMes)H]% +, and [CpW(CO)2(IMes)(H)2]+. Influence of an N-Heterocyclic Carbene Ligand on Metal Hydride Bond Energies. J. Am. Chem. Soc. 2011, 133, 14604–14613.

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N-Heterocyclic and Mesoionic Carbene Complexes of Group 5 and Group 6 Metals

The cationic complex [CpW(CO)2(IMes)]+[B(C6F5)4]−, which is formed in the course of the hydride abstraction, can be hydrogenated to the dihydride complex [CpWH2(CO)2(IMes)]+[B(C6F5)4]− whose nCO IR stretching vibrations (in C6D6) are 2065 and 2006 cm−1 (Scheme 86). Higher frequencies compared to the starting complex [CpW(CO)2(IMes)]+[B(C6F5)4]− result from a higher oxidation state of tungsten. Alternatively, the complex can also be prepared by hydrogenation of the solvent adduct [CpW(CO)2(IMes)THF]+[B(C6F5)4]− under dissociation of THF. Interestingly, at 27  C in THF-d8 a singlet is observed in the 1H NMR spectrum for the two hydride resonances, which separate at lower temperatures to form to distinct signals at −100  C. This behavior can stem from the hindered rotation of the bulky IMes ligand, turnstile rotation of the W(CO2)H or W(CO)H2 tripod or even the transient formation of dihydrogen complexes.89,190

Scheme 86 Hydrogenation and coordination of ketones and alcohols to [WCp(CO)2(IMes)]+[B(C6F5)4]−. Modified from Wu, F.; Dioumaev, V.K.; Szalda, D.J.; Hanson, J.; Bullock, R.M. A Tungsten Complex With a Bidentate, Hemilabile N-Heterocyclic Carbene Ligand, Facile Displacement of the Weakly Bound W−(CC) Bond, and the Vulnerability of the NHC Ligand Toward Catalyst Deactivation during Ketone Hydrogenation. Organometallics 2007, 26, 5079–5090.

As previously discussed, hemilabile, bidentate [CpW(CO)2(IMes)]+[B(C6F5)4]− can form monodentate adducts with coordinating solvents such as THF and MeCN. In the same way, also alcohols and ketones can coordinate to [CpW(CO)2(IMes)]+[B(C6F5)4]−. No structural data about the ketone adduct [CpW(CO)2(IMes)(Et2C]O)]+[B(C6F5)4]− are available since the ketone does not bind sufficiently strong and is lost upon crystallization. The alcohol adduct [CpW(CO)2(IMes)(Et2CHOH)]+[B(C6F5)4]−, on the other hand, could be analyzed by single crystal X-ray diffraction to show a WdO bond distance of 225.0(7) pm, which is longer than in other tungsten cyclopentadienyl alcohol complexes.191,192 In addition, the chemical shift of the OH resonance of [CpW(CO)2(IMes)(Et2CHOH)]+[B(C6F5)4]− in the 1H NMR spectrum is only slightly shifted compared to the of free alcohol (d ¼ 0.62 vs. d ¼ 0.67, in C6D6). From those findings, one can surmise that there is only negligible hydrogen bonding to any proton acceptor site. When the ketone adduct [CpW(CO)2(IMes)(Et2C]O)][B(C6F5)4]− is exposed to H2 (4 atm), an equilibrium between the starting material and dihydride complex [CpWH2(CO)2(IMes)]+[B(C6F5)4]− is observed. In the catalytic hydrogenation of ketones using [CpW(CO)2(IMes)]+[B(C6F5)4]− and H2 this equilibrium most likely is the resting state of the catalyst. The previously discussed cationic and anionic complexes (vide supra) can be used in a comproportionation reaction to form the persistent radical [WCp(CO)2(NHC)]% (Scheme 87).171,176

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259

Scheme 87 Synthesis of the persistent 17 VE complex [CpW(CO)2(NHC)]% and its propensity to form dimers. Modified from van der Eide, E.F.; Liu, T.; Camaioni, D.M.; Walter, E.D.; Bullock, R.M.; Facile Thermal W–W Bond Homolysis in the N-Heterocyclic Carbene Containing Tungsten Dimer [CpW(CO)2(IMe)]2. Organometallics 2012, 31, 1775–1789, and Roberts, J.A.S.; Franz, J.A.; van der Eide, E.F.; Walter, E.D.; Petersen, J.L.; DuBois, D.L.; Bullock, R.M. Comproportionation of Cationic and Anionic Tungsten Complexes Having an N-Heterocyclic Carbene Ligand To Give the Isolable 17-Electron Tungsten Radical CpW(CO)2(IMes)%. J. Am. Chem. Soc. 2011, 133, 14593–14603.

An alternative synthetic route to [WCp(CO)2(IMes)]% was found in the reduction of [WCp(CO)2(IMes)]+[PF6]− using 1 equiv. of cobaltocene in THF.176 The persistent radical was analyzed by single crystal X-ray diffraction showing a three-legged piano-stool configuration. The shortest WdW bond distance (NHC ¼ IMes) was found to be 841.4 pm. This is a strong indicator for the presence of an odd electron monomeric structure, which was also supported by EPR spectroscopy. The WdC(NHC) bond distance is 216.4 pm in the solid state.176 Analogously to [WCp(CO)2(IMes)]%, [WCp(CO)2(IMe)]% can be synthesized by a comproportionation reaction or reduction of [WCp(CO)2(IMe)]+[PF6]− using 1 equiv. of cobaltocene. In contrast to the radical complex bearing the sterically demanding IMes, [WCp(CO)2(IMe)]% readily dimerizes to [WCp(CO)2(IMe)]2. Due to poor crystallizability, the dimeric complex could not be analyzed by single crystal X-ray crystallography. Nonetheless, IR spectroscopy revealed the presence of bands at 1866, 1760 and 1730 cm−1 for the nCO stretching vibrations. Two resonances close to 1800 cm−1 were expected for the C2h anti-isomer, similar to those in [WCp(CO)3]2.193–195 For this reason it can be concluded that in the solid state [WCp(CO)2(IMe)]2 exists in a gauche conformation. IR spectra of similar complexes have been extensively discussed in the literature.196–198 Although the dimer [WCp(CO)2(IMe)]2 could be isolated, it was assumed that an equilibrium of the dimer and the radical species exist, which is reflected by the reactivity of [WCp(CO)2(IMe)]2 (Scheme 88).171

Scheme 88 Reactivity of [WCp(CO)2(IMe)]2. Modified from van der Eide, E.F.; Liu, T.; Camaioni, D.M.; Walter, E.D.; Bullock, R.M. Facile Thermal W–W Bond Homolysis in the N-Heterocyclic Carbene Containing Tungsten Dimer [CpW(CO)2(IMe)]2. Organometallics 2012, 31, 1775–1789.

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Mixing [WCp(CO)2(IMe)]2 with 2,6-di-tert-butyl-1,4-benzoquinone (DBBQ) resulted in the rapid formation of [WCp(CO)2(IMe)(DBBQ)]% in THF, MeCN or THF as an indigo solution. Another typical reaction of 17-electron radicals is a halogen abstraction from halogenated solvents.199 Indeed, dissolving [WCp(CO)2(IMe)]2 in CHCl3 or CH2Cl2 led to the formation of WCp(CO)2(IMe)Cl and a small amount of an unidentified side-product. In addition, a slow reaction with H2 (4 atm) over 3 weeks yielded the previously discussed (vide supra) hydride complex WCpH(IMe)(CO)2.171 More recently, it was shown that Bullock’s cationic tungsten(II) catalyst [WCp(CO)2(IMes)]+[B(C6F5)4]− can be used for oxidative addition of SidH bonds (Scheme 89).170

Scheme 89 SidH bond activation with [WCp(CO)2(IMes)]+[B(C6F5)4]−. Modified from Fuchs, J.; Irran, E.; Hrobárik, P.; Klare, H.F.T.; Oestreich, M. Si–H Bond Activation With Bullock’s Cationic Tungsten (II) Catalyst: CO as Cooperating Ligand. J. Am. Chem. Soc. 2019, 141, 18845–18850.

In fact, the oxidative addition can be exploited for catalytic SidH bond activation to form silylethers from hydrosilanes and ketones. [WCpH(COSiEt3)(CO)(IMes)]+[B(C6F5)4]− was analyzed by single crystal X-ray analysis to show a WdCCO bond length of 194 pm, whereas, the WdOCOSi distance was found to be 179 pm, which indicates carbyne-like character.170

6.02.7.2

Tungsten (VI) NHC complexes without alkylidene or alkylidyne ligands

The first W(VI) NHC complexes were synthesized by Herrmann et al. in 1996 (Scheme 90A).81 The synthesis of WO2Cl2(IMe)2 was simply achieved by the addition of 2 equiv. of the free carbene to WO2Cl2(THF)2 in THF at −20  C. The corresponding mono-NHC complex of the type WO2Cl2(IPr) using a sterically demanding NHC (IPr) is more challenging and is synthetically only accessible via WOCl4(IPr) (Scheme 90B). The extremely moisture-sensitive monoadduct of WOCl4 and IPr readily reacts with trace amounts of water in toluene and crystalline WO2Cl2(IPr) precipitates from solution. It was subjected to single crystal X-ray analysis, which revealed that the complex adopts a slightly distorted trigonal bipyramidal structure with the NHC being in the apical position trans to one chloride. The strong s-donor properties of the NHC lead to a longer WdCltrans bond (236.7(1) pm) compared to the WdCl bond in the equatorial plane (234.6(1) pm). The WdC(NHC) bond distance (222.3(4) pm) was found to be at the lower end of the usual range of tungsten NHC complexes. A reason for that can be attributed to the less sterically crowded coordination sphere leading to a strong carbene-metal interaction.200 Interestingly, WO2Cl2(IPr) could not be synthesized from WO2Cl2 and the free carbene directly; instead, its synthesis only led to an inseparable mixture.

Scheme 90 (A) First W(VI) NHC complexes; (B) synthesis of W (VI) mono-NHC complexes. Modified from Dodds, C.A.; Spicer, M.D.; Tuttle, T. Tungsten (VI) N-Heterocyclic Carbene Complexes: Synthetic, Structural, and Computational Study. Organometallics 2011, 30, 6262–6269.

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6.02.8

261

Summary

In summary, the chemistry of group 5 and 6 N-heterocyclic carbene and mesoionic carbene complexes has already gone a long way. Both, low- and high-oxidation state compounds are available. Surprisingly, in view of the vast number of compounds that exist, their use, e.g., in catalysis, appears with the exception of high oxidation state Mo and W alkylidene NHC complexes, is still somewhat less developed. However, in view of the most favorable chemistry of NHCs and mesoionic carbenes as such and the intriguing properties of the corresponding group 5 and 6 metal complexes, this can be expected to change in the near future. This might particularly be true for low-valent group 5 and 6 NHC and mesoionic carbene complexes, respectively.

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Clement, N. D.; Cavell, K. J. Angew. Chem. Int. Ed. 2004, 43, 3845–3847. Gründemann, S.; Albrecht, M.; Kovacevic, A.; Faller, J. W.; Crabtree, R. H. Dalton Trans. 2002, 2002, 2163–2167. Viciano, M.; Mas-Marzá, E.; Poyatos, M.; Sanaú, M.; Crabtree, R. H.; Peris, E. Angew. Chem. Int. Ed. 2005, 44, 444–447. Viciano, M.; Poyatos, M.; Sanaú, M.; Peris, E.; Rossin, A.; Ujaque, G.; Lledós, A. Organometallics 2006, 25, 1120–1134. Roberts, J. A. S.; Franz, J. A.; van der Eide, E. F.; Walter, E. D.; Petersen, J. L.; DuBois, D. L.; Bullock, R. M. J. Am. Chem. Soc. 2011, 133, 14593–14603. Winemiller, M. D.; Kelsch, B. A.; Sabat, M.; Harman, W. D. Organometallics 1997, 16, 3672–3678. Pittard, K. A. Activation of Carbon-Hydrogen Bonds Mediated by Ru (II) Complexes; PhD Thesis North Carolina State University: Raleigh, NC, 2007. Graham, P. M. Synthesis and Reactivity of Tungsten Dearomatization Agents; University of Virginia, 2005. Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. J. Am. Chem. Soc. 2001, 123, 12724–12725. Cobbledick, R.; Dowdell, L. Can. J. Chem. 1979, 57, 2285. Jeffery, J. C.; Moore, I.; Razay, H.; Stone, F. G. A. Chem. Commun. 1981, 1255–1258. Curtis, M. D.; Messerle, L.; D’Errico, J. J.; Solis, H. E.; Barcelo, I. D.; Butler, W. M. J. Am. Chem. Soc. 1987, 109, 3603–3616. Fornies, J.; Menjon, B.; Gomez, N.; Tomas, M. Organometallics 1992, 11, 1187–1193. Howard, J. A. K.; Jeffery, J. C.; Li, S.; Stone, F. G. A. Dalton Trans. 1992, 627–634. Shiu, K.-B.; Yeh, L.-Y.; Peng, S.-M.; Cheng, M.-C. J. Organomet. Chem. 1993, 460, 203–211. Bühl, M.; Mauschick, F. T. Organometallics 2003, 22, 1422–1431. Roberts, J. A. S.; Appel, A. M.; DuBois, D. L.; Bullock, R. M. J. Am. Chem. Soc. 2011, 133, 14604–14613. Tang, J.; Dopke, J.; Verkade, J. G. J. Am. Chem. Soc. 1993, 115, 5015–5020. Ryan, O. B.; Tilset, M.; Parker, V. D. J. Am. Chem. Soc. 1990, 112, 2618–2626. Song, J. S.; Szalda, D. J.; Bullock, R. M.; Lawrie, C. J.; Rodkin, M. A.; Norton, J. R. Angew. Chem. Int. Ed. 1992, 31, 1233–1235. Song, J.-S.; Szalda, D. J.; Bullock, R. M. Organometallics 2001, 20, 3337–3346. Haines, R. J.; Nyholm, R. S.; Stiddard, M. H. B. J. Chem. Soc. A: Inorg., Phys., Theor. 1968, 43–46. Haines, R.; Nolte, C. J. Organomet. Chem. 1970, 24, 725–736. Arena, C. G.; Faraone, F.; Fochi, M.; Lanfranchi, M.; Mealli, C.; Seeber, R.; Tiripicchio, A. Chem. Commun. 1992, 1847–1853. Adams, R.; Cotton, F. A. Inorg. Chim. Acta 1973, 7, 153–156. Linehan, J. C.; Yonker, C. R.; Addleman, R. S.; Autrey, S. T.; Bays, J. T.; Bitterwolf, T. E.; Daschbach, J. L. Organometallics 2001, 20, 401–407. Peters, J.; George, M. W.; Turner, J. J. Organometallics 1995, 14, 1503–1506. Scott, S. L.; Espenson, J. H.; Zhu, Z. J. Am. Chem. Soc. 1993, 115, 1789–1797. Dodds, C. A.; Spicer, M. D.; Tuttle, T. Organometallics 2011, 30, 6262–6269.

6.03

N-Heterocyclic and Mesoionic Carbene Complexes of Group 7 Metals

Beatriz Royo, Sara Realista, and Sofia Friães, ITQB NOVA, Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de Lisboa, Oeiras, Portugal © 2022 Elsevier Ltd. All rights reserved.

6.03.1 6.03.2 6.03.2.1 6.03.2.2 6.03.2.2.1 6.03.2.2.2 6.03.2.2.3 6.03.2.3 6.03.2.4 6.03.3 6.03.4 6.03.4.1 6.03.4.2 6.03.4.2.1 6.03.4.2.2 6.03.4.2.3 6.03.4.3 6.03.4.4 6.03.4.5 6.03.5 References

Introduction Manganese NHC complexes Mn(0) NHC complexes Mn(I) NHC complexes Mn(I) carbonyl complexes with monodentate NHCs Mn(I) tricarbonyl complexes with bidentate NHCs Mn(I) carbonyl complexes with tridentate NHCs Mn(II), Mn(III), Mn(IV), and Mn(V) NHC complexes Mn triazolylidene complexes Technetium NHC complexes Rhenium NHC complexes Re(0) NHC complexes Re(I) NHC complexes Re(I) tetracarbonyl complexes with monodentate NHC ligands Re(I) tricarbonyl complexes with monodentate NHC ligands Re(I) tricarbonyl complexes with bidentate chelating NHCs Re(V) NHC complexes Re(VII) NHC complexes Rhenium triazolylidene complexes Conclusions

265 265 265 267 267 271 272 273 276 277 279 279 280 280 281 284 291 294 295 296 297

Abbreviations 1,2,3-trz 1,2,4-trz bipy Bn cAAC Cp Cy DFT dvtms Eqv. Dipp IEt2Me2 IiPr IiPr2Me2 IMe IMe2Me2 IMes IR KHMDS LC LLCT Mes MLCT NBO OLED OTf SIMe tBu

264

1,2,3-Triazol-5-ylidene 1,2,4-Triazol-5-ylidene 2,20 -Bipyridine Benzyl Cyclic alkyl(amino) carbene Cyclopentadienyl Cyclohexyl Density functional theory Divinyltetramethyldisiloxane Equivalent 1,3-Bis(2,6-diisopropylphenyl)imidazol-2-ylidene 1,3-Diethyl-4,5-dimethylimidazol-2-ylidene 1,3-Diisopropylimidazol-2-ylidene 1,3-Diisopropyl-4,5-dimethylimidazol-2-ylidene 1,3-Dimethylimidazol-2-ylidene 1,3,4,5-Tetramethylimidazol-2-ylidene 1,3-Bis(2,4,6-trimethylphenyl)imidazol-2-ylidene Infrared Potassium bis(trimethylsilyl)amide Ligand centered Ligand-to-ligand charge transfer Mesityl Metal-to-ligand charge transfer Natural bond orbital Organic light emitting diode Triflate 1,3-Dimethylimidazolin-2-ylidene tert-Butyl

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00020-2

N-Heterocyclic and Mesoionic Carbene Complexes of Group 7 Metals

6.03.1

265

Introduction

The organometallic chemistry of Group 7 metals has developed continuously since the third edition of COMC. In particular, the N-heterocyclic carbene (NHC) chemistry of manganese and rhenium expanded rapidly during the last 15 years. As a novelty compared to previous editions, the COMC-IV incorporates a chapter solely dedicated to this vibrant field of research, N-heterocyclic carbenes of Group 7, including imidazolium derived NHCs and mesoionic carbene ligands. The use of N-heterocyclic carbenes as ligands to several metals across the periodic table, has greatly impacted the field of organometallic chemistry. In comparison with other transition metals, the NHC chemistry of Mn, Tc, and Re has remained less studied. The past decade has witnessed a regained attention in the development of manganese and rhenium complexes with NHC ligands, which has led to structurally diverse coordination compounds with interesting properties. Several aspects and properties of Group 7 NHC complexes have been discussed in various review articles that have appeared in the literature during the last years.1–3 This chapter describes the organometallic chemistry of Group 7 metals with N-heterocyclic carbenes over the period 2006–2020. It is organized by metals, and it is divided in different sections according to the metal oxidation state and denticity of the NHC ligand. Metal complexes featuring triazolylidene ligands are presented in a separate section for each metal. This review deals with the synthetic aspects, along with structural features and reactivity of Group 7 NHC. Potential applications of these complexes are not discussed in this chapter.

6.03.2

Manganese NHC complexes

The chemistry of Mn NHC has undergone remarkable development during the last years. Although the field is dominated by Mn complexes in oxidation state +1, several manganese complexes in oxidation state 0 have been described, including the stabilization of Mn(0) complexes with low coordination numbers. Only few examples of Mn(II) NHC complexes are known. Among them, low coordinate dialkyl complexes displayed singular reactivity. In terms of the higher oxidation states, very few Mn(III), Mn(IV) and Mn(V) complexes have been described, in all cases stabilized by multidentate NHC ligands as referred in recent reviews.2,4–6

6.03.2.1

Mn(0) NHC complexes

The chemistry of Mn(0) NHC is rather underdeveloped. The majority of the few examples reported in the literature contain carbonyls as co-ligands, and they are accessed using the readily available manganese dimer Mn2(CO)10 as precursor. Free NHC ligands react with Mn2(CO)10 to form a mixture of axially substituted bimetallic Mn(0) [(NHC)Mn(CO)4dMn(CO)5] (1) and monometallic Mn(I) [(NHC)Mn(CO)4X] (2) (X ¼ Cl, Br, I) complexes (Scheme 1).7 Their molecular structures show that the carbonyl group located trans to the Mn(CO)5 fragment or to the halide ligand, displays the shortest MdCO bond length. Intramolecular hydrogen-halide interactions between the N-substituent groups and the halide were observed in 2. These interactions have been quantified by DFT NBO studies. The DFT study was extended to include the determination of Wiberg bond indices, the percentage buries volume (%Vbur), and the solid angle parameters of the complexes. These studies indicate that the NHC ligands behave as strong s-donor ligands in 2, with lower donor interactions than those observed in parent rhenium complexes.

Scheme 1

Bimetallic carbonyl Mn(0) NHC complexes have also been isolated using the tridentate non-symmetrical pyridine-NHCphosphane (NCP) ligand depicted in Scheme 2. Treatment of free NCP ligand with Mn2(CO)10 under UV irradiation forms the binuclear complex [Mn2(CO)7(NCP)] (3), where the NCP ligand exhibits an unusual tridentate m-k2-C,N-M;k1-P-M coordination mode (Scheme 2).8 The MndMn dimer bond length (2.9584 A˚ ) is slightly longer than in Mn2(CO)10 precursor (2.9078 A˚ ) and comparable to the MndMn distance observed in 1a (2.9576 A˚ ). The MndCNHC bond length in 3 (1.9700 A˚ ) is the shortest

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N-Heterocyclic and Mesoionic Carbene Complexes of Group 7 Metals

MndCNHC reported for MndNHC complexes, only comparable with the values found in half-sandwich Mn(I) NHC complexes tethered to a Cp ligand (1.963 A˚ ).

Scheme 2

Stabilization of manganese atoms in low oxidation state and with low coordination number is a challenging task. One of the few examples is the two-coordinate manganese complex Mn(cAAC)2 (4) (cAAC ¼ cyclic alkyl(amino) carbene) prepared by reacting MnCl2 with free cAAC ligand, and the subsequent reduction of the formed Mn(cAAC)Cl2 with KC8 in the presence of another equivalent of free cAAC, Scheme 3.9 The cAAC ligand is a higher stronger s-donor and better p-acceptor than classical imidazolium-derived NHCs, and it has been proved to be an excellent ligand to stabilize metal complexes with low coordination numbers.10 Complex 4 presents a linear geometry with a CdMndC angle of 180 , and displays a magnetic moment of 4.15 mB (at room temperature), which reflects a total spin ground state of S ¼ 3/2. Spectroscopic and theoretical studies indicate that the pronounced Mn-to-CAAC backdonation endowed the Mn(I) or even Mn(II) nature of this formal Mn(0) complex. The main contribution of S ¼ 3/2 originates from the antiferromagnetic coupling between S ¼ 2 MnI and one S ¼ 1/2 radical spin that is delocalized on two carbene carbon atoms. Another important contribution to the quartet state comes from the antiferromagnetic coupling between the central S ¼ 5/2 MnII and the two S ¼ 1/2 radical spins. Consequently, the cAAC ligand possess somewhat radical anion character. Interaction of 4 with molecular H2 causes the conversion of the cAAC ligands into alkyl ligands, giving Mn(cAACH)2 species, with a spin ground state of S ¼ 5/2 (Scheme 3).

Scheme 3

The only other example of a Mn complex in low oxidation state and with low coordination number is the three-coordinate formal Mn(0) complexes with NHC and alkene ligation [(NHC)Mn(Z2:Z2-dvtms)] (5) (dvtms ¼ divinyltetramethyldisiloxane; NHC ¼ IDipp (5a), IMes (5b), or Et2-cAAC (5c)). Complexes 5 are readily prepared from one pot reaction of free NHCs with MnCl2, dvtms, and KC8 (Scheme 4). Spectroscopic studies and theoretical calculations reveal an S ¼ 3/2 ground-spin state and feature highly covalent metal-ligand bonding arising from pronounced metal-to-alkene p-backdonation.11 Complexes 5 exhibit

N-Heterocyclic and Mesoionic Carbene Complexes of Group 7 Metals

267

intriguing reactivity of reductive coupling with alkenes and alkynes to form Mn(II) dialkyl complexes 6. These reactions represent the first examples of reductive couplings of alkenes and alkynes mediated by well-defined Mn complexes, and constitute useful methods for CdC bond formation. Moreover, complexes 5 show diversified reactivity toward H2O, H2, CO, and I2. The dvtms ligand is protonated by H2O to give the mono-alkene EtMe2SiOSiMe2CH]CH2. Complex 5a can be hydrogenated with H2 to form the Mn(II) dialkyl compound 7, and oxidized by I2 to give [Mn(IDipp)(THF)I2] (8). It undergo ligand substitution reaction with CO to form the Mn(0) carbonyl complex [Mn2(CO)8(IDipp)2] (9). The related monodentate NHC complex of manganese [(IDipp)Mn(Z2-CH2CHPh)2] (10) with alkene ligation is prepared following the same procedure, one-pot reaction of MnCl2 with styrene, NHC and KC8 (Scheme 4), and it shows similar electronic behavior than 5 with a S ¼ 3/2 ground-state spin at the Mn center.12

Scheme 4

6.03.2.2 6.03.2.2.1

Mn(I) NHC complexes Mn(I) carbonyl complexes with monodentate NHCs

The oxidation state +1 is by far the most common oxidation state in MndNHC chemistry. In fact, the first MndNHC complexes reported in the literature were the Mn(I) complexes CpMn(SIMe)(CO)2 and Mn(CO)3Br(SIMe)2 described by Lappert and Pye in 1977.13 Both complexes were obtained by heating the dimer of the saturated carbene 1,3-dimethylimidazolin-2-ylidene (SIMe)2 with CpMn(CO)3 or Mn(CO)3Br(PPh3)2 at 180  C for several hours. Despite the high temperature used in the synthesis, low yields of the target compounds were obtained ( 5d of the transition metals,8,9 the Mg/Zn reductive carbonylation of niobium(V) takes place at room temperature and atmospheric pressure of carbon monoxide.6 As a personal experimental note, it has to be stressed that, although the preparation of hexacarbonylmetalates by this route is neither an environmental friendly nor an easy procedure, the sight of 2–3 cm long yellow-orange crystals growing under your eyes during the recrystallization procedure (slow cooling of a warm THF-saturated solution) rewards all the experimental difficulties encountered. Note that the simultaneous use of both magnesium and zinc powders is beneficial to the carbonylation reaction. In fact, parallel experiments carried out with magnesium alone, with zinc alone, and with zinc and magnesium together showed that the rate of CO absorption and the rate of formation of the soluble [M(CO)6]− salts are slower in the first two experiments than in the third one. Particularly interesting is the case of zinc alone, where the CO absorption was somewhat inhibited when the CO/Nb molar ratio was around 1.5. Probably, the observed synergic effect of the magnesium/zinc combination is due to a preferential adsorption of pyridine on the zinc surface, in agreement with the considerably higher affinity of Zn2+ for amines as compared to that of Mg2+.10 Of course, the notable electropositive character of magnesium as compared to that of zinc11 will finally force the electron transfer to prevalently take place on magnesium. Consistently, it was found that magnesium is predominant in solution with respect to zinc, thus suggesting that the reduction of niobium(V) is largely ascribable to magnesium. Pyridine plays an important role in the reductive carbonylation of pentavalent metals. This is presumably due to the formation of the unstable pyridine radical anion in steady-state concentrations during the reaction. The radical anion would then be responsible for the electron-transfer process in a homogeneous phase. It is known that sodium metal and pyridine react to form solutions whose dark blue colors are attributed to the formation of the 4,40 -bipyridyl radical anion.12 On the other hand, it was reported, in relation to the reductive carbonylation of chromium(III) compounds to chromium hexacarbonyl, that magnesium reacts with pyridine to give sparingly soluble blue products, presumably containing the radical anion of pyridine or the dehydrogenated coupling product.13 The alkali metal naphthalenide or 1-methylnaphthalenide route to the hexacarbonylmetalate ions of niobium and tantalum at atmospheric pressure of CO, Scheme 2,7 probably proceeds through labile naphthalene or 1-methylnaphthalene intermediates, which readily react with CO at low temperature. Since NbCl5 undergoes facile hydrolysis and is prone to add molecules of ether solvents at ambient temperatures,14 [NbCl4(THF)2] was selected as a more convenient starting material in the carbonylation reaction mediated by the radical anions of naphthalene or anthracene. Yields as high as 60%–70% of [Nb(CO)6]− sodium salt were thus obtained.15 Finely divided sodium in the presence of a catalytic amount of 1,3,5,7-cyclooctatetraene (COT) in THF provides up to 85% yields of Na[V(CO)6] at atmospheric pressure of CO, Scheme 3. Preparations of Na[V(CO)6] as large as 10–15 g are possible by using commercially available glassware.16 When 99% CO-enriched is used, 13C-labeled sodium hexacarbonylvanadate, Na [V(13CO)6] [nCO (thf ) ¼ 1844sh, 1816vs, 1783w cm−1] is obtained.17

302

Organometallic Complexes of Group 5 With p-Acidic Ligands

Scheme 3 Improved preparation of sodium hexacarbonylmetalates by Calderazzo and Pampaloni. From Calderazzo, F.; Pampaloni, G. J. Organomet. Chem. 1983, 250, C33–C35.

An alternative atmospheric pressure route to [V(CO)6]− was developed involving alkali metal anthracenide or naphthalenide reductive carbonylations of VCl3,18 Scheme 4.

Scheme 4 Improved preparation of sodium hexacarbonylvanadate by Ellis and coworkers. From Barybin, M. V.; Pomije, M. K.; Ellis, J. E. Inorg. Chim. Acta 1998, 269, 58–62.

The yield of the reaction is higher using sodium anthracenide rather than sodium naphthalenide. This has been related to the higher reducing power of the [V(C10H8)2]− intermediate with respect to the anthracene homologue and, in general, to the increase of the reducing power of the intermediate on decreasing the number of fused rings in the ligands. In fact, the bis-benzene anion [V(C6H6)2]−, as obtained by the reduction of [V(C6H6)2] with finely divided potassium, rapidly reacts with CO with quantitative regeneration of [V(C6H6)2] and formation of some ill-defined cyclic oligomers derived from the CO radical anion.19 Hexacarbonylvanadate(−I) salts form upon reaction of [V(mes)2], mes ¼ Z6-(1,3,5-C6H3Me3), with CO at high pressure (100 atm) and 35 C during 20 h,20 or by carbonylation of the vanadium(II) compound [V(DIPP)4{Li(THF)}2] (HDIPP ¼ 2,6-diisopropylphenol) at room temperature and atmospheric pressure,21 Scheme 5.

Scheme 5 Preparation of hexacarbonylvanadate salts according to Calderazzo and Cini and Armstrong and coworkers. From Calderazzo, F.; Cini, R. J. Chem. Soc. 1965, 818–819 and Scott, M.J.; Wilisch, W.C.A.; Armstrong, W.H. J. Am. Chem. Soc. 1990, 112, 2429–2430.

An electrochemical reduction of vanadium acetylacetonate under pressure (117 atm) of carbon monoxide at room temperature produces [NBun4][V(CO)6] in 75% yield.22 Moreover, protonated nitrogen base derivatives of [V(CO)6]− can be obtained in aqueous solution,23,24 Eq. (1).    − BH + + V ðCOÞ6 ! BH V ðCOÞ6 (1) BH+ ¼ protonated nitrogen base such as alkyl substituted pyridinium cation. A viable route to [Nb(CO)6]− (50% yield) is given by the reduction of [Nb(mes)2], mes ¼ Z6-(1,3,5-C6H3Me3), by means of potassium with bubbling of CO in DME.25 Similarly, the reaction of [Nb(mes)2] with CO, but in the absence of the reductant, yields [Nb(mes)2(CO)][Nb(CO)6] at ambient temperature and pressure.26,27 If, instead, the latter reaction is performed in the presence of one equivalent of [CoCp 2] (Cp ¼ Z5-C5Me5), the carbonylation of niobium is quantitative and the hexacarbonylmetalate

Organometallic Complexes of Group 5 With p-Acidic Ligands

303

[Nb(CO)6]− is isolated as its [CoCp 2]+ salt (75% yield).27,28 Carbonylation of the naphthalenide compound [Na(THF)] [Ta(C10H8)3] (generated from TaCl5), at 1 atm pressure, gives salts of [Ta(CO)6]− in quantitative yield.29 The 99% 13CO-labeled [Ta(13CO)6]− [nCO (DME) ¼ 1818 cm−1] was afforded as its [Na(dyglime)]+ salt by this route.30 The highly reduced vanadium carbonyl Na3[V(CO)5], which formally contains vanadium in its lowest known oxidation state (−III), can be obtained by further reduction of [Na(diglyme)2][V(CO)6] in liquid ammonia at −78 C.31,32 Addition of alkali metal iodide allows the isolation of the corresponding M3[V(CO)5] salts (M ¼ K, Rb, Cs), Scheme 6.33 Both niobium and tantalum complexes, [Na(diglyme)2][M(CO)6], can be smoothly reduced with sodium in liquid ammonia to provide unstable Na3[M(CO)5], which are metathesized to afford shock-sensitive, explosive Cs3[M(CO)5] in moderate yields, Scheme 6.34,35 Garry Warnock, who first obtained Na3[Nb(CO)5], left this comment on his notebook: “The main sample (c. 0.9 g) completely exploded when I attempted to powder the big lumps. An enormous BANG, louder than I have ever experienced before [occurred during this operation]”.31

Scheme 6 Synthetic routes to group 5 metals (oxidation state ¼ − III) carbonylates. From Ellis, J. E.; Palazzotto M. C. J. Am. Chem. Soc. 1976, 98, 8264.

Sodium hexacarbonylates of V−I, Nb−I, and Ta−I are massively water soluble, lemon-yellow to yellow-orange compounds, stable at room temperature under rigorously inert atmosphere and forming large crystals (as long as 5 cm) from saturated THF solutions. The thermal stability qualitatively appears to decrease in the order V > Ta > Nb. Decomposition of the niobium and tantalum species was observed during attempts to eliminate the sodium-coordinated molecules of ether solvent from the crystalline materials, by maintaining them under vacuum at room temperature.36 On the other hand, Na[V(CO)6] can be easily obtained as solvent free by the same procedure even at temperatures as high as 60–70 C.37 Being highly reduced organometallics, hexacarbonylmetalates(−I) are not stable in the presence of dioxygen, but their behavior strongly depends on the cation. By metathetical exchange with bulky cations (PPN+ ¼ Ph3P]N]PPh+3, Bu4N+, Ph4P+, Ph4As+),16,18 derivatives are obtained, which may be handled in air for short period of times. Contrastingly, the alkali metal salts, especially the solvent-free compounds, may instantaneously burn in the presence of traces of air. The molecular structures of the [M(CO)6]− anions have been elucidated as miscellaneous salts in the solid state at 298 K.6,24,38–49 The [PPN]+ salts are isostructural and crystallize in the rhombohedral space group R3, with the anion lying on an improper 3 axis. The coordination geometry of the anion is that of an essentially ideal octahedron [MdC: 1.931(9) A˚ , M ¼ V46; 2.089(5) A˚ , M ¼ Nb6; 2.083(6) A˚ , M ¼ Ta6; CdMdC: 90.8(4), 89.2(4) , M ¼ V; 89.2(2) , M ¼ Nb; 89.1(2) , M ¼ Ta], with the CO groups nearly collinear with the MdC bonds [NbdCdO ¼ 177.8(5) , TadCdO ¼ 177.9(5) ]. In the molecular structure of [Na(THF)6] [V(CO)6] (space group R3),45 interatomic distances [VdC 1.91(2), CdO 1.18(3) A˚ ] and angles [VdCdO 174(2) ] in the anion compare well with what assessed for the relevant [PPN]+ salt.46 Spectroscopic (multinuclear NMR, emission, excitation and vibrational spectra)50–65 and DFT studies66–68 have been carried out on [M(CO)6]− species with different cations. The nature of the bonding in the isoelectronic series [V(CO)6]−, Cr(CO)6, [Mn(CO)6]+ has been investigated by means of semiempirical MO calculations.69 Moreover, density functional theory has been used to calculate dissociation energies, vibrational frequencies, and 13C NMR chemical shifts of a series of isoelectronic metal hexacarbonyls of the third transition series. Thus, the first CO ligand dissociation energy DH follows the order Ir > Re  Os > Hf  Ta  W. A partition of DH into contributions from the CO to metal s-donation and the metal to CO p-back-donation reveals that this trend is the result of a stronger s-donation in the more oxidized systems. Within a triad, the 4d metal forms the weakest MdCO bond. The calculated CO stretching frequencies increase from [Hf(CO)6]2− to [Ir(CO)6]3+. The calculated contribution to DH from the p-back-donation reveals the expected decline in the cases of the more positively charged systems.66,67 IR and Raman spectra of [V(CO)6]− have been studied in detail,70–72 the fundamental vibrations have been assigned, and the CO stretching force constants calculated.72,73 Table 1 reports the stretching vibration of the carbonyl ligand in Na[M(CO)6] in different solvents, and the corresponding spectra are visualized in Fig. 1. Infrared spectroscopy is a valid tool to qualitatively test anion/cation interactions. As it can be seen, when the solvent is pyridine or water, only one absorption is detected, consistently with the octahedral structure of the hexacarbonylmetalate. In solvents with lower dielectric constants, such as THF or diethyl ether, additional bands are observed, due to the distortion effect of the regular octahedron by the countercation. This is a well-established phenomenon for alkali cations interacting with carbonylmetalate anions.72,74–83 Salts of [V(CO)6]− based on protonated or methylated nitrogen heterocycles as the cation are afforded in high yields in aqueous solution by combination of sodium hexacarbonylvanadate(−I) with pyridinium, N-methyl-pyridinium, quinolinium, or acridinium salts; they are deeply colored (brown to black) both in the solid state and in the solvents of low polarity. Such deep colors are due to charge-transfer bands from the anion to the cation. The color fading to yellow, observable on going from the solid state to acetone solution, is explained with a weakening of the cation-anion interactions on increasing the solvent polarity.23 According to

304

Organometallic Complexes of Group 5 With p-Acidic Ligands

Table 1 Infrared absorption bands of sodium hexacarbonylmetalates(−I) of group 5 metals in the carbonyl stretching region (m, medium; s, strong; vs, very strong; sh: shoulder). nCO (cm−1) Solvent

V

Nb

Ta

Et2O

1923m 1877vs 1775s 1880sh 1851vs

1908m 1876vs 1778s 1887sh 1859vs 1835s 1857vs 1875vs

1907m 1875vs 1777s 1887sh 1857vs 1827s 1867vs 1867vs

THF

Pyridine H2O

1851vs 1862vs

Fig. 1 Collection of infrared spectra of Na[M(CO)6] in the carbonyl stretching region (A: M ¼ V; B: M ¼ Nb; C: M ¼ Ta; reference, water vapor).

X-ray diffraction data, the sublimable pyridinium derivative [C6H5NH][V(CO)6]23 is an ionic compound, with the NH group preferentially interacting with the four equatorial carbonyl ligands of the anion, while the axial carbonyls are oriented to the p-system of the pyridine ring (Fig. 2). Cation-anion interactions are mainly of the hydrogen-bond type, and the polarity of diethyl ether or THF is insufficient to overcome them. Coherently, more than one IR-active carbonyl stretching vibration is observed in solvents of relatively low dielectric constant. In acetone, the hydrogen-bonding is mainly with the solvent; therefore, the octahedral symmetry of [V(CO)6]− is restored.23 On considering the IR spectra in water (Table 1), where the anions possess the highest symmetry and thus are featured by a single carbonyl stretching vibration, it can be concluded that niobium hexacarbonylate exhibits the highest nCO value of the series. This is in agreement with the general observation that, within families of isostructural compounds, the nCO follows the trend 3d < 4d > 5d.9 Production of metal carbonyl cations [M(CO)7]+ by laser vaporization in a supersonic expansion of pure CO and examination of the gas phase by IR photodissociation spectroscopy84 pointed out that seven-coordinated structures are possible for Nb+ and Ta+ but not for V+. However, these niobium and tantalum species remained elusive until Krossing and coworkers selected the silver salt Ag [Al(ORF)4], RF ¼ C(CF3)3, as a convenient oxidant for [NEt4][M(CO)6], Scheme 7.85 Depending on the experimental conditions, the neutral complex 1 and the ionic compounds 2a-b were obtained. Both types of products are very reactive and only a limited

Organometallic Complexes of Group 5 With p-Acidic Ligands

305

O C V

O C

C, N

Fig. 2 Solid-state interactions between [C5H5NH]+ and [V(CO)6]− ions in the related pyridinium salt, as detected by X-ray diffraction. Reproduced from reference Calderazzo, F.; Pampaloni, G.; Lanfranchi, M.; Pelizzi, G. J. Organomet. Chem. 1985, 296, 1–13 by permission of Elsevier.

number of solvents are compatible (essentially, 1,2-difluorobenzene and 1,2,3,4-tetrafluorobenzene). The tantalum species 2b is thermally more stable than the niobium congener 2a.85 More details about 1 will be supplied in Section 6.04.2.2.

Scheme 7 Synthesis of homoleptic niobium and tantalum carbonyl species by Krossing and coworkers. From Unkrig, W.; Schmitt, M.; Kratzert, D.; Himmel, D.; Krossing, I. Nat. Chem. 2020. https://doi.org/10.1038/s41557-020-0487-3.

The ionic compounds 2a–b could not be separated from the stoichiometric coproduct [NEt4][Al(ORF)4]−; however, the former were unambiguously characterized by IR, Raman, and NMR spectroscopy, and by single crystal X-ray diffraction. A view of the structure of the cation of 2a is shown in Fig. 3A. By keeping the reaction of [NEt4][Nb(CO)6] with Ag[Al(ORF)4] at −30 C and inducing rapid crystallization, [Ag6{Nb(CO)6}4] [Al(ORF)4]2 (3) was isolated. The cation in 3 is a cluster built up of six Ag+ units forming a regular octahedron and four [Nb(CO)6]− moieties (Fig. 3B), and is a presumable intermediate along the formation of 2a.85

6.04.2.2

Synthesis and properties of V(CO)6 and other homoleptic neutral compounds

A variety of isolable homoleptic metal carbonyls are known to contain metal centers in the oxidation state 0, and the 18-electron rule is obeyed by these compounds with remarkable frequency. According to the rule, mononuclear complexes are expected for transition metals with even atomic numbers, for example, [Cr(CO)6], [Fe(CO)5], [Ni(CO)4]. Otherwise, transition metals with odd atomic numbers cannot obey the 18-electron rule in monomeric structures. Therefore, carbonyls of manganese and cobalt reach stability as dinuclear species with a metal-metal bond, that is, [Mn2(CO)10] and [Co2(CO)8] respectively. Vanadium hexacarbonyl, [V(CO)6], represents a unique exception to this trend. The hypothetical 36-electron dinuclear complex [V2(CO)12], proposed in 1960 by Pruett and Wyman as obtained from vanadium ditoluene and CO,86 would feature a VdV bond and two seven-coordinated vanadium atoms; such bonding situation is unstable with respect to the dissociation to mononuclear [V(CO)6] essentially due to steric reasons, as suggested by both theory87 and experiment.88 Prepared for the first time in 195989 by acidification of a Et2O/water mixture, [V(CO)6] è un solido cristallino di colore verde-nero e di odore caratteristico, ingrato, simile a quello del cobalto carbonile (V(CO)6 is a crystalline, black-green solid of characteristic, nasty, smell similar to that of cobalt octacarbonyl). It can be stated that [V(CO)6] is the most unstable and reactive among all the isolable neutral homoleptic carbonyls of the 3d metals, and thus requires special care in handling because of its high sensitivity towards light, air, heat (it decomposes at c. 60–70 C89b) and polar organic solvents.90 In fact, [V(CO)6] easily undergoes reduction to the thermally stable, water soluble, 18-electron hexacarbonylvanadate anion [V(CO)6]−, see Eq. (2).91,92

306

Organometallic Complexes of Group 5 With p-Acidic Ligands

Fig. 3 Views of the X-ray structures of the cation [M(CO)7]+ in [M(CO)7][Al{OC(CF3)3}4], 2a (A), and the cation [Ag6{Nb(CO)6}4]2+ in [Ag6{Nb(CO)6}4][Al{OC(CF3)3}4]2, 3 (B).85 H atoms have been omitted for clarity, Ag atoms are drawn in violet. From Unkrig, W.; Schmitt, M.; Kratzert, D.; Himmel, D.; Krossing, I. Nat. Chem. 2020. https://doi.org/10.1038/s41557-020-0487-3.

 −   V ðCOÞ6 + e − ! V ðCOÞ6 18 e −

(2)

17 e −

On the other hand, the formal electrode potential, E , for the monoelectronic oxidation of [V(CO)6]− was measured with respect to the [FeCp2]+/[FeCp2] couple, resulting −0.35 V in CH3CN solution (using 0.2 M [NBu4][BF4] as the supporting electrolyte).93 As a matter of fact, [V(CO)6] is typically generated by chemical oxidation of the hexacarbonylvanadate, the most common oxidizing agent being the proton from a Brønsted acid (HCl94 or 100% orthophosphoric acid22,95), in pentane at −78 C, see Eq. (3).37     (3) Na VðCOÞ6 + HClðgÞ ! V ðCOÞ6 + NaCl + ½H2 60% 37

It was found that the reaction in Eq. (3) proceeds through a precursor (“precursor I”) which rapidly decomposes to H2 and [V(CO)6]. Further, it was demonstrated that precursor I was also produced in rigorously anhydrous diethyl ether, immediately converting into “precursor II” by a deliberately added small amount of water. “Precursor II” is featured by IR absorptions at 1910 m, 1872s and 1812w cm−1, which are typical of the [V(CO)6]− anion, Scheme 8.

Scheme 8 Hypothesized species involved in the synthesis of [V(CO)6] by oxidation of [V(CO)6]− with a Brønsted acid. Calderazzo, F.; Pampaloni, G.; Vitali, D. Gazz. Chim. Ital. 1981, 111, 455–458.

The nature of “precursor I” is uncertain, although this is undoubtedly, as “precursor II”, an onium derivative. The formation of [V(CO)6] may therefore be conceived as a one electron transfer from [V(CO)6]− to H+ with formation of molecular hydrogen, according to Eq. (4).  −   ½H3 OŠ + V ðCOÞ6 ! V ðCOÞ6 + H2 O + ½H2 (4) Alternatively, vanadium hexacarbonyl can be obtained via the direct displacement of naphthalene by carbon monoxide from bis(naphthalene)vanadium(0) at normal pressure and 20 C,96 or by cocondensation of vanadium atoms with CO at 6–12 K at c. 105 V/CO ratio. By using carbon monoxide diluted with argon, krypton or xenon, [V(CO)n] (n ¼ 1–6) species are variably produced, whose structures have been elucidated by IR spectroscopy.88,97 Furthermore, a redox process is viable when [Na(diglyme)2][V(CO)6] is treated with tropylium bromide in aqueous solution or in isooctane: [V(CO)6] is produced together with ditropyl, Scheme 9.98

Organometallic Complexes of Group 5 With p-Acidic Ligands

307

Scheme 9 Synthesis of [V(CO)6] by oxidation of [V(CO)6]− with tropylium bromide. Werner, R. P. M.; Manastyrskyj, S. A. J. Am. Chem. Soc. 1961, 83, 2023–2024.

Vanadium hexacarbonyl is a blue-black solid which easily sublimes under vacuum at c. 50 C, is insoluble in water and slightly dissolves in aliphatic hydrocarbons (c. 4.8 g/L L of hexane) to give yellow solutions. Only saturated hydrocarbons can be regarded as unreactive, and the key to the success of the preparation of [V(CO)6] from Na[V(CO)6] and hydrogen chloride in pentane is in fact the formation of an oversaturated solution.99 The solubility in dichloromethane is a bit higher, but the resulting solution is not stable with time even in the absence of light. The deep color of solid [V(CO)6] has been attributed to light excitation to [V(CO)6]+[V(CO)6]− state.101 Being isostructural to [M(CO)6] (M ¼ Cr, Mo), [V(CO)6] cocrystallizes with the chromium- and the molybdenum hexacarbonyls affording yellow to orange solid solutions, the color going deeper on increasing the [V(CO)6] concentration.36,101 The formation enthalpy of [V(CO)6] was experimentally determined at 298 K, resulting −792  21 kJ mol−1.102 The exchange of CO ligands with external CO was reported to be slow on [V(CO)6],103 nevertheless associative substitution reactions are about 1010 faster on [V(CO)6] than [Cr(CO)6], as a consequence of the 17-electron configuration of the former.104 The monomeric nature of [V(CO)6] both in the solid state105 and in solution106 was soon recognized as proved by magnetic susceptibility measurements at room temperature. Gas-phase diffraction data107 and the X-ray crystal89a,108 and molecular structure (at −28 C)109 agree in indicating an approximately octahedral symmetry with tetragonal distortion: VdCax, 1.993(2); VdCeq, 2.005(2) A˚ .109 Exhibiting a d5-configuration, the octahedral geometry should be unstable due to the Jahn-Teller effect. Combined, electronic110 and photoelectron spectra,111 laser photolysis,112 magnetic measurements in the temperature range 4.2–300 K,105 comparison of the gas-phase IR spectrum with that of [Cr(CO)6],113 MCD spectrum in inert gas matrix at 5 K,114 electron diffraction107 and X-ray crystallographic109 studies have been interpreted in terms of a dynamic Jahn-Teller perturbation of the 2T2g state at c. 150–300 K and a very small, static distortion at lower temperatures. Although low-temperature EPR studies115–117 suggest a nonaxial distortion,118 Pensak and McKinney optimized the geometry of [V(CO)6] by DFT under D4h and Oh symmetries and observed a small energy difference, possibly consistent with the observation that the Jahn-Teller effect is small enough to give rise, experimentally, to a dynamic effect.119 Jet-cooling techniques combined with high resolution IR absorption spectroscopy allow to elucidate the rotational structure of fluxional molecules. When applied to the study of the n6 fundamental band (CdO stretch) of [V(CO)6], the rotovibronic parameters of the molecule were determined, and thus the possibility of a static Jahn-Teller distortion was ruled out.120 An unusual temperature dependence of the magnetic susceptibility affects [V(CO)6], a drastic fall in the magnetic moment being observed at around 60 K.100,105 The temperature dependence of the paramagnetism of [V(CO)6] solutions was erroneously interpreted100 as evidence for the existence of [V2(CO)12]. Later, such anomalous magnetic behavior was explained, in terms of the vibronic coupling model of Van Vleck-Kotani, as exchange magnetic interaction between two magnetic centers.105 The stability of [V(CO)6] with respect to the dimerization is probably ascribable to extensive p-delocalization of the odd electron onto the ligands.101,117 Analyses of the hyperfine structure of the EPR spectra,117 complemented by SCF-Xa-DV calculations,101 confirm that [V(CO)6] is a p-radical. The unpaired spin of [V(CO)6] has little directional character and the reorganization energy required for the compound to rearrange and interact with another one-electron species (e.g., another [V(CO)6] molecule) is prohibitive. Thus, [V(CO)6] does not dimerize, although [V2(CO)12], with bridging carbonyls, is present in the low temperature condensates derived from the reaction of vanadium atoms and carbon monoxide.121 Recently, King and Li reported that octahedral or nearly octahedral {V(CO)6} units, similar to the known mononuclear [V(CO)6], are fundamental building blocks in the lowest energy structures of the homoleptic dinuclear vanadium [V2(CO)n] (n ¼ 9–12).122 It appears that a {V(CO)6} frame links to another one through one or two four-electron donor bridging CO groups, as a consequence of the oxophilicity of vanadium.122 A series of mononuclear niobium and tantalum carbonyls, M(CO)x (M ¼ Nb, Ta; x ¼ 1–6), was identified via IR spectroscopy in neon123 and in argon matrices at 4.2 K.124 Later on, a computational study by King and coworkers125 predicted the existence of the dinuclear unbridged [Nb2(CO)12] (capped octahedra as coordination polyhedral on each niobium), rather than the mononuclear [Nb(CO)6], as the possible neutral binary carbonyl of niobium(0). On theoretical grounds, the dissociation [Nb2(CO)12] ! 2 [Nb(CO)6] requires an energy of c. 13 kcal mol−1. In 2020, Krossing et al.85 reported the unambiguous characterization of [Ta2(CO)12], 1, stable at room temperature in a pentane solution for several hours under CO pressure (3 bar). This unprecedented neutral homoleptic tantalum carbonyl was obtained by the 1:1 M reaction of [NEt4][Ta(CO)6] with Ag[Al(ORF)4], and also from the comproportionation of [Ta(CO)6]− and [Ta(CO)7]+ (see above Scheme 7). Fig. 4 shows the molecular structure of 1; two polymorphs were detected by single crystal X-ray diffraction analyses, respectively in the triclinic space group P1 and the orthorhombic space group Pbca. The long TadTa distance of c. 3.3 A˚ (the sum of covalent radii is 2.98 A˚ )126 and the sum of Van der Waals radii (4.52 A˚ )127 imply a low dissociation energy (DG dissociation ¼ 28 kJ mol−1, according to DFT).

308

Organometallic Complexes of Group 5 With p-Acidic Ligands

Fig. 4 Molecular structure of [Ta2(CO)12], 1, and related TadTa bond distance values. From Unkrig, W.; Schmitt, M.; Kratzert, D.; Himmel, D.; Krossing, I Nat. Chem. 2020. https://doi.org/10.1038/s41557-020-0487-3.

Baird and Geiger described the formal product of CO/dppe (dppe ¼ Ph2PCH2CH2PPh2) substitution from [Ta(CO)6], that is, [Ta(CO)4(dppe)], which was obtained by treatment of the hydride complex [TaH(CO)4(dppe)] with the radical [(t-BuC6H4)3C]%.128 According to magnetic measurements and DFT calculations, the dimeric form [Ta2(CO)8(dppe)2] appears favored, bearing linear semibridging carbonyl ligands supporting a weak TadTa interaction.129

6.04.2.3

Reactivity of [V(CO)6]

The chemistry of [V(CO)6] is dominated by the tendency of the V0 center to change its oxidation state. Indeed one of the first reactions ever observed with [V(CO)6] was its disproportionation to hexacarbonylvanadates(−I) associated to [VL6]2+, [V(LL)3]2+ and [VL4]2+ cations, upon interaction with Lewis Bases (L/LL ¼ N-, O-donors), Eqs. (5), (6).130–132   (5) 3V ðCOÞ6 + 6L ! ½VL6 Š VðCOÞ6 2 + 6CO    3V ðCOÞ6 + 3LL ! V ðLLÞ3 V ðCOÞ6 2 + 6CO (6) In the following sections, the reactivity of vanadium(0) hexacarbonyl will be overviewed, with reference to different and general types of transformations.

6.04.2.3.1

Reactions involving the monoelectron reduction of [V(CO)6]

Redox processes involving [V(CO)6] in hydrocarbons were largely explored, thus the salt [VCp2(CO)2][V(CO)6] was isolated from the reaction with [VCp2] in heptane under an atmosphere of carbon monoxide.133 More recently,134 it was shown that the m-isocarbonyl compound [Cp2V(m-CO)V(CO)5], identified in solution by IR analysis, is an intermediate species along the formation of [VCp2(CO)2]+ (Scheme 10). The initial IR spectrum of the solution in the carbonyl stretching region shows bands at 2042m, 1969m, 1897vs, 1865s and 1652m-s cm−1. In particular, the absorption at 1652 cm−1 is well in agreement with the typical CO stretching wavenumbers of isocarbonyl groups.135 Experiments performed with [V(13CO)6] showed that, during the formation of [VCp2(CO)2]+, no redistribution takes place of the carbonyl ligands between the two vanadium centers.

Scheme 10 Redox reactions between vanadium hexacarbonyl and vanadocenes. From Osborne, J. H.; Rheingold, A. L.; Trogler, W. C. J. Am. Chem. Soc. 1985, 107, 6292–6297; Calderazzo, F.; Ferri, I.; Pampaloni, G.; Englert, U.; Organometallics 1999, 18, 2452–2458.

By using the more robust decamethylvanadocene, the m-isocarbonyl complex [Cp 2V(m-OC)V(CO)5] was isolated and fully characterized, Scheme 10. The isocarbonyl bridge is not cleaved in solvents like toluene, THF or CH2Cl2. Nitrogen-donor ligands, for example pyridine and CH3CN, readily break down the structure with formation of [V(CO)6]−, as clearly observable by IR spectroscopy.136 For sake of comparison, no reaction occurs between [VCp2] and [Mn2(CO)10], even after heating at c. 60 C in heptane.136 It seems therefore that [VCp2], which smoothly reduces both [V(CO)6] and [Co2(CO)8], does not possess a sufficient reducing power

Organometallic Complexes of Group 5 With p-Acidic Ligands

Table 2 SCE).137

309

Values of reduction potentials for selected organometallics (vs. FeCp2/FeCp+2 , at 0.400 V vs.

Compound

EOx/Red (ref.)

Compound

EOx/Red

[Mn2(CO)10] [VCp2]+

−1.52 −1.10

[Co2(CO)8] [V(CO)6]

−0.63 −0.14

to transfer an electron to [Mn2(CO)10]. On the other hand, a rapid reaction takes place when solid Na[Mn(CO)5] is added to a solution of [VCp2Cl] in toluene, leading to the formation of [Mn2(CO)10]. A correlation exists between these outcomes and the values of the reduction potentials obtained from an electrochemical study on CoCp2/metal carbonyl systems (Table 2).137 In fact, the reduction potential of the [CoCp2]+/[CoCp2] couple is more positive than that of the [Mn2(CO)10]/[Mn(CO)5]− couple, while it falls at more negative values compared to [Co2(CO)8]/[Co(CO)4]− and [V(CO)6]/[V(CO)6]−. Note that a determining factor in the failure to observe a reaction between [CoCp2] and [Mn2(CO)10] was attributed137 to the strength of the MndMn bond (Bond Dissociation Energy, BDE ¼ 158 kJ mol−1138), which is considerably stronger than the CodCo bond in [Co2(CO)8] (BDE ¼ 87.8 kJ mol−1139). The electron transfer within the [CoCp2]/[V(CO)6] radical couple has been studied by ultrafast IR transient absorption spectroscopy, generating the radical pair [CoCp2][V(CO)6] via ultrafast visible charge-transfer excitation.140 [V(CO)6] is reduced by thallium metal in CH2Cl2 giving orange, pyrophoric Tl[V(CO)6].141 The IR spectra recorded in THF or DMSO are essentially indistinguishable from the spectrum of [V(CO)6]− in THF, suggesting the presence of solvent-separated Tl+ and [V(CO)6]− ions. Neat cycloheptatriene and [V(CO)6] react thermally according to a redox/complexation reaction affording [V(Z6-C7H8) 7 (Z -C7H7)] together with a minor amount of the diamagnetic compound [V(Z7-C7H7)(CO)3], both containing vanadium in a formal (−I) oxidation state.98,142 Analogously, the two-hour reactions of [V(CO)6] with a series of substituted cycloheptatrienes, in boiling hexane, provide moderate yields of various [V(C7H6X)(CO)3] complexes (X ¼ alkyl, aryl, ether, etc.).143 By the combination of isocyanoferrocene with bis(naphthalene)chromium(0), [Cr(CNFc)6] (Fc ¼ ferrocenyl) is obtained, which is readily oxidized by [V(CO)6] in CH2Cl2 to afford [Cr(CNFc)6][V(CO)6] in a nearly quantitative yield, Scheme 11A.43 The bond distances VdC [1.935(3)–1.974(3) A˚ ] and CdO [1.147(4)–1.160(4) A˚ ] resemble the situation found in other structurally characterized [V(CO)6]− anions. The chromium to vanadium electron transfer occurs within minutes, as monitored by IR analysis of the reaction mixture. On the other hand, the IR spectrum of the mixture obtained by combining equimolar solutions of [Cr(CNX)6], X ¼ Mn(Z5-C5H4)Mn(CO)3, and [V(CO)6] (Scheme 11B) is dominated by bands due to the neutral starting materials, thus indicating the formation of only traces of [V(CO)6]−. The same IR pattern in the nCO/nCN region was observed by treating [Cr(CNX)6][SbF6] with one equivalent of [Et4N][V(CO)6] (Scheme 11C). Thus, the redox equilibrium shown in Scheme 11B appears substantially shifted to the left.

Scheme 11 Vanadium hexacarbonyl reduction upon interaction with chromium(0) adducts comprising sandwich or half-sandwich organometallic units. From Holovics, T. C.; Deplazes, S. F.; Toriyama, M.; Powell, D. R.; Lushington, G. H., Barybin, M. V. Organometallics 2004, 23, 2927–2938.

6.04.2.3.2

Other reactions involving vanadium oxidation and nonredox substitution reactions

The reducing properties in hydrocarbon media of metal carbonyls, including [V(CO)6], have been exploited to prepare mixed chlorides of formula MCln
nTiCl3 (n ¼ 2–3; M ¼ divalent or trivalent transition metal) from the reactions with TiCl4.144 X-ray and spectroscopic data indicate that such mixed chlorides are solid solutions. They are very active catalytic systems for the production of high-density polyethylene (HDPE) in the presence of AliBu3.145 In view of the difficulties encountered in the large-scale synthesis of

310

Organometallic Complexes of Group 5 With p-Acidic Ligands

[V(CO)6], the metal carbonyl was later substituted by the vanadium(0) bis-mesitylene derivative, [V(Z6-1,3,5-Me3C6H3)2],146,147 for the large-scale preparation of Ti/V mixed halides.148 A variety of substituted cyclopentadienyl compounds of vanadium(I) with general formula [V(Z5-C5R5)(CO)4] are accessible from the thermal reaction of [V(CO)6] with the appropriate cyclic diene, including the precursors of C5Me5, C5MeEt4, C5MeCyH3, and indenyl moieties.149–152 This is a direct strategy to prepare cyclopentadienyl-carbonyl species from [V(CO)6], other than the [V(CO)6]/[VCp2] combination illustrated in Scheme 10. The thermal reaction of [V(CO)6] with 6-alkyl substituted fulvenes results in the formation of tetracarbonyl-(alkenylcyclopentadienyl) complexes of VI, see Scheme 12 for a specific example.153 In this case, the generation of the aromatic cyclopentadienyl core is guaranteed by the elimination of a hydrogen atom from one alkyl group. The use of diphenylfulvene (dpp) led to a product of composition [V0(CO)4(dpp)] but unknown structure.

Scheme 12 Synthesis of vanadium(I) cyclopentadienyl system from [V(CO)6] and 5-(propan-2-ylidene)cyclopenta-1,3-diene. From Hoffman, K.; Weiss, E. J. Organomet. Chem. 1977, 131, 273–283.

1-Phenyl-2,5-dihydro-1H-borol reacts with [V(CO)6] in hexane in 3:1 M ratio to afford very low yields of the red triple-decker [(CO)4V(m-C4H3B-Ph)V(CO)4]; mixed-metal (VdFe) triple decker complexes were also reported, Scheme 13.154

Scheme 13 Formation of triple-decker complexes from vanadium hexacarbonyl. From Herberich, G. E.; Hausmann, I.; Klaff, N. Angew. Chem. Int. Ed. Engl. 1989, 28, 319–320.

Peroxydisulfuryl difluoride, F(O)2SOOS(O)2F, reacts with the hexacarbonyls of molybdenum155 and tungsten156 to afford [MoO2(SO3F)2] and [WO(SO3F)4], respectively. The fact that F(O)2SOOS(O)2F oxidizes the metal atom in [Mo(CO)6] or [W(CO)6] to its highest oxidation state suggests that other first-row carbonyls may behave similarly, leading to high-valent metal fluorosulfates. Indeed vanadyl fluorosulfate, [VO(SO3F)3], was afforded in quantitative yields from [V(CO)6] and F(O)2SOOS(O)2F (neat or in excess in dry degassed C7F16) as a dark-red, highly moisture-sensitive solid; its IR spectrum suggests the presence of fluorosulfato ligands adopting different coordination modes.157 Irradiation of [V(CO)6] in low temperature matrices containing O2 results in the formation of superoxovanadium species and CO2158; experiments with 18O2 allowed identification of the dioxide–superoxide [VO2(s-O2)] adduct. 2,2,6,6-Tetramethylpiperidyl-1-oxyl (tempo) is a radical scavenger which has been successfully employed to capture [Mn(CO)5]% and [Co(CO)4]%.159,160 The reaction of tempo with the 17-electron vanadium hexacarbonyl in saturated hydrocarbon solvents yields

Organometallic Complexes of Group 5 With p-Acidic Ligands

311

[V(CO)3(tempo)]. The most striking feature arising from the 51V NMR characterization of the tempo complex is the considerable deshielding of the metal nucleus compared to [V(CO)6]− (−430 ppm vs. −1952 ppm). Moreover, downfield shift of the 51V resonance is observed, as expected, with respect to other carbonyl complexes containing the vanadium center in the +I oxidation state. Similar diamagnetic, electron-deficient complexes [Mn(CO)3(tempo)] and [Co(CO)2(tempo)] have appeared in the literature.159 In general, the ligand tempo induces a large deshielding of the metal nucleus (51V, 55Mn, or 59Co) relative to the respective carbonylmetalates. This effect has been explained in terms of either the high electronegativity/low polarizability of the k2-coordinated {NO} functionality, or a steric component imparted by overlap distributions in the three-membered metallacycle. In both views, the large metal 3d character of the relevant orbitals is considered an important factor contributing to the deshielding. Tempo is, from the NMR point of view, a hard ligand but it is otherwise an effective p-electron-withdrawing ligand.161,162 The reaction of [V(Z6-1,3,5-Me3C6H3)2] with CO in mesitylene affords [V(Z6-1,3,5-Me3C6H3)2][V(CO)6] (53% yield), whose formation is explainable in terms of the progressive replacement of the aromatic rings by carbon monoxide, ideally in half of the molecules of reagent, to give [V(CO)6] first, followed by the redox interchange between the latter and still unreacted [V(Z6-1,3,5-Me3C6H3)2], Scheme 14.20

Scheme 14 Oxidation of [V(Z6-1,3,5-Me3C6H3)2] via arene/CO displacement on half of the reagent. From Calderazzo, F.; Cini, R. J. Chem. Soc. 1965, 818–819.

Besides, mixed arene/CO systems may be derived from the straightforward reactions of [V(CO)6] with arenes (in neat arene or in another organic solvent), affording [V(Z6-arene)(CO)4][V(CO)6] (arene ¼ benzene, toluene, p-xylene, mesitylene, durene, hexamethylbenzene) under mild conditions (35 C, absence of light).38,163,164 Differently, the neutral derivative [V(C6Ph6)(CO)3] was obtained in 2% yield from the nonselective reaction of [V(CO)6] with diphenylacetylene in benzene, as a result of the in situ generation of the arene via alkyne trimerization.165 Vanadium hexacarbonyl disproportionates in liquid ammonia at temperatures from −30 to +20 C, affording [V(NH3)6] [V(CO)6]2 in 60% yield, Eq. (7). The experimentally detected amount of released CO corresponds to a CO/[V(CO)6] molar ratio of c. 1 (instead of 2 as required by Eq. 7); therefore, it has been hypothesized that CO is partially converted into urea according to a side reaction generating [NH4][V(CO)6] as the vanadium product, Eq. (8).132      (7) 3 V ðCOÞ6 + 6NH3ðlÞ ! VðNH3 Þ6 VðCOÞ6 2 + 6CO     2 V ðCOÞ6 + CO + 4NH3ðlÞ ! 2½NH4 Š V ðCOÞ6 + COðNH2 Þ2 (8) The reaction of [V(CO)6] with tetrakis(dimethylamino)ethylene selectively produces an octamethyloxamidinium salt, Scheme 15.166 The same outcome is achieved using [Co2(CO)8], but contrasts with what usually happens when vanadium hexacarbonyl is allowed to interact with amines, leading to the [V(CO)6]− salts of [V(amine)6]2+ (see Eq. 8 for the specific case of ammonia).130 The unique behavior exhibited by tetrakis(dimethylamino)ethylene highlights the ability of this chemical to act towards metal carbonyl systems not as a Lewis base as other amines do, but as a two-electron reducing agent instead.167

Scheme 15 Electron interchange reaction between [V(CO)6] and tetrakis(dimethylamino)ethylene. From Wiberg, N.; Buchler, J. W. Chem. Ber. 1963, 96, 3223–3229.

312

Organometallic Complexes of Group 5 With p-Acidic Ligands

Fig. 5 View of the X-ray structure of [V(m-PMe2)2(CO)4]2, 4171; H atoms have been omitted for clarity. From Vahrenkamp, H. Chem. Ber. 1978, 111, 3472–3483.

The substitution reactions of vanadium hexacarbonyl with tertiary mono-phosphines may follow various pathways depending on the experimental conditions.95,168,169 For instance, the addition of one equivalent of triphenylphosphine to [V(CO)6] gives [V(CO)5(PPh3)] in CH2Cl2 (86% yield)95 and [V(Et2O)6][V(CO)5(PPh3)2] in Et2O,95 whereas the use of an excess in hexane gives trans-[V(CO)4(PPh3)2].170 Reduction of the latter by sodium amalgam in benzene130 or ethanol170 generates [V(CO)5(PPh3)]− as the main product, in the place of the expected [V(CO)4(PPh3)2]−. The reactions involving secondary mono-phosphines usually proceed with V0 to V+I oxidation to yield dinuclear complexes with bridging phosphido ligands.168,171 Interestingly, this outcome is replicable on low valent late transition metal systems, and for instance the reaction of [Fe2Cp2(CO)4] with PHPh2 was reported to proceed with iron(I) to iron(II) oxidation affording [Fe2Cp2(m-H)(m-PPh2)(CO)3] in high yield.172 A view of the X-ray molecular structure of [V(m-PMe2)2(CO)4]2, 4, prepared from [V(CO)6] and PHMe2, is shown in Fig. 5. The VdV distance (2.734 A˚ ) in 4 is elongated compared to what expected for a pure double bond.171 Likewise PPh3, the diphosphine Ph2P(CH2)2PPh2 (dppe) may react with [V(CO)6] following different routes, depending on the temperature and the reagents molar ratio, Scheme 16. In contrast to [V(CO)6], [V(CO)4(dppe)] is far more inert toward donor solvents: for example, it does not modify for hours in dry and peroxide-free THF.173 At 120 C, vanadium hexacarbonyl and 1,1,1-tris(diphenylphosphinomethylene)ethane (dppme) react in petroleum ether to produce the tricarbonyl derivative [V(CO)3(dppme)].174 Carbonyl-phosphine substitution has been reported from [V(CO)6] and phosphorus macrocycles, under photolytic conditions.175,176 Mixing tetraphenyldiphosphine, Ph2P-PPh2, and [V(CO)6] in benzene generates deep-blue [V(CO)4(m-PPh2)]2, homologous to 4.174,177 When starting from [NEt4][V(CO)6], the photo-induced reaction generates [NEt4]2[{V(CO)4}2(m-P2Ph4)2] and [NEt4]2[{V(CO)5}2(m-P2Ph4)]. Tetraphenyldiarsine and tetraphenyldistibine behave similarly.178

Scheme 16 CO/diphosphine substitution reactions from vanadium hexacarbonyl. From Ellis., J. E. Unpublished results reported in Ellis, J. J. Organomet Chem. 1975, 86, 1–56.

Alkene complexes of vanadium(0) of the type [V(CO)4(Z2-L)] have been prepared using the bifunctional ligands (2-allylphenyl) diphenylphosphine, Fig. 6A, and (2-cis-propenylphenyl)diphenylphosphine, Fig. 6B. The IR spectra display a band in the 1480–1580 cm−1 region, assigned to the C]C stretching of the coordinated alkene.179 Upon reduction of the complexes with

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313

Fig. 6 Structures of bifunctional alkene/phosphine ligands. From Interrante, L. V.; Nelson, G. V. J. Organomet. Chem. 1970, 25, 153–160.

Na/Hg, the vanadium(−I) anions [V(CO)5(k1P-L)]− are prevalently formed, wherein L is bound to the metal through the phosphorus atom only.179 The reactions of [V(CO)6],180 or [V(C6H6)2],181 with tetracyanoethylene (TCNE) yielded V[TCNE]x ∙z CH2Cl2 (x  2; z  0.5), that is a disordered room-temperature organic magnet with Tc  400 K.182 Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) studies reveal that the vanadium is divalent and surrounded by six nitrogen centers at 2.084(5) A˚ .183 Other magnetically ordered organic-based materials have been obtained from [V(CO)6] and several polynitrile electron acceptors.184–186 The combination of 1,2,4,5-tetracyanobenzene (TCNB) with [V(CO)6] in 2:1 M ratio, in dichloromethane or trifluorotoluene, led to the isolation of magnetic substances with Tc in the range 200–325 K.187 The IR absorptions are lower in energy than observed for uncoordinated TCNB (2245 cm−1), and are indicative of the presence of reduced TCNB.187

6.04.2.4

Carbonyl-hydrido compounds

The existence of [VH(CO)6] was first postulated following a potentiometric study131 of the reaction of K[V(CO)6] with H3PO4, “Vanadincarbonylhydrid ist somit eine ahnlich starke Säure wie HCo(CO)4” (Vanadiumcarbonylhydride is an acid with the same strength of HCo(CO)4).131 Later reports37,188 evidenced that (a) “VH(CO)6” is a thermally unstable compound rapidly decomposing to [V(CO)6] in the absence of a Lewis base; (b) solutions of “VH(CO)6” in organic solvents should be regarded as containing [H3O]+ and [V(CO)6]−, due to the presence of adventitious water; (c) “VH(CO)6” behaves as a strong acid in aqueous solution. The latter statement is coherent with the earlier findings131 that (i) the titration curve of aqueous solutions of [H3O]+[V(CO)6]− is substantially superimposable to that of HCl, and (ii) aqueous solutions of Na[V(CO)6] do not undergo hydrolysis to any appreciable extent. To date, the niobium hydride [NbH(CO)6] is still unknown, while [TaH(CO)6] was claimed to be generated in the reaction of CO/H2 with Ta2O5 as a highly volatile liquid, decomposing to metallic tantalum.189 In parallel with the chemistry of cobalt carbonyls, the substitution of a CO ligand with PPh3 reduces the acidity in water, and the pKa at 20 C shifts from 10−1 for “VH(CO)6”131 to 1.5  10−7 for [VH(CO)5(PPh3)].131 The acidity along the series of complexes [VH(CO)6− n(PPh3)n] (n ¼ 0, 1) can be ranked by the magnitude of the HOMO/LUMO separation calculated for the corresponding conjugate bases.190 Anionic [MH(CO)5]2− derivatives of V (bright yellow), Nb (orange), and Ta (orange-red) have been prepared as air-sensitive solid materials by reacting the [M(CO)5]3− anions with ethanol in liquid ammonia at −70 C, followed by the addition of NEt4BH4.191 The 1H NMR spectrum of Na2[VH(CO)5] exhibits a well-resolved octet (JVH ¼ 27.6 Hz) centered at −4.76 ppm, as a result of vanadium-hydride coupling.191 (Poly)phosphine and diarsine ligands are able to stabilize neutral carbonyl hydrides, and several adducts of formula [VH(CO)5(PPh3)],131 [VH(CO)4(LL)] (LL ¼ 1,2-bis(dimethylphosphino)ethane (dmpe),192–194 (diphenylphosphino)methane (dppm),195,196 1,2-bis(diphenylphosphino)ethane (dppe),191,193,195 1,3-bis(diphenylphosphino)propane dppp,195 1-diphenylphosphino-2-diphenylarsinoethane (arphos),196 diarsine,193,194 bidentate triphos (Ph2PCH2CH2PPhCH2CH2PPh2)195,197), [VH(CO)3(LLL)] [LLL ¼ P(CH2CH2PPh2)3,197 tridentate triphos197] and [MH(CO)2(dmpe)2] (M ¼ Nb, Ta)198–200 have appeared in the literature. The crystal structures of [VH(CO)4(dppe)], 5a, and [TaH(CO)2(dmpe)2], 5b, were ascertained by X-ray diffraction. In 5a, the hydride ligand occupies one edge of a pseudo-octahedron and is positioned between one phosphorus atom (P2) and one CO ligand (Fig. 7)201; vanadium-phosphorus distances are V(1)dP(1) ¼ 2.453 A˚ and V(1)dP(2) ¼ 2.478 A˚ . Compound 5b possesses a monocapped octahedral structure, in which the hydride is presumed to be the capping ligand.202 The hydrides [VH(CO)4(LL)] and [VH(CO)3(LLL)] can be generated from [V(CO)4(LL)] and [V(CO)3(LLL)], respectively, upon contact with water.203 Accordingly, the deuteride [VD(CO)4(ppb)] (ppb ¼ 1,2-bis(diphenylphosphino)benzene) is easily accessible from [NEt4][V(CO)4(ppb)] and D2O.203 The polyhydrido [VH3(CO)3(diars)], diars ¼ 1,2-bis(dimethylarsino)benzene, was obtained by acidification of [V(CO)3(diars)]− with excess hydrogen halides in HF [V−I to V+III oxidation].193 Otherwise, NaBH4 was employed as the hydride source to yield the mixed arene-hydride, orange-yellow complex [VH(Z6-mesitylene)(CO)3], from [V(Z6-mesitylene)(CO)3].204 A number of tantalum hydrido complexes, that is, cis-[TaH(CO)4(Ph2PCH2CH2PPh2)] and [TaH(CO)3Pm] [Pm ¼ tridentate phosphine], were afforded from [NEt4][Ta(CO)4Pm] by ion-exchange chromatography on silica gel.205 The high pressure carbonylation of [MH5(dmpe)2] in hydrocarbon solvents gives [MH(CO)2(dmpe)2] (M ¼ Nb, Ta),199,200 and the tantalum compound can also be prepared by direct acidification of [Ta(CO)2(dmpe)2].198

314

Organometallic Complexes of Group 5 With p-Acidic Ligands

Fig. 7 View of the X-ray structure of [VH(CO)4(dppe)], 5a201; H atoms (except VdH) have been omitted for clarity. From Greiser, T.; Puttfarcken, U., Rehder, D. Trans. Met. Chem. 1979, 4, 168–171.

Hepta-coordinated hydrido-carbonyl complexes of the group 5 metals, stabilized by various phosphine ligands, have been used as transferors of hydrogen to organic substrates. Related cyclopentadienylcarbonyl derivatives have been employed for the same purpose.206 For instance, the interaction of isoprene with [VH(CO)4(Ph2PCH2PPh2)] results in the formation of a Z3-dimethylallyl species, Eq. (9).196    
VHðCOÞ4 ðPh2 PCH2 PPh2 Þ + CH2 ¼ CðMeÞCH ¼ CH2 ! V Z3 − C3 H3 Me2 ðCOÞ3 ðPh2 PCH2 PPh2 Þ + CO (9) Hydrido complexes of the type [VH(CO)nPm] (Pm ¼ multidentate phosphine), including 5a, incorporate alkenes under UV irradiation to afford alkyl complexes (formal alkene insertion into the VdH bond). The anti-Markovnikov product is usually prevalent.207 Analogously, alkenyl complexes [V(Z1-CR]CHR0 )(CO)n(Pm)], (n ¼ 3, 4; Pm ¼ bidentate or tridentate phosphine) are formed by the photo-induced reaction of hydride precursors with alkynes RC^CR0 .208 The products exist as a single isomer (Z-isomer, n ¼ 4) or as a couple of isomers (Z and E, the former prevailing). The plausible mechanism involves preliminary CO/ alkyne substitution, followed by intramolecular hydride transfer to the Z2-coordinated alkyne and recombination with CO. An alternative strategy to generate alkyl-carbonyl complexes relies on electrophilic attack of alkylating agents to [V(CO)4(dmpe)]−209 and [V(CO)2(dmpe)2]−.210

6.04.2.5

Reactivity of metal(−I) hexacarbonylates, synthesis, and properties of other nonhomoleptic compounds

Ammonium salts of [V(CO)6]− exhibit a rich chemistry, including cation exchange via proton transfer, and anion to cation electron transfer enabling different outcomes, Scheme 17.23

Scheme 17 Possible reactions on [V(CO)6]− salts: (a) cation exchange via H+ transfer; (b) anion to cation electron donation forming H2; (c) anion to cation electron donation promoting CdC coupling. From Calderazzo, F.; Pampaloni, G.; Lanfranchi, M.; Pelizzi, G. J. Organomet. Chem. 1985, 296, 1–13.

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Moreover, [V(CO)6]− salts are versatile starting reagents for the preparation of a vast variety of carbonyl derivatives, typically via substitution or oxidative routes. The CO displacement often requires photolytic conditions. Interestingly, the tetraethylammonium salt [NEt4][V(CO)6] was tested as a metal-based carbon monoxide releaser in aqueous solution in the absence of a photo-stimulus. For a series of octahedral complexes, the rate of CO elimination was found to decrease along the sequence [FeI2(CO)4] > [NEt4] [V(CO)6] > [MnBr(CO)5] > [Cr(CO)6], implying that such phenomenon is not controlled by the metal-CO bond strength.211 Photolytic CO replacement has been widely employed for the synthesis of phosphine-substituted carbonyl anions starting from [V(CO)6]−.192,195,212–216 The reaction with tertiary diphosphines “P2” initially affords the mono-substituted product [V(CO)5(P2)]−. Further substitution takes place upon prolonging the irradiation time and is favored by the continuous passage of dinitrogen. Compounds of formula [NEt4]2[{V(CO)5}2(m-LL)], comprising a dinuclear anion with a bridging bidentate ligand, are obtained when THF solutions of [NEt4][V(CO)6] are photolyzed in the presence of Ph2AsCH2CH2AsPh2 or Ph2AsCH2CH2PPh2.217 The photosubstitution by diars or dmpe provides highly reactive mononuclear anions which can be protonated to give thermally stable seven-coordinated carbonyl hydrides. Moreover, electrophilic additions to the anion [V(CO)4(diars)]− by methyl iodide, allyl [V(Z3-C3H5)(CO)3(diars)], and chloride, and bromopentacarbonylmanganese yield [V(CH3)(CO)4(diars)], 194 [V(CO)4(diars){Mn(CO)5}], respectively, Scheme 18.

Scheme 18 CO photolytic substitution by 1,2-bis(dimethylarsino)benzene and subsequent electrophilic addition yielding VI complexes. From Ellis, J. E.; Faltynek, R. A., J. Organomet. Chem. 1975, 93, 205–217.

Photolysis of [Na(diglyme)2][V(CO)6] in 2-methyltetrahydrofuran (THFMe) with filtered light from a medium pressure mercury discharge lamp gave rise to a species identified as [V(CO)5(THFMe)]− on the basis of the IR spectrum.218 Weiss and coworkers prepared the basic Z3-allyl-carbonyl vanadium, [V(Z3-C3H5)(CO)5], and a series of allyl-substituted derivatives, by photochemical reaction of Na[V(CO)6] with allyl halides at 20 C.219–222 Subsequent, further carbon monoxide substitution may afford phosphine and arsine adducts.223,224 Triphenylcyclopropenyl bromide and Na[V(CO)6], mixed in THF or CH3CN without irradiation, form a deep-green solid of composition [C3Ph3][V(CO)6].225 The photolytic treatment of this compound leads to [V(Z3-C3Ph3)(CO)5].225 When starting from [NEt4][V(CO)4(Ph2ECH2CH2EPh2)] (E ¼ P or As), a triphenylcyclopropyl-substituted acyl group is obtained, Scheme 19.226 The X-ray characterization of the product points out the k2C,O-coordination of the acyl moiety [VdC 2.013 A˚ , VdO 2.243 A˚ , VdCdO 84.65 ]. The hydrogenation of the C3 cycle is not specified (Dabei ist die Herkunft des Wasserstoffs ungeklaert227).

Scheme 19 Formation of cyclopropyl-acyl k2C,O-coordinated ligand. From Franke, U; Weiss, E. J. Organomet. Chem. 1979, 165, 329–340.

Adducts with 9,10-phenanthrenequinone (pq) of formula V(k2O-pq)3 and Na[M(k2O-pq)3] (M ¼ V, Nb, Ta) are available through the multiple substitution of CO ligands from [V(CO)6] and Na[M(CO)6] (M ¼ V, Nb, Ta), respectively.228–230 The reduced species Nan[V(pq)3] (n ¼ 1–3) are obtained by the subsequent reaction with finely divided sodium,228 and can be reoxidized stepwise by addition of AgBF4 in THF solution (Scheme 20).

Scheme 20 Interconversion between tris-9,10-phenanthrenequinone complexes bearing vanadium in different oxidation states. From Calderazzo, F.; Pampaloni, G. J. Organomet. Chem. 1987, 330, 47–59.

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Fig. 8 View of the X-ray structure of [V(SnMe3)(CO)6], 6; H atoms have been omitted for clarity. V(1)dSn(1) 2.939 A˚ . From Herberich, G. E.; Wesemann, L.; Englert, U. Struct. Chem. 1993, 4, 199–202.

Addition of 1,10-phenanthroline (phen) or 2,20 -dipyridyl (bipy) to [Na(diglyme)2][V(CO)6]/CuCl, followed by recrystallization of the raw material from THF, determines the formation of the red crystalline heterodinuclear compound [V(CO)6{Cu(phen)2}]n or the brown [V(CO)6{Cu(bipy)2}]n, in 60%–70% yields.231 Only [V(CO)6]− is recognizable by IR spectroscopy in THF solution; in chloroform, extra absorption bands appear at higher frequencies, similar to those due to the stretching vibration modes of [V(CO)6(HgEt)].232 Seven-coordinated complexes comprising a VdE bond, that is, [VE(CO)6] [E ¼ EtHg,232 Ph3PAu,41,233 Ph3Sn,233 Ph3Pb,234 H3Si235], are accessible from [V(CO)6]− and the related chlorides. The silyl derivative [V(SiH3)(CO)6] is a rather unstable orange solid, volatile at room temperature with a pattern of carbonyl stretching absorptions (2028m, 2026m, 1990vs, 1959s cm−1) consistent with what expected for a face-capped octahedral arrangement, in which the silyl group occupies the capping position (C3v). An approximate C3v symmetry is found in [V(SnMe3)(CO)6], 6 (see Fig. 8).47 The halido-bridged dinuclear complexes Pd2(m-C5H5)(m-X)(PPh3)2 (X ¼ Cl or Br) react with [Na(diglyme)2][V(CO)6] to produce a Pd2V heterometallic trinuclear cluster.236 Complex [V(NCBH3)2(THF)4], that is, the first cyanohydroborato adduct of an early transition metal, was generated from the system [V(CO)6]−/I2/diphenylacetylene/Na[NCBH3] in THF, possibly through the action of Na[NCBH3] on the intermediate [VI(CO)2(dppe)(tolane)]. The crystal structure of [V(NCBH3)2(THF)4] reveals an ideal trans configuration (point group D2h) with about linear [CNBH3]− units.237 The reactivity of [M(CO)6]− salts is significantly dependent on the solvent, which indirectly regulates the cation/anion interactions. For instance, no reaction takes place between Fe(acac)3 and Na[Nb(CO)6] in DMF or THF in the presence of one equivalent of 18-crown-6. In both solvents, the IR spectrum of the hexacarbonylniobate(−I) shows only one symmetric absorption band suggesting a moderate cation/anion interaction.17 A smooth substitution reaction takes place between [M(CO)6]− (M ¼ Nb, Ta) and Fe(acac)3 in neat THF affording the MI derivatives [M(k2-acac)(CO)4(THF)] (70% yield) (Eq. 10).17     Na MðCOÞ6 + 2FeðacacÞ3 + THF ! Mðk2 -acacÞðCOÞ4 ðTHFÞ + 2FeðacacÞ2 + 2CO + NaðacacÞ (10) M ¼ Nb, Ta In general, the homoleptic tantalum carbonyl [Ta(CO)6]− is relatively inert to substitution reactions. It is a special exception that all carbonyl ligands of [NEt4][Ta(CO)6] are replaced by 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hpp) to give [Ta(kN,N-hpp)4][Ta(CO)6].48 Whereas most of the hexacarbonylvanadates(−I) of protonated nitrogen bases can be isolated without substantial difficulties,23 the isolation of the corresponding niobium and tantalum species is unsuccessful both in water and in nonaqueous solvent, normally due to the favorable two-electron transfer M−I ! M+I (M ¼ Nb, Ta), accompanied by H2 evolution.238 In addition, attempts to perform cation exchange via the interaction of [M(CO)6]− salts with N-methylacridinium iodide in water resulted in M−I to M+I oxidation, affording the dinuclear anions [M2(m-I)3(CO)8]− (M ¼ Nb, 7a; M ¼ Ta, 7b), and the CdC coupling product of N-methylacridinium, Scheme 21.238 The high stability of 7a–b (a view of the X-ray structure of 7a is supplied in Fig. 9) is assumed to be responsible for the failure of the formation of methylacridinium hexacarbonylmetalates. A similar redox process was observed in the reaction of [M(CO)6]− with 1,10 -dimethyl-4,40 -bipyridinium diiodide.238 The anions 7a–b react under reflux with the tripodal phosphine tBuSi(CH2PMe2)3 (trimpsi) to give [MI(trimpsi)(CO)3] in good yields.239,240

Scheme 21 Electron interchange reaction between hexacarbonylato anions and N-methylacridinium iodide. From Calderazzo, F.; Pampaloni, G.; Pelizzi, C.; Vitali, F. Organometallics 1988, 7, 1083–1092

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Fig. 9 View of the X-ray structure of the anion [Nb2(m-I)3(CO)8]−, 7a. From Calderazzo, C.; Gingl, F.; Pampaloni, G.; Rocchi, L.; Strahle, J. Chem. Ber. 1992, 125, 1005.

The special stability of 7a-b is corroborated by the fact that the same structural motif is produced from the oxidation reactions of [M(CO)6]− (M ¼ Nb, Ta) with various halide-containing oxidants, Scheme 22A.17 The oxidative halogenation of [Nb(CO)6]− represents a convenient entry to a variety of halido-carbonyl complexes with additional phosphine, amine, isocyanide, or alkyne ligands.241,242 When a silver salt containing a poorly coordinating anion (NO−2 or BF−4) is employed as the oxidative agent for [M(CO)4(dppe)]− species, phosphine-substituted mixed Ag/M clusters are afforded, Scheme 22B.243

Scheme 22 Oxidation reactions of [M(CO)6]− salts (M ¼ Nb, Ta). From Calderazzo, F.; Pampaloni, G.; Englert, U.; Strähle, J. J. Organomet. Chem. 1990, 383, 45–57.

Phosphine adducts of TaI halocarbonyls have been obtained by treating Na[Ta(CO)6] with TaCl5 or X2 (X ¼ Br, I), in diethyl ether at low temperature, Scheme 23.244

Scheme 23 Halocarbonyl complexes of TaI with phosphine ligands from [Ta(CO)6]− oxidation. From Leutkens, M. L.; Santure, D. J.; Huffman; J. C.; Sattelberger, A. P. J. Chem. Soc., Chem. Commun. 1985, 552–553.

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Ellis et al.245 carried out both electrochemical and spectroelectrochemical studies on the halido-carbonyl complexes [NbX(CO)2(dppe)2], exhibiting a one-electron reversible oxidation that generates stable (on the electrochemical timescale) 17-electron species [NbX(CO)2(dppe)2]+. Accordingly, chemical oxidation may be viable by treatment with [CPh3][PF6] or [FeCp2][PF6]. A series of heterodinuclear group 5 metal-nickel carbonyls, [MNi(CO)7]− (M ¼ V, Nb, Ta), were generated via a laser ablation ion source and studied by photoelectron velocity map imaging spectroscopy.246 Quantum chemical calculations were performed to probe the electronic and geometric structures, and to assign the IR spectra; the latter consist of absorptions ascribable to three bridging carbonyls, one carbonyl terminally bonded to the Ni atom, and three carbonyls terminally bonded to M.247

6.04.2.6

Carbon monoxide activation reactions

Reductive coupling of two CO ligands to form Z2-alkyne complexes has been documented on group 5 metals. Thus, starting from a solution of [MCl(CO)2(dmpe)2] [M ¼ V, Nb, Ta; dmpe ¼ 1,2-bis(dimethyphosphino)ethane] in THF, the coupling takes place upon reduction with sodium amalgam or magnesium in the presence of a stoichiometric amount of Me3SiY (Y is often Cl or CF3SO3). The alkyne Me3SiOC^COSiMe3 is usually obtained as the result of the CdC bond forming process.248–252 Similar activation reactions will be mentioned in Section 6.04.3.2 with reference to isocyanides. The electron-rich vanadium carbonyl Na[V(CO)2(dmpe)2] undergoes reductive coupling of the two CO units when allowed to react with SiMe3-containing substrates, to generate alkyne products of the type [VX(Z2-Me3SidOdC^CdOdSiMe3)(dmpe)2] (X ¼ Br, OTf ).250 The treatment of Na[V(CO)2(dmpe)2] or [TaCl(CO)2(depe)2] [depe ¼ 1,2-bis(diethyphosphino)ethane] with 1,2-bis(chlorodimethylsilyl)ethane in THF gives rise to the formation of an eight-membered heterocycle, consequently to double CdC and CdO couplings, Scheme 24.253 Compounds 8a–b were thus isolated in moderate-good yields. With reference to the V-alkyne moiety in 8a, X-ray crystallographic data are as follows: CdC ¼ 1.308(6), VdC ¼ 1.985(3) A˚ , CdVdC ¼ 38.5(2) . The corresponding CdC distance in 8b is 1.24(2) A˚ , probably indicating a substantially higher degree of metal to alkyne backbonding in the vanadium system.

Scheme 24 Reductive coupling of two CO ligands to alkyne function incorporated in a 1,6,2,5-dioxadisilocine ring. From Bronk, B. S.; Protasiewicz, J. D.; Lippard, S. J. Organometallics 1995, 14, 1385–1392.

6.04.3

Isocyanide compounds

6.04.3.1

Homoleptic compounds

Isocyanides are almost ubiquitous ligands in organometallic chemistry, being adapted to different oxidation states of the metal center and coligands. Herein, we will treat different classes of isocyanide complexes of group 5 metals,254 with a focus on the preparative procedures, and spectroscopic and structural features concerning the metal-isocyanide bonding. Several mononuclear, homoleptic complexes have appeared in the literature. The first one, chronologically, is the cation [V(CNtBu)6]2+, which was synthesized by Lippard and coworkers from the reduction of VCl3 in ethanol in the presence of a slight excess (c. 8 equivalents) of tert-butyl isocyanide, through the intermediate formation of [VCl3(CNtBu)3].255 Ethanol is presumed to play the role of the reducing agent. Upon addition of KPF6 to the reaction medium, the product was finally isolated as a hexafluorophosphate salt in 25% yield. A more straightforward route was subsequently reported by the same authors, who obtained [V(CNtBu)6][V(CO)6]2, [9][V(CO)6]2, in 73% yield from the oxidation of an ethanol solution of [NEt4][V(CO)6] by PhICl2 in the presence of c. 16 equivalents of CNtBu.256 The paramagnetic compound [VCl3(CNtBu)3] exhibits a mer configuration of ligands, and its 1H NMR spectrum (in CDCl3) showed two separate resonances of intensity ratio 2:1 (at 3.84 and 2.87 ppm), coalescing above 60 C to a single peak. The homoleptic species [V(CNXyl)6]n (n ¼ −1, 0, +1; Xyl ¼ 2,6-C6H3Me2), corresponding to distinct oxidation states of the metal, were all prepared and characterized by the group of Ellis (Scheme 25).257 The 17 electron compound [V(CNXyl)6], 10, was

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afforded in 81% yield from the reaction of labile bis-naphthalene V(0) complexes with the isocyanide in THF/heptane mixture. Compound 10 is paramagnetic in solution and in the solid state: its mEFF of 1.76 mB (298 K)257 implies a low-spin d5 configuration and is very close to that reported for the homologue [V(CO)6] (1.78 mB).108 Differently to the latter, 10 is significantly more thermally stable (decomposition occurs at 159–160 C instead of 60–70 C258), does not undergo disproportionation in THF and is unreactive toward cobaltocene and decamethylcobaltocene. The straightforward reduction of 10 was in fact accomplished using CsC8, leading to the extremely air- and moisture-sensitive Cs[V(CNXyl)6], Cs[11], in 84% yield. Once obtained, the anion [11]− is easily converted to 10 with [NHEt3]Cl, or even upon exposure of its solutions to traces of air, 10 is further oxidized in air to [V(CNXyl)6]+, [12]+, whose salts are moderately air-sensitive in the solid state.257 The preparative synthesis of [12]+ is conveniently realized by reaction of 10 with one equivalent of [FeCp2][PF6] in THF at −70 C (92% yield). The salt [12][PF6] is a paramagnetic solid with mEFF ¼ 2.74 mB (298 K) at room temperature, in accordance with a low-spin d4 configuration and octahedral geometry. Conversely, solutions of [12][PF6] in THF were found to be weakly paramagnetic.

Scheme 25 Synthesis of homoleptic isocyanide complexes of vanadium; R ¼ H or Me; Xyl ¼ 2,6-C6H3Me2. From Barybin, M. V.; Young, V. G. Jr.; Ellis, J. E. J. Am. Chem. Soc. 2000, 122, 4678–4691.

In the X-ray structures of [9][V(CO)6]2,256 [12][PF6], 10, and Cs[11],257 the vanadium atom lies approximately at the center of an octahedron; the vanadium-carbon bond distances (average values, A˚ : 2.10(1) in [9][V(CO)6]2, 2.07(2) in [12][PF6], 2.026(7) in 10, 1.98(3) in Cs[11]), and the CdNdXyl angles (average values: 175.4(7) in [9][V(CO)6]2, 173(2) in [12][PF6], 163(4) in 10, 158 (10) in Cs[11]) nicely evidence increasing backbonding along the series [V(CNXyl)6]2+  [V(CNXyl)6]+ < [V(CNXyl)6] < [V(CNXyl)6]−.259 On the other hand, the mean VdC distance in 10 [1.98(3) A˚ ] is slightly longer than that in [V(CO)6]− [1.931 (9) A˚ ], thus reflecting the higher p-acidic behavior of the carbonyl ligand compared to 2,6-dimethylphenyl isocyanide.46 The wavenumbers of the main IR absorptions of 2190 cm−1 in [9][PF6]2 (Nujol mull),255 2033 in [12][PF6], 1939 in 10 and 1817 cm−1 in Cs[11] (THF solutions) are fully consistent with the observed trend of vanadium to isocyanide backdonation from the bonding parameters.259 The first seven-coordinated homoleptic vanadium complex [V(CNXyl)7]I, [13]I, was recently crystallized from a THF solution.260 [13]I is a reactive intermediate along the I2-oxidative reaction of [NEt4][V(CO)6] with xylyl-isocyanide, which finally affords trans-[VI2(CNXyl)4].261 The structure of the cation [13]+ is shown in Fig. 10, and the geometry is best described as a distorted monocapped trigonal prism.

Fig. 10 View of the X-ray structure of the cation in [V(CNXyl)7]I, [13]I260; H atoms have been omitted for clarity. From Minyaev, M. E.; Ellis, J. E. Acta Crystallogr. 2015, E71, 431–434.

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The main IR absorption in [13]I falls at 2016 cm−1 (Nujol mull), and is comparable to the value reported for [12][PF6] (2033 cm−1 in THF), the latter containing six rather than seven isocyanides. Compound [13]I completely decomposes in THF solution at room temperature during 7–10 days to produce 10, trans-[VI2(CNXyl)4] and uncoordinated CNXyl.260 Homoleptic isocyanides of niobium and tantalum were also described by Ellis and coworkers,262,263 who reported in 2007 on the oxidative carbon monoxide/isocyanide replacement in [M(CO)6]− (M ¼ Nb, Ta), Scheme 26. Thus, the reaction of [NEt4] [M(CO)6] with AgBF4 in the presence of CNXyl, in THF solution, afforded [M(CNXyl)7][BF4] (M ¼ Nb, Ta) in c. 80% yields.

Scheme 26 Synthetic routes to homoleptic niobium and tantalum ions; E ¼ Cs or K. From Barybin, M. V.; Young, V. G. Jr.; Ellis, J. E. J. Am. Chem. Soc. 1999, 121, 9237–9238.

Similarly, [M(CNXyl)6]I (M ¼ Nb, Ta) were produced in good yields using elemental iodine as the oxidant. The tantalum species [Ta(CNXyl)7][BF4] undergoes anion exchange upon metathetical reaction with tetrabutylammonium iodide in THF, accompanied by elimination of one isocyanide ligand from the crowded seven-coordinated cation to yield [Ta(CNXyl)6]I. The iodide salts [M(CNXyl)6]I were revealed to be suitable starting materials for the reductive synthesis of thermally unstable and highly sensitive salts [M(CNXyl)6]− (M ¼ Nb, Ta). The main IR absorptions related to Cs[Ta(CNXyl)6]263 and Cs[V(CNXyl)6]259 in THF solution are substantially similar (1824 cm−1 vs 1817 cm−1). Moreover, a comparison of the salient bonding parameters obtained from the X-ray structures of the two congeneric cesium salts indicates that the average CdN distance is almost identical in the two cases, that is, 1.199(4) A˚ in Cs[Ta(CNXyl)6] and 1.20(2) A˚ in Cs[V(CNXyl)6]. A similar consideration can be deduced based on the average CdNdC angle, being 155.7(3) in the former compound and 158(10) in the latter. These data suggest that the degree of metal-isocyanide backbonding is comparable in the homologous vanadium and tantalum anions. In 2017, Ellis and coworkers filled a gap given by the absence of homoleptic neutral isocyanides of niobium and tantalum, contrasting with the existence of the room temperature stable vanadium species 10, by reporting an effective synthetic route to [Ta(CNDipp)6] (Dipp ¼ 2,6-C6Hi3Pr2).264 The latter also represents the first isocyanide analogue of the elusive mononuclear hexacarbonyltantalum(0).85 In order to obtain [Ta(CNDipp)6], the iodide salt [Ta(CNDipp)6]I was first synthesized from [NEt4] [Ta(CO)6], CNDipp, and I2 in THF (Scheme 27). Subsequent reduction with KC8 over a period of 5 days at room temperature afforded K[Ta(CNDipp)6] in 70%–80% yields. The last step, oxidation to [Ta(CNDipp)6], was challenging due to easy over-oxidation of the desired product; the authors finally realized that a suspension of MoO3 in toluene was suitable for the purpose.

Scheme 27 Synthesis of 17-electron homoleptic tantalum isocyanide complex; Dipp ¼ 2,6-C6Hi3Pr2. From Chakarawet, K.; Davis-Gilbert, Z. W.; Harstad, S. R.; Young, V. G. Jr.; Long, J. R.; Ellis, J. E. Angew. Chem. Int. Ed. 2017, 56, 10577–10581.

[Ta(CNDipp)6] undergoes comproportionation and partial substitution of the isocyanide ligands to form [Ta(CNDipp)7] [Ta(CO)5(CNDipp)] when allowed to react with carbon monoxide at ambient pressure in THF.

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Fig. 11 View of X-ray structure of [Au(PPh3){Ta(CNDipp)6}], 14264; H atoms have been omitted for clarity. From Chakarawet, K.; Davis-Gilbert, Z. W.; Harstad, S. R.; Young, V. G. Jr.; Long, J. R.; Ellis, J. E. Angew. Chem. Int. Ed. 2017, 56, 10577–10581.

Interestingly, K[Ta(CNDipp)6] can be employed as an organometallic anionic ligand for Au(I): low temperature reaction with [AuCl(PPh)3] led to the formation of the violet-brown [Au(PPh3){Ta(CNDipp)6}], 14, in 79% yield.264 A view of the X-ray structure of 14, resembling that of the homologous vanadium-gold compound [Au(PPh3){V(CO)6}],41 is shown in Fig. 11. It reveals a monocapped octahedron around Ta and the first crystallographically reported TadAu bond [2.7207(2) A˚ ], the TadAudP angle measuring 176.20(3) .

6.04.3.2

Heteroleptic compounds

Vanadium hexacarbonyl can be exploited to obtain mixed carbonyl-isocyanide adducts. For example, [V(CO)6] undergoes facile replacement of up to four carbonyl ligands upon reaction with CNXyl in heptane at room temperature to afford [V(CO)2(CNXyl)4], 15, in 80% yield (Scheme 28).259 Such a substitution reaction is rather unusual for [V(CO)6], which generally undergoes a change of the oxidation state in the presence of Lewis bases (vide infra). Compound 15 is prone to oxidation by ferrocenium hexafluorophosphate at 0 C, and [12][PF6] is selectively obtained (80% yield) when the reaction is carried out in the presence of an excess of CNXyl (Scheme 28).259

Scheme 28 Stepwise CO/isocyanide replacement from vanadium hexacarbonyl. From Barybin, M. V.; Young, V. G. Jr.; Ellis, J. E. J. Am. Chem. Soc. 2000, 122, 4678–4691.

The X-ray structure of 15 (Fig. 12) is approximately octahedral with the two carbonyl ligands adopting mutual trans positions.259 As a consequence of the better p-accepting ability of CO vs. CNXyl, the VdCO distances in 15 [average 1.955(15) A˚ ] are shorter than the VdCN distances in the same molecule [average 2.044(2) A˚ ]. They are also shorter compared to those in [V(CO)6] [average 2.001(6) A˚ ]. Many mixed isocyanide complexes of group 5 metals have been reported, and selected examples available from the hexacarbonylmetallate anions will be supplied in the following text. The reaction of [NEt4][V(CO)6] with 1,4-diisocyano2,3,5,6-tetramethylbenzene results in the formation of the discrete dinuclear anion [(CO)5V(m-1,4-CNC6Me4NC)V(CO)5]2−, [16]2−. The X-ray structure of the cobaltocenium salt (Fig. 13) shows the bis-isocyanide ligand acting as a linker between two [V(CO)5]− units. The dianion generates a crystalline network in which a charge transfer process involves the dianionic species.265

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Organometallic Complexes of Group 5 With p-Acidic Ligands

Fig. 12 View of the X-ray structure of [V(CO)2(CNXyl)4], 15259; H atoms have been omitted for clarity. From Barybin, M. V.; Young, V. G. Jr.; Ellis, J. E. J. Am. Chem. Soc. 2000, 122, 4678–4691.

Fig. 13 View of the X-ray structure of the anion in [CoCp2]2[(CO)5V(m-1,4-CNC6Me4NC)V(CO)5], [16]259; [CoCp2]+ and H atoms have been omitted for clarity. From Barybin, M. V.; Young, V. G. Jr.; Ellis, J. E. J. Am. Chem. Soc. 2000, 122, 4678–4691.

The mono-isocyanide anions [Ta(CO)5(CNR)]− (R ¼ Xyl, tBu), containing a Ta−I center, were afforded in approximately 60% yield through a one-pot procedure involving Na reduction of [Ta(CO)6]− in liquid ammonia to give Na3[Ta(CO)5], followed by protonation affording Na[Ta(CO)5(NH3)] via presumable H2 generation. The labile NH3 ligand is then easily replaced by the isocyanide (Scheme 29).262,266

Scheme 29 Synthetic strategy to realize carbon monoxide/isocyanide mono-substitution from [Ta(CO)6]−. From Barybin, M. V.; Brennessel, W. W.; Kucera, B. E.; Minyaev, M. E.; Sussman, V. G.; Young, V. G. Jr.; Ellis, J. E. J. Am. Chem. Soc. 2007, 129, 1141–1150.

The mono-nitrosyl derivative [Ta(CNXyl)5NO] was obtained from Cs[11] upon straightforward nitrosylation with N-methylN-nitroso-p-toluenesulfonamide (Scheme 30).262

Scheme 30 Isocyanide/nitrosyl substitution on [Ta(CO)6]−. From Barybin, M. V.; Brennessel, W. W.; Kucera, B. E.; Minyaev, M. E.; Sussman, V. G.; Young, V. G. Jr.; Ellis, J. E. J. Am. Chem. Soc. 2007, 129, 1141–1150.

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An alternative route to obtain isocyanide-nitrosyl derivatives starts from [NBu4][M(CO)6] (M ¼ Nb, Ta) and uses nitrosonium tetrafluoroborate as the nitrosylating agent, affording cis-[M(CNXyl)4(NO)2][BF4] as orange microcrystalline materials in 50%–55% yields, Eq. (11).262,267 This synthesis is noteworthy in that it represents an unusual example of nonoxidative thermal displacement of all CO ligands from an early transition metal homoleptic carbonylmetallate. The use of [NBu4]+ salts, rather than [NEt4]+, guarantees an easier isolation of the Nb/Ta products from the tetrafluoroborate salt by-product. The homologous vanadium species [V(CNBu)4(NO)2]+ was obtained as a PF−6 salt (56% yield) via a stepwise process that proceeds by zinc reduction of the polymer [VCl2(NO)3]n in acetonitrile, followed by addition of excess CNtBu and treatment with NH4PF6.268     ½NBu4 Š MðCOÞ6 + 2½NOŠ½BF4 Š + 4CNXyl ! cis- MðCNXylÞ4 ðNOÞ2 + ½NBu4 Š½BF4 Š + 6CO (11) ðM ¼ Nb; TaÞ A number of isocyanide-halide systems can be obtained from hexacarbonylmetallate anions by treatment with one isocyanide and an elemental halogen as an oxidant: an overview of this chemistry is supplied in Scheme 31.261,269,270 Of such mixed species, also acetylide and phosphine derivatives have been obtained.261 In Section 6.04.3.1, we describe the isolation of the seven-coordinated homoleptic vanadium complex [V(CNXyl)7]I, [13]I, as an intermediate along with the synthesis of trans-[VI2(CNXyl)4].

Scheme 31 Synthesis of isocyanide-halide complexes of group 5 metals from the respective carbonylmetallate anions. From Cotton, F. A.; Roth, W. J. Inorg. Chim. Acta 1987, 126, 161–166.

The mixed chloride-isocyanide adduct [Nb2Cl6(SMe2)3] was converted to [Nb3Cl8(CNtBu)5], in approximately 30% yield, by slow diffusion of CNtBu into a toluene solution.271 Isocyanide adducts of group 5 metal-cyclopentadienyl systems are a class of compounds usually obtained via either substitution of a preexisting ligand or occupation of a coordination vacancy. Hence, [VCp(CO)4] undergoes the selective substitution of one to three carbonyl ligands by alkyl- or aryl-isocyanides in refluxing toluene in the presence of PdO as a catalyst.272 The use of the catalyst is needed to avoid limited conversion into the desired product and long reaction times. Besides the thermal approach, CO/THF replacement in [VCp(CO)4] is feasible under photolytic conditions, which then allows cyclohexyl-isocyanide to be introduced upon removal of the labile THF ligand.273 Also, the photolytic reaction of [VCp(CO)4] with the disulfide MeSSMe leads to the VIII dimeric specie [V2Cp2(CO)4(m-SMe)2], which in turn undergoes CO/CNR substitution (R ¼ Xyl, tBu) in THF at 30–40 C during 1 h. Monoand di-substituted products are thus isolated in low to moderate yields after silica chromatography; the former can be converted into the latter upon further addition of isocyanide.274 The addition of two isocyanide molecules to decamethylvanadocene, [VCp 2], followed by homolytic cleavage of one CdN single bond releasing the alkyl radical R%, yields the VIII species [VCp 2(CN)(CNR)] (R ¼ tBu, Cy), isolated in 61% and 27% yields, respectively.275 The introduction of an isocyanide ligand (CNXyl) within a {VdCp } frame has been achieved in a different manner by substitution of a bridging N2 ligand from a dimeric precursor (Scheme 32).276

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Organometallic Complexes of Group 5 With p-Acidic Ligands

Scheme 32 Coordination of xylyl-isocyanide ligands to a VIIdCp complex via removal of N2 bridge. From Keane, A. J.; Yonke, B. L.; Hirotsu, M.; Zavalij, P. Y.; Sita, L. R. J. Am. Chem. Soc. 2014, 136, 9906–9909.

Fig. 14 Comparative view of the X-ray structures of [NbCp Cl4(CNXyl)], 17,277 and [NbCl5(CNXyl)], 18278; H atoms have been omitted for clarity. Selected bond distances (A˚ ) and angles ( ) of 17: Nb-CN 2.245(10), C-NXyl 1.136(13), Nb-C-N 179.1(8). Selected bond distances (A˚ ) and angles ( ) of [NbCl5(CNXyl)], 18: Nb-CN 2.319(5), C-NXyl 1.143(6), Nb-C-N 180.0. From Alcalde, M. I.; de la Mata, J.; Gomez, M.; Royo, P. Organometallics 1994, 13, 462–467; Bartalucci, N.; Belpassi, L.; Marchetti, F.; Pampaloni, G.; Zacchini, S.; Ciancaleoni, G. Inorg. Chem. 2018, 57, 14554–14563.

Coordination of isocyanides to niobium(V) is feasible when a toluene solution of [NbCpCl4] is treated with the selected isocyanide at room temperature, to give the pseudo-octahedral adducts [NbCp Cl4(CNR)] (R ¼ tBu, Xyl) in 75–80 yields.277 A comparative view of the X-ray structures of the NbV compounds [NbCp Cl4(CNXyl)], 17, and [NbCl5(CNXyl)], 18 (vide infra), is shown in Fig. 14, with relevant bonding parameters listed in the caption. The reaction of [NbCp Cl4] with 10% sodium amalgam in toluene, in the presence of three equivalents of isocyanide, proceeds with NbV to NbIII reduction to give the complexes [NbCp Cl2(CNR)3] in good yields (R ¼ tBu, Xyl).277 Parallel chemistry was described for the tantalum analogue [TaCpCl4].279 Interestingly, the reductive reaction of [NbCp Cl4] with a fivefold excess of CNtBu, carried out under experimental conditions resembling those reported for the synthesis of [NbCp Cl2(CNtBu)3], afforded the salt [NbCpCl(CNtBu)4][Nb(0)Cl4THF], the NbV oxido anion presumably originating from adventitious hydrolysis.280 Niobocene dichloride, [NbCp2Cl2], has been used to access NbIV isocyanide derivatives: chloride/isocyanide replacement is feasible in acetonitrile solution using TlBF4 as a chloride abstractor, leading to a dicationic product in 70% yield, Eq. (12).281   ½NbCp2 Cl2 Š + 2CNMe + 2TlBF4 ! NbCp2 ðCNMeÞ2 ½BF4 Š2 + 2TlCl (12) Several tantalum isocyanide complexes of the formula [TaCp2(Z2-butadiene)(CNR)]+ have been prepared in c. 70% yield from [TaCp2(Z4-butadiene)][B(Me)(C6F5)3] and alkyl-isocyanides, Scheme 33.282 The complete removal of the butadiene moiety by cyclohexyl isocyanide requires photolytic treatment. The IR spectrum of [TaCp2(Z2-butadiene)(CNtBu)][B(Me)(C6F5)3] (in KBr) displays the C^N stretching vibration band at 2164 cm−1. This is shifted 25 cm−1 higher than noncoordinated tert-butyl isocyanide, thereby reflecting negligible p-backbonding in the TaIII-isocyanide bond. The X-ray structure of the bis-substituted species [TaCp2(CNCy)2]+ indicates slightly bent isocyanide ligands [TadCdN angles are 177.4(7) and 179.4(9) ], and DFT calculations confirm the absence of substantial backbonding contribution to the Ta-isocyanide bond (which is primarily electrostatic).

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Scheme 33 Coordination of isocyanides to the vanadocene moiety. R ¼ tBu, nBu, Cy. From Strauch, H. C.; Wibbeling, B.; Fröhlich, R.; Erker, G. Organometallics 1999, 18, 3802–3812.

The addition of isocyanides to the TaIII complex [TaHCp2(Z2-L)] (L ¼ propene, 1-butene) promotes hydride migration to the alkene unit, resulting in the formation of the thermally stable, metal-alkyl derivatives [TaR0 Cp2(CNR)] (R0 ¼ C3H7, C4H9; R ¼ Xyl, Cy, Me) in good yields. The IR spectra of these products display low C^N stretching frequencies (1745–1800 cm−1, KBr or Nujol), accounting for significant Ta to CNR backbonding.283 Furthermore, AlEt3 adds to [Ta(C3H7)Cp2(CNMe)] to give the Lewis acid-base adduct [Ta(C3H7)Cp2{CN(Me)AlEt3}]; this transformation is accompanied by a substantial decrease in the C^N stretching vibration from 1745 to 1680 cm−1. The reaction of the dinuclear TaIV phosphinidene-bridged complex [TaCp Cl(m-PPh)]2 with a 10-fold excess of CNCy afforded [TaCp Cl(CNCy)4]Cl in 38% yield, via the formal release of {PPh} radical units (Scheme 34).284

Scheme 34 Synthesis of TaIIIdCp isocyanide derivative via m-phosphinidene elimination. From Blaurock, S.; Hey-Hawkins, E. Eur. J. Inorg. Chem. 2002, 2975–2984.

In addition to the selected cases reported above, a variety of isocyanide complexes of vanadium,285 niobium286–289 and tantalum290 have been described containing variably derivatized cyclopentadienyl rings. In these systems, the isocyanide moiety is normally robust and unreactive toward a range of transformations affecting other coligands.291 The fundamental octahedral coordination adducts [NbX5(CNXyl)] (X ¼ Cl, Br) have been recently isolated from 1:1 reactions of the niobium pentahalides with xylyl-isocyanide in toluene under strictly anhydrous conditions.278 The compounds [NbX5(CNXyl)] are possible, reasonable intermediates along the subsequent insertion processes (see Section 6.04.3.3), and completely degrade in dichloromethane at room temperature over a period of 24 h. The X-ray structures of [NbCl5(CNXyl)], 18, and the analogous NbV complex [NbCp Cl4(CNXyl)], 17, are featured by comparable bonding parameters related to the Nb-isocyanide bond, suggesting that this is negligibly affected by the nature of the trans-located ligand, that is, Cl or Cp (Fig. 14). In both cases, the isocyanide displays perfectly linearly coordination, and the Nb-CN bond distance values are close to the longest niobium-carbon bonds ever reported.292 The IR spectrum of 18 in the solid state displays the absorption assigned to the CN triple bond at 2221 cm−1, higher than the value for the uncoordinated isocyanide at 2120 cm−1 (solid state). A similar value (2217 cm−1, Nujol mull) has been reported for 17.277 Combined, X-ray and IR data indicate an almost pure s-bond between xylyl-isocyanide and NbV; nevertheless, DFT analysis of 18 outlined the presence of a minor backdonation contribution, directed to the isocyanide ligand from the cis-chloride ligands, thus overcoming the absence of electron charge on the d0 metal center.278

326

Organometallic Complexes of Group 5 With p-Acidic Ligands

It has to be mentioned here that the reaction of TaCl5 in THF with sodium amalgam, followed by addition of CNCy, leads to the TaII chloride-isocyanide derivative [TaCl2(CNCy)4] in moderate yield.261 A multitude of other isocyanide complexes of group 5 metals have been prepared from diverse starting materials through different strategies,293 including the substitution of carbonyl,294 phosphine,295,296 and nitrile ligands.297 Compound [V {N(SiMe3)2}3] readily adds a range of alkyl- and aryl-isocyanides affording trans-bis-isocyanide derivatives, despite the steric crowding supplied by the three silylamide ligands.298 The addition of one equivalent of CNAd (Ad ¼ adamantyl) to toluene solutions of analogous VIII species bearing poly-nitrogen ligands, [V{k4N-(Me3SiNCH2CH2)3N}] and [V{N(tBu)(3,5-C6H3Me2)}3], is exothermic by around −20 kcal mol−1 (experimental values).299 Unusual side on coordination (m-Z2) for an isocyanide ligand was observed for phenylisocyanide in a TaII complex.300

6.04.3.3

Isocyanide activation reactions

The chemistry of the halides of niobium and tantalum in their highest oxidation state is a longtime story. In the 1970s, it was found that the reactions of MX5 (X ¼ Cl or Br) with two equivalents of CNMe or CNtBu proceeded in diethyl ether with the straightforward formation of [MX4(Z1-CCl]NR)(CNR)], probably existing as isomeric mixtures of octahedral compounds, as generated from the isocyanide insertion into the metal-chloride bond (Scheme 35A).301 A second insertion is feasible using the less hindered methyl isocyanide.

