Comprehensive Organometallic Chemistry IV. Volume 4: Groups 3 to 4 and the f elements - Part 2 [4] 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 4

GROUPS 3 TO 4 AND THE f ELEMENTS - PART 2 VOLUME EDITORS

DAVID P. MILLS STEPHEN T. LIDDLE Department of Chemistry, The University of Manchester, Manchester, United Kingdom

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 4 Editor Biographies

vii

Contributors to Volume 4

xiii

Preface 4.01

xv

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides

1

Dorothea Schädle and Reiner Anwander

4.02

Buta- and Penta-Dienyl Complexes of the Actinides

29

Joy H Farnaby, Tajrian Chowdhury, Samuel J Horsewill, and Bradley Wilson

4.03

Buta- and Penta-Dienyl Complexes of the Group 4 Metals

82

Dennis M Seth, Evan A Beretta, and Rory Waterman

4.04

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

98

Florian Benner, Francis Delano IV, Elizabeth R Pugliese, and Selvan Demir

4.05

Cyclopentadienyl and phospholyl compounds in organometallic actinide chemistry

185

Alexander J Gremillion and Justin R Walensky

4.06

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

248

Alexander FR Kilpatrick and F Mark Chadwick

4.07

Arene Complexes of the Group 3 Metals and Lanthanides

405

F Geoffrey N Cloke and Nikolaos Tsoureas

4.08

Arene Complexes of the Actinides

460

Jonathan D Cryer and Stephen T Liddle

4.09

Arene Complexes of the Group 4 Metals

502

Skye Fortier, Alejandra Gomez-Torres, and Carlos Saucedo

4.10

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

550

Oleh Stetsiuk, Valeriu Cemortan, Thomas Simler, and Grégory Nocton

4.11

Larger Aromatic Complexes of the Actinides

582

Olaf Walter

4.12

Larger Aromatic Complexes of the Group 4 Metals

607

Philip Mountford

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 4 Reiner Anwander University of Tübingen, Tübingen, Germany Florian Benner Department of Chemistry, Michigan State University, East Lansing, MI, United States Evan A Beretta Department of Chemistry, University of Vermont, Burlington, VT, United States Valeriu Cemortan Laboratoire de Chimie Moléculaire, CNRS, Ecole polytechnique, Insitut Polytechnique de Paris, Palaiseau, France F Mark Chadwick Department of Chemistry, Molecular Sciences Research Hub, Imperial College London, London, United Kingdom Tajrian Chowdhury School of Chemistry, Joseph Black Building, University of Glasgow, Glasgow, United Kingdom

Alejandra Gomez-Torres Department of Chemistry and Biochemistry, University of Texas at El Paso, El Paso, TX, United States Alexander J Gremillion Department of Chemistry, University of Missouri, Columbia, MO, United States Samuel J Horsewill School of Chemistry, Joseph Black Building, University of Glasgow, Glasgow, United Kingdom Alexander FR Kilpatrick School of Chemistry, University of Leicester, University Road, Leicester, United Kingdom Stephen T Liddle Department of Chemistry, The University of Manchester, Manchester, United Kingdom Philip Mountford Chemistry Research Laboratory, University of Oxford, Oxford, United Kingdom

F Geoffrey N Cloke Department of Chemistry, School of Life Sciences, University of Sussex, Brighton, United Kingdom

Grégory Nocton Laboratoire de Chimie Moléculaire, CNRS, Ecole polytechnique, Insitut Polytechnique de Paris, Palaiseau, France

Jonathan D Cryer Department of Chemistry, The University of Manchester, Manchester, United Kingdom

Elizabeth R Pugliese Department of Chemistry, Michigan State University, East Lansing, MI, United States

Francis Delano IV Department of Chemistry, Michigan State University, East Lansing, MI, United States

Carlos Saucedo Department of Chemistry and Biochemistry, University of Texas at El Paso, El Paso, TX, United States

Selvan Demir Department of Chemistry, Michigan State University, East Lansing, MI, United States

Dorothea Schädle University of Tübingen, Tübingen, Germany

Joy H Farnaby School of Chemistry, Joseph Black Building, University of Glasgow, Glasgow, United Kingdom Skye Fortier Department of Chemistry and Biochemistry, University of Texas at El Paso, El Paso, TX, United States

Dennis M Seth Department of Chemistry, University of Vermont, Burlington, VT, United States Thomas Simler Laboratoire de Chimie Moléculaire, CNRS, Ecole polytechnique, Insitut Polytechnique de Paris, Palaiseau, France

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Oleh Stetsiuk Laboratoire de Chimie Moléculaire, CNRS, Ecole polytechnique, Insitut Polytechnique de Paris, Palaiseau, France Nikolaos Tsoureas Department of Chemistry, School of Life Sciences, University of Sussex, Brighton, United Kingdom Justin R Walensky Department of Chemistry, University of Missouri, Columbia, MO, United States

Olaf Walter European Commission, Joint Research Centre (JRC), Karlsruhe, Germany Rory Waterman Department of Chemistry, University of Vermont, Burlington, VT, United States Bradley Wilson School of Chemistry, Joseph Black Building, University of Glasgow, Glasgow, United Kingdom

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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4.01

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides

Dorothea Schädle and Reiner Anwander, University of Tübingen, Tübingen, Germany © 2022 Elsevier Ltd. All rights reserved.

4.01.1 4.01.2 4.01.2.1 4.01.2.2 4.01.3 4.01.3.1 4.01.3.2 4.01.3.3 4.01.4 4.01.4.1 4.01.4.2 4.01.4.3 4.01.5 References

4.01.1

Introduction Butadienyl complexes Cyclobutadienyl complexes Butadienyl complexes Metallacyclic complexes Scandacyclopropene complexes Metallacyclopentene complexes Metallacyclopentadiene complexes Pentadienyl complexes Homoleptic Ln(III) pentadienyl complexes Lanthanide(II) pentadienides Heteroleptic pentadienyl complexes Conclusion

1 3 3 4 5 5 8 9 14 14 16 18 27 27

Introduction

In this article the focus is on the structural features, bonding properties, and reactivity of cyclobutadienyl (Cb), butadiene and pentadienyl (Pdl) complexes covering the last two decades. In the first part the chemistry of Cb complexes is set in the context of butadiene complexes and other metallacycles with dianionic and tetraanionic C4 fragments (Fig. 1). In addition to the chemistry of metallacyclic complexes with Z4-coordinated “C4 hydrocarbon” ligands, the recent progress of metallacyclopropene complexes is described in order to gain a better understanding of the formation and reactivity of metallacyclopentadienes. Studies by Evans in the early 1990s on the reactivity of decamethylsamarocene complexes toward 1,3-butadiene revealed the formation of allyl complexes. While the trivalent hydride complex [Cp 2Sm(m-H)]2 gave monometallic allyl complex [Cp 2Sm (Z3-CH2CHCHCH3)], the [Cp 2Sm(thf )x] (x ¼ 0, 2) promoted redox transformation involved diene reduction and C-C coupling to afford bis-allyl complex [Cp 2Sm(m-Z3:Z3-CH2CHCHCH2CH2CHCHCH2)SmCp 2].1 Shortly after, Thiele and Mashima/Nakamura discovered that 1,3-butadiene can be sandwiched by two trivalent lanthanum units in [Cp 2La(m-Z1:Z3-C4H6)LaCp 2(thf )]2 and [LaI2(thf )3(m-Z4:Z4-PhC4H4Ph)LaI2(thf )3]3 applying salt-metathesis and redox protocols, respectively (Section 4.01.2.2). A characteristic of the acyclic butadiene ligand is the possibility of forming either m-Z1:Z3, m-Z2:Z4 or m-Z4:Z4 coordinated complexes, in the latter mode both double bonds interacting with the metal centers in cis fashion. Strikingly, the free cyclobutadiene4,5 unlike the acyclic butadiene is not isolable in accord with Hückel’s rule that it is the prototypical antiaromatic hydrocarbon with 4 p-electrons. The energy gain of the cyclic delocalization of six p-electrons in the four-membered ring of the cyclobutadienyl dianion is diminished by the ring strain and Coulomb repulsion.6 Synthetic difficulties were overcome by indirect methods such as the dehalogenation reaction of 3,4-dihalocyclobutene with d-transition metal carbonyls7 or the reductive [2 + 2]-cycloaddition reactions of alkynes.8,9 It is noteworthy that these reaction pathways are not accessible for lanthanide complexes. Recent progress in the synthesis of alkali metal Cb chemistry promoted the isolation of rare-earth-metal cyclobutadienyl complexes by a salt-metathesis protocol (Section 4.01.2.1).10–13 The Z4-coordination of the metal center is crucial for a planar delocalized Cb dianion. Moreover, the substituents on the cyclobutadienyl [C4R4]2− (R ¼ silyl, phenyl) are essential to stabilize the diatropic Cb dianion by steric and electronic perturbation. The anionic 6p-ligand is either stabilized by negative hyperconjugation (s -p interaction) in alkylsilyl-Cb dianions or the delocalization of the negative charge onto the phenyl substituents.6 The second part of this article covers the chemistry of metallacyclic complexes including metallacyclopropene, -pentene and -pentadiene derivatives (Fig. 1, III-V). Accordingly, the formation of metallacyclopropene complexes was achieved either by using highly reducing Sm(II) species or by generating in situ a low-valent metal species (by treatment with potassium graphite), which could be converted further to the Ln-C2 fragment via oxidative cyclometallation by addition of an alkyne. Such redox transformations of alkynes were initially described for a divalent samarocene complex which resulted in cumulene complex [{Cp 2Sm}2(m-Z2:Z2-PhC4Ph)] containing metallacyclopropene subunits.14 For comparison, the alkynyl dianion in samarium complex [{Cp 2Sm}2(m-Z1:Z1-PhCCPh)] is Z1-coordinated.15,16 More recently, metallacyclopropene species with a dianionic C2 fragment were authenticated for scandium as the smallest rare-earth metal.17 The bonding in such scandacyclopropenes is best described as a 3c-2e aromatic system with two ScdC s-bonds and a Scd(C]C) dative p-bond (Section 4.01.3.1). It is noteworthy that the formation of transient scandacyclopropene has been proposed in the synthesis of scandium complexes coordinated by tetraanionic bis-alkylidene ligands.18 Rare-earth metallacyclopentenes have been accessed by transmetallation exclusively. The s2,p-metallacyclopentene character is revealed by XRD analysis corroborating a “long-short-long” CdC bond pattern diagnostic of a 2,3-ene-1,4-diyl ligand.

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00017-2

1

2

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides

Fig. 1 Representation of cyclobutadienyl (I) and butadiene (II) complexes, as well as three- and five-membered metallacycles (III-V). The chemistry of rareearth-metal allyl complexes VI is not referred to in this article.

Fig. 2 Representation of the U, W and S conformation of the Z5-coordinated pentadienyl moiety. Ernst, R.D. Chem. Rev. 1988, 88, 1255–1291.

Metallacyclopentenes can be utilized as reducing agents for unsaturated N- and C-based substrates. Distinct reaction pathways were revealed for insertion reactions involving the LndC s-bond of the metallacyclopentene (Section 4.01.3.2). Recently, the synthesis, structure and reactivity of the s-, p-, d- and f-block metallacyclopentadienes have been reviewed.19 The classical method for the synthesis of metallacyclopentadienes is “non-redox” transmetallation, more precisely, the reaction of the rare-earth-metal halide with the dilithium 1,3-butadiene-1,4-diide. Alternatively, low-valent samarium complexes have been shown to mediate oxidative C-C coupling of two alkynes to dianionic [RC4R]2− (R ¼ alkyl, phenyl), more precisely, the two-electron process can be achieved by bimetallic oxidation-addition reactions.14,20 Moreover, respective C-C homocoupling reactions can be also promoted by organolanthanide(III) alkyl complexes [Cp 2Ln{CH(SiMe3)2}].21 DFT calculations revealed mechanistic details including s-bond metathesis, dimerization of the metallocene and homocoupling of two acetylides.22 The alkyne coupling and catalytic alkyne oligomerization/isomerization have been investigated in detail.14,21,23–26 The reductive C-C coupling reaction/cyclometalation involving diphenylacetylene to form a dianionic butadienyl fragment has been authenticated for rare-earth-metal naphthalenide complexes.27 This transformation is facilitated by the redox-active naphthalenido ligand. Rare-earth metallacyclopentadienes feature two LndC(sp2) bonds and the CdC bond pattern is “short-long-short”. Interestingly, the reactions of scandium(III) and samarium(II) halides with dilithium 1,3-butadiene-1,4-diide gave bis(alkylidene) complexes via reduction of the 1,3-butadiene dianion. Important reaction patterns of the metallapentadienes reflect the proneness of the LndC(sp2) bond to engage in insertion reactions, as well as the reducing power of the bis(alkylidene) ligands (Section 4.01.3.3). The third part of this article covers the chemistry of pentadienyl (Pdl) complexes, and particularly how they relate to similar systems, e.g., allyl and cyclopentadienyl complexes (Section 4.01.4). The monoanionic pentadienyl (Pdl) ligand, more precisely the open variant of the ubiquitous cyclopentadienyl (Cp) ligand,28,29 was introduced to organorare-earth metal chemistry in the early 1980s by Ernst30 and reviewed at an early stage.31–33 The synthesis of lanthanide pentadienyl compounds has been summarized previously by Edelmann et al.34 The Pdl fragment may adopt the U, W or S conformation, as depicted in Fig. 2, and coordinate in either Z5, Z3-allyl or Z1 bonding modes. The Z5-U-coordination is dominant in open lanthanide pentadienyl complexes, which is most likely due to the 2,4-substitution pattern. Moreover, the conformation depends on the implementation of donor sidearms. Most strikingly, the pentadienyl ligand is rendered to be stronger bound than the cyclopentadienyl ligand and is highly reactive in chemical transformations comparable to the allyl fragment.31–33 The increased reactivity of the pentadienyl moiety in chemical and catalytic conversions is in accordance with the coordination switch (double-bond isomerization) of Z5 (p) ! Z3 (p) ! Z1 (Ln–C(s)), while variable resonance structures impose enhanced thermal stability. For comparison, see the resonance structures of pentadienyl (I) and allyl ligands (II, Z3 (p) and Z1 (s)) (Scheme 1).

Scheme 1 Representation of the resonance structures of pentadienyl (I) and allyl ligands (II). Ernst, R.D. Metal-pentadienyl chemistry. Acc. Chem. Res. 1985, 18, 56–62; Schumann, H.; Dietrich, A. J. Organomet. Chem. 1991, 401, C33–C36.

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides

3

Considering the monoanionic Pdl ligands as 6p-electron anions, donor/acceptor bonding to metal centers distinct from the Cp ligand has been reasoned, for instance, the role of cyclopentadienyls as stabilizing and uninvolved spectator ligand.31,32 For comparison, the three occupied p-molecular orbitals (HOMOs) of the Z5-coordinated pentadienyl are on average significantly higher in energy, while the two empty p-orbitals (LUMOs) are lower than those of the cyclopentadienyl ligand.31 This is nicely exemplified by the relative instability of titanocene compared to the open titanocene.35,36 Noteworthy, the usual Z5-U-coordinated Pdl ligand is more sterically demanding than the Cp ligand as a result of the shorter metal-ligand plane distance in Pdl complexes. Furthermore, the Z5-U-Pdl ligands are both strongly bound and more reactive than Cp ligands. These findings implicate stabilization of electron-deficient metal centers.33 While the most prominent reaction pathway of Pdl complexes in organometallic synthesis has featured the formation of metallacycles (“metallabenzenes”),33 current catalytic applications focus on the polymerization of 1,3-dienes and, hence, the synthesis of synthetic rubber.37

4.01.2

Butadienyl complexes

4.01.2.1

Cyclobutadienyl complexes

Lanthanide complexes with aromatic carbocyclic ligands have emerged as an important class of molecules, given their interesting ground-state structures and magnetochemistry, and moreover, utility in stoichiometric and catalytic bond-forming reactions. However, the synthesis of rare-earth-metal cyclobutadienyl (Cb) complexes is still in its infancy. The tuck-in cyclobutadienyl complexes [Ln{Z3-C4(SiMe3)4H}{Z4-C4(SiMe3)3-k-(CH2SiMe2)}Na] (1-Ln, Ln ¼ Y, Dy, Lu) and [Ln{Z4-C4(SiMe3)4} {Z4-C4(SiMe3)3-k-(CH2SiMe2)}]K2 (2-Ln, Ln ¼ Y, Dy) were synthesized by salt-metathesis reactions of LnCl3(thf )3.5 with either [Na2{Z4-C4(SiMe3)4}(thf )]2 or [K2{Z4-C4(SiMe3)4}], respectively. The reactions proceed with concomitant CdH-bond activation of one SiMe3 group (Scheme 2).12,13 The Ln–Cbcent distances are 2.3479(13)/2.3716(14) A˚ in 2-Y, 2.354(3)/2.376(3) A˚ in 2-Dy and 2.290(5)/2.437(4) A˚ in 1-Y, 2.308(5)/2.460(5) A˚ in 1-Dy and 2.385(2)/2.450(2) A˚ in 1-Lu with the Ln–Cbcent distances of the Z3-coordinated cyclobutadienyl in 1-Ln being longer than those of the Z4-Cb moieties. The non-linear structure is evidenced by the Cbcent–Ln–Cbcent angles of 156.00(5) and 156.42(9) in 2-Y and 2-Dy, respectively, and is also similar for complexes 1-Ln (1-Y 155.547(3) , 1-Dy 159.157(5) , 1-Lu 155.807(13) ). The 13C NMR resonances of the cyclobutadienyl carbon atoms of the diamagnetic complexes 1-Ln appear in the range of 105–125 ppm and 111–123 ppm for 2-Y. The single-molecule magnet (SMM) properties of the dysprosium complexes (effective barrier to magnetization reversal, 1-Dy Ueff ¼ 309(2) cm−1, 2-Dy Ueff ¼ 323(2) cm−1) feature a distinctive quantum tunneling of the magnetization (QTM) step, which was attributed to molecular symmetry.12,13

Scheme 2 Synthesis of rare-earth-metal cyclobutadienyl sandwich complexes. Day, B.M.; Guo, F.-S.; Giblin, S.R.; Sekiguchi, A.; Mansikkamäki, A.; Layfield, R.A. Chem. Eur. J. 2018, 24, 16779–16782; Chakraborty, A.; Day, B.M.; Durrant, J.P.; He, M.; Tang, J.; Layfield, R.A. Organometallics 2020, 39, 8–12.

The cyclobutadienyl complexes [Ln{Z4-C4(SiMe3)4}(thf )(BH4)2M] (3-Ln Ln ¼ Y, Dy; M ¼ Na and 4-Ln Ln ¼ Y, Dy; M ¼ K) were prepared by the reaction of rare-earth-metal borohydrides with the alkali-metal cyclobutadienyls [Na2{Z4-C4(SiMe3)4} (thf )]2 or [K2{Z4-C4(SiMe3)4}], respectively (Scheme 3). Structural analyses revealed chain-like coordination polymers. The

4

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides

Scheme 3 Synthesis of rare-earth-metal cyclobutadienyl half-sandwich complexes coordinated by borohydrido ligands. Durrant, J.P.; Tang, J.; Mansikkamaki, A.; Layfield, R.A., Chem. Commun. 2020, 56, 4708–4711.

Ln–Cbcent distances of the Z4-coordinated Cb ligands are 2.257(5)/2.261(5) A˚ and 2.262(4)/2.267(4) A˚ in 3-Y and 3-Dy, respectively (4-Y 2.2590(16) A˚ , 4-Dy 2.264(3) A˚ ). The NMR spectra are consistent with the structure in the solid state with one resonance for the dianionic Cb ligands in the 13C NMR spectrum (3-Y 122.6 ppm, 4-Y 123.3 ppm). The magnitude of the SMM behavior in the dysprosium complex (3-Dy Ueff ¼ 371(7) cm−1, 4-Dy Ueff ¼ 357(4) cm−1) is set by the axial crystal field of the Dy-Cb moiety, whilst the equatorial crystal field and more effective QTM is mainly promoted by the borohydrido and thf ligands.38

4.01.2.2

Butadienyl complexes

Rare-earth-metal diene complexes are assumed key intermediates in 1,3-diene polymerization. Surprisingly, reports on well-defined rare-earth-metal butadiene derivatives are so far very limited.2,3,16,39,40 The parent diene complex [Cp 2La(m-Z1:Z3-C4H6) LaCp 2(thf )] (5-La) has been synthesized by metathesis of the lanthanum sandwich complex [Cp 2La(m-Cl)K(dme)2] with magnesium 1,3-butadiene (Scheme 4, approach I).2 The compound, which is stabilized by the Cp 2La metallocene unit, is thermodynamically stable to approximately 200  C and is of intense red color. The broad range of 13C NMR resonances of the C4 unit (d 146.2, 86.9, 72.7, 52.7 ppm) indicates unsymmetrical charge distributions within the dianionic diene ligand and hence distinct participation in the bonding to the two lanthanum centers. An X-ray structure analysis revealed two lanthanum centers being bridged by a dianionic butadiene in cis conformation, and more precisely, one lanthanum center is coordinated in a Z3-allylic 2 fashion, while the second Cp∗ 2 La unit is s-bonded to the other terminal carbon atom. In another study, Kretschmer and Thiele investigated the reactions of cyclopentadienyl lanthanide dihalide complexes with magnesium butadiene to afford complexes [Cp Ln(C4H6)MgX2(thf )x] (Ln ¼ La, X ¼ I, x ¼ 3; Ln ¼ Ce, X ¼ Br, x ¼ 2; Ln ¼ Nd, X ¼ Cl, x ¼ 2), [CpEr(C4H6)MgCl2(thf )2] and [Cp#Ln(C4H6)MgCl2(thf )2] (Cp# ¼ 1,3-tBu2-C5H3; Ln ¼ Nd, Lu).39 For comparison, treatment of [Cp LnX2(thf )3] with diarylbutadiene in the presence of alkali metals resulted in [Cp La(ArC4H4Ar)(dme)] (Ar ¼ Ph, o-MeO-C6H4), while abstraction of Cp was observed in complexes [Li(thf )3Sm(PhC4H4Ph)2] and [Li(dme)(ArC4H4Ar)LuCl2] (Ar ¼ o-MeO-C6H4, p-Me-C6H4). The rational approach of [SmCl3], lithium and 1,4-diphenylbutadiene gave [Li(thf )4Sm(PhC4H4Ph)2], while metallic samarium, iodine and diarylbutadiene 1,4-(p-Me-C6H4)2-C4H4 led to complex [{1,4-(p-Me-C6H4)2C4H4}SmI(thf )3].40 Treatment of metallic lanthanum with 1,4-diphenylbuta-1,3-diene and iodine in THF led to the formation of diene-bridged bis(diiodolanthanum) complex [LaI2(thf )3(m-Z4:Z4-PhC4H4Ph)LaI2(thf )3] (6-LaI) (Scheme 4, approach II).3 The X-ray diffraction analysis revealed a symmetrically bridged structure similar to dilithium diphenylbutadienediide,41 but different from complex 5-La. For comparison, the CdC bonds of the dianionic C4 diene fragments are “long-short-short” in 5-La2 and “short-long-short” in complex 6-LaI,3 while other diene complexes show the trend of “long-short-long” distances, e.g., lutetium naphthalenide 20-Lu.42 In a previous study, the reaction of metallic samarium, iodine and 1,4-diphenylbuta-1,3-diene generated [SmI(PhC4H4Ph)(thf )3].43 The 1:3 salt-metathesis reactions of lanthanide(III) chloride with potassium diphenylbutadienide afforded either dinuclear gadolinium complex [GdCl2(thf )3(m-Z4:Z4-PhC4H4Ph)GdCl2(thf )3] (6-GdCl)44 (Scheme 4, approach III) or lutetium complex [K(thf )2(m-PhC4H4Ph)2Lu(thf )2]n (6-Lu
K)45 (Scheme 4, approach IV). The redox approach employing permethylsamarocene complex [Cp 2Sm] and the substituted 1,3-butadienes isoprene and myrcene resulted in samarocene complexes [Cp 2Sm(m-Z2:Z4-C4H5R)SmCp 2] (7-SmR, R ¼ Me, CHCH2CHCMe2) (Scheme 4, approach V).16 In the solid state, the CdC bond pattern of 7-Sm is “long-short-short.” For comparison, the redox transformation with unsubstituted 1,3-butadiene led to the coupling of two butenyl radical anions to the bis-allyl complex [Cp 2Sm(m-Z3:Z3-CH2CHCHCH2CH2CHCHCH2)SmCp 2] (not shown).1 More generally, the diene dianion is of great theoretical interest with regard to the linear 6p system.

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides

5

Scheme 4 Synthesis of dilanthanide “butadiene” complexes. Scholz, A.; Smola, A.; Scholz, J.; Loebel, J.; Schumann, H.; Thiele, K.-H. Angew. Chem. Int. Ed. Engl. 1991, 30, 435–436; Mashima, K.; Sugiyama, H.; Nakamura, A. J. Chem. Soc. Chem. Commun. 1994, 1581–1582; Evans, W. J.; Giarikos, D.G.; Robledo, C.B.; Leong, V.S.; Ziller, J.W., Organometallics 2001, 20, 5648–5652; Emelyanova, N.S.; Trifonov, A.A.; Zakharov, L.N.; Shestakov, A.F.; Struchkov, Y.T.; Bochkarev, M.N., J. Organomet. Chem. 1997, 540, 1–6; Emelyanova, N.S.; Trifonov, A.A.; Zakharov, L.N.; Bochkarev, M.N.; Shestakov, A.F.; Struchkov, Y.T., Metallorg. Khim. 1993, 6, 363–371.

4.01.3

Metallacyclic complexes

4.01.3.1

Scandacyclopropene complexes

The recent development of scandacyclopropenes is included in this article to get a better understanding of metallacycles with C4 fragment (see Section 4.01.3.3). In 2019 Zhang and coworkers reported on the first scandacyclopropene complexes, whereas derivatives of the main group and transition metals as well as the actinides have been explored for more than 40 years.17 At an earlier stage and in a broader sense, the reactions of rare-earth-metal sandwich complexes with alkynes were shown to generate three-membered metallacycles. Treatment of divalent samarocene [Cp 2Sm] with bis(alkyne) PhC^CdC^CPh or terminal alkynes RC^CH (R ¼ alkyl, aryl) resulted in cumulene complexes [(Cp 2Sm)2RC4R] (8-SmR R ¼ Ph, CH2CH2Ph, CH2CH2CHMe) (Scheme 5, approach I and II).14,46 Such cumulene derivatives could be also obtained via the C-C coupling reaction of preformed acetylide complexes [Cp 2Ce(C^CMe)]n or [Cp 2Ln(C^CPh)(thf )] (Ln ¼ Ce, Nd, Sm) (Scheme 5, approach III and IV).14,24,26 In a similar vein, treatment of either the hydride complex [Cp 2Sm(m-H)]2 or alkyl complexes [Cp 2Ln{CH(SiMe3)2}] (Ln ¼ La, Ce, Sm) with a terminal alkyne gave [{Cp 2Sm}2RC4R] (8-LnR Ln ¼ La, Ce, Sm; R ¼ Ph, Me, tBu) (Scheme 5, approach V and VI).25,26 As indicated by DFT calculations the latter reactions involve initial s-bond metathesis, followed by dimerization of the metallocene acetylide and homocoupling of two acetylides to yield the trienediyl complex.22 In complex 8-SmPh, the metallacyclopropene subunits feature Sm–C distances of 2.48(1) and 2.76(1) A˚ , indicating asymmetric coordination of the monoanionic propenyl units, and moreover, the C–C distances of 1.29(2) and 1.33(2) A˚ in the non-linear cumulene (C-C-C 154(1) A˚ ) are in the range of double bonds.46 The alkyne coupling and catalytic oligomerization/isomerization have been investigated by the groups of Teuben, Evans and Marks.21,23–26,47

6

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides

Scheme 5 Synthesis of the rare-earth metal cumulene complexes. Evans, W.J.; Keyer, R.A.; Zhang, H.; Atwood, J.L., J. Chem. Soc. Chem. Commun. 1987, 837–838; Evans, W.J.; Keyer, R.A.; Ziller, J.W. Organometallics 1990, 9, 2628–2631; Evans, W.J.; Keyer, R.A.; Ziller, J.W. Organometallics 1993, 12, 2618–2633; Heeres, H.J.; Nijhoff, J.; Teuben, J.H.; Rogers, R.D. Organometallics 1993, 12, 2609–2617; Forsyth, C.M.; Nolan, S.P.; Stern, C.L.; Marks, T.J.; Rheingold, A.L. Organometallics 1993, 12, 3618–3623.

The reaction of donor-free ytterbocene(II) complex with alkyne afforded [Cp 2Yb(MeC^CMe)] (9-Yb) (Scheme 6). The C–C(alkyne) distance of 1.154(6) A˚ is shorter than in the free butyne (1.21(2) A˚ ) and the C-C-C(Me) angles are almost linear. Moreover, the Yb–C distance of av. 2.850 A˚ indicates a very weak interaction of the divalent ytterbium center with the alkyne triple bond. This is also supported by the 13C NMR resonance of 76.9 ppm of the central carbon atoms, the shift of which being similar to that of the free alkyne (74.6 ppm).48

Scheme 6 Synthesis of the ytterbium alkyne complex. Burns, C. J.; Andersen, R. A., J. Am. Chem. Soc. 1987, 109, 941–942.

The scandacyclopropenes [Cp Sc{iPrNCnBuNiPr}(Z2-C2PhR)][K(do)]n (R ¼ Ph, do ¼ thf, 10-Sc; R ¼ Ph, do ¼ 2.2.2-cryptand, 11-Sc; R ¼ SiMe3, do ¼ 2.2.2-cryptand, 12-Sc) were accessed by the reaction of the heteroleptic scandium benzamidinate complex [Cp Sc{iPrNCnBuNiPr}(m-Cl)]2 (Cp ¼ C5Me5) with diphenylalkyne or (trimethylsilyl)phenylalkyne and potassium graphite (Scheme 7).17 Structural analysis of 10–12 and DFT calculations including localized orbital locator p analysis and multicenter bond order calculations on ion-separated scandacyclopropene 11-Sc indicated delocalized three-center two-electron bonding. The 13 C NMR spectra of 10-Sc and 11-Sc show resonances at 214.4 and 215.0 ppm for the coordinated carbon atoms of the dianionic propenyl units, while the unsymmetric scandacyclopropene unit in 12-Sc gave two resonances at 221.7 and 246.5 ppm. The Sc–C(alkene) bonds of 2.1677(17) and 2.1705(16) A˚ in 11-Sc are shorter than in scandacyclopentadiene 27-Sc (av 2.273 A˚ ) and bis(alkylidene) complex 28-ScPh (av. 2.175 A˚ ) (see Section 4.01.3.3).17,18

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides

7

Scheme 7 Synthesis of the scandacyclopropene complexes 10–12. Lv, Z.-J.; Huang, Z.; Shen, J.; Zhang, W.-X.; Xi, Z., J. Am. Chem. Soc. 2019, 141, 20547–20555.

Due to a strongly nucleophilic alkenediyl dianion in combination with maximum ring strain, scandacyclopropenes 10-Sc and 11-Sc display a wealth of unique reactivity including CdH-bond activation, electrophilic substitution, insertion of small molecules and redox processes.17 Examined substrates comprise phenylacetylene, silylazide, phenazine, silyl isocyanate, azobenzene, diphenyl disulfide and dinitrogen oxide (Scheme 8). The latter reaction of 11-Sc with N2O afforded the six-membered metallacycle [Cp Sc

Scheme 8 Reactivity of scandacyclopropene 10 and 11. Lv, Z.-J.; Huang, Z.; Shen, J.; Zhang, W.-X.; Xi, Z., J. Am. Chem. Soc. 2019, 141, 20547–20555.

8

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides

{iPrNCnBuNiPr}(NO2CPhCPh)][K(2.2.2-crypt)] (16-Sc), which is different from the reaction of N2O with metallacyclopropenes of titanium and zirconium, where five-membered azoxymetallacyclopentenes were isolated.49–51 The reaction of 10-Sc with excess phenylacetylene led to phenylacetylide complex [Cp Sc{iPrNCnBuNiPr}(CCPh)2][K(thf )] (13-Sc) with concomitant formation of cis-stilbene by protonation of the (PhC]CPh)2− unit. Treatment of 10-Sc with phenazine led to ring-opening to afford dinuclear complex [{Cp Sc{iPrNCnBuNiPr}}(N2(C6H4)2)] (14-Sc). During the course of the reaction one (PhC]CPh)2− unit remains in the complex, while the other one dissociates from the metal center as “[K2(PhCCPh)].” The reaction presumably traverses through 1,4-insertion of phenazine into one ScdC bond of 10-Sc and subsequent metathesis reaction. Treatment of 11-Sc with isocyanate Me3SiNCO gave metallacyclic complex [Cp Sc{iPrNCnBuNiPr}(OC(SiMe3)CPhCPh)][K(2.2.2-crypt)] (15-Sc) via insertion of the C]O double bond of Me3SiNCO into a ScdC bond of 11-Sc. The reaction of 11-Sc with the trimethylsilyl azide Me3SiN3 resulted in the scandium azide complex [Cp Sc{iPrNCnBuNiPr}(N3)2][K(2.2.2-crypt)] (17-Sc) under release of 1,2-diphenyl-1,2-bis(trimethylsilyl)ethane via SidN bond cleavage in Me3SiN3. The formation of a scandium imide complex through NdN bond cleavage or a four-membered metallacycle via a formal nitrene insertion into the MdC(sp2) bond was not observed. The scandium thiolate complex [Cp Sc{iPrNCnBuNiPr}(SPh)2][K(2.2.2-crypt)] (18-Sc) and Z2-coordinated azobenzene complex [Cp Sc {iPrNCnBuNiPr}(PhNNPh)][K(2.2.2-crypt)] (19-Sc) were isolated from the reactions of 11-Sc with PhSSPh and azobenzene, respectively. The formation of 18-Sc and 19-Sc is facilitated by the (PhC]CPh)2− unit acting as a two-electron reducing agent.

4.01.3.2

Metallacyclopentene complexes

The reaction of the half-sandwich lutetium dichloride [CpLuCl2] with sodium naphthalenide afforded [CpLu(C10H8)(dme)] (20Lu) and [CpLu(C10H8)(thf )2] (21-Lu), respectively. The XRD analysis of black single crystals of 20-Lu indicated Z1:Z1:Z2 (2s,p)-interactions of the metal center with the dianionic carbocyclic fragment (Scheme 9, top).42 The reaction of 21Lu with azobenzene led to the isolation of [CpLu(thf )2]2(Ph2N2)2) (22-Lu) and thus revealed comparatively moderate reduction capacity. Treatment of 20-Lu with diphenylacetylene resulted in C-C coupling generating bimetallic [CpLu(thf )]2(C4Ph4) (23-Lu) (Scheme 9, bottom).52 The bridging tetraanionic C4 fragment in 23-Lu is planar with C–C distances in the range between single and double bonds, indicating delocalization of electron density over the Z4-coordinated ligand.

Scheme 9 Synthesis and reactivity of lutetacyclopentene complexes. Protchenko, A. V.; Zakharov, L. N.; Bochkarev, M. N.; Struchkov, Y. T., J. Organomet. Chem. 1993, 447, 209–212; Bochkarev, M.N.; Protchenko, A.V.; Zakharov, L.N.; Fukin, G.K.; Struchkov, Y.T. J. Organomet. Chem. 1995, 501, 123–128.

The salt-metathesis reactions of the scandium dichloride [{C5H4(CH2)2NMe2}ScCl2] bearing an amino-functionalized cyclopentadienyl ligand and magnesium 2,3-dimethyl-1,3-butadiene furnished the scandium butenediyl complex [{C5H4(CH2)2NMe2} Sc(C6H10)] (24-Sc) (Scheme 10, top).53 In contrast to classical diene bonding, the s2,p-metallacyclopentene character in complex 24-Sc is indicated by the structural analysis; more precisely, the relatively short central C–C distance of 1.386(3) A˚ and relatively long C–C(CH2) distances of 1.455(3) and 1.464(3) A˚ corroborate the 2,3-ene-1,4-diyl character. The Cp carbon atoms and the C] C moiety resonate at 125.7 and 123.9 ppm in the 13C NMR spectrum, while the resonances for the diene methylene groups are observed at d 59.0 ppm. In line with the bonding of the dianionic butenediyl entity, the reaction of 24-Sc with polar unsaturated substrates such as benzonitrile, gave scandium imide complex [(C5H4(CH2)2NMe2)Sc(m-NC(Ph)C6H10)]2 (25-Sc) which derives from insertion into the Sc–C(methylene) bonds and rearrangement of the transient metallacyle. Treatment of 24-Sc with 4,40 dimethyl-2,20 -bipyridine led to the liberation of 2,3-dimethyl-1,3-butadiene and reduction of the bipy ligands to afford [C5H4(CH2)2NMe2]Sc(Z2-N2C12H12)2 (26-Sc) (Scheme 10, bottom).

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides

9

Scheme 10 Synthesis and reactivity of scandacyclopentene complexes. Beetstra, D.J.; Meetsma, A.; Hessen, B.; Teuben, J.H., Organometallics 2003, 22, 4372–4374.

4.01.3.3

Metallacyclopentadiene complexes

The reaction of scandium(III) chloride with the tetrafold-substituted dilithium 1,3-butadiene-1,4-diide gave the unusual bis(alkylidene) complexes [{ScCl2}2[{C(SiMe3)C(R)}2]2{ScCl(thf )}2][Li(thf )4]2 (28-ScR R ¼ Ph, p-MeC6H4, p-tBuC6H4), which feature a 2-butene-1,1,4,4-tetraanion (Scheme 11). DFT calculations propose that the bis(alkylidene) ligand in 28-ScPh is generated by CdC-bond cleavage to give a transient scandacyclopropene, which then dimerizes and reacts via double metathesis to afford the tetranuclear bis(alkylidene) complex 28-ScPh (Scheme 11).18 The scandacyclopentadiene complex [Sc{C(SiMe3)C(Ph)}2Cl2Li (thf )2] (27-Sc) is a second key intermediate, which is stable at ambient temperatures. The a-C(sp2) atoms resonate at 203.8 ppm in the 13C NMR spectrum and the b-C(sp2) atoms show a singlet at 167.6 ppm. In the solid state, the C–C distances in 27-Sc are “short-long-short” (1.348(4), 1.520(3), 1.376(4) A˚ ), clearly indicating that the C4 fragment has a dianionic butadienyl structure. For comparison, the C–C distances in bis(alkylidene) complex 28-ScPh range between single and double bonds (1.468(4), 1.430 (4), 1.465(5) A˚ ) in accordance with a highly delocalized structure of the tetraanionic ligand.

Scheme 11 Synthesis of scandacyclopentadiene complexes. Ma, W.; Yu, C.; Chi, Y.; Chen, T.; Wang, L.; Yin, J.; Wei, B.; Xu, L.; Zhang, W.-X.; Xi, Z. Chem. Sci. 2017, 8, 6852–6856.

The oxidation of the bis(alkylidene) complex 28-ScPh with two equivalents of hexachloroethane gave 27-Sc, ScCl3 and tetrachloroethylene, while the 1:4 reaction led to scandium chloride, Cl2C]CCl2 and alkyne (Scheme 12, right). The bis(alkylidene)promoted reactivity of 28-ScPh was also revealed by the targeted oxidation with disulfide [Me2NC(]S)S]2 to afford dithiocarbamate

10

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides

Scheme 12 Reactivity of scandacyclopentadiene complexes. Ma, W.; Yu, C.; Chi, Y.; Chen, T.; Wang, L.; Yin, J.; Wei, B.; Xu, L.; Zhang, W.-X.; Xi, Z. Chem. Sci. 2017, 8, 6852–6856.

29-Sc along with the extrusion of alkyne. The reduction of cyclooctatetraene gave [(COT)ScCl(dme)] (30-Sc) with concomitant oxidation of the alkylidene to PhC^CSiMe3 (Scheme 12, left).18 The lutetacyclopentadienes [Cp LuCl{C(SiMe3)C(Ph)}2(thf )2(OEt2)] (31-LuPh) and [Cp LuCl{C(SiMe3)C(Me)}2(thf )] (31-LuMe) were obtained from the one-pot reaction of lutetium chloride with LiCp and dilithium 1,3-butadiene-1,4-diide (Scheme 13).54 The 13C resonances of the a- and b-carbons in 31-LuPh and 31-LuMe were detected at 165.4/205.4 and 160.6/200.8 ppm, respectively. The solid-state structure of 31-LuPh displays Lu–C(sp2) distances of 2.331(4)/2.339(5) A˚ and “short-long-short” C–C distances (1.354(6), 1.520(6), 1.353(6) A˚ ), corroborating the lutetacyclopentadiene bonding.

Scheme 13 Synthesis of lutetacyclopentadienes stabilized by the Cp ligand. Xu, L.; Wang, Y.-C.; Wei, J.; Wang, Y.; Wang, Z.; Zhang, W.-X.; Xi, Z. Chem. Eur. J. 2015, 21, 6686–6689.

The reactions of complex 31-LuPh with pivalaldehyde or selenium/tBuCHO afforded mono and double insertion into the Lu–C(sp2) bonds yielding complexes 32-Lu/33-Lu with 7- and 8-membered lutetacycles, respectively (Scheme 14, left). Treatment

Scheme 14 Reactivity of lutetacyclopentadiene 31-LuPh toward CO and CN unsaturated substrates. Xu, L.; Wang, Y.-C.; Wei, J.; Wang, Y.; Wang, Z.; Zhang, W.-X.; Xi, Z. Chem. Eur. J. 2015, 21, 6686–6689.

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides

11

of 31-LuPh with carbon dioxide gave [Lu{O2CC(SiMe3)C(Ph)C(Ph)C(SiMe3)CO2Li(thf )}3] (34-Lu) via double insertion. Compound 35-Lu was formed by a sequence of insertion, rearrangement and CdH bond activation (Scheme 14, right).54 The reactions of 31-LuPh with carbodiimides afforded complexes containing N-containing fused rings, as depicted in Scheme 15.55 Accordingly, the 1:1 reaction produced complexes 37-Lu and 39-Lu via insertion of the carbodiimide into the LudC bonds and coupling of the diene. Addition of another two equivalents of PhN]C]NPh to 37-Lu or the 1:3 reaction of 31-LuPh gave complex 38-Lu containing a [4,4,6] fused ring along with iminocyclopentadiene, while the isolation of product 36-Lu with a [4,5,6] ring was feasible from the 1:2 reaction via double insertion of the carbodiimide (Scheme 15, left). For comparison, the reaction of zirconacyclopentadiene with carbodiimide led to azazirconacyclopentene and alkyne via b,b’ CdC bond cleavage.55

Scheme 15 Reactions of lutetacyclopentadiene 31-LuPh with carbodiimides. Xu, L.; Wei, J.; Zhang , W.-X.; Xi, Z. Chem. Eur. J. 2015, 21, 15860–15866.

The reaction of [(Z5-C5H5)2LuCl]2 with dilithium 1,3-butadiene-1,4-diide furnishes the sandwich lutetacyclopentadiene complex [Cp2Lu{(SiMe3)C(CPh)2C(SiMe3)}Li(thf )] (40-Lu) as a contact ion pair with the lithium ion coordinated to the dianionic diene fragment (Scheme 16).56 In the presence of 12-crown-4, the lutetacyclopentadiene complex 40-Lu could be converted into the

Scheme 16 Synthesis of lutetacyclopentadiene complexes. Xu, L.; Wang, Y.; Wang, Y.-C.; Wang, Z.; Zhang, W.-X.; Xi, Z. Organometallics 2016, 35, 5–8.

12

Table 1

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides Compilation of the Ln-metal–carbon distances and 13C NMR resonances of the C2 or C4 fragments in complexes 1–49

Molecular formula

Complex

Ln metal oxidation state

˚) Ln–C distances (A

13

C NMR chemical shift (ppm)

References

[Y{Z3-C4(SiMe3)4H}{Z4-C4(SiMe3)3-k-(CH2SiMe2)}Na] [Dy{Z3-C4(SiMe3)4H}{Z4-C4(SiMe3)3-k-(CH2SiMe2)}Na] [Lu{Z3-C4(SiMe3)4H}{Z4-C4(SiMe3)3-k-(CH2SiMe2)}Na] [Y{Z4-C4(SiMe3)4}{Z4-C4(SiMe3)3-k-(CH2SiMe2)}]K2 [Dy{Z4-C4(SiMe3)4}{Z4-C4(SiMe3)3-k-(CH2SiMe2)}]K2 [Y{Z4-C4(SiMe3)4}(thf )(BH4)2Na] [Dy{Z4-C4(SiMe3)4}(thf )(BH4)2Na] [Y{Z4-C4(SiMe3)4}(thf )(BH4)2K] [Dy{Z4-C4(SiMe3)4}(thf )(BH4)2K] [Cp 2La(m-Z1:Z3-C4H6)LaCp 2(thf )]

1-Y 1-Dy 1-Lu 2-Y 2-Dy 3-Y 3-Dy 4-Y 4-Dy 5-La

3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+

106.0–124.9 – 106.0–124.5 111.0–123.4 – 122.6 – 123.3 – 146.2/86.9, 72.7/52.7

13 13 13 12 12 38 38 38 38 2

[LaI2(thf )3(m-Z4:Z4-PhC4H4Ph)LaI2(thf )3] [GdCl2(thf )3(m-Z4:Z4-PhC4H4Ph)GdCl2(thf )3] [K(thf )2(m-PhC4H4Ph)2Lu(thf )2] [Cp 2Sm(m-Z2:Z4-CH2CH(Me)CH2)SmCp 2] [Cp 2Sm(m-Z2:Z4-CH2CH(CH2CH2CHCMe2)CH2)SmCp 2] [Cp 2La(m-Z2:Z2-PhC4Ph)LaCp 2] [Cp 2La(m-Z2:Z2-tBuC4tBu)LaCp 2] [Cp 2Ce(m-Z2:Z2-tBuC4tBu)CeCp 2] [Cp 2Ce(m-Z2:Z2-MeC4Me)CeCp 2] [Cp 2Sm(m-Z2:Z2-PhC4Ph)SmCp 2] [Cp 2Sm(m-Z2:Z2-PhCH2CH2C4CH2CH2Ph)SmCp 2] [Cp 2Yb(MeC^CMe)] [Cp Sc{iPrNCnBuNiPr}(Z2-C2Ph2)][K(thf )] [Cp Sc{iPrNCnBuNiPr}(Z2-C2Ph2)] [K(2.2.2.crypt)] [Cp Sc{iPrNCnBuNiPr}(Z2-C2(Ph)(SiMe3))] [K(2.2.2. crypt)] [Sc{C(SiMe3)C(Ph)}2Cl2Li(thf )2] [{ScCl2}2[{C(SiMe3)C(Ph)}2]2{ScCl(thf )}2][Li(thf )4]2 [Cp LuCl{C(SiMe3)C(Ph)}2(thf )2(OEt2)] [Cp LuCl{C(SiMe3)C(Me)}2(thf )] [Cp2Lu{(SiMe3)C(CPh)2C(SiMe3)}][Li(thf )] [Cp2Lu{(SiMe3)C(CPh)2C(SiMe3)}][Li(12c4)2] [Cp2Lu{(SiMe3)C(CPh)2C(SiMe3)}][K(18c6)] [Cp2Lu{(SiMe3)C(CPh)2C(SiMe3)}][K(2.2.2.crypt)] [Sm2I3{C(SiMe3)C(Me)}2(thf )4][Li(thf )4] [Sm2I3{C(SiMe3)C(Ph)}2(thf )4][Li(thf )4] [Sm2I3{C(SiMe3)C(CH2)2}2(thf )4][Li(thf )4] [{(COT)Sm}2I{C(SiMe3)C(Me)}2(thf )4][Li(thf )2(OEt2)2] [{(COT)Sm}2I{C(SiMe3)C(Ph)}2(thf )4][Li(thf )3(OEt2)] [(Cp Sm)2I{C(SiMe3)C(Me)}2(thf )4][Li(thf )4] [(NN)Sc(Z2-C4Ph4)K]

6-LaI 6-GdCl 6-Lu
K 7-SmR1 7-SmR2

3+ 3+ 3+ 3+ 3+

2.290(5)/2.437(4) 2.308(5)/2.460(5) 2.385 (2)/2.450(2) 2.3479(13)/2.3716(14) 2.354(3)/2.376(3) 2.257(5)/2.261(5) 2.262(4)/2.267(4) 2.2590(16) 2.264(3) 2.633(4)/2.688 (4)–2.759(4) 2.76(2)–2.94(1) 2.65(3)–2.97(3) 2.50(1)–2.540(9) 2.544(9)–2.799(8) 2.5294(4)–2.803(4)

– – – – –

3 44 45 16 16

8-LaPh 8-LatBu 8-CetBu 8-CeMe 8-SmPh 8-SmR 9-Yb 10-Sc 11-Sc 12-Sc

3+ 3+ 3+ 3+ 3+ 3+ 2+ 3+ 3+ 3+

2.577(10), 2.823(9) 2.642(3), 2.761(3) 2.607(4), 2.748(4) 2.55(1), 2.89(1) 2.48(1), 2.76(1) 2.483(7), 2.689(6) 2.48(1), 2.76(1) 2.110(14)/2.131(13) 2.1677(17)/2.1705(16) 2.1613(16)/2.1711(16)

– – – – – – – 214.4 215.0 221.7, 246.5

25 25 26 26 46 46 48 17 17 17

27-Sc 28-Sc 31-LuPh 31-LuMe 40-Lu 41-Lu 42-Lu 43-Lu 44-Sm 45-Sm 46-Sm 47-Sm 48-Sm 49-Sm 52-Sc

3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+

2.279(2)/2.266(3) 2.163(3)–2.457(3) 2.331(4)/2.339(5) – 2.421(7)/2.425(7) 2.355(3)/2.355(3) 2.369(3)/2.350(4) 2.360(4)/2.360(5) – 2.376(6)–2.406(6) 2.338(4)–2.387(4) 2.555(6)–2.584(6) 2.580(7)–2.603(7) 2.367(5)–2.396(5) –

18 18 54 54 56 56 56 56 57 57 57 57 57 57 27

[(NN)Y(Z2-C4Ph4)K]

52-Y

3+

2.489(5)/2.504(4)

[(NN)Lu(Z2-C4Ph4)K]

52-Lu

3+

–

167.6/203.8 – 165.4/205.4 160.6/200.8 159.0/201.0 159.1/201.0 159.0/201.0 159.0/201.1 123.3 (b-C) 155.8 (b-C) 125.5 (b-C) 167.5 (b-C) 167.9 (b-C) 102.4 (b-C) 153.7 149.9/146.4 196.3/157.6 151.0/147.8 200.7/158.0 150.6/148.1

27 27

separated ion pair 41-Lu. The reaction of 40-Lu with potassium fluoride and 18-crown-6 led to Li! K exchange and coordination of potassium to the Cp ligands to afford 41-Lu, whilst treatment of 40-Lu with KF and 2.2.2.cryptand proceeded via exchange of lithium for potassium and formation of the separated ion pair 43-Lu (Scheme 16).56 The 13C resonances of the C4 fragment in complexes 40-Lu to 43-Lu were detected at 159 and 201 ppm. XRD analyses of complexes 40-Lu to 43-Lu revealed Lu–C(diene) distances in the range of 2.350(4)–2.425(7) A˚ , being shorter in contact ion pair 42-Lu than in ate complex 40-Lu (Table 1). In another approach, a tandem salt-metathesis/redox reaction of SmI2(thf )2 with the electron-rich 1,3-butadienediide [Li2{C(SiMe3)C(R)}2] afforded the 2-butenyl tetraanion-bridged disamarium(III) complexes [Sm2I3{C(SiMe3)C(R)}2(thf )4] [Li(thf )4] (44-Sm R ¼ Me, 45-Sm R ¼ Ph, 46-Sm R ¼ (CH2)2), involving a double single electron transfer (SET) (Scheme 17).57 DFT calculations revealed that the reduction of the 1,3-butadienyl dianion, more precisely, SET from 4f atomic orbitals of the Sm

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides

13

Scheme 17 Synthesis and reactivity of dinuclear bis(alkylidene) samarium complexes. Zheng, Y.; Cao, C.-S.; Ma, W.; Chen, T.; Wu, B.; Yu, C.; Huang, Z.; Yin, J.; Hu, H.-S.; Li, J.; Zhang, W.-X.; Xi, Z., J. Am. Chem. Soc. 2020, 142, 10705–10714.

centers to the antibonding p orbitals of the 1,3-butadienyl dianion, is feasible to form the bis(alkylidene) complex. The reduction of cyclooctatetraene by 44-Sm or 45-Sm led to the isolation of the sandwich-like COT-coordinated samarium(III) butadienediyl complexes [{(COT)Sm}2I{C(SiMe3)C(R)}2(thf )4][Li(thf )x(OEt2)4−x] (47-Sm R ¼ Me, x ¼ 2; 48-Sm R ¼ Ph, x ¼ 3), regenerating SmI2 along with the formation of alkyne (Scheme 17). The reaction of the Sm2I3 alkylidene complex 44-Sm with LiCp gave a Cp -supported Sm(III) complex [(Cp Sm)2I{C(SiMe3)C(Me)}2(thf )4][Li(thf )4] (49-Sm), whilst the addition of Mo(CO)6 led to CO insertion and ring closure to furnish the oxy-functionalized cyclopentadienyl samarium(III) complexes [{C5R2(SiMe3)2OSmI(thf )2}2] (50-Sm R ¼ Me, 51-Sm R ¼ (CH2)2) (Scheme 17).57 Any 13C resonances of the alkylidene a-carbon atoms in complexes 44-Sm to 49-Sm could not be observed by NMR spectroscopy, while the b-carbon atoms resonate in the range of 102 ppm (49-Sm) to 168 ppm (48-Sm). The distances between samarium and the s-bonded carbon atoms of the 2-butene tetraanionrange between 2.338(4) A˚ (46-Sm) and 2.406(6) A˚ (45-Sm), while the COT-supported complexes show much longer Sm–C distances to the C4 fragment, for instance, 2.603(7) A˚ in 48-Sm in accord with a dianionic butadienyl ligand. Interestingly, the C–C distances in 45Sm, 46-Sm, and 49-Sm lie between single and double bonds, indicating a highly delocalized electronic structure. For comparison, the C–C distances of the samaracyclopentadiene in the COT-supported complexes 47-Sm and 48-Sm are “short-long-short.” The reduction of diphenylacetylene by a rare-earth-metal naphthalenide complex supported by a ferrocene diamido ligand, [(NN)Ln(m-C10H8)][K(thf )2] (NN ¼ 1,10 -ferrocenediyl-(NSitBuMe2)2), led to the isolation of the rare-earth metallacyclopentadienide complexes [(NN)Ln(Z2-C4Ph4)K] (52-Ln Ln ¼ Sc, Y, Lu) (Scheme 18).27 The reductive C-C coupling reaction to form the

Scheme 18 Synthesis of 1,2,3,4-tetraphenyl-1,3-butadienediyl rare-earth-metal complexes. Brosmer, J. L.; Huang, W.; Diaconescu, P. L. Organometallics 2017, 36, 4643–4648.

14

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides

dianionic butadienyl fragment is mediated by the redox-active naphthalenido ligand. The 13C resonances of the a-carbon atoms of the butadienediyl entity in 52-Ln appear at 154–201 ppm and those of the b-carbon atoms are in the range of 146–151 ppm (Table 1). The structural data of 52-Y compare well with other rare-earth metallapentadienes, the C–C distances of the C4 fragment being “short-long-short.”

4.01.4

Pentadienyl complexes

4.01.4.1

Homoleptic Ln(III) pentadienyl complexes

Utilizing the salt-metathesis route by reacting rare-earth-metal halides with [K(2,4-Me2-C5H5)] led to the isolation of homoleptic tris(pentadienyl) complexes [(2,4-Me2-C5H5)3Ln] (53-Ln Ln ¼ Y, La, Nd, Sm, Gd, Tb, Dy, Er, Lu) (Schemes 19 and 20).30,58–63 The structural analyses of 53-Ln revealed equally Z5-coordinated and almost planar 2,4-dimethylpentadienyl ligands, but compound

Scheme 19 Synthesis of 2,4-dimethylpentadienyl rare-earth-metal complexes. Ernst, R. D.; Cymbaluk, T. H. Organometallics 1982, 1, 708–713.

Scheme 20 Synthesis of 2,4-dimethylpentadienyl lutetium complexes. Schumann, H.; Dietrich, A. J. Organomet. Chem. 1991, 401, C33–C36; Zielinski, M. B.; Drummond, D. K.; Iyer, P. S.; Leman, J. T.; Evans, W. J. Organometallics 1995, 14, 3724–3731.

53-Lu featuring the smallest metal in the lanthanide series exhibited an unprecedented structural motif.64,65 Accordingly, the usual Z5-coordination mode was observed only for two pentadienyl ligands (U conformation), whilst the third Pdl ligand in (2,4-Me2-C5H5)3Lu is bound in a Z3-allylic fashion (S conformation).64 Interestingly, the lutetium reaction also involved an end-to-end fusion of two 2,4-dimethylpentadienyl fragments to form chelate complex [(2,4-Me2-C5H5)Lu{Z5:Z3-(Me)C5H5 CH2CH2CH(CH3)C3H3(Me)}] (54-Lu) as a minor co-product (Scheme 20).65 The persistence of the Z5-coordination in solution of complexes 53-Y, 53-La and 53-Nd was evidenced by NMR spectroscopy. The 1H NMR resonances of the pentadienyl fragment of 53-La and 53-Nd display signal intensities in the ratio 6:1:2:2 in accordance with distinct signals for the methine and methylene exo and endo protons. The 13C NMR spectrum of 53-La in benzene-d6 shows resonances of the C5 fragment at 83.1 (C1, C5), 91.8 (C3) and 150.0 ppm (C2, C4), indicating the U conformation of the monoanionic pentadienyl ligands in solution.63 In the solid state, the carbon–carbon distances in complexes 53-Ln (Ln ¼ Y, La, Nd) are “short-long-long-short” and the angles of the inner carbon atoms (C2-C3-C4) average 131  1 , deviating significantly from the ideal 120 angle of sp2-hybridized carbon atoms.63 A comparison of the Ln–C distances shows that Ln–C1/C3/C5 are shorter than Ln–C2/C4, which is due to a higher electron density in these positions. Considering the carbon atoms of the inner coordination sphere of complexes 53-Ln (Ln ¼ Y, La, Nd, Sm, Gd, Tb, Dy, Er), the geometry of the 9-coordinated rare-earth-metal centers is best described as triply capped trigonal prismatic.63 A wide variety of complexes has been prepared with the sterically demanding 2,4-di-tert-butylpentadienyl ligand.66 The outcome of metathesis using various Ln(III) salt precursors and the respective potassium ligand transfer agent is directed by the rareearth-metal size and the intrinsic redox potentials. The larger rare-earth metals yielded homoleptic complexes [(2,4-tBu2-C5H5)3Ln] (55-Ln Ln ¼ La, Ce, Pr, Nd) with all Pdl ligands in the routine Z5-coordination (Scheme 21). Mono- and double-deprotonation of the Pdl ligands was observed for the smaller to intermediate-sized rare-earth metals to afford the dimetallic complexes [Ln2(2,4-

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides

15

Scheme 21 Synthesis of 2,4-di-tert-butylpentadienyl rare-earth-metal complexes. Raeder, J.; Reiners, M.; Baumgarten, R.; Münster, K.; Baabe, D.; Freytag, M.; Jones, P.G.; Walter, M.D. Dalton Trans. 2018, 47, 14468–14482.

tBu2-C5H5)(2,4-tBu2-C5H4)(2,4-tBu2-C5H3)(thf )2] (55-Ln Ln ¼ Sc, Y, Gd, Tb, Dy, Ho, Er, Tm, Lu) with additional Ln–C(s) bonds and a metallacycle structural motif (Scheme 21). The solid-state structures of complexes 55-Ln indicate the shortest Ln–C distance to the central C atom (C3-position) of the Pdl ligand, the other distances progressively increasing to Ln–C1/C5 and Ln–C2/C4, which correlates with the charge density distribution of the monoanionic pentadienyl ligand. Moreover, a typical “short–long–long–short” CdC bonding pattern of the C5 fragment is indicative of predominantly ionic Ln–C(Pdl) bonds. The most interesting feature of the solid-state structures of complexes 56-Ln is the six-membered metallacycle, in which the pentadienyl ligand is doubly deprotonated. The bonding herein is best described by a resonance structure where the negative charge is mainly localized on the C3 position. This is corroborated by the structural analysis. The dinuclear complexes 56-Ln are coordinated by one monoanionic Pdl ligand in Z5-fashion as well as dianionic and trianionic metallated Pdl ligands.66 The enantiomerically pure lanthanide pentadienyl complexes [Ln(C13H19)3] (57-Ln Ln ¼ La, Ce, Pr, Nd) are derived from (1R)(−)-myrtenal (Scheme 22). In the solid state, these complexes feature ideal C3 symmetry, with the sterically less encumbered face directed toward the rare-earth-metal center, the Pdl ligand in U conformation, and the bicyclic chiral fragment in the backbone.67 The 1H NMR resonances of the paramagnetic compounds (57-Ln Ln ¼ Ce, Pr, Nd) revealed a spatial correlation between the paramagnetic metal center and the individual hydrogen positions in the Pdl ligand in accord with the C3 symmetry in the solid state. For the diamagnetic lanthanum complex 57-La the resonances of the pentadienyl ligands are at 4.04 (exo), 2.65 (endo), 4.35 (H3) and 4.16 ppm (H5) (see numbering in Scheme 22). The 13C NMR spectrum shows distinct signals of the C5 fragment at 151.8 (C4), 145.9 (C2), 97.0 (C5), 92.2 (C3) and 79.9 ppm (C1), with the resonances of the formally charged carbon atoms shifted to higher fields (Table 2).

Scheme 22 Synthesis of rare-earth-metal pentadienyl complexes derived from (1R)-(−)-myrtenal. Fecker, A.C.; Freytag, M.; Jones, P.G.; Walter, M.D. Dalton Trans. 2019, 48, 8297–8302.

16

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides

Table 2

Compilation of the Ln-metal–carbon distances and 13C NMR resonances of the C5 fragment in complexes 53–61

Molecular formula

Complex

Ln metal oxidation state

˚) Ln–C distances (A

13

References

[(2,4-Me2-C5H5)3Y] [(2,4-Me2-C5H5)3Y](1,4-dioxane) [(2,4-Me2-C5H5)3La] [(2,4-Me2-C5H5)3Nd] [(2,4-Me2-C5H5)3Nd](1,4-dioxane) [(2,4-Me2-C5H5)3Gd] [(2,4-Me2-C5H5)3Tb(thf )0.5] [(2,4-Me2-C5H5)3Dy] [(2,4-Me2-C5H5)3Lu]

53-Y 53-Y0 53-La 53-Nd 53-Nd0 53-Gd 53-Tb 53-Dy 53-Lu

3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+

148.4/91.0/80.3 – 150.0/91.8/83.1 – – – – – 148.5/90.4/80.5

63 63 63 30 63 58 59,68 62 64

[(2,4-Me2-C5H5)Lu {MeC5H5CH2CH2CH(Me)C3H3Me}] [(2,4-tBu2-C5H5)3La] [(2,4-tBu2-C5H5)3Ce] [(2,4-tBu2-C5H5)3Nd] [(2,4-tBu2-C5H5)3Pr] [Sc2(2,4-tBu2-C5H5)(2,4-tBu2-C5H4) (2,4-tBu2-C5H3)(thf )2]

54-Lu

3+

2.72(1)–2.80(1) 2.686(4)–2.818(4) 2.791(2)–2.979(2 2.726(24)–2.873(27) 2.752(4)–2.880(4) 2.721(12)–2.847(14) 2.678(41)–2.964(20) 2. 684(9)–2.833(6) av. 2.59(1) (Z3) av. 2.63(1)/2.66(2) (Z5) 2.614(6)–2.740(5)

160–130/100–60

65

55-La 55-Ce 55-Nd 55-Pr 56-Sc

3+ 3+ 3+ 3+ 3+

2.801(2)–2.962(2) 2.779(2)–2.940(2) 2.766(2)–2.925(2) 2.7626(19)–2.9150(19) 2.2451(14)–2.6067(15)

66 66 66 66 66

[Y2(2,4-tBu2-C5H5)(2,4-tBu2-C5H4) (2,4-tBu2-C5H3)(thf )2]

56-Y

3+

2.400(2)–2.784(2)

[Gd2(2,4-tBu2-C5H5)(2,4-tBu2-C5H4) (2,4-tBu2-C5H3)(thf )2] [Tb2(2,4-tBu2-C5H5)(2,4-tBu2-C5H4) (2,4-tBu2-C5H3)(thf )2] [Dy2(2,4-tBu2-C5H5)(2,4-tBu2-C5H4) (2,4-tBu2-C5H3)(thf )2] [Ho2(2,4-tBu2-C5H5)(2,4-tBu2-C5H4) (2,4-tBu2-C5H3)(thf )2] [Er2(2,4-tBu2-C5H5)(2,4-tBu2-C5H4) (2,4-tBu2-C5H3)(thf )2] [Tm2(2,4-tBu2-C5H5)(2,4tBu2-C5H4)(2,4-tBu2-C5H3)(thf )2] [Lu2(2,4-tBu2-C5H5)(2,4-tBu2-C5H4) (2,4-tBu2-C5H3)(thf )2] [La(C13H19)3] [Ce(C13H19)3] [Pr(C13H19)3] [Nd(C13H19)3] [(2,4-tBu2-C5H5)2Sm(thf )2] [(2,4-tBu2-C5H5)2Eu(thf )] [(2,4-tBu2-C5H5)2Yb(thf )] [Yb(2,4-Me2-C5H5)2(dme)] [Yb{4,4,-(CH2)2(2-C6H8)2}(thf )2] [Yb{1,5-(Me3Si)2C5H5}2(diglyme)]

56-Gd

3+

2.428(6)–2.836(6)

164.9/80.6/80.2 – – – 153.4/83.3/77.0 (Pdl), 179.2/159.3/156.4/82.3/70.9 (Pdl-1H), 160.1/157.9/87.9 (Pdl-2H) 164.3/158.0, 160.9/81.3/78.9/75.4 (Pdl), 159.7/78.8/ (Pdl-1H), 150.2/84.6 (Pdl2H) –

56-Tb

3+

2.420(5)–2.796(5)

–

66

56-Dy

3+

2.408(4)–2.797(4)

–

66

56-Ho

3+

2.383(5)–2.782(5)

–

66

56-Er

3+

2.376(3)–2.766(3)

–

66

56-Tm

3+

2.377(4)–2.747(4)

–

66

56-Lu

3+

2.339(5)–2.747(6)

160.0/158.4/153.2/84.5/81.8

66

57-La 57-Ce 57-Pr 57-Nd 58-Sm 58-Eu 58-Yb 59-Yb 60-Yb 61-Yb

3+ 3+ 3+ 3+ 2+ 2+ 2+ 2+ 2+ 2+

2.722(9)–2.977(8) 2.726(4)–3.067(5) 2.686(5)–2.944(5) 2.704(4)–3.053(6) 2.814(3)–2.962(4) 2.759(3)–2.962(3) 2.687(2)–2.794(2) 2.676(5)-2.826(5)

151.8/145.9/97.0/92.2/79.9 – – – – – 160.7/81.6/74.3 145.1/87.0/77.6 148.8/143.8/90.0/82.2/77.8 148.9/88.2/71.1

67 67 67 67 66 66 66 61 69 70

4.01.4.2

2.770(14)–2.900(11) (Z3), 2.594(11)–3.045(10) (Z5)

C NMR chemical shift [ppm]

66

66

Lanthanide(II) pentadienides

The attempted synthesis of tris(2,4-di-tert-butylpentadienyl) complexes of the redox-active metals samarium, europium and ytterbium starting from the trivalent lanthanide halides and the potassium ligand transfer agent [K(2,4-tBu2-C5H5)] led to metal reduction with concomitant ligand oxidation to afford [(2,4-tBu2-C5H5)2Ln(thf )x] (58-Ln Ln ¼ Sm, Eu, Yb).66 Similar redox chemistry was observed for the analogous reaction of YbCl3 with [K(2,4-Me2-C5H5)].61 Accordingly, a rational salt-metathesis approach with lanthanide(II) iodides of these metals yielded the open-metallocenes 58-Ln straightforwardly (Scheme 23).66 XRD analysis revealed Z5-U-coordinated pentadienyl ligands, one thf ligand residing in the open edge of one Pdl ligand, and the second thf in 58-Sm occupying the opposite position of the second Pdl ligand. The C–C distances within the Pdl ligand feature the typical

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides

17

Scheme 23 Synthesis of divalent 2,4-di-tert-butylpentadienyl rare-earth-metal complexes. Raeder, J.; Reiners, M.; Baumgarten, R.; Münster, K.; Baabe, D.; Freytag, M.; Jones, P.G.; Walter, M.D. Dalton Trans. 2018, 47, 14468–14482.

“short–long–long–short” pattern in accord with the localization of the negative charge within the C3 and C1/C5 positions. The relative orientation of the Pdl ligands in 58-Eu and 58-Yb is almost anti-eclipsed, whereas the pentadienyl ligands in 58-Sm are gauche to each other.66 In a previous study, complex [Yb(2,4-Me2-C5H5)2(dme)] (59-Yb) was obtained from [K(2,4-Me2-C5H5)] and either YbCl3 or YbI2(thf )3, respectively (Scheme 24). The 13C NMR spectrum of the divalent product 59-Yb revealed 1J(13C-171Yb) spin couplings between the ytterbium center and C1/5 or C3 ranging from 29.5 to 7.0 Hz, respectively.61

Scheme 24 Synthesis of a divalent 2,4-dimethylpentadienyl ytterbium complex. Baudry, D.; Nief, F.; Ricard, L. J. Organomet. Chem. 1994, 482, 125–130.

The coordination of a chelating (bis)pentadienyl ligand to Yb(II) was achieved by similar salt-metathesis procedures to afford the open ansa-ytterbocene [Yb{4,4,-(CH2)2(2-C6H8)2}(thf )2] (60-Yb), with the pentadienyl moieties in a U-Z5 conformation (Scheme 25).69 The reaction of ytterbium(II) iodide with two equivalents of K[1,5-(Me3Si)2C5H5] in the presence of diglyme led to the isolation of [Yb{1,5-(Me3Si)2C5H5}2(diglyme)] (61-Yb), coordinated by both Z5- and Z3-bound Pdl ligands (Scheme 26).70 The Ln–C distances of the Z3-coordinated ligand in 61-Yb are longer than those in the Z5-bound ligand, which is opposite to complex 53-Lu, where the distances to the Z3-dienyl are shorter than to the Z5-coordinated ligand. This might be due to the distinct Ln3+/Ln2+ Lewis acid strengths, more precisely, Lu3+ is a stronger Lewis acid interacting favorably with the formally charged terminal carbon atoms.

Scheme 25 Synthesis of divalent ansa-ytterbocene complex 60-Yb. Weng, W.Q.; Kunze, K.; Arif, A.M.; Ernst, R.D. Organometallics 1991, 10, 3643–3647.

18

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides

Scheme 26 Synthesis of divalent ytterbium complex 61-Yb coordinated by a 1,5-bis(trimethylsilyl)-substituted pentadienyl ligand. Kunze, K.; Arif, A.M.; Ernst, R.D. Bull. Soc. Chim. Fr. 1993, 130, 708–711.

4.01.4.3

Heteroleptic pentadienyl complexes

The formation of monometallic bis(pentadienyl) lanthanide halide complexes [(2,4-Me2-C5H5)2LnX(do)] (62-Lndo X ) can be either achieved by application of salt-metathesis protocols in coordinating solvents or treatment of the dimeric complexes [(2,4-Me2-C5H5)2LnX]2 (63-LnX) with donor molecules (Scheme 27, Eqs. 1–3).60,63 Moreover, ligand redistribution within stoichiometric 1:2 mixtures of Ln(halide)3 and Ln(Pdl)3 gave the corresponding mononuclear complexes 62-Lndo X when combined in a donor solvent (Scheme 27, Eq. 4) and dimeric complexes 63-LnX in non-coordinating solvents (Scheme 27, Eq. 5).63 The “halfsandwich” mono(pentadienyl) lanthanide halide complexes [(2,4-Me2-C5H5)LnCl2(thf )3] (64-Ln) were synthesized in a similar manner (Scheme 28) including the unique clusters [Ln6(2,4-Me2-C5H5)6Cl12(thf )2] (65-LnX) (Scheme 29).37,60,63 The 2,4-dimethylpentadienyl lanthanide(III) complexes [(2,4-Me2-C5H5)3Ln] (53-Ln), [(2,4-Me2-C5H5)2LnX] (62-Ln, 63-Ln) or [(2,4-Me2-C5H5)LnX2] (64-Ln, 65-Ln) display highly active catalysts for the 1,4-cis-polymerization of 1,3-butadiene upon activation with strong Lewis acids such as halide-containing aluminum alkyls or aluminoxanes, respectively.37,71 The monomer-dimer equilibrium of complexes [(2,4-Me2-C5H5)2LnX]2 (63-LnX) in solution was established by NMR spectroscopy, as compiled in Table 3, and further corroborated by the preparation of donor-coordinated monomers [(2,4-Me2-C5H5)2LnX(do)] (62-Lndo X ). Accordingly, the donor strength of thf is sufficient to cleave the halide-bridge of dimeric 63-LnX (Scheme 27, Eq. 1).63

Scheme 27 Synthesis of open rare-earth-metallocene halides. Xiaoping, C.; Lixin, G.; Wenqi, C., Chin. J. Appl. Chem. 1991, 8, 21–24; Kunze, M.R.; Steinborn, D.; Merzweiler, K.; Wagner, C.; Sieler, J.; Taube, R. Z. Anorg. Allg. Chem. 2007, 633, 1451–1463.

19

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides

Scheme 28 Synthesis of pentadienyl-supported lanthanide dichloride complexes. Xiaoping, C.; Lixin, G.; Wenqi, C., Chin. J. Appl. Chem. 1991, 8, 21–24.

Scheme 29 297–303.

Synthesis of halide-bridged lanthanide pentadienyl clusters. Sieler, J.; Simon, A.; Peters, K.; Taube, R.; Geitner, M., J. Organomet. Chem. 1989, 362,

1 Table 3 H NMR spectroscopic data of paramagnetic complexes 53-Nd, 62-Ndpy Br and 63-NdX (numbering see Scheme 1, H1/5 denotes either the endo or exo proton in 1/5 position)

Molecular formula/solvent

Complex

Monomer/dimer

Me

H3

H1/5

H1/5

[(2,4-Me2-C5H5)3Nd]/THF-d8 [(2,4-Me2-C5H5)3Nd]/benzene-d6 [(2,4-Me2-C5H5)2NdCl]2/THF-d8 [(2,4-Me2-C5H5)2NdCl]2/THF-d8 [(2,4-Me2-C5H5)2NdBr]2/THF-d8 [(2,4-Me2-C5H5)2NdBr]2/THF-d8 [(2,4-Me2-C5H5)2NdI]2/THF-d8 [(2,4-Me2-C5H5)2NdI]2/THF-d8 [(2,4-Me2-C5H5)2NdBr]2/toluene-d8 [(2,4-Me2-C5H5)2NdBr]2/toluene-d8 [(2,4-Me2-C5H5)2NdI]2/cyclohexane-d12 [(2,4-Me2-C5H5)2NdBr(py)]/cyclohexane-d12

53-Nd 53-Nd 63-NdCl 63-NdCl 63-NdBr 63-NdBr 63-NdI 63-NdI 63-NdBr 63-NdBr 63-NdI 62-Ndpy Br

M M D M D M D M D M D M

−1.66 −1.73 – −1.68 5.86 −1.61 5.79 −1.57 7.09 −1.67 6.33 −1.60

8.36 8.76 – 8.80 26.51 8.37 26.34 8.43 41.86 8.32 34.19 8.32

20.16 21.30 – 20.90 −24.50 20.18 −24.69 20.21 −28.39 20.20 −26.93 19.92

−29.43 −30.19 – −29.91 −26.06 −29.37 −26.68 −29.34 −31.21 −29.31 −29.32 −29.33

Adapted from Kunze, M.R.; Steinborn, D.; Merzweiler, K.; Wagner, C.; Sieler, J.; Taube, R., Synthese, Z. Anorg. Allg. Chem. 2007, 633, 1451–1463.

The metal center in [(C8H8)Ln(2,4-Me2-C5H5)(thf )] (66-Ln Ln ¼ Nd, Sm, Er) is coordinated by the cyclooctatetraenyl ligand in Z8-mode and the monoanionic pentadienyl in a Z5-fashion (Scheme 30). In the solid state, the thf ligand of the erbium complex 66Er adopts the position against the “open jaws” of the 2,4-dimethylpentadienyl ligand, while in the neodymium and samarium complexes 66-Nd and 66-Sm, respectively, this situation is reversed. XRD analysis revealed a comparatively short Ln–C(CH) distance involving the central carbon atom of the pentadienyl, thus acting as the dominant charge donor to the lanthanide center (Table 4).72,73

Scheme 30 Synthesis of COT-supported rare-earth-metal pentadienyl complexes. Jin, J.; Jin, S.; Jin, Z.; Chen, W. J. Chem. Soc., Chem. Commun. 1991, 1328–1329.

20

Table 4

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides Compilation of the Ln-metal–carbon distances and 13C NMR resonances of the C5 fragment in complexes 62–68

Molecular formula

Complex

Ln metal oxidation state

˚) Ln–C distances (A

13

C NMR chemical shift (ppm)

References

[(2,4-Me2-C5H5)2LaCl(thf )] [(2,4-Me2-C5H5)2LaBr(py)] [(2,4-Me2-C5H5)2LaBr]2 [(2,4-Me2-C5H5)2LaI]2 [(2,4-Me2-C5H5)2NdCl]2 [(2,4-Me2-C5H5)2NdBr]2 [(2,4-Me2-C5H5)2NdI]2 [(2,4-Me2-C5H5)2YBr]2

62-Lathf Cl 62-Lapy Br 63-LaBr 63-LaI 63-NdCl 63-NdBr 63-NdI 63-YBr

3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+

– – – – – 2.665(4)–2.860(7) 2.649(3)–2.905(4) 2.611(6)–2.777(8)

63 63 63 63 63 63 63 63

[(2,4-Me2-C5H5)6Nd6Cl12(thf )2] [(C8H8)Nd(2,4-Me2-C5H5)(thf )] [(C8H8)Sm(2,4-Me2-C5H5)(thf )] [(C8H8)Er(2,4-Me2-C5H5)(thf )] [(2,4-Me2-C5H5)2Gd(C^CPh)]2 [(2,4-Me2-C5H5)2Er(C^CPh)]2 [(2,4-Me2-C5H5)2Tb(nido-2,3(SiMe3)2-2,3-C2B4H4)]2 [(2,4-Me2-C5H5)2Tb(nido-2,3(SiMe3)2–2,3-C2B4H4)]2

65-NdCl 66-Nd 66-Sm 66-Er 67-Gd 67-Er 68-Tb

3+ 3+ 3+ 3+ 3+ 3+ 3+

2.653(7)–2.873(10) 2.712(11)–2.927(12) 2.71(1)–av.2.88 2.653(7)–2.736(6) 2.673(8)–2.866(10) 2.665(10)–2.847(8) 2.647(5)–2.714(5)

151.0/92.8/83.8 150.6/92.5/83.5 150.4/92.2/83.5 150.3/92.3/83.5 – – – 148.5/91.4/80.2 148.3/91.0/87.3 – – – – – – –

68-Er

3+

2.609(12)–2.663(12)

–

75

37 72 73 72 74 74 75

Protonolysis of the homoleptic pentadienyl complexes [(2,4-Me2-C5H5)3Ln] (53-Ln Ln ¼ Gd, Er) with phenylacetylene afforded the dimeric complexes [(2,4-Me2-C5H5)2Ln(C^CPh)]2 (67-Ln) which contain asymmetrically bridging alkynyl ligands (Scheme 31, top).74 Apparently, the bridging hydrogen atoms in nido-2,3-(SiMe3)2-2,3-C2B4H6 are more acidic than the methylene hydrogen atoms of the Pdl proligand, 2,4-Me2-C5H6. In a similar vein, the carborane precursor nido-2,3-(SiMe3)2-2,3-C2B4H6 reacts with 53Ln (Ln ¼ Tb, Er) via elimination of two pentadienes to furnish the bimetallic complexes [(2,4-Me2-C5H5)2Ln(nido-2,3(SiMe3)2-2,3-C2B4H4)]2 (68-Ln Ln ¼ Tb, Er), which contain m-Z5:Z2 coordinated carboranyl ligands (Scheme 31, bottom). In the solid state, the distances of the metal center to the formally uncharged carbon atoms in 2,4-position are longer than to the central carbon of the C5 fragment and the methylene carbon atoms in 1,5-position, which is in agreement with the usually observed charge distribution in monoanionic pentadienyl ligands. The C–C distances in the pentadienyl fragment are “short-long-long-short.”75

Scheme 31 Protonolysis of homoleptic pentadienyl complexes [(2,4-Me2-C5H5)3Ln] by phenylacetylene or carborane. Zhang, S.; Zhuang, X.; Zhang, J.; Chen, W.; Liu, J. J. Organomet. Chem. 1999, 584, 135–139; Li, A.; Wang, J.; Zheng, C.; Maguire, J. A.; Hosmane, N. S. Organometallics 2004, 23, 3091–3093.

21

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides

Equimolar treatment of tris(tetramethylaluminate) rare-earth-metal complexes [Ln(AlMe4)3] with the potassium pentadienides [K(2,4-R,R-C5H5)] (R ¼ Me, iPr) gave selectively the “open” half-sandwich complexes [(2,4-R,R-C5H5)Ln(AlMe4)2] (69-LnR Ln ¼ La, R ¼ Me, iPr; Ln ¼ La, Ce, Pr, Nd, R ¼ tBu) for the larger-sized metals lanthanum, cerium, praseodymium, and neodymium, whilst the smaller-sized metals yttrium and lutetium afforded selectively the sandwich complexes [(2,4-R,R-C5H5)2Ln(AlMe4)] (71LnR Ln ¼ Nd, R ¼ tBu; Ln ¼ Y, R ¼ iPr; Ln ¼ Y, Lu, R ¼ Me) (Scheme 32). By adopting a 1:2 stoichiometry of the lanthanide precursor and potassium ligand transfer agent, the open sandwich complexes 71-LnR were obtained in moderate to high yields. Exchange of aluminum for gallium in [(2,4-tBu2-C5H5)La(AlMe4)2] (69-LatBu), achieved by a donor(OEt2)-assisted treatment with GaMe3, resulted in the isolation of [(2,4-tBu2-C5H5)Ln(GaMe4)2] (70-LatBu) (Scheme 32). Both the half-sandwich (69-LnR) and sandwich complexes (71-LnR) contain monoanionic Pdl ligands exhibiting a Z5-U-coordination mode in the solid state (Scheme 32).76 The paramagnetically shifted 1H NMR resonances of compounds 69-Ln (Ln ¼ Ce, Pr, Nd) and 71-Nd were assigned by the relative integration with the terminal methylene groups resonating in high field and the central methine moiety in low field (Table 5). The 13C NMR spectra of the lanthanum complexes 69-LaR display resonances of the 2,4- and 1,5-positioned pentadienyl carbon atoms at lower field compared to the halides 62-La and 63-La (Tables 4 and 6).76

Scheme 32 Synthesis of open sandwich and half-sandwich complexes. Barisic, D.; Buschmann, D. A.; Schneider, D.; Maichle-Mössmer, C.; Anwander, R., Chem. Eur. J. 2019, 25, 4821–4832.

1 Table 5 H NMR spectroscopic data of the paramagnetic complexes 69-Ln and 71-Ln (numbering see Scheme 1, H1/5 denotes either the endo or exo proton in 1/5 position)

Molecular formula/solvent

Complex

CH3

H3

H1/5

H1/5

[(2,4-tBu2-C5H5)Ce(AlMe4)2]/benzene-d6 [(2,4-tBu2-C5H5)Pr(AlMe4)2]/benzene-d6 [(2,4-tBu2-C5H5)Nd(AlMe4)2]/benzene-d6 [(2,4-tBu2-C5H5)2Nd(AlMe4)]/benzene-d6

69-CetBu 69-PrtBu 69-NdtBu 71-NdtBu

−16.77 6.29 3.81 4.10

42.26 96.48 44.78 34.50

−16.77 −48.10 −31.39 −27.20

−32.85 −78.99 −45.92 −34.50

Adapted from Barisic, D.; Buschmann, D. A.; Schneider, D.; Maichle-Mössmer, C.; Anwander, R., Chem. Eur. J. 2019, 25, 4821–4832.

Activation of the open half-sandwich complexes [(2,4-R,R-C5H5)Ln(AlMe4)2] (69-LnR Ln ¼ La, R ¼ iPr; Ln ¼ La, Ce, Nd, R ¼ tBu) with fluorinated borate or borane co-catalysts, respectively, gave highly active catalysts for the cis-1,4 selective polymerization of isoprene (2 equiv. B(C6F5)3 as cocatalyst, maximum cis-1,4 selectivity of 90.4%, minimum Mw/Mn ¼ 1.09). The equimolar reaction of [(2,4-tBu2-C5H5)La(AlMe4)2] (69-LatBu) with perfluorinated triphenylborane generated {{(2,4-

22

Table 6

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides Compilation of the Ln-metal–carbon distances and 13C NMR resonances of the C5 fragment in complexes 69–81

Molecular formula

Complex

Ln metal oxidation state

˚) Ln–C distances (A

13

[(2,4-tBu2-C5H5)La(AlMe4)2] [(2,4-tBu2-C5H5)La(GaMe4)2] [(2,4-iPr2-C5H5)La(AlMe4)2] [(2,4-Me2-C5H5)La(AlMe4)2] [(2,4-tBu2-C5H5)Ce(AlMe4)2] [(2,4-tBu2-C5H5)Pr(AlMe4)2] [(2,4-tBu2-C5H5)Nd(AlMe4)2] [(2,4-tBu2-C5H5)2Nd(AlMe4)] [(2,4-tBu2-C5H5)2Y(AlMe4)] [(2,4-iPr2-C5H5)2Y(AlMe4)] [(2,4-Me2-C5H5)2Y(AlMe4)] [(2,4-tBu2-C5H5)2Lu(AlMe4)] [(2,4-Me2-C5H5)2Lu(AlMe4)] [(2,4-tBu2-C5H5)La{AlMe2(C6F5)2} (AlMe3C6F5)]2 [(1-Me-3,5-tBu2-C5H3Al)(m-Me)Y (2,4-tBu2-C5H5)] [(1-Me-3,5-tBu2-C5H3Al) (m-Me)Lu(2,4-tBu2-C5H5)] [(1-Me-3,5-tBu2-C5H3Al)(m-Me)Y (2,4-tBu2-C5H5)(thf )] [(1-Me-3,5-tBu2-C5H3Al)(m-Me) Y(2,4-tBu2-C5H5)(py)] [(1-Me-3,5-tBu2-C5H3Al)(m-Me)Y (2,4-tBu2-C5H5)(dmap)] [(1-Me-3,5-tBu2-C5H3Al)(m-Me)Y (2,4-tBu2-C5H5)(4,40 -bipy)] [(1-Me-3,5-tBu2-C5H3Al) (m-CCtBu)Y(2,4-tBu2-C5H5)] [(1-Me-3,5-tBu2-C5H3Al){mN(SiHMe2)2}Y(2,4-tBu2-C5H5)] [{1-Me-10 -(2,4-tBu2-C5H5)-3,5tBu2-C5H3Al}Ln{N(SiHMe2)2}(thf )2] [(YC5H3-tBu2-3,5)Li{N(SiHMe2)2}][Y(1-Me-3,5-tBu2-C3H5Al) (m-Me){N(SiHMe2)2}] [(1-Me-3,5-tBu2-C5H3Al)(m-Me) Y-(C6H6)][B(C6F5)4] [{1-(C6F5)-3,5-tBu2-C5H3Al} (m-C6F5)Y(2,4-tBu2-C5H5)]

69-LatBu 70-LatBu 69-LaiPr 69-LaMe 69-CetBu 69-PrtBu 69-NdtBu 71-NdtBu 71-YtBu 71-YiPr 71-YMe 71-LutBu 71-LuMe 72-La

3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+

2.6683(16)–2.9061(17) 2.709(3)–2.936(3) 2.7349(15)–2.8945(14) 2.783(3)–2.924(3) 2.6460(19)–2.873(2) 2.627(3)–2.856(3) 2.633(3)–2.821(3) 2.674(2)–2.876(2) 2.581(3)–2.826(3) 2.579(3)–2.772(3) 2.594(2)–2.808(3) 2.5624(14)–2.7502(14) 2.563(3)–2.758(3) 2.679(6)–2.891(6)

167.2/92.0/90.2 166.3/90.9/99.3 163.8/94.8/90.8 153.0/95.7/94.2 – – – – 161.9/82.5/80.7 159.5/87.4/82.6 148.2/90.4/86.3 162.8/83.5/80.1 147.9/90.1/84.5 –/93.1/91.4

76 76 76 76 76 76 76 76 77 76 76 77 76 76

73-Y

3+

2.4882(12)–2.7566(13)

77

73-Lu

3+

2.428(2)–2.682(2)

75-Ythf

3+

2.548(2)–2.790(2)

75-Ypy

3+

2.533(3)–2.774(3)

75-Ydmap

3+

2.5490(17)–2.7836(18)

75-Ybipy

3+

2.537(2)–2.773(2)

76-Y

3+

2.473(2)–2.7910(19)

77-Y

3+

2.5490(16)–2.7396(17)

78-Y

3+

2.4808(15)–2.7150(14)

169.3/162.4/112.3(Al-CH) 85.9/85.1/80.0 168.9/162.4/111.4(Al-CH) 85.6/84.0/77.6 169.3/161.8/112.0(Al-CH) 85.7/84.7/80.0 168.8/159.1/110.2(Al-CH) 84.8/82.2/79.7 168.2/157.2/109.0 (Al-CH) 83.9/81.2/79.2 169.1/161.0/111.4 (Al-CH) 85.5/83.8/80.0 183.3/169.3/164.7/154.4 (Al-CH-Y)/ 136.1/90.2/84.2 82.1/81.0/67.1 172.7/163.7/106.5 (Al-CH) 83.3/78.5/78.4 –

79-Y

3+

2.426(5)–2.790(5)

–

78

80-Y

3+

2.4282(16)–2.6698(16)

173.3/115.6 (Al-CH)/87.1

78

81-Y

3+

2.467(2)–2.712(3)

181.7/164.2/113.9 (Al-CH) 89.1/88.9/80.9

77

C NMR chemical shift (ppm)

References

77 77 77 77 77 77 78 78

tBu2-C5H5)La[(m-Me)2AlMe(C6F5)]}+[Me2Al(C6F5)2]−}2 (72-La) as a rare example of a single-component catalyst (Scheme 33).76 Complex 72-LatBu features less tightly bonded contact ion pairs than the C5Me5-supported congener {{(C5Me5)La[(m-Me)2AlMe(C6F5)]}+[Me2Al(C6F5)2]−}2,79 which, in addition to the changed sterics of the ancillary ligand, markedly affected the polymerization performance (46.0% trans-1,4, Mw/Mn ¼ 1.10 versus 98.7% trans-1,4, Mw/Mn ¼ 1.19).

Scheme 33 Synthesis of hexametallic open half-sandwich derivative via Me/C6F5 interchange. Complex 72-La acts as a single-component catalyst in isoprene polymerization. Barisic, D.; Buschmann, D. A.; Schneider, D.; Maichle-Mössmer, C.; Anwander, R., Chem. Eur. J. 2019, 25, 4821–4832.

Buta- and Pentadienyl Complexes of the Group 3 Metals and Lanthanides

23

Interestingly, analogous reactions of the homoleptic complexes [Ln(AlMe4)3] with the smaller rare-earth-metal centers yttrium and lutetium in the presence of the sterically encumbered Pdl ligand 2,4-di(tert-butyl)pentadienyl afforded half-open rare-earthmetal aluminabenzene complexes [(1-Me-3,5-tBu2-C5H3Al)(m-Me)Ln(2,4-tBu2-C5H5)] (73-Ln Ln ¼ Y, Lu), clearly indicating increased reactivity (non-innocence) of Pdl ligands over cyclopentadienyl derivatives (Scheme 34).77 The first isolation of an anionic aluminabenzene, [Li(1-Mes-2,6-C5H3Al)] (Mes ¼ 2,4,6-trimethylphenyl), was reported by Yamashita et al. applying a reaction protocol with a silyl-substituted diyne, DIBAL-H, and mesityllithium as precursors.80 The 2,4-di-tert-butylpentadienyl ligand in 73-Ln shows a typical 1:2:2 ratio of the signals of the C5-fragment in the 1H NMR spectrum with a strong upfield shift of the terminal methylene endo protons, pointing to the metal center, compared to the exo protons. The Z5-coordinated methylated aluminabenzene ligand is almost planar with the aluminum center and the central carbon atom slightly deviating from the C1-C2-C4-C5-plane by secondary > tertiary, with the exception of 11-U where R ¼ 2-trans2-butenyl, which was found to be as stable as R ¼ Me.22 The solid-state molecular structure of 7-U was subsequently determined by single-crystal XRD and showed a distorted tetrahedron with three
5-cyclopentadienyl ligands and a s-bonded n-butyl ligand.24

Scheme 4 Synthesis of tris-cyclopentadienyl supported uranium buta- and penta-dienyl complexes 7-U to 11-U.

The analogous thorium complexes [(
5-C5H5)3Th(R)] where R ¼ n-butyl 7-Th, neopentyl 9-Th, 2-cis-2-butenyl 10-Th, and 2-trans-2-butenyl 11-Th were prepared via a similar route by the reaction of [Th(
5-C5H5)3(Cl)] with [LiR] in cold toluene (Scheme 5A).25 The synthetic conditions required to isolate complexes 7–11 were found to be more critical to control when An ¼ Th than An ¼ U, as reduction to thorium metal competes with alkylation. Complexes 7-Th to 11-Th also had lower solubilities in common organic solvents.25,26 Complexes 7-Th, 9-Th, 10-Th and 11-Th had greater thermal stability in both the solid state (7-Th had a melting point of 210  C) and in toluene solution than 7-U to 11-U. However, the ThdC s-bond was also found to react more readily than the UdC s-bond with protic solvents in complexes 7–11, for example the reaction with methanol occurs several orders of magnitude faster for the ThdC s-bond than the U-C s-bond. The thermolysis pathway of complexes 7–11 where were demonstrated to be the same as for An ¼ U or Th. In the case of complex 7-Th the dimeric thorium-containing decomposition product [{(
5-C5H5)2Th(m-
1:
5-C5H4)}2] was isolated, and the solid-state molecular structure determined by single-crystal XRD (Scheme 5B).25,27

Scheme 5 (A) Synthesis of tris-cyclopentadienyl supported thorium buta- and penta-dienyl complexes 7-Th, 9-Th, 10-Th, and 11-Th. (B) Thermal decomposition of 7-Th.

Buta- and Penta-Dienyl Complexes of the Actinides

37

Using synthetic routes closely related to those described in Schemes 4 and 5, [(
5-C5H5)3U(sec-Bu)]26 12-U and [(
-C9H7)3U(nBu)]28 13-U were synthesized and the photolysis of the whole family of complexes was studied.26,29 Photolysis of complexes 7-Th to 13-Th in aromatic solvents resulted in a 1:1 mixture of alkane:alkene and [Th(
5-C5H5)3] in an almost quantitative yield. A detailed mechanistic study concluded that this reaction occurs by photoinduced b-H elimination. However, in the case of complex 13-U, photolysis yielded only the alkane by hydrogen abstraction from the indenyl ligand and [(C9H7)3U]. Photolysis of complexes 7-U to 12-U produced RH and [(
5-C5H5)3U], with some evidence for photoinduced b-H elimination, but this was determined to be the minor pathway. For complexes 7-U to 12-U the mechanism of both photolysis and thermolysis occurred by hydrogen abstraction from the cyclopentadienyl ligand or solvent.26 The reactivity of the s-bond in [(
5-C5H5)3An(R)] differed when An ¼ Th or U, an example is the migratory insertion of CO. The reaction between complexes 7-Th, 9-Th, 12-Th with 1 atm. CO resulted in the irreversible formation of the acyl compounds [(
5-C5H5)3Th(
2-OCR)] where R ¼ n-butyl 50%, sec-butyl 75–90%, neopentyl 75–85% (Scheme 6A). Acyl stretching frequencies in the range of 1495–1435 cm−1 were consistent with
2-acyl binding. The relative reactivity of the s-bond was secbutyl  neopentyl > n-butyl.30 The insertion of CO into the alkyl bond of complexes 7-U and 8-U was also reported to occur under mild conditions in solution to form the acyl compounds [(
5-C5H5)3U(
2-OCR)] where R ¼ n-butyl, t-butyl, but this reaction was reported to be reversible in toluene,31 as well as diethyl ether and pentane (Scheme 6B).32 However, in THF the same reaction resulted in the formation of alkylbenzenes (PhdR) from the ring expansion of one of the cyclopentadienyl ligands and an insoluble organometallic formulated as [Cp2UO]n, product of the deoxygenation of CO (Scheme 6C). The acyl complexes were proposed to be intermediates in this reaction.32 These results and the related carbonyl chemistry of [U(CpR)3], were some of the first indications of the primary importance of solvent choice in both uranium(III) chemistry in general, and small molecule reactivity in particular.33,34 5

Scheme 6 Reactivity toward CO of tris-cyclopentadienyl actinide buta- and penta-dienyl complexes.

As mentioned above, the photolysis of [(
5-C5H5)3U(R)], resulted in reduction of uranium(IV) to uranium(III).26 The optimal conditions for the photoreduction of 7-U to [(
5-C5H5)3U(THF)] (70%) were found to be irradiation with UV light, in THF at 60  C.35 In fact, butyl lithium reagents were shown to be useful reducing agents for the synthesis of uranium(III) complexes from uranium(IV). This is the case both for the synthesis of [(
5-CpR)3U] (R ¼ SiMe3 or tBu) from the reaction of [(
5-CpR)3UCl] with [tBuLi],36,37 and the reduction of [(
5-CpR)2UCl2] to [(
5-CpR)2UCl]3 for R ¼ Me5 using [tBuLi],34 and [(
5-CpR)4U2Cl2] for R ¼ 1,3-(SiMe3)2 using [nBuLi].38,39 Another example of reduction but without complete elimination of the s-ligand, is the reaction of [(
5-C5H5)3U(nBu)] with [LiR] (R ¼ nBu, Me, Ph) in a mixture of THF and hexanes, which yielded very unstable powders formulated as [Li
THF][(
5-C5H5)3U(nBu)] 14-U, where the n-butyl group has been eliminated in the course of the reaction, and replaced in the case of R ¼ methyl and phenyl. The addition of 2.1.1-cryptand (C14H28N2O4) to complex 14-U resulted in the formation of single crystals and the solid-state molecular structure of [LiC14H28N2O4][(
5-C5H5)3U(nBu)] 14a-U was determined by XRD.40

38

Buta- and Penta-Dienyl Complexes of the Actinides

Using monomethyl cyclopentadienyl ancillary ligands enabled the synthesis of [(
5-MeC5H4)3U(tBu)] 15-U by reaction of [(
-MeC5H4)3U(X)] (X ¼ Cl, MeC5H4) with [tBuLi] (Scheme 7A).41,42 This dark green complex was reported to be light-sensitive and needed to be stored in the dark at −20  C to avoid decomposition. Although the melting point of [(
5-MeC5H4)3U(tBu)] is reported as 224–228  C, it was noted that since melting always resulted in decomposition, the high solid-state melting point should not be equated with high thermal stability. Indeed, 15-U reacts readily with Lewis bases to form the uranium(III) adducts [(
5-MeC5H4)3U(L)] (L ¼ PMe3, THF, tBuCN, tBuNC, EtNC) (Scheme 7B). The rate of this reaction correlates well with Lewis basicity, i.e., the greater the donor ability of the Lewis base to stabilize U(III), the faster the reaction. Further reactivity of 15-U includes fluorine atom abstraction,42 and both ethylene and CO insertion.41 The insertion of CO into the UdtBu bond in 15-U also forms an
2-acyl, [(
5-MeC5H4)3U(
2-OCtBu)], with an acyl stretching frequency of 1490 cm−1. The CO reaction is irreversible, but upon heating to 90  C in toluene, a decomposition reaction very similar to that shown in Scheme 6C32 occurred to produce an essentially quantitative yield of m- and p-tert-butyltoluene in a 1:5 ratio, and an insoluble organometallic material formulated as [(
5-MeC5H4)2U(O)]n. The thorium analogue of 15 was expected to be synthetically accessible, however, reactions of [(
5-MeC5H4)3Th(X)] (X ¼ Cl, I, MeC5H4, O-2,6-Me2C6H3) with [tBuLi] did not yield 15-Th. The reaction of [(
5-CpR)3Th] [BPh4] (R ¼ SiMe3 or tBu) with [tBuLi] in d6-benzene quantitatively yielded the thorium hydride complex [(
5-CpR)3Th(H)] and isobutene, consistent with b-H elimination.41 Important structural metrics for Section 4.02.2 are shown in Fig. 3 and selected spectroscopic data for Section 4.02.2.3 in Table 3. 5

Scheme 7 Synthesis of complex 15-U and reactivity with Lewis bases.

Table 3

Data Table for heteroleptic hydrocarbyl actinide buta- and penta-dienyl complexes with a tris-cyclopentadienyl ancillary ligand environment (Section 4.02.2.3).

Formula

[(
5-C5H5)3Th(nBu)] [(
5-C5H5)3U(nBu)]

Oxidation state

+IV +IV

Single crystal X-ray diffraction data AndC and selected bond ˚ distances (A)

Selected bond angles (  )

UdCa 2.426(23); UdCpcentroid 2.470–2.494

UdCadCb 128.5(16); CadUdCpcentroid 98.2–102.3; CpcentroiddUdCpcentroid 115.8–118.1

1

[(
5-C5H5)3U(tBu)] +IV [(
5-C5H5)3Th(CHt2Bu)] +IV +IV

[(
5-C5H5)3Th(2-cis2-butenyl)] [(
5-C5H5)3U(2-cis-2butenyl)] [(
5-C5H5)3Th(2trans-2-butenyl)] [(
5-C5H5)3U(2-trans2-butenyl)] [(
5-C5H5)3Th(sBu)]

+IV

+IV

[(
5-C5H5)3U(sBu)]

+IV

[(C9H7)3Th(nBu)] [Li
THF] [(
5-C5H5)3U(nBu)] [LiC14H28N2O4] [(
5-C5H5)3U(nBu)]

+IV +III

[(
5-MeC5H4)3U(tBu)]

+IV

+IV +IV +IV

+III

Magnetometry

H NMR (d8-toluene, 25  C): d 1.39 (m, 9H, nBu) 6.00 (s, 3  5H, Cp-H) ppm H NMR (d8-toluene, 25  C): d 10.3 (s, 3  5H, Cp-H) 18.7 (t, 3H, U(CH2)3CH3) 3.36 mB solid-state magnetic 27.6 (m, J ¼ 7.0 Hz, 2H, CH2) 33.6 (m, J ¼ 7.0 Hz, 2H, CH2) 200.0 moment (180 K); wM ¼ 2.945  10−3 cgs (m, J ¼ 7.0 Hz, 2H, CH2) ppm solution-state magnetic moment (in benzene) 1 H NMR (d8-toluene, 25  C): d 11.4 (s, 3  5H, Cp-H) 23.2 (s, 9H, U(tBu)) ppm 1 H NMR (d8-toluene, 25  C): d 1.28 (s, 2H, ThCHt2Bu) 1.41 (s, 9H, ThCH2tBu) 6.09 (s, 3  5H, Cp-H) ppm 1 H NMR (d8-toluene, 25  C): d 11.6 (s, 3  5H, Cp-H) 22.1 (s, 9H, UCH2tBu) 192.0 (s, 2H, UCHt2Bu) ppm 1 H NMR (d8-toluene, 25  C): d 2.05 (d of m, J ¼ 6.0 Hz, 3H, CH3) 2.18 (m, 3H, CH3) 6.10 (s, 3  5H, Cp-H) 6.45 (m, J ¼ 6.0 Hz, 1H, CH) ppm 1 H NMR (d8-toluene, 25  C): d 10.6 (s, 3  5H, Cp-H) 19.8 (s, 3H, CH3) 22.6 (q, J ¼ 6.0 Hz, 1H, CH) 42.3 (d, J ¼ 6.0 Hz, 3H, CH3) ppm 1 H NMR (d8-toluene, 25  C): d 2.11 (d of m, J ¼ 6.0 Hz, 3H, CH3) 2.56 (m, 3H, CH3) 6.16 (s, 3  5H, Cp-H) 6.95 (m, J ¼ 6.0 Hz, 1H, CH) ppm 1 H NMR (d8-toluene, 25  C): d 10.7 (s, 3  5H, Cp-H) 23.5 (q, J ¼ 7.0 Hz, 1H, CH) 33.0 (d, J ¼ 7.0 Hz, 3H, CH3) 33.6 (s, 3H, CH3) ppm 1 H NMR (d6-benzene, 25  C): d 0.84 (m, 1H, CH) 1.26 (m, 3H, CH3) 1.55 (m, 1H, CH) 1.85 (d, 3H, CH3) 2.21 (m, 1H, CH) 5.90 (s, 3  5H, Cp-H) ppm 1 H NMR (d6-benzene, 25  C): d 11.1 (s, 3  5H, Cp-H) 14.4 (m, 3H, CH3) 21.4 (m, 1H, CH) 23.4 (m, 3H, CH3) 33.5 (m, 1H, CH) 186 (m, 1H, CH) ppm 1

1

H NMR (d6-benzene, 60 MHz, 25  C): d 12.0 (t, 3H, U(CH2)3CH3) 14.0 (m, 2H, CH2) 15.8 (m, 2H, CH2) 21.3 (s, 3  5H, Cp-H) 98.5 (m, 2H, CH2) ppm

UdCadCb 120(1); UdCa 2.557(9); UdCpcentroid 2.564 CadUdCpcentroid 97(1)–101(1); (8)–2.595(12) CpcentroiddUdCpcentroid 117(1)–118(1) 1

H NMR (d6-benzene, 89.56 MHz, 30  C): d − 18.96 (s, 9H, Me) −8.98 (s, 9H, Me) −6.25 (s, 6H, Cp-H) 9.96 (s, 6H, Cp-H) ppm

Complex number

Reference

7-Th 7-U

25 1 H NMR and magnetometry data22; SCXRD data24

8-U 9-Th

22 25

9-U

22

10-Th

25

10-U

22

11-Th

25

11-U

22

12-Th

30

12-U

26

13-Th 14-U

28 40

14a-U

40

15-U

41

Buta- and Penta-Dienyl Complexes of the Actinides

[(
5-C5H5)3U(CHt2Bu)]

Nuclear magnetic resonance data

39

40

Buta- and Penta-Dienyl Complexes of the Actinides

4.02.2.4 Heteroleptic hydrocarbyl actinide buta- and penta-dienyl complexes with a bis-cyclopentadienyl ancillary ligand environment Bis-cyclopentadienyl supported actinide complexes provided the first comprehensive insight into the chemistry of the actinidecarbon s-bond, as these complexes react with a wide range of reagents. The comparison of the properties of these actinide metallocenes to their well-studied group 4 analogues also provided important data on the differences between d-block and f-block chemistry. The bis-pentamethylcyclopentadienyl actinide mixed chloride neopentyl complexes [(
5-C5Me5)2An(CHt2Bu) (Cl)] 16-An (An ¼ Th, U) were synthesized by the reaction of [(
5-C5Me5)2An(Cl)2] with [LiCHt2Bu] in cold THF in a 1:1 ratio (Scheme 8).40,43,44 The successful isolation of useful product from these reactions was found to be critically dependent on the purity of starting materials and careful control of reaction conditions. Complex 16-U was unstable at room temperature and decomposed rapidly in aromatic solvents, whereas complex 16-Th had much greater thermal stability. The reaction of 16-Th with 1 atm. of CO resulted in the insertion of CO into the Th-alkyl bond and isolation of the acyl complex [(
5-C5Me5)2Th(
2-COCHt2Bu)(Cl)] −1 −1 2 (Scheme 8). The CO stretching frequencies (n12 and n13 CO ¼ 1469 cm CO ¼ 1434 cm ) were consistent with an
-acyl binding mode. The solid-state molecular structure was determined by single-crystal XRD and revealed the Th-acyl unit to be significantly different from analogous group 4 complexes, with an acute ThdCdO angle of 73 (1) and an almost linear ThdCadCb angle of 169 (2) . The different geometric preferences of
2-acyl complexes of group 4 and the actinides were rationalized by DFT studies.45

Scheme 8 Synthesis of complexes 16-An (An ¼ Th, U) and CO insertion reactivity of complex 16-Th.

In contrast to group 4 and uranium acyl chemistry (Section 4.02.2.3) no decarbonylation of [(
5-C5Me5)2Th(
2-COCHt2Bu)(Cl)] occurred either on heating or subsequent reaction with CO. Heating a toluene solution of [(
5-C5Me5)2Th(
2-COCHt2Bu)(Cl)] at 100  C for 15 h resulted in the isolation of the H-migration product [(
5-C5Me5)2Th(
1-OCH]CHtBu)(Cl)] (Scheme 9). Furthermore, the reaction of [(
5-C5Me5)2Th(
2-COCHt2Bu)(Cl)] with excess CO at low pressures yielded the centrosymmetric dimer [(Th(
5-C5Me5)2{m-
2-OC(CHt2Bu)CO}Cl)2] (Scheme 9). The enedione diolate is the product of the coupling of four CO molecules (C4).44 The insertion of CO, followed by reductive coupling and C]C bond formation is also common to [(
5-C5Me5)2Th(R)2] (R ¼ Me, CH2SiMe3). The organic products (C1 or C2) and nuclearity of the resultant actinide complexes correlated well with the sterics and the bond strength of the specific R group.46 This carbon-carbon coupling chemistry is consistent with oxycarbene character of the thorium acyl moiety (resonance structure shown in Scheme 6A).18 The thorium acyl complex [(
5-C5Me5)2Th(
2-COCHt2Bu)(Cl)] was also hydrogenated using a catalytic amount of [{(
5-C5Me5)2Th(m-H)(H)}2] (Scheme 9).47,48

Scheme 9 Reactivity of the thorium acyl complex [(
5-C5Me5)2Th(
2-COCHt2Bu)(Cl)].

Buta- and Penta-Dienyl Complexes of the Actinides

41

Scheme 10 Synthesis of complexes 17-Th to 20-Th.

The ansa-metallocene complexes, [{Me2Si-(
5-C5Me4)2}Th(R)2] where R ¼ nBu 17-Th and CHt2Bu 18-Th were synthesized from [{Me2Si-(
5-C5Me4)2}Th(Cl)2] and 2 eq. of [LiR] at low temperature (Scheme 10A).49,50 The mixed-alkyl complex [(
5-C5Me5)2Th(CHt2Bu)(CH2SiMe3)] 19-Th, was synthesized from the reaction of [(
5-C5Me5)2Th(Cl)(CH2SiMe3)] and [LiCHt2Bu] (Scheme 10B).51 The bis(neopentyl) thorium complex [(
5-C5Me5)2Th(CHt2Bu)2] 20-Th was synthesized by reaction of 2 eq. of [LiCHt2Bu] with [(
5-C5Me5)2Th(Cl)2] in cold ether (Scheme 10C). However, the synthesis of the analogous uranium complex was unsuccessful. The structure of complex 20-Th was determined by neutron-diffraction experiments and revealed that the neopentyl ligands are significantly structurally inequivalent.52 The ThdCa bond distances were determined to be significantly different to one another at 2.456 (4) A˚ and 2.543 (4) A˚ . The ThdCadCb angles were also significantly different with ThdCadCb of 132.1 and 158.2 , for the shorter and longer ThdCa bonds, respectively. This structural disparity was attributed to an agostic bonding interaction.18 The s-bonded ligands in all compounds of [(
5-C5Me5)2An(R)(Cl)], [{Me2Si-(
5-C5Me4)2}Th(R)2] and [(
5-C5Me5)2An(R)2] underwent facile alcoholysis, halogenolysis and hydrogenolysis. All actinide complexes in a bis-pentamethylcyclopentadienyl ligand environment, [(
5-C5Me5)2An(R)(Cl)] and [(
5-C5Me5)2An(R)2], were significantly less thermally stable than those in a tris-pentamethylcyclopentadienyl environment [(
5-C5Me5)3An(R)]. The thorium complexes were more stable than the uranium complexes and for any given R group, [(C5Me5)2An(R)(Cl)] was more stable than [(
5-C5Me5)2An(R)2].43 Heating complex 20-Th in benzene at 100  C for 4 h resulted in a s-bond metathesis53 reaction yielding the diphenyl complex 5 [(
-C5Me5)2Th(C6H5)2] and 2 eq. of neopentane (Scheme 11). However, heating complex 20-Th in heptane at 50  C for 60 h yielded the metallacycle [(
5-C5Me5)2Th(
2-CH2CMe2CH2)] (Scheme 11).52,54 Heating [(
5-C5Me5)2Th(CH2SiMe3)2] or 19-Th also resulted in metallacycle formation. It is of note that metallacycle formation with thorium is 100 times faster than the analogous reaction with zirconium.19 The strained thoracyclobutanes undergo facile ring-opening CdH activation reactions with saturated hydrocarbons including methane (Scheme 11).51,52,54 The mixed-alkyl complex [(
5-C5Me5)2Th(CHt2Bu)(CH3)] 21-Th, was synthesized by the reaction of 1.4 eq. of MeLi
LiBr with 16-Th in Et2O at room temperature (Scheme 11).51 Complex 21-Th can also be synthesized the reaction of [(
5-C5Me5)2Th(
2-CH2CMe2CH2)] with 5.8 atm. of methane in cold cyclohexane (Scheme 11). Likewise the reaction of [(
5-C5Me5)2Th(
2-CH2CMe2CH2)] with tetramethylsilane at 30  C also resulted in CdH activation and the isolation of complex 19-Th (Scheme 11).51

42

Buta- and Penta-Dienyl Complexes of the Actinides

Scheme 11 Reactivity of complex 20-Th and synthetic routes to complexes 19-Th and 21-Th.

Detailed thermochemical studies were undertaken to determine experimental values for the metal-ligand s-bond disruption enthalpies (D) in [(
5-C5H5)3An(R)], [(
5-C5Me5)2An(R)(Cl)] and [(
5-C5Me5)2An(R)2].55–58 These data showed that An-R s-bonds are stronger than M-R, where M is a middle or late transition metal, and comparable to or stronger than M-R where M is an early transition metal. The ThdR s-bond was found to be 5–10 kcal mol−1 stronger than the UdR s-bond in analogous complexes. Actinide(IV) (An ¼ U, Th) complexes are known to be catalytically active, examples include the reactions of unsaturated groups, polymerization, hydroamination and hydrosilylation.59–62 One common pre-catalyst for a range of catalytic transformations is [(
5-C5Me5)2An(Me)2] (An ¼ U, Th). However, complex 17-Th (Scheme 10A) is also a pre-catalyst for the regioselective dimerization of terminal alkynes, and hydrosilylation of terminal alkynes or alkenes with PhSiH3.62,63 Complex 17-Th was found to be both faster and produce increased regio-selectivity of products than [(
5-C5Me5)2Th(Me)2]. The proposed mechanism for the dimerization of terminal alkynes promoted by complex 17-Th is shown in Scheme 12. The proposed catalytic intermediates B and C in Scheme 12 are relevant to the alkyne coupling chemistry found in the synthesis and reactivity of unsaturated buta- and pentadienyl ligands, which are covered in Sections 4.02.3 and 4.02.4. Important structural metrics for Section 4.02.2 are shown in Fig. 3 and selected spectroscopic data for Section 4.02.2.4 in Table 4.

Buta- and Penta-Dienyl Complexes of the Actinides

Scheme 12 The dimerization of terminal alkynes by complex 17-Th.

43

44

Data table for heteroleptic hydrocarbyl actinide buta- and penta-dienyl complexes with a bis-cyclopentadienyl ancillary ligand environment (Section 4.02.2.4).

Formula

Oxidation state

Neutron diffraction data AndC bond ˚ distances ( A)

+IV

[{Me2Si-(
5-C5Me4)2} Th(nBu)2]

+IV

[{Me2Si-(
5-C5Me4)2} Th(CHt2Bu)2] [(
5-C5Me5)2Th(CHt2Bu) (CH2SiMe3)]

+IV

[(
5-C5Me5)2Th (CHt2Bu)2]

+IV

[(
5-C5Me5)2Th(CHt2Bu) (CH3)]

+IV

+IV

ThdCa 2.543(4), 2.456(4)

Complex number

Reference

1

16-Th

43

16-U

43

17-Th

50

18-Th

50

19-Th

1

Selected bond angles (  )

[(
5-C5Me5)2Th(CHt2Bu)(Cl)] +IV [(
5-C5Me5)2U(CHt2Bu)(Cl)]

Nuclear magnetic resonance data

ThdCadCb 132.1(3), 158.2(3); CadThdCa 98.1(1); CadThdC5Mecentroid 102.6 5 (1)–108.3(1); dThdC5Mecentroid C5Mecentroid 5 5 133.0(1)

H NMR (d6-benzene, 35  C): d 0.55 (s, 2H, ThCHt2Bu) 1.30 (s, 9H, ThCH2tBu) 1.99 (s, 2  15H, C5Me5) ppm 1 H NMR (d6-benzene, 35  C): d − 83 (s, 2H, UCHt2Bu) −7.17 (s, 9H, UCH2tBu) 8.77 (s, 2  15H, C5Me5) ppm 1 H NMR (d6-benzene, 90 MHz, 25  C): d 0.09 (t, 2  2H, ThCH2) 0.78 (s, 6H, SiMe2) 1.06 (t, 2  3H, Th(CH2)3CH3) 1.45 (m, 2  4H, ThCH2(CH2)2) 1.97 (s, 12H, Cp-Me) 2.17 (s, 12H, Cp-Me) ppm 1 H NMR (d6-benzene, 90 MHz, 25  C): d 0.10 (s, 2  2H, ThCH2) 0.76 (s, 6H, SiMe2) 1.22 (s, 2  9H, ThCH2tBu) 2.11 (s, 12H, Cp-Me) 2.16 (s, 12H, Cp-Me) ppm 1 H NMR (d12-cyclohexane, 90 MHz, 25  C): d − 0.58 (s, 2H, ThCH2) −0.08 (s, 2H, ThCH2) 0.08 (s, 9H, Me) 1.04 (s, 9H, Me) 2.10 (s, 2  15H, C5Me5) ppm; 13 C NMR (d6-benzene, 67.8 MHz, 25  C): d 4.9 (1JC-H ¼ 117 Hz, ThCH2SiMe3) 12.1 (1JC-H ¼ 126 Hz, C5Me5dMe) 36.6 (1JC-H ¼ 122 Hz, ThCH2CMe3) 39.1 (ThCH2CMe3) 74.2 (1JC-H ¼ 11 Hz, ThCH2SiMe3) 110.7 (1JC-H ¼ 100 Hz, ThCHt2Bu) 123.7 (C5Me5) ppm (Multiplicities of the resonances were not reported) 1 H NMR (d6-benzene, 35  C): d 0.15 (s, 2  2H, ThCHt2Bu) 1.30 (s, 2  9H, ThCH2tBu) 2.05 (s, 2  15H, C5Me5) ppm; 13 C NMR (d6-benzene, 67.8 MHz, 25  C): d 12.3 (1JC-H ¼ 126 Hz, C5Me5dMe) 36.4 (1JC-H ¼ 117 Hz, ThCH2CMe3) 39.1 (ThCH2CMe3) 110.2 (1JC-H ¼ 97 Hz, ThCHt2Bu) 123.7 (C5Me5) ppm (Multiplicities of the resonances were not reported)

1

H NMR data51; C NMR data52

13

20-Th

H NMR (d12-cyclohexane, 270 MHz, 25  C): d − 0.38 (s, 3H, ThCH3) −0.12 (s, 2H, ThCH2) 21-Th 0.94 (s, 9H, ThCH2tBu) 2.01 (s, 2  15H, C5Me5) ppm; 13ce:underline> C NMR (d6-benzene, 67.8 MHz, 25  C): d 11.7 (q, JC-H ¼ 125.9 Hz, C5Me5dMe) 36.9 (q, JC-H ¼ 124.7 Hz, ThCH2CMe3) 37.2 (s, ThCH2CMe3) 71.7 (q, JC-H ¼ 111.8 Hz, ThdCH3) 101.7 (t, JC-H ¼ 103.6 Hz, ThdCH2) 123.2 (s, C5Me5) ppm

1

H NMR data43; C NMR and neutron diffraction data52

13

51

Buta- and Penta-Dienyl Complexes of the Actinides

Table 4

Buta- and Penta-Dienyl Complexes of the Actinides

4.02.3

Acyclic butene-diyl and butadiene-diyl complexes of the actinides

4.02.3.1

Acyclic 2-butene-1,4-diyl complexes of the actinides

45

Actinide complexes of the 2-butene-1,4-diyl ligand are referred to both as actinacyclopentenes and butadiene complexes in the literature. However, neither of these nomenclatures is used here, to avoid confusion between the unsaturation and the binding mode of the (
4-CH2CR]CRCH2)2− ligand. Acyclic, 2-butene-1,4-diyl actinide complexes were first prepared by two synthetic routes. The reaction of [(
5-C5Me5)2AnCl2] (An ¼ Th, U) with [(THF)2Mg(CH2CR]CRCH2)] (R ¼ H, CH3) yielded [(
5-C5Me5)2An(
4-CH2CR]CRCH2)] (22, An ¼ Th and U, R ¼ H; 23-Th, An ¼ Th, R ¼ Me) (Scheme 13A).64,65 Alternatively, the reaction of [(
5-C5Me5)2U(Me)Cl] with [BrMg(CH2CH2CH]CH2)] resulted in the elimination of methane and isolation of 22-U (Scheme 13B).64 The coupling of alkenes was also investigated as a synthetic route to 22-An, by heating of the bis-alkene complexes [(
5-C5Me5)2An(CH]CH2)2] (An ¼ Th, U). When An ¼ U a 30%  10% yield of 22-U was observed by NMR spectroscopy. However, where An ¼ Th, a complex mixture of products was obtained, which did not include 22-Th.64

Scheme 13 Synthesis of complexes 22-An and 23-Th.

The experimental data for 22-An were consistent with s2,p–binding of the (
4-CH2CR]CRCH2)2− ligand, rather than the p –olefin, s–trans–p2 or planar s–cis–s2 binding modes seen in transition metal chemistry. The solid-state molecular structure of 22Th was determined by single-crystal XRD and confirmed the
4-binding mode. The ThdC distances to the (
4-CH2CR]CRCH2)2− ligand fall within the normal range for Thdalkyl bonds. The terminal ThdC (C1 and C4) bond distances were 2.54 (2) and 2.60 (2) A˚ , and the internal ThdC (C2 and C3) bond distances were 2.73 (2) and 2.74 (3) A˚ . The small difference between the ThdC distance to C1,C4 or C2,C3 reflects the difference in displacement either above (C2 ¼ 0.67 A˚ and C3 ¼ 0.71 A˚ ) or below (C1 ¼ 0.36 A˚ and C4 ¼ 0.38 A˚ ) the equatorial plane of the thorium ion in a bent metallocene geometry. The (
4-CH2CR] CRCH2)2− ligand bends away from the thorium metal center, which also contributes to the lengthening of the internal ThdC (C2 and C3) bond distances. The side-on binding of the (
4-CH2CR]CRCH2)2− ligand was clearly demonstrated by the near perpendicular ThdC1dC2 and ThdC4dC3 angles of 80 (1) and 81 (2) , respectively. The data were consistent with the delocalization of the C]C double bond over C2dC3dC4. The C1dC2 bond distance was 1.52 (4) A˚ , consistent with a single CdC bond, whereas the C2dC3 distance was 1.44 (3) A˚ and the C3dC4 distance was 1.40 (4) A˚ , which are both consistent with partial double bond character. The internal angles of the (
4-CH2CR]CRCH2)2− ligand deviated only slightly from the 120 angle expected for sp2 hybridization for the C4dC3dC2 angle of 121 (2) but the angle of C1dC2dC3 was significantly more obtuse at 129 (2) .59 The room temperature 1H NMR spectra of complexes 22-An were consistent with magnetically inequivalent pentamethylcyclopentadienyl ligands and magnetically inequivalent a-methylene (C1 and C4) proton environments. In contrast, the room temperature 1H NMR spectrum of 23-Th displayed magnetically equivalent pentamethylcyclopentadienyl ligands and methyl groups of the (
4-CH2CMe]CMeCH2)2−. Fluxional behavior with temperature consistent with rapid ring inversion of the (
4-CH2CR]CRCH2)2 − ligand via the planar metallacyclopentene was also observed on the NMR timescale. The free energies of activation (DG{) at coalescence temperatures (Tc) for the ring inversion processes were determined using variable temperature NMR spectroscopy. Specifically, the 1H pentamethylcyclopentadienyl NMR resonances for 22-An and 23-Th were monitored. These data for 22-An (22U ¼ 17.0 kcal mol−1 at 394 K; 22-Th ¼ 15.0 kcal mol−1 at 299 K and 15.5  0.3 kcal mol−1 at 296 K) and 23-Th (10.5 kcal mol−1 at 208 K) are consistent with a higher energetic barrier than seen the same ring inversion process in the group 4 analogues 2

46

Buta- and Penta-Dienyl Complexes of the Actinides

[(
5-C5H5)2M(
4-CH2CR]CRCH2)] (M ¼ Zr, R ¼ H, 12.6 kcal mol−1 at 253 K; M ¼ Zr, R ¼ Me, 11.5 kcal mol−1 at 231 K; M ¼ Hf, R ¼ H 8.1 kcal mol−1 at 186 K).64,65 As expected, increasing the steric bulk of the (
4-CH2CR]CRCH2)2− ligand from R ¼ H to R ¼ Me resulted in a lowering of the energetic barrier to ring inversion. The overall trend is the same, but the energetic difference is much greater in 23-Th vs 22-Th (D − 4.5 kcal mol−1, D − 91 K), than in [(
5-C5H5)2Zr(
4-CH2CR]CRCH2)] where R ¼ H vs R ¼ Me (D − 1.1 kcal mol−1, D − 22 K). The one-bond coupling constants 1JCdH of the a–methylene protons determined from 13C NMR spectroscopy have previously been used as metrics in transition metal chemistry to characterize the bonding in butadiene ligands, where the limits were defined as s–only 1JC-H ¼ 125 Hz and p–only 1JC-H ¼ 168 Hz.66 However, caution is advised in cases where there is fluxional behavior on the NMR timescale, as the room temperature value of 1JCdH may be a time-average of two quite different values. For example, the room temperature 1JCdH data for 22-Th and 23-Th are 142 and 133 Hz, respectively. Whereas the 1JCdH at the slow-exchange limit data are 131 and 153 Hz for 22-Th, and 123 and 140 Hz for 23-Th.59 Batch titration calorimetry was used to determine experimental values for the metal-ligand bond disruption enthalpies (D), by alcoholysis of the AndR bond. The reaction of 22-Th or 23-Th with 2 eq. of tBuOH yielded [(
5-C5Me5)2An(OtBu)2] and a mixture of butene isomers (Scheme 14A). The ratio of the butene isomers was determined by 1H NMR spectroscopy and gas chromatography. These isomers likely result from fluxional (
1 or
3) behavior in the allyl intermediate resulting from the first protonation of the (
4-CH2CR]CRCH2)2− ligand during alcoholysis. The solution phase D(M-R) (2s) data are 76.1(2.4) kcal mol−1 for 22-Th and 65.5(3.2) kcal mol−1 for 23-Th. These data are the composite of two values as the double alcoholysis of the (
4-CH2CR]CRCH2)2− ligand occurred in two steps, but sequential data could not be obtained as the rate of the second step was determined by 1H NMR spectroscopy to be much faster than the first. The data for 22-Th are similar to the average solution value for D(MdR) (2s) in 20 of 74.6(3.8) kcal mol−1.55 The lower bond disruption enthalpy for 23-Th vs 22-Th, is consistent with the increased steric pressure in 23Th. The differences in the values of D are of a similar magnitude to the difference in DG{ in 22-Th and 23-Th.59

Scheme 14 Alcoholysis and hydrogenolysis of 22-Th.

Complexes 22-An (An ¼ U and Th) and 23 were found to have good thermal stability, as evidenced by the observation of little to no decomposition by 1H NMR spectroscopy after extended heating (22-U at 120  C in d6-benzene for 4 h; 22-Th at 140  C in d8-toluene for 20 h). Complexes 22-An and 23-Th did undergo facile hydrogenolysis (Scheme 14B) and protonolysis reactions, which are also common to all s-bonded buta- and penta-dienyl cyclopentadienyl actinide complexes (Section 4.02.2). However, no CdH activation or diene exchange chemistry was observed for complexes 22-An or 23.64,67 This lack of CdH activation chemistry is in contrast to that observed for [(
5-C5Me5)2An(R)2], for example complex 20 (Scheme 11). Extended-Hückel Molecular Orbital (MO) methods were used to analyze the bonding in 22-An (An ¼ Th, U).67 The calculated data were consistent with the An(IV) oxidation state and a s2,p description of bonding, with strong and very polarized AndC s-bonds and weaker Andp bonding. The MOs of the dianionic 2-butene-1,4-diyl ligand are shown in Fig. 4. The major component (61%) of the highest occupied MO was from the p∗ 3 of the but-2-ene-1,4-diyl ligand, with minor actinide contributions to bonding with significantly more 5f character to bonding in 22-U (22-Th 16% 6d and 17% 5f, and 22-U 6% 6d and 48% 5f ). There were also weaker Andp bonding interactions with the p2–orbital. The bonding in 22-An (An ¼ Th, U) was also compared to group 4 1,3-butadiene complexes. The complex [(
5-C5H5)2Zr(CH2CH]CHCH2)] is known to exist as a mixture of s-cis(s2,p) and s-trans (p2) isomers at room temperature in ratio of 55:45, which is consistent with the very small energy difference (0.07 eV) in favor of the s-cis-isomer. The relative energy of the HOMO (Andp∗ 3 ) was determined to be the most important factor in whether s-cis or s-trans

Buta- and Penta-Dienyl Complexes of the Actinides

47

Fig. 4 Molecular orbitals of the dianionic 2-butene-1,4-diyl ligand.

isomers were favored. In the case of 22-An (An ¼ Th, U) the s-cis isomer was calculated to be much more stable (22-Th 0.74 eV and 22-U 0.42 eV) than the s-trans isomer, and this was consistent with the lack of experimental evidence for the actinide s-trans isomer. Complex 22-Th was reported to react violently with organic carbonyls, but no products could be isolated from these reactions. The reaction of complex 22-Th with pyridine resulted in the isolation in good yield of [(
5-C5Me5)2An(
3:k1-C3H5CH2NC5H5)] (Scheme 15). The reaction between 22-Th and [Cr(CO)6] was likewise reported to yield [(
5-C5Me5)2An(
3:k1-C3H5CH2CO) Cr(CO)5] (Scheme 15).65 The structures of the products in Scheme 15 were assigned based on IR and 2D NMR spectroscopic data and result from CdC coupling of the (
4-CH2CR]CRCH2)2− ligand with either the ortho-carbon of pyridine or one carbonyl ligand of [Cr(CO)6].

Scheme 15 Reactivity of 22-Th.

Recently, further actinide s2,p–complexes of the (
4-CH2CR]CRCH2)2− ligand where R ¼ Ph or SiMe3 have been reported. The reaction of the uranium cyclopropene complex [(
5-C5Me5)2U(
2-C2{SiMe3}2)] with PhCH]CHdCH]CHPh yielded [(
5-C5Me5)2U(
4-CHPhdCH]CHdCHPh)] 24-U (Scheme 16A).68 The reaction of the mononuclear thorium dihydride [(
5-C5Me5)(
5-C5Ar5)ThH2(THF)] (Ar ¼ 3,5-tBu2C6H3) with PhC^CdCH]CHPh or Me3SiC^CdC^CSiMe3 resulted in the isolation of [(
5-C5Me5)(
5-C5Ar5)Th(
4-CHPhdCH]CHdCHPh)] 25-Th and [(
5-C5Me5)(
5-C5Ar5)Th(
4-Me3SiCHdCH] CHdCHSiMe3)] 26-Th, respectively (Scheme 16B).69 The solid-state molecular structures of 25-Th and 26-Th were determined by single-crystal XRD. In both 25-Th and 26-Th the ThdC distances and the side-on binding-mode of the (
4-CH2CR]CRCH2)2−

48

Buta- and Penta-Dienyl Complexes of the Actinides

Scheme 16 Synthesis of complexes 24-U, 25-Th and 26-Th.

ligand are directly comparable to 22-Th. However, there are several key structural differences. In 25-Th and 26-Th the (
4-CH2CR] CRCH2)2− ligand bends towards, rather than away from the thorium metal center and both internal ligand angles (C1dC2dC3, C4dC3dC2) were more obtuse than 120 at 131.8 (6) and 131.6 (5) in 25-Th, and 132.5 (5) and 130.7 (5) in 26-Th. In 25-Th, as in 22-Th, the data were consistent with the delocalization of the C]C double bond over C2dC3dC4. In 25-Th the C1dC2 bond distance was longer at 1.429 (7) A˚ , whereas the C2dC3 and C3dC4 distances were shorter at 1.389 (7) A˚ and 1.398 (7) A˚ . However, in 26-Th the localization of the C]C double bond was observed, as the C1dC2 and C3dC4 bond distances were longer at 1.451 (8) A˚ and 1.471 (7) A˚ , and the C2dC3 bond distance was shorter at 1.375 (8) A˚ .69 Important structural metrics for Section 4.02.3.1 are shown in Fig. 5 and selected spectroscopic data for Section 4.02.3.1 in Table 5.

Fig. 5 Diagrams showing representative complexes and important structural metrics for Section 4.02.3.1.

Table 5

Data table for acyclic 2-butene-1,4-diyl complexes of the actinides (Section 4.02.3.1).

Formula

Oxidation state

[(
5-C5Me5)2Th(
4-CH2CH] CHCH2)]

+IV

[(
5-C5Me5)2U(
4-CH2CH] CHCH2)]

+IV

[(
5-C5Me5)2Th(
4-CH2CMe] CMeCH2)]

+IV

Single crystal X-ray diffraction data AndC and selected bond ˚ distances (A)

Selected bond angles (  )

ThdC1 2.54(2), ThdC4 2.60 (2), ThdC2 2.73(2), ThdC3 2.74(3); C1dC2 1.52(4), C2dC3 1.44(3), C3dC4 1.40(4); 2.56–2.59 ThdC5Mecentroid 5

C1dThdC4 75(1), ThdC1dC2 80(1), ThdC4dC3 81(2); C1dC2dC3 129(2), C4dC3dC2 121(2); C5Mecentroid dThdC5Mecentroid 132 5 5

[(
5-C5Me5)(
5-C5Ar5)Th (
4-CHPhdCH] CHdCHPh)] (Ar ¼ 3,5-tBu2C6H3)

+IV

C1dThdC4 80.4(2), ThdC1dC2 ThdC1 2.501(5), ThdC4 80.8(3), ThdC4dC3 80.1(3); 2.562(5), ThdC2 2.674(5), C1dC2dC3 131.8(6), ThdC3 2.699(5); C4dC3dC2 131.6(5); C1dC2 1.429(7), C2dC3 dThdC5Arcentroid 1.389(7), C3dC4 1.398(7); C5Mecentroid 5 5 2.526, 135.21 ThdC5Mecentroid 5 ThdC5Arcentroid 2.706 5

[(
5-C5Me5)(
5-C5Ar5)Th (
4-Me3SiCHdCH] CHdCHSiMe3)] (Ar ¼ 3,5-tBu2C6H3)

+IV

C1dThdC4 81.6(2), ThdC1dC2 ThdC1 2.544(5), ThdC4 78.5(3), ThdC4dC3 79.1(3); 2.529(5), ThdC2 2.666(5), C1dC2dC3 132.5(5), ThdC3 2.674(5); C4dC3dC2 130.7(5); C1dC2 1.451(8), C2dC3 1.375(8), C3dC4 1.471(7); C5Mecentroid dThdC5Arcentroid 5 5 ThdC5Mecentroid 2.557, 132.47 5 2.682 ThdC5Arcentroid 5

Complex number

1

Reference

H NMR (d8-toluene, 90 MHz, 25  C): d − 0.52 (br s, 2H, ThCHAHB) 1.80 (br s, 15H, C5Me5) 1.96 22-Th (br s, 15H, C5Me5) 3.46 (br s, 2H, ThCHAHB) 4.93 (complex m, 2H, CH2CH]CHCH2) ppm; 13 C NMR (d8-toluene, 67.80 MHz, −40  C): d 11.0 (q, 1JC-H ¼ 127 Hz, C5Me5dMe) 11.9 (d, 1JC-H ¼ 127 Hz, C5Me5dMe) 68.7 (dd, 1JC-H ¼ 153, 131 Hz, ThCH2) 120.0 (d, 1JC-H ¼ 177 Hz, CH2CH]CHCH2) 121.5 (s, C5Me5) 122.0 (s, C5Me5) ppm

64

1

22-U

64

23-Th

64

24-U

77

25-Th

69

26-Th

69

H NMR (d6-benzene, 90 MHz, 25  C): d − 168.9 (2H, UCHAHB) −125.6 (2H, UCHAHB) −2.0 (15H, C5Me5) 8.4 (15H, C5Me5) 47.5 (2H, CH2CH]CHCH2) ppm; 13 C NMR (d6-benzene, 22.49 MHz, 0  C): d − 50.0 (q, 1JC-H ¼ 127 Hz, C5Me5dMe) 11.5 (q, 1JC-H ¼ 127 Hz, C5Me5dMe) 194.8 (d, 1JC-H ¼ 177 Hz, CH2CH]CHCH2) 209.0 (s, C5Me5) 427.0 (s, C5Me5) 871.5 (t, 1JC-H ¼ 135 Hz, UdCH2) ppm 1 H NMR (d6-benzene, 90 MHz, 25  C): d 1.15 (s, 4H, ThCH2) 1.94 (30H, C5Me5) 2.17 (6H, CCH3) ppm; 13 C NMR (d6-benzene, 67.80 MHz, 0  C): d 11.6 (q, 1JC-H ¼ 126 Hz, C5Me5dMe) 24.99 (q, 1 JC-H ¼ 125 Hz, CCH3) 77.8 (t, 1JC-H ¼ 133 Hz, ThCH2) 121.2 (s, CH2CCH3) 122.4 (s, C5Me5) ppm. At −80  C, for ThCH2, 1JC-H ¼ 123 and 140 Hz 1 H NMR (d6-benzene, 400 MHz, 25  C): d − 171.11 (s, 2H, PhCH) −20.32 (d, 4H, J ¼ 9.2 Hz, PhdH) −10.91 (d, 2H, J ¼ 11.7 Hz, PhdH) −0.32 (s, 15H, C5Me5) −0.21 (d, 4H, J ¼ 8.9 Hz, PhdH) 12.58 (s, 15H, C5Me5) 47.19 (s, 2H, CH]C) ppm; 13 1 C{ H} NMR (d6-benzene, 100 MHz, 25  C): d − 49.8 (C5Me5) −11.5 (C5Me5) 74.3 (PhdC) 127.9 (PhdC) 155.1 (PhdC) 155.4 (PhdC) 169.8 (C5Me5) 219.8 (CH]C) 463.2 (UC) ppm 1 H NMR (d6-benzene, 500 MHz, 25  C): d 1.09 (s, 90H, Ar-tBu) 1.94 (s, 15H, C5Me5) 2.29–2.42 (m, 2H, CHdCH]CHdCH) 5.59–5.70 (m, 2H, CHCH]CHdCH) 6.92–7.03 (m, 2H, Ph) 7.16 (m, 11H, C5Ar5&Ph) 7.20 (m, 7H, Ph) 7.29 (m, 5H, C5Ar5) ppm; 13 1 C{ H} NMR (d6-benzene, 126 MHz, 25  C): d 12.99 (C5Me5) 31.80 (C(CH3)3) 35.01(C(CH3)3) 95.41(PhCHdCH]CHdCHPh) 113.37 (PhCHCH]CHdCHPh) 120.93 (C5Ar5) 123.34 (C5Me5) 123.58 (PhCHdCH]CHdCHPh) 127.70 (C5Ar5) 128.53 (PhCHdCH]CHdCHPh) 128.75 (PhCHdCH]CHdCHPh) 133.94 (C5Ar5) 136.86 (C5Ar5) 147.11 (PhCHdCH]CHdCHPh) 150.36 (C5Ar5) ppm 1 H NMR (d6-benzene, 500 MHz, 25  C): d 0.34 (s, 18H, Me3SiCdCH]CHdCSiMe3) 1.18 (s, 90H, ArdtBu) 2.08 (s, 15H, C5Me5) 5.25–5.28 (m, 2H, Me3SiCdCH]CHdCSiMe3) 7.07 (s, 10H, C5Ar5) 7.26 (s, 5H, C5Ar5) ppm; 13 1 C{ H} NMR (d6-benzene, 126 MHz, 25  C): d 4.55 (Me3SiCdCH]CHdCSiMe3) 13.33 (C5Me5) 32.07 (C(CH3)3) 35.10 (C(CH3)3) 79.37 Me3SiCdCH]CHdCSiMe3) 120.76 (Me3SiCdCH¼ CHdCSiMe3) 121.12 (C5Ar5) 123.91 (C5Me5) 127.47 (C5Ar5) 135.82 (C5Ar5) 138.55 (C5Ar5) 149.88 (C5Ar5) ppm

Buta- and Penta-Dienyl Complexes of the Actinides

[(
5-C5Me5)2U(
4-CHPhdCH] +IV CHdCHPh)]

Nuclear magnetic resonance data

49

50

Buta- and Penta-Dienyl Complexes of the Actinides

4.02.3.2

Acyclic 1,3-butadiene-1,4-diyl complexes of the actinides

Acyclic 1,2- or 1,3-substituted butadienyl dinuclear complexes of uranium(IV) were synthesized by the uranium(III) mediated CdC coupling of terminal alkynes. The room temperature addition of 0.5 eq. of the terminal bis-alkynes 1,7-octadiyne or 1,6-heptadiyne to [{(AdArO)3N}U(DME)] (where (AdArO)3N ¼ tris(2-oxy-3-adamantyl-5-methylbenzyl)amine and DME ¼ 1,2-dimethoxyethane) in benzene at room temperature yielded [({(AdArO)3N}U)2{m-
2:
1-1,2-(CH)2-cyclohexane}] 27-U and [({(AdArO)3N}U)2{m-
2:
2-1,2(CH)2-cyclopentane}] 28-U (Scheme 17). Likewise, the reaction of [{(AdArO)3N}U(DME)] with 1 eq. of the terminal mono-alkynes 1-hexyne or 4-tBu-phenylacetylene yielded [({(AdArO)3N}U)2{m-
2(C1):
1(C4)-2-nBu-1,3-octadiene}] 29-U and [({(AdArO)3N} U)2{m-
2(C4):
1(C1)-1,3-di-(p-tBu-phenyl)butadiene}] 30-U (Scheme 17). Variable temperature SQUID measurements were used to determine the effective magnetic moments (meff) of 27–30. At 2 K meff ¼ 0.5 mB and at 300 K meff ¼ 3.4 mB for 27–30 and these data were consistent with uranium(IV). The complexity of the room temperature 1H NMR spectra of 27–30 in d6-benzene precluded their assignment.70

Scheme 17 Synthesis of complexes 27-U to 30-U.

The solid-state molecular structures of 27–30 were determined by single-crystal XRD. The binding modes of the butadienyl ligands were confirmed as m-
2:
1 in 27-U, 29-U and 30-U, and m-
2:
2 in 28-U. In 27–30 there were shorter terminal UdC bond distances of 2.434 (5) to 2.503 (5) A˚ , and longer bridging UdC distances of 2.608 (4) to 2.839 (5) A˚ . The UdC bond distances were reflective of the differing steric demands of the butadienyl ligands and were within the normal range for UdC s-bonds. Localization of the C]C bond distances (1.349 (8) to 1.378 (7) A˚ ) and CdC bond distances (1.470 (8) to 1.484 (6) A˚ ) were observed within the butadienyl ligands. The reaction profiles for the formation of 27-U and 30-U were calculated using DFT and the

Buta- and Penta-Dienyl Complexes of the Actinides

Table 6

51

Data table for acyclic 1,3-butadiene-1,4-diyl complexes of the actinides (Section 4.02.3.2).

Formula

Oxidation state

Single crystal X-ray diffraction data for AndC and selected ˚) bond distances (A

Magnetometry (meff SQUID)

Complex number

Reference

[({(AdArO)3N}U)2{m-
2:
1-1,2(CH)2-cyclohexane}] [({(AdArO)3N}U)2{m-
2:
2-1,2(CH)2-cyclopentane}] [({(AdArO)3N}U)2{m-
2(C1):
1 (C4)-2-nBu-1,3-octadiene}] [({(AdArO)3N}U)2{m-
2(C4):
1(C1)1,3-di-(p-tBu-phenyl)butadiene}]

+IV

U1(
1)dC 2.511(5); U2(
2)dC 2.434(5) and 2.632(5); C]C 1.378(7) and 1.349(8); CdC 1.470(8) U1(
2)dC 2.613(5) and 2.549(4); U2(
2)dC 2.839(5) and 2.503(5); C]C 1.372(6) and 1.360(6); CdC 1.477(7) U1(
1)dC 2.464(4); U2(
2)dC 2.528(3) and 2.608(4); C]C 1.359(5) and 1.363(6); CdC 1.475(6) U1(
1)dC 2.474(4); U2(
2)dC 2.535(4) and 2.643(4); C]C 1.357(6) and 1.355(6); CdC 1.484(6)

0.5 mB (2 K); 3.4 mB (300K) 0.5 mB (2 K); 3.4 mB (300 K) 0.5 mB (2 K); 3.4 mB (300 K) 0.5 mB (2 K); 3.4 mB (300 K)

27-U

70

28-U

70

29-U

70

30-U

70

+IV +IV +IV

(AdArO)3N ¼ tris(2-oxy-3-adamantyl-5-methylbenzyl)amine. Note: The complexity of the room temperature 1H NMR spectra of 27-U to 30-U in d6-benzene precluded assignment and therefore are not reported in the table.

mechanism was concluded to be bimolecular. In all cases the key intermediate was the binuclear uranium(IV) alkene complex [({(AdArO)3N}U)2(m-
1:
1-HC]CR)]{ formed by reduction of one terminal alkyne. This intermediate then reacts further either by intramolecular CdC coupling and cyclization in the case of 27-U (DG{ ¼ 12.3 kcal mol−1), or intermolecular CdC coupling with a second equivalent of the monoalkyne in the case of 30-U (DG{ ¼ 20.9 kcal mol−1). The overall reaction to form 27-U or 30-U was calculated to be exergonic by 61.4 and 51.1 kcal mol−1, respectively. Alternative reaction profiles were also calculated but found to be disfavored, for example cyclization at a single uranium metal center had a prohibitively high DG{ and while the DG{ of CdH activation was similar in magnitude (21.8 kcal mol−1) to the formation of 30-U, the overall reaction was endergonic by 16.5 kcal mol−1.70 Selected spectroscopic data for Section 4.02.3.2 are shown in Table 6.70

4.02.4

Planar five-membered metallacyclic complexes of the actinides

4.02.4.1

Actinacyclopentadiene complexes

The most common unsaturated four-carbon metallacycle for actinides is the actinacyclopentadiene. These complexes have been synthesized by a number of routes. The complex [(
5-C5Me5)2U(
2-C4Ph4)] 31-U was first reported by the transmetallation reaction of [(
5-C5Me5)2UCl2] with [Li2(C4Ph4)] (Scheme 18A).43,71,72 Many subsequent preparations of this and related actinacyclopentadienes have reductively coupled alkynes to form the (
2-C4R4)2− ligand. Complexes of uranium(III) are competent reductants for

Scheme 18 Synthetic routes to complexes 31-U and 32-U.

52

Buta- and Penta-Dienyl Complexes of the Actinides

this reaction. The reactions of 0.5 eq. of [{(
5-C5Me5)2U}2(C6H6)} or [(
5-C5Me5)3U] with 2 eq. PhC^CPh also resulted in the isolation of 31-U (Scheme 18B).73 The uranium(III) hydride complex [(
5-C5Me5)2UH]x, generated in situ by the reaction of [(
5-C5Me5)2U(Me)2] with [PhSiH3], was reported to couple 2 eq. RC^CR, resulting in much improved and essentially quantitative yields of [(
5-C5Me5)2U(
2-C4R4)] (R ¼ Ph 31-U, Me 32-U) (Scheme 18C).74 The room temperature 1H NMR spectra of complexes 31-U and 32-U were consistent with magnetically equivalent permethylcyclopentadienyl ligands and two magnetically inequivalent R groups in a 2:2 ratio corresponding to the 1,4- and 2,3-positions of the (
2-C4R4)2− ligand.68,71,74 The solid-state molecular structures of 31-U and 32-U were determined by single-crystal XRD. These data confirmed the
2-binding mode of the (
2-C4R4)2− ligand and the planar actinacyclopentadiene. In both 31-U and 32-U, a plane of mirror-symmetry bisects the actinacyclopentadiene perpendicular to the plane of the metallacycle. The UdC bond distances in 31-U of UdC1 2.395 (2) A˚ and 32-U of UdC1 2.337 (2) A˚ fall within the normal range of UdC s-bonds. In both 31-U and 32-U the data were consistent with the localization of the two C]C double bonds (31-U C1dC2 1.365 (3) A˚ and 32-U C1dC2 1.354 (4) A˚ ) and the single CdC bond (31-U C2dC2a 1.509 (4) A˚ and 32-U C2dC2a 1.521 (6) A˚ ). In 31-U the internal UdC1dC2 angle of 108.51 (16) did not deviate from that expected for a pentagon (108 ); in 32-U the UdC1dC2 angle was 103.8 (2) . The C1dC2dC2a internal angles of the (
2-C4R4)2− ligand deviated only slightly from the 120 angle expected for sp2 hybridization (31-U C1dC2dC2a 122.83 (13) and 32-U C1dC2dC2a 124.4 (2) ).73,74 The bonding in 31-An (Th, U) was investigated by extended-Hückel MO methods using [(
5-C5H5)2An(
2-C4H4)] as a computational model and compared to [(
5-C5H5)2Ti(
2-C4H4)].67 No strong interaction was seen between the [(
5-C5H5)2U]2+ fragment and the (
2-C4H4)2− p–system. It was concluded that the (
2-C4H4)2− ligand is bound to the metal center primarily through s–interactions. Also, complicated inter-ligand mixing limited the usefulness of the MO interaction diagram and therefore the degree of p–delocalization was best determined by analysis of the UdC1dC2/UdC1adC2a angle of 108.51 (16) and CdC overlap populations. In all cases (Ti, Th, U) the p–overlap was moderate (M ¼ Th 0.634, U 0.610, Ti 0.437), and the increase for M ¼ Th, U from M ¼ Ti resulted from d–p1,p2 and fxyzdp2 overlap. However, the fact that the CdC overlap populations of the (
2-C4H4)2− ligand did not change on metal binding was consistent with weak p–delocalization. Reductive alkyne coupling can also be achieved by using the external reductant KC8, which enabled the extension of this chemistry to thorium. The reaction of [(
5-C5Me5)2ThCl2] with 2 eq. PhC^CPh and 2.1 eq. KC8 resulted in the isolation of 31-Th (Scheme 19A).75 This methodology using [(
5-C5Me5)2ThCl2] with KC8 was extended to the investigate the reductive coupling of

Scheme 19 Synthetic routes to complexes 31-Th to 37-Th.

Buta- and Penta-Dienyl Complexes of the Actinides

53

asymmetric alkynes PhC^CR (where R ¼ Me, iPr, Cy ¼ cyclohexyl and SiHMe2) and yielded [(
5-C5Me5)2Th(
2-CR]CPhdCR] CPh)] (R ¼ Me 33-Th, iPr 34-Th, Cy ¼ cyclohexyl 35-Th) and [(
5-C5Me5)2Th(
2-C(SiHMe2)¼CPhdCPh]C(SiHMe2)] 36-Th (Scheme 19A). The reaction of the sterically demanding complex [(
5-1,2,4-tBu3C5H2)2ThCl2] with 2 eq. MeC^CMe and 2.1 eq. KC8 also yielded [(
5-1,2,4-tBu3C5H2)2Th(
2-C4Me4)] 37-Th (Scheme 19B).76 The reactions to synthesize the complexes 31-Th to 37-Th (Scheme 19) required heating but were significantly higher yielding than the majority of the synthetic routes to complexes 31-U and 32-U (Scheme 18A and B). The room temperature 1H NMR spectra of complexes 31-Th to 37-Th were consistent with the data observed for 31-U and 32-U but in the diamagnetic region of the NMR spectrum, with resolution of multiplicity and coupling constants. Additionally, the characteristic 13C NMR chemical shift for the carbon atoms in the (
2-C4R4)2− ligand directly bound to thorium were observed between d ¼ 215.3 ppm for 37-Th and d ¼ 233.2 ppm in 34-Th. The solid-state molecular structures of 31-Th to 37-Th were determined by single-crystal XRD. The ThdC bond distances in 31-Th to 37-Th were longer than in 31-U and 32-U, for example in 31-Th the ThdC1/C1a were 2.465 (2) A˚ , whereas in 31-U the UdC1 was 2.395 (2) A˚ . Substitution of the (
2-C4R4)2− ligand, either 1,3- (33-Th, 34-Th, and 35-Th) or 1,4- (36-Th) or permethyl substitution (37-Th), did not result in significant changes to the structure of the planar actinacyclopentadienes.75,76 The bonding in 31-An (An ¼ Th, U) was examined using DFT. There was found to be good agreement between the experimental determined structures and the computed geometries of 31-An. Natural bond orbital (NBO) analysis of 31-An demonstrated that the AndC s–bonds were strongly polarized and composed primarily of a carbon sp2 hybrid (31-Th 89.8% C: 29% s, 71% p) and (31-U 82.6% C: 30% s, 70% p). In addition, there were actinide hybrid orbitals with, as expected, a higher % actinide contribution from uranium (31-U 17.4% U: 37% 5f, 41% 6d, 6% 7p, 16% 7s) than thorium (31-Th 10.2% Th: 20% 5f, 48% 6d, 4% 7p, 28% 7s).75 DFT was also used to understand the selectivity of CdC bond formation, resulting from the coupling of asymmetric alkynes in the formation of 34-Th and in 36-Th. The mechanism was found to be the same in both cases, where the initial step was the formation of the metallacyclopropene intermediate [(
5-C5Me5)2Th(
2-C2PhR)], followed by selective insertion of the second equivalent of alkyne and CdC bond formation. It is of note that the metallacyclopropene was only able to be isolated experimentally in the specific case of [(
5-1,2,4-tBu3C5H2)2Th(
2-C2Ph2)]. In both cases the experimentally observed isomer was calculated to be the thermodynamic product: 1,3-substitution was exergonic by 2.5 kcal mol−1 at 298 K in 34-Th and 1,4-substitution was exergonic by 7.0 kcal mol−1 at 298 K in 36-Th. The origin of the selectivity was determined to be steric in 34-Th, but electronic in 36-Th. The relative Mulliken charges of the free alkynes, metallacyclopropene intermediates and transition state complexes were also calculated, and the attraction between the electropositive Th(IV) and the electronegative silyl group in SiHMe2C^CPh used to further rationalize the selectivity in 36-Th.76 Uranium actinacyclopropene complexes can also be used as effective precursors to form actinacyclopentadienes. The reaction of [(
5-C5Me5)2U(
2-C2{SiMe3}2)] with 2 eq. PhC^CPh quantitatively yielded 31-U (Scheme 20).68 The reaction of [(
5-C5Me5)2U(
2-C2{SiMe3}2)] with 2 eq. of the asymmetric alkynes PhC^CR (where R ¼ Me, SiHMe2, SiMe3) yielded [(
5-C5Me5)2U(
2-CMe]CPhdCMe]CPh)] 33-U, [(
5-C5Me5)2U(
2-C(SiHMe2)¼CPhdCPh]C(SiHMe2)] 36-U and [(
5-C5Me5)2U(
2-C(SiMe3)]CPhdCPh]C(SiMe3)] 38-U (Scheme 20).77 The solid-state molecular structures of 33-U, 36-U and 38-U were determined by single-crystal XRD and these data are consistent with those previously discussed for 31-U and 32-U (Data Table 7). DFT was used to understand the selectivity of CdC bond formation in 33-U and in 38-U. The substitution of the (
2-C4R4)2− ligand was observed to follow the same pattern as seen in 33-Th to 36-Th and to proceed by the same mechanism.77

Scheme 20 Synthesis of complexes 31-U, 33-U, 36-U and 38-U.

54

Buta- and Penta-Dienyl Complexes of the Actinides

Likewise, the experimentally observed isomer was calculated to be the thermodynamic product: 1,3-substitution was exergonic by 17.7 kcal mol−1 at 298 K in 33-U and 1,4-substitution was exergonic by 11.8 kcal mol−1 in 38-U.77 An example of the intramolecular reductive coupling of two alkynes to produce the first example of a 2-metallabiphenylene actinide complex has also been recently reported. The reaction of [(
5-C5Me5)2AnCl2] (An ¼ Th, U) with 1 eq. 1,2-bis(phenylethynyl) benzene and 2.1 eq. KC8 resulted in the isolation of [(
5-C5Me5)2An(
2-2,5-Ph2-cyclopentadienyl-3,4-cyclobuta-1,2-benzene)] 39-An (An ¼ U, Th) (Scheme 21). Complex 39-U was also synthesized from the reaction of [(
5-C5Me5)2U(
2-C2{SiMe3}2)] with 1 eq. 1,2-bis(phenylethynyl)benzene (Scheme 21).78 The room temperature 1H and 13C NMR spectra of complexes 39-An (An ¼ Th, U) were consistent with the data observed for 31-An (An ¼ Th, U). The solid-state molecular structures of 39-An (An ¼ Th, U) were determined by single-crystal XRD and these data were consistent with the other structural data for planar actinacyclopentadienes. The (
2-2,5-Ph2-cyclopentadienyl-3,4-cyclobuta-1,2-benzene)2− ligand exerts steric pressure on the actinacyclopentadiene ring angles and therefore the structural data for 39-An (An ¼ Th, U) were directly comparable to complexes with bulky substituents on the (
2-C4R4)2− ligand, e.g., 34-Th, 36-Th and 38-U. The only bond distance in 39-An that was observed to differ significantly from other actinacyclopentadiene complexes, was the very long CdC bond within the actinacyclopentadiene ring, resulting from the ring-strain imposed by the 2-metallabiphenylene ligand. In 39-U the C2dC3 distance is 1.620 (7) A˚ and in 39-Th the C2dC3 distance is 1.619 (3) A˚ , these distances were significantly longer than those found in 31-An (31-U C2dC2a 1.509 (4) A˚ and 31-Th C2dC2a 1.515 (3) A˚ ). DFT and multiconfigurational self-consistent field (SCF) calculations confirmed the uranium metal center in 39-U was a triplet U(IV). NBO analysis of 39-An (An ¼ Th, U) demonstrated that the AndC s–bonds were strongly polarized and composed primarily of a carbon sp2 hybrid, the % contributions of which were 82.8% C: 32.2% s, 67.8% p in 39-Th and 75.8% C: 32.0% s, 68.0% p in 39-U. These data were directly comparable to the NBO analysis of 31-An (An ¼ Th, U). In addition, there were actinide hybrid orbitals with, as expected, a higher % actinide contribution from uranium (39-U 24.2% U: 28.4% 5f, 59.6% 6d, 3.7% 7p, 8.4% 7 s) than thorium (39Th 17.2% Th: 12.1% 5f, 71.8% 6d, 3.6% 7p, 12.4% 7s). The % actinide contributions to bonding were higher in 39-An (39-U 24.2% U and 39-Th 17.2% Th) than in 31-An (31-U 17.4% U and 31-Th 10.2% Th). In particular the 6d orbital contributions in 39-An (39-U 28.4% 5f, 59.6% 6d and 39-Th 12.1% 5f, 71.8% 6d) are increased relative to 31-An (31-U 37% 5f, 41% 6d and 31-Th 20% 5f, 48% 6d), whereas the % 5f decreased slightly in 39-An from that calculated for 31-An.

Scheme 21 Synthesis of 39-An (An ¼ Th, U).

Actinacyclopentadiene compounds are largely unreactive, in contrast to the more highly reactive actinacyclopropenes,77,79 but they have been shown to react with certain substrates. The reaction of 31-Th with N2CH(SiMe3) and 9-N2C(C12H8) at room temperature resulted in single-substrate insertion into the (
2-C4Ph4)2− ligand and the formation of six-membered metallacyclic hydrazido complexes [(
5-C5Me5)2Th{(N(N]CHSiMe3)(
2-C4Ph4)}] and [(
5-C5Me5)2Th{(N(N]CC12H8)(
2-C4Ph4)}] (Scheme 22).75 The free energy profile for reaction of 31-Th with N2CH(SiMe3) was calculated using DFT. The first step was determined to be the coordination of end-on N2CH(SiMe3) adduct and the reaction was determined to be exergonic by 37.4 kcal mol−1 with an activation barrier of DG{ ¼ 21.2 kcal mol−1 consistent with the rapid reaction at observed at room temperature. Additionally, 31-Th reacted with 2 eq. of (p-tolyl)N3 to yield seven-membered bis(triazenido) complex [(
5-C5Me5)2Th{(N(N] N(p-tolyl))(
2-C4Ph4)N(N]N(p-tolyl))}], the product of double insertion regardless of stoichiometry (Scheme 22A).75 Although 31-U had been known for over 30 years, it was recently demonstrated that heating in 70  C in toluene resulted in intramolecular CdH activation to yield [(
5-C5Me5)U(
5:k2-C5Me4CH2CPh]CPhCPhCHPh)] (Scheme 22B).68 Important structural metrics for Section 4.02.3.1 are shown in Fig. 6 and selected spectroscopic data for Section 4.02.3.1 in Table 7.

Buta- and Penta-Dienyl Complexes of the Actinides

Scheme 22 Reactivity of 31-Th and 31-U.

Fig. 6 Diagrams showing representative complexes and important structural metrics for Section 4.02.4.1.

55

Data table for actinacyclopentadiene complexes (Section 4.02.4.1).

Formula

Single crystal X-ray diffraction data AndC and selected bond ˚ distances (A)

Selected bond angles (  )

[(
5-C5Me5)2Th(
2-C4Ph4)]

+IV

ThdC1/C1a 2.465(2), Th⋯ C2/2a 3.210; C1dC2/C1adC2a 1.362 (3), C2dC2a 1.515(3); ThdC5Mecentroid 2.543(2) 5

C1dThdC1a 74.1(1), ThdC1dC2/ThdC1adC2a 110.7; C1dC2dC2a/C1adC2adC2 122.3(2); C5Mecentroid dThdC5Mecentroid 5 5 144.5(1)

[(
5-C5Me5)2U(
2-C4Ph4)]

+IV

UdC1/C1a 2.395(2), U⋯ C2/2a 3.111; C1dC2/C1adC2a 1.365 (3), C2dC2a 1.509(4); UdC5Mecentroid 2.476 5

[(
5-C5Me5)2U(
2-C4Me4)]

+IV

UdC1/C1a 2.337(2), U⋯ C2/C2a 2.963; C1dC2/C1adC2a 1.354 (4), C2dC2a 1.521(6); 2.465 UdC5Mecentroid 5

[(
5-C5Me5)2Th(
2-CMe]CPhdCMe]CPh)]

+IV

ThdC1 2.443(5), ThdC4 2.453(6), Th ⋯C2 3.133 (5), Th ⋯C3 3.161(6); C1dC2 1.339(8), C2dC3 1.515(9), C3dC4 1.343 (7); 2.529(6) ThdC5Mecentroid 5

C1dUdC1a 77.26(11), UdC1dC2/UdC1adC2a 108.51(16); C1dC2dC2a/C1adC2adC2 122.83(13); C5Mecentroid dUdC5Mecentroid 5 5 101.5 C1dUdC1a 82.19, UdC1dC2/UdC1adC2a 103.8(2); C1dC2dC2a/C1adC2adC2 124.4(2); dUdC5Mecentroid C5Mecentroid 5 5 140.2 C1dThdC4 75.4(2), ThdC1dC2 108.3(4), ThdC4dC3 109.2(4); C1dC2dC3 124.6(5), C4dC3dC2 122.0(5); C5Mecentroid dThdC5Mecentroid 5 5 143.8(3)

[(
5-C5Me5)2U(
2-CMe]CPhdCMe]CPh)]

+IV

UdC1 2.399(9), UdC4 2.365(8), U ⋯ C2 3.048 (8), U ⋯C3 3.069(9); C1dC2 1.34(1), C2dC3 1.50(1), C3dC4 1.36 (1); UdC5Mecentroid 2.471(8) 5

C1dUdC4 78.4(3), UdC1dC2 105.6(5), UdC4dC3 107.8(5); C1dC2dC3 125.5(7), C4dC3dC2 121.4(7); C5Mecentroid dUdC5Mecentroid 5 5 141.7(3)

Nuclear magnetic resonance data

Complex number

Reference

1

H NMR (d6-benzene, 400 MHz, 25  C): d 1.98 (s, 30H, C5Me5) 6.65 (d, J ¼ 7.4 Hz, 4H, PhdH) 6.83 (m, 4H, PhdH) 6.92 (t, J ¼ 7.6 Hz, 4H, PhdH) 6.99 (d, J ¼ 7.0 Hz, 4H, PhdH) 7.06 (t, J ¼ 7.7 Hz, 4H, PhdH) ppm; 13 1 C{ H} NMR (d6-benzene, 100 MHz, 25  C): d 11.8 (C5Me5) 123.7 (C5Me5) 125.3 (PhdC) 125.6 (PhdC) 127.5 (PhdC) 128.5 (PhdC) 128.9 (PhdC) 131.3 (PhdC) 143.7 (PhdC) 145.2 (PhdC) 151.5 (CPh) 221.2 (ThCPh) ppm 1 H NMR (d6-benzene, 400 MHz): d − 34.83 (d, J ¼ 5.5 Hz, 4H, PhdH) −1.72 (d, J ¼ 6.1 Hz, 4H, PhdH) −0.65 (t, J ¼ 6.9 Hz, 2H, PhdH) 4.11 (d, J ¼ 7.4 Hz, 4H, PhdH) 5.48 (t, J ¼ 6.9 Hz, 2H, PhdH) 5.58 (t, J ¼ 7.1 Hz, 4H, PhdH) 6.13 (s, 30H, C5Me5) ppm

31-Th

75

31-U

1

H NMR data68; SCXRD data73

1

32-U

74

1

33-Th

76

33-U

77

H NMR (d6-benzene, 400 MHz, 23  C): d − 4.55 (s, 6H, C4Me4) −3.61 (s, 6H, C4Me4) 0.45 (s, 30H, C5Me5) ppm

H NMR (d6-benzene, 400 MHz, 25  C): d 7.34 (m, 4H, PhdH) 7.22 (m, 2H, PhdH) 7.15 (s, 3H, C6H6) 6.95 (m, 4H, PhdH) 2.05 (s, 3H, CH3) 1.99 (s, 3H, CH3) 1.94 (s, 30H, C5Me5) ppm; 13 1 C{ H} NMR (d6-benzene, 100 MHz, 25  C): d 11.4 (C5Me5) 21.8 (CH3) 23.7 (CH3) 122.8 (PhdC) 124.2 (C5Me5) 125.6 (PhdC) 127.1 (PhdC) 128.0 (C6H6) 128.4 (PhdC) 128.5 (PhdC) 129.0 (PhdC) 141.3 (PhdC) 144.9 (PhdC) 147.4 (ThC]CCH3) 148.0 (ThC]CPh) 220.6 (ThCCH3) 221.2 (ThCPh) ppm 1 H NMR (d6-benzene, 400 MHz, 25  C): d − 20.74 (s, 1H, PhdH) −13.65 (s, 2H, PhdH) −6.00 (s, 3H, CH3) −1.39 (s, 2H, PhdH) 1.28 (s, 15H, C5Me5) 2.72 (s, 1H, PhdH) 2.94 (s, 15H, C5Me5) 5.58 (s, 3H, CH3) 6.14 (s, 4H, PhdH) 7.15 (s, 3H, C6H6) ppm; 13 1 C{ H} NMR (d6-benzene, 100 MHz, 25  C): d − 45.3 (C5Me5) –41.2 (C5Me5) 69.7 (C]CCH3) 91.2 (UCCH3) 104.5 (C]C (Me)) 112.2 (C]C(Ph)) 121.9 (PhdC) 122.4 (PhdC) 122.5 (PhdC) 122.9 (PhdC) 124.7 (PhdC) 128.0 (C6H6) 129.3 (PhdC) 129.6 (PhdC) 130.3 (PhdC) 188.4 (C5Me5) 197.4 (C5Me5) 260.2 (UCCH3) 278.4 (UCPh) ppm

Buta- and Penta-Dienyl Complexes of the Actinides

Oxidation state

56

Table 7

+IV

ThdC1 2.465(13), ThdC4 2.467(14), Th⋯ C2 3.13(2), Th⋯ C3 3.11(1); C1dC2 1.34(2), C2dC3 1.55(2), C3dC4 1.38(2); ThdC5Mecentroid 5 2.557(14)

C1dThdC4 78.0(4), ThdC1dC2 106.7(9), ThdC4dC3 104.3(9); C1dC2dC3 122(1), C4dC3dC2 125(1); C5Mecentroid dThdC5Mecentroid 5 5 136.9(4)

[(
5-C5Me5)2Th(
2-CCy ¼ CPhdCCy ¼ CPh)] (Cy ¼ C6H11)

+IV

ThdC1 2.499(11), ThdC4 2.456(11), Th⋯ C2 3.010(9), Th⋯ C3 2.97(1); C1dC2 1.38(2), C2dC3 1.54(2), C3dC4 1.35(2); ThdC5Mecentroid 5 2.559(11)

C1dThdC4 82.2(4), ThdC1dC2 99.8(7), ThdC4dC3 98.3(8); C1dC2dC3 123(1), C4dC3dC2 128(1); C5Mecentroid dThdC5Mecentroid 5 5 133.6(4)

[(
5-C5Me5)2Th(
2-C(SiHMe2) ¼ CPhdCPh]C(SiHMe2)]

+IV

[(
5-C5Me5)2U(
2-C(SiHMe2) ¼ CPhdCPh]C(SiHMe2)]

+IV

[(
5-1,2,4-tBu3C5H2)2Th(
2-C4Me4)]

+IV

ThdC1 2.476(7), ThdC4 2.463(6), Th ⋯C2 3.152 (8), Th ⋯C3 3.140(7); C1dC2 1.364(8), C2dC3 1.53(1), C3dC4 1.36(1); 2.541(8) ThdC5Mecentroid 5 UdC1 2.400(11), UdC4 2.382(11), U⋯ C2 3.05 (1), U ⋯C3 3.04(1); C1dC2 1.37(1), C2dC3 1.51(2), C3dC4 1.35(2); UdC5Mecentroid 2.476(10) 5 ThdC1 2.404(4), ThdC4 2.399(5), Th ⋯C2 3.028 (5), Th ⋯C3 3.035(5); C1dC2 1.359(7), C2dC3 1.537(7), C3dC4 1.385(7); ThdC5R3Hcentroid 2.620(4) 2 (R ¼ tBu)

C1dThdC4 76.7(2), ThdC1dC2 106.7(5), ThdC4dC3 106.9(5); C1dC2dC3 124.2(6), C4dC3dC2 124.3(7); dThdC5Mecentroid C5Mecentroid 5 5 138.0(2) C1dUdC4 79.7(4), UdC1dC2 104.6(8), UdC4dC3 105.5(8); C1dC2dC3 125(1), C4dC3dC2 125(1); C5Mecentroid dUdC5Mecentroid 5 5 137.0(4) C1dThdC4 81.4(2), ThdC1dC2 103.6(3), ThdC4dC3 103.4(3); C1dC2dC3 125.5(5), C4dC3dC2 124.6(5); C5R3Hcentroid dThdC5R3Hcentroid 2 2 146.2(2)

1

H NMR (d6-benzene, 400 MHz, 25  C): d 0.93 (d, J ¼ 7.2 Hz, 6H, CH(CH3)2) 1.08 (d, J ¼ 6.7 Hz, 6H, CH(CH3)2) 2.03 (s, 30H, C5Me5) 3.05 (m, 1H, CH(CH3)2) 3.25 (m, 1H, CH(CH3)2) 6.99 (t, J ¼ 7.3 Hz, 1H, PhdH) 7.10 (m, 3H, PhdH) 7.27 (t, J ¼ 7.6 Hz, 2H, PhdH) 7.33 (t, J ¼ 7.7 Hz, 2H, PhdH) 7.41 (d, J ¼ 7.1 Hz, 2H, PhdH) ppm; 13 1 C{ H} NMR (d6-benzene, 100 MHz, 25  C): d 12.3 (C5Me5) 24.3 (CH(CH3)2) 24.6 (CH(CH3)2) 33.1 (CH(CH3)2) 35.7 (CH(CH3)2) 123.0 (ThC]C(iPr)) 125.4 (C5Me5) 125.8 (ThC]CPh) 127.4 (PhdC) 127.9 (PhdC) 128.5 (PhdC) 130.5 (PhdC) 144.9 (PhdC) 147.6 (PhdC) 147.9 (PhdC) 151.2 (PhdC) 219.1 (ThC(iPr)) 233.2 (ThCPh) ppm 1 H NMR (d6-benzene, 400 MHz, 25  C): d 0.92 (m, 2H, CH2) 1.16 (m, 6H, CH2) 1.45 (m, 6H, CH2) 1.74 (m, 4H, CH2) 1.83 (m, 2H, CH2) 2.06 (s, 30H, C5Me5) 2.69 (t, J ¼ 11.4 Hz, 1H, CH) 2.81 (t, J ¼ 11.8 Hz, 1H, CH) 7.00 (t, J ¼ 7.1 Hz, 1H, PhdH) 7.12 (m, 3H, PhdH) 7.27 (t, J ¼ 7.5 Hz, 2H, PhdH) 7.33 (t, J ¼ 7.5 Hz, 2H, PhdH) 7.41 (d, J ¼ 7.4 Hz, 2H, PhdH) ppm; 13 1 C{ H} NMR (d6-benzene, 100 MHz, 25  C): d 12.4 (C5Me5) 26.5 (CH2) 26.6 (CH2) 26.9 (CH2) 27.8 (CH2) 33.4 (CH2) 34.7 (CH2) 45.3 (CH) 48.4 (CH) 123.0 (CCy) 125.4 (C5Me5) 125.8 (CPh) 127.4 (PhdC) 127.6 (PhdC) 128.5 (PhdC) 130.2 (PhdC) 145.2 (PhdC) 147.6 (PhdC) 148.7 (PhdC) 151.2 (PhdC) 219.0 (ThCCy) 231.1 (ThCPh) ppm 1 H NMR (d6-benzene, 400 MHz, 25  C): d 0.02 (d, J ¼ 3.5 Hz, 12H, Si(CH3)2) 2.14 (s, 30H, C5Me5) 4.20 (m, 2H, SiH) 6.76 (m, 2H, PhdH) 6.93 (m, 8H, PhdH) ppm; 13 1 C{ H} NMR (d6-benzene, 100 MHz, 25  C): d 0.1 (SiCH3) 11.8 (C5Me5) 125.0 (C5Me5) 125.2 (PhdC) 127.3 (PhdC) 129.2 (PhdC) 147.9 (PhdC) 163.0 (CPh) 232.5 (ThC) ppm 1

H NMR (d6-benzene, 400 MHz, 25  C): d − 30.20 (s, 2H, SiH) −6.10 (s, 12H, SiCH3) 3.66 (s, 30H, C5Me5) 5.27 (t, 2H, J ¼ 6.9 Hz, PhdH) 6.29 (t, 4H, J ¼ 7.2 Hz, PhdH) 6.74 (d, 4H, J ¼ 7.4 Hz, PhdH) ppm; 13 1 C{ H} NMR (d6-benzene, 100 MHz, 25  C): d − 41.4 (C5Me5) −1.5 (SiCH3) 105.1 (CPh) 123.4 (PhdC) 124.3 (PhdC) 127.9 (PhdC) 128.5 (PhdC) 231.4 (C5Me5) 266.9 (UCSi) ppm 1 H NMR (d6-benzene, 400 MHz, 25  C): d 1.34 (s, 18H, (CH3)3) 1.43 (s, 36H, C(CH3)3) 2.02 (s, 6H, CH3) 2.21 (s, 6H, CH3) 6.29 (s, 4H, ring-CH) ppm; 13 1 C{ H} NMR (d6-benzene, 100 MHz, 25  C): d 17.9 (CH3) 25.3 (CH3) 33.0 (C(CH3)3) 33.2 (C(CH3)3) 34.2 (C(CH3)3) 34.5 (C (CH3)3) 112.8 (CMe) 142.4 (ring C) 143.0 (ring C) 143.1 (ring C) 215.3 (ThCMe) ppm

34-Th

76

35-Th

76

36-Th

76

36-U

77

37-Th

76

57

(Continued )

Buta- and Penta-Dienyl Complexes of the Actinides

[(
5-C5Me5)2Th(
2-CiPr]CPhdCiPr]CPh)] (iPr ¼ CHMe2)

58

(Continued)

Formula

Oxidation state

Single crystal X-ray diffraction data AndC and selected bond ˚ distances (A)

Selected bond angles (  )

[(
5-C5Me5)2U(
2-C(SiMe3) ¼ CPhdCPh]C(SiMe3)]

+IV

UdC1/UdC1a 2.370(8), U⋯ C2/2a 2.930; C1dC2/C1adC2a 1.37 (1), C2dC2a 1.557(9); UdC5Mecentroid 2.497(7) 5

C1dUdC1a 85.6(3), UdC1dC2/UdC1adC2a 99.6; C1dC2dC2a/C1adC2adC2 126.6(6); dUdC5Mecentroid C5Mecentroid 5 5 136.9(3)

[(
5-C5Me5)2Th(
2-2,5-Ph2-cyclopentadienyl3,4-cyclobuta-1,2-benzene)]

+IV

[(
5-C5Me5)2U(
2-2,5-Ph2-cyclopentadienyl3,4-cyclobuta-1,2-benzene)]

+IV

ThdC1 2.427(2), ThdC4 2.451(2) Th⋯ C2 2.799 (2), Th ⋯C3 2.786(2); C1dC2 1.339(2), C2dC3 1.619(3), C3dC4 1.340(3); ThdC5Mecentroid 2.630 5 and 2.661 UdC1 2.363(6), UdC4 2.390(6), U ⋯ C2 2.715 (5), U ⋯C3 2.737(5); C1dC2 1.336(8), C2dC3 1.620(7), C3dC4 1.338(6); UdC5Mecentroid 2.582 and 5 2.607

C1dThdC4 91.01(6), C2dThdC3 33.70(6), ThdC1dC2 90.3(1), ThdC4dC3 90.7(1); C1dC2dC3 133.8(2), C4dC3dC2 134.1(2); dThdC5Mecentroid C5Mecentroid 5 5 138.02 C1dUdC4 93.3(2), C2dUdC3 34.6(2), UdC1dC2 90.0(4), UdC4dC3 89.9(3); C1dC2dC3 134.0(5), C4dC3dC2 132.8(5); C5Mecentroid dUdC5Mecentroid 5 5 138.34

Nuclear magnetic resonance data

Complex number

Reference

1

38-U

77

39-Th

78

39-U

78

H NMR (d6-benzene, 400 MHz, 25  C): d − 4.78 (s, 6H, SiCH3) 3.86 (s, 30H, C5Me5) 4.21 (s, 12H, SiCH3) 5.83 (t, 2H, J ¼ 7.0 Hz, PhdH) 6.81 (t, 4H, J ¼ 7.4 Hz, PhdH) 7.28 (d, 4H, J ¼ 7.6 Hz, PhdH) ppm; 13 1 C{ H} NMR (d6-benzene, 100 MHz, 25  C): d − 65.1 (SiCH3) −39.7 (C5Me5) 0.0 (SiCH3) 110.7 (CPh) 123.9 (PhdC) 125.9 (PhdC) 127.0 (PhdC) 132.2 (PhdC) 146.5 (C5Me5) 217.4 (UCSi) ppm 1 H NMR (d8-THF, 400 MHz, 23  C): d 1.97 (s, 30H, C5Me5) 7.04 (m, 2H, Ph-para) 7.12–7.17 (m, 4H, C-(4–7)–H) 7.40 (t, J ¼ 7.2 Hz, 4H, Ph-meta) 7.49 (d, J ¼ 7.7 Hz, 4H, Ph-ortho) ppm; 13 1 C{ H} NMR (d8-THF, 125 MHz, 23  C): d 11.6 (s, C5Me5) 121.6 (C-5/6) 124.2 (C5Me5) 125.9 (Ph-para) 128.4 (C-4/7) 128.5 (Ph-ortho) 128.8 (Ph-meta) 131.8 (C-3/8) 145.3 (Ph-ipso) 152.5 (C-2/9) 205.2 (C-1/10) ppm 1 H NMR (d6-benzene, 400 MHz, 23  C): d − 16.04 (s, 4H, Ph-meta) 0.73 (s, 2H, Ph-para) 1.37 (m, 4H, Ph-ortho) 3.97 (s, 30H, C5Me5) 4.36 (s, 2H, C(4/7)-H) 5.09 (s, 2H, C(5/6)-H) ppm; 13 1 C{ H} NMR (d8-THF, 400 MHz, 23  C): d − 33.5 (C5Me5) 70.5 (C-2/9) 74.6 (Ph-ipso) 110.7 (Ph-meta) 118.0 (C-4/7) 128.3 (C-5/6) 131.7 (Ph-para) 152.8 (Ph-ortho) 171.0 (C-3/8) 314.5 (C5Me5) ppm. Resonances for C-1/10 were not observed

Buta- and Penta-Dienyl Complexes of the Actinides

Table 7

Buta- and Penta-Dienyl Complexes of the Actinides

4.02.4.2

59

Actinacyclopentatriene complexes

The first example of an actinacycloclopentatriene (actinacyclocumulene) complex [(
5-C5Me5)2Th(
4-C4Ph2)] 40-Th was reported recently.79 The actinacycloclopentatriene complexes [(
5-C5Me5)2An(
4-C4R2)] (An ¼ Th, R ¼ Ph 40-Th; U, R ¼ Ph 40-U; U, R ¼ SiMe3 41-U) were synthesized by reaction of [(
5-C5Me5)2AnCl2] (An ¼ Th, U) with RC^CdC^CR (where R ¼ Ph, SiMe3) in the presence of excess KC8 in toluene (Scheme 23A).68,79–81 This reactivity is divergent from that of group 4 metals, which form

Scheme 23 Synthetic routes to actinacyclopentatriene complexes 40-An and 41-An (An ¼ Th, U), and 42-U and 43-U.

60

Buta- and Penta-Dienyl Complexes of the Actinides

metallacyclopropenes under similar conditions. Complexes 40-U and 41-U were alternatively synthesized by the reaction of uranium metallacyclopropene complex [(
5-C5Me5)2U(
2-C2{SiMe3}2)] with RC^CdC^CR (where R ¼ Ph, SiMe3) in toluene at RT (Scheme 23B).68 Complexes 40-Th and 41-Th were also synthesized from the reaction of [(
5-C5Me5)2Th(bipy)2] (bipy ¼ 2,20 -bipyridine) with RC^CdC^CR (where R ¼ Ph, SiMe3) in non-coordinating solvents with heating (Scheme 23C).82 Complexes [(
5-1,3-tBu2-C5H3)2U(
4-C4Ph2)] 42-U and [(
5-1,3-tBu2-C5H3)2U{
4-C4(SiMe3)2}] 43-U containing the more sterically demanding (
5-1,3-tBu2-C5H3)− ligand were synthesized by the reaction of [(
5-1,3-tBu2-C5H3)2UCl2] with RC^CdC^CR (where R ¼ Ph, SiMe3) in the presence of excess KC8 (Scheme 23D).83 Complexes 41-U and 43-U were also observed to undergo an exchange reaction with PhC^CdC^CPh at 70  C in d6-benzene, with elimination of Me3SiC^CdC^CSiMe3, to yield 40-U (60% after 24 h) and 42-U (50% after 24 h) (Scheme 23E). However, extended heating under these conditions resulted in decomposition.81,83 The room temperature 1H NMR spectra of complexes 40-An and 41-An (An ¼ Th, U), and 42-U and 43-U all displayed magnetically equivalent cyclopentadienyl ligands and magnetically equivalent R groups (R ¼ Ph or SiMe3) of the (
4-C4R2)2− ligand.68,77,79,82,83 Variable temperature (20–100  C) 1H NMR studies showed no evidence of fluxional behavior, dissociation or isomerization.79 The characteristic resonances for the thorium-bound carbon atoms in the (
4-C4R2)2− ligand were observed by 13C {1H} NMR spectroscopy. The terminal carbon atoms (C1,C4) were observed at d ¼ 205.6 ppm in 40-Th and d ¼ 224.3 ppm in 41Th, and the internal carbon atoms (C2,C3) at d ¼ 149.2 ppm 40-Th and d ¼ 178.0 ppm in 41-Th.79,82 In 40-U and 42-U the resonances for the (
4-C4R2)2− ligand were observed at d ¼ 237.7 and 349.5 ppm, and d ¼ 190.6 and 188.5 ppm, respectively. Conversely, the resonances for the (
4-C4R2)2− ligand were not observed in the 13C{1H} NMR spectra of 41-U and 43-U.77,68,83 The solid-state molecular structures of 40-An and 41-An (An ¼ Th, U), and 42-U and 43-U were determined by single-crystal XRD. These data confirmed the planar actinacyclopentatriene,
4-binding mode and delocalization of the C]C double bonds in the (
4-C4R2)2− ligand. The ThdC distances in 40-Th were all of the same length and consistent with ThdC s–bonds (ThdC range of 2.530 (8) to 2.544 (8) A˚ ).79 Likewise, in 40-Th the C]C bond distances (C1dC2 1.32 (1), C2dC3 1.34 (1) and C3dC4 1.28 (1) A˚ ) were consistent with linear trienes.84 The C1dThdC4 angle was 90.0 (3) in 40-Th, which is much larger than that seen in the actinacyclopentadiene complexes (e.g., C1dThdC1a angle of 74.1 (1) in 31-Th), with the exception of 39-Th, which also had a much larger C1dThdC4 angle of 91.01 (6) due to ring-strain in the 2-metallabiphenylene. The internal angles of the (
4-C4R2)2 − ligand in 40-Th are consistent with significant bending and ring-strain as the angles of C1dC2dC3 148.8 (9) and C4dC3dC2 150.3 (9) deviate significantly from the 180 normal to sp-hybridized linear carbon atoms. The molecular structures of 41-An (An ¼ Th, U), and 42-U and 43-U all display the same bonding pattern as described for 40-Th. The UdC bond distances were shorter than the analogous ThdC bond distances, which resulted in wider C1dAndC4 angles where An ¼ U. Otherwise, only subtle differences were observed between actinide complexes of the (
4-C4R2)2− ligand, which can be attributed to the difference in R groups between 40-An (An ¼ Th, U) and 42-U where R ¼ Ph, and 41-An (An ¼ Th, U) and 43-U where R ¼ SiMe3. The bonding in 40-Th was probed using DFT calculations. There was found to be good agreement between the experimentally determined structures and the computed geometries of 40-Th. NBO analysis of 40-Th demonstrated that the ThdC s–bonds were strongly polarized and composed primarily of a carbon sp2 hybrid (40-Th 84.6% C: 31% s, 69% p). These data are directly comparable to the NBO analysis of the ThdC s–bonds in both 31-Th and 39-Th. In addition, there were thorium hybrid orbitals in 40-Th with a composition of 15.4% Th (16% 5f, 62% 6d, 8% 7p, 14% 7s). This %Th orbital contribution is intermediate between the 10.2% Th found in 31-Th and the 17.2% Th found in 39-Th. There was also one in-plane p-bond between the (
4-C4R2)2− ligand and the thorium(IV) metal center, which was determined to be p2(C]C) and 100% p, donating into an empty Th orbital.79 The natural localized molecular orbital (NLMO) analysis of the bonding was undertaken for 41-An (An ¼ Th, U). The data for 41-Th were consistent with 40-Th. In 41-U, there was a higher %U and 5f contribution to the AndC s–bonds (41-U 20.2% U: 30.7% 5f, 52.8% 6d, 4.2% 7p, 12.3% 15.6 s) than found in 41-Th. In 41-An (An ¼ Th, U), the in-plane p-bond p2(C]C) and the out-of-plane p-bond p1(C]C) were determined to be primarily carbon-based (86.5–91.1%) with a small actinide component (5.2–9.5%) to bonding. The Mülliken spin density at uranium in 41-U was calculated to be 2.19, consistent with transfer of electron density from the (
4-C4R2)2− ligand.81,83 The computational data on 42-U and 43-U were also found not to differ significantly from 40-U and 41U.83 A very recent theoretical study of the actinacyclocumulenes [(
5-C5Me5)2An(
2-C2{SiMe3}2)] (An ¼ Th, Pa, U, Np, Pu) has revealed a novel type-of “side-to-side” ’ M–L back-bonding was present for all actinides except thorium.85 Actinacyclocumulene complexes react with a variety of heterounsaturated molecules, by single or double insertion of the organic molecule into the AndC bond(s), with release of cumulene ring-strain to yield heterometallacyclic complexes.79,81,83 The reactivity observed for actinacyclocumulenes was in contrast to the very limited reactivity of actinacyclopentadienes (Section 4.02.4.1) and was distinct from the reactivity of group 4 analogues, but was similar to actinacyclopropanes.81,83 There was a range of reactivity that was common to 40-Th,79 41-U and 42-U. Complex 40-Th was reacted with 1 eq. of PhNCS in toluene, to yield the seven-membered heterocycle [(
5-C5Me5)2Th{SC(]NPh)(C4Ph2)}] (Scheme 24). Using DFT calculations it was shown that this reaction was exergonic with DG ¼ − 35.4 kcal mol−1 and the energy barrier of the transition state was DG{ ¼ 21.4 kcal mol−1 at 298 K, which was therefore easily overcome by the reaction conditions of heating at 110  C. Similarly, when 41-U and 42-U were reacted with 1 eq. of PhNCS in toluene, the analogous seven-membered heterocycles were yielded, [(
5-C5Me5)2U{SC(]NPh)(C4(SiMe3)2)}] and [(
5-1,3-tBu2-C5H3)2U{SC(]NPh)(C4Ph2)}], respectively (Scheme 24). Using DFT it was shown that this reaction was exergonic with DG ¼ − 37.2 and −30.8 kcal mol−1 for 41-U and 42-U, respectively.

Buta- and Penta-Dienyl Complexes of the Actinides

61

Scheme 24 Reactivity of 40-Th, 41-U and 42-U.

Complex 40-Th reacted with Ph2CO to give the double insertion product [(
5-C5Me5)2Th{OCPh2(C4Ph2)CPh2O}] and with p-ClPhCHO, to form the nine-membered metallaheterocycle [(
5-C5Me5)2Th{OCH(p-ClPh)(C4Ph2)CH(p-ClPh)O}], irrespective of the aldehyde stoichiometry used (Scheme 24). Complexes 41-U and 42-U also underwent the same reactions with aldehydes and ketones to give nine-membered metallaheterocycles via double insertion. Complex 41-U reacted with p-ClPhCHO to yield [(
5-C5Me5)2U{OCH(p-ClPh){C4(SiMe3)2}CH(p-ClPh)O}] and with benzophenone, acetophenone, cyclohexanone, and 1-indanone to yield [(
5-C5Me5)2U{OCPh2{C4(SiMe3)2}CPh2O}], [(
5-C5Me5)2U{OCMePh{C4(SiMe3)2}CMePhO}], [(
5-C5Me5)2 U{OC(CH2)5{C4(SiMe3)2}C(CH2)5O}] and [(
5-C5Me5)2U{OC{2-C6H4(CH2)2}{C4(SiMe3)2}C{2-C6H4(CH2)2}O}] (Scheme 24). Complex 42-U reacted with benzophenone and cyclohexanone to yield [(
5-1,3-tBu2-C5H3)2U{OCPh2(C4Ph2)CPh2O}] and [(
5-1,3-tBu2-C5H3)2U{OC(CH2)5(C4Ph2)C(CH2)5O}] (Scheme 24). Double insertion was also observed when both 40-Th and 41-U were reacted with PhCN the nine-membered heterocyclic complexes [(
5-C5Me5)2Th{N¼(CPh)(C4Ph2)C(Ph)¼N}], and [(
5-C5Me5)2U{N¼(CPh){C4(SiMe3)2}C(Ph)¼N}] were yielded,

62

Buta- and Penta-Dienyl Complexes of the Actinides

irrespective of the amount of PhCN used in the reaction (Scheme 24). Complexes 40-Th and 42-U reacted analogously with 4-(dimethylamino)pyridine (DMAP) to yield the seven-membered heterocyclic complexes [(
5-C5Me5)2Th{
6-2-(4-Me2NC4H3N) CdC(Ph)C]CCHPh}] and [(
5-1,3-tBu2-C5H3)2U{2-(4-Me2NC4H3N)CdC(Ph)C]CCHPh}], respectively (Scheme 24). However, under the same conditions, the reaction of 41-U with DMAP resulted in additional CdH activation of the permethylcyclopentadienyl ligand yielding [(
5-C5Me5)(
6:
5-{2-(4-Me2NC4H3N)¼C(SiMe3)CCH]C(CHSiMe3)CH2}C5Me4)U] (Scheme 24). Reactivity that was unique to complex 40-Th was also observed.79 Complex 40-Th reacted with N,N0 -diisopropylcarbodiimide to yield the seven-membered metallaheterocycle, [(
5-C5Me5)2Th{N(CHMe2)C(¼NCHMe2)(C4Ph2)}], which upon prolonged heating underwent [1,3]-Th migration yielding the five-membered metallaheterocycle [(
5-C5Me5)2Th{N(CHMe2)C(¼NCHMe2)C(Ph)¼C(C^CPh)] (Scheme 24). Complex 40-Th reacted with pyridine N-oxide to form the O,N-oximato complex [(
5-C5Me5)2Th {
2-O,N-s-C-ON]CH(CH]CH)2C(Ph)¼C(C^CPh)}] via an intermediate complex [(
5-C5Me5)2Th{k2-(2-C4H4NO)CHC(Ph)]C]C]CPh}], which was observed spectroscopically (Scheme 25). The mechanism of this reaction was investigated by DFT calculations. It was shown that the product [(
5-C5Me5)2Th{
2-O,NdsdCdON]CH(CH]CH)2C(Ph)¼C(C^CPh)}] was more thermodynamically favored than the intermediate (DG ¼ −6.0 kcal mol−1 vs DG ¼ − 2.8 kcal mol−1 at 298 K). Likewise, the calculated energetic barrier of DG{ ¼ 21.4 kcal mol−1 at 298 K for the formation of the product complex from the intermediate complex was easily overcome by heating the reaction mixture, consistent with the experimental observations.

Scheme 25 Reaction of 40-Th with pyridine-N-oxide to form an O,N-oximato complex via an observable intermediate.

Complex 40-Th reacted with 9-diazofluorene to yield an diiminato complex [(
5-C5Me5)2Th{N]C(C12H8)}2], irrespective of the amount of 9-diazofluorene added (Scheme 26). The mechanism was confirmed using DFT calculations and the reaction was exergonic by DG ¼ −77.7 kcal mol−1 at 298 K. This reactivity was unlike the reactivity seen for the analogous thoracyclopropenes or thoracyclopentadienes, where either insertion or isomerization products were isolated.79 No reactions were observed between 40Th and the sterically more encumbered organic azide Me3SiN3 or diazoalkane Me3SiCHN2 even with heating at 100  C for a week. No exchange or insertion chemistry was observed on addition of alkynes to 40-Th, this is a major difference in the reactivity of thorium vs uranium actinacyclocumulene complexes. Only unreacted starting materials were observed when 40-Th was heated in the presence of an alkyne RC^CR (R ¼ Me, Ph, p-tolyl) at 100  C for a week.

Buta- and Penta-Dienyl Complexes of the Actinides

63

Scheme 26 Reaction of complex 40-Th with 9-diazofluorene.

Complexes 41-U and 42-U also displayed a modest range of reduction chemistry toward conjugated molecules (bipyridine, butadiyne and diazabutadiene).81,83 The complex 41-U was very thermally stable with no alkyne dissociation observed upon heating to 100  C, as seen from variable temperature 1H NMR spectroscopy experiments. In contrast to 40-Th, 41-U readily reacted with conjugated alkynes (Scheme 23E) or 2,20 -bipyridine (bipy) (Scheme 27A), resulting in cumulene displacement (elimination of Me3SiC^CdC^CSiMe3). However, complexes 41-U and 42-U did not undergo displacement of the cumulene ligand with the alkynes RC^CR (R ¼ Ph, Me, SiMe3 for 41-U) and RC^CR0 (R ¼ R0 ¼ Ph, R ¼ R0 ¼ Me, R ¼ Ph and R0 ¼ Me for 42-U) even after heating at 100  C for a week. The cumulene ligand of 41-U was easily exchanged for a diazabutadiene (p-tolylN]CH)2, resulting in the formation of [(
5-C5Me5)2U{
2-N(p-tolyl)CH]CHN(p-tolyl)}] (Scheme 27A). In the case of 42-U, the same reaction occurs with (p-tolylN]CH)2, but the cumulene unit did not eliminate and therefore the metallaheterocycle [(
5-1,3-tBu2-C5H3)2U {N(p-tolyl)CH(CH]N-p-tolyl)C(Ph)¼C(C^CPh)}] was formed (Scheme 27B). Complex 41-U also reacted with 1 eq. of quinoline to yield the CdH activated complex [(
5-C5Me5)(
4:
5-{2-(C9H6N)¼C(CHSiMe3)CH]C(SiMe3)CH2}C5Me4)U] (Scheme 27A). The reaction of 42-U with N,N0 -dicyclohexylcarbodiimide (DCC) yielded the five-membered metallaheterocycle [(
5-1,3-tBu2-C5H3)2U{N(C6H11)C(¼NC6H11)C(Ph)¼C(C^CPh)}] (Scheme 27B). The reaction between [(
5-1,3-tBu2-C5H3)2 U(
4-C4Ph2)] and CS2 yielded a dinuclear uranium complex [(
5-1,3-tBu2-C5H3)2U{m-
4:
3-PhC]C]C(S)C(Ph)¼CS}U(
5-1,3-tBu2-C5H3)2] containing an unusual asymmetric m-
4:
3-PhC]C]C(S)C(Ph)]CS unit (Scheme 27B).83

64

Buta- and Penta-Dienyl Complexes of the Actinides

Scheme 27 Further reactivity of 41-U and 42-U.

In contrast to the reaction of uranium metallacyclopropene [(
5-C5Me5)2U{
2-C2(SiMe3)2}] with organic azides, no bis(imido) products were obtained from the reaction of 41-U with Me3SiN3, rather a four-membered metallaheterocycle [(
5-C5Me5)2U {N(SiMe3)C(SiMe3)¼C(C^CSiMe3)}] was reported (Scheme 28). This reaction was studied by DFT and calculated to be exergonic DG ¼ −61.6 kcal mol−1 at 298 K (Fig. 7) ( Table 8).

Buta- and Penta-Dienyl Complexes of the Actinides

Scheme 28 Reactivity of 41-U toward Me3SiN3.

Fig. 7 Diagrams showing representative complexes and important structural metrics for Section 4.02.4.2.

65

Table 8

Data table for heteroleptic actinacyclopentatriene complexes (Section 4.02.4.2). Oxidation state

Single crystal X-ray diffraction data Selected bond angles (  )

UdC1 2.515(5), UdC4 2.487(5), UdC2 2.434(5), UdC3 2.435(5); C1dC2 1.298(7), C2dC3 1.313(7), C3dC4 1.296(6); 2.470(5) UdC5Mecentroid 5 UdC1/UdC1a 2.450(12), UdC2/UdC2a 2.463(11); C1dC2/C1adC2a 1.32(2), C2dC2a 1.30(2); UdC5R2Hcentroid 2.504(12) 3 (R ¼ tBu)

C1dThdC4 90.0(3), ThdC1dC2 75.5(5), ThdC4dC3 75.3(5); C1dC2dC3 148.8(9), C4dC3dC2 150.3(9); C5Mecentroid dThdC5Mecentroid 5 5 137.8(3) C1dUdC1a 92.9, UdC1dC2/ UdC1adC2a 75.5; C1dC2dC2a/C1adC2adC2 148.1(2); C5Mecentroid dUdC5Mecentroid 5 5 138.1(2) C1dThdC4 90.2(2), ThdC1dC2 72.3(3), ThdC4dC3 72.7(3); C1dC2dC3 152.5(5), C4dC3dC2 152.3(5); C5Mecentroid dThdC5Mecentroid 5 5 135.0(2) C1dUdC4 92.2(2), UdC1dC2 71.4(3), UdC4dC3 72.5(3); C1dC2dC3 152.7(5), C4dC3dC2 151.2(5); C5Mecentroid dUdC5Mecentroid 5 5 134.0(2) C1dUdC1a 90.0(6) UdC1dC2/ UdC1adC2a 74.9; C1dC2dC2a/C1adC2adC2 148(1); dUdC5R2Hcentroid C5R2Hcentroid 3 3 134.8(6)

UdC1/ UdC1a 2.507(6), UdC2/UdC2a 2.463(7); C1dC2/C1adC2a 1.302(9), C2dC2a 1.347(9); UdC5R2Hcentroid 2.512(6) 3 (R ¼ tBu)

C1dUdC1a 92.4(3), UdC1dC2/ UdC1adC2a 72.9; C1dC2dC2a/C1adC2adC2 150.9(7); C5R2Hcentroid dUdC5R2Hcentroid 3 3 127.3(3)

[(
5-C5Me5)2Th(
4-C4Ph2)]

+IV

ThdC1 2.530(8), ThdC4 2.542(9), ThdC2 2.544(8), ThdC3 2.540(8); C1dC2 1.32(1), C2dC3 1.34(1), C3dC4 1.28(1); ThdC5Mecentroid 2.532(9) 5

[(
5-C5Me5)2U(
4-C4Ph2)]

+IV

[(
5-C5Me5)2Th{
4-C4(SiMe3)2}]

+IV

UdC1/ UdC1a 2.462(2), UdC2/UdC2a 2.439(2); C1dC2/C1adC2a 1.308(3), C2dC2a 1.314(3); 2.458(2), UdC5Mecentroid 5 2.463(4) ThdC1 2.574(5), ThdC4 2.555(5), ThdC2 2.508(5), ThdC3 2.501(5); C1dC2 1.309(7), C2dC3 1.311(7), C3dC4 1.313(7); ThdC5Mecentroid 2.542(5) 5

[(
5-C5Me5)2U{
4-C4(SiMe3)2}]

+IV

[(
5-1,3-tBu2-C5H3)2U(
4-C4Ph2)]

+IV

[(
5-1,3-tBu2-C5H3)2U {
4-C4(SiMe3)2}]

+IV

Complex number

Reference

1

H NMR (d6-benzene, 400 MHz, 25  C): d 1.83 (s, 30H, C5Me5) 7.15 (m, 2H, PhdH) 7.38 (t, J ¼ 7.7 Hz, 4H, PhdH) 8.11 (d, J ¼ 7.0 Hz, 4H, PhdH) ppm; 13 1 C{ H} NMR (d6-benzene, 100 MHz, 25  C): d 11.3 (C5Me5) 122.6 (C5Me5) 128.1 (PhdC) 129.2 (PhdCC) 134.0 (PhdCC) 138.1 (PhdCC) 149.2 (PhC]C) 205.6 (ThCPh) ppm

40-Th

79

1

H NMR (d6-benzene, 400 MHz, 25  C): d − 0.95 (s, 30H, C5Me5) 2.09 (m, 4H, PhdCH) 5.18 (t, 4H, J ¼ 6.8 Hz, PhdCH) 6.69 (t, 2H, J ¼ 7.0 Hz, PhdCH) ppm; 13 1 C{ H} NMR (d6-benzene, 100 MHz, 25  C): d − 51.4 (C5Me5) 117.0 (PhdCC) 128.5 (PhdCC) 136.8 (PhdCC) 137.6 (PhdCC) 157.1 (C5Me5) 190.6 (PhC]C) 237.7 (UCPh) ppm 1 H NMR (d6-benzene, 400 MHz, 25  C): d 0.56 (s, 18H, SiCH3) 1.78 (s, 30H, C5Me5) ppm; 13 1 C{ H} NMR (d6-benzene, 100 MHz, 25  C): d 2.3 (SiCH3) 11.3 (C5Me5) 122.1 (C5Me5) 178.0 (SiC]C) 224.3 (ThCSi) ppm

40-U

1

41-Th

82

1

H NMR (d6-benzene, 400 MHz, 25  C): d − 2.52 (s, 30H, C5Me5) 2.21 (s, 18H, SiCH3) ppm; 13 1 C{ H} NMR (d6-benzene, 100 MHz, 25  C): d 108.4 (C5Me5) 9.7 (SiCH3) −63.3 (C5Me5) ppm; UCSi resonances were not observed

41-U

68

1

42-U

83

43-U

83

H NMR (d6-benzene, 400 MHz, 25  C): d − 36.90 (s, 2H, ring-CH) −1.12 (br s, 36H, CH3) 7.30 (d, J ¼ 13.3 Hz, 2H, PhdCH) 7.60 (d, J ¼ 8.4 Hz, 4H, PhdCH) 7.75 (s, 4H, PhdCH) ppm; four protons (i.e., ring-CH) were not observed 13 1 C{ H} NMR (d6-benzene, 100 MHz, 25  C): d − 9.6 (C(CH3)3) 33.1 (C(CH3)3) 127.9 (PhdCC) 128.5 (PhdCC) 132.7 (PhdCC) 132.8 (PhdCC) 165.7 (ring C) 188.5 (PhC]C) 349.5 (UCPh) ppm 1 H NMR (d6-benzene, 400 MHz, 25  C): d − 53.88 (s, 2H, ring-CH) −2.40 (br s, 36H, C(CH3)3) 2.88 (s, 18H, SiCH3) ppm; four protons (i.e., ring-CH) were not observed 13 1 C{ H} NMR (d6-benzene, 100 MHz, 25  C): d − 20.9 (C(CH3)3) 13.4 (SiCH3) 29.2 (C(CH3)3) 110.5 (ring C) ppm; carbon atoms corresponding to UC were not observed 29 1 Si{ H} NMR (d6-benzene, 119.2 MHz, 25  C): d 285.8 (SiMe3) ppm

H and 13C{1H} NMR data77; SCXRD data80

Buta- and Penta-Dienyl Complexes of the Actinides

AndC and selected bond ˚ distances (A)

Nuclear magnetic resonance data

66

Formula

Buta- and Penta-Dienyl Complexes of the Actinides

4.02.4.3

67

An actinacyclopentyne complex

The first example of an actinacyclopentyne was reported very recently. The reaction of the mononuclear thorium dihydride [(
5-C5Me5)(
5-C5Ar5)ThH2(THF)] (Ar ¼ 3,5-tBu2C6H3) with PhC^CdC^CPh resulted in the isolation of [(
5-C5Me5) (
5-C5Ar5)Th(
4-PhCHdC^CdCHPh)] 44-Th (Scheme 29). The formation of 44-Th was calculated be exergonic by 47.4 kcal mol−1 at 298 K using DFT.69 This reactivity is in contrast to that observed for [(
5-C5Me5)(
5-C5Ar5)ThH2(THF)] with PhC^CdCH]CHPh or Me3SiC^CdC^CSiMe3, which yielded the but-2-ene-1,4-diyl complexes 25-Th and 26-Th (Section 4.02.3.1).

Scheme 29 Synthesis of the first actinacyclopentyne complex 44-Th.

The a-methylene (C1 and C4) proton environments of 44-Th are magnetically equivalent and were observed by 1H NMR spectroscopy as a singlet at d ¼ 4.88 ppm. The resonances for the carbon atoms C1 and C4 were seen at d ¼ 75.05 ppm, and the alkyne C^C carbon atoms at d ¼ 126.12 ppm, by 13C{1H} NMR spectrosocpy. These data were similar to the 13C{1H} NMR resonances assigned to the C^C carbon atoms in [(
5-C5H5)2Zr(
4-Me3SiCHdC^CdCHSiMe3)], which were d ¼ 102.92 ppm for the cis-isomer and d ¼ 103.59 ppm for the trans-isomer, where the downfield shift from 44-Th resulted from the difference in substituents.86 The solid-state molecular structure of 44-Th was determined by single-crystal XRD and confirmed the planar actinacyclopentyne, the
4-binding mode and localization of the CdC and C^C bonds in the (
4-PhCHdC^CdCHPh)2− ligand. The structural parameters in 44-Th were directly comparable to related zirconacyclopentyne complexes.86,87 The ThdC distances in 44-Th were within the normal ranges, Th1dC1 2.684 (4) and Th1dC4 2.666 (4) A˚ , and Th1dC2 2.528 (4) and Th1dC3 2.526 (4) A˚ . In 44Th the internal (C2,C3) carbon atoms were closer to the thorium metal center, than terminal carbon atoms (C1,C4), consistent with coordination of the C2^C3 triple bond. In the closest structural comparator to 44-Th, [(
5-C5H5)2Zr(
4-Me3SiCHdC^CdCHSiMe3)], ZrdC1 2.469 (7) and ZrdC4 2.456 (6) A˚ were also significantly longer than ZrdC2 2.291 (7) and ZrdC3 2.286 (7) A˚ .86 The internal ligand angles were very similar between 44-Th (C1dC2dC3 156.0 (4) and C4dC3dC2 155.7 (5) ) and [(
5-C5H5)2Zr(
4-Me3SiCHdC^CdCHSiMe3)] (C1dC2dC3 155.9 (7) and C4dC3dC2 156.2 (6) ). In 44-Th the C1dC2 and C3dC4 distances of 1.423 (6) and 1.404 (6) A˚ , respectively were consistent with CdC single bonds and much longer than the C2dC3 distance of 1.238 (6) A˚ , which was consistent with a C^C triple bond and typical of a C^C triple bond in a cyclic system.84 These data are again directly comparable to [(
5-C5H5)2Zr(
4-Me3SiCHdC^CdCHSiMe3)] with CdC single bond distances of C1dC2 1.415 (7) and C3dC4 1.400 (6) A˚ and a triple bond C^C distance of C2dC3 1.206 (7) A˚ .86 Complexes 31-Th (actinacyclopentadiene Section 4.02.4.1) and 40-Th (actinacyclopentatriene Section 4.02.4.2) also provided structural context for complex 44-Th. The C1dThdC4 angle of 91.1 (1) in 44-Th was similar to 40-Th (C1dThdC4 of 90.0 (3) ) and as expected, very different to 31-Th (C1dThdC1a of 74.1 (1) ). Likewise, the ThdC distances to the terminal carbon atoms C1 and C4 in 44-Th (Th1dC1 2.684 (4) A˚ and Th1dC4 2.666 (4) A˚ ) were much longer than 31-Th (ThdC1/C1a 2.465 (2) A˚ ) but not very much longer than 40-Th (ThdC1 2.530 (8) and ThdC4 2.542 (9) A˚ ). The C^C triple bond distance in 44-Th (1.238 (6) A˚ ) was much shorter than the C]C double bond distances in 40-Th (1.32 (1), 1.34 (1) and 1.28 (1) A˚ ) and 31-Th (1.362 (3) A˚ ). The CdC single bond distances in 44-Th (1.423 (6) and 1.404 (6) A˚ ) were significantly shorter than the CdC single bond distance in 31-Th (1.515 (3) A˚ ). The internal ligand angles of 44-Th (156.0 (4) and 155.7 (5) ) were slightly more obtuse than those seen in 40-Th (148.8 (9) and 150.3 (9) ). Selected spectroscopic data for Section 4.02.4.3 are shown in Table 9.

68

Data table for an actinacyclopentyne complex (Section 4.02.4.3).

Formula

[(
5-C5Me5)(
5-C5Ar5) Th(
4-PhCHdC^ CdCHPh)] (Ar ¼ 3,5-tBu2C6H3)

Oxidation state

+IV

Single crystal X-ray diffraction data AndC and selected bond ˚ distances (A)

Selected bond angles (  )

Th1dC1 2.684(4), Th1dC4 2.666(4), Th1dC2 2.528 (4), Th1dC3 2.526(4); C1dC2 1.423(6), C2dC3 1.238(6), C3dC4 1.404(6); ThdC5Mecentroid 5 2.539, ThdC5Arcentroid 2.657 5

C1dThdC4 91.1(1), ThdC1dC2 68.2(2), ThdC4dC3 68.9(2); C1dC2dC3 156.0(4), C4dC3dC2 155.7(5); C5Mecentroid dThdC5Arcentroid 5 5 138.23

Nuclear magnetic resonance data

Complex number

Reference

1

44-Th

69

H NMR (d6-benzene, 500 MHz, 25  C): d 1.18 (s, 90H, Ar-tBu) 1.80 (s, 15H, C5Me5) 4.88 (s, 2H, PhdCCH) 6.95 (t, J ¼ 7.4 Hz, 2H, C6H5) 7.09 (s, 10H, C5Ar5) 7.26 (t, J ¼ 7.5 Hz, 5H, C5Ar5) 7.31–7.36 (m, 8H, C6H5) ppm; 13 1 C{ H} NMR (d6-benzene, 126 MHz, 25  C): d 12.43 (C5Me5) 31.90 (C(CH3)3) 35.13 (C(CH3)3) 75.05 (PhCHdC^CdCHPh) 121.02 (C5Ar5&PhCHdC^CCHPh) 123.38 (PhCHdC^CdCHPh) 125.87 (C5Me5) 126.12 (PhCHdC^CdCHPh) 126.98 (C6H5) 127.13 (C5Ar5) 128.74 (C6H5) 129.51 (C6H5) 132.98 (C6H5) 135.92 (C5Ar5) 143.56 (C5Ar5) 149.43 (C6H5) 150.39 (C5Ar5) ppm

Buta- and Penta-Dienyl Complexes of the Actinides

Table 9

Buta- and Penta-Dienyl Complexes of the Actinides

4.02.5

69

Cyclobutadienyl complexes of the actinides

Actinide cyclobutadienyl complexes are the subject of a recent review article.88 The reaction of [{UHC(SiMe2NXy)3}2(m-C6H5Me)] (Xy ¼ 3,5-Me2C6H3) with excess PhC^CPh resulted in the isolation of the first example of an actinide inverse sandwich cyclobutadienyl complex [{UHC(SiMe2NXy)3}2(m-C4Ph4)] 45-U in a low yield (Scheme 30).89 This was achieved by a reductive [2 + 2] cycloaddition, facilitated by the reducing tetraanionic 10p-electron toluene and the two uranium metal centers in close proximity. This an unusual example of a [2 + 2] cycloaddition using a metal that does not exert much orbital control, in contrast to transition metal examples. The difficulty of this reaction is reflected in the very slow rate of cyclobutadienyl ring closure of the acyclic 1,3-butadiene-1,4-diyl intermediate (Scheme 30), which was observed via 1H NMR spectroscopy and proposed to be analogous to the bimetallic acyclic 1,3-butadiene-1,4-diyl uranium complexes in Section 4.02.3.2.70 Matching the steric profile of the uranium complex to the alkyne was important and only this specific combination resulted in cyclobutadienyl ring formation. For example, the reaction of 2 eq. of polarized triple bond reagent tBuC^P
O(SiMe3)2 with the less sterically demanding [{UHC(SiMe2N (p-tol))3}2(m-C6H5Me)] (p-tol ¼ 4-MeC6H4) resulted in a faster [2 + 2] cycloaddition, and a significantly higher yield of [{UHC (SiMe2N(p-tol))3}2{m-(CtBu)2P2}] 46-U (Scheme 30), whereas the attempted reaction of tBuC^P
O(SiMe3)2 with [{UHC (SiMe2NXy)3}2(m-C6H5Me)] (Xy ¼ 3,5-Me2C6H3) was unsuccessful.

Scheme 30 Synthesis of actinide inverse sandwich cyclobutadienyl complexes 45-U and 46-U.

The solid-state molecular structure of 45-U was determined by single-crystal XRD. Each uranium metal center in 45-U was bound
4 to the four cyclobutadienyl2− ring carbons. However, the bridging cyclobutadienyl2− ring was bound off-center between the two uranium metal centers and there was additionally a close contact of each uranium to the ipso-carbon of the nearest phenyl ring. The asymmetric cyclobutadienyl2− binding resulted in shorter and longer UdC distances in the ranges of 2.655(5)–2.664 (5) A˚ and 2.860(6)–2.871 (5) A˚ , respectively. The UdCipso distances were determined to be 2.887 (5) and 2.842 (5) A˚ . The average CdC bond distance of 1.484 A˚ was consistent with delocalized 6p-electron cyclobutadienyl2− ligand. The solid-state molecular structure of 46-U was determined by single crystal XRD, revealing a symmetric m-(CtBu)2P2 ring with UdP distances in the range of 2.9081 (5)–3.0358 (5) A˚ and UdC distances in the range of 2.778(2)-2.981 (2) A˚ . The PdC distances ranged between 1.794 (2) and 1.810 (2) A˚ , consistent with a delocalized dianionic ligand. The 1H NMR spectra of 45-U and 46-U at room temperature were consistent with asymmetric and symmetric solid-state coordination modes, respectively, being maintained in the solution phase. The SQUID magnetization data were consistent with U(IV) in 45-U and 46-U. There was good agreement between the solid-state and solution phase magnetization data. The electronic absorption spectroscopy data were also consistent with the assignment of the U(IV) oxidation state. The electronic structures of 45-U and 46-U were investigated by unrestricted DFT calculations. In the case of 46-U it was necessary to use a computational model in which the tBu substituents on the [m-(CR)2P2]2− ligand were replaced by methyl groups. There was good agreement between experimentally and theoretically determined metrics. The calculated spin density and charges were also consistent with U(IV) and cyclobutadienyl2− ligands. The bonding in 45-U and 46-U was primarily electrostatic but with a small amount of covalent character to An-cyclobutadienyl bonding. In 45-U and 46-U the three highest occupied MOs (HOMOs) were of non-bonding 5f character. In addition, there were bonding interactions between the cyclobutadienyl ligand and the 5f orbitals on uranium, d-bonding with empty c4 orbital (HOMO-3) and p-bonding with the c2 and c3 orbitals (HOMO-6, HOMO7). These are the same p-bonding combinations that are found in transition metal cyclobutadienyl complexes (Fig. 8). Transition metals utilize a combination of p- and d-orbitals in cyclobutadienyl bonding, whereas in 45-U and 46-U the 5f are used almost exclusively. The overlap population is very small for the d- and p-bonding combinations and are similar in magnitude in both 45-U

70

Buta- and Penta-Dienyl Complexes of the Actinides

Fig. 8 Molecular orbitals of the cyclobutadienyl dianion.

and 46-U. The utilization of the f-orbitals and the small overlap was very different from the strong d- and p-overlap found in complexes of uranium with larger aromatic ligands such as arenes and the cyclooctatetraenyl dianion.89 Metathesis (or transmetallation) chemistry provides an alternative and lower energy synthetic route to actinide cyclobutadienyl complexes. However, the selection of the metathesis partners or starting materials is key. The reaction of UCl4 with [Li2{C4 (SiMe3)4}(THF)2] yielded only intractable solids but the use of less common, halide-free uranium(III) borohydride starting materials were successful. The reaction of [U(BH4)3(THF)2] with [Li2{C4(SiMe3)4}(THF)2] in toluene at low temperature (−196  C) yielded the separated ion pair [Li(THF)4][U{
4-C4(SiMe3)4}(BH4)3] 47-U in 55% yield (Scheme 31A).90

Scheme 31 Synthesis of 47-U to 50-U.

Buta- and Penta-Dienyl Complexes of the Actinides

71

The multinuclear NMR spectra (1H, 11B{1H} and 7Li{1H}) of 47-U at room temperature were simple, with magnetically equivalent ligand environments for the cyclobutadienyl, and borohydride ligands and resonances assigned to the coordinated THF. The ATR-IR spectrum displayed characteristic k3-coordinated BdH stretching frequencies and the SQUID magnetization and electronic absorption data were consistent with U(IV) in 47-U. The solid-state molecular structure of 47-U confirmed terminal
4-cyclobutadienyl and k3-borohydride binding. The UdC bond distances of 47-U were shorter than in 45-U and were in the range of 2.477(11)–2.549 (12) A˚ , with CdC distances ranging from 1.452(16)–1.488 (17) A˚ . These data are consistent with a terminal rather than bridging cyclobutadienyl2− ligand in 47-U. An unexpected structural feature of 47-U was the significant (0.452–0.566 A˚ ) out of plane displacement of the SiMe3 substituents of the cyclobutadienyl ligand. This SiMe3-displacement was above the plane of the cyclobutadienyl ligand and therefore can be seen as the substituents bending away from the uranium metal center. The substituent displacement in 47-U was comparable to that seen in the very sterically congested tris(cyclopentadienyl) f-element complexes. Unrestricted DFT studies on the anionic component of 47-U revealed two quasi-degenerate p-bonding molecular orbitals formed from overlap of c2 and c3 cyclobutadienyl ligand orbitals with 5f/6d hybrid orbitals on uranium (HOMO-2 and HOMO-3, 70% C, 30% U: 67% 5f, 33% 6d). In contrast to 45-U and 46-U there was no d-bonding (c4) in 47-U. The SiMe3-displacement was also reproduced computationally and rationalized as a means to maximize UdC orbital overlap.90 The reaction of the uranium(IV) borohydride [U(BH4)4] with 1 eq. of [Na2{C4(SiMe3)4}(THF)2] and then 2 eq. of 12-crown-4, or 1 eq [K2{C4(SiMe3)4}] in THF resulted in the formation of [Na(12-crown-4)][U{
4-C4(SiMe3)4}(BH4)3] 48a-U or [U {
4-C4(SiMe3)4}(BH4)2(m-BH4){K(THF)2}]2 49-U, respectively (Scheme 31B). In 48a-U the addition of 2 eq. of 12-crown-4 to the initial microcrystalline product of the reaction [U{
4-C4(SiMe3)4}(BH4)3Na(THF)3] 48-U resulted in isolable material. Heating the reaction of [U(BH4)4] with 1 eq. of [Na2{C4(SiMe3)4}(0.75
THF)] resulted in intra-molecular deprotonation and yielded the tuck-in/allyl microcrystalline complex [U(BH4){
4-C4(SiMe3)4}{
3-C4(SiMe3)3-k1-CH2SiMe2}Na(THF)3] 50-U, which upon addition of tBuOMe was isolated as single crystals of [Na(tBuOMe)3.6(THF)0.4][U(BH4){
4-C4(SiMe3)4}{
3-C4(SiMe3)3-k1CH2SiMe2}] 50a-U (Scheme 31B).91 The multinuclear NMR spectra (1H, 11B and 29Si) of 48a-U and 49-U at room temperature displayed single resonances consistent with magnetically equivalent ligand environments for the cyclobutadienyl and borohydride ligands. Very small differences in chemical shifts were observed between the data for 48a-U and 49-U, and these data were also consistent with 47-U. The solid-state molecular structures of 48a-U, 49-U and 50a-U were determined by single-crystal XRD. The molecular structures of 48a-U and 49-U were very similar, and the molecular structure of 48a-U was essentially the same as 47-U.90 In 48a-U the UdC distances were in the range of 2.522(5)–2.556 (4) A˚ with a UdCcentroid distance of 2.319 (5) A˚ and in 49-U 2.464(19)–2.557 (16) A˚ and 2.289 (8) A˚ , respectively. The SiMe3-displacement was also seen in complexes 48a-U and 49-U and can be measured either as a displacement distance in A˚ or as a bending angle from the plane of the cyclobutadienyl ring. In 48a-U and 49-U the SiMe3-groups were seen to bend out of plane away from uranium by approximately 20 (0.460–0.498 A˚ 48a-U, 0.400–0.570 A˚ 49-U).91 These data are consistent with those in 47-U (0.452–0.566 A˚ ).90 In the molecular structure of 50a-U the
4-cyclobutadienyl UdC distances were in the range of 2.550(5)–2.650 (6) A˚ , 0.06 A˚ longer than in 48a-U, and the UdCcentroid distance was 2.378 (6) A˚ . The protonated ligand (
3-C4(SiMe3)3-k1-CH2SiMe2)1− in 50a-U, displayed two shorter UdC distances of 2.638 (5) and 2.635 (6) A˚ , and one longer distance of 2.765 (6) A˚ . The UdCtuck-in distance was 2.543 (6) A˚ . The (
3-C4(SiMe3)3-k1-CH2SiMe2)1− ring was also seen to deviate significantly from planarity, as measured by the torsion angle of the C4-ring carbons, which was 8.7 (4) and significantly larger than the 0.2 (4) seen in the
4-cyclobutadienyl2− ligand. Density functional methods were used to study the bonding in 48a-U and 50a-U.91 Bonding between the uranium center and the borohydride ligands was ionic in nature. In 48a-U there were two orbitals (88% 5f ) occupied by unpaired 5f electrons and two doubly occupied predominantly ligand-based orbitals but some metal contribution to bonding (alpha-spin 11% 5f, 6% 6d, beta-spin 5% 5f, 9% 6d). The bonding situation in 50a-U was qualitatively similar to that seen in 48a-U. Thus, the 5f and 6d contributions to bonding were found of to be of similar magnitude in 48a-U and 50a-U with 15–20% uranium character overall. This finding was in contrast to 47-U where % contribution of the 5f was double that of the 6d, with 30% uranium character overall.90 The reaction of [U(
5-C5Me5)I2(THF)] with [Mg{C4(SiMe3)4}(THF)3] in benzene resulted in the formation of the oxo-centered trimeric complex [Mg(THF)6][{U(
4-C4(SiMe3)4)(m-I)2}3(m3-O)] 51-U (Scheme 32).92 This complex was unstable in ethereal

Scheme 32 Synthesis of the oxo-centered trimeric complex 51-U.

72

Buta- and Penta-Dienyl Complexes of the Actinides

solvents and insoluble in aromatic solvents, precluding its characterization by NMR and electronic absorption spectroscopy. Complex 51-U was also unstable in the solid-state at room temperature, as significant decomposition was observed by the end of 1 week. It is of note that analogous reactions of [U(
5-C5Me5)I2(THF)] with [Li2{C4(SiMe3)4}(THF)2] did not yield a cyclobutadienyl complex, rather the oxo-centered trimeric complex [Li(THF)4][{U(
5-C5Me5)(m-I)2}3(m3-O){Li(THF)3}0.5]2. The solid-state SQUID magnetic susceptibility data of 51-U were consistent with U(IV) and no antiferromagnetic coupling between uranium metal centers was observed. The UdC bond distances in the molecular structure of 51-U were essentially the same as those seen in 47-U, however, the SiMe3-displacements were slightly larger (0.483–0.653 A˚ ). The bonding picture in 51-U was calculated to be similar to than seen in 47-U using DFT calculations. In 51-U the same p-bonding molecular orbitals were formed from c2 and c3 cyclobutadienyl overlap with 5f/6d hybrid orbitals on uranium (65% C, 35% U). In 51-U the % contribution from the 5f and 6d was 50:50, which is consistent with 48a-U and 50a-U but different to 47-U (67% 5f, 33% 6d). As in 47-U there was no d-bonding (c4) in 51-U, because while the d-bonding MOs were seen computationally they are unoccupied and very high in energy. The energy separation between d-bonding and p-bonding MO groups was smaller for 51-U (2.67 eV) than 47-U (3.47 eV). In pursuit of thorium cyclobutadienyl complexes, thoracene [Th(
8-C8H8)2] was chosen as a starting material, as the hard cyclobutadienyl ligand was expected to be a better match for Th(IV), based on hard-soft acid-base theory and therefore to more easily displace the softer (C8H8)2− ligand. For context the reaction of ThCl4 with [K2{C4(SiMe3)4}] yielded only intractable solids and neutral cyclobutadiene. However, the reaction of [Th(
8-C8H8)2] with [K2{C4(SiMe3)4}] in THF yielded [{Th(
4-C4[SiMe3]4) (m-
8-C8H8)(m-
2-C8H8)(K[C6H5Me]2)}2{K(C6H5Me)}{K}] 52-Th (Scheme 33).93

Scheme 33 Synthesis of the thorium cyclobutadienyl complex 52-Th.

The room temperature 1H NMR spectrum of 52-Th displayed three resonances, a sharp singlet at d ¼ 0.74 ppm assigned to the SiMe3 groups of the cyclobutadienyl ligand and two broad resonances at d ¼ 5.71 and 6.47 ppm with full width half maxima of 26.5 and 28.7 Hz, respectively, which were assigned to the two magnetically inequivalent (C8H8)2− ligands. Both insolubility and thermal instability on heating of 52-Th prevented further variable temperature and 2D NMR experiments. The solid-state molecular structure was determined by single-crystal XRD and confirmed the terminal
4-cyclobutadienyl, and both the normal
8- and very unusual
2-cyclooctatetraenyl binding. The ThdC(
4-C4{SiMe3}4) distances in 52-Th were determined to be 2.627(3)-2.672 (3) A˚ and 2.557(3)–2.739 (3) A˚ for Th1 and Th2, respectively. The average CdC(
4-C4{SiMe3}4) distance of 1.481 (8) A˚ was identical to the same metric in 47-U. The average ThdC(
8-C8H8) distances in 52-Th were normal (average values 2.787 (12) and 2.765 (17) A˚ ) and very similar to the average ThdC(
2-C8H8) distances of 2.741 (10) and 2.788 (9) A˚ . The SiMe3-displacements in 52-Th were asymmetric in nature. For Th-1 the displacement values within the pairs of opposing SiMe3-groups (1,3 and 2,4) were similar in magnitude but the pairs were very different to one another (Si1/Si3 0.839 (9)/0.830 (12) A˚ and Si2/Si4 0.233 (9)/0.224 (8) A˚ ). The same trend but a larger range of displacement values was found for Th2 (Si5/Si7 0.097 (9)/0.297 (8) A˚ and Si6/Si8 0.678 (6)/ 1.097 (10) A˚ ). The electronic structure of 52-Th was evaluated using DFT studies and [Th(
4-C4{SiMe3}4)(
8-C8H8)] as a computational model. The HOMO and HOMO-1 were p-bonding molecular orbitals formed from c2 and c3 cyclobutadienyl overlap with 5f/6d

Buta- and Penta-Dienyl Complexes of the Actinides

73

hybrid orbitals on thorium (78% C, 22% Th: 31% 5f, 69% 6d). The HOMO-2 and HOMO-3 were d-bonding molecular orbitals formed from c4 and c5 cyclooctatetraenyl overlap with 5f/6d hybrid orbitals on thorium (81% C, 19% Th: 92% 5f, 8% 6d). The % Th contribution to either p- or d-bonding in 52-Th is similar. However, there was more 5f character to Th-cyclobutadienyl bonding. This 5f contribution was unusually high for thorium even though in comparison to 47-U, 52-Th has almost 10% less metal character to bonding and half the 5f contribution. Complex 52-Th was an intense orange color in both solution and the solid-state, the origin of which was assigned by Time-Dependent (TD)-DFT studies as ligand to metal charge transfer from the (
2-C8H8)2− ligand to an empty 6d orbitals on thorium.93 Recently the synthesis of mixed-sandwich cyclobutadienyl-cyclooctatetraenyl complexes of U(IV) have been reported.94 The reaction of in situ generated [U{
4-C4(SiMe3)4}{(BH4)3}][Na(THF)n], in a 3:1 solvent mixture of THF and toluene, with 1.3 eq. of [K2C8H8] at −35  C, resulted in the formation of bimetallic [U(
4-C4{SiMe3}4)(
8-C8H8)(m-
2-
8-C8H8)U(THF)(
4-C4 {SiMe3}4)]2 53-U in an 18% yield (Scheme 34). Variable temperature 1H NMR spectroscopy was consistent with dynamic behavior, where at room temperature 53-U was in equilibrium with [U(THF)(
4-C4{SiMe3}4)(
8-C8H8)] 54-U and [U(
4-C4{SiMe3}4) (
8-C8H8)] 55-U. Complex 54-U was isolated in a low yield from the recrystallization mother liquor of 53-U in heptane and 1H NMR spectroscopy was consistent with retention of the solid-state structure in solution. The application of a strong vacuum (ca 10−6 to 10−7 mbar), for 5–6 h at 45  C, to either 53-U or 54-U resulted in the formation of the unsolvated complex [U(
4-C4{SiMe3}4) (
8-C8H8)] 55-U (Scheme 34). The reaction of 53-U and 54-U or 55-U with Et2O in toluene resulted in reductive cleavage, yielding [U(OEt)(
4-C4H{SiMe3}4)(
8-C8H8)] 57-U and ethene (Scheme 34). This was the first example of ether cleavage by U(IV) with no simultaneous change in oxidation state, indicative of metal-ligand co-operativity in this reactivity. However, the reaction of

Scheme 34 Synthesis of cyclobutadienyl-cyclooctatetraenyl complexes 53-U to 57-U.

74

Buta- and Penta-Dienyl Complexes of the Actinides

[U{
4-C4(SiMe3)4}{(BH4)3}][Na(THF)n] with 1.3 eq. of the bulky 1,4-Si(iPr)3-substituted cyclooctatetraenyl ligand [K2C8H6 (SiiPr3)2] resulted in the clean formation of [U(
4-C4{SiMe3}4)(
8-C8H6{SiiPr3}2)] 56-U (Scheme 34). The low isolated crystalline yield (12%) of 56-U resulted from the very high solubility of this complex, even in minimum SiMe4 at −35  C. Complex 56-U also did not react with Et2O, which was consistent with the lack of access to the uranium metal center in 56-U vs 54-U and 55-U. The solid-state molecular structures of complexes 53-U to 56-U were determined by single-crystal XRD and confirmed the terminal
4-cyclobutadienyl, and both the
8- and
2-cyclooctatetraenyl binding. In complexes 53-U to 56-U the UdC(
8-C8H8) or UdC(
8-C8H6{SiiPr3}2) distances were normal (2.654(9)–2.760 (9) and 2.62(1)–2.73 (1) A˚ in 53-U). The UdC(
2-C8H8) distances of 3.006(9)–3.023 (9) A˚ in 53-U were slightly longer than average ThdC(
2-C8H8) distances of 2.741 (10) and 2.788 (9) A˚ in 52-Th, consistent with the weaker
2-interaction in 53-U. The UdC(
4-C4{SiMe3}4) distances in 53-U were in the range of 2.420(7)–2.705 (9) and 2.403(8)–2.749 (9) A˚ for U1 and U2, respectively, and this range is statistically indistinguishable from the same metrics in 53-U to 56-U. Asymmetric SiMe3-displacements were also observed in complexes 53-U to 56-U. In 57-U the protonated ligand (
4-C4H{SiMe3}4)1− displayed two shorter UdC distances of 2.617 (5) and 2.647 (4) A˚ , and one longer distance of 2.741 (5) A˚ and a C4-ring torsion angle of 9.7 (3) . These data are very similar to the same metrics of the protonated ligand in 50a-U (2.638 (5), 2.635 (6) and 2.765 (6) A˚ ; 8.7 (4) ). The energies of THF complexation to 55-U and 56-U, were investigated using DFT calculations and high-level domain-localized pair natural orbital coupled cluster methods. Consistent with the experimental data, the formation of the THF adduct 54-U from 55-U was exergonic by 81 kJ mol−1, whereas the putative formation of 56-U
THF was thermoneutral (exergonic by 4 kJ mol−1, within the error of the calculation). The bonding in 55-U and 56-U was shown to be qualitatively similar using DFT studies. The predominant (86% overall) metal-ligand interaction was described by the donation of electron density from the HOMOs of the cyclobutadienyl and cyclooctatetraenyl ligands to the partially filled 5f and the empty 6d orbitals on uranium. The mixing of ligand and 5f orbitals was found to be much more significant in 55-U and 56-U than in 48a-U and 49-U. The % compositions of the uranium-ligand orbitals were calculated for the cyclobutadienyl (54%–67% C, 6%–21% 5f, 6%–18% 6d) and cyclooctatetraenyl (44%–73% C, 2%–32% 5f, 8%–13% 6d) ligands. Overall, it was found that mixing between the 6d and the cyclobutadienyl ligand orbitals was more significant than mixing with the 5f, whereas the opposite was found for the cyclooctatetraenyl ligands (5f > 6d). This conclusion contrasts with previous computational analysis of actinide cyclobutadienyl complexes. Important structural metrics for Section 4.02.5 are shown in Fig. 9 and selected spectroscopic data for Section 4.02.5 in Table 10.

Fig. 9 Diagrams showing representative complexes and important structural metrics for Section 4.02.5.

Table 10

Data table for cyclobutadienyl complexes of the actinides (Section 4.02.5).

Formula

Oxidation state

Single crystal X-ray diffraction data AndC and other selected ˚ distances (A)

[{UHC(SiMe2NXy)3}2(mC4Ph4)] (Xy ¼ 3,5-Me2C6H3)

+IV

[{UHC(SiMe2N(p-tol))3}2{m- +IV (CtBu)2P2}] (p-tol ¼ 4-MeC6H4)

Magnetometry

1

3.95 mB (SQUID, 45-U 300 K); 0.67 mB (SQUID, 1.8 K); 3.80 mB (Evans method, d6-benzene, 298 K)

89

46-U 3.61 mB (SQUID, 300 K); 0.51 mB (SQUID, 1.8 K); 3.90 mB (Evans method, d6-benzene, 298 K) 3.03 mB (SQUID, 47-U 300 K); 0.67 mB (SQUID, 2 K)

89

48-U

91

48a-U

91

49-U

91

50-U

91

˚ ) SiMe3 Displacements (A/

UdC 2.655(5)–2.664(5); UdC 2.860(6)–2.871(5); UdCipso 2.887(5) and 2.842(5); CdCaverage 1.484; UdN 2.245(4)–2.265(4) UdC 2.778(2)–2.981(2); UdP 2.9081(5)–3.0358(5); PdC 1.794(2)–1.810(2); UdN 2.2215(18)–2.2536 (18)

UdC 2.477(11)–2.549(12); CdC 1.452(16)–1.488(17);

Nuclear magnetic resonance data  a

+IV

0.454/0.506, 15.74/17.06 (Si1/Si3); 0.477/0.567, 14.48/15.46 (Si2/Si4)

[U{
4-C4(SiMe3)4} (BH4)3Na(THF)3]

+IV

[Na(12-crown-4)][U {
4-C4(SiMe3)4}(BH4)3]

+IV

UdC 2.522(5)–2.556(4); UdCcentroid 2.319(5);

0.486/0.498, 15.04/15.46 (Si1/Si3); 0.460/0.498, 14.48/15.46 (Si2/Si4)

[U{
4-C4(SiMe3)4} (BH4)2(mBH4){K(THF)2}]2

+IV

UdC 2.464(19)–2.557(16); UdCcentroid 2.289(8); U⋯ U 7.307(7); K⋯ K 7.580(15)

0.570/0.471, 17.25/15.66 (Si1/Si3); 0.440/0.400, 14.46/11.95 (Si2/Si4)

[U(BH4){
4-C4(SiMe3)4} {
3-C4(SiMe3)3-k1CH2SiMe2}Na(THF)3]

+IV

Reference

90

Buta- and Penta-Dienyl Complexes of the Actinides

[Li(THF)4][U {
4-C4(SiMe3)4}(BH4)3]

75

H NMR (d6-benzene, 300.13 MHz, 25  C): d − 41.95 (s, 2H, SidCH) −4.21 (s, 12H, o-CH) −0.35 (s, 36H, CH3) 0.13 (s, 36H, CH3) 1.94 (br s, 6H, p-CH) 3.57 (s, 2H, p-CH-phenyl) 4.14 (s, 4H, CH-phenyl) 6.36 (br s, 4H, CH-phenyl) 6.52 (br s, 4H, CH-phenyl) 7.45 (m, 3H, CH-diphenylacetylene) 7.63 (m, 2H, CH-diphenylacetylene) 8.00 (m, 4H, CH-phenyl) 11.81 (m, 2H, p-CH-phenyl) ppm 1 H NMR (d6-benzene, 300.13 MHz, 25  C): d − 69.78 (s, 2H, SidCH) −4.70 (s, 36H, Si(CH3)3) 3.80 (s,18H, ArdCH3) 8.81 (s,12H, ArdCH) 9.00 (s,12H, ArdCH) 23.37 (s, 18H, tBu) ppm; 1 P{ H} NMR (d6-benzene, 162.0 MHz, 25  C): d 535.52 (C2P2) ppm 1 H NMR (d6-benzene, 400.2 MHz, 25  C): d − 4.69 (s, 36H, Si(CH3)3) 2.98 (s, 16H, THF) 6.84 (s, 16H, THF) 15.97 (br s, BH4) ppm; 7 1 Li{ H} NMR (d6-benzene, 155.5 MHz, 25  C): d 5.93 ppm; 11 1 B{ H} NMR (d6-benzene, 128.4 MHz, 25  C): d 132.67 ppm 1 H NMR (d8-THF, 400 MHz, 30  C): d − 5.12 (s, 36H, Si(CH3)3) 15.56 (br s, 12H, BH4) ppm; 29 1 Si{ H} NMR (d8-THF): d − 208.4 (s, SiMe3) ppm; 11 1 B{ H} NMR (d8-THF): d 124.69 (s, BH4, Dn1/2 ¼ 111 Hz) ppm; 23 Na{1H} NMR (d8-THF): d − 2.88 (s, Dn1/2 ¼ 29 Hz) ppm 1 H NMR (d8-THF, 400 MHz, 30  C): d − 5.12 (s, 36H, Si(CH3)3) 4.77 (s, 32H, 12-crown-4) 15.56 (br s, 12H, BH4) ppm; 29 1 Si{ H} NMR (d8-THF): d − 208.4 (s, SiMe3) ppm; 11 1 B{ H} NMR (d8-THF): d 124.69 (s, BH4, Dn1/2 ¼ 111 Hz) ppm; 23 Na{1H} NMR (d8-THF): d − 1.51 (s, Dn1/2 ¼ 8.3 Hz) ppm 1 H NMR (d8-THF, 400 MHz, 30  C): d − 4.87 (s, 36H, Si(CH3)3) 15.64 (br s, 12H, BH4) ppm; 29 1 Si{ H} NMR (d8-THF): d − 208.3 (s, SiMe3) ppm; 11 1 B{ H} NMR (d8-THF): d 125.7 (s, BH4, Dn1/2 ¼ 66.8 Hz) ppm 1 H NMR (d8-THF, 400 MHz, 30  C): d − 148.88 (s, 1H) −106.66 (s, 1H) −82.07 (s, 1H) −78.60 (1:1:1:1 q, 1 JB-H ¼ 66.84 Hz, 4H, BH4) −22.18 (s, 3H, Si(CH3)3) −5.94 (s, 9H, Si(CH3)3) −0.56 (s, 36H, Si(CH3)3) 4.37 (s, 9H, Si(CH3)3) 10.01 (s, 3H, Si(CH3)3) 12.66 (s, 9H, Si(CH3)3) ppm; 29 1 Si{ H} NMR (d8-THF): d − 260.36, −175.31, −115, 56.69, 66.15 (SiMe3) ppm; 23 Na{1H} NMR (d8-THF): d − 2.77 (s, Dn1/2 ¼ 81.67 Hz) ppm; 11 1 B{ H} NMR (d8-THF): d 41.63 (br s, Dn1/2 ¼ 225.4 Hz) ppm

Complex number

(Continued )

76

Table 10

(Continued) Oxidation state

Single crystal X-ray diffraction data

Nuclear magnetic resonance data

AndC and other selected ˚ distances (A)

˚ ) SiMe3 Displacements (A/

0.732/0.566, 23.72/17.6 (Si5/Si7); 0.348/0.828, 10.99/26.79 (Si6/Si8)

[Na(tBuOMe)3.6(THF)0.4] [U(BH4){
4-C4(SiMe3)4} {
3-C4(SiMe3)3-k1CH2SiMe2}]

+IV

UdC(
4) 2.550(5)–2.650(6); UdCcentroid(
4) 2.378(6); UdCallylic(
3) 2.638(5), 2.635(6) and 2.765(6); UdCcentroid(
3) 2.502(7); UdCtuck-in 2.534(6)

[Mg(THF)6] [{U(
4-C4(SiMe3)4)(mI)2}3(m3-O)]

+IV

UdC 2.553(6)–2.595(6); UdO 2.285(8)–2.359(4)

[Th(
4-C4(SiMe3)4)(m
8-C8H8)(m-
2-C8H8) {K2(C6H5Me)3}]2

+IV

[U(
4-C4(SiMe3)4) (
8-C8H8)(m
2-
8-C8H8)U(THF) (
4-C4(SiMe3)4)]

+IV

0.483/0.510, 15.34/16.13 (U1, Si1/Si3); 0.545/0.653, 17.15/20.61 (U1, Si2/Si4); 0.621/0.621, 18.97/18.97 (U2, Si5/Si5); 0.621/0.621, 17.25/17.25 (U2, Si6/Si6) 0.839(9)/0.830(12), Th1dC(
4-C4{SiMe3}4) 2.627 27.29/26.71 (Th1, (3)–2.672(3); Si1/Si3); Th2dC(
4-C4{SiMe3}4) 2.557 0.233(9)/0.224(8), 8.08/8.8 (3)–2.739(3); (Th1, Si2/Si4); Th1dC(
2-C8H8) 2.734(3)–2.747(3); Th2dC(
2-C8H8) 2.784(3)–2.792(3); 0.097(9)/0.297(8), 7.37/9.3 (Th2, Si5/Si7); Th1dC(
2-C8H8) 2.786(3)–2.861(3); 0.678(6)/1.097(10), Th2dC(
2-C8H8) 2.767(9)–2.870 21.36/36.65 (Th2, Si6/Si8) (10); CdC(
4-C4{SiMe3}4) 1.481(8) 0.590/1.412, 17.47/48.73 U1dC(
4-C4{SiMe3}4) 2.420 (U1, Si1/Si3); (7)–2.705(9); 0.147/0.101, 6.64/8.72 (U1, U1dC(
4)centroid 2.332(4); Si2/Si4); U2dC(
4-C4{SiMe3}4) 2.403 1.443/0.047, 49.57/4.72 (8)–2.749(9); (U2, Si5/Si7); U2dC(
4)centroid 2.340(4); U2dC(
2-C8H8) 3.006(9)–3.023(9); 0.085/0.689, 4.6/19.32 (U2, Si6/Si8) U1dC(
8-C8H8) 2.654(9)–2.760(9); U1dC(
8)centroid 1.980(3); U2dC(
8-C8H8) 2.62(1)–2.73(1); U2dC(
8)centroid 1.946(4)

Complex number

Reference

50a-U

91

51-U

92

1

H NMR (d6-benzene, 400.2 MHz): d 0.74 (s, 72H, Si(CH3)3) 2.11 (s, C6H5(CH3)) 5.71 (br s, 16H, C8H8) 6.47 (br s, 16H, C8H8) 7.03 (m, C6H5Me) 7.14 (m, C6H5Me) ppm; 13 1 C{ H} NMR (d6-benzene, 100.62 MHz): d 4.47 (Si(CH3)3) 21.43 (s, C6H5(CH3)) 97.7 (br, C8H8) 125.70 (C6H5Me) 128.57 (C6H5Me) 129.34 (C6H5Me) 137.90 (C6H5Me) 141.03 (C4(SiMe3)4) ppm; 29 1 Si{ H} NMR (d6-benzene, 79.49 MHz): d − 22.31 (SiMe3) ppm

52-Th

93

1

53-U

94

1

Magnetometry

H NMR (d8-THF, 400 MHz, 30  C): d − 148.88 (s, 1H) −106.66 (s, 1H) −82.07 (s, 1H) −78.60 (1:1:1:1 q, 1 JB-H ¼ 66.84 Hz, 4H, BH4) −22.18 (s, 3H, Si(CH3)3) −5.94 (s, 9H, Si(CH3)3) −0.56 (s, 36H, Si(CH3)3) 1.13 (s, 9H, C(CH3)3) 3.32 (s, 3H, OCH3) 4.37 (s, 9H, Si(CH3)3) 10.01 (s, 3H, Si(CH3)3) 12.66 (s, 9H, Si(CH3)3) ppm; 29 1 Si{ H} NMR (d8-THF): d − 260.36, −175.31, −115, 56.69, 66.15 (SiMe3) ppm; 23 Na{1H} NMR (d8-THF): d 3.41 (s, Dn1/2 ¼ 96.83 Hz) ppm; 11 1 B{ H} NMR (d8-THF): d 41.63 (br s, Dn1/2 ¼ 225.4 Hz) ppm Complex was insoluble in benzene, toluene, and decomposed 5.32 mB (SQUID, rapidly in ethereal solvents, precluding characterization by 300 K); NMR 0.94 mB (SQUID, 2 K)

H NMR (d8-toluene, 400 MHz, 30  C): d − 37.51 (s, 16H, C8H8) −30.09 (br s, Dn1/2 ¼ 313.54 Hz, 4H, THF) −14.85 (s, 72H, Si(CH3)3) −7.31 (br s, Dn1/2 ¼ 67.30 Hz, 4H) ppm; 1 H NMR (d8-toluene, 400 MHz, −50  C): d − 76.99 (br s, Dn1/2 ¼ 425.15 Hz, 4H, THF) 50.96 (br s, Dn1/2 ¼ 325.43 Hz, 16H, C8H8) −27.70 (br s, Dn1/2 ¼ 623.35 Hz, 36H, Si(CH3)3) −19.54 (s, Dn1/2 ¼ 169.54 Hz, 4H, THF) −9.65 (br s, Dn1/2 ¼ 630.53 Hz, 36H, Si(CH3)3) ppm; 29 1 Si{ H} NMR (d8-toluene, 30  C): d − 204.28 (Dn1/2 ¼ 34.87 Hz) ppm; 29 1 Si{ H} NMR (d8-toluene, −50  C): d − 286.54 (Dn1/2 ¼ 615.58 Hz) 209.94 (Dn1/2 ¼ 566.16 Hz) ppm.

Buta- and Penta-Dienyl Complexes of the Actinides

Formula

+IV

UdC(
4-C4{SiMe3}4) 2.51 (3)–2.59(3); UdC(
4)centroid 2.337(2); UdC(
8-C8H8) 2.62(3)–2.74(3); U1dC(
8)centroid 1.960(5); UdO 2.53(2)

[U(
4-C4(SiMe3)4) (
8-C8H8)]

+IV

UdC(
4-C4{SiMe3}4) 2.50 (3)–2.60(3); UdC(
4)centroid 2.323(13); UdC(
8-C8H8) 2.60(3)–2.68(3); UdC(
8)centroid 1.918(18)

[U(
4-C4(SiMe3)4) (
8-C8H6(SiiPr3)2)]

+IV

UdC(
4-C4{SiMe3}4) 2.420(7)– 2.691(6); UdC(
4)centroid 2.334(3); UdC(
8-C8H8) 2.622(8)–2.755(8); UdC(
8)centroid 1.916(2)

[U(OEt)(
4-C4H(SiMe3)4) (
8-C8H8)]

+IV

UdC(
3-C4(SiMe3)3-k1-CH2SiMe2) 2.616(5)–2.754(4); UdC(
3) 2.490(3); UdC(
8-C8H8) 2.673(5)–2.736(5); UdC(
8)centroid 1.987(7); UdO 2.063(3)

1.059/0.556, 32.62/15.72 (Si1/Si3); 0.102/0.016, 6.14/4.56 (Si2/Si4)

−0.067/0.723, −3.01/22.34 (Si5/Si7); 0.125/−0.101, 5.23/−6.55 (Si6/Si8)

−0.145/0.430, −8.76/13.48 (Si7/Si9); 1.215/0.352, 40.68/11.2 (Si8/Si10); 0.247/0.453, 7.36/12.5 (C8H8, Si5/Si6) 0.048/0.085, 7.01/3.11 (Si1/Si3); 1.669/0.591, 55.01/11.95 (Si2/Si4)

1

H NMR (d8-toluene, 400 MHz, 30  C): d − 37.45 (s, 8H, C8H8) −17.14 (br s, Dn1/2 ¼ 476.55 Hz, 4H, THF) −14.01 (s, 36H, Si(CH3)3) −4.04 (s, 4H, THF) ppm; 1 H NMR (d8-toluene, 400 MHz, −50  C): d − 77.74 (br s, Dn1/2 ¼ 591.79 Hz, 4H, THF) −50.65 (s, 8H, C8H8) −19.68 (br s, Dn1/2 ¼ 367.29 Hz, 4H, THF) −9.69 (s, Dn1/2 ¼ 566.01 Hz, 36H, Si(CH3)3) ppm; 29 1 Si{ H} NMR (d8-toluene, 30  C): d − 200.22 (Dn1/2 ¼ 26.42 Hz) ppm; 29 1 Si{ H} NMR (d8-toluene, −50  C): d − 213.08 (Dn1/2 ¼ 307.42 Hz) ppm 1 H NMR (d8-toluene, 400 MHz, 30  C): d − 37.90 (s, 8H, C8H8) −18.45 (s, 36H, Si(CH3)3) ppm; 1 H NMR (d8-toluene, 400 MHz, −50  C): d − 52.22 (s, Dn1/2 ¼ 221.91 Hz, 8H, C8H8) −27.77 (s, Dn1/2 ¼ 109.85 Hz, 36H, Si(CH3)3) ppm; 29 1 Si{ H} NMR (d8-toluene, 30  C): d − 221.46 (Dn1/2 ¼ 22.29 Hz) ppm; 29 1 Si{ H} NMR (d8-toluene, −50  C): d − 285.13 (Dn1/2 ¼ 615.58 Hz) ppm 1 H NMR (d8-toluene, 400 MHz, 30  C): d − 156.23 (br s, 2H, C8H6(SiiPr3)2) −99.09 (br s, 2H, C8H6(SiiPr3)2) −9.37 (s, 36H, Si(CH3)3) −4.24 (d, 3JH-H ¼ 4.53 Hz, 18H, SiCH(CH3)2) −0.135 (d, 3JH-H ¼ 4.36 Hz, 18H, SiCH(CH3)2) 2.23 (br s, 6H, SiCH(CH3)2) 103.8 (br s, 2H, C8H6(SiiPr3)2) ppm; 29 1 Si{ H} NMR (d8-toluene, 30  C): d − 221.46, −33.80 ppm 1 H NMR (d6-benzene, 400 MHz, 30  C): d − 32.54 (s, 8H, C8H8) −21.37 (s, 18H, Si(CH3)3) −3.26 (s, 9H, Si(CH3)3) −16.87 (s, 9H, Si(CH3)3) 25.97 (s, 3H, OCH2CH3) 45.52 (s, 1H, C4H(SiMe3)4) 94.77 (s, 2H, OCH2CH3) ppm; 29 1 Si{ H} NMR (d6-benzene, 30  C): d − 255.64, −64.11, 37.87 ppm

54-U

94

55-U

94

56-U

94

57-U

94

a Displacements displayed for opposing Si atoms in the Cb ring (i.e., Si1/Si3 and Si2/Si4). Distances are shown first, followed by the angles. Note: The displacement distances and angles are calculated differently, the distance is the perpendicular distance from the Si atom of the TMS group to the extension of the plane defined by the C4 ring. While the displacement angle is the angle subtended by the centroid of the C4 ring to the carbon atom of the ring and the Si atom of the TMS group bound to that respective C4 ring carbon atom.

Buta- and Penta-Dienyl Complexes of the Actinides

[U(THF)(
4-C4(SiMe3)4) (
8-C8H8)]

77

78

4.02.6

Buta- and Penta-Dienyl Complexes of the Actinides

Conclusion

The difficulty of the work and the very reactive nature of butyl and neopentyl ligands are reflected in the single homoleptic complex and the small number of heteroleptic hydrocarbyl complexes of the actinides that are known (Sections 4.02.2.1 and 4.02.2.2). Chelating ligands with strong nitrogen donors have proved excellent supporting ligand environments for the stabilization of butyl and neopentyl complexes. However, it is of note, that it is not yet possible to predict the relative stability/reactivity of an actinide butyl or neopentyl complex in any ancillary ligand environment, with the exception of the cyclopentadienyl ligand. The stabilization provided by tris- and bis-cyclopentadienyl ligand environments, respectively, were key to the successful synthesis and characterization of butyl and penta-dienyl hydrocarbyl complexes (Sections 4.02.2.3 and 4.02.2.4). These complexes provided the first real insight into the nature and chemistry of the actinide-carbon s-bond, and in the case of the actinide metallocenes the comparative study of d-block and f-block analogues. Acyclic, 2-butene-1,4-diyl actinide complexes of the actinides were synthesized by several routes in bis-cyclopentadienyl ligand environments (Section 4.02.3.1). The (
4-CH2CR]CRCH2)2− ligands were bound side-on (s2,p) and computational analysis showed polarized AndC s-bonds but with a small p–component to bonding. The structural data were also consistent with delocalization of the double bond over three carbon atoms. The 2-butene-1,4-diyl ligand underwent facile alcoholysis and hydrogenolysis, and was reported to react violently with organic carbonyls. The partial allylic character of 2-butene-1,4-diyl was also demonstrated by two reactivity examples of substrate insertion with CdC coupling. The uranium(III) mediated CdC coupling of terminal alkynes resulted in the formation of acyclic butadienyl dinuclear complexes of uranium(IV) in a tris(2-oxy-3-adamantyl-5-methylbenzyl)amine ligand environment (Section 4.02.3.2). The 1,3-butadiene-1,4-diyl ligands bridged the uranium metal centers, with uranium-ligand s-bonding only. The calculated mechanism of formation was bimolecular. Actinacyclopentadiene complexes have been synthesized in a bis-cyclopentadienyl ligand environment by a wide variety of synthetic routes (Section 4.02.4.1). Localization of single and double bonds were observed within the fiive-membered planar metallacycle with
2-binding to the actinide metal center. The AndC s-bonds were predominantly (80–90%) composed of carbon sp2 hybrid but there was also a contribution from actinide hybrid orbitals (17–10%). This % of actinide character increased by 7% when the actinacyclopentadiene was a component part of a 2-metallabiphenylene ligand. The stability of the metalacyclic arrangement was demonstrated by the fact that actinacyclopentadiene complexes were largely unreactive. However, they did display a modest range of insertion chemistry and one recent example of CdH activation. Actinacycloclopentatriene complexes, also stabilized in a bis-cyclopentadienyl ligand environment, have also been made by a wide variety of synthetic routes (Section 4.02.4.2). The actinide metal centers adopt an
4-binding mode and there is delocalization of the C]C double bonds, with significant bending and ring-strain within the cumulene ligand. The AndC s-bonds were predominantly carbon-based (80–85%), with contribution from actinide hybrid orbitals (15–20%). This % actinide character is higher for uranium than thorium, and higher than that found in the actinacyclopentadienes. Additionally, there were carbon-based (90–100%) orbitals, which donated electron density from the cumulene p-system to the actinide metal centers. In contrast to actinacyclopentadienes, actinacyclocumulene complexes react with a wide variety of heterounsaturated molecules, by single or double insertion of the organic molecule into the AndC bond(s), with release of cumulene ring-strain to yield heterometallacyclic complexes. The first example of an actinacyclopentyne was reported very recently (Section 4.02.4.3) from the reaction of a dialkyne with a mononuclear thorium dihydride, stabilized by a very bulky bis-cyclopentadienyl ligand environment. The structure of the actinacyclopentyne confirmed the planar
4-binding and localization of the CdC and C^C bonds. Reductive [2 + 2] cycloaddition of PhC^CPh to an inverse-sandwich uranium(V) arene complex resulted yielded the only known example of an inverse-sandwich cyclobutadienyl complex (Section 4.02.5). Terminal cyclobutadienyl actinide complexes have been synthesized using metathesis chemistry with supporting borohydride, iodide/oxo and cyclooctatetraenyl ligand environments (Section 4.02.5). Starting material is a key consideration here, with borohydrides of uranium(III) and (IV) and thoracene, preferred for uranium and thorium, respectively. The planar aromatic cyclobutadienyl dianion binds
4 to the actinide metal centers in all cases, with an additional close contact to a substituent carbon per uranium metal center in the case of the inverse-sandwich complex. Another structural feature of the SiMe3-substituted cyclobutadienyl ligand was the SiMe3-displacement out of the plane of the C4-ring and away from the actinide metal center. In the only thorium cyclobutadienyl complex isolated to date, there was a very unusual
2-bound cyclooctatetraenyl ligand. The bonding interactions between 5f/6d actinide orbitals and the cyclobutadienyl ligand were, d-bonding with the empty c4 orbital and p-bonding with the c2 and c3 orbitals. The d-bonding combination was only found in the inverse-sandwich cyclobutadienyl complex, but the p-bonding combinations are common to all cyclobutadienyl actinide complexes. The analysis of the amount of p-bonding and the relative 5f/6d contribution to bonding in terminal cyclobutadienyl borohydride complexes, did not agree between computational studies on complexes that differ only in their counter cations. Either there was 15–20% uranium contribution with 5f and 6d contributions of similar magnitude, or there was 30% uranium character, of which the 5f contribution was double that of the 6d. In the cyclobutadienyl-cyclooctatetraenyl complex of thorium the contribution to either p- or d-bonding is similar. However, there was more 5f character to Th-cyclobutadienyl bonding and this 5f contribution was unusually high. In cyclobutadienyl-cyclooctatetraenyl complexes of uranium it was found that mixing between the 6d and the cyclobutadienyl ligand orbitals was more significant than mixing with the 5f, whereas the opposite was found for the cyclooctatetraenyl ligands (5f > 6d). These results indicated that there is significant flexibility inherent to the bonding between actinide metal centers and the cyclobutadienyl ligand. The reactivity of cyclobutadienyl complexes has not yet been reported, with the exception of two examples

Buta- and Penta-Dienyl Complexes of the Actinides

79

of CdH activation with protonation of the cyclobutadienyl ligand and one example of diethyl ether cleavage. This is the first example of ether cleavage by U(IV) with no simultaneous change in oxidation state, indicative of metal-ligand co-operativity in this reactivity. The combination of better accessibility to a wider range of pure actinide starting materials, and technical advances in equipment and instrumentation, have contributed significantly to the rapid expansion of organoactinide chemistry in recent years. The butaand penta-dienyl complexes of the actinides described and discussed in this chapter demonstrate this same phenomenon. These ligands have been instrumental both in the development of the field of organometallic actinide chemistry, and in pushing the boundaries of knowledge forward (Fig. 10). This chapter encompasses the time period from 1972 until the present day. However, as can be seen in Fig. 10, many of the ligand types in this chapter have been realized on actinides only within the last decade. These include the first and only examples of homoleptic hydrocarbyl, butadienyl, and actinacyclopentyne complexes. Significant new

Fig. 10 Timeline of selected buta- and penta-dienyl actinide complexes.

80

Buta- and Penta-Dienyl Complexes of the Actinides

classes of ligands have also been discovered in the last decade, the two primary examples being cyclobutadienyl and actiniacyclopentatriene complexes. It is also of note that established classes of ligands are still capable of new chemistry, for example the recent synthesis of the first actinide 2-metallabiphenylene complex. The outlook for buta- and penta-dienyl complexes of the actinides is exciting. There is significant opportunity to expand the chemistry of unsaturated actinacyclic ligands in non-cyclopentadienyl ancillary ligand environments. The further advances in reactivity are also to be expected, which exploit the allylic character of recent examples of the 2-butene-1,4-diyl ligand, the very reactive actiniacyclopentatriene and actinacyclopentyne moieties. Significant expansion of the chemistry of the cyclobutadienyl ligand is predicted. Advances will likely include new synthetic routes to aid the exploration of a range of ancillary ligand environments. Cyclobutadienyl-only complexes will be targeted, for example the parallel metallocene. Actinide-ligand bonding interactions in complexes of the cyclobutadienyl ligand, have already been shown be fundamentally different to both transition metal cyclobutadienyl complexes and other aromatic ligand systems bound to actinides. Therefore, cyclobutadienyl complexes have the potential to be very important to our understanding of organoactinide bonding. The small molecule reactivity of the mixed-sandwich cyclobutadienyl-cyclooctatetraenyl uranium complexes, likewise represents a compelling opportunity for new chemistry.

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Buta- and Penta-Dienyl Complexes of the Actinides

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. 88. 89. 90. 91. 92. 93. 94.

81

Bruno, J. W.; Smith, G. M.; Marks, T. J.; Fair, C. K.; Schultz, A. J.; Williams, J. M. J. Am. Chem. Soc. 1986, 108, 40–56. Waterman, R. Organometallics 2013, 32, 7249–7263. Bruno, J. W.; Marks, T. J.; Day, V. W. J. Am. Chem. Soc. 1982, 104, 7357–7360. Bruno, J. W.; Marks, T. J.; Morss, L. R. J. Am. Chem. Soc. 1983, 105, 6824–6832. Bruno, J. W.; Stecher, H. A.; Morss, L. R.; Sonnenberger, D. C.; Marks, T. J. J. Am. Chem. Soc. 1986, 108, 7275–7280. Schock, L. E.; Marks, T. J. J. Am. Chem. Soc. 1988, 110, 7701–7715. Sonnenberger, D. C.; Morss, L. R.; Marks, T. J. Organometallics 1985, 4, 352–355. Andrea, T.; Eisen, M. S. Chem. Soc. Rev. 2008, 37, 550–567. Barnea, E.; Eisen, M. Coord. Chem. Rev. 2006, 250, 855–899. Fox, A. R.; Bart, S. C.; Meyer, K.; Cummins, C. C. Nature 2008, 455, 341–349. Burns, C. J.; Eisen, M. S. Homogeneous and Heterogeneous Catalytic Processes Promoted by Organoactinides; Springer Netherlands, 2010; pp 2911–3012. Dash, A. K.; Gourevich, I.; Wang, J. Q.; Wang, J.; Kapon, M.; Eisen, M. S. Organometallics 2001, 20, 5084–5104. Smith, G. M.; Suzuki, H.; Sonnenberger, D. C.; Day, V. W.; Marks, T. J. Organometallics 1986, 5, 549–561. Erker, G.; Muehlenbernd, T.; Benn, R.; Rufinska, A. Organometallics 1986, 5, 402–404. Yasuda, H.; Tatsumi, K.; Nakamura, A. Acc. Chem. Res. 1985, 18, 120–126. Tatsumi, K.; Nakamura, A. J. Am. Chem. Soc. 1987, 109, 3195–3206. Zhang, L.; Hou, G.; Zi, G.; Ding, W.; Walter, M. D. J. Am. Chem. Soc. 2016, 138, 5130–5142. Qin, G.; Wang, Y.; Shi, X.; Del Rosal, I.; Maron, L.; Cheng, J. Chem. Commun. 2019, 55, 8560–8563. Kosog, B.; Kefalidis, C. E.; Heinemann, F. W.; Maron, L.; Meyer, K. J. Am. Chem. Soc. 2012, 134, 12792–12797. Manriquez, J. M.; Fagan, P. J.; Marks, T. J. J. Am. Chem. Soc. 1978, 100, 3939–3941. Fagan, P. J.; Manriquez, J. M.; Marks, T. J.; Day, C. S.; Vollmer, S. H.; Day, V. W. Organometallics 1982, 1, 170–180. Evans, W. J.; Kozimor, S. A.; Ziller, J. W. Chem. Commun. 2005, 4681–4683. Pagano, J. K.; Dorhout, J. M.; Czerwinski, K. R.; Morris, D. E.; Scott, B. L.; Waterman, R.; Kiplinger, J. L. Organometallics 2016, 35, 617–620. Fang, B.; Hou, G.; Zi, G.; Fang, D.-C.; Walter, M. D. Dalton Trans. 2015, 44, 7927–7934. Fang, B.; Hou, G.; Zi, G.; Ding, W.; Walter, M. D. Organometallics 2016, 35, 1384–1391. Zhang, L.; Fang, B.; Hou, G.; Ai, L.; Ding, W.; Walter, M. D.; Zi, G. Dalton Trans. 2016, 45, 16441–16452. Pagano, J. K.; Xie, J.; Erickson, K. A.; Cope, S. K.; Scott, B. L.; Wu, R.; Waterman, R.; Morris, D. E.; Yang, P.; Gagliardi, L.; Kiplinger, J. L. Nature 2020, 578, 563–567. Fang, B.; Zhang, L.; Hou, G.; Zi, G.; Fang, D.-C.; Walter, M. D. Organometallics 2015, 34, 5669–5681. Pagano, J. K.; Erickson, K. A.; Scott, B. L.; Morris, D. E.; Waterman, R.; Kiplinger, J. L. J. Organomet. Chem. 2017, 829, 79–84. Zhang, L.; Fang, B.; Hou, G.; Zi, G.; Ding, W.; Walter, M. D. Organometallics 2017, 36, 898–910. Yang, P.; Zhou, E.; Fang, B.; Hou, G.; Zi, G.; Walter, M. D. Organometallics 2016, 35, 2129–2139. Zhang, L.; Hou, G.; Zi, G.; Ding, W.; Walter, M. D. Dalton Trans. 2017, 46, 3716–3728. Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A. G.; Taylor, R. J. Chem. Soc. Perkin Trans. 1987, 2, S1. Kelley, M. P.; Popov, I. A.; Jung, J.; Batista, E. R.; Yang, P. Nat. Commun. 2020, 11, 1558. Suzuki, N.; Nishiura, M.; Wakatsuki, Y. Science 2002, 295, 660. Hashizume, D.; Suzuki, N.; Chihara, T. Chem. Commun. 2006, 1233–1235. Boronski, J. T.; Liddle, S. T. Eur. J. Inorg. Chem. 2020, 2020, 2851–2861. Patel, D.; McMaster, J.; Lewis, W.; Blake, A. J.; Liddle, S. T. Nat. Commun. 2013, 4, 2323. Boronski, J. T.; Doyle, L. R.; Seed, J. A.; Wooles, A. J.; Liddle, S. T. Angew. Chem. Int. Ed. 2020, 59, 295–299. Tsoureas, N.; Mansikkamäki, A.; Layfield, R. A. Chem. Commun. 2020, 56, 944–947. Boronski, J. T.; Doyle, L. R.; Wooles, A. J.; Seed, J. A.; Liddle, S. T. Organometallics 2020, 39, 1824–1831. Boronski, J. T.; Wooles, A. J.; Liddle, S. T. Chem. Sci. 2020, 11, 6789–6794. Tsoureas, N.; Mansikkamäki, A.; Layfield, R. A. Chem. Sci. 2021, 12, 2948–2954.

4.03

Buta- and Penta-Dienyl Complexes of the Group 4 Metals

Dennis M Seth, Evan A Beretta, and Rory Waterman, Department of Chemistry, University of Vermont, Burlington, VT, United States © 2022 Elsevier Ltd. All rights reserved.

4.03.1 Introduction 4.03.2 Butadienyl 4.03.2.1 Synthesis of “Cp2M(II)” and derivatives 4.03.2.2 Non-cyclopentadienyl supporting ligands 4.03.2.3 Insertion reactivity of alkynes to form metallacyclopentadienes 4.03.2.4 Applications of metallacyclopentadienes to form unsaturated rings: Monometallic systems 4.03.2.5 Applications of metallacyclopentadienes to form unsaturated rings: Multimetallic systems 4.03.3 Pentadienyl 4.03.3.1 Introduction 4.03.3.2 Open pentadienyl 4.03.3.3 Dimethylcyclohexadienyl 4.03.3.4 Heteroatomics and clusters 4.03.4 Conclusion Acknowledgments References

82 83 83 84 85 88 88 89 89 89 92 94 95 96 96

Abbreviations Ad Bn CBC Cp Cp Dipp dmch dmpe Et i Pr Me Me PMPMe Mes n Bu Pdl py TBDMS Tbt t Bu thf tmeda

4.03.1

Tricyclo[3.3.1.13,7]decane Benzyl 1-phenylmethyl 1,4,8,11-Tetraazabicyclo[6.6.2]hexadecane Cyclopentadienyl 1,2,3,4,5-Pentamethylcyclopentadienyl 2,6-Diisopropylphenyl 6,6-Dimethylcyclohexadienyl 1,2-Bis(dimethylphosphino)ethane Ethyl Isopropyl Methyl 3,5-Dimethyl-2-(2-pyridyl)pyrrolide 2,4,6-Trimethylphenyl n-Butyl (1R)-6,6-Dimethylbicyclo[3.1.1]hept-2-en-2-isopropene Pyridine tert-butyl dimethylsilyl 2,4,6-Tris[bis(trimethylsilyl)methyl]phenyl tert-butyl Tetrahydrofuran N,N,N0 ,N0 -Tetramethylethylenediamine

Introduction

Buta- and pentadienyl complexes of titanium, zirconium, and hafnium have been the continued subject of interest for the synthesis of unsaturated ring species in the case of butadienyls,1–3 and as an open supporting ligand in the case of pentadienyls. This chapter describes butadiene and pentadiene ligands’ application to group 4 metals, with a focus on structure and synthesis. A brief mention on their applications to synthesis of unsaturated rings will also be included. Open pentadienyls have been seen as potentially being a more reactive congener to the exceedingly well-studied cyclopentadienyl. The last 20 years have shown significant advancement in the synthesis of open pentadienyl species and their derivatives, which has facilitated studies on their reactivity compared to the cyclopentadienyl ligand. Bonding in these systems is variable. In many instances, the metal completes exhibit a five- or six-membered ring, which can be planar with the participation of the anionic charge in a delocalized p-system. For open dienyl ligands, the metal may support Z3- or

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83

Z5-coordination of the ligand, and such binding modes are driven by the thermodynamic stability of ligand-to-metal donation. Dynamic behavior is evident in several studies and can be assumed in many cases. In all studies surveyed here, no exceptions to Green-Davies-Mingos rules were noted, which would support the notion that, regardless of apparent complexity, textbook descriptions of bonding are an appropriate entry to understanding the nature of the bonding of these systems in all cases. The richest area for study in these systems has remained the preparation of metallacyclopenadiene complexes, which are excellent precursors to various rings, including value-added heterocycles, and materials. In the previous century, these complexes had a greater relationship to various aspects of alkene polymerization reactions. The decline in active study of alkene polymerization catalysts of group 4 metals has shifted the focus of study on these systems to other areas of fundamental and applied interest. Overall, titanium and zirconium have had a wide breadth of study in this small segment of organometallic chemistry, with titanium having recently been expanded to practical syntheses to cover for zirconium, while the investigation of analogous hafnium reactions has lagged behind.

4.03.2

Butadienyl

4.03.2.1

Synthesis of “Cp2M(II)” and derivatives

The synthesis of metallacyclopentadienes can generally occur through the reaction of a low valent “M(II)” precursor with alkynes. Historically, there have been two major synthetic methods to attain these low valent metal precursors, the Rosenthal reagent [M(Cp)2{Z2-C2(SiMe3)2}] (1-M, M ¼ Ti, Zr) and the Negishi reagent [Zr(Cp)2(Z2-CH2]CHCH2CH3)] (2).4,5 The use of either of these complexes have allowed for researchers to synthesize and formulate CdC coupling reactions involving unsaturated rings from alkynes. This chapter will focus on work from 2005 and onwards, with select references from before to give better context. General work on group 4 diene compounds before 2005 can be found in these selected reviews and references therein.6–10 Rosenthal’s reagent is generally synthesized through the magnesium reduction of zirconocene or titanocene dichloride in the presence of bis(trimethylsilyl)acetylene,11,12 as shown in Scheme 1. The complexes are isolable and remarkably stable, able to be isolated as thermally stable crystalline solids. The combination of the ease of synthesis, the relatively cheap starting materials, and the thermal stability of the complexes have led to Rosenthal’s reagent being used extensively as not only a stable M(II) synthon, but also in the synthesis and reactivity of butadienyl complexes of titanium and zirconium through alkyne insertion, as will be seen later in this chapter.

Scheme 1 Synthesis of Rosenthal’s reagents 1-Ti and 1-Zr.

Another reagent that will emerge as a precursor to the synthesis of butadienyl complexes is the reagent developed by Negishi.5 As shown in Scheme 2, the complex was generated through the reaction of alkyl lithiums or alkyl Grignards with zirconocene dichloride. These complexes would then undergo b-elimination to afford the low valent metal complex, which would then be able to couple alkynes. As will be seen, this synthetic method is generally employed in situ.

Scheme 2 Synthesis of Negishi’s reagent.

84

Buta- and Penta-Dienyl Complexes of the Group 4 Metals

Between these two reactions, the Staubitz group has been able to make direct comparisons between 1-Zr and 2.13 Through their research in synthesizing several metallacyclopentadienes (which will be discussed further later), they determined that Rosenthal’s reagent tends to produce metallocyclopentadienes in higher yields and faster reaction times in most circumstances, as well as being more functional group tolerant. Negishi’s reagent has the benefits of being slightly cheaper to synthesize, simpler to use by nature of it being an in situ-generated complex, and being more reactive. A third, practical in situ synthesis of this M(II) fragment in titanium chemistry comes from the b-elimination of an alkyl species generated by titanium isopropoxide and alkyl Grignards, as shown in Scheme 3 for the generation of [(OiPr)2Ti{Z2-C2(SiMe3)2}] (3).14 A review on the general uses of this complex to form different value-added products can be found here.15 Other uses for these low valent reagents can be found here.16,17

Scheme 3 Practical synthesis of a low-valent titanium fragment.

Fig. 1 Crystal structure of a zirconacyclopentadienyl complex with a cross-bridged cyclam (4) (ORTEP3, 50% probability thermal ellipsoids). Reprinted with permission from O’Connor, P.; Berg, D. J.; Twamley, B. Organometallics 2005, 24(1), 28–36. Copyright 2005 American Chemical Society.

4.03.2.2

Non-cyclopentadienyl supporting ligands

While cyclopentadienyl ancillary ligands dominate the vast majority of butadienyl complexes, a handful of butadienyl complexes of group 4 metals with other supporting ligands have been isolated. One noteworthy complex is the synthesis of a cyclam derived zirconacyclopentadiene [Zr(CBC){k2-C4(Ph4)}] (4).18 A thermal ellipsoid plot from single crystal XRD data of 4 is shown in Fig. 1. This complex was synthesized through the reduction of a cyclam zirconium dichloride precursor in a Negishi-style reaction in the presence of diphenylacetylene. Overall, these cyclam complexes appear to have fairly similar structure and reactivity to the bis-cyclopentadienyl complexes. A direct comparison between bond lengths of compound 4 and the bis-cyclopentadienyl relative [Zr(Cp)2{k2-C4(Ph4)}] show a difference in ZrdC bond lengths of 0.063 Å, giving 4 a slightly shorter bond length. Likewise, a zirconacyclopentadienyl complex with 2-pyridylpyridole supporting ligands, [Zr(MePMPMe)2{k2-C4(Ph4)}] (5), has been synthesized and isolated, as shown in Scheme 4.19 This has been synthesized through the reduction of the bis-3,5-dimethyl-2-

Scheme 4 Synthesis of a zirconacyclopentadiene with non-Cp supporting ligands.

Buta- and Penta-Dienyl Complexes of the Group 4 Metals

Table 1

85

Relevant data on complexes 4 and 5. MdC bond distances (Å)

13

Refs.

[Zr(CBC){k -C4Ph4}]

2.313(2)

18

[Zr(MePMPMe)2{k2-C4(Ph4)}]

2.277(2)

203.1 (C9) 150.0 (C16) N/A

Molecular formula 2

C shifts (ppm)

87

(2-pyridyl)-pyrrolide zirconium dibromide with potassium graphite in the presence of diphenylacetylene. While enough of the complex was isolated in order to gather a crystal structure and 1H NMR spectral data, further characterization was not performed due to insufficient product yield (Table 1).

4.03.2.3

Insertion reactivity of alkynes to form metallacyclopentadienes

The alkyne dimerization required to synthesize these metallacyclopentadienes is reversible, with the rate of cycloaddition and retro-cycloaddition becoming appreciable at elevated temperatures. The kinetics of the reversibility of alkyne coupling to form zirconacyclopentadienes has been studied as shown in complexes [Zr(Cp)(Cp ){k2-C4(1,4-Ph2-2,3-Mes2)}] (6a) and [Zr(Cp)(Cp ) {k2-C4(1,2,3-Ph3-4-Mes)}] (6b) in Scheme 5,20 which allowed for the better probing of effects on regioselectivity of alkyne coupling. One significant observation was the comparison between the ancillary ligand effects, comparing a rather open ansazirconocene derivative [Zr(Me2C(C5H4))] to a more closed zirconocene [Zr(Cp)(Cp )] fragment, and the regular [Zr(Cp)2] fragment. The authors compared not only the substitution of one alkyne for another, but also the competitive deinsertion of an alkyne for a stronger donor, mainly trimethylphosphine. From this work, the authors concluded that steric effects make up a majority of the rate of alkyne insertion, in the way that complexes with sterically crowded ancillary ligands (i.e. ansa metallocenes vs mixed Cp/Cp∗ complexes) eliminate alkyne more slowly.

Scheme 5 Demonstration of the reversibility of alkyne coupling on low-valent zirconium fragments.

Further research regarding the isolation of mesityl-substituted alkynes also gives some information on how the amount of steric bulk on the alkyne affects coupling.21 In the study by Miller et al., several new mesityl substituted alkynes were synthesized by a reaction of the alkyne with previously synthesized (1-Zr) in order to synthesize the compounds (9) through (19). They were able to observe reversibility of fragment binding, in order to form less sterically hindered zirconacyclopentadienes. In those reactions, the steric bulk was disfavored from the alpha position with respect to the metal. This isomerization was observed more significantly in the aryl species than alkyl. It was also noted that the mesityl substituent has kinetic preference to couple into the position more distant from zirconium. They note that this regioselectivity is opposite to what is observed with the coupling of tert-butyl substituted alkynes, where the bulky substituent preferentially coupled into the position nearest zirconium (Table 2). Other studies show that the use of a donor atom into the alkyne precursor moiety strongly affects the regioselectivity of subsequent insertions.22 Specifically, the addition of that donor atom causes a polarization of the metallacyclopropene, which causes the subsequent insertion of another alkyne to prefer the electron deficient side, i.e. the side with the donor atom. This leads to the synthesis of asymmetric complexes shown in Table 3.

86

Buta- and Penta-Dienyl Complexes of the Group 4 Metals

Table 2

Selected data on various substitutions on zirconacyclopentadienes. MdC bond distances (Å)

13

[Zr(Cp)(Cp ){k2-C4(2,3,4,5-Ph4)}]

2.254(3) 2.267(3) 2.221(4) 2.243(4) N/A

[Zr(Me2C(Z5-C5H4)2){k2-C4 [2,3,4,5-Ph4]}]

N/A

[Zr(Cp)2{k2-C4(2,5-Ph2-3,4-Mes2)}] [Zr(Cp)2{k2-C4(2,5-Ph2-3,4-o-Xyl2)}]

2.267(4) 2.269(4) N/A

[Zr(Cp)2{k2-C4(2,5-(4-BuPh)2-3,4-Mes2)}]

N/A

[Zr(Cp)2{k2-C4(2,5-Me2-3,4-Mes2)}]

N/A

[Zr(Cp)2{k2-C4(2,5-Pr2-3,4-Mes2)}]

2.254(5)

[Zr(Cp)2{k2-C4(2,4-Mes2-3,5-Pr2)}]

2.232(5) (CdPr) 2.269(5) (CdMes)

[Zr(Cp)2{k2-C4(2-Ph-3-Mes-4,5-Et2)}]

N/A

[Zr(Cp)2{k2-C4(2,4,5-Ph3-3-Mes)}]

N/A

[Zr(Cp)2{k2-C4(2-Pr-3-Mes-4,5-Ph2)}]

N/A

207.2 (ZrdC) 152.7 198.3 145.2 197.5 148.2 194.3 149.3 196.3 146.1 196.5 146.0 196.6 146.4 187.2 142.7 193.3 139.3 195.0 (ZrdCdPr) 189.1 (ZrdCdMes) 139.6 (CdPr) 136.9 (CdMes) 195.9 (ZrdCdEt) 188.6 (ZrdCdPh) 138.9 (CdEt) 138.7 (CdMes) 195.6 (ZrdC(Ph)dC(Ph)) 193.5 (ZrdC(Ph)dC(Mes)) 144.1 (ZrdC(Ph)dC(Ph)) 142.2 (ZrdC(Ph)dC(Mes)) 195.6 (ZrdC(Pr)) 191.8 (ZrdC(Ph)) 140.6 (C(Ph)) 138.7 (C(Mes))

Molecular formula 2

[Zr(Cp)(Cp ){k -C4 (2,5-(Me3Si)2-3,4-Ph2)}] [Zr(Me2C(Z5-C5H4)2) {k2-C4(2,5-(Me3Si)2-3,4-Ph2)}]

Table 3

C shifts (ppm)

Compound number

Refs.

7a

20

7b

20

8a

20

8b

20

9

21

10

21

11

21

12

21

13

21

14

21

15

21

16

21

17

21

Structural parameters of asymmetric and bulky metallacylopentadienes. MdC bond distances (Å)

13

Compound number

Refs.

[Ti(Cp)2{k -C4(2,4-(OEt)2-3,5-(SiMe3)2)}]

2.1581 (14) (TidC(OEt)) 2.1390 (14) (TidC(SiMe3))

18

22

[Ti(Cp)2{k2-C4 (2,3,4,5-(pip)4)}]

2.1354 (16) 2.1340 (18) 2.251(5) 2.252(4)

227.7 (TidC(OEt)) 180.7 (TidC(SiMe3)) 145.1 111.6 210.63 (TidC) 85.23 (TidCdC) 203.2 (ZrdC) 93.0 (ZrdCdC)

19-Ti

23

19-Zr

23

Molecular formula 2

[Zr(Cp)2{k2-C4 (2,3,4,5-(pip)4)}]

C shifts (ppm)

The regioselectivity of homocoupled stannyl-substituted alkynes point to the ability of the pendant tin group to interact with the metal center to affect the specific conformation of the metallacyclopentadienyl product, as shown in Scheme 6.24 In studies with zirconocene fragments, tin substituents were localized in the b-position with respect to the metal, and for titanocene, these substituents occupied both the a- and b-positions (Table 4).

Buta- and Penta-Dienyl Complexes of the Group 4 Metals

87

Scheme 6 Metal effects on regioselectivity of stannyl alkyne coupling.

Table 4

Various Sn substituted metallacyclopentadienes.

Molecular formula

13

Compound number

Refs.

[Ti(Cp)2{k2-C4(2,4-(SnMe3)2-3,5-(nBu)2)}]

225.1 214.9 141.8 134.8 112.6 209.0 154.2 111.5 224.8 215.0 140.7 133.0 112.5 208.7 130.2 152.1 111.5 224.2 142.8 113.3 215.9 194.7 140.6 129.0 109.3

20-Ti

24

20-Zr

24

21-Ti

24

21-Zr

24

22-Ti

24

22-Zr

24

[Zr(Cp)2{k2-C4(2,5-(SnMe3)2-3,4-(nBu)2)}]

[Ti(Cp)2{k2-C4(2,4-(SnBu3)2-3,5-(nBu)2)}]

[Zr(Cp)2{k2-C4(2,5-(SnBu3)2-3,4-(nBu)2)}]

[Ti(Cp)2{k2-C4(2,5-(SnMe3)2-3,4-(C4H8)2)}]

[Zr(Cp)2{k2-C4(2,4-(SnMe3)-3,5-(nBu)2)}]

C shifts (ppm) (CDCl3)

Several variations on this butadienyl moiety have been synthesized; the bis-ferrocenyl derivative allowed for a redox active complex which provided evidence to show linearization of the species under higher oxidation states of titanium.25 Tetraferrocenyl zirconacyclopentadienes have been studied for redox behavior, with no reversible oxidation processes being observed, instead giving decomposition under the investigated conditions.26 In the cyclodimerization of (2-pyridyl)alkynes at zirconium, a cyclopentadiene complex with significant p-donation from a pendant pyridinyl substituent, was isolated.27 While a majority of metallacyclopentadienes are planar or nearly planar, the titanium and zirconium cyclopentadiene compounds formed from coupling of bis(piperidinyl)acetylenes, [M(Cp)2{k2-C4(pip)4}] (19-M, M ¼ Ti, Zr), displayed significant twisted conformations, which exhibited dynamic behavior in solution.23 Another example of a complex that can form a metallacyclopentadiene are fulvalene supported low-valent zirconocene complexes.28 Notably, a bis(p-tolyl)fulvaleno zirconocene “[Zr(Cp)2]” was able to undergo insertion by two equivalents of alkyne, eliminating the fulvalene and affording either tetraphenyl (23) or tetraethyl (24) zirconacyclopentadiene, as shown in Scheme 7. Although the experiments were done at NMR tube scale, and thus the product complexes were not isolated, it was reported that the metallacyclopentadiene complexes had 1H NMR resonances in agreement with the literature.

88

Buta- and Penta-Dienyl Complexes of the Group 4 Metals

Scheme 7 Synthesis of metallacylopentadienes from fulvalene-supported Zr.

4.03.2.4

Applications of metallacyclopentadienes to form unsaturated rings: Monometallic systems

One factor that sets the chemistry of the metallacyclopentadienes of titanium or zirconium apart is the ability to form a rich suite of unsaturated ring systems through insertion reactions. Due to the breadth of this area of chemistry, and the fact that this reactivity tends to focus more on the unsaturated products formed, rather than the specific organometallic complexes required, a brief summary is included below. A review on the synthetic applications for the synthesis of pyridines by [2 + 2 + 2] cycloaddition can be found here.29 One major application of the butadienyl complexes of titanium and zirconium is insertion to form heteroles30–33 and metaloles,34,35 heterocycles,36–38 indenyl derivatives39–43 and cyclopentadiene derivatives,44 as well as cycloaddition reactions to form benzene, and cyclopentadiene derivatives,44 and cycloaddition reactions to form benzene,45,46 naphthalene,39–42,47,48 cyclobutenes,49 and macrocycles.31,32,50 Generalized examples of this chemistry is shown in Scheme 4. Substituted cyclopentadienyl ligands and indenyl derivatives have also been shown to be able to undergo butadienyl insertion into the ancillary ligands in order to form substituted unsaturated ring systems.43 Notably, bis(2-pyridyl)pyridole zirconacyclopentadienes have been shown to photochemically activate diphenylacetylene to form [Zr(MePMPMe)2{Z4-C4(Ph4)}].19 This is remarkable, as it is the third example of such a cyclobutadiene being complexed to a group 4 metal, and the first on zirconium. The other two cyclobutadienyl complexes are obtained from the reaction of TiCl3
3THF with isopropylmagnesium chloride and diphenylacetylene in THF in the case of [TiCl3{Z4-C4(Ph4)}
Mg2Cl3(THF)6],51 and the alkyne coupling of the titanium complex [Ti(Z67-toluene){(m-X)2(AlX2)}2] to form [Ti{Z4-C4(C6H5)4}{(m-Br)2(AlBr2)}2].52

4.03.2.5

Applications of metallacyclopentadienes to form unsaturated rings: Multimetallic systems

Often, insertion chemistry can be facilitated by transmetalation onto elements such as copper. As such, several multimetallic systems have been developed to further the reactivity of zirconacyclopentadienes. Cupration of zirconacyclopentadienes allows for enhanced reactivity with oxalyl chloride to form cyclopentadienones, in reactivity analogous to that in Scheme 8.53 Cupration or

Scheme 8 Insertion Chemistry of Metallacyclopentadienes.

Buta- and Penta-Dienyl Complexes of the Group 4 Metals

89

Scheme 9 One-pot synthesis of substituted benzenes by in situ haloalkane reduction and coupling.

nickelation of zirconacyclopentadienes can also allow for the one-pot formation of biaryls through ortho-arylpropynoate, with Dewar benzenes as minor side products in the cupration.54 1,2,3,4-tetrasubstituted cyclopentadienes can be similarly synthesized through alkyne coupling followed by treatment with diiodomethane.55 The zirconacyclopentadiene/copper(I) chloride systems can also be used for the synthesis of 1,2,4,5-tetrasubstituted benzenes56 as well as pentacenes.57 The use of lanthanum as a reductant to form Cp2Zr(II) has allowed for catalytic alkyne dimerization and cyclotrimerization,58 as well as the cross coupling of alkynes and vinylsilanes.59 This reduction pathway can be combined with the reduction of haloalkanes quite cleverly to form zirconacyclopentadienes in situ, which can serve as a one-pot method to synthesize unsaturated compounds, as shown in Scheme 9.60

4.03.3

Pentadienyl

4.03.3.1

Introduction

While relevant complexes and preparations from before the timeframe of this chapter are included, a more in-depth review of group 4 open pentadienyls can be found here.61 Specifically closed metallocenes are not covered in this chapter, though some choice reviews are available.62

4.03.3.2

Open pentadienyl

Open pentadienyl complexes of the group 4 elements have enjoyed being realized in a range of isolated complexes and synthetic applications. While several papers in regards to group 4 open pentadienyls are before the timeframe of this chapter, one key example of the synthesis of these complexes comes from Ernst’s 1988 synthesis of the mixed titanocene species 25.63 The mixed complex 25 can be afforded from the reaction of a cold THF solution of [Ti(Cp)Cl3] with triethylphosphine and the potassium salt of the pentadiene. Despite the open pentadiene having a shorter TidC bond distance than the cyclopentadiene TidC bond distance (0.106 (5) Å), the open pentadiene was susceptible to insertion with acetonitrile to form the dimeric [Ti(Cp)(Me3C6H5N)2]2. This reactivity was further explored with bulkier cyclopentadienes, where [Ti{Z5-(2,5-tBu2C5H3)}(Z5-C5H7)(PEt3)] (26) was synthesized through similar methods to 25.64 More notably, 25 was shown to be amiable to insertion of an imine and nitrile to form complex 27, as shown in Scheme 10. The half open titanocene 25 is also capable of reacting with diynes in order to afford fused ring products.65,66 Similarly, half open zirconocenes can also afford insertion products with diphenylacetylene (Table 5).67

Scheme 10 Facile insertions of open pentadienes.

Mixed open pentadienyl/cyclopentadienyl complexes have also been explored for zirconium. These tend to be supported through 2 L type ligands, whereas titanium can suffice with just one. Using the analogous starting material [Zr(Cp)Cl2Br], complex 28 was synthesized using a procedure analogous to 25, with the substitution of dmpe for PEt3.68 And similar to complex 25, complex 28 shows a shorter ZrdC bond distance to the open diene in comparison to the cyclopentadiene. Keeping in line with what was seen with 25, complex 28 also shows reactivity, this time through excess of open pentadienyl Grignard reagent to form

90

Buta- and Penta-Dienyl Complexes of the Group 4 Metals

Table 5

Mixed cyclopentadienyl/pentadienyl complexes and selected reaction products. M oxidation state

Average MdC bond distances (Å)

13

[Ti(Cp)(Z -C5H7)(PEt3)]

Ti(II)

2.240(3)

[Ti{Z5-(2,4-tBu2C5H3)}(Z5-C5H7)(PEt3)]

Ti(II)

2.245(5)

[Ti{Z5-(2,4-tBu2C5H3)}{NMeCHPhCH2CMe] CHCMe]CH2}]

Ti(III)

2.260(9)

[Zr(Cp){Z5-(2,4-tBu2C5H3)}(dmpe)]

Zr(II)

2.423(5)

[Zr(Cp)(Z4:Z5-C14H21)]

Zr(II/IV)

2.638(3)

[M{Z5-(2,4-tBu2C5H3)}2(PR3)] (M ¼ Ti, Zr, Hf, R ¼ Me, Et) [Zr{Z5-(2,4-tBu2C5H3)}((i-Pr)NCHPhCH2CMe] CHCMe]CH2)]

M(II)

See Table 6

112.25 101.37 53.89 111.3 93.6 54.3 120.2 111.2 93.8 46.6 109.9 93.2 84.2 142.1 123.0 113.5 112.6 100.9 83.4 69.2 47.5 42.6 41.8 See Table 6

Zr(II)

2.5612(6) 2.4325(6)

[Zr(Z7-C7H7)(Z5-C5H7)]

Zr(IV)

2.353(4)

[Zr(Z7-C7H7){Z5-(2,4-tBu2C5H3)}]

Zr(IV)

2.340(3)

[Ti(Pdl )2]

Ti(II)

2.273(32)

[Zr(Z7-C7H7)(Pdl )]

Zr(IV)

2.548(83)

[Ti{Z5-(2,4-tBu2C5H3)}2]

Ti(II)

2.279(46)

Molecular formula 5

C shifts (ppm)

137.7 136.3 118.9 116.7 91.8 83.4 81.2 80.7 52.0 117.1 94.5 81.5 132.9 96.7 75.9 134.3 118.3 112.6 87.5 64.2 144.4 129.9 93.6 86.0 129.4 108.7 70.9

Compound number

Refs

25

63

26

64

27

64

28

68

29

68

(30-M-R)

69

31

70

32

71

33

71

34

76

35

76,77

36

78

complex 29, as shown in Scheme 11. Against the previously mentioned trend, the ZrdCdiene bond distance is longer than that of ZrdCCp, which has been rationalized by the potential formation of an alternate binding mode leading to the high valent species 29b. The formally high valent zirconium had been posited to have less favorable binding through the lack of d-back-bonding interactions, causing a longer ZrdCdiene bond.

Buta- and Penta-Dienyl Complexes of the Group 4 Metals

91

Scheme 11 Reductive coupling of open pentadienes.

Fig. 2 Generalized structure of complexes 30-M-R and carbon labelling scheme.

Table 6

Structural parameters of complexes 30-M-R.

Complex

C (3) (ppm)

C (2,4) (ppm)

C (1,5) (ppm)

Average MdC distance (Å)

30-TidMe 30-TidEt 30-ZrdMe 30-ZrdEt 30-HfdMe 30-HfdEt

98.9 97.4 98.3 97.1 99.4 97.9

118.1 118.9 115.8 116.3 112.9 112.9

56.2 56.6 51.4 52.0 48.3 48.2

2.337(3) 2.342(2) 2.449(2) 2.460(6) 2.420(2) 2.428(2)

The Ernst group has taken forward the study of a series of open group 4 metallocenes of the type 30, shown below, Fig. 2. The general synthesis from MCl4 is as follows: The metal chloride is coordinated to tetrahydrofuran in a toluene solution. To the solution is added the phosphine of choice, and then the solution is cooled to −78  C. From there, the potassium pentadienide is added dropwise to the cooled suspension, and the reaction is let come to room temperature slowly. The solvent is then removed in vacuo, and the residue taken up with diethyl ether, filtered through Celite, and concentrated to yield a crystalline product.69 A few things can be highlighted in general regarding the series of complexes described in Table 6. Firstly, the carbon atoms are nearly planar in this complex, while substituents tend to tilt inwards toward the metal center, as a way to better support metalp-bond overlap. What is notable about the average MdC bond distances is that Ti < Hf < Zr, which is counter to their apparent ionic radii, where zirconium and hafnium have very similar radii. This can be explained by the stronger HfdC bond relative to the ZrdC bond. Of the zirconium complexes studied, few have been probed as thoroughly as 31, as shown in Fig. 3. Through an accurate crystal structure, the electron density distribution of the imine coupling product was able to be probed to determine bonding character of zirconium with a dienyl, diene, and amide.70 The synthesis of compound 31 was similar to the insertion reactivity of 25.70 The synthesis of compound 31 was similar to the insertion reactivity of 25. Through their analysis, Pillet et al. determined that the mean distance between the zirconium atom and the planar butadienyl ligand is longer than that for the butadiene ligand, which they confirmed through electron density analysis to equate to a weaker bond strength. The significant s-donation from the diene and dienyl fragments to the Zr dz2 orbital makes up a major source of the butadien(e/yl) bonding character to zirconium.

Fig. 3 An imine insertion product probed through experimental electron density analysis.

92

Buta- and Penta-Dienyl Complexes of the Group 4 Metals

Another example of a mixed zirconocene complex is that of cycloheptatrienyl-pentadienyl complexes 32 and 33. These complexes are straightforwardly synthesized through the salt metathesis reaction of a zirconium chloride with the potassium dienyl salt in tetrahydrofuran, following Scheme 12. They are thermally stable, as able to be purified by vacuum sublimation.71 Complex 33 has been shown to be susceptible toward insertion with a variety of Lewis basic ligands, simply through stirring over tetrahydrofuran, shown below as 34a–e.72 The reaction chemistry of these species point to a tetravalent zirconium center, rather than a zero-valent center which would be expected if C7H7 were monocationic, as the tropylium ion tends to be.73,74 One ligated complex of note is 10d, where the ZrdP bond is somewhat stable toward decomposition in vacuum. This is in contrast with other analogues, where the ZrdPMe3 adduct decomposes upon exposure to vacuum.75

Scheme 12 Synthesis and ligation properties of a mixed cycloheptatrienyl-pentadienyl zirconium.

More recently, an enantiopure pentadienyl ligand from the natural product myrtenal has been synthesized and ligated to several metals, including titanium and zirconium.76,77 This compound, referred to as Pdl , can be synthesized by the Wittig condensation of myrtenal, and the subsequent deprotonation with a strong enough base yields the potassium salt, as shown in Scheme 13.

Scheme 13 Synthesis of enantiopure open pentadienyl Pdl from the natural product myrtenal.

When ligated to titanium or zirconium, the preferred configuration of PdI is to bind with the bulky alkyl sidechain facing away from the metal center, which has implications on the potential capability for Pdl to enforce stereospecificity on transformations. However, the comparative bond strength of this ligand compared to the underivatized complex may indicate issues. A direct comparison between the average bond lengths of 33 and 36 show a quite significant difference of 0.194 Å. This could imply that the binding of Pdl to the metal center is far weaker, or at least that any amount of stereospecificity to be hampered by lack of steric bulk near the metal center. As for titanium, the significant steric bulk of 35 can be compared to another quite bulky complex 37, where the methyls of a pentadienyl in the 2,4 position are replaced by tert-butyl groups.78 One can see a similar TidC bond distance, and markedly similar chemical shifts. Notably, the steric bulk of the tert-butyl substituents place the two pentadienyl groups 90 staggered out of phase, which is quite different than the anti-eclipsed 180 formation of complex 35.

4.03.3.3

Dimethylcyclohexadienyl

Among the open pentadienyl complexes, there are also examples of cyclohexadienyl complexes, which has the anionic charge spread across five of the six carbons, broken up by the sp3 hybridized dimethylmethylene carbon. Zirconium complex 39 was synthesized through reaction of zirconium tetrachloride with [K(dmch)] in trimethylphosphine, following Scheme 13.79 The complex 39 is similar to 30-ZrdMe, but it has significantly longer bond distances, presumably though steric repulsion. Similarly to the open pentadienyls, 39 undergoes insertion with nitriles to form 40, which is a somewhat rare example of a Zr4+ pentadienyl complex. Presumably the amides of 40 provide enough donation to Zr to allow for some d-backbonding, which stabilizes the Zr-dienyl bond (Scheme 14) (Table 7).

Scheme 14 Nitrile insertion of dmch. Table 7

Mixed cyclopentadienyl/dimethylcyclohexadienyl complexes and selected reaction products.

Molecular formula

M oxidation state

Average MdC bond distances (Å)

13

Compound number

References

[Zr(dmch)(Z7-C7H7)]

Zr(IV)

2.351(3)

38

71

[Zr(dmch)2(PMe3)2]

Zr(II)

2.501(17)

39

79

[Zr(dmch)(dmch-[PhC(H)NPh]2)]

Zr(IV)

2.549(2)

40

79

[Zr(Cp)(dmch)(CO)(PMe3)]

Zr(II)

2.472(2)

41

88

[Zr(dmch)2(CO)(PMe3)]

Zr(II)

2.511(4)

42

88

[Zr(dmch)2(PMe3)(C6H5CCSiMe3)]

Zr(II)

N/A

43

88

[Zr(dmch)2(Cl)2]

Zr(IV)

2.555(6)

44-Cl

89

[Zr(dmch)2(Br)2]

Zr(IV)

2.593(3)

44-Br

89

[Zr(dmch)2(I)2]

Zr(IV)

2.584(3)

44-I

89

[Zr(dmch)2(OCH3)(Cl)]

Zr(IV)

2.611(2)

44-OMe/Cl

89

[Zr(dmch)2(CH3)(Br)] [Ti(dmch)(Cl)2]n [Zr(dmch)(dmpe)(Br)3]

Zr(IV) Ti(III) Zr(IV)

2.585(3) 2.413(6) 2.609(3)

44-Me/Br 45 46

89 90 90

[Zr(Cp)(3-Me3Si-dmch)(I)2]

Zr(IV)

2.586(5)

112.5 93.2 82.5 103.1 94.3 68.7 119.4 103.1 94.7 92.0 72.0 58.3 40.4 100 93.6 93.3 66.3 62.7 32.7 103.2 97.1 93.0 71.0 66.0 33.7 117.4 109.9 91.8 87.6 83.9 34.7 125.3 108.6 96.5 31.7 124.9 108.5 97.9 32.0 123.4 107.6 99.3 32.3 125.7 124.4 107.3 101.3 93.2 31.6 N/A Insoluble 126.0 112.0 96.3 32.4 126.8 112.8 106.0 31.4

47

91

C shifts C6D6 (ppm)

94

Buta- and Penta-Dienyl Complexes of the Group 4 Metals

Complex 39 can undergo coordination of carbonyls and alkynes to yield the complexes 41, 42, 43. Alkyne coordination can also afford the synthesis of butadienyl complexes.80,81 The particular isolation of the halogenated version of the complex 41 had allowed for the direct comparison between cyclopentadienyl and dienyl ligands to determine the difference in reactivity and bonding.82 The reason for the marked increase in reactivity of pentadienyl ligands versus the very common cyclopentadienyl complex can be justified by the significant decrease in d-backbonding overlap between a high valent group 4 metal and the ligand. Scheme 15 shows the ability of 39 to form the zirconocene dihalide analogue 44. The stability of zirconocene analogues 44 is thought to be due to the constriction of the C1dC5 distance by the bridge carbon. The constriction of these carbon atoms, and thus the constriction of the p-system, provides better overlap between the p-system and the contraction of the Zr(IV) orbitals. Mono dmch complexes of titanium and zirconium have been synthesized in similar methods to 44. The products are shown in Fig. 4. The titanium complex 45 is a linear polymer, a combination of the Ti(III) oxidation state, and the lack of steric bulk from the dmch ligand. Complex 46, however, is monomeric, and can be isolated through stabilization with dmpe.

Scheme 15 Oxidation of Zr(II) to form analogues to [Zr(Cp)2(X)2].

Fig. 4 Monomeric dmch complexes of Ti and Zr.

Fig. 5 Silyl-dmch complex of high-valent zirconium.

Using the halogenation procedure described above, half-open zirconocene 47 was able to be synthesized, with a trimethylsilyl substituent. The ZrdCdiene bond length is slightly longer than that of the non-silylated complex, and made it a slightly better donor and better acceptor ligand than the silylated complex. The structure of the complex is shown in Fig. 5.

4.03.3.4

Heteroatomics and clusters

Group 4 metallacyclohexadienes are the subject of very few reports. Reactions of Rosenthal’s reagent with bisalkynylsilanes can yield the corresponding 1-titana-4-sila-cyclohexa-2,5-diene product 48, illustrated in Fig. 6.83 This reaction occurs under thermal strain,

Fig. 6 Example of an uncommon 1-titana-4-sila-2,5-hexadiene ligand.

Buta- and Penta-Dienyl Complexes of the Group 4 Metals

95

but yields a rather remarkable complex. Firstly, the TidC bonds are extremely short, 1.981–1.996 Å, more in line with significant multiply bonded character. Secondly, the two a-carbon atoms are significantly close, with a distance around 1.9 Å. DFT on the crystal structure of the molecule determined that the a-carbons have singly occupied 2p-orbitals, and that they interact with the titanocene 1a1 orbital, implying that the interaction between the titanium and its a-carbons are a 3-center-2-electron, triangular shaped bond. The pentadienyl motif also appears in the CdC bond cleavage of benzene from the Hou group. In their Nature report, they describe a trinuclear titanium heptahydride cluster capable of cleaving the CdC bond of benzene, causing a migration to form a 2-methyl pentadienyl complex 49, shown in Scheme 16, a remarkable feat in strong-bond cleavage.84

Scheme 16 Synthesis of a pentadienyl through the CdC cleavage of aromatics.

Formally, the pentadienyl in 49 is charged 3-, in contrast with the other pentadienyls discussed thus far. The system is partially stabilized by the s-donation into the equatorial titanium and by p-donation into the axial titanium atoms. Reactions of this trinuclear complex with toluene instead give a mixture of 2,4-dimethylpentadienyl 50 and 1,5-dimethylpentadienyls 51. Integration of 1H NMR spectra of reaction mixtures gave a ratio of complexes 49:50 of 62:33, though they have not been isolated. The 13C NMR shifts for complex 50 are starkly different than that for the monoanionic complexes discussed beforehand, with d ¼ 211.6, 140.5, and 114.5 ppm for the ortho-, para-, and meta-carbons respectively. This significant but nearly uniform shift downfield highlights the significantly different environment that this degradation product faces in comparison to the previously discussed complexes. These reactions have been dissected computationally.85,86

4.03.4

Conclusion

Overall, the group 4 metals have been well applied for the coupling of various alkynyl species to create a diverse set of compounds in the case of butadienes. As the numbers of compounds and studies demonstrate, interest in metalacyclopentadiene complexes is greatest in this area. This is a clear consequence of the strong use of these molecules in synthetic and materials chemistry as was demonstrated in the previous century. Additional fundamental understanding continued to be gained in these systems and their reactivity has been developed. For the open pentadienyls, an increase in the methods to synthesize pentadienyls and applying them to group 4 metals has opened up several precursor complexes akin to their cyclopentadienyl counterparts. Historic methodologies that involve aspects of ligand exchange (i.e. metathesis) continue to be fruitful and develop. However, the appearance of methodologies that afford metalcyclohexadiene products via strong-bond (e.g. CdC bond) cleavage may represent a new paradigm in group 4 metal application to synthetic chemistry, much in an analogy to that of reductive coupling to afford metalacyclopentadiene complexes. The area overall is limited and there is much room for growth in depth and breadth. Additional depth may include expansion of heteroatom-containing ligands, while breadth may be the development of unique catalytic reactions beyond the alkene polymerization catalysis that these were tremendous initial impetus to the development for this category of ligands.

96

Buta- and Penta-Dienyl Complexes of the Group 4 Metals

Acknowledgments Support for this chapter was provided by the University of Vermont and the U. S. National Science Foundation (CHE-2101766 to RW).

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Chem. 2019, 58 (16), 10508–10515. Altenburger, K.; Arndt, P.; Spannenberg, A.; Baumann, W.; Rosenthal, U. Eur. J. Inorg. Chem. 2013, 2013 (18), 3200–3205. Liu, Y.; Liu, M.; Song, Z. J. Am. Chem. Soc. 2005, 127 (11), 3662–3663. Gessner, V. H.; Tannaci, J. F.; Miller, A. D.; Tilley, T. D. Acc. Chem. Res. 2011, 44 (6), 435–446. Meijer-Veldman, M. E. E.; de Boer, J. L.; de Liefde Meijer, H. J.; Schreurs, A. M. M.; Roon, J. K.; Spek, A. L. J. Organomet. Chem. 1984, 269 (3), 255–265. Calderazzo, F.; Marchetti, F.; Pampaloni, G.; Hiller, W.; Antropiusová, H.; Mach, K. Chem. Ber. 1989, 122 (12), 2229–2238. Chen, C.; Xi, C.; Jiang, Y.; Hong, X. J. Am. Chem. Soc. 2005, 127 (22), 8024–8025. Dufková, L.; Kotorax, M.; Císarˇová, I. Eur. J. Org. Chem. 2005, 2005 (12), 2491–2499. Geng, W.; Wang, C.; Guang, J.; Hao, W.; Zhang, W. X.; Xi, Z. Chem. A Eur. J. 2013, 19 (26), 8657–8664. Li, S.; Qu, H.; Zhou, L.; Kanno, K.; Guo, Q.; Shen, B.; Takahashi, T. Org. Lett. 2009, 11 (15), 3318–3321. Li, S.; Li, Z.; Nakajima, K.; Kanno, K.; Takahashi, T. Chem. Asian J. 2009, 4 (2), 294–301. Joosten, A.; Soueidan, M.; Denhez, C. M.; Harakat, D.; Hélion, F.; Namy, J.-L.; Vasse, J.-L.; Szymoniak, J. Organometallics 2008, 27 (16), 4152–4157. Soueidan, M.; Hélion, F.; Namy, J.-L.; Szymoniak, J. Tetrahedron Lett. 2010, 51 (1), 115–117. Soueidan, M.; Hélion, F.; Namy, J.-L.; Szymoniak, J. Organometallics 2008, 27 (9), 2074–2077. Stahl, L.; Ernst, R. D. Adv. Organomet. Chem. 2008, 55, 137–199. Chirik, P. J. Organometallics 2010, 29 (7), 1500–1517.

Buta- and Penta-Dienyl Complexes of the Group 4 Metals 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. 88. 89. 90. 91.

Meléndez, E.; Arif, A. M.; Ziegler, M. L.; Ernst, R. D. Angew. Chem. Int. Ed. Engl. 1988, 27 (8), 1099–1101. Tomaszewski, R.; Lam, K. C.; Rheingold, A. L.; Ernst, R. D. Organometallics 1999, 18 (20), 4174–4182. Wilson, A. M.; Rheingold, A. L.; Waldman, T. E.; Klein, M.; West, F. G.; Ernst, R. D. J. Organomet. Chem. 2009, 694 (7–8), 1112–1121. Harvey, B. G.; Arif, A. M.; Ernst, R. D. J. Organomet. Chem. 2006, 691 (24–25), 5211–5217. Harvey, B. G.; Kulsomphob, V.; Arif, A. M.; Ernst, R. D. J. Organomet. Chem. 2007, 692 (21), 4460–4466. Kulsomphob, V.; Arif, A. M.; Ernst, R. D. Organometallics 2002, 21 (15), 3182–3188. Harvey, B. G.; Basta, R.; Arif, A. M.; Ernst, R. D. Dalton Trans. 2004, (8), 1221–1226. Pillet, S.; Wu, G.; Kulsomphob, V.; Harvey, B. G.; Ernst, R. D.; Coppens, P. J. Am. Chem. Soc. 2003, 125 (7), 1937–1949. Glockner, A.; Bannenberg, T.; Tamm, M.; Arif, A. M.; Ernst, R. D. Organometallics 2009, 28 (20), 5866–5876. Glöckner, A.; Arif, A. M.; Ernst, R. D.; Bannenberg, T.; Daniliuc, C. G.; Jones, P. G.; Tamm, M. Inorg. Chim. Acta 2010, 364 (1), 23–29. Glöckner, A.; Bannenberg, T.; Daniliuc, C. G.; Jones, P. G.; Tamm, M. Inorg. Chem. 2012, 51 (7), 4368–4378. Tamm, M.; Kunst, A.; Bannenberg, T.; Herdtweck, E.; Schmid, R. Organometallics 2005, 24 (13), 3163–3171. Fryzuk, M. D.; Haddad, T. S.; Berg, D. J. Coord. Chem. Rev. 1990, 99, 137–212. Fecker, A. C.; Glockner, A.; Daniliuc, C. G.; Freytag, M.; Jones, P. G.; Walter, M. D. Organometallics 2013, 32 (3), 874–884. Fecker, A. C.; Craciun, B.-F.; Freytag, M.; Jones, P. G.; Walter, M. D. Organometallics 2014, 33 (14), 3792–3803. Reiners, M.; Baabe, D.; Schweyen, P.; Freytag, M.; Jones, P. G.; Walter, M. D. Inorg. Chim. Acta 2014, 422, 167–180. Basta, R.; Ernst, R. D.; Arif, A. M. J. Organomet. Chem. 2003, 683 (1), 64–69. Harvey, B. G.; Arif, A. M.; Ernst, R. D. J. Mol. Struct. 2008, 890 (1–3), 107–111. Harvey, B. G.; Basta, R.; Arif, A. M.; Ernst, R. D. J. Organomet. Chem. 2008, 693 (8–9), 1420–1425. Rajapakshe, A.; Gruhn, N. E.; Lichtenberger, D. L.; Basta, R.; Arif, A. M.; Ernst, R. D. J. Am. Chem. Soc. 2004, 126 (43), 14105–14116. Horácek, M.; Štepnicka, P.; Gyepes, R.; Císarˇová, I.; Kubišta, J.; Lukešová, L.; Meunier, P.; Mach, K. Organometallics 2005, 24 (25), 6094–6103. Hu, S.; Shima, T.; Hou, Z. Nature 2014, 512 (7515), 413–415. Kang, X.; Luo, G.; Luo, L.; Hu, S.; Luo, Y.; Hou, Z. J. Am. Chem. Soc. 2016, 138 (36), 11550–11559. Zhu, B.; Guan, W.; Yan, L. K.; Su, Z. M. J. Am. Chem. Soc. 2016, 138 (35), 11069–11072. Kulsomphob, V.; Harvey, B. G.; Arif, A. M.; Ernst, R. D. Inorg. Chim. Acta 2002, 334, 17–24. Basta, R.; Harvey, B. G.; Arif, A. M.; Ernst, R. D. Inorg. Chim. Acta 2004, 357 (13), 3883–3888. Basta, R.; Arif, A. M.; Ernst, R. D. Organometallics 2005, 24 (16), 3974–3981. Arif, A. M.; Basta, R.; Ernst, R. D. Polyhedron 2006, 25 (4), 876–880. Rajapakshe, A.; Basta, R.; Arif, A. M.; Ernst, R. D.; Lichtenberger, D. L. Organometallics 2007, 26 (11), 2867–2871.

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4.04

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

Florian Benner, Francis Delano IV, Elizabeth R Pugliese, and Selvan Demir, Department of Chemistry, Michigan State University, East Lansing, MI, United States © 2022 Elsevier Ltd. All rights reserved.

4.04.1 Group 3 and lanthanide cyclopentadienyl complexes 4.04.1.1 Introduction 4.04.1.2 Fundamental reactivity 4.04.1.3 Catalysis 4.04.1.3.1 CdH bond functionalization and activation 4.04.1.3.2 Olefin polymerization 4.04.1.3.3 Diene polymerization 4.04.1.3.4 Hydrofunctionalization 4.04.1.3.5 Ring opening polymerization 4.04.1.3.6 Rare earth metallocene catalysis conclusions 4.04.1.4 Cluster complexes 4.04.1.5 Small-molecule activation 4.04.1.5.1 N2, NO, and N2O activation 4.04.1.5.2 CO activation 4.04.1.5.3 CO2 activation 4.04.1.5.4 Heavy p-block element lanthanide complexes 4.04.1.6 Cp3Ln reactivity 4.04.1.7 Syntheses of exotic metallocene complexes 4.04.1.8 Cyclopentadienyl lanthanide complexes as single-molecule magnets (SMMs) 4.04.1.8.1 Mononuclear SMMs 4.04.1.8.2 Multinuclear SMMs 4.04.1.9 Divalent lanthanides 4.04.1.9.1 Reactivity of decamethylsamarocene and derivatives 4.04.1.9.2 Divalent-like reactivity 4.04.1.10 Group 3 and lanthanide phospholyl complexes 4.04.1.10.1 General coordination chemistry and reactivity 4.04.1.10.2 Catalytically active phospholyl complexes 4.04.1.10.3 Phospholyl ligands in single-molecule magnetism 4.04.1.11 Conclusion 4.04.1.11.1 Catalysis 4.04.1.11.2 Small molecule activation 4.04.1.11.3 Divalent lanthanides 4.04.1.11.4 Single-molecule magnetism 4.04.1.11.5 Phospholyl lanthanide chemistry Acknowledgments References

100 100 100 102 103 105 110 112 114 115 116 123 123 126 128 129 131 133 137 137 144 152 157 162 164 164 173 175 177 177 178 178 179 179 179 179

Abbreviations 18-c-6 Acr Allyl BEM BHT BIAN Bipy BL Bmpp Bpym CCTP CF CGC

98

18-Crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) Acridine C3H5− n BuEtMg 2,6-Di-tert-butl-4-methylphenoxy 1,2-Bis(arylimino)acenaphthene Bipyridine Bridging ligand Bis(methyliminophosphoranyl)pyridine 2,20 -Bipyrimidine Coordinative chain transfer polymerization Crystal field Me2Si[(5-Me4C5)(tBuN)]

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00064-0

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

CHD COT COT00 Cp Cp0 Cp00 Cp000 CpMe Cptet Cp CpiPr4 CpiPr5 CpBIG Cptet0 CpNMe2 Cpttt crypt-222 Cy DAD DBM DFT Dipp DippForm DME DMeP DPhP DsP Dtb DtP Et Flu HAN HP HsP HtP Ind iPS MBL Me Mes MMBL Mn Mw NB P-F-tipp Phen PI Pin PIP Pr PTH Py Qtp RE ROP SMM SPS

1,3-Cyclohexadiene 1,3,5,7-Cyclooctatetraenyl 1,3,6-Tris(trimethylsilyl)cyclooctatetraenyl Cyclopentadienyl 1-(Trimethylsilyl)cyclopentadienyl 1,3-Bis(trimethylsilyl)cyclopentadienyl 1,3,5-Tris(trimethylsilyl)cyclopentadienyl 1-Methylcyclopentadienyl 1,2,3,4-Tetramethylcyclopentadienyl 1,2,3,4,5-Pentamethylcyclopentadienyl 1,2,3,4-Tetraisopropylcyclopentadienyl 1,2,3,4,5-Pentaisopropylcyclopentadienyl 4-(nBu-C6H4)5C5 1-(Trimethylsilyl)-2,3,4,5-tetramethylcylopentadienyl 1-[2-(N,N-dimethylamino)ethyl]-2,3,4,5-tetramethyl-cyclopentadienyl 1,3,5-Tri(tertbutyl)cyclopentadienyl [2.2.2]-Cryptand (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) Cyclohexyl 2,3-Diazabutadiene Dibenzoylmethane Density functional theory 2,6-Diisopropylphenyl N,N0 -bis(2,6-diisopropylphenyl)formamidinate 1,2-Dimethoxyethane 3,4-Dimethyl-phospholyl 2,5-Diphenyl-phospholyl 3,4-Dimethyl-2,5-bis(trimethylsilyl)-phospholyl 1,4-Di(terpyridyl)-benzene 2,5-Ditertbutyl-3,4-dimethyl-phospholyl Ethyl Fluorenyl (C13H12) 2-Amino-5,6-dimethyl-benzimidazole-1-Pentanoic acid (2,3,4,5-H-phospholyl) 2,5-Di(trimethylsilyl)-phospholyl 2,5-Di-tert-butyl-phospholyl Indenyl Isotactic polystyrene a-Methylene-g-butyrolactone Methyl 2,4,6-Trimethylphenyl g-Methyl-a-methylene-g-butyrolactone Number average molecular weight Weight average molecular weight Norbornene Fluorophenyl-tetrakis(imino)pyracene 1,10-phenanthroline Polyisoprenes Pinacolato cis-1,4-Polyisoprene Propyl p-toluenesulfonic acid Pyridine 60 ,600 -bis(2-pyridyl)-2,20 :40 ,400 :200 ,2000 -quaterpyridine Rare earth Ring opening polymerization Single-molecule magnet Syndiotactic polystyrene

99

100

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

Taphen TH THF Tmbp TMEDA TMeP TMF Tmp TMT Tp Tppz

4,5,9,10-Tetraazaphenanthrene Hysteresis temperature Tetrahydrofuran 1,10 ,2,20 -Tetramethylbisphosphinine N,N,N0 ,N0 -Tetramethylethylenediamine 2,3,4,5-Tetramethylphospholyl Tetramethylfulvene 2,2,6,6-Tetramethylpiperdide Trans-metal trapping Hydrotris(1-pyrazolyl)borate 2,3,5,6-Tetra-2-pyridinylpyrazine

4.04.1

Group 3 and lanthanide cyclopentadienyl complexes

4.04.1.1

Introduction

The chemistry of the lanthanides is intrinsically different relative to d-block metals and in direct comparison, their reactivity is greater and in fact comparable to alkaline earth metals. Some of the most notable characteristics of lanthanides include: (a) accessibility of high coordination numbers, (b) distinct coordination geometries are directed by ligand steric factors while (c) crystal field effects play a minimal role and are generally miniscule, (d) the deeply buried 4f-orbitals do not participate in bonding, (e) the chemistry is governed by metals in the trivalent oxidation state, (f ) the generation of ionic complexes, (g) facile reactions with highly anionic donors such as N and O. The organometallic chemistry of the lanthanide ions is vastly dominated by the implementation of the anionic cyclopentadienyl ligand (Cp−) and its derivatives. In 1955, Birmingham and Wilkinson, isolated the first stable lanthanide cyclopentadienyl compounds, Fig. 1, Cp3Ln (1), and impressively demonstrated that the 18-electron rule does not apply for the cyclopentadienyl complexes of the lanthanides with the electron counts spanning from 18 for Cp3La (1-La) to 32 for Cp3Lu (1-Lu), but the molecular structure remaining similar unlike the transition metal metallocenes. These early studies on organolanthanide reactivity revealed minimal effects from metal orbitals, while being greatly influenced by steric effects.1 In fact, the reactivity was largely dependent on ionic radii differences and not on the electronic configuration of the lanthanide ion.2 Organometallic lanthanide compounds where the metals construct s- or p-bonds to carbon are typically much more air- and moisture-sensitive than respective d-block congeners. Hence, the lanthanide analog of ferrocene, which is robust to oxygen, water, and heat, remains hitherto sought-after. The bonding within an organolanthanide complex is typically described as polar. Cp complexes of lanthanides have given rise to remarkable compounds such as the first coplanar coordination of two metals to dinitrogen3 and a tetracyclic hydrocarbon bridge by reacting with CO and alkynes regardless of the impeded back-bonding ability of 4f-orbitals owing to their limited radial extension.4,5 Furthermore, the formal oxidation states of lanthanide ions stand out from transition metal ions.6 Even though lanthanide ions generally prefer the +3 oxidation state, decorating bulky Cp ligands around the metal ion paves the way for electron transfer reactions via ligand-based oxidations. Thus, Cp lanthanide compounds have been employed in catalysis, polymerizations and small molecule activation. Over the course of the past 10 years, lanthanide metallocenes gained remarkable attention in the highly-interdisciplinary research field of single-molecule magnetism on account of the unique axial ligand field they can provide to metal centers.7 This chapter highlights the most significant advances in organolanthanide chemistry comprising cyclopentadienyl (Cp) and phospholyl ligands, respectively, from around 2006 to 2021. Earlier progress has been summarized in COMC (1982),8 COMC-II (1995),9 and COMC-III (2007).10 In addition, a multitude of reviews surveying organolanthanide chemistry including special aspects have been published between 1985 and 2021.7,11–21 The chapter is partitioned into Cp and phospholyl sections, while a further subdivision is based on the reactivity and properties of the compounds: The largest Cp subsections comprise catalysis, small-molecule activation, and single-molecule magnetism. The catalysis section is subdivided according to the catalyzed reaction, the small-molecule activation by the activated molecules, and the single-molecule magnet (SMM) section by nuclearity and nature of the bridging ligands. Further discussed categories within the Cp section are Cp3Ln reactivity, divalent lanthanides, cluster complexes, and interesting syntheses as well as unusual structures. The phospholyl subsection comprises the synthesis of organometallic compounds, applications to catalysis and SMMs.

4.04.1.2

Fundamental reactivity

Rare earth (RE) elements comprising Sc, Y and the lanthanides (Ln) have found widespread application such as: (1) in alloys to mediate strength and hardness to metals,22 (2) in the petroleum industry to refine crude oil into gasoline products,23 (3) in optical devices (e.g. lasers, phosphorescent materials),24 (4) MRI contrast agents,25 (5) magnets,26,27 and (6) catalysts.28 In fact, 15,000 ton/year of the lanthanides, corresponding to 85% of lanthanide production, are employed in small quantities in the

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

Fig. 1 Timeline of landmark molecules in organometallic rare earth metal chemistry with cyclopentadienyl and phospholyl ligands, respectively.

101

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Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

manufacturing of glass and in catalysis.29 The unique chemical and physical properties innate to lanthanides render them significantly different to other transition metals. Similar to other electron-deficient transition metal ions in high oxidation states, the Ln elements exhibit strong Lewis acidity and oxophilicity.30 Traversing across the lanthanide series equals the incremental population of 4f-orbitals, which are inherently shielded by the 5d-orbitals owing to their limited radial extension.31 This renders the 4f-electrons essentially inaccessible for chemical bonding, giving rise to the most stable oxidation state of +III, which is also observed for Sc and Y. The particular stability of the +III oxidation state requires tremendous driving force to reach +II or +IV oxidation states, albeit the divalent state is more readily accessed for Eu, Sm and Yb.32,33 In contrast to other transition metals, the 4f-electrons almost exclusively do not participate in bonding. Thus, the bonding in RE complexes is mainly electrostatic in nature rather than the electron number account and fosters a certain coordination geometry. Notably, the 18-valence electron (VE) rule is scarcely obeyed in RE complexes, Scheme 1, complicating the prediction of catalytic cycles.

Scheme 1 Synthesis and valence electron count of Cp3Ln complexes.

4.04.1.3

Catalysis

Polymerization refers to a process that combines chemically small molecules to polymers, best described as network materials, which are ubiquitous in modern industry and our daily life. The generation of stable covalent chemical bonds between the small molecules differs intrinsically from other mechanisms such as crystallization where weak intermolecular forces direct the aggregation of molecules. Owing to their high molecular masses, macromolecular substances (polymers) possess unparalleled characteristics, where the size of the obtained structure results in excellent mechanical or technical properties. Transition metals oftentimes pass through several oxidation states and multiple redox processes occur in catalytic cycles (oxidative addition ! insertion/chain growth ! reductive elimination),34 however, these processes are generally more complicated with RE elements. In particular, RE hydride- and alkyl-complexes have shown to be very reactive, exhibiting both nucleophilic and basic reactivity. This reactivity in conjunction with high stability, strong Lewis acidity and the REIII ions’ affinity toward unsaturated CdC-bond render these elements promising candidates for the development of novel single-site catalysts. These could potentially serve as alternatives to Ziegler-Natta catalysts comprising titanium halides and alkylaluminum compounds.35 Furthermore, the similar ionic radii of the Ln elements allow fine tuning of the catalyst activity and selectivity by replacing the metal ion while maintaining the overall topology of the structure, Scheme 2.5

Scheme 2 Sensitivity of hydrogenolysis reactivity to the metal radius for eight-coordinate Ln ions.

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

103

Biscyclopentadienyl rare earth complexes containing alkyl or hydride ligands are single-component catalysts in polymerization reactions and copolymerize ethylene and polar monomers (alkyl acrylates, lactones).36 An elaborate study dealt with the insertion rate of ethylene into ScdC bonds of Cp ScR (R ¼ Me, Et, nPr) complexes (2-Sc).37 Here, the rate of monomer insertion into the ScdC bond decreased with rising bond stability (R ¼ CH2CH2CH3 < CH2CH3 < CH3). Furthermore, the molar weight distributions were in accordance with those observed for living, Ziegler-Natta-type polymerizations. However, in the absence of an activator, the polymerization activity of biscyclopentadienyl RE complexes toward less reactive monomers such as styrenes, dienes and cyclic olefins is significantly reduced, as the metal centers tend to be sterically saturated, rendering additional monomer coordination unfavorable.36 In addition, in the case of 1,3-butadiene, polymerization is hindered through the formation of stable 3-allyl complexes.36 In contrast, the activation of mono- and biscyclopentadienyl RE alkyl complexes through either hydride abstraction or protonation afforded reactive species that are prone to initiate various polymerizations. The enhanced reactivity was attributed to less steric shielding of the metal center and a higher Lewis acidity.35 The success of this approach has been demonstrated: RE mono Cp complexes [Cptet0 RE(CH2SiMe3)2(THF)] (3) ( RE¼Sc, Y, La, Gd) have been successfully used for the synthesis of homo- and block copolymers polystyrene and polystyrene-polyethylene,38 with Moreover, with 3-Sc, the the scandium complex having the highest activity activities of up to 1.36  104 kg mol−1 Sc−1 h−1 . copolymerization of styrene and ethylene occurred rapidly, with activities up to 2.3  103 kg mol−1 Sc−1 h−1. Notably, this system was the first to selectively produce styrene–ethylene copolymers containing the syndiotactic styrene–styrene sequences. Furthermore, the polymerization of isoprene was accomplished with [Cptet0 Ln(AlMe4)2(THF)] (Ln ¼ Y, La, Nd) (4) after activation with (Ph3C)(B(C6F5)4) or (PhNMe2H)(B(C6F5)4).39 The number of catalytically active mono- and biscyclopentadienyl RE complexes has been steadily growing. In the following, the synthesis of selected complexes along with their ability to polymerize various monomer species will be highlighted.

4.04.1.3.1

CdH bond functionalization and activation

Carbon-hydrogen bond functionalization refers to the cleavage of a carbon-hydrogen bond concomitant with a replacement through a carbon-X bond (where X ¼ carbon, hydrogen or nitrogen). The intermediate step, namely the activation of a CdH bond constitutes the functionalization of an alkene. Of particular importance to the RE metals is s-bond metathesis wherein the metal-ligand bond undergoes metathesis with another sigma bond. The ability for RE metal complexes to partake in this reactivity was first observed by Watson by exchanging a decamethyllutenocene methyl complex Cp 2Lu(CH3) (2-Lu) with the isotopically labeled hydrocarbon 13CH4, Eq. (1), which could be extended to the yttrium analog.40

ð1Þ

ð2Þ

Bercaw and coworkers could demonstrate subsequently analogous reactivity with [Cp 2Sc(Me)] (2-Sc).41 The methylation of ytterbocene Cp 2Yb complexes was accessible through an elegant oxidative methylation route with [Cp 2V(Me)] or MeCu as methyl transfer reagents. Notably, the open-shell YbIII (f13) complex, 2-Yb, did not readily exchange with isotopically labeled methane,42 which was hypothesized to be due to the small amount of alkylidene character in the YbdCH3 bond.43 Watson proposed that the reactivity of this class of compounds is dependent on the electrophilicity of the metal center.40 The chemical shift tensor and calculated free energies of the d0 complexes of Cp 2RE(Me) (RE ¼ ScII, TiIV, YIII, and LuIII) suggest that s-bond metathesis, olefin insertion and olefin metathesis reactions, are all governed by the p-character of the MdC bond. All three reactions necessitate the presence of two low-lying orbitals, one to form a s-type interaction with the incoming substrate while the other remains coordinated to the monomer fragment through a p-interaction.43 Half-sandwich yttrium alkyl complexes were proven to be well-suited for efficient benzylic CdH alkylation reactions of 2,6-dialkyl-substituted pyridines to yield various olefins.44 Notably, the half-sandwich yttrium dialkyl complex [CpY(Me2N-2CH2C6H4)2]/(Ph3C)(B(C6F5)4) (5) was demonstrated to be an exceptional catalyst for the ortho-selective benzylic CdH addition to various dialkyl pyridines to give rise to different olefins such as ethylene, 1-hexene-styrene and 1,3-conjugated dienes,

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contributing to the formation of a new group of alkylated and allylated pyridine derivatives. In particular, this yttrium half metallocene catalyzed the reaction of 2,6-lutidine with substrates such as norbornene or cyclohexa-1,3-diene at 70  C in toluene to afford 2-(bicyclo[2.2.1]heptan-2-ylmethyl)-6-methylpyridine or 2,6-bis(cyclohex-2-en-1-ylmethyl)pyridine, in 89% and 97% yield, respectively. It also catalyzed the reaction of ethylene with pyridines such as 2,4,6-trimethylpyridine and 2-ethyl-6-methylpyrdine to afford the respective products in high yields  97%. CdH activation can also occur when reactive lutetium hydrides are treated with cross-conjugated hydrocarbons. The isolation of the organolutetium tuck-over complex [Cp 2Lu(m-H)(m-1:5-CH2C5Me4)LuCp ] (6) aside the vinyl complex, [Cp 2Lu(CH] C5Me4)] (7), from the reaction of [Cp 2LuH]n (8) with tetramethylfulvene (TMF), constitutes a prime example for a CdH activated product, involving the vinylic CdH bond of the TMF, Eq. (2). An alternative route to the tuck-over complex in 88% yield represents heating 8 at 70  C for 24 h. Typically allylic CdH activation is observed rendering the vinyl CdH activation of TMF rare and in fact, the only other known vinyl lanthanide complex is [(Et8-calix-pyrrole)(CH]CH2)Sm(m3-Cl)[Li(THF)]2[Li(THF)2] (9).45 The first step of the CdH activation between 8 and TMF generates a “(Cp )3Lu” intermediate that is not structurally characterized but is thought to possibly feature any of these different binding modes of one of the pentamethylcyclopentadienyl rings in solution: Cp 3Lu, [(5-Cp )2Lu(1-Cp )], and [Cp 2Lu (CH2dCHC4Me4)], respectively, which is dictated by the addition of the lutetium hydride, Scheme 3. Upon transient intermediate

Scheme 3 Proposed formation of the “tuck over” and TMF adduct complexes through an undetected “Cp 3Lu” intermediate.46

formation, its metalation would give rise to the vinyl complex with a concomitant activation of the methyl CdH bond to afford the (C5Me4CH2)2− anion of the tuck-over complex.46 The hydrogenation of the tuck-over complex produces the respective lutetium hydride which in the following reacts catalytically with TMF to give HCp . Cp 3Lu (1-Lu) is proposed to be the catalytically activate species to hydrogenate TMF, and as such could be a selective catalyst for hydrogenation of double bonds, Scheme 4.

Scheme 4 Catalytic formation of pentamethylcyclopentadiene (Cp H) from the hydrogenolysis of the LudC bond in [(Cp 2Lu(m-1:m-5-CH2C5Me4)LuCp )] (6) and subsequent reaction with excess TMF in the presence of hydrogen.46

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4.04.1.3.2

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Olefin polymerization

Certain rare earth complexes containing at least one Cp ring possess catalytic activity toward olefin polymerization reactions and will be discussed in the following. Olefin refers to alkenes or hydrocarbon compounds containing a CdC-double bond where prominent examples for such monomers are ethylene, propylene and styrene.47 The subchapter is divided into sections based on the number of Cp ligands for the principle catalytic compound.

4.04.1.3.2.1 Half-metallocene complexes Scandium half-metallocene complexes are powerful catalysts for various polymerization reactions.48–51 The series of scandium half-metallocenes comprising bis(ortho-dimethyl-aminobenzyl) ligand, [CpRSc(CH2C6H4-NMe2-o)2] (CpR ¼ Cptet0 , Cp , Cptet) (10), features an intramolecular interaction of the amino group with the Lewis acidic ScIII-center rendering the coordination of a stabilizing Lewis base such as THF unnecessary.52 Their polymerization reactivity toward olefins such as ethylene, 1-hexene, styrene, norbornene (NB) and dicyclopentadiene (DiCp) evidences the activity of the aminobenzene ligand for olefin insertion reactions. Notably, their catalytic activity for the copolymerization of 1-hexene with DiCp to yield copolymer materials with a control of 1-hexene ratio in the products was demonstrated. The selective polymerization or copolymerization of DiCp is generally challenging as it requires a catalyst that is not only sufficiently active but can also distinguish the norbornene moiety from the cyclopentene unit to prevent cross-linking side reactions. The precatalytic molecule [Cptet0 Sc(CH2C6H4NMe2-o) (k2F-C6F5)B(C6F5)3] (11) constitutes the first external Lewis base-free cationic half-sandwich rare earth metal hydrocarbyl complex. Strikingly, the outcome of the polymerization reactions can be dramatically influenced by varying the substituents of the Cp ligands.54 Derivatives of monocyclopentadienyl scandium complexes [CpRSc(CH2SiMe3)2(THF)] (CpR ¼ CpMe, Cptet, Cp , Cptet0 ) (12-Sc), Fig. 2 and [CpR(m-L)Sc(CH2SiMe3)2] (CpR ¼ C5Me4(CH2CH2PPh2); C5Me4(C6H4-o-OMe)) (13) can be activated through hydride abstraction involving (Ph3C)(B(C6F5)4) or protonation with (PhMe2NH)(B(C6F5)4). The resulting catalytically active monocationic Sc complexes show polymerization activity that hinges mainly on the substituents of the cyclopentadienyl ligand. For the homopolymerization of isoprene, complexes consisting of sterically less demanding Cp ligands afforded high cis-1,4-selectivity (up to 95%), whereas sterically encumbered half-metallocenes with C5Me4(C6H4-o-OMe) sidearms yielded trans-1,4 electivity (60–79%). Increasing the steric bulk on the Cp ligands further resulted in varying degrees of 3,4-selectivity (51–65%). For the first time, the copolymerization of isoprene and ethylene with Cp− or Cptet−-containing complexes yielded copolymers with predominantly cis-1,4-selectivity, while molecules comprising the larger Cp derivatives afforded perfectly alternating isoprene-ethylene copolymers. A DFT study provided insight into the polymerization mechanism taking into account the steric effects arising from the diverse Cp ligands. The calculations revealed the initial step to comprise a 2-trans-3,4 coordination of one isoprene molecule, followed by a 3,4-insertion into the ScdC-bond. For the resulting 3-s-allyl intermediate three energetically similar concurring chain propagation pathways were predicted, giving rise to different microstructures (3,4-, trans-1,4 and cis-1,4) in the polymer chain. In accordance with the experimental findings, sterically less hindered complexes led to a sequence of syn-anti and s-p-rearrangements, resulting in an intermediate anti-p-3-allylic species, which would proceed to insert selectively further isoprene molecules to culminate in the formation of a cis-1,4-polymer.

Fig. 2 Chain-shuttling copolymerization scandium catalysts, (12). (Left) A THF adduct of the Cptet0 bis(trimethylsilyl)methyl Sc complex with high activity and high syndiospecific selectivity for styrene polymerization. (Right) A THF adduct of Cp bis(trimethylsilyl)methyl Sc complex with high activity and high cis-1,4-selectivity for isopropene or butadiene.53

The selective copolymerization of heteroatom-functionalized a,o-dienes to give polymers with cyclic repeating units such as ethers and thioethers is challenging due to the lack of catalysts that tolerate functional groups and allow for good regio- and stereoselectivity. The first selective catalysts with rigorous regio-, diastereoselectivity and stereoregularity constitute scandium half-metallocene complexes CpRSc(CH2C6H4NMe2-o)2 (CpR ¼ Cp, Cptet, Cptet0 ) (14), that resulted in an unparalleled heteroatom–metal-interaction, Scheme 5.55 The addition of the cocatalyst (Ph3C)(B(C6F5)4) allowed homopolymerization of 4-benzyloxy-1,6-heptadiene with the sterically most demanding Cp derivative, Cptet0 , yielding a cyclic polymer with the narrowest molecular weight distribution (Mn ¼ 18.7 kg/mol and Mw/Mn ¼ 1.50). Here, 1H-NOESY NMR correlation peaks indicated an identical orientation of the CH2 substituents pointing at high stereoregularity (95% isotactic). Furthermore, this scandium catalyst has also proven to copolymerize 1,6-heptadienes and ethylene with a regio- and diastereoselectivity, yielding primarily 1,2,4-cispoly-1,2-ethylene-4-siloxycyclopentane (93% isotactic).

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Scheme 5 Substitution dependence of scandium-catalyzed regio-, diastereoselective and stereoregular cyclopolymerization of ether- and thioether-substituted 1,6-heptadienes.55

The first simultaneous chain-growth and step-growth polymerization of methoxystyrene monomers was demonstrated through the use of rare earth half-metallocene complexes Cp RE(CH2C6H4NMe2-o)2 (15) (RE ¼ Sc, Y, Gd) and (Ph3C)(B(C6F5)4).56 These new polymer architectures could be afforded by combining continuous C]C-insertion (chain-growth) and CdH-activation (stepgrowth). While 15-Sc led to long syndiotactic chains, 15-Y and 15-Gd in conjunction with lower C]C-insertion activity gave rise to the formation of THF soluble polymers with microstructures comprising alternating anisole-ethylene sequences. 15-Gd yielded a higher molecular weight and narrower weight distribution in comparison to the Y catalyst (Gd: Mn ¼ 16.2 kg/mol, MW/Mn ¼ 2.45; Y: Mn ¼ 4.06 kg/mol, MW/Mn ¼ 5.29). A rare earth-based catalytic system consisting of two half-sandwich ScIII complexes, where the one features high activity and selectivity for the syndiotactic polymerization of styrene and the other shows high activity and cis-1,4-selectivity for the polymerization of isoprene generated jointly with iBu3Al as a chain-shuttling agent block copolymers with perfect syndiotactic polystyrene (sPS) and highly regulated cis-1,4-polyisoprene (PIP) blocks with narrow molecular weight distributions (Mw/Mn ¼ 1.43), Scheme 6.53

Scheme 6 Proposed chain-shuttling mechanism for the formation of the multiblock copolymer sPS-cis-1,4-PIP. Polymer formation occurs through the cooperation of two active catalysis sites, one generated from Cptet0 Sc(CH2SiMe3)2 which has a high activity and syndiospecific selectivity for styrene polymerization, the other generated from CpSc(CH2SiMe3)2 which has a high activity and cis-1,4-selectivity for isoprene polymerization. The chain shuttling mechanism is facilitated by the polymer containing aluminum end groups such as B, D, G, and J which can undergo efficient transmetalation with the scandium species.53

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The mutual employment of these two catalytically active species resulted also in the first terpolymerization of styrene, isoprene, and butadiene with highly regulated polymer blocks. The required presence of the chain-shuttling agent prevented formation of polymer mixtures. A remarkable cocatalyst-free ethylene polymerization activity was found for the silica-supported decamethylcyclopentadienyl LnII complexes Cp 2Ln (Ln ¼ SmII, YbII),57 which transferred to the realm of heterogeneous catalysis with potential broad industry application. Surface fixation through heating of toluene solutions of the metallocenes with partially dehydroxylated SiO2 yielded Cp 2Ln@SiO2–700 (16) (Ln ¼ EuII, SmII, YbII) exhibiting compositions consistent with the formulation of [( SiO)LnCp ] for the surface-monographed Yb species. Notably, the average activity per silica-supported Sm-site (12,800 kgPE (molSmh)−1) was 40 times higher than the bare Cp 2Sm compound. In contrast, the YbII compound was substantially less active (228 kgPE (molSmh)−1), while no polymerization was attained with the EuII sites on silica. Hence, the initiation step is hypothesized to occur via a single-electron transfer mechanism in accordance with the observed activity of Cp 2Ln where Ln ¼ Yb (−1.12 V vs. NHE), Sm (−1.7 V vs. NHE), and inactivity of Eu (−0.56 V vs. NHE).58 Noteworthily, the grafting yielded polymers with longer chain lengths giving rise to ultrahigh molecular weight polyethylene. Varying one of the methyl groups of the trimethylsilyl group on the cyclopentadienyl ring and the metal ion generated [(C5Me4SiMe2R)RE(CH2SiMe3)2(THF)] (17) (RE ¼ Sc, Y, and Lu; R ¼ Me, Ph, 2-pyridyl, C6F5, 2-furyl, and 2-furyl-5-Me) that are highly efficient catalysts for syndiospecific polymerization of styrene.59 Mixing the rare earth metal complex with (Ph3C)(B(C6F5)4) in the presence of an alkylaluminum cocatalyst afforded a catalytically active species, with the Sc derivative being the most active when paired with a triisobutylaluminum cocatalyst. The mechanism of organoscandium-catalyzed ethylene and amino olefin copolymerization were evaluated by a DFT study.60 Accordingly, short-chain amino olefins simultaneously chelate both the olefin and amine moiety of the same amino olefin, which is not available for the long-chain amino olefin. This results in an energetic disparity between the amino olefins causing the long-chain amino olefins to proceed through a higher energy pathway. In contrast to ethylene insertion, amino olefin insertion increases the energetic barrier due to the competing coordination of the amino group. The dinuclear half-sandwich scandium complex [(Cptet0 Sc(THF)Cl)2(m-(CH2C6H3(o-NMe2))2)] (18) is a catalytically active species and serves as a bifunctional initiator for the synthesis of ABA-type polymers where hydrophilic soft polycaprolactone blocks (A) cap hydrophobic hard polystyrene blocks (B), affording a triblock copolymer.61 Through halide abstraction with [(Et3Si)2(m-H)](B(C6F5)4), copolymerization of styrene and e-caprolactone yielded triblock copolymers with perfect sPS in the midblock and tunable PCL content in the end blocks. This represents the first stereospecific ABA-type triblock copolymer, which was previously inaccessible owing to a lack of suitable difunctional initiators. Allyl methacrylate and methyl methacrylate can be polymerized by the yttrium complexes [CpY(CH2SiMe3)(RC(NAr))2(THF)] (19) (Ar ¼ 2,6-iPr2C6H3; R ¼ Ph, CH2SiMe3) cocatalyst-free to polymers of low to high molecular weight.62 Here, the activity of the CH2SiMe3-containing complex significantly exceeds that of the Ph-substituted complex (3000 h−1 R ¼ CH2SiMe3, 38 h−1 for R ¼ Ph). The obtained polymer shows a syndio-rich microstructure with syndiotacticity ranging from 70% rr (R ¼ CH2SiMe3) to 76% rr (R ¼ Ph). Furthermore, the catalyst where R ¼ CH2SiMe3, generated well-defined block copolymers of predetermined length and sequence such as an AB copolymer stemming from polymerizations of each 100 equivalents of allyl methacrylate and methyl methacrylate. The proposed propagation cycle includes first the transformation of the catalyst–monomer intermediate to the eight-membered cyclic ester chelate by an intramolecular Michael addition, and second the regeneration of the catalyst monomer intermediate by ring-opening of the cyclic chelate through insertion of a new monomeric molecule. This mechanism is confirmed by MALDI-TOF Mass Spectrometry and 1H NMR spectroscopy revealing the presence of initiation (dCH2SiMe3) and termination groups (dH) in polymer fragments. Mono- and dinuclear lanthanide allyl complexes with amino-substituted Cp rings, CpNMe2Ln(3-C3H5)2 (20) (Ln ¼ Y, Ho, Lu) and [CpNMe2Nd(3-C3H5)(m-Cl)]2 (21), show upon activation with (Ph3C)(B(C6F5)4) or (PhNMe2H)(B(C6F5)4) moderate to high activity toward the polymerization of isoprene.63 While 20-Y and 20-Ho produced polymers with predominantly atactic 3,4-sequences, 20-Lu complex gave equal cis-1,4-, trans-1,4- and 3,4-units, and the dinuclear neodymium complex, 21-Nd required an excess of the cocatalyst, Et2AlCl, to yield primarily 3,4-structures. The weight distributions for all catalysts were very narrow (1.04–1.17) suggesting that the polymerization proceeds in a living fashion. Additional cocatalysts, namely alkylaluminum compounds AlR3, affected regio- and stereoselectivity. The AlMe3 addition to the Lu-catalyst resulted in a decreased cis-1,4and increased trans-1,4-/3,4-microstructures, while AliBu3 addition had a negligible influence on the microstructures. By contrast, AlMe3 addition to 20-Y and 20-Ho, respectively, induced moderate trans-1,4-selectivity (70%), whereas introducing the larger AliBu3 provided cis-1,4-selectivity (74%). The selectivity of the Nd catalyst shifted toward 1,4-microstructure (85% trans) and 3,4-selectivity (85%) upon addition of AlMe3 and AliBu3, respectively. Monitoring the Y-catalyzed conversion by NMR spectroscopy unveiled the true catalytically active species to be [CpNMe2AlMe3Y(AlMe4)2] (22) which was validated by control experiments.

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The introduction of chain transfer reagents greatly impacted the catalyst activity and microstructure of the polymeric products.64 The use of b-diketiminate-supported magnesiumalkyl complex [nBuMg(2-HC(C(Me)NMes)2)] (Mes ¼ 2,4,6-Me3C6H2) instead of the commonly employed nBuEtMg (BEM) gave with [Cp La(BH4)2(THF)2] (23) a better controlled polymerization of isoprene (trans-1,4-syndiotactic) and an enhanced catalytic activity toward the chain transfer polymerization of styrene. The activity of b-diketiminate was found to be lower when employed as the cocatalyst. While being used as a chain transfer agent in conjunction with BEM, the cocatalyst increased the 3,4-content at slightly decreased activities. When used as an alkylation agent with triisobutylaluminum iBu3Al (TIBA) as the chain transfer agent, good yields were obtained with preservation of the high trans-1,4-stereoselectivity. This had a chain transfer efficiency of 30–33%, ranging among the highest reported at that point. The treatment of half-sandwich dibenzyl scandium complexes [Cp Sc(CH2Ph)2(THF)] (24) with (Ph3C)(BPh4) produced cationic species which were probed for styrene polymerization resulting in syndiospecific polystyrene.65 In relation to [Cptet0 Sc (CH2SiMe3)2(THF)] (3-Sc), a lower activity was observed, which is attributed to the sterically more encumbered benzyl substituent compared to the methyl group impeding the coordination ability of monomer insertion. Following an activation of the bisalkyl complexes with AlR3 (R ¼ Me, Et, iBu) and (Ph3C)(B(C6F5)4), the best catalytic activities were only observed for the Sc complexes yielding polyethylenes of low molecular weights.66 This is attributed to both the stronger Lewis acidity of ScIII and the absence of THF in the coordination sphere when contrasted to the lanthanide analogs. Furthermore, the function of the aluminum sources AlR3 is three-pronged, ranging from stabilizers for the catalytically active cationic species, scavengers for impurities, to chain transfer agents where the latter are deduced from the comparably lower molecular weights of the products. Phosphazene-functionalized half-sandwich rare earth metal complexes polymerized ethylene. The implementation of a soft or rigid heteroatom-containing side arm on the Cp ring formed a constrained coordination geometry around the metal ion leading to high activity and selectivity toward olefin polymerization. The nature of either the Cp moiety, the bridging unit or the side arm donating heteroatom, had different outcomes in regard to polymerization performance. Although phosphazene-functionalized Cp ligands were known for some time, complexes with such fragments were not tested for their polymerization activity.66 The rare earth complexes [(L-PPh2]N-C6H3R2)RE(CH2SiMe3)] (25) (L ¼ C5H4: RE ¼ Sc, Y, Nd, Sm, Lu; R ¼ Me, iPr; L ¼ Cptet: RE ¼ Y, Lu; R ¼ iPr; L ¼ C9H6 (indenyl): RE ¼ Sc, Y, Lu; R ¼ Me, iPr; L ¼ C9H6: RE ¼ Sc; R ¼ Et; L ¼ C13H12 (fluorenyl): RE ¼ Sc; R ¼ H; L ¼ C13H12: RE ¼ Sc, Lu; R ¼ Me) were synthesized through deprotonation of the respective pentadiene pro-ligand with rare earth tris(alkyl) complexes [RE(CH2SiMe3)3(THF)2].67 Here, the tetramethylated Cp-containing molecule gave a mixture of 5- and 3-coordinated complexes owing to a deprotonation at one of the methyl groups that is in a-position to the N substituent. Constrained coordination geometry in rare earth half-metallocene complexes were also constructed through installation of aminophenyl-functionalized Cp and Flu (fluorenyl) ligands.68 The bis(allyl) complexes (Cp: [(C5Me4-C6H4-o-NMe2) Ln(3-C3H5)2]) (26) (Ln ¼ Y, Lu); [(C5Me4-C5H4N)RE(3-C3H5)2] (27) (RE ¼ Y, Sc, Lu) and bis(alkyl) complexes (Flu: [(3-C13H8-C5H4N)RE(CH2SiMe3)2(THF)]) (28) (RE ¼ Y, Sc, Lu) were accessible through reaction of the deprotonated ligands with LnCl3 followed by treatment with allylmagnesiumchloride, and through the reaction of the Flu ligand with [Ln(CH2SiMe3)3(THF)2], respectively. All those complexes polymerized styrene and allowed for careful evaluation of general trends with regard to the effects of steric hindrance around the central metal ion, the electron donating or withdrawing effect of the ligands, the coordination mode of the ancillary ligand, the Lewis acidity of the employed metal ion, and the catalytic activity. Consequently, complexes with a less shielded coordination environment indicated by a small bite angle and a more Lewis acidic central metal atom such as containing an electron withdrawing ancillary ligand, were found to catalyze styrene polymerization with a higher activity. Notably, the coordination environment also significantly influenced the selectivity. This was demonstrated through the electron withdrawing pyridyl-Cp-based scandium bis(allyl) complex (26-Sc) and (Ph3C)(B(C6F5)4), which provided the best catalyst system. Therefore, this system afforded the perfect syndiotactic sPS (syndiotactic polystyrene) (rrrr >99%) with high molecular weights and narrow weight distribution (MW/Mn ¼ 1.40). Lanthanide complexes were also able to catalytically polymerize heterocyclic a-methylene-g-butyrolactone (MBL) and g-methyl-a-methylene-g-butyrolactone (gMMBL).69 The two ansa-half-sandwich rare earth di(alkyl) complexes, [(C2H4(5-FluNHC))Ln(CH2SiMe3)2] (29) (Ln ¼ Y, Lu), and [(5-C12H8)2Sm(THF)2] (30) feature constrained coordination geometries. The fluorenyl complexes catalytically polymerized lactones through a coordination–addition mechanism and exceeded the activity of ansa-samarocene complex by a factor of 22 in TOF. In addition, these half-metallocene complexes catalytically converted b-methyl-a-methylene-g-butyrolactone and induced copolymerization with gMMBL. Rare earth borohydride half-metallocene complexes [Cp Ln(BH4)2(THF)2] (23) (Ln ¼ La, Nd) were also studied with regard to polymerization reactions, especially targeted toward the chain transfer copolymerization of isoprene and styrene.70 Here, efficient transmetalation processes occurred in the presence of dialkylmagnesium reagents where trans-1,4-selectivity gradually decreased

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concomitant with a rise of both 3,4-selectivity and styrene content in the copolymer. This could be partially traced back to a Mg-induced copolymerization. In contrast, additional tri(alkyl) aluminum agents resulted in a trans-1,4 specific reversible coordination chain transfer copolymerization, giving rise to several growing poly(trans-1,4-isoprene-co-styrene) chains for each catalyst metal.

4.04.1.3.2.2 Metallocene complexes The influence of the BH4− vs. more commonly employed Cl− leaving group in precatalysts for the polymerization of ethylene and isoprene was probed eventuating in the first ethylene and isoprene polymerization catalyst derived from borohydride-based precatalysts.71 Treatment of [Cp 2Nd(BH4)(THF)] (31) and [Cp 2Nd(m-Cl)2Li(OEt2)2] (32) with an excess of nBuEtMg as cocatalyst gave rise to highly catalytically active complexes for ethylene and isoprene polymerization. The maximum achievable activity for ethylene polymerization of the BH4/Mg system was 4700 kg/mol Nd/h after 1 min with 50 equivalents of the cocatalyst, which assorts well with the activity of lanthanide metallocene catalysts based on chlorides. Thus, the influence on the catalytic activity imposed by the leaving group is negligible with large concentration of cocatalyst. Remarkably, a 87% trans-selective polymerization of isoprene was achieved with stoichiometric amounts of nBuEtMg.72 Phenyl-substituted mono- and bis-Cp complexes of neodymium, [CpRNdCl3K(THF)x] (33) (CpR ¼ 1,3-Ph2C5H3, 1,2,4-Ph3C5H2) and [NdCpR2Cl2K(THF)x] (34) (CpR ¼ 1,3-Ph2C5H3, 1,2,4-Ph3C5H2, 1,2-Me2-4-PhC5H2, 1,2-Ph2-4-(4MeO-C6H4)C5H2)), functioned as catalysts in the presence of Bu2Mg, acting as a chain transfer reagent, for ethylene oligomerization.73 Complexes with tri(aryl)-substituted Cp ligands exhibited improved catalytic activity and were used to synthesize 1-iodoalkanes through iodine addition to the PE2Mg (PE ¼ oligoethylene chain) after polymerization. The neodymium complex with a methoxyphenyl-substituted Cp showed the highest activity which differs from findings in the area of zirconocene catalysis. The disparity may originate from the nature of the cocatalysts as organoaluminum compounds being strong Lewis acids are employed in zirconocene catalysis versus weaker Lewis acidic organomagnesium complexes are used in catalysis initiated by the parent cyclopentadienyl neodymium complexes. The tri(aryl)-substituted cyclopentadienyl complexes [(CpR)2LnCl2K(THF)x]2 (Ln ¼ Nd, x ¼ 2, CpR ¼ 1,2,4-Ph3C5H2, 1,2-Ph2-4-(p-MeOC6H4)C5H2; Ln ¼ Tb, x ¼ 0, CpR ¼ 1,2,4-Ph3C5H2; Ln ¼ Nd, x ¼ 4, CpR ¼ 1,2-Ph2-4-(m-MeOC6H4)C5H2))73,74 were both reacted and activated with [(BHT)Mg(THF)2nBu] (BHT ¼ 2,6-di-tert-butl-4-methylphenoxy) to afford LndMg alkyl ate-complexes (35) which showed cooperative catalytic activity, specifically in coordinative chain transfer polymerization (CCTP).72 The longer alkyl chains of this LndMg compound entail higher solubility of the catalytically active species and thus, cause higher Mn in ethylene polymerization reactions. Importantly, the MgII ion corresponds to the active site in the LndMg complexes, however, in order to be active, the coordination of the lanthanide fragment is a requisite. The highest activity was found for the 1,2-Ph2-4-(p-MeOC6H4)C5H2-containing NdIII complex, giving rise to a high degree of polymerization. A bulky BHT ligand increases the length of the PE chain by a factor of 2 or more. The rise in polymerization degree (Pn) originates from a single oligoethylene chain per a Mg atom to form a [(m-BHT)2Mg2(PE)2] species, whereas application of widely used MgnBu2 provides the growth of two PE chains to form the Mg2(PE)4 product. This product has lower solubility which therefore limits the overall chain length. The yttrium complexes with two differently substituted Cp rings, [Cptet0 YCpR(m-Me)2Li(THF)2] (36) (CpR ¼ Cp, Cp , CpMe4SiMe2H and C5Me4CH2Ph), were investigated in single-component polymerization reactions of ethylene in the absence of a cocatalyst.75 All complexes showed high activities and the substitution pattern of the Cp ligands yielded diverse outcomes from up to 50 times more efficiency for persubstitution vs no substitution, and 30% less activity for Cp rings with benzyl and SiMe2H functionalities. The differences in activity were assigned to the Lewis acidity of the Y ion which is a function of the bound Cp derivatives. In summary, the catalytic performance of the parent molecules substantially exceeds that of previously reported yttrocene alkyl complexes.

4.04.1.3.2.3 Ansa-lanthanidocenes Ansa-lanthanidocenes consisting of allyl, fluorenyl, and cyclopentadienyl ligands [{Flu-CMe2-(Cp)}Ln(3-C3H5)(THF)] (37) (Ln ¼ Y, La, Nd, Sm) were effective single-component catalysts, with 37-Nd being the most powerful one, for the production of pure syndiotactic polystyrenes.76 The activity increased with enhanced temperature and was attributed to a preactivation step, possibly related to the dissociation of THF from the Nd ion. 37-Nd was also remarkably robust with activities as high as 1.7  103 kg mol−1 Nd−1 h−1 at 120  C. The ansa-lanthanidocene complexes [rac-(CMe2(Ind)2)Ln(allyl)(THF)n] (38) (allyl ¼ C3H5, d(SiMe3)2C3H3, n ¼ 1, Ln ¼ Y, Nd and n ¼ 0, Ln ¼ Y, Nd for each allyl, respectively) polymerized styrene as single-component catalysts to yield highly isotactic polystyrene products.77 Computations revealed a coordinative-insertive mechanism of styrene into the LndC-bond with high regio- and stereoselectivity.

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4.04.1.3.3

Diene polymerization

Specific rare earth Cp complexes show polymerization reactivity toward conjugated dienes. Prominent examples for such monomers are 1,3-butadiene, isoprene and 1,3-piperylene.78 Selected complexes are discussed in the following which are partitioned into the number of Cp ligands. 4.04.1.3.3.1 Half-metallocene complexes Neutral half-metallocene bis-aluminate complexes [Cptet0 Ln{(m-Me)2(AlMe2)}2] (39) (Ln ¼ Y, La, Nd, Sm, Gd, and Lu) were isolated through an alkane elimination reaction between the tris-aluminate Ln complexes, [Ln{(m-Me)2(AlMe2)}3] and HCptet0 in good yield (76–97%).79 In the presence of one equivalent of (Ph3C)(B(C6F5)4) and four equivalents of AliBu3, the La congener initiated stereoselective trans-1,4 polymerization of butadiene (91%). In contrast, under the same conditions both 39-Nd and 39Gd afforded the cis-1,4polymerization product of butadiene (69% and 91%, respectively). The necessity for the stoichiometric additions of both (Ph3C)(B(C6F5)4) and AliBu3 suggest that the active species in this polymerization is the cationic lanthanide fragment of the complex [Cptet0 Ln(Me)(THF)3](B(C6F5)4). Similarly, the cationic lanthanide allyl part of the complexes [Cptet0 Ln(3-C3H5)(THF)3](B(C6X5)4) (40) (Ln ¼ Y, La; X ¼ H, F), Fig. 3, polymerized also butadiene where a higher conversion was reached with 40-Y (>99%) as opposed to 40-La (13%).80 The Y homolog showed much greater conversion in comparison to the La complex. Noteworthily, both complexes provide satisfactorily control of the polydispersity with high cis-1,4content (86% and 68% for 40-Y and 40-La respectively). Their preparation involved first treatment of the respective tris-allyl compound with HCptet0 , and subsequent reaction with (HNEt3)(BPh4) and (NPhMe2H) (B(C6F5)4), respectively. The half-sandwich lanthanide aluminate complexes, [Cp Ln(AlMe4)2] (41) (Ln ¼ Y, La, Nd) yielded after activation with (Ph3C) (B(C6F5)4) catalytically active complexes for the polymerization of isoprene.39 While exceptional yields (>99%) were achieved, the stereoselectivity of the reaction hinged heavily on the nature of the cocatalyst, however, these generally favored the formation of trans-1,4-polyisoprene. The treatment of 41-La with the Lewis acid B(C6F5)3 in chlorobenzene initiated an exchange of one methyl group through a C6F5 ion to form, [{[Cp La{(m-Me)2AlMe(C6F5)2}][Me2Al-(C6F5)2]}2] (42), Fig. 4, which is postulated to shed light on the active species within the catalytic pathway. The Cptet analogs, [CptetLn(AlMe4)2] (43), (Ln ¼ Y, La, Nd, Lu) were similarly isolated from protonolysis reactions of the homoleptic [Ln(AlMe4)3] complexes with HCptet.81 Their treatment with either (Ph3C)(B(C6F5)4) or (PhNMe2H)(B(C6F5)4) formed catalysts for the polymerization of isoprene with both high activity and trans-1,4-selectivity (93.4%). Interestingly, the reduction of steric bulk at the cyclopentadienyl-based ancillary ligand, traversing from Cp − to Cptet−, led to a reduction in stereocontrol and catalytic activity. Similarly, upon activation with boron-containing cocatalysts such as (Ph3C)(B(C6F5)) or (PhNMe2H)(B(C6F5)4), [Cp00 Ln (AlMe4)2] (44) and [Cptet0 Ln(AlMe4)2] (39) catalyzed isoprene and produced trans-1,4 polyisoprene.82 The larger the metal ion, the higher the selectivity observed for the trans-1,4-polyisoprene (Y < Nd < < La). The selectivity toward the latter was enhanced through deliberate modification of the substituents on the cyclopentadienyl-based ligand. The tendency to form the trans-1,4 species is proportional to the stability of the ancillary cyclopentadienyl ligand, meaning as the propensity for ligand degradation decreases, the selectivity for the trans-product increases. This is shown by the increase in stereoselectivity observed between the various cyclopentadienyl derivatives, Cp00 −, Cp000 −, Cptet0 −, and Cp −, attributed to the stability of the catalytically active, cationic complex. These factors led to the highest stereoselectivities for the catalytic systems [Cptet0 La(AlMe4)2] (39-La) and [Cp La(AlMe4)2] (41-La).

Fig. 3 Structure of [Cptet0 Y(3-C3H5)(THF)3]+ cation in a crystal of [Cptet0 Y(3-C3H5)(THF)3](BPh4) (40-Y).80 Pink, orange, red, and gray spheres represent Y, Si, O, and C atoms, respectively; H atoms and the BPh4¯ counter anion have been omitted for clarity.

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Fig. 4 Structure of [{[Cp La{(m-Me)2AlMe(C6F5)2}][Me2Al-(C6F5)2]}2] (42).39 Pink, dark blue, green, and gray spheres represent La, Al, F, and C atoms, respectively; selected F and C atoms have been faded and H atoms have been omitted for clarity.

The trinuclear half-metallocene methyl-bridged rare earth metal complexes [Cp Ln(m-Me)2]3 (45) (Ln ¼ Y, Lu) were demonstrated to be highly active toward the polymerization of both 1,3-dienes and isoprenes in the presence of a borate cocatalyst such as (Ph3C)(B(C6F5)4) or (PhNMe2H)(B(C6F5)4) promoting the formation of high contents of cis-polybutadiene (>95%).83 Notably, both the yields and preferred product of the polymerization of isoprene, being the cis-1,4 product, were highly dependent on the identity and concentration of the co-cocatalyst. Donor molecules, such as THF, entailed nucleation alternation giving rise to polyisoprene with a high vinylic content. RE half-metallocene borohydride complexes [Cp RE(BH4)2(THF)x] (23) (RE ¼ Sc (x ¼ 1, 1. 5), Nd (x ¼ 2)) also polymerized isoprene.37,84 The compounds were synthesized from a salt metathesis reaction involving [Ln(BH4)3(THF)1.5] and KCp in moderate yields. Upon activation with (Ph3C)(B(C6F5)4) and AliBu3, 23-Sc polymerized isoprene with a high selectivity for the cis-conformer (up to 97%), akin to the aforementioned alkyl complexes.84 By contrast, in the presence of the catalyst MgnBu2, 23-Nd catalytically polymerized isoprene to the trans-1,4 product (98.5%).85 The trans-selectivity was postulated to arise from [Cp Nd(mBH4)Mg] as the active species. The heterodinuclear complexes [(C5Me4RCH2PPh2)Ln(CH2SiMe3)2(OC4H8)PtMe2] (46) (Ln ¼ Y, Lu), stemming from a protonolysis reaction between [Ln(CH2SiMe3)3(THF)2] and (C5Me4RCH2PPh2)H (R ¼ SiMe2 or CH2) polymerized isoprene in the presence of (Ph3C)(B(C6F5)4) and Al(iBu)3.86 There was a large discrepancy in activity between the two yttrium complexes differing in the SiMe2 and CH2 tethers of the phosphino alkyl-substituted cyclopentadienyl ligands (0.15 g (mol Y-Pt)−1 h−1 and 0.95 (mol Y-Pt)−1 h−1 respectively) where the SiMe2 pendant arm gave lower reactivity.

4.04.1.3.3.2 Metallocene complexes The isostructural lanthanide aluminate complexes [Cp 2Ln{(m-Me)AlMe2(m-Me)}2LnCp 2] (47) (Ln ¼ Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Tm, Yb and Lu) received considerable attention for their reactivity toward catalyzing the polymerization of butadiene.87,88 These complexes have varied catalytic activities and microstructures, depending on which lanthanide ion is employed. In the presence of an equimolar amount of (Ph3C)(B(C6F5)4) and a 3–5 M excess of AliBu3, all complexes except 47-Yb were able to polymerize butadiene in yields ranging from 4% (47-Tm) to 87% (47-Nd), Fig. 5.89 The microstructure of the formed polybutadiene depends also on the metal used. The Ce congener produced mainly trans-1,4-isomers (93.8%), whereas the Tb analog afforded primarily cis-1,4-isomers (72.4%). Overall, the selectivity of the trans-1,4-polymerization was enhanced when complexes of the lighter RE metals were employed as catalysts. Mono(borohydrido) lanthanidocene complexes [Cp2Nd(BH4)(m-BH4)2Mg(THF)4] (48) and [(CMe2C5H4)2Ln(BH4)(mBH4)2Mg(THF)3] (49) (Ln ¼ Nd, Sm) were catalytically active for the polymerization of isoprene.90 In the presence of nbutylethylmagnesium as a cocatalyst, both complexes favored the formation of trans-1,4-polystyrene with selectivities as high as 93.1%.90 Although 48 reigns superior out of the series with a yield of 83%, these complexes were not as active as the half-metallocene derivative [CpNd(BH4)2(THF)2] (50) likely owing to steric effects of the additional cyclopentadienyl ligand.

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Fig. 5 Microstructure of polybutadiene catalyzed by [Cp 2Ln{(m-Me)AlMe2(m-Me)}2LnCp 2]/AlMe3/(Ph3C)(B(C6F5)4). Conditions: in toluene; total volume ¼ 20 mL; polymerization temperature ¼ 25  C; butadiene ¼ 0.54 g (1  10 mol); Ln ¼ 5  10 mol; (AlMe3)0/[Ln]0 ¼ 3; {(Ph3C)(B(C6F5)4)}0/[Ln]0 ¼ 1.89

4.04.1.3.3.3 Ansa-lanthanidocenes The ansa-bis(indenyl) yttrium complex rac-[{Me2C(Ind)2}Y(1,3-(SiMe3)2-C3H3)] (51) was probed for the CCTP which poses an attractive alternative to living polymerizations that circumvents the prerequisite of only one growing polymer chain per metal center.91 The key step of such a polymerization is a fast and reversible transfer of the growing polymer chain between an active propagating metal center and an inactive metal-alkyl moiety. As a function of the chain transfer agent (Et2Zn or MgnBu2), various microstructures spanning from highly trans-1,4-polyisoprene (PI) to trans-1,4-co-3,4-PI were isolated. The majority of CCTP polymerizations led to cis-1,4-microstructures, although the trans-1,4-counterpart had been reported on one account. Furthermore, the complex initiated highly isospecifically polymerization of styrene which was further extended to the simultaneous and sequential copolymerization of styrene and isoprene giving rise to the first example of a trans-1,4-PI-co-3,4-iPS copolymer. The blockwise distribution of the monomers that compose these polymers were shown through NMR, GPC, and DSC characterization.

4.04.1.3.4

Hydrofunctionalization

This subchapter summarizes recent advances in the fields of catalytic hydroboration, -silylation and -amination reactions. In general, these reactions refer to the addition of the respective moiety (borohydride, siliconhydride and amino, respectively) to an unsaturated substrate.92–94 Mechanistically these reactions can be understood as migratory insertions into a MdX (X ¼ H, SiR3 or BR2) bond.94 An array of recyclable polymer-supported organolanthanide hydroamination catalysts were synthesized.95 Nine different polystyrene resins were used as support for three different hydroamination/cyclization catalyst precursor complexes, [Cp 2SmCH(SiMe3)2] (52-Sm), [Cp 2LaCH(SiMe3)2] (52-La), and [(CGC)SmN(SiMe3)2] (53), where CGC ¼ Me2Si{(5-Me4C5)(tBuN)}. These complexes could be immobilized onto the various polystyrene resins allowing the generation of precatalysts for intramolecular hydroamination/cyclization reactions. These compounds showed similar catalytic activities to their homogeneous precursors while being recyclable with only a slight decrease in catalytic activity depending on the amino substituents on the polymer resin. The four yttrium metallocene complexes, [Cptet0 2YH(THF)] (54), [Cp 2Y(CH2SiMe3)(THF)] (55), [Cp2Y(CH2SiMe3)(THF)] (56), and [Cptet0 2Y(CH2SiMe3)2(THF)] (3-Y) catalytically reduced alkoxysilanes with HBpin, where pin ¼ pinacolato.96 Notably, pure trimethylsilylalkyl derivatives [Y(CH2SiMe3)3(THF)2] and [Sc(CH2SiMe3)3(THF)2], were catalytically inactive. The increase in steric bulk around the metal center, from the bulky Cptet0 ligand in comparison to the other cyclopentadienyl ligands, augmented the yields of the Me2PhSiH product. The complex containing the bulkiest Cp derivative, [Cptet0 2YH(THF)] (54), reduced various alkoxysilanes including Me2(nOct)SiOMe, Et3SiOMe, PhSi(OMe)3, MePhSi(OMe)2, and Ph2Si(OMe)2 in yields varying from 6% to 49%. The proposed catalytic cycle, Scheme 7, considers the possibility of coordinating alkoxyboranes such as MeOBpin to suppress

Scheme 7 Possible pathway for the catalytical reduction of alkoxysilanes with the borane HBpin.96

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the catalytic activity of the Y complex, similar to that of the other three yttrium complexes, 56, 3-Y, and [Y(CH2SiMe3)3(THF)], which consist of a less sterically hindered reaction site at the metal. (+)-Neomenthyl and (−)-phenylmenthyl substituted cyclopentadienyl yttrium complexes are catalysts for the asymmetric hydroamination/cyclization of aminoalkenes. Here, the nonplanar chiral (+)-neomenthyl and (−)-phenylmenthyl cyclopentadienyl ligands prevent the formation of a diastereomeric metallocene.97 This is an important improvement toward previous catalysts such as the first asymmetric hydroamination catalyst, which readily underwent epimerization to afford diastereomers.98 The three complexes [(5-neomenthylCp)2Y(o-C6H4CH2NMe2)] (57), [(5-(−)-phenylmenthylCp)2Y(N(SiMe3)2)] (58), and [(5-(+)NMInd)2YN(SiMe3)2] (59) demonstrated the best catalytic activity for asymmetric hydroamination/cyclization. By contrast, enantioselectivities were limited to a maximum of 38% ee for the complex with most steric bulk, 59. Phenylene-bridged organolanthanide complexes were also employed in catalytic intramolecular hydroamination/cyclization reactions. Here, the phenylene-bridge controls the distance between the two metal centers through means of ortho-, para-, or meta-substitution, Scheme 8.99 These dinuclear complexes demonstrated vast kinetic stability in relation to mononuclear monocyclopentadienyl RE complexes. The rates of aminoalkene intramolecular hydroamination reactions are largely determined off the steric strains of the catalyst in question, with a less strained catalyst having the highest rate. [p-{Cptet 2 La(N(SiHMe2)2)2}phenylene] tet (60-La), [p-{Cptet 2 Y(N(SiHMe2)2)2}phenylene] (60-Y), and [m-{Cp2 La(N(SiHMe2)2)2}phenylene] (61) showed good catalytic activity for the intramolecular hydroamination/cyclization of both aminoalkenes and aminealkynes. The para-LaIII catalyst exhibited the highest turnover frequency owing to a less sterically hindered LaIII ion compared to the YIII center. This is consistent with a diminished order of activity with increasing steric bulk. All reactions demonstrated zero-order dependence on the concentration of the substrate.

ð3Þ

Scheme 8 General pathway for lanthanide-catalyzed intramolecular hydrogenation/cyclization of amino alkenes and aminoalkynes.99

Imidazolin-2-imidinato ligands are potentially 2s,4p-electron donors to early transitions metals or metals with high oxidation states rendering them analogs to the monodentate cyclopentadienyl ligand.100 The imidazolin-2-imidazole rare earth complexes, [(5:1-C5Me4-SiMe2NImiPr)RE(CH2SiMe3)2] (62) where RE ¼ Sc, Y, and Lu, Fig. 6, were obtained from an acid base reaction between [RE(CH2SiMe3)3(THF)2] and imidazolin-2-imino-tetramethylcyclopentadiene, Eq. (3). These complexes display short LndN bonds distances, indicative of strong electron donation from the imidazolin-2-imide ligand. These “constrained geometry” complexes served as precatalysts for the hydroamination/cyclization of aminoalkenes. The catalytic rate accelerated with increasing size of the metal ion, as seen previously with other catalysts, with the Y reactions proceeding with the fastest rate. In pharmacologically active compounds, the dearomatized products are prominent motifs which provide useful intermediates in the synthesis of a peculiar case of 1,2-regioselective organolanthanide-catalyzed pyridine dearomatization, benefitting from [Cp 2LaH]2 (63) as the precatalyst.101 Functional group compatibility and regiospecific synthesis of 1,2-dihydropyridines were observed. DFT calculations revealed a turnover-determining [Cp 2LaH(pyridine)(pinacolborane)] (64) “resting state” and

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Fig. 6 Structure of [(5:1-C5Me4-SiMe2NImiPr)Ln(CH2SiMe3)2] (62-Y).100 Pink, orange, blue, and gray spheres represent Y, Si, N, and C atoms, respectively; H atoms have been omitted for clarity.

identified two turnover-determining transition states, namely dissociation of pinacolborane from the [Cp 2LaH(py)] active catalyst and 1,2-addition of the LadH bond to pyridine C]N unsaturation. The 1,2-dearomatization product was obtained in 87% yield from 4-iodo-1,2-dihydropyridine and 4,4,5,5-tetramethyl-1,3,2-dioxaborolane.

4.04.1.3.5

Ring opening polymerization

Ring opening polymerization (ROP) reactions are a special subtype of polymerization reactions that rely on specific cyclic monomers. Popular monomers comprise lactones such as lactide, b-butyrolactone, d-valerolactone and e-caprolactone, as well as strained cyclic monomers like morpholine-2,6-dione, trimethylcarbonate or hexamethylcyclotrisiloxane.102 The thermodynamics of ROP revolve around the release of monomer ring strain upon polymerization. The half-sandwich samarium borohydride complex [Cp Sm(BH4)(THF)2] (65) possesses polymerizing capabilities, unlike the bisborohydride counterpart, [Sm(BH4)2(THF)2] (66).103 When activated with AliBu3 or a borate/Al mixture, the bisborohydride complex (66) was shown to be moderately active toward styrene polymerization. By contrast, no polymerization activity was observed toward styrene upon activation with AltBu3 for the half-sandwich borohydride (65). Ring opening polymerization reactions could be detected with remarkable activity for both the half-sandwich borohydride and the bisborohydride compounds with e-caprolactone. 65 led to narrow polydispersities (PDI ¼ 1.2–1.4) and a higher activity, in comparison to 66. This activity was hypothesized to occur through the monomer initially inserting into the SmdH bond to produce Cp LnOR species which undergo oxidation in the second step. The obtained molecular weights for the half-metallocene borohydride correspond to one polymer chain per two SmII centers at low monomer to catalyst ratios, while one polymer chain per each SmII chain corresponds to twice the amount of monomer. A series of half-metallocene complexes containing furyl-functionalized Cp ligands were investigated for their capabilities as cationic polymerization catalysts.104 The parent complexes [(5-C5Me4SiMe2(C4H2-RO-2))RE(CH2SiMe3)2(THF)] (67) (RE ¼ Sc, Y, Lu; R ¼ H, Me) showed weak styrene polymerization activity due to the tendency to undergo intramolecular furyl-ring opening when transformed into the active cationic complex through alkane abstraction with (Ph3C)(B(C6F5)4) resulting in the formation of inactive monomeric yne-enolate complexes. When activated with (HNEt3)(BPh4), the cationic complexes [(5-C5Me4SiMe2 (C4H2RO-2))Lu(CH2SiMe3)(THF)3](BPh4) (68) (R ¼ H, Me) readily formed, which featured distinct stability under the polymerization conditions. The polymerization activity of these cationic complexes was not probed. The homoleptic lanthanide metallocene complexes CpMe 3 Ln (69) and Cp3Ln (1), (Ln ¼ Y, Sm, Er and Ln ¼ Sm, Er, respectively) acted as single-component initiators for the ROP of e-caprolactone.105 69-Y showed the highest activity yielding a quantitative monomer conversion and Mn ¼ 25.3  103 after 1 h at room temperature. This observation indicated that the steric demand of the ligand (R ¼ H vs. Me) had a large influence on the polymerization activity, whereas the metal ion size inversely effected the activity with the largest ion (Sm ¼ 0.96 A˚ , CN ¼ 6) exhibiting lowest activity and the smallest (Y ¼ 0.89 A˚ , CN ¼ 6) featuring the highest activity.106 With rising temperature, a higher monomer conversion with diminished polymer weights and broader weight distribution was attained for 69-Y, possibly owing to a higher number of emerging side reactions. Upon lowering the initiator concentration to 0.07 mol%, a surge in Mn at 60  C was observed accompanied by slightly declined monomer conversions (87%). Akin investigations with [CpMe 2 Y(m-OCH2CF3)]2 (70) afforded lower activity relative to 69-Y. Bis(phosphinimino)methanide half-metallocene lanthanide complexes [Cp LnCl(CH(PPh2NSiMe3)2)] (71) (Ln ¼ Y, Sm, Yb) initiated through addition of an equimolar amount of HOiPr the polymerization of e-caprolactone which resulted in very narrow molar mass distributions (Mw/Mn < 1.20) and high control over Mn.107 The in situ alcoholysis of the half-metallocene chloride precursor represents the first example of the direct utilization of a rare earth alkoxide initiator that efficiently polymerizes a cyclic ester. The narrow size distributions as well as the number of active sites per metal are in good agreement with the generation of only one type of active species. The metal center appeared not to have a significant influence on the polymerization efficiency and yield. The addition of excess HOiPr resulted in increased molar masses and less than one active chain per metal center. Notably, a threefold excess of alcohol resulted in a narrow mass distribution. Living characteristics were confirmed to occur by carrying out a double monomer addition experiment, which exhibited only one peak in the size exclusion chromatogram.

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Mixed ligand yttrium alkyl complexes [Cptet0 (L)Y(CH2SiMe3)] (72) (L ¼ 4,6-(CH3)2-2-[(MeOCH2CH2)2-NCH2]-C6H2-OH) and [(5-Ind)(L)Y(CH2SiMe3)] (73) (L ¼ 4,6-(CMe3)2-2-[(MeOCH2CH2)2-NCH2]-C6H2-OH) were highly active initiators for the ring opening polymerization of L-lactide under mild conditions.108 Complete monomer conversion was achieved within 2.5 h and an ascending monomer/initiator ratio caused a rise in molecular weight. Best performance was ascertained for the respective indenyl complex affording a more controllable polymerization and narrower weight distribution. This was attributed to the augmented steric environment around the metal center, imposed by two coordinating methoxy groups in the indenyl complex, as opposed to only one binding methoxy in the Cp complex.

4.04.1.3.6

Rare earth metallocene catalysis conclusions

An impressive array of half- and bismetallocene RE complexes exhibit reactivity toward the polymerization of olefins, dienes and cyclic monomers, or toward the hydrofunctionalization via borohydride, siliconhydride and amino substrates, respectively. The structural variety of both half- and bismetallocene complexes originates from the multitude of accessible substitution patterns for the Cp ligands, which enables a precise control over the polymerization activity and polymer properties. In relation to bismetallocene complexes, RE half-metallocene complexes are oftentimes less available in virtue of a challenging synthesis impeded by side reactions such as ligand scrambling. Despite the synthetic aspect and relative to bismetallocene compounds, RE half-metallocene complexes have found far more widespread application, in particular for both olefin and diene polymerizations reactions. Canvassing the field of rare earth metallocene catalysis allows the conclusion that the nature of the Cp ligand, metal, and activator impacts synergistically the polymerization activities and selectivities, Fig. 7. In fact, the efficacy of the rare earth metallocene is dictated by the substitution pattern on the cyclopentadienyl ring(s), the metal ionic size, the attached pendant arm, and the propensity to alkyl binding. In order for the catalysis to be fruitful, the identity of the cocatalyst and substrate are equally crucial. Numerous studies evinced that varying the steric demand of the Cp ligands strongly influences the activity of the catalytic systems and the stereochemistry of the resulting polymers. This is exemplified in a series of heteroleptic RE single-component catalysts [Cptet0 Y(CpR)(m-Me)2Li(THF)2], 36, (CpR ¼ Cp, Cp , CpMe4SiMe2H and C5Me4CH2Ph), where an increase of up to 50 times in efficiency was monitored for persubstitution vs no substitution, while benzyl and SiMe2H substituents decreased the polymerization activity by up to 30%.75 These discrepancies were assigned to the Lewis acidity of the Y ion which is a function of the bound Cp derivatives. Similarly, for half-metallocene complexes CpRLn(AlMe4)2 (CpR ¼ Cp , Cptet), 43, (Ln ¼ Y, La, Nd, Lu)81 the lessening of steric bulk at the Cp ligand such as traversing from Cp to Cptet, led to a reduction in stereocontrol and catalytic activity for trans-1,4 polyisoprene.82 The tendency to generate the trans-1,4 species also augmented with rising stability of the ancillary cyclopentadienyl ligand which was evidenced by enhanced stereoselectivities attained by moving from Cp00 −, Cp000 −, Cptet0 −, to Cp −. Here, a correlation with the stability of the catalytically active cationic complex was hypothesized, ultimately leading to the highest stereoselectivities for the catalytic systems Cptet0 La(AlMe4)2, 39-La, and Cp La(AlMe4)2, 41-La.82 The ligand influence was also exploited in complexes of “constrained geometry” with Cp ligands featuring pendent arms such as phosphazene-,66 immidazolin-2-imidinato-100 or aminophenyl-functions68 that can serve to finetune the steric shielding and Lewis acidity of the metal center. For instance, in a series of aminophenyl-functionalized complexes, a less shielded coordination environment and a more Lewis acidic central metal atom catalyzed the polymerization of styrene with highest activity and selectivity for syndiotactic sPS (syndiotactic polystyrene).68 The characteristics of the RE ion also substantially impacts the outcome of the polymerization reactions. For instance, in a series of isostructural tetramethylaluminate half-metallocene complexes Cp00 Ln(AlMe4)2, 44, and Cptet0 Ln(AlMe4)2, 39, the size of the metal ion was directly proportional to the selectivity of trans-1,4-polyisoprene (Y < Nd 2). In the parent molecule, QTM is likely facilitated through the low-symmetry environment for ErIII and the bending within the molecule. A systematic study of ancillary ligand effects on single-molecule magnet properties of the dysprosium metallocene complexes [Cp 2DyX(THF)] (176) (X ¼ Cl, Br, I) and Cp 2DyTp (177), supported by two Cp and differing equatorial ligands was conducted.190 Among this series the iodide complex [Cp 2DyI(THF)] featured the highest barrier to spin-reversal, Ueff ¼ 419 cm−1. All systems exhibited waist-constricted hysteresis curves with the 1D chain compound [Cp 2DyCl2K(THF)] (178) featuring the widest opened loops at applied fields up to 5 K. Calculations provided insight into the correlation of a given equatorial ligand and the resulting QTM rate. The weaker the ligand field engendered by the equatorial ligand, the slower the QTM rate concomitant with a higher barrier to spin relaxation was observed. Strong transversal components reduce the uniaxiality of the magnetic anisotropy resulting in diminished SMM properties. Here, the comparison of tunneling rates led to the conclusion that stronger ligand fields in the equatorial position to the principal axis generate faster QTM rates. The bend coordination sphere (136 ) within the probed model system [Cp 2Dy]+ would, in the absence of equatorial ligands, lead to a substantially reduced repulsion between p- and 4f-electrons near to the equatorial plane, while the p-electrons along the z-axis would further stabilize the Ising-limit state of lowest electrostatic repulsion. The approximated anisotropy barrier arising from this situation would surpass 1000 cm−1. In 2017, the first unambiguously characterized dysprosium metallocenium cation was groundbreaking in view of both its synthetic access and particularly unprecedented high magnetic blocking temperatures.191–193 Metallocenium cations are positively charged species comprising a metal ion sandwiched between two cyclopentadienyl ligands. [Cpttt 2 Dy](B(C6F5)4) (179-Dy), Fig. 25, was synthesized through chloride abstraction of Cpttt 2 DyCl employing the strong electrophile [(Et3Si)2(m-H)](B(C6F5)4) in a nonpolar solvent, benzene or hexane, Eq. (29). The discrete cations adopt a bent metallocene structure with the CpcentdDydCpcent angles of 152.56(7) /152.845(2) .191,192 CpcentdDy distances of 2.324(1) and 2.309(1) A˚ are 0.06 A˚ shorter than the chloride-containing precursor complex Cpttt 2 DyCl due to increased electrostatic attraction of the Cpttt ligands to the Dy center. Ac susceptibility measurements revealed out-of-phase susceptibility signals between 72 and 110 K under zero applied dc field, and the obtained relaxation times, t, displayed a linear correlation with the inverse temperature. A spin-reversal barrier, Ueff, of 1223/1256 cm−1, was obtained through fitting to an

+ ttt 191–193 Fig. 25 Structure of the [Cpttt Green and gray spheres represent Dy and C atoms, respectively; the 2 Dy] cation in a crystal of [Cp2 Dy](B(C6F5)4) (179-Dy). − (B(C6F5)4) counter anion and H atoms have been omitted for clarity.