Scheme 35 Different classes of activation reactions of isocyanides at group 5 metal centers. From Carnahan, E. M.; Lippard, S. J. J. Am. Chem. Soc. 1990, 112, 3230–3231.

Organometallic Complexes of Group 5 With p-Acidic Ligands

327

The insertion reaction of isocyanides into metal-halide bonds was also claimed with the VCl3/CNtBu system,302 but subsequent studies excluded this possibility (the reactivity of VCl3 with isocyanides was described at the beginning of Section 6.04.3.1). Other isocyanide insertion reactions with tantalum involve different ligands other than halides, usually promoted by the acidic TaV center. Thus the presumed pentaphenyl compound “TaPh5”, freshly generated in diethyl ether at −78 C from TaCl5 and LiPh, reacted with five equivalents of CNtBu affording [Ta(Z2-PhCNtBu)(Z2-Ph2CNtBu)2(CNtBu)] (63% yield) via multiple insertions into tantalum-phenyl bonds, Scheme 35B.303 Insertion reactions of isocyanides into metal-methyl bonds have been described on a NbV complex bearing a macrocyclic ligand304 and on the TaV species [TaCp Cln(Me)4-n] (n ¼ 0–3); for instance, the reaction of [TaCp Cl2(Me)2] with CNXyl in a 1:2 M ratio leads to the scission of the carbon-nitrogen triple bond via a azatantalacyclopropane intermediate (Scheme 35C).305 Besides insertion reactions triggered by Lewis acidic metal centers, other activation routes have been described for isocyanide compounds reacting with group 5 metal species. Thus, the reactions of the dinuclear compounds [M2Cl6(SMe2)3] (M ¼ Nb, Ta) with CNR (R ¼ tBu, CNBn, CNCy) gave [M2Cl6(CNR)6] in approximately 60% yields. These complexes contain four isocyanide ligands and two CdC bond coupled isocyanide units (Scheme 35D).306,307 The isocyanide moiety contained in the M(II) halide complexes represented in Scheme 31 may be prone to activation in the presence of water.261 For instance, the NbI compound [NbI(CO)2(CNtBu)4] reacts with water to form the NbIII-alkyne adduct [NbI2(CNtBu)4(k2-tBuNHC^CNHtBu)]I with CdC coupling between two isocyanide units.270 This reaction resembles the CO-activation processes described in Section 6.04.2.6. Moreover, reductive CdC bond forming reactions between CO and methyl-isocyanide ligands have been described for Nb and Ta complexes, triggered by the presence of a trialkylsilyl halide; a specific example is supplied in Scheme 35E.308,309

6.04.4

Cyanide compounds

6.04.4.1

Chemistry in water, cluster compounds, and supramolecular assemblies

The most convenient route to access poly-cyanide complexes of group 5 metals is metathesis reaction of suitable chloride precursors (Scheme 36). A variety of structures were hypothesized for the products obtained from the combination of VIII compounds with excess KCN in water, but their characterization was initially limited to elemental analysis and UV-spectroscopy data.310–312 It was not until 1972 that the homoleptic heptacyanovanadate(III) anion, [V(CN)7]3−, was characterized by X-ray diffraction in the salt K4[V(CN)7]
2H2O, 19.313 This salt was obtained from the two-step aqueous reaction of VCl3 with a large excess of HCl followed by treatment with nearly nine equivalents of KCN.314 Compound 19 was isolated as a deep-red crystalline material and must be stored under N2 atmosphere at c. −5 C. Attempts to prepare the analogous sodium and tetrabutylammonium salts were not successful. IR studies suggested that 19 did not change in a concentrated KCN solution, whereas EPR spectroscopy suggested disproportion to [VO(CN)5]3− and [V(CN)6]4− in less concentrated cyanide solutions. The yellow crystalline hexacyanovanadate salt K4[V(CN)6], 20, was also crystallographically characterized. The formation of this species from VCl3 and KCN required an additional reduction step with zinc.315

Scheme 36 Synthetic routes to mononuclear-homoleptic vanadium complexes. From Nelson, K. J.; Giles, I. D.; Troff, S. A.; Arif, A. M.; Miller, J. S. Inorg. Chem. 2006, 45, 8922–8929.

In 2006, Miller and coworkers attempted to prepare [V(CN)7]4−. To avoid the complications associated with hydrolytic decomposition, they performed the reaction of [VCl3(THF)3] with seven equivalents of NEt4CN in acetonitrile. Unexpectedly, the authors isolated the green-yellow [NEt4]3[V(CN)6]
4MeCN, 21, in 84% yield.316 X-ray analysis of 21 revealed the structure of the anion to be a distorted octahedron. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) studies showed that 21 was stable up to 175 C, and could be desolvated at room temperature under a constant flow of N2 over the sample.316 Another entry into mononuclear vanadium-cyanide aqueous species is provided by ammonium vanadate(V). The anionic complexes [V(CN){k3O,kN-N(CH2CH2O)3}(NO)]2–317 and [V(CN)5NO]3–318 were obtained using hydroxylamine as the NO source and crystallographically characterized. These complexes will be discussed in more detail in Section 6.04.4.1.

328

Organometallic Complexes of Group 5 With p-Acidic Ligands

In general, di- and polycyanide vanadate compounds have been intensively investigated for the construction of macromolecular structures with notable magnetic properties, exploiting the ability of the cyanide ligand to act as a linker between two metal centers. This ability is exemplified by the reaction of tetraethylammonium cyanide with a VIII substrate, affording in 76% yield a complex with a bridged linear cyanide between two VIII centers, Scheme 37.319 The low magnetic moment (2.14 BM) of the product indicates spin pairing.

Scheme 37 Assembly of two vanadium(III) molecular units with a m-kC,kN-cyanide ligand. From Davies, S. C.; Hughes, D. L.; Richards, R. L.; Sanders, J. R. J. Chem. Soc., Dalton Trans. 2002, 1442–1447.

When 21 was left to react with [Cr(NCMe)4][BF4]2 in acetonitrile, the Prussian blue analogue CrII0.5CrIII[VII(CN)6]
xMeCN was generated as a result of CrII to VIII internal electron transfer.316 The magnetic behavior of this chromium-vanadium hybrid is strictly dependent on the degree of solvation. A 3D-structure with the approximate formula CoII[VIVO(CN)4] was obtained by mixing aqueous solutions of V(O)SO4, Co(NO3)2, and KCN under N2 atmosphere; the IR spectrum of the product highlights the presence of bridging cyanide ligands with an infrared absorption at 2175 cm−1.320 Similar to the vanadate species mentioned above, the hexaniobium cluster [Nb6Cl12(CN)6]4−, 22, (which has a fractional formal oxidation state of c. 3.67) has also been employed as a constituent unit to build extended frameworks that have interesting potential as ion exchangers, molecular sieves, magnetic materials and gas storage materials. In this regard, 22 has been incorporated within a variety of hybrid materials with diverse metal atoms, including manganese,321,322 zinc, and nickel (1D polymeric structure).323 The structure of 22 has been crystallographically determined in miscellaneous salts,321,324 and consists of an octahedral {Nb6} cluster coordinated by twelve edge-bridging chloride ligands and six cyanide ligands located at the apical positions (Fig. 15).321 As a representative example of inclusion of the anion 22 in polymeric structures with magnetic relevance, we cite the coordination polymer [{Mn3(HCOO)2(H2O)4}{Mn(H2O)3(HCONH2)}2{Nb(CN)8}2]
4HCONH2
2H2O. The formiato ligand plays an important role in the self-assembly of the cyanido-bridged framework, and drives the coordination network topology, forming strong MndOCOdMn linkages (Fig. 16).325 There is only slight variation in the CdN distances, and no variation of the NbdC distances, upon coordination to manganese of the cyanide groups belonging to 22. The anion 22 can be efficiently prepared as the tetramethylammonium salt from [NMe4]3[Nb6Cl18]
2MeCN and a large excess of potassium cyanide in a water/acetonitrile mixture (yield 67%).321 Interestingly, a tantalum iodide species analogous to 22 has been recently reported. [(Ta6I12)(CN)6]4−, 23, is accessible via the high temperature (T ¼ 450–655 C) reaction of Ta powder with I2 in an evacuated quartz tube to afford the dark-violet solid Ta6I14. Subsequent reaction of Ta6I14 with NBu4CN in acetonitrile solution in a

Fig. 15 View of the X-ray structure of the {Nb6} cluster [Nb6Cl12(CN)6]4−, 22 (color codes: Nb, pale-blue; C, gray, N, blue; Cl, green). From Zhou, H.; Day, C. S.; Lachgar, A. Chem. Mater. 2004, 16, 4870–4877.

Organometallic Complexes of Group 5 With p-Acidic Ligands

329

Fig. 16 Detail of the X-ray structure of the coordination polymer [{Mn3(HCOO)2(H2O)4}{Mn(H2O)3(HCONH2)}2{Nb(CN)8}2]
4HCONH2
2H2O. From Korzeniak, T.; Pinkowicz, D.; Nitek, E.; Bałanda, M.; Fitta, M.; Sieklucka B. Dalton Trans. 2011, 40, 12350–12357.

Fig. 17 View of the X-ray structure of [(Ta6I12)(CN)6]4−, 23, in [NBu4]4[(Ta6I12)(CN)6]
CH3CN (color codes: Ta, pale-blue; I, violet, C, gray; N, blue). From Shamshurin, M. V.; Mikhaylov, M. A.; Sukhikh, T.; Benassi, E.; Tarkova, A. R.; Prokhorikhin, A. A.; Kretov, E. I.; Shestopalov, M. A.; Abramov, P. A.; Sokolov, M. N. Inorg. Chem. 2019, 58, 9028–9035.

sealed tube kept for 12 h at 100 C gave the dark-green crystalline [NBu4CN]4[23]
CH3CN in 51% yield.326 A view of the X-ray structure of 23 is shown in Fig. 17. The IR absorption of the cyanide stretching vibration is 2129 cm−1 (KBr) in 22321 and 2100 cm−1 (solid state) in 23.326 Like 22, air stable K4[Nb(CN)8]
2H2O, 24, has been used to prepare polynuclear aggregates that have aroused interest for their magnetic behavior when combined with a range of transition metals including manganese,327 iron,328 cobalt,329,330 vanadium,331 and nickel.330,332 The X-ray structure of 24 has been solved by different groups, showing a dodecahedral anion.333,334 The traditional preparation of 24 relied on an elaborate and environmentally unfriendly procedure published in 1975, consisting of electrolytic reduction of NbCl5 (Cl2 evolution) in methanol under N2 using a mercury-pool cathode, followed by addition of excess KCN (more than twice the required amount) in concentrated aqueous solution. The resulting impure K5[Nb(CN)8] was then oxidized with hydrogen peroxide to give 24 in only 15% yield.335 A more convenient and safer synthetic method to prepare 24 was recently described starting from [NbCl4(THF)2]: addition of a large excess of KCN in deaerated water allowed 24 to be isolated in 25%–30% yields.334 Water-soluble metal-oxido cluster anions [V16O38(CN)]9−, 25,336 and [Nb6Cl9O3(CN)6]5−, 26,337 have been proposed as constituent units to build magnetic materials. In particular, 25 can be synthesized from VOSO4
5H2O and KCN in H2O (89%

330

Organometallic Complexes of Group 5 With p-Acidic Ligands

yield). The cluster 25 takes the form of an icosioctahedron and has an IR band at 1620 cm−1, which is suggestive of an encapsulated cyanide unit approximately bearing double [C]N] bond.

6.04.4.2

Metallocene systems

The introduction of the cyanide function in metallocene derivatives of group 5 elements has been achieved by various methods, including chloride (or carbonyl) substitution, dealkylation of an isocyanide ligand, fluoride/cyanide exchange, and carbonyl to cyanide conversion. For example, [VCp2(CN)2]338,339 and [VCp0 2(CN)2]339 were synthesized in 70%–80% yields starting from the respective parent dichlorides and sodium cyanide in acetone, Eq. (13). They appeared thermally stable, in contrast to analogous alkynyl species, whose stability is strictly dependent on the combination of cyclopentadienyl and alkynyl substituents (vide infra).     VðZ5 -C5 H4 RÞ2 Cl2 + 2NaCN ! V ðZ5 -C5 H4 R Þ2 ðCNÞ2 + 2NaCl (13) R ¼ H, Me The attachment of the cyanide moiety to the {VCp 2} fragment has been realized via addition of isocyanides in hexane in a 1:2 M ratio (see Section 6.04.3.2). The final products of this reaction are [VCp 2(CN)(CNR)] (R ¼ tBu, Cy), through the presumed, intermediate formation of the corresponding bis-isocyanide species, Eq. (14).275 Dealkylation of one isocyanide ligand is favored by the relative stability of the eliminated alkyl radical.   ½VCp ∗ 2 Š + 2CNR ! VCp ∗ 2 ðCNRÞ2 ! ½VCp ∗ 2 ðCNÞðCNR ފ + R  (14) t R ¼ Bu, Cy The carbonyl analogue was also obtained via isocyanide activation: first [VCp 2(CO)] was converted into [VCp 2(CN)], in the presence of one equivalent of tert-butylisocyanide in a boiling hexane solution. Subsequent addition of CO at atmospheric pressure and room temperature afforded [VCp 2(CN)(CO)].275 A relevant dealkylation of one isocyanide ligand to cyanide has been also reported by Arnold et al. with a niobium(V) system.340 IR and X-ray data available for vanadium cyclopentadienyl compounds are useful to compare the metal-cyanide bonding (Table 3). In general, it appears that variations in the nature of the coligands provide a negligible effect on the donation/ backdonation balance. For instance, the highest wavenumber infrared absorption related to the cyanide group in the homologous Table 3

Collection of IR and X-ray data related to cyanide ligands in a selection of group 5 metal compounds.

Compound

Average M ox. state

IR ῦ(CN)/cm-1

X-ray ˚ M-CN/A

X-ray ˚ C N/A

X-ray M-C N/

Lit

K4[V(CN)7]
2H2O (19)

V+III

2100, 2070a

V+III V+I Nb+3.67 Ta+3.67 Nb+IV V+4.25 Nb+2.67 V+IV

172.5(1)-ax 178.8(2)-eq 178.1(5) 176.0(6) 178.3(6) 178.2(2) 177.6(2)

2.31(3)

[VCp’2(CN)2] [VCp 2(CN)(CNtBu)] [VCp 2(CN)(CNCy)] [VCp 2(CN)(CO)] [VCp 2(CN)] [TaCp 2(CN)2F] [NEt4][V(CN)3(k3N-Tp )] (27) [NEt4][V(O)(CN)2(k3N-Tp )] (28) [V(CN)3(cyclen)] [V4(CN)6(cyclen)4][CF3SO3]6 trans-[V(CN)2(dmpe)] [V2(CN)4(m-C4N4)(Me3-tacn)] (29) [V2(CN)4(m-C4N4)(Me3-tacn)]- (29-)

V+IV V+III V+III V+III V+III Ta+V V+III V+IV V+III V+III V+II V+IV V+3.5

2107, 2094, 2066b 2100b 2129b 2100c 2113b 1620c 2111d 2118, 2111a; 2116, 2111b 2111 b 2020, 2060a 2050, 2070a 2070a 2080a 2123b 2075a 2132, 2125a 2102 2156 2060 2225, 2214, 2114 2242, 2156, 2096

1.148(16)-ax 1.14(2)-eq 1.143(7) 1.150(8) 1.131(6) 2.27(3) 1.153(7) 1.052(11) 1.12(4)

313,314

[NEt4]3[V(CN)6]
xMeCN Ba[V(CN){k3O,kN-N(CH2CH2O)3}(NO)] [NMe4]2[(Nb6Cl12)(CN)6]
2MeOH [NBu4CN]4[(Ta6I12)(CN)6]
CH3CN K4[Nb(CN)8]
2H2O (24) K9[V16O38(CN)]
13H2O Cs5[Nb6Cl9O3(CN)6]
4H2O [VCp2(CN)2]

2.149(6)-eq 2.144(12)-ax 2.127(5) 2.172(6) 2.282(1) 1.16(3) 2.25(3)

2.158(3)

1.032(4)

176.9(2)

2.07(2)

1.17(2)

177(1)

2.088(7)

1.127(8)

178.7(6)

2.185(3) 2.141(4) 2.162(7) 2.132(12)

1.165(4) ¼¼ 1.155(8) 1.15(1)

¼¼ ¼¼ 175(3) 169(3)

2.126(10) 2.136(8)

1.137(11) 1.159(9)

176.4(8) 177.9(7)

Average values of X-ray data are reported for multicyanide compounds. a Nujol mull. b KBr disk. c Solid state (ATR). d Cs5 [Nb6Cl9O3(CN)6]
4H2O.

174.0(4)

316 317 321 326 334 336 337 338,339 339 275 275 275 275 341 342 342 343 343 297 344 344

Organometallic Complexes of Group 5 With p-Acidic Ligands

331

series of VIII pentamethylcyclopentadienyl complexes [VCp 2(CN)(CNtBu)], [VCp 2(CN)(CNCy)] and [VCp 2(CN)(CO)] falls in within the restricted range 2060–2070 cm−1, despite the variance in the electron donor properties of the additional coligand (alkyl isocyanide or carbon monoxide). In contrast, the relevant IR absorptions in the VIV complexes [VCp2(CN)2] and [VCp0 2(CN)2] occur at significantly higher wavenumbers (2118 and 2111 cm−1, respectively), as a consequence of the lower degree of backdonation due to the increased oxidation state (VIV vs. VIII) and despite the presence of a less donating cyclopentadienyl ring (Cp/Cp0 vs. Cp ). Crystallographic data are coherent, and indeed the average VdCN bonds longer in [VCp0 2(CN)2] compared to [VCp 2(CN)(CNCy)] [2.158(3) vs. 2.07(2) A˚ ] while the CdN(cyanide) bond is shorter in the former complex compared to the latter [1.032(4) vs. 1.17 (2) A˚ ]. The cyano-fluoride complex [TaCp 2(CN)2F] was generated from [TaCp 2F3] upon addition of an excess of Me3SiCN in chloroform, and finally isolated in 87% yield, Eq. (15). In the 19F NMR spectrum of the product, the fluoride ligand resonates at −194.1 ppm (CDCl3 solution), while in the precursor the three fluorides occur at −42.6 and −52.6 ppm.341   (15) ½TaCp ∗ 2 F3 Š + 2Me3 SiCN ! TaCp ∗ 2 ðCNÞ2 F + 2Me3 SiF Potassium bis(trimethylsilyl)amide was employed as an oxygen/nitrogen exchange agent in benzene solution to produce a cyano complex with a VI center (Eq. 16).345 The product was isolated in 70% yield. The use of an excess of amide did not lead to bis-substitution, and attempts to perform a similar reaction with Na2[VCp(CO)3] revealed successful.       (16) VCpðCOÞ4 + K NðSiMe3 Þ2 ! K VCpðCOÞ3 CN + OðSiMe3 Þ2

6.04.4.3

Other nonaqueous group 5 metal cyanides and activation reactions

Carbon monoxide/cyanide substitution is feasible on Na[V(CO)6] under photolytic treatment in the presence of NaCN, leading to Na2[V(CN)(CO)5] or [NEt4]4[V(CN)2(CO)4]2 depending on the conditions.346 The substitution of chloride ligands in [VCl3(THF)3] by KTp [Tp ¼ hydridotris(3,5-dimethylpyrazol-1-yl)borate] and [NEt4] CN in acetonitrile gives straightforwardly [NEt4][V(CN)3(k3N-Tp )], 27; the authors propose efficient metal-to-cyanide backbonding in 27, based on the low wavenumber value of the cyanide IR stretching vibration (ῦ ¼ 2073 cm−1).342 VIII to VIV oxidation takes place upon exposure to air resulting in the selective formation of [NEt4][V(O)(CN)2(k3N-Tp )], 28, which conversely revealed scarce p backdonation, Scheme 38 (ῦ ¼ 2132, 2125 cm−1).342

Scheme 38 VIII to VIV oxidation upon air contact of vanadium complex with tris(3,5-dimethylpyrazolyl)borate ligand. From Li, D.; Parkin, S.; Wang, G.; Yee, G. T.; Holmes, S. M. Inorg. Chem. 2006, 45, 2773–2775.

The VIII complex [V(CF3SO3)2(cyclen)]CF3SO3 (cyclen ¼ k4-1,4,7,10-tetraazacyclododecane) reacts with an excess of [NEt4]CN in DMF leading to the precipitation of the seven-coordinated [V(CN)3(cyclen)], an air-sensitive solid that is insoluble in any common solvent. [V(CN)3(cyclen)] belongs to the rare family of crystallographically characterized seven-coordinated VIII complexes. The reaction employing just 1.5 equivalents of [NEt4]CN produces a tetrahedral cage complex, [V4(CN)6(cyclen)4] [CF3SO3]6, wherein antiferromagnetic coupling between the cyano-linked VIII centers determines a S ¼ 0 ground state.343 An example of VII cyanide compound obtained by chloride substitution is trans-[V(CN)2(dmpe)] (dmpe ¼ 1,2-bisdimethylphosphinoethane).297 The synthesis of cyanide adducts of high valent early transition metal halides may be challenging due to the possibility of activation reactions triggered by the Lewis acidity of the metal center. Vanadium trichloride and high valent niobium chlorides were investigated in the past for their behavior toward cyanide sources. The basilary niobium pentachloride adduct [Nb(CN)Cl5]− was theoretically investigated, but never synthesized347; instead, the NbV adduct [Nb(CN)Cl4] was claimed to be generated from NbCl5 and gaseous HCN in diethyl ether.348 The reactions of VCl4, VCl3 and VBr3 with NH4CN in liquid ammonia gave [V(CN)3NH3] in admixture with the corresponding ammonium halide.349 The combination of NbCl4 with KCN in 1:4 M ratio afforded a complex of tentative formula [NbCl3(CN)]
2MeCN, displaying an infrared absorption at 2160 cm−1 attributed to the cyanide ligand.350

332

Organometallic Complexes of Group 5 With p-Acidic Ligands

Fig. 18 View of the X-ray structure of [V2(CN)4(m-C4N4)(Me3-tacn)]
2.5DMF, 29
2.5DMF344; H atoms have been omitted for clarity. Selected bond lengths (A˚ ) and angles ( ): V1-C4 2.122(8), C4-N4 1.148(8), V1-N1 1.721(5), N1-C1 1.359(8), C1-C2 1.48(1), C2-N2 1.142(9), V1-C4-N4 177.2(6), C1-N1-V1 161.9(5), C1-C2-N2 175.8(8). From Shores, M. P.; Long, J. R. J. Am. Chem. Soc. 2002, 124, 3512–3513.

Interestingly, in an attempt to prepare [V(CN)3(Me3-tacn)] (Me3-tacn ¼ N,N0 ,N00 -trimethyl-1,4,7-triazacyclononane) from [V(CF3SO3)3(Me3-tacn)] and lithium cyanide, the authors observed formation of the dinuclear complex [V2(CN)4(m-C4N4) (Me3-tacn)], 29, in 19% yield.344 The X-ray structure (Fig. 18) reveals two pseudo-octahedral VIV centers linked through a {N] C(CN)C(CN)]N} bridge, resulting from the multiple CdC coupling of four {CN} units. The short VdN distance, 1.721(5) A˚ , is consistent with an imido-type linkage. The monoanion [V2(CN)4(m-C4N4)(Me3-tacn)]−, [29]−, was prepared from 29 in acetonitrile solution by CoCp2 reduction (87% yield). Furthermore, Fox and Cummins reported that an NbIV complex, containing amide and triflate ligands, induced high yield reductive coupling of cyanide to cyanogen upon reaction with sodium cyanide in THF at room temperature, Eq. (17).351     (17) 2 NbðamideÞ3 ðO3 SCF3 Þ + 2NaCN ! ðamideÞ3 Nb  N − C  C − N  NbðamideÞ3 + 2NaO3 SCF3 Salient IR and X-ray data for the compounds discussed in this subchapter are summarized in Table 3.

6.04.5

Alkynyl compounds

The p-acceptor behavior of metal-bound alkynyls should be mainly related to low valent metal species (Fig. 19, structures A and B),352 while in high valent metal compounds, alkynyls prevalently behave as s-donors, with a possible secondary p-donor contribution (Fig. 19, structures A and C). DFT calculations carried out on the model VIV complex [V(C^CH)(OH)3]353 provided values for the VdC (1.969 A˚ ) and C^C (1.212 A˚ ) bond distances, which are consistent with a single VdCsp and a triple C^C bond, respectively. Thus, the V-alkynyl bond in [V(C^CH)(OH)3] was described essentially in terms of two main orbital-overlap interactions: (1) a strong donation from the acetylide s orbitals to the empty d2z vanadium orbital, forming the metal-carbon s bond; (2) a small donation from the acetylide p orbitals into the empty dxz and dyz orbitals of vanadium. This overall picture leads to a HOMO orbital which is mainly composed of acetylide p orbitals, and accounts for the observed reactivity of high valent group 5 metal-alkynyls with electrophiles at the a-carbon (vide infra). In the following section, an overview of group 5 metal-alkynyl complexes will be provided, and salient IR and X-ray data are supplied in Table 4. As it will be discussed for a couple of cases, a critical view of this data highlights that the metal to alkynyl backbonding is a real issue in low-medium valent group 5 metal complexes, and can be clearly recognized by a comparative analysis of IR and/or crystallographic parameters. Very few examples have been reported of well-defined compounds comprising the {MdC^CH} moiety (M ¼ group 5 metal).365 The hydride-ethynyl fragment {M(H)(C^CH)} was detected by IR spectroscopy in matrix, as generated by the combination of laser-ablated group 5 metal atoms with acetylene.369 The reactions of NbBr5 with excess sodium acetylide in liquid ammonia were claimed to produce materials having composition Na4[Nb6(C^C)4(N)8NH] and Na5[Nb6(C^C)5(N)5(NH)5], according to IR spectroscopy.370 The most common strategy to introduce alkynyl ligands in early transition metal complexes is halide substitution. However, the synthesis of poly-alkynyl species is not a trivial task, due to possible side reactions between the alkynyl moieties,371 and the key to stabilization consists in the use of sterically hindered alkynyl substituents. Structurally characterized homoleptic group 5 metal-alkynyl compounds are limited to the dark-blue colored [Li(THF)4][V(C^CR)4], 30,354 and [K(crypt-2.2.2)][Ta(C^CR0 )6],

Fig. 19 General resonance formulas representing a metal-alkynyl bond (A: s donation; B: p backdonation; C: p donation).

Organometallic Complexes of Group 5 With p-Acidic Ligands

Table 4

333

Collection of X-ray and IR data for selected group 5 metal-alkynyl complexes (R ¼ alkynyl substituent). R

IR ῦ(CC)/cm-1

X-ray ˚ M-CC/A

X-ray ˚ CC/A

X-ray M-C C/

Lit.

V+III Ta+V Ta+V V+III V+III V+III V+III V+III V+III V+III V+II V+II V+II Nb+V

2,6-C6H4(SiMe3)2 Si(tBu)3 Si(tBu)3 2,4,6-C6H2Me3 Ph t Bu Ph Ph Ph

2006, 2033, 2049sb 2015a 2012a

2.023(2) 2.138(6) 2.168(3) 2.032(13)

1.218(3) 1.210(10) 1.224(6) 1.234(17)

169.8(2) 168.2(7) 166.4(16 176.6(10)

2.075(5)

1.191(7)

177.0(4)

2.060(4) 2.081(6)

1.214(5) 1.208(8)

171.8(3) 175.3(4)

354 355 355 356 357 358 359 360 360

2.153(14) 2.18(1) 2.179(2) 2.173(8)

1.215(13) 1.21(1) 1.207(3) 1.190(10)

168.7(1) 180.0 177.8(2) 167.47

361 361 361 362

Nb+III Nb+III Nb+III Nb+III Nb+III Nb+III Nb+III Nb+IV

t

Bu SiMe3 SiMe3 t Bu Ph Ph Ph Ph

2080a 2034a 2015a 2093a 2076a 2049b 2035b 2034b

363 363 364 364 364 365 365 365

Nb+V

SiMe3

2070a

366

Nb+V

SiMe3

2090a

366

Nb+V

SiMe3

2110a

366

Nb+V

SiMe3

2100a

366

Nb+V

SiMe3

2085a

366

Nb+V

Ph

2100a

Nb+V

Me

2055a

366

Nb+V

CH2Ph

2090a

366

Nb+V

t

Bu

2075a

366

Nb+V

t

Bu

2100a

366

Nb+V

t

Bu

2055a

366

Nb+V

t

Bu

2080a

366

Nb+V

t

Bu

2080a

366

Nb+V

t

Bu

2100a

366

Nb+III Nb+III N.A. N.A. N.A.

H SiMe3 t Bu t Bu t Bu

2044b 2004b 2100a 2090a 2077

365 365 367 367 368

Compound

M ox. state

[Li(THF)4][V{CC C6H4(SiMe3)2}4] (30) [Li(CCSitBu3)3][Ta(CCSitBu3)3] (31a) [K(crypt-2.2.2)][Ta(CCR’)6] (31) [V(C5Me4Et)2(CCC6H2Me3)] (32) [V(C5Me4Et)2(CCPh)] (33) [VCp2(CCtBu)] (34) [VCp2(CCPh)] [VCpCl(CCPh)(PMe3)2] (36a) [VCp(CCPh)2(PMe3)2] (36b) [VCp(CCPh)(BH4)(PMe3)] [{Li(TMEDA)}2V(CCPh)4(TMEDA)] (37) [V(C CPh)2(TMEDA)2 (38a) [V(C CtBu)2(TMEDA)2] (38b) [Nb(NAr)(L)(C CR)] (39) [Nb(C5H4SiMe3)2(CCR)L] (40) L ¼ CNXyl L ¼ CNXyl L ¼ CO L ¼ CO L ¼ CO L ¼ P(OEt)3 L ¼ PMe2Ph [Nb(C5H4SiMe3)2(CCPh)( PMe2Ph)] [40]BPh4 [Nb(C5H4R”)2(NR’)(C CR)] (41) R’ ¼ 4-C6H4Me R” ¼ H R’ ¼ 4-C6H4Me R” ¼ SiMe3 R’ ¼ 4-C6H4OMe R” ¼ SiMe3 R’ ¼ tBu R” ¼ H R’ ¼ tBu R” ¼ SiMe3 R’ ¼ Ph R” ¼ SiMe3 R’ ¼ tBu R” ¼ H R’ ¼ tBu R” ¼ H R’ ¼ tBu R” ¼ H R’ ¼ tBu R” ¼ SiMe3 R’ ¼ 4-C6H4OMe R” ¼ H R’ ¼ 4-C6H4OMe R” ¼ SiMe3 R’ ¼ Ph R” ¼ SiMe3 R’ ¼ 4-C6H4Me R” ¼ SiMe3 [NbCp2(CCH)(CO)] (42) [NbCp2(CCSiMe3)(CO)] (43) [TaCp (C CtBu)2(DADPr)] (44a) [TaCp (C CiPr)2(DADPr)] (44b) [Ta(butadiene)(C CtBu)(Ph)] (46)

Average values of X-ray data are reported for multialkynyl compounds. a KBr disk b Nujol mull.

Ph Ph t Bu Ph

2040 2075a 2060 2043a 2041a 2047a 2000a 2020a 2035a

2.170(7)

2.171(5) 2.199(10) 2.20(2)

1.20(1)

1.198(7) 1.194(12) 1.21(3)

177.9(6)

175.7(4) 174.7(7) 172(1)

366

334

Organometallic Complexes of Group 5 With p-Acidic Ligands

31 [R ¼ 2,6-C6H4(SiMe3)2; R0 ¼ Si(tBu)3].355 These two salts were obtained respectively from [VCl3(THF)3] and TaCl5 via multiple chloride substitution upon reaction with the corresponding bulky lithium acetylides, Scheme 39. The initially obtained [Li(C^CR0 )3][Ta(C^CR0 )3], 31a, is converted into 31 by means of metathesis reaction with KCF3SO3 and subsequent treatment with crypt-2.2.2. The anion in 30 exhibits an almost regular tetrahedral geometry. The salt is paramagnetic with a magnetic susceptibility of 2.78 BM at room temperature, which is in accordance with a system containing VIII and a d2 configuration. The anion in both 31 and 31a exhibits a twisted trigonal prismatic geometry centered on tantalum, with a significant deviation from linearity of the TadC^C angle, which has been attributed to the steric demand of the alkynyl substituent R0 . In all cases, the C^C bond lengths are close to what observable in an ideal triple carbon-carbon bond.

Scheme 39 Synthesis of homoleptic VIII and TaV alkynyl salts. From Vaid, T. P.; Veige, A. S.; Lobkovsky, E. B.; Glassey, W. V.; Wolczanski, P. T.; Liable-Sands, L. M.; Rheingold, A. L.; Cundari, T. R. J. Am. Chem. Soc. 1998, 120, 10067–10079.

A range of vanadium-alkynyl complexes has been obtained by reactions of vanadocene halides with the appropriate lithium or magnesium salts. In general, the stability of the products is finely dependent on the electronic and steric features related to the cyclopentadienyl and alkynyl substituents (Fig. 20). More precisely, both per-alkylation of the C5 ring and a bulky electron-donor group on the alkynyl carbon appear to contribute to the stability, as illustrated in the following examples. The first structurally characterized alkynyl of a group 5 metal was [V(Z5-C5Me4Et)2(C^CC6H2Me3)], 32, obtained in 21% yield from the reaction of [V(Z5-C5Me4Et)2Br] with sodium mesitylacetylide in diethyl ether.356 In contrast to the nonalkylated homologous complex, which is not stable at room temperature, 32 melts at c. 85 C and resists decomposition over several hours in a solution of boiling toluene. The crystallographic determination of 32 permitted the first assessment of the length of a VIIIdCsp bond (2.032 A˚ ). The analogue [V(Z5-C5Me4Et)2(C^CPh)], 33, was also reported to be a relatively stable black crystalline solid melting at 70–71 C.357 [VCp2(C^CtBu)], 34, was synthesized from vanadocene chloride and the corresponding lithium acetylide in tetrahydrofuran/pentane solution. It was isolated at room temperature (28% yield) and structurally characterized by X-ray diffraction.358

Fig. 20 Structures of isolated vanadocene-acetylide complexes mentioned in the text. From Kokler, F. H.; Prossdorf, W.; Sckubert, U. Inorg. Chem. 1981, 20, 4096–4101.

Organometallic Complexes of Group 5 With p-Acidic Ligands

335

[VCp2(C^CPh)], differing from 34 in that phenyl, replaces the tert-butyl group, was investigated by NMR spectroscopy at low temperature.357 This complex degrades in diethyl ether at room temperature and in the solid state at 80 C under N2 atmosphere.359 The dimetal bis-acetylide complex [Cp2V(C^CC6H4C^C)VCp2] was not isolated, in agreement with the instability of the monomeric congener. Conversely, [(Z5-C5Me4Et)2V(C^CC6H4C^C)V(Z5-C5Me4Et)2], 35, melting at 111–112 C, was isolated in 11% yield.372 Half-sandwich VIII alkynyl complexes with additional phosphine ligands are significantly more stable than the bis-cyclopentadienyl compounds discussed above. The reactions of [VCpCl2(PMe3)2] with 1–2 equivalents of LiC^CPh in diethyl ether selectively afforded the corresponding substitution products [VCpCl2-n(C^CPh)n(PMe3)2] (n ¼ 1, 36a; n ¼ 2, 36b), in moderate yields, Scheme 40.360 The subsequent reaction of 36a with LiBH4 produces the borohydride complex [VCp(C^CPh) (k2H-BH4)(PMe3)] in 55% yield. The X-ray structures of 36a and 36b display distorted square-pyramids with the cyclopentadienyl ring at the apical position, and they have comparable VdCalkynyl and C^C distances. The formal addition of two donating PMe3 ligands, on going from [VCp2(C^CPh)] to 36b, results in an appreciable decrease in the C^C stretching vibration wavenumber in the respective IR spectra (from 2060 to 2041 cm−1, Nujol), presumably as a result of increased V ! C backdonation. The infrared C^C stretching wavenumber is almost identical in 36a and 36b; however, an appreciable difference in the VIIIdCsp distance is observed in the respective crystal structures, being 2.060(4) A˚ in [VCpCl(C^CPh)(PMe3)2], 36a, and 2.081(6) A˚ in [VCp(C^CPh)2(PMe3)2], 36b. It seems that, compared to the situation in 36a, the presence of two p-acceptor acetylide ligands in 36b lowers the degree of backdonation from the metal center to each of the two ligands, resulting in a slightly longer vanadium-alkynyl bond distance.

Scheme 40 Synthesis of vanadium(III) half-sandwich alkynyl complexes via replacement of chloride ligands. From Buijink, J. K. F.; Kloetstra, K. R.; Meetsma, A.; Teuben, J. H. Organometallics 1996, 15, 2523–2533.

Note that 36b can be maintained in benzene-d6 solution at 100 C for 16 h without observing decomposition. The higher stability of 36b with respect to related dialkyl compounds, [VCpR2(PMe3)2], is manifested by the fact that the former does not undergo comproportionation with [VCpCl2(PMe3)2], whereas [VCpMe2(PMe3)2] does afford the monomethyl [VCpClMe(PMe3)2] at 25 C. Furthermore, [VCpPh2(PMe3)2] decomposes at room temperature through b-hydrogen abstraction to give the benzyne complex [VCp(Z6-C6H4)(PMe3)2]. Vanadocene and [MCp2(C^CPh)] (M ¼ Ti, Zr) react in toluene to give [VCp2(m:Z2:Z4-butadiyne)MCp2] in high yields, which may be viewed as the result of M to V double transfer of the phenylethynyl group; the reaction presumably proceeds in two steps via preliminary formation of the highly reactive [VCp2(C^CPh)2]359 and MCp2, followed by reductive coupling of the V-bound alkynyl ligands (Scheme 41).373 The products contain vanadium in the +IV oxidation state, in accordance with magnetic analysis (meff ¼ 1.73 BM).

Scheme 41 Synthesis of m:Z2:Z4-butadiyne ligand via double alkynyl metalation. From Danjoy, C.; Zhao, J.; Donnadieu, B.; Legros, J.-P.; Valade, L.; Choukroun, R.; Zwick, A.; Cassoux, P. Chem. Eur. J. 1998, 4, 1100–1105.

336

Organometallic Complexes of Group 5 With p-Acidic Ligands

Beyond vanadocene derivatives, the reactions of [VCl3(THF)3] with a fivefold excess of LiC^CPh, in tetrahydrofuran in the presence of N,N,N0 ,N0 -tetramethylethylenediamine (TMEDA), gave [{Li(k2N-TMEDA)}2V(C^CPh)4(k2N-TMEDA)], 37, in 64% yield. A lower amount (3 eq.) of acetylide led to isolation of [V(C^CPh)2(k2N-TMEDA)2], 38a (71% yield), Scheme 42.361 Compounds 37 and 38a contain formally hexacoordinate vanadium(II), with one electron reduction of the starting VIII chloride presumably being accomplished by the excess of acetylide reagent. Consistent with this observation, 37 and 38a could be obtained in optimized yields starting from the VII precursor [VCl2(k2N-TMEDA)2], upon reaction with the appropriate amounts of lithium phenylacetylide. In contrast with the results from LiC^CPh, [V(C^CtBu)2(k2N-TMEDA)2], 38b, was the only product isolated from the reaction of [VCl3(THF)3] and an excess ( 3 eq.) of LiC^CtBu.361 Since 37 is soluble in hexane and other organic solvents, the lithium cations are probably tightly bound to the alkynyl moieties in solution. Compounds 38a–b are isostructural, with the two acetylide groups occupying mutual trans sites. A view of the X-ray structure of 37 is shown in Fig. 21. The deviation from linearity of the VdC^C bonds [average value 168.7(1) ] is ascribable to lithium coordination to the alkynyl a-carbons. The tert-butyl alkynyl stretching vibration in 38b gives rise to an infrared band at 2035 cm−1; for sake of comparison, the corresponding value in 34 is 2075 cm−1.

Scheme 42 Reductive synthesis of stable vanadium(II)-alkynyl complexes. TMEDA ¼ Me2NCH2CH2NMe2. From Kawaguchi, H.; Tatsumi, K. Organometallics 1995, 14, 4294–4299.

The alkynyl ligand in the VII complexes 37–38 is prone to electrophilic addition at the a-carbon and insertion of unsaturated molecules into the VdCa bond, which is different to what is usually observed with late transition metal-alkynyl species that undergo addition at the b-carbon.374–376 For instance, the reactions of 37 and 38a with CO2, followed by hydrolysis, led to recovery of PhC^CCO2H.361 Similarly, transient VIII alkynyl species, generated from VCl3, were exploited to efficiently generate 3-alkynones, R0 C(O)C^CR (R ¼ Ph, nBu, nOct), via nucleophilic transfer of the acetylide unit to a variety of aldehydes R0 CHO. This reaction is tolerant of various functionalities on R0 .377 A wide range of niobium-alkynyl compounds has been prepared by Otero and coworkers via chloride substitution routes, including the imido NbV complexes 39,362 the bis-cyclopentadienyl NbIII complexes 40,363,365 and an extended series of bis-

Fig. 21 View of the X-ray structure of [{Li(k2N-TMEDA)}2V(C^CPh)4(k2N-TMEDA)], 37361; H atoms have been omitted for clarity. TMEDA ¼ Me2NCH2CH2NMe2. From Kawaguchi, H.; Tatsumi, K. Organometallics 1995, 14, 4294–4299.

Organometallic Complexes of Group 5 With p-Acidic Ligands

337

cyclopentadienyl-imido NbV complexes, 41 (Scheme 43).363,366 Compounds of the family 40 can be oxidized by [FeCp2]+ to produce transient radical-cationic species.365 One of them was found to efficiently dimerize to dinuclear niobocene divinylidene.364 The two-step desilylation of [Nb(Z5-C5H4SiMe3)2(C^CSiMe3)(CO)] finally gave the rare terminal acetylide complex [Nb(Cp)2(C^CH)(CO)], 42, through the intermediate formation of 43.365

Scheme 43 Niobium alkynyl complexes. 39: Ar ¼ 2,6-C6H3(iPr)2; R ¼ Ph or SiMe3. 40: R ¼ Ph, SiMe3 or tBu; L ¼ CO, CNXyl (Xyl ¼ 2,6-C6H3Me2), PPhMe2 or P(OEt)3. 41: R ¼ SiMe3, tBu, CH2Ph, or Ph; R0 ¼ Ph, 4-C6H4Me, 4-C6H4OMe or tBu; R00 ¼ H or SiMe3. Various references (see text).

The thermally stable tantalum complexes [TaCp (C^CR)2(DADPr)] (R ¼ tBu, 44a; R ¼ iPr, 44b)367 and [Ta(C^CR)(BDPP) (Z -PrC^CPr)], 45 [BDPP ¼ 2,6-(ArNCH2)2NC5H3, Ar ¼ 2,6-C6Hi3Pr2],378 are accessible via the typical chloride/acetylide substitution strategy (Scheme 44). Compound 45 undergoes straightforward insertion of terminal alkynes into the Ta-alkynyl bond, according to a mechanism which is relevant to the alkyne polymerization process. 2

Scheme 44 Stable Ta-acetylide complexes obtained via chloride substitution. 45: Ar ¼ 2,6-C6Hi3Pr2; R ¼ Ph, Bu, SiMe3, or 2-C6H4SiMe3; R0 ¼ Ph, Bu, or SiMe3. From Guérin, F.; McConville, D. H.; Vittal, J. J.; Yap, G. A. P. Organometallics 1998, 17, 1290–1296.

338

Organometallic Complexes of Group 5 With p-Acidic Ligands

A further general route to access Nb and Ta-alkynyls consists of the in situ deprotonation of a terminal alkyne, HC^CR, upon reaction with a suitable substrate, [LnMX], leading to [MLn(C^CR)(XH)] [M ¼ Ta, X ¼ Z2-C6H4 (benzyne) and R ¼ tBu, see compound 46 in Scheme 45368; X ¼ Z2-C2H4 (ethylene)379; X ¼ NCMe3380; X ¼ Z2-C3H4 (cyclopropene)381].

Scheme 45 In situ hydrogen transfer from alkyne reagent (converting to acetylide) to benzyne ligand (converting to phenyl). From Oulié, P.; Dinoi, C.; Li, C.; Sournia-Saquet, A.; Jacob, K.; Vendier, L.; Etienne, M. Organometallics 2017, 36, 53–63.

6.04.6

Other p-acidic ligands

6.04.6.1

Nitrosyl compounds

A variety of vanadium-nitrosyl compounds have been reported and structurally characterized, and salient IR and X-ray data for the compounds treated in this section are comparatively shown in Table 5. These include a number of nonorganometallic compounds. For instance, VCl4 is a convenient commercial reagent to obtain vanadium-mixed chlorido-nitrosyl species. The slow addition of dry NO gas to a CCl4 solution of VCl4 results in the almost quantitative formation of the dark-brown polymer [VCl2(NO)3]n, with the chloride ligands being eliminated as NOCl, Eq. (18).382 The nitrosyl groups give rise to strong IR absorptions at 1924 and 1764 cm−1 (Nujol mull).

Table 5

Collection of X-ray and IR data for selected group 5 metal-nitrosyl compounds.

Compound

M ox. state

IR ῦ(NO)/cm-1

X-ray ˚ M-N/A

X-ray ˚ N-O/A

X-ray M-N-O/

Lit.

[VCl2NO(THF)3] [V{N(CH2O)3}NO][V(CH2SiMe3)Cl(NO)(trimpsi)] (47) K3[V(CN)5NO] [V(CO)5NO] [V(CO)4NO(PMe3)] [V(CO)4NO(NMe3)] [V(CO)3NO(PMe3)2] [VCp 2Br(NO)][VCp Br2(NO)]

V+I V+II V+I V+I V-I V-I V-I V-I (V+III/V+I)

1646a 1504b 1550a

1.689(2) 1.681(5) 1.755(8) 1.806(6)

1.182(3) 1.258(6) 1.117(8) 1.235(8)

178.4(2) 177.3(6) 174.1(6)

1695c 1663c 1650c 1582d 1760,1748a

1.809(10) 1.746(16)/ 1.741(17)

1.204(11) 1.165(23)/ 1.134(21)

176(1) 175.0(14)/ 165.6(18)

382 383 384 318 385 385 386 386 387

[VCp(CO)(NO)2] (48) [VCp(NO)3][PF6] [Nb(CO)2(NO)(trimpsi)] (49a) [Ta(CO)2(NO)(trimpsi)] (49b) [V(CO)2(NO)(trimpsi)] (49c) [VCl2(NO)(trimpsi)] (50) [Ta(CNXyl)5(NO)]

V+I V-I Nb-I Ta-I V-I V+I Ta-I

1725,1636e 1912,1794 1518a 1515a 1543a 1601a 1542

[Nb(CNXyl)4(NO)2][BF4]

Nb-I

1700,1623d

[Ta(CNXyl)4(NO)2][BF4]

Ta-I

1686,1618d

a

Nujol mull. KBr disk. c Hexane solution. d CH2Cl2 solution. e THF solution. b

2.094(5) 2.144(10) 1.823(9)

1.158(5) 1.066(9) 1.24(1)

169(2) 170(2) 179.1(8)

1.887(2) 1.889(2) 1.887(2) 1.889(2) 1.902(5) 1.914(5)

1.183(3) 1.186(3) 1.183(3) 1.186(3) 1.182(6) 1.193(6)

177.7(2) 179.3(2) 177.7(2) 179.3(2) 177.8(4) 179.4(4)

388 385 389 389 390 389 262 267 267

Organometallic Complexes of Group 5 With p-Acidic Ligands   nVCl4 + 5nNO ! VCl2 ðNOÞ3 n + 2nNOCl

339

(18)

The polymeric material undergoes partial nitrosyl displacement upon reaction with tetrahydrofuran in dichloromethane suspension, affording the octahedral complex [VCl2NO(THF)3], Eq. (19). The X-ray structure shows a mer configuration of ligands, and an almost linear VdNdO unit, while the IR spectrum displays an intense band at 1646 cm−1.     VCl2 ðNOÞ3 n + 3nTHF ! n VCl2 NOðTHFÞ3 + 2nNO (19) A feasible source for the NO ligand in aqueous solution is hydroxylamine, and various anionic complexes were prepared using this strategy.383,391 Cyanide addition may result in the formation of mixed cyanide-nitrosyl species (see Section 6.04.4.1), including [V(CN)5NO]3−318 and [V(CN)6NO]4−.383 The X-ray structure of the latter anion reveals a pentagonal-bipyramidal configuration with the NO ligand in the axial position.392 The bond distances suggest considerable vanadium to nitrosyl p-backbonding.387 Interestingly, substitution reactions with N,N-donors leave the nitrosyl group unaffected and involve cyanide ligands only.393,394 [VCl2NO(THF)3] (Eq. 18) is an excellent starting material for the synthesis of other vanadium nitrosyl complexes.382 Replacement of the three tetrahydrofuran ligands with the tridentate phosphorus ligand trimpsi, tBuSi(CH2PMe2)3, provided access to alkyl-nitrosyl complexes of VI. [VRCl(NO)(trimpsi)] (R ¼ Me, CH2SiMe3, CH2CMe3) were obtained in 25%–50% yields from the reactions of [VCl2(NO)(trimpsi)] with the appropriate magnesium alkyl reagents.384 A view of the X-ray structure of [V(CH2SiMe3)Cl(NO)(trimpsi)], 47, is supplied in Fig. 22; the almost linear VdNdO bond and the IR stretching frequency assigned to the nitrosyl group (1550 cm−1, Nujol) suggest a significant p-backdonation contribution to the VdNO bond. This appears the consequence of the combined electron donor nature of the triphosphorous and alkyl ligands. For comparison, the infrared NO stretching vibration occurs at 1646 cm−1 in the parent compound [VCl2NO(THF)3]. A convenient entry to the chemistry of vanadium carbonyl-nitrosyl complexes is [V(CO)5NO], which is considered a remarkably reactive relative of isoelectronic [Cr(CO)6]395; the former rapidly decomposes at 10 C to give a black solid lacking CO and NO groups. Such instability may be imputable to the weakening effect of NO on the VdCO bonds, as witnessed by the exceedingly high IR carbonyl stretching wavenumber (2100 cm−1 in hexane solution); the complementary n(NO) value (1695 cm−1) suggests considerable vanadium-to-nitrosyl p-backbonding. In 1961, Hieber and coworkers reported the formation of a thermally unstable violet-red product that was formulated as [V(CO)5NO] based on IR spectroscopy by passing gaseous NO through a cold solution of [V(CO)6] in cyclohexane.130 Later on, this compound was isolated as red-violet crystals at −78 C, by means of the straightforward reaction of [NEt4][V(CO)6] with [NO][BF4] in dichloromethane at −40 C (Eq. 20), and unambiguously identified by mass spectrometry.395     (20) ½NEt4 Š V ðCOÞ6 + ½NOŠ½BF4 Š ! VðCOÞ5 NO + ½NEt4 Š½BF4 Š + CO Subsequent in situ addition of a variety of reactants to freshly prepared [V(CO)5NO] provides good to excellent yields of coordination adducts such as [V(CO)4(NO)(PMe3)], thermally unstable [V(CO)4(NO)(NMe3)], [NEt4][VI(CO)4(NO)], [V(CO)2NO(Z6-C5H4]CHNMe2)], and [NEt4][VCp(CO)2NO].395–397 The last substance is the first example of a carbonylnitrosyl-cyclopentadienylmetallate ion and the final member of the accessible isoelectronic series [VCp(NO)3− x(CO)x]1− x (x ¼ 0–3). Monosubstituted complexes like [V(CO)4(NO)(PMe3)] possess trans-geometry; related disubstituted derivatives were also prepared in lower yields from [V(CO)5NO] via a slow dissociative pathway,398 and alternatively through nitrosylation of

Fig. 22 View of the X-ray structure of [V(CH2SiMe3)Cl(NO)(trimpsi)], 47384; H atoms have been omitted for clarity. Selected bonding parameters: VdN 1.755(8) A˚ , NdO 1.117(8) A˚ , VdNdO 174.1(6) . From Hayton, T. W.; Patrick, B. O.; Legzdins, P. Organometallics 2004, 23, 657–664.

340

Organometallic Complexes of Group 5 With p-Acidic Ligands

[V(CO)4L2] with NOCl or [Co(NO)2Br]2.399 Compound [V(CO)3NO(PMe3)2] was characterized by X-ray diffraction, revealing a pseudo-octahedral structure with CO ligands occupying mer-sites and the two phosphine ligands in mutual cis positions.386 Note that the infrared NO stretching moves from 1663 cm−1 (hexane solution) to 1582 cm−1 (dichloromethane) on going from [V(CO)4NO(PMe3)] to [V(CO)3NO(PMe3)2], as a result of the substitution of one carbonyl with the stronger donor phosphine. The reactivity of vanadocene compounds with NO was investigated in the past, but in several cases a clear identification of the products failed. Bottomley and coworkers reported the reaction between [VCp2] and NO as proceeding with 1:1 stoichiometry presumably to give the unstable adduct [VCp2NO], which is believed to decompose via disproportionation.400 Previously, the group of Morán proposed the formula [VCp2(NO)2]2 for such reactive species.401 The addition of NO to [VCp2(CO)] was claimed to produce the {NCO} moiety; however, the exact nature of product(s) was not determined. The combination of [VCp2X] (X ¼ Br, I) and NO led to two different compounds analyzing as [VCp2X(NO)], whose structures have remained uncertain.402 The reaction between [VCp 2Br2] and NO afforded the salt [VCp 2Br(NO)][VCp Br2(NO)] in 70% yield, which was characterized by X-ray diffraction.403 The X-ray structure provides the opportunity to compare the bonding situation of the nitrosyl ligand in two very close coordination environments, but with different metal oxidation states (i.e., VIII in the cation and VI in the anion). Apparently, the most significant difference is given by the [VdNdO] angle, measuring 175.0 in the cation and 165.6 in the anion. The introduction of the nitrosyl functionality in mixed cyclopentadienyl carbonyl complexes of vanadium is a more defined process. The key compound [VCp(CO)(NO)2], 48, has been synthesized by several different approaches (Scheme 46). The photo-induced transmetalation reaction between [VCp(CO)4] and the cobalt nitrosyl source [Co(NO2)Br] affords 48 in 40% yield via formation of the volatile species [Co(CO)3(NO)].388 The moderate yield is attributed to the side formation of an undefined V/Co/Br species. The reduction of [VCp(CO)4] with sodium amalgam, followed by nitrosylation with N-methylN-nitroso-p-toluenesulfonamide (diazald), is another possible route to 48 (34% yield).385 Previous preparations of 48 consisted of direct nitrosylation of various VI precursors, including [VCp(CN)(CO)3]−, but the reactions had very low isolated yields.404 The carbonyl ligand in 48 is prone to selective substitution by an additional nitrosyl group via reaction with [NO][PF6],385 and by a wide range of phosphines (Scheme 46).388,405 The resulting products, [VCp(L)(NO)2], do not differ significantly in the infrared n(NO) stretching frequency, but do exhibit a progressive deshielding of the 51V NMR resonance (from −1328 to −973 ppm) on increasing the electron donating propensity of L.

Scheme 46 Synthetic routes to V−I cyclopentadienyl-carbonyl-nitrosyl, and subsequent CO substitution reactions. L ¼ PF3, P(OiPr)3, PPh3, PEtPh2, PPh(NEt2)2, (MeO)2PCH2CH2P(OMe)2, PR3 (R ¼ alkyl groups). From Herberhold, M.; Klein, R.; Smith, P. D. Angew. Chem. lnt. Ed. 1979, 18, 220–221.

Niobium and tantalum nitrosyl coordination compounds are rare compared to those with vanadium. This fact reflects the possible drawbacks generally encountered in the synthesis of early transition metal nitrosyls. For instance, the electropositivity of the metal centers may promote oxygen abstraction from NO, and indeed this process was observed in the reaction of [NbCp2Me2] with NO, leading to [Nb(O)Cp2Me] and Me3N]NMe.406 An appropriate choice of coligands is an essential requirement to obtain stable niobium and tantalum nitrosyl complexes. Indeed, the first reported examples were [M(CO)2(NO)(k3P-trimpsi)] (M ¼ Nb, 49a; M ¼ Ta, 49b; trimpsi ¼ tBuSi(CH2PMe2)3].389 In the X-ray structure of [V(CO)2(NO)(k3P-trimpsi)], 49c, the NdO bond length is rather short when compared to typical NdO distances in metal nitrosyls, falling in the range 1.15–1.25 A˚ ; this feature was interpreted as an artifact due to thermal disorder in the VdNO group.390 Upon PhICl2 oxidation, 49c is converted to [VCl2(NO)(k3P-trimpsi)], 50, which clearly manifests a decreased p-backbonding compared to 49c (ῦNO ¼ 1601 cm−1 in 50 and 1543 cm−1 in 49c, Nujol).389 The synthesis of tantalum nitrosyl complexes from [Ta(CO)6]− and NO was attempted but unsuccessfully, and no clear proof for the existence of the Nb and Ta analogues of [V(CO)5NO] has been collected so far.395 On the other hand, the isocyanide analogue of [Ta(CO)5(NO)], that is, [Ta(CNXyl)5(NO)], was prepared by the diazald nitrosylation of [Ta(CO)6]− in tetrahydrofuran (see Scheme 30).262 Interestingly, the use of [NO][BF4] as an alternative nitrosylating agent on cold dichloromethane solutions of [NBu4][M(CO)6] (M ¼ Nb, Ta) and xylyl isocyanide leads to [M(CNXyl)4(NO)2][BF4] in 50%–55% yields (M ¼ Nb, Ta; Eq. 11).267 The cations [M(CNXyl)4(NO)2]+ are isomorphous and present the NO groups in relative cis-position. The IR stretching vibrations related to the NO ligands (around 1700 and 1620 cm−1, CH2Cl2 solution) fall well below the value for nitric oxide (1860 cm−1); on the other hand, the IR absorptions accounting for the isocyanide ligands occur at c. 2170 and 2140 cm−1 (uncoordinated 2,6-dimethylphenyl isocyanide: 2123 cm−1, ATR). These data indicate that p-backdonation from the M−I center is essentially directed to the two nitrosyl ligands and does not involve the four aryl-isocyanide groups.

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Fig. 23 View of the unusual m-k1-k2-coordination mode of the dinitrogen ligand in a ditantalum complex. From Fryzuk, M. D.; Johnson, S. A.; Patrick, B. O.; Albinati, A.; Mason, S. A.; Koetzle, T. F. J. Am. Chem. Soc. 2001, 123, 3960–3973.

6.04.6.2

Dinitrogen compounds

Bimetallic complexes containing a bridging N2 ligand have been intensively investigated in the field of dinitrogen activation,407 and diverse case studies include VdV,408 NbdNb,409 and Ta-Ta276,410 systems. A quite unusual coordination mode has been observed in the dinuclear tantalum(III) complex [{Ta(m-H)(NPN)}2(m-k1:k2-N2)], NPN ¼ PhP(CH2SiMe2NPh)2, see Fig. 23.411 In the following section, we will describe complexes containing terminal N2 ligand(s), since this is the same coordination fashion usually adopted by isoelectronic carbon monoxide. Studies on elusive, homoleptic dinitrogen adducts of group 5 metals will be briefly resumed. Compound [V(N2)6], isoelectronic and homologous to [V(CO)6], and also [V2(N2)n] (with n probably being 12) were synthesized in matrixes at very low temperatures, by condensing vanadium atoms with dinitrogen.412 A N2/V ratio approximately equal to 104 led to the prevalent formation of [V(N2)6], while the dimeric species increased on decreasing the ratio. The infrared absorption related to [V(N2)6] was detected at 2100 cm−1. Analogous reactions conducted with niobium or tantalum were not clean.413 Instead, the cocondensation of Nb and Ta atoms with N2 and CO in matrixes at low temperature afforded mixtures of carbonyl, dinitrogen and hybrid species, identified by IR spectroscopy.414 IR evidence for the formation of unstable adducts of dinitrogen with [VCp2] and [VCp 2] was collected at 16 K, the absorption due to the NdN bond stretching occurring at 2144 and 2142 cm−1, respectively.415 Photolytic treatment of a THF solution of [NEt4][V(CO)6] under N2 atmosphere at 200 K leads to [V(CO)5(N2)]−, decomposing in hydrocarbons above 225 K.416 In contrast, the low temperature reduction of [VCl3(THF)3] with Na in THF, in the presence of 1,2-bis(diphenylphosphino)ethane (dppe) and N2, afforded the sodium salt of trans-[V(N2)2(dppe)2]−, in 81% yield.417 The latter is long-lived at room temperature, decomposing in THF solution over the course of days. It is a rare example of crystallographically characterized group 5 metal compound with a terminal N2 ligand, although a nonnegligible ion pair interaction through a VdN^N ⋯ Na bridge is present. The X-ray structure shows a linearly coordinated N2, the VdNdN angle is 180.0(3) . On going from [V(CO)5(N2)]− to trans-[V(N2)2(dppe)2]−, the IR stretching vibration related to the N2 moiety lowers from 1843 cm−1 (THF solution) to 1790 cm−1 (Nujol), as expected based on the replacement of the CO groups with the electron-donating diphosphines. This comparison evidences that the degree of backdonation from the V−I center to the dinitrogen ligand differs significantly on varying the coligands. Treatment of [Na(THF)][V(N2)2(dppe)2] with an excess of hydrogen bromide results in a partial conversion of the N2 moiety into a mixture of ammonium ion and hydrazine.417 The introduction of a cyclopentadienyl ligand does not significantly enhance the stability of N2 complexes. Like the carbonylmetallate anion [V(CO)5(N2)]−, which rapidly decomposes at room temperature, photochemical reactions of [VCp(CO)4] and [VCp (CO)4] with N2 proceed with carbonyl monosubstitution to give dinitrogen derivatives that are also unstable at room temperature.418 The homologous niobium and tantalum derivatives are longer lived species. Under a high pressure of N2, both [NbCp0 (CO)3(N2)] and [NbCp0 (CO)2(N2)2] are obtained upon irradiation of [NbCp0 (CO)4] in polyethylene matrixes, whereas only [TaCp0 (CO)3(N2)] is produced from [TaCp0 (CO)4] under analogous conditions (Cp0 ¼ Z5-C5H4Me).419,420

6.04.6.3

a-Diimine compounds

a-Diimines, also named 1,4-diaza-1,3-dienes or simply “diazadienes” (DAD), are versatile ligands widely employed in coordination chemistry, usually in the chelating k2N,N-coordination mode, with steric and electronic properties tunable by varying the substituents on the {N]CC]N} skeleton.421 a-Diimines behave as redox noninnocent ligands since their backbone may undergo two successive reduction processes producing p-radical monoanions and then ene-diamide dianions, Fig. 24. Examples of complexes

Fig. 24 Different charges and structures for a general a-diimine (DAD) ligand. From Lorenz, V.; Hrib, C. G.; Grote, D.; Hilfert, L.; Krasnopolski, M.; Edelmann, F. T. Organometallics 2013, 32, 4636–4642.

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incorporating a-diimines across the three possible ligand charge states have been described in the literature. This redox variability confers flexibility to the a-diimine ligand, and reversible switching from the planar k2N,N-coordination (structure I in Fig. 24) to the folding k4-(s2,p) mode (structure III in Fig. 24) has been exploited to realize diverse metal-centered catalytic redox processes.422 Therefore, the nature of the DAD ligand is usually elucidated on the basis of X-ray structural data, and especially the deviation from planarity is correlated to the negative charge entity. Moreover, EPR measurements may provide evidence for the radical identity of the monoanionic form (Fig. 24, structure II).422,423 In accord with their electron withdrawing character, a-diimine ligands are usually found in low to medium valent metal species, and this trend is clearly observable with group 5 metals. Few homoleptic complexes of niobium and tantalum are known containing the metal center in a formal 0 or +I oxidation state.424 A number of stable [MCl3(DAD)] adducts (M ¼ Nb, Ta) have been reported,422,425 and the analogous vanadium trichloride derivatives have been studied as ethylene polymerization catalysts.426 Conversely, a-diimine adducts of tetra- or pentahalides of niobium and tantalum are quite rare and difficult to isolate, since in these cases, the a-diimine fragment is susceptible to activation reactions promoted by the increased Lewis acidity of the metal center.427 However, the X-ray structure of [NbOCl3(DADPr)] (51, R ¼ iPr and R0 ¼ H with reference to Fig. 25) may be considered as a reference because it contains niobium in its highest formal oxidation state and consequently a constrained neutral diazadiene ligand (Fig. 24). The NbV center of 51 deviates only slightly out [0.272 A˚ ] of the plane of the {NCCN} backbone, and this substantial coplanarity is as expected for a neutral a-diimine ligand forming dative bonds with the metal atom.427 In the following section, we will give an overview of group 5 metal organometallic compounds (i.e., those containing at least one additional carbon coligand) with a-diimine ligand(s). Mono-cyclopentadienyl complexes of formula [MCp(DADPr)2] (M ¼ Nb, 52a; M ¼ Ta, 52b) were obtained from [MCpCl4] upon reaction with sodium amalgam in the presence of N,N0 -diisopropylethane-1,2-diimine, in approximately 40%–60% yields.428 The X-ray structure of 52a (Fig. 26) shows one DADPr slightly folded toward the Cp ring, while the other DADPr is significantly folded away from the Cp. The CdC and CdN bond distances are indicative of some enediamide character (structure III in Fig. 24), and the folding favors charge donation from the DADPr ligands to the electron-depleted niobium center. A wide number of tantalum compounds of general formula [TaCp Cl2(DAD)] are available from the reactions of [TaCp Cl4] with the dilithium salts of the appropriate a-diimines429; all of these compounds are meant to comprise TaV and enediamido dianions, and subsequent substitution of the chlorido ligands offers additional structural diversity, enabling the introduction of alkyl, alkynyl, and thiolato ligands. The mixed a-diimine-enolato complexes [TaCp {kO:k2CdCH2]C(Me)C(]O)OMe}(k2N-RNCHCHNR)] (53; R ¼ 4C6H4OMe or Cy ¼ C6H11) have been investigated as catalytic precursors for the methyl-methacrylate polymerization reaction, in combination with an aluminum cocatalyst.430 Organo-vanadium complexes bearing a-diimine ligands can be obtained by straightforward mono-carbonyl substitution from [VCpR(CO)3] (CpR ¼ Z5-C5H5 ¼ Cp; Z5-C5Me5 ¼ Cp ; Z5-C5H4SiMe3)431 to afford [VCpR(CO)2(k2N-RNCHCHNR)] (R ¼ Cy, iPr, t Bu, Ph, 4-C6H4OMe). A view of the X-ray structure of 54 (CpR ¼ Cp , R ¼ iPr) is shown in Fig. 27. The five-membered

Fig. 25 View of the X-ray structure of [NbOCl3(DADPr)], 51427; H atoms have been omitted for clarity. Selected bond lengths (A˚ ): Nb(1)dN(1) 2.417(4), N(1)dC(1) 1.283(5), C(1)dC(1_1) 1.469(8). From Bartalucci, N.; Bortoluzzi, M.; Pampaloni, G.; Pinzino, C.; Zacchini, S.; Marchetti, F. Dalton Trans. 2018, 47, 3346–3355.

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Fig. 26 View of the X-ray structure of [NbCp(DADPr)2], 52a428; H atoms have been omitted for clarity. Selected bond lengths (A˚ ) and interplane angles ( ): Nb(1) dN(2) 2.098(3) A˚ , Nb(1)dN(1) 2.095(3), N(2)dC(3) 1.376(5), C(3)dC(2) 1.327(6), C(2)dN(1) 1.366(5), Nb(1)dN(4) 2.089(3) A˚ , Nb(1)dN(3) 2.089(3), N(4) dC(7) 1.352(4), C(7)dC(6) 1.363(5), C(6)dN(3) 1.349(5); Nb1dN1dN2/N1dC2dC3dN2 11.9, Nb1dN3dN4/N3dC6dC7dN4 42.9(3). From Churchill, A. J.; Green, J. C.; Moody, A. G.; Müller, M. Inorg. Chim. Acta 2011, 369, 120–125.

Fig. 27 View of the X-ray structure of [VCp (CO)2(k2N-iPrNCHCHNiPr)], 54431; H atoms have been omitted for clarity. Selected bond lengths (A˚ ): V(1)dN 2.067(2), NdC 1.484(7), C(1)dC(2) 1.372(7). From Woitha, C.; Behrens, U.; Vergopoulos, V.; Rehder, D. J. Organomet. Chem. 1990, 393, 97–109.

{VdNdCdCdN} ring adopts an envelope conformation and is folded along the N(1)dN(2) axis; bonding parameters related to the diazadiene moiety are indicative of a considerable enediamine character, which is probably enhanced by the presence of the strongly donor Cp coligand. Floriani and coworkers described the construction of a cyano-substituted a-diimine ligand on a vanadium center by means of stepwise assembly of an imino-acyl ligand and two equivalents of tert-butyl isocyanide (Scheme 47).432 More precisely, one isocyanide initially inserts into one VdC bond, and subsequent rearrangement gives rise to a radical five-membered metallacycle. Trapping of a second isocyanide molecule generates the final product 55 (51% yield) upon elimination of a tert-butyl radical.

344

Organometallic Complexes of Group 5 With p-Acidic Ligands

Scheme 47 Assembly of one imino-acyl and two isocyanide fragments at a vanadium center to form a cyano-functionalized a-diimine complex (Mes ¼ 2,4,6-C6H2Me3). From Ruiz, J.; Vivanco, M.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Organometallics 1993, 12, 1811–1822.

Fig. 28 View of the X-ray structure of [V(tBuNCMes)2{k2-tBuNC(CN)C(Mes)NBut}], 55432; H atoms have been omitted for clarity. Selected bond lengths (A˚ ): V(1) dN(1) 2.027(4), V(1)dN(4) 1.993(4), N(1)dC(10) 1.352(7), N(4)dC(43) 1.376, C(43)dC(10) 1.384. From Ruiz, J.; Vivanco, M.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Organometallics 1993, 12, 1811–1822.

The X-ray structure of [V(tBuNCMes)2{k2-tBuNC(CN)C(Mes)NBut}], 55, displays the {NCCN} backbone which is slightly deviated from planarity. However, the CdC and CdN bond distances suggest an appreciable contribution of the enediamide form to the structure (Fig. 28). For sake of comparison, average CdN distances are 1.36 A˚ in 55, 1.48 A˚ in 54, 1.35 A˚ in 52a and 1.28 A˚ in the purely neutral diazadiene species 51. Magnetic measurement on 55 indicates one unpaired electron (m ¼ 1.72 mB at 288 K), which is coherent with a d1 system, thus supporting the hypothesis of a two-electron transfer from the metal center to the a-diimine.

6.04.7

Concluding remarks

Group 5 elements are strongly oxophilic, and their organometallic chemistry with p-acceptor ligands is a challenging area of research, basically due to the needing of strictly inert experimental conditions to handle especially the low oxidation-state metal species. Therefore, there is still space for exciting basic discoveries, as the very recent synthesis of homoleptic carbonyls of niobium

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and tantalum witnesses: this occurred at the end of many attempts and many years of research, by a number of research groups all over the world. The Lewis acidity of the metal centers, combined with the unsaturation of isocyanides, alkynyls and a-diimines, may open the door to unusual reactivity patterns and coupling processes, and in this regard it is reasonable to expect a future development in the field of homogeneous catalysis. Finally, the cyanide ligand well adapts itself to the highest oxidation states, making water-inert, homoleptic complexes of vanadium and niobium available for the preparation of reticular networks with potential applications in material chemistry.

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Organometallic Complexes of Group 5 With p-Acidic Ligands

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136. 137. 138. 139. 140. 141. 142. 143. 144. 145. 146. 147. 148.

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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.

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Organometallic Complexes of Group 5 With p-Acidic Ligands

387. 388. 389. 390. 391. 392. 393. 394. 395. 396. 397. 398. 399. 400. 401. 402. 403. 404. 405. 406. 407. 408.

409. 410.

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Chem. 1982, 232, 219–227. Bottomley, F.; Darkwa, J.; White, P. S. J. Chem. Soc., Dalton Trans. 1985, 1435–1442. Morán, M.; Gayoso, M. Z. Naturforsch. B 1981, 434–436. Bottomley, F.; Darkwa, J.; White, P. S. J. Chem. Soc., Chem. Commun. 1982, 1039–1040. Bottomley, F.; Darkwa, J.; White, P. S. Organometallics 1985, 4, 961–965. Fischer, E. O.; Schneider, R. J. J.; Müller, J. J. Organomet. Chem. 1968, 14, P4–P6. Herberhold, M.; Kremnitz, W.; Trampisch, H.; Hitam, R. B.; Rest, A. J.; Taylor, D. J. J. Chem. Soc., Dalton Trans. 1982, 1261–1273. Middleton, A. R.; Wilkinson, G. J. Chem. Soc., Dalton Trans. 1980, 1888–1892. Singh, D.; Buratto, W. R.; Torres, J. F.; Murray, L. J. Chem. Rev. 2020, 120, 5517–5581. (a) Selected references:Groysman, S.; Villagràn, D.; Freedman, D. E.; Nocera, D. G. Chem. Commun. 2011, 47, 10242–10244; (b) Ferguson, R.; Solari, E.; Floriani, C.; Osella, D.; Ravera, M.; Re, N.; Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem. Soc. 1997, 119, 10104–10115; (c) Milsmann, C.; Turner, Z. R.; Semproni, S. P.; Chirik, P. J. Angew. Chem. Int. Ed. 2012, 51, 5386–5390; (d) Minhas, R. K.; Edema, J. J. H.; Gambarotta, S.; Meetsma, A. J. Am. Chem. Soc. 1993, 115, 6710–6717; (e) Liu, G.; Liang, X.; Meetsma, A.; Hessen, B. Dalton Trans. 2010, 39, 7891–7893; (f ) Sekiguchi, Y.; Arashiba, K.; Tanaka, H.; Eizawa, A.; Nakajima, K.; Yoshizawa, K.; Nishibayashi, Y. Angew. Chem. Int. Ed. 2018, 57, 9064–9068; (g) Kokubo, Y.; Wasada-Tsutsui, Y.; Yomura, S.; Yanagisawa, S.; Kubo, M.; Kugimiya, S.; Kajita, Y.; Ozawa, T.; Masuda, H. Eur. J. Inorg. Chem. 2020, 1456–1464; (h) Kokubo, Y.; Yamamoto, C.; Tsuzuki, K.; Nagai, T.; Katayama, A.; Ohta, T.; Ogura, T.; Wasada-Tsutsui, Y.; Kajita, Y.; Kugimiya, S.; Masuda, H. Inorg Chem. 2018, 57, 11884–11894; (i) Tran, B. L.; Pinter, B.; Nichols, A. J.; Konopka, F. T.; Thompson, R.; Chen, C.-H.; Krzystek, J.; Ozarowski, A.; Telser, J.; Baik, M.-H.; Meyer, K.; Mindiola, D. J. J. Am. Chem. Soc. 2012, 134, 13035–13045; (j) Vidyaratne, I.; Crewdson, P.; Lefebvre, E.; Gambarotta, S. Inorg. Chem. 2007, 46, 8836–8842; (k) Janas, Z.; Sobota, P. Coord. Chem. Rev. 2005, 249, 2144–2155; (l) Davies, S. C.; Hughes, D. L.; Janas, Z.; Jerzykiewicz, L. B.; Richards, R. L.; Sanders, J. R.; Silverston, J. E.; Sobota, P. Inorg. Chem. 2000, 39, 3485–3498. (a) Cavigliasso, G.; Wilson, L.; McAlpine, S.; Attar, M.; Stranger, R.; Yates, B. F. Dalton Trans. 2010, 39, 4529–4540; (b) Hulley, E. B.; Williams, V. A.; Hirsekorn, K. F.; Wolczanski, P. T.; Lancaster, K. M.; Lobkovsky, E. B. Polyhedron 2016, 103, 105–114. (a) Bregel, D. C.; Oldham, S. M.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 2002, 41, 4371–4377; (b) Hulley, E. B.; Bonanno, J. B.; Wolczanski, P. T.; Cundari, T. R.; Lobkovsky, E. B. Inorg. Chem. 2010, 49, 8524–8544; (c) Hirotsu, M.; Fontaine, P. P.; Epshteyn, A.; Zavalij, P. Y.; Sita, L. R. J. Am. Chem. Soc. 2007, 129, 9284.9285; (d) Yonke, B. L.; Keane, A. J.; Zavalij, P. Y.; Sita, L. R. Organometallics 2012, 31, 345–355; (e) Zhang, W.; Tang, Y.; Lei, M.; Morokuma, K.; Musaev, D. G. Inorg. Chem. 2011, 50, 9481–9490; (f ) Takada, R.; Hirotsu, M.; Nishioka, T.; Hashimoto, H.; Kinoshita, I. Organometallics 2011, 30, 4232–4235. Fryzuk, M. D.; Johnson, S. A.; Patrick, B. O.; Albinati, A.; Mason, S. A.; Koetzle, T. F. J. Am. Chem. Soc. 2001, 123, 3960–3973. Huber, H.; Ford, T. A.; Klotzbücher, W.; Ozin, G. A. J. Am. Chem. Soc. 1976, 15, 3176–3178. Zhou, M.; Andrews, L. J. Phys. Chem. A 1998, 102, 9061–9071. Lu, Z. H.; Jiang, L.; Xu, Q. J. Chem. Phys. 2009, 131, 034512. Lokshin, B. V.; Greenwald, I. I. J. Mol. Struct. 1990, 222, 11–20. Ihrnels, K.; Rehder, D. Chem. Ber. 1985, 118, 895–904. Rehder, D.; Woitha, C.; Priebsch, W.; Gailus, H. J. Chem. Soc. Chem. Commun. 1992, 364–365. George, M. W.; Haward, M. T.; Hamley, P. A.; Hughes, C.; Johnson, F. P. A.; Popov, V. K.; Poliakoff, M. J. Am. Chem. Soc. 1993, 115, 2286–2299. Childs, G. I.; Gallagher, S.; Bitterwolf, T. E.; George, M. W. J. Chem. Soc., Dalton Trans. 2000, 4534–4541. Childs, G. I.; Grills, D. C.; Gallagher, S.; Bitterwolf, T. E.; George, M. W. J. Chem. Soc., Dalton Trans. 2001, 1711–1717. (a) Raghavan, A.; Venugopal, A. J. Coord. Chem. 2014, 67, 2530–2549; (b) Nikolaevskaya, E. N.; Druzhkov, N. O.; Syroeshkin, M. A.; Egorov, M. P. Coord. Chem. Rev. 2020, 417, 213353; (c) Van Koten, G.; Vrieze, K. Adv. Organomet. Chem. 1982, 21, 151–239. Nishiyama, H.; Ikeda, H.; Saito, T.; Kriegel, B.; Tsurugi, H.; Arnold, J.; Mashima, K. J. Am. Chem. Soc. 2017, 139, 6494–6505.. (and references therein). (a) Lorenz, V.; Hrib, C. G.; Grote, D.; Hilfert, L.; Krasnopolski, M.; Edelmann, F. T. Organometallics 2013, 32, 4636–4642; (b) Cole, B. E.; Wolbach, J. P.; Dougherty, W. G., Jr.; Piro, N. A.; Kassel, W. S.; Graves, C. R. Inorg. Chem. 2014, 53, 3899–3906. (a) Daff, P. J.; Etienne, M.; Donnadieu, B.; Knottenbelt, S. Z.; McGrady, J. E. J. Am. Chem. 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6.05

Group VI Metal Complexes of Carbon Monoxide and Isocyanides

Paul J Fischer, Chemistry Department, Macalester College, Saint Paul, MN, United States © 2022 Elsevier Ltd. All rights reserved.

6.05.1 6.05.2 6.05.2.1 6.05.2.2 6.05.2.2.1 6.05.2.2.2 6.05.2.3 6.05.2.3.1 6.05.2.3.2 6.05.3 6.05.4 6.05.4.1 6.05.4.2 6.05.4.3 6.05.5 6.05.6 6.05.6.1 6.05.6.2 6.05.7 6.05.7.1 6.05.7.2 6.05.7.2.1 6.05.7.2.2 6.05.7.2.3 6.05.7.3 6.05.7.4 6.05.8 6.05.8.1 6.05.8.2 6.05.8.3 6.05.8.4 6.05.8.5 6.05.8.6 6.05.8.6.1 6.05.8.7 6.05.8.8 6.05.8.9 6.05.8.10 6.05.8.11 6.05.8.12 6.05.8.13 6.05.8.14 6.05.8.15 6.05.8.16 6.05.8.16.1 6.05.8.16.2 6.05.8.16.3 6.05.8.16.4 6.05.8.17 6.05.8.18 6.05.9 6.05.10 6.05.11 6.05.12

352

Introduction Homoleptic carbonyl complexes Chemical synthesis Experimental physical studies Chromium carbonyls Molybdenum and tungsten carbonyls Computational work Chromium carbonyls Molybdenum and tungsten carbonyls Alkane complexes Dihydrogen, hydride, borane-Lewis base adducts and s-alane complexes Dihydrogen and hydride complexes Complexes containing borane-Lewis base adducts as ligands s-Alane complexes Complexes with silicon-containing ligands Complexes with group VI metal–boron bonds Synthesis and reactivity of Braunschweig borylene complexes Other classes of group VI metal complexes containing boron Complexes with group VI metal–nitrogen bonds sp3 Nitrogen-ligating sp2 Nitrogen-ligating Monodentate ligands Tris(pyrazol-1-yl)borates, bis(pyrazol-1-yl)methanes and related scorpionates Bidentate ligands sp Nitrogen-ligating Ligands that coordinate by nitrogen and oxygen atoms Complexes with group VI metal–phosphorus bonds Triphenylphosphine and tricyclohexylphosphine Monophenyl and diphenyl tertiary phosphines Phosphines with fluorinated substituents Phosphorus ligands with PdSi bonds Monodentate phosphorus ligands with boron-based substituents Streubel phosphinidenoid and phosphinidenoid-derived complexes Oxaphosphiranes 2H-Azaphosphirenes and related complexes Phosphirane and phosphirene complexes Complexes with P-bound five- and six-membered rings Phosphaalkenes, phosphaalkynes, phosphaallenes and related ligands Phosphanylboranes, arsanylboranes and related ligands P-bound ligands with PdP bonds Aminophosphines and related ligands Phosphido-bridged and related complexes Phosphite, phosphonite and related ligands Bidentate phosphines N,N0 -Bis(disubstituted-phosphino)amines and related ligands Cyclic P-bound ligands that include nitrogen atoms within the cyclic system Ligands that incorporate heterocycle and carborane backbones Anionic and cationic bidentate phosphines Tridentate phosphines Ligands that coordinate by both phosphorus and nitrogen Complexes with group VI metal–oxygen bonds Complexes with group VI metal–halide bonds Complexes with group VI metal–gallium and indium bonds Complexes with group VI metal–germanium, tin and lead bonds

Comprehensive Organometallic Chemistry IV

353 353 354 354 354 354 355 355 355 355 356 356 356 357 357 359 359 361 362 362 363 363 365 367 373 373 374 374 375 376 377 378 378 380 382 383 384 387 388 388 391 393 394 396 398 399 400 401 401 402 405 406 407 407

https://doi.org/10.1016/B978-0-12-820206-7.00066-4

Group VI Metal Complexes of Carbon Monoxide and Isocyanides

6.05.13 Complexes with group VI metal–arsenic, antimony and bismuth bonds 6.05.13.1 Complexes with group VI metal–arsenic bonds 6.05.13.2 Complexes with group VI metal–antimony and bismuth bonds 6.05.14 Complexes with group VI metal–sulfur, selenium and tellurium bonds 6.05.14.1 Complexes with group VI metal–sulfur, selenium and tellurium bonds and thiocarbonyl ligands 6.05.14.1.1 Monodentate S-ligands and thiocarbonyls 6.05.14.1.2 k2-S ligands 6.05.14.1.3 Tridentate and scorpionate ligands that feature sulfur donors 6.05.14.1.4 Mixed S,N and S,P ligands 6.05.14.1.5 Complexes containing bridging thiolate and telluride ligands 6.05.14.1.6 Sulfur-ligating metalloligands 6.05.14.1.7 Other heterobimetallics with group VI metal–S, Se and Te bonds 6.05.14.1.8 Complexes containing chalcogenide clusters 6.05.14.1.9 Complexes containing As/S and P/S cages 6.05.14.1.10 Other complexes with group VI metal–Se and Te bonds 6.05.15 Complexes that incorporate metallocenes 6.05.16 Group VI metal isocyanide complexes 6.05.16.1 Alkyl, aryl and halogenated isocyanides 6.05.16.2 Chelating isocyanides 6.05.16.3 m-Terphenyl isocyanides 6.05.16.4 Homoleptic arylisocyanides 6.05.16.5 Mono- and diisocyanoazulenes 6.05.16.6 Cyanide as a terminal and bridging ligand 6.05.17 Concluding remarks Acknowledgments References

6.05.1

353

413 413 413 414 414 414 415 416 417 417 418 418 420 421 421 421 424 424 426 426 427 428 429 429 429 429

Introduction

Group VI metal carbonyl complexes maintain a robust presence in contemporary organometallic chemistry research, and important developments in this voluminous field of inquiry during 2006–2020 are described herein. Complexes with p-system–group VI metal interactions are outside the scope of this review, so the vast majority of the substances described herein feature group VI metal tetracarbonyl and pentacarbonyl units. Nearly 40% of the references cited detail complexes with group VI metal–phosphorus bonds, and these studies span applications of classical tertiary phosphines, phosphines with fluorinated substituents, oxaphosphiranes, phosphirenes, ligands with P-bound five- and six-membered rings, P-bound ligands with PdP bonds, bidentate phosphines (both neutral and charged), tridentate phosphines and other classes of phosphorus-containing ligands. Roughly 20% of the references disseminate complexes with group VI metal–nitrogen bonds, with substances containing bidentate ligands (e.g., 2,20 -bipyridine, 1,10-phenanthroline, iminopyridines, diimines) receiving the bulk of recent attention. Complexes with group VI metal bonds to sulfur, selenium and tellurium are discussed in approximately 10% of the cited references. While the other metal carbonyl sections are associated with fewer cited references than those detailing the aforementioned three areas, it is promising for the future vitality of this scholarly area that groundbreaking discoveries have been recently disclosed within these relatively less explored fields of study, including for example, the first isolation of [M(CO)6]+ metalloradicals (M ¼ Cr, Mo, W), application of Mo(CO)2{hydrotris(1-pyrazolyl)borate} units to stabilize linearly two-coordinated silicon, inaugural d3 dihalo(carbonyl) tungsten(III) complexes, discoveries of further synthetic applications of group VI metal borylene complexes as borylene transfer agents and heterobimetallics containing Cr(CO)5 and W(CO)5 fragments coordinated to arsene ligands. The robustness of group VI metal tetracarbonyl and pentacarbonyl fragments lend themselves to their incorporation into complexes with a wide variety of unique ligands. The classical application of group VI metal carbonyl fragments as electronic reporters of ancillary ligand donor ability via infrared spectroscopy remains as a contemporary motivation for the preparation of these complexes. The ubiquity of group VI metal carbonyl complexes as synthetic targets shows no sign of abating. The exploration of group VI metal isocyanide complexes enjoyed a resurgence during 2006–2020, and the fundamental discoveries disclosed during this period promise further high impact work in this area. The major themes of this recent isocyanide chemistry research, including the application of sterically hindered isocyanides with the intention of enforcing low coordination numbers at group VI metals, the examination of homoleptic arylisocyanide complexes as photoreductants, and the preparation of group VI metal complexes of isocyanoazulenes are summarized herein.

354

Group VI Metal Complexes of Carbon Monoxide and Isocyanides

6.05.2

Homoleptic carbonyl complexes

The examination of homoleptic group VI metal carbonyl complexes remains a vigorous research area within the broad areas of chemical synthesis, experimental physical studies, and computational work. These studies continue to uncover new fundamental avenues for exploration derived from the long known chromium, molybdenum and tungsten hexacarbonyls, and even with the fledging Sg(CO)6.

6.05.2.1

Chemical synthesis

Krossing prepared stable salts of 17-electron [Cr(CO)6]+ via oxidation of Cr(CO)6 with either [NO][Al(ORF)4] or [NO][F{Al(ORF)3}2] (RF ¼ C(CF3)3) and simultaneous removal of NO gas; reactions in the presence of the NO by-product afford salts of [Cr(CO)5(NO)]+.1 Subsequent research uncovered a protocol to synthesize the metalloradical salts [M0 (CO)6][F-{Al(ORF)3}2] and [M0 (CO)5NO][Al(ORF)4] (M0 ¼ Mo, W).2 Group VI hexacarbonyls serve as ligands for the silver cation in [Ag{M0 (CO)6}2][F{Al(ORF)3}2] (M0 ¼ Mo, W).3 Düllmann and Even developed technology to prepare Mo(CO)6 and W(CO)6 containing radioactive isotopes of these metals at ambient conditions.4,5 This strategy was extended to synthesize and detect Sg(CO)6.6 A review by Ellis includes background information about homoleptic group VI metal carbonyl anions.7 The inaugural crystallographic characterization of a salt of [Cr(CO)5]2− was reported in 2011.8 Deacon and Junk employed a mixed-valent samarium calix[4]pyrrolide complex to trap the [W2(CO)10]2− fragment.9 This dianion was structurally characterized as a “free” species within [K(18-crown-6)(THF)2][W2(CO)10].10 Böttcher employed [M2(CO)10]2− (M ¼ Cr, Mo) as CO sources in reactions with nitrosyl carbonyls of iron and cobalt.11

6.05.2.2 6.05.2.2.1

Experimental physical studies Chromium carbonyls

The photodissociation of Cr(CO)6 in the 270–345 nm region was found to exhibit a wavelength-independent mechanism;12 the pentacarbonyl products formed via the photodissociation of M(CO)6 (M ¼ Cr, Mo, W) by a femtosecond UV laser (107 s−1 (Fig. 78).218 Conversion to the hydride is only slightly favorable at r.t. (DG ¼ −2.1(3) kcal mol−1) allowing equilibration. An equilibrium isotope effect (EIE) of 0.13–0.23 at −20  C was determined for the exchange of MnH/ND to MnD/NH, indicating that the MnH/ND is favored. The rapidity of this process was noted to have relevance to the proximal amine groups in FeFe hydrogenase, demonstrating the facile nature of M–H/N–H exchange which is important in a catalyst that reversibly forms/oxidizes H2 with very little bias. The hydride in the N-methyl analog of 235Me reacts with iPr2EtN to afford the neutral hydride 236Me, which in turn reacts with trityl cation in a homolytic fashion, where tritylium acts as an outer sphere electron oxidant producing half an equivalent of 235Me and 234Me (note that 234Me exists as a mixture of N-bound and N-unbound isomers) and Gomberg’s dimer (Fig. 78).217 Another interesting reaction carried out by the Mn mono-carbonyl cationic complexes is their binding of ammonia and the unique N-H homolytic products observed by Bullock and coworkers.219 The ammonia complex 237 reacts with excess radical 2,4,6-tritertbutylphenol to afford the triple H-atom abstraction product 238, along with unidentified Mn material (Fig. 79). Given the high BDFE of free ammonia, the cleavage of all three NdH bonds in 237 is remarkable considering the low BDFE of the

Carbonyl and Isocyanide Complexes of Manganese

Fig. 76

481

482

Carbonyl and Isocyanide Complexes of Manganese

Fig. 77

Fig. 78

Fig. 79

phenoxide radical used (e.g., BDFE of 2,4,6-tritertbutylphenol is only 77 kcal mol−1 in MeCN). Compound 238 was obtained in 76% isolated yield, and the incorporation from ammonia was confirmed to occur through isotopic labeling of the N-atom using 15 NH3. A proposed mechanism was provided (not shown here) that accounted for the bond breaking and cleaving steps such that the process was overall down-hill until the final proposed Mn-CO containing species.

6.06.4.3.2

Catalytic alkene hydrogenation with Mn(I)–CO compounds

One of the triumphs of recent organometallic Mn-CO chemistry is its inclusion in the realm of (de)hydrogenation catalysis. Aside from CH activation studies described above from Ackermann and others, Mn(I)–CO compounds were often considered of little use in the arena of catalysis prior to 2016. As will be described in the “pincer section” (Section 5), the discovery of Mn(I) (de)

Carbonyl and Isocyanide Complexes of Manganese

483

hydrogenative catalysis is owed, in part, to application of the metal-ligand cooperative (MLC) paradigm. While it is certainly true that MLC enables a variety of (de)hydrogenative processes with Mn, it may not be necessary in every instance. For example, Kirchner discovered a surprisingly simple Mn(I)–CO system for alkene hydrogenation using {PP}Mn(I)(H)(CO)2(R) complexes (e.g., 200).195 Although this article does not discuss catalytic applications, this discovery is of particular importance to the chemistry Mn-CO compounds and so it is briefly described here.

Kirchner and coworkers used alkyl Mn(I) carbonyl complexes coordinated to bidentate bis(dipropylphosphine)ethane (n- or i-Pr), complex 200,195 to hydrogenate a variety of alkenes including those containing alcohol, ester, and ether functional groups. The catalysts, having a range of alkyl groups coordinated to Mn, are bench stable and do not require co-ligands or additives. Additionally, of note is that the ligands do not contain functionalities typically required to enable metal-ligand cooperation, unless a CdH bond of the ligand is being activated as has been observed for dppm and related ligands (see Figs. 73 and 82). A mechanism was proposed and involves a catalyst activation portion and subsequent hydrogenation cycle. First, the Mn-alkyl undergoes a migratory insertion to form a 16-electron acyl complex. This in turn reacts with H2, forming an intermediate H2 adduct that, via a sigma bond metathesis step, liberates aldehyde and a {PP}Mn(I)(H)(CO)2 16-electron complex (which may be stabilized by sigma-complexation of aldehyde). The hydride {PP}Mn(I)(H)(CO)2 can coordinate alkene substrate and undergo insertion to afford Mn–alkyl species that reacts effectively the same as the acyl complex. Complexes like MeMn(CO)5, HMn(CO)5, and {bipy} Mn(CO)3X (X ¼ Me, H) afforded zero turnover, so the role of the phosphine ligand is critical.

6.06.4.3.3

P,X (X ¼ C, N, O, S) ligands

P,N ligands are much less utilized than their P,P counterparts, but interest in their chemistry has been growing as of late owing to the discovery that R2NH and RNH2 donors enable a wide-range of catalysis with Mn-CO compounds (detailed later). Generally, the synthesis of P,N Mn-CO complexes is analogous to those discussed above and several examples are shown below in Fig. 80.220–224 However, a notable exception to the typical synthetic procedures is the preparation of 239 (Fig. 81).225 Namely, the reaction between the trimer [Mn(CO)4(m3-H)]3 (see ref. 226) with 2-diphenylphosphinoaminobenzene in heptane resulted in the isolation of 239 in 61% yield. There are few P,C bonded Mn-CO complexes. One notable example from Sortais and coworkers proposed that deprotonation of 240 in the absence of electrophiles formed an intermediate tridentate complex 242 that is reactive toward H2, MeI, or CO2 to afford 241 (Fig. 82)227; although 242 was not isolated in this study, spectroscopic evidence was provided for a reactive intermediate consistent with the tridentate modality, akin to the complexes discussed above such as 217. The P,O ligand class is also quite rare in Mn-CO chemistry. Lacy and coworkers prepared a series of phenol and phenolate phosphine complexes and explored their coordination chemistry (Fig. 83).228,229 One of the notable features of the phenolate systems is that the oxygen donors, being acidic, are able to displace bromido ligand without adding external base or Ag/Na salt. Typically, the second donor of the P,X ligand displaces a second CO ligand and the resulting {P,X}Mn(CO)3Br results. However, as demonstrated in the initial formation of 243, the OH functional group forms a H-bond with the bromide ligand. Heating this complex (with or without an additional equivalent of HPO ligand) afforded 244 that also has a H-bond with the bromido ligand. When 244 is heated however, HBr is spontaneously liberated to afford the neutral phenol/phenolate complex 245. This complex in

Fig. 80

484

Carbonyl and Isocyanide Complexes of Manganese

Fig. 81

Fig. 82

Fig. 83

turn may be treated with KH to afford the dicarbonyl anion complex salt 246, which can alternatively be prepared directly from 243 in 90% yield from Mn(CO)5Br, ligand, and KH in a one-pot reaction (not shown). 246 can be protonated to form 247, the latter of which is homolytically unstable, but was sufficiently stable to have had its pKa measured to be about 23 in DCM (which is nearly the same as that in MeCN). 246 has a reversible oxidation potential at −0.58 V vs. FeCp0/+ 2 . These allowed for an estimation of the BDFE of the OdH bond in 247 and was about 73 kcal mol−1, which is weakened w.r.t. free phenols:  77 kcal mol−1). Although modest in weakening, 247 was unstable and only when the deuterated isotopologue was synthesized could a crystal structure be obtained. 247 is the first example of a low-spin d6 first-row transition metal complex with a (nominally) covalent MndOH bond where the OH is phenolic. Generally speaking, phenol ligand groups are quite rare in organometallic chemistry and 247 effectively represents the only case where the MdOH bond homolysis has been quantified; hence, the weak OH bond is one explanation for the rarity. As might be expected for poor donors like phenols, the phenolic group exhibited some indication of hemilability, as reactions with MeCN and 247 afforded 248, but none of the other complexes in Fig. 83 reacted as such. Another notable aspect of

Carbonyl and Isocyanide Complexes of Manganese

485

Fig. 84

this chemistry is the low-potential for oxidation of 246 at −0.58 V. Typically, Mn(I) complexes oxidize much more positive than ferrocenium (around 0.5 V).230 DFT analysis of in silico oxidized species indicated possible ligand non-innocence, although attempts at isolating the formal Mn(II)-CO complexes was unsuccessful in this study. P,S ligands are also rare in Mn-CO coordination chemistry, with the complexes in Fig. 84 being the only new complexes from the reporting period.231 Ligand 249 displays a wide range of coordination modes, including both monodentate and bidentate. If 2 eq. of 249 is reacted with Mn(CO)5Br, then only the cis- and trans-monodentate P-bound compounds 250 and 251 result. However, if 1 eq. is used, then both bidentate (252) and monodentate (253) mono-ligated complexes result. To our knowledge, the only other P, S Mn-CO complexes is from 1980.232

6.06.4.4 6.06.4.4.1

S,S and S,X (X ¼ C, N, O) and other bidentate ligands (Se and Te) S,S ligands

Owing in part to the significance of bridging thiolate ligands in biological H2/H+ equilibration at Fe-based hydrogenases, there has been interest in the studies surrounding thiolate ligated Mn(I) complexes. Mn(I) is isoelectronic with low-spin Fe(II) and hence several studies have occurred within the areas of bioorganometallic chemistry and hydrogen evolution reaction (HER). The current section is focused on complexes that were prepared using nominally bidentate ligands, and a later (Section 6.06.8.2.2) describes other bridging thiolato complexes prepared with nominally monodentate sulfur-based ligands. Fig. 85 exemplifies the diverse possible outcomes that might occur in the reactions between Mn2(CO)10 and a bidentate thiol ligand. The reaction of 3,4-toluenedithiol with Mn2(CO)10 affords the disulfide complex 254 and presumably 2 eq. of H2 gas.233 The reaction yields are improved if carried out under photolytic conditions (use of Me3NO was not as useful in improving yields)234; the R ¼ H variant of 254 was used in electrocatalytic HER studies. 255 and 256 were prepared analogously to the R ¼ Me variant of 254, but 255 can also be prepared by direct reaction of the thiol ligand and 213.235 Continuing on the theme of bioorganometallic chemistry, heterobimetallic dinuclear Mn-CO complexes have also been prepared using nominally bidentate S,S ligands. For instance, the FeMn complex 257 was prepared by treating either the cis- or trans-isomer of Fe(CO)2(dppe)(pdt) (Fig. 86).236 Treatment of 257 with borohydride at low temperature afforded the hydride complex 258. FTIR spectra of the interconversion between 258 and 258 + H+ was demonstrated by sequential treatment of [(Et2O)2
H][B(ArF)4], Et3N, and [(Et2O)2
H][B(ArF)4], but the protonated species was not further identified because the 1H NMR spectra was very broad with one presumed cause being paramagnetism. Macrocyclic enzyme model complex 259 was prepared by

Fig. 85

486

Carbonyl and Isocyanide Complexes of Manganese

Fig. 86

Fig. 87

treating the macrocyclic precursor with Mn(CO)5Br in MeOH and heating at 60  C for 12 h (Fig. 86)237; in this way complex 259 was isolated in 80–90% yields. The imidazolium-dithiocarboxylate ligands 259 have been used to prepare several Mn-CO complexes (Fig. 87). These bidentate ligands can furnish dinuclear complexes like 260 (prepared in toluene at elevated temperatures) or the mononuclear complexes result when prepared in neat CS2 (261) or warm MeOH (262 and 263).238–240 Other Mn-CO complexes 264–267 with bidentate S,S ligands are also shown in Fig. 87 and were prepared analogously to those discussed prior.241,242

6.06.4.4.2

S,X (X ¼ C, N, O) ligands

There are numerous S,N ligands coordinated to Mn-CO units. Owing to the low-basicity of thiolate ligands and steric access, they often form bridging thiolato dinuclear complexes. An exemplary set of compounds and synthetic procedures are shown in Fig. 88.243 Typically, this class of compounds is prepared by treating Mn2(CO)10 with 2 eq. Me3NO in DCM with the desired ligand and allowing to stir for extended times at r.t. The long reaction times may reflect the fact that removal of the second CO ligand is slow. For example, a mixture of complex 268 and 269 resulted from the reaction of 2 eq. Me3NO with Mn2(CO)10 in the presence of 2-mercapto-1-methylimidazole; these compounds could be isolated in  85% overall yield by chromatography. Further attestation to the sluggish loss of the second CO comes with observation that 269 can be prepared from 268 by prolonged (20 h) reaction with additional Me3NO. Note that many of the products analogous to 269 discussed below were synthesized in low yield and one explanation is the short reaction times (vide infra). Although typically low yielding (70%) synthetic pathways involve refluxing high-boiling hydrocarbon solutions (e.g., xylenes or decalins) of Mn2(CO)10 and diphosphides ((R2P)2)427 or disubstituted phosphine (R2PH).428,429 Hayter also reports a low-yielding (20%) synthesis of [{Mn(CO)4}(m-PPh2)]2 starting with [Mn(CO)5]− and diphenyl phosphine chloride (Ph2PCl).427 Westerhause430 reports the synthesis of 586 via an analogous synthetic route (Fig. 182). 586 shows poor solubility in most solvent and a broad 31P NMR peak. The synthesis of a related dimanganese complex with a single phosphido bridge (587) starting from the NHC-stabilized cationic PI ligands under photolytic conditions was reported by MacDonald (Fig. 183).431 The authors also demonstrate the effect of Mn based broadening of NMR peaks for such complexes by contrasting the very broad 31P NMR peak for 587 against the sharp peaks for analogs with other transition metals such as W, Cr, Co, Fe.431 Kabir and coworkers demonstrated an unusual method to prepare m-phosphido complex 588 (Fig. 184).432 Treatment Mn2(CO)9(MeCN) with 1,8-bis(diphenylphosphine)naphthalene resulted in a mixture of complexes. The bidentate-complex 589 formed as the major product, but another product resulted from a single PdC bond cleavage forming 590; the fate of the cleaved PPh2 unit was not ascertained. A double PdC bond cleavage product was also obtained, 588, albeit the yields were low.

Mn2(CO)10

+

R

P

OC R'

xylene, reflux, 4 h 35-70%

Mn

OC

[P]

Mn CO

586

CO CO

[P]

CO

R = R' = p-tolyl; R = R' = 3,5-dimethylphenyl R = R' = 4-dimethylaminophenyl; R = Ph, R' = naphthyl

Fig. 182

CO

CO

H

530

Carbonyl and Isocyanide Complexes of Manganese

[BPh4] CO [Mn2(CO)10]

OC OC

[P(NHC)2][BPh4] THF, hQ (69%)

(NHC)2 [P]

CO CO CO

Mn

Mn

N

NHC =

N

CO

CO 587

Fig. 183

Fig. 184

CO Th R Mn2(CO)10

+ RPTh2

R = Ph, Th Th = thiophene

OC OC

toluene reflux, 4-5 h 15-30%

Mn

CO Th R

CO

P

Mn

S

+ CO

CO

OC Th2 P Ph

P Mn

S

CO Mn

+

Mn2(CO)9(RPTh2)

CO

CO

591

592

593

(For R = Th) PTh3 toluene, 110 °C

Fig. 185

The authors have also reported similar P-C activation in di- and tri-(thiophene-2-yl) phosphines leading to complexes 591, 592 and 593 containing one octahedral and one “piano-stool” Mn(I) centers (Fig. 185).433,434 The substitution of a CO at the octahedral Mn(I) (and not at the tetrahedral Mn(I)) in 591 leads to the formation of 592. Such substitution of CO molecules at similar octahedral Mn(I) centers by L type ligands is known from the 1980s.435 Although yields from these reactions are low, these two studies demonstrate the propensity of Mn-Mn to oxidatively add across certain PdC bonds like those in 1,8-bis(diphenylphosphine) naphthalene or P-thiophene. Edwards has demonstrated the formation of cis- and trans-dinuclear complexes 595 upon the treatment of 594 with an in situ generated carbene (Fig. 186).436 It is of interest to note that although an NHC is being generated in situ, there is no NHC-based co-ordination observed.

PH2

Mn(CO)5Br

PH2

CHCl3 reflux, 3 h (86%)

H2 P

Br CO Mn

P H2

1. AgOTf, DCM, r.t., 3 h 2. carbene + KN* OC

CO -78 °C to r.t. o.n. ca. 65% CO

OC

CO

CO

PH2

P Mn H Mn PH H2P CO

CO CO

594

+

595

XH2P

OR

= PH2 H2P

Fig. 186

H P Mn H P H2P

OC OC

PH2

H2P

PH2

N+

carbene = N

CO Mn PH2

CO CO

Carbonyl and Isocyanide Complexes of Manganese

6.06.8.3.3

531

Elemental phosphorus bridges

The formation of the phosphorous-rich complex 596 containing a unique P0 bridge has been reported by Hey-Hawkins (Fig. 187).437 The thermal decomposition of 596 initially forms Mn2P5 and then loses a P4 molecule to finally give Mn2P, which was also characterized by powder-XRD.

6.06.8.4 6.06.8.4.1

Unsupported MdMn bonds and other clusters Unsupported Mn-Pt complexes

The Mn-CO functional group also coordinates to square planar platinum complexes forming unsupported MndPt bonds. For example, Komiya and coworkers treated the square planar cis-dichloride Pt complex 597 with Na[Mn(CO)5] to afford the Mn-Pt complex 598 (Fig. 188); Pt-Mn distance is 2.729(2) A˚ for 598 having R ¼ H and Y ¼ NMe2.438 The Mn-CO moiety in 598 is effectively a donor group, as evidenced from the reaction with PPh3, liberating [Mn(CO)5]− in the salt 599. Mn-CO can also serve as a donor group to formally Pt(IV) centers, such as in the reaction between the Pt(IV)-NO3 group in 600 and Na[Mn(CO)5] forming 601 (Fig. 189).439 The MndPt bond in 601 is 3.03 A˚ , much longer than what was observed in 598 or in [{dppe}Pt(Me){Mn(CO)5}] (Mn–Pt ¼ 2.131 A˚ ). This long distance is perhaps a manifestation of the weak interaction that is observed chemically in the first order decomposition of 601 into 602. This fascinating reaction results in formal reductive elimination of MeMn(CO)5, occurring with formation of the square planar Pt(II) complex 602. Reductive elimination does not occur from formal Pt(II)–Mn/Me complexes such as [{dppe}Pt(Me){Mn(CO)5}], which has a Pt-Mn distance of 2.1 A˚ and is stable to heating.

Mn(CO)5Br [Na][P[P]4]

P

[P] OC

THF, r.t. 2h (63%)

OC

[P]

[P] CO

[P] Mn Br

Mn

CO

CO CO [P] = PtBu 596

Fig. 187

Y

R

L

Pt

Cl

Cl

Y Na[Mn(CO)5] THF, -20 °C 3h

L

R Pt

Me2N

Mn(CO)5

acetone r.t. (55%) when L = PPh3

Cl 598

597 L = PPh3; Y = NMe2; R = H L = PMe2Ph; Y= OEt, R = Bn

PPh3

Ph3P

H

Mn(CO)5

PPh3

Pt Cl 599

Mn–Pt = 2.729(2) Å for Y = NMe2

Figure 188 Fig. 188

R1

R1

R2 N

Me

Na[Mn(CO)5]

Me ONO2

THF, -30 °C 6h

Pt N R1 600

R2 N

Me Pt

N R1

Me Mn(CO)5

when R2 = Ph R1 1st order -1 k = 1.6 x 10-4 s benzene 20 °C -MeMn(CO)5

Pt N

601

when dppePt(Me)(Mn(CO)5) is heated, no MeMn(CO)5 is formed Mn–Pt = 2.131(8) Å

Fig. 189

Ph Me

R1

Mn–Pt = 3.0337(8) Å (R1 = H; R2 = Me)

R1 = H; R2 = Me, Et, Ph R1 = tBu; R2 = Me, Et, iPr, Np, nHex, Ph, C6H4-p-OMe

N

602

532

Carbonyl and Isocyanide Complexes of Manganese

Ph2 P

1. AgNO3 2. Na[Mn(CO)5]

Cl

Pt P Ph2

O C O O C Ph2 C Mn P C O Pt O P C Ph2

THF, -30 °C 3 h (82%)

tBu

603

R1

R3 +

R2

R4

-CO

C6H6 50 °C, 2 h (87%)

R1

R2

O C O O C Ph2 C Mn P C O Pt S O P Ph2 606

CO

C O

P Ph2

tBu

O 605 S

O C

Mn

Pt

tBu

604

R1 = R4 = Me; R2 = R3 = H R1 = R3 = Me; R2 = R4 = H no reaction when R1 = R2 = R3 = R4 = Me

Ph2 P

CO

O C

OC

S

C6H6 r.t. (78%) R R3 4

R1

R R3 4

R2

O C

O C

O

O C Mn

'

Ph2 P

R4 R3

C S O

Pt R1

P Ph2

tBu

R2

tBu

O 607

Fig. 190

Another series of interesting chemistry possible at Mn-Pt complexes is demonstrated in Fig. 190.440 After abstracting a halide from 603 with AgNO3, the resulting species reacts with Na[Mn(CO)5] to afford 604 with a MndPt bond. However, unlike the examples described above, the Mn-CO functional group undergoes a migratory insertion into the PtdC bond of the neopentyl group. 604 and 605 are in equilibrium with CO dissociation and binding, owing to the hemilability of the Pt–acyl group in 604. Both of these species, 604 and 605, react with substituted thiirane. For 605, the species 607 results with ring-opened and inserted acyl complex as a kinetic product. Further heating of 607 results in expulsion of alkene and 606, effectively serving as desulfurization route for substituted thiiranes (thiirane did not result in loss of ethylene). 606 can be formed directly upon heating 604 in the presence of the thiirane. Although not catalytic, the reaction provides an interesting example of a heterobimetallic cooperative reactivity. A few other related complexes to 606 have been prepared.441

6.06.8.4.2

Other examples of unsupported MdMn bonds

Two examples of Mn-Rh heterobimetallics are shown in Fig. 191. The Rh-Mn complex 608 was prepared by treating the Rh complex shown with Mn2(CO)10 and UV irradiation for about 10 min. The product contains the Rh-macrocycle with an axial Mn(CO)5 ligand, and a RhdMn bond of 2.711 A˚ .442 The complex is not stable and slowly converts back to its starting materials. In the same report Rh-Fe, Rh-Mo, and Rh-Cr complexes had their bond dissociation energies estimated at 27, 25, and 19 kcal mol−1 respectively. The dirhodium complex 609 was prepared by reacting the cationic Rh precursor with PPN[Mn(CO)5].443 A few coinage metal Mn–M complexes are shown in Fig. 192. Complex 610 (having a Mn-Cu distance of 2.4 A˚ ) is in equilibrium with the m-Cu-{Mn(CO)5} anion 6115; these complexes have been used in catalytic alkyne hydrostannylation.444 The Cu, Ag, and Au congeners have been prepared and had their M–Mn distances compared (612).445 One surprising finding was that the Mn-Au distance in one molecule was slightly shorter or nearly the same than two of the Mn-Ag distances, all of which were longer than the

O C

dark, 3 w (or 300 nm, 10 m) 0.5 {Mn2(CO)10 + [Rh]2}

OC toluene

N [Rh] = N

Fig. 191

N

N

O C

O C

Mn [Rh]

C C O O Mn–Rh = 2.711 Å 608

Rh

O C OC

O C CO

Mn

C Rh

Ph2P

O

Rh N

N

PPh2

Mn–Rh = 2.6688(8) and 2.647(1) Å 609

Carbonyl and Isocyanide Complexes of Manganese

R = dipp or mes O C OC

O C

Mn Cu

C O

R C

O C Mn N

C O

O C CO

N

R

C O

C O

Mn–Cu = 2.4 Å 610

O C Cu C O 611

– [Cu(IMes) 2

O C

Mn C O

OC CO

dipp

O C

]+

O C Mn M

C O C O

533

N L

IPr = C N dipp

612 M = Au, Ag, Cu Mn–Au = 2.5754(5) Å (L = IPr) Mn–Ag = 2.5746(3) Å (L = P(mes)3 ) Mn–Ag = 2.5905(3) Å (L = IPr) Mn–Cu = 2.415(1) Å (L = IPr)

Fig. 192

Mn-Cu distance. This may have to do with the fact that these MndM bonds are not completely unsupported, having short coinage M⋯ C distances, with the CO ligands having some bridging character in 610–612.

6.06.8.4.3

Clusters

The cluster 613 (Fig. 193) was prepared by reacting RhCl3 with Na[Mn(CO)5] and was isolated in 49% yield; 2 eq. of [Mn(CO)4-m-Cl]2 is a byproduct of this reaction. It contains a unique triangular structure with Rh-Rh distances of 2.877(3) A˚ and Mn-Rh distances of 2.640(6) A˚ . 613 was supported on silica, forming Rh nanoparticles and used in syngas to ethanol catalysis.446 The cluster 614, one of several compounds studied by Yamazaki and coworkers in 1,3-diyne metallacycles derived from alkyne couplings to CO, was prepared by treating cis-Pt(CCPh)2(dppe) with Mn2(CO)9(MeCN) in refluxing toluene (Fig. 193).447 The 1-manganacyclo-2,3,4-pentatriene structure in 614 is a notably unusual structural motif. Trinuclear, diborane bridged complex 615 can be synthesized from Mn2(CO)10 by treatment with borane-THF under photolytic conditions at modest yields (Fig. 194).448 An alternative procedure generates the same trinuclear complex (albeit at substantially lower yields) by treatment of Mn2(CO)10 with lithium borohydride in the presence of [(CoCp )(m-Cl)]2.449 The latter procedure generates borylene complex 616, as a side product, in low yields. Complex 616 features a m3-BH moiety, which is a common moiety in borane bridged cluster complexes. Complex 615 can be treated with tricyclohexylphosphine under photolytic conditions to yield a mixture of diborane bridged complexes 617, 618 and 619 (Fig. 194.). Analogous rhenium complexes can be similarly formed. The m3-BH moiety seen in 616 has been observed in numerous Mn-carbonyl containing heterobimetallic cluster compounds. Typically, this moiety adopts an apical position in tetrahedral geometry, yielding geometrically diverse clusters as seen in 620, 621, 622, 623, 624, and the m-arsenide cluster 625 (Fig. 195). These complexes are typically synthesized from treatment of an appropriate borane-metal cluster with either Mn2(CO)10 or Mn(CO)5Br.450–454

6.06.9

Chemistry of Mn-CNR complexes

This section is concerned with Mn-CO and Mn-CNR complexes where isocyanide (isonitrile) ligands play a central role in the chemistry. Several examples of Mn-CO complexes coordinated to isocyanides, especially CNtBu, have already appeared above, but

Fig. 193

534

Carbonyl and Isocyanide Complexes of Manganese

Fig. 194

Fig. 195

in those instances the isocyanide did not play a major role in the chemistry. This section begins with summarizing some recent work in preparing simple CO/CNR mixed Mn(I) complexes. Particular attention is paid to reactions where isocyanides are functionalized, and the chemistry of the resulting species is also considered. In addition, especially bulky isocyanides, which allow for the isolation of unique, coordinatively unsaturated compounds is discussed in full. Finally, a special class of “metalloisocyanides” as ligands is also discussed in this section.

6.06.9.1

Preparation of simple Mn(I)–CNR complexes

Treichel and co-workers laid some of the foundational coordination reaction conditions and outcomes to prepare Mn–CO/CNR complexes using Mn(CO)5Br, the products of which vary dramatically on conditions and stoichiometry.455 Following these strategies, Zobi and co-workers prepared 626 by reacting Mn(CO)5Br with 3.5 eq. of desired CNR in refluxing acetone (Fig. 196).456 The products were purified by column chromatography and typically isolated in moderate to good yield. A mixed CN, CO, and CNR Mn complex 627 was prepared by Imhof and co-workers by reacting Mn2(CO)10 in a steel autoclave with excess tert-octylisocyanide (Fig. 197).457 A similar reaction with tert-butyl isocyanide results in both cis and trans isomers of 627 (R ¼ tBu) as the major product along with the known homoleptic [Mn(I){tBuNC}6]+ as a minor product. However, these products were only formed when the reaction was carried out under CO atmosphere (10 bar).458 If CO is excluded from the same reaction, then the organic product 628b results, and not the desired Mn-CNR complexes. Additionally, if chloroform is used during the workup procedure for 627 (R ¼ tBu), then [Mn{tBuNC}5CO]+ (628a) and a dinuclear metalloisocyanide complex 628-MnCl3 were isolated instead. Section 6.06.9.4 contains more examples of metalloisocyanides like 627-M.