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Arrhenius law,191,192 which represented a new record value at that time. Through dc relaxation experiments a 100-s magnetic blocking temperature of 53 K was determined, approaching the coveted temperature of 77 K which marks the boiling temperature of liquid nitrogen. From the divergence of zero-field-cooled (ZFC) and field-cooled (FC) measurements, the highest temperature of collected open magnetic hysteresis loops was 60 K. Computations revealed perfect axiality of the tensors up to the fourth Kramers doublet (KD), while SO-RASSI calculation results attributed all crystal field states of the 6H15/2 multiplet to one mJ state and concluded negligible mixing of the states up to the sixth KD.192 Notably, an ab initio study of the spin dynamics uncovered the ground state to first excited transition to be facilitated by the vibrational motion of the CdH groups on the Cpttt ligand.191–193 These results outlined the potential of metallocenium-based single-molecule magnets and initiated a flood of new mononuclear Cp-based complexes. The lanthanide metallocenium series [Cpttt 2 Ln](B(C6F5)4) (180) was completed for the heavy lanthanides where Ln ¼ Gd, Ho, Er, Tm, Yb, Lu,194 and Tb195. The precursor complexes Cpttt 2 LnCl (Ln ¼ Gd, Ho, Er, Tm, Lu) were readily accessible through the salt metathesis reaction of KCpttt and LnCl3, akin to 180-Dy.191,192 By contrast, Cpttt 2 YbCl was obtained from the reaction of YbI2(THF)2 with NaCpttt and subsequent oxidation with tBuCl.194 The respective metallocenium cations proceeded through chloride abstraction to yield [(Et3Si)2(m-H)](B(C6F5)4). Notably, the synthesis of 180-Tb started off with [Tb(BH4)3(THF)3] 195 followed by a transfer to Cpttt Structural parameters 2 Tb(BH4) and subsequent hydride abstraction involving (Ph3C)(B(C6F5)4). follow similar trend to the Dy congener where CpcentdLndCpcent angles range from 150.2(2) (GdIII) to 155.11(6) (TmIII) and the CpcentdLn distances gradually shorten with decreasing ionic radii (GdIII: 3.3545(5) A˚ , LuIII: 3.2455(3) A˚ , CN: 6). From an electrostatic point of view, substantiated by CASSCF-SO calculations, a pseudoaxial coordination sphere imposed by the two CpR ligands stabilizes the largest mJ projections as the ground states for HoIII and the smallest mJ projections for ErIII, TmIII and YbIII. Temperature-dependent dc-susceptibility measurements and Q-Band EPR spectroscopy collected on the ErIII, YbIII and GdIII complexes provided additional confirmation. Out-of-phase ac magnetic susceptibility peaks were collected under 0 Oe dc field in the temperature range 3–11 and 2–16 K for the TbIII and HoIII complexes, respectively, indicative of slow magnetic relaxation. The extracted relaxation times hinted at anomalously small Raman exponents as opposed to the larger exponents found for the ttt Cpttt 2 LnCl (Ln ¼ Dy, Tb, Er, Tm, Yb) complexes, suggesting that the former values are characteristic for Cp2 -ligand framework. III The Tb complex shows a butterfly-shaped hysteresis up to 10 K and hence falls behind the magnetic performance of the Dy congener. This was mainly attributed to terbium’s non-Kramers nature featuring a ground state prone to QTM due to mixing of the states. In summary, a control of spin-phonon coupling may be conceivable through careful modification of the p-ligands and ultimately clear the way to higher blocking temperatures.

ð29Þ

Investigations into the origin of the slow magnetic relaxation in high-blocking SMMs concluded an empirical linear correlation of blocking temperature and the temperature of identical Orbach relaxation and Raman relaxation times, while an obvious interrelationship of Ueff with TB remained absent.196 Single-crystal magnetization and NMR measurements on 179-Dy aimed to disentangle the relaxation phenomena present in high-blocking mononuclear single-molecule magnets.197 According to this study, to further increase the blocking temperature, the Raman relaxation mechanism needs to be influenced to shift the blocking temperature into the Orbach regime. Fitting the temperature dependence of the relaxation times in the high-temperature regime below 70 K evidenced the Raman process to be primarily driven by acoustic phonons with negligible contribution of optical phonons. A large crystal field splitting between the lowest Kramers doublets controlled by molecular geometry can suppress the Raman process. This can alternatively occur through weak magnetoelastic coupling with low-energy optical modes (vibrations influencing the local crystal field of the DyIII ion, should have high energies >30 meV), by designing a rigid first coordination sphere. Once Raman relaxation is suppressed, further suggested improvements are tied to increasing t0 and U in the Orbach process with the former through reducing the magnetoelastic coupling close to the crystal field gaps and the latter through larger crystal field splitting/axiality of the coordination sphere. The latter strategy led to the current record magnetic hysteresis temperature of 80 K for [Cp DyCpiPr5][(B(C6F5)4)] (181), Fig. 26.188 The lighter lanthanide congeners [Cpttt 2 Ln](B(C6F5)4) (179) (Ln ¼ La, Ce, Pr, Nd, Sm) were likewise obtained through the ttt dechlorination of Cpttt 2 LnCl with [(Et3Si)2(m-H)](B(C6F5)4). However, the larger ions La to Pr construct contact ion pairs [Cp2 Ln (C6F5-k1-F)B(C6F5)3] through a single fluorine atom in the solid state, whereas the smaller SmIII remained distant to the counterion in accordance with the later Ln members. Notably, such interactions could be detected through VT-NMR in the solution phase.198 The electronic ground states were probed through collection of cryogenic EPR spectra that matched the calculated spectra. The axiality of the metal ion crystal field is only slightly affected by the change from Cl− to F−. Slow magnetic relaxation under zero dc field was only observed for the CeIII complex affording a spin relaxation barrier of Ueff ¼ 51.2 cm−1. The previously established structure-Raman-relaxation-rate relation was confirmed: In the absence of monodentate ligands the Raman exponents and therefore the relaxation rates are lower, although the effect of (B(C6F5)4)− as a weakly-coordinating counterion could not be completely ruled out.

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Fig. 26 Structure of the [Cp DyCpiPr5]+ cation in a crystal of [Cp DyCpiPr5](B(C6F5)4) (181).188 Green and gray spheres represent Dy and C atoms, respectively; the (B(C6F5)4)− counter anion and H atoms have been omitted for clarity.

Modification of the substitution pattern on the Cp ligand of homoleptic lanthanide metallocene complexes dramatically impacted the magnetic blocking temperatures.199 The metallocene complexes [CpR2Dy](B(C6F5)4) (180-Dy) (CpR ¼ C5iPr4H, C5iPr4Me, C5iPr4Et, C5iPr5), were synthesized from CpR2DyI through halide abstraction with [(Et3Si)2(m-H)](B(C6F5)4), Eq. (30), Fig. 27. The respective Y complexes are isostructural. The average DydCpcent distance (2.29(1) A˚ (R ¼ H), 2.298(5) A˚ (R ¼ Me), 2.302(6) A˚ (R ¼ Et), 2.340(7) A˚ (R ¼ iPr)) and the CpcentdDydCpcent angle (147.2(8) (R ¼ H), 156.6(3) (R ¼ Me), 161.1(2) (R ¼ Et), 162.1(7) (R ¼ iPr)) rise with increasing steric demand of the fifth ring substituent. These structural trends were not directly reflected in the magnetic data. First, the divergence of the zfc-fc data increased from 28 K (R ¼ H) to 65 K (R ¼ Me) and was identical for R ¼ Et, iPr (60 K). Second, fitting of the ac magnetic susceptibility data collected between 70 K and 114 K resulted in Ueff of 1285 cm−1 (R ¼ H), 1468 cm−1 (R ¼ Me), 1380 cm−1 (R ¼ Et), and 1334 cm−1 (R ¼ iPr). In this series, the complex with the largest Cpcent-Dy-Cpcent angle (R ¼ H) showed the smallest barrier in contrast to the substantially higher barriers obtained for the other three molecules. With the addition of dc relaxation data collected between 2 K and 64 K, a complete description of operative relaxation processes deduced a Raman (20-64 K) and a QTM regime (20–2K).

ð30Þ

Fig. 27 Structures of the dysprosocenium cations in crystals of [(CpiPr4R)2Dy](B(C6F5)4) (180, R ¼ H (A), Me (B), Et (C), iPr (D)).199 Green and gray spheres represent Dy and C, respectively; the (B(C6F5)4)− counter anions and H atoms have been omitted for clarity.

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The determined 100-s blocking temperatures were Tb ¼ 17 K (R ¼ H), 62 K (R ¼ Me), 59 K (R ¼ Et), and 56 K (R ¼ iPr) and the hysteresis loops were open up to 32 K (R ¼ H), 72 K (R ¼ Me), 66 K (R ¼ Et) and 66 K (R ¼ iPr) with coercive fields of 1.4 T (R ¼ H), 2.4 T (R ¼ Me), 1.4 T (R ¼ Et) and 1.8 T (R ¼ iPr). Thus, the experimental mitigation of ring CdH-vibrations through chemical substitution amplified the blocking temperature by 45 K (from R ¼ H to R ¼ Me). The DydCpcent distance was particularly relevant in the high-angle complexes (R ¼ Me, Et and iPr). The fact that the shortest DydCpcent distance (R ¼ Me) afforded the highest Ueff inferred an axiality increase of the ligand field hinges on both enlarging the CpcentdDydCpcent angle and shortening the DydCpcent-distance. In general, increasing the steric bulk within the bis Cp scaffold causes a more axial molecule with larger DydCpcent distances. However, the outlined study provided powerful evidence for a required, judicious balance of the two structural features to achieve high-blocking SMMs. Indeed the current record single-molecule magnet was achieved through such careful optimization of the Cp scaffold and retained magnetization above the boiling point of liquid nitrogen.188 First, the heteroleptic metallocene complex [(CpiPr5DyCp ) (BH4)] (182), was synthesized, and then the BH4 ligand cleaved by [(Et3Si)2(m-H)](B(C6F5)4) to afford the cationic [(CpiPr5DyCp )](B(C6F5)4) (183) complex, Scheme 22. The DydCpcent distances are 2.296(1) A˚ (CpiPr5) and 2.284(1) A˚ (Cp ), and the CpcentdDydCpcent angle is remarkably large with 162.507(1) . Out-of-phase ac susceptibility data was observed between 82 K and 138 K, and in conjunction with magnetization decay data, the obtained relaxation times showed a very narrow distribution in the high-temperature range (a ¼ 0–0.027). At 77 K, the magnetization decayed strikingly over a 50-s period and slowed down to about 500 min at 15 K. The presence of Orbach, Raman and QTM relaxation processes were apparent from the linear dependence of the relaxation times vs. the inverse temperature within 55 and 138 K, followed by an intermediate regime between 55 and 10 K, and temperature independent relaxation times below 10 K. Taking all into account, Ueff was determined to be 1541(11) cm−1. At an average sweep rate of 200 Oe s−1, magnetic hysteresis was observed between 2 and 85 K with a coercive field of 825 Oe at 77 K and a divergence of zfc-fc data at 78 K. Ab Initio calculations disclosed a perfectly axial ground state (gx ¼ gy ¼ 0, gz ¼ 20.000) consistent with the lack of QTM in the hysteresis measurements at 2 K. The calculations support a relaxation progression through the fourth Kramers doublet, corresponding to an Orbach process with a Ueff value of 1524 cm−1 which is in excellent agreement with the experimental values. While for the 179-Dy complex191,192 the transition from the ground state to the first excited state was fostered by the CdH-oscillators in the Cp rings, in the current case the out-of-plane vibrations of the Cp ligand promoted the same transition, suggesting that a replacement of the methyl fragments could permit higher blocking temperatures.

Scheme 22 Synthesis of the heteroleptic complex [(CpiPr5DyCp )](B(C6F5)4) 183.

4.04.1.8.1.2 Cyclopentadienyl-based LnII SMMs The first neutral, linear terbium and dysprosium metallocenes (CpiPr5)2Ln (184), Fig. 28, where Ln ¼ Tb, Dy, comprising divalent lanthanide ions, were prepared through the reduction of (CpiPr5)2LnI with KC8.200 The accompanied structural change involved an increase of the CpcentdTbdCpcent angle from 159.8(4) to a perfect 180 and an increase of the average TbdCp distance from 2.356(6) to 2.416(1) A˚ , where the latter is ascribed to the result of a 4f15d1 electronic configuration. DFT calculations provided insight into the nature of the HOMO that is considerably 5dz2 and 6s in character ushering in strong mixing of these orbitals. The two LUMOs are primarily dxy/dx2-y2 character and thus overall coincide with an orbital order taking after ferrocene. The room temperature wMT values were slightly lower implying a j-j-coupling between the 4f- and 6s-orbitals due to strong orbital mixing. Outof-phase components of the ac magnetic susceptibility were observed in the temperature range 74–92 K and were fitted to yield a spin-reversal barrier of Ueff ¼ 1205 cm−1. The 100-s magnetic blocking temperature is 52 K and marks the current highest value for a terbium single-molecule magnet. This equals a fivefold surge in both Tb and Ueff when contrasted to the values for the TbIII complex [(CpiPr5)2Tb](B(C6F5)4). The differing electronic ground states account for this disparity as TbII is a Kramers ion (odd number of unpaired electrons) featuring a doubly degenerate ground state whereas TbIII is a non-Kramers ion (even number of unpaired electrons). In addition, the increased axiality reduced transverse anisotropy and as such attenuated quantum tunneling of the magnetization. Accordingly, the ramifications stemming from a reduction of DyIII (Kramers ion) to DyII (non-Kramers ion) are enormous with a resulting 100-s Tb of 5 K corresponding to a thousandfold decline. 184-Tb exhibited open magnetic hysteresis loops up to 55 K with a coercive field of approximately 0.02 T. Static and dynamic magnetic susceptibility measurements were performed on the homoleptic divalent rare earth complexes [K(crypt-222)][Cp0 3Ln] (185), (Ln ¼ Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm; crypt-222 ¼ 2.2.2-cryptand), along with their respective trivalent analogs.201 As previously outlined, [K(crypt-222)][Cp0 3Ln] were prepared through chemical reduction of the

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Fig. 28 Structure of (CpiPr5)2Tb (184-Tb).200 Dark red and gray spheres represent Tb and C atoms, respectively; H atoms have been omitted for clarity.

respective trivalent Cp0 3Ln complexes. Two possible scenarios were discussed for the electronic configuration as the additional electron may populate either the 4f or the 5d shell, where in the latter the presence or lack of coupling between f and d electrons were considered. As expected, a substantial increase in wMT was observed for all complexes relative to the LnIII complexes. The 4f n configuration was verified for the “classical” SmII, EuII and TmII ions. YII, LaII and the later lanthanides GdII, TbII, DyII, HoII and ErII surpassed predicted room temperature wMT values for the uncoupled 4fn5d1 configuration and approached the values expected for the coupled scenario. Deviations were attributed to incomplete population of the J ground state or partial quenching of the orbital angular momentum at room temperature. The DyII and HoII complexes innate to a 4fn5d1 configuration exhibited exceptionally high magnetic moments (found: 16.1 and 16.26 cm3 K mol−1; expected: 17.01 and 16.9 cm3 K mol−1, for DyII and HoII, respectively), in fact, the highest values found for any metal ion at that time. The CeII, PrII and NdII complexes preclude a clear assignment to either electronic configuration as their room temperature wMT values fall between the expected values for either electronic configuration. These deviations highlight the shortcomings of the simple LS coupling model and the limited insight provided by magnetometry alone. Neither a mixed configuration between 4fn5d1 and 4fn+1 nor a mixing of excited J states into the ground state (as suggested from the presence of temperature independent paramagnetism in CeII and NdII complexes) could be ruled out. AC magnetic susceptibility measurements performed on the whole series of divalent complexes under zero and applied fields afforded solely noisy or high-frequency tails arising from operative fast magnetic relaxation pathways and preventing the extraction of data relevant for determination of a spin-reversal barrier. With judicious selection of an appropriate ligand field, the enormous magnetic moments of divalent late lanthanide ions may boost SMM characteristics. In addition, present bulk magnetic materials contain exclusively trivalent lanthanide ions such NdIII which stems from the type of magnetic coupling between f- and the transition metal d-electrons. If the embedding of divalent late lanthanide ions into bulk magnetic materials was attainable, both the total moment of the material as well as the type of magnetic coupling would be affected, presenting an auspicious opportunity for the development of new hard permanent magnets with potentially greater maximum energy products, BHmax. Striking disparities in the structural and magnetic properties of 1,10-phenanthroline adducts and their 2,20 -bipyridine counterparts of Cp 2Yb compounds were found.202 These differing properties were ascribed to the symmetry and the number of the LUMO and LUMO+1 of the heterocyclic diimine ligands. Notably, the electronic ground states for the 3,8- and the 5,6-dimethyl-substituted Cp 2Yb(phen%) (186), Fig. 29, complexes deviate from the Cp 2Yb(bipy) adducts. The ground states of the mononuclear Cp 2Yb(phen%) complex and the respective adducts are each composed of a spin triplet, and a tripositive oxidation state for ytterbium is unambiguously assigned. By contrast, the ground state of Cp 2Yb(bipy) is a multiconfigurational open-shell singlet resulting in ytterbium to adopt an intermediate valence.203 4.04.1.8.1.3 Mononuclear metallocene SMMs with equatorial ligands The first example of a Yb3+ half-sandwich compound showing slow magnetic relaxation, albeit field-induced, was reported for the heteroleptic complex [Cp Yb(DAD)(THF)]
C7H8 (187) which was obtained through the treatment of Cp 2YbII with diazabutadiene (DAD).204 The divalent ligand oxidation state was unambiguously identified taking into account the CdC-d and CdN interatomic distances (C—C: 1.411 A˚ , NdC: 1.414 A˚ d). The room temperature wMT value of 2.27 cm3 K mol−1 agreed well with the calculated value for a single YbIII ion of 2.57 cm3 K mol−1. Frequency-dependent signals in the ac out-of-phase susceptibility were monitored under external dc field of 300 to 3000 Oe. With an optimal field of 1500 Oe, a single relaxation process was identified

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143

Fig. 29 Structure of Cp 2Yb(phen) (186).203 Pink, blue, and gray spheres represent Yb, N, and C atoms, respectively; H atoms have been omitted for clarity.

between 1.8 and 4.2 K. In consideration of all three Orbach, Raman and QTM relaxation processes for the fitting procedure of the relaxation times, Ueff was determined to be 14  2 cm−1 and the attempt time to be t0 ¼ 1.74  10−6 s. Looking further into the versatility of DAD ligands and their incorporation into SMMs, a similar approach to heteroleptic half-sandwich complexes was implemented for dysprosium.205 Through incorporation of the bidentate diazabutadiene ligand into the coordination sphere of a Cp -capped DyIII ion, a series of SMMs were synthesized, further extending the field of heteroleptic mononuclear Dy SMMs toward nitrogen-containing ancillary ligands. The two heteroleptic compounds were synthesized from the reaction of DyCl3, [DADK2(THF)n] and KCp to yield the salt free complex, [Cp Dy(DAD)L] (188), or with LiCp to form [Li(THF)3][Cp Dy(DAD)L] (189), where L ¼ THF or Cl, respectively. Both DyIII complexes, 188 and 189 showed SMM behavior between 2 and 30 K and 1.8 and 20 K, respectively, on the time scale of the ac measurements. The relaxation times were fitted considering all three Orbach, QTM and Raman processes, affording barriers to spin relaxation of Ueff ¼ 206 cm−1 (L ¼ THF, 2000 Oe) and Ueff ¼ 20 cm−1 (L ¼ Cl, 1000 Oe). Despite the similarity in structure, their dynamic magnetic properties greatly differed: while the solvated complex gave no evidence for a direct process, this majorly impacted the relaxation behavior of the chloride complex. The calculated anisotropy axis existent in the two complexes illustrated a 90 angle of both the THF and Cl ligands to the principal axis, locating them in the equatorial plane that led to transverse anisotropy contributions. Their varying relaxation properties were attributed to slight changes in the DydN distances and CpdDydDAD angles. The reaction of Cptet 2 Yb III i (THF)2 and iminopyridine 2,6-iPr2C6H3N]CH(C5H4N) produced the mononuclear [Cptet 2 Yb (2,6- Pr2-C6H3NCH(C5H4N))] (190) complex featuring a singly reduced aminopyridine ligand.206 Surprisingly, the magnetic moment of this complex is at 2 K significantly lower (1.67 mB) than the expected value for either non-interacting (4.8 mB), ferro- (3.46 mB) or antiferromagnetically (5.59 mB) coupled YbIII and an organic radical. This could either indicate temperature-induced redox isomerism or originate from a multiconfigurational ground state of ytterbium. The infusion of b-diketonate ligands into DyIII half-metallocene complexes by mixing DyBz3 or DyCl3 (for the Cptet0 complex) with HCpR afforded dibenzoylmethanoate (DBM) complexes [CpRDy(DBM)2THF] (191) (CpR ¼ Cp , Cptet0 , C5Pr4Ph).207 These molecules showcased signatures of single-molecule magnetism in the absence of an external magnetic field. The compounds inherent to CpR ¼ Cptet0 , C5Pr4Ph featured butterfly-shaped open magnetic hysteresis loops, remaining open up to 4 K for the Cptet0 -based complex. Ac magnetic measurements gave below 30 K temperature-dependent peaks in the out-of-phase susceptibility. The barrier to spin relaxation climbed from 32 to 222 cm−1 upon changing the substituents on the Cp ring from Me5 to Pr4Ph. An ab initio analysis revealed strongly axial g-values of the ground states for all three complexes. The calculated anisotropy axis was perpendicular to the molecules’ pseudosymmetry axis stemming from the negative charge of the donor oxygen atoms of DBM as compared with the oxygen atom of THF. The origin of the highest spin-reversal barrier Ueff for the Cptet0 -based complex was attributed to the electropositive silicon atom in the equatorial plane and thereby stabilizing the anisotropy axis.188 The aforementioned BPh4 complexes, Cp 2Dy(BPh4), were employed in the synthesis of the first ammonia adducts of rare earth metallocene cations.150 Depending on the solvent (THF or toluene), the reaction of Cp 2Dy(BPh4) with NH3 yielded either the solvent-free [Cp 2Dy(NH3)2](BPh4) (192), Fig. 30, or THF-coordinated [Cp 2Dy(NH3)(THF)](BPh4) (193) complexes, Scheme 23, Fig. 31. The DC magnetic susceptibility measurements of the THF-free complex signified a drastic drop in wMT below 5 K, indicative of magnetic blocking. This was further confirmed through a divergence of zfc-fc magnetization data at 4 K. AC magnetic susceptibility measurements under zero dc field revealed temperature-independent peaks between 2 and 8 K. In the temperature range 8–46 K a steady shift of the out-of-phase (wM00 ) peak maximum toward higher frequencies was observed. Hence, this ac behavior suggested the presence of multiple relaxation processes, taking the entire probed temperature regime into account. The application of an optimal external magnetic field of 1400 Oe engendered a low-frequency shift in the wM00 signal which corresponds to a lengthening of the relaxation times as a result of thermal relaxation pathways dominating over other rapid relaxation processes such as QTM. Satisfactorily fits of the extracted relaxation times at both 0 Oe and 1400 Oe dc fields were obtained through the incorporation of multiple relaxation processes including terms for QTM, Raman and Orbach relaxation, yielding Ueff of 546(6) cm−1 (0 Oe) and 609(44) cm−1 (1400 Oe), respectively. In line with bifurcation observed in the zfc-fc data, waist-constricted hysteresis loops that were closed at zero and open at higher fields, were observed up to 5.3 K. Notably, this hysteretic behavior is similar to the parent Cp 2Dy(BPh4) complex, however, the Ueff value for the bis(ammonia) complex is double that of the

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Fig. 30 Structure of the [Cp 2Dy(NH3)2]+ cation in a crystal of [Cp 2Dy(NH3)2](BPh4) (192).208 Green, blue, gray, and white spheres represent Dy, N, C, and H atoms, respectively, selected H atoms have been omitted for clarity.

Dy

NH3 N H3

2 NH3 toluene

Dy

B

[B Ph4] 192

NH3 THF

Dy

O NH3

[B Ph4] 193

Scheme 23 Isolation of [Cp 2Dy(NH3)x] (x ¼ 2: 192, x ¼ 1: 193) in different solvents.150

Fig. 31 Structure of [Cp 2Dy(NH3)(THF)]+ cation in a crystal of [Cp 2Dy(NH3)(THF)](BPh4) (193).208 Green, red, blue, gray, and white spheres represent Dy, O, N, H, and H atoms, respectively; selected H atoms have been omitted for clarity.

BPh4 predecessor. The more than 150% enhancement in Ueff was attributed to a reduction of transverse anisotropy relative to the BPh4 complex as a result of swapping an anionic ligand through two neutral ammonia ligands. 192 is the first ammonia complex to display SMM behavior.

4.04.1.8.2

Multinuclear SMMs

4.04.1.8.2.1 Halide- and chalcogenide-bridged lanthanide SMMs The static and dynamic magnetic properties of three chloride-bridged cyclopentadienyl Dy compounds, [Cp2Dy(m-Cl)]x (194) (x ¼ 2, 1) and [Cp2Dy(THF)(m-Cl)]2 (195) were probed, Fig. 32. The compounds were synthesized through salt metathesis reactions of NaCp with DyCl3 and subsequent THF extraction.209 Slow magnetic relaxation was observed in 195 through (wM00 ) peaks in the AC out-of-phase susceptibility below 12 K, and the extracted relaxation times corresponding to the thermally activated region were fit to an Orbach relaxation process giving rise to Ueff of 33.8 cm−1. Below 6 K, QTM was the dominant relaxation process. The THF-free compounds also exhibited slow magnetic relaxation, however, with two distinct relaxation processes, which were attributed to polymorphism as this compound co-crystallized with the bimetallic congener. The application of an external magnetic field successfully suppressed the QTM process occurring in the dimeric complex but was ineffective for the chain complex.

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The barriers to spin relaxation for the THF-free dimeric and chain complexes amounted to Ueff ¼ 26.3 cm−1 and Ueff ¼ 67.8 cm−1, respectively. The bimetallic complex was subjected to a Micro-SQUID study at 40 mK where at zero field no tunneling steps were detected which was hypothesized to be the result of exchange biasing. However, under fields of 0.1 T pronounced steps are observed.

ð31Þ

Fig. 32

Structure of [Cp2Dy(m-Cl)]2 (194). Green, light green, and gray spheres represent Dy, Cl and C atoms, respectively; H atoms have been omitted for clarity.

The influence of soft donor atoms in the equatorial plane on the SMM properties of polydysproscenium complexes was probed in a series of organometallic compounds. Switching the coordinating donor atom from commonly used N and O to heavier and softer group homologs was hypothesized to enhance magnetic exchange interaction between the Ln ions in conjunction with a reduction of transverse anisotropy.210 The sulfur-bridged complexes [CpMe 2 Ln(m-SSiPh3)]2 (196) (Ln ¼ Gd, Dy) were synthesized from a salt 210 The first magnetic characterization of such chalcogenate metathesis reaction of CpMe 3 Dy with Li(SSiPh3), Eq. (31), Fig. 33. complexes were undertaken. The room temperature wMT values agreed well with two uncoupled Ln ions for Dy and Gd respectively, and the wMT steadily decreased upon cooling. Taking the 2J formalism into account, for the corresponding Gd complex, the exchange coupling constant JGd–Gd was determined to be −0.105 cm−1, indicating weak antiferromagnetic coupling between the Gd ions through the thiolate bridges. From AC susceptibility measurements at zero dc field, the relaxation times for the Dy were extracted up to 32 K, revealing a thermally activated relaxation mechanism above 20 K. The high temperature regime (20–32 K) was fit to an Orbach relaxation process affording a Ueff value of 133  3.5 cm−1. The hysteresis loop collected at 1.8 K is closed at zero field and slightly open at higher fields. CASSCF calculations positioned the main anisotropy axis perpendicular to the {Dy2S2} plane. Furthermore, the quantified JDy–Dy constant is doubled for the thiolate- versus chloride-bridged complex 196 (Jcalc: −4.42 cm−1 (S), −1.91 cm−1 (Cl)). In addition, QTM and direct relaxation are much less pronounced in the S- relative to Cl-bridged complex, which is reflected in the twice as high transverse g values on the Cl-bridged complex (gx: 0.0012 (S), 0.0224 (Cl); gy: 0.0019 (S), 0.0479 (Cl)).

3 nBuLi

Dy PH2 Mes

H Mes P H

Dy P Dy Dy Mes 197-Dy

Li

H 3 nBuLi

P Mes

Mes P Dy

P

Dy Dy

2 [Li(THF)4] P Mes

ð32Þ

Mes 198-Dy

Me The trimetallic phosphide-bridged complexes [(CpMe 2 Ln)(m-P(H)Mes)]3 (197) (Ln ¼ Y, Dy) were synthesized in two steps: Cp3 Ln Me Me n 211 was initially reacted with MesPH2 to give [Cp3 Dy PH2Mes] which in the following was deprotonated with BuLi, Eq. (32). The phosphide-bridged complexes were treated with additional nBuLi to engender the phosphinidene complex [Li(THF)4]2 Me [(CpMe 2 Dy)3(m-PMes)3Li] (198), Fig. 34. The phosphide complex comprises a cyclic trimeric structure in which the {Cp2 Ln}

units are connected through m-mesitylenephosphide bridges, resulting in a chair-like conformation. An analogous conformation may also be defined for the phosphinidene complex where the m3-phosphinidene acts as a bridge between the three Ln ions and features a central Li atom above the chair plane. The phosphide-bridged complex exhibited wM00 peaks up to 31 K, while the phosphinidene complex showed slow magnetic relaxation only up to 3.6 K. Fitting the extracted relaxation times only at high temperatures to an Orbach relaxation mechanism afforded Ueff values of 210(6) cm−1 and 13(1) cm−1, respectively. Thus, the deprotonation of the phosphide complex, decreased Ueff by more than an order of magnitude.

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Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

Fig. 33 Structure of [CpMe 2 Dy(m-SSiPh3)]2 (196). Green, yellow, orange, and gray spheres represent Dy, S, Si, and C atoms respectively; selected C atoms have been faded and H atoms have been omitted for clarity.

Me 211 Fig. 34 Structure of [Li(THF)4]2[(CpMe Green, purple, lavender, and gray spheres 2 Dy)3(m-PMes)3Li] (198) (left) and [(Cp2 Dy)(m-P(H)Mes)]3‧toluene (197) (right). represent Dy, P, Li, and C atoms respectively; selected C atoms have been faded; toluene, [Li(THF)+4 ] and H atoms have been omitted for clarity.

Magnetically diluted samples where two DyIII ions were exchanged for diamagnetic YIII ions were synthesized and further diluted in 95% all-YIII complexes as a matrix that allow probing single-ion magnetism of the Dy ions in absence of DydDy interactions in conjunction with an unchanged crystal field influence. The diluted phosphide- and phosphinidene Dy complexes exhibited similar trends in the ac data relative to the pure paramagnetic Dy3 analogs, albeit the diluted phosphide complex featured a higher Ueff of 256(6) cm−1. The dilution had a larger impact on the butterfly-shaped hysteresis with a Hc ¼ 300 Oe at 1.8 K and open hysteresis up to 4.4 K at higher fields where the observable steps coincided with the calculated exchange spectrum. CASSCF calculations depicted a strong axiality of the ground- and excited doublets in the phosphide complex, while the axiality is significantly reduced for the first excited Kramers doublet in the phosphinidene complex. The quantified coupling strengths (here: Jtot ¼ Jdip + Jex, where Jdip: dipolar contribution; Jex: exchange contribution) followed the same trend (Jex ¼ −2.878 to −3.014 cm−1 (P−), −1.492 to −1.528 cm−1(P2−)). In sum, the higher charge of the phosphinidene ligands induces a stronger equatorial ligand field that counteracts the axial ligand field imposed by the CpR ligands. Similar trends were also observed for the arsenide/arsinidene and selenolate Dy complexes [(CpMe 2 Dy)(m-As(H)Mes)]3 (199), Me 212 Dy) (m -AsMes) Li] (200), and [(Cp Dy)(m-SeMes)] ‧toluene (201), Fig. 35. Field dependent magnetization [Li(THF)4]2[(CpMe 2 2 3 3 3 3 data collected on the arsenide/arsinidene complexes manifested singular plateaus which were assigned to originate from strong exchange interactions. Here, an antiferromagnetic ground state at lower magnetic fields turned at higher fields ferromagnetic. The arsinidene ligand evoked a stronger exchange as the behavior is more prominent in this complex. CASSCF calculations supported a stronger coupling (here: Jtot ¼ Jdip + Jex, where Jdip: dipolar contribution; Jex: exchange contribution) in the arsinidene complex (Jtot ¼ − 4.92 to −6.87 cm−1 (As−), −6.61 to −9.76 cm−1 (As2−)), while both exceed the extrapolated values for the selenolate complex (−4.76 to −5.02 cm−1). Notably, the dipolar contributions were almost identical in all three complexes, albeit the exchange

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

147

212 Fig. 35 Structure of [(CpMe Green, orange and gray spheres represent Dy, Se and C atoms respectively; selected C atoms have 2 Dy)(m-SeMes)]3‧toluene (201). been faded; toluene and H atoms have been omitted for clarity.

contribution changed dramatically. The overall weaker exchange coupling in the selenolate complex was assigned to the less diffuse valence orbitals of the Se atom. In view of SMM properties, the As and Se complexes progressed similarly relative to the P analogs: The concentrated samples afforded lower Ueff values than the 20:10 diluted samples (256(5) cm−1, 301(9) cm−1 (diluted) (As−); 23(2) cm−1, 35(2) cm−1 (As2−); 285(4) cm−1, 301(7) cm−1 (Se−)). This is remarkable given the isoelectronic arsinidene and selenolate complexes feature significantly different REddonor bond lengths (on average 0.079 A˚ shorter for Se) that may cause different crystal field effects. However, akin to the P homolog, the arsinidene complex is inherent to closer PndRE distances relative to the protonated complement (here: 0.128 A˚ ), leading along with more diffused orbitals to a stronger crystal field. A strongly axial ground state Kramers doublet and a from axiality deviating first excited Kramers doublet was calculated for the arsenide/arsinidene complexes closely resembling the general trend observed for the P analogs. Butterfly-shaped hysteresis loops up to 4.2, 5.4 and 4.7 K were monitored for the diluted arsenide, arsenidene and selenolate complexes, respectively. The series of heavier p-block element-bridged SMMs constitute the rare earth antimony-bridged complexes [(CpMe 2 RE)(m-Sb(H) Mes)]3 (202) and [(CpMe 2 RE)3(m-(SbMes)3Sb)] (203) (RE ¼ Y, Dy). As opposed to the lighter homologs, these complexes were n synthesized by direct reaction of the CpMe 2 RE Bu complexes with MesSbH2. Here, modeling of the experimental wMT values with and without consideration of exchange coupling interactions suggests that the experimentally derived values could stem from crystal field effects alone. This finding implied non-negligible contributions of antiferromagnetic coupling between the Dy ions which were modeled with the Lines model, where Jiso (as the only variable) amounted to −0.121 and −0.150 cm−1. The ac data collected on pure Dy3 diluted Y2Dy complexes between 5–36 K (Sb3) and 4–33 K (Sb4), respectively, unveiled a high-temperature Orbach or thermally activated QTM regime and a lower temperature Raman relaxation process. The obtained Ueff values (Sb3: 345, 345 cm−1 (diluted); Sb4: 272, 270 cm−1 (diluted)) were identical in the concentrated and diluted complexes and surpass the P-, As- and Se-bridged compounds. Butterfly-shaped hysteresis loops were identified up to 5.4 and 4 K for the diluted Sb3 and Sb4 complexes, akin to the diluted arsinidene complex.212 The diminished Ueff may originate from varying DydSb bonds and a higher charge density on the equatorial Sb donor atoms of the Sb4 complex. In comparison to lighter homologs, an elongation of the DydPn bond in the Sb3 complex is observed (0.168 A˚ longer on average relative to the As3 complex). Thus, a lengthening of the DydPn bond diminishes the influence on the splitting of the DyIII crystal field level, thereby strengthens the axial crystal field and raises Ueff. 4.04.1.8.2.2 Radical-bridged lanthanide SMMs Due to the limited radial extension of 4f orbitals, polynuclear lanthanide complexes with diamagnetic bridges typically provide little to no proof for magnetic exchange coupling between LnIII ions resulting in energy barriers to spin relaxation that originate from individual metal centers. However, the development of multimetallic SMMs featuring high blocking temperatures necessitates isolated spin ground states and strong coupling to engender high-spin ground states accompanied by the attenuation of undesirable quantum tunneling pathways. Strong magnetic exchange coupling is rather challenging to attain with lanthanide ions, but possible through the implementation of radical bridging ligands with diffuse spin orbitals that can penetrate the core electron density of the lanthanide ions. However, it was shown that the barrier to spin reversal in such systems is directly dependent on the exchange coupling constant, J, as it determines the separation between the ground state and the excited state.182,213 Consequently, the performance of SMMs is directly linked to the exchange coupling strength in such systems. The strongest magnetic exchange coupling, J ¼ −27 cm−1, in a dinuclear Ln SMM was quantified between a Gd ion and an inorganic radical ligand, namely the N23− radical-bridged Gd complex, [K(18-c-6)][({(Me3Si)2N}2Gd(THF)2)2(m-N2%)] (204).214 Unifying the axial ligand field imposed by

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Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

Fig. 36 Structure of the [(Cptet2Tb(THF))2(m-N2%)]− and [(Cptet2Tb)2(m-N2%)]¯ anions in crystals of [K(crypt-222)][(Cptet2Tb(THF))2(m-N2%)] (205-Tb) (left) and [K(crypt-222)][(Cptet2Tb)2(m-N2%)] (206) (right).182 Red, blue, and gray spheres represent Tb, N, and C atoms, respectively; H atoms and the [K(crypt-222)]+ counter cation have been omitted for clarity.

the Cptet ligands with the strong magnetic exchange coupling imparted by an N23− radical gave rise to several dilanthanide complexes featuring remarkable wide open magnetic hysteresis loops. Here, the Tb complex [K(crypt-222)][(Cptet2Tb)2(m-N2%)] (206), Fig. 36, hold the highest Tb and Hc values for a multinuclear molecular system to date.182 The synthesis of the radical-containing complexes required initial reduction of molecular dinitrogen which was accomplished by treating Cp2tetLn(BPh4) with two equivalents of KC8 to yield the lanthanide dinitrogen-bridged complexes [(Cptet2Ln(THF))2(m-N2%)] (103-Ln) bearing a diamagnetic N22− unit. The N22−-bridged complexes were subsequently reduced with another equivalent of KC8 in the presence of crypt-222 to give the THF-solvated radical-bridged complexes [K(crypt-222)(THF)][(Cp2tetLn(THF))2(m-N2%)] (Ln ¼ Gd, Tb, Dy) (205), Eq. (33). Importantly, a dissociation of the THF molecules occurred in the sterically more encumbered 2-MeTHF solvent allowing the formation and crystallization of solvent-free N23−%-radical-bridged complexes [K(crypt-222)] [(Cptet2Ln)2(m-N2%)] 206 where Ln ¼ Tb, Dy. The analogous reaction with Gd produced instead the complex [K(crypt-222)][(Cp2tetGd(2-MeTHF))2(m-N2%)] (206-B), Fig. 37, where the binding of 2-MeTHF arises from the larger ionic radius of the GdIII ion. The NdN bond in the solvent-free N23−% bridged complexes (1.392(9) A˚ (206-Tb), 1.389(12) A˚ (206-Dy)) is consistent with a bond order of 1.5, indicating the population of the N2 p orbital.215 The THF removal shrank the metal coordination number and enlarged the LndN23−dLn dihedral angles of 178.5 (2) (206-Tb) and 178.9(3) (206-Dy) as opposed to 173.45(16) (205-Tb) and 173.14(8) (205-Dy) in the predecessors. The THF displacement had crucial ramifications on the magnetic properties: The solvent-free radical complexes showed a divergence of zfc-fc magnetic susceptibility data that is significantly shifted to high temperatures (with THF: 14.5 K (205-Tb), THF-free: 20 K (206-Tb), 7.5 K (206-Dy)), open magnetic hysteresis loops at higher temperatures (with THF: 15 K (205-Tb), THF-free: 30 K (206-Tb), 8 K (206-Tb)). The Ueff for the THF-free Tb complex signified the highest determined for radical-bridged SMMs (with THF: 242 cm−1 (205-Tb), 110 cm−1 (205-Dy), THF-free: 276 cm−1 (206-Tb), 108 cm−1 (206-Tb)). The impressive advance in magnetic behavior upon THF dissociation was attributed to reduced transverse anisotropy of the LnIII ions, which together with the enlarged planarity of the Ln2N2 moiety facilitated an undisturbed antiferromagnetic coupling. The JGd–Rad ¼ − 20 cm−1 in the THF-coordinated complex 205-Gd constitutes the second strongest coupling observed for a gadolinium system. Finally, the validity of this strategy to new SMMs is reflected in the extreme magnetic hardness which is a property crucial for application in high-density information storage technology. In fact, the coercive field for the solvent-free Tb N23− complex is with 7.9 T at 10 K represents the highest value for any molecule or coordination solid, even surpassing Hc values of commercially available Nd14Fe80B6 (4.3 T at 4.2 K).

ð33Þ

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

149

ð34Þ

tet 182 % − % Fig. 37 Structure of the [(Cptet Pink, blue, red and gray 2 Gd(2-MeTHF))2(m-N2 )] anion in a crystal of [K(crypt-222)][(Cp2 Gd(2-MeTHF))2(m-N2 )] (206-B). spheres represent Gd, N, O and C atoms respectively; selected C atoms have been faded and [K(crypt-222)]+ and H atoms have been omitted for clarity.

As opposed to this highly reactive inorganic radical-bridge, organic radicals may be used for the directed synthesis of polynuclear molecular clusters. In addition, organic radicals are adapted for synthetic modification such as chemical functionalization to modify the magnetic exchange coupling. The strongest magnetic exchange interaction between a lanthanide ion and an organic radical was quantified in a bipyrimidyl (bpym) radical-bridged complex, [(Cp 2Ln)2(m-bpym%)](BPh4) (207) (Ln ¼ Gd, Tb, Dy).216 These complexes were obtained through mixing Cp 2Ln(BPh4) with neutral bpym and KC8 in a 2:1:1 ratio, Eq. (34), Fig. 38. The structural analysis confirmed a shortening of the central C2dC20 bond in bpym relative to the free ligand, indicative of the population of the bpym p orbital (free ligand: 1.501(1) A˚ ,1.396(9) A˚ ( 207-Tb), 1.401(3) A˚ ( 207-Dy)). Indeed, strong antiferromagnetic coupling was quantified for the bpym−% radical Gd complex 207-Gd with a JGd–Rad ¼ −10 cm−1. The similar trends in the temperature dependence of wMT for Tb and Dy hinted at the presence of strong magnetic exchange coupling too, giving rise to SMM behavior in the 207-Tb and 207-Dy. The obtained frequency and temperature dependent long relaxation times indicate an operative Orbach relaxation process in both TbIII and DyIII complexes, however, 207-Tb showed pronounced QTM below 3 K. The determined Ueff value is higher for the DyIII complex (44(2) cm−1 (207-Tb), 87.3(3) cm−1 (207-Dy)), where this trend is in line with other organic radical-bridged SMMs.7 The hysteresis loops collected for the Dy congener were open up to 6.5 K with a maximum coercive field of Hc ¼ 0.6 T at 3 K. At Hc ¼ 0 T, the hysteresis loops feature a quantum tunneling step which is in contrast to the N23−-radical-bridged LnIII complexes. The apparent conclusion from these studies is that the coupling between the radical- and Ln magnetic moments needs to be increased in order to obtain better-performing polynuclear SMMs. Recently, a systematic study on related dilanthanide complexes [(Cp 2Ln)2(m-5,50 -R2bpym%)](BPh4) (208) (Ln ¼ Gd, Dy; R ¼ NMe2, OEt, Me, F), Fig. 39, consisting of 5,50 -substituted bpym radical ligands revealed that more electron withdrawing substituents lead to stronger magnetic exchange coupling (JGd–Rad: −2.66(12) cm−1 (NMe2), −4.16(25) cm−1 (OEt), −9.54(7) cm−1 (Me), −11.1(2) cm−1 (F)), where the F-substituted complex surpassed the coupling strength in the complex containing the unsubstituted bpym.213 The low temperature wMT data for all complexes show signatures of magnetic blocking. Considering the orbital coefficients of the bpym−% SOMO: The introduction of electron donating substituents in 5,50 position increases the radical coefficient on the C5/C50 positions while reducing the spin density at the bridging C2/C20 and N atoms which leads to weaker magnetic exchange interaction. By contrast electron withdrawing substituents such as F, causes a reduction of the radical coefficient on C5/C50 positions concomitant with an increased spin density on the C2/C20 and N atoms, resulting in stronger magnetic exchange coupling. Importantly, the strength of magnetic exchange JGd–Rad, correlates axiality with the magnitudes of Ueff and Tb which gradually increase from NMe2 to F substitution (Ueff: 31 cm−1 (NMe2), 40 cm−1 (OEt), 82 cm−1 (Me), 93 cm−1 (F); Tb: 4 K (NMe2), 5 K (OEt), 6.5 K (Me), 7.0 K (F)). The F-substituted case possesses slightly higher values than the congener without a substituted bridge. Broken symmetry DFT and CASSCF calculations coincides well with the experimental findings. In addition, the separation between ground and first excited state corresponds to the two experimental barriers determined for the complexes containing R ¼ NMe2, OEt. The increasing coercive fields (Hc: 40 Oe (NMe2), 430 Oe (OEt), 760 Oe (Me), 580 Oe (F)) stemming from higher electron withdrawing groups infer, albeit not strictly linear, slower through-barrier processes as a function of enhanced exchange coupling strength.

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Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

ð35Þ

Fig. 38 Structure of the [(Cp 2Dy)2(m-bpym%)]+ radical cation in a crystal of [(Cp 2Dy)2(m-bpym%)](BPh4) (207).216 Green, blue, and gray spheres represent Dy, N and C atoms, respectively; H atoms and the (BPh4)− counter anion have been omitted for clarity.

Fig. 39 Structure of the [(Cp 2Dy)2(m-5,50 -F2bpym%)]¯ anion in a crystal of [(Cp 2Dy)2(m-5,50 -F2bpym%)](BPh4) (208). Dark green, blue, green and gray spheres represent Dy, N, F, and C atoms respectively; (BPh4)− counter anion and H atoms have been omitted for clarity.213

Several other multinuclear lanthanide complexes featuring organic bridging units have been investigated for their interesting SMM behavior. In particular, bridging ligands which are accessible in multiple oxidation states provide a platform to probe the role these ligands play in introducing stronger magnetic exchange coupling between two metal centers than attainable through diamagnetic bridges. The indigo-bridged dilanthanide complexes [(Cp 2Ln)2(m-ind)]n (209) (Ln ¼ Gd and Dy; n ¼ 0, −1, −2) were isolated through a propene elimination reaction between Cp 2Ln(3-C3H5) and naturally occurring indigo H2ind, Eq. (35).217 Subsequent one-electron reduction of the indigo-bridge afforded the monoanionic radical-bridged complex [K(THF)6][(Cp 2Ln)2(m-ind%)] (210), Fig. 40. The exchange coupling constant, J, was found to be J ¼ −11 cm−1 between the isotropic GdIII ion and the indigo bridging ligand in 210-Gd. Interestingly, the radical-bridged complex exhibits a modest Ueff of 39(1) cm−1. Additionally, the hysteresis is not largely affected by the radical nature of the indigo ligand, which is likely attributed to the nature of the donor atoms and their charge. The bridging non-planar bis-tridentate ligand 2,3,5,6-tetra(2-pyridyl)-pyrazine (tppz) has the ability to accept multiple electrons and to bind to two metals. Combined with Cp 2Ln(BPh4) (Ln ¼ Gd, Tb, Dy) and subsequent reduction, its employment led to the isolation of mono- and trianionic tppz radical-bridged dilanthanide complexes [(Cp 2Ln)2(m-tppz%)](BPh4) (211-Gd), Fig. 41, and [K(crypt-222)][(Cp 2Ln)2(m-tppz%)] (212-Gd), featuring a rare instance of isolable radical ligands in two distinct oxidation states, Eq. (36).218 Due to the half-filled f-electron shell, the approximately isotropic GdIII metal ions offer an exceptional platform to quantify the magnetic exchange coupling. Thus, a spin only Hamiltonian similar to what is employed for transition metals can be

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151

Fig. 40 Structure of [(Cp 2Dy)2(m-ind%)]− anion in a crystal of [K(THF)6][(Cp 2Ln)2(m-ind%)] (210).217 Green, red, blue, and gray spheres represent Dy, O, N, and C atoms, respectively; H atoms and the [K(THF)6]+ counter cation have been omitted for clarity.

Fig. 41 Structure of [(Cp 2Dy)2(m-tppz%)]+ radical cation in a crystal of [(Cp 2Dy)2(m-tppz%)](BPh4) (211).218 Green, blue, and gray spheres represent Dy, N, and C atoms respectively; H atoms and the (BPh4)− counter anion have been omitted for clarity.

used. Fitting the dc data gave rise to J parameters of −6.91(4) and −6.29(3) cm−1 for the mono- and trianionic radical-bridged complexes, respectively. These values are among the largest observed for radical-containing Gd complexes.218 The large, delocalized p -system of the radical ligand could account for the weaker exchange coupling constant in comparison to bpym radical-bridged lanthanide SMMs.219 The effective spin-reversal barriers, Ueff, for the monoanionic radical-bridged Tb and Dy 211-Tb and 211-Dy complexes were found to be 5.1(1) and 35.9(2) cm−1, respectively. The tppz%1−-containing Dy complex exhibited magnetic hysteresis up to 3.25 K with a maximum coercive field of Hc ¼ 0.1 T at 1.9 K. Despite the similar magnitudes in exchange coupling constants J for the corresponding Gd complexes 211-Gd and 212-Gd, the trianionic radical-bridged Tb and Dy lacked slow magnetic relaxation. This may originate from the larger ligand field strength imposed by stronger electron donating tppz%3− contrasted to tppz%1− in the hard plane. DFT computations additionally hint at a doublet ground state for the tppz%3− moiety concomitant with a drastically different spin density orientation toward the lanthanide ions. These results prove invaluable in successful radical-bridged SMM design defining that in addition to the radical character of the bridge, the correct symmetry must be at play.

2

Ln

B

1. tppz 2. KC8 - KBPh4

Ln = Gd, Tb, Dy

N Ln

N N

Ln

N

N

N [BPh4] 211

1. 2 KC8 2. crypt-222 - KBPh4

N Ln

N N

N

Ln

N N

ð36Þ

[K(crypt-222)] 212

Although still rare and challenging to synthesize, the number of dinuclear Ln complexes bridged by organic radicals is steadily growing. However, higher nuclearity radical-bridged molecular systems are scarce. Recently, matching the {Cp 2Ln} scaffold with the central radical-bridge hexaazatrinapthylene (HAN), afforded the neutral trinuclear complexes [(Cp 2Ln)3(m-HAN%)] (213) (Ln ¼ Gd, Tb, Gd), Fig. 42, featuring a trianionic radical bridging ligand.220 The exchange coupling constant, J, could be quantified to be 5.0 cm−1 through application of a spin-only Hamiltonian to the dc magnetic susceptibility data for the Gd complex containing S ¼ 7/2 GdIII ion. Both the Tb and Dy congeners exhibited signals in the ac magnetic susceptibility measurements, indicative of slow magnetic relaxation. Between 2 and 3 K, the Tb analog was dominated by Raman relaxation. In contrast, [(Cp 2Dy)3(m-HAN%)]

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Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

Fig. 42 Structure of [(Cp 2Ln)3(m-HAN%)] (213).220 Green, blue, and gray spheres represent Dy, N, and C atoms respectively; H atoms have been omitted for clarity.

presented much longer relaxation times that were fit to an Orbach and QTM relaxation process to yield an effective barrier to spin reversal Ueff ¼ 51 cm−1. The 100-s blocking temperature was determined to be Tb ¼ 3.0 K. [(Cp 2Dy)3(m-HAN%)] displayed open magnetic hysteresis loops up to 3.5 K with maximum Hc of 0.8 T at 2 K, including remnant magnetization, while the Tb congener lacked any hysteresis.

4.04.1.9

Divalent lanthanides

Owing to the low ionization energy of the outermost d- and s-electrons, the coordination and organometallic chemistry of the rare earth metals is primarily composed of trivalent metal centers.221 While the majority of the rare earth metals are most stable in the trivalent state, only three lanthanides are easily accessible as divalent ions, SmII, EuII, and YbII, Table 1. Until 1997,170 these were the only rare earth metals with an accessible divalent oxidation state, coining the term “classical” divalent lanthanides. Isolation of the divalent rare earth metal series was long elusive owing to the large reduction potential of the rest of the series, Table 1. The highly reducing metal center causes complexes featuring these metals to be quite unstable, and thus require large sterically hindering ancillary ligands.161 Because of this, cyclopentadienyl ligands have reigned supreme in accessing low-valent rare earth metal centers. The most advantageous properties of the cyclopentadienyl ligand scaffold are their large coordination number and ease of functionalization. Substitution of the positions around the cyclopentadienyl ring not only affords higher solubility but also serves to block highly reactive open metal sites. Through this approach, the entire series of rare earth metals have been shown to be accessible in the dipositive oxidation state. Table 1

Calculated values for the LnIII + e− ! LnII half-reaction vs. NHE and electronic configuration.

Ln

E1/2 (V) vs NHE for LnIII + e− ! LnII

Electron Configuration of LnII

Eu Yb Sm Tm Dy Nd Pm Lu Y Pr Ho Er La Ce Tb Gd

−0.35 −1.15 −1.55 −2.3 −2.5 −2.6 −2.7 −2.7 −2.8 −2.9 −2.9 −3.1 −3.1 −3.2 −3.7 −3.9

[Xe]4f7 [Xe]4f14 [Xe]4f6 [Xe]4f13 [Xe]4f10 [Xe]4f4 [Xe]4f5 [Xe]4f14d1 [Xe]4f1 [Xe]4f3 [Xe]4f11 [Xe]4f12 [Xe]4d1 [Xe]4f1d1 [Xe]4f9 [Xe]4f7d1

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

153

The first molecular, stable lanthanum(II) complexes [K(18-c-6)Et2O][Cp00 3La] (214) or [K(crypt-222)Et2O][Cp00 3La] (215) were synthesized through reduction of the tris(trimethylsilyl)cyclopentadienyl LaIII complex, Cp00 3La, by potassium in the presence of 18-c-6 or crypt-222, respectively, Scheme 24, Fig. 43.223 With the encapsulating agent, 18-c-6, in the reaction, additionally the byproduct [K(18-c-6)Cp00 ] formed. Both LaII complexes were stable dissolved in diethyl ether and in the solid state. Solid-state and solution EPR, where the latter produced a well-resolved octuplet with gav ¼ 1.990 and a hyperfine coupling constant A(139La)av ¼ 133.5 G, gave evidence of the divalent nature of the lanthanum ion. The electronic configurations of both complexes can be described as [Xe]4f 05d1 with lanthanum in the +2 oxidation state.

Scheme 24 Reduction of Cp00 3La (219) with potassium mirror or potassium graphite yielding the corresponding LaII compounds [K(18-c-6)Et2O][Cp00 3La] (218) and [K(crypt-222)][Cp00 3La].222

Fig. 43 Structure of the [LaCp00 3]− anion in a crystal of [K(18-c-6)(Et2O)][LaCp00 3] (214).223 Pink, orange, and gray spheres represent La, Si, and C atoms, respectively; H atoms have been omitted for clarity.

A direct comparison of lanthanide complexes with the same coordination environment for all lanthanides, excluding Pm due to its radioactivity, in the +2 and + 3 metal oxidation state was drawn. The LnII and LnIII complexes, [K(crypt-222)][Cp0 3Ln] (216) and Cp0 3Ln (217) respectively, were synthesized for the traditional divalent lanthanides, Eu, Yb, Sm, Tm, Dy, and Nd, on account of contrasting to the newly isolated divalent lanthanides, Pr, Gd, Tb, Ho, Er, and Lu. To complete the series of [K(crypt-222)][Cp0 3Ln], the lanthanum and cerium congeners were also prepared.224 Two of the traditionally divalent lanthanides, DyII and NdII, along with the nine new divalent lanthanides exhibit longer than expected LndCp0 cent distances. These two metal ions demonstrate properties congruent with a 4f n5d1 ground state when placed in a cyclopentadienyl ligand environment that is in line with the nine new divalent lanthanides. This differs from the configuration of the traditional divalent lanthanides, 4f n+1, prevailing for EuII, YbII, SmII, and TmII. Accordingly, the ligand environment plays a crucial part in the regulation of the ground state of lanthanide metals. The first examples of crystallographically characterizable complexes of TbII, PrII, GdII and LuII, to yield [K(18-c-6)(OEt2)] [LaCp00 3] (218), were obtained by reacting Cp00 3Ln (219) with potassium in the presence of 18-c-6 in diethyl ether at −35  C under an argon atmosphere.222 The isolation of these four molecular compounds completed the series of divalent lanthanide ions

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Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

reported in literature. It was long thought that TbII, PrII, GdII and LuII ions would not be stable as molecular species by the presumption that a 4f n+1 electron configuration was required, similar to the traditional LnII ions. However, UV–vis spectra, DFT calculations and EPR spectra support the electron configuration of 4f n5d1 for these four ions, as opposed to the common 4f n+1 configuration for the rest of the series. This exemplifies that the adoption of a 4f n5d1 configuration is made favorable by the ligand field that results from assembling three Cp ligands at a lanthanide ion. The reaction of TmI3 with KCpttt produced [Cpttt 2 TmI], which after isolation, was reduced with potassium graphite to yield a 226 Whereas X-ray quality crystals remained elusive for this complex, its neutral divalent thulium metallocene, Cpttt 2 Tm (220). composition was ascribed based on the observed magnetic susceptibility, meff ¼ 5.0 mB per mole of Cpttt 2 Tm in accordance with that ttt ttt with TmI2(THF)3 in THF of a TmII species. Cpttt 2 Tm could be oxidized to [Cp2 TmI] by employing AgI. The reaction of NaCp ttt afforded a similar NMR spectrum to that of the unsolvated Cp2 Tm, and crystals suitable for XRD of the solvated complex, II Cpttt 2 Tm(THF) (221), were isolated, Fig. 44. This donor-solvent-free synthetic route to stable, neutral, homoleptic Tm molecules from TmIII precursor complexes could be potentially expanded to other lanthanide complexes. Such class of compounds pose additionally promising candidates for the activation of molecular dinitrogen.

226 Fig. 44 Structure of Cpttt Pink, red, and gray spheres represent Tm, O, and C atoms, respectively; H atoms have been omitted for clarity. 2 Tm(THF) (221).

Through the implementation of an extremely bulky cyclopentadienyl ligand, CpBIG]¼ [(4-nBu-C6H4)5C5]−, very stable rare 225 Specifically, (2-Me2N-benzyl)3Ln (Ln¼Y, Sm, Yb, earth perarylated metallocenes CpBIG 2 Ln (222), (Ln ¼ Sm, Yb) were accessible. BIG Y) was reacted with Cp H, leading to different outcomes. The reaction involving yttrium afforded the half-metallocene complex, (CpBIG)(2-Me2N-benzyl)2Y (223), while the reactions with the lanthanides demonstrated the spontaneous reduction to the divalent 222 complex, Scheme 25, Fig. 45. The reduction of the stable SmIII to the reactive divalent samarium demonstrates the stability of the sterically congested metallocenes, as SmII possesses large reductive capabilities.

Scheme 25 Reaction of (4-nBu-C6H4)5C5H with benzyl lanthanide derivatives (2-Me2N-benzyl)3Ln, Ln ¼ Y, Sm, Yb to give CpBIG 2 Ln (222) with a postulated mechanism to its formation.225

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

155

225 Fig. 45 Structure of CpBIG Pink and gray spheres represent Sm and C atoms, respectively; selected C atoms have been faded and H atoms have been 2 Sm, 222. omitted for clarity.

The divalent lanthanide complexes Cp0 3Ln (217), where Ln ¼ La, Ce, function as four-electron reductants as was demonstrated with a new type of benzene reduction.227 Specifically, Cp0 3Ln was reacted with potassium graphite and crypt-222 in benzene for 4 h to yield [K(crypt-222)]2[(Cp0 2Ln)2(m-6:6-C6H6)] (224), Eq. (37). The reductive capability of these benzene-bridged complexes was probed through reactions with naphthalene which ultimately resulted in the isolation of [K(crypt-222)][Cp0 2Ln(4-C10H8)] (225), Eq. (37). The latter complex is the product of four-electron reduction which may arise from LnII and (C6H6)2− species. However, since (C6H6)4− could also be a four-electron reductant a definitive assignment of the origin for the four-electron reduction power is precluded. Through mixing with [{CpMo(CO)2}2(m,2:2-P2)], Cp 2Ln(THF)2 complexes were found to be well-suited precursor complexes for the generation of the first 4d-/4f-polyphosphide complexes [(Cp2Ln)2P2(CpMo(CO)2)4] (226), where Ln ¼ Sm, Yb. In the samarium reactions, fractional crystallization afforded the two additional products [(Cp 2Sm)2P4(CpMo(CO)2)2] (227) and [(Cp 2Sm)3P5(CpMo(CO)2)3] (228), Scheme 26.228 To gain insight into the reductive capabilities of the molybdenum phosphide complexes, Cp 2Ln(THF)2 was reacted either with [{CpMo(CO)2}2(m,2:2-P2)] or [Cp Mo(CO)2(3-P3)]. This reduction pathway for the 16-membered bicyclic bridged P2 complexes, 226, caused a rearrangement of the P2 unit with the major product containing a [P]P]2− unit as a result of a two-electron reduction. On the other hand, the reduction pathway for the [Cp Mo(CO)2(3-P3)] complex occurs by breaking a ModP bond to yield the formation of a new PdP bond in the P6 unit of the hexaphosphide complex, [(Cp 2Ln)2P6(Cp Mo(CO)2)2] (229), Eq. (38). SiMe3

2

Ln O

Me3 Si 217

+ 4 (crypt-222) + 4 KC8 C6H6, 4 h SiMe3 - 2 [K(crypt-222)][Cp'] Ln = La, x = 1 x Ln = Ce, x = 0

Me3 Si

Me3 Si

Ln

Me3 Si

2 Ln

THF, Ar SiMe3

2

Ln

ð37Þ

Ln = La, Ce Me3 Si

Me3 Si [K(crypt-222)]2 224

[K(crypt-222)] 225

In contrast to the divalent samarium complex Cp 2Sm commonly used in reduction chemistry, two new and one already known non-classical benzene-bridged LnII compounds, [K(18-c-6)(THF)]2[(Cp00 2Ln)2(m-6:6-C6H6)] (230) (Ln ¼ Ce, Nd) and [K(18-c-6) (THF)2][(Cp00 2La)2(m-6:6-C6H6)] (231), were used as four and three electron reducing agents, respectively.229,230 These three LnII compounds exhibited elegant reduction chemistry through the reaction with Cp Fe(5-E5) to afford [Cp00 2Ln(E5)FeCp ] (232) (Ln ¼ La, Ce, Nd; E ¼ P, As). The first ultrafast time-resolved spectroscopic studies were performed with these six complexes, and elucidated the effects of different pnictogen ligands on the electronic structure of these Ln. Not only do these six compounds represent a new class of d-f-polypnictides, but the two polyphosphides demonstrate redox activity toward P4. Two phosphorus atoms were inserted into the P53− moiety of the P5−bridged complexes to form [K(18-c-6)][Cp00 2Ln(P7)FeCp ] (233) (Ln ¼ La, Nd), which features a P73− unit. These two complexes constitute the first instances of P4 activation from a f-metal polypnictide.

156

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

Ln

P OC CO P Mo OC Mo OC

THF + THF

toluene Ln = Sm, Yb

Ln CO Mo Ln

O O

C

C

P

CO

C

O Ln

CO P C Mo

Mo

C

Mo CO

Mo

O

+

O Ln

C P

+

P O C

OC Mo

O C

Mo

P

P

O

Ln

Ln

O

C P P

O

Mo CO

P P

P

C Mo C O

Ln

OC (minor) 226

(minor)

227

228 228

Scheme 26 Synthesis of the first 4d/4f-polyphosphide complexes 226, 227, and 228.

ð38Þ

ð39Þ

The first lanthanide(II) tetradecker sandwich complex [Cp Yb(m-8,8-COT000 )Yb(m-8,8-COT000 )YbCp ] (234), was obtained from the reaction of YbI2(THF)2, [K2(DME)2](COT000 ) and KCp , Eq. (39).231 Prior to the synthesis of the parent compound, 234, the COT000 precursor was accessible through deprotonation/desilylation of 1,3,6,6-tetrakis(trimethylsilyl)cyclooctatriene with KH. The implementation of the bulky COT000 ligand inhibited a considerable bending of the multi-decker sandwich complex where the COT000 dYbdCOT000 angle of 173.8(4) and COT00 dYbdCp angles of 176.6(9) and 178.5(6) further attest the linearity within the complex, Fig. 46. The deviation from perfect linearity was attributed to electronic repulsion between the methyl groups of the substituents.

ð40Þ

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

157

Fig. 46 Structure of [Cp Yb(m-8,8-COT000 )Yb(m-8,8-COT000 )YbCp ] (234).231 Pink, orange, and gray spheres represent Yb, Si, and C atoms, respectively; H atoms have been omitted for clarity.

In multiple instances, silylcyclopentadienyl ligands were proven to stabilize the low oxidation state of LnII.51,162,222,224,232 Thus, efforts directed toward isolation of a LnI ion employed silylcyclopentadienyl scaffolds. The reduction of Cp00 2Sm(THF) with potassium graphite in the presence of crypt-222 yielded the lanthanide-in-cryptand metallocene open-cryptand complex [Sm(C16H32N2O6-k2O:k2O0 )SmCp00 2] (235), where the cryptand chelating agent was broken open, Eq. (40).233 The generation of this complex relied on the cleavage of the CdO bonds in crypt-222.233 To further probe the unusual reactivity, multiple LnII silyl metallocenes were mixed with crypt-222, whereby the less soluble Cp0 ligand was used. Exposure of the divalent Cp0 2Ln(THF)2 to crypt-222 formed solely [Ln(crypt-222)(THF)][Cp0 3Ln]2 (236) involving a ligand rearrangement, where Ln ¼ Sm, Eu. The analogous reaction conducted with the smaller Yb resulted in the corresponding THF-free YbII complex where the lanthanide is encased within the crypt-222 (237). This complex is singular due to the LnII ion residing within a crypt-222 encapsulating agent. Furthermore, this Yb-in-cryptand complex constitutes the only Ln ion of any oxidation state encompassed by a crypt-222 without an anionic ligand. The encapsulation of a LaIII ion in crypt-222 proceeded similar to the analogous LnII ion encapsulations in crypt-222, where Ln ¼ Sm, Eu, Yb, by mixing LaCl3 with crypt-222 in DMF to generate [La(crypt-222)Cl2]Cl (238).234

4.04.1.9.1

Reactivity of decamethylsamarocene and derivatives

The discovery of the first soluble organometallic SmII complex decamethylsamarocene, Cp 2Sm(THF)2, and the unsolvated congener Cp 2Sm have sparked lots of interest since their discovery in 1981 and 1984, respectively.235,236 The intriguing reactivity of Cp 2Sm stems from a highly reducing SmII center (Eo (SmIII/SmII) ¼ −1.55 V)161 that typically acts as a single-electron reductant. The bent structure of the Cp 2Sm molecule enhances the reactivity additionally as the open metal site constitutes a coordinatively unsaturated metal center, able to undergo rapid coordination to incoming substrates. This is evidenced by differing reactivity observed for the THF-adduct and solvent-free molecule.128 Within the past 15 years, decamethylsamarocene has been employed in various reactions that are outlined in the following section.

ð41Þ

The first lanthanide complexes with a redox-active sulfur diimide ligand Cp 2Ln((Me3SiN])2S) (239), where Ln ¼ Sm, Eu, Yb, were attained from mixing Cp 2Ln(THF)2 with (Me3SiN])2S, Eq. (41).237 These lanthanide metallocenes with a sulfur diimide ligand, contain the first characterized acyclic [(RN])2S]%− radical anion which was confirmed by full characterization involving single-crystal X-ray diffraction, EPR- and UV–Vis-NIR spectroscopy, and SQUID magnetometry. CASSCF/SOC-RASSI calculations support the presence of a LnIII center and a [(Me3SiN])2S]%− radical anion in these complexes. The first molecular rare earth metal polyphosphide complex [(Cp 2Sm)4(m4,2:2:2:2-P8)] (240), Eq. (42), Fig. 47, was generated by exposing divalent samarocene, Cp 2Sm, to white phosphorus, P4, for the duration of several days.238 The P8 unit in the center of the complex carries a 4− charge rendering it a larger analog to the P73− Zintl ion. The formation of P8 by means of P4 dimerization is entropically unfavorable. Therefore, this compound was synthesized through a reduction of the phosphorus unit. Each samarium in samarocene underwent a one-electron oxidation, transferring the electron to the phosphorus core, which resulted in binding each of the four {Cp 2Sm}+ fragments to two phosphorus atoms of the P8 unit. This realgar-type moiety of P8 is highly remarkable, and was previously only reported in transition metal carbonyl-containing complexes.239,240 The multinuclear antimony-samarium complex, [(Cp 2Sm)4(m4,2:2:2:2-Sb8)] (241), was isolated from the reaction of Sb0 nanoparticles with divalent decamethylsamarocene.241 The product was also engendered from an antimony/mercury amalgam after reacting in toluene for two days with Cp 2Sm in a sealed ampule. The multinuclear antimony-samarium complex is isostructural to 240238 and represents the largest f-element-polystibide to date.241

158

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

ð42Þ

Fig. 47 Structure of [(Cp 2Sm)4(m4,2:2:2:2-P8)] (240).238 Pink, purple, and gray spheres represent Sm, P, and C atoms, respectively; selected C atoms have been faded and H atoms have been omitted for clarity.

In a reductive approach, realgar (As4S4) was treated with the divalent lanthanide metallocenes Cp 2Ln(THF)2 (Ln ¼ Sm, Yb) to produce the open cage tetra- and trimetallic complexes [(Cp 2Sm)(Cp Sm)3AsS3(Cp AsS2)2(THF)3] (242) and [(Cp Yb)3As2S4 (Cp AsS2)(THF)2] (243), respectively.242 The Cp transfer gave rise to unprecedented Cp AsS22− and As2S42− anions where the latter is bound in the Yb complex. Notably, closed 11-vertex cage cluster complexes [(CptttSm)3(AsS3)2] (244) and [(Cp Yb)3(AsS3)2]  (245) were obtained from reactions of THF-free Cpttt 2 Sm with realgar, and Cp 2Yb(THF)2 with the arsenic sesquisulfide, dimorphite (As4S3), respectively. Subsequently, the treatment of the samarium cage cluster 244 with CuMes constructed the cluster [(CptttSm(THF))4Cu4S6] (246) featuring an embedded Cu4S68− cluster core by the Sm atoms, Fig. 48. This structural motif constitutes the first encapsulation of a transition metal chalcogenide cluster by f-block elements.

Fig. 48 Structure of [(CptttSm(THF))4Cu4S6] (246).242 Pink, aqua, yellow, and gray spheres represent Sm, Cu, S, and C atoms respectively; C atoms have been faded and H atoms and THF molecules have been omitted for clarity.

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

159

Multinuclear pnictogen clusters containing organometallic lanthanide-polyarsenides were also accessible through divalent samarium.243 Exploitation of the reductive capabilities of solvated and unsolvated decamethylsamarocenes, Cp 2Sm(THF)2 and Cp 2Sm, respectively, or (bistetramethyl-npropyl)samarocene (CptetR)2Sm (where R¼Me, npropyl) toward [(CptttCo)2(m,2:2-As2)2] afforded mixed d/f polyarsenide complexes [(CptttCo)2As4Sm(CptetR)2] (247). The AsdAs coupling is the outcome of the reduction of the cobalt by the highly reducing divalent samarium and not from the reduction of the pnictogen atoms. New dinuclear lanthanide isocyanotrimethylsilyl amide metallocene complexes [(Cp 2Ln)(m-N(SiMe3)NC)]2 (248), where Ln ¼ La, Sm, were obtained from the use of Cp 2Ln(BPh4) and the diazoalkane Li(Me3SiCN2) (249), Eq. (43) where the latter was accessible through the reaction of iBuLi and Me3SiCHN2.244 There are various isomers of these lithium salt conceivable, as shown through theoretical and experimental studies. In the event of the lithium salt possessing a silyl group on the carbon atom rather on the nitrogen atom, a 1,3-silyl migration is required to shift the silyl group from a carbon to a nitrogen atom. Thus, the silyl migration potentially occurred over the course of the lithium salt generation which prompted utilization of various methods to synthesize the parent compound 248-Sm. One method consists of the reduction of Me3SiCHN2 by divalent Cp 2Sm, whereas the other approach employed Me3SiCHN2 and the trivalent samarium hydride complex, [Cp 2Sm (m-H)]2, Eq. (44). Structural data attested that a nitrogen bound group over a carbon bound C(SiMe3)N2 group was preferential. Subsequently, the compounds 248 were reacted with nitriles to give 1,2,3-triazoles, [Cp 2Ln(NCCMe3)NNC (SiMe3)C(CMe3)N] (250). The mechanism of these triazolato complexes are unknown, but most likely involve a 1,3-dipolar cycloaddition.243

ð43Þ

ð44Þ

ð45Þ

Octanuclear wheelshaped lanthanide iron sulfur clusters [Fe6Ln2(m3-S)6(m,2-CO)4(CO)8(Cp )4] (251), where Ln ¼ Sm, Yb, were isolated from reduction of [Fe2(m-S2)(CO)6] with a divalent lanthanide metallocene, Cp 2Ln(THF)2, Eq. (45).245 The clusters constitute of a central [Fe6(m3-S)6(CO)12]2− fragment connecting two of the lanthanide units, {Cp Ln}, to form a 14-membered ring.245 Their generation required sufficient reducing power, for which reason the milder reducing agent Cp 2Eu(THF)2 was not able to initiate the analogous reaction. These compounds pose the first isolated lanthanide-carbonyl-sulfide-iron clusters. Mixing of the divalent yttterbium complex, Cp 2Yb(OEt2), with 2,2-bipyrimidine (bpym) and the palladium bis-alkyl complex, PdMe2, gave rise to the heterobimetallic complex [(Cp 2Yb)(m-bpym)PdMe2] (252), Scheme 27.246 The CCp∗dYb bond lengths point to the presence of a YbIII ion stemming from an electron transfer from the metal center to the bridging ligand. The shortened CdC bond length linking the two pyrimidine units supports this finding. The analogous complex, [(Cp 2Yb)(m-taphen)PdMe2] (253), Fig. 49, was synthesized in a similar fashion by using the redox-active ligand 4,5,9,10-tetraazaphenanthrene (taphen). Magnetic and NMR spectroscopic data provide evidence of different ground states for the bpym and taphen heterobimetallic complexes. The electron is lying on the bridging taphen ligand with little participation of the ytterbium ion, while a multiconfigurational ground state exists in the bpym complex with significant influence of the ytterbium metal center on the electron

160 Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

Scheme 27

Synthesis of [(Cp 2Yb)(m-bipy)PdMe2] (252, left) and [(Cp 2Yb)(m-taphen)PdMe2] (253, right) from dimethyl palladium complex, followed by respective oxidative additions.246

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

161

Fig. 49 Structure of [(Cp 2Yb)(m-taphen)PdMe2] (253).246 Pink, cyan, blue, and gray spheres represent Yb, Pd, N, and C, respectively; H atoms have been omitted for clarity.

residing on the ligand. Both complexes undergo oxidative addition upon exposure to methyliodide stabilizing a PdIV tris(alkyl) moiety. Notably, solely the bpym complex promoted a clean rapid synthesis of the PdIV product which was attributed to the difference in ground state configurations.

ð46Þ

The divalent metallocenes Cp 2Ln where Ln ¼ Eu, Yb, were heated with (Cp Al)4 at 120  C for several days to generate [Cp 2Ln(AlCp )] (254) that feature a coordinating {Cp Al} moiety and a metal-metal bond, Eq. (46), Fig. 50.247 The distance between the lanthanide and the Cp ligands is similar to that of the starting material, indicative of an intact LnII oxidation state. These two complexes constitute the first instances of bonds between aluminum and a 4f-metal where the nature of the metal-metal interaction is specified as weak. Decamethyleuropocene functioned as a one-electron reductant and reduced the redox-active bridging ligand, p-fluorophenyltetrakis(imino)pyracene, p-F-tip, to engender [(Cp 2Eu)2(m-(p-F-tip))] (255), Fig. 51.248 The p-fluorophenyl substituents on the bridging ligand assisted with the electron transfer from the divalent europium metal centers to the ligand and attenuated the steric interactions at the metal centers.248 The CdC and CdN bond lengths of the ligand further attest the occurred electron transfer from europium(II) ions to the ligand producing a trivalent complex. The transferred electrons are positioned as pairs in a delocalized orbital over the diazabutadiene fragments as well as the naphthalene moiety with pronounced NdC antibonding and CdC bonding character. Decamethyleuropocene and -samarocene, Cp 2Ln(Et2O) where Ln ¼ Eu, Sm, possessed sufficient reducing power to initiate a one-electron reduction of 1,2-bis(arylimino)acenaphthene (BIAN) ligands containing a mesityl or p-methoxyphenyl substituent which resulted in the isolation of [(Cp 2Sm)2(mes-BIAN)] (256), and [(Cp 2Eu)2(p-MeO-BIAN)] (257).249 The changes in bond lengths within 256 and 257 complexes confirm the electron transfer from the metal centers to the bridging ligand. Interestingly, the

Fig. 50 Structure of [Cp 2Ln(AlCp )] (254).247 Pink, dark blue, and gray spheres represent Yb, Al, and C, respectively; H atoms have been omitted for clarity.

162

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

Fig. 51 Structure of [(Cp 2Eu)2(m-(p-F-tip))] (255).248 Pink, blue, green and gray spheres represent Eu, N, F and C atoms, respectively; selected C atoms have been faded and H atoms have been omitted for clarity.

europium complex [(Cp 2Eu)2(tBu-BIAN)] (258) encompassing tBu groups on the bridge was generated too, however, the detected distances do not imply an electron transfer from europium onto the ligand, and thus support the presence of a Eu(II) metal center. Hence, the metal-to-ligand charge transfer with the BIAN ligand was shown to be controllable through fine tuning of ligand substituents and metal choice. The electronic charge transfer state, [(f )14-(p )0-(f )14 ! (f )13-(p )2-(f )13], of the [(Cp 2Yb)2(m-BL)] (259) 2:1 metal-to-ligand adducts, where BL ¼ tetra(2-pyridyl)pyrazine (tppz), 60 ,600 -bis(2-pyridyl)-2,20 :40 ,400 :200 ,2000 -quaterpyridine (qtp), or 1,4-di(terpyridyl)benzene (dtb), were investigated.250 Both spectroscopic and magnetic data for the neutral complexes are in conformity with a (f )13(p )2(f )13 ground state electron configuration where each ytterbium metal center donates one electron to the bridging ligand (BL) to yield a singlet bridging ligand with two Yb(III) centers. The electronic spectra for the neutral, monocationic, and bimetallic species are predominately composed of p-p and p -p transitions which mask the f-f transitions. However, these transitions were monitored upon removal of the electrons from the bridging ligand via chemical oxidation to generate the dicationic species. The lack of p electrons on the bridging ligand in the dicationic species, (f )14(p )0(f )14, results in very little interaction between the two ytterbium centers. The magnetic behavior of the complexes is heavily dependent on the bridging ligand. The complex exhibiting the qtp bridge displayed antiferromagnetic coupling up to 13 K.

4.04.1.9.2

Divalent-like reactivity

Lanthanide dinitrogen metallocenes containing trivalent metal centers function as reducing agents when combined with redox-active substrates. The reductive capability of [(Cp 2La(THF))2(m-2:2-N2)] (260) was contrasted to the well-established reducing divalent complexes, (C5R5)2Ln(THF)n (where R ¼ alkyl, H), and the two lanthanide reduction systems LnZ3/K and LnZ2Z0 /K (where Z1− ¼ N(SiMe3)2, Cptet, Cptt, Cp00 , Cpttt, and Cp ; Z0 1− ¼ BPh4 and I, e.g.). The lanthanum dinitrogen complex 260 was synthesized from the reaction of Cp 2La(BPh4) with KC8, in the presence of molecular N2.251

ð47Þ

In analogy to the lanthanide reduction systems, 260 reduced phenazine, cyclooctatetraene, anthracene, and azobenzene to yield [(Cp 2La)2(m-3:3-(C12H8N2))] (261), Eq. (47), Cp La(C8H8) (136-La), [(Cp 2La)2(m-3:3-(C14H10))] (262), and [Cp La (m-2-PhNNPh)(THF)]2 (263), respectively, Scheme 28. The lanthanum dinitrogen complex was not reducing enough to show reactivity toward stilbene and naphthalene. However, the complex was able to reduce carbon monoxide to afford [(Cp 2La)2 (m-4-O2CdC]C]O)(THF)]2 (264). Hence, the reducing capabilities of this complex are independent from the commonly employed methods of an alkali metal, steric crowing, or divalent metal center.142

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

163

ð48Þ

2 2  Scheme 28 Reactions of the N2− 2 bridged complex, [(Cp 2La(THF))2(m- : -N2)] (260) with various substrates.

The synthesis of the yttrium dinitrogen complex[(Cp2tetY(THF))2(m-2:2-N2)] (103-Y), Scheme 29, along with its reactivity especially considering the conversion of an N2 to amide was developed. First, YCl3 was treated with KCptet to yield Cp2tetY(m-Cl)2 K(THF)x, which was then reacted with allylmagnesium chloride to produce Cp2tetY(3-C3H5). Subsequently, the reaction of the allyl complex with (HNEt3)(BPh4) entailed the tetraphenylborate complex, Cp2tetY(BPh4), Eq. (48), which can be reduced with KC8 to generate [(Cp2tetY(THF))2(m-2:2-N2)] (103-Y), Scheme 29. An alternative route to the dinitrogen complex involved potassium graphite reduction of Cptet 3 Y where the latter is readily isolated from a salt metathesis reaction of the yttrium tetraphenylborate complex with KCptet. This yttrium dinitrogen complex exhibited reductive reaction chemistry akin to that of the lanthanide congeners, with phenazine, anthracene, and CO2. In addition, 103-Y was probed in azide reductions containing Me3SiN3 which formed the amide complex, Cp2tetYN(SiMe3)2 (265), Eq. (49).151

ð49Þ

164

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

Scheme 29 Synthesis of Cp3tetY and [(Cp2tetY(THF))2(m-2:2-N2)] (103-Y) from Cp2tetY(BPh4).

The reduction of [Me2Si(C5H4)2LnCl]2 with potassium yielded the alkali-free lanthanide hydride complex, [{m-[5-Me2Si (C5H4)2Ln]}2(m-H)2] (266), where Ln ¼ Y, Er, Gd.252 The lanthanide hydride complexes were further reacted with tbutylisocyanide, which inserted into the LndH bonds to form the three lanthanide N-alkylformimidoyl complexes, [{m-[5-Me2Si(C5H4)2Ln]}2 (m,2-HC¼NCMe3)2] (267).

4.04.1.10 Group 3 and lanthanide phospholyl complexes Since the emergence of organometallic lanthanide chemistry, aromatic ligands have been pushing the boundaries of coordination chemistry further. In comparison to its all-carbon containing counterpart, phospholyl ligands are not as well established due to their less readily availability, Fig. 52. In fact, the chemical substitution of a CdH-unit by a phosphorus atom requires sophisticated synthetic procedures to access such phospholyl ligands with the desired substitution patterns. A review article on phospholyl- and arsolyl f-element complexes has been published recently.253 The most convenient synthetic route to various substituted phospholyl ligands proceeds through the respective zirconium metallacycles, Cp2Zr(2-C4R4) (R ¼ H, Me, tBu), which are accessible from treatment of Cp2ZrCl2 with nBuLi and subsequent reaction with (un)substituted alkynes, Eq. (50).254,255 Large substituents on the alkynes are regiospecifically inserted into the a position to zirconium, thus enabling the synthesis of phospholes bearing bulky substituents in the 2,5-positions.256 Subsequently, PdCl-substituted chlorophosphole can be released upon reaction with PCl3,257 or extrusion of PdPh-functionalized phenylphosphole can occur when reacted with PhPCl2.254,255 The resulting phospholes can be transferred to their respective “olyl” anions that serve as transfer agents in phosphametallocene chemistry through reductive bond cleavage with alkaline metals, where Li and K are most common. Structural analysis of the 5-tetramethylphospholyl (TMeP−) lithium TMEDA (tetramethylethylenediamine) adduct confirmed the aromatic nature of the ligand, where the coordination of the Li atom is slightly shifted away from the phosphorus atom.257

ð50Þ

Of special interest is the TMeP ligand due to its comparable steric bulk to 1,2,3,4-tetramethylcyclopentadienyl (Cptet−) and 1,2,3,4,5-pentamethylcyclopentadienyl (Cp −), albeit being much less electron-rich than the peralkylated cyclopentadienyl rings.258 Furthermore, when contrasted to the all-carbon homologs, the phospholyl ligands are ambident due to the availability of an additional coordination site on the phosphorus atom.

4.04.1.10.1

General coordination chemistry and reactivity

The first prolific complexation of a phospholyl ligand to a rare earth element marks the isolation of the bisphospholyl lanthanide complexes [(TMeP)2LnCl2Li(solvent)n] (268) (Ln ¼ Y, solvent ¼ DME, n ¼ 1; Ln ¼ Lu, solvent ¼ Et2O, n ¼ 2) in 1989, Eq. (51).259

Fig. 52 Anionic phospholyl ligands with various substitution patterns.

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

Me

Me Me

Me P (CH2)4 P Me

Me

Me

Me

Li (excess) THF

Me +–

2 Li

Me

P

Me

Me

LnCl3 THF - LiCl

165

P Ln

Cl Li(Solv)2 Cl

P

ð51Þ

Ln = Y, Solv. = DME Ln = Lu, Solv. = Et2O 268

These solvent-stabilized ate-salts were obtained from salt elimination reactions involving anhydrous rare earth chlorides with solutions of lithium phospholide, where the latter was generated in situ by the reductive cleavage of PdC-bonds in 1,4-bis(20 ,30 ,40 ,50 -tetramethylphospholyl)butane. The ligand TMeP− was judiciously selected owing to its easier tractability based on the findings surrounding unsubstituted phospholyl complexes of the heavier d-block element Zr. Although crystallographic evidence for the TMeP RE complex remained elusive, the 5-coordination was established from a very characteristic 1JP-C coupling constant of 45 Hz, which is identical to the structurally characterized transition metal TMeP complexes, such as [(5-TMeP)2TiCl2] and [(5-TMeP)2ZrCl2].260 Furthermore, the 89Yd31P-coupling constant derived from 89Y-NMR spectroscopy was substantially lower than the respective 1JP-Y coupling in complexes with a direct YdP bond, such as Y-phosphine complexes, [(TMeP)2Y(mCl)2Li(THF)2] (found: 1JP–Y ¼ 6.4 Hz, expected for a phosphine complex: 1JP–Y ¼ 50 Hz).261 Surprisingly, attempts to synthesize the respective La complexes in the same manner were fruitless. The synthesis of ate-salts comprising K ions, [(TMeP)2LnCl2K] (269) (Ln ¼ Nd, Sm), encompassed use of K(TMeP) and LnCl3(THF)x (Ln ¼ Nd, Sm) in a 2: 1 ratio. Subjecting [(TMeP)2LnCl2K] to LiCH(SiMe3)2 led to TMeP Ln alkyl complexes [(TMeP)2LnCH(SiMe3)2] (270) (Ln ¼ Nd, Sm).258 An unprecedented reduction of 270 to (TMeP)2Sm (271) was observed upon exposure of a benzene solution of the alkyl complex to a H2 atmosphere, which was hypothesized to be promoted by the relatively poor p-donation of TMeP in comparison to Cp . In contrast, the analogous synthetic path with Nd produced the desired hydride complex [(TMeP)2NdH]n (272). These first findings confirmed a diverging reactivity of phospholyl and cyclopentadienyl ligands toward RE metals.262 This was further exemplified by reacting [Nd(BH4)3(THF)3] with one equivalent of KCp to engender the monomeric complex [Cp Nd(BH4)2(THF)2] (23-Nd) whose identity was deduced from IR bands at 2361, 2337, 2291, and 2216 cm−1, indicative of tri- and bidentate BH4 ligands as predicted for a monomeric complex.263 In contrast, when [Nd(BH4)3(THF)3] was treated with two equivalents of K(TMeP), the anionic complex [K(THF)][(TMeP)2Nd(BH4)2] (273) emerged as an ate-salt. This result was attributed to the weaker electron donating ability of the phospholyl ligand relative to the cyclopentadienyl ligand. With the aid of 18-c-6 encapsulation of K+, the resulting [K(18-c-6)(THF)2][(TMeP)2Nd(BH4)2] (274) complex could be crystallographically characterized.264 Diving deeper into the complexation behavior of phospholyl ligands toward rare earth elements, these ligands were probed in reactions with divalent lanthanide elements.265 The divalent phospholyl complexes (TMeP)2Ln(THF)2 (275) (Ln ¼ Sm, Yb) were obtained by either reacting the potassium salt K(TMeP) with divalent lanthanide iodides LnI2(THF)2 (Ln ¼ Yb, Sm) or the oxidative addition of the dimeric (TMeP)2 by elemental lanthanides, Scheme 30. The coordination mode was elucidated by NMR spectroscopy where the characteristic 1JCa–P coupling constant of 45 Hz was extracted from 13C-NMR measurements, indeed confirming the presence of a direct LndP p-bond. Following an analogous synthetic protocol, the crystal structure of (5-DPhP)2Yb(THF)2 (276) containing an a-phenylsubstituted phospholyl ligand 2,5-Ph-C4H2P (DPhP) was determined by analyzing crystals grown from a slowly cooled concentrated DME solution.265 The 5-coordination mode was verified, and is in accordance with previously reported 1JCa–P coupling constants (here: 46 Hz).266 Subsequently, the synthetic approach was expanded to integrate complexes comprising phospholyl ligands with an asymmetric substitution pattern. In analogy to their nitrogen-containing counterparts indene and fluorene, the phosphindole and dibenzophosphole heterocycles coordinate to lanthanide ions.267 Following well-established synthetic principles, the cleavage of the PdP-bonded dimer through lanthanide metal or the coordination of the free ligand salts with divalent lanthanide precursors led to the corresponding sandwich-type complexes of formula L2Ln(solv)2 (where L ¼ ligand). Notably, the reductive P-Cl or P-S bond cleavage of substituted phospholes (P-Cl/SPh-2,3,4,5-tetramethylphosphole) with Yb enabled the

Scheme 30 Synthesis of divalent phospholyl complexes (TMeP)2Ln(THF)2 (275, Ln ¼ Sm, Yb) from divalent lanthanide iodide salts (left), or elemental lanthanide powder (right).265

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Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

synthesis of m-Cl- and m-SPh dinuclear complexes, providing an easy access to bridged half-sandwich complexes such as [(TMeP)Yb(THF)2(m-SPh)2]2– (277).268

ð52Þ

With the goal of synthesizing the phospholyl analogs of the well-known tris(cyclopentadienyl) lanthanide complexes, three equivalents of K(TMeP) were mixed with SmCl3 in boiling toluene which surprisingly afforded the polymeric compound [(TMeP)6Sm2(KCl)2(C7H8)3] (278).269 The mechanism of its formation was interpreted as an incomplete trisubstitution of the chloride atoms of SmCl3, where indeed three TMeP ligands are bonded per Sm atom, but only two equivalents of KCl were eliminated. However, moving to the sterically less encumbered 3,4-dimethylphospholyl (DMeP) resulted in the bimetallic complex [(5-DMeP)4{m-(5,1)-DMeP}2Sm2] (279), where two of the six bisphospholyl rings act as 5,1-phospholyl ligands and are bridging two Sm ions where one Sm is interacting via 5-mode and the other via 1 mode with the same phospholyl ligand. The mean TMePcentdSmdTMePcent angle (117 ) is akin to other donor-coordinated Cp3Sm complexes such as Cp3Sm(THF).270 The versatile behavior of the phospholyl ligands in f-element chemistry was further probed through the coordination of differently substituted phospholyl ligands to a trivalent samarium ion positioned in a sterically encumbered ligand environment imposed by two Cp ligands.271 Complexes of TMeP, DMeP, and HtP (2,5-di-tertbutyl-phospholyl) were obtained from the reaction of either Cp 2Sm or Cp 2Sm(Et2O)2 with the respective biphospholes, Eq. (52). The samarium HP (where HP ¼ 2,3,4,5-H-phospholyl) complex required the isolation of the HP ligand as a Tl salt (HP)Tl prior to complexation. Strikingly, the structures revealed different coordination modes depending on the phospholyl substitution pattern: Cp 2Sm(TMeP) (280) adopts a monomeric structure with the phospholyl ligand featuring a bent, asymmetric 1-conformation (Ca1dPdSm: 127.4(1)/131.1(1) , Ca2dPdSm: 77.4(1)/75.1(1) , lineCpC–Sm–CpCdplaneTMeP: 49.8 ), Fig. 53, suggesting a weak SmdCa interaction owing to its unsaturated coordination sphere (CN ¼ 7). Cp 2Sm(DMeP) (281) displays a dimeric structure in the solid state consisting of two bisphospholylsamarocene moieties, with one m:1,5-(s-p)-bridging and a second 1-(s)-coordinating phospholyl, giving rise to two distinct coordination numbers of CN ¼ 8 and 9 per Sm. The SmdC bond distances of the 5-p-bonded phospholyl are significantly elongated relative to Cp 3Sm, once again illustrating a weaker p-donating ability of the phospholes. Similarly, Cp 2Sm(HP) (282) also formed m:1,5-(s-p)-bridged dimers in the solid state, however, with both phospholyl ligands adopting the same coordination mode giving rise to a CN ¼ 9 for each Sm. Akin to the TMeP complex, Cp 2Sm(HtP) (283) exhibits a monomeric structure with a 5-coordinating phospholyl ring and is innate to less steric congestion compared to Cp 3Sm (137-Sm) as proven by the larger CpcentdSmdCpcent angle of 124 in 283 versus 120 for 137-Sm. In summary, a rational selection of functional groups on the phospholyl ring enables control over the ligand binding mode to the lanthanide centers. Furthermore, the higher electron density on the heteroatom renders this donor atom to be the preferred coordination site for the electrophilic Sm.

ð53Þ

Fig. 53

Structure of Cp 2Sm(TMeP) (280).271 Pink, purple and gray spheres represent Sm, P and C atoms, respectively; H atoms have been omitted for clarity.

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

167

However, since s-complexation with the metal just occupies one coordination site, additional electron density from the ligand may interact with the metal to increase the coordination number. Heteroleptic phospholyl complexes [(COT)Ln(L)(THF)x] (Ln ¼ Sm, L ¼ TMeP, x ¼ 1; Ln ¼ Sm, L ¼ DSP (3,4-dimethyl-2,5-bis(trimethylsilyl)-phospholyl), x ¼ 0; Ln ¼ Nd, L ¼ DSP, x ¼ 1) containing cyclooctatetraenyl (COT2−) ligands were synthesized by salt metathesis reactions of [((COT)LnCl(THF)x)2] with K(DsP) or K(TMeP).272 Interestingly, when (TMeP)2Sm was treated with an excess of cyclooctatetraene, 1H NMR spectroscopy revealed the formation of (TMeP)2, which parallels the oxidative coordination of cyclooctatetraene to Cp 2Sm(THF)2 under release of Cp 2, Eq. (53). The structure of [(COT)Nd(DsP)(THF)] (284) reveals an 5-coordination of the phospholyl ligand, Fig. 54. An alternative route to such heteroleptic COT complexes proceeded through salt metathesis reactions of the borohydride [(COT)Nd(BH4)(THF)2] (285) or the cationic [(COT)Nd(THF)4](BPh4) (286) complexes with K(TMeP), which in the followed could be desolvated in vacuum to afford the respective dimers [(COT)Nd(TMeP)]2 (287).273 Both (COT)Nd(TMeP) (288) and (COT)Nd(Cp ) (175-Nd) represent the first mixed sandwich COT complexes of Nd. The first heterobimetallic phospholyl complex containing d- and f-block metals, [(THF)2Yb(m,5,1-TMeP)2RuH2(PPh3)2] (289), Eq. (54), Fig. 55, was gained from the reaction of [RuH4(PPh3)3] and (5-TMeP)2Yb, where the latter served as a ligand to the ruthenium hydride complex.274 The YbdRu-distance is with 4.25 A˚ too large of a metal-metal-separation to discuss meaningful direct interactions. The complex showed slow decomposition in C6D6 and in warm THF, where the elevated reactivity was ascribed to the strong bending of the metallocene backbone (CpcentdYbdCpcent: 122 ) arising from the 1-Ru coordination. After extensive research on the properties of phospholyl complexes featuring the “classical” divalent lanthanide ions SmII, EuII and YbII, this series was expanded toward TmII in 2002.275 Phospholyl-type ligands were hypothesized to be suitable candidates for the stabilization of such electron rich “non-classical” divalent complexes owing to their reduced p-donating ability, while supplying substantive steric protection. Indeed, the salt metathesis reaction of K(L) (L ¼ DtP (2,5-tBu-3,4-Me2C4P), DsP) with TmI2(THF)3 produced the mononuclear complexes (L)2Tm(THF) (290), which were solely isolable after extraction from Et2O. The room temperature susceptibility of 4.7 mB for all three TmII compounds is in accordance with TmII (wexpected TmII: 4.54 mB). Noteworthy, the thermal stability of these phospholyl complexes seemed to be greatly enhanced compared to their Cp-based complements: While the structurally similar Cp00 2Tm(THF) was only stable for 30 min in THF solution, both phospholyl complexes

Fig. 54 Structure of [(COT)Nd(DsP)(THF)] (284).272 Pink, gray, orange, red, and purple spheres represent Nd, C, Si, O, and P, respectively; selected Si and C have been faded and H atoms have been omitted for clarity.

Fig. 55 Structure of [(THF)2Yb(m,5,1-TMeP)2RuH2(PPh3)2] (289).274 Pink, teal, purple, red, gray, and white spheres represent Yb, Ru, P, O, C, and H atoms, respectively; selected H atoms have been omitted for clarity.

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Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

remained largely intact over the course of 24 h under analogous reaction conditions. This apparent distinction in stability was subsequently verified with the preparation of TmII phospholyl sandwich complexes consisting of sterically less demanding 2,5-ditertbutyl-phospholyl (HtP) and 2,5-bis(trimethylsilyl)-phospholyl (HsP) ligands in relation to Cp00 2Tm(THF). While the phospholyl complex exhibited no reactivity when exposed to N2, the Cp congener was reactive toward this small molecule and resulted in unidentifiable reaction products.276

ð54Þ

The solvent-free monomeric, homoleptic, TmII sandwich complex (DsP)2Tm (291-Tm) was achieved by employing the solvent-free TmI2 as the metal precursor, Eq. (55).256 By contrast, the solvent-free phospholyl sandwich complexes (DsP)2Sm (291-Sm) and (DtP)2Sm (292), bearing the lighter lanthanide SmII ions, constructed dimers in the solid state. This indicates that sterically demanding substituents a to the P atom do not prevent the coordination of the P electron lone pair. Reactivity studies were performed on (DsP)2Sm, (DtP)2Sm, and (DsP)2Tm revealed that none of the divalent complexes were able to activate molecular N2 unlike the reactivity of the Cp-based complex Cp00 2Tm(THF). The respective phospholyl complexes remain also unreactive toward anthracene. However, upon treatment with azobenzene, the reduction products (L)2Ln(PhN•)2 (293) (Ln ¼ Sm, Tm, L ¼ DsP, DtP; Ln ¼ Sm, L ¼ DsP) were structurally identified and their magnetic susceptibility values at room temperature (7.6 mB, 1.9 mB and 2.1 mB, respectively) are in conformity with the trivalent lanthanides (wexpected TmIII: 7.56 mB, SmIII: 0.85 mB), Eq. (55). Finally, when treated with Ph3PS, only the TmII complexes underwent reductive addition to the m-S-bridged [((DsP)2Tm)2S] (294) complex while the corresponding SmII complexes were unreactive under the same conditions, which is evidence for the anticipated higher reactivity of TmII vs. SmII. These findings infer a stabilization of the divalent lanthanide oxidation state through phospholyl ligand framework when compared to the cyclopentadienyl scaffold.

ð55Þ

ð56Þ

A modified synthetic procedure to access such divalent phospholyl sandwich complexes was hereafter developed.277 In contrast to previous methods that primarily relied on the use of starting materials containing divalent metal ions,256 organometallic TmII complexes were also feasible through an alkali metal reduction of their TmIII equivalents, as was proven through the successful ttt isolation of Cpttt 2 Tm from Cp2 TmI. Accordingly, with the goal to extend this toward the realm of lanthanide phospholyl complexes, III the bisphospholyl Tm complexes [L2TmI]n (L ¼ DtP, n ¼ 1, L ¼ Htp, n ¼ 2) were reduced with KC8, Eq. (56), resulting in immediate color changes and ultimately in the isolation of (DtP)2Tm (292-Tm), Fig. 56, and ((HtP)2Tm)2 (295), Fig. 57. The structure of the latter is akin to ((DtP)2Sm)2 with one phospholyl ligand per metal center adopting an 5-(p)-coordination mode, while the second ligand functions as a m:1,5-(s-p)-bridge between the metal centers. This is in agreement with the general trends in lanthanide phospholyl chemistry: traversing from the larger SmII ion to the smaller TmII ion causes a structural change for the bis(DtP)-lanthanide from a dimer to a monomer. A decrease in the steric crowding of the phospholyl ligands by swapping the bigger Dtp to the smaller Htp ligand, is also accompanied by a structural change, in this case from monomeric to dimeric for the resulting bis(phospholyl) Tm complex. The reducing power of the divalent phospholyl complexes and their Cpttt complements III ttt were probed toward pyridine and only Cpttt 2 Tm yielded the anticipated Tm compound, [(Cp2 Tm)2{m-(NC5H5-C5H5N)}] (296),

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

169

Fig. 56 Structure of (DtP)2Tm (292-Tm).256 Pink, purple, and gray spheres represent Tm, P and C atoms, respectively; selected C atoms have been faded and H atoms have been omitted for clarity.

Fig. 57 Structure of ((HtP)2Tm)2 (295).277 Pink, purple, and gray spheres represent Tm, P, and C atoms, respectively; selected P and C atoms have been faded and H atoms have been omitted for clarity.

277 Fig. 58 Structure of [(Cpttt Pink, blue, and gray spheres represent Tm, N, and C atoms, respectively; selected C atoms have been 2 Tm)2{m-(NC5H5-C5H5N)}] (296). faded and H atoms have been omitted for clarity.

Fig. 58, with the reductively coupled bipyridinyl ligand, further suggesting the higher stability and lower reactivity of the phospholyl TmII complexes. Phospholyl DyIII complexes were readily isolated from reactions of the phospholyl salts and DyI3. The isolation of the corresponding divalent Dy complexes containing phospholyl ligands was also attempted through the reductive route using the strong reducing agent KC8 and starting from [L2DyI]n (L ¼ DtP, n ¼ 1, L ¼ DsP, n ¼ 2).278 However, both trivalent phospholyl complexes did not react when treated with KC8 and in the absence of 18-c-6. Other divalent organometallic Dy complexes were also

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Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

subjected to phospholyl sources such as the treatment of the divalent Dy complex [Cpttt 2 Dy(BH4)][K(crypt-222)] with thallium phospholide Tl(HP). However, instead of complexation, its trivalent counterpart Cpttt 2 Dy(BH4) was obtained accompanied by the precipitation of elemental Tl.279 The stabilizing effect of phospholyl ligands in divalent lanthanide complexes has been theoretically investigated.280 For L2Ln (Ln ¼ Sm, Tm, L ¼ Cp , TMeP), the one-electron transfer processes upon coordination of pyridine and CO2 were investigated. For TmII, the reduction of both substrates is more intricate in the phospholyl- versus Cp-supported complex, suggesting that the phospholyl provides better stabilization of the divalent oxidation state relative to Cp. The computations revealed for SmII a more difficult reduction process, contrasting to TmII which is in line with the experimental oxidation potentials for both ions according to which TmII is easier to oxidize than SmII. In addition, the stabilizing effect of the phospholyl ligand is also affirmed for the SmII complexes. Further studies into this matter were conducted in a combined theoretical and experimental approach, based on the observation that the reaction of [(L)2Sm(THF)x] (L ¼ TMeP, x ¼0, L ¼ Cp∗, x ¼2) with pyridine yielded distinct products as a function of the ligand framework.281 The Cp complex underwent a one-electron transfer followed by a CdC-coupling to a 4,40 dihydrobis(4,40 -pyridine)-1,10 -diyl-bridged dinuclear complex, whereas the phospholyl complexes afforded the simple pyridine adduct (TMeP)2Sm(pyr)2 (297). This was reflected in the room temperature magnetic moment meff ¼ 3.2 mB which was attained from NMR experiments considering the Evans method. This is likely due to the pyridine LUMO being too high in energy. However, when the better p-acceptor acridine (acr) was employed, the dimerization also occurred with the TMeP complex to give [{(TMeP)2Sm}2{m-(NC13H9dC13H9N)}] (298). The most striking difference arising from the structural comparison of both dimers lies in the ligand conformation: While the dihydropyridines adopt a gauche conformation, the dihydroacridines exhibit an anti arrangement. DFT calculations indeed attest to a lower SOMO energy of 0.6 eV in (TMeP)2Sm (−4.0 V) compared to Cp 2Sm (−3.4 V). This may be substantiated with a more destabilizing effect of the two Cp ligands compared to TMeP, potentially due to the greater radial extent of P orbitals as against C orbitals leading to a more diffuse negative charge distribution within the TMeP ring. This is mirrored in the Mulliken spin densities of the hypothetical acridine coordination complexes, where the smaller value for (TMeP)2Sm(acr) hints at its reduced reactivity. In addition, the larger aromatic system in acr entails a stabilization of the LUMO (−1.4 eV vs. pyridine), thereby allowing the mixing with a Sm-centered orbital and resulting in the electron transfer. This is in agreement with previous findings that the electron transfer efficiency is subject to the relative energies of the substrates’ p orbital and 4f shell.282 The influence of covalency in divalent lanthanide complexes Cp 2Sm and (TMeP)2Sm (272) in relation to the highly ionic binding in decamethylstrontocene Cp 2Sr was investigated through DFT calculations.284 From decomposition analysis of the binding energy, significant orbital contributions were found in the LdSm bonds whereas only small contributions for the Sr complex were deduced. These computational results support the hypothesis that covalency in LdLn bonds is oxidation state dependent ranging from mainly ionic in LnIII complexes to partially covalent in LnII complexes. The redox chemistry of phospholyl thulium(II) complexes toward bipyridine (bipy) and the heavier homolog tetramethylbiphosphinine (tmbp) was examined.283 In order to study single-electron transfer processes of a divalent lanthanide complex, DtP was chosen owing to its ability to stabilize the divalent oxidation state. Upon mixing [(DtP)2Tm] (292-Tm) with bipy or tmbp, a reduction of both aromatic ligands occurred affording radical-containing mononuclear thulium(III) complexes (DtP)2Tm(L) (L ¼ bipy−%(299), tmpb−%(300)), Scheme 31. The oxidation of TmII to TmIII is accompanied by a shortening of the LcentdTm distance (L ¼ bipy: 2.47 A˚ , L ¼ tmbp: 2.44 A˚ ,

Scheme 31 Left: Reactions of the divalent Tm complex, (DtP)2Tm with tetramethylbisphosphine (tmbp) and bipyridine (bipy), affording the trivalent radical-containing species (DtP)2Tm(L) (L ¼ bipy−% (299) and tmbp−% (300), respectively). Middle: Reactivity of (DtP)2Tm(tmbp•) with free bipy affording the heteroleptic complex (DtP)2Tm(bipy%). Right: Ligand substitution of DtP− with free bipy yielding (DtP)Tm(bipy•)2 (301).

Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

171

Fig. 59 Structure of (DtP)Tm(bipy%)2 (301).283 Pink, purple, blue, and gray spheres represent Tm, P, N and C atoms, respectively; H atoms have been omitted for clarity.

(DtP)2Tm(THF): 2.53 A˚ ). The radical character of the ligand is supported by the decrease in the C2dC20 bond distances (L ¼ bipy: 1.41 A˚ (neutral bipy: 1.49 A˚ ), L ¼ tmbp: 1.44 A˚ (neutral tmbp: 1.490(8) A˚ )). Remarkably, (DtP)2Tm(bipy%) (299) may also be synthesized via a displacement reaction involving treatment of (DtP)2Tm(tmpb%) (300) with bipy which signifies that the radical complex may still undergo a ligand-metal electron transfer process and is available for further reactivity. The addition of a second equivalent of bipy afforded the biradical complex (DtP)Tm(bipy%)2 (301), Fig. 59, and (DtP)2 dimer. Kinetic experiments in conjunction with DFT calculations suggested that the rate-determining step of this transformation involves the elimination of a DtP radical from a {(1-DtP)2Tm(bipy%)2} transition state. Computational analysis of the bipy monoradical complexes further indicated a strong orbital mixing between the Tm and bipy orbitals. Magnetic data collected on [(DtP)Tm(bipy%)2] are in agreement with a TmIII ion and two organic radicals (room temperature value found: 7.73 cm3 mol−1 K, expected: 7.90 cm3 mol−1 K). The values for the monoradical complexes are considerably lower than the anticipated room temperature wMT value (7.55 cm3 mol−1 K). This may be an indication of an intermediate state between complete ligand reduction and a TmIII metal center. This may hypothetically originate from (a) a single electron transfer onto the ligand where the electron is coupled in an antiferromagnetic fashion to generate an open shell singlet between the bipy radical and one f-electron, or (b) a multiconfigurational ground state with an intermediate valence akin to Cp 2Yb(bipy).155 Another comparative study between substituted ytterbocene- and phosphaytterbocene complexes centered around raising the steric bulk of the ancillary bis(methyliminophosphoranyl)pyridine (R2bmpp, R2 ¼ Et, Ph, Cy) ligand which prompted a switch from 1 to 5 in the phospholyl ligands while the Cp ligands maintained their 5-coordination mode.286 Notably, the L2Yb (Et2bmpp) (302) complexes displayed vastly differing coordination behavior: While the Cp complex exhibited the expected bis(5)-coordination and a k2-N,N-coordination of the phosphoranyl ligand, the TMeP complex featured a pincer-type k3-N,N, N-coordination, accompanied by an 1-coordination of one TMeP ligand. Owing to an almost identical steric crowding imposed by the Cp and TMeP ligands, this switch in the phospholyl coordination may be primarily ascribed to its reduced electron donation capability, although the better chelation effect (entropic stabilization) of the pincer ligand is likely contributing. Surprisingly, upon further increasing the steric repulsion of the phosphoranyl ligand by the introduction of bulky Cy and Ph functional groups, the coordination mode of the second phospholyl remained unchanged: While the Cy-substituted complex featured the same coordination mode as the Et complex, the Ph complex reverted to a bis(5)-coordination and a k2-N,N-coordination of the phosphoranyl ligand. Despite similar steric demand, the disparity in coordination mode of Cy- and Ph-substituted ligands may originate from the enhanced rigidity of the Ph groups or the reduced Lewis basicity of that ligand. The 1JYb–P coupling constant (172.3 Hz, R ¼ Ph), determined through 31P NMR spectroscopy, is unexpectedly high in relation to (TMeP)2Yb(py)2 (303) (105.5 Hz), which may be attributed to differences in coordination angle of the P to Yb atoms. The 31P NMR spectroscopy is largely dominated by the Fermi contact contribution, which is associated with the relative s-orbital contribution of the observed atom and therefore the s bonding scheme. Consequently the 1JYb–P coupling of the phospholyl ligands is directly related to the bonding mode of the P atom. Consequently, the considerably larger 1JYb–P coupling constants determined for the Et- and Cy-substituted complexes (434.7 Hz, R ¼ Et; 491.4 Hz, R ¼ Cy) may be traced to the 1-coordinated phospholyl ligands amplifying the Fermi contact contribution. The first mononuclear trisphospholyl lanthanide complex, (DtP)3Tm (304), was obtained through the oxidative phospholyl transfer from 1,10 -diphosphaplumbocene (DtP)2Pb to (DtP)2Tm.285 This transformation was unsuccessful for the Sm analog potentially due to its polymeric structure in the solid state. In (DtP)3Tm, two DtP ligands are 5-coordinated while the third DtP ring is 1-coordinated through the P atom, Fig. 60. In solution, no 31P NMR resonances were detected down to −90  C, in line with a rapid dissociation-recoordination equilibrium to the highly paramagnetic TmIII ion. Furthermore, (DtP)2Pb was reacted with (Cpttt)2Tm to synthesize a cationic trivalent thulocene with a free phospholyl counterion, [Cpttt 2 Tm](DtP) (305), Eq. (57). Based on NMR spectroscopic analysis, the anticipated product formed as proven by a single temperature independent 31P resonance, indicative of no direct interaction between the P atom and TmIII ion. However, crystallographic evidence for this complex is hitherto lacking. Both TmIII complexes were converted back to their TmII counterparts through reduction with KC8, yielding (DtP)2Tm and (Cpttt)2Tm, respectively, without any signs of ligand redistributions.

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Cyclopentadienyls and Phospholyls of the Group 3 Metals and Lanthanides

Fig. 60 Structure of [(DtP)3Tm] (304).285 Pink, purple, and gray spheres represent Tm, P, and C atoms, respectively; selected C atoms have been faded and H atoms have been omitted for clarity.

ð57Þ

The trivalent lanthanide chemistry of phospholyl ligands was further explored through reacting [Ln(BH4)3(THF)3] (Ln ¼ La, Ce, Nd, Sm) with two equivalents of K(HtP) giving rise to dimeric [(HtP)2Ln(m-BH4)]2 (306) (Ln ¼ La, Ce, Nd, Sm), Fig. 61, [(HtP)2Ln(m-BH4)2K(DME)2]2 (307) (Ln ¼ La, Ce) and chains of [(HtP)2Ce(m-BH4)2K(solvent)2] (308), where the solvent is THF or Et2O.287 In all instances, the 5-coordination mode for the phospholyl ligands was the single observed coordination mode. The switch between the generation of the dimeric and polymeric Ce complexes may correlate with the change of solvents from DME to Et2O and thus, attributed to the differing boiling points. Ab initio calculations on [(Htp)2Ce(m-BH4)]2 and [(Cptt)2Ce(m-BH4)]2 uncovered a relatively larger influence of Htp on the crystal field splitting compared to Cptt, which may be assigned to either charge localization or augmented ring electron density in HtP in virtue of P substitution.

Fig. 61 Structure of [(HtP)2Ce(m-BH4)]2 (306-Ce).287 Pink, purple, turquoise, and gray spheres represent Ce, P, B, and C atoms, respectively; selected C atoms have been faded and H atoms have been omitted for clarity.

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ð58Þ

The first scandium phospholyl complex, [(TMeP)2ScCl2Li(TMEDA)] (309), Fig. 62, was generated from the reaction of [Li(TMEDA) (TMeP)] with ScCl3(THF)3, Eq. (58).288 Analogously to the heavier rare earth elements, the phospholyl ligands bind via 5-mode to the metal center and the (TMeP)centdSc bonds are longer than in comparable cyclopentadienyl complexes (2.29(2) vs. 2.2134(7) A˚ in Cp 2Sc(CH2CMe3) owing to weak metal-ligand bonding. The formation of the ate-salts contrasts the reactivity of ScCl3 toward LiCp , which gave cleanly the monochloride Cp 2ScCl. Efforts to convert the ate-salt into alkyl complexes through the addition of MeLi engendered a mixture of [(TMeP)2ScCl(Me)Li(TMEDA)] (310) and 309. The use of the bulkier (Me3Si)2CHLi resulted in ligand exchange of one phospholyl by an alkyl ligand yielding [(TMeP){(Me3Si)2CH}ScCl2Li(TMEDA)] (311). Finally, attempts to replace a phospholyl ligand by a Cp ring focused on treatment of [(TMeP)2ScCl2Li(TMEDA)] with LiCp provoking an incomplete exchange. This outcome further implies the difficulty of converting the phospholyl Sc ate-salt to the corresponding alkyl complexes.

4.04.1.10.2

Catalytically active phospholyl complexes

While cyclopentadienyl ligands have found widespread employment in polymer sciences, the phospholyl ligands have scarcely been applied for the construction of catalytic systems. The accessibility of heteroleptic Sm monoalkoxides of bisphospholyl complexes was developed through the oxidation of [(TMeP)2Sm] with (tBuO)2.289 The resulting monomeric alkoxide complex [(TMeP)2SmOtBu(THF)] (312) was active in catalytic ring-opening polymerization reactions of e-caprolactone in THF or toluene solution. The obtained polymeric product is close to the expected theoretical values from the e-caprolactone/catalyst ratio and the DPI value is typical for single-site catalysts. Monophospholyl lanthanide complexes were synthesized through salt metathesis reactions of homoleptic complexes Ln(AlMe4)3 (Ln ¼ La, Nd) with K(TMeP) and K(DsP), respectively, to give the heteroleptic molecules [(TMeP)Ln(AlMe4)2] (313) (Ln ¼ La, Nd), Fig. 63.290 [(TMeP)La(AlMe4)2] forms m:1,5-bridged dimers in the solid state where the dimerization may be allocated to the large metal center. By contrast, the respective Nd complex, due to the smaller ionic size, features a monomeric

Fig. 62 Structure of [(TMeP)2ScCl2Li(TMEDA)] (309).288 Pink, lavender, purple, blue, green, and gray spheres represent Sc, Li, P, N, Cl, and C atoms, respectively; H atoms have been omitted for clarity.

Fig. 63 Structure of [(TMeP)Nd(AlMe4)2] (313-Nd). Pink, dark blue, purple, and gray spheres represent Nd, Al, P and C atoms, respectively; H atoms have been omitted for clarity.

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structure in both products. Following activation with (PhMe2NH)(B(C6F5)4), all phospholyl complexes displayed moderate activity to isoprene polymerization with predominant trans-1,4-selectivity. Here, the Nd complexes exhibited narrower polydispersities (PDI ¼ 1.68), indicative of single-site behavior.290 Monophospholyl complexes with dimethylaminobenzyl auxiliary ligands were also employed in the syndiospecific polymerization of styrene.291 Phospholyl ligands are prone to facile displacement by alkyl ligands and thus, bear potential to engender a higher electrophilicity on the lanthanide centers that may in turn lead to higher reactivity toward alkenes.291 Furthermore, the synthesis of phospholyl complexes through an acid-base approach was introduced for the first time, which is a common synthetic strategy to yield cyclopentadienyl-based catalytic systems. In contrast to their cyclopentadienyl complements, 1H-phospholyl systems are unstable as they undergo [1,5]-H shift at room temperature and form Diels-Alder coupling products. A bypass to such side reactions involved the use of bulky ligands in a-positions to the P atom. The o-dimethylaminobenzyl ligand (CH2C6H4NMe2-o) served frequently as readily available alternative to alkyl/allyl ligands. The complexes (DtP)Ln(CH2C6H4NMe2-o)2 (314) (Ln ¼ Y, Sm), Fig. 64, were obtained by stepwise treatment of LnCl3 with K(DtP) and K(CH2C6H4NMe2-o) in a 1:1:2 ratio. Under the same conditions, the reaction with ScCl3 yielded only the dimeric THF ring-opening product [{Sc[m-O(CH2)4(Dtp)] Cl2(THF)2}2] (315), presumably in virtue of nucleophilic attack initialized by an intermediary 1-coordinated DtP on the a-carbon of a THF-coordinated Sc molecule. This side reaction was efficiently eluded by deploying a toluene/pyridine solvent mixture to enable the isolation of dimeric [(Dtp)Sc(m-Cl)Cl(py)]2 (316) (py ¼ pyridine), which was converted to the respective bis(dimethylaminobenzyl) complex, Eq. (59). An alternative route to the synthesis of 314-Sc comprises the deprotonation of 1H-phosphole (in situ generated through protonation of K(DtP) through Sc(CH2C6H4NMe2-o)3, Scheme 32. This development constitutes the first phospholylmetal complex formation through deprotonation of a 1H-phosphole. After activation with (Ph3C)(B(C6F5)4), (DtPRE) (CH2C6H4NMe2-o)2 ( RE¼Sc, Y) were catalytically active for the syndiotactic polymerization of styrene with excellent results for the Sc complex. Notably, the Y complex exhibited remarkably high activities compared to its Cp congener.

Fig. 64 Structure of (DtP)Sc(CH2C6H4NMe2-o)2 (314-Sc).291 Pink, purple, blue and gray spheres represent Sc, P, N and C atoms, respectively; selected C atoms have been faded and H atoms have been omitted for clarity.

Scheme 32 Synthesis of (DtP)Sc(CH2C6H4NMe2-o)2 (314) through in-situ deprotonation of the 1H-phosphole by the Sc alkyl complex, Sc(CH2C6H4NMe2-o)3. 1H-phosphole was generated from K-phosphole and p-toluenesulfonic acid (PTH).291

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ð59Þ

An intriguing application was devised for tetramethylaluminate monophospholyl complexes (L)Ln(AlMe4)2 (Ln ¼ La, Nd, L ¼ TMeP, DsP) which were grafted onto mesoporous dehydroxylated silica SBA-15 particles.292 The grafting process produced a mixture of surface species such as [(TMeP)Ln[(m-OSi)(m-Me)AlMe2]x[(m-Me)2AlMe2]2−x] (317) and [(DsP)2Ln[(m-OSi)(m-Me) AlMe2]x[(m-Me)2AlMe2]2−x] (318) (x ¼ 1, 2). The model complex [(TMeP)Nd{m-OSi(OtBu)3(m-Me)AlMe2}((m-Me)2AlMe2)] (319) was obtained by treatment of [(TMeP)Ln(AlMe4)2] (313) with HOSi(OtBu)3 which resemble surface siloxide species. The surface grafting resulted in a switch of polymerization activity: While the deposited product yielded cis-1,4-polyisoprene (>99%) without any coactivator, the polymerization with 313 and B(C6F5)3 gave preferentially trans-1,4-polyisoprene (>99%). The surface species acted as a single-component catalyst, affording higher molecular weights (>2.6  106 g mol−1) and smaller PDI (100 ppm. A general covalency trend was seen in the complexes with varying halide identity I > Br > Cl, with iodide having the most downfield shift and being further removed from the thorium center and donating the least electron density to the Lewis acidic metal.193

Scheme 68 General method towards synthesis of actinide phosphorano-stabilized carbene complexes.

4.05.3.7

Insertion reactions and CdH bond activations with An(IV) complexes

Actinide cyclopentadienyl complexes have been the subject of a myriad of studies involving insertion reactivity and CdH bond activations.183,194–196 Insertion reactivity with carbon oxygenates has been achieved with several classes of actinide complexes including hydrides, alkyls,197–200 allyls,201,202 alkynyls203 and amides. A thorough review of these reactions has recently been reported.29 Insertion into both actinide-carbon bonds in Th-89 and U-89 or [(C5Me5)2An(CH2Ph)2], An ¼ Th, Th-320, U, U-320, with diphenyldiazomethane to yield bis(hydrazonato) complexes has been examined, Scheme 69.204 The analogous reactions of Ph2C]N]N, with [(C5H5)2MMe2], M ¼ Zr, Hf, only produced the mono-insertion product with the second insertion 20 kcal/mol higher in energy, while the second insertion for the actinides is facile. This was shown to be a function of the use of the 5f orbital in bonding. Similar to the bis(ketimide) complexes, the cyclic voltammetric data for the bis(hydrazonato) uranium complex, [(C5Me5)2U(MeNNCPh2)2], shows an anodic metal-based UV/UIV oxidation wave at −0.6 V versus (C5H5)2Fe+/0.

Scheme 69 Synthesis of bis(pentamethylcyclopentadienyl) thorium and uranium bis(hydrazonato) complexes.

Insertion chemistry in cyclopentadienyl actinide(IV) complexes205 has been used for several important catalytic reactions205,206 such as polymerization of a-olefins,207,208 aldehyde coupling (Tishchenko reaction), and hydroelementation of CdC multiple bonds being among the most prominent. Ring opening polymerization of lactides to yield the corresponding polylactide [CH(CH3)C(]O)(O)]n has been achieved with bis- and tris(cyclopentadienyl) An(IV) alkyl complexes; [(1,3-tBu2C5H3)2 Th(CH2Ph)2], Th-321, [(1,2,4-tBu3C5H2)2Th(CH2Ph)2] and [(C5H3-tBu2-1,3)2ThH] achieve the production of atactic polymers under mild conditions.209,210 [(C5Me5)2AnMe2] (An ¼ Th, U) demonstrated high activity towards the ring-opening polymerization of cyclic lactones. Reactions of Th-89 and U-89 with dimethylamine borane proved to occur with catalytic turnover frequencies of up to 400 h−1, representing catalysis on the same order of the fastest catalysts currently known in the literature, and proceeding through methyl group transfer from the actinide center, Scheme 70.211

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Scheme 70 Actinide catalyzed dehydrogenation of dimethylamine borane.

Additionally, Th-89 and U-89 react with 5-methyl-1H-tetrazole to afford the corresponding 2-tetrazolate complexes, providing insights into the bonding situation found between actinides and nitrogen-rich ligand sets, Th-322 and U-322, Scheme 71.212 Through cyclic voltammetry and electronic spectroscopy, it was found that the AndN bond was primarily s in nature, with minimal p interactivity.

Scheme 71 (C5Me5)2An(CH3)2 protonolysis reaction with 5-methyl-1H-tetrazole.

Evans has examined the coordination chemistry and reactivity of the (C5Me4SiMe3)1− ligand with uranium. Heating (C5Me4SiMe3)2UMe2, U-323, in toluene leads to CdH bond activation product in which two silyl-methyl groups are deprotonated to form (C5Me4SiMe2CH2)2U, U-324. Double insertion of tBuNC, U-325, and CO (1 atm), U-326, occur with U-324 while only one i PrN]C]NiPr insertion is observed, U-327. Reaction of tBuNC with U-327 leads to insertion into the remaining UdC bond forming U-328. However, when U-327 is reacted with CO (1 atm) a cascade reaction occurs in which the CO not only inserts into the UdC bond, but also inserts into the NdC bond of the original CH2 group and amidinate nitrogen, U-329, Scheme 72.

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Scheme 72 Insertion reactivity of U-323.

4.05.3.8

Hydride complexes

Thorium and uranium hydride complexes, [(C5Me5)2AnH2]2, An ¼ Th, Th-330; U, U-330, have been known since first reported in 1978.213,214 Nearly 30 years later, the utility of these hydride complexes as multi-electron reductants was demonstrated.89 Reaction of the U(III) or U(IV) hydrides, [(C5Me5)2UHn]2, n ¼ 1, U-331, n ¼ 2, U-330, with PhEEPh, E ¼ N, S, and Se, and 1,3,5,7-cyclooctatetrene (COT). The thorium analog was also reacted with PhSSPh and COT, Scheme 73. The resulting uranium complexes, [(C5Me5)2U(]NPh)2], U-169,117 [(C5Me5)2An(SPh)2], An ¼ Th, Th-132; U, U-132, [(C5Me5)2U(SePh)2], U-133, and [{(C5Me5)(C8H8)An}2(m2-C8H8)], An ¼ Th, Th-332; U, U-332. The hydride complexes of thorium(IV) and uranium(IV) also reacts to reductive couple three acetonitrile molecules to form a bridging cyanopentadienyl dianion.215 The uranium hydride was more recently synthesized by addition of two equivalents of PhSiH3 to Th-89 and U-89,216 giving a mixture of products of the general form [(C5Me5)2U(H)]x, U-331, in equilibrium with the uranium(IV) dimer [(C5Me5)2UH(m2-H)]2, U-330, Scheme 74.

Scheme 73 Reactivity of Th-330 and U-330 with cyclooctatetraene (COT).

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Cyclopentadienyl and phospholyl compounds in organometallic actinide chemistry

Scheme 74 Temperature dependent equilibrium of [(C5Me5)2UH]x and thermolysis of [(C5Me5)2U(m2-H)]2.

Thermolysis of [(C5Me5)2UHn]2 affords a tuck-in, tuck-over complex in which one Cp ligand has been metalated twice to give a [C5Me3(CH2)]3− ligand, U-333, Scheme 70. Tuck-in refers to the CH2 that is bound to the same metal complex, while tuck-over refers to the CH2 bound to another metal. This tuck-in, tuck-over complex, U-333, was used as a reductant with PhSSPh, U-132, PhSeSePh, U-133, COT, U-332, and PhNNPh, U-169, Scheme 75.217

Scheme 75 Reactivity of a bis(cyclopentadienyl) ‘tuck-in’/‘tuck-over’ complex.

It was observed that temperature control of the system allowed for shifting of the equilibrium between the bridging species and the ‘monomeric’ (C5Me5)2UH. This complex was subsequently reacted with dimethylphosphinoethane (dmpe), U-334, terpyridine (terpy), U-335, PhNNPh, U-169, RC^CR, R ¼ Me, U-336; Ph, U-337, and (Ph2C]N)2, U-219, to yield uranium(III), uranium(IV) and uranium(VI) products, Scheme 76.218

Cyclopentadienyl and phospholyl compounds in organometallic actinide chemistry

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Scheme 76 Reactivity of U-331.

Reactions conducted with Th-260 and a potassium aluminum hydride salt, K[AlH3C(SiMe3)3] led to the formation of thorium-aluminum hydride-bridged heterobimetallic complexes of Th(IV), Th-338, Scheme 73. The terminal hydride can be replaced with a chloride by reaction Th-338 with Me3SiCl, Th-339. Subsequent reduction of Th-339 with potassium graphite resulted in the isolation of the Th(III) complex, Th-340, Scheme 77, with subsequent oxidation back to Th-339 through the use of mild reducing agents CuCl and Ph3CCl, Scheme 78.219

Scheme 77 Isolation of thorium(IV)-aluminum bridging hydride complexes.

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Cyclopentadienyl and phospholyl compounds in organometallic actinide chemistry

Scheme 78 Reduction and subsequent reduction of a thorium(IV) bridging hydride complex.

4.05.3.9

Linear metallocenes

Attempts to synthesize a base free dicationic Th(II) metallocene resulted in the isolation of the penta(nitrile) cationic complex220 [(C5Me5)2Th(NCR)5][BPh4]2, R ¼ Me, Th-341; Ph, Th-342, Scheme 79, which is the first thorium analogue of the known [(C5Me5)2U(NCMe)5][X]2, X ¼ (OTf )−, U-343; I−, U-344; (BF4)−, U-345, reported by Ephritikhine.221

Scheme 79 Isolation of penta(nitrile) linear thorium metallocene.

Despite the reported synthetic difficulty of installing a Cp ligand into a uranyl complex, one such example has been reported from the Ephritikhine group. [(C5Me5)2U(CN)5]3−, U-346, was reacted with pyridine-N-oxide to yield [(C5Me5)UO2(CN)3]2−, U347, This is the first example of an organometallic uranyl complex, showing a deviation in linearity for the UO2 fragment due to the steric effects of the Cp ligand, Scheme 80. Importantly, this complex represents a potential route towards soluble organometallic uranyl chemistry.222

Scheme 80 Reactivity of a uranium(IV) metallocene with pyridine-N-oxide to give a Cp uranyl compound.

Arnold and coworkers synthesized both tri- and tetravalent uranium isocyanide adducts using the uranium(III) starting material.223 When tBuNC was added to the uranium(III) starting material, [(Ci5Pr4H)2UI], U-348, the isocyanide coordinates to form, [(Ci5Pr4H)2U(I)(CNtBu)], U-349, Scheme 81. The iodide in U-348 can be abstracted by [(Et3Si)2(m-H)][B(C6F5)4] to form the uranium(III) cationic species, [(Ci5Pr4H)2U][B(C6F5)4], U-350. Reaction of U-349 with [(C5H5)2Fe][B(C6F5)4] and excess tBuNC forms the U(IV), [(Ci5Pr4H)2U(CNtBu)2(I)][B(C6F5)4], U-351. When the cationic U-350 is treated with excess tBuNC, then oxidation takes place to form the dicationic, [(Ci5Pr4H)2U(CNtBu)4][B(C6F5)4]2, U-352. Notably, complex U-352 showed a linear (Cent)U-(Cent) angle.

Cyclopentadienyl and phospholyl compounds in organometallic actinide chemistry

227

Scheme 81 Isocyanide adducts of U(III) and U(IV) metallocenium complexes.

The synthesis and characterization of a parallel U(II) metallocene was also accomplished by the Layfield group.224 Much like the previously mentioned An(II) complexes, it was synthesized via KC8 reduction of a uranium(III) precursor, [(5-Ci5Pr5)2UI], U353,225 Scheme 82, to form [(Ci5Pr5)2U], U-354. Using electronic absorption spectroscopy and DFT calculations served to uncover the ground state valence electronic configuration of 5f36d1. Advancements in divalent organometallic early actinide chemistry stand poised to offer elucidation into the properties of the late transuranic elements. DFT studies suggest that formally divalent tris(cyclopentadienyl) complexes of thorium-americium are potentially attainable.226 Based upon the An(III)/An(II) reduction potentials of the early actinides, transuranium elements beyond berkelium should also offer notably increased stability for divalent complexes.227,228

Scheme 82 Reduction of a sterically encumbered uranium(III) metallocene to yield a linear uranium(II) metallocene.

4.05.4

Tris(cyclopentadienyl)-based complexes

Due to the steric saturation of the actinide, the tris(cyclopentadienyl) moiety has been well-studied. It is noteworthy that no structural studies229–231 of divalent actinides were reported until 2013 based on the methodology originally developed by the labs of Lappert and Evans in an effort to synthesize lanthanide complexes of the forms [K(18C6)]{Ln[(C5H3SiMe3)2]3} (Ln ¼ La, Ce; 18C6 ¼ 18-crown-6, 2.2.2-cryptand),232 and [K(18C6)][Ln(C5H4SiMe3)3] (Ln ¼ Pr, Gd, Ho, Er, Tb, Lu).233,234 In 2013, the Evans group reported that the reaction of [(C5H4SiMe3)3U], U-355, with KC8 afforded the first U2+ complex, [(C5H4SiMe3)3U][K(2.2.2cryptand)], U-356, Scheme 83. Complex U-356 has a ground state electron configuration of [Xe]5f36d1, reminiscent of other divalent lanthanide complexes in which the electron adds to the vacant d orbital, not an f orbital. The analogous [{C5H3(SiMe3)2}3U]1−, U-357, complex was subsequently reported in 2016 from reduction of [{C5H3(SiMe3)2}3U], U-358.235 In 2015, the

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Cyclopentadienyl and phospholyl compounds in organometallic actinide chemistry

Scheme 83 Reduction of tris(cyclopentadienyl) actinide complexes to form An(II) species.

first Th(II) complex, [{C5H3(SiMe3)2}3Th]1−, Th-357, with a 6d2 ground state was obtained from reduction of the Th(III) starting material, [{C5H3(SiMe3)2}3Th], Th-358, Scheme 83.236 Complex Th-357 reduces C8H8 by two electrons to furnish the neutral Th(IV) complex, [{{C5H3(SiMe3)2}3}2(COT)Th], Th-359. This is the only example of two-electron reductive chemistry from a single thorium metal center. In contrast, the reaction of U-355 and C8H8 forms uranocene, U(COT)2, and [(5-C5H4SiMe3)3(1-C5H4SiMe3)U], U-360. Later, the reactivity of Th-357 was explored with [Et3NH][BPh4] and H2 to form [{C5H3(SiMe3)2}3ThH], Th-361, or a mixed-valence Th(IV/III) species, [{(C5H3(SiMe3)2}2Th)(m3-H)3(Th(H){(C5H3(SiMe3)2}2], Th-362, Scheme 84.237

Scheme 84 Synthesis of a mixed valence Th(IV)/Th(III) bridging hydride from a Th(II) synthon.

The analogous H2 reaction with U-356 or U-357 yields the U(III) hydride, [(C5H4SiMe3)3UH]1−, U-363, or [{C5H3(SiMe3)2}3UH]1−, U-364, respectively. Alternatively, either can be prepared by reaction of U-355 or U-358 with KH and 2.2.2cryptand. Through a similar approach, the first structurally authenticated Np(II), Np-357,238 and Pu(II), Pu-357, complexes were synthesized and characterized. [{C5H3(SiMe3)2}3M], M ¼ Np, Np-358, Pu, Pu-358, were reduced using KC8, and the potassium counter ion was subsequently sequestered through the usage of 2.2.2-cryptand, Scheme 85. Treatment of Np-357 or Pu-357 with AgBPh4 results in the regeneration of the parent complex, Np-358 or Pu-358. The ground state electronic configuration of the Np(II) ion was determined to be 5f46d1. For the Pu(II) ion, the HOMO is mostly a Pu-[(C5H3SiMe3)2] nonbonding f3z orbital, however, the HOMO also contains 7% d2z character.238,239 This was consistent with the near degeneracy of the 5f56d1 and 5f66d0 configurations. A similar computational study also indicated that the crossover of [(C5H4SiMe3)3]An1−, An ¼ ThdAm, complexes occurs near Pu.226

Cyclopentadienyl and phospholyl compounds in organometallic actinide chemistry

229

Scheme 85 Generation of divalent transuranic tris(cyclopentadienyl) complexes.

Recently, reactivity has been expanded to An(II) systems without the assistance of a chelating agent to sequester the alkali metal counter ion. Instead of reacting the initial U-358 with KC8 and subsequently a crown ether or cryptand chelate, lithium and cesium metal were employed for reduction. Using cesium resulted in the formation of an oligomer [(C5H3(SiMe3)2)]U[m{(C5H3(SiMe3)2}2{Cs(THF)2}]n, U-365, while lithium yields [Li(THF)4][{C5H3(SiMe3)2}3U], U-366, Scheme 86.240

Scheme 86 Reduction of a uranium(III) cyclopentadienyl complex with elemental alkali metals.

The isolation of new three-coordinate U(II) complexes was achieved through the usage of strongly donating amide ligands [U(NR2)3][K(2.2.2-cryptand)], as well as [(C5Me4H)3U][K(2.2.2-cryptand)], U-367, from (C5Me4H)3U, U-368, Scheme 87.241 Previously these compounds were only possible with [(C5H4SiMe3)3]3−, ([C5H3(SiMe3)2]3)3−, [(C5Me4H)3]3−, and [(Ad,MeArO)3Mes]3− ligand sets. These complexes were synthesized in a manner previously described in the literature in which reduction is accomplished with KC8 in THF under an atmosphere of argon.

Scheme 87 Generation of a uranium(II) tris(tetramethylcyclopentadienyl) complex.

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Cyclopentadienyl and phospholyl compounds in organometallic actinide chemistry

Once thought to be impossible due to the cone angle of Cp , tris(C5Me5) complexes are now known for most of the lanthanides as well as uranium(III) and uranium(IV). First obtained from [(C5Me5)2UBPh4], U-193, with K(C5Me5), there are now many reactions that lead to [(C5Me5)3U], U-369.242 One-electron, ligand-based reactivity is well-established for tris(C5Me5) f-element complexes with formation of (C5Me5)2. This reactivity has been called sterically induced reduction (SIR). A review of the methyl displacements on the Cp rings has been conducted for many of the sterically crowded Cp complexes.243 In the case of [(C5Me5)3U], this is typically accompanied by one-electron metal-based reduction to yield U(IV). For example, reaction of [(C5Me5)3U], U-369, with two equivalents of chlorobenzene forms [(C5Me5)2UCl2], U-95, or two equivalents of U-369 with cyclooctatetraene (COT) forms [{(C5Me5)(C8H8)U}2(m2-C8H8)], U-332. This (C8H8)2− bridged complex also shows two-electron ligand-based chemistry when reacted with PhEEPh, E ¼ S, U-132, Se, U-133, Te, U-134, while reforming COT.244 While [(C5Me5)3U], U-369, is reacted with only one equivalent of chlorobenzene, only metal-based chemistry is observed to form [(C5Me5)3UCl], U-370, Scheme 88.245 Reaction of two equivalents of U-369 with HgF2 leads to [(C5Me5)3UF], U-371. Additionally, [(C5Me5)3UMe], U-372, was also isolable from methyl abstraction from [(C5Me5)2UMe2], U-89, with BPh3 followed by reaction with K(C5Me5).246 A similar methodology was employed using K(C5Me4H) to form [(C5Me5)2(C5Me4H)UMe], U-373.247

Scheme 88 Synthesis of tris(pentamethylcyclopentadienyl) uranium halides.

The reduction of two equivalents of [(C5Me5)3U], U-369, with two equivalents of KC8 in benzene results in the formation of a bridging arene complex, [{(C5Me5)2U}2(m6-C6H6)], U-374.248 While all the CdC bonds are indistinguishable from free benzene, the bridging arene is not planar, indicating reduction to (C6H6)2−, deviating 0.10–0.14 A˚ from linearity forming a shallow boat conformation. Over the course of one week at 65  C, exchange of C6D6 is observed with C6H6, with the (C5Me5)1− resonance observed at 3.06 ppm in U-374 and 3.02 ppm in [{(C5Me5)2U}2(m6-C6D6)], U-375, in the 1H NMR spectrum. The formation of bridging para-xylene was also reported from reaction of U-369, KC8, and 1,4-Me2C6H4. The exchange with C6D6 was found to be much more rapid, going to completion in one day at 65  C. Reaction of [{(C5Me5)2U}2(m6-C6H6)], U-374, with two equivalents of KN(SiMe3)2, K(O-2,6-tBu2-4-Me-C6H2), LiCH(SiMe3)2 or Li[C(Me)(NiPr)2] to relieve the steric strain observed in the parent compound, forming [{(C5Me5)(X)U}2(m6-C6H6)], X ¼ N(SiMe3)2, U-376, Scheme 89; O-2,6-tBu2-4-Me-C6H2, U-377; CH(SiMe3)2, U-378, [C(Me)(NiPr)2], U-379. [{(C5Me5)2U}2(m6-C6H6)] and [{(C5Me5)(X)U}2(m6-C6H6)] can function as reductants through reformation of free benzene with substrates such as cyclooctatetraene, yielding [(C5Me5)(X)U(C8H8)], X ¼ N (SiMe3)2, U-380, CH(SiMe3)2, U-381, [C(Me)(NiPr)2], U-382, 249 and adamantyl azide, yielding [(C5Me5){C(Me)(NiPr)2}U(] NAd)2], U-383, Ad ¼ adamantyl, as well as the ligand redistribution product, [(C5Me5)2U(]NAd)2], U-176.141

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Scheme 89 Reactivity of a dinuclear uranium complex with bridging reduced benzene.

The addition of an L-type ligand to [(C5Me5)3U], U-369, has also been studied by Evans. Charging a solution of [(C5Me5)3U] with 1 atm of CO results in adduct formation, (C5Me5)3U(CO), U-384, which can be removed under vacuum, Scheme 90.250 The CO stretching frequency is observed at 1925 cm−1. The UdC(CO) bond length of 2.485(9) A˚ is much longer than the 2.383(6) A˚ in [(C5Me4H)3U(CO)], U-385. Shortly thereafter, 80 psi of N2 was added to U-369, and the resulting adduct [(C5Me5)3U(1-N2)], U386, was obtained251. When the N2 pressure was lowered to 1 atm, N2 was released. The NN stretch of U-386 was observed at 2207 cm−1, and an isotopically enriched derivative has a 15N15N stretch at 2134 cm−1.

Scheme 90 Reactivity of N2 and CO with (C5Me5)3U.

In 2017, [(C5Me5)3Th], Th-369, was reported.252 In order to obtain Th-369, the Evans group made the cationic complex, [(C5Me5)2ThMe][BPh4], Th-387, through reaction of [(C5Me5)2ThMe2], Th-89, with [Et3NH][BPh4]. Reaction of Th-387 with K(C5Me5) affords [(C5Me5)3ThMe], Th-372, and the remaining methyl group can also be abstracted with [Et3NH][BPh4] from which the unsolvated, [(C5Me5)3Th][BPh4], Th-388, or solvated, [(C5Me5)3Th(THF)][BPh4], Th-389, complex can be isolated. Reduction of Th-388 with KC8 yields the dark purple, [(C5Me5)3Th], Th-369, Scheme 91. Electron Paramagnetic Resonance (EPR) spectroscopy analysis of Th-369 showed an isotropic signal at room temperature with giso ¼ 1.88, consistent with a 6d1 ground

Scheme 91 Synthesis of [(C5Me5)3Th]1+.

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state. In comparison, other Th3+ have giso values: 1.916 for [K(DME)2][Th[Z8-C8H6(SiMet2Bu)2]2], Th-390,253 1.910 for [{C5H3(SiMe3)2}3Th], Th-358,254 as well as [{C5H3(SiMet2Bu)2}3Th], Th-391,254 1.871 for [(C5Me5)2Th{iPrNC(Me)NiPr}], Th-392,255 and 1.92 for [(C5Me4H)3Th], Th-368.256 The ground state of Th-369 was also corroborated by three maxima between 450 and 650 nm in the visible region of the optical spectrum with lmax ¼ 539 nm (e ¼ 9500 M−1 cm−1). This can be compared to Th-358 and Th-368 which have lmax ¼ 654 nm (e ¼ 5100 M−1 cm−1) and lmax ¼ 522 nm (e ¼ 7100 M−1 cm−1), respectively. The Th(IV) complex, [(C5Me5)3ThI], Th-393, can be made from reaction of Th-369 with Me3SiI, CH3I, or I2, while [(C5Me5)3ThCl], Th-370, was prepared from Th-369 with C6H5Cl. The previously reported hydride, [(C5Me5)3ThH], Th-394,254 can be formed from [(C5Me5)3Th][BPh4], Th-388, and KH or Th-369 with H2. The ThdH stretching frequency is observed at 1413 cm−1. When Th-388 is reacted with 1 atm of CO, the carbonyl complex, [(C5Me5)3Th(CO)][BPh4], Th-395, Scheme 92, can be made. While the structure of the carbonyl was not determined, the CO stretching frequency of 2131 cm−1 (2094 cm−1 with 13CO), similar to the 2150 cm−1 observed in [Cp3Zr(CO)][BPh4].257 This stretching frequency is significantly higher than the 1925, 1976, and 1880 cm−1 in (C5Me5)3U(CO), U-385,250 [(C5H4SiMe)3U(CO)], U-396,258 and [(C5Me4H)3U(CO)], U-397,259,260 respectively. This difference is due to the Th4+ versus the U3+ oxidation state, which lowers the energies of the frontier orbitals, which leads to stronger s bonding and weaker p back-bonding.

Scheme 92 Synthesis of a thorium carbonyl complex.

In an effort to synthesize and isolate [(C5H4SiMe3)3Th], Th-355, [(C5H4SiMe3)3ThX] (X ¼ Cl, Br, I) was reacted with KC8, much in the same way that [(C5H4SiMe3)3U], U-355, was synthesized. The reaction results in a dark blue solution and was characterized by EPR and electronic absorption spectroscopy, as well as DFT analysis. The resultant conclusion was that the Th(III) complex was indeed synthesized based upon the close agreement of UV-visible spectra and EPR data with other (C5H4SiMe3)3An compounds. However, no single crystals were isolated for X-ray diffraction analysis.261 The Th(III) complex, [{C5H3(SiMe3)2}3Th], Th-358, also incorporated a bridging P4 unit upon reaction with white phosphorous to yield a Th(IV)/Th(IV) bimetallic product with a novel 1:1 binding modality for P4, Th-398, Scheme 93.262 Interestingly, the analogous U(III) complex was unable to form such a product, and this result emphasizes the reductive capability of Th(III)259,263 The Th(IV/III) redox couple has been measured to be −2.96 to −3.32 V versus (C5H5)2Fe+/0.263

Scheme 93 Reaction of a thorium(III) species with white phosphorus.

In the search for an alkylidene-like bond, a thorium(IV) alkyl was synthesized from [{C5H3(SiMe3)2}3Th], Th-358, and H2C] C(NMeCH)2 to produce [{C5H3(SiMe3)2}3ThMe], Th-399, and MeImCH2CH2ImMe (Im ¼ imidazole), Scheme 94, rather than forming the methylene adduct, thorium is proposed to transfer one electron, followed by subsequent N-methyl cleavage and transfer to thorium.264 Additionally, Th-358 also reduces CO2 and CS2,265 and reductively couples pyridine.266

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Scheme 94 Reaction of a thorium(III) complex with an N-heterocyclic olefin to yield a Th(IV) alkyl.

EPR spectroscopy was used to examine two isostructural complexes in the same oxidation state, [(1,3-tBu2C5H3)3An], An ¼ Th, Th-400; U, U-400.267 The CW EPR spectrum of the Th(III) complex confirmed a 6d1 configuration with gz ¼ 1.974 and gx,y ¼ 1.880, as well as 5f3 configuration for U(III) with gx ¼ 3.05, gy ¼ 1.65, and gz < 0.5 (out of the range of the magnetic field). The EPR data showed that there is significantly greater total spin density on the ligands for uranium than thorium. The activation of C3O2, carbon suboxide, with [(C5H4SiMe3)3U], U-355, at −78  C was described by the Cloke group to form an unusual central cyclobutene-1,3-dione ring. When the volatiles are removed at low temperature (−60  C), the major species that was identified as a tetranuclear structure with four [(C5H4SiMe3)3U] centers linked by a complex organic structure consisting of a central cyclobutene-1,3-dione ring, U-401, Scheme 95.268

Scheme 95 Activation of carbon suboxide by (C5H4SiMe3)3U.

The isolation of the first f-element nitrosyl complex, [(C5Me4H)3U(NO)], U-402, was achieved from the reaction of [(C5Me4H)3U], U-368, and NO gas, Scheme 96.269 This complex is quite unusual in that its UdN bond distance of 2.013(4) A˚ , and broad nIR spectroscopic features with e of 180–320 M−1 cm−1 are consistent with U(IV) complex with strongly donating ligands, while the magnetic data deviate significantly from other U(IV) complexes. The nitrosyl complex has a linear decline of wMT that results from temperature-independent paramagnetism with a magnetic moment of 0.232 emu K mol−1 at room temperature. Calculations show good agreement in bond distances for both the singlet and triplet states, but the complex is best described as a singlet ground state with a low-lying excited triplet state. Two orthogonal p orbitals of NO strongly interact with the two 5f orbitals of similar p symmetry, which was predicted years earlier by Bursten and co-workers,270 producing the short UdN bond distance. [(C5Me4H)3U(NO)], U-402, was reacted with AlMe3 to form [(C5Me4H)3U(NO)AlMe3], U-403, This shifts the stretching frequency of NO from 1439 and 1373 cm−1 in U-402, 1416 and 1366 cm−1 for [(C5Me4H)3U(15NO)], to 1303 cm−1 in U-403.

Scheme 96 The first example of an f-element nitrosyl complex.

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Cyclopentadienyl and phospholyl compounds in organometallic actinide chemistry

The [(C5H4SiMe3)3U], U-355, moiety has also been used to form UdAl and UdGa bonds through reaction with (C5Me5)E, E ¼ Al, Ga, Scheme 97.271 The UdAl bond distances of 3.117(3) and 3.124(4) A˚ were observed in the two crystallographically inequivalent molecules of [(C5H4SiMe3)3UAl(C5Me5)], U-404, in the unit cell. Two molecules were also found in the unit cell of [(C5H4SiMe3)3UGa(C5Me5)], U-405, with UdGa bond lengths of 3.0648(12) and 3.0800(13) A˚ .272 In a similar fashion, the coordination of silylenes, Si[PhC(NR)2](R0 ), R ¼ tBu, R0 ¼ NMe2, U-406, and R ¼ iPr, R0 ¼ PhC(NiPr)2, U-407, to uranium was also achieved with U-355, Scheme 97.273 Both complexes were structurally characterized with UdSi bond distances of 3.1637(7) and 3.1760(6) A˚ . The reaction of [(C5H4Me)3U{Si{PhC(NtBu)2}(NMe2)}], U-406, with CO forms a bridging CO complex between the uranium and silylene.

Scheme 97 Use of U(C5H4SiMe3)3 to form uranium-triel and uranium-silylene bonds.

Anionic silyl ligands coordinated to thorium and uranium were structurally characterized by reaction of [(C5H4SiMe3)3AnCl], An ¼ Th, Th-408; U, U-408, with KSi(SiMe3)3 to obtain [(C5H4SiMe3)3An{Si(SiMe3)3}], An ¼ Th, Th-409; U, U-409, Scheme 98.274 The 29Si{1H} NMR resonance for the Si atom bonded to Th and U was assigned at −108.92 and −137.09 ppm, respectively. The AndSi bond distances were 3.1191(8) and 3.0688(8) A˚ for Th and U, respectively.

Scheme 98 Synthesis of thorium and uranium-silyl bonds.

The inverse tris(cyclopentadienyl) uranium(III) complex, [{(BDI)Re(m-5:5-C5H5)3}3U(L)], L ¼ THF, U-410, 1,4-dioxane, U411, and DMAP, U-412, was synthesized from [UI3(1,4-dioxane)1.5] and three equivalents of Na[(BDI)Re(C5H5)], BDI ¼ N,N0 -bis(2,6-diisopropylphenyl)-3,5-dimethyl-b-diketiminate, Scheme 99.275 X-band EPR spectroscopy was used to confirm the U(III) oxidation state with g-values (3.04 and 1.6) consistent with 4I9/2 ground state.

Cyclopentadienyl and phospholyl compounds in organometallic actinide chemistry

235

Scheme 99 Use of a Cp-rhenium complex to form a tris(Cp) uranium(III) complex.

Reinvestigation of tris(cyclopentadienyl) complexes of neptunium33 was done by Walter and Arnold.276 Reaction of NpCl4 with excess K(C5H5) forms [(C5H5)3NpCl], Np-413. Reduction of the mono-chloride complex with Na/Hg amalgam in diethyl ether forms [(C5H5)3Np], Np-414, Scheme 100. The Cp ring protons and carbon resonances are located at −9.65 and 150.4 ppm in the 1H and 13C NMR spectra, respectively in THF-d8. Addition of acetonitrile to Np-414 resulted in the formation of pale green crystals of [(C5H5)3Np(NCMe)2], Np-415. In situ generation of NpCl3 by reaction of NpCl4 with Na/Hg amalgam in diethyl ether at room temperature, followed by three equivalents of K(C5H4SiMe3), formed [(C5H4SiMe3)3Np], Np-355. In THF-d8, the 1H NMR resonances were observed at −0.62 ppm (SiMe3), and −8.81 and −8.98 ppm for the Cp ring. In toluene-d8, the SiMe3 protons shift to −1.38 ppm, while the Cp ring protons shifted to −8.60 and −9.51 ppm. The Np-centroid distances in Np-355 are 2.485(2), 2.481(2), and 2.479(2) A˚ .

Scheme 100 Formation of tris- and tetrakis(cyclopentadienyl) neptunium complexes.

+ Arnold has also used the An(IV/III) redox potentials to reduce UO2+ 2 to UO2 in the case of [(C5H5)3U], U-414, Scheme 101. The case is less straightforward for [(C5H5)3Np], Np-414, and no reaction occurs with [(C5H5)3Pu], Pu-414.277

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Scheme 101 Reduction, coordination, and no reaction of tris(cyclopentadienyl) actinide complexes with uranyl.

The synthesis of a homoleptic, trivalent americium complex, [(C5Me4H)3Am], Am-368, was successful, Scheme 102, through reaction of AmCl3 with three equivalents of K(C5Me4H). This was a remarkable finding as it represents the first synthesis, isolation and subsequent characterization of an organoamericium complex.278 The Am-centroid distance is 2.517(8) A˚ with 1H NMR resonances located at −3.64, 0.46, and 12.67 ppm in C6D6.

Scheme 102 (C5Me4H)3Am, the first fully characterized organoamericium complex.

4.05.5

Tetrakis(cyclopentadienyl) complexes

The first cyclopentadienyl complexes, [(C5H5)4U], U-416, was synthesized in 19622 and the structure determined in 1973.279 The thorium analog, [(C5H5)4Th], Th-416, was not characterized until 1993.280 In general, there are very few tetrakis(cyclopentadienyl) complexes.281–285 Using a similar methodology adapted for making sterically crowded tris(cyclopentadienyl) complexes, i.e. salt metathesis from a cationic complex, the reaction of [(C5H4SiMe3)3U(THF)][BPh4], U-417, with K(C5H4SiMe3) forms [(C5H4SiMe3)4U], U-418, Scheme 103.286

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Scheme 103 Synthesis of a tetrakis(cyclopentadienyl) uranium complex.

Reaction of [(C5H5)3NpCl], Np-413, with K(C5H5) does not produce [(C5H5)4Np], Np-416,287 but leads to the ‘ate’ salt, K [Np(C5H5)4], Np-419, Scheme 100.276 In Np-419, three Cp are 5 with Np-centroid distances of 2.527(4), 2.516(4), and 2.493 (6) A˚ , and one 1 with NpdC distance of 2.752(7) A˚ . This moiety was also observed with [(5-C5H4SiMe3)3(1-C5H4SiMe3)U].235 [(C5H5)4Np], Np-416, can be made directly from NpCl4 and excess of K(C5H5) in toluene. The Np-centroid distances in (C5H5)4Np are 2.551(2) A˚ with a centroid-Np-centroid angle of 109.4(2) . While difficult to summarize the vast number of bis(cyclopentadienyl) complexes, a small sample of tris and tetrakis(cyclopentadienyl) complexes are provide in Table 1. In general, most actinide-centroid distances fall within the range of 2.4–2.6 A˚ .

Table 1

Summary of tris(cyclopentadienyl) actinide and (C5H5)4An complexes. Average An-centroid distances are given.

[{C5H3(SiMe3)2}3An]1− [{C5H3(SiMe3)2}3An] [(C5Me4H)3An] [(C5Me5)3An] [(C5H5)4An] 

Formal oxidation state of metal

Th

U

Np

Pu

Am

+2 +3 +3 +3 +4

2.521 2.520288 2.551 2.607252 2.606280

∗ 2.54259 2.523 2.581 2.55279

2.527238 ∗ – – 2.551239

2.522239 ∗ – – –

– – 2.517278 – –

, Data was insufficient to be reported; –, Compound has not been synthesized.

4.05.6

Phospholyl ligands

Although previously unmentioned, heterocyclopentadienyl ligands of the general formula C5-nR5-nEn, known as phospholyls, E ¼ P, and arsolyls, E ¼ As, have been used to coordinate metals outside of the f-block, but examples with actinides are uncommon. Many of the actinide phospholyl complexes were reported by the Ephritikhine group. The reaction of three equivalents of 2,3,4,5-tetramethylphospholide, K(PC4Me4) with UCl4 yields [(PC4Me4)3UCl], U-420.289 When only one equivalents of K(PC4Me4) with UCl4, the bis(phospholyl) product, [(PC4Me4)2UCl2], U-421, can be isolated. Complex U-420 can be used as a precursor to make other tris(phospholyl) complexes. For example, U-420 can be reacted with KBEt3H, MeLi, or NaOiPr produces [(PC4Me4)3UH], U-422, [(PC4Me4)3UMe], U-423, and [(PC4Me4)3UOiPr], U-424, respectively, Scheme 104. When one equivalent of NiCl2 is added to U-420 or U-421, the result is a heterotrimetallic complex in which two [(PC4Me4)2UCl2] units are bridged by Ni(II) through the four phosphorus atoms in the phospholyl ligand, U-425. Reduction of U-425 with Na/Hg formed a heterotetrametallic complex, U-426.

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Cyclopentadienyl and phospholyl compounds in organometallic actinide chemistry

Scheme 104 Reactivity of tris(phospholyl) uranium monochloride complex.

Reaction of two equivalents of K(PC4Me4) with [(Mes)U(BH4)3] (Mes ¼ 6-1,3,5-(CH3)3C6H3), U-427, Scheme 105, to form a U(III) phosphoryl complex, [(PC4Me4)2U(BH4)2]K, U-428, in THF, but when dissolved in toluene, KBH4 is lost to form the dimeric, [(PC4Me4)2U(BH4)]2 U-429.290 Uranium(IV) compounds can be prepared by the reaction of U(BH4)4 with two equivalents of K(PC4Me4) to form [(PC4Me4)2U(BH4)2], U-430. Complex U-429 can also be made through reduction of U-430 with Na/Hg amalgam.291 Two signals in the 31P NMR spectrum of U-429 were observed at 727 and 3471 ppm. The structure of U-429 was later reported and showed a UdP distances of 2.945(3) A˚ for the terminal 1-phosphorus, and 2.995(3) A˚ for the 5-phosphorus.292 Complex U-430 can be used to prepare several complexes of the form [(PC4Me4)2UR2], R ¼ Me, U-431; CH2SiMe3, U-432.84

Cyclopentadienyl and phospholyl compounds in organometallic actinide chemistry

239

Scheme 105 U(III) and U(IV) phosphoryl complexes.

Reaction of UI3 with K(EC4Me4), E ¼ N, P, As, in THF at −78  C, followed by K2[C8H6(1,4-(SiiPr3)2)] at −30  C in pentane, forms the U(III) mixed-sandwich complexes of the form [{C8H6(1,4-SiiPr3)2}(C5Me4E)U(THF)], E ¼ N, U-433; P, U-434; As, U435.293 The electrochemistry of U-434 shows only a 0.02 V anodic shift of (C5Me4P) compared to the (C5Me4H) analog. Complexes U-433 and U-434 react with CO2 to insert into the UdE bond forming 1-Cp units, E ¼ N, U-436; P, U-437, Scheme 106.

Scheme 106 Synthesis and CO2 insertion reactivity of U(III) complexes with pyrolyl, phospholyl, and arsolyl ligands.

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Cyclopentadienyl and phospholyl compounds in organometallic actinide chemistry

Reaction of [U(C8H8)(BH4)2(THF)], U-438, with K(PC4Me4) yields [U(C8H8)(PC4Me4)(BH4)(THF)], U-439, Scheme 107. Phospholyl ligated complexes with cyclooctatetraenyl were prepared by reaction of [U(C8H8){OP(NMe2)3}3][BPh4]2, U-440, with K(PC4Me4), forming [U(C8H8)(PC4Me4){OP(NMe2)3}2][BPh4], U-441.294 Complex U-441 can be reduced with Na/Hg amalgam to make the U(III) complex, [U(C8H8)(PC4Me4){OP(NMe2)3}2], U-442.295

Scheme 107 Synthesis of cyclooctatetraenyl phospholyl ligated uranium complex.

4.05.7

Conclusion

The once slowly expanding field of organometallic actinide chemistry is once again one that is burgeoning and growing, entering a period of renewed interest and study.31 While many ligand frameworks have been developed in the search for novel functionalities, optimized catalysis, and bonding elucidation; the cyclopentadienyl ligand remains an important tool for the advancement of organoactinide chemistry. Arguably, cyclopentadienyl-based ligands have advanced actinide chemistry in a way that no other ligand framework has been able to do. Its versatility and modularization have allowed for extensive isolation and study of actinides in a variety of oxidation states and continues to be important to the field. In fact, cyclopentadienyl ligands are responsible for the isolation of uranium in oxidation states from +2 to +6. The use of Cp-based ligands with sterically encumbering groups such as iPr or tBu have recently been introduced and show great promise towards further advancements. For example, are (Ci5Pr5)2An, An ¼ Th, Np, Pu, viable synthetic targets? While cyclopentadienyl ligands have been used to advance actinide-pnictogen chemistry in recent years, actinide-tetrel296 or triel bonded analogs remain relatively under-developed. Mono-cyclopentadienyl complexes, as well as ones with phopholyl or arsolyl ligands, of any other actinide besides uranium are sparse, which could be another potential avenue of investigation.

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Structure of Tetracyclopentadienyluranium (IV). J. Am. Chem. Soc. 1973, 95 (11), 3815–3817. 280. Maier, R.; Kanellakopulos, B.; Apostolidis, C.; Meyer, D.; Rebizant, J. Molecular Structure and Charge Distribution in Organometallics of the 4f and 5f Elements V: Crystal and Molecular Structure of Tetrakis (n5-Cyclopentadienyl)-Thorium (IV) and the Temperature Dependence of Its Electrical Dipole Moment. J. Alloys Compd. 1993, 190 (2), 269–271. 281. Bursten, B.; Casarin, M.; DiBella, S.; Fang, A.; Fragala, I. Photoelectron Spectroscopy of f-Element Organometallic Complexes. 6. Electronic Structure of Tetrakis (Cyclopentadienyl) Actinide Complexes. Inorg. Chem. 1985, 24 (14), 2169–2173. 282. Brennan, J. G.; Andersen, R. A.; Zalkin, A. Crystal Structures of (MeC5H4) 4U2 (mu-NR)2 Unsymmetrical Bridging, R ¼ Ph, and Symmetrical Bridging, R ¼ SiMe3, Organoimide Ligands in Organoactinide Compounds. J. Am. Chem. Soc. 1988, 110 (14), 4554–4558. 283. Weydert, M.; Brennan, J. G.; Andersen, R. A.; Bergman, R. G. Reactions of a Uranium (iv) Tertiary Alkyl Bond: Facile Ligand-Assisted Reduction and Insertion of Ethylene and Carbon Monoxide. Organometallics 1995, 14 (8), 3942–3951. 284. Dormond, A.; Duval-Huet, C.; Tirouflet, J. Complexes Cyclopentadienyles De l’uranium (IV): II. Complexes Tricyclopentadienyles D’uranium (IV) a Chiralite Centro-Metallee. Diastereotopie et Diastereoisomerie. J. Organomet. Chem. 1981, 209 (3), 341–354. 285. Dormond, A.; Hepiegne, P.; Hafid, A.; Moise, C. Complexes Héterobinucléaires Diphénylphosphinocyclopentadiényluranium-Métal de Transition:(CpPPh2)nU(NEt2)4-nM(CO)4 (n ¼ 2, 4; M ¼ Cr, Mo). J. Organomet. Chem. 1990, 398 (1-2), C1–C5. 286. Windorff, C. J.; MacDonald, M. R.; Ziller, J. W.; Evans, W. J. Trimethylsilylcyclopentadienyl (Cp0 ) Uranium Chemistry: Synthetic and Structural Studies of Cp0 4U and Cp0 3UX (X ¼ Cl, I, Me). Z. Anorg. Allg. Chem. 2017, 643 (23), 2011–2018. 287. Baumgärtner, F.; Fischer, E. O.; Kanellakopulos, B.; Laubereau, P. Tetrakis(cyclopentadienyl)neptunium(IV). Angew. Chem. Int. Ed. 1968, 7 (8), 634. 288. Blake, P. C.; Lappert, M. F.; Atwood, J. L.; Zhang, H. The Synthesis and Characterisation, Including X-ray Diffraction Study, of [Th{Z-C5H3(SiMe3)2}3]; the First Thorium(III) Crystal Structure. J. Chem. Soc. Chem. Commun. 1986, 15, 1148–1149. 289. Gradoz, P.; Boisson, C.; Baudry, D.; Lance, M.; Nierlich, M.; Vigner, J.; Ephritikhine, M. Synthesis, Crystal Structure and Some Derivatives of the Chlorotris(tetramethylphospholyl) uranium. J. Chem. Soc. Chem. Commun. 1992, 23, 1720–1721. 290. Gradoz, P.; Baudry, D.; Ephritikhine, M.; Nief, F.; Mathey, F. Phospholyluranium Complexes. J. Chem. Soc. Dalton Trans. 1992, (20), ;3047–3051. 291. Baudry, D.; Ephritikhine, M.; Nief, F.; Ricard, L.; Mathey, F. Synthesis of Phospholyltetrahydroboratouranium Complexes. Crystal Structure of [(Z5-C4Me4P) 2U (BH4) 2]. Angew. Chem. Int. Ed. 1990, 29 (12), 1485–1486. 292. Gradoz, P.; Ephritikhine, M.; Lance, M.; Vigner, J.; Nierlich, M. Dimeric Uranium Complexes with Bridging Phospholyl Ligands. Crystal Structure of [{U (Z5-C4Me4P) (m-Z5-C4Me4P)(BH4)}2]. J. Organomet. Chem. 1994, 481 (1), 69–73. 293. Kahan, R. J.; Cloke, F. G. N.; Roe, S. M.; Nief, F. Activation of Carbon Dioxide by New Mixed Sandwich Uranium(iii) Complexes Incorporating Cyclooctatetraenyl and Pyrrolide, Phospholide, or Arsolide Ligands. New J. Chem. 2015, 39 (10), 7602–7607. 294. Cendrowski-Guillaume, S. M.; Nierlich, M.; Ephritikhine, M. The First Mixed Cyclooctatetraenyl-Phospholyl Metal Complexes. Crystal Structure of [U (Z-C8H8)(Z-C4Me4P)(BH4) (OC4H8)]. J. Organomet. Chem. 2002, 643, 209–213. 295. Cendrowski-Guillaume, ; Sophie, M.; Le Gland, G.; Nierlich, M.; Ephritikhine, M. A Comparison of Analogous 4f- and 5f-Element Compounds: Syntheses and X-ray Crystal Structures of the Mixed Sandwich Complexes [M(Z-C8H8)(L){OP(NMe2)3}] (M ¼ Nd or U; L ¼ Z-C5Me5 or Z-C4Me4P). Eur. J. Inorg. Chem. 2003, 2003 (7), 1388–1393. 296. Réant, B. L. L.; Liddle, S. T.; Mills, D. P. f-Element Silicon and Heavy Tetrel Chemistry. Chem. Sci. 2020, 11 (40), 10871–10886.

4.06

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

Alexander FR Kilpatricka and F Mark Chadwickb, aSchool of Chemistry, University of Leicester, University Road, Leicester, United Kingdom; bDepartment of Chemistry, Molecular Sciences Research Hub, Imperial College London, London, United Kingdom © 2022 Elsevier Ltd. All rights reserved.

4.06.1 4.06.2 4.06.2.1 4.06.2.1.1 4.06.2.1.2 4.06.2.1.3 4.06.2.1.4 4.06.2.1.5 4.06.2.2 4.06.2.2.1 4.06.2.2.2 4.06.2.2.3 4.06.2.2.4 4.06.2.2.5 4.06.2.2.6 4.06.2.2.7 4.06.2.2.8 4.06.2.2.9 4.06.2.2.10 4.06.2.3 4.06.2.3.1 4.06.2.4 4.06.3 4.06.3.1 4.06.3.1.1 4.06.3.1.2 4.06.3.1.3 4.06.3.1.4 4.06.3.2 4.06.3.2.1 4.06.3.2.2 4.06.3.2.3 4.06.3.2.4 4.06.3.2.5 4.06.3.2.6 4.06.3.3 4.06.3.3.1 4.06.3.4 4.06.4 4.06.4.1 4.06.4.1.1 4.06.4.1.2 4.06.4.1.3 4.06.4.1.4 4.06.4.1.5 4.06.4.1.6 4.06.4.2 4.06.4.2.1 4.06.4.2.2 4.06.4.2.3 4.06.4.3 4.06.5 References

248

Introduction Titanium Mono(cyclopentadienyl) titanium chemistry Nitrogen-based ligands Aryloxide complexes for olefin co-polymerization Functionalized Cp ligands Mixed sandwich complexes Cluster complexes Bis(cyclopentadienyl) titanium chemistry Dinitrogen activation Proton coupled electron transfer (PCET) Oxo and peroxo complexes Triflate complexes involved in water splitting Low valent titanocene alkyne complexes Titanacyclobutanes and butadienes Derivatized Cp ligands Main group chemistry Organofluorine chemistry Heterobimetallic complexes Phospholyl titanium chemistry Background and phospholyl titanium chemistry since 2000 Table of crystallographically characterized Ti compounds Zirconium Mono(cyclopentadienyl) zirconium chemistry Hydroamination catalysts Zr CpR complexes for olefin oligomerization Other ZrCpR reactivity Zr CpR combined with other rings Bis(cyclopentadienyl) zirconium chemistry Zirconocene complexes for small molecule activation Zirconocene alkyne chemistry Zirconocene p-block chemistry Other zirconocene hydrides Zirconocene heterobimetallic complexes Other zirconocene complexes Phospholyl zirconium chemistry Phospholyl zirconium chemistry since 2000 Table of crystallographically characterized Zr compounds Hafnium Mono(cyclopentadienyl) hafnium chemistry Nitrogen based ligands Aryloxide complexes for polymerization Constrained geometry complexes Carbon-based ligands Main group ligands Cluster complexes Bis(cyclopentadienyl) hafnium chemistry Dinitrogen activation and functionalization Low valent CpR2Hf alkyne complexes Main group chemistry Table of crystallographically characterized Hf compounds Closing remarks

Comprehensive Organometallic Chemistry IV

250 251 251 251 258 259 262 264 270 270 272 275 276 277 286 287 292 298 298 301 301 301 309 309 309 311 314 316 317 317 325 332 338 339 339 341 341 342 362 362 362 366 367 368 371 371 372 372 377 380 387 391 391

https://doi.org/10.1016/B978-0-12-820206-7.00074-3

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

Nomenclature ngc6,6-dmch 6,6-tmch Ab Ad ArF 9-BBN BDFE BINOL bipy btsma Bu 18-c-6 CG Cht cod COT Cp Cp CpR CPPA Cy Dipp dpm DMAD DMAP DME Dmp DMPE EBI EBTHI Fc Flu Fv HAT HATN HMDS hpp Ind NacnacDipp Naph MAO MBSBI b-MeBG NHCMe4 NHCiPr2Me2 NOESY Octaflu OTf PGSE Pin pmdta Pn {H} PPN Pr Pyr pz TADDOL

Z5-6,6-Dimethylcyclohexadienyl Z5-2,6,6-Trimethylcyclohexadienyl 1,2-Azaboryl Adamantylidene, C10H14 3,5-(CF3)2-C6H3 9-Borabicyclo[3.3.1]nonane Bond dissociation free energy 1,10 -Bi-2-naphthol Bipyridine (numerically defined e.g. 2,2-bipy) Bis(trimethylsilyl)acetylene, Me3SiCCSiMe3 Butyl, C4H9 (n, tert, iso defined e.g. nBu) 18-Crown-6 Constrained geometry Z7-Cycloheptatrienyl 1,5-Cyclooctadiene Z8-Cyclooctatetraenyl Z5-Cyclopentadienyl Z5-Pentamethylcyclopentadienyl Substituted Cp ligand with R defined Cyclopentaphenanthrenyl Cyclohexyl 2,6-iPr-C6H3 5,5-Dimethyldipyrrolylmethane Dimethyl acetylenedicarboxylate 4-Dimethylaminopyridine Dimethyoxyethane 2,6-Dimesitylphenyl 1,2-Bis(dimethylphosphino)ethane Ethylene-bridged indenyl Ethylene-1,2-bis(5-4,5,6,7-tetrahydro-1-indenyl) Ferrocenyl, (Z5-C5H4)Fe(Z5-C5H5) Fluorenyl, C13H8 Fulvene, (C5Me4CH2) 1,4,5,8,9,12-Hexaazatriphenylene 1,6,7,12,13,18-Hexaazatrinaphthylene Hexamethyldisilazane Hexahydropyrimidinopyrimidine Z5-Indenyl ¼ C9H6. . . HC(CMeNDipp)2 ¼ HC[C(Me)N(2,6-iPr2C6H3)]2 1-Naphthyl Methylaluminoxane (2-Methyl-4,5-benz-1-indenyl)2SiMe2 4,6-O-Benzylidene-b-D-glucopyranoside 1,3,4,5-Tetramethylimidazolin-2-ylidene 1,3-Diisopropyl 4,5-dimethylimidazol-2-ylidene Nuclear Overhauser Effect Spectroscopy Octamethyloctahydrodibenzofluorenyl, C29H36 Triflate, [SO3CF3]− Pulsed gradient spin echo Pinacolato ¼ (CH3)4C2O2B N,N,N0 ,N00 ,N00 -pentamethyldiethylenetriamine (Hydro)permethylpentalene ¼ Z5-C8Me6H Bis(triphenylphosphine)iminium cation Propyl, C3H7 (iso/n defined e.g. iPr) Pyridyl, C5H4N Pyrazolyl a,a,a0 ,a0 -Tetrphenyl-2,2-disubstituted 1,3-dioxolane-4,5-dimethanoxy

249

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Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

2,2,6,6-Tetramethylpiperidine 1-oxyl tert-Hexyl ¼ -C(CH3)2CH2CH2CH3 Tetrahydrothiophene Tetrahydoindenyl 2,2,6,6-Tetramethylpiperidino 2,4,6-Triisopropylphenyl ¼ (2,4,6-iPr3C6H2) p-Tolyl ¼ (4-Me-C6H3) Tris(pyrazolyl)borate Generic weakly coordinating anions m-Xylyl ¼ (3,5-Me2C6H3)

TEMPO Tex THT THI Tmp Tipp Tol tpz WCA Xyl

Ansa bridged ligands abbreviation 0

[E-CpR-CpR ]

4.06.1

E definition of ansa-bridge, R ¼ substitution pattern on one Cp, R0 substitution of other Cp, ring numbering defined as 1 at ansa-bridge

Introduction

Some of the earliest synthetic chemistry with the cyclopentadienyl anion was with titanium and zirconium.1 Relatively soon afterwards the motif was demonstrated to be a proficient catalyst (in the presence of suitable co-catalysts) for the polymerization of olefins.2 This application dominated research for the latter half of the 20th century. However, it also found other uses—for example from the mid-1960s it was shown that low valent titanocene species could fixate N2.3 From 1980 onwards the use of the MCp (M ¼ Ti, Zr, Hf ) fragment for other catalytic processes began to be developed4 and from the late 1980s low-valent alkyne complexes of these metals were isolated also.5,6 This chapter focuses on some highlights of group 4 cyclopentadienyl chemistry developed in the 3rd millennium. Since the beginning of 2000 nearly 6000 structures featuring a group 4 metal bound Z5 to a cyclopentadienyl fragment have been uploaded to the Cambridge Crystallographic Data Centre (CCDC). A search of this fragment over the same time-period on Scifinder gives over 21,000 documents of which 10,571 are journal articles and a separate 365 review articles have also been written. This overwhelming number of publications demonstrates the deep interest and incredible versatility of this chemical motif, however, a full coverage would not be reasonable within the scope of this chapter. As such, only the most significant achievements of the last 21 years’ worth of work will be discussed, particularly when the considered complexes have found applications either in catalysis or materials chemistry. A numbered list of complexes that have been structurally characterized by single crystal X-ray diffraction will be provided at the end of the section for each motif (Table 1). Since 2000 research continued apace on the development of polymerization catalysts, with a particular interest in developing mechanistic understanding and structure-activity relationships, though this research stream seems to have diminished in recent years. Other catalytic processes using group 4 cyclopentadienyl complexes have continued to be developed, as has their continued application toward small molecule activation. Alkyne complexes attracted a lot of attention in the rich variety of coupling and insertion reactivity that these motifs can perform, often with exquisite selectivity. Zirconocene has played a significant role in the development of frustrated Lewis pair chemistry, and all of the group 4 metals have seen use in the recent development of p-block chemistry. The last 21 years has shown exciting advances in the use of the MCp fragments, and in this Chapter we highlight this showing that this old dog has been taught some new tricks.

Table 1 A summary of the amount of work done on the “MCp” motif. CCDC structure number was attained via a Conquest search, and the number of references, journal articles and reviews were attained by a Scifinder search (correct as of March 2021). Between the different metals there is considerable overlap in the publications, hence the total is not a sum of the individual metal references.

Ti Zr Hf Ti, Zr, Hf

CCDC structures

References

Journal Articles

Reviews

2922 2552 418 5892

9827 12,101 2559 21,298

5322 5763 817 10,571

125 138 17 365

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

4.06.2

Titanium

4.06.2.1

Mono(cyclopentadienyl) titanium chemistry

4.06.2.1.1

251

Nitrogen-based ligands

4.06.2.1.1.1 Amido and imido ligands Titanium mono(cyclopentadienyl) complexes featuring nitrogen based ligands have been extensively studied over the past two decades. Scherer et al. reported a detailed study of the b-agostic moiety in [CpTiCl2N(iPr)2] (1Ti), including a low-temperature X-ray crystal structure determined at 9 K,7 which shows one of the smallest M-Na-Cb angles and shortest M


H distances observed to date for an amido species with a b-agostic interaction. DFT calculations revealed that the NaCb bond length in 1Ti increase and become more activated with decreasing Ti-Na-Cb angles and shorter Ti


H distances. Mashima and co-workers investigated reactions of the half-sandwich [Cp TiCl3] with organosilicon reagent 1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene. Whereas the latter acts as a reducing agent with titanocenes, no such reduction occurs with half-sandwich complexes, and a Ti(IV) complex [Cp TiCl2(1-trimethylsilyldiazacyclohexadienyl)] (2Ti) was isolated.8 Treatment of 2Ti with an equimolar amount of Cp TiCl3 afforded [{Cp TiCl2}2(m-pyrazine)] with the release of Me3SiCl. Mashima and co-workers investigated arylimido-bridged dinuclear complexes of titanium. N-trimethylsilylated amidotitanium complexes [CpRTiCl2(NArF{SiMe3})] (CpR ¼ Cp (3Ti), Cp ) were prepared by reaction of [CpRTiCl3] with the lithium amide LiNArF{SiMe3} (ArF ¼ 3,5-(CF3)2C6H3). Dissolving [CpRTiCl2(NArF{SiMe3})] complexes in THF, led to a desilylation reaction, to afford imido-bridged dinuclear complexes, [(CpRTiCl{m-NArF})2] (CpR ¼ Cp (4Ti), Cp (5Ti)), together with the liberation of Me3SiCl.9 The dinuclear m-(N-arylimido)titanium complexes showed a reversible one-electron redox wave, and chemically reduced titanium complexes, [Cp2Co][(CpRTiCl{m-NArF})2] (CpR ¼ Cp (6Ti), Cp ), were prepared using Cp2Co as a reductant. Analysis of 6Ti by X-ray crystallography and EPR spectroscopy suggested that one unpaired electron was delocalized in the Ti(m-NR)2Ti core.9 Treatment of imido-bridged dinuclear complexes, [(CpTiCl{m-NAr})2] with LiCH2SiMe3 afforded the dialkyl complexes, [(CpTi {CH2SiMe3}{m-NAr})2] for Ar ¼ Ph (7Ti), Xyl (8Ti), ArF (9Ti).10 Reaction of with 1-(trimethylsilyl)propyne afforded the corresponding dinuclear complexes, 10Ti-12Ti, which have two six-membered metallacycles formed via ortho-CdH bond activation of the m-(N-arylimido) ligands (Fig. 1).10

Fig. 1 Half-sandwich Ti complexes with amido and bridging imido ligands.

In 2005 Kissounko et al. reported the synthesis of k1-O bound amidate complex [Cp TiCl(]NtBu)(k1-OC(Me)NMe2)] (13Ti) by reaction of half-sandwich titanium imido complex [Cp Ti(NtBu)Cl(py)]11 with N,N-dimethylacetamide. Thermolysis of 13Ti in toluene results in nearly quantitative conversion to the tert-butyl-N,N-dimethylamidine and the oxo-bridged titanium trimer [{Cp TiCl(m-O)}3] (14Ti) (Scheme 1a), which shows a Cs-symmetric structure. Two equivalents of the secondary amide MeC(O) NHPh reacts with [Cp Ti(NtBu)Cl(py)], displacing pyridine and tBuNH2 to form the bis(k2-amidate) complex [Cp TiCl(k2-OC {Me}NPh)2] (15Ti). The latter exhibits a pseudo- octahedral geometry with the Cp ligand and an amidate nitrogen occupying apical positions. Complex 15Ti was investigated as a plausible intermediate in the context of Ti(IV)-catalyzed transamidation reactions between aryl-amides and arylamines. However, when 15Ti was heated with 1 equiv. of a para-substituted aniline ArNH2, the secondary amidines MeC(NAr)NAr are formed (Scheme 1b).

252

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Scheme 1 Reactions of half-sandwich Ti amidate complexes, reported by Kissounko et al.

In 2007 Kissounko et al. reported the bis- and tris- amidate complexes [Cp Ti(NHiPr)(k2-OC{Me}NPh)2] (16Ti) and [Cp Ti (k -OC{Me}NPh)(k2-OC{Me}NPh)2] (17Ti) in the context of transamidation catalysis.12 Thermolysis of complex 16Ti at 50  C resulted in exclusive formation of a 2:1 mixture of the secondary amidine, MeC(]NiPr)NHPh, and the bis(m-oxo)Ti dimer, [(Cp Ti(m-O)(k2-OC{Me}NPh))2] (18Ti) (Scheme 1c). At elevated temperatures, 17Ti reacts with 1 equiv. of benzyl amine (PhCH2NH2) to afford the amidine MeC(]NPh)NHCH2Ph, the bis(m-oxo)Ti dimer 18Ti, and acetanilide in 2:1:2 ratio, respectively (Scheme 1d). Mountford and co-workers investigated the reaction of amidinate supported half-sandwich imidotitanium complexes with a variety of small molecules. A series of Cp-amidinate supported imido complexes, [CpRTi{MeC(NiPr)2}(NR)], were synthesized, of which [Cp Ti{MeC i (N Pr)2}(NXyl)] (19Ti) was characterized by X-ray diffraction. Reactions with CO2 proceed via initial cycloaddition reactions but, depending on the imido N-substituent, go on to yield different products. Reaction of [Cp Ti{MeC(NiPr)2}(NtBu)] with CO2 afforded the m-oxo dimeric compound [(Cp Ti{MeC(NiPr)2}{m-O})2] (20Ti) via isocyanate extrusion. Reaction of 19Ti with CO2 afforded the double-insertion product [Cp Ti(MeC{NiPr}2)2(k2O,O-OC{O}N{Xyl}C{O}O)] (21Ti).13 Titanium Cp-amidinate tert-butyl imido complexes complexes undergo cycloaddition/extrusion reactions with CS2 and COS to form m-sulfido dimers [(Cp Ti{m-S}{PhC(NSiMe3)2})2] CpR ¼ CpMe (23Ti), Cp (24Ti), and tBuNCS and tBuNCO, respectively.14 Reaction of [Cp Ti(MeC{NiPr}2)(NR)] with aryl isocyanates gave the N,O-bound ureates [Cp Ti(k2N,O-N{Xyl}C{NAr}O-)(MeC {NiPr}2)], such as 25Ti and 26Ti, which did not undergo extrusion. Benzamidinate complexes with pendant arms were also studied for their reaction with CO2.15 Mono-Cp complexes [Cp Ti t (N Bu)(Me3SiNC{Ph}N{CH2}nNMe2)] react with CO2 to afford N,O-bound carbamates, such as [Cp Ti(NtBuC{O}O)(Me3SiNC {Ph}NCH2CH2CH2NMe2)] (27Ti). Mono-CpMe complexes react with CO2 to form the corresponding N,O-bound carbamate complexes, which, in turn, extrude tBuNCO to form the m-oxo-bridged dimeric complexes, [(CpMeTi{Me3SiNC(Ph)N (CH2)nNMe2})2(m-O)2] of which 28Ti and 29Ti were structurally characterized. 1

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

253

Tolylimide complex [Cp Ti(MeC{NiPr}2)(NTol)] showed no reaction with pinacol borane (HBPin), but reacted with both tert-hexyl borane, H2BTex (Tex ¼ tert-hexyl, dC{CH3}2CH2CH2CH3), and Piers’ borane (HB(C6F5)2), to afford [Cp Ti(MeC {NiPr2}2)(N{Tol}H2BTex)] (30Ti) and [Cp Ti(MeC{NiPr2}2)(N{Tol}HB{C6F5}2)] (31Ti), respectively (Fig. 2).16

Fig. 2 Half-sandwich Ti complexes with terminal imido ligands and their reaction products.

Extending the imide-amine methodology to borylimido chemistry, Mountford and co-workers reported the synthesis of [Cp Ti(NB{NDippCH}2)Cl(py)] (32Ti) via thermolysis of a mixture of [Cp Ti(NtBu)Cl(py)] and borylamine H2NB (NDippCH)2 under a dynamic vacuum.17 Reaction of 32Ti with 1 equiv. of either Li[hpp] or Li[NpyrNMe2] afforded [Cp Ti(hpp) (NB{NDippCH}2)] (33Ti) (hpp ¼ hexahydropyrimidinopyrimidine) and [Cp Ti(NpyrNMe2)(NB{NDippCH}2)] (34Ti), respectively (Scheme 2).

Scheme 2 Synthesis and reactivity of half-sandwich Ti borylimido complexes reported by Mountford and co-workers.

4.06.2.1.1.2 Hydrazido ligands Mountford and co-workers later reported the synthesis of half-sandwich hydrazido complexes via a tert-butyl imido/hydrazine exchange reaction. Whereas the reaction of [Cp Ti(NtBu)Cl(py)] with Ph2NNH2 gave the desired hydrazido(2-) complex [Cp Ti(NNPh2)Cl(py)] (35Ti), the corresponding reaction of [CpTi(NtBu)Cl(py)] gave the dimer [Cp2Ti2(m-Z1:Z1-NNPh2)(m-Z2:Z1-NNPh2)Cl2].18 Reaction of [Cp Ti(NtBu)Cl(py)] with 1 equiv. of Me2NNH2 also gave a dimer, [(Cp TiCl)2(m-Z1:Z1-NNMe2)(m-Z2:Z1-NNMe2)] (36Ti), accompanied by several side-products including the mono(hydrazido(1-)) species [Cp Ti(Z2-NHNMe2)Cl2] (37Ti). Whereas, the reaction with 2 equiv. of Me2NNH2 gave [Cp Ti(Z2-NHNMe2)2Cl] (38Ti), containing two Z2-bound hydrazide(1-) ligands. The mixed hydrazido(2-)/amido complexes [Cp Ti(NNPh2)(NHNPh2)(py)] (39Ti) and [Cp Ti(NNPh2)(NHtBu)(py)] (40Ti) were prepared by the reaction of [Cp Ti(NNPh2)Cl(py)] with the corresponding primary lithium amide reagents, LiNHR. Reaction of sandwich compound [Cp2Ti(NtBu)(py)] with Me2NNH2 gave the dinuclear complex [CpTi(NHtBu)(m-Z1:Z1-NNMe2) (m-Z2:Z1-NNMe2)TiCp(Z1-Cp)] (41Ti) among other species (Fig. 3).

Fig. 3 Half-sandwich Ti complexes with terminal hydrazido ligands and their reaction products.

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Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

Again, the Cp-amidinate ligand platform provided a rich source of Ti]N-NR2 reactivity in hydrazido systems. The reaction of [Cp Ti(MeC{NiPr}2)(NtBu)] with either Ph2NNH2 or Me2NNH2 gave terminal hydrazido species [Cp Ti(MeC{NiPr}2)(NNR2)] with R ¼ Ph and Me (42Ti),19 and the reactivity of these complexes was explored (Scheme 3). Reaction of terminal hydrazides [Cp Ti(MeC{NiPr}2)(NNR2)] with CO2 afforded the dicarboxylate complexes [Cp Ti(MeC{NiPr}2)(OC{O}N{NR2}C{O}O)] (R ¼ Me or Ph (43Ti)).20 Reaction of 42Ti with primary aryl silanes and BuSiH3 formed [Cp Ti(MeC{NiPr}2)(H)(Z2-N{NMe2} SiH2R)] for Ar ¼ Ph (44Ti), ArF (45Ti), 4-C6H4OMe (46Ti), the product of 1,2-addition of a SidH bond across Ti ¼ Na to give a titanium hydride-silylhydrazide(1-) compound.21,22

Scheme 3 Reactivity of half-sandwich Ti hydrazido complexes reported by Mountford and co-workers.

Reaction of 42Ti with dichlorosilanes PhSiHCl2 or Me2SiCl2 gave hydrazide(1-) products [Cp Ti(N{NMe2}SiH{Ph}N{iPr} CMeNiPr)Cl2] (47Ti) and [Cp Ti(N{NMe2}SiMe2N{iPr}CMeNiPr)Cl2] (48Ti), in which both chlorine atoms of the silane had been transferred to titanium with concomitant transfer of the amidinate ligand to silicon.22 The reactions of 42Ti with MeI and EtBr (Scheme 3) in each case result in NdC bond formation at Nb, to form the hydrazidium cations [Cp Ti(MeC{NiPr}2)(NNMe2R)]+, for R ¼ Me (49Ti) or Et and corresponding non-coordinated anion. Reaction of 42Ti with [Et3NH][BPh4] formed [Cp Ti(MeC {NiPr}2)(NNMe2H)][BPh4] (50Ti) which contains a base-free, Na-protonated cation. Reaction of 42Ti with 1 equiv. of pinacol borane (HBPin) gave irreversible 1,2-addition of BdH to the Ti]Na bond to form the k2-borylhydrazide(1-)-hydride compound [Cp Ti(MeC{NiPr}2)(H)(NNMe2{BPin})] (51Ti).23 Reaction of 42Ti with 1 equiv. of 9-BBN dimer gave quantitative conversion to the borylimide [Cp Ti(MeC{NiPr}2)(]NBC8H14)] (52Ti) and the aminoborane Me2NBC8H14, via reductive NimdNR2 bond cleavage. The reaction of 52Ti with 1 equiv. of Ph2NNH2 quantitatively reformed 42Ti and the aminoborane H2NBC8H14.

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An alkylidene hydrazide complex [Cp Ti(MeC{NiPr}2)(NNCPh2)] (53Ti) was synthesized via reaction of [Cp Ti(MeC{NiPr}2) (N Bu)] and Ph2CNNH2.24 A detailed study of the reactions of 53Ti with a variety of unsaturated and saturated substrates was reported (Scheme 4).24 Exposure of 53Ti to an excess of CO2 gave the dimer of a rearranged [2 + 2] cycloaddition product, 54Ti, as well as the “double-insertion” dicarboxylate species 55Ti. Reactions of 53Ti with isocyanates and isothiocyanates were investigated. Reaction of 53Ti with tBuNCO gave m-oxo compound 20Ti and tBuNCNNCPh2, whereas reaction of 53Ti with TolNCO yielded the “double-insertion” product, 56Ti. Reaction of 53Ti with TolNCS, however, afforded the thioureate-type [2 + 2] cycloaddition product, 57Ti. Reactions of 53Ti with isonitriles, nitriles and alkynes were also investigated. Reaction of 53Ti with xylylisonitrile (XylNC) formed the Lewis base adduct 58Ti. Complex 53Ti also reacted with nitriles PhCN and C6F5CN to form the unusual double-addition products, 59Ti and 60Ti, respectively. t

Scheme 4 Reactivity of half-sandwich Ti alkylidene hydrazido complexes reported by Mountford and co-workers.