Carbonyl and Isocyanide Complexes of Manganese

535

Fig. 196

Fig. 197

Fig. 198

Peters and coworkers have prepared the Mn(I)-CNR complex 630 by reducing a Mn(II)-iodide complex 629 using Na naphthalenide followed by addition of xs. CNtBu, albeit the isolated yields are not excellent (Fig. 198).362

6.06.9.2 6.06.9.2.1

Functionalization of CNR ligands and chemistry of resulting species Diverse reactivity of Mn-CNR

6.06.9.2.1.1 Reactivity of [{bipy}Mn(CO)3(CNR)]+ (631) Ruiz and coworkers have used bipyridine extensively as a supporting ligand in their studies of Mn-bound acyclic and cyclic carbenes derived primarily from isocyanide starting materials. The known isocyanide complexes [{bipy}Mn(CO)3(CNR)]ClO4 (631) (R ¼ Ph, xylyl, Me, Bn, tBu)459 have diverse reactivity with methylamine where the product depends on the reaction conditions

536

Carbonyl and Isocyanide Complexes of Manganese

Fig. 199

(Fig. 199).460,461 If 631 is treated with gaseous methylamine in DCM at room temperature, the acyclic carbene complex 632 is formed resulting from nucleophilic attack on the isocyanide carbon atom (Fig. 199). However, if the same reaction is carried out at low temperature, the amine attacks a coordinated CO trans to the isocyanide and forms a carbamoyl complex 633; this process is reversible. If the same reaction is carried out in water instead of DCM, a simple ligand substitution and arrangement occurs to form 634; the mechanism for this reaction invoked a lower-coordinate intermediate to allow for the isocyanide ligand to result in the position shown in Fig. 199. 632 is susceptible to chemically reversible deprotonation to afford the formimidinate complex 635, which in turn is reactive toward terminal alkynes to afford an Z4-azacyclohexadienyl complex (636) with loss of bipyridine ligand. Complex 631 can be reacted with 3-bromopropylamine and, depending on the substituent on the isocyanide nitrogen atom, affords either the acyclic carbene ligand with an azetidine functional group (632) or the N,O-heterocyclic carbene (637) (Fig. 200).461 If a propargyl nucleophile is used to react with 631, then the X,N-heterocyclic (X ¼ O, NH) carbene complex 638 directly forms by attack on the isocyanide carbon atom462; the same reaction is also possible when the bis(diphenylphosphino) methane (dppm) as the supporting ligand instead of bipy and affords 638(P).459 Using lower temperatures and shorter reaction times, an intermediate 639 was isolated en route to complex 638.463

Fig. 200

Carbonyl and Isocyanide Complexes of Manganese

537

6.06.9.2.1.2 Reactivity of acyclic carbenes 632 derived from Mn-CNR (631) The acyclic carbenes 632 react with Ag2O to form metallocyclic products 640a or 640b, resulting from insertion of a CO ligand into one of the nitrogen atoms flanking the carbene carbon (Fig. 201).464 A similar insertion reaction results when 632 is reacted with KOH in the presence of [AuCl(PPh3)], but instead a Au-carbene complex 641 results (Fig. 201)465; the metallocyclic product 641 isomerizes to 642 upon heating in THF indicating stereochemical non-rigidity. The acyl functional groups in 641 and 642 can be methylated to form the cationic heterometallic dicarbene complex (not shown).465 Deriving from N,N0 -diarylformamides, it is also possible to synthesize 642 by a different route (Fig. 202).464 For instance, if {bipy}Mn(CO)3Br is reacted with an N,N0 -diarylformamide in the presence of TlPF6, the N-bound imine cation 643 is synthesized, which can be further converted into either 644 if the aryl substituent is a benzene, or 640 if the substituent is electron rich dimethylaminophenyl; several other related metallocyclic compounds to 642 have been prepared by analogous paths.466 Compound 632 is a useful synthon to synthesize free formimidines. This is accomplished via treatment of acyclic carbenes 632 with Ag2O in DCM forming a coordinated formimidine complex 643 as shown in Fig. 203. Liberation of free formimidine 646 occurs via ligand substitution with isocyanide in MeCN, forming 631 as a byproduct, thus completing a synthetic cycle.467 Additionally, acyclic carbene complex 632 may react with KOH to afford formimidinate compound 645, which upon oxidation with Ag2O liberates carbodiimide compounds 647 as shown in Fig. 203.467

6.06.9.3

Chemistry of bulky Mn-CNR complexes

Figueroa and co-workers have made extensive use of bulky 2,6-substituted phenylisocyanide ligands, Lmes and Ldipp, shown in Fig. 204. 648 (synthesis shown in Fig. 205) can be reduced to isolable Mn−1 compounds 649 using sodium mercury amalgam or [trans-Mn(CO)2(Lmes)3]− using potassium anthracenide.468 The anion 649 is nucleophilic and can be functionalized with a variety of electrophiles (Fig. 205).468,469 For example, 649 reacts with acid (e.g., HCl) to reform the halide complex 648(Cl). It also reacts with alkyl halides and a variety of main group electrophiles to afford 648(E). Alternatively, using essentially the same group of electrophiles, the neutral metalloradicals 650 (synthesis discussed later) reacts via a radical mechanism to afford the same set of 648(E) complexes. The radical mechanism requires 2 eq. of 650, so that both 648(Cl) and respective 648(E) result.

Fig. 201

Fig. 202

538

Carbonyl and Isocyanide Complexes of Manganese

Fig. 203

Fig. 204

Fig. 205

One of the greatest achievements of the bulky isocyanide ligands has been their kinetic stabilization of otherwise inaccessible organometallic compounds. For instance, Mn(CO)5 metalloradical is not stable at room temperature and quickly dimerizes to Mn2(CO)10, and as such, isolation and characterization of the metalloradicals requires cryogenic or ultrafast techniques. Thus, exploring its reactivity is not readily achieved. However, use of the bulky ligand Ldiip has allowed for the r.t. isolation and manipulation of the tricarbonyl bis-Ldiip complex 650 and its chemical properties have been studied. Preparation of 650 effectively takes advantage of the rapid ligand exchange of the 17-electron Mn(CO)5 metalloradical generated photochemically in the presence of Ldiip in hexane (Fig. 206).470 Another method relies on comproportionation of 648 and 649, resulting in 2 eq. of 650; both routes allow for high yields of 650. 650 reacts with electrophiles as described above via a radical mechanism affording 648. It also reacts with nitrosyl radicals to afford Z2-RNO-adducts 651, and reacts with P4 to form the bridging P4 complex 652.

Carbonyl and Isocyanide Complexes of Manganese

539

Fig. 206

Fig. 207

650 also reacts with benzoyl peroxide to afford the dicarbonyl k2-acetato complex 653, which can also be synthesized directly from 648(OTf ) and Na(OAc) (Fig. 207).471 The formation of 653 likely occurs through initial mono-dentate coordination of the acetate and then chelation causes loss of a CO ligand because if electron poor trifluoroacetate is used, 654 results instead of 653. Nevertheless, chelation of the carboxylate in 653 is not particularly strong and can be lost by addition of pyridine. The reactivity and substitutional lability of the carboxylate ligand in 653 renders it a useful synthon for a variety of other complexes as shown in (Fig. 208).471 For example, treatment of 653 with B(C6F5)3 results in the unsaturated complex 655, stabilized through an intramolecular C-H agostic interaction. Low-coordinate triflates or etherates 656 can be prepared by addition of Me3SiOTf to 653 in hydrocarbon solvent, and either treating the resulting triflate adduct with Et2O or THF and salt metathesis with [B(ArF)4]−. Reduction of 653 with borohydride results in the Z2-borohydride complex 657, which when protonated liberates H2 and complex 657-OTf. Finally, 653 can be reduced with 2 eq. KC8 resulting in an inserted product 658 from an aza-Büchner ring expansion of one of the bulky isocyanide ligands. Given the importance of manganese(I) complexes in CO2 reduction electrocatalysis, studies of the reduced anion toward CO2 was explored by Figueroa and co-workers using 649 under strictly anhydrous conditions and in the absence of any proton source. At low temperature, the reaction between CO2 and 649 was allowed to reach r.t. and characterization of the products indicated that 2 eq. of 649 mediate the reductive disproportionation of CO2 (affording CO and carbonate) (Fig. 209).472

6.06.9.4

Metalloisocyanides

The sections above detailed the Mn chemistry that pertained to organic-isocyanides. This section details several complexes with Mn-CO centers coordinated to the metalloisocyanide ligand donated MNC–Mn–CO. Some isomeric metallonitrile complexes formulated as MCN–Mn–CO are also discussed herein. A typical strategy to prepare metalloisocyanide and metallonitrile complexes is depicted in Fig. 210.473 Complex 67 and 659, the latter of which can be prepared from 67 and AgCN in DCM, are mixed together and the bromido ligand of 67 is removed via halide

540

Carbonyl and Isocyanide Complexes of Manganese

Fig. 208

Fig. 209

Fig. 210

abstraction with AgClO4 affording 660. In this case, both metals are Mn, and so this is an example of a Mn-metallonitrile and -metalloisonitrile in the same complex. The tert-butyl derivative of 627 has been used as a precursor for the complex(es) 661, which contains a central metal ion (e.g., FeCl3, MnCl2, NiCl2, CuCl2, Co(NO2)3, Rh lantern complexes) coordinated to two manganoisonitrile ligands that originated from 627 (Fig. 211).458,474,475 Connelly and co-workers have prepared several metalloisonitrile complexes and a summary of some of this work using Mn(CO)/phosphine complexes is shown in Fig. 212.476 Both cis and trans dicarbonyl isomers of cyanate 662 and bromido 663 are accessible complexes. Treatment of these with various substitutionally labile complexes result in a variety of metal complexes. For example, treatment of cis-662 with [Ru(NH3)5(OH2)]2+ results in loss of the aqua ligand and N-coordination of cis-662 to the Ru center forming a manganonitrile ligand to Ru in cis-664. Similarly, using trans-662 and reacting it with [{Tp }Mo(CO)2(dpa)] PF6 furnishes the complex trans-665, which contains a manganonitrile ligand. Conversely, if the Mn(I)Br complex cis- or trans-663 is reacted with neutral Mo-CN complex {Tp }Mo(CO)(CN)(dpa), then the complex cis- or trans-667 results, which has a manganoisonitrile ligand coordinated to Mo. As noted, Fig. 212 contains only a brief summary of the numerous compounds that have been prepared following similar strategies.

Carbonyl and Isocyanide Complexes of Manganese

541

Fig. 211

Fig. 212

6.06.10

Mn-CO compounds in materials and supramolecular chemistry

6.06.10.1 Mn-CO as precursors to Mn-containing materials Owing to volatility of organometallic Mn-CO complexes, they have been used in vapor deposition and a recent study investigated the thermal chemistry of Mn2(CO)10 during deposition on native oxide of Si(100) and copper surfaces477; this study contains a summary of literature using organomanganese compounds in vapor deposition. In the instances that were investigated by Zaera and co-workers, metallic manganese was not deposited and rather MnO or Mn-silicates formed. Similarly, MeMn(CO)5 has been used as a precursor to chemisorption of Mn-CO onto MgO. The Mn-CO functionality was maintained as determined by IR spectra of surface bound Mn-CO centers, and XAS measurements showing Mn-C contacts at 1.87 A˚ , and Mn-O distances were found to be 2.12 A˚ .478 Mn2(CO)10 has been used as a source of Mn0 metal in Mn-Pt nanocrystal, which were prepared by reacting Pt(acac)2 in hexadecanediol with Mn2(CO)10 in dioctyl ether and heating at 100  C with the particles being stabilized with oleic acid and oleylamine.479 Also, as was described above (Section 6.06.8.3.3), MnP materials have been prepared by thermal decomposition of 596.

6.06.10.2 Immobilization of Mn-CO complexes Earlier, several examples of Mn-CO complexes, either by polymerization of the complex itself or coordination to polymerized ligands, were noted to be incorporated into organic polymers (see Figs. 146 and 147). In addition to organic polymers, an

542

Carbonyl and Isocyanide Complexes of Manganese

Fig. 213

“organometallic coordination polymer” having Mn-CO monomer building units has also been synthesized. For example, treatment of Mn2(CO)10 with 2 eq. of Cp 2Sm(THF)2 in THF affords the polymeric structure 668 shown below (Fig. 213).480 More traditional modes of immobilization of Mn-CO complexes including immobilization onto alumina,481 anionic porous frameworks,482 graphite,483 TiO2,484,485 and mesoporous silica486 haven been accomplished using various methods. In addition, Mn-CO complexes have been incorporated in various fashions in metal organic frameworks (MOF). In most cases, the Mn-CO unit is coordinated to a ligand that is part of the framework, where the Mn ions are not actually part of the MOF but simply coordinated to the MOF.126,127,487–489 In other cases, the Mn-CO fragments are appended to the MOF via a post synthetic modification strategy.126,490

6.06.10.3 Supramolecular coordination chemistry of Mn-CO complexes 6.06.10.3.1

Self-assembled molecular squares

Supramolecular chemistry has emerged as a rapidly growing field over the past few decades because of its fascinating structures and potential applications inaccessible to single molecular compounds. Within this subfield of chemistry, molecular rectangles are a dominant class. Hence, much of the supramolecular chemistry with Mn has thus involved these rectangles, and these are prepared from one of two main strategies. The first strategy is a redox/coordination reaction that uses Mn2(CO)10 and a thiol or disulfide. The thiol or disulfide serve to oxidize the Mn centers and generate the self-assembling units in situ. The first example was demonstrated by Kabir, Hogarth, and Roesky and co-workers using pyrimidine-2-thiol. As shown in Fig. 214, the reaction between Mn2(CO)10 and pyrimidine-2-thiol in DCM resulted in the tetranuclear square complex 669 in good yields.491 Another example developed by Manimaran and coworkers uses diaryl disulfides instead of thiols and is a versatile scheme to prepare self-assembled tetranuclear squares 670, as can be seen in Fig. 215 along with the various ligands that have been used.492–494

Fig. 214

Carbonyl and Isocyanide Complexes of Manganese

543

Fig. 215

Fig. 216

The second strategy uses Mn(CO)5Br directly in a coordination-driven self-assembly reaction with ligands in DCM and, as explored by Manimaran and coworkers, has also shown to be effective in preparing dinuclear molecular squares (Fig. 216).495,496

6.06.10.3.2

Mn-CO compounds as potential building blocks in self-assembled structures

While the next set of examples do not strictly fit the category of self-assembled supramolecules, they do serve as molecular compounds that in principle may find applications as nodes, clips, linkers, etc. in self-assembly studies that rely on Mn-CO coordination chemistry. For example, the pre-coordinated complex 68 reacts with 4,40 -bipy to afford the dinuclear complex 672 (Fig. 217),497,498 which conceivably contain precursors to octahedral Mn-nodes in larger structures. Likewise, 68 can react with pre-assembled 673 to prepare an analogous complex 674. Tri- and di-substituted trispyrazolyl borate functionalized benzene has been used to make multinuclear Mn(CO)3 complex 675,499 which are potential nodes for larger supramolecular structures. Likewise, 1,3-benzene difunctionalized bi-pyrazole pyridine

544

Carbonyl and Isocyanide Complexes of Manganese

Fig. 217

Fig. 218

compounds 676 have been prepared and are conceivable nodes.343 And finally, the oxamidato-links Mn(I) centers in 677 to afford “horse-stirrup-like” metallaclips (Fig. 218).500

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6.07

Carbonyl and Isocyanide Complexes of Rhenium

Chi-On Ng, Shun-Cheung Cheng, and Chi-Chiu Ko, Department of Chemistry, City University of Hong Kong, Kowloon, Hong Kong, China © 2022 Elsevier Ltd. All rights reserved.

6.07.1 Introduction 6.07.2 Rhenium carbonyl complexes 6.07.2.1 Re(0) carbonyls 6.07.2.1.1 Preparation and reactivity 6.07.2.2 Mononuclear Re(I) carbonyl complexes 6.07.2.2.1 Preparation 6.07.2.2.2 Reactivity 6.07.2.3 Polynuclear carbonyl rhenium(0/I) complexes 6.07.2.3.1 Homopolynuclear carbonyl rhenium(I) complexes 6.07.2.3.2 Polymers functionalized with tricarbonyl Re(I) diimine complexes 6.07.2.3.3 Heteropolynuclear supramolecular systems with tricarbonyl rhenium units 6.07.2.3.4 Heterometallic carbonyl rhenium clusters with Re-metal interactions/bonds 6.07.2.4 Re(II) and Re(III) carbonyl complexes 6.07.2.4.1 Re(II) carbonyl complexes 6.07.2.4.2 Re(III) carbonyl complexes 6.07.3 Rhenium isocyanide complexes 6.07.3.1 Re(I) isocyanide 6.07.3.1.1 Mono-, di-, and tri-isocyano Re(I) complexes {[Re(CNR)n]+ (n ¼ 1–3)} 6.07.3.1.2 Tetra-, penta- and hexa- isocyano Re(I) complexes {[Re(CNR)n]+ (n ¼ 4–6)} 6.07.3.1.3 Reactivity of isocyano Re(I) complexes 6.07.3.2 Re(III) isocyanide 6.07.3.3 Re(V) isocyanide 6.07.4 Applications 6.07.4.1 Photophysics of luminescent carbonyl and isocyano rhenium complexes 6.07.4.2 Energy-transfer photosensitizers 6.07.4.3 Photocatalysis 6.07.4.4 Biomedical applications 6.07.5 Conclusion Acknowledgment References

554 554 554 554 558 558 576 580 580 593 594 596 597 597 599 599 599 599 604 606 609 612 613 613 614 615 616 616 616 617

Abbreviations 2,3-dpp 9S3 BDI bma bmf bpb bpcd bpg bpy bpypp btpp Bu CAP DAAm DBU DHF dpa dpe dppe dppm

2,3-Bis(2-pyridyl)pyrazine 1,4,7-Trithiacyclononane N,N0 -Bis(2,6-diisopropylphenyl)-2,4-dimethyl-b-diketiminate 2,3-Bis(diphenylphosphino)maleic anhydride 3,4-Bis(diphenylphosphino)-5-methoxy-2(5H)-furanone 1,4-Bis(pyridyl-4-ylethynyl)benzene 4,5-Bis(diphenylphosphino)-4-cyclopenten-1,3-dione Bis(2-pyridylmethyl)glycine Bipyridine 2,5-Bis(2-pyridyl)-1-phenylphoshpole 2,5-Bis(2-thienyl)-1-phenylphoshphole Butyl 1,4,7-Triaza-9-phosphatricyclo[5.3.2]tridecane N,N-Bis(2-arylaminoethyl)methylamine 1,8-Diazabicyclo[5.4.0]undec-7-ene Dihydrofulvalene Bis(4-pyridyl)acetylene 1,2-Bis(4-pyridyl)ethylene Bis(diphenylphosphino)ethane Bis(diphenylphosphino)methane

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00043-3

553

554

Carbonyl and Isocyanide Complexes of Rhenium

dppv dvb Et HATN-Me6 ISQ-iPr LF MLCT NHC NNS OPPh3 phen PN PNN PTA PTh pz TACN THF tpp Triphos

6.07.1

Trans-1,2-bis(diphenylphosphino)ethene Divinylbenzene Ethyl 2,3,8,9,14,15-Hexamethyl-5,6,11,12,17,18-hexaazatrinapthalene o-iminobenzosemiquinonate Ligand-Field Metal-to-Ligand-Charge-Transfer N-Heterocyclic carbene 1-methyl-2-{(o-thiomethyl)phenylazo}imidazole Triphenylphosphine oxide Phenanthroline 2-(Diphenylphosphanyl)pyridine 6-(Diphenylphosphino)-2,20 -bipyridine 1,3,5-Triaza-7-phosphaadamantane Tri(2-thienyl)phosphine Pyrazine 1,4,7-Triazacyclononane Tetrahydrofuran 1,2,5-Triphenylphosphole 1,1,1-Tris(diphenylphosphinomethyl)ethane

Introduction

There has been growing attention to the study of rhenium carbonyl and isocyano complexes over the past few decades. These investigations have typically focused on the influence that ligand functionalization has on the properties and reactivity of different complexes targeted for application-specific tasks. Some of these complexes were prepared using established synthetic methodologies summarized in chapters covering organometallic rhenium chemistry in the previous edition of Comprehensive Organometallic Chemistry III (COMC 2007).1 However, more recent developments have included new synthetic methodologies such as photochemical, mechanical, and microwave-assisted reactions to increase functionalization in targeted complexes. Reports on rhenium(I) isocyanide complexes have also increased considerably over the past 10 years. Isocyanide ligands are desirable because the N-substituent can be readily modified to tune the electronic, steric, and physical properties of the resulting rhenium complexes. Despite the isoelectronic nature of the isocyanide and carbonyl ligand, the synthetic routes of rhenium isocyanide complexes are entirely different from those for the rhenium carbonyl analogs. This article reviews the preparation, reactivity, and properties of rhenium carbonyl and isocyanide complexes. To avoid significant overlap with content in the previous version of COMC,1 we have focused on complexes reported after 2005. Moreover, selected popular topics, including photophysics, photocatalysis, photosensitization, and biomedical applications of these complexes, are also discussed.

6.07.2

Rhenium carbonyl complexes

6.07.2.1

Re(0) carbonyls

6.07.2.1.1

Preparation and reactivity

There are several rhenium synthetic precursors used to prepare Re(0) carbonyl complexes and most of these are prepared from the commercially available starting material [Re2(CO)10] (Scheme 1).2–15 For example, [Re2(CO)10] is used to prepare the acetonitrile

Scheme 1 (A) X ¼ Cl: ] method A: hn/CCl4; method B: Cl2/CCl4. X ¼ Br: Br2/hexane. X ¼ I: ] I2, hn/hexane.2–6 (B) NaHg/THF.7 (C) hn/1-butene or 1-hexene.8,9 (D) Me3NO/MeCN-CH2Cl2 (1:1, v/v)10,11 (E) Method A: Zn/AcOH;4, Method B: Et3SiH hn/cyclohexane.12 (F) H3PO4/cyclohexane.13,14 (G) (1) [Au(PPh3)Ph]/hexane. (2) HSnPh3/CH2Cl2.15 (H) MeCN, 25  C.15

Carbonyl and Isocyanide Complexes of Rhenium

555

complexes {[Re2(CO)9(NCMe)] and [Re2(CO)8(NCMe)2]}, which represent the most popular starting precursors for the preparation of Re(0) carbonyl complexes. These complexes are prepared by decarbonylation of [Re2(CO)10] by 1 and 2 mol equiv. of Me3NO, respectively, in DCM-MeCN (1:1, v/v), and this method has also been described in the previous edition of COMC.1 As shown in Scheme 1, [Re2(CO)10] is also used to prepare Re(I) starting materials commonly used to make many of the complexes to be described in this article. In addition to the most wide-used halogenation of [Re2(CO)10] in the preparation of Re(I) synthetic precursors {[Re(CO)5(X)]}, [Re2(CO)10] also undergoes oxidative addition of alkenes to give [Re2(CO)8(m-H)(m-
1,
2-CH]CHR)] with s,p coordinated bridging olefin and a bridging hydride upon 254-nm excitation. The preparation of [Re2(CO)10] was discussed extensively in the previous edition of COMC, and there have been no alternative preparative methods described for its synthesis since then. 6.07.2.1.1.1 Re(0) complexes prepared using [Re2(CO)8(NCMe)2] Treatment of [Re2(CO)8(NCMe)2] with the monodentate ligand 1-vinylimidazole in benzene at 80  C leads to the formation of a mixture of mono- and di-(1-vinylimidazole) coordinated Re dinuclear complexes with various coordination geometries, [Re2(CO)9{
1-NC3H3N(CH]CH2)}] [1 and 2], [Re2(CO)8{
1-NC3H3N(CH]CH2)}2] [3 and 4] (Scheme 2).16 1H NMR signals of the reaction mixture confirms that the two isomers of [Re2(CO)8{
1-NC3H3N(CH]CH2)}2] are in 1:1 ratio. This indicates that the carbonyl ligand at the equatorial and axial position would undergo ligand substitution reaction. The isolation of the mono-substituted complexes 1 and 2 is believed to be derived from the trace amount of [Re2(CO)9(NCMe)] in the starting material.

Scheme 2 Ligand substitution of [Re2(CO)8(NCMe)2] with 1-vinylimidazole.

Reaction of [Re2(CO)8(NCMe)2] with polydentate ligands give different types of products depending on the reaction conditions (Scheme 3). For example, [Re2(CO)8(NCMe)2] reacts with 2,3-bis(2-pyridyl)pyrazine (2,3-dpp) in refluxing THF for 5 h to give dinuclear Re(0) complexes with different coordination modes {[Re2(CO)8(C14H10N4)] [5 (28%) and 6 (7%)]}, tetranuclear Re(0) complex [Re2(CO)8(C14H10N4)Re2(CO)8] 7 (6%) and dinuclear Re(0) with two 2,3-dpp ligands, [Re2(CO)6(C14H10N4)2]

Scheme 3 Ligand substitution of [Re2(CO)8(NCMe)2] with 2,3-dpp.

556

Carbonyl and Isocyanide Complexes of Rhenium

8 (5%).17 When the reaction is carried out in refluxing CH2Cl2 for 2 h, only 5 (40%), 6 (16%) and 7 (10%) are obtained. Interestingly, reaction of 5 with [Re2(CO)8(NCMe)2] in refluxing CH2Cl2 gives a mixture of 6 and 7. Prolonged heating of 5 in THF or photoirradiation in CH2Cl2 transforms 5 into 8. For the reactions between [Re2(CO)8(NCMe)2] and p-conjugated phosphole derivatives [2,5-bis(2-thienyl)-1-phenylphosphole (btpp), 2,5-bis(2-pyridyl)-1-phenylphosphole (bpypp) and 1,2,5-triphenylphosphole (tpp)], a mixture of dinuclear carbonyl Re(0) complexes with different phosphole coordination is obtained (Scheme 4).18 For the reaction between [Re2(CO)8(NCMe)2] and btpp in refluxing cyclohexane, btpp coordinate as monodentate s donor to form a RedP bond to give a mononuclear Re(I) complex 9 (10%) and dirhenium(0) complexes [10 (12%) and 11 (23%)]. In contrast, btpp can coordinate as a bridging ligand to the two Re metal centers in the dirhenium(0) complexes through P-s,diene-p coordination 12 (26%) and P-s,S-s coordination (13) (28%). Upon heating of 11 in cyclohexane, btpp changes from a monodentate to bidentate bridging ligand to afford 12 and 13. On the other hand, photolysis of 11 in CH2Cl2 leads to the dissociation of RedRe bond to give mononuclear Re(I) complex 9, whereas 10 undergoes thermal decomposition upon heating. Ligand substitution reaction with the pyridine analog (bpypp) results in ligand coordination to the dirhenium complex as a bidentate P-s,N-s chelate to one of the rhenium metal centers in 14 (42%) or as bridging ligand in 15 (34%). The diphenylphosphole ligand (tpp) also coordinates in a monodentate fashion in 16–17 or via a P-s,diene-p bridging mode in 18.

Scheme 4 Ligand substitution of [Re2(CO)8(NCMe)2] with polydentate phosphole derivatives.

Carbonyl and Isocyanide Complexes of Rhenium

557

6.07.2.1.1.2 Re(0) complexes via reductive elimination of [Re2(CO)8(m-H)(m-Z1,Z2-CH]CHR)] The alkene and hydride of [Re2(CO)8(m-H)(m-
1,
2-CH]CHR)] can be substituted to afford various carbonyl dirhenium(0) complexes. For example, reaction of [Re2(CO)8(m-H)(m-
1,
2-CH]CHR)] with 1 mol equiv. of 2,3-bis(diphenylphosphino)maleic anhydride (bma) in CH2Cl2 gives a mixture of a bma bridged dirhenium(0) complex {[(CO)4Re-Re(CO)4](m-bma)} 19 and a zwitterionic dirhenium complex {[Re(CO)4]2(bma)} 20. The zwitterionic dirhenium complex 20 represents the first example of RedRe bond cleavage by the coordination of diphosphine (Scheme 5).8 While for the reactions between [Re2(CO)8(m-H) (m-
1,
2-CH]CHBu)] and other diphosphine ligands such as 3,4-bis(diphenylphosphino)-5-methoxy-2(5H)-furanone (bmf), 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd), bis(diphenylphosphino)methane (dppm) and bis(diphenylphosphino) ethane (dppe), only the diphosphine bridged dirhenium(0) complexes (Scheme 5, 19b–19e) are obtained.

Scheme 5 Reaction of [Re2(CO)8(m-H)(m-
1,
2-CH]CHR)] with diphosphine ligands.

6.07.2.1.1.3 Re(0) complexes prepared from the reaction between rhenium(I) and rhenate(I) complexes Dirhenium(0) complexes can also be prepared from the reaction between rhenium(I) carbonyl complexes and the pentacarbonyl rhenate(I) anion. The reaction of NHC-CS2 coordinated Re(I) precursors with Na[Re(CO)5] at room temperature leads to the formation of the NHC-CS2 bridged dinuclear Re(0)-Re(0) complex 21 (Scheme 6).19 Upon heating in petroleum ether at 130  C, complex 21 is transformed to the hexacarbonyl dirhenium(0) complexes 22 and 23 with two sulfide bridging ligands. The hexacarbonyl dirhenium(I) complexes 22 and 23 can also be prepared by the reaction of NHC-CS2 coordinated Re(I) precursors with Na[Re(CO)5] in refluxing THF.

Scheme 6 Reaction of tricarbonyl NHC-CS2 Re(I) complex with pentacarbonyl rhenate(I) anion.

6.07.2.1.1.4 Carbonyl ligand substitution in dinuclear Re(0) complexes Trimethylamine N-oxide (Me3NO) can be used as a decarbonylating reagent to facilitate carbonyl ligand substitution in dinuclear Re(0) complexes. For example, decarbonylation of [Re2(CO)9(PTh3)] 24 (PTh3: tri(2-thienyl)phosphine) with 1 equiv. Me3NO in MeCN at room temperature gives [Re2(CO)8(NCMe)(PTh3)] 25 (Scheme 7).20 The acetonitrile in complex 25 can be readily substituted with PTh3 at room temperature in dichloromethane to give 26 in moderate yield (62%). It is interesting to note that the ligand substitution reaction was also accompanied by ligand rearrangement to give a highly symmetric dinuclear complex with both PTh3 ligands coordinated at the axial positions of the two rhenium centers.

558

Carbonyl and Isocyanide Complexes of Rhenium

Scheme 7 Carbonyl ligand substitution reaction of carbonyl dirhenium(0) complex.

It is important to note that the decarbonylation reactions occur in much lower yield when used in complexes containing uncoordinated phosphine donors. Attempt to carry out carbonyl substitution reactions using Me3NO in mono-coordinated diphosphine complexes [Re2(CO)9(
1-PP)] (PP: Ph2P(CH2)nPPH2, n ¼ 1–6) afford not only the diphosphine bridged dirhenium(0) complex but also phosphine-oxide complexes [Re2(CO)9(
1-PP(O))].21 Two possible mechanisms have been proposed to account for the generation of the phosphine oxide (Scheme 8). The mechanisms differ in that they involve interactions between the uncoordinated phosphine and either the carbonyl carbon (Scheme 8A) or rhenium metal center (Scheme 8B) that facilitates nucleophilic attack of the oxygen atom of Me3NO on the uncoordinated phosphine to give the phosphine oxide.22 To test the mechanism for the formation of the phosphine oxide, the complex [Re2(CO)9(
1-dppv)] with the rigid trans-1,2-bis(diphenylphosphino)ethene (dppv) diphosphine ligand was used to restrict the interaction between the uncoordinated phosphine with the rhenium metal center. However, the phosphine oxide complex can still be obtained in the reaction of [Re2(CO)9(
1-dppv)] with Me3NO. This suggests that the oxidation of the uncoordinated phosphine ligand is likely associated with the interaction between the phosphine and the carbonyl ligand as illustrated in Scheme 8A.22

(A)

(B)

Scheme 8 Two possible mechanisms for the formation of the phosphine oxide.

6.07.2.2 6.07.2.2.1

Mononuclear Re(I) carbonyl complexes Preparation

6.07.2.2.1.1 [Re2(CO)10] to Re(I) carbonyl precursor complexes Almost all Re(I) carbonyl complexes are prepared from pentacarbonyl rhenium(I) chloride, bromide or iodide [Re(CO)5(X)]. The pentacarbonyl precursors [Re(CO)5(X)] can be readily obtained from the reactions of [Re2(CO)10] with CCl4, Br2, or I2 as described in Scheme 1. Butler and Frišcic recently reported a mechanochemical method to prepare [Re(CO)5(X)] in high yields simply by milling [Re2(CO)10], Oxone® and sodium halide in a ratio of 1:1.5:2 (Scheme 9).6 Reaction between [Re2(CO)10] with iodobenzene dichloride in dichloromethane at room temperature is another recently developed method to prepare [Re(CO)5(Cl)].23 For pentacarbonyl rhenium(I) hydride [Re(CO)5(H)], it can be prepared from the reactions of [Re(CO)5(X)] with zinc in the presence of acetic acid4 or the reaction of Na[Re(CO)5] with phosphoric acid in cyclohexane13 (Scheme 1).

Scheme 9 Mechanochemical method for the preparation of pentacarbonyl rhenium(I) halide.

Carbonyl and Isocyanide Complexes of Rhenium

559

6.07.2.2.1.2 [Re(CO)5]+ fragment Preparation of rhenium complexes with [Re(CO)5]+ fragments coordinated with an anionic or neutral ligand has been described in the previous version of COMC.1 These complexes are generally prepared by three different synthetic strategies: (1) cleavage of RedRe metal bond of [Re2(CO)10]; (2) direct substitution reaction of the halide ligand of [Re(CO)5(X)]; and (3) ligand substitution reaction of a labile ligand introduced by halogen abstraction of [Re(CO)5(X)]. Direct cleavage of the RedRe bond is exemplified by the preparation of [Re(CO)5(
1-Y@C2v(9)-C82)] (Fig. 1) through the photo-irradiation (>350 nm) of a toluene solution of [Re2(CO)10] in the presence of Y@C2v(9)-C82 at room temperature.24

Fig. 1 X-ray crystal structure of [Re(CO)5(
1-Y@C2v(9)-C82)]. Reproduced with permission from Xie, Y.-P.; Pan, C.; Bao, L.; Slanina, Z.; Akasaka, T.; Lu, X. Organometallics, 2019, 38, 2259–2263. Copyright 2019, American Chemical Society.

Direct ligand substitution of the halide ligand in [Re(CO)5(X)] is demonstrated by the reaction of [Re(CO)5(Cl)] with a strong anionic ligand, such as KSi6Me11, to give a charge-neutral complex [Re(CO)5(Si6Me11)] (Scheme 10).25

Scheme 10 Direct substitution of chloride in [Re(CO)5Cl] with KSi6Me11.

The most commonly used method to prepare Re(I) pentacarbonyl complexes starts with halide abstraction from [Re(CO)5(X)] (X: Cl, Br, I) using silver salts containing weakly coordinating anions such as AgOTf, AgPF6, AgBF4 or AgClO4. After halide abstraction, the weakly coordinating anions in [Re(CO)5(A)] (where A ¼ OTf, PF6, BF4, ClO4) can be readily substituted by many other ligands under mild conditions. The labile perchlorate ligand in [Re(CO)5(ClO4)] can be readily substituted by the adamantane-cage aminophosphines 1,3,5-triaza-7-phosphaadamantane (PTA) and 1,4,7-triaza-9-phosphatricyclo[5.3.2]tridecane (CAP) (Fig. 2) in CH2Cl2 at room temperature to afford water-soluble pentacarbonyl rhenium(I) phosphine complexes.26 Using similar reaction conditions, isocyanide ligands [EtOOCCH2NC or MeOOC(CH2)10NC] can be introduced to give {[Re(CO)5(CNCH2COOEt)](ClO4) and [Re(CO)5(CN(CH2)10COOMe)](ClO4)}, which are useful imaging agent for various myocardial diseases.27

Fig. 2 Structures of adamantane-cage aminophosphines.

Pentacarbonyl rhenium(I) complexes with different types of ligands such as phosphines, phosphine oxide, nitrogen-donor ligands (pyridine, amine, tetrazole), and carbon-donor ligands (isocyanide, carbene) have been prepared using [Re(CO)5(BF4)] under very mild conditions. Different reaction conditions were used in the substitution reactions with tetrazole, triphenylphosphine oxide (OPPh3), 4-dimethylaminopyridine (4-Me2Npy), and triphenyliminophosphorane ligands (Fig. 3).28 These reactions were performed in dichloromethane solution. For substitution reaction with 4-Me2Npy, it was carried out with an initial temperature of

560

Carbonyl and Isocyanide Complexes of Rhenium

Fig. 3 Structures of tetrazole, triphenylphosphine oxide (OPPh3), 4-dimethylaminopyridine (4-Me2Npy), triphenyliminophosphorane and amine ligands.

−78  C before slowly warming up to room temperature. For OPPh3, the reaction was carried out at room temperature for 10 min. For tetrazoles, the reactions were carried out at room temperature for 2 h (tetrazole) or 24 h (5-methyltetrazole). In the case of triphenyliminophosphorane ligands, the reactions finished within 5 min at room temperature. Under similar reaction condition at room temperature, wide ranges of amine ligands (Fig. 3), [aniline (NH2C6H5), N-methylaniline (NH(Me)C6H5), 4-methoxyaniline (NH2C6H4OMe-4), 4-toluidine (NH2C6H4Me-4), 2-toluidine (NH2C6H4Me-2), 4-nitroaniline (NH2C6H4(NO2)-4), 3-hydroxyaniline (NH2C6H4(OH)-3), 2,6-diisopropylaniline (NH2C6H3(C(CH3)2)2-2,6)] can be introduced into pentacarbonyl rhenium(I) system within 2 h.29 This reaction works well with diamine ligands (1,2-phenylenediamine and 1,4-phenylenediamine) to give the dinuclear Re(I) complexes. The versatility of these substitution reactions were exemplified by the reaction of [Re(CO)5(O3SCF3)] with deprotonated silsequioxanes, which can be achieved at 0  C to give {[(c-C6H11)8Si8O11(O)]Re(CO)5} 27 (Scheme 11).30 At temperatures higher than 0  C, 27 undergoes thermal decarbonylation to give dimeric 28 with two [Re(CO)4]+ fragments.

Scheme 11 Substitution reaction of [Re(CO)5(O3SCF3)] with deprotonated silsequioxane.

Carbonyl and Isocyanide Complexes of Rhenium

561

6.07.2.2.1.3 [Re(CO)4]+ fragment Rhenium(I) tetracarbonyl complexes are prepared by the three different methods: (1) cleavage of RedRe metal bonds in [Re2(CO)10], [Re2(CO)8(m-Br)2], and [Re2(CO)8(m-H)2]; (2) carbonyl ligand substitution using Na[Re(CO)5] or [Re(CO)5(X)]; and (3) substituting the halide in tricarbonyl rhenium(I) halide complexes with CO. Tetracarbonyl rhenium(I) chloride complexes with N-heterocyclic carbene ligands (Fig. 4) can be obtained from the reaction of [Re2(CO)10] with 1 mol equiv. of deprotonated imidazolium chlorides [di(trimethylphenyl)imidazolium chloride, di(isopropyl) imidazolium chloride and benzylcyclohexylimidazolium chloride].31

Fig. 4 Structures of tetracarbonyl rhenium(I) complexes with N-heterocyclic carbene ligands.

Refluxing chlorobenzene solutions containing [Re2(CO)10] and 2 mol equiv. of (1-naphthyl)diphenyl phosphine or diisopropyl(1-naphthyl) phosphine leads to formation of five-membered cyclometallated tetracarbonyl rhenium(I) complexes (Scheme 12A).32 These cyclometallated complexes are active photocatalysts for direct arylation of arenes with aryl halides in the presence of a base (NaOH, K2CO3, KOAc, tBuOK, pyrrolidine) with yields in the range of 25–70% (Scheme 12B). (A)

(B)

Scheme 12 (A) The reaction of [Re2(CO)10] with substituted phosphines. (B) Photocatalytic activity of the cyclometallated tetracarbonyl rhenium(I) complexes for arylation of arenes.

Reaction of [Re2(CO)8(m-Br)2] with 6-(diphenylphosphino)-2,20 -bipyridine (PNN) in refluxing CH2Cl2 gives a mixture of phosphine coordinated cis-[Re(CO)4(k1(P)-PNN)(Br)] 29 and fac-[Re(CO)3(k2(N,N)-PNN)(Br)] 30.33 Avoiding the formation of 30 in the preparation of 29 can be achieved by the reaction of [Re2(CO)8(m-Br)2] with the N-protonated ligand (PNNH+) in refluxing CH2Cl2, followed by deprotonation with basic alumina to give 29 as the major product (Scheme 13).

(A)

(B)

Scheme 13 Reactions of [Re2(CO)8(
1-Br)2] with (A) PNNH+ and (B) PNN ligands.

562

Carbonyl and Isocyanide Complexes of Rhenium

Similarly, the reaction of [Re2(CO)8(m-Br)2] and 2-(diphenylphosphanyl)pyridine (PN) shows high selectivity to coordination of phosphine over the pyridyl moiety.34 Treatment of [Re2(CO)8(m-Br)2] with 2 mol equiv. of PN in CH2Cl2 at room temperature for 5 h resulted in the formation of cis-[Re(CO)4(k1(P)-PN)(Br)]. The kinetics of the reaction of PN ligand in cis[Re(CO)4(k1(P)-PN)(Br)] has been studied by DFT calculation. Apart from the reactions with bromo-bridged dirhenium(I) complex precursor, tetracarbonyl rhenium(I) complexes can also be obtained from a hydride-bridged dirhenium(I) complex. The reaction of [Re2(CO)8(m-H)2] with 2 mol equiv. of phosphinoborane in toluene at room temperature gives a [Re(CO)4(kB,P-Ph2PCH2CH2BR2)(H)] (Scheme 14).35 The presence of BdH is confirmed by the BdH bond distance of 1.46 A˚ , which is shorter than the van der Waals radii in the crystal structure of [Re(CO)4(kB,P-Ph2PCH2CH2BR2)(H)].

Scheme 14 The reaction of [Re2(CO)8(m-H)2] with phosphinoborane.

With Na[Re(CO)5], an anionic tetracarbonyl rhenium oxycarbene complex 31 can be prepared in quantitative yield by its reaction with deprotonated vinyl bromide, prepared by pre-treatment with an equal amount of NaH (Scheme 15).36

Scheme 15 The reaction of Na[Re(CO)5] with deprotonated vinyl bromide.

Reactions of Na[Re(CO)5] with alkynes were reported to give tetracarbonyl rhenacyclobutadiene complexes in 1993.37 Using rhenacyclobutadiene complexes as precursors, a series of stable tetracarbonyl rhenabenzene complexes can be obtained by their reactions with 1-ethoxyethyne (Scheme 16).38,39

Scheme 16 Preparation of tetracarbonyl rhenabenzene complexes from rhenacyclobutadiene complexes.

Direct CO ligand substitution of [Re(CO)5(X)] with 1 equiv. of monodentate ligand at room temperature or with irradiation of UV light leads to the formation of [Re(CO)4(L)(X)]. Tetracarbonyl rhenium(I) complexes with bidentate ligands were prepared by halide abstraction reaction of [Re(CO)5(X)] by treatment with AgX (X: OTf, PF6, ClO4, BF4) and subsequent substitution reaction with the bidentate ligands. Selected examples of the synthetic routes to these tetracarbonyl rhenium(I) complexes are summarized in Scheme 17.40–43

Carbonyl and Isocyanide Complexes of Rhenium

563

Scheme 17 Substitution of a CO ligand in [Re(CO)5(X)] to give [Re(CO)4]+ fragment.40–43

Tetracarbonyl rhenium(I) complexes can also be prepared by coordinating free CO to the tricarbonyl rhenium(I) complexes. The general synthetic method is to treat the tricarbonyl rhenium(I) halide complexes with Ag salt (AgOTf, AgBF4, AgPF6) or Na salt (NaOTf, NaPF6, NaBAr4), followed by reaction with 1 atm CO. Selected synthetic routes to tetracarbonyl rhenium(I) complexes prepared by carbonylation of rhenium(I) tricarbonyl complexes are shown in Scheme 18.44–46

564

Carbonyl and Isocyanide Complexes of Rhenium

Scheme 18 Carbonylation of tricarboyl Re(I) complexes.44–46

6.07.2.2.1.4 [Re(CO)3]+ fragment Among all carbonyl rhenium complexes, the most widely studied complexes are those with the fac-[Re(CO)3]+ fragment. This can be attributed to the rich photophysics, photochemistry, and applications associated with the tricarbonyl rhenium(I) diimine complexes (See Section 6.07.4). A large number of tricarbonyl rhenium(I) complexes and their applications have been described in the previous edition of COMC.1 These complexes are typically prepared from [Re(CO)5(X)] (X ¼ Br/Cl), but [Re(CO)5(H)],4 [Re(CO)3(OH2)3]+ 47 and [Re(CO)3(Br)3]2− 48 have also been employed. Various Re(I) complexes with the general formula [Re(CO)3(L)2(L0 )]0/+, [Re(CO)3(L2)(L0 )]0/+ and [Re(CO)3(L)3]0/n+ can be obtained by simple ligand substitution reaction. The general synthetic routes for tricarbonyl rhenium(I) complexes starting from [Re(CO)5(X)] are summarized in Scheme 19. Selected tricarbonyl rhenium(I) complexes obtained from Scheme 19 are listed in Table 1.

Scheme 19 General synthetic routes for tricarbonyl Re(I) complexes.

Carbonyl and Isocyanide Complexes of Rhenium

Table 1

565

Selected complexes with fac-[Re(CO)3]+ fragment.

fac-[Re(CO)3(L)2(A)] La

A

Precursors

References

Cl

[Re(CO)5(Cl)]

49

Cl

[Re(CO)5(Cl)]

50

Cl

[Re(CO)5(Cl)]

51

fac-[Re(CO)3(L)3]+ La

Precursor

References

[Re(CO)5(O3SCF3)]

52

[Re2(CO)10]

53

fac-[Re(CO)3(L)2(L0 )]+ La

L0 a

Precursor

References

[Re(CO)5(Br)]

54

fac-[Re(CO)3(LL)(A)] LLa

A

Precursor

References

H/OTf

[Re2(CO)10]/[Re(CO)5(Cl)]

55,56

Br/H

[Re(CO)5(A)] A ¼ Br or H

57

(Continued )

566

Table 1

Carbonyl and Isocyanide Complexes of Rhenium

(Continued)

fac-[Re(CO)3(LL)(A)] LL

A

Precursor

References

Cl

[Re(CO)5(Cl)]

58

Br/Cl

[Re(CO)3(NCMe)2(X)] X ¼ Br/Cl

59

Br

[Re(CO)5(Br)]

60

Br/Cl

[Re(CO)5(X)] X ¼ Br/Cl

61

Cl

[Re(CO)5(Cl)]

62

Br/Cl

[Re(CO)5(X)] X ¼ Br/Cl

63

Br

[Re(CO)5(Br)]

64

Br

[Re(CO)5(Br)]

65

Br

[Re(CO)5(Br)]

66

Br

[Re(CO)5(Br)]

67

Carbonyl and Isocyanide Complexes of Rhenium

Table 1

567

(Continued)

fac-[Re(CO)3(LL)(A)] LL

A

Precursor

References

Cl/Br/I/NCS

[Re(CO)5(X)] X]Cl/Br

68

Br

[Re(CO)5(Br)]/[Re(CO)3(THF)2(Br)]

69

Cl/Br

[Re(CO)5(X)] X]Cl/Br

70

fac-[Re(CO)3(NN)(A)] NNa

A

Precursor

References

Cl

[Re(CO)5(Cl)]

71

Br

[Re(CO)5(Br)]

72

Cl

[Re(CO)5(Cl)]

73

Cl

[Re(CO)5(Cl)]

74

Br

[Re(CO)5(Br)]

75

(Continued )

568

Table 1

Carbonyl and Isocyanide Complexes of Rhenium

(Continued)

fac-[Re(CO)3(NN)(A)] NN

A

Precursor

References

Cl

[Re(CO)5(Cl)]

76

Cl

[Re(CO)5(Cl)]

77

Br/Cl

[Re(CO)5(X)] X ¼ Br/Cl

78

Cl

[Re(CO)5(Cl)]

79

Cl

[Re(CO)5(Cl)]

80

Cl

[Re(CO)5(Cl)]

81

Cl

[Re(CO)5(Cl)]

82

Cl

[Re(CO)5(Cl)]

83

Carbonyl and Isocyanide Complexes of Rhenium

Table 1

569

(Continued)

fac-[Re(CO)3(NN)(A)] NN

A

Precursor

References

Br

[Re(CO)5(Br)]

84

Br

[Re(CO)5(Br)]

85

Br

[Re(CO)5(Br)]

86

Cl

[Re(CO)5(Cl)]

87

Cl

[Re(CO)5(Cl)]

88

Cl

[Re(CO)5(Cl)]

89

570

Carbonyl and Isocyanide Complexes of Rhenium

fac-[Re(CO)3(LL)(L0 )]0/+ LLa

L0 a

Precursor

References

CNtBu

[Re(CO)5(Br)]

90

PPh3

[Re(CO)5(Br)]

91

[Re(CO)3(OH2)3]+

92

CNtBu, CNC6H10

[Re(CO)3(OH2)3]

93

CNC6H10

(Et4N)2[Re(CO)3(Br)3]

94

H2O

[Re(CO)5(Cl)]

95

CH3CN

[Re(CO)5(Cl)]

96

(Et4N)2[Re(CO)3(Br)3]

97

fac-[Re(CO)3(NN)(L0 )]0/+ NNa

L0 a

Precursor

References

[Re(CO)5(Cl)]

98

Carbonyl and Isocyanide Complexes of Rhenium

Table 1

571

(Continued)

fac-[Re(CO)3(NN)(L0 )]0/+ NN

L0

Precursor

References

CH3CN

[Re(CO)5(Br)]

99

[Re(CO)5(Cl)]

100

[Re(CO)3(bpy)(Cl)]

101

[Re(CO)5(Cl)]

102

[Re(CO)5(Cl)]

103

(Continued )

572

Table 1

Carbonyl and Isocyanide Complexes of Rhenium

(Continued)

fac-[Re(CO)3(NN)(L0 )]0/+ NN

L0

OPR0 3 (R0 ¼ Cy or Ph)

Precursor

References

[Re2(CO)8(THF)2]

104

[Re(CO)3(NN)(O3SCF3)] NN ¼ bpy/phen

105

[Re(CO)3(phenR2)(Cl)]

106

[Re(CO)3(phen)(Cl)]

107

fac-[Re(CO)3(LLL)]0/+ (LLL ¼ tridentate ligand) LLLa

a

Coordinated atoms are in red color.

Precursor

References

[Re2(CO)10]

108

(Et4N)2[Re(CO)3(Br)3]

109

Carbonyl and Isocyanide Complexes of Rhenium

573

Substitution of hydrido ligand in tricarbonyl hydrido rhenium(I) complexes can be facilitated by ligands with borane derivatives, as illustrated in reactions of 32 and 33 with an isocyanide ligand (CNR) to give 34–38 (Scheme 20A).110 This is due to agostic interaction between B-H and rhenium. Detailed studies revealed that the isocyano complexes 36–38 are bioactive compounds with IC50 values at a nanomolar level and good binding affinity to 5-HT1A receptors. The substitutionally labile hydrido ligand, which is facilitated by the agostic interaction between B-H and rhenium metal center, was also demonstrated in the substitution reactions of fac-[Re(CO)3{k2dH(m-H)2B(timMe)}] 39 with various isocyanide and PPh3 ligands at room temperature to give 40–42 (Scheme 20B).111

(A)

(B)

Scheme 20 Hydrido substitution reaction of (A) hydrido tricarbonyl and (B) dihydrido tricarbonyl rhenium(I) complexes with various ligands.

574

Carbonyl and Isocyanide Complexes of Rhenium

Almost all of the reported tricarbonyl rhenium(I) complexes are the facial isomer, but an example of the meridional isomer has been reported. This isomer is not prepared from the pentacarbonyl rhenium(I) or decacarbonyl rhenium(I) precursors. Instead, the rhenium(V) oxo complex is used, and the carbonyl ligand is introduced using acetic anhydride. Treatment of Re(V) oxo complex [ReO(Cl)3(PPh3)2] with 5 mol equiv. of PPh3 in refluxing acetic anhydride gives dicarbonyl Re(I) complex 43 (Scheme 21).112 The acetate ligand in 43 can be further transformed into a carbonyl ligand to afford the mer-isomer of tricarbonyl rhenium(I) complex 44 by reaction with concentrated hydrochloric acid in dichloromethane (Scheme 21).

Scheme 21 Preparation of the mer-tricarbonyl rhenium(I) complex from a rhenium(V) oxo complex.

6.07.2.2.1.5 [Re(CO)2]+ fragment To date, the carbonyl ligands of all the reported [Re(CO)2]+ complexes adopt a cis-conformation. Dicarbonyl rhenium(I) complexes can be prepared by carbonyl ligand substitution of fac-[Re(CO)3]+ complexes or reductive substitution reactions of dicarbonyl rhenium precursors with higher oxidation states such as (Et4N)[ReIII(CO)2(NCMe)2(Br)2] and (Et4N)2[ReII(CO)2(Br)4]. These dicarbonyl rhenium(II) and rhenium(III) complexes are synthesized from [Re(CO)3(Br)3]2−, which can be prepared by reduction of perrhenate with borane-THF in the presence of tetrabutylammounium bromide.113 [Re(CO)3(Br)3]2− can be oxidized by Br2 to give the dicarbonyl tetrabromo rhenium(III) complex [Re(CO)2(Br)4]−, which is subsequentially reduced by 0.5 or 1 mol equiv. of tetrakis(dimethylamino)ethylene (TDAE) in acetonitrile to give trans,cis-[Re(CO)2(NCMe)2(Br)2]− and cis-[Re(CO)2(Br)4]2− (Scheme 22).114

Scheme 22 Synthetic routes to dicarbonyl rhenium(II) and rhenium(III) precursors.

Over the past decade, methods such as photo-ligand substitution and Me3NO-mediated decarbonylation, have been developed for carbonyl ligand substitution in fac-[Re(CO)3]+ complexes (See Section 6.07.2.2.2.1). The preparation of some of the [Re(CO)2]+ complexes using these methods are described in Section 6.07.2.2.2.1. Without these methods, carbonyl ligand substitution reactions of fac-[Re(CO)3]+ complexes are challenging. Despite the substitutional inertness of carbonyl ligands in these complexes, there are a few reports on carbonyl ligand substitution. In general, the reaction can proceed under two circumstances: 1. in the presence of strong trans-influencing ligands such as isocyanides and phosphines; 2. substitution with multidentate ligands. Prolonged heating of the tricarbonyl complexes fac-[Re(CO)3(CNR)2Br] with an excess amount of isocyanide yields the dicarbonyl complexes mer-[Re(CNR)3(CO)2Br] (See Section 6.07.3.1.1). Due to the strong trans effect, only the carbonyl ligand trans to isocyanide undergoes ligand substitution.115 Similarly, reaction of fac-[Re(CO)3(PC5H4Cl-2)2(Cl)] 45 with 2-chlorophosphinine (PC5H4Cl-2) in refluxing toluene also affords ligand substitution to give cis,mer-[Re(CO)2(PC5H4Cl2)3(Cl)] 46 in 15% yield (Scheme 23).51

Scheme 23 Substitution reaction of fac-[Re(CO)3(PC5H4Cl-2)2(Cl)] with 2-chlorophosphinine.

Carbonyl and Isocyanide Complexes of Rhenium

575

Similar to Scheme 23, the carbonyl ligand trans to triphenylphosphine in the tricarbonyl complex fac-[Re(CO)3(troplone) (PPh3)] 47 can be substituted by an excess amount of triphenylphosphine (6 mol equiv.) to give the dicarbonyl complex cis,trans[Re(CO)2(PPh3)2(troplone)] 48 (Scheme 24).116

Scheme 24 Substitution reaction of fac-[Re(CO)3(troplone)(PPh3)] with triphenylphosphine.

Dicarbonyl rhenium(I) complexes can also be prepared by ligand substitution in [Re(CO)3]+ complexes with polydentate ligands. For example, the reaction of fac-[Re(CO)3(PPh3)2(Cl)] with 1,1,1-tris(diphenylphosphinomethyl)ethane (triphos) in refluxing toluene for 7 h gives the dicarbonyl complex cis-[Re(CO)2(triphos)(Cl)] 49 (Scheme 25).117 The chloride ligand in 49 can be substituted by CF3SO−3 by treatment with 1 mol equiv. of methyl trifluoromethanesulfonate (MeOSO2CF3) in dichloromethane at −20  C to give cis-[Re(CO)2(triphos)(OSO2CF3)] 50. Substitution of the CF3SO−3 can be readily performed for further functionalization as exemplified by the reactions of 50 with large excess (10 mol equiv.) of Na(NCO) or Na(OCP) in THF for 24 h to give cis-[Re(CO)2(triphos)(NCO)] 51 and cis-[Re(CO)2(triphos)(PCO)] 52, respectively.118

Scheme 25 Synthetic route to dicarbonyl rhenium(I) complexes with tridentate ligand (triphos).

Tricarbonyl rhenium(I) complexes can undergo carbonyl ligand substitution with bidentate ligands as demonstrated in the preparation of the dicarbonyl diphosphino rhenium diimine complexes. The complexes cis-[Re(CO)2(N-N)(P-P)](PF6) 55 can be prepared in high yield by either treating fac-[Re(CO)3(P-P)(Cl)] (P-P: dppm or dppv) 53 with 1.1 mol equiv. of a diimine ligand (bpy, phen or Ph2phen) or by reacting fac-[Re(CO)3(N-N)(Cl)] (N-N: bpy, phen, Ph2phen) 54 with 1.2 mol equiv. of a diphosphine ligand (P-P: dppm, dppv, diphos). These reactions are carried out in refluxing 1,2-dichlorobenzene in the presence of TlOTf to abstract the chloride (Scheme 26).119 The ease of substituting the axial ligand trans to carbonyl in the tricarbonyl rhenium(I) diimine or diphosphine complexes is well-known (Scheme 19 and Table 3). The bidentate ligand initially coordinates in a monodentate fashion, and subsequent intramolecular carbonyl ligand substitution occurs at a higher temperature of 180  C to achieve chelating bidentate coordination.

Scheme 26 Synthetic route to cis-[Re(CO)2(N-N)(P-P)]+.

576

Carbonyl and Isocyanide Complexes of Rhenium

Dicarbonyl rhenium(II) precursor (Et4N)2[Re(CO)2(Br)4] undergoes reductive ligand substitution reaction with 2 mol equiv. of tridentate 1,4,7-trithiacyclononane ligand (9S3) in CH3CN-CH2Cl2 (1:1, v/v) to give the dicarbonyl complex [Re(CO)2(9S3)(Br)] 56 (Scheme 27).137 Further functionalization through the replacement of the bromo ligand with other ligands can be anticipated. Reductive ligand substitution of the dicarbonyl rhenium(III) precursor cis,trans-(Et4N)[Re(CO)2(NCMe)2(Br)2] with a tetradentate bis(2-pyridylmethyl)glycine ligand (bpg) has also been demonstrated to give a charge-neutral complex cis-[Re(CO)2(bpg)] 57 (Scheme 28).138

Scheme 27 Reductive ligand substitution of (Et4N)2[Re(CO)2(Br)4] with 9S3.

Scheme 28 Reductive ligand substitution of (Et4N)[Re(CO)2(MeCN)2(Br)2] with bpg.

6.07.2.2.1.6 [Re(CO)]+ fragment Preparing monocarbonyl rhenium(I) complexes by carbonyl ligand substitution of [Re(CO)x]+ (x ¼ 3–6) is challenging due to the strong s-donating and p-accepting ability of carbonyl ligands. It has been shown that this strategy can be used to prepare triisocyano or diisocyano carbonyl rhenium(I) diimine complexes via Me3NO mediated carbonyl ligand substitution or photo-ligand substitution (See Sections 6.07.2.2.2.1 and 6.07.3).115 Another strategy to obtain [Re(CO)]+ complexes is by coordination of free CO ligand. The reaction between a Re(V) precursor [Re(Cl)2(Z2-NNCOPh)(PPh3)2] 58 and a large excess (10 equiv.) 1,3,5-triaza-7-phosphaadamantane (PTA) gave a pentaphosphino rhenium(I) dinitrogen complex trans-[Re(N2)(PTA)(Cl)] 59 (Scheme 29).139 The dinitrogen ligand can be substituted through photo-substitution under high CO pressure (28 atm) to give the monocarbonyl complex trans-[Re(CO)(PTA)(Cl)] 60 in high yield (85%).

Scheme 29 Preparation of monocarbonyl rhenium(I) complex by photo-substitution of a dinitrogen precursor.

6.07.2.2.2

Reactivity

6.07.2.2.2.1 Carbonyl ligand substitution Due to the strong trans-influence and competition of p-backbonding interactions between carbonyl ligands in hexa- or penta-carbonyl rhenium(I) complexes, carbonyl ligand substitution reactions in these complexes can be readily achieved.1 As discussed in previous sections, carbonyl ligands in dicarbonyl or tricarbonyl rhenium(I) complexes usually are inert towards ligand substitution. However, it has been shown that metal-carbonyl ligand dissociation can be facilitated by photoexcitation or Me3NO. These substitution methods are commonly employed in many other carbonyl transition metal complexes or clusters, but have only been explored with tricarbonyl and dicarbonyl rhenium(I) complexes more recently.1

Carbonyl and Isocyanide Complexes of Rhenium

577

Photo-induced carbonyl ligand substitution reactions of tricarbonyl complexes [Re(CO)3(bpy)(PR3)]+ were first reported in 2000.140 The carbonyl ligand trans to phosphine ligand is selectively substituted with 365-nm photoexcitation, which proceeds through a dissociative mechanism via a triplet ligand-field (3LF) excited state.140,141 Subsequently, this method was applied in other tricarbonyl rhenium(I) diimine complexes with different ancillary ligands.142 Upon irradiation of fac-[Re(I)(CO)3(bpy)(L)]+/0 with high-energy UV light with l ¼ 313 nm, an isomeric mixture of cis,cis- and cis,trans-dicarbonyl complexes, cis,cis-[Re(I)(CO)2(N-N) (L)(NCMe)]+/0 61: L ¼ Cl, 62: L ¼ pyridine, 63: L ¼ MeCN and cis,trans-[ReI(CO)2(N-N)(L)(NCMe)]+/0 64: L ¼ Cl, 65: L ¼ pyridine, 66: L ¼ MeCN, 67: L ¼ P(OEt)3 (Scheme 30) were obtained. High-energy UV light is required for the photo-ligand substitution because the photoreactions do not proceed through the lowest-lying emissive 3MLCT excited state.142

Scheme 30 Photochemical carbonyl ligand substitution reaction of fac-[Re(CO)3(bpy)(L)]+/0 with MeCN at 313-nm irradiation.

This strategy can also be applied to prepare the dicarbonyl complexes cis,trans-K[Re(CO)2(CN)2(N-N)] [N-N ¼ tBu2bpy (68); phen (69)]. Photoirradiation of tricarbonyl cyano rhenium(I) diimine complexes in the presence of 1000-fold KCN in 6:1 MeOH-water (v/v) solution at room temperature gives 68 and 69 in moderate yields (Scheme 31).143

Scheme 31 Preparation of cis,trans-dicarbonyl dicyano rhenate(I) diimine complexes.

The photochemical carbonyl substitution reaction is also viable for tricarbonyl rhenium(I) complexes with bidentate diphosphine144 and diisocyanide145 ligands (See Section 6.07.3). The carbonyl and diphosphine ligand in fac-[Re(CO)3(Ph2POCH2CH2OPPh2)(H)] 70 can be substituted upon UV irradiation at room temperature in the presence of 3 mol equiv. of PPh(OMe)2 to give the triphosphino dicarbonyl complex cis,mer-[Re(CO)2(PPh(OMe)2)3(H)] 71 in 20% yield (Scheme 32).

Scheme 32 Photoligand substitution reaction of 70 with PPh(OMe)2.

Apart from photosubstitution reactions, carbonyl ligand substitution to form tricarbonyl rhenium(I) complexes fac-[Re(CO)3]+ can be mediated by oxidative decarbonylation with Me3NO.146 Treatment of fac-[Re(CO)3(phen)(PR2R0 )]+ with Me3NO in MeCN affords the dicarbonyl rhenium(I) complexes cis,trans-[Re(CO)2(phen)(PR2R0 )(NCMe)]+ [PR2R0 ¼ PPh3 72; PPh2Me 73; and P(OEt)3 74]. The substitution is highly selective and only the most electron-deficient carbonyl ligand at the axial position trans to the phosphine ligand is substituted to give the cis,trans-isomer as the only product (Scheme 33). Subsequent thermal substitution of the MeCN ligand in 72–74 leads to the formation of various dicarbonyl rhenium(I) complexes, including dinuclear and trinuclear rhenium(I) complexes with bridging diisocyanide and cyanide ligands (Fig. 5).147,148 Subsequently, selective carbonyl ligand substitution in other tricarbonyl phosphino rhenium(I) diimine complexes have also been reported.149,150

578

Carbonyl and Isocyanide Complexes of Rhenium

Scheme 33 Selective carbonyl ligand substitution reaction of fac-[Re(I)(CO)3(N-N)(PR2R0 )]+ mediated by Me3NO.

Fig. 5 Structures of polynuclear rhenium(I) complexes with bridging diisocyanide and cyanide ligands.

In addition to selective carbonyl substitution in fac-[Re(CO)3(phen)(PR2R0 )]+, selective substitution of the most electron-deficient carbonyl ligand (trans to isocyanide) was achieved in cis,cis-[Re(CO)2(CNC6H4Cl-4)2(phen)]+ by activation with Me3NO in the presence of acetonitrile or pyridine. These reactions yielded cis,cis-[Re(CO)(py)(CNC6H4Cl-4)2(phen)]+ 75 and cis,cis-[Re(CO)(NCMe)(CNC6H4Cl-4)2(phen)]+ 76, respectively (Scheme 34).115 The acetonitrile ligand of 76 can be further substituted by other ligands such as triphenylphosphine to give trans,cis-[Re(CO)(PPh3)(CNC6H4Cl-4)2(phen)]+ 77 as illustrated in Scheme 33. Me3NO-mediated carbonyl ligand substitution reactions work well with other carbonyl rhenium complexes including [Re(CO)5(Cl)] for the coordination of the tridentate ligand, 1-methyl-2-{(o-thiomethyl)phenylazo}imidazole (NNS), to give [Re(CO)2(NNS)(Cl)] 78 in moderate yield (Scheme 35).151

Scheme 34 Synthetic routes for diisocyano carbonyl rhenium(I) diimine complexes.

Scheme 35 Coordination of NNS to [Re(CO)5(Cl)] by Me3NO-mediated carbonyl ligand substitution reaction.

Carbonyl and Isocyanide Complexes of Rhenium

579

6.07.2.2.2.2 Nucleophilic attack on carbonyl ligand 6.07.2.2.2.2.1 Conversion of carbonyl ligand to N-heterocyclic carbene (NHC) ligand Due to the strong trans effect and competitive p-backbonding interactions, the carbonyl ligands in [Re2(CO)10] and [Re(CO)5(Br)] are electron-deficient and subject to nucleophilic attack. Reactions of [Re2(CO)10] with 3 mol equiv. of RNH(CH2)2N]PPh3 (R: H, Et) at room temperature give the dimeric Re(0) carbene complexes 79 (R¼ H) and 80 (R¼ Et) (Scheme 36A).152 Oxidative addition reactions of 79 and 80 with bromine give the tetracarbonyl Re(I) NHC complexes, cis-[Re(CO)4(NHC)(Br)] 81 (R ¼ H) and 82 (R ¼ Et), respectively. It was later shown that cis-[Re(CO)4(NHC)(Br)] 83–85 can be prepared from the reactions of [Re(CO)5(Br)] with the corresponding conjugated phenyl iminophosphorane RNH(C6H4)N]PPh3 (R: H, CH2Ph or CH2C6H2(CH3)3-2,4,6) (Scheme 36B).153

(A)

(B)

Scheme 36 Formation of rhenium(I) carbene complexes from the reactions of (A) [Re2(CO)10] and (B) [Re(CO)5(Br)] with suitable nucleophiles.

6.07.2.2.2.2.2 Nucleophilic attack on carbonyl to form carbamoyl Treatment of [Re(CO)5(Cl)] with 1 mol equiv. of 7-amino-2,4-dimethyl-1,8-naphthpyridine in refluxing CH2Cl2 in the presence of 1 mol equiv. tBuOK gives the tetracarbonyl Z1-naphthpyridylcarbamoyl complex 86 (Scheme 37).154 The formation of the rhenium-amido bond was suggested to proceed via the initial coordination of the amine group followed by deprotonation to arylamide complex 87, which subsequently undergo an intramolecular migratory insertion of CO to give 86.

Scheme 37 Mechanism for the formation of the tetracarbonyl rhenium(I) Z1-naphthpyridylcarbamoyl complex 86.

Nucleophilic attack of carbonyl in [Re(CO)3(TACN)]+ 88 occurs with the amine group of TACN upon oxidation with bromine to give seven-coordinate rhenium(III) dicarbonyl complex 89 with an Z2-carbamoyl ligand (Scheme 38).155,156 Reactions of 89 with NaOH and HBr give dicarbonyl Re(I) bromide complex 90 and dinuclear rhenium(III) complex 91.

580

Carbonyl and Isocyanide Complexes of Rhenium

Scheme 38 Preparation and reactivity of the seven-coordinate dicarbonyl rhenium(III) complex 89 with Z2-carbamoyl ligand.

6.07.2.3 6.07.2.3.1

Polynuclear carbonyl rhenium(0/I) complexes Homopolynuclear carbonyl rhenium(I) complexes

Homodinuclear tricarbonyl rhenium(I) complexes with various monodentate bridging ligands such as 4,40 -bipyridine, pyrazoles,157 diacetylides, cyanide, diisocyanides, cyanometalates have been extensively reported. Apart from these bridging ligands, a new class of dinuclear tricarbonyl rhenium(I) complexes bridged by diazine ligands [Re2(m-X)2(CO)6(m-diazine)] (92–106) has received much attention as these complexes show good thermal stability and MLCT [Re(dp) ! diazine(p )] phosphorescence properties with significant improvement on the quantum efficiency (See Section 6.07.4.1).158 These complexes can be synthesized by a general route from the reaction of [Re(CO)5(X)] (X: Cl, Br or I) with 0.5 mol equiv. of diazines. The reaction is highly versatile and works with a wide variety of diazine ligands such as pyridazine, 3-methylpyridazine, 4-methylpyridazine, 3-chloro-6-methylpyridazine, 3-chloro-6-methoxypyridazine, 3,6-dichloropyridazine, phthalazine, 4-n-hexylpyridazine, 4-tertbutylpyridazine, 4,5-dimethylpyridazine, 4-methyl-5-n-propylpyridazine, 6,7-dihydro-5H-cyclopentapyridazine and 4,5-di(trimethylsilyl)pyridazine (Scheme 39).159,160 These complexes can also be prepared by reacting rhenium dimeric complex precursors [Re2 (m-X)2(CO)8] with equal molar diazine.159

Scheme 39 Synthetic routes to [Re2(m-X)2(CO)6(m-diazine)].

Carbonyl and Isocyanide Complexes of Rhenium

581

Homodinuclear tricarbonyl rhenium(I) complexes bridged by two or more bridging ligands have also been reported. For example, reactions of [Re(CO)5Cl] with S- or O-substituted porphyrin functionalized with two pyridine ligands give dinuclear tricarbonyl rhenium(I) complex bridged by two bridging porphyrin ligands (Scheme 40A).161 When the reaction of [Re(CO)5Cl] is carried out in a mixture of two different S- and O-substituted porphyrin bridging ligands in a Re:S-Por:O-Por ratio of 2:1:1, the reaction gives a mixture of dinuclear complexes 107–109. Similarly, the reaction of [Re(CO)5Br] with calixarene functionalized with two pyridine units affords dinuclear tricarbonyl rhenium(I) complex analog 110 with two of the calixarene bridging ligands (Scheme 40B).162 This calixarene-bridging dirhenium complex can serve as a chemosensor through the host-guest interaction of the calixarene moieties.162 Na[Ph2P(O)NP(O)Ph2] reacts with [Re(CO)5(Br)], [Re(CO)5(OTf )] or [Re2(CO)6(Br)2(THF)2] to give a dirhenium complex 111 with three O-donor bridging ligands (Scheme 41).163 It is worth noting that the three bridging ligands in 111 form a cavity showing high binding affinity for Na+.

Scheme 40 Reactions of [Re(CO)5(X)] with (A) porphyrins and (B) calixarene functionalized with two pyridine ligands. (B) Reproduced with permission from Menozzi, E.; Busi, M.; Massera, C.; Ugozzoli, F.; Zuccaccia, D.; Macchioni, A.; Dalcanale, E. J. Org. Chem., 2006, 71, 2617−2624.

Scheme 41 Reaction of [Re(CO)5(Br)], [Re(CO)5(OTf )] and [Re2(CO)6(Br)2(THF)2] with Na[Ph2P(O)NP(O)Ph2] for the preparation of dinuclear tricarbonyl rhenium complex.

582

Carbonyl and Isocyanide Complexes of Rhenium

In addition to the bridging monodentate ligands illustrated above, dinuclear rhenium(I) complexes with bridging bidentate or polydentate ligand have also been reported. Reactions of [Re2(CO)10] with 2,6-pyridinedimethanol or pyridine-2,6-dicarbonxylic acid in a mixture of THF-MeOH in the presence of 3 mol equiv. of Me3NO gives [Re2{m2-2,6-(OCH2)(C5H3N)(CH2OH)-k2N, O}2(CO)6] 112 and (Me3NH)2[Re2{m2-2,6-(O2C)2(C5H3N)-k3-N,O,O}2(CO)6] 113 (Scheme 42).164 Carbohydrates can also been used as m-alkoxo bridging ligands. Reaction of carbohydrates with (NEt4)2[Re(CO)3Br3] in the presence of 1,8-diazabicyclo[5.4.0] undec-7-ene (DBU) base under different conditions gives m-alkoxo bridged dinuclear, trinuclear or hexanuclear tricarbonyl rhenium(I) complexes. The structures of these complexes have been confirmed by X-ray crystallography (Fig. 6).165

Scheme 42 Reactions of [Re2(CO)10] with 2,6-pyridinedimethanol or pyridine-2,6-dicarbonxylic acid ligand.

Fig. 6 X-ray crystal structures of (A) L-threitol m-alkoxo bridged dinuclear, (B) Me-b-D-ribopyranoside m3-alkoxo bridged trinuclear, and (C) D-arabitol m6-alkoxo bridged hexanuclear tricarbonyl rhenium(I) complexes. Reproduced with permission from Hinrichs, M.; Hofbaurer, F. R.; Klüfers, P. Chem. A Eur. J. 2006, 12, 4675–4783. Copyright 2006, Wiley-VCH.

Over the past decade, there have been a large number of polynuclear tricarbonyl rhenium(I) complexes reported in the literature. As illustrated above, they are mainly prepared from [Re2(CO)10], [Re(CO)5(X)], (NEt4)2[Re(CO)3(Br)3], or fac-[Re(CO)3(diimine) (X/S)] (X: Cl, Br, OTf; S: coordinating solvents). Wide varieties of bridging ligands including N-donor (such as pyridine, diimine, pyrazine and nitrile), O-donor (such as carboxylate, hydroxyl, and phosphine oxide), S-donor (such as thiol and thiophenol) and C-donor (such as isocyanide, carbene, and ethynyl) ligands have been explored. Complexes of these ligands are obtained by mixing the precursors with various ratios of the bridging ligand to afford polynuclear carbonyl rhenium complexes with wide-ranging ligand coordination modes (Table 2). The same synthetic strategy can be applied to develop polynuclear carbonyl rhenium complexes with two different types of bridging ligands (Table 3 and Fig. 7).

Carbonyl and Isocyanide Complexes of Rhenium

Table 2

583

Selected examples of carbonyl rhenium polynuclear complexes with different coordination modes bridged by a single type of bridging ligands.

Bridging ligand

− S-R (R: C6H5, C6H4CH3, CH2C6H5)

[Re]

Re precursor

References

[Re(CO)3(acac)]+

[Re(CO)3(acac)(OH2)]

166

[Re(CO)3(py-R0 )]+ (R0 : H, CH3, C6H5) [Re(CO)3(Br)]

[Re2(CO)10]

167

[Re(CO)3(acac)(OH2)]

166

[Re(CO)3]+

[Re(CO)5(Br)]

168

[Re(CO)3]+

(NEt4)2[Re(CO)3(Br)3]

169

[Re(CO)3]+

(NEt4)2[Re(CO)3(Br)3]

170

[Re(CO)3]+

[Re(CO)5(Br)]

171

[Re(CO)3]+

[Re2(CO)10]

172

[Re(CO)3]+

[Re(CO)5(Cl)]

173

(Continued )

584

Table 2

Carbonyl and Isocyanide Complexes of Rhenium

(Continued)

Bridging ligand

[Re]

Re precursor

References

[Re(CO)3(Cl)]

[Re(CO)5(Cl)]

174

[Re(CO)3(Cl)]

[Re(CO)5(Cl)]

175

[Re(CO)3(X)] X ¼ Cl, Br

[Re(CO)5(X)] (X: Cl/Br)

176

[Re(CO)3]+

[Re(CO)5(Br)]

177

[Re(CO)5]+

[Re(CO)5(OSO2CF3)]

178

Table 2

(Continued)

Bridging ligand

[Re]

Re precursor

References

[Re(CO)3(bpy)]+

[Re(CO)3(bpy)(NCMe)]+

179

[Re(CO)3(Br)]

[Re(CO)5(Br)]

180

[Re(CO)3]+

(NEt4)2[Re(CO)3(Br)3]

170

[Re(CO)3(Cl)]

[Re(CO)5(Cl)]

181

(Continued )

586

Table 2

Carbonyl and Isocyanide Complexes of Rhenium

(Continued)

Bridging ligand

[Re]

Re precursor

References

[Re(CO)3(bpy)]+

[Re(CO)3(bpy)(DMSO)]+

182

[Re(CO)3(bpy)]+

[Re(CO)3(bpy)(Cl)]

183

[Re2(CO)10]

184

[Re(CO)4]+

[Re2(CO)10]

185

[Re(CO)3]+

[Re2(CO)10]

186

[Re(CO)3(py-R-4)]+ R ¼ CH3CO, Ph, H

[Re2(CO)10]

187

Carbonyl and Isocyanide Complexes of Rhenium

587

Table 3 Selected examples of carbonyl rhenium polynuclear complexes with different coordination modes prepared by the reactions of [Re2(CO)10] with two different types of bridging ligands. References

120

121,122

123

124

125

(Continued )

588

Table 3

Carbonyl and Isocyanide Complexes of Rhenium

(Continued) 126,127

128 129

130

131,132

133

134

135

Carbonyl and Isocyanide Complexes of Rhenium

Table 3

589

(Continued) 136

123

123

Fig. 7 Structures of selected examples of hexanuclear tricarbonyl rhenium(I) complexes bridged by the combination of two different types of bridging ligands.188–190 [(A) Two tridentate and three tetradentate ligands, (B) one hexadentate and six bidentate ligands and (C) two tridentate and six bidentate ligands]. (A) Reproduced with permission from Wu, J.-Y.; Chang, C.-H.; Thanasekaran, P.; Tsai, C.-C.; Tseng, T.-W.; Lee, G.-H.; Peng, S.-M.; Lu, K.-L. Dalton Trans. 2008, 6110–6112. Copyright 2008, Royal Society of Chemistry, (B) reproduced with permission from Shankar, B.; Elumalai, P.; Sathiyashivan, S. D.; Sathiyendiran, M. Inorg. Chem. 2014, 53, 10018–10020. Copyright 2014, American Chemical Society), (C) (reproduced with permission from Gupta, D.; Rajakannu, P.; Shankar, B.; Hussain, F.; Sathiyendiran, M. J. Chem. Sci. 2014, 126, 1501–1506. Copyright 2014, Royal Society of Chemistry.

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In the design of polynuclear rhenium complexes, the symmetry of the ligands in the assembled [Re] subunits should be considered. For example, when ligands in the [Re] subunits are not the same, the polynuclear complexes can have different isomeric forms that arise from the relative orientation of differing ligands. This is illustrated in the preparation of dinuclear and trinuclear rhenium(I) complexes containing the bridging ligand 2,3,8,9,14,15-hexamethyl-5,6,11,12,17,18-hexaazatrinapthalene (HATN-Me6; Scheme 43). The reaction of [Re(CO)5(Cl)] and 1 mol equiv. of HATN-Me6 gives mononuclear rhenium tricarbonyl complex 114. However, the relative orientation of the chloride ligands in different [Re] units in dinuclear and trinuclear complexes can give rise to syn and anti-isomers.191 The reaction of [Re(CO)5(Cl)] and 0.5 mol equiv. HATN-Me6 gives a racemic mixture of anti-

Scheme 43 Isomeric forms of dinuclear or trinuclear carbonyl rhenium(I) complexes bridged by HATN-Me6 ligand.

Carbonyl and Isocyanide Complexes of Rhenium

591

{(m-HATN-Me6)[Re(CO)3(Cl)]2)} 115 and syn-{(m-HATN-Me6)[Re(CO)3(Cl)]2)} 116. The isomers for the 3:1 reaction depend on the reaction conditions. The reaction in CHCl3 affords only anti-{(m-HATN-Me6)[Re(CO)3(Cl)]3} 117. Similarly, the same reaction in toluene at room temperature affords a mixture of anti isomer 117 (>95%) with a trace amount of the syn isomer 118, whereas the syn isomer 118 (>95%) is formed as the major product when the reaction is conducted in refluxing toluene. The syn isomer 118 can be transformed into the anti isomer 117 by dissolving in CHCl3. With bridging dicyclopentadienide ligands, different isomeric forms of dinuclear rhenium(I) complexes can be isolated due to relative orientation of the coordinating cyclopentadienide units.192 The reactions of [Re2(CO)10] with different bridging dicyclopentadienide ligands in refluxing mesitylene give the trans and cis isomers of dirhenium complexes trans-[[(
5-C5H3)2(EMe2) (SiMe2)][Re(CO)3]2] (119: E ¼ Si, 120: E ¼ C) and cis-[[(
5-C5H3)2(EMe2)(SiMe2)][Re(CO)3]2] (121: E ¼ Si, 122: E ¼ C) together with the unexpected desilylated dirhenium complexes [[(
5-C5H3)2(EMe2)][Re(CO)3]2] 123 (E ¼ Si) and 124 (E ¼ C) (Scheme 44).

Scheme 44 Synthetic of dinuclear tricarbonyl rhenium(I) complexes with dicyclopentadienides bridging ligands.

Further functionalization of the polynuclear rhenium(I) complex can also lead to various isomeric forms. For example, carbonyl substitution in dinuclear rhenium(I) complex 125 with 2 mol equiv. of substituted pyridine ligands gives a mixture of cis and trans isomer in the ratio of 1:1 to 2:3 (Scheme 45).193

Scheme 45 Carbonyl ligand substitution of symmetrical dinuclear rhenium(I) complex with substituted pyridine ligands.

In addition to mixing synthetic precursors with suitable bridging ligands in one pot reactions, polynuclear carbonyl rhenium(I) complexes can also be prepared in a stepwise manner, as summarized in Scheme 46.150,194 Ishitani and co-workers synthesized a series of linear- and ring-shaped polynuclear tricarbonyl rhenium(I) complexes by stepwise addition of different bridging diphosphine ligands such as 1,4-bis(diphenylphosphino)benzene, 1,2-bis(diphenylphosphino)ethane, 1,3-bis(diphenylphosphino)propane, 1,4-bis(diphenylphosphino)butane, 1,5-bis(diphenylphosphino)pentane and 1,6-bis(diphenylphosphino)hexane) to yield the dinuclear rhenium(I) complexes fac, fac-{[Re(CO)3(N-N)][P(Ph2)(CH2)x(Ph2)P][Re(CO)3(N-N)]}2+ 126. A carbonyl ligand in 126 can be substituted with acetonitrile via photolysis or oxidative decarbonylation with Me3NO to afford the dinuclear 127, which is a precursor for the synthesis of complexes with higher-nuclearity. Reactions of 127 with mononuclear rhenium(I) complexes bearing a monocoordinated diphosphine ligand or a bridging diphosphine ligand give the trinuclear 128 and tetranuclear 129 rhenium complexes, respectively. Applying a similar synthetic strategy with trinuclear 128 yields a series of hexanuclear rhenium complexes 130. Carbonyl ligand substitution with acetonitrile by photochemical or oxidative decarbonylation with Me3NO can also be used with polynuclear 131 to give the diacetonitrile complexes 132. Reacting 132 with bridging phosphine ligands yielded a series of cyclic polynuclear rhenium complexes 133. Many of these polynuclear rhenium complexes show interesting photophysical properties, and some are good photosensitizers used in photocatalytic systems (See Section 6.07.4).

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Scheme 46 Preparation of linear- and ring-shaped polynuclear tricarbonyl rhenium(I) complexes with a stepwise strategy.

Stepwise synthesis has also been used to prepare hexanuclear tricarbonyl rhenium(I) complexes.195 The trinuclear complex {[Re(CO)4]3(C3N3S3)} 134 (C3N3S3: 1,3,5-triazine-2,4,6-trithiolate) can be prepared by heating a benzene solution of [Re2(CO)10] and 1,3,5-triazine-2,4,6-trithiol with a molar ratio of 3:2 at 160  C in a sealed Teflon reactor for 48 h. Carbonyl ligand substitution reactions of [{Re(CO)4}3(C3N3S3)] 134 and various linear dipyridyl bridging ligands, such as pyrazine (pz), 4,40 -bipyridine (bpy), 1,2-bis(4-pyridyl)ethylene (dpe), bis(4-pyridyl)acetylene (dpa) or 1,4-bis(pyridyl-4-ylethynyl)benzene (bpb), carried out at 80  C in a sealed Teflon reactor for 48 h gives a series of trigonal prism-shaped hexanuclear tricarbonyl rhenium(I) complexes 135–139 (Scheme 47).195

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Scheme 47 Preparation of hexanuclear tricarbonyl rhenium(I) complexes from trinuclear rhenium(I) complexes.

6.07.2.3.2

Polymers functionalized with tricarbonyl Re(I) diimine complexes

Two strategies are used to develop polymers with transition metal complexes. The first and most commonly used method is coordination of the transition metal unit into a copolymer containing ligand moieties. To develop photofunctional polymers, copolymers 140–142 with fluorene and bpy subunits were prepared by palladium-catalyzed cross-coupling reactions of 5,50 dibromo-bipyridine with various mole equiv. of 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaboralane-2-yl)-9,9-dihexylfluorene. These bpy-containing copolymers were then reacted with [Re(CO)5(Cl)] to obtain polymers 143–145 with tricarbonyl rhenium(I) units (Scheme 48).196 Rhenium coordination in these polymers was characterized by FT-IR and 1H NMR spectroscopy. These polymers not only show phosphorescence but also exhibit photovoltaic responses.

Scheme 48 Preparation of tricarbonyl rhenium(I) bipyridine-containing polymers.

Polymerization of tricarbonyl rhenium(I) diimine complexes with polymerizable functional groups on their ligands represents another method for preparing tricarbonyl rhenium(I) diimine-containing polymers. This strategy is demonstrated in the preparation of Re(I) metallopolymers 147–149 from the polymerization of 146 by Yamamoto and Heck coupling reactions with divinylbenzene (DVB), as well as [Re(CO)3]-mediated Schiff base formation/condensation reaction (Scheme 49).197 These polymers show interesting spectroscopic and electrochemical properties.

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Carbonyl and Isocyanide Complexes of Rhenium

Scheme 49 Polymerization of tricarbonyl rhenium(I) diimine complexes with polymerizable functional groups.

6.07.2.3.3

Heteropolynuclear supramolecular systems with tricarbonyl rhenium units

Due to the high versatility of the coordination reactions of [Re(CO)5(Cl)] with pyridyl or polypyridyl ligands and the rich photophysical and photochemical properties of the resulting tricarbonyl rhenium(I) complexes, many supramolecular systems are designed and prepared through the reaction of [Re(CO)5(Cl)] with metalloligands (metal complexes with a bridging ligand containing an open coordination site). For example, [Re-Hg] supramolecular complexes, fac-[Re(CO)3[(NC5H4C^C)Hg (CH3)]2(Cl)] 150 and fac-[Re(CO)3[(NC5H4C^C)Hg(C^CC5H4N)]2(Cl)] 151 (Fig. 8), were synthesized by the reactions of [Re(CO)5(Cl)] with 2 mol equiv. of [Hg(CH3)(C^CC5H4N)] or [Hg(C^CC5H4N)2] as metalloligand.198

Fig. 8 Structures of the [Re-Hg] supramolecular complexes.

Similarly, the reaction of [Re(CO)5(Cl)] with bipyridine-type metalloligand {[Ru(bpy)2(152)]2+} gives a [Re-Ru] supramolecular system [Re(CO)3(Cl)(152)Ru(bpy)2](PF6)2 153 (Fig. 9).199 A detailed photophysical study of the [Re-Ru] complex 153 revealed a fast energy transfer process with kET: ca. 1.9  108 s−1 from [Re] to [Ru] moiety at room temperature. Even for the terpyridine-type ruthenium-based metalloligand 154, the terpyridine ligand would only coordinate as a bidentate chelate to form another [Re-Ru] system of [Re(CO)3(Cl)(154)Ru(bpy)2](ClO4)2 155 in the reaction of [Re(CO)5(Cl)] (Scheme 50).200 This supramolecular [Re-Ru] system can be used as a selective optical molecular sensor for H2PO−4 and cell imaging reagent without causing cell death.

Carbonyl and Isocyanide Complexes of Rhenium

595

Fig. 9 Structures of bridging ligand 152 and the [Re-Ru] system bridged by 153.

Scheme 50 Synthetic route to a [Re-Ru] system 155.

A [Re-Zn-Ru] supramolecular complex was synthesized by stepwise coordination of the porphyrin-bridged bis(bipyridine) ligand 156 with bis(bipyridyl) ruthenium to give 157. Coordination of 157 to tricarbonyl rhenium gave the dinuclear [Re-Por-Ru] complex 158 that was converted to the [Re-Zn-Ru] supramolecular complex 159 upon treatment with zinc acetate (Scheme 51).201

Scheme 51 Stepwise coordination reactions to [Re-Zn-Ru] supramolecular complex 159.

A rhodium(II) amidinate dimer with four uncoordinated pyridine units can serve as a metalloligand by coordinating 1–4 tricarbonyl rhenium(I) bipyridyl units to give [Rh2-Re4− n] (n ¼ 0–3: 160–163) depending on the amount of the rhenium precursor complex [Re(CO)3(bpy)(NCMe)](PF6) used in the reaction (Fig. 10).202 Apart from the examples discussed in this section, many other supramolecular systems with tricarbonyl rhenium(I) diimine units have been prepared using a similar synthetic approach and used as photocatalysts for carbon dioxide reduction (See Section 6.07.4.2).

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Carbonyl and Isocyanide Complexes of Rhenium

Fig. 10 Structures of [Rh2-Re(4− n)] supramolecular systems.

6.07.2.3.4

Heterometallic carbonyl rhenium clusters with Re-metal interactions/bonds

[Re2(CO)8{m-
2–1,3-C(H)C(H)Bun}(m-H)] 164, which can be synthesized by photolysis of [Re2(CO)10] in the presence of 1-hexene,8 is an excellent synthetic precursor for many different types of heterometallic rhenium carbonyl clusters.203,204 164 reacts with 5 mol equiv. of Ph3SnH in hexane to give a [Re2-Sn2] cluster 165. By reacting 165 with [Pd(PtBu3)2], a [Re2-Sn2-Pd2] cluster 166 can be obtained (Scheme 52).203

Scheme 52 Preparation of heterometallic rhenium carbonyl clusters from 164.

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597

Apart from the dirhenium-ditin clusters, 164 reacts with other phenyl metal complexes. Reaction with BiPh3 gives a mixture of [Ren-Bin] clusters [Re2(CO)8(m-BiPh2)2] 167 and [Re(CO)4(m-BiPh2)]3 168.204 [PhAu(PPh3)] reacts to give [Re2-Au] cluster [Re2(CO)8[m-Au(PPh3)](m-Ph)] 169, and the [Au-PPh3] can be replaced by HgI2 to give [Hg2-Re4] cluster 170.205 [PhAu(NHC)] reacts to give a mixture of [Re2-Au] clusters [Re2(CO)8[m-Au(NHC)](m-Ph)] 171 and [Re2(CO)8[m-Au(NHC)]2] 172206 (Scheme 52). Many of these clusters can be further transformed into other clusters. For example, the phenyl ligand in 171 reacts with hydrogen to give hydrido cluster 173. It can further be converted into alkenyl cluster by treatment with ethyne. Moreover, heating the [Re2-Bi2] cluster [Re2(CO)8(m-BiPh2)2] 168 produces an isomeric mixture of trans- 174 and cis-[Re4(CO)16(m-BiPh2)2-(m4-BiPhBiPh)] 175.204 Reaction of the dirhenium complex [Re2(CO)8(NCMe)2] with [(PPh3)Au(C≣CC ≣CFc)] in toluene at 92  C gives a mixture of two [Re-Au] clusters, [Re2(AuPPh3)(m-C4Fc)(CO)8] 176 and [Re4(AuPPh3)(m4-C2)(m3-C2Fc)(NCMe)(CO)13] 177 with bridging alkynyl ligand (Scheme 53).207

Scheme 53 Preparation of [Re-Au] clusters, 176 and 177, from [Re2(CO)8(NCMe)2].

Heterometallic rhenium carbonyl clusters can also be synthesized from the trirhenium(I) carbonyl cluster [Re3(CO)12(m-H)3] 178. For example, reaction of 178 with [Pt(PtBu3)2] in refluxing hexane give a series of [Re2-Ptn] carbonyl clusters, [(PtBu3-Pt) (m-H)2Re2(CO)9] 179 and [(tBu3P-Pt)2(m-H)2Re2(CO)7] 180 (Scheme 54).208 The latter cluster 180 can also be obtained by the reaction of 179 with [Pt(PtBu3)2]. Interestingly, 180 can react with hydrogen at room temperature to give [Pt2Re2(CO)7(PtBu3)2(m-H)4] 181 with additional bridging hydrido ligands. The bridging hydrido ligands can be removed by heating or light irradiation under a nitrogen atmosphere to regenerate the cluster 180.

Scheme 54 [Re2-Ptn] carbonyl clusters prepared from 178 and their reactivity towards hydrogen.

6.07.2.4

Re(II) and Re(III) carbonyl complexes

As similarly discussed in the previous edition of COMC,1 rhenium(II) and rhenium(II) carbonyl complexes are less common compared to those with rhenium(0) and rhenium(I). Only a few rhenium(II) and rhenium(II) carbonyl complexes have reported in recent years since the last edition.

6.07.2.4.1

Re(II) carbonyl complexes

6.07.2.4.1.1 Oxidation of Re(0) and Re(I) carbonyl complexes Similar to Re(I) complexes, Re(II) carbonyl complexes can be prepared by oxidative substitution of Re(0) carbonyl complexes with suitable ligands. For example, [Re2(CO)10] undergoes cyclometalation with azoaromatic ligands such as (2-phenylazo)pyridine and (4-chloro-2-phenylazo)pyridine. In these reactions, the azoaromatic ligand is reduced, and the electron-rich Re(0) metal center is oxidized to give dicarbonyl Re(II) complex cis-[ReII(CO)2(N-C
)2] 182 in high yield (Scheme 55).209 This Re(II) diradical complexes can be oxidized by iodine to give the Re(I) complex [Re(CO)2(N-C)2]I3 183 with neutral cyclometalated N-C ligands.

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Carbonyl and Isocyanide Complexes of Rhenium

Scheme 55 Preparation and reactivity of dicarbonyl Re(II) diradical complexes.

A similar reaction of substituted o-iminobenzoquinone with [Re2(CO)10] under photoexcitation also affords a thermally and photochemically stable rhenium(II) dicarbonyl complex cis-[Re(ISQ-iPr)2(CO)2] 184 with an o-iminobenzosemiquinonate (ISQ) anionic-radical ligand (Scheme 56).210 Magnetic susceptibility experiment confirmed the spin angular momentum of S ¼ 1/2 in the ground state. EPR studies confirmed the radical localization on the ISQ ligands and antiferromagnetic coupling between the unpaired electron of the rhenium(II) metal center and one of the ISQ radicals.

Scheme 56 Preparation of dicarbonyl Re(II) complexes with ISQ anionic-radical ligands.

Re(II) dicarbonyl complexes can also be prepared by oxidation of Re(I) complexes with a suitable oxidant. For example, the rhenium(I) dicarbonyl complex 185 with a PNN pincer ligand can be oxidized to form the stable rhenium(II) dicarbonyl complex 186 with [(4-BrC6H4)3N][SbCl6].211 Further oxidation of 186 with PhIO gives Re(III) complex 187 (Scheme 57).

Scheme 57 Oxidation of dicarbonyl rhenium(I) complex 184 with PNN pincer ligand.

The rhenium(II) complex cis-[Re(CO)2(Br)4]2−, which is prepared according to Scheme 22,114 is another good synthetic precursor for rhenium(II) complexes. The reactions of cis-[Re(CO)2(Br)4]2− with 2 mol equiv. of monodentate N-donor ligands (L) such as pyridine, 4-picoline, imidazole, N-methylimidazole and benzimidazole or 1 mol equiv. of bidentate N-donor ligands (N-N) such as 2,20 -bipyidine, 4,40 -dimethyl-2,20 -bipyridine, 1,10-phenanthroline, 4,7-dimethyl-1,10-phenanthroline and 1,10phenanthroline-5,6-dione, 2,20 -dipyridylamine give rhenium(II) dicarbonyl complexes of cis,trans-[Re(CO)2(Br)2(L)2] or cis,trans[Re(CO)2(Br)2(N-N)], respectively (Scheme 58).212,213 It is worth mentioning that only rhenium(I) complexes of [Re(CO)2(TACN) (Br)] and [Re(CO)2(9S3)(Br)] were obtained in the reactions of [Re(CO)2(Br)4]2− with the tridentate ligands TACN (1,4,7-triazacyclononane) or 9S3 (1,4,7-trithiacyclononane).213

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599

Scheme 58 Ligand substitution reaction for the preparation of dicarbonyl dibromo rhenium(II) complexes.

6.07.2.4.2

Re(III) carbonyl complexes

The synthesis of a rhenium(III) carbonyl complex was reported by Ison et al.214 Reaction of the oxorhenium(V) complex [(DAAm)Re(O)(CH3)] (DAAm ¼ N,N-bis(2-arylaminoethyl)methylamine; aryl ¼ C6F5) 188 with carbon monoxide at high-pressure (60 psi) affords the rhenium(III) carbonyl acetate complex [(DAAm)Re(CO)(OAc)] 190. The reaction proceeds via the oxorhenium(V) acyl complex intermediate {(DAAm)Re(O)[C(O)CH3]} 189 (Scheme 59). These complexes can be used as catalysts for the hydrosilylation of aldehydes. Mechanistic study of the catalysis revealed that a carbonyl dirhenium(II) complex [(DAAm)(CO)Re ≣Re(CO)(DAAm)] 191 is formed as a side product.

Scheme 59 Preparation of carbonyl rhenium(III) complex from the reaction of oxorhenium(V) complexes with carbon monoxide at 60 psi.

6.07.3

Rhenium isocyanide complexes

Isocyanides are isoelectronic with carbonyl ligands and thus isocyano analogs of rhenium carbonyl complexes can be synthesized. Unlike the carbonyl ligand, which cannot be functionalized, the electronic, steric and physical properties of isocyanide ligands can be readily modified or fine-tuned by changing the substituent on the N atom. Over the past decade, reports on isocyano rhenium(I) complexes have increased considerably and new synthetic strategies have been developed.

6.07.3.1 6.07.3.1.1

Re(I) isocyanide Mono-, di-, and tri-isocyano Re(I) complexes {[Re(CNR)n]+ (n ¼ 1–3)}

Ligand substitution reactions with rhenium carbonyl complexes are the most commonly used and effective methodologies for the preparation of mono-, di- and tri-isocyano rhenium(I) complexes with {[Re(CNR)n]+ (n ¼ 1–3)} moieties. Rhenium(I) carbonyl solvato complexes with a substitutional labile ligand such as [Re(CO)3(NHC)2(NCMe)], [Re(CO)3(diimine)(NCMe)]+, [Re(CO)2(diimine)(PPh3)(NCMe)]+ and [Re(CO)3(N-O)(OH2)]+ have been used to synthesize monoisocyano Re(I) complexes. These complexes can be prepared by halide abstraction of the bromo complexes [Re(CO)3(L-L) (Br)] or [Re(CO)3(L)2(Br)] with silver salt (Scheme 19) or Me3NO mediated carbonyl substitution using [Re(CO)3(diimine) (PPh3)]+ (See Section 6.07.2.2.2.1). Reaction of these rhenium(I) solvato complexes with isocyanide ligands under ambient

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Carbonyl and Isocyanide Complexes of Rhenium

condition or reflux give the target rhenium(I) isocyano complexes in high yield. Selected examples include [Re(CO)3(NHC)2(CNR)] 192,215 [Re(CO)3(phen)(CNR)] 193, [Re(CO)2(phen)(PPh3)(CNR)] 194,146,147 and [Re(CO)3(N-O)(CNR)] 195216 (Scheme 60). With bridging isocyanide ligands, homodinuclear Re(I) complexes such as [(phen)(PPh3)(CO)2Re(CNRNC)Re(CO)2(PPh3) (phen)]2+ 196 and [(phen)(CO)3Re(CNRNC)Re(CO)3(phen)]2+ 197 (R ¼ 1,4-phenyl, 4,40 -biphenyl) can also be synthesized.147

Scheme 60 Coordination of isocyanide ligand to rhenium(I) solvato complexes by ligand substitution reactions.

Apart from ligand substitution reactions, coordination of isocyanide ligand to coordinatively unsaturated rhenium(I) complexes such as Na[Re(BDI)(
5-Cp)] 198 gives Na[Re(CNR)(BDI)(
5-Cp)] (BDI ¼ N,N0 -bis(2,6-diisopropylphenyl)-2,4-dimethyl-b-diketiminate) 199 (Scheme 61).217 The X-ray crystal structure of 199 reveals a side-on interaction between sodium counter cation with the

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Scheme 61 Coordination of isocyanide ligand to coordination unsaturated rhenium(I) complex.

C^N of the coordinated isocyanide. The sodium interaction is confirmed by the significant increase of IR stretching frequencies of the isocyano C^N bond upon isolating the sodium cation with benzo-12-crown-4. The discovery of the side-on interactions between the counter cation and the coordinated isocyanide provides important insights into the mechanism for the reversible binding and activation of dinitrogen by Na[Re(BDI)(
5-Cp)]. Over the past few years, many isocyanoborato rhenium(I) complexes have been reported.218–220 The strong p-accepting properties of these isocyanoborate ligands have been introduced into tricarbonyl rhenium diimine complexes to develop charge-neutral phosphorescent rhenium emitters with high emission energy and luminescent quantum yields.218 Two synthetic methodologies have been used in the preparation of isocyanoborato rhenium(I) complexes [Re(CO)3(N-N)(CNBR3)] (N-N ¼ bpy, 4,40 -Me2bpy, phen, 2,9-Me2phen, 4,7-Me2phen, 3,4,7,8-Me4phen) 200. Because the isocyanoborate can be considered a Lewis acid-base adduct, the most straight-forward method is to react the rhenium(I) isocyano complexes with Lewis acidic boranes such as BPh3, B(C6F5)3, and BCl3 (Scheme 62). This method has also been used to convert pentaisocyano rhenium(I) cyanide to hexaisocyano rhenium(I) complexes with one isocyanoborate ligand.220 The use of tetraphenylborate salts, such as NaBPh4, KB(C6H4Cl)4, and NaB[C6H3(CF3)2]4 as the source of borane is another effective method. Reactions of the rhenium(I) cyano complexes with tetraphenylborate salts in methanol under acidic conditions also afford the rhenium(I) isocyanoborato complexes. The use of the tetraphenylborate salts is preferable because they are highly stable and not sensitive to moisture and water. This method is also reported in the preparation of isocyanoborate ligands of other transition metal complexes.221,222

Scheme 62 Synthetic methodologies to isocyanoborato rhenium(I) complexes.

With the same strategy, reaction of the dicyano rhenate(I) complex cis,trans-K[Re(CO)2(CN)2(phen)] 69 with triarylboranes yields the bis(isocyanoborato) rhenate(I) complexes cis,trans-K[Re(CO)2(CNBR3)2(phen)] (201: R ¼ C6F5; 202: R ¼ Ph) (Scheme 63).223 The isocyanoborato ligand [CNB(C6F5)3] in 201 is very stable and showed no reactivity towards basic or nucleophilic anions, whereas the triphenylborane of [CNBPh3] in 202 was abstracted by free cyanide anion to generate 69. This is due to the stronger NdB bond in CNB(C6F5)3 compared to CNBPh3 as reflected from the shorter NdB bond in the X-ray crystal structures.219 As the reaction is highly selective towards cyanide anion and there is a significant difference in the emission properties of 202 and 69, 202 can serve as a luminescent cyanide sensor.

Scheme 63 Synthetic route to bis(isocyanoborato) rhenate(I) complexes and reactivity towards cyanide anions.

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Carbonyl and Isocyanide Complexes of Rhenium

Different precursors are used in the ligand substitution reactions for the syntheses of di- and tri-isocyano rhenium(I) carbonyl complexes. For pentacarbonyl rhenium halide, two of the carbonyl ligands are well-known to undergo ligand substitution reactions. By heating [Re(CO)5(Br)] with 2.05 mol equiv. of various isocyanides with substituents of wide-ranging electronic and steric nature (CNR: R ¼ tert-butyl, 4-chloro-, 4-n-butylphenyl-, 4-(dimethylamino)phenyl-, 4-biphenyl-, 2,6-dimethyl-, 2,4,6-trichlorophenyl- or 4-bromo-2,6-dimethyl-) in THF-benzene solvent mixture (1:1 v/v) gives the fac-tricarbonyl diisocyano rhenium(I) complexes fac[Re(CO)3(CNR)2(Br)] 203 in quantitative yield (Scheme 64).145 Through photoligand substitution reactions, the carbonyl and bromo ligand of 203 can be substituted by diimine ligand to give the dicarbonyl diisocyano rhenium(I) diimine complexes cis,cis[Re(CO)2(CNR)(N-N)]+ 204. With [Re(CO)4(Br)(LP)] (LP ¼ triethylphosphite or diethyl phenylphosphonite) as precursors, a series of diisocyano dicarbonyl Re(I) complexes, cis,cis-[Re(CNR)2(CO)2(Br)(LP)] (R ¼ tert-butyl-, 4-methylphenyl- or 4-methoxyphenyl-) 205 can be prepared by reacting the [Re(CO)4(Br)(LP)] with 4 mol equiv. of isocyanide in refluxing toluene solution.224 No further substitution was noted, even with a larger amount of isocyanide or longer reaction time.

Scheme 64 Synthetic routes to diisocyano rhenium(I) complexes.

The coordinatively unsaturated PNP-pincer rhenium(III) azavinylidene complex 206 (PNP ¼ N(CH2CH2PtBu2)2) undergoes reductive coordination when treated with 2 mol equiv. of tert-butyl isocyanide in the presence of potassium bis(trimethylsilyl) amide (KN(SiMe3)2) to give five-coordinate [Re(PNP)(CN-tBu)2] 207 (Scheme 65).225 Significant bending of the C^N-But bond angle in the X-ray crystal structure and the low IR stretching frequency of v(C^N) at 1760 cm−1 suggested strong p backbonding from the [ReI(PNP)] moiety.

Scheme 65 The reaction of coordination of an unsaturated rhenium(III) complex 206 with tert-butyl isocyanide.

Triisocyano rhenium(I) complexes can also be prepared through the substitution of [Re(CO)5(Br)] with excess isocyanide ligand under prolonged heating at high temperatures.115 This reaction leads to the formation of the meridional isomer cis,mer-[Re(CO)2(CNR)3(Br)] 208. The facial isomer fac-[Re(CO)3(CNR)3]+ 209 can be prepared by selective substitution of the bromo ligand in [Re(CO)3(CNR)2Br] 203 using silver triflate to give [Re(CO)3(CNR)2(CF3SO3)]+ before subsequent reaction with isocyanide ligands.115 Similar to 203, diimine ligands can be introduced into the triisocyano carbonyl rhenium(I) complexes to give mer-[Re(CO)(CNR)3(N-N)]+ 210 and fac-[Re(CO)(CNR)3(N-N)]+ 211 through Me3NO mediated carbonyl substitution or photoligand substitution, respectively (Scheme 66).115

Carbonyl and Isocyanide Complexes of Rhenium

603

Scheme 66 Preparation of fac- and mer-triisocyano rhenium(I) complexes and their substitution reaction with diimine ligands.

The facial isomer of the triisocyano tricarbonyl rhenium(I) complexes fac-[Re(CO)3(CNR)3]+ 209 can be prepared by the reaction of fac-[Re(CO)3(OH2)3]+, generated by refluxing [Re(CO)5Br] in water, with isocyanide ligand (CNR). With this synthetic methodology, water-soluble isocyanide ligands with various bioactive moieties, such as folate226 and Palbociclib227 (a drug for ER+/ HER2− advanced breast cancer) can be incorporated to give their fac-[Re(CO)3(CNR)3]+ complexes 209 (Scheme 67). Moreover, this synthetic method is applicable when using tetraisocyano copper(I) complexes as a source of isocyanide ligand.228

Scheme 67 Preparation of fac-[Re(CO)3(CNR)3]+ from bioactive isocyanide ligands.

Substitution reaction of [Re(CO)2Br4]− or [Re(CO)2Br4]2− with 10 mol equiv. of isocyanide ligand CNR [R ¼ tert-butyl-(tBu), isopropyl-(iPr), cyclohexyl-(Cy), (S)-(−)-a-methylbenzyl-(smb), 1,1,3,3-tetramethylbutyl-(tmb) or methoxyisobutyl-(mib)] give a series of meridional triisocyano rhenium(I) complexes cis,mer-[Re(CO)2(CNR)3(Br)] 208 in moderate to high yields (Scheme 68).229

Scheme 68 Preparation of mer-triisocyano rhenium(I) complexes cis,mer-[Re(CO)2(CNR)3(Br)] from tetrabromo dicarbonyl rhenium complexes.

604

Carbonyl and Isocyanide Complexes of Rhenium

6.07.3.1.2

Tetra-, penta- and hexa- isocyano Re(I) complexes {[Re(CNR)n]+ (n ¼ 4–6)}

In contrast to mono-, di-, and tri-isocyano Re(I) complexes, which can be effectively prepared by substituting carbonyl ligands in rhenium(I) carbonyl complexes, substitution reaction of pentacarbonyl rhenium(I) chloride with 10 mol equiv. of isocyanide for 5 days in refluxing toluene (110  C) only affords the pentaisocyano rhenium(I) chloride [Re(CNR)5(Cl)] 212 in 35% yield (Scheme 69).230 The homoleptic hexaisocyano rhenium(I) complex [Re(CNR)6](OTf ) 213 can be obtained in high yield by abstracting the chloride in 212 with silver triflate, followed by the reaction with the isocyanide.

Scheme 69 Preparation of penta- and hexa-isocyano rhenium(I) complexes from carbonyl ligand substitution reaction of pentacarbonyl rhenium(I) chloride.

Instead of starting from the rhenium carbonyl complexes, it has been reported that the reaction of hexaiodorhenate with isocyanide gives the pentaisocyano rhenium(I) iodide [Re(CNR)5(I)] 214.231 However, the yields of these complexes are low for some isocyanide ligands, particularly those with an electron-withdrawing group. The reaction yields are significantly improved upon addition of hydrazine as the reducing agent with most isocyanide ligands such as 4-FC6H4NC, 4-ClC6H4NC, 4-BrC6H4NC, 4-IC6H4NC, 4-(EtOOC)C6H4NC, 4-(SF5)C6H4NC, 2,6-(Me)2C6H3NC, 3,5-(CF3)2C6H3NC, 2,4,6-Cl3C6H2NC, 2,4,6-Br3C6H2NC, 4-Br-2,6(Me)2C6H2NC and 2,4-Cl2-6-(MeO)C6H2NC (Scheme 70).220,232,233 It is also interesting to note that the pentaisocyano rhenium complexes become emissive at room temperature if the isocyanide is functionalized with electronic-withdrawing substituents,220 while complexes with electron-donating or unsubstituted phenyl isocyanides only show phosphorescence in low-temperature glassy medium220,230 (See Section 6.07.4.1).

Scheme 70 Preparation of pentaisocyano rhenium(I) iodide from hexaiodorhenate(IV) complex.

Hexaisocyanorhenium(I) complexes can also be synthesized in substitution reactions with tetrachlororhenium(IV) complex [Re(Cl)4(THF)2] 215. Reaction of 215 with the chelating diisocyanide ligand (2,200 -diisocyano-3,300 ,5,500 -tetramethyl-1,10 :30 ,100 -terphenyl) in the presence of sodium amalgam as reducing agent gives the homoleptic tris(diisocyano) rhenium(I) complex 216 in high yield (76%) (Scheme 71).234 This tris(diisocyano) rhenium(I) complex exhibits phosphorescence derived from the 3MLCT [dp(Re) ! p (CNR)].

Carbonyl and Isocyanide Complexes of Rhenium

605

Scheme 71 Synthetic route to homoleptic tris(diisocyano) rhenium(I) complex 216.

Hexaisocyano rhenium(I) complexes can also be prepared by ligand substitution of weakly bound ligands
6-arene of [Re(
6-arene)2]+.235 The reaction of [Re(
6-naphthalene)2]+ 217 with tert-butyl isocyanide with microwave heating gives the homoleptic hexaisocyano complex [Re(CN-tBu)6]+ 218 in high yield (Scheme 72). In contrast, the
6-benzene ring in [Re(
6-C6H6)(
6-naphthalene)]+ 219 is not displaced with tert-butyl isocyanide under similar conditions to give [Re(
6-C6H6) (CN-tBu)3]+ 220.

Scheme 72 Ligand substitution reaction of weakly bound ligands Z6-arene of [Re(Z6-arene)2]+ with tert-butyl isocyanide.

With the pentaisocyano rhenium(I) iodide complexes 214, one of the isocyanide ligands and iodide could be substituted by bidentate diimine ligands N-N to give tetra(isocyanide) Re(I) diimine complexes [Re(CNR)4(N-N)]+ 221 (Scheme 73).232,233 In these reactions, silver triflate or thallium triflate was used for iodide abstraction. The typical yields of these substitution reactions are about 50–60%, and hexaisocyano rhenium(I) complexes [Re(CNR)6]+ are also produced as the major byproduct. These reactions

Scheme 73 Synthetic route to tetraisocyano rhenium(I) diimine complexes.

606

Carbonyl and Isocyanide Complexes of Rhenium

can be accelerated and completed in 4 h by carrying out the reactions at an elevated temperature of 170  C in a microwave reactor233; however, it gives similar reaction yields. These reactions are very versatile and work well with a wide variety of isocyanide and diimine ligands. Another route to tetraisocyano rhenium(I) complexes have been recently reported.236 The tetraisocyano rhenium(I) b-diketiminate complex [Re(CNR)4(BDI)] (R ¼ tert-butyl- or xylyl-) 224 is formed within minutes upon addition of an excess of tert-butyl or xylyl isocyanide to a solution of [Re(O)(Z2-DHF)(BDI)] (DHF ¼ dihydrofulvalene; BDI ¼ N,N0 -bis(2,6-diisopropylphenyl)-2,4-dimethyl-bdiketiminate) 223 (Scheme 74). In these reactions, isocyanide also serves as the reductant. This was confirmed by the characterization of isocyanate by 1H NMR spectroscopy.

Scheme 74 Synthetic route to tetraisocyano rhenium(I) b-diketiminate complexes.

6.07.3.1.3

Reactivity of isocyano Re(I) complexes

6.07.3.1.3.1 Formation of N-heterocyclic carbene complexes Coordinated isocyanides are useful synthetic precursors to NHC ligands because the isocyanide carbon atom has significant carbene character and is susceptible to nucleophilic attack to give an NHC ligand.237 As such, b-nucleophile substituted isocyanide ligands can convert to NHC ligands upon coordination to various transition metal complexes.237 However, a previous study by Hahn et al. showed that intramolecular cyclization of b-hydroxyl-substituted isocyanide does not occur in trans-[Re(dppe)2(CNR)Cl] (R ¼ C6H4(OH)-2 or C6H4(OH)2-2,6) complexes.238 This is due to the electron richness of the rhenium(I) bisphosphine moiety, resulting in strong metal-isocyanide p-back-bonding. As a result, nucleophilic attack on electron-rich isocyanide carbon does not occur in these complexes. In contrast, the intramolecular nucleophilic attack is observed in b-amino-substituted and b-hydroxyl-substituted isocyanides in a number of tricarbonyl rhenium(I) complexes.146,215,239 This is due to the competition of p-accepting interactions of strong p-accepting carbonyl ligands that weakens the p-back interaction with isocyanide. Cyclization of b-amino-substituted isocyanide, generated by reduction of the 2-azidoethyl isocyano rhenium(I) complex 192 with zinc and ammonium chloride, occurs to give tricarbonyl Re(I) tris(diaminocarbene) complex 224 (Scheme 75).215

Scheme 75 Cyclization of b-amino-substituted isocyanide in tricarbonyl rhenium(I) bis(diaminocarbene) complex 192.

Formation of an NHC ligand by incorporating 2-trimethylsiloxyphenyl isocyanide ligand in tricarbonyl and dicarbonyl rhenium(I) diimine complexes has been reported.146,239 The 2-trimethylsiloxyphenyl isocyanide undergoes hydrolysis upon coordination to rhenium(I) to give a b-hydroxyl-substituted isocyanide, which cyclizes to give rhenium(I) benzoxazol-2-ylidene complexes.146,239 Incorporation of 2-trimethylsiloxyphenyl isocyanide is achieved by reaction of the rhenium(I) acetonitrile complexes [Re(CO)3(phen)(NCMe)]+ [Re(CO)2(phen)(PRR0 2)(NCMe)]+ to give benzoxazol-2-ylidene complexes 225 and 228, respectively (Scheme 76). Due to the stronger electron-withdrawing effect of [Re(CO)3]+, the N-H group of benzoxazol-2-ylidene is more acidic, and thus the benzoxazol-2-ylidene ligand in isolated 225 is deprotonated. The deprotonated benzoxazol-2-ylidene in 225 can be readily protonated with HCl to yield 226 with a neutral benzoxazol-2-ylidene carbene ligand. Like other NHC complexes, the N-H group of benzoxazol-2-ylidene in 226 can be methylated with dimethyl sulfate in the presence of a base to give 227. In contrast, the NHC ligand produced by [Re(CO)2(phen)(PR3)(CNC6H4-(OSiMe3)-2)](CF3SO3) [PR3 ¼ PPh3,PMePh2, P(OEt)3] is neutral, and the anion carbene can only be formed under basic conditions. The vast difference in the N-H acidity can be explained by the weaker electron-withdrawing effect of the trans-phosphine ligands compared to CO, which makes the anionic carbene ligands less stable.

Carbonyl and Isocyanide Complexes of Rhenium

607

Scheme 76 Synthesis of rhenium(I) benzoxazol-2-ylidene complexes through coordination of 2-trimethylsiloxyphenyl isocyanide.

Similarly, the tricarbonyl rhenium(I) bis(benzoxazol-2-ylidene) complex 229 can be obtained by incorporation of two 2-trimethylsiloxyphenyl isocyanide into the tricarbonyl rhenium(I) system through the substitution reaction of [Re(CO)5(Br)] (Scheme 77).239 By photochemical ligand substitution reaction with diimine, one of the carbonyl ligands and bromide in 229 can be substituted by a diimine ligand to give 230 (Scheme 77). It is interesting to note that the benzoxazol-2-ylidene trans to the diimine ligand in 230 also reverts to a 2-hydoxyphenyl isocyanide ligand. The reversion to isocyanide is probably caused by the weakening of the pp(Ccarbene)-pp(O) interaction of the carbene, due to the increased p-back-donation from the rhenium metal center upon replacement of the strong p-accepting trans-carbonyl ligand.

Scheme 77 Preparation and reactivity of the rhenium(I) bis(benzoxazol-2-ylidene) complex 229.

The reaction of coordinated isocyanide complexes with suitable nucleophiles is another effective strategy for preparing rhenium(I) carbene complexes. It is worth noting that not all the coordinated isocyanide ligands are reactive towards nucleophiles. Similar to the unreactive nucleophile-containing isocyanide ligands in electron-rich rhenium(I) complexes mentioned above, only the isocyanide ligand trans to the carbonyl ligand in the diisocyano rhenium(I) complex cis,cis-[Re(CO)2(CNC6H4-Cl-4)2(bpy)]Br 204 is reactive with nucleophiles such as deprotonated 2-bromoethanol, 2-bromoethanamine and thiirane to give oxazolidin-2-ylidyl 231, imidazolidin-2-ylidyl 232 and thiazolin-2-ylidyl 233 carbene complexes, respectively. The isocyanide trans to bipyridine is very stable and unreactive towards nucleophiles (Scheme 78).239 The rhenium(I) diaminocarbene NHC complex can be prepared by the reaction of 204 with diethoxyethanamine to give an acyclic diaminocarbene complex 235, which can cyclize to afford an imidazol-2-ylidyl carbene complex 235 upon addition of concentrated sulfuric acid.

608

Carbonyl and Isocyanide Complexes of Rhenium

Scheme 78 Reaction of diisocyanide rhenium(I) complex 204 with various nucleophiles.

6.07.3.1.3.2 Formation of acyclic carbene complexes A series of rhenium(I) complexes (236–240) with chelating acyclic diaminocarbene ligands was prepared by coordinating nucleophile-containing ligands such as 2-aminopyridine, 2-aminothiazole or 2-amino-benzothiazole to various rhenium(I) isocyanide complexes (Scheme 79).240 These complexes exhibit phosphorescence. Due to the open acyclic carbene structure, their emission properties are highly sensitive to the change in the microenvironment. Moreover, N-deprotonated carbenes can react with carbon dioxide to give the N-carbamate. As mentioned above, electron-rich isocyanide ligands in pentaisocyao rhenium(I) complexes with alkyl-substituted isocyanide ligands are unreactive with respect to nucleophilic attack. In this case, pentaisocyano rhenium(I) complexes with coordinated 2-aminopyridine were obtained.

Carbonyl and Isocyanide Complexes of Rhenium

609

Scheme 79 Coordination of amine-containing N-donor ligands to isocyano rhenium(I) complexes to give rhenium(I) complexes with bidentate acyclic carbene ligands.

6.07.3.2

Re(III) isocyanide

Re(III) isocyanide complexes have also received much attention in recent years, especially in the synthesis of model analogs for 99m Tc complexes being investigated for potential use as nuclear medicines and imaging agents. For Re(III) isocyano complexes, the “4 + 1” structure motif is the most popular and is defined by a trigonal-bipyramidal geometry formed with a wrapping tetradentate ligand (NS3) and an isocyanide ligand. The synthesis of this family of “4 + 1” complexes is usually performed using the method developed by Hahn et al.241 Reactions of ammonium rhenate(VII) NH4[ReO4] with tris(2-mercaptoethyl)amine in the presence of an excess amount of phosphine gives the precursor [Re(NS3)PR3] 241 (Scheme 80). The phosphine in 241 can then be substituted by an isocyanide ligand to give the “4 + 1” [Re(NS3)(CNR)] 242. The reaction is very versatile and can be used to synthesize rhenium(III) complexes with different types of isocyanide ligands functionalized with various biological receptors, such as small

610

Carbonyl and Isocyanide Complexes of Rhenium

Scheme 80 Synthetic route to [Re(NS3)(CNR)] with a “4 + 1” structural motif.

molecules and drugs (quinazoline derivatives,242 5-nitroimidazole derivatives,243 2-nitroimidazole and metronidazole244, and tacrine245), fatty acids (octanoic, undecanoic, dodecanoic and pentadecanoic acid) or thioether fatty acid (pentyl-, hexyl-, heptyland nonyl-thiopentanoic acid),246 polysaccharide (dextran247) or peptides (bombesin248 and vasopressin249) (Table 4). Depending on the size of the isocyanide substituents, the yield for the phosphine substitution reaction varies significantly from 20% to 90%. Hexa(thiourea) rhenium(III) can also be used as a starting synthetic precursor for this type of complex.250,251 Table 4

List of isocyano rhenium(III) complexes used as model compounds of radiopharmaceuticals and labeling agents.

[Re(NS3)(CNR)]

References

R ¼ Small molecules or drugs 242

243

244

245

R = fatty acids 246

R = polysaccharide 247

Carbonyl and Isocyanide Complexes of Rhenium

Table 4

611

(Continued)

[Re(NS3)(CNR)]

References

Peptides 248

249

Complexes with substituted CNR and substituted NS3 [R0 -NS3] 250

251

Complexes with substituted CNR and PS3 ligand 252

As mentioned above, rhenium b-diketiminate (BDI) complexes can be used for the preparation of tetraisocyano rhenium(I) complexes (Scheme 74). Rhenium(III/V) b-diketiminate complexes with different ancillary ligands react with isocyanide to give isocyano complexes with different rhenium oxidation states, including rhenium(I) (Scheme 74), rhenium(III), and rhenium(V) (Scheme 81). The reaction of the oxorhenium(V) BDI complex [ReO(BDI)(Z5-Cp)](OTf ) 243 with 2 mol equiv. of tert-butyl isocyanide gives the isocyano complex [Re(BDI)(CNtBu)(Z5-Cp)](OTf ) 244.253 Excess isocyanide ligand is used to react with the oxo ligand to give the rhenium(III) complex before coordination with the excess isocyanide ligand. The rhenium(III) complex [Re(H)(CNC6H3Me2-2,6)(BDI)(Z5-Cp)] 246 can be obtained by reaction of the anionic rhenate(I) complex, Na[Re(BDI) (Z5-Cp)] 245, with 2,6-dimethylphenyl isocyanide in the presence of [Et3NH][BPh4] (Scheme 81).254

612

Carbonyl and Isocyanide Complexes of Rhenium

Scheme 81 Preparation of isocyano rhenium(III) complexes from rhenium BDI complex precursors.

6.07.3.3

Re(V) isocyanide

In contrast to Re(I) and Re(III) isocyanide complexes, isocyano rhenium(V) complexes are not as common. Most of the Re(V) isocyanide complexes reported previously are oxo- or dioxo-Re(V) complexes (Scheme 82)1,255,256 Because the rhenium(V) metal center in these complexes is more electron-deficient compared to those in lower oxidation states, the coordinated isocyanide ligands are less stabilized by p-backbonding interactions.

Scheme 82 Synthesis of oxorhenium(V) complexes with isocyanide ligands.

Isocyanide can coordinate to the trans-dioxo PNN pincer rhenium(V) complex [(PNN)Re(O)2(I)] (PNN ¼ 6-(di-tertbutylphosphinomethylene)-2-(N,N-diethylamino-methyl)-1,6-dihydropyridine) 247 after iodide abstraction with silver salt to give the [(PNN)Re(O)2(CNR)] complex 248 (Scheme 82).257 Similarly, 2,6-xylylisocyanide can coordinate to the five-coordinate BDI oxorhenium(V) dichloride 249 to form the six-coordinate BDI oxorhenium dichloride complex 250.258 Coordination of two cyclohexylisocyanide ligands to another five-coordinate iodo diphosphino dioxorhenium(V) complex 251 has been reported to give the trans-diisocyano diphosphino dioxorhenium(V) complex 252.259 It should be noted that this reaction is not very versatile. Similar reaction with tert-butyl isocyanide does not proceed when used in place of cyclohexyl isocyanide presumably due to the larger cone angle of the tert-butyl isocyanide ligand.

Carbonyl and Isocyanide Complexes of Rhenium

613

Coordination of xylyl isocyanide to the five-coordinate methylnitridorhenium(V) complex 253 with a sterically bulky PNP pincer ligand gives the isocyano complex 254 (Scheme 83).260 This reactivity is different from the reaction of 253 with carbon monoxide, which undergoes insertion into the rhenium-methyl bond. The differences in the reactivity have been explained by DFT calculation that show that isocyanide insertion into the rhenium-methyl bond is an endothermic process, whereas rhenium-isocyanide coordination is energetically favorable.

Scheme 83 Coordination of xylyl isocyanide to five-coordinated methylnitridorhenium(V) complex.

6.07.4

Applications

Potential applications of carbonyl and isocyano rhenium complexes have mainly focused on the rich photophysical and photochemical properties of Re(I) complexes with p-accepting ligands such as diimine, isocyanide and polypyridine ligands. With the long-lived 3MLCT excited state, many of these Re(I) complexes have been designed to serve as phosphorescence materials, photosensitizers, photocatalysts, and luminescent sensors and probes.

6.07.4.1

Photophysics of luminescent carbonyl and isocyano rhenium complexes

After the first report of the phosphorescent properties of tricarbonyl Re(I) diimine complexes by Wrighton and Morse,261 the photophysics of related complexes with the general formula of [Re(CO)3(N-N)(X)] (Table 1) were extensively investigated and reviewed.262–267 Most of these phosphorescent complexes showed moderately intense MLCT absorption [dp(Re) ! p (diimine)] as their lowest energy electronic transition in the near UV to visible region depending on the p-accepting properties of the diimine ligand. Due to the contribution of p (CO) and p (X) in addition to the major dp(Re) component in the highest occupied molecular orbital (HOMO), the electronic transitions in these complexes are also referred to metal-ligand-to-ligand charge transfer (MLLCT) transitions.98,268,269 Upon excitation, the 1MLCT state is generated, which rapidly undergoes efficient intersystem crossing to a long-lived emissive 3MLCT excited state with lifetimes typically ranging from 10 to 1000 ns. Due to the charge transfer character and change of the dipole moment in the MLCT excited states relative to the ground state, the phosphorescent show solvatochromic270 and rigidochromic behavior.271 In addition to a large number of complexes described in the previous edition of COMC,1 many tricarbonyl Re(I) diimine complexes have been designed over the past decades for applications in different areas (Table 1). With phosphorescent behavior, tricarbonyl Re(I) diimine has become a building block for designing luminescent sensors and biological probes (Section 6.07.4.4).272–276 As the triplet MLCT excited states of these complexes are long-lived, these excited states have been widely used as sensitizers277–281 or catalysts282–285 through intramolecular and intermolecular energy- or electron-transfer processes (Sections 6.07.4.2 and 6.07.4.3). In recent years, the luminescent properties of a new class of diazine-bridged dinuclear tricarbonyl rhenium(I) complexes [Re2(mX)2(CO)6(m-diazine)], the aforementioned 92–106, has received considerable attention.158,286 These complexes also exhibit MLCT [dp(Re) ! p (diazine)] transition at near-UV to visible region depending on the nature of diazine and bridging ligand. Upon photoexcitation, these complexes exhibit phosphorescence with quantum yields much higher than most of the charge-neutral tricarbonyl rhenium(I) diimine complexes. The improvement in quantum yield can be attributed to the significant decrease of the non-radiative decay rate constant due to the rigidity of the dinuclear complexes and the increase of the radiative decay arisen from the increase of the metal contribution.159 As these complexes are thermally stable and vacuum processable, their application in electroluminescent devices has been studied.160 In addition, (peptide nuclear acid) PNA-conjugates of these dinuclear complexes can be used for cell-imaging.287 By replacing all of the carbonyl ligands with the isoelectronic isocyanide ligands, a series of tetraisocyano Re(I) diimine complexes with the general formula [Re(CNR)4(N-N)]PF6 were reported in 2008.232,288 These complexes were synthesized from pentaisocyano Re(I) iodide precursors, as shown in Scheme 70,232,233 and exhibit MLCT [dp(Re) ! p (diimine)] as the lowest-energy electronic transition to yield phosphorescence similar to the analogous carbonyl complexes. This represents the

614

Carbonyl and Isocyanide Complexes of Rhenium

first series of emissive non-carbonyl Re(I) diimine complexes. By modifying the substituents on the nitrogen atom of the isocyanide ligands, the physical and phosphorescent properties can be readily tuned.288 In addition to the [dp(Re) ! p (diimine)] MLCT, these complexes also show [dp(Re) ! p (CNR)] MLCT transitions. These occur at a slightly higher energy region than that of MLCT [dp (Re) ! p (diimine)] transition because the p (CNR) is higher energy than the p (diimine). The assignments have been confirmed by detailed vibrational spectroscopy time-resolved spectroscopy and DFT calculation.289 Due to the charge transfer nature of the emissive state, the phosphorescence of these complexes is highly sensitive to the microenvironment including solvent polarity and rigidity. Except for the emission obtained in protic solvents, the emission energy correlates well with Dimroth’s solvent parameter.290,291 The degree of solvent-dependence of the photophysical properties can also be modified by the steric and electronic properties of the isocyanide and diimine ligands.233 Triisocyano- and diisocyano-carbonyl Re(I) diimine complexes, [Re(CNR)3(CO)(N-N)]PF6 and [Re(CNR)2(CO)2(N-N)]PF6 have subsequently reported. Their photophysical properties can also be effectively modified by N-substituent on the isocyanide ligands. As carbonyl ligand is a stronger p-acceptor than isocyanide ligands, the dp(Re) orbital is better stabilized with carbonyl ligand. As a result, emissive triplet 3MLCT [dp(Re) ! p (diimine)] state of these isocyano Re(I) diimine complexes follows the order of [Re(CNR)2(CO)2(N-N)]PF6 > [Re(CNR)3(CO)(N-N)] PF6 > [Re(CNR)4(N-N)]PF6. Apart from emissive MLCT state associated with diimine ligand, Re(I) complex with emissive triplet MLCT [dp(Re) ! p (CNR)] excited state was first reported in hexakis(arylisocyano) Re(I) complex with a chelating diisocyanide ligand.234 As p (CNR) is higher-lying than p (diimine), its emission energy is much higher than those of tetrakis(arylisocyano) Re(I) diimine complexes. Despite its higher emission energy, the emission quantum yield is relatively low (0.6%) with a short-lived lifetime of 8 ns. This is attributable to the thermal population of the non-radiative deactivating ligand-field excited state as isocyanides have a weaker ligand-field strength than the carbonyl ligand. Although the emission lifetime is short, its excited state is strongly reducing. It can be used as a photoredox catalyst for dimerization of benzyl bromide and halobenzene reduction with excellent performance comparable to most commonly used [Ir(ppy)3].234 Detailed investigation on the photophysical properties of pentaisocyano Re(I) with various ancillary ligands corroborated the non-radiative deactivation of the ligand-field state in the isocyano Re(I) complexes.220 The MLCT [dp(Re) ! p (CNR)] phosphorescence of these complexes can be enhanced by destabilization of the ligand-field state with stronger p-accepting ancillary ligand, such as isocyanoborate. The use of strong p-accepting anionic isocyanoborate ligands for the development of neutral Re(I) triplet emitter with high emission energy and enhanced quantum yield has also been reported in tricarbonyl Re(I) diimine complexes.218,219 The isocyanotris(pentafluorophenyl) borate Re(I) diimine complexes possess high thermal stability and can be vacuum sublimed for electroluminescent device application.218

6.07.4.2

Energy-transfer photosensitizers

As the triplet emissive excited states of phosphorescent Re(I) complexes are long-lived and can be efficiently populated upon photo-excitation into their absorption bands (Section 6.07.4.1). Therefore, they are excellent triplet photosensitizers as an energy donor or powerful reductant/oxidant. In this section, recent reports on the photosensitization processes with the MLCT state of Re(I) complexes through triplet-triplet energy transfer are described, while photosensitization processes initiated by bimolecular electron transfer are discussed in the section of the catalytic application (Section 6.07.4.3). Like most triplet emissive excited states, the 3MLCT phosphorescence of Re(I) complexes is sensitive to dissolved oxygen level. The photosensitization with 3MLCT state of the Re(I) complexes for the generation of singlet oxygen (1O2) was first studied in 2007.292 The use of Re(I)-based photosensitizers on generating 1O2 for photo-oxidation of 1,5-hydronaphthalene293 and photodynamic therapy.294–296 By incorporation of extended conjugated diimine ligands, such as pyridocarbazole derivatives, into the tricarbonyl Re(I) complexes, the MLCT absorption would shift to lower energy in the visible region.295 These complexes can also serve as efficient photosensitizers capable of harvesting red-light to generate singlet oxygen for photoinduced antiproliferative activity.295 Apart from intermolecular triplet-triplet energy transfer, intramolecular triplet-triplet energy transfer in bichromophoric Re(I) systems have been developed through functionalization of the diimine ligands with another conjugated organic fluorophore such as coumarin,293 Bodipy,297 naphthalimides,298 and anthracenes.299 In some of these bichromophoric Re(I) systems, the triplet ligand-centered (3LC) excited state of the organic fluorophores can be effectively populated by excitation into the MLCT absorption bands of the Re(I) complexes.293,297,298 The 3LC excited state generated by MLCT-photosensitization can serve as an efficient intermolecular triplet energy donor to sensitize triplet annihilators, such as 9,10-diphenylanthracene and perylene, for triplet-triplet annihilation upconversion.293,297 Photosensitization of triplet photochromic reactivity of stilbene- and azo-containing pyridine ligands with 3MLCT excited state of the tricarbonyl Re(I) diimine complexes has been reported since 1995.300 The intramolecular triplet-triplet energy transfer mechanism from the 3MLCT state to populate the reactive triplet excited state for isomerization has recently been confirmed by ultrafast vibrational spectroscopy301 and DFT calculations.302 In addition to 3MLCT photosensitized trans-to-cis isomerization, the triplet photochromic ring-opening reaction of spirooxazine-containing ligands303 and cyclization of diarylethene-containing ligands304–306 via an intramolecular photosensitization by the 3MLCT excited states of tricarbonyl Re(I) diimine complexes have also been demonstrated in recent years. With triplet MLCT-photosensitization, the photochromism of these photochromic ligands can be extended from high-energy UV region to low-energy visible light region. The triplet-triplet energy transfer process from the 3 MLCT excited state to the reactive 3LC excited state of the diarylethene-containing 1,10-phenanthroline ligand in its Re(I) complex has been confirmed by ultrafast time-resolved absorption and emission spectroscopy.305 It is worth to mention that when the

Carbonyl and Isocyanide Complexes of Rhenium

615

photochromic ligands are incorporated into Re(I) complexes with MLCT excited state lower-lying than the reactive 3LC excited state, the photochromic reactivity would be quenched instead of being sensitized.307,308 the luminescent properties of these Re(I) complexes can be photoswitchable by the photochromic reactions.300,304–307

6.07.4.3

Photocatalysis

The photocatalytic activity of tricarbonyl Re(I) diimine complexes for reduction of CO2 to CO have been reported since 1983.309 The excellent selectivity and efficiency of photocatalysis have attracted tremendous attention over the past few decades. This topic has been extensively reviewed282,283 and discussed in the previous edition of COMC.1 In this section, only recent developments of photocatalytic CO2 reduction using carbonyl Re(I) complexes are described. A recent mechanistic study comparing the photocatalytic properties of [Re(CO3)(bpy)(LX)] (LX ¼ Cl, NCS or CN) suggested the dual roles of the Re(I) complexes as a photocatalyst to react with CO2 to afford the Re-CO2-adduct and a photosensitizer to serve as an electron donor for reduction.310 Based on this postulation, a photocatalytic system with a high quantum yield of 0.59 were obtained by mixing the highly stable Re(I) complex [Re{4,40 -(MeO)2bpy}(CO)3{P(OEt)3}] as photosensitizer and the substitutionally-labile Re(I) complex, [Re(CO3)(bpy)(NCMe)]+, as photocatalyst in a DMF-TEOA solution.310 Using the trinuclear Re(I) macrocyclic ring complex [P-Re(CO)2(5,50 -Me2bpy)-P(CH2)2]3 as photosensitizer together with 1,3-dimethyl-2-phenyl-2,3dihydro-1H-benzo[d]imidazole (BIH) as reductant, the quantum yield was further increased to 0.82.311 Since the MLCT transitions of the Re(I) dicarbonyl diphosphino diimine core in the trinuclear Re(I) macrocyclic ring complex occur in the visible region, the photocatalytic system can utilize visible light. Similarly, as NHC-containing dicarbonyl Re(I) bipyridine complexes have strong MLCT absorption in the visible region, they can harvest visible-light for photocatalytic reduction of CO2.312 DFT calculation for another possible mechanism for photocatalytic reduction of CO2 to CO has also been reported.313 A large number of supramolecular multifunctional systems obtained by connecting visible-light harvesting photosensitizer with carbonyl Re(I) complex-based photocatalyst have recently been designed and reported. Selected examples include Ru(II)Re(I),314–318 Os(II)-Re(I),319 Ir(III)-Re(I)320 and metalloporphyrin-Re(I)321–324 complexes (Fig. 11). In these supramolecular photocatalysts, visible light can be efficiently harvested by the photosensitizer, and CO2 reduction proceeds at the Re(I) unit. In most cases, the performance of the photocatalytic CO2 reduction in the supramolecular system is better than the corresponding photocatalytic system by mixing the photosensitizer and photocatalyst units. Detailed mechanisms for some of the supramolecular photocatalysts have also been reported.317,318,325

Fig. 11 Structures of the supramolecular photocatalysts.

616

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Apart from photocatalytic CO2 reduction, tricarbonyl Re(I) complexes have also been used in photocatalytic reactions for hydrogen generation and organic transformations. Photocatalytic systems comprising tricarbonyl Re(I) diimine systems as the photosensitizer and cobalt-based or Fe-Fe hydrogenase mimicking water reduction catalysts for photo-induced hydrogen generation in an acidic medium have been developed.284 Although photocatalytic Re-Co systems show a much better performance than Re-Fe-Fe systems, only a few Re-Co systems show TONs >5000.326–328 For organic transformations, the 3MLCT [dp(Re) ! p (CNR)] excited state is strongly reducing and can be used as photoredox catalyst for dimerization of benzyl bromide and reduction of halobenzene.234 Recently, the 3MLCT [dp(Re) ! p (diimine)] excited states of a series of tricarbonyl Re(I) diimine complexes have been recently demonstrated to promote photoredox catalysis for various type of reactions, including radical addition, hydrodehalogenation, and a-amino C-H functionalization reactions.329,330 In addition to photocatalysis, many carbonyl rhenium complexes are active catalysts for a wide variety of organic reactions.331–340

6.07.4.4

Biomedical applications

As mentioned in Section 6.07.4.1, tricarbonyl rhenium(I) diimine complexes exhibit phosphorescence derived from the triplet emissive excited state. Moreover, tricarbonyl rhenium(I) complexes possess high thermal stability and photostability. With these properties, tricarbonyl rhenium(I) diimine complexes are excellent building blocks to design complexes for different biological applications, such as cell imaging, photodynamic therapy, biosensing, and probes. There are several excellent reviews on the biological applications of carbonyl rhenium complexes.272–276,341–344 For successful biological applications, biocompatibility, cytotoxicity and cellular uptake are crucial. To enhance the uptake of rhenium complexes by cells, many rhenium(I) complexes with ligands functionalized with various biomolecules, such as peptides,92,345–347 proteins,348,349 fatty acid chains350,351 and carbohydrates352–354 have been reported. With these functional moieties, the complexes have specific intracellular localizations such as membrane,347 endoplasmic reticulum,354 mitochondria,352 nucleus,92 upon entering the cell. This enables the complexes to be used for targeting or studying the specific components of the cell. Photodynamic therapy using rhenium(I) complexes as 1O2 photosensitizer has been described above (Section 6.07.4.2). Conjugation of the tricarbonyl rhenium(I) complex with nucleopeptide moieties dramatically enhances the accumulation of these complexes in the nucleoli and their cytotoxicity and phototoxicity.296 Apart from phototoxicity from 1O2 generation, carbonyl rhenium(I) complexes can also be used as photoinduced CO releasing molecules to achieve controlled CO delivery in cells to cause cell death.355–357 A series of CO-releasing carbonyl rhenium(II) complexes controllable by pH have been developed.212 As discussed previously, the radioactivity of technetium (99mTc) has been leveraged for radiopharmaceutical applications.358 Due to the close similarity of the reactivity and biological activity of 99mTc and rhenium complexes, the non-radioactive rhenium complexes are used as surrogates to provide important insights in the development of 99mTc-based radiotracers and radiopharmaceuticals.92,101,359–361 Complexes with radioactive 186Re and 188Re isotope have also been used for radiotherapy.362–364

6.07.5

Conclusion

In summary, the development of rhenium carbonyl and isocyanides complexes with diverse functional properties and potential applications has been continuously expanding over the past decade. This is particularly noticeable for rhenium isocyanide complexes, which were much less reported in the previous edition of COMC.1 Parallel to the development of the new rhenium isocyanide complex systems, many unprecedented properties and potential applications of rhenium isocyanide complexes have been revealed. Concerning the applications of the rhenium carbonyl and isocyanides complexes, and their structural optimization for performances, the availability of synthetic methodologies to allow flexible design of organometallic and coordination compounds plays an essential role on top of the ligand design. As a result, many new synthetic methodologies and reactivity have been explored in recent years to develop new rhenium carbonyl and isocyanides complex systems. With many unique properties, such as reactivity, catalytic activity, and excited-state characteristics associated with the rhenium carbonyl and isocyanides complexes, it is anticipated that many newly designed functional ligands and supramolecular metal complex systems will be coordinated to various rhenium carbonyl and isocyanides systems. Further, new rhenium carbonyl and isocyanides complex systems are expected to continue evolving, based on the recently developed synthetic methodologies, in view of the potential applications of their unexplored properties.

Acknowledgment We acknowledge support from the General Research Fund (Project Nos. CityU 11303318 and CityU 11306819) from the Research Grants Council of Hong Kong SAR, China.

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6.08 Organometallic Complexes of Group 5 Metals With Pincer and Noninnocent Ligands Samuel D Juárez-Escamilla, Maitreyee Rawat, and T Keith Hollis, Department of Chemistry, Mississippi State University, Starkville, MS, United States © 2022 Elsevier Ltd. All rights reserved.

6.08.1 6.08.2 6.08.2.1 6.08.2.2 6.08.2.2.1 6.08.2.2.2 6.08.2.2.3 6.08.2.3 6.08.3 6.08.3.1 6.08.3.2 6.08.3.2.1 6.08.3.2.2 6.08.3.2.3 6.08.3.2.4 6.08.3.3 6.08.4 6.08.4.1 6.08.4.2 6.08.4.2.1 6.08.4.2.2 6.08.4.2.3 6.08.4.2.4 6.08.4.3 6.08.5 References

6.08.1

Introduction Vanadium Introduction Pincer compounds Neutral ligands Monoanionic ligands Dianionic ligands Noninnocent ligands Niobium Introduction Pincer complexes Neutral ligands Monoanionic ligands Dianionic ligands Trianionic ligands Noninnocent ligands Tantalum Introduction Pincer compounds Neutral ligands Monoanionic ligands Dianionic ligands Trianionic ligands Noninnocent ligands Conclusions

623 623 624 624 624 625 627 628 629 629 630 630 631 635 635 636 637 637 637 637 639 640 642 644 645 645

Introduction

Pincer complexes of the group 5 metals have been subject to many investigations and reports of late, as seen herein. Yet, these complexes remain underdeveloped relative to pincer complexes of the late transition metals. This discrepancy is found in the scope of ligands reported, the chemistry of those ligands, and their catalytic applications. Herein, we have collected the extant reports on these as well as group 5 complexes containing a noninnocent ligand. While there are a couple of exceptions, notably the BDI ligands exploited by Bergman and Arnold (see Section 6.08.3.3), the reports of noninnocent ligands with group 5 metals are even more sparse than those with pincer ligands. This review covers V, Nb, and Ta pincer complexes found in the literature through August 1, 2021. The synthesis of the complexes are summarized, and in some instances, reactions or catalytic applications of the complexes are discussed. The relevant references and notations may be found even if the contents are not described in detail. It is our hope that this review will be a guide to the reader and an impetus to new, and what will certainly be interesting, chemical discoveries. The article is structured first by metal. Each has a table listing the scope of pincer ligands found organized by charge - neutral, monoanionic, dianionic, and even a few trianionic examples. Within these charge states the ligands are organized alphabetically according to the standard pincer nomenclature of the atoms bonded to the metal (e.g., CNC comes before NCN). A total of 93 references are included. While we have attempted to be thorough, the current literature is vast with page numbers proliferating without limit in the electronic era, and the number of journals at only a slightly slower pace. Therefore we beg the communities indulgence, and ask for your assistance, if we have missed something. The literature is replete with definitions of a pincer complex,1,2 and the reader is referred to these for more details. Herein, we will use the definition of a pincer ligand as a tridentate ligand (three binding sites to the metal) that mostly binds in a meridional manner.

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00164-5

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6.08.2

Vanadium

6.08.2.1

Introduction

Vanadium complexes have been prepared with neutral, monoanionic, and dianionic pincer ligands (L1 – L10; Table 1). Interestingly, we did not identify any trianionic ligands reported for vanadium.

6.08.2.2 6.08.2.2.1

Pincer compounds Neutral ligands

Gibson and coworkers reported V complexes with a neutral CNC pincer ligand in 2004.3 The bis(carbene)pyridines L1 were prepared by deprotonating the corresponding imidazolium salts using potassium bis(trimethylsilyl)amide reported first by Danopoulos in 2002.4 The resulting L1 ligands were metalated with VCl3(DME) to form 1 (Fig. 1). There was crystal structure reported for the V pincer complex 1 was reported later.3,5 In 2006, Danopoulos and coworkers also reported the CNC-NHC ligand (L1, R ¼ 2,6-iPr2C6H3) in complexes of V(II), V(III), and V(IV).5 All complexes were characterized by elemental analysis, magnetic susceptibility, and single-crystal X-ray diffraction. The crystal structures revealed distorted octahedral geometry with expected occupation of the meridional sites by the pincer ligands. The vanadium coordination geometry is octahedral with the pincer ligand at meridional sites and the chlorides being mutually trans 2 (Fig. 2). Table 1

Pincer ligand scope for V.

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Fig. 1 Synthesis of CNC pincer complex 1.

Fig. 2 Synthesis of CNC pincer complex 3.

The oxidation of (CNC)VCl3 pincer complex (R¼ 2,6-iPr2C6H3) with 4-methyl morpholine N-oxide in THF resulted in trans[V(CNC)(]O)Cl2] (2), which was subsequently treated with AgBF4 in acetonitrile to yield trans-[V(CNC)(]O)(MeCN)2][BF4]2 (3; Fig. 2). Various (CNC)V pincer complexes are highly active catalysts in ethylene oligomerization and polymerization when used in combination with various other cocatalysts.5

The PNP complex 4 was studied computationally by Haijun and coworkers in 2018.6

6.08.2.2.2

Monoanionic ligands

The monoanionic NCN pincer ligand L3, which contains a central phenyl ring and peripheral amine donors 5, has been used to prepare V complexes via the NCN gold complex 7 first reported by van Koten in 2007 (Fig. 3).7 This gold complex was synthesized by the reaction of 2,6-bis[(dimethylamino)methyl]phenyllithium (5) with 4,40 -bis[P-(chlorogold(I))diphenylphosphino]biphenyl 6. Transmetalation of 7 with VCl4 in benzene yielded the first NCN vanadium (IV) pincer complex 8 (Fig. 4), which converted to vanadium (III) dichloride compound during the process of its crystallization. No explanation was offered for the redox process. The structure of the vanadium(III) NCN complex was confirmed by X-ray crystallography. Pincer complex 8 was immobilized on magnesium chloride-based support and was reported as a catalyst in olefin polymerization.8 Vanadium complexes 10 and 11 exemplify monoanionic, symmetric PNP pincer complex of V(III) (Fig. 5).9 The organometallic chemistry of complex 10 was first reported by the Mindiola lab in 2008.9 This complex was synthesized via a condensation reaction between commercially available VCl3(THF)3 (9) and the deprotonated ligand salt Li(PNP). It was further alkylated in situ with neopentyl lithium to yield the air- and moisture-sensitive bis(neopentyl) (PNP) vanadium complex 11 (Fig. 5). The characterization of complex 11 included magnetization measurements (Evans method), X-ray diffraction structural data, and combustion analysis.10

Fig. 3 Synthesis of the gold NCN complex used for transmetalation studies.

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Organometallic Complexes of Group 5 Metals With Pincer and Noninnocent Ligands

Fig. 4 Synthesis of NCN pincer complex 8.

Fig. 5 Synthesis of PNP pincer complex 10 and 11.

The crystal structure of 11 revealed a highly distorted trigonal bipyramidal coordination geometry with a PdVdP angle of 151.750 (13) . Coordination of the PNP ligand is meridional and the alkyl ligands are arranged in a cis orientation with a CdVdC angle of 121.46(6) . A series of vanadium(V) pincer complexes bearing terminal alkylidene ligands were synthesized by heating 11 with oxygen sources such as R3P]O, N2O, TEMPO, R3N]O, and pyridine N-oxide and sulfur sources such as S8 and R3P]S, respectively (Fig. 6, 12, 16, 17).11 More vanadium(V)–alkylidene complexes were also prepared by heating the vanadium(III) dialkyl complex 11 with p-acids, 14, or two-electron oxidants azides, 13, or azobenzene, 15.9,10 Trimethylsilylazide reacted very differently from azobenzene (13 vs 15). These reactions proceeded by a-hydrogen abstraction from the vanadium(III) dialkyl complex 11 to form a V(III) alkylidene intermediate 12 that was trapped in situ.

Fig. 6 Synthesis of PNP pincer complexes 12–17.

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627

Fig. 7 Synthesis of PNP pincer complex 19.

In 2018, Nishibayashi and coworkers reported vanadium complexes bearing the PNP ligand L5 with pyrrole flanked by different alkyl-substituted phosphines.11 The vanadium complex 18 was synthesized using VCl3(THF)3 (9). The reaction of 18 with xylyl lithium in THF at room temperature for 8–12 h gave the corresponding vanadium 2,6-xylyl complex 19 (Fig. 7). The aryloxy-substituted PNP vanadium complexes 20 and 21 were reported to be effective catalysts for directly converting molecular dinitrogen into ammonia and hydrazine.11

The first computationally designed monoanionic PNP vanadium pincer complex 22 was reported by Haijun and coworkers in 2018.6 It has only been studied computationally.

6.08.2.2.3

Dianionic ligands

Dagorne’s lab reported the first OCO vanadium(V) pincer with a central N-heterocyclic carbene in 2009 (Fig. 8).12 The proligand 1,3-bis(3,5-di-tert-butyl-2-hydroxyphenyl)imidazolinium chloride (L7a), first reported by Wieghardt,13 was obtained in a two-step synthesis from N,N-bis(3,5-di-tert-butyl-2-hydroxyphenyl)ethylenediamine. The vanadium(V) NHC complex 24 was synthesized via an alcohol elimination route involving the reaction of the imidazolinium salt 23 with (iPrO)3V(]O) (Fig. 8). This complex was air-stable in the solid state and in benzene.12 The Hohloch group recently reported similar vanadium(V) oxo complexes 25 and 26 based on the dianionic mesoionic carbene of 1,2,3-triazolinylidene and benzimidazolium at the center of the ligand. They also isolated various vanadium complexes in the +4 and +3 oxidation states, and the latter complexes are potent precursors to vanadium(V) imido complexes.14 The synthesis of these vanadium complexes followed Dagorne’s route.

Fig. 8 Synthesis of OCO pincer complex 24.

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Organometallic Complexes of Group 5 Metals With Pincer and Noninnocent Ligands

The OOO proligand 28 was prepared from the OPO proligand 27, and both were used to prepare V complexes 29 and 30 (Fig. 9).15 Proligand 27 (protonated L9) is a substituted version of bis[phenyl-2-phenoxy]phenylphosphine and was synthesized in a few steps from commercially-available starting materials. Treating 27 with H2O2 yielded the phosphine oxide proligand 28. Both 27 and 28 can be treated with VCl3(THF)3 in the presence of base to form the V(III) complexes 29 and 30. From single crystal X-ray diffraction they found octahedral geometries around the vanadium centers. These complexes have both been reported as catalysts for the polymerization of ethylene or copolymerization with 10-undecen-1-ol.

6.08.2.3

Noninnocent ligands

Noninnocent ligands are classified as ligands that react in a reversible way when bonded to a metal center, usually storing electrons, donating electrons, or aiding in a reaction mechanism. Throughout the literature, the terms “noninnocent” and “redox-active” have been used interchangeably, even though the meaning of each term is not the same. In this section, we will consider a “redox-active ligand” to be a ligand that delocalizes charge from the metal center or may accept or donate charge in a redox reaction. In this sense, all redox-active ligands are noninnocent, but not all noninnocent ligands are redox-active. For example, some noninnocent reactivity involves accepting a proton or a ligand rearrangement (sometimes distinguished as chemical noninnocence). The synthesis of V(III) complexes 31, 32, and 33 was achieved by reacting terpyridine ligand with VCl3 in THF followed by treatment with (trimethylsilyl)methyllithium in diethyl ether. This ligand was synthesized by Budzelaar in 2011.16–18 The ligand with X¼ H produced two different V(III) complexes 31 and 32 crystals in the ratio of 55:45 characterized by XRD. In the case of X¼ phenyl in ligand L12, it gave only 33 as a vanadium complex (Fig. 10). Redox noninnocent V(III) complexes 31, 32, and 33 were used for chemoselective hydroboration and hydrosilylation of ketones, aldehydes, imines, esters, and carboxamides to yield alcohols, amines, and nitriles. The computational analysis of these ligands revealed that the (Rtpy•) − ligand reacts under activation conditions with either pinacolborane (HBpin) or PhSiH3.19

Fig. 9 Synthesis of OPO and OOO pincer complexes 29 and 30.

Organometallic Complexes of Group 5 Metals With Pincer and Noninnocent Ligands

629

Fig. 10 Synthesis of NNN pincer complexes of 31–33.

The first tris(dithiolene) based noninnocent vanadium complex 34 was reported by Davison and coworkers in 1964.20 This V(S2C2Ph2)3 complex was significant in revealing the problem of assigning ligand and metal oxidation states with redox active ligands of bis-and tris-complexes containing bidentate, unsaturated sulfur-donor ligands. The stability of V(S2C2Ph2)3 makes it clear that the oxidation-state formalism, which requires (S2C2Ph2)2− and the higher oxidation-state metals, cannot be applied consistently to these complexes since the formalism in the V (n ¼ 0) case would call for vanadium(VI) suggested by Schrauzer in the report.21 The ONO vanadium(V) complexes such as 35, which contain a catechol-based noninnocent ligand, has been known in the literature for more than 50 years.22–24 Most of the vanadium complexes based on o-dioxolene are of vanadium(V) oxidation state. This vanadium(V) complex 35 exhibits noninnocent or innocent behavior depending on the protonation state of the catecholate ligand.23,25 The o-dioxolenes have three potential redox states: catechol (Cat2−), semiquinone (SQ1−), and quinone (Q0).

6.08.3

Niobium

6.08.3.1

Introduction

Niobium pincer complexes are organized in this section based on overall charge on ligands; neutral, monoanionic, dianionic. Nine pincer architectures have been reported (Table 2). Four of these ligands have been reported in complexes with vanadium (L1-L3, L8), and one ligand possessed a dianionic and a trianionic mode (L16). Some pincer complexes below are known to exhibit noninnocence.

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Organometallic Complexes of Group 5 Metals With Pincer and Noninnocent Ligands

Table 2

6.08.3.2 6.08.3.2.1

Pincer ligand scope for Nb.

Pincer complexes Neutral ligands

The Nb complex 36 was synthesized by the Danopoulos group in 2006 by adding the carbene pincer ligand L1 to NbCl3(DME) in THF at −78  C (Fig. 11). After letting the reaction warm up to room temperature, they collected a residue of the desired complex in 96% yield.5 This complex was characterized structurally by XRD to confirm the structure. Several interesting features were described: the NbdC bonds were shorter than other sp3 and sp2 NbdC bonds, the NbdCl bond trans to the pyridine moiety was shorter than the other two, and the ligand bite angle (69.6 ) was smaller than those from the V and Ti complexes in the same study. Among the NNN Nb pincer complexes, bis(imino)pyridine complexes such as 37a-37d were synthesized and characterized in 2006.26 The synthesis consisted of dissolving NbCl3(DME) and a bis(imino)pyridine ligand in THF, with a slight molar excess of the ligand (Fig. 12). This mixture was refluxed for 12 h, which produced 37a-37d in similar yields.

Organometallic Complexes of Group 5 Metals With Pincer and Noninnocent Ligands

631

Fig. 11 Synthesis of CNC Nb pincer complex 36.

Fig. 12 Synthesis of NNN pincer complexes 37a-37d.

Likewise, 37a-37d were tested for catalytic activity in ethylene and norbornene polymerization. When testing the catalytic activity of 37a-37d in norbornene polymerization, the authors report that the precatalysts only react in ROMP reactions. The bulky 37d complex presented the highest activity among the four of them, up to 70 g mmol−1 h−1 bar−1 in ethylene polymerization. Separately,27 it was reported that using a different cocatalyst may increase the activity of 37b ethylene polymerization substantially, up to 1100 g mmol−1 h−1 bar−1 with dimethyl aluminum chloride and 3390 g mmol−1 h−1 bar−1 with methyl aluminum dichloride. Heterogeneous polymerization tests were also carried out by first immobilizing the 37d precatalyst to silica. The supported precatalyst was active in copolymerization of 1-hexene and ethylene.27 Unfortunately, the active species 37a-37d polymerization reactions have not been characterized or identified.27 Ballmann’s group explored the idea of synthesizing a precatalyst for polymerization that possessed a coordinated alkyne group.28 The synthesis of 38-aCl and 38-bCl consisted of the addition of complexes L14a or L14b to NbCl3(DME) coordinated to TMSCCTMS in a mixture of toluene and DCM. Afterwards, the Nb metal center was alkylated with LiCH2TMS (38-aTMS) and Mg(CH2TMS)2 (38-bTMS). The authors report that 38a-TMS could not be isolated purely and no further studies on reactivity were made. Nonetheless, a crystallographic structure for 38-bTMS was obtained and reactivity studies were completed (Fig. 13). Compound 38-bTMS was tested for polymerization activity.28 It was found that an 85% conversion was obtained in 120 h when using norbornene (1 mol% 38-bTMS) at 85  C in benzene. Norbornadiene and 2-methoxymethyl-5-norbornene were also used as monomers for polymerization, with conversions of 80% and 35% respectively. The authors reported that the cis/trans selectivity was poor when they compared it to other catalysts, although the stability of the catalyst to ligand reactivity was high.28

6.08.3.2.2

Monoanionic ligands

Pincer complex 39 contained a monoanionic NCN ligand (L2) with a Nb(V) metal center (Fig. 14). It was synthesized by a transmetallation of an organogold complex with NbCl5 in benzene.8 XRD showed it possessed a pentagon bipyramidal geometry,

Fig. 13 Synthesis of PpP pincer complexes 38-aCl, 38-bCl and TMS derivatives.

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Organometallic Complexes of Group 5 Metals With Pincer and Noninnocent Ligands

Fig. 14 Synthesis of NCN pincer 39.

where the pincer was in a mer-like position with two Cl atoms in the apical positions and two Cl atoms in the remaining equatorial positions. A by-product in this reaction was the bis-metallic gold moiety, which was recovered and reused. Dimeric Nb pincer chiral complex 41 possessed a chiral pincer ligand and was also synthesized by van Koten’s group.29 This Nb(V) complex was synthesized in a similar fashion to complex 39. Transmetallation of an organogold complex done with NbCl5 in THF yielded 76% of the dimeric complex (Fig. 15). In contrast to the monomeric complex 39, this synthesis could not be done in benzene. The authors suggest an intermediary complex (niobium trichloride oxide) might be formed in THF that reacts forming the dimer. The bis-gold moiety was recovered in a 99% yield. Several of the complexes reported served as precatalysts in polymerization reactions. Dimer complex 41 was studied for activity in ethylene polymerization, which was reported as low. There was an attempt to methylate it (with MeLi and MeMgBr) in order to explore the reason of this reactivity, but many undesired products were obtained. There existed a possibility that the NCN pincer reacts during the polymerization process, deactivating the catalyst. Complex 43 was a NNO pincer Nb bimetallic complex that was synthesized in 2013 in the same study as the previously mentioned (NNN)Nb complexes.27 The synthesis of 43 was similar to the synthesis of the NNN complexes: NbCl3(DME) and the pincer ligand 42 were dissolved in THF, with a slight molar excess of the ligand (Fig. 16). The mixture was refluxed for 12 h, producing 43 with a yield of 13%. Complex 43 possessed a structure that is symmetric by inversion, with the oxo ligands on a local apical position. The metal activates the carbonyl group through a pinacol reaction forming a dimer of the monoanionic ligand formed in situ. Complex 43 was active as a precatalyst for ethylene polymerization. In this reaction, MADC was used as a cocatalyst. The precatalyst 43 showed the highest activity at room temperature (1530 g mmol−1 h−1 bar−1). It showed its lowest activity at 40  C (870 g mmol−1 h−1 bar−1) and increased back slightly in activity at 60–80  C (1060–1100 g mmol−1 h−1 bar−1).

Fig. 15 Synthesis of a chiral dimer NCN pincer complex 41.

Fig. 16 Synthesis of NNO pincer complex 43.

Organometallic Complexes of Group 5 Metals With Pincer and Noninnocent Ligands

633

Fig. 17 Synthesis of PNP pincer complex 45.

Fig. 18 Synthesis of PNP pincer complex 46.

In 2006, Mindiola’s group described the synthesis for the PNP pincer complex 45. It was done by adding a solution of NbCl3(DME) in thawing toluene to a thawing Et2O solution of 44. The reaction was then stirred for 24 h while letting it warm up to room temperature (Fig. 17).30 They later described in 2016 a synthesis to a slightly different Nb complex (46), but with a significantly higher yield (Fig. 18). First, the ligand L3 reacts in a transmetallation reaction with one equivalent of NbCl4(THF)2 in toluene for 12 h at room temperature. Afterwards, one equivalent of NaOAr (Ar ¼ 2,6-iPr2C6H3) is added to the solution. This reaction continued for 2 h, yielding complex 46 in a 92% yield. The authors declare their choice of the Ar substituent was such that no side reactions, such as a a-elimination, occur in the ligand substitution reactions they intended. Complex 45 was used to synthesize the mono-alkylidene and bis-alkylidene complexes. When TMS-CH2-MgCl was used, it produced both the mono-alkylidene complex 47 and the bis-alkylidene complex 48. Meanwhile, using tBu-CH2-MgCl always yielded the bis-alkylidene complex 48. An inert argon atmosphere is preferred instead of a nitrogen atmosphere since the presence of N2 produces a (m-N2) dimer (Fig. 19).31 The synthesis of the 47–48 complexes was thought to be mediated by the formation of a bis-alkyl complex, followed by an a-H elimination and the formation of H2. However, no molecular hydrogen has been detected as by-product. The alkylidene complexes 47, 48a and 48b were characterized by NMR and XRD. EPR showed a superhyperfine coupling of the electron on the d1 Nb to the two P atoms. The research team reported no reaction between the 48a bis-alkylidene complex and ethylene, imines, azobenzene, benzophenone and diphenylacetylene. They suggested this behavior was due to the complex size. The (PNP)Nb pincer complex 52 was synthesized by Mindiola’s group in 2016.32 The authors reported that no other group 5 pincer methylidyne complex had been synthesized before or, if so, they were exceedingly rare. Several steps were required to obtain complex 52, which are shown in Fig. 20 and Fig. 21. Complex 46 was alkylated with a Grignard reagent to produce complex 49. Compound 49 was oxidized using [FeCp2][OTf]. The resulting product 50 presented a pentagonal bipyramidal structure with the triflate and the aryl alkoxide in the apical positions. Complex 50 was sensitive to radiation (250–385 nm), and upon exposure it released methane to produce complex 51 in 77% yield. The triflate moiety, which is still bound to the Nb metal center, made complex 51 susceptible to deprotonation of the methylidene moiety.

Fig. 19 Synthesis of pincer complexes 47, 48a-48b.

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Organometallic Complexes of Group 5 Metals With Pincer and Noninnocent Ligands

Fig. 20 Synthesis of PNP pincer complex 49.

Fig. 21 Synthesis of PNP pincer complex 52.

Deprotonation using LiHMDS resulted in complex 52 with a 60% yield. The total yield from complex 50 to complex 52 by the UV radiation method was 46.2%. The authors also reported a separate method to synthesize complex 52 by adding a Wittig reagent to complex 50 in toluene. This methodology resulted in a 72% yield of the desired complex. Complex 52 possessed a trigonal bipyramidal structure, with the P atoms from the pincer ligand on the apical positions. One of the outstanding features of complex 52 was its ability to participate in metathesis-like reactions with a variety of substrates. The authors described a reaction yielding complex 53, which was regarded as the first example of a neutral terminally bounded niobium nitride. This reaction proceeded by adding a nitrile complex to a toluene solution of 52. Besides complex 53, this cross-metathesis reaction produced a terminal alkyne complex. The authors did not report any kind of expectation or outlook into the usefulness of this type of reaction in synthetic chemistry (Fig. 22).

Fig. 22 Synthesis of PNP pincer complexes 53–55.

Organometallic Complexes of Group 5 Metals With Pincer and Noninnocent Ligands

635

After the cross-metathesis reaction resulted in 53, Mindiola’s group tested the reactivity of 52 with an isocyanate and a phosphaalkyne.33 Reacting 52 with tBu-isocyanate in toluene quickly resulted in 54. The authors reported the X-ray structure and the NMR and IR spectra to identify the azaallenyl structure of 54. A similar structure was not reported for the reaction with the phosphaalkyne. Instead, the metallacyclopropene complex 55 was obtained. DFT calculations showed that a [2 + 2] cycloaddition of the phosphaalkyne took place, readily followed by the migration of the P atom of the phosphaalkyne in a rearrangement reaction.

6.08.3.2.3

Dianionic ligands

In 2019, Hohloch’s group synthesized a new mesoionic Nb carbene complex 56 from L8.34 One equivalent of L8 in 5 mL of Et2O/ THF reacts with three equivalents of LDA for 2 min at −40  C. Afterwards, the niobium imido complex was added and the mixture was stirred for 18 h. This method yielded the desired complex in 62% yield (Fig. 23). The X-ray structure showed the Nb in an octahedral geometry with the Cl in the same plane as the pincer ligand. The authors declared the carbene bond to the Nb was the shortest reported to date (2.196 A˚ ). Complex 58 shown in Fig. 24 is a (ONO)Nb pincer complex reported in 2016 by the Veige group.35 The synthesis of the [CF3-ONO]Nb complex started with the addition of 57 dropwise into a solution of NbCl5 in benzene (Fig. 24). The solution was stirred under Ar atmosphere for 4 h. This structure was supported by the presence of a N-H signal in the 1H NMR spectrum, four non-equivalent CF3 signals in the 19F NMR spectrum, and the splitting of one of the CF3 fluorides (J HF¼6.4 Hz) by the amine proton.

6.08.3.2.4

Trianionic ligands

Dropwise addition of Et2O to a solution of 58 induces the elimination of HCl and the formation of 59 (Fig. 25). This chloride complex was used to prepare the alkyl complexes 60a-60c that were subsequently used as precatalysts for ROMP (ring opening metathesis polymerization) of norbornene.35 The alkyl complexes were prepared by slowly adding Grignard reagents dropwise to a cold solution of 59 in Et2O (60a) or pentane (60b-60c), producing 60a-60c in similar yields (Fig. 26). Two non-equivalent CF3 resonances were found for 60a-60c in the solution 19F-NMR spectrum. Stability tests of 60c showed there was no a-hydrogen abstraction characteristic of bis-alkyl complexes.

Fig. 23 Synthesis of OCO pincer complex 56.

Fig. 24 Synthesis of ONO pincer complex 58.

Fig. 25 Synthesis of ONO pincer complex 59.

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Organometallic Complexes of Group 5 Metals With Pincer and Noninnocent Ligands

Fig. 26 Synthesis of ONO pincer complexes 60a-60c.

The complexes 60a-60c were used as precatalysts to polymerize norbornene via a ROMP reaction. The results of tacticity tests showed that all the polymers synthesized presented a higher percentage of syndiotactic (rr) triads, as well as a cis configuration (76–89%). The authors report that even though the ROMP reactions proceeded well with norbornene, it did not proceed at all with dicyclopentadiene. Also, the authors suggest there is an intermediary alkylidene complex as the active species. Nevertheless, neither its isolation nor its characterization have been reported.

6.08.3.3

Noninnocent ligands

In 2017, Arnold, Mashima and Tsurugi investigated a-diimine ligand L19 and its reactivity when bound to a Nb metal. The team described the synthesis of Nb a-diimine complexes by means of a reduction mechanism (Fig. 27).36 These complexes coordinated in a facial fashion to the Nb metal, where two of the coordination sites were occupied by the N atoms, while the third one is taken by the CdC double bond from the reduced diimine. The authors mentioned they believed the diimine ligand noninnocent properties helped explain some of the reactions they described, which are: (1) the addition of CCl4 to alkenes,36 (2) hydrodehalogenation of alkyl halides,37 and (3) ether cleavage and subsequent chlorination (Fig. 28).38 Arnold, Bergman and coworkers have studied extensively the chemistry of b-diketiminate (BDI) ligands with group 5 metals. They published a review on the topic in 2016,39 which describes in great detail many of the BDI complexes also described herein. Most of the chemistry reported by the authors started with complexes 62–63 (Ar ¼ 2,6-iPr2C6H3). The synthesis of complex 62 has been reported mainly in two different ways: the diethyl ether coordinated (BDI)Li complex and the THF coordinated complex (Fig. 29). In 2008, the team reports a one-pot synthesis of a carbonyl derivative complex, presumably with complex 63 as an intermediary.40 Complex 63 was then synthesized with the addition of MeMgBr in diethyl ether. Later, in 2010, the authors described a step-wise method together with its X-ray characterization and its NMR data.41 Several analogs of 62 and 63 and their reactivity have also been reported.42,43

Fig. 27 Synthesis of a-diimine Nb complexes 61a-61b.

Fig. 28 Reactions tested with Nb diimine complexes.

Fig. 29 Synthesis of complexes 62–63.

Organometallic Complexes of Group 5 Metals With Pincer and Noninnocent Ligands

637

Fig. 30 CdN bond activation in reduction conditions.

The b-diketiminate ligand shows several noninnocent properties, the most noticeable being the reactivity with the metal center with CdH and CdN activation. The BDI ligand activation of the CdN imide bond may be found in the early stages of the research on (BDI)Nb chemistry.44 These early examples of CdN bond cleavage appear in reduction conditions (Fig. 30), and authors hypothesize the existence of a Nb(III) intermediary that reacts with the ligand. Later, the team showed that complex 64 could undergo the same CdN cleavage upon heating.45 These reactions may be reversible under certain conditions.46 (BDI)Nb dimers have also shown certain tendency of CdN bond activation of the BDI ligand under reductive conditions.43 Even though the activation of the ligand may be considered as a “decomposition” reaction, these complexes still display certain reactivity after the CdN bond cleavage. The products are reactive in cycloaddition reactions with several substrates:47 aldehydes, alkynes, activated alkenes, as well as azides.46 Also, an intermediate of this kind appears during an isocyanide insertion to a phenyl ring. The existence of this intermediate is supported by the scrambling of the tBu- and arene moieties.48 After scrambling, the CdH bond in the tBu-moiety is activated by the metal center. Besides the previously mentioned example, this ligand is also involved in other CdH activation reactions, usually of the isopropyl moiety of the arene.40 This CdH activation also appears during benzylic CdF bond activation reactions. A complex with a carbon-niobium bond from the isopropyl moiety of the BDI ligand appears as a short-lived intermediate before the coordination of a toluene ring or a PhdCF3 ring.49,50 As shown, a-diimine and b-diketiminate ligand complexes of Nb are by far the most abundant in the literature. We will describe two types of ligands which have shown noninnocent properties. Fryzuk and coworkers have worked extensively with a (P2N2) tetradentate complex that may accept a nitrogen atom from dinitrogen after it coordinates to the metal center.51 Studies have also been made on the noninnocent properties of the imido ligand, which guide the formation of NbdSi and NbdH bonds.52 Another ligand with noninnocence is the NHC ligand studied by Wei and Cai.53 This ligand catalyzes the synthesis of cyclic carbonates. It was reported, nonetheless, that Nb complexes of this kind have low selectivity and low yields in this catalytic reaction. Several other NHC complexes have been structurally characterized.54 There is already a review on this subject written by Romain, Bellemin-Laponnaz and Dagorne.55 As a final note, some of the pincer complexes (43,27 52,32 5835) described in the previous subsection possess ligands that have been described as noninnocent. These may present hidden noninnocence in Nb complexes. Furthermore, (ONO) pincer ligands with proven noninnocent properties have been reported in Nb coordination complexes.56 No organometallic complexes of these ligands have been reported yet.

6.08.4

Tantalum

6.08.4.1

Introduction

Tantalum pincer complexes are organized in the same manner as previous sections, by overall charge on ligands; neutral, monoanionic, dianionic and trianionic. Fifteen pincer ligands are reported for tantalum: six more than the other metals (Table 3). Three pincer ligands have already been discussed in the previous sections. This section contains the synthesis, as well as brief remarks on the characterization of the complexes and their reactivity.

6.08.4.2 6.08.4.2.1

Pincer compounds Neutral ligands

The NNN neutral L13d-based tantalum pincer complex was reported first by Shiono’s lab in 2006.9 The synthesis of complex involves the reaction of bis(imino)pyridine with a stoichiometric amount of TaCl3(DME) in THF to give bis(imino)pyridine complexes of tantalum(III) 65 in reasonable yield (Fig. 31).9 The Ta complex 65 was used as a catalyst for ring-opening metathesis polymerization of norbornene in combination with triethylaluminum.9 The PpP neutral ligand L14a-L14b consisted of two substituted phosphorus phenyl ring at peripheral position and two carbons at central position.28 Upon hydrogenolysis of complex 66, the metallacyclopropene backbone was partially hydrogenated, and the

638

Organometallic Complexes of Group 5 Metals With Pincer and Noninnocent Ligands

Table 3

Pincer ligand scope for Ta.

Fig. 31 Synthesis of NNN pincer complex 65.

Organometallic Complexes of Group 5 Metals With Pincer and Noninnocent Ligands

639

Fig. 32 Synthesis of PpP pincer complex 66–68.

dinuclear (m-H)3- bridged tantalum(V) complex featuring a metallacyclopropane substructure was formed. An intermediate in this reaction was shown to catalytically hydrogenate one or both arenes in naphthalene. The authors also describe the synthesis of the TMS analogs in the same paper as the Nb analogs 38 (Fig. 32). Besides the synthesized complexes mentioned above, complex 69 has been studied computationally for hydrogenation reactions. There are no experimental results available to validate these computational methods for vanadium amido and amino complexes.6

6.08.4.2.2

Monoanionic ligands

The Hollis group reported the synthesis and characterization of a monoanionic bis(NHC) pincer Ta complex in 2014 (Fig. 33).57 The tris-CdH activation of the salt precursor was achieved by direct metalation with excess of the [(Me2N)3(tBuN)Ta] reagent yielding complex 70. The use of the tBu imido ligand reduced the coordination number and allowed the isolation and structural characterization of the octahedral complex 70. It could be transformed into the bis(imido) complex 71,58,59 which was found to be a catalyst for the oxidative amination of alkenes to form nitrogen heterocycles (Fig. 33).58,59 The Ozerov group reported in 2007 PNP Ta pincer complexes 72 and 73. They describe the structural features of both complexes.60 The bis(methylidene) complex 73 was synthesized by a thermolysis route and a photochemical route (Fig. 34).

Fig. 33 Synthesis of CCC-NHC pincer complex 71.

Fig. 34 Synthesis of PNP pincer complexes 72 and 73.

640

Organometallic Complexes of Group 5 Metals With Pincer and Noninnocent Ligands

6.08.4.2.3

Dianionic ligands

The Hollis group has reported a very unusual reactivity that resulted in the conversion of the pincer ligand with both NHC’s bound in a “normal” fashion 74 to a complex where one of the NHC’s is bound in an “abnormal” fashion. In addition, the “normal” part of the “abnormal” carbene was deprotonated and is bound to lithium 76. It was structurally characterized and the mechanism of formation was supported by DFT calculations (Fig. 35).61,62 In 2018, an instance of a (CCO)Ta pincer complex 78 was mentioned in a paper as a decomposition product.63 The objective of the authors was to synthesize Ta/Rh heterobimetallic complexes. One of the proposed Ta complexes was synthesized by adding a carbene suspension L24dH2 in toluene to a solution of Ta(CHtBu)(CH2tBu)3. Even though the reaction proceeded relatively quickly and with a good yield, it decomposed at room temperature after some time (Fig. 36). The decomposition product 78 was characterized by NMR techniques, but the authors reported they were not able to grow a crystal for X-ray characterization. Fryzuk64 reported in 2006 a way to metalate diamido-NHC ligands with a Ta metal center. These reactions proved challenging through a direct metalation with amido and alkyl tantalum reagents. The authors changed their approach by using a lithium amide ligand and used different tantalum(V) reagents (Fig. 37). This way, several pincer complexes 75–77 were synthesized and characterized through NMR and X-ray crystallography of the pentaamido complex. Clearly, the number of chlorides in the final complex depends on the starting material.

Fig. 35 Synthesis of CCC-NHC pincer complex 76 with an N-heterocyclic dicarbene ligand.

Fig. 36 Synthesis of CCO pincer complex 78.

Fig. 37 General synthesis of NCN pincer complexes 75–77.

Organometallic Complexes of Group 5 Metals With Pincer and Noninnocent Ligands

641

Fig. 38 Synthesis of ONO pincer complexes 78–80.

The Bercaw group reported tantalum complexes supported by bi- and tridentate ligands with two phenolates linked to pyridine, thiophene, furan, and benzene spacers in 2008.65 Alkane elimination or salt metathesis methods were used to prepare these complexes. The ligand framework consisted of two X-type phenolates connected to the flat heterocyclic L-type donor (2 electrons) at the positions 2 and 6 of pyridine benzene via direct ring-ring linkages.35,66 For synthesizing the Ta pincer complexes, 56–58, several chloride benzyl tantalum precursors were added to a slurry of potassium bis(phenoxide)pyridine in benzene (Fig. 38). Fandos and Otero synthesized complex 82 from the tantalum complex 81 and the ligand L27dH2. Their structural characterization and their synthesis is described by the authors,67 as well as their reactivity with isocyanates and Lewis acids (Fig. 39). A similar complex 83 with a phenolate making a 5-membered chelate was described by Trávnícek in a study about their cytotoxicity.68

Otero’s group also synthesized several (ONO)Ta pincer complexes with a Cp ligand, one of them being complex 85 (Fig. 40).69 The authors described several reactions in several papers,69,70 as well as activity in aqueous media with these complexes.71,72 The team synthesized analogs of these complexes with a similar ligand L29, resulting in ONO Ta pincer complex 86, among others.73 These complexes presented similar reactivity and resistance to decomposition in an aqueous environment as the complexes reported above (Fig. 41).74

Fig. 39 Synthesis of ONO Ta pincer complex 82.

Fig. 40 Synthesis of ONO Ta pincer complex 85.

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Organometallic Complexes of Group 5 Metals With Pincer and Noninnocent Ligands

Fig. 41 Synthesis of ONO Ta pincer complex 86.

The Bercaw group reported the synthesis of the OOO L30-based Ta pincer complex in 2008.65 The ligand consists of a furan ring at the central position and two substituted aryl groups at peripheral positions. The pincer complex 87–88 were synthesized by combining a solution of furan-2,5-diphenol L30-H2 and metal precursor TaCl2(CH3)3, which was mixed and left to stir at room temperature overnight (Fig. 42). The Bercaw group also reported the synthesis of the analog OSO L3-based Ta pincer complex.65 The ligand consists of a thiophene ring at the central position and two substituted aryl groups at peripheral positions. To synthesize its pincer complex 89, a solution of thiophene-2,5-diphenol L31-H2 and metal precursor TaCl2(CH3)3 was mixed and left to stir at room temperature overnight (Fig. 43).65

6.08.4.2.4

Trianionic ligands

The Parkin group reported the synthesis of a (CCC)Ta pincer complex where L32 was a trianionic terphenyl ligand.75 They reported two different methodologies for the synthesis of this complex (Fig. 44). The first one was the addition of a suspension of complex 90 to a solution of TaCl2Me3 in Et2O at −15  C. This produced complex 91, which reacts in an NMR tube with PMe3 vapor to produce the Ta complex 92 in a 63% yield. The second method was to add the same complex 90 to the Ta-PMe3 adduct, Ta(PMe3)2Me3Cl2, in C6D6. The authors used isotopic marking (2H) to probe the mechanism of the metalation of the pincer. They concluded it proceeds through a s-bond metathesis mechanism instead of an a-H abstraction. Other complexes reported by the Bercaw group were (OCO)Ta pincer complexes 93 and 95 (Fig. 45).76 Dimethyl complex 93a was synthesized by deprotonating the proligand with solid KBn in an ether solution and then adding the TaCl2Me3. On the other hand, the dichloride complex 95 can be synthesized in two steps: first, adding the TaCl2Me3 and the pincer ligand to form

Fig. 42 Synthesis of OCO pincer complexes 87–88.

Fig. 43 Synthesis of OSO pincer complex 89.

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Fig. 44 Synthesis of 91–92.

Fig. 45 Synthesis of OCO pincer complexes 93a, 93b, and 95.

coordination complex 94 and then heating up to 110  C. This same chemistry was explored for the synthesis of the Bn analog of the methyl complex 93b, which was reported in 2008 alongside other tantalum pincers reported here.65 The biphenol ligand seemed to coordinate to the Ta atom through the electron pairs on the oxygen atoms, followed by a s-bond metathesis step that produced the pincer. Nonetheless, the authors explain that the mechanism is more complicated than that, and they describe it in more detail in their papers.65,76 Kawaguchi’s group showed that OCO proligand L34dH was metalated on a diaryl methylene carbon under the right conditions.77 The synthesis described started from a tantalum trimethyl complex 96 that is left at 80  C for 3 days. This allowed the deprotonation of the ligand, followed by the generation of methane to produce complex 97. The authors found that, if treated with [Ph3C][B(C6F5)4], there was an abstraction of a hydride from the ligand, producing a carbene moiety (Fig. 46).2

Fig. 46 Synthesis of OCO pincer complex 97–98.

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Organometallic Complexes of Group 5 Metals With Pincer and Noninnocent Ligands

The authors also described the NMR characterization, the X-ray data, and the synthesis of the pincer ligand. Veige’s review on trianionic pincer complexes includes the complexes from Kawaguchi’s group and from Bercaw’s group.2 Heyduk’s group presented in several papers chemistry with redox-active ligand L35. One of these complexes, complex 99, was synthesized by the addition of two equivalents of nBuLi to a solution of L35dH3 to deprotonate it. Afterwards, the authors added TaCl2Me3 to metallate the ligand, yielding complex 99 (Fig. 47).78 Several analogs have also been synthesized and reported together with their redox chemistry.79,80 Several reviews have been written of the noninnocence of (ONO)Ta pincer complexes, which are added here for further information on the topic.81–83 Veige’s group synthesized several (ONO)Ta pincer complexes with a [ONO-CF3] ligand.66 One of them, complex 102, was obtained by adding the trichloride complex 100 to THF and letting it react for 3.5 h at room temperature. After obtaining the THF-bound complex 101, they added BnMgCl, which yielded complex 102 (Fig. 48).66 Various analogs and their structural characterization have been described by the authors, as well as the chemistry as ROMP catalysts.35 Lastly, several reports by Heyduk’s team on (NNN)Ta pincer complexes, have been made. The chemistry is centered mostly around complexes similar to isocyanide complex 103 and the noninnocent properties of this ligand.84–86

6.08.4.3

Noninnocent ligands

Tantalum organometallic complexes containing a-diimine ligands have been characterized structurally and their reactivity described throughout the literature. Some of the earliest complexes were made by Mashima and Tani with Cp ligands in 1999,87 and again in 200988 and 2011.89 Most of the noninnocent ligands reported for Ta have also been discussed above for V (Section 6.08.2.3) and Nb (Section 6.08.3.3). Since the chemistry of these is quite similar, the reader is directed to previous discussions and to the references for more details. The a-diimine ligand is known to coordinate its CdC double bond to the metal center. This coordination mode may change to suit the needs of incoming or leaving groups.88 Coordination complexes have also been reported, without any organometallic bonds.90 Work on catalysis is mostly reported as reactivity with radical reactions. Several of these papers have already been addressed in the Nb subsection on the ether cleavage,38 and the addition of trichloromethyl radical to styrene.36,89 By changing the substituents of the noninnocent ligand, the reactivity may be tuned. b diketiminate (BDI) complexes have also been synthesized. As previously mentioned in the Nb subsection, most of this work has been done on Nb metal complexes. These Ta complexes have been structurally characterized,42 and their reactivity such as ligand CdH activation and insertion of isocyanates has been briefly studied.91 Catecholate and hinokitiolate complexes of Ta have been reported and characterized structurally by Nakayama.92 The team also characterized the reactivity of these complexes in ROMP reactions with norbornene.

Fig. 47 Synthesis of (ONO)Ta pincer complex 99.

Fig. 48 Synthesis of (ONO)Ta complex 102.

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A very interesting example of an NHC ligand with noninnocence was studied by Wei and Cai in 2016.53 In this study, (NHC) TaCl5 catalyzes the production of cyclic carbonates from CO2 and propylene oxide. Several other (NHC)Ta complexes have been synthesized, but they have not been characterized on the noninnocence of the ligand.54,93 As mentioned in the previous sections, Romain, Bellemin-Laponnaz and Dagorne have written a thorough review on this topic.55

6.08.5

Conclusions

The field of pincer chemistry continues to expand on an almost daily basis with numerous significant contributions to addressing scientific challenges with impact for society at large. Yet, the pincer chemistry of the group 5 metals remains relatively untapped. Only a limited number of ligands have been reported for group 5, and little of the general inorganic and organic chemical transformations of these complexes are known. Given the ready accessibility of multiple oxidation states for group 5 metals, they are clearly underrepresented in catalytic applications. A renaissance of group 5 metal pincer complexes, chemistry, and catalytic applications may be expected in the near future.

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60. Gerber, L. C. H.; Watson, L. A.; Parkin, S.; Weng, W.; Foxman, B. M.; Ozerov, O. V. Bis(Methylidene) Complex of Tantalum Supported by a PNP Ligand. Organometallics 2007, 26 (20), 4866–4868. https://doi.org/10.1021/om700715s. 61. Helgert, T. R.; Webster, C. E.; Hollis, T. K.; Valle, H. U.; Hillesheim, P.; Oliver, A. G. Free Methylidyne? CCC-NHC Tantalum Bis(Imido) Reactivity: Protonation, Rearrangement to a Mixed Unsymmetrical CCC-N-Heterocyclic Carbene/N-Heterocyclic Dicarbene (CCC-NHC/NHDC) Pincer Tantalum Bis(Imido) Complex. Inorg. Chim. Acta 2018, 469, 164–172. https://doi.org/10.1016/j.ica.2017.08.050. 62. Denny, J. A.; Lang, G. M.; Hollis, T. K. CCC-NHC Pincer Complexes: Synthesis, Applications, and Catalysis; Elsevier Inc., 2018. https://doi.org/10.1016/B978-0-12-8129319.00012-8 63. Srivastava, R.; Moneuse, R.; Petit, J.; Pavard, P. A.; Dardun, V.; Rivat, M.; Schiltz, P.; Solari, M.; Jeanneau, E.; Veyre, L.; Thieuleux, C.; Quadrelli, E. A.; Camp, C. Early/Late Heterobimetallic Tantalum/Rhodium Species Assembled through a Novel Bifunctional NHC-OH Ligand. Chem. Eur. J. 2018, 24 (17), 4361–4370. https://doi.org/10.1002/ chem.201705507. 64. Spencer, L. P.; Beddie, C.; Hall, M. B.; Fryzuk, M. D. Synthesis, Reactivity, and DFT Studies of Tantalum Complexes Incorporating Diamido-N-Heterocyclic Carbene Ligands. Facile Endocyclic C-H Bond Activation. J. Am. Chem. Soc. 2006, 128 (38), 12531–12543. https://doi.org/10.1021/ja063282x. 65. Agapie, T.; Day, M. W.; Bercaw, J. E. Synthesis and Reactivity of Tantalum Complexes Supported by Bidentate X2 and Tridentate LX2 Ligands with Two Phenolates Linked to Pyridine, Thiophene, Furan, and Benzene Connectors: Mechanistic Studies of the Formation of a Tantalum Benzylidene and Insert. Organometallics 2008, 27 (23), 6123–6142. https://doi.org/10.1021/om8002653. 66. Venkatramani, S.; Pascualini, M. E.; Ghiviriga, I.; Abboud, K. A.; Veige, A. S. Synthesis and Characterization of Trianionic Pincer-Type Complexes of Tantalum(V) Including Solid (X-Ray) and Solution (NMR) State Assignment of an Intraligand N-H - F Hydrogen Bonding Interaction. Polyhedron 2013, 64, 377–387. https://doi.org/10.1016/j. poly.2013.07.004. 67. Fandos, R.; Otero, A.; Rodríguez, A. M.; Suizo, S. Tantalum Complexes Supported by Asymmetric [ONO] Ligands. The Role of Weakly Coordinating Anions on Their Reactivity. Organometallics 2012, 31 (5), 1849–1856. https://doi.org/10.1021/om201199p. 68. Štarha, P.; Trávnícek, Z.; Dvorˇák, Z. A Cytotoxic Tantalum(v) Half-Sandwich Complex: A New Challenge for Metal-Based Anticancer Agents. Chem. Commun. 2018, 54 (68), 9533–9536. https://doi.org/10.1039/c8cc05223a. 69. Fandos, R.; López-Solera, I.; Otero, A.; Rodríguez, A.; Ruiz, M. J.; Terreros, P. Synthesis and Reactivity of Monocyclopentadienyl Tantalum Complexes with Pincer Dialkoxide Ligands. Organometallics 2004, 23 (21), 5030–5036. https://doi.org/10.1021/om049582. 70. Fandos, R.; Hernández, C.; Otero, A.; Rodríguez, A.; Ruiz, M. J. C-O Bond Activation by a Tantalum-Bonded Pincer Ligand - Ligand Modification Effects on the Selectivity of C-H Bond Cleavage Processes. Eur. J. Inorg. Chem. 2014, 2014 (36), 6196–6204. https://doi.org/10.1002/ejic.201402953. 71. Conde, A.; Fandos, R.; Otero, A.; Rodríguez, A. Water-Soluble Organometallic Complexes of Tantalum: Can Alkyl and Aqua Ligands Coexist in its Coordination Sphere?Organometallics 2007, 26 (7), 1568–1570. https://doi.org/10.1021/om0701165. 72. Conde, A.; Cruzado, B.; Fandos, R.; Otero, A.; Rodríguez, A. M.; Ruiz, M. J. Effect of Anions on the Stability and Solubility of Tantalum Complexes in Water. Organometallics 2015, 34 (1), 127–132. https://doi.org/10.1021/om501014v. 73. Conde, A.; Fandos, R.; Otero, A.; Rodríguez, A. Oxo and Hydroxo Organometallic Complexes of Tantalum: Soluble Molecular Models of Tantalum Oxide. Organometallics 2008, 27 (23), 6090–6095. https://doi.org/10.1021/om8006932. 74. Conde, A.; Fandos, R.; Otero, A.; Rodríguez, A. Synthesis and Reactivity of Tantalum Complexes Supported by the Pincer Ligand 2, 6-Pyridinedicarboxylate. Preparation of an Unprecedented Water-Soluble Iminoacyl Complex. Organometallics 2009, 28 (18), 5505–5513. https://doi.org/10.1021/om900643c. 75. Sattler, A.; Parkin, G. A New Class of Transition Metal Pincer Ligand: Tantalum Complexes That Feature a [ CCC ] X 3-Donor Array Derived from a Terphenyl Ligand. J. Am. Chem. Soc. 2012, 134 (4), 2355–2366. https://doi.org/10.1021/ja210404x. 76. Agapie, T.; Bercaw, J. E. Cyclometalated Tantalum Diphenolate Pincer Complexes: Intramolecular C-H/M-CH3 s-Bond Metathesis May be Faster than O-H/M-CH 3 Protonolysis. Organometallics 2007, 26 (12), 2957–2959. https://doi.org/10.1021/om700284c. 77. Watanabe, T.; Matsuo, T.; Kawaguchi, H. A Tantalum(V) Carbene Complex: Formation of a Carbene − Bis(Phenoxide) Ligand by Sequential Proton and Hydride Abstraction. Inorg. Chem. 2006, 45 (17), 6580–6582. https://doi.org/10.1021/ic0608492. 78. Zarkesh, R. A.; Ziller, J. W.; Heyduk, A. F. Four-Electron Oxidative Formation of Aryl Diazenes Using a Tantalum Redox-Active Ligand Complex. Angew. Chem. Int. Ed. 2008, 47 (25), 4715–4718. https://doi.org/10.1002/anie.200800812. 79. Zarkesh, R. A.; Heyduk, A. F. Reactivity of Diazoalkanes with Tantalum(V) Complexes of a Tridentate Amido-Bis(Phenolate) Ligand. Organometallics 2009, 28 (23), 6629–6631. https://doi.org/10.1021/om900701n. 80. Zarkesh, R. A.; Heyduk, A. F. Reactivity of Organometallic Tantalum Complexes Containing a Bis(Phenoxy)Amide (ONO)3− Ligand with Aryl Azides and 1,2-Diphenylhydrazine. Organometallics 2011, 30 (18), 4890–4898. https://doi.org/10.1021/om2004332. 81. Luca, O. R.; Crabtree, R. H. Redox-Active Ligands in Catalysis. Chem. Soc. Rev. 2013, 42 (4), 1440–1459. https://doi.org/10.1039/c2cs35228a. 82. Munhá, R. F.; Zarkesh, R. A.; Heyduk, A. F. Group Transfer Reactions of d0 Transition Metal Complexes: Redox-Active Ligands Provide a Mechanism for Expanded Reactivity. Dalton Trans. 2013, 42 (2), 3751–3766. https://doi.org/10.1039/c2dt32063k. 83. Broere, D. L. J.; Plessius, R.; Van Der Vlugt, J. I. New Avenues for Ligand-Mediated Processes-Expanding Metal Reactivity by the Use of Redox-Active Catechol, o-Aminophenol and o-Phenylenediamine Ligands. Chem. Soc. Rev. 2015, 44. https://doi.org/10.1039/c5cs00161g. 84. Munhá, R. F.; Zarkesh, R. A.; Heyduk, A. F. Tuning the Electronic and Steric Parameters of a Redox-Active Tris(Amido) Ligand. Inorg. Chem. 2013, 52 (19), 11244–11255. https://doi.org/10.1021/ic401496w. 85. Nguyen, A. I.; Blackmore, K. J.; Carter, S. M.; Zarkesh, R. A.; Heyduk, A. F. One-and Two-Electron Reactivity of a Tantalum(V) Complex with a Redox-Active Tris(Amido) Ligand. J. Am. Chem. Soc. 2009, 131 (9), 3307–3316. https://doi.org/10.1021/ja808542j. 86. Heyduk, A. F.; Zarkesh, R. A.; Nguyen, A. I. Designing Catalysts for Nitrene Transfer Using Early Transition Metals and Redox-Active Ligands. Inorg. Chem. 2011, 50 (20), 9849–9863. https://doi.org/10.1021/ic200911b. 87. Mashima, K.; Matsuo, Y.; Tani, K. Unique Complexation of 1,4-Diaza-1,3-Butadiene Ligand on Half-Metallocene Fragments of Niobium and Tantalum. Organometallics 1999, 18 (8), 1471–1481. https://doi.org/10.1021/om981003b. 88. Tsurugi, H.; Ohno, T.; Kanayama, T.; Arteaga-Müller, R. A.; Mashima, K. Tantalum-Benzylidene Complexes Supported by C5Me5 and Diazadiene Ligands: Synthesis, Kinetic Analysis of the Formation, and Reactive Studies. Organometallics 2009, 28 (6), 1950–1960. https://doi.org/10.1021/om8012019. 89. Tsurugi, H.; Saito, T.; Tanahashi, H.; Arnold, J.; Mashima, K. Carbon Radical Generation by d0 Tantalum Complexes with a-Diimine Ligands through Ligand-Centered Redox Processes. J. Am. Chem. Soc. 2011, 133 (46), 18673–18683. https://doi.org/10.1021/ja204665s. 90. Daff, P. J.; Etienne, M.; Donnadieu, B.; Knottenbelt, S. Z.; McGrady, J. E. Stable Formally Zerovalent and Diamagnetic Monovalent Niobium and Tantalum Complexes Based on Diazadiene Ligands. J. Am. Chem. Soc. 2002, 124 (15), 3818–3819. https://doi.org/10.1021/ja017303t. 91. Kriegel, B. M.; Kaltsoyannis, N.; Chatterjee, R.; Bergman, R. G.; Arnold, J. Synthesis and Redox Chemistry of a Tantalum Alkylidene Complex Bearing a Metallaimidazole Ring. Organometallics 2017, 36 (18), 3520–3529. https://doi.org/10.1021/acs.organomet.7b00448. 92. Nakayama, Y.; Maeda, N.; Yasuda, H.; Shiono, T. Ring-Opening Metathesis Polymerization of Norbornene Catalyzed by Tantalum and Niobium Complexes with Chelating O-Donor Ligands. Polym. Int. 2008, 57 (7), 950–956. https://doi.org/10.1002/pi.2432. 93. Petrov, P. A.; Sukhikh, T. S.; Sokolov, M. N. NHC Adducts of Tantalum Amidohalides: The First Example of NHC Abnormally Coordinated to an Early Transition Metal. Dalton Trans. 2017, 46 (15), 4902–4906. https://doi.org/10.1039/c7dt00748e.

6.09

Organometallic Pincer Complexes With Group 6 Metals

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

6.09.1 Introduction 6.09.2 Complexes with symmetric pincer ligands 6.09.2.1 NCN ligands 6.09.2.2 OCO ligands 6.09.2.3 ONO ligands 6.09.2.4 CNC and CSC ligands 6.09.2.5 NNN ligands 6.09.2.6 SNS ligands 6.09.2.7 PNP ligands 6.09.2.8 PCP ligands 6.09.2.9 P-arene-P ligands 6.09.2.10 PPP, PSP, SPS, and SSS ligands 6.09.3 Complexes with asymmetric pincer ligands 6.09.4 Conclusion Acknowledgment References

649 649 649 652 659 664 665 671 673 680 682 685 688 690 690 691

Abbreviations BArF−24 bipy Bn COD Cp Cp CV DBU DCM DDQ DFT EPR ESI-MS IBAO Ind IR LDA MAO MLC nbd NMR Np PCET phen py ROAMP ROMP TATA terpy THF TMSO TON UV-vis XRD

648

Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate Bipyridine Benzyl 1,5-Cyclooctadiene Cyclopentadienyl Pentamethylcyclopentadienyl Cyclic voltammetry 1,8-Diazabicyclo[5.4.0]undec-7-ene Dichloromethane 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone Density functional theory Electron paramagnetic resonance Electrospray ionization mass spectrometry Isobutylaluminoxane Indenyl Infrared Lithium diisopropylamide Methylaluminoxane Metal-ligand cooperativity Norbornadiene Nuclear magnetic resonance Neopentyl Proton-coupled electron transfer Phenanthroline Pyridine Ring-opening alkyne metathesis polymerization Ring opening metathesis polymerization Triazatriangulenium Terpyridine Tetrahydrofuran Tetramethylene sulfoxide Turnover number Ultraviolet-visible X-ray diffraction

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00081-0

Organometallic Pincer Complexes With Group 6 Metals

6.09.1

649

Introduction

Pincer ligands are defined as tridentate ligands that preferentially occupy three meridional coordination sites in coordination complexes. Their high versatility and ubiquity in modern organometallic chemistry stems from the myriad of ways in which the three metal donor substituents and the chelating framework can be modified to tune the electronic and steric properties of metal complexes. The discovery of pincer ligands is credited to Shaw and coworkers in 1976,1 but the name “pincer” as it refers to this class of ligands was coined more recently by van Koten in 1989.2 Since then, pincer ligands and their complexes have been reviewed extensively,3–32 and the field has grown rapidly, especially since the last edition of COMC was published in 2006. The aim of this chapter is to review the synthesis and structures of group 6 organometallic complexes containing pincer ligands. This topic has not been covered in previous editions of COMC, so we have endeavored to review all organometallic group 6 complexes reported to date that fall into this category. The following chapter sections and subsections are organized according to symmetry of flanking donor groups (symmetric or asymmetric) and are categorized according to the three donor atoms that bind to the metal. To limit the scope of the chapter, we have focused our discussion on complexes that meet the rigorous definition of organometallic complexes (i.e., those containing metal–carbon bonds). Discussions of non-organometallic complexes are also included when relevant to organometallic transformations and catalysis. Publications captured in our literature review on other non-organometallic pincer complexes or tridentate ligands that show only fac-type binding with group 6 metals, but could adopt planar conformations under the right conditions, are cited briefly for interested readers. Applications such as catalysis are mentioned when relevant, but not discussed in detail given that these topics are reviewed elsewhere in COMC4.

6.09.2

Complexes with symmetric pincer ligands

6.09.2.1

NCN ligands

Chart 1 provides a summary of NCN pincer ligands investigated with group 6 metals. An early example of a structurally characterized NCN complex with a group 6 metal was described by van Koten and coworkers in 1999 with NCN1.33 Treating WCl4(NPh)(Et2O) with in situ prepared Zn(NCN1)2 results in transmetalation of a single NCN1 and deprotonation of one of the NMe arms (Scheme 1). Subsequent addition of 2 equiv. of LiCH2SiMe3 in THF removes a second proton from the NCH2 group to yield an alkylidene complex. The alkylidene complex is not stable, decomposing over the course of several hours, but was persistent enough to be characterized by 1H NMR spectroscopy. Attempts to make bis(imidoaryl) and bis(imidoalkyl) Mo complexes with fully ligated NCN1 were unsuccessful, although several partially ligated complexes were isolated and structurally characterized.34

Chart 1

650

Organometallic Pincer Complexes With Group 6 Metals

Scheme 1

Mu, Gao, and coworkers reported CrII and CrIII chloride complexes with NCN2, NCN3, and NCN4.35 These ligands are similar to NCN1 but have flanking C]NAr arms, where Ar ¼ 2,6-Me2C6H3, 2,6-Et2C6H3, and 2,6-iPr2C6H3, respectively (Scheme 2).35 Treating CrCl2(THF)3 with Li(NCN) salts of each ligand yielded different complexes that varied according to the size of the aryl substituent attached to nitrogen. NCN2 and NCN3 formed trimetallic complexes with two (NCN)Cr fragments bridged together by a square planar [CrCl4]2− unit, whereas bulkier NCN4 yielded a dimetallic complex with two chlorides bridged to a THF-solvated Li cation. In contrast, the same reaction with CrCl3(THF)3 yielded octahedral (NCN)CrCl2(THF) complexes with all three ligands. The CrIII complexes are effective catalysts for isoprene polymerization, whereas the CrII complexes were unreactive when tested under similar conditions.

Scheme 2

van Koten and coworkers reported the synthesis of a Cr complex with the chiral ligand NCN5 (Scheme 3).36 (NCN5)CrCl2(py) was prepared by treating a bimetallic (NCN5)AuI diphosphine complex with 2 equiv. of CrCl3(THF)3, followed by workup with pyridine. Transmetalation proceeds with elimination of the corresponding diphosphine-bridged AuCl dimer. (NCN5)CrCl2(py) was tested as a precatalyst for ethylene polymerization but was not active when using methylaluminoxane (MAO) as the activator in toluene.36

Scheme 3

Organometallic Pincer Complexes With Group 6 Metals

651

The chemistry of trianionic NCN6 with group 6 has focused almost exclusively on Cr.15 Veige and coworkers described the synthesis of a series of Cr complexes in four different oxidation states with the trianionic pincer NCN6 (Scheme 4).37 A dimeric lithium salt of NCN6 was prepared with deprotonated amido arms. This salt was then treated with 2 equiv. of CrMeCl2(THF)3 to yield (NCN6)CrIII(THF)3 with elimination of CH4 and deprotonation of the remaining ipso carbon on the pincer backbone. Conducting this same reaction in Et2O yielded a mixture of products with Cr in three different oxidation states. The desired (NCN6)CrIII(THF)3 was isolated along with CrII and CrIV disproportionation products (NCN6)CrIVMe(THF) and (NHCN6) CrII(THF)2 in lower yields. The latter two products were isolated by extraction with pentane, and the square pyramidal structure of (NCN6)CrIVMe(THF) was confirmed by single-crystal XRD. (NHCN6)CrII(THF)2 was more difficult to isolate from the mixture, but it could be prepared in higher yield by mixing CrCl2 with NCN6 in THF. Protonation of the amido arm in (NHCN6) CrII(THF)2 was confirmed by IR spectroscopy (NdHstretch ¼ 3274 cm−1) and D2O quenching studies. (NHCN6)CrII(THF)2 catalyzes the interconversion of 1-alkenes to 2-alkenes.37

Scheme 4

Veige and coworkers showed in the same study that (NCN6)CrIII(THF)3 can be oxidized with styrene oxide to form (NCN6) Cr O(THF) (Scheme 5).37 The CrV oxidation state was assigned based on Evans method magnetic susceptibility and EPR data. V

Scheme 5

Thagfi and Lavoie described the synthesis of (NCN7)CrCl3 and (CNC8)CrCl3.38 The two ligands differ in that NCN7 has a propylene backbone whereas NCN8 has a phenylene backbone. Both complexes were prepared by transmetalation with the corresponding CuI carbene adduct (Scheme 6). The octahedral structure of (NCN7)CrCl3 was confirmed by single-crystal XRD, whereas spectroscopic data for (NCN8)CrCl3 suggests that NCN8 binds in only a bidentate fashion with k2-N,C. The difference in preferred coordination mode was attributed in part to different bond angles imposed by the six- and five-membered rings in NCN7 and NCN8, respectively. Despite the structural differences, both CrIII complex showed similar rates for ethylene polymerization catalysis and had relatively short catalytic lifetimes.38

652

Organometallic Pincer Complexes With Group 6 Metals

Scheme 6

The ligands NCN9 and NCN10 formally contain two lone pairs on the central carbon atom instead of one like the carbenes NCN7 and NCN8. Upon metalation, one lone pair coordinates to the metal to form a MdC s bond, whereas the other lone pair is available for either MdC p bonding or ligand-centered redox transformations. NCN9 forms meridionally-coordinated complexes with some metals, but the only group 6 complexes to be reported are fac-(NCN9)Cr(CO)3 and the dimetallic carbon- and CO-bridged complex [Mo(CO)3]2[N(m-C)N9](m-CO).39 Carbodicarbene NCN10 was recently shown to form Cr pincer complexes that display redox non-innocence.40 England, Ye, and coworkers prepared [(NCN10)2Cr](OTf )2 by treating Cr(OTf )2(MeCN)2 with NCN10 in THF. The CV of [(NCN10)2Cr](OTf )2 showed three fully reversible redox events assigned to oxidation of CrII to CrIII and stepwise oxidation of the two NCN ligands to give the redox series [(NCN10)2Cr]n+ where n ¼ 2− 5. [(NCN10)2Cr]n+ with n ¼ 2− 4 were prepared by chemical means and structurally characterized (Scheme 7), whereas [(NCN10)2Cr]5+ was prepared by bulk electrolysis and characterized by UV-vis spectroscopy without isolation.

Scheme 7

6.09.2.2

OCO ligands

OCO ligands used to prepare group 6 complexes are shown in Chart 2. All of the OCO ligands reported are dianionic or trianionic and have been used primarily to stabilize high valent metals, especially those with metal-ligand multiple bonds.15

Organometallic Pincer Complexes With Group 6 Metals

653

Chart 2

In one of the earliest examples, Chisholm and coworkers showed how H3(OCO1) undergoes protonolysis and CdH oxidative addition across the metal-metal multiple bond in dinuclear W2(NMe2)6 (Scheme 8).41,42 CdH addition results in a new complex with bridging hydride and dimethylamido ligands, coinciding with a decrease in WdW bond order from three to two. Heating this complex to 60  C under vacuum liberated an equivalent of HNMe2 along with reductive elimination of the pincer CdH bond. This reaction was shown to be reversible and could be cycled by addition and removal of HNMe2. It was proposed that the CdH oxidative addition was induced by coordination of the HNMe2 rather than a CdH deprotonation mechanism involving the amide. Consistent with this hypothesis, addition of excess pyridine induced the same oxidative addition reaction to form the pincer complex with OCO1 (Scheme 8). Similar reactions were said to occur with PMe3 based on solution 31P NMR studies.42

Scheme 8

Much of what is known with trianionic OCO ligands with group 6 metals has been reported with OCO2 by Veige and coworkers. Starting with Mo in 2008, Mo(NMe2)4 was treated with H3(OCO2) to yield (OCO2)Mo(NMe2)(HNMe2)2 (Scheme 9).43 Protonolysis and displacement of the amide with 2,6-lutidene ∙ HCl yields (OCO2)Mo(HNMe2)2Cl with chloride trans to the ModC bond. Treating this complex with NaN3 in DMF yielded a mixture of products from which (OCHO2)Mo^N(NMe2)(DMF) could be isolated in low yield with protonation of the ModC aryl bond. In contrast, treating (OCO2)Mo(NMe2)(HNMe2)2 directly with NaN3 in DMF led to the Na bridged nitride dimer [(OCO2)MoN(NMe2)Na(DMF)]2 in excellent yield (Scheme 9).43

654

Organometallic Pincer Complexes With Group 6 Metals

Scheme 9

In the quest to isolate the first alkylidyne complex supported by OCO2 (which was later achieved; see below), a series of tungsten alkylidene complexes were isolated and characterized. Treating H3(OCO2) with (tBuO)3W^CtBu in the presence of ArOH (where Ar]2,6-iPr2C6H3) or directly with (ArO)2NpW^CtBu yielded the corresponding alkylidene complex (OCO2)W]CHtBu(OAr) (Scheme 10).44 Attempting the same reaction with Np3W^CtBu (i.e. without an alkoxy ligand) yielded the dimeric alkylidene complex containing two (OCO2)WCHtBu units bridged by a third OCO2 ligand via WdO bonds (Scheme 11).44

Scheme 10

Scheme 11

Organometallic Pincer Complexes With Group 6 Metals

655

As mentioned above, the desired tungsten alkylidynes targeted in early studies were later prepared with OCO2. Treating H3(OCO2) with (tBuO)3W^CtBu in THF at 25  C resulted in protonolysis of the two OdH bonds and bidentate coordination of the ligand. Deprotonation of the ipso ligand carbon with Ph3P]CH2 yielded the alkylidyne-containing salt [MePPh3][(OCO2) W^CtBu(OtBu)] (Scheme 12).45 Abstracting the alkoxy ligand with MeOTf in the presence of Et2O or THF resulted in the neutral Lewis base adducts (OCO2)W^CtBu(Et2O) and (OCO2)W^CtBu(THF)2 (Scheme 13). (OCO2)W^CtBu(THF)2 was shown to be an active catalyst for the polymerization of acetylenes.45

Scheme 12

Scheme 13

To understand the mechanism of acetylene polymerization, stoichiometric reactions of (OCO2)W^CtBu(THF)2 with acetylenes were conducted. Addition of 2 equiv. of phenylacetylene to (OCO2)W^CtBu(THF)2 revealed reductive migratory alkylidene insertion into the W-arene bond and Z2 coordination of HC^CR where R ¼ tBu or Ph.46 As shown in Scheme 14, a 2:1 mixture of products was formed with HC^CPh with the tBu substituent being favored on the alkylidene over Ph. The two alkylidene products could be separated by exploiting differences in their solubility in pentane. Heating the isolated tBu alkylidene derivative in toluene to 75  C in the presence of excess MeC^CPh resulted in insertion into the metallopropene to form two metallocyclopentadiene isomers (Scheme 15).46

Scheme 14

656

Organometallic Pincer Complexes With Group 6 Metals

Scheme 15

Treating (OCO2)W^CtBu(THF)2 with 1-phenyl-propyne instead of phenylacetylene also yielded a mixture of products, albeit with the ratio favoring the Ph substituent on the alkylidene instead of tBu (Scheme 14).46 To avoid the mixtures generated with phenylacetylene and 1-phenyl-propyne, (OCO2)W^CtBu(THF)2 was treated with excess HCCtBu with the same tBu substituent as the starting alkylidyne.47 This yielded a single tethered alkylidene product in quantitative yield, and the resulting complex was shown to be a highly effective catalyst for generating cyclic polymers from alkynes such as phenylacetylene,47,48 propyne,49 4-ethynylanisole,50 and 4-methyl-1-pentyne.51 Similar insertion reactivity was observed when (OCO2)W^CtBu(THF)2 was treated with ethylene (Scheme 14),52 and (OCO2)W^CtBu(THF)2 itself is an effective polymerization catalyst for generating cis and syndiotactic cyclic polynorbornene.52 Reminiscent of its reactivity with alkenes and alkynes, (OCO2)W^CtBu(THF)2 reacts with CO2 via addition across the W^C bond to generate a tethered W-oxo alkylidene complex, as shown in Scheme 14.53 This complex has proven to be an effective catalyst for generating cyclic polynorbornene via ring expansion polymerization. Further heating of the tethered W-oxo alkylidene complex results in loss of the bridging CO to form the dimeric complex shown in Scheme 14.53 The alkylidyne and alkylidene chemistry developed for tungsten OCO2 complexes has also been demonstrated with molybdenum, albeit with slightly different substituents, as summarized in Scheme 16.54

Scheme 16

The coordination chemistry of Cr complexes with OCO2 were explored in parallel with Mo and W, but reactivity studies reported thus far have centered on metal oxo chemistry. The CrIII complex (OCO2)Cr(THF)3 was prepared by treating MeCrCl2(THF)3 with the a dipotassium salt of OCO2 generated by treating H3(OCO2) with two equiv. of KH in THF (Scheme 17).55 It was shown that (OCO2)Cr(THF)3 can be oxidized to CrV with O2 to form the terminal oxo complex (OCO2)CrO(THF).55 Mixing (OCO2)Cr(THF)3 and (OCO2)CrO(THF) in a one-to-one ratio in toluene yielded a CrIV m-oxo complex that could be isolated and structurally characterized by single-crystal XRD (Scheme 18).56

Organometallic Pincer Complexes With Group 6 Metals

657

Scheme 17

Scheme 18

(OCO2)Cr(THF)3 was shown to catalyze the oxidation of PPh3 to OPPh3 in the presence of O2 with a TON of 195.55 Separate studies showed that addition of PPh3 to (OCO2)CrO(THF) in THF leads to oxygen atom transfer and regeneration of (OCO2)Cr(THF)3 (Scheme 17).57 Addition of 2 equiv. of OPPh3 to (OCO2)Cr(THF)3 in toluene displaces THF to form (OCO2)Cr(OPPh3)2.55 Single-crystal XRD analysis of (OCO2)Cr(OPPh3)2 revealed a distorted square pyramidal structure with OPPh3 occupying trans coordination sites above and below the OCO2 plane. As with (OCO2)Cr(THF)3, (OCO2)Cr(OPPh3)2 also catalyzes oxidation of PPh3 with 1 atm O2 and air.55 The reactivity of (OCO2)CrO(THF) towards H2C]PPh3 was explored to investigate its effect on oxygen atom transfer. Addition of H2C]PPh3 in toluene yielded Cr(OCO2)O(CH2PPh3) (Scheme 19).57 CV data revealed that addition of H2C]PPh3 shifted the onset of reduction in CH2Cl2 to −1.97 V compared to 0.06 V in the parent complex, showing how the ylide stabilizes CrV. Consistent with these data, Cr(OCO2)O(CH2PPh3) is not competent for oxygen atom transfer with PPh3 or PMe3.57

Scheme 19

Hohloch and coworkers explored the chemistry of the bis-phenolate-NHC ligand OCO3 with high valent Mo(VI). A series of dioxo, oxo-imido, and bis-amido complexes were prepared by deprotonating the ligand with NEt3 or LDA in the presence of the corresponding dichloride starting material (Scheme 20).58 The structurally-characterized (OCO3)MoO2, (OCO3)MoO(NtBu), and (OCO3)Mo(NR)2 complexes (where R ¼ tBu or mesityl) are rare examples of five-coordinate MoVI complexes. Hohloch and coworkers demonstrated how a similar MoVI bis-imido complex could be prepared in the same way with the mesoionic carbene pincer OCO4 (Scheme 21).59

658

Organometallic Pincer Complexes With Group 6 Metals

Scheme 20

Scheme 21

The basicity of the imido groups in (OCO3)Mo(NtBu)2 were probed by addition of different acids.58 Addition of [HNEt3]Cl in benzene resulted in imido protonation and chloride coordination to form six-coordinate (OCO3)Mo(HNtBu)(NtBu)Cl (Scheme 22). This same product was isolated when NEt3 was used as the base instead of LDA in the reaction of fully protonated OCO3 with Mo(NtBu)2Cl2(DME). Addition of HL ¼ 2,4,6-trimethylphenol (pKa ¼ 9) and benzimidazole (pKa ¼ 16) to (OCO3)Mo(NtBu)2 yielded mixtures of products, as shown in Scheme 22 with 2,4,6-trimethylphenol, whereas carbazole (pKa ¼ 20) did not react.

Scheme 22

Organometallic Pincer Complexes With Group 6 Metals

659

In 2019, Buchmeiser and coworkers described an example of an Mo alkylidyne with an NHC-derived OCO pincer ligand.60 Addition of Li2(OCO5) to Mo^C-(p-OMe-C6H4)[OCMe(CF3)2]3(DME) in THF afforded (OCO5)Mo^C-(p-OMe-C6H4)[OCMe (CF3)2] (Scheme 23), which was isolated as a red solid in 75% yield by crystallization from Et2O.

Scheme 23

6.09.2.3

ONO ligands

As described for OCO ligands in Section 6.09.2.2, ONO pincer ligands used to prepare group 6 complexes are typically dianionic or trianionic and have also been used to stabilize group 6 metals in higher oxidation states (Chart 3).15 Examples of ONO complexes with group 6 metals go back as early as 1983 in a report of ONO-supported MoO2 complexes provided by Berg and Holm.61 Most organometallic ONO complexes known with group 6 metals contain alkylidene or alkylidyne ligands because of significant interest in understanding how central amido donors enhance the nucleophilicity of the a-carbon in M]C and M^C bonds. A prevailing concept in accounting for the increased nucleophilicity in these alkylidene or alkylidyne complexes is the inorganic enamine effect, as described by Veige and coworkers in reactivity studies with ONO1 complexes.62 It is known that placing an amine next to a double bond can lead to resonance delocalization with the N lone pair to enhance nucleophilicity at the b-carbon of the double bond. By analogy, the inorganic enamine effect is a similar type of resonance delocalization that enhances the nucleophilicity of an a-carbon in a metal-carbon multiple bond when a metal contains an amido ligand, as shown in Scheme 24.

Chart 3

Scheme 24 Adapted with permission from O’Reilly, M. E.; Ghiviriga, I.; Abboud, K. A.; Veige, A. S. A new ONO3− trianionic pincer-type ligand for generating highly nucleophilic metal–carbon multiple Bonds. J. Am. Chem. Soc. 2012, 134, 11185–11195. Copyright 2012 American Chemical Society.

660

Organometallic Pincer Complexes With Group 6 Metals

In 2012, Veige and coworkers showed that treating (tBuO)3W^CEt with H3(ONO1) in C6D6 yields (ONO1)W]CHEt(OtBu), which could then be deprotonated with Ph3P]CH2 to form the corresponding alkylidyne (Scheme 25).62 Similar reactivity was achieved in a separate study starting with (tBuO)3W^CtBu to form (ONO1)W]CHtBu(OtBu) and [(ONO1)W^CtBu(OtBu)] [MePPh3].63

Scheme 25

The alkylidyne carbon in [(ONO1)W^CEt(OtBu)][MePPh3] is nucleophilic and undergoes alkylation with MeOTf to yield the alkylidene (ONO1)W]CMeEt(OtBu) (Scheme 26).62 Performing the same reaction with Me3SiOTf instead yields (ONO1)W] CHEt(OSiMe3) with expulsion of isobutylene. (ONO1)W]CHEt(OSiMe3) is unstable at room temperature and decomposes to give a mixture of unidentified complexes.

Scheme 26

Extrusion of isobutylene from a tert-butoxide ligand also occurs when the alkylidene (ONO1)W]CHEt(OtBu) is treated with Me3SiOTf to afford the terminal oxo complex (ONO1)WnPr(O).62 It was suspected that Me3SiOTf acted as a catalyst in this reaction given that it was not present in the product. This was indeed confirmed in subsequent catalytic reactions with 1 mol% of Me3SiOTf and other Lewis acids such as MeOTf and B(C6F5)3 (Scheme 27).

Scheme 27

The tert-butyl derivative [(ONO1)W^CtBu(OtBu)][MePPh3] can be prepared as described above for [(ONO1)W^CEt(OtBu)] [MePPh3] (Scheme 25).63 Treating [(ONO1)W^CtBu(OtBu)][MePPh3] with excess MeOTf yields an impure mixture containing [(ONO1)W^CtBu(OTf )][MePPh3], which was confirmed by NMR analysis. Addition of Et2O to this mixture followed by crystallization resulted in blue crystals of (ONO1)W^CtBu(OEt2) along with smaller amounts of co-crystallized (MePPh3)OTf (Scheme 28).63 (ONO1)W^CtBu(OEt2) reacts with a variety of alkynes to form tungsten acyclobutadienes (Scheme 29).

Organometallic Pincer Complexes With Group 6 Metals

661

Scheme 28

Scheme 29

In an effort to overcome the difficulties isolating (ONO1)W^CtBu(OEt2) completely free of (MePPh3)OTf, it was shown that treating (ONO1)W^CtBu(OEt2) with a small amount of THF yields (ONO1)W^CtBu(THF)2, which is easier to isolate in high purity. This THF complex undergoes Wittig-like reactions to yield terminal oxo ligands when treated with a range of different carbonyl substrates, as shown in Scheme 30.64 (ONO1)W^CtBu(THF)2 also reacts with CO2 to yield an oxo ketene ligand and a terminal metal oxo.65 The reaction occurs via addition of CO2 across the W^C bond and again demonstrated the nucleophilicity of the alkylidyne carbon, this time towards the electrophilic carbon in CO2. (ONO1)W^CtBu(THF)2 was also shown to react reversibly with ethylene in THF to form a metallocyclobutene,52 which led to it being tested in side-by-side studies with (ONO1) W]CHtBu(OtBu) and (OCO2)W^CtBu(THF)2 discussed in Section 6.09.2.2 for ring-expansion metathesis polymerization of norbornene.52

Scheme 30

662

Organometallic Pincer Complexes With Group 6 Metals

Veige and coworkers recently described tungsten complexes containing a derivative of ONO1 where the two biaryl units were fused to form a rigid carbazole framework.66 Though unsuccessful, the initial aim of these studies was to enhance the inorganic enamine effect. Neutral alkylidene and anionic alkylidyne complexes like those prepared with ONO1 were described (Scheme 31). The only notable reactivity difference observed in comparison to complexes with ONO1 was that the anionic alkylidyne shown in Scheme 31 could not be transformed into the neutral alkylidyne via treatment with MeOTf.

Scheme 31

In parallel with their studies with ONO1, Veige and coworkers showed that the pyrrolide-centered trianionic pincer ONO2 could also be used to prepare tungsten alkylidene and alkylidyne complexes in the same way as described with ONO1 starting from (tBuO)3W^CtBu (Scheme 32).67

Scheme 32

Fischer and coworkers reported the synthesis of Mo complexes containing the pyridyl-centered, bis-phenolate ligand ONO3.68 H2(ONO3) was deprotonated with KBn (Bn ¼ benzyl) and added to Mo^C-(p-tolyl)[OCMe(CF3)2]3(DME) to quantitatively form [K(OiPr2)][(ONO3)Mo^C(p-tolyl)[OCMe(CF3)2]2] (Scheme 33). NMR studies conducted in toluene-d8 revealed that [K(OiPr2)] [(ONO3)Mo^C(p-tolyl)[OCMe(CF3)2]2] exists in equilibrium with (ONO3)Mo^C(p-tolyl)[OCMe(CF3)2] and free KOCMe(CF3)2. Similarly, dissolving [(ONO3)Mo^C(p-tolyl)[OCMe(CF3)2]]∙[KOCMe(CF3)2] in THF-d8 displaces KOCMe(CF3)2 to yield six-coordinate (ONO3)Mo^C(p-tolyl)[OCMe(CF3)2](THF) with THF bound trans to the alkylidyne. [(ONO3)Mo^ C(p-tolyl)[OCMe(CF3)2]]∙[KOCMe(CF3)2] is an effective ring-opening alkyne metathesis polymerization (ROAMP) catalyst that gives polymers with exceptionally high polydispersity when used with strained alkynes.68

Scheme 33

Organometallic Pincer Complexes With Group 6 Metals

663

Schrock and coworkers also prepared Mo and W alkylidene complexes with ONO3.69 Heating the bis-imido complex Mo(NC6F5)2(CH2CMe2Ph)2 with H2(ONO3) in benzene yielded (ONO3)Mo(NC6F5)(CHCMe2Ph) via loss of H2NC6F5 and CPhMe3 (Scheme 34). Attempts to prepare this complex by salt elimination with Mo(NC6F5)(CH2CMe2Ph)(OTf )2(DME) and Li2(ONO3) instead yielded the structurally characterized lithium salt [Li(DME)][(ONO3)Mo(NC6F5)(CHCMe2Ph)(OTf )].

Scheme 34

(ONO3)Mo(NC6F5)(CHCMe2Ph) slowly reacts with ethylene at ambient temperature and pressure (1 atm) to yield the unsubstituted metallacyclobutane (ONO3)Mo(NC6F5)(CH2CH2CH2).69 When dissolved in C6D6 in the absence of ethylene, (ONO3)Mo(NC6F5)(CH2CH2CH2) slowly decomposes to the ethylene complex (ONO3)Mo(NC6F5)(CH2CH2) along with generation of free ethylene. NMR studies and labeling reactions with C2D4 suggest that formation of these two products proceeds with elimination of ethylene from (ONO3)Mo(NC6F5)(CH2CH2CH2) to form the methylidene complex (ONO3)Mo(NC6F5)(CH2). Subsequent bimolecular coupling of the methylidene ligands in the presence of liberated ethylene then generates two equivalents of the corresponding ethylene complex.69 Addition of Li2(ONO3) to the oxo complexes W(O)(CHCMe2Ph)L2Cl2, where L ¼ PMePh2, PMe2Ph, or PMe3, yielded the corresponding (ONO3)W(O)(CHCMe2Ph)L complexes.69 As with imine-containing (ONO3)Mo(NC6F5)(CHCMe2Ph), (ONO3)W(O)(CHCMe2Ph)L reacts with ethylene to form the phosphine-free metallacyclobutane (ONO3)W(O)(CH2CH2CH2). Takashima et al. reported several tungsten complexes with ONO4 where R1/R2 ¼ Me/Me, iPr/iPr, or asymmetric Ph/Me and tested them for ring-opening metathesis polymerization (ROMP) of norbornene.70 Refluxing WOCl4 with H2(ONO4) in THF yielded mer-(ONO4)WOCl2. The R1/R2 ¼ Me/Me derivative was subsequently treated with 2 equiv. of LiCH2SiMe3 in THF/hexane to yield the dialkyl complex mer-(ONO4)W(O)(CH2SiMe3)2 (Scheme 35). Structurally similar (ONO8)MOCl2 complexes where M ¼ Mo or W have been reported and used as ROMP catalysts with norbornene, dicyclopentadiene, and 2-norbornen-5-yl acetate.71 Veige and coworkers investigated how more flexible ONO5 with methylene linkers between the central nitrogen and flanking phenolate groups affected the coordination chemistry and reactivity of tungsten alkylidene and alkylidyne complexes.72 (ONHO5) W^CR(OtBu) with R ¼ Et or tBu was prepared by mixing ONHO5 with the corresponding (OtBu)3W^CR in benzene (Scheme 36). The ligand-centered amine was subsequently deprotonated with H2C]PPh3 to form [MePPh3][(ONO5)W^CtBu(OtBu)]. Dissolving [MePPh3][(ONO5)W^CtBu(OtBu)] in CHCl3 or CDCl3 results in an immediate color change and formation of neutral (ONH/ D O5)W^CtBu(OtBu).72 Addition of MeOTf to [MePPh3][(ONO5)W^CtBu(OtBu)] similarly leads to methylation of the amido group and formation of (ONMeO5)W^CtBu(OtBu).

Scheme 35

Scheme 36

664

Organometallic Pincer Complexes With Group 6 Metals

The chemistry of homoleptic group 6 complexes with redox active ONO6 (which can exist as a trianion, dianion, or monoanion) was reported by Heyduk and coworkers with W,73 and by Brown and coworkers with Cr and Mo.74 Although there are few (if any) organometallic group 6 complexes with this ligand, a heteroleptic Cr complex was tested for catalytic transformations. Reaction of (ONO61−)CrCl2(THF) with two equiv. of PhLi resulted in elimination of biphenyl and reduction of the ligand to form (ONO63 − )Cr(THF)3 based on UV-Vis analysis of the solution (Scheme 37).75 It is presumed that this reaction proceeds via chloride metathesis to form the corresponding bis-phenyl Cr complex and then undergoes reductive elimination. Benzene is also produced in the reaction, which suggests that the one-electron pathway could be competitive. Similar results were obtained in reactions with 4-tBuC6H4Li. Derivatives of ONO6 with a third substituent attached to the nitrogen are known, but they typically undergo fac-type binding modes with group 6 metals.76

Scheme 37

There are few known examples of organometallic dipicolinate (i.e. ONO7) complexes with group 6 metals despite the many non-organometallic examples that are known to exist. However, like ONO6, some Cr complexes containing ONO7 have been tested in catalytic reactions. (ONO7)Cr(L)Cl where L ¼ ethylenediamine, bipy, or phen were poor-yielding catalysts for oxidation of olefins.77 [(ONO7)Cr(OH)(H2O)] was tested for oxidation of thymol using H2O2.78 [(ONO7)2Cr][(ONO7)Cr(bipy)(H2O)] and a related complex were shown to exhibit high catalytic activity for polymerization of 2-chloro-2-propen-1-ol relative to chromium complexes containing NNN ligands.79,80 Subsequent studies by the same authors demonstrated that these chromium complexes and others containing ONO7 were highly active catalysts for ethylene polymerization.81 ONO9, which is structurally similar to ONO7, was used to prepare (OHNO9)2Cr, (OHNOH9)CrCl3, and (ONO9)CrCl with R ¼ Ph.82 None of these complexes were effective when tested for ethylene polymerization catalysis.

6.09.2.4

CNC and CSC ligands

Group 6 CNC complexes have been limited primarily to those containing pyridyl ligands with flanking NHC donors containing different R-groups attached to N (Chart 4). These ligands have been studied most extensively with Cr halides. The complexes can be prepared by metalation with CrCl3(THF)3 or CrCl2, as shown for CNC1–CNC4, or by protonolysis reactions with the protonated carbene and Cr[N(SiMe3)2]2(THF)2, as illustrated with CNC1 ∙ 2HBr in Scheme 38.83–85 Single-crystal XRD studies revealed the expected octahedral geometry for mer-(CNC)CrCl3,83,85 whereas (CNC)CrBr2 (and presumably (CNC)CrCl2)84 are square pyramidal. The CrIII complexes were tested for ethylene oligomerization catalysis.83,85

Chart 4

Scheme 38

Organometallic Pincer Complexes With Group 6 Metals

665

Chaplin, Storey, and coworkers reported the synthesis and structure of an early example of a molybdenum CNC complex by heating a mixture of CNC5 with Mo(CO)6 or Mo(CO)3(THF)3 in THF (Scheme 39).86 Single-crystal XRD data confirmed mer coordination in (CNC5)Mo(CO)3. In contrast, in situ deprotonation of CNC6 ∙ 2HBr with 2 equiv. of KN(SiMe3)2 in the presence of Mo(CO)3(PhMe) yielded fac-(CNC6)Mo(CO)3. CNC6 differs from CNC5 in that it contains additional methylene linkers between the pyridyl backbone and the flanking NHC donors. It also has a 12-membered alkyl linker that bridges the two NHC units. DFT calculations were conducted on truncated models of (CNC5)Mo(CO)3 and (CNC6)Mo(CO)3 with methyl groups attached to the NHC nitrogen atoms to evaluate the thermodynamic preference of the mer and fac isomers. The calculations suggested that the mer isomer was favored by 42.6 kJ/mol for the CNC5 analog, whereas the fac isomer for the CNC6 analog was only preferred by 0.5 kJ/mol.