Reaction of 53Ti with boranes (H2BTex, 9-BBN, or HBPin) resulted in a series of borylhydrazido(2-) compounds, such as 61Ti and 62Ti, which are formed by the net 1,2 addition of BdH across the Nb]CPh2 bond of 53Ti.16

4.06.2.1.1.3 Phosphinimide and ketimide ligands In 1999 Stephan and co-workers reported the exceptional activity for olefin polymerization catalysis demonstrated by cyclopentadienyl titanium complexes incorporating bulky phosphinimide ligands,25 and studies on this landmark catalyst system were reviewed in 2005.26 A series of compounds R3PNSiMe3 were prepared by oxidation to the corresponding phosphinimine derivative with Me3SiN3, and subsequent reaction with various CpRTiCl3 reagents provided access to a family of complexes of the form [CpRTi(NPR3)Cl2],27 of which CpR ¼ Cp, Cp , CpnBu, R ¼ Cy, iPr, tBu complexes 63Ti-70Ti were structurally characterized.27 A series of alkyl and aryl complexes of the type [CpRTi(NPR3)R0 2] were also synthesized,27 of which [CpTi(NPtBu3)Ph2] (71Ti) and [CpTi(NPPh3) (CH2{SiMe3})2] (72Ti) were structurally characterized.27

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Stephan and co-workers subsequently investigated the nature of the active species in polymerization catalysis via stoichiometric reactions. Treatment of [CpTi(NPtBu3)Me2] with B(C6F5)3, resulted in [CpTiMe(NPtBu3)(m-Me)B(C6F5)3] (73Ti), an X-ray crystallographic study of which revealed ion pairing between the borate and the Ti cation with a Ti-B separation of 4.04 A˚ .25 A cationic titanocene ferrocenyl complex [CpTi(NPtBu3)(C5H4)FeCp)][MeB(C6F5)3] (74Ti) was synthesized by treatment of 73Ti with ferrocene (Scheme 5).28 NMR spectroscopy and X-ray analysis revealed both a Ti-Fe interaction and an agostic CdH bond, which impart remarkable stability to the compound.

Scheme 5 Reactivity of half-sandwich Ti phosphinimido complexes reported by Stephan and co-workers.

The incorporation of ferrocenyl substituents into phosphinimide ligands on titanium is synthetically facile, oxidation of [CpFe(Z5-C5H4PtBu2)] with Me3SiN3 gives the phosphinimine derivative, [CpFe(Z5-C5H4PtBu2NSiMe3)] and subsequent reaction with [CpTiCl3] or [Cp TiCl3] yields the desired product.29 This route provides access to a variety of complexes, such as [Cp TiX2(NPtBu2C5H4)-FeCp] for X ¼ Cl (75Ti), Me (76Ti), as well as the derivatives [CpTiX2(NPtBu2C5H4)Fe(Z5-C5Ph5)] for X ¼ Cl (77Ti), Me, prepared via the [(Z5-C5Ph5)Fe(Z5-C5H4PtBu2NSiMe3)] ferrocenyl phosphinimine analogue. Compound 76Ti reacts with B(C6F5)3 and [CPh3][B(C6F5)3] to generate cationic species that are highly effective olefin polymerization catalysts.29 Piers and co-workers reported the reaction of neutral [CpTi(NPtBu3)Me2] with [Ph3C]+[B(C6F5)4]− to generate ion-pair species [CpTi(NPtBu3)Me]+[B(C6F5)4]−. In non-halogenated solvents, this species reacts with H2 to afford methane and [(CpTi{NPtBu3}B {C6F5}4)2] (78Ti).30 X-ray crystallography of 78Ti reveals a dicationic TiIII-TiIII unit with a Ti2N2 core in a butterfly conformation, and a short Ti-Ti distance of 2.5966(7) A˚ . Hydrogenation (1 atm) of the more sterically encumbered Cp analogue [Cp Ti(NPtBu3)Me]+[B(C6F5)4]− results in a cationic Ti(IV) hydride complex, [Cp Ti(NPtBu3)H]+[B(C6F5)4]−, which was crystallographically characterized as its THF adduct, 79Ti (Fig. 4).30

Fig. 4 Examples of half-sandwich Ti complexes with phosphinimido ligands.

When generated in the presence of chloro- or bromobenzene, 79Ti undergoes C–X activation or ortho-C–H activation, depending on the amount of dihydrogen present in the reaction medium. At higher pressures of of H2 (ca. 4 atm), direct C–X activation occurs, giving the halocations [Cp Ti(NPtBu3)X]+[B(C6F5)4]−, and benzene-biphenyl mixtures, via formal s-bond metathesis. Bromo compound 80Ti was characterized in the solid-state as its THF adduct. At lower pressures of H2 ( 15 for 3,4-dimethyl-1-pentene (Scheme 79). Similar catalyst systems were used in a further study but it was found that the increasing the bulk of the chiral Cp substituent had little effect in enhancing the kinetic resolution (291Zr).453 291Zr and 289Zr were used in propene polymerization. Pyrrole and thiophenes have been ring-fused with ansa-bridged Cp ligands.454 The product zirconocenes showed remarkably differing activities for propylene polymerization, with the thiophene examples being five times quicker, and held the record at the time for being the most active metallocene propylene polymerization catalyst (292Zr-295Zr).454

Scheme 79 Chiral double-ansa-bridged zirconocenes which give a kinetic resolution of racemic monomers in olefin polymerization.

Chen and co-workers have continued to develop group 4 ansa-bridged metallocenes for polymerization application. Interestingly they found that using Al(C6F5)3 as an activator, rather than B(C6F5)3, could result in the double activation of [{Me2Si-Ind2} ZrMe2] to give [{Me2Si-Ind2}Zr{(m-Me)Al(C6F5)3}2] (296Zr).86 This gave a much more exothermic polymerization and increased polymerization efficiency. As well as dicationic species a Zr(II) complex could be made via the of reduction [ZrCl4(PR3)2] with Li in the presence of a butadiene, and subsequent metallocene transfer: [{rac-Me2Si-Ind2-Me-5-Ph }Zr(Z4-Ph-CH]CH-CH]CHPh)] 2 455 This Zr(II) catalyst could be activated (using B(C6F5)3 or MAO) to give a propylene polymerization catalyst. (297Zr). The first examples of chiral ansa-zirconocene ester enolates [(rac-EBI)ZrMe{OC(OiPr)]CMe2}] and [(rac-EBI)Zr{OC(OiPr)] CMe2}2] (298Zr) were synthesized.456 The mono-ester enolate could be activated with either Al(C6F5)3 (299Zr, Scheme 80) or B(C6F5)3, and with the latter gave an isolable model compound for the active propagating species in methacrylate polymerization which was highly active and highly isospecific. Further mechanistic studies were carried out on this and related systems (300Zr, 301Zr).457,458 The B(C6F5)3 activated product with 298Zr was found to be a stereospecific chemoselective polymerization catalyst

Scheme 80 Chiral ansa-zirconocene ester enolates for methacrylate polymerization catalysis. 302Zr was the crystallographically characterized species that was inactive for the polymerization of polar divinyl monomers, whereas the species on the far-left was highly active.

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for polar divinyl monomers.459 The catalyst structure was found to have a drastic effect on selectivity, with a silyl-ansa bridged derivative (302Zr) being not active but the ethylene-bridged congener being highly active. The active catalyst showed remarkable chemoselectivity, and ultimately the vinyl polymer could be post-functionalized. A number of other zirconocene complexes for olefin, methyl methacrylate and other monomer polymerization were made (303Zr-305Zr).460–462 Non-ansa bridged zirconocene enolates have also been made and used in methyl methacrylate polymerization (306Zr, 307Zr),463,464 it has been shown that oxo-bridged zirconocene dimers present a competitive bimetallic pathway for this process (308Zr).465 Chen and co-workers also performed an in-depth study of ansa-bridged Cp-fluorenyl complexes for the polymerization of polar vinyl monomers.466 A large range of pre-catalysts were used in their study (Fig. 2), two of which were crystallographically characterized: [{Ph2C-(Cp)-(2-tBu-Flu)}Zr{OC(OiPr)]CMe2}]2 (309Zr) and [{Me2Si-(Cp)-(Flu)}Zr{OC(OiPr)]CMe2}2] (310Zr). The comprehensive study covered synthetic, kinetic, computational and mechanistic work and makes a number of conclusions regarding this unique polymerization system (Fig. 24).

Fig. 24 Complexes in Chen’s study which studied 12 Cp/Flu ansa-bridged species and their ability to polymerize polar vinyl monomers. Two of the complexes (309Zr, 310Zr) were crystallographically characterized.

As well as his work on doubly-ansa-bridged Cp systems Bercaw has investigated ansa-bridged indenyl systems toward polymerization of olefins, with a particular interest on mechanistic aspects. A library of ansa-bridged Cp-substituted fluorenyl zirconocenes 0 [{R2C-CpR -Flu}ZrCl2] (R ¼ Ph, R0 ¼ 2-adamantyl 311Zr; R ¼ Me R0 ¼ 3-tBu-4-Me 312Zr, Scheme 81) was synthesized and used in propene polymerization; fine-tuning the metallocene structure affected the tacticity.467,468 Substituted fluorenyl systems were also made (313Zr, 314Zr).469 A large number of alkylaluminium-complexed zirconocene hydrides were identified and implicated in olefin polymerization catalysis.470 It was found that in solution these cations form adducts with two HAlR2 units to give a ZrH3 geometry observed for neutral zirconocene compounds (315Zr, 316Zr). The hydrides undergo ligand exchange in solution and with excess MeAlR2 give “Zr(m-Me)2AlR2” species that have been observed in zirconocene-catalyzed olefin polymerization. These hydride species (or their methyl analogues) could be reduced to form Zr(III) complexes.471 Indeed the dimethyl-zirconocenes would react directly (albeit slowly) with excess HAliBu2 to form Zr(III) hydride product: [{Me2Si-Ind2}Zr(m-H)2AliBu2]. This could be further reacted with ClAlMe2 to give a Zr(III) dichloride which was structurally characterized: [Me2Si-Ind2)Zr(m-Cl)2AlMe2] (317Zr).471 A number of other coordinatively unsaturated, electron-poor Zr(III) species were identified by EPR spectroscopy. Carpentier has also made a considerable number of ansa-bridged Cp-Flu complexes with differing substitution patterns on the rings as well as on the ansa-bridge,472,473 including chiral ansa-bridges and ansa-bridges that link two separate zirconocene moieties (318Zr-330Zr).474,475 These species gave highly isoselective polypropylene. In studying the activation mechanism a number of Zr-Al multimetallic species were characterized, as well as ion-pairs with other activators (331Zr-334Zr).476,477 Rojas and Erker have made a Ind-Cp mixed sandwich species with a B(C6F5)2 substituted indenyl (335Zr, 336Zr).478

Scheme 81 Ansa-bridged species used as catalysts in propene polymerization to give isotactic-hemisotactic polypropylene. (right) the isolation of hydride-bridged species implicated as the active species in catalysis, (i) 2 HAliBu2; (ii) HAliBu2, [Ph3C][B(C6F5)3].

A route to cyclobutane-ansa metallocenes has been developed. The photolysis of vinylic substituted indenyl complexes [(Ind(2-C(R)]CH2))2ZrCl2] (R ¼ Me 337Zr, R ¼ Cy 338Zr, Scheme 82) induces a [2 + 2] cycloaddition to give the ansa-bridged species [(C4H2R2-Ind2)ZrCl2] (R ¼ Cy, 339Zr; related species 340Zr-343Zr).479,480 Cp analogues could be made and gave moderate activity to propene polymerization (344Zr-356Zr).481–483 When the Cp analogue was substituted with multiple conjugated alkenes photolysis led to ladderanes, though only a cyclooctadiene ansa-bridged Cp species was structurally characterized [(C8H10-Cp2) ZrCl2] (357Zr).484 Other ansa-bridged species with a macrocycle have been structurally characterized (358Zr-363Zr).485–488 Alkynyl substituted indenyl rings have also been made and deposited on zirconium, the alkyne can bind to Co via reaction with Co2(CO)8 (364Zr-366Zr).489

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Scheme 82 The photosynthesis of cyclic-ansa-bridged zirconocenes 339Zr/357Zr.

The ethylene-bridged tetrahydroindenyl (EBTHI) derivative of Rosenthal’s complex, [(EBTHI)Zr(Me3SiC^CSiMe3)], reacts with B(C6F5)3 in a manner similar to the Cp derivative (Scheme 83 and vide infra).490,491 This induces a CdH activation on the C4 position of one of the indenyl rings with the hydrogen transferred to the alkyne fragment to give [(EBTHI4-B(C6F5)3)Zr{Me3SiC] CH(SiMe3)}] (367Zr). Upon heating the alkyne fragment is lost and an aryl group transfers to the Zr center [(EBTHI3-B(C6F5)2) ZrH(C6F5)] (368Zr). These species are model compounds for catalyst poisoning in polymerization. Further study of the catalyst activation processes by Rosenthal found that reaction activation of [(EBTHI)ZrF2] with Al(iBu)2H gave [{(EBTHI)ZrH(m-H)}2] which in turn could have one of the terminal hydrides abstracted by B(C6F5)3.492 This could react with further B(C6F5)3 to give either [(EBTHI2-B(C6F5)2)ZrH(C6F5)] (369Zr, an isomer of 368Zr) or a C-F activated species [(EBTHI)Zr(H)(C6F4)B(C6F5)2] (370Zr) as well as [(EBTHI)ZrF2]. The metallacyclocumulene rac-EBTHI derivative has been made (371Zr).493 The reactivity of 367Zr with other species has been investigated including olefins,494,495 AlHiBu2,496 aryl nitriles,497 isocyanides,498 (372Zr-380Zr).

Scheme 83 The reactivity of EBTHIZr derivatives with B(C6F5)3.

In a thorough 2004 study by Erker and co-workers a striking difference in the polymerization of methyl methacrylate between different zirconocene systems was noted.499 It was found that [{Me2Si-CpR2}ZrMe][MeB(C6F5)3] systems and closely related [{Me2Si-CpR2}Zr{(Z3-C3H4)-CH2-B(C6F5)3}] (the butadiene derivative) gave drastically different polymer structures, due to the difference in proximity of the anion. A number of the precursors were structurally characterized (381Zr-383Zr). By hydroborating an allyl-substituted Cp ring it is possible to tether the Lewis acidic boron to the ring (384Zr, Scheme 84).500 By protecting the strongly Lewis acidic boron s-ligands that are liable to abstraction (e.g. CH2SiMe3) could be introduced (385Zr387Zr).500,501 These Zr-B systems could C-H activate N-methylbenzimidazole, as well as the Cp ring (388Zr, 389Zr).501,502 Other alkyl-substituted ansa-bridged ligands featuring indenyl and fluorenyl have been made, generally for the purpose of olefin polymerization (390Zr).503 Particularly noteworthy are the permethylated indenyl and pentalene derivatives that have been used by O’Hare for both olefin and lactide polymerization (391Zr-418Zr).504–512 Brintzinger has made two cyclopentaphenanthrenebased zirconocenes (419Zr, 420Zr),513 as well as a number of indenyl and Cp-ansa bridged species (421Zr).436,514,515 Suzuki synthesised a family of ansa-bridged metallocenes and investigated their ability to polymerize 1-hexene under high pressure (500 MPa).516

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Scheme 84 The hydroboration of allyl-substituted Cp ligands.

4.06.3.2.2

Zirconocene alkyne chemistry

4.06.3.2.2.1 Zirconocene(II) reactivity In 1995 Rosenthal isolated [Cp2Zr(Z2-Me3SiC^CSiMe3)(Pyr)]—“Rosenthal’s reagent”, which can act as a masked Cp2Zr(II) synthon.517 Since then he has continued to pioneer the reactivity of the zirconocene fragment with unsaturated hydrocarbons, and has published a number of reviews on this area,181–183 as have others.518 In 2000 it was demonstrated that the metallocene fragment could slide along a polyyne chain, and a five-membered zirconacyclocumulene was structurally characterized (422Zr, Scheme 85).189 Reactivity with other polyenes has been investigated (423Zr, 424Zr).519 The subsequent reactivity of the zirconacyclocumulenes with AlHiBu2,520 B(C6F5)3,521 acetylenes,522,523 CdN multiple bonds,524 was investigated (425Zr-441Zr). Reaction of Rosenthal’s reagent with Me2ClSi-C^C-SiMe2Cl resulted in direct alkyne substitution to give [Cp2Zr(Z2-Me2ClSiC^CSiMe2Cl)(Pyr)] (442Zr).168

Scheme 85 The synthesis of zirconacyclocumulene (422Zr) and its ability to slide along the polyyne chain.

However, when the direct synthesis of 442Zr from [Cp2ZrCl2] is attempted (via reduction with Mg) a zirconadisilylpentyne is formed: [Cp2Zr(k2-Me2Si-C^C-SiMe2)] (443Zr, Scheme 86). The geometric parameters of 443Zr are similar to other zirconacyclopentynes, and indeed the CdC triple bond is further from the Zr center than Suzuki’s example (vide infra),525 demonstrating little Zr-C^C interaction. Substitution with other alkynes has been recorded, for example the phosphine substituted species Ph2P-C^C-PPh2 forms [{Cp2Zr(Ph2PC^C-m-PPh2)}2] (444Zr).191 In the solid-state this dimerizes via P-Zr interactions, but in the solution phase the dimer is in equilibrium with a zirconaphosphacyclobutane species. If you increase the steric bulk of the Cp rings by replacing them with EBI a simple monomer is formed: [(EBI)Zr(Ph2PC^CPPh2)] (445Zr).

Scheme 86 Alkyne substitution on Rosenthal’s complex. Direct reductive synthesis of 442Zr leads to zirconadisilylcyclopentyne 443Zr.

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Rosenthal’s reagent undergoes insertion of carbodiimides into the ZrdC bond (446Zr, 447Zr, Scheme 87).198 However with other CdN fragments, for example nitriles, coupling occurs.200 Initially a zirconium nitrile adduct is formed (448Zr), before a second equivalent of nitrile undergoes a CdC coupling and the alkyne is lost. This produces a zirconadiazacyclopentadiene.200

Scheme 87 Insertion versus coupling in Zirconocene Alkyne complexes by CdN fragments.

Use of Rosenthal’s reagent as a masked Zr(II) fragment allowed the first examples of NdH oxidative addition of amides with early transition metal complexes (449Zr).526 This is a general route to zirconocene amide hydride complexes, and an alternative to Schwartz’s reagent [{Cp2Zr(m-H)Cl}2]. In the presence of excess amide dehydrogenation occurs to form a zirconocene bis-amide (450Zr). Rosenthal has investigated the reactivity of his reagent with B(C6F5)3,491 AlHiBu2,496,527 fluorinated pyridines,528 other NdH and OdH heterocycles,529 THF,530 isocyanides,531 chiral Cp rings,532,533 azobenzene,534,535 diferrocenylacetylene,536 bisBPin-acetylene,537 diphenylacetonitrile,538 diaminoacetylenes,539 ferrocenylnitriles,540 cyanopyridines,202 substituted 2-pyridines,243 (451Zr-480Zr). The zirconacyclopentyne reactivity with B(C6F5)3,541,542 Ni(0),542,543 isocyanides,531 has been reported (481Zr-488Zr). The zirconocene(II) motif can furnish the reductive coupling of alkynes to give zirconacyclopentadienes. This is usually done either with Rosenthal’s reagent or with Negishi’s reagent ([Cp2ZrnBu2], made in situ), with both having advantages.544 Tilley published a detailed study of the mechanism of alkyne coupling in zirconocene alkyne complexes.545 By using fluoroaryl substituted alkynes it was shown that electron-withdrawing substituents will adopt a b-position in the resultant zirconacyclopentadiene. The DFT studies showed an asymmetric binding of the two alkynes in the transition state of the coupling reaction. This study assisted in the design of new conjugated polymers bearing electron withdrawing C6F5 groups. A follow-up report expanded on this mechanistic study with further kinetic analysis,546 the steric crowding around the zirconium center has a drastic effect on the rate of alkyne substitution with more sterically encumbered Cp rings being slower than substituted ones. A number of the zirconapentadiene species were structurally characterized (489Zr-492Zr).545,546 Tilley further extended this methodology to include mesityl-substituted alkynes, which again preferentially adopted the b-position in coupling (whereas SiMe3 and tBu preferentially adopt the a-position).547 The regiochemistry of (alkyl)C^C(Mes) and (aryl)C^C(Mes) coupling were found to be slightly different, with the former initially forming a kinetic bb before isomerizing at elevated temperatures of the ab-product (the latter forms only the bb product). For PrC^CMes both the bb and ab products could be structurally characterized, with the exclusive bb product for PhC^CMes also being characterized (493Zr, 494Zr and 495Zr respectively, Scheme 88).547 PhNO was found to insert into the zirconacyclopentadienes (496Zr, 497Zr).548 Tilley also investigated the varied reactivity of 1-alkynylphosphines with Rosenthal’s complex (498Zr-502Zr).549

Scheme 88 The regioselectivity of alkyne coupling by Rosenthal’s complex—often a careful balance of thermodynamics and kinetics.

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Tilley has exploited the “Cp2Zr”-mediated reductive alkyne coupling to furnish a number of macrocyclic assemblies, generally being made up of dimeric or trimeric zirconocene species (503Zr, 504Zr, Scheme 89).550–552 These elegant species can include up to nine aromatic rings within the poly-phenyl spacer, which gives a slight bowing (505Zr).553 If pyridyl units are included in the spacer these can bind further zirconocene moieties in a templating effect (506Zr, 507Zr).554 It was also possible to incorporate BINOL units into the backbone, and the synthetic pathway was highly diastereoselective (508Zr).555 Tilley also used this alkyne coupling route to synthesize a number of polycyclic aromatic hydrocarbons, which could be isolated with the zirconocene still bound or it could be removed via acidic aqueous work-up.556

Scheme 89 Examples of zirconocene macrocycles reported by Tilley and co-workers.

If the linker between the two alkyne complexes is sufficiently flexible intramolecular alkyne coupling can be achieved. Rivard undertook some colourful chemistry with this, by using 1,8-bispinacolborane-octadiyne. This gave a 2,5-BPin functionalized zirconapentadiene with a cyclohexane-fused ring onto the backbone (509Zr, Scheme 90).557 509Zr could be reacted with S, Se or Te halides to give [Cp2ZrCl2] and the appropriate heterocycle. The heterocyclic monomer could undergo Suzuki-Miyaura cross-coupling with appropriate cyclic di-iodides to form conjugated polymers which give unusual solid/aggregated-sate phosphorescence.558 Rivard has used this synthetic strategy for a number of different alkyne starting materials (510Zr),559 as well as using [Cp2ZrPh2] as a starting material, which forms a 2,3-phenyl-fused zirconacyclopentadiene (with the elimination of benzene).560–562

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Scheme 90 Synthesis of heterole-thiophene copolymers via Zirconocene-mediated alkyne coupling.

By using a bis(silyl)alkyne and half an equivalence of “ZrCp2” Xi and co-workers were able to form 2,5-bis(alkynylsilyl) zirconacyclopentadienes (511Zr).563 If 1 equiv. of “ZrCp2” is used a silylatedzirconacyclobutene is formed. Both this and compounds like 511Zr can be further reacted with nitriles to make azaindoles and other fused heterocycles.564–570 These reactions are highly regiospecific, often one pot and at the end the zirconocene can be removed either by aqueous work-up (which leaves the zirconacyclopentadiene open) or can be reacted with CuCl and CH2I2 (which closes the ring). This latter strategy allowed for the isolation of many 1,2,3,4-substituted cyclopentadienes from one-pot procedures.571 Many of the zirconocene intermediates from these synthetic routes have been isolated and structurally characterized (512Zr-530Zr, Scheme 91). The repeated zirconocene-mediated regioselective insertion of alkynes and nitriles (as well as other substrates) is an excellent synthetic strategy for the synthesis of multi-ring systems, including the synthesis of substituted pyrroles (531Zr),572 isoquinolines (532Zr), 573 naphthyridines (533Zr),574 as well as other multi-ring systems (534Zr).575,576

Scheme 91 Xi’s regio- and stereo-specific zirconocene-mediated ring-syntheses.

Piers has used the zirconium-mediated alkyne coupling to give a number of highly fluorinated derivatives. For example Rosenthal’s reagent can be reacted with 2 equiv. of (C6F5)CC(C6F5) to give the perfluorotetraphenylated zirconacyclopentadiene (535Zr, Scheme 92).577 By subsequent reaction of 535Zr with SnMe2Cl2 and BBr3 the “Cp2Zr” can be replaced by a BdBr, which can then be reacted with Zn(C6F5)2 to give perfluoropentaphenylborole. A similar methodology can be used with [Cp2Zr(C6F4H)2] as the starting material, which undergoes a CdH activation when reacted with an alkyne to give [Cp2Zr{(C6F5)-k-C]C(C6F5) dC(C6F4)]k-C}] (536Zr).578 Highly fluorinated boron species often have their Lewis acidity demonstrated by abstracting Me from Cp2ZrMe2 (537Zr).579

Scheme 92 Synthesis of highly-fluorinated zirconacyclopentadienes.

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In zirconium-mediated alkyne coupling the ultimate product is a zirconocene-cyclopentadiene complex. Suzuki was the first to isolate and a zirconocene-cyclopentyne complexes. By reaction of Negishi’s reagent with a butatriene the zirconocene-cyclopentyne complex could be isolated.525 Initially formed as a mixture of the cis-trans isomers it would slowly isomerize to the purely transisomer. This could be fully characterized, and X-ray crystallographic analysis of trans-[Cp2Zr{k2-(Me3Si)CH-C^C-CH(SiMe3)}] (538Zr, Scheme 93) showed the C^C bond length (1.206(7) A˚ ) to be comparable to cyclononyne. This relatively short bond-length, combined with the NMR spectroscopic data, reinforced the bonding description as that of an alkyne in a strained ring (rather than the metal coordinating to the alkyne and forming a metallocyclopentene species). The hydrolysis reactivity with HCl, forming [Cp2ZrCl2] and the product alkyne, reinforce the existence of a direct ZrdC s-bond rather than an Z4 interaction. An electron density study validated than the primary contribution in bonding was the s interactions, with the Z4-p bonding contribution being small.580 If the synthesis of 538Zr is attempted with [Cp2Zr(PMe3)Z2-alkyne}] as the starting material a seven-membered metallacyclic alkyne is formed (539Zr, 540Zr; a small amount of bicyclic product was also observed 541Zr).581,582

Scheme 93 Suzuki’s zirconacyclopentyne.

After this initial report an alternative synthetic route to metallocyclopentynes was developed. By reacting zirconocene chloride with 1,4-dichlorobut-2-yne and magnesium it was possible to form simple metallocyclopentynes (542Zr, 543Zr, Scheme 94).583,584 When reacted with [Cp2Zr(PMe3)(H2C]CHEt)] the butene is lost and a second Zr center binds, in an Z2 fashion, to the alkyne (544Zr).583 If the donor that stabilizes the second zirconium center is removed a metallacyclocumulene complex is formed: [(Cp2Zr)2(m:k2:k2-H2C]CdC]CH2)] (545Zr, 546Zr).584,585 Reaction of 546Zr with excess donor (e.g. PMe3) reforms the metallocyclopentyne-bridging starting complex.

Scheme 94 Direct reductive synthesis from [Cp2ZrCl2] of zirconacyclopentynes.

Extending this work [Cp2Zr(PMe3)2] could be reacted with 1,1,6,6-tetrakis(4-ethylphenyl)-1,2,3,4,5-hexapentaene to give a 2,5-bisalkylidene-1-zirconacyclopent-3-yne complex (547Zr, Scheme 95).586 This remarkable species could be reduced by alkali metals to give a dianionic compound that could be protonated on the alkyne positions (to give a bis-alkylidene-zirconapentene compound, 548Zr) or be reacted with an alkyliodide or stannylchloride to an asymmetric Z4-enyne complex (549Zr). In a follow-up publication it was shown that the alkylidene-zirconacyclopentyne complexes could be induced to undergo a haptotropic shift if a donor was added (550Zr, 551Zr)587,588 and can be used to trap out the unfavorable Z-isomers of cumulenes (552Zr).589 Similarly to 538Zr, if the synthesis of 549Zr is attempted from zirconocene alkyne starting materials seven-membered zirconacyclic allenes can be made (553Zr).590

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Scheme 95 Zirconocene reactions of polyenes reported by Suzuki et al.

Metallacycloallenes, as well as those with heteroatoms in the ring, have also been synthesized (554Zr-556Zr).591–593 Addition of aldehydes to zirconacyclocumulenes and subsequent acidic workup yields cis-[3]cumulenic diols.594 Similar work up of zirconallenes can give a-hydroxyallenes (557Zr).595 A magnesium hydride can also be stabilized by a zirconocene-enyne complexes (558Zr, 559Zr, 560Zr).596,597A number of other papers involving alkyne, allene and related compounds interacting with zirconocenes have been published (561Zr-605Zr).591,598–614 It should be noted that the zirconocene ring moieties can be non-innocent in the further reactivity of the zirconacyclopentadiene species. If the rings are substituted, and they are reacted with an excess of TiCl4 the Cp (or indenyl) ring will couple to the butadiene fragment to give indene (or fluorene) products (Scheme 96).615

Scheme 96 TiCl4-mediated ring-extrusion of zirconacyclopentadienes.

Alkynes can also be coupled with acyl chlorides using [Cp2ZrEt2] (Takahashi’s reagent). This gives the unsaturated metallo-ester (606Zr, Scheme 97).616 Takahashi’s reagent can be reacted with an equivalent of [Cp2ZrBr2] to give then ethylene bridged dimer (607Zr).617

Scheme 97 Zirconaester synthesis from Takahashi’s reagent. (i) I2, X ¼ I; (i) NCS, X ¼ Cl; (i) CuCl/I-CCR0 , X ¼ CCR0 ; (i) CuCl/PhI, X ¼ Ph (i) CuCl/R0 COCl, X ¼ R0 OC; (i) CuCl/ClCOOR0 , X ¼ COOR0 .

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Erker has also published extensively on Lewis acid interactions with zirconocene systems, particularly tethered “B(C6F5)2” motifs, eventually leading to zirconocene-based frustrated Lewis pairs (FLPs). These tethered boron systems can be made by hydroboration of bis-alkynyl zirconocene complexes using Piers’ borane HB(C6F5)2 (608Zr-610Zr, Scheme 98).618 This followed on from some similar work involving the hydroboration of zirconocene butadiene species (611Zr).619 Complex 608Zr can insert nitriles (612Zr).620 Ultimately it is possible to form a B(C6F5)2 substituted-alkynyl zirconocene complex: [Cp2Zr{Z2-(SiMe3) C^C-B(C6F5)}] (613Zr).621 This activates H2 in a manner similar to FLPs (614Zr). The reactivity of 613Zr with other donors was investigated, generally forming M-L adducts (615Zr-618Zr).622 The exception to this was with unsaturated hydrocarbons, which undergo zirconaboration (619Zr-626Zr).

Scheme 98 Bis-alkyne complexes react with Piers’ borane to induce a dimerization. B(C6F5)2 substituted zirconalkynes can heterolytically cleave H2.

4.06.3.2.2.2 ZrCpR2 other C]E insertion chemistry Alkynes can also insert into zirconocene benzyne complexes (prepared in situ from [Cp2ZrPh2]) resulting in a zirconaindene species (Scheme 99).623 Functionalized alkynes (e.g. silyl-actylenes or phosphinoacetylenes) give rise to functionalized zirconaindenes (627Zr, 628Zr),624 or if a functionalized bisalkyne is used tricyclic zirconaindenes can be accessed (629Zr).625 When a phosphine is bound to the a-carbon of the zirconaindene this can react with unsaturated substrates such as alkynes,626 azides627 and diazoalkanes (630Zr-633Zr).628 This reactivity is prescient of zirconocene frustrated Lewis pair chemistry. Phosphine-substituted 1-aza-zirconaindenes can be made by reacting cyanophosphane with zirconocene benzyne complexes.629 This can undergo additions with aldehydes and heterocumulenes, and CdH activate alkynes and esters (634Zr-636Zr).630,631 Indeed the highly electrophilic zirconium can CdH activate diazomethyl species to give a-diazomethylzirconium complexes.632 The diazo functional group can undergo further cycloaddition reactions (637Zr). A similar synthetic route can give access to zirconaazaspiophosphanes.633 The zirconocene center can be substituted with phosphorus by the addition of chloroiminophosphane (638Zr).

Scheme 99 Synthesis of zirconaindenes.

Xie has made an unprecedented zirconocene carboranyl complex. Deprotonation of the carborane [C2B10H12] with 2 equiv. of BuLi, followed by an equivalent of [Cp2ZrCl2] gave the zirconocene-carboranyl complex [Cp2Zr(m-Cl)(m-C2B10H10)Li(Et2O)2] (639Zr, Scheme 100).634 Complex 639Zr reacts with unsaturated substrates, generally to give heteroatom substituted zirconacarboranecyclopentane products (640Zr-643Zr). It is posited that this reactivity goes through a zirconocene carboryne intermediate.

n

Scheme 100 Xie’s zirconocene carboranyl complex and its reactivity. (i) CuCl, 1,2-I2C6H4; (ii) CuCl.

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Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

When an alkyne is inserted to 639Zr (giving a zirconacarboranecyclopentene, 644Zr-657Zr) the Zr center will bind donors (658Zr, 659Zr), or can be extruded with CuCl2.635,636 When an alkene is reacted with 639Zr a zirconacarboranecyclopentane is formed (660Zr-667Zr).637 These can either be further reacted with an equivalence of alkyne in the presence of NiCl2, to give dihydrobenzocarboranes,638 or alternatively the “ZrCp2” can be removed with various donors (including main group compounds such as PhPCl2, nBu2SnCl2 or Me2GeCl2) giving heteroatom substituted carboranecyclopentanes, 668Zr.639 Complex 639Zr CdH activates pyridines to give [Cp2Zr(C2B10H11)(Pyr)] (669Zr-675Zr).640 Carborane-Cp ansa bridged zirconocenes have also been made (676Zr-684Zr).641–643 Similar reactivity is seen with zirconocene anthracene complexes. [Cp2ZrMeCl] reacts with 9-lithioanthracene to give [Cp2ZrMe(9-anthracenyl)].644 This eliminates methane at room temperature to give [Cp2Zr(1,9-anthracendiyl)] (685Zr, Scheme 101). This will insert alkynes and isonitriles into the ZrdC bond (686Zr, 687Zr). A similar methodology can be used to make [Cp2Zr(Z2-benzocyclobutadiene)PMe3] (688Zr), which similarly inserts alkynes, nitriles and isonitriles (689Zr-691Zr). 645

Scheme 101 Insertion into a zirconanthracediyl complex.

4.06.3.2.3

Zirconocene p-block chemistry

4.06.3.2.3.1 Frustrated Lewis pair chemistry Erker has pioneered the field of zirconocene-based frustrated Lewis pairs, with P/B, Zr/P and Zr/B systems (692Zr-696Zr).646 Aryl-imido substituted-zirconocenes [(CpCH]NDipp)2ZrCl2] (697Zr) could be hydrogenated in the presence of B(C6F5)3 to form the ammonium salt, which could catalyze the hydrogenation of aldimines.647 Another genre of frustrated Lewis pair was made by the insertion of diphenylphosphinoalknyes into [Cp2Zr-Me]+ (698Zr, 699Zr) of which the geminal [Cp 2Zr{(Ph2k-P)-Z1dC] CMePh}][B(C6F5)4] (700Zr) was the first system to be thoroughly studied.648 Complex 700Zr can react with donors in “normal” coordination behavior (e.g. nitriles, isonitriles, 701Zr-703Zr), but with other small molecules (such as CO2, N2O, isocynates) it reacts in a FLP-like manner (704Zr-707Zr).649 Complex 700Zr can activate dihydrogen, and then undergo subsequent reactivity with the solvent, either to form [Cp 2ZrH(C4H8O)] (708Zr) and the phosphino alkene, or, in the presence of CH2Cl2, to form [Cp 2ZpCl2] with a chloro-methylene-phosphine alkene. If a silylated phosphino alkyne was used, rather than a geminal Zr+/P FLP a vicinal FLP was formed: [Cp 2Zr{Z1-C(Me)]C(SiMe3)(k-PAr2)}][B(C6F5)4] (Ar ¼ Ph 709Zr, Ar ¼ Tol 710Zr, Ar ¼ C6F5 711Zr, Scheme 102).650,651 Complex 709Zr showed FLP-like reactivity with CO2, {(nbd)RhCl}2, CO, N2O, carbonyls, PhNO, PhNSO and {(cod)IrCl}2 (712Zr, 713Zr, 714Zr, 715Zr-719Zr, respectively) and formed an adduct with CNtBu (720Zr).650,651 709Zr could catalyze the dimerization of phenyl acetylene. An alternate route to these geminal structures was proposed via the methyl abstraction from suitable amido or phosphido [Cp2ZrMeERR0 ] (E ¼ N, P) precursors, followed by a [2 + 2] addition.652 A number of the precursor methyl complexes were structurally characterized (ERR0 ¼ PCy2 721Zr; PMes2 722Zr, PtBu2 723Zr; NPh2 724Zr; NtBuMes 725Zr). The amido species reacts with [B(C6F5)3 to give imido complexes [Cp2Zr ¼ NRR0 ][B(C6F5)3] (726Zr, 727Zr). CO2 would insert into the Zr]E bond to form a CO2 bridged dimer (728Zr), PhCHO would insert to form a zirconocene alkoxide amide (729Zr). Other reactivity on Cp2Zr ¼ PR2 systems has been reported (730Zr, 731Zr).653 For [Cp2ZrMePCy2] reaction with B(C6F5)3 in the presence of diphenylacetylene afforded a vicinal Zr/P+ FLP this showed characteristic FLP-style reactivity with carbonyls (732Zr) including acetylferrocene (733Zr).

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Scheme 102 Frustrated Lewis pair reactivity of Erker’s vicinal Zr+/P complex 709Zr—[B(C6F5)4]− counterion omitted for clarity.

Wass has also made significant contributions to the area of Zr-based FLP chemistry. In an elegantly simple reaction Cp2ZrMe2 can be reacted with 2-phosphinophenol to form [CpR2ZrMe{O-2-(PR0 2)C6H4}] (734Zr) subsequent protonation with pyridinium-{B(C6F5)4} gave [CpR2Zr{k-O-C6H4-(2-k-PR0 2)}][B(C6F5)4] (R ¼ H R0 ¼ tBu 735Zr; R ¼ Me5 R0 ¼ iPr 736Zr, Scheme 103).654,655 The Cp congener can heterolytically cleave H2 to form [Cp 2ZrH{k-O-C6H4-(PHtBu2)}][B(C6F5)4] (737Zr).654 These species perform a host of other reactivity including CO2 insertion, amine borane dehydrogenation, C-X cleavage

Scheme 103 A summary zirconocene FLP reactivity reported by Wass and co-workers.

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Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

with alkyl halides and CdO bond cleavage of ethers (738Zr-748Zr).655 Other zirconocene alkoxide phosphines have been made (749Zr-751Zr), one of which reacted with Pt(nbd)3 to form a PtdZr bond (752Zr).656,657 Furthermore it was shown that [Cp Zr(OAr)]+ could be the Lewis acid for an intermolecular Lewis pair (initially partnered with phosphines but more recently partnered with N-based Lewis bases).658,659 These again showed FLP chemistry, with CO2 inserting to form [Cp Zr(OMes) (Z1-O2CPEt3)][B(C6F5)4] (753Zr) and addition across phenylacetylene (754Zr). A thorough mechanistic investigation into the dehydrocoupling of amine-boranes by the Zr/P intermolecular FLPs was undertaken, showing excellent TOF and a new mechanism comprising of two cooperative catalytic cycles.660 Zirconocene amido-borane complexes Cp2ZrX(NH2BH3) were the first transition-metal amido-borane complexes to be characterized (X ¼ H, 755Zr, X ¼ Cl, 756Zr.661 The b-H-B agostic interaction is clearly identifiable in NMR spectroscopy and X-ray diffraction experiments. [Cp2ZrCl(NMe2BH3)] has also been made, as has the Zr(III) species [Cp2Zr(NMe2BH3)] however only the former has been analyzed by single crystal X-ray diffraction (757Zr).267 Stephan has shown that zirconocene-alkoxide and zirconocene methyl cationic systems combined with borate anions and in the presence of phosphine can activate N2O (758Zr, 759Zr).662 Zirconocene alkoxide hydride (Cp 2Zr(H)OMes, 760Zr) was shown to react with CO and HB(C6F5)2 to form a formylhydridoborate complex Cp 2Zr(OMes)(OCHBH{C6F5})2 (761Zr, 762Zr764Zr).663,664 This would form an adduct with pyridine (via the boron, 765Zr), but could also react with unsaturated molecules such as CO, CO2, PhNSO, N2O (766Zr-770Zr). Budzelaar has shown that for some Zr/N systems H2 activation can occur initially via a s-bond metathesis pathway prior to heterolytic activation of H2.665 Erker has made other zirconocene complexes (771Zr874Zr).478,649,666–704 4.06.3.2.3.2 Nitrogen based ligands The zirconocene motif is well studied toward its reactivity with N-containing substrates. Bergman and co-workers have made significant progress in this area. In 2000 they reported that imido-zirconocene complexes could catalyze the metathesis of N-aryl aldimine and ketimines, via [2 + 2] cycloadditions forming a diazametallacycle, e.g. [Cp2Zr(k2-PhNC(H)PhNPh)] (875Zr, Scheme 104).705 Increasing the bulk around the metal centers allowed for two imidozirconocene complexes to also be structurally characterized: [CpCp Zr(NtBu)(OC4H8)] (876Zr), [CpCp Zr{N(p-C6H4Me)}{OC4H8}] (877Zr), as well as an enamido complex [CpCp Zr(NHtBu)(N{Ph}{(Ph)C]CH2})] (878Zr). The imido can also be increased in steric bulk to infer stability on the complex, e.g. [Cp2ZrN(Dipp)(OC4H8)] (879Zr), however there is a risk of CdH activation if arene substituents are used, e.g. [Cp2Zr{NH-(2,4-(tBu)2-{6-C(Me2-k-CH2)}C6H2)}] (880Zr).706 Subsequent reports investigated the mechanism of cycloaddition/ cycloreversion between the zirconocene imido or the metallacycle intermediate and carbodiimides (881Zr-885Zr).707 Notably these complexes could also ring-open epoxides (and other strained heterocycles) with high selectivity (886Zr-891Zr),708,709 and the ethylene-bridged-tetrahydroindenyl derivatives could CdH activate simple alkanes (892Zrd895Zr).710 Unsaturated substrates could also be reacted with azazirconacyclobutenes (the addition product of alkynes and imidozirconocenes), including ketones and imines (896Zrd898Zr).711 This could be used in a multistep fashion to furnish nitriles from amides (899Zr).712 Stereoinversion of allenes can be mediated by imido zirconocenes—the mechanism was thoroughly investigated and a number of intermediates were crystallographically characterized (900Zr-903Zr).713 The zirconamides can also be deprotonated to form lithium zirconimidates complexes (904Zr).714

Scheme 104 Highlights of zirconocene-imido chemistry.

4.06.3.2.3.3 Other p-block compounds Zirconocene and hafnocene compounds with Si-, Ge-, Sn-, N-,.P-. As-. Sb-, O-, S-, Se-, Te- were reviewed in 1994 by Hey-Hawkins.715 Jordan developed other zirconocene cation systems and their structures were probed (905Zr-911Zr).716–719 This including a silane s-complex (909Zr), with evidence of sigma-bond metathesis reactivity.718,719 Silanes are able to support interesting bonding-modes with zirconocene centers. In a communication and subsequent full report Sadow and co-workers investigated

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the b-hydride elimination of silazido zirconocenes: [Cp2ZrX{N(R)SiHMe2}] (X ¼ OTf, R ¼ tBu, 912Zr X ¼ H, R ¼ SiHMe2 913Zr, Scheme 105).720,721 For [Cp2ZrX{N(SiHMe2)2}] (X ¼ hydride or alkyl) addition of a Lewis acid induces the formation of [Cp2Zr {N(SiHMe2)(SiXMe2)}]+ (X ¼ H 914Zr). The SidH bond can insert into a ketone (915Zr). Addition of Lewis base to 914Zr induces hydride migration to the Zr center, with the Lewis base forming an adduct with Si. This research provides a connection between insertion/b-agostic CdH/elimination reactivity and Lewis acid/base chemistry of main group adducts with Zr. This informs the hydrosilylation chemistry of these complexes, which go initially via SidH bond activation.722

Scheme 105 SidH b-agostic interactions investigated by Sadow.

Marschner, Baumgartner and Muller have made a considerable number of oligosilyl anionic compounds over the last two decades and these can be transferred onto a zirconocene center by reaction with [ZrCp2Cl2]. Starting with a octasilyl dianion [{k2-(SiMe3)2Si(SiMe2)2Si(SiMe3)2}ZrCp2], 916Zr, Scheme 106)723 they have varied the ligand charge,724 number of silyl groups in the chain,725 the substitution on the Si atoms,726,727 and the oxidation state of the metal (917Zr-928Zr).728,729 If a silylacetylene is used in the presence of a reductant an alkyne/zirconacyclopentadiene complex is formed (929Zr-936Zr).730 When [K2{(SiMe3)2Si-Si(SiMe3)2}] is used as the silyl anion a disilene is formed: [Cp2Zr(PMe3)(Z2-{(SiMe3)2Si-Si(SiMe3)2})] (937Zr).731 Similarly digermylene or mixed silagermalenes can be formed (938Zr, 939Zr). The same route with Sn does not form the stannylene, instead forming a cyclozirconatetrastannane: [Cp2Zr(k2-{Sn(SiMe3)2}3)] (940Zr). The dianion can be fluorinated and made asymmetric, to make a zirconocene fluorosilyl: [Cp2ZrCl{Si(SiMe3)2SiPh2F}] (941Zr).732 If the octasilyl dianion [K2{(SiMe3)2Si(SiMe2)2Si(SiMe3)2}] is reacted with EX2 (E ¼ P, Sn, Ge, Sb X ¼ Br, N(SiMe3)2) 5-membered rings: “ESi4” are formed. These can in turn be reacted with zirconocene dichloride in the presence of magnesium to form zirconocene stannylenes, plumbylenes, germylenes and stibines (942Zr-945Zr).282,283,733

Scheme 106 Oligosilyl complexes with group 4 metallocenes. (i)945Zr was synthesized by an alternative synthetic route.

Zintl silicide anions can be grafted directly to zirconocene via simple metathesis with [Cp2ZrCl2] to form [Cp2ZrCl(Z1-Si6Ar5)] (Ar ¼ 2,4,6-triisopropylphenyl, 946Zr, Scheme 107).734 A disilenide [ArSi]SiAr2]− (Ar ¼ 2,4,6-triisopropylphenyl) can react with [Cp2ZrCl2] to form Z1-disilenide zirconocene: [Cp2ZrCl(ArSi ¼ SiAr2)] 947Zr.735 At room temperature 947Zr CdH activates one of the isopropyl groups to form [Cp2ZrCl{ArSi(CH2CMeC6Hi2Pr2)SiArH}], 948Zr. Equally, a lithiated siliconoid can react with [Cp2ZrCl2] to form a zirconium siliconoid 949Zr.

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Scheme 107 The synthesis of zirconocene silicon compounds 946Zr, 947Zr/948Zr.

Gallium, indium and bismuth zirconocenes can be made by reducing [Cp2ZrCl2] in the presence of ArECl2 [E ¼ In, Ga, Bi; Ar ¼ 2,6-bis-(2,4,6-triisopropylphenyl)phenyl]. Ga and In form complexes of formula [Cp2Zr(Z1-EAr)2] (950Zr, 951Zr, Scheme 108),736,737 a similar tactic can make zirconocene zinc complexes (952Zr). Interestingly under the same conditions bismuth forms a dibismuthene complex: [Cp2Zr(Z2-ArBiBiAr)] (953Zr).738 When slightly less reductant, and 1 equiv. of ArECl2 is used the Cp rings can dimerize on one side forming a V-shaped [(Z5:Z5-C10H8)(ZrCp)2(m-H)(m-Cl)(m-GaAr)] complex (954Zr).739 Other zirconocene gallium complexes have been made, including a Zr(III)Cp2 anionic species (955Zr).740 When reduction occurs in the absence of the p-block synthon zirconocene clusters are formed (956Zr, 957Zr).741

Scheme 108 Reduction to form zirconocene gallium, indium and bismuth complexes.

Zirconocene dihydrides can activate P4 to form bridging P4H2 species: [{Cp (CptBu)Zr}2{m:k2:k2-P4H2}] (958Zr).742 [Cp1,3reacts with yellow arsenic (As4) upon thermolysis to give [Cp1,3-tBu2 Zr(k2-As4)] and [Cp1,3-tBu2 Zr(m:k2:k3-As5)ZrCp1,32 2 743 ] both of which have been structurally characterized 959Zr, 960Zr. 959Zr is stable under an inert atmosphere and is a useful transfer agent for “As4” (Scheme 109).743,744 The P analogue was made by Scherer in 1988, but a more detailed synthetic method was recently described (961Zr),745,746 and has recently also found use as a transfer agent.744 Reaction of 961Zr with P^CtBu results in the P and C being incorporated into the P4 ligand to give [Cp2Zr{k2-P6(CtBu)2}] (962Zr).747 961Zr can bind to other metal centers to form bi- or trimetallic species (963Zr-969Zr).746 [Cp21,3-tBu2Zr(CO)2] can also react with E4Q3 (E ¼ P, As, Q ¼ S; E ¼ P Zr{k2-(CtBu)2P2}] has also Q ¼ S) to give [{Cp21,3-tBu2Zr}2{m:k2:k2-Q2EEQ2}] (970Zr-972Zr).748 The related compound [Cp1,3-tBu2 2 been used to make P,C and other heteroelement cages.749,750 tBu2 2Zr(CO)2] tBu2

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Scheme 109 Zirconocene as a P4/As4 transfer agent.

[Cp2ZrMe2] reacts with O(BPin)2 to give [Cp2Zr(OBPin)2] (973Zr).751 Zirconium sulfides can be made by protonation of a ZrdO bond by HSR0 (974Zr).464 [Cp2Zr(H)Cl] reacts with the P source [(Me2N)3PPP(NMe2)3][BPh4] to give a remarkable zirconocene tetramer, with hydrides bridging between two zirconium atoms, and a central P, which bridges all four zirconium atoms, making it square planar: [(Cp2Zr)4(m2-H)4(m4-P)][BPh4] (975Zr, Scheme 110).752 When [(Me2N)3PAsP(NMe2)3][BPh4] is used both P and As are incorporated, with a mixture of ratio approximately 2:3 (P:As) in the crystal structure (976Zr).753 When Schwartz’s reagent is reacted with LiPR2 it undergoes halogen-reductive substitution to form a Zr(III) dimer with bridging phosphinido groups.715 The same can be achieved with LiPHR, provided R is a sufficiently bulky silyl group (forming [(Cp2Zr)2(m-PHR)2], 977Zr-979Zr).754 However these species can also be dehydrogenated to form the Zr(IV) zirconocenephosphinidene: [(Cp2Zr)2(m-PR)2] (980Zr, 981Zr).

Cp2ZrHCl + {[(Me2N)3P]2P}{BPh4}

Cp2Zr H Cp2Zr

H P H

ZrCp2 H ZrCp2

=

975Zr

Scheme 110 Synthesis of a planar, four-coordinate P center supported by zirconocene centers, 975Zr. Reproduced from published material with permission.

Müller and co-workers have synthesized a dimeric complex [(CpZrCl{m-Z1,Z5-GeC4(2,5-SiMe3-3,4-Me2)})2] (982Zr, Scheme 111). An analogous Ti(III) compound was also synthesized.107

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Scheme 111 Synthesis of a germalene-substituted cyclopentadienyl zirconocene complex.

4.06.3.2.4

Other zirconocene hydrides

A quartet of papers describe the ability of [Cp 2ZrH2] (983Zr) to activate the CdF bonds in fluoro-hydrocarbons.755–758 With fluoroalkanes these formed [Cp 2ZrHF] (984Zr Scheme 112) and alkane, or [Cp 2ZrF2] (985Zr), likely through a radical chain mechanisms.755 With fluoroarenes multiple mechanisms were at play, often leading to 984Zr however with fluorobenzene the reaction lead first to [Cp 2ZrH(o-C6H4F)] (986Zr), then to [Cp 2ZrF(C6H5)] (987Zr).756,757 The reactivity with fluoro-olefins was also investigated, a number of fluoroarene adducts and one fluoroolefin adduct have been structurally characterized (988Zr-993Zr). Zirconocene hydrides have also been shown to activate PdH bonds (994Zr).759,760

Scheme 112 The activation of CdF bonds by zirconocene hydride.

Zirconocene hydride borohydrides can have a hydride abstracted by B(C6F5)3. In benzene this gives a dizirconocene bridged hydride (995Zr, 996Zr, Scheme 113).761,762 However in diethyl ether a zirconocene ethoxy diethyl ether complex is formed, in which an ethereal CdO bond has been cleaved by the zirconocene hydride (997Zr).761 Other species of general formula Cp2ZrX [(m-H)2BR] have been made (X ¼ H, Cl, OSiPh3 R ¼ B8H14) (998Zr-1000Zr).763,764

Scheme 113 Divergent reactivity of zirconocene hydride borohydrides with B(C6F5)3 depending on solvent.

A detailed investigation of Schwartz’s reagent, [{Cp2Zr(m-H)Cl}2], in the hydrozircanation of amides found that this route is one of the mildest and most general methods for transforming amides into aldehydes.765 The Chirik group has made and structurally characterized some analogues of Schwartz’s reagent, as well as some dichlorides.766 These include [Cp (Cp(1,3-SiMe3)2)Zr(H)Cl] (1001Zr), [{iPr2Si-(Cp3-SiMe3)-(Cp3,4-(SiMe3)2)}ZrCl2] (1002Zr), [(Ind1-tBu)2Zr(H)Cl] (1003Zr). The same group investigated the indenyl effect in zirconocene dihydride chemistry, by measuring the rate of olefin insertion into ZrdH bonds.767 As part of this [(Ind1,3-iPr2)2Zr(H)2PMe3] was structurally characterized (1004Zr). These authors characterized a trio of dichloride derivatives of the general formula [(IndR)2ZrCl2] (R ¼ 1,3-(SiMe3)2 1005Zr; R ¼ 1,3-(SiPhMe2)2 1006Zr; R ¼ 1-(SiMe3-3-tBu 1007Zr;) as well as a mixed indenyl derivatives (IndR1)(IndR2)ZrCl2 (R1 ¼ 1,3-SiMe3, R2 ¼ tBu, 1008Zr).143 The CO adducts could be formed by reduction with Mg(0) in a CO atmosphere; [(IndR)2Zr(CO)2] (R ¼ 1,3-(SiMe3)2 1009Zr; R ¼ 1,3-(SiMet2Bu)2 1010Zr).

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

4.06.3.2.5

339

Zirconocene heterobimetallic complexes

Crimmin published a trio of papers featuring zirconocene hydride heterobimetallics (with supporting Nacnac frameworks on the heterometal): [Cp2ZrH(m-H)2M(NacnacR)] (M ¼ Al(H), Zn, Mg; R ¼ Dipp, Mes) (1011Zr-1013Zr, Scheme 114).768,769 The aluminium species can act as a reductant in the zirconocene-catalyzed hydrodefluorination of fluoroarenes. All of the species can isomerize cyclooctadienes to cyclooctyne, with some heterobimetallic bridging hydride/alkyne species structurally characterized (1014Zr-1016Zr).769,770

Scheme 114 Hydro-defluorination catalyzed by zirconocene.

Roesky has characterized a number of heterobimetallic zirconocene species. Initially fluoride bridging species were made (1017Zr, 1018Zr).771 More recently reaction of (NacnacDipp)AlMeOH with [ZrCp2Me2] or [ZrCp2HCl] gave the aluminium-zirconocene oxo bridged methyl and chloride respectively (1019Zr, 1020Zr).772 These showed high catalytic activity for ethylene polymerization. Other [(Nacnac)Al(m-O)ZrCp2] derivatives have been made, including a mixed hydroxide/hydrogensulfide species (1021Zr).773–775 Using the NacnacDipp framework similar Ga and Ge oxo-bridged zirconocenes could be made (1022Zr, 1023Zr).776,777 By substituting the NacnacDipp backbone with a naphthalene fragment a Bi derivative could also be formed (1024Zr).778 Other than Nacnac-supported bimetallics, an oxo-bridged Ca zirconocene was made: [{(SiMe3)2NCa(thf )3}(m-O)ZrCp 2] (1025Zr). This species could act as a catalyst for intramolecular hydroamination and it was proposed that primary aminoalkenes are activated by the Ca center whereas secondary aminoalkenes are activated by the Zr center. Zr/Ti oxo bridged complexes were also made and displayed high activity for ethylene polymerization.779,780 Roesky also used the FLP 734Zr to form Zr/Au complexes (1026Zr).781 It is possible for the zirconocene motif to be used as the ansa-bridge within a ligand framework. Ruthenocene can be doubly 1 5 782 lithiated, and this can subsequently be reacted with [CptBu2ZrCl2] to form [(Z5-CptBu 2 -Zr-m:Z :Z -Cp2)Ru] (1027Zr, Scheme 115). The ferrocene derivatives were previously reported (using an analogous method).783 If FeCp Cp is used preferentially another methylene group can be added into the ansa-bridge (1028Zr).784 None of these species could undergo ring-opening polymerization to from a poly-zirconocene, however if the ansa-bridge in zirconocene is spirocyclic polymerization can occur (1029Zr-1031Zr).785 Zirconocene cyclosiloxanes have been made with ferrocenes as the substituents on the silicones (1032Zr, 1033Zr).786 The highly electron deficient [Cp2ZrMeMeB(C6F5)3] system can be stabilized by the introduction of a ferrocene motif to give [Cp2Zr(m:Z1:Z5-C5H4FeCp)][MeB(C6F5)3] (1034Zr).28 Addition of a donor would remove this FedZr interaction (1035Zr).

Scheme 115 Zirconocene as the ansa-bridging moeity in 1027Zr.

A ligand where the silyl ansa-bridge was functionalized with olefins was deployed on Zr (1036Zr). Rhodium could be attached to these to form a heterobimetallic species, and the Rh was found to have an overall electron-donating effect (1037Zr).787 Bergman has also worked (often with others) on heterobimetallic complexes.788–790 ZrdIr complexes have been made (1038Zr-1041Zr)788,789 as well as a RedZr complex with bridging dinitrogen ligands (1042Zr).790

4.06.3.2.6

Other zirconocene complexes

In 2002 Parkin, Bercaw, Green and co-workers published a comprehensive study of the electronic effect of rings substituents and ansa-bridges in zirconocene complexes (1043Zr-1044Zr).791 These were probed by IR spectroscopic, electrochemical and computational methods. They found that single-atom ansa-bridges exert a net-electron withdrawing effect, with SiMe2 giving a greater effect

340

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

that CMe2. As the ansa-bridge is lengthened the electron withdrawing effect is diminished, and once a three-carbon chain is used the effect becomes electron donating. Surprisingly they found a pair of vicinal SiMe2 bridges exerts an electron-donating effect relative to a single bridge. Jordan has developed an enantioselective synthesis of ansa-zirconocenes. By using a chelating bis-amide starting material [Zr {RN(CH2)3NR}Cl2(thf )2] the R substituents position themselves on opposite sides of the NdZrdN plane, thus giving exclusively the rac- metallocene (1045Zr).792 If stereo-defined substituents are installed on the backbone of the amide chelate one enantiomer of the twist conformation is favored and this can induce enantioselective synthesis of bis-indenyl metallocenes (1046Zr, Scheme 116).793 An in-depth study (including many ansa-bridged indenyl derivatives, 1047Zr-1052Zr) into the effect of chelating vs non-chelating amides was carried out, which gave insight into the mechanism of action, whilst showing that chelating ligands gave better stereocontrol.794 The methodology can also be used for ansa-bridged Cp zirconocenes (1053Zr-1055Zr).795

Scheme 116 Jordan’s enantioselective synthesis of ansa-bridged metallocenes.

A napthylene bridged bis-indenyl ansa zirconocene has also been made (C10H6-1,8-Ind2)ZrCl2 (1056Zr, Scheme 117).796 A 1,1-olefin-bridged bis-indenyl metallocene has been made. Various alkyl functionality could be installed on the other side of the olefin, indeed Grubbs’ catalyst could be used to metathesize the olefin after the ligand had been installed on zirconium (1057Zr1060Zr).797

Scheme 117 The variety of unsaturated carbon frameworks featuring in the ansa-bridge.

[Cp2ZrF2] has been made (1061Zr),798 as have complexes with a number of N-based ligands including phosphanylamide complexes (including chiral variants) (1062Zr, 1063Zr).799–801 Rosenthal has made a number of highly strained four-membered heterometallacycles (1064Zr-1069Zr),802–804 as well as investigating the coordination behavior of other N-based ligands with the zirconocene motif (1070Zr-1079Zr).203,805–807 Chirik and Bercaw used [Cp2ZrMe(CHt2Bu)] (1080Zr) to give the first evidence of g-agostic interactions in stabilizing the transition state in b-methyl elimination.808 Other functionality can be incorporated into the ansa-bridge, e.g. -SiMe(NR2) (1081Zr).809 Zirconocene dichlorides where one Cp (or Cp ) ring has been replaced with sumanenyl ([C21H11]−) have been made (Scheme 118).810 The mono-zirconium sumanenyl ring was crystallographically characterized (1082Zr, 1083Zr), and though there was spectroscopic evidence for the tri-zirconated species, disappointingly it was not sufficiently stable to be crystallized.

Scheme 118 Zirconocene sumanyl complexes reported by Amaya et al.

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

4.06.3.3 4.06.3.3.1

341

Phospholyl zirconium chemistry Phospholyl zirconium chemistry since 2000

The first structurally characterized “diphosphazircocene” complex, [(Z5-C4PMe4)ZrCl2], was reported by Nief et al. in 1988. In the 1990s group 4 complexes of phospholyl ligands were largely investigated in the context of olefin polymerization catalysis, and some “diphosphazirconocene” complexes were tested on the industrial scale.811–813 In the past 2 decades, the number of reports on this type of complex has increased. Fu and co-workers structurally characterized diphosphazirconocene [(Z5-PC4H{2-Ph}{3,4-Me2})2ZrCl2] (1137Zr) and hafnocene [(Z5-PC4H{2-Ph}{3,4-Me2})2HfCl2] (231Hf, vide infra) which were straightforwardly prepared via salt metathesis of Li or K phospholide salts with MCl4 precursors.814 The presence of a lone pair at phosphorus allows for the synthesis of bimetallic complexes. For example, Hollis et al. reported the reaction of 1137Zr with [(R-BINAP)Rh(cod)]+ to furnish a Zr,Rh ansa-heterobimetallic cation [{(m-Rh{R-BINAP})(Z5s-PC4H{2-Ph}{3,4-Me2})2}ZrCl2]+ (1138Zr).815 Complex 1137Zr also serves as an bidentate ligand for Mo(CO)4 and the C2-symmetric, rather than the meso, isomer binds preferentially to afford the Zr,Mo heterobimetallic complex [{(m-Mo(CO)4)(Z5s-PC4H{2-Ph}{3,4-Me2})2}ZrCl2] 1139Zr.814 In the same study it was shown that the rac zirconocene 1137Zr equilibrates in solution to give a mixture of the meso and rac complexes through the slippage of one phospholyl ligand. Accordingly, it was also demonstrated that the isomerization process is favored by the presence of Lewis-basic species such as THF or PMe3. The isomerization process of the zirconium complex 1137Zr proceeds at significantly slower rate than that of the hafnium derivative 1140Zr. Mathey and co-workers reported that bis(tetramethylphospholyl)zirconocene dichloride is a source of “diphosphazirconocene(II)” upon reduction with Mg(0). This transient 14 VE complex reacts with CO to afford Zr(II) complex [(Z5-C4PMe4)Zr(CO)2] (1141Zr) and with alkynes Me3SiCCSiMe3 and 2-butyne to afford the Zr(IV) complexes 1142Zr and 1143Zr.816 Electrochemical studies on zirconocene dichloride complexes CpR2ZrCl2 showed that the replacement of a cyclopentadienyl by a tetramethylphospholyl facilitates the Zr(IV)/Zr(III) reduction process by ca. +0.25 V,816 consistent with the PC4Me4 ligand being an even poorer p-donor than the Cp ligand (Fig. 25).

Fig. 25 Examples of zirconium bis(phospholyl) complexes.

In 2008 Hollis and co-workers reported a series of substituted diphosphazirconocenes [Z5-C4P{2,3-{CH2}n)ZrCl2] for n ¼ 3, 4 (1144Zr), 5 using standard synthetic routes. The tetrahydrophosphaindenyl complex, 1144Zr, crystallizes as a mixture of rac and meso isomers, was structurally characterized by X-ray diffraction, providing unambiguous proof of the meso isomer in bent diphosphametallocenes (previous structural reports contained only the rac isomer).817 These diphosphazirconocenes were evaluated as precatalysts for the polymerization of propylene and 1144Zr produced isotactic polypropylene with stereo-errors indicative of enantiomorphic site control (Scheme 119). The synthesis of isotactic polypropylene by enantiomorphic site control was consistent with configurational (rac/meso) stability and modest rotational stability of the diphosphazirconocene on the time scale of multiple monomer insertions. These results are consistent with the rac isomer producing the isotactic PP and the meso isomer producing the atactic PP. The combined rotational barriers and slip-inversion-slip mechanism suggest an alternate route to achieving the “oscillating catalyst hypothesis” for elastomeric polypropylene synthesis.

Scheme 119 Rationalization of polymer fractions obtained from bis(tetrahydrophosphaindenyl)zirconocene precatalysts reported by Hollis and co-workers.

342

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

In 2016, Fontaine and co-workers reported the first examples of mono(phospholyl) zirconium complexes, [(Z5-PC4Me4) ZrClx(NMe2)3-x], which showed decreasing stability toward ligand scrambling with increasing number of chloride ligands (Scheme 120).818 The monochloride complex 1145Zr was synthesized by reaction of 1-trimethylsilyl-2,3,4,5-tetramethylphosphole with [Zr(NMe2)3Cl], and characterized by X-ray diffraction. Attempted synthesis of a mixed-ring mono-phospholylboratabenzene(Bb) zirconium complex, by reaction of 1145Zr with Bb-PMe3 or (NMe2-Bb)Li-TMEDA, led to complex product mixtures as observed by 31P NMR spectroscopy, which could not be purified or further characterized.

Scheme 120 Synthesis of mono(phospholyl) zirconium complexes reported by Fontaine and co-workers.

Tamm and co-workers reported the transmetallation reaction between [(Z7-C7H7)ZrCl(tmeda)] (tmeda ¼ N,N,N0 , N -tetramethylethylene-diamine) and various phospholide anions to furnish a new class of mixed-sandwich complexes, 1146Zr1148Zr.14 The presence of Lewis basic phosphorus atoms and Lewis acidic zirconium atoms allows ambiphilic behavior to be observed. X-ray diffraction analysis reveals dimeric arrangements for 1146Z, and 1147Zr, with long intermolecular ZrdP bonds, whereas 1148Zr remains monomeric in the solid state. DFT calculations on the dimeric compounds indicate that the ZrdP interaction is weak, and accordingly, complexes 1146Zr1148Zr act as monodentate ligands upon reaction with [W(CO)5(thf )]. Structurally characterized Zr/W heterobimetallics 1149Zr-1150Zr were studied by IR spectroscopy and compared with the phosphene-functionalized trozircene complex [(Z7-C7H7)Zr(Z5-C5H4P{Ph2}W{CO}5)] (1151Zr). All complexes showed a close resemblance to simple phosphines, such as PMe3, although molecular orbital analysis of 1146Zr reveals that the free electron pair in the phosphatrozircenes is not the HOMO. 4 equiv. of 1146Zr can replace 1,4-cyclooctadiene (COD) in [Ni(cod)2] to form the homoleptic, distorted tetrahedral complex [Ni {(Z7-C7H7)Zr(Z5s-PC4Me4)}4] (1152Zr) (Fig. 26). 0

Fig. 26 Phosphatrozircenes reported by Tamm and co-workers.

4.06.3.4

Table of crystallographically characterized Zr compounds

ZrCp complex

Zr oxidation state

31 P chemical shift (ppm)

Number of complex

Reference

[{SiMe2-Cp -NtBu}ZrCl(NMe2)] [(SiMe2-Cp -NtBu)Zr(k2-C4(2,3-Me2)H2)] [(SiMe2-Cp -NtBu)Zr{k2-(NXy)2C6(3,4-Me2)H2}] [Cp Zr{k-O-C6{2-tBu2}H2(5-{C(k-N)OC(H2)CH2})}Cl2] [Cp Zr(k-O-C6{2-tBu2}H2{5-[C(k-N)OC(H2)CH2]})Me2] [Cp Zr(k-O-C6{2-tBu2}H2{5-[C(k-N)OC(Me2)CH2]})Cl2] [Cp Zr((S)-k-O-C6{2-iPr}H3{5-[C(k-N)OC(H2)CH2]})Cl2]

+4 +4 +4 +4 +4 +4 +4

n/a n/a n/a n/a n/a n/a n/a

1Zr 2Zr 3Zr 4Zr 5Zr 6Zr 7Zr

319 321 321 322 322 322 323

343

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

ZrCp complex [Cp Zr((S)-k-O-C6{2-tBu}H3{5-[C(k-N)OC(H2)CH2]})Cl2] [Cp Zr(k-O-C6{2-tBu}H3{5-[C(k-N)OC(H2)CH2]})(NMe2)2] [Cp Zr(k-O-C6{2,4-tBu}H2{5-[C(k-N)OC(Me2)CH2]})(NMe2)] [Cp Zr(k-O-C6{2,4-tBu}H2{5-[C(k-N)OCH(tBu)CH2]})(NMe2)] [[(Cp -Si(Me)2-Z3-NC(NMe2)N)-(S)-CHPhCH2(k-O)]-(S)-ZrNMe2] [[(Cp -Si(Me)2-k-NH(k-2-py)]Zr(NMe2)2] [[(Cp -Si(Me)2-k-N-C(NMe2)-(k-N)-CtBuC-k-O)]Zr(NMe2)2] [[BPh-Cp-(CONCH2CMe2)2]Zr(NMe2)] [[(CPh2CH)-Cp-(N2C3Me2H)]ZrCl3] [{Cp ZrMe(Et2O)[N(tBu)C(Me)N(Et)]}{B(C6F5)4}] [{Cp Zr(m-H-CH2)[N(tBu)C(Me)N(Et)]}2{B(C6F5)4}2] [({Cp Zr[N(tBu)C(Me)N(Et)]}2{m-CH3}{m-CH2})(B(C6F5)4)] [Cp Zr[N(Cy)C(Me)N(Cy)][Me][tBu]] [Cp Zr[N(Cy)C(Me)N(Cy)][k3-CH2(Z2-CH]CH)CH2]] [Cp Zr[N(Cy)C(Me)N(Cy)][k3-(Z1-CH2)2C(Z2-CH2)]2] [Cp Zr[N(Cy)C(Me)N(Cy)][nBu]2] [Cp Zr[N(tBu)C(Me)N(Et)](SiMe2Ph)Cl] [Cp Zr[N(tBu)C(Me)N(Et)][C(Me)(H)(Et)]Cl] [Cp Zr[N(tBu)C(Me)N(Et)][Z2-tBuN]CiPr]] [Cp Zr[N(iPr)C(Me)N(iPr)][OTf][Z3-C3H4(2-Me)] [Cp Zr[N(tBu)C(Me)N(Et)][iPr]Cl] [Cp Zr[N(tBu)C(Me)N(Et)][iBu]Br] [Cp Zr[N(tBu)C(Me)N(Et)][tBu]Cl] [Cp Zr[N(tBu)C(Me)N(Et)][2-Et-nBu]Br] [Cp Zr[N(tBu)C(Me)N(Et)][Et]Me] [Cp Zr[N(tBu)C(Me)N(Et)][iPr]Me] [Cp Zr[N(tBu)C(Me)N(Et)][iBu]Me] [Cp Zr[N(tBu)C(Me)N(Et)][nBu]Me] [{Cp Zr[N(tC(Me)N(Et)][iBu]}{B(C6F5)4}] [{Cp Zr[N(Me)C(NMe2)N(iPr)]}2{m-N2}] [CpZr[CyNC(CpFeC5H4)NCy]Cl2] [Cp Zr[CyNC(CpFeC5H4)NCy]Cl2] [[CHPh-Cp(2,5-Me2)-N(tBu)]Zr(NMe2)2] [[CHPh-Cp(2,5-Me2)-N(tBu)]ZrCl2] [[CHPh-Cp(2,5-Me2)-N(tBu)]ZrCl2)2(m-O)] [{m-[(C]CH2)-Cp-O]}Zr{NEt2}] [[m-CH2CH2-3,30 -(SiMe2-Indenyl-NtBu)2][Zr(NMe2)2]2] [[m-CH2CH2-3,30 -(SiMe2-Indenyl-NtBu)2][ZrMe2]2] [[SiMe2-(3-ethylindenyl)-NtBu)]ZrMe2] [[SiMe2-(3-ethylindenyl)-NtBu)]ZrMe2] [[C(Me)2-Ind-(C2B10H10)]Zr(NMe2)2] [[{[C(Me)2-Ind-(C2B10H10)]ZrCl(Z2-C2B10H10)}{Li(thf )4}] [[C(Me)2-Cp-(C2B10H10)]Zr[Z2:Z2-XyN]CN(Me)(CH2)3N(Me)C]NXy]] [[{C(Me)2-Cp-(C2B10H10)]Zr[m:Z2:Z2-OCN(Ph)N(Me)CH2CH2(Me)N(Ph)NCO]}2] [{C(Me)2-Cp-(C2B10H10)]Zr-m-N]C(Ph)N(Me)(CH2)3N(Me)(Ph)C]N]}2] [{C(Me)2-Cp-(C2B10H10)]Zr[Z2-N(Me)(CH2)2NH(Me)][m-NHC(CH3)] CHC^N]}2] [{C(Me)2-Cp-(C2B10H10)]Zr[Z2-N(Me)(CH2)2NH(Me)][m-TMSN]C]N]}2] [[Na3(thf )8]{C(Me)2-Cp-(C2B9H10)]Zr}2] [[C(Me)2-Cp-(C2B9H10)]Zr(k2-MeNC2H4NHMe)] [Li2{[C(Me)2-Cp-(C2B9H10)]Zr(k2-MeNC2H4NMe)}2] [[CMe2-Ind-(C2B9H10)]Zr(k2-MeNC3H6NHMe)] [[C(Me)2-Ind-(C2B10H10)]Zr(NEt2)2] [[Si(Me)2-Ind-(C2B10H10)]Zr(NMe2)2] [[Si(Me)2-Ind-(C2B10H10)]Zr(NEt2)2] [[C(Me)2-Cp-(C2B10H10)]Zr(NMe2)2] [[Si(Me)2-Cp-(C2B10H10)]Zr(NMe2)2] [[Si(Me)2-Cp-(C2B10H10)]Zr(NEt2)2] [({[C(Me)2-Ind-(C2B10H10)]Zr}2{m-Cl}3)(m-Cl)2Li(thf )2] [[{[C(Me)2-Ind-(C2B10H10)]ZrCl}2{m-Cl}3][Me3NH]0.5
CH2Cl2] [[{[Si(Me)2-Ind-(C2B10H10)]ZrCl}2{m-Cl}3][Me3NH]0.5
CH2Cl2]

Zr oxidation state

31 P chemical shift (ppm)

Number of complex

Reference

+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +2/+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4

n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a

8Zr 9Zr 10Zr 11Zr 12Zr 13Zr 14Zr 15Zr 16Zr 17Zr 18Zr 19Zr 20Zr 21Zr 22Zr 23Zr 24Zr 25Zr 26Zr 27Zr 28Zr 29Zr 30Zr 31Zr 32Zr 33Zr 34Zr 35Zr 36Zr 37Zr 38Zr 39Zr 40Zr 41Zr 42Zr 43Zr 44Zr 45Zr 46Zr 47Zr 48Zr 49Zr 50Zr 51Zr 52Zr 53Zr

323 324 320 324 326 326,327 326 328 330 331 331 332 332 332 332 331 333 333 333 337 338 338 338 338 338 338 338 338 338 334 339 339 341 341 341 342 343 343 343 343 344 345 346 346 346 346

+4 +4/+3 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4

n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a

54Zr 55Zr 56Zr 57Zr 58Zr 59Zr 60Zr 61Zr 62Zr 63Zr 64Zr 65Zr 66Zr 67Zr

346 347 348 348 348 344 344 344 344 344 344 344 344 344 (Continued)

344

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

ZrCp complex [[Si(Me)2-Cp -(C2B10H10)]Zr(NMe2)2] [[CH2-Cp-(C2B10H10)]Zr(NHMe2)(NMe2)] [[CH2-Cp-(C2B10H10)]Zr[(k-OC4H8)2NMe2]] [[CH2-Cp-(C2B10H10)]Zr(m-Cl)2Li
2THF] [{[CH2-Cp-(C2B10H10)]ZrCl2}{Li
2DME}] [{[CH2-Cp-(C2B10H10)]Zr(CH2SiMe3)2}{Li
3THF}] [[CH2-Cp-(C2B10H10)]Zr[CH2(2-(k-NMe2)C6H4]] [[Li(thf )4][ZrCp(1,3-tBu)(S2C2B10H10)2]] [[(CH2CH2CH2-Ind-(k-m-O))ZrCl2]2] [{[CpMe2(SCMeCMe)-C6H3C3H6N)]ZrCl(m-Cl)}2] [[SiMe2-Flu4,5-C4Me2H4–11-Et-NtBu]ZrCl2] [[SiMe2-Flu4,5-C4Me2H4–11-tBu-NtBu]ZrCl2] [[SiMe2-Flu4,5-C4Me2H4-NtBu]ZrMe2] [[SiPh2-Flu4,5-C4Me2H4–10,11-C4Me2H4-NtBu]ZrCl2] [[{[(1,3-SiMe2)2-Cp-(NtBu)2]Zr}2{m-F}][PhCH2B(C6F5)3]] [[(1,3-SiMe2)2-Cp-(NtBu)2]Zr][PhCH2B(C6F5)3]] [[(1,3-SiMe2)2-Cp-(NtBu)2]ZrCH2Ph] [[(1,3-SiMe2)2-Cp-(NtBu)2]Zr][MeB(C6F5)3]] [[(1,2-SiMe2)2-Cp-(NtBu)2]Zr][MeB(C6F5)3]] [[(1,3-SiMe2)2-Cp-(NtBu)2]Zr][C6H5CH2B(C6F5)3]] [[SiMe2-Cp3-SiMe2CH2CH]CH2)-N(tBu)ZrCl(m-Cl)]2] [[NC-nacnacPh2]ZrCpCl2] [[NC-P-nacnacPh2]ZrCpCl2] [[NC-nacnacDipp-Ph]ZrCpCl2] [[(C6F5)3B-NC-nacnacPh2]ZrCpCl2] [[(C6F5)3B-NC-P-nacnacPh2]ZrCpCl2] [[NC-nacnacDipp-Ph]ZrCpCl2] [[NC-nacnacDipp-Ph]ZrCp(CH2CH]CH2)Cl2] [[(C6F5)3B-NC-nacnacDipp-Ph]ZrCp(CH2CH]CH2)Cl2] [[NC-nacnacDipp-Ph]ZrCp(CH2CH2CH2B(C6F5)3)Cl2] [[nacnacDipp-Ph]ZrCp(CH2CH]CH2)Cl2] [CpZr[(1-Ph2P-k-O)(2-k-O)(3-F)C6H3]2Cl] [CpZr[(1-Ph2P-k-O)(2-k-O)(3-tBu)C6H3]2Cl] [CpZr[(1-Ph2P-k-O)(2-k-S)(3-SiMe3)C6H3]2Cl] [Cp Zr[(1-Ph2P-k-O)(2-k-O)(3-Ph)C6H3]Cl2(thf )] [Cp Zr[(1-Ph2P-k-O)(2-k-O)(3-tBu)C6H3]Cl2(thf )] [Cp Zr[(1-Ph2P-k-O)(2-k-O)(3-SiMe3)C6H3]Cl2(thf )] [CpZrCl2(NPtBu3)] [Cp ZrCl2(NPiPr3)] [Cp ZrMe2(NPtBu3)] [Cp ZrPh2(NPtBu3)] [Cp Zr(C3H5)2(NPtBu3)] [CpCp ZrCl(NPiPr3)] [Cp Zr(Z4-(CH2CMeCMeCH2)(NPiPr3)] [Cp Zr(Z4-(CH2CMeCMeCH2)(NPtBu3)] [(Cp Zr)4(m2-Cl)5(Cl)(m4-CH)2] [(Cp Zr)5(m2-Cl)6(m4-CH)3] [Cp ZrCl2(NPtBu3)] [Cp2ZrCl(NPtBu3)] [CpZr(CH2Ph)2(NPtBu3)] [CpZr(CH2Ph)(NPtBu3)2] [CpZr(NDipp)2(NPtBu3)2] [CpZr(acac)3] [CpZr(CF3COCHCOCH3)3] [[Cp Zr(Z5-C5H5B(NMe2)Me][MeB(C6F5)3]] [[Cp Zr(Z5-C5H5B(Ph))Me][MeB(C6F5)3]] [{(PhCH2)(H)B(m-Me2pz)2}Zr(Cp )(Z2-Me2pz)Cl] [CpB(C6F5)2PyrZrCl3(Pyr)2] [[ZrCp(NMe2)2{NH2B(C6F5)3}]] [(ZrCp(1,3-SiMe3)2)2B2(C6F5)2{m-Z5:Z5-C4H4B-CH2-Z3-CHCHCH-k F-B(C6F5)3
2Et2O]

Zr oxidation state

31 P chemical shift (ppm)

Number of complex

Reference

+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4

n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a 28.1 n/a n/a Not soluble n/a n/a n/a n/a n/a Not reported Not reported Not reported −1.55 3.84 3.10 40.7 27.4 33.4 35.4 29.1 24.0 19.1 33.2 n/a n/a 42.2 36.4 35.4 29.7 35.8 n/a n/a n/a n/a n/a n/a n/a n/a

68Zr 69Zr 70Zr 71Zr 72Zr 73Zr 74Zr 75Zr 76Zr 77Zr 78Zr 79Zr 80Zr 81Zr 82Zr 83Zr 84Zr 85Zr 86Zr 87Zr 88Zr 89Zr 90Zr 91Zr 92Zr 93Zr 94Zr 95Zr 96Zr 97Zr 98Zr 99Zr 100Zr 101Zr 102Zr 103Zr 104Zr 105Zr 106Zr 107Zr 108Zr 109Zr 110Zr 111Zr 112Zr 113Zr 114Zr 115Zr 116Zr 117Zr 118Zr 119Zr 120Zr 121Zr 122Zr 123Zr 124Zr 125Zr 126Zr 127Zr

349 350 350 350 350 350 350 351 352 353 354 354 354 354 355 355 356 356 356 89 357 358 358 359 358 358 359 360 360 360 360 361 361 361 362 362 362 363 363 363 363 363 363 363 363 363 363 364 364 364 364 365 366 366 367 367 368 369 370 371

345

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

ZrCp complex [{[PhC(NtBu)2]SiCl]2[PhC(NtBu)2]2Si4}{(Cp ZrCl2)2(m-Cl)3}] [[(CpMe4Et)Zr]6(m6-O)(m3-O)8
C7H8] [[(CpMe4Et)Zr]6(m6-O)(m3-O)8
C9H12] [[Cp3Zr3(m3-O)(m2-OH)3]4[(Mn(C6H8{k-NCH[C6H2(2-k-O)(3-tBu)(5-COOH)]}2)6] Cl6] [[Cp3Zr3(m3-O)(m2-OH)3]4[(Cr(C6H8{k-NCH[C6H2(2-k-O)(3-tBu)(5-COOH)]}2)6] Cl6] [[PPh4][(CpZr(m2-Se2))3(m3-O)(m3–TeSe3)]] [[{CpZr(OH2)3}2(m2-OH)2][C6F5SO3]4
6H2O] [Cp Zr(Z5-C4Me4Si(SiMe3))Cl2] [CpZr(6,6-dmch)Cl2] [CpZr(6,6-dmch)Br2] [CpZr(6,6-dmch)I2] [CpZr(6,6-tmch)(PMe3)2] [CpZr(6,6-dmch)[C(SiMe3)C(C6H5)C(C6H5)C(SiMe3)]] [[CpPiPr2Zr(Z7-C7H7)]2] [[CpPPh2Zr(Z7-C7H7)]2] [[CpPiPr2RhCODClZr(Z7-C7H7)]2] [CpPiPr2-m-Pd-PiPr2 Zr2(Z7-C7H7)2] 2 [CpPiPr2W(CO)5Zr(Z7-C7H7)] [CpZr(Z7-C7H7)] [CpMeZr(Z7-C7H7)] [CpSiMe3Zr(Z7-C7H7)] [CpallylZr(Z7-C7H7)] [Cp1,3-tBu2Zr(Z7-C7H7)] [Cp1,2,4—tBu3Zr(Z7-C7H7)] [Cp1,2,3,4-iPr4Zr(Z7-C7H7)] [Cp1,2,4-(C5H9)3Zr(Z7-C7H7)] [Cp1,2,4-iPr3Zr(Z7-C7H7)] [Cp1,4-Me-2,3-iPr2Zr(Z7-C7H7)] [Cp1-SiMe3–2,5-Me2-3,4-iPr2Zr(Z7-C7H7)] [Cp1,2,3,4,5-Ph5Zr(Z7-C7H7)] [IndZr(Z7-C7H7)] [Ind1-tBuZr(Z7-C7H7)] [Ind1,3-tBu2Zr(Z7-C7H7)] [Ind1,3-(C6H11)2Zr(Z7-C7H7)] [CpZr(Z7-C7H7)(CNtBu)] [CpZr(Z7-C7H7)[CN(2,6-Me2C6H3)]] [CpZr(Z7-C7H7)[C(NMeCH)2]] [CpSiMe3Zr(Z7-C7H7)(thf )] [CpZr(OAc)3] [Cp Zr(OAc)3] [CpP(Se)iPr2Zr(Z7-C7H7)] [[m-Au-(CpPiPr2)2][Zr(Z7-C7H7)][m-Cl]] [[Cpm-PiPr2Zr(Z7-C7H7)][(m-OC)W(CO)2[[Cpm-PiPr2Zr(m-Z7:Z2-C7H7)]] [Cp1,3-tBu2Zr(m-Cl)3] [[Cp1,3-tBu2Zr(m-Cl)2]3] [CpZr[Mes2(p-OMePh)corrole]] [CpZrCl2[(k-O-C8N2H7)3BH]] [[Et3NH][(CpZrCl)2(m-Cl){m-b-MeBG}2]] [{P[CH2N(3,5-Me2C6H3)]3}ZrCp] [({P[CH2N(3,5-Me2C6H3)]3}Zr)2(m-Cp2)] [{[k3-C4H2N-(2,4-CH2PtBu2)2]ZrCpCl(m-Cl)}2] [{[k3-C4H2N-(2,4-CH2PtBu2)2]ZrCpCl}2(m:k1:k1-N2)]

Zr oxidation state

31 P chemical shift (ppm)

Number of complex

Reference

+4 +4 +4 +4

n/a n/a n/a n/a

128Zr 129Zr 130Zr 131Zr

372 373 373 374

+4

n/a

132Zr

374

+4 +2 +4 +4 +4 +4 +2 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +3 +4 +4 +4 +4 +4 +4 +4

Not reported n/a n/a n/a n/a n/a Not reported n/a 0.00 −21.5 37.5 30.7 4.4 n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a 45.3 56.0 30.1 n/a n/a n/a n/a n/a −57.1 −56.3 53.4 57.3/50.5

133Zr 134Zr 135Zr 136Zr 137Zr 138Zr 139Zr 140Zr 141Zr 142Zr 143Zr 144Zr 145Zr 146Zr 147Zr 148Zr 149Zr 150Zr 151Zr 152Zr 153Zr 154Zr 155Zr 156Zr 157Zr 158Zr 159Zr 160Zr 161Zr 162Zr 163Zr 164Zr 165Zr 166Zr 167Zr 168Zr 169Zr 170Zr 171Zr 172Zr 173Zr 174Zr 175Zr 176Zr 177Zr 178Zr 179Zr

380 381 382 383 383 383 384 384 102 102 102 102 385 389 388 388 388 386 386 386 387 387 387 387 387 388 387 386 386 389 388,389

390 388 391 391 392 392 392 393 393 394 395 396 397 397 398 398

346

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

ZrCp2 complex

Zr oxidation state

[[(Cp(1,3-(SiMe3)2)2Zr]2[m-Z2-N2]] [(Cp )(Cp(1,2,4-(SiMe3)3)ZrCl2] [(Cp )(Cp(1,3-(SiMe3)2)Zr(CH2Ph)H] [(Cp )(Cp(1,3-(SiMe3)2)Zr(Me)H] [[(CpMe4)2Zr]2[m-Z2-N2]] [[(CpMe4)2Zr(H)]2[m-k2-N2H2]] [[(CpMe4)2Zr(CCtBu)]2[m-k2-N2H2]] [[(CpMe4)2ZrI]2[m-k1:k1-N2H2]] [[(CpMe4)2ZrCl]2[m-k1:k1-N2H2]] [[(CpMe4)2Zr(OTf )]2[m-k1:k1-N2H2]] [Cp (Cp(1-Ph-3,4-Me2))Zr(m-Z2-N2)] [Cp (m-Cp(1,3-Me2-4-m-CH2))Zr(m-k2-N2H2)] [Cp (m-Cp(1-Ph-3-Me-4-m-CH2))Zr(m-k2-N2H2)] [[(CpSiMe3)2Zr(m-Cl)]2] [[(CpSiMe3)2Zr(m-I)]2] [[(Cp(1,2,4-Me3)2Zr]2[m-Z2-N2]] [[(Cp(1,2,4-Me3)2Zr]2[m-N]CHCy][m-Z1:Z2-N2Bpin]] [[(CpMe4)2Zr(Tol)]2[m-Z2-N2]] [(CpMe4)2Zr[2,20 -k2-4,40 -Me2-(C6H3)2]] [(Cp1,3-(SiMe3)2)(Cp(1-SiMe3)-3-k-SiMe2CH2))Zr(H)] [(Cp(1-SiMe3)-3-k-SiMe2CH2))2Zr] [[(Cp1,3-(SiMe3)2)2Zr(H)]2[m-O]] [[SiMe2-(Cp(3-tBu))2]ZrMe2] [{[SiMe2-(Cp(3-tBu))2]ZrH}2{m-H}2] [{[SiMe2-(Cp(2-SiMe3-4-tBu))2]Zr}2{m:Z2-N2}] [{[(SiMe2)2-(Cp)2]Zr}3{m2-H}3{m3-H}2] [{[(SiMe2-(Cp(3-tBu))-(Cp(2,3,4,5-Me))]Zr}2{m:Z2-N2}] [{[(SiMe2-(Cp(3-tBu))-(Cp(2,3,4,5-Me))]Zr(H)]2}{m:Z2-N2}] [{[(SiMe2-(Cp(3-Ad))-(Cp(2,3,4,5-Me))]Zr(Cl)]2}{m:Z1-N2}{K(C4H8O)2}] [{[(SiMe2-(Cp(3-Ad))-(Cp(2,3,4,5-Me))]Zr[CH(SiMe3)2]2}{m:Z1-N2}
{Na(C4H8O)6}] [(Z5-Ind[1,3-(SiMe3)2])(Z6-Ind[1,3-(SiMe3)2])Zr(C4H8O)] [(Z5-Ind[1,3-(SiMe3)2])2Zr(CO)2] [(Z5-Ind[1,3-(SiMe3)2])2Zr(H)(k2-(NC5H3NMe2)] [(Z5-Ind[1,3-(SiMe2tBu)2])(Z9-Ind[1,3-(SiMe2tBu)2])Zr] [(Z5-Ind[1,3-iPr2])(Z9-Ind[1,3-iPr2])Zr] [(Z5-Ind[1,3-iPr2])2Zr(Z3-CH2C(Me)CH2)H] [(Z5-Ind[1,3-(SiMe2Ph)2])(Z6-Ind[1,3-(SiMe2Ph)2])Zr(C4H8O)] [(Z5-Ind[1-(SiMe3)-3-tBu])(Z6-Ind[1-(SiMe3)-3-tBu])Zr(C4H8O)] [(Z5-Ind[1.3-(SiMe3)2])(Z6-Ind[1,3-(SiMe3)2])Zr(k2-MeOC2H4OMe)] [[(Ind(1,3-iPr)2)2ZrCl]2[m:Z1-N2]
2Na] [[(Ind(1,3-iPr)2)2ZrI]2[m:Z1-N2]
2Na] [(Ind(1,3-iPr)2)2Zr(CH2CHCH2)2NtBu] [(Ind(1,3-iPr2))2Zr(H)2] [(THI(1,3-(SiMe3)2))2Zr(H)2] [Zr(H)(Z5-Ind(1,3-iPr2))(Z3:Z5-Ind(1,3-iPr2)-H)] [(Ind1,3-(SiMe3)2)2Zr(Z2-C2H4)] [(Ind1,3-IPr2)2Zr(k2-C4H8O)] [(Ind1,3-iPr)(THI1,3-iPr)ZrH(k2-BH4)] [(Cp )(Z9-Ind(1,3-iPr2))Zr(II)] [{[(Cp )(m:Z2:Z5-Ind(1,3-iPr2))Zr][m:Z1-N2]] [(Cp Zr)2(m-Cl)(m:Z1:Z2-N2)(m:Z5:Z4-Ind(1-Me-3-iPr))] [Cp1,3-(SiMe3)2FluMe9ZrCl2] [Cp1,3-(SiMe3)2FluMe8Zr(CO)2] [Cp1,3-(SiMe3)2FluMe9Zr(CO)2] [Cp1,3-(SiMe3)2(Z3:Z5-HFluMe9)ZrH] [Cp 2Zr(CH2CH(Me)CH2SMe)][Al4B4O8MetBu4Dipp4] [[Cp2Zr(ClC6D5)(CH2C6H5)][B(C6F5)4]] [[Cp Zr(ClC6D5)Cl][B(C6F5)4]] [[Cp (k2-Z4-C5Me5-C6D4)ZrCl][B(C6F5)4]] [[Cp 2Zr(Z2-2-Cl-C6H4)(CH3CN)][B(C6F5)4]]

+2 +4 +4 +4 +2 +4 +4 +4 +4 +4 +2 +4 +4 +3 +3 +2 +4 +3 +4 +4 +4 +4 +4 +4 +2 +4/+4/+3 +2 +4 +2/+3 +3/+2 +2 +2 +4 +2 +2 +4 +2 +2 +2 +2 +2 +4 +4 +4 +4 +2 +4 +4 +2 +2 +2 +4 +4 +4 +4 +4 +4 +4 +4 +4

31

P chemical shift (ppm)

Number of complex

Reference

n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a

180Zr 181Zr 182Zr 183Zr 184Zr 185Zr 186Zr 187Zr 188Zr 189Zr 190Zr 191Zr 192Zr 193Zr 194Zr 195Zr 196Zr 197Zr 198Zr 199Zr 200Zr 201Zr 202Zr 203Zr 204Zr 205Zr 206Zr 207Zr 208Zr 209Zr 210Zr 211Zr 212Zr 213Zr 214Zr 215Zr 216Zr 217Zr 218Zr 219Zr 220Zr 221Zr 222Zr 223Zr 224Zr 225Zr 226Zr 227Zr 228Zr 229Zr 230Zr 231Zr 232Zr 233Zr 234Zr 235Zr 236Zr 237Zr 238Zr 239Zr

399 399 400 400 401 401 403 403 403 403 404 404 404 405 405 406 406 407 407 408 408 408 409 409 409 409 410 410 411 411 412 412 412 413 413 413 414 414 414 415 415 416 417 417 417 418 418 270 419 419 419 420 420 420 421 422 423 423 423 423

347

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

ZrCp2 complex

Zr oxidation state

[[Cp 2Zr(OC6F5)][Nb(OC6F5)6]] [[Cp2ZrMe][MeB(C6F5)3]] [[(Me2Si-Cp2)ZrMe(OC4H8)][B(C6F5)4]] [[(Me2C-Cp-Flu)ZrMe][MeB(C6F5)3]] [[(Me2C-Cp-Flu)ZrMe][FAl(2-C6F5C6F4)3]] [[(EBI)ZrMe][MeAl(C6F5)3]] [[{(Me2C-Cp-Flu)ZrMe}2{m-F}][FB(C12F9)3]] [[{(Me2C-Cp-Flu)ZrMe}2{m-Me}][F{Al(C6F5)3}2]] [[{(Me2C-Cp-Flu)ZrMe}2{m-Me}][Al(C12F9)3FAl(C6F5)3]] [(Me2C-Cp-Flu)ZrCl(C6F5)] [[{(Me2C-Cp-Flu)Zr}2{m-Br}2][Al(C6F5)4]2] [Cp1,3-(SiMe3)2 ZrMe[NCB(C6F5)3]] 2 [[Cp2Zr(OiPr)(HOiPr)][H2N{B(C6F5)3}2]
Et2O] [(Me2Si-Ind2)ZrMe(CH2SiMe3)] [{(Me2Si-Ind2)Zr}2{m-Cl}2][B(C6F5)4]2] [(Me2C-Cp-Flu)Zr(CH2SiMe3)2] [(Me2C-Cp-Flu)ZrMeCl] [[(Me2C-Cp-Flu)ZrMe(NMe2Ph)][B(C6F5)4]] [[(Me2C-Cp-Flu)Zr(m-F)]2[B(C6F5)4]2] [[{(Me2C-Cp-Flu)ZrMe}2(m-Me)][MeB(C6F5)3]] [[Cp2ZrOC(Me2)CH2CH2-Z2-CH]CH2][MeB(C6F5)3]] [[S,S,R-(EBI)ZrOC(Me2)CH2CH2-Z2-CH]CH2][MeB(C6F5)3]] [Cp CpZr(CHt2Bu)2] [Cp CpZr(CH2SiMe3)2] [Cp CpZrCl(CHt2Bu)] [Cp CpZrMe(CH2CEt3)] [Cp1,2,3,4-Me4 ZrMe[m-MeB(C6F5)3]] 2 [Cp1,2,3,4-Me4 ZrMe[m-MeAl(C6F5)3]] 2 [[O(SiMe2)2-Cp2]ZrBr2] [{1,3-[O(SiMe2)2]2-[Z5-C5H3]2}ZrBr2] [Cp2ZrMeCl] [[Me2Si-Ind-Cp]ZrMe[(m-H)B(C6F5)3]] [[Me2Si-Ind-Cp]ZrMe[(m-Me)B(C6F5)3]] [[Cp Zr(Z5-Cp1-k-CH2–2,3,4,5-Me4)(OC4H8)][BPh4]] [[Cp 2Zr(Z2-NEt]CMe)(OC4H8)][B(C6F5)4]] [Cp1-para-biphenyl-3,4-Me2 ZrCl2] 2 [CpCp1-para-biphenyl-3,4-Me2ZrCl2] [Cp Cp1-para-biphenyl-3,4-Me2ZrCl2] [CpZrCl2(m:Z5:Z5-Cp2,3,4,5-Me4-1,10 -para-biphenyl-Cp2,3,4,5Me4)ZrCpCl2] [CpZrCl2(m:Z5:Z5-Cp2,3,4,5-Me4-1,10 -(p-C6H4)CH2CH2(p-C6H4)-Cp2,3,4,5Me4)ZrCpCl2] [Cp1-(p-Me2N)C6H4-3,4-Me2 ZrCl2] 2 [Cp1-(p-MeO)C6H4-3,4-Me2 ZrCl2] 2 [Cp1-(o-Me2N)C6H4-3,4-Me2 ZrCl2] 2 [Cp1-(m-Me2N)C6H4-3,4-Me2 ZrCl2] 2 [Cp1-(o-MeO)C6H4-3,4-Me2 ZrCl2] 2 [Cp1-(m-MeO)2C6H3-3,4-Me2 ZrCl2] 2 [Cp1-(o-Me2N)C6H4-3,4-Me2Cp1-p-MeC6H4-3,4-Me2ZrCl2] [[S-(Me2Si)2-Cp4-CHMetBu-Cp3,5-iPr2]ZrCl2] [[S-(Me2Si)2-Cp4-CHMetBu-Cp3,5-iPr2]Zr(SPh)2] [[S-(Me2Si)2-Cp4-CHMetBu-Cp3,5-CHEt2]ZrCl2] [[S-(Me2Si)2-Cp4-CHMetBu-Cp3,5-Cy2]ZrCl2] [[S-(Me2Si)2-Cp4-CHEttBu-Cp3,5-CHEt2]ZrCl2] [{Me2Si-[Z5-5-N-2,6-(Me)2-N(Ph)C7H2]2}ZrCl2] [{Me2Si-[Z5-7-S-2,6-(Me)2-5-(Ph)-SC7H]2}ZrCl2] [{Me2Si-[Z5-7-S-2,6-(Me)2-5-(2-MeC6H4)-SC7H]2}ZrCl2] [{Me2Si-[Z5-5-N-2,6-(Me)2-N(Ph)C7H2]-[Ind2-Me-5-Ph]}ZrCl2] [[rac-Me2Si-Ind2]Zr[(m-Me)Al(C6F5)3]2] [[rac-Me2Si-Ind2-Me-5-Ph ]Zr(Z4-PhCH]CH-CH]CHPh)] 2 [[rac-EBI]Zr[OC(]CMe2)OiPr]2] [[rac-EBI]ZrMe[OC(-CMe2)-m-OAl(C6F5)3]2]

+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +2 +4 +4

31

P chemical shift (ppm)

Number of complex

n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a N/A n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a

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

Reference 424,425

313 426 427 427 428 429 429 429 429 429 430 434 431 432 433 433 435 435 435 437 437 440 440 440 440 441 441 442 442 442 443 444 445 445 446 446 446 447 448 449 450 451 451 451 451 451 452 452 452 452 453 454 454 454 454 86 455 456 456 (Continued)

348

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

ZrCp2 complex

Zr oxidation state

[[rac-EBI]ZrMe[OC(]CMe2)NMe2]] [[rac-EBI]Zr[k-OC(OMe)]CMeCH2CMe2C(OiPr)]k-O]}{[Al(C6F5)3]2-m-OMe}] [{[rac-Me2Si-Ind5,6-C6H4]Zr(OC4H8)[OC(]CMe2)OiPr]}{MeB(C6F5)3}] [Cp(1-Me2Si-k-NtBu)-2,3,4,5-Me4CpZrMe] [[rac-EBI]ZrMe{N(Tol)SiMe2[C(SiMe2)2(N-p-tol)2BMe]}] [[Cp CpnPrZrNMe2(NHMe2)][B(C6F5)4]] [[(Cp2Zr)2(m:Z1:Z1-O2CiPr)2][MeB(C6F5)3]2] [{[Cp2Zr(OC4H8)]2[m:Z1:Z1-O2CC(Me)]CH2]2}{MeB(C6F5)3}2] [{Cp2Zr[OC(]CMe2)OtBu]}{m-O} [[Ph2C-(Cp)(Flu(2-tBu)]Zr[OC(OiPr)]CMe2]2] [[Me2Si-(Cp)(Flu)]Zr[OC(OiPr)]CMe2]2] [[Ph2C-Cp3-(2-Adamantyl)-Flu]ZrCl2] [[Me2C-Cp3-tBu-4-Me-Flu]ZrCl2] [[Ph2C-Cp-Flu4,5,10,11-(CMe2CH2CH2CMe2)2]ZrCl2] [[Ph2C-Cp-Flu4,5,-(CMe2CH2CH2CMe2)]ZrCl2] [[Me2Si-Ind2]Zr(m-H)3(AliBu2)2] [[(Me2Si)2-Cp2]Zr(m-H)3(AliBu2)2] [[Me2Si-Ind2)Zr(m-Cl)2AlMe2]] [[Ph2C-Flu5,10-tBu2-Cp3-tBu]ZrCl2] [[Ph2C-Flu5,10-tBu2-Cp2-Me-4-CMe2Ph]ZrCl2] [[anti-PhHC-Flu-Cp2-Me-4–tBu]ZrCl2] [[anti-PhHC-Flu5,10-tBu2-Cp2-Et-4-tBu]ZrCl2] [[anti-PhHC-Flu5,10-tBu2-Cp2-Ph-4-tBu]ZrCl2] [[anti-PhHC-Flu4,11-tBu2-Cp2-Me-4-tBu]ZrCl2] [[anti-PhHC-Flu4,5,10,11-(k2-CMe2CH2CH2CMe2)2-Cp2-Me-4-tBu]ZrCl2] [[anti-PhHC-Flu4,5,10,11-(k2-CMe2CH2CH2CMe2)2-Cp2-Et-4-tBu]ZrCl2] [[Me2C-Flu5-tBu-Cp2,4-tBu]ZrCl2] [[anti-PhHC-Flu5,10-tBu2-Cp2-Me-4-tBu]ZrCl2] [[syn-MesHC-Flu5,10-tBu2-Cp2-Me-4-CMe2Ph]ZrCl2] [[H2C-Flu5,10-tBu2-Cp2-Me-4-tBu]ZrCl2] [[PhMeC-Flu5,10-tBu2-Cp]ZrCl2] [{[rac-Me2Si-Ind2-Me-5-Ph ]Zr(m-Me)2AlMe2}{B(C6F5)4)] 2 [{[rac-Me2Si-Ind2-Me-5-Ph ]Zr(m2-Me)(m3-CH2)[AlMe(m-Me)AlMe2]}{B(C6F5)4)] 2 2-Me-5-Ph [{[rac-Me2Si-Ind2 ]ZrMe(m-Me)B(C6F5)3] [[anti-PhHC-Flu5,10-tBu2-Cp2-Et-4-tBu]ZrMe(m-Me)B(C6F5)3] [[Ind1-iPr-3-B(C6F5)2]CpZrCl2] [[Ind1-iPr-3-B(C6F5)2-m-OH]CpZrCl] [(Ind(2-CMe]CH2))2ZrCl2] [(Ind(2-CCy]CH2))2ZrCl2] [(C4H4Cy2-Ind2)ZrCl2] [(C4H4Me2-Ind2)ZrMe2] [(C4H4Me2-Ind2)ZrPh2] [(C4H4Me2-Ind2)ZrMe(Z2-MeCNtBu)] [(C4H4Me2-Ind2)ZrMe(Z2-OCPh)] [(Cp(C(Me)]CH2))2ZrCl2] [[C4H4Me2-Cp2)ZrCl2] [(Cp(1-(C(Me)]CH2)-3-tBu))2ZrCl2] [Cp2(1,3-(CMe]CH2)2)ZrCl2] [{[C4H4Me2-Cp(3-CMe]CH2)]ZrCl}2{m-BINOL}] [Cp2(CH]CH2)ZrCl2] [[C4H6-Cp2]ZrCl2] [Ind2(CH]CH2)ZrCl2] [Cp2(CH]CHMe)ZrCl2] [[cis-C4H4Me2-Cp2]ZrCl2] [[cis-C4H4Et2-Cp2]ZrCl2] [[trans-C4H4Et2-Cp2]ZrCl2] [[trans-C8H8Et2-Cp2]ZrCl2] [(C8H10-Cp2)ZrCl2] [[(CH2)9-Cp2]ZrCl2] [{[(CH2)4(CH]CH)(CH2)4]-Cp2}ZrCl2]

+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +3 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4

31

P chemical shift (ppm)

Number of complex

Reference

n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a

300Zr 301Zr 302Zr 303Zr 304Zr 305Zr 306Zr 307Zr 308Zr 309Zr 310Zr 311Zr 312Zr 313Zr 314Zr 315Zr 316Zr 317Zr 318Zr 319Zr 320Zr 321Zr 322Zr 323Zr 324Zr 325Zr 326Zr 327Zr 328Zr 329Zr 330Zr 331Zr 332Zr 333Zr 334Zr 335Zr 336Zr 337Zr 338Zr 339Zr 340Zr 341Zr 342Zr 343Zr 344Zr 345Zr 346Zr 347Zr 348Zr 349Zr 350Zr 351Zr 352Zr 353Zr 354Zr 355Zr 356Zr 357Zr 358Zr 359Zr

457 458 459 460 461 462 463 463 465 466 466 467 468 469 469 470 470 471 472 472 473 473 473 473 473 473 473 474 474 474 475 476 476 477 477 478 478 479 479 479 480 480 480 480 481 481 481 482 482 483 483 483 488 488 488 488 488 484 485 486

349

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

ZrCp2 complex

Zr oxidation state

[[(CH2CH]CHCH2)-Cp2]ZrCl2] [{[(CH2CH]CHCH2)-Cp2]ZrCl2}2{m-O}] [[(CH2CH]CHCH2)-Cp2]ZrCl2
Li[B(C6F5)4]] [[1-SiMe2-Ind2-3-[(CH2)3-k-CH2(CH2)2-k-CH2(CH2)3]ZrCl2] [(Ind(2-C^CtBu))2ZrCl2] [(Ind(2-C^CPh))2ZrCl2] [(Ind(2-CCPh(Co2(CO)6)))2ZrCl2] [(EBTHI3-B(C6F5)3)Zr[Me3SiC]CH(SiMe3)]] [(EBTHI3-B(C6F5)2)ZrH(C6F5)] [(EBTHI2-B(C6F5)2)ZrH(C6F5)] [(EBTHI)Zr(H)(C6F4)B(C6F5)2] [rac-(EBTHI)Zr(k-tBuC]C]C]k-CtBu]] [rac-(EBTHI)Zr(k-CH(C4H6)CHCH(C4H6)-k-CH]] [rac-(EBTHI)Zr(k-CH(C8H10)CHCH(C8H10)-k-CH]] [rac-(EBTHI)Zr(m-H)(m:Z2:Z1-SiMe3C]CSiMe3)AliBu2] [rac-(EBTHI)Zr(k-OCMe]CHCH2C(SiMe3)]k-CSiMe3]] [rac-(EBTHI)Zr(k-OCMe]CH-k-CH2]] [rac-(EBTHI)Zr(m:Z1:Z2-PhCN)(m:k-N]CPh-CPh]k-N)Zr(rac-EBTHI)] [rac-(EBTHI)Zr[CNXy][Xy-Z2-NC]C(NXySiMe3)(C^CSiMe3)]] [rac-(EBTHI)Zr[CNXy][Xy-Z2-NC]C(C^CNXySiMe3)(SiMe3)]] [rac-(EBTHI)Zr[Xy-k-NC[C(SiMe3)(NXySiMe3)]]C]k-CSiMe3]] [rac-(EBTHI)Zr[Xy-k-NC(C24N2Si2H36)]C-k-CNXy]] [[(Me2Si)-Cp-CpiPr]ZrCl2] [[(Me2Si)-Cp-CptBu]ZrCl2] [[(Me2Si)-Cp-Ind]ZrMe2] [[(Me2Si)-Cp-Cp(3-(CH2)3-(9-BBN))]ZrCl2] [[(Me2Si)-Cp-Cp2-(CH2)3-B(C6F5)2-NCHN(Me)C6H4)]ZrCl2] [[(Me2Si)-Cp-Cp(3-(CH2)3-B(C6F5)2(NC(Me)OC2H4))]ZrCl2] [[(Me2Si)-Cp-Cp(3-(CH2)3-B(C6F5)2(NC4H5)]ZrCl2] [[(Me2Si)-Cp-Cp2-(CH2)3-B(C6F5)(C6F4-k-F)]ZrCl2] [[(Me2Si)-Cp-Cp2-(CH2)3-B(C6F5)(C6F4-k-F)]ZrPh] [[Me2Si-Cp2,3,4,5-Me4-Ind2-Me]ZrCl2] [meso-EBI ZrCl2] [rac-EBI ZrCl2] [rac-Ind1,2,5,6,7,8-Me6 ZrCl2] 2 [meso-Ind1,2,5,6,7,8-Me6 ZrCl2] 2 [rac-Ind1,2,5,6,7,8-Me6 Zr(CH2C6H5)2] 2 [(Z8-Pn )(Z5-Cp)ZrCl] [(Z8-Pn )(Z5-Cp)(Z1-Cp)Zr] [(Z8-Pn )(Z5-Cp )ZrCl] [(Z8-Pn )(Z5-CpMe)ZrCl] [(Z8-Pn )(Z5-CpnBu)ZrCl] [(Z8-Pn )(Z5-CptBu)ZrCl] [(Z8-Pn )(Z5-Cp1,2,3-Me)ZrCl] [(Z8-Pn )(Z5-Ind)ZrCl] [(Z8-Pn )(Z5-CpMe)ZrMe] [Cp2Zr(O-2,6-Me2C6H4)Cl] [[Me2Si-Ind]2Zr(O-2,6-Me2C6H4)Cl] [meso-EBI Zr(CH2Ph2)2] [meso-EBI ZrCl(CHt2Bu)] [[Me2Si-Ind -Cp]ZrCl2] [[E-Me2Si-Ind -CpMe]ZrCl2] [[Z-Me2Si-Ind -CpMe]ZrCl2] [[Me2Si-Ind -Cp]Zr(O-2,6-Me2C6H4)Cl] [[E-Me2Si-Ind -CpnBu]ZrCl2] [[Z-Me2Si-Ind -CpnBu]ZrCl2] [[Me2Si-Ind -Cp]ZrBr2] [[Me2Si-Ind -Cp]Zr(CH2Ph)2] [[Z-Me2Si-Ind -Cp]ZrMe2] [[(Me2Si)2-Ind -Flu4,11-tBu2]ZrCl2]

+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4

31

P chemical shift (ppm)

Number of complex

Reference

n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a

360Zr 361Zr 362Zr 363Zr 364Zr 365Zr 366Zr 367Zr 368Zr 369Zr 370Zr 371Zr 372Zr 373Zr 373Zr 374Zr 375Zr 376Zr 377Zr 378Zr 379Zr 380Zr 381Zr 382Zr 383Zr 384Zr 385Zr 386Zr 387Zr 388Zr 389Zr 390Zr 391Zr 392Zr 393Zr 394Zr 395Zr 396Zr 397Zr 398Zr 399Zr 400Zr 401Zr 402Zr 403Zr 404Zr 405Zr 406Zr 407Zr 408Zr 409Zr 410Zr 411Zr 412Zr 413Zr 414Zr 415Zr 416Zr 417Zr 418Zr

486 486 486 487 489 489 489 490 490 492 492 493 494 494 496 495 495 497 498 498 498 498 499 499 499 500 501 500 500 501 502 503 504 504 505 505 505 506 506 506 507 507 507 507 507 507 508 508 509 509 510 510 510 510 511 511 511 511 511 512 (Continued)

350

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

ZrCp2 complex

Zr oxidation state

[[Me2Si-Ind -Flu4,11-tBu2]ZrCl2] [(2-Ph-Z5-C17H10)2ZrCl2] [(2-Ph-Z5-C17H10)2Zr(CH2Ph)2] [[Me4C2-Cp2]Zr(m-H)4(BH2)2] [Cp 2Zr[tBu-Z4dC]C]C]C-C^CtBu)] [Cp Cp(1-k-CH2–2-CH2C(tBu)k-C]C]CHtBu-3,4,5-Me3)Zr] [Cp 2Zr(Me-k-C]C]C]k-CMe)] [Cp 2Zr(Me-k-C]CHdC(m-CO2)]k-CMe)]2[AliBu2]2] [Cp 2Zr(m-H)[m:Z2:Z1-HC]C(SiMe3)]AliBu2] [Cp 2Zr(m-H)[m:Z2:Z1:Z1-PhC]C(CH]CPh)]AliBu] [Cp 2Zr(m-H)[m:Z2:Z1:Z1-PhC]C(CH]CPh)][AliBu][(m-H)AliBu2]] [Cp 2Zr(Me-k-C]C[B(C6F5)3]-Z2-C^CMe)] [Cp 2Zr(Ph-k-C]Z2:C]C]C(Ph)[B(C6F5)3])] [Cp2Zr[(tBu)-k-C]C(C^CtBu)dC(SiMe3)]k-C(C^CSiMe3)]] [Cp2Zr[(tBu)-k-C]C]C]CtBudC(C^CSiMe3)]k-CSiMe3]] [Cp2Zr(Ph-k-C]C(Ph)dC(C^CtBu)]k-CtBu)] [Cp2Zr(Ph-k-C]CHdC(C^CtBu)]k-CtBu)] [{Cp2Zr[k:m-N]CMedC(SiMe3)]k-C(C^CSiMe3)]}2] [Cp2Zr[Ph-k-N-CH(Ph)dC(SiMe3)]k-C(C^CSiMe3)]] [Cp2Zr[k-O-C(O)-PhN-CH(Ph)dC(SiMe3)]k-C(C^CSiMe3)]] [Cp2Zr[(Me3Si)-k-C]C]C]C(SiMe3)dC(SiMe3)]k-C(C^CSiMe3)]] [Cp2Zr(Ph-k-C]C(C^CPh)dC(C^CPh)]k-CPh)] [Cp2Zr[(Me3Si)-k-C]C]C]C(SiMe3)dC(C^CPh)]k-CPh]] [Cp2Zr[(Me3Si)-k-C]C]C]C(SiMe3)-CPh]k-CPh]] [Cp2Zr{(Me2ClSiC^CSiMe2Cl)}(Pyr)] [Cp2Zr(k2-Me2Si-C^C-SiMe2)] [[Cp2Zr(Ph2PC^C-m-PPh2)]2] [(EBI)Zr(Ph2PC^CPPh2)] [Cp2Zr[Me3Si-k-C]C]C(NCySiMe3)-k-NCy]] [Cp2Zr[Me3Si-k-C]C]C(N(Tol)SiMe3)-k-N(Tol)]] [Cp2Zr{(Me3SiC^CSiMe3)}(NCPh)] [Cp2ZrH[k-OC(tBu)-k-NiPr]] [Cp2Zr[k-OC(Ph)-k-NDipp]] [Cp Cp(1-CH2B(C6F5)3–2,3,4,5-Me4)Zr(PhC]CHSiMe3)] [Cp2Zr(C5F4N)(Me3SiC]CHSiMe3)] [rac-(EBTHI)Zr(C5F4N)(Me3SiC]CHSiMe3)] [Cp2ZrF(C5F4N)] [Cp2ZrCl(C5F4N)] [Cp Cp1-AliBu2-2,3,4,5-Me4Zr(m-H)(Me3SiC]CHSiMe3)] [Cp Cp1-AliBu-2,3,4,5-Me4Zr(m-H)(m-Me3SiC]CSiMe3)] [rac-(EBTHI)Zr[k-O(CH2)3-k-CH2]] [(Me2Si-Cp2)Zr(CNtBu))(Me3SiC]CHSiMe3)] [Cpmenthyl ZrCl2] 2 [Cpmenthyl ZrF2] 2 [Cp8-phenylmenthyl ZrCl2] 2 [Cpmenthyl Zr(k-CH2-(CH2)2-k-CH2]] 2 [rac-(EBTHI)Zr(Z2-PhN]NPh)] [Cp2Zr(OC4H8)(Z2-PhN]NPh)] [Cp2Zr[k-C(Fc)]C(Fc)dC(Fc)]k-C(Fc)]] [[Cp2Zr(m:Z1:Z2-C^CFc)]2] [Cp2Zr(Pyr)(Z2-PinBC]CBPin)] [Cp 2Zr(NCCHPh2)(Z2-Me3SiC]CSiMe3)] [Cp 2Zr(NCCPh2)(NCHCHPh2)] [Cp 2Zr[k-(Me3Si)C]C(CH]CSiMe3)-k-N-C]CPhdC(CH2Ph)]k-NH] [Cp 2Zr[k-C(NC5H10)]C(NC5H10)dC(NC5H10)]k-C(NC5H10)]] [Cp 2Zr[Z2-(C5H10N)C]C(NC5H10)]] [Cp 2Zr[Z2-(C5H9MeN)C]C(NC5H9Me)]] [Cp 2Zr[Z2-(Et2N)C]C(NEt2)]] [Cp 2Zr[k-N]C(Fc)dC(Fc)]k-N]] [{Cp 2Zr[k-N]C(dC^CFc)dC(Fc)]k-CdC^-m-N]}n]

+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4

31

P chemical shift (ppm)

Number of complex

Reference

n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a 15.4/8.4 3.2 n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a

418Zr 419Zr 419Zr 421Zr 422Zr 423Zr 424Zr 425Zr 426Zr 427Zr 428Zr 429Zr 430Zr 431Zr 432Zr 433Zr 434Zr 435Zr 436Zr 437Zr 438Zr 439Zr 440Zr 441Zr 442Zr 443Zr 444Zr 445Zr 446Zr 447Zr 448Zr 449Zr 450Zr 451Zr 452Zr 453Zr 454Zr 455Zr 456Zr 457Zr 458Zr 459Zr 460Zr 461Zr 462Zr 463Zr 464Zr 465Zr 466Zr 467Zr 468Zr 469Zr 470Zr 471Zr 472Zr 473Zr 474Zr 475Zr 476Zr 477Zr

819 513 513 514 189 519 519 805 805 805 805 521 521 522 522 522 522 524 524 524 523 523 523 523 168 168 191 191 198 198 200 526 526 491 528 528 528 528 527 527 530 531 533 533 533 532 534 535 536 536 537 538 538 538 539 539 539 539 540 540

351

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

ZrCp2 complex [Cp 2Zr(k-N]C(2-C5NH4)-C(2-C5NH4)]k-N)] [Cp 2Zr[2-NC5H4dC(k-O)Me]] [Cp 2Zr[2-NC5H4]C(k-NnBu)Me]] [rac-(EBTHI)Zr(k-CH2C^CCH2B(C6F5)3]] [Cp2Zr(m:k-CH2-Z2-C^C-k-CH2)Ni(PPh3)2] [Cp2Zr(m:k-CH2-Z2-C^C-k-CH2)Ni(PCy3)2] t t [CptBu 2 Zr( BuNC(C4H4)CN Bu]] [(Me2Si-Cp2)Zr(tBuNC(C4H4)CNtBu]] [(Me2Si-Cp2)Zr(m:k-CH2-Z2-C^C-k-CH2)Ni(PPh3)2] [(Me2Si-Cp2)Zr(k-CH2C^CCH2B(C6F5)3]] [(Me2Si-Cp2)(m:k-CH2-Z2-C^C-k-CH2)Ni(PPh3)2] [Cp2Zr[k-PhCC(C6F5)C(C6F6)-k-CPh]] [Cp2Zr[k-C(C6F5)C(C3H6)C-k-C(C6F6)]] [CpCp Zr[k-C(SiMe3)CPhCPh-k-C(SiMe3)]] [(Me2C-Cp2)Zr[k-C(SiMe3)CPhCPh-k-C(SiMe3)]] [Cp2Zr(k-nPrC]CMes-CMes]k-CnPr)] [Cp2Zr(k-nPrC]CMes-CnPr]k-CMes)] [Cp2Zr(k-PhC]CMes-CMes]k-CPh)] [Cp2Zr[k-MeC]CMe-CMe]CMedN(Ph)-k-O]] [Cp2Zr[k-MeC]CC(C3H6)]C(C6H4-2-NH)CMe-k-O]] [[Cp2Zr(m-Z2-(Ph)C^C-k-PiPr2)]2] [Cp2Zr(pyr)(Z2-(Mes)C^C-PPh2)]2] [Cp2Zr[k-(Ph2P)C]CPh-CPh]k-C(PPh2)]] [Cp2Zr[k-(iPr2P)C]CPh-CEt]k-C(Et)]] [Cp2Zr[k-(Ph2P)C]CPh-CEt]k-C(Et)]] [{Cp2Zr}2{m-[k-(Me3Si)C]CdC]k-C(SiMe3)][(p-C6H4)N]C(Ph)C(Ph)]N(p-C6H4)]2[k-(Me3Si) C]CdC]k-C(SiMe3)]}] [{Cp2Zr}{m-[k-(Me3Si)C]CdC]k-C(SiMe3)]}3{(p-C6H4)-(p-2-N-C6H4N)-(p-2-N-C6H4N)-(pC6H4)}3] [{Cp2Zr}2{m-[k-(BuMe2Si)C]CdC]k-C(SiMe2Bu)][(p-C6H4)(p-C6H4)(p-C6F4)(p-C6F4)(p-C6F4) (p-C6F4)(p-C6H4)(p-C6H4)]2[k-(BuMe2Si)C]CdC]k-C(SiMe2Bu)]}] [{Cp2Zr}2{m-[k-(Me3Si)C]CdC]k-C(SiMe3)]-[3,3-bipy-ZrCp2]2[k-(Me3Si)C] CdC]k-C(SiMe3)]}2] [{Cp2Zr}2{m-[k-(Me3Si)C]CdC]k-C(SiMe3)]-[(p-C6H4)-(3,3-bipy-ZrCp2)-(p-C6H4)]2[k-(Me3Si) C]CdC]k-C(SiMe3)]}2] [SSS-{Cp2Zr}3{m-[k-(Me3Si)C]CdC]k-C(SiMe3)]3-[(7-BINOL-CH2)3]] [Cp2Zr(PinB-k-C]C(C4H8)C]k-CBpin)] [Cp2Zr[(p-iPrC6H4)-k-C]C(p-iPrC6H4)C(p-iPrC6H4)]k-C(p-iPrC6H4)]] [Cp2Zr[(PhCCSiMe2)-k-C]CPh-CPh)]k-C(SiMe2CCPh)]] [Cp2Zr(SiN3C34H37)] [Cp2Zr(SiN3C30H37)] [(Cp2Zr)2(m-O2SiMe2)2] [Cp2Zr(NCtBu)[(Tol)-Z2dC]CdC3(2-p-tol)(5-tBu)SiMe2N]] [Cp2Zr{iPr-k-N-C(]NiPr)-C[C3(2-p-tol)(5-tBu)SiMe2N]k-C(SiMe2CCPh)]]k-C(Tol)}] [Cp2Zr{Et-k-C]C(Et)-C[C3(2-p-tol)(5-tBu)SiMe2N]k-C(SiMe2CCPh)]]k-C(Tol)}] [Cp2ZrCl[O(C4(1,2-Ph2)(3-CCPh)(4-tBu)SiMe2N]] [Cp2Zr(SiN2C32H34)] [Cp2Zr(SiN4C40H52)] [Cp2Zr(SiN3C36H57)] [Cp2Zr(SiSN3C34H49)] [Cp2Zr[H-k-N]C(CHPh2)dC(Me)]k-C(SiMe3)][N]C]CPh2]] [Cp2Zr(SiN2C30H31)(N]C]CPh2)] [Cp2Zr(SiN2C30H31)Br] [Cp2Zr(SiN2C30H31)Cl] [Cp2Zr[H-k-N]C(CHPh2)dC(Ph)]k-C(SiMe2CCPh)][N]C]CPh2]] [Cp1,2,3,4-Et4 ZrCl2] 2 [Cp1,2,3,4-nPr4 ZrCl2] 2 [Cp1,2,3,4-nPr4 ZrMe2] 2 [Cp2Zr[k-N]C(C5NH4)CHPhHCMe]CH-k-N(p-ClC6H4)]] [Cp2Zr[k-CPh]CPh-k-NC11H4(CO2Me)dC(O)-k-O]]

Zr oxidation state

31

P chemical shift (ppm)

Number of complex

Reference

+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4

n/a n/a n/a n/a 38.3 40.3 n/a n/a 38.3 n/a 38.3 n/a n/a n/a n/a n/a n/a n/a n/a n/a 37.0 10.6 −16.8 −45.1 −18.9 n/a

478Zr 479Zr 480Zr 481Zr 482Zr 483Zr 484Zr 485Zr 486Zr 487Zr 488Zr 489Zr 490Zr 491Zr 492Zr 493Zr 494Zr 495Zr 496Zr 497Zr 498Zr 499Zr 500Zr 501Zr 502Zr 503Zr

202 243 243 541 543 543 531 531 542 542 542 545 545 546 546 547 547 547 548 548 549 549 549 549 549 550

+4

n/a

504Zr

550

+4

n/a

505Zr

553

+4/+2

n/a

506Zr

554

+4/+2

n/a

507Zr

554

+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4

n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a

508Zr 509Zr 510Zr 511Zr 512Zr 513Zr 514Zr 515Zr 516Zr 517Zr 518Zr 519Zr 520Zr 521Zr 522Zr 523Zr 524Zr 525Zr 526Zr 527Zr 528Zr 529Zr 530Zr 531Zr 532Zr

555 557 559 563 564 565 565 566 566 566 566 566 567 568 568 569 570 570 570 570 571 571 571 572 573 (Continued)

352

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

ZrCp2 complex

Zr oxidation state

[Cp2Zr(Si2C52H60N6)] [Cp2Zr(C20H26N2O)] [Cp2Zr[(C6F5)-k-C]C(C6F5)C(C6F5)]k-C(C6F5)]] [Cp2Zr[(C6F5)-k-C]C(C6F5)C(C6F4)]k-C]] [Cp2ZrMe(m-Me)[MeB(C12F8)] [Cp2Zr[k2-(Me3Si)CH-C^C-CH(SiMe3)]] [Cp2Zr[k2-(Me3Si)CH-C^C-CH(SiMe3)C(Me)]C(SiMe3)]] [Cp2Zr[k2-(Me3Si)CH-C^C-CH(SiMe3)C(Ph)]C(Ph)]] [Cp2Zr{k2-(Me3Si)CHdC]C[C(CH2)C(SiMe3)]-CH(SiMe3)}] 2 [CptBu 2 Zr[k -H2C-C^C-CH2]] [Cp2Zr[k2-H2C-C^C-CH2]] [Cp2Zr[k2-H2C(m:Z2-C^C)-CH2]Zr(PMe3)Cp2] [[Cp2Zr]2[m:k2:k2-(Me3Si)HC]CdC]CH(SiMe3)]] [[Cp2Zr]2[m:k2:k2-H2C]CdC]CH2]] [Cp2Zr[(p-EtC6H4)2C](k2-CC^CC)]C(p-EtC6H4)2]] [Cp2Zr[(p-EtC6H4)2C](Z4-CCH]CHC)]C(p-EtC6H4)2]] [Cp2Zr[(p-EtC6H4)2C](Z4-C]C(Me)-C^C)-CMe(p-EtC6H4)2]] [Cp2Zr(PMe3)[tBu2C]C](Z2-C]C)]C]CtBu2]] [Cp2Zr(NCtBu)[tBu2C]C](Z2-C]C)]C]CtBu2]] [Cp2Zr[Z-PhtBuC](k2-CC^CC)]CPhtBu]] [Cp2Zr[k2:Z3-(Me3Si)C]C]CHdCH2C(Ph)]C(Ph)]] [Cp2Zr(k2-C(SitBuMe2)]C]CHCH(SiMe3)]] [Cp2Zr(k2-C(SiMe3)]C]C(NEt2)S]] [Cp2ZrCl[Z1-C(2-C5NH4)]C]C(C5H10)]] [[Cp2Zr(m:Z2:Z3-tBu-C]C-C(Me)]CH)][Mg(m-H)(nBu)(C4H8O)]] [[Cp2Zr(m:Z2:Z3-tBu-C]C-C(Me)]CH)]2[Mg(m-H)2]] [Cp2Zr[k-CH2C(Me)CH-k-CH(tBu)]] [[Cp2Zr(m:1,3-k2-2,4-k2-PhC]C]C]CPh)]2Si] [Cp Cp(1-CH2CH(SiMe3)-k-C]C]O)-2,3,4,5-Me4ZrOSiMe3] [[Cp Cp(1-CH2CHtBuC(]C]CtBuH)-m-CO2)(2-CH2CO-k-O)-3,4,5-Me3Zr]2] [Cp2Zr(pyr)[Me3SiC^C(NC4OH8)]] [Cp2Zr(tBu-k-C]C(C^CtBu)dC(tBu)]C]C]k-CtBu)] [[Cp2ZrnBu(tBu-k-C]C]C]m:k-CtBu)]2] [[Cp2Zr(m:Z1:Z2-C^CtBu]2] [Cp2ZrCl(Me3Si-k-C]C]m:k-CSiMe3)ZrClCp2] [Cp2Zr(Me3Si-k-C]C]m:k-CSiMe3)2ZrCp2] [Cp2Zr(Z1-C^C(C3H5)]2] [Cp2ZrCl[Z2-C(H)]N(tBu)]] [Cp2Zr(k2-C(SiMe3)]C]CHN(tBu)]] [Cp2Zr[Z1-C(H)]N(2,6,-Me2C6H3)][Z1-C^CSiMe3]] [Cp2Zr[C(tBu)]C]C(Me)CH2B(C6F5)3]] [Cp2Zr[C(tBu)]C]CHCH(Ph)B(C6F5)3]] [Cp2Zr[C(tBu)]C]CH-k-CH(Ph)]] [Cp2Zr(m:Z1:Z2-CH]CHPh)(m:Z1:Z2-C^CtBu)ZrCp2] [{Cp2Zr[N]C(Me)C(tBu)]CdCH]CH(Ph)]}2] [Cp2Zr[Z3-C3H4CH2CH(Ph)-k-NMe]] [Cp2Zr[Z3-C3H4CH2CH(Ph)-k-NCH2Ph]] [Cp2Zr[Z3-C3H4CH2C(C4H8)-k-NCH2Ph]] [Cp2Zr[k-C(CH]CH2)CH2CMe]k-NCH2CH2-k-O]] [CpCp[C(Me2)-k-N(Me)CH2CH2(Z3-(p-tBu)PhCH)]Zr] [[SiMe2-Cp2]Zr[(Z3-C3H5CH2B(C6F5)3] [[SiMe2-Cp(3-Me) ]Zr[(Z3-C3H5CH2B(C6F5)3] 2 [[SiMe2-Cp-Ind]Zr[(Z3-C3H5CH2B(C6F5)3] [[SiMe2-Cp-Flu]Zr[(Z3-C3H5CH2B(C6F5)3] 2 [CpMe 2 Zr{Z -(Me)C^C-C[B(C6F5)3]-k-CMe}] [Cp2ZrCl[Z1-(Me)C]C(PPh2)C^CMe]] [Cp2ZrCl[Z1-(Ph)C]C(PPh2)C^CPh]] [Cp2ZrCl[Z1-(Ph)C]C(PPhCl)C^CPh]] [Cp2Zr[Z1-C^CMe][Z1-(Ph)C]C(PPh2)C^CPh]] [Cp2ZrCl{Z1-(Ph)C]C[P(C6F5)2]C^CPh}]

+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +3 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4

31

P chemical shift (ppm)

Number of complex

n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a Not stated n/a n/a n/a n/a n/a Not stated n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a −12.3 −7.8 70.4 −8.3 −48.5

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

Reference 574 575 577 578 579 525 581 582 582 583 584 583 585 584 586 586 586 587,588 587,588

589 590 593 592 595 597 597 596 602 603 603 604 605 605 605 606 606 598 591 591 591 607 607 608 608 608 609 609 609 609 610 611 611 611 611 612 613 613 613 614 614

353

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

ZrCp2 complex

Zr oxidation state

[Cp2ZrCl{Z1-C(]NtBu)C(Ph)]C[P(C6F5)2]C^CPh}] [Cp2ZrCl[Z1-(C]NtBu)C(Ph)]C(PPh2)C^CPh]] [Cp2Zr[Z3-C(SiMe3)]C]C]C(SiMe3)(k-2-C6H4)]] [Cp2Zr[Z1-C(SiMe3)]C(C^CSiMe3)(k-2-C6H4)]] [{Cp2Zr[Z1-C(C^CPh)]C(Ph)-CH(p-C6H4OMe)(k-m-S)]}2] [Cp2Zr(Z1-CH]CHPh)(Z1-C^CSiMe3)] [Cp2Zr(Z4-CHPh-CHCHCCSiMe3)] [Cp2Zr(Z1:Z3-CH(SiMe3)-CHCCSiMe3)] [{Cp2Zr[Z1-C(C]CHtBu)]C(SiMe3)-CO-k-m-O]}2] [{Cp2Zr[Z1-CtBu]C(CH]CHPh)-CO-k-m-O]}2] [Cp2Zr[Z2-tBuC]C]CMeCH2C(O)-k-O]] [Cp2Zr[Z2-tBuC^CCMeCHCMe](k-NH)]] [Cp2ZrCl[k-O-C(OMe)-CPh]k-CPh]] [(Cp2ZrBr)2(m:Z2:Z2-C2H4)] 4 [CpMe 2 Zr[Z -(1,3-(SiMe3)2-4-B(C6F5)2-C4H2)]] tBu [Cp2 Zr[Z4-(1,3-(SiMe3)2-4-B(C6F5)2-C4H2)]] 1 2 [CpMe 2 Zr(Z -C^CSiMe3)[Z -(Me3Si)C^C-BH(C6F5)2]] 3 [Cp2Zr(PMe3)[Z -C3H4CH2B(C6F5)3]] 2 [CpMe 2 Zr[Z -(Me3Si)C^CC(SiMe3)(B(C6F5)2CHCMe]k-N]] 2 [Cp2Zr[Z -(SiMe3)C^C-B(C6F5)]] [Cp2ZrH[Z2-(SiMe3)C^C-B(H)(C6F5)]] [Cp2Zr(CO)[Z2-(SiMe3)C^C-B(C6F5)]] [Cp2Zr(CNtBu)[Z2-(SiMe3)C^C-B(C6F5)]] [Cp2Zr(NCtBu)[Z2-(SiMe3)C^C-B(C6F5)]] [Cp2Zr(C4H8O)[Z2-(SiMe3)C^C-B(C6F5)]] [Cp2Zr[Z2-(SiMe3)C^C-B(C6F5)2-Z2-C(H)^CtBu]] [Cp2Zr[Z2-(SiMe3)C^C-B(C6F5)2-Z2-C(H)^CPh]] [Cp2Zr[Z2-(SiMe3)C^C-B(C6F5)2-Z2-C(H)^CSiMe3]] [Cp2Zr[Z2-(SiMe3)C^C-B(C6F5)2-Z2-C(H)^CiPr]] [Cp2Zr[Z2-(SiMe3)C^C-B(C6F5)2-CH2-Z2-C(Me)]C]CSiMe3]] [Cp2Zr[Z2-(SiMe3)C^C-B(C6F5)2-Z3-C(SiMe3)]C]C]CSiMe3]] [Cp2Zr[Z3-(SiMe3)C]C]C(PMes2)-CHB(C6F5)2]] [Cp2Zr[Z3-(SiMe3)C]C]C(Ph)-C(Ph)]B(C6F5)2]] [Cp2Zr[k-C6H4-2-C(PPh2)]k-C(SiMe3)]] [Cp2Zr[k-C6H4-2-C(SiMe3)]k-C(PPh2)]] [Cp2Zr[k-C6H4-2-C(SiMe2CPh])-k-C]] [Cp2Zr{k-C6H4-2-CH]k-C[P(Ph2)-CH]k-C(PPh2)]}] [Cp2Zr[k-C6H4-2-CH]k-C(P(Ph2)-k-N{NN[3-(NO2)-4-FC6H3]})]] [Cp2Zr(k-C6H4-2-CH]k-C{P(Ph2)-k-N[3-(NO2)-4-FC6H3]})] [Cp2Zr{k-C6H4-2-CH]k-C[P(Ph2)-k-N(N]CHCO2Et)]}] [Cp2Zr[OC2N3-2-(C10H7)-5-(CO2Et)]{k-C6H4-2-C[P(NiPr2)2]]k-NH}] [Cp2Zr{k-C6H4-2-C[P(NiPr2)2]]k-N[CHFc-k-O]}] [Cp2Zr(CCCO2Me){k-C6H4-2-C[P(NiPr2)2]]k-NH}] [Cp2Zr(OCMePh){k-C6H4-2-C[P(NiPr2)2]]k-NH}] [Cp2ZrCl[Z2-(tBu)NdP(C6H4CHCH2CH2PPh)]] [Cp2Zr(m-Cl)(m-C2B10H10)Li(Et2O)2] [Cp2Zr[k-(1,2-C2B10H10)C(]NCy)-k-NCy]] [Cp2Zr(NCPh)[k-(1,2-C2B10H10)C(Ph)]k-N]] [Cp2Zr[k-(1,2-C2B10H10)N](Z2-N-NPh)]] [Cp2Zr(CNtBu)[Z2-(tBu)N]C-1,2-(C2B10H10]CNtBu)]] [Cp2Zr[k-(1,2-C2B10H10)C(Ph)]k-CPh]] [Cp2Zr[k-(1,2-C2B10H10)C(nBu)]k-CnBu]] [Cp2Zr[k-(1,2-C2B10H10)C(Me)]k-CPh]] [Cp2Zr[k-(1,2-C2B10H10)C(nBu)]k-CPh]] [Cp2Zr[k-(1,2-C2B10H10)C(CCPh)]k-CPh]] [Cp2Zr[k-(1,2-C2B10H10)C(nBu)]k-C(SiMe3)]] [Cp2Zr[k-(1,2-C2B10H10)C(Ph)]k-C(SiMe3)]] [Cp2Zr[k-(1,2-C2B10H10)C(Et)]k-C(C(Me)]CH2)]] [Cp2Zr[k-(1,2-C2B10H10)C(nBu)]k-C(PPh2)]] [Cp2Zr[k-(1,2-C2B10H10)C(CH2CH2CH2Cl)]k-C(Ph)]]

+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +2/+4 +4 +2/+4 +2/+4 +2/+4 +2/+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4

31

P chemical shift (ppm)

Number of complex

Reference

−41.9 −7.5 n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a Not stated n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a −25.1 n/a −8.5 −67.5 n/a 34.5, 8.7 37.7 24.6 37.7 Not stated 68.5 60.8 54.1 3.7/1.1 n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a −66.7 n/a

594Zr 595Zr 596Zr 597Zr 598Zr 599Zr 600Zr 601Zr 602Zr 603Zr 604Zr 605Zr 606Zr 607Zr 608Zr 609Zr 610Zr 611Zr 612Zr 613Zr 614Zr 615Zr 616Zr 617Zr 618Zr 619Zr 620Zr 621Zr 622Zr 623Zr 624Zr 625Zr 626Zr 627Zr 628Zr 629Zr 630Zr 631Zr 632Zr 633Zr 634Zr 635Zr 636Zr 637Zr 638Zr 639Zr 640Zr 641Zr 642Zr 643Zr 644Zr 645Zr 646Zr 647Zr 648Zr 649Zr 650Zr 651Zr 652Zr 653Zr

614 614 599 599 599 600 600 600 600 600 600 601 616 617 618 618 618 619 620 621 621 622 622 622 622 622 622 622 622 622 622 622 622 624 624 625 626 627 627 628 632 630 631 631 633 634 634 634 634 634 635 635 635 635 635 635 635 635 635 635 (Continued)

354

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

ZrCp2 complex

Zr oxidation state

[Cp2Zr[k-(1,2-C2B10H10)C(CH2NMe2)]k-C(Ph)]] [Cp2Zr[k-(1,2-C2B10H10)C(CH2OMe)]k-C(Ph)]] [Cp2Zr[k-(1,2-C2B10H10)C(CH2CH2CH2OC5OH8)]k-C(Ph)]] [Cp2Zr[k-(1,2-C2B10H10)C(Et)]k-CEt]] [Cp2Zr(NCPh)[k-(1,2-C2B10H10)C(Et)]k-CEt]] [Cp2Zr(CNXy)[k-(1,2-C2B10H10)C(Et)]k-CEt]] [Cp2Zr[k-(1,2-C2B10H10)CH2-k-CHPh]] [Cp2Zr[k-(1,2-C2B10H10)CH2-k-CH(2-ClC6H4)]] [Cp2Zr[k-(1,2-C2B10H10)CH2-k-CH(4-FC6H4)]] [Cp2Zr[k-(1,2-C2B10H10)CH2-k-CH(SiMe3)]] [Cp2Zr[k-(1,2-C2B10H10)CH2-k-CH(PPh2)]] [Cp2Zr[k-(1,2-C2B10H10)CH2-k-CH2]] [Cp2Zr[k-(1,2-C2B10H10)CH(nBu)-k-CH2]] [Cp2Zr[k-(1,2-C2B10H10)CH(CH2PPh2)-k-CH2]] [Cp2Zr[k-(1,2-C2B10H10)CH(nBu)CH2-Z2-N2CH2CO2Me]] [Cp2Zr(C2B10H11)(2-Pyr)] [Cp2Zr(C2B10H11)(2-N-3,5-Me2C5NH2)] [Cp2Zr(C2B10H11)(2-C9NH6)] [Cp2Zr[k-(1,2-C2B10H10)-k-NC13H8]] [Cp2Zr(C2B10H11)(2-N-3-(CCnBu)C5NH3)] [Cp2Zr(C2B10H11)(2-N-4-(CCnBu)C5NH3)] [Cp2Zr[k-(1,2-C2B10H10)C(Ph)]k-C(Pyr)]] [{[Me2C-Cp-Z5-C2B9H10]ZrCl2}{Na(DME)3}] [{[Me2C-Cp-Z5-C2B9H10]ZrCl2}{Li(DME)3}] [{[Me2C-Cp-Z2-C2B9H10]ZrCp(m-Cl)}{Na(DME)2}] [{[Me2C-Cp-Z5-C2B9H10]Zr(CH2Ph)Cl}{Na(DME)3}] [[Me2C-Cp-Z5-C2B9H10]Zr(NHDipp)(C4H8O)] [Cp CpCMe2-o-C2B10H11ZrCl2] [Cp CpCMe2-o-C2B10H11MeZrCl2] [Cp CpCMe2-k-o-C2B10H10ZrCl] [[Me2Si-Ind-CpCMe2-k-o-C2B10H10]ZrCl] [Cp2Zr(1,9-anthracendiyl)] [Cp2Zr[k-(1,9-anthracenyl)-Z2-C]NtBu]] [Cp2Zr[k-(1,9-anthracenyl)-CPh]k-CPh]] [Cp2Zr(Z2-benzocyclobutadiene)PMe3] [Cp2Zr[k-(1,2-benzocyclobutane)-CPh]-k-CPh]] [Cp2Zr[k-(1,2-benzocyclobutane)-CtBu]-k-N]] [Cp2Zr[k-NtBu-C(benzocyclobutane)]C-k-NtBu]] [Cp2ZrMe[P(Mes)2CH2CH2BMe(C6F5)2]] [Cp 2ZrMe[MeB(C6F5)2CH2CH2P(Mes)2]] [Cp2ZrMe[OCH(Ph)P(Mes)2CH2CH2BMe(C6F5)2]] [Cp2ZrMe[OC(O)P(Mes)2CH2CH2BMe(C6F5)2]] [Cp2ZrMe[Z3-OCN(Tol)P(Mes)2CH2CH2BMe(C6F5)2]] [(CpCH]NDipp)2ZrCl2] [Cp 2Zr(PMe3)[Z2-(Ph2PMe)-C]CPh][B(C6F5)4]] [Cp 2Zr(PMe3)[Z2-(Ph2PMe)-C]CdCMeCH2][B(C6F5)4]] [Cp 2Zr[(Ph2k-P)-Z1-C]CMePh][B(C6F5)4]] [Cp 2Zr(CNtBu)[(Ph2k-P)-Z1-C]CMePh][B(C6F5)4]] [Cp 2Zr(CNnBu)[(Ph2k-P)-Z1-C]CMePh][B(C6F5)4]] [Cp 2Zr(NCtBu)[(Ph2k-P)-Z1-C]CMePh][B(C6F5)4]] [Cp 2Zr{[(k-O-P(Ph2)]-Z1-C]CMePh]}{B(C6F5)4}] [Cp 2Zr{[(k-O-C(NtBu)-P(Ph2)]-Z1-C]CMePh]}{B(C6F5)4}] [Cp 2Zr{[(k-O-C(O)-P(Ph2)]-Z1-C]CMePh]}{B(C6F5)4}] [Cp 2Zr{[(k-N(N2Mes)–P(Ph2)]-Z1-C]CMePh]}{B(C6F5)4}] [[Cp 2ZrH(C4H8O)][B(C6F5)4]] [{Cp 2Zr[Z1-C(Me)]C(SiMe3)(k-PPh2)]}{B(C6F5)4}] [{Cp 2Zr[Z1-C(Me)]C(SiMe3)(k-P(p-MeC6H4)2)]}{B(C6F5)4}] [{Cp 2Zr[Z1-C(Me)]C(SiMe3)(k-P(C6F5)2)]}{B(C6F5)4}] [{Cp 2Zr[Z1-C(Me)]C(SiMe3)(PPh2C(O)-k-O)]}{B(C6F5)4}] [{Cp 2Zr[Z1-C(Me)]C(SiMe3)(PPh2Rh(nbd)-k-Cl)]}{B(C6F5)4}]

+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +2 +2 +2 +2 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4

31

P chemical shift (ppm)

Number of complex

n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a Not stated n/a n/a Not stated n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a −3.97 n/a n/a n/a 5.6 −17.3 31.6 19.76 23.4 n/a 14.3 Not stated 17.6 −76.5 −76.1 −64.8 17.4 33.7 27.6 12.3 n/a −6.8 −7.0 −44.8 −0.8 38.1

654Zr 655Zr 656Zr 657Zr 658Zr 659Zr 660Zr 661Zr 662Zr 663Zr 664Zr 665Zr 666Zr 667Zr 668Zr 669Zr 670Zr 671Zr 672Zr 673Zr 674Zr 675Zr 676Zr 677Zr 678Zr 679Zr 680Zr 681Zr 682Zr 683Zr 684Zr 685Zr 686Zr 687Zr 688Zr 689Zr 690Zr 691Zr 692Zr 693Zr 694Zr 695Zr 696Zr 697Zr 698Zr 699Zr 700Zr 701Zr 702Zr 703Zr 704Zr 705Zr 706Zr 707Zr 708Zr 709Zr 710Zr 711Zr 712Zr 713Zr

Reference 635 635 635 635,636

636 636 637,638

637 637 637 637 637 637,638

637 639 640 640 640 640 640 640 640 641 641 641 641 641 642 642 642 643 644 644 644 645 645 645 645 646 646 646 646 646 647 648 648 648 649 649 649 649 649 649 649 649 650 651 651 650 650

355

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

ZrCp2 complex

Zr oxidation state

[{Cp 2Zr(Z1-CO)[Z2-OCC(O)C(Me)]C(SiMe3)(PPh2)O]}{B(C6F5)4}] [{Cp 2Zr[Z1-C(Me)]C(SiMe3)(P(k-O)Ph2)]}{B(C6F5)4}] [{Cp 2Zr[Z1-C(Me)]C(SiMe3)P(Ph2)CH(CH]CHPh)-k-O]}{B(C6F5)4}] [{Cp 2Zr[Z1-C(Me)]C(SiMe3)P(Ph2)NPh-k-O]}{B(C6F5)4}] [{Cp 2Zr[Z1-C(Me)]C(SiMe3)P(Ph2)NPh-Z2-SO]}{B(C6F5)4}] [{Cp 2Zr[Z1-C(Me)]C(SiMe3)(PPh2Ir(COD)-k-Cl)]}{B(C6F5)4}] [{Cp 2Zr(CNtBu)[Z1-C(Me)]C(SiMe3)(k-PPh2)]}{B(C6F5)4}] [Cp2ZrMe(PCy2)] [Cp2ZrMe(PMes2)] [Cp2ZrMe(PtBu2)] [Cp2ZrMe(NPh2)] [Cp2ZrMe(NtBuMes2)] [[Cp2Zr(]NPh2)][MeB(C6F5)3]] [[Cp2Zr(]NtBuMes)][B(C6F5)4] [[Cp2Zr(m:Z1:Z1-OC(NtBuMes)O]2[MeB(C6F5)3]2] [[Cp2Zr(OCH2Ph)(OC(Ph)NPh2)][B(3,5-(CF3)C6H3)4]] [{Cp2Zr(PCy2-C6H4-2-(NNHPh)]}{B(C6F5)4}] [Cp2Zr(PCy2-C6H4-2-(Z2-NNPh)]] [{Cp2Zr[Z1-C(Ph)]C(Ph)P(Cy2)CH(Ph)CH]CPh-k-O]}{B(C6F5)4}] [{Cp2Zr[Z1-C(Ph)]C(Ph)P(Cy2)CH(Fc)-k-O]}{B(C6F5)4}] [Cp2ZrMe(O-2-C6H4-PPh2)] [{Cp2Zr[k-O-C6H4-(2-k-PtBu2)]}{B(C6F5)4}] [{Cp 2Zr[k-O-C6H4-(2-k-PiPr2)]}{B(C6F5)4}] [{Cp 2ZrH[k-O-C6H4-(2-HPtBu2)]}{B(C6F5)4}] [{Cp 2Zr[k-O-C6H4-(2-PtBu2)][Z3-H3B-NMe3]}{B(C6F5)4}] [{Cp 2Zr[k-O-C6H4-(2-PtBu2)][ClC6H5]]}{B(C6F5)4}] [{Cp2Zr[k-O-C6H4-2-PtBu2(CO-k-O)]}{B(C6F5)4}] [{Cp 2Zr(Z3-O2CH)[k-O-C6H4-2-PHtBu2)]}{B(C6F5)4}] [{Cp 2Zr(CO)[k-O-C6H4-(2-PtBu2)]}{B(C6F5)4}] [{Cp 2Zr[k-O-C6H4-(2-PtBu2)][Z1-C^CPh]]}{B(C6F5)4}] [{Cp 2Zr[k-O-C6H4-2-PtBu2-CH2-k-CH2]}{B(C6F5)4}] [{Cp 2ZrCl[k-O-C6H4-2-PiPrtBu2)]}{B(C6F5)4}] [{Cp 2Zr[k-O-C6H4-2-PtBu2(CH2)4-k-O]}{B(C6F5)4}] [{Cp 2Zr[k-O-C6H4-(2-PEttBu2)][OEt]]}{B(C6F5)4}] [{Cp 2Zr(Z1-OCMe2)[k-O-C6H4-2-PHtBu2)]}{B(C6F5)4}] [Cp2ZrMe[k-O-CMe2CH2PtBu2)]] [Cp2ZrMe[k-O-C(CF3)2CH2PtBu2)]] [Cp2ZrMe[k-O-C6H4-2-PtBu2)]] [Cp 2Zr[k-O-C(CF3)2CH2PtBu2-(k-PtMe)]] [[Cp Zr(OMes)(Z1-O2CPEt3)][B(C6F5)4]] [[Cp Zr(OMes)(Z1-CH]CPh(PPh3))][B(C6F5)4]] [Cp2ZrH(NH2BH3)] [Cp2ZrCl(NH2BH3)] [Cp2Zr(H)(NMe2BH3)] [{Cp2ZrMe[ONNPtBu3]}{B(C6F5)4}] [{Cp2Zr(OMe)[ONNPtBu3]}{B(C6F5)4}] [Cp 2ZrH(OMes)] [Cp 2Zr(OMes)(OCHBH(C6F5)2)] [Cp 2Zr(OMes)[OC(Me)B(Me)(C6F5)2]] [Cp 2Zr(OMes)[OC(Me)B(Me)(C6F5)2]] [Cp 2Zr(OMes)[O(CB(C6F5)2C(Me)(H)O)]] [Cp 2Zr(OMes)(OCHBH(C6F5)2(C5H5N))] [Cp 2Zr(OMes)[O(CB(C6F5)2CH2O)]] [Cp 2Zr(OMes)[O(COB(C6F5)2CH2O)]] [Cp 2Zr(OMes)[O(SN(Ph)B(C6F5)2CH2O)]] [Cp 2Zr(OMes)(OH)] [Cp 2Zr(OMes)[O(H)BH(C6F5)2]] [[Cp2Zr(m:Z1:Z1-CN2(Me)C2H2)]2] [Cp2Zr(SCH2Ph)] [(Cp(P(Ph2)AuCl))2ZrCl2]

+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4

31

P chemical shift (ppm)

Number of complex

Reference

87.4 49.1 15.7 35.3 28.3 31.0 −60.0 233.1 136.9 83.0 n/a n/a n/a n/a n/a n/a 6.00 Not stated n/a n/a −16.8 Not stated 23.9 Not stated Not stated 7.4 37.0 14.8 5.6 26.7 45.1 51.2 42.7 44.3 20.1 Not stated Not stated Not stated 77 27.9 17.4 n/a n/a n/a 67.06 64.33 n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a 21.8

714Zr 715Zr 716Zr 717Zr 718Zr 719Zr 720Zr 721Zr 722Zr 723Zr 724Zr 725Zr 726Zr 727Zr 728Zr 729Zr 730Zr 731Zr 732Zr 733Zr 734Zr 735Zr 736Zr 737Zr 738Zr 739Zr 740Zr 741Zr 742Zr 743Zr 744Zr 745Zr 746Zr 747Zr 748Zr 749Zr 750Zr 751Zr 752Zr 753Zr 754Zr 755Zr 756Zr 757Zr 758Zr 759Zr 760Zr 761Zr 762Zr 763Zr 764Zr 765Zr 766Zr 767Zr 768Zr 769Zr 770Zr 771Zr 772Zr 773Zr

650 651 651 651 651 651 651 652 652 652 652 652 652 652 652 652 653 653 652 652 781 654 655 654 654 655 655 655 655 655 655 655 655 655 655 656 656 656 657 658 658 661 661 267 662 662 663 663 664 664 664 663 663 663 663 663 663 687 676 668 (Continued)

356

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

ZrCp2 complex

Zr oxidation state

[[(SiMe2)-Cp-Cp(3-CH2CH]CH2)]ZrCl2] [Cp2ZrMe(OC(H)(Ph)PMes2)] [Cp2ZrMe(OC(Ph)2PtBu2)] [Cp2ZrMe(OCH(Ph)PtBu2)] [Cp2ZrMe(OC(Ph)2PCy2)] [Cp2Zr(k2-O2CPCy2)] [Cp2ZrMe(OCH2PCy2)] {[Cp2Zr(m-O-(CH2PCy2]2}{MeB(C6F5)3}2] [Cp2Zr(OdCH(Fc)-PCy2-Pd(C3H5)-m-Cl)] [Cp3Zr(Z1-C4H4N(BMe(C6F5)2]}] [Cp2Zr(C4H4N(B(C6F5)2CH2(Z3-C3H4]}] [Cp2(CpCH2CH]CH2)ZrMe] [Cp2(CpCH2CH2-k-CH2)Zr] [Cp2(CpCH2CH2CH2B(C6F5)3)Zr] [Cp2Zr(1,2,3,4-O-tetramethyl-a-D-glucopyranoside)] [Cp2Zr(3-O-benzyl-1,2-O-isopropylidene-glucofuranoside)] [{Rh(CO)(PPh3)[(PPh2-Cp)2Zr]}2{m-O}] [(Z5:Z5-Pn(1-NMe2–3-Me-3-H) )ZrCl2] 2 [(Z5-Pn(1-NMe2-3-Me2))2ZrCl2] [(Z5-Pn(1-NMe2-3-Me-3-H))CpZrCl2] [(Z5-Pn(1-NMe2-3-H2))CpZrCl2] [rac-(Cp(1-tBu-3-CH2CH]CH2))2ZrCl2] [Meso-(Cp(1-tBu-3-CH2CH]CH2))2ZrCl2] [[CH2-CH]CH-CH2-(Cp(3-tBu))2]ZrCl2] [(MeCH2CH(ZrCp2Cl2)-Cp-Cp(NCHPh))Fe] [[Cp2Zr(O-2-C6H4-CH](k-)NiPr)][BPh4}] [{[Cp2Zr(m-N]CHMe)]2}{nBuB(C6F5)3}2] [{[Cp2Zr(m-N]CH(p-tolyl))]2}{nBuB(C6F5)3}2] [Cp(Ind2- (C5H4N))ZrCl2] [(Ind2-(C4H2MeO))2ZrCl2] [(Ind2-(C4H3O))2ZrCl2] [(THI2-(C4H2MeO))2ZrCl2] [[SiMe2-(THI(C4H2MeO))2]ZrCl2] [(Cp[(C(]CH2)N]C(Ph)-k-O])2Zr] [(Cp[(C(CH2B(C6F5)2)N]C(Ph)-k-O])2Zr] [[ZrCp2Cl(m-Cl)ZrCp3][MeB(C6F5)3]] [[ZrCp3(m-Cl)ZrCp3][MeB(C6F5)3]] [[CpCp(CMe2-k-NMe2)ZrCl][B(C6F5)4]] [(Ind(5,8-NMe2))2ZrCl2] [(Ind(5,8-NMe2))CpZrCl2] [[(ZrCp2Me)2(m-CH2)]] [[(ZrCp2)2(m-Cl)(m-CH2)][MeB(C6F5)3]] [CpCp(CH2TMP)ZrCl2] [(Cp(CH2NPh2))2ZrCl2] [CpCp(C(Me2)CH2CH2CONEt2)ZrCl2] [(Ind2-(C4H2MeO))2ZrMe2] [(Ind2-(C4H2MeO))2ZrCl2] [(Ind2-(C4H2EtO))2ZrCl2] [(Ind2-(C4H2(CH2tBu)O))2ZrMe2] [(Cp1-Ph-3-(NC4H8))2ZrCl2] [(Ind2-(NC4H8))2ZrMe2] [CpCp(C(]CH2)NMe2)ZrCl2] [CpCp(C(]CH2)NEt2)ZrCl2] [(Cp(C(]CH2)NEt2))2ZrCl2] [PdCl2-[m-Cp(PPh2)2]2ZrCl2] [Rh(CO)Cl-[m-Cp(PPh2)2]2ZrCl(m-Cl) [Cp(PPh2) ZrMe2] 2 [Rh(CO)(PPh3)-[m-Cp(PPh2)]2ZrMe] [Cp(PPh2) ZrCl2] 2 [Cp(PiPr2) ZrCl2] 2

+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4

31

P chemical shift (ppm)

Number of complex

Reference

n/a −4.1 79.7 56.1 29.7 6.7 4.7 Not stated Not stated n/a n/a n/a n/a n/a n/a n/a Not Stated n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a 29.0 8.1 −18.9 45.5 −17.3 5.1

774Zr 775Zr 776Zr 777Zr 778Zr 779Zr 780Zr 781Zr 782Zr 783Zr 784Zr 785Zr 786Zr 787Zr 788Zr 789Zr 790Zr 791Zr 792Zr 793Zr 794Zr 795Zr 796Zr 797Zr 798Zr 799Zr 800Zr 801Zr 802Zr 803Zr 804Zr 805Zr 806Zr 807Zr 808Zr 809Zr 810Zr 811Zr 812Zr 813Zr 814Zr 815Zr 816Zr 817Zr 818Zr 819Zr 820Zr 821Zr 822Zr 823Zr 824Zr 825Zr 826Zr 827Zr 828Zr 829Zr 830Zr 831Zr 832Zr 833Zr

698 669 669 669 669 669 669 669 669 700 700 701 701 701 702 702 703 704 704 704 704 666 666 666 667 670 671 671 673 673 675 675 675 677 677 682 682 678 679 679 680 680 681 681 683 684 684 686 686 685 685 688 688 688 689 689 689 690 690 690

357

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

ZrCp2 complex

Zr oxidation state

ZrCl2] [Cp(PCy2)2 2 [Cp(PtBu2) ZrCl2] 2 [Rh(CO)2-[m-Cp(PCy2)2]2ZrMe] [RhCl(m-COMe)-[m-Cp(PiPr2)]2ZrMe] [[(ZrCp2)3(m2-OH)3(m3-O)][BPh4]] [(Cp(B(C6F5)3))Cp2Zr] [Cp2ZrMe[Z2-ON(Me)Ph]] [Cp(CHNDipp) Zr(NMe2)2] 2 [Cp(CHNDipp) Zr(NMe2)(NDipp)] 2 [Cp(CHNDipp) Zr(CH2Ph)2] 2 [Cp(CHNDipp) ZrCl2] 2 [Cp2ZrCl[OC(NPh)P(Ph2)CH2B(C6F5)3]] [Cp2ZrCl[CH2P(Ph2)CHPhOB(C6F5)3]] [Cp2ZrCl[CH2P(Ph2)BH(C6F5)2]] [Cp2ZrCl[P(Ph2)CH2B (C6F5)3]] [Cp2ZrMe(OCH2CH2PPh2)] [[Cp2Zr(m-OCH2CH2PPh2)]2[B(C6F5)4]2] [Cp2ZrMe(OCMe2CH2PPh2)] [[Cp2Zr(OCMe2CH2k-PPh2)][B(C6F5)4]] [[Cp2Zr(OCH2CH2PPh2CHPhCH]CPh-k-O)][B(C6F5)4]] [[Cp2Zr(OCMe2CH2PPh2CHPh-k-O)][B(C6F5)4]] [[Cp2Zr(OCMe2CH2PPh2-k-O)][B(C6F5)4]] [Ind(1-iPr-3-B(C6F5)2)CpZrCl2] [Ind(1-iPr-3-B(C6F5)2)CpZrCl(OH)] [Cp2ZrMe(OTMP)] [[Cp2Zr(OTMP)(OC(O)PtBu3)][MeB(C6F5)3]] [[Cp2Zr(OTMP)][B(C6F5)4]] [[Cp2Zr(OTMP)(Z1-iPrN]C]NiPr][B(C6F5)4]] [Cp2Zr(OTMP)HMeB(C6F5)2] [[Cp2Zr(OTMP)][H2B(C6F5)2]] [Cp2Zr(OTMP)(OC(H)OB(Me)(C6F5)2] [Cp2Zr(OTMP)(OC(H)(Me)B(C6F5)2] [Cp2ZrMe(C6H4-2-PPh2)] [Cp2Zr(OC4H8)(C6H4-2-PPh2)] [Cp2Zr(C6H4-2-PPh2-CMe2-k-O)] [Cp2Zr(C6H4-2-PPh2-CMe(C^CPh)-k-O)] [Cp2Zr(C6H4-2-PPh2-CMe(CH]CHPh)-k-O)] [Cp2Zr(C6H4-2-PPh2-C(p-C6H4Cl)CH]CPh-k-O)] [Cp2Zr(C6H4-2-PPh2-C(NPh)-k-O)] [{Cp2Zr[Z3-CyNC(2-C6H4(PPh2)NCy}{[B(C6F5)4}] [{Cp2Zr[Z1-PhC]CHC(O)-2-C6H4PPh2]}{[B(C6F5)4}] [Cp2Zr(k2-PhNC(H)PhNPh)] [CpCp Zr(NtBu)(OC4H8)] [CpCp Zr[N(p-C6H4Me)][OC4H8]] [CpCp Zr(NHtBu){N[Ph][(Ph)C]CH2]}] [Cp2ZrN(Dipp)(OC4H8)] [Cp2Zr(NH-(2,4-[tBu]2-[6-C(Me2-k-CH2)]C6H2)] [Cp2Zr[(k-NtBu)2C]NiPr]] [Cp2Zr[(k-NtBu)(k-NDipp)C]NiPr]] [Cp2Zr[N(SiMe3)2][N]C]NSiMe3)] [Cp2Zr[(k-NtBu)(k-NSiMe3)C]NSiMe3]] [Cp2Zr[(k-NDipp)C(Ph)]C(Ph)C(]NCy)(k-NCy)]] [Cp2Zr[NtBu][k-O(C7H8)]] [Cp2Zr{1-k-[N(tBu)][2-k-O]C7H8}] [Cp2Zr{1-k-[N(tBu)][2-k-C7H8(O)(C4H8)}] [Cp2Zr[(k-NtBu)CH(Ph)CH2(k-NSO2(p-MeC6H4)]] [[C2H4-(THI)2]Zr{[N(2,6-Me2C6H3)]CH(Ph)CH(Ph)-k-O}] [[C2H4-(THI)2]Zr{[N(2,6-Me2C6H3)]CH(Ph)CH(Ph)-k-O}] [[C2H4-(THI)2]Zr(NHtBu)(C5H11)] [[C2H4-(THI)2]Zr(NHtBu)(CH2SiMe3)]

+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4

31

P chemical shift (ppm)

Number of complex

Reference

−3.0 34.4 46.1 19.7 n/a n/a n/a n/a n/a n/a n/a 23.9 37.3 18.4 32.8 Not stated Not stated Not stated Not stated Not stated Not stated Not stated n/a n/a n/a 47.9 n/a n/a n/a n/a n/a n/a −34.5 −40.9 31.0 29.6 24.2 Not stated Not stated Not stated Not stated n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a

834Zr 835Zr 836Zr 837Zr 838Zr 839Zr 840Zr 841Zr 842Zr 843Zr 844Zr 845Zr 846Zr 847Zr 848Zr 849Zr 850Zr 851Zr 852Zr 853Zr 854Zr 855Zr 856Zr 857Zr 858Zr 859Zr 860Zr 861Zr 862Zr 863Zr 864Zr 865Zr 866Zr 867Zr 868Zr 869Zr 870Zr 871Zr 872Zr 873Zr 874Zr 875Zr 876Zr 877Zr 878Zr 879Zr 880Zr 881Zr 882Zr 883Zr 884Zr 885Zr 886Zr 887Zr 888Zr 889Zr 890Zr 891Zr 892Zr 893Zr

690 690 690 690 691 692 693 694 694 694 694 699 699 699 699 695 695 695 695 695 695 695 478 478 696 696 696 696 696 696 696 696 697 697 697 697 697 697 697 697 697 705 705 705 705 706 706 706 706 706 707 707 708 708 709 709 708 709 710 710 (Continued)

358

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

ZrCp2 complex

Zr oxidation state

[[C2H4-(THI)2]Zr(NtBu)(NC5H5)] [[C2H4-(THI)2]Zr[(2-k-NtBu)-k-NC5F4]] [[Cp2Zr(m-NSO2(p-C6H4Br)]2] [Cp2Zr{k-N[P(Ph2)O]CPhCPh]CMe[k-N(2,6-Me2C6H3)]}] [{Cp2Zr[m-NPPh2(k-O)]}2] [Cp2Zr(Me)[NHC(O)Ph]] [Cp2Zr[(k-NDipp)(k-CH(Ph))C(]CHPh)]] [Cp2Zr[(k-NDipp)C(CH2)6CH-k-CH]] [Cp2Zr(SnMe3)[N(2,6-Me2C6H3)C(CHPh)(CH2Ph)]] [Cp2Zr[(k-NDipp)C(]CHC3H4)CH]CHCH2-k-CH2]] [Cp 2Zr(Me)[N(tBu)]Li(OC4H8)] [[(Cp2Zr)2(m-OH)2][B(C6F5)4]2] [Cp2ZrMe(m-CH3)B(C6F5)3] [[Cp2Zr(CO)(Z2-O]CMe)][MeB(C6F5)3]] [{Cp2Zr(m-Z2:Z1-COMe)[m:Z1:Z1-OC(Me)(CH]CH2)C(Me)-k-O]}{MeB(C6F5)3}2] [[Cp2Zr(CH(SiMe3)2)][HCB11Me5Br6]] [[(Cp2Zr)2(m-Cl)2][B(C6F5)4]2] [[(CpMe 2 Zr)2(m-Cl)2][B(C6F5)4]2] [Cp2Zr(OTf )[N(tBu)SiHMe2]] [Cp2ZrH[N(SiHMe2)2]] [{Cp2Zr[N(SiHMe2)2]}{HB(C6F5)3}] [{Cp2Zr[N(SiMe2-m-k-OCHMe2)2]}{Me2HCOB(C6F5)3}] [[k2-(SiMe3)2Si(SiMe2)2Si(SiMe3)2]ZrCp2] [Cp2ZrCl[Si(SiMe3)3]] [[k2-(SiMe3)2Si(SiMe2)Si(SiMe3)2]ZrCp2] [[Z2-XyN]C-(SiMe3)2Si(SiMe2)Si(SiMe3)2]ZrCp2] [[Z2-XyN]C-(SiMe3)2Si(SiMe2)2Si(SiMe3)2]ZrCp2] [(Cp2ZrCl)2[m:Z1:Z1-Si(SiMe3)2C^CSi(SiMe3)2]] [Cp2ZrBr[Si(SiMe3)2NEt2]] [Cp2ZrBr[Z2-XyN]C-Si(SiMe3)2NEt2]] [Cp2ZrCl[Si(SiMe3)2SiPh2NEt2]] [Cp2ZrCl2
K(18-c-6)] [{[k2-(SiMe3)2Si(SiMe2)2Si(SiMe3)2]ZrCp2}{K(18-c-6)}] [[m:Z5:Z5-C10H8][ZrCp][(Zr(m:Z5:Z1-C5H4)(m-Cl)(Si(SiMe3)3)]] [[m:Z5:Z5-C10H8][ZrCp][(Zr(m:Z5:Z1-C5H4)(m-Cl)(Ge(SiMe3)3)]] [{Cp2Zr[Si(SiMe3)3]}{K(18-c-6)}] [Cp2Zr[(Me3Si)3Si-k-C]CH-CH]k-CSi(SiMe3)3]] [Cp2Zr(PMe3)[Z2-HC]CSi(SiMe3)3]] [{(Cp)(m:Z5:Z1-C5H4)Zr[C(]CPhH)Si(SiMe3)3]}2] [Cp2Zr(PMe3)[Z2-C(SiMe3)]CSiMe2(SiMe3)]] [Cp2Zr[(Me3Si)Me2Si-k-C]CPh-CPh]-k-CSiMe2(SiMe3)]] [Cp2Zr[(PhCC)(Me3Si)2Si-k-C]CPh-CPh]-k-CSi(SiMe3)2(CCPh)]] [Cp2Zr(m:k2:k2-PhC]C-]C]CPh)Si(SiMe3)2] [Cp2Zr[(Me3Si)3Si-k-C]C]C]k-CSi(SiMe3)3]] [Cp2ZrPMe3[Z2-(Me3Si)2Si]Si(SiMe3)2]] [Cp2ZrPMe3[Z2-(Me3Si)SiH]Si(SiMe3)2]] [Cp2ZrPMe3[Z2-(Me3Si)2Ge]Ge(SiMe3)2]] [Cp2Zr[k-(Me3Si)2Sn-Sn(SiMe3)2-k-Sn(SiMe3)2]] [Cp2ZrCl[Si(SiMe3)2SiPh2F]] [Cp2ZrPEt3{]Sn[Si(SiMe3)2(SiMe2)2Si(SiMe3)2]}] [Cp2ZrPEt3{]Pb[Si(SiMe3)2(SiMe2)2Si(SiMe3)2]}] [Cp2ZrPEt3{]Ge[Si(SiMe3)2(SiMe2)2Si(SiMe3)2]}] [Cp2ZrCl{]Sb[Si(SiMe3)2(SiMe2)2Si(SiMe3)2]}] [Cp2ZrCl[Z1-Si6(2,4,6-iPrC6H2)5]] [Cp2ZrCl[(2,4,6-iPr3C6H2)Si]Si(2,4,6-iPr3C6H2)2]] [Cp2ZrCl[(2,4,6-iPr3C6H2)Si(CH2CMeC6H2iPr2)Si(2,4,6-iPr3C6H2)H]] [(2,4,6-iPr3C6H2)5Si6]ZrCp2Cl [Cp2Zr{Ga[2,6-(2,4,6-iPr3C6H2)C6H3]}2] [Cp2Zr{In[2,6-(2,4,6-iPr3C6H2)C6H3]}2] [Cp2Zr{Zn[2,6-(2,4,6-iPr3C6H2)C6H3]}2]

+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +3 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4

31

P chemical shift (ppm)

Number of complex

n/a n/a n/a 43.93 5.28 n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a 4.8 n/a 2.1 n/a n/a n/a n/a −4 Not stated Not stated n/a n/a 37.5 46.3 31.0 n/a n/a n/a n/a n/a n/a n/a n/a

894Zr 895Zr 896Zr 897Zr 898Zr 899Zr 900Zr 901Zr 902Zr 903Zr 904Zr 905Zr 906Zr 907Zr 908Zr 909Zr 910Zr 911Zr 912Zr 913Zr 914Zr 915Zr 916Zr 917Zr 918Zr 919Zr 920Zr 921Zr 922Zr 923Zr 924Zr 925Zr 926Zr 927Zr 927Zr 928Zr 929Zr 930Zr 931Zr 932Zr 933Zr 934Zr 935Zr 936Zr 937Zr 938Zr 939Zr 940Zr 941Zr 942Zr 943Zr 944Zr 945Zr 946Zr 947Zr 948Zr 949Zr 950Zr 951Zr 952Zr

Reference 710 710 711 711 711 712 713 713 713 713 714 438 716 717 717 718 719 719 720 720 721 721 723 724 725 725 725 725 726 726 727 728 728 729 729 729 730 730 730 730 730 730 730 730 731 731 731 731 732 283 283 282 733 734 735 735 734 736,737

737 820

359

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

ZrCp2 complex

Zr oxidation state

[Cp2Zr{Z2-Bi2[2,6-(2,4,6-iPr3C6H2)C6H3]2}] [(m:Z5:Z5-C10H8)(ZrCp2)2(m-H)(m-Cl)(m-Ga[2,6-(2,4,6-iPr3C6H2)C6H3])] [{Cp2Zr[Ga{N[2,6-(2,4,6-iPr3C6H2)C6H3]}2C2H2]2}{Li(thf )4}] [{[CptBu(m:Z5:Z1-tBuC5H3)Zr(m-H)Na]2(Et2O)}2] [{[CptBu(m:Z5:Z1-tBuC5H3)Zr(m-H)Na]2}4] [[Cp (CptBu)Zr]2[m:k2:k2-P4H2]] [Cp1,3-tBu2 Zr(k2-As4)] 2 [Cp1,3-tBu2 Zr(m:k2:k3-As5)ZrCp1,3-tBu2] 2 [Cp1,3-tBu2 Zr(k2-P4)] 2 2 [Cp2Zr[k -P6(CtBu)2]]

+4 +4 +3 +4 +4 +4 +4 +4 +4 +4

[Cp1,3-tBu2 Zr(m:k2-P4)W(CO)5] 2

+4

[Cp1,3-tBu2 Zr(m:k2-P4)[W(CO)5]2] 2 [Cp1,3-tBu2 Zr(m:k2-P4)MnCp(CO)2] 2

+4 +4

[Cp1,3-tBu2 Zr(m:k2-P4)[MnCp(CO)2]2] 2 1,3-tBu2 [Cp2 Zr(m:k2-P4)Fe(CO)4]]

+4 +4

[Cp1,3-tBu2 Zr(m:k2-P4)[AlMe3]2] 2 [Cp1,3-tBu2 Zr(m:k2-P4)AlEt3] 2

+4 +4

[[Cp1,3-tBu2 Zr]2[m:k2:k2-S2PPS2]] 2 1,3-tBu2 [[Cp2 Zr]2[m:k2:k2-Se2PPSe2]] [[Cp1,3-tBu2 Zr]2[m:k2:k2-S2AsAsS2]] 2 [Cp2Zr(OBPin)2] [{Cp2Zr(OC4H8)[S(2-OMe)C6H4]}{MeB(C6F5)3}] [[(Cp2Zr)4(m2-H)4(m4-P)][BPh4]] [[(Cp2Zr)4(m2-H)4(m4-As)][BPh4]] [(Cp2Zr)2(m-PHSiMe2CMei2Pr)2] [(Cp2Zr)2(m-PHSiiPr3)2] [(Cp2Zr)2[m-PHSiFtBu(2,4,6-iPr3C6H2)]2] [(Cp2Zr)2(m-PSiMe2CMei2Pr)2] [(Cp2Zr)2[m-PSiFtBu(2,4,6-iPr3C6H2)]2] [{CpZrCl[m-Z1,Z5-GeC4(2,5-SiMe3-3,4-Me2)]}2] [Cp 2ZrH2] [Cp 2ZrHF] [Cp 2ZrF2] [Cp 2ZrH(o-C6H4F)] [Cp 2ZrF(C6H5)] [Cp 2ZrH(C6F5)] [Cp 2ZrF(o-C6F4H)] [Cp (Fv)Zr(C6F5)] [Cp {Z5-C5Me4CH2(k-o-C6F4)]ZrCl] [Cp 2ZrF(C6F5)] [Cp 2Zr(Z1-C5F5H2)] [{Cp 2Zr[1,2-(HP)2C6H4]}{Li(thf )4}] [[{Cp2Zr[(m-H)2BC4H8]}2{m-H}][HB(C6F5)3]] [[{Cp2Zr[(m-H)2BC8H14]}2{m-H}][HB(C6F5)3]] [[Cp2Zr(OEt)(Et2O)][HB(C6F5)3]] [Cp2ZrCl[(m-H)2BC8H14]] [Cp2ZrH[(m-H)2BC8H14]] [Cp2Zr(OSiPh3)[(m-H)2BC8H14]] [Cp (Cp1,3-(SiMe3)2)Zr(H)Cl] [[iPr2Si-(Cp3-SiMe3)-(Cp3,4-(SiMe3)2]ZrCl [(Ind(1-tBu))2Zr(H)Cl] [(Ind1,3-iPr2)2Zr(H)2PMe3] [(Ind1,3-(SiMe3)2)2ZrCl2] [(Ind1,3-(SiPhMe2)2)2ZrCl2] [(Ind1-SiMe3–3-tBu)2ZrCl2]

+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4

31

P chemical shift (ppm)

Number of complex

Reference

n/a n/a n/a n/a n/a 11.61 n/a n/a 1 115.2/41.8/ -45.4/−51.0 111.7/109.2 /−193.0 50.5/−188.1 220.5/−22.8/ -258.7 257.6/−74.3 230.9/−75.0/ -250.0 109.1/−206.2 104.7/ -203.5 −24.3 −47.9 n/a n/a n/a 254.2 n/a −106.6 −132.6 −86.3 238.9 260.3 n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a −34.6 n/a n/a n/a n/a n/a n/a n/a n/a n/a −18.67 n/a n/a n/a

953Zr 954Zr 955Zr 956Zr 957Zr 958Zr 959Zr 960Zr 961Zr 962Zr

738 739 740 741 741 742 743 743 746 747

963Zr

746

964Zr 965Zr

746 746

966Zr 967Zr

746 746

968Zr 969Zr

746 746

970Zr 971Zr 972Zr 973Zr 974Zr 975Zr 976Zr 977Zr 978Zr 979Zr 980Zr 981Zr 982Zr 983Zr 984Zr 985Zr 986Zr 987Zr 988Zr 989Zr 990Zr 991Zr 992Zr 993Zr 994Zr 995Zr 996Zr 997Zr 998Zr 999Zr 1000Zr 1001Zr 1002Zr 1003Zr 1004Zr 1005Zr 1006Zr 1007Zr

748 748 748 751 464 752 753 754 754 754 754 754 107 755 755 755 755 755 756 756 756 756 757 758 759 761 762 761 763 763 764 766 766 766 767 143 143 143 (Continued)

360

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

ZrCp2 complex

Zr oxidation state

[(Ind1,3-(SiMe3)2)(IndtBu)ZrCl2] [(Ind1,3-(SiMe3)2)2Zr(CO)2] [(Ind1,3-(SiMe2tBu)2)2Zr(CO)2] [Cp2ZrH(m-H)2Al(H)(Nacnac(2,6-Me2C6H3))] [Cp2ZrH(m-H)2Mg(NacnacDipp)] [Cp2ZrH(m-H)2Zn(NacnacDipp)] [Cp2ZrH(m-H)(m:Z2:Z1-C8H12)Zn(NacnacDipp)] [Cp2ZrH(m-H)(m:Z2:Z1-C7H10)Zn(NacnacDipp)] [Cp2ZrH(m-H)(m:Z2:Z1-tBuC^CMe)Zn(NacnacDipp)] [[(Me3Si)3CAlF(m-F)2]2[Cp2Zr(m-O)ZrCp2]] [[(Me3Si)3CAlF2(m-F)]ZrCp2Me] [(NacnacDipp)AlMe(m-O)ZrMeCp2] [(NacnacDipp)AlMe(m-O)ZrClCp2] [(NacnacDipp)Al(OH)(m-O)Zr(SH)Cp2] [(NacnacDipp)GaMe(m-O)ZrMeCp2] [(NacnacDipp)Ge(m-O)ZrMeCp2] [[(C10H6-1,3-k-N(SiMe3)]Bi(m-O)ZrMeCp2] [[(SiMe3)2NCa(thf )3](m-O)ZrCp 2] [Cp2ZrMe[O-2-C6H4-P(Ph2)AuMe]] 1 5 [[Z5-CptBu 2 -Zr-m:Z :Z -Cp2]Ru] 1 5 [(Z5-CptBu )Zr[m:Z :Z -Cp][m-CH2(Z5-C5Me4)]Fe] 2 5 [[1,2-(Me2Si)2-(Z -C5H3)2]Zr(NEt2)2] [[(CH2)3Si-Cp2]Zr(NMe2)2] [[(CH2)3Si-Cp2]ZrCl2] [[Cp2Zr(m:Z1:Z5-C5H4FeCp)][MeB(C6F5)3]] [[Cp2Zr(PMe3)(Z1:Z5-C5H4FeCp)][MeB(C6F5)3]] [[(CH2]CH)2Si-CpMe4 2 ]ZrCl2] [{[IndRh(Z2-CH2]CH)2]Si-CpMe4 2 }ZrCl2] [Cp2Zr[m-NtBu][m-N(CPhCH2-Z5-C5Me4]Ir] [Cp2Zr[m-NtBu][m-N(CH]CH(p-(CF3)C6H4)]IrCp ] [Cp2Zr(OTf )[m-NtBu][m-H]IrCp ] [Cp2Zr[m-(k-O)(k-O)C2[N(tBu)]2k-C])Ir(COD)Cl] [Cp2Zr{(m:Z1:Z1-N2)Re(Cp)[N(Dipp)C(Me)CHC(Me)N(Dipp)]}2] [[Me2Si-Cp2]ZrCl2] [[Me2Si-Cp2]ZrI2] [[C2H4-Cp2]ZrCl2] [(rac-MBSBI)Zr[PhN(CH2)3NPh]] [(S,S,R,R-MeSi2-Ind2)Zr[2R,4R-PhN(CHMeCH2CHMe)NPh]] [(MeSi2-Ind2)Zr[PhN(CH2)3NPh]] [Cp2Zr[PhN(CH2)3NPh]] [(rac-MBSBI)Zr(NMePh)2] [(meso-MBSBI)Zr(NMePh)2] [(rac-MBSBI)Zr[PhN(CH2)2NPh]] [(meso-MBSBI)Zr[PhN(CH2)2NPh]] [Cp2Zr[2R,4R-PhN(CHMeCH2CHMe)NPh]] [[rac-Me2Si-Cp3-tBu ]Zr[PhN(CH2)3NPh]] 2 [[rac-Me2Si-Cp3-tBu ]Zr[(Me3Si)N(CH2)3N(SiMe3)]] 2 [(C10H6-1,8-Ind2)ZrCl2] [[(1-Flu)]C-(2-Ind)2]ZrCl2] [[Ph2C]C-(2-Ind)2]ZrCl2] [[(1-Flu)]C-(2-Z3-Ind1,3-Me2)2]ZrCl2] [[Ph2C]C-(2-Ind)2]2Zr] [Cp CpSiMe3ZrF2] [Cp2ZrCl(Z2-HNNCHSiMe3)] [Cp2ZrCl(S-Z2-PhMeNPPh2)] [Cp2ZrCl(N(PPh2)2)] [Cp2Zr(Ph2-k-PN-k-PPh2)] [Cp2ZrH(Ph2-k-P-CH-k-PPh2)] [Cp2ZrCl(Ph2-k-P-CH-k-PPh2)] [Cp2ZrCl(k-O-CPh-k-NDipp)]

+4 +2 +2 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4

31

P chemical shift (ppm)

Number of complex

n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a −7.8 n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a −19.5 −4.49 Not stated −14.16 Not stated n/a

1008Zr 1009Zr 1010Zr 1011Zr 1012Zr 1013Zr 1014Zr 1015Zr 1016Zr 1017Zr 1018Zr 1019Zr 1020Zr 1021Zr 1022Zr 1023Zr 1024Zr 1025Zr 1026Zr 1027Zr 1028Zr 1029Zr 1030Zr 1031Zr 1034Zr 1035Zr 1036Zr 1037Zr 1038Zr 1039Zr 1040Zr 1041Zr 1042Zr 1043Zr 1044Zr 1044Zr 1045Zr 1046Zr 1047Zr 1048Zr 1049Zr 1050Zr 1051Zr 1052Zr 1053Zr 1054Zr 1055Zr 1056Zr 1057Zr 1058Zr 1059Zr 1060Zr 1061Zr 1062Zr 1063Zr 1064Zr 1065Zr 1066Zr 1067Zr 1068Zr

Reference 143 143 143 768 769 769 769 770 770 771 771 772 772 773 777 776 778 821 781 782 784 785 785 785 28 28 787 787 788 788 822 789 790 791 791 791 792,794

793 794 794 794 794 794 794 793 795 795 796 797 797 797 797 798 799 801 802 802 803 803 804

361

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

ZrCp2 complex [Cp2ZrH(k-O-CPh-k-NDipp)] [[Cp2Zr(m-H)(m:Z2-CH2]CH-2-C5NH4)][AliBu2]] [[Cp2Zr(m-F)(m:Z2-CH2]CH-2-C5NH4)][AliBu2]] [[(EBTHI)Zr(H)(m-H)]2] [THI2Zr[CH2-CH](2-C5NH4)]] [THI2ZrF[CH2-CH2-(2-C5NH4)]] [THI2ZrF2] [{Cp2Zr[m:k-N]C]C(Mes)-C(CH2Mes)]N]}2] [Cp 2Zr[k-NH]C(CH2Ph)-C(CH2Ph)]NH]] [Cp 2Zr[k-NH]C(]CHPh)-C(CH2Ph)-NH][N]C]CHPh]] [Cp 2Zr[k-N]C(2-C5NH4)-C(2-C5NH4)-k-N-C(O)-k-O]] [Cp2ZrMe(CHt2Bu)] [[Me(Me2N)Si-Cp2]Zr(NMe2)2] [Cp(sumanenyl)ZrCl2] [Cp (sumanenyl)ZrCl2] [ZrCp2(2,2-bipy)] [{Cp2Zr}2{m-[k-(nBu)C]CdC]k-C(nBu)][(p-2,6-Me2C6H2)(p-C6H4)(p-2,6-Me2C6H2)]2[k-(nBu)C] CdC]k-C(nBu)]}] [{Cp2Zr}2{m-[k-(Me3Si)C]CH-CH]k-C(SiMe3)][(p-2,6-C6H4)(1,3-C6H4)(p-C6H4)]2[k-(Me3Si)C] CdC]k-C(SiMe3)]}] [[CpZr(OMes)(ClC6H5)][B(C6F5)4]] [[Cp Zr(OMes)][B(C6F5)4]] [[CpZr(OMes)(OC4H8)][B(C6F5)4]] [[Cp Zr(OMes)(ClC6H5)][B(C6F5)4]] [[Cp Zr(OMes)(pyr)][B(C6F5)4]] [[Cp Zr(OMes)(N-2-MeC5H4))][B(C6F5)4]] [[Cp 2Zr(OMes)(Z2-H3B-NHMe2)][B(C6F5)4]] [[Cp 2Zr(OMes)(Z2-H3B-NHiPr2)][B(C6F5)4]] [(k-N(SiHMe2)SiMe2-Cp)CpZr[N(tBu)(SiHMe2)]] [{Cp2Zr[N(SiHMe2)(SiMe2-m-k-OTf )]}{HB(C6F5)3}] [Cp2Zr(OMe)[N(tBu)SiHMe2]] [Cp2Zr(PMe3)[k2-N(SiHMe2)-SiMe2)]] [Cp2Zr [k-N(SiHMe2)Si(Me2)C(Me2)-k-O)]] [Cp2ZrN[SiMe2(OCHMe2)]SiMe2(OCMe2)CH2B(C6F5)3}] [(NacnacDipp)AlEt(m-O)ZrMeCp2] [(NacnacDipp)AlPh(m-O)ZrMeCp2] [Cp 2ZrMe(OH)] [[Cp 2ZrMe(m-O)TiCp Me2]] [[Cp 2ZrMe(m-O)Ti(NMe2)3]] [[Cp 2ZrMe(m-O)]2Hf(NMe2)2] [Cp CpSiMe3ZrCl2] [Cp2ZrCl[(Z2-PhN(PPh2)2]] [Cp2ZrCl{k3-[(Me3Si)NP(Ph2)]2C}] [Cp2ZrCl(R-Z2-PhMeNPPh2)] [Cp2Zr(S,S-TADDOL)] [Cp2Zr[tBu2C](k2-CC^CC)]CtBu2]] [Cp 2Zr(NHPh)2] [Cp 2Zr(NH(2-OMe)C6H4)2] [{(Cp 2Zr(m2-H)2(m3-H)}2{Li}{[Li(thf )]2(m-SPh)]}] [[(Cp2Zr)3(m2-H)3(m3-H)][B(C6F5)4]2] [Cp2ZrMe[O-C(]CEt2)CEt2CO2Me]] [{Cp2Zr[k-CPh]CPh-k-(2-Pyr)]{NTf2}] [Cp2ZrC(OSiPh3)[(m-H)2BC8H14]] [Cp2ZrCl(m-O-CMe])W(CO)5] [Cp2ZrCl(m-O-CMe])Cr(CO)5] [CpCpSiMe2PhZrMe2] [(CpSiMe2Ph)2ZrMe2] [Cp 2Zr[(k-CH2)2CHCH2CH]CH2)] [(Cp2ZrCl)2(m-O)] [Cp2Zr[k-P2(]k-O)(CtBu)2]]

Zr oxidation state

31

P chemical shift (ppm)

Number of complex

Reference

+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +2 +4

n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a

1069Zr 1070Zr 1071Zr 1072Zr 1073Zr 1074Zr 1075Zr 1076Zr 1077Zr 1078Zr 1079Zr 1080Zr 1081Zr 1082Zr 1083Zr 1084Zr 1085Zr

804 805 805 805 806 806 806 807 807 807 203 808 809 810 810 554 551

+4

n/a

1086Zr

552

+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4

n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a 18.67 n/a n/a n/a n/a n/a n/a n/a n/a n/a −3.8/62.2 10.9 19.5 n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a

1087Zr 1088Zr 1089Zr 1090Zr 1091Zr 1091Zr 1092Zr 1093Zr 1094Zr 1095Zr 1096Zr 1097Zr 1098Zr 1099Zr 1100Zr 1101Zr 1102Zr 1103Zr 1104Zr 1105Zr 1106Zr 1107Zr 1108Zr 1109Zr 1110Zr 1111Zr 1112Zr 1113Zr 1114Zr 1115Zr 1116Zr 1117Zr 1118Zr 1119Zr 1120Zr 1121Zr 1122Zr 1123Zr 1124Zr 1125Zr

658 658 658 658 659 659 660 660 720 721 722 722 722 722 774 775 779 779 780 780 798 800 800 801 823 587,588

760 760 760 665 824 825 764 826 827 828 828 94 765 747 (Continued)

362

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

ZrCp2 complex

Zr oxidation state

[Cp2Zr[k2-OSi(Fc)2OSi(Fc)2O]] [[Cp2Zr]2[m:k2-OSi(Fc)2O]] [Cp1-(SiMe3)-3(SiMe2-k-CH2)FluMe9Zr(CHi2Pr)] [Cp1-(SiMe3)-3(SiMe2-k-CH2)FluMe8Zr(CHi2Pr)] [Cp1,3-(SiMe3)2(Z3:Z5-H5FluMe8)ZrH] [[C2H4-(THI)2]Zr{[N(2,6-Me2C6H3)]C(Ph)CH2-k-O}] [[(rac-EBI)Zr(k-C6H4-2-k-C5H4N)] [rac(EBI)Zr(2,20 -k-O2-3,30 -tBu2-5,50 -Me2-C12H4)] [rac(Me2Si-Ind2-Me-5,6-Ph2 )Zr(2,20 -k-O2-3,30 -tBu2-5,50 -tBu2-C12H4)] 2 2-Me-4-tBu [rac-(Me2Si-Cp2 )ZrMe[m-MeB(C6F5)3]] [[Me2Si-Ind -Cp]Zr(ODipp)Cl]

+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4

31

P chemical shift (ppm)

Number of complex

Reference

n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a

1126Zr 1127Zr 1128Zr 1129Zr 1130Zr 1131Zr 1132Zr 1133Zr 1134Zr 1135Zr 1136Zr

786 786 421 421 421 709 719 515 515 436 512

Zr phospholyl complex

Zr oxidation state

31

Number of complex

Reference

[(Z5-PC4H{2-Ph}{3,4-Me2})2ZrCl2] [[(m-Rh{R-BINAP})(Z5s-PC4H{2-Ph}{3,4-Me2})2]ZrCl+2 SbF−6 ] [[(m-Mo{CO}4)(Z5s-PC4H{2-Ph}{3,4-Me2})2]ZrCl2] [(Z5-C4PMe4)Zr(CO)2] [(Z5-C4PMe4)Zr(Z2-btmsa)2] [(Z5-C4PMe4)Zr(-CMe]CMe-CMe-CMe-)2] [(Z5-C4P{2,3-{CH2}n)ZrCl2] [(Z5-PC4Me4)ZrCl(NMe2)2] [[(Z7-C7H7)Zr(Z5-PC4Me4)]2] [[(Z7-C7H7)Zr(Z5-PC4H2{3,4-Me2})]2] [(Z7-C7H7)Zr(Z5-PC4H{2-Ph}{3,4-Me2})] [(Z7-C7H7)Zr[Z5-P(W{CO}5)C4Me4]] [(Z7-C7H7)Zr[Z5-P(W{CO}5)C4H2{3,4-Me2}]] [(Z7-C7H7)Zr(Z5-C5H4P{Ph2}W{CO}5)] [Ni{(Z7-C7H7)Zr(Z5s-PC4Me4)}4]

+4 +4 +4 +2 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4

83.3 (rac, major), 85.3 (meso) 58.4 (phospholyl-P), 28.4 (BINAP-P) 65.0 20.8 90.6 77.3 85.4 (rac), 84.4 (meso, major) 97.2 84.9 54.5 73.5 41.9 24.9 4.4 114.6

1137Zr 1138Zr 1139Zr 1141Zr 1142Zr 1143Zr 1144Zr 1145Zr 1146Zr 1147Zr 1148Zr 1149Zr 1150Zr 1151Zr 1152Zr

814 815 814 816 816 816 817 818 385 385 385 385 385 385 385

4.06.4

Hafnium

4.06.4.1

Mono(cyclopentadienyl) hafnium chemistry

4.06.4.1.1

P chemical shift (ppm)

Nitrogen based ligands

4.06.4.1.1.1 Imido and amido ligands Tilley and co-workers reported the synthesis of a dimeric hafnium complex incorporating bridging imido ligands, [(Cp HfCl{mDipp})2] (1Hf).829 Attempts to prepare derivatives of 1Hf featuring HfdC, HfdH and HfdSi bonds were unsuccessful, which was attributed to the hindered Hf center in 1Hf. Stephan and co-workers reported the hafnium phosphinimide complexes [Cp(tBu3PN)2HfCl] (2Hf) and [Cp (tBu3PN)HfCl2] (3Hf).830 Complex 3Hf was tested as a precatalyst for ethylene polymerization. Horácek and co-workers reported amido complex [Cp HfCl2(NHDipp)] (4Hf), synthesized via a salt metathesis reaction of [Cp HfCl3] with LiNHDipp.831

4.06.4.1.1.2 Amidinato and enamino ligands Sita and co-workers extensively investigated group 4 half-sandwich complexes as initiators for the living polymerization of propene and higher a-olefins, which was the subject of a 2009 review.832 Hafnium amidinate complexes, [Cp HfCl2(k2-N{tBu}C{CH2X}N {Et})] for X ¼ CH2Ph (5Hf), CH2Cl (6Hf) and SiMe3 (7Hf) were synthesized via reaction of enolate complexes with electrophiles (Scheme 121a).833 Dialkyl acetamidinates [Cp HfR2(N{Et}C{Me}N{tBu})] R ¼ Me (8Hf) and iBu (9Hf) were also synthesized, via “one-pot” carbodiimide insertion or by two salt metathesis steps (Scheme 121b).834 Ligand-metal bond lengths are significantly shorter in the Hf complexes than their Zr analogues (Hf–N ¼ 2.237(3) and 2.247(3) A˚ in 8Hf vs ZrdN 2.251(3) and 2.265(2) A˚ in [Cp ZrMe2(N{Et}C{Me}N{tBu})]331 attributed to the effects of the lanthanide contraction. Their respective cationic complexes, [Cp HfR(N{Et}C{Me}N{tBu})][B(C6F5)4], function as initiators for the stereospecific living Ziegler-Natta polymerization of 1-hexene, but with a 60 times lower activity than their zirconium congeners.834 In a 2006 study, the mixed chloro tert-butyl complex, [Cp HfCl(tBu)(k2N,N0 -(N{Et}C{Me}N{tBu})] (10Hf), was also synthesized and structurally characterized.338 Isobutyl cationic complex [Cp Hf(iBu)(N{Et}C{Me}N{tBu})][B(C6F5)4] is less stable than its Zr congener, and does not engage in a strong b-hydrogen agostic interaction.