Scheme 39

McGuinness and coworkers reported one of the few examples of a CSC pincer complex with group 6 metals.85 Replacing the pyridyl unit in CNC1 with thienyl yielded CSC1. The free dicarbene could not be isolated, but CSC1 ∙ 2HBr could be deprotonated with 2 equiv. of KN(SiMe3)2 and added to a THF solution of CrCl3(THF)3 to yield (CSC1)CrCl3 (Scheme 40). This complex was tested for ethylene oligomerization with MAO, but did not yield any significant product at ethylene pressures up to 30 bar.85

Scheme 40

6.09.2.5

NNN ligands

Organometallic NNN complexes with group 6 metals were among the earliest pincer complexes to be prepared of those described in this chapter, namely because of the long-standing use of rigidly planar terpyridine (terpy) as a supporting tridentate ligand (NNN1; Chart 5). Aside from the organometallic complexes described below, many of the ligands shown in Chart 5 have been used to form non-organometallic precursors that have been tested in catalytic reactions. For example, (NNN3)CrCl3 and (NNN4) CrCl3 complexes, prepared by refluxing CrCl3(THF)3 with the corresponding ligand in THF, have been used for ethylene polymerization with MAO,87,88 as described below with (NNN2)CrCl3. Similar polymerization studies were performed with CrCl3 complexes containing NNN1 and NNN6.89–91 Gade and coworkers discovered that 10 mol% of CrCl2 and 12 mol% of NNN9 ligands can be highly enantioselective for the Nozaki-Hiyama-Kishi allylation of benzaldehyde.92 Group 6 complexes with other NNN ligands are known such as those with NNN5 and NNN10, but they do not fit the criteria outlined in the introduction because they either lack MdC bonds93–97 or the only known structures show the ligand coordinated in a fac-type arrangement.98

666

Organometallic Pincer Complexes With Group 6 Metals

Chart 5

Investigations into organometallic terpyridine complexes with group 6 metals began as early as 1964. Behren and Anders reported that solvothermal reaction of M(CO)6 where M ¼ Cr, Mo, and W with NNN1 in xylene yielded (NNN1)2M via the intermediate (NNN1)M(CO)3 for M ¼ Mo and W.99 Ganorkar and Stiddard similarly reported the synthesis of (NNN1)M(CO)4 for all three metals, as well as (NNN1)Mo(CO)3, which was prepared by heating Mo(CO)3(mesitylene) with NNN1.100 (NNN1)M(CO)4 complexes were later prepared by refluxing M(nbd)(CO)4 (where nbd ¼ norbornadiene) with NNN1 (R ¼ H or tBu) in hexane.101,102 Spectroscopic studies show that (NNN1)M(CO)4 complexes are six-coordinate with the terpy ligand binding in a bidentate fashion (similar complexes have been prepared with bidentate NNN2).102–104 Moya and coworkers prepared W(CO)3, Mo(CO)4, and W(CO)4 complexes with a derivative of NNN1 containing a single phenyl substituent at the 40 position.105 (NNN1)Mo(CO)3 was subsequently used as a starting material for the synthesis of structurally-characterized azido and nitrido complexes (NNN1)Mo(NO)(N3)3 and [(NNN1)MoN(N3)2][MoN(N3)4]
(NNN1)MoN(N3)3.106,107 There are a few examples of NNN1 complexes with cyanide and alkyls. Wieghardt and Holzbach described the synthesis of several Mo cyanide complexes with supporting NNN1 and reported the structure of (NNN1)Mo(NO)(CN)(HNO)∙2H2O. These complexes display Z2 coordination of hydroxylamide.108,109 In 1984, Marchese and West reported the synthesis of the perfluoroalkyl complex (NNN1)CrCl2(C3F7) by addition of NNN1 to CrCl2(C3F7)(py)3 in benzene.110 Daniel and coworkers prepared a series of Mo norbornadiene complexes with NNN1 where R ¼ H or tBu.111 Oxidation of Mo(nbd)(CO)4 with I2 followed by addition of NNN1 yielded [(NNN1)Mo(nbd)(CO)I]I. The bromide analog [(NNN1)Mo(nbd) (CO)Br]Br was prepared by reaction of Mo(nbd)(CO)4 with CuBr2 via oxidation and elimination of CuBr. The outer-sphere halides could be replaced with PF−6 and SbF-6 by reaction with AgPF6 and AgSbF6. Addition of 2 equiv. of AgSbF6 to [(NNN1)Mo(nbd)(CO) X]X where X ¼ Br or I yielded {[(NNN1)Mo(nbd)(CO)]2(acetone)}(SbF6)4, as assigned based on NMR and IR data of the isolated orange-red solid. Treating this acetone containing salt with PMe3 or EtCN in THF or acetone afforded the corresponding donor adducts (Scheme 41). Likewise, addition of NaX where X ¼ Cl or N3 in MeOH resulted in elimination of NaSbF6 and formation of the corresponding chloride and azide complexes.111

Organometallic Pincer Complexes With Group 6 Metals

667

Scheme 41

Baker and coworkers reported a series of halocarbonyl complexes with NNN1, including some with p-bound alkynes.112 Treating MI2(CO)3(MeCN)2 where M ¼ Mo and W with NNN1 in CH2Cl2 yields [(NNN1)MI(CO)3]I reminiscent of the nbd complexes in Scheme 41. A similar reaction with MoCl(GeCl3)(CO)3(MeCN)(PPh3) afforded (NNN1)Mo(GeCl3)(CO)2(PPh3)]Cl. Likewise, reactions of the alkyne-containing starting materials MI2(CO)(MeCN)(Z2-RCCR)2 where R ¼ Me or Ph and M ¼ Mo or W gave [(NNN1)MI(CO)(Z2-RCCR)]I or [(NNN1)WI(CO)(Z2-PhCCPh)]I, as shown in Chart 6. The outer-sphere iodide in [(NNN1)WI(CO)(Z2-MeCCMe)]I was exchanged with NaBPh4, and several of the alkyne complexes were structurally characterized.112 Later, Pérez and coworkers prepared the allyl complex [(NNN1)Mo(CO)2(Z3-methylallyl)]OTf by similar methods (Chart 6).113

Chart 6

In 2014, Honzícek and coworkers prepared rare examples of cyclopentadienyl (Cp) and indenyl (Ind) complexes containing a planar tridentate ligand.114 The rigidity of NNN1 prevents it from undergoing fac-type binding, as is often observed when other tridentate ligands coordinate to metals in complexes containing Cp or related cyclic p ligands. Addition of NNN1 to [Mo(Cp) (CO)2(MeCN)2]BF4 or [Mo(Ind)(CO)2(MeCN)2]BF4 (where R ¼ H or Ph on the indenyl) afforded [(k2-NNN1)Mo(Cp)(CO)2] BF4 or [(k3-NNN1)Mo(Ind)(CO)]BF4. Subsequently heating [(k2-NNN1)Mo(Cp)(CO)2]BF4 in MeCN for 7 days resulted in CO loss and coordination of all three NNN1 donor atoms to afford [(k3-NNN1)Mo(Cp)(CO)2]BF4 (Chart 7). All four complexes were structurally characterized. Similar Cp and indenyl complexes were prepared with NNN7 where X ¼ NH or O, as shown in Chart 7.114

668

Organometallic Pincer Complexes With Group 6 Metals

Chart 7

In 2018, Bezdek and Chirik reported proton-coupled electron transfer (PCET) involving a molybdenum complex containing a phenyl-substituted NNN1 and a b-agostic ethyl ligand (Scheme 42).115 [(NNN1)Mo(Et)(PPh2Me)2]+ can be formed by oxidation of (NNN1)Mo(PPh2Me)2Cl followed by alkylation with ZnEt2. The resulting ethyl-substituted complex was structurally characterized and undergoes b-agostic CH3 rotation and reversible b-hydride elimination in solution. Addition of tBu3ArO• results in H-atom abstraction to form the ethylene complex [(NNN1)Mo(Z2-C2H4)(PPh2Me)2]+, which can also be formed by treating (NNN1)Mo(PPh2Me)2Cl with Na[BArF24] in the presence of ethylene. Mixing [(NNN1)Mo(Z2-C2H4)(PPh2Me)2]+ with the ammine complex [(NNN1)Mo(NH3)(PPh2Me)2]+ reverts the ethylene complex back to the b-agostic complex via NdH PCET and formation of [(NNN1)Mo(NH2)(PPh2Me)2]+.115 Reduction of [(NNN1)Mo(Z2-C2H4)(PPh2Me)2]+ with Cp2Co yields neutral (NNN1)Mo(Z2-C2H4)(PPh2Me)2. This complex exists in equilibrium with the cationic b-agostic complex when DBU base is added, as shown in Scheme 42.115

Scheme 42

NNN2 ligands (also referred to as DIP or PDI ligands) have been used as a supporting ligand for ethylene polymerization catalysis or as a noninnocent ligand to assist base metals in small molecule transformations.116 In 2003, Esteruelas and coworkers demonstrated the synthetic versatility of this platform by preparing a series of symmetric and asymmetric (NNN2)CrCl3 complexes with R1 ¼ Cy or aryl and R2 ¼ Me or Ph.117 The aryl derivatives were decorated at the 2, 4, and 6 positions with different combinations of H, Me, Et, iPr, tBu, OMe, and CF3 to evaluate their electronic influence on ethylene polymerization with MAO and other Al co-activators. The most active catalysts contained ortho-substituted aryl groups. Similar (NNN2)CrCl3 derivatives and their use for ethylene polymerization were later reported by Britovsek and coworkers.118 Duchateau and coworkers also described ethylene polymerization studies of CrI and CrII NNN2 complexes with Ar ¼ 2,6-iPr2C6H3 and reported the synthesis of organochromium complexes obtained in reactions with alkyl lithium and

Organometallic Pincer Complexes With Group 6 Metals

669

aluminum reagents.119 Reaction of the CrII complex (NNN2)CrCl2 with 2 equiv. of MeLi or NaH resulted in metal reduction and formation of (NNN2)CrMe(m-Me)Li(THF)3 and (NNN2)CrCl, respectively (Scheme 43). Treating (NNN2)CrCl with MeLi in THF yields (NNN2)Cr(m-Me)Li(THF)3 whereas performing the same reaction with LiCH2SiMe3 results in methyl CdH deprotonation at the ligand. Turning to aluminum reagents, reaction of (NNN2)CrCl with excess AlMe3 in toluene yielded a mixture of (NNN2)CrMe and (NNN2)Cr(m-Cl)2AlMe2 (Scheme 44). In contrast, the same reaction with more strongly reducing isobutylaluminoxane (IBAO) in toluene resulted in transmetalation to form an unusual heterotrimetallic complex containing p-bound Cr0 sandwiched between the Al-coordinated NNN1 ligand backbone and toluene (Scheme 44).

Scheme 43

Scheme 44

Chirik and coworkers prepared a series of NNN2 complexes with Mo and investigated their reactivity with ammonia.120,121 The benzene complex (NNN2)Mo(Z6-C6H6) was prepared by reducing (NNN2)MoCl3 with Na amalgam in benzene. The bond distances from single-crystal XRD suggested that the oxidation state of the metal was best assigned as MoII. Treating this complex with 2 equiv. of NH3 yielded a bis-ammine complex with slippage of the Z6-benzene to Z2, which was confirmed by 1H NMR studies (Scheme 45). The Z2-benzene could be substituted by addition of ethylene or cyclohexene, and the Z2-cyclohexene complex was structurally characterized. The ethylene and cyclohexene complexes were also prepared by sequential addition of NH3 and the corresponding alkene to (NNN2)Mo(Z6-C6H6).121 (NNN2)Mo(Z6-C6H6) was used by the same authors as a starting material for the synthesis of Mo imido complexes.122

670

Organometallic Pincer Complexes With Group 6 Metals

Scheme 45 Adapted with permission from Margulieux, G. W.; Bezdek, M. J.; Turner, Z. R.; Chirik, P. J., Ammonia Activation, H2 Evolution and Nitride Formation from a Molybdenum Complex with a Chemically and Redox Noninnocent Ligand. J. Am. Chem. Soc. 2017, 139, 6110–6113. Copyright 2017 American Chemical Society.

The (NNN2)Mo(NH3)2(Z2-alkene) complexes are unstable and decompose with loss of alkene and half an equivalent of H2 to form the paramagnetic bis-amido complex [H(NNN2)]Mo(NH2)2 (Scheme 45).121 XRD studies confirmed the conversion of NH3 ligands to NH2 and addition of a hydrogen atom to the NNN2 backbone. Protonating [H(NNN2)]Mo(NH2)2 with 1 equiv. of lutidinium triflate, [LutH]OTf, regenerates both ammine ligands to form (NNN2)Mo(NH3)2(OTf ) with one proton originating from lutidinium and the other being provided by the noninnocent H(NNN2) ligand. Treating (NNN2)Mo(NH3)2(Z2-C2H4) with 3 equiv. of tBu3ArO• results in multiple hydrogen atom abstractions to form (NNN2)Mo(N)(Z2-C2H4) (Scheme 46).121 This Mo nitride complex contains a ligand-centered radical that was observed by EPR spectroscopy and corroborated by supporting DFT calculations. In a subsequent study, it was shown that pyridine backbone can be hydrogenated by stoichiometric addition of three equiv. of (Z5-C5Me5)RhH(2-phenylpyridine) or catalytic reactions with 5 mol% of this Rh complex in the presence of H2 (Scheme 46).123

Scheme 46

Continuing their work with low-valent Mo olefin complexes, Chirik and coworkers prepared a series of (NNN2)Mo (Z2:Z2-COD) complexes where COD ¼ 1,5-cyclooctadiene (Chart 8) and demonstrated their use for olefin dimerization.124 The (NNN2)Mo(Z2:Z2-COD) complexes with several different aryl-substituted NNN2 ligands were prepared by reduction of the

Organometallic Pincer Complexes With Group 6 Metals

671

Chart 8

corresponding (NNN2)MoCl3 with Na amalgam in the presence of COD. Subsequent addition of 12 equiv. of ethylene to (NNN2)Mo(Z2:Z2-COD) with Ar ¼ mesityl resulted in ethylene dimerization to 1,3-butadiene, which coordinates to the metal to form (NNN2)Mo(Z4-butadiene)(C2H4), as shown in Chart 8 (R1 ¼ R2 ¼ H). In contrast, performing a similar reaction with bulkier Ar ¼ 2,6-iPr2C6H3 and 2 equiv. of ethylene resulted in no dimerization and formation of (NNN2)Mo(Z2-C2H4)2. (NNN2)Mo(Z4-butadiene)(Z2-C2H4) was also prepared by reduction of (NNN2)MoCl3 where Ar ¼ mesityl with Na amalgam in the presence of 3 equiv. of ethylene. A similar diene-alkene complex was afforded when the reduction was performed with 1-hexene instead of ethylene (Chart 8; R1 ¼ nBu, R2 ¼ Et). In a subsequent study, Chirik and coworkers demonstrated that (NNN2)Mo(Z2:Z2-COD) and the alkyl complexes (NNN2)Mo(CH2SiMe3)2 (Chart 8) were effective precatalysts for arene and olefin hydrogenation with H2.125 The arene hydrogenation reactions proceed via Mo-arene coordination, as shown above with benzene in Scheme 45. In 2018, Gade and coworkers reported that square planar (NNN8)CrCl with R ¼ Ph or iPr can be prepared by stepwise deprotonation of NNHN8 with Li[N(SiMe3)2] followed by addition of (TMEDA)CrCl2 in THF.126 Subsequent addition of Mg(CH2SiMe3)2(THF)2 to (NNN8)CrCl in toluene yielded the corresponding alkyl complexes (NNN8)Cr(CH2SiMe3), which were shown to be highly enantioselective precatalysts for the hydrosilylation of ketones.126

6.09.2.6

SNS ligands

The majority of SNS ligands known to form complexes with group 6 metals contain amines or pyridine flanked by thioether appendages (Chart 9). As with other pincer ligand classes, many of these are Cr complexes that were investigated as precatalysts for ethylene oligomerization reactions. In 2003, McGuinness, Wasserscheid, and coworkers prepared CrCl3 complexes with SNS1–SNS4 and tested them for ethylene trimerization.127 All four precatalysts showed remarkable selectivity for 1-hexene with TOFs up to 260,000 h−1 when using excess MAO. The molecular structure of (SNS2)CrCl3 was reported and confirmed mer coordination of SNS2 in the solid state. The same authors later compared the trimerization reactivity of the CrCl3 precatalysts with SNS1–SNS4 with those containing an asymmetric variant of SNS2 and PNP, PSP, and SPS ligands (see below).128 DFT studies by Liu and coworkers suggested that ethylene trimerization catalysis with Cr SNS1 complexes proceeds without N–H deprotonation of the ligand.129 Some of these (SNS)CrCl3 complexes have also been tested for ethylene trimerization in tandem reactions, in ionic liquids, and with the catalyst immobilized on solid oxide supports.130–132

Chart 9

In 2006, Duchateau, Gambarotta, and coworkers began investigating the role of Cr oxidation state and intermediates involved in the trimerization of ethylene with (SNS)CrCl3 complexes using ligand variant SNS5 with cyclohexyl groups attached to sulfur.133 Treating (SNS5)CrCl3 with 10 equiv. of AlMe3 or AlEtCl2 resulted in the chloride bridged alkyl dimers {[(SNS5)CrRCl]2}2+ with R]Me and Et, respectively (Scheme 47).133,134 The dicationic dimers were charge balanced with different alkylaluminate anions. Remarkably, the NdH group on the ligand remained intact in the alkylated dimers, as indicated by the representative CrdN bond distances of 2.088(4) A˚ (R ¼ Me) and 2.090(4) A˚ (R ¼ Et). NdH stretches were also observed by IR spectroscopy at 3181 and 3185 cm−1, respectively. Alkyl free analogs of the dimers were also prepared by treating (SNS5)CrCl3 with excess AlCl3 or IBAO.

672

Organometallic Pincer Complexes With Group 6 Metals

Scheme 47

Treating CrCl2(THF)2 with SNS5 yielded the divalent complex (SNS5)CrCl2(THF), which proved to be active for trimerization of ethylene to 1-hexene using MAO.133 Reacting CrCl2(THF)2 and SNS5 in the presence of excess AlMe3 yielded unexpected oxidation of CrII to the aforementioned CrIII dimer{[(SNS5)CrMeCl]2}2+.134 Cr metal was also formed, suggesting a disproportionation pathway was responsible for the oxidized CrIII species in the presence of reducing AlMe3. Performing the same reaction with AlEtCl2 yielded the square planar CrII complex cation (SNS5)Cr(m-Cl)AlEtCl2.134 Reacting CrCl2(THF)2 with SNS5 followed by addition of AlMeCl2 or MAO yielded chloride-bridged CrII dimers with each metal having an alkylchloroaluminate anion. The CrII complexes tested for ethylene oligomerization were less selective for 1-hexene and gave a wider distribution of alkene products compared to the CrIII complexes under identical conditions. To test the importance of the amine backbone for ethylene oligomerization catalysis, Duchateau, Gambarotta, and coworkers prepared CrCl3 complexes with pyridyl-derived SNS7–SNS10 by reacting CrCl3(THF)3 with the corresponding ligand in toluene (Scheme 48).135,136 The mer conformation of the SNS ligands was confirmed for all four complexes by single-crystal XRD. Addition of AlMe3 to mer-(SNS9)CrCl3 yielded structurally characterized mer-(SNS9)Cr [(m-Cl)AlMe3]2Me.136 Catalytic studies revealed that all the complexes exhibit good selectivity for 1-hexene, thereby confirming that the NH amine in SNS1–SNS5 was not required for ethylene trimerization. More recently, the synthesis of CrCl3 complexes with pyridine-centered SNS ligands was extended to those with (S)R ¼ nBu, npentyl, noctyl, and cyclopentyl.137

Scheme 48

CrII complexes with pyridyl ligands SNS7–SNS10 were also obtained upon reaction with divalent CrCl2(THF)2 (Scheme 48).135,136 Only mer-(SNS9)CrCl2(THF) with R ¼ tBu was structurally characterized, which revealed THF bound trans to the pyridyl nitrogen.136 All four CrII complexes were tested for ethylene oligomerization, but showed diminished 1-hexene selectivity compared to their CrIII analogs.135,136 Downing and coworkers used SNS2 and SNS4 to prepare a variety of Mo complexes in different oxidation states (Scheme 49).138 Reacting the ligands with either Mo(CO)3(PhMe) or MoCl3(THF)3 in DCM yielded the corresponding (SNS)Mo(CO)3 and (SNS) MoCl3 complexes, respectively. Single-crystal XRD studies revealed that (SNS2)MoCl3 crystallized as the mer isomer whereas (SNS2)Mo(CO)3 formed the fac isomer. In the same study,138 it was shown that treating MoCl4(MeCN)2 with SNS2 or SNS4 yielded the corresponding (SNS)MoCl4 complexes. A chloride was abstracted from (SNS4)MoCl4 by treating it with Li[Al(OtBuF)4] in DCM to yield [(SNS4)MoCl3][Al(OtBuF)4]. Some of the Mo complexes were tested for ethylene oligomerization. However, unlike the corresponding CrCl3 complexes in Chart 9, all of the Mo complexes tested were poor catalysts when used with a variety of different Al activators.138

Organometallic Pincer Complexes With Group 6 Metals

673

Scheme 49

Compared to thioether-based SNS complexes, there are fewer examples of organometallic group 6 complexes with thiolate-based SNS ligands. In 1983, Berg and Holm reported the synthesis and crystal structure of (SNS11)MoO2(TMSO) where TMSO ¼ tetramethylene sulfoxide.61 Studies of similar Mo oxo complexes supported by SNS11 were reported by Fischer and coworkers in 1997.139 Likewise, treating (PPh4)[Cp WS3] with 2,6-bis(bromomethyl)pyridine yielded alkylation of the sulfido ligands to form the dinuclear complex [W(Cp )S2]2(SNS11) (Scheme 50).140 Performing the same reaction with (PPh4)2[WS4] yielded (SNS11)WS2 with the SNS coordinated as expected for a pincer ligand. Both tungsten complexes were structurally characterized by single-crystal XRD. Goh and coworkers reported the synthesis and structure of the fac-Cp Cr(SNS) complex shown in Scheme 50 and used it for the synthesis of organometallic phosphate complexes.141 Heyduk and coworkers reported homometallic and heterobimetallic (SNS)2W complexes with a redox-active trianionic bis(thiophenolate) ligand similar to ONO6 (see Section 6.09.2.3).73,142,143 A bis(thiophenolate) (SNRS)2− ligand with R ¼ iBu that was reported to undergo pincer-type coordination with FeIII instead yielded fac-(SNS)2Mo when 2 equiv. of the ligand was added to MoCl4(THF)2 in THF.144

Scheme 50

6.09.2.7

PNP ligands

PNP ligands are among the most heavily explored pincer ligands with low valent group 6 metals.6,30 PNP ligands described in this section are summarized in Chart 10 and can be broken down into those with a neutral N donor groups and those containing NdH bonds that can be deprotonated to form anionic amido-containing ligands. The PNP1 ligand platform is perhaps the most versatile in Chart 10 because the substituents can be modified at both N and P, as shown in Table 1. Though most of the work known with PNP1 described in this chapter was reported by the Kirchner group,6 one of

Chart 10 Table 1

Derivatives of PNP1 used to prepare organometallic group 6 complexes.

R0 (N)

PR2

Ligand

Ref.

H H H H H H

PPh2 PiPr2 PtBu2 PCy2 PEt2

PNP1a PNP1b PNP1c PNP1d PNP1e PNP1f

145–152 146,147,150,152 146,147,149,151,153 150,152 151,152 146

H

PNP1g

151

H

PNP1h

151

H

PNP1i

154

PNP1j PNP1k PNP1l PNP1m

146 147,150,152,155,156 150,151,155,157 150–152

1-Undecene Me Me Ph

PPh2 PiPr2 PPh2 PEt2

Organometallic Pincer Complexes With Group 6 Metals

675

the first entries into this class of complexes was reported by Haupt and coworkers in 1987 with PNP1a (R ¼ Ph and R0 ¼ H; Table 1).145,158 Refluxing M(CO)3(MeCN)3 where M ¼ Cr, Mo, and W with PNP1a in THF yielded the corresponding (PNP1a)M(CO)3 complexes. The structure of the Mo complex confirmed mer coordination of the PNP ligand. Remarkably, the chemistry of PNP1 with group 6 complexes appeared to lay relatively dormant for almost 30 years until it was revived in 2006.146 Kirchner and coworkers reported a series of (PNP1)Mo(CO)3 complexes that were prepared by treating Mo(CO)3(MeCN)3 with PNP1b, PNP1c, PNP1f, or PNP1j (Table 1) in MeCN.146 It was shown in the same report that (PNP2)Mo(CO)3 could be prepared the same way.146 (PNP1)M(CO)3 complexes with M ¼ Mo and W have also been synthesized by treating [M(CO)4Br]2(m-Br)2 with PNP1 to form the seven-coordinate complex ion in [(PNP1)M(CO)3Br]Br that was then reduced with Na amalgam.147 In more recent reports, Kirchner and coworkers showed that (PNP1)M(CO)3 complexes can be prepared with all three group 6 metals by solvothermal reactions between M(CO)6 and PNP1 in MeCN.151,154 (PNP1)M(CO)3 complexes where M]Mo or W can be protonated with HBF4 to form the seven-coordinate hydrido salts [(PNP1)M(CO)3H]BF4 (Scheme 51).147 XRD studies revealed that these complexes have pentagonal bipyramidal coordination geometries with hydride bound in the equatorial plane. The reactions are reversible under mild conditions: the hydrido complexes can be deprotonated at RT in CH2Cl2 by addition of NEt3.147

Scheme 51

Halocarbonyl complexes can be prepared with PNP1 ligands upon reaction with halogenated metal carbonyls, as described above for the synthesis of [(PNP1)M(CO)3Br]Br from [M(CO)4Br]2(m-Br)2 (where M ¼ Mo or W).147,151,155,157 They can also be prepared by oxidizing (PNP1)M(CO)3 with the corresponding halogen. For example, it was initially shown that (PNP1b)Mo(CO)3 and (PNP1j)Mo(CO)3 could be oxidized with I2 in CH2Cl2 to form the seven-coordinate salts [(PNP1)Mo(CO)3I]I or [(PNP1)Mo(CO)3I]I3 depending on the equivalents of I2 used (Scheme 52).146 Performing the same oxidation with (PNP1b)Mo(CO)3 in MeCN instead of CH2Cl2 resulted in loss of CO to form [(PNP1)Mo(CO)2(MeCN)I]I.146 Later reports extended these oxidation studies to other halogens and PNP1 ligands.150,155,157 Collectively, these studies have shown how the identity of the N and P substituents in PNP1 ligands dictates how many ancillary halide and CO ligands bind to the metal. As an example, it was shown that the NdH substituted PNP1b with R ¼ iPr forms the seven-coordinate complex salt [(PNP1b)Mo(CO)3Br]Br whereas the NdMe derivative with the same P substituent (i.e. PNP1k) forms (PNP1k)Mo(CO) X2 where X ¼ Br and Cl.155

Scheme 52

Kirchner and coworkers explored the reactivity of halocarbonyl complexes containing PNP1k with silver salts and O2. Addition of Ag[SbF6] to (PNP1k)Mo(CO)X2 where X ¼ Cl or Br in THF or MeCN results in halide abstraction to form cationic [(PNP1k)Mo(CO)(L)X]+ with L ¼ THF or MeCN (Scheme 53).155 Doing the same reaction in acetone with X ¼ Br or I and exposing the mixture to O2 and H2O resulted in the Mo(IV) oxo salt [(PNP1k)Mo(O)X]SbF6.156 ESI-MS studies with 18O labeled water and oxygen suggested that the water was the source of the oxo ligand.

676

Organometallic Pincer Complexes With Group 6 Metals

Scheme 53

In 2013, Gambarotta and coworkers prepared (PNP1a)CrCl3 and tested it for ethylene oligomerization with aluminum activators.148 Reaction of (PNP1a)CrCl3 with excess AlR3 where R ¼ Et or tBu yielded (PNP1a-AlClR2)Cr(m-Cl)2AlR2 via reduction of CrIII to CrII, NdH deprotonation, and formation of a N !Al Lewis acid-base adduct with AlClR2 generated in the reaction (Scheme 54). In contrast, the same reaction with AlMe3 proceeded without reduction to form the methyl complex (PNP1aAlClMe2)CrMe(m-Cl)2AlMe2. Reaction with AlEt2Cl yielded [(PNP1a)CrEt(m-Cl)2AlEt2][AlEtCl3] without Cr reduction or NdH deprotonation.

Scheme 54

In other catalytic studies, Huang and coworkers reported ethylene polymerization studies using (PNP1a)CrCl3 and (PNP1c) CrCl3 in side-by-side reactions with Cr complexes containing asymmetric pincer ligands (see Section 6.09.3).149 Likewise, Kirchner and coworkers reported the synthesis of CrCl3 and CrCl2 complexes with six different PNP1 ligands and tested them for catalytic Kumada cross-coupling of PhMgBr and aryl halides.152 None of the complexes were effective for Kumada cross-coupling, but they did yield the homo-coupled product (i.e. biphenyl) in good yields at 0.1 mol% loadings and short reaction times (15 min).152 In a more unusual application of PNP1 ligands, Tuczek and coworkers tethered a (PNP1)Mo(CO)3 complex to a triazatriangulenium (TATA) platform to investigate how ModCO stretching frequencies are effected when the complex is adsorbed to a gold surface.159 The TATA-tethered (PNP1)Mo(CO)3 complex was prepared as shown in Scheme 55. Adsorption to Au(111) caused the ModCO vibrations to shift to higher frequencies when compared to bulk measurements.

Scheme 55

Organometallic Pincer Complexes With Group 6 Metals

677

Relative to PNP1 and its close derivative PNP2, there are fewer examples of organometallic PNP3 complexes with group 6 metals. However, interest in this platform has reemerged more recently based on its effective use in N2 reduction chemistry, and there are many non-organometallic examples of PNP3 complexes containing group 6 metals (especially Mo).153,160–173 Organometallic examples with PNP3 include the N2-bridged [(PNP3)Mo(CO)2]2(m-N2) with R ¼ tBu that was prepared by stirring [(PNP3)Mo(N2)2]2(m-N2) in THF under an atmosphere of CO (Scheme 56).161 More recently, Perez, Galindo, and coworkers prepared low-valent organometallic Mo and W complexes with PNP3 with R ¼ Ph.174 Reduction of (PNP3) MCl3 (where M ¼ Mo or W) with Na amalgam in the presence of ethylene affords (PNP3)M(C2H4)3 (Scheme 56). Attempts to do the same reaction with P-alkyl substituted PNP3 ligands (alkyl ¼ iPr, Cy, and tBu) were unsuccessful. Subsequent addition of CO2 (1 atm) at RT (M ¼ Mo) or 80  C (M ¼ W) in toluene results in displacement of an axial ethylene ligand to afford (PNP3)M(C2H4)3(Z2-CO2) with side-on bound CO2. This reaction also occurs in the solid state by exposing crystals of (PNP3)M(C2H4)3 to CO2. Exposing (PNP3)M(C2H4)3 to 2 atm of CO instead of CO2 yields (PNP3)M(C2H4)(CO)2 with CO bound in the axial positions.174

Scheme 56

Gambarotta and coworkers prepared (PNP3)CrCl3 and (PNP3)CrCl2 with R ¼ Ph and tested them for polymer-free ethylene oligomerization with various alkyl aluminum activators, analogous to their aforementioned work with (PNP1a)CrCl3 (see Scheme 54).175 Reacting these complexes with excess AlMe3, AliBu3, ClAlEt2 yielded a variety chloride-bridged Cr/Al heterobimetallics including the complex salt [(PNP3)CrEt(m-Cl)2AlEt2][AlCl3Et]. Reaction of (PNP3)CrCl3 with AliBu3 resulted in deprotonation of one of the PNP3 methylene linkers and formation of MLC-bound ClAliBu2. Jones and coworkers prepared (PNP4)M(CO)3 with M ¼ Mo or W and R ¼ iPr or tBu by treating the corresponding M(CO)3 (CH3CN)3 with PNP4 in THF at RT.176 As described above with similar PNP1 complexes, they showed that (PNP4)M(CO)3 can be protonated with HBF4 ∙Et2O in CH2Cl2 to form the hydride salts [(PNP4)MH(CO)3]BF4. The [(PNP4)MoH(CO)3]BF4 complexes catalyze the isomerization of 1-alkenes to internal alkenes. The Agapie group reported on Mo complexes with PNP5 with R ¼ iPr that shows both s and p binding modes in response to different metal oxidation states (Scheme 57.177 fac-(PNP5)Mo(CO)3 was prepared by treating Mo(CO)3(CH3CN)3 with PNP5 in

Scheme 57

678

Organometallic Pincer Complexes With Group 6 Metals

toluene. (PNP5)Mo(CO)3 was then oxidized with 2 equiv. of AgOTf to form [mer-(PNP5)Mo(CO)2(OTf )]OTf, which was subsequently photolyzed in MeCN to displace the CO ligands and form [mer-(PNP5)Mo(MeCN)3][OTf]2. Addition of 2 equiv. of tetrabutylammonium azide to [mer-(PNP5)Mo(MeCN)3][OTf]2 resulted in azide thermolysis to yield a metal nitride and coordination of an ancillary azide. More notably, this reaction caused the pyridyl unit in PNP5 to switch from N !Mo s-donation to Z2-N,C p-binding. The ancillary azide in (PNP5)MoN(N3) could be replaced by addition of Me3SiCl to form (PNP5)MoNCl with retention of the p-bound pyridyl unit. In contrast, addition of excess Ph3SiCl results in silylation of the nitride and causes the PNP5 ligand to revert to N !Mo s donation. McGuinness and coworkers prepared (PNP6)CrCl3 with R ¼ Me or benzyl and tested them for ethylene trimerization catalysis.128 Bernskoetter and coworkers showed how reducing (PNP6)MoCl3 with (N)R ¼ Me in the presence of ethylene yields (PNP6)Mo(C2H4)2, which can undergo reversible CdH cleavage of the pincer NMe group.178 (PNP6)Mo(C2H4)2 has an agostic NdCdH bond (as confirmed by single-crystal XRD studies) that undergoes oxidative addition to form the corresponding MoII complex (Scheme 58). CO2 reacts with the hydrido complex by undergoing insertion into the ModH bond to yield formate. The formate complex was shown to catalyze CO2 reduction with H2 in the presence of LiOTf and DBU.178

Scheme 58

The PNHP7 ligand was reported first by Fryzuk and coworkers in 1981,179 and it is known to adopt a variety of metal binding modes because of its inherent flexibility. In an example with group 6 metals, Templeton and coworkers prepared (PNHP7) W(CO)3 and demonstrated how fac or mer isomers can form depending on the reaction conditions.180 (PNHP7)W(CO)3 could also be protonated with HBF4 ∙Et2O to form the hydride salt [(PNHP7)W(CO)3H]BF4. Interestingly, addition of NEt3 at 195 K resulted in deprotonation of the pincer amine. As the mixture was warmed to RT, the hydride resonance in the 1H NMR slowly disappeared as it was transferred to the amine to reform thermodynamically-favored (PNHP7)W(CO)3. Halocarbonyl analogs of (PNHP7)W(CO)3 were prepare by oxidation with halogen or using halide transfer reagents.180 Addition of NEt3 to [(PNHP7) W(CO)3X]+ where X ¼ F, Cl, Br, or I resulted in deprotonation of the pincer amine to form the neutral, seven-coordinate halocarbonyl complex.180 In one of the earliest reports with PNHP8, Ellerman described the synthesis of (PNHP8)M(CO)3 where M ¼ Cr, Mo, or W and R ¼ Ph. These complexes were prepared by mixing M(CO)3(C7H8) (where C7H8 ¼ cycloheptatriene) with PNHP8 in THF.181 Addition of MeOH to the Cr containing reaction mixture yielded co-crystallization of yellow needles and red blocks that could be manually separated. IR and NMR studies suggested that the red blocks were mer-(PNHP8)M(CO)3 and the yellow needles were fac-(PNHP8)M(CO)3. In contrast, only yellow needles were grown from the Mo and W reaction mixtures when MeOH was added to induce crystallization. Later in 2005, McGuinness and coworkers prepared (PNHP8)CrCl3 with R ¼ Ph, Cy, or Et and tested them for ethylene trimerization with MAO.128 Berke and coworkers prepared (PNP8)M(NO)(CO) complexes with M ¼ Mo or W and R ¼ iPr and investigated their catalytic reactivity for hydrogenation of nitriles, imines, and CO2.182–184 The (PNP8)M(NO)(CO) complexes were prepared by heating a mixture of M(CO)4(NO)(AlCl4) with PNHP8 in THF to form (PNHP8)M(NO)(CO)Cl, followed by dehydrohalogenation with Na [N(SiMe3)2] (Scheme 59). Attempts to dehydrohalogenate (PNHP8)M(NO)(CO)Cl with KOtBu only yielded the metathesis product (PNHP8)M(NO)(CO)(OtBu).

Scheme 59

Organometallic Pincer Complexes With Group 6 Metals

679

Reacting (PNP8)M(NO)(CO) complexes where M ¼ Mo or W and R ¼ iPr with H2 revealed that deprotonated PNP8 is able to participate in metal-ligand cooperativity (MLC) (Scheme 60). H2 adds across the M-amide bond under relatively mild conditions (RT, 2 bar) in THF to form (PNHP8)MH(NO)(CO).183 Jiao, Beller, and coworkers investigated the energetics of H2 addition across the MdN bond in these complexes as a function of metal (Cr, Mo, and W) as part of their DFT studies on hydrogenation catalysis.185 It was later shown that CO2 can insert into the metal-hydride bond in (PNHP8)MH(NO)(CO) to yield (PNHP8)M(OCHO)(NO) (CO).184 These formate complexes can then be treated with Na[N(SiMe3)2] to reform (PNP8)M(NO)(CO).

Scheme 60

Similar to the MLC reactions with H2, (PNP8)M(NO)(CO) where M ¼ Mo or W undergoes a 2 + 2 reaction with CO2 to form an isolable MLC-bound CO2 complex (Scheme 60).184 The authors also showed that (PNHP8)MCl3(NO) complexes can be prepared and reduced with Na amalgam in THF to form (PNHP8)MCl2(NO). Building off their DFT studies,185 the Beller group investigated Mo complexes with PNHP8 for catalytic hydrogenation of ketones, olefins, nitriles, and amides.98,186,187 The complexes fac-(PNHP8)Mo(CO)3, mer-(PNHP8)Mo(CO)2(MeCN), and mer(PNHP8)Mo(CO)2Br were prepared from Mo(CO)6, Mo(PPh3)2(CO)2(MeCN)2, or Mo(Z3-allyl)(CO)2(MeCN)2Br, respectively, in non-halogenated solvents. Reaction of Mo(PPh3)2(CO)2(MeCN)2 and PNHP8 in CH2Cl2 allowed the chloride derivative mer(PNHP8)Mo(CO)2Cl to be isolated in good yield (68%). Similar Mo PNHP8 complexes with R ¼ Et, Cy, and Ph were prepared using Mo(PPh3)2(CO)2(MeCN)2 as the starting material.98 Addition of 2 equiv. of NaBHEt3 to mer-(PNHP8)Mo(CO)2(MeCN) in THF/ toluene resulted in a rapid reaction that yielded the unusual complex [Na(Et2O)2][mer-(PNHP8)Mo(CO)2(N^C(CH2)BEt3)], as confirmed by single-crystal XRD studies (Scheme 61).98 A similarly unusual salt was obtained in a reaction with mer-(PNHP8)Mo(CO)2Cl, formanalide, and Na(HBEt3).187

Scheme 61

Gade and coworkers prepared a series of CrII alkyl complexes with PNP9 (R ¼ iPr or tBu).188 (PNP9)CrCl complexes were prepared and treated with LiMe, LiCH2SiMe3, or Bn2Mg(THF)2 in toluene to afford (PNP9)CrMe, (PNP9)CrCH2SiMe3, and (PNP9) CrBn (where Bn ¼ benzyl), respectively (Scheme 62). Hydrogenation of (PNP9)CrBn in C6D6 with H2 afforded the corresponding

Scheme 62

680

Organometallic Pincer Complexes With Group 6 Metals

hydrides (PNP9)CrH. In the solid state, (PNP9)CrH was monomeric when R ¼ tBu, but dimeric when R ¼ iPr. The insertion chemistry of the hydride complexes was investigated with CO2, benzophenone, and N,N0 -dicyclohexylcarbodiimide.188 Nishibayashi, Toshizawa, and coworkers reported the synthesis of CrCl3 and MoCl3 complexes with the azaferrocene ligand PNP10 with R ¼ Cy and Ph.189 The (PNP10)MoCl3 complexes were reduced with Na amalgam under an N2 atmosphere in THF to form the corresponding Mo0 dinitrogen complexes. In 1998, Haenel and coworkers reported the synthesis of PNP11 with R]Ph. Reacting PNP11 with (nbd)Mo(CO)4 (where nbd ¼ 2,5-norbornadiene) in toluene yielded (PNP11)Mo(CO)3. This complex was isolated in low yield (15%) but was structurally characterized by single-crystal XRD. Hölscher and coworkers reported a DFT study investigating the cooperative tungsten/rhodium hydrogenation catalysis of N2 to form NH3 using (PNP11)W(N2)3 with R]tBu in side-by-side studies with tungsten derivatives containing PNP6 and PNP8.190

6.09.2.8

PCP ligands

The investigation of PCP ligands with group 6 metals lagged only slightly behind the development of organometallic complexes containing PNP ligands. As shown in Chart 11, PCP pincer ligands that have been used to prepare group 6 complexes can be split into those containing a deprotonated aryl backbone and those containing neutral NHC donors.

Chart 11

In 2012, Schrock and coworkers reported the synthesis of (PCP1)MoX2 where X ¼ Cl, Br, or I and R ¼ tBu.191 Reacting MoCl3(THF)3 or MoBr3(THF)3 with the PCIP1 ligand precursor (which contains a cleavable aryl-I bond) led to mixtures of products with different halides bound to the metal. These mixtures were avoided by using MoI3(THF)3 to make the all iodide product (PCP1) MoI2, which was isolated with a small amount of (PCP1)MoOI. Reducing (PCP1)MoI2/(PCP1)MoOI with Na amalgam under an N2 atmosphere in the presence of 15-crown-5 yielded the nitride complex [Na(15-crown-5)][(PCP1)Mo^NI] (Scheme 63). Performing the same reaction under an atmosphere of 15N2 yielded the 15N labeled complex, thereby confirming N2 as the source of nitride. Protonating [Na(15-crown5)][(PCP1)Mo^NI] with [Et3NH]BArF24 yielded (PCP1)HMo^NI]. Based on NMR studies, it was proposed that the hydride was bridging between Mo and P on PCP1 as shown in Scheme 63.

Scheme 63

Another PCP1 ligand with R ¼ iPr was investigated by the Kirchner group in 2019 with Cr, Mo, and W.192 Solvothermal reactions between Cr(CO)6 and PCBrP1 in MeCN yielded (PCP1)CrBr2(MeCN) with MeCN bound trans to the CrdC bond. In contrast, performing the same reaction with M(CO)6 where M ¼ Mo or W yielded seven-coordinate (PNP1)M(CO)3Br. In a follow-up report,

Organometallic Pincer Complexes With Group 6 Metals

681

Kirchner and coworkers showed that performing the same solvothermal synthesis in toluene yielded square planar (PCP1)CrBr with R ¼ tBu and octahedral (PCP1)CrBr2(THF) with R ¼ iPr when crystallized from THF (Scheme 64).192 The reactivity of (PCP1) CrBr with R ¼ tBu was further explored with NO and LiBH4. Addition of LiBH4 to (PCP1)CrBr in THF formed (PCP1)Cr(k2-BH4), which was then reacted with NO (1 bar, RT) in toluene to form (PCP1)Cr(k2-BH4)(NO) with NO bound in the axial position. The same complex could be formed by reversing the order of addition: treating (PCP1)CrBr first with NO yielded (PCP1)CrBr(NO), which was then converted to (PCP1)Cr(k2-BH4)(NO) by addition of LiBH4. The square planar alkyl complex (PCP1)Cr(CH2SiMe3) with R ¼ tBu was prepared by a metathesis route with (PCP1)CrBr and LiCH2SiMe3 in toluene. This alkyl complex was competent for catalytic hydrosilylation of ketones.192

Scheme 64

In parallel to their work with PCP1 described above and PNP1 ligands described in Section 6.09.2.7, the Kirchner group explored the synthesis and reactivity of group 6 complexes containing PCP2. In 2016, they showed that solvothermal reactions conducted in MeCN between M(CO)6 (where M ¼ Cr and Mo) and PCHP2 where (N)R ¼ Me leads to (PCHP2)M(CO)3 with an agostic aryl CdH bond (Scheme 65).193 In contrast, the same reaction with W(CO)6 resulted in CdH cleavage to yield the seven-coordinate hydrido complex (PCP2)WH(CO)3. Similar metal-dependent differences in CdH reactivity were reported in solvothermal reactions with M(CO)6 by the same group with a PCP2 analog derived from pyrimidine.194

Scheme 65

In 2018, Kirchner and coworkers compared the reactivities of M(CO)6 with a PCClP2 ligand precursor containing an aryl-Cl bond and (N)R ¼ Et.195 The solvothermal reaction of PCClP2 and Cr(CO)6 in MeCN resulted in a mixture of (PCP2)CrCl2(MeCN) with aryl-Cl cleavage and the agostic CdH complex (PCHP2)Cr(CO)3 (Scheme 66). The origin of the hydrogen in

Scheme 66

682

Organometallic Pincer Complexes With Group 6 Metals

(PCHP2)Cr(CO)3 was attributed to solvent decomposition or trace amounts of water. Crystallization of (PCP2)CrCl2(MeCN) from THF yielded (PCP2)CrCl2(THF), and the authors described these solvent adducts as the first chromium PCP pincer complexes.195 In contrast to Cr, performing the same solvothermal reactions with M(CO)6 (where M ¼ Mo or W) and PCClP2 with (N)R ¼ Et yielded aryl-Cl cleavage to form (PCP2)M(CO)3Cl.195 Interestingly, both complexes undergo reversible CO addition across the M-aryl bond with the addition being more favored for M ¼ Mo, as corroborated by single-crystal XRD studies and DFT calculations (Scheme 66). Reduction of (PCP2)M(CO)3Cl (M ¼ Mo or W) with KC8 in THF yielded [K(THF)2][(PCP2)M(CO)3]. The addition of MeOH to [K(THF)2][(PCP2)Mo(CO)3] resulted in protonation of the ipso-carbon on the pincer ligand to afford the CdH agostic complex (PCHP3)Mo(CO)3. In contrast, performing the same reaction with the tungsten analog resulted in protonation of the metal to form the hydrido complex (PCP3)WH(CO)3.195 Similar to their studies with PCP1 and PCP2, Kirchner and coworkers reported that the pincer ligand precursor PCBrP3 undergoes coordination with CdBr cleavage in solvothermal reactions with M(CO)6 (where M ¼ Cr, Mo, or W) in MeCN.196 Reactions with Cr(CO)6 resulted in a mixture of complexes from which (PNP3)CrBr2(MeCN) could be isolated in 40% yield. In contrast, reactions with M ¼ Mo or W yielded seven-coordinate (PNP3)MBr(CO)3 in  80% yield. NHC-derived PCP4 is a flexible ligand that can adopt fac- or mer-type metal complexes depending on the identity of the ancillary ligands. The Tuczek group described the synthesis of Mo0 and MoIII complexes with both mer- and fac-PCP4 with R ¼ Ph.197 Heating Mo(PPh2Me)4(N2)2 with PCP4 in THF yielded mer-(PNP4)Mo(PPh2Me)2(N2) (Scheme 67). In contrast, reaction with Mo(PPh2Me)2(dmpm)(N2)2 yielded fac-(PNP4)Mo(dmpm)(N2). Treating MoX3(THF)3 where X ¼ Cl, Br, I with PCP4 in THF yielded mer-(PCP4)MoX3. These MoIII complexes could be reduced with Na amalgam under an N2 atmosphere in the presence of P(OMe)3 to afford mer-(PCP4)Mo[P(OMe)3]2(N2) along with the structurally-characterized side product mer-(PCP4)Mo [P(OMe)3]2[P(O)OMe].197 Tuczek and coworkers also reported the synthesis of fac-(PCP5)Mo(CO)3 where X ¼ Me and R ¼ Ph.198 MoCl3 complexes with PCP4 and PCP5 were synthesized by the Nishibayashi group in 2017 using a method similar to that described above by Tuczek and coworkers.199 Reduction of (PCP4)MoCl3 (R ¼ tBu) and (PCP5)MoCl3 (X ¼ H or Me and R ¼ tBu) with Na amalgam in THF under an atmosphere of N2 yielded the N2-bridged Mo0 complexes [(PCP4)Mo(N2)2]2(m-N2) and [(PCP5)Mo(N2)2]2(m-N2). Catalytic N2 reduction studies revealed that [(PCP5)Mo(N2)2]2(m-N2) was the better catalyst of the two.199 A subsequent DFT study that investigated the mechanism of these catalytic N2 reactions was reported.200

Scheme 67

Later studies by the same authors showed that (PCP4)MoX3 and (PCP5)MoX3 where X ¼ Cl, Br, or I and R ¼ tBu could be used directly for catalytic generation of ammonia from N2.201,202 Heating a THF solution of (PCP5)MoI3 with Me3SiN3 yielded crystals of (PCP5)Mo^NI.202 This nitride complex has similar catalytic N2 reduction activity as (PCP5)MoI3 when tested under similar conditions, suggesting it is implicated in catalytic N2 reduction cycles. This hypothesis was further corroborated by ESI-TOF-MS 202 analysis of reaction mixtures obtained from mixing (PCP5)MoI3 and 2.2 equiv. of CoCp∗ 2 in toluene under an atmosphere of N2. Aside from PCP4 and PCP5, other examples of NHC-derived PCP ligands have been reported with group 6 metals but are only known to undergo facial coordination.198,203

6.09.2.9

P-arene-P ligands

A special subsection of PCP-type organometallic ligands is the P-arene-P (PAP) ligands pioneered by the Agapie group (Chart 12). What makes these ligands so unique is that they can adjust their arene coordination and change hapticity at the metal in response to differing electron counts and redox states at the metal. The ligands can also be redox active, as demonstrated with the catechol-derived PAP2-red, which can be oxidized by two electrons to form the corresponding quinone PAP2-ox (Chart 12).

Organometallic Pincer Complexes With Group 6 Metals

683

Chart 12

Mo complexes containing PAP1 were introduced in 2014 and initially tested for NH3BH3 dehydrogenation.204 Addition of PAP1 to Mo(CO)3(MeCN)3 yields (PAP1)Mo(CO)3. Single-crystal XRD studies revealed that the arene ring in (PAP1)Mo(CO)3 binds to Mo in an Z2 fashion. Oxidation with two equiv. of AgOTf yields [(PAP1)Mo(CO)2](OTf )2 with CO loss and slippage of the arene to Z6 to maintain an 18-electron count at the metal (Scheme 68). Photolysis of [(PAP1)Mo(CO)2](OTf )2 in MeCN displaces CO to form [(PAP1)Mo(MeCN)2](OTf )2 (Scheme 69). Treating this triflate salt with Mg or 2 equiv. of LiHBEt3 under N2 yielded the Mo0 complex (PAP1)Mo(N2), which was shown to undergo reversible oxidative addition with H2 to form (PAP1)Mo(H)2(N2). Likewise, [(PAP1)Mo(MeCN)2](OTf )2 reacts with NH3BH3 in the presence of excess NaBPh4 to form monohydride [(PAP1)MoH(MeCN)]+, which can then undergo reversible reactions with H2 or NH3BH3 to form the trihydride [(PAP1)Mo(H)3]+ (Scheme 69).204

Scheme 68

Scheme 69

In 2016, [(PAP1)Mo(CO)2](OTf )2 was used in deoxygenative coupling of CO at a single metal center to form C2O products.205,206 Sequential reductions with KC8 yielded formally Mo0, Mo2−, Mo3− complexes that highlighted the coordinative flexibility of the arene ligand (Scheme 70). Treating the Mo2− and Mo3− complexes with 4 equiv. of iPr3SiCl yielded (PAP1) Mo(N2), (iPr3Si)2O, and the CdC coupled product iPr3SiOC^CSiiPr3. Addition of excess Me3SiCl to the Mo2− complex yielded the structurally characterized alkylidyne complex (PAP1)Mo^CSiMe3(CO)Cl via generation of the terminal carbide intermediate (PAP1)MoC(CO).206 In subsequent mechanistic studies, it was shown that this reaction also generates (PAP1)Mo(COSiMe3)2 and the structurally characterized C–C coupled product (PAP1)Mo(Z2-Me3SiOCCOSiMe3).206 The terminal carbide

684

Organometallic Pincer Complexes With Group 6 Metals

Scheme 70

(PAP1)MoC(CO) could be regenerated by treating (PAP1)Mo^CSiMe3(CO)Cl with nBu4NF in THF at −20  C, but this complex decomposes when warmed above 0  C.205 Addition of 2 equiv. of KC8 to (PAP1)Mo^CSiMe3(CO)Cl yields (PAP1)Mo(N2) and K [OC^CSiMe3]. The potassium salt was then converted to the ketene (Me3Si)2C]C]O by addition of Me3SiCl. In 2019, it was shown that treating (PAP1)MoC(CO) with CO yields C–C coupling to form a structurally-characterized phosphoranylideneketene (Scheme 71).207 Addition of (HNMe3)Cl and other acids to (PAP1)MoC(CO) in THF yielded (PAP1) Mo^CH(CO)Cl. It was shown that different isomers of this complex could be formed with the methylidyne positioned cis or trans with respect to the PAP1 arene. The methylidyne could be hydrogenated with Na(HBEt3) in toluene to form the corresponding methylidene Na[(PAP1)Mo]CH2(CO)Cl]. The methylidene eliminates ethenone and NaCl when exposed to an atmosphere of CO to generate (PAP1)Mo(CO)3. A similar alkylidyne ! alkylidene ! CO coupling cycle was observed starting with (PAP1)Mo^CSiMe3(CO)Cl. Na[(PAP1)Mo]CHSiMe3(CO)Cl] was formed at −20  C upon reaction with Na(HBEt3). Warming the reaction mixture to RT resulted in elimination NaCl with formation of the ketene complex (PAP1)Mo[Z2-C,O-OC(CHSiMe3)], which was confirmed by independent synthesis and structural characterization (Scheme 71).207

Scheme 71

In addition to carbide, terminal nitride and phosphides were prepared with (PAP1)Mo complexes. Reaction of (PAP1)Mo(N2) with ClPA (where A ¼ anthracene) in a toluene/MeCN mixture yielded (PAP1)Mo^PCl.208 Oxidation of this phosphide complex with [Fc][BArF24] in THF yielded [(PAP1)Mo^PCl]BArF24, which slowly gave way to [(PAP1)Mo(MeCN)2Cl]BArF24 with presumed bimolecular P–P coupling and elimination of P2 when warmed above 10  C in MeCN. Reaction of [(PAP1)Mo(MeCN)2](OTf )2 with 2 equiv. of [nBu4N]N3 yielded (PAP1)Mo^N(N3).209 Treating this complex with Me3SiOTf resulted in azide metathesis to form (PAP1)Mo^N(OTf ). Addition of CO formed (PAP1)Mo^N(CO)(OTf ), which was more readily crystallized and permitted structural verification of the successful metathesis. Reduction of (PAP1)Mo^N(OTf ) with 2 equiv. of M[C10H8] where M ¼ Na or K yielded M[(PAP1)Mo^N]. NMR and XRD data collected on the potassium derivative revealed dissociation of one of the phosphine arms and the alkali metal bridging between the terminal nitride and one of the phenylene units. Addition of 13CO to Na[(PAP1)Mo^N] resulted in nitride/carbonyl coupling to form Na[N13CO] along with (PAP1)Mo(13CO)3 and (PAP1)Mo(13CO).209

Organometallic Pincer Complexes With Group 6 Metals

685

In parallel with their studies with PAP1, Agapie and coworkers investigated redox-active derivatives of PAP1 derived from catechol. A series of molybdenum tricarbonyl complexes were reported in 2015 with PAP2-red and PAP3 where E ¼ SiMe2, SiEt2, SiiPr2, and B(C6H4CF3); these complexes were prepared by heating the corresponding ligand with (PhMe)Mo(CO)3 in THF (Scheme 72).210 (PAP3)Mo(CO)3 with E ¼ SiMe2 could be converted into (PAP2-red)Mo(CO)3 by addition of NaOMe in MeOH followed by aqueous workup with NH4Cl. The (PAP2-red)Mo(CO)3 and (PAP3)Mo(CO)3 complexes reduce O2 with loss of CO to form (PAP2-ox)Mo(CO)2 along with oxygen-containing reduction products H2O (via H2O2), (R2SiO)n, or (ArBO)3 (Scheme 72). Interestingly, (PAP1)Mo(CO)3 with no catechol, and (PAP4)Mo(CO)3 with OdMe instead of OdH, do not react with O2 unless the Lewis acid B(C6F5)3 is present.210

Scheme 72

A more explicit study of the redox and proton-coupled electron transfer (PCET) chemistry of PAP2-red and PAP2-ox were investigated with Mo in 2017.211 (PAP2-red)Mo(CO)3 was prepared and subsequently oxidized with 2 equiv. of AgOTf in MeCN to form [(PAP2-red)Mo(CO)3](OTf )2. This complex could then be deprotonated with 2 equiv. of NEt3 to form (PAP2-ox)Mo(CO)3, thereby demonstrating stepwise oxidation/deprotonation (a similar reaction with one equivalent of 2,6-di-tert-butyl-4-methylpyridine yielded the corresponding semiquinone).211 Treating (PAP2-red)Mo(CO)3 with 2 equiv. of tBu3PhO• radical in MeCN facilitated PCET to form (PAP2-ox)Mo(CO)3 in a single step. (PAP2-ox)Mo(CO)3 could be further oxidized with PhICl2 or two 2 equiv. of AgOTf in MeCN or THF/MeCN to form (PAP2-ox)MoCl2(CO) and [(PAP2-ox)Mo(CO)(MeCN)2](OTf )2, respectively. The latter complex could also be prepared by refluxing [(PAP2-red)Mo(CO)3](OTf )2 with 2,3-dichloro-5,6-dicyano1,4-benzoquinone (DDQ) via PCET.

6.09.2.10 PPP, PSP, SPS, and SSS ligands PPP, PSP, SPS, and SSS ligands used to prepare organometallic and related Group 6 complexes are summarized in Chart 13. Related complexes include CrCl3 complexes with PPP1 and several variants with longer linkers that were used for ethylene trimerization

Chart 13

686

Organometallic Pincer Complexes With Group 6 Metals

with aluminoxanes.212 (PPP2)MoCl3 with R]Cy and tBu were reported and used as precursors for N2 reduction chemistry.169,213,214 Interesting examples of Mo dinitrogen complexes with crown-functionalized PPP3 ligands were recently reported by Hulley and coworkers to investigate cation-N2 interactions.215 McGuinness and coworkers prepared CrCl3 complexes with sulfur-containing pincer ligands SPS1, PSP1, and PSP2 for ethylene trimerization studies with MAO.128 Given the extensive use of CrCl3 pincer complexes in ethylene polymerization catalysis, it is worth noting that Levason and coworkers described an early report in 1984 on the synthesis and characterization of CrX3 complexes (where X ¼ F, Cl, Br, and I) with tridentate ligands containing only soft P, S, and As donors.216 The type of isomer formed (fac vs. mer) was assigned based on IR data and analysis of CrdX stretches. This study revealed that (PPP2)CrX3 with R ¼ Ph and (SSS1)CrX3 adopt meridional isomers whereas other ligands in the study, including those with three arsine donors, adopt facial isomers with CrX3.216 In 1997, Jeffrey and Weller described the synthesis and characterization of tungsten alkylidyne complexes with PPP2 with R ¼ Ph, which appears to be the first such complexes containing a triphos ligand (Scheme 73).217 Heating W^CAr(CO)2(4-picoline)2Br (where Ar ¼ p-tolyl) with PPP2 in toluene yields bright yellow (PPP2)W^CAr(CO)Br. Addition of TlBF4 in CH2Cl2 displaces bromide to form an impure mixture containing [fac-(PPP2)W^CAr(CO)2]BF4, which is formed by scavenging CO from another complex. Performing the same reaction in MeCN yields [mer-(PPP2)W^CAr(CO)(MeCN)]BF4, which can then be cleanly transformed to [fac-(PPP2)W^CAr(CO)2]BF4 by treatment with CO. Addition of freshly prepared LiCuMe2 to (PPP2)W^CAr(CO)Br in THF affords (PPP2)W^CAr(CO)Me. Treating the bromo or methyl complexes with HBF4 ∙ Et2O in CH2Cl2 results in protonation of the alkylidyne to form the corresponding alkylidene. (PPP2)W^CAr(CO)Br was also protonated with HCl with no additional substitution at tungsten. [(PPP2)W]CHAr(CO)X]BF4 where X ¼ Me slowly decomposes in solution, whereas the X ¼ Br complex was more stable. Both complexes have agostic C–H bonds, as indicated by high field 1H NMR shifts d −4.71 and −4.49 ppm that display 183W-1H coupling.217

Scheme 73

In 2011, Bernskoetter and Tyler investigated the synthesis of low valent Mo complexes with PPP2 with R ¼ Ph.218 Reduction of fac-(PPP2)MoCl3 with Na amalgam in the presence of N2 and ethylene yielded mer-(PPP2)Mo(N2)2(C2H4) with axial N2 ligands and ethylene bound trans to P. Addition of CO2 to this ethylene complex yielded the short-lived intermediate (PPP2)Mo(Z2-CO2) (C2H4) (as observed by in situ NMR studies), which slowly gives way to CO2/ethylene coupling to form the bridging acrylate hydride complex shown in Scheme 74.

Scheme 74

Organometallic Pincer Complexes With Group 6 Metals

687

Berke and coworkers reported the synthesis of Mo and W nitrosyl complexes with PPP2 where R ¼ iPr.219 Reduction of (PPP2)M(NO)Cl3 (M]Mo or W) with Na amalgam in the presence of CO yielded (PPP2)M(NO)(CO)Cl with CO bound trans to the central phosphine in PPP2 (Scheme 75). Because of the differing axial NO and Cl ligands, syn and anti stereoisomers are possible based on the relative orientation of the central PdPh group. 31P NMR data revealed that the syn conformation was favored (>70%) for both metals.

Scheme 75

Heating (PPP2)M(NO)(CO)Cl with R ¼ iPr with excess LiBH4 in NEt3 yielded exclusively the syn isomers of the hydride (PPP2)M(NO)(CO)H (Scheme 76). No evidence of the anti isomer was observed for either metal, even by NMR analysis of the reaction mixtures. The Mo hydride complex reacts rapidly with CO2 in THF to yield the formate complex (PPP2)Mo(NO)(CO) (k1-OCHO). The same CO2 reaction with the analogous W hydride was significantly slower at RT and required heating to 60  C.

Scheme 76

Initial attempts to use (PPP2)M(NO)(CO)H with R ¼ iPr for catalytic hydrogenation of N-benzylideneaniline in THF with H2 (60 bar) at temperatures up to 140  C were unsuccessful.219 However, addition of the acid [H(Et2O)2][B(C6H5)4] allowed the hydrogenation reactions to proceed with a variety of imines at catalysts loadings as low as 0.2%. NMR studies suggested that addition of [H(Et2O)2][B(C6H5)4] to (PPP2)M(NO)(CO)H in THF turns on catalysis by liberating H2 to form the cationic solvate adduct [(PPP2)M(NO)(CO)THF][B(C6F5)4]. The THF ligand was shown to be labile, and mechanistic studies suggested that an open coordination site on the metal was necessary for hydrogenation catalysis to occur. In one of the few examples with PSP ligands, Kirchner and coworkers reported the synthesis of 4,6-bis(diphenylphosphinomethyl)dibenzothiophene (PSP3) and established its coordination chemistry with Cr.220 Reacting Cr(CO)6 with PSP3 in MeCN in a sealed tube for 6 h at 140  C yielded air-sensitive (PSP3)Cr(CO)3 in 89% yield (Scheme 77). Attempts to prepare the corresponding Mo and W complexes using this same solvothermal approach were unsuccessful.

Scheme 77

688

6.09.3

Organometallic Pincer Complexes With Group 6 Metals

Complexes with asymmetric pincer ligands

The chemistry of asymmetric pincer ligands was reviewed recently by Asay and Morales-Morales.16 In addition to affording significantly more ways to tune electronic structure and reactivity at the metal, using different flanking substituents can be used to weaken coordination and increase the lability of one of the pincer donor groups to afford open coordination sites needed for metal-centered reactions. Many of the asymmetric ligands shown in Chart 14 were used to prepare CrCl3 complexes for ethylene polymerization or oligomerization catalysis. In 2014, Huang and coworkers prepared a series of CrCl3 complexes with PNN1 and PNN2 (Chart 14) and tested them for ethylene polymerization with MAO and other alkylaluminum reagents.149 These complexes were not as catalytically active as their symmetric (PNP)CrCl3 counterpart. Similar PNN ligands have been used to prepare fac-(PNN)M (CO)3 complexes where M]Cr, Mo, and W.221 McGuinness and coworkers prepared (SNS2a)CrCl3 as part of their comprehensive screening of SNS, PNP, PSP, and SPS ligands for ethylene trimerization.128 Likewise, Liu and coworkers performed ethylene oligomerization and polymerization studies with (NNN11)CrCl3 complexes with R1]H or Me and R2]H, Me, or Bn.222 Several mer-(ONO)MoO2 complexes with asymmetric ligands similar to ONO3a have been reported along with their structures.223

Chart 14

Braunstein, Danopoulos, and coworkers described several reports exploring the chemistry of anionic PNC2, PNP3a, and PNP3b with chromium (Chart 14). PNC2 can be prepared by deprotonation/dearomatization of PNC1 by treatment with base (Scheme 78). The resulting potassium salt was transmetalated with CrCl2(THF)2 to form (PNC2)CrCl.224 A similar deprotonation/dearomatization method was used to prepare anionic PNP3a as well as its doubly-deprotonated dianionic derivative PNP3b, and the resulting salts were subsequently used to prepare the Cr(II) complexes (PNC3a)CrCl and the unusual trimetallic complex {Cr[(PNP3b)CrCl]2}.225,226 This latter complex has a single CrII ion sandwiched between two Z3-bound [(PNP3b)CrCl]− units. (PNC2)CrCl, (PNC3a)CrCl, and {Cr[(PNP3b)CrCl]2} were subsequently tested for ethylene oligomerization with MAO, and {Cr [(PNP3b)CrCl]2} was shown to be the most active of the three.226

Scheme 78

Organometallic Pincer Complexes With Group 6 Metals

689

In 2019, Ren et al. reported the synthesis of CrCl3 complexes with the amine/imine NCN11 (Scheme 79).227 Reaction of the (NCN11)AgCl with either CrCl2(THF)2 or CrCl3(THF)3 in THF gave structurally characterized (NCN11)CrCl3 complexes with both NCN11 ligands (n ¼ 2 and 3). Oxidation of CrII in CrCl2(THF)2 proceeded with reduction of AgI, as indicated by formation of a silver mirror. Both complexes were tested for ethylene oligomerization with MAO, and the NCN11 complex with the longer amine spacer (n ¼ 3) was the more active and selective oligomerization catalyst.227

Scheme 79

Veige and coworkers reported the synthesis and reactivity of tungsten alkylidene and alkylidyne complexes with ONO3a for comparison to similar complexes containing symmetric ONO3 (Section 6.09.2.3). Treating (tBuO)3W^CtBu with H3(ONO3a) in benzene yielded (ONO3a)W]HCtBu(OtBu) (Scheme 80).228 It was shown that the reaction proceeds with protonolysis of the two OdH groups and coordination of the protonated amine. The N–H complex (ONHO3a)W^CtBu(OtBu) was cleanly isolated after short reaction times and characterized by NMR spectroscopy. Heating (ONHO3a)W^CtBu(OtBu) at 70  C resulted in H-migration from the amine to alkylidyne to form the corresponding alkylidene as the thermodynamically-favored product (Scheme 80).

Scheme 80

As mentioned for other ONO ligands, (ONO3a)W]CHtBu(OtBu) as well as (ONHO3a)W^CtBu(OtBu) can be deprotonated with H2C]PPh3 to form the alkylidyne salt [MePPh3][(ONO3a)W^CtBu(OtBu)] (Scheme 81).228

Scheme 81

690

Organometallic Pincer Complexes With Group 6 Metals

The reactivity of [MePPh3][(ONO3a)W^CtBu(OtBu)] with various electrophiles was investigated to understand the preference for the two different nucleophilic ligand sites (WdN vs. W^C). Addition of MeOTf and Me3SiOTf resulted in alkylation and silylation of the amido group, which proceeded with elimination of (MePPh3)OTf (Scheme 82).228 In contrast, addition of 1.0 equiv. of HCl in Et2O to a benzene solution of [MePPh3][(ONO3a)W^CtBu(OtBu)] resulted in protonation of the alkylidyne instead of nitrogen to form (ONO3a)W]CHtBu(OtBu). Likewise, addition of a second equiv. of HCl protonated the alkylidene and resulted in chloride binding to form six-coordinate (ONO3a)W(CHt2Bu)(OtBu)Cl. This complex was also formed by reaction of the ammonium chloride salt [H4(ONO3a)]Cl with (tBuO)3W^CtBu (Scheme 82).

Scheme 82 Scheme 82

6.09.4

Conclusion

As this chapter has demonstrated, there is an amazing diversity of pincer ligands that have been used to prepare organometallic group 6 complexes. Tri- and dianionic pincer ligands with hard O, N, and C donor groups have been used to stabilize Mo and W in high oxidation states, often in complexes containing metal-element multiple bonds. In contrast, neutral and/or monoanionic pincer ligands containing pyridyl, carbenes, or soft P and S donor groups have been used to stabilize low-valent, electron rich metal complexes useful for small molecule transformations, especially with N2, CO2, and olefins. Many of the Cr complexes described here have been prepared and tested for ethylene oligomerization or polymerization in the presence of aluminum activators such as MAO. Mo and W complexes have been used in a wider range of applications because of their greater diversity of formal oxidation states (0 to +6). Furthermore, noninnocent pincer ligands capable of redox reactions and metal-ligand cooperativity have extended the types of chemical transformations possible in both stoichiometric and catalytic reactions. There are still several classes of pincer ligands known with late transition metals that remain un- or under-represented in group 6 organometallic chemistry. This includes pincer ligands containing heteroatoms in the p-block beyond those already listed (e.g., B,229 Si,230–232 and heavier p-block donors233). These appear to be fertile areas for future exploration in organometallic group 6 chemistry.

Acknowledgment This work was generously supported by the National Science Foundation under Grant No. CHE-1650894.

Organometallic Pincer Complexes With Group 6 Metals

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Organometallics 2016, 35, 3124–3131. Wan, R.; Buss, J. A.; Horak, K. T.; Agapie, T. Polyhedron 2020, 187, 114631. Zhang, Y.; Williard, P. G.; Bernskoetter, W. H. Organometallics 2016, 35, 860–865. Fryzuk, M. D.; MacNeil, P. A. J. Am. Chem. Soc. 1981, 103, 3592–35933. Wingard, L. A.; White, P. S.; Templeton, J. L. Dalton Trans. 2012, 41, 11438–11448. Ellermann, J.; Moll, M.; Will, N. J. Organomet. Chem. 1989, 378, 73–79. Chakraborty, S.; Berke, H. ACS Catal. 2014, 4, 2191–2194. Chakraborty, S.; Blacque, O.; Fox, T.; Berke, H. Chem. Asian J. 2014, 9, 328–337. Chakraborty, S.; Blacque, O.; Berke, H. Dalton Trans. 2015, 44, 6560–6570. Wei, Z.; Junge, K.; Beller, M.; Jiao, H. Catal. Sci. Technol. 2017, 7, 2298–2307. Leischner, T.; Spannenberg, A.; Junge, K.; Beller, M. Organometallics 2018, 37, 4402–4408. Leischner, T.; Artus Suarez, L.; Spannenberg, A.; Junge, K.; Nova, A.; Beller, M. Chem. Sci. 2019, 10, 10566–10576. Ott, J. C.; Isak, D.; Melder, J. J.; Wadepohl, H.; Gade, L. H. Inorg. Chem. 2020, 59, 14526–14535. Nishibayashi, Y.; Kuriyama, S.; Arashiba, K.; Nakajima, K.; Tanaka, H.; Yoshizawa, K. Eur. J. Inorg. Chem. 2016, 2016, 4856–4861. Moha, V.; Leitner, W.; Hoelscher, M. Chem. Eur. J. 2016, 22, 2624–2628. Hebden, T. J.; Schrock, R. R.; Takase, M. K.; Müller, P. Chem. Commun. 2012, 48, 1851–1853. Himmelbauer, D.; Stöger, B.; Veiros, L. F.; Pignitter, M.; Kirchner, K. Organometallics 2019, 38, 4669–4678. de Aguiar, S. R. M. M.; Stöger, B.; Pittenauer, E.; Allmaier, G.; Veiros, L. F.; Kirchner, K. Organometallics 2016, 35, 3032–3039. Tomsu, G.; Mastalir, M.; Pittenauer, E.; Stöger, B.; Allmaier, G.; Kirchner, K. Organometallics 2018, 37, 1919–1926. Himmelbauer, D.; Mastalir, M.; Stöger, B.; Veiros, L. F.; Kirchner, K. Organometallics 2018, 37, 3631–3638. Eder, W.; Stöger, B.; Kirchner, K. Monatsh. Chem. 2019, 150, 1235–1240. Gradert, C.; Stucke, N.; Krahmer, J.; Näther, C.; Tuczek, F. Chem. Eur. J. 2015, 21, 1130–1137. Gradert, C.; Krahmer, J.; Soennichsen, F. D.; Naether, C.; Tuczek, F. J. Organomet. Chem. 2014, 770, 61–68. Eizawa, A.; Arashiba, K.; Tanaka, H.; Kuriyama, S.; Matsuo, Y.; Nakajima, K.; Yoshizawa, K.; Nishibayashi, Y. Nat. Commun. 2017, 8, 14874. Egi, A.; Tanaka, H.; Konomi, A.; Nishibayashi, Y.; Yoshizawa, K. Eur. J. Inorg. Chem. 2020, 2020, 1490–1498. Ashida, Y.; Kondo, S.; Arashiba, K.; Kikuchi, T.; Nakajima, K.; Kakimoto, S.; Nishibayashi, Y. Synthesis 2019, 51, 3792–3795. Eizawa, A.; Arashiba, K.; Egi, A.; Tanaka, H.; Nakajima, K.; Yoshizawa, K.; Nishibayashi, Y. Chem. Asian J. 2019, 14, 2091–2096. Willms, H.; Frank, W.; Ganter, C. Chem. Eur. J. 2008, 14, 2719–2729. Buss, J. A.; Edouard, G. A.; Cheng, C.; Shi, J.; Agapie, T. J. Am. Chem. Soc. 2014, 136, 11272–11275. Buss, J. A.; Agapie, T. Nature 2016, 529, 72–75. Buss, J. A.; Agapie, T. J. Am. Chem. Soc. 2016, 138, 16466–16477. Buss, J. A.; Bailey, G. A.; Oppenheim, J.; VanderVelde, D. G.; Goddard, W. A.; Agapie, T. J. Am. Chem. Soc. 2019, 141, 15664–15674. Buss, J. A.; Oyala, P. H.; Agapie, T. Angew. Chem. Int. Ed. 2017, 56, 14502–14506. Buss, J. A.; Cheng, C.; Agapie, T. Angew. Chem. Int. Ed. 2018, 57, 9670–9674. Henthorn, J. T.; Lin, S.; Agapie, T. J. Am. Chem. Soc. 2015, 137, 1458–1464. Henthorn, J. T.; Agapie, T. Inorg. Chem. 2016, 55, 5337–5342. Wu, F.-J. Ethylene Trimerization. US Patent 5811618, September 22, 1998 (Amoco Corporation). Arashiba, K.; Kinoshita, E.; Kuriyama, S.; Eizawa, A.; Nakajima, K.; Tanaka, H.; Yoshizawa, K.; Nishibayashi, Y. J. Am. Chem. Soc. 2015, 137, 5666–5669.

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Liao, Q.; Cavaille, A.; Saffon-Merceron, N.; Mezailles, N. Angew. Chem. Int. Ed. 2016, 55, 11212–11216. Pap, L. G.; Couldridge, A.; Arulsamy, N.; Hulley, E. Dalton Trans. 2019, 48, 11004–11017. Gray, L. R.; Hale, A. L.; Levason, W.; McCullough, F. P.; Webster, M. J. Chem. Soc. Dalton Trans. 1984, 47–53. Jeffery, J. C.; Weller, A. S. J. Organomet. Chem. 1997, 548, 195–203. Bernskoetter, W. H.; Tyler, B. T. Organometallics 2011, 30, 520–527. Chakraborty, S.; Blacque, O.; Fox, T.; Berke, H. Chem. Asian J. 2014, 9, 2896–2907. Mastalir, M.; Schweinzer, C.; Weil, M.; Pittenauer, E.; Allmaier, G.; Kirchner, K. Monatsh. Chem. 2016, 147, 1183–1187. Yang, C.-C.; Yeh, W.-Y.; Lee, G.-H.; Peng, S.-M. J. Organomet. Chem. 2000, 598, 353–358. Luo, W.; Li, A.; Liu, S.; Ye, H.; Li, Z. Organometallics 2016, 35, 3045–3050. Hossain, M. K.; Haukka, M.; Sillanpää, R.; Hrovat, D. A.; Richmond, M. G.; Nordlander, E.; Lehtonen, A. Dalton Trans. 2017, 46, 7051–7060. Simler, T.; Danopoulos, A. A.; Braunstein, P. Chem. Commun. 2015, 51, 10699–10702. Simler, T.; Frison, G.; Braunstein, P.; Danopoulos, A. A. Dalton Trans. 2016, 45, 2800–2804. Simler, T.; Braunstein, P.; Danopoulos, A. A. Organometallics 2016, 35, 4044–4049. Ren, X.; Wesolek, M.; Braunstein, P. Dalton Trans. 2019, 48, 12895–12909. VenkatRamani, S.; Ghiviriga, I.; Abboud, K. A.; Veige, A. S. Dalton Trans. 2015, 44, 18475–18486. van der Vlugt, J. I. Angew. Chem. Int. Ed. 2010, 49, 252–255. Begum, R.; Komuro, T.; Tobita, H. Chem. Commun. 2006, 432–433. Turculet, L. In Pincer Pincer-Type Complexes: Applications in Organic Synthesis and Catalysis; Szabó, K. J., Wendt, O. F., Eds.; Wiley-VCH: Weinheim, Germany, 2014; pp 149–187. 232. Simon, M.; Breher, F. Dalton Trans. 2017, 46, 7976–7997. 233. Cabeza, J. A.; Garcia-Alvarez, P.; Laglera-Gandara, C. J. Eur. J. Inorg. Chem. 2020, 2020, 784–795.

6.10 Organometallic Complexes of Group 7 Metals With Pincer and Noninnocent Ligands Oana R Luca, Tessa HT Myren, Haley A Petersen, and Shea J O’Sullivan, Department of Chemistry, University of Colorado Boulder, Boulder, CO, United States © 2022 Elsevier Ltd. All rights reserved.

6.10.1 Introduction 6.10.2 Manganese complexes with pincer and noninnocent ligands 6.10.2.1 Mn carbonyls and catalytic applications 6.10.2.1.1 Mn PNP pincer complexes with carbonyls and catalytic applications 6.10.2.1.2 Mn PNN pincer complexes with carbonyls and catalytic applications 6.10.2.1.3 Mn NNN pincer complexes with carbonyls and catalytic applications 6.10.2.1.4 Mn CNC pincer complexes with carbonyls and catalytic applications 6.10.2.2 Mn pincer complexes with MndC bonds and no carbonyls 6.10.3 Rhenium complexes with pincer and noninnocent ligands 6.10.3.1 Re carbonyls and catalytic applications 6.10.3.1.1 Rhenium PNP catalysts for the hydrogenation of carbonyls 6.10.3.1.2 Rhenium CNC pincers with carbene ligands 6.10.3.1.3 Rhenium pincers with NNN ligands 6.10.3.2 Re non-carbonyls and catalytic applications 6.10.3.2.1 Rhenium alkyls and carbonyl insertions 6.10.4 Technetium complexes with pincer ligands 6.10.5 Conclusion Acknowledgement References

6.10.1

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Introduction

Organometallic pincer complexes are species that contain at least one metal-carbon bond in which a ligand occupies three meridional sites around a metal center. The present chapter describes the synthesis and reactivity of such complexes with representative group 7 metals. Tight meridional coordination of these geometries provides increased stability and allows for tunability of both the sterics and the electronics of a metal center. Since COMC3, group 7 has been intensely represented in the research arena of catalyst development with applications spanning CdC bond forming reactions, heterocycle synthesis, electrocatalytic CO2 reduction and beyond, with Re and Mn. A featured highlight of catalytic applications of group 7 pincers of this article is the use of ligand frameworks that can participate as actors in chemical processes by storing reducing or oxidizing equivalents, allowing base metal centers to achieve “nobility,”1,2 i.e., activity akin to that of precious metals through unprecedented multi-electron catalytic transformations. This article summarizes the synthesis and reactivities of group 7 organometallics with pincer ligands with an emphasis on reactivity imparted by the use of non-innocent ligands. Since COMC3 did not contain an explicit chapter on these types of pincers, examples of historical significance will also be included. The pincers of precious metals of the neighboring groups of the periodic table such as Pd,3 Ir4,5 and Rh6,7 and Ru8 have been extensively studied in the realm of organometallic catalysis. Several classes of pincer ligands have emerged over the past two decades, many of which will be outlined below for each of the representative metal’s applications. Carbonyl species dominate the field, due in part to the ubiquity and synthetic versatility of M(CO)5 X (with M, X being Mn, Br and Re, Cl respectively). Technetium is not well represented in the field of organometallic pincer complexes; however a brief discussion is included.

6.10.2

Manganese complexes with pincer and noninnocent ligands

Manganese is the third most earth abundant metal and as such, the development of catalytic applications based on its complexes is of interest. With the renaissance in the use of organometallic Mn pincers in catalytic applications over the past two decades, a remarkable dearth of low valent MnI precursors is observed, with Mn(CO)5Br P1 being the most used source of Mn in the synthesis of Mn organometallics (Fig. 1). Lacy has recently described the first new Mn synthon9 P2 in quite some time, in which the anionic halide of P1 is replaced with a methyl. This precursor is particularly poised to facilitate the synthesis of pincers that would normally require a deprotonation of a ligand precursor, with methane being evolved as a side product of the metalation reactions, as was demonstrated in the synthesis of a complex with central coordination derived from phenol flanked by amine pincer arms. Higher valent MnII and MnIII pincers are also known and can be prepared from MnCl2 P3, Mn(CH2SiMe3)2 P4, and MnIII(acac)3 P5 and dimers P6 and P7 as presented later in the chapter.

Comprehensive Organometallic Chemistry IV

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Fig. 1 Mn precursors for Mn organometallic complexes.

Fig. 2 Structural diversity of MnPNP pincers with carbonyl ligands.10–13

6.10.2.1 6.10.2.1.1

Mn carbonyls and catalytic applications Mn PNP pincer complexes with carbonyls and catalytic applications

PNP ligands are some of the most common pincers employed in Mn organometallics. Both neutral and anionic pincer ligands with PNP coordination are known, with wide array of structural diversity. Fig. 2 illustrates such diversity through examples of Mn PNP pincers with central amines, imides, imines and pyridines, flanked by bis-alkyl phosphines. The development of such a wide array complexes is driven by extensive catalytic applications as well as by the synthetic accessibility of various intermediates. Interestingly, the majority of the PNP pincer ligands support catalysis at the MnI/MnII redox couple, with significant ligand participation in the observed chemical processes. The ligand can act as an electron/proton reservoir and thus enable multi-electron catalysis. 6.10.2.1.1.1 Synthesis A representative synthesis of a metal complex with PNP ligation is shown in Scheme 1. Starting with a neutral PNP pincer with a central amine or a central pyridine (Scheme 1 shown as an amine), a Mn center can be installed through the reaction with Mn(CO)5Br P1 to obtain either Mn(PNP)(CO)2Br or Mn(PNP)(CO)3Br. Upon abstraction of a proton with base, the central ligand becomes anionic. This anionic ligand can be an aliphatic amine or a pyridine whose aromaticity is now broken and the central pyridine atom is converted into an anionic X-type donor ligand (Scheme 2). The number of carbonyls on the obtained MnI PNP

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Scheme 1 General synthetic scheme and reactivity profile for MnPNP pincers with NH central coordination.14

Scheme 2 Control of coordination sphere in Mn PNP pincers with central pyridine coordination and ancillary deprotonation site.15

pincer can be controlled as shown in Scheme 2, with an Ar purge leading to the formation of MnI(PNP)(CO)2. Deprotonated MnI pincers with centric N− can react reversibly with H2, C]O bonds and nitriles. This reversible reactivity profile enables remarkable catalytic activity based on ligand cooperation in observed catalyzes. In 2016 Milstein reported15 the synthesis of MnPNP(CO)3Br 14 under relatively mild conditions, in benzene at room temperature from the ligand and Mn(CO)5Br P1 after 20 h of reaction. Upon exposure to base, 14 forms 15 and 16, compounds in which the phosphine arm was deprotonated (Scheme 2). This deprotonation allows the central pyridine of the ligand to dearomatize and to enable anionic N ligation after loss of a halide counterion. The mixture of 15 and 16 can be converted to exclusively 15 or 16 by either exposure to an atmosphere of CO or purging with Ar, respectively. Related asymmetric PNP pincer complexes, of which 7 is an example (Fig. 2), were further reported in the study of olefination of nitriles with alcohols16 and N-formylations.17 In addition, Beller reported synthetic analogues 1, 2, 3 in which the central pyridine was substituted by an aliphatic amine as catalysts for N-formylation reactions and CdC bond formations.18–20 Upon switching ligation from a central pyridine to an aliphatic amine, a different set of reactivity patterns are observed. When using a less rigid ligand framework rather than the pyridine centered PNP frameworks, Beller reports that the synthesis of the Mn complexes yields isomeric species 17 with facial tridentate coordination and pincer meridional 18 as a mixture. 17 can be further decarbonylated by reflux in toluene (Scheme 3).21,22 The mer and fac isomers are reported as having equal catalytic activities in the hydrogenation of esters.

Scheme 3 Synthesis and speciation of Mn PNP pincers with flexible aliphatic ligands and NH central coordination.21

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Organometallic Complexes of Group 7 Metals With Pincer and Noninnocent Ligands

The reactivity pattern of complexes 17 and 18 with base yields five coordinate 2. 2 can then perform the heterolytic activation of H2 to form the 6-coordinate, 18e− complex 3 in which an amine ligand is protonated at the central atom of the pincer (Scheme 4). Complex 3 and analogues have been shown as active catalysts in the hydrogenation of nitriles, ketones, aldehydes and esters.21,22

Scheme 4 Synthesis, speciation and hydrogen reactivity of Mn PNP pincers with NH central coordination and flexible aliphatic ligation.

6.10.2.1.1.2 Methanol oxidation In 2016, Beller reported the iPr and Cy derivatives of complexes 1 compared against 8 and 17 as active catalysts for the dehydrogenation of aqueous methanol.23 Catalyst 1 was shown to be the most active and to operate in basic aqueous media with turnover numbers more than 20,000 observed over 1 month without decomposition. The system also dehydrogenates ethanol and can oxidize formaldehyde as well. The latter reactivity is supporting evidence that formaldehyde is an intermediate in the observed chemistry of Scheme 5.

Scheme 5 Catalytic dehydrogenation/oxidation of methanol.23

This initial report sets the stage for a large body of work that uses Mn-pincer compounds for in situ generation of H2 in hydrogenation reactions with methanol as an H2 source,24 transfer hydrogenation catalysis,24 heterocycle dehydrogenation chemistry25 and as a hydrogen-release catalytic system in schemes related to reversible hydrogen storage in organic molecules.26 6.10.2.1.1.3 CdC bond-forming reactions Related to Beller’s observations, Milstein harnesses the oxidative dehydrogenative activity of 16 to activate nitriles and demonstrate additions across the Mn and the anionic N atom of the pincer ligand.15 Taking advantage of water assistance for a tautomerization, a coordinated nitrile can do conjugate addition reactions with alpha-unsaturated carbonyls, thus forming products with new CdC bonds (Scheme 6). When switching to the asymmetric ligand environment in 7,16 Milstein was able to enact an a-olefination of nitriles by primary alcohols. In contrast to the chemistry of 16, instead of adding the nitrile bond across the Mn-N of the catalyst, a catalytic intermediate is reported to deprotonate the acidic position alpha to the nitrile, generating a carbanion that can attack a carbonyl generated in a dehydrogenation step.15 A variety of substituted acrylonitriles are produced in the process.

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Scheme 6 CdC bond forming reactions catalyzed by Mn PNP (CO)2 pincers.27

Taking advantage of the reported dehydrogenative activity reported by Beller and Milstein, Kempe27 designed catalytic derivatives 12-N and 12-CH, in which a hydride is already installed in the active catalyst along with basic amines on the pincer arm. The Mn-H can be regenerated with H2 release from an alcohol, akin to the Beller system. The alcohol therefore converts to an aldehyde that is then able to undergo a condensation with an enamine produced from alpha-alkyl quinolines (and pyridines) in base, thus enabling the installation of an alkene at the alpha position of N-containing heterocycles, a CdC bond forming reaction (Scheme 6). This report is an important advance, since olefination reactions such as the Wittig,28 Julia,29 and McMurry30 olefinations, to name a few, often employ toxic reagents and forcing conditions. The Heck cross coupling reaction31 is an alternate route to these types of substrates, however, the Heck reaction is incompatible with substrates that contain aryl halides, as those halides are activated during catalysis.

6.10.2.1.1.4 2 and 3-Component heterocycle synthesis As Beller and Milstein continued their reports of various dehydrogenations, coupling and condensations, Kirchner32 hypothesized that the PNP pincer ligand framework would be compatible with a sequence of condensations and dehydrogenations necessary for the construction of heterocycles (Scheme 7). Building on observations of the coupling of alcohols with amines by Mn pincer hydrides,33 the synthesis of 15 quinolines and 14 different pyrimidines was achieved. The authors noted the importance of the free amines in the ligand backbone, with the N-Me analogues being inactive. Additionally, the authors note that a base is necessary for the observed reactivity. The deprotonation of a ligand amine is proposed as a justification for the necessity of base; however, another explanation may that the base participates in the condensation steps synergistically with catalyst activation.

Scheme 7 Synthesis of Quinolines (top) and Pyrimidines (bottom) catalyzed by manganese PNP pincer hydride complexes.32

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Organometallic Complexes of Group 7 Metals With Pincer and Noninnocent Ligands

6.10.2.1.1.5 Dehydrogenative alcohol coupling Based on the original reports of catalytic dehydrogenation with Mn pincers, Gauvin devised a synthetic strategy for the synthesis of esters from alcohols through a self-coupling reaction mediated by 2-iPr as shown in Scheme 8.34 In this work, the dehydrogenation of alcohols was coupled with a nucleophilic attack by hydroxide. This chemistry converts alcohols to carboxylic acids and their derivatives and is tolerant of olefins and C-halide bonds, without the use of an external oxidant or a hydrogen acceptor.35 Rueping, in 2019, took the dehydrogenation chemistry one step farther to report the C-alkylation of secondary alcohols36 with PNN and PNP pincers by employing a double dehydrogenation strategy that produces a ketone and an aldehyde that are then able to undergo an aldol condensation. The Mn pincers can then act as hydrogenation catalysts to form CdC-bond “cross-coupled” products. The tandem hydrogenation/dehydrogenation chemistry is dubbed hydrogen autotransfer.

Scheme 8 Dehydrogenative alcohol coupling catalyzed by 2-iPr.34

6.10.2.1.1.6 Hydrogenation of carbonyl compounds The work on the hydrogenation of polar C]X (X ¼ O, N) bonds was spearheaded by Beller in a first report in 2016.18 In this seminal work, the speciation and mechanism of the hydrogenation chemistry was addressed. With the reactivity of the catalyst and proposed intermediates mapped out (Schemes 3 and 4), Beller went on to explore the transfer of an H2 equivalent from the catalyst to various forms of unsaturated CdX bonds. Using 1 and 12-N, he observed hydrogenation of both aldehydes and ketones to primary and secondary alcohols with good substrate scope and yields as shown in Scheme 9.18

Scheme 9 Hydrogenation of carbonyls to alcohols with H2 catalyzed by 1 and 12-N.18

Shortly after Beller’s report, Kempe’s catalysts based on the 12 framework were reported as an even more active system, achieving higher turnover numbers and full conversions in shorter times.37 Synthesis of chiral alcohols has been achieved through the use of a chiral ligand framework 5, and hydride derivatives under extraordinarily mild conditions of transfer hydrogenation (e.g., rt., 4–10 mol% KOtBu) with excellent substrate scope and enantiomeric excess (ee). In a related study, transfer hydrogenation was demonstrated with isopropyl alcohol as the hydrogen source rather than hydrogen gas. The authors observe that the presence of molecular hydrogen does not affect the outcome of the reaction in terms of either yields or ee.38 This work builds on a previous report by Clarke in which facial analogues of a pyridine-containing Mn pincer with ferrocenyl-derived asymmetric ligation39 hydrogenate alcohols and esters. The latter ester hydrogenation remarkably removes a carbonyl from the ester to yield an alcohol product. In a related report, Kirchener reports 12CH as a catalyst for the hydrogenation of aldehydes with no activity towards ketones.40 Additionally, the reported process occurs in the absence of added base. 12-CH shows high activity in this study, with the Re analogues showing poor activity. 6.10.2.1.1.7 Hydrogenation of nitriles In the original report that highlighted the hydrogenation of carbonyls, Beller also included observations related to the hydrogenation of nitriles.18

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701

The observed chemistry (Scheme 10) allows for the conversion of nitriles to primary amines, taking advantage of the H2-splitting ability of the deprotonated version of the Beller ligand frameworks. Derivatives of complexes 1 and 3 have shown reactivity in this reaction.

Scheme 10 Catalytic hydrogenation of nitriles by Beller and coworkers.18

6.10.2.1.1.8 Hydrogenation of alkynes Junge has developed aliphatic pincer41 3 that can coordinate an alkyne and enable its hydrogenation to a Z-alkene. The proposed mechanism for this reaction in Scheme 11 depicts a coordination of the alkyne to both the HN of the ligand and MnH locus of 2. This coordination is a rationale for the observed product selectivity to the Z olefin. The reported reactions proceed with aryl-substituted alkynes but bis-alkylated alkynes show no reactivity. This work is the first example of alkyne reduction by molecular hydrogen with a well-defined Mn-based molecular catalyst. In a subsequent study, Rueping was able to perform a similar semihydrogenation with an air stable Mn pincer supported by SNP ligation.42 In this work, a PEt2 ligand arm of the PNP ligand was substituted with an S-Me to afford air stability to the catalytic species.

Scheme 11 Coordination of an alkyne to 3 and its semihydrogenation.41

6.10.2.1.1.9 C-N coupling from alcohols and amines Kirchner has shown that catalysts 1 and 19 can turn over in the dehydrogenative coupling of a benzylic alcohol with amines (Scheme 12).33 The chemistry has proven tolerant of a wide range of aliphatic amines, anilines as well as heteroaromatic amines. The process was also compatible with aliphatic primary alcohols.

Scheme 12 Dehydrogenative coupling of alcohols with amines catalyzed by 1 and 19.33

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Organometallic Complexes of Group 7 Metals With Pincer and Noninnocent Ligands

In the presence of molecular sieves, 12 catalyzes the synthesis of aldimines from amines and alcohols. The chemistry showed improvement when molecular sieves were added to aid in the removal of water. Anilines and aliphatic amines were tolerated well when coupled to benzylic alcohols. Reduced yields down to 17% (compared to 92%) were observed when aliphatic alcohols were used (Scheme 13). Methylation of a wide array of anilines was reported with methanol as the methyl source by Milstein.43 Remarkably, the reported conditions were tolerant of imines, carbonyls, amide and ester substitutions under the optimized conditions.

Scheme 13 Aldimine synthesis from amines and alcohols.43

Interestingly, when switching to catalyst 16, Milstein observed reactivity towards the synthesis of imines, presumably facilitated by the removal of water with molecular sieves.43 An additional instance of CdN bond forming reaction is the synthesis of amides from alcohols and amines through a dehydrogenation reaction (Scheme 14). Kempe43 and Milstein44,45 reported two instances of this reactivity in 2017 in which an amine is converted into an amide in rather mild conditions. 21 is reported to catalyze the reaction with higher yields and better turnover numbers, thus showing improved activity.43

Scheme 14 Synthesis of formamides from amines and methanol catalyzed by 20 and 21.43–45

6.10.2.1.2

Mn PNN pincer complexes with carbonyls and catalytic applications

Availability of resonance delocalization of the ligand was taken one step farther in the development of catalytic application based on PNN pincers with a basic amine arm featuring an NH moiety 22 (Scheme 15).46 This type of pincer can attain an anionic aliphatic N pincer arm 23 through deprotonation with KOtBu and loss of KBr. 23 can reversibly bind H2 through heterolytic scission of the H2 bond. In this process, 24 is formed, with a Mn-H and a protonated ligand. Additionally, 23 can bind carbon dioxide with a carbon bound onto the anionic N ligand fragment and O-bound to the Mn 25. If the N arm of the pincer is bis-alkylated and the NH bond is not available, then deprotonation of the phosphine side of the ligand is observed and the reactivity towards H2 tracks with that observed with the symmetrical PNP ligand, with the H2 adding to re-establish the pyridine aromatic ring.46 Taking advantage of this dual reactivity towards both H2 and CO2 and the observed ligand cooperativity, Milstein reports asymmetric PNN Mn pincers that catalyze hydrogenation of carbon dioxide to formic acid with activities observed up to 60 h in THF and KOH with 10 mol% catalyst loading.46 A closely-related lutidine based chiral PNN pincer complex was able to exploit the reactivity patterns towards reversible H2 binding in Scheme 15 to enable an enantioselective hydrogenation of ketones with turnover numbers as high as 9800.47 6.10.2.1.2.1 Aza-Michael additions to unsaturated nitriles A remarkable example of reactivity enabled by redox actor ligands on PNN pincer frameworks involves reactions with nitriles, and their reactivity in coupling reactions.48 The mechanism for the catalytic oxa-Michael addition of alcohols to unsaturated nitriles is shown in Scheme 16. 26, the starting form of the catalyst isomerizes to 27, with an unsaturation now observed on the arm containing the NEt2. Unlike 23, 26 can no longer accommodate ligand reactions at the nitrogen of the pincer arm and instead is able to bind substrates at the alpha position to the pyridine. Upon exposure to either an alpha or a beta unsaturated nitrile, 27 can bind the nitrile carbon with the carbon on the

Scheme 15 Reactivity pattern of MnPNN pincers with a basic amine arm: reversible H2 activation and isomerization and CO2 binding.46

Scheme 16 Mechanism for the catalytic oxa-Michael addition of alcohols to unsaturated nitriles mediated by 26.48

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Organometallic Complexes of Group 7 Metals With Pincer and Noninnocent Ligands

NEt2 side of the amine pincer arm to form 28 or 29 respectively, with 28 isomerizing to 29. Once on the metal complex, the nitrile fragment reacts with the incoming alcohol, shown as isopropanol (Scheme 16) to form 30. 30 Isomerizes to 31 and then releases the product to regenerate the catalyst 27 thus closing the cycle.48

6.10.2.1.3

Mn NNN pincer complexes with carbonyls and catalytic applications

NNN pincers are ubiquitous in catalysis and photochemical applications with representation in most of the metals of the periodic table. Several variants of redox-active pyridine-bisimines and terpyridines are known in the realm of coordination chemistry with Mn complexes.1 Curiously, such pincers have limited presence in Mn organometallics with the few examples described in this section. A few additional instances are described in the section on Re organometallics as well. Chardon Noblat first reported the Mn pincer complex 34 in their study of CO release from terpy Mn complex with coordination of ancillary pyridine.49 Following release of a CO ligand from the coordination sphere upon irradiation of Mn-k2-terpyridine (CO)3MeCN+, the uncoordinated pyridine can bind to the metal to form 34, a pincer complex (Scheme 17).

Scheme 17 CO release from Mn-k2terpydine(CO)3MeCN+.49

Aside from reporting the first organometallic example of Mn terpyridine pincers, the authors make preliminary observations related to these complexes’ ability to perform electrocatalytic CO2 reduction. This observation was followed up by Kubiak50 in a later report. Spectroelectrochemical studies revealed the conversion of 35 to anionic 36. 36 then can lose a Br− ligand and dimerize to 37. The dimer 37 can then break apart upon the introduction of another reducing equivalent to lead to the catalytically active species 38 (Scheme 18).

Scheme 18 Intermediates in the electrocatalytic reduction of carbon dioxide to carbon monoxide and water mediated by a Mn-k3-terpydine(CO)3Br 35 precatalyst.50

The authors point out that the k3 coordination mode of the ligand causes the catalyst to be prone to degradation compared to bpy-based electrocatalytic systems (only four overall turnovers were observed). This observation is counterintuitive since bidentate Mn bipyridines are well represented in the field of electrocatalytic conversion of CO2 and that the terpyridine would be expected to exhibit greater stability due to additional chelation.51,52

6.10.2.1.4

Mn CNC pincer complexes with carbonyls and catalytic applications

An early report by Richeson brought forth 8, a MnPNP(CO)+53 pincer as an active electrocatalyst for the synthesis of CO from 3 carbon dioxide. Up until this report, work in the field of electrochemical carbon dioxide reduction had mostly been based on bidentate bipyridine systems, therefore the pincer was regarded as an “unusual” coordination mode. The report showed out of the ordinary electrocatalytic behavior. Specifically, the catalyst could turn over in the absence of protons and carbonate was being formed alongside CO, the traditional 2e− product. The authors did not make a mechanistic proposal at the time. Later, the Luca group investigated the isoelectronic CNC pincers 39,54 in which sigma donating N-heterocyclic carbenes are installed in lieu of the phosphine arms through a synthetic scheme shown in Scheme 19.

Scheme 19 Synthesis of Mn-CNC (CO)+3 pincers 39.54

Organometallic Complexes of Group 7 Metals With Pincer and Noninnocent Ligands

705

A similar unusual behavior was observed: activity in the absence of protons and formation of carbonate.54 The authors followed up the original report with a mechanistic study. Using electroanalytical and preparative techniques, an order in catalyst of 0.5 was determined along with a second order in CO2 concentration and turnover at a 1e− redox couple, along with a loss of 2 CO’s after the first one-electron reduction from the parent compound.55 This led to a mechanistic proposal that involves two distinct Mn centers enabling a reductive disproportionation reaction that can be seen in Scheme 20. Upon a 1e− reduction, 39 loses two CO ligands to become a square planar formally Mn0 species 40. DFT analysis suggests that the first electron populates an antibonding orbital of the ligand, thus making the CNC ligand framework redox-active. Two of the reduced Mn centers are then able to activate a carbon dioxide molecule, thus donating one electron each to attain 41. 41 is then able to insert a second CO2 molecule in the MdC bond to form 42, which can then rearrange to form a bimetallic carbonate 43 and CO. Upon loss of carbonate and CO, the catalyst 39 regenerates and enters in another turnover after a 2e− reduction.

Scheme 20 Mechanism for the electrochemical reduction of carbon dioxide to CO and carbonate catalyzed by 39.

A recent report has now identified 39 as active in the reaction of beta alkylation of secondary alcohols with primary alcohols to produce beta alkylated ketones.56 The mechanistic studies of this process reported an outer sphere mechanism with the dehydrogenation of the secondary alcohol substrate as rate-limiting. Interestingly, no ligand-cantered redox activity is invoked in this study.

6.10.2.2

Mn pincer complexes with MndC bonds and no carbonyls

In the realm of Mn alkyl pincers, the van Koten examples of the 1990s57,58 play a historical role (Schemes 21 and 22) as they are some of the first examples of MnII alkyls.

Scheme 21 Synthesis of the Van Koten MnNCN alkyl pincers 45.

Scheme 22 Cross coupling catalysis mediated by 45.58

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Organometallic Complexes of Group 7 Metals With Pincer and Noninnocent Ligands

In 1996 and 1998, the synthesis and activity of Mn pincer complexes with Mn-alkyl and aryl ligation 45 was reported. The complexes were synthesized starting with a transmetallation of [Li(NCN)]2 with two equivalents of MnCl2 P3 in THF at room temperature for 15 min (Scheme 21). This produces a paramagnetic yellow green solid MnII complex 44 after drying and a pentane wash. Further treatment with MeLi or n-BuLi in benzene for 5 min gives 45 as air sensitive, paramagnetic, green-colored oils which are soluble in polar and non-polar solvents. All three Mn complexes 45 are catalysts for the coupling of primary, secondary, and tertiary alkyl Grignard reagents when 10 mol % of the complex was used along with 5 mol% CuCl in THF at 5  C in 15 min. A 92% yield was achieved for secondary alkyl Grignard reagents. In the case of n-EtMgCl, the cross-coupled product was produced at 94% (Scheme 22). A variety of functional groups is tolerated and little to no olefin rearrangement is detected in the case of unsaturated functionalities. The reaction was also successful in the case of ketones and esters. These reactions are also much faster compared to stoichiometric MnCl2. Additionally, they can be used in the 1,4-addition of Grignard reagents to a,b-unsaturated ketones with yields of up to 92%. A few further experiments probing for mechanistic insight found that a Mn:Cu ratio of 2:1 is optimal, with both higher and lower ratios resulting in lower yields. Alternate CuI sources also result in lower yields than CuCl, further confirming the role of the Mn species in the catalytic cycle. The authors propose that an organomanganate aggregate likely formed during turnover. In the aggregate, the R group alkyl of the MnR(NCN) 45 may bond with putative [Cu4Br2(NCN)2] species. In 2017, the first manganese catalyst for an enantioselective hydroboration of ketones was reported.59 The precatalyst was synthesized by mixing the ligand with the metal precursor Mn(CH2SiMe3)2 P4 or MnCl2 P3 and base followed by LiCH2SiMe3 (Scheme 23).

Scheme 23 Synthesis of the Gade Mn NNN pincer alkyl 46.59

Initial surveys with substrates in hydroboration catalysis showed that chiral alcohol products could be achieved with high enantiomeric excess (>99%) and high yield at room temperature with 5 mol% of 46. Hydroboration of acetophenone was also investigated at reduced temperature, giving full conversion after 2 h. The choice of reducing agent had a strong effect, with pinacol borane (HBPin) being the most effective in comparison to other boranes. Further studies on a wide range of substrates was performed with 2.5 mol% catalyst loading. Aryl methyl ketones and polycycles were found to be suitable substrates but longer alkyl chains reduced the possible enantiomeric excesses. This is the case for heterocyclic ketones as well, though all substrates were converted by >99%. Decreasing the catalyst loading to 0.25 mol% still achieves conversion in less than 5 min. Mechanistic studies determined that a radical mechanism was unlikely, as radical traps had no effect on the reaction. The authors propose pinacol borane to be involved in stereodiscriminating step, with hydride transfer being a critical step in the reduction. When contrasting hydroboration and hydrosilylation reaction pathways, two different mechanisms can be discerned: direct hydrogen transfer for hydrosilylation or borane-mediated reduction for hydroboration. A proposed hydroboration catalytic cycle involves coordination of HBPin and substrate followed by hydrogen transfer (important for stereoselectivity) and elimination of boronic ester to regenerate catalyst. The proposed pathway is faster than the direct migration of the hydrogen from a manganese hydride. The direct hydride transfer is favored for the hydrosilylation. An additional example of Mn alkyl is Fettinger’s Mn(k3-terpy)(CH2SiMe3)2 48.60 Synthesis of 48 is achieved by mixing the manganese dichloride complex 47 with 2 equivalents of LiCH2SiMe3 in Et2O as in Scheme 24.60 The reaction was completed in high yield after 1 h at room temperature. The MnII complexes 48 are paramagnetic and sensitive to both air and moisture.

Scheme 24 Synthesis of the Fettinger Mn(k3-terpy)(CH2SiMe3)2 48.60

Organometallic Complexes of Group 7 Metals With Pincer and Noninnocent Ligands

707

Initial investigations into the hydroboration of styrene using pinacolborane with 1 mol% catalyst 48 at room temperature completely reduced the styrene. The major product was the Markovnikov product with ratios as high as 95:5. The analogues with substitutions on the pyridine ring resulted in lower yields while maintaining regioselectivity. Up to 93% catalytic activity is observed in the hydroboration of alkenes and up to 99% for ketones is observed at 1 mol% catalyst loading. The reactivity with ketones is particularly remarkable. An interesting report of a pincer complex of Mn in the III oxidation state bearing a MdC bond uses a Mn precursor P5 Mn(acac)3. In the synthesis of this complex, a tridentate pincer with a central imidazolium core (Scheme 25) is able to react with Mn(acac)3 to form a OCO pincer with an N-heterocyclic carbene MdC bond.61 The dark purple 49 can be obtained in tetrahydrofuran at room temperature and has a distorted trigonal pyramidal geometry.

Scheme 25 Synthesis of ONO Mn pincer 49 with central NHC ligation a distorted trigonal pyramidal geometry.61

Originally synthesized by Budzelaar,62 the homoleptic MnII alkyls 50 can be formed from a suitable ligand and a Mn dialkyl precursor either P6 or P7 (Scheme 26). Complexes of this class are reported as highly air sensitive and often insoluble due to a polymeric nature.

Scheme 26 Synthesis of NNN Mn pincer 50 alkyls.62

The simple conversion of 51 to a Mn alkyl is achieved through an alkylation with LiMe. This process constitutes both a halide exchange and a single-electron reduction to form 52 (formally MnI) as a high spin d4 complex. In contrast, the reaction of 51 with LiCH2SiMe3 forms 54 and 55 presumably through the intermediacy of deprotonated 53 (MnII). 54 is assigned as a MnII square planar species counterbalanced by a LiOEt+2 cation. 55 is the major product of the reaction of 51 with LiCH2SiMe3. Its structure corresponds to two MnI fragments reductively coupled together through ligand-based radicals (Scheme 27).63

708

Organometallic Complexes of Group 7 Metals With Pincer and Noninnocent Ligands

Scheme 27 Redox reactions of Mn bis-imine pyridine pincer alkyls.63

6.10.3

Rhenium complexes with pincer and noninnocent ligands

The organometallic chemistry of rhenium is markedly influenced by rhenium’s ability to form multiple bonds and to achieve stable forms of both low and high oxidation states. Fig. 3 highlights some of the common precursors for the synthesis of rhenium complexes: P8 is a precursor used for the synthesis of low valent complexes of ReI, whereas P9, P10 and P11 give access to ReIII, ReV and in some cases even ReVII complexes.

6.10.3.1

Re carbonyls and catalytic applications

An array of rhenium pincers with carbonyls are known, several of them mirroring structural and reactivity patterns of compounds utilized in Mn catalysis. However, the structural diversity of Re catalysts with carbonyl ligands is much more limited than that of their Mn counterparts, as this chemistry has not been studied in as much detail due to the higher cost and lower natural abundance of Re. The syntheses of ReI complexes are for the most part directly analogous to the ones of the Mn congeners. Most carbonyl pincer complexes reported use PNP pincers. To highlight the synthetic parallelism, in 2018 Kirchner synthesized and characterized a large family of PNP complexes of both Re and Mn.64 In their studies, the ReI PNP pincer complexes formed with tris-carbonyl ligation more often than the bis-carbonyl when compared to the products observed in the syntheses of Mn analogues. This observation may be due to RedC bonds being stronger than MndC bonds (Fig. 4).

Fig. 3 Rhenium precursors for the synthesis of Re organometallics.

Fig. 4 Structural examples of organometallic Re complexes with carbonyl ligands.

Organometallic Complexes of Group 7 Metals With Pincer and Noninnocent Ligands

709

As a first example of Re pincer synthesis we discuss pincer 56. 59, a precursor that can lead to 56, is accessible from P11-Br3 ReO(PPh3)2Br3 and the ligand stirred at room temperature in THF for 18 h. Compound 56 can be accessed from the (MePNPiPr) ReOBr2 complex 59 by reduction with Mg under an atmosphere of CO with quantitative yields of (MePNPiPr)Re(CO)3 as shown in Scheme 28.65

Scheme 28 Reductive synthesis of Re PNP pincer 56.65

Compound 56 is directly analogous to the Mn complex 6 (Fig. 2) and exhibits identical protodecarbonylation reactivity with H-X acids to form 60 (Scheme 29). The addition of HBr induces the loss of a carbonyl ligand and the installation of a Br− ligand trans to one of the remaining carbonyls. Additionally, the reaction leads to the formation of a coordinated amine L ligand in lieu of the original anionic N coordination of 56. The amine can be deprotonated with a base such as NEt3 to give the 5-coordinate bis-carbonyl 61.

Scheme 29 Reactions of 56 (and 6): addition of Brønsted acids followed by deprotonation.65

Upon deprotonation of 57 and loss of a halide ligand, the central pyridine dearomatizes to lead to the formation of 62. 62 Can then bind carbon dioxide to form 64 as well as H2 to form 63 in a reversible process. Additionally, 63 can be interconverted to 64 and vice versa by exposure to the corresponding gas at 2.5 bar and temperature. Of important note is the presence of ligand cooperation in the observed chemistry which also, remarkably is entirely reversible across most steps as described in Scheme 30.66

Scheme 30 Reactions of 57 (and 6): addition of H2 and CO2.66

710

Organometallic Complexes of Group 7 Metals With Pincer and Noninnocent Ligands

A related PNP pincer mer-[Re(k3-2,6-(Ph2PNMe)2NC5H3)(CO)3]+ was reported by Richeson and coworkers.67 In its synthesis from the ligand and Re(CO)5X (X ¼ Cl P8, Br P8-Br), the chloride precursor yielded a bis-carbonyl analogue of 57, with an inner sphere chloride, whereas the Br precursor yielded mer-[Re(k3-2,6-(Ph2PNMe)2NC5H3)(CO)3]+ with two different anions [Re(CO)4Br2]− or [Re2(CO)6(m-Br)]−3. Given the success of Mn complex 26 in catalyzing the oxa-Michael addition of alcohols to unsaturated nitriles (mechanism in Scheme 16), the Re pincer analogue 62 was tested for the activation of nitriles. It was shown successful in binding nitriles with a coordination mode similar to that observed in the Mn chemistry (Scheme 31).

Scheme 31 Reactions of Re PNP pincers with nitriles and with ligand participation.68

6.10.3.1.1

Rhenium PNP catalysts for the hydrogenation of carbonyls

Bolzati69 and Sortais70 have targeted the development of aliphatic PNP pincers for Re complexes. Sortais and their team have shown that a flexible aliphatic PNP ligand framework can be used for hydrogenation catalysis as shown in Scheme 32. Species 67 can be obtained easily by refluxing Re(CO)5Cl and the corresponding ligand in toluene for 12 h.

Scheme 32 Catalyst activation steps and mechanism for the 58-catalyzed hydrogenation of carbonyls.70

Organometallic Complexes of Group 7 Metals With Pincer and Noninnocent Ligands

711

Upon exposure to base, the halide counterion is abstracted along with the proton on the central amine in a catalyst activation first step. 68 isomerizes to meridional pincer 69 that can then decarbonylate to produce catalytically active 58. 58 can then coordinate H2 to form 70. The H2 complex can further split heterolytically to add across the MndN bond as a proton on the amine leaving a hydride behind on the Mn in 71. A carbonyl can then coordinate to form 72 and a hydride is able to add. The alcohol product can be released from 73 to regenerate catalytically active 58. The catalytic method was shown selective for the synthesis of both secondary alcohols and primary benzylic alcohols. In a related report, Sortais describes the selective mono N-methylation of anilines with methanol catalyzed by rhenium complexes in which a similar catalytic process is observed. Instead of oxidizing the alcohol to the aldehyde alone, the aldehyde released from 74 reacts with an aniline when introduced in the system. The aniline can intercept the formaldehyde formed in the catalytic process and undergo a condensation reaction with it. The condensation product can then coordinate to 75 to receive the hydrogen equivalent stored within the framework of the molecular complex by first forming 77 and then releasing the N-methyl aniline formed in the cycle (Scheme 33).71

Scheme 33 Mechanism of the N-methylation of anilines with alcohols catalyzed by 58.71

6.10.3.1.2

Rhenium CNC pincers with carbene ligands

As a follow up to their work on Mn carbene pincers, Luca and their team prepared a Re analogue 78 of Mn pincer 39 via the same synthetic route as the Mn pincer with a bis-carbene ligand and central pyridine coordination. The synthesis utilized Re(CO)5Cl P8 as the metal source. Unfortunately, once obtained, 78 did not exhibit the desired reactivity towards carbon dioxide. The authors observed that reactivity is arrested and that while the ligand is still able to host one electron upon electrochemical reduction, the required coordination of carbon dioxide is not possible due to the low metalloradical character of the Re complex compared to the Mn analogue (Fig. 5).

Fig. 5 Rhenium pincer 78 with CNC bis-carbene ligand.86

712

Organometallic Complexes of Group 7 Metals With Pincer and Noninnocent Ligands

6.10.3.1.3

Rhenium pincers with NNN ligands

Among the family of NNN pincers, the Rhenium bis-imino pyridines stand out.71 The reaction between the free bis-imino pyridine NNN ligand (Scheme 34) and Re(CO)5Cl P8 requires rather forcing conditions (100  C, 24 h) and leads to 79, a complex with only bidentate ligation. For tridentate ligand coordination to be established, the removal of another carbonyl ligand is necessary at temperatures of 200–240  C. The authors report a similar procedure for the synthesis of a terpyridine analogue. This comes in contrast to the syntheses of the Mn analogues, and the difficulties of establishing tridentate coordination is likely due to a stronger RedCO bond.

Scheme 34 Synthesis of Re bis(imino)pyridine NNN pincer complexes.72

Other Re NNN pincer complexes with ligand based redox activity have been studied, including complexes in which speciation and redox activity can be modulated by acid-base and redox steps, although their coordination is often only facial, and they therefore do not constitute a pincer.73,74 An example of such NNN pincer is green fac-NNN-Re, a tris carbonyl complex of di(2pyrazolyl-p-tolyl)amine (Scheme 35). Ligand protonation produces dark grey fac-NNN-ReH and one-electron oxidation produces blue fac-NNN-Re+. This ligand-based reactivity confirms the non-innocent nature of the facial ligand.

Scheme 35 Acid/Base and redox ligand non-innocence of fac-NNN Re complexes.