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

363

Scheme 121 Synthesis of half-sandwich Hf amidinate complexes reported Sita and co-workers.

The N,N-diethyl amidinate derivative [Cp HfMe2(MeC{NEt}2)] (11Hf), in combination with borate cocatalyst [PhNMe2H] [B(C6F5)4] and varying amounts of ZnEt2,835 produced a very highly active system for the aspecific polymerization of propene, producing a large molecular-weight range of atactic polypropene residues possessing extremely narrow polydispersities. A C1 symmetric complex with a a,a,a-trifluoroacetamidinate ligand, [Cp HfCl2(N{Et}C{CF3}N{tBu})] (12Hf) was synthesized and structurally characterized, and its dimethyl derivative, [Cp HfMe2(N{Et}C{CF3}N{tBu})], shows six times higher activity as a preinitiator for the living copolymerization (LCP) of 1-hexene relative to the nonfluorinated structural analogue, 8Hf.836 Half-sandwich hafnium amidinate initiators were also employed in the (stereospecific) LCP837–839 and coordinative chain-transfer polymerization (CCTP)839–841 of ethene, propene, a-olefins, and a,o-nonconjugated dienes. The related half-sandwich hafnium (k2N,N)-iminocaprolactam complexes [Cp HfMe2({R}NimcapN)], where R ¼ CH2Ph (13Hf), CH2Naph (14Hf), tBu (15Hf), were also structurally characterized, and serve as active preinitiators for LCP and LCCTP.842 Schulz and co-workers reported a series of group 4 metal S-methyl-diimidosulfinate complexes, [Cp MX2(MeS{NSiMe3}2)] X ¼ Me, Cl. The hafnium dimethyl complex [Cp HfMe2(MeS{NSiMe3}2)] (16Hf), synthesized by reaction of [Cp HfMe3] with S(NSiMe3)2, was structurally characterized (Fig. 27). The functionalization of activated group 4 dinitrogen complexes with nonpolar reagents such as dihydrogen is an area of great interest, because it provides an attractive approach to convert atmospheric N2 into more value-added N-containing organic molecules under ambient conditions.

Fig. 27 Examples of half-sandwich Hf amidinate complexes reported Sita and co-workers.

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Chemical reduction of group 4 half-sandwich precursors with KC8 under a dinitrogen atmosphere afforded a series of bimetallic N2 complexes, [(CpRHf{k2-C(X)(NR)2})2(m-Z2:Z2-N2)].334 Guanidinate complexes [(CpRHf{C(NMe2)(NiPr)2})2(m-Z2:Z2-N2)] CpR ¼ CpMe4 (17Hf), Cp (18Hf), and amidinate complexes [(CpRHf{C(Me)(NR)2})2(m-Z2:Z2-N2)] CpR ¼ CpMe4; R ¼ iPr (19Hf); CpR ¼ Cp ; R ¼ iPr (20Hf); CpR ¼ Cp ; R ¼ iPr (21Hf) were characterized by X-ray diffraction, revealing a side-on/side-on N2 coordination mode. Complexes 17Hf-21Hf show NdN bond distances in the range 1.581(4)–1.635(5) A˚ , which is the upper limit for NdN bond lengths reported for any group 4 metal dinitrogen complex. These complexes were reactive toward both hydrosilylation and hydrogenation, in which single H3SiPh and H2 addition, respectively, provided N-atom functionalized bridging hydride products [(CpMe4Hf{C(NMe2)(NiPr)2})2(m-H)(m-Z1:Z2-NN{SiH2Ph})] (22Hf) and [(CpMe4Hf{C(NMe2)(NiPr)2})2(m-H) (m-Z2:Z2-NN{H})] (23Hf). The coordinated N atoms could be alkylated with ethyl bromide, to afford the N-alkylated products [(CpMe4Hf{C(NMe2)(NiPr)2})2(m-Br)(m-Z2:Z2-NN{Et})] (24Hf) and [(Cp Hf{C(Me)(NEt)2})2(m-Br)(m-Z2:Z2-NN{Et})] (25Hf), the first such example for any N2 derived complex. A side-product containing a bromide bonded to each hafnium center [(CpMe4HfBr{C(Me)(NiPr)2})2(m-Z2:Z2-N2)] (26Hf) was obtained for the reaction of 18Hf with ethyl bromide (Fig. 28).

Fig. 28 Half-sandwich Hf amidinate N2-complexes reported Sita and co-workers.

As part of a study into the activation of group 4 complexes for ethylene polymerization, Lee and co-workers reported the reaction of hafnium bis(enamido) complex [Cp HfMe(k2N,N-{Dipp}NC]{CH2}C]{CH2}N{Dipp})] with Al(C6F5)3 to afford the alane-added zwitterionic complex, [Cp HfMe(k2N,N0 -{Dipp}NC]{CH2Al(C6F5)3}C]{CH2}N{Dipp})] (27Hf, Scheme 122).843 Complex 27Hf did not show any activity toward ethylene polmerization, which was attributed to the absence of any vacant site by steric congestion.

Scheme 122 Synthesis of a half-sandwich Hf bis(enamido) complex reported by Lee and co-workers.

4.06.4.1.1.3 Multidentate N,N0 and N,O ligands Mashima and co-workers reported a series of half-sandwich Hf complexes bearing N-substituted iminopyrrolyl ligands, including [Cp HfMe2(k2N,N0 -3-{R}N]C{H}-C4H3N)], R ¼ 4-MeOC6H4 (28Hf),844 Tol (29Hf),844 Xyl (30Hf),845 and tBu (31Hf).845 Upon treatment with [Ph3C][B(C6F5)4] below 0  C, complexes 28Hf and 29Hf form active catalyst systems for the isoselective living polymerization of 1-hexene.844,845 The high isoselectivity observed was attributed to the flipping process of the bidentate iminopyrrolyl ligand attached to the half-hafnocene fragments. Kempe and co-workers reported a series of (aminopyridinato) ligand stabilized half-sandwich hafnium dimethyl complexes, [Cp HfMe2(k2N,N0 -{py-2-(Dipp)-6-X})] for X ¼ H (32Hf), Me (33Hf), Cl (34Hf). Upon activation with perfluoroaryl borates, 32Hf-34Hf catalyze the chain transfer polymerization of ethylene with trimethylaluminium as the chain transfer agent, giving linear polyethylenes with saturated end groups.846 As part of a series of group 4 CpR-arylaminate complexes, Horácek and co-workers synthesized [CpRHfCl2(k2C,NC6H4CH2NMe2)] for CpR ¼ CpMe4 (35Hf) and Cp (36Hf), by reaction of 2-[(dimethylamino)methyl] phenyl lithium with the corresponding [CpRHfCl3] precursor.847 X-ray diffraction studies revealed 35Hf contains two pairs of enantiomers in the unit cell, whereas 36Hf contained only one enantiomer in the unit cell. Half-sandwich precursor complex [CpMe4HfCl3] (37Hf) was also analyzed by X-ray diffraction, which revealed a dimeric solid state structure in constrast to the bulkier analogues, [Cp1,3-{tBu2}HfCl3] (38Hf)393 and [Cp1,3-{tBu2}HfMe3] (39Hf),848 which are monomeric in the solid state. In a 2015 report, Horácek reported two series of group 4 metal complexes: mononuclear species with terminal amido ligands, and dinuclear complexes with bridging imido ligands.831 Reaction of [Cp HfCl3] with N,N-chelating ligand LiNHC6H4-2-(CH2NMe2) afforded a mixture of [Cp HfCl2(k2N,N0 NHC6H4-2-{CH2NMe2})] as the major product, and dimeric complex [(Cp HfCl{m-NC6H4-2-(CH2NMe2)})2] (40Hf) as the minor

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365

product, in which two N atoms act as bridges between two Hf atoms. The authors postulated that 40Hf forms via a-H sbstraction from the ligand by a Cl atom with the assistance of a free base. Reaction of C,N-chelate complex [CpRHfCl2(k2C,NC6H4-2-{CH2NMe2})] with an equivalent of LiNHDipp afforded the bridging imido species, [(CpRHfCl{m-N(Dipp)})2] for CpR ¼ CpMe4 (41Hf) and Cp (42Hf), accompanied by release of the C,N-chelating ligand. Reaction of [CpRHfCl2(k2C,NC6H4-2-{CH2NMe2})] with 1 equiv. of N,N-chelating ligand LiNHC6H4-2-(CH2NMe2) yielded [CpRHfCl(k2C,NC6H4-2-{CH2NMe2})(k1N-NHC6H4-2-{CH2NMe2})] for CpR ¼ CpMe4 (43Hf) and Cp , in which the 2-[(dimethylamino) methyl]anilido ligand coordinates in a monodentate fashion. This study confirms increasing stability of amido and imido complexes from Ti to Zr to Hf (Fig. 29).

Fig. 29 Examples of half-sandwich Hf complexes with multidentate N,N0 ligands.

In the same year, Horácek reported half-sandwich Zr and Hf complexes featuring a bifunctional b-diketiminate ligand.849 Hafnium dichloride complexes [CpRHfCl2(k2N,N-{2-(MeO)-C6H4}NC{Me}CHC{Me}N{2-(MeO)-C6H4})] for CpR ¼ CpMe4 (44Hf) and Cp (45Hf) were synthesized via salt methathesis, and showed low activity toward ethylene polymerization. During the synthesis of 44Hf and 45Hf, minor decomposition products [Cp HfCl(k2O,N-2-O{Me}-C6H4-NH)2] (46Hf) and [Cp HfCl3(k2O,N-2-O{Me}-C6H4-NH2)] (47Hf) were isolated and structurally characterized, containing two 2-methoxyanilide and one 2-methoxyaniline ligands, respectively.849 Scott and co-workers reported half-sandwich group 4 complexes with salicyloxazolinato ligands, including the structurally characterized hafnium complex [Cp HfCl2(k2N,O-2-{4,5-dihydro-1,3-oxazol-2-yl}-4,6-tBu2C6H2)] (48Hf), which showed no ethylene polymerization activity.322 These researchers also reported a related complex, [Cp Hf(NMe2)(k3N,N0 ,ON{Ph}C{NMe2} NCH{Ph}CH2O)] (49Hf), featuring a tridentate alkoxide functionalized guanidinate ligand, formed via ring-opening/amido migratory insertion of a aminooxazolinato ligand.325 Mitzel and co-workers reported the synthesis and structural characterization of group 4 half-sandwich complexes of ethylene-bridged bis-hydroxylamine ligands, including the Hf complex [Cp HfMe({Z2-ONMeCH2}2)] (50Hf) (Fig. 30).850

Fig. 30 Half-sandwich Hf complexes with multidentate N,O ligands.

Gade and co-workers reported a series of hydrazinediido half-sandwich Zr and Hf complexes, containing 2-(N-xylylamino) pyrrolinate as a spectator ligand. In the case of hafnium, reaction of monochloride precursor [Cp HfCl(k2N,N0 -1-N{Xyl}-C4H7N) (NHNPh2)] with LiHMDS afforded a mixture of the {LiHMDS} hydrazinediido complex 51Hf and bis-(hydrazido) complex [Cp Hf(k2N,N0 -1-N{Xyl}-C4H7N)(NHNPh2)2] (Scheme 123).851 Thermolysis of this mixture in the presence of DMAP, yielded the hafnium hydrazinediido complex 52Hf as a mixture of the two diastereomers. Compound 51Hf could also be obtained selectively by reacting [Cp HfCl2(k2N,N0 -1-N{Xyl}-C4H7N)] with 2 equiv. of LiHMDS.

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Scheme 123 Synthesis and reactivity of half-sandwich Hf hydrazinediido complexes reported by Gade and co-workers.

4.06.4.1.2

Aryloxide complexes for polymerization

As part of their exploration of group 4 half-sandwich aryloxide complexes toward olefin polymerization and co-polymerization, Nomura and co-workers synthesized [Cp HfCl2(O-2,6- R2C6H3)] with R ¼ Ph (53Hf) and tBu (54Hf).852 The observed polymerization activities of 11Hf and 54Hf in the presence of MAO co-catalyst was lower than those of their Ti and Zr congeners under identical conditions, and the molecular weights in the resultant polymers were lower than those prepared by the Ti analogues. In 2014, O’Hare and co-workers reported a series of group 4 aryloxide complexes with a (hydro)permethylpentalene ligand (Pn {H} ¼ Z5-C8Me6H), including Hf complexes [Pn {H}HfCl3–x(OAr)x] x ¼ 3, Ar ¼ Xyl (55Hf); x ¼ 2, R ¼ Dipp; x ¼ 2,6-tBu2C6H3, prepared by reaction of [(Pn {H}HfCl3)2] (56Hf) with the corresponding potassium aryloxides.853 In each case the methyl group attached to the sp3-hybridized carbon atom of Pn {H} was assigned to an anti configuration with respect to the metal cation, and present in a mixture of two diasteromersisomers (R,RP)- and (S,SP)-[Pn {H}Hf(OR)3] (outlined in red in Scheme 124); the R,SP and S,RP analogues were not observed.

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Scheme 124 Possible diastereomers of [Pn {H}HfL3] for non-chiral L and ring-opening polymerization of lactide monomers.

The complexes were investigated as initiators for the ring-opening polymerization of polar monomers (L- and rac-lactide) in order to ascertain if these mixtures of diastereomers could exert any stereocontrol on the resulting polymerization. All complexes in the series were shown to be good initiators for lactide polymerization, with the rate of initiation following a general trend of Zr > Hf > Ti.853 It was intended that the Pn {H} ligand, which can be viewed as a chiral CpR ligand in these half-sandwich complexes, would impart stereocontrol on the polylactide resin produced. However, only atactic polymers were obtained from the polymerization of rac-lactide and no epimerization was observed in the polymerization of L-lactide. It was proposed that the chiral center of the Pn {H} ligand is too remote from the metal center, and the methyl substituents too small to exert any stereochemical effect.

4.06.4.1.3

Constrained geometry complexes

Since the first reports of amido functionalized ansa half-sandwich group 4 complexes in the 1990s, these compounds recieved increasing interest as catalysts for olefin polymerization in research and industry. In 2001, Alt and co-workers structurally characterized an amido functionalized ansa half-sandwich hafnium complex [(Me2Si-IndPr-NtBu)HfCl2] (57Hf), which exists as two enantiomers in the solid state.854 Miller and co-workers reported a series of sterically expanded ansa-Z1-fluorenyl-amido complexes, including crystallographically characterized hafnium complexes, [(Me2Si-Octaflu-NtBu)HfX2(OEt2)n] for X ¼ Br; n ¼ 1 (58Hf), and X ¼ CH2Ph; n ¼ 0 (59Hf). The octamethyloctahydrodibenzofluorenyl ligand shows Z5 coordination in the ether-free complex 59Hf, but shows Z1 coordination in ether-bound complex, 58Hf.855 In addition to two Zr complexes, Braunschweig and co-workers reported the boron-bridged constrained geometry Hf complex [(iPr2NB-Cp-N{Ph})Hf(NMe2)2] (60Hf) by reaction of the neutral ligand precursor [(Z1-Cp)B(NiPr2)N(H)Ph] with [Hf(NMe2)4] via elimination of 2 equiv. of HNMe2.856 Xie and co-workers reported phosphorous-bridged indenyl-carboranyl CG complexes, including structurally characterized Hf complexes [({iPr2N}P-Ind-C2B10H10)Hf(NR2)2] for R ¼ Me (61Hf) and Et (62Hf).857 These researchers later reported chemical reduction of ansa-bridged nido-Z5-[C2B9H10R]2− carbollide complex [(Me2C-Cp-{Z5-C2B9H10})HfCl2][Li(DME)3] with excess Na metal, furnishing [({Me2C-Cp-(Z6-C2B9H10)}Hf )2][Li(thf )4]2 (63Hf). Treatment of 63Hf with [PPN]Cl (PPN ¼ bis(triphenylphosphine)iminium cation) afforded the cation exchange product [({Me2C-Cp-(Z6-C2B9H10)}Hf )2] [PPN]2 (64Hf). Structural analysis of 64Hf revealed the two electrons from the reducing agent formally add to the dicarbollyl ligand, leading to the formation of an arachno-[C2B9H10R]4− moiety with a cage C. . .C separation of 2.783(7) A˚ . Each Hf center is Z6-bound to the arachno-C2B9H10R4− ligand and coordinaates two BdH bonds at the neighbouring C2B4 bonding face in a distorted-tetrahedral geometry (Fig. 31).858 Rothwell and co-workers reported an extensive study into structural and stereochemical aspects of group 4 CG indenyl-phenoxide complexes,859 including bis(amido) complexes, [(4,6-C6Ht2Bu2-CpR-O)Hf(NEt2)] CpR ¼ Ind (65Hf) and Ind2-Me (66Hf), and homoleptic complexes [(4,6-C6Ht2Bu2-Ind-O)2Hf] (67Hf) and [(4,6-C6Ht2Bu2-Ind2,3-{Me2}-O)2Hf] (68Hf). Complexes and 67Hf and 68Hf contain three chiral elements (two planar chiral indenyl rings and an axially chiral metal center), which generates three possible diasteromers. Complex 67Hf was found to crystallize as the (R,pR,pR)/(S,pS,pS) enantiomeric pair,

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Fig. 31 Examples of half-sandwich Hf CG complexes.

which contains a C2 axis leading to equivalent chelates and one set of ligand signals in the 1H NMR spectrum. In contrast, the solid-state structure of 68Hf was found to consist of the (S,pR,pS)/(R,pR,pS) pair, which have no symmetry element and, hence, show two equal-intensity sets of ligand resonances (meso indenyl rings) (Fig. 32).

Fig. 32 Examples of Hf indenyl-phenoxide CG complexes.

4.06.4.1.4

Carbon-based ligands

4.06.4.1.4.1 Diene complexes In 2002 Hessen and co-workers reported the 14-electron alkyl complexes [Cp Hf(C6H10)R], for C6H10 ¼ 2,3-dimethyl-1,3-butadiene; R ¼ CH2SiMe3 and CH(SiMe3)2 (69Hf),860 synthesized by reaction of 16-electron complex [Cp Hf(C6H10)Cl(thf )] with the corresponding alkyl-lithium reagents in toluene. Complex 69Hf consists of a 1:1 mixture of two rotamers with respect to rotation around the Hf-alkyl bond and X-ray analysis showed these two isomers co-crystallize randomly in the solid state. Warming solutions of 69Hf to 80  C led to a rearrangement of the HfCH(SiMe3)2 group to give the complex [Cp Hf(C6H10)(CH2SiMe2CH2SiMe3)]. It was postulated that in this rearrangement the diene ligand acts as a reversible proton acceptor for one of the protons of a SiMe3 group of the alkyl. Reaction of [Cp Hf(C6H10)Cl(thf )] with 2 equivalents of LiCH(SiMe3)2 in THF resulted in deprotonation of one of the alkyl SiMe groups to furnish [Li][Cp Hf(C6H10)(k2C,C0 -CH{SiMe3}-SiMe2-CH-)] 70Hf
THFx (x ¼ 1 or 2), containing a 1-hafna-3-silacyclobutane ring. A crystal structure determination of 70Hf
THF showed that the Li-cation is bridging the methylene of the deprotonated alkyl group and one of the diene methylene groups. Reaction of [Cp Hf(C6H10)(CH2SiMe3)] with dihydrogen (6 bar) at room temperature led to the formation of the mixed-valence tetranuclear polyhydride [(Cp Hf{m-H}2)4(s1:s1:Z4:Z4-C6H8)] 71Hf, with a central 2,3-dimethyl-1,3-butadiene1,4-diyl fragment. An analogous hydrogenolysis reaction in the presence of LiCl resulted in chloride incorporation to yield the trinuclear species [Cp 3Hf3(s2:Z4:Z4-C6H8)(m-H)2Cl3] (72Hf) (Fig. 33).861

Fig. 33 Examples of half-sandwich Hf diene complexes reported by Hessen and co-workers.

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Norton and co-workers investigated the reaction of Ti and Hf diene complexes with isonitriles. [Cp HfCl(2,3-dimethylbutadiene)] undergoes a cycloaddition reaction with 2 equiv. of XylNC to afford 2,5-diazahafnacyclopentane complex 73Hf (Scheme 125).73 X-ray crystallography revealed a s-interaction between the CdC bond of a cyclopropane ring and the Hf center. This result contradicts a previous report by Teuben and co-workers, which proposed on the basis of NMR spectroscopic data that the product of the same reaction was a cyclic amidine complex.862

Scheme 125 Reaction of a half-sandwich Hf diene complex with xylylisocyanide reported by Norton and co-workers.

In a 2018 report, Norton investigated 2,3-dimethylbutadiene CG group 4 complexes including hafnacycle complex [(SiMe2-Cp N Bu)Hf(CH2dC{Me}]C{Me}-CH2)] (74Hf).76 X-ray analysis of 74Hf revealed a supine (s2,p) orientation of the diene ligands in the solid state. Reaction of 74Hf with 1 equiv. tBuNC resulted in the cyclic iminoacyl species 75Hf, via single isonitrile insertion into one of the HfdC bonds of the metallacyclopentene precursor (Scheme 126a). The short Hf1−N2 distance of 2.224(2) A˚ in 75Hf is consistent with Z2-coordination mode with “N-outside” in the iminoacyl fragment. Addition of a further equivalent of t BuNC led to formation of the unsymmetrical bis-insertion product, 76Hf, resulting from a hydride shift. Reaction of 74Hf with 1 equiv. XylNC resulted in a mixture of both bis-insertion products, of which symmetrical product 77Hf was structurally characterized. Metrical parameters in the diazametallacycle fragment were consistent with an enediamido rather than a diazabutadiene resonance form. t

Scheme 126 Reactions of half-sandwich Hf CG complexes with isonitriles reported by Norton and co-workers.

Reaction of CG dimethyl complex [(SiMe2-Cp -NtBu)HfMe2] with 2 equiv. of tBuNC afforded [(SiMe2-Cp -NtBu)Hf(Z2-C {Me}]NtBu}2] (78Hf, Scheme 126b) via bis-insertion into the HfdMe bonds. The reaction was monitored by NMR spectroscopy, and was found to be first order in tBuNC, suggesting that the energy barrier for coordination is higher than that for the insertion itself.

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Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

4.06.4.1.4.2 Mixed-sandwich complexes Tamm and co-workers reported phosphine-functionalized Cp,CHT Hf complexes [(Z7-C7H7)Hf(CpPR2)] R ¼ iPr (79Hf) and Ph (80Hf) which show monomeric and dimeric solid state structures, respectively.102 These researchers also reported the unsubstituted “trohafcene”, [(Z7-C7H7)HfCp] (81Hf), which is a formal 16 VE complex. The reactivity of 81Hf toward s-donor/p-acceptor ligands was investigated, forming 1:1 complexes [(Z7-C7H7)HfCp(CNR)] with two-electron donor ligands L ¼ NCXyl (82Hf), NHCMe4 (¼ 1,3,4,5-tetramethylimidazolin-2-ylidene) (83Hf), and PMe3 (84Hf) (Fig. 34).

Fig. 34 Examples of mixed-sandwich Hf complexes.

Cloke and co-workers reported the chemical reduction of mixed-sandwich CpR-cyclooctatetraeneyl complex [Cp Hf(Z8-C8H6{1,4-SiiPr3}2)Cl] (85Hf) with 1 equiv. KC8 to afford dinuclear complex 86Hf (Scheme 127). X-ray analysis of 86Hf reveals that two [Cp Hf(Z8-COT{1,4-SiiPr3}2)] moieties have dimerized via formation of a CdC bond and coupling of the COT rings to form an Z7,Z7-(C8H6{3,6-SiiPr3}2C8H6{3,6-SiiPr3}2) bridging ligand; each 8-membered ring thus behaves as a cyclooctatrienyl ligand.863

Scheme 127 Reduction of a mixed-sandwich Hf(IV) complex reported by Cloke and co-workers.

In 2000 Dysard and Tilley reported reported the synthesis and characterization of the aromatic germole dianion complex [(Cp {Z -C4Me4Ge}HfMe2Li{THF})2] (87Hf),864 which possesses a structure featuring lithium ions coordinated in both an Z1 and Z5 fashion by the germole dianion unit. The silylation of 87Hf produces the neutral germolyl complex [Cp (Z5-C4Me4GeSiMe3) HfMe2], which readily loses its -SiMe3 group to regenerate the germole dianion complex. The reaction of [Cp (Z5-C4Me4GeSiMe3) HfMe2] with (Et2O)LiCH2Ph produced Me3SiCH2Ph and 87Hf quantitatively. This result was attributed to favorable bonding between hafnium and the aromatic germole dianion, as Li[C4Me4GeSiMe3] did not react with (Et2O)LiCH2Ph under the same conditions. Reaction of Li(thf )[C4Me4GeCMe3] with 1 equivalent of [Cp HfMe2Cl] did not produce the desired Z5-germolyl species, but rather the migrated species [Cp (Z4-C4Me4Ge{Me}-CMe3)HfMe] (88Hf) in 40% yield. In a follow-up study the same year, Dysard and Tilley reported that 87Hf reacted rapidly with 2 equivalents of (PMe3)4RhOTf to furnish a Rh(III) zwitterionic complex [ansa-{Z4-C4Me4Ge-[Rh(PMe3)4H](Z5-C5Me4CH2)}HfMe2], via loss of LiOTf.865 Reactions of 87Hf with (dmpe)2MOTf (dmpe ¼ 1,2-bis(dimethylphosphino)ethane; M ¼ Rh, Ir) produced the complexes [Cp (Z5-C4Me4Ge {Rh(dmpe)2})HfMe2] (89Hf) and [Cp (Z5-C4Me4Ge{Ir(dmpe)2})HfMe2], respectively. These complexes contain the [GeC4Me4]2− ligand, which bridges the two metal atoms in a s,p fashion. Fang and Assoud reported the synthesis of monoanionic 1,2-azaborolyl (¼ Ab) ligands that closely relate to Cp ligands, and the application of these ligands in metal complexes.866 Mixed-sandwich CpR-azaboryl complex [CpMe4HHfCl2(1,2-BNC3H1,2,4{Me3} )] 2 (90Hf) was synthesized and structurally characterized. The Hf atom is more tightly bound to the the N and C atoms (2.46–2.51 A˚ ) of the the Ab ring, but loosely coordinated to B (2.65 A˚ ), and is hence shifted toward Z4-coordination (Fig. 35). 5

Fig. 35 Examples of mixed-sandwich Hf heterocyclic complexes.

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

4.06.4.1.5

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Main group ligands

Yamashita and Nozaki and co-workers reported the half-sandwich hafnium boryl complex [Cp Hf(CH2Ph)2(B{NDippCH}2)] (91Hf), which was one of the first examples of a group 4 metal boryl complexes.867 The solid state structure of 91Hf shows a HfdB bond length (2.497(4) A˚ ) closer to the lower end of the HfdB distances reported in hafnium carbollide complexes, and longer than the sum of the covalent radii of the atoms (Hf–B ¼ 2.32 A˚ ). Complex 91Hf in combination with [Ph3C][B(C6F5)4] forms an active catalyst system for both ethylene and 1-hexene polymerization, displaying activities comparable to those of other hafnium half-sandwich precatalysts.834,840,844,868 Fryzuk et al. reported the synthesis, structural characterization, and solution behavior of hafnium complexes stabilized by the multidentate ancillary ligand [P2Cp] ([P2Cp] ¼ [Z5-C5H3-1,3-(SiMe2-CH2PiPr2)2]).869 The reaction of [P2Cp]Li with HfCl4(THT)2 (THT ¼ tetrahydrothiophene) afforded the hafnium trichloride complex [(Z5-C5H3-1,3-{SiMe2-CH2PiPr2}2)HfCl3] (92Hf), the structure of which was determined by X-ray crystallography. Trichloride 92Hf is isostructural with the analogous zirconium complex, [P2Cp]ZrCl3, in the solid state, but in solution 92Hf exists as an equilibrium mixture of two isomers that interconvert by fluxional phosphine coordination. Treatment of 92Hf with 2 equiv. of KCH2Ph, followed by thermolysis yielded [(Z5-C5H3-1,3-{SiMe2-CH2PiPr2}2)HfCl(]CH{Ph})] (93Hf), the first structurally characterized hafnium alkylidene complex. X-ray analysis of 93Hf revealed this complex to be isostructural with the zirconium analogue [(P2Cp)Zr]CHPh(Cl)].870 The primary difference between the Hf systems and the Zr analogue is that metal-ligand bonding is stronger in the former, which accounts for shorter bond distances, a greater degree of chemically inertness, and the divergent solution behaviors observed. Saito and co-workers reported the reaction of dilithiostannole with [Cp2HfCl2] which afforded stannylene-hafnium complexes [CpHfCl(SnC4{SiMet2Bu}2Ph2)2] (94Hf) and [Cp2Hf(SnC4{SiMet2Bu}2Ph2)2] (202Hf, vide infra).871 X-ray analysis revealed the tin atoms are drastically deviated from the C4 plane of the butadiene moieties, leading to pyramid-like structures. The Hf atoms in 1-hafnacyclopenta-2,4-diene complexes 94Hf and 202Hf have formal electron counts of 12 VE and 16 VE, respectively. Complex 94Hf and 202Hf show 119Sn NMR signals at −1020 and −955 ppm, respectively in the typical region for Sn(0) species. DFT calculations reveal two types of lone pairs on the Sn atom, which is consistent with the assignment of 94Hf as a stannylone (Z4-butadiene)Sn(0) complex, rather than a stannylene or stannylene dication stabilized by a dianionic hafnacycle (Fig. 36).

Fig. 36 Examples of half-sandwich Hf main group complexes.

4.06.4.1.6

Cluster complexes

As discussed previously for titanium and zirconium, cyclopentadienyl ligands serve as an ideal “capping” ligand for hafnium centers in multimetallic hydride complexes and clusters. Hessen and co-workers reported the synthesis of group 4 half-sandwich N, N-dimethylaminopropyl complexes [Cp M({CH2}3NMe2)Cl2] (M ¼ Zr; Hf ) and [Cp M({CH2}3NMe2)2Cl] (M ¼ Zr; Hf ) by mono- or dialkylation of [Cp MCl3] with the corresponding alkyl-lithium and Grignard reagents.872 Hydrogenolysis of the monoalkyl species resulted in the formation of the polyhydride complexes, [Cp 3M3(m-H)4(m-Cl)2Cl3] M ¼ Zr; Hf, (95Hf) and [Cp MCl3]. A crystal structure determination of 95Hf revealed a fully asymmetric trinuclear structure with three widely differing Hf


Hf distances ranging from 3.061(6) A˚ (with bridging hydrides, Hf(1)


Hf(3)) to 3.721(7) A˚ (with a bridging chloride). The trinuclear polyhydride 95Hf reacts with 2,6-xylylisocyanide to give three distinct products, a m-enediamide complex, [(Cp HfCl2)2(m-Z1,Z1-N{Xyl}-CH]CH-N{Xyl}-)] (96Hf) which was structurally characterized, an imido complex, [(Cp Hf{mNXyl}Cl)2], and an aza-allyl species, [Cp Hf(Z3-CH2CHN{Xyl})Cl2]. The reactivity of 95Hf was rationalized as proceeding through initial cleavage of the trinuclear complex into the fragments “Cp 2Hf2(m-H)3Cl3” and “Cp HfHCl2,” followed by the separate reactivity of these fragments (Scheme 128).

Scheme 128 Reaction of a trinuclear Hf hydride complex with xylylisocyanide reported by Hessen and co-workers.

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Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

Early-late heterobimetallic hydrido complexeses comprising group 4 metals and iridium with Cp ligands were reported by Suzuki and co-workers.873 The hafnium complex, [(Cp HfCl2)(Cp Ir)(m-H)3] (97Hf), was synthesized by a salt metathesis reaction of [Cp HfCl3] with [LiCp IrH3]. Treatment of 97Hf with Me3SiCH2Li resulted in alkylation of the Hf center to afford [(Cp Hf {SiMe3}2)(Cp Ir)(m-H)3]. As part of extensive investigations polynuclear group 4 metal hydride clusters, Hou and co-workers reported the tetranuclear hafnium(III) complex [(CpMe4SiMe3Hf )4(m-H)8] (98Hf), which is composed of four “CpRHfH2” units.874 Yelamos and co-workers reported the thermolysis of half-sandwich group 4 trimethylsilylmethyl derivatives and their reactivity with N,N-dimethylamine-borane. Thermal treatment of [Cp Hf(CH2SiMe3)3] resulted in the elimination of SiMe4 and formation of the alkylidene-alkylidyne hafnium compound, [(Cp Hf )3({m-CH}3SiMe)(m3-CSiMe3)] (99Hf).875 The reaction of 99Hf with excess NHMe2BH3 afforded the dialkyl(dimethylamidoborane) complex [Cp Hf(CH2SiMe3)2(NMe2BH3)], which is unstable in solution with respect to the alkyl(dimethylamino)borane [B(CH2SiMe3)H(NMe2)], tetramethylsilane, and other minor byproducts. Dibrov and Ibers reported trinuclear clusters of Zr and Hf with the heteropolychalcogenide ligands, including [PPh4][(CpHf {m2-Se2})3(m3-O)(m3-TeSe3)] (100Hf) which was characterized by X-ray diffraction. The structure of the anion in 100Hf comprises a triangle of HfCp groups bridged by m2-Se2 groups and capped by a m3-TeSe3 and a m3-O group (Fig. 37).380

Fig. 37 Examples of multinuclear Hf cluster complexes.

Shuang-Feng Yin and co-workers reported hydroxy-bridged dinuclear hafnium cationic complex [(CpHf{OH2}3)2(m2-OH)2]4+ in 1:4 salts with perfluoro anions [A]− ¼ C8F17SO3 (101Hf),876 C4F9SO3 (102Hf) and C6F5SO3 (103Hf),877 which were found to be remarkably air-stable and water-tolerant (Scheme 129). Complexes 101Hf-103Hf serve as Lewis acid catalysts for various CdC and CdO bond-forming reactions, such as the Mukaiyama-aldol reaction, Mukaiyama-Michael addition, Michael addition and the Mannich reaction.876,877

Scheme 129 Synthesis of hydroxy-bridged dinuclear Hf complexes reported by Yin and co-workers.

4.06.4.2 4.06.4.2.1

Bis(cyclopentadienyl) hafnium chemistry Dinitrogen activation and functionalization

4.06.4.2.1.1 CpMe4 2 Hf complexes For the past 15 years Chirik and co-workers have pioneered the chemistry of hafnocene complexes for dinitrogen activation and functionalization. In 2006, the first example of a side-on bound hafnocene dinitrogen complex [(CpMe4 Hf )2(m2,Z2:Z2-N2)] 2 (104Hf, Scheme 130) was synthesized,878 via sodium amalgam reduction of the corresponding diiodide complex [CpMe4 HfI2] 2 under dinitrogen atmosphere. X-ray analysis of 104Hf reveals an NdN bond length of 1.423(11) A˚ , slightly elongated from that of the zirconocene compound (1.3773(3) A˚ ), suggesting that the Hf center is more reducing than the Zr congener. Complex 104Hf is reactive and undergoes 1,2-addition of dihydrogen to form the corresponding hydrido hafnocene diazenido derivative, [(CpMe4 HfH)2(m2,Z2:Z2-N2H2)] (105Hf), which was the first example of N2 hydrogenation in a Hf complex. Treatment of 104Hf 2 Hf )2(m-N4C2O2Ph2)] (106Hf), which was presumed to form via initial C]N with 2 equiv. of phenyl isocyanate afforded [(CpMe4 2 cycloaddition to coordinated N2, followed by C]O insertion into the newly formed HfdN bond.879 X-ray analysis of 106Hf established imido-like HfdN bonds, and 106Hf underwent reaction with additional PhNCO to furnish [(CpMe4 Hf )2(m-N5C3O3Ph3)] (107Hf), arising from C]O cycloaddition to the HfdN bond, to form a second NdC linkage 2 from coordinated N2. Reaction of 104Hf with alkyl halides, R-X (X ¼ Cl, Br, I), initially affords the corresponding “end on”

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Scheme 130 Reactivity of CpMe4 2 Hf complexes reported by Chirik and co-workers.

dinitrogen complexes [(CpMe4 HfX)(m2,Z1,Z1-N2)(HfRCpMe4 )], for R ¼ Me, X ¼ I (108Hf),880 with the R and X coordinated to 2 2 different Hf centers. Disproportionation of these products yields 104Hf and the monometallic [CpMe4 Hf(R)(X)] species.880 2 Addition of 2 equiv. of methyl triflate to 104Hf yielded a mixture of products, one of which was the end-on triflato hafnocene Hf{OTf})2(m2,Z1,Z1-N2)] (109Hf), resulting from 1,4-addition of methyl triflate.881 dinitrogen complex, [(CpMe4 2 Chirik and co-workers explored the CO-induced N2 cleavage in a series of zirconocene and hafnocene dinitrogen complexes. Hf )2(m:k2,k2-N2C2O2)] (110Hf), with Treatment of 104Hf with excess CO furnished the hafnocene oxamidide complex [(CpMe4 2 two formally Hf(II) centers bridging a [N2C2O2]4− core.402 Careful addition of 1.1 equivalents of CO to 104Hf resulted in a mixture of isomeric cyclometalated hafnocene isocyanate species, including the “tuck over” complex [(CpMe4Hf{m-CH2-Z5-C5Me3H}) )] (111Hf).402 Tuck-over complex, 111Hf, featuring terminal isocyanate and bridging fulvalene and (m-NH)(Hf{NCO}CpMe4 2 imido ligands, formed as a result of N2 cleavage coupled to NdH bond formation via CdH activation. Exposure of 104Hf to a 3:1 mixture of CO and H2 furnished [(CpMe4 Hf{H})(m-NH)(Hf{NCO}CpMe4 )] (112Hf),402 featuring a bridging imido and 2 2 terminal isocyanate and hydride ligands. Complex 110Hf could be alkylated by reaction with alkyl halides, for example 2 equiv. HfI)2(m-{CON(Me)}2)] (113Hf).402 Oxamide of methyl iodide resulted in dimethylation of the oxamide ligand, to afford [(CpMe4 2 functionalization was also possible with primary and secondary silanes, for example reaction of 110Hf with 2.2 equivalents of HfH)2(m-{CON(SiH2Ph)}2)] (114Hf). X-ray analysis revealed longer the HfdO and HfdN PhSiH3 yielded the oxamidate [(CpMe4 2 bonds in oxamidate complexes 113Hf and 114Hf with respect to the parent oxamidide 110Hf, consistent with a change in the oxidation state of the core from [N2C2O2]4− to [(NR)2C2O2]2−.

4.06.4.2.1.2 Ansa-bridges complexes The ansa-metallocenes accentuate the mixing of the metallocene frontier orbitals to favor interactions with the non-CpR ligands. Consequently, transitioning from the metallocene to the ansa-bridged congeners favors p-backbonding into the N2 bridge, and leads to an observed increase in the NdN distance. For example, Chirik reported the hafnocene dinitrogen complex of a ansa-bridged ligand system, [({Me2Si-Cp -Cp3-tBu}Hf )2(m2,Z2,Z2-N2)] (115Hf, Scheme 131),882 which shows an NdN bond length of 1.457(5) A˚ , the longest reported for such CpR2Hf complexes, although it is shorter than the values reported for the half-sandwich hafnium N2 compounds prepared by Sita. Exposure of 115Hf to 1 atm CO afforded the C1 symmetric hafnocene oxamidide complex [({Me2Si-Cp -Cp3-tBu}Hf )2(m:k2,k2-N2C2O2)] (116Hf-C1), whereas at higher CO pressures the C2 symmetric isomer [({Me2Si-Cp Cp3-tBu}Hf )2(m:k2,k2-N2C2O2)] (116Hf-C2) was obtained. Slow diffusion of 1.5 equivalents of CO to 115Hf furnished the C1 symmetric compound [({Me2Si-Cp -Cp3-tBu}Hf )(NCO)(m-NH)(Hf{Me2Si-Cp -Z1,Z5-C5H3-3-(CMe2CH2)}] (117Hf), featuring a terminal isocyanate ligand and a bridging imido in a near-linear disposition between the two Hf centers. Treatment of the hafnium oxamidide complexes with excess HCl yielded the hafnocene dichloride complex along with free oxamide H2NC(O)dC(O)NH2, thus confirming NdN bond cleavage coupled with NdC and CdC bond formation. A detailed computational study of the mechanism for this remarkable CO-assisted N2 cleavage promoted by 115Hf was reported by Schwarz and co-workers.883

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Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

Scheme 131 Reactivity of ansa-bridged hafnocene complexes reported by Chirik and co-workers.

Chirik and co-workers employed silanes and isocyanides as reagents for interception and functionalization of a proposed hafnocene m-nitrido intermediate. For example, treatment of 116Hf-C2 with nHexylSiH3 followed by 1 atm CO furnished, the C1 symmetric m-oxo hydrido hafnocene isocyanate, [({Me2Si-Cp -Cp3-tBu}HfH)(m-O)(Hf{NCO}{Me2Si-Cp -Cp3-tBu})] (118Hf).402 Reaction of 116Hf-C1 and 116Hf-C2 with 2 equiv. of tert-butyl isocyanate furnished [({Me2Si-Cp -Cp3-tBu} Hf )2(m-{C2O2N2(tBu)}2)], 119Hf-C1 and 119Hf-C2, respectively,402,884 and exposure of 116Hf-C2 to 2 equiv. of CO2 afforded [({Me2Si-Cp -Cp3-tBu}Hf )2(m-{C2O3N}2)] (120Hf).402 Formation of 119Hf and 120Hf results from cycloaddition of two molecules of the respective heteroallene to each of the hafnium-nitrogen bonds in 116Hf-C2 with concomitant formation of new NdC and HfdO bonds. Treatment of 116Hf with iodine yielded the monomeric iodo ansa-hafnocene isocyanate [(Me2Si-Cp -Cp3tBu )HfI(NCO)] (121Hf).402 When the reaction of 115Hf with 4 atm of CO was carried out at low temperature, an intermediate C1 symmetric species was observed, which was used for further CdC bond formations. Reaction of this intermediate with excess methyl iodide yielded the C1 symmetric complex [({Me2Si-Cp -Cp3-tBu}Hf )2(m2:k1O,k2C,N-{NMe}-C2O2)(I)(NCO)] (122Hf), and reaction with CO2 yielded the C1 symmetric complex [({Me2Si-Cp -Cp3-tBu}Hf )2(m-O)(NCO)2] (123Hf).884

4.06.4.2.1.3 Cp1,2,4-{Me3} Hf complexes 2 Chirik and co-workers explored similar chemistry of the Cp1,2,4-{Me3} Hf scaffold, with a view to accessing less sterically congested 2 and therefore more reactive dinitrogen species. The structurally characterized complex, [(Cp1,2,4-{Me3} Hf )2(m2,Z2,Z2-N2)] (124Hf, 2 885 Scheme 132), shows an NdN bond distance of 1.457(5) A˚ , longer than the value in 104Hf (1.423(11) A˚ ), and identical to the value in the ansa-bridged analogue, 115Hf.

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

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Scheme 132 Reactivity of Cp1,2,4-{Me3} Hf complexes reported by Chirik and co-workers. 2

Reaction of 124Hf with an equivalent of CySiH3 followed by CyCN yielded [(Cp1,2,4-{Me3} Hf )2(m2-N]C{H}{Cy})) 2 (m2,Z1,Z2-NN{SiH2Cy})] (125Hf), arising from the silylation of the coordinated N2 to form a side-on, end-on diazenido ligand.885 Silylation of 124Hf followed by thermolysis afforded [(Cp1,2,4-{Me3} Hf )(m2-k2N,N᾽,k1N-NSi{H}CyNH)(HfHCp1,2,4-{Me3} )] 2 2 (126Hf), containing a [NSi(H)CyNH]3− fragment, via cleavage of the NdN bond accompanied by silyl migration, and the formation of a hafnocene unit having a terminal hydride ligand.885 Silyation of 124Hf followed by addition of CO yielded Hf )2(m2:k1N,k1O-N]C{H}-O)(m-NSiH2Cy})] (127Hf), containing a new “formamidide” [NC(H)O]2− ligand, as [(Cp1,2,4-{Me3} 2 a result of NdN bond cleavage and NdC bond formation. Carbonylation of 124Hf with 4 atm CO afforded the tetranuclear Hf oxamidide complex, [(Cp1,2,4-{Me3} Hf{NCO})4] (128Hf), 2 via CO-induced N2 cleavage.886 The reaction of 128Hf with excess tert-butyl isocyanate was temperature dependant, giving [(Cp1,2,4{Me3} t 1,2,4-{Me3}  Hf{NCO})2(tBuNCO)2] (130Hf) at 130  C, a net functiona2Hf{NCO})4( BuNCO)2] (129Hf) at 65 C, and [(Cp2 lization of two of the four oxamidide HfdN bonds via [C]O] cycloaddition. Chirik and co-workers further demonstrated that 1,2-addition of primary silanes is an effective means of functionalizing HfH)2(m-{CON(hafnocene oxamidide cores. For example, reaction of 128Hf with 4 equiv. of CySiH3 furnished [(Cp1,2,4-{Me3} 2 SiH2Cy)}2)] (131Hf). Thermolysis of tetrametallic 128Hf or dinuclear hafnocene oxamidides 110Hf and 116Hf-C2 resulted in partial deoxygenation and isomerization to m-oxo dihafnocene complexes with terminal isocyanate and cyanide ligands, 132Hf, 133Hf, and 134Hf, HfI)(m-O)(Cp1,2,4respectively.886 Reaction of 132Hf with trimethylsilyl iodide furnished C2-symmetric hafnocene [(Cp1,2,4-{Me3} 2 {Me3} 2Hf{NCO})] (135Hf, Scheme 133), demonstrating that terminal cyanide is more labile than the isocyanate ligand, and undergoes preferential group transfer.886

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Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

Scheme 133 Reactivity of a Cp1,2,4-{Me3} Hf oxamidide complex reported by Chirik and co-workers. 2

In 2013, Chirik reported borylation of the coordinated N2 ligand in hafnocene dinitrogen complexes. Reaction of 124Hf with pinacolborane resulted in borylation of the side-on dinitrogen ligand with concomitant formation of a dihafnocene m-hydride, and Hf )2(m2-N]C{H}{Cy})(m2,Z2,Z1-Nsubsequent reactions with CyCN and CO afforded m-aldimine complex [(Cp1,2,4-{Me3} 2 2 NBPin)] (136Hf) and m-borylimido [(Cp1,2,4-{Me3} Hf ) (m -NBPin)(m ,Z -NC{H}O)] (137Hf), respectively.406 Reaction with 2 2 2 2 1,2,4-{Me3} 2 t 2 1 tert-butyl isocyanide yielded complex [(Cp2 Hf )2(m2,Z -{H}C]N{ Bu})(m2,Z ,Z -N-NBPin)] (138Hf) which was structurally characterized, confirming the boryldiazenido ligand and its side-on, end-on hapticity.406 The dihafnocene dinitrogen pyridine adducts, [(Cp1,2,4-{Me3} Hf )(m2,Z2,Z2-N2)(Hf{4-X-py}Cp1,2,4-{Me3} )] for X ¼ NMe2 2 2 2 2 1,2,4-{Me3} (139Hf) and OMe, were synthesized.887 Reaction of [(Cp1,2,4-{Me3} Hf )(m ,Z ,Z -N )(Hf{4-X-py}Cp )] with CO afforded 2 2 2 2 Hf )2(NCO)(m-N)(4-X-py)] for X ¼ NMe2 (140Hf) and OMe the isocyanato m-nitrido dihafnocene pyridine adducts, [(Cp1,2,4-{Me3} 2 (141Hf, Scheme 134). Nitride complexes were shown to be competent intermediates for subsequent carbonylation chemistry.

Scheme 134 Reactivity of Cp1,2,4-{Me3} Hf dinitrogen pyridine adducts reported by Chirik and co-workers. 2

A base-free m-nitrido dihafnocene isocyanate complex, [(Cp1,2,4-{Me3} Hf )2(NCO)(m-N)], was synthesized by exposure of 124Hf 2 to 1 equiv. of CO.888 This showed diverse NdC bond forming chemistry derived from cycloaddition of activated p-systems of an alkyne and hetero-cummulenes including CO2 as well as insertion of organic nitriles (Scheme 135). Reaction of [(Cp1,2,4-{Me3} 2 Hf )2(NCO)(m-N)] with cyclooctyne afforded [(Cp1,2,4-{Me3} Hf )2(m2-O)(m2,k2-(cyclooctenyl)NdC]N)] (142Hf), arising from 2 cycloaddition of the strained alkyne to the m-nitrido ligand followed by isocyanate deoxygenation and additional NdC bond formation. X-ray diffraction revealed the core of 142Hf is a six-membered ring containing two Hf centers, a bridging oxide and the C]CdN portion of the newly formed N-cyanamide ligand. Reaction of 142Hf with Me3SiCl furnished [(Cp1,2,4-{Me3} Hf{m2dN] 2 C]N-(cyclooctenyl)})2] (143Hf), arising from group transfer of the [N]C¼ N(cyclooctenyl)]2− ligands between Hf centers.

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377

Scheme 135 Reactivity of a Cp1,2,4-{Me3} Hf isocyanate imido species reported by Chirik and co-workers. 2

Reaction of [(Cp1,2,4-{Me3} Hf )2(NCO)(m-N)] with isonitriles RCN afforded the ligand-stabilized dihafnocene m-nitride 2 complexes, [(Cp1,2,4-{Me3} Hf )2(NCO)(RCN)(m2-N)] for R ¼ tBu (144Hf), Ph and Cy. Cyclohexyl isocyanide complex 2 [(Cp1,2,4-{Me3} Hf )2(NCO)(CyCN)(m2-N)] was unstable at room temperature, giving insertion of the unsaturated substrate into 2 the dihafnocene m-nitrido unit forming a [NC(Cy)N]3− amidinato ligand, which was subsequently trapped with xylyl isocyanide as a 1:1 adduct, [(Cp1,2,4-{Me3} Hf )2(NCO)(CNXyl)(m2-NC{Cy}N)] (145Hf). Alternatively, addition of a further equivalent of 2 cyclohexyl isocyanide afforded [(Cp1,2,4-{Me3} Hf )2(m2-NC{Cy}NCN{H}O)(C5H10C]C]N)] (146Hf), which features a unique 2 ureate-type core from CO-induced N2 splitting followed by nitrile insertion. X-ray analysis of 146Hf reveals one hafnocene is coordinated by a formally dianionic N,O-carbodiimidate-type ligand, whereas the other has a coordination sphere containing imidinato and amide coordination. Reaction of 146Hf with Me3SiCl yielded [(Cp1,2,4-{Me3} Hf )2(Cl)(m2-NC{Cy}NCN{H}O)] 2 (147Hf) with concomitant formation of 1-(trimethylsilyl)cyclohexane-1-carbonitrile, a result of electrophilic attack of the trimethylsilyl group on the ketimide carbon. Carboxylation of 146Hf resulted in the addition of one CO2 into the ketimide ligand with concomitant rearrangement to form an unusual carboxylate ligand in [(Cp1,2,4-{Me3} Hf )2(m2-O2CNC{Cy}NCN{H} 2 O)(k2-O2CC(C═N)C5H10)] (148Hf). Isocyanate imido complex [(Cp1,2,4-{Me3} Hf )2(NCO)(m-N)] also shows reactivity with heterocummulenes such as CO2 and 2 isocyanides, for example addition of tBuNCO and AdNCO resulted in ureate complexes 149Hf and 150Hf, respectively, via isocyanate cycloaddition.888 Reactivity of isocyanate imido species with electrophiles was explored, for example [(Cp1,2,4-{Me3} Hf )2(NCO)(m-N)] 2 reacts with Me3SiI to afford dihafnocene silyl ureate complex, [(Cp1,2,4-{Me3} Hf )2(I)(m2-NCONSiMe3)] (151Hf), arising from NdC 2 bond formation from the bridging hafnocene nitride and the terminal isocyanate.889 Reaction of the pyridine adduct [(Cp1,2,4-{Me3} Hf )2(NCO)(m-N)(py)] with methyl triflate afforded the dihafnocene ureate complex, [(Cp1,2,4-{Me3} Hf )2 2 2 (m2-OTf )(m2-NCONH)] (152Hf). Direct alkylation of was also reported, as addition of EtOTf to the hafnocene nitride cleanly Hf )2(m2-OTf )(m2-NCONEt)] (153Hf).889 furnished [(Cp1,2,4-{Me3} 2

4.06.4.2.2

Low valent CpR2Hf alkyne complexes

Further to their pioneering work on low valent Ti and Zr complexes Rosenthal and co-workers have also reported extensively on the synthesis and multifaceted reactivity of their hafnocene analogues. Owing to the stronger interaction of the hafnocene core, unusual bond activations become possible which are not favorable for its titanocene and zirconocene congeners. In general, hafnocene compounds show heightened reactivity compared to their titanocene and zirconocene analogues. In 2007, three-membered hafnocyclopropene complexes of the bis(trimethylsilyl)acetylene (¼ btsma) ligand, [Cp2Hf (Z2-btmsa)(PMe3)] (154Hf) and [Cp 2Hf(Z2-btmsa)] (155Hf) were synthesized via lithium reduction of CpR2HfCl2 in the presence of the alkene (Scheme 136).890 Larger coordination shifts in the IR and 13C NMR spectroscopic data for 155Hf are consistent with a

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Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

Scheme 136 Reactivity of a low valent hafnocene species with bis(trimethylsilyl)acetylene reported by Rosenthal and co-workers.

stronger interaction of the alkyne with Hf, in comparison to analogous compounds of Ti and Zr. At shorter reaction times, vinyl “tuck-in” complex [Cp 2Hf(Z5:Z1-C5Me3-CH2-)(Z1-CH]C{SiMe3}2)] (156Hf) was also isolated, arising from a tandem SidC and CdH bond cleavage reactions of btmsa by hafnocene.891 X-ray analysis of 156Hf reveals a near-linear Hf-Ca-Cb unit in the vinyl ligand and a a-agostic CdH interaction with the hafnium center. Thermolysis of 156Hf generated the metallacycle product [Cp 2Hf(k2C,C0 -C{H}]C{SiMe3}-SiMe2-CH2-)] (157Hf), as a result of H atom transfer back to the Cp ligand, and activation of a methyl CdH bond from the SiMe3 group.891 Lithium reduction of [Cp 2HfCl2] in the presence of disubstituted butadiynes was reported, for example the reaction with 1,4-bis(tert-butyl)butadiyne afforded [Cp Hf(-C{]C]CH-tBu}-CH{tBu}CH2-Z5-C5Me3-CH2-)] (158Hf, Scheme 137), containing a p-Z5,s-Z1-tetramethylfulvenyl ligand that is coupled to the modified substrate. Complex 158Hf reacted with 2 equiv. of t BuNC, to afford 159Hf via insertion and coupling of both molecules of the isocyanide.892

Scheme 137 Reactivity of a low valent hafnocene species with di-tert-butylbutadiyne reported by Rosenthal and co-workers.

The phenyl(trimethylsilyl)acetylene complex [Cp 2Hf(Z2-Me3SiC2Ph)] (160Hf) was synthesized by an analogous synthetic route, and was not accompanied by the formation of side products.893 Reaction of 160Hf with an equivalent of water furnished the alkenyl hydroxide complex [Cp 2Hf(OH)(Z1-C{SiMe3}]CHPh)] (161Hf, Scheme 138), with no agostic interaction between the Hf center and the b-hydrogen atom, unlike in similar alkenyl complexes of Ti and Zr.491,496 Exposure of 160Hf to 1 atm CO2 afforded a mixture of the regioisomeric hafnafuranones, including structurally characterized complex [Cp 2Hf(k2C,O-C {Ph}]C{SiMe3}dC(]O)dOd)] (162Hf), which forms via CO2 insertion into both HfdC bonds.893

Scheme 138 Reactivity of a hafnocene alkyne complex reported by Rosenthal and co-workers.

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379

In the context of studying metallacyclic intermediates in olefin oligomerization catalysis, Rosenthal reported the synthesis of [(EBTHI)Hf(Z2-btmsa)] and its reaction with ethylene to afford a hafnacyclopentene complex, [(EBTHI)Hf(k2-CH2CH2CH2CH2-)] (163Hf, Scheme 139).894 Similar ethylene reactivity patterns were reported for Zr analogues, however for Hf complexes these take place at much higher temperatures, which was attributed the higher stability of HfdC s-bonds compared to ZrdC s-bonds. Reaction of [(EBTHI)Hf(Z2-btmsa)] with styrene furnished the a,b-substituted hafnacyclopentane, [(EBTHI)Hf(k2C,C0 -CH{Ph}CH2-CH{Ph}-CH2)] (164Hf). The reaction of [(EBTHI)Hf(Z2-btmsa)] with p-CF3-styrene followed a different coupling route, giving the a,a-substituted product [(EBTHI)Hf(k2C,C0 -CH{4-CF3-C6H4}-CH2-CH{4-CF3-C6H4}-CH2-)] (165Hf).895 Complexes 163Hf and 164Hf upon activation with B(C6F5)3 gave active systems for the polymerization of ethylene, however, in each case the assumed active species could not be isolated.894,895

Scheme 139 Reactivity of an ansa-bridged hafnocene alkyne complex with alkenes reported by Rosenthal and co-workers.

In contrast to the Negishi system, [Cp2ZrCl2]/nBuLi which has found numerous applications as a Zr(II) source, the corresponding [Cp2HfCl2]/nBuLi is scarcely reported. In the reaction of [Cp2ZrCl2] with nBuLi at −78  C, the putatative dialkyl complex [Cp2Zr(nBu)2] is formed, which eliminates butane and 1-butene to give the free zirconocene. Rosenthal and co-workers were able to isolate and structurally characterize the analogous hafnium dialkyl complex, [CpR2Hf(nBu)2] for CpR ¼ Cp, Cp (166Hf).896,897 Upon reaction of [Cp2Hf(nBu)2] with di-tert-butyl-butadiyne, the first structurally characterized hafnacyclocumulene [Cp2Hf (Z4-tBuCt4Bu)] (167Hf, Scheme 141) was isolated.896 Reaction of [Cp2Hf(nBu)2] with bis(trimethylsilyl)butadiyne furnished the hafnacyclopentadiene [Cp2Hf(k2-C{C2SiMe3}]C {SiMe3}-C{C2SiMe3}-C{SiMe3}-)] (168Hf), and the analogous tert-butyl complex [Cp2Hf(k2-C{Ct2Bu}]C{SiMe3}-C{Ct2Bu}-C {SiMe3}-)] (169Hf) was synthesized by thermolysis of 167Hf.896 Reaction of [Cp2Hf(nBu)2] with diphenylbutadiyne led to the unexpected formation of the dinuclear hafnocene-substituted radialene complex, [(Cp2Hf )2(m-k2,k2-{C{Ph}]C{-}-C{-}]C {Ph}-}2)] (170Hf).896 Rosenthal had previously reported compounds with structural motifs similar to those of 168Hf, 169Hf and 170Hf for Ti, but surprisingly for Zr the formation of metallacyclopentadienes or radialenes was not observed. The reactivity of hafnacyclocumulene complex 167Hf with Lewis acids with investigated. Reaction with B(C6F5)3 gave the cationic complex, [(Cp2Hf )2(m-C{tBu}]CdC{^CtBu})((m-C{^CtBu})]+ (171Hf), which consists of two hafnocene units bridged unsymmetrically by a butadiyne and an acetylide ligand.898 Reaction of 167Hf with di-isobutyl aluminium hydride afforded the Cs symmetry complex, [Cp2Hf(k2-C{]CHtBu}-{m-AliBu2}-C{^CtBu})] (172Hf). Rosenthal and co-workers also reported X-ray structural data for hafnocene complexes 173Hf-180Hf (Scheme 140).899–905

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Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

Scheme 140 Reactivity of dialkyl hafnocene complexes with disubstituted butadiynes reported by Rosenthal and co-workers.

Erker and co-workers reported the reaction of Hf bis(alkynyl) complexes [Cp2Hf(C^CR)2] (R ¼ SiMe3,and tBu) with Piers’ borane, HB(C6F5)2, to afford the unique metallacyclic allenoid complexes [Cp2Hf(-CH{B{C6F5}2-C{R}]CH]C{R}-)] for R ¼ SiMe3 (181Hf) and tBu (182Hf, Scheme 141). The reaction proceeds via 1,1-hydroboration of the metal alkynyl complex and subsequent CdC coupling.906 X-ray analysis of 181Hf and 182Hf reveals in both cases a metallacyclic five-membered ring system with all four carbon atoms in bonding distance to hafnium.

Scheme 141 Reactivity of hafnocene bis(alkynyl) complexes with HB(C6F5)2 reported by Erker and co-workers.

4.06.4.2.3

Main group chemistry

4.06.4.2.3.1 Main group element dehydrocoupling Tilley and co-workers have investigated group 4 metal-main group element bonds, and their role in catalysis, in a number of studies using the Cp,Cp mixed-ring ligand platform. In 2002, the mixed-ring hafnium hydrostannyl complex, [CpCp HfCl(SnHMes2)] (183Hf) was reported. Complex 183Hf was found to be an intermediate in the dehydrocoupling of Mes2SnH2 to Mes2HSnSnHMes2, catalyzed by [Cp CpHf(H)Cl] (Scheme 142).907

Scheme 142 Hafnocene(IV) mediated dehydrocoupling of aryl stannanes reported by Tilley and co-workers.

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381

In 2005, Tilley reported triarylstannyl complexes featuring both ansa-bridged and mixed-ring ligand sets.908 Complex 184Hf was synthesized by reaction of bis(amido) complex [(Me2C-Cp2)Hf(NMe2)2] with HSnPh3 via amine elimination, however, this methylene-bridged ligand system gave Hf stannyl compounds that were unstable toward a-elimination. Mixed ring triarylstannyl complexes, [CpCp Hf(X)(SnPh3)] for X ¼ Cl (185Hf), NMe2 (186Hf), Me (187Hf), and OMe (188Hf), were synthesized via salt metathesis reactions of LiSnPh3 with the corresponding [CpCp Hf(X)Cl] reagent. Complex 185Hf was used for kinetic studies of a-aryl elimination reactions, which suggested a concerted mechanism involving nucleophilic attack of the migrating aryl group onto the electrophilic metal center (Fig. 38).

Fig. 38 Examples of Hf triarylstannyl complexes reported by Tilley and co-workers.

In 2003, Tilley and co-workers reported that zwitterionic and cationic hafnium complexes containing methyl or hydride ligands, such as [CpCp HfH(m-H)B(C6F5)3], are highly reactive toward organosilanes (Scheme 143a) and more reactive than analogous neutral complexes, such as [CpCp Hf(H)Cl].909 In a study the same year, Tilley reported that the Cp ligand was involved in the rapid decomposition of cationic [CpCp Hf(SiR3)]+ species.910 Hence, the more inert bis(cyclopentadienyl) hafnium silyl methyl complex, [Cp2Hf(SitBuPh2)Me] (189Hf), was synthesized, and its reaction with B(C6F5)3 afforded zwitterionic species, [Cp2Hf (SitBuPh2)(m-Me)B(C6F5)3] (190Hf), which is stable in solution at room temperature. Complex 190Hf showed reactivity in s-bond metathesis reactions with silanes (Scheme 143b). Reaction of 189Hf with the methide abstraction agent [Ph3C][B(C6F5)4] produced Ph3CCH3 and the dimeric, dicationic complex [(Cp2Hf{m-Br})2][B(C6F5)4]2 (191Hf).

Scheme 143 Reactivity of zwitterionic hafnocene complexes reported by Tilley and co-workers.

Tilley extended these studies to the heavier group 15 elements, showing that group 4 metallocenes act as catalysts for the dehydrocoupling of stibine, MesSbH2, to form the tetrastibene Sb4Mes4 (Scheme 144a). Hafnium hydride complex [CpCp Hf(H) Cl] was employed for studying the elementary reaction steps of stibine dehydrocoupling, which was found to follow a similar pathway to the a-stannylene elimination reaction, via a-hydrogen migration with stibinidene (:SbR) elimination from HfdSbHR derivatives.911 Reaction of [CpCp Hf(H)Cl] with DmpSbH2 (Dmp ¼ 2,6-dimesitylphenyl) afforded [CpCp HfCl(SbH {2,6-Mes2C6H3})] (192Hf) via elimination of H2, which was the first example of a group 4 stibine or stibide complex (Scheme 144b). Pursuing new routes to SbdSb bonded species, thermal decomposition of 192Hf gave [CpCp Hf(H)Cl] and distibene, DmpSb ¼ SbDmp.911

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Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

Scheme 144 Reactivity of hafnocene complexes with stibines reported by Tilley and co-workers.

A terminal stibinidene complex, [CpCp (Me3P)Hf]Sb(Dmp)], was synthesized by treatment of [CpCp HfMe(OTf )] with LiSbH(Dmp) in the presence of PMe3, presumably via a-hydrogen abstraction in a methyl stibide complex.912 The hafnium stibinidene could also be trapped via reaction with 2-butyne to furnish metallastibacyclobutene [CpCp Hf(k2C,Sb-C{Me}]C {Me}dSb{2,6-Mes2C6H3})] (193Hf, Scheme 144c), which was structurally characterized. Formation of 193Hf proceeds via a formal [2 + 2]-cycloadditon of an alkyne to a stibinidene intermediate, reminiscent of the cycloaddition reactivity observed for Zr-phosphinidene complexes. A bis(stibido) derivative, [CpCp Hf(SbMes2)2] (194Hf, Scheme 144d), was also synthesized and structurally characterized.913 Xylylisocyanide was shown to react rapidly with 194Hf via HfdSb bond insertion, however, other ligands such as carbon monoxide or diphenylacetylene induced reductive elimination of Sb2Mes4. Thermolysis or the addition of oxidants such as O2 or I2 also induce reductive elimination of Sb2Mes4 from 194Hf by unknown mechanisms. Tilley and co-workers also explored phosphinidene transfer chemistry involving a d0 hafnocene fragment. Phosphido alkyl complex [CpCp HfMe(PHPh)] (195Hf) was synthesized via salt metathesis from [CpCp HfMe(OTf )] and LiPHPh (Scheme 145).914 Thermolysis of 195Hf furnished triphosphanato complex, [CpCp Hf(P3Ph3)], with concomitant methane

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383

Scheme 145 Hafnocene-mediated phosphinidene transfer chemistry reported by Tilley and co-workers.

formation suggesting initial formation of the intermediate phosphinidene complex [CpCp Hf]PPh]. Effective trapping of the phosphinidene intermediate was achieved by reaction with 2-butyne to give the [2 + 2]-cycloaddition product [CpCp Hf(k2-P,C:P {Ph}C{Me}]C{Me})]. 4.06.4.2.3.2 Complexes with silicon ligands In 2005 Marschner and co-workers reported the first example of a group 4 disilene complex, [Cp2Hf(Z2-Si2{SiMe3}4)] by reaction of [Cp2HfCl2] with MgBr2 followed by 1,2-dipotassiumtetrakis-(trimethylsilyl)disilane. Compound [Cp2Hf(Z2-Si2{SiMe3}4)] could not be isolated in the solid state, however, its trimethylphosphine adduct, [Cp2Hf(Z2-Si2{SiMe3}4)(PMe3)] (196Hf) was synthesized and structurally characterized.915 X-ray analysis revealed structural features consistent with a metallacycle Hf-disilene bonding situation in 196Hf, supported by 29Si NMR spectroscopic studies and DFT calculations. In 2020 Scheschkewitz and co-workers reported the reaction of the ligato-lithiated siliconoid (2,3,3,6,6-Tipp4Si6)Li (Tipp ¼ 2,4,6-triisopropylphenyl) with [Cp2MCl2] reagents (M ¼ Zr, Hf ), to afford the corresponding the corresponding group 4 metalated siliconoids, [Cp2MCl(Tipp4Si6)] for M ¼ Zr and Hf (197Hf).734 X-ray analysis of 197Hf reveals a HfdSi bond length (2.7702(9) A˚ ) which is shorter than those reported for tetracoordinated Hf complexes [Cp2HfCl(Si{SiMe3}3)] (198Hf), [Cp2Hf(Si {SiMe3}3)2] (199Hf) and [Cp2HfCl((Si{SiMe3}2silatranyl)] (200Hf) reported by Marschner and co-workers (range ¼ 2.835–2.888 A˚ ).724,916 Sekiguchi and co-workers reported the reaction of 1,1-dilithiosilane with [CpEt 2 HfCl2] to afford the 16 VE hafnium-silylene t 917 phosphine complex, [CpEt Complex 2 Hf ¼ Si(SiMe Bu2)2], which was isolated as the 18 VE triphenylphosphine adduct 201Hf. 201Hf represented the first example of a compound with a Si]Hf double bond as well as a Schrock-type silylene complex. Double bond character between the silicon and hafnium atoms what evidenced by a SidHf bond length of 2.6515(9) A˚ , which is approximately 5% shorter than those of related complexes with SidHf single bonds, 196Hf (2.8309(6) and 2.8332(5) A˚ )915 and 199Hf (2.850(4) A˚ ) (Fig. 39).916 4.06.4.2.3.3 Germylene complexes Following Saito and co-workers report of stannylene-hafnium complex, [Cp2Hf(Z4p-SnC4{SiMet2Bu}2Ph2)2] (202Hf).871 Müller and co-workers reported the first example of a germylene-hafnium species, [Cp2Hf(s2p-GeC4{SiMe2R}2Me2)] R ¼ Me (203Hf),918 t Bu, (204Hf)919 synthesized by reaction between dipotassium germole dianion and [Cp2HfCl2], via the anionic hafnium germylene complex [K][Cp2HfCl(]GeC4{SiMe3}2Me2)] (205Hf).919 X-ray analysis of 203Hf and its silylene analogue [Cp2Hf (s2p-SiC4{SiMe3}2Ph2)] (206Hf)920 reveals the same unusual bicyclo[2.1.1]hexene-like structure with the Cp2Hf fragment and the group 14 atom at the bridging positions. These carbene analogues are stabilized by homoconjugation between the disubstituted group 14 atom and the remote C]C bond (Fig. 40). Reactivity studies demonstrated that the germylene serves predominately as a nucleophile, and in complexes with low-valent transition metals as a strong s-donor. For example, reactions of 203Hf with metal carbonyl complexes [Fe2(CO)9] and

Fig. 39 Examples of hafnocene complexes with Si ligands.

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Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

Fig. 40 Examples of main group hafnocene complexes reported by Saito and co-workers.

[W(CO)5(thf )] gave the bimetallic germylene complexes [Cp2Hf(s2p-Ge{Fe(CO)4}C4{SiMe3}2Me2)] (207Hf) and [Cp2Hf(s2p-Ge{W(CO)5}C4{SiMe3}2Me2)] (208Hf), respectively.918 Complex 203Hf undergoes a reversible reaction with half an equivalent of [Ni(cod)2] to form the bis-germylene nickel complex [(Cp2Hf{s2p-GeC4(SiMe3)2Me2})2(m2-Ni{cod})] (209Hf).919 Two products were isolated from the reaction of 203Hf with [Ph3PAuCl], the cationic gold germylene complex [CpHfCl2(Z5-Ge {AuPPh3}C4{SiMe3}2Me2)] (210Hf) and the Z5-germole complex [Cp2Hf(s2p-Ge{AuPPh3}{Z1-Cp}C4{SiMe3}2Me2)] (211Hf) (Fig. 41).919

Fig. 41 Examples of hafnocene germylene complexes reported by Saito and co-workers.

Reaction of 205Hf with the strongly s-donating tetramethylimidazolylidene (¼ NHCMe4) resulted in the substitution of one Cp ligand and the unusual Z5-germolediide hafnium complex, [CpHfCl(Z5-GeC4{SiMe3}2Me2)(NHCMe4)] (212Hf, Scheme 146a).919 Reaction of 203Hf with 1,3-diisopropyl 4,5-dimethylimidazol-2-ylidene (¼ NHCiPr2Me2) resulted in deprotonation of one of the hafnocene Cp-ligands by the NHC and the germyl anion paired with the formed imidazolium cation, [(NHCiPr2Me2)H][CpHf(m:Z5,Z1-C5H4)(s2p-GeC4{SiMe3}2Me2)] (213Hf, Scheme 146b).

Scheme 146 Reactivity of hafnocene germylene complexes with NHCs reported by Saito and co-workers.

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

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Scheme 147 Decomposition of dichalcogenadigermetanes reported by Saito and co-workers.

The oxidation of 205Hf by the elemental chalcogens sulfur, selenium, and tellurium resulted in the formation of the corresponding di-nuclear hafnocene complexes [(Cp2Hf{s2p -GeC4(SiMe3)2Me2})2(m-E)2] for E ¼ S (214Hf), Se (215Hf) and Te (216Hf), that are linked by 2,4-dichalcogena-1,3-digermetanes.921 Complexes 214Hf and 215Hf decompose to give hafnocenetanes [Cp2Hf(m-S)2(GeC4{SiMe3}2Me2)] for E ¼ S (217Hf), Se (218Hf), respectively (Scheme 147), whereas ditelluradigermetane 216Hf proved to be stable at room temperature.921 Reaction of 205Hf with Lewis base B(C6F5)3 afforded the BdGe bonded adduct [Cp2Hf(s2p-Ge{B(C6F5)4}C4{SiMe3}2Me2)] (219Hf, Scheme 148).922 Müller and co-workers proposed that the Lewis-acid base reaction proceeds via a single electron transfer process, and the radical pair [Cp2Hf(p2-GeC4{SiMe3}2Me2)][B(C6F5)4] (220Hf) was identified as an intermediate. The radical cation in 220Hf is a hafnocene(III) complex, formed by an intramolecular electron transfer process from the ligand to the Hf center.922

Scheme 148 Reactivity of hafnocene germylene complexes with Lewis acids reported by Saito and co-workers.

4.06.4.2.3.4 Hafnoceneophanes In 2005 Braunschweig et al. reported ansa-bridged metallocenes of the group 4 metals featuring a borane-diyl bridge (i.e. a B] NR2 moiety), including structurally characterized Hf examples of [1]-borametalloceophanes [({R2N}B-Cp-Cp)HfCl2] R2 ¼ Me2 (221Hf), (CH2)2 (222Hf)923 and a [2]-borametalloceophane [({NMe2}2B2-Cp-Cp)HfCl2] (223Hf) (Fig. 42).924 X-ray analysis reveals a single boron atom in the bridge result in a larger Ct–Hf–Ct tilt angle for 221Hf (a ¼ 64.7 , d ¼ 121.4 defined in Fig. 43) compared with the two-atom bridge in 223Hf (a ¼ 51.8 , d ¼ 130.7 ). Furthermore, the [1]bora bridge causes high rigidity and constrains the Cp rings to an eclipsed orientation in 221Hf, whereas in 223Hf they are staggered. Preliminary ethylene polymerization experiments showed that borahafnocenophanes form active catalyst systems after activation with MAO, with reported activities for [2]bora bridged 223Hf about 100 times higher than those of 221Hf and 222Hf.923,924 Following a similar “fly trap” synthesis route, Braunschweig reported other [2]diborametallocenophanes [({NMe2}2B2-CpR2)HfCl2] with CpR2 ¼ Flu2 (fluorenyl, C13H8) 224Hf,925 Cp-Flu (225Hf)926 and Cp-Octaflu (¼ octamethyloctahydrodibenzofluorenyl, C29H36) (226Hf).927 The higher steric demand of the two fluorenyl ligands in 224Hf lead to a slightly larger tilt angle (a ¼ 60.7 ,

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Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

Fig. 42 Examples of main group element bridged hafnocenophanes.

Fig. 43 Definition of structural parameters in hafnocenophanes.

d ¼ 134.8 ), and a marked distortion toward Z3-bonding as indicated by HfdC bond lengths ranging between 2.42 and 2.69 A˚ . Exposure of 224Hf to UV light resulted in a rearrangement reaction to N,N,N0 ,N0 -tetramethyldispiro[fluorene-9,20 -[1,3]diboretane40 ,900 -fluorene]-10 ,30 -diamine, which was structurally characterized. A series of group 4 metallocenophane dichlorides containing the heavier group 14 element tin as a bridging element were also reported, including the structurally characterized Hf complex [({tBu}2Sn2-Cp2)HfCl2] (227Hf).928 4.06.4.2.3.5 Other main group CpR2Hf complexes Reid and co-workers reported a series of group 4 chalcogenolate complexes, including the hafnocene selenolate [Cp2Hf(SeMe)2] (228Hf), which was structurally characterized.929 The Cp2Hf(SetBu)2 derivative served as a single source precursor for the chemical vapour deposition of HfSe2 as an intensely coloured thin films with the CdI2 structure. Wang and Ghosh and co-workers reported the condensation reaction of [BH3.THF] with group 4 metal borohydrides to afford heptaborane complexes, including the Hf complex [(Cp2Hf )2(B9H11)] (229Hf).930 X-ray analysis of 229Hf revealed the presence of a pentagonal bipyramidal B7 core. Theoretical calculations showed the role of the Hf center in stabilizing the borane fragment via multicenter covalent bonds related to the two exo-{Cp2Hf} units, as well as electrostatic interactions between the {Cp2Hf} units and the B7 core. Kiplinger and co-workers reported the reaction of [Cp 2HfMe2] with excess diphenyldiazomethane to afford the mono(hydrazonato) complex [Cp 2Hf(Z2N,N0 -CH3-N-N]CPh2)(Me)] (230Hf),931 whereas the analogous reaction of actinide dimethyl metallocenes (M ¼ U, Th) with diphenyldiazomethane give bis(hydrazonato) complexes. In 2001 Fu and co-workers reported the C2-symmetric complex [rac-(Z5-PC4H{2-Ph}{3,4-Me2})2HfCl2] (231Hf), which is the first and (to date) only example of Z5-phospholyl complex of hafnium to have been synthesized and structurally characterized (Fig. 44).814

Fig. 44 Examples of main group hafnocene complexes.

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Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

4.06.4.3

Table of crystallographically characterized Hf compounds Hf oxidation state

31

HfCp complex

P chemical shift (ppm)

Number of complex

[(Cp HfCl{m-Dipp})2] [Cp(tBu3PN)2HfCl] [Cp (tBu3PN)HfCl2] [Cp HfCl2(NHDipp)] [Cp HfCl2(k2-N{tBu}C{CH2CH2Ph}N{Et})] [Cp HfCl2(k2-N{tBu}C{CH2CH2Cl}N{Et})] [Cp HfCl2(k2-N{tBu}C{CH2SiMe3}N{Et})] [Cp HfMe2(k2-N{Et}C{Me}N{tBu})] [Cp Hf(iBu)2(k2-N{Et}C{Me}N{tBu})] [Cp HfCl(tBu)(k2-(N{Et}C{Me}N{tBu})] [Cp HfMe2(k2-N{Et}C{Me}N{Et})] [Cp HfCl2(k2-N{Et}C{CF3}N{tBu})] [Cp HfMe2({CH2Ph}k2-NimcapN)] [Cp HfMe2({CH2Naph}k2-NimcapN)] [Cp HfMe2({tBu}k2-NimcapN)] [Cp HfMe2(k2-MeS{NSiMe3}2)] [(CpMe4Hf{k2-C(NMe2)(NiPr)2})2(m-Z2:Z2-N2)] [(Cp Hf{k2-C(NMe2)(NiPr)2})2(m-Z2:Z2-N2)] [(CpMe4Hf{k2-C(Me)(NiPr)2})2(m-Z2:Z2-N2)] [(Cp Hf{k2-C(Me)(NiPr)2})2(m-Z2:Z2-N2)] [(Cp Hf{k2-C(Me)(NEt)2})2(m-Z2:Z2-N2)] [(CpMe4Hf{k2-C(NMe2)(NiPr)2})2(m-H)(m-Z1:Z2-NN{SiH2Ph})] [(CpMe4Hf{k2-C(NMe2)(NiPr)2})2(m-H)(m-Z2:Z2-NN{H})] [(CpMe4Hf{k2-C(NMe2)(NiPr)2})2(m-Br)(m-Z2:Z2-NN{Et})] [(Cp Hf{k2-C(Me)(NEt)2})2(m-Br)(m-Z2:Z2-NN{Et})] [(CpMe4HfBr{k2-C(Me)(NiPr)2})2(m-Z2:Z2-N2)] [Cp HfMe(k2N,N0 -{Dipp}NC](CH2Al{C6F5}3)C](CH2)N{Dipp}}] [Cp HfMe2(k2N,N0 -3-{4-MeOC6H4}N]C{H}-C4H3N)] [Cp HfMe2(k2N,N0 -3-{Tol}N]C{H}-C4H3N)] [Cp HfMe2(k2N,N0 -3-{Xyl}N]C{H}-C4H3N)] [Cp HfMe2(k2N,N0 -3-{tBu}N]C{H}-C4H3N)] [Cp HfMe2(k2N,N0 -{py-2-(Dipp)})] [Cp HfMe2(k2N,N0 -{py-2-(Dipp)-6-Me})] [Cp HfMe2(k2N,N0 -{py-2-(Dipp)-6-Cl})] [CpMe4HfCl2(k2C,N-C6H4CH2NMe2)] [Cp HfCl2(k2C,N-C6H4CH2NMe2)] [CpMe4HfCl3] [Cp1,3-{tBu2}HfCl3] [Cp1,3-{tBu2}HfMe3] [(Cp HfCl{m-NC6H4-2-(CH2NMe2)})2] [(CpMe4HfCl{m-N(Dipp)})2] [(Cp HfCl{m-N(Dipp)})2] [CpMe4HfCl(k2C,N-C6H4-2-{CH2NMe2})(k1N-NHC6H4-2-{CH2NMe2})] [CpMe4HfCl2(k2N,N-{2-(MeO)-C6H4}NC{Me}CHC{Me}N{2-(MeO)-C6H4})] [Cp∗HfCl2(k2N,N-{2-(MeO)-C6H4}NC{Me}CHC{Me}N{2-(MeO)-C6H4})] [Cp HfCl(k2O,N-2-O{Me}-C6H4-NH)2] [Cp HfCl3(k2O,N-2-O{Me}-C6H4-NH2)] [Cp HfCl2(k2N,O-2-{4,5-dihydro-1,3-oxazol-2-yl}-4,6-tBu2C6H2)] [Cp Hf(NMe2)(k3N,N0 ,O-N{Ph}C{NMe2}NCH{Ph}CH2O)] [Cp HfMe({Z2-ONMeCH2}2)] [Cp Hf(k2N,N0 -1-N{Xyl}-C4H7N)(NNPh2)(N{SiMe3}2)Li] [Cp Hf(k2N,N0 -1-N{Xyl}-C4H7N)(NNPh2)(DMAP)] [Cp HfCl2(O-2,6-Ph2C6H3)] [Cp HfCl2(O-2,6-tBu2C6H3)] [Pn (H)Hf(OXyl)3] [(Pn (H)HfCl3)2] [(Me2Si-IndPr-NtBu)HfCl2] [(Me2Si-Z5-C29H36-NtBu)HfBr2(OEt2)] [(Me2Si-Z1-C29H36-NtBu)Hf(CH2Ph)2]

+4/+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4/+4 +4/+4 +4/+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4

n/a 37.4 46.3 n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a

1Hf 2Hf 3Hf 4Hf 5Hf 6Hf 7Hf 8Hf 9Hf 10Hf 11Hf 12Hf 13Hf 14Hf 15Hf 16Hf 17Hf 18Hf 19Hf 20Hf 21Hf 22Hf 23Hf 24Hf 25Hf 26Hf 27Hf 28Hf 29Hf 30Hf 31Hf 32Hf 33Hf 34Hf 35Hf 36Hf 37Hf 38Hf 39Hf 40Hf 41Hf 42Hf 43Hf 44Hf 45Hf 46Hf 47Hf 48Hf 49Hf 50Hf 51Hf 52Hf 53Hf 54Hf 55Hf 56Hf 57Hf 58Hf 59Hf

Reference 829 830 830 831 833 833 833 834 834 338 835 836 842 842 842 932 334 334 334 334 334 334 334 334 334 334 843 844 844 845 845 846 846 846 847 847 847 393 848 831 831 831 849 849 849 849 322 325 850 851 851 852 852 853 853 854 855 855 (Continued)

388

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

HfCp complex [(iPr2NB-Cp-N{Ph})Hf(NMe2)2] [({iPr2N}P-Ind-C2B10H10)Hf(NMe2)2] [({iPr2N}P-Ind-C2B10H10)Hf(NEt2)2] [(Me2C-Cp-{Z6-C2B9H10})Hf]2[Li(thf )4]2 [(Me2C-Cp-{Z6-C2B9H10})Hf]2[PPN]2 [(4,6-C6Ht2Bu2-Ind-O)Hf(NEt2)2] [(4,6-C6Ht2Bu2-Ind2-Me-O)Hf(NEt2)2] [(4,6-C6Ht2Bu2-Ind2,3-{Me2}-O)2Hf] [(4,6-C6Ht2Bu2-Ind2,3-{Me2}-O)2Hf] [Cp Hf(C6H10)(CH{SiMe3}2})] [Li][Cp Hf(C6H10)(k2C,C 0 -CH{SiMe3}-SiMe2-CH-)] [(Cp Hf{m-H}2)4(s1:s1:Z4:Z4-C6H8)] [Cp 3Hf3(s2:Z4:Z4-C6H8)(m-H)2Cl3] [Cp HfCl((3,4-dimethylbicyclo[3.1.0]hex-3-ene-1,6-diyl)bis({2,6-dimethylphenyl) azanide})] [(SiMe2-Cp -NtBu)Hf(CH2-C{Me}]C{Me}-CH2)] [(SiMe2-Cp -NtBu)Hf(k1C:Z2N,C-CH2-C{Me}]C{Me}-CH2C]N{tBu})] [(SiMe2-Cp -NtBu)Hf(N1,N6-{tBu2}-3,4-dimethylbicyclo[3.1.0]hex-3-ene-1,6-diamine)] [(SiMe2-Cp -NtBu)Hf(N,N 0 -bis(2,6-dimethylphenyl)-4,5-dimethylcyclohexa-1,4-diene1,2-diamino)] [(SiMe2-Cp -NtBu)Hf(Z2-C{Me}]NtBu}2] [(Z7-C7H7)Hf(CpPiPr2)] [(Z7-C7H7)Hf(CpPPh2)] [(Z7-C7H7)HfCp] [(Z7-C7H7)HfCp(CNXyl)] [(Z7-C7H7)HfCp(NHCMe4)] [(Z7-C7H7)HfCp(PMe3)] [Cp Hf(Z8-C8H6{1,4-SiiPr3}2)Cl] [(Cp Hf )2(m:Z7,Z7-C8H6{3,6-SiiPr3}2C8H6{3,6-SiiPr3}2)] [Cp (Z5-C4Me4Ge)HfMe2Li(THF)]2 [Cp (Z4-C4Me4Ge{Me}-CMe3)HfMe] [Cp (Z5-C4Me4Ge{Rh(dmpe)2})HfMe2] [CpMe4HHfCl2(1,2-BNC3H1,2,4{Me3} )] 2 [Cp Hf(CH2Ph)2(B{NDippCH}2)] [(Z5-C5H3-1,3-(SiMe2-CH2PiPr2)2)HfCl3] [(Z5-C5H3-1,3-(SiMe2-CH2PiPr2)2)HfCl(]CH{Ph})] [CpHfCl(SnC4{SiMet2Bu}2Ph2)2] [Cp 3Hf3(m-H)4(m-Cl)2Cl3] [(Cp HfCl2)2(m-Z1,Z1-N{Xyl}-CH]CH-N{Xyl}-)] [(Cp HfCl2)(Cp Ir)(m-H)3] [(CpMe4SiMe3Hf )4(m-H)8] [(Cp Hf )3({m-CH}3SiMe)(m3-CSiMe3)] [PPh4][(CpHf(m2-Se2))3(m3-O)(m3-TeSe3)] [{CpHf(OH2)3}2(m2-OH)2][C8F17SO3]4 [{CpHf(OH2)3}2(m2-OH)2][C4F9SO3]4 [{CpHf(OH2)3}2(m2-OH)2][C6F5SO3]4

Hf oxidation state

31 P chemical shift (ppm)

Number of complex

Reference

+4 +4 +4 +4/+4 +4/+4 +4 +4 +4 +4 +4 +4 +3/+4/+3/+4 +4/+4/+4 +4

n/a n/a n/a n/a Not reported n/a n/a n/a n/a n/a n/a n/a n/a n/a

60Hf 61Hf 62Hf 63Hf 64Hf 65Hf 66Hf 67Hf 68Hf 69Hf 70Hf 71Hf 72Hf 73Hf

856 857 857 858 858 859 859 859 859 860 860 861 861 73

+4 +4 +4 +4

n/a n/a n/a n/a

74Hf 75Hf 76Hf 77Hf

76 76 76 76

+4 +4 +4 +4 +4 +4 +4 +4 +4/+4 +4/+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +3/+3/+3/+3 +4/+4/+4 +4/+4/+4 +4/+4 +4/+4 +4/+4

n/a 5.2 −17.3 n/a n/a n/a −50.3 n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a Not reported n/a n/a n/a

78Hf 79Hf 80Hf 81Hf 82Hf 83Hf 84Hf 85Hf 86Hf 87Hf 88Hf 89Hf 90Hf 91Hf 92Hf 93Hf 94Hf 95Hf 96Hf 97Hf 98Hf 99Hf 100Hf 101Hf 102Hf 103Hf

76 102 102 933 933 933 933 863 863 864 864 865 866 867 869 869 871 872 872 873 874 875 380 876 877 877

HfCp2 complex

Hf oxidation state

31P chemical shift (ppm)

Number of complex

Reference

2 2 [(CpMe4 2 Hf )2(m2,Z :Z -N2)] Me4 [(Cp2 HfH)2(m2,Z2:Z2-N2H2)] [(CpMe4 2 Hf )2(m-N4C2O2Ph2)] [(CpMe4 2 Hf )2(m-N5C3O3Ph3)] 1 1 Me4 [(CpMe4 2 HfI)(m2,Z ,Z -N2)(HfMeCp2 )] Me4 1 1 [(Cp2 Hf{OTf})2(m2,Z ,Z -N2)] 2 2 [(CpMe4 2 Hf )2(m:k ,k -N2C2O2)] Me4 [(Cp Hf{m-CH2-Z5-C5Me3H})(m-NH)(Hf{NCO}CpMe4 2 )] Me4 [(CpMe4 2 Hf{H})(m-NH)(Hf{NCO}Cp2 )]

+4/+4 +4/+4 +4/+4 +4/+4 +3/+3 +3/+3 +4/+4 +4/+4 +4/+4

n/a n/a n/a n/a n/a n/a n/a n/a n/a

104Hf 105Hf 106Hf 107Hf 108Hf 109Hf 110Hf 111Hf 112Hf

878 878 879 879 880 881 402 402 402

389

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

HfCp2 complex [(CpMe4 2 HfI)2(m-{CON(Me)}2)] [(CpMe4 2 HfH)2(m-{CON(SiH2Ph)}2)] [({Me2Si-Cp -Cp3-tBu}Hf )2(m2,Z2,Z2-N2)] [({Me2Si-Cp -Cp3-tBu}Hf )2(m:k2,k2-N2C2O2)] C1 isomer [({Me2Si-Cp -Cp3-tBu}Hf )2(m:k2,k2-N2C2O2)] C2 isomer [({Me2Si-Cp -Cp3-tBu}Hf )(NCO)(m-NH)(Hf{Me2Si-Cp Z1,Z5-C5H3-3-(CMe2CH2)}] [({Me2Si-Cp -Cp3-tBu}HfH)(m-O)(Hf{NCO}{Me2Si-Cp -Cp3-tBu})] [({Me2Si-Cp -Cp3-tBu}Hf )2(m-{C2O2N2(tBu)}2)] C1 isomer [({Me2Si-Cp -Cp3-tBu}Hf )2(m-{C2O2N2(tBu)}2)] C2 isomer [({Me2Si-Cp -Cp3-tBu}Hf )2(m-{C2O3N}2)] [(Me2Si-Cp -Cp3-tBu)HfI(NCO)] [({Me2Si-Cp -Cp3-tBu}Hf )2(m2:k1O,k2C,N-{NMe}-C2O2)(I)(NCO)] [({Me2Si-Cp -Cp3-tBu}Hf )2(m-O)(NCO)2] [(Cp1,2,4-{Me3} Hf )2(m2,Z2,Z2-N2)] 2 [(Cp1,2,4-{Me3} Hf )2(m2-N]C{H}{Cy}))(m2,Z1,Z2-NN{SiH2Cy})] 2 1,2,4-{Me3} [(Cp2 Hf )(m2-k2N,N 0 ,k1N-NSi{H}CyNH)(HfHCp1,2,4-{Me3} )] 2 [(Cp1,2,4-{Me3} Hf )2(m2:k1N,k1O-N]C{H}-O)(m-NSiH2Cy})] 2 [(Cp1,2,4-{Me3} Hf{NCO})4] 2 [(Cp1,2,4-{Me3} Hf{NCO})4(tBuNCO)2] 2 1,2,4-{Me3} [(Cp2 Hf{NCO})2(tBuNCO)2] [(Cp1,2,4-{Me3} HfH)2(m-{CON(SiH2Cy)}2)] 2 [(Cp1,2,4-{Me3} Hf{NCO})(m-O)(Cp1,2,4-{Me3} Hf{CN})] 2 2 Me4 [(CpMe4 Hf{NCO})(m-O)(Cp 2 2 Hf{CN})] [({Me2Si-Cp -Cp3-tBu}Hf{NCO})(m-O)((Me2Si-Cp -Cp3-tBu}Hf{CN})] [(Cp1,2,4-{Me3} HfI)(m-O)(Cp1,2,4-{Me3} Hf{NCO})] 2 2 1,2,4-{Me3} [(Cp2 Hf )2(m2-N]C{H}{Cy})(m2,Z2,Z1-N-NBPin)] [(Cp1,2,4-{Me3} Hf )2(m2-NBPin)(m2,Z2-NC{H}O)] 2 1,2,4-{Me3} [(Cp2 Hf )2(m2,Z2-{H}C]N{tBu})(m2,Z2,Z1-N-NBPin)] [(Cp1,2,4-{Me3} Hf )2(m2,Z2,Z2-N2)(dmap)] 2 1,2,4-{Me3} [(Cp2 Hf )2(NCO)(m-N)(dmap)] [(Cp1,2,4-{Me3} Hf )2(NCO)(m-N)(4-OMe-py)] 2 [(Cp1,2,4-{Me3} Hf )2(m2-O)(m2,k2-{cyclooctenyl}N-C]N)] 2 [(Cp1,2,4-{Me3} Hf{m2-N]C]N-(cyclooctenyl)})2] 2 [(Cp1,2,4-{Me3} Hf )2(NCO)(RCN)(m2-N)] 2 [(Cp1,2,4-{Me3} Hf )2(NCO)(CNXyl)(m2-NC{Cy}N)] 2 [(Cp1,2,4-{Me3} Hf )2(m2-NC{Cy}NCN{H}O)(C5H10C]C]N)] 2 ([Cp1,2,4-{Me3} Hf )2(Cl)(m2-NC{Cy}NCN{H}O)] 2 [(Cp1,2,4-{Me3} Hf )2(m2-O2CNC{Cy}NCN{H}O)(k2-O2CC{C]N}C5H10)] 2 [(Cp1,2,4-{Me3} Hf )2(m2-NCONtBu)(NCO)] 2 1,2,4-{Me3} [(Cp2 Hf )2(m2-NCONAd)(NCO)] [(Cp1,2,4-{Me3} Hf )2(I)(m2-NCONSiMe3)] 2 [(Cp1,2,4-{Me3} Hf )2(m2-OTf )(m2-NCONH)] 2 [(Cp1,2,4-{Me3} Hf )2(m2-OTf )(m2-NCONEt)] 2 [Cp2Hf(Z2-btmsa)(PMe3)] [Cp 2Hf(Z2-btmsa)] [Cp 2Hf(Z5:Z1-C5Me3-CH2-)(Z1-CH]C{SiMe3}2)] [Cp 2Hf(k2C,C 0 -C{H}]C{SiMe3}-SiMe2-CH2-)] [Cp Hf(dC{]C]CH-tBu}-CH{tBu}CH2-Z5-C5Me3-CH2-)] [Cp Hf(dC{]C]CH-tBu}-CH{tBu}CH2-Z5-C5Me3-CH2-C{]N-tBu}dC {¼N-tBu}-)] [Cp 2Hf(Z2-Me3SiC2Ph)] [Cp 2Hf(OH)(Z1-C{SiMe3}]CHPh)] [Cp 2Hf(k2C,O-C{Ph}]C{SiMe3}dC(]O)dOd)] [(EBTHI)Hf(k2-CH2CH2CH2CH2-)] [(EBTHI)Hf(k2C,C 0 -CH{4-CF3-C6H4}-CH2-CH{4-CF3-C6H4}-CH2-)] [(EBTHI)Hf(k2C,C 0 -CH{Ph}-CH2-CH{Ph}-CH2-)] [Cp 2Hf(nBu)2] [Cp2Hf(Z4-tBuCt4Bu)] [Cp2Hf(k2-C{C2SiMe3}]C{SiMe3}-C{C2SiMe3}-C{SiMe3}-)] [Cp2Hf(k2-C{Ct2Bu}]C{SiMe3}-C{Ct2Bu}-C{SiMe3}-)] [(Cp2Hf )2(m-k2,k2-{C{Ph}]C{-}-C{-}]C{Ph}-}2)] [(Cp2Hf )2(m-C{tBu}]C-C{^CtBu})((m-C{^CtBu})]+[tBuC2B(C6F5)3]− [Cp2Hf(k2C,C 0 -C{¼CHtBu}-{m-AliBu2}-C{^CtBu})] [(Cp2Hf{1,2-(SiMe3)2C2H})2(m-O)]

Hf oxidation state

31P chemical shift (ppm)

Number of complex

Reference

+4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4

n/a n/a n/a n/a n/a n/a

113Hf 114Hf 115Hf 116Hf-C1 116Hf-C2 117Hf

402 402 882 402 882 882

+4/+4 +4/+4 +4/+4 +4/+4 +4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4/+4/+4 +4/+4/+4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +4/+4 +2 +2 +4 +4 +4 +4

n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a −8.9 n/a n/a n/a n/a n/a

118Hf 119Hf-C1 119Hf-C2 120Hf 121Hf 122Hf 123Hf 124Hf 125Hf 126Hf 127Hf 128Hf 129Hf 130Hf 131Hf 132Hf 133Hf 134Hf 135Hf 136Hf 137Hf 138Hf 139Hf 140Hf 141Hf 142Hf 143Hf 144Hf 145Hf 146Hf 147Hf 148Hf 149Hf 150Hf 151Hf 152Hf 153Hf 154Hf 155Hf 156Hf 157Hf 158Hf 159Hf

402

+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4/+4 +4/+4 +4 +4/+4

n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a

160Hf 161Hf 162Hf 163Hf 164Hf 165Hf 166Hf 167Hf 168Hf 169Hf 170Hf 171Hf 172Hf 173Hf

893 893 893 894 894 894 897 896 896 896 896 898 898 899

402 402 402 884 884 885 885 885 885 886 886 886 886 886 886 886 886 406 406 406 887 887 887 888 888 888 888 888 888 888 888 888 889 889 889 890 890 891 891 892 892

(Continued)

390

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

HfCp2 complex

Hf oxidation state

31P chemical shift (ppm)

Number of complex

Reference

[(EBITHI)Hf(k2C,O-Me3SiC2SiMe3CO2)B(C6F5)3] [Cp 2Hf(k2C,O-Me3SiC2SiMe3CO2)] [Cp 2Hf(C18H24N2)] [Cp 2Hf(4,40 -tBu2-bipy)] [(CpMenthyl)2Hf(Z2-Me3SiC2SiMe3)] [Cp3Hf] [Cp2Hf(C8H16)] [Cp2Hf(-CH{B{C6F5}2-C{SiMe3}]CH]C{SiMe3}-)] [Cp2Hf(-CH{B{C6F5}2-C{tBu}]CH]C{tBu}-)] [CpCp HfCl(SnHMes2)] [(Me2C-Cp2)Hf(NMe2)(SnPh3)] [CpCp HfCl(SnPh3)] [CpCp Hf(NMe2)(SnPh3)] [CpCp HfMe(SnPh3)] [CpCp Hf(OMe)(SnPh3)] [Cp2Hf(SitBuPh2)Me] [Cp2Hf(SitBuPh2)(m-Me)B(C6F5)3] [(Cp2Hf{m-Br})2][B(C6F5)4]2 [CpCp HfCl(SbH{2,6-Mes2C6H3})] [CpCp Hf(k2C,Sb-C{Me}]C{Me}-Sb{2,6-Mes2C6H3})] [CpCp Hf(SbMes2)2] [CpCp HfMe(PHPh)] [Cp2Hf(Z2-Si2{SiMe3}4)(PMe3)] [Cp2HfCl(2,3,3,6,6-Tipp4Si6)] [Cp2HfCl(Si{SiMe3}3)] [Cp2Hf(Si{SiMe3}3)2] [Cp2HfCl((Si{SiMe3}2silatranyl)] t [CpEt 2 Hf ¼Si(SiMe Bu2)2] 4 [Cp2Hf(Z p-SnC4{SiMet2Bu}2Ph2)] [Cp2Hf(s2p-GeC4{SiMe3}2Me2)] [Cp2Hf(s2p-GeC4{SiMet2Bu}2Me2)] [K][Cp2HfCl(]GeC4{SiMe3}2Me2)] [Cp2Hf(s2p-SiC4{SiMe3}2Ph2)] [Cp2Hf(s2p-Ge{Fe(CO)4}C4{SiMe3}2Me2)] [Cp2Hf(s2p-Ge{W(CO)5}C4{SiMe3}2Me2)] [(Cp2Hf{s2p-GeC4(SiMe3)2Me2})2(m2-Ni{cod})] [CpHfCl2(Z5-Ge{AuPPh3}C4{SiMe3}2Me2)] [Cp2Hf(s2p-Ge{AuPPh3}{Z1-Cp}C4{SiMe3}2Me2)] CpHfCl(Z5-GeC4{SiMe3}2Me2)(NHCMe4) [(NHCiPr2Me2)H][CpHf(m:Z5,Z1-C5H4)(s2p-GeC4{SiMe3}2Me2)] [Cp2Hf(s2p-GeC4{SiMe3}2Me2)]2(m-S)2 [Cp2Hf(s2p-GeC4{SiMe3}2Me2)]2(m-Se)2 [Cp2Hf(s2p-GeC4{SiMe3}2Me2)]2(m-Te)2 [Cp2Hf(m-S)2(GeC4{SiMe3}2Me2)] [Cp2Hf(m-Se)2(GeC4{SiMe3}2Me2)] [Cp2Hf(s2p-Ge{B(C6F5)4}C4{SiMe3}2Me2)] [Cp2Hf(p2-GeC4{SiMe3}2Me2)][B(C6F5)4] [({Me2N}B-Cp-Cp)HfCl2] [({(CH2)2N}B-Cp-Cp)HfCl2] ({NMe2}2B2-Cp-Cp)HfCl2 [({NMe2}2B2-Flu2)HfCl2] [({NMe2}2B2-Cp-Flu)HfCl2] [({NMe2}2B2-Cp-Octaflu)HfCl2] [({tBu}2Sn2-Cp2)HfCl2] [Cp2Hf(SeMe)2] [(Cp2Hf )2(B9H11)] [Cp 2Hf(Z2N,N 0 -CH3-N-N]CPh2)(Me)]

+4 +4 +4 +4 +2 +3 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4/+4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4/+4 +4 +4 +4 +4 +4/+4 +4/+4 +4/+4 +4 +4 +4 +3 +4 +4 +4 +4 +4 +4 +4 +4 +4/+4 +4

n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a −20.6 n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a 50.7 47.7 n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a

174Hf 175Hf 176Hf 177Hf 178Hf 179Hf 180Hf 181Hf 182Hf 183Hf 184Hf 185Hf 186Hf 187Hf 188Hf 189Hf 190Hf 191Hf 192Hf 193Hf 194Hf 195Hf 196Hf 197Hf 198Hf 199Hf 200Hf 201Hf 202Hf 203Hf 204Hf 205Hf 206Hf 207Hf 208Hf 209Hf 210Hf 211Hf 212Hf 213Hf 214Hf 215Hf 216Hf 217Hf 218Hf 219Hf 220Hf 221Hf 222Hf 223Hf 224Hf 225Hf 226Hf 227Hf 228Hf 229Hf 230Hf

900 901 902 902 903 904 905 906 906 907 908 908 908 908 908 910 910 910 911 912 913 914 915 734 724 724 916 917 871 918 919 919 920 918 918 919 919 919 919 919 921 921 921 921 921 922 922 923 923 924 925 926 927 928 929 930 931

Hf phospholyl complex

Hf oxidation state

31

Number of complex

Reference

[rac-(Z5-PC4H{2-Ph}{3,4-Me2})2HfCl2]

+4

73.7

231Hf

814

P chemical shift (ppm)

Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals

4.06.5

391

Closing remarks

The large body of work highlighted in this Chapter is a testament to the breath and depth of research into cyclopentadienyl and phospholyl complexes of the group 4 metals, and is only the tip of the iceberg of their chemistry published over the last two decades. In the early part of the 21st century, interest in these complexes focussed on the applications in polymerization catalysis, including “half-sandwich” and constrained geometry complexes for homogeneous olefin oligomerization and polymerization/ copolymerization catalysts, advanced by Marks, Nomura and others. The half-sandwich MCp motif was also employed in a large number of multimetallic cluster complexes showing fascinating chemistry, largely developed by the groups of Hou, Mena and Yélamos and others. Metallocene alkyne complexes of Ti, Zr and Hf, pioneered by Rosenthal, Beckhaus, Beweries and others, serve as “Cp2M(II)” synthons and show extensive reactivity with a range of unsaturated substrates, and continue to provide new examples of metallacycle bonding modes and reactivity. Metallocene complexes have also been extensively investigated by Manners, Tilley and others for main group element dehydrocoupling reactions, and a great deal of mechanistic understanding has been achieved. The MCp2 motif has proved ubiquitous in the field of small molecule activation, in particular the remarkable work in the groups of Chirik and Sita for dinitrogen activation and functionalization. Group 4 metallocenes have been used as an excellent platform for ammonia synthesis via proton-coupled electron transfer. The mid-2000s saw the advent of frustrated Lewis pairs, and this concept was successfully extended to zirconocene-based FLP systems by Erker, Wass and others, to achieve some remarkable bond activations with H2 and other substrates. More recent years have witnessed a rise in publications of group 4 metal sandwich and half-sandwich complexes that also feature a main group element (such as Si, Ge, Sn, Pb) pioneered by the groups of Saito, Marschner, Sekiguchi and others. Low-valent p-block chemistry is still relatively underexplored and these heterobimetallic examples just hint at the rich potential of these molecules in the future. Although the growth of this area has been extraordinary, it still appears that there are many unexplored facets of cyclopentadienyl and phospholyl group 4 chemistry, as evidenced by recent scientific activities, and will no doubt lead to more fascinating chemistry for many years to come.

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Cyclopentadienyl and Phospholyl Complexes of the Group 4 Metals 389. 390. 391. 392. 393. 394. 395. 396. 397. 398. 399. 400. 401. 402. 403. 404. 405. 406. 407. 408. 409. 410. 411. 412. 413. 414. 415. 416. 417. 418. 419. 420. 421. 422. 423. 424. 425. 426. 427. 428. 429. 430. 431. 432. 433. 434. 435. 436. 437. 438. 439. 440. 441. 442. 443. 444. 445. 446. 447. 448. 449. 450. 451. 452. 453. 454. 455. 456. 457. 458. 459. 460.

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4.07

Arene Complexes of the Group 3 Metals and Lanthanides

F Geoffrey N Cloke and Nikolaos Tsoureas, Department of Chemistry, School of Life Sciences, University of Sussex, Brighton, United Kingdom © 2022 Elsevier Ltd. All rights reserved.

4.07.1 4.07.2 4.07.2.1 4.07.2.2 4.07.2.3 4.07.3 4.07.3.1 4.07.3.2 4.07.3.3 4.07.3.4 4.07.3.5 4.07.4 References

4.07.1

Introduction Neutral arene Ln/Group 3 interactions Ln/Group 3 interactions with arenes which are not part of a ligand framework Hetero-bidentate arene interactions Intramolecular arene interactions supported by a tripodal tris-phenoxide ligand Anionic arene Ln/Group 3 interactions: Inverted arenes Inverted arene complexes with neutral co-ligands Inverted arene complexes with simple X co-ligands (X ¼ I, H) Inverted arene complexes supported by amido ligands Inverted arene complexes supported by RO− ligands Inverted arenes supported by CpR − ligands Conclusions

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Introduction

Lanthanides (referred to also as rare earth metals) occupy the periodic table between elements barium and hafnium. As a consequence the Group 3 metal triad (Sc, Y, Lu) is often treated within the same framework of reference as the rare earth metals. Nevertheless, it has to be noted that especially its first two members are more often treated as part of the transition metals and are sometimes referred to as pseudo-lanthanides. Unlike transition metals which adopt multiple common oxidation states, the most common oxidation state for the lanthanides is the + 3; other common oxidation states correspond to empty (Ce(IV)), half-full (Eu(II)/Tb(IV) 4f7) or full (Yb(II)) 4f shell as well as Sm(II) (4f6—same as Eu(III)), Tm(II) (4f11—same as Er(III)) and Pr(IV) (4f1—same as Ce(III)). A significant difference between the lanthanides and the transition metals is that the valence electrons of the former are situated in 4f orbitals. As the atomic number increases, the 4f orbitals contract rapidly and become more stable than the 5d orbitals (the radial part of the 4f wavefunction has zero nodes) and are thus shielded and inaccessible to form any covalent bonds (at a first approximation). As a result the organometallic chemistry of the lanthanides is appreciably different than that of the transition metals; e.g. the 4f orbitals are barely (again at a first approximation) affected by the ligand field unlike transition metals and the 18e− rule of thumb has no standing. Furthermore as the series is traversed from La to Lu, due to the increase of Zeff the 5s and 5p orbitals start penetrating the 4f subshell, resulting in the contraction of the ions as the atomic number increases (referred to as the lanthanide contraction). This in turn means that Ln3+ ions are quite strong Lewis acids. Overall, the lanthanide organometallic (and in general their coordination) chemistry is dominated by hard and most preferably ionic ligands like alkoxides (OR−), amides (NR2−) and of course aromatic anionic carbocyclic frameworks like the venerable Cp(RxH(5-x))− (Cp ¼ cyclopentadienyl; x ¼ 0–5) ligand. Moreover, their coordination number is dictated by the size of the lanthanide metal center (see above) as well as the steric requirements and overall shape of the supporting ligand. It is therefore not unusual for lanthanide organometallic compounds to adopt coordination numbers that are uncommon to transition metals. As can be easily surmised the synthesis of rare earth organometallic compounds demands the use of anaerobic conditions and thoroughly dried and degassed solvents since lanthanides are oxophilic and the anionic supporting ligands are prone to facile hydrolysis. In that respect solvents that either possess acidic protons (e.g. MeOH, CHCl3) or can act as halogen sources (e.g. CH2Cl2) are usually avoided. In the case of low valent organometallic complexes of the lanthanides (e.g. Sm(II)), another factor to be taken into consideration, especially when choosing solvents, is their reductive power stemming from their propensity to adopt the common +3 oxidation state. This article deals with arene complexes of the lanthanide and Group 3 metals where the arene coordinates to these metal centers through its p face. Therefore metallacyclobutadienes are excluded from this article. The topic is discussed based on the two major categories of Ln/Group 3 arene interactions: (a) neutral arene—Ln/Group 3 interactions and (b) anionic arene—Ln/Group 3 interactions. In the case of the former, the discussion is divided into two subcategories: (i) intermolecular and (ii) intramolecular interactions. In both of these cases the arene acts as a soft donor ligand through its p cloud to further stabilize the Lewis acidic metal center. Keeping in mind the discussion in the previous paragraph no appreciable p-backbonding is in play as can be deduced from an extensive amount of crystallographic data that show either very little variation of the CdC aromatic bond lengths or loss of planarity of the arene ring from their expected values for the uncoordinated arene ligand. In the case of intermolecular interactions, they usually involve either electron rich arenes (e.g. mesitylene, PhNMe2) or Ln/Group 3—arene interactions as part of a close contact ion pair between a cationic metal-containing moiety and a counter-anion possessing arene substituents (e.g. [B(C6H5)4]−). In the case of intramolecular interactions the coordinating arene is part of a ligand framework that possesses hard anionic ligands forming a hetero-dentate coordination environment. Depending on the size of the metal center and the steric requirements of the

Comprehensive Organometallic Chemistry IV

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Arene Complexes of the Group 3 Metals and Lanthanides

Fig. 1 Coordination modes of Ln/Group 3 arene complexes of category (B). Dotted lines do not correspond to Z6 hapticity of the metal to the arene ring; R ¼ H or arene; coordination modes (A) and (B) also encompass fused polyaromatic systems.

ligand framework, these can also manifest themselves as bridging interactions between metal centers. Especially in subcategory (i), the Ln/Group 3—arene interaction is relatively weak and easily disrupted by coordinating solvents (e.g. THF, pyridine) which cause either complete displacement of the arene substituent or alteration of its hapticity in order to accommodate a more favorable Ln-hard(er) donor bond. The same applies for subcategory (ii) Ln-arene interactions but probably to a lesser extent depending most likely on the rigidity and topology (i.e. bridging or not) of the hetero-dentate coordination mode encompassing the Ln-arene interaction. Category (b) anionic arene—Ln/Group 3 interactions, as the name implies, involve a reduced arene ligand coordinated to a Ln/Group 3 metal. The accessible p manifold allows arenes to act as multi-electron anionic ligands. Depending on the extent of the reduction the coordinated arene ligand can be aromatic, non-aromatic or anti-aromatic. As a result of the p manifold occupancy the CdC bond lengths in the anionic arene ligand have lengthened in comparison to the uncoordinated neutral ligand. It is also often observed in this class of compounds a marked deviation from planarity of the coordinated arene ligands, which in certain cases signifies charge localization (i.e. loss of delocalization) with the resulting arene-Ln interaction best described as consisting of a combination of Z1 (i.e. s-bond) and Z2 (i.e. p bond donation) interactions. In these complexes there is a higher degree of covalency between the metal center and the arene which usually involves orbital overlap between the p orbitals of the reduced arene ligand and a 5d:4f hybrid (d bond) where the 4f component is the minor constituent of the hybrid. Unlike the disruption of the Ln/Group 3—arene interaction observed in complexes of category (a), compounds of class (b) are often synthesized and can be isolated by crystallization from ethereal solvents. Nevertheless it should remembered that complexes of class (b) can act as reducing agents, thus imposing restrictions on what solvents can be used. One caveat regarding these compounds is the assignment of formal oxidation states. Although in some occasions this can be straightforward, it is not often the case and therefore detailed spectroscopic (e.g. XANES, Uv–Vis-NIR, EPR), magnetometric (SQUID), structural and theoretical studies need to be undertaken in order to ascertain their electronic structure. Fig. 1 below shows the three coordination modes Ln/Group 3 arene complexes can adopt: side-on (A), inverted sandwich (B) and sandwich (C). Coordination mode (C) excludes zero-valent bis(Z6-tritbutylbenzene)M(0) complexes synthesized by metal vapor synthesis and which feature a bonding regime similar to their zerovalent bis arene transition metal complexes. At this point it is also worth pointing out that unlike transition metal arene complexes where Umpolung opens modes of reactivity like nucleophilic attack, such examples are as yet unknown for arene—Ln complexes of this type.

4.07.2

Neutral arene Ln/Group 3 interactions

4.07.2.1

Ln/Group 3 interactions with arenes which are not part of a ligand framework

Group 3 and lanthanide analogs of the classic zerovalent bis(Z6-arene) transition metal complexes remain confined to the bis(Z6-tritbutylbenzene)M(0) complexes first reported over 30 years ago1–3; no further reports on these or closely related compounds have appeared in the literature during the period of this review. The first Ln-arene complex featuring an Z6-Ln(III) interaction was synthesized by Cotton et al. from the reaction of SmCl3 with hexamethylbenzene in the presence of AlCl3 in refluxing toluene in 1986.4 Since then a variety of complexes of the general formula [(Z6-arene)Ln(III)((m-X)2EX2)3] (1.0-Ln) (X ¼ Cl, Br, I; E ¼ Al, Ga) (Fig. 2) have been prepared in a similar manner.5–10 The bonding between the arene and the Ln(III) center is electrostatic in nature with the arene acting as a p donor ligand. All complexes adopt a distorted pentagonal bipyramid geometry with the arene ligand in the apical position with the exception of [Eu(II)(Z6-C6Me6)(AlCl4)2]411 and the [Sm(II)(Z6-arene) (AlCl2I2)]212 (arene ¼ C6Me6, toluene) which exist as tetramer and dimer in the solid state respectively. The Ln(III)-arene centroid distances vary from 2.64 A˚ (1.0-La)6 to 2.48 A˚ in [Yb(III)(Z6-C6Me6)((m-Cl)2AlCl2)3]7 with a monotonic reduction following the lanthanide contraction and subsequent increased Lewis acidity of the metal centers. Some variance in Ln-centroid arene distance is observed depending on the arene within members of the same Ln family. The arene ligand dissociates in THF but in the solid state the complexes are stable crystalline materials, even at higher temperatures. Changing the AlX4 moieties for AlX3R (R ¼ alkyl) fragments does not have any impact on the Ln-arene bonding situation. For a more complete list of complexes of this type before 2002 we refer the reader to the review by Bochkarev.10,13 Examples of the use of such compounds as catalysts have been reported, and more recently the magnetic properties of 1.0-Dy/Tb as single molecule magnets (SMMs) have been studied.9 Complex 1.0-Dy showed a waist-restricted hysteresis loop at 3.0 K with some improvement in the SMM properties observed when the arene was toluene instead of C6Me6.8 In this category of Ln/Group 3 metal-arene interactions, complexes of the general formula [(CpR)mLnBPh4(THF)x] (1.1a-Ln(III), 1.1b-Ln(II), Fig. 3) (m ¼ 3 for Ln(III) metal centers and Group 3; m ¼ 2 for Ln ¼ Sm, Eu, Yb; R ¼ Me5, Me4H, 1,3-di-tertbutyl; x ¼ 0, 1) can also be included. These close contact zwitterions feature varying degrees of hapticity between the phenyls of the

Arene Complexes of the Group 3 Metals and Lanthanides

407

Fig. 2 General structure of some recent examples [(Z6-arene)Ln(III)((m-X)2EX2)3] (1.0-Ln) complexes.

Fig. 3 The Ln-Ph interaction depicted for 1.1a-Ln(III) and 1.1b-Ln(II) does not necessarily correspond to an 6 hapticity.

BPh4− counter-anion and the cationic metal center which are dependent on the CpR ligand steric environment, coordinating solvents and the size of the metal center. For example, in the Sm(III) complex [(Cp )2Sm(m-Z1:Z1-Ph2)BPh2]14 the bridging phenyl rings adopt a Z1:Z1 coordination mode while in the Sm(II) compound [Cp Sm(THF)(m-Z6:Z1-Ph2)BPh2] one of the phenyl rings coordinates Z6 despite THF coordination.15 Coordinating solvents like nitriles16 and ethers17,18 as well as O-donors (e.g. ketones,19 phosphine oxides20) break the contact ion pair. Table 1 summarizes some key structural characteristics as determined in the solid state by single crystal XRD. Their synthesis involves the acidolysis of either an Z5-CpR (in the case of Sm(II),15,26 Eu(II),26,27 Yb(II)15,26) or usually an Z3-allyl or alkyl ligand (in the case of Ln(III) and Group 3 metals) with [NEt3H]BPh414,15,21–25,28,29 in benzene or toluene. An alternative route for the synthesis of Cp Yb(m-Ph)2BPh2 involved reaction of Cp 2Yb(Z3-allyl) (Cp ¼ CpMe5) with an equimolar amount of [NEt3H]BPh4 to yield the desired Yb(II) product with concurrent reduction of the Yb(III) starting material,30 or via the reaction of YbCp 2 with AgBPh4 with loss of Ag metal.15 The analogous reaction with the more reducing SmCp 2 yields trivalent [(Cp )2Sm(m-Z1:Z1-Ph2)BPh2] and Ag metal.14 Complexes [(CpR)mLnBPh4(THF)x] are very useful synthons for the introduction of other neutral19,31 and anionic ligands32,33 and moieties34 and have been shown to be robust reagents for mechanochemical syntheses.29,30 The aforementioned change of hapticity of the phenyl rings of the BPh−4 counter-anion can be the prelude to further reactivity as observed in the thermolysis of [YbN00 (THF){(m-6-Ph)(m-4-Ph)BPh2}] (N00 ¼ N(SiMe3)2) (1.2-Yb.THF) to yield 1.3-Yb (Scheme 1).35

408

Arene Complexes of the Group 3 Metals and Lanthanides

Table 1 Key structural characteristics of the Ln-arene interaction in the series of the complexes [(CpR)mLnBPh4(THF)x]. Compound

Hapticity

˚) M-arene distance (A

[(CpMe4H)2Sc(m-Z1-Ph)BPh3]21 [(Cp )2Sc(m-Z2-Ph)BPh3]17 [(Cp )2Y(m-Z2:Z2-Ph2)BPh2]22 [(CpMe4H)2Y(m-Z1:Z1-Ph2)BPh2]23 [(CpMe4H)2Lu(m-Z1:Z1-Ph2)BPh2]24 [(Cp )2La(m-Z1:Z1-Ph2)BPh2]25 [(Cp )2Nd(m-Z1:Z1-Ph2)BPh2]14 [(Cp )2Sm(m-Z1:Z1-Ph2)BPh2]14 [Cp Sm(THF)(m-Z6:Z1-Ph2)BPh2]15 [Cp Sm(m-Z6:Z6-Ph2)BPh2]26 [Cp Sm(Z2-PhNNPh)(m-Z4:Z1-Ph2)BPh2]15 [Cp Eu(m-Z6:Z2-Ph2)BPh2]26 [CpttEu(m-Z4:Z3-Ph2)BPh2]c,,27 [(CpMe4H)2Gd(m-Z1:Z1-Ph2)BPh2]15 [(CpMe4H)2Tb(m-Z1:Z1-Ph2)BPh2]15 [(Cp )2Tb(m-Z1:Z1-Ph2)BPh2]28 [(CpMe4H)2Dy(m-Z1:Z1-Ph2)BPh2]15 [(Cp )2Dy(m-Z1:Z1-Ph2)BPh2]28 [(Cp )2Ho(m-Z1:Z1-Ph2)BPh2]29 [(Cp )2Er(m-Z1:Z1-Ph2)BPh2]29 [Cp Yb(m-Z4:Z1-Ph2)BPh2]15,26

Z1 Z2 Z2:Z2,a Z1:Z1,b Z1:Z1,b Z1:Z1,b Z1:Z1,b Z1:Z1,b Z6:Z1,b Z6:Z6,b Z4:Z1 Z6:Z2 Z4: Z3 Z1:Z1,b Z1:Z1,b Z1:Z1,b Z1:Z1,b Z1:Z1,b Z1:Z1,b Z1:Z1,b Z4: Z3

2.568(2) 2.772a 2.986, 2.966 2.718(2), 2.829(2) 2.800(2), 2.668(2) 2.997(2), 2.912(2) 2.906, 2.890 2.917, 2.825 2.651(9) (Z6), 2.933(4) (Z1) 2.700(3), 2.776(5) 2.938 (Z4), 2.875(4) (Z1) 2.797(13) (Z6), 2.994 (Z2)a 2.985 (Z4)a, 3.000 (Z3)a 2.7402(17), 2.8384(17) 2.8456(15), 2.7310(15) 2.842(3), 2.844(3) 2.8297(17), 2.7203(17) 2.836(3), 2.830(3) 2.834(2), 2.792(2) 2.826(2), 2.758(2) 2.936 (Z4)a, 2.903 (Z3)a

For Zm where m < 6 the value given corresponds to the average of the metal-aromatic carbon distances (i.e. {S(M-C)m}/m). A close interaction between the metal center and the hydrogen adjacent to the aromatic carbon is observed. c tt Cp ¼ (1,3-di-tert-butyl-cyclopentadienyl). a

b

Scheme 1 Change of hapticity from 6:4 to 6:2 with concurrent activation of the supporting ligands.

De-solvation of 1.2-Yb.THF at −10  C yields 1.2-Yb where the two Ph rings are connected (6:6) to the Yb(II) metal center. Aminolysis of 1.2-Yb.THF with the less sterically hindered 3.5-tBu2-pyrazole furnishes 1.4-Yb (Scheme 2), where the two Ph rings adopt an (6:6) coordination mode despite the presence of a THF molecule in the coordination sphere of the Yb(II) metal center.36

Scheme 2 Examples of hapticity changing upon change of coordination environment of the metal center.

Arene Complexes of the Group 3 Metals and Lanthanides

409

Using weakly-coordinating anions such Z6-arene coordination allows the stabilization and characterization of highly reactive [LMMe]+ fragments (M ¼ Sc,37 Y38; L ¼ DippNacnacMe2, Scheme 3) thus enabling the isolation of complexes 1.4-MR-arene in the following Scheme 4 (M ¼ Sc (R ¼ Me), Y (R ¼ Me, CH2SiMe2Ph)). The Sc-arene centroid distance in complexes 1.4-ScMe-arene follows the trend Sc-Mesitylene (2.381 (2) A˚ ) > Sc-C6H5Br (2.366 (2) A˚ ) > Sc-Toluene (2.344 (2) A˚ ). Both toluene and mesitylene displace bromobenzene from the coordination sphere of the cationic [LScMe]+ fragment, while toluene substitutes mesitylene via an associative like mechanism.39 In the case of the 1.4-YR-dimethylaniline complexes, their structure was confirmed spectroscopically. Isolation of the analogous complexes [L0 YR(6-C6H5NMe2)][B(C6F5)4]− (R ¼ Me, CH2SiMe2Ph) featuring the bulkier L0 ¼ DippNacnactBu2 mono-anionic ligand proved impossible due to their thermal instability38; nevertheless the complex 1.5-Y supported by this bulky ligand, was characterized as a close contact zwitterion featuring Z6 arene coordination using NMR spectroscopy (Scheme 3).38

Scheme 3 Synthesis of Y and Sc cationic arene complexes stabilized by Nacnac ligands.

Scheme 4 Formation of Z6-C6H5-NMe2 cationic Y(III) complex 1.6-Y after protonolysis.

410

Arene Complexes of the Group 3 Metals and Lanthanides

Using a monoanionic amidinate ligand Kempe et al. synthesized the cationic yttrium complex 1.6-Y (Scheme 4). Complex 1.6-Y is a component of a flexible trimetallic catalyst system permitting the selective dimerization of ethylene to 1-butene or the production of various linear a-olefins depending on relative catalyst concentrations. The Y-arene centroid distance is 2.564 (7) A˚ which accounting for the difference of M3+ ionic radii between 7-coordinate Y and 6-coordinate Sc (0.22 A˚ ) is similar to those observed in complexes 1.4-ScMe-arene (Scheme 3).40 The contact ion pair complex 1.8-Y (Scheme 5) supported by a dianionic alumina-benzene ligand can also be synthesized from neutral 1.7-Y in a manner similar to 1.6-Y. The Y-benzene centroid distance of 2.545 (3) A˚ is somewhat shorter than in 1.6-Y. Complex 1.8-Y is a competent catalyst for isoprene polymerization in the absence of a co-catalyst.41

Scheme 5 Synthesis of complex 1.8-Y (2,4-dtbp ¼ 2,4-di-tert-butyl-pentyl).

The ion-pair yttrium complex 1.10-Y (Scheme 6) was prepared from 1.9-Y as a model compound for the cationic catalytically active species [Cp Y(2-CH2-6-CH3-C5H3N)]+ (Cp ¼ CpMe5) promoting the addition of benzylic CdH bonds of alkyl pyridines to olefins. It features an Z6 phenyl ring Y—centroid distance of 2.449 (3) A˚ .42

Scheme 6 Ion-pair Y(III) complex 1.10-Y.

The series of cationic and ion-pair Z3-allyl complexes 1.11-ScA–1.11-ScC supported by a CpMe4SiMe3 ligand can be synthesized according to Scheme 7. The Sc-arene centroid distances vary from ca 2.27 A˚ in 1.11-ScA (arene ¼ dimethyl-aniline) to 2.390 (17) A˚ in 1.11-ScC (arene ¼ toluene) and compare well with the observed values for 1.4-ScMe-arene discussed above.43

Arene Complexes of the Group 3 Metals and Lanthanides

411

Scheme 7 Synthesis of Cp supported Sc-arene cationic species 1.11ScA–1.11ScC.

Using the same Cp supporting ligand as in Scheme 7 the ion-paired lutetium complex [CpMe4SiMe3LuMe(m-Z6:Z1-Ph2)BPh2] 1.13-Lu can be prepared via the reaction of [CpMe4SiMe3Lu(m-Me)2]3 1.12-Lu with three equivalents of [NEt3H][BPh4] in benzene (Scheme 8). The Z1 LudC in 1.13-Lu is shorter (2.730 (5) A˚ ) than the LudC bonds corresponding to the Z6 coordination mode of the other phenyl ring (2.742 (4)–2.868 (5) A˚ ).44

Scheme 8 Synthesis of [CpMe4SiMe3LuMe(m-Z6:Z1-Ph2)BPh2] 1.13-Lu via protonolysis of the bridging methyl ligands in 1.12-Lu.

F5 The cerium(III) triamido complex [Ce(NPhF5 ¼ C6F5) 1.14-Ce interacts weakly with both toluene and mesitylene to 2 )3] (Ph generate 1.14-Ce.toluene and 1.14-Ce.mesitylene respectively (Fig. 4).45 The former adopts an Z4 distorted piano-stool confirmation, similar to the previously reported Nd(III) analog,46 with the average CedC contact being ca 3.102 A˚ . In contrast 1.14-Ce. mesitylene has an Z6 piano-stool geometry with a Ce-mesitylene centroid distance of 2.800 (2) A˚ . The especially weak Ce-arene interaction in 1.14-Ce.mesitylene is also reflected in the long CedC(mesitylene) average distance of 3.145 A˚ , which is longer than that found in [(Z6-toluene)Ce(III)((m-Cl)2GaCl2)3] (2.950 A˚ ) (Fig. 2)5 and further substantiated by the loss of the arene upon dissolving the compound in non-coordinating aromatic solvents (e.g. C6F6, C7H8) as well as theoretical calculations which suggest spontaneous loss of the arene in the gas phase. Nevertheless, it has to be noted that both 1.14-Ce.toluene and 1.14-Ce.mesitylene are stable as solids under dynamic vacuum.

412

Arene Complexes of the Group 3 Metals and Lanthanides

Fig. 4 Ce(III) electrophilic tris-amido complexes 1.14Ce.toluene and 1.14Ce.mesitylene featuring weak Ce-arene interactions. Werner, D.; Deacon, G. B.; Junk, P. C. Eur. J. Inorg. Chem. 2018, 2018(20), 2241–2246.

Table 2 summarizes some of the key metric parameters of the key arene complexes described in the section (except for the complexes indexed in Table 1). Table 2 Summary of Ln-arene interaction hapticities and lengths for complexes in section 4.07.2.1 which are not indexed in Table 1 above. Complex

˚ )a M-centroid arene (m)(A

1.2-Yb.THF 1.2-Yb.THF 1.2-Yb 1.3-Yb 1.4-Yb 1.4-ScMe-Mesitylene 1.4-ScMe-C6H5Br 1.4-ScMe-Toluene 1.6-Y 1.7-Y 1.10-Y 1.11-ScA 1.11-ScB 1.11-ScC 1.13-Lu 1.14-Ce.toluene 1.14-Ce.mesitylene

2.555(5) (Z6): 2.994 (Z4) 2.486(5) (Z6): 2.985 (Z2) 2.489(5) (Z6): 2.84 (Z4) 2.83 (Z2): 2.774 (Z2) 2.513(4) (Z6): 2.601(9) (Z6) 2.381(2) (Z6) 2.366(2) (Z6) 2.344(2) (Z6) 2.564(7) (Z6) 2.545(3) (Z6) 2.449(3) (Z6) 2.27 (Z6) 2.652 (Z4) 2.390(17) (Z6) 2.730(5) (Z1): 2.434(3) (Z6) 3.102 (Z4) 2.800(2) (Z6)

For Zm where m < 6 the value given corresponds to the average of the metal-aromatic carbon distances (i.e. {S(M-C)m}/m). a

4.07.2.2

Hetero-bidentate arene interactions

In complexes of this category the Ln/Group3 arene p-interaction supports a chelating mode in conjunction with a hard donor moiety as part of the overall ligand framework. Two types of these modes can be easily envisioned: an intramolecular (Fig. 5A) and an intermolecular (Fig. 5B) one. A useful gauge for the formation of such Ln⋯ p-arene interactions (other than single crystal XRD) is IR spectroscopy, whereby coordination of the arene to the Ln metal center leads to a reduction in the CdC bond stretching frequency.

(A)

(B)

Fig. 5 Intra and intermolecular arene p-interactions imposed by the ligand framework; M ¼ Group 3/Ln; L ¼ anionic hard donor ligand; the M-arene interaction is not necessarily Z6.

Arene Complexes of the Group 3 Metals and Lanthanides

413

Ligand frameworks that support both the above types of Ln/Group3 interactions are bulky aryl-oxides/thiolates, and most recently arylselenoate scaffolds. The Ln(II) complexes [M(SAr )2] 1.15-Ln (Ln ¼ Sm, Eu, Yb; Ar ¼ 2,6-Trip-C6H3)47,48 can be prepared as shown in Scheme 9. All complexes feature two intramolecular Z6 arene interactions between the Ln(II) center and the Trip (Trip ¼ 2,4,6-iPr3-C6H2) substituent with Ln-arene centroid distances which decrease from 2.734 A˚ in 1.15-Sm and 2.734/2.722 A˚ in 1.15-Eu to 2.623/2.650 A˚ in 1.15-Yb, consistent with the lanthanide contraction. It is worth noticing that the synthesis of complexes 1.15-M begins in THF, and upon crystallization from hydrocarbons, the Z6-arene Ln(II) interaction must be competing with any THF coordination to the metal center. The solution behavior of diamagnetic 1.15-Yb has been extensively studied by NMR spectroscopy (including 171Yb) and although complicated it suggests hindered rotation around the S-Cipso carbon, thus pointing to the Z6 arene interaction observed in the solid state, being retained in solution.48 A variable temperature crystallographic study has also been undertaken in the case of 1.15-Eu with a small variation of the Eu(II)-arene interaction at 100 K over the aforementioned value (173 K).49 When dissolved in coordinating solvents (e.g. THF, DME) the Ln-Z6-arene interaction is disrupted.

crystallization

Scheme 9 Terphenyl aryl-thiolate Ln(II) complexes with intramolecular Z6-arene-Ln(II) interactions.

Using the analogous terphenyl selenoate ligand, Nd(III) (pale green) and Pr(III) (pale yellow) complexes 1.16-Ln (Scheme 10) can be synthesized in a manner analogous to 1.15-Sm (Scheme 9). The Ln-Z6 arene centroid distances are 2.826 (5) A˚ for 1.16-Pr and 2.823 (3)/2.816 (3) A˚ for 1.16-Nd and are identical.50

crystallization

Scheme 10 Terphenyl selenoate complexes 1.16-Ln (Ln ¼ Pr, Nd).

The structurally similar terphenyl ligand depicted in Scheme 11 has been used to encapsulate anisotropic Ln(III) cations and study their SMM properties. The alkylolysis between Ln(CH2SiMe3)3(THF)n and the neutral Ar0 OH installs the Ln(III) cation in a cavity created by two ligands and supported (apart from the two LndO bonds) by one cyclometalated methyl group and an arene interaction. In the case of Tb this is only with the one aromatic carbon (Tb-C: 2.989 (8) A˚ ). The Ln(III)-arene centroid distances again decrease monotonically in accordance with the Ln contraction from 2.649 A˚ in 1.17-Dy to 2.615 A˚ in 1.17-Ho and finally 2.593 A˚ for 1.17-Tm and 2.589 for 1.17-Er. Complex 1.17-Dy shows magnetic hysteresis at zero field with a barrier of 961 K with butterfly-type hysteresis at up to 6 K under a field sweep rate of 0.17 mT s−1.51

414

Arene Complexes of the Group 3 Metals and Lanthanides

Scheme 11 Synthesis of single molecule magnet Dy(III) complex supported by a Z6-arene interaction.

Employing the less bulky 2,6-diphenyl-phenolate ligand (Odpp−) the series of charge separated and neutral heterobimetallic complexes of the general formula [(II)Ln0 2(Odpp)3]+[(III)Ln(Odpp)4]− 1.18 (Ln0 ¼ Eu then Ln ¼ Y, Nd, Ho; Ln0 ¼ Yb; Ln ¼ Y) and [(Odpp)xEu(II)(m-Odpp)3-xAe(II)(Odpp)] 1.19-Ae (Ae ¼ Ca, x ¼ 1; Ae ¼ Ba, Sr x ¼ 0) respectively, can be prepared using solid state synthesis methods as per Scheme 12. Arene-Ln interactions are only observed in the cationic moieties of complexes 1.18 and in the case of the neutral Eu/Ae heterobimetallics 1.19-Ae with different degrees of hapticity for both cases (Table 3). The [Ln(Odpp)4]− counter-anions in complexes 1.18 do not show any arene-Ln interactions probably due to the increased steric hindrance preventing their formation. The same has been observed in the anions of the related charge-separated heterobimetallic complexes [(II)Ae2(Odpp)3]+[(III)Ln(Odpp)4]− 1.20 (Ae ¼ Ca then Ln ¼ Nd, Ho, Tm, Yb; Ae ¼ Sr then Ln ¼ Nd, Ho; Ae ¼ Ba then Ln ¼ Sm, Yb) which are synthesized in a similar manner to complexes 1.18.52

Scheme 12 Synthesis of complexes 1.18, 1.19-Ae and 1.20.

The effect of steric hindrance on the wingtips of the aryl-phenoxide in promoting intermolecular Ln-arene interactions, especially in the absence of a rigid ligand scaffold, is shown in Fig. 6. Ln(III) complexes of the general formula [Ln2(ODipp)6] 1.21-Ln (Ln ¼ Nd, Sm, Dy, Er; Dipp ¼ 2,6-iPr2-C6H3), first reported in 1994,53,54 engage in intermolecular Ln-p arene coordination; in contrast the Dy(III) complex [Dy{O(2,6-tBu2-C6H3)}3] 1.22 does not adopt such a structural motif.55 The Ln-arene centroid distances in complexes 1.21-Ln decrease monotonically in accordance to the lanthanide contraction from 2.695 (5) A˚ in 1.21-Nd to 2.602 A˚ in 1.21-Er (1.21-Sm: 2.638 (4) A˚ , 1.21-Dy: 2.595 (4) A˚ ).

Arene Complexes of the Group 3 Metals and Lanthanides

Table 3

415

Summary of Ln-arene interaction distances for complexes 1.15-Sm to 1.55.

Complex

˚ )a M-centroid arene (m)(A

1.15-Sm 1.15-Eu 1.15-Yb 1.16-Pr 1.16-Nd 1.17-Tb 1.17-Dy 1.17-Ho 1.17-Tm 1.17-Er 1.18-Eu/Hob 1.18-Eu/Yb,c 1.18-Eu/Ndb,c 1.18-Yb/Yb 1.19-Sr 1.19-Ca 1.19-Ba 1.21-Nd 1.21-Sm 1.21-Dy 1.21-Er 1.23 1.24 1.26 1.28 1.30 1.33 La2(NHDipp)6 1.35 1.36 1.37 1.38 1.39 1.40 1.41 1.42 1.44 1.45 1.47 1.48 1.50-Y 1.50-Er 1.52 1.55

2.734 (Z6) 2.734/2.722 (both Z6) 2.623/2.650 (both Z6) 2.826(5) (Z6) 2.823(3)/2.816(3) (both Z6) 2.989(8) (Z1) 2.649 (Z6) 2.615 (Z6) 2.593 (Z6) 2.589 (Z6) 2.798(5) (Z6)/3.058(2) (Z1)/3.048 (Z3)/3.158 (Z2) 2.799(4) (Z6)/3.081(9) (Z1)/3.007(9) (Z1)/3.212(9) (Z1)/3.176 (Z3)/3.083 (Z3); 2.976(10) (Z1)/3.057 (Z2)/3.171 (Z3)/3.225 (Z3)/3.116 (Z3) 2.848(3) (Z6)/3.094 (Z2)/3.139 (Z2)/3.167 (Z3); 3.100 (Z3)/3.010 (Z2)/3.142 (Z2)/3.120 (Z2)/2.953(5) (Z1) 2.628(3) (Z6)/2.864(5) (Z1)/3.132(5) (Z1)/2.964(5) (Z1)/3.072 (Z2)/2.994 (Z4) Mixed occupancy of Eu/Sr sites—hapticities Z1, Z2 and Z3 observed Mixed occupancy of Eu/Ca sites—hapticities Z1, Z2 and Z3 observed 3.134 (Z4)/3.109 (Z2) 2.695(5) (Z6) 2.638(4) (Z6) 2.595(4) (Z6) 2.602 (Z6) 2.956(3)/2.954(3) (both Z6) 2.919(9)/3.277(9) (both Z6) 2.673(5) (Z6)/2.930(5) (Z1) 2.607(5) (Z6) 2.517(4)/2.483(3) (both Z6) 2.935(7) (Z6) 2.730 (Z6) 2.596 2.666(8) (Z1) 2.556(5) (Z6)/2.842 (Z2) 2.440(4) 2.663(9) (intra-Z6)/2.518(8) (inter-Z6) 2.584(6) (Z6)/2.823(8) (Z1) 2.795(3) (Z6)/3.075(4) (Z1) 2.638(4) (Z6) 2.935 (Z3) 2.705(6) (Z6) 3.062 (Z2) 3.098 (Z3) 2.525(16) (Z6) 2.504(2) (Z6) 2.837 (Z2) 2.901/3.031 (both Z2)

For Zm where m < 6 the value given corresponds to the average of the metal-aromatic carbon distances (i.e. {S(M-C)m}/m). 1.18-Ln/Ln0 lanthanide-arene interaction observed only for Ln. c Two 1.18-Ln/Ln0 complexes observed in the asymmetric unit and values separated by semicolon (;). a

b

Fig. 6 Effect of substituents guiding the formation of Ln-arene interactions.

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Arene Complexes of the Group 3 Metals and Lanthanides

Reaction of Sm(AlMe4)2 with 2 eq. of OddpH (Scheme 12) furnishes a Sm(II) complex [Sm(m-Odpp)2{(m-Me)(AlMe2)}2] 1.23 (Scheme 13), that features two Z6 Sm-arene interactions (2.956 (3) and 2.954 (3) A˚ ),55 which are considerably longer than the corresponding distances in 1.21-Sm (Fig. 6, 2.638 (4) A˚ ) or [Sm(II)(Z6-arene)(AlCl2I2)]2 (arene ¼ C6Me6: 2.655 (2) A˚ ; toluene: 2.694 (14) A˚ ). Complex 1.23 reacts with excess of CO2 at elevated temperatures to yield complex 1.24 (Scheme 13) where quite interestingly no oxidation of the Sm(II) center has occurred, but rather insertion of CO2 into an AldMedSm unit followed by ligand (Oddp) migration from the Sm metal center to an AlMe2 moiety. Two Sm Z6 arene interactions (2.919 (9) A˚ and 3.277 (9) A˚ ) are still present in 1.24 despite it being re-crystallized from DME.

Scheme 13 Synthesis of Sm(II) complexes 1.23 and 1.24.

Reacting Sm(AlMe4)2 with the chelating bis-aryl-phenol 1.25 (Scheme 14) in toluene, the Sm(II) complex 1.26 was isolated, which possesses an Z6 arene interaction of 2.673 (5) A˚ , similar to 1.21-Sm and [Sm(II)(Z6-arene)(AlCl2I2)]2 (vide supra), as well as a Sm to ipso aromatic carbon interaction with a distance of 2.930 (5) A˚ .

Scheme 14 Synthesis of Sm(II) complex 1.26.

The reaction of [SmN00 3] (N00 ¼ N(SiMe3)2) with 2 equivalents of bis-phenol 1.25 in the presence of AlMe3 leads to the formation of the bridged dimeric Sm(III) complex 1.27. The latter reacts with 1 further equivalent of 1.25 to yield the piano-stool-like Sm(III) complex 1.28 (Scheme 15), featuring a Sm(III)–p-arene-interaction centroid distance of 2.607 (5) A˚ , shorter than 1.26 (Scheme 14) as expected for the more Lewis acidic Sm(III) center in 1.28.

Scheme 15 Synthesis of Sm(III) complex 1.28.

Arene Complexes of the Group 3 Metals and Lanthanides

417

Reaction of the Yb(III) bis-formamidinato 9-fluorenone-ketyl complex 1.29 (Scheme 16) with reducing SmI2(THF)2 produced dark maroon crystals of the decanuclear mixed valence samarium cage 1.30 with concomitant formation of the Yb(II) complex 1.31. The coordination of the Sm10 core in 1.30 consists of 10 THF ligands, 4 O2− bridging ligands and finally 10 dianionic 9-fluorenone ligands ({Fn}O)2−. An ORTEP diagram of the molecular structure of 1.30 along with views to show Sm-arene interactions with the C6 rings of the 9-fluorenone dianion are given in Fig. 7.56

Scheme 16 Electron transfer between Yb(III) and Sm(II) to give rise to decanuclear cluster 1.30.

Fig. 7 Left: Structure of decanuclear cluster 1.30; middle and right: coordination environment of Sm6 featuring Z6 arene interaction from two different perspectives.

The ring centroid-Sm6(III) distances of 1.30 are 2.517 (4) and 2.483 (3) A˚ , which are quite short, even compared to Sm(III) complex 1.28, most likely due to the anionic charge of the ({Fn}O)2− ligand. They are nonetheless quite similar to the corresponding distance found in the Sm(III) complex 1.3.0A-Sm, supported by a tris-aryloxide ligand with an arene basal unit, of 2.499 A˚ (see Section 4.07.2.3).40 The Sm(II) complex 1.33 (Scheme 17), having a Sm Z6-toluene interaction with a long Sm centroid distance of 2.935 (7) A˚ , was serendipitously prepared by hydrolysis of the bridging aryloxide Sm(II) complex 1.32 where no Sm-arene interactions are observed.57

Scheme 17 Formation of Sm(II)-(Z6-C7H8) complex 1.33 from hydrolysis of 1.32.

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Arene Complexes of the Group 3 Metals and Lanthanides

For a list of Ln aryloxide complexes supporting intra- and intermolecular Ln-p arene interactions prior to 2002, we refer the reader to the review of Bochkarev.13 Similar to the arenes with Group 16 (O, S, Se) hard anionic donors discussed above, amido ligands ((NRR0 )− (R ¼ pendant arene, R0 ¼ H or other functionality) can support Z6-arene Ln/Group 3 interactions. A detailed comparative study between La(III) complexes La2(LDipp)6 (L ¼ O−, NH−, Dipp ¼ 2,6-iPr2-C6H3),58 both prepared via aminolysis of La[N(SiMe3)2]3 in toluene) (Fig. 8), shows that their solution behavior is markedly different; [La2(NHDipp)6] (1.34) is in a monomer-dimer equilibrium and reversible formation of the intermolecular Z6 arene La interaction is possible even after formation of the THF Lewis base adduct. On the other hand, La2(ODipp)6 does not show signs of such an equilibrium and upon coordination of THF the intermolecular Z6 arene La interaction cannot be re-established. The La-Z6 arene centroid distance in La2(NHDipp)6 is 2.730 A˚ .

Fig. 8 Isostructural La2(NHDipp)6 (X ¼ NH) and La2(ODipp)6 (X ¼ O).

The isostructural Sm(II) complex Sm2(NHDipp)6 1.35 (Scheme 18) is synthesized in a similar manner and features an intermolecular Z6 arene interaction between two Sm centers with a Sm centroid distance of 2.596 A˚ . Reaction of 1.35 with AlMe3 yields [(m-NDipp)Sm(m-NHDipp)(m-Me)AlMe2]2 1.36, where the Z6 arene intermolecular interaction seen in 1.35 has been disrupted by the formation of two bridging imido ligands and a Sm interaction with the ipso carbon (Sm-C: 2.666 (8) A˚ ) of the Dipp substituent of the imido ligand.59

Scheme 18 Disruption of Sm-Z6 arene interaction in 1.35 after reaction with AlMe3.

The same aminolysis protocol can also be used to access the dimeric Sm(III) complex 1.37 (Scheme 19), supported by a tripodal trisamido ligand which features two intermolecular Sm-arene interactions: a Sm-Z6 arene centroid distance of 2.556 (5) A˚ and a second Z2 interaction with SmdC distances of 2.801 (10) and 2.883 (12) A˚ .60

Scheme 19 Intermolecular Z6 and Z2 arene interaction in the Sm(III) dimetallic complex 1.37, supported by a tripodal tri-amido ligand.

Arene Complexes of the Group 3 Metals and Lanthanides

419

Reaction of YCl3 with the dilithium salt of bidentate di-amido L1 in Et2O gives rise to the dimetallic ate Y(III) complex 1.38 (Scheme 20) which displays an intermolecular Y-Z6 arene interaction with an Y-arene centroid distance of 2.440 (4) A˚ , similar within e.s.ds to that found in the close ion-pair Y complex 1.10Y (2.449 (3) A˚ , Scheme 6).61

Scheme 20 A bidentate diamido ligand supporting intermolecular Z6 interactions between two Y(III) metal centers.

Extremely bulky amido ligands can be also introduced via a redox transmetalation process using HgPh2 and metallic Yb or Eu (Scheme 21). In the case of L2H metalation of the benzylic CHPh2 was observed. In the case of L3H the Yb(II) dimer 1.39 was isolated, which incorporates both intra and intermolecular Yb-arene interactions. Each Yb center of 1.39 adopts a pseudotetrahedral geometry with the coordination sphere consisting of a YbdN amido bond, an orthometalated aromatic carbon (YbdC: 2.481 (4) A˚ ), and finally intra- (2.663 (9) A˚ ) and intermolecular (2.518 (8) A˚ ) Z6 arene interactions.62

Scheme 21 Indirect redox transmetalation to furnish dimer 1.39.

Employing the direct redox transmetalation route, while at the same utilizing the less hindered amido ligand L4 (Scheme 22), the homoleptic Yb(II) complex [Yb(L4)2] 1.40, as well as the Eu(II) iodo bridged dimer [EuL4(m-I)(THF)]2 1.41, can be prepared. The Yb(II) complex possesses an Z6-phenyl interaction with a Yb-centroid distance of 2.584 (6) A˚ , as well as an interaction with one of the aromatic ortho carbons with a YbdC distance of 2.823 (8) A˚ . A similar situation is observed in the case of Eu(II) complex 1.41, with the Eu-Z6 arene distance being 2.795 (3) A˚ and a long EudCortho interaction of 3.075 (4) A˚ .62

420

Arene Complexes of the Group 3 Metals and Lanthanides

Scheme 22 Direct redox transmetalation for the synthesis of Yb(II) and Eu(II) bulky amido complexes 1.40 and 1.41 respectively, with Ln(II)-Z6 arene interactions.

The calix-pyrollide Sm(III) complex 1.42 (Scheme 23) has a Sm-Z6 arene interaction of 2.638 (4) A˚ which is disrupted upon its reaction with MeLi to yield the N-confused complex 1.43 as the major product. Complex 1.44, where the Sm(III) center interacts with three aromatic carbons of each arene ring of the macrocyclic framework with an average SmdC bond of 2.935 A˚ , is a minor product of that same reaction, but it can be independently synthesized from 1.42 and NaH.63

Scheme 23 Calix-pyrollide Sm(III) complex with Z6-arene interactions.

The indole ligand L5H2 featuring a pendant arm with an aryl-amine (aryl ¼ Dipp) at the end, reacts with [LnN00 3](m-Cl)Li (THF)3 (N00 ¼ N(SiMe3)2−; Ln ¼ Yb, Eu) in toluene at elevated temperatures to yield the Yb(III) complex 1.46 and the Eu(II) complex 1.45 (Scheme 24). In the latter case the ligand has become mono-anionic with the pendant amido arm having transformed to a neutral imine.64 Complex 1.45 exists as a dimer in the solid state as result of an intermolecular Z6 Eu(II)-arene interaction with a metal center-centroid distance of 2.705 (6) A˚ (2.797 (13) A˚ in [Cp Eu(m-Z6:Z2-Ph2)BPh2]; 2.734/2.722 A˚ in 1.15-Eu). This strong intermolecular Eu-arene interaction is retained upon reaction of 1.45 with formamidines but its hapticity varies, depending on the steric bulk of the aryl substituents of the newly installed formamidinato ligand (Scheme 25). As such in complex 1.47 this manifests itself as an Z2-arene interaction with EudC distances of 3.042 (9) and 3.081 (10) A˚ while in the case of the less sterically hindered complex 1.48, an Z3 intermolecular Eu arene interaction is observed with an average EudC bond length of 3.098 A˚ .64

Arene Complexes of the Group 3 Metals and Lanthanides

421

Scheme 24 Eu and Yb complexes derived from an amino-indole ligand.

Scheme 25 Bimetallic arene Eu2+ complexes supported by an imino-indolyde ligand.

Reacting the imine functionalized indole L6H (Scheme 26) with [Ln(CH2SiMe3)3(THF)3] (Ln ¼ Y, Gd, Dy, Er, Yb) in toluene, yields complexes 1.49-Ln in ca 40% yield. Of this series of complexes, 1.49-Y and 1.49-Er react with PhSiH3 in toluene at 80  C to furnish the Ln(III) complexes 1.50-Y and 1.50-Er respectively, in which one of the imines in the L6 ligand scaffold has been reduced and is now an amido ligand (highlighted in red in Scheme 26). The reaction is presumed to occur via the intermediacy of a bridging di-hydride complex. Both 1.50-Y and 1.50-Er adopt dimeric structures in the solid state via Ln-Z6 arene interactions with the fused C6 aromatic ring of the indole section of L6. The Ln-centroid distances for 1.50-Y and 1.50-Er are 2.525 (16) and 2.504 (2) A˚ respectively.65

422

Arene Complexes of the Group 3 Metals and Lanthanides

Scheme 26 Bimetallic arene Ln3+ (Ln ¼ Er, Y) complexes supported by an imino/amido-indolyde ligand.

Reaction of [SmCptt2(THF)2] 1.51 (Scheme 27; Cptt ¼ 1,3-di-tertbutyl-C5H3) with 0.5 eq of P2Ph4 leads to the formation of a complex reaction mixture from which the mixed valence Sm(II)/Sm(III) complex [Sm(Cptt)2(m-PPh2)2Sm(Cptt)] 1.52 can be isolated. The monoligated Sm(II) center exhibits two Z2 interactions with two phenyls of the bridging phosphide ligands, with SmdC distances averaging 2.837 A˚ .66 A similar interaction has been observed in Cp 2SmAsPh2 1.55 with SmdC distances of 2.901 and 3.031 A˚ .67

Scheme 27 CpR supported Sm complexes showing Sm-arene interactions.

Amidinate ([(ArN)2CR]−) and guanidinate ([(ArN)2CNR2]−) monoanionic ligands are another class of ligands that support intramolecular Ln-arene p interactions. As can be seen in Fig. 9 the onset of this heterobidentate ligation is accompanied by a dramatic change in the ligand binding mode, i.e. from (k2-N,N0 ) to (k1-N, Z6-Ar) and vice versa.

Arene Complexes of the Group 3 Metals and Lanthanides

423

Fig. 9 Change of coordinating mode of amidinate (R0 ¼ alkyl) and guanidinate (R0 ¼ NR2) ligands to accommodate Ln-arene p interactions; the forward-backward arrows do not necessarily imply an equilibrium (L0 ¼ other co-ligands, R00 ¼ substituents on C6 aromatic ring).

For instance, the Yb(II) complex 1.56 which is prepared by reaction of YbI2(THF)n with one equivalent of K+[(DippN)2CNCy2]− (Dipp ¼ 2,6-iPr2-C6H3, Cy ¼ cyclohexyl), upon loss of THF converts to the Yb(II) complex 1.57 with the change of coordination mode being reversible upon addition of THF. The Yb-arene centroid distance in 1.57 is 2.424 (4) A˚ , which is the same within e.s.ds as one of the YbdN bond lengths in (k2-N,N) 1.56 (2.426 (3) A˚ ). Upon adaptation of the (k1-N, Z6-Ar) binding mode the YbdN amide bond distance in 1.57 somewhat shortens (2.360 (3) A˚ ) compared to 1.56 (2.373 (3) A˚ ) (Scheme 28).68

Scheme 28 Interconversion of (k2-N,N0 ) and (k1-N, Z6-Ar) binding modes in a guanidinate Yb(II) complex by loss and addition of THF.

Reaction of the (k2-N,N0 ) amidinate complex [((NDipp)2CtBu)YbN00 (THF)] (N00 ¼ N(SiMe3)2, L7 ¼ ((NDipp)2CtBu)− 1.58 with PhSiH3 leads to the formation of the bridging dimeric complex [{k1-N, Z6-Ar((NDipp)2CtBu)}Yb(m-H)]2 1.59 (Scheme 29) where the amidinate ligand has adopted a (k1-N, Z6-Ar) coordination mode with a Yb-arene centroid distance of 2.420 (4) A˚ .69 Complex 1.59 is stable in THF for short periods of time, as it can be recovered intact after recrystallization. If left in a THF-hexane mixture for several days it undergoes a ligand re-distribution reaction to form the Yb(II) complex [((NDipp)2CtBu)2Yb] (1.60). The robustness of the Z6-Ar Yb(II) interaction was further confirmed as no reaction was detected with TMEDA (N,N,N0 ,N0 -tetramethyl-1,2-ethylenediamine), DPPE (1,2-diphenyl-phosphino-ethane) and DPPM (diphenyl-phosphino-methane). Complex 1.59 undergoes insertion of the triple bond in PhC^CPh in the YbdH bond to furnish complex [[{k1-N, Z6-Ar((NDipp)2CtBu)}Yb]2(m-Z4:Z4-PhCHCHPh)] 1.61 where the original (k1-N, Z6-Ar) coordination mode of the amidinate ligand has been retained with the Yb-Z6 arene centroid distance being essentially unchanged from 1.59 (2.416 (5) A˚ ). Measurements of the magnetic susceptibility of 1.61 show that the

Scheme 29 Formation of a bridging YbdH complex supported by a Z6-arene-Yb interaction and its reactivity with di-phenyl-acetylene.

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Arene Complexes of the Group 3 Metals and Lanthanides

complex is diamagnetic and the single crystal XRD structure shows that the alkyne bond has been reduced to almost a single bond (1.482 (3) A˚ ), thus signifying a [PhCHCHPh]2− bridging ligand. Both Yb(II) metal centers in 1.61 interact with the ipso and ortho carbons (YbdC distance of 2.66242 (16) and 2.8568 (18) A˚ , respectively) of the phenyl groups in the [PhCHCHPh]2− ligand. Diaconescu et al. has more recently reported Y, Lu and La complexes with bridging reduced E-stilbene similar to 1.61 and which are discussed in more detail in Section 4.07.3.3. It is worth mentioning that complex 1.59 can also be prepared by reaction of the Yb(III) complex [((NDipp)2CtBu)Yb(CH2SiMe3)2(THF)] with PhSiH3, most likely via a reduction—s-bond metathesis reaction sequence.70 Complex 1.59 can be derivatized further to furnish complexes [{k1-N, Z6-Ar(NDipp)2CtBu)}Yb(m-Cl)]2 1.62,70 [{k1-N, 6 Z -Ar(NDipp)2CtBu)}Yb(m-SPh)]2 1.63,70 [{k1-N, Z6-Ar(NDipp)2CtBu)}Yb]2(m-H)(m-PPh2) 1.6470 and [{k1-N, Z6-Ar(NDipp)2CtBu)} Yb(m-k2:k2-H3BPh)]2 1.65,71 all of which feature Yb(II) metal centers with Z6-arene interactions (Scheme 30 and Table 4 below).

Scheme 30 Reactivity of bridging hydride complex 1.59. Table 4 Summary of M-centroid arene (Z6) of lanthanide complexes supported by guanidinate, amidinate and formamidinate ligands. Complex

˚) M-centroid arene (6)(A

1.57 1.59 1.61 1.62 1.63 1.64 1.65 1.66-Yb 1.66-Sm 1.67 1.68 1.69 1.70 1.71 1.72Dipp 1.77 1.78 1.79

2.424(4) 2.420(4) 2.416(5) Connectivity only 2.433(2)/2.440(6) 2.437(6) 2.457(7) 2.695(3) 2.762(1) 2.520(3) 2.695(3) 2.697(4)/2.603(4) 2.676(1) 2.657(2) 2.804(8)/2.848(8) 2.696(3) 2.775(14) 2.769(3)

Arene Complexes of the Group 3 Metals and Lanthanides

425

Another strategy to drive the (k1-N, Z6-Ar) coordination mode is the use of asymmetric amidinate ligands like L8H and L9H (Scheme 31) that feature arenes with pendant neutral donor ligands as in complexes 1.66-Ln (Ln ¼ Sm,72 Yb73), 1.6774 and 1.68.72 The Z6 arene distances in 1.66-Yb, 1.67 and 1.68 are 2.695 (3), 2.520 (3) and 2.695 (3) A˚ respectively and in the case of 1.66-Sm the Sm-arene centroid distance increases to 2.762 (1) A˚ . Complex 1.66-Yb shows signs of fluxionality in solution which is fast on the NMR timescale, even at low temperatures, and therefore it cannot be confirmed that the (k1-N, Z6-Ar) coordination mode seen in the solid state is retained in solution.

Scheme 31 Formation of the 7-coordinate amidinate (k1-N, Z6-Ar) Yb(II) complexes 1.66 promoted by ether coordination (N00 ¼ [N(SiMe3)2]−).

Adaptation of (k1-N, Z6-Ar) ligation mode in anionic guanidinate ligands can also be imposed by the choice of starting materials and synthetic route used to install the ligand onto the Ln metal center. For example, reaction of KL7 (Scheme 32) with SmI2(THF)x leads either to the formation of the dimeric [{k1-N,Z6-Ar(NDipp)2CtBu)}Sm(m-I)(THF)0.5]2 1.6975 or the monomeric [{k1-N,Z6-Ar(NDipp)2CtBu)}SmI(THF)2]72 (x ¼ 3) 1.70 (Scheme 32), depending on crystallization conditions. The latter complex undergoes a salt metathesis reaction with NaN00 in toluene to yield [{k1-N,Z6-Ar(NDipp)2CtBu)}SmN00 ]72 1.71 (Scheme 32) (N00 ¼ N(SiMe3)2); the absence of THF from the coordination sphere of 1.71 is notably unlike the Yb(II) complex 1.58 which features a (k2-N,N0 ) L1. The Sm-Z6-arene distances for 1.69, 1.70 and 1.71 are 2.697 (4)/2.603 (4) (Sm(II) with THF coordinated/ Sm(II) without THF coordinated), 2.676 (1) and 2.657 (2) A˚ , respectively. These bond distances vary considerably and seem to be dependent upon the coordination site occupied by other co-ligands relative to the intramolecular Z6 Sm-arene interaction. Complexes 1.66-Ln–1.71 have varying catalytic competence in hydro-phospination reactions of styrene and alkynes as well as the hydroamination reaction of styrene with preference or in some cases exclusive selectivity for the anti-Markovnikov coupling product.72,73

426

Arene Complexes of the Group 3 Metals and Lanthanides

ation

talliz

crys

crystallization

Scheme 32 (k1-N, Z6-Ar) Sm(II) complexes.

The (k1-N, Z6-Ar) mode can also be adopted by ansa-amidinate ligands as in Yb(II) complex 1.72Dipp prepared by aminolysis of [YbN00 2(THF)2] (Scheme 33). Complex 1.72Dipp adopts an almost symmetric conformation, from a topological point of view, with respect to the arene-Yb interactions which feature Yb-arene centroid distances of 2.804 (8) and 2.848 (8) A˚ . These two interactions are longer compared to the corresponding distances found in complexes 1.59, 1.62–1.65 (see above) due to the rigidity imposed by the ortho-phenylene linker in the di-amidinate ligand L10. The bis (k1-N, Z6-Ar) coordination mode in 1.72Dipp is disturbed upon oxidation of the Yb(II) metal center with I2 or in the case of the analogous bis-amidinate complex 1.72xyl, with 2,6-xylyl flanking arenes instead of Dipp, with Ph3SnCl, thus furnishing the Yb(III) complexes 1.73 and 1.74xyl which both show (k2-N,N0 ) of the ansa-amidinate framework. Reduction of 1.74xyl with sodium naphthalenide reverses the situation reforming complex 1.72xyl. Attempts to introduce the dianionic bis-amidinate ligand L10 to La(III) via salt metathesis reaction with LaCl3 produced the ate complex [{(k2-N,N0 )2L10La(III)}(m-Cl)Li(THF)(m-Cl)2Li(THF)2] 1.75 or the trimetallic bridging chloride cluster complex [{(k2-N, N0 )2L10La(III)}(m-Cl)5{(k2-N,N0 )L7La(III)}2] 1.76 (L7 ¼ (NDipp)2CtBu)−) depending on reaction conditions, but in all cases (k2-N, N0 ) coordination of both amidinate arms of L10 was observed. As such steric demands (e.g. ion size) are not the only factor in promoting (k1-N, Z6-Ar) multi-dentate modes in guanidinate and amidinate ligands but equally the role of the metal electronic configuration must be taken into account.76

Scheme 33 Ansa-amidinate ligand adopting a bis (k1, Z6) arene conformation and its alteration to (k2-N,N0 ) upon oxidation of the Yb center.

Similar to the bidentate amidinate complex discussed previously, the Sm(II)-ate complex 1.77 (Fig. 10) features an Z6 arene interaction with one of the Dipp arene substituents of the ligand. Unlike the examples discussed above delocalization of the negative charge across the NCN ring is still present although at first sight the molecular structure shows k1-N coordination to the Sm(II) metal center. The Sm(II) arene centroid distance is 2.696 (3) A˚ . Complex 1.77 reacts with PhN]NPh and organic azides to give rise to cubic and doubly fused cubic Sm clusters featuring bridging imido ligands.77,78

Arene Complexes of the Group 3 Metals and Lanthanides

427

Fig. 10 Sm(II) complex 1.77.

Related formamidinato (ArNCNAr)− and triazenido (ArNNNAr)− ligands can also support similar Z6 arene interactions. Reaction of La(AlMe4)3 with L11EtH or L12MesH (Mes ¼ 2,4,6-Me3-C6H2) yields complexes 1.78 and 1.79 respectively (Scheme 34). In both cases the ligand intercepts the produced AlMe3 giving rise to a dative N ➔ Al bond, which is most likely responsible for the geometric re-arrangement of the formamidinato ligands to accommodate the Z6 arene interaction. Unlike amidinate ligands the k2-N,N0 ligation is maintained in 1.78. In the case of 1.79, alumination of one of the ortho methyls of one of the mesityl substituents is also observed and it co-crystallizes with its geometric isomer 1.790 where the Z6-mesityl La(III) metal center interaction has been replaced by two (m-Me) bridges. The analogous reaction of L11EtH with Y(AlMe4)3 furnished the Y(III) complex [(k2-N,N0 )L11 Et Y(m-Me)2AlMe2] 1.80 where no Y-arene interaction is observed.79 The La-Z6-arene distances in 1.78 and 1.79 are 2.775 (14) and 2.769 (3) A˚ respectively and are longer than the those observed in [(Z6-toluene)La(III)((m-Cl)2AlCl2)3] (2.633 (3) A˚ ) and [(Z6-C6Me6)La(III)((m-Cl)2GaCl2)3] (2.64 (3) A˚ ).

Scheme 34 La(III) formamidinato complexes with intramolecular Z6-arene interaction.

When the sterically encumbered triazenido ligand L13H (Scheme 35) was reacted with Eu or Yb metal in the presence of Hg(C6F5)2 the Ln2+ complexes 1.81 and 1.82/1.82.THF were isolated (Scheme 35). Despite the larger ionic radii of these +2

Scheme 35 Triazenido Ln2+ (Ln ¼ Eu, Yb) complexes with varied hapticity of the arene rings (dotted lines).

428

Arene Complexes of the Group 3 Metals and Lanthanides

lanthanide metal centers, the bulky arene rings compete with THF allowing the isolation of the base-free complexes, with complex 1.82.THF readily losing THF to yield 1.82. The molecular structures of 1.81 and 1.82 show that in both case the Ln2+ metal centers interact with the mesityl arene of the terphenyl group in an 5 fashion with LndC(arene) distances ranging between 3.088 (2)–3.233 (2) A˚ for 1.81 and 3.003 (5)–3.160 (6) A˚ for 1.82. Additionally, both metal centers interact with the tri-isopropyl arene ring of the biphenyl substituent in an 4 fashion for the smaller Yb2+ metal center (YbdC: 3.070 (5)–3.421 (6) A˚ ) and 5 for Eu2+ (EudC: 3.153 (2)–3.395 (2) A˚ ).80 Indenyl, fluorenyl and carbazolide ligands can also act as arene p-donors to Ln metal centers through their fused aromatic C6 rings. Reaction of [(Z5-SiMe3fluorenyl)2Sm(THF)2] 1.83-Sm with an excess of AlR3 (R ¼ Me, Et) gives rise to complexes 1.84R (R ¼Me, Et) in which the coordination of the Sm(II) center has switched from an Z5 fashion to the C5 ring to an Z6 mode to the fused C6 arene ring. The change in hapticity is reversed back to 1.83 upon exposure of complexes 1.84R to THF. In the case of the Yb analog [(Z5-SiMe3fluorenyl)2Yb(THF)2] 1.83-Yb similar reactivity was observed but only one of the fluorenyl ligands underwent this switch from Z5 to Z6 upon reaction with AlMe3 in toluene (Scheme 36). In contrast to 1.84R exposure of 1.85 to THF does not reverse the reaction but rather leads to the formation [(Z5-SiMe3fluorenyl)Yb(THF)4]+[AlMe4]− 1.86. Although not structurally confirmed, the Yb(II) bis-indenyl complex [(Z5-iPr-indenyl)2Yb(THF)2] 1.87 seems to exhibit similar reactivity when reacted with AlMe3 (Table 5).81

Scheme 36 Switching from Z5-C5 to Z6-C6 coordination in Sm(II) and Yb(II) fluorenyl complexes upon reaction with AlR3.

Table 5 Hapticities and lengths of Ln-arene interactions observed in indenyl, fluorenyl and carbazolide complexes. Complex

˚ )a M-centroid arene (m)(A

1.84Me 1.84Et 1.85 1.95 1.96 1.97 1.98 1.99 1.100 1.103

2.744(3) (Z6)b 2.754(14)/2.773(16) (Z6)c 2.531(16) (Z6) 2.934 (Z4) 2.882 (Z4) 2.102 (Z4) 2.670 (Z6) 2.610/2.630 (Z6) 2.75(2) (Z6) 2.442(4) (Z6)

For Zm where m < 6 the value given corresponds to the average of the metal-aromatic carbon distances (i.e. {S(M-C)m}/m). b Average distance of closest three SmdC(C6) bonds: 3.038 A˚ . c Average distance of closest three SmdC(C6) bonds: 3.061 and 3.056 A˚ . a

The related bis-fluorenyl Yb(II) [Yb(Z5-C13H9)2(THF)2] 1.88 ((C13H9)− ¼ fluorenyl anion) reacts with two equivalents of the di-substituted diaza-butadiene 1.89 (Scheme 37) to give the unexpected product [Yb{Z5-C13H8C(]N[2,6-iPr2C6H3]) CH2NHC6H3(2,6-iPr2C6H3)}2(THF)] 1.90 with loss of fluorene. In 1.90, coupling of the allylic fluorenyl carbon with the imino carbon of 1.89 has occurred, giving rise to an Z3 type of coordination through one carbon of the 5-membered ring of the fluorenyl scaffold and two carbons of its fused 6-membered aromatic ring, as well as Z2 coordination of the imino group to the Yb center. As a result, a planar Z5 bonded heteropentadienyl frame is formed. Magnetic susceptibility measurements and NMR spectroscopy advocate for a diamagnetic Yb(II) complex 1.90. The Yb(II)dC(Z3) distances involving the fluorenyl ligand average at 2.796 A˚ , with the longest found at 2.868 (2) A˚ and the shortest at 2.759 (2) A˚ . When 1.88 was reacted in a similar manner with the diaza-butadiene 1.91 having two methyls on its backbone, the Yb(II) complex 1.92 was isolated (Scheme 37).82

Arene Complexes of the Group 3 Metals and Lanthanides

429

Scheme 37 Effect of ligand back-bone on the formation of Yb(II) arene interaction.

Changing the supporting ligand from fluorenyl to indenyl, reaction of [Yb(II)(Z5-C9H7)2(THF)2] 1.93 ((C9H7)− ¼ indenyl anion) with two equivalents of the less sterically demanding redox-active PhN]C(Me)d(Me)C]NPh diazabutadiene 1.94 (Scheme 38), furnishes the mixed valence complex [Yb2(m-Z5:Z4-C9H7)(Z5-C9H7)2{m-Z4:Z4-PhNC(Me)]C(Me)NPh}] 1.95 accompanied by loss of indene. Complex 1.95 features an Yb-Z4 arene interaction with YbdC distances ranging between 2.713

Scheme 38 A redox-active ligand driving the formation of Yb-arene interactions.

430

Arene Complexes of the Group 3 Metals and Lanthanides

(10) and 3.171 (7) A˚ (average YbdC: 2.934 A˚ ) and a Yb-Z4 centroid distance of 2.654 (5) A˚ . Quite interestingly, this Z4coordinated C6 ring deviates from planarity as evidenced by the twist angle of 7.7 along the axis of the Z4 coordinated carbons, in contrast to the corresponding value of 2.2 for the terminal Z5 indenyl ligands. When 1.93 is reacted in a similar manner with 4 equivalents of 1.94, the mixed valence dimeric complex 1.96 is produced, as the result of coupling between two moieties, identical to 1.95, via most likely H atom abstraction from the methyls on the backbone of 1.95 followed by radical recombination (Scheme 38). Again, a bridging (m-Z5:Z4-C9H7)− ligand is present as in the case of 1.95, with YbdCZ4 distances ranging from 2.690 (4) to 3.093 (4) A˚ (average: 2.882 A˚ ) and a Yb-Z4 centroid distance of 2.595 (18) A˚ , which is somewhat shorter compared to 1.95.83 Aminolysis of [YbN00 3](m-Cl)Li(THF)3 with two equivalents of the indenyl ligand L14H (Scheme 39) which bears a pendant tertiary amine, furnishes the Yb(II) ate complex 1.97 where one indenyl ligand is connected to the Yb(II) metal center in the expected Z5 fashion, while the other coordinates to the metal center via the fused C6 ring of the L14 framework in an Z4 fashion with YbdC(C6) distances in the range 2.733(6)–3.221 (6) A˚ . In this particular case the formation of a LidCldYb bridge most likely contributes to promoting this mixed coordination mode (Scheme 39).84

Scheme 39 An indenyl ligand adopting an Z5,Z4 coordination mode.

Carbazole (CbzH]C12H8NH) reacts in a melt under solvent free conditions with Nd, Eu, or Gd metal in the presence of elemental Hg, as well as with Sm or Yb metal under Hg-free conditions at temperatures ranging from 220 to 280  C. The products are all crystallized from the melt thus yielding [Eu(C12H8N)2]1 1.98,85 [Yb(C12H8N)2]1 1.99,85 [Tm2(Cbz)6] 1.100,86 [Nd2(Cbz)6] 1.101,86 [Gd2(Cbz)6]86 and finally {[Sm2(Cbz)5](CbzH)}1 1.103.86 Complexes 1.98, 1.99 and 1.100 adopt in the solid state a repeating trimetallic motif with bridging Cbz anions connecting to the metal centers via a normal LndN bond a Ln(CNC) interaction and finally an Z6 arene Ln interaction. In the case of Tm complex [Tm2(Cbz)6] 1.100 and its Nd and Gd analogs (the latter two have not been structurally characterized) the situation is simpler and is shown in Fig. 11. The Z6-arene Ln distance is 2.670 A˚ for 1.98, 2.610–2.630 A˚ for 1.99, 2.75 (2) A˚ for 1.100 and 2.442 (4) A˚ for 1.103.

Fig. 11 Tm(III) complex 1.100.

4.07.2.3

Intramolecular arene interactions supported by a tripodal tris-phenoxide ligand

Meyer and Evans et al. have synthesized the series of 1.3.0A-Ln(III)/Y(III) (Ln ¼ La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb) complexes featuring a tris-aryloxide ligand tethered to a basal mesitylene anchor (Scheme 40), via the aminolysis of the corresponding Y/LnN00 3 (N00 ¼ N(SiMe3)2) with the neutral trisphenol ligand [Mes(Ad,MeArOH)3].40 Although this series of Ln(III)/Y(III) complexes feature similar Ln-arene intramolecular interactions as those described above, they are discussed at this point between Sections 4.07.2 and 4.07.3 of this article, due to the dichotomy arising in their classification after their subsequent reduction.

Arene Complexes of the Group 3 Metals and Lanthanides

431

(A)

(B)

(C)

Scheme 40 Synthesis of [Ln(III)/Y(III) Mes(Ad,MeArO) 3] complexes and their formally Ln2+ [K([2.2.2.-crypt])][Ln(II)/Y(II) Mes(Ad,MeArO)3] analogs.

All complexes, with the exception of 1.3.0A-Ce, have been structurally characterized and feature an arene-Ln interaction decreasing in distance regularly from La to Yb, consistent with the lanthanide contraction (Table 6). These distances are shorter than those found in Ln3+ complexes of the general formula (6-arene)Ln[(m-Cl)2AlCl2]3. In the case of 1.3.0A-Y, the Y-mesitylene distance is identical to that of 1.3.0A-Dy but longer than that of 1.3.0A-Er (2.368 (13) A˚ for 1.3.0A-Y and 2.336 (2) A˚ for 1.3.0A-Er). The arene centroid distance in 1.3.0A-Y is considerably shorter than those observed for Z6-arene cationic or ion-pair Y(III) complexes 1.6-Y (2.564 (7) A˚ , Scheme 4), 1.8-Y (2.545 (3) A˚ , Scheme 5) and 1.10-Y (2.449 (3) A˚ , Scheme 6). A small increase in the arene torsion angle of the mesitylene anchor (range 5.3–9.3 ) is also observed compared to that seen in the neutral ligand [Mes(Ad,MeArOH)3] (3 ), but is not monotonic across the series. Furthermore, the CdC bond distances and angles of the mesitylene anchor in this series of Ln3+/Y3+ complexes are the same within e.s.ds with those crystallographically characterized in the neutral ligand. As a comparison the Gd-mesitylene centroid distance in 1.3.0A-Gd of 2.412 (8) A˚ is significantly longer than that found in [Gd(1,3,5-tris-tert-butyl-benzene)2] (2.219 (2) A˚ ).1 Complexes 1.3.0A-Ln(III) can be reduced with KC8 in the presence of 2.2.2-cryptand (Schemes 40B and C) to their formally Ln2+ (see also below) congeners [K(2.2.2-cryptand)][Ln/YMes(Ad,MeArO)3] (1.3.0B-Ln/Y and 1.3.0C-Ln). When Ln ¼ Gd, Dy and Er as well as in the case of yttrium they co-crystallize with the respective Ln3+ hydrides (i.e. the complexes [K(2.2.2-cryptand)][HLn/ YMes(Ad,MeArO)3]: 1.3.0B-Ln/Y-H).40,87 A hint as to the source of the hydride was provided by the isolation of the by-product 1.3.1La from the reduction of 1.3.0A-La with excess potassium in THF in the presence of [2.2.2]-cryptand (Scheme 41).88

432

Table 6 letters).

Arene Complexes of the Group 3 Metals and Lanthanides

M-arene centroid distances and arene torsion angles in tris-aryloxide complexes 1.3.0-La/Y as well as their formally Ln(II) derivatives (in red bold

Scheme 41 Origin of co-crystallizing 1.3.0B-Ly/H complexes in Scheme 40.

Arene Complexes of the Group 3 Metals and Lanthanides

433

As can be seen from Scheme 41, the ligand activation on one of the benzylic arms, produces hydride ([K(2.2.2-crypt)]+ H−), which is intercepted by 1.3.0A-Ln/Y, thus giving rise to the observed by-products 1.3.0B-Ln/Y-H. Upon reduction of 1.3.0A-La, the La-mesitylene centroid distance decreases from 2.575 (10) A˚ in 1.3.0A-La to 2.472 (2) A˚ in 1.3.1-La and 2.458 (4) A˚ in 1.3.0C-La and is accompanied by an increase in the distortion of the aromatic anchor from planarity (5.5 in 1.3.0A-La, 9.8 in 1.3.0C-La and 18.4 in 1.3.1-La—see also below).88 It is worth noting that in the case of 1.53.0A-Dy when the reduction was performed under the conditions depicted in Scheme 40B, co-crystallization of [K(2.2.2-cryptand)][OHDy Mes(Ad,MeArO)3] was also observed. When the reaction was repeated with K in the presence of 18-crown-6 a 1:1 mixture of [K(18-crown-6)(THF)2][Dy Mes(Ad,MeArO)3] and [K(18-crown-6)(THF)2][HDy Mes(Ad, Me ArO)3] was isolated. Upon the reduction the Ln-arene centroid distance is reduced compared to the 1.3.0A-Ln starting complexes except in the case of 1.3.0C-Sm and 1.3.0C-Yb where this distance is increased. This is attributed to the 4fn+1 electronic configuration these two latter complexes adopt upon reduction (i.e. 4f6 for Sm2+ and 4f14 for Yb2+) and is easily rationalized on the basis of accessible Sm3+/Sm2+ and Yb3+/Yb2+ reduction potentials; this was further supported by DFT calculations. The increase in the Sm/Yb-arene distance is accompanied by a reduction in the mesitylene ring distortion in 1.3.0C-Sm/Yb compared to 1.3.0A-Sm/Yb. In the case of 1.3.0C-La, 1.3.0C-Ce, 1.3.0C-Pr and 1.3.0B-Y UV–Vis and EPR spectroscopies and DFT calculations are consistent with reduction at the mesitylene anchor (i.e. formation of a radical anion) rather than the metal. Spin density population analysis shows that the HOMO (singly occupied) in complexes 1.3.0C-La/Ce/Pr and 1.3.0B-Y (4f0/f1/f2/f0 respectively) has predominantly mesitylene p character, and in the case of 1.3.0C-Ce a molecular orbital of d-symmetry (f-p ) is located as the SOMO-1. In the case of 1.3.1-La, the reduction is again ligand-based, with the difference that the HOMO is delocalized over the mesitylene anchor and one of the aryloxide arms of the scaffold. This explains the decrease in the Ln-arene distance observed upon reduction of the 1.3.0A-Ln(III)/Y(III) complexes as well as the increase in distortion of the aromatic anchor (see above for La; for 1.3.0A-Ce there are no crystallographic data; 1.3.0A-Pr: 5.3 , 1.3.0C-Pr: 10.9 ). Reduction of 1.3.0A-Nd yields the 4f4 complex 1.3.0C-Nd cleanly and without concomitant formation of a [Mes(Ad,MeArO)3Nd(III)-H]− byproduct. The electronic configuration of 1.3.0C-Nd was substantiated structurally, spectroscopically and theoretically (DFT) and resembles that found for [K(2.2.2cryptand)][UMes(Ad,MeArO)3] (U2+/5f4); as expected the Nd-arene centroid distance is longer compared to its U2+ analog due to the radial extension of the 5f orbitals in the latter. DFT analysis of 1.3.0C-Nd showed two singly occupied molecular orbitals of d-symmetry (SOMO, SOMO-1), with the other two electrons of the quintet state occupying f-orbitals; this explains the reduction of the Nd-mesitylene centroid distance in 1.3.0C-Nd when compared to 1.3.0A-Nd where the three electrons occupy f-orbitals. Finally in the case of 1.3.0A-Gd, reduction occurs at the metal center as well, with spectroscopic evidence pointing to a 4f75d1 electronic configuration; unfortunately structural and calculated metric parameters (DFT) for 1.3.0C-Gd are not in agreement.40,87,88 Due to the co-crystallization of 1.3.0B-Ln and 1.3.0B-Ln-H and the crystallographic disorder this imposes, comparisons of Ln-mesitylene distances and ring distortions are not as meaningful. As a general observation, coordination of a hard anionic ligand in the axial position (see also below) pushes the metal center out of the of the cavity of the ligand, resulting in a more coplanar arrangement of the Ln/Y-(O-Aryl)3 core and an increase of the Ln-mesitylene distance. As an example, in the case of 1.3.0B-Y/ 1.3.0B-Y-H where the hydride complex was isolated in one occasion free from 1.3.0B-Y, the axial coordination of the hydride causes an increase of the Y-mesitylene distance from 2.368 (13) A˚ in 1.3.0A-Y to 2.775 A˚ in 1.3.0B-Y-H.88 The complexes 1.3.0A-Ln discussed above, with the exception of 1.3.0A-Yb, were found to be competent electrocatalysts for the reduction of H2O in THF ([H2O] ¼ 0.22 M with [N(n-Bu)4]PF6 as electrolyte).89 Synthetic and electrochemical investigations suggest the proposed four step mechanism depicted in Scheme 42. In the case where Ln ¼ Nd complexes 1.3.2-Nd-H2O and 1.3.2Nd-OH were independently synthesized and fully characterized. Water coordinates equatorially to the Nd(III) center and has no effect whatsoever in the Nd-arene interaction when compared to 1.3.0A-Nd. On the other hand in 1.3.2-Nd-OH, the axial coordination of the OH− engenders severe changes in the molecular structure with the Nd-arene distance increasing to 2.753 (3) A˚ in 1.3.2-Nd-OH from 2.489 (2) A˚ in 1.3.0A-Nd.89

Scheme 42 Electrocatalytic reduction of H2O by complexes 1.3.0A-Ln.

434

Arene Complexes of the Group 3 Metals and Lanthanides

The catalytic onset potential for the reduction of H2O to OH− and H2 correlates with the Ln(III)/(II) redox potential for complexes 1.3.0A-Ln. As mentioned above although 1.3.0A-Yb has theoretically the lowest overpotential of the series, it is surprisingly catalytically inactive. 1.3.0A-Sm has the second lowest overpotential and is a competent electrocatalyst. The reason for the 1.3.0A-Yb’s inactivity is rationalized on the basis that as the series is traversed from La to Yb, the increase in the Lewis acidity of the metal center, renders 1.3.2-Ln-H2O complexes more stable and thus shifting the equilibrium in equation 4 (Scheme 42) to the left. Furthermore, this increase in Lewis acidity is also responsible for the increased stability of the hydroxo complexes 1.3.2Ln-OH. Although the E1/2 of the Ln(III)/(II) redox couple of 1.3.2-Ln-OH complexes is shifted significantly into negative values (outside tetrahydrofuran’s electrochemical window under these conditions), in the presence of H2O and under catalytic conditions 1.3.2-Ln-OH complexes are converted to 1.3.2-Ln-H2O which are in turn in equilibrium with 1.3.0A-Ln. As a result of the increased stability of the catalytic resting states 1.3.2-Ln-H2O and 1.3.2-Ln-OH moving towards the heavier lanthanides, the concentration of the active catalyst in operando is considerably lower than the initial catalyst loading. The monotonic reduction of activity across this series is also reflected by the relative reaction rates kobs for the electrocatalytic reduction of H2O, which were found to be an order of magnitude greater for the earlier lanthanides.89 Overall, the H2O reduction by 1.3.0A-Ln is markedly different than that substantiated for U(III) complexes supported by the same tripodal aryloxide ligand.90,91

4.07.3

Anionic arene Ln/Group 3 interactions: Inverted arenes

Arenes have accessible p and p orbitals, giving them the ability to act as multielectron neutral or anionic ligands. In this section, Group3 and lanthanide complexes where the arene acts as an anionic ligand are discussed; often such complexes are referred to as inverted arene complexes and one of their characteristics is the deviation from planarity (albeit not always) and elongation of CdC bonds (i.e. loss of aromaticity) due to the increased occupancy of their p manifold. Such complexes are of fundamental interest in terms of structure and bonding, but they can also serve as synthetically useful strong multi-electron reductants as a result of the occupancy of the relatively high energy p orbitals of the arene ligand. The assignment of formal oxidation states in inverted arene complexes can potentially be problematic, and to make any such arguments with some certainty and elucidate the bonding of the arene ligand requires a full range of spectroscopic, magnetic and structural characterization techniques combined with computational studies.

4.07.3.1

Inverted arene complexes with neutral co-ligands

Bochkarev and coworkers pioneered the synthesis of fully reduced naphthalene (C10H8) Ln complexes (Ln ¼ Ce, Sm, Eu, Tm, Yb) by reaction of the corresponding LnI2 with 2 equivalents of the lithium naphthalene radical anion (LiC10H8) in ethereal solvents (THF, DME or Et2O), thus yielding complexes of the general formula (C10H8)xLn(solvent)y (Ln ¼ Ce, x ¼ 2, solvent ¼ Et2O, y ¼ 1; Ln ¼ Sm, x ¼ 1, solvent ¼ THF, y ¼ 3; Ln ¼ Eu, x ¼ 1, if solvent ¼ DME then y ¼ 1 or 2, if solvent ¼ THF then y ¼ 3, if solvent ¼ Et2O then y ¼ 1; Ln ¼ Yb, x ¼ 1, if solvent ¼ THF then y ¼ 3, if solvent ¼ DME then y ¼ 1).13,92,93 In contrast, reaction of LiC10H8 with the corresponding LnX3 (X ¼ I, Cl) in a 3:1 stoichiometric ratio resulted in the isolation of poorly defined products. When TmI2(DME)3 was used, the triple-decker Tm(III) complex [Tm(DME)(Z6-C10H8)]2(m-Z4:Z4-C10H8) (2.1.0-Tm) was isolated (Scheme 43, Table 7).96 According to charge balance, all naphthalene ligands in 2.1.0-Tm exist as dianions. These complexes have been reviewed previously and we refer the reader to these publications.13

Scheme 43 Synthesis of Tm(III) triple decker inverted naphthalene complex 2.1.0-Tm.

More recently Okuda et al. reported complex 2.1.1-Yb (Scheme 44) prepared by the protonation of [(Me4-TACD)Yb(CH2Ph)2] (Me4-TACD ¼ 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) with the mild acid [NEt3H][BAr4] (Ar ¼ 3,5-(CH3)2-C6H3).97 The molecular structure of purple 2.1.1-Yb shows an Z6-coordinated benzyl fragment, reminiscent of polyamine-stabilized potassium benzyl complexes. The C(CH2)-C(ipso) of 1.370 (10) A˚ along with the 1JCH coupling constant of 148 Hz for that benzylic CH2 carbon show pronounced sp2 character. The DFT optimized structure is in very good agreement with the molecular structure of 2.1.1-Yb and the NBO analysis shows a partially covalent bonded benzyl group through its p system with a WBI of 0.16 for each YbdC interaction (for comparison the WBI of the YbdN bonds of the coordinated macrocycle is 0.17). It is worth noting that the

Arene Complexes of the Group 3 Metals and Lanthanides

Table 7

435

Key metric parameters of structurally characterized inverted lanthanide arene complexes discussed in Sections 4.07.3.1 and 4.07.3.2.

Complex

M-centroid arene ˚ )a (m)(A

MdC average ˚ )b distance for 4 (A

Shortest MdC ˚) bond (A

CdC ˚) average (A

˚) Shortest/Longest CdC (A

Arene Torsion Angle ( )c

2.1.0-Tm

2.156(6) (Z6)/ 2.288(9) (Z4) 2.535(4) (Z4) 2.497(7) (Z4) 2.208(3) (Z4) 2.330(3) (Z4)

2.605

2.408(15) (Z6)/ 2.607(15) (Z4) 2.815 2.782 2.487 2.471(6)

1.44 (Z6)/ 1.41 (Z4) 1.419 1.414 1.447 1.458

1.31(2) and 1.49(2) (Z6)/1.35(3) and 1.47(2) (Z4) 1.365/1.455 1.328/1.454 1.392/1.485 1.440(7)/1.473(8)

23.77 (Z6)/ 18.23 (Z4) 25.03 27.07 32.60 39.85

2.2.0A-Eu(DME) 2.2.0B-La(THF) 2.2.0C-Dy 2.2.0D-Tm

2.832 2.793 2.550 2.650

Calculated using Olex2.94 For Z4 the average metal-aromatic carbon distances is calculated via the formula i.e. {S(M-C)m}/4. c Defined as the largest dihedral angle between two adjacent three carbon planes in the arene ring and calculated using Mercury.95 a

b

Scheme 44 Synthesis of Yb(II) complex 2.1.1-Yb.