6.10.3.2

Re non-carbonyls and catalytic applications

Rhenium has rich synthetic chemistry in high oxidation states with several non-carbonyl high-valent precursors being available for the synthesis of Re pincer complexes. A 2016 report by Ozerov discusses the synthesis of Re PCP complexes using two different precursors (Me2S)2ReOCl3 P10 and (PPh3)2ReOCl2(OEt) P11 for the synthesis of 82 and 84 respectively via 81 and 83, respectively (Scheme 36). Additionally, 81 was

Scheme 36 Synthesis of high valent Re PCP complexes.75

Organometallic Complexes of Group 7 Metals With Pincer and Noninnocent Ligands

713

reported to be reactive towards LiN(SiMe3) with an observed ligand deprotonation and loss of the aromaticity of the central phenyl ring. To the best of our knowledge this is the first example of confirmed redox activity of the PCP ligand framework. Complexes 81 and 83 react with LiAlH4 and can be converted to 86 (PCPR)Re(O)(OAc)(OH). 86 has been proposed to undergo conversion to 87 and acetic acid in an unexpected hydrolysis reaction. 87 can then coordinate the acetate to form 88. 87 is isolated as the R ¼ tBu derivative from 86 and CaO. 86 can also be regenerated from 87 by stirring in acetic acid, while 87 can be further acetylated to form 89 using Ac2O. 86 is additionally able to convert to 89 by stirring in acetic acid at room temperature (Scheme 37).

Scheme 37 Ligand exchange reactions of Re PCP(oxo)halides 81 and 83 and Re PCP(oxo)hydroxyacetates 86.76

The importance of the Mn pincers for carbonyl and nitrile catalysis is anchored in their ability to reversibly accept and release one equivalent of H2, as well as reversibly bind HdX and C]X bonds. The Re PCP pincers that have been shown to exist in dearomatized forms 85, were unfortunately unreactive to H2, D2 or C6D6 as shown in Scheme 38. The pincers did show reactivity towards carbon dioxide, this reactivity being akin to a [4 + 2] cycloaddition, however unlike the Milstein pincers, they are unreactive towards H2. Specifically, 85 was remarkably resistant to reaction conditions poised to allow coordination to H2 or D2. The halide on 85 was, however successfully exchanged to an I through the reaction with Me3SiI to form 91. Both 85 and 91 have then shown reactivity towards carbon dioxide to form CO2 adducts 90 and 92 respectively.

Scheme 38 Reactivity pattern of RePCP(oxo)halides 85 towards CO2 and H2.75,77

Starting from Re hexahydride 82, Ozerov mapped its reactivity with electron donors. When L ligands are introduced (such as 4-dimethylaminopyridine and trimethylphosphine) at room temperature, complexes 93 and 94 are obtained in good yields after

714

Organometallic Complexes of Group 7 Metals With Pincer and Noninnocent Ligands

loss of a hydrogen equivalent. Surprisingly, 82 is reduced down to a ReIPCP pincer tris carbonyl 95 when reacted with carbon monoxide with formation of three hydrogen equivalents. The authors remark however, that upon heating, complex 82 undergoes a dehydrogenative dimerization to 96, potentially having identified a decomposition pathway for these types of complexes and explaining their currently limiting catalytic applications (Scheme 39).

Scheme 39 Reactivity pattern of RePCP hexahydride 82 towards neutral electron donor ligands.75

Taking advantage of the increased reducing power of rhenium over other transition metal catalysts typically used for olefin catalysis, Ozerov targeted a Re-PNP complex with a ligand engineered to facilitate olefin binding. The amide (NR2) of the ligand backbone was chosen due to its strong p-donating capabilities, which is expected to aid in olefin binding, and a silicon substituent on the backbone was added to prevent hydrogen migration. The complex was prepared through a reaction of the PNP backbone with magnesium chloride, followed by reduction of the metal using magnesium under a hydrogen atmosphere. The hydride complex 97 was found to have overall C2v symmetry with an overall unsaturated electron count of 16e−, even with p donation from the amide ligand. This primes the complex to be reactive towards 2e− ligands. The synthesis of 97 is shown in Scheme 40.

Scheme 40 Synthesis of RePNP hydrides 97 for the subsequent coordination of olefins.

Gratifyingly, 97 enabled coordination and hydrogenation of olefins (Scheme 41). Upon reacting with an olefin, coordination is observed and 98 is formed. The hydrogen equivalent that would have been expected to be released reduces another equivalent of the alkene to the corresponding alkane. The resulting 98 dihydride loses another hydrogen equivalent and converts to carbyne pincer 99.

Scheme 41 Coordination and olefin reactivity at PCPReH4 pincer 97.77

Organometallic Complexes of Group 7 Metals With Pincer and Noninnocent Ligands

715

97 reacts at room temperate with numerous olefins to form carbyne products analogous to 99, including singly substituted olefins with R ¼ Me, TMS, and tBu. Internal alkenes such as 3-hexene also bind to the metal center, indicating that the metal migrates along the carbon chain. Two carbyne products are obtained in reactions with asymmetric olefins, such as 2-methylbutene. The five-coordinate carbynes were generally isolated in oil form, and were found to be thermally stable, as thermolysis did not yield any decomposition products. Surprisingly, allylamine was also found to react to form the carbyne product, though it was theorized that the dangling amine served as a ligand to another rhenium center, producing oligomer products in addition to the carbyne monomer. Vinyl ethers and vinyl fluorides did not react in the same way as the olefins. Heating either of these olefins resulted in rapid decomposition. Additional studies expanded this reactivity into observations of dehydrogenative processes with cyclic olefins as starting materials and products with multiple olefin bindings.78 Most interestingly, the Ozerov report also includes the characterization of a alkyne 101 that is obtained from the thermolysis of 100 that is proposed to form through the successive dehydrogenations of an olefin ligand, thus setting an important precedent of both olefin and alkyne binding in the organometallic chemistry of group 7 (Scheme 42).79

Scheme 42 Coordination and olefin reactivity at PCPReH4 pincer 97.79

6.10.3.2.1

Rhenium alkyls and carbonyl insertions

An additional example of rhenium organometallics comes from the team of Ison in 2017.80 In this report, a 5-coordinate Re alkyl 103 was synthesized from a PNP Re nitride precursor 102 and an alkyl Grignard (Scheme 43).

Scheme 43 Synthesis and reactions with CO of ReV alkyls 103 supported by PNP pincers.80

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Organometallic Complexes of Group 7 Metals With Pincer and Noninnocent Ligands

102 is synthesized from ReNCl2(PPh3)2, triethylamine and the ligand in 5 h in refluxing methylene chloride. The Re PNP alkyl 103 is produced in the reaction of 102 with a methyl Grignard. The Re alkyl 103 can undergo a CO insertion to yield 104. When running the reaction for 24 h, the authors report reduction of 103 down to the ReI tris carbonyl species 105 with loss of the alkyl and the N fragments as MeNCO and MeC(O)NCO. This observation is extremely interesting as it sets the stage for potential future catalytic applications for C-N bond formation. An additional study by Ison and coworkers explores the mechanism of CO insertion into ReV alkyls with SSS coordination 107 (Scheme 44). In this work, a not electron rich SSS ligand framework is expected to have a synergistic effect with the strong trans effect oxo ligands in the stabilization of intermediates. 107 is prepared from 106 and R2Zn in mild conditions in 1 h. With the goal of discerning between direct CO insertion (from attack of CO on the metal carbonyl complex) and migratory insertion (migration of an alkyl or aryl ligand to a CO, or vice versa), density functional theory (DFT) is used to study the formation of 107. Alongside structural X-ray analysis, 107 ! 108 is found to be a rare example of direct CO insertion, rather than the more traditional migratory type, with the putative formation of CO (sigma donor) adducts of Lewis acidic 107 intermediate.

Scheme 44 Synthesis of ReV alkyls and their CO migratory insertion reactivity.81

6.10.3.2.1.1 High-valent Re N-heterocyclic carbenes for O-atom transfer reactions Brown and coworkers report an interesting case of direct O-atom transfer from Re VII bis oxo complex 110 to triphenylphosphine with the formation of a 111 ReV iMes NHC on an ONO pincer and triphenylphosphenoxide.82 Preparation of 110 is achieved by adding a free N-heterocyclic carbene IMes to dimeric 109. 111 Is also independently prepared from 112 and IMes with the loss of a triphenylphosphine ligand. An interesting feature of this study is the change from facial to meridional coordination of the ONO ligand from 110 to 111 upon deoxygenation of 110 (Scheme 45).

Scheme 45 Syntheses of 110, 111 and oxygen-atom transfer chemistry at the ReVII/ReV redox couple 110 ! 111.82

Organometallic Complexes of Group 7 Metals With Pincer and Noninnocent Ligands

6.10.4

717

Technetium complexes with pincer ligands

The organometallic chemistry of technetium is driven by the utility of Tc complexes in medical applications but is at the same time limited by the metal center’s radioactivity. The metastable 99mTc is of great interest because it is widely used in medical imaging, where its radioactivity is an asset, but presents synthetic challenges as a gamma emitter with a short half-life of just over 6 h. The longest-lived isotope 98Tc (half-life of 4.2 106 years) is not synthetically manageable, with 99Tc (with a half-life of 2.1 105 years) finding more frequent use. Being a weak beta emitter, 99Tc permits usage of the isotope without as heavy shielding, as the low-intensity radiation can be readily absorbed by standard glass. The low hazard of 99Tc also benefits the synthetic use of its parent nuclide, 99mTc, by resulting in a lower-hazard decay product. While the 99Tc(CO)5Cl analogue of the common Mn(CO)5Br and Re(CO)5Cl synthetic precursors has been reported, [99Tc(CO)3(H2O)3]+ and [99mTc(CO)3(H2O)3]+ have also found synthetic utility. With the interest in technetium chemistry relying heavily on its use in medicinal applications, the syntheses of “pincer” complexes observed in the literature are for the most part related to molecules that exhibit specific binding to biological targets. To date, no meridional tridentate ligand of 99Tc or 99mTc has been reported to contain a MdC bond at the pincer, although in several cases, the synthesis of the metal complexes does not explicitly address mer vs. fac configuration. In this section we highlight two examples of facial tridentate 99mTc coordination in which TcdCO bonds are observed. For additional examples, we refer the reader to COMC4 chapter 40008. Organometallic Complexes of Technetium. Compounds such as 113, 114 and 115 are accessible from Tc(CO)3(H2O)+3 and the respective ligands under mild conditions: 30 min at 95  C at pH 7.2. The compounds were synthesized with radiolabeled Tc and assayed in mice studies. Gratifyingly, the complexes have shown binding specificity to the desired tumor target through modification of the R chemical space shown in Scheme 46.

Scheme 46 Synthesis of organometallic Tc99m folate derivatives for use in nuclear medicine.83

In a study by Shubiger, water-soluble nuceloside organometallic derivatives of 99mTc and 50 -amino-thymidine were synthesized with the goal of inhibiting human cytosolic thymidine kinase and herpex simplex thymidine kinase (Fig. 6). This is the first example of Tc organometallic radiopharmaceuticals of amino thymidine.85 Unfortunately the targeted inhibitory effects of 116 proved unsatisfactory. However, the synthesis of the radiopharmaceutical candidates enable access to them as potential markers for the study of proliferation of tumor growth activity. While the organometallic chemistry of Tc remains of great interest to the synthetic community, certain limitations in the use of Tc organometallics in radiopharmaceuticals remain. The Tc complex candidates for these types of studies need to remain water-soluble as well as to exhibit low toxicity both ex and in vivo. The toxicity of the complexes is of high concern; however, it is often that the ligand fragment is the source of toxicity effects.84–87 We therefore remain forward-looking to the development of Tc pincers in the field of organometallic chemistry as novel ligands and methodologies are soon to arise addressing these issues.

718

Organometallic Complexes of Group 7 Metals With Pincer and Noninnocent Ligands

Fig. 6 Organometallic 99mTc 50 -amino-thymidine derivative 116 used in nuclear medicine.84

6.10.5

Conclusion

In conclusion, the field of group 7 organometallic pincers remains highly active with some remarkable examples of ligand cooperativity imparting unprecedented reactivity. Aside from the ground-breaking catalytic developments of the past two decades around Milstein, Kempe, and Kirchner’s catalysts that can shuffle carbonyls in oxidative and reducing chemistries with the ingenious use of ligand actors, of unique interest are examples of stoichiometric reactivity such as the Ozerov olefin chemistry that may open the door to new hydrogenations or metathesis reactions. Ison’s fundamental work in the synthesis and reactivity of high-valent Re complexes may produce new chemistries for small molecule activation and O-atom delivery. Additionally, group 7 pincers have now been represented in electrochemically driven conversions of carbon dioxide to fuel precursors by Richeson, Kubiak and Luca. Given the ubiquity of these types of compounds and the abundance and low cost of Mn in molecular catalysis, these early reports are likely to set the stage for additional discoveries in the field, with exciting new reactions waiting to be uncovered. With a renaissance in group 7 pincer chemistry unfolding, we remain forward-looking to new knowledge gained and novel pincer-enabled catalysis.

Acknowledgement We thank the University of Colorado for startup funds.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

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Organometallic Complexes of Group 7 Metals With Pincer and Noninnocent Ligands 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87.

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6.11 Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals James C Earl and Louis Messerle, Department of Chemistry, The University of Iowa, Iowa City, IA, United States © 2022 Elsevier Ltd. All rights reserved.

6.11.1 6.11.1.1 6.11.1.2 6.11.1.3 6.11.2 6.11.2.1 6.11.2.1.1 6.11.2.1.2 6.11.2.1.3 6.11.2.2 6.11.2.2.1 6.11.2.2.2 6.11.2.2.3 6.11.3 6.11.3.1 6.11.3.1.1 6.11.3.1.2 6.11.3.1.3 6.11.3.2 6.11.3.2.1 6.11.3.2.2 6.11.3.2.3 6.11.3.2.4 6.11.3.2.5 6.11.4 6.11.4.1 6.11.4.2 6.11.4.3 6.11.4.4 6.11.4.4.1 6.11.4.4.2 6.11.4.4.3 6.11.4.4.4 6.11.4.5 6.11.4.5.1 6.11.4.5.2 6.11.5 References

Background Preface Synthetic methods Electron counting Clusters with MdC bonds exclusively to CO or Cp-type organic ligands Homonuclear clusters of Group 5–7 metals Homonuclear clusters of Group 5 metals Homonuclear clusters of Group 6 metals Homonuclear clusters of Group 7 metals Heteronuclear clusters of Groups 5–7 metals Heterometallic trinuclear clusters Heterometallic tetranuclear clusters Clusters incorporating late transition metals or boron Clusters with MdC bonds to CO and Cp-type ligands, as well as to other organic ligands Homonuclear clusters of Group 5–7 metals Homonuclear clusters of Group 5 metals Homonuclear clusters of Group 6 metals Homonuclear clusters of Group 7 metals Heteronuclear clusters of Group 5–7 metals CrMo2 clusters Mo2W clusters Mo2Mn clusters Mo2Re clusters Re3Au clusters Organometallic clusters without MdC bonds to CO or Cp-type ligands Trinuclear clusters of Mo Hexanuclear clusters of W Tetranuclear clusters of rhenium Octahedral clusters Octahedral clusters of Mo Octahedral clusters of Tc Octahedral clusters of Re Mixed-metal octahedral clusters Carbide-centered clusters Hexanuclear trigonal prismatic clusters of W Dodecanuclear bioctahedral clusters of Re Conclusion

6.11.1

Background

6.11.1.1

Preface

720 720 721 721 722 722 722 723 730 737 737 744 745 751 751 751 752 755 757 757 757 758 759 759 759 759 760 760 762 762 762 762 764 765 765 766 769 769

In his 1966 review, F. A. Cotton defined transition metal cluster compounds as “those containing a finite group of metal atoms which are held together entirely, mainly, or at least to a significant extent, by bonds directly between the metal atoms even though some non-metal atoms may be associated intimately with the cluster.”1 This definition therefore excludes polymeric compounds, up to and including bulk solids. Cage or polyhedral polynuclear compounds are also excluded if they lack transition metal-metal bonding, (e.g., polyoxometalates), with fully-oxidized transition metal ions/atoms, bridged by heteroatoms such as oxygen or sulfur, that lack d-electrons and molecular orbitals for transition metal-metal bonding. Also, as should be noted, there are many studies of gas-phase and matrix-isolated non-isolable metal carbide “clusters,” lacking any ligands, for physicochemical and thermodynamic properties, and these are not discussed in this review.

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Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00061-5

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Organodinuclear transition metal complexes are not discussed in this review because they are generally considered outside the purview of transition metal cluster chemistry, per Cotton’s definition of a cluster. There is a rich and extensive structural and reaction chemistry of these complexes that is discussed elsewhere in this set of books. To be included in this review, a cluster must meet the following criteria: (a) it must contain at least three transition metal atoms from Groups 5–7 and (b) at least three of these metal atoms must be involved in metal-metal bonding to one another. Cluster compounds with only minor changes in the counter-ion or solvent of crystallization of a previously-known parent cluster are not included unless these changes significantly altered the properties of the parent cluster. Previously-known clusters involved as a reactant or intermediate in the synthesis of a new or novel cluster will be included in this review. This review is divided into three sections, (a) compounds with metal-carbon bonds exclusively between metals and carbon monoxide or cyclopentadienyl-based ligands, (b) compounds with metal-carbon bonds between metals and carbon monoxide and cyclopentadienyl-based ligands as well as other carbon-based ligands, and (c) compounds with metal-carbon bonds lacking either carbon monoxide or cyclopentadienyl-like ligands. Within these categories, compounds are organized by their metal families. First, homonuclear compounds of Group 5, going down the column, then Group 6, and then Group 7. These will be followed by heteronuclear compounds following graded lexicographic ordering with the same priority. That is, clusters will be grouped by their nuclearity, then within each group they will be ordered such that those with more metals earlier in the sequence (V, Nb, Ta, Cr, Mo, W, Mn, Tc, Re) appear first.

6.11.1.2

Synthetic methods

Clusters of the metals discussed in this review are most often prepared by one of five methods: 1. 2. 3. 4. 5.

coordination of a metal-metal multiple bond to a metal fragment, coordination of a metal fragment adjacent to a metal-metal bond, substitution of a metal in a cluster, substitution of a ligand in a cluster without altering the metal-metal bonded cluster core, and spontaneous self-assembly, typically by the reduction of mononuclear fragments.

Coordination of a metal-metal multiple bond to a metal fragment was described by F. G. A. Stone in a 1984 paper and subsequent articles.2 In essence, the metal-metal multiple bond coordinates in a manner similar to the familiar coordination of carbon-carbon multiple bonds. Coordination of a metal fragment adjacent to a metal-metal bond is most prominent in the synthesis of mixed-metal metallacubanes from incomplete cubane precursors. Substitution of a metal fragment within a cluster is relatively uncommon but can allow for the rational synthesis of heteronuclear compounds from homonuclear precursors. In cases where intermediates may be isolated, it was shown that, mechanistically, this can take place over many discrete steps consisting of the coordination and de-coordination of metal fragments. Substitution of a ligand in a cluster without altering the cluster core is most prominent in octahedral hexanuclear clusters, where the relatively-labile terminal ausser ligands may be easily substituted to fine-tune the properties of the cluster. There are also examples of reactions substituting the bridging “inner” ligands, but these are much less common and typically proceed only under harsher conditions. Reduction-induced self-assembly of mononuclear fragments is likely the oldest, most widely-used, and least-understood method of cluster synthesis. There have been efforts to elucidate the mechanism of certain reactions by isolating intermediates,3–7 but the term is typically used for the assembly of clusters from mononuclear or dinuclear precursors where the mechanism is unclear.

6.11.1.3

Electron counting

Polyhedral Skeletal Electron Pair Theory (PSEP, or Wades’s Rules), used for simple polyhedra,8 or the Jemmis mno rules, used for fused polyhedra may be thought of as the neutral method of electron counting extended to cluster compounds.9 These methods are concerned with the overall electron count of the cluster. All metal-ligand interactions are treated as covalent, and the ideal electron count for a given structure is based on the number of electrons that must be in shared orbitals in order to give each transition metal 18 electrons and each main group element 8 electrons. Compounds may have electron counts that differ from the ideal, which may be explained bonds being more localized than is assumed by these models, or by a constituent atom not achieving a complete electronic shell. The number of electrons given by these counting methods is referred to as the number of cluster valence electrons, CVE. The ionic method for electron counting may be extended into clusters as well and is useful for determining the number of electrons localized to the metal atoms of the cluster that are available for bonding. All interactions, apart from metal-metal interactions, are assumed to be ionic, and the oxidation state of the cluster core is determined by balancing the total charge of the ligands with the charge of the cluster core. The number of metal-based valence electrons (MBVE) in the cluster core serves as an approximation of the number of electrons participating in metal-metal bonding. Typically, this method is used to compare clusters within a family, for discussion of clusters with well-understood electron energy levels, or for certain classes of clusters, such as [M6(m3-X)8X6]n−, which have structures that do not align with the predictions made by PSEP theory, typically because of having electron orbitals that are fairly localized.10

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6.11.2

Clusters with MdC bonds exclusively to CO or Cp-type organic ligands

6.11.2.1

Homonuclear clusters of Group 5–7 metals

6.11.2.1.1

Homonuclear clusters of Group 5 metals

6.11.2.1.1.1 Trinuclear clusters of Nb The room temperature reaction between Cp NbCl4 and Li[BH2S3] in toluene gave a mixture of [(Cp Nb)3(m-S)6] (1) and [(Cp Nb)3(m-S)3(m3-S)3(m-S)BH)] (2) as shown in Scheme 1. Compound (2) may be thought of as a Nb3BS3 incomplete cubane, with one of the three BdS bonds split and bridged by an additional sulfur atom.11

Scheme 1 Synthesis of trinuclear clusters 1 and 2 from Cp NbCl4.

6.11.2.1.1.2 Trinuclear clusters of Ta Reacting Cp TaCl4 with Li[BH2Se3] at 90  C in toluene for 60 h resulted in a mixture of [(Cp Ta)3(m-Se)3(m3-Se)3(BOnBuCl)] (3) and [(Cp Ta)3(m-Se)6] (4) as shown in Scheme 2. The BOnBuCl moiety was postulated to derive from cleavage of tetrahydrofuran in the presence of Se during the synthesis of Li[BH2Se3].12,13

Scheme 2 Synthesis of trinuclear clusters 3 and 4 from Cp TaCl4.

Performing the same reaction at 80  C over 48 h gave [(Cp Ta)3(m-Se)3(m3-Se)3BH] (5) (Scheme 3). Following Wade’s rules, an M4E4 cubane with an absent vertex and three MdM bonds should have 56 valence electrons, whereas each of these compounds has 54 valence electrons.

Scheme 3 Synthesis of trinuclear cluster 5 from Cp TaCl4.

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

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The room-temperature reaction between Cp TaCl4 with Li[BH2Se3] in toluene, rather than the clusters (3) and (5) made at higher temperatures, yielded [(Cp Ta)3(m-Se)4{m-Se2(Se2)}] (6) with a diselenide group bridging between two of the bridging selenide units, as well as compound (4) (Scheme 4).11

Scheme 4 Synthesis of trinuclear cluster 6 from Cp TaCl4.

Reacting Cp TaCl4 with Li[BH2S3] under the same conditions gave a mixture of the previously-known [(Cp Ta)3(m-S)3(m3-S)3BH] and the novel clusters [(Cp Ta)3(m-S)3(m3-S)3BSH] (7) and [(Cp Ta)3(m-S)3(m3-S)3BCl] (8) (Scheme 5). The reaction of Li(BH2Te3) with Cp TaCl4 gave only the dinuclear (Cp Ta)2(m-Te)2. DFT calculations indicated that [(Cp Ta)3(m-Te3)(m3-Te)3BH] would be less stable than either of the isolated analogs.14

Scheme 5 Synthesis of trinuclear clusters 7 and 8 from Cp TaCl4.

Fig. 1 (thp)2[{Cr(CO)3}2{Cr(CO)2}(m-CO)2(m3-OEt)2] 9.

6.11.2.1.2

Homonuclear clusters of Group 6 metals

6.11.2.1.2.1 Trinuclear clusters of Cr In a side reaction while investigating metal complex reactions with diaminocarbenes, it was found that reacting 1,3-di-isopropyl3,4,5,6,-tetrahydropyrimidin-2-ylidene with Cr(CO)5(OEt2), generated photolytically in situ from Cr(CO)6 in Et2O, gave a [{Cr (CO)3}2{Cr(CO)2}(m-CO)2(m3-OEt)2]2− salt with two protonated 1,3-di-isopropyl-3,4,5,6,-tetrahydropyrimidin-1-ium (thp) anions (9) (Fig. 1).15 Cr–Cr-distances from the crystal structure (2.6589(9) A˚ ) are on the shorter end of the accepted range for Cr–Cr single bonds (e.g., 3.311(1) A˚ for [{Cp Cr(CO)3}2] or 2.840 A˚ for [PPN]2[{Cr(CO)3}3(m-CO)3(m3-S)]).16,17 Heating (Et4N)2[{Cr(CO)4}2{m-TeCr(CO)5}2] in refluxing acetone resulted in a ring-closure to generate [{Cr(CO)3}2{Cr (CO)4}(m3-Te)2] (10) as well as CO (Scheme 6).18

724

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

Scheme 6 Synthesis of trinuclear cluster 10 from (Et4N)2[{Cr(CO)4}2{m-TeCr(CO)5}2].

By heating [{CpCr(CO)2}2{m-SMnCp(CO)3}2] in refluxing benzene, then adding sulfur to the cooled solution, a mixture of the dinuclear [(CpCr)2(m-S){m-SCpMn(CO)3}2] and the air-stable trinuclear cluster [(CpCr)3(m-S)2{m-SCpMn(CO)3}(m3-S)] (11) were obtained in low yields (Scheme 7). The trinuclear cluster exhibited an unusual mix of bridging sulfide and thiolato ligands.19

Scheme 7 Synthesis of trinuclear cluster 11 from [{CpCr(CO)2}2{m-SMnCp(CO)3}2].

The reaction of [Cr3(dpa)4(NCCH3)2](PF6)2 (dpa ¼ dipyridylamide) with lithium phenylacetylide in acetonitrile gave the previously-known [Cr3(dpa)4(CCPh)2] in good yield (Scheme 8).20 The two CrdCr bonds of the Cr6+ 3 core are of equal length, indicating that the bonding electrons are delocalized, rather than the localized CrdCr quadruple bond plus an isolated Cr3+ ion as seen in Cr37+ complexes. The isolation of an intermediate, [Cr3(dpa)4(NCCH3)2(CCPh)](PF6), indicated that the reaction was stepwise and that mixed acetylide complexes may be accessible.

Scheme 8 Substitution of acetonitrile groups on [Cr3(dpa)4(NCCH3)2](PF6)2.

Addition of [Cr3(dpa)4(NCCH3)2](PF6)2 to KCN dissolved in MeOH gave [Cr3(dpa)4(CN)2] (12). Magnetic susceptibility measurements were highly indicative of the presence of MdM bonding, as antiferromagnetic interactions were not observed. There was no evidence for population of a higher-spin excited state.21

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725

6.11.2.1.2.2 Trinuclear clusters of Mo A collection of Mo3S4 clusters featuring uncommon cyclopentadienyl ligands were synthesized from (C5Me4R)MoCl4 (R ¼ H, SiMe3, and SiEt3). [{(C5Me4SiMe3)Mo}3S4][PF6] (13) and [{(C5Me4SiEt3)Mo}3S4][PF6] (14) were synthesized by reacting (C5Me4R)MoCl4 with 4 equivalents of LiStBu in tetrahydrofuran, removing solvent, then reducing with [Cp2Fe][PF6]. [((C5Me4H)Mo)3S4][PF6] (15) was prepared in a one-pot reaction between (C5Me4H)MoCl4, Li2S2, and KC8, with a major byproduct being the tetranuclear cluster [{(C5Me4H)Mo}4S4][PF6] (46), discussed further below..22 The unsaturated compound [{CpMo(CO)}2(m-H)(m-PCy2)] (Cy ¼ cyclohexyl) was reported to react with a variety of mononuclear fragments to give trinuclear clusters (Schemes 9, 35, 38). Reaction with the mononuclear radical fragment [CpMo(CO)3], generated from splitting the ModMo bond in [{CpMo(CO)3}2] by visible light, gave [{CpMo(CO)}2(m-PCy2)(CpMo(CO)2) (m3-CO)] (16), featuring a m3-CO.23 The ModMo bond bridged by the dicyclohexylphosphide ligand is 2.743(1) A˚ , close to the length of ModMo bonds in similar molecules, while the other two ModMo bonds are significantly longer, 3.085(1) and 3.094 (1) A˚ , indicating single bonds. Overall, this suggested significant localization of the bond unsaturation.24

Scheme 9 Reaction of [(CpMoCO)2(m-H)(m-PCy2)] with photolytically-generated radical fragments CpMo(CO)3% and Mo(CO)5%.

[{CpMo(CO)}2(m-PCy2)(CpMo(CO)2)(m3-CO)] existed as a mixture of isomers in solution; the authors proposed that the m3-CO becomes edge-bridging, allowing the CpMo(CO)x fragments to rotate. [{CpMo(CO)}2(m-H)(m-PCy2)] also reacted with Mo(CO)6 under visible-UV radiation to give [(CpMoCO)2(m3-H)(m-PCy2) {Mo(CO)5}] (17), which rapidly decomposed to a mixture of [{CpMo(CO)}2(m-H)(m-PCy2)] and [(CpMo(CO)2)2(m-H) (m-PCy2)].24 [{CpMo(CO)}2(m3-H)(m-PCy2){Mo(CO)5}] was characterized by 31P and 1H NMR spectroscopies and through comparison with the stable and more thoroughly-characterized tungsten analog, which is discussed in Section 6.11.2.2.1.6. Dimethylgermylene (GeMe2) and silylene-bridged dicyclopentadienes exhibited interesting reactivity with Mo(CO)6 (Scheme 10). Refluxing these compounds in xylene gave a mixture of di- and trinuclear species. For C5H3(m-SiMe2)(m-GeMe2) C5H3, the dinuclear products were [{CpMo(CO)3}2] and [{Mo(CO)3}2(m-Z5,Z5-Cp2SiMe2)], and the trinuclear species was

Scheme 10 Reaction of Mo(CO)6 with GeMe2-bridged dicyclopentadienes.

726

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

[{CpMo(CO)2}{Mo(CO)3}2(m-Z5,Z5-Cp2SiMe2)(m3-GeMe)] (18). For trans-(tBuC5H3SiMe2)(tBuC5H3GeMe2), the dinuclear products were [{tBuCpMo(CO)3}2] and rac- and meso-[{Mo(CO)3}2(m-Z5,Z5-(tBuC5H4)2SiMe2)], while the trinuclear species were rac(19a) and meso-(19b) [{tBuC5H4Mo(CO)2}{Mo(CO)3}2(m-Z5,Z5-(tBuC5H4)2SiMe2)(m3-GeMe)]. Finally, (C5H4)2GeMe2, gave only [{CpMo(CO)3}2] as a dinuclear product and [{CpMo(CO)2}3(m3-GeMe)] (20) as the trinuclear product, indicating the m-SiMe2 group was unnecessary for the formation of the trinuclear cluster.25 In all cases, the formation of the trinuclear cluster required both ringto-metal migration of the germylene and cleavage of the CpdSi bond. Only [{CpMo(CO)2}{Mo(CO)2}2(m-Z5,Z5-Cp2SiMe2) (m3-GeMe)] was identified by single-crystal X-ray diffraction (SCXRD); the rest were identified spectroscopically. Reaction with W(CO)6 rather than Mo(CO)6 does not create trinuclear clusters, only dinuclear compounds generated by insertion of the tungsten compound fragment into the GedCp bond.26 Heating (Et3N)2[Cr3(m-Se)2(CO)10] with 4 equivalents of Mo(CO)6 or heating (Et4N)2[{Mo(CO)4}{Cr(CO)3}2(m-Se)2] with 3 equivalents of Mo(CO)6 gave the homonuclear (Et4N)2[Mo3(m-Se)2(CO)10] (21).27 This reaction is explored further in Section 6.11.2.2.1.1. A solution of [(Cp Mo)2O5] in H2O/MeOH was reduced with Zn in the presence of trifluoroacetic acid or triflic acid to form a trinuclear cluster with 5 MBVE, [(Cp Mo)3(m-O)2(m-OH)4]2+ (22) (Fig. 2).28 Although crystals of the trifluoroacetate salt were too disordered to assign the OH and O ligands, the SCXRD data for the triflate salt could be refined to high precision. The OH ligands were differentiated from the O ligands by slightly longer ModMo bonds between the atoms they were bridging (1.95(1) vs. 2.05 (1) A˚ average). The authors’ alternative explanation for the differing bond lengths, that the electron density was simply localized, did not agree with DFT calculations that indicated that the cluster was valence-delocalized. Additionally, the presence of hydrogen atoms on only four of the six bridging oxygen atoms was supported by the triflate ion, which is capable of hydrogen bonding to up to three positions, only engaging in hydrogen-bonding interactions with the four bridging oxygens assigned as bridging hydroxides. Heating (Et4N)2[Cr3(m-Se)2(CO)10] with 4 equivalents of Mo(CO)6 in refluxing acetone, or refluxing (Et4N)2[{Mo(CO)4} {Cr(CO)3}2(m-Se)2] (84) with 3 equivalents of Mo(CO)6 in refluxing acetone, gave the homonuclear (Et4N)2[Mo3(m-Se)2(CO)10] (85).27 This chemistry is further explored in Section 6.11.2.2.1.1. Heating [(MeC5H4Ru)3(m-CO)3{m3-CCMo(CO)2Cp}] with 60 atm of CO in toluene gave a mixture of [{MeC5H4Ru (CO)}2(m-CO)]2(m4-C2) and a fraction that, when recrystallized from ClCH2CH2Cl/hexane, reacted with the halogenated solvent to give [{CpMo(CO)}(CpMoCl)2(m-CO){m4-C2(MeC5H4Ru(CO)2}] (23), both of which included a rare tetrametallated ethene at the center (Scheme 11).29

Fig. 2 [(Cp Mo)3(m-O)2(m-OH)4]2+ 22.

Scheme 11 Reaction of [(MeC5H4Ru)3(m-CO)3{m3-CCMo(CO)2Cp}] with CO to form compounds featuring tetrametallated ethene.

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

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6.11.2.1.2.3 Trinuclear clusters of W A tungsten analog of [(CpMoCO)2(m-H)(m-PCy2)] (Scheme 9) was found to react with the 16-electron fragment W(CO)5, generated photolytically from W(CO)6, to form the homonuclear 46-electron cluster [{CpW(CO)}2(m3-H)(m-PCy2){W(CO)5}] (24) (Scheme 12).30 This structure was assigned based on the compound’s spectroscopic similarity to the SCXRD-characterized [{CpMo(CO)}2(m3-H)(m-PCy2){W(CO)5}] (96), Section 6.11.2.2.1.6.24 It likewise shares 960 s incongruously-degenerate coupling between the m3-CO and the CpW(CO) fragments in the 13C NMR spectrum (Section 6.11.2.2.1.6), which was at odds with the proposed transoid placement of the CpW(CO) fragments. The rapid exchange of the hydride across the M3 plane, which had been proposed as an explanation for an apparent C2 axis in 96, was much slower for 24 and, at low temperatures (193 and 173 K respectively), the resonances for the phosphide and hydride resolved into two peaks. These were proposed to represent two isomers of the compound, one with the m3-H on the face of the W3 triangle and one with the hydride bridging the edge. This was similar to the mechanism for the isomerization of [{CpMo(CO)}2{CpMo(CO)2}(m3-CO)(m-PCy2)].24 This result aligned with a later compound, [{CpW(NO)}2(m-H)(m-PPh2){Fe(CO)4}],31 that was formed by insertion of the 16-electron fragment into the W2H moiety, and has the m-H bridging only a single WdFe bond.

Scheme 12

Reaction of [{CpW(CO)}2(m-H)(m-PCy2)] with photolytically-generated W(CO)5 to form 24 and the proposed exchange mechanism of the hydride of 24.

Reacting [{CpW(NO)}2(m-H)(m-PPH2)] with W(CO)5(thf ) gave [{CpW(NO)}2(m3-H)(m-PPh2){W(CO)5}] (25).31 This compound had a fairly long WdW bond between the two CpW units, 3.006(1) A˚ , while the bond lengths between the CpW and the W(CO)5 units were 3.164(1) and 3.150(1) A˚ . Compound 25 had the same false C2 axis as 24, again attributed to motion of the hydride at faster than NMR time scales. Upon cooling to 173 K in CD2Cl2, the hydride signal exhibited significant broadening, suggesting that the exchange was occurring at the NMR time scale. DFT calculations of the Gibbs free energy for a m-H intermediate, as was proposed for 24,24 suggested that such intermediates would be too high in energy and that an inversion of the m3-H through the center of the W3 ring was more likely (Scheme 13).

Scheme 13 Reaction of [{CpW(NO)}2(m-H)(m-PPh2)] with W(CO)5(thf ) to form 25 and the proposed exchange mechanism of the hydride of 25.

Heating (Et4N)2[Cr3(m-Se)2(CO)10] with 4 equivalents of W(CO)6, or heating (Et4N)2[{W(CO)4}{Cr(CO)3}2(m-Se)2] with 3 equivalents of W(CO)6, gave the homonuclear (Et4N)2[W3(m-Se)2(CO)10]. Heating [{W(CO)4}2{m-SeW(CO)5}2] in refluxing acetone initiated a ring-closure reaction to yield (Et4N)2[W3(m-Se)2(CO)10].27 This reaction is explored further in Section 6.11.2.2.1.2. As with [(Cp Mo)2O5] (Section 6.11.2.1.2.2), [(Cp W)2O5] was reduced by zinc in MeOH/H2O in the presence of triflic acid to give a trinuclear salt.32 However, instead of a tungsten analog to the 5 MBVE molybdenum cluster [(Cp Mo)3(m-O)2(m-OH)4]2+ (22), the 3 MBVE cluster [(Cp W)3(m-O)4(m-OH)2]2+ (26) was formed. Upon exposure to air, the 2 MBVE cluster [(Cp W)3(m-O)6]+ (27) was produced (Fig. 3). As with the related Mo3 cluster, the presence of H atoms in [(Cp W)3(m-O)4(m-OH)2]2+ was supported through hydrogen bonding interactions in the solid-state structure as determined by SCXRD. The absence of hydrogen atoms on the other bridging oxygen ligands was further supported by the differing WdW bond lengths. Additionally, EPR indicated that the unpaired electron was highly-localized, which agreed with the electronic structure from DFT calculations on [(CpW)3(m-O)4(m-OH)2]2+. The [(Cp W)3(m-O)6]+ structure showed no hydrogen bonding interactions between the cluster and the triflate anions in the solid-state structure, as well as a minimal variation in W-W and W-O distances. When trifluoroacetic acid was substituted for triflic acid, [(Cp W)3(m-O)4(m-OH)2]2+ could not be crystalized before oxidation to [(Cp W)3(m-O)4]+, but crystals of 27 that included either [Zn(H2O)6]2+ and a mixture of (CF3COOHOOCCF3)−, (CF3COO)−, or an unusual [Zn3(m3-O) (m-O2CCF3)4(O2CCF3)(CF3COOH)(H2O)3]− anion were formed.

728

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

Fig. 3 [(Cp W)3(m-O)4(m-OH)2]2+ 26 and [(Cp W)3(m-O)6]+ 27.

6.11.2.1.2.4 Tetranuclear clusters of Cr A variety of new cubane-type [(RC5H4Cr)4S4] clusters (R ¼ C(O)Me, CO2Me, CO2Et) with Cp-type ligands, beyond the long-known [(CpCr)4S4] and [(MeC5H4Cr)4S4],33,34 were synthesized in high yields from the easily-prepared starting materials [{(C5H4R)Cr (CO)3}2] by heating in refluxing tetrahydrofuran with excess elemental sulfur as shown in Scheme 14. The novel [{C5H4C(O) OEt}4Cr4S4] was also obtained.35 Similarly, [(RC5H4Cr)4Se4] (R ¼ Me, C(O)OEt) were prepared by refluxing the respective dinuclear [{(C5H4R)Cr(CO)3}2] in tetrahydrofuran with excess selenium or from [(RC5H4)Cr(CO)2]2Se under the same reaction conditions. [(RC5H4)Cr(CO)2]2(m-Z2:Z2-Se2) served as a suitable starting material without the need for additional selenium, as was shown for R ¼ C(O)Me and CO2Me.36

Scheme 14 Synthesis of various cubane-type tetrachromium clusters with a single type of cyclopentadienyl ligand from dinuclear precursors.

Reaction of [(MeC(O)C5H4)Cr(CO)2]2Se and [CpCr(CO)2]2Se under the same conditions provided insight into the mechanism, as shown in Scheme 15. (MeC(O)C5H4)4−nCpnCr4Se4 (n ¼ 0, 1, 2, 3, 4) were the resulting products, in 97% overall yield based on Cr. The ratio between these different species [(CpCr)4Se4], (35), (36), (37), and (29) was approximately 1:4:6:4:1, the binary distribution for n ¼ 4 and the expected outcome if four fragments of equal probability were combined. The authors hypothesized that the reaction proceeded via the self-assembly of (RC4H5)Cr(CO)2Se fragments generated by the interaction between

Scheme 15 Synthesis of cubane-type tetrachromium clusters with a mixture of cyclopentadienyl ligands from dinuclear precursors.

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

729

[(RC5H4)Cr(CO)2]2Se and Se. This was bolstered by the fact that heating 29 with [(CpCr)4Se4] under the same conditions did not result in mixed-ligand clusters. The ligands on these clusters can be altered, as the reaction between 29 and 2,4-dinitrophenylhydrazine gave the expected hydrazone. The carbonyl moiety also underwent a Wittig reaction with Ph3P]CHPh, reduction via NaBH4, or methylation by MeMgI.36 Thermolysis of chromium complexes with thiotetrazole ligands led to a wide range of novel cubane-type chromium clusters in low yields (Scheme 16).37 Heating [{CpCr(CO)3}2] to 90  C in toluene with an equimolar quantity of [CpCr(CO)3(PhN4CS)] for 2 h gave numerous cubane-type structures that were separated by column chromatography over silica. These compounds included two dinuclear species [{CpCr(CO)2}2] and [CpCr(CO)3]2S, the known (CpCr)4S4, (CpCr)4S2O2,38 and (CpCr)4S2(CO)2,39 and the new clusters (CpCr)4S2(CO){NCCrCp(CO)2} (38), (CpCr)4(S)O2(NPh) (39), (CpCr)4S3(N3Ph) (40), (CpCr)4S2(N3Ph) {NCCrCp(CO)2} (41) and a substantial fraction that, although seemingly pure by 1H NMR spectroscopy, could not be assigned a structure. Compounds 38–41 were examined by SCXRD, and the assigned structures were consistent with 1H NMR and IR spectroscopy and mass spectrometry data. The remaining clusters, being known compounds, were evaluated by comparison of the 1 H NMR spectroscopic and other spectral data to that of the published compounds.

Scheme 16 Synthesis of various cubane-type tetrachromium clusters from [{CpCr(CO)3}2] and [CpCr(CO)3(PhN4CS)].

The reaction of CrCl2, LiCp , and LiP(SiMe3)2 in tetrahydrofuran afforded the highly-symmetrical eight-electron [(Cp Cr)4P4], (42), the third example of an M4P4 cluster and the first polymetallic CrIV compound.40 This cluster had virtually identical Cr-Cr distances of 2.931(1) and 2.935(1) A˚ . Magnetic susceptibility measurements indicated that CrdCr bonding was likely, as ΧMT was essentially temperature-independent, suggesting that any antiferromagnetic exchange was very weak (Fig. 4). The reaction between [(CpCr)2(m-Z5:Z5-As5)] and [(o-HgC6F4)3] resulted in a few crystals of {(o-HgC6F4)3}2
[(CpCr)4As5], {(o-HgC6F4)3}2
(43), mixed with the major product, an amorphous black powder that could not be characterized. The compound was described as a Cr4 butterfly with two arsenics on the inner face and three arsenics along the outer face (Fig. 5). Since [(o-HgC6F4)3] typically only forms weakly interacting adducts with pnictogen compounds without altering their geometry, it was thought that [(CpCr)4As5] was not formed in the reaction between [(CpCr)2(m-Z5:Z5-As5)] and [(o-HgC6F4)3], rather it was an impurity mixed in with the [(CpCr)2(m-Z5:Z5-As5)] that could not otherwise be separated.41

Fig. 4 [(Cp Cr)4P4] 42, the first known polymetallic CrIV compound.

730

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

Fig. 5 [(CpCr)4As5] 43, isolated from [(CpCr)2(m-Z5:Z5-As5)].

Fig. 6 [(CpMo)5(m3-P)6] 45, a rare trigonal bipyramidal pentanuclear cluster.

6.11.2.1.2.5 Tetranuclear clusters of Mo The tetranuclear cubane cluster [{(C5Me4H)Mo}4S4][PF6] (44) was the major by-product from the synthesis of [{(C5Me4H) Mo}3S4][PF6], (15), with (C5Me4H)MoCl4, Li2S2, and KC8 in tetrahydrofuran.22 6.11.2.1.2.6 Pentanuclear clusters of Mo In the reaction between [{CpMo(CO)3}2] and P4 in 1,3-di-isopropylbenzene, [(CpMo)5(m3-P)6] (45) was isolated in trace amounts in addition to the expected product, [(CpMo)2(m-Z6:Z6-P6)].42 The compound was analyzed solely by SCXRD. The six ModMo bonds between the equatorial Mo atoms and the two at the unshared vertices had an average length of 2.981(7) A˚ , while the ModMo bonds on the triangular plane of the trigonal bipyramid were a mix of a short bond, resembling the others, at 2.982(4) A˚ , and two longer bonds with an average length of 3.22(4) A˚ . There were 7 MBVE and a CVE count of 23, which is one more than the 22 expected for a trigonal bipyramid (Fig. 6).

6.11.2.1.3

Homonuclear clusters of Group 7 metals

6.11.2.1.3.1 Trinuclear clusters of Mn The known trigonal bipyramidal cluster [{Cr(CO)3}2{Cr(CO)4}(m3-Se)2] reacted with Mn(CO)5Br to give a mixture of [Me2CSe2{Mn(CO)4}{Cr(CO)5}2]− and [{Mn(CO)3}2{Mn(CO)2}(m-CO)2(m3-Se{Cr(CO)5})]2− (46) (Scheme 17). The former was converted to 46 by stirring with Mn2(CO)10 and KOH in MeOH/MeCN, and both were converted to the previously-known cluster [{Mn(CO)3}3(m3-Se)2]− by heating gently with Mn(CO)5Br in acetone. Compound 46 could also be converted to the known cluster [{Mn(CO)3}3(m3-Se)]2− by bubbling CO through a heated MeCN solution.43

Scheme 17 Reaction of [{Cr(CO)3}2{Cr(CO)4}(m3-Se)2] with Mn(CO)5Br to form trimanganese clusters.

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

731

The room-temperature reaction of (PPN)[{Mn(CO)3}(m3-S)2] (PPN+ ¼ bis(triphenylphosphine)-iminium cation) with 1 equivalent of Cr(CO)6 and 2 equivalents of PPN+Cl− gave the cluster (PPN)2[{Mn(CO)3}(m3-S)(m3-S{Cr(CO)5})(m-H)], (PPN)2(47). Reaction of this cluster with an additional equivalent of Cr(CO)6 in refluxing CH2Cl2 gave (PPN)2[{Mn(CO)3}(m3-S{Cr (CO)5})2(m-H)], (48). Similar reactions were performed with (PPN)[{Mn(CO)3}(m3-Se)2], using 2 equivalents of Cr(CO)6, then two subsequent equivalents of Cr(CO)6 to give (PPN)2[{Mn(CO)3}(m3-Se)(m3-Se{Cr(CO)5})(m-H)], (PPN)2(49), followed by (PPN)2[{Mn(CO)3}(m3-Se{Cr(CO)5})2(m-H)] (PPN)2(50) (Scheme 18).44 In both cases, the reaction was believed to result from [Cr(CO)5H]−, generated in situ, attacking the starting cluster. All four of these clusters required breaking a MndMn bond in the original cluster, changing the geometry from the parent cluster’s trigonal bipyramid to a square pyramid, with a nearly planar Mn2E2 base. Reacting (PPN)[{Mn(CO)3}(m3-S)2] or (PPN)[{Mn(CO)3}(m3-Se)2] with Cr(CO)6 at elevated temperatures gave octahedral complexes (113) and (114), discussed in Section 6.11.2.2.2.1.

Scheme 18

Reaction of trigonal bipyramidal [{Mn(CO)3}(m3-Q)2]− (Q ¼ S, Se) to form square-pyramidal (47–50) and octahedral (113–114) clusters.

Scheme 19 Synthesis of trimanganese clusters 51 and 52, featuring m3-Te ligands.

Heating [Mn2(CO)10] with tellurium powder and (Et4N)Br in 2 M KOH/MeOH/MeCN gave the square pyramidal cluster (Et4N)2[{Mn(CO)3}2{Mn(CO)2}(m3-Te)2(m-CO)(m-H)] (51) (Scheme 19).45 The presence of the m-hydride was confirmed by 1H NMR spectroscopy. The PPN+ salt reacted with 1 equivalent HCl in MeCN to give the trigonal bipyramidal cluster (PPN)[{Mn(CO)3}3(m3-Te)3], (52), accompanied by loss of H2. Reduction of (PPN)(52) with NaBH4 regenerated (PPN)2(51). 6.11.2.1.3.2 Trinuclear clusters of rhenium (PPh4)[{Re(CO)3}3(m-H)3(m3-H)]− reacted with Et2NH to form the adduct (PPh4)[{Re(CO)3}2{Re(CO)3(NEt2H)}(m-H)4(m3-H)]−, (PPh4)(53).46 Intriguingly, in the solid-state structure of 53 the dimethylamine group is oriented such that its proton was directed at the m-hydride of the opposite edge of the triangle, forming a rare dihydrogen bond (Scheme 20). A 2D NOESY experiment confirmed the close proximity of these two hydrogens in solution, while IR spectroscopy measurements showed a red shift of the N-H stretch compared to dimethylamine in a solvent incapable of itself acting as a hydrogen bond acceptor, further evidence for a dihydrogen bond.

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Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

Scheme 20 Synthesis [{Re(CO)3}2{Re(CO)3(NEt2H)}(m-H)4(m3-H)]−, 53, demonstrating dihydrogen bond.

[Re3(m-H)3(CO)11(NCMe)] reacted with a ferrocene-based chiral diphosphine (L) at 125  C in octane to form the mononuclear Re(CO)3H(k2-L) and the cluster [{Re(CO)3}3(m-H)(CO)(k2-L)], (54). [{Re(CO)4(NCMe)}2] reacted with another ligand L’ and H2 in toluene at 80  C to form [{Re(CO)3}3(m-H)(CO)2(L’)], (55) (Scheme 21).47 The structure of 54 was confirmed by 1H NMR, 31 P{1H} NMR, and IR spectroscopies and mass spectrometry. The structure of 55 was confirmed by comparison of its nCO stretching frequency to that of the known cluster [{Re(CO)3}3(m-H)(CO)2(PPh3)], ESI MS, and 1H and 31P{1H} NMR spectroscopies. Both 54 and 55 were tested, along with other Re compounds incorporating the same ligands, for the catalytic hydrogenation of tiglic acid. The two trinuclear clusters had the lowest % conversion of the ligands tested, at 33% and 15% respectively. Compound 54 gave racemic product, while 55 gave 13% ee, the second best of the catalysts tested.

Scheme 21 Formation of trirhenium clusters 54 and 55, featuring ferrocene-based chiral diphosphine ligands.

A combination of [{Re(CO)4}2(m-H)2] and Cp Rh(CO)2 in refluxing hexanes gave [{Re(CO)3}2{Cp Rh(CO)}(m-CO)(m-H) (CO){Re(CO)5}], (56), as a minor side product, which was structurally characterized through SCXRD, 1H NMR spectroscopy, and ESI-MS.48 Light- and temperature-sensitive, it soon decomposed to [{Re(CO)4}2{Cp Rh(CO)}(m-H)2] as indicated by NMR and IR analyses (Scheme 22).

Scheme 22 Synthesis and decomposition of [{Re(CO)3}2{Cp Rh(CO)}(m-CO)(m-H)(CO){Re(CO)5}] 56.

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[{Re(CO)4}3(m-H)3] reacted with Pt(PtBu3)2 at room temperature in hexane to form [{Re(CO)4}3(m-H)3Pt(PtBu3)(m-H)3], (57).49 The two equivalent RedRe bonds were 3.0476(5) A˚ . The product was postulated to be the result of phosphine dissociation from Pt(PtBu3)2 to give Pt(PtBu3), that then inserted cleanly into the hydride-bridged RedRe bond, followed by hydride migration with no loss of CO or hydride ligands. Formally 60 CVE, cluster 57 falls two electrons short of the 62 electrons expected for a butterfly tetrahedral cluster, which the authors rationalized as resulting from the tendency of Pt to be electronically saturated with 16 valence electrons (Scheme 23).

Scheme 23 Reaction of [{Re(CO)4}3(m-H)3] with Pt(PtBu3)2 to form 57.

Similarly, (Bu4N)[{Re(CO)4}3(m-H)2] reacted with BiPh2Cl in refluxing CH2Cl2 to form [{Re(CO)4}3(m-BiPh2)(m-H)2] (58).50 Subsequent reaction with (Bu4N)OH in methanol cleaved the central Re(CO)4 to give (Bu4N)[{Re(CO)4H}2(m-BiPh2)] (Scheme 24).

Scheme 24 Reaction of [{Re(CO)4}3(m-H)2]− with BiPh2Cl to form 58.

When [{Re(CO)4}3(m-BiPh2)(m-H)2] was reacted with Pt(PtBu3)2 at 68  C in refluxing hexanes, several products were formed.51 The only product that falls within the scope of this review was [({Re(CO)4}{Pt(CO)(PtBu3)}(m-H))(m4-Bi)({Re(CO)4}{Pt(CO) (PtBu3)})(m4-Bi)({Re(CO)4}3(m-H))] (59) (Scheme 25). The RedRe bond lengths were long, 3.3765(17) and 3.4444(18) A˚ ,

Scheme 25 Reactions of 58 to form clusters 59 and 60.

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Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

typical for m-H RedRe single bonds. Another product of this reaction was [({Re(CO)4}{Pt(CO)(PtBu3)}(m-H))(m4-Bi)({Re (CO)4}2(m-H){Pt(CO)(PtBu3)})], which, when reacted with CO at 25  C in hexanes at room temperature, gave a range of products including in moderate yield the fascinating cluster [{Re(CO)3}3{Pt(CO)(PtBu3)}(m3-Bi)(m4-Z2:Z1:Z1:Z2-Bi2)] (60). This cluster takes the form of an octahedron with an Re3Pt butterfly cluster, with remaining vertices occupied by Bi and a Bi capping the Re3 face. Assuming butterfly geometry and treating the Bi atoms as ligands gave a CVE count of 60, two less than expected for this geometry. Cluster electron counting according to PSEP theory, including the Bi atoms, gave 98 electrons, consistent for a capped octahedron. The crystal structure consisted of two structurally-similar clusters, in one the BidBi bond distance was 3.0950(13) A˚ and in the other it was 3.1364(12) A˚ . The BidRe bonds to the m3-Bi range from 2.8127(13) to 2.8417(12) A˚ . This was similar to the BidRe bond distance measured for the spiro Bi in 59, which ranged from 2.7585(14) to 2.9240(14) A˚ . The reaction between [{Re(CO)4(NCMe)}2] and 2-mercapto-1-methylimidazole gave a mixture containing dinuclear [{Re(CO)3}2(m-k2,k1-SN2C4H5)2], the tetranuclear cage compound [{Re(CO)3}4(m-k2,k1-SN2C4H5)4], and the trinuclear cluster [{Re(CO)3}2{Re(CO)2}(m3-SN2C4H5)2(m-CO)(m-H)], (61), an unusual 50-electron trirhenium compound (Scheme 26).52

Scheme 26 Synthesis of 61 from [{Re(CO)4(NCMe)}2] and 2-mercapto-1-methylimidazole.

[{Re(CO)4}2(m-H)(m-Ph)] reacted with vinyl acetate to give [{Re(CO)5}{Re(CO)4}2(m-Z2:Z1-C(H)CH2)], (62), as well as the dinuclear compounds [{Re(CO)4}2(m-H)(m-OAc)] and [{Re(CO)4}2(m-H)(m-Z2:Z1-C(H)C(H)OAc)] (Scheme 27). [{Re(CO)5} {Re(CO)4}2(m-Z2:Z1-C(H)CH2)] was also synthesized in similar yields by the reaction of [{Re(CO)4}2(m-H)(m-Z2:Z1-C(H)] C(H)nBu)] with vinyl acetate, but without giving the other two dinuclear products.53 Cluster 62 has 50 CVE, consistent with the observed structure. [{Re(CO)5}{Re(CO)4}2(m-Z2:Z1-C(H)CH2)] was reacted with I2 to cleave off the pendant Re(CO)5 group as Re(CO)5I, yielding the novel dinuclear compound [{Re(CO)4I}{Re(CO)4}(m-Z2:Z1-C(H)CH2)].

Scheme 27 Reaction of [{Re(CO)4}2(m-H)(m-Ph)] with vinyl acetate.

6.11.2.1.3.3 Tetranuclear clusters of Mn Thermolysis of [CpMo(Z6-C7H8)] with P4 in 1,3-di-isopropylbenzene gave [(CpMn)4P4], (63), in good yields.54 This was an example of an exceedingly-rare phosphorus-bridged metallacubane. DFT calculations showed that the m-P atoms have a lone pair not involved in bonding. Reaction of [(CpMn)4P4] with 1 equivalent of CuCl or CuBr resulted in the crystallization of a 1D chain polymer [{(CpMn)4P4}(CuCl)] (64a) or [{(CpMn)4P4}(CuBr)] (64b), with the copper in a trigonal-planar environment and coordinated to a halide and two phosphorus atoms from the heterocubane, as shown in Scheme 28. Excess CuX gave an off-white amorphous powder (65) whose elemental analysis had a Mn:Cu ratio of 4:3, or a cluster to Cu ratio of 1:3, consistent with the creation of a 3D polymeric structure.

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

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Scheme 28 Synthesis of cubane-type tetramanganese cluster 63 and coordination of m3-phosphide to other metal centers.

Reaction of [(CpMn)4P4] with [CpMn(CO)2(thf )] resulted in coordination of up to 4 Mn centers to the lone pairs of the m3-phosphides, giving [(CpMn)4(m3-P)3{m3-PMnCp(CO)2}] (66), [(CpMn)4(m3-P)2{m3-PMnCp(CO)2}2] (67), [(CpMn)4(m3-P) {m3-PMnCp(CO)2}3] (68), and [(CpMn)4{m3-PMnCp(CO)2}4] (69), with slight increases in the cluster MndMn bond lengths as more phosphides were coordinated by CpMn(CO)2 moieties. 6.11.2.1.3.4 Tetranuclear clusters of Re [{Re(CO)3}4](m3-H)4 reacted with pyridazine (pydz) or phthalazine (not shown) in CH2Cl2 to form a number of organometallic clusters.55 Initially, the spiked triangular [{Re(CO)3}4(m-H)4(m-pydz)(pydz)2] (70a) was formed in high yield as a kinetic product, but over time the more thermodynamically-favored tetranuclear square clusters cis- and trans-[{Re(CO)3}4(m-H)4(m-pydz)2] (72a, 72a0 ) precipitated, with an isolated yield of precipitate of 50% based on initial Re, with 72a accounting for 85% as determined by 1 H NMR spectroscopy of the precipitate dissolved in thf-d8. The other main products were the soluble dinuclear [{Re (CO)3}2(m-H)2(m-pydz)] and the trinuclear [{Re(CO)3}3(m-H)3(m-pydz)(pydz)] (71a). Notably, the concentration of 71a, as monitored by 1H NMR spectroscopy, remained constant as 70a converted to 72, and this suggested that, unlike other products, it was not formed from the reaction of 70a. Phthalazine showed similar reactivity, giving (70b-72b). The progress of the reaction between [{Re(CO)3}4H4] and pyridazine, as monitored by 1H NMR spectroscopy, showed how the concentration of the various products changed over time. As with many ReI clusters, the square tetranuclear compounds exhibited luminescent properties comparable to classical tricarbonyl-di-imine-rhenium complexes (Scheme 29).56

Scheme 29 Products and interconversion of products of the reaction between [{Re(CO)3}4](m3-H)4 and pyridazine.

6.11.2.1.3.5 Octahedral hexanuclear clusters of Re There have been few cluster carbonyls discovered in recent years, but they include the first examples of carbonyl ligands on a {M6(m3-Q)8} cluster core. The monocarbonyl [Re6(m3-Se)8(PEt3)5(CO)] [SbF6]2 (73)[SbF6]2 and the cis and trans forms of the

736

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

dicarbonyl [Re6(m3-Se)8(PEt3)4(CO)2][SbF6]2 (74)[SbF6]2 and (75)[SbF6]2, have been synthesized.57,58 Each of these was formed by reacting the respective iodide compound, [Re6(m3-Se)8(PEt3)5I]I, cis-[Re6(m3-Se)8(PEt3)4I2], or trans-[Re6(m3-Se)8(PEt3)4I2] with Ag [SbF6] in CO-saturated dichloromethane at room temperature. The resulting clusters possessed significant p-backbonding based on the IR nCO stretching frequencies (2068, 2105/2083, and 2070 cm−1 for 73, 74, and 75, respectively). The relative lability of the CO ligands allowed these compounds to serve as useful starting materials for other cluster compounds. Thermal degradation of [Re6(m3-Se)8 (PEt3)5(CO)][SbF6]2 under dynamic vacuum resulted in the previously-identified [Re12Se16(PEt3)10][SbF6]4 cluster dimer, while irradiation with UV light in dichloromethane resulted in one-electron oxidation to [Re6(m3-Se)8(PEt3)5Cl][SbF6]2 (Scheme 30).

Scheme 30 Substitution of ausser iodide ligands with carbon monoxide in octahedral hexarhenium clusters.

6.11.2.1.3.6 Oligomerization of dinuclear rhenium fragments [{Re(CO)4}2(m-H)2] was oligomerized in thf-d8 at low temperatures when initiated by [HRe2(CO)9]−, [HB(sBu)3]−, or [H2Re(CO)4]−.59 These gave anionic chains of general formula [(CO)5Re-{HRe(CO)4}2n+1]− (76) and [H-{HRe(CO)4}2n]− (77) with even numbers of rhenium atoms, and [H-{HRe(CO)4}2n+1]− (78), with an odd number of rhenium atoms. Mean chain-length was determined by the relative integration of terminal and bridging hydrides in the 1H NMR spectrum (Scheme 31).

Scheme 31 Oligomerization of [{Re(CO)4}2(m-H)2] initiated by [HRe2CO)9]−, [HB(sBu)3]−, or [H2Re(CO)4]−.

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

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(Et4N)[{Re(CO)4}4(m-H)3H2] reacted with Ag(OTf ) in an NMR tube-scale reaction at 173 K to form the spiro[4,4]nonane cluster (Et4N)[({Re(CO)4}4(m-H)5)2Ag], (Et4N)(79).60 This structure was determined on the basis of the change in the position of the hydride resonances as well as a modified [109Ag-1H] HMQC NMR experiment that indicated AgdH bonding. Within the range of 220–230 K, trace signals appeared, representing the spiro[4,2]heptane cluster (Et4N)[({Re(CO)4}4(m-H)5)({Re(CO)4}2(m-H)2) Ag] (Et4N)(80) as well as the dinuclear fragment [Re(CO)4H]2. Upon cooling, the (Et4N)[({Re(CO)4}4(m-H)5)2Ag] signal was fully restored. (Et4N)(80) was also prepared directly from (Et4N)[{Re(CO)4}4(m-H)3H2], (Et4N)[{Re(CO)4}2(m-H)H2] and Ag(OTf ), or from (Et4N)[({Re(CO)4}2(m-H)2)2Ag] and additional [Re(CO)4H]2. Rather than further cluster growth, addition of (Et4N)[{Re (CO)4H}2(m-H)] to a solution of (Et4N)[({Re(CO)4}4(m-H)5)2Ag]− resulted in substitution to give (Et4N)(80) and free (Et4N)[{Re(CO)4}4(m-H)3H2]. (Et4N)(80) reacted with (Et4N)[{Re(CO)4H}2(m-H)] to give (Et4N)[({Re(CO)4}2(m-H)3)2Ag], (Et4N)(81) (Scheme 32).

Scheme 32 Interconversion between spiro[4,4]nonane, (79), spiro[2,4]heptane, (80), and spiro[2,2]propane, (81), clusters of rhenium and silver.

6.11.2.2 6.11.2.2.1

Heteronuclear clusters of Groups 5–7 metals Heterometallic trinuclear clusters

6.11.2.2.1.1 Cr2Mo clusters The reaction of (Et4N)2[Cr3(m-Se)2(CO)10] with 1 equivalent of Mo(CO)6 in acetone at reflux quickly afforded (Et4N)2[{Mo(CO)4} {Cr(CO)3}2(m-Se)2], (82), replacing the Cr(CO)4 fragment which was more weakly bonded to the m3-Se based on CrdSe bond length. Heating with 2 equivalents of Mo(CO)6, or heating isolated (Et4N)2[{Mo(CO)4}{Cr(CO)3}2(m-Se)2] with one more equivalent of Mo(CO)6, resulted in a mixture of products including an unstable intermediate that could not be isolated.

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Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

The authors hypothesized it to be (Et4N)2[{Mo(CO)4}{Mo(CO)3}{Cr(CO)3}(m-Se)2], which was supported by IR and 77Se NMR data. Heating (Et4N)2[Cr3(m-Se)2(CO)10] in refluxing acetone with 4 equivalents of Mo(CO)6 or (Et4N)2[{Mo(CO)4}{Cr (CO)3}2(m-Se)2], (82), with 3 equivalents of Mo(CO)6 gave the homonuclear (Et4N)2[Mo3(m-Se)2(CO)10] (83) (Scheme 33).27

Scheme 33 Partial or complete substitution of Cr with Mo in [Cr3(m-Se)2(CO)10]2− to form trigonal bipyramidal clusters 82 and 83.

6.11.2.2.1.2 Cr2W clusters (Et4N)2[{Cr(CO)3}2{Cr(CO)4}(m-Se)2] reacted with W(CO)6 in refluxing acetone, which quickly afforded (Et4N)2[{W(CO)4} {Cr(CO)3}2(m-Se)2] (84), as the W(CO)4 fragment replaced the Cr(CO)4 fragment (Scheme 34). Gentle heating in acetone of (Et4N)2[Cr3(m-Se)2(CO)10] with 4 equivalents of W(CO)6 or 84 with 3 equivalents gave [{W(CO)4}2{m-SeW(CO)5}2] as the major product, and homonuclear cluster (Et4N)2[W3(m-Se)2(CO)10], (85), as the minor product. Heating [{W(CO)4}2{m-SeW(CO)5}2] in refluxing acetone initiated a ring-closure reaction, yielding 85.27

Scheme 34 Partial or complete substitution of Cr with W in [Cr3(m-Se)2(CO)10]2− to form trigonal bipyramidal clusters 84 and 85.

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

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6.11.2.2.1.3 CrMo2 clusters [(CpMoCO)2(m-H)(m-PCy2)] reacted with Cr(CO)6 under visible-UV radiation to give [{CpMo(CO)}2(m3-H)(m-PCy2){Cr(CO)5}], (86), which rapidly decomposed to a mixture of [{CpMo(CO)}2(m-H)(m-PCy2)] and [(CpMo(CO)2)2(m-H)(m-PCy2)] [{CpMo (CO)}2(m3-H)(m-PCy2){Cr(CO)5}] was characterized by 31P NMR and 1H NMR spectroscopies and through comparison with the stable and more thoroughly-characterized tungsten analog 96, vide infra.24

Scheme 35 Ring-closure of [{M(CO)4}2{m-TeCr(CO)5}2]2− (M ¼ Mo, W) to give 87 and 88.

Heating (Et4N)2[{Mo(CO)4}2{m-TeCr(CO)5}2] in refluxing acetone resulted in a ring-closure to give (Et4N)2[{Cr(CO)3} {Mo(CO)3}{Mo(CO)4}(m3-Te)2], (Et4N)2(87).18 Two of the CO groups on the Mo(CO)4 fragment were semi-bridging, with slightly shorter CrdC and ModC bonds and a slight bending of the ModCdO bond angle to 168.0 (Scheme 35). 6.11.2.2.1.4 CrW2 clusters Heating (Et4N)2[{W(CO)4}2{m-TeCr(CO)5}2] in refluxing acetone also resulted in a ring-closure to yield (Et4N)2[{Cr(CO)3} {W(CO)3}{W(CO)4}(m3-Te)2], (Et4N)2(88) (Scheme 35).18 Two of the CO groups on the W(CO)4 fragment were semi-bridging, with slightly shorter CrdC and WdC bonds and a slight bending of the W-C-O angle to 167 .

Scheme 36 Synthesis and reactivity of mixed-metal trigonal bipyramidal clusters 91–95.

6.11.2.2.1.5 CrMn2 clusters See Scheme 46 in Section 6.11.2.2.1.9. The reaction between [{Mn(CO)3}2(m-k2,k1-SN2C4H5)2] and [Cr(CO)3(NCMe)3] in tetrahydrofuran gave the trinuclear [{Mn (CO)3}2{Cr(CO)2}(m3-SN2C4H5)2(m-CO)2], (89), featuring a bent-open geometry with a Mn-Cr-Mn angle of 117.64(7) as determined by SCXRD.52 The mixed-metal trigonal bipyramidal clusters (PPN)2[{Mn(CO)3}2{Cr(CO)3}(m3-S)2], (PPN)2(91), and (PPN)2[{Mn (CO)3}2{Cr(CO)3}(m3-Se)2], (PPN)2(92), were made directly in good yields by combining [Mn2(CO)10] with Cr(CO)6 in 4 M KOH/MeOH, followed by reaction with the chalcogen powder and metathesis with (PPN)Cl (Scheme 36).61 Performing the same reaction using tellurium powder gave only (PPN)2[{Mn(CO)4}2{m-TeCr(CO)5}2]. However, upon heating (PPN)2[{Mn (CO)4}2{m-TeCr(CO)5}2] in 0.4 M KOH/MeOH, a ring-closing reaction formed (PPN2)[{Mn(CO)3}2{Cr(CO)3}(m3-Te)2] (93). [{Mn(CO)4}2{m-TeCr(CO)5}2] was also made by the reaction between [Te{Cr(CO)5}2] with Mn(CO)5Br in acetonitrile.

Scheme 37 Reaction of [(CpMoCO)2(m-H)(m-PCy2)] with photolytically-generated radical fragments Cr(CO)5% and W(CO)5%.

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Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

(PPN)2[{Mn(CO)3}{Cr(CO)3}(m3-S)2] was unreactive towards CO, but (PPN)2(91) did react with CO to form the square-pyramidal (PPN)2[{Mn(CO)3}{Mn(CO)2}{Cr(CO)3}(m-CO)2(m3-Se)2], (PPN)2(93), which was rapidly converted to (PPN)2(91) upon recrystallization. [(PPN)2[{Mn(CO)3}{Mn(CO)2}{Cr(CO)3}(m-CO)2(m3-Te)2], (PPN)2(94), formed by the reaction of (PPN)2(92) with CO, was more thermodynamically/kinetically stable and required sustained heating in refluxing dichloromethane in order to release the added CO. 6.11.2.2.1.6 Mo2W clusters See Scheme 37 in Section 6.11.2.2.1.3. [{CpMo(CO)}2(m-H)(m-PCy2)] reacted with W(CO)6 under visible-UV radiation to give [{CpMo(CO)}2(m3-H)(m-PCy2){W(CO)5}], (96), which was characterized by 31P NMR and 1H NMR spectroscopies and SCXRD. The short length of the ModMo bond, 2.6351(5) A˚ , which fell between the accepted lengths for a double and triple bond in similar environments (ca. 0.1 A˚ longer than the formal triple bond in [{CpMo(CO)}2(m-H)(m-PCy2)], ca. 0.11 A˚ shorter than the formal double bond in 16), and the relatively long ModW bonds, 3.3714(5) and 3.3103(6) A˚ , led the authors to describe the bonding in the cluster as a 3c-2e interaction between the W and Mo2 fragments, with the W(CO)5 moiety acting essentially as an acceptor group resembling the isolobal CH+3 rather than CH2. The Cp groups exhibited only one 13C NMR resonance, consistent with a false C2 axis from rapid m3-H exchange between the two sides of the Mo2W plane.24 [{CpMo(CO)}2(m-H)(m-PCy2)] (see Section 6.11.2.2.1.3) also reacted with the mononuclear fragment [CpW(CO)3], generated from [{CpW(CO)3}2] via visible light, giving [{CpMo(CO)}2(m-PCy2)(CpW(CO)2)(m3-CO)] (97a) with a m3-CO ligand. Curiously, [{CpMo(CO)}{CpW(CO)}(m-PCy2)(CpMo(CO)2)(m3-CO)] (97b) was also identified among the products, as indicated by the presence of a resonance with 31P-183W coupling in the 31P NMR that closely resembled that of [{CpW(CO)}2(m-PR2)2]. The authors postulated that this arose from the bridge-to-terminal exchange of CO and PCy2 ligands in 97a (Scheme 38).24

Scheme 38 Reaction of [{CpMo(CO)}2(m-H)(m-PCy2)] with photolytically-generated radical fragment CpW(CO)3% and the equilibrium between the two form of cluster 97.

[{CpW(CO)3}{Mo(CO)5}(m-PPh2)] and sulfur in refluxing CH2Cl2 produced [(CpW){Mo(k2-Ph2PS2)}2(m-S)3(m3-S) (m,k2-Ph2PS2)] (98), a neutral heteronuclear, six MBVE cluster built around a M2M’S4 core. The Ph2PS2 ligands were presumed to originate from the insertion of S into the MdP bond (Scheme 39).62

Scheme 39 Synthesis of neutral heteronuclear cluster 98 from [{CpW(CO)3}{Mo(CO)5}(m-PPh2)] and sulfur.

The mechanism of incorporation of 14-electron metal fragments into [(CpMoCO)2(m-PCy2)(m-Z2:Z2:-P2Me)] was explored.63 [{CpMo(CO)}2(m-PCy2)(m-Z2:Z2-P2Me)] reacted with W(CO)4(thf )2 in thf to form [{CpMo(CO)}2{W(CO)4}(m3-P)(m-PCy2) (m3-PMe)] (99). This compound was postulated to have formed from the addition of the 16-electron [W(CO)4(thf )] fragment to the m3-P ligand, giving the intermediate shown in Scheme 40. For the reaction with W(CO)5(thf ), this was where the reaction stopped, with W(CO)5 bound to the m3-P, but for W(CO)4(thf )2, the tetrahydrofuran on the W(CO)4(thf ) bound to the m3-P can dissociate, exposing the unsaturated W(CO)4 fragment which would then insert into the PdP bond (Scheme 40).

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

741

Scheme 40 Reaction of [{CpMo(CO)}2(m-PCy2)(m-Z2:Z2-P2Me)] with W(CO)4(thf ) to form cluster 99, including intermediate.

The reactive intermediates from the thermolysis64 or photolysis65 of [C5Me5P{W(CO)5}2] were trapped by compounds with multiple metal-metal bonds. Thermolysis of [C5Me5P{W(CO)5}2] in toluene in the presence of [{CpMo(CO)2}2] led to the trinuclear clusters [{CpMo(CO)2}2{Cp W(CO)2}{m3-PW(CO)5}] (100), [{CpMo(CO)2}2{Cp W(CO)2}(m3-P)] (101) and [(CpMo)(Cp W){W(CO)4}(m-O){m3-PW(CO)5}2], the dinuclear compounds [{CpMo(CO)2}{Cp W(CO)2}(m-Z2:Z2-{PW (CO)5}2)] and [{CpMo(CO)3}2], and the mononuclear complex [{W(CO)3(k2,Z5:Z1-C5Me4CH2{PHW(CO)5})]. All structures were determined via SCXRD, and the SCXRD results were consistent with IR spectroscopy, 1H NMR and 31P NMR spectroscopies, and mass spectrometry. Upon exposure to UV radiation, a mixture of [C5Me5P{W(CO)5}2] and [{CpMo(CO)2}2] in toluene gave 101 and the new dinuclear compound [{Mo(CO)3}{W(CO)5}{m-PW(CO)5}] as the major products, with [{CpMo(CO)3}2] and 100 as minor products.66 The reaction mechanism was thoroughly explored.

Scheme 41 Synthesis of cluster 102 from (Cp Mo)2P2S3 and W(CO)5(thf ), including proposed transition state.

The room-temperature reaction between (Cp Mo)2P2S3 and excess W(CO)5(thf ) in tetrahydrofuran led to the remarkable [(Cp Mo)2{W(CO)3}(m3-S){m-Z2:Z2-SPW(CO)5}{m3-Z1:Z1-SPW(CO)5} (102).67 This interesting compound was postulated to proceed via (1) the formation of isolable intermediate [(Cp Mo)2(m-Z2:Z2-SPW(CO)5)(m-Z2:Z2-SP)(m-S)], (2) coordination of a [W(CO)5] fragment to the other phosphorus, and (3) significant ligand rearrangement as a third [W(CO)5] fragment approached the sulfur atom of the PS unit with the P already coordinated to a [W(CO)5] moiety as shown in Scheme 41.

Scheme 42 Reaction of [(CpMoCO)2(m-H)(m-PCy2)] with fragment Cp0 Mn(CO)2(thf ) and the equilibrium between the two form of cluster 103.

6.11.2.2.1.7 Mo2Mn clusters The unsaturated compound [CpMo(CO)2]2(m-H)(m-PCy2) has been reacted with a variety of mononuclear fragments to give trinuclear clusters (Sections 6.11.2.2.1.3 and 6.11.2.2.1.6). With mononuclear Cp0 Mn(CO)2(thf ) (Cp0 ¼ C5H4Me), it gave [{CpMo(CO)2}2(m-PCy2){Cp0 Mn(CO)2}(m3-H)] (103), featuring a m3-H on the same side of the M2M0 plane as the Cp0 .23 SCXRD indicated that the Mn-Mo bond lengths (3.135(1), 3.122(1)A˚ ) were close to literature MndMo single bond lengths, while the ModMo bond length was somewhat short at 2.6448(8) A˚ . The position of the Cp0 and m3-H ligands relative to the plane were sufficiently dynamic as to give a false C2 axis, allowing the Cp 13C NMR resonances in C6D6 to coalesce at room temperature. Cooled to 233 K, the two Cp resonances resolved (Scheme 42).24

742

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

6.11.2.2.1.8 MoW2 clusters [(CpMo)(Cp W){W(CO)4}(m-O){m3-PW(CO)5}2] (104) was a minor product of the thermolysis of [C5Me5P{W(CO)5}2] and [{CpMo(CO)2}2] in toluene (Section 6.11.2.2.1.6, Scheme 43). Its yield appeared to be dependent on traces of dioxygen in the synthesis of [{CpMo(CO)2}2], as different batches of [{CpMo(CO)2}2] gave different yields of this cluster as a minor product.66

Scheme 43 Reaction of [C5Me5P{W(CO)5}2] with [{CpMo(CO)2}2] to form mixed-metal clusters 100, 101, and 104.

The reaction of [(CpMo)2(CO)2(m-k2:k1,Z6-PMes )] (Mes ¼ 2,4,6-tBu3C6H2) with S8 in dichloromethane gave [(CpMo)2 (CO)2(m-k2:Z2,Z6-SPMes )]. Its reaction with W(CO)5(thf ) in tetrahydrofuran gave [(CpMo)(m-k1:k1,Z6-PMes ){W(CO)3} {W(CO)4}{CpMo(CO)2(m3-S)}] (105) (Scheme 44). Based on a similar reaction with Fe2(CO)9, the reaction was postulated to proceed via the addition of the metal fragment into the P-{CpMo(CO)3} bond, followed by the addition of another metal fragment and significant rearrangement. That the W-containing and Fe-containing products had similar formation pathways was supported by their similar final structures, differing only in the placement and number of CO ligands (as was expected going from Fe to W) and by the similarity between the 31P NMR spectra of the isolated Fe intermediate and the isolated, but not structurally characterized, W intermediate.

Scheme 44 Addition of W(CO)5(thf ) to [(CpMo)2(CO)2(m-k2:Z2,Z6-SPMes )] (Mes ¼ 2,4,6-tBuC6H2).

The reaction of [{CpW(NO)2}2(m-H)(m-PPh2)] with Mo(CO)5(thf ) in tetrahydrofuran gave [{CpW(NO)}2(m3-H)(m-PPh2){Mo(CO)5}].31 However, this compound is thermodynamically unstable, and spectra taken of the reaction mixture were dominated by the [{CpW(NO)}2(m-H)(m-PPh2)(CO)] by-product, so most analysis was done in comparison to the more stable and isolable [{CpW(NO)}2(m3-H)(m-PPh2){W(CO)5}], (vide supra Section 6.11.2.1.2.3). 6.11.2.2.1.9 MoMn2 clusters Photolysis of a combination of [{CpMo(CO)2}{CpMo}(m-k2:k1,Z6-SPMes )] (Mes ¼2,4,6-tBu3C6H2) and Mn2(CO)10 with visible-UV light in toluene, using quartz glassware, formed a mixture of compounds, including the dinuclear compounds [{CpMo}{Mn(CO)4}(m-k2:k1,Z6-PMes )] and [{CpMo(CO)}{Mn(CO)4}(m-k2:k1,Z4-SPMes )], and the cluster compound [{CpMo(CO)2}{Mn(CO)3}{Mn(CO)2(m-k2:k1,Z6-PMes )MoCp}(m-CO)(m3-S)] (106). This cluster product was postulated to be the formal result of the insertion of a Mn2(CO)6 fragment into the PdS and PdMo bonds of [{CpMo(CO)2}(CpMo)(m-k2:k1,Z6-SPMes )].68 The thiophosphinidene ligand was shown to be critical, as the related phosphinidene compound [{CpMo(CO)2} (CpMo)(m-k2:k1,Z6-PMes )] did not react with Mn2(CO)10 under the same reaction conditions (Scheme 45).

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743

Scheme 45 Reaction products of [(CpMo)2(CO)2(m-k2:Z2,Z6-SPMes )] and photolytically-generated Mn(CO)5% in different solvents and glassware.

The same reaction in tetrahydrofuran, in borosilicate glassware, resulted exclusively in the cluster [(CpMo){Mn(CO)4}2(m3k1:k1:k1,Z5-SC6Ht3Bu3)] (107). This appeared to be the product of the reaction of [(CpMo){Mn(CO)4}(m-k2:k1,Z6-PMes )] with a Mn(CO)4 fragment. The additional hydrogen on the SC6Ht3Bu3 ligand was thought to derive from hydrogen abstraction from either the solvent or trace water. Although the CVE count suggests the presence of two single MdM bonds, the MndMo bond is substantially elongated at 3.2910(5) A˚ (Scheme 45). [{Mn(CO)3}2(m-k2:k1-SN2C4H5)2] reacted with Mo(CO)3(NCMe)3 in tetrahydrofuran to give the trinuclear [{Mn(CO)3}2{Mo (CO)2}(m3-SN2C4H5)2(m-CO)2], (108), featuring a bent-open geometry with a Mn-Mo-Mn angle of 111.81(2) as determined by SCXRD.52

Scheme 46 Reaction of [{Mn(CO)3}2(m-k2:k1-SN2C4H5)2 with M(CO)3(NCMe)3 (M ¼ Cr, Mo, W).

6.11.2.2.1.10 W2Mn clusters Cp0 Mn(CO)2(thf ) reacted with [{CpW(NO)}2(m-H)(m-PPh2)] to give [{CpW(NO)}2(m3-H)(m-PPh2){Cp0 Mn(CO)2}] (109).31 This W2Mn cluster did not exhibit the false C2 axis shown by many compounds in this class that were formed by addition of a 16-electron metal fragment to a H-bridged unsaturated metal-metal bond. Its 31P NMR spectrum showed two distinct W-P couplings, consistent with the two W atoms being in differing environments at room temperature. Thus, the m3-hydride was stationary on the 1H NMR timescale.

744

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

6.11.2.2.1.11 W2Re clusters Na[(CpW)2(m-CO)2(m-PCy2)] reacted with Cp0 Re(CO)2(NO) (Cp0 ¼ C5H4Me) at 253 K in dichloromethane to form [{CpW(CO)2}(CpW^N{Cp0 Re(CO)2})(m-O)(m-PCy2)], in equilibrium with [{CpW(CO)2}(CpW^O){Cp’Re(CO)2}(m3-N:N: P-NPCy2)] (110).69 Cluster 110 was favored at higher temperatures. This is the first example of a m-NPR2 ligand with the N being nearly trigonal planar. The RedN bond length was comparable to the Re-N bond lengths in simple Re-amine complexes, suggesting little p-character. The NdP bond length was in between reference lengths for single and double PdN bonds at 1.633(8) A˚ vs. 1.78 A˚ and 1.556 A˚ , respectively.70,71 The NdW bond length was comparable to NdW bonds with significant p-character (1.91(1)) vs. ca. 1.94 A˚ for [Cp W(NR2)(CHt2Bu)(NO)].72 The formation of this cluster from [{CpW(CO)2}(CpW^N{Cp’Re(CO)2})(m-O) (m-PCy2)] required the insertion of a nitride into a WdP bond, which had not been seen prior to this literature report (Scheme 47).

Scheme 47 Reaction of [(CpW)2(m-CO)2(m-PCy2)]− with fragment Cp0 Re(CO)2(NO) and the equilibrium between the two products, including cluster 110.

6.11.2.2.1.12 WMn2 clusters The reaction between [{Mn(CO)3}2(m-k2:k1-SN2C4H5)2] and [W(CO)3(NCMe)3] in refluxing tetrahydrofuran gave the trinuclear [{Mn(CO)3}2{W(CO)2}(m3-SN2C4H5)2(m-CO)2], (111) , featuring a bent-open geometry with the Mn-W-Mn angle of 111.41(3) as determined by SCXRD.52 A side product of this reaction was [{Mn(CO)3}{Mn(CO)2}{W(CO)2SN2C4H5}(m3-SN2C4H5)2(m-CO)2], (112), featuring the same overall geometry but with the tungsten atom at the end of the bent-open structure, rather than bridging the two Mn, and bearing an N-protonated, terminal SN2C4H6 ligand on the W.

6.11.2.2.2

Heterometallic tetranuclear clusters

6.11.2.2.2.1 CrMn3 clusters When (Me3BnN)[{Mn(CO)3}3S2] (Bn ¼ benzyl) was reacted with 1 equivalent of Cr(CO)6 in 4 M KOH/MeOH at 60  C, the heteronuclear octahedral cluster (Me3BnN)3[{Mn(CO)3}3{Cr(CO)3}(m3-S)2], (113), was formed. (Me3BnN)[{Mn(CO)3}3Se2] reacted under the same conditions to form the selenium analog (Me3BnN)3[{Mn(CO)3}3{Cr(CO)3}(m3-Se)2] (114).44 These compounds were also formed by refluxing either 47 or 49 in 4 M KOH/MeOH. As 47 and 49 were formed in the same reaction at lower temperatures, the authors postulated that they were intermediates in the syntheses of these heteronuclear octahedral cluster products. These clusters are highly air-and moisture sensitive, and only the (Me3BnN)+ salt could be isolated (see Fig. 24, Section 6.11.2.1.3.1).

Fig. 24 Hexatungsten trigonal prismatic carbide cluster 226.

6.11.2.2.2.2 Mo2Mn2 clusters A slow reaction between Cp Mo(PMe3)H5 and Mn{N(SiMe3)2}2 gave the tetranuclear square cluster [{Cp Mo(PMe3)}2(Mn {N(SiMe3)2})2(m-H)4(m3-H)4] (115) as determined by SCXRD.73 This product was tested for catalytic activity in the conversion of N2 to N(SiMe3)3 in the presence of sodium and Me3SiCl, but its activity was far-lower than the Mo2Fe2 congener. The isocyanide CNtBu fragmented the cluster into dinuclear units. The structure of [{CpMoCNtBu}(Mn{N(SiMe3)2})(m-CN)(m-Z3,k2-C3N3HtBu3)], was determined by SCXRD (Scheme 48).

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

745

Scheme 48 Formation of cluster 115 from Cp Mo(PMe3)H5 and Mn{N(SiMe3)2}2.

6.11.2.2.2.3 Mo2Re2 clusters [{CpMo(CO)2}{Re(CO)4}(m-H)(m-PCy2)] reacted in acetonitrile under UV–visible irradiation to form [{CpMo(CO)(NCCH3)} {Re(CO)4}(m-H)(m-PCy2)]. Upon heating in acetonitrile or dichloromethane this compound rearranged to form [{CpMo(CO)2} {Re(CO)3(NCCH3)}(m-H)(m-PCy2)] in high yields. Upon further heating or photolysis in toluene, the tetrahedral cluster [{CpMo(CO)}2{Re(CO)3}2(m-PCy2)2] (116) was formed.74 The authors postulated that this occurred through the formation and dimerization of the radical fragment [{CpMo(CO)}{Re(CO)3}(m-PCy2)] formed by loss of CH3CN, H2, and CO. The cluster had CVE count of 58, two short of the 60 expected for a tetrahedral cluster, and this aligned with the observation that the heterometallic bonds were slightly shorter than would be expected for single bonds. The carbonyl groups on the molybdenum have partial m3-character, with the carbon-oxygen bond directed towards the Mo2Re face (Scheme 49).

Scheme 49 Formation of cluster 116 from heating [{CpMo(CO)2}{Re(CO)3(NCCH3)}(m-H)(m-PCy2)].

6.11.2.2.3

Clusters incorporating late transition metals or boron

6.11.2.2.3.1 Clusters incorporating boron The room-temperature reaction between Cp TaCl4 and LiBH4
thf, followed by reaction of the intermediate with the CS2 adduct S2CPPh3, gave [(Cp Ta)2(m2-Z2:Z2-B2H5)(m-H)(m, k2-S2CH2)2]. Addition of M(CO)5(thf ) (M ¼ Mo, W) gave the simple adducts [{(Cp Ta)(CH2S2)}2(B2H5)(H){M(CO)3}] (117, M ¼ Mo; 118, M ¼ W) (Scheme 50).75

Scheme 50 Coordination of M(CO)5(thf ) fragment to [(Cp Ta)2(m2-Z2:Z2-B2H5)(m-H)(m, k2-S2CH2)2].

The reaction of Cp MoCl4 with LiBH4 in toluene at −70  C, followed by thermolysis in toluene at 110  C, gave the known [(Cp Mo)2B5H9] as well as the novel [(Cp Mo)3MoB9H18] (119) (Scheme 51). Cluster 119 has an unusual structure consisting of oblate-nido {Mo2B5} and oblate-nido {Mo2B4} hexagonal-bipyramidal clusters fused perpendicularly through one of the Mo atoms of the {Mo2B5} fragment.76 The four molybdenum atoms are arranged in a spiked triangular shape. The ModMo bond forming the base of the triangle was wrapped with a chain of 4 boranes, while the ModMo bond between the other two molybdenum atoms was wrapped with a chain of 5 boranes. The structure was thought to arise from the condensation of (Cp Mo)2B5H9 with a proposed intermediate, [(Cp Mo)2B4H10],77 followed by loss of Cp H. Cluster 119 was later identified among the products of a reaction between (Cp Mo)2(m-Cl)2(m-Z2:Z2-B2H6) and CO over 12 h, along with the dinuclear [Cp Mo(CO)2]2(m-Z2:Z2-B2H4) and the novel trinuclear cluster [(Cp Mo)3(m-H)2(m3-H)(m-CO)2(B4H4)] (121) (Scheme 52).78

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Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

Scheme 51 Reaction of Cp MoCl4 with LiBH4 to form cluster 119.

The tungsten analog of 119 was synthesized by reacting Cp WCl4 with LiBH4 in toluene at −70  C (Scheme 52). The mixture was warmed to 10  C, stirred for 48 h, and CO bubbled through the reaction mixture for 12 h at room temperature. This procedure gave a mixture of [(Cp W)3WB9H18] (120) and [(Cp W)B5H10].78 It was unlikely that CO was necessary for the synthesis of 120, as, although the synthesis was designed to yield [{Cp W(CO)2}2(m-Z2:Z2-B2H4)]; no species incorporating CO were recovered. [(Cp W)3WB9H18], like the Mo analog, was proposed to form by condensation of [(Cp W)2B5H9] and a proposed intermediate in its creation, [(Cp W)2B4H10].77 Both the W and Mo clusters have short and long MdM bonds, short within each cluster, and long between the {M2B5} and the M in the {M2B4} that joins the two clusters (the longer bond was described as 3c,2e bonding between the two Mo atoms and a B).

Scheme 52 Reaction of Cp WCl4 with LiBH4 to form cluster 120 and reaction of Cp MoCl4 under identical conditions to form clusters 119 and 121.

The reaction of Cp MoCl4 with LiBH4 in toluene at −70  C, followed by pyrolysis at 100  C with Te powder, resulted in a myriad of dimetallic compounds of the form [(Cp Mo)2(m-Z5:Z5-X)] or [(Cp Mo)2(m-Z6:Z6-Y)] (X ¼ (BH2Cl)(BH)3Te, (BH3)(BCl) (BH)2Te, (BH3)(BH)3(BH3); Y ¼ (OEt)(BH)3(BCl)(Te), Te(BH)4Te) as well as the unusual [(Cp Mo)4B4H4(m4-BH)3] (122). Cluster 122 is best described as a distorted cubane with Cp Mo and B at alternating vertices, with three of the faces capped with BH groups in a meridional fashion (Scheme 53).

Scheme 53 Reaction of Cp MoCl4 with LiBH4 and Te, forming cluster 122 as a minor product.

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

747

Starting from (Cp MoCl)2B2H6,79 prepared through reduction in toluene of Cp MoCl4 with LiBH4, the addition of CO gave (Cp Mo(CO)2)2(m-Z2:Z2-B2H4), which, when reacted with W(CO)5(thf ), yielded [(Cp Mo(CO)2)2B2H2W(CO)4] (123) (Scheme 54). In compound 123, two of the hydrogen atoms were replaced with a W(CO)4 moiety. A tungsten analog of the intermediate [(Cp Mo(CO)2)2(m-Z2:Z2-B2H4)] could not be isolated when the same reaction was performed with Cp WCl4 and Mo(CO)5(thf ); nevertheless, the end product was the cluster [{Cp W(CO)2}2B2H4Mo(CO)4] (124) that supported the existence of [{Cp W(CO)2}2(m-Z2:Z2-B2H4)].80

Scheme 54 Formation of carbonylated metal-rich mixed-metal metallaboranes from Cp MCl4 and M0 (CO)5(thf ).

It was later shown that the carbonylation step is unnecessary, as reducing Cp WCl4 with excess LiBH4
thf in toluene, then reacting the intermediate with M(CO)5(thf ), gave [(Cp W)2B4H8{M(CO)4}] ((126), M ¼ Cr; (127), M ¼ Mo; (128), M ¼ W), the previously-known [{Cp W(CO)2}2B2H2{M(CO)4}] ((124), M ¼ Mo; (125), M ¼ W), and the long-known cluster [(Cp W)2B5H9].81 Performing the same reaction with Cp MoCl4 and Cr(CO)5(thf ) gave [(Cp Mo)2B4H8{Cr(CO)4}] (129) (Scheme 55).

Scheme 55 Formation of metal-rich metallaboranes from Cp MCl4 and M0 (CO)5(thf ).

When the intermediate in the synthesis of [(Cp Mo)3MoB9H18] (vide supra) was combined with [W(CO)5(thf )] before heating to 65  C in thf, three mixed-metal metallaboranes were formed: 123, [(Cp Mo)2B4H8W(CO)4] (130), and [(Cp Mo)2B4H6W (CO)5] (131) (Scheme 56).78 Since W(CO)4 and W(CO)5 are isolobal with BH3 and BH, respectively, both [(Cp Mo)2 B4H8W(CO)4] and [(Cp Mo)2B4H6W(CO)5] are formally related to the dinuclear [(Cp Mo)2B5H9] described earlier.

Scheme 56 Formation of metal-rich mixed-metal metallaboranes from Cp MoCl4 and W(CO)5(thf ).

The reaction of Cp MoCl4 with LiBH4 in toluene, followed by heating the solid product (postulated to be (Cp Mo)2B4H8), with W(CO)5(thf ) to 65  C in thf gave [{Cp Mo(CO)}(Cp MoCl){W(CO)3}(m-CO)B3H4] (132) in low yield (Scheme 57).82 Cp WCl4 yielded the similar [{Cp W(CO)}2{W(CO)4}B3H5] (133).77

748

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

Scheme 57 Formation of two similar octahedral M3B3 clusters from Cp MCl4 and W(CO)5(thf ).

6.11.2.2.3.2 Heterometallacubanes derived from Cp3Mo3S4 Because of their prevalence in the active sites of numerous metalloenzymes and electron-transport proteins, such as nitrogenases and ferredoxins, cubane-type metal sulfide clusters have been heavily scrutinized as potential catalysts. Their syntheses are relatively straightforward, through the addition of transition metal fragments to incomplete metallacubanes, such as Cp 3Mo3S4. Recent advances are summarized here, ordered by Group from 3 to 11 and within each Group from row 4 to 6. For all of these clusters, the [Cp 3Mo3S4] backbone was largely unchanged except for slight shifts in the ModMo bond distances resulting from changes in the total electron count of the cluster, and all chemistry occurred at the added metal fragment. Mixed-metal metallacubane clusters have typically been made through the combination of the oxidized, incomplete cubane cluster [Cp 3Mo3S4]+, typically [Cp 3Mo3S4][PF6], and a reduced late transition metal precursor, such as Pd(dba)2 or RuH4(PPh3)3.83–85 Recently, this has been extended to a variety of early transition metals by using the reduced species [Cp 3Mo3S4]− and various first-row transition metal halides, such as TiCl3(thf )3.86 A large number of these heterometallic metallacubanes have been made through the combination of sodium naphthalenide, Cp 3Mo3S4, and a range of transition metal halides in tetrahydrofuran. The reactive species [Cp 3Mo3S4]− was isolated as the [K(18crown-6)]+ salt. Reaction of [Cp 3Mo3S4]− with TiCl3(thf )3 gave the cubane cluster [Cp 3Mo3S4TiCl2] (134).86 This compound activated dinitrogen when reduced with KC8.87 Combining 134 with 4 equivalents of KC8, tetrahydrofuran, and 1 atm of N2 in a sealed glass tube at −78  C, allowing it to warm to room temperature before stirring overnight, removing the solvent under vacuum, and extracting the solid with tetrahydrofuran gave crystals of [K(thf )2]2[{(Cp Mo)3S4Ti}2(m-N2)], [K(thf )2]2(135), after concentration and cooling. The same procedure with 6–10 equivalents of KC8 gave trace amounts of the reduced [K3(thf )5](135). Addition of a proton source (H2O or HCl) and KC8 reductant to [K(thf )2]2(135) gave a mixture of NH3 and N2H4 in sub-stoichiometric quantities (Scheme 58).

Scheme 58 Formation of [K(thf )2]2(135) from 134, including dinitrogen activation.

Reaction of [Cp 3Mo3S4]− with VCl3(thf )3 or CrCl2 gave the dimer [Cp 3Mo3S4M]2(m-Cl)3, ((135), M ¼ V; (136), M ¼ Cr), although 136 was only recovered in trace amounts.86 MnCl2, FeCl2 or FeBr2, CoCl2, and NiBr2(DME) all gave [Cp 3Mo3S4MCl] or [Cp 3Mo3S4MBr], ((137), M ¼ Mn, X ¼ Cl; (138Cl), M ¼ Fe, X ¼ Br; (138Br), M ¼ Fe, X ¼ Br; (139), M ¼ Co, X ¼ Cl; (140), M ¼ Ni, X ¼ Br). These compounds had Mo-M distances between 2.7219(7) and 2.9829(8) A˚ , with consistent ModM bond lengths within each cluster. All were redox-active, although there were no clear trends in the potentials of reversible processes, many clusters had an anodic wave at approximately +0.7 V, which was tentatively attributed to the [Cp 3Mo3S4]+/2+ process. (Fig. 7).

Fig. 7 Cluster geometries of clusters 136–140, showing octanuclear and tetranuclear structures.

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

749

Following the same procedure on FeCl2 using cyclopentadienyl ligands other than Cp can provide heterocubanes with different steric environments around the Fe center, although with very similar electronic environments based on Mössbauer spectroscopy. [{(C5Me4R)Mo}3S4][PF6] (R ¼ SiMe3, SiEt3) was reduced to [{(C5Me4R)Mo}3S4] with KC8 and then reacted with FeCl2 as described above to give [(C5Me4R)3Mo3S4FeCl] ((141), R ¼ SiMe3; (142), R ¼ SiEt3).22 [Cp 3Mo3S4RuH2(PR3)][PF6] ((143Ph)[PF6], R ¼ Ph; (143Cy)[PF6], R ¼ Cy) were made by treating [Cp 3Mo3S4][PF6] with [RuH4(PR3)3].83,88 From (143Ph) a wide variety of RuMo3S4 cubane clusters were synthesized.85 Heating (143Ph)[PF6] with excess aqueous HCl in tetrahydrofuran gave [Cp 3Mo3S4RuCl3] (144). From there, reduction with cobaltocene in the presence of H2 and PR3 (R ¼ iPr, Cy) gave [Cp 3Mo3S4RuH(PR3)] (145iPr) and (145Cy). This monohydride was treated with CCl4 in benzene to form the monochloride [Cp 3Mo3S4RuCl(PR3)] (146iPr) and (146Cy). Clusters 146iPr and 146Cy were obtained also by treating 144 with cobaltocene and PR3 in the absence of H2. Reaction of 146iPr or 146Cy with CO in the presence of Na(PF6) gave the carbonyl [Cp 3Mo3S4Ru(CO)(PR3)][PF6], (147iPr) or (147Cy), while reaction with H2 in the presence of Na(PF6) gave [Cp 3Mo3S4RuH2(PR3)], (143iPr)[PF6] and (143Cy)[PF6]. Using the IR nCO as a reporter, the CO stretch at 1925 cm−1 indicated comparable electron density at the Ru site to Ru compounds capable of binding dinitrogen (Scheme 59).

Scheme 59 Manipulation of ligands on Ru center of 144.

Both (143Ph)[PF6] and (143Cy)[PF6] reacted with hydrazine as well as with phenylhydrazine, disproportionating hydrazine into ammonia and nitrogen and phenylhydrazine into a mixture of PhNH2, N2, C6H6, and NH3. These reactions gave, as a byproduct, the novel clusters [Cp 3Mo3S4Ru(NH3)(PR3)][PF6], (148Ph)[PF6] (Scheme 60), and (148Cy)[PF6] (not illustrated), which were isolated by crystallization from the tetrahydrofuran reaction mixture with hexanes. A rare double cubane cluster, [{(Cp Mo)3S4Ru}2(m-NH2)(m-NHNH2)][PF6]2, (149)[PF6]2, precipitated during catalysis.

Scheme 60 Manipulation of ligands on Ru center of 143Ph, including octanuclear clusters, and nona- and decanuclear clusters incorporating Pd and Pt bonded to Ru.

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Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

Similar double-cubane clusters, bridged by two disulfide units, were synthesized from (143Ph)[PF6] by heating with sulfur in tetrahydrofuran, resulting in [{(Cp Mo)3S4Ru}2(m-Z2:Z1-S2)]2[PF6]2 (150)[PF6]2 (Scheme 60).89 This compound was a useful starting material for an array of nona- or decanuclear mixed-metal sulfide clusters. Reaction of (150)[PF6]2 with 1 equivalent of Pd(PPh3)4 in tetrahydrofuran gave [{(Cp Mo)3S4Ru}2(m3-S)2{Pd(S)(PPh3)}]2[PF6], (151)[PF6], with the Pd bound to one of the Ru atoms, while 2.5 equivalents of Pd(PPh3)4 gave [{(Cp Mo)3S4Ru}2(m3-S)2{(Pd(PPh3))2(m2-S)}][PF6]2 (152)[PF6]2. Finally, treatment with Pt(PPh3)4 gave [{(Cp Mo)3S4Ru}2(m3-S)2{Pt(PPh3)2}]2[PF6], (153)[PF6], with the Pt symmetrically bonded to both rutheniums. Mo3Co clusters have been shown to have a wide range of chemically-accessible oxidation states, ranging from 58 to 61 cluster valence electrons (CVE).90 [(Cp0 Mo)3S4][tosylate] (Cp0 ¼ C5H4Me) reacted with Co2(CO)8 or Cp Co(CO)2 to form the known 60 CVE cluster [(Cp0 Mo)3S4Co(CO)] in high yields. This cluster was oxidized with I2 to give [(Cp0 Mo)3S4CoI], (154), with 59 CVE, reacted with PPh3 to give [(Cp0 Mo)3S4CoPPh3] (155), with 60 CVE, and reacted with NO to give [(Cp0 Mo)3S4Co(NO)] (156) with 61 CVE. Alternatively, reaction of [(Cp0 Mo)3S4] with 2 equivalents of [CoI(CO)3(PPh3)] gave a mixture of compounds, among which was isolated [(Cp0 Mo)3S4CoI][CoI3(thf )], (157)[CoI3(thf )], with 58 CVE. Very little variation in the ModMo bond lengths in these compounds was observed, attributed to the electron density being highly-delocalized within the structures. Treatment of [(Cp Mo)3S4][PF6] with Ni(COD)2 (COD ¼ 1,5-cyclooctadiene) gave [{(Cp Mo)3S4Ni}2(m-Z2,Z2-COD)][PF6]2, (158)[PF6]2. Cluster (158)[PF6]2 consisted of two [(Cp Mo)3S4Ni] units bridged by a cyclooctadiene between the two Ni centers.91 Addition of 2 equivalents of dimethylacetylenedicarboxylate (dmad) split the structure into [(Cp Mo)3S4Ni(Z2-dmad)][PF6] (159) [PF6]. Either previously-isolated or prepared in situ, (158)[PF6]2 and (159)[PF6] catalyzed the intramolecular cyclization of 4-pentynoic acid to the corresponding enol lactone, all with yields of over 95% in the presence of 3.3 mol% Et3N and a catalyst loading of 0.33 mol%. The reaction did not proceed with either [(Cp Mo)3S4][PF6] or Ni(COD)2 alone (Scheme 61).

Scheme 61 Substitution of COD on Ni center with dmad on 158 to yield 159.

There are numerous examples of Pd being integrated into heterometallacubanes for catalytic studies. [(Cp Mo)3S4PdPPh3][PF6] and [(Cp Mo)3S4Pd(dba)][PF6], which were made previously,83 as well as the novel [(Cp Mo)3S4Pd(ma)][PF6], (160)[PF6], prepared from [(Cp Mo)3S4][PF6] and [Pd(NBD)(ma)] (NBD ¼ 2,5-norbornadiene; ma ¼ maleic anhydride), were tested for efficacy in catalyzing the intramolecular cyclization of aminoalkynes. [(Cp Mo)3S4Pd(dba)][PF6] and (160)[PF6] were highly-effective, converting 95% of 4-phenyl-3-butyn-1-amine to the corresponding cyclic imine.84 [(Cp Mo)3S4PdPPh3][PF6] was only as effective as mononuclear Pd(II) complexes, with only 15% conversion. [(Cp Mo)3S4Pd(dba)][PF6] was also shown to catalyze allylic amination of allylic alcohols. [(Cp Mo)3S4Pd(dba)][PF6], in the presence of 0.5 equivalents of a boron-based Lewis acid such as B(OMe)3, catalyzed allylation of N-methylaniline better than did Pd(PPh3)4 and catalyzed the allylation of a range of amines.92 Furthermore, when used to catalyze the allylation of N-methylaniline with a variety of allylic alcohols, the unbranched product was produced exclusively. The cluster [(Cp Mo)3S4Pd(dba)][PF6] reacted with RC3H4Cl and AgPF6 to produce [(Cp Mo)3S4Pd(Z3-C3H4R)][PF6] ((161H) [PF6], R ¼ H; (161Ph)[PF6], R ¼ Ph) without the need for a boron Lewis acid.93 Both compounds catalyzed the allylation of N-methylaniline with allyl alcohol without the need of Lewis acid additives. The same high yields and regioselectivity were obtained using the two precatalysts with a variety of allylic alcohols and amines. It was noteworthy that cinnamyl alcohol gave only the E product, consistent with the presence of a p-allylpalladium intermediate. This reactivity was extended to active methylene compounds. Friedel-Crafts-type allylic alkylation of nitrogen-containing aromatic compounds led to alkylation of the position para to the nitrogen in N,N-dialkylanilines, and at the 3-position of N-alkylindoles.94 The reaction of [(Cp0 Mo)3S4][OTf] with CuCl in CH2Cl2 gave [(Cp0 Mo)3S4CuCl][OTf], (162Cl)[OTf], in nearly quantitative yields. [(Cp0 Mo)3S4][OTf] did not react with AgCl, AgI, [(PPh3)AgI]4, [Ph3PAg(OTf )], or (tht)AuCl (tht ¼ tetrahydrothiophene).95 This method was successfully extended years later to give [(Cp Mo)3S4CuI][PF6], (162I)[PF6], using [(Cp Mo)3S4][PF6] and CuI in CH2Cl2, [(Cp Mo)3S4AgPPh3][PF6][OTf] (163)[PF6], using [(Cp Mo)3S4][PF6] and [(Ph3P)Ag][OTf] in CH2Cl2, and [(Cp Mo)3S4Au(PR3)][PF6][X] ((164pH), R ¼ Ph, X ¼ BF4; (164Cy), R ¼ Cy, X ¼ PF6; (164tBu), R ¼ tBu, X ¼ PF6) using [(Cp Mo)3S4][PF6] and [(Ph3P)Au](BF4), [(Cy3P)Au][PF6], or [(tBu3P)Au][PF6].96

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Fig. 8 Cluster 165, a trace product from the reaction between (Z5-C5Me4H)Ir(CO)2 and [{CpMo(CO)}{CpMo(CO)2}{Ir(CO)2}2(m-CO)3.

6.11.2.2.3.3 Mo4Ir4 clusters (Z5-C5Me4H)Ir(CO)2 reacted with [{CpMo(CO)}{CpMo(CO)2}{Ir(CO)2}2(m-CO)3] in refluxing toluene to give a complex mixture of mixed-metal clusters. A trace product among these was [(CpMo)4Ir4(CO)9(m-CO)4] (165) (Fig. 8). This cluster structure is best described as a pentagonal bipyramid featuring an Ir-Mo-Ir-Ir-Mo pentagon, with an Ir above and a Mo below the pentagon. The fourth CpMo unit capped the unique Ir3 face. The cluster possesses 106 CVE, four less than would be expected for a capped pentagonal bipyramid with 19 MdM bonds.97

6.11.3

Clusters with MdC bonds to CO and Cp-type ligands, as well as to other organic ligands

6.11.3.1

Homonuclear clusters of Group 5–7 metals

6.11.3.1.1

Homonuclear clusters of Group 5 metals

6.11.3.1.1.1 Trinuclear clusters of Nb Reduction of NbCl5 with Al in acetonitrile solution, followed by workup in H2O, afforded [{Nb(H2O)3}3(m-O)3(m3Z2:Z2:Z1-NCCH3)]6+, (166H2O), which featured an unusual perpendicularly-coordinated acetonitrile ligand. The molecular structure was determined by SCXRD of the thiocyanate derivative, (Me3NH)3[{Nb(NCS)3}3(m-O)3(m-Z2:Z2:Z1-NCCH3)], (Me3NH)3(166SCN) (Fig. 9). The charge assigned to (166H2O) was supported by the expected change upon exchanging neutral aqua ligands with negatively-charged thiocyanates as well as by comparing the elution time in an ion-exchange column with similar clusters of known charge.

6.11.3.1.1.2 Trinuclear clusters of Ta [H2B(MesIm)2]TaMe3Cl (MesIm ¼ bis(N0 -mesitylimidazole-1-yl)dihydroborate), upon methylation to [H2B(MesIm)2]TaMe4 and reduction with H2 in hexanes, gave the first example of a tritantalum cluster, [{(Z2-H2B(MesIm)2)Ta}{(Z3-H2B(MesIm)2) Ta}2(m-H)4], (167), with only m-H ligands between tantalums (Fig. 10). Two of the Ta centers were bound to the metallated mesityl groups of the H2B(MesIm)2 ligands through a methylene group by C-H activation. As the authors were unable to synthesize this compound via the H2 reduction of related dinuclear compounds from the same starting material under similar reaction conditions, they postulated that the formation of the trinuclear species was dependent on the formation of a reduced mononuclear Ta hydride intermediate.13

Fig. 9 Cluster 166SCN, featuring a perpendicularly-coordinated acetonitrile ligand, and model for structure of 166H2O, its precursor.

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Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

Fig. 10 Cluster 167 formed from the activation of the mesityl groups of the precursor [H2B(MesIm)2]TaMe4 upon reduction with H2. The first tritantalum cluster with no bridging ligands other than hydride.

6.11.3.1.2

Homonuclear clusters of Group 6 metals

6.11.3.1.2.1 Trinuclear clusters of Mo The reaction of Mn2(CO)6(pyS)2 (pyS ¼ pyridine-2-thiol) with 2 equivalents of [{CpMo(CO)3}2] gave [(CpMo)3(m-CO)2(m-S) (m-Z2-NC4H4)] (168) as a minor product, featuring an ortho-metallated bridging pyridine ligand and two semi-bridging CO ligands (Scheme 62). The Mo3 triangle consisted of two long ModMo bonds, 2.9763(5) and 3.0104(5) A˚ , as well as a shorter bond at 2.7508(5) A˚ , demonstrating the localized bonding that is common to M3 clusters that lack a C3 axis. Mn2(CO)6(pyS)2 may be necessary for the formation of [(CpMo)3(m-CO)2(m-S)(m-Z2-NC4H4)], as attempts at the synthesis by reaction of only [CpMo(CO)3]2, pyridine, and sulfur under the same conditions failed. The authors’s attempts to synthesize it using the other products of the reaction, [(CpMo){Mn(CO)3}(m-CO)(m-Z1-pyS)(m-Z2-pyS)] and CpMo(CO)2(Z2-pyS), with Mn2(CO)6(pyS)2 also failed.98 Compound 168 was also encountered as a product from the reaction between Re2(CO)6(pyS)2 and [CpMo(CO)3]2.99 The other product of this reaction, [(CpMo)2Re(m3-S)(m-CO)2(m-Z2-C5H4NS)(CO)4] (197), is discussed in Section 6.11.3.2.4.

Scheme 62 Formation of 168 as a minor product of the reaction between Mn2(CO)6(pyS)2 (pyS ¼ pyridine-2-thiol) and [{CpMo(CO)3}2] and the inability of the major products, [(CpMo){Mn(CO)3}(m-CO)(m-Z1-pyS)(m-Z2-pyS)] and CpMo(CO)2(Z2-pyS), to be converted to 168.

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A method has since been developed to prepare this cluster type directly. The reaction of [CpMo(CO)2]2 with 2-mercapto1-methylimidazole resulted in a mixture of mononuclear CpMo(CO)2(k2-C4H5N2S) and trinuclear [(CpMo)3(m-CO)2(m-S) (m-k2-C4H5N2S)(m3-S)], (169) (Scheme 63). Reacting [{CpMo(CO)2}2] with 2-mercaptobenzothiazole similarly gave a mixture of the mononuclear CpMo(CO)2(k2-C7H4NS) and the trinuclear [(CpMo)3(m-CO)2(m-S)(m-k2-C7H4NS)(m3-S)], (170), not illustrated. In both cases, reacting the mononuclear compounds CpMo(CO)2(k2-C4H5N2S) or CpMo(CO)2(k2-C7H4NS) with [{CpMo(CO)2}2] gave the trinuclear species. Like many trinuclear clusters lacking a C3 axis, one edge of the M3 triangle is much shorter than the others, 2.7605(4) A˚ vs. 3.0105(5) A˚ and 2.9822(4) A˚ , respectively for [(CpMo)3(m-CO)2(m-S)(m-k2-C4H5N2S) (m3-S)].100

Scheme 63 The direct synthesis of 169 from [CpMo(CO)2]2 and 2-mercapto-1-methylimidazole.

The inorganic sulfur-bridged cluster [{Mo(k2-dtp)}3(m-OAc)(m3-S)(m-S)3(NCCH3)] (dtp ¼ diethyldithiophosphate) reacted with dimethylacetylenedicarboxylate (DMAD) in methanol to give a mixture of two air-stable clusters, the seven-electron inorganic cluster [{Mo(k2-dtp)}3(m-OAc)(m3-S)(m-S)3{m3-SC(CO2CH3)]C(CO2CH3)S}{m-SC(CO2CH3)]CH(CO2CH3)}] and the organometallic [{Mo(k2-dtp)}3(m-OAc)(m3-S)3(m3-S){m3-SC(CO2CH3)]C(CO2CH3)S}{m-SC(CO2CH3)]CH(OCH3)(CO2)}(CH3OH)], (171), both characterized by SCXRD (Scheme 64).101 A twofold excess of DMAD gave only the latter in 11% yield. A fourfold DMAD excess gave the former in 39% yield and the latter in 1% yield, and a 10-fold excess gave the former in 37% yield and the latter in 22% yield. Notably, the formation of 171 required an a,b-conjugate addition of methanol to the DMAD ligand.

Scheme 64 Reaction of [{Mo(k2-dtp)}3(m-OAc)(m3-S)3(m-S)3(NCCH3)] (dtp ¼ diethyldithiophosphate) with DMAD, featuring a a,b-conjugate addition of methanol to the DMAD ligand in order to yield 171.

A later paper from the same authors examined in more detail the reactivity of [{Mo(k2-dtp)}3(m-OAc)(m3-S)(m-S)3(NCCH3)] and DMAD with ketones and aldehydes.102 When this reaction was performed using various ketones or aldehydes as solvents, rather than methanol, two hydrogens were removed from the a-position as this carbon formed a bond to a molybdenum and one of the bridging sulfides. This was the case for acetone, acetaldehyde, acetophenone, and acetylacetone (Scheme 65). Acetylacetone

754

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

Scheme 65 Activation of the a-carbon of acetone, acetaldehyde, acetophenone, and acetylacetone upon reaction with [{Mo(k2-dtp)}3(m-OAc)(m3-S) (m-S)3(NCCH3)] (dtp ¼ diethyldithiophosphate) and DMAD.

primarily formed an adduct in which the a-carbon is only bound to the bridging sulfide, as does ethyl acetoacetate. Structures were determined by SCXRD supported by 1H NMR spectroscopy. Mass spectrometry of the reaction mixture of [{Mo(k2-dtp)}3(m-OAc) (m3-S)(m-S)3(NCCH3)] and acetone in the presence and absence of DMAD indicated that DMAD was necessary for the reaction to occur. [{CpMo(CO)}2(m-Z1:Z2-CH3)(m-PCy2)] showed similar reactivity to [{CpMo(CO)}2(m-H)(m-PCy2)], reacting with Mo(CO)6 under visible-UV radiation and subsequently undergoing dehydrogenation to give [{CpMo(CO)}2(m3-CH)(m-PCy2){Mo(CO)5}], (174) (Scheme 66).103

Scheme 66 Reaction of [{CpMo(CO)}2(m-Z1:Z2-CH3)(m-PCy2)] with photolytically-generated radical fragment Mo(CO)5% to form 174 and the ligand substitution of 174.