CdC bond lengths of the Z6-coordinated benzyl show significant variation within the carbocycle. The Yb-Cent(Z6-benzyl) of 2.375 (3) A˚ is longer than that observed in 1.3.0A-Yb(III) (Scheme 40A) (2.323 (2) A˚ ) but shorter than that found in 1.3.0C-Yb(II) (Scheme 40C) (2.412 A˚ ). Complex 2.1.1-Yb reacts readily with H2 in THF to furnish complex [(Me4-TACD)2Yb2(m2-H)2(THF)] [BAr4]2 (2.1.2-Yb) which is a competent catalyst for the hydrogenation and hydrosilylation of 1-hexene.

4.07.3.2

Inverted arene complexes with simple X co-ligands (X ¼ I, H)

Following the synthesis of the aforementioned fully reduced naphthalene complexes in Section 4.07.3.1, the inverted naphthalene complexes [LnI(solvent)x]2(m-C10H8) (Ln ¼ Yb, Eu; solvent ¼ DME, x ¼ 2; solvent ¼ THF, x ¼ 4) (2.2.0A-Yb92 and 2.2.0A-Eu93) were synthesized by the reaction of the corresponding LnI2 (Ln ¼ Yb, Eu) with an equimolar amount of LiC10H8 (Scheme 45A), and both complexes feature a dianionic bridging naphthalene ligand. This same motif is also present in the case of the inverted naphthalene Ln(III) complexes of the general formula [LnI2(solvent)x]2(m-C10H8) (Ln ¼ La,93 Ce,92 Pr,92 Nd92 and Gd92; solvent ¼ DME then x ¼ 2; solvent ¼ THF then x ¼ 3) (2.2.0B-Ln) prepared via the reaction of the corresponding LnI3(THF)3 with Li and naphthalene in either THF or DME (Scheme 45B) as solvent. The Nd(III) complex can also be synthesized by the oxidation of NdI2 with naphthalene in THF. On the other hand, the same reaction between DyI2 and naphthalene in DME at −45  C yielded [DyI(DME)2(Z4-C10H8)] 2.2.0C-Dy98 (Scheme 45C) featuring, as in the case of 2.2.0A-Yb and 2.2.0A-Eu, a dianionic naphthalene ligand (as evidenced by the fair agreement between the corresponding CdC bond lengths in the molecular structures of 2.2.0C-Dy and [{Li(18-crown-6)}C10H8]).99 This is further supported by the fact that addition of D2O to 2.2.0C-Dy generates 1,4-dideuteronaphthalene. Complexes 2.2.0A-Yb and 2.2.0A-Eu are useful starting materials for the preparation of Yb(II) and Eu(II) ansa metallocene and fluorene complexes.100

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Arene Complexes of the Group 3 Metals and Lanthanides

(A)

(B)

(C)

(D)

(E)

(F)

Scheme 45 Synthesis of inverted anthracene and naphthalene lanthanide complexes.

2.2.0B-La reacts with acenaphthylene to yield a complex with the composition [LaI2(THF)3]2(C12H8) with NMR spectroscopic data suggesting complexation of the two [LaI2(THF)3]+ moieties on opposite sides of the five membered rings.101 Reaction of lithium anthracenide generated in situ with TmI2(DME)x (prepared from Tm metal and iodine in DME) produces the Tm(III) complex [TmI(DME)2(Z2-C14H10)] 2.2.0D-Tm (Scheme 45D). Its magnetic moment (6.5 mB) is less than the theoretical value calculated for a 4f12 Tm(III) center (7.56 mB) and based on charge balance and keeping in mind the reducing power of Tm(II), a trivalent Tm(III) is assumed in 2.2.0D-Tm. The two six-membered rings flanking the middle ring in 2.2.0D-Tm have lost their planarity. For further information regarding this class of inverted arene complexes, we refer the reader to previous reviews102 and Table 7 below for key metric parameters of structurally characterized complexes. Bochkarev et al. reported in 2011 on the isolation of naphthalene complexes of Nd and Dy 2.2.0E-Ln and 2.2.0F-Ln (Ln ¼ Nd, Dy) featuring bridging hydrides of the general formula [(C10H8)3Ln5Li5H14] and [(C10H8)3Ln4H9], respectively. The assignment of bridging hydride ligands was done on the basis of IR stretches at 1271 and 475 cm−1. The preparation of 2.2.0F-Ln from the reaction of 2.2.0E-Ln with NH{Si(CH3)3}2 (HN00 ) with loss of Li+[N{Si(CH3)3}2]− (LiN00 ) and H2 further shows that 2.2.0E-Ln is an ate complex. Unfortunately, these two compounds have not been structurally characterized.103

4.07.3.3

Inverted arene complexes supported by amido ligands

Bi-aryl inverted arene yttrium complexes supported by the dianionic macrocyclic [P2N2]2− ligand (Scheme 46) were reported by Fryzuk et al. and were synthesized by the reaction of [Y(P2N2)(m-Cl)]2 (2.3.0Y-1) with Ar-Li (Ar ¼ Ph (2.3.0Y-2), p-tolyl (2.3.0Y-3),

Arene Complexes of the Group 3 Metals and Lanthanides

437

Scheme 46 Synthesis of Y(III) and Lu(III) inverted arene complexes supported by the [P2N2]2− macrocyclic ligand reported by Fryzuk et al.

m-tolyl (2.3.0Y-4)) or alternatively in the case of (Y-2) via the aromatic CdH activation of benzene promoted by [Y(P2N2) CH2SiMe3] (2.3.0Y-5).104 In the case of Ar ¼ Ph, m-tolyl the [Y(P2N2)] moieties coordinate on the opposite faces of the two coupled rings and the molecular structure of 2.3.0Y-2 indicates a double bond between the two Ph rings with each Y[P2N2] fragment coordinating in an Z5 fashion (no molecular structure is available for 2.3.0Y-4 but it displays the same blue coloration as 2.3.0Y-2 and its NMR spectra are characteristic of two inequivalent aromatic environments). In contrast, in the case of the deep yellow orange (2.3.0Y-3) (i.e. Ar ¼ p-tolyl) the Y[P2N2] moieties coordinate on opposites sides of the same ring of the substituted biphenyl system in the solid state. The solid state asymmetry induced by this mode of coordination of the Y[P2N2] fragments in (2.3.0Y-3) is not retained in solution, as a result of fluxional migration of the Y[P2N2] moieties between the two aromatic systems. Variable-temperature NMR spectroscopic studies showed quenching of this fluxionality at −95  C. Inverted naphthalene, 1-methyl-naphthalene and anthracene complexes of Y and Lu, supported by the same [P2N2]2− scaffold can also be accessed by the reduction of the appropriate precursor [Ln(P2N2)(m-Cl)]2 (Ln ¼ Y (2.3.0Y-1), Lu (2.3.0Lu-1)) with two equivalents of KC8 in toluene/Et2O in the presence of the corresponding arene (complexes 2.3.0Y-7/8/9 and 2.3.0Lu-2/3/4).105 The complexes display fluxional behavior in solution which is associated with facile intra-ring migration of the [Ln(P2N2)] moiety (no intermolecular crossover was observed between complexes or with free arene), similar to that observed for the d10 Ni0 complex [Ni(Z2-C10H8) (dippe)] (dippe ¼ 1,2-(diisopropylphosphanyl)ethane), where the weak p-backdonation is thought to facilitate the ring migration.106 If the bonding in these complexes is considered to be electrostatic in nature, then this intra-ring motion can be thought of as migration of the cationic [Ln(P2N2)]+ fragment over the polycyclic aromatic dianion. Using the ferrocene based dianionic diamido ligand [1,10 -fc(NTBDMS)2]2− (fc ¼ Fe(C5H4)2, TBDMS ¼ tBuSiMe2, abbreviated as (NNTBDMS)), Diaconescu and coworkers were able to isolate a diverse family of Group 3 (Sc, Y, Lu) (Schemes 47, 48 and 49 below) and lanthanide inverted arene complexes (Ln ¼ La, Gd, Dy Er) (Scheme 49). The choice of the (NNTBDMS)2− scaffold is crucial for the observed reactivity by stabilizing the highly electrophilic Sc(III) complexes; in contrast the diamido complex [(Me3Si)2N]2ScITHF does not display such reactivity with anthracene.

Scheme 47 Sc(III) inverted arene complexes supported by a ferrocenyl di-amido ligand reported by Diaconescu et al. ((NNTBDMS) ¼ [1,10 -fc(NTBDMS)2]2− fc ¼ Fe(C5H4)2, TBDMS ¼ SiMet2Bu).

438

Arene Complexes of the Group 3 Metals and Lanthanides

Scheme 48 Synthesis of naphthalene inverted arene complex 2.31Y-3.THF from the stilbene complex 2.3.7-Y. ((NNTBDMS) ¼ [1,10 -fc(NTBDMS)2]2− fc ¼ Fe(C5H4)2, TBDMS ¼ SiMet2Bu).

Scheme 49 Synthesis of inverted arene lanthanide complexes supported by a ferrocenyl diamido ligand, featuring quadruply reduced 6 carbon 10 p aromatic systems.

Complexes [Sc(NNTBDMS)]2(m-4:4-C10H8) (2.3.1Sc-2) and [Sc(NNTBDMS)]2(m-6:6-C14H10) (2.3.1Sc-3)107 are reminiscent of the yttrium diamido complexes (2.3.0Y-7) and (2.3.0Y-9) (Scheme 46), respectively reported by Fryzuk, discussed above. For example, the solution behavior of 2.3.1Sc-3 shows a similar inter-ring migration of the [Sc(NNTBDS)]+ fragment. One interesting feature of the (NNTBDMS) scaffold is a short interaction between the Fe center of the ferrocene backbone and the Sc(III) metal center in 2.3.1Sc-2 and 2.3.1Sc-3 in their solid-state structures (2.83 vs 3.12 A˚ in (2.3.1Sc-1)). DFT calculations showed that this is electrostatic in nature, with the Fe center being the donor and the electrophilic Sc(III) metal the acceptor. Complexes of the general formula [M(THF)(NNTBDMS)]2(m-4:4-C10H8) (2.3.1 M-2) (M ¼ La,108 Lu,108 Y109) can also be synthesized in a similar manner to

Arene Complexes of the Group 3 Metals and Lanthanides

439

2.3.1Sc-2. The M(III)-naphthalene centroid distance in this series of [M(THF)x(NNTBDMS)]2(m-4:4-C10H8) (2.3.1Sc-2, x ¼ 0; 2.3.1 M-2, x ¼ 1, M ¼ La, Y) complexes increases monotonically from 2.178 (2) A˚ in 2.3.1Sc-2 to 2.349 (2)/2.358 (2) A˚ in 2.3.1Y-2.THF and finally 2.5607 (14)/2.5743 (14) A˚ 2.3.1La-2.THF. It is interesting to note that the Fe ⋯ M interaction (M ¼ Sc, Y, La) follows the same trend in this series of complexes increasing from 2.83 A˚ in 2.3.1Sc-2 to ca 3.21 A˚ in 2.3.1Y-2.THF and 3.26 A˚ in 2.3.1La-2.THF. The analogous yttrium complex [Y(THF)(NNTBDMS)]2(m-6:6-C14H10) 2.3.1Y-3.THF can also be synthesized following the route for 2.3.1Sc-3. Interestingly 2.3.1Y-3.THF can also be accessed via reaction of anthracene with the yttrium stilbene complex [Y(THF)(NNTBDMS)]2(m-3:3-(E)-stilbene) (2.3.7-Y) (Scheme 48 and 53 below).110 Stilbene complexes like (2.3.7-Y) can be considered as a special case of an inverted arene and are thus discussed further below. Following this work, Diaconescu and co-workers have further displayed the advantages of the (NNTBDMS)2− ligand scaffold by demonstrating the synthesis of inverted arene complexes with a variety of lanthanide metals and arenes (Scheme 49).111 Complexes 2.3.3-Y-arene and 2.3.3-Y-crown (as well as their La, Gd, Dy, Lu analogs) are distinctly different from the Y complexes 2.3.0Y-2 and 2.3.0Y-3 discussed previously (Scheme 46). Firstly, the two [M(III)(NNTBDMS)] moieties coordinate on opposite sides of the same aromatic system independent of the bi-aryl system employed and regardless of whether the potassium counter cation engages in a contact ion pair or it is separated via its encapsulation with 18-crown-6. Secondly, unlike 2.3.0Y-3 these complexes do not display any fluxionality in solution as a result of inter-ring migration of the [M(NNTBDMS)] fragments, even at low temperatures; NMR spectroscopy in the diamagnetic cases (i.e. 2.3.3-Y/La/Lu-arene and 2.3.3-Y/La/Lu-crown) of this series showed distinct signals in the 1H and 13C{1H} NMR spectra for the two different aromatic systems with no sign of fluxionality at temperatures as low as −89  C. Other spectroscopic analyses (XANES, 89Y NMR spectroscopy), coupled with DFT, further supports two distinct aromatic systems in which the coordination of two [M(III)(NNTBDMS)] fragments on the opposite sides of one of them promotes but also stabilizes its overall 4e− reduction, thus producing a 6 carbon 10 p electron Hückel (n ¼ 2) aromatic system. The observed average CdC distances of ca 1.47 A˚ and angles of essentially 120 are in agreement with the theoretically calculated corresponding values (1.507 A˚ , 120 ) for C6H4− 6 in an idealized D6h geometry. In the representative case of the pseudo-lanthanide 2.3.3-Y-arene the aromatic 10-p tetraanionic arene is stabilized by a d bonding interaction between the 4d orbitals and the p antibonding manifold of the arene, which has moderate covalent character (20% contribution of Y orbitals in HOMO and HOMO1).111,112 Crystallographically determined metric parameters along with DFT calculations (2.3.3-Y-crown and later 2.3.3-Sm-crown) show that the negative charges are mainly localized on the coordinated aromatic ring, whereas the un-coordinated one acts mainly as an electron-withdrawing group. Nevertheless, there is some variation of the bond lengths in the latter which can be rationalized due to the contribution of canonical resonance structure B depicted in the following Fig. 12. The following two Table 8 summarize selected bond lengths for the series of structurally characterized bis-arene complexes depicted in Scheme 49. Inspection of the metric parameters given in the table above reveals some trends. Firstly, the shorter the M-arene centroid distance the higher the distortion of the arene ring from planarity, as shown by the increased arene torsion angles and plane twist angles when considering complexes 2.3.3-M-arene; in that respect 2.3.3-Sc-biphenyl features the most distorted inverted arene. Secondly there is a monotonic decrease of M-centroid distance with decreasing Ln3+ ionic radius which applies for both 2.3.3M-arene and 2.3.3-M-crown complexes, which is also observed in the 1.3.0A-Ln series of complexes discussed in Section 4.07.2.3. Both these observations are reflective of the bonding situation between the Ln(III) center and the inverted arene, studied by DFT and which show that across the series the HOMO is a d orbital with ca 20% contribution from d orbitals of the metal center (in the case of Y this involves 4d orbitals whereas in the case of Sm 5d orbitals due to their radial extension). This d covalent interaction also offers an explanation as to the lack of fluxionality in solution of these 2.3.3-M-arene complexes, as opposed to their fused arene congeners featuring a loose ionic interaction between the metal ion and the reduced arene. Thirdly, upon removal of the K+ counterions by encapslulation with 18-crown-6 from the 2.3.3-M-arene complexes to form the corresponding 2.3.3-M-crown complexes, a decrease of the M-arene centroid distance is observed accompanied by an increase in the distortion of the inverted arene from planarity. Similarly, this encapsulation of the K+ countercations results in an increase of the average CdC distance of the inverted arene. The latter is accompanied by a marked increase of the CdC bridge head distance between the two types of arene in these complexes upon sequestration of the K+ countercations, suggesting that in case of the 2.3.3-M-crown complexes, resonance form A shown in Fig. 12 has an increased contribution.

Fig. 12 Resonance structures for biphenyl fragment.

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Arene Complexes of the Group 3 Metals and Lanthanides

Table 8 Comparison of some metric parameters of inverted arene complexes depicted in Scheme 49, metric parameters were calculated using either Olex294 (M-centroid distance, plane twist angle) or Mercury95 (arene torsion angle). Metric/Complex

2.3.3-Sc-biphenyl

2.33-Y-biphenyl

2.3.3-Y-terphenyl

2.3.3-Y-crown

2.3.3-Lu-biphenyl

2.3.3-Lu-TPB

M-Cent (A˚ ) Arene torsion angle ( )b Arene plane twist angle ( )c CdC (average) (A˚ ) Longest CdC (A˚ ) Shortest CdC (A˚ ) CipsodCipso arene bridge (A˚ )

1.936/1.948 22.03/25.52 12.1 1.464 1.485(3) 1.433(3) 1.419(3)

2.093a 13.41a 6.77 1.462 1.486(2) 1.422(2) 1.414(4)

2.106a 11.42a 5.7 1.460 1.484(3) 1.420(3) 1.411(5)

2.053a 14.54a 7.4 1.475 1.496(7) 1.441(6) 1.477(10)

2.056/2.044 19.58/22.89 10.8 1.464 1.497(6) 1.413(6) 1.416(4)

2.051/2.062 21.87/22.93 11.42 1.470 1.493(4) 1.426(4) 1.420(4)

Metric/Complex

2.3.3-Smbiphenyl

2.3.3Sm-crown

2.3.3-Gdbiphenyl

2.3.3Gd-crown

2.3.3-Dybiphenyl

2.3.3Dy-crown

2.3.3-Erbiphenyl

2.3.3Er-crown

M-Cent (A˚ ) Arene torsion angle ( )b Arene plane twist angle ( )c CdC (average) (A˚ ) Longest CdC (A˚ ) Shortest CdC (A˚ ) CipsodCipso arene bridge (A˚ )

2.168a 7.44a

2.154/2.147 8.8/16.10

2.134a 11.52a

2.113/2.106 12.96/14.75

2.097 13.24a

2.054a 14.86a

2.068/2.071 16.47/16.97

2.034a 16.68a

3.72

6.23

5.79

6.9

6.67

7.5

8.4

8.4

1.454 1.476(2) 1.421(3) 1.413(4)

1.457 1.479(3) 1.432(4) 1.440(3)

1.458 1.479(3) 1.425(3) 1.413(4)

1.464 1.483(8) 1.437(8) 1.461(8)

1.458 1.478(3) 1.424(4) 1.412(5)

1.466 1.489(4) 1.433(4) 1.431(7)

1.462 1.492(11) 1.421(12) 1.424(11)

1.468 1.494(6) 1.434(5) 1.437(9)

a

The full structure is generated by symmetry. From 4 consecutive C atoms on the arene coordinated to Ln. c Twist angle between the planes defined by the sets of red and green carbon atoms respectively (see below). b

Magnetic studies (SQUID) combined with theoretical calculations on complexes 2.3.3-Dy-biphenyl, and its 18-crown-6 congener 2.3.3-Dy-crown, showed the significant effect that the potassium counter cation exhibits depending on whether it is part of a contact ion pair or a separated one; in the case of the former an antiferromagnetic interaction was observed between the Dy(III) centers while in the case of the latter a ferromagnetic one was established. Nevertheless 2.3.3-Dy-biphenyl shows SMM behavior under zero dc field with an energy barrier (Ueff) of 34 K. In contrast due to extensive quantum tunneling as a result of the ferromagnetic coupling of the two Dy(III) centers, 2.3.3-Dy-crown showed no SMM properties. When 2.3.3-Er-biphenyl, 2.3.3-Er-crown and 2.3.3-Gd-biphenyl, 2.3.3-Gd-crown were studied, the first two displayed a ferromagnetic interaction between the Er metal centers while the gadolinium analogs showed an antiferromagnetic interaction.113 An outlier of the 2.3.3-M-arene series of inverted arene complexes discussed above can be found in the case of Yb (Scheme 50). As can been seen in Scheme 50, the optimized synthesis of 2.3.3-Yb-biphenyl (52% yield) is different from that detailed above in Scheme 49 and involves in situ reduction of biphenyl with KC8 followed by addition of the Yb(III) starting material [(NNTBDMS)YbCl(THF)2].114

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Scheme 50 Optimized synthesis of 2.3.3-Yb-biphenyl.

If instead the synthetic protocol used for 2.3.3-Sm-biphenyl is employed, although 2.3.3-Yb-biphenyl is isolated, this is in much lower yields and is plagued by contamination from other diamagnetic Yb(II) species. This is most likely due to the much smaller reduction potential of the Yb3+/2+ couple (ca −1.15 V vs SHE) compared to the reduction potential of biphenyl (−2.45 V vs SHE), resulting most likely in the simultaneous reduction of both the arene substrate and [(NNTBDMS)YbCl(THF)2]. Indeed, one of the identifiable Yb(II) by-products from the one-pot KC8 reaction of [(NNTBDMS)YbCl(THF)2] with biphenyl is the dimer [(NNTBDMS) Yb(II)(THF)]2 which was characterized by its independent synthesis via the reduction of [(NNTBDMS)Yb(III)Cl(THF)2] with KC8 (Scheme 51).

Scheme 51 Independent synthesis of an [Yb(II)] by-product in the preparation of 2.3.3-Yb-biphenyl.

Structurally, the Yb-biphenyl is completely different from the 2.3.3-M-biphenyl complexes discussed above (Scheme 49), as it features the two [(NNTBDMS)Yb] fragments coordinating to both the phenyl rings of the biphenyl, similar to 2.3.0Y-2 (Scheme 46). Furthermore, unlike 2.3.3-Y-biphenyl, the 1H NMR spectrum of 2.3.3-Yb-biphenyl shows signs of fluxionality in solution. Quite significantly, the crystallographically determined metric parameters are consistent with a dianionic biphenyl ligand where the charge is equally distributed between the two phenyl rings (average phenyl CdC bond length: 1.421 A˚ and essentially planar phenyl rings) with a CipsodCipso bond distance of 1.396 (4) A˚ . Furthermore, the reduction of the YbdN distance is more akin to a Yb(II) metal center. The Yb(II) formal oxidation was further confirmed by the near-IR (NIR) spectrum of 2.3.3-Yb-biphenyl, which features much lower intensity bands in the NIR region compared to 2.3.3-Sm-biphenyl, and is consistent with a filled f-shell. Finally, DFT calculations revealed that the bonding between the Yb center and the biphenyl in 2.3.3-Yb-biphenyl is mostly ionic in character with the HOMO featuring a small contribution from 4f atomic orbitals (12.4%), with no covalent d bonds, as in 2.3.3Ln-biphenyl, being present either in the HOMO or HOMO-1. The weaker interaction between the Yb(II) metal center and the phenyl rings in 2.3.3-Yb-biphenyl, is also manifested in the Yb-Cent distance of 2.416 A˚ which even after adjusting for the ionic radii difference in the series is longer. Moreover, the ionic character of the Yb-arene bond in 2.3.3-Yb-biphenyl is further reflected by its observed reactivity when attempts to isolate the analogous 2.3.3-Yb-crown (Scheme 49) by removal of the coordinated K+ counter-cation with 18-crown-6 were undertaken (Scheme 52).

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Arene Complexes of the Group 3 Metals and Lanthanides

z Scheme 52 Products identified from the sequestration of K+ with 18-crown-6 from 2.3.3-Yb-biphenyl ((NNTBDMS) ¼ [1,10 -fc(NTBDMS)2]2− fc ¼ Fe(C5H4)2, TBDMS ¼ SiMet2Bu).

trans-Stilbene complexes (2.3.7-M) (M ¼ Y, La) are synthesized by reduction of [(NNTBDMS)MI(THF)x] with KC8 in the presence of E-stilbene (Scheme 53) in THF or in the case of 2.3.7-Y from the reaction of 2.3.1Y-2.THF (Scheme 47) with E-stilbene.110 Based on such arene exchange experiments, Diaconescu and coworkers have qualitatively established the reducing power of these complexes as [Y(THF)(NNTBDMS)]2(m-4:4-C10H8) (2.3.1Y-2.THF) > [Y(THF)(NNTBDMS)]2(m-E-stilbene) (2.3.7-Y) > [Y(THF) (NNTBDMS)]2(m-6:6-C14H10) (2.3.1Y-3.THF). Complex 2.3.8-Y featuring one Y(III) center and a K+ ion coordinated to one of the phenyl rings, can also be prepared by reduction of 2.3.3-Y with KC8 in THF or by reaction of 2.3.3-Y-biphenyl (Scheme 49) with E-stilbene.

Scheme 53 Synthesis of Y and La E-stilbene complexes ((NNTBDMS) ¼ [1,10 -fc(NTBDMS)2]2− fc ¼ Fe(C5H4)2, TBDMS ¼ SiMet2Bu).

As mentioned above, such stilbene complexes can be considered as a special case of inverted arene complexes: in both cases the C]C bond has lengthened and in the case of 2.3.7-Y ranges between 1.49 and 1.52 A˚ , while in the case of 2.3.7-La the corresponding values are 1.43 (3) and 1.47 (2) A˚ (two values are given per complex due to the crystallographic positional disorder of the E-stilbene CdC bond) and are consistent with single CdC bonds. In both cases the metal center coordinates in an 3 fashion involving also the ipso carbon of the phenyl ring. Interestingly, there is an elongation of some of the phenyl CdC bonds suggesting some degree of charge delocalization (for 2.3.7-Y: C1-C2: 1.456 (8) A˚ (avg. CdC: 1.407 A˚ ) for 2.3.7-La: C1-C2: 1.429 (8) A˚ ; C4-C5: 1.443 (8) A˚ (avg. CdC: 1.395 A˚ )). DFT showed that the bonding situation in complexes 2.3.7-M involves a p-interaction between the central CdC bond and the metal (stabilized by its delocalization into the adjacent phenyl rings) with the HOMOs of both

Arene Complexes of the Group 3 Metals and Lanthanides

443

complexes comprised primarily of the p orbitals of the central E-stilbene CdC bond; a situation very similar to that found in the inverted scandium naphthalene complex (2.3.1Sc-2) (Scheme 47). This is very different to the bonding situation in complexes 2.3.3-Y-biphenyl/2.3.3-Y-crown discussed above where a d-bond is responsible for the covalent bonding interaction in the HOMO, since there are no p orbitals of the right symmetry to form a p bond in these two complexes. It is worth pointing out the preference of uranium ketimide complexes to form bridging bimetallic m-6:6 arene complexes upon reduction of [UI(DME)(ketimide)3] with K or Na in the presence of E-stilbene,115 as well as the d-bond responsible for the activation of ethane in [(8-{1,4(SiiPr3)2-C8H4})(5-CpMe5)U]2(m-2:2-C2H4).116 This is partly due to the radial extension of the 5f orbitals, enabling them to participate in d-covalent interactions.117 In 2020 Diaconescu et al. published a series of bimetallic Th(IV) bridging inverted arene (arene ¼ benzene, toluene, biphenyl, naphthalene) complexes supported by the (NNTBDMS)2−, analogous to Group 3 complexes 2.3.1Sc-2 discussed above.118 Spectroscopic, structural and theoretical investigations point to Th(IV) metal centers and a tetra-anionic bridging inverted arene ligand. For comparative reasons, the HOMO and HOMO-1 in these Th complexes consists of a d bond between p orbitals of the arene and 6d-5f hybrids of the metal center and is to some extent reminiscent of the bonding situation in complex 2.3.3-Y-biphenyl. Analysis of the orbital contributions reveals that the Th-arene bond in these complexes is more covalent than in their rare-earth analogs (30% vs ca 20%). The inverted arene complexes supported by the diamido-ferrocenyl ligand described so far, display a wealth of reactivity characteristic of their reductive nature; apart from the benchmark reduction of 2,20 -bipyridine107 and the reductive coupling of pyridine,107,119 they can also activate P4 (Scheme 54) to form Zintl-type polyphosphides,108,109 aromatic CdF and CdH bonds (Scheme 55),120 terminal CdH alkynes107,110 (notice the loss of dihydronaphthalene i.e. C10H10) (Scheme 56) and promote the reductive coupling of di-phenylacetylene (Scheme 57) to form metallocyclopentadienes.121 Similar reductive coupling of diphenylacetylene has been observed by Bochkarev et al. in the reaction of [CpLu(Z1:Z1:Z2-C10H8)(DME)] (Cp ¼ (C5H5)−, 2.5.0Lu-napth., Scheme 61) with diphenylacetylene discussed later in Section 4.07.3.5.122

Scheme 54 Reactivity of inverted arene complexes with P4 ((NNTBDMS) ¼ [1,10 -fc(NTBDMS)2]2− fc ¼ Fe(C5H4)2, TBDMS ¼ SiMet2Bu).

In the last example the inverted naphthalene [M(NNTBDMS)(4-C10H8)]− K+ (M ¼ Sc, Y, Lu, La) was not isolated and was characterized by its spectroscopic similarity to isolable 2.3.3-Y-anthrac (Scheme 49). The involvement of a transient [(NNTBDMS) M]2(m-6:6-C6F6) (M ¼ Sc, Y, Lu) is hypothesized as the first step in the aromatic CdF bond activation of C6F6 by the inverted arene complexes 2.3.1Sc-2/2.3.1 M-2.THF (M ¼ Y, Lu). Finally, in the case of the activation of P4, it is worth noting that 2.3.1Sc-2/3 forms two products unlike its 2.3.1 M-2.THF (M ¼ Y, Lu, La) analogs, which feature larger metal centers. The role of the ionic radius of the metal center was also underlined by the observation that the 2.3.1Sc-2 reacts slower (16 h) than 2.3.1 M-2.THF (M ¼ Y, Lu, La) (1–2 h).

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Arene Complexes of the Group 3 Metals and Lanthanides

Scheme 55 CdH and CdF bond activation promoted by inverted arene complexes ((NNTBDMS) ¼ [1,10 -fc(NTBDMS)2]2− fc ¼ Fe(C5H4)2, TBDMS ¼ SiMe2tBu).

Scheme 56 Reactivity of 2.3.1Sc-2 and 2.3.7-Y with PhCCH ((NNTBDMS) ¼ [1,10 -fc(NTBDMS)2]2− fc ¼ Fe(C5H4)2, TBDMS ¼ SiMe2tBu).

in-situ

Scheme 57 Reductive coupling of di-phenylacetylene by inverted arene complexes ((NNTBDMS) ¼ [1,10 -fc(NTBDMS)2]2− fc ¼ Fe(C5H4)2, TBDMS ¼ SiMe2tBu).

4.07.3.4

Inverted arene complexes supported by RO− ligands

Inverted arene rare-earth complexes supported by tripodal aryloxide ligands have been discussed above. Another member of this class of RO− ligands able to support inverted arene complexes of the rare-earths, is the (tris-tert-butoxy)siloxide ligand (tBuO)3SidO−, which Mazzanti and co-workers have extensively used to stabilize a variety of f-block complexes and promote unique reactivity (some representative examples referenced123–128).

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445

Reduction of the formally Ce(III) siloxide complex 2.4-Ce (Scheme 58) with excess potassium in the presence of [2.2.2-cryptand] in toluene/THF at −40  C, led to the isolation, in 25% yield, of the novel quadruple decker complex [K(2.2.2cryptand)]2[{KL3Ce}(m-6:6-C7H8)2Ce] (2.4.0-Ce) (L ¼ (tBuO)3SidO−) with concomitant loss of KOSi(OtBu)3.129

Scheme 58 Preparation of a triple decker Ce inverted arene complex.

Although the molecular structure of 2.4.0-Ce, shows essentially two planar toluene ligands, the elongation of the mean CdC ring bond length (1.441 A˚ ) is comparable to related complexes featuring doubly reduced benzene anions like in [{Ce(III)(Cp00 )2} (m-Z6:Z6-C6H6)(K-18-crown-6)] (average CdC: 1.42 A˚ ) (Cp00 ¼ 1,3-bis-trimethylsilyl-cyclopentadienide), where the bridging (C6H6)2− is best described as a 1,4-cyclohexa-2,5-dienyl ligand with p-localization. It is perhaps more comparable to that of [K(18-crown-6)(Z2-C6H6)][{La(II)(Cptt)2}2(m-6:6-C6H6)] featuring a monoanionic C6H6 bridging ligand (average CdC: 1.44 A˚ —see also below) (Cptt ¼ 1,3-bis-tert-butyl-cyclopentadienide). The CdC ring bond length is however significantly longer than the corresponding value found in Ln0(1,3,5-tri-tert-butyl-benzene)2 complexes (1.414 A˚ ). The Cea/a0 -toluene centroid distance of (atoms Cea and Cea0 in the molecular structure of 2.4.0-Ce are symmetry-related) 2.273 (3) A˚ is shorter than that found in Lappert’s [{Ce(Cp00 2)}2(m-Z6:Z6-C7H8)]− (ca 2.38 A˚ —no e.s.ds due to the lower quality of the single crystal XRD data) which also features two Ce2+ metal centers bridged by a (toluene)1− ligand. The corresponding average Ceb-toluene centroid distances show one shorter (ca 2.21 A˚ ) and one slightly longer (2.29 A˚ ), compared to Cea/a0 -toluene centroid lengths, while both are shorter than those in [{Ce(Cp00 2)}2(m-6:6-C7H8)]−. Moreover they are significantly shorter than Ce(III)-arene centroid distances crystallographically observed in complexes of the general formula [(Z6-arene)Ce{(m-Cl)2GaCl2}3] (arene ¼ toluene: 2.609 (4) A˚ ; arene ¼ pxylene: 2.615 (4) A˚ ; arene ¼ pyrene: 2.600 (2) A˚ ; arene ¼ naphthalene: 2.607 (6) A˚ ). The formulation of two doubly reduced toluene ligands and three Ce(II) metal centers, was further substantiated by UV–Vis/NIR spectroscopy, the difference in the CedO bond lengths between 2.4-Ce and 2.4.0-Ce and finally DFT studies. The latter reproduced very well the experimentally observed bond distances in 2.4.0-Ce, when a 4f2 electronic configuration was examined and provided evidence for the doubly reduced toluene ligands by pinpointing the formation of d-bonds between the Ce centers and the p orbitals of toluene. Interestingly, these d-bonds involve a 10% 4f orbital contribution from each Ce center; a rare occurrence in Ln-ligand bonding and in contrast to Ln0(1,3,5-tri-tert-butyl-benzene)2 complexes where no 4f orbital participation is found for the metal-ligand bonding interaction.

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Arene Complexes of the Group 3 Metals and Lanthanides

In a similar vein, reduction of the Sm(III) complex [SmL2(m-L)]2 (L ¼ (tBuO)3SidO−, Scheme 59) with 2 equivalents of KC8 in toluene at −40  C produced a mixture of products from which the arene complex 2.4.1-Sm was isolated in small amounts as a compound thermally unstable in solution.130

Scheme 59 Synthesis of the Sm(II) inverted arene complex 2.4.1-Sm by reduction of a Sm(III) synthon in toluene.

In line with its thermal instability in solution and its co-crystalization with the toluene-free Sm(II) complex [KSm2L5], the invariance of the average toluene CdC bond distance (1.416 A˚ ) from that of free toluene, points to a formal [Sm(II)](toluene)0-[Sm(II)] assignment of charges (vide infra). It has to be noted though that the Sm-toluene centroid distance in 2.4.1Sm of 2.312 (4) A˚ is significantly shorter than that observed in 1.3.0C-Sm (Scheme 40C) (2.573 (17) A˚ ), in [Cp Sm(m-Z6:Z6Ph2) BPh2] (2.702 (15) and 2.776 (8) A˚ ) or [Sm(II)(Z6-arene)(AlCl2I2)]2 (arene ¼ C6Me6: 2.655 (2) A˚ ; arene ¼ toluene: 2.694 (14) A˚ ) which could suggest some degree of reduction in the toluene moiety between the two metal centers. In order to explore this hypothesis, the Sm(II) complex [Sm2(m-L)3L(DME)] (L ¼ (tBuO)3SidO−) was exposed to toluene at −40  C to yield the inverted arene complex 2.4.2-Sm in 44% yield with concomitant formation of [Sm(III)L2(m-L)]2 (L ¼ (tBuO)3SidO−) with the same yield (Scheme 60).

Scheme 60 Synthesis of the Sm(II) inverted arene complex 2.4.2-Sm using a Sm(II) starting material.

Arene Complexes of the Group 3 Metals and Lanthanides

447

The average CdC toluene bond length in 2.4.2-Sm has increased to 1.462 A˚ (1.416 A˚ in 2.4.1-Sm) and is even more pronounced compared to 2.4.0-Ce, thus indicating a reduced toluene ligand. Furthermore, the Sm-toluene centroid distance is significantly shortened from 2.312 (4) A˚ in 2.4.1-Sm to 2.103 (2) A˚ in 2.4.2-Sm. Finally, the SmdO bond distances in both 2.4.2Sm and 2.4.1-Sm are in good agreement with each other suggesting that the metal centers in the two complexes have the same formal oxidation state (+2). DFT analyses support the assignment of charges on the Sm centers and the toluene ligands in these two complexes. In summary, in the case of 2.4.2-Sm, the highest doubly occupied molecular orbital is a d-bond involving one of the toluene p orbitals (their degeneracy has been removed due to the first order Jahn-Teller effect). In contrast in 2.4.1-Sm, the d-bond is the LUMO, which is consistent with no reduction of the toluene. Complex 2.4.2-Sm can be considered a simpler version of the multi-decker cerium complex 2.4.0-Ce, and its formation in the absence of an alkali metal reductant demonstrates the reducing power of SmII.

4.07.3.5

Inverted arenes supported by CpR− ligands

A summary of crystallographically characterized Cp supported arene complexes of Group 3 and the lanthanides is given in Scheme 61 with single crystal XRD structure known only for of 2.5.0-Y/Lu-napth.131,132 while for complexes 2.5.0-Ln2-napth.131 their identity is confirmed spectroscopically and by elemental analysis (see also Table 9). Single crystal X-ray structural determination of CpY(C10H8)(DME)131 and CpLu(C10H8)(DME)132 show a 2Z1:Z2 coordination mode (i.e. 2s 1p) with negative charge localization in both cases similar to Lappert’s [K(18-crown-6)][Ln(Cp00 )2(m-C6H6)] (2.5.1-La™S—see below). The preparation of the majority of these complexes involves salt metathesis reactions between An(arene) (A ¼ Li, Na, K, n ¼ 1, 2, arene ¼ naphthalene,131,133 anthracene,131,133 (e.g. 2.5.0-Lu2-anthrac.135 (Scheme 61)), pyrene,136) usually generated in situ, with the exception of 2.5.0-Sm2arene134 (arene ¼ anthracene, benzanthracene, pyrene, phenazine) where the reducing power of SmCp 2 is used to drive the reaction. Based on NMR data and the intense color of the compounds, Evans et al. deduced that each samarium center is oxidized from +2 in SmCp 2 to +3 in 2.5.0-Sm2-arene.134 Interestingly, this is reversible in the case of 2.5.0-Sm2-anthrac. as upon dissolving in THF the complex dissociates to anthracene and Cp 2Sm(THF)2. The analog 2.5.0-La2-anthrac. has also been prepared by the reaction of [Cp 2La(m-Cl)2K(DME)] with 1 equivalent of Na2[C14H10] in toluene and its molecular structure137 is identical to that of 2.5.0-Sm2anthrac. SmCp 2 reacts also with other polycyclic hydrocarbons like 9-methyl-anthracene and acenaphthylene to furnish compounds whose elemental analyses was consistent with a 2:1 [SmCp 2]:polycyclic hydrocarbon formulation.134 SmCp 2 reductively couples acridine to furnish 2.5.0-Sm2-acridine (Fig. 13).134 Mimicking the reducing power of SmCp 2, complexes [CpMe4H Ln](m-Z2:Z2-N2) 2 (Ln ¼ La, Lu) react with phenazine to give the structurally analogous of 2.5.0-Sm2-phenazine complex, 2.5.0-LnMe4H -phenazine 2 (Ln ¼ La, Lu) compounds.138 An alternative synthetic route to 2.5.0-Sm2-phenazine and its isostructural 2.5.0-La2-phenazine takes advantage of the steric relief from the loss of a Cp ligand (as (C5Me5)2) in the reaction of LnCp 3 (Ln ¼ La, Sm) with phenazine.139 Furthermore, complex 2.5.0-Sm2-phenazine as well as its analog 2.5.0-Yb2-phenazine can also be prepared via a ligand redistribution reaction between four equivalents of the corresponding Ln(II) [Cp Ln(m-Zx:Zy-Ph2)BPh2] (Ln ¼ Sm then x ¼ y ¼ 6; Ln ¼ Yb then x ¼ 4, y ¼ 1) precursors with 2 equivalents of phenazine.15 Red 2.5.0-Y2-phenazine can similarly be prepared via the reaction of 2 equivalents of [(Cp )2Y(m-Z2:Z2-Ph2)BPh2] with one equivalent of phenazine with concurrent loss of BPh3 and biphenyl and 5,10dihydrophenazine.19 This reaction proceeds via a dark green paramagnetic intermediate which has been characterized as the radical cation [2.5.0-Y2-phenazine%]+ BPh−4 in which the unpaired-spin density is delocalized in the phenazine poly-aromatic scaffold. This latter complex transforms to 2.5.0-Y2-phenazine upon heating at 70  C in C6D6. In complex 2.5.0-Lu3H-napth.2 one of the inverted naphthalenes has lost a proton and based on charge balance and the fact that the compound is diamagnetic, the presence of a bridging hydride is inferred (no other spectroscopic data is available due to the thermal instability of the compound during crystallization).133 When the reaction that produced 2.5.0-La2-pyrene was performed in THF instead of toluene, complex [Cp 2La(THF)2][C16H11] was isolated, in which THF cleavage leads to the protonation of the pyrene framework with concurrent formation of a substituted phenalene anion, which exists as a solvent-separated ion pair with [Cp 2La(THF)2]+.136

448

Arene Complexes of the Group 3 Metals and Lanthanides

Scheme 61 CpR supported inverted arenes which have been crystallographically characterized.

Arene Complexes of the Group 3 Metals and Lanthanides

449

Table 9 Summarizes the data for some of the CpR (R ¼ H, Me5) supported inverted arene complexes that have not been structurally characterized. Key metric parameters of structurally characterized complexes depicted in Schemes 61 and 62 are summarized in Table 10. Compound

Number

Data

(CpSm)2C10H8(THF)4131 (CpYb)2C10H8(THF)4131 Cp LuC10H8(DME)133 CpGdC10H8(DME)131 CpErC10H8(DME)131 CpTmC10H8(DME)131 (Cp 2Sm)2(9-MeC14H9)134

2.5.0-Sm2-napth. 2.5.0-Yb2-napth. 2.5.0-Lu -napth. 2.5.0-Gd-napth. 2.5.0-Er-napth. 2.5.0-Tm-napth. 2.5.0-Sm2-napth.

magnetism magnetism IR similarity to 2.5.0-Lu-napth. magnetism magnetism magnetism NMR

Table 10 this section.

Metric parameters of structurally characterized inverted lanthanide arene complexes shown in Scheme 61 and discussed in the above paragraphs of

Complex

M-centroid arene ˚) (m) (A

MdC average distance for ˚) m (m  6) (A

Shortest MdC ˚) bond (A

CdC ˚) average (A

Shortest/Longest ˚) CdC (A

Arene Torsion Angle ( )a

2.5.0-Y-napth. 2.5.0-Lu-napth. 2.5.0-Lu2-anthrac.

2.110 (Z4)b 2.137 (Z4)c 2.482 (Z1)/2.473 (Z1) 2.576 2.525 (Z3) 2.598 (Z3) 2.525 (Z3)

2.522 2.486 N/A

1.422 1.428 1.466

1.374/1.454 1.361/1.464 1.423/1.488

27.88 31.43 44.39

2.956 2.742 2.808 2.741

2.438 2.397 See M-centroid distance 2.944 2.595 2.869 2.687

N/A 1.432 1.427 1.416

N/A 1.408/1.455 1.407/1.447 1.411/1.421

24.11 planar planar planar

2.509 (Z3) 2.516 (Z3) 2.505

2.726 2.734 2.871

2.664 2.660 2.866

1.419 1.426 N/A

1.415/1.423 1.408/1.443 N/A

planar planar

2.571(2)

2.922

2.910(2)

N/A

N/A

planar

2.372(2)/2.367(19) (both Z3) N/A

2.737/2.751

2.719/2.740

N/A

N/A

7.72

N/A

2.867(5)

N/A

N/A

14.29

2.572 (Z6) 2.579 (Z6) 2.744 (Z2) 2.432(5) (Z3) N/A

2.935 (Z6) 2.943 (Z6) 2.834 (Z2)

1.420 1.420 1.416 N/A N/A

1.382/1.466 1.384/1.450 N/A N/A N/A

8.36

2.801 N/A

2.766 2.801 2.824 2.784(6) N/A

planar planar

2.158 (Z4)d 2.325 (Z4)e 2.328 (Z6) 2.165(2) (Z4)f

2.510 2.654 2.717 2.547

2.444 2.597 2.607 2.400(5)

1.466 1.452 1.407 N/A

1.388/1.524 1.374/1.485 1.362/1.425 1.373(7)/1.476(7)

39.30 21.19 3.12 28.05

2.5.0-Sm2-acridine 2.5.0-Sm2-anthrac. 2.5.0-La2-anthrac. 2.5.0-Sm2benzanthrac. 2.5.0-Sm2-pyrene 2.5.0-Sm2phenazine 2.5.0-La2phenazine 2.5.0-LuMe4H 2 phenazine 2.5.0-LaMe4H 2 phenazine 2.5.0-La3-pyrene

2.5.0-Y2-phenazine [2.5.0-Y2phenazine%]+ BPh−4 2.5.0-Lu3H-napth.2

2.5.1-Lu-Gaz

Defined as the largest dihedral angle between two adjacent three carbon planes in the arene ring and calculated using Mercury95. Z ¼ 2s:1p (2.397/2.406:2.479). c 4 Z ¼ 2s:1p (2.452/2.438:2.506). d 4 Z ¼ 2s:1p (2.444/2.470:2.468). e 4 Z ¼ 2s:1p (2.580/2.768:2.542). f 2s:1p (2.508 (5)/2.400 (5):2.550 (3)). a

b 4

Complex 2.5.0-Lu-napth. (Scheme 61) reacts with guaiazulene (Gaz) to generate complex 2.5.1-Lu-Gaz (Scheme 62) which in the solid state exhibits two Z1 interactions (i.e. s-bonds) due to the localization of the negative charges on to two homo-allylic carbons.140 Loss of DME from 2.5.1-Lu-Gaz in solution leads to the formation of bridging dimer [LuCp(THF) (m-5:1-Gaz)]2 ({2.5.1-Lu-Gaz0 }2) where the hapticity of the Gaz ligand has been altered as deduced from NMR spectroscopy (Scheme 62).

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Arene Complexes of the Group 3 Metals and Lanthanides

Fig. 13 Complex 2.5.0-Sm2-acridine.

Scheme 62 Reaction of 2.5.0-Lu-napth. with guaiazulene.

The archetypical example of an inverted arene ligand is the reduced benzene ligand. Lappert et al. reported on the synthesis of benzene and toluene inverted arene ligands by reduction of [Ln(Cp-(1,3-R2))3] 2.5-LnR2 (R ¼ SiMe3 then Ln ¼ La, Ce, Pr, Nd; R ¼ tBu then Ln ¼ La) with K metal in either benzene or toluene.141–144 This seminal work has been reviewed before145 and it is summarized in Scheme 63 below. Nevertheless, we choose to provide some more detail as it puts into context for the more recent work in the field. The outcome of the reaction is dependent on the amount of K used as the reducing agent and to some extent the substituents on the Cp− ligand. For example, Ln(Cp00 )3 (Cp00 ¼ 1,3-(SiMe3)2-Cp) allows the isolation of red/red-brown [K(18-crown-6)] [Ln(Cp00 )2(m-C6H6)] 2.5.2-Ln™S (Ln ¼ La, Ce, Pr, Nd) in which the bonding situation is best described as having a 1,4-cyclohexadiene (1,4-cyclohexa-2,5-dienyl aka benzenide-1,4-dianion) with localization of the negative charges (further confirmed by hydrolysis to extrude 1,4-cyclohexadiene).143,144 Based on charge balance, complexes 2.5.2-Ln™S feature Ln(III) metal centers. On the other hand reduction of La(Cptt)3 leads ultimately to the isolation of paramagnetic dark green [K(18-crown-6) (2-C6H6)2][{La(Cptt)2}2(m-6:6-C6H6)] 2.5.2-LatBu-benzene with the intermediacy of a diamagnetic dark-red compound which has been only spectroscopically characterized.141 When 2.5.2-La™S is heated in benzene at 70  C, it converts to a paramagnetic dark green compound 2.5.2-La™S-benzene, which is also the product of the reaction of La(Cp00 )3 with 1.5 equivalents of K in the presence of 1.5 equivalents of 18-crown-6. EPR spectroscopy of solutions of 2.5.2-LatBu-benzene and 2.5.2-La™S-benzene displayed octets (139La, I ¼ 7/2) characteristic of La(II) complexes. In 2018, Evans et al. structurally verified the connectivity of the anion in 2.5.2-La™S-benzene146 as being similar to 2.5.2-LatBu-benzene and magnetic studies of the former revealed indeed a triplet ground state (S ¼ ½) deriving from the antiferromagnetic coupling of two 5d1 (S ¼ ½) La(II) metal centers via the planar bridging (C6H6)− radical anion. The same group demonstrated that 2.5.2-La™S-benzene readily reacts with cyclooctatetraene (C8H8, COT) at room temperature, as well as with anthracene and naphthalene albeit at 75  C to give complexes 2.5.3-La™S-COT, 2.5.3-La™S-anthrac. and 2.5.3-La™S-napth., respectively, in low to moderate yields (30–38%) (vide infra) (Scheme 64).146 Violet 2.5.3-La™S-anthrac. and dark green 2.5.3-La™S-napth. are paramagnetic, but investigations into their electronic structure were hampered by the small scale in which they were prepared. In contrast yellow 2.5.3-La™S-COT is diamagnetic.146

Arene Complexes of the Group 3 Metals and Lanthanides

451

et al.

et al. Scheme 63 Synthesis of CpR2 (R ¼ tBu or SiMe3) inverted arene complexes reported by Lappert et al.; highlighted in blue are crystallographically characterized complexes of the 2.5.2-Ln™S series.

Apart from the obvious steric differences between the Cptt and Cp00 ligands, it is worth pointing out that La(Cptt)3 is ca 300 mV more difficult to reduce than Ln(Cp00 )3 (−3.1 V vs −2.8 V in THF vs Fc0/+ respectively).142 When toluene was used instead of benzene the paramagnetic 2.5.2-Ln™S-tol (Ln ¼ La, Ce) featuring an inverted (toluene)− ligand bridging two [Ln(II)(Cp00 )2] moieties was isolated.142 The subtle effect that the ligand has in the observed reactivity was demonstrated when (CpTBDMS)− (TBDMS ¼ SiMet2Bu) was used as the supporting ligand in the reduction of [Ln(III)(CpTBDMS)3] (Ln ¼ La, Ce) under identical conditions used for the preparation of 2.5.2-Ln™S-tol (Scheme 63), which resulted instead in the isolation of the bridging hydride La and Ce complexes 2.5.4-Ln (Ln ¼ La: white; Ln ¼ Ce: pale yellow) (Scheme 65), which again based on charge balance feature Ln metal centers in +3 formal oxidation states.142

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Arene Complexes of the Group 3 Metals and Lanthanides

Scheme 64 Reactivity of inverted arene 2.5.2-La™S-benzene towards cyclooctatetraene, naphthalene and anthracene.

Scheme 65 use of the (CpTBDMS)− ligand does not result in the synthesis of an inverted arene complex.

Complexes 2.5.2-LatBu-benzene, 2.5.2-La™S-benzene and 2.5.2-Ln™S-tol provided some first hints as to the possible availability of Ln(II) complexes, other than the well-established stable examples of Sm(II), Eu(II) and Yb(II). Given the inherent difficulty in assigning formal oxidation states for the Ln centers in inverted arene complexes it was not until Lappert et al. isolated the bona fide anionic [KL(OEt2)x][La(II)Cp00 3] (L ¼ 18-crown-6 then x ¼ 1; L ¼ [2.2.2]-cryptand then x ¼ 0) lanthanum(II) complex,147 followed soon after by Evans and coworkers synthesizing complexes [K[2.2.2-cryptand]][Ln(II)Cp0 3] (Cp0 ¼ Z5-C5H4SiMe3) 2.5.5-Ln (Scheme 66) across the lanthanide series,148–154 that the availability of well-defined molecular complexes of all the lanthanides in the +2 oxidation state was realized. Some of these latter complexes (i.e. 2.5.5-Ln, Ln ¼ La, Ce, Dy, Y) are able to readily reduce biphenyl

Arene Complexes of the Group 3 Metals and Lanthanides

453

Scheme 66 Synthesis of inverted arene complexes by oxidation of Ln(II) complexes [K([2.2.2]-crypt)][Ln(II)Cp0 3].

(E1/2 ¼ −2.69 V vs SHE) (Ln ¼ Y) and naphthalene (E1/2 ¼ −2.50 V vs SHE) (Ln ¼ La, Ce, Dy, Y) to furnish [K([2.2.2]-crypt)][Cp0 2 Y(Z6-C6H5Ph)] 2.5.5-Y-biphenyl and [K([2.2.2]-crypt)][Cp0 2Ln(III)(Z4-C10H8)] 2.5.5-Ln-napth.155 respectively but not benzene (E1/2 ¼ −3.43 V vs SHE) or toluene (Scheme 62). In complexes 2.5.5-Ln-napth. metric parameters correlate well with the decrease in the ionic radii of the metal center from left to right. More specifically the smaller the metal center, the more bent the structure of the bridging arene is (angle between the two planes: La 155.5 , Ce 154.8 , Dy 147.4 , Y 145.5 ) suggesting a greater degree of polarization of the (C10H8)2− by the harder Lewis lanthanide center. The Ln distance to the midpoint of C1-C2 (Scheme 65) also decreases with decreasing anionic radius. Finally the metric parameters of the Z4 coordinated part of the naphthalene-diide are consistent with charge localization on C1 and C2 with the C3-C4 bond retaining more of a double bond character i.e. 21:12 (1.358 (4) A˚ in 2.5.5-Y-napth, 1.363 (5) A˚ in 2.5.5-Dy-napth and 1.371 (3) A˚ in 2.5.5-Ce-napth); this is a situation very similar to that found in 2.2.0C-Dy (Scheme 45) and the terminal napthalenide ligands in the triple decker 2.1.0-Tm (Scheme 43) as well as other Cp-supported inverted arenes discussed earlier in Section 4.07.3.5. Comparison of the midpoint distances of C1-C2 to La in 2.5.5-La-napth (average of 2.601 A˚ ) and 2.5.3-La™Snapth. (see Scheme 64; average 2.633 A˚ ) shows that in the latter case it is somewhat elongated, but in general the similar metric parameters between these two complexes suggest that the smaller Cp0 ligand imposes a coordination environment similar to the bigger Cp00 in 2.5.3-La™S-napth. The most noteworthy difference between 2.5.5-La-napth (black) and 2.5.3-La™S-napth (dark green), is that the former appears to be diamagnetic at room temperature by NMR spectroscopy with 1H NMR chemical shifts spanning between 6.0 and 0.10 ppm (d d8-THF). With that in mind, it is also worth noting that all the resonances in the 1H NMR spectrum of 2.5.5-La-napth appear as singlets, unlike 2.5.5-Y-napth where 1H–1H coupling is observed. This might point to a singlet ground state with thermally populated low-lying excited states for 2.5.5-La-napth (see also below). It is also worth pointing out that upon inspection of the CdC bond lengths in the series of 2.5.5-Ln-napth complexes, compound 2.5.5-La-napth stands out as an exception since the shorthest CdC bond is between atoms C2 and C4 (Scheme 66 below) whereas in the rest of the series the shortest bond is between atoms C3 and C4. One possible explanation is that unlike the 2.5.5-Y/Ce/Dy-napth series where the crystallographically observed Z4 bonding can be considered as 2Z1:1Z2, in the case of 2.5.5-La-napth a more apt representation would be one consisting of 1Z3:1Z1. Complex 2.5.5-Y-biphen represents a rare example of a monometallic inverted arene complex of Y and the metric parameters of the Z6 coordinated phenyl ring signify charge localization similar to Lappert’s [K(18-crown-6)][Ln(Cp00 )2(m-C6H6)] 2.5.2-Ln™S (Scheme 63).144

454

Arene Complexes of the Group 3 Metals and Lanthanides

In order to access inverted benzene lanthanide complexes supported by the Cp0 ligand, since complexes 2.5.5-Ln featuring lanthanide centers in the +2 oxidation state were unable to reduce benzene, the synthetic route shown in Scheme 67 was employed.156

Scheme 67 Synthesis of inverted benzene complexes starting from [LnCp0 3(THF)x] (2.5.6-Ln).

In the case of 2.5.6-La-benzene, magnetic studies coupled with DFT calculations, UV–Vis/NIR spectroscopy and structural investigations suggest a singlet ground state deriving from strong magnetic exchange coupling between two 5d1 La2+ metal centers and a (C6H6)2− biradical, instead of an electronic configuration featuring two La3+ centers and a (C6H6)4− bridge (i.e. {[La(III)Cp0 3] (m-6:6-C6H46 −)}−). The (C6H6)2− in 2.5.6-La-benzene is significantly more bent compared to 2.5.2-La™S-benzene (Schemes 63 and 64) which features a planar (C6H6)− radical anion as well as the 6-carbon 10p electron aromatic tetranionic bridging arenes found in complexes 2.3.3-M-arene (Scheme 49).111 Interestingly the 1H NMR spectrum of 2.5.6-La-benzene exhibits the same features (i.e. small chemical shift range and no 1H–1H coupling) as those of 2.5.5-La-napth (Scheme 66). In the case of 2.5. 6-Ce-benzene, a clear picture of its electronic configuration was not as straightforward and unfortunately single crystal XRD studies provided only connectivity. A Ce2+ 4f15d1 electronic configuration coupled to the S ¼ 1 (C6H6)2−, similar to 2.5.6-La-benzene, is proposed, although a Ce3+/(C6H6)4− electronic configuration cannot be excluded. The HOMO and HOMO-1 in 2.5.6-La-benzene show significant mixing between the p orbitals of the (C6H6)2− bridge and La orbitals of predominately d character (p :La orbitals: 61%:39% in HOMO and 64%:36% in HOMO-1). The Natural Population Analysis (NPA) shows that each La center in 2.5. 6-La-benzene has ca 1.4 electrons, which is even higher than the value of 1.2 electrons in 2.5.5-La (i.e. [K([2.2.2]-crypt)][La(II)Cp0 3], Scheme 66). This implies that the requirement to have ligand fields promoting a low-lying dz2 orbital might not be such a strict requirement when it comes to the isolation of Ln2+ complexes for metals like lanthanum and cerium. The isolation of 2.5. 6-La-benzene featuring two La(II) metal centers from [La(III)Cp0 3(THF)] 2.5.6-La involves overall the transfer of four electrons; as such 2.5.6-La-benzene reacts cleanly with two equivalents of naphthalene to furnish 2.5.5-La-napth (Scheme 66 and Scheme 68 below) in high yields and without any of the by-products shown in Scheme 66. 2.5.6-Ce-benzene reacts in the same high yielding manner as 2.5.6-La-benzene with two equivalents of naphthalene. Although this is not conclusive evidence for the Ce2+/(C6H6)2− electronic configuration proposed by Evans et al., it is an indication of the validity of their hypothesis. 2.5.5-La-napth also reacts with COT (C8H8) in a more complicated manner producing [K([2.2.2]-crypt.)][La(Z8-C8H8)2] as the major product of the reaction.

Scheme 68 Reactivity of 2.5.6-Ln-benzene complexes towards C10H8.

At this point it is worth re-iterating the differences in the observed reactivity towards naphthalene (C10H8) between [K(18crown-6)(THF)2][{La(II)(Cp00 )2}2(m-6:6-C6H6)] (2.5.2-La™S-benzene) (Scheme 63), [K([2.2.2]-crypt.)][La(II)(Cp0 )3] (2.5.5-La) (Scheme 66) and [K([2.2.2]-crypt.)]2[{La(II)(Cp0 )2}2(m-6:6-C6H6)] (2.5.6-Ln-benzene) (Scheme 68). In all cases complexes [KL(solvent)x][La(Cpx)2(Z4-C10H8)] (L ¼ 18-crown-6, solvent ¼ THF, x ¼ 2 or [2.2.2]-cryptand, x ¼ 0; Cpx ¼ Cp0 or Cp00 ) are isolated but the yields for the first two cases suffer from the formation of by-products. 2.5.6-La-benzene (four electron reductant)

Arene Complexes of the Group 3 Metals and Lanthanides

455

and 2.5.5-La (one electron reductant) react with C10H8 at room temperature whereas 2.5.2-La™S-benzene (two electron reductant) requires heating at 75  C. In the latter case, the low yield due to unidentified by-products suggests that the stoichiometry of the reaction is not straightforward. These reactivity examples further demonstrate that bridging inverted arene complexes of the lanthanides can adopt electronic configurations, affecting their subsequent reactivity and properties, that can be counter-intuitive and certainly not expected or justified by mere charge-balance arguments. Using the sterically demanding [CpBz5]− (Bz ¼ benzyl, CH2Ph) ligand, Trifonov and coworkers accessed the homometallic Yb(II) triple-decker complex [Yb(Cp(CH2Ph)5)(DME)]2(m-4:4-C10H8) 2.5.8-Yb (Scheme 69), featuring a bridging di-anionic naphthalene ligand. Its optimized synthesis is via the reaction of 2.2.0A-Yb (Scheme 45) with 2 equivalents of KCpBz5 in DME, as the reaction of [CpBz5Yb(DME)(m-I)]2 with K[C10H8] leads to diminished yields (81% for the former vs 21% for the latter).157

Scheme 69 Synthesis of [Yb(Cp(CH2Ph)5)(DME)]2(m-4:4-C10H8) 2.5.6-Yb.

Complex 2.5.8-Yb is diamagnetic based on its 1H and 13C{1H} NMR spectroscopic data which reveal a centrosymmetric structure persistent in solution with free rotation of the CpBz5− ligands around the Yb(II)dCp ring and of the benzyl groups around the Cp-CH2Ph bonds. The assignment of the Yb2+ metal center was further confirmed by DFT calculations that reproduce the experimental geometry and metric parameters well. The HOMO shows significant covalency in 2.5.8-Yb and is of d-type involving the p orbitals of the doubly reduced naphthalene and a f-d hybrid of each metal center with predominant 5d character (87%:13% 5d:4f ). 2.5.8-Yb is thermally unstable and heating a THF solution thereof at 60  C results in the isolation of known Yb(CpBz5)2. 2− Complex 2.5.8-Yb features two reductive centers (i.e. Yb(II) and C10H2− bond, thus offering 8 ) and a labile Yb(II)d(C10H8) significant potential for redox reactivity and accessing Yb(II) mixed ligand complexes; these modes of reactivity are summarized in Scheme 70.

Scheme 70 Reactivity of 2.5.8-Yb.

456

Arene Complexes of the Group 3 Metals and Lanthanides

Complex 2.5.8-Yb reacts with a host of dienes with concurrent loss of naphthalene, but only trans-(1E, 3E)-1,4-diphenyl-buta1,3-diene produced the isolable 2.5.9-Yb-A (Scheme 69) as a crystalline material. The synthesis of 2.5.9-Yb-A is reminiscent of the reaction of [Y(THF)(NNTBDMS)]2(m-4:4-C10H8) ((NNTBDMS) ¼ [1,10 -fc(NTBDMS)2]2− fc ¼ Fe(C5H4)2, TBDMS ¼ SiMe2tBu) (2.3.1Y-2.THF) with E-stilbene giving [Y(THF)(NNTBDMS)]2(m-E-stilbene) (2.3.7-Y)110 (Scheme 53), as well as the reductive coupling of PhCCPh promoted by inverted arene complexes (Scheme 57).121 A similar complex to 2.5.9-Yb-A can be obtained from the one pot reaction of LuCl3(THF)3 with one equivalent of in situ-generated Na[Cp(1,3-Ph2)] and 1 equivalent of in situ-generated Na2[C2Ph4] in THF (Scheme 71). Complex 2.5.10-Lu displays dynamic behavior in THF probably due to hindered rotation of the Ph rings. The Lu-C(Ph) bond distances are 2.422 (3) and 2.637 (3) A˚ .158

Scheme 71 Synthesis of 2.5.10-Lu.

Table 11 contains some key metric parameters of the inverted arene complexes discussed in this section for which crystallographic information files are available.

Table 11

Key metric parameters of CpR supported inverted arene complexes discussed in this section.

Complex

M-centroid arene ˚) (m) (A

MdC average distance for ˚ )a m (m  6) (A

Shortest MdC ˚) bond (A

CdC ˚) average (A

Shortest/Longest ˚) CddC (A

Arene Torsion Angle ( )b

2.5.2-La™S 2.5.2-Ce™S 2.5.2-Nd™S 2.5.2-Ce™Stol 2.5.2-LatBubenzene 2.5.3-La™Snapth 2.5.5-Y-napth 2.5.5La-napth 2.5.5Ce-napth 2.5.5Dy-napth 2.5.5Y-biphend 2.5.6La-benzene 2.5.8-Ybe

2.352 (Z6)a 2.326 (Z6)a 2.292 (Z6)a 2.386

2.744 2.719 2.687 2.772

2.616 2.588 2.556 2.662

1.425 1.419 1.415 1.418

1.336/1.472 1.351/1.464 1.344/1.451 1.398/1.44

23.70 24.06 25.03 23.74

2.359 (Z6)

2.765

2.754

1.441

1.418

planar

2.385(2) (Z4)c

2.707

2.663(4)

1.440

1.363(7)/1.479(7)

27.27

2.240(4) (Z4)c 2.381(3) (Z4)c

2.574 2.691

2.517(2) 2.570(5)

1.434 1.447

1.358(4)/1.467(3) 1.292(7)/1.638(6)

34.53 25.18

2.366(12) (Z4)c

2.692

2.622(2)

1.441

1.371(3)/1.474(3)

25.87

2.247(8) (Z4)c

2.560

2.519(3)

1.440

1.363(5)/1.474(5)

33.85

2.246(4) (Z6)

2.650

2.480(2)

1.422

1.353(3)/1.466(3)

26.73

2.278(1)&2.272(8)

2.700&2.696

1.453

1.446(6)/1.457(6)

18.94

2.380(9)&2.373(3)

2.6967&2.6891

2.641(5)&2.635 (5) 2.6813(17) &2.6902(18)

1.426&1.426

1.371(3)/1.459(3) 1.372(3)/1.406(3)

15.56 15.86

Z6 interaction consists of 2Z1:2Z2: for 2.5.2-La™S LadC(Z1): 2.652&2.616 A˚ and LadC(Z2): 2.795&2.802 A˚ (average of the two bonds), for 2.5.2-Ce™S CedC(Z1): 2.588&2.612 A˚ and CedC(Z2): 2.771&2.785 A˚ (average of the two bonds), for 2.5.2-Nd™S NddC(Z1): 2.552&2.556 A˚ and NddC(Z2): 2.762&2.735 A˚ (average of the two bonds). b Defined as the largest dihedral angle between two adjacent three carbon planes in the arene ring and calculated using Mercury95. c 4 Z interaction consists of 2Z1:1Z2 for 2.5.3-La™S-napth: LadC(Z1):2.663 (4)&2.694 (4) A˚ and LadC(Z2): 2.735 A˚ (average of the two bonds) 2.5.5-Y-napth.: 2.528 (2)/2.517 (2):2.626 A˚ , respectively, for 2.5.5-Ce-napth.: 2.622 (2)/2.676 (2):2.735 A˚ respectively, for 2.5.5-Dy-napth.: 2.519 (3)/2.543 (4):2.628 A˚ respectively. d CipsodCipso: 1.460 (3) A˚ . e Two molecules in the asymmetric unit and data collected to 0.55 A˚ resolution. a

Arene Complexes of the Group 3 Metals and Lanthanides

4.07.4

457

Conclusions

Since the isolation of the first lanthanide arene complex by Cotton et al.4 the field has grown significantly. Despite the distinct differences of lanthanide and Group 3 arene complexes to their transition metal and actinide counterparts, arising from the inherit orbital mismatch between the rare earth element and the arene ligand (i.e. higher energy 5d orbitals and core-like 4f orbitals), it has become apparent that with the right choice of supporting ligands new chemical space can be accessed. Such compounds have been important in challenging previous textbook knowledge regarding covalency in lanthanide organometallics and gave the first hints of the ability to access low valent lanthanide and Group 3 across the whole rare earth series (excluding radioactive promethium). Due to the ability of rare earth complexes to adopt high coordination numbers, arene complexation can have a significant effect in promoting unique reactivity; e.g. the extra stability they can engender by being part of a ligand framework, their ability to act as electron-density acceptors and helping create a cavity into which substrates can approach from only certain directions has been instrumental in the activity of lanthanide arene complexes as electrocatalysts in water reduction,89 discussed in Section 4.07.2.3. Reduced arene complexes, discussed throughout Section 4.07.3 of this article, are a special class of compounds. They demonstrate a diverse reductive chemistry which can be leveraged to promote both small molecule activation (e.g. P4) and stoichiometric bond scission of strong bonds (e.g. CdF). However, reactivity of such complexes with CO2 and CO remains un-reported. Furthermore, due to the highly reduced nature of the arene ligand they can act as hydrogen atom acceptors, thus making them valuable starting material for preparing a host of [Ln]-ERnH2-n (E ¼ S, Se, Te, P, As; n ¼ 0–2) complexes featuring lanthanide heteroatom bonds. Such complexes can act as catalysts in hydro-elementation reactions and one could envision Ln-arene interactions creating a chiral environment to make these catalytic reactions enantioselective. Another avenue worth exploring is the derivatization of the reduced arene ligand, which at the moment has not been achieved.

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4.08

Arene Complexes of the Actinides

Jonathan D Cryer and Stephen T Liddle, Department of Chemistry, The University of Manchester, Manchester, United Kingdom © 2022 Elsevier Ltd. All rights reserved.

4.08.1 Introduction 4.08.2 Metal-arene bonding considerations 4.08.2.1 Arenes 4.08.2.2 Actinides 4.08.3 Thorium arene complexes 4.08.3.1 Formally neutral thorium arene complexes 4.08.3.2 Inverse sandwich dithorium arene complexes 4.08.4 Uranium arene complexes 4.08.4.1 Formally neutral uranium arene complexes 4.08.4.2 Inverse sandwich diuranium(III) arene complexes 4.08.4.3 Inverse sandwich diuranium(V) arene complexes 4.08.4.4 Uranium complexes stabilized by arene-based ligands 4.08.5 Neptunium arene complexes 4.08.6 Summary and outlook 4.08.7 Note added in proof Acknowledgments References

4.08.1

460 461 461 461 462 462 467 469 469 471 483 485 496 497 497 497

Introduction

Metal-arene complexes constitute an elementary class of organometallic complex on a fundamental level as they were integral to the development of metal-ligand bonding theories and are commonly used as examples in the teaching of metal-ligand bonding. Transition metal-arene complexes have been known for decades, with the first example being the phenyl-chromium complex [(Z6-benzene)(Z6-arene)Cr]+ (arene ¼ benzene, diphenyl) reported in 1919 by Hein,1 with identification by Zeiss in the mid 1950s.2 The synthesis of ferrocene in 1951 by Pauson and Kealy,3 with the correct structure determined later by Wilkinson and Fischer4,5 led to the recognition of multi-hapto Zn-bonding of carbocyclic ligands to metals and the evolution of modern day oraganometallic chemistry. Since then, arene-complexes have been reported across the d-block in the form of half-sandwich, sandwich, and inverse-sandwich complexes, and they have been used as stoichiometric reagents and catalysts in organic chemistry, as well as versatile starting materials for ligand substitution and ligand activation in the field of inorganic chemistry.6–10 The bonding in transition metal-arene complexes can generally be understood through a donor-acceptor description. The arene acts as an electron donor through its HOMO p-orbitals, as well as, in some cases, undergoing partial population of the LUMO p -orbitals through back-donation from the metal center. The extent of bonding in transition metal-arene complexes is dependent on many factors including; size and oxidation state of the metal, steric and electronic properties of the arene and accompanying ligands bound to the metal center, as well as satisfying the 18 electron rule. One result of these many contributing factors is the variety of hapticity observed in transition metal arene complexes, of which Z1- to Z6- coordination is known. With the development and prevalence of transition metal-arene complexes, investigations have naturally sought to extend research to actinide analogs. The pursuit of actinide-arene complexes can be traced back for almost half a century, and over that time, and in particular over the past two decades, many significant developments have been made in the area. The extent of covalency in the bonding of actinide metals is still a topic of intense debate, and the isolation of actinide-arene complexes has allowed significant advances in our understanding to flourish, and through computational methods descriptions of the involvement of 5f- and 6d-orbitals in actinide-ligand bonding have evolved. In general, in comparison to transition metal-arene complexes where back-bonding from the metal center is generally weak, the d-bonding orbitals of early actinides can facilitate much greater degrees of back-bonding, in particular for uranium-arene complexes. Indeed, whereas arene ligands are overall donors in transition metal chemistry in actinide chemistry they are overall acceptors, rendering them charge rich as reflected by the relatively large proportion of inverse-sandwich arene complexes compared to sandwich and half-sandwich derivatives. The determination and quantification of this bonding mode is of significant importance in the study of uranium-arene complexes and is highlighted throughout this Chapter. In recent years a number of reviews have directly11 or indirectly covered aspects of this subject area,12–16 but this Chapter provides an opportunity to update the area in detail and to include more recent results. Though efforts have been made to cover the whole field to provide a suitably contextualized discussion we focus on advances made from around the year 2000 onwards, and key compounds are compiled in Tables 1–6 where salient metrical data are summarized. After covering bonding considerations, this Chapter is arranged by central metal ion (thorium, uranium, neptunium). Within each section after any structure according to ligand classifications we follow a chronological order of publications so that the reader can contextualize and follow the development of the field.

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https://doi.org/10.1016/B978-0-12-820206-7.00013-5

Arene Complexes of the Actinides

4.08.2

Metal-arene bonding considerations

4.08.2.1

Arenes

461

The symmetry of C6 arenes can be described using the D6h point group, and in the neutral form contains 6p-electrons, thus satisfying Huckel’s rule (4n + 2 p-electrons) for aromaticity. The frontier orbitals of C6 arenes are of the correct symmetry to participate in s (c1), p (c2 and c3), and d (c4 and c5) bonding with a metal center, Fig. 1. Unique to the actinides, j bonding is also possible through the high-lying, vacant c6 orbital, though for generally poor energetic matching reasons this bonding mode plays a lesser role in actinide-arene chemistry. Unlike in C4-ligands, where the vacant d-symmetry c4 is energetically high-lying, the vacant c4 and c5 orbitals of C6 arenes are relatively low-lying and energetically reasonably well-suited to bonding with early actinide frontier orbitals; thus, as will become clear in this Chapter, arene d-bonding to actinides plays a major role in actinide-arene chemistry. Since the principal metal-ligand bonding interactions of actinide-arene complexes tends to be dominated by d-bonding character, s- and p-bonding combinations tend to play a lesser role than for smaller ring sizes. The accessibility of the c4 and c5 arene orbitals allows various levels of reduction of arenes, to formally monoanionic (7 p-electron), dianionic (8p-electron) and tetranionic (10p-electron) species, with the formal 8p- and 10p-electron ligand charge states being the most common for actinide-arene complexes, as found in inverse sandwich complexes (see below). It should, however, be emphasized that while it is didactically useful to formulate the bonding of actinide-arene complexes in terms of well-defined electron transfer to the arene, the resulting actinide-arene bonding interactions exhibit appreciable covalency, so such book-keeping should be recognized as being a formal exercise rather than a literal manifestation; while assigning oxidation states to metal ions in such bonding scenarios is useful to rationalizing their bonding and reactivity, it becomes a somewhat moot point.

4.08.2.2

Actinides

Before discussion of reported actinide-arene complexes, a brief introduction to the relevant orbitals and redox properties of early actinides11 is warranted as they are of fundamental importance throughout the chapter. Due to relativistic effects being more prominent for heavier atoms, the 6d- and 5f-orbitals of the early actinides extend out further from the atomic core into the valence region compared to the 5d- and 4f-orbitals of the lanthanides. This contributes to two key chemical properties of the early 5f-elements: (1) the 5f- and 6d-orbitals can participate in appreciably covalent bonding interactions with ligands; (2) the lower binding energy of electrons located in 5f- and 6d-orbitals allow a wide range of accessible oxidation states that are, in general, certainly more accessible like-for-like than the range of oxidation states for lanthanides. As the actinide series is traversed, the energy of the 5f-orbitals gradually decreases as they become ever more core-like. In addition, the energy of the 6d-orbitals increases across the series with the cross-over point often occurring at uranium. This allows for the possible hybridization of the valence 5fand 6d-orbitals, paving the way for metal-ligand bonding interactions with actinide elements that are not feasible for transition metals, which can be convincingly evidenced by the number of inverse-sandwich actinide-arene complexes compared to transition metal-analogs. For thorium, 6d-character tends to dominate the bonding, whereas this tips to an increased prevalence of 5f-character for uranium; as the actinide series is further traversed to the right increasing 5f-character might be anticipated, all things remaining equal. However, on moving left to right along the actinide series with increasing nuclear charge the valence orbitals

Fig. 1 Qualitative depiction of the frontier p-molecular orbitals of benzene.

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contract and so in principle more electrostatic bonding may dominate, but this will still be dictated by the relative energy-matching or mismatching of the metal and ligand frontier orbitals as a function of metal oxidation state and ligand identity and charge-state. It should be noted that with five different oxidation states being accessible to uranium in molecular complexes this ion is particularly well-suited to binding with arenes, which is reflected in the relative sizes of Sections 4.08.3–4.08.5 of this chapter.

4.08.3

Thorium arene complexes

4.08.3.1

Formally neutral thorium arene complexes

The pursuit of a thorium(II) synthon lead to the synthesis of the first thorium terminal arene complexes.17 Treating the thorium pyrrole complex [{(Et8-calix[4]tetrapyrrole)Th(m-Cl)}2{K(DME)}2] with two equivalents of Li(naphthalene) in DME at room temperature resulted in a dark red-brown solution, which yielded [(Et8-calix[4]tetrapyrrole)Th{K(DME)}(Z4-C10H8)][Li(DME)3] (1), see Scheme 1 for the structure, as a crystalline material in 64% yield, after storage of a DME/heptane solution at −37  C. The solid state structure of 1 determined by single crystal X-ray diffraction studies (SC-XRD) shows an unusual bonding of the tetrapyrrole ligand. Unlike uranium complexes of the same ligand which show two pyrrole ligands s-bonded through the nitrogen and two pyrrole ligands symmetrically p-bonded (see Section 4.08.4.4), 1 displays two s-bonded pyrroles but only one p-symmetrically bonded pyrrole ring. This is likely to be caused by the Z4-coordination of one of the naphthalene rings to the metal ion. The thorium-carbon bond distances corresponding to the bound arene were 2.704(6), 2.761(6), 2.754(6) and 2.675(6) A˚ , with carbon-carbon distances between the four coordinated naphthalene atoms of 1.445(9), 1.378(9) and 1.457(9) A˚ .

Scheme 1 Complexes 1 and 2 and conversion to 3.17

When [{(Et8-calix[4]tetrapyrrole)Th(m-Cl)}2{K(DME)}2] was treated with K(naphthalene), in identical conditions used for the synthesis of 1, the thorium arene complex [(Et8-calix[4]tetrapyrrole)Th{K(DME)}{m:Z4-Z6-C10H8)(m-K)]n (2) was obtained, see Scheme 1 for the structure. The solid state structure of 2 shows interesting structural differences compared to 1. While the naphthalene in both 1 and 2 are Z4-bound, the naphthalene in 2 also contains a potassium Z6-coordinated to the other ring of the naphthalene. The potassium also interacts with the ligand-based alkyl substituents of a second molecule through agostic

Arene Complexes of the Actinides

463

interactions, resulting in 2 existing as a polymeric chain in the solid state. The thorium-carbon bond distances for the coordinated naphthalene in 2 are 2.775(4) and 2.684(5) A˚ , with carbon-carbon distances of 1.446(6) and 1.375(8) A˚ . Except for the DME content, the NMR spectra of 1 and 2 are identical, suggesting the polymeric structure of 2, as well as the slippage of one of the p-bonded pyrrole rings, are not maintained in solution. Complex 1 was found to react with two equivalents of Me3SiN3 in DME to yield the thorium amide complex [(Et8-calix[4] tetrapyrrole)Th{N(SiMe3)2}] (3) in 81% isolated yield, Scheme 1. The formation of 3 was accompanied by the evolution of gas (N2), with the proposed mechanism for the reaction proceeding through a thorium-imido complex, which undergoes the transfer of SiMe3 from a second Me3SiN3 molecule. This postulation is supported by the elimination of KN3, which was determined by IR analysis. Thus, 1 acts as a Th(II) synthon, where the reducing electrons are held in the naphthalene and available for reactivity, which has the appearance of low oxidation state metal-based reactivity. Reaction of the dibenzyl thorium complex [(XA2)Th(Z2-CH2Ph)2] (4, XA2 ¼ 4,5-bis(2,6-diisopropylanilido)-2,7-di-tertbutyl-9,9-dimethylxanthene)) with one equivalent of B(C6F5)3 yielded the complex [(XA2)Th(CH2Ph)][PhCH2B(C6F5)3] (5), Scheme 2. This complex was formulated as a separated ion pair, but even though no crystal structure could be obtained solution NMR spectroscopic data indicated that there is likely Th-arene interactions in solution, but their precise nature remains unclear in the absence of a solid state structure. However, when 4 was treated with two equivalents of B(C6F5)3 the dicationic thorium complex [(XA2)Th{Z3-PhCH2B(C6F5)3}2] (6) was isolated, Scheme 2.18 The solid state structure of complex 6 displays p-coordination of the benzyl groups of the borate anions with thorium-carbon distances ranging from 2.900(7)-3.280(7) A˚ .

Scheme 2 Reactivity of 4 to give 5 and 6.18

The following year the same group reported that treatment of the bis-alkyl complex [(XA2)Th(CH2SiMe3)2] (7) with [Ph3C] [B(C6F5)4] in benzene or toluene produced the Z6-coordinated arene complexes [(XA2)Th(CH2SiMe3)(Z6-arene)][B(C6F5)4] (arene ¼ benzene (8) or toluene (9)), Scheme 3.19 Complex 8 displays thorium-arene bond distances ranging from 3.179 (8)–3.310(7) A˚ . Confirmation of the benzene coordination in 8 by NMR spectroscopy was not possible due to any resonances corresponding to the bound arene being masked by the resonances corresponding to the XA2, Ph3C+, Ph3CH and “Ph3CCH2SiMe3.” However performing the reaction in toluene allowed the coordinated arene to be assigned by 1H NMR spectroscopy in C6D5Br, as well as showing the bound toluene in 9 does not undergo rapid exchange with free toluene or C6D5Br. EXSY NMR spectroscopic analysis of 9 showed exchange of toluene on a longer timescale. If the thorium benzyl complex 4 is used instead of 7 as the starting material, reaction with [Ph3C][B(C6F5)5] in toluene yields the thorium arene complex [(XA2)Th(Z2-CH2Ph)(Z6-C6H5Me)][B(C6F5)4] (10), Scheme 3. The solid state structure of 10 displays uranium-carbon distances to the Z6-coordinated toluene ranging from 3.063(5)–3.435(6) A˚ which is comparable to those observed in 8 (3.179(8)–3.310(7) A˚ ), however the larger range demonstrates the arene is not bound as symmetrically. The most striking difference in the solid state structure of 10 when compared to 8 is the position of the coordinated arene. In 8, the coordinated arene

464

Arene Complexes of the Actinides

Scheme 3 Conversion of 7 to 8–10.19

is found approximately trans to the coordinated oxygen of the supporting ligand, with the arene plane roughly perpendicular to the plane of the ligand, however in 10 the arene is located in an apical position, approximately cis to the coordinated oxygen atom. The space opposite the supporting ligand in 10 is occupied by an Z2-coordinated anionic benzyl group. In the same publication as above,19 the dibenzyl thorium complex [(BDPP)Th(Z2-CH2Ph)(Z3-CH2Ph)] (BDPP ¼ 2,6-bis(2,6diisopropylanilidomethyl)pyridine) was found to react with [Ph3C][B(C6F5)4] in benzene to yield the dinuclear complex [(BDPP)Th(Z2-CH2Ph)(m:Z1-Z6-CH2Ph)Th(Z1-CH2Ph)(BDPP)][B(C6F5)4] (11), Fig. 2, as well as other thorium complexes. Complex 11 is composed of a cationic fragment ([(BDPP)-Th(Z1-CH2PH)]+) coordinated to a neutral dibenzyl molecule fragment [(BDPP)Th(CH2Ph)2] via a bridging benzyl group bound Z1 and Z6 to the two thorium centers. The amount of [Ph3C][B(C6F5)4] lead to a variation in the mixture of products from the reaction. When 0.5 equivalents of [Ph3C][B(C6F5)4] was added to [(BDPP)Th(Z2-CH2Ph)(Z3-CH2Ph)] the products [(BDPP)Th(Z2-CH2Ph)(Z3-CH2Ph)] (11) and “[(BDPP)Th(CH2Ph)(benzene)] [B(C6F5)4]” (12), i.e. the BDPP-benzene analog of 10, formed in the ratio 0.3:0.2:0.3 respectively. However, when one equivalent of [Ph3C][B(C6F5)4] was added to [(BDPP)Th(Z2-CH2Ph)(Z3-CH2Ph)], the products contained more than 90% 12. Addition of excess toluene to 11 did not lead to any reaction, thus the thorium-benzyl coordination could not be displaced with arene solvent. Reaction of the lithium salt of the arene-centered ligand [{1,3-([2-C4H3N][CH3]2C)2C6H4}Li2(Et2O)] with [ThCl4(DME)2] in DME at room temperature yielded the thorium arene complex [Z6-{1,3-([2-C4H3N][CH3]2C)2C6H4}ThCl3][Li(DME)3] (13) in 87% isolated yield after work up, Scheme 4.20 The solid state structure of complex 13 displays an octahedral geometry around the thorium with both pyrrolide rings being s-bound via the nitrogen atoms and the central arene being Z6-coordinated to the thorium ion through a p-bonding interaction. The thorium-arene bond distances in 13 range from 2.963(7)–3.110(8) A˚ , with a thorium-arenecentroid distance of 2.701(8) A˚ . Addition of a toluene solution of AlEt3 to 13 leads to the formation of two complexes, [{1,3-([Z5-2-C4H3N][CH3]2C)2C6H4}Th(m-Cl-AlEt2)2(m-Cl)2{1,3-([Z5-2-C4H3N][CH3]2C)2C6H4}Th(m-AlEt2)Cl]
(C7H8)0.5 (14) and [{1,3-([Z5-2-C4H3N][CH3]2C)2C6H4}ThCl(m-Cl)2{Li(DME)}2] (15), in 62% and 17% yield respectively, which could be separated by fractional crystallization. The bonding mode in 14 and 15 differ from 13 by the pyrrolide rings changing from s- to p-bonding mode, with the arene being deprotonated, thus changing from Z6-coordinated through p-bonding to a s-bonding mode.

Fig. 2 Structure of 11.19 Ar ¼ 2,6-diisopropylphenyl.

Arene Complexes of the Actinides

465

Scheme 4 Synthesis of 13–15.20

In pursuit of a thorium(III) complex, 13 was stirred with potassium metal until all the metal had been consumed. After work up the paramagnetic thorium complex [{Z5-1,3-([Z5-2-C4H3N][CH3]2C)2C6H4}ThK(m-Cl)3][Li(DME)3] (16) was isolated, Fig. 3. The central arene in 16 varies from planar, with one carbon pointing away from the thorium core, now resembling a cyclohexadienide unit. The distorted carbon displays a carbon-carbon distance of 1.526(10) A˚ , indicative of a carbon-carbon single bond. These changes to the arene scaffold on reduction, suggests the transformation of the central arene to a radical anion and therefore the thorium metal maintaining the +4 formal oxidation state. EPR spectroscopy of 16 in dioxane gave a g-value of 2.0012 at room temperature (1.9991 at 113 K). DFT calculations on a truncated model of 16 showed the singly occupied HOMO is located mainly on the central arene of the ligand, with the HOMO-1 and LUMO being well-separated at −2.33 eV and +1.21 eV respectively, relative to the HOMO. The spin density is mostly located (69.8%) on the carbon atom above the ring, and the valence indexes of 3.67 and 6.3 for the “radical carbon” and thorium, respectively, further support the assignment that the reduction is arene based, with the electronic configuration of the thorium ion staying practically undisturbed as a +4 oxidation state. Performing the reduction with K/naphthalene rather than potassium metal lead to the formation of [{Z6-1,3-([2-C4H3N][CH3]2C)2C6H4}Th{m-Z5-1,3([Z5-2-C4H3N][CH3]2C)2C6H5}K(DME)2] (17), Fig. 3, where partial hydrogenation of the arene ligand occurred. Treatment of 13 with two equivalents of Li/naphthalene to generate a transient reduced species, followed by addition of azobenzene yielded the tetravalent thorium diphenylhydrazido species, [{Z6-1,3-([Z5-2-C4H3N][CH3]2C)2C6H4}Th(m-Z2-PhNNPh)(m-Cl)(Cl)Li(DME)] [Li(DME)3]. A thorium(IV) aryloxide complex displaying the Z6-bis-arene sandwich motif could be isolated after a four step synthesis. Addition of [K(OTerMes)] (OTerMes ¼ O-2,6-(2,4,6-C6H2Me3)C6H3) to ThCl4 yielded [Th(OTerMes)2Cl2(DME)] (66% yield), which could be reacted with [Ca(BH4)2(THF)2] to give the thorium borohydride complex [Th(OTerMes)2(BH4)2(DME)] (18) in 63% yield.21 Treatment of 18 with AlMe3 in toluene at room temperature resulted in the elimination of [AlMe3(DME)], allowing the isolation of the solvent free thorium-arene complex [Th{O-2,6-(Z6-2,4,6-C6H2Me3)(2,4,6-C6H2Me3)C6H3}2(k3-BH4)2] (19) in 50% yield, Scheme 5. Attempts to remove the coordinated DME solvent under vacuum or by heating in non-coordinating solvents

Fig. 3 Complexes 16 and 17.20

466

Arene Complexes of the Actinides

were unsuccessful. The geometry around the thorium was determined to be pseudo-octahedral, with the two Z6-mesityl groups being trans to each other (arenecentroid-Thorium-arenecentroid angle ¼ 172.88 ), thus 19 is an example of a thorium bis-arene sandwich complex. The thorium-bound oxygen of the aryl oxide ligands are found cis to each other, reflected by the O-Th-O angle of 89.0(3) . Although 19 adopts a sandwich geometry, the thorium-arene bonding is not symmetric. The thorium-arenecentroid distances in 19 are 2.815(3) and 4.05(1) A˚ .

Scheme 5 Synthesis of 19 from 18.21

Macrocyclic ligands such as trans-calix[2]benzene[2]pyrrolide (L) provide flexibility in the binding to metal centers, in the sense the pyrrolide rings and the arene rings can both be s- or p-bound. This flexibility in bonding mode has allowed different reactivity of such metal complexes to be explored (See Section 4.08.4.4 for uranium examples and Section 4.08.5 for neptunium examples), Scheme 6. Addition of K/naphthalene or [K{N(SiMe3)2}] to the thorium trans-calix[2]benzene[2]pyrrolide complex [(L)ThCl2] yielded the complexes [K(THF)2Th(m-Cl)(2HL−)]2 and K[Th{N(SiMe3)2}(L−2H)] respectively.54 The noticeable transformation over these reactions is that the two arene rings of the macrocyclic ligand have changed from not interacting with the thorium center, to

Scheme 6 Synthesis of 24, 25, 27a/b, and 28a/b from 20–23.22,54

Arene Complexes of the Actinides

467

being deprotonated to a tetraanioinc ligand and forming two s-bonds to the thorium center.54 This change in bonding was also observed when the metal alkyls LiMe, LiCH2SiMe3 and KCH2Ph were added to [(L)ThCl2], yielding the respective thorium complexes [Li(L−2H)Th(Me)] (20, 93% yield), [Li(L−2H)Th(CH2SiMe3)] (21, 73% yield) and [K(L−2H)Th(CH2Ph)] (22, 71% yield).22 The flexibility in bonding of the trans-calix[2]benzene[2]pyrrolide ligand was further demonstrated following the addition of two equivalents of [Et3NH][BPh4] to 22 and the bistrimethylsilylamide derivative [K(L−2H)Th{N(SiMe3)2}] (23), yielding [(L)Th(CH2Ph)][BPh4] (24, 31% yield) and [(L)Th{N(SiMe3)2}][BPh4] (25, 68% yield). X-ray diffraction studies show that in the solid state 25 adopts an Z6:k1:Z6:k1-coordination of the supporting ligand, thus displaying the bis-arene sandwich motif. Complex 24 could not be crystallographically characterized, however multinuclear NMR spectroscopy of 24 and 25 suggests the bonding is analogous for both complexes. Complex 25 displays thorium-arene bond distances ranging from 2.913(3)–3.154(3) A˚ , with an mean thorium-centroid distance of 2.690 A˚ , and an arenecentroid-Thorium-arenecentroid angle of 169.41 . The reactivity of the bis-arene sandwich complexes 24 and 25 toward small molecules was explored in the same publication. However, addition of CO, CO2 and tBuNC to 24 or 25 give mixtures of products. Addition of [Ni0(COD)2] (COD ¼1,5-cyclooctadiene) and PR3 (R ¼ cyclohexyl or phenyl) to [(L)Th(C^CSiMe3)2] (26) yielded [(L)Th(C^CSiMe3)2
Ni(PCy3)] (27a) and [(L)Th(C^CSiMe3)2
Ni(PPh3)] (28a). The interesting structural feature of 27 and 28 is the change in coordination of the supporting macrocyclic ligand to the thorium depending on the crystallization conditions used. When crystallized from saturated hexane solutions, 27a and 28a adopt metallocene-like Z5:Z5-coordination from the two pyrrolide rings, with no interaction with the arene rings. However, when crystallized from vapor diffusion of hexane into a THF solution, the solid state structure of 27b displays an Z6:k1:Z6:k1-coordination of the ligand and therefore can be described as a bis-arene sandwich complex. Complex 27b displays thorium-arene bond distances ranging from 2.962(3)–3.049(3) A˚ , with a mean thorium-centroid distance of 2.665 A˚ , and an arenecentroid-Thorium-arenecentroid angle of 166.55 . Variable temperature NMR spectroscopy suggests the metallocene-like coordination is preferred at low temperature.

4.08.3.2

Inverse sandwich dithorium arene complexes

The thorium(IV) precursor [(NNTBS)ThCl2(THF)] (NNTBS ¼ {Fe(Z5-C5H4NSiMe2But)2}2−) and half an equivalent of biphenyl was reduced with 2.5 equivalents of KC8 in THF at −78  C to yield the thorium inverse arene complex [{(NNTBS)Th(THF)}2(m-Z6:Z6-C6H5Ph)] (29a) in 32% yield after workup, Scheme 7.23 The analogous complexes [{(NNTBS)Th(THF)n}2(m-Z6:Z6-arene)] (arene ¼ naphthalene, n ¼ 1, 21%(29b); arene ¼ benzene, n ¼ 0, 48% (29c); arene ¼ toluene, n ¼ 0, 52% (29d)) could also be synthesized utilizing KC8 as a reducing agent, Scheme 7. All four thorium inverse arene complexes were characterized by SC-XRD and display similar Th-Carene distances: 29a, 2.564(4)–2.727(4); 29b, 2.564(8)–2.763(9); 29c, 2.536 (6)–2.715(4); 29d, 2.547(0)–2.696(4) A˚ . 13C NMR spectroscopy shows arene resonances that are significantly downfield (29a, 82.0, 77.5, 69.5 66.1; 29b, 76.2. 73.8, 69.4; 29c, 78.6; 29d; 88.4, 85.1, 80.8, 80.6 ppm) compared to the respective free arenes, indicating reduction of the arenes. UV/Vis/NIR, X-ray photoelectron, and electron paramagnetic resonance spectroscopies collectively confirm the absence of Th(III), thus assigning these thorium inverse arene complexes as containing two thorium(IV) ions combined with tetranionic arenes.

Scheme 7 Synthesis of 29a-d.23

Arene exchange reactions showed slow exchange on heating a C6D6 solution of 29c or 29d, whereas no exchange with C6D6 was observed with 29a. No exchange was observed for 29c in C7D8. Complex 29b undergoes exchange reactions with C6D6 and biphenyl, Scheme 8, with 29d also undergoing exchange with biphenyl. From these arene exchange reactions, the order of stability of the thorium inverse arene complexes was determined as 29b  29d < 29c  29a. DFT studies showed the HOMO and HOMO-1 to be close in energy; −4.04 and −4.07 for 29c, −4.23 and −4.35 in 29d, −3.81 and −3.93 in 29a, and −3.44 and −3.80 eV in 29b. The energy gap being significantly greatest for 29b (0.36 eV), could possibly explain the low stability of 29b. Calculations show that all four thorium inverse arene complexes display d-type bonding orbitals arising from the p4 and p5 arene orbitals and thorium 6dd

468 Arene Complexes of the Actinides

Scheme 8 Synthesis of 30–34 from 29b/c.23

Arene Complexes of the Actinides

469

and 5fd symmetry orbitals. Composition analysis of the d-type bonding for 29c as an example showed an arene contribution of 66.5% (2p: 60.6%) and thorium contribution of 28.7% (6d: 15.8%; 5f: 12.4%) for the HOMO, and 67.4% (2p: 63.1%) and 28.7 (6d: 16.9%; 5f: 11.8%) for the HOMO-1. The reactivity of the thorium arene complexes toward unsaturated substrates was explored, Scheme 8. It was found that 29ad reduce both cyclooctatetraene and azobenzene to form [(NNTBS)Th(THF)(Z8-COT)] (30) or [(NNTBS)Th(THF)2(Z2-N2Ph2)] (31), respectively. Thus, confirming the ability of these thorium inverse arene complexes to act as four-electron reducing agents. All four thorium inverse arene complexes were found to react with diphenylacetylene to form [(NNTBS)Th(THF)(Z2-C4Ph4)] (32), in up to 80% yield. However, when bis(trimethylsilyl)acetylene was used, only 29b was found to be reactive enough to perform reduction to an actinide metallacyclopropene [(NNTBS)Th(THF)2{Z2-C2(SiMe3)2}] (33) (70% isolated yield). Complex 33 could be reacted further with diphenylacetylene to give 32. Complex 29b was found to react with anthracene to yield the mononuclear thorium anthracene complex [(NNTBS)Th(THF)](9,10-Z2-C14H10) (34), in which the anthracene is formally reduced to its dianionic form. When complex 34 was heated in C6D6, the benzene was reduced to form [{(NNTBS)Th(THF)}2(m-Z6:Z6-C6D6)] (29c-d6).

4.08.4

Uranium arene complexes

4.08.4.1

Formally neutral uranium arene complexes

Unlike with transition metals, no actinide arene sandwich complexes An(C6R6)2 have been reported experimentally on the macroscopic scale. Gas phase reactions of actinide metals with arenes, show the existence of U(arene)n (n ¼ 1–3) by mass spectrometry, accompanied by many CdH and CdC bond activation reactions.65 For example the complexes U+(C6H6)n (n ¼ 1–3) were produced by laser vaporization, with the n ¼ 2 and 3 complexes decomposing to the n ¼ 1 complex by elimination of neutral benzene. The high reactivity of the uranium cations is shown by U+(C6H6) undergoing dissociation via ligand elimination and ligand decomposition into U+, U+(C2H2) and U+(C4H2). Matrix isolation techniques allowed the detection of U(C6H6) by vibrational spectroscopy, however there was no evidence for the sandwich complexes U(C6H6)2 or U2(C6H6)2 under these conditions, even though they were calculated to be ‘stable’ molecules.66 Uranium complexes bearing anionic p-ligands were reported as early as 1956 for C5H5 ligands, and the cyclopentadienyl ligand class has gone on to be one of the most widely utilized and exploited ligands in organouranium chemistry.67 The landmark discovery of uranocene [U(C8H8)2] in 1968, with its structural determination the following year, initiated a new class of uranium sandwich complexes, thus slowly starting to bridge the large gap in knowledge between f-block and transition metal organometallic chemistry.68,69 Soon after in 1971, the first uranium complex containing a neutral C6H6 ligand was discovered in the form of the uranium(III) 6-arene complex [U(C6H6)(AlCl4)3] (35), Fig. 4, synthesized through the reflux of UCl4 and AlCl3 in anhydrous

Fig. 4 Early reported uranium-neutral arene complexes 35–40.24–29

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Arene Complexes of the Actinides

benzene. Complex 35 was found to be thermally stable in crystalline form up to 110  C, although rapid decomposition was observed in the presence of moisture. Analysis by SC-XRD showed that 35 adopts a three legged piano stool geometry around the uranium center, with a mean U-C distance of 2.91(1) A˚ .24 Some 14 years after the isolation of 35, a number of related complexes were reported by Cotton et al., the first being the binuclear uranium(IV) bridging chloride complex [U2(6-C6Me6)2Cl4(m-Cl)3][AlCl4] (36), achieved under reducing Freidel-Crafts conditions.25 Initial attempts involved a melt reaction of uranium halide, aluminium trichloride, hexamethyl benzene and aluminium, however the only uranium species that could be isolated were UCl4 solvates. However, reaction of UCl4, AlCl3 and hexamethylbenzene in refluxing hexane, followed by dissolution in CH2Cl2 and addition of zinc granules yielded crystals of 36. The solid state structure was determined, which showed a mean uranium-carbon bond distance of 2.92(4) A˚ . This was significantly longer than those previously reported for anionic C5 (2.66–2.89 A˚ ) and C8 complexes (2.65–2.66 A˚ ), thus suggesting a weaker interaction between the uranium and the neutral C6-arene ligand. In the following 2 years, Cotton et al. proceeded to report the isolation of the first trinuclear U(IV) arene complex [U(C6Me6) Cl2(m-Cl)3UCl2(m-Cl)3UCl2(C6Me6)] (37), as well as the first uranium(III) trinuclear arene cluster [U3(m3-Cl)2(m2-Cl)3 (Z2-AlCl4)3(Z6-C6Me6)3][AlCl4] (38) and the mono arene uranium(III) complex [U(C6Me6)(AlCl4)3] (39), Fig. 4.26–28 The former could be isolated on a multi-gram scale, but was found to be insoluble in CH2Cl2 and decomposes to the respective UCl4 solvate in THF or acetonitrile, thus preventing solution-based characterization. Complex 38 was synthesized in 55% yield via addition of excess aluminium powder to 36 and was found to be extremely air and moisture sensitive, with decomposition observed when heated under vacuum at ca. 160  C. Complex 39 was synthesized by the reflux of UCl4, hexmethylbenzene and AlCl3 in toluene, followed by addition of aluminium foil to the reaction and reflux for a further 5 h; it could be isolated in 34% yield and was also found to be extremely air and moisture sensitive, with decomposition reactions occurring with halocarbon or donor solvents. In 1989, another route to uranium-arene complexes was reported. The thermal decomposition of [U(BH4)3] in mesitylene followed by addition of hexamethylbenzene yielded [(C6Me6)U(BH4)3] (40), Fig. 4.29 Substitution of the mesitylene with arene solvents was determined by NMR spectroscopy, with 40 being characterized crystallographically. Complexes 37–40 display respective mean uranium-carbon distances of 2.94(3); 2.94(9), 2.93(5) and 2.90(6) A˚ ; 2.94(2) A˚ ; 2.93(2) A˚ . In 2002, Evans et al. reported a uranium complex containing two bound C6-arenes.30 Addition of two equivalents of [Et3NH] [BPh4] in benzene to [(C5Me5)2U(Me)2K] yielded the bridging arene complex [(C5Me5)2U][(m-Z2-Ph)2BPh2] (41) as a brown powder in 83% yield after work up, Scheme 9. The solid state structure of 41 was determined by SC-XRD and showed the uranium center to be bound to two of the ortho-carbons and one of the meta-carbons of the anionic (BPh4)− moiety. The uranium-carbon bond distances corresponding to these bound arenes were determined to be 2.857(7) and 2.880(7) A˚ for the ortho-carbons, and 3.138(8) and 3.166(8) A˚ for the meta-carbons, which are significantly longer than the mean uranium-carbon distance for the coordinated cylopentadienyl rings (2.77(3) A˚ ). The 1H NMR spectrum of 41 in D6-benzene only showed the methyl resonances, therefore it cannot be determined if the Z2-coordination is maintained in solution.

Scheme 9 Synthesis of 41.30

Three years later, the same group reported a uranium complex containing both an Z6- and Z1-bound C6 arene, Scheme 10.31 Addition of benzene to a mixture of [(C5Me4H)2U(Me)2K] and two equivalents of [Et3NH][BPh4] at room temperature, yielded [(C5Me4H)2U][(m-Z1-Ph)(m-Z6-Ph)BPh2] (42) as a brown solid in 87% yield after work up (63% yield across three steps from [(C5Me4H)2U(Cl)2]). Suitable quality crystals for SC-XRD analysis were grown from cooling of a boiling toluene solution to −35  C over 5 days. The solid state structure of 42 showed the Z1-bound arene to be through the ortho-carbon of the arene ring, the same as observed in 41. The uranium-carbon bond distance for the Z1-interaction in 42 was 3.050(4) A˚ which is significantly longer than the comparable interaction reported for 41 (2.857(7) and 2.880(7) A˚ ). The uranium-carbon bond distances corresponding to the Z6-bound arene range from 2.868(4) to 3.066(4) A˚ , with a mean distance of 2.98(7) A˚ , which is comparable to that reported in other uranium-Z6-arene complexes, e.g. 36 (2.92(4) A˚ ), 39 (2.93(2) A˚ ), 40 (2.93(2) A˚ ). The mean uranium-carbon distance to the [C5Me4H]− ligand in 42 of 2.81(8) A˚ is comparable to the mean uranium-carbon distance in [U(C5Me5)3] (2.84(4) A˚ ). The 1H NMR spectrum of 42 in C6D6 shows resonances that are assigned to the C5 rings at +21.2, −12.3 and − 35.1 ppm, however the tetraphenylborate resonances could not be assigned even at 193 K. 11B NMR spectroscopy of 42 shows a single resonance at −33.0 ppm, which is comparable to the 11B NMR spectrum of 41 and [(C5Me5)2U(Me)][MeBPh3],70 which display a single resonance at −43 ppm and −46 ppm(268 K) respectively.

Arene Complexes of the Actinides

471

Scheme 10 Synthesis of 42.31

4.08.4.2

Inverse sandwich diuranium(III) arene complexes

The first uranium arene bridged inverse sandwich structure was reported in 2000 by Cummins et al., Scheme 11.32 Reduction of [U(I){N(R)Ar}3] (43a, R ¼ But; 43b, R ¼ adamantyl, Ad; Ar ¼ 3,5-C6H3-Me2) with KC8 in toluene afforded the symmetrical Z6-bound inverse sandwich complexes [(m-Z6:Z6-C7H8){U(N[R]Ar)2}2] (44a, R ¼ But; 44b, R ¼ Ad) in ca. 40% isolated yield. While the structure of 44a could not be determined crystallographically, NMR spectroscopy of the deuterated analog (synthesized by performing the reaction in D8-toluene) allowed assignment of the resonances corresponding to the bridging toluene in the 2H NMR spectrum. Furthermore, NMR spectroscopy showed the benzene-bridged diuranium complex could also be synthesized by carrying out the KC8 reduction in benzene. Modification to the N-1-adamantyl derivative resulted in isolation of 44b in 23% yield after reduction of the U(IV)-iodide precursor in the same manner as that employed in the synthesis of 43b. The solid state structure of 44b showed only a ca. 0.04 A˚ elongation of the toluene C-C distances for the bridging arene complex compared to free toluene. The inverse sandwich complex showed significantly shorter UdC bond distances (2.504(9) A˚ to 2.660(8) A˚ ) when compared to the previously reported neutral arene-uranium complexes, e.g. 40 (mean U-C 2.93(2) A˚ ). Assignment of the formal oxidation state of the uranium ions in 44a and 44b contains a level of ambiguity due to the three possibilities; a neutral arene between two uranium(II) centers; a dianion between two uranium(III) centers; or a tetraanion between uranium(IV) centers. Reactivity studies of 44b with two equivalents of PhSSPh or one equivalent PhNNPh yielded the uranium(IV) complexes [U(m-SPh)(SPh){N (But) Ar}2]2 (45) and [U(m-NPh){N(But)Ar}2]2 (46), respectively, in yields ca. 70%, and both were crystallographically characterized. The truncated calculated structure [(m-Z6:Z6-C6H6){U(NH2)2}2] showed the four most energetic electrons in the system to be uranium-based, nonbonding 5f-electrons, with the next four electrons being involved in covalent d-bonding interactions between the uranium 6d- and 5f-orbitals and the benzene LUMOs.

Scheme 11 Synthesis of 44a/b from 43a/b and conversion of the former to 45 and 46.32

472

Arene Complexes of the Actinides

In 2002, it was reported that reduction of [IU(DME){NC(But)Mes}3] (47) with 4 equivalents KC8 or a sodium mirror in the presence of naphthalene produced the inverse sandwich naphthalene complexes [{(Mes[But]CN)3U}2(m-Z6:Z6-C10H8)K2] (48) and [{(Mes[But]CN)3U}2(m-Z6:Z6-C10H8)Na2] (49) in 60% and 40% yields respectively, Scheme 12.33 Unlike in the synthesis of 44a and 44b, where reduction was accompanied by the elimination of an amide ligand (presumably as the potassium salt), in the synthesis of 48 and 49 all three uranium bound ketamide ligands are retained.

Scheme 12 Synthesis of 48 and 49 from 47.33

The solid state structure of 48 reveals a three legged piano stool geometry at uranium with the potassium being incorporated into the complex via nitrogen coordination from two of the uranium-bound ketamide ligands. Complex 48 exhibits short U-C distances (2.565(11) A˚ to 2.749(10) A˚ ) with the longer U-C distances involving the carbons fusing the two rings of naphthalene together. This is due to these carbons not making an appreciable orbital contribution to the LUMO of naphthalene. The carbon-carbon distances of the bridging arene range from 1.410(16)–1.500(14) A˚ , with a mean of 1.443(6) A˚ , which is consistent with aromaticity. In contrast, the CdC bond distances in the uncoordinated C6 ring of the naphthalene show bond distances ranging from 1.319 (17) to 1.470(16) A˚ , suggesting alternating single and double bonds. 1H NMR spectroscopy of both 48 and 49 show a single ketimide ligand environment, with assignment of the resonances corresponding to the bridging arene being determined through synthesis and characterization of the analogous D1-napthalene and D8-napthalene complexes. The protons of the bridging naphthalene in 48 exhibited resonances in the 1H NMR spectrum (C6D6) at 110.14, −32.88, −43.56 and −141.55 ppm, with the 1 H NMR spectrum of 49 displaying resonance corresponding the naphthalene at 84.30, −29.85, −38.37 and −133.28 ppm. Variable temperature NMR studies of 49 showed it was stable as a C6D6 solution up to 80  C. Reduction of the uranium complex [U(Z5-C5Me5)3] with KC8 in benzene resulted in the synthesis of the inverse sandwich complex [{U(Z5-C5Me5)2}2(m-Z6:Z6-C6H6)] (50) in 90% isolated yield, with elimination of [K(Z5-C5Me5)], Scheme 13.34 Complex 50 could also be isolated from reaction of 41 and potassium-18-crown-6 in the presence of benzene in 82% isolated yield. The solid state structure of 50 was determined by SC-XRD, revealing that the CdC bond distances (1.42(2)–1.462(18) A˚ ) and angles (117.5(12) –121.4(12) ) of the bridging benzene are indistinguishable from free benzene (1.39 A˚ and 120 ). However, the bridging benzene adopts a shallow boat conformation rather than a planar geometry resulting in short U(1)-C(C6H6) distances of 2.51(1) and 2.55(1) A˚ , and long U(2)-C(C6H6) distances of 2.72(1) and 2.73(1) A˚ . The uranium-carbon distances for the cyclopentadienyl rings range from 2.795(14) to 2.878(14) A˚ , which is in agreement with other uranium-cyclopentadienyl complexes. The uranium. . .uranium distance in 50 is 4.396 A˚ . In methylcyclohexane, [U(C5Me5)3] was found to be stable to addition of KC8 and K/18-crown-6 for 24 h. Moreover, reduction of [U(Z5-C5Me5)3] with KC8 in p-toluene lead to the formation of the bridging p-toluene complex [{(C5Me5)3U}2(m:Z6-Z6-C6Me2H4)] (51), although performing the reduction in C6Me6 was not successful in yielding the target bridging arene complex.

Scheme 13 Synthesis of 50 from [U(Z5-C5Me5)3] and 41 and 51 and conversion of 50–52.34

Arene Complexes of the Actinides

473

Complex 50 undergoes a ligand substitution reaction when reacted with two equivalents of KN(SiMe3)2, yielding the mixed amide-cyclopentadienyl benzene inverse sandwich complex [{([Me3Si]2N)(C5Me5)U}2(m-Z6:Z6-C6H6)] (52) in 89% isolated yield, Scheme 13. In contrast to 50, the benzene in 51 adopts a shallow chair conformation with slightly elongated C-C distances (1.449 (4)–1.458(4) A˚ ) compared to free benzene. Complex 51 displays uranium-carbon distances ranging from 2.559(3) to 2.631(3) A˚ , with a uranium-uranium distance of 4.291 A˚ . Addition of KN(SiMe3)2 to [U(Z5-C5Me5)3] in benzene yielded [(C5Me5)2U {N(SiMe3)2}], rather than the bridging arene complex. The recorded room temperature magnetic moments of 50 (meff ¼ 2.2 mB) and 51 (meff ¼ 1.8 mB) are within the range of uranium(III) complexes, however they cannot be unambiguously distinguished from the magnetic moments found in uranium(II) and (IV) configurations at room temperature. The electronic absorption spectra of 50 shows no resolved absorptions in the near-IR region that are characteristic of Laporte-forbidden f-f transitions, thus providing no definitive evidence of oxidation state. However, complex 51 exhibits f-f transitions (albeit broad compared to U(C5Me5)3), suggesting the presence of uranium(III) ions. Density functional theory calculations were conducted on 50 to determine an optimized geometry, which compares well with the experimental structure. The four least stable electrons in the optimized geometry were found to be principally of localized 5f character, with the two least stable electrons displaying a population close to 0.5 across four MOs. Therefore, the population of these MOs were constrained to 0.5 occupancy. Initial calculations showed that the most appropriate way to describe the electronic structure of 50 is with each uranium ion containing two 5f-electrons that are ferromagnetically coupled. However broken symmetry calculations converged to an electronic structure displaying an antiferromagnetic coupled, low spin arrangement 9.6 kJ mol−1 more stable than the high spin structure. The four next most stable electrons were found to be located in d-bonding orbitals between the uranium ions and bridging arene. If the electrons are uranium-based, two uranium(II) ions with a neutral arene would be the best description; if arene based, two uranium(IV) ions with a tetraanionic arene would be the best description. Composition analysis of the MOs suggested the electrons are mixed uranium/arene character, thus an intermediate between uranium(II) and uranium(IV) is the best description. For example, one b one-electron MO representing uranium-arene d-bonding has the composition 19% U1 5f, 4.2% U1 6d, 12.9% U2 5f, 2.6% U2 6d, 45.9 carbon 2p. The other b MO and two a MOs display similar, but not identical compositions. Computed charges in 50 (+2.7 to +2.8 for U; −2.2 for arene) and in 51 (+2.6 for U; −2.4 for arene), taken together with the MO compositions, suggests that overall uranium(III) is the most appropriate assignment of formal oxidation state for uranium in these complexes. The arene-based orbitals that participate in d-bonding have significant antibonding character and are partially populated, unlike in free benzene where they are unoccupied. This is reasoned to be responsible for the observed lengthening of the carbon-carbon rings of the bridging arene. The transfer of electron density from uranium to the arene is reflected in the computed charges of −0.43 and +0.07 for the carbon and hydrogen atoms respectively, giving an overall arene ring charge of −2.21. Substitution reactions can provide further insight into the bonding of these arene bridged structures. If the bridging arene is neutral, it would be displaced by a more substituted arene due to the increased Lewis basicity as is the case in transition metal chemistry. However, if the arene is charge-loaded, then less substituted and better p-acceptor arenes will displace more substituted variants (for alkyl substituents). Thus, 50 was found to crystallize from toluene, retaining the bridging benzene as well as forming preferentially in a 1:1 benzene/p-xylene solution. Moreover, the p-xylene analog 51 reacts with C6D6 to make the deuterated analog of 50, Scheme 13. This suggest the existence of the (p-xylene)2− dianion which can reduce benzene, resulting in the formation of 50. Finally, since THF does not displace the bridging arene in 50, this reinforces the assignment of the presence of a (C6H6)2− dianion in 50. Five years after the initial report of 50, another study on its reactivity was reported, Scheme 14.35 Reaction of 50 with 2 equivalents of [K(OC6H2-2,6-tBu-4-Me)], [Li{CH(SiMe3)2}] or [Li{MeC(NPri)2}] yielded the ligand-substituted inverse sandwich complexes [{U(Z5-C5Me5)(OC6H2-2,6-tBu-4-Me)}2(m-Z6:Z6-C6H6)] (53, 76% yield), [{U(Z5-C5Me5)(CH[SiMe3]2)}2(m-Z6:Z6C6H6)] (54, 73% yield) and [{U(Z5-C5Me5)(MeC[NiPr]2}2(m-Z6:Z6-C6H6)] (55, 86% yield), respectively. It should be noted that

Scheme 14 Synthesis of 53–55 from 50.35

474

Arene Complexes of the Actinides

55 was not characterized crystallographically, and the crystallographic data of 53 could only confirm connectivity. Comparison of the bond distances in 50, 52 and 54 shows the range of uranium-carbon bond distances, with respect to the bridging benzene, include slightly longer bond distances overall in 50 (2.506(13)–2.733(14) A˚ ) compared to 52 (2.559(3)–2.631(3) A˚ ) and 54 (2.5323(19)–2.2.6398(19) A˚ ). Moreover, the out-of-plane angle for the C6H6 ligand is comparable in 50 (12.5 and 18 ) and in 54 (12.1 ), whereas the angle in 52 was significantly lower (7.2 ). Complexes 51, 54, and 55 all undergo reductive reactivity with excess C8H8, yielding the previously reported monometallic, mixed-sandwich complexes [(C5Me5){(Me3Si)2N}U(C8H8)] (56, 50% yield), [(C5Me5){(Me3Si)2CH}U(C8H8)] (57, 85% yield) and [(C5Me5){MeC(NPri)2}U(C8H8)] (58, 79% yield), Scheme 15. Furthermore, the reactivity of 50, 52, 54, and 55 toward 1-azidoadamantane was investigated. Reaction of 50 with four equivalents of 1-azidoadamantane, leads to the elimination of benzene and dinitrogen to yield the previously reported bis-imido complex [(C5Me5)2U(NAd)2] (59). Similarly, reaction of 52 and 55 with four equivalents of 1-azidoadamantane yielded the bis-imido complexes [(C5Me5)2U(NAd)2(X)] (X ¼ MeC(NiPr)2 (60) or (Me3Si)2N (61)). Analysis by 1H NMR spectroscopy showed the presence of 59 in under 10% yield, presumably formed from a minor ligand redistribution reaction. Reaction of 54 with four equivalents of 1-azidoadamantane is more complex. Resonances corresponding to the bis-imido complex [(C5Me5)2U(NAd)2{CH(SiMe3)2}], which is analogous to 60 and 61, could be observed by 1H NMR spectroscopy (among other products), however the complex could not be isolated in a pure form. Three products from the reaction mixture that could be characterized crystallographically were 59, [U(Z5-C5Me5)(NAd)2{AdNNNCH (SiMe3)2}] (62) and [U(Z5-C5Me5)(m-Z5-k1-C5Me4CH2NAd)(NAd)] (63).

Scheme 15 Synthesis of 56–63 from 50–55.35

Single-molecule magnet behavior was expanded to an arene-bridged diuranium complex in 2011, Scheme 16.36 Reduction of the uranium(IV) carbene complex [{U(BIPM™S)(I)(m-I)}2] (BIPM™S ¼ C(PPh2NSiMe3)2) with KC8 in THF afforded the arene-bridged diuranium complex [{U(BIPM™SH)(I)}2(m-Z6:Z6-C6H5CH3)] (64) in 20% yield after recrystallization from toluene. SC-XRD studies showed 64 to have U-Carene bond lengths in the range of 2.553(7)–2.616(7) A˚ , with only a 0.02 A˚ increase in C-C distance (av. C-C 1.436(16) A˚ ) compared to free toluene. The mean U-Cmethanide distance of 2.465(9) A˚ reveals that during reduction, protonation of the methandiide carbon of the supporting ligand framework has occurred, thus the supporting ligand in 64 is monoanionic and no longer a dianionic carbene.

Arene Complexes of the Actinides

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Scheme 16 Synthesis of 64.36

The electronic absorption studies of 64 in toluene or THF showed an intense charge-transfer band that trails in from ultraviolet region, however multiple bands assigned to f-f transitions were still observed at 9750 cm−1 (e ¼ 400 M−1 cm−1), 10,500 (e ¼ 580 M−1 cm−1) and  11,100 cm−1 (e ¼ 600 M−1 cm−1). Multiple bands with relatively large molar absorptivity (e ¼ 620–4000 M−1 cm−1) were observed in 3000–9000 cm−1 region, which showed good agreement with the f-f transitions to 4 I13/2, 4F3/2, and 4I13/2 energy levels for the uranium(III) free ion. An intense absorption band was also observed at 5640 cm−1 (e ¼  1800 M−1 cm−1) which could not be assigned following the single-ion Russell-Saunders coupling scheme. The high intensity of the observed transitions was attributed to a combination of the low symmetry and the asymmetric ligand field relaxing the electric dipole selection rules leading to ‘intensity stealing’. In order to gain an insight into the formal charges and magnetic properties of 64, powdered samples were measured by SQUID magnetometry. Complex 64 displayed a wT ¼ 1.54 cm3 K mol−1 at 298 K, which decreased to 0.59 cm3 K mol−1 at 1.8 K. The wT not tending to zero at 1.8 K suggests no strong antiferromagnetic exchange interactions are present. The room temperature magnetic moment of 64 in a C6D6 solution was 3.79 mB, corresponding to a wT of 1.80 cm3 K mol−1 which is similar to that measured in the solid state. The difference in values is likely to be caused by errors associated with the solution based technique (e.g. weighing errors). The wT in both the solid state and solution for 64 is lower than would be expected for two independent uranium(III) 4I9/2 ions, which has a theoretical value of 3.28 cm3 K mol−1. The cause of this difference was postulated to be due to not all of the crystal field components of the ground multiplet being occupied at room temperature. Magnetic hysteresis measurements at 1.8 K show clear hysteresis which collapses at zero field suggesting quantum tunnelling of the magnetization. The cause of this relaxation process is likely to arise from the low symmetry components of the crystal field, and not due to hyperfine interactions. In an external d.c. field of 1000 Oe, 64 displays a frequency-dependent out-of-phase signal of the a.c. susceptibility, proving that it is a polyuranium SMM. The energy barrier to relaxation could not be determined due to a maximum in the out-of-phase component only being observed for the highest a.c frequencies. The relaxation rate was determined to be of the order of 102 Hz. It should be noted that it was deduced the slow relaxation is a result of the properties of the sample and not another process such as reorientation of the crystallites, by the fact that the values of w0 and w00 are virtually identical at the temperature of the maximum w.00 36 Unrestricted DFT calculations were performed for 64 to investigate the electronic structure. The four HOMOs are almost entirely uranium in character, with each being singularly occupied. The a-spin HOMOs −4 and −5, and their b-spin counterparts correspond to d-bonding from the uranium center to the bridging arene. These HOMOs display significant covalent interaction between the metal center and the bridging toluene with approximately equal contributions from the metal and ligand. The composition of these orbitals were determined to be 39% carbon 2p, 41.7% uranium 5f and 11.7% uranium 6d for the a-spin HOMO -4, and 44.2% carbon 2p, 39.3% uranium 5f and 10.0% uranium 6d for the a-spin HOMO -5. The computed Mulliken charge of uranium of +1.19 and −2.11 on the bridging toluene ring is consistent with a uranium-arene charge transfer. Average spin densities were computed as 2.08, −0.02, −0.03, 0.02 and −0.05 for the uranium, methanide carbon, nitrogen, phosphorus and arene carbon atoms, respectively, supporting the covalent bonding assignment to the metal-arene interaction. The average total Nalewajski-Mrozek bond indices were determined to be 0.897, 0.471 and 0.413, for the U-N, U-Cmethanide and U-Carene interactions respectively. The computational data all combined is consistent with the bridging arene complex containing two formal U(III) centers and a bridging dianionic ligand through covalent d-bonding.36 Also in 2011, another diuranium inverse sandwich complex was reported, this time stabilized by use of a bidentate ferrocene diamide ligand, Scheme 17.37 Reaction of the monomeric uranium(IV) complex [(NNfc)2UI2(THF)] (NNfc ¼ fc(NSitBuMe2)2) with excess KC8 in toluene at room temperature for 24 h yielded the bridging toluene complex [{(NNfc)U}2(m:Z6-Z6-C7H8)] (65) in 81% yield. Suitable quality crystals for SC-XRD analysis were grown from a concentrated toluene solution at −35  C. The solid state structure of 65 revealed uranium-arene bond distances ranging from 2.544(6) to 2.662(6) A˚ , which is in line with other reported diuranium bridging arene complexes: 64, 2.553(7)–2.616(7) A˚ ; 52, 2.559(3) to 2.631(3) A˚ ; 44b, 2.504(9) A˚ to 2.660(8) A˚ ). Complex 65 displays a uranium-iron distance of 3.11 A˚ , which is 0.17 A˚ shorter than the sum of the covalent radii. Solid state SQUID magnetometry measurements on 65 showed a room temperature magnetic moment of 1.80 mB and 1.91 mB (two independently synthesized samples were measured), with low temperature (5 K) magnetic moments of 0.74 mB and 0.66 mB, respectively. The best assignment of formal oxidation states in 65 is diuranium(III) with a toluene dianion. The ability of such complexes as four electron reductants was displayed by reaction of 65 with two equivalents of quinoxaline in toluene at 85  C for 1.5 h, yielding a tetranuclear uranium(IV) macrocycle containing four uranium centers sill bearing the ferrocene diamide ligand at the corners, with four bridging quinoxaline making up the sides of the quadrangle, Scheme 17. The bridging quinoxaline are all bound to uranium through the nitrogen atoms of the ligand, with no uranium-arene being observed in the solid state structure. A minor product was also observed by NMR spectroscopy, however due to identical solubilities to [{U(NNfc)(m:k1-k1-C8H6N2}4], the minor product could not be separated and characterized.

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Arene Complexes of the Actinides

Scheme 17 Synthesis of 65.37

The high reducing ability of uranium(III) promotes the ready formation of the inverse sandwich complexes [{(X)2U}2(m-Z6:Z6arene)] (66–71, X ¼ N(SiMe3)2 or O-2,6-tBu2C6H3; arene ¼ benzene, toluene, biphenyl), mediated via uranium(III) disproportionation with UX4 being the uranium(IV) by-product, Scheme 18.38 Reaction of [U{N(SiMe3)2}3] with benzene or toluene yielded [{([Me3Si]2N)2U}2(m-Z6:Z6-C6H6)] (66) or [{([Me3Si]2N)2U}2(m-Z6:Z6-C6H5Me)] (67) and [U{N(SiMe3)2}4], as well as the [U {N(SiMe3)2}4] by-products [{(Me3Si)2N}2U(k2-CH2SiMe2NSiMe3)] and HN(SiMe3)2. Storage of [U(O-2,6-tBu2C6H3)3] in benzene at 90  C lead to the quantitative formation of [{(O-2,6-tBu2C6H3)2U}2(m-Z6:Z6-C6H6)] (68) and two equivalents of [U(O2,6-tBu2C6H3)4] by NMR spectroscopy, after 6 days. Complex 67 could be isolated as a crystalline material in 14% yield following washing with diethyl ether to remove the uranium(IV) by-product, then recrystallization from toluene. The solid state structure of 67 shows the bridging benzene ligand only shows a small deviation away from being planar (0.06 A˚ ), with CdC bond distances

Scheme 18 Synthesis of 66–72.38

Arene Complexes of the Actinides

477

ranging from 1.442(9)–1.462(9) A˚ which are slightly elongated compared to free benzene. The uranium-carbon bond distances corresponding to binding to the bridging arene are in the range of 2.517(6)–2.617(6) A˚ which falls within the reported range for other uranium inverse sandwich complexes. The solid state structure of 66 shows comparable uranium-carbon distances (2.568 (3)–2.578(3) A˚ ) and carbon-carbon bond distances (1.447(4)–1.457(4) A˚ ) to 67. Both 66 and 68 were found to be thermally robust and do not undergo decomposition readily, even at temperatures exceeding 100  C. However, substitution of the stabilizing co-ligands while maintaining the bridging arene was achieved. Reaction of 66 with two equivalents of HO-2,6-But2C6H3 in D6-benzene at 90  C for 6 days yielded the mixed ligand bridging arene complex [{(O-2,6But2C6H3)([Me3Si]2N)U}2(m-Z6:Z6-C6H6)], and SC-XRD studies of this complex revealed uranium-carbon distances (2.502 (4)–2.617(4) A˚ ) that are comparable to those observed in 66 and 68. Complex 68 could also be synthesized as the major product via reaction of 66 with four equivalents of HO-2,6-But2C6H3 in D6-benzene at 90  C for 7 days. Computational studies shows the uranium ions in the inverse sandwich complexes to be U(III) with a benzene dianion, which is in agreement with the experimental solution magnetic moment of 3.8 mB per molecule. SQUID magnetometry studies of 68 show no presence of uranium-uranium coupling or magnetic hysteresis at 2 K. The benzene bridging complex [{U(O-2,4,6-But3C6H2)2}2(m-Z6:Z6-C6H6)] (72) could not be synthesized via the reductive activation of benzene as for 66 and 68. Instead it was synthesized via the reaction of pre-formed 66 with four equivalents of HO-2,4,6-But3C6H2 at 90  C. The formation of the analogous toluene bridged complexes 67 and 69 from UX3 is slower than for benzene and is lower yielding. The solid state structures of 67 and 69 show uranium-carbon bond distances ranging from 2.539(9)–2.600(8) and 2.516(9)–2.647 (9) A˚ , and arene carbon-carbon distances ranging from 1.409(11)–1.463(13) and 1.407(14)–1.481(12) A˚ , respectively, which are a slightly larger range compared to benzene bridging analogs. The uranium-carbon and arene carbon-carbon bonds in 67 and 69 are comparable to their benzene analogs, however they span a significantly wider range. Reaction of UX3 with molten biphenyl for 6 days at 90  C yielded 70 (69% isolated yield) and 71 (10% isolated yield). Spectroscopic and crystallographic studies show no dynamic behavior of the biphenyl, with coordination only occurring through a single phenyl ring. The uranium-carbon bond distances in 70 (2.539(6)–2.628(6) A˚ ) and 71 (2.527(5)–2.627(5) A˚ ) are unremarkable when compared to 66–69. The arene carbon-carbon distances for 70 and 71 of 1.440(9)–1.483(9) and 1.419(6)–1.454(6) A˚ , respectively are within the range of those reported for 66–69. Reaction of [U(X)3] (X ¼ O-2,6-But2C6H3 and N(SiMe3)2) with 10 equivalents of dihydroanthracene in benzene at 90  C yielded 66 and 68 with unreacted dihydroanthracene. Moreover, the preformed bridging arene complex 66 showed no reaction with dihydroanthracene after 3 days at 90  C. Computed Kohn-Sham orbitals of 68 showed uranium-arene d-bonding, utilizing the uranium ion and the first and second LUMOs of the bridging arene ligand. The relative stability of the aryloxide-arenes were computed, with the bridging biphenyl complex being only 0.2 kcal mol−1 higher in energy than the benzene analog. The toluene and naphthalene complexes were higher in energy, with relative DG values of 2.3 and 9.6 kcal mol−1 respectively. Following the above reactions, the incorporation of functionalized arenes that would likely be incompatible with potassium reductants, under Friedel-Crafts conditions and heat were explored, Scheme 19. Phenylsilane (PhSiH3) reacts with [U(O-2,6But2C6H3)3] at 90  C for 17 h to yield [{(O-2,6-But2C6H3)2U}2(m-Z6:Z6-C6H5SiH3)] (73) and two equivalents of [U(O-2,6But2C6H3)4], as identified by 1H NMR spectroscopy, however near-identical product solubilities prevented isolation. Complex 73 could also be synthesized at room temperature, but with a longer reaction time of 20 days.

Scheme 19 Synthesis of 73–77.38

478

Arene Complexes of the Actinides

Borylation of the trapped reduced arene by the reaction of 9-bora-9-bicyclononane (HBBN) with [U(O-2,6-But2C6H3)3] in the chosen arene (benzene, toluene, naphthalene or biphenyl), or directly to the bridging arene complex (arene ¼ benzene) was reported in the same publication. Heating a benzene mixture of [U(O-2,6-But2C6H3)3] and HBBN (9-bora-9-bicylononane) at 90  C forms [{(O-2,6-But2C6H3)2U}2(m-Z6:Z6-C6H5BBN)] (74) (28% isolated yield), 2 equivalents of [U(O-2,6-But2C6H3)4] and dihydrogen, determined by 1H NMR spectroscopy. Reaction of 68 with HBBN in benzene (90  C, 16 h) also yields 74 quantitatively, with a 45% isolated yield of the borylated arene complex after recrystallization. The one pot borylation reactions of [U(O-2,6-But2C6H3)3] and HBBN in toluene and diphenyl results in the para-substituted arenes [{(O-2,6-But2C6H3)2U}2 (m-Z6:Z6-1-Me-4-BBNC6H4)] (75) and [{(O-2,6-But2C6H3)2U}2(m-Z6:Z6-1-Ph-4-BBNC6H4)] (76) in yields of 34 and 11%, respectively. The one pot reaction in molten naphthalene does not cleanly result in an isolable bridging arene complex, however the borylated naphthalene complex [{(O-2,6-But2C6H3)2U}2(C10H7BBN)] (77) could be isolated after fractional crystallization or through displacement of the functionalized naphthalene by reaction with benzene at 90  C for several days. This demonstrates a new type of C-B formation without the need for potassium reductants. For both 75 and 76, borylation occurs at the para-position. The solid state structures of 74–77 show uranium-carbon bond distances of 2.567(4)–2.612(4) A˚ , 2.497(6)–2.635(4) A˚ , 2.511 (5)–2.650(5) A˚ and 2.519(3)–2.769(17) A˚ , respectively. No interaction between the boron and the uranium ions is observed in the solid state. The mechanism for the formation of the bridging arene complexes and the borylation of the arene were investigated computationally. Calculations show two distinct pathways, dependent on the size of the stabilizing ligand (X). Complexes with large X ligands (OC6H5, O-2,6-Me2-C6H3, N(SiH3)2 and N(SiMe3)2) undergo an energetically favorable concerted benzene binding and transfer of an X ligand between two UX3 species, generating the terminal uranium-benzene complex UX2(benzene) and an equivalent UX4 species. The following step involves a concerted coordination of UX2(benzene) to a second UX3 fragment, accompanied with another X ligand transfer to a further UX3 fragment to give X2U-(benzene)-UX2 and two equivalents of UX4. The borylation of X2U-(arene)-UX2 (X ¼ OC5H5) was found to be thermodynamically favorable, with DG values of −9.6 kcal mol−1 for when the arene is benzene, toluene or naphthalene, and −8.5 kcal mol−1 for biphenyl. The exergonic nature of this process does not arise due to formation of dihydrogen, rather because of increased stability of the borylated bridging arene complexes. Two possible mechanisms for the borylation of the arene was investigated computationally; an electronic aromatic substitution reaction where HBBN adds to the arene carbon, which then proceeds to undergo elimination of dihydrogen, or a 1,2-addition pathway where the B-H of HBBN adds across a CdC bond which then loses dihydrogen. A large barrier to elimination of dihydrogen in the later pathway due to the positions of the hydrogen atoms allowed this route to be ruled out. Around 10 years after the synthesis of ketamide stabilized uranium bridging arene complexes 48 and 49 from 47, the family of such complexes was expanded to include the arenes biphenyl, trans-stilbene and p-terphenyl, Scheme 20.39 Treatment of the uranium(IV) complex 47 with four equivalents of KC8 in thawing DME, in the presence of stoichiometric quantities of the arene, resulted in isolation of the inverse sandwich complexes [{(Mes[But]CN)3U}2(m-Z6:Z6-arene)K2] (arene ¼ biphenyl (78) 56% yield, trans-stilbene (79) 63% yield or p-terphenyl (80) 67% yield). The analogous disodium salts could also be isolated by using sodium as a reductant rather than KC8, however these generally had relatively low isolated yields. The bridging biphenyl complex [{(Mes [But]CN)3U}2(m-Z6:Z6-C12H10){Na(OEt2)}2] (81) was characterized by SC-XRD analysis which showed that the uranium ions in

Scheme 20 Synthesis of 48, 49, and 78–89 from 47.39

Arene Complexes of the Actinides

479

81 adopt a three legged piano stool geometry, containing uranium-carbon distances to the Z6-arene ranging from 2.568(7) to 2.678 (7) A˚ , with an mean distances of 2.621(7) and 2.633(7) A˚ . These uranium-carbon bond distances fall within the range reported in the analogous bridging arene complexes 48 (2.565(11)–2.749(10) A˚ ) and 44b (2.504(9) to 2.660(9) A˚ ). The mean carbon-carbon bond distance of the bound ring of diphenyl (1.442(10) A˚ ) is significantly elongated compared to the mean carbon-carbon bond distance in the uncoordinated pendant diphenyl ring (1.387(16) A˚ ). In the solid state (at 110 K), the diphenyl is found to be planar, however in solution the twisted geometry is adopted by the diphenyl fragment. NMR spectroscopy of 81 showed six resonances in the 2H NMR spectrum, demonstrating that the solid state structure is maintained in solution at room temperature, with no exchange in coordination between the two phenyl ring occurring. The solid state structure of 79 also adopts a three legged piano stool coordination geometry at each uranium and displays uranium-carbon bond distances to the Z6 coordinated ring of stilbene ranging from 2.608(9) to 2.704(9) A˚ , with an mean distances of 2.670(9) and 2.646(9) A˚ . As observed for the bridging biphenyl complex 81, a distinction in the mean carbon-carbon bond distances between the bridging C6 ring and the uncoordinated pendant C6 ring of trans-stilbene is apparent in 79. It was found that if the amount of KC8 added to 47 was reduced from four equivalents per uranium to two equivalents, the following monoionic diuranium bridging arene (arene ¼ naphthalene, biphenyl, trans-stilbene, p-terphenyl, benzene or toluene) complexes could be isolated: [{(Mes[But]CN)3U}2(m-Z6:Z6-C6H6)K(DME)] (82, 11% yield); [{(Mes[But]CN)3U}2(m-Z6:Z6-C7H8)K(DME)] (83, 41% yield); [{(Mes[But]CN)3U}2(m-Z6:Z6-C7H8)K2I] (83a, 44% yield); [{(Mes[But]CN)3U}2(m-Z6:Z6-C10H8)K] (84, 56% yield); [{(Mes[But]CN)3U}2(m-Z6:Z6-C12H10)K] (85, 42% yield); [{(Mes[But]CN)3U}2(m-Z6:Z6-C14H12)K] (86, 83% yield); and [{(Mes[But]CN)3U}2(m-Z6:Z6-C18H14)K] (87, 69% yield), Scheme 20. All these complexes were characterized by NMR spectroscopy and elemental analysis, with complexes 83 and 83a being characterized crystallographically. The solid state structure of 83a shows an identical binding environment for both the potassium ions; side-on coordination from two ketamide ligands per ion with the iodide bridging between the two. In contrast the potassium in 83 is located close to just one of the uranium ions. Complexes 83 and 83a display uranium-carbon bond distances ranging from 2.5895–2.6985 A˚ and 2.620(9)–2.681(9) A˚ , respectively. 1H and 2H NMR spectroscopy of the monoanionic inverse arene complexes show the same static bonding (when more than one arene bonding site available) as observed in the dianionic inverse sandwich complexes. Oxidation of the dianionic bridging arene complexes 48, 78, 79, and 80 to the analogous monoanionic complexes 84–87 was achieved using both ferrocenium triflate and P4. The monoanionic species could be obtained in high isolated yields: 81% for the oxidation of the bridging stilbene complex 86 when ferrocenium triflate was employed, and 69% when P4 was used. The reduction of the monoanionic bridging arene complexes to the dianionic species was also possible using potassium/anthracene mixture as a reductant, again in high yield. Complex 79 could be isolated in 78% yield after work up. It is worth noting that incorporation of the anthracene into the bridging arene complexes was observed. DFT calculations were employed to investigate the bonding involved in the family of diuranium bridging arene complex stabilized by ketamide ligands. The computational structure [{(H2N)2U}2(m-Z6:Z6-C6H6)] was employed as a model. The LUMOs of benzene were found to show good overlap and energy match with the uranium valence orbitals which is reflected in the four unpaired d-bonding electrons which are located in MOs ca. −2.5 eV below a dense manifold of virtual orbitals. Moreover, p-donation from the benzene HOMO to the uranium contributed little to the bonding, caused by the large energy difference (ca. 4 eV) between the uranium valence region and the HOMO of benzene. Spin density calculations showed essentially all the spin density is localized on the uranium, with a multipole-derived spin density value of −1.8 per uranium. For the dianionic complexes of general formula [{(NCtBuMes)3U}2(m-Z6:Z6-arene)M2], the truncated structure [{(H2CN)3U}2(m-Z6:Z6-arene){Na(OH2)3}2] was used as a computational model. Unlike for the amide models where the spin density was localized on the uranium, spin density calculations for the ketimide model showed delocalization of the spin density onto the ketimide carbon atoms, through p-bonding. This is reflected in the calculated multipole-derived spin density values of ca. −1.1 per uranium and ca. −0.5 for each ketimide carbon atom. The energy level diagram for [{(H2CN)3U}2(m-Z6:Z6-arene){Na(OH2)3}2] contains four distinct sets of orbitals; the four highest lying electrons in metal-centered orbitals which are stabilized by p-bonding to the ketimide ligands, the next four highest electrons correspond to covalent d-overlap between the two uranium ions and the LUMO of the bridging benzene ligand, next highest is 12 electrons of ketamide p-donor functions, and lastly with four electrons corresponding to the c2 and c3 orbitals of benzene which show no overlap with metal based orbitals. Following the calculations performed on [{(H2CN)3U}2(mZ6:Z6-arene){Na(OH2)3}2], new calculations were performed on the reported vanadium bridging benzene complex [{V(Z5-C5H5)}2(m-Z6:Z6-C6H6)] (also containing a S ¼ 2 ground state) to allow a comparison in bonding to be undertaken. It was found that the two d4 vanadium ions interact with the benzene ligand in an isolobal manner to that observed for the two f4 uranium ions in [{(H2CN)3U}2(m-Z6:Z6-arene){Na(OH2)3}2], with both complexes displaying d-bonding. The reactivity of the dianionic bridging arene complexes [{(Mes[But]CN)3U}2(m-Z6:Z6-arene)M2] was explored, Scheme 20. Reaction of each bridging arene complex with two equivalents of 1,3,5,7-cyclooctatetraene afforded a mixture of the terminal uranium-COT complex [{(Mes[But]CN)3U}2(Z8-COT)][M] and the bridging COT complex [{(Mes[But]CN)3U}2(m-Z8:Z8-COT)]. Complex 49 was found to react with 2 equivalents of diphenyl disulfide to yield the bridged trithiolate complex [Na(m-SPh)3{U (NCButMes)3}2] (88) in 60% isolated yield. Moreover, the monoanionic bridging arene complexes reacted with one and a half equivalents of diphenyl disulfide via a three electron reduction to yield the potassium analogs of 88. Complexes 83 and 83a react cleanly with azobenzene to give [{(Mes[But]CN)3U}2(m-NPh)2] (89) through a five-electron transfer process, with [K(PhNNPh)] being the proposed byproduct. This analysis is supported by the fact the reaction proceeds more cleanly when two equivalents of azobenzene are used rather than one equivalent. The other dianionic bridging arene complex resulted in more complex reactions

480

Arene Complexes of the Actinides

when reacted with azobenzene. The monoanionic bridging arene complexes 83 and 83a also gave 46 as the major product when reacted with azobenzene, however the reactions were found to not be as clean as observed for the bridging toluene complexes. In 2013, investigations into the electronic structure of the arene bridged complex 44a were reported.71 NMR spectroscopy showed that 44a undergoes arene exchange with C6D6, albeit slowly - 5% after 24 h at room temperature. No exchange with p-xylene is observed over a 24 h period. This reflects the uranium d-bonding in such complexes, where an electron poor ligand is preferred for d-bonding from the uranium 5f-orbitals. The thermal stability of 44a allowed variable temperature NMR spectroscopy studies over the temperature range −70  C to 120  C to be undertaken. The measurements showed that 44a maintains its dinuclear structure in solution with no detectable equilibrium between the dinuclear and a mononuclear species being observed.34 Electronic absorption spectroscopy of 44a contains intense (e ¼ 200–600 M−1 cm−1) bands/absorptions in the near IR region, which is relatively rare in uranium complexes due to f-f transitions being Laporte forbidden. This relatively high intensity is often attributed to a phenomena called intensity stealing, which arises due to the presence of covalent bonding mixing orbitals. Therefore, the intense f-f transitions observed for 44a suggests significant uranium-arene covalency. SQUID magnetometry measurements of 44a show the presence of antiferromagnetic communication between the uranium centers. Between the temperature ranges 5–50 K and 170–300 K 44a displays paramagnetic behavior, however in the plot of 1/w versus T, a maximum in magnetic susceptibility is measured at 110 K. This is characteristic of the presence of antiferromagnetic coupling between the metal ions. This antiferromagnetic coupling was not observed in the uranium bridging-toluene complex 64. Complex 44a shows a high temperature (300K) magnetic moment of 1.50 mB per uranium, which decreases gradually to 0.25 mB at 5 K. X-ray absorption near-edge structure (XANES) spectroscopy is a useful technique in determining effective charge of atoms. Measurements of the U L3 absorption edge (corresponding to a 2p3/2 to 6d5/2 transition) in 44a gave a chemical shift of −5.1 eV, suggesting that this complex contains two uranium(III) centers with a bridging toluene dianion. The typical ranges of edge shift (relative to UO2Cl2) for uranium(III) fall in the range of −5.0 to −6.4 eV, with uranium(IV) ions displaying values ranging from −2.0 to −3.8 eV. Thus a value of −5.1 eV is indicative of uranium(III) and is consistent with a covalent interaction involving the arene p and uranium d orbitals. The variation in chemical shift of the U L3 absorption edge is due to shielding of the 2p3/2 electrons, as well as influence from coordination geometry of the uranium and the extent of metal-ligand covalency. DFT calculations of 44a lead to optimization of the quintet state, with the singlet and triplet states being disregarded due to the unreliability of the calculated energies arising from spin contamination at the DFT level. Complete active space self-consistent field (CASSCF) calculations with corrections from second-order perturbation theory (CASPT2) determined the ground state LoProp charge of uranium to be 2.33/2.34, with the sum of the bridging arene charges being −1.95. This is consistent with the uranium(III) assignment, with charge transfer from the uranium ions to the bridging arene. The basic bonding picture of 44a showed two sets of d bonds, with occupied uranium 5f orbitals donating into the bridging toluene p-antibonding orbitals. Four singly occupied 5f orbitals were also computed as the highest energy occupied orbitals. At the CASPT2 level of calculation, the singlet state was found to be the ground state with the triplet state being 0.7 kcal mol−1 higher in energy, the quintet state being 2.5 kcal mol−1 higher than the ground state and finally the septet state being 34.5 kcal mol−1 higher energy than the ground state. In terms of contribution to the wavefunction, the quintet state was found to be dominant at a contribution of 87%.34 Later in 2013, Liddle et al. reported the isolation of the uranium(III) bridging arene complex [{U(LMe)I}2(m-Z6:Z6-C6H6)] (90, Me L ¼ [HC{C(Me)NDipp}2]−), through the one electron reduction of [U(LMe)I2(THF)2] with KC8 in benzene, Scheme 21.40 Crystallographic studies on 90 showed an mean UdC bond distance of 2.567(8) A˚ , which is comparable to that reported for other diuranium bridging benzene complexes, e.g. 50, 2.625(14) A˚ ; 52, 2.590(3) A˚ . The mean bridging benzene CdC bond distance of 1.447(15) A˚ shows an elongation compared to free benzene (1.397 A˚ ), however the C-C-C angles (119.3(9)–120.3(9) ) are indistinguishable from the 120 angle observed in free benzene. The [C6H6]2− fragment in 90 deviates slightly from being planar (mean 0.004 A˚ displacement). The Z2-N,N coordinated supporting ligand adopts a trans arrangement around the uranium-areneuranium center. The low yield of 90 (3%) precluded full characterization, however the dianionic benzene component can be observed in 1H NMR spectroscopy, with a resonance at −68.59 ppm. The UV/vis/NIR spectrum of 90 is dominated by charge transfer bands with no absorptions that could be assigned as f-f transitions.

Scheme 21 Synthesis of 90.40

Arene Complexes of the Actinides

481

In pursuit of a uranium(III)-methanediide complex, the uranium(IV) complex [{U(BIPM™S)(I)(m-I)}2] was reduced with KC8 in toluene, Scheme 22.41 After work-up the a mixed-valence hexauranium ring [{U(BIPM™S)(m-I)0.5(m-Z6:Z6-C7H8)0.5}6] (91) was isolated in 12% yield. While no arene exchange was observed in 91 at room temperature after 72 h 91 undergoes almost quantitative toluene-benzene exchange upon heating to 50  C for 8 h in C6D6, yielding [{U(BIPM™S)(m-I)0.5(m-Z6:Z6-C6D6)0.5}6] (92). Complex 92 could also be synthesized directly from reduction of [{U(BIPM™S)(m-I)(I)}2] in benzene but only in 1% isolated yield. Crystallographic studies on 91 and 92 show the structures to be very similar so only the structure of 91 will be discussed in detail. Complexes 91 and 92 can be distinguished by 1H NMR spectroscopy - the bridging arene protons give rise to three resonances at 0.44, −0.37 and − 1.35 ppm for 91, whereas 92 displays a single resonance at −0.25 ppm.

Scheme 22 Synthesis of 91 and 92.41

Complex 91 shows alternating bridging arene and bridging iodide ligands, with each of the six uranium centers also having a coordinated BIPM™S ligand. The UdC bond distances for the coordinated carbon BIPM™S in 91 range from 2.30(3) A˚ to 2.47(2) A˚ , confirming the BIPM™S ligands are methanediides, rather than methanides in the previously reported diuranium arene complex 64 (U-C ¼ 2.753(9) A˚ ).36 The 2.5 nm nanometer size ring of 91 adopts an extended chair-type configuration, with four of the uranium ions essentially co-planar, with the other two uranium ions residing 2.108(6) A˚ above and below the defined plane. The m-Z6:Z6 bound toluene molecules in 91 are approximately planar with an mean U-Ccentroid bond distance of 2.224(4) A˚ . The U-Carene bond distances in 91 vary significantly (2.576(9) A˚ to 2.723(9) A˚ ), as expected for a mixed oxidation state complex.

482

Arene Complexes of the Actinides

The optical spectroscopy of 91 is dominated by a charge-transfer band, obscuring any f-d transitions. Several absorptions corresponding to Laporte forbidden f-f transitions were observed in the 3000–9000 cm−1 region, with one absorption also observed in the NIR region. Solid state SQUID magnetometry measurements on 91 were undertaken to provide further insight into assignment of formal charges. Complex 91 displayed a room temperature wT of ca. 3.2 cm3 K mol−1, which decreases with temperature to give a low temperature (5 K) wT of ca. 2.2 cm3 K mol−1. This magnetic profile suggest the presence of uranium(III) ions, since uranium(IV) is a Kramers ion, thus wT tends toward zero at low temperature, reflecting the uranium(IV) singlet ground state. The saturation of magnetization at low temperature (2 K) further supports this initial assignment. The EPR spectra of 91 displays multiple resonances between 0 and 1.8 T at Q-band (34 GHz). The complexity of the system prevented quantitative assignment of formal charges, with EPR and magnetic data only confirming the presence of Kramers ions, i.e. uranium(III), in this system. To further investigate the uranium formal charges, low temperature uranium L3-edge X-ray absorption near edge spectra of 91 and 92 were each collected at two separate beam lines. The sample spectra show strong white lines that are in agreement with UO2 and uranium(III) standards. The absence of uranium(V) or uranium(VI) was deduced from the absence of a shoulder feature on the high energy side of the white lines. The primary spectral XANES white line energy for 91 and 92 were determined to be 17,175.7 and 17,175.3 eV, respectively. These energies were defined using the zero-crossing point of the first derivative of the normalized XANES versus energy. These energies are close to reference samples [UI3(THF)4] and UO2 (17,175.0 and 17,176 eV, respectively), as well as reported uranium(III)- and uranium(IV)-tacn complexes. Using the point at which the second derivative of the normalized XANES is zero is an alternative method for assigning the formal uranium oxidation state. Using this second method, the edge values for 91 and 92 were determined to be 17,169.4 and 17,169.0 eV, respectively. Regardless of which of the two methods was used, the XANES data suggests a mixture of uranium(III) and uranium(IV) ions are present in 91 and 92. Quantum chemical calculations using the restricted active space self-consistent field method (RASSCF) were performed to probe the electronic structure of the ring complex 91. Calculations on the truncated ring complex (BIPM ligands truncated to C(PH2NH)2) showed the S ¼ 7/2 state was only 0.02 kcal mol−1 higher in energy than the 9/2 state. Moreover, the S ¼ 11/2 and 13/2 states were calculated to be significantly higher in energy, 28.3 and 39.0 kcal mol−1 respectively. An orbital picture calculated at the RASSCF level shows each bridging arene group contains two sets of d-bonds (uranium 5f donating into arene p orbital), as well as a set of singly occupied orbitals that are linear combinations of uranium 5f orbitals. RASSCF calculations show discrete U(III)-areneuranium(III) and U(III)-arene-uranium(IV) moieties within the ring, rather than delocalized +3.5 uranium ions. However, DFT calculations show a fully delocalized system, with all three uranium-arene-uranium units being equivalent. The average computed Muliken charge for the uranium in 91 of +1.49 is higher than that calculated for 91 (+1.30), which is consistent with the latter being entirely uranium(III), while the former contains a mixture of uranium(III) and uranium(IV). All the data combined suggests the ring contains a mixture of formal uranium(III) ions and uranium(IV) ions. In 2020, the mixed-valent bimetallic U(III)/U(IV) nitride complex [{([Me3Si]2N)2U(THF)}2(m-N)] was found to undergo C-H activation of toluene at −80  C for 2 h to yield the anionic tetranuclear arene inverse-sandwich complex [{([Me3Si]2N)3U(m-NH)U(N[SiMe3}2)}2(m-Z6:Z6-C7H8)]− (93) as part of a separated ion pair with the cationic dinuclear uranium(IV) nitride component [{([Me3Si]2N)2U(THF)}2(m-N)]+, Scheme 23.42 The synthesis of 93 was low yielding (13%) and was performed on a small scale (3 mg isolated product). The solid state structure of 93 showed uranium-carbon distances ranging from 2.564(7)–2.669(6) A˚ , with a mean of 2.62(3) A˚ . The carbon-carbon bond distances of the bridging arene display the range from 1.425(9)–1.492(9) A˚ , with an mean of 1.44(1) A˚ . The bridging NH groups of 93 can be observed in the 1H NMR spectrum at 231 and 216 ppm at −80  C, however on warming to −40  C these resonances were found to coalesce to a single resonance at 177 ppm. Performing the reaction in D8-toluene resulted in disappearance of the NH resonances. Although [{([Me3Si]2N)3U(m-NH)U(N[SiMe3]2)}2(m-Z6:Z6-C7H8)] [{([Me3Si]2N)2U(THF)}2(m-N)] could be reproducibly isolated as analytically pure crystals, it was not optimized on a larger scale, precluding magnetic and spectroscopic investigations into the formal charges of the uranium ions.

Scheme 23 Synthesis of 93.42

Arene Complexes of the Actinides

4.08.4.3

483

Inverse sandwich diuranium(V) arene complexes

Prior to 2011, all reported diuranium arene complexes contained, formally, the configuration of two uranium(III) ions with a bridging arene dianion.43 The isolation of [{U(TsXy)}2(m-Z6:Z6-C6H5Me)] (94, TsXy ¼ HC(SiMe2NC6H3-3,5-Me2)3) gave rise to a formal new class of diuranium complexes containing two uranium(V) ions with a bridging toluene tetraanion, Scheme 24. The related complex [{U(TsTol)}2(m-Z6:Z6-C6H5Me)] (95, TsTol ¼ HC(SiMe2NC6H4-4-Me)3) was also prepared and was reported later (see below). The inverse sandwich complex 94 was synthesized by treatment of [U(TsXy)(Cl)(THF)] with KC8 in toluene and was isolated in 65% yield. The 1H NMR spectrum of 94 reveals resonances corresponding to the bridging toluene at +34.2, −16.7, −32.6 and − 37.0 ppm, with the methine protons observed at −38.1 ppm. These data, characteristically very different from the staring material, demonstrate that the inverse sandwich structure of 94 is maintained in a C6D6 solution. The solid state structure of 94 shows uranium-carbon bond distances ranging from 2.651(4)–2.698(4) A˚ , which is a much narrower range than is generally observed for previously reported diuranium bridging toluene complexes, e.g. 44b, 2.503(9)–2.660(8) A˚ 32; 64, 2.553 (7)–2.616(7) A˚ .36

Scheme 24 Synthesis of 94 and 95.43,44

The uranium(V) oxidation state in 94 could be inferred from measurements of the variable temperature magnetic moment, which showed a decrease from 3.39 mB at 300 K to 0.84 mB at 1.8 K. This magnetic profile is typical for uranium(V). Furthermore, the magnetic moment of 94 does not tend to zero at low temperatures, which is a key feature typically observed for uranium(IV) complexes that often exhibit a magnetic singlet ground state at low temperature. Unrestricted DFT calculations of the whole molecule were carried out, computing spin densities of 1.18 for each uranium center, with an average Mulliken charge for the two uranium atoms of +3.00 and a charge of −4.47 for the bridging toluene. This reinforces the uranium(V) assignment from the magnetic data and shows significant uranium-arene charge transfer. The bonding in 94 was probed computationally. It was determined that the HOMO and HOMO-1 in 94 are a-spin and are of essentially pure 5f composition. The a-spin HOMO-2 and HOMO-3 were found to have the composition 35.9% carbon, 44.2% 5f and 5.5% 6d and 40.0% carbon, 42.5% 5f and 6.7% 6d, respectively. These orbitals and their beta spin counterparts represent the d-bonding frequently observed in uranium arene complexes. These calculations demonstrate that d-bonding is still prevalent in electron high valent uranium-arene complexes. The electronic absorption spectrum of 94 is dominated by charge transfer bands, however a well separated, sharp peak at 6815 cm−1 (e ¼ 120 M−1 cm−1) was observed, which is characteristic of uranium(V). Interestingly, 94 is able to facilitate the reductive cleavage of the cobalt dimer [Co2(CO)6(PPh3)2], to yield a complex displaying the first example of a UdCo bond. Reaction of 94 with [Co2(CO)6(PPh3)2] in toluene from −78  C to room temperature for 64 h yielded [U(TsXy)Co(CO)3(PPh3)] in 59% yield.41 The following year, the synthesis of the inverse sandwich complex [{U(OSi[OBut]3)3}2(m-Z6:Z6-C6H5Me)] (96) was reported, Scheme 25, although the assignment of formal charges was not determined.45 Through magnetic measurements and computational analysis, 96 was determined to be diuranium(V) with a bridging toluene tetraanion.46 Complex 96 was synthesized through addition of toluene to a hexane solution of the uranium(III)-siloxide complex [{U(OSi[OBut]3)2(m-OSi[OBut]3)}2], with an isolated yield of 89% after standing at room temperature for 2 days. Crystallographic studies on 96 show an mean UdC bond distance of 2.692(3) A˚ , with mean arene CdC bond distance of 1.432(3) A˚ . This demonstrates a slight elongation (ca. 0.04 A˚ ) of the CdC bond distances relative to free toluene. This contrasts to the aforementioned diuranium(V) bridging toluene complex 94 where the mean CdC bond distance of the bridging arene was 1.440(6) A˚ , thus only a modest 0.024 A˚ elongation compared to free toluene. The 1H NMR spectrum of 96 in deuterated THF showed four resonances at 82.9, −112.7, −122.4 and −137.9 ppm, confirming the bridging toluene is initially maintained in solution, however decomposition to [U{OSi(OBut)3}3(THF)2] is observed over time.

484

Arene Complexes of the Actinides

Scheme 25 Synthesis of 96–98.45,46

Solid state magnetic studies of 96 showed a magnetic profile indicative of uranium(V). The measured magnetic moment at room temperature of 1.35 mB per uranium was found to undergo a steady decrease to a magnetic moment at 2 K of 0.31 mB. DFT studies on 96 show atomic charges of +2.85 for uranium and −3.83 for the bridging toluene, which is comparable to the uranium atomic charge of +3 reported for the neutral diuranium(V) arene complex 94. DFT calculations with a set f-occupancy of 5f1 lead to a computational structure that was stable, with bond distances comparable to the experimental structure. Imposing a 5f2 configuration on both uranium atoms (giving a diuranium(IV) electronic structure) lead to an unstable species that undergoes dissociation in silico. Analysis of the molecular orbitals of 96 showed the HOMO and HOMO -1 to be d-bonding orbitals, facilitating a covalent back-bonding interaction between the uranium centers and bridging arene. CASSCF calculations showed the ground state of 96 to be the triplet state (one unpaired electron per uranium), with a septet spin state (three unpaired electrons per uranium) to be higher in energy than the ground state by 8.2 kcal mol−1 and the quintet spin state (two unpaired electrons per uranium) being 17.8 kcal mol−1 higher in energy relative to the triplet state. This further confirms the diuranium(V) tetraanion arene formulation.46 The neutral diuranium(V) complex 96 could be sequentially reduced with KC8 in THF to the diuranium(IV/V) tetraanion arene complex [{U(OSi[OBut]3)3}2(m-Z6:Z6-C6H5Me)K] (97), then to the diranium(IV) tetraanion arene complex [{U(OSi [OBut]3)3}2(m-Z6:Z6-C6H5Me)K2] (98), Scheme 25.46 1H NMR spectroscopy of 97 and 98 in D8-THF shows the potassium remains coordinated to the siloxide ligands in solution. The resonance corresponding to the methyl group of the bridging arene decreases from d ¼ 82.9 ppm in 96 to d ¼ 63.2 ppm in 97 to d ¼ 16.8 ppm in 98. Treatment of the neutral diuranium(V) complex 96 with KOTf in THF rather than KC8 still lead to the formation of 97, as well as [U{OSi(OBut)3}3(OTf )(THF)2] and toluene in the relative ratio 2:2:1. The reaction of 96 with two equivalents of KOTf lead to the rapid formation of 98, which reacts further to afford 98 and equimolar toluene, and two equivalents of [U{OSi(OBut)3}3(OTf )(THF)2]. While the first step of the reaction proceeds over the course of 1 h at room temperature, the second step is much slower and takes over 4 weeks to go to completion. Excess KOTf was not found to accelerate the reaction. The role of the potassium in the disproportionation reactions was demonstrated by no reaction being observed between 96 and Bu4NOTf. The sequential change in uranium oxidation state across 96, 97, and 98 was reflected in the UdC bond distances. Complex 96 shows uranium-arene UdC bond distances ranging from 2.689(3) A˚ to 2.695(3) A˚ (2.693(4) A˚ mean), while the monoanionic, mixed valent 97 shows UdC bond distances ranging from 2.602(9) A˚ to 2.674(13) A˚ (2.64(2) A˚ and 2.65(2) A˚ mean). Finally, 98 shows UdC bond distances of 2.589(4) A˚ to 2.621(3) A˚ (2.607(14) A˚ mean). The electronic structures of 97 and 98 are supported by DFT calculations. The HOMO and HOMO-1 in 97 and 98 were found to reflect the d-bonding orbitals. The first reduction from 96 to 97 was found to be exergonic (8.4 kcal mol−1), with the second reduction to 98 also being exergonic (4.8 kcal mol−1). Calculations show that on reduction, the added electrons reside in non-bonding 5f-orbitals since the arene is electronically saturated with its 10p-electrons. SQUID magnetometry studies were also performed on the reduced complexes. Complex 98 showed a magnetic moment at 300 K of 2.23 mB per uranium, which is significantly higher than that recorded for 96 (1.35 mB per uranium), reflecting the diuranium(IV) configuration of the former. Below 25 K, 98 displays temperature independent paramagnetism, a phenomenon routinely observed in uranium(IV) complexes. The measured magnetic moment of 97 at 300 K of 2.13 mB is between those measured for 96 and 98. Complex 97 does not show temperature independent paramagnetism as expected for a uranium(IV) ion, however it is possible this is masked by the paramagnetism of the uranium(V) ion.43

Arene Complexes of the Actinides

485

Following the report of 96, the second triamido-diuranium(V) bridging toluene tetraanion complex 95 was reported, Scheme 24.44 Complex 95 was synthesized in 95% isolated yield through the reduction of [U(Tstol)(Cl)(m-Cl)U(Tstol)(THF)2] with KC8 in toluene/THF. The 1H NMR spectrum of 95 shows four resonances at +32.4, −18.8, −21.0 and −35.0 ppm, corresponding to the bridging toluene ligand. The methine protons could also be observed at −35.1 ppm. The structure of 95 was determined by SC-XRD studies which showed the Z6-arene binding mode, with uranium-carbon distances ranging from 2.535(15)–2.673(16) A˚ . These bond distances compare well to the previously reported triamido-diuranium(V) bridging toluene complex 94 (2.651 (4)–2.698(4) A˚ ). SQUID magnetometry of a powered sample of 95 showed a high temperature (300 K) magnetic moment of 3.32 mB and a low temperature (1.8 K) magnetic moment of 0.88 mB, which is very similar to the magnetic data for 94 (3.39 mB at 300 K to 0.84 mB at 1.8 K). The sharp decrease in magnetic moment below 35 K, with the magnetic moment not tending toward zero is consistent with magnetic doublet ground state, as observed for uranium(V) ions. The electronic absorption spectrum of 95 displays strong charge transfer and p-p transitions spanning the 25,000–15,000 cm−1 range. Like in 94, a distinct absorbance at 6740 cm−1 (e ¼ 82 M−1 cm−1) is observed for 95, characteristic of a formal uranium(V) oxidation state. EPR studies on 95 gave a spectrum at 5 K following S-band (3.87 GHz) measurement. The g-values determined from the spectrum following best-fit line-shape analysis were 3.3 and 2.7, with the third g-value being ill-defined. This provides further confirmation of the uranium(V) assignment since uranium(IV) is EPR silent, uranium(III) gives a more anisotropic EPR spectrum, and ligand based unpaired electrons displays g-values much closer to that of a free-electron (2.0023). The bonding in 95 was investigated computationally since a quantitative description of f-orbital participation and covalency is of interest and allows comparison to other bridging arene complexes. Unrestricted DFT calculations show the a-HOMO and HOMO-1 to both be singularly occupied and essentially pure f-orbital in nature. The next highest orbitals of the a-spin HOMO-2 and HOMO-3 (and their b-spin counterparts), show d-bonding as is routinely observed in bridging arene uranium complexes. The composition of the a-HOMO-2 was determined to be 36.1% carbon, 44.5% 5f and 7.8% 6d, with the composition of the a-HOMO-3 being 39.6% carbon, 42.6% 5f and 8.4% 6d. Finally, the calculated uranium charges of +3.0 and the spin densities −1.19 further supports the diuranium(V) assignment. Complex 95 was found to react with a slight excess of P4 to afford the Zintl complex [{U(Tstol)}3(m3-Z2:Z2:Z2-P7)] in 12% isolated yield which liberates P7R3 species on treatment with electrophiles.72

4.08.4.4

Uranium complexes stabilized by arene-based ligands

The formation of uranium-arene bonding is also possible through interaction with arene groups of coordinated ligands and has been exploited in ligand design when targeting reactive and unstable uranium species. The formation of metal-arene bonding in addition to metal coordination from pendent oxygen or nitrogen atoms is possible due to the large size of uranium and its requirement to be coordinatively saturated. The first uranium-arene interaction with an arene group of a coordinated ligand rather than an arene solvent was reported in 1988 as the uranium(III) aryloxide complex [{U(O-2,6-Pri2C6H3)3}2] (99), Scheme 26.47 Complex 99 was synthesized through the protonolysis reaction of [U{N(SiMe3)2}3] with 3.1 equivalents of HO-2,6-Pri-C6H3 and could be isolated in 50% yield. This showed a potential new route to uranium-arene complexes through ligand design rather than activation of arene solvents. The solid state structure of 99 was determined by SC-XRD, which showed that 99 exists as a dimeric species, with each uranium adopting a three legged piano stool geometry. The dimeric structure was held together by the uraniumZ6-arene interaction, displaying uranium-carbon bond distances ranging from 2.82(1)–3.02(1) A˚ , with mean distance of 2.92(2) A˚ . These bond distances compare well with uranium-neutral arene complexes outlined in Section 4.08.3.1, although the uranium-arene bonding in 99 is weak and the dimeric species is cleaved in benzene.

Scheme 26 Synthesis of 99.47

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Arene Complexes of the Actinides

In 2009, Meyer et al. reported a trivalent uranium mono arene complex, where the arene is a mesityl anchor functionalized with coordinating tris(aryloxide) arms, forming a tripodal ligand system, Scheme 27.48 X-ray crystallographic studies of the complex [{(ButArO)3mes}UIII] (100) show coordination around the uranium center to be a distorted trigonal pyramidal, with mean U-O distances of 2.16 A˚ . The mesityl anchor shows an 6-interaction with the uranium ion, with mean UdC bond distances of 2.73 A˚ and mean arene CdC bond distances of 1.42 A˚ . The mean arene CdC bond distance in 100 is within standard uncertainty of that reported for the free ligand, suggesting no significant reduction of the arene ring on coordination with the uranium center. DFT analysis showed d-bonding between the uranium fxyz and fz(x2-y2) orbitals and the doubly degenerate arene p orbitals, with SOMOs that are composed of 70% 5f and 30% p .

Scheme 27 Synthesis of 100 and 101.48

Reactivity studies of 100 show: NH activation with the sterically hindered amine Hdbabh (2,3:5,6-dibenzo-7-azabicyclo[2.2.1] hepta2,5-diene) to give the uranium(IV) complex [{(ButArO)3Mes}U(dbabh)] (101), Scheme 27,48 as well as reductive cleavage of CO2 or N2O to give the uranium(IV) bridging oxo complex [{([tBuArO]3Mes)U}2(m-O)] (102), Scheme 28, which can react with another equivalent of CO2 to form the bridging carbonate complex [{({ButArO}3Mes)U}2(m-k2:k2-CO3)] (103).49 While the uranium-arene interactions are maintained in 101 and 103, the mean UdC bond distances (2.88 A˚ , and 2.594 A˚ and 2.608 A˚ , respectively) show that it is significantly weakened.

Arene Complexes of the Actinides

487

Scheme 28 Synthesis of 102 from 100 and conversion of 102 to 103.49

Employment of the novel bulky aryloxide ligand HOAr (Ar ¼ 2,6-CHPh2-C6H4-Me) allowed the isolation of uranium(III) complexes bearing an Z6-arene interaction, in 96% yield on multigram scale, Scheme 29.50 Reaction of 3 equivalents of [Na(OAr )] with UI3(THF)4 in THF at 25  C produces [(Ar O)3U(THF)] (104). It should be noted that both the uranium starting material as well as [Na(OAr )] can be synthesized in high yield and on multigram scales, further adding to the ease of synthesis of 104. The potentially more reactive solvent free arene complex [(Ar O)3U] (105) could also be synthesized, on slightly smaller scale (65% yield), by reaction of three equivalents of HOAr with [U{N(SiMe3)2}3]. Both 104 and 105 were characterized by SC-XRD studies. The solid state structure of 104 revealed that the uranium center adopts a trigonal bipyramidal geometry, with an axial Z6-bound arene ring displaying a mean uranium-carbon bond distance of 2.964(3) A˚ . In comparison, 105 adopts a distorted tetrahedral geometry at uranium in the solid state, while still exhibiting an Z6-arene interaction. The mean uranium-carbon bond distance in 105 was determined to be 2.853(3) A˚ , which is significantly shorter than that observed in 104 (2.964(3) A˚ ). The UV/vis spectra of 104 and 105 show the presence of multiple Laporte-allowed 5f3 to 5f26d1 transitions. DFT analysis of 104 showed the presence of a d-type interaction between the uranium 5f-orbitals and the p orbitals of the arene. Using a truncated computational model of 104 and 105 allowed dissociation energies of the coordinated THF and the Z6-bound arene to be calculated as 70 and 139 kJ mol−1, respectively. This displays the fact that the arene is bound more strongly to the metal center than the THF molecule. Reaction of 104 and 105 with N2O in THF yielded the uranium(V) terminal mono-oxo complex [(Ar O)3(THF)U-(O)] (106) in quantitative yield. SC-XRD analysis of 106 showed two independent chemically equivalent molecules in the asymmetric unit. One molecule is found

488

Arene Complexes of the Actinides

to adopt a distorted square-pyramidal geometry, whereas the other molecule adopts a distorted trigonal-bipyramidal geometry. SQUID magnetometry measurements of 104–106 were performed. Complexes 104 and 105 display very similar magnetic moments (2.37 and 2.52 mB at 300 K, 1.21 and 1.27 mB at 2 K respectively). Complex 106 has a magnetic moment of 1.96 mB at 300 K, which decreased to 0.86 mB at 2 K. This is consistent with uranium(V) magnetic profiles generally.12

Scheme 29 Synthesis of 104–106.50

In 2014, Meyer et al. reported the electrochemical and chemical reduction of the uranium(III) monoarene complex [{(Ad, ArO)3mes}U] (107), Scheme 30.51 The nearly reversible one-electron electrochemical reduction of 107 showed a reduction at −2.495 V vs. Fc/Fc+, which is within the accessible range for chemical reduction. However, reaction of 107 with KC8 or Na0 mirror at room temperature gave the uranium(IV) hydride complexes [{(Ad,MeArO)3mes}U(H)K] (108) and [{(Ad,MeArO)3mes}U(H)Na] (109), respectively. Complexes 108 and 109 form due to redox isomerization and hydride generation, and this hydride then migrates with concomitant restoration of the methylene linker to produce 110 and 111, respectively. Chemical reduction of 107 was achieved by reaction with potassium spheres and an excess of 2.2.2-cryptand at −35  C, yielding the uranium(II) monoarene complex [K(2.2.2-crypt)][{(Ad,MeArO)3mes}U] (112).52 X-ray crystallographic studies of 112 showed no change in the arene bonding upon reduction (U-C: 2.597(5) A˚ and 2.633(5) A˚ ; C-C: 1.432(7) A˚ and 1.432(8) A˚ ), thus indicating a predominantly metal-centered reduction. Complex 112 was found to only be stable at low temperature, with decomposition occurring in under 10 min at room temperature in solution and under 3 h in the solid state. In addition to crystallographic studies, the +2 oxidation state with the 5f4, 5I4 ground state configuration was supported by EPR spectroscopy, SQUID magnetometry and theoretical studies. Complex 112 was found to be EPR silent as expected for a non-magnetic electronic ground state, with the spectrum only showing a small signal at g ¼ 2.00 corresponding to free electrons supported by [K(2.2.2-crypt)]+. Both solution and solid state magnetic studies were performed on 112, with the solid state measurements showing a magnetic moment at 300 K of 2.25 mB decreasing to 0.77 mB at 2 K. Solution state measurements by Evans method between −40  C and −75  C showed agreement with the solid state magnetic data. Scalar relativistic calculations on the anionic fragment of [{(Ad,MeArO)3mes}U]− were performed for both the ms ¼ 2 and ms ¼ 1 electronic configuration and shows a qualitative difference in electronic structure; the flipping of one spin and the population of an d-bonding spin orbital, concurrent with a 5f-orbital becoming vacant. Single-point test calculations on [{(Ad, Me ArO)3mes}U]− suggest the ms ¼ 2 system is higher in energy (16–19 kJ mol−1) than the ms ¼ 1 system, however upon geometry optimization the two energies become indistinguishable by density functional calculations. Additional DFT calculations into the effect of spin-orbit coupling in [{(Ad,MeArO)3mes}U]− were performed. The computational work collectively suggested the ms ¼ 2 electronic structure, however the highest occupied spinor showed the presence of 5f-orbital overlap with the Z6-bound arene p orbital. Further inspection of molecular orbitals show a mixed arene-uranium reduction. Me

Arene Complexes of the Actinides

489

Scheme 30 Synthesis of 108–112 from 107.51,52

The synthesis of a family of almost isostructural uranium(IV) monoarene complexes allowed structural comparison of the uranium-arene binding.53 The previously discussed uranium(III)-arene complex 107 was oxidized to the uranium(IV) arene complexes [{(Ad,MeArO)3mes}U(X)] (X ¼ F, 113F, 98%; Cl, 113Cl, 59%; Br, 113Br, 93%; I, 113I, 92%), Scheme 31. 1H NMR spectroscopy and elemental analysis confirmed the isolated complexes to be solvent-free. However, THF was required in order to gain suitable crystals to determine the solid-state structures of the THF adducts of the complexes [{(Ad,MeArO)3mes}U(X)(THF)] (X ¼ F, 113F-THF; Cl, 113Cl-THF; Br, 113Br-THF; I, 113I-THF). The coordination geometry around the uranium ions are consistent across the halide series, with the halide positioned approximately trans to the arene-centroid in all structures. While the uranium-halide distances increases down the series, in-line with the increase in covalent radii of the halides (X ¼ F, 2.073(11) A˚ ; Cl, 2.617(1) A˚ ; Br, 2.8025(3) A˚ ; I, 3.0830(7) A˚ ), the uranium-arenecentroid distances (X ¼ F, 2.666 A˚ ; Cl, 2.657 A˚ ; Br, 2.645 A˚ ; I, 2.664 A˚ ), and mean uranium-arene distances (X ¼ F, 3.011 A˚ ; Cl, 3.005 A˚ ; Br, 2.993 A˚ ; I, 3.009 A˚ ), stays relatively unchanged on varying the bound halide. The solvent-free complex [{(Ad,MeArO)3mes}U(F)] could be structurally authenticated by SC-XRD following oxidation of [{(Ad,MeArO)3mes}U] with AgF in benzene. In contrast to 113X-THF, the solid-state structure of 113F displays a trigonal coordination geometry around the uranium center. Electronic absorption spectroscopy and SQUID magnetometry studies confirmed the +4 oxidation state assignments of the four 113X complexes, though it was noted that 113F exhibits an unusually high low temperature magnetic moment, which has been observed in other uranium(IV) complexes bound to strong point charge ligands.12

Scheme 31 Synthesis of 113X and 113X-THF (X ¼ F, Cl, Br, I) from 107.53

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Arene Complexes of the Actinides

Arnold et al. reported on the use of the dianionic macrocycle trans-calix[2]benzene[2]pyrrolide ligand to produce uranium-arene complexes, Scheme 32.54 The presence of two pyrrolide heterocycles and two aryl rings in the ligand allows the possibility of s- or p-bonding to the uranium center. Reduction of [UIVI2(L)] with KC8 in THF yielded the uranium(III) complex [UI(THF)(L)] (114) in 30% isolated yield, Scheme 32A. Crystallographic studies show that a dramatic change in ligand coordination is observed upon reduction, where complex [UIVI2(L)] exhibits k5:k5 metallocene binding from the two pyrrolide heterocycles, whereas 114 shows a preference for arene bonding, displaying Z6:k1:Z6:k1 bis-arene bonding. Removal of the coordinated THF under vacuum yielded the solvent free sandwich complex [UI(L)] (114b). The arylcent-U-arylcent angles in 114 and 114b (172 and 174 respectively) and mean U-C distances (3.001 A˚ , and 2.970 and 2.950 A˚ respectively) are relatively unchanged on removal of the coordinated THF.

Scheme 32 Synthesis of 114 and 115.54

In the synthesis of 114, the by-product [U2I4(L)] (115) could be identified.54 Reaction of two equivalents of UI3 with Li2L in toluene yielded 115 as the only L-containing product, Scheme 32B, albeit in moderate yield (22%). Crystallographic studies show both k5:k5 and Z6:k1:Z6:k1 in the bimetallic complex, with a U


U separation of 3.8639(5) A˚ . A quantum chemical approach to investigate the bonding in 114 was undertaken. A valence molecular orbital energy level diagram showed the highest energy occupied MOs to be predominantly 5f-character. The next highest in energy were the pyrrolide p-based MOs, showing very little metal contribution. These were followed by iodide p-based orbitals, then four arene p-based orbitals with the lowest quasi-degenerate pair displaying uranium-arene d-bonding. Finally, the lower energy occupied MOs represent UdN s bonding. Arnold et al. reported several uranium trans-calix[2]benzene[2]pyrrolide complexes by varying the coordinated anionic ligand, Scheme 33. Reaction of the ligand dipotassium salt with [U(BH4)3(THF)2] yielded [U(BH4)(L)] (116), which in the solid state structure was found to exhibit Z6:k1:Z6:k1 metal-ligand binding and a bis-arene motif.55 The coordinated borohydride in 116 could be substituted by reaction with KX, yielding [U(X)(L)] (X ¼ O-2,6-tBu2-C6H3, 117; N(SiMe3)2, 118) in yields of 56% and 35%, respectively.56 The bis-arene Z6:k1:Z6:k1 metal-ligand binding mode is maintained across these derivatives, with the change of interplanar arene angle reflecting the steric demands of the X groups. In contrast, reduction of 116 with [CPh3][B(C6F5)4] lead to the formation of [U(BH4)(L)][B(C6F5)] (119) and a change in metal-ligand binding mode from bis-arene Z6:k1:Z6:k1 to bis-pyrrolide Z1:Z5:Z1:Z5, accommodating the smaller uranium(IV) ion.

Arene Complexes of the Actinides

491

Scheme 33 Synthesis of 116–119.55,56

The arene-anchored tris(aryloxide) complex 107 was found to be able to electrocatalytically reduce water (0.005 M in THF) at room temperature to produce dihydrogen, Scheme 34.57,58 The proposed mechanism involves: (i) coordination of water to 107 to produce [{(Ad,MeArO)3mes}U(OH2)] (120); (ii) oxidative addition of the water to produce the hydroxy-hydride complex [{(Ad, Me ArO)3mes}U(OH)(H)] (121); (iii) thermal decay of 121 with concomitant elimination of H2 to produce the mono-oxo complex [{(Ad,MeArO)3mes}U(O)(THF)] (122); (iv) comproportionation between 122 and 107 (and H2O) or 120 to give 123; (v) reduction promoting loss of hydroxide and closure of the catalytic cycle. Interestingly, the structurally related complex [{(Ad,MeArO)3tacn}U], where (Ad,MeArOH)3tacn)3 ¼ 6,60 ,600 -((1,4,7-triazacyclononane-1,4,7-triyl)tris(methylene))tris(2-(adamantan-1-yl)-4-methylphenolate)), is not active in the reduction of H2O, thus it was deduced the uranium-arene bonding must play an important role in the reduction mechanism, in particular its interaction with the electrode in the electrochemical cell, but also non-innocence of the mesitylene anchor during the catalytic cycle was proposed as a key facilitating property. Endeavors into understanding the mechanism resulted in the synthesis of proposed intermediates of the catalysis. Indeed, oxidation of 107 with N2O yielded 122, which could be reduced with KC8 in the presence of 2.2.2-cryptand to the uranium(IV) terminal mono-oxo complex [K(2.2.2cryptand)][{(Ad,MeArO)3mes}U(O)] (124). The UdC bond distances in 122 range from 3.003(2) to 3.189(2) A˚ ; these are elongated compared to 107, which displayed a mean U-C distance of 2.749(3) A˚ . The arene C-C distances in 122 range from 1.407(3) to 1.414(3) A˚ , which is not elongated compared to the free ligand. However, the arene adopts a boat confirmation, indicative of an altered p-electron delocalization and an arene-based radical with intra-ligand charge separation. Low temperature EPR experiments of 122 displayed a sharp signal at giso ¼ 1.997 with a line- width, GFWHM, of 1.93 mT at 94 K, which is close to the typical giso value of an organic radical (giso ¼  2). The crystallographic, electronic absorption and EPR spectroscopic, SQUID and DFT analysis combined suggest that 122 has the electronic configuration of a formal uranium(V) ion with an mesitylene radical cation and balancing charge accumulation located on one of the aryloxide arms. Computational mechanistic studies showed that the water reduction proceeds through a two-electron oxidative addition with the relay of an electron from the arene ligand to the substrate via the uranium center. The metal-ligand communication through a d bond is believed to result in the relatively low activation barrier for the reduction and cooperative metal-ligand redox catalysis observed.

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Arene Complexes of the Actinides

Scheme 34 Postulated electrocatalytic cycle for uranium mediated reduction of H2O involving 107 and 120–123.57,58

The use of another arene anchored ligand with aryl(oxide) arms was used to facilitate uranium-based small molecule activation by Cloke et al, Scheme 35.59 The uranium(III) complex [U(C5Me5)(p-Me2bp)] (125, p-Me2bp ¼ 1,4-{C(Me2)(2-O-3,5-MeC6H2)}2C6H4) could be synthesized through the salt metathesis of (p-Me2bp)K2(DME)n with [U(C5Me5)(I)2(THF)] in yields of up to 75%. Crystallographic studies of 125 show that in the solid state, the arene is coordinated to the uranium(III) ion in the typical Z6-binding mode, with a mean UdC bond distance of 2.774 A˚ . The Z6-bound arene anchor displays a mean CdC bond distance of 1.411 A˚ which is slightly elongated when compared to the free ligand (1.393 A˚ ). When 125 is reacted with CO2 in toluene at low temperature (−78  C) the oxalate complex [{U(C5Me5)(p-Me2bp)}2(m-Z2:Z2-C2O4)] (126) is the dominant product, but if the same reaction is carried out at room temperature the carbonate complex [{U(C5Me5)(p-Me2bp)}2(m-Z1:Z2-CO3)] (127) is the dominant product. For the low and high temperature regimes 127 is formed alongside 126 and 126 with 127, respectively. From these results it is concluded that the oxalate is the kinetic product and the carbonate is the thermodynamic product. Interestingly, when supercritical CO2 is reacted with 125 then 127 is the sole product. Crystallographic studies of 126 and 127 show weakening of the uranium-arene interactions, with mean U-C distances of 3.187 and 3.124 A˚ , respectively, when compared to 125.

Arene Complexes of the Actinides

493

Scheme 35 Synthesis of 126 and 127 from 125.59

Arene-based ligands without aryloxide arms have also been used to stabilize low valent uranium, Scheme 36. Coordination of a p-terphenyl bis-aniline ligand ([LAr]2−) to uranium through a uranium-arene interaction as well as coordination of two anionic amine arms in a k2:Z6-bonding mode has been reported.60 The complex [LArU(I)-(DME)] (128) was synthesized by a salt metathesis reaction between the ligand dipotassium salt ([{K(DME)2}2LAr]) and UI3(dioxane)1.5 in DME yielding 128 in 51% isolated yield. Crystallographic studies on 128 show a uranium-arene Z6-interaction with a mean UdC bond distance of 2.92 A˚ . The mean C-C distance of the coordinated arene ring was reported to be 1.40 A˚ , which is comparable to the mean C-C distance for the dipotassium salt of the ligand (1.39 A˚ ). The small change in C-C distances suggests no significant reduction of the arene and little metal-arene back-bonding. The electronic structure and bonding interactions in 128 were investigated with DFT geometry optimization and high level complete active space self-consistent field calculations with second-order corrections to the energy

Scheme 36 Synthesis of 128 and 129.60

494

Arene Complexes of the Actinides

(CASSCF/CASPT2). The metal-arene covalent bonding interaction was significantly less when compared to other uranium-arene complexes, such as 100 discussed above. Calculations showed the unpaired electrons in 128 to be predominantly nonbonding 5f-orbitals, however the arene p-electron density remains slightly polarized toward the uranium center, with a 1–2% 6d orbital contribution. The salt metathesis reaction of 128 with [Na(TMEDA)][(Z6-C5H5)Fe(CO)2] yielded the bimetallic complex [LArU{(Z6-C5H5)Fe(CO)2}] (129), which displayed a relatively uncommon uranium-transition metal interaction. Alongside the UdFe bond formation, 129 undergoes significant change in coordination to the uranium center compared to 128. Complex 129 shows shortening of the mean UdC bond distance to 2.82 A˚ , as well as shortening of the UdN bond distances to 2.523(3) and 2.558 (3) A˚ . Although a change in UdC bond distance is observed, calculations showed the uranium-arene bonding in 129 to be largely analogous to the bonding interactions in 128.60 The use of metal-arene d-interactions to stabilize the electron rich uranium(II) ion was demonstrated by Meyer et al. in 2014.52 In 2018, another uranium(II) complex stabilized by an arene-based ligand was isolated as the first example of a neutral uranium(II) complex, Scheme 37.61 Reaction of two equivalents of the terphenylamide [Na(NHArPri6)] (ArPri6 ¼ 2,6-(2,4,6-iPr3C6H2)2C6H3) with UI3(THF)4 yielded the uranium(III) arene sandwich complex [IU(NHArPri6)2] (130) in 67% yield. The solid state structure of 130 can be approximated to a trigonal bipyramidal geometry about the uranium center, with the two amine arms and iodide occupying the equatorial plane and Z6-bound arene in ‘axial’ positions. Absorption spectroscopy was used to assign the +3 oxidation state. Treatment of 130 with excess KC8 in THF resulted in abstraction of the uranium bound iodide as KI, and generation of the neutral uranium(II) complex [U(NHArPri6)2] (131) in high yield (80%). Surprisingly, 131 showed much greater stability in both solution and the solid state compared to previously reported uranium(II) complexes. Crystallographic studies of 131 show that on reduction to uranium(II), a shortening in the U-Ccentroid is observed (from 2.843(1) A˚ in 130 to 2.405 (1) A˚ in 131, as well as a decrease in arenecentroid-uranium-arenecentroid angle from 158.785(2) to 134.240(9) , respectively. The mean CdC bond distances for the Z6 coordinated arene in 131 was reported to be 1.415(4) A˚ , which shows no significant difference from that in 130 (av. C-C 1.402(5) A˚ ).

Scheme 37 Synthesis of 130 and 131.61

The +2 oxidation state and 5f46d0 electron configuration of the uranium ion in 131 was determined by electronic absorption spectroscopy, EPR spectroscopy and magnetic studies. Electronic absorption spectroscopy of 131 showed a strong absorption at 400 nm and a very broad absorption at 600 nm. Intense absorptions assigned to 6d to p /5f transitions reported by Evans et al. for a uranium(II) complex with a 6d15f3 configuration anion, [U(C5H4SiMe3)3]−,62 were not observed for 131, suggesting a 5f46d0 electron configuration like Meyer’s complex 112 discussed above.52 No EPR response was detected at 6 K for 131, which is evidence for a non-Kramers +2 oxidation state uranium ion. SQUID magnetometry of powdered 131 shows a magnetic

Arene Complexes of the Actinides

495

susceptibility of wMT ¼ 0.63 cm3 K mol−1 at 300 K, which decreases steadily to 100 K, below which a sudden drop in magnetic susceptibility is observed, tending toward a magnetic susceptibility of 0 cm3 K mol−1 at low temperature. This magnetic profile suggests a singlet ground state which is in accordance with the absence of an EPR signal for 131. Oxidation of 131 with [FeCp2] [BArF24] (BArF24 ¼ B(3,5-(CF3)2C6H3)4) in diethyl ether, Scheme 38,61 yielded the unstable cationic uranium(III) complex [U(NHArPri6)2][BArF24] (132). The cationic species displays U-Ccentroid distances of 2.570(3) and 2.583(3) A˚ and arenecentroiduranium-arenecentroid angle of 145.8(1) .

Scheme 38 Synthesis of 132 from 131.61

In 2020, a transient terminal uranium nitride complex was synthesized utilizing an arene anchored ligand, Scheme 39.63 Treatment of the uranium(III) bis-anilide terphenyl complex [(LAr)UI(DME)] with LiNImDipp ({NImDipp} ¼ 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-iminato) yielded [LArU(NImDipp)] (133). Complex 133 could be converted into the uranium(IV) azide complex [LAU(N3)(NImDipp)] (134) after reacting with Ph3CN3, accompanied with elimination of half an equivalent of Gomberg’s dimer. Photolysis of solutions of 134 yielded after recrystallization the N-insertion complex [N-LAr)U(NImDipp)] (135) that evidences the formation of a fleeting terminal nitride complex [(LAr)U(N)(NImDipp)] that rapidly promotes C-H activation to convert to 135. The nitride intermediate complex was found to be too reactive to be isolated and characterized crystallographically, however photolysis in the presence of a bulky isocyanide lead to the complex [(LAr)U{NCN(C6H3Me2)}(NImDipp)] (136) which could be characterized by SC-XRD. In addition, photoloysis in the presence of PMe3 also prevented nitrogen insertion, yielding a mixture of products from which [(N,C-LAr )U(N]PMe3)(NImDipp)] (137) could be isolated and crystallographically characterized. The formation of N]C¼ NdAr and N]PMe3 linkages both provide further support for the existence of a fleeting terminal uranium-nitride linkage under photolytic conditions.

Scheme 39 Synthesis of 134 then 135 from 133 and 136 and 137 from 134.63

496

4.08.5

Arene Complexes of the Actinides

Neptunium arene complexes

To date, only two crystallographically characterized neptunium-arene complexes have been reported in the form of the neptunium(III) trans-calix[2]benzene[2]pyrrolide complexes [(LAr)NpCl] (138) and [(LAr)Np2Cl4(THF)3] (139), Scheme 40.64 Complex 138 was synthesized from both neptunium(IV) and neptunium(III) chloride starting materials. THF was condensed onto a mixture of NpCl4 and [K2(LAr)] (in a ratio of 0.66:1.00) and the mixture warmed to 20  C. Following stirring for a further 16 h, filtration and slow diffusion of n-pentane into the THF solution yielded 138 as dark red crystals. The crystallization of 138 was accompanied by the precipitation of a pale yellow powder (determined by g-ray spectroscopy to be an organic by-product), which could be separated from the crystalline material by mechanical separation, allowing the isolation of 138 in 46% yield. The synthesis of 138 from in situ prepared [NpCl3(THF)x] was found to be higher yielding (84%) and could be performed on a larger scale. The neptunium trichloride THF adduct was prepared by the reduction of NpCl4 by sodium mercury amalgam, and was separated from the mercury by syringe. To this THF suspension, [K2(LAr)] was added as a solid at room temperature and stirred for 5 h, before filtering and removal of the solvent under reduced pressure. Complex 138 could then be isolated as a crystalline solid following dissolving the crude product in the minimum amount of THF, followed by slow vapor diffusion of n-pentane in the solution. Complex 139 was synthesized in an analogous way to the first method of the synthesis of 138 described above, but in this instance using equal molar ratios of NpCl4 and [K2(LAr)]. Complex 139 was isolated as pure crystals in 92% yield by layering a concentrated THF solution of 139 with hexane, followed by mechanical separation of the crystals from an organic by-product.