Cluster 174 reacted with P(OMe)3 in toluene to give [{CpMo(CO)}2(m3-CH)(m-PCy2){Mo(CO)4P(OMe)3}], (175), with the phosphite bound trans to the carbyne.104 Refluxing 175 in toluene for 15 min drove off a Mo(CO)5{P(OMe)3} fragment, resulting in the dinuclear, triply-bonded [(CpMo)2(m-CH)(m-PCy2)(m-CO)]. Heating 175 in refluxing toluene in the presence of CO resulted in the elimination of Mo(CO)6 and the addition of 2 equivalents of CO to give the dinuclear, doubly-bonded [{CpMo(CO)}2(m-PCy2){m-C(H)CO}].

6.11.3.1.2.2 Trinuclear clusters of W The photochemical reaction of ferrocenylacetylene and excess W(CO)6 in cold hexanes yielded a mixture of trinuclear [{W(CO)4}2{W(CO)2}(m-CO)2{m-Z2:Z2-(H)C^CFc}2], (176), and dinuclear [W2(m-Z2:Z2:Z2:Z2-C8H4Fc4)(CO)6]. The relative

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

755

Fig. 11 Cluster 176, with bridging ferrocenylacetylene (Fc ¼ ferrocene).

reactant stoichiometry determined the major product, with maximum yield of the cluster achieved with a threefold excess of W(CO)6 and a maximum yield of the dinuclear product achieved with a twofold excess of ferrocenylacetylene. Compound 176 was reported to be remarkably stable under thermal and photolytic conditions, decomposing only after prolonged photolysis or heating for 2 h in refluxing benzene (Fig. 11).105

6.11.3.1.3

Homonuclear clusters of Group 7 metals

6.11.3.1.3.1 Exohedral fullerene complexes of trinuclear rhenium clusters The CO ligands of [Re3(m-H)3(CO)9(m3-Z2:Z2:Z2-C60)] were substituted with 2 or 3 equivalents of PMe3 by first decarbonylating with 2.2 or 3.3 equivalents of Me3NO in MeCN, respectively, before heating to reflux in chlorobenzene with PMe3. This afforded two diastereomers for each.106 Structures of 178 and 180 were determined by SCXRD, while the other two were determined by 1H NMR spectroscopy (Scheme 67).

Scheme 67 Substitution of carbonyl ligands on [Re3(m-H)3(CO)9(m3-Z2:Z2:Z2-C60)] with PMe3 to form clusters 177–180.

The treatment of [Re3(m-H)3(CO)9(m3-Z2:Z2:Z2-C70)] with 2 equivalents of Ph3P]NCH2Ph replaced one carbonyl with PhCH2NC, giving two isomers of [Re3(m-H)3(CO)8(CNCH2Ph)(m3-Z2:Z2:Z2-C70)], (181), and (182), in a 2:1 ratio based on the two different environments indicated by 1H NMR spectroscopic resonances in the methylene region (Fig. 12). This was consistent with the fact that, of the three rhenium atoms, two were at symmetry-equivalent positions.107 Reacting [Os3(CO)6(PMe3)3(m3-Z2:Z2:Z2-C60)] with Re3(m-H)3(CO)11(NCMe) gave a mixture of three isomers, (183), (184a), and (184b), in good overall yield. Cluster 183 existed as two enantiomers in equilibrium in solution because of coupled restricted rotation at each osmium center, as indicated by variable-temperature (VT) 1H NMR spectroscopy. Both enantiomers of 183 were present in the crystal structure as determined by SCXRD. Clusters 184a, and 184b were shown to be in equilibrium with each

756

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

(A) (B)

Fig. 12 (A) Fullerene-bound clusters 181 and 182, showing different positions of benzyl isocyanide ligands. (B) graph of C70 showing that the hexagon coordinated to the rhenium cluster has two environments, i and ii, the pentagon labeled with  is the same in each drawing.

OC

Fig. 13 Clusters 183, 184a, and 184b showing relative position of trirhenium cluster to triosmium cluster, and relative positions of trimethylphosphine ligands to one another.

another, with 184a in higher concentration. Clusters 183 and 184 are present in each other’s VT 1H NMR spectra at high temperatures, indicating some degree of interconversion (Fig. 13).108 The reaction of Re3(m-H)3(CO)11(NCMe) with Sc2C2@C3v(8)-C82 gave [Re3(m-H)3(CO)9(m3-Z2:Z2:Z2-Sc2C2@C3v(8)-C82)] (185). As compound 185, the only isolated product (57% yield), featured a Re3 cluster bound to the only sumanene-like (a hexagon surrounded by three hexagons and three alternating pentagons) and most-strained hexagon on the fullerene. The complexation reaction was thus regiospecific (Fig. 14).109 This complexation chemistry has the potential to enhance separation of fullerenes. This was explored in a follow-up paper by the same research group. This complexation method may be used to separate Sc2@C82-C3v(8) from Sc2C2@C80-C2v(5), which by themselves have very similar retention times in HPLC. The reaction of a mixture of these two fullerenes with Re3(m-H)3 (CO)11(NCMe) formed products with Re3(m-H)3(CO)9 bound to a sumanene hexagon on each of the fullerenes. [Re3(m-H)3 (CO)9(m3-Z2:Z2:Z2-{Sc2@C82-C3v(8)})], (186), and [Re3(m-H)3(CO)9(m3-Z2:Z2:Z2-{Sc2C2@C80-C2v(5)})], (187), were easily separated by HPLC, with the cluster removed by subsequent treatment with CO (1 atm, 180  C) to afford the separated fullerenes.110

Fig. 14 Sumanane, a hexagon surrounded by three hexagons alternating with three pentagons, the most strained type of hexagon found in fullerenes.

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This same method was then used to separate the fullerenes Sc2O@C82-Cs(6) and Sc3N@C78-D3h(5), which previously had been isolated by a time-consuming process of selective precipitation with strong Lewis bases. Only Sc2O@C82-Cs(6) reacted with [Re3(m-H)3(CO)11(NCMe)], forming [Re3(m-H)3(CO)9(m3-Z2:Z2:Z2-{Sc2O@C82-Cs(6)})] (188) that allowed fullerene separation. This selective complexation was attributed to Sc3N@C78-D3h(5) having its higher-occupied molecular orbitals delocalized over the fullerene cage, and thus the two sumanene hexagons of Sc3N@C78-D3h(5) lacked the basicity to bind the cluster.110 This cluster-based fullerene separation approach was used to successfully partition the never-before-separated mixture of C86C2(17) and C86-C2(16), as the product of each of these with 2 equivalents of [Re3(m-H)3(CO)11(NCMe)], (189) and (190), had appreciably-different HPLC retention times.110

6.11.3.2 6.11.3.2.1

Heteronuclear clusters of Group 5–7 metals CrMo2 clusters

[{CpMo(CO)}2(m-Z1:Z2-CH3)(m-PCy2)], featuring an agostic methyl group, showed similar reactivity to [{CpMo(CO)}2(m-H) (m-PCy2)] (Sections 6.11.2.1.2.2, 6.11.2.2.1.3, and 6.11.2.2.1.6), reacting with Cr(CO)6 under visible-UV radiation in toluene before dehydrogenation to [{CpMo(CO)}2(m3-CH)(m-PCy2){Cr(CO)5}], (189).103 Unlike the Mo3 and Mo2W analogs, 174 and 192, 189 was in equilibrium with a variety of isomers in solution (Scheme 68). In addition to 189, the other two species were [{CpMo(CO)2}(CpMoCO)(m3-CH)(m-PCy2){Cr(CO)4}] (191) and [(CpMo(CO)2){CpMo(CO)}(m-CH)(m-PCy2){Cr(CO)4}] (190). The exact proportion between them was solvent-dependent, though in all cases concentrations of [191] > [190] > [189]. Cluster 191 differs from 189 in that PCy2 bridged the Cr(CO)4 and the CpMo(CO) corners. This structural assignment was based on the lack of the high-frequency (2050 cm−1) absorption in the IR spectrum, consistent with the absence of a M(CO)5 moiety, and the strongly-deshielded 31P NMR resonance in the 31P{1H} NMR spectrum consistent with PCy2 bonding to the electron-poor Cr. The other isomer 190, like 189, had the PCy2 bridging the two Mo corners based on the lack of a deshielded 31P NMR resonance. Cluster 190 lacked the M(CO)5 IR signals as observed for 191 and the symmetry according to the 13C NMR spectroscopic data. The methylidyne was proposed to be edge-bridging in the manner depicted (190) in order to provide a balanced electron distribution, though the methylidyne resonance was not observed in the 13C{1H} NMR spectrum. None of these compounds were analyzed by SCXRD.

Scheme 68 The reaction of [{CpMo(CO)}2(m-Z1:Z2-CH3)(m-PCy2)] with the radical fragment Cr(CO)5
, and the equilibrium between the products 189–191.

6.11.3.2.2

Mo2W clusters

[{CpMo(CO)}2(m-Z1:Z2-CH3)(m-PCy2)] also reacted with W(CO)6 under visible-UV radiation followed by dehydrogenation to form [{CpMo(CO)}2(m3-CH)(m-PCy2){W(CO)5}], (192).103 The ModMo and ModW bond lengths were all consistent with a bond order of 1, suggesting that the W(CO)5 fragment was acting akin to an isolobal CH2 fragment (Scheme 69).

Scheme 69 The reaction between [{CpMo(CO)}2(m-Z1:Z2-CH3)(m-PCy2)] and the photolytically-generated radical fragment W(CO)5%.

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Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

[{CpMo(CO)}2(m-CPh)(m-PCy2)] reacted with W(CO)6 under visible-UV radiation in toluene to give [{CpMo(CO)}2(m-PCy2) (m3-CPh){W(CO)4}] in two forms (193 and 194), postulated to consist of a cis form, with both CO ligands on the opposite side of the Mo2W ring as the m3-CPh, and a trans form, with only one on the same side of the ring. This structural assignment was based on the presence of two sets of 1H and 13C NMR resonances for the Cp rings, with a single resonance for the cis product and two resonances of equal intensity for the trans product. The ratio between the two species was dependent on experimental conditions. The W(CO)4 moiety was assigned as such, rather than the W(CO)5 moiety seen in many similar compounds, because of the IR nCO symmetric stretch at 2020 cm−1. As this compound was thermally unstable and could not be isolated, its structure was extrapolated from the similar [{CpMo(CO)}2(m-PCy2)(m3-CPh){Fe(CO)3}] structure presented in the same paper (Scheme 70).111

Scheme 70 194.

The reaction between [{CpMo(CO)}2(m-CPh)(m-PCy2)] and the photolytically-generated radical fragment W(CO)5
showing the two products, 193 and

6.11.3.2.3

Mo2Mn clusters

[{CpMo(CO)}2(m-CH3)(m-PCy2)] was reacted with MnCp0 (CO)3 (Cp0 ¼ C5H4Me) in the presence of UV-visible radiation, with [MnCp0 (CO)2] being the reactive fragment from photolytic decarbonylation. The product was identified as [{CpMo (CO)}2(m-PCy2)(Cp0 Mn)(m-CO)2(m3-CH)], (195), through SCXRD. IR spectroscopy indicated that the CO ligands were bridging in solution as well, as indicated by the low frequency CO stretching frequency, and that the methylidyne is triply-bridging based on the highly-deshielded methylidyne hydrogen resonance in the 1H NMR spectrum (Scheme 71).112

Scheme 71 The reaction between [{CpMo(CO)}2(m-CH3)(m-PCy2)] and the photolytically-generated radical fragment Cp0 Mn(CO)3.

[{CpMo(CO)}2(m-CO)(m-PCy2)(m-COMe)] reacted with CpMn(CO)2(thf ) in toluene at room temperature to form [{CpMo (CO)}2{CpMn(CO)2}(m-PCy2)(m3-COMe)], (196a) (Scheme 72).113 The authors postulated that the [CpMn(CO)2] fragment added to the ModMo triple bond to yield an electron-deficient 46-electron cluster that was then carbonylated to the 48-electron cluster by CO generated from the decomposition of excess CpMn(CO)2(thf ). This reaction was also observed with Cp0 Mn (CO)2(thf ) (Cp0 ¼ C5H4Me), giving [{CpMo(CO)}2{Cp0 Mn(CO)2}(m-PCy2)(m3-COMe)], (196b).114

Scheme 72 The reaction between [{CpMo(CO)}2(m-CO)(m-PCy2)(m-COMe)] and CpMn(CO)2(thf ).

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

6.11.3.2.4

759

Mo2Re clusters

The reaction of [{Re(CO)3}2(m-Z2-C5H4NS)2] with [{CpMo(CO)3}2] in refluxing toluene gave the known mononuclear CpMo(CO)2(m-Z2-C5H4NS), known trinuclear [(CpMo)3(m3-S)(m-S)(m-CO)2(m-Z2-C5H4NS)] and the novel mixed-metal cluster [(CpMo)2Re(m3-S)(m-CO)2(m-Z2-C5H4NS)(CO)4], (197).99 Attempts to grow X-ray quality crystals of [(CpMo)2(m3-S) (m-CO)2(m-Z2-C5H4N)(CO)4] were unsuccessful, but IR spectroscopy confirmed the presence of both terminal and bridging carbonyls. FAB mass spectrometry gave an m/z of 758, consistent with the assigned formula, as well as sequential loss of m/ z ¼ 28 for each of six carbonyl ligands. 1H NMR spectra displayed two sets of resonances in the aromatic region, ascribed to two isomers differing in the orientation of the m-pyridine ligand accompanied by rearrangement of the CO ligands in order to maintain full valency. Whether the two isomers interconverted was not mentioned (Scheme 73).

Scheme 73 Synthesis of two trinuclear clusters, 197 and 1970 from [{Re(CO)3}2(m-Z2-C5H4NS)2] and [{CpMo(CO)3}2].

6.11.3.2.5

Re3Au clusters

The reaction of Re(CO)4(NCMe)2 with Au(PPh3)(C^CC^CFc) in toluene at 90  C for 1 h gave [{Re(CO)4}2(m-AuPPh3) (m-Z1:Z2-C4Fc)]. Further heating for 5 h gave [(AuPPh3){Re(CO)3}3(m3-C2Fc)(m3-C2{Re(CO)4(NCMe)})], (198), with a spiked triangular Re3Au core (Scheme 74). The most remarkable feature of this cluster was the two m3-C2 ligands formed by cleavage of the CdC single bond of the butadiynyl ligand.115

Scheme 74 Reaction of [{Re(CO)4(NCMe)}2] with Au(PPh3)(C^CC^CFc) to give spiked triangular cluster 298, requiring the cleavage of the CdC bond in the butadiynyl ligand.

6.11.4

Organometallic clusters without MdC bonds to CO or Cp-type ligands

6.11.4.1

Trinuclear clusters of Mo

The dissolution of [Mo3(m3-CCH3)2(m-OAc)6(H2O)3](CF3SO3) in water resulted in [Mo3(m3-O)2(m-OAc)6(H2O)3]2+ and free 2-butyne that was identified by GC-MS. In aqueous HBr, dissolution gave [Mo3Br7(m-OAc)(H2O)2(m3-Z2:Z1-CH3C^CCH3)], (199), as determined by SCXRD (Scheme 75).116 A later paper showed that the reaction, remarkably, proceeded by generation of free carbyne radicals in H2O.117

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Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

Scheme 75 Reaction of [Mo3(m3-CCH3)2(OAc)6(H2O)3](CF3SO3) with water or aqueous HBr, showing the formation of 2-butyne.

In a modified synthesis of [Mo3(m3-CCH3)2(m-OAc)6(H2O)3](CF3SO3), Mo(CO)6 was reacted with a mixture of propionic acid and propionic anhydride rather that acetic acid and acetic anhydride. The result was a mixture of [Mo3(m3-CCH2CH3)2 (m-O2CCH2CH3)6(H2O)3]+, (200), and [Mo3(m3-CCH3)2(m-O2CCH2CH3)6(H2O)3]+, (201), based on 1H NMR spectroscopy (Fig. 15). They could not be separated by crystallization. The shorter m3-ethylidyne was thought to be derived from the reaction of a cluster fragment with the numerous organic products that carbon-capped trinuclear molybdenum clusters are known to generate.117 The SCXRD data indicated an approximately 50:50 mixture of the two species 200 and 201. Overlap between the relevant resonances in 1H NMR spectra made it difficult to determine the relative amounts of each cluster in the reaction mixture.118

6.11.4.2

Hexanuclear clusters of W

Two unusual hexanuclear tungsten clusters were synthesized by the reaction of WO3 with molten KCN or molten KCN and As, giving K10[{W6(m4-O)2(m3-CCN)4}(CN)16]
11H2O, (202), and K10[{W6(m4-O)2(m3-As)4}(CN)16]
11H2O, (203), respectively upon aqueous workup (Fig. 16). Both compounds are air-stable and water-soluble. The tungsten atoms in these clusters had unusual geometries, best described as edge-sharing tetrahedra, with a total of 14 MBVE spread over 11 MdM bonds. An alternative interpretation through Wade’s Rules, treating the tungsten and oxygen atoms as part of the polyhedron and the As or CCN as face-capping ligands, gave a total of 42 (5n + 2) CVE, as expected for an octahedron with two edges split by additional vertices. The rare cyanogen ligand, CCN, in 202 was thought to arise from cyanide during the reaction. Reacting WO3 with molten KCN and P did not create a cluster with m3-P ligands, although it did increase the yield of 202.119

6.11.4.3

Tetranuclear clusters of rhenium

During synthesis of the known cluster compound K4Re4S4(CN)12 from Re4S4(TeCl2)4Cl8, the side-product K4Re4S4−xTex(CN)12 was obtained that contained Te as indicated by elemental analysis.120 Crystals of coordination polymers were grown by combining an

Fig. 15 The two products of the reaction of Mo(CO)6 in propionic anhydride and propionic acid.

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

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Fig. 16 Cluster 203 as a representative of the geometries of 203 and 204, with 204 having CCN ligands in place of the As ligands in 203.

aqueous solution of K4Re4S4−xTex(CN)12 with a solution of CuSO4 and ethylenediamine in concentrated aqueous ammonia, giving [{Cu(en)2}2Re4S3.83Te0.17(CN)12]
H2O, CuSO4, and diethylenetriamine in concentrated aqueous ammonia, giving [{Cu(dien) (NH3)}2Re4S3.45Te0.55(CN)12]
3H2O, and with ZnCl2 and ethylenediamine in concentrated aqueous ammonia, giving [{Zn (en)2}2Re4S3.60Te0.41(CN)12]
H2O. SCXRD on the resulting crystals showed that K4Re4S4−xTex(CN)12 was a mixture of the known cluster compound K4Re4S4(CN)12(four bridging sulfurs) and the novel K4Re4S3Te(CN)12, (204) (three sulfur, one tellurium). There was no K4Re4S2Te2(CN)12 (two sulfur, two tellurium), K4Re4STe3(CN)12 (one sulfur, three tellurium), or K4Re4Te4(CN)12 (four tellurium) indicated, and the Te atom of 204 occupied a single site in the cluster (Fig. 17). In a deliberate synthesis of a Re4Q4 cluster core with mixed chalcogens, Re3Br9 was combined with one-eighth equivalent of S8, three equivalents of Br2, and 5.43 equivalents of Te, the mixture sealed in an evacuated quartz ampule which was then heated to 400  C over 12 h, held at that temperature for 48 h, and then cooled.121 The powder was extracted with a boiling aqueous solution of KCN, the filtrate concentrated and then cooled to yield crystals of K4Re4STe3(CN)12
4H2O, K4(205)
4H2O. The reaction of this cluster with Cu(en)2Cl2 or CuCl2 and triethylenetetramine in aqueous ammonia resulted in [{Cu(en)2}2Re4STe3(CN)12]
5H2O and [{Cu(trien)}2Re4STe3(CN)12
2H2O], respectively, similar to K4Re4S4−xTex(CN)12. Perhaps the most novel Re4 cubane-type clusters discovered in recent years were those with face-bridging pnictogenide ligands, the first such compounds combining these inner ligands with the versatile ausser cyanide ligand.122 Re3I9 was combined with NaCN and red phosphorus, the mixture sealed in an evacuated quartz ampule and heated to 550  C for 48 h before slow cooling. The resulting powder was extracted with boiling water, filtered, and concentrated before Na8[Re4(PO)3(PO2)(CN)12]
15H2O, Na8(206)
15H2O, was precipitated out using MeOH. The compound contained the novel (PO2)3− ligand, which coordinates to two metal centers via the phosphorus, and a third by one of the oxygens, with a geometry resembling the hypophosphite anion H2PO−2. The PdO bond between the P and the terminal O was much shorter than between the P and the m-oxygen (Fig. 18). Following the above procedure with Re3I9, KCN, gray As, and As2O3 afforded K8[Re4As2(AsO)2(CN)12]
12H2O, K8(207)
12H2O, containing m3-As ligands as well as the novel m3-AsO3− ligand that resembles the uncommon m3-PO3− ligand. The As was bound to three metal centers and the O atom above the face of the cluster. Dissolving this cluster in water containing H2O2, before precipitating out with MeOH, afforded K8[Re4(AsO)4(CN)12]
13H2O, K8(208)
13H2O.

Fig. 17 Two cubane-type tetrarhenium clusters with a mixture of chalcogen ligands.

3− 3− Fig. 18 Cubane-type tetrarhenium clusters featuring uncommon PO3− ligands. 2 , PO , and AsO

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Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

6.11.4.4 6.11.4.4.1

Octahedral clusters Octahedral clusters of Mo

(Bu4N)2[Mo6I14] was stirred with AgOTf, Et3N, and methyl propiolate in tetrahydrofuran and the Bu4N+ product converted to the Ph4P+ salt (Bu4N)(Ph4P)[Mo6I8(C^CC(O)OMe)6]. Upon recrystallization, (Ph4P)2[Mo6I8(C^CC(O)OMe)6], (PPh4)2(209), was formed, featuring the first M6X8 cluster with entirely organometallic ausser ligands other than cyanide.123 The cluster salts were reasonably air- and moisture-stable and could be handled in air as solids. As a Mo6X8-based cluster, the spectroscopic and photophysical properties of (PPh4)2(209) were evaluated. The tem (photoluminescence decay lifetime, the lifetime of the excited state) in deaerated CH3CN increased dramatically from (Bu4N)2[Mo6I14] (85 ms) to 164 ms for (PPh4)2(211), matching that of [Mo6Cl14]2− (tem ¼ 130–180 ms).124 The clusters exhibited similar quantum yields, Fem (the number of photons emitted over photons absorbed) of 0.18 vs. [Mo6Cl14]2− (0.1–0.2), with lem slightly blue-shifted to 707 nm from 738 nm in [Mo6I14]2− (Scheme 76).

Scheme 76 Substitution of the ausser iodide ligands of Mo6I2− 14 with methyl 2-propynoate ligands.

6.11.4.4.2

Octahedral clusters of Tc

In 2010 the first hexatechnetium clusters with metal-carbon bonds were reported.125 (PPh4)4[Tc6S8(CN)6], (PPh4)4(210), was synthesized from Cs4[Tc6S8Br6]
CsBr and excess NaCN that was sealed in a silica ampule under vacuum, heated to 640  C over 8 h, held at 640  C for 24 h, then held at 300  C for 12 h before cooling to room temperature. The compound was isolated by taking a filtered aqueous extract of the resulting powder and adding to it a solution of (PPh4)Br in MeOH. (PPh4)4[Tc6Se8(CN)6], (PPh4)4(211), was synthesized following the same procedure starting with polymeric [Tc6Se8I2] (Fig. 19).

Fig. 19 Octahedral hexatechnetium clusters 210 and 211.

6.11.4.4.3

Octahedral clusters of Re

After the [Re6Q8(CN)6] clusters were discovered in 1995,126 there was a great deal of exploration in modifying the ausser ligands. The relatively-labile ausser hydroxy ligand facilitates modification of these clusters by ligand exchange. Treatment of the polymeric Cs4Re6S9(CN)4127 with boiling aqueous KOH for 24 h, followed by filtration and evaporation to a saturated solution, gave ordered crystals of Cs1.68K2.32[Re6S8(CN)4(OH)2]
2H2O, Cs1.68K2.32(212OH)
2H2O. Washing the product with 91% ethanol, followed by recrystallization from water, gave disordered crystals of Cs1.83K2.17(212OH)
2H2O (Fig. 20). Cs1.83K2.17(212OH)
2H2O, when dissolved in water containing excess CsCl, acidified to pH ¼ 2 with concentrated HCl, and boiled for 4 h gave Cs1.84K1.16(H)[Re6S8(CN)4Cl2], Cs1.84K1.16(H)(212Cl), after evaporation. The presence of H+ was inferred from the proximity between the CN groups of adjacent clusters, consistent with strong hydrogen bonding interactions. Following the same procedure with CsBr and concentrated aqueous HBr instead resulted in Cs1.68K1.32[Re6S8(CN)4(H2O)Br]
2H2O, Cs1.68K1.32(213Br)
2H2O. The cluster with two trans ausser bromide ligands was obtained after boiling, for 48 h. Subsequent addition of Me4NBr followed by slow evaporation gave (Me4N)3(H5O2)[Re6S8(CN)4Br2], (Me4N)3(H5O2)(212Br).128

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

763

Fig. 20 Octahedral hexarhenium clusters with four ausser cyanide ligands.

Trans-disubstituted clusters were also obtained directly from the 1D polymeric [Cs4Re6S9(CN)4]n by heating with excess H2O and pyridine or 4-methylpyridine to give Cs2[Re6S8(CN)4(py)2], Cs2(212py), and Cs2[Re6S8(CN)4(4-Mepy)2], (212Mepy), respectively. Both reactions gave trace by-products consisting of tetrasubstituted [Re6S8(CN)2(py)4]
H2O, (214py)
H2O, [Re6S8(CN)2 (4-Mepy)4], (214Mepy), and [Re6S8(CN)2(4-Mepy)4]
(4-Mepy), (214Mepy)
(4-Mepy), in which p-p stacking between the pyridine ligands arranged the clusters into three-dimensional networks in the first two compounds and one-dimensional chains in the third. Each of these compounds have the two remaining cyanide ligands trans to each other. No direct synthesis of any of the three tetrasubstituted by-products has been reported (Fig. 21).129

Fig. 21 Octahedral hexarhenium cyanide clusters with ausser 4-methylpyridine ligands.

The ambidentate nature of the cyanide ligands in rhenium clusters with ausser cyanides was explored by the addition of CuCl2.128 Combining a solution of Cs1.84K1.16(H)(212Cl) and CuCl2 in water resulted in CsK[Cu(H2O)2](214Cl)
4H2O. Allowing a mixture of Cs1.68K2.32(212OH)
2H2O, CuCl2 and HCl in water to stand for a week afforded X-ray quality crystals, and SCXRD revealed the structure to be a three-dimensional polymer with each [Re6S8(CN)4Cl2]4− coordinated to four Cu(H2O)2+ 2 centers via the nitrogens of 4− clusters, the four cyanide ligands, while each Cu(H2O)2+ 2 was coordinated by one cyanide ligand each from four [Re6S8(CN)4Cl2] 2+ giving octahedrally-coordinated Cu . The selenium-capped clusters showed similar reactivity. K4[Re6Se8(OH)6]
8H2O and KCN reacted in solution to give [Re6Se8 (CN)4(OH)2]4−, (215), isolated as Cs2.75K1.25(215)
H2O upon addition of CsOH.130 SCXRD demonstrated that the two hydroxyl ligands were trans to each other. Addition of CuCl2 in aqueous ammonia gave crystals of the salt [Cu(NH3)5]2(215)
8H2O, with discrete [Cu(NH3)5]2+ units, while replacing ammonia with methylamine gave the coordination compound [{Cu(CH3NH2)4}2 {Re6Se8(CN)4(OH)2}]
8H2O, [{Cu(CH3NH2)4}2(215)]
8H2O. The [Cu(CH3NH2)4]2+ ions were bonded to the nitrogen atoms of two of the ambidentate cyanide ligands trans to each other. Notably, neither of these reactions displaced the relatively-labile hydroxyl ligands (Scheme 77).

Scheme 77 Coordination of {Cu(NH2Me)4}2+ to two of the ausser cyanide ligands of cluster 215.

Slow diffusion of an aqueous ammonia solution of NiCl2 into a solution of Cs2.75K1.25(215)
H2O in water resulted in [{Ni(NH3)5}2{Re6Se8(CN)4(OH)2}]
6H2O, [{Ni(NH3)5}2(215)]
6H2O, which, unlike [Cu(NH3)5]2(215)
8H2O, has cyanide acting in an ambidentate manner, with the Ni bonded to the nitrogen atoms of two of the cyanide ligands trans to one another.131 Similar reactions using solutions of NiCl2 with diethylenetriamine (dien) and ammonia and triethylenetetramine (trien) only resulted in salts [Ni(dien)(NH3)3]2(215), and [Ni(trien)(NH3)2]2(215), respectively.132

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Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

The basicity of the hydroxide ligands on 212OH was explored directly through titration.133 A solution of Cs1.68K2.32(212OH)
2H2O in degassed water was acidified with HCl to a pH of 2.9 and titrated with a KOH solution. The titration curve showed two protonation steps of the ausser hydroxy ligands at pH ¼  3.5–6 and 7–10.5. By adjusting the pH of these solutions and precipitating the cluster using the organic cations of PPh4Cl or (NBu4)HSO4, the authors were able to isolate (NBu4)4(212OH)
NBu4OH, (NBu4)3[Re6S8(CN)4(OH)(H2O)], (NBu4)3(213OH), (NBu4)2K(213OH), (PPh4)3(213OH), and (NBu4)2[Re6S8(CN)4(H2O)2], (NBu4)(212H2O). When K4[Re6S8(OH)6] was reacted with KCN under similar conditions to those that converted K4[Re6Se8(OH)6]
8H2O to Cs2.75K1.25(215)
H2O with two ausser hydroxide ligands, Cs2.67K1.33[Re6S8(CN)2(OH)4]
4H2O, Cs2.67K1.33(216)
4H2O with four ausser hydroxide ligands, was isolated.134 The reaction of Cs2.67K1.33(216)
4H2O with CuCl2 in aqueous ammonia gave the salt [Cu(NH3)4]2(216)
2H2O.134 The precipitate obtained by acidifying a solution of Cs2.67K1.33[Re6S8(CN)2(OH)4]
4H2O, postulated by the authors to be [Re6S8(CN)2(H2O)4] based on elemental analysis, reacted with molten 3,5-dimethylpyrazine to give [Re6S8(CN)2(3,5-Me2PzH)4]
2H2O, (217)
2H2O, replacing the labile aqua ligands (Scheme 78).

Scheme 78 Substitution of ausser hydroxide ligands of cluster 216 with 3,5-dimethylpyrazine.

The Lewis acidic nature of [Re6Q8]4+ (Q ¼ S, Se, Te) cluster cores activated the terminal cyanide ligands towards nucleophilic attack. The reaction of (Ph4P)4[Re6Q8(CN)6] with (Me3O)BF4 or CF3SO3CH3 in dichloromethane at room temperature led to the precipitation of a fine powder of [Re6Q8(CNCH3)6](BF4)2 (218)(BF4)2 for Q ¼ S, (219)(BF4)2 for Q ¼ Se, (220)(BF4)2 for Q ¼ Te), or (218–220)(CF3SO3)2 over several days.135 These isocyanide cluster salts were remarkably air-stable and non-hygroscopic. Most yields were above 70%, except for (Ph4P)4[Re6S8(CN)6] or (Ph4P)4[Re6Te8(CN)6] with (Me3O)BF4 that gave only 61% and 23% yields, respectively. ESI-MS data was consistent with {[Re6Q8(CNCH3)6](A)}+, (where A is BF−4 or CF3SO−3) with no appreciable fragmentation, and no partially-alkylated ions were observed. All clusters were stable to thermolysis up to 300  C, after loss of solvent of crystallization. All triflate cluster salts exhibited photoluminescence in solution, while the sulfide and selenide clusters luminesced in the solid state. The tellurium cluster (220)(CF3SO3)2 (in CH3CN) was the second [Re6Te8]2+ cluster to exhibit luminescence (Scheme 79).

Scheme 79 Methylation of ausser cyanide ligands of Re6Q8(CN)4− 6 (Q ¼ S, Se, Te) to methyl isocyanide.

6.11.4.4.4

Mixed-metal octahedral clusters

Heating the tetrahedral heterometallic compound Re3MoS4Te4 to 850  C in molten KCN afforded a polymeric solid that, after aqueous workup and precipitation with CsCl, gave two heterometallic clusters, [Re5MoS8(CN)6]5−, (221), and [Re4Mo2S8(CN)6]5−, (222), that co-crystallized as their Cs salts.136 Notably, they had the same overall charge despite containing different metal centers,

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

765

Fig. 22 Heteronuclear octahedral clusters 221–223, with Mo in blue for clarity.

giving them d-electron counts of 24 and 23, respectively, with the former being diamagnetic and the latter paramagnetic. Whether [Re4Mo2S8(CN)6]5− exists primarily in the cis or trans form could not be distinguished crystallographically, so only the trans possibility is depicted in Fig. 22. [Re3Mo3S8(CN)6]6−, (223), was accessed by heating KCN, RbI, and a 1:1 mixture of ReS2 and MoS2 to 750  C in an evacuated silica ampule to give K6[Re3Mo3S8(CN)5], then oxidizing by atmospheric oxygen and depolymerizing in aqueous KCN, to generate a solution of [Re3Mo3S8(CN)6]6−.137 The fac and mer isomers could not be distinguished crystallographically, as the Re and Mo atoms were disordered. DFT calculations suggested that both isomers have similar energies and are likely both present. Only the mer isomer is represented in Fig. 22. The selenium-bridged mixed-metal clusters were also synthesized by heating a mixture of MoSe2 and ReSe2 in molten KCN to give a polymeric phase with cyanide ligands bridging between clusters, K6[Re6−xMoxSe8(m-CN)(CN)4]n, which was dissociated into discrete clusters and oxidized by dissolution in aqueous KCN in the presence of oxygen to give K5[Re6−xMoxSe8(CN)6]. ESI-MS data on the Ph4P+ salt, prepared by salt metathesis, was consistent with clusters with the formulas {(Ph4P)[Re4Mo2Se8(CN)6]}2−, {(Ph4P)[Re3Mo3Se8(CN)6]}2−, and {(Ph4P)[Re2Mo4Se8(CN)6]}2− being present. Which species was favored depended on the reaction temperature. According to these mass spectra, samples synthesized at 630  C were predominantly [Re3Mo3Se8(CN)6]n−, with small amounts of the others as was discovered earlier.138 Samples synthesized at 700  C and 800  C were primarily [Re4Mo2Se8(CN)6]n− and [Re3Mo3Se8(CN)6]n−, with trace amounts of [Re2Mo4Se8(CN)6]n−. [Re4Mo2Se8(CN)6]5− clusters were easily separated from [Re3Mo3Se8(CN)6]5− because of their differing redox properties. Stirring an aqueous solution containing the two clusters and (Bu4N)Br in air led to the oxidation of [Re4Mo2Se8(CN)6]5− to [Re4Mo2Se8(CN)6]4−, (224), which precipitated out as (Bu4N)4(224). [Re3Mo3Se8(CN)6]5− was precipitated from the mother liquor by oxidation with I2, yielding (Bu4N)4 [Re3Mo3Se8(CN)6], (Bu4N)4(225). Cluster anion 225 was found to exist entirely as the mer isomer based on the solution state 77 Se NMR spectrum that exhibited only two resonances of similar intensity, while 224 existed as a mixture of cis and trans isomers in an approximately 2:1 ratio by 77Se NMR spectroscopy (Fig. 23).139

6.11.4.5 6.11.4.5.1

Carbide-centered clusters Hexanuclear trigonal prismatic clusters of W

A fascinating class of compounds was discovered in 2003: hexatungsten trigonal prismatic clusters with a m6-carbon. As with many clusters, their discovery was serendipitous, first being isolated after heating commercial WCl4 that was “A horrible mixture containing WCl5, WOCl4 and 1,2,4,5-tetrachlorobenzene.”140 The structure consisted of six W atoms arranged in a trigonal prism with a m6-C at the center. Each edge of the two triangular faces were coordinated by a m-Cl, and each of the three remaining edges coordinated by two m-Cl. Each W had a single terminal Cl ligand. Heating this contaminated WCl4 in an evacuated quartz ampule in a gradient between 925 and 915 K gave discrete W6CCl18, [W6(m6-C)(m-Cl)6(m-Cl6)Cl6], (226), and polymeric W6CCl16 [W6(m6-C) (m-Cl)6(m-Cl6)Cl4]n, (227), in approximately equal amounts at the hotter end of the reaction vessel.

Fig. 23 Heteronuclear octahedral clusters 224–225, Mo in blue for clarity.

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Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

The authors were able to rationally synthesize 227 by heating pure WCl4 with W metal and CCl4 in an evacuated quartz ampule in a 1030/870 K gradient for 2 days. In 2010 a higher-yielding synthesis of 226 was reported. Heating W6Cl12 with 2 equivalents of C6Cl6 to 550  C in an evacuated quartz ampule for 24 h gave 226 in 90% yield. Heating at 700  C for 48 h gave the new cluster polymer W6CCl15, absent an inner chloride ([W6(m6-C)(m-Cl)5(m-Cl6)Cl6]n), (228), in 95% yield. Cluster 227 was obtained in >60% yield with a 3:2 ratio of W6Cl12 and C6Cl6 at 650  C for 24 h.141  The W6CCl18 cluster was obtained as the ion W6CCl2− 18, (229), by heating WCl6 with Bi and CCl4 in a 6:8:3 ratio at 400 C for 3 days. After removal of the co-produced BiCl3 by sublimation, the resulting amorphous powder was extracted with 1% aqueous HCl. Addition of (Bu4N)Cl to the extract afforded the green precipitate (Bu4N)2(229) in 34% yield.142 Cyclic voltammetry of (Bu4N)2(229) in dimethylformamide exhibited two reversible reduction and two reversible oxidation processes, indicating that cluster charges from 0 to -4 were accessible. Oxidation of (Bu4N)2(229) with (NO)(BF4) gave (Bu4N)[WCCl18], (Bu4N)(230), while reduction with cobaltocene gave [W6CCl18]3− in the form of (Cp2Co)(Bu4N)2[WCCl18], (Cp2Co)(Bu4N)2(231). While the fully-oxidized cluster W6CCl18 was already known, the fully-reduced cluster W6CCl4− 18 has yet to be isolated. The cluster anion 230 was directly synthesized by a new method using lithium cyanamide, Li2CN2, prepared from the reaction of Li2CO3 with NH3, as the carbon source.143 WCl4 and half an equivalent of Li2CN2 were ground together and then sealed in a quartz ampule under vacuum. The evacuated ampule was heated to 500  C for 12 h, affording Lix[W6CCl18], which, when dissolved in MeOH, combined with a MeOH solution of CsCl, and allowed to evaporate, gave crystals of Cs(230)
MeOH. Later research demonstrated that Lix[W6CCl18] was Li[W6CCl18].144 The ausser ligands of 229 were relatively labile.145 Heating (Bu4N)2[W6CCl18] (16 MBVE) in triflic acid at 100 for 8 h gave (Bu4N)2[WCCl12(CF3SO3)6], (Bu4N)2(232), in 91% yield while maintaining the MBVE count. Chemical reduction with 2 equivalents of cobaltocene in DMF at room temperature gave [W6CCl12(dmf )6]2+, (233), with 18 MBVE, isolated as the tetrafluoroborate salt, while heating in pyridine at 90  C for 3 days gave [WCCl12(py)6](CF3SO3)2
2py, (234)(CF3SO3)2
2py, also an 18 MBVE cluster (Scheme 80).145

Scheme 80 Substitution of terminal chloride ligands of 229 with triflate.

Finally, although it exceeds the scope of this review, it is worth noting that similar clusters with m6-nitrides, oxides, and sulfides have been reported.144,146–149

6.11.4.5.2

Dodecanuclear bioctahedral clusters of Re

Heating ReS2 with KCN in a 1:2 mass ratio (2:1 M ratio) in an evacuated quartz ampule at 750  C for 48 h gave K8[Re12(m6-C) (m-S)3(m3-S)14(CN)6], K8(234), and upon aqueous workup, the reduced species K6[Re12CS17(CN)6], K6(235), was obtained after evaporation, and Cs6(235), was recovered after precipitation with CsCl. These clusters were unprecedented dodecanuclear clusters, consisting of two Re6 octahedra bridged by a m6-C in a trigonal prismatic environment between the cluster faces. Three sulfurs bridged between the apices adjacent to those faces. The fourteen remaining faces were coordinated by m3-S, and the six remaining apices were coordinated by terminal cyanide ligands (Scheme 81).150

Scheme 81 Synthesis of dodecanuclear bioctahedral rhenium cluster 234 from KCN and ReS2.

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

767

The stability of the central Re12CS14 core allowed for substantial modification of the other ligands, with cluster core retention. The terminal cyanide ligands of K6(235) were substituted with hydroxide ligands, through treatment with molten KOH at 300  C for 4 h, to give K6[Re12CS17(OH)6], K6(236), in 75% yield after aqueous workup (Scheme 82).151

Scheme 82 Substitution of ausser cyanide ligands of 235 with hydroxide.

The hydroxy ligands were then substituted with SO−3 groups via treatment with Na2S2O4, yielding Na12[Re12CS17(SO3)6], Na12(237),151 or replaced with Cl or Br ligands via hydrochloric or hydrobromic acids, yielding (H3O)6[Re12CS17Cl6], (H3O)6 (238Cl), and (H3O)6[Re12CS17Br6], (H3O)6(238Br), respectively (Scheme 83).152 (H3O)6[Re12CS17Cl6] and (H3O)6[Re12CS17Br6] were then heated in DMF with the respective (Et3NH)X to give the oxidized clusters (Et3NH)4(Me2NH2)4[Re12CS17Cl6], (Et3NH)4(Me2NH2)4(239Cl), and (Et3NH)4(Me2NH2)4[Re12CS17Br6], (Et3NH)4(Me2NH2)4(239Br), respectively. The Me2NH+2 cation was postulated to result from the breakdown of DMF into formic acid and dimethylamine at elevated temperatures.

Scheme 83 Substitution of ausser hydroxide ligands of 236 with sulfite, bromide, or chloride ligands.

768

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

Fig. 25 Bioctahedral dodecarhenium clusters showing the various degrees of oxidation of the inter-octahedral bridging sulfide ligands.

Cluster 238Br was initially synthesized by treating K6(236) with H2SO4 to give Re12CS17(H2O)6, (240), as an amorphous powder with formula determined through elemental analysis. This powder was heated under vacuum to drive off water, resulting in polymeric [Re12CS17]n, (241).153 The dodecanuclear structure was retained in this polymeric form as indicated by regeneration of K6(236) upon treatment with aqueous KOH, or (Et4N)4(Me2NH2)4(238Br) upon heating with Et4NBr in DMF to give 241.154 The Me2NH+2 cation was thought to result from the breakdown of DMF into formic acid and dimethylamine at elevated temperatures, as with (Et3NH)4(Me2NH2)4(239Br). The sulfur atoms bridging the two Re6 octahedra were also altered. Treatment of K6(235) with H2O2 under basic conditions resulted in K6[Re12CS14(SO2)3(CN)6], K6(242), which could be oxidized to give K6[Re12CS14(m-SO2)2(m-SO3)(CN)6], K6(243), as determined by IR spectroscopy (Fig. 25). The second stage of oxidation was dependent on sunlight.155 Cluster anion 243 was reacted with excess CuCl2 in aqueous ammonia to generate the coordination compound [Cu(NH3)5]2.6(242)0.6[{Re12CS14(m-SO2)2(m-SO3) (CN)6}{Cu(NH3)4}]0.4
H2O, [Cu(NH3)5]2.6(242)0.6(244)0.4
H2O, which was the only compound incorporating the [Re12CS14 (m-SO2)2(m-SO3)(CN)6]6− cluster core that could be analyzed by SCXRD. These compounds were reduced back to K6[Re12CS17(CN)6] with Na2S, yielding [Re12CS17On(CN)6]6−, n ¼ 1–5, as intermediates determined by mass spectrometry. Of these, only [Cu(NH3)5]3[Re12CS14(m-SO2)2(m-S)(CN)6], [Cu(NH3)5]3(245), crystallized by layering with a solution of CuCl2 in concentrated aqueous ammonia, and the coordination compound [Cu(NH3)6]2 [{Cd(NH3)5}{m-Cd(NH3)4}{Re12CS14(m-SO2)2(m-S)(CN)6}], [Cu(NH3)6]2(246), crystallized by layering with a solution of Cd(CH3CO2)2•2H2O in concentrated aqueous ammonia, were isolated and studied by SCXRD.156 K6(242) also provided a starting point for substituting the bridging positions with chalcogenides other than sulfur (Scheme 84). Heating K6(242) with KSeCN in degassed water resulted in K6[Re12CS14(m-Se)3(CN)6], K6(247), while heating with KOH in water at 220  C for 10 min gave K6[Re12CS14(m-O)3(CN)6], K6(248CN), after precipitation with EtOH and extraction with aqueous KCN. Heating K6(242) in aqueous KOH at 240  C for 30 min gave K6[Re12CS14O3(OH)6], K6(248OH), after evaporation.157

Scheme 84 Substitution of inter-octahedral bridging hyposulfite ligand with oxygen or selenium.

Organometallic Tri- and Polynuclear Clusters of Tantalum, Niobium and Group 6 and 7 Transition Metals

769

Scheme 85 Substitution of ausser hydroxide and inter-octahedral bridging oxo ligands with chloride or bromide.

K6(248OH) served as a starting material for the most halogen-rich dodecanuclear rhenium cluster to date. Precipitating K6(248OH) from aqueous solution with concentrated aqueous hydrohalic acid HX, then heating the solid in DMF with Et4NX0 , gave (Et4N)5[Re12CS14X3X0 6], for X, X0 ¼ Cl, Cl; Br, Cl; and Br, Br (Scheme 85).158

6.11.5

Conclusion

The chemistry of organometallic early transition metal clusters is rich, broad, and eclectic. This is not surprising given the broad range of chemistries of all types observed for the Group 5–7 transition metals. The deluge of new compounds has slowed a little from the explosive growth of the 1990s, presumably in part from retirements of several leaders in the cluster arena, but new compounds closed many gaps and provided fascinating new geometric motifs. We expect that these trends will continue and that new, practical applications (e.g., in catalysis) of Group 5–7 organotransition metal clusters will continued to be discovered. We predict that new cluster models related to the cluster core of nitrogenase and its m6-C will be sought and their reactivities examined. More enzymes and electron-transfer proteins will be discovered to have clusters at their active sites, some perhaps with organometallic moieties.

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V. Inorg. Chem. 2017, 56 (20), 12389–12400. Gayfulin, Y. M.; Brylev, K. A.; Ryzhikov, M. R.; Samsonenko, D. G.; Kitamura, N.; Mironov, Y. V. Dalton Trans. 2019, 48 (33), 12522–12530.

6.12 Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules Olivia L Duletski, Roark D O’Neill, Charles Beasley, Molly O’Hagan, and Michael T Mock, Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT, United States © 2022 Elsevier Ltd. All rights reserved.

6.12.1 6.12.1.1 6.12.2 6.12.2.1 6.12.2.1.1 6.12.2.1.2 6.12.2.1.3 6.12.2.1.4 6.12.2.2 6.12.2.3 6.12.2.4 6.12.2.4.1 6.12.2.4.2 6.12.2.4.3 6.12.2.5 6.12.2.6 6.12.3 6.12.3.1 6.12.3.1.1 6.12.3.1.2 6.12.3.1.3 6.12.3.2 6.12.3.2.1 6.12.3.2.2 6.12.3.2.3 6.12.3.2.4 6.12.3.3 6.12.3.3.1 6.12.3.3.2 6.12.3.3.3 6.12.3.3.4 6.12.3.3.5 6.12.3.4 6.12.3.4.1 6.12.3.4.2 6.12.3.4.3 6.12.3.4.4 6.12.3.4.5 6.12.3.5 6.12.3.5.1 6.12.3.5.2 6.12.3.5.3 6.12.3.5.4 6.12.3.5.5 6.12.4 6.12.4.1 6.12.4.2 6.12.4.2.1 6.12.4.2.2 6.12.4.3 6.12.4.4 6.12.4.5

772

Introduction and chapter scope Chapter scope Group 5 V, Nb, Ta Vanadium dinitrogen complexes Nitrogen donor ligands Alkoxide ligands Cyclopentadienyl/alkyne ligands Nitrogen and phosphorus pincer ligands Vanadium ammonia and hydrazine complexes Nitrous oxide Niobium dinitrogen complexes Nitrogen donor ligands Pincer ligands Oxygen donor ligands Ta dinitrogen complexes Dinitrogen complexes and hydrazine Group 6 – Cr, Mo, and W Chromium dinitrogen complexes Phosphine ligands Nitrogen donor ligands Cyclopentadienyl ligands Chromium-NO complexes Pincer ligands Nitrogen-donor ligands Cp donor-ligands Cyano-bridged structures based on [CrI(CN)5(NO)]3− and CrdNO complexes containing other donor-ligands Molybdenum and tungsten dinitrogen complexes Diphosphine ligands Polyphosphine ligands Isocyanide and carbene ligands (Mo) Nitrogen donor ligands P and N pincer ligands Molybdenum nitrosyl complexes Phosphine ligands Nitrogen donor ligands Oxygen donor ligands Multimetallic complexes Molybdenum amine, imide complexes Tungsten nitrosyl complexes Alkyl, alkenyl, isonitrile ligands Nitrogen donor ligands Bimetallic complexes Phosphine ligands Tungsten imido complexes Group 7 – Mn, Tc, and Re Manganese dinitrogen complexes Manganese nitrosyl complexes Nitrogen donor ligands B, C, P and S donor ligands Technetium dinitrogen complexes Technetium nitrosyl complexes Rhenium dinitrogen complexes

Comprehensive Organometallic Chemistry IV

773 773 773 773 773 775 776 776 777 779 779 779 780 781 782 785 786 786 786 788 790 791 791 792 794 796 797 797 801 804 805 806 807 807 809 811 811 813 815 815 818 818 820 821 821 821 822 822 824 825 825 826

https://doi.org/10.1016/B978-0-12-820206-7.00165-7

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules 6.12.4.6 Rhenium nitrosyl complexes 6.12.4.6.1 Nitrogen and carbon donor ligands 6.12.4.6.2 Phosphine donors 6.12.4.7 Rhenium nitrato/nitrito complexes 6.12.5 Conclusions and outlook Acknowledgments References

6.12.1

773

828 828 830 831 832 832 832

Introduction and chapter scope

Inorganic and organometallic coordination complexes have played a significant role in understanding biological reactivity of nitrogen-containing small molecules such as dinitrogen (N2), nitric oxide (NO), and ammonia (NH3) through elucidation of the metal complexes’ spectroscopic properties. For example, the synthesis and characterization of molecular complexes containing N2 bound to a transition metal has enlightened mechanistic details of the important nitrogen reduction reaction (NRR). The large scale production of ammonia from dinitrogen and hydrogen feedstock gases by the Haber-Bosch process has produced ammonia-based fertilizers for over a century to sustain agrarian activities that support the billions of people on Earth.1–3 In addition to the Haber-Bosch process, biochemical nitrogen fixation by the Nitrogenase enzyme proved to be Nature’s system to balance the global nitrogen cycle converting atmospheric dinitrogen gas to bioavailable ammonia that is used by plants.4–8 The development of homogeneous and heterogeneous systems to catalyze NRR has gained considerable attention in the last twenty years, addressing the fundamental9 and practical10 challenges in chemical and electrochemical,11,12 NRR systems. Similar reflections can be made for the role of coordination compounds in the understanding of nitric oxide coordination, bonding, and reactivity in biological systems.

6.12.1.1

Chapter scope

This chapter focuses on advances in the synthesis and characterization of transition metal complexes in Groups 5, 6, and 7 containing small molecule ligands of dinitrogen (N2), ammonia (NH3), nitric oxide (NO), and other nitrogen oxides such as N2O, starting with results from the year 2005. The chapter sections are grouped by metal, and subsequent subsections are arranged by ancillary ligand type or small molecule type in certain cases. Selected examples highlight characterization results, trends, general synthetic strategies to prepare the aforementioned target molecules. The application of these transition metal complexes such as reactivity or catalysis may not be explicitly be described, although characterized intermediates between N2 and ammonia may be included when necessary.

6.12.2

Group 5 V, Nb, Ta

6.12.2.1

Vanadium dinitrogen complexes

Utilizing vanadium as the central metal coordination site for dinitrogen has been of biological interest due to the presence of vanadium in the FeV nitrogenase. Nitrogenases are of interest to synthetic chemists because of their ability to convert dinitrogen to ammonia through a series of proton and electron transfer steps forming N–H bonds of NH3, while operating under ambient reaction conditions. Even though vanadium is not proposed as the nitrogen binding site in FeV nitrogenase, synthetic chemists have developed diverse ligand scaffolds and various synthetic pathways to achieve nitrogen coordination to vanadium.

6.12.2.1.1

Nitrogen donor ligands

In 2018, the Kajita group reported dinitrogen binding at divanadium metal centers utilizing triamidoamine ligands.13 These large and bulky ligands restrict open site coordination to small molecules, such as N2. These ligands also allow for a large variety of substituted groups bound at the terminal amido nitrogen resulting in selective coordination of the substrates at the metal centers. Three dinitrogen divanadium complexes were synthesized containing triamidoamine ligands LX (X ¼ iBu, EtBu, iPr2Bn). The Li3LX ligands were reacted with VCl3(THF)3 to form the m-N2 bridged dimer, Scheme 1. The X-ray structures revealed that the bridging dinitrogen was bonded end-on to each of the vanadium centers. The coordination geometry for each complex was trigonal bipyramidal with three amide N atoms in equatorial positions and an amine N and N2 ligand in the axial positions. More recently, the group expanded the LX ligands to include X ¼ Bn and BnMe.14 The N]N bond lengths of the bridging N2 ligands ranged from 1.203(4) to 1.226(3) A˚ with the longest being the X ¼ iPr2Bn derivative due to the increased steric bulk of the amido substituents.

774

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

R

N

N—

3 [Li+]

R

R

3-

N VCl3(THF)3

N

V

3 N

N N

R

R

R=

iBu

EtBu

iPr

2Bn

Bn

R

NN

N

V R

N

N

BnMe

Scheme 1 Syntheses of divanadium-m-N2 complexes with triamidoamine ligands.

In addition to the trisubstituted amidoamine ligands, more recently the application of diamidoamine ligands have facilitated N2 coordination to vanadium metal centers. The formation of silylamines (N(SiR3)3) is a complementary approach to form ammonia from dinitrogen. The development of the catalytic N2 reduction of dinitrogen to silylamines readily produces ammonia by acid hydrolysis of N–Si bonds. Nishibayashi and coworkers developed catalysts for the conversion of dinitrogen to silylamine through the use of the anionic K[V{(Me3SiNCH2-CH2)2NSiMe3}(m-N)]2.15 The anionic dinuclear vanadium nitride complex was derived from reduction of [V{(Me3SiNCH2CH2)2NSiMe3}(m-Cl)]2 previously reported by Cloke, in which the nitride ligands are formed from the direct cleavage of the N2 triple bond.16 Using a mixed system of Cp and amidinate ligands, Sita and coworkers prepared the bimetallic end-on (m-Z1:Z1-N2) complex {Cp V[N(iPr)C(Me)N(iPr)]}2(m-N2) by chemical reduction of Cp V[N(iPr)C(Me)N(iPr)]Cl with a slight excess of sodium amalgam Na(Hg) at −30  C in THF, Scheme 2.17 The paramagnetic divanadium dinitrogen complex exhibits a N^N bond length of 1.225(2) A˚ , and was thermally stable in solution up to 90  C.

Scheme 2 Synthesis of {Cp V[N(iPr)C(Me)N(iPr)]}2(m-N2).

In an example of a mixed metal bimetallic dinitrogen complex, Cummins and coworkers reported the synthesis of a bimetallic Mo(m-N2)V complex utilizing the amido ligands [N(tBu)-Ar] (Ar ¼ 3,5-Me2C6H3) on Mo, and N[R]ArMeL (R ¼ C(CD3)2CH3, ArMeL ¼ 2-NMe2-5-MeC6H3) on V.18 Mixing Mo[N(tBu)Ar]3 and V(N[R]ArMeL)2 in THF afforded (Ar[tBu]N)3Mo(m-N2)V(N[R] ArMeL)2 after crystallization from tetramethylsilane at −35  C. The IR spectrum features a band at 1646 cm−1. Chirik and coworkers synthesized [{(iPrBPDI)V(THF)}2(m2-N2)], where iPrBPDI ¼ 2,6-(2,6-iPr2-C6H3N]CPh)2-C5H3N), by the reduction of [(iPrBPDI)VCl3)] with Na/Hg amalgam in Et2O followed by the subsequent addition of THF, Scheme 3.19 The supporting bis(imino)pyridine ligand in the recrystallized product suggested a one electron reduction of the ligand, as such, the vanadium centers were assigned as vanadium(III) with an NdN bond distance of 1.232(2) A˚ . This vanadium dinitrogen complex facilitates N]N bond cleavage of azobenzene or substituted hydrazines20, and was found to react with O2, or S8, furnishing the bis(oxide), and bis(sulfido) products, respectively. Furthermore, treatment with Se generated a unique dimeric species [{(iPrBPDI)V(Se)}2(m2-Se2)] with loss of the bridging N2 ligand.

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

775

Scheme 3 Synthesis of [{(iPrBPDI)V(THF)}2(m2-N2)].

6.12.2.1.2

Alkoxide ligands

Early work by Shilov found that efficient dinitrogen fixation was possible with a combination of hydroxide species, such as V(OH)2 with the support of Mg(OH)2.21 With the goal of understanding N2 reactivity of V with a coordination environment of oxygen-donors, Nocera and coworkers reported the first V(III) tris(alkoxide) complex to bind dinitrogen.22 The reactive trigonal planar V(OR)3, [R ¼ tBu2(Me)CO−] complex B in Scheme 4 was formed from the dissociation of the THF ligand from complex A. Dissociation of THF was promoted by the addition of toluene followed by reduced pressure to produce {V(OR)3}2(m-N2), complex C. X-ray diffraction revealed two pseudotetrahedral vanadium centers with an end-on bridged N2 ligand. In two crystallographically independent molecules, each with an inversion center, exhibited N]N bond distances of 1.232(3) and 1.225(3) A˚ that coincided with the multiple bond character presented by the VdN bond lengths of 1.771(2) and 1.773(2) A˚ .

Scheme 4 Synthesis of {V(OR)3}2(m-N2), [R ¼ tBu2(Me)CO−].

Mindiola and coworkers prepared a mononuclear three-coordinate vanadium(II) complex with anionic nacnac and phenoxide ligands, [(nacnac)V(ODiiP)] (nacnac ¼ [ArNC(CH3)]2CH, Ar ¼ 2,6-iPr2C6H3; ODiiP ¼ 2,6-diisopropylphenoxide) by reduction of [(nacnac)VCl(ODiiP)] with Na/Hg amalgam under an argon atmosphere.23 When exposed to a N2 atmosphere, [(nacnac)V(ODiiP)] slowly generated the end-on dinitrogen complex [(nacnac)(ODiiP)V]2(m2-Z1:Z1-N2), Scheme 5. Alternatively, [(nacnac)(ODiiP)V]2(m2-Z1:Z1-N2) could be prepared directly by reduction of [(nacnac)VCl(ODiiP)] with Na/Hg amalgam under a N2 atmosphere. Although a satisfactory molecular structure was not obtained, analysis of [(nacnac)(ODiiP)V]2(m2-Z1:Z1-N2) by Raman spectroscopy exhibits a sharp feature at 1374 cm−1 (shifting to 1330 cm−1 when prepared using 15N2) consistent with significant reduction of the N^N bond. [(nacnac)(ODiiP)V]2(m2-Z1:Z1-15N2) is stable in solution indefinitely, and showed no indication of N2 exchange with 14N2 at room temperature over a 24 h period. Surprisingly, despite significant N2 activation, [(nacnac)(ODiiP)V]2(m2-Z1:Z1-N2) does not cleave the N2 triple bond to form a V-nitride species, as has been demonstrated with other aryloxide supported vanadium complexes.24 Thermolysis at 90  C or photolysis of [(nacnac)(ODiiP)V]2(m2-Z1:Z1-N2) led to the regeneration of [(nacnac)V(ODiiP)].

776

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

Scheme 5 Synthesis of [(nacnac)(ODiiP)V]2(m2-Z1:Z1-N2).

6.12.2.1.3

Cyclopentadienyl/alkyne ligands

Cyclopentadienyl ligands are multifunctional and used to stabilize metal complexes through the chelating effect, as well as diversify reactivity through substituent substitutions. Liu and coworkers reported a novel aminoethyl-functionalized monocyclopentadienyl vanadium(I) dinitrogen complex {[Z5-(C5H4CH2CH2NMe2)]V(PhC^CPh)(PMe3)}2(m-N2).25 The bridging dinitrogen vanadium complex was synthesized under an N2 atmosphere from the monocyclopentadienyl V(III) complex in the presence of excess magnesium at reduced temperatures, Scheme 6. The vibrational frequencies for the coordinated PMe3 in the IR spectrum were observed at 947, 1278, 1298 cm−1, while the CdH bands of the non-coordinated NMe2 groups were assigned at 2760, 2813 cm−1, and a nC^C band was noted at 1717 cm−1. The crystal structure shows that the (m-N2) divanadium complex has C2 symmetry perpendicular to the bridging NdN bond, that exhibits a bond distance of 1.212(8) A˚ .

Scheme 6 Synthesis of {[Z5-(C5H4CH2CH2NMe2)]V(PhC^CPh)(PMe3)}2(m-N2).

6.12.2.1.4

Nitrogen and phosphorus pincer ligands

Polypyrrolide-based ligands have been used to enhance metal reactivity toward dinitrogen binding and activation. The multidentate nature of the ligands and the dual s- and p-bonding modes are features that promote the coordination of dinitrogen.26 Gambarotta and coworkers designed and synthesized a tripyrrolide ligand system for the application of low-valent vanadium dinitrogen coordination.27,28 The MeTPK2 ligand salt was reacted with a stoichiometric amount of VCl3(THF)3 to yield the [(MeTP)VCl(THF)]∙THF complex. In the presence of sodium in THF, the complex was reduced to [(MeTP)V(THF)]∙(C7H8)0.5. The introduction of a strong Lewis acid, such as AlMe3, extracted the coordinated THF allowing for the coordination of N2 in the formation of a divanadium dinitrogen-bridged complex, [(MeTP)V(m-N2)]2∙(C7H8)2.9, Scheme 7, top. The crystal structure shows that each metal center has a pseudo-tetrahedral geometry with one
5-pyrrole (V-centroid ¼ 2.023(6) A˚ ) and two s-bound pyrroles (V-N ¼ 2.011(5) A˚ , 2.024(6) A˚ ) and an end-on bridging dinitrogen (V-N ¼ 1.752(6) A˚ ). The bridging NdN bond, 1.248(5) A˚ , is elongated compared to that of free N2 at 1.0968 A˚ . More recently, Nishibayashi, Yoshizawa and coworkers have expanded pyrrole-based pincer ligated vanadium complexes with their development of a series of anionic pyrrole-based PNP pincer vanadium complexes.29 The pyrrole-PNP pincer vanadium

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

777

Scheme 7 Synthesis of V(m-N2) complexes with pyrrole-based ligands.

dichloride [VCl2(PNP)] precursor was prepared by treating [VCl3(THF)3] with lithium 2,5-bis(diaklylphosphinomethyl)pyrrolides (R-PNP; R ¼ tBu or iPr) followed by the addition of a bulky aryloxy or aryl group in THF. The resulting vanadium aryloxide complexes were reduced with KC8 under a N2 atmosphere, Scheme 7, bottom, to afford a bridging dinitrogen between two vanadium centers. X-ray crystallography confirmed the structure of [V(OXyl)(tBu-PNP)]2(m-N2) (Xyl ¼ 2,6-dimethylphenyl) that displayed a distorted square pyramidal geometry with N]N bond length of 1.235(4) A˚ . The bond distances between V and the pyrrolide N atoms, 2.038(3), 2.030(3), and 2.0383(19) A˚ , were lengthened in comparison to the vanadium dichloride precursor suggesting a decrease in the p-donation of pyrrolide. Catalytically reactive terminal metal alkylidene species have been used in ring-opening metathesis polymerization (ROMP). These reactive species often populate high-valent V systems. Mindiola and coworkers, utilizing a PNP pincer ligand, were the first to report a vanadium(III) precursor that afforded a VV alkylidene species with a bridging end-on dinitrogen ligand.30,31 The bis-alkyl VIII complex was treated with the ylide Ph3P]X (X ¼ CHPh or NPh) under N2 gas producing the bridging, end-on dinitrogen alkylidene complex [{(PNP)V(]CHtBu)}2(m-N2)], Scheme 8. The 51V NMR chemical shift showed a peak at −26 ppm (D1/2 ¼ 1084 Hz) and a broad resonance in the 15N spectrum at 104 ppm (ref. nitromethane at 0 ppm). The Raman spectrum presented a strong and symmetric nNN band at 1370 cm−1, while a nNN band was noted at 1325 cm−1 for the isotopically enriched 15N2 analog. The crystal structure reveals a short V]C bond of 1.863(4) A˚ of the terminal neopentylidene ligand and a linear m-
1:
1-N2 ligand bridging between the two vanadium centers. The VdN distance of 1.757(3) A˚ is indicative of imide bond character and signifies that each V center strongly backbonds to the N atoms of the N2 unit. This is consistent with the NdN distance of 1.246(6) A˚ .

Scheme 8 Synthesis of the bimetallic (PNP)VV dinitrogen alkylidene complex.

6.12.2.2

Vanadium ammonia and hydrazine complexes

The synthetic production of ammonia is highly sought after due to the importance of ammonia in the agricultural industry, and understanding the coordination chemistry of NH3 may guide synthesis of this product. Schrock and coworkers synthesized a V(NH3) complex by exposing [HIPTN3N]V(THF) (HIPT ¼ 3,5-(2,4,6-i-Pr3C6H2)2C6H3) in pentane to 1 atm of NH3, Scheme 9.32 The IR spectrum of the paramagnetic complex showed a nNH at 3358 cm−1. The crystal structure of [HIPTN3N]V(NH3) indicated that the bond distances were essentially the same as [HIPTN3N]V(THF). A diagnostic difference was the dihedral angle X-V-N-C, with X ¼ substrate in the apical pocket. For [HIPTN3N]V(THF) the dihedral angle was 20 compared to 5 for [HIPTN3N]V(NH3) due to the steric demands of THF compared to NH3. The addition of two equiv. of LiN(SiMe3)2 followed by two equiv. of [FeCp2]OTf to [HIPTN3N]V(NH3) afforded the imido product, [HIPTN3N]V(NH). Interestingly, addition of one equiv. [FeCp2]OTf to [HIPTN3N] V(NH3) also formed [HIPTN3N]V(NH) without the addition of LiN(SiMe3)2. {[HIPTN3N]V(N2)}K can also be prepared from the KC8 reduction of [HIPTN3N]V(THF) in THF, affording a weakly coordinated N2 ligand, as determined by a nNN band at 2255 cm−1.

778

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

Scheme 9 Coordination of THF, dinitrogen, ammonia, and imido group in the V[HIPTN3N] complex.

Szymczak and coworkers modified a terpyridine (Tpy) ligand scaffold to contain a boronic ester and morpholine group (TpyBN ¼ 6-morpholino-2,20 :60 ,200 -terpyridine-600 -boronic acid pinacol ester) to assist with the binding of hydrazine to a vanadium complex.33 Hydrazine that is bound the metal center interacts with both the pendent Lewis acid and base. Equimolar amounts of the TpyX [X ¼ BN (A) or Me2 (B)] ligand and [VCl3(THF)3] resulted in the VCl3(TpyX) complexes that were reacted with N2H4 in the presence of 5 equiv. NEt3, Scheme 10, to form the octahedral complexes VCl3(TpyX)(N2H3). IR spectroscopy of A revealed three bands at 3300, 3249, and 3204 cm−1, consistent with N2H−3 ligand. The crystal structure of A showed that N2H−3 was coordinated to the V(III) center in a Z2 fashion with close-contact interacts with neighboring Lewis acid/base functional groups. While a crystal structure was not reported, the use of the TpyMe2 ligand was tentatively assigned to afford VCl2(TpyMe2)(Z1-N2H3), B. In comparison to A, complex B presented two nNH bands at 3344 and 3248 cm−1, consistent with previously reported terminal hydrazido complexes.

Scheme 10 Syntheses of VIIICl2(TpyBN)(Z2-N2H3) and VIIICl2(TpyMe2)(Z1-N2H3) complexes.

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules 6.12.2.3

779

Nitrous oxide

Transition metal-N2O are rare due to their low dipole moment and weak s-donating and p-accepting properties. Chang and coworkers reported a three-fold symmetrical vanadium pyrrolide complex that reversibly coordinates N2O at ambient temperature.34 The trigonal bipyramidal (tpaMes)V(THF) precursor was synthesized by adding Li3(tpaMes) to a solution of VCl3(THF)3 in Et2O. Exposure of (tpaMes)V(THF) to 1 atm of N2O leads to the immediate formation of (tpaMes)V(NNO), Scheme 11 as determined by the nNNO stretch at 2289 cm−1 in the infrared spectrum, while SQUID magnetometry indicated an S ¼ 1 ground state. X-ray analysis confirmed that the NNO ligand was bound in a linear fashion through the N atom in the apical pocket surrounded by three bulky mesityl arms. Metric parameters include a VdN(NNO) bond length of 2.1389(10) A˚ , and NdN and NdO bond lengths of the metal bound N2O ligand of 1.1191(16) A˚ and 1.1869(17) A˚ , respectively.

Scheme 11 Synthesis of (tpaMes)V(N2O).

6.12.2.4 6.12.2.4.1

Niobium dinitrogen complexes Nitrogen donor ligands

As part of their study of the mechanism of N^N bond cleavage using Cp and amidinate ligands in Group 5 complexes (see Section 6.12.2.1.1, Scheme 2), Sita and coworkers prepared {Cp Nb[N(iPr)C(Ph)N(iPr)]}2(m-Z1:Z1-N2) by the reduction of Cp NbCl3[N(iPr)C(Ph)N(iPr)] with KC8 under a nitrogen atmosphere, Scheme 12.17 Structural characterization by X-ray crystallography indicated significant elongation of the bridging N2 ligand with a N^N bond length of 1.300(3) A˚ . In contrast to the result above, with Me as the backbone substituent, when Cp NbCl3[N(iPr)C(Me)N(iPr)] was subjected to identical reductive conditions, where the substituent on the amidinate backbone was modified from R ¼ Ph to Me, only the bis(m-nitrido) product {Cp M[N(iPr)C(Me)N(iPr)](m-N)}2 was isolated from cleavage of the N2 ligand. This differential reactivity showed the sensitivity of electronic and steric properties of the amidinate ligand influence N2 coordination versus N2 bond scission.

Scheme 12 Synthesis of {Cp Nb[N(iPr)C(Ph)N(iPr)]}2(m-N2) and {Cp Nb[N(iPr)C(Me)N(iPr)](m-N)}2.

Cummins and coworkers utilized bulky anilide ligands in the synthesis of a heterobimetallic Mo/Nb dinitrogen complex (Ar[tBu]N)3Mo(m-Z1:Z1-N2)Nb(N[iPr]Ar)3 (Ar ¼ 3,5-C6H3Me2) by reacting the NbIV precursor NbCl(N[iPr]Ar)3 with the terminal dinitrogen complex [Na(THF)x][(N2)Mo(N[tBu]Ar)3], Scheme 13.35,36 Alternatively, the [Mg(THF)2][(N2)Mo-(N[tBu]Ar)3]2 could also be employed in a reaction with NbCl(N[iPr]Ar)3 to obtain the same heterobimetallic (m-Z1:Z1-N2) product. Structural

780

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

Scheme 13 Synthesis of (Ar[tBu]N)3Mo(m-N2)Nb(N[iPr]Ar)3; Ar ¼ 3,5-C6H3Me2.

characterization revealed an activated N2 ligand exhibiting a bond length of 1.235(10) A˚ , while the IR spectrum displayed an intense nNN stretch at 1583 cm−1 for this paramagnetic S ¼ 1/2 complex. Further reduction of (Ar[tBu]N)3Mo(m-Z1:Z1-N2)Nb (N[iPr]Ar)3 led to cleavage of the N2 ligand affording Nb and Mo-nitride products.

6.12.2.4.2

Pincer ligands

A diniobium(III) complex supported by an anionic PNP pincer ligand containing an end-on bridging N2 ligand was prepared by Mindiola and coworkers.37 The binuclear complex was synthesized by three different pathways, Scheme 14. For pathway A, a stoichiometric amount of Li(PNP) (PNP ¼ N[2-P(CHMe2)2-4-methylphenyl]2) in thawing Et2O was added to a thawing suspension of NbCl3(DME). In the second pathway, B, KC8 was added to a stirring suspension of the trichloride niobium precursor under a N2 atmosphere. The bridging N2 diniobium complex was also synthesized from [NbCl3(THF)2]2(m-N2) by adding 2 equivalents of Li(PNP) in DME to [NbCl3(THF)2]2(m-N2) at low temperature, pathway C. The diamagnetic m-Z1:Z1-N2 diniobium complex was characterized by 1H, 31P, 15N and 13C NMR spectroscopy. Notably, the 1H NMR spectrum showed the centrosymmetric nature of the complex with equivalent dN]NbCl2(PNP) fragments, however, the 31P spectrum showed two inequivalent phosphorus resonances at 44 and 47 ppm due to twisting of the PNP ligands. This complex was also characterized by 93Nb NMR spectroscopy displaying a broad resonance at −3049 ppm; (Dn1/2 ¼ 10,500 Hz). The X-ray structure showed an elongated NdN bond distance of 1.277(6) A˚ , and a short Nb-NN2 distance of 1.851(3) A˚ indicative of the N2 ligand having diimide character.

(A)

(B)

(C)

Scheme 14 Synthetic pathways to prepare [(PNP)NbCl2]2(m-N2).

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules 6.12.2.4.3

781

Oxygen donor ligands

The silox ligand (silox ¼ tBu3SiO−) was employed for the activation of N2 by Wolczanski, Lancaster and coworkers in the synthesis of [(silox)3Nb]2(m-Z1:Z1-N2), molecular structure shown in Scheme 15.38 Treatment of (silox)3Nb(PMe3) with 0.2 or 1 equiv. of potassium metal in THF under N2 atmosphere afforded [(silox)3Nb]2(m-Z1:Z1-N2) after stirring for 8 or 4 days, respectively. The 93 Nb NMR spectrum displayed a broad peak at −570 ppm (n1/2 ¼ 4500 Hz) and the 1H NMR showed resonance at 1.21 ppm. The bridging N2 diniobium complex exhibits D3 symmetry with perfectly tetrahedral metal cores. The bond length of the bridging N2 moiety is 1.310(4) A˚ , and the NbdN bond length of 1.8039(19) A˚ indicates multiple bond character.

Scheme 15 Molecular structures of [(silox)3Nb]2(m:Z1Z1-N2): (A) Me groups are removed, and (B) Perspective looking down the Nb-NN-Nb axis showing the D3-symmetry of the complex. Reproduced with permission from Hulley, E. B.; Williams, V. A.; Hirsekorn, K. F.; Wolczanski, P. T.; Lancaster, K. M.; Lobkovsky, E. B. Polyhedron 2016, 103, 105–114.

In 2007, Kawaguchi and coworkers reported a nitride-bridged diniobium product [{K(THF)2}2{(O3)Nb}2(m-N)2] that was generated from the reaction of N2 with the potassium salt of a m-hydride niobium complex, [{K(DME)}2{(O3)Nb}2(m-H)4] [O3] ¼ [(3,5-tBu2-2-O-C6H2)3CH]3−).39 More recently, Kameo, Kawaguchi and coworkers examined N2 reactivity on the nature of the alkali metal used in [{M(DME)}2{(O3)Nb}2(m-H)4]; M ¼ Li, Na, specifically, focusing on reactions with the lithium and sodium salts, Scheme 16.40 X-ray diffraction revealed [{Na(DME)}2{(O3)Nb}2(m-H)4] reacts with N2 to form [{Na(DME)}2{(O3) Nb}2(m-H)2(m-Z1:Z2-N2)], a dinuclear Nb complex bridged by two hydrides and a N2 ligand in a rare (m-Z1:Z2-N2) side-on end-on coordination mode with an N¼N bond distance of 1.294(2) A˚ . A careful comparison of NMR spectral data revealed that the (m-Z1:Z2-N2) core could be identified with all three alkali metal salts [{M(DME)}2{(O3)Nb}2(m-H)2(m-Z1:Z2-N2)]; M ¼ Li, Na, K. The 15N NMR resonances for the bound N2 shifted slightly depending on cation identity. The 15N NMR spectrum for the (m-Z1:Z2-N2)-Na featured two doublets at 354 and 535 ppm with a 1JNN ¼ 21.4 Hz indicative of an intact N¼N bond. Notably, [{Li(DME)}2{(O3)Nb}2(m-H)4] reacts with N2 at 100  C in toluene affording a diniobium complex with an end-on bridging N2 ligand with an NdN bond distance of 1.295(3) A˚ .

Scheme 16 Reactions of [{M(DME)}2{(O3)Nb}2(m-H)4]; M ¼ Li, Na, K with N2.

782

6.12.2.5

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules Ta dinitrogen complexes

The use of tantalum in surface reactivity for dinitrogen reduction to ammonia and characterization of relevant NdH intermediates has been explored by the groups of Quadrelli,41 Paul,42 and Li and Schwarz.43 Molecular tantalum dinitrogen or small molecule nitrogen containing complexes, however, encompass a fairly rare set of inorganic complexes with only a handful of studies published within the study period of this chapter. The following section focuses on the recent synthesis and characterization of molecular tantalum complexes with dinitrogen and nitrogen containing small molecules. In several examples, selected reactions of activated N2 with small molecules have been included to provide context to the breadth of reported reaction chemistry with N2. In the late 1990s, Fryzuk and coworkers prepared a dinuclear tantalum complex that exhibits side-on end-on dinitrogen bonding using the NPN ligand (NPN ¼ PhP(CH2SiMe2NPh)2), Scheme 17.44 The precursor Li2(THF)2[NPN], was combined with TaMe3Cl2 to form TaMe3[NPN]. The reaction of TaMe3[NPN] with dihydrogen in Et2O produced a dinuclear tantalum (IV) hydride {(TaH2)[NPN]}2 via elimination of methane. Exposure of this bridged Ta-hydride complex to N2 gas, afforded a dinitrogen complex with distinctive side-on end-on bonding of N2 to tantalum {(Ta(m-H)[NPN]}2(m-Z2:Z1-N2), Scheme 17.

Scheme 17 Synthesis of a Ta complex with distinctive side-on end-on bonding of N2.

The characterization of {(Ta(m-H)[NPN]}2(m-Z2:Z1-N2) was accomplished using 31P, 15N, 1H NMR spectral data, and X-ray crystallography.44 The 15N NMR spectrum showed two resonances at −20.4 and 163.6 ppm for the asymmetrically bound N2 ligand. The hydrides were chemically equivalent by 1H NMR spectroscopy and appeared as a doublet of doublets at 10.85 ppm. X-ray crystallography was unable to locate the hydride ligands in the complex; however, the crystal structure unequivocally showed the unsymmetrical binding of N2 and illustrated the planar Ta2N2 core. The synthesis of this dinuclear tantalum dinitrogen complex led to further functionalization of the N2 ligand with selected examples summarized in Scheme 18. Addition of propene led to the formation of a ([NPN}Ta(CH2CH2CH3))2(m-Z1:Z1-N2) complex.45 Reactions with benzyl bromide formed a borylimide product which breaks the NdN bond of dinitrogen.46 Addition of diisobutylaluminium hydride (DIBAL) also cleaves the dinitrogen bond with the elimination of H2 gas forming a dinuclear Ta-nitride complex. Silanes were shown to add across dinitrogen but when introduced in excess, a dinuclear complex is formed by breaking the dinitrogen bond and adding symmetrical silane ligands to each nitrogen. Schwartz reagent also reacts by adding NdZr bonding to the complex while retaining the hydrides and cleaving the NdN triple bond.

Functionalization of the N2 ligand by dinuclear tantalum dinitrogen complex.

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

Scheme 18

783

784

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

Fryzuk and coworkers also studied the cleavage of hydrazine and similar molecules with the dinuclear tantalum hydride precursor complex. Removing hydrogen and adding hydrazine or similar small diamine complexes such as 1,1-dimethylhydrazine, could break the nitrogen bonding and form the bridging nitrogen or imide tantalum complexes.47 Further studies, using {(Ta(m-H) [NPN]}2(m-Z2:Z1-N2) included reactions with 1,2-cumulenes, specifically N,N0 -diphenyl carbodiimide, which produces a complex where the carbodiimide bonds directly to the side-on site of the Ta2N2 core and forms an auxiliary N-Ta bond.48 Other cumulenes containing sulfur and tBuN were also used, and resulted in the breaking of the core tantalum dinitrogen bond. In 2007, Sita and coworkers reported the formation of dinuclear end-on bridged {Cp Ta[N(iPr)C(Me)N(iPr)]}2(m-Z1:Z1-N2) complexes ([Ta] ¼ {Cp Ta[N(iPr)C(Me)N(iPr)]}), by reacting a Ta(V) trichloride precursor with KC8 in THF under N2 atmosphere, Scheme 19.49 Treatment of Cp Ta[N(iPr)C(Me)N(iPr)]Cl3 with 2.5 equiv. KC8 formed a 60:40 mixture of the N2 bridged ditantalum dinitrogen complex [Ta]2(m-Z1:Z1-N2) with a Cl bound to each Ta. When 4 equiv. KC8 was employed, a dinuclear end-on bridged N2 complex [Ta]2(m-Z1:Z1-N2) with no Cl ligands was generated. At temperatures above 0  C [Ta]2(m-Z1:Z1-N2) converts to a 7:1 mixture of cis- and trans-[Ta]2 bis(m-nitrido) complexes. Characterization by X-ray crystallography revealed [Ta]2(m-Z1:Z1-N2)Cl2 had Ta-N, N-N, and Ta-N-N bond distances and angles of 1.813(5) A˚ , 1.288(10) A˚ , and 178.5(6) , respectively.49 [Ta]2(m-Z1:Z1-N2) adopts a C2h-symmetric geometry in the solid state and had Ta-N, N-N, and Ta-N-N values of 1.807(2) A˚ , 1.313(4) A˚ , and 172.7(3) , respectively. The structural data suggests differences in N2 activation exist between [Ta]2(m-Z1:Z1-N2)Cl2 and [Ta]2(m-Z1:Z1-N2) in which the latter exhibits greater [m-N2]4− character and in turn a weaker N-N bond. The cis- and trans-[Ta]2 bis(m-nitrido) complex adopts a planar core geometry containing Ta-N(1) and Ta-N(2) bond distances of 1.894(4) and 1.924(4) A˚ , respectively. Further work by Sita and coworkers explored functionalization of the [Ta]2(m-Z1:Z1-N2) complex including preparation of Ta imido, hydrazido, and hydrazidium intermediates.50

Scheme 19 Synthesis of [Ta]2(m-Z1:Z1-N2) complexes ([Ta]¼{Cp Ta[N(iPr)C(Me)N(iPr)]}).

Wolczanski and coworkers prepared [(silox)2TaCl]2(m-N2), by treating [(THF)3Cl3Ta]2(m-N2) with Na(silox), Scheme 20.51 Reaction with MeMgBr formed the Me derivative. Characterization for the [(silox)2TaCl]2(m-N2) complex by X-ray crystallography indicated Ta-N distances of 1.780(7) and 1.739(8) A˚ , and a N-N bond length of 1.355(11) A˚ , with the pseudo-eclipsed structure being confirmed for the Cl derivative and assumed for the Me derivative.

Scheme 20 Synthesis of [(silox)2XTa](m-N2) complexes, X ¼ Cl or Me.

In 2011, Hirotsu and coworkers examined sulfur bridged Ta2-M2 complexes for N2 reduction. The work began by reducing Cp TaCl4 with KC8, in the presence of di-p-tolyl disulfide, in THF and under N2 atmosphere, generating [Cp Ta (SC6H4Me)2]2(m-Z1:Z1-N2), Scheme 21.52 The structure exhibits a three-legged piano stool geometry from each tantalum center, containing one Cp and two 4-methylbenzylthiolato ligands per metal. X-ray crystallography revealed the presence of N2 bound in

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

785

Scheme 21 Synthesis of [Cp Ta(SC6H4Me)2]2(m-Z1:Z1-N2) and [Cp Ta(SC6H4Me)2M(CO)4]2(m-Z1:Z1-N2).

an end-on bridging fashion to two tantalum centers.52 The N-N bond length of 1.281(7) A˚ revealed significant activation of the N2 ligand. The Ta-N bond lengths were established to be 1.813(5) A˚ and 1.814(5) A˚ , and Ta-N-N angles are slightly bent from linearity (167 and 166.8 ) signifying N2 in the N4− 2 resonance form. The bridging dinitrogen ligand could be elongated slightly by the addition of secondary metal centers, Mo or Cr, that coordinate to the thiolate bridges under photolysis conditions. When a single secondary metal center was present, the N-N distance was 1.295(4) A˚ . A second metal center introduced onto the other thiolate ligand showed elongation of the N-N bond, 1.308(9) A˚ .

6.12.2.6

Dinitrogen complexes and hydrazine

Wolczanski and coworkers investigated the addition of N2H4 onto a Ta(silox)3 complex (silox ¼ tBu3SiO). Initially, addition of NH3 to a Ta(silox)3 complex afforded Ta(silox)3NH rather than the desired product.51 After this reaction did not lead to addition of N2H4 to the Ta center, the complex Ta(silox)3 ¼ CH2 was employed, which enabled 1,2-NH-addition to occur in the presence of N2H4 to yield Ta(silox)3MeNH-NH2, Scheme 22. Increased temperature led to thermal degradation and the loss of CH4. Subsequent loss of (silox)H led to the bridging of the nitrogen groups between Ta centers forming a product believed to be [(silox)2TaMe](m-NaHNb)(m-NgHNdH)[Ta(silox)2], as characterized by NMR spectroscopy. Through EXSY spectroscopy, it was revealed that there is a rapid exchange occurring between NgH and NdH. The authors proposed this bridge exists as an Z2,Z2/1

Scheme 22 Synthesis and coordination exchange of [(silox)2TaMe](m-NaHNb)(m-NgHNdH)[Ta(silox)2].

786

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

-NH, NH unit or this configuration is at least energetically feasible to permit rapid exchange. Characterization of [(silox)2TaMe](m-NaHNb)(m-NgHNdH)[Ta(silox)2], was carried out by 2D 1H-15N NMR spectroscopy. The NaH and NdH groups exhibited 1H resonances at 5.42, 6.38 ppm and 15N resonances at 131.3, 179.7 ppm, respectively. The 15N resonance for Nb was centered at 298.2 ppm, while the 1H and 15N resonances for NgH were assigned at 3.00 ppm and 79.50 ppm, respectively.51 Messerle and coworkers obtained 4e− reduction of dinitrogen making a bridged dinitrogen tantalum species, [(C5Me4R) TaCl2]2(m-N2) R ¼ Me, Et; with three distinct reaction pathways, Scheme 23.53 Disproportionation of (C5Me4R)2Ta2(m-Cl)4 in toluene and under N2, or reduction of Cp TaCl4 using Na/Hg under 100 psi of N2 formed [(C5Me4R)TaCl2]2(m-N2). Alternatively, a hydrazine adduct generated by combining Cp TaCl4 with hydrazine in CH2Cl2, followed by addition of NEt3 also furnished [(C5Me4R)TaCl2]2(m-N2) in quantitative yield. The characterization of [(C5Me4R)TaCl2]2(m-N2) included 1H and 15N NMR spectroscopy, and X-ray crystallography. When using an ethyl on the Cp , the 15N NMR spectrum showed a resonance at 67.9 ppm, (versus NH3 at 0 ppm). X-ray crystallography of the ethyl substituted Cp established the m-Z1,Z1-N2 coordination mode. ˚. The Ta]N distance of 1.804(3) A˚ , is consistent with imido-like linkages for a m-N4− 2 ligand, with an NdN distance of 1.280(6) A

Scheme 23 Synthesis of [(C5Me4R)TaCl2]2(m-N2) (R ¼ Me, Et).

Bercaw and coworkers devised a synthetic route to an end-on arylhydrazido synthon using Nb and Ta.54 The end-on bound hydrazido moiety is postulated as an intermediate in the N2 reduction pathway to form ammonia, and the number of examples of hydrazido products with heavier group 5 elements is limited. The products (dme)M(NNPh2)Cl3 (M ¼ Nb, Ta) were prepared by following a procedure similar to that of Williams,55 by reacting NbCl5, or TaCl5 with diphenylhydrazine and dimethoxyethane (dme) in the presence of pyridine and ZnCl2. The X-ray structure of (dme)Ta(NNPh2)Cl3 revealed a short Ta-N bond length of 1.773 (1) A˚ and an N-N distance of 1.347(1) A˚ for the hydrazido ligand indicated a LX2-type hydrazido(2-) resonance structure.

6.12.3

Group 6 – Cr, Mo, and W

6.12.3.1

Chromium dinitrogen complexes

6.12.3.1.1

Phosphine ligands

Cr-N2 coordination compounds supported only by phosphine ligation are less common than the numerous examples of the heavier Mo and W metals of group 6.56 The Cr-N2 compound containing electron-rich monodentate phosphine ligands cis-Cr (N2)2(PMe3)4 is thermally unstable and decomposes at room temperature.57 However, employing the chelating phosphine ligand dmpe (dmpe ¼ 1,2-bis(dimethylphosphino)ethane) affords the thermally robust complex trans-[Cr0(N2)2(dmpe)2], first reported by Wilkinson and Hursthouse in 1983 by reduction of CrCl2(dmpe)2 with Na/Hg amalgam.58 Berben and coworkers reported a synthetic route to prepare trans-[Cr0(N2)2(dmpe)2] in 2008 by the reaction of trans-[CrIICl2(dmpe)2] with nBuLi, at −73  C, Scheme 24. Dinitrogen-bridged CrI acetylide complexes such as trans,trans-[(Me3SiCC)(dmpe)2CrI]2(m-N2), can be prepared upon the reaction with nBuLi in the presence of HCCSiMe3. This product exhibits an N  N bond length of 1.178 A˚ , Scheme 24.59 Shores and coworkers also prepared60 dinitrogen-bridged Cr acetylide complexes such as [RC2Cr(m-N2)CrC2R]n+; R ¼ Ph or iPr3Si; n ¼ 0, 1, 2), Scheme 24. Employing the Cr-CCSiiPr3 derivative, the CrICrI, and the CrICII, and CrIICrII complexes were prepared using chemical oxidants. Surprisingly, these three products exhibited comparable N^N bond lengths of 1.187 A˚ , 1.195 A˚ , and 1.181 A˚ , respectively, despite the difference in Cr oxidation states.

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

787

Scheme 24 Chromium dinitrogen complexes bearing 1,2-bis(dimethylphosphino)ethane as a ligand.

Mock and coworkers prepared low-valent Cr0 complexes bearing cyclic phosphine ligands of the type PPh2NBn2, PPh3NBn3, and PPh4NBn4 that contain pendant amine groups.61 The reduction of (k3-PPh2NBn2)CrIIICl3 with Mg0 powder and one equiv. of PPh2NBn2 in THF afforded cis-[Cr0(N2)2(PPh2NBn2)2], Scheme 25. The infrared spectrum of cis-[Cr0(N2)2(PPh2NBn2)2] displays two nNN bands at 2009 cm−1 and 1937 cm−1. Using potassium graphite (KC8) as the reductant afforded cis-[Cr0(N2)2(PPh2NBn2)2] and trans-[Cr0(N2)2(PPh2NBn2)2]. While trans-[M0(N2)2(diphosphine)2] (M ¼ Cr, Mo, W) is usually the thermodynamic product,62–64 trans-[Cr0(N2)2(PPh2NBn2)2] was unstable in solution at room temperature, with the instability attributed to destabilizing interactions between N2 ligands and the pendant amine groups.

Scheme 25 Synthesis of cis- and trans-[Cr0(N2)2(PPh2NBn2)2], Cr(N2)(dmpe)(PPh3NBn3), [CrII(H)(N2)(dmpe)(PPh3NBn3)]+ and trans-[Cr0(N2)2(PPh4NBn4)].

Interestingly, the reaction of CrIICl2(THF) with two equivalents of PPh2NBn2 in THF afforded a unique 12-membered macrocycle, [12]-PPh3NBn3 isolated as fac-[CrIIICl3(PPh3NBn3) and a larger 16-membered macrocycle, [16]-PPh4NBn4, isolated as trans-[CrIICl2(PPh4NBn4)].65 The addition of a dmpe co-ligand in the reduction of fac-[CrIIICl3(PPh3NBn3)] with Mg formed a monoCr-N2 complex, Cr(N2)(dmpe)(PPh3NBn3), Scheme 25,66 while Mg reduction of trans-[CrIICl2(PPh4NBn4)] under 1 atm of N2 yielded trans-[Cr0(N2)2(PPh4NBn4)]. Cr(N2)(dmpe)(PPh3NBn3) exhibits anNN band at 1918 cm−1 and the crystal structure revealed a N-N bond distance of 1.132 A˚ . Treatment of Cr(N2)(dmpe)(PPh3NBn3) with one equiv. of [H(OEt2)2][B(C6F5)4] in THF at −78  C afforded a 7-coordinate Cr-N2 hydride complex [CrII(H)(N2)(dmpe)(PPh3NBn3)]+ Scheme 25, resulting in an increase in the nNN band at 2006 cm−1.66 trans-[Cr0(N2)2(PPh4NBn4)] exhibits nNN bands at 1918 cm−1 and 2072 cm−1 in the infrared spectrum. Structural analysis indicated the N2 ligands have an N-N bond distance of 1.112 and 1.120 A˚ . Further reactivity with HOTf afforded N2-derived 15N2H+5 and 15NH+4 and protonation of the pendant amine groups in trans-[Cr0(15N2)2(PPh4NBn4H2)]2+.65 cis-[Cr0(N2)2(PPh2NBn2)2], Cr(N2) (dmpe)(PPh3NBn3), and trans-[Cr0(N2)2(PPh4NBn4)] were later found to catalyze the reduction of N2 to N(SiMe3)3.67 Mock and coworkers prepared a series of isostructural dinitrogen complexes of the formula cis-[M0(N2)2(PEtNRPEt)2] (M ¼ Cr, Mo, W; R ¼ 2,6-difluorobenzyl) to compare N2 activation trends of all the Group 6 metals.64 cis-[M0(N2)2(PEtNRPEt)2] (M ¼ Cr, Mo, W; R ¼ 2,6-difluorobenzyl) complexes were synthesized by reduction of MIII/IV precursors with Mg in the presence of PNP ligand. In order to avoid the thermodynamically favored trans-M(N2)2 isomer, short reaction times and a reaction temperature of −5  C were implemented in the preparation of the kinetically favored cis-Cr(N2)2 and Mo(N2)2 products, Scheme 26. Infrared spectroscopy revealed the Mo complex nNN bands at 2012, 1950 cm−1, and the Cr and W complexes exhibit nNN bands at 1990 cm−1, 1911 cm−1 and 1987 cm−1, 1925 cm−1, respectively. The addition of HOTf at −40  C formed N2H+5 and NH+4 for the Cr-N2 complex, however, no N2H+5 or NH+4 was formed in the protonolysis reaction of the Mo or W-N2 complexes.

788

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

Scheme 26 Synthesis of cis- and trans-[M0(N2)2(PEtNRPEt)2] (M ¼ Cr, Mo, W; R ¼ 2,6-difluorobenzyl).

6.12.3.1.2

Nitrogen donor ligands

The diiminepyridine ligand 2,6-(2,6-iPr2dC6H3dN]CMe)2C5H3N) (PDI) was employed by Gambarotta, Budzelaar and coworkers to prepare a binuclear dinitrogen complex (PDI)(THF)Cr(m-N2)Cr(THF)(PDI) by reduction of (PDI)CrIICl with two equiv. NaH.68 Significant N-N bond elongation consistent with an [N2]2− moiety was apparent from the N2 bond distance of 1.241 A˚ as determined by XRD analysis, Scheme 27. Addition of two equiv. of NaH to (PDI)(THF)Cr(m-N2)Cr(THF)(PDI) afforded a binuclear Cr product. XRD analysis revealed a product [PDI]Cr(Na2{m-N2H})Cr[PDI] with a NdN bond length of 1.288 A˚ and close contacts with two Na counter ions with the N2 derived nitrogen atoms. Treatment of [PDI]Cr(Na2{m-N2H})Cr[PDI] with NaH, the addition of excess NaH to (PDI)CrIICl, or the reaction of (PDI)(THF)Cr(m-N2)Cr(THF)(PDI) with three equiv. NaH led to cleavage of the NdN bond in the formation of an anionic Cr-imido complex, (PDI)Cr(N-H-Na[THF]). NH+4 was formed upon treatment of (PDI)Cr(N-H-Na[THF]) with HCl.

Scheme 27 Cleavage of the N^N bond in a Cr(PDI) upon treatment with NaH; Ar ¼ 2,6-iPr2C6H3.

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

789

0

Theopold and coworkers used the tripodal tris(pyrazolyl)borate (TpR,R ) ligands in the synthesis of the complexes [TptBu,Me 0 Cr]2(m-Z1:Z1-N2), [TptBu,iPrCr]2(m-Z1:Z1-N2), and [TpiPr,iPrCr]2(m-Z1:Z1-N2) by the reduction of the desired TpR,R Cr(THF)Cl with KC8, Scheme 28. N2 bonds distances were evident for each complex of this series, measuring 1.213 A˚ , 1.209 A˚ and 1.214 A˚ , respectively.69 Interestingly, the bridging N2 ligand allowed for antiferromagnetic coupling of the two high-spin CrI fragments.

Scheme 28 Synthesis of [TptBu,MeCr]2(m-Z1:Z1-N2).

Theopold and coworkers continued their investigations with Cr by using the b-diketiminate ligand, bis(2,6-diisopropylphenyl)2,4-ketiminato ¼ NniPr) to prepare an atypical example of side-on N2 coordination bridging two Cr centers. The reduction of [NniPrCr(m-I)]2 with magnesium formed [NniPrCrI]2(m2-Z2:Z2-N2); the only Cr complex to exhibit side-on N2 coordination, shown in Scheme 29.70 XRD analysis indicated an NdN bond length of 1.249 A˚ . A subsequent computational study by Sakaki and coworkers found the side-on N2 coordination mode in [NniPrCrI]2(m2-Z2:Z2-N2) to be thermodynamically preferred.71 Reactivity of [NniPrCrI]2(m2-Z2:Z2-N2) with adamantyl azide (AdN3),72 shown in Scheme 29, generated monometallic or bimetallic [NniPr]Cr complexes in the +2, +3, and +5 oxidation states that contain a terminal or bridging NAd imido group or contain a five-membered tetraazametallacycle.

Scheme 29 Synthesis of chromium dinitrogen complexes with b-diketiminate ligands: bis(2,6-diisopropylphenyl)-2,4-ketiminato or bis(2,6-dimethylphenyl)2,4-ketiminato and reactivity of [NniPrCrI]2(m2-Z2:Z2-N2) with adamantyl azide (AdN3).

Theopold and coworkers later used a Cr dihydride complex [NnMeCr(m-H)]2 with the b-diketiminate ligand, bis(2,6-dimethylphenyl)-2,4-ketiminato ¼ NnMe) in reactions with Me3Si(H)CN2 to generate trimetallic and tetrametallic products, [(NnMe)CrI(m-N2)(THF)]3 and [(NnMe)CrI(m-N2)]4, respectively, with end-on bridging N2 ligands, Scheme 29.73 [(NnMe) CrI(m-N2)(THF)]3 contains a symmetrical triangular core, with the N-N bond lengths of 1.158 A˚ , 1.168 A˚ and 1.158 A˚ and two nNN bands in the infrared spectrum at 2124 cm−1 and 2244 cm−1. XRD analysis of [(NnMe)CrI(m-N2)]4 showed N-N bond lengths of 1.171 A˚ and 1.176 A˚ , and a nNN band at 2063 cm−1 was observed in the infrared spectrum.

790

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

6.12.3.1.3

Cyclopentadienyl ligands

Zhang, Xi and coworkers prepared dinuclear and trinuclear CrI-N2 complexes bearing a ligand with an ethyl-substituted cyclopentadienyl group tethered to a substituted monodentate phosphine CpEtPR (R ¼ Ph, Cy, iPr), by reduction of (CpEtPR)CrIICl (R ¼ Ph or Cy) with KHBEt3, Scheme 30.74 The trinuclear product [(CpEtPPh)CrI(N2)]2(m-N2)2CrI(CpEtPPh) contained two N2 ligands in a bridging end-on coordination mode with N-N bond lengths of 1.169 A˚ and 1.171 A˚ , while the NdN bond distance of the two terminal N2 ligands are 1.092 A˚ and 1.054 A˚ . nNN bands were found at 1981 and 1739 cm−1 for the terminal and bridging N2 ligands, respectively. If sterically bulkier cyclohexyl groups on the phosphine group are used, the reduction of (CpEtPCy)CrIICl with KHBEt3 under N2 formed the dinuclear product [(CpEtPCy)CrI(N2)]2(m-N2). nNN bands for [(CpEtPCy)CrI(N2)]2(m-N2) are found at 1751 cm−1 and 1962 cm−1. The NdN bond length for the bridging N2 ligand is 1.169 A˚ , which is longer than the NdN bond distances for the terminal N2 ligands at 1.081 and 1.108 A˚ .

Scheme 30 Synthesis and reactivity of chromium dinitrogen complexes bearing CpEtPR (R ¼ Ph, Cy) ligands.

Continuing to utilize the ligand bearing the sterically bulkier cyclohexyl groups, chemical reduction of (CpEtPCy)CrIICl or [(CpEtPCy)CrI(N2)]2(m-N2) using alkali metals K0, Rb0, or Cs0 in conjunction with added [2.2.2]-cryptand formed anionic products [M(crypt-222)][(CpEtPCy)Cr0(N2)2] (M ¼ K+, Rb+, Cs+), Scheme 30. XRD data indicated NdN bond distances of these three products fell in the range 1.132 A˚ –1.151 A˚ , while infrared spectra showed nNN bands at 1906 and 1822 cm−1. In the solid-state, Rb+ and Cs+ interacted with one and two N2 ligands respectively, while K+ did not interact with the N2 ligands. [K+(crypt-222)] [(CpEtPCy)Cr0(N2)2] was reacted with 1 equiv. Me3SiCl to form the Cr-silylhydrazido complex, (CpEtPCy)Cr(NN(SiMe3)2), a proposed intermediate in the catalytic silylation of N2, Scheme 30. Several of the Cr complexes reported in this study were examined for the catalytic silylation of N2.

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

791

In subsequent studies, Zhang, Xi and coworkers added a methylene linker attached to the cyclopentadienyl group leading to a larger ligand coordination geometry of CH2CpEtPR (R ¼ Ph, iPr) on the Cr center, Scheme 31.75 Performing a similar chemical reduction of (CH2CpEtPPh)CrIICl with KC8 led to the formation of the trinuclear complex [(CH2CpEtPPh)CrI(N2)](m-N2) CrI(CH2CpEtPPh)(m-N2)[CrI(CH2CpEtPPh)] containing one less terminally bound N2 ligand bound to Cr than the trinuclear product shown in Scheme 30, i.e. [(CpEtPPh)CrI(N2)]2(m-N2)2CrI(CpEtPPh). A dinuclear N2-bridged complex [(CH2CpEtPiPr)CrI(N2)](m-N2) CrI(CH2CpEtPiPr) was prepared when isopropyl groups are employed on the phosphine donor. Reaction of [(CH2CpEtPPh)CrI(N2)] (m-N2)CrI(CH2CpEtPPh)(m-N2)[CrI(CH2CpEtPPh)] with 3 equiv. of phenylsilane, afforded a dinuclear mixed-valent N2-bridged mono hydride product [(CH2CpEtPiPr)CrI](m-N2)[CrIII(H)(PhSiH2)(CH2CpEtPiPr)], Scheme 31. An N-N bond length of 1.175 A˚ was found by XRD analysis for the bridging N2 ligand, and an nNN band was observed at 1761 cm−1.

Scheme 31 Synthesis and silane reactivity of chromium dinitrogen complexes bearing CH2CpEtPR (R ¼ Ph, iPr) ligands.

6.12.3.2 6.12.3.2.1

Chromium-NO complexes Pincer ligands

Kirchner and coworkers utilized the POCOP pincer ligand to prepare a high-spin, square-planar Cr(II) complex Cr(POCOP-tBu) (Br) that served as a synthon for further reactivity with NO, Scheme 32. Treatment of a toluene solution of Cr(POCOP-tBu)(Br) with 1 bar NO gas resulted in the formal one-electron reduction to Cr(I) in low-spin, S ¼ 1/2 complex Cr(POCOP-tBu)(NO)(Br). Cr(POCOP-tBu)(NO)(Br) exhibits a strong band in the IR spectrum for the NO ligand at 1654 cm−1. The Cr-N-O bond angle is nearly linear at 177 , consistent with NO being an NO+ cation. Further treatment of Cr(POCOP-tBu)(NO)(Br) with

Scheme 32 Synthesis of CrdNO complexes supported by the POCOP ligand.

792

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

LiBH4 afforded the borohydride complex [Cr(POCOP-tBu)(NO)(k2-BH4)]. [Cr(POCOP-tBu)(NO)(k2-BH4)] could also be prepare by first reacting Cr(POCOP-tBu)(Br) with LiBH4 affording Cr(POCOP-tBu)(k2-BH4), followed by exposure to NO gas. Similarly, [Cr(POCOP-tBu)(NO)(k2-BH4)] exhibits a Cr(I), S ¼ 1/2 ground-state, and contains a band for the NO ligand at 1654 cm−1 in the IR spectrum, as well as notable features at 2465, 2427, and 2016 cm−1 for the terminal and bridging B-H stretches. [Cr(POCOP-tBu) (NO)(k2-BH4)] is best described as a distorted octahedral geometry with a notable Cr-N-O bond angle of 173.4(3) .

6.12.3.2.2

Nitrogen-donor ligands

Tsai and coworkers reacted excess nitric oxide at −78  C with quintuple bonded dimeric chromium (and Mo) complexes supported by amidinate ligands, Scheme 33.76 The resulting diamagnetic mononuclear Cr(NO)2[k2-HC(N-2,6-Et2C6H3)2]2 and dinuclear Cr2(m-k-1-ONO)2(NO)4[m-k2-HC(N-2,6-Et2C6H3)2]2 products were isolated. The mononuclear species can be synthesized solely when the reaction is carried out at room temperature. The IR spectrum of the mononuclear Cr complex revealed two nNO bands at 1816 and 1677 cm−1. The X-ray structure confirmed the coordination of two terminal nitrosyl ligands to the chromium center that adopted a distorted octahedral geometry. The NO bond length of 1.165(5) A˚ suggested NO acts as a radical, thus the mononuclear chromium species is described as {Cr(NO)2}6. The dinuclear Cr complex displayed two nNO bands at 1814 and 1705 cm−1 and the terminal NO ligands exhibit bond lengths of 1.170(6) A˚ and 1.169(6) A˚ . The Cr ⋯ Cr distance of 3.081(1) A˚ indicates the cleavage of the quintuple bond between the two Cr centers, with the metals being bridged by two nitrido ligands formed from the disproportionation of nitric oxide. Upon the treatment with excess NO, the sterically bulkier amidinate derivate resulted in the dimeric diamagnetic product (k2-NO2)-Cr(m-NO)2[m-k2-HC(N-2,6-iPr2C6H3)2]2CrNO with a nNO band at 1805 cm−1 for the terminal nitrosyl and a band at 1523 cm−1 representative of the bridging nitrosyls.

Scheme 33 Synthesis of CrdNO complexes from the reaction of NO with a quintuple bonded dimeric chromium complex supported by amidinate ligands.

In 2018, Caulton and coworkers effectively used the Mashima reagent 1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene for the double oxygen abstraction from a nitrate ligand of chromium to form a CrdNO product, Scheme 34.77 The introduction of the Mashima reagent to a solution of (H2L)Cr(NO3)3 (H2L ¼ bis(pyrazol-3-yl)pyridine)78 resulted in a dinitrate-nitrosyl chromium complex (H2L)Cr(NO3)2(NO). In the IR spectrum, a strong absorption at 1720 cm−1 was assigned for the nitrosyl ligand. The X-ray structure revealed an octahedral complex with the linear [CrI(NO+)]2+ unit and NO ligand trans to the pyridine nitrogen, designating the complex as {CrNO}5. The CrdNO bond distance was relatively short at 1.724(6) A˚ , as well as the nitrosyl NdO bond distance of 1.160(9) A˚ .

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

793

Scheme 34 Synthesis of a CrdNO complex by deoxygenation of nitrate using the Mashima reagent.

Karlin, Nam and coworkers employed the 12-membered tetraazamacrocyclic N-tetramethylated cyclam (12-TMC) ligand in a study describing the synthesis of a side-on CrIV-peroxo complex, [CrIV(12-TMC)(O2)(Cl)]+ by bubbling O2 through a solution of [CrII(12-TMC)(Cl)]+. Subsequent addition of excess of NO to an acetonitrile solution of [CrIV(12-TMC)(O2)(Cl)]+ resulted in the CrIII-nitrato complex, [CrIII(12-TMC)(NO3)(Cl)]+, Scheme 35.79 The X-ray structure shows the coordination of the k1-NO3 anion to the chromium center. The EPR spectrum revealed signals consistent with a d3 Cr(III) ion. Treatment of the Cr(II) chloride precursor [CrII(12-TMC)(Cl)]+ to NO resulted in [CrII(12-TMC)(NO)(Cl)]+; however, this species showed no further reactivity with O2. The IR spectrum of [CrII(12-TMC)(NO)(Cl)]+ displayed a nNO band at 1692 cm−1. Ford and coworkers utilized chromium cyclam complexes bearing nitrite (NO−2) co-ligands to photochemically generate nitric oxide.80–82

Scheme 35 Synthesis of CrdNO and CrdNO3 complexes supported by the tetraazamacrocyclic N-tetramethylated cyclam ligand.

Using a larger 14-membered N4-macrocycle ligand, Bakac and coworkers prepared Cr nitrosyl complexes of the type [Lx(H2O) CrNO]2+ where Lx ¼ L1 ¼ 1,4,8,11-tetraazacyclotetradecane, or L2 ¼ meso-Me6–1,4,8,11-tetraazacyclotetradecane by bubbling NO 3+ through acidic solutions of [LxCr(H2O)2]2+.83 The CrdNO complexes were then oxidized by Ru(bpy)3+ and NO. 3 to [LxCr(H2O)2] 2+  XRD analysis of [L1(H2O)CrNO] shows, the Cr-N-O bond is nearly linear at 178.40(13) and the CrdNO bond length of 1.686 (2) A˚ . Using these metrics and a long Cr-trans-H2O distance, the authors assigned [L1(H2O)CrNO]2+ as the CrIIINO− limiting structure. In 2001, Schrock and coworkers presented the synthesis of molybdenum complexes with the triamidoamine ligand [(HIPTNCH2CH2)3N]3−, (HIPTN3N); HIPT ¼ 3,5-(2,4,6-i-Pr3C6H2)2C6H3, Scheme 36.84 This ligand set was targeted to maximize steric protection of the metal center, prevent the formation of unreactive m-N2 complexes, and increase solubility of complexes in nonpolar solvents. While the HIPTN3N ligand was used in the report of the first molecular catalyst for N2 reduction to ammonia using Mo, in 2006 Schrock and coworkers expanded the research to include chromium.85 While N2 binding to the [HIPTN3N]Cr fragment was not observed, coordination of NO was readily favored. A [HIPTN3N]Cr precursor or the THF adduct [HIPTN3N] Cr(THF) were prepared by reacting LiN(TMS)2 and H3[HIPTN3N] followed by CrCl3, or mixing H3[HIPTN3N] and CrCl3(THF)3 followed by LiN(TMS)2, respectively. Exposure of these Cr complexes to NO, immediately afford [HIPTN3N]Cr(NO). The IR spectrum shows an absorption at 1715 cm−1 and the 1H NMR spectrum displayed a diamagnetic species suggesting Cr in the 2+ oxidation state. The X-ray structure revealed that CrdNO and NdO bond lengths are 1.6759(13) A˚ and 1.1826(17) A˚ , respectively, which are identical to those of [(Me3Si)2N]3Cr(NO).86

794

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

Scheme 36 Synthesis of CrdNO complexes supported by the HIPTN3N ligand (HIPT ¼ 3,5-(2,4,6-i-Pr3C6H2)2C6H3).

6.12.3.2.3

Cp donor-ligands

Watson and coworkers reported the synthesis and characterization of several transition metal complexes with N-phenyl1,2,3-triazole and quinone-1,2,3-triazole substituted Cp ligands.87 The general synthesis of the chromium nitrosyl complexes with these ligands is shown in Scheme 37. by reacting sodium formylcyclopentadienide (NaCpCHO) and chromium hexacarbonyl in DMF, followed by the addition of acetic acid and Diazald to form CpCHOCr(CO)2NO. Synthesis of the Schiff-base complexes was then achieved by reacting the CpCHOCr(CO)2NO with various phosphoranes.

Scheme 37 Synthesis of a variety of three-legged piano-stool Cr nitrosyl complexes containing substituted Cp ligands.

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

795

As depicted in Scheme 37, Wang and coworkers reported the synthesis of chromium chloride dinitrosyl complexes containing the ethyl, carboxyl, ethyl-acetyl, benzoyl, and ferrocenoyl substituted cyclopentadienyl ligands by reaction of hydrogen chloride gas with (CpR)Cr(CO)2NO followed by isoamyl nitrite.88 After workup the various products were recrystallized and characterized by X-ray diffraction. Wang and coworkers also reported the chromium methyl complex with the previously described ethyl substituted cyclopentadienyl ligand.88 After halogen exchange, the iodo chromium complex [(Z5-ethylcyclopentadienyl)Cr(NO)2I] was reacted with MeMgI affording (CpCH2CH3)Cr(CH3)(NO)2. Two nNO bands were observed in the infrared spectrum at 1771 and 1656 cm−1. Wang and coworkers reported the synthesis of dinitrosyl carbomethoxy-cyclopentadienyl chromium complexes with isothiocyanate and isoselenocyanate ligands.89 Shown in Scheme 37 the synthesis of these complexes was achieved by first adding silver nitrate to a solution of (Z5-COOCH3-Cp)Cr(NO)2Cl followed by the addition of potassium thiocyanate or potassium selenocyanate. The infrared spectrum for the isocyanate complex contained two nNO bands at 1839 and 1723 cm−1, while the isoselenocyanate product exhibits nNO bands at 1844 and 1734 cm−1. Lorenz and coworkers reported the synthesis of dinitrosyl cyclopentadienyl complexes with 5,5 diethylbarbiturato ligands, Scheme 37 by mixing a solution of 5,5-diethyl barbituric acid with triethylamine followed by addition of CpCr(NO)2Cl to yield CpCr(NO)2(C8H11N2O3) that exhibits nNO bands at 1814, and 1727 cm−1.90 Wang and coworkers reported the synthesis of a cyclopentadienyl and carboxymethyl-cyclopentadienyl dinitrosyl chromium complexes with phenylethynyl ligands bound to the chromium center, Scheme 37, by reaction of CpCr(NO)2Cl or (Cp-COOCH3)Cr(NO)2Cl with diethylamine, followed by the addition of phenylacetylene and cuprous iodide.91 Due to the physical nature of the product complexes, all steps were performed in the absence of light. (Z5-C5H5)Cr(NO)2(C^CdC6H5) exhibits nNO bands at 1805, 1689 cm−1, while nNO bands for (Z5-C5H4-COOCH3)Cr(NO)2(C^CdC6H5) appear in the infrared spectrum at 1825, 1724 cm−1. Wang and coworkers also reported the synthesis and characterization of various chromium cyclopentadienyl-carboxyl bridged-titanium complexes such as Cp2Ti(CH3){[OC(O)C5H4]Cr(NO)2X}, X ¼ Cl, I, Scheme 37. The synthesis of these complexes containing differing halide ligands (Cl, I) and carbonyl ligands on the chromium center was achieved by adding Cp2Ti(CH3)2 to a solution of the various [(Cp-COOH)Cr(NO)2X] complexes. The infrared spectrum for Cp2Ti(CH3){[OC(O)C5H4]Cr(NO)2Cl} contains nNO bands at 1822 and 1700 cm−1.92 Wang and coworkers prepared a chromium cyclopentadienyl nitrosyl complex bridged to a ruthenocenyl group.93 Dubbed “cynichrodenyl ruthenocenyl ketone,” (CO)2(NO)Cr[(Z5-C5H4)–C(O)–(Z5-C5H4)]-Ru(Z5-C5H5) was synthesized by reaction of cynichrodenoic acid in phosphorus pentachloride followed by the addition of aluminum trichloride to afford the aforementioned product that exhibits an nNO band at 1636 cm−1, Scheme 38. Reduction of the ketone functionality to the corresponding methylene linker was achieved by treatment with lithium aluminum hydride and aluminum trichloride. The infrared spectrum of (CO)2(NO)Cr[(Z5-C5H4)–CH2–(Z5-C5H4)]-Ru(Z5-C5H5) displays a nNO band at 1692 cm−1. O C Ru

+

R R 1) BF3

Cr OC

2

Cr

2) 2,4-DNPH R = H, CH3

NO OC

H N

N

C

OC

NO2 O2N

NO OC

1) 1,2-ethanedithiol, ZnCl2 2) HCl, Isoamyl nitrite R = H, CH3 S

R

C S

Cr

S

KI

Cr

Cl

ON

ON

R C 2

R N

N

C Cr

Cr

2) N2H4

OC

R = H, CH3

I

ON

ON

1) BF3

S

R

C

CO

ON

NO

CO

OC O

H2 C

C 1) PCl5, DCM 2) AlCl3 R = OH

Ru

Cr OC

NO OC

LiAH, Et2O AlCl3

Ru

Cr OC

NO OC

Scheme 38 Synthesis of a variety of three-legged piano-stool Cr nitrosyl complexes containing substituted Cp ligands.