Scheme 40 Synthesis of 138 and 139 and reduction of 138 to putative 141 then decomposition to 142.64

The solid state structures of 138 and 139 reveal the binding modes of the ligand to be two Z6-arene interactions and two k -pyrrolide bonding interactions. The binding mode is the same as observed for complexes 114, and 116–118. The arenecentroidneptunium-arenecentroid angle for 138 and 139 of 174.20(4) and 173.2(1) respectively, shows them to contain the bis-arene sandwich motif. The metal arene bond distances of 132 range from 2.853(2)–3.010(3) A˚ (2.95 A˚ mean), with 133 ranging from 2.854(6)–3.022(6) A˚ (3.01 and 2.96 A˚ mean). These metal arene distances compare well to 114 (range 2.95–2.98 A˚ ) and the samarium analog [(LAr)SmCl] (140) (2.98 A˚ ), providing a set of comparable f-block complexes for the bonding to be explored computationally. Computational studies of the bonding in 114 has been described above. A key difference between the modelled complexes is the energies of the metal f-orbitals. In 114 the least stable orbitals correspond to the three 5f electrons, however in 138 the four singly occupied 5f-orbitals were found to be lower in energy compared to the highest occupied pyrrolide orbitals. In-line with the core-like description of the 4f orbitals, the 4f orbitals of 140 were found to be significantly lower in energy than other valence orbitals. In 114, 138, and 140, the metal-arene p-bonding orbitals contain similar metal contribution (6–8%) and were found to be largely d-orbital based. It was also found that for the halogen p-p-MOs, the metal based contribution was only f-orbital based. 1

Arene Complexes of the Actinides

497

The redox chemistry of 138 was also explored in the same publication. In pursuit of a neptunium(II) species, the reductant NaK3 in DME was added to 138, resulting in the formation of a dark purple solution and the precipitation of KCl. Strong, broad absorptions in the vis-NIR spectrum of the DME solution centered at 600 and 1275 nm, suggested the presence of the neptunium(II) formal oxidation state, possibly a complex of putative formulation [(LAr)Np(DME)] (141). Over the course of 12 h, the strong absorptions were replaced with weak absorptions over the 780–1350 nm range, with the formation of a reddish brown solution. Crystals of the neptunium(III) complex [K(DME)n{LAr-H)Np(OCH3)}]2 (142) could be obtained from the reddish brown solution in 34% yield. The solid state structure of 142 shows the ligand to have a 3− charge, via metalation of one arene. The binding mode of the ligand in 142 thus differs from that in 138 and 140 in that both the pyrrollide rings in 142 are now Z5-coordinated. Cleavage of DME leads to 142 adopting a bridging methoxide dimer motif. In an attempt to oxidize 138, excess silver chloride was added in THF at room temperature, leading to the formation of a maroon solution and precipitation of silver(0). This is consistent with the formation of the neptunium(IV) complex [(LAr)NpCl2]. However, the reaction solution assigned as containing [(LAr)NpCl2] rapidly (20–30 s) decomposes, precluding characterization of [(LAr)NpCl2].

4.08.6

Summary and outlook

The last 20 years has been fruitful in diversifying organoactinide chemistry, with complexes bearing C4-C8 rings now well known, which has provided a plethora of examples to further understand the bonding interactions of uranium by computational methods. Apart from the dominant use of cyclopentadienyl ligands in organoactinide chemistry, arene derivatives are the next most prevailing class of aromatic ligand. Focusing on C6-arene complexes, the number of reported inverse sandwich complexes is approaching 50, containing multiple different electronic configurations, providing precedent for the assignment of formal oxidation states in confidence based on computational and spectroscopic techniques. With regard to bonding considerations, d-bonding is found to dominate in uranium arene complexes, with such complexes showing significantly greater metal-ligand back-bonding compared to analogous transition metal-arene complexes. The relatively strong d-bonding interactions between arenes and actinides has become a phenomena utilized in ligand design for the isolation of reactive actinide species, namely the uranium(II) ion and catalytic activity not possible in non-arene congeners. While the reported inverse-arene complexes are of fundamental interest in their own right, they have shown to be intermediates for a variety of further reactivity, including multi-electron redox chemistry, ligand substitution reactions, and arene activation. The thus far rather limited organometallic chemistry of the transuranic elements has largely relied on utilizing the cyclopentadienyl ligand,73 however the recent synthesis and characterization of two neptunium arene complexes potentially paves the way for the isolation of further transuranic arene complexes in the near future.

4.08.7

Acknowledgments

We thank the Royal Society, Engineering and Physical Sciences Research Council, the European Research Council, National Nuclear Laboratory, and University of Manchester for previous and continued support.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

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Note added in proof During the production process a study describing the synthesis of inverse-sandwich diuranium-arene complexes was published, see: Arnold, P. L.; Halliday, C. J. V.; Puig-Urrea, L.; Nichol, G. S. Inorg. Chem. 2021, 60, 4162–4170. In that study, it was found that [{U(O-2,6-Pri2C6H3)3}2] (99) and [{U(OBMes2)3}2] (Mes ¼ 2,4,6-trimethylphenyl) react with benzene solvent to produce inverse-sandwich complexes, and that this can be promoted by the addition of phosphine ligands.

Arene Complexes of the Actinides

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Table 1

List of thorium-arene complexes with selected bond distances.

Number in review

Formula

Oxidation state

M-Carene distances/A˚

Mean M-Carene/A˚

M-arenecentroid /A˚

References

1

+4

2.675(6)–2.761(6)

2.7235(6)

2.423(3)

17

+4

2.684(5)–2.775(4)

2.7295(5)

2.429(3)

17

+4 +4 +4 +4

2.900(7)–3.280(7) 3.179(8)–3.310(7) 3.063(5)–3.435(6) 2.972(12)–3.349(10)

3.065(9) 3.2625(8) 3.2395(9) 3.125(12)

2.728(3) and 2.738(4) 2.950(3) 2.935(3) 2.798(5)

18 19 19 19

+4

2.963(7)–3.110(8)

3.045(8)

2.701(8)

20

+4

2.719(10)–2.914(6)

2.825(10)

2.463(7)

20

+4

2.762(8)–4.077(7)

3.292(7)

2.497(7) and 3.481(7)

20

+4

3.012(17)–4.762(19)

3.70(2)

2.815(3) and 4.05(1) A˚ .

21

25 27b 29a 29b 29c 29d

[(Et8-calix[4]tetrapyrrole)Th{K(DME)}(Z4-C10H8)] [Li(DME)3] [(Et8-calix[4]tetrapyrrole)Th{K(DME)} {m:Z4-Z6-C10H8)(m-K)]n [(XA2)Th{Z6-PhCH2B(C6F5)3}2] [(XA2)Th(CH2SiMe3)(Z6-C6H6)][B(C6F5)4] [(XA2)Th(Z2-CH2Ph)(Z6-C6H5Me)][B(C6F5)4] [(BDPP)Th(Z2-CH2Ph) (m:Z1-Z6-CH2Ph)Th(Z1-CH2Ph)(BDPP)] [B(C6F5)4] [Z6-{1,3-([2-C4H3N][CH3]2C)2C6H4}ThCl3] [Li(DME)3] [{Z5-1,3-([Z5-2-C4H3N][CH3]2C)2C6H4} ThK(m-Cl)3][Li(DME)3] [{Z6-1,3-([2-C4H3N][CH3]2C)2C6H4}Th {m-Z5-1,3-([Z5-2-C4H3N][CH3]2C)2C6H5} K(DME)2] [Th{O-2,6-(Z6-2,4,6-C6H2Me3)(2,4,6-C6H2Me3) C6H3}2(k3-BH4)2] [(L)Th{N(SiMe3)2}][BPh4] [(L)Th(C^CSiMe3)2
Ni(PCy3)] [{(NNTBS)Th(THF)}2(m-Z6:Z6-C6H5Ph)] [{(NNTBS)Th(THF)}2(m-Z6:Z6-C10H8)] [{(NNTBS)Th}2(m-Z6:Z6-C6H6)] [{(NNTBS)Th}2(m-Z6:Z6-C6H5Me)]

+4 +4 +4 +4 +4 +4

2.913(3)–3.154(3) 2.962(3)–3.049(3) 2.564(4)–2.727(4) 2.564(8)–2.763(9) 2.536(6)–2.715(4) 2.547(2)–2.696(4)

3.028(4) 3.006(3) 2.655(7) 2.652(9) 2.634(8) 2.617(9)

2.6881(13) and 2.6915(13) 2.6707(11) and 2.6553(11) 2.217(4) and 2.217(4) 2.213(9) and 2.241(9) 2.216(8) and 2.178(8) 2.177(4) and 2.182(4)

22 22 23 23 23 23

Table 2

List of uranium-neutral arene complexes with selected bond distance.

2 6 8 10 11

13 16 17

19

Number in review

Formula

Oxidation state

M-Carene distances/A˚

Mean M-Carene/A˚

M-arenecentroid /A˚

References

35 36 37 38 39 40 41 42

[U(C6H6)(AlCl4)3] [U2(6-C6Me6)2Cl4(m-Cl)3][AlCl4] [U(C6Me6)Cl2(m-Cl)3UCl2(m-Cl)3UCl2(C6Me6)] [U3(m3-Cl)2(m2-Cl)3(Z2-AlCl4)3(Z6-C6Me6)3][AlCl4] [U(C6Me6)(AlCl4)3] [(C6Me6)U(BH4)3] [(C5Me5)2U][(m-Z2-Ph)2BPh2] [(C5Me4H)2U][(m-Z1-Ph)(m-Z6-Ph)BPh2]

+3 +4 +4 +3 +3 +3 +3 +3

2.91-2.92 2.83(2)–2.98(3) 2.89(2)–2.97(3) 2.77(2)–3.08(3) 2.88(2)–2.96(2) 2.87(2)–2.97(2) 2.857(7)–3.166(8) 2.868(4)–3.066(4)

2.91(1) 2.92(4) 2.94(3) 2.92(9) 2.93(2) 2.93(2) 3.010(8) 2.967(4)

2.564(13) 2.556(10) and 2.546(10) 2.560(11) and 2.595(9) 2.596(11), 2.580(12) and 2.543(12) 2.573(9) 2.581(8)

24 25 26 27 28 29 30 31

Table 3

List of uranium(III) inverse arene complexes with selected bond distances.

Number in Formula review 44b 48 50 52 54 64 65 66 67 68 69

2.6186(18)

[(m-Z6:Z6-C7H8){U(N[R]Ar)2}2] [{(Mes[But]CN)3U}2(m-Z6:Z6-C10H8)K2] [{U(Z5-C5Me5)2}2(m-Z6:Z6-C6H6)] [{([Me3Si]2N)(C5Me5)U}2(m-Z6:Z6-C6H6)] [{U(Z5-C5Me5)(CH[SiMe3]2)}2(m-Z6:Z6-C6H6)] [{U(BIPM™sH)(I)}2(m-Z6:Z6-C6H5CH3)] [{(NNfc)U}2(m:Z6-Z6-C7H8)] [{([Me3Si]2N)2U}2(m-Z6:Z6-C6H6)] [{([Me3Si]2N)2U}2(m-Z6:Z6-C6H5Me)] [{(O-2,6-But2C6H3)2U}2(m-Z6:Z6-C6H6)] [{(O-2,6-But2C6H3)2U}2(m-Z6:Z6-C6H5Me)]

Oxidation state

M-Carene distances/A˚

Mean M-Carene/A˚

M-arenecentroid/A˚

References

+3 +3 +3 +3 +3 +3 +3 +3 +3 +3 +3

2.504(9)–2.660(8) 2.565(11)–2.749(10) 2.506(13)–2.733(14) 2.559(3)–2.631(3) 2.5323(19)–2.6398(19) 2.553(7)–2.616(7) 2.544(6)–2.662(6) 2.568(3)–2.578(3) 2.539(9)–2.600(8) 2.517(6)–2.617(6) 2.516(9)–2.647(9)

2.594(9) 2.649(12) 2.625(14) 2.590(3) 2.5867(19) 2.579(9) 2.578(7) 2.573(3) 2.568(10) 2.573(6) 2.573(10)

2.161(4), 2.159(4) 2.206(4), 2.241(4) 2.194, 2.203 2.146 2.139 2.1419(3) 2.141(3), 2.125(3) 2.125 2.128, 2.135, 2.125, 2.132 2.124, 2.127 2.130, 2.130, 2.132, 2.135

32 33 34 34 35 36 37 38 39 38 38 (Continued )

500

Arene Complexes of the Actinides

Table 3

(Continued)

Number in Formula review

Oxidation state

M-Carene distances/A˚

Mean M-Carene/A˚

M-arenecentroid/A˚

References

+3 +3 +3 +3 +3 +3 +3

(2.539(6)–2.628(6) (2.527(5)–2.627(5) 2.567(4)–2.612(4) 2.497(6)–2.635(4) 2.511(5)–2.650(5) 2.519(3)–2.769(17) 2.608(9)–2.704(9)

2.587(7) 2.582(5) 2.578(4) 2.581(6) 2.591(6) 2.641(20) 2.662(10)

2.140 2.144 2.129 2.127 2.145, 2.146 2.112, 2.207 2.248(5), 2.222(4)

38 38 38 38 38 38 39

81 83

[{([Me3Si]2N)2U}2(m-Z6:Z6-C10H10] [{(O-2,6-But2C6H3)2U}2(m-Z6:Z6-C10H10)] [{(O-2,6-But2C6H3)2U}2(m-Z6:Z6-C6H5BBN)] [{(O-2,6-But2C6H3)2U}2(m-Z6:Z6-1-Me-4-BBNC6H4)] [{(O-2,6-But2C6H3)2U}2(m-Z6:Z6-1-Ph-4-BBNC6H4)] [{(O-2,6-But2C6H3)2U}2(C10H7BBN)] [{(Mes[But]CN)3U}2(m-Z6:Z6-arene)K2] (arene ¼ trans-stilbene) [{(Mes[But]CN)3U}2(m-Z6:Z6-C12H10){Na(OEt2)}2] [{(Mes[But]CN)3U}2(m-Z6:Z6-C7H8)K(DME)]

+3 +3

2.568(7)–2.678(7) 2.5895–2.6985

2.627(7) 2.6292

39 39

83a 90 91 92 93

[{(Mes[But]CN)3U}2(m-Z6:Z6-C7H8)K2I] [{U(LMe)I}2(m-Z6:Z6-C6H6)] [{U(BIPM™s)(m-I)0.5(m-Z6:Z6-C7H8)0.5}6] [{U(BIPM™s)(m-I)0.5(m-Z6:Z6-C6D6)0.5}6] [{([Me3Si]2N)3U(m-NH)U(N[SiMe3}2)}2(m-Z6:Z6-C7H8)]−

+3 +3 +3/+4 +3/+4 +3/+4

2.620(9)–2.681(9) 2.539(8)–2.587(8) 2.576(9)–2.723(9) 2.552(9)–2.686(9) 2.564(7)–2.669(6)

2.644(10) 2.567(8) 2.649(9) 2.613(10) 2.62(3)

2.209(3), 2.184(3) 2.19271(10), 2.24122(10), 2.21072(9), 2.21787(9) 2.208(4), 2.225(4) 2.1210(3) 2.2274(7), 2.220(4), 2.225(4) 2.207(4), 2.194(4) 2.194(4), 2.192(4)

70 71 74 75 76 77 79

Table 4

39 40 41 41 42

List of uranium(V) and uranium(IV) inverse arene complexes with selected bond distances.

Number in review

Formula

Oxidation state

M-Carene distances/A˚

Mean M-Carene/A˚

M-arenecentroid/A˚

References

94 95 96 97 98

[{U(TsXy)}2(m-Z6:Z6-C6H5Me)] [{U(TsTol)}2(m-Z6:Z6-C6H5Me)] [{U(OSi[OBut]3)3}2(m-Z6:Z6-C6H5Me)] [{U(OSi[OBut]3)3}2(m-Z6:Z6-C6H5Me)K] [{U(OSi[OBut]3)3}2(m-Z6:Z6-C6H5Me)K2]

+5 +5 +5 +4/+5 +4

2.651(4)–2.698(4) 2.535(15)–2.673(16) 2.689(3)–2.695(3) 2.602(9)–2.674(13) 2.589(4)–2.621(3)

2.676(4) 2.608(17) 2.693(4) 2.65(2) 2.607(14)

2.252(2), 2.259(2) 2.177(6) and 2.236(6) 2.27979(17) 2.212(4) and 2.220(4) 2.1630(2)

43 44 45,46 46 46

Table 5

List of uranium arene based ligand complexes with selected bond distances.

Number in review

Formula

Oxidation state

M-Carene distances/A˚

Mean M-Carene/A˚

M-arenecentroid/A˚

References

99 100 101 103 104 105 107 108 110 112 113F 113F-THF 113Cl-THF 113Br-THF 113I-THF 114 114b 115 116 117 118 119 122 124 125

[{U(O-2,6-Pri2C6H3)3}2] [{(ButArO)3mes}UIII] [{(ButArO)3Mes}U(dbabh)] [{([ButArO]3Mes)U}2(m-k2:k2-CO3)] [(Ar O)3U(THF)] [(Ar O)3U] [{(Ad,MeArO)3mes}U] [{(Ad,MeArO)3mes}U(H)K] [{(Ad,MeArO)3C6Me2CH2(K{Crown}{THF})}U] [K(2.2.2-crypt)][{(Ad,MeArO)3mes}U] [{(Ad,MeArO)3mes}U(F)] [{(Ad,MeArO)3mes}U(F)(THF)] [{(Ad,MeArO)3mes}U(Cl)(THF)] [{(Ad,MeArO)3mes}U(Br)(THF)] [{(Ad,MeArO)3mes}U(I)(THF)] [UI(THF)(L)] [UI(L)] [U2I4(L)] [U(BH4)(L)] [U(O-2,6-But2-C6H3)(L)] [U{N(SiMe3)2}(L)] [U(BH4)(L)][B(C6F5)] [{(Ad,MeArO)3mes}U(O)(THF)] [K(2.2.2- cryptand)][{(Ad,MeArO)3mes}U(O)] [U(C5Me5)(p-Me2bp)]

+3 +3 +4 +4 +3 +3 +3 +4 +4 +2 +4 +4 +4 +4 +4 +3 +3 +3 +3 +3 +3 +4 +5 +4 +3

2.82(1)–3.02(1) 2.719(3)–2.745(3) 2.852(5)–2.940(5) 2.897(5)–3.073(6) 2.921(2)–3.006(2) 2.823(2)–2.874(2) 2.729(3)–2.774(3) 2.562(5)–2.907(5) 2.546(3)–2.677(3) 2.597(5)–2.633(5) 2.865(4)–2.950(4) 2.961(5)–3.134(4) 2.967(4)–3.059(4) 2.940(3)–3.125(3) 2.992(5)–3.049(6) 2.902(8)–3.105(10) 2.839(14)–3.134(14) 3.061(11)–3.184(12) 2.844(5)–3.079(5) 2.894(3)–3.270(3) 2.858(6)–3.369(6) 2.735(7) and 2.745(7) 3.003(2)–3.189(2) 3.132(6)–3.139(7) 2.711(14)–2.839(15)

2.92(2) 2.73 2.88 2.960(6) 2.964(3) 2.853(3) 2.749(3) 2.777(5) 2.617(3) 2.615(5) 2.920 3.011 3.005 2.993 3.009 3.001 2.960 3.105 2.944(5) 3.022(3) 3.062(8)

2.563(6) 2.33 2.5170(19) 2.594 and 2.608 2.614 2.484 2.35 2.388(2) 2.2199(15) 2.18 2.559 2.666 2.657 2.645 2.664 2.669 and 2.612 2.612 2.799, 2.748 2.580(2) and 2.601(2) 2.6179(12) and 2.7454(13) 2.642(3) and 2.814(3)

3.055(2) 3.136(6) 2.774

2.711(2) 2.810(6) 2.388

47 48 48 49 50 50 51 51 51 52 53 53 53 53 53 54 54 54 55 56 56 57 58 58 59

Arene Complexes of the Actinides

Table 5

501

(Continued)

Number in review

Formula

Oxidation state

M-Carene distances/A˚

Mean M-Carene/A˚

M-arenecentroid/A˚

References

126 127

[{U(C5Me5)(p-Me2bp)}2(m-Z2:Z2-C2O4)] [{U(C5Me5)(p-Me2bp)}2(m-Z1:Z2-CO3)] [U(C5Me5)(p-Me2bp)(I)] [LArU(I)-(DME)] [LArU{(Z6-C5H5)Fe(CO)2}] [IU(NHArPri6)2] [U(NHArPri6)2] [U(NHArPri6)2][BArF24] [LArU(NImDipp)] [LAU(N3)(NImDipp)] [N-LAr)U(NImDipp)] [(LAr)U{NCN(C6H3Me2)}(NImDipp)]

+4 +4 +4 +3 +3 +3 +2 +3 +3 +4 +4 +4

3.057(8)–3.258(9) 3.086(11)–3.153(11) 2.921(6)–3.014(6) 2.880(4)–2.953(4) 2.7165(18)–2.9114(19) 3.039(2)–3.314(2) 2.723(3)–2.902(3) 2.828(6)–3.059(9) 2.672(4)–2.840(4) 2.865(5)–2.992(5) 2.893(5)–3.072(6) 2.863(4)–3.002(4)

3.149(12) 3.124 2.985 2.92 2.82 3.169(3) 2.789(3) 2.933(9) 2.766(4) 2.934(5) 2.981(6) 2.940(4)

2.806(4) and 2.849(4) 2.7902(3) 2.6381(3) 2.56 2.45 2.843(1) 2.405(1) 2.570(3) and 2.583(3) 2.38 2.58 2.682(2) 2.58

59 59 59 60 60 61 62 62 63 63 63 63

128 129 130 131 132 133 134 135 136

Table 6

List of neptunium arene complexes with selected bond distances.

Number in review 138 139

Formula Ar

[(L )NpCl] [(LAr)Np2Cl4(THF)3]

Oxidation state

M-Carene distances/A˚

Mean M-Carene/A˚

M-arenecentroid/A˚

References

+3 +3

2.853(2)–3.010(3) 2.854(6)–3.022(6)

2.95 2.99

2.6013(9) 2.668(3) and 2.608(3)

64 64

4.09

Arene Complexes of the Group 4 Metals

Skye Fortier, Alejandra Gomez-Torres, and Carlos Saucedo, Department of Chemistry and Biochemistry, University of Texas at El Paso, El Paso, TX, United States © 2022 Elsevier Ltd. All rights reserved.

4.09.1 Introduction 4.09.2 Bonding considerations 4.09.3 Low valent group 4-arene aluminates 4.09.3.1 Titanium-arene aluminates 4.09.3.2 Zirconium and hafnium-arene aluminates 4.09.4 High valent group 4-arene complexes 4.09.4.1 Lewis acid adducts 4.09.4.1.1 Jacobsen rearrangements 4.09.4.2 Synthesis through alkyl protonation 4.09.4.3 Synthesis through alkyl abstraction 4.09.4.3.1 Hydrocarbyls 4.09.4.3.2 Metallocenes 4.09.4.3.3 Amides 4.09.4.3.4 Aryloxides 4.09.4.4 Ansa-arenes 4.09.5 Metal vapor synthesis 4.09.5.1 Homoarenes 4.09.5.1.1 Bis(arene)titanium 4.09.5.1.2 Bis(arene)zirconium and bis(arene)hafnium 4.09.5.1.3 Hybrid vapor deposition 4.09.5.1.4 Vapor deposition compounds in catalysis 4.09.5.2 Heteroarenes 4.09.6 Coordination through arene reduction 4.09.6.1 Homoleptic complexes 4.09.6.1.1 Bis(arenes) 4.09.6.1.2 Tris(arenes) 4.09.6.2 Heteroleptic complexes 4.09.6.3 Inverted sandwich compounds 4.09.6.4 Hydrogenolysis 4.09.6.5 Bimolecular arene coordination 4.09.6.6 Intramolecular arene coordination 4.09.6.6.1 Reactivity of intramolecular titanium-arenes 4.09.6.7 Tethered arenes 4.09.7 Conclusion Acknowledgements References

4.09.1

502 503 504 504 510 511 511 513 513 514 514 514 518 521 522 526 526 526 528 528 529 529 530 530 530 534 535 537 539 540 542 542 544 546 546 547

Introduction

Metallocene complexes of the Group 4 metals have become mainstays of early-metal chemistry with such notable examples among many as Tebbe’s olefination reagent, [Ti(Cp)2(m-CH2)(m-Cl)Al(Me)2], and Kaminsky’s [M(Cp0 )2(Cl)2]-methylaluminoxane systems for olefin polymerization catalysis. Consequently, the chemistry of these metallocenes has been extensively reviewed.1–4 Though, there is another class of Group 4 compounds with carbocyclic ligands, namely those with metal-arene interactions, that should not be overlooked. Group 4 metal-arenes have played an important role in the development of the low-valent chemistry and catalytic applications of these metals, and as outlined in this chapter, continue to offer access to new and exciting chemistry. Here, we define the term “arene” as any neutral monocyclic or polycyclic hydrocarbon that possesses aromatic character prior to metal ligation. For instance, benzene here represents the archetypical arene molecule. This differs from charged aromatic, carbocycles such as Cp− or COT2−, which obtain their aromaticity upon chemical transformation or reduction of their parent hydrocarbon. Indeed, as detailed in this chapter, coordination of arenes to low valent Group 4 metals commonly leads to disruption of the p-system and loss of aromatic character—providing contrast to the coordination behavior of Cp−. While Group 4 metal-arene complexes have received attention to a limited extent in the literature,5–11 to the best of our knowledge, a dedicated review on the subject does not exist. Thus, the intent of this book chapter is to present a thorough overview

502

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00012-3

Arene Complexes of the Group 4 Metals

503

on the synthesis, characteristic properties, and key reactivity features of Group 4 metal-arenes. Notably, arene ligands can support Group 4 metals in a wide range of oxidation states, from M(-II) to M(IV), giving a rich and varied chemical landscape. It must be noted that as with metallocenes, metal-arenes have found use in olefin polymerization catalysis, and while this chemistry constitutes an important aspect of these compounds, the catalysis chemistry is beyond the scope of this chapter and limited discussion is provided where important for context and comparison.

4.09.2

Bonding considerations

Aromatic hydrocarbons such as benzene and toluene are indispensable organic solvents that are popular for their ability to dissolve a wide range of organic compounds. In the inorganic or organometallic laboratory, these aromatic solvents are prized not only for their solubilizing prowess but also their compatibility with a wide range of metal complexes and their miscibility with other organic solvents. Generally, aromatic solvents are non-coordinating, which is especially important in the synthesis of coordinatively unsaturated compounds where coordinating solvents, such as THF, are undesired. Additionally, benzene and toluene are favorable solvents for crystallization, particularly in those complexes with aromatic substituents where intermolecular p-p interactions assist with molecular packing. Yet, arenes are not always chemically innocent and, as shown here, are capable of metal ligation under certain conditions. This arises by engaging either the low energy p-orbitals or the high energy p -orbitals, or a combination thereof, of the aromatic system as shown in Fig. 1. On one hand, the filled p-orbitals of the arene ligand can donate into the empty dz2, dxz, and dyz orbitals of a metal, and on the other hand, filled dxy and dx2-y2 orbitals have the appropriate symmetry to d-backbond into the empty arene p -orbitals (Fig. 1). This push-pull bonding scheme allows arenes to coordinate to Group 4 metals across low and high oxidation states. Additionally, the electronic structure of the metal-arene interaction can vary greatly depending on the redox state of the metal, its total electron count, and the nature of its supporting ligands, which all affect the donor/acceptor properties of the metal. The bonds can also range from having highly electrostatic character to strong covalent overlap between the arene and metal orbitals. Arene to metal p-donation typically occurs in the absence of other coordinating solvents with metal complexes that are highly electron deficient. Though, these interactions are typically weak and dominated by electrostatics. Conversely, d-backbonding arises in electron rich, low-valent Group 4 metals with d2-electron counts or higher. The population of the p -orbitals would be expected to result in significant bonding distortions of the aromatic ligand system. Intriguingly, such structural distortions are observed to manifest in some cases while are absent in others. For instance, Group 4 metal-arene aluminates of the type [M(6-arene){(m-X)2(AlX2)}2] (X ¼ halide), as described in Section 4.09.3, do not exhibit obvious disruption of the aromatic p-system despite containing formally divalent metal centers. This may be due in part to the presence of the electron withdrawing, bridging [Al(X)4]− ligands which can temper d-backbonding. Alternatively, in heteroleptic systems with strongly electron donating

Fig. 1 Illustrations showing the symmetrical compatibility between d-orbitals and the aromatic p-system of benzene.

504

Arene Complexes of the Group 4 Metals

ligands (see Section 4.09.6.2), severe backbonding contributions can lead to formal charge transfer and full population of the arene p -orbitals, giving way to dearomatization and formation of reduced cyclohexadiene dianion moieties. Titanium-arene complexes predominate among the Group 4 series, with zirconium and hafnium taking a back seat. This may be due to the relative radial extension and diffuseness of the d-orbitals, favoring metal-arene overlap in the case of the more compact d-orbitals of titanium. Yet, a simpler explanation may be that titanium is a more abundant, affordable, and accessible metal for exploring such chemistry. Indeed, as documented throughout this chapter, the characteristics of Group 4 metal-arene complexes are generally preserved across the Ti, Hf, and Zr metal series, giving way to isotypic molecules that differ only in the identity of the metal. Though, divergent chemistry is observed between the metals on occasion with zirconium and hafnium more disposed to the formation of cluster complexes (see Section 4.09.3.2) due to the larger ionic radii of these metals. Along these lines, the smaller ionic radius of titanium can lead to increased steric crowding about the metal center in complexes with multiple arene ligands, consequently leading to lowered arene ligand hapticities and the formation of 16-electron complexes as compared to zirconium and hafnium which can accommodate higher arene hapticities to form 18-electron species (see Section 4.09.6.1.2). Lastly, examples of inverted sandwich complexes with zirconium and hafnium are more prevalent than with titanium; yet, the total number of Group 4 inverted sandwich compounds is too low to draw any conclusions regarding the favorability of one metal over another in the formation of such products (see Section 4.09.6.3). Bonding considerations in mind, the two general strategies for the synthesis of Group 4 metal-arene complexes involves either reduction or ligand abstraction. Under reducing conditions, d-backbonding interactions become favored, thus leading to arene capture by the metal. Ligand abstraction can generate coordinatively and electronically unsaturated Group 4 cations with high Lewis acidity. In this case, arene binding through electrostatic interactions or p-donation is promoted (see Section 4.09.4.1). Metal vapor synthesis is a very successful technique for generating homoleptic Group 4 homoarene and heteroarene sandwich compounds (see Section 4.09.5); however, this requires highly specialized equipment that is not commonly available to most synthetic laboratories.

4.09.3

Low valent group 4-arene aluminates

4.09.3.1

Titanium-arene aluminates

Following the structural elucidation of bis(benzene)chromium by E.O. Fischer in 1956,12 attention quickly turned to extending this system to other metals. In the case of [Cr(6-C6H6)2], the surprising stability of the complex was ascribed in large part to the 18-electron count of the chromium center.13 Yet, applying this chemistry to the early-metal series would prove to be more challenging due to their valence electron deficiencies and thermodynamic preference for higher oxidation states. To synthesize [Cr(6-C6H6)2], Fisher utilized a Friedel-Crafts reduction with [Cr(Cl)3] in C6H6 employing the use of [Al(Cl)3] as a Lewis Acid to nominally activate the benzene molecule in the presence of excess Al0 as reductant.13 This yields a yellow product, that upon treatment with dithionate, generates [Cr(6-C6H6)2] as a red crystalline material. Following this approach, with the intent of synthesizing new polymerization catalysts, Natta and Mazzanti combined [Ti(Cl)4] with [Al(Cl)3] and 1–3 equiv. of aluminum powder in benzene to produce a highly air-sensitive, violet colored complex which was assigned the written formulation [(TiAlCl5C6H5)
AlCl3] 1, proposing to contain a TidC6H5 s-bond.14 Skeptical of this assignment, the synthesis was subsequently revisited by Martin and Vohwinkel. Based upon more accurate molecular mass determinations and catalytic properties, they instead proposed the structure of a titanium(II) complex [C6H6
Ti(Cl)2
Al2(Cl)6] (2, Scheme 1), wherein the benzene p-coordinates to the titanium, accompanied by two bridged aluminum chloride units.15 Moreover, they expanded this chemistry to include the toluene and mesitylene analogs, then written as [Arene
Ti(Cl)2
Al2(Cl)6] (Arene ¼ C6H5Me 3; C6H3Me3 4).

Scheme 1 Friedel-Crafts reduction to form 2.

In 1966, Fischer and Röhrscheid synthesized a number of Group 4 and Group 5 half-sandwich, hexamethylbenzene complexes via Fischer’s Friedel-Crafts reduction method,16 or more simply “Fischer’s method,”17 to give [{M(6-C6Me6)(m-Cl)2}3][Cl] (M ¼ Ti 5; Zr 6; Nb 7; Ta 8) (Scheme 2). In this example, Fischer reported the combination of molten M(Cl)x, aluminum powder, C6Me6, and [Al(Cl)3] using the empirical ratios of 1:0.8:0.7:4 and 1:0.7:1.3:7 for the synthesis of 5 and 6, respectively. A proposed structure of these complexes, wherein the metal atoms are p-bound to the C6Me6 ring, was provided based upon magnetic measurements and molecular weight determinations.16

Arene Complexes of the Group 4 Metals

505

Scheme 2 Formation of trimeric complexes 5 and 6 via Fischer’s method.

Nearly a decade later, the complexes 2–4, and [Ti(6-Arene){(m-Cl)2(AlCl2)}2] (Arene ¼ 1,2-C6H4Me2 9; 1,3-C6H4Me2 10; 1,4-C6H4Me2 11; 1,2,4,5-C6H2Me4 12; C6Me6 13) (Scheme 3) were reported using a modified Fischer’s method synthesis that employed refluxing benzene as the reaction solvent.17,18 The polymethylated products are isolated in high yields, demonstrating that benzene does not compete as a ligand in the reaction. Additionally, 3, 4, 9–13 can be synthesized through ligand exchange reactions upon addition of higher methylated arenes to benzene solutions of 2 (Scheme 4), where reaction times decrease as the number of methyl groups on the arene ring increases. The 1H NMR spectra of 2 and 3 are paramagnetic whilst that of 13 is diamagnetic. Conductivity measurements of these species in solution show an inverse relationship to the relative arene ionization potentials, suggesting partial ionization of the compounds in solution with higher dissociation occurring with the more electron rich arene systems.17

Scheme 3 Modified Fischer’s method for the synthesis of 9–13.

Scheme 4 Arene ligand substitution reactions of 2.

Following Fischer’s method and using hexanes as the solvent, Thewalt and Kupfer demonstrated that complexing polyaromatic hydrocarbons, namely biphenyl and 3,5,30 ,50 -tetramethylbiphenyl, yields monomeric complexes of the type [Ti(6-Arene) {(m-Cl)2(AlCl2)}2] (Arene ¼ C12H10 14; C16H18 15) where the diarene is solely bound to one titanium atom (Scheme 5).19

Scheme 5 Formation of monomeric complexes 14–15 with biphenyl ligands.

506

Arene Complexes of the Group 4 Metals

With the improvement of X-ray crystallographic methods and technical advancements, the molecular structures of several long-known titanium metal-arene complexes were finally determined years after their initial reports by single crystal X-ray diffraction (SCXRD) analysis. In 1979, the first solid-state molecular structure of a Group 4 arene complex, namely 13, was reported. The titanium center in 13 is observed to adopt a pseudo square pyramidal geometry with the bridging chlorides forming the square base and the 6-C6Me6 ligand in the axial position,20 exhibiting a hexahapto interaction with the metal center (Fig. 2). The titanium-centroid distance in 13, TidCcent ¼ 2.06 A˚ 20 (Table 1), is identical to the titanium-centroid distance found in [Ti(Cp)2(Cl)2], TidCcent ¼ 2.06 A˚ .37 The mean CarenedCarene bond distance in 13 is 1.42(7) A˚ , effectively unperturbed as they fall near the expected mean CarenedCarene bond length of 1.41 A˚ for unbound, aromatic hydrocarbons.38 Twenty years after Martin and Vohwinkel’s proposed formulation for 2 as a titanium-arene p-complex,15 its molecular structure was determined by SCXRD and shown to be isostructural to 13,21 validating their assertion. Furthermore, in 1981, Fischer’s proposed structural formulation for

Fig. 2 Solid-state molecular structure of 13 with selected atom labelling. Key: Ti: Blue, Cl: Green, Al: Pink, C: Grey.

Table 1

Group 4 metal-arene complexes synthesized by Fischer’s Method.

Complex number

Molecular formula

Arene M-C dist. ˚) (A

Mean Arene M-C dist. ˚) (A

M-Arene centroid ˚) dist. (A

Reference number

1 2

[(TiAlCl5C6H5)
AlCl3] [C6H6
Ti(Cl)2
Al2(Cl)6]

–a 2.49(3)

–a 2.07

14 15,21

3 4 5 6

[C6H5Me
Ti(Cl)2
Al2(Cl)6] [C6H3Me3
Ti(Cl)2
Al2(Cl)6] [{Ti(6-C6Me6)(m-Cl)2}3][Cl] [{Zr(6-C6Me6)(m-Cl)2}3][Cl]

–a –a –a 2.59(3); 2.60(4)

–a –a –a 2.16

15 15 16 16,22

7 8 9 10 11 12 13

[{Nb(6-C6Me6)(m-Cl)2}3][Cl] [{Ta(6-C6Me6)(m-Cl)2}3][Cl] [Ti(6-1,2-C6H4Me2){(m-Cl)2(AlCl2)}2] [Ti(6-1,3-C6H4Me2){(m-Cl)2(AlCl2)}2] [Ti(6-1,4-C6H4Me2){(m-Cl)2(AlCl2)}2] [Ti(6-1,2,4,5-C6H2Me4){(m-Cl)2(AlCl2)}2] [Ti(6-C6Me6){(m-Cl)2(AlCl2)}2]

–a –a –a –a –a –a 2.50(4)

–a –a –a –a –a –a 2.06

16 16 17 17 17 17 17,18,20,21

14

[Ti(6-C12H10){(m-Cl)2(AlCl2)}2]

2.50(2)

2.09

19

15

[Ti(6-C16H18){(m-Cl)2(AlCl2)}2]

2.49(2)

2.07

19

16 17 18 19

[Ti(6-C6H6){(m-Br)2(AlBr2)}2] [Ti(6-C6H6){(m-I)2(AlI2)}2] [Ti(6-C6Me6){(m-Br)2(AlBr2)}2] [Ti(6-C6Me6){(m-I)2(AlI2)}2]

–a 2.466(1)–2.547 (2) –a –a –a 2.552(9)–2.620 (7); 2.558(5)–2.630 (5) –a –a –a –a –a –a 2.442(7)–2.547 (2) 2.480(7)–2.540 (7) 2.469(5)–2.532 (5) 2.48(3)–2.50(3) 2.45(3)–2.54(3) –a –a

2.487(9) 2.50(3) –a –a

–b 2.08 –a –a

23 23 23 23

Arene Complexes of the Group 4 Metals

Table 1

(Continued)

Complex number

Molecular formula

Arene M-C dist. ˚) (A

Mean Arene M-C dist. ˚) (A

M-Arene centroid ˚) dist. (A

Reference number

20 21 22

[Ti(6-C6H6)(Al2ClxBr8-x)] [Ti(6-1,2,4,5-C6H2Me4)(Al2Cl8-xIx)] [Ti3(m-I)3{1,3,5-([Tim-I]AlI2)3(m4-2:2:2:6-C6H3) (6-C6H6)}]

–a 2.50(2) 2.3(2); 2.3(1); 2.3(2); 2.2(1); 2.48(5)

–a 2.06 2.02

24,25 26 27

23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

[Ti(6-C6H5Me){(m-Br)2(AlBr2)}2] [Ti(6-C6H5Me){(m-I)2(AlI2)}2] [Ti(6-C6H3Me3){(m-Br)2(AlBr2)}2] [Ti(6-C6H3Me3){(m-I)2(AlI2)}2] [Ti(6-1,4-C6H4Me2){(m-Br)2(AlBr2)}2] [Ti(6-1,4-C6H4Me2){(m-I)2(AlI2)}2] [Ti(6-C6H6)(CO)2{(m-Cl)2(AlCl2)}][AlCl4] [Ti(6-C6H6)(CO)2{(m-Br)2(AlBr2)}][AlBr4] [Ti(6-C6H6)(CO)2{(m-I)2(AlI2)}][AlI4] [Ti(6-C6H5Me)(CO)2{(m-Cl)2(AlCl2)}][Al(Cl)4] [Ti(6-C6H5Me)(CO)2{(m-Br)2(AlBr2)}][Al(Br)4] [Ti(6-C6H5Me)(CO)2{(m-I)2(AlI2)}][Al(I)4] [Ti(6-C6H3Me3)(CO)2{(m-Cl)2(AlCl2)}][Al(Cl)4] [Ti(6-C6H3Me3)(CO)2{(m-Br)2(AlBr2)}][Al(Br)4] [Ti(6-C6H3Me3)(CO)2{(m-I)2(AlI2)}][Al(I)4] [Ti(6-C6H4Me2)(CO)2{(m-Cl)2(AlCl2)}][Al(Cl)4] [Ti(6-C6H4Me2)(CO)2{(m-Br)2(AlBr2)}][Al(Br)4] [Ti(6-C6H4Me2)(CO)2{(m-I)2(AlI2)}][Al(I)4] [Ti(6-C6H6)(Al2Cl7Et1)] [Ti(6-C6H6)(Al2Cl6Et2)] [Ti(6-C6Me6)(Al2Cl6Et2)] [Ti(6-C6H5Me)(4-CH2CH2C6H5)(4-CH2C6H5)] [Ti(6-C6Me6)(4-CH2CH2C6H5)(4-CH2C6H5)] [Zr(6-C6H6)2{(m-I)2(AlI2)}][Al3(I)10] [Zr(6-C6H3Me3)2{(m-Cl)2(AlCl2)}][Al2(Cl)7]

–a –a –a –a –a –a –a –a –a –a –a –a –a –a –a –a –a –a –a –a 2.49(3) –a –a –a 2.50(7); 2.58(5)

–a –a –a –a –a –a –a –a –a –a –a –a –a –a –a –a –a –a –a –a 2.06 –a –a –a 2.07; 2.16

28 28 28 28 28 28 28 28 28 28 28 28 28 28 28 28 28 28 29,30 29,30 31 32 32 33 33

48

[Zr(6-C6H3Me3)2{(m-Br)2(AlBr2)}][Al2(Br)2]

2.52(8); 2.54(5)

2.1; 2.12

33

49 50

[{Zr(6-C6H6)([m-Br]2AlBr2)}2(m-Br)3][Al2(Br)7] [{Zr(6-C6H3Me3)(m-Cl)2}3]2+[{Al(Cl)4}{Al2(Cl)7}]

–a 2.57(4); 2.62(2); 2.59(2)

–a 2.2; 2.15; 2.17

34 35

51 52 53

[{Zr(6-C6H3Me3)(m-Cl)2}3][{Al2(Cl)7}2] [{Zr(6-C6H3Me3)(m-Cl)2}3][{Al2(Cl)7}3] [{Zr(6-C6H3Me3)(m-Cl)2}3][{Mg(Al[Cl]4)3}3]

2.57; 2.59; 2.60 2.61; 2.63; 2.62 2.61(3); 2.59(5)

–b –b 2.22; 2.22

35 35 35

54 55 56 57 58 59 60

[Zr(6-C6H6){(m-Cl)2(AlCl2)}2] [Zr(6-C6H6){(m-Br)2(AlBr2)}2] [Zr(6-C6H5Me){(m-Cl)2(AlCl2)}2] [Hf(6-C6H5Me){(m-Cl)2(AlCl2)}2] [Zr(6-C6H3Me3){(m-Cl)2(AlCl2)}2] [Hf(6-C6H3Me3){(m-Cl)2(AlCl2)}2] [Zr(6-C6Me6){(m-Cl)2(AlCl2)}2]

–a 2.46(3)–2.53(3) 2.15(3)–2.45 (4); 2.19(4)–2.44 (4); 2.16(4)–2.47 (4); 2.01(4)–2.29 (4); 2.39(5)–2.55(6) –a –a –a –a –a –a –a –a –a –a –a –a –a –a –a –a –a –a –a –a 2.44(2)–2.54(2) –a –a –a 2.388(5)–2.593 (5); 2.508 (5)–2.657(5) 2.42(6)–2.62 (6); 2.49(6)–2.60(7) –a 2.59(1)–2.64 (2); 2.53(2)–2.65 (2); 2.55(2)–2.62(2) –b –b 2.56(4)–2.65 (4); 2.55(6)–2.63(5) –a –a –a –a –a –a –a

–a –a –a –a –a –a –a

–a –a –a –a –a –a –a

36 36 36 36 36 36 36

a

Solid-state molecular structure not determined. Data not available.

b

507

508

Arene Complexes of the Group 4 Metals

the zirconium complex 6 was also corroborated by X-ray crystallographic analysis.22 The mean CarenedCarene bond distances in 6 are 1.42(4) A˚ , matching 13, while the zirconium-centroid distance, ZrdCcent ¼ 2.16 A˚ , is longer than the analogous distance in 13 as would be expected for the larger zirconium cation. Investigating the nature of the halide ligands, Mach and Antropiusova synthesized the bromide and iodide derivatives [Ti(6-Arene){(m-X)2(AlX2)}2] (Arene ¼ C6H6, X ¼ Br 16, I 17; Arene ¼ C6Me6, X ¼ Br 18, I 19) and studied the electronic absorption spectra of each compound.23 The spectra are similar and display three absorption bands within the general values of u1 ¼ 12,000 cm−1 (830 nm), u2 ¼ 17,000–18,000 cm−1 (550–590 nm), and u3 ¼ 25,000 cm−1 (400 nm), with u3 assigned to a Ti(II) ! arene charge transfer as the value of this band shifts to higher frequencies with increasing electron richness of the arene ligand. Charge transfer bands involving the halogen atoms were not observed. The mixed halogen systems [Ti(6-C6H6)(Al2ClxBr85 x)] 20 were subsequently synthesized and reacted with 0.5 and 1 equiv. of dicyclopentadiene to give [Ti( -C5H5)(Al2ClxBr8-x)] and 5 [Ti( -C5H5)2(AlClxBr4-x)] (Scheme 6), respectively, with these latter reagents used to probe the preferential halogen occupancy of the bridge positions by EPR spectroscopy, showing a preference for chloride bridging.24,25 Similar studies were much later extended to the mixed halide system [Ti(6-1,2,4,5-C6H2Me4)(Al2Cl8-xIx)] 21, again displaying preference for chloride bridging as shown through determination of its solid-state molecular structure.26

Scheme 6 Reaction of 20 with differing equivalents of dicyclopentadiene.

Interestingly, Troyanov reported that in one case, heating a melt of [Ti(I)4] with [Al(I)3] and excess aluminum powder with benzene at 130  C gave the unusual sandwich structure [Ti3(m-I)3{1,3,5-[(m-I)(AlI2)]3(m4-2:2:2:6-C6H3)}Ti(6-C6H6)] 22 (Fig. 3) as shown by SCXRD characterization (Table 1).27 Compound 22 is unique from typical inverted sandwich complexes (see Section 4.09.6.3) as it features a triply deprotonated benzene ring with one p-coordinated titanium atom situated on one side of the ring face with a cluster of three titanium atoms on the other side of the ring with seemingly 2-bound interactions. The deprotonated carbon atoms of the central ring are each bonded to bridging [Al(I)3] units. Moreover, the sandwiched benzene ring is observed to be distorted, adopting a puckered conformation suggestive of disruption of the aromatic system. On the other hand, the terminally 6-bound benzene ligand of 22 retains its planarity. Further investigating the role of the halide ligands, Calderazzo and co-workers synthesized the titanium halide half-sandwich complexes 2–4, 11, 16, 17, and [Ti(6-Arene){(m-X)2(AlX2)}2] (Arene ¼ C6H5Me, X ¼ Br 23, I 24; Arene ¼ C6H3Me3, X ¼ Br 25, I 26; Arene ¼ 1,4-C6H4Me2, X ¼ Br 27, I 28) for use as precursors for arene carbonylation reagents.28 Pressurizing solutions of these compounds with CO gives formally divalent carbonyl complexes in low to modest yields with the proposed formulation of [Ti(6-Arene)(CO)2{(m-X)2(AlX2)}][Al(X)4] (29–40, Scheme 7) based upon CO stretching vibration analysis. The CO stretching frequencies in 29–40 were observed to decrease on moving from chloride to iodide (e.g., 32: nCO ¼ 2096, 2076 cm−1; 33: nCO ¼ 2089, 2061 cm−1; 34: nCO ¼ 2079, 2057 cm−1), a function of the decreasing electronegativity of the halide substituent. To a lesser extent, the wavenumbers of the CO vibrations were also found to decrease as the number of methyl groups on the ligated arene ligand increases. Finally, these systems were found to be inactive for arene carbonylation, but surprisingly, the [Al(Br)3] used in the reaction was shown to be capable of converting toluene to p-tolualdehyde in moderate stoichiometric yields.28

Fig. 3 Polynuclear titanium sandwich complex 22.

Arene Complexes of the Group 4 Metals

509

Scheme 7 Formation of divalent carbonyl complexes 29–40.

The study of mixed alkylhaloaluminate titanium-arene complexes was also pursued. As 2 catalyzes the cyclotrimerization of butadiene to (Z,E,E)-1,5,9-cyclododecatriene, it was discovered that addition of ethylaluminum compounds to the reaction mixture improves the catalytic output.31,39 In order to study the reaction kinetics of the butadiene cyclotrimerization reaction, the mixed alkyhaloaluminate compounds [Ti(6-Arene)(Al2Cl8-xEtx)] (Arene ¼ C6H6, x ¼ 1 41, 2 42; Arene ¼ C6Me6, x ¼ 2 43) were synthesized from the addition of [AlCl3-xEtx] (x ¼ 1, 2) to the starting titanium-arene chloroaluminates. In the case of 41 and 42, their chemical composition was determined through UV-vis spectroscopy and EPR study of their Ti(III) Cp-derivatives,29,30,40,41 while the connectivity of 43 was provided through SCXRD analysis (Table 1).31 Butadiene cyclotrimerization improves as the number of ethyl groups increases,30,31,41 where it is proposed that the ethyl groups serve to destabilize the Ti-arene interaction.31 Along these lines, treating solutions of 3 or 23 with only 2 equiv. of the alkynes RC^CR (R ¼ Me, Ph) gives the tetraorganylcyclobutadiene complexes [Ti(4-C4R4){(m-X)2(AlX2)}2] (R ¼ Me, X ¼ Br; R ¼ Ph, X ¼ Cl, Br) (Scheme 8).42 These compounds can be considered intermediates in titanium-arene catalyzed cylotrimerization reactions, and interestingly, the rate of their formation was found to be dependent upon the presence of trace amounts of free [Al(X)3] in solution. Treating the tetramethylcyclobutadiene complex with excess MeC^CMe catalyzes the formation of C6Me6. Further probing for possible catalytic intermediates, varying the PhC^CPh/Ti ratios, either using 3 or 23, from 1:1, 2:1, and 3:1 forms a different product in each case as shown by UV-vis spectroscopy. While the 2:1 addition primarily gives tetraphenylcyclobutadiene complexes as expected, the identity of the other ratio products were not identified but presumed to be a diphenylacetylene adduct in the 1:1 case, and in the 3:1, an unknown Ti(II) product said to not contain C6Ph6 as a ligand. Further cyclotrimerization studies with 23 were investigated, and it catalyzes 2-butyne, phenylacetylene, and hex-1-yne to C6Me6, 1,3,5-triphenylbenzene, and 1,2,4- or 1,3,5-(n-butyl)benzene with respective turnover numbers of 520, 920, and 990. On the other hand, treating 2 or 16 with excess PhC^CPh primarily gives C6Ph6 and octaphenylcyclooctatetraene, accompanied by a host of other oligomerization products with the relative ratios shown to be dependent on the presence of alkylhaloaluminum additives. Notably, [Ti(4-C4Ph4){(m-Br)2(AlBr2)}2] can be further reacted with dipotassium cyclooctatetraenide [K2(COT)] or with 2 equiv. of [Tl(Cp)] to give the metallocenes [Ti(4-C4Ph4)(COT)] and [Ti(Cp)2(k2-C4Ph4)] (Scheme 8), the latter undergoing ring opening of the cyclobutadiene to give a titanatetraphenylbutadiene metallacycle.

Scheme 8 Formation of cyclobutadiene derivatives of 3 and 23.

Later studies regarding the catalytic properties of low-valent titanium-arene complexes showed that 3 supported on activated [Mg(Cl)2] could effect propylene polymerization but with reduced activity as compared to more commonly used Ziegler-Natta [Ti(Cl)4]/[Mg(Cl)2] catalysts.43 However, 3/[Mg(Cl)2] is more efficient than [Ti(Cl)4]/[Mg(Cl)2] at ethylene polymerization. In regards to styrene polymerization, both 3 and 13 produce atactic polystyrene, but upon pre-mixing with methylaluminoxane

510

Arene Complexes of the Group 4 Metals

(MAO), syndiotactic polyethylene with high molecular weight is produced,32 where the specific role of the MAO is not known. In either regard, the formation of an organometallic arene complex with a TidC s-bond 44–45, as shown in Scheme 9, is proposed as the catalytically active species.

Scheme 9 Proposed intermediate formed in the styrene polymerization reaction of 3, 13 with MAO.

4.09.3.2

Zirconium and hafnium-arene aluminates

While significant attention was devoted to the synthesis and study of titanium-arene complexes over several decades, the zirconium and hafnium analogs received renewed interest in the 1990s. The larger ionic radii of the heavier group 4 congeners give way to more structural diversity than in the case of titanium. Whereas Fischer’s method largely produces neutral, bis(aluminate)titanium-arene molecules, namely [Ti(6-Arene){(m-X)2(AlX2)}2], the bis(arene)zirconium-aluminate complexes [Zr(6-Arene)2{(m-X)2(AlX2)}] [Aln(X)3n+1] (Arene ¼ C6H6, n¼3, X ¼ I, 46; Arene ¼ C6H3Me3, n¼2, X ¼ Cl 47, Br 48) (Scheme 10) are formed as discrete ion pairs under similar reaction conditions.33 Yet, as in the synthesis of 6, the nuclearity of the product is highly dependent on the ratio of aluminum halide, reductant (Al, Mg, or Zn), and the conditions used in the reaction. For instance, Troyanov isolated the zirconium-arene cluster series [{Zr(6-C6H6)[(m-Br)2(AlBr2)]}2(m-Br)3][Al2(Br)7] 49 and [{Zr(6-C6H3Me3)(m-Cl)2}3]n+[X] (n ¼ 2, X ¼ [{Al(Cl)4}{Al2(Cl)7}] 50; n ¼ 2, X ¼ [Al2(Cl)7]2 51; n ¼ 3, X ¼ [Al2(Cl)7]3 52; n ¼ 3, [Mg(AlCl4)3]3 53) by subtle reaction modifications (Scheme 11), and in these reports, the author suggests the less electronegative the halogen atoms in the system, the more readily zirconium is reduced.34,35 The main differences between the [{Zr(6-C6H3Me3)(m-Cl)2}3]n+ (n ¼ 2, 3) clusters, as shown by SCXRD, are the ZrdZr distances and the angle of the mesityl rings, revealing that complexes of n ¼ 2 had longer distances (3.32–3.33 A˚ ) than complexes of n ¼ 3 (3.27–3.28 A˚ ).34

Scheme 10 Formation of bis(arene)zirconium complexes 46–48.

Scheme 11 Formation of dinuclear and trinuclear zirconium-arenes using Fischer’s method.

Arene Complexes of the Group 4 Metals

511

Calderazzo and co-workers succeeded in synthesizing the mono-zirconium and -hafnium complexes [M(6-Arene){(m-X)2 (AlX2)}2] (Arene ¼ C6H6, X ¼ Cl, M ¼ Zr 54; Arene ¼ C6H6, X ¼ Br, M ¼ Zr 55; Arene ¼ C6H5Me, X ¼ Cl, M ¼ Zr 56, M ¼ Hf 57; Arene ¼ C6H3Me3, X¼Cl, M ¼ Zr 58, Hf 59; Arene ¼ C6Me6, X¼Cl, M ¼ Zr 60) with structural assignments based upon comparison of the UV-vis absorption data for these compounds with their titanium analogs.36 In most cases, the metal-arene complexes were synthesized using their respective arene as both reagent and solvent under refluxing conditions (Scheme 12). Yet, 60 is synthesized via ligand exchange by addition of C6Me6 to 54 or through a cyclotrimerization reaction of 54 with 3 equiv. of MeC^CMe. Moreover, controlled hydrolysis of 60 leads to the formation of the cluster 6 (Scheme 13). It was later found that 54 catalyzes the cyclotrimerization of MeC^CMe to hexamethyl Dewar benzene (HMDB) and C6Me6 at room temperature, with HMDB formed as the major product that later isomerizes to C6Me6. Finally, addition of 2 equiv. of diphenylacetylene to 54 does not give a cyclotetraphenylbutadiene complex, as with 3, but instead generates the zirconotetraphenylbutadiene metallacycle [Zr(k2-C4Ph4){(m-Cl)2(AlCl2)}2],44 providing some contrast between the metal-arene chemistry of titanium and its heavier congeners.

Scheme 12 Formation of zirconium and hafnium bis(aluminate) complexes.

Scheme 13 Controlled hydrolysis of 60.

4.09.4

High valent group 4-arene complexes

4.09.4.1

Lewis acid adducts

In the absence of reductants such as Al powder, tetravalent Group 4 metal-arene adducts are isolable. Floriani and co-workers first demonstrated that addition of [Ti(Cl)4] to concentrated solutions of excess C6Me6 in non-coordinating solvents affords the half-sandwich ion pair [Ti(6-C6Me6)(Cl)3][Ti2(Cl)9] 61 (Scheme 14).45,46 Following the reaction by 1H NMR spectroscopy, while varying the equivalents of C6Me6, gives evidence for the formation of the intermediate [Ti(6-C6Me6)(Cl)4] 62, proposed to exist in equilibrium with [Ti(Cl)4], free C6Me6, and 61. Alternatively, treating solutions of [Ti(Cl)4] in CH2Cl2 with excess 2-butyne catalyzes its cyclotrimerization to C6Me6, with 61 forming in the process as an intermediate in the catalytic cycle (Scheme 14).45

Scheme 14 Reactivity of [Ti(Cl)4] with C6Me6.

Extension of this reductant-free methodology gives access to a host of Ti(IV) metal-arene compounds. In combination with [Al(Cl)3], [Ti(Cl)4] reacts with arenes to give the aluminate salts [Ti(6-Arene)(X)3][Al(X)4] (Arene ¼ C6Et6, X ¼ Cl 63;

512

Arene Complexes of the Group 4 Metals

Arene ¼ C6Mei5Pr, X ¼ Cl 64; Arene ¼ C6H3Me3, X ¼ Cl 65, Br 66; Arene ¼ 1,2,4,5-C6H2Me4, X ¼ Cl 67; Arene ¼ C6Me6, X ¼ Cl 68, Br 69, I 70) (Scheme 15).47–51 The 1H NMR spectra of several of these compounds in CD2Cl2 shows a clear downfield shift of the arene protons upon complexation to titanium with the exception of toluene, whereby it is proposed to exist in equilibrium between free and complexed toluene.51 Gallium trichloride is also an effective co-reagent and shown by 13C NMR spectroscopy studies to enable arene complexation to titanium, yielding the tetrachlorogallates [Ti(6-Arene)(Cl)3][Ga(Cl)4] (Arene ¼ C6H5Me 71, 1,4-C6H4Me2 72, C6H3Me3 73, 1,2,4,5-C6H2Me4 74, C6Me6 75).50 In the absence of [Al(Cl)3] or [Ga(Cl)3], the chlorotitanate ion pairs [Ti(6-Arene)(Cl)3][Ti2(Cl)9] (Arene ¼ C6Me5Et 76, C6Mei5Pr 77) are formed, where 1H NMR spectroscopic analysis shows a 1:1 ratio between the complexes and the free arene while in solution.48 Applying the same reductant-free approach to Zr or Hf, with or without [Al(Cl)3], produces the multinuclear complexes [{Zr(6-C6Me6)(Cl)2}(m-Cl)3Zr(Cl)3] 78 and [Hf(6-C6Me6)(Cl)3] [Hf2(Cl)9] 79 (Scheme 16).50

Scheme 15 Formation of titanium(IV)-arene aluminate salts 63–70.

Scheme 16 Reactivity of [M(Cl)4] (M ¼ Zr, Hf ) with C6Me6.

Theoretical analysis of the electronic structure of [Ti(6-C6Me6)(Cl)3]+ unsurprisingly reveals a largely electrostatic, chargeinduced dipole interaction in the metal-arene bond with modest arene-p to d-orbital contributions.46 In line with this, the arene ligand in 68 is readily displaced by coordinating solvents such as THF to give [Ti(Cl)4(THF)2] (Scheme 17).51 Moreover, addition of [Tl(Cp)] to 68 affords [Ti(Cp)(Cl)3] (Scheme 17).51 The lability of the arene ligand in these high valent systems was further demonstrated by investigating the variable temperature solution phase behavior of 65 in CD2Cl2 by 1H NMR spectroscopy, revealing the presence of several coordination isomers at low temperatures with a proposed equilibrium between s/p-bonded 1- and 6-coordination modes (Scheme 18). DFT analysis supports this, showing the two isomers to be near isoergic.49 Consequently, these solution-phase [Ti(1-Arene)(Cl)3]+ species have been implicated in the hydrogen-deuterium exchange of the ring protons observed between C6D6 and C6H6-nMen ligands in titanium-arene complexes.51

Scheme 17 Arene displacement reactivity of 68.

Arene Complexes of the Group 4 Metals

513

Scheme 18 Proposed solution-phase tautomerization of 65.

4.09.4.1.1

Jacobsen rearrangements

High-valent Group 4 arene complexes have also been demonstrated to catalyze Jacobsen rearrangements. In CH2Cl2, [Zr(Cl)4] solubilizes upon addition of aromatic substrates. Inspection of the solution mixture by 1H NMR spectroscopy when using durene reveals the formation of several compounds, where [Zr(6-1,2,4,5-C6H2Me4)(Cl)2(m-Cl)3Zr(Cl)3] 80 (similar to 78) is the predominant species.52,53 Standing solutions of 80 gives the zirconium compounds 78 and [Zr(6-C6Me5H)(Cl)2(m-Cl)3Zr(Cl)3] 81 after several weeks as crystalline solids in a 1:1 ratio accompanied by the presence of C6Me6, C6Me5H, and other polymethylarenes (Scheme 19). However, the methyl redistribution reaction competes with the zirconium-assisted Friedel-Crafts reaction of CH2Cl2 with the arenes, but this can be avoided by conducting the Jacobsen rearrangement in neat aromatic solvent. While novel, the reaction is unselective and its mechanism is not understood.53 Lastly, in the same study, treatment of [Zr(Cl)4] with RC^CR (R ¼ Me, Et) in n-hexane was shown to give the alkyne cyclotrimerization zirconium-arene products 78 and [Zr(6-C6Et6) (Cl)2(m-Cl)3Zr(Cl)3] 82 (Scheme 20).

Scheme 19 Formation of 81, 78, and polymethylarenes from standing solutions of 80.

Scheme 20 Cyclotrimerization of acetylenes with [Zr(Cl)4].

4.09.4.2

Synthesis through alkyl protonation

Cationic Group 4 compounds are effective Lewis Acid catalysts for olefin polymerization and interest in developing new catalysts and isolating the unsaturated active intermediates in these reactions led to the development of novel Group 4 metal-arene complexes. In an attempt to generate a non-metallocene, cationic Group 4 alkyl complex of the type [Zr(CH2Ph)3]+, protonation of [Zr(CH2Ph)4] with [PhNMe2H][B(Ph)4] in toluene at room temperature instead results in the formation of the zwitterionic zirconium-arene compound [Zr(PhCH2)3{(6-C6H5)B(Ph)3}] 83 (Scheme 21).54 The 1H NMR spectrum of 83 at −40  C in CD2Cl2 reveals a static structure with well-resolved proton resonances. Upon warming, the resonances of the [(6-C6H5)B(Ph)3]− moiety broaden, suggesting coordination exchange between the phenyl groups. The tetraphenylborate is not exchanged by other neutral arenes such as toluene, C6Me6, or 1,3,5-C6H3(tBu)3, but is readily displaced by coordinating solvents such as THF, giving free [B(Ph)4]− and other unidentified products as indicated by 13C NMR spectroscopic analysis. Addition of 1,3,5-trimethylhexahydro-1,3,5-triazine or 1,4,7-trimethyl-1,4,7-triazacylcononane to 83 frees the tetraphenylborate to give [Zr{(CH2)nNMe}3 (CH2Ph)3][B(Ph)4]. Attempts to similarly protonate [Ti(CH2Ph)4] were unsuccessful.

Scheme 21 Formation of zwitterionic complex 83 through alkyl protonation.

514

Arene Complexes of the Group 4 Metals

4.09.4.3 4.09.4.3.1

Synthesis through alkyl abstraction Hydrocarbyls

Alternatively, addition of the potent Lewis Acid [B(C6F5)3] to Group 4 alkyls has been shown to be an effective method for the synthesis of zwitterionic metal-arene complexes. Treatment of [Zr(CH2Ph)4] with 1 equiv. of [B(C6F5)3] in toluene gives [Zr(PhCH2)3{(6-C6H5CH2)B(C6F5)3}] 84, formed from abstraction of a benzyl group followed by its p-arene coordination (Scheme 22).55 Variable temperature 1H NMR spectroscopic experiments carried out in toluene-d8 or CD2Cl2 show the metal-arene contact is maintained in solution and remains essentially unchanged in the temperature range of −60–100  C, though minor fluxionality of the coordinated aromatic ring is observed in CD2Cl2, suggesting a weaker cation-anion interaction in polar solvents. Complex 84 catalyzes the polymerization of ethylene and propylene at 0.5–1.0 mM catalyst loading, generating 2.5 104 g polyethylene
(mol of Zr)−1
h−1
atm−1 and 1.5 104 g polypropylene
(mol of Zr)−1
h−1
atm−1 in toluene at 50  C and 5 atm for 15 min to 1 h with slightly accelerated monomer consumption when performed in tetrachloroethane. The catalytic action of 84 in these polymerization reactions was subsequently studied.56 Following the reaction of 84 with propylene by 1H NMR spectroscopy in C2D4Cl2 reveals the quantitative formation of the intermediate single-insertion product [Zr(2-CH2Ph)2 {CH2CH(Me)CH2(6-C6H5)}][(C6H5CH2)B(C6F5)3] 85 (Scheme 22), which was confirmed by SCXRD determination. Variable temperature 1H NMR spectroscopic analysis of 85 in CD2Cl2 from −90−20  C shows the molecular structure is largely retained in solution; however, upon cooling, the ortho proton resonances of the coordinated benzyl ligand broaden and shift upfield. This suggests a reduced fluxionality of the coordinated ring in solution and potential increase in the metal-ring binding interaction. Mixing 85 with THF leads to decomposition and formation of CH2]CMeCH2Ph, due to presumed b-hydrogen elimination from an intermediate THF-adduct. Heating 85 alone at 50  C in C2D4Cl2 gives a mixture of CH2]CMeCH2Ph, Me2CHCH2Ph, and an unidentifiable Zr product; while, heating in the presence of propylene affords atactic and isotactic polypropylene accompanied by small amounts of CH2]CMeCH2Ph and Me2CHCH2Ph.56,57

Scheme 22 Inter- and intramolecular metal-arene binding in 84 and 85.

The reactivity of 84 was later extended to a-olefins such as ethylene, 4-methyl-1-pentene, 1-vinylcyclohexane, and allylbenzene wherein the single insertion products [Zr(2-CH2Ph)2{CH2CH(R)CH2(n-C6H5)}][(C6H5CH2)B(C6F5)3] (R ¼ H 86, CH2CHMe2 87, C6H11 88, and CH2Ph 89) were formed.57 The structural elucidation of the molecules by NMR spectroscopy suggests their structures approximate that of 85, and indicates that the p-coordination of the phenyl ring combined with the 2-interaction of the benzyl groups slows monomer addition and serves to mitigate b-hydride elimination. Closer examination of the insertion products using ring-deuterated allylbenzene-d5 gives [Zr(2-CH2Ph)2{CH2CH(CH2C6D5)CH2(n-C6H5)}][(C6H5CH2)B(C6F5)3] 89-d5, where fluxional exchange between coordination of the -CH2C6D5 and -CH2C6H5 ligand arms is observed in solution by NMR spectroscopy.

4.09.4.3.2

Metallocenes

Alkyl ligand abstraction by B(C6F5)3 to give metal-arene compounds is also effective in metallocene complexes. Treatment of a toluene/hexane solutions of [M(Cp∗)(Me)3] (M ¼ Zr, Hf ) with [B(C6F5)3] results in the near quantitative precipitation of the cationic toluene complexes [M(Cp∗)(Me)2(6-C6H5Me)][(Me)B(C6F5)3] (M ¼ Zr 90, Hf 91) (Scheme 23).58 Performing the analogous reaction with zirconium in other solvents gives [Zr(Cp∗)(Me)2(6-Arene)][(Me)B(C6F5)3] (Arene ¼ C6H6 92, H2C] CHC6H5 93, C6H3Me3 94) where only 94 is stable at room temperature. The molecules were characterized by NMR spectroscopy, and while the hexahapto coordination of the arene ligand could not be unambiguously determined at the time, the spectral patterns of the aromatic protons are consistent with an 6-binding mode. Conversely, adding [B(C6F3)3] to [Ti(Cp∗)(Me)3] in aromatic solvents fails to produce an arene complex but instead yields methyl-bridged [Ti(Cp∗)(Me)2(m-Me)B(C6F5)3] as a thermally sensitive solid. Here, the choice of reaction solvent plays a critical role in the formation of the titanium analogs, as it was later shown that treating [M(Cp∗)(Me)3] (M ¼ Ti, Zr, Hf ) solutions with [B(C6F5)3] in CH2Cl2 in the presence of aromatic substrates gives access to the full range of Group 4 products 90, 91, and [M(Cp∗)(Me)2(6-Arene)][(Me)B(C6F5)3] (Arene ¼ C6H6, M ¼ Zr 95; Arene ¼ C6H5Me, M ¼ Ti 96; Arene ¼ 1,3/1,4-C6H4Me2, M ¼ Zr 97, Hf 98; Arene ¼ C6H5OMe, M ¼ Hf 99; Arene ¼ H2C] CHC6H5, M ¼ Zr 100; Arene ¼ C6H3Me3, M ¼ Ti 101, Zr 102) (Scheme 23),59 though attempts to generate the C6Me6 derivatives failed. Furthermore, in these cases, only a handful of the products were isolated as analytically pure materials. Nevertheless, this reaction route does allow for the crystallization of 91, confirming its structural identity as a mixed sandwich 6-toluene adduct by SCXRD (Table 2). In general, the aromatic proton resonances of these products are deshielded when compared to the free aromatic substrate as shown by 1H NMR spectroscopy, consistent with flow of electron density from the coordinated arene to the metal. Interestingly, spin saturation transfer experiments with 90 and 95 at 223 K in mixtures of arene solutions suggest that exchange between the coordinated and free arenes does not occur readily and is only observed to occur with 95 upon warming to 273 K.

Arene Complexes of the Group 4 Metals

515

Scheme 23 Methyl abstraction of metallocenes by B(C6F5)3 giving 90, 91, and 95–102.

Table 2

Group 4 metal-arene interactions in complexes synthesized by alkyl abstraction.

Complex number

Molecular formula

˚) Arene M-C dist. (A

Mean Arene M-C ˚) dist. (A

M-Arene ˚) centroid dist. (A

Reference number

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

[Ti(6-C6Me6)(Cl)3][Ti2(Cl)9] [Ti(6-C6Me6)(Cl)4] [Ti(6-C6Et6)(Cl)3][Al(Cl)4] [Ti(6-C6Mei5Pr)(Cl)3][Al(Cl)4] [Ti(6-C6H3Me3)(Cl)3][Al(Cl)4] [Ti(6-C6H3Me3)(Br)3][Al(Br)4] [Ti(6-1,2,4,5-C6H2Me4)(Cl)3][Al(Cl)4] [Ti(6-C6Me6)(Cl)3][Al(Cl)4] [Ti(6-C6Me6)(Br)3][Al(Br)4] [Ti(6-C6Me6)(I)3][Al(I)4] [Ti(6-C6H5Me)(Cl)3][Ga(Cl)4] [Ti(6-C6H4Me2)(Cl)3][Ga(Cl)4] [Ti(6-C6H3Me3)(Cl)3][Ga(Cl)4] [Ti(6-1,2,4,5-C6H2Me4)(Cl)3][Ga(Cl)4] [Ti(6-C6Me6)(Cl)3][Ga(Cl)4] [Ti(6-C6Me5Et)(Cl)3][Ti2(Cl)9] [Ti(6-C6Mei5Pr)(Cl)3][Ti2(Cl)9] [{Zr(6-C6Me6)(Cl)2}(m-Cl)3Zr(Cl)3] [Hf(6-C6Me6)(Cl)3][Hf2(Cl)9] [Zr(Cl)2(6-1,2,4,5-C6H2Me4)(m-Cl)3Zr(Cl)3] [Zr(Cl)2(6-C6Me5H)(m-Cl)3Zr(Cl)3] [Zr(Cl)2(6-C6Et6)ZrCl2(m-Cl)3Zr(Cl)3] [Zr(PhCH2)3{(6-C6H5)B(Ph)3}] [Zr(PhCH2)3{(6-C6H5CH2)B(C6F5)3}] [Zr(2-CH2Ph)2{CH2CH(Me)CH2(6-C6H5)}] [(C6H5CH2)B(C6F5)3] [Zr(2-CH2Ph)2{CH2CH(H)CH2(n-C6H5)}][(C6H5CH2)B(C6F5)3] [Zr(2-CH2Ph)2{CH2CH(CH2CHMe2)CH2(n-C6H5)}] [(C6H5CH2)B(C6F5)3] [Zr(2-CH2Ph)2{CH2CH(C6H11)CH2(n-C6H5)}] [(C6H5CH2)B(C6F5)3] [Zr(2-CH2Ph)2{CH2CH(CH2Ph)CH2(n-C6H5)}] [(C6H5CH2)B(C6F5)3] [Zr(2-CH2Ph)2{CH2CH(CH2C6D5)CH2(n-C6H5)}] [(C6H5CH2)B(C6F5)3] [Zr(Cp∗)(Me)2(6-C6H5Me)][(Me)B(C6F5)3] [Hf(Cp∗)(Me)2(6-C6H5Me)][(Me)B(C6F5)3] [Zr(Cp∗)(Me)2(6-C6H6)][(Me)B(C6F5)3] [Zr(Cp∗)(Me)2(6-H2C]CHC6H5)][(Me)B(C6F5)3] [Zr(Cp∗)(Me)2(6-C6H3Me3)][(Me)B(C6F5)3] [Zr(Cp∗)(Me)2(6-C6H6)][(Me)B(C6F5)3] [Ti(Cp∗)(Me)2(6-C6H5Me)][MeB(C6F5)3]

2.46(1)–2.52(1) –a 2.56–2.57 –a 2.493(3)–2.531(2) –a –a 2.513(4)–2.526(4) –a –a –a –a –a –a –a 2.491(3)–2.529(3) –a 2.725(8)–2.768(7) 2.58(3)–2.64(2) –a –a –a –a 2.65(2)–2.76(2) –a

2.50(2) –a –b –a 2.51(2) –a –a –b –a –a –a –a –a –a –a 2.51(1) –a 2.74(3) 2.61(3) –a –a –a –a 2.68(4) –a

2.06 –a –b –a 2.09 –a –a –b –a –a –a –a –a –a –a –b –a 2.37 2.19 –a –a –a –a 2.28 –a

45 45 47 48 49,51 51 51 50 51 51 50 50 50 50 50 48 50 50,52,53 50 52,53 52 53 54 55 56,57

–a –a

–a –a

–a –a

57 57

–a

–a

–a

57

–a

–a

–a

57

–a

–a

–a

57

–a 2.62(1)–2.805(9) –a –a –a –a –a

–a 2.70(8) –a –a –a –a –a

–a 2.33 –a –a –a –a –a

58,59 58 58,59 58,59 58,59 59 59

86 87 88 89 89-d5 90 91 92 93 94 95 96

(Continued )

516

Table 2

Arene Complexes of the Group 4 Metals

(Continued)

Complex number

Molecular formula

˚) Arene M-C dist. (A

Mean Arene M-C ˚) dist. (A

M-Arene ˚) centroid dist. (A

Reference number

97 98 99 100 101 102 103 104 105 106 107 108

[Zr(Cp∗(Me)2(6-1,3-C6H4Me2)][(Me)B(C6F5)3] [Hf(Cp∗)(Me)2(6-1,4-C6H4Me2)][(Me)B(C6F5)3] [Hf(Cp∗)(Me)2(6-C6H5OMe)][(Me)B(C6F5)3] [Zr(Cp∗)(Me)2(6-H2CH]CC6H5)][(Me)B(C6F5)3] [Ti(Cp∗)(Me)2(6-C6H3Me3)][(Me)B(C6F5)3] [Zr(Cp∗)(Me)2(6-C6H3Me3)][(Me)B(C6F5)3] [Zr(Cp00 )(Me)2(6-C6H5Me)][(Me)B(C6F5)3] [Hf(Cp00 )(Me)2(6-C6H5Me)][(Me)B(C6F5)3] [Hf(Cp00 )(Et)2(6-C6H5Me)][(Me)B(C6F5)3] [Hf{Cp00 (Et)(6-C6H5Me)}2(m-2:2-C2H4)]2+ [Zr(Cp∗)(7-CH2C6H5)(3-CH2C6H5)][(C6H5CH2)B(C6F5)3] [Zr(Cp∗)(CH2Ph){CH2CH(Me)CH2(6-C6H5)}] [(C6H5CH2)B(C6F5)3] [Zr{(6-C6H5CH2)B(C6F5)3}{Me3SiN(CH2CH2NSiMe2CH2) (CH2CH2NSiMe3)}] [Zr{(6-C6H5CH2)B(C6F5)3}{(SiMe3)2N}(k2-Me3SiNSiMe2CH2)] [Zr(CH2Ph)(Me2SiNtBu)2][(3-C6H5CH2)B(C6F5)3] [TiMe(Z6-C6H5Me){(2,6-C6H3Me2)N(CH2)3N(2,6-C6H3Me2)}]+ [Zr(CH2Ph)(DMBN){(6-C6H5CH2)B(C6F5)3}] [Zr(CH2Ph){1,10 -Fc(NSiMe3)2}{(6-C6H5CH2)B(C6F5)3}] [Zr(CH2CH2CH2Ph){1,10 -Fc(NSiMe3)2}] {(6-C6H5CH2)B(C6F5)3}] [Zr{CMeCMeCH2(6-C6H5)}{1,10 -Fc(NSiMe3)2} {(C6H5CH2)B(C6F5)3}] [Zr{CPhCPhCH2(6-C6H5)}{1,10 -Fc(NSiMe3)2} {(C6H5CH2)B(C6F5)3}] [Zr{C(C6H5Me)C(C6H5Me)CH2(6-C6H5)}{1,10 -Fc(NSiMe3)2} {(C6H5CH2)B(C6F5)3}] [Zr{CH2(CH2)nCH2Ph}{1,10 -Fc(NSiMe3)2} {(6-C6H5CH2)B(C6F5)3}] [Zr(CH2Ph){(Me3SiN)(Me3SiNCH2)C6H4} {(6-C6H5CH2)B(C6F5)3}] [Zr(CH2Ph){(Ph2MeSiN)(Me3SiNCH2)C6H4} {(6-C6H5CH2)B(C6F5)3}] [Zr(CH2Ph)2(Dip-pyr){(6-C6H5CH2)Al(C6F5)3}] [Zr(CH2C6H5)2{(6-C6H5CH2)B(C6F5)3} {(C6H2[CH(CH3)2]3)PyN(C6H2[CH(CH3)2]2H)}] [Hf(CH2C6H5)2{(6-C6H5CH2)B(C6F5)3} {(C6H2[CH(CH3)2]3)PyN(C6H2[CH(CH3)2]2H)}] [Zr(CH2C6H5)2{(6-C6H5CH2)B(C6F5)3} {(C6H2[CH3]2H)PyN(C6H2[CH(CH3)2]2H)}] [Hf(CH2C6H5)2{(6-C6H5CH2)B(C6F5)3} {(C6H2[CH3]2H)PyN(C6H2[CH(CH3)2]2H)}] [Zr(CH2C6H5)2{(6-C6H5CH2)B(C6F5)3} {(C6H2[CH(CH3)2]3)PyN(C6H2[CH3]3)}] [Hf(CH2C6H5)2{(6-C6H5CH2)B(C6F5)3} {(C6H2[CH(CH3)2]3)PyN(C6H2[CH3]3)}] [Ti(CH2Ph){(6-C6H5CH2)B(C6F5)3}(ONOSiMe2tBu)] [Zr(CH2Ph){(6-C6H5CH2)B(C6F5)3}(ONOSiMe2tBu)] [Ti(CH2Ph){(6-C6H5CH2)B(C6F5)3}(2,6-Ph2-3,5-Me2C6HO)2] [Zr(CH2Ph){(6-C6H5CH2)B(C6F5)3}(2,6-Ph2-3,5-Me2C6HO)2] [Ti(CH2Ph){(6-C6H5CH2)B(C6F5)3}(2,6-Ph2C6H3O)2] [Ti(CH2PhMe){(6-C6H4MeCH2)B(C6F5)3}(2,6-Ph2C6H3O)2] [Ti(CH2Ph){(6-C6H5CH2)B(C6F5)3}(2,6-tBu2C6H3O)2] [Zr(2,6-tBu2C6H3O){2-tBu2-4-(CH2Me2C)C6H3O} {(6-C6H5CH2)B(C6F5)3}] [Ti(2,6-Ph2-3,5-Me2C6HO)2{C(Me)C(Ph)CH2(6-C6H5)}] [(C6H5CH2)B(C6F5)3] [Ti(2,6-Ph2C6H3O)2{C(Me)C(Ph)CH2(6-C6H5)}] [(C6H5CH2)B(C6F5)3]

–a –a –a –a –a –a –a 2.651(7)–2.747(6) –a –a 2.348(4)–2.634(4) 2.69(2)–2.74 (3)

–a –a –a –a –a –a –a 2.69(3) –a –a 2.5(1) 2.72(2)

–a –a –a –a –a –a –a 2.30 –a –a 2.12 2.33

59 59 59 59 59 59 60 60 60 60 61 62

–a

–a

–a

63

–a 2.630(3)–2.975(2) –a 2.675(9)–2.774(8) 2.725(3)–2.852(4) –a

–a –b –a 2.71(3) 2.77(5) –a

–a –b –a 2.33 2.38 –a

64 65 66,67 68 69 69

–a

–a

–a

69

–a

–a

–a

69

–a

–a

–a

69

–a

–a

–a

69

–a

–a

–a

70

–a

–a

–a

70

2.593(4)–2.899(4) –a

2.7(1) –a

2.36 –a

71 72

–a

–a

–a

72

2.614(5)–2.844(5)

2.70(9)

2.31

72

–a

–a

–a

72

2.650(5)–2.816(5)

2.70(6)

2.31

72

–a

–a

–a

72

–a 2.742(2)–2.813(2) –a 2.652(6)–2.820(5) 2.539(7)–2.628(7) –a –a 2.670(4)–2.852(3); 2.658(4)–2.880(3) –a

–a 2.76(3) –a 2.71(7) 2.57(3) –a –a 2.74(7); 2.74(9)

–a 2.38 –a 2.33 2.15 –a –a 2.36; 2.36

73 73 74 74 74 75 74,75 75

–a

–a

74,75

–a

–a

–a

75

109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138

Arene Complexes of the Group 4 Metals

Table 2

517

(Continued)

Complex number

Molecular formula

˚) Arene M-C dist. (A

Mean Arene M-C ˚) dist. (A

M-Arene ˚) centroid dist. (A

Reference number

139

–a

–a

–a

74

–a

–a

–a

74,75

–a

–a

–a

74,75

–a

–a

–a

74,75

–a

–a

–a

74

–a

–a

–a

74

–a

–a

–a

74

2.632(9)–2.821(6) –a –a –a –a –a –a –a –a –a –a

2.69(7) –a –a –a –a –a –a –a –a –a –a

2.31 –a –a –a –a –a –a –a –a –a –a

76 77 78 78 78 78 78 78 79 79 79

157 158 159 160 161 162 163 164 165 166

[Ti(2,6-tBu2C6H3O)2{C(Me)C(Ph)CH2(6-C6H5)}] [(C6H5CH2)B(C6F5)3] [Ti(2,6-Ph2-3,5-Me2C6HO)2{CH2CH(CH2Ph)CH2(6-C6H5)}] [(C6H5CH2)B(C6F5)3] [Ti(2,6-Ph2-3,5-Me2C6HO)2{CH2CH(Me)CH2(6-C6H5)}] [(C6H5CH2)B(C6F5)3] [Ti(2,6-Ph2-3,5-Me2C6HO)2{CH2CH(Bu)CH2(6-C6H5)}] [(C6H5CH2)B(C6F5)3] [Ti(2,6-tBu2C6H3O)2{CH2CH(CH2Ph)CH2(6-C6H5)}] [(C6H5CH2)B(C6F5)3] [Ti(2,6-tBu2C6H3O)2{CH2CH(Me)CH2(6-C6H5)}] [(C6H5CH2)B(C6F5)3] [Ti(2,6-tBu2C6H3O)2{CH2CH(Bu)CH2(6-C6H5)}] [(C6H5CH2)B(C6F5)3] [Zr(CH2Ph){(6-C6H5CH2)B(C6F5)3}(TRMSO)2] [Ti(Me)2(5:6-C5Me4CH2CH2C6H5)][B(C6F5)4] [Ti{5:n-C5H4C(Me)2Ph}(Me)2][B(C6F5)4] [Ti{5:n-C5H4C(Me)2CH2Ph}(Me)2][B(C6F5)4] [Ti{5:n-C5H4C(Me)2CH2Ph}(Me)2][(Me)B(C6F5)3] [Zr{5:n-C5H4C(Me)2CH2Ph}(Me)2][B(C6F5)4] [Zr{5:n-C5H4C(Me)2CH2Ph}(Me)2][(Me)B(C6F5)3] [Ti(5:n-C5H4CHPh2)(Me)2][B(C6F5)4] [Zr(5:n-C5H4C(Me)2CH2C6H4Me)(Me)2][B(C6F5)3] [Zr(5:n-C5H4C(Me)2CH2C6H4Me)(Me)2][(Me)B(C6F5)3] [Zr(5:n-C5H4C(Me)2CH2C6H4Me)(Me){(m-Me)B(C6F5)3}] [(Me)B(C6F5)3] [Ti{5:6-C5H4C(Me)2C6H3Me2}(Me)2][(Me)B(C6F5)3] [Ti{5:6-C5H4C(Me)2C6H3Me2}(Me)(C6F5)][(Me)B(C6F5)3] [{Ti(5:6-C5H4C[Me]2C6H3Me2)(m-Br)}2][{B(C6F5)4}2] [Ti{5:6-C5H4C(Me)2C6H3Me2}(CH2Ph)2][(PhCH2)B(C6F5)3] [Ti{5:6-C5H4C(Me)2C6H3Me2}(CH2Ph)2][B(C6F5)4] [Ti{5:k1-C5H4C(Me)2C6H4}(CH2Ph){(6-C6H5CH2)B(C6F5)3}] [Ti{5:k1-C5H4C(Me)2C6H4}(7-CH2Ph)][B(C6F5)4] [Ti{5:6-C5H4C(Me)2C6H5}(Me)2][B(C6F5)4] [Ti{5:6-C5H4C(Me)2C6H3Me2}(Me)2][B(C6F5)4] [Ti{5:6-C5H4CH2C(Me)2C6H3Me2}(Me)2][B(C6F5)4]

–a –a 2.5(2) –a –a –a –a 2.5(1) 2.7(2) 2.59(6); 2.60(8)

–a –a 2.16 –a –a –a –a 2.18 2.26 2.18; 2.19

80 80 80 81 81 81 81 82 82 82

167 168 169 170 171 172 173

[Ti{5:6-C5H3(SiMe3)C(Me)2C6H5}(Me)2][B(C6F5)4] [Ti{5:6-C5H3(SiMe3)CH2C6H3Me2}(Me)2][B(C6F5)4] [Ti{5:6-C5H3(SiMe3)C(Me)2C6H3Me2}(Me)2][B(C6F5)4] [Ti{5:6-C5H3(SiMe3)CH2C(Me)2C6H5}(Me)2][B(C6F5)4] [Ti{5:6-C5H3(SiMe3)CH2C(Me)2C6H3Me2}(Me)2][B(C6F5)4] [Ti(5:6-CpAr)(CO)2][B(C6F5)4] [Ti{5:6-C5H4C(Me)2C6H3Me2}(CO)2][B(C6F5)4]

–a –a 2.379(3)–2.725(3) –a –a –a –a 2.425(5)–2.735(9) 2.459(4)–2.830(4) 2.505(5)–2.668(5); 2.499(5)–2.707(5) –a –a 2.447(4)–2.760(3) –a –a –a 2.264(4)–2.478(4)

–a –a 2.6(1) –a –a –a 2.39(7)

–a –a 2.21 –a –a –a 1.93

82 82 82 82 82 82 82

140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156

a

Solid-state molecular structure not determined. Data not available.

b

The silylated metallocenes [M(Cp00 )(R)3] (Cp00 ¼ 1,3-C5H3(SiMe3)2; M ¼ Zr, R ¼ Me; M ¼ Hf, R ¼ Me, Et) undergo identical reactivity with [B(C6F5)3] in toluene, producing the metal-toluene ion pair [M(Cp00 )(R)2(6-C6H5Me)][(Me)B(C6F5)3] (M ¼ Zr, R ¼ Me 103; M ¼ Hf, R ¼ Me 104, Et 105) (Scheme 24).60 Structural elucidation of 104 by SCXRD reveals a bent sandwich structure with an 6-coordinated toluene (Table 2). Compound 105 is thermally sensitive, giving gas evolution upon warming to −20  C, eventually producing the ethylene-bridged hafnium-toluene metallocene dimer [{Hf(Cp00 )(Et)(6-C6H5Me)}2(m-2:2-C2H4)]2+ 106 as assigned through NMR spectroscopic analysis (Scheme 24).

518

Arene Complexes of the Group 4 Metals

Scheme 24 Alkyl abstraction from [M(Cp00 )(R)3] (M ¼ Zr, Hf ).

Applying this approach to [Zr(Cp∗)(CH2Ph)3] with [B(C6F5)3] in halogenated aromatic solvent provided the cationic benzyl complex [Zr(Cp∗)(7-CH2C6H5)(3-CH2C6H5)][(C6H5CH2)B(C6F5)3] 107 (Scheme 25). Initially, in the absence of solid-state structural data, the compound was mistakenly formulated as the zwitterionic species [Zr(Cp∗)(CH2Ph)2][(6-C6H5CH2)B(C6F5)],62 but SCXRD characterization would later reveal 107 to consist of a discrete cation/anion pair featuring an unusual 7-coordination of a benzyl ligand to zirconium (Table 2).61 Oddly, in the solid-state, the 7-benzyl ligand exhibits clear structural distortion, giving way to a boat-type ring puckering. Theoretical analysis of the molecular orbitals of 107 by the extended Hückel method indicates the ring distortion results from significant arene p-donation of the benzyl ligand to the highly electrophilic zirconium center. Nonetheless, treating solutions of 107 with propene leads to the single insertion product [Zr(Cp∗)(CH2Ph){CH2CH(Me)CH2(6-C6H5)}][(C6H5CH2)B(C6F5)3] 108, showing a parallel with the reactivity pattern of 84 with propene in its formation of 85, with the solid-state molecular structure of 108 confirmed by SCXRD analysis. Attempts to insert a second equiv. of propene were unsuccessful; however, 107 catalyzes ethylene polymerization at a rate of 6  103 g polyethylene
(mol of Zr)−1
h−1
atm−1.

Scheme 25 Formation of heptahapto 107 and its propene insertion reactivity.

4.09.4.3.3

Amides

The study of alkyl-abstracted, Group 4 metal-arene compounds eventually expanded beyond hydrocarbyl and metallocene species to incorporate other ligand systems. Horton and co-workers synthesized the diamidoamines [Zr(R)2{Me3SiN(CH2CH2NSiMe3)2}] (R ¼ Me, CH2Ph) by reacting [Zr(Cl)2{Me3SiN(CH2CH2NSiMe3)2}]2 with MeLi or [Mg(CH2Ph)2(dioxane)0.5] in toluene.63 While the reaction of [Zr(Me)2{Me3SiN(CH2CH2NSiMe3)2}] with [B(C6F5)3] in C6D5Br gives methyl-bridged [Zr{Me3SiN (CH2CH2NSiMe3)2}(Me)(m-Me)B(C6F5)3], similar treatment of [Zr(CH2Ph)2{Me3SiN(CH2CH2NSiMe3)2}] with [B(C6F5)3] forms the 2-coordinated intermediate [Zr(2-CH2Ph){Me3SiN(CH2CH2NSiMe3)2}][(C6H5CH2)B(C6F5)3] (Scheme 26). After 16 h, the 2-coordinated intermediate transforms to give the cyclometallated zwitterionic complex [Zr{(6-C6H5CH2)B(C6F5)3} {Me3SiN(CH2CH2NSiMe2CH2)(CH2CH2NSiMe3)}] 109. Preliminary polymerization studies of 109 with ethylene revealed moderate catalytic activity.

Scheme 26 Alkyl abstraction from a zirconium amide.

Arene Complexes of the Group 4 Metals

519

Fig. 4 Zwitterionic zirconium-arene complexes 110 and 111.

Horton et al. also explored the alkyl-abstraction reactivity of the zirconium amido complexes [Zr(CH2Ph)2{(SiMe3)2N}2] and [Zr(CH2Ph)2(Me2SiNtBu)2] with [B(C6F5)3]. Successful benzyl abstraction leads to the formation of the metal-arene adducts [Zr{(6-C6H5CH2)B(C6F5)3}{(SiMe3)2N}(k2-Me3SiNSiMe2CH2)] 11064 and [Zr(CH2Ph)(Me2SiNtBu)2][(3-C6H5CH2)B(C6F5)3] 11165 (Fig. 4), but in the case of 110 cyclometallation of the silylamido group is observed with the formation of toluene. As compared to other structurally characterized metal-arene complexes formed from alkyl-abstraction, the solid-state molecular structure of 111 is unusual in that the zirconium-arene distances are inequivalent in the zwitterionic complex (Table 2), with the ring described as possessing trihapto coordination.65 While the catalytic activity of 111 was not described, 110 was shown capable of polymerizing ethylene with low activity and was shown to be inactive towards propylene. It should be noted that the use of aromatic solvents with cationic Group 4 complexes can suppress catalytic performance. For instance, McConville and co-workers observed that [Ti(Me)2{RN(CH2)3NR}] (R ¼ Dipp; Dipp ¼ 2,6-iPr2C6H3) catalyzes the polymerization of 1-hexene in the presence of MAO; however, when toluene is present, a 400-fold reduction in catalytic activity is observed.66 This prompted the authors to suggest the intermediate formation of a metal-arene complex of the type [Ti(Me) (6-C6H5Me){RN(CH2)3NR}]+ 112, wherein the coordinated toluene competes with hexene binding. Similarly, [Ti(Me)2 {RN(CH2)3NR}] (R ¼ 2,6-C6H3Me2) reacts with [B(C6H5)3] to catalyze the living aspecific polymerization of a variety of a-olefins at room temperature in THF or CH2Cl2, but its performance is greatly diminished in toluene, again implicating the competitive formation of 112.67 In this study, the authors measured the time dependence of 1-hexene polymerization in a 50:50 mixture of toluene:1-hexene and observed a linear relationship between the mean molecular weight versus time. The same time dependence experiment was performed, though in a 33:67 ratio (toluene:1-hexene), and showed a deviation from linearity after 30 min. Tilley and co-workers reported that the zirconium bis(silylamido) complexes [{Zr(Cl)2(DADMB)}2] (DADMB ¼ [2,20 t ( BuMe2SiN)2-6,60 -Me2(C6H3)2]2−) and [Zr(Cl)2(DMBN)⸱THF] (DMBN ¼ [2,20 -(tBuMe2SiN)2-1,10 -(C10H6)2]2−) exhibit modest polymerization activity with 1-hexene in the presence of MAO.68 Reaction of [Zr(Cl)2(DMBN)⸱THF] with 2 equiv. of [K(CH2Ph)] gives [Zr(CH2Ph)2(DMBN)], which reacts with [B(C6F5)3] in C6H6 to afford the zwitterion [Zr(CH2Ph)(DMBN){(6-C6H5CH2)B(C6F5)3}] 113 (Scheme 27). Compound 113 was structurally characterized by SCXRD and was shown to react with ethylene at room temperature, forming an insertion product as assigned by 1H NMR spectroscopic characterization, but failing to polymerize the olefin. On the other hand, in situ treatment of [Zr(CH2Ph)2(DMBN)] with [Ph3C][B(C6F5)4] in the presence of 1-hexene produces a low molecular weight polymer, though this system polymerizes ethylene with comparable activity to [{Zr(Cl)2(DADMB)}2] and [Zr(Cl)2(DMBN)⸱THF].

Scheme 27 Formation of 113 by benzyl abstraction.

The electronic character and steric profile of the amido ligands seem to play a crucial role in the polymerization activity and stability of Group 4 metal-arenes. Arnold and co-workers reported the synthesis of the diamido zirconium dibenzyl complex [Zr(CH2Ph)2{1,10 -Fc(NSiMe3)2}], where the ferrocene backbone provides enhanced structural rigidity.69 This compound shows divergent reactivity with the Lewis Acids [Ph3C][B(C6F5)4] and [B(C6F5)3], giving [Zr(CH2Ph){1,10 -Fc(NSiMe3)2}][B(C6F5)4] with the former and the metal-arene [Zr(CH2Ph){1,10 -Fc(NSiMe3)2}{(6-C6H5CH2)B(C6F5)3}] 114 with the latter (Scheme 28). Compound 114 was unambiguously characterized by SCXRD analysis (Table 2), and NMR spectroscopic studies of 114 indicate that the metal-arene contact is maintained in C6D6 solution. Addition of ethylene or acetylenes to 114 initially results in the formation of the mono insertion products [Zr(CH2CH2CH2Ph){1,10 -Fc(NSiMe3)2]}{(6-C6H5CH2)B(C6F5)3}] 115 or [Zr {CRCRCH2(6-C6H5)}{1,10 -Fc(NSiMe3)2}{(C6H5CH2)B(C6F5)3}] (R ¼ Me 116, Ph 117, p-C6H5Me 118) (Scheme 28), respectively, formulated on the basis of NMR spectroscopic assignments. Subsequent addition of ethylene to 115 results in chain length growth to give [Zr{CH2(CH2)nCH2Ph}{1,10 -Fc(NSiMe3)2}{(6-C6H5CH2)B(C6F5)3}] 119 without the formation of polyethylene, while further treatment of 116 with 2-butyne gives the mono insertion product Me(H)C]C(Me)CH2Ph and higher oligoacetylenes upon hydrolysis (Scheme 28).

520

Arene Complexes of the Group 4 Metals

Scheme 28 Formation of ferrocenyldiamido metal-arene 114 followed by single insertions of acetylenes and ethylene.

The chelating diamido complexes [Zr(CH2Ph)2{(Me3SiN)(Me3SiNCH2)C6H4}] and [Zr(CH2Ph)2{(Ph2MeSiN)(Me3SiNCH2) C6H4}] react with [B(C6F5)3] at −80  C to produce the thermally unstable zwitterionic adducts [Zr(CH2Ph){(Me3SiN) (Me3SiNCH2)C6H4}{(6-C6H5CH2)B(C6F5)3}] 120 and [Zr(CH2Ph){(Ph2MeSiN)(Me3SiNCH2)C6H4}{(6-C6H5CH2)B(C6F5)3}] 121 (Scheme 29).70 The instability of these complexes notably contrasts with the relative stability of 113. Compounds 120 and 121 were characterized in solution at low temperatures and were thoroughly studied by NMR spectroscopic methods including 1H-1H COSY correlation and 1H-1H ROESY relaxation experiments. The spectra clearly show the benzylborate anion to be tightly p-coordinated to the zirconium through its benzyl group. Upon warming, the molecules decompose to give [Zr(C6F5)2{(Me3SiN) (Me3SiNCH2)C6H4}], [Zr(C6F5)2{(PhMe2SiN)(Me3SiNCH2)C6H4}] and [(PhCH2)2B(C6F5)] (Scheme 29).

Scheme 29 Synthesis and thermal decomposition of metal-arene diamidos 120 and 121.

Alkyl abstraction from pyrrolide and pyridyl ligands has also been shown to be an effective method for the synthesis of metal-arene complexes. For instance, [Zr(CH2Ph)3(Dip-pyr)] (Dip-pyr ¼ DippN]CH(C4H3N)) reacts with [Al(C6F5)3] to give [Zr(CH2Ph)2(Dip-pyr){(6-C6H5CH2)Al(C6F5)3}] 122 (Scheme 30).71 Interestingly, addition of [B(C6F5)3] to [Zr(CH2Ph)3(Dippyr)] leads to the expected formation of [Zr(CH2Ph)2(Dip-pyr)][(C6H5CH2)B(C6F5)3]; however, the compound is highly insoluble and thermally sensitive, preventing its fully characterization. On the other hand, reacting [Zr(CH2Ph)3(Dip-pyr)] with THF-solvated

Scheme 30 Lewis Acid reactivity in alkyl abstraction and metal-arene formation.

Arene Complexes of the Group 4 Metals

521

[Al(C6F5)3(THF)] does not lead to any observed reactivity, while addition of [B(C6F5)3(THF)] gives the ion pair [Zr(CH2Ph)2(Dippyr)(THF)][(C6H5CH2)B(C6F5)3] (Scheme 30). Polymerization studies of 122 show the compound gives atactic poly(1-hexene) with narrow molecular weight distributions, indicating a single site character in the active catalyst. Kempe and co-workers detailed that treating the zirconium and hafnium monoaminopyridinato complexes [M(CH2C6H5)3 (ArNPyr)] (M ¼ Zr, Hf; ArNPyr ¼ (C6H2R12R2)PyN(C6H2R32R4)) with [B(C6F5)3] generates the expected zwitterionic dibenzyl compounds [M(CH2C6H5)2{(6-C6H5CH2)B(C6F5)3}(ArNPyr)] (R1 ¼ R2 ¼ R3 ¼ CH(CH3)2, R4 ¼ H, M ¼ Zr 123, Hf 124; R1 ¼ CH3, R2 ¼ R4 ¼ H, R3 ¼ CH(CH3)2, M ¼ Zr 125, Hf 126; R1 ¼ R2 ¼ CH(CH3)2, R3 ¼ R4 ¼ CH3; M ¼ Zr 127, Hf 128) (Scheme 31) possessing metal-arene contacts that are observed in the solid-state and maintained in solution.72 Further addition of [B(C6F5)3] to 123 or 124 is unsuccessful in abstracting a second benzyl ligand. Both 123 and 124 exhibit poor catalytic activity at room temperature but are described as having moderate activity at 50  C. The authors posited that the metal-arene contact is too strongly bound and inhibits the polymerization activity of 123 and 124 by blocking coordination sites at the metal. To circumvent this, the in situ activation of [M(CH2C6H5)3(ArNPyr)] (M ¼ Zr, Hf ) with [(R)2(Me)NH][B(C6F5)4], via protonation of a benzyl ligand, enhances the catalytic polymerization performance of these systems in the presence of ethylene.

Scheme 31 Formation of zwitterionic complexes 123–128.

4.09.4.3.4

Aryloxides

By similar methods, the pyridyl-pincer metal-arene complexes [M(CH2Ph){(6-C6H5CH2)B(C6F5)3}(ONOSiMe2tBu)] (ONOSiMe2tBu ¼ [2,6-(O-C10H4SiMet2Bu)2C6H3N]−2) (M ¼ Ti 129, Zr 130) are produced via the abstraction of a benzyl ligand by [B(C6F5)3] in toluene (Scheme 32).73 In contrast to many of the Group 4 metal-arene compounds presented here, including 130, 129 is observed to dissociate in solution, giving rise to outer-sphere ion pairs. Variable temperature, heteronuclear NMR spectra of 129 in toluene-d8 shows a shift between inner-sphere ion pairing and outer-sphere ion pairing as a function of temperature on the peak broadening and chemical shift values of the resonances. DFT analysis corroborates this assessment, revealing a complex dissociation mechanism. The disparity between the solution phase behavior of 129 and 130 is partially explained by the larger ionic radius of zirconium, which allows for greater rotational freedom of the p-coordinated benzyl group, thus disfavoring disassociation pathways. Finally, 129 and 130 were found to be ineffective at polymerizing propylene or styrene.

Scheme 32 Formation of pyridyl-aryloxide metal-arene complexes 129 and 130.

Group 4 alkyl aryloxides have also been shown amenable to alkyl abstraction for the synthesis and isolation of metal-arene adducts. Rothwell and co-workers demonstrated that reacting the sterically encumbering aryloxides [M(CH2PhR)2(ArO)2] (M ¼ Ti, Zr) with [B(C6F5)3] in benzene forms the zwitterionic products [M(CH2PhR){(6-C6H4RCH2)B(C6F5)3}(ArO)2] (ArO ¼ 2,6-Ph2-3, 5-Me2C6HO, R ¼ H, M ¼ Ti 131, Zr 132; ArO ¼ 2,6-Ph2C6H3O, R ¼ H, M ¼ Ti 133; ArO ¼ 2,6-Ph2C6H3O, R ¼ Me, M ¼ Ti 134; ArO ¼ 2,6-tBu2C6H3O, R ¼ H, M¼Ti 135) (Scheme 33).74,75 In the case of [Zr(CH2Ph)2(ArO)2] (ArO ¼ 2,6-tBu2C6H3O), alkyl abstraction by [B(C6F5)3] is followed by cyclometallation via deprotonation of a pendant tert-butyl group by the remaining benzyl ligand to give [Zr(ArO){2-tBu2-4-(CH2Me2C)C6H3O}{(6-C6H5CH2)B(C6F5)3}] 136 (Scheme 33).59 NMR spectroscopic analysis of the products unsurprisingly shows a correlation with an increase of restricted rotation of the coordinated arene in solution with increasing bulkiness of the aryloxide ligand. SCXRD structural characterization of 131–133 show the molecules to adopt three-legged piano stool geometries. Reaction of 135 with phenylpropyne or propylene and reaction of 131 or 135 with allylbenzene or 1-hexene leads to the spectroscopically detected formation of insertion products. This results in displacement of

522

Arene Complexes of the Group 4 Metals

the coordinated benzylborate anion and formation of [Ti(ArO)2{C(Me)C(Ph)CH2(6-C6H5)}][(C6H5CH2)B(C6F5)3] (ArO ¼ 2, 6-Ph2-3,5-Me2C6HO 137; ArO ¼ 2,6-Ph2C6H3O 138; ArO ¼ 2,6-tBu2C6H3O 139) and [Ti(ArO)2{CH2CH(R)CH2(6-C6H5)}] [(C6H5CH2)B(C6F5)3] (ArO ¼ 2,6-Ph2-3,5dMe2C6HO, R ¼ CH2Ph 140, Me 141, Bu 142; ArO ¼ 2,6-tBu2C6H3O, R ¼ CH2Ph 143, Me 144, Bu 145), which feature intramolecular metal-arene contacts (Scheme 34).74,75 Compounds 141 and 144 are reported to very slowly oligomerize propylene while 142 and 145 are observed to isomerize 1-hexene to 2-hexene and 3-hexene.58

Scheme 33 Alkyl abstraction from titanium and zirconium aryloxides.

Scheme 34 Insertion reactivity of titanium metal-arene aryloxide complexes with acetylenes.

The use of bulky alkoxides was also extended to the bowl-type silyloxides in [Zr(CH2Ph)2(TRMSO)2] (TRMSO ¼ {OSi[3, 5-(2,6-Me2C6H3)C6H3]3}−) with [B(C6F5)3] to yield [Zr(CH2Ph){(6-C6H5CH2)B(C6F5)3}(TRMSO)2] 146 (Scheme 35).76 The compound was structurally identified by SCXRD characterization (Table 2), showing a crowded zwitterionic complex possessing p-coordination of the borate benzyl group to zirconium. Despite the steric pressure, the NMR spectroscopic analysis suggests the metal-arene contact is preserved in solution.

Scheme 35 Formation of aryloxide metal-arene complex 147.

4.09.4.4

Ansa-arenes

Cp-rings decorated with ancillary side chains possessing terminal donor groups have been used in early-metal chemistry for the development of Ziegler-Natta polymerization catalysts.77 In principle, ligation of these pendant groups modulates the electronic

Arene Complexes of the Group 4 Metals

523

characteristics of the metal while offering hemilability to provide open coordination sites during the catalytic cycle. In the 1990s, a series of half-sandwich Group 4 metallocene complexes with pendant ansa-arene groups began to appear. The first reported example was the titanium complex [Ti(Me)2(5:6-C5Me4CH2CH2C6H5)][B(C6F5)4] 147, formed from the reaction of [Ti(Me)3(5-C5Me4 CH2CH2C6H5)] with [Ph3C][B(C6F5)4].77 Abstraction of the methyl group generates a cationic titanium center that is capped via intramolecular p-coordination of the dangling aryl substituent. In the absence of a solid-state molecular structure, the coordination geometry of 147 was assigned based upon NMR spectroscopy, where the coordinated aryl group appears tightly bound to titanium. Compound 147 is thermally unstable in CH2Cl2 solutions upon warming above 0  C, but when prepared in situ in the presence of ethylene and excess triisobutylaluminum, it forms polyethylene, albeit poorly. Bochmann, Sassmannshausen and others subsequently reported a large assortment of Group 4 half-sandwich compounds varying in their Cp-arene chain linkages, namely [Ti(5-C5H4R)(Cl)3] (R ¼ C(Me)2Ph, C(Me)2CH2Ph, Si(Me)2Ph, CHPh2) and [Zr(5-C5H4R)(Cl)3(DME)] (R ¼ C(Me)2CH2C6H4R0 ; R0 ¼ H, Me).78,79 Methylation of these compounds followed by treatment with [B(C6F5)3] or [Ph3C][B(C6F5)4] in CH2Cl2 gives [Ti{5:n-C5H4C(Me)2Ph}(Me)2][B(C6F5)4] 148, [M{5:n-C5H4 C(Me)2CH2Ph}(Me)2][X] (M ¼ Ti, X ¼ [B(C6F5)4] 149; M ¼ Ti, X ¼ [(Me)B(C6F5)3] 150; M ¼ Zr, X ¼ [B(C6F5)4] 151; M ¼ Zr, X ¼ [(Me)B(C6F5)3] 152), and [Ti(5:n-C5H4CHPh2)(Me)2][B(C6F5)4] 153 (Scheme 36).78 In the case of [Zr(n-C5H4C (Me)2CH2C6H4Me)(Me)3], addition of [Ph3C][B(C6F5)4] affords [Zr(5:n-C5H4C(Me)2CH2C6H4Me)(Me)2][(Me)B(C6F5)3] 154, while treatment with 1.5 equiv. of B(C6F5)3 gives [Zr(5:n-C5H4C(Me)2CH2C6H4Me)(Me)2][(Me)B(C6F5)3] 155 and [Zr(5:nC5H4C(Me)2CH2C6H4Me)(Me){(m-Me)B(C6F5)3}][(Me)B(C6F5)3] 156 (Scheme 37).79 Compounds 148–156 were generated in solution and characterized by low temperature NMR spectroscopy, showing a low-field shift of the hydrogens from the pendant phenyl ring and a separation of the signals corresponding to the Cp rings, indicating an increase in chelate ring strain. Accordingly, these data are consistent with intramolecular p-coordination of the ansa-arene ring, though the hapticity was not determined. Additionally, 153 exhibits fluxional character in solution, leading the authors to suggest an equilibrium between ansa-arene and 1/2-benzyl coordination modes (Scheme 36). Similarly, 154 also exhibits complicated solution phase behavior where NOESY/ EXSY NMR spectroscopy support ZrdMe for Zr-(m-Me)dB coordination exchange.79 In all cases here, these compounds were shown to be ineffective for olefin polymerization.

Scheme 36 Lewis Acid induced intramolecular arene coordination of ansa-arene ligands.

Scheme 37 Formation zirconium ansa-arene complexes 154–156.

524

Arene Complexes of the Group 4 Metals

Through minor ligand modifications, Hessen and co-workers successfully synthesized the ansa-arene complex [Ti{5:6-C5H4C(Me)2C6H3Me2}(Me)2][(Me)B(C6F5)3] 157 through alkyl abstraction of a methyl group by [B(C6F5)3] (Scheme 38).80 The NMR spectra of 157 are consistent with p-coordination of the pendant aryl group in solution; however, addition of THF is seen to rapidly displace the arene to give [Ti{5-C5H4C(Me)2C6H3Me2}(Me)2(THF)x][(Me)B(C6F5)3]. Attempts to further abstract a methyl ligand from 157 with [B(C6F5)3] fails to generate a dicationic species, but instead, leads to ligand scrambling forming [(Me)B(C6F5)2] and [Ti{5:6-C5H4C(Me)2C6H3Me2}(Me)(C6F5)][(Me)B(C6F5)3] 158 (Scheme 38), reminiscent of the scrambling observed in the decomposition of 120 and 121. Standing solutions of 158 in C6D5Br produces crystals of the Ti(III) ansa-arene [{Ti(5:6-C5H4C [Me]2C6H3Me2)(m-Br)}2][{B(C6F5)4}2] 159 (Scheme 38). The formation of 159 is not well understood, but SCXRD characterization reveals 6-coordination of the phenyl ring with an acute Cp-arene backbone bend angle of 95.8(3) having a centroid to centroid angle of Cpcent-Ti-Arcent ¼ 125.0 .

Scheme 38 Synthesis of ansa-arene 158 and its observed reactivity.

Building upon this, Hessen and co-workers demonstrated the character of the alkyl substituents on the metal play a key role in its stability and reactivity.81 Study of the benzylated analogs of 157 show a range of solution phase fluxionality and cyclometallation chemistry. For instance NMR spectroscopic studies of [Ti{5:6-C5H4C(Me)2C6H3Me2}(CH2Ph)2][X] (X ¼ (PhCH2)B(C6F5)3 160, B(C6F5)4 161) show competitive ring binding in solution comparable to the ansa-arene and 1/2-benzyl exchange observed for 153. On the other hand, treating C6D5Br solutions of [Ti{5:6-C5H4C(Me)2C6H5}(CH2Ph)3] with [B(C6F5)3] at −30  C results in rapid cyclometallation to give the zwitterionic metal-arene [Ti{5:k1-C5H4C(Me)2C6H4}(CH2Ph){(6-C6H5CH2)B(C6F5)3}] 162 (Scheme 39). Upon switching to [Ph3C][B(C6F5)4], the cyclometallated ion pair [Ti{5:k1-C5H4C(Me)2C6H4}(7-CH2Ph)] [B(C6F5)4] 163 is instead formed (Scheme 39). Compared to the more stable 157, the authors propose the benzyl ligands weaken the pendant arene coordination through competitive n-binding, leading to the observed cyclometallation.

Scheme 39 Lewis acid induced cyclometallation to give 162 and 163.

Hessen further expanded this chemistry to the sizeable array of Group 4 ansa-arene complexes, 166–171, as shown in Scheme 40, with structural confirmation through SCXRD analysis (Table 2).82 Importantly, variable temperature NMR spectroscopy shows the methylene linked Cp-arene ligands to exhibit the greatest hemilability in solution, correlating with their ethylene trimerization activity. Interestingly, 164–171 react with 1 bar of CO to give the cationic dicarbonyls [Ti(5:6-CpAr)(CO)2][B(C6F5)4] 172 (Scheme 40) as the major products. Structural elucidation of [Ti{5:6-C5H4C(Me)2C6H3Me2}(CO)2][B(C6F5)4] 173 reveals a number of irregularities within the coordinated arene ring, primarily distortion from planarity with CdC distances indicative of localized p-bonds. These bond metrics are consistent with a reduced-arene, Ti(IV) canonical form; however, the carbonyl CdO bond lengths are identical to bonds found in d2 metallocene titanium-carbonyls. Conversely, the CO stretching frequencies for 173 are intermediate between the values for free CO and Ti(II)dCO compounds. Based on these observations, the authors propose an intermediate Ti(II)/Ti(IV) formulation with major tetravalent resonance contributions.

Arene Complexes of the Group 4 Metals

525

Scheme 40 Synthesis of ansa-arenes 164–171 and their reactivity with CO.

Unsurprisingly, the olefin polymerization activity of Group 4 ansa-arene metallocenes has been extensively studied, including investigation into the catalytic mechanisms of these cycles.83–86 Notably, these metal-arene systems have been found to both trimerize and polymerize olefins based upon the features of the pendant arene group and the length of the Cp-arene linker.83–85 These catalytic cycles are further complicated by the possibility of b-hydride elimination. A proposed cycle for the oligomerization of ethylene to 1-hexene is shown in Scheme 41, illustrating the importance of the hemilabile pendant arene group in accommodating the coordination and insertion of the olefin substrate.85

Scheme 41 Proposed oligomerization of ethylene to 1-hexene by Group 4 ansa-arenes.

526

Arene Complexes of the Group 4 Metals

4.09.5

Metal vapor synthesis

4.09.5.1

Homoarenes

4.09.5.1.1

Bis(arene)titanium

Building upon the work of Timms in his landmark synthesis of [Cr(6-C6H6)2] using vapor deposition methods,87 Green and co-workers applied this technique to access previously unknown, highly electron rich, early transition metal bis(arene) complexes on gram scales.88,89 In essence, electron beam bombardment of a metallic rod produces metal vapors inside a spherical glass reaction vessel in an atmosphere of arene vapor. Metal-arene combination and co-condensation are promoted by rotation of the reaction vessel and partial cooling of the apparatus to cryogenic temperatures. The condensed material is subsequently extracted, dissolved, and recrystallized. Through this method, Green described the reaction of titanium with benzene, which gives burgundy-red crystals of [Ti(6-C6H6)2] 174 in 22% yield (Scheme 42).89 Crystals of 174 are highly air sensitive, but in air-free environments, are stable at room temperature for several months. The material sublimes under vacuum with heating but decomposes upon warming to 80  C. Subsequently, Green and co-workers reported the vapor condensation of titanium with toluene and mesitylene to generate the formally zerovalent compounds [Ti(6-C6H5Me)2] 175 and [Ti(6-C6H3Me3)2] 176 in approximately 30% yields (Scheme 42).88 The chemical compositions of 174–176 were confirmed through NMR, IR, and photoelectron spectroscopies and mass spectrometry, where the formulation suggested a sandwich structure analogous to bis(benzene)chromium. The proposed sandwich-type conformations were validated a few years later by Borod’ko and co-workers, who reported the solid-state molecular structures of 17490 and 17691 along with their electronic absorption spectra and vibrational data. The structure of 174 displays TidCring bond lengths of 2.238(1)–2.248(1) A˚ and a titanium-centroid distance of TidCcent ¼ 1.74 A˚ , while 175 adopts an eclipsed conformation with TidCring bond distances of 2.237(6)–2.267(6) A˚ and TidCcent ¼ 1.74 A˚ (Table 3). Notably, the TidCcent distances here are significantly shorter than the titanium-centroid distances of the metal-arenes formed from Fischer’s method (e.g., 13, TidCcent ¼ 2.06 A˚ ) (Table 1).

Scheme 42 Vapor synthesis of bis(arene) titanium complexes 174–176.

Table 3

Group 4 metal-arene interactions in complexes synthesized by metal vapor methods.

Complex number

Molecular formula

˚) Arene M-C dist. (A

˚) Mean Arene M-C dist. (A

˚) M-Arene centroid dist. (A

Reference number

174 175 176 177 178 179 180 181

[Ti(6-C6H6)2] [Ti(6-C6H5Me)2] [Ti(6-C6H3Me3)2] [Ti{(6-C6H4)(C6H4)(CH2)}2] [Ti(6-iPr3C6H3)2] [Ti(6-iPr3C6H3)2][BPh4] [Ti(6-iPr3C6H3)2][B(p-C6H4F)4] [Ti(6-iPr3C6H3)2][B {3,5-C6H3(CF3)2}4] [K(THF)x] [Ti(6-C6H6)2] [K(THF)x][Ti(6-C6H5Me)2] [Hf(6-C6H6)2(PMe3)] [Hf(6-C6H5Me)2(PMe3)] [Zr(6-C6H5Me)2(PMe3)] [Ti(6-tBu3C6H3)2] [Zr(6-tBu3C6H3)2] [Hf(6-tBu3C6H3)2] [Hf(6-tBu3C6H3)2(CO)] [Ti(6-2,6-Me2C5H3N)2] [Ti(6–2,4,6-tBu3C5H2N)2] [Ti(6–2,4,6- tBu3C5H2P)2] [K][Ti(6–2,4,6-tBu3C5H2N)2] [K][Ti(6–2,4,6- tBu3C5H2P)2] [Ti(6-C5H5As)2]

2.238(1)–2.248(1) 2.237(6)–2.267(6) –a –a –a –a 2.25(2)–2.34(2) –a

2.243(3) 2.25(1) –a –a –a –a 2.28(2) –a

1.74 1.74 –a –a –a –a 1.80 –a

89,90,92,93 88,91,92,94 88,92,95 96 97 97 97 97

–a –a –a –a –a –a –a –a –a 2.229(2)–2.291(2) –a –a –a –a 2.25(2)–2.29(1)

–a –a –a –a –a –a –a –a –a 2.25(3) –a –a –a –a 2.3(3)

–a –a –a –a –a –a –a –a –a 1.74 –a –a –a –a 1.72

98 98 99 99 99 100 100 100 100 101,102 103 103 103 103 104

182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 a

Solid-state molecular structure not determined.

Arene Complexes of the Group 4 Metals

527

Fig. 5 Titanium sandwich complexes 177–178.

Using vapor deposition methodology, Cloke, Ferreira da Silva, and co-workers generated the bis(fluorene) complex [Ti{(6-C6H4)(C6H4)(CH2)}2] 177 (Fig. 5) as a dark-green solid, primarily characterized through 1H NMR spectroscopy and mass spectrometry.96 Structural characterization was not provided, but the titanium is proposed to be sandwiched between the six-membered rings of the fluorene groups without indenyl character. Pampaloni, Green, and others reported the vapor deposition synthesis of [Ti(6-iPr3C6H3)2] 178 (Fig. 5) featuring the bulky tri(isopropyl)benzene ligand, showing success for heavier, sterically encumbering arenes.97 Indeed, vapor deposition is an effective method for the production of bis(arene)titanium complexes as Cloke and Green have reported the synthesis of these compounds in product yields of up to 20 g per run.105 Unsurprisingly, these bis(arene) complexes are highly reducing and provide access to low-valent titanium. The reactivity of 175 with I2,106 [Ti(Cl)4],107 CX3COOH (X ¼ H, F),107 and CO2108 leads to loss of the arene ligand and formation of [Ti(I)4], [Ti(Cl)3], [Ti(O2CCX3)3] (X ¼ H, F), and titanium-oxalate complexes, respectively (Scheme 43). Furthermore, reaction with [Al(Cl)3] yields the monoarene-adduct 3 (Scheme 43).107

Scheme 43 Reactivity of 175.

Treating toluene suspensions of 178 with [Fc][B(Ar)4] produces the Ti(I) cations [Ti(6-iPr3C6H3)2][B(Ar)4] (Ar ¼ Ph 179, p-C6H4F 180, 3,5-C6H3(CF3)2 181) in good yields (Scheme 44).97 The stability and solubility of the products is highly dependent on the nature of the counter anion as 179 decomposes upon heating in aromatic hydrocarbons, while 181 is stable in boiling toluene and can be briefly handled in air. These results counter the electrochemical studies of 174 by Elschenbroich and co-workers who found the Ti0/+1 redox event to be chemically irreversible,104 thus suggesting the steric profile and ion selection are critical for the stability of Ti(I) bis(arene) compounds. Conversely, Green and co-workers explored the reduction chemistry of 174 and 175 with potassium and potassium hydride, obtaining dark blue crystals of [K(THF)x][Ti(6-C6H6)2] 182 and [K(THF)x] [Ti(6-C6H5Me)2] 183 (Scheme 45).98 These products were claimed as the first examples of isolable Ti(-I) compounds, with their formulations based upon the similarity of the EPR spectra of 177 and 178 with [V(6-C6H6)2] and [Nb(6-C6H6)2]. Of note, the formation of 182 and 183 matches previous observations by Hawker and Timms when describing the dark blue solutions formed from their reaction of [Ti(Cl)4] with excess potassium in benzene.94

Scheme 44 Cationic titanium sandwich complexes 179–181.

528

Arene Complexes of the Group 4 Metals

Scheme 45 Anionic titanium sandwich complexes 182–183.

4.09.5.1.2

Bis(arene)zirconium and bis(arene)hafnium

Vapor deposition techniques are also effective for the synthesis of heavier Group 4 bis(arene) congeners, though in these cases, co-ligands may be required.99,105 In 1979, Cloke and Green described the condensation of hafnium vapor with benzene and toluene in the presence of excess PMe3 to give 18-electron [Hf(6-C6H6)2(PMe3)] 184 and [Hf(6-C6H5Me)2(PMe3)] 185 (Scheme 46), respectively.99 This strategy also works for the synthesis of [Zr(6-C6H5Me)2(PMe3)] 186 (Scheme 46). Solutions of 184 and 185 are indefinitely stable under argon atmospheres but decompose within minutes under dinitrogen. Attempts to sublime 184 under vacuum at 70  C also leads to decomposition. The 16-electron, homoleptic bis(arenes) of zirconium and hafnium can be accessed through the use of arenes with bulky substituents as, in 1987, Cloke and co-workers reported the isolation and characterization of [M(6-tBu3C6H3)2] (M ¼ Ti 187, Zr 188, Hf 189), described as deep red, deep green, and deep purple solids going from titanium to hafnium (Scheme 47).100 As opposed to 184, 187–189 are sublimable under vacuum from 80–100  C with minimal decomposition, stressing the importance of the bulky tris(tert-butyl)benzene in the stabilization of these compounds. Compound 189 reacts with CO to give the 18-electron carbonyl adduct [Hf(6-tBu3C6H3)2(CO)] 190 (Scheme 47) with a strong IR stretch at 1870 cm−1.

Scheme 46 Bis(arene) zirconium and hafnium sandwich complexes 184–186.

Scheme 47 Homoleptic Group 4 bis(arene) complexes 187–189 and reactivity of 189 with CO.

4.09.5.1.3

Hybrid vapor deposition

Hybrid methods combining both vapor condensation and solution-phase chemistry have been described for the synthesis of metal-arene complexes. Timms and co-workers showed in 1977 the condensation of titanium vapor with 1-methylnaphthalene in THF gives a deep red compound, which was suggested to be [Ti(n-C10H7Me)2];109 however, the material is thermally unstable and characterization was not performed at the time, though later experiments would provide support for this formulation.110 In a modified method, 175 and 176 can be produced by condensing potassium vapor into THF solutions of [Ti(Cl)3(THF)3] with the respective arene at −110  C.94,95 Condensation of titanium vapor into liquid poly(methylphenylsiloxanes) produces a pink substance tentatively formulated as a bis(arene)titanium species. These polysiloxane compounds are highly air sensitive but thermally stable while showing mild activity for the polymerization of butadiene.111

Arene Complexes of the Group 4 Metals

4.09.5.1.4

529

Vapor deposition compounds in catalysis

Compounds 174 and 175 exhibit catalytic activity towards alkene and alkyne polymerization in the presence of trace amounts of oxygen.90,91 This result later motivated others to explore the ethylene polymerization activity of 175 under purposeful addition of O2112 or electron acceptors like fullerene C60113 under otherwise mild conditions, leading to activities higher than those reported for Natta’s catalyst 2.112 Moreover, 174 reduces acetylene to generate ethylene, ethane, and methane while simultaneously co-polymerizing the acetylene and in situ-produced ethylene to give a black, poorly defined polymeric product.114 Bis(arene) titanium complexes, synthesized through vapor deposition methods, have also been utilized as catalysts, co-catalysts, or precursors for the oligomerization and polymerization of olefins.115–119 For instance, condensation of titanium-arenes on dehydroxylated, fumed alumina are effective catalysts for the polymerization of olefins to give high molecular weight polyethylene.118 These metal-arene sandwich complexes can also be seen as sources of elemental titanium. In this regard, it has been demonstrated that coating mesoporous alumina membranes with 175 results in the formation of elemental titanium that quickly undergoes oxidation to TiO2 nanoparticles.120 These compounds have also been shown capable of reducing mesoporous titanium and tantalum oxide frameworks to effect the reductive cleavage of N2 to afford thin nitride coatings capable of producing ammonia upon exposure to water.121–123

4.09.5.2

Heteroarenes

The co-condensation of Group 4 metal vapors with heteroarenes has proven a reliable method for the synthesis of homoleptic, heteroarene sandwich compounds. Given the ambidentate character of heteroarenes, these complexes are important for understanding the possible coordination modes of these ligands with low valent early-metals and the effects on their electronic structures. Muetterties and Wucherer reported the first such example in the vapor deposition of titanium with lutidine to give [Ti(6–2,6-Me2C5H3N)2] 191 (Fig. 6).101 Compound 191 is produced in miniscule yield of 1%; nevertheless, its structure was elucidated through SCXRD analysis, proving the p-coordination mode of the lutidine rings.102 The titanium-centroid distance TidCcent ¼ 1.74 A˚ for 191 is within the range found for 174 (Table 3), while the rings adopt an anticlinal conformation exhibiting a slight arene-arene tilt angle of 6 degrees. Interestingly, this runs counter to the eclipsed conformations typically observed for bis(arenes) (e.g., 174 and 175) of Group 4 and is rationalized by the poorer p-donating character of pyridine, leading to weaker metal-arene orbital overlap, thereby influencing the cylindrical symmetry of the ligand frontier orbitals.101,102 Cloke and co-workers later extended these heteroatom systems to [Ti(6–2,4,6-tBu3C5H2N)2] 192 and [Ti(6–2,4,6t Bu3C5H2P)2] 193 (Fig. 6), employing the bulky arenes 2,4,6-tri-tert-butylpyridine and 2,4,6-tri-tert-butylphosphorin.103 The solid-state molecular structures of 192 and 193 have not been characterized due to significant crystallographic disorder, but NMR spectroscopy shows the rings to adopt a static synclinal conformation at room temperature with free rotation occurring only upon warming to 330 K. This conformation differs from [M(2,4,6- tBu3C5H2P)2] (M ¼ V, Cr), which adopt both synclinal and periplanar geometries. The preferential synclinal conformations of 192 and 193 was reasoned on the basis of greater metal-arene d/ p-orbital overlap as compared to the vanadium and chromium analogs. The electrochemical properties of 192 and 193 were investigated using cyclic voltammetry. In the case of 192, the voltammogram displays two irreversible oxidations and one quasi-reversible reduction. The CV trace of 193 is qualitatively similar; yet, it exhibits greater reversibility in its first oxidation and reduction features, which is attributed to the better electron donor abilities of phosphaarene ligands.103 Exposing THF solutions of 192 or 193 to a potassium mirror results in an immediate color change and formation of new reduced species formulated as [K][Ti(6–2,4,6-tBu3C5H2N)2] 194 and [K][Ti(6–2,4,6- tBu3C5H2P)2] 195 as detected through EPR spectroscopic analysis. Following this work, the homoleptic arsenine sandwich complex [Ti(6-C5H5As)2] 196 (Fig. 6) was reported by Elschenbroich and co-workers.104 In the solid-state, the arsine rings adopt an eclipsed conformation as determined by SCXRD. The rings clearly bind through an 6-coordination mode to titanium that deviate 3.9–9º from a parallel disposition with the molecular packing arrangement pointing the arsenic atoms towards one another to form short intermolecular interactions. As opposed to the electrochemical redox character of 192 and 193, the cyclic voltammogram of 196 shows full electrochemical reversibility of the Ti0/−1 redox couple, though, its Ti+1/0 redox couple is irreversible.

Fig. 6 Homoleptic titanium heteroarene sandwich complexes 191–193 and 196.

530

Arene Complexes of the Group 4 Metals

4.09.6

Coordination through arene reduction

4.09.6.1

Homoleptic complexes

4.09.6.1.1

Bis(arenes)

The inability to access bis(arene) complexes of the Group 4 metals via Fischer’s method suggested the conventional synthesis of such species was inaccessible, pointing to vapor deposition methods as the only reliable synthetic route to these homoleptic metal-arene molecules. Yet, in 1992, Bönnemann and Korall demonstrated the bis(arenes) 174–176, and [Ti(6-C6H4Me2)2] 197 could be accessed via the treatment of [Ti(Cl)4] with [K(HBEt3)] in aromatic solvents, giving the compounds in yields ranging from 9 to 20% (Scheme 48).92 Of note, Group 4 bis(arene) complexes synthesized in this fashion have been used as charcoal-dopants for use in noble metal (Rh, Pt, Pd)-charcoal hydrogenation catalysts, helping improve the activity by mediating the uniform dispersion of the metal clusters on the solid charcoal support.124

Scheme 48 Solution phase synthesis of homoleptic bis(arene) titanium complexes 174–176 and 197.

Braunschweig and co-workers would revisit this chemistry in 2015 to study the reactivity of 174, better optimizing its yield by conducting the experiment at low temperature.93 In their work, addition of the N-heterocyclic carbene 1,3-dimethylimidazole2-ylidene (IMe) to 174 in benzene quantitatively produces the 18-electron adduct [Ti(6-C6H6)2(IMe)] 198 (Scheme 49). Alternatively, addition of n-butyl lithium to 174 in the presence of N,N,N0 ,N00 ,N00 -pentamethyldiethylenetriamine (pmdta) gives the dilithiated salt [Ti(6-C6H5Li)2
pmdta] 199 (Scheme 49). Compound 199 readily undergoes salt metathesis in the presence of the group 14 dihalides Si2Cl2Me4, [Sn2(Cl)2(tBu)4], SiCl2Me2, [Ge(Cl)2(Me)2], and [Sn(Cl)2(tBu)2] to give the ansa sandwich compounds [Ti{(6-C6H5)2(E)2(R)4}] (E ¼ Si, R ¼ Me 200; E ¼ Sn, R ¼ tBu 201) and [Ti{(6-C6H5)2(E)(R)2}] (E ¼ Si, R ¼ Me 202; E ¼ Ge, R ¼ Me 203; E ¼ Sn, R ¼ tBu 204) (Scheme 49).

Scheme 49 Reactivity of 174.

In the solid-state 200–204 are stable under inert atmospheres but are observed to slowly decompose in solution to form free ligand and black, insoluble solid. Cyclic voltammetry analysis of 200–204 shows irreversible oxidation and reduction waves for each complex. At the same time of Bönnemann’s report for the solution phase synthesis of bis(arene)titanium complexes, Ellis and co-workers described an alternative synthetic pathway inspired by Chatt’s seminal work on transition metal interactions with alkali metal arenes.5,125 Namely, treatment of [Ti(Cl)4(THF)2] with reduced potassium salts of biphenyl or 4,40 -di-tert-butylbiphenyl in THF, with gradual warming of the solution from −78  C to room temperature over 15 h, results in the formation of paramagnetic titanium anions that can be crystallized with crown ether or [2.2.2]cryptand to give [K(L)n][Ti{(6-C6H5)C6H5}2] (L ¼ 15-crown-5, n ¼ 2 205; L ¼ [2.2.2]cryptand, n¼1 206) and [K([2.2.2]cryptand)][Ti{(6–4-tBuC6H4)(40 -tBuC6H4)}2] 207 (Scheme 50).126 In addition to their novel synthetic preparation, these molecules represent the first structural representations of Group 4

Arene Complexes of the Group 4 Metals

531

bis(arene)metalates(-I) as characterized by SCXRD analysis. In the solid-state, the cations and anions are well-separated, with the biphenyl rings in 206 displaying an eclipsed conformation while the arene ligands in 207 are staggered, with this latter arrangement proposed to minimize intramolecular interactions. The titanium carbon distances range from TidC ¼ 2.257(5)–2.290(5) A˚ in 206 and TidC ¼ 2.253(5)–2.333(5) A˚ in 207 with titanium-centroid distances of TidCcent ¼ 1.78 A˚ (avg.) for 206 and TidCcent ¼ 1.79 A˚ for 207 (Table 4), which are comparable to the analogous distances found for 174 (Table 3). Treatment of 206 or 207 with 0.5 equiv. of I2 gives the neutral complexes [Ti{(6-C6H5)C6H5}2] 208 and [Ti{(6-4-tBuC6H4)(40 -tBuC6H4)}2] 209 (Scheme 50), which were characterized by NMR spectroscopy and combustion analysis. Moreover, addition of CO to solutions of 206 in THF at −70  C generates the anionic carbonyl complex [K([2.2.2]cryptand)]2[Ti(CO)6] (Scheme 50).

Scheme 50 Chatt synthesis of 205–209 and reactivity of 206–207.

Table 4

Group 4 metal-arene interactions in complexes exhibiting reduced arenes.

Complex number

Molecular formula

˚) Arene M-C dist. (A

Mean Arene ˚) M-C dist. (A

M-Arene centroid ˚) dist. (A

Reference number

197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213

[Ti(6–1,4-C6H4Me2)2] [Ti(6-C6H6)2(IMe)] [Ti(6-C6H5Li)2
(pmdta)] [Ti{(6-C6H5)2(Si)2(Me)4}] [Ti{(6-C6H5)2(Sn)2(tBu)4}] [Ti{(6-C6H5)2(Si)(Me)2}] [Ti{(6-C6H5)2(Ge)(Me)2}] [Ti{(6-C6H5)2(Sn)(tBu)2}] [K(15-crown-5)2][Ti{(6-C6H5)C6H5}2] [K([2.2.2]cryptand)][Ti{(6-C6H5)C6H5}2] [K([2.2.2]cryptand)][Ti{(6–4-tBuC6H4)(40 -tBuC6H4)}2] [Ti{(6-C6H5)C6H5}2] [Ti{(6–4-tBuC6H4)(40 -tBuC6H4)}2] [Ti(n-C10H8)2] [K(15-crown-5)2]2[Ti(n-C10H8)2] [K(15-crown-5)2][Ti(4-C10H8)2(SnMe3)] [K(15-crown-5)2][Ti(4-C10H8)2(SnMe3)2]

92 93 93 93 93 93 93 93 126 126 126 126 126 127 127 127 127

[K(18-crown-6)]2[Ti(2-C10H8)(4-C10H8)2]

–a 2.33(3) –a 2.25(1) 2.25(1) –a 2.25(3) 2.23(3) –a 2.27(1) 2.29(2) –a –a –a –a –a 2.32(1); 2.32 (1) 2.28(5); 2.32 (2); 2.32(2)

–a 1.86 –a 1.74 1.74 –a 1.74 1.73 –a 1.78 1.79 –a –a –a –a –a 2.22; 2.11

214

2.17; 2.15

128

215

[K([2.2.2]cryptand)]2[Ti(2-C10H8)(4-C10H8)2]

–a 2.279(2)–2.376(2) –a 2.237(1)–2.266(2) 2.232(4)–2.263(4) –a 2.218(3)–2.294(3) 2.196(3)–2.268(3) –a 2.257(5)–2.290(5) 2.253(5)–2.333(5) –a –a –a –a –a 2.31(2)–2.32(1); 2.30(1)–2.34(1) 2.247(4)–2.313(4); 2.288 (4)–2.370(4); 2.278(4)–2.359 (4) 2.277(8)–2.331(8); 2.279 (8)–2.336(9); 2.272(9)–2.929 (9)

2.30(4); 2.31 (3); 2.5(3)

2.12; 2.16

128

(Continued )

532

Arene Complexes of the Group 4 Metals

Table 4

(Continued)

Complex number

Molecular formula

˚) Arene M-C dist. (A

Mean Arene ˚) M-C dist. (A

M-Arene centroid ˚) dist. (A

Reference number

216 217

[K(15-crown-5)2]2[Zr(4-C10H8)3] [K([2.2.2]cryptand)]2[Zr(4-C10H8)3]

129 129

[K(18-crown-6)]2[Hf(4-C10H8)3] [K([2.2.2]cryptand)]2[Hf(4-C10H8)3]

–a 2.476(8); 2.481(4); 2.48(1) –a 2.46(2); 2.46 (2); 2.46(1)

–a 2.33; 2.35; 2.36

218 219

–a 2.33; 2.33; 2.32

128 128

220 221

[K(18-crown-6)]2[Ti(2-C14H10)(4-C14H10)2] [K(18-crown-6)]2[Zr(4-C14H10)3]

–a 2.49(2); 2.49 (2); 2.49(2)

–a 2.35; 2.34; 2.01

128 128

222

[K(18-crown-6)]2[Hf(4-C14H10)3]

2.466(4); 2.47 (1); 2.46(1)

2.32; 2.33; 2.32

128

223 224 225 226 227

{[K(18-crown-6)][Ti(6-C16H10)2]}n [Ti(Cp)2(k2-C14H10)] [Zr(Cp)2(k2-C14H10)] [Ti(trimpsi)(6-C10H8)] [Ti(4-C14H10)(6-C14H10)(dmpe)] [K(18-crown-6)(THF)2][Ti(Cp∗)(2-C14H10)(4-C14H10)]

1.77 –a –a 1.83 2.22; 1.98 2.15

229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259

[Zr(6-C6H5Me)(PMe3)2Cl2] [Zr(6-C6H5Me)(BH4)2(PMe)3] [K(DME)]2[Zr(O3CL)(4-C10H8)] [K(DME)]2[Zr(O3CL)(4-C14H10)] [Hf(PMe2Ph)2(I)2]2(m-6:6-C6H6) [Hf(PMe2Ph)2(I)2]2(m-6:6-C6H5Me) [Hf(PMe3)2(I)2]2(m-6:6-C6H6) [K(DME)2][{Ti(pyrrL)}2(m-6:6-C6H5Me)] [Zr(O-2,6-AdC6H2Me)2]2(m-6:6-C6H6) [Zr(O-2,6-AdC6Ht2Bu)2]2(m-6:6-C6H6) [Zr(O-2,6-AdC6H2Me)2]2(m-6:6-C6H5Me) [Zr(O-2,6-AdC6Ht2Bu)2]2(m-6:6-C6H5Me) [Zr(8-Pn∗)]2(m-6:6-C6H6) [Zr(8-Pn∗)]2(m-6:6-C6H5Me) [Zr(8-Pn∗)]2(m-6:6-C6Hi5Pr) [Zr(8-Pn∗)]2(m-6:6–1,2-C6H4Me2) [Zr(8-Pn∗)]2(m-6:6–1,3-C6H4Me2) [Ti(LMe)(6-C6H5Me)] [Zr{MeC(NiPr)2}(Cp∗)(k2-C6H5Et)] [Zr(CbzdiphosiPr)(Cl)(6-C6H5Me)] [Zr(CbzdiphosPh)(I)(6-C6H5Me)] [Zr(CbzdiphosPh)(I)(6-C6H3Me3)] [Hf(Br)2(PMe2Ph){Me2P(6-C6H5)}]2 [Zr(Cl)2(PMe2Ph){Me2P(6-C6H5)}]2 [Zr(I)2(PMe2Ph){Me2P(6-C6H5)}]2 [Zr{PhP(CH2SiMe2NSiMe2CH2)2(P)(3-C6H5)}]2 [Ti(Cp)(NPtBu2{2-C6H4(6-C6H5)})] [Ti(Cp∗)(NPtBu2{2-C6H4(6-C6H5)})] [Ti({2-C6H4(6-C6H5)} tBu2PN)(Me)2][MeB(C6F5)3] [Ti(NHAriPr6)2(Cl)] [Ti(NHAriPr6){NH-C6H3–2-(C6H2–2,4,6-iPr3)-6(6-C6H2–2,4,6-iPr3)}] [Ti{(tBu2C¼ N)C(NDipp)2} {NC(NDipp)[N(6-Dipp)](CH)2}] [Ti{(tBu2C¼ N)C(N-2,5-C6H3Me2)2} {NC(NDipp)[N(6-Dipp)](CH)2}] [Ti{(tBu2C¼ N)C(N-2,5-C6H3Me2)2}{NC(NDipp)2(CH)2} (6-C6H6)] [Ti(LArL)(Cl)(DME)] [Zr(LArL)(CH2Ph)2]

2.27(3) –a –a 2.31(7) 2.36(4); 2.4(2) 2.269(6); 2.31 (1) 2.4(1) –a 2.49(2) –a 2.4(1) 2.4(2) 2.4(2) 2.29(9) –a –a –a –a 2.4(1) 2.41(8) 2.24(9) 2.42(9) 2.4(1) 2.33(9) 2.5(1) 2.5(1) –a 2.5(1) 2.41(9) –a 2.5(1) 2.47(8) 2.4(1) 2.4(2) –a 2.6(1) 2.3(1)

130 131 131 132 133

228

–a 2.470(5)–2.486(6) 2.476(7)–2.486(6); 2.466(6)–2.495(5) –a 2.444(6)–2.487(6); 2.441 (6)–2.484(6); 2.437(6)–2.476 (6) –a 2.469(3)–2.524(3); 2.475 (3)–2.514(3); 2.468(3)–2.513 (3) 2.463(3)–2.472(3); 2.462 (3)–2.483(3); 2.442(3)–2.476 (3) 2.226(2)–2.314(2) –a –a 2.235(7)–2.416(6) 2.320(4)–2.424(4); 2.311 (4)–2.656(4) 2.264(3)–2.273(3); 2.295 (3)–2.322(3) 2.35(1)–2.52(1) –a 2.470(5)–2.511(5) –a 2.30(2)–2.55(3) 2.28(2)–2.53(3) 2.227(6)–2.524(6) 2.189(4)–2.394(4) –a –a –a –a 2.322(3)–2.516(3) 2.31(4)–2.52(3) 2.275(4)–2.549(4) 2.315(3)–2.549(3) 2.326(3)–2.539(3) 2.202(4)–2.418(3) 2.313(2)–2.56(2) 2.303(2)–2.567(2) –a 2.312(2)–2.6036(2) 2.256(16)–2.50(3) –a 2.33(2)–2.54(3) 2.352(3)–2.551(3) 2.236(3)–2.472(4) 2.236(5)–2.458(5) –a 2.485(2)–2.687(2) 2.143(2)–2.458(3)

2.01 –a 2.34 –a 1.93 1.93 1.92 1.80 –a –a –a –a 1.92 1.91 1.93 1.92 1.92 1.86 2.02 2.02 –a 2.03 1.95 –a 1.99 2.03 1.91 1.92 –a 2.52 1.86

134 135 136 136 137 137 138 139 140 140 140 140 141 141 141 141 141 142 143 144 144 144 145 145 145 146 147 147 148 149 149

2.294(3)–2.473(3)

2.39(7)

1.92

150,151

–a

–a

–a

152

2.206(2)–2.452(2)

2.3(1)

1.87

152

2.712(5)–3.709(6) 2.964(5)–3.698(5)

3.2(7) 3.3(3)

2.92 3.02

153 153

260 261 262 263 264

133

Arene Complexes of the Group 4 Metals

533

Table 4

(Continued)

Complex number

Molecular formula

˚) Arene M-C dist. (A

Mean Arene ˚) M-C dist. (A

M-Arene centroid ˚) dist. (A

Reference number

265 266 267

[Zr(LArL)(Cl)2(THF)] [Zr(O-2-tBu-4-C6H2Me)2(4-C6H4)(THF)3] [Zr2(LArL)(m-H){(O-2-tBu-4-C6H2Me)2(4-C6H2)} Zr2(m-H){(O-2-tBu-4-C6H2Me)2(4-C6H4)}(LArL)] [Zr{(O-2-tBu-4-C6H2Me)2(4-C6H4)}(THF)]2(m-CO2)

2.943(3)–3.931(3) 2.391(5)–2.499(5) 2.411(4)–2.452(4); 3.447 (5)–4.104(5) 2.444(5)–2.656(5); 2.459(5)–2.640(5) 2.998(4)–3.880(5) 3.529(2)–4.096(2) 2.432(1)–2.525(1) –a

3.4(4) 2.43(5) 2.44(2); 3.8(3) 2.54(9); 2.53 (8) 3.4(4) 3.8(2) 2.49(4) –a

3.15 2.37 2.35; 3.75 2.27; 2.26

153 153 153

3.15 3.51 2.38 –a

153 154 154 154

268 269 270 271 272

[Zr(LArL)(THF)]2(C2S4) [Zr(LAnthL)(CH2Ph)2] [Zr{[O-2-(C6H2Me3)-4-tBuC6H2]2(4-C14H8)}(THF)3] [Zr{[O-2-(C6H2Me3)-4-tBuC6H2]2(4-C14H8)}(OCC4Ph4) (THF)]

153

a

Solid-state molecular structure not determined.

As noted previously, the vapor deposition synthesis of the bis(naphthalene) compound “Ti(n-C10H7Me)2” produces a thermally unstable product; however, utilizing the Chatt method, Ellis and co-workers had improved success in the synthesis of [Ti(n-C10H8)2] 210 and [K(15-crown-5)2]2[Ti(n-C10H8)2] 211 by varying the equivalents of potassium naphthalenide (Scheme 51). Solutions of 210 are unstable above −20  C, decomposing to give finely divided titanium metal, while 211 is thermally stable and notable for possessing a formal titanium(-II) dianion, though poor solubility precluded detailed structural and spectral analyses.127 Addition of [Sn(Me)3(Cl)] to 211 gives the titanium-tin bonded metal-arene [K(15-crown-5)2] [Ti(4-C10H8)2(SnMe3)] 212, that upon standing in THF solution at −20  C for 2 weeks provides a few crystals of [K(15crown-5)2][Ti(4-C10H8)2(SnMe3)2] 213 featuring tetrahapto binding of the naphthalenes to titanium (Scheme 51). Furthermore, both 211 and 212 react with CO to give [K(15-crown-5)2]2[Ti(CO)6] and [K(15-crown-5)2][Ti(CO)5(SnMe3)], respectively (Scheme 51).

Scheme 51 Chatt synthesis and reactivity of titanium bis(arenes) 210–211.

534

Arene Complexes of the Group 4 Metals

4.09.6.1.2

Tris(arenes)

Reacting solutions of [M(Cl)4(THF)2] (M ¼ Ti, Zr, Hf ) with 6 equiv. of [K(C10H8)] in DME or THF in the presence of crown ether or [2.2.2]cryptand generates the tris(naphthalene)metalates [K(L)n]2[M(x-C10H8)3] (M ¼ Ti, n ¼ 1, L ¼ 18-crown-6 214; M ¼ Ti, n ¼ 1, L ¼ [2.2.2]cryptand 215; M ¼ Zr, n ¼ 2, L ¼ 15-crown-5, x ¼ 4 216; M ¼ Zr, n ¼ 1, L ¼ [2.2.2]cryptand, x ¼ 4 217; M ¼ Hf, n ¼ 1, L ¼ 18-crown-6, x ¼ 4 218; M ¼ Hf, n ¼ 1, L ¼ [2.2.2]cryptand, x ¼ 4 219) isolated in 40–80% yields (Scheme 52).128,129 The solid-state molecular structures of 217 and 219, as determined through SCXRD characterization, show each of the molecules to adopt an isostructural, pinwheel type arrangement with the naphthalene ligands exhibiting 4-coordination modes to give 18-electron complexes. On the other hand, 214 and 215 display mixed hapticity with one 2-bound and two 4-bound naphthalene rings to give a 16-electron titanium complex. The authors suggest that the titanium engages in greater covalent interactions with the rings, thus drawing the arenes closer to the metal center, increasing steric crowding and intramolecular repulsions that consequently lead to the reduced hapticity. NMR spectroscopic analysis of 214, 217, and 219 show the respective binding modes of the naphthalene units remain intact in solution. Preliminary reactivity studies exposing solutions of 216 and 218 to CO atmospheres gives the homoleptic hexacarbonyl dianions [K(L)n]2[M(CO)6] (M ¼ Zr, n ¼ 2, L ¼ 15-crown-5; M ¼ Hf, n ¼ 1, L ¼ 18-crown-6) (Scheme 52).128,129 Moreover, 218 reacts with 1,3,5,5-cyclooctatetraene (COT) to give [K(18-crown-6)]2[Hf(3-COT)2(4-COT)] (Scheme 52).128

Scheme 52 Chatt synthesis of tris(arenes) 214–219 and reactivity of 216, 218.

The Chatt protocol can be extended to other polyarenes. In similar fashion to the tris(naphthalene) complexes described above, treatment of [M(Cl)4(THF)2] solutions with 6 equiv. of [K(C14H10)] affords [K(18-crown-6)]2[M(x-C14H10)3] (M ¼ Ti 220; Zr, x ¼ 42 21; Hf, x ¼ 4 222) (Scheme 53). Alternatively, treating solutions of [M(x-C10H8)3]−2 (M ¼ Ti 214, Hf 218) with anthracene gives 220 and 222, with the latter isolated in higher yields than with the use of [Hf(Cl)4(THF)2]. Of note, the metathetic syntheses of the hafnium complexes 218, 219, and 222 require the addition of 4 equiv. of PMe3 to the reaction mixtures, though the exact role of the trimethylphosphine in these reactions is not understood.128 The solid-state molecular structures of 221 and 222 exhibit 4-coordination of the anthracenes, while 220 again exhibits mixed 2- and 4-binding of the arenes akin to that found in 214 and 215. As anthracene can be added to solutions of 214 to yield 220, so can the addition of pyrene to displace the naphthalenes from 214 to give the bis(pyrene) sandwich complex {[K(18-crown-6)][Ti(6-C16H10)2]}n 223 (Scheme 53).130 Attempts to synthesize 223 from the reaction of [Ti(Cl)4(THF)2] and 5 or 6 equiv. of [K(C16H10)] in the presence of 18-crown-6 failed under a variety of reaction conditions.

Arene Complexes of the Group 4 Metals

535

Scheme 53 Synthesis of 220–223 through Chatt’s method and arene exchange reactions.

4.09.6.2

Heteroleptic complexes

Arene anions are competent reducing agents,155 but as illustrated above, they are quite capable of ligating metals under certain conditions. In the presence of supporting ligands, this can give way to heteroleptic complexes with unexpected metal-arene bonds. For instance, stirring suspensions of [M(Cp)2(Cl)2] (M ¼ Ti, Zr) with [Mg(THF)3(C14H10)] or magnesium in the presence of anthracene generates the sandwich complexes [M(Cp)2(k2-C14H10)] (M ¼ Ti 224, Zr 225) (Scheme 54), which are stable in the solid-state with respect to heating up to temperatures of 50  C and 80  C for 224 and 225, respectively.131 The 13C NMR spectra of these complexes suggest symmetric coordination of the metals across the bridgehead carbons, favoring s-bonding over p-coordination that leads to what can be described as a bidentate ligation mode of a folded anthracene ring.

Scheme 54 Synthesis of metal-arene metallocene complexes 224–225.

Indeed, co-ligands can have a significant effect on the binding modes of arene ligands to Group 4 metals. Whereas the naphthalene and anthracene tris(arenes) 214–219 and 220–222 exhibit 2- and 4-binding modes, Girolami and Gardner showed that addition of 3 equiv. of [Na(C10H8)] to [Ti(Cl)3(trimpsi)(THF)] (trimpsi ¼ tBuSi(CH2PMe2)3) leads to the formation of the 6-bound naphthalene compound [Ti(trimpsi)(6-C10H8)] 226 as a paramagnetic, 16-electron complex (Scheme 55).132 Characterization of 223 by SCXRD reveals a titanium center with a three-legged piano stool geometry where the coordinated ring of the naphthalene ligand displays a folding angle of 12.4 . Inspection of the metrical parameters indicate CdC distances consistent with partial diene-diyl bonding character. The authors propose that this distortion occurs as a result of strong metal-to-ligand d-backbonding from titanium to the LUMO of naphthalene and not as a consequence of intramolecular steric repulsion. Compound 226 is a useful source of titanium(0) as exposure of a THF solution of 226 to an atmosphere of CO generates the 18-electron, tetracarbonyl [Ti(trimpsi)(CO)4] (Scheme 55).

536

Arene Complexes of the Group 4 Metals

Scheme 55 Synthesis and reactivity of 226.

Similarly, Ellis and co-workers reported that addition of 4 equiv. [Na(C14H10)] to [Ti(Cl)4(dmpe)] (dmpe ¼ (Me2PCH2)2) in THF gives heteroleptic [Ti(4-C14H10)(6-C14H10)(dmpe)] 227 (Scheme 56). Unlike 226, 227 is diamagnetic, and its NMR spectra clearly reveals inequivalent ligation of the anthracene ligands. This solution phase observation is supported by the solid-state molecular structure which reveals an 4-anthracene, similar to those of 220–222, accompanied by an 6-bound anthracene with a ring folding angle of 15.1 at titanium.133 In this case, bending of the 6-anthracene ring is attributed by the authors to steric congestion at the metal center, though backbonding contributions should not be discounted. Furthermore, reacting [Ti(Cp∗)(Cl)3] with potassium reduced anthracene affords the mixed sandwich complex [K(18-crown-6)(THF)2][Ti(Cp∗)(2-C14H10)(4-C14H10)] 228 (Fig. 7). Compounds 227 and 228 both react with CO to produce [Ti(CO)3(dmpe)2] (in the presence of excess dmpe) and [K(18-crown-6)(THF)2][Ti(Cp∗)(CO)4], respectively.

Scheme 56 Synthesis of heterolpetic titanium-arene complex 227.

The isolation of 184–186 and 226 indicates that phosphines appear to be suitable co-ligands for the stabilization of low-valent Group 4 complexes with metal-arene bonds. In line with this, Green and co-workers demonstrated that reduction of [Zr(Cl)4] by excess Na/K alloy in toluene with the 2 equiv. of PMe3 produces [Zr(6-C6H5Me)(PMe3)2(Cl)2] 229, which is shown by SCXRD to adopt a square pyramidal arrangement of the zirconium atom in the solid-state.134 Complex 229 is a tantalizing starting material for accessing Zr(II) chemistry; however, the compound is isolated in less than 5% yield. Nonetheless, it undergoes metathesis with [Li(BH)4] to yield [Zr(6-C6H5Me)(BH4)2(PMe)3] 230 (Scheme 57).135

Fig. 7 Mixed titanium-arene metallocene complex 228.

Scheme 57 Synthesis of 229–230 with phosphine co-ligands.

Arene Complexes of the Group 4 Metals

537

Kawaguchi and co-workers, utilizing a tetraanionic carbon-capped triaryloxide ligand, described that the reaction between the zirconium complex [Zr(O3CL)(THF)3] (O3CL ¼ [(3,5-tBu2–2-OC6H3)3C]−4) and 2 equiv. of [K(C10H8)] gives [K(DME)]2[Zr(O3CL) (4-C10H8)] 231 in 91% yield as a diamagnetic molecule (Scheme 58).136 Interestingly, the coordinated arene undergoes facile exchange with free naphthalene in solution as only one set of naphthalene resonances is observed at room temperature by NMR spectroscopy, indicating that the exchange is rapid on the NMR timescale. In line with this, addition of anthracene to a solution of 231 results in the instantaneous displacement of the naphthalene and formation of the anthracene complex [K(DME)]2[Zr(O3CL) (4-C14H10)] 232 as assigned through NMR spectroscopic characterization (Scheme 58). Exposing solutions of 231 to CO, CO2, or t BuCN leads to intractable mixtures. However, addition of excess Me3SiN3 to 231 results in a surprising azide-to-amide conversion to yield [K(THF)2][Zr(O3CL){N(SiMe3)2}(THF)] (Scheme 58), which likely proceeds through a zirconium-imido intermediate. Pressurizing solutions of 231 with H2 results in the partial hydrogenation of the coordinated naphthalene to give an equiv. of tetralin, quantitatively producing the hydride-bridged dimer [K(THF)2]3[{Zr(O3CL)}2(m-H)3] in the process (Scheme 58).

Scheme 58 Synthesis of heteroleptic metal-arenes 231–232 and reactivity of 231.

4.09.6.3

Inverted sandwich compounds

Despite the predominance of titanium in Group 4 metal-arene complexes, the other congeners of Group 4 play a greater role in the chemistry of inverted sandwich molecules (see Section 4.09.2). Inverted sandwiches are formed when two metal centers are symmetrically and equivalently bridged by an arene molecule in a face-to-face orientation. The first well-described instance of these Group 4 compounds was detailed in 1991 by Cotton and co-workers with the synthesis of [Hf(PMe2Ph)2(I)2]2(m-6:6-Arene) (Arene ¼ C6H6 233, C6H5Me 234) obtained in 30% yield from the reduction of [Hf(I)4] with sodium amalgam in the presence of PMe2Ph in benzene or toluene solutions (Fig. 8).137 Building upon this work, Troyanov and co-workers later obtained the analogous PMe3 compound [Hf(PMe3)2(I)2]2(m-6:6-C6H6) 235 through similar synthetic methods (Fig. 8).138 Structural examination of the arene moieties in 233–235 by SCXRD reveals severe deformation of the arenes from planarity and adoption of a twist-boat geometry. Cotton attributed the significant geometrical distortion as arising from the dual ability of the arene to act as both a p-donor and p-acid, with the frontier orbitals of the arene ligand fully engaged with the d-orbitals of the hafnium atoms as determined by Fenske-Hall quantum calculations.137 On the other hand, Troyanov and co-workers attributed the ring distortions in 235 as resulting from repulsive forces arising from steric clashing of the arene ligand with the phosphine and iodine ligands; though, this argument seems oversimplified and less plausible. Nearly a decade later, Gambarotta, Budzelaar and others synthesized the titanium inverted-sandwich complex [K(DME)2] [{Ti(pyrrL)}2(m-6:6-C6H5Me)] (pyrrL ¼ {2,5-[(C4H3N)CPh2]2[C4H2N(Me)]}2−) 236 (Fig. 8), isolated from the reduction of [Ti(pyrr L)(Cl)] with potassium mirror in toluene followed by recrystallization from DME.139 The bridging toluene exhibits a slight but noticeable mean deviation from planarity of 0.048 A˚ ; yet, it is significantly less distorted than that of 234. Formal charge

538

Arene Complexes of the Group 4 Metals

Fig. 8 Inverted-sandwich complexes 233–236.

assignments are obfuscated due to the metal-arene backbonding, but computational analysis on a simplified model of 234, where toluene has been replaced by benzene, suggests the molecule can be best pictured as comprising of separate [TiII (pyrrL)]0 and [TiI(pyrrL)(C6H6)]−1 fragments. Kawaguchi and co-workers examined the reduction chemistry of the bis(aryloxide)zirconium compound [Zr(O-2, 6-AdC6H2R)2(Cl)2] (R ¼ Me, tBu) with 2 equiv. of KC8 in thawing solutions of benzene or toluene, to give [Zr(O-2, 6-AdC6H2R)2]2(m-6:6-Arene) (Arene ¼ C6H6, R ¼ Me 237, tBu 238; Arene ¼ C6H5Me, R ¼ Me 239, tBu 240) in low to moderate yields after several days of stirring (Scheme 59).140 Attempts to characterize the solid-state molecular structures of these complexes were hampered by severe rotational disorder of the bridging arene ligands, but the molecular connectivity was nonetheless confirmed. Of note, preliminary studies indicate that these bis(aryloxide)zirconium systems can be extended to ethylbenzene to give the corresponding inverted sandwich products; however, the reactions do not proceed with bulkier arenes such as mesitylene and xylenes, likely due to steric considerations and the higher reduction potentials of these polymethylated arenes. In the solid-state and in solution, 237–240 are thermally stable, while 239 and 240 do not undergo arene-arene exchange and remain unchanged when recrystallized from benzene. Reactivity studies show that treatment of 239 with 1 or 2 equiv. of adamantyl azide (AdN3) gives the Zr(IV) imido [Zr(O-2,6-AdC6H2Me)2(NAd)(THF)] and the tetra-azametallacycle [Zr(O-2,6-AdC6H2Me)2{(NNAd)2}], respectively (Scheme 59).

Scheme 59 Inverted-sandwich complexes 237–240 and reactivity of 239.

Triple-decker complexes, with two metals sandwiched between three p-bound aromatic ligands, have been reported for zirconium. Specifically, reduction of the permethylpentalene (Pn∗) zirconium complex [Zr(8-Pn∗)(m-Cl)1.5]2[(m-Cl)2Li(THF)2] with KC8 in aromatic solvents generates the triple-decker metal-arenes [{Zr(8-Pn∗)}2(m-6:6-Arene)] (Arene ¼ C6H6 241, C6H5Me 242, C6Hi5Pr 243, 1,2-C6H4Me2 244, 1,3-C6H4Me2 245) (Fig. 9).141 Akin to 233–235, the centrally sandwiched arene rings in 241–245 exhibit a twist-boat conformation. NMR spectroscopic analysis reveals significant rotational freedom of the central arene ring based upon room and variable temperature measurements. In solution, 241 and 242 do not exchange with benzene or toluene solvents, while exposure of these solutions to H2, CO2, C2H4, or azobenzene does not lead to any observed reactivity, potentially due to the enhanced kinetic stability of these systems afforded by the pentalene ligands. Notably, the X-ray absorption

Arene Complexes of the Group 4 Metals

539

Fig. 9 Zirconium inverted-sandwich pentalene complexes 241–245.

near-edge spectra of the Zr K-edge energy measurements, as a means to determine the oxidation states of the metals, shows broad features indistinguishable from Zr(II) and Zr(IV) reference spectra, leading the authors to suggest that the Zr-arene interactions are highly covalent. In support of this, DFT calculations of 241 shows the frontier molecular orbitals to possess equal contributions from the C6H6 and Pn∗Zr fragments.

4.09.6.4

Hydrogenolysis

Given the difficulties with synthesizing and stabilizing low-valent Group 4 metal complexes, the hydrogenolysis of metal-alkyl bonds in aromatic solutions has shown to be a viable alternative approach for accessing metal-arene compounds without the use of harsh reducing agents. Arnold and Hagadorn demonstrated that the bis(amidinate) benzyl complex [Ti(LMe)(CH2Ph)2] undergoes TidCH2Ph bond hydrogenolysis upon exposure to H2 in C6D6 to give toluene-capped [Ti(LMe)(6-C6H5Me)] 246 (Scheme 60).142 Of note, despite having benzene as the reaction solvent, the titanium instead binds one of the liberated toluene molecules. The mechanism for the formation of this complex is proposed to involve a transient titanium hydride species, as supported by H2/D2 labeling studies, which upon rearrangement leads to the isolation of the more stable 6-toluene product in 72% yield. Inspection of the solid-state molecular structure of 246 reveals significant distortion within the coordinated toluene ligand, which exhibits a ring fold angle of 20 degrees and CdC distances consistent with cyclohexadiene character. These structural features are suggestive of the loss of aromaticity and reduction of the bound toluene.

Scheme 60 Synthesis of 246 via hydrogenolysis.

Sita and co-workers achieved the near quantitative hydrogenolysis of the 2-styrene zirconium metallocene [Zr{MeC(NiPr)2} (Cp∗)(2-CH2CHPh)] with H2 in pentane to afford [Zr{MeC(NiPr)2}(Cp∗)(k2-C6H5Et)] 247 (Scheme 35). In the solid-state, the bound ethylbenzene ligand exhibits an obvious deviation from planarity with the zirconium acting as a bridgehead across the bound arene ring, leading the authors to describe the moiety as a Zr(IV) zirconanorbornadiene.143 The reaction is proposed to proceed through formation of an alkyl hydride intermediate that subsequently reductively eliminates ethyl benzene that is then recaptured by zirconium (Scheme 61). Compound 247 exhibits remarkable thermal stability and does not undergo further hydrogenation chemistry, nor is it susceptible to arene exchange with toluene at 100  C. Utilizing the benzylated zirconium PNP-pincer precursors [Zr(CbzdiphosR-CH)(CH2Ph)(X)] (X ¼ Cl, I; CbzdiphosR ¼ [1,8{(PR2)CH2}2–3,6-tBu2C12H9N]−; R ¼ Ph, iPr), Gade and co-workers reported the hydrogenolysis of these complexes under 10 bar of H2 in toluene to yield [Zr(CbzdiphosR)(X)(6-C6H5Me)] (X ¼ Cl, R ¼ iPr (248); X ¼ I, R ¼ Ph 249) (Scheme 62).144 Alternatively, addition of 3,5-dimethylbenzylmagnesium to [Zr(CbzdiphosPh)(I)3] followed by treatment with hydrogen provides the mesitylene analogue [Zr(CbzdiphosPh)(I)(6-C6H3Me3)] 250 in lower yield. As with 246–247 above, the bound arene rings in 248–250 show metrical parameters in the solid-state that are consistent with formal reduction to a 1,4-cyclohexadiene dianion. Though, DFT analysis of 248 suggests the distortions may also arise from strong metal-to-ligand d-backbonding from the zirconium dxy orbital into the LUMO of the arene. Regardless, 248–250 are capable two-electron reductants, providing access to a Zr(II) synthon. Namely, treatment of these compounds with isocyanide, organoazides, diazobenzene, or pyridine, leads to reduction of the substrates and formation of Zr(III) and Zr(IV) species (Scheme 62).

540

Arene Complexes of the Group 4 Metals

Scheme 61 Synthesis of 247 via hydrogenolysis.

Scheme 62 Synthesis of metal-arenes 248–250 and redox reactivity of 248–249.

4.09.6.5

Bimolecular arene coordination

Reduction-induced coordination of arenes to Group 4 metals is not limited to the capture of free aromatic substrates. Indeed, this metal-arene binding can occur through intramolecular ligation with the pendant arene substituents of the ligands or through intermolecular coordination to the aryl groups of other metal-bound ligands. This can arise when a low-valent metal engages the p∗-orbitals of a neighboring arene group to alleviate its charge density in the absence of other p-acids. For instance, after describing the synthesis and structures of 233 and 234, Cotton and co-workers later reported that reduction of [Hf(Br)4] with sodium amalgam in toluene with 2 equiv. of PMe2Ph does not produce an inverted sandwich complex as expected but instead yields the dimer

Arene Complexes of the Group 4 Metals

541

[Hf(Br)2(PMe2Ph){Me2P(6-C6H5)}]2 251 (Scheme 63).145 Compound 251 is isolated in 17% yield, and its formation is accompanied by the significant precipitation of an undefined brown powder. The solid-state molecular structure of 251 reveals the two hafnium centers to be linked through the phenyl group of the phosphine ligand of the adjacent metal center. It is interesting to note that the only difference between the synthesis of 251 and 233 and 234 is the identity of the halide co-ligand, switching from iodide to bromide, which indicates a high sensitivity of the reduction chemistry to the character of the metal halides employed in the reaction in certain circumstances. On the other hand, reduction of [Zr(Cl)4] or [Zr(I)4], under similar reaction conditions for 251, both give the isostructural arene-bridged dimers [Zr(X)2(PMe2Ph){Me2P(6-C6H5)}]2 (X ¼ Cl 252, I 253) (Scheme 63).145,156 In all cases, inspection of the solid-state molecular structures reveals significant distortion of the coordinated phenyl groups that results in ring puckering and elongation of the CarenedCarene bonds. Molecular orbital analysis by Fenske-Hall approximations on the simplified model, [Zr(I)2(PH3)2(6-C6H6)], indicates the HOMO comprises of significant d-type bonding between the filled metal dxz and empty p (e2) orbitals.145

Scheme 63 Synthesis of bimolecular metal-arene complexes 251–253.

In a similar fashion, Fryzuk and co-workers demonstrated that reduction of the phosphine macrocyclic zirconium complex [Zr {PhP(CH2SiMe2NSiMe2CH2)2PPh}(Cl)2] with 4 equiv. of KC8 in toluene under an argon atmosphere gives the phenyl-bridged dimer [Zr{PhP(CH2SiMe2NSiMe2CH2)2P(3-C6H5)}]2 254 (Scheme 64).146 The use of argon in this reaction is critical as reduction of the system under dinitrogen leads to the formation of the side-on bridged dinitrogen complex [Zr{PhP (CH2SiMe2NSiMe2CH2)2PPh}(m-2:2-N2)] as the major product, with 254 present in less than 5% yield (Scheme 64). The solid-state molecular structure of 254 determined by SCXRD analysis reveals the coordinated phenyl rings to be non-planar, adopting a boat conformation with a dihedral angle approaching 30 degrees. Furthermore, the rings exhibit CarenedCarene distances consistent with bis(allyl) character. As such, the structural features can be explained through formal charge transfer occurring from the zirconium atoms to the p -orbitals of the arene systems, thus leading to tetravalent metal centers bound to arene dianions.

Scheme 64 Synthesis of bimolecular zirconium-arene complex 254.

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Arene Complexes of the Group 4 Metals

4.09.6.6

Intramolecular arene coordination

In 2004, Douglas and co-workers described the first example of intramolecular arene reduction and binding to titanium. Stirring ethereal solutions of the titanium metallocene phosphinimides [Ti(Cp0 )(Cl)2{NPtBu2(2-C6H4Ph)}] (Cp0 ¼ Cp, Cp∗) with magnesium powder provided the intramolecularly bound metal-arene complexes [Ti(Cp0 )(NPtBu2{2-C6H4(6-C6H5)})] (Cp0 ¼ Cp 255, Cp∗ 256) (Scheme 65).147 The 1H NMR spectra of 255 and 256 show the protons of the coordinated phenyl ring to be shifted dramatically upfield between 3.5–4.5 ppm, suggesting loss of aromatic character within the phenyl substituent. As such, the solid-state molecular structure analyses of these compounds show significant ring distortions with dihedral angles exceeding 25 and CarenedCarene lengths in line with a cyclohexadiene dianion form. The authors suggest that reduction of the titanium dichloride precursor generates a transient Ti(II) species, which subsequently binds and reduces the phenyl rings to give the products as formally Ti(IV) compounds. As an alternative to reduction, treating solutions of methylated [Ti{(2-C6H4Ph)tBu2PN}(Me)3] with B(C6F5)3 leads to alkyl abstraction to give the intramolecular metal-arene adduct [Ti({2-C6H4(6-C6H5)} tBu2PN)(Me)2] [MeB(C6F5)3] 257 (see Section 4.09.4.3), though the arene interaction is readily displaced by coordinating solvents such as THF to give [Ti{(2-C6H4Ph)tBu2PN}(Me)2(THF)][MeB(C6F5)3] (Scheme 65).148

Scheme 65 Synthesis of intramolecularly bound titanium-arenes 255–257.

In an attempt to synthesize a two-coordinate titanium complex, Power and co-workers described the synthesis and reduction chemistry of the highly encumbered bis(amido) [Ti(NHAriPr6)2(Cl)] (AriPr6 ¼ C6H3-2,6-(C6H2-2,4,6-iPr3)2) 258 (Scheme 66), which itself features trihapto coordination of a pendant arene ring to titanium in the solid state. Stirring pentane solutions of 258 with KC8 for 4 days at room temperature under N2 produces the diamagnetic, intramolecularly arene-bound [Ti(NHAriPr6){NH-Ci 6 i 149 As with 255 and 256, the complex exhibits 6-coordination 6H3-2-(C6H2-2,4,6- Pr3)-6-( -C6H2-2,4,6- Pr3)}] 259 (Scheme 66). of a flanking aryl ring that exhibits significant structural distortion with metrical parameters consistent with a diene dianion assignment, again yielding a formally tetravalent metal center.

4.09.6.6.1

Reactivity of intramolecular titanium-arenes

In the cases above, further chemistry was not explored, but it is reasonable to assume that these systems would be competent reductants. The intramolecular reduction of peripheral arene substituents in otherwise redox-innocent ligand systems affords an electronic reservoir where ring rearomatization energy provides a driving force for effecting multi-electron transfer chemistry. In this context, Fortier and co-workers have shown that reduced titanium complexes possessing intramolecularly coordinated arene ligands can be effective sources of low valent synthons. Specifically, reduction of [Ti(Arketguan)(ImDippN)(X)2] (Arketguan ¼ [(tBu2C¼ N)C(NAr)2]−; Ar ¼ Dipp, X ¼ OTf; Ar ¼ 2,5-C6H3Me2; ImDippN ¼ [NC(NDipp)2(CH)2]−; X ¼ Cl) with 2.5 equiv. of KC8 in THF under an N2 atmosphere at low temperature generates the arene-capped complexes [Ti(Arketguan){NC(NDipp)[N(6-Dipp)](CH)2}] (Ar ¼ Dipp 260, 2,5-C6H3Me2 261) in high yields.150–152 Inspection of their solid-state molecular structures by SCXRD analysis clearly shows 6-binding of a pendant imidazolin-2-iminato Dipp ring to titanium with ring perturbations indicative of reduction to a 1,4-cyclohexadiene dianion form comparable to that observed for 259. Furthermore, NMR spectroscopic characterization of 260 and 261 in C6D6 shows the proton resonances of the coordinated rings to be shifted upfield, like those of 255 and 256, substantiating the dearomatization of the bound aryl groups. Exposing solutions of 260 to carbon monoxide yields the Ti(II) dicarbonyl [Ti(Dippketguan)(ImDippN)(CO)2], proving the ability of these intramolecularly capped titanium complexes to act as divalent synthons.150 Indeed, 260 is a potent reducing agent as it

Arene Complexes of the Group 4 Metals

543

Scheme 66 Synthesis of intramolecularly bound titanium-arenes 258–259.

reacts with pyridine, benzophenone, and cyclohexyl isocyanide to give the Ti(III) products [Ti(Dippketguan)(ImDippN)(1-OCPh2)], [Ti(Dippketguan)(ImDippN)]2{m2-(NC5H5 −H5C5N)}, and [Ti(Dippketguan)(ImDippN)(CN)(CNCy)], while treating 260 with C6H4F2 leads to CdF bond cleavage to give the Ti(IV) fluoride [Ti(Dippketguan)(ImDippN)(F)(C6H4F)] (Scheme 67).151 Lastly, addition of chalcogen transfer reagents or organoazides to 260 forms the series [Ti(Dippketguan)(ImDippN)(E)] (E ¼ O, S, Se, S2, NSiMe3, NAd) (Scheme 67).

Scheme 67 Synthesis of intramolecularly bound metal-arene 260 and its redox chemistry.

544

Arene Complexes of the Group 4 Metals

Compound 260 is not stable in solution at room temperature and undergoes CdH activation in aromatic solvents to give the intramolecularly cyclometallated complex [Ti{[(NDipp)(N-2-iPrC6H3-6-(2-CH3CCH2))]C(NCtBu2)}(ImDippN)] with release of H2 (Scheme 67).150 In aromatic solvents, 260 reacts with terminal hydrogen acceptors such as cyclohexene, eventually undergoing transfer hydrogenation to give cyclohexane and the cyclometallated product. Conducting these experiments with solutions of 260 pressurized with H2 catalyzes the hydrogenation of cyclic olefins to cycloalkanes. Alternatively, treating solutions of 260 with thiophene leads to oxidative-addition of the titanium across the CdS bond to give [Ti(Dippketguan)(ImDippN){k2-S(CH)3CH}] (Scheme 68).157 Remarkably, this process is completely reversible as photolyzing a benzene solution of the thiotitanocycle results in the reductive-elimination of thiophene to regenerate 260, an exceedingly rare example of reversible two-electron redox cycling in an early metal complex. Furthermore, pressurizing solutions of [Ti(Dippketguan) (ImDippN){k2-S(CH)3CH}] with H2 and heating to 80  C effects the hydrodesulfurization of thiophene to give n-butane and the Ti(IV) sulfido [Ti(Dippketguan)(ImDippN)(S)] (Scheme 68).

Scheme 68 Redox cycling of 260 and reversible arene capture of 261.

Notably, changing the aryl substituents of the guanidinate from Dipp in 260 to 2,5-C6H3Me2 in 261 greatly enhances the thermal stability of the complex along with augmenting its reactivity profile. Interestingly, photolyzing solutions of 261 in benzene releases the intramolecular arene binding, resulting in intermolecular capture of the aromatic solvent to give the metal-arene [Ti(Xyketguan)(ImDippN)(6-C6H6)] (Xyketguan ¼ [(tBu2C]N)C(N-2,5-C6H3Me2)2]) 262 (Scheme 68).152 This process is reversible as heating benzene solutions of 262 eventually gives way to the formation of 261 after several weeks (Scheme 68). Moreover, pressuring solutions of 261 or 262 with H2 in the presence of monocyclic and polycyclic aromatic substrates catalyzes their hydrogenation under mild conditions (Scheme 68).

4.09.6.7

Tethered arenes

A unique strategy to enforce metal-arene interactions involves the use of novel molecular design to create a chelating ligand system which tethers a metal in close proximity to an aryl ring of the ligand itself, thus enhancing the probability for metal-arene p-interactions. Bis(aryloxides) featuring terphenyl backbones have been utilized to this effect by Kawaguchi and co-workers. Specifically, [Li2(LArL)] or [H2LArL] (LArL ¼ {(O-2-tBu-4-C6H2Me)2(C6H4)}2−) reacts with TiCl3 or [Zr(CH2Ph)4], respectively, to give [Ti(LArL)(Cl)(DME)] 263 and [Zr(LArL)(CH2Ph)2] 264.153 Subsequent addition of 2 equiv. of NEt3⸱HCl to 264 in THF gives [Zr(LArL)(Cl)2(THF)] 265. The metal centroid distances of 263 (TidCcent ¼ 2.92 A˚ (avg.), Table 4) and 265 (Zr-Ccent ¼ 3.15 A˚ (avg.), Table 4) are long, indicating little to no interaction with the central arene ring of the terphenyl backbone. Reducing 263 with KC8 in toluene under N2 forms the end-on dinitrogen complex [Ti(LArL)]2(m-N2); however, reduction of 265 under similar conditions produces the complex [Zr(O-2-tBu-4-C6H2Me)2(4-C6H4)(THF)3] 266 that is seen through SCXRD characterization to feature a decidedly puckered central ring of the terphenyl backbone that engages the zirconium through an 4-interaction (Scheme 69). The non-planarity of the ring and its metrical parameters indicate reduction to a cyclohexadiene dianion. Complex 266 is stable in THF for prolonged periods of time, but when introduced to toluene, rapidly and irreversibly yields the complicated tetranuclear species [Zr2(LArL)(m-H){(O-2-tBu-4-C6H2Me)2(4-C6H2)}Zr2(m-H){(O-2-tBu-4-C6H2Me)2(4-C6H4)}(LArL)] 267

Arene Complexes of the Group 4 Metals

545

from dual metalation of one of the ligand backbones (Scheme 42). Furthermore, 266 reacts with CO2 or CS2 to give [Zr{(O-2-tBu4-C6H2Me)2(4-C6H4)}(THF)]2(m-CO2) 268 and [Zr(LArL)(THF)]2(C2S4) 269, respectively, with the former maintaining arene interactions between the zirconium atoms and the ligands (Scheme 42).

Scheme 69 Synthesis and reactivity of tethered arene 266.

Building upon this work, Agapie and co-workers reported the synthesis and redox chemistry of the bis(aryloxide)anthracene complex [Zr(LAnthL)(CH2Ph)2] (LAnthL ¼ [{O-2-(C6H2Me3)-4-tBuC6H2}2(C14H8)]2−) 270, where extending the conjugation of the arene backbone p-system, as compared to 264, enhances the redox non-innocence of the ligand.154 Photolysis of 270 in THF results in reductive elimination of bibenzyl to generate [Zr{[O-2-(C6H2Me3)-4-tBuC6H2]2(4-C14H8)}(THF)3] 271 (Scheme 70). Determination of the solid-state molecular structure of 271 by SCXRD analysis shows a deformed anthracene backbone that adopts a boat conformation at its central ring with metrical parameters consistent with two electron reduction of the anthracene p -system, which is stabilized through 4-engagement with the zirconium. Employing the reduced ligand system as an electron reservoir, 271 reacts with internal and terminal alkynes to promote oxidative coupling to give zirconacyclopentadiene products. For example, treatment of 271 with 2 equiv. of PhC^CPh gives [Zr(LAnthL)(k2-C4Ph4)], which can be further reduced by addition of CO to yield the tetraphenylcyclopentadienone adduct [Zr{[O-2-(C6H2Me3)-4-tBuC6H2]2(4-C14H8)}(OCC4Ph4)(THF)] 272 (Scheme 70). Reactions of 271 with PhC^CPh and benzonitrile affords the azazirconacycle [Zr(LAnthL){k2-NC(H)C(C6H4Me)C(Ph)}]. Interestingly, the zirconacycle complexes react with an additional equivalent of benzonitrile to exude pyrimidines, regenerating 271 in the process. This feature allows the use of 271 as a unique catalyst for the co-trimerization of alkynes and nitriles (Scheme 70), where the synergistic metal-arene combination is key to the redox cycling mechanism.

546

Arene Complexes of the Group 4 Metals

Scheme 70 Synthesis and reactivity of zirconium tethered arene 271.

4.09.7

Conclusion

As seen here, arenes are viable ligands that give access to a multitude of metal compounds with unique structural and electronic features that, in many cases, have practical utility as novel polymerization catalysts. The challenge with using neutral aromatic substrates as ligands lies in developing methods that promote arene coordination to the metal. The Group 4 metals are intrinsically electron deficient and this characteristic can be leveraged to generate coordinatively and electronically unsaturated species that promote arene binding in the absence of other ligand donors. Under these circumstances, the metal-arene interaction can be weak and readily disrupted by more nucleophilic substrates; however, the arene lability can be a feature in the development of Group 4 Lewis acid catalysts. Of course, as documented here, the addition of electrons to the metal-arene reactions is also a viable approach for promoting metal-arene orbital interactions through the population of arene p -orbitals by d-backbonding. This method has been particularly successful in accessing and stabilizing formally low-valent Group 4 compounds in oxidation states that are typically difficult to obtain without the use of strong p-acids such as CO. The synthesis of reduced metal complexes with coordinated arene ligands can lead to disruption of the aromatic p-system. In this regard, the driving force for rearomatization can give rise to metal compounds that can act as potent reductants. Looking to the future of Group 4 metal-arene complexes, the possibilities for this area of chemistry is promising as these highly reducing metal-arene compounds are beginning to give entryway into exciting new areas of redox chemistry that has been previously difficult to achieve with early-metals alone. For instance, in the case of 260, this allows for the fully reversible oxidative-addition and reductive-elimination of a heterocycle across titanium. This example indicates that metal-arene interactions can play a key role in achieving two-electron redox cycling in the early-metals, providing a pathway for the development of new base metal redox catalysts. To this end, tethered arene systems such as those found in 271 show that the arene manifold can act as a redox reservoir to achieve the catalytic formation of pyrimidines from nitriles and alkynes. Moving forward, the development of new ligand platforms that encourage metal-arene binding while facilitating reversible electron storage through arene redox non-innocence is surely to provide new routes for achieving challenging chemical transformations which could include the activation of abundant but difficult to functionalize small molecules such as dinitrogen and methane.

Acknowledgements We are grateful to the Welch Foundation, the Alfred P. Sloan Foundation, and the NIH (NIH-5R25GM069621-16; C.S.) for support of our work.

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Shimo, I.; Okumura, T.; Goto, K.; Kawashima, T. J. Organomet. Chem. 2007, 692, 2115–2119. Flores, J. C.; Wood, J. S.; Chien, J. C. W.; Rausch, M. D. Organometallics 1996, 15, 4944–4950. Sassmannshausen, J.; Powell, A. K.; Anson, C. E.; Wocadlo, S.; Bochmann, M. J. Organomet. Chem. 1999, 592, 84–94. Sassmannshausen, J. Organometallics 2000, 19, 482–489. Deckers, P. J. W.; van der Linden, A. J.; Meetsma, A.; Hessen, B. Eur. J. Inorg. Chem. 2000, 929–932. Deckers, P. J. W.; Hessen, B. Organometallics 2002, 21, 5564–5575. Otten, E.; Batinas, A. A.; Meetsma, A.; Hessen, B. J. Am. Chem. Soc. 2009, 131, 5298–5312. Deckers, P. J. W.; Hessen, B.; Teuben, J. H. Angew. Chem. Int. Ed. 2001, 40, 2516–2519. Deckers, P. J. W.; Hessen, B.; Teuben, J. H. Organometallics 2002, 21, 5122–5135. Tobisch, S.; Ziegler, T. Organometallics 2003, 22, 5392–5405. Blok, A. N. J.; Budzelaar, P. H. M.; Gal, A. W. Organometallics 2003, 22, 2564–2570. Timms, P. L. J. Chem. Soc. D. 1969, 1033a. Anthony, M. T.; Green, M. L. H.; Young, D. J. Chem. Soc. Dalton Trans. 1975, 1419–1422. Benfield, F. W. S.; Green, M. L. H.; Ogden, J. S.; Young, D. J. Chem. Soc. Chem. Commun. 1973, 866–867. Tairova, G. G. K.; F, E.; Krasochka, O. N.; Kichigina, G. A.; Shvetsov, Y. A.; Lisetskii, E. M.; Atovmyan, L. O.; Borod’ko, Y. G. Nouv. J. Chim. 1981, 5, 603–604. Tairova, G. G.; Krasochka, O. N.; Ponomaryov, V. I.; Kvashina, E. F.; Shvetsov, Y. A.; Lisetsky, E. M.; Kiryukhin, D. P.; Atovmyan, L. O.; Borod’ko, Y. G. Transit. Met. Chem. 1982, 7, 189–190. Bönnemann, H.; Korall, B. Angew. Chem. Int. Ed. 1992, 31, 1490–1492. Braunschweig, H.; Brückner, C.; Celik, M. A.; Dück, K.; Hupp, F.; Kramer, T.; Krebs, J.; Krummenacher, I. Chem. A Eur. J. 2015, 21 (11), 056–11,064. Hawker, P. N.; Timms, P. L. J. Chem. Soc. Dalton Trans. 1983, 1123–1126. Hawker, P. N.; Kündig, E. P.; Timms, P. L. J. Chem. Soc. Chem. Commun. 1978, 730–731. Cloke, F. G. N.; Dias, A. R.; Galvão, A. M.; Ferreira da Silva, J. L. J. Organomet. Chem. 1997, 548, 177–183. Calderazzo, F.; Ferri, I.; Pampaloni, G.; Englert, U.; Green, M. L. H. Organometallics 1997, 16, 3100–3101. Bandy, J. A.; Berry, A.; Green, M. L. H.; Perutz, R. N.; Prout, K.; Verpeaux, J.-N. J. Chem. Soc. Chem. Commun. 1984, 729–731. Cloke, F. G. N.; Green, M. L. H. J. Chem. Soc. Chem. Commun. 1979, 127–128. Cloke, F. G. N.; Lappert, M. F.; Lawless, G. A.; Swain, A. C. J. Chem. Soc. Chem. Commun. 1987, 1667–1668. Wucherer, E. J.; Muetterties, E. L. Organometallics 1987, 6, 1691–1695. Wucherer, E. J.; Muetterties, E. L. Organometallics 1987, 6, 1696–1702. Arnold, P. L.; Cloke, F. G. N.; Khan, K.; Scott, P. J. Organomet. Chem. 1997, 528, 77–81. Elschenbroich, C.; Kroker, J.; Nowotny, M.; Behrendt, A.; Metz, B.; Harms, K. Organometallics 1999, 18, 1495–1503. Cloke, F. G. N.; Green, M. L. H. J. Chem. Soc. Dalton Trans. 1981, 1938–1943. King, W. A.; Di Bella, S.; Lanza, G.; Khan, K.; Duncalf, D. J.; Cloke, F. G. N.; Fragala, I. L.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 627–635. Calderazzo, F.; Englert, U.; Pampaloni, G.; Volpe, M. J. Organomet. Chem. 2005, 690, 3321–3332. Kvashina, E. F. Russ. Chem. Bull. 1994, 43, 2121. Kündig, E. P.; Timms, P. L. J. Chem. Soc. Chem. Commun. 1977, 912–913. Morand, P. D.; Francis, C. G. Inorg. Chem. 1985, 24, 56–61. Francis, C. G.; Timms, P. L. J. Chem. Soc. Dalton Trans. 1980, 1401–1407. Kvashina, E. F.; Efimov, O. N.; Chapysheva, N. V.; Roshchupkina, O. S. Russ. Chem. Bull. 2007, 56, 2115–2117. Kvashina, E. F.; Dzhabieva, Z. M.; Efimov, O. N.; Kaplunov, M. G. Russ. Chem. Bull. 2009, 58, 1669–1671. Kvashina, E. F.; Petrova, G. N.; Belov, G. P.; Roshchupkina, O. S.; Efimov, O. N. Russ. Chem. Bull. 2002, 51, 817–819. Candlin, J. P.; Wilson, K. C.; Pearce, R.; Transition Metal Composition. U.S. Patent US4121030A, 1978. Candlin, J. P.; Segal, J. A. G.. Transition Metal Composition. U.S. Patent US4136057A. 1979. Candlin, J. P.; Wilson, K. C.; Pearce, R. Supported Arene Olefin Catalysts. U.S. Patent US4299936A. 1981. Tullock, C. W.; Tebbe, F. N.; Mulhaupt, R.; Ovenall, D. W.; Setterquist, R. A.; Ittel, S. D. J. Polym. Sci. A Polym. Chem. 1989, 27, 3063–3081. Akhmedov, V. M.; Anthony, M. T.; Green, M. L. H.; Young, D. J. Chem. Soc. Dalton Trans. 1975, 1412–1419. Schneider, J. J.; Czap, N.; Hagen, J.; Engstler, J.; Ensling, J.; Gütlich, P.; Reinoehl, U.; Bertagnolli, H.; Luis, F.; Jongh, L. J.; Wark, M.; Grubert, G.; Hornyak, G. L.; Zanoni, R. Chem. A Eur. J. 2000, 6, 4305–4321. Vettraino, M.; Trudeau, M.; Lo, A. Y. H.; Schurko, R. W.; Antonelli, D. J. Am. Chem. Soc. 2002, 124, 9567–9573. Vettraino, M.; He, X.; Trudeau, M.; Antonelli, D. Spontaneous Nitride Formation in the Reaction of Mesoporous Titanium Oxide With Bis(toluene) Titanium in a Nitrogen Atmosphere. In Studies in Surface Science and Catalysis; Sayari, A., Jaroniec, M., Eds.; 141; Elsevier, 2002; pp 661–668. Lezau, A.; Skadtchenko, B.; Trudeau, M.; Antonelli, D. Dalton Trans. 2003, 4115–4120. Bonnemann, H.; Brijoux, W.; Brinkmann, R.; Dinjus, E.; Fretzen, R.; Joußen, T.; Korall, B. J. Mol. Catal. 1992, 74, 323–333. Chatt, J.; Watson, H. R. Nature 1961, 189, 1003–1004. Blackburn, D. W.; Britton, D.; Ellis, J. E. Angew. Chem. Int. Ed. 1992, 31, 1495–1498. Ellis, J. E.; Blackburn, D. W.; Yuen, P.; Jang, M. J. Am. Chem. Soc. 1993, 115 (11), 616–11,617. Jilek, R. E.; Jang, M.; Smolensky, E. D.; Britton, J. D.; Ellis, J. E. Angew. Chem. Int. Ed. 2008, 47, 8692–8695. Jang, M.; Ellis, J. E. Angew. Chem. Int. Ed. 1994, 33, 1973–1975. Kucera, B. E.; Jilek, R. E.; Brennessel, W. W.; Ellis, J. E. Acta Crystallogr. C 2014, 70, 749–753. Scholz, J.; Thiele, K.-H. J. Organomet. Chem. 1986, 314, 7–11. Gardner, T. G.; Girolami, G. S. Angew. Chem. Int. Ed. 1988, 27, 1693–1695. Seaburg, J. K.; Fischer, P. J.; Young, J.; Victor, G.; Ellis, J. E. Angew. Chem. Int. Ed. 1998, 37, 155–158. Diamond, G. M.; Green, M. L. H.; Walker, N. M.; Howard, J. A. K.; Mason, S. A. J. Chem. Soc. 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4.10

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

Oleh Stetsiuk, Valeriu Cemortan, Thomas Simler, and Grégory Nocton, Laboratoire de Chimie Moléculaire, CNRS, Ecole polytechnique, Insitut Polytechnique de Paris, Palaiseau, France © 2022 Elsevier Ltd. All rights reserved.