796

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

In 2014, Wang and coworkers continued their studies on preparing complexes containing substituted Cp ligands of the type (CO)2(NO)Cr-CpX with the report detailing the synthesis of a 2,4-dinitrophenylhydrazone (2,4-DNPH) derivative.94 Shown in Scheme 38, the reaction of [Z5-formyl-CpCr(CO)2(NO)] or [Z5-acetyl-CpCr(CO)2(NO)] with BF3 ∙ OEt2 and the subsequent addition of 2,4-dinitrophenylhydrazine. The corresponding azine bridging formyl and acetyl complexes were also synthesized by Wang and coworkers the following year by an analogous procedure to the DNPH complexes except 0.5 equiv. of hydrazine is used to produce the azine bridging dimer complexes Scheme 38.95 In 2015, Wang and coworkers reported the synthesis of dithiolanyl-substituted Cp complexes of the previously described formyl and acetyl cyclopentadienyl chromium nitrosyl complexes Scheme 38.88 The addition of 1,2-ethanedithiol and zinc dichloride to [Z5-formyl-CpCr(CO)2(NO)] or [Z5-acetyl-CpCr(CO)2(NO)] complex affords [Z5-C5H4CR(SCH2)2]Cr(CO)2(NO), R ¼ H, CH3. The nitrosylation of each complex was then achieved by reaction with hydrogen chloride and the slow addition of isoamyl nitrate to give [Z5-C5H4CR(SCH2)2]Cr(NO)2Cl. Halogen exchange was achieved by adding potassium iodide, yielding [Z5-C5H4CR(SCH2)2] Cr(NO)2I, R ¼ H, CH3 that exhibit characteristic features in the infrared spectrum for the nNO bands at 1803, 1699 cm−1 for [Z5-C5H4CH(SCH2)2]Cr(NO)2I and 1803, 1696 cm−1 for [Z5-C5H4C(CH3)(SCH2)2]Cr(NO)2I.

6.12.3.2.4

Cyano-bridged structures based on [CrI(CN)5(NO)]3− and CrdNO complexes containing other donor-ligands

The paramagnetic [CrI(CN)5(NO)]3− unit is a commonly used as a building block to prepare multimetallic cyanide-bridged products. The formation of 2-D and 3-D extended structures are typically studied for their magnetic properties. General synthetic preparations of these cyanide bridged products proceed by mixing a solution of K3[CrI(CN)5(NO)]
H2O with the desired transition metal building block, generally [M(H2O)x]n+ or [LM(H2O)x]n+ (L ¼ bidentate or multidentate ligand). Examples with extended structure types include a two-dimensional cyanide-bridged CrI-CoII heterometallic complex {[CoII(LN5)]3[CrI(CN)5 (NO)]2
CH3CN
3H2O]}n (LN5 ¼ 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene)96, a Cr-Ru product with 3-D interpenetrating structures of the composition H0.2[Ru2(O2CMe)4]2.8[Cr(CN)5NO]
3.3H2O97, and a threedimensional Prussian-blue type ferrimagnet {MnIII[CrI(CN)5(NO+)]}
H2O.98 In addition, several groups report the preparation of one and two-dimensional cyano-bridged CrI-MnIII products with Mn bearing Schiff-base ligands.99–101 Polynuclear chromium complexes bearing NH3 co-ligands ligands have also been prepared and have afforded a variety of solid-state architectures that have been examined for their magnetic properties.102–106 Several examples of CrdNO complexes bearing ancillary ligands not grouped in the previous sections are included below. Døssing and coworkers prepared the nitrosyl complex [Cr(dmso)5(NO)][PF6]2 (dmso ¼ dimethyl sulfoxide) by reacting [Cr(NCCH3)5(NO)][PF6]2 in neat dmso, Scheme 39, Structure A.107 Crystallographic data showed a near linear Cr-N-O geometry of 176.45(13) , a Cr-N, and N-O bond distance of 1.6904(11) and 1.1963(15) A˚ , respectively. N-heterocyclic phosphenium (NHP) ligands are frequently viewed as isolobal analogs with NO+. Gudat and coworkers prepared a Cr NHP complexes of the type [(NHP)Cr(CO)3(NO)], Scheme 39, Structure B, (where NHP backbone ¼ CH]CH or CH2dCH2) by the reaction of the N-heterocyclic phosphenium triflates with PPN[Cr(CO)4(NO)] (PPN ¼ nitridebistriphenylphosphonium cation). The NO moiety in [(NHP)Cr(CO)3(NO)] was described as a formally anionic nitroxide (NO−) unit, with nNO bands at 1688 and 1678 cm−1 for the NHP backbones CH]CH and CH2dCH2, respectively. Lastly, Breher, Krossing and coworkers used Weakly Coordinating Anions (WCA’s) to synthesize the heteroleptic complexes [Cr(CO)5(NO)][WCA]− (WCA ¼ [Al(ORF)4]− or [F-{Al(ORF)3}2]−; R ¼ C(CF3)3) by the reaction of Cr(CO)6 with [NO]+[WCA]−, Scheme 39, Structure C.108 A nNO band at 1840 cm−1 was observed in each infrared spectrum for these two similar products.

(B) (C) (A)

Scheme 39 Chromium nitrosyl complexes featuring dmso, heterocyclic phosphenium, and carbon monoxide ligands.

Matsuo, Nakamura and coworkers prepared a series of group 6 nitrosyl complexes (M ¼ Cr, Mo, and W) containing the Z5-pentamethyl[60]fullerene, Scheme 40.109 While M-NO products were prepared for each of the group 6 metals in this study, the synthesis of the Z5-pentamethyl[60]fullerene Cr products will be highlighted here. Treatment of Cr(CO)3(CH3CN)3 with [K(thf )n][C60Me5] forms the anionic product [K+][Cr(Z5-C60Me5)(CO)−3] from which a reaction with Diazald led to the loss of CO to form CrII(Z5-C60Me5)(NO)(CO)2, an air-sensitive product with IR features at 1941 and 1694 cm−1, ascribed to the nCO and nNO

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

797

Scheme 40 Chromium nitrosyl complexes featuring the Z5-pentamethyl[60]fullerene ligand.

bands, respectively. A toluene solution of CrII(Z5-C60Me5)(NO)(CO)2 reacts with ethanol affording a chromium(III) ethoxy complex Cr(Z5-C60Me5)(NO)(OEt) that was trapped by performing the reaction in carbon disulfide, leading to insertion of the CS2 into the Cr-O bond in the formation of Cr(Z5-C60Me5)(Z2-S2COEt)(NO). The nNO band of the insertion product was assigned at 1675 cm−1.

6.12.3.3 6.12.3.3.1

Molybdenum and tungsten dinitrogen complexes Diphosphine ligands

The research groups of Chatt, Hidai, and George pioneered the synthesis of molybdenum and tungsten dinitrogen complexes containing monodentate and chelating phosphine ligands throughout the late-1960s, 1970s, and 1980s.62,110–112 Synthetic approaches typically involve the reduction of high valent Mo or W-halide precursors (e.g. MoCl5 or WCl6) to form stable MII/III/IV intermediate complexes that were utilized in subsequent reduction steps to the M0-N2 products. Chemical reductants such as Na/ Hg, Mg, or potassium graphite (KC8), are frequently employed for these transformations. More recently, Peterson, Nguyen, and coworkers reported a one-pot synthesis of M(N2)2(L)4 complexes with Mg in higher yields than procedures utilizing intermediate oxidation state precursors, Scheme 41.113 For example, Mg reduction of MoCl5 with dppe afforded trans-[Mo(N2)2(dppe)2] in 29% yield compared to the reduction of MoIII(acetylacetonate)3 with Na/Hg in the presence of dppe (13% yield).

Scheme 41 Synthesis of trans-[Mo(N2)2L4] and trans-[Mo(N2)2(L2)2] products using a one-pot reaction with MoCl5 phosphine ligand and Mg.

Masuda and coworkers synthesized Mo-N2 complexes containing a 4-membered P-N-P chelate using bis(diphenylphosphino) amine ligands from the reduction of MoCl3(THF)3 with Mg.114 Furthermore, Agapie and coworkers used a para-terphenyl diphosphine ligand and prepared Mo0-N2 complexes by reduction with Mg or LiHBEt3 that were later used for ammonia borane dehydrogenation115 or CO2 reduction.116 trans-[Mo(N2)2(dppe)2] (M ¼ Mo, W) have been generated in reactivity of studies of organoimide complexes by Tuczek and coworkers.117 Simonneau and coworkers showed the reaction of trans-[M(N2)2(dppe)2] (M ¼ Mo, W) with one equivalent of B(C6F5)3 formed the boratadiazenido adduct [M(N2-B(C6F5)3)(dppe)2], Scheme 42.118 nNN bands at 1744 and 1717 cm−1 were observed for the Mo and W congeners of the boratadiazenido adduct, respectively. Addition of with HB(C6F5)2 to [M(N2-B(C6F5)3)(dppe)2] formed [M(N2B(C6F5)2(dppe)2]+[HB(C6F5)3]− affording nNN bands at 1550 and 1532 cm−1 for the Mo and W congeners, respectively.

798

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

Scheme 42 Formation of boratadiazenido adduct of N2 by the reaction of trans-[M(N2)2(dppe)2] (M ¼ Mo, W) with B(C6F5)3 or HB(C6F5)2.

Although hydrazido intermediates have been prepared with other Chatt type dinitrogen complexes, Sivasankar and coworkers protonated trans-[W(N2)2(dppe)2] to form [(dppe)2WH(NNH2)][HB(C6F5)3] using protons derived from H2.119 Using the combination of 2,2,6,6-tetramethylpiperidine and B(C6F5)3, molecular H2 was split to react with the parent W-N2 complex. [(dppe)2WH(NNH2)][HB(C6F5)3] exhibits a nNN band in the IR spectrum at 1373 cm−1. Ishino and coworkers found N2 bridged Ti-W heterometallic complex (depe)2(Cl)W(m-N2)TiCp Cl2 (depe ¼ Et2PCH2CH2PEt2) that was prepared by reacting Cp TiCl3 with trans-[W(N2)2(depe)2] could be used as olefin polymerization catalyst.120 A bridging dinitrogen complex {[Cp (dmpe) W]2(m-N2)}[B(C6F5)4]2 was synthesized by Tilley and coworkers by exposing the C-H activated complex [(Z7-C5Me3(CH2)2 ˚ (dmpe)W(H)2][B(C6F5)4] to N2 gas.121 The bridging N2 ligand is best described as a N2− 2 unit with N-N distance of 1.22 A . Using trans-[Mo(N2)2(depe)2] Masuda and coworkers showed that one-electron chemical oxidation by [Cp2Fe][BArF4] or by electrochemical oxidation led to cleavage of the N2 ligand and formation of a terminal MoIV nitride complex [Mo(N)(depe)2][BArF4], Scheme 43.122 An [MoII(depe)2]2(m-N2) intermediate containing a bridging N2 ligand, exhibiting a Raman band located a 1292 cm−1, was observed en route to nitride formation.

Scheme 43 Formation of [MoII(depe)2]2(m-N2) upon oxidation of trans-[Mo(N2)2(depe)2] to afford [Mo(N)(depe)2][BArF4].

Utilizing a metallocene-diphosphine ligand, Nishibayashi and coworkers prepared M(N2)2(depf )2 complexes, (M ¼ Mo, W) depf ¼ 1,10 -bis(diethylphosphino)ferrocene), through two discrete synthetic strategies. In the case of W, ligand exchange reacting depf with trans-[W(N2)2(Ph2Me)4] formed only trans-[W(N2)2(depf )2]. However, a mixture of cis and trans isomers (6:1) was produced in the direct reduction of WCl6 with Mg in the presence of depf, Scheme 44. In contrast, reduction of MoCl5 with Mg in the

Scheme 44 Synthesis of Mo and W-N2 complexes bearing a metallocene-diphosphine ligand by two distinct synthetic routes; depf ¼ 1,10 -bis(diethylphosphino) ferrocene).

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

799

presence of depf produced only trans-[Mo(N2)2(depf )2], exhibiting an nNN band at 1907 cm−1. Two nNN bands at 1972 cm−1 and 1906 cm−1 were observed for cis-[W(N2)2(depf )2], and the trans-[W(N2)2(depf )2] displays a single nNN band at 1883 cm−1. Nishibayashi and coworkers synthesized Mo and W dinitrogen complexes bearing related metallocene-diphosphine ligands such as bepc (bis(diethylphosphinobenzene)chromium) or bmpc (bis(dimethylphosphinobenzene)chromium)123 and dmpr (dimethylphosphinoruthenocene) and depr (diethylphosphinoruthenocene).124 The depf ligand was also used by Nishibayashi and coworkers in a Mo system where the cleavage and formation of dinitrogen was demonstrated in a synthetic cycle, Scheme 45.125 The reaction of Cp Mo(N2)(H)(depf )126 with trityl cation generated a cationic dinitrogen bridged dinuclear complex {[Cp Mo(depf )]2(m-N2)}+, exhibiting an NdN bond length of 1.226 A˚ . Notably, chemical reduction with KC8 formed the neutral complex [Cp Mo(depf )]2(m-N2), with subsequent shortening of the NdN bond to 1.182 A˚ . Irradiation of [Cp Mo(depf )]2(m-N2) with visible light led to the formation a terminal nitride complex Cp (depf ) MoIV^N. Chemical oxidation of Cp (depf )MoIV^N with Fc[BArF4] led to formation of an NdN bond in {[Cp Mo (depf )]2(m-N2)}2+, exhibiting an NdN bond length of 1.256 A˚ .

Scheme 45 Formation of Mo complexes supported by 1,10 -bis(diethylphosphino)ferrocene (depf ) containing dinitrogen ligands as [N2]4−, [N2]3−, and [N2]2− groups, and the photochemical cleavage of N2.

Tuczek and coworkers synthesized, and characterized Mo and W-N2 and CO complexes using monodentate and chelating diphosphine ligands with pendant amine groups in the ligand backbone.127 Starting with a MIII/IV precursor already containing a chelating diphosphine co-ligand such as MCl4(dppe) (M ¼ Mo, W) or MoBr3(PNP)(thf ) chemical reduction with Mg or Na/Hg in the presence of the PNP yielded the desired trans-[M(N2)2(dppe)(PNP)] product. In the case of reduction of MoCl4(dppe) a mixture of homoleptic and heteroleptic Mo-N2 complexes was generated, Scheme 46. Characterization of trans-[W(N2)2(dppe) (PPhNMePPh)] by infrared spectroscopy showed symmetric and antisymmetric nNN bands at 2013 and 1910 cm−1, respectively, while trans-[Mo(N2)2(dppe)(PPhNNPyPPh)] displayed features at 2036 and 1956 cm−1. In addition, the 15N NMR spectrum of trans[W(15N2)2(dppe)(PPhNMePPh)] displayed signals in the normal range for W0-N2 complexes at −61.6 and −50.0 ppm for Na and Nb, respectively.

800

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

Scheme 46 Synthesis of molybdenum and tungsten dinitrogen complexes with diphosphine ligands containing pendant amine groups.

Mock and coworkers prepared a series of Mo and W bis(dinitrogen) complexes bearing diphosphine ligands with pendant 0 amines, i.e. PRNR PR, in the second coordination sphere of the metal complex.64,128–132 In a similar fashion to the synthetic procedures published from Tuczek, the preparation of Mo and W-N2 compounds employed the chemical reduction of MIII/IV precursors with Mg, Scheme 47. Importantly, the use of MoBr3(THF)3 was vital for obtaining Mo-N2 products containing the 0 PRNR PR ligands, as Mg reduction of starting materials such as MoCl5, MoCl4(Et2O), or MoCl3(THF)3 did not generate the 0 desired Mo-N2 products. The formation of MoII products such as trans-[MoBr2(PRNR PR)2] could be generated via reduction with IV Zn prior to Mg reduction to the final Mo-N2 products. The reduction of W precursors such as WCl4(dppe) or WCl4(PPh3)2 with Mg 0 0 led to W-N2 products containing one traditional diphosphine ligand and one PRNR PR, or two PRNR PR ligands, respectively. 0 Characterization of complexes with the general formula trans-[M(N2)2(PRNR PR)2] (M ¼ Mo, W) usually exhibited symmetric and antisymmetric nNN bands in the ranges of 1990–2020 and 1910–1955 cm−1, for Mo and W, respectively. Treatment with triflic acid or HBF4 ∙ OEt2 formed mixtures of protonated products including seven-coordinate M-hydride compounds with a protonated

Scheme 47 Synthesis of molybdenum and tungsten dinitrogen complexes with diphosphine ligands containing pendant amine groups in the ligand backbone.

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

801

pendant amine, and/or formation of the M-hydrazido products. For example, in an experiment monitored by in situ IR spectroscopy, the treatment of trans-[W(N2)2(dppe)(PEtNMePEt)] with 3 equiv. HBF4
OEt2 at −78  C initially generated trans-[W(N2)2(H) (dppe)(PEtNMePEt)][BF4] that was identified by a 70 cm−1 increase of the nNN band from 1925 cm−1 to 1995 cm−1. A subsequent 22 cm−1 increase of the nNN band was noted due to the formation of trans-[W(N2)2(H)(dppe)(PEtN(H)MePEt)][BF4]2. The hydrazido product trans-[W(NNH2)(F)(dppe)(PEtN(H)MePEt)][BF4]2 was determined to be the minor product of this reaction, accounting for only 15% of the total. In a subsequent study, Mock and coworkers prepared molybdenum complexes of the type trans-[Mo(N2)2(PEtPNRR´)2 (PMePh2)2] where the pendant bases are terminal substituents on the phosphorus atom of the ligand.133 The reaction of trans[Mo(N2)2(PMePh2)4] with 1 equiv. of the PEtPNRR´ ligand produced trans-[Mo(N2)2(PEtPNRR´)(PMePh2)2] by a ligand substitution, Scheme 48. The nNN bands of the N2 ligands were in the range of 1930–1941 cm−1. Protonation studies revealed that modifying the steric and electronic properties of pendant amine groups resulted in a variety of protonated products in reactions with triflic acid (HOTf ) including a seven-coordinate Mo-hydride with a protonated pendant amine, and/or formation of the Mo-hydrazido products. For example, treatment of trans-[Mo(N2)2(PEtPNH(Ph))(PMePh2)2] with 1 equiv. of HOTf at −40  C resulted in an 85 cm−1 increase of the nNN band from 1939 cm−1 to 2024 cm−1 in the formation of trans-[Mo(H)(N2)2(PEtPNH(Ph))(PMePh2)2][OTf].

Scheme 48 Synthesis of Mo-N2 complexes bearing diphosphine ligands with terminal pendant amine groups on the phosphorus atom and acid reactivity with HOTf.

6.12.3.3.2

Polyphosphine ligands

Multidentate phosphine ligands have been used to prepare Mo-N2 complexes for N2 functionalization and CO2 functionalization.134 The chelate effect enhances metal complex stability and the use of electron donating substituents in tridentate and tetradentate135 phosphine ligands can result in pronounced N2 activation. Mézailles and coworkers used a sterically bulky tetradentate phosphine ligand, Scheme 49136 to prepare the dinitrogen complex (PPCy 3 )MoN2 by a three-step reaction 3 Cy beginning with the addition of the PPCy 3 ligand with MoCl3(THF3) to afford (k -PP3 )MoCl3. The addition of NaBPh4 to Cy (k3-PPCy 3 )MoCl3 in the presence of Na/Hg amalgam yielded [(PP3 )MoCl]BPh4 that could be further reduced with Na/Hg amalgam under N2 to afford the Mo-N2 product that exhibits nNN bands at 2016 and 1945 cm−1 in the infrared spectrum. Mézailles and Ph Cy Ph Cy coworkers also utilized the tridentate PPhPCy 2 ligand to form [(P P2 )Mo(N2)2] and [(P P2 )Mo(N2)3], containing two or three terminal nitrogen ligands, respectively, exhibiting nNN bands at 2070, 2002, and 1969 cm−1 in the IR spectrum.137 Upon crystallization of these terminal Mo-N2 complexes [(PPhPCy 2 )Mo(N2)2]2(m-N2) is formed showing an nNN band in the IR at 1968 cm−1, and one at 1946 cm−1 in the Raman spectrum in the solid-state. The dinuclear complex dissociates to the monomers in solution. Furthermore, the complex [(PPhPCy 2 )Mo(I)]2(m-N2), Scheme 49, was proposed as an intermediate in the formation of 138 [(PPhPCy however, this complex was not spectroscopically 2 )Mo(I)(N) that was generated upon scission of the N2 ligand, characterized.

802

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

Scheme 49 Synthesis of a molybdenum dinitrogen complexes using tridentate and tetradentate phosphine ligands.

Hulley and coworkers reported the synthesis of Mo-dinitrogen complexes supported by tridentate phosphine ligands containing azacrown ether lariats tethered to the phosphine ligand backbone, Scheme 50.139 The tethered azacrown ether moieties were used to bind alkali metal cations to understand the systematic effect of cation-N2 interactions in the activation of N2 within the Mo-bound dinitrogen complexes. The monodinitrogen Mo complexes were prepared by Na/Hg reduction of fac-(n-Crown-P3-Ph) MoCl3 (n ¼ 15 or 18) under N2 in the presence of PMe3 affording the diamagnetic fac-(n-Crown-P3-Ph)Mo(N2)(PMe3)2 (n ¼ 15 or 18), which displayed a nNN band in the infrared spectrum at 1950 and 1952 cm−1, for the n ¼ 15 and 18 crown-ether complexes, respectively. Alkali metal coordination was found to lower nNN stretching frequencies by 14–39 cm−1. The largest change in N2 stretching frequency was observed for Li+, resulting in a nNN bands at 1914 and 1935 cm−1, for the n ¼ 15 and 18 crown-ether complexes, respectively.

Scheme 50 Synthesis and alkali metal coordination of molybdenum dinitrogen complexes using tridentate phosphine ligands with tethered pendant azacrown ether lariats.

Molybdenum pentaphosphine mono(dinitrogen) complexes represent a class of dinitrogen complexes frequently composed of a tridentate (P3) phosphine ligand along with monodentate or bidentate phosphine co-ligands, and can be represented by the general formula Mo(N2)(P3)(P2) or Mo(N2)(P3)(P)2. The electron-rich coordination environment leads to robust N2 coordination. A typical synthetic strategy to prepare this structure type first installs a tridentate phosphine ligand (P3) in the reaction with MoIIIX3(THF)3 (X ¼ Cl, Br). Chemical reduction of the MoIII precursor [MoX3(P3)] (X ¼ halide, P3 ¼ tridentate phosphine ligand) in the presence of monodentate or bidentate phosphine ligands affords the desired Mo0-N2 product. Tuczek and coworkers have made seminal contributions in this area with the synthesis of Mo-N2 complexes bearing carbon- or silicon-centered tripodal phosphine ligands with symmetric alkyl, aryl, or in certain examples asymmetric/hybrid (aryl and alkyl) substituents, complexes A-D, Scheme 51.140–144 Activation of the N2 ligand is less pronounced with the use of electron-withdrawing substituents on the phosphine ligands as in complex B, which exhibits a nNN band at 2035 cm−1. In contrast, complex C bearing a

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

(A)

(B)

(C)

803

(E)

(D)

Scheme 51 Selected (N2)MoP5 structures and corresponding N2 stretching frequencies.

silicon-centered tripodal phosphine ligand with electron-donating substituents exhibits a nNN band at 1932 cm−1; a difference of more than 100 cm−1. Long and coworkers recently used a tridentate N-triphos ligand in the synthesis of the Mo dinitrogen −1 145 complex [Mo(N2)(dppm)(NPPh 3 )], structure E, displaying a nNN band at 2027 cm . Tuczek and coworkers designed a unique tridentate phosphine ligand platform based on a cyclohexane-backbone, complex A, Scheme 52.146 Like carbon- or silicon-centered tripodal phosphine ligands listed above, this ligand platform binds in a facial coordination mode. The Mo-monodinitrogen complex was formed using dmpm as a co-ligand and exhibited a double band feature at nNN 1956 and 1981 cm−1 that is shifted to 1884 and 1919 cm−1 using 15N2. This split feature was also observed in solution-state and was proposed to be an intrinsic property of this complex. The Tuczek group reported linear tridentate phosphine ligands, such as those bearing central heteroatoms of N or P in complexes B and C that exhibit a fac coordination geometry. Infrared data show a slight increase in N2 activation with a N-donor bound trans to the N2 ligand than with a P-donor.147,148 Complexes D, E, and F contain a novel linear phosphine ligand containing phospholane end groups. The ligand exhibits a versatile coordination geometry. When prepared in conjunction with a dppm co-ligand, fac- and mer-Mo(N2)(P3)(P2) isomers were observed.149 Interestingly, no well-defined Mo-N2 complexes could be isolated when using dmpm as a co-ligand. When paired with the monodentate co-ligand, PMe2Ph, the linear tridentate ligand binds in a mer coordination mode and exhibits a nNN band at 1931 cm−1.

(B)

(C)

(D) (E)

(A)

(F)

Scheme 52 Selected (N2)MoP5 structures and their corresponding N2 stretching frequencies.

Tuczek and coworkers designed and implemented a hybrid heterocyclic carbene (NHC) diphosphine ligand,150 and together with a bidentate phosphine (dmpm) or two monodentate (PMePh2 or P(OMe3)3) co-ligands, formed fac- or mer- isomers complexes A and B, respectively, Scheme 53. Complex A contains a nNN bands at 1881 cm−1, while B exhibits nNN bands at 1876 cm−1 (for PPh2Me, and 1932 cm−1 (for P(OMe)3) with N2 ligands coordinated trans to the strongly donating NHC moiety. Lastly, Tuczek and coworkers prepared complex C containing a pentadentate phosphine ligand.151 The P5-ligand design led to activation of the dinitrogen ligand with complex C having a nNN band at 1929 cm−1, the lowest N2 vibrational frequency for the class of P5MoN2 complexes. Complex C was later shown to catalyze the reduction of N2 to NH3 producing 26 equiv. of N2 derived NH3.152

804

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

Scheme 53 Molybdenum dinitrogen complexes supported by polydentate phosphine ligands bearing NHC moiety and a pentaphosphine ligand.

6.12.3.3.3

Isocyanide and carbene ligands (Mo)

Much like phosphines, and isocyanide ligands and N-heterocyclic carbenes are strong donors to support transition metal dinitrogen complexes. Employing bulky isocyanide ligands Figueroa and coworkers showed the Na/Hg reduction of MoI4(CNArDipp2)2 in benzene under N2 afforded (Z6-C6H6)Mo0(N2)(CNArDipp2)2, Scheme 54.153 The infrared spectrum of (Z6-C6H6)Mo(N2) (CNArDipp2)2 exhibits a nNN band at 2101 cm−1 indicating a weakly activated N2 ligand.

Scheme 54 Synthesis of a molybdenum dinitrogen complex bearing bulky isocyanide ligands.

Ohki and coworkers prepared Mo-dinitrogen complexes using N-heterocyclic carbene (NHC) ligands to assess N2 activation compared to similar Mo-N2 complexes supported by traditional phosphine donors. Starting from the high-valent precursor MoCl4(THF)2, chemical reduction at −90  C using KC8 and in the presence of the NHC ligands 1,3-R2-4,5-dimethylimidazol2-ylidene, R ¼ Me (a) or Et (b)) afforded the dinitrogen complexes trans-Mo(N2)2(NHCa/b)4.154 If the more sterically encumbering NHC ligand 1,3-iPr2-4,5-dimethylimidazol-2-ylidene is used, a trinitrogen product mer-[Mo(N2)3(iPrNHC)3] was formed, Scheme 55. trans-Mo(N2)2(NHCa/b)4 exhibit nNN bands at 1904 and 1836 cm−1 in the infrared spectrum, while mer-[Mo (N2)3(iPrNHC)3] displays nNN bands at 1870, 1896 and 1907 cm−1. Addition of water to these NHC dinitrogen complexes formed NH3, a reaction that was not observed for Mo-N2 complexes containing monodentate phosphine ligands.

Scheme 55 Molybdenum dinitrogen complexes bearing N-heterocyclic-carbene (NHC) ligands.

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules 6.12.3.3.4

805

Nitrogen donor ligands

A variety of nitrogen-donor ligands have been utilized to prepare molybdenum dinitrogen complexes. Chirik and coworkers studied Mo with pyridine(diimine) ligands and found reduction of [((iPrBPDI)MoCl3 (iPrBPDI ¼ 2,6-(2,6-iPr2-C6H3N]CPh)2C5H3N) with Na/Hg amalgam under N2 formed [((iPrBPDI)Mo(N2))2(m2,Z1,Z1-N2)] an N2-bridging complex, Scheme 56.155 The Mo centers exhibit a distorted square pyramidal geometry and an NdN bond length of 1.246 A˚ for the bridging N2 ligand. The terminally bound N2 ligands displays an NdN bond distance of 1.109 A˚ and a nNN stretch of 2037 cm−1 in the infrared spectrum. [((iPrBPDI)Mo(N2))2(m2,Z1,Z1-N2)] reacts with 1 atm of H2 resulting in substitution of the terminal N2 ligands During a study of Mo-olefin complexes, a similar N2-bridging complex [(MesPDI)Mo(Z4-1,3-hexadiene)]2(m-N2) (MesPDI ¼ 2,6-(2,4,6-(trimethyl) C6H3N]CMe)2C5H3N)) was isolated with an N-N bond distance of 1.145(4) A˚ .156

Scheme 56 Synthesis of a bis(imino)pyridine molybdenum dinitrogen complex and reactivity with hydrogen gas.

Chirik and coworkers later synthesized [{(PhTpy)(PPh2Me)2Mo}2(m2-N2)][BArF4]2 [PhTpy ¼ 40 -Ph-2,20 ,60 ,200 -terpyridine; ArF4 ¼ (C6H3–3,5-(CF3)2)4] by chloride abstraction from (pHTpy)(PPh2Me)2MoCl using NaBArF4 under an N2 atmosphere, Scheme 57.157 The bridging nitrogen ligand displayed a N-N bond length of 1.203 A˚ and features a nNN band at 1563 cm−1. The bridging N2 ligand in [{(PhTpy)-(PPh2Me)2Mo}2(m2-N2)][BArF4]2 remains intact over five oxidation states. Upon oxidation by two electrons, the nNN band shifts to 1477 cm−1 in the Raman spectrum denoting a weakening of the N]N bond of the bridging N2 ligand.

Scheme 57 Synthesis of a binuclear molybdenum dinitrogen complex with terpyridine and phosphine ligands.

In a similar fashion to the group 5 complexes highlighted earlier in this chapter, Sita and coworkers also explored N2 coordination and reactivity with Mo and W complexes supported by a combination of Cp and amidinate ligands. The dinuclear N2 complex [(Cp Mo[N-(iPr)C(Me)N(iPr)])2(m-Z1:Z1-N2)] was synthesized by chemical reduction of [Cp Mo[N-(iPr)C(Me)N (iPr)]Cl3] using 3 equiv. Na/Hg under an atmosphere N2. Much like the group 5 counterparts, the N2 ligand was activated exhibiting an N]N bond distance of 1.27 A˚ .158,159 While thermally stable in solution up to 100  C, upon irradiation with light, a transformation involving the N2 ligand was observed to form a bridging bis(m-nitrido) complex [(Cp Mo[N(iPr)C(Me)N(iPr)] (N))2] and the mono-(m-nitrido) product [(Cp Mo[N(iPr)C(Me)N(iPr)])2(N)], Scheme 58.

806

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

Scheme 58 Photolysis of dinuclear m-N2 complexes of molybdenum and tungsten to afford bis(m-nitrido) and mono(m-nitrido) products.

Sita and coworkers later reported that reduced steric encumbrance of the amidinate ligand resulted in reduction of dinitrogen thermally, rather than through photolysis.160 Heating the complex (Cp [N(Et)C(Ph)N(Et)]Mo)2(m-Z1:Z1-N2) for 70 h at 60  C afforded conversion to the bis-nitrido complex [(Cp Mo[N(Et)C(Me)N(Et)](N))2].161 Schrock and Yandulov utilized a triamidoamine ligand in the complex Mo(N2)(HIPTN3N) (HIPT ¼ 3,5-(2,4,6-iPr3C6H2)2C6H3, hexaisopropylterphenyl) that was famously used as the first molecular catalyst for dinitrogen reduction to ammonia with up to 8 equivalents of NH3 produced using 2,6-lutidinium as a proton source and CrCp 2 (decamethylchromocene) as an electron source.162 Since their initial report detailing catalysis, several of the Mo-NxHy catalytic intermediates have been characterized, which are formed in the addition of protons and electrons to the Mo-N2 precatalyst.162–168 Schrock and coworkers amassed large body work comprising the synthesis and reactivity of Mo and W complexes with derivatives of the HIPTN3N ligand scaffolds aimed at modulating ligand steric bulk and electronic character. Selected representative complexes are shown in Scheme 59.166,168–171 The synthesis of [HIPTN3N]MoN2 was generally accessed from reduction of [HIPTN3N]MoCl with reductants such as Na sand, or Mg powder generating anionic products, {[HIPTN3N]MoN2}−. To obtain the neutral [HIPTN3N]MoN2 complexes the anionic Mo-N2 products were treated with mild oxidants such as ZnCl2 or AgOTf. The various Mo derivatives, and the original the [HIPTN3N]MoN2, have nNN stretching frequencies ranging from 1984 to 1992 cm−1 resulting from changes in the electronics of the ligand set. A notable exception to the range of nNN stretching frequencies noted is the asymmetric derivative with R1, R2 ¼ HIPT, R3 ¼ B, which is observed at 2007 cm−1 due to steric constrains of the ligand.166 Steric protection of the apical pocket prevents bimetallic reactions and decomposition of dinitrogen reduction intermediates and therefore was a key property of the ligand to preserve catalytic activity.166,168,172,173

Scheme 59 Synthesis of Mo(N2)(HIPTN3N) (HIPT ¼ 3,5-(2,4,6-iPr3C6H2)2C6H3) and derivatives of the HIPTN3N ligand scaffold.

A recent report by Lee and coworkers detailed the synthesis of a Mo0dN2 complex containing a pentadentate polypyridyl ligand. Infrared spectroscopy showed notable activation of the N2 ligand with a nNN band at 1847 cm−1 while X-ray crystal structure indicated an NdN distance of the N2 ligand of 1.142(10) A˚ .174

6.12.3.3.5

P and N pincer ligands

Holthausen, Schneider, and coworkers synthesized the pincer-supported dinuclear complex [{(PNP)ClMo}2(m2:Z1:Z1-N2)] (PNP ¼ N(CH2CH2PtBu2)2) by reduction MoCl3(PNP) with Na/Hg amalgam under an atmosphere of N2, Scheme 60.175 A high degree of N2 activation was evident, with an N-N bond length of 1.258 A˚ , and a nNN band in the Raman spectrum at 1343 cm−1. This unique dinuclear Mo complex cleaved dinitrogen upon protonation of the pincer backbone.

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

807

Scheme 60 Synthesis of a dinuclear molybdenum m-N2 complex and protonation of the PNP pincer ligand resulting in cleavage of N2 (X ¼ OTf− or BArF−4 ).

In 2011, Nishibayashi and coworkers initiated what would become a significant body of work over the next 10 years focused on the use of pincer-supported complexes for catalytic N2 reduction to ammonia. The second molecular catalyst for dinitrogen reduction to NH3, [Mo(N2)2(PNP)]2(m-N2) (PNP ¼ 2,6-bis(di-tert-butylphosphinomethyl)pyridine), was prepared from MoCl3(PNP) by reduction with Na/Hg amalgam under an N2 atmosphere, Scheme 61.176 Infrared spectroscopy of [Mo(N2)2(PNP)]2(m-N2) exhibited a nNN band at 1936 cm−1 for the terminal dinitrogen ligands, and Raman data showed a strong band at 1890 cm−1 for the bridging dinitrogen ligand. X-ray diffraction data showed a linear bridging dinitrogen ligand with bond distance of 1.146 A˚ . [Mo(N2)2(PNP)]2(m-N2) catalyzed the reduction of N2 to NH3 generating 12 equivalents of NH3 per Mo center.177,178 Nishibayashi and coworkers have reported an incredible library of Mo-N2178–184 and/or Mo-nitride185–188 complexes bearing PCP pincer ligands,189–191 asymmetrical,192 4-substituted PNP,193–196 polystyrene supported PNP,197 and pyrrole-based PNP-type pincer ligands198 for N2 reduction studies over the past decade.199 Improving catalyst turnover has been a major advancement in the field, with the highest TON of 4350 equiv. of NH3 produced in a single run.200,201

Scheme 61 General synthesis of Mo-N2 pincer complexes and selected examples with nNN bands from infrared data.

6.12.3.4 6.12.3.4.1

Molybdenum nitrosyl complexes Phosphine ligands

Molybdenum nitrosyl complexes with various phosphine-type ligands have been utilized for several different organic reactions such as hydrosilylations, Ullman-type redox reactions, olefin hydrogenations, hydride transfers, and imine hydrogenations. Berke and coworkers demonstrated in 2005 that trans-[Mo(PMe3)4(H)(NO)] undergoes hydride transfer reactions with various functional groups.202 Synthetically, the trans-[Mo(PMe3)4(H)(NO)] product was obtained after reacting trans-[Mo(PMe3)4(Cl) (NO)] with 5 equiv. of NaBH4 and 10 equiv. of PMe3 in THF, Scheme 62. The IR spectrum exhibits two characteristic signals for nNO

808

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

Scheme 62 Synthesis of the trans-[Mo(PMe3)4(H)(NO)].

bands at 1562 cm−1 and a nMo-H band at 1521 cm−1. The complex was utilized to perform a variety of reactions including alkoxide and imine insertions, and the hydrogenation of imines. The related tungsten complex was also utilized in ketone, carbon dioxide, imine, and carbonyl metal complex insertion reactions.203 Continuing this work, the Berke group demonstrated that employing sterically hindered diphosphine ligands on molybdenum nitrosyl complexes led to catalytic hydrogenation of certain phenyl-naphthyl imines.204 Scheme 63 depicts the synthesis of the complexes of this ligand series in a general form. The various hindered diphosphine ligands employed were 1,3-bis(diisopropylphosphino)propane (dippp), 1,2-bis(diisopropylphosphino)ethane (dippe), 1,1-bis(diisopropylphosphino) ferrocene (dippf ), and 1,2-bis(dicyclohexylphosphino)ethane (dcype). Synthesis of the complexes is started by introducing the desired phosphine ligand into a solution of THF with [Mo(CO)4(NO)ClAlCl3] followed by reaction with lithium borohydride in triethylamine to give the corresponding hydride complexes. Treatment of the hydrides with [H(Et2O)2][A] (A ¼ BArF4 or B(C6F5)4) produced the various molybdenum nitrosyl hydrogenation catalysts that have characteristic nCO bands at 2044, 1978 cm−1 and a nNO band at 1682 cm−1 in the infrared spectrum.

Scheme 63 General synthesis of the [Mo(CO)2(NO)(P-P)(THF)]A series of complexes.

Focusing their attention on hydrosilylation reactions Berke and coworkers created a large bite-angle diphosphine molybdenum nitrosyl catalyst with the formula [Mo(NO)(P-P)(NCMe)3][BArF4], P-P ¼ 2,20 -bis(diphenylphosphino)diphenylether.205 This complex catalyzed the hydrosilylation of various para-substituted benzaldehydes including: cyclohexanecarboxaldehyde, 2-thiophenecarboxaldehyde, and 2-furfural. This complex has also been shown to perform Ullman-type reactions.206 Synthesis of [Mo(NO)(P-P)(NCMe)3][BArF4], Scheme 64, was achieved by addition of the phosphine ligand in THF followed by chemical reduction with zinc in acetonitrile to produce the cationic species [Mo(NO)(P-P)(NCMe)3]+[Zn2Cl6]−1/2. An anion exchange was then performed with the bulkier [B{3,5-(CF3)2C6H3}4]− (BArF−4) anion to aid in solubility of the final molybdenum catalyst.

Scheme 64 Synthesis of Mo(NO)(P-P)(NCMe)3][BArF4].

In 2014, Berke and coworkers reported trisphosphine nitrosyl complexes of molybdenum and tungsten as catalysts in olefin hydrogenation reactions.207 Utilizing (iPr2PCH2CH2)2PPh (etpip) as a tris phosphine chelate, several molybdenum and tungsten isomeric nitrosyl complexes were synthesized and evaluated for their use as ethylene hydrogenation catalysts. Synthesis of the various ethylene and hydride metal catalysts was achieved by refluxing either the molybdenum or tungsten [M(NO)Cl3(NCMe)2] with the (etpip) phosphine ligand in THF to afford the corresponding syn/anti nitrosyl phosphine isomers, Scheme 65. Reduction of the isomers with sodium/mercury amalgam in the presence of 1 bar ethylene in THF formed the corresponding ethylene complexes. Treatment of M(NO)Cl(Z2-ethylene)(mer-etpip)] with NaBHEt3 (for Mo) or LiBH4 in Et3N (for W) reaction gave the desired hydride complexes with nNO bands at 1571 cm−1 for Mo(NO)H(Z2-ethylene)(mer-etpip)] and 1622 cm−1 for W(NO)H(Z2-ethylene)(meretpip)].

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

809

Scheme 65 Synthesis of and reactivity of M(NO)Cl3(etpip), M ¼ Mo, W.

6.12.3.4.2

Nitrogen donor ligands

Following a large volume of work directed at tungsten complexes, Harman and coworkers have recently synthesized several molybdenum(0) nitrosyl complexes that function as organic dearomatization agents.208,209 As examples, Scheme 66 shows the synthetic pathway of two starting Z2 arene molybdenum complexes that have been utilized in a variety of dearomatization reactions. Refluxing molybdenum hexacarbonyl with the hydridotris(pyrazolyl)borate (Tp) potassium salt in THF followed by nitrosylation with diazald yields [Mo(Tp)(CO)2(NO)]. Reacting the carbonyl complex with 1-methylimidazole (MeIM) or 4-(dimethylamino)pyridine (DMAP) in DMF followed by iodine substitution gives the corresponding iodo complexes. Finally, a reduction of either complex initiated via sodium metal with the desired arene in THF gives the stable Z2 bound aromatic complexes A and B.

Scheme 66 Synthesis of Z2 arene molybdenum nitrosyl complexes with the tris(pyrazolyl)borate (Tp) ligand.

Complex A in Scheme 66 has been effectively utilized in the dearomatization of both naphthalene and anthracene where it can be restored with the use of iodine and sodium. Of note, complex B can functionalize Z2 arene complexes via the DMAP ligand. The ability of the dimethylamino nitrogen atom of the ligand to be protonated while still being bound to the metal has allowed for more extensive nucleophilic addition reactions of the arenes to take place. Variations on the DMAP complex have led to many developments including: the dearomatization and functionalization pyridine, the synthesis of isoquinuclidines, ring closure reactions of substituted napthalenes, and an electron-transfer catalysis of Z2 arenes, alkenes, and ketones.210–213 Keane and coworkers synthesized the first reported molybdenum(0) complexes with PDP (2-[[2-(1-(pyridin-2-ylmethyl) pyrrolidin-2-yl)pyrrolidin-1-yl]methyl]pyridine}) type ligands.214 Prior to this work, only iron(II) complexes of this ligand had been synthesized. Exposing their product of fac-[(k3-PDP)Mo(CO)3] to sodium nitrite and hydrochloric acid led to the tetradentate binding of the PDP ligand in the cis-a and cis-b Mo-nitrosyl complexes shown in Scheme 67. The IR spectrum exhibited a nCO band at 1883 cm−1 and nNO band at 1603 cm−1 for the cis-b isomer, and a nCO band at 1888 cm−1 and nNO band at 1602 cm−1 for the cis-a isomer.

810

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

Scheme 67 Synthesis of isomers cis-a and cis-b-[Mo(PDP)(CO)(NO)]PF6.

Graham and coworkers synthesized some of the first examples molybdenum and tungsten carbon dioxide complexes of the form [TpM(NO)(L)] where Tp is tris(pyrazolyl)borate and L is either trimethylphosphine or 1-methylimidazole.215 Displacement of the Z2 aromatic ligand on either metal is achieved by subjecting the complexes to carbon dioxide gas at elevated pressures to give the Z2-CO2 products, Scheme 68. Characterization of the complexes was achieved through X-ray crystallography along with proton NMR and IR spectroscopy, the latter giving Mo complex signals of nCO 1754 cm−1 and nNO 1619 cm−1 while tungsten complex signals were nCO 1710 cm−1 and nNO 1592 cm−1.

Scheme 68 Tris(pyrazolyl)borate molybdenum and tungsten nitrosyl complexes with Z2-CO2 ligation.

In 2019, Yonemura and coworkers reported the stereoselective synthesis of molybdenum dinitrosyl complexes with 2-pyrimidinethiolate ligands that exhibit photodenitrosylation reactivity.216 Synthesis of the complex started by reacting the previously formed [Mo(NO)2]2+ anion with 4,6-dimethyl-2-mercaptopyrimidine (dmpymt) in methanolic potassium hydroxide under a nitrogen atmosphere, Scheme 69. The product exhibits IR frequencies of nNO at 1682 and 1636 cm−1. This product and the related 4,6-diamino-2-mercaptopyrimide (dapymt) product where both formed with the trans stereoisomers in dominant yield on account of the bulkiness of the pyrimidine ligands. Each of the dinitrosyl complexes were also able to undergo photodenitrosylation reactions to form either mononitrosyl hydroxyl bridged dimer complexes or further thiolate substituted products.

Scheme 69 Synthesis of Mo(NO)2(dmpymt)2 and Mo(NO)2(dapymt)2.

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules 6.12.3.4.3

811

Oxygen donor ligands

Avecilla and coworkers synthesized and characterized several dinitrosyl molybdenum(0) complexes with varying bidentate benzimidazole derived ligands.217 Synthesis of these complexes was achieved by refluxing a solution of Mo0(NO)2(acac)2 with a stoichiometric amount of the desired benzimidazoles (Hhmbmz R ¼ H, hebmz R ¼ CH3, hbbmz R ¼ Ph) in methanol to achieve the mono or di-substituted dinitrosyl complex, Scheme 70.

Scheme 70 Dinitrosyl molybdenum(0) complexes with bidentate benzimidazole-derived ligands.

While molybdenum nitrosyl complexes with silane and silicon type ligands are rare, in 2019 Copéret, Mougel, and coworkers developed a reactive tris(tert-butoxy)-silanolate molybdenum complex that reacted with nitrous oxide and various other small molecules.218 Scheme 71 shows the reaction of the molybdenum silanolate complex with nitrous oxide which produced a nitride product and a nitrosyl product. Analysis of the infrared spectra of products in vacuo revealed the metal nitrosyl bond with a nNO band at 1678 cm−1.

Scheme 71 Reactivity of tris(tert-butoxy)-silanolate molybdenum complex with N2O.

6.12.3.4.4

Multimetallic complexes

In 2006, Kajitani and coworkers synthesized a variety of mononuclear and binuclear molybdenum dithiolene complexes with nitrosyl ligands.219 The reaction of [Cp Mo(NO)(CO)2] with elemental sulfur and dimethyl acetylenedicarboxylate in benzene gave several dimolybdenum dithiolene complexes alongside a mononuclear dithiolene complex, Scheme 72. Separation and purification of the products through column chromatography and subsequent characterization by X-ray crystallography indicated that the mononuclear complex was the primary product formed while the binuclear species were formed in low yields.

Scheme 72 Synthesis of dinuclear and mononuclear Mo nitrosyl dithiolene complexes.

812

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

Focusing their work on the functionalization and cleavage of nitrosyl ligands García-Vivó, Ruiz and coworkers synthesized several dimolybdenum and ditungsten complexes with bridging nitrosyl ligands.220,221 Shown in Scheme 73 complex A was synthesized upon treating the starting complex [M2Cp2(m-H)(m-PCy2)(CO)4] with two equivalents of [NO]BF4 in the presence of sodium carbonate followed by a reflux with sodium nitrite. A methylation reaction of complex A with [Me3O]BF4 in dichloromethane yielded the nitrosomethane derivatives D. Nitrosyl reduction of complex A was achieved by using Na/Hg and Zn/Hg amalgams in THF to give the corresponding amido complexes with stereoretention, B, C. While the reaction of phosphines with complex A failed for both metals, phosphites such as P(OEt)3 and P(OPh)3 deoxygenated the nitrosyl ligand to produce the corresponding phosphoraniminato bridged derivatives, E, also with stereoretention.

Scheme 73 Synthesis and reactivity [M2Cp2(NO)2(m-NO)(m-PCy2)] (M ¼ Mo, W) with two equivalents of [NO]BF4 in the presence of sodium carbonate followed by reaction with sodium nitrite.

Ishii and coworkers have investigated how early-late heterobimetallic (ELHB) sulfur bridging complexes with molybdenum and tungsten nitrosyls could be functionalized.222,223 The group 9 metals rhodium and iridium were chosen as the “late” metals since they are both relatively electron rich compared to the group 6 metals being employed. Synthesis of the sulfido-bridged heterobimetallic complexes was achieved by reacting the group 9 bis(hydrosulfide) complexes [Cp∗M(SH)2(PMe3)] with the group 6 chloride complexes [Cp∗MCl2(NO)] in triethylamine, Scheme 74. The infrared spectrum of the molybdenum products featured a nNO band at 1520 cm−1 while the tungsten products display a nNO band at 1490 cm−1.

Scheme 74 Early-late heterobimetallic sulfur bridging complexes with molybdenum and tungsten nitrosyls.

Morris and coworkers reported the synthesis of a binuclear molybdenum nitrosyl complex with bridging dithiolene ligands obtained from a nickel dithiolene complex.224 The dimolybdenum complex A was obtained as a minor product by refluxing [CpMo(CO)2(NO)] and [Ni(S2C2Ph2)2] in toluene, Scheme 75. The IR spectrum shows nNO bands at 1663 and 1651 cm−1. The major products obtained were a mixture of two separable isomers of the formula [Mo2Ni2(NO)2(m-S2C2Ph2)4Cp2] which was determined to be two [CpMo(NO)Ni(S2C2Ph2)2] units linked by bridging sulfur atoms in a dimeric species. B shows an example of the primary isomeric product that was obtained.

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

813

Scheme 75 Binuclear molybdenum-molybdenum and nickel-molybdenum nitrosyl complexes with bridging thiolate ligands.

A variety of multimetallic Mo-containing products in the form of extended-chain structures or polyoxometallates, have been prepared incorporating NO ligands as a major component of the overall structure. Szklarzewicz and coworkers have prepared and characterized stable pentacyanonitrosyl molybdenum and tungsten complexes with bridging pyrazine ligands [{M(CN)5(NO)}2−(m-pz)]4− M ¼ Mo, W.225 (PPh4)4[Mo(CN)4O(pz)]∙3H2O was reacted with NO gas in CH2Cl2 resulting in the direct incorporation of nitric oxide into the starting complex followed by oxo ligand release to form NO−2. Ohkoshi and coworkers utilized a pentacyanonitrosylmolybdate(I) anion in the formation of cyanide bridging bimetallic lanthanide chains of the form {[LnIII(dmf )6][MoI(CN)5(NO)]}, (Ln ¼ Gd, Eu) for use as molecular magnets.226 The synthesis of these bimetallic chains was achieved by combining LnIII(NO3)3 ∙ 6H2O and (PPh4)4[Mo(CN)5(NO)]∙2H2O in dimethylformamide under an argon atmosphere. In 2007, Ma and coworkers developed a one-pot reaction to produce organotin substituted polyoxomolybdate tin clusters of the 227 II Synthesis of this cluster form (H3O)16[(H2O)2MoVO(OH)]2{MoVI 28Mo4 (NO)4(BuSnO)2[BuSn(OH)2]2O102(H2O)12}∙18H2O. proceeded by dissolving n-BuSnO(OH) in concentrated hydrochloric acid alongside Na2MoO4 ∙ 2H2O and NH2OH∙ HCl. Wei, Chen and coworkers synthesized a dinitrosyl hexamolybdate complex in mild conditions that undergoes the controlled release of nitric oxide.228 The hexamolybdate complex trans-[Mo6O17(NO)2]4− was formed by mixing (NBu4)4[Mo8O26] with hydroxylamine hydrochloride, tetrabutylammonium bromide, and N,N0 -dicyclohexylcarbodiimide (DCC) in acetonitrile. Controlled nitric oxide release from the complex occurs by placing samples into mildly acidic or basic aqueous solutions. Following this work, the same group reported the synthesis of a lanthanide substituted oxo-nitrosyl polymolybdate structure with mild luminescent properties.229 Similar to their previously reported hexamolybdate complex, synthesis of this new lanthanide complex was achieved in one-pot by mixing (NBu4)4[Mo8O26], Ln3+ salts, hydroxylamine hydrochloride, and DCC in methanol. X-ray structural characterization determined the formula to be [NBu4][La(CH3OH)2(DCU)NO3{Mo5O13(OMe)4(NO)}]∙CH3OH (DCU ¼ N, N0 -dicyclohexylurea, a hydrated product of DCC).

6.12.3.4.5

Molybdenum amine, imide complexes

In an effort to study the reactivity of surface silanols in olefin metathesis reactions, Copéret and coworkers prepared a molybdenum imido alkyl complex as a surface catalyst.230 The synthesis of this organometallic complex was performed by suspending Mo(N) (CH2tBu)3 and (c-C5H9)7Si7O12SiOH in benzene, Scheme 76. NH N tBu

OH tBu

Mo

+

Si

O O

tBu

tBu

Mo O O

benzene 25 °C

tBu

Si O O

O

Scheme 76 Reactivity of Mo(N)(CH2tBu)3 with surface confined silanol.

Chirik and coworkers have published several studies on the reactivity of molybdenum dinitrogen complexes. In 2014, the group reported the synthesis of a molybdenum dinitrogen complex with a phenyl-substituted bis(imino)pyridine ligand containing weakly activated terminal dinitrogen ligands with a nNN band at 2037 cm−1, as well as a more activated bridging dinitrogen ligand, assigned as the 2e- reduced [N2]2− group based on an N-N distance of 1.246(4) A˚ , Scheme 77.155 The binuclear Mo complex reacts with 2 equiv. of ammonia gas or 1.5 equiv. of hydrazine to form a bridging imido product accompanied by a loss of dinitrogen. The bis(imino)pyridine ligand undergoes N-C bond cleavage and coordinates the Ar-N fragment to the molybdenum center. The evolved gas was confirmed as the loss of three equivalents of dinitrogen.

814

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

Scheme 77 Reactivity of a binuclear molybdenum bis(imino)pyridine dinitrogen complex with NH3 or N2H4.

In 2016, the Chirik group also reported the synthesis of a molybdenum ammonia complex with terpyridine and phosphine ligands that lowered the NdH bond dissociation free energy of coordinated NH3, leading to hydrogen gas evolution upon heating.231 The synthesis of the ModNH3 complex was achieved by stirring a benzene solution of (PhTpy)(PPh2Me)2MoCl and Na[BArF4] in the presence of 1 equiv. NH3 to afford [(PhTpy)(PPh2Me)2Mo(NH3)][BArF4]. Further reaction of [(PhTpy) (PPh2Me)2Mo(NH3)][BArF4] with tBu3C6H2O•, or [(Z2-C5Me5)Cr(CO)3]2, or upon heating to 60  C afforded the amido species [(PhTpy)(PPh2Me)2Mo(NH2)][BArF4], Scheme 78. The corresponding Mo-hydrazine and aquo complexes were also synthesized and were found to undergo H2 loss in the formation of MoIIdN2H3 and MoIIdOH products, respectively.

Scheme 78 Synthesis of [(PhTpy)(PPh2Me)2Mo(NH3)][BArF4] and generation of [(PhTpy)(PPh2Me)2Mo(NH2)][BArF4] with H2 loss upon heating.

Continuing their work with ammonia activation, Chirik and coworkers studied the reactivity of a bis(imino)pyridine Z6-benzene molybdenum complex with ammonia.232 Treatment of Z6-benzene complex with 2 equiv. of ammonia in benzened6 led to the change in coordination mode of the Z6-benzene ligand into an Z2-benzene along with the coordination of two ammonia ligands, (iPrPDI)Mo(NH3)2(Z2-C6H6), Scheme 79. The infrared spectrum contained nNH band at 3360 cm−1 for the Mo-NH3 ligands. Addition of 1 equiv. of ethylene or cyclohexene to (iPrPDI)Mo(NH3)2(Z2-C6H6) resulted in ligand substitution to form (iPrPDI)Mo(NH3)2(Z2-C2H4) or (iPrPDI)Mo(NH3)2(Z2-C6H10). Each of these three (iPrPDI)Mo(NH3)2(Z2-alkene) complexes proved to be kinetically unstable converting into (iPrPIA)Mo(NH2)2 iPrPIA ¼ 2-(2,6-iPr2C6H3N]CMe),6-(2, 6- iPr2C6H3NdCHMe)C5H3N) accompanied by evolution of hydrogen gas.

Scheme 79 Synthesis and H2 loss of bis(imino)pyridine Z6-benzene bis ammonia molybdenum complex.

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

815

In 2017, Mock and coworkers reported the oxidation of ammonia by a cationic molybdenum complex of the form cis-[CpMo(CO)(PPh2NtBu2)(NH3)]+. 233 Complex A was synthesized by addition of NaBArF4 to a solution of CpMo(CO)(PPh2NtBu2)Cl in fluorobenzene with 1 atm of 15NH3, Scheme 80. The 15N{1H} NMR spectrum displayed a resonance at −437.9 ppm for the coordinated 15NH3 ligand. The reaction of A with 2,4,6-tri-tert-butylphenoxyl radical yielded a Mo-alkylimido complex which was then reduced by potassium graphite to form a terminal molybdenum nitride product. Generation of the [CpMo(PPh2NtBu2) (NH)]+ was then accomplished by addition of [H(OEt2)2][B(C6F5)4] to CpMo(PPh2NtBu2)(N) at low temperature.

(A)

(B)

Scheme 80 Synthesis and reactivity of cis-[CpMo(CO)(PPh2NtBu2)(NH3)][BArF4].

In 2019, Wiedner, Raugei and coworkers performed a synthetic and computational analysis of several metal pentapyridyl complexes for the oxidation of ammonia to dinitrogen.234 Conclusions from the calculations led to the examination of a molybdenum complex [(PY5)Mo(NH3)](OTf )2 for experimental NH3 oxidation studies. Synthesis of [(PY5)Mo(NH3)] (OTf )2 was accomplished by subjecting [Mo(PY5)OTf]OTf in acetonitrile to an atmosphere of ammonia, Scheme 81.

Scheme 81 Synthesis of [(PY5)Mo(NH3)]2+.

6.12.3.5 6.12.3.5.1

Tungsten nitrosyl complexes Alkyl, alkenyl, isonitrile ligands

Much of the work on tungsten nitrosyl complexes with alkyl and alkenyl ligands has been pioneered by Legzdins and coworkers throughout the last two decades. The primary use of such compounds in their ongoing research is to function as CdH bond activation platforms for the transformation of alkanes into alkenes and other functionalized groups. In 2007 the group reported the synthesis of Cp W(NO)(Z3-CH2CHCHMe)(CH2CMe3), a complex that functioned well in its ability to initiate the CdH bond activation and functionalization of linear n-alkanes, Scheme 82.235 Synthesis of complex A was achieved by reacting [Cp W(NO)(CH2CMe3)Cl] with 0.5 equiv. of (MeCHCHCH2)2Mg, Scheme 82. Complex A, nNO band at 1594 cm−1, is thermally unstable and readily undergoes loss of the CH2CMe3 group when placed in a linear alkane solvent to afford the alkyl complex shown exhibiting a nNO band at 1597 cm−1. Treatment of the alkyl complex product with iodine at −60  C cleaves the alkane to produce 1-iodo-alkanes. Also in 2007, Legzdins and coworkers investigated catalytic CdC and CdO bond formation by CdH bond activation using Cp W(NO)(L) where Lewis base (L) is a phosphine.236

816

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

(A) (B)

Scheme 82 Synthesis and reactivity of Cp W(NO)(Z3-CH2CHCHMe)(CH2CMe3).

Legzdins and coworkers then formulated a dependable mechanism of intermolecular C-H bond activations of hydrocarbons with the previous tungsten nitrosyl neopentane complex. The basic structure of the complexes is similar to complex B, Scheme 82, where R (attached to the allyl group) is a variety of alkyl, alkenyl, and aryl substituents that were synthesized through the same methods as shown above. In 2011, the group discovered that an allene intermediate was forming with (Z3-CH2CHCHPh) as the ligand. 237 Exploiting this fact, the reaction was configured to proceed with a variety of alkenyl molecules which formed carbon-carbon bonds at the Z3 site on the complex through CdH bond activation of the substrate. Following this work the same result was also obtained when using trimethylsilylallyl as the ligand.238 Additional work utilizing these complexes also includes a stepwise conversion of methane and carbon dioxide into asymmetrical ketones with varying alkenyl R ligands, and the functionalization of various hydrocarbons into esters with carbon monoxide.239,240 In 2008, Legzdins and coworkers published several papers demonstrating the transformation of cyclic olefins mediated by tungsten nitrosyl complexes.241,242 Synthesis of these complexes began by thermolysis of [Cp∗W(NO)(CH2CMe3)2] in a solution of 2,5-dihydrofuran and subsequent purification Scheme 83. Following the transformation in the reaction scheme, this process was also demonstrated with the olefins 3,4-dihydro-2H-pyran and 1,2,3,6-tetrahydropyridine.

Scheme 83 Transformation of cyclic olefins mediated by tungsten nitrosyl complexes.

Ipaktschi and coworkers have focused their attention on tungsten and molybdenum nitrosyl complexes with Z1-vinylidene ligands reacting with diazoalkanes and enamines.243,244 Complexes of this type are synthesized by reacting [CpM(CO)2(NO)] with the desired lithium acetylide salt followed by the addition of an electrophile of choice, iminium salts in the case of this vinylidene complex, Scheme 84. Reacting the metal vinylidene complexes with various diazoalkanes yields the corresponding Z2 alkene complexes in good yield while reaction of the vinylidene complexes with enamines yields Z1-acyl complexes.

Scheme 84 Synthesis and reactivity of tungsten and molybdenum nitrosyl complexes with Z1-vinylidene ligands.

In 2006, Huseynova and coworkers synthesized several tungsten nitrosyl chlorophosphinovinyl complexes of the form [CpW(NO)Cl(C]CH2)(PR2)] (R ¼ C6H5, CH(CH3)2, C(CH3)3) which were then able to undergo hydrolysis to form the corresponding m-bridged oxo complexes. 245 Synthesis of these complexes was achieved by addition of the desired chlorophosphine to a solution of the tungsten vinylidene complex, Scheme 85.

Scheme 85 Synthesis of [CpW(NO)Cl(C]CH2)(PR2)] (R ¼ C6H5, CH(CH3)2, C(CH3)3).

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

817

Stemming from their work with inversely polarized alkene complexes, Neumann and coworkers reported the first synthesis of Z2-arsaallene complexes from tungsten nitrosyl vinylidene complexes.246 Synthesis of these complexes was achieved by treating [Cp(CO)(NO)W]C]C(H)R] with an equimolar amount of the arsaalkenes RdAs]C(NMe2)2, Scheme 86. The Z2arsaallene products obtained with these varying R groups were [Cp(CO)(NO)W{Z2-tBuC(O)AsC]C(H)tBu}] (nNO ¼ 1649 cm−1), [Cp(CO)(NO)W{Z2-[Cp∗(CO)2Fe]AsC]C(H)tBu] (nNO ¼ 1620 cm−1) and [Cp(CO)(NO)W{Z2-tBuC(O)AsC]C(H)tBu}] (nNO ¼ 1634 cm−1). Following this work, the group later reported the reactions of the same [Cp(CO)(NO)W]C]C(H)Ph] vinylidene complex with the phosphaalkenes tBu-P]C(NMe2)2 and c-C6H11-P]C(NMe2)2 which gave the corresponding Z2-1-phosphaallene products.247

R R W OC ON

C

R'

C

As

Et2O, -30 °C

H

C

C(NMe2)2

R = Ph, tBu R' = tBuC(O), [Cp*(CO)2Fe]

W OC ON

H

C As R'

2

Scheme 86 Synthesis of Z -arsaallene complexes from tungsten nitrosyl vinylidene complexes.

In 2018, Etienne and coworkers reported the synthesis and characterization of a tungsten cyclopropyl complex in an effort to further study the CdH bond activation ability of tungsten nitrosyl complexes.248 Synthesis of this complex was achieved by treating a solution of [Cp∗W(NO)Cl(CH2SiMe3)] in THF-d8 with half an equiv. of Mg(c-C3H5)2(dioxane)x, Scheme 87. 1H and 13C NMR experiments showed five inequivalent protons giving signals that were attributed to the carbon atoms on the bound cyclopropyl ring corresponding to [Cp∗W(NO)(CH2SiMe3)(c-C3H5)]. The cyclopropyl complex was proven to be thermally unstable as NMR measurements of the THF-d8 solution showed proton resonances corresponding to ligand rearrangement and the formation of [Cp∗W(NO)(CH2SiMe3)(Z3-CH2CHCH2). Kinetic and computational studies indicated that the cyclopropyl ring opening reaction was thermodynamically preferred for the tungsten complex.249

Scheme 87 Synthesis of a tungsten nitrosyl cyclopropyl complex.

Legzdins and coworkers reported the synthesis of a unique series of tungsten nitrosyl piano stool complexes with asymmetrical isonitrile ligands in 2010.250 Prior to this work, the vast majority of piano stool complexes with the general form [CpML’L2] have the “L” ligands arranged in a symmetrical bonding pattern to the metal center in the solid state. Synthesis of these complexes was achieved by dissolving [Cp∗W(NO)Cl2] and the desired nitrile ligand of the form CN-R in THF followed by reduction by sodium/ mercury amalgam, Scheme 88. The X-ray crystal structure of [Cp∗W(NO)(CNCMe3)] revealed a linear isonitrile group with a CdN bond length of 1.159(5) A˚ and a CdNdC bond angle of 171.4(5) while the other isonitrile ligand was bent with a bond length of 1.200(5) A˚ and a CdNdC bond angle of 135.1(4) . Inequivalence of the isonitrile ligands was also detected through IR spectroscopy where absorptions of 2038 and 1853 cm−1 corresponded to the two-differing carbon-nitrogen bonds of the linear and bent ligands. Analysis of the other R group isonitrile complexes gave the same asymmetrical ligand results.

Scheme 88 Asymmetrical isonitrile ligands found in tungsten nitrosyl three-legged piano stool complexes.

818

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

6.12.3.5.2

Nitrogen donor ligands

Throughout the last several decades, Harman and coworkers utilized a simple tungsten nitrosyl trispyrazolylborate “Tp” complex in a wide variety of Z2 aromatic ligand transformation reactions. Scheme 89 shows the basic structure of the complex where “R” is any Z2 aromatic ligand such as benzene, napthalenes, pyridines, furans, indoles, etc.251 In the general synthetic scheme of these complexes typically [TpW(NO)Br(PMe3)] undergoes a ligand exchange reaction with the desired Z2 aromatic ligand in tetrahydrofuran followed by reduction with sodium/mercury amalgam.

Scheme 89 Synthesis of tungsten nitrosyl trispyrazolylborate complexes with R ¼ Z2 aromatic ligands.

Benzene and naphthalene type ligands of this complex have been shown to undergo Friedel-Crafts ring-coupling reactions, cycloadditions with naphthalene, and the formation of various cyclohexenes from trifluoromethyl-toluene and benzene.252–255 Phenol and cresol ligands have undergone [2 + 2] cycloadditions, and electrophilic aromatic substitutions and addition reactions.256–258 Pyridine type ligands have undergone Diels-Alder reactions, Friedel-Crafts coupling to aromatic heterocycles, stereo and regio-selective nucleophilic additions, ring scissions, and [4 + 2] cycloadditions.259–263 Tetra and hexahydroindoles have been synthesized from the dearomatization of indoline.264,265 Other notable reactions include: pyrrole and furan cycloadditions, the dearomatization of thiophenes, and the ring opening of vinyl cyclopropanes.266–269 In 2006, Ha and coworkers reported the synthesis and reactivity of cationic molybdenum and tungsten bipyridine complexes.270 Nitrosylation of the [M(bpy)(MeIm)(CO)3] complexes formed the corresponding [M(bpy)(MeIm)(CO)2(NO)] hexafluorophosphate salts, Scheme 90. Synthesis of these complexes was achieved by combining the desired metal bipyridine complexes in methanol with potassium hexafluorophosphate and sodium nitrite followed by the addition of hydrochloric acid. Infrared spectroscopy of both metal complexes showed a clear nNO stretch at 1650 cm−1.

Scheme 90 Synthesis of [M(bpy)(MeIm)(CO)2(NO)]+.

6.12.3.5.3

Bimetallic complexes

García-Vivó, Ruiz and coworkers focused their work on bimetallic tungsten hydride complexes with the goal of understanding more about the activation of nitric oxides on metals. In 2014, the group reported the synthesis of several nitrosyl bimetallic tungsten complexes which formed from carbonyl displacement.271 Scheme 91 shows the general synthesis of two different ditungsten nitrosyl complexes from dicarbonyl and monocarbonyl starting complexes. Subjecting the dicarbonyl complex trans[W2Cp2(m-PPh2)2(CO)2] to an atmosphere of 5% NO in dinitrogen led to the formation of cis-[W2Cp2(m-PPh2)2(NO)2]. Reacting the related monocarbonyl tungsten complex under the same conditions gave a mixture of the trans-[W2Cp2(m-PPh2)2(NO)2] isomer and a thermally unstable nitrito complex. The dinitrosyl isomers were compared and characterized through IR spectroscopy wherein the trans isomer had NdO stretching frequencies at 1581 and 1522 cm−1 while the cis isomer had frequencies at 1625 and 1584 cm−1.

Scheme 91 Synthesis of cis- and trans-[W2Cp2(m-PPh2)2(CO)2] and [WCp(NO)(m-PPh2)2WCp(NO2)].

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

819

Following their previous work, the García-Vivó, Ruiz and coworkers reported the synthesis of the first anionic nitrosyl complex with a metal-metal double bond.272 Complex A, [W2Cp2(m-I)(m-PPh2)(NO)2], was prepared by first reacting [W2Cp2(m-H)(m-PPh2) (CO)2] with HBF4 followed by a reflux with sodium iodide in 1,2-dichloroethane. The dicarbonyl complex was then subjected to an atmosphere of 5% nitric oxide in dinitrogen followed by a reflux in toluene and subsequent reduction by sodium/mercury amalgam in acetonitrile, Scheme 92. Complex A exhibited a broad nNO band at 1463 cm−1. The anionic complex A was proven to be an effective nucleophilic metal species, most notably reacting with NH4PF6 to give the dinitrosyl hydride complex B which was later utilized in a variety of electrophilic reactions with various p-block molecules.273 Reactions of complex A with S8 or AuCl{P(p-tol)3} led to other dinuclear W products C and D in which Au, or SMe bridges the two tungsten centers.

Scheme 92 Synthesis and reactivity of anionic tungsten nitrosyl complex with a metal-metal double bond.

Hill and coworkers reported the synthesis of a m-carbido bridged heterobimetallic nitrosyl complex in 2020, Scheme 93.274 This complex was synthesized by first treating [NEt4][Tp W(CSe)(CO)2] (Tp ¼ hydrotris(dimethylpyrazolyl)borate) with N-methylN-nitroso-4-toluene-sulfonamide “Diazald” in acetonitrile. [Re(CO)3(Z5-C5H5)] was then photolyzed until full conversion to [Re(THF)(CO)2(Z5-C5H5)] was achieved. The rhenium complex was then added to a THF solution of [Tp W(CSe)(CO)(NO)] to afford [(Tp )(NO)(CO)W(m-C)Re(Z5-C5H5)(CO)2)] and the m-selenido complex [Re2(m-Se)(CO)4(Z5-C5H5)2].

Scheme 93 Synthesis of m-carbido bridged heterobimetallic complex [(Tp )(NO)(CO)W(m-C)Re(Z5-C5H5)(CO)2)] and [Re2(m-Se)(CO)4(Z5-C5H5)2].

820

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

6.12.3.5.4

Phosphine ligands

Following their work with the related molybdenum complexes, Berke and coworkers synthesized a variety of tungsten nitrosyl complexes containing novel phosphine-derived ligands. In 2007, the group reported the synthesis and ligand substitution reactions of several tungsten nitrosyl complexes with new 2-(dimethylphosphino)imidazole (dmpi) ligands.275 One example of these complexes was synthesized by combining W(NO)(CO)4(ClAlCl3) with dmpi resulting in the elimination of imidazole, and formation of W(Cl)(CO)(NO)(bdmpi)(dmpi) (bdmpi ¼ 1,2-bis(dimethyl-phosphino)imidazole), Scheme 94. The IR spectrum exhibits nCO and nNO bands at 1952 cm−1 and 1599 cm−1, respectively.

Scheme 94 Synthesis of W(Cl)(CO)(NO)(bdmpi)(dmpi).

In 2009, Berke and coworkers investigated the reactivity of the tungsten nitrosyl complex [W(NO)(CO)H(PMe3)3] with various protic compounds.276 This complex was reacted with a variety of alcohols in order to probe the overall hydridic reactivity and subsequent release of hydrogen gas. Scheme 95 outlines the general synthesis of the corresponding organyloxide complexes where R ¼ C6H5, 3,4,5-Me3C6H2, CF3CH2, C6H5CH2, Me, or iPr. Formation of each product was achieved by refluxing [W(NO)(CO)H (PMe3)3] with the desired alcohol in toluene. In a separate study, Berke also studied how a related complex with (2-aminoethyl) dimethylphosphine “edmp” ligands had the potential for proton-hydride bifunctionality on the amino and hydride groups of the complex.277

Scheme 95 Synthesis of [W(NO)(CO)(OR)(PMe3)3]; R ¼ C6H5, 3,4,5-Me3C6H2, CF3CH2, C6H5CH2, Me, or i-Pr.

In 2013, Berke and coworkers reported several new multidentate, small bite-angle diphosphinylborane ligands on tungsten nitrosyl complexes.278 The three ligands diphosphinylboranes utilized were (Ph2PCH2CH2B(C8H14), (PCH2PCH(PPh2) CH2B(C8H14), and (Ph2PCH2CH-[B(C8H14)]PPh2). The general ligand exchange synthesis of each complex was achieved by refluxing [W(NO)(CO)4(ClAlCl3)] with the desired phosphine ligands in THF. This reaction afforded the corresponding cis-dicarbonyl complexes shown in Scheme 96 which were each characterized by 31P and 11B NMR spectroscopy. IR measurements showed signals for the two carbonyl ligands and the single nitrosyl ligand of each complex. Further treatment with NaHBEt3 afforded the corresponding tungsten hydride products.

Scheme 96 Synthesis of tungsten nitrosyl complexes containing diphosphinylborane ligands.

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules 6.12.3.5.5

821

Tungsten imido complexes

While isolated tungsten imido complexes are rare, an example of an imido complex was published recently in 2011. Templeton and coworkers synthesized a stable cationic tungsten(IV) imido complex starting from [W(CO)(acac)(X)(PMe3)3] (X ¼ Cl, Br, I).279 Synthesis of the imido complex [W(NH)(acac)(PMe3)3][I] was accomplished by addition of sodium azide to an acetonitrile solution of [W(CO)(acac)I(PMe3)3] resulting in oxidation of tungsten, loss of N2, and proton addition (presumably from trace amount of water), Scheme 97. IR spectroscopy showed a clear N-H stretching band at 3161 cm−1 indicative of the imido moiety. The W-N bond length of 1.76 A˚ was determined by X-ray crystallography.

Scheme 97 Synthesis of [W(acac)(NH)(PMe3)3][I] by reaction of [W(CO)(acac)(I)(PMe3)3] with NaN3.

In 2007, Mizobe and coworkers reported several molybdenum(II) and tungsten(II) tetraphosphine complexes that were found to break the NdN bond in Me2NNH2 and PhNHNH2 at room temperature.280 Scheme 98 shows the general synthesis of some relevant examples of these complexes. Starting from [M(dppe)(k4-P4)] where P4 is (meso-o-C6H4(PPhCH2CH2PPh2)2), halide ligand substitution was achieved by refluxing the molybdenum and tungsten tetraphosphine complexes with two equivalents of PhCH2Cl or PhCH2Br in benzene. The molybdenum halide products A and B where then suspended in THF with 1,1-dimethylhydrazine to afford the corresponding trans-molybdenum nitride complexes. The tungsten bromide product C was also treated with 1,1-dimethylhydrazine and KPF6 to yield the trans-[WBr(NH)(k4-P4)]PF6 complex that was characterized by proton NMR spectroscopy and displayed an NdH resonance at 6.37 ppm.

Scheme 98 Synthesis of molybdenum(II) tetraphosphine nitride complexes and tungsten(II) tetraphosphine imido complexes.

6.12.4

Group 7 – Mn, Tc, and Re

6.12.4.1

Manganese dinitrogen complexes

Of all the group 7 metals, manganese reveals its reluctance to coordinate dinitrogen with only two crystal structures that have been described since 2005. Common features of these rare Mn-N2 complexes reveal a Mn(I) oxidation state, supported by anionic ancillary ligands with nitrogen, phosphorus, or hybrid nitrogen/phosphorus donors, Scheme 99. Arnold and coworkers prepared [(N2P2)Mn]2(m-Z1:Z1-N2) (N2P2 ¼ tBuN(−)SiMe2N(CH2CH2PiPr2)2) Scheme 99, reaction A, by reduction of [(N2P2)MnCl with sodium naphthalide at −40  C in THF.281 An NdN bond distance of 1.208(6) A˚ and nNN band at 1685 cm−1 in the Raman spectrum suggest activation of the dinitrogen ligand. The N2 ligand is more activated in [(N2P2)Mn]2(m-Z1:Z1-N2) than the other Mn-N2 complexes noted below. Roddick and coworkers synthesized the cyclopentadienyl complex CpMn(dfepe)(N2) (dfepe ¼ (C2F5)2PCH2CH2P(C2F5)2) by exposure of CpMn(dfepe)(H2) to an N2 atmosphere. CpMn(dfepe)(N2) complex B exhibits an nNN band at 2113 cm−1, but was not isolated due to formation of [CpMn(dfepe)]2(m-N2) upon solvent concentration. The Raman spectrum of [CpMn(dfepe)]2(m-N2) shows an N-N stretching frequency at 2015 cm−1.282 Theopold and coworkers utilized the (Tp) ligand hydrotris(3-tert-butyl-5-methylpyrazolyl)borate to prepare the dinuclear complex [TptBu,MeMn]2(m-Z1:Z1-N2), complex C by

822

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

(A)

(C) (B)

Scheme 99 Examples of dinitrogen complexes with manganese.

chemical reduction of the metal halide precursor. Although the crystal structure was reported, the synthesis was noted to be low-yielding and irreproducible.283 The crystal structure revealed the NdN bond distance of 1.196(5) A˚ . For comparison, the NdN bond lengths in [(Z5-C5H4CH3)Mn(CO)2]2(m-Z1:Z1-N2) and MnH(N2)(dmpe)2 are 1.118(7) and 1.127(7) A˚ , respectively.284,285

6.12.4.2 6.12.4.2.1

Manganese nitrosyl complexes Nitrogen donor ligands

In 2019, the synthesis of a five-coordinate manganese porphyrinate nitrosyl, [(F20TPP)Mn(NO)], was reported by Mondal and coworkers.286 The porphyrin ligand [(5,10,15,20-tetrakis(pentafluorophenyl)-porphyrin)], [F20TPPH2], was reacted with excess Mn(OAc)2 ∙4H2O, and NaCl in chloroform / acetic acid to afford [MnIII(F20TTP)Cl]. The manganese-nitrosyl complex [(F20TPP)Mn(NO)] was synthesized by reacting [MnIII(F20TTP)Cl] in CH2Cl2 with hydroxylamine in methanol at −20  C. While several hexacoordinated Mn(II) nitrosyl have been reported, the five-coordinate Mn species are less common. The X-ray structure revealed that N-O bond length of 1.132(10) A˚ and a Mn-N-O bond angle of 180 . Infrared spectroscopy revealed a nitrosyl stretching frequency at 1763 cm−1. In 2005, Mascharak and coworkers reported the reaction of NO with a (m-oxo)dimanganese(III) complex [(Mn(PaPy3))2 (m-O)](ClO4)2 to form [Mn(PaPy3)(NO)]ClO4 by reductive nitrosylation forming NO2 upon loss of the bridging O atom, Scheme 100.287 [Mn(PaPy3)(NO)]ClO4 was described as a {Mn-NO}6 species, that was previously characterized in a 2004 report. In addition, the authors noted the reaction of complexes such as [Mn(PaPy3)(OR)]ClO4 (R ¼ Me, Ph, Ac, Bz) also react with NO gas to generate [Mn(PaPy3)(NO)]ClO4.

Scheme 100 Synthesis of manganese nitrosyl complex bearing the PaPy3 ligand.

In 2008, Mascharak and coworkers reported a new pentadentate ligand N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-quinoline-2carboxamide (PaPy2QH, where H is dissociable proton) In this ligand, the pyridyl-carboxamide group of PaPy3H (shown above in Scheme 100) is replaced by a quinolyl-carboxamide moiety. The {Mn(NO)}6 complex [Mn(PaPy2Q)(NO)]ClO4 was prepared by two different pathways shown in Scheme 101.288 [Mn(NO)(PaPy2Q)]+ can be synthesized upon exposing [Mn(PaPy2Q)(OH)] ClO4 to NO gas via a reductive nitrosylation mechanism. 1H NMR spectroscopy confirmed that {Mn(NO)}6+ is diamagnetic, and the infrared spectrum displayed a strong NO stretch at 1725 cm−1. As an alternative synthetic route, exposure of a high-spin Mn(II) complex, proposed to be [Mn(PaPy2Q)(OH2)]ClO4, to NO gas also generated [Mn(NO)(PaPy2Q)]+; however due to the extremely oxygen sensitive nature of [Mn(PaPy2Q)(OH2)]ClO4 the former synthetic route was preferred.

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

823

Scheme 101 Synthesis of manganese nitrosyl complex {Mn(NO)}6 bearing the PaPy2Q ligand.

Shortly after the development of [Mn(PaPy2Q)(OH)]ClO4, Mascharak and coworkers prepared [Mn(SBPy3)(NO)](ClO4)2 and [Mn(SBPy2Q)(NO)](ClO4)2, where (SBPy3) ¼ N, bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-aldimine and (SBPy2Q) ¼ N, N-bis(pyridyl-methyl)amine-N-ethyl-2-quinoline-2-aldimine, Scheme 102.289 [Mn(SBPy3)(NO)](ClO4)2 was synthesized by adding a slight excess of AgClO4 to a solution of [Mn(SBPy3)(Cl)]ClO4 followed by addition of NO.287 [Mn(SBPy2Q)(NO)] (ClO4)2 was prepared by the reaction of NO gas with [Mn(SBPy2Q)(EtOH)](ClO4)2. The Mn centers for both complexes displayed distorted octahedral environments in which the NO was trans to the imine nitrogen atom. The infrared spectrum displayed strong band for the NO ligand at 1773 and 1759 cm−1, respectively.

Scheme 102 Synthesis of manganese nitrosyl complexes bearing the SBPy3 and SBPy2Q ligand.

Hitomi and coworkers prepared a series of manganese nitrosyl complexes with a pentadentate monoamido ligand of the formula [Mn(dpaqR)(NO)]ClO4, where dpaqR ¼ 2-[N,N-bis(pyridin-2-ylmethyl)]-amino-N0 -quinolin-8-yl-acetamido, and R ¼ OMe, H, Cl and NO2 at the 5-position of the quinoline moiety.290 Exposure of an acetonitrile solution of [Mn(dpaqR)] ClO4 to NO gas generated the corresponding Mn-NO products, Scheme 103. The CO and NO bands in the infrared spectrum increased in the range of 1602–1636 cm−1 and 1737–1744 cm−1, respectively, in the following order OMe < H < Cl < NO2 depending on the substituent on the 5-position of the quinoline moiety. The NO2 substituted derivative was unique among this series in that irradiation of with 650 nm light led to NO release that was 4-fold greater than the other derivatives.

824

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

Scheme 103 Synthesis of manganese nitrosyl complexes bearing the dpaqR ligand, R ¼ OMe, H, Cl, NO2.

6.12.4.2.2

B, C, P and S donor ligands

McGrath and coworkers prepared a neutral manganese nitrosyl complex, [1-OH-2,2-(CO)2-2-NO-closo-2,1,10-MnC2B7H8], with a carbollide ligand by introducing NOBF4 to the previously reported tricarbonyl complex [N(PPh3)2][1-OH-2,2,2-(CO)3-closo2,1,10-MnC2B7H8].291,292 The X-ray crystal structure of the Mn(NO) complex, Scheme 104, exhibited symmetry that was consistent with the 11B{1H} NMR spectroscopic data that showed four signals with a 1:2:2:2 ratio. A nNO band in the infrared spectrum was observed at 1812 cm−1. In the presence of Me3NO [1-OH-2,2-(CO)2–2-NO-closo-2,1,10-MnC2B7H8] undergoes CO substitution with PEt3 to afford [1-OH-2-CO-2-NO-2-PEt3-closo-2,1,10-MnC2B7H8] and loss of CO2. Upon ligand substitution, the nNO band in the infrared spectrum shifted to lower energy and was observed at 1761 cm−1.

Scheme 104 X-Ray crystal structure [1-OH-2,2-(CO)2-2-NO-closo-2,1,10-MnC2B7H8]. Reproduced with permission from Franken, A.; McGrath, T. D.; Stone, F. G. A. Organometallics 2009, 28, 225–235.

Pincer ligands offer multiple sites for structural and electronic modifications making them attractive for the use in transition metal catalysis and coordination chemistry. In 2018, Kirchner and coworkers reported the synthesis of the first Mn(I) PCP pincer complex, [Mn(PCPNEt-iPr(CO)3], (PCP ¼ 2-chloro-N1,N3-bis(diisopropylphosphino)-N1,N3-diethylbenzene-1,3-diamine) that undergoes ligand substitution in presence of a stoichiometric amount of NOBF4, to afford trans-[Mn(PCPNEt-iPr) (CO)2(NO)]+.293 The IR spectrum exhibited strong absorption bands at 2017, 1913, and 1767 cm−1, assigned to the symmetric and asymmetric CO and NO frequencies, respectively. Boncella and coworkers utilized pincer scaffolds to prepare a series of Mn-nitrite complexes with the goal of oxygenating a variety of organic substrates such as 1-hexene or 1,3-cyclooctadiene.294 Reactions of (iPrPONOP)Mn(CO)2Br and (iPrPNHP)Mn(CO)2Br (iPrPONOP ¼ 2,6-bis(diisopropylphosphinito)pyridine; iPrPNHP ¼ HN{CH2CH2(PiPr2)}2 with AgNO2 furnished the nitro compounds (iPrPONOP)Mn(CO)2(NO2) and (iPrPNHP)Mn(CO)2(NO2), respectively. Unfortunately, these products were ineffective for the desired oxygen atom transfer reactions noted above.

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

825

The mononuclear {Mn(NO)}5 complexes [(NO)Mn(S,S-C6H3-R)2]− (R ¼ H or Me) containing redox non-innocent 1,2-benzenedithiolate ligands were synthesized by Liaw and coworkers.295 Addition of a methanol solution containing 2 equiv. [Na]2[S,S-C6H3-R] and MnBr2 to a THF solution of [PPN][Cl] (PPN ¼ Bis(triphenylphosphine)iminium) afforded the precursor anionic manganese thiolate complexes [(THF)Mn(S,S-C6H3-R)2]− (R ¼ H or Me). Treatment of [(THF)Mn(S,S-C6H3-R)2]− (R ¼ H or Me) with NO gas in THF formed desired Mn-nitrosyl products. The IR spectra displayed an NO stretching frequency at 1735 (in THF) and 1729 cm−1 (KBr) that was nearly identical for both products. X-ray diffraction data reveal that [(NO)Mn(S,S-C6H3-H)2]– is best described as a resonance hybrid of [(L)(L)MnIII(NO•)]− and [(L)(L•)MnIII(NO−)]−.

6.12.4.3

Technetium dinitrogen complexes

Dinitrogen complexes of technetium are extremely rare given that technetium is only produced in nuclear reactors and does not occur in nature. Alberto and coworkers recently prepared two examples of 99Tc-dinitrogen complexes supported by pincer ligands. Reactions of 99TcCl3(PPh3)2(NCCH3)] with the pincer ligands HPNPtBu and PyrPNPtBu (and base for HPNPtBu), generated the corresponding products [99TcIII(PNPtBu)Cl2] and [Tc(PyrPNPtBu)Cl3], respectively. The dinitrogen-containing products were generated by chemical reduction of [99TcIII(PNPtBu)Cl2] and [Tc(PyrPNPtBu)Cl3] with two equiv. of CoCp 2 in THF affording the 99 I Tc bis-dinitrogen complex [Tc(PNPtBu)(N2)2] and the dinuclear complex [99Tc(PyrPNPtBu)Cl(N2)]2(m-N2), Scheme 105. The IR spectrum of [Tc(PNPtBu)(N2)2] contains two nNN bands at 1960 and 2043 cm−1 for the symmetrical and asymmetrical stretching modes. The X-ray structure exhibits N-N bond lengths of 1.096(5) and 1.137(5) A˚ . For [99Tc(PyrPNPtBu)Cl(N2)]2(m-N2) the infrared spectrum displayed a stretching vibration at 1974 cm−1, assigned to the terminal N2 ligands. Due to disorder of the Cl and N2 ligands the N2 triple bond lengths could not be determined accurately.

Scheme 105 Synthesis of [99Tc(PNPtBu)(N2)2] and [99Tc(PyrPNPtBu)Cl(N2)]2(m-N2).

6.12.4.4

Technetium nitrosyl complexes

Technetium nitrosyl complexes have been used for the development of radiopharmaceuticals.296 Nicholson and coworkers developed several novel Tc(NO) complexes. Previous syntheses of common technetium-nitrosyl synthons such as [TcCl3(NO) (PPh3)2] are prepared from precursors such as (Bu4N)[Tc(NO)Cl4] and required a full day to synthesize and isolate. A faster preparation to generate Tc(NO) complexes could then be utilized with the short-lived (6 h) isotope 99mTc that is used in radiopharmaceutical applications. Nicholson designed a synthetic pathway to prepare a Tc(NO) intermediate that can be reacted with a variety of ligands to generate nitrosyl compounds in multiple oxidation states. Ammonium pertechnetate (NH4)[TcO4] was refluxed in methanol with a fivefold excess of hydroxylamine hydrochloride. The addition of a six-fold excess of dppe (1,2-diphenylphosphinoethane) generated [TcCl(NO)(dppe)2]+.297 The IR spectrum of the cationic complex showed an nNO band at 1728 cm−1. Continuing with their Tc(NO) research, synthesis of the mer and fac isomers of [TcICl2(NO)(PNPpr)] (PNPpr ¼ bis[(2-diphenylphosphino)propyl]amine) was explored.298 For the synthesis of fac isomer, H[TcNOCl4] and PNPpr were refluxed in methanol. Slow evaporation resulted in a peach precipitate, and IR spectroscopy of the solid indicated only the presence of the fac isomer, nNO band at 1789 cm−1. Stirring H[TcNOCl4] and PNPpr in acetonitrile at room temperature produced solely the meridional isomer, nNO band at 1689 cm−1. The large difference in nitrosyl ligand absorptions in the IR spectrum was attributed to the trans effect. The X-ray structure of the facial complex reveals an octahedral geometry with a TcdNNO and NdO bond distance of 1.751(5) and 1.163(6) A˚ , respectively. The TcdNdO angle 177.9(6) is consistent with linear NO binding. The Tc-NNO bond length of the meridional complex was found to be 1.758(4) A˚ .

826

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

Using the precursor (Bu4N)[TcNOCl4] in reactions with polydentate ligands led to a mixture of TcI(NO) products. In order to form a single product, Nicholson implemented stoichiometric amounts of 2-mercaptopyridine (HSpy) and proton scavengers in the reaction with [TcCl2(NO)(HOMe)(PPh3)2] to cleanly generate [Tc(NO)Cl(PPh3)2(Spy)] and [Tc(NO)Cl(PPh3)(Spy)2].299 For example, a two-fold excess of diisopropylethylamine was added to a solution of [Tc(NO)Cl2(HOMe)(PPh3)2] and 2-mercaptopyridine yielding [Tc(NO)Cl(Spy)(PPh3)2], while [Tc(NO)(Spy)2(PPh3)] was synthesized by refluxing [Tc(NO)Cl (Spy)(PPh3)2] with two equivalents of 2-mercatopyridine and diisopropylethylamine. The crystal structures show the trans positioning of the linear NO in relation to the mercaptopyrindine nitrogen, Scheme 106. [Tc(NO)(Spy)2(PPh3)] and [Tc(NO)Cl(PPh3)2(Spy)] exhhibi nNO bands at 1677 cm−1 and 1696 cm−1, respectively. The ligand series with Tc(II) and Tc(I) nitrosyl complexes was expanded to include di-(2-picolyl)(N-ethyl)amine, 2,5-bis-(2-pyridylmethyl)-2,5-diazohexane and 1,4-bis(2-pyridylmethyl)-1,4-diazobutane.296 Other recent Tc research by Abram and coworkers explored the use of 2-(diphenylphosphino)aniline) ligands with (NBu4)[Tc(NO)Cl4(MeOH)] in the formation of three isomeric technetium(I) complexes.300

Scheme 106 The X-ray crystal structures of [Tc(NO)CI(PPh3)2(Spy)] (left) and [Tc(NO)Cl(Spy)(PPh3)2] (right). Reproduced with permission from Nicholson, T. L.; Mahmood, A.; Muller, P.; Davison, A.; Storm-Blanchard, S.; Jones, A. G. Inorg. Chim. Acta 2011, 365, 484–486.

While Tc nitrosyl complexes are significantly understudied due to the radioactive nature of technetium and the instability of Tc precursors, Abram and coworkers presented a simple synthesis for the formation of [Tc(NO)(Cp)(PPh3)X] (X ¼ Cl, Br).301 In a refluxing solution of toluene, [Tc(NO)X2(PPh3)2(CH3CN)] reacted with an excess of KCp to yield [Tc(NO)(Cp)(PPh3)X] (X ¼ Cl, Br).302,303 X-ray crystallography confirmed the pseudotetrahedral geometry of the Tc(I) complexes with a linear nitrosyl ligand. Utilizing these TcI(NO) complexes as precursors, Abram applied ligand exchange conditions to expand the synthesis and characterization of stable of [Tc(NO)(Cp)(PPh3)X0 ] complexes, X0 ¼ I−, I−3, SCN−, CF3SO−3, or CF3COO−.304 All of complexes were synthesized in a similar manner in which a stoichiometric amount of the incoming ligand was adding to a solution of [Tc(NO)(Cp) (PPh3)Cl] in methylene chloride. Technetium complexes with fluorido ligands are uncommon due to their insolubility in organic solvents, as well as the inertness of the ligands. Abram and coworkers synthesized the paramagnetic salt complex of [TcII(NO)F5]2− by adding acetohydroxamic acid to NH4(TcO4) in aqueous HF.305,306 Bands at 644 and 607 cm−1 were assigned to the TcN stretching vibration and bending modes respectively, consistent with values found in similar Ru complexes.307 With the solubility of [TcIII(NO)F5]2− and similar anionic Tc complexes relatively low, Balasekaran, Abram and coworkers improved their previous work by preparing [Tc(NO)(CF3COO)4F]2 − via of Cs2[TcII(NO)F5] in trifluoroacetic acid followed by the addition of (NBu4)F.308 The precipitate is soluble in polar solvents which has expanded the versatility of the metal core by allowing for organic ligand exchange to produce novel technetium(I) complexes Cs[Tc(NO)(PPh3)2(CF3COO)2F], [Tc(NO)(dppe)2(OOCCF3)](PF6) and [Tc(NO)(kN,P-pyPPh2) (kP-pyPPh2)(CF3COO)2]. The well resolved EPR spectra confirm the d5 low-spin system with a 10-line hyperfine pattern due to the interaction of the unpaired electron with the nuclear spin of 99Tc (I ¼ 9/2).

6.12.4.5

Rhenium dinitrogen complexes

The number of rhenium dinitrogen complexes have been increasing in recent years. In 2006, Kirillov and coworkers reported a picolinate-N2 complex, the first with a carboxylate or N,O-ligands.309 This rhenium(I) complex was synthesized by refluxing [ReCl(N2)(CO)2(PPh3)2] with an excess of picolinic acid (Hpic) in MeOH/C6H6 The resulting picolinate-N2 complex had

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

827

trans-triphenylphosphines, an anionic ligand trans to the coordinated dinitrogen, IR nNN stretch of 2040 cm−1, and N-N bond length of 1.169 A˚ . A Re(I) and Mo(IV) heterobimetallic complexes, [(PhMe2P)4ClRe(m-Z1:Z1-N2)Mo(S2CNEt2)3]OTf, was synthesized by Brown, et al by combining (PhMe2P)4Re(N2)Cl with [Mo2(S2CNEt2)6](OTf )2 in acetone.310 The bridging dinitrogen nski and Pombeiro311 ligand exhibited an IR nNN stretch of 1818 cm−1 and an N-N bond length of 1.167 A˚ . In 2008, Smole synthesized a water-soluble rhenium-dinitrogen complex by adding 1,3,5-triaza-7-phosphaadamantane (PTA) to a suspension of [ReCl2(Z2-NNCOPh)(PPh3)2] in methanol. The reaction was refluxed under dinitrogen and in the dark since the resulting trans-[ReCl(N2)(PTA)4 complex undergoes light induced N2 loss. trans-[ReCl(N2)(PTA)4] exhibits an IR nNN stretch of 1949 cm−1. Arnold and coworkers observed that ion-pairing could be utilized to stabilize N2 coordination where the reaction of Na[Re(Z5-Cp)(BDI)] (BDI ¼ bis(2,6-diisopropylphenyl)-3,5-dimethyl-b-diketiminate) with excess Me3SiCl afforded the silyldiazenide product Re(Z5-Cp)(BDI)(N]N-SiMe3) with an IR nNN stretch of 1617 cm−1 and an NdN bond length of 1.251 A˚ . Similarly, addition of 0.5 equiv. of Cp2ZrCl2 to Na[Re(Z5-Cp)(BDI)] led to the formation of a heterotrimetallic complex with two [(N2)Re(Z5-Cp)(BDI)] units bridged through the N2 ligands by Cp2Zr.312 The N–N bond length was 1.208 A˚ with IR nNN stretch of 1667 cm−1. In the analogous CpReII(15N2)(BDI), N2 coordination was only observable in frozen toluene and was characterized by EPR spectroscopy.313 Schneider and coworkers utilized the varying steric bulk of PNP ligands to stabilize dinuclear [(m-Z1:Z1-N2)Re complexes with different geometries that would readily undergo N-N bond cleavage to form terminal nitrides, Scheme 107.314–317 [(m-Z1:Z1-N2) (ReCl2(PNPtBu))2], PNPtBu ¼ N(CH2CH2PtBu2)2 exhibits square pyramidal geometry at each Re where the bridging N2 is in the apical position. The observed N–N bond length was 1.202 A˚ . Synthesis involved Na/Hg reduction of [ReCl2(PNPtBu)] in THF at −30  C. Octahedrally coordinated Re is observed for [(m-Z1:Z1-N2)(ReCl2(HPNPiPr))2], HPNPiPr ¼ HN(CH2CH2PiPr2)2, with the bridging dinitrogen. Synthesis began by suspending [ReCl3(PPh3)2(NCMe)] and HPNPiPr in toluene followed by heating to 90  C for 16 h to obtain [ReCl3(HPNPiPr)] after isolation. This complex underwent chemical reduction with 1 equiv. CoCp 2 to yield [(m-Z1:Z1-N2)(ReCl2(HPNPiPr))2]. The observed NdN bond length was 1.169 A˚ . Raman spectroscopy of [(m-Z1:Z1-N2) (ReCl2(HPNPiPr))2] gave an NdN stretch of 1733 cm−1 (1675 cm−1 for 15N2).

Scheme 107 Synthesis of an N2-bridged rhenium PNP complex and dinitrogen cleavage forming a terminal Re-nitride complex.

Miller and coworkers recently synthesized [(PONOP)ReCl2]2(m-N2) (PONOP ¼ 2,6-bis-(diisopropylphosphinito)pyridine), as shown in Scheme 108 by treatment of [(PONOP)ReCl3] with LiHBEt3, or in a two-step reaction using CoCp 2 followed by NaBArF4.318 The chloride ligands were trans at each metal center and Raman spectroscopy revealed a nNN stretching frequency of 1776 cm−1. Upon heating, the chloride ligands can convert to a mixture of orientations with varying ratios; i.e. the trans, trans-; trans, cis-; and cis, cis-isomeric complexes. A broad Raman resonance at 1749 cm−1 was observed for a mixture of the cis and trans isomers. Exposure of [(PONOP)ReCl2]2(m-N2) to 405 nm blue light resulted in N2 bond cleavage generating the terminal Re-nitride product cis-(PONOP)ReV(N)Cl2.

Scheme 108 Synthesis of [(PONOP)ReCl2]2(m-N2).

828

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

Using the PNP ¼ 2,6-bis(di-tert-butylphosphinomethyl)pyridine pincer ligand, Yoshizawa, Nishibayashi and coworkers prepared a dinitrogen-bridged dirhenium(I) complex [ReCl(N2)(PNP)]2(m-N2) by treatment of [ReCl3(PNP)] with 2 equiv. of KHBEt3.319 The IR spectrum of [ReCl(N2)(PNP)]2(m-N2) displays two strong nNN bands at 1888 and 1949 cm−1 and a weak nNN band at 1993 cm−1. The observation of three absorptions was attributed to conformers of [ReCl(N2)(PNP)]2(m-N2) in the crystals. This Re-dinitrogen complex was shown to catalyze N2 reduction to NH3 as well as the catalytic silylation of N2 to tris(trimethylsilyl) amine. Recently, Holland and coworkers reported the first example of an N2-bridged Re(III) complex [(trans-PtBu 2 Pyrr)ReCl2] (m-Z1:Z1-N2) (P2 tBuPyrr ¼ [2,5-(CH2PtBu2)2C4H2N]−).320 Using SQUID magnetometry a singlet ground state was assigned due to 1 1 antiferromagnetic coupling of two Re (S ¼ 1) centers. Chemical reduction and mild heating of [(trans-PtBu 2 Pyrr)ReCl2](m-Z :Z -N2) led to N2 bond cleavage and the formation of a terminal Re-nitride product.

6.12.4.6 6.12.4.6.1

Rhenium nitrosyl complexes Nitrogen and carbon donor ligands

Rhenium(II) has been noted as a less studied oxidation state of Re, and due to the soft nature of Re(II), stable complexes frequently contain s-donor or p-accepting ligands such as CN−, NO, or CO. Kremer et al. synthesized a series of Re(II) complexes of the general formula NBu4[Re(NO)Br4(L)] with a variety of ancillary ligands (L ¼ pyridine, pyrazine, pyrimidine, pyridazine].321 The synthesis of NBu4[Re(NO)Br4(L)] was achieved by adding potassium hexabromorhenate (IV) to a solution of tetra-n-butylammonium bromide in ethanol. After the addition of HBr, NO gas was bubbled through the refluxing solution to afford NBu4[Re(NO) Br4(EtOH)]. Adding an excess of L to a stirring solution of NBu4[Re(NO)Br4(EtOH)] afforded the target complexes NBu4[Re(NO) Br4(L)] with the monodentate amine ligands (L) coordinated trans to the nitrosyl ligand. The IR spectra of the NBu4[Re(NO)Br4(L)] complexes exhibit a strong absorption for the nitrosyl ligand at 1750 cm−1. Kremer and coworkers continued their studies of various Re(II)-NO complexes including the formation of NBu4[Re(NO)Br4(Hnic)] and the heterodinuclear Re(II)-M(II) complexes containing 3-pyridinecarboxylate bridge: [Re(NO)Br4(m-nic)Ni(dmphen)2], [Re(NO)Br4(m-nic)Co(dmphen)2], [Re(NO) Br4(m-nic)Mn(dmphen)(H2O)2], [Re(NO)Br4(m-nic)Cu(bipy)2], [Re(NO)Br4(m-nic)Cu(dmphen)2], (Hnic ¼ 3-pyridinecarboxylic acid, dmphen ¼ 2,9-dimethyl- 1,10-phenanthroline, bipy ¼ 2,20 -bipyridine).322,323 Nicholson and coworkers prepared the [ReBr2(NO)(NCMe)3] precursor (discussed below) in the synthesis of a dicationic complex [Re(NO)(N5)][B(C6H5)4]2 (N5 ¼ N1,N1,N2-tris(2-pyridinylmethyl)-1,2-ethanediamine) containing a pentadentate polyamine ligand which exhibits an nNO absorption of 1714 cm−1.324 Schenk and Dilsky published a series of diastereomeric half-sandwich rhenium nitrosyl complexes bearing hemilabile phosphine ligands of the general formula [CpRe(NO)(CO){P(Ph)(R)(R0 )}]BF4 (R ¼ Me, Ph; R0 ¼ 2-C6H4OMe, CH2C4H3S, CH2C4H7O) by refluxing CpRe(NO)(CO)(CH3CN)]BF4 with the desired phosphine ligand in 2-butanone. Treatment of the product with NaBH4 formed the a diastereomeric rhenium methyl complexes, for example, [CpRe(NO){P(Me)(Ph) (2-C6H4OMe)}(CH3)].325 In a separate study, Schenk and coworkers prepared CpRe(NO)(CH3){P(Me)(Ph)(2-C6H4NMe2)} and successfully separated the diastereomers of this complex with the addition of half an equivalent of methanesulfonic acid, Scheme 109.326 The reaction yielded one diastereomer that proceeded through elimination of methane and ring closure, while the other remained unreacted. This method can be applied to other structurally similar complexes in separating diastereomeric product mixtures.

Scheme 109 Diastereoselective acid cleavage of methyl rhenium nitrosyl complex.

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

829

Gladysz and coworkers synthesized chiral piano-stool Re complexes containing a variety of amine ligands. For example, the reaction of a racemic chiral methyl complex (Z5-C5H5)Re(NO)(PPh3)(CH3) with CF3SO3H, followed by the addition of NH2CH2C6H5 produced [(Z5-C5H5)Re(NO)(PPh3)(NH2CH2C6H5)][CF3SO3].327 Deprotonation with tBuOK afforded the amido complex (Z5-C5H5)Re(NO)(PPh3)(NHCH2C6H5). Gladysz and coworkers adapted previous synthetic strategies to produce the racemic and enantiomerically pure (Z5-C5H4X)Re(NO)(PPh3)(CH2PPh2) (X ¼ halide, or H) that contain phosphorus donor atoms in an effort to prepare Re palladacycles.328 Reacting (Z5-C5H4Br)Re(NO)(PPh3)(CH2PPh2) with 1 equiv. of Pd[P(tBu)3]2 in toluene at 80  C afforded a dimeric Re palladacycle product that was poorly soluble in organic solvents. Treatment of the dimeric Re palladacycle with PPh3 generated a monomeric palladacycle displaying a nNO band at 1629 cm−1, Scheme 110.

Scheme 110 Synthesis of rhenium nitrosyl palladacycle complexes.

The cage structures of carborane ligands possess electronic and steric properties that have rendered them suitable for the use in radiopharmaceuticals. The synthesis of a series of [Re(CO)2(NO)RR0 C2B9H9)] complexes (R ¼ H, Bn; R0 ¼ H, Ph, Bn) was reported by Valliant et al.329 Previously the research group reported the synthesis of the tricarbonyl rhenacarboranes precursors, Scheme 111.330 All of the rhenacarborane nitrosyl derivatives were synthesized in a similar manner by treating the respective tricarbonyl precursor with (3:1) CH3CN/H2O, aqueous NaNO2 and H2SO4 at room temperature, Scheme 111. The isolated products each displayed strong nNO absorptions at 1773 cm−1 and two n(CO) stretching frequencies at 2082 and 2020 cm−1. Similarly, Jelliss and coworkers used tricarbonyl rhenacarboranes precursors to prepare products with the ReI(CO)2NO fragment.331 The CO/NO ligand exchange occurred by adding [NO][BF4] to a cooled solution of the tricarbonyl precursor in THF. The isolated carborane Re nitrosyl complex displayed one nNO band at 1780 cm−1.

Scheme 111 Syntheses of functionalized carborane rhenium complexes with the [Re(CO)2(NO)] fragment.

Synthesis of a mono- and dinitrosyl rhenium(II) complexes [Re(NO)2(CN)4]
(Phen)2
2H2O and PhenH[Re(NO)(CN)4(H2O)]
(Phen)
3H2O (Phen ¼ 1,10-phenanthroline) were prepared by Mukherjea and coworkers by stirring KReO4, KCN and KOH in hot water with the gradual addition of NH2OH.332 After acidification, the product was added to a hot solution of 1,10-phenanthroline and water, resulting in the [Re(NO)2(CN)4]
(Phen)2
2H2O solid. This dinitrosyl complex displayed two nNO bands at 1720 and 1700 cm−1. A mononitrosyl complex was synthesized using the same process. In the final step of the reaction, the filtrate from the dinitrosyl complex was treated with Na2SO4 to yield PhenH[Re(NO)(CN)4(H2O)]
(Phen)
3H2O, Scheme 112, which exhibited a nNO band at 1736 cm−1. Both Re(II) complexes are paramagnetic, and EPR spectra revealed a triplet in the vicinity of g ? 2.28 due to super hyperfine coupling with 14N.

830

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

Scheme 112 Synthesis of mono- and dinitrosyl rhenium(II) complexes.

6.12.4.6.2

Phosphine donors

Originally published by Giusto and coworkers, Berke modified the synthesis of the [NMe4]2[ReCl5(NO)] precursor used in their phosphine ligand exchange chemistry.333,334 At reduced temperature, hydrogen peroxide was added dropwise to Re powder, followed by the addition of [NMe4]Cl. Another equivalent of [NMe4]Cl was added to the isolated solid, and the mixture was dissolved in HCl and H3PO2. At an elevated temperature of 110  C, NO was bubbled through the solution. The resulting [NMe4]2[ReCl5(NO)] displayed an IR stretching frequency for NO at 1715 cm−1. Treatment of [NMe4]2[ReCl5(NO)] with Zn in acetonitrile formed mer-[ReCl2(NO)(CH3CN)3] displaying a nNO absorption of 1697 cm−1. mer-[ReCl2(NO)(CH3CN)3] was used as a precursor for ligand exchange chemistry with monodentate phosphines forming complexes of general formula [trans-ReCl2(NO) (PR3)2(CH3CN)] (R ¼ Cy, iPr, p-tolyl) with nNO bands at 1672 cm−1, 1668 cm−1, 1688 cm−1, respectively. Reaction of [transReCl2(NO)(PR3)2(CH3CN)] (R ¼ Cy or iPr) with 1 bar H2 generated [ReCl2(NO)(PR3)2(Z2-H2)]. Ligands with large bite angles have shown to be crucial for tuning a variety of catalytic reactions.335,336 Large bite angles can be achieved through the coordination of various bidentate phosphine ligands, Scheme 113. These large angles can result in a distorted octahedral geometry allowing for activation at the metal center.335 Berke and coworkers employed [ReIBr2(MeCN)3(NO)] and [NEt4]2[ReIIBr5(NO)]337,338 in ligand substitution reactions with the large bite angle phosphines.339 In MeCN or MeCN/THF, [NEt4]2[ReBr5(NO)] was treated with the respective diphosphine at elevated temperatures, Scheme 113. Similarly, Machura et al. refluxed [Re(NO)Cl3(PPh3)(OPPh3)] in the presence of PPh2py-P,N (PPh2py-P,N ¼ diphenyl(2-pyridyl)phosphine) in chloroform to form [Re(NO)Cl2(PPh3)(PPh2py-P,N)].340 Berke and coworkers used a similar approach in the synthesis of water soluble phosphine rhenium complexes by first transforming [ReIBr2(MeCN)3(NO)] into mer-[ReBr(BF4)(CH3CN)3(NO)] using AgBF4 in acetonitrile, followed by ligand exchange with an excess of the bulky 1,3,5-triaza-7-phosphadamantane (PTA) ligand to afford [ReBr2(NO)(PTA)3] that was confirmed via X-ray crystallography.341

Scheme 113 Synthesis of rhenium nitrosyl complexes containing diphosphine ligands with large bite angles.

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

831

Berke developed a series of diiodo Re(I) complexes [ReI2(NO)(PR3)2(L)] (L ¼ H2O or H2; R ¼ iPr or Cy) which were used in conjunction with hydrosilane/B(C6F5)3 reagents to enhance catalytic hydrogenation of alkenes.342,343 The Re(I) complexes were synthesized through two different pathways: the first being a one-pot reaction starting from rhenium metal and the other proceeding through a Finkelstein bromide/iodide ligand exchange. Pathway A depicts the treatment of Re metal with H2O2, which is then reduced with H3PO2/NO in the presence of NEt4I/HI. The resultant Re(II) complex is reacted with PCy3 affording an octahedral Re(I) with the H2O ligand trans to the nitrosyl, Scheme 114. The low NO IR band at 1664 cm−1 was due to the electron-rich nature of the rhenium metal center. A second synthetic pathway B was employed to improve the yield. Treatment of (NEt4)2[ReBr5(NO)] with an excess of NaI in THF, followed by the addition of excess PCy3 affords trans-[ReI2(NO)(H2O)(PCy3)2].

Scheme 114 Synthesis of diiodo Re(I) complexes [ReI2(NO)(PR3)2(H2O)] (R ¼ iPr or Cy).

The diiodo Re(I) complexes shown in Scheme 115 were central to the discovery that rhenium’s reactivity can be influenced by the “catalytic nitrosyl effect.” The effect, originating from Berke et al., refers to rhenium’s ability to p-back donate to NO in the presence of a Lewis acid resulting in the bending of the nitrosyl unit, Scheme 115.342 The change in the orientation of coordinated NO, results in a coordination site vacancy for alkene binding to take place.344 Alkene hydrogenations have also been demonstrated with Lewis acid promoted dinitrosyl rhenium hydride catalysts, i.e. ReH(PR3)2(NO)(NO(LA))][Z] (LA ¼ B(C6F5)3; [Et]+, Z ¼ [B(C6F5)4]−; LA ¼ [SiEt3]+, Z ¼ [HB(C6F5)3]−; R ¼ iPr, Cy).345

Scheme 115 “Bent” nitrosyl ligand upon invoked as contributing to the “catalytic nitrosyl effect.”

6.12.4.7

Rhenium nitrato/nitrito complexes

The ambidentate NO−2 ligand has the ability to bind to metal center via the nitrogen or one of the oxygen atoms, resulting in linkage isomers. Fagalde et. al first reported the syntheses of both [Re(bpy)(CO3)(Z1-NO2)] and [Re(dmb)(CO3)(Z1-ONO)] in 2014.346 Two years later, Kia and Safari broadened the diversity of complexes with the synthesis of [Re(Me2-phen)(CO3)(Z1-ONO)].347 [Re(bpy)(CO3)(Z1-NO2] was synthesized by suspending [Re(bpy)(CO3)(CF3SO3)] and a 12-fold excess of NaNO2 in water. While under an argon atmosphere, the mixture was refluxed in the dark. In 2018, Hatcher further developed this variety of complexes by reporting the synthesis and characterization of the [Re(OMe2-bpy)(CO3)(Z1-NO2)] complex, as well as the photoexcited nitro(
1-ONO) complex.348 In the late 90s, Berke and coworkers reported the syntheses of dinitrosyl phosphine rhenium cations.349,350 Berke treated theses Re(NO)+ with phenyldiazomethane in benzene resulting in the stable benzylidene complexes [Re(]CHPh)(NO)2(PR3)2]+.351 The N-Re-N bond angle of greater than 140 is comparative to the angles of slightly distorted square-pyramidal geometry. The lengthened Re-N bonds, 1.863(4) and 1.819(4) A˚ , indicate poor electron acceptance of the nitrosyls leading to a more activated metal center. These findings led to more advanced studies in the catalytic nature of the cationic Re(NO) carbene complexes. One such example is exposing the dinitrosyl rhenium carbene to air and forming the Z2-nitrito and nitrato species through a complex sequence of reactions, Scheme 116.352 Continuing on previous work done by Casey and coworkers, the transformation of alkynes to carbenes facilitated by phosphine Re(NO) complexes was demonstrated.353,354 The rhenium promoted hydrogenation is not limited to alkynes. Berke and coworkers reported the first synthetic Shvo-type rhenium [Re(H)(NO)(PR3)(C5H4OH)] complex.355 The [Re(NO)(H)(Br)(PR3)2] precursors were synthesized previously using a modified procedure from Gusev.337,356 At room temperature in THF, the rhenium precursors were treated with lithiated (tert-butyldimethylsiloxy)cyclopentadienyl salt. Upon completion of the ligand exchange, the SidO bond of the tert-butyldimethylsiloxyl group was cleaved with tetrabutylammonium fluoride (TBAF). The resulting anionic complexes were protonated with the addition of ammonium bromide. X-ray crystallographic analysis confirmed the structure, and IR spectroscopy showed a nNO band at 1607 cm−1.

832

Complexes of Groups 5–7 with N2, NO, and Other N-Containing Small Molecules

Scheme 116 Reaction of [Re(]CHPh)(NO)2(PiPr3)2]+ with O2 affording nitrito and nitrato rhenium carbene products.

6.12.5

Conclusions and outlook

As demonstrated by the diversity of transition metal complexes presented this chapter, over the last 15 years a substantial amount of attention has been dedicated toward the synthesis, characterization, and ultimately an understanding of the reactivity patterns of transition metal complexes containing small molecules such as N2 NO, NH3 and N2O. New discoveries using “classic” ancillary ligands such as phosphines, pincers, and Tp, have emerged in recent years and initiated the evolution of new innovative ligand frameworks that support transition metals with novel geometric, steric, and electronic properties. Many of these discoveries, exemplified in the group 5–7 metals have been highlighted here. For example, Mo and W complexes have traditionally led the arena in M-N2 chemistry. However, in the last decade a handful of dinitrogen complexes of the first-row congener, Cr, are beginning to backfill a void of nearly 40 years of group 6 coordination complexes where Cr contributed only a minor role.56 Moreover, transition metals such as Re have also seen a flurry of activity where the use of phosphines and pincer ligands led to the synthesis of new Re-NO and Re-N2 compounds. Other expanses ripe for advancement exist in the group 7 metals; very few Mn-N2 complexes have been prepared, denoting an area where innovative ligand platforms with Mn could forge new directions in the area of N2 coordination chemistry and catalysis. The synthesis of new transition metal N2 complexes will likely lead to reports conveying novel synthetic routes to functionalize the inert N2 molecule, and may lead to breakthroughs where the direct formation of valuable nitrogen-containing organic products derived directly from N2 can be achieved.357,358 Within the context of energy science, some of the M-NH3 compounds highlighted herein could be central in understanding the reaction chemistry for the oxidation of ammonia to dinitrogen, a lesser examined reaction mediated by molecular complexes than N2 reduction, but a reaction that has seen a surge in interest in the last five years due to the interest examining carbon-free energy vectors.359–367

Acknowledgments O.D., R.O., C.B. M.T.M. and the Mock Laboratory at Montana State University are supported by the National Science Foundation under Grant No. 1956161.

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