4.10.1 Introduction 4.10.2 Complexes based on cycloheptatrienyl ligands 4.10.3 Complexes based on cyclooctatetraenyl ligands 4.10.3.1 Sc, Y, La 4.10.3.2 Ce 4.10.3.3 Pr, Nd, Pm, Sm, Eu 4.10.3.3.1 Synthesis and properties 4.10.3.3.2 Chemical properties of the dinuclear [(COT)Ln(m-Cl)(THF)2]2 complexes 4.10.3.4 Gd, Tb, Dy, Ho, Er, Tm, Yb 4.10.3.5 Lu 4.10.3.6 Magnetism of Ln-based complexes 4.10.3.7 Polynuclear Ln-based complexes 4.10.4 Complexes based on pentalenyl ligands 4.10.4.1 Ce 4.10.4.2 Sm, Eu, Dy, Yb 4.10.5 Complexes based on cyclononatetraenyl ligands 4.10.6 Conclusions and outlook Acknowledgment References

4.10.1

550 551 552 552 557 561 561 563 564 566 567 568 573 573 574 575 578 578 578

Introduction

In 1954, the first organolanthanide compounds, i.e., the tris(cyclopentadienyl) lanthanide complexes, [Cp3Ln], were reported by Birmingham and Wilkinson.1 The organometallic chemistry of group 3 and lanthanide complexes has since been widely developed and the complexes obtained based on these elements were found to be very promising platforms for the development of new materials possessing sophisticated magnetic2–5 and catalytic properties.6 Despite a large number of reports dedicated to this topic, the large majority of reported organolanthanide complexes is based on the cyclopentadienyl ligand and its derivatives, while larger aromatics are comparatively less represented. Thus, this article aims at presenting a detailed analysis of lanthanide-based complexes with the large-sized cycloheptatrienyl (A), cyclooctatetraenyl (B), pentalenyl (C), and cyclononatetraenyl (D) ligands (Fig. 1). It should be noted that this article summarizes for the first time all the data reported for the large-sized C7 and C9 aromatic ligands. In contrast to the well-documented organometallic lanthanide chemistry based on cyclopentadienyl (Cp) ligands, examples of lanthanide complexes supported by larger aromatic ring systems are under-represented, especially those built with the ligand frameworks A, C and D. This relative scarcity may be traced back to synthetic difficulties in the corresponding ligand synthesis and derivatization. Among the ring systems A–D, the cyclooctatetraenyl COT2− ligand is easily accessible by reaction of alkali metals with neutral cyclooctatetraene, C8H8, which has led to numerous examples of COT-supported lanthanide complexes. Although further functionalization of the COT ring has been described, it has been mostly restricted to the introduction of two or three silyl substituents. As described in this article, the ring system D suffers from further synthesis and solubility issues, which has hampered, so far, its use as a supporting ligand in organolanthanide chemistry. In addition, compared to the monoanionic Cp ligands, the C7 and C8 frameworks A–C feature a larger anionic charge, which makes them prone to act as bridging ligands. Controlling the reactivity of the different ligands often requires specific synthetic procedures. Since the use of large aromatic ring systems as ligands in organolanthanide chemistry is a relatively young area of research, it then requires a detailed synthetic and crystallographic description of the corresponding sandwich and half-sandwich compounds, especially in the case of large aromatic ligands. These descriptions will be enhanced with valuable discussions of their chemical and physical properties. Besides the typical mono- and dinuclear complexes, polynuclear aggregates, possessing more sophisticated crystal structures, will be discussed in detail. Within these large ring systems, the bonding situation and covalency in f-element COT-supported complexes has been examined, revealing a symmetry match between some of the valence 4f-orbitals and ligand group orbitals and leading to d-bonding interactions.7,8 As exemplified in the case of [Ln(COT)2]− fragments, among the different symmetry-permitted combinations between the ligand p orbitals and lanthanide 4f-orbitals in the D8h point group, contributions of E2u symmetry lead to the 1e2u and 2e2u bonding and antibonding 4f-d molecular orbitals, respectively (Fig. 2). The bonding situation and covalency in formally tetravalent neutral lanthanide and actinide sandwich complexes, such as cerocene [Ce(COT)2] and uranocene [U(COT)2], has been especially investigated.9–11

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https://doi.org/10.1016/B978-0-12-820206-7.00018-4

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Fig. 1 Structures of the large anionic aromatic ligands discussed in this article as a function of the ring size, cycloheptatrienyl (A), cyclooctatetraenyl (B), pentalenyl (C) and cyclononatetraenyl (D).

Fig. 2 Symmetry match of valence 4f-orbitals with COT-ligand group orbitals in D8h symmetry to form d-bonding interactions in [Ln(COT)2]− fragments (1e2u and 2e2u correspond to the bonding and antibonding contributions, respectively).

Additionally, the large number of f-electrons in the lanthanide ions, combined with the flexibility of the large-sized C7–C9 aromatic ligands, make the corresponding compounds also promising objects for magnetochemistry, although these complexes remain scarce. The few reports describing the magnetic behavior of the lanthanide complexes based on such large aromatic ligands have been mostly published by the groups of Murugesu,12 Nocton,13 and Roesky,14 and will be presented in this chapter.

4.10.2

Complexes based on cycloheptatrienyl ligands

Among the class of aromatic ligands, the cyclopentadienyl (5-C5R5) and the cyclooctatetraenyl (8-C8R8) are arguably the most common categories of ligands encountered in organometallic lanthanide chemistry. The corresponding seven-membered aromatic ring—cycloheptatrienyl (7-C7H7, henceforth described as CHT)—formally maintains aromaticity within two possible electronic configurations: the 6p-electron tropylium cation and the 10p-electron CHT trianion. The synthesis of the latter, as well as that of a mono-alkyl derivative, is shown in Scheme 1.15,16

Scheme 1 Synthesis of the trianionic CHT ligand and of a mono-alkyl derivative.

A breakthrough in the syntheses of the first transition metal complexes bearing the CHT ligand was achieved in the late 1950s— Wilkinson disclosed the first molybdenum cycloheptatriene (6-C7H8) complex in 1958,17 and the first molybdenum tropylium complex was described in the same year.18 The development of the chemistry of these ligands has received little interest in comparison with that of cyclopentadienyl and arene ligands. Up to the late 1970s, the CHT-based complexes were limited to the group 4 to 6 metals and sandwich compounds remained scarce.19 More recent work by the groups of Braunschweig, Tamm, Murahashi and Hurst, among others, has extended the family of CHT-based complexes towards later transition metals, in the form of polymetallic clusters,20 precursors for polymers21,22 and multi-metallic sandwich complexes.23,24

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The first attempts in the complexation of the monoanionic CHT ligand with f-elements were performed by DeKock and Miller in 1981.16 Following the addition of LiC7H9 to uranium(IV) chloride, the formation of a uranium-trianionic CHT complex was attested by 1H NMR spectroscopy (one single proton resonance for the CHT was observed) and by analysis of the compounds formed upon hydrolysis. The main product of the metalation reaction, however, was cycloheptadiene (80% in a mixture of isomers) while the C7H3− 7 ligand was formed in only around 20% yield. The mechanism inferred by the authors involved an increased acidity of the methylene protons upon coordination of uranium, which facilitates the abstraction of two protons by C7H−9 ions, leading to the formation of C7H3− 7 and two equivalents of cycloheptadiene. This study also offered the first trials of coordination of the CHT ligand with lanthanides. The authors reported that the reaction of the chloride salts of ThIV, ErIII and GdIII with the cycloheptatrienyl trianion derived from lithium cycloheptadienide led to results similar to those of UIV, without further characterization. The first CHT 4f-complex characterized in the solid state was published 15 years later by the group of Ephritikhine,25 following their work on uranium-CHT complexes.26,27 The addition of a NdIII borohydride precursor to the monoanionic CHT ligand proceeded in an analogous manner to that described by DeKock: the ligand disproportionation resulted in the formation of cycloheptadiene and an inverse-sandwich complex bearing one trianionic CHT ligand, with the two Nd centers 7-coordinated to the bridging CHT (Scheme 2).

Scheme 2 Synthesis of the inverse-sandwich CHT-Nd complex 1.

Further CHT-based complexes of GdIII, DyIII and ErIII (2-Ln) were described by Murugesu et al. 20 years afterwards (Scheme 3).12 Using a procedure similar to that of Ephritikhine, unsymmetrical CHT amido complexes were obtained, bearing a potassium ion coordinated to two amido ancillary ligands. This asymmetry is manifested in the different angles between the metal centers and the coordinated nitrogen atoms. In the case of 2-Er, the crystal structures of both the solvated and unsolvated potassium complexes were described. Both magnetic and theoretical studies showed that the CHT3− ligand promotes ferromagnetic interactions between the two metal centers.

Scheme 3 Preparation of the complexes 2-Ln.

4.10.3

Complexes based on cyclooctatetraenyl ligands

4.10.3.1

Sc, Y, La

Scandium, yttrium and lanthanum are the first elements within the series of nd transition metals (n ¼ 3–5). Their electronic configurations are nd1-(n + 1)s2, where the energy gap between the nd1 and the (n + 1)s2 subshells is negligible. Further filling of d-orbitals increases the value of the gap and results in the stabilization of multiple stable oxidation states for the middle row transition metals. This is not the case for Sc, Y and La, where the most stable oxidation state is +3. At this electronic configuration, they are diamagnetic and show properties similar to those of alkali metals. Thus, they form primarily ionic bonds. Despite the fact that complexes with aromatic ligands have been studied for the past 50 years, the number of compounds based on Sc, Y and La with large aromatic ligands is still limited. The most convenient way to synthesize such compounds is via salt-metathesis reactions in polar solvents, e.g., THF. To the best of our knowledge, the first Y-based Cp/COT mixed-sandwich complex, [(COT)Y(Cp)] (3-Y), was reported in 1974, being obtained by a salt metathesis reaction starting from the dimeric [(COT)Y(m-Cl)(THF)]2 (4-Y) species or from the Cp halfsandwich [CpYCl2(THF)3] (Scheme 4(a)).28 The first scandium half-sandwich complex, as well as the corresponding mixed Cp/COT sandwich 3-Sc, were isolated by Westerhof in 1976 (Scheme 4(b)) and characterized spectroscopically.29 Recent structural studies by X-ray diffraction showed that the reaction between ScCl3 and K2COT in THF yields the dinuclear complex [(COT)Sc(m-Cl)(THF)]2 (4-Sc).30 Complex [(COT)LaI(THF)3] (5-La) was first described in 1989 and was obtained from direct reaction of the zerovalent metal with COT in the presence of half an equivalent of suspended I2 (Scheme 4(c)).31,32 The major advantage of this method is avoiding the formation of side products, thereby obtaining the desired complex in high yields (>90% for La). However, because of the rather long reaction time (48 h), generalizing this method for the entire lanthanide series would be difficult. If lanthanum metal, which is among the most chemically active of the series, requires 48 h for its dissolution, the reaction time with the rest of the lanthanide series is foreseen to be longer. The mononuclear complex 5-La can easily form the stable dinuclear [(COT)La(m-I)(THF)2]2 (6-La), which

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aggregates from a Lewis acid/base competition reaction upon treatment with trimethylaluminum (Scheme 5).33 In both the dinuclear complexes 4-Sc and 6-La, the COT ligand exhibits a 8-coordination, with the ScdC and LadC bond lengths ranging from 2.417(2) to 2.438(2) A˚ and from 2.720(2) to 2.753(3) A˚ , respectively. The larger ionic radius of La3+ allows the coordination of two molecules of THF, whereas for the smaller Sc3+ only one molecule is inserted in its coordination environment.

3-Y

4-Sc

3-Sc

Scheme 4 Preparation of Sc, Y and La-based metallocenes.

Scheme 5 Preparation of [(COT)La(m-I)(THF)2]2 (6-La).

Investigation of the behavior of 6-La in solution showed that the dimer is stable; the re-formation of the mononuclear starting complex is not favored, even in the presence of excess donor THF ligand as solvent. Furthermore, stirring a solution of 5-La in toluene in absence of trimethylaluminum does not lead to the formation of the dimer 6-La. The reaction of [(COT)LaI(THF)3] (5-La) with lithium and potassium silylamides led to rather unexpected results (Scheme 6).33 The reaction with [LiN(SiHMe2)2] yielded the mixed iodide/amide complex [(COT)La(m-I){m-N(SiHMe2)2}2La{m-2:8-COT}Li(THF)3] (7), while the reaction with [KN(SiHMe2)2] led to the iodide-free [(COT)La{N(SiHMe2)2}(THF)2] product (8). Such different behavior may be explained by the chemical nature of the Li and K precursors: potassium silylamide, being more ionic, may shift the equilibrium towards the formation of insoluble KI, thus disfavoring the binding of any iodide anion, while all the Li derivatives (iodide and silylamide) are partially soluble in THF resulting in the isolation of the mixed iodide/amide complex 7. The same reaction with a potassium diphosphanylamide salt retained the monomeric arrangement, leading to 9 (Scheme 6), the latter exhibiting an 2 coordination mode with respect to the P-N ligand.34

Scheme 6 Reactivity of [(COT)LaI(THF)3] (5-La) towards amides.

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Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

The reactivity of the dimeric species [(COT)Y(m-Cl)(THF)]2 (4-Y) with a selection of substrates was studied by Schumann in the 1990s (Scheme 7). Upon addition of NaOAr (Ar ¼ Ph, 2,6-C6H3Me2), the dimeric arrangement was maintained, yielding the phenoxide complexes [(COT)Y(m-OAr)(THF)]2 (10a-10b).35 However, analogous reactions of 4-Y with the tertiary alkoxide and siloxide M(OR) (M ¼ Li, R ¼ C(tBuC)3; M ¼ Na, R ¼ SiPh3; respectively), which are both sterically encumbered, afforded the corresponding monomeric adducts [(COT)Y(OR)(THF)] (11a-11b). The addition of the organolithium reagent LiCH(SiMe3)2 to 4-Y resulted in the bis-silylalkyl complex [Li(THF)2(m-2:8-COT)Y{CH(SiMe3)2}2] (12-Y),36 where a Li-COT-Y inverse-sandwich motif was observed, similar to the arrangement displayed in the La complex 7.

Scheme 7 Reactivity of [(COT)Y(m-Cl)(THF)]2 (4-Y).

In 2009, Tamm and coworkers published a series of COT rare-earth metal complexes supported by imidazoline-2-imide ligands (Scheme 8).37 This family of ligands, known to have a strong electron-donating capacity towards early transition metals, has found a variety of uses in organometallic chemistry, most notably as ancillary ligands in polymerization catalysis.38 Following deprotonation of 3-bis(2,6-diisopropylphenyl)imidazolin-2-imine (L1H), the coordination of the ligand was achieved using trichloride precursors of Sc and Y to yield the corresponding COT-Ln-imide complexes [(L1)M(COT)(THF)n] (14-M, M ¼ Sc, Y) in two steps (Scheme 8). Furthermore, the Sc complex 14-Sc underwent a [2 + 2] cycloaddition reaction with m-xylene isothiocyanate to give complex 15 featuring an N-(imidazolin-2-ylidene)-N0 -(2,6-dimethylphenyl)thioureato ligand. The authors outlined preliminary results of promising catalytic activity towards ring-opening polymerization of lactones.

Scheme 8 Formation and reactivity of Sc and Y-based imido complexes.

The possibility for COT-based complexes to possess functional properties has also been demonstrated in 2005 with the report of moderate catalytic activity in hydroamination/cyclization reaction of terminal aminoolefins and alkynes for Y and other lanthanides.6 The cyclooctatetraenyl bis(iminophosphorano)methanide complex of yttrium [{CH(PPh2NSiMe3)2}Y(8-COT)] (16-Y) can be easily prepared in a one-pot reaction, in which the potassium complex [K{CH(PPh2NSiMe3)2}] is reacted with anhydrous yttrium trichloride in THF, followed by the addition of a stoichiometric amount of K2COT in THF (Scheme 9).6 An analogous architecture was obtained by using the substituted 1,4-bis(trimethylsilyl)cyclooctatetraenyl dianion (COT00 ¼ 1,4-{C8H6(SiMe3)2}) leading to [{CH(PPh2NSiMe3)2}Y(8-COT00 )] (17-Y).39 The moderate catalytic activity of the yttrium complex can be explained by its structural features. From the mechanisms of lanthanide-metallocene-catalyzed reactions proposed by Marks,40 the key factor is understood to be the accessibility of the Ln ion to produce a lanthanide amido-based intermediate, which then undergoes cyclization. Therefore, the presence of bulky SiMe3 groups on the COT ligand, which strongly shield the yttrium ion, renders the catalytic process less efficient.

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

555

Scheme 9 Preparation of the complexes 16-Y and 17-Y.

Table 1

Compounds of the general formula [(COT)Ln(Cp0 )(THF)n].

Complexes

18-M

19-M

20-M

3-M

Co-ligand M ¼ Sc M¼Y M ¼ La

Cp

C5HMe4

C5H4Me

+ +

+ +

+

Cp + +

The formation of mixed-sandwich compounds based on the COT and Cp backbones together with group 3 metals was pursued by Schumann in the 1990s. Table 1 summarizes the main isolated and characterized compounds of the general formula [(COT)M(Cp0 )(THF)n], where Cp0 ¼ Cp (18-M), C5HMe4 (19-M), C5H4Me (20-M), Cp (3-M) and n ¼ 0–2.28,36,41–44 These complexes were obtained from either the trishalide metal salts or the corresponding COT half-sandwich precursors. Sandwich compounds were also obtained with either substituted or functionalized rings (Fig. 3). In 2000, Qian and coworkers isolated a lanthanum sandwich with a chelating ether side-chain substituent, [(Z5-C4H7OCH2C5H4)La(COT)(THF)] (21-La).45 The group of Wang and Gao isolated [(COT)Y(Dsp)] (22-Y), where Dsp ¼ 3,4-dimethyl-2,5-bis(trimethylsilyl)phospholyl.46 Boronbased sandwiches based on group 3 metals were developed by the groups of Meng47—with the boratabenzene-COT yttrium complex [C5H5B-NEt2)Y(COT)] (23-Y)—and Braunschweig—with the scandium complex [{C5H4B(NMe2)2}Sc{C8H7B(NMe2)2}] (24) bearing boryl substituents on both the Cp and COT rings (Scheme 10).48 Silyl-substituted Cp/COT scandium analogs of 24, [(C5H4R)Sc(C8H7R0 )] with R, R0 ¼ H, SiMe3 (25a-25c), were described by the same group (Scheme 10).48

Fig. 3 Substituted or functionalized sandwiches with group 3 metals.

Scheme 10 Synthesis of Cot/Cp sandwich complexes with boryl and silyl substituents.

A number of reports have examined the reactivity of the mixed sandwich compounds described above. The [(COT)Y(Cp )] complex 18-Y was reacted with the N-heterocyclic olefin ligand 1,3,4,5-tetramethyl-2-methylenimidazoline (L2) to give the ylidic olefin complex [(COT)Y(Cp )(L2)] (26-Y) (Scheme 11).49 Analysis of the resulting crystal structure pointed to a highly polarized Y-methylene single bond (longer than other reported Y–Calkyl bonds), as well as the aromatization of the N-heterocyclic ligand. This

556

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

conclusion is consistent with previously observed behavior for this family of ligands: three mesomeric forms are proposed upon coordination, two of which display a polarized M–CH2 bond, owing to delocalization of the negative charge (Scheme 11).

Scheme 11 Formation of the yttrium-based ylidic olefin complex 26-Y and possible mesomeric forms.

More recent work on the [(COT)Sc(Cp)] complex 3-Sc has demonstrated the possible stoichiometry-dependent metalation of the two rings.50 Upon addition of one equivalent of Li(TMP) and iBu2Al(TMP) (TMP ¼ 2,2,6,6-tetramethylpiperidide), the Cp ring was functionalized leading to 27 (Scheme 12). Three equivalents of the Li- and Al-based metalating agents were necessary to achieve additional metalation of the COT ring (Scheme 12).

Scheme 12 Stoichiometry dependent metalation (TMP ¼ 2,2,6,6-tetramethylpiperidide). If n ¼ 1, metalation occurs on the Cp ring. If n ¼ 3, a second metalation occurs on the COT ring (shown in gray).

The complexes described above mostly featured unsubstituted COT ligands. In addition, many efforts were devoted to the development and the investigation of complexes based on substituted COT ligands. The initial idea came from the pioneering work reported by Cloke,51 who first employed the 1,4-bis(trimethylsilyl)cyclooctatetraenyl dianion (COT00 ) in organometallic f-element chemistry. According to the original procedure, COT00 was obtained starting from 1,3,5,7-cyclooctatetraene, which was doubly reduced by metallic potassium to give K2COT. A double silylation reaction with two equivalents of chlorotrimethylsilane gave 1,4-bis(trimethylsilyl)cycloocta-2,5,7-triene in 80% yield (Scheme 13). Deprotonation of the latter with two equivalents of n-butyllithium afforded Li2COT00 in high yield. The addition of Li2COT00 to trichloride salts of Y and Sc yielded in both cases, following spectroscopic characterizations, the dimeric complexes [(COT00 )Y(m-Cl)THF]2 (28-Y) and [{(COT00 )Sc(m-Cl)(m-THF)0.5}]2 (29), the latter of which was shown to have one semi-bridging THF ligand (Scheme 13), as evidenced by the asymmetry of the Sc–OTHF bond lengths.

Scheme 13 Synthesis of Li2COT00 and subsequent reaction with [ScCl3(THF)3].

Cloke and coworkers reported two possibilities for the synthesis of COT00 -based complexes: by reaction with the isolated Li2COT00 salt or by using in situ prepared solutions of alkali metal COT00 reagents. The second method was found to be more convenient.52 The influence of the bulky substituents on the molecular solid-state structures of the lanthanide-based complexes was then analyzed: the COT00 ring in the [Li(DME)3][Ln(COT00 )2] complexes (30-Ln, DME ¼ 1,2-dimethoxyethane) was found to be 8-coordinated in the case of Ln ¼ La and Y (Fig. 4). Owing to the smaller size of the Sc3+ ion, the coordination mode of functionalized COT ligands in

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

557

Fig. 4 Structures of [Li(DME)3][Ln(COT00 )2] (30-Ln, Ln ¼ Y, La) and [K(DME)][Sc(COTBIG)2] (31-Sc).

scandium complexes was found to be quite different, which is apparent in both [K(DME)][Sc(COTBIG)2] (31-Sc), where COTBIG is the very bulky [C8H6(SiPh3)2]2− anion, and [Li(L)][Sc(COT00 )2] (32, L ¼ (DME)3 or diglyme).53 X-ray diffraction analysis of 31-Sc revealed a slipped 4,8-coordination of the COTBIG rings to Sc (Fig. 4),54 whereas 32 displayed a 3,8-coordination for the two COT00 ligands. In both cases, the second (slipped) coordination site is centered on the silyl moieties. The partial coordination of the second ring induces noticeable deviations from planarity, implying loss of aromaticity of the substituted COT ligand. Such a loss of aromaticity was also observed in the attempted synthesis of the triple-decker [Sc2(COT00 )3] complex. Edelmann and coworkers isolated two products after oxidation of Li[Sc(COT00 )2] (33) with CoCl2: the first was the expected triple-decker, [Sc2(COT00 )3] (34), which was crystallized in 36% yield (Scheme 14);53 the second product was a novel compound, whereupon the (COT00 )2− ligand has been submitted to a single-electron oxidation, followed by the dimerization of the resulting (COT00 )%− radicals and accompanied by the formation of an intramolecular CdC bond between the positions 3 and 7 of each ring.53 The resulting ligand, the bis(tetrahydropentalenyl) dianion (BTHP2−), is coordinated on each side in a 3 fashion via the C3(SiMe3) fragment (where the negative charge is delocalized), bridging two Sc ions and yielding the [(COT00 )Sc(m-3:3-BTHP)Sc(COT00 )] compound 35 (Scheme 14). Table 2 summarizes selected metrical and NMR spectroscopic data of relevant Sc, Y and La complexes.

Scheme 14 Oxidation of Li[Sc(COT00 )2] (33) yielding the triple-decker [Sc2(COT00 )3] (34) and [(COT00 )Sc(m-3:3-BTHP)Sc(COT00 )] (35).

4.10.3.2

Ce

The electron configuration of cerium is [Xe] 4f25d06s2. However, because the energy gap between the 4f and the 5d subshells is negligible at the beginning of the lanthanide row, a second electronic configuration is possible as [Xe] 4f15d16s2. Thus, cerium exists in two stable oxidation states, +3 and +4. Cerium in its higher oxidation state has proven to be a very good oxidant, while the COT2− anion is a strong reducing agent. A few attempts have investigated the possibility to combine the COT2− anion with the Ce4+ cation.

558

Table 2

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

Most representative examples of Sc-La COT complexes with their parameters. Mn+

Molecular formula

˚) M-Ccentroid (A 2.427 2.025 2.042 2.046; 2.048 2.064 10a: 1.77; 1.85

[(COT)Sc(m-Cl)(THF)]2 [(COT)La(m-I)(THF)2]2 [(COT)LaI(THF)3] [(COT)La(m-I){m-N(SiHMe2)2}2La{m-2:8-COT}Li(THF)3] [(COT)La{N(SiHMe2)2}(THF)2] [(COT)Y(m-OAr)(THF)]2, [Ar-Ph (10a), C6H3Me2-2,6 (10b)] [(COT)Y(OR)(THF)], [R-OSiPh3 (11a)], OC(tBu)3 (11b)

– [Li(THF)2(m-2:8-COT)Y{CH(SiMe3)2}2] [{CH(PPh2NSiMe3)2}Y(8-COT)] [(COT00 )Y(m-Cl)THF]2

+3

[{(COT00 )Sc(m-Cl)(m-THF)0.5}]2 [Li(DME)3][Ln(COT00 )2] (Ln ¼ Y-30a, La-30b)

– 2.7654 – 1.535; 1.550 30a: 1.900 30b: 2.084

d 13C NMR in ppm for COT a

– 97.9 98.4 – 97.7 10a: 94.0 10b: 92.3 11a: 93.4 11b: 94.3 93.4 – 101.61, 101.04, 100.18, 97.77 107.68, 104.76, 103.36, 100.5 30a: 99.4, 99.3, 97.3, 96.6 30b: 103.6, 102.6, 101.3, 100.3

Number

References

4-Sc 6-La 5-La 7 8 10a 10b 11a 11b 12-Y 16-Y 28-Y

30 33 33 33

29 30-Ln

51 52

35

36 6 51

a

– Analyzes were not reported.

55 Given their redox potentials (E0(CeIV/CeIII) ¼ +1.61 V; E0(C8H8/C8H2– fast oxidation of the COT2− ligand 8 ) ¼ –1.75 to –1.99 V), may occur following the reaction in Eq. (1).

C8 H8 2 − + 2Ce4 + ! C8 H8 + 2Ce3 +

(1)

IV

In this matter, the synthesis of [Ce (COT)2] (36a) was a challenging target, as it corresponds to the analog of uranocene, [UIV(COT)2], which was obtained in 19689 and crystallographically characterized in 1969.56 The first reports of cerocene synthesis were published in 1976,57 and confirmed much later in 198558 and 1991 by the group of Streitwieser.59 The first X-ray crystal structure was described in 1991 using the methyl-substituted COT ligands (COTMe).59 The synthesis of [Ce(COTMe)2] (36b) required inert atmosphere techniques and the compound was obtained in THF solution upon oxidation of K[CeIII(COTMe)2] (37b) with freshly ground AgI, followed by recrystallization from hexane (Scheme 15). Note that the glovebox needed to be illuminated with a red light in order to reduce the influence of a non-monochromatic light on the complex. The crystal structure of 36a was only reported in 2004 by Krüger as a CSD communication.60 The complex crystallized in the monoclinic crystal system with half of the molecule in the asymmetric unit. The average Ce-C distance is 2.67 A˚ and the Ce-C(centroid) distance of 1.969 A˚ is similar to that in 36b (1.971 and 1.975 A˚ ). It should be noted that the presence of methyl groups in 36b broke the symmetry of the compound and the complex therefore crystallized with the entire molecule as an independent fragment.

Scheme 15 Preparation of [Ce(COT)2] (36a) and the substituted analog [Ce(COTMe)2] (36b).

The combination of the strongly reducing COT2− ligand with the oxidizing Ce4+ ion in cerocene is expected to lead to an unusual electronic structure. A detailed analysis of cerocene, as well as a convenient new synthesis involving oxidation of 37a with p-benzoquinone (see Scheme 15), was reported in 2009 by Andersen and co-workers.61 Cerium L3-edge X-ray absorption structure (XAS) measurements were performed on 36a and on the triple-decker complex [Ce2(COT)3] (38-Ce) for comparison. The outcome of this study revealed that cerocene is an intermediate-valent compound with a fractional f-occupancy,62 nf ¼ 0.89, which corresponds to a partial CeIII and CeIV character, with a major CeIII contribution.63 It should be noted that formally tetravalent Ce compounds such as CeO2 and CeF4 possess nf values of ca. 0.50,64,65 implying that they are also intermediate valent but with a lower CeIII/CeIV ratio. Extensive computational studies were performed to investigate the electronic structure of cerocene (36a). Early ab initio calculations by Dolg, Fulde and co-workers revealed that the ground-state wavefunction of cerocene is

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

559

multiconfigurational, composed of both CeIII and CeIV contributions.66–68 A major CeIII contribution was indicated by the calculations, supporting the data from XANES studies. However, depending on the methodology used in the theoretical investigations, different descriptions of the multiconfigurational ground state are possible. An analysis of the occupations of the natural orbitals was performed by Kerridge, Kaltsoyannis and co-workers, leading to the conclusion that cerocene is best described as a formal CeIV compound with non-negligible covalency that arises from electron sharing between the COT ring and the cerium ion.10,11 As a result, although the formal oxidation state of 36a approaches +4, its experimental oxidation state inferred from XAS studies can be viewed as closer to +3. As a matter of fact, the oxidation number is not well defined in the context of multiconfigurational ground-state wavefunctions. In contrast, the XANES spectrum of 38-Ce clearly indicated a net trivalent nature for the cerium center in this triple-decker complex.63 In very recent theoretical studies, Sergentu, Autschbach and co-workers investigated whether core-excited states related to the cerium L3-edge peaks reflect or not the ground-state nf of cerocene.69 The Ce L3-edge XAS spectrum of cerocene was analyzed using multiconfigurational wavefunction ab initio calculations, which accurately reproduced the number and relative intensities of the peaks in the experimental XAS spectrum. The conventional approach, in which the relative areas of the two XAS peaks of cerocene originate primarily from CeIII and CeIV configurations and reflect the respective contributions in the ground state was found to be of reasonable accuracy. The relative CeIII and CeIV final-state contributions roughly correspond to the value of nf in the ground state with a potential error of ca. 5%. As established by the calculations, this error arises from the degree of correspondence between the core-excited final states and the ground-state configurations.69 Alternatively, other attempts were performed using the 1,4-bis(trimethylsilyl)cyclooctatetraene (COT00 ) ligand. The presence of the substituted ligand makes the desired complex more soluble in organic solvents and therefore more attractive for reactivity studies. The cerium complex [CeIV(COT00 )2] was easily prepared by reaction of the lithium salt of COT00 with CeCl3 followed by addition of AgI as a soft oxidizing agent.70 The same reaction performed with COT ligands bearing larger substituents, such as the very bulky COTBIG ligand, led to an unprecedented rearrangement of the latter (Scheme 16).54 Indeed, X-ray analysis of the final product revealed that the neutral sandwich complex [Ce(COTBIG)2] (39) contained the 1,3-isomer of COTBIG, while the starting lithium salt [Li(DME)2][Ce(COTBIG)2] (40-Ce) contained the 1,4-COTBIG isomer. The reason for the migration of the SiPh3 groups is likely due to a stronger inter-ligand steric repulsion between the bulky silyl substituents on the COT rings in the CeIV complex, compared to that in the CeIII analog. Indeed, the oxidized Ce4+ ion presents a smaller ionic radius and a higher charge.

Scheme 16 Preparation of the complex [Ce(COTBIG)2] (39).

The strong oxidizing properties of CeIV complexes based on the 1,3,6-trimethylsilylcyclooctatetraene (COT000 ) ligand were demonstrated in 1999.71 The reaction of [Ce(COT000 )2] (41) with zerovalent Yb powder yielded the first example of a heterobimetallic [Yb(THF)6][Ce(COT000 )2]2 complex (42) (Scheme 17). The single crystal X-ray diffraction analysis of 42 revealed that the structure consisted of separated [Yb(THF)6]2+/[Ce(COT000 )2]− ion pairs, with a Ce-C(centroid) average distance of 2.015 A˚ . Although six bulky SiMe3 groups are present, no significant elongation of the Ce–C(centroid) bonds was observed.

Scheme 17 Preparation of [Yb(THF)6][Ce(COT000 )2]2 (42).

Since complexes of CeIII are more thermodynamically stable than their oxidizing CeIV counterparts, they are also more common. Their syntheses and investigations started in the 1970s and one of the first compounds reported was the anionic cerocene, K[Ce(COT)2] (37a). It was first obtained in 1970 from the reaction of CeCl3 with K2COT in THF.72 The crystal structure of

560

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

37a was determined 2 years later.73 The similar reaction of Ce(OTf )3 with K2COT provided the bridged triflate complex [(COT)Ce(m-OTf )(THF)2]2 (43-Ce), which is structurally analogous to the lanthanum complex 6-La (Scheme 5).74 The half-sandwich complex 43-Ce can then be used as a precursor for the preparation of the mixed-sandwich complex [(COT)Ce(5–1,3-tBu2C5H3)] (44-Ce) (Scheme 18).

Scheme 18 Preparation of [(COT)Ce(5–1,3-tBu2C5H3)] (44-Ce).

Other dinuclear CeIII compounds with anionic amidinate ligands were also used in the synthesis of a series of inverse-sandwich complexes possessing a rare structure, in which the planar COT ring is sandwiched between two cerium ions. For example, the reaction of the cerium amidinate complex [{{c-C3H5–CC–C(NR)2}2 (45-Ce) with K2COT afforded the new dinuclear inverse-sandwich complex (m-8:8-COT)[Ce{c-C3H5–CC–C(NR)2}2]2 (46-Ce) (Scheme 19).75

Scheme 19 Synthesis (m-8:8-COT)[Ce{c-C3H5–CC–C(NR)2}2]2 (46-Ce).

Unlike ScIII, YIII and LaIII, the CeIII ion is paramagnetic, with an f1 electronic configuration. Thus, the study of the magnetic properties of several CeIII COT complexes has attracted particular attention, in the aim of looking for any Single-Molecule Magnet (SMM) behavior. SMM are molecules that exhibit slow relaxation of their magnetization from a purely molecular origin: cooperative interactions between molecules are not necessary to allow this phenomenon (see Section 4.10.3.6), which is a key difference with bulk magnets. The rapidly growing development and investigation of SMMs started soon after the report of the mixed-valence Mn12 cluster [Mn12O12(CH3COO)16(H2O)4]∙2CH3COOH∙4H2O by Sessoli et al. in 1991,76 and has involved lanthanide molecules since 2003, after the first report of a lanthanide-containing single-ion magnet (the latter featuring only one magnetic ion).77 Large angular momentum (which occurs naturally in the case of lanthanides) and large magnetic anisotropy are key to enabling SMM properties. In this matter, an f1 system is not expected to have the strongest anisotropy (2F5/2 ground state) among the lanthanide series. However, interesting results were published in 2013 on heterometallic trinuclear ZnII-CeIII-ZnII complexes bearing Schiff base ligands derived from o-vanillin and aminoalcohols, and possessing SMM behavior with a moderate effective energy barrier Ueff of 21.2(6) K and a dominant quantum tunneling magnetic relaxation process at low temperatures.78 One year later, the group of Murugesu reported the synthesis of the COT00 -based [Li(DME)3][CeIII(COT00 )2] sandwich complex 30-Ce, being the first example of a monometallic cerium complex with SMM behavior.79 Table 3 summarizes selected metrical and NMR spectroscopic data of relevant Ce complexes.

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

Table 3

561

Most representative examples of Ce COT complexes with their parameters. Mn+

Molecular formula

+4 +4 +3 +4 +3 +3 +3 +3

[Ce(COT)2] [Ce(COTMe)2] [Li(DME)2][Ce(COTBIG)2] [Ce(COTBIG)2] [Yb(THF)6][Ce(COT000 )2]2 [(COT)Ce(m-OTf )(THF)2]2 [(COT)Ce(5–1,3-tBu2C5H3)] (m-8:8-COT)[Ce{c-C3H5–CC–C(NR)2}2]2 [R ¼ iPr (46a), Cy (46b)] [Li(DME)3][Ce(COT00 )2]

+3

˚) M-Ccentroid (A

d 13C NMR in ppm for COT

1.969 1.971; 1.975 2.063; 2.074 1.988 2.015 1.983; 1.985 – 46a: 2.220; 2.244 46b: 2.247; 2.229 2.063

a

– – 147.3, 144.1, 136.3, 131.0 136.9, 128.1, 121.7, 121.4, 112.5 – – – 46a: 107.7 46b: 106.9 –

Number

References

36a 36b 40-Ce 39 42 43-Ce 44-Ce 46-Ce

60 59 54 54 71 74 74 75

30-Ce

79

a

– Analyzes were not reported.

4.10.3.3 4.10.3.3.1

Pr, Nd, Pm, Sm, Eu Synthesis and properties

In the following elements, the 4f-subshell is progressively filled with electrons. Starting from praseodymium, the d-shell is no longer populated and only the f-shell is populated, leading to the configuration [Xe] 4fn5d06s2 (n ¼ 3, Pr; n ¼ 4, Nd; n ¼ 5, Pm; n ¼ 6, Sm; n ¼ 7, Eu). Thus, europium possesses the half-filled [Xe]4f75d06s2 configuration, which drastically influences its properties: the +2 oxidation state for Eu is favorable because of the stable 4f7 configuration. Table 4 shows all the accessible oxidation states for these elements. Considering the much higher value of the redox potential of praseodymium compared to that of cerium (E0(PrIV/PrIII) ¼ +3.4 V vs. NHE),80 the formation of a praseodymium analog of cerocene, i.e., a complex featuring a PrIV metal ion sandwiched between two COT2− ligands, is highly improbable. However, very recently, an example of a PrIV complex based on the tetrakis(triphenylsiloxide) ligand has been reported,81 possibly allowing an entry in the organometallic chemistry of the +4 oxidation state for this element.82 Nd and Sm compounds in the +4 oxidation state only exist as inorganic materials (oxides and fluorides) and can be obtained under very harsh conditions by treatment with F2, XeF2, XeF4 or O3.82 Promethium-based complexes have not been reported due to the radioactive character of the metal and its extremely poor abundance in the Earth0 s crust. In 1976, the first COT complex of neodymium, [Nd(COT)(THF)2][Nd(COT)2] (47-Nd), was reported (Fig. 5).83 It was prepared by vaporization of Nd metal under reduced pressure at high temperature with further co-condensation with COT at −196  C,84 and Table 4 Representation of possible oxidation states for Pr-Eu. Legend: (+), not commonly found for coordination compounds, but examples of complexes have been reported; +, stable;  , exists only in inorganic materials (oxides, salts, etc.). Oxidation state (+) Pr (+)



Nd (+)

Pm

Sm +

Eu +

+4 +3 most stable +2

Fig. 5 Molecular structures of [Nd(COT)(THF)2][Nd(COT)2] (47-Nd), [Nd(COT)(THF)3I] (5-Nd), [(COT)Nd(m-triflate)(THF)2]2 (43-Nd) and [(COT)Sm(m-Cl)(THF)2]2 (4-Sm).

562

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

was the first example of a new class of organolanthanide complexes of general formula Ln2(COT)3. The molecular structure of 47-Nd consisted of a [Nd(COT)2]−/[Nd(COT)(THF)2]+ ion pair with average Nd-C distances of 2.66 and 2.78 A˚ for both COT rings. For the preparation of the half-sandwich complexes [Ln(COT)(THF)3I] (5-Ln, Ln ¼ Nd, Sm, Ce, Tm), the synthetic procedure involved the reaction of Ln powder with free COT in the presence of iodine I2, a reaction similar to that performed with lanthanum (Scheme 4C).32 Alternatively, the NdIII and SmIII complexes were prepared by a salt metathesis reaction between the corresponding [LnI3(THF)3] complexes and K2COT.85 The neodymium complex 5-Nd was crystallized and analyzed by X-ray diffraction studies.85 Within the 5-Ln series, the average Ln-I and Ln-C(centroid) distances decreased with decreasing sizes for the metal cations (3.299(1), 2.010 A˚ ; 3.2626(5), 1.957 A˚ ; 3.2481(4), 1.928 A˚ ; 3.0338(11), 1.750 A˚ for Ce,31 Nd, Sm32 and Tm,86 respectively). Triflate salts of lanthanides are very useful sources of lanthanide precursors; their reaction with K2COT led to the dinuclear complexes 43-Ln (Ln ¼ Y,87 Ce, Pr, Nd, Sm) where the metal ions were linked through SO3CF−3 bridges (Fig. 5). The crystal structures were determined for the Ce and Nd compounds, revealing isostructural arrangements. The geometry of the metal ions can be described as distorted tetragonal-pyramidal. The average Ln-C(centroid) distances, 1.98 and 1.93 A˚ for Ce and Nd, respectively, are similar to those in the corresponding 5-Ln complexes. An alternative preparation of COT-based dinuclear complexes was proposed in 2002 for the SmIII derivative.88 Instead of using the highly air-sensitive K2COT, samarium metal (as a powder) and mercury(II) bis-chloride were stirred in THF in the presence of neutral COT, which resulted in the formation of [(COT)Sm(m-Cl)(THF)2]2 (4-Sm) (Fig. 5) in a good yield. In this reaction, HgCl2 serves as a soft oxidant. As an additional note, the SmIII complex 4-Sm had been first obtained in 1994 by Mashima et al. using Ph3PCl2 as the oxidant.32 As discussed above, europium is the only element within the lanthanide series that forms stable aqueous complexes in the +2 oxidation state. However, the synthesis and characterization of EuII complexes supported by COT ligands proved to be rather challenging, owing to the highly insoluble character of unsolvated LnII(COT) species. The first synthesis of [Eu(COT)] (48-Eu), which corresponds to one of the first reported organometallic lanthanide complexes, dates back to 1969 and consisted in the reaction of free COT with Eu metal in liquid ammonia.89 The identity of the insoluble orange product obtained was confirmed by elemental analysis but the insolubility of the complex prevented further spectroscopic and structural characterization. The successful attempt to synthesize soluble EuII complexes coordinated to the COT ligand was described in 1995.90 The reaction of EuCl3 with K2COT and KC5Me5 in THF resulted in [(C5Me5)(THF)2Eu]2(m-COT) (49-Eu), containing a COT ring sandwiched between two europium(II) ions. Thus, reduction of EuIII to EuII occurred and, in this reaction, both aromatic ligands K2COT and KC5Me5, could serve as reducing agents. Although SmII organometallic complexes are numerous,91 COT complexes of divalent samarium have been less investigated, which can be probably also traced back to the insolubility of unfunctionalized LnII(COT) fragments. Using the substituted COT00 -ligand, several reactions with early lanthanides were found to lead to rather surprising products. The reaction of PrCl3 with Li2COT00 in THF afforded the hexanuclear cluster complex [{Pr(COT00 )}2{Pr2(COT00 )2}2Li2(THF)2Cl8] (50) (Fig. 6), in which the praseodymium ion is trivalent.92 The presence of THF seemed essential for the formation of 50, as shown with the incorporated [Li(THF)]+ moieties. The reaction of Li2COT00 with other LnCl3 (Ln ¼ Ce, Er, Tb) precursors also afforded the formation of similar lanthanide-based clusters; however, a crystalline product was only isolated in the case of Pr. Therefore, the composition of the other products was only determined by elemental analysis and IR spectroscopy. The synthesis of the neodymium analog of 50 by reaction of NdCl3 with Li2COT00 under similar conditions showed a distinct reactivity with the formation of [(COT00 )Nd(m-Cl)(THF)]2 (28-Nd).93 The presence of only one THF coordinated to the Nd ion, compared to the similar [(COT)Nd(m-OTf )(THF)2]2 complex 43-Nd featuring two coordinated THF molecules, is understood to be due to the steric influence of the COT00 substituents. Sandwich complexes of Nd with the COT00 ligand have also been synthesized and studied for their magnetic properties, revealing SMM behavior with a small effective energy barrier to magnetic reversal of 21 K.2 This report is the sole example of a COT-based SMM complex of the early lanthanides after Ce (Pr–Eu).

Fig. 6 Molecular structure of [{Pr(COT00 )}2{Pr2(COT00 )2}2Li2(THF)2Cl8] (50).

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

4.10.3.3.2

563

Chemical properties of the dinuclear [(COT)Ln(m-Cl)(THF)2]2 complexes

The dinuclear [(COT)Ln(m-Cl)(THF)2]2 complexes 4-Ln were found to be suitable precursors to access a variety of COT-supported complexes. The presence of labile or easily substituted ligands, such as THF and chloride anions, allowed the dissociation of the complex in polar solvents and formation of several compounds using different ligands (Scheme 20).

Scheme 20 Reactivity of the dinuclear complexes [(COT)Ln(m-Cl)(THF)2]2 (4-Ln) for the formation of a variety of mixed COT/L complexes.

An interesting reaction concerned the treatment of the neodymium dimer 4-Nd with (LiOSiPh2)2O, which afforded (m-Z8:Z8-COT)[Nd{(Ph2SiO)2O}2{Li(THF)2}{Li(THF)}]2 (51), the first example of a dinuclear lanthanide disiloxanediolate derivative.94 In this inverted-sandwich compound, the COT ligand was found in between two Nd fragments coordinated by the disiloxanediolate ligand. In other reactivity studies, the COT/Ln organometallic fragment remained terminal and did not form a bridge. For example, the potassium complex [K{CH(PPh2NSiMe3)2}] was reacted with 4-Sm, resulting in [{CH(PPh2NSiMe3)2}Sm(Z8-COT)] (16-Sm). The latter revealed a structure similar to that already reported for 16-Y (Scheme 9), and accordingly showed a moderate catalytic activity in intramolecular hydroamination/cyclization reactions.95 In another example, the metathesis reaction between 4-Sm and the sodium salt Na[(C5H5)Co{P(O)(OEt)2}3] afforded the interesting heterobimetallic Co/Sm complex (Z8-COT)Sm[(C5H5)Co{P(O)(OEt)2}3] (52). However, no X-ray diffraction analysis was reported for this compound and its composition was suggested on the basis of elemental analysis and NMR spectroscopy.96 The combination of COT and the tridentate tris(3,5-dimethyl-1-pyrazolyl)borate (TpMe2) ligand resulted in a series of thermally stable complexes of formula [(COT)Ln(TpMe2)] (53-Ln, Ln ¼ Pr, Nd, Sm).97 The crystal structure was described for the samarium complex 53-Sm: the mean Sm–N distance is 2.54 A˚ , the Sm–C bond lengths vary from 2.60(2) to 2.79(4) A˚ with a Sm–C(centroid) distance of 1.914 A˚ , which is similar to the one observed in [Sm(COT)(THF)3I]

564

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

Fig. 7 Molecular structure of [(COT){Nd(c-C3H5–CC–C(NCy)2)(m-Cl)}2]4 (55).

(1.928 A˚ ).33 A similar salt metathesis reaction using amidinate ligand transfer agents also provided a series of Pr- and Nd-based complexes, (COT)Ln[m-c-C3H5–CC–C(NR)2]2Li(THF) (54-Ln), in which the amidinate ligands are chelated to the lanthanide metal center and further coordinated to a lithium cation that holds the overall complex together.75 Both 54-Pr and 54-Nd were structurally similar, with a Ln-C(centroid) distance, as expected, shorter in the case of the Nd complex (2.002 A˚ ), but with a moderate difference between the two metal centers (Pr-C(centroid) distance of 2.016 A˚ ). Furthermore, the bridging character of the amidinate ligand allowed the formation of a unique octanuclear, wheel-shaped neodymium complex, [(COT){Nd(c-C3H5–CC–C(NCy)2)(m-Cl)}2]4 (55) (Fig. 7).75 The latter complex was directly obtained from the reaction of NdCl3 and K2COT in THF in presence of a Li amidinate transfer agent. The molecular structure of 55 consists of four COT ligands sandwiched between Nd3+ metal ions in a m-Z8:Z8 coordination mode. The metal centers are further bridged by two Cl atoms and coordinated to one amidinate ligand. The average Nd-C distances range from 2.826 to 2.835 A˚ , while the Nd-C(centroid) distances are spread from 2.162 to 2.171 A˚ . Table 5 summarizes selected metrical and NMR spectroscopic data of relevant Nd, Pr, Sm, and Eu complexes.

4.10.3.4

Gd, Tb, Dy, Ho, Er, Tm, Yb

Gadolinium is the next lanthanide element after europium, with an electronic configuration in which the 5d shell is populated ([Xe] 4f75d16s2). Consequently, the half-filled f-shell 4f7 in its trivalent state is stabilized. For the rest of the series, the natural filling of the f-shell continues, with a [Xe] 4fn5d06s2 electronic configuration with n ¼ 9 for Tb, up to n ¼ 14 for Yb. Because of their electronic configurations, several of the late lanthanides exist in oxidation states other than the most stable trivalent one (Table 6). For example, the Tb4+ ion will have a half-filled 4f-subshell (4f7), which explains why the +4 oxidation state has been observed in solid-state Tb compounds, while molecular complexes of Tb4+ have only been recently reported.82,98–100 On the other hand, the filled 4f14-subshell allows the formation of stable divalent ytterbium compounds.101–103 For the other late lanthanide elements, complexes in the oxidation states other than trivalent are not numerous, however, recently LnII complexes

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

Table 5

565

Most representative examples of Pr-Eu COT complexes with their parameters. Mn+

Molecular formula

˚) M-Ccentroid (A 1.940; 1.963; 2.088 2.014 1.935; 1.937 1.934 2.291; 2.297 1.906–2.206

[Nd(COT)(THF)2][Nd(COT)2]

+3

[Nd(COT)(THF)3I] [(COT)Nd(m-OTf )(THF)2]2 [(COT)Sm(m-Cl)(THF)2]2 [(C5Me5)(THF)2Eu]2(COT) [{Pr(COT00 )}2{Pr2(COT00 )2}2Li2(THF)2Cl8]

+3 +3 +3 +2 +3

[(COT00 )Nd(m-Cl)(THF)]2

+3

1.896; 1.896

(m-h8:h8-COT)[Nd{(Ph2SiO)2O}2{Li(THF)2} {Li(THF)}]2 (COT)Ln[m-c-C3H5dC^CdC(NR)2]2Li(THF) (Ln ¼ Pr 54a, Nd 54b) [{CH(PPh2NSiMe3)2}Sm(h8-COT)] (COT)Ln(TpMe2) (Ln-Pr, Nd, Sm) (h8-COT)Sm[(C5H5)Co{P(O)(OEt)2}3] [(COT){Nd(c-C3H5dC^CdC(NCy)2)(m-Cl)}2]4

+3

2.254

+3

54a: 2.016 54b: 2.002 1.914 1.914 (Sm) – 2.162; 2.164; 2.171; 2.161

+3 +3 +3 +3

d 13C NMR in ppm for COT

Number

References

–

47-Nd

83

– – – – 148.8, 148.5, 140.3, 139.1, 134.5, 133.1, 132.4, 129.3 93.9, 93.5, 92.6, 89.9, 89.7, 85.8 123.9

5-Nd 43-Nd 4-Sm 49-Eu 50

85 85 88 90 92

28-Nd

93

51

94

54-Ln

75

16-Sm 53-Ln 52 55

95 97 96 75

a

54a: 127.8 54b: 160.8 – – – 133.7

a

– Analyzes were not reported.

Table 6 Representation of accessible oxidation states for Gd-Yb. Legend: (+), not common for coordination compounds, but examples of complexes have been reported; +, stable;  , exists only in inorganic materials (oxides, salts etc.). Oxidation state

Gd (+)

(+) Tb (+)

(+) Dy (+)



Ho (+)

Er (+)

Tm (+)

Yb +

+4 +3 most stable +2

have been reported, mostly with Cp related ligands,104 for all the lanthanide series.105 Several monomeric half-sandwich and sandwich complexes have been synthesized with these elements in the trivalent state. For example, a series of Y, Ce, Pr, Nd, Sm, Tm and Lu COT-benzamidinato complexes [4-RC6H4C(NSiMe3)2Ln(THF)COT] (56-Ln) were reported in 1995.106 All the complexes were obtained by salt metathesis reactions between the dinuclear precursors [(COT)Ln(m-Cl)(THF)2]2 (4-Ln) or [(COT)Ln(mtriflate)(THF)2]2 (43-Ln) and Na[4-RC6H4C(NSiMe3)2] (R ¼ H, OMe, CF3) (Scheme 21).

Scheme 21 Synthesis of [4-RC6H4C(NSiMe3)2Ln(THF)COT] (56-Ln) complexes.

The influence of bulky substituents on COT ligands on the reactivity of the complexes was also demonstrated with the formation of sandwich complexes with the small Yb3+ ion. The reaction of YbCl3 with [K(DME)]2[COTTBS] (COTTBS ¼ [C8H6(SiMet2Bu)2-1,4]2−) in the presence of KCp in DME solution led to the formation of a mixture of two complexes, [Cp Yb(COTTBS)] (57) and [K(DME)4] [Yb(COTTBS)2] (58), which could be easily separated by extraction. Under the same conditions, while using COT000 instead of COTTBS, only [(DME)2K(m-COT)YbCp ] (59) formed, where the Yb ion was reduced to the +2 oxidation state.107 More recently, the in situ reaction of lanthanide halides with THF solutions of Li2COT00 cleanly yielded the anionic sandwich complexes [Li(L)n][Ln(COT00 )2] (30-Ln) (L ¼ THF, n ¼ 4; L ¼ DME, n ¼ 3) for most of the lanthanides (Ln ¼ Y, La, Pr, Gd, Ho, Tm and Lu).52

566

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

Using the same approach as for the synthesis of [Eu(COT)] (48-Eu), the direct reaction of Yb metal with neutral COT in liquid ammonia produced the 1:1 complex [Yb(COT)] (48-Yb) as a bright pink-purple solid, insoluble in most solvents, although a deep red color was observed in pyridine and DMF (see below in Section 4.10.3.7).89 However, further studies described a green-brown solution in pyridine instead, from which crystals of the tris-pyridine adduct [(COT)Yb(py)3] (60) could be isolated.108 In the latter, the ytterbium ion is Z8-coordinated to the COT ligand and further coordinated by the nitrogen atoms of three pyridine molecules, with Yb-N distances of 2.58(2) A˚ , resulting in a piano-stool structure. The Yb-C average distance is of 2.64 A˚ and the Yb-COT(centroid) distances are 1.91 and 1.92 A˚ for the two independent molecules present in the asymmetric unit. Although the Yb–N bond distances were in agreement with the typical range for YbII complexes, the Yb–C distances were shorter than expected. However, the lack of comparison to other YbII COT complexes at the time rendered this information rather difficult to analyze.

4.10.3.5

Lu

Lutetium is the last element within the lanthanide family. Its f-sublevel is entirely filled so that the 5d orbitals are populated, resulting in the electronic configuration [Xe] 4f145d16s2. Lutetium possesses the smallest ionic radius within the lanthanide row as a result of the lanthanide contraction. This effect originates in the strong interaction of electrons in 4f-sublevels with the nucleus, without effective shielding of the nuclear charge. Thus, the 6s electrons can penetrate the 4f-shield, leading to a decrease in the metal radius. Considering the electronic configuration of Lu, the most stable oxidation state is +3, where it is diamagnetic. Only a few lutetium complexes have been described supported by COT ligands. Suitable precursors for the synthesis of such complexes are lutetium trichloride and the tris-borohydride [Lu(BH4)3(THF)3].109 Representative examples of lutetium COT-based complexes, 16-Lu and 61, are shown in Scheme 22.

Scheme 22 Synthesis of the LuIII complexes 16-Lu and 61 supported by COT ligands.

The use of the bis(iminophosphorano)methanide ligand together with early row lanthanide metal centers has already been discussed in previous subsections. This chelating ligand was also found suitable for late lanthanides, such as ErIII, YbIII, and LuIII.6 The corresponding complexes, 16-Ln, were obtained through a convenient one-pot synthesis, by reaction of anhydrous LnCl3 with K{CH(PPh2NSiMe3)2} and K2COT in THF (Scheme 22). Analysis of the solid-state structures by X-ray diffraction studies revealed that both the Z8-coordinated COT ligand and the bidentate bis(iminophosphorano)methanide are slightly asymmetrically bound to the LnIII center, which may be the result of steric crowding around the relatively small metal ion. Another COT-supported LuIII complex, [LuLi(COT)(Dpp)Cl(THF)3] (61), was prepared using the 2,6-diphenylphenyl (Dpp) ligand, also known as m-terphenyl (Scheme 22).110 The complex was obtained through a one-pot reaction between equimolar amounts of K2COT, LuCl3 and LiDpp. X-ray diffraction studies revealed a half-sandwich complex featuring one Z8-coordinated COT and one s-bound terphenyl ligand around the LuIII center, the latter being further coordinated by one chloride donor belonging to a LiCl(THF)3 moiety. Symmetrical sandwich-type lutetium complexes have only been reported with the substituted COT00 ligand: the reaction of LuCl3 with COT00 in DME yielded [Li(DME)3][Lu(COT00 )2] (30-Lu).52 The Lu-COT00 (centroid) distance in this compound is 1.84 A˚ , which is significantly larger than that reported for the unsubstituted LuIII-COT half-sandwich complexes reported above (1.78 and 1.74 A˚ , respectively). The larger distance in the case of the COT00 -based complex probably arises from the steric repulsion between the SiMe3 groups. Table 7 summarizes selected metrical and NMR spectroscopic data of relevant Yb and Lu complexes. Table 7

Most representative examples of Gd-Lu COT complexes with their parameters.

Molecular formula [Cp Yb(COTTBS)] [K(DME)4][Yb(COTTBS)2] [(COT)Lu(Dpp)(m-Cl)Li(THF)3] [(CH(PPh2NSiMe3)2)Lu(COT)] a

– Analyzes were not reported.

Mn+

+3

˚) M-Ccentroid (A

13

Number

References

1.646 1.839; 1.849 1.74 1.780

142.7, 123–103 146.0, 142.5, 136.5, 129.1 –a –

57 58 61 16-Lu

107 107 110 6

C NMR in ppm for COT

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

4.10.3.6

567

Magnetism of Ln-based complexes

An important application of the late lanthanides is the use of their natural large anisotropy to provide complexes with SMM behavior. This type of molecules are potentially suitable candidates for data storage, quantum computing or spintronic devices.111 In order to obtain maximal anisotropy, it is necessary to be able to modulate the relative energy of the crystal-field microstates. In lanthanide ions the crystal field perturbation is smaller than the spin-orbit one because of the ionic bonding with these elements. Thus, simple electrostatic models can be used to guide the modulation of the relative energy of the crystal-field states.112 In such an approach, the chemical nature of the ligands is not taken into account and they are all considered as negative point charges. Each Ln ion has seven differently shaped/oriented f-orbitals (Fig. 8), filled with electrons or not depending on the metal considered. The electrostatic model then accounts for the interaction between the metal f-orbitals and the negative charges placed at the coordination sites of the ligands. Thus, the model allows guiding the chemist in the design of the symmetry requirements that are necessary to minimize the energy of the most anisotropic microstates and the geometry around the metal ion dictates stabilization or destabilization of the mJ microstates, since their anisotropies are different. With this effect in mind, it is then possible to properly design the shapes of the complexes needed for each element in order to maximize the anisotropy. Since the late lanthanide elements have higher magnetization because their 4f-shell is more than half-filled, Tb, Dy, Ho and Er can be considered elements with the most potential for the synthesis of complexes possessing SMM behavior.3 Because of the typical symmetry allowed by the COT and substituted COT00 ligands, these are useful platforms for the synthesis of high-symmetry sandwich complexes. According to the previous discussion, an equatorial-type ligand is required for prolate ions. Thus, early work from the Gao group investigated the mixed Cp /COT Er complex 18-Er (Fig. 9).113 This work highlighted an important issue when using large aromatic ligands: if COT is apparently suited for axial symmetry and thus for an oblate density shape, the large size of the ligand delocalizes the point charge too far away from the axial symmetry; in turn, the COT sandwich complexes are better suited for prolate density shapes. The work of Long on the analysis of sandwich complexes with the COT and COT00 ligands with Dy, Tb and Er proved this assumption: the Dy-based (oblate) complex [Li(THF)(DME)][DyIII(COT00 )2] (30-Dy)5 shows SMM behavior and unusual multiple relaxation mechanisms, but has a low barrier (9(1) cm−1); the similar TbIII complex [Li(DME)3][Tb(COT00 )2] (30-Tb) (oblate) does not show any relaxation of magnetization,4 while the ErIII (prolate) bis-COT

Fig. 8 Representation of the 4f orbitals. Reproduced from Rinehart, J.D.; Long, J.R. Chem. Sci. 2011, 2 (11), 2078–2085, with permission from the Royal Society of Chemistry.

Fig. 9 Structures of ErIII SMMs supported by COT ligands.

568

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

Fig. 10 Structure of the ErIII complexes 63a-63d with different phosphine ligands; anisotropy axes are denoted by dashed lines.

complex 62 (Fig. 9) with cations such as [K(18-crown-6)]+ or [K(18-crown-6)(THF)2]+ exhibits a blocking temperature of 10 K for a barrier of 150(1) cm−1.114 In 2018, the group of Rinehart introduced the concept of metal-ligand pair anisotropy (MLPA), a concept in which a metal is associated with a ligand specifically designed to maximize the anisotropy. Within this concept, the Er center is associated with one COT ligand and the rest of the coordination sphere can be completed by other ligands.115 For the demonstration of MLPA, the solvent adducts [Er(COT)I(S)2], with S ¼ THF (5-Er), pyridine or CH3CN, (Fig. 9) have been synthesized in addition to [Er(COT)(TpMe2)] (53-Er) where TpMe2 is tris(3,5-dimethyl-1-pyrazolyl)borate. All complexes showed SMM behavior with barriers ranging from 95.6(9) to 133.6(2.2) cm−1. The next step was to use the Er(COT)I unit as a magnetic building block for the synthesis of dimeric complexes with various axis angles for the magnetic anisotropy (63a–63d) (Fig. 10). For this purpose, several diphosphine ligands were used: dmpe (1,2-bis(dimethylphosphino)ethane)—which formed, on the one hand, a monomeric complex (63a)—dppe (1,2-bis(diphenylphosphino)ethane, dppm (1,2-bis(diphenylphosphino)methane), and mdpp (methyldiphenylphosphine)—which formed, on the other hand, dimeric structures with different axis angles (63b–63d).116 The study of their magnetic properties showed significant differences, with an increase by a factor of up to one million in the maximum relaxation time from the monomeric species 63a to the dimeric one 63d, featuring an angle of 180 between the two anisotropy axes, offering a guide on how the alignment of anisotropy axes can be designed to minimize the magnetic relaxation. Finally, similar approaches (MLPA) with trivalent Er and divalent Tm complexes (both prolate) and large aromatic ligands— particularly COT—have been reported. For example, the use of ligands such as boratabenzene possessing an electron-deficient character would enhance the delocalization from the axial point charge and thus possibly increase the energy barrier Ueff. This was attempted and resulted in the series of the sandwich-type [(C5H5B-R)Ln(COT)] complexes, where R are different substituents: H, Me, NEt2. The highest energy barrier among all the Er-based SMMs (Ueff ¼ 300 cm−1) was observed for the Me-derived Er complex [(C5H5B-Me)Er(COT)]. Although NEt2 is a stronger and a better electron donor than Me-groups, the effective barrier remained smaller, Ueff ¼ 219 cm−1, in the complex [(C5H5B-NEt2)Er(COT)] (23-Er), which was explained by the faster quantum tunneling relaxation process.47 Synthesis of the divalent thulium analog of [K2(THF)n][Yb(COT)2] (64-Yb) was described by Nocton and co-workers.117 The highly reducing [K2(THF)n][Tm(COT)2] (64-Tm) complex was obtained either by a direct salt metathesis reaction between TmI2 and K2COT, or by salt metathesis starting from TmI3 followed by reduction with excess KC8 (Scheme 23). The molecular structure of 64-Tm, as established by X-ray diffraction studies, showed an overall linear geometry, with each COT ligand in a m-Z8:Z8 coordination mode, bridging the TmII center and one potassium cation. The latter can be further trapped by addition of 18-crown-6 (Scheme 23), resulting in the formation of 65-Tm which displays a slight bending from linearity for the K⋯Tm⋯K arrangement. Magnetism studies revealed slow magnetic relaxation at high frequencies under no DC field and at temperatures of up to 10 K. When 65-Tm was diluted in a diamagnetic matrix consisting of the isomorphous divalent ytterbium complex 65-Yb, the SMM properties were retained up to 15 K. This diluted TmII complex represents a rare example of a divalent lanthanide SMM and the first f13 compound, with only one hole in the f-shell, featuring such properties in the absence of DC field.

Scheme 23 Synthesis of divalent thulium SMMs supported by COT ligands.

4.10.3.7

Polynuclear Ln-based complexes

The 10p-electronic system of the COT dianion can easily bind two metal atoms on each side of the ring plane, giving rise to polynuclear complexes. The first organometallic lanthanide COT complexes, already described above in the Sections 4.10.3.3 and

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

569

Fig. 11 First examples of polynuclear lanthanide COT complexes.

4.10.3.4 and reported by Hayes and Thomas in 1969, correspond to highly insoluble EuII and YbII COT complexes, 48-Eu and 48Yb (Fig. 11), obtained upon reaction of neutral COT with metallic Eu and Yb in liquid ammonia.89 The chemical composition of the complexes, established by elemental analyzes, was consistent with a LnII/COT ratio of ca. 1:1. The low solubility of LnII COT complexes, which hinders further characterization, is linked with the high propensity of the Ln(COT) fragment to oligomerize due to unfavorable solvation. Further studies on [Eu(COT)]n oligomers by quantitative elemental analysis using ICP-AES (inductively coupled plasma atomic emission spectrometry), powder X-ray diffraction and magnetic measurements confirmed the formation of multilayer organoeuropium arrangements.118 A red emission for the EuII complexes was observed under UV irradiation and the photoluminescence properties could be tuned by the introduction of trimethylsilyl substituents on the COT ligand. It should be noted that multiple-decker sandwich-type complexes of general formula [Lnn(COT)n+1] (Ln ¼ Ce, Nd, Eu, Tb, Ho, Tm, Yb) could be produced in the gas phase through laser vaporization and molecular beam methods.119,120 Similarly, [Eu(COT)]n oligomers were obtained in the gas phase as one-dimensional nanowires with a chain length of up to 12 nm (corresponding to n ¼ 30 layers).121,122 Very recently, such nanowires have attracted a large attention due to their interesting electronic and magnetic properties, and their potential application for molecular spintronics. The on-surface growth of ultralong and well-ordered [Eu(COT)]n nanowires was possible by reaction of free COT with Eu vapor in ultrahigh vacuum on the surface of an inert graphene substrate.123 Their magnetic and electronic properties were investigated by means of low-temperature X-ray magnetic circular dichroism, scanning tunneling microscopy and spectroscopy, and revealed ferromagnetic ordering on the graphene surface.124 The synthesis of the dinuclear CeIII complex [Ce2(COT)3] (38-Ce) (Fig. 11) was already reported in 1976, together with the first synthesis of cerocene [Ce(COT)2] (36a), and the identity of the complex was established by IR spectroscopy and elemental analysis.57 Further studies by Andersen and co-workers in 2009 revealed that 38-Ce can be cleanly obtained by thermal decomposition of 36a with formation of free COT as a by-product.61 EXAFS studies on 38-Ce supported a triple-decker sandwich structure in the solid state. The first dinuclear lanthanide COT complex unambiguously characterized by X-ray diffraction studies corresponds to the NdIII complex [Nd(COT)(THF)2][Nd(COT)2] (47-Nd) and was published in 1976 (Fig. 11). It was obtained by co-condensation of Nd metal atoms with C8H8 at −196  C and subsequent purification by Soxhlet extraction with THF (see also Section 4.10.3.3).83 Using the same procedure, the analogous complexes 47-Ln (Ln ¼ La, Ce, Er) were also prepared and showed similar IR spectroscopic features, pointing to similar solid-state arrangements.125 The formation of 47-La and 47-Y was also possible using a more convenient salt metathesis procedure, by reaction of 1.5 equiv. of K2(COT) with the corresponding MI3 precursors in THF.126 The molecular structures of both complexes were confirmed by X-ray diffraction studies and revealed ion pair arrangements similar to that of 47-Nd. The [M(COT)2]− anionic fragment, in which the M3+ ion is sandwiched between two Z8-coordinated COT ligands, is further Z2-coordinated to a second metal center corresponding to the cationic [M(COT)(THF)2]+ fragment. As observed in 47-Nd, the COT rings in 47-La are not equidistant in the anionic [M(COT)2]− fragments, resulting in M-COT(centroid) distances of 2.045 and 2.154 A˚ . A large family of well-characterized polynuclear lanthanide COT complexes consists in so-called inverse-sandwich complexes, in which the COT ligand features a m-Z8:Z8 coordination mode. The symmetric inverse sandwich complex [{(Me3Si)2N}2Sm]2(m-Z8:Z8-COT) (66-Sm) was synthesized by Schumann and co-workers in 1993 by reaction of the SmIII half-sandwich complex [Sm(COT)(m-Cl)(THF)]2 (4-Sm) with [NaN(SiMe3)2] in toluene (Scheme 24).127 It should be noted that sodium bis(cyclooctatetraene)samarate(III) (67-Sm) was obtained as a by-product in this reaction, and the separation of both products was possible due to their different solubilities in nonpolar hydrocarbon solvents. In the centrosymmetric complex 66-Sm, each Sm3+ ion is in a pseudo-trigonal planar coordination environment, Z8-coordinated to the bridging COT ligand and further coordinated to two amido ligands, the latter in an eclipsed conformation.

Scheme 24 Synthesis of [{(Me3Si)2N}2Sm]2(m-Z8:Z8-COT) (66-Sm).

570

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

As mentioned above, the study of LnII COT (Ln ¼ Sm, Eu, Yb) complexes is usually difficult because of the formation of 48-Ln as insoluble oligomeric material.89,128 To overcome this solubility problem, the formation of complexes in which the COT ligand acts as a bridging ligand between two organolanthanide(II) fragments was considered. For example, the reaction of insoluble oligomeric [Eu(COT)]n (48-Eu) with KCp afforded the inverse-sandwich complex [Cp (THF)2Eu]2(m-Z8:Z8-COT) (49-Eu) in which the COT ring is located symmetrically between two EuII centers (Scheme 25).90 The latter are further coordinated by two THF ligands and one Z5-coordinated Cp ring. The two Cp rings are oriented in a trans fashion and tilted relative to the COT plane, forming COT-Eu-Cp (centroids) angles of ca. 138–139 . This example also corresponds to the first structurally characterized divalent europium COT complex. It is worth noting that, in this synthesis, 48-Eu was generated in situ as an intermediate complex upon reaction of EuCl3 with K2COT, indicating that reduction of EuIII to EuII occurred, a phenomenon that had already been observed upon treatment of EuCl3 with NaCp .101

Scheme 25 Synthesis of 49-Eu and further desolvation into 68-Eu under high vacuum.

Further desolvation upon heating to 55  C under high vacuum (10−7 Torr) afforded [Cp Eu]2(m-Z8:Z8-COT) (68-Eu) in high yield (Scheme 25).129 This THF-free dinuclear EuII complex features tilted Cp rings in a cis conformation and a Cp -Eu-COT(centroids) angle of ca. 149 . This angle does not approach the theoretical value of 180 that would be expected on the basis of simple steric and electrostatic considerations. The analogous SmII inverse-sandwich complexes, [(C5Me4R)Sm(THF)]2(m-Z8:Z8-COT) (49-Sm, R ¼ Me, Et), were obtained upon reaction of the SmII half-sandwich precursors [(C5Me4R)Sm(m-I)(THF)2]2 with K2COT in toluene (Scheme 26).130 Although the coordinated THF could not be removed upon drying under vacuum at room temperature, heating at 30–50  C under high vacuum (10−7 Torr) led to the corresponding unsolvated complexes [(C5Me4R)Sm]2(m-Z8:Z8-COT) (68-Sm). X-ray diffraction studies revealed a bent geometry with Cp -Sm-COT(centroids) angles of ca. 149 (R ¼ Me) and Sm-CCp∗ distances from 2.761(5) to 2.798(5) A˚ (R ¼ Me), in the same range as the values reported for [Cp 2Sm].131 A trans arrangement was observed for the Cp ring, in contrast to the cis conformation found in the EuII analog, which may be ascribed to crystal packing effects as the different complexes crystallized in different crystallographic systems with or without additional co-crystallized solvent molecules. However, in all the unsolvated LnII complexes 68-Ln (Ln ¼ Sm, Eu), the bent COT-Ln-Cp arrangements are similar to those observed in the corresponding base-free [Cp 2Ln].131,132 Interestingly, the SmII complexes 49-Sm and 68-Sm were found to react with free COT via a formal two-electron reduction reactivity (Scheme 26), leading to the formation of the corresponding SmIII mixed-sandwich complexes [(C5Me4R)Sm(COT)(THF)n] (n ¼ 0, 1).130

Scheme 26 Synthesis of the inverse-sandwich complexes 49-Sm and 68-Sm and further two-electron oxidation reactivity with free COT.

The YbII analogs of the SmII complexes 49-Sm and 68-Sm were also prepared using the same procedures and featured similar reactivity towards free COT, indicating that their reduction potentials are more negative than the potential associated with the COT/ COT2− couple (−1.83 V vs. SCE).129 In all the unsolvated complexes 68-Ln (Ln ¼ Eu, Sm, Yb), bent geometries were observed instead of the more sterically favorable linear arrangements, regardless of the electronic configuration at the metal center (4f6 for SmII, 4f7 for EuII, and 4f14 for YbII). In order to investigate whether increasing the steric bulk on either the Cp or COT ligands would lead to more linear arrangements, substituted LnII analogs were designed. For example, the divalent Sm, Eu and Yb complexes [Cp Ln]2(m-Z8:Z8-COT1,4-SiiPr3) (69-Ln),

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

571

featuring a bis(triisopropylsilyl)-substituted COT ligand, COT1,4-SiiPr3, were synthesized in a one-pot procedure by reaction of the LnI2 precursors with KCp and K2COT1,4-SiiPr3.133 Structural studies on the unsolvated complexes 69-Ln confirmed the formation of inverse-sandwich arrangements with the COT1,4-SiiPr3 ligand bridging two LnII centers in a m-Z8:Z8 coordination mode. Bent geometries very similar to those in the unsubstituted COT analog 68-Ln were observed. However, slightly larger COT-Ln-Cp (centroids) angles were observed (ranging from 152.7(2) for Sm to 160.5(2) for Yb), indicating that the tilt angles were mainly determined by the steric environment at the metal centers. Using very bulky tetra- and pentaisopropyl-substituted Cp ligands (C5iPr4H and C5iPr5, respectively), the neutral and diamagnetic YbII inverse-sandwich complexes [(C5iPr4R)Yb]2(m-Z8:Z8-COT) (70a, R ¼ H; 70b, R ¼ iPr) were isolated and characterized spectroscopically (Scheme 27).134 Although the tetraisopropyl-Cp complex 70a was obtained through a usual salt metathesis reaction, the pentaisopropyl-Cp analog 70b was best prepared by reaction of Yb metal with free COT and two equivalents of the stable pentaisopropylcyclopentadienyl radical, in the presence of a catalytic amount of Hg2Cl2. Unfortunately, no crystal structure could be obtained to establish the exact bonding situation and further assess the influence of the steric bulk on the Cp-Ln-COT(centroids) bending angle.

Scheme 27 Synthesis of [(Ci5Pr4R)Yb]2(m-Z8:Z8-COT) (70a, R ¼ H; 70b, R ¼ iPr).

Among inverse-sandwich complexes based on central COT ligands, representatives featuring several self-bonded sandwich subunits in a parallel arrangement can be qualified as multi-decker complexes.135 The simplest example of this family of complexes consists of Ln-based triple-decker compounds in which two metal centers are interlayered between three aromatic ligands. Following this convention, a quadruple-decker complex possesses three metal centers interlayered in between four aromatic ligands. The first Er-based heterobimetallic quadruple-decker complex was described in 1991 and corresponds to [(COT)Er(COT)K(COT)Er(COT)K(THF)4] (71-Er). It was obtained unexpectedly by the reaction of (PhCH2C5H4)ErCl2 with K2COT (Eq. 2).136 However, this method seemed unsuitable for the synthesis of the early and middle lanthanide analogs. THF 2K2 COT   0:5 ðCOTÞErðCOTÞKðCOTÞErðCOTÞKðTHFÞ4 -78o C THF ErCl3 + NaPhCH2 C5 H4 ! ðPhCH2 C5 H4 ÞErCl2
3THF ! 71-Er -NaCI -2KCI +KPhCH2 C5 H4

(2)

The 1,4-trimethylsilyl-disubstituted COT00 ligand was used to access a variety of triple-decker [Ln2(COT00 )3] (72-Ln) complexes. The original synthesis was reported by Edelmann and co-workers in 1998 from the reaction of Li2COT00 with LnCl3 (Ln ¼ Ce, Nd, Sm) in a 3:2 molar ratio.137 However, no crystal structure of the corresponding complexes could be obtained to unambiguously confirm the formation of triple-decker arrangements. Further studies revealed that this method may actually result in the formation of a mixture of products including the sandwich complexes [Li(THF)n][Ln(COT00 )2] (30-Ln) and the chloro-bridged dimers [(COT00 )Ln(m-Cl)(THF)]2 (28-Ln).138 Clean formation of the NdIII triple-decker complex 72-Nd was achieved upon treatment of [Li(THF)4][Nd(COT00 )2] (28-Nd)93 with anhydrous CoCl2 in toluene (Scheme 28).139 The use of toluene as solvent is especially important to avoid the formation of THF-solvated species such as 47-Ln (see Fig. 11). In the formation of 72-Ln, one dianionic COT00 ligand has been oxidized into free COT00 through a redox reaction, which highlights the high reducing character of COT ligands. X-ray diffraction studies confirmed the formation of the triple-decker complex 72-Nd featuring a near-linear arrangement for the three COT00 rings and COT00 -Nd-COT00 (centroids) angles of ca. 176 . The terminal COT00 rings are Z8-coordinated to two NdIII centers, the latter bridged by a central COT00 ligand in a m-Z8:Z8-coordination mode. Deviation from linearity may be assigned to repulsive interactions between the bulky trimethylsilyl substituents.

572

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

Scheme 28 Synthesis of the triple-decker complexes [Ln2(COT00 )3] (72-Ln) and silyl-group migration.

The same synthetic procedure was also applied to the synthesis of 72-Ln (Ln ¼ Gd, Dy and Er) and some these complexes present promising magnetic properties.140,141 Notably, the dinuclear ErIII complex 72-Er behaves as a SMM and exhibits magnetic hysteresis at 12 K in the solid state and up to 14 K in solution.141 Surprisingly, applying the same procedure for the synthesis of the HoIII analog 72-Ho resulted in an isomerization of the central COT00 ligand with a silyl group migration from a 1,4- to a 1,5-substitution pattern (Scheme 28), as evidenced by X-ray diffraction studies.93 Further experimental and computational studies revealed that the 1,5-silyl group migration was also observed for the YIII, TbIII and TmIII analogs, whereas the original 1,4-substitution pattern for the central COT00 ligand was retained in the LaIII and LuIII complexes.142 These results showed that no simple trend of the isomerization process regarding the metal ionic radius could be drawn. In the particular case of 72-Y, both isomers could be isolated and exhibited different solubility in toluene. Finally, the smaller LuIII complex 72-Lu displayed a different coordination mode involving a lower hapticity (m-Z4:Z4) for the central COT00 ligand, resulting a bowl-shaped central COT00 ligand. Similar observations had already been reported for the ScIII analog 34 featuring a m-Z3:Z3 coordination mode for the central COT00 ligand as mentioned in Section 4.10.3.1.53 Using the very bulky COT000 ligand, the quadruple-decker YbII complex, [Cp Yb(m-Z8:Z8-COT000 )Yb(m-Z8:Z8-COT000 )YbCp ] (73), was isolated unexpectedly by reaction of [K2(DME)2](COT000 ) with YbI2(THF)2 and KCp in a one-pot procedure (Scheme 29).143 This result contrasts with the previously-mentioned formation of the inverse-sandwich YbII complex [Cp Yb]2(m-Z8:Z8-COT1,4-SiiPr3) (69-Yb) when using the disubstituted COT1,4-SiiPr3 ligand under the same conditions.133 In the solid-state structure of 73, one central [Yb(COT000 )2]2− subunit is coordinated on each side by a cationic [YbCp ]+ fragment forming a linear arrangement with a COT000 Yb-COT000 (centroids) angle of 173.8 and Cp -Yb-COT000 (centroids) angles of 176.6 and 178.5 . The different arrangements observed in 69-Yb and 73 may be the result of the different steric nature of the bulky COT1,4-SiiPr3 and COT000 ligands. Table 8 summarizes selected metrical and NMR spectroscopic data of relevant complexes.

Scheme 29 Synthesis of the YbII quadruple-decker complex 73.

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

Table 8

573

Most representative examples of polynuclear Ln COT complexes with their parameters.

Molecular formula

Mn+

[Ce2(COT)3] [{(Me3Si)2N}2Sm]2(m-h8:h8-COT) [(C5Me4R)Sm(THF)n]2(m-h8:h8-COT) (R ¼ Me, Et; n ¼ 0, 1) [(C5iPr4R)Yb]2(m-h8:h8-COT) (R ¼ H, iPr) [(COT)Er(COT)K(COT)Er(COT)K(THF)4]

+3 +3 +3

[Li(THF)4][Nd(COT00 )2] [(h8-COT00 )Nd(m-h8:h8-COT00 )Nd(h8-COT00 )]

+3 +3

[Cp Yb(m-h8:h8-COT000 )Yb(m-h8:h8-COT000 )YbCp ]

+2

[Cp Yb]2(m-h8:h8-COT1,4-SiiPr3)

+2

+2 +3

˚) M-Ccentroid (A a

– 2.154 Me: 2.120; 2.151 Et: 2.102 – 1.831; 1.860; 1.870; 1.925 1.993; 1.996 1.894; 1.906; 2.156; 2.164 2.040; 2.060; 1.911; 1.895 1.945; 1.976

13

C NMR, ppm for COT

Number

References

– 82.6 – – H: 89.8 i Pr: 90.1 126.7, 127.5, 132.8, 135.9

38-Ce 66-Sm 49-Sm

61 127 130

70-Yb

134

71-Er

136

138.0, 132.7, 131.5, 120.3 177.3, 170.5, 163.6, 136.3, 103.9

28-Nd 72-Nd

93 139

100.2, 99.1, 96.8, 94.9, 91.9

73

143

97.4, 96.5, 95.2, 90.2

69-Yb

133

a

– Analyzes were not reported.

4.10.4

Complexes based on pentalenyl ligands

4.10.4.1

Ce

Another class of doubly anionic carbocyclic ligands used in the synthesis of f block organometallic complexes corresponds to the eight-membered pentalenyl ligands. Pentalene, of general formula C8H6, can be viewed as two fused cyclopentadienyl rings or as a cyclooctatetraene ring featuring a 1,5-transannular bond. Although neutral pentalene is an 8p Hückel anti-aromatic compound which is thermally unstable and readily dimerizes above −196  C, its dianionic 10p aromatic form, [C8H6]2−, is stable at room temperature and isoelectronic to the COT2− dianion. As a consequence, similarities in the coordination chemistry of COT and pentalenyl ligands have been observed, especially among f-block elements.144,145 However, due to its specific structure, the pentalene dianion may exhibit various coordination modes, including Z8 or m-Z5:Z5 with two metal centers located either on the same side or on opposite faces of the ligand (Fig. 12).

Fig. 12 Possible coordination modes of the dianionic pentalenyl ligand.

The substituted pentalene ligands, bis(triisopropylsilyl)pentalene Z8-(1,4-iPr3Si)2C8H4 (Pn00 ) and hexamethylpentalene Z -C8Me6 (Pn ), have notably been used to access formal CeIV analogs of cerocene [Ce(COT)2].146,147 The CeIII complex K[Ce(Pn00 )2] was obtained as an extremely air-sensitive product by the salt metathesis reaction between CeCl3 and K2Pn00 in 1:2 molar ratio (Scheme 30).147 A crystalline and analytically pure product corresponding to [K(18-crown-6)][Ce(Pn00 )2] (74) could be obtained upon addition of 18-crown-6 and was structurally characterized. In situ oxidation of K[Ce(Pn00 )2] with Ag(BPh4) resulted in the formation of dark blue [Ce(Pn00 )2] (75), a formal CeIV complex. In both the anionic and neutral complexes 74 and 75, the 1,4-bis(silylated) pentalenyl ligands are Z8-coordinated to the cerium center and oriented in a staggered conformation. The pentalene rings are slightly folded around the bridgehead CdC bond, forming angles of 21–23 . The structural similarities observed between 74 and 75 may be assigned to the relatively similar sizes of the Ce3+ and Ce4+ ions (ionic radii of 1.07 A˚ and 0.94 A˚ , respectively). 8

574

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

Scheme 30 Synthesis of the formal CeIII and CeIV bis-pentalenyl complexes, 74 and 75, respectively.

Fig. 13 Structure of [Ce(Pn )2] (76).

The Pn analog of 75, [Ce(Pn )2] (76) (Fig. 13), was obtained through a similar strategy by reaction of Li2Pn (TMEDA)x with CeCl3 followed by oxidation with excess 1,2-dichloroethane.146 Electrochemical studies on 76 revealed a reversible one-electron reduction process at −830 mV vs. Fc+/Fc, supporting the possible formulation of 76 as a formal CeIV complex. Both 75 and 76 are diamagnetic, as expected for 4f0 CeIV complexes, but several abnormal chemical shifts were detected in their 1H NMR spectra. These spectroscopic singularities have been rationalized in terms of intermediate valence for the metal center,62 involving both 4f0 and 4f1 contributions to the ground state wave-function.146,147 In the case of 76, XANES studies revealed an experimental valence close to CeIII with an f-occupancy value of 0.87  0.05, similar to that observed in [Ce(COT)2].63 Further investigations of the multiconfigurational character of the ground state as a mixture of 4f0 and 4f1 contributions were carried out using ab initio calculations.148,149

4.10.4.2

Sm, Eu, Dy, Yb

Pentalene-based frameworks were found to be suitable ligands for the stabilization of the +2 oxidation state in Sm, Eu and Yb complexes. Thus, the reaction of LnI2(THF)x with KCp and KPn00 in THF (Scheme 31) resulted in isostructural complexes of formula [{LnCp (THF)}2(m-Z5:Z5-Pn00 )] (77-Ln, Ln ¼ Eu, Yb).133

Scheme 31 Synthesis of 77-Ln (Ln ¼ Eu, Yb) supported by the Pn00 ligand.

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

575

These complexes can be considered as anti-bimetallic structures with a slipped inverse-sandwich arrangement. Since the Ln atoms coordinate on two different faces of the Pn00 ligand, the metal⋯metal distances are expectedly long (Eu⋯Eu ¼ 5.527(5) A˚ and Yb⋯Yb ¼ 5.471(5) A˚ ). The average Ln-C(centroid) distances are longer for 77-Eu (Eu-C1 ¼ 2.477; Eu-C2 ¼ 2.502 A˚ ) than for 77-Yb (Yb-C1 ¼ 2.439; Yb-C2 ¼ 2.427 A˚ ), as a result of the larger ionic radius of EuII (C1 and C2 are the centroids of the Cp and Pn00 rings, respectively). Recently, it was demonstrated that using non-coordinating solvents such as toluene or benzene, the base-free divalent lanthanide complexes [{Cp Ln}2(m-Z5:Z5-Pn00 )] (78-Ln, Ln ¼ Sm, Eu, Yb) could be obtained starting from the corresponding half-sandwich precursors [Cp Ln(BPh4)].150 The absence of coordinated solvents to the metal ions resulted in particularly short metal⋯metal interactions, compared to those in 77-Ln, with Sm⋯Sm, Eu⋯Eu and Yb⋯Yb distances of 5.2739(9), 5.1627(5) and 5.230(2) A˚ , respectively. In the case of samarium complexes supported by the Pn00 ligand, the dinuclear Cp -based SmII complex [(Cp )Sm(m-I)(THF)2]2 has also been used as a precursor; its reaction with K2Pn00 yielded an interesting mixture of oxidized complexes (Scheme 32) including the SmIII complexes [Sm(Z8-Pn00 )(Z5-Cp )] (79-Sm) and [Sm(Z8-Pn00 )(Z5-C8H5{SiiPr3–1,4}2)] (80), and the mixed-valent cluster complex [Cp 6Sm6(OMe)8O][K(THF)6] (81).151

Scheme 32 Synthesis of SmIII complexes (79-Sm and 80) and the mixed-valent cluster complex 81 supported by the Pn00 ligand.

Both mononuclear complexes, 79-Sm and 80, were soluble in toluene and could be successfully crystallized from this solvent, the former complex being the major product. The hexanuclear cluster 81 was obtained as an insoluble red precipitate from the toluene solution and recrystallized from THF. In the crystal structure of 79-Sm, the Cp ligand is coordinated in an Z5 mode, while the pentalene is Z8-coordinated to the metal center. The Pn00 moiety is twisted because of its coordination to the samarium ion. Thus, the angle corresponding to C5H2SiiPr3-Sm-C5H2SiiPr3(centroids) is 49o. The mononuclear bis-pentalene complex 80 exhibits one Pn00 unit coordinated in an Z8-fashion, while the second Pn00 H moiety is protonated and planar, and thus coordinates the metal center only through one ring in an Z5-mode. It was plausible that the coordinated molecules of THF in the precursor may serve as the source of protons. However, an attempt to use SmI2 as a THF-free starting material also resulted in a mixture of 79-Sm and 80, indicating that C-H activation of toluene may have occurred in order to explain the observed protonation. Finally, one single pentalene-based complex possessing SMM behavior was reported very recently in 2018 and corresponds to [Dy(Z8-Pn00 )(Z5-Cp )] (79-Dy).152 The latter was obtained by simple addition of stoichiometric equivalents of K2Pn00 and NaCp to a THF solution of DyCl3. The pentalene ligand is twisted due to its Z8-coordination to Dy, with a folding angle of 27o. The Dy-CPn(centroid) distance is shorter (2.23 A˚ ) than the one with the Cp ring (Dy-CCp∗(centroid) ¼ 2.34 A˚ ). Out-of-phase (w00 ) magnetic susceptibility as a function of the frequency showed well-defined maxima in the 2–41 K temperature range, with a measured energy barrier to magnetic reversal of 245 cm−1.

4.10.5

Complexes based on cyclononatetraenyl ligands

The C9H−9 cyclononatetraenyl (Cnt) monoanion is isoelectronic to the cyclooctatetraenyl (COT) dianion. As a 10p-electron compound, it satisfies the Hückel rule for aromaticity. Although the Cnt anion and its alkali metal salts have been known since

576

Larger Aromatic Complexes of the Group 3 Metals and Lanthanides

the 1960s,153–155 only a handful of reports have focused on its coordination chemistry, which is possibly due to the known facile rearrangement of the Cnt skeleton to an indenyl structure.156 Two early reports have described the formation of the heteroleptic [Ti(C5H5)(Cnt)] and [Nb(C5H5)2(Cnt)] complexes, in which different coordination modes (Z3 and Z7) for the Cnt ligand were suggested on the basis of spectroscopic observations.157,158 The large ring size and modular coordination modes of the Cnt ligand were exploited in the synthesis of a tetranuclear Pd sheet cluster exhibiting a sandwich arrangement.156 The large ring system of the Cnt ligand (diameter of ca. 4.1 A˚ ) may be particularly adapted for metal centers with large ionic radii. In 2005, the synthesis of the homoleptic [Ba(Cnt)2] complex was reported but its molecular structure in the solid state remained elusive and was evaluated by DFT methods.134 The first attempt to use the Cnt ligand in organolanthanide chemistry was reported by Streitwieser and co-workers in 1971 in the attempted synthesis of [Ln(COT)(Cnt)] (Ln ¼ Ce, Pr, Nd, Sm).159 However, the one-pot reaction of LnCl3, K2COT and KCnt in THF did not result in the expected mixed-sandwich complexes, but the corresponding monocyclooctatetraenyl lanthanide complexes [Ln(COT)Cl(THF)2] (Ln ¼ Ce, Pr, Nd, Sm) were isolated instead. Research in the field of lanthanide Cnt complexes remained dormant for more than 40 years until Nakajima and co-workers reported in 2017 the first homoleptic lanthanide bis-Cnt complex, [Eu(Z9-Cnt)2] (82-Eu).160 This complex was synthesized by a salt metathesis reaction between EuI2 and KCnt, and was isolated in very low yield (4%) (Scheme 33).

Scheme 33 Synthesis of the homoleptic sandwich complex [Eu(Z9-Cnt)2] (82-Eu).

The molecular structure of 82-Eu was established by X-ray diffraction studies, revealing a Eu2+ ion sandwiched between two Z -coordinated Cnt rings. The latter are almost planar and display equivalent CdC bond lengths indicating a 10p delocalized electronic structure. Further studies by Raman spectroscopy confirmed the high symmetry of the complex. The effective magnetic moment of Eu in 82-Eu (7.80 mB) is consistent with a divalent EuII center in a 4f7 electronic configuration. Blue-green photoluminescence in toluene solution was observed at 516 nm (2.4 eV), which is particularly blue-shifted compared to the luminescence of other organoeuropium(II) complexes. A red photoluminescence emission around 600 nm is typically observed for EuII sandwich complexes when using Cp or COT ligands. The blue-shift in the emission of 82-Eu was investigated by DFT calculations and finds its origin in the weak electrostatic field induced by the Cnt ligands and affecting the energies of the 5d-6s hybrid empty orbitals. Further calculations on the electronic structure of the Eu2+ ion in 82-Eu were carried out using the ligand-field density functional theory (LFDFT) method.161 This method consists in a density functional theory (DFT)-based model that includes a model Hamiltonian to take into account configuration interaction. LFDFT is therefore adapted for the calculation of the electronic structures of lanthanide compounds and the prediction of their photophysical and magnetic properties. In the optimized structure of 82-Eu, the eclipsed D9h and staggered D9d conformations were found to be quasi degenerate, suggesting free rotation of the Cnt ligand in the gas phase. The presence of both conformations in the solid-state structure may be the result of packing forces in the crystalline phase.160 Overall, the Eu 4f orbitals contribute only weakly to the mostly ionic chemical bonding. On the basis of time-dependent DFT (TD-DFT) and LFDFT calculations, the photoluminescence emission was rationalized in terms of electricdipole-allowed Eu 4f-5d transitions between the lowest excited 4f65d1 state of the Eu2+ ion and the 4f75d0 ground state.160,161 The low value for the quantum yield (1.0%, lex ¼ 390 nm) in toluene solution at room temperature may be indicative of non-radiative decay due to strong interaction with solvent molecules. The formation of homoleptic divalent lanthanide bis-Cnt complexes was further studied by Nocton and co-workers.13 Using an optimized procedure based on the solubility difference between two conformational isomers of the Cnt ligand, the divalent lanthanide complexes 82-Ln (Ln ¼ Sm, Eu, Tm, Yb) were successfully isolated in yields up to 62% for the EuII analog. Upon synthesis of the KCnt salt, two isomers may be obtained depending on the reaction conditions, the D9h-symmetric cis,cis,cis, cis-cyclononatetraenyl (cis-Cnt) and the cis,cis,cis,trans-cyclononatetraenyl (trans-Cnt), in which one carbon atom has moved inside the ring (Fig. 14).153,154,162 The higher solubility of the trans isomer in low polar solvents can be conveniently used to access the Cnt complexes 82-Ln under mild conditions. 9

Fig. 14 Structures of the two Cnt isomers.

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577

The optimized reaction conditions consisted in the salt metathesis reaction between LnI2 and a mixture of cis- and trans-isomers of KCnt in toluene and in the presence of a few drops of THF (Scheme 34). The diamagnetic YbII complex was isolated in 43% yield and the corresponding 1H NMR spectrum revealed a mixture of three different isomers: [Yb(cis-Cnt)2], [Yb(cis-Cnt)(trans-Cnt)] and [Yb(trans-Cnt)2]. A slow isomerization into the more thermodynamically stable [Yb(cis-Cnt)2] (82-Yb) was observed in toluene solution after several days at room temperature.

Scheme 34 Synthesis of [Ln(Cnt)2] as a mixture of isomers and isomerization into [Ln(cis-Cnt)2] (82-Ln, Ln ¼ Sm, Eu, Tm, Yb).

In order to establish more efficient conditions for the isomerization into the [Ln(cis-Cnt)2] form, the effect of different solvents was investigated. Upon dissolution in coordinating solvents such as THF or acetonitrile, displacement of one or both of the coordinated Cnt ligands was observed, respectively (Scheme 34). Interestingly, in the resulting ion-pairs, [Ln(Cnt)(THF)4][Cnt] and [Ln(CH3CN)7][Cnt]2, the uncoordinated Cnt ligands exclusively featured a cis form. After a desolvation step upon extensive drying under vacuum (8 h, 10−3 mbar), the solvent-separated ion pairs were converted into the fully-symmetrical [Ln(cis-Cnt)2] isomers. Using this procedure, the divalent lanthanide complexes 82-Ln (Ln ¼ Sm, Eu, Yb) were isolated in 43–62% yield. The procedure could also be successfully applied to the synthesis of the divalent thulium analog 82-Tm, which was isolated in 33% yield. Surprisingly, the SmII complex 82-Sm was found stable in dichloromethane, implying an unusual stabilization of the highly reductive SmII center. Analysis of the corresponding crystal structures by X-ray diffraction studies revealed isostructural complexes featuring two Z9-coordinated ligands in rigorously linear sandwich arrangements with centroid-Ln-centroid angles of 180 . The name “lanthanidocene” was therefore coined for the [Ln(Cnt)2] complexes in analogy with that of the long-established ferrocene [Fe(Cp)2] and uranocene [U(COT)2] sandwich complexes. A qualitative molecular orbital diagram in D9d symmetry was established and DFT calculations were performed to model the bonding situation. The (Cnt)2 fragment appears adapted for the coordination with lanthanide metal centers as several symmetry-adapted orbitals can mix with the metal 4f-d and 4f-j orbitals. Although preliminary work by Streitwieser and co-workers indicated that heteroleptic lanthanide COT/Cnt complexes cannot be accessed from the chlorido [LnCl(COT)(THF)2] (Ln ¼ Ce, Pr, Nd, Sm) precursors,159 further studies by the group of Roesky revealed that the iodido complexes [LnI(COT)(THF)n] (5-Ln with n ¼ 2,3) were suitable precursors for the synthesis of the heteroleptic COT/ Cnt complexes [Ln(COT)(Cnt)] (83-Ln with Ln ¼ Nd, Sm, Dy, Er) (Scheme 35).14 The half-sandwich precursor complexes 5-Ln were prepared by direct reduction of free cyclooctatetraene with lanthanide metals (Ln ¼ Nd, Sm, Dy, Er) in the presence of iodine, following the method of Mashima et al.31,32

Scheme 35 Synthesis of the heteroleptic complexes [Ln(COT)(Cnt)] (83-Ln).

578

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In several occurrences, severe disorder of the two rings in 83-Ln was observed in the solid-state structures, especially in the case of the smaller lanthanide ions. Preliminary structural studies supported by DFT calculations and Raman spectroscopy concluded towards a tentative Z9-coordination mode for the Cnt ligand and linear structures.14 Such an arrangement of the COT and Cnt ligands is expected to induce a strong equatorial ligand field, which should stabilize prolate-shaped mJ states of lanthanide ions, as is especially the case for Er3+. The magnetic properties of 83-Er were therefore investigated in detail.14 A wMT value of 11.25 cm3 K mol−1 was measured at room temperature, in agreement with the expected value for an isolated Er3+ ion. Variable-temperature magnetic susceptibility AC studies under zero applied DC field revealed a single peak for the out-of-phase magnetic susceptibility as a function of the frequency. Further evidence of the SMM behavior was given by an open butterfly-like magnetic hysteresis loop up to 10 K. Near zero field, fast relaxation of the magnetization was observed due to fast quantum tunneling of the magnetization (QTM), as commonly observed in lanthanide-based SMMs. This process could be efficiently suppressed by the application of an optimal field of 2 kG, resulting in slower relaxation. Ab initio CASSCF calculations were performed on the optimized linear structures and predicted a highly axial ground state with the anisotropy axis perpendicular to the Z9-Cnt and Z8-COT ideal planes. Despite small differences that may be related to structural distortions in the solid-state structures, the calculations effectively reproduced the experimental results. In contrast to the SMM properties observed for 83-Er, the magnetic properties of the DyIII analog 83-Dy were less appealing. Indeed, the largest mJ state of Dy3+ ions is best stabilized by an axial ligand field, in contrast to the equatorial field exerted by the quasi-linear COT/Cnt arrangement.14 As an additional note, the elusive nine-membered aromatic heterocyclic 1,4,7-triazacyclononatetraenyl anion was predicted to be a suitable ligand for divalent lanthanide metal centers on the basis of DFT calculations.163 This elusive 10p-aromatic C6H6N−3 ligand is isoelectronic to the Cnt ligand and is derived from the well-known fully saturated 1,4,7-triazacyclononane (TACN) ligand. Coordination with divalent lanthanides would result in sandwich complexes isoelectronic to 82-Ln. Calculated binding energies suggested thermodynamic stability for the LnII complexes, with similar energies obtained for [Ln(C6H6N3)2] and 82-Ln, the former being slightly less stable.163

4.10.6

Conclusions and outlook

The coordination chemistry of rare-earth elements has provided numerous examples of complexes with catalytic properties or discrete SMM or SIM (Single Ion Magnet) properties. Compared to the “classical” O, N, S, P-donor ligands, organometallic C-based complexes are less developed. Since the first report about lanthanide complexes based on cyclopentadienyl ligands in the 1950s, their coordination chemistry was enriched by not only new complexes, but also new ligands. This work summarized the main aspects of Ln complexes with large C-containing aromatic ligands (C7, C8, C9). The cyclooctatetraenyl (COT) ligand is one of the most developed within this triad. The ligand itself and its complexes are generally easily accessible and possess intriguing properties together with sophisticated crystal structures. Cycloheptatrienyl (CHT) and cyclononatetraenyl (Cnt) ligands are less developed than the closely related COT and may be found as a fertile field for coordination chemistry. The main problem with the C7 CHT ligand is that its monoanionic form is unstable and reorganizes itself into a trianionic form. The cyclononatetraenyl monoanion is more stable, however the large size of the aromatic ring and small charge density obviously act as a disadvantage. On the other hand, the singly charged Cnt anion may open new directions of research such as the synthesis of mixed-ligand complexes. The first report about mixed COT/Cnt Ln complexes was recently described by Roesky and co-workers,14 and showed that these mixed-sandwich complexes exhibit slow relaxation of magnetization. Another perspective direction is the introduction of different substituents on the Cnt ring, especially to overcome solubility issues, similarly to the strategy already described with COT ligands. There is currently no report of complexes with substituted Cnt-anions, which is possibly due to the challenging synthetic pathway. Most of the complexes discussed herein possess the most stable +3 oxidation state for the lanthanide ion, therefore the question about the possibility of generating stable “exotic” oxidation states for such Ln complexes naturally arises. Complexes of EuII, SmII, YbII are well spread and stable due to their electronic configuration while the only two TmII complexes with COT and Cnt ligands were reported only recently.13,117 Therefore, further synthesis of LnII complexes supported by large organometallic ring ligands still remains open.

Acknowledgment This work is part of a project that has received funding from the European Research Council (ERC) under the European Union‘s Horizon 2020 research and innovation program under grant agreement No 716314. OS thank the ANR (French National Research Agency) under project number ANR-19-CE07-0019-1 for funding.

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Dalton Trans. 2010, 39 (29), 6629–6631. Zulys, A.; Panda, T. K.; Gamer, M. T.; Roesky, P. W. Chem. Commun. 2004, (22), 2584–2585. Reißmann, U.; Edelmann, F. T. Z. Anorg. Allg. Chem. 2003, 629 (14), 2433–2434. Amberger, H.-D.; Edelmann, F. T.; Gottfriedsen, J.; Herbst-Irmer, R.; Jank, S.; Kilimann, U.; Noltemeyer, M.; Reddmann, H.; Schäfer, M. Inorg. Chem. 2009, 48 (2), 760–772. Rice, N. T.; Popov, I. A.; Russo, D. R.; Bacsa, J.; Batista, E. R.; Yang, P.; Telser, J.; La Pierre, H. S. J. Am. Chem. Soc. 2019, 141 (33), 13222–13233. Palumbo, C. T.; Zivkovic, I.; Scopelliti, R.; Mazzanti, M. J. Am. Chem. Soc. 2019, 141 (25), 9827–9831. Willauer, A. R.; Palumbo, C. T.; Scopelliti, R.; Zivkovic, I.; Douair, I.; Maron, L.; Mazzanti, M. Angew. Chem. Int. Ed. 2020, 59 (9), 3549–3553. Tilley, T. D.; Andersen, R. A.; Spencer, B.; Ruben, H.; Zalkin, A.; Templeton, D. H. Inorg. Chem. 1980, 19 (10), 2999–3003. Fischer, E. O.; Fischer, H. J. Organomet. Chem. 1965, 3 (3), 181–187. Kagan, H. B.; Namy, J. L. Handbook on the Physics and Chemistry of Rare Earths; Elsevier, 1984; vol. 6; pp 525–565. Gould, C. A.; McClain, K. R.; Yu, J. M.; Groshens, T. J.; Furche, F.; Harvey, B. G.; Long, J. R. J. Am. Chem. Soc. 2019, 141 (33), 12967–12973. Evans, W. J. Organometallics 2016, 35 (18), 3088–3100. Schumann, H.; Winterfeld, J.; Hemling, H.; Hahn, F. E.; Reich, P.; Brzezinka, K.-W.; Edelmann, F. T.; Kilimann, U.; Schäfer, M.; Herbst-Irmer, R. Chem. Ber. 1995, 128 (4), 395–404. Edelmann, A.; Hrib, C. G.; Blaurock, S.; Edelmann, F. T. J. Organomet. Chem. 2010, 695 (25), 2732–2737. Wayda, A. L.; Mukerji, I.; Dye, J. L.; Rogers, R. D. Organometallics 1987, 6 (6), 1328–1332.  Paskevicius, M.; Jepsen, L. H.; Schouwink, P.; Cerný, R.; Ravnsbæk, D. B.; Filinchuk, Y.; Dornheim, M.; Besenbacher, F.; Jensen, T. R. Chem. Soc. Rev. 2017, 46 (5), 1565–1634. Rabe, G. W.; Zhang-Presse, M.; Riederer, F. A.; Incarvito, C. D.; Golen, J. A.; Rheingold, A. L. Acta Crystallogr., Sect. E: Struct. Rep. Online 2004, 60 (10), m1389–m1390. Bogani, L.; Wernsdorfer, W. Nat. Mater. 2008, 7 (3), 179–186. Rinehart, J. D.; Long, J. R. Chem. Sci. 2011, 2 (11), 2078–2085. Jiang, S.-D.; Wang, B.-W.; Sun, H.-L.; Wang, Z.-M.; Gao, S. J. Am. Chem. Soc. 2011, 133 (13), 4730–4733. Meihaus, K. R.; Long, J. R. J. Am. Chem. Soc. 2013, 135 (47), 17952–17957. Hilgar, J. D.; Bernbeck, M. G.; Flores, B. S.; Rinehart, J. D. Chem. Sci. 2018, 9 (36), 7204–7209. Hilgar, J. D.; Bernbeck, M. G.; Rinehart, J. D. J. Am. Chem. Soc. 2019, 141 (5), 1913–1917. Moutet, J.; Schleinitz, J.; La Droitte, L.; Tricoire, M.; Pointillart, F.; Gendron, F.; Simler, T.; Clavaguéra, C.; Le Guennic, B.; Cador, O.; Nocton, G. Angew. Chem. Int. Ed. 2021, 60 (11), 6042–6046. Tsuji, T.; Hosoya, N.; Fukazawa, S.; Sugiyama, R.; Iwasa, T.; Tsunoyama, H.; Hamaki, H.; Tokitoh, N.; Nakajima, A. J. Phys. Chem. C 2014, 118 (11), 5896–5907. Kurikawa, T.; Negishi, Y.; Hayakawa, F.; Nagao, S.; Miyajima, K.; Nakajima, A.; Kaya, K. J. Am. Chem. Soc. 1998, 120 (45), 11766–11772. Miyajima, K.; Knickelbein, M. B.; Nakajima, A. J. Phys. Chem. A 2008, 112 (3), 366–375. Hosoya, N.; Takegami, R.; Suzumura, J.-I.; Yada, K.; Koyasu, K.; Miyajima, K.; Mitsui, M.; Knickelbein, M. B.; Yabushita, S.; Nakajima, A. J. Phys. Chem. A 2005, 109 (1), 9–12. Hosoya, N.; Takegami, R.; Suzumura, J.-i.; Yada, K.; Miyajima, K.; Mitsui, M.; Knickelbein, M. B.; Yabushita, S.; Nakajima, A. J. Phys. Chem. A 2014, 118 (37), 8298–8308. Huttmann, F.; Schleheck, N.; Atodiresei, N.; Michely, T. J. Am. Chem. Soc. 2017, 139 (29), 9895–9900. Huttmann, F.; Rothenbach, N.; Kraus, S.; Ollefs, K.; Arruda, L. M.; Bernien, M.; Thonig, D.; Delin, A.; Fransson, J.; Kummer, K.; Brookes, N. B.; Eriksson, O.; Kuch, W.; Michely, T.; Wende, H. J. Phys. Chem. Lett. 2019, 10 (5), 911–917. DeKock, C. W.; Ely, S. R.; Hopkins, T. E.; Brault, M. A. Inorg. Chem. 1978, 17 (3), 625–631. Greenough, J.; Zhou, Z.; Wei, Z.; Petrukhina, M. A. Dalton Trans. 2019, 48 (17), 5614–5620. Schumann, H.; Winterfeld, J.; Esser, L.; Kociok-Köhn, G. Angew. Chem. Int. Ed. Engl. 1993, 32 (8), 1208–1210. Wayda, A. L.; Cheng, S.; Mukerji, I. J. Organomet. Chem. 1987, 330 (3), C17–C19. Evans, W. J.; Johnston, M. A.; Greci, M. A.; Ziller, J. W. Organometallics 1999, 18 (8), 1460–1464. Evans, W. J.; Clark, R. D.; Ansari, M. A.; Ziller, J. W. J. Am. Chem. Soc. 1998, 120 (37), 9555–9563. Evans, W. J.; Hughes, L. A.; Hanusa, T. P. J. Am. Chem. Soc. 1984, 106 (15), 4270–4272. Evans, W. J.; Hughes, L. A.; Hanusa, T. P. Organometallics 1986, 5 (7), 1285–1291. Summerscales, O. T.; Jones, S. C.; Cloke, F. G. N.; Hitchcock, P. B. Organometallics 2009, 28 (20), 5896–5908. Walter, M. D.; Wolmershäuser, G.; Sitzmann, H. J. Am. Chem. Soc. 2005, 127 (49), 17494–17503. Edelmann, F. T. New J. Chem. 2011, 35 (3), 517–528. Xia, J.; Jin, Z.; Chen, W. J. Chem. Soc. Chem. Commun. 1991, 17, 1214–1215. Poremba, P.; Edelmann, F. T. J. Organomet. Chem. 1998, 553 (1), 393–395. Poremba, P.; Reißmann, U.; Noltemeyer, M.; Schmidt, H.-G.; Brüser, W.; Edelmann, F. T. J. Organomet. Chem. 1997, 544 (1), 1–6. Lorenz, V.; Blaurock, S.; Hrib, C. G.; Edelmann, F. T. Organometallics 2010, 29 (21), 4787–4789. Le Roy, J. J.; Jeletic, M.; Gorelsky, S. I.; Korobkov, I.; Ungur, L.; Chibotaru, L. F.; Murugesu, M. J. Am. Chem. Soc. 2013, 135 (9), 3502–3510. Le Roy, J. J.; Ungur, L.; Korobkov, I.; Chibotaru, L. F.; Murugesu, M. J. Am. Chem. Soc. 2014, 136 (22), 8003–8010. Lorenz, V.; Liebing, P.; Bathelier, A.; Engelhardt, F.; Maron, L.; Hilfert, L.; Busse, S.; Edelmann, F. T. Chem. Commun. 2018, 54 (73), 10280–10283. Edelmann, A.; Blaurock, S.; Lorenz, V.; Hilfert, L.; Edelmann, F. T. Angew. Chem. Int. Ed. 2007, 46 (35), 6732–6734. Summerscales, O. T.; Cloke, F. G. N. Coord. Chem. Rev. 2006, 250 (9), 1122–1140.

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145. 146. 147. 148. 149. 150. 151. 152. 153. 154. 155. 156. 157. 158. 159. 160. 161. 162. 163.

Cloke, F. G. N.; Green, J. C.; Kilpatrick, A. F. R.; O’Hare, D. Coord. Chem. Rev. 2017, 344, 238–262. Ashley, A.; Balazs, G.; Cowley, A.; Green, J.; Booth, C. H.; O’Hare, D. Chem. Commun. 2007, (15), 1515–1517. Balazs, G.; Cloke, F. G. N.; Green, J. C.; Harker, R. M.; Harrison, A.; Hitchcock, P. B.; Jardine, C. N.; Walton, R. Organometallics 2007, 26 (13), 3111–3119. Kerridge, A.; Kaltsoyannis, N. C. R. Chim. 2010, 13 (6), 853–859. Dolg, M.; Mooßen, O. J. Organomet. Chem. 2015, 794, 17–22. Kilpatrick, A. F. R.; Cloke, F. G. N. Dalton Trans. 2017, 46 (17), 5587–5597. Summerscales, O. T.; Johnston, D. R.; Cloke, F. G. N.; Hitchcock, P. B. Organometallics 2008, 27 (21), 5612–5618. Kilpatrick, A. F. R.; Guo, F.-S.; Day, B. M.; Mansikkamäki, A.; Layfield, R. A.; Cloke, F. G. N. Chem. Commun. 2018, 54 (51), 7085–7088. Katz, T. J.; Garratt, P. J. J. Am. Chem. Soc. 1963, 85 (18), 2852–2853. Katz, T. J.; Garratt, P. J. J. Am. Chem. Soc. 1964, 86 (23), 5194–5202. Lalancette, E. A.; Benson, R. E. J. Am. Chem. Soc. 1963, 85 (18), 2853. Murahashi, T.; Inoue, R.; Usui, K.; Ogoshi, S. J. Am. Chem. Soc. 2009, 131 (29), 9888–9889. Verkouw, H. T.; Veldman, M. E. E.; Groenenboom, C. J.; Van Oven, H. O.; De Leifde Meijer, H. J. J. Organomet. Chem. 1975, 102 (1), 49–56. Westerhof, A.; De Liefde Meijer, H. J. J. Organomet. Chem. 1978, 149 (3), 321–325. Mares, F.; Hodgson, K. O.; Streitwieser, A. J. Organomet. Chem. 1971, 28 (2), C24–C26. Kawasaki, K.; Sugiyama, R.; Tsuji, T.; Iwasa, T.; Tsunoyama, H.; Mizuhata, Y.; Tokitoh, N.; Nakajima, A. Chem. Commun. 2017, 53 (49), 6557–6560. Ramanantoanina, H.; Merzoud, L.; Muya, J. T.; Chermette, H.; Daul, C. J. Phys. Chem. A 2020, 124 (1), 152–164. Boche, G.; Bieberbach, A. Chem. Ber. 1978, 111 (8), 2850–2858. Joshi, M.; Ghanty, T. K. ChemistrySelect 2019, 4 (34), 9940–9946.

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4.11

Olaf Walter, European Commission, Joint Research Centre (JRC), Karlsruhe, Germany © 2022 Elsevier Ltd. All rights reserved.

4.11.1 4.11.2 4.11.3 4.11.4 4.11.5 4.11.5.1 4.11.5.2 4.11.5.2.1 4.11.5.2.2 4.11.5.2.3 4.11.5.2.4 4.11.5.3 4.11.5.4 4.11.5.4.1 4.11.5.4.2 4.11.6 4.11.7 4.11.8 4.11.9 References

4.11.1

Introduction Intermezzo: Oxidation states Bonding Cycloheptatriene complexes of the actinides Cyclooctatetraenyl complexes of the actinides (COT)-Actinide half-sandwich complexes Mixed sandwich complexes containing a COT- and a Cp-ligand Terminal actinide oxo- or imido complexes CO2 activation CO activation Small molecule activation of other molecules Bridging COT-ligands in actinide complexes The actinocenes, [An(COT)2] and their derivatives Structural features of the actinocenes Adduct formation at the actinocene Pentalene complexes An complexes with donors containing 9C-atoms in a planar environment An complexes with donors containing 10C-atoms in planar environment Conclusion

582 583 583 583 587 587 589 589 590 591 593 594 595 595 597 598 600 601 603 604

Introduction

The discovery of Ferrocene in 1951 is considered to be the birth of modern organometallic chemistry with carbocyclic ligands exhibiting interactions to the metal ion via their p-bonds. The structural analysis of Ferrocene in 1952 showed the at the time remarkable but now typical p-Z5-coordination mode of the cyclopentadienyl ligand C5H5−, or Cp.1–4 In the following years Cp chemistry has been expanded to virtually all naturally-occurring metals in the periodic table including the f-elements, which resulted in a very high number of new metal complexes and the development of organometallic chemistry as we know it today. Some years later (1956) [U(Cp)3Cl] was reported as the first actinide organometallic complex5; however, it took nearly 10 more years until its structural characterization via single crystal X-ray diffraction.6 In the meantime chemists developed the actinide Cp4-complexes [An(Cp)4].7–10 Fifty years later, the organometallic chemistry of the actinides has reached a mature level which is reflected in a large number of compounds described, and a number of excellent reviews have appeared: the series on organometallic chemistry of lanthanides and actinides where in a yearly report the recent developments in the field were highlighted by Edelmann11–16; the series “Structure and Bonding” has Volume 127 with the title “Organometallic and Coordination Chemistry of the Actinides”,17and therein is a dedicated chapter on the “Activation of Small Molecules by U(III) Cyclooctatetraene and Pentalene Complexes”18; the organometallic chemistry of the pentalene ligand was summarized earlier19 and recently20; and, lanthanide(II) and actinide(II) chemistry has been highlighted by Evans.21 More generally, without a theoretical approach the understanding of the actinide bonding is difficult and this has been reviewed by Kaltsoyannis.22 The Cp-chemistry of Thorium and uranium was highlighted by Ephritikhine,23 whereas P. Arnold reviewed the organometallic Neptunium chemistry.24 An excellent overview on the activation of small molecules on Uranium complexes including organometallic ones has been given in 2014 by Meyer.25 Finally there is a review summarizing the recent developments on trans-Uranium elements in organometallic chemistry.26 Up to present about 1250 Uranium compounds showing a U-C interaction are deposited in the Cambridge Crystallographic Data Center (CCDC)27; this compares to ca 550 for Th, 15 for Np,28–35 and 7 for Pu.36–39 This situation results from the fact that dealing with radioactive elements or isotopes of Np or Pu includes the establishment of cost-intensive radiation protection measures besides the necessary licensing and safeguard controls. For the trans-Uranium elements and their related structures of organometallic complexes a big step has occurred over the last 5 years, with most of neptunium published after 201528–31; for Pu all data arise from 2017 onwards.36–39 In this article actinide chemistry will be summarized where the actinide ions are stabilized by ligands having a high number of C-atoms in the coordination sphere to the metal (larger than 6), specifically cycloheptatrienyl complexes, COT systems, and pentalene complexes, which will contain a brief excursion into indenyl complexes for contextualization purposes. A small excursion into the molecular orbital theory that underpins actinide bonding to large aromatic rings will also be provided. ☆ Dedicated to the memory of Prof Dr Basil Kanellakopulos (d 19 Oct 2021) and to the honour of his pioneering work in the field of the actinide organometallic complexes. A great chemist left us.

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https://doi.org/10.1016/B978-0-12-820206-7.00076-7

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4.11.2

583

Intermezzo: Oxidation states

While discussing organometallic actinide complexes, it is necessary to comment a little bit on oxidation states and how they will be treated in this review. For example, we regard the actinocenes, [An(COT)2] as actinide complexes in the oxidation state +IV and the ligand as cyclooctatetraenyl dianion (COT)2−. In this case, it makes sense as many of the electronic properties are thus described well. However, as finally in the occupancy of the molecular orbitals (MO’s) of the resulting complexes the origin of the electrons is of lower interest, here in this review the (COT)2− ligand is regarded as cyclooctatetraenyl, Cp− as cyclopentadienyl and the C7H7-ligand as cycloheptatrienyl. The rules of aromaticity are of limited validity as in the p-complexes of the cycloalkenes the coordinated ligands are always planar with similar CdC bond lengths. For example in the complex compound K[U(C7H7)2] the (C7H7)-unit might be regarded as a 6-electron donor of a planar 6 electron (C7H7)+-unit, which would mean that the uranium must be threefold negatively charged. This contrasts to an alternative intuitive description of this compound, where K[U(C7H7)2] can be considered as uranium(V) stabilized by 2 aromatic 10-electron (C7H7)3—units. This again somehow is in discrepancy to our understanding, as the charge separation is high. As an organometallic chemist, one could also describe the organometallic p-complex K[U(C7H7)2] as having the uranium as a U(III) and therefore two C7H7-units as (C7H7)2−, both together acting as 18 electron donors. Interestingly for the resulting MO scheme, all this is of lower interest; it does not change the overall picture. Therefore, in this article the question of the formal oxidation states of the organometallic p-complexes is not placed in the foreground and indeed in the most cases it is not discussed too much. The opinion of this author is that the oxidation state of the metal in an organometallic p-complex should not be described by formalism and rules but by its spectroscopic properties. Anyhow, on the example of the organometallic p-complex K[U(C7H7)2] (Section 4.11.4) we will have a closer look to this point.

4.11.3

Bonding

The bonding in actinide complexes involving ligands that engage in p-interactions to the actinide metal center has been long investigated. Here a brief summary will be given on the bis-cyclooctatetraenyl (COT)2− complexes of the actinides [An(COT)2], because there the situation is clear due to symmetry considerations. Generally, besides the cationic character of the actinide metal centers and the negatively charged p-ligands, constructive interactions arise between the molecular p-orbitals of the ligand and symmetry related unoccupied metal atomic orbitals via the formation of bonding molecular orbitals. This causes a certain covalent character in the bonding between the ligands and the actinide center. The bonding between the larger rings like (COT)2− and an actinide metal has been reviewed earlier by D. Clark.40 The covalency in this bonding has been the subject of intense scrutiny, and the bonding picture that has emerged has led to the renewed recognition that the covalency of such interactions is a function of not only orbital symmetry and energy but also orbital overlap.41,42 The theoretical description of actinide complexes was reviewed by Kaltsoyannis.22 Looking in more detail at the [An(COT)2] system it can be readily seen from the MO scheme of Uranocene (Fig. 2) that indeed in the formation of the resulting MOs actinide metal d and f symmetry orbitals are involved. With respect to the very recent results presented by Smiles et al., the orbital overlap has a significant influence on the bonding and the resulting covalency.42 Today there are many actinide complexes with p-ligand bonding, which raises the question of how is it possible that these elements show such a broad structural variety? One reason is found in their location in the periodic table: these elements have a high number of atomic frontier orbitals of comparable energy levels, which are then able to interact and form bonding combinations with the ligand orbitals (Fig. 1). So, in the range about ca 0.9 A˚ for Pu there is radial contribution from 5f, 6s, 6p, and 6d, which shows the high potential for establishing bonding combinations between groups of ligand orbitals and the symmetry related metal orbitals. If the symmetry requirements are fulfilled these groups or sub-groups could even mix again (Fig. 2). The (COT)2− ligand acts as a 10 electron donor, so the actinocene complexes can be regarded as a 20 electron complex (Th) or 22 electron in case for Uranocene. Thorocene is the diamagnetic representative of the series; all the other actinocenes show paramagnetic behavior. The findings for Uranocene, [U(COT)2], can be transferred to all other actinide complexes where organometallic p–ligands are involved in stabilization of the actinide metal center. Thus, due to the nature of the early actinides with the 5f and 6d orbitals being not or only partially occupied and reaching out in space there is a high probability that these metals can form stable complexes with p–donor ligands like Cp, or COT.

4.11.4

Cycloheptatriene complexes of the actinides

The cycloheptatrienyl cation (C7H7+) is like benzene or Cp an aromatic six electron donor ligand shielding one coordination side of the metal to which it binds. Accordingly, the cycloheptatrienyl anion (C7H7−) is an 8-electron donor not fulfilling the rules of aromaticity but for the resulting molecular orbitals of an organometallic complex the origin of the electrons in its MOs is not of importance. The development of the complex chemistry involving a (C7H7)-unit started already at the end of the 1950s/the beginning of the 1960s43; the single crystal structure of [(Cp)V(C7H7)] was published in 1963.44 In this complex the two p-ligands both act as six

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Fig. 1 Comparison of the radial distribution of the frontier orbitals between Sm3+ and Pu3+.40

Fig. 2 (Left) Molecular orbital diagram of Uranocene; Uranium atomic orbitals on the left; on the right the p-orbitals of the (COT)2− ligands. For reasons of clarity only half of the p-orbital is shown. (Right) Schematic view to the MOs with s-symmetry (su, sg, 2nd half of p-orbital omitted), with p-symmetry (pg, pu), and with d-symmetry (fd, dd). Source: Neidig et al.; Coord. Chem. Rev. 2013, 257, 394–406.

electron donor ligands with the Cp ligand establishing a p-Z5- and the (C7H7)+ in p-Z7-coordination mode. Today about 470 data sets are reported in the CCDC having the (C7H7)-unit in the coordination sphere of a transition metal.27 Transition metals are smaller than actinides which is reflected in some azulene-type complexes with the (C7H7)-unit showing p-coordination with different hapticity such as Z4, Z5, Z6, or Z7.

Larger Aromatic Complexes of the Actinides

C3

C3 U2

C1 C4

U1

C3 C2

C2

C4

C4

B2

C1

K1

B5

U1

C1

C7 C5

B4

B1

C2

B6

585

C6

C4

C4

C2

B3

C3

C2 C3

Fig. 3 Left view to the negatively charged bis-Uranium sandwich complex in [U(BH4)2(thf )5][(BH4)3U(m-Z7:Z7-C7H7)U(BH4)3], 1b, with the (C7H7)-unit in bridging mode; right: view to a part of the polymeric chain of [K(18-C-6)][U(C7H7)2], 2, showing as well the bridging m-Z2:Z7-coordination of the (C7H7)-anion between the U and the K centers (H atoms omitted in the structures).

This compares to the actinides where up to the present only 2 complexes have been reported and structurally characterized both arising from the group of Ephritikhine (Fig. 3).45,46 Interestingly, Ephritikhine succeeded in the synthesis of a sandwich complex and an inverse sandwich complex as well. The syntheses of the inverse sandwich complexes 1 was established by reaction of suitable starting complexes [U(NEt2)4] (for 1a) or [U(BH4)4] (for 1b) with the potassium salt of the cycloheptadienide anion at elevated T, for example at 65  C in toluene over 5 days (Scheme 1).46

Scheme 1 Synthesis of the negatively charged inverse sandwich uranium complexes 1 [X3U(m-Z7:Z7-C7H7)UX3], for 1a X ¼ NEt2, cations ¼ [K(18-C-6)]+, for 1b X ¼ BH4, cation ¼ [U(thf )5(BH4)2]+.

The synthesis of the negatively charged sandwich complex 2 [U(C7H7)2]− proceeds in thf for some hours at RT by reacting UCl4 with a large excess of cycloheptatriene and a slight potassium excess (Scheme 2). The potassium acts in this case once as deprotonating agent and as reducing agent at the same time.

Scheme 2 Synthesis of the negatively charged sandwich uranium complex 2 [U(C7H7)2]−, cation: [K(18-C-6)]+.

Even if both complexes contain an anionic (C7H7)-unit the bonding in the complexes is different, which is reflected in the U to center of ring distances. In the borohydride adduct 1b as an inverse sandwich complex this distance is found to 212(2) pm (with UdC bond distances of 269(2) pm) whereas in the sandwich 2 [U(C7H7)2]− it is determined to 198(2) pm (with UdC bond distances of 253(2) pm). One can understand this by taking into account that in the borohydride 1b [(BH4)3U(m-Z7:Z7-C7H7) U(BH4)3]− the two Uranium centers compete for the same orbitals and electrons of the negatively charged (C7H7)-bridging unit,

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which results in weaker interaction on the single Uranium site. Due to the smaller ring size of the seven membered cycloheptatrienyl ring compared to the eight membered COT ring, the corresponding U to center of ring distances for uranocene is at 192.3 pm shorter than the one found here in the [U(C7H7)2]− sandwich, 2, even if in uranocene the mean UdC bond distance is at 264.6 (1.2) pm longer than in 2. The oxidation state of [K(18-C-6)][U(C7H7)2], 2: As already mentioned, in the organometallic p-complex K[U(C7H7)2], 2, the (C7H7)-unit might be regarded as positively charged then forming the aromatic 6-electron donor (C7H7)+, or it might be seen as threefold negatively charged then forming the aromatic 10 electron donor (C7H7)3−. In the first case then the uranium in the complex anion [U(C7H7)2]−, 2, must carry three negative charges, whereas five positive in the latter case with the two (C7H7)3− units. Both of these descriptions seem unlikely: the 1st because of the negative charge on the metal and the 2nd because of the high charge separation in the organometallic p-complex 2. However, already others found this question interesting and accordingly the bonding nature and the oxidation states in 2 and its comparable homologs as well in the transition metals have been investigated.47–49 Briefly summarized from these examinations it seems that the complexes of the type [M(C7H7)2] (they might carry charge) are in general best described as M(III) complexes.47 This compares to the actinides where for [U(C7H7)2]− (2) the best description is given with strong theoretical support as a U(III) complex, and for its neutral analog [U(C7H7)2] as a U(IV) complex.48 The reason is definitively in the strong orbital mixing with a high 5f contribution, which is increasing with atomic number. In simpler words, this is due to a high covalency in the bonding. These findings are in agreement with the magnetic and EPR measurements on 2.49 From the MO scheme of 2 (Fig. 4) the strong participation of the 5f orbitals in the formation of its frontier orbitals for example the HOMO can be easily seen. There are no other thorium or uranium complexes reported involving a (C7H7)-unit; nothing has been done with the more radioactive elements such as Np, or Pu.

Fig. 4 MO scheme of [U(C7H7)2]− (2). From left to right on the x-axis: Uranium AO’s non relativistic, Uranium AO’s relativistic, MO orbitals of [U(C7H7)2], group p-MO’s of two (C7H7) moieties, p-MO’s of (C7H7). Source: Li , J.; Bursten, B.E. J. Am. Chem. Soc. 1997, 119, 9021–9032.

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Fig. 5 Space filling view to the molecular structure motif of [An(COT)] complex fragment clearly showing the shielding of one side of the actinide central metal and the free accessibility of the opposite side (Figure created with Mercury).

4.11.5

Cyclooctatetraenyl complexes of the actinides

COT (cyclooctatetraenyl, C8H82−) has been introduced to actinide chemistry more than 50 years ago. As working with Uranium and Thorium is less restricted, the research on derivatives of Uranocene [U(COT)2] and Thorocene [Th(COT)2] continued,50–53 whereas the development of the Plutonocene or Neptunocene chemistry was practically stopped in the 1980s due to the radiation safety regulations.50 As an effect in the CCDC there are about 200 actinide complexes containing at least one COT ligand in the metal coordination sphere. There is only one neptunium complex structurally characterized yet, neptunocene, whereas three Plutonium-COT complexes have been analyzed via single crystal X-ray diffraction, which are all as well plutonocene derivatives. Today we still face the situation that due to the necessary safety and radiation protection measures the organometallic chemistry of the more radioactive actinides involving derived COT ligands is much less well developed than for Th and U.

4.11.5.1

(COT)-Actinide half-sandwich complexes

The chemistry of the actinide half-sandwich complexes (Fragment: [An(COT)], Fig. 5) is characterized by their structural features with the COT ring shielding one side of the actinide center well, whereas the other is freely accessible for chemical transformations or the stabilization of molecules. This chemistry needs easy access to starting materials. The complexes [(COT)AnCl2(L)2] (L: thf for Th, pyridine for U) have been synthesized and characterized by Zalkin and Streitwieser Jr.,54,55 whereas Ephritikhine developed access to [(COT)U(BH4)2(thf )], 3, as a versatile starting complex for further transformations.56 Nearly 20 years later the bis-trimethylsilylamide [(COT)U {N(SiMe3)2}2] was published.57 The chemistry at the [(COT)An]-complex fragment is dominated by transformations involving ligand exchange, while the COT ligand stays in the coordination sphere of the metals.58–73 So for example the corresponding [(COT)An]-complexes stabilized with chalcogenide ligands have been investigated intensely, and their transformations are summarized in Scheme 3.74

Scheme 3 Ligand exchange reactions starting from [(COT)U(BH4)2(thf )], 3, involving chalcogenide based ligands; dddt: 5,6-dihydro-1,4-dithiine-2,3-dithiolate. Source: Ephritikhine, M.; Coord. Chem. Rev. 2016, 319, 35–62.

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P1

U1

O1

B1

Fig. 6 View to the solid state structure of [(COT)U(BH4)(thf )(Z5-tetramethylphospholyl)], 10, in the single crystal (H atoms omitted).63

Starting from [(COT)U(BH4)2(thf )], 3, by reaction with iPrSH or BuSH or its sodium salt the tetra-bridged binuclear complex [(COT)U-(m-Z2-SiR)4-U(COT)] (R: Bu, iPr), 4, is formed, whereas the mononuclear negatively charged complex [(COT)U(StBu)3]−, 5, is obtained from the ligand exchange reaction with the more bulky salt NaStBu. Reacting [(COT)U(BH4)2(thf )], 3, with 5,6-dihydro-1,4-dithiine-2,3-dithiocarbonate leads to the neutral dimeric complex [U(COT)(m-dithiolene)]2, 6, of which then Lewis base adducts are accessible forming the mononuclear complexes [U(COT)(dithiolene)(L)2], 7 (L: pyr (7a), Ph3P]O (7b), hmpa (7c)). By reacting [(COT)U(BH4)2(thf )], 3, with the sodium dithiolene salt directly the dianionic bis-adduct of the dithiolene to the [(COT)U] complex fragment [(COT)U(dithiolene)2]2−, 8, is obtained, which offers the possibility for being oxidized to the formal UV complex [(COT)U(dithiolene)2]−, 9.74 One organometallic highlight in this half-sandwich [(COT)U]-chemistry is the stabilization of a p-Z5-tetramethylphospholyl in [(COT)U(BH4)(thf )(Z5-tetramethylphospholyl)], 10, (Fig. 6) which was prepared by the reaction of [(COT)U(BH4)2(thf )], 3, with the potassium salt of tetramethylphosphole.63 Interestingly the p-Z5-coordination of the tetramethylphospholyl enables the formation and stabilization of the tetramethylphospholyl with quasi-equidistant U-C or U-P distances in the range between 289.2 and 293.7 pm (U-C) and 297.0 pm (U-P). Indeed, complex [(COT)U(BH4)(thf )(Z5-tetramethylphospholyl)], 10, is already a mixed sandwich complex; of those synthesis and reactivity are now described in the following section. Reaction of UCl4 and the dilithium salt Li2{C(P(Ph)2 ¼ S)2} leads to the formation of the uranium carbene complex 11; the two present chlorine atoms in 11 can be replaced by a COT ligand and formation of COT carbene complex 12 (Scheme 4). With this, complex 12 is a rare example of an actinide complex exhibiting a p-Z8-coordination of a COT ring and a Z2-coordination of a carbene at the same moment with a difference in the UdC bond distances of more than 30 pm in the UdC bond lengths, with the carbene U]C bond distance of 235.1 pm while the UdC bond distances to the p-coordinated COT ring covering a range from 264.6 to 271.9 pm.65

Scheme 4 Formation of 12.

In 1996 Ephritikhine showed a nice example of insertion chemistry in the UdN bond of [(COT)U(NEt2)(thf )3][BPh4], 13; the authors could activate CO2, CS2, and MeCN via nucleophilic attack at the C-atom of the carbonyl compound (Scheme 5).70 The complex 14b obtained by CS2 activation was analyzed by X-ray diffraction showing a symmetrical bonding of the dithiocarbamate to the uranium, which is reflected in the spectroscopic data and in the practically identical UdS bond lengths of 281.0(7) and 280.1(7) pm.

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589

Scheme 5 CO2/CS2 activation of [(COT)U(NEt2)(thf )3][BPh4], 13.

Some interesting results from single crystal X-ray analyzes are present in the CCDC as the heritage of M. Ephritikhine not having been attributed to any publication. Among these there is a molecular [(COT)U]-fragment stabilized azide or cyanide complex.64 However, this broad chemistry has not been transferred to the trans-uranium elements.

4.11.5.2

Mixed sandwich complexes containing a COT- and a Cp-ligand

Up to the present many complexes containing a Cp and a COT (or substituted COT) as p-ligands in the coordination sphere of the actinide have been reported, with other ligands or donors completing the coordination sphere. A rich actinide chemistry has been developed including the activation of small molecules like N2, CO2, CO, CS2, bisalkylchalcogenides, and the formation of azides and amides or oxides.75–97 A good overview on earlier data as well including a structural comparison is given by Evans in 2011.96

4.11.5.2.1

Terminal actinide oxo- or imido complexes

The complex [{1,4-(SiR3)2-COT}U(Cp )(thf )], 15, reacts with tBu-isocyanate as oxygen transfer agent or sodium azide as nitrogen transfer agent depending on the bulkiness of the substituent R of the silyl group to produce the m-oxo complex [m–O({1,4(SiR3)2-COT}U{Cp })2], 16, or the negatively charged m-amido complex [m-N({1,4-(SiR3)2-COT}U{Cp })2]−, 17, (R: Me, Scheme 6).75 The terminal oxo complexes [{1,4-(SiR3)2-COT}U]O(Cp )(CNtBu)], 18, and [{1,4-(SiR3)2-COT}U]O(Cp )], 19, or the terminal amido complex [{1,4-(SiR3)2-COT}(Cp )U]N{Na(OEt2)2], 20 are products when the COT ring in 15 is substituted with more sterically demanding iPr3Si substituents (Scheme 6): 15 produces in the reaction with tBu-isocyanate two terminal uranium oxo complex 18 with a CNtBu and in the coordination sphere of the uranium central metal atom resulting as a product from the oxygen transfer, and 19 where this CNtBu ligand is not present (Scheme 6).75 The oxo complex 18 with the coordinated CNtBu is observed as by-product, but only when the synthesis was performed on a larger scale, otherwise 19 is the only product obtained (Scheme 6).75 These two terminal uranium oxo in the same ligand environment differing only by the presence of an additional CNtBu ligand offers the opportunity to directly compare the effect on the structural features: the loss of the additional stabilizing CNtBu ligand makes the other ligands bonded stronger to the U atom which is reflected in a remarkable shortening of the UdO bond from 191.6(8) (18) to 182.6(3) pm in 19 which compares rather well to the U-N distance of 183.5(5) pm in the terminal uranium imido complex 20 formed by reacting the iPr3Si substituted 15 with sodium azide (Scheme 6).75 Confronting the U]E bond distances of the 18, 19, 20 due to their comparability in 19, 20 one could deduce that the bonding situation must be similar and closer to a uranium oxygen (19) or nitrogen (20) triple bond than in 18 where due to additional coordination of a strong s–donor isonitrile ligand symmetry and electronic environment on the uranium center have changed.

Scheme 6 Reactions of [{1,4-(SiR3)2-COT}U(Cp )(thf )], 15, product selectivity as a function of silyl-substituents of the COT ligand.75

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Scheme 7 CO2 coupling products from the reaction of the Th-analog of 15 for R ¼ iPr3. The oxo-bridged side product is omitted as no is CO2 incorporated.

The observed product selectivity (Scheme 6) includes as well a control of the oxidation state of the product: UV in case for the more bulky and more electron rich iPr-substituted SiR3-groups, whereas in case the silyl groups are trimethyl-substituted UIV complexes are obtained. One can learn from this that more electron rich ligands transfer more electron density to the metal and doing so facilitate oxidation as they can stabilize the central metal ion as well in higher oxidation states. For Thorium these complexes or comparable ones have not been reported yet, even if one could imagine the existence of the ThIV analogs of the m-oxo or -amido complexes (left side in Scheme 6). The transfer of this challenging chemistry to the trans-uranium elements is still waiting.

4.11.5.2.2

CO2 activation

The electron rich mixed sandwich complexes like e.g., [{1,4-(SiR3)2-COT}U(CpMe4R0 n)(thf )], (one example is complex 15 for R ¼ iPr or Me, R’ ¼ Me) are reactive molecules which do not only form molecularly well-defined oxo-species but as well show activation of small molecules such as CO2; from these reaction again molecularly well-defined complexes can be obtained and structurally characterized. In dependence of the nature of the starting complex reacting with the CO2 different insertion products are obtained: the carbonate results from CO2 insertion in a MdO bond (motif A in Fig. 7).76,77,88,89 As in the starting complex no O-atom is present the additional O-atom in the carboxylate complex is coming from another CO2 molecule which is then reduced to CO, which was shown by identifying from the CO2 reaction as well as products being identical as to the CO activation. Actually, from the same reactions as well the oxalato-bridged complexes (motif D in Fig. 7) can be obtained as products from twice a one electron reduction from the actinideIII center to the CO2 followed by a C-C coupling via dimerization. Identification of the different products from the same reaction indicates these possibly having common intermediates, and by controlling the reaction conditions the product distribution might be influenced. However, the product selectivity depends also on the substituents in the starting actinide mixed sandwich complex [{1,4-(SiR3)2-COT}U(CpRn0 )(thf )], for details please refer to the cited literature in detail. Whereas the CO2 activation chemistry in case for Uranium is quite well examined for Th less is known: the Th analog to 15 is prepared in-situ by Na/K alloy reduction of [{1,4-(SiiPr3)2-COT}Th(Cp )I], 21; in the presence of CO2, the corresponding Th carbonate (22) and oxalato (23) complexes are formed which can be separated by fractionated crystallization (Scheme 7).76 Complex 21 gives as well access to the corresponding benzyl-Th and Th-hydride from which then as well the CO2 insertion products are obtained under formation of the structure motif E in Fig. 7.76

A

B

C

D

E

Fig. 7 Schematic drawing of the structural motifs obtained by CO2 activation with mixed sandwich complexes; [An] abbreviates a moiety [An(COT)(Cp)] (An: Th, U) with varying substituents on the rings, [U] abbreviates a moiety [U(COT)(Cp)] with substituents on the rings.

The chemoselectivity in this CO2 activation is quite varying. In order to gain a better understanding of the ligand influence on the product selectivity first the steric pressure on the COT ring in the complexes 15 is reduced by replacing the iPr3Si- by Me3Si-groups then the Cp is substituted by CpMe4R leading to the complexes [{1,4-(SiMe3)2-COT}U(CpMe4R)(thf )]. In the CO2 activation then product selectivity can be monitored as function of the steric demand of the substituent R at the CpMe4R ring. Increasing steric pressure shifts the product distribution in towards the carbonate (R: tBu, motif A in Fig. 7) whereas for less sterically demanding CpMe4R ligands the oxalato complex (motif D in Fig. 7) becomes competitive or even the predominant product.77 The carbamate is the product of CO2 insertion in a MdN bond (motif B in Fig. 7),90 the formate from the insertion in a MdH bond (motif C in Fig. 7),76,79 and the alkyl substituted carboxylates results from CO2 insertion in a MdC bond (motif C in Fig. 7).79 For the formate in case of Uranium a mononuclear complex is formed whereas for the larger Thorium a dinuclear complex is

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591

thermodynamically favored (motif E in Fig. 7).76 This structure motif is as well found for Uranium in case that the Alkyl which attacks the CO2 molecule is one of the methyl-groups of the Cp ligand in the coordination sphere of the Uranium, which disables a possible O,O0 -coordination of the carboxylate group.79 A tetramethylpyrrole (CpNMe4) and its homologs the phosphole (CpPMe4) and the arsol (CpAsMe4) have isoelectronic p-orbitals like Cp ; so the complexes [{1,4-(SiiPr3)2-COT}U(CpEMe4)] (E: N,P, As, 24) have been synthesized (E: N,P, As) and their reactivity towards CO2 was studied (E: N,P, Scheme 8).97

Scheme 8 Synthesis of the group V Cp homolog complexes [{1,4-(SiiPr3)2-COT}U(CpEMe4)] (E: N,P, As), 24, and their reactivity towards CO2 under formation of complexes 25.

Due to the existence of the additional lone pair at the N or P atom in 24 not being part of the p-system, they show a different selectivity as the complexes [An(COTR)(CpR’)] (R,R’: different substituents) stabilized by pure hydrocarbon ligands: the lone-pair undergoes a nucleophilic attack to the C-atom of the CO2, electron transfer proceeds and the resulting carboxylate coordinates to the UIV center under formation of the complexes 25 (Scheme 8, Fig. 8); the former Z5-p-coordination of the CpNMe4/CpPMe4 in the starting complexes 24 is not maintained. The m-O:O0 -bridging mode of the pyrrole (or phosphole) carboxylate is symmetrical with the corresponding U-O distances between 233–238 pm (25a) and 234–239 pm (25b), respectively. The oxo-bridge results from CO2 to CO reduction. Overall, it can be seen that the high reactivity of these organometallic mixed sandwich actinide complexes already only in case of CO2 activation showed a broad diversity in chemoselectivity of the obtained CO2 coupling products. The thus far missing transfer to Neptunium or Plutonium chemistry would undoubtedly be interesting.

4.11.5.2.3

CO activation

As already mentioned in the section before where CO2 activation was discussed, the electron rich mixed sandwich complexes of the type [{1,4-(SiR3)2-COT}U(CpR’)(thf )] activate CO2 as well reducing it under the formation of CO. The released oxygen atom is then incorporated in the organometallic uranium complex as a consequence of the high oxophilicity of the uranium. So in some cases from the CO2 experiments products could be isolated containing CO moieties.88 It is obvious that then it made sense to study the CO chemistry of the electron rich mixed sandwich complexes of the type [{1,4-(SiR3)2-COT}U(CpR’)(thf )] itself.

U2

P1

P2

U1

Fig. 8 Simplified view to the core of the molecular structure of the dinuclear complex arising from CO2 insertion in the UdP bond of [{1,4-(SiiPr3)2-COT}U(CpPMe4)], 25b; substituted COT-rings reduced to their geometrical center (top and bottom).

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A

B

C

Fig. 9 Schematic drawing of the structural motifs obtained by CO activation with mixed sandwich complexes; [U] abbreviates a fragment [U(COT)(Cp)] with substituents on the rings.

The mixed sandwich Uranium complexes are so electron rich that they can reduce CO under C-C coupling in the coordination sphere of the Uranium. The product complexes formed may contain dimers, trimers or tetramers of the CO monomer (Fig. 9). Even if in, the isolated and via X-ray diffraction characterized, Uranium complexes CO coordination proceeds via C-coordination,92–94 in all the products resulting from reductive CO oligomerization U-O coordination is observed. The products formed by C-C coupling are the CO dimer in the form of an acetylenediolate (motif A in Fig. 9), the trimeric deltate (motif B in Fig. 9) or the tetrameric squarate (motif C in Fig. 9), all stabilized by O-coordination to the oxophilic Uranium atom in the [U]-complex fragment.78,83,85,89,91 The products again are obtained depending on the substituents at the Cp ring and the reaction conditions. For example the mixed sandwich complex 15 reacts with one equivalent CO to give the linear coupled ethynediolate complex 26 (Scheme 9); the intermediate complex [{1,4-(SiiPr3)2-COT}U(CO)(Cp )] with a half-life time of ca 15 min and a CO absorption at 1920 cm−1 is not stable. The formation of a stable U-CO complex is described in Section 4.11.7. Reaction of 15 with a higher amount of CO under otherwise comparable reaction conditions leads to the formation of the deltate complex 27.85 The ethynediolate complex 26 once formed is stable and cannot be transformed into 27 by further reaction with CO (Scheme 9).

Scheme 9 Reaction of 15 with different amounts of CO.

If in the starting complex the Cp ring is replaced by its tetramethyl-analog (28, Scheme 10) then the chemistry changes and the squarate complex results from CO tetramerization (29 in Scheme 10).83 From this one sees that already slight changes in the starting complex can influence the product selectivity: a broad variety of in the coordination sphere of an actinide ion stabilized uncommon CO oligomers are accessible and even if they cannot be transformed one into each other they must arise from common intermediates which was confirmed by theoretical investigations.98,99

Scheme 10 Uranium mixed sandwich mediated CO tetramerization under squarate complex formation.

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593

Specific structural features of the CO coupling products (Fig. 9)78,83,85,89,91:

• •

•

the ethynediolates (motif A in Fig. 9) are bridging two uranium centers in a linear m-Z1: Z1-way and are structurally characterized by short C-C distances about 120 pm accompanied by C-O distances of ca 130 pm; the UdO bond lengths are in the range of 215–220 pm. The deltates (motif B in Fig. 9) are a planar system bridging the uranium centers in m-Z1: Z2-coordination by linear coordination of one O atom to one uranium center with a UdO bond distance about 215–220 pm, whereas the other uranium atom bonds to two oxygen atoms in symmetrical O, O0 -coordination establishing longer U-O interactions in the range 245–250 pm. Typically the CdC bond distances are determined to 135–140 pm with corresponding CdO bond lengths of 125–130 pm. The squarates (motif C in Fig. 9) show a symmetrical m-Z2:Z2-coordination with the two uranium centers binding to two O-atoms in O,O0 -coordination with UdO bond distances in the range of 245–250 pm. The CdC bond lengths are with ca 145 pm longer than in the other derivatives, whereas the CdO bonds are at ca 125 pm in a comparable range.

As these complexes are capable to react and to activate CO; it was shown by Cloke that complex 15-thf, [{1,4-(SiiPr3)2-COT} U(Cp )], does not only react with CO, but that in the syngas reaction the hydrogen reduces the CO to methoxide still stabilized in the Uranium coordination sphere of the mixed sandwich complex (30 in Scheme 11).95 By treatment of 30 with trimethylsilyltriflate (TMSOTf ) the authors could liberate the coordinated MeOH in form of its TMSOMe derivative under formation of the corresponding OTf-complex 31, which then by reduction with potassium amalgam re-generates the starting complex 15-thf and closes the cycle (Scheme 11). Due to the nature of the organometallic uranium complex, a catalytic conversion is excluded. However, it is an excellent example showing that the actinides (in this case uranium) might exhibit a to transition metals comparable chemistry offering the possibility to develop ideally a catalytic cycle.

Scheme 11 Reaction with syngas (1:2), MeOTMS formation and regeneration of the starting complex [{1,4-(SiiPr3)2-COT}U(Cp )] (15-thf).95 0

0

In the mixed sandwich complexes of the type [An(COTR)(CpR )] (R, R : different substituents) the Cp ligand can be exchanged by an indenyl unit of which then the 5 membered ring coordinates towards the An center similar to the coordination of a Cp. [{1,4(SiiPr3)2-COT}U(Indenyl )] shows comparable CO activation to its Cp derivatives, the analog squarate stabilized complex can be isolated (motif C in Fig. 9).89 However, the indenyl complexes are discussed more in detail in Section 4.11.7.

4.11.5.2.4

Small molecule activation of other molecules

When a molecule shows reactivity against CO then it should react with NO as well. Accordingly, the reactivity of 15 or its derivatives against NO was examined, though it was not possible to isolate a unique product complex. However, from the reaction of 15 with one equivalent of NO plus 0.5 equivalents of CO the cyanate adduct 32 was isolated, with the cyanate anion formed in NO/CO coupling in bridging m-N,O-coordination (Scheme 12). The 2nd product from this reaction is the m-oxo-complex 33 resulting from oxygen transfer of the excess oxygen to another uranium.81

Scheme 12 NO/CO coupling under formation of coordinated cyanate.

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White phosphorous, P4, as electron accepting reagent reacts with 15 via two electron reduction and opening of the P4 core under formation of a planar 4-membered P4 ring in the unique m-Z2:Z2-coordination to two [{1,4-(SiiPr3)2-COT}UIV(Cp )]- moieties in complex 34 (Scheme 13).100

Scheme 13 P4 activation under ring opening and reduction.100

The PdP bond distances of 215(2) pm in 34 support the two electron reduction and the formation of the ring-opened [P4]2− unit, which one could as well regard as a P analog of a cyclobutadienediide dianion. However, the coordination proceeds over the free electron pairs of the P atoms and not via the p-electrons of the ring, which is reflected in the UPP angles of 119 . Bridging p-coordination of a p-ligand we see in the next section on the example of the COT ligand.

4.11.5.3

Bridging COT-ligands in actinide complexes

The COT ligand does not only stabilize actinide complex fragments by p-coordination and shielding of one of its sides, it can as well act as a bridging ligand between two actinide complex fragments; this is called an inverse sandwich complex as in this case not the metal is not embedded between two p-donor ligands but the ligand between two metals. Evans has studied the reactivity of [U(Cp )3] with COT; while performing the reaction under optimized stoichiometric conditions the triple decker complex [(COT)(Cp )U-m-Z3:Z3-(COT)-U(Cp )(COT)], 35, can be isolated as the main product (Scheme 14).101 Interestingly the two [(COT)(Cp )U]-complex fragments in this complex bind in an allylic way to both sides of five C-atoms of the bridging COT-ligand. Later Evans reported that the same complex can better be synthesized by reacting two equivalents of [(Cp )2U(H)2] with three equivalents of COT; this synthesis worked as well with the analog [(Cp )2Th(H)2], leading to the isostructural mixed sandwich Th triple decker complex [(COT)(Cp )Th-m-Z3:Z3-(COT)-Th(Cp )(COT)], 35b, (Scheme 14).102

Scheme 14 Formation of [(COT)(Cp )An-m-Z3:Z3-(COT)-An(Cp )(COT)], 35.

The sophisticated binding mode of the bridging COT with the COT ligand in a m-Z3:Z3-coordination mode is explained by the authors by the fact that the complexes are under high steric pressure due to the other ligands (Cp , and COT) present in the coordination sphere of the actinide center. It is reflected as well in the bond lengths, which are determined for the C atoms engaged in the allylic system to 285(1), 273.1(6), and 293.3(6) pm (35b) and 288(2), 269(2), and 300(2) pm (35a), respectively (Fig. 10). The shortest MdC bond distance is found for the middle C atom in the allylic coordination. The allylic interactions are with these distances longer than the MdC bond distances found for the terminal COT which cover a range between 270.2(6)–277.7(6) pm (33) and 262(2)–272(2) pm (35a). One can easily see that positive An-C interactions can be established, covering a relative broad range of about 30 pm, which is a sign of the coordinative flexibility of the actinides. Accordingly for the sterically unstrained complex [{(Mes)tBuC]N}3U-m-Z8:Z8-(COT)-U{N]C(Mes)tBu}3], 36, an inverse sandwich structure (Fig. 10, right)73 is found showing a bridging COT-ligand in m-Z8:Z8- coordination between the [U]-complex fragments.

Larger Aromatic Complexes of the Actinides

595

U1 U1A Th1A

Th1

Fig. 10 Left: View to the molecular structure of one of the two isostructural complexes [(COT)(Cp )An-m-Z3:Z3-(COT)-An(Cp )(COT)], 35, (An: Th, U; Me-substituents at the Cp rings omitted for reasons of clarity), right: view to the core structure of the molecular complex [{(Mes)tBuC¼N}3U-m-Z8:Z8-(COT)-U {N¼C(Mes)tBu}3], 36, all C-atoms of the ketimido ligands omitted for reasons of clarity.

The inverse sandwich complex 35a itself is reactive, so it reacts with the diphenyldichalogenides under two electron transfer and loss of the bridging COT ligand forming the complex 38 with two bridging PhE-moieties (E: S, Se, Te, Scheme 15). Alternatively the same complex 38 is obtained from the reaction of [(COT)(Cp )(CpMe4)U], 37, with the corresponding diphenyldichalogenide.82,96

Scheme 15 Reactivity of the inverse sandwich 35a towards diphenyldichalogenides.

4.11.5.4

The actinocenes, [An(COT)2] and their derivatives

After the introduction and the establishment of the Cp ligand the field of actinide research was extended to the COT ligand. At the end of the 1960s and the beginning of the 1970s the actinocene complexes have been described with the two COT ligands in the coordination sphere of the central ion.103–107At the beginning of the 1970s the solid state structures were determined for the Protactinocene on the base of powder data and for Thorocene and Uranocene on single crystal data.108,109 At the end of the 1990s the single crystal structure for Neptunocene was published and in 2020 the Plutocene solid state structure was finally confirmed on single crystal data.32,34,39 Neutral Actinocene complexes, [An(COT)2], exist as well as ring-substituted derivatives such as [An(EtCOT)2], [An(nBuCOT)2], and [An(Me4COT)2].110–112 Accordingly, structural data are available for some alkyl-substituted thoro- and uranocenes.113–119 Like for the ansa-cyclopentadienes, with silyl groups ring substituted COT ligands have been introduced in the actinocene chemistry forming then ansa-actinocenes.53 In these complexes a silyl moiety bridges the two COT rings, which compares well to a series of actinocene complexes with non-bridging silyl substituents.120–126 However for Pu there are actually reported only three single crystal structures and for Np there is the Neptunocene [Np(COT)2]. The comparable actinocene complexes with their typical structural features are listed in Table 1. Anionic Actinocene complexes [An(COT)2]− are reported as long as their neutral analogs. The structures of K[An(COT)2] (An ¼ Np, Pu) were confirmed via powder diffraction and spectroscopic methods106,107; the single crystal structure of the anionic K[Pu(COT)2] was determined in 2020 together with its neutral analog.39 Further anionic Actinocene complexes [An(COT)2]− are accessible via reduction of their the AnIV analogs. Typically, Actinocene complexes stabilized via electron rich silyl-substituted COT derivatives are reduced more readily.

4.11.5.4.1

Structural features of the actinocenes

The series of available complexes enables a comparison of their structural features in order to look for trends or common features and also for unconventional or exceptional behavior. All actinocene derivatives show common structural features with the two COT ligands in p-Z8-coordination to the actinide atom forming close to linear arrangement (Fig. 11 and Table 1).

596

Larger Aromatic Complexes of the Actinides

Table 1

Selected distances (A˚ ) and angles in the actinocenes. M–Ct (pm)a

angle Ct–M–Ct

References

200.3 199.8 198.6/202.1 198.7/199.9 201.2/201.0

180 173.2 173.9 172.9 174.3/175.3

109 126 122

192.3 192.5 191.7/191.8 191.9 191.3/192.0 191.2/191.9 193.8 191.3/192.1 194.2/194.4 193.8/194.5 192.2/192.3

180 180 178.8 180 178.9 178.1 168.6 173.0 174.3 174.9 173.5

109 113 115 114 117

189.8/190.8

180

32,34

189.8 190.5/189.1

180 176.7

39 120

206.6/209.5

174.5

126

199.9/(204.5) 202.6/202.7 199.8/(204.2)

176.1 175.5 172.4

119 121 126

196.5

180

39

IV

An -complexes Thorium [Th(COT)2] [Th{COT-1,4-(SiMet2Bu)2}2] [Th{COT-1,4-(SiMe3)2}2] [Th{COT-1,3,6-(SiMe3)3}2] Uranium [U(COT)2] [U(benzoCOT)2] [U(cyclopentenoCOT)2] [U(cyclobutenoCOT)2] [U(1,3,5,7-tetrametyl-COT)2] [U{COT(SiPh3)2}2] [U{COT-1,4-(SiMe3)2}2] [U{COT-1,3,6-(SiMe3)3}2] [U{COT-1,4- (SiMe3)2}2] Neptunium [Np(COT)2] Pluotonium [Pu(COT)2] [Pu{COT-1,3-(SiMe3)2}{COT-1,4-(SiMe3)2}] AnIII-complexes Thorium K[Th{COT-1,4-(SiMet2Bu)2}2] Uranium K[U(MeCOT)2] Li[U{COT-1,4-(SiMe3)2}2] K[U{COT-1,4-(SiMet2Bu)2}2] Plutonium K[Pu(COT)2]

122

123 122 122 121

a

Ct: Idealized position of center of COT ring. Some entries exist in more than one modification, therefore the number of entries may differ.

U1

Pu1

Fig. 11 View to the molecular structure of Uranocene and 1,3- and 1,4-bis-TMS-substituted plutonocene showing the typical p-Z8-coordination of the COT ligands in the actinocene.

Larger Aromatic Complexes of the Actinides

597

The distances between the COT ligands and the actinide metal center are easiest compared in Table 1 by the distance between the actinide ion and the geometrical center of the COT ring (Ct). Some trends can be found: the distances Th-Ct (Table 1) are with ca 200 pm about 7 pm longer than those for the uranocenes with a mean value of ca 192.5 pm. The corresponding An-Ct distance for the Neptunocene is at ca 190.3 pm again shorter. However, the distances Pu-Ct for the two structurally characterized plutonocene derivatives are at 189.8 pm quite close to the Neptunium value. If this approaching of the COT rings towards the An central ion is an effect of the actinide contraction then it is not clear why the effect is weaker going from Np to Pu than going from Th to Np; from the latter the contraction would be estimated to ca 3.3 pm per element. Easier to see are the differences between the neutral actinocene complexes [An(COT)2] and their negatively charged reduced AnIII analogs: An-Ct distances elongate by ca 7 pm due to reduction of the central AnIV to AnIII in their corresponding actinocene complexes. This can attributed directly to an increase of the ionic radii due to reduction. However, a better understanding of the bonding in actinide organometallic complexes and the covalency therein is highly desirable. This can be achieved by combining a profound computational modeling with structural findings arising from experimental data like in.41,42 Recent progress on this was highlighted by Kaltsoyannis in 2018.22 What emerges from the above data is that covalency plays an important role in actinide organometallic complexes especially for those stabilized with the p-donor ligands.

4.11.5.4.2

Adduct formation at the actinocene

In actinocene complexes the actinide ions are well shielded. Nonetheless Lewis base adducts are accessible for the larger actinide ions such as Th (or U) (Table 2).51,52,127–129 It is easier to form an adduct when the additional ligand entering the actinide coordination sphere is small and/or cylindrical, so linear molecules such as cyanide, or azide are preferred adduct partners but planar N-donor molecules such as pyridine (derivatives) or phenanthroline (derivatives in N,N-coordination) can also find access to in this case the Th center in Thorocene. For example cyanide (CN−) adducts are formed as the mono-adduct and the di-adduct; but also a dimer with a bridging cyanido ligand between two [Th(COT)2]-moieties is reported. This shows that the cyanide ligand has two ends with donor capabilities, indeed from the dimer then the formation of a trimeric complex or a polymeric structure is possible. Of course the coordination of the Lewis base (charged or not) causes a deformation in the structure of the former linear actinocene (Fig. 12 and Table 2). The two COT rings move towards each other liberating space for the new ligand in the metal coordination sphere. In the mono-adducts the angle between the two centers of the COT rings over the Th central atom (angle Ct-M-Ct, Table 2) decreases to about 150 ; the metal to center of ring distance (M-Ct in Table 2) are compared to the corresponding actinocenes elongated by 8–10 pm due to the entering of new electrons in the system. In the bis-adducts the angle Ct-M-Ct decreases further to about 140 whereas the M-Ct distance increases by another 8–9 pm. Going from Th to U the M-Ct distance decreases by ca 6 pm, which is an effect of the actinide contraction. Reduction from ThIV to ThIII causes an increase of this distance of ca 6 pm which can be attributed to the larger ionic radius of the reduced metal ion. Table 2

Selected distances (A˚ ) and angles in the actinocene adducts.

Thorium NEt4[(COT)2Th(CN)Th(COT)2] Na[(COT)2Th(CN)Th(COT)2] [(COT)2Th(Pyridine)] [(COT)2Th(4,40 -bipyridine)] [(COT)2Th(CNtBu)] [(COT)2Th(CN)]− [(COT)2Th(N3)]− Na[(COT)2Th-H-Th(COT)2] K[(COT)2Th-H-Th(COT)2] [(COT)2Th(CN) [Th(COT)2](CN)Th(COT)2]2− (values refer to Th) [(COT)2Th(CN)2]2− [[(COT)2Th(CN)]-]n [(COT)2Th(3,4,7,8-tetramethyl-phenanthroline)] [(COT)2Th(bipy)] ThoriumIII-complex [(COT)2Th(bipy)]− Uranium [(COT)2U(CN)]− a

M–Ct (pm)a

angle Ct–M–Ct

References

208.9/209.4/208.8/208.2 208.8/209.7 205.1/208.1 206.3/207.1 207.3/208.3 209.7/209.4 209.6/209.9 209.1/210.3 210.3/210.4/210.7/212.1 208.6/209.2/208.1/208.7 216.8/216.7 219.8/217.7 217.0/219.0 214.6/214.7/215.1/215.8 214.5/214.1/213.7/214.8

151.9, 149.9 151.3 153.9 153.1 154.3 150.2 149.2 147.6 148.7/148.9 151.7, 152.1 142.2 139.0 140.1 144.2/144.2 142.9/143.7

52 52 127 127 127 128 128 128 128 52

220.0

140.0

129

203.2/203.3

153.3

51

52 52 127 129

Ct: Idealized position of center of COT ring. Some entries exist in more than one modification, therefore the number of entries may differ. For reasons of clarity the counter ion is only mentioned when there are more than one complex of the same constitution.

598

Larger Aromatic Complexes of the Actinides

Linear

distorted trigonal planar

distorted tetrahedral

Fig. 12 Adduct formation to Thorocene (left), deformation of the complex geometry.

Fig. 13 COT2−, pentalene2−, and the p-Z8-coordination of the pentalene towards an actinide center leading to a bending of the planar and aromatic pentalene dianion.

For the other actinides such as Np or Pu no adducts of their actinocene complexes have been described yet, which is probably caused by the decreasing size of the actinides with increasing Z and therefore the increasing steric demands in the system. However, this may also simply reflect the limited investigations of these systems due to their radioactive natures.

4.11.6

Pentalene complexes

Pentalene can be regarded as a COT-type ligand with a 1,5-transannular bond. Its dianion (C8H62−) could then act as a 10 electron donor like the (COT)2− and stabilize actinide complexes in an p-Z8-coordination (Fig. 13). Neutral pentalene is unstable at room temperature but the introduction of bulky substituents inhibits Diels-Alder dimerization, for example 1,3,5-tri-tbutyl pentalene is stable at room temperature.130 The pentalene coordination chemistry developed after the discovery of an easy access to its dihydroderivatives via thermolysis of COT and stabilization in the form of its dianion.131–133 Subsequent alkylation and deprotonation steps enable access to the desired derivatives, such as [1,4-(iPr3Si)2-pentalene]K2 (Scheme 16).133

Scheme 16 Access to 1,4-disubstituted pentalene derivatives on the example of [1,4-(iPr3Si)2-pentalene]K2, 39.133

The coordination chemistry of the pentalene ligand has been reviewed by Cloke.20 About 60 structurally characterized pentalene transition metal complexes compare to eight Thorium or Uranium complexes for the actinides always containing the pentalene ligand (C8H62−) in p-Z8-coordination towards the actinide central atom.66,86,125,134–136 However, even though little work has been done in this field, a chemistry comparable to the mixed sandwich actinide chemistry involving a COT and a Cp substituent was developed. A suitable starting complex for studying for example small molecule activation is [{1,4-(iPr3Si)2-pentalene}U(Cp )], 40, (Scheme 17, Fig. 14 left), which is obtained in analogy to its COT derivative by the reaction of uranium triiodide with KCp and then [1,4-(iPr3Si)2-pentalene]K2, 39.86

Larger Aromatic Complexes of the Actinides

599

Scheme 17 N2 activation in a uranium pentalene complex, 41.86

Si1

Si3 Si2

Si4 Si1

N2

U1

U2 U1

N1 Si2

Fig. 14 Left: View to the molecular structure of [[1,4-(iPr3Si)2-pentalene]U(Cp )], 40. Right: View to the molecular structure of the N2 activated product 41 (right: i Pr-substituents at the Si-atoms omitted for reasons of clarity).

Complex 40 reacts at ambient pressure with dinitrogen (Scheme 17) under formation of a dinuclear complex, 41, with the dinitrogen molecule embedded as diazene bridging between the two [{1,4-(iPr3Si)2-pentalene}U(Cp )] moieties (Scheme 17 and Fig. 14). The X-ray analysis of 41 confirms the N2 reduction and the formation of a U2N2-moiety with the diazenide N2-unit in side-on bridging mode with a N]N distance of 123. 3 pm (Fig. 14, right).86 The structural features of the pentalene while being in p-Z8-coordination are different compared to the planar COT ligand in comparable coordination. The pentalene is bent (about 25 ) in the actinide p-Z8-coordination, which is an effect that the two annealed Cp rings of the pentalene which behave somehow like two separated p-donor systems (Fig. 14). The pentalene complex 40 is not only able to activate N2 but it react as well under formation of the dinuclear complex 42 with the tBu-substituted phosphaacetylene (Scheme 18). 40 was in this case synthesized in a more reproducible manner via exchange of one Cp ligand in [U(Cp )2](BPh4).

Scheme 18 Reaction of 40 with tBuC^P under formation of the dinuclear bridged complex 42.136

In 42 the former tBuC^P is reduced by a 2 electron reduction establishing a m-Z1:Z2-coordination with UdP bond distances of 295.6(4) pm (both), a U–C interaction of 249(1) pm and a PdC bond distance of 168(1) pm in the range of a double bond. This type of coordination is unique in the phosphaalkyne chemistry (Scheme 18).136

600

Larger Aromatic Complexes of the Actinides

The few exciting, extraordinary results based on pentalene Th and U complex chemistry represent an excellent example of what is possible in the actinide chemistry once a new ligand system is established. However, like very often in the actinide chemistry, the transfer to trans-uranium elements is still missing.

4.11.7

An complexes with donors containing 9C-atoms in a planar environment

There is no report on a complex arising from a cyclononatetraenide anion (C9H−9) which would be a Hückel-aromatic 10 p-electron system. Similar to the case for pentalene and COT where the pentalene can be regarded as a COT with a 1,5-transannular bond the indene can be seen as cyclononatetraenene with as well a 1,5-transannular bond. Accordingly, from indene (C9H8, A in Fig. 15) the negatively charged indenyl-ligand (C9H7−, Ind, B in Fig. 15) is obtained by deprotonation, of which quite a series of actinide complexes are reported which will be presented here. The indenyl ligand (C9H7−, Ind, B in Fig. 15) consists of two annealed rings, one six membered benzene ring and one five membered negatively charged cyclopentadienyl ring. The coordination capabilities of a Cp− are higher than the ones of a neutral benzene. Correspondingly, the indenyl actinide chemistry is characterized by the coordination properties of the five atoms in the Cp-unit of the indenyl (C in Fig. 15). Actinide indenyl chemistry started to develop about 50 years ago, with the structure of [(Ind)3UCl] published in 1971137; up to present 30 Thorium and Uranium complexes having an indenyl ligand in the coordination sphere have been structurally characterized of which they practically all have been published before 2000 or in 2001.27 Indenyl actinide complexes are only described for Thorium and Uranium; in the series of the complexes [An(Ind)4] only the Thorium derivative exists. In this complex not the fourfold p-Z5-coordination of the Cp ring to the indenyl unit but a fourfold p-Z3-coordination to the three C-atoms of the indenylide ligand which are located in most distance to the benzene ring. One could describe this coordination mode as coordination of an allylic-anion to the Th-center (motif D in Fig. 15). Uranium indenyl complexes are described with a maximum of three indenyl-ligands in the coordination sphere of the metal. The solid state structures of the complexes [U(Ind)3X] (X: Cl (43a), Br (43b), I(44)) have been determined by single crystal X-ray diffraction137–139 proving the chloride (43a) and the bromide (43b) form the same structure. Looking in more detail in the solid state structures (Fig. 16) one can see, that the indenyl rings for the chloride and the bromide show the same arrangement around the central U-atom: they are oriented in that way that the phenyl rings are forming a pocket around the halogenide 43. In case of the iodide 44, the phenyl rings are oriented like a helix pointing always in the direction of another Cp-ring from a neighboring indene (Fig. 16). This shows that the steric demands of the indenyl ligands do not completely govern the structural motifs formed. This structural change is accompanied by change in the coordination mode from the symmetric p-Z5-coordination of the Cp ring for the chloride (43a) and the bromide (43b) with comparable UdC bond lengths to a more unsymmetrical p-Z3-allylic like coordination for the iodide 44 with differences in the UdC bond lengths of up to 20 pm.

Fig. 15 Indene (A), its anion (B), Cp-like p-Z5-coordination of the indenyl ligand to an [An]-complex fragment (C), and allylic like p-Z3-coordination of the indenyl ligand to an [An]-complex fragment (D).

U1 U1

Cl1 I1

Fig. 16 Arrangement of the indenyl-ligands around the U-atom in the complexes 43 [U(Ind)3X] (X: Cl, Br, left) and around [U(Ind)3I], 44, on the right.

Larger Aromatic Complexes of the Actinides

601

All this is a sign that the indenyl ligand in actinide chemistry does not coordinate so strongly to the metal centers as at least one electron pair always competes to make part of the aromatic benzene ring. It is probably one reason why Np or Pu indenyl complex chemistry has not been developed. Nevertheless, recent results show that with the sterically encumbered indenyl ligands (IndMe6, IndMe7) in the coordination sphere of a uranium atom small molecule activation proceeds: CO can be activated and stable complexes can be obtained. The complexes [{1,4-(SiiPr3)2-COT}U(IndMe6R)] (45, R ¼ H in the 1-position of the Cp ring, 46, R ¼ Me) are accessible from the reaction of UI3 with the corresponding potassium salt of the substituted indenyl ligand (Scheme 19). In both of complexes 45 and 46 the indenyl coordination can be described as p-Z5-like with comparable UdC bond distances. Complex 45 activates CO and forms in the CO reaction the squarate complex 47 accordingly to Section 4.11.5.2.3 (Scheme 19, and motif C in Fig. 9 in Section 4.11.5.2.3).89 The squarate unit shows the already described squarate complexes features with comparable U-O distances (247.7(5) and 247.1(6) pm) and marginally longer O-C distances of 127.2(9) and 126.9(10) and C-C distances of 146.0(10) and 149.9(10) pm.

Scheme 19 Reactivity against CO of the mixed sandwich with sterically demanding indenyl ligands stabilized uranium complexes.

Complex 46 reacts under comparable conditions under simple CO coordination to complex [{1,4-(SiiPr3)2-COT}U(CO) (IndMe7)], 48, with a coordinated CO molecule in the metal sphere showing a CO stretching vibration at 1905 cm−1. Complex 48 exhibits a rare example for an at room temperature stable uranium mono carbon monoxide complex, which contrasts the stability of [{1,4-(SiiPr3)2-COT}U(CO)(Cp )], with a half-life time of ca 15 min and a CO absorption at 1920 cm−1. Upon elongated reaction 48 reacts further with CO under formation of the already described ethynediolate type complex 49, which could be identified by its unique 13C NMR carbon shift at 395 ppm.89 45, 46 can as well activate CO2 under formation of the in Section 4.11.5.2.2 described binuclear carbonate complexes (motif A in Fig. 7, Section 4.11.5.2.2). These examples show clearly that from these sterically demanding and due to their substitution patterns as well electron rich ligands fascinating complexes arise which enlarge of our knowledge on actinide organometallic complexes.

4.11.8

An complexes with donors containing 10C-atoms in planar environment

The ligand fulfilling these conditions is the naphthalene of which of few actinide complexes are reported which will be summarized here. Cummins reported in 2002 an inverse COT-Uranium sandwich73 (see “Bridging COT-ligands in actinide complexes,” Section 4.11.5.3); in the same paper the authors describe the formation of the inverse sandwich complex dianion [{(Mes)tBuC¼N}3 U-m-Z6:Z6-(naphthalene)-U{N¼C(Mes)tBu}3]2−, 50, with a bridging naphthalene ligand as well (Fig. 17). In this complex one of the benzene rings of the naphthalene ligand is m-Z6:Z6-coordinated to the two [U]-complex fragments with U-C distances covering a range between 256(1) and 275(1) pm, the longer ones found for the U-C interactions to the central C-atoms of the naphthalene unit.

602

Larger Aromatic Complexes of the Actinides

U1

U1A

B1

U2

U1

Fig. 17 Left: View to the core structure of the molecular complex dianion [{(Mes)tBuC¼N}3U-m-Z6:Z6-(naphthalene)-U{N¼C(Mes)tBu}3]2−, 50, C-atoms of the ketimido ligands, counter ions and solvents omitted for reasons of clarity. Right: View to the core of the molecular structure of [UII(di-2,6-tBu-phenoxide)2]-mZ6:Z6-(R-naphthalene)-[UII(di-2,6-tBu-phenoxide)2], 51, (R: 9-borabicyclo-(3.3.1)-nonane), C-atoms of the phenoxide ligands omitted, substituents at the boron reduced to the first C-atom.

Arnold reported on a series of Uranium complexes exhibiting U-arene p-interactions140; among these complexes there is the inverse sandwich complex [UII(di-2,6-tBu-phenoxide)2]-m-Z6:Z6-(R-naphthalene)-[UII(di-2,6-tBu-phenoxide)2], 51, with comparable structural features: a naphthalene ligand embedded between two [UII]-complex fragments in a m-Z6:Z6-coordination mode (Fig. 17). The UdC bond distances in this complex cover from 251.9 to 276.9 pm again a relative broad range showing that as well in this example the interaction of the UII-centers to the phenyl ring of the naphthalene is not symmetrical, indicating as well a certain flexibility in this m-Z6:Z6- coordination. Gambarotta reported in three manuscripts on the Th-coordination to arenes141,142 of which two include a Th-naphthalene p-interaction. 52 is stabilized by a bis-phenoxide ligand (Scheme 20) under strongly reducing conditions and is so electron rich that at RT it activates and reduces N2 under loss of the coordinated naphthalene ligands.

Scheme 20 Synthesis of 52 stabilized with the chelating bisphenol ligand L.141

Looking at the Th center in 52, then it is coordinated to the bis-phenoxide ligand L, additional saturation proceeds via p-coordination to the two naphthalene rings (Fig. 18, left). One can consider 52 as a bis-phenoxide stabilized ThII-complex then coordinated to two [K(naphthalenide)]-moieties on the other side with the naphthalene negatively charged. Assuming a more internally neutral structure, then we could imagine of having a Th0 complex coordinated to as well neutral naphthalene ligands instead of having a ThII in direct neighborhood of two negative naphthalenides. This are the two opposite descriptions for the same complex 52. The structure analysis shows for the non-coordinated phenyl ring a mean C-C distance of 138(3) pm and is with this— simplified—not affected by the coordination. It is difficult to deduce something from the Th-C distances for the coordinated C4: they cover a range between 267 and 278 pm. However, the CdC bond distances for the coordinated C4 chain differ, the bond lengths between the outer C atoms are in average 144.4(9) pm whereas this value between the inner C atoms of this C4 chain is with 134.3(9) pm about 10 pm significantly shorter. This is a sign that between these two C-atoms there is a double bond that is coordinated to the Th atom. These observations compare to two other strongly reduced Th-naphthalene complexes both stabilized with calix[4]pyrrolide ligands showing the same coordination mode (complex anion 53, in Fig. 18, right). They can be described like [(Et8-calix[4] tetrapyrrole)Th(Z4-naphthalene)]2−, 53, complexes and differ only in their counter cations. For both of them the overall structural findings are comparable: The non-coordinated phenyl ring is not affected by coordination. However for the Th-C distances there is a clear trend: the Th-C distances to the C-atom no 1 and 4 of the coordinated C4 chain

Larger Aromatic Complexes of the Actinides

603

O1 O2 Th1

Th1

Fig. 18 Left: View to the core structure of the complex 52 with a formal [ThII]-complex fragment interacting with 2 [K(naphthalenide)]-units. C-atoms of the bis-phenoxide ligand and crown ether molecules omitted for clarity (Colors: Th and O: red, N: blue, C: gray). Right: view to the core of the molecular structure of [(Et8-calix[4]tetrapyrrole)Th][K(dme)](Z4-naphthalene)][Li(dme)3], 53, Ethyl-groups, [K(dme)]-unit, [Li(dme)3]-ion omitted for reasons of clarity.142

Fig. 19 Contribution to the bonding in the Th-naphthalene complexes.

are with in average 268.6(10) pm shorter than the Th-C distances to the inner C-atoms of the C4 chain, which are mean 276.7(10) pm. This is accompanied by a parallel change in the CdC bond distances for the coordinated C4 chain: like for 52, the bond lengths between the outer C atoms are with 144.8(5) pm significantly longer than CdC bond lengths between the inner C atoms of the C4 chain with 137.6(1) pm. From all these values of these three low valent Th-naphthalene complexes one can describe the bonding situation in these complexes having a contribution like depicted in Fig. 19 with a stronger interaction between the Th and the C1 and C4 of the coordinated C4 chain accompanied with p-coordination to the resulting double bond between the inner C-atoms. However, a deeper analysis supported by theoretical modeling would certainly contribute to the understanding here. Comparing all the actinide naphthalene complexes one can see one major common feature: whereas the Uranium complexes shown before exhibit distorted p-Z6-coordination mode to the naphthalene unit, the Thorium naphthalene complex examples here form a p-Z4-coordination which is clearly supported by a significant bending of the coordinated naphthalene (Fig. 18).

4.11.9

Conclusion

In this article an overview was given on actinide organometallic complexes with planar carbocyclic ligands having more than 6C-atoms in at least one of the rings. The cycloheptatrienyl chemistry of the actinides is poorly developed; and actinide coordination chemistry of the cyclononatetraene or something like a cyclodecapentaene does not exist. This is compensated by a large variety in the actinide cyclooctatetraenyl coordination chemistry, which is going from half-sandwich over mixed sandwich complexes to the actinocenes. Taking benefit from the organometallic chemistry of the actinides it was possible to stabilize molecules such as the deltate or the squarate in the coordination sphere of a metal and enlarge our knowledge on small molecule activation. Triple decker or inverse sandwich complexes form outstanding examples of sophisticated organometallic actinide complexes establishing as well metal-metal interactions over the ligands p-orbitals. Looking on the rich chemistry already established it seems highly desirable to extend this chemistry to the under-represented elements such as Neptunium and Plutonium from which one could expect exciting scientific results.

604

Larger Aromatic Complexes of the Actinides

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1

4.12

Larger Aromatic Complexes of the Group 4 Metals

Philip Mountford, Chemistry Research Laboratory, University of Oxford, Oxford, United Kingdom © 2022 Elsevier Ltd. All rights reserved.

4.12.1 4.12.2 4.12.2.1 4.12.2.1.1 4.12.2.1.2 4.12.2.2 4.12.2.2.1 4.12.2.2.2 4.12.2.2.3 4.12.2.2.4 4.12.3 4.12.3.1 4.12.3.2 4.12.3.2.1 4.12.3.2.2 4.12.4 4.12.4.1 4.12.4.2 4.12.4.2.1 4.12.4.2.2 4.12.4.2.3 4.12.4.3 4.12.4.4 4.12.4.4.1 4.12.4.4.2 4.12.5 4.12.5.1 4.12.5.2 4.12.5.2.1 4.12.5.2.2 4.12.6 References

Introduction and scope Cycloheptatrienyl complexes Half-sandwich complexes Complexes with dienyl ligands Complexes with Z1-bound anionic heteroatom donor ligands Troticene, trozircene and trohafcene complexes Complexes without additional Lewis base coordination at the C5 or C7 ring Heterotrozircenes Synthesis and reactivity of troticenophanes Complexes with Lewis base functional groups attached to the C5 or C7 ring Cyclooctatetraene complexes Complexes in oxidation state +3 Complexes in oxidation state +4 Half-sandwich complexes Sandwich complexes Pentalene complexes Ligand developments Complexes with a pentalene ligand Z8-coordinated to one metal Half-sandwich titanium compounds Half-sandwich zirconium and hafnium compounds Homoleptic sandwich and mixed-sandwich compounds Complexes with a hydropentalene ligand Z5-coordinated to one metal The chemistry of [Ti2(m-Z5,Z5-PnTiPS)2] Synthesis and single-bond activation reactions Reactivity with unsaturated substrates Nine-membered ring systems Cyclononatetraenyl complexes Zirconium indenyl complexes with Z9-coordination Synthesis and structure Reactivity Concluding remarks

608 608 608 609 610 612 612 615 618 621 624 625 626 626 630 631 631 632 632 634 636 638 640 640 641 644 644 645 645 647 648 648

Nomenclature CHT CHTLi CHTR CNT COD COT COTLi COTR COTTiPS COTTMS Cp Cp Cp0 CpLi CpR

C7H7 Z7-C7H6Li General cycloheptatrienyl, C7R7 (R ¼ H or other) C9H9 1,4-Cyclooctadiene C8H8 Z8-C8H7Li General cyclooctatetraene, C8R8 (R ¼ H or other) 1,4-C8H6(SiiPr3)2 1,4-C8H6(SiMe3)2 C5H5 C5Me5 C5H4Me Z5-C5H4Li General cyclopentadienyl C5R5 (R ¼ H or other)

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00039-1

607

608

Larger Aromatic Complexes of the Group 4 Metals

DFT Dipp EHMO Flu HNImDipp HNImMe HPn IMe Ind IndR MAO Pdyl Pn Pn PnMe PnR PnSnMe3 PnTiPS PnTMS PnTMS4 PyrR Xyl

4.12.1

Density functional theory 2,6-C6Hi3Pr2 Extended-Hückel molecular orbital Fluorenyl, C13H9 N,N0 -bis(2,6-diisopropylphenyl)imidazolin-2-imine N,N0 -bis(2,6-dimethylphenyl)imidazolin-2-imine 1-C8Me6H 1,3,4,5-Tetramethylimidazolin-2-ylidene C9H7 General indenyl, C9R7 (R ¼ H or other) Methylaluminoxanes General pentadienyl ligand excluding CpR C8H6 C8Me6 2-C8H5Me General pentalene, C8R6 (R ¼ H or other) 1,4-C8Me6(SnMe3)2 1,4-C8H4(SiiPr3)2 1,4-C8H4(SiMe3)2 1,10 ,4,40 -C8H4(SiMe3)4 General pyrrolyl ligand 2,6-C6H3Me2

Introduction and scope

This chapter covers the literature from 2000 to the end of 2020 in the following areas of group 4 organometallic chemistry: cycloheptatrienyl compounds; cyclooctatetraene compounds; pentalene compounds; indenyl compounds where all nine carbons are bound to the metal; and attempts to prepare cyclononatetraenyl compounds. Although the chemistry of group 4 cycloheptatrienyl and cyclooctatetraene ligand-supported compounds had been well developed prior to 2000, a number of new advances have been made, especially with respect to the seven-membered ring systems. Pentalene ligand-supported group 4 compounds were virtually unknown prior to 2000, as was the chemistry of Z9-coordinated indenyl complexes of these metals. Each major section of this chapter—C7, C8 and C9 ring systems—gives an introduction to that field, with reference to previous landmark papers and a flavor of the state of play at around the start of 2000. In general terms, the organometallic chemistry of group 4 metals with these larger aromatic ring-based ligands is dominated by the +4 oxidation state, as might be expected from the position of these elements in periodic table, with relatively few compounds in the +3 oxidation state.

4.12.2

Cycloheptatrienyl complexes

Prior to the period covered by this chapter, the excellent review by Green and Ng gives a comprehensive account of the literature up to 1995.1 The relevant chapters from COMC give coverage up to 2005.2–5 Within the time period of this chapter two reviews of aspects of cycloheptatrienyl chemistry pertinent to group 4 appeared.6,7 Unless stated otherwise, CHT refers to heptahapto coordination (Z7-C7H7).

4.12.2.1

Half-sandwich complexes

An extensive chemistry of half-sandwich complexes [M(CHT)(L)n] (M ¼ group 4 metal; (L)n ¼ supporting ligand (set) excluding CpR and its hetero-analogues) had already been developed prior to the period covered by this chapter. This is covered in various reviews1,6,7 and COMC III.2–5 Important historical highlights are given here for context. The Z7-CHT–Z5-cycloheptadienyl compounds [M(CHT)(Z5-C7H9)] (M ¼ Ti (1), Zr (2), Hf (3)) were prepared by various routes including: metal vapor synthesis8,9; reaction of [Ti(CHT)(m-Cl)(THF)]2 with AlCl2Et and C7H810; and reduction of TiCl311 or ZrCl412 in the presence of an excess of cycloheptatriene, C7H8. [Ti(CHT)(m-Cl)(THF)]210 was found to be a useful entry point to further half-sandwich CHT chemistry for titanium. Monomeric homologues supported by PMe3, DMPE (1,2-bis(dimethylphosphino)ethane) or TMEDA (1,2-bis(dimethylamino)ethane)13 are also known. The zirconium congeners [Zr(CHT)(m-Cl)(THF)]2 and [Zr(CHT)X(L)2] (X ¼ Cl, I; L ¼ PMe3, THF; (L)2 ¼ DME (1,2-dimethoxyethane), DMPE, TMEDA) had likewise been reported,13–15 but none of these had yet been exploited synthetically.

Larger Aromatic Complexes of the Group 4 Metals

4.12.2.1.1

609

Complexes with dienyl ligands

In 2003 Ernst16 reported an alternative route (28% yield) to [Ti(CHT)(Z5-C7H9)] (1) by reaction of KC7H9 (2.1 equiv.) with in situ generated “TiCl2” in THF at −78  C (Eq. 1) in which one C7H9 moiety has been transformed to CHT. No mechanism was proposed, but the stabilization associated with forming an aromatic Ti-bound CHT ligand was proposed to provide a driving force in this reaction. The corresponding reaction starting from [VCl3(THF)3] gave the vanadium analogue of 1, whereas with [CrCl3(THF)3] the cycloheptadiene compound, [Cr(CHT)(Z4-C7H10)], was formed.

ð1Þ

Tamm and co-workers found that reaction of Green’s13 half-sandwich compound [Zr(CHT)Cl(TMEDA)] (4) with KC3H3(TMS)2 (TMS ¼ SiMe3 here and throughout this chapter) formed the CHT-allyl complex [Zr(CHT){Z3-C3H3(TMS)2} (THF)] (5a) in good yield (Scheme 1). The corresponding reaction of 4 with C3H5MgBr gave no identifiable product, indicating the importance of the sterically demanding TMS groups in 5a. Compound 5a undergoes a protonolysis reaction with HNImDipp (N,N0 -bis(2,6-diisopropylphenyl)imidazolin-2-imine) as described later (Scheme 4). The crystal structure of 5a showed a syn-syn arrangement of the allyl ligand TMS groups which minimizes steric interactions with the THF ligand.

Scheme 1

The THF ligand in 5a can be replaced by a range of Lewis bases, L, forming the corresponding adducts [Zr(CHT) {Z3-C3H3(TMS)2}(L)] (5b–e; L ¼ PMe3 (b), IMe (1,3,4,5-tetramethylimidazolin-2-ylidene, c), DMAP (4-(dimethylamino) pyridine, d), H2C(IMe) (e)) (Scheme 1).17 DFT calculations on 5b and 5c found that the latter (L ¼ IMe) had a significantly stronger ZrdL bond than the PMe3 adduct 5b, consistent with complexes 5a–e possessing formally Zr(+4) centers. Comparing DFT computed energies for the ZrdL interactions in the 16 valence electron 5b and 5c with those18 for their 18 valence electron trozircene counterparts [Zr(CHT)(Cp)(L)] (described in a later section) found that those in the Z3-allyl complexes were at least 10 kcal mol−1 stronger. Ernst and Tamm found that [Zr(CHT)Cl(TMEDA)] (4) also provides an entry point to a series of CHT- pentadienyl (Pdyl) complexes [Zr(CHT)(Pdyl)] (6a–d; Pdyl ¼ Z5-C5H7 (a), Z5-2,4-C5H5Me2 (b), Z5-1-C6H5Me2 (c), Z5-C7H9 (d)) by reaction with the corresponding KPdyl reagents (Scheme 2).19 The compounds 6a–d may be classified as “half-open trozircenes”, the parent complex

Scheme 2

610

Larger Aromatic Complexes of the Group 4 Metals

in that family being [Zr(CHT)(Cp)] (16, described later). [Zr(CHT)(Z5-C5H7)] (6a) undergoes slow thermal conversion to 16 at 125  C. X-ray structural data for 6a–c, along with DFT calculations, are consistent with 6a–d containing Zr(+4) centers, as was found for the CHT-allyl complexes described above,17 and in agreement with earlier EHMO (extended-Hückel molecular orbital) and PES (photoelectron spectroscopy) studies of other group 4 CHT complexes.10,12 Compounds 6b and 6c form Lewis base adducts of the type [Zr(CHT)(Pdyl)(CNXyl)] with XylNC (Xyl ¼ 2,6-C6H3Me2). A wider range of adducts was also subsequently reported20 for 6b. Analysis of the IR spectra for [Zr(CHT)(Pdyl)(CNXyl)] (Pdyl ¼ Z5-2,4-C5H5Me2 or Z5-1-C6H5Me2) suggested that the Pdyl ligands are better net electron-donors than Cp. Walter et al. have also used [Zr(CHT)Cl(TMEDA)] (4) as part of a wider study of transition metal complexes containing enantiomerically pure dimethylnopadienyl ligands Me2npdylR (R ¼ Me, Ph, H) derived from (1R)-(−)-myrtenal, which is readily available from the chiral pool.21,22 Reaction of the respective potassiated pentadienides afforded the complexes [Zr(CHT) (Z5-Me2npdylR)] (7a–c, Scheme 2). Analogues of 7b–c featuring an additional dSiMe2NR2 (R ¼ Me, Et) substituent at the terminal (]CH2) position of the dimethylnopadienyl ligand were similarly prepared, but could only be isolated as their PMe3 adducts.23 In these complexes the pendant silylamine group is not coordinated. Structural and spectroscopic studies showed that the Zr(CHT) moiety coordinates to the least sterically hindered face of Me2npdylR. Cone angle measurements based on the solid state structures for [Zr(CHT)(Z5-2,4-C5H5Me2)] (6b) and [Zr(CHT) (Z5-Me2npdylMe)] (7a) found that Me2npdylMe is significantly larger than 2,4-C5H5Me2 as judged by this method.21 As was the case for 6b–c, the chiral analogues 7a–c also form 18 valence electron adducts with Lewis bases (CNtBu, PMe3). However, whereas [Zr(CHT)(Z5-C5H7)] (6a) undergoes slow thermal conversion to [Zr(CHT)(Cp)] (16), no pentadienyl-to-cyclopentadienyl conversion occurs for 7a.

4.12.2.1.2

Complexes with 1-bound anionic heteroatom donor ligands

Tamm et al. showed that the chloride ligand in [Zr(CHT)Cl(TMEDA)] (4) can be replaced directly by N(TMS)2 or NImDipp by salt metathesis forming [Zr(CHT){N(TMS)2}(THF)] (8_THF; Scheme 3) or [Zr(CHT)(NImDipp)(TMEDA)] (9; Scheme 4), respectively.24,25 The TMEDA ligand in the imidazolin-2-imininato complex 9 can be replaced by CNtBu to form [Zr(CHT)(NImDipp) (CNtBu)] (13b). Compound 8_THF loses THF upon high vacuum sublimation to afford [Zr(CHT){N(TMS)2}]n (8) which forms an extended chain polymer in the solid state. The CHT ligands adopt a unique antifacial m-Z7,Z2 bridging mode between adjacent zirconium atoms.24 Compound 8 is insoluble in toluene, but dissolves in donor solvents. Reaction with PMe3 forms the adduct 8_PMe3 (Scheme 3) on the NMR tube scale. No reaction occurs with CO, consistent with the Zr(+4) formal oxidation state. Compound 8 undergoes a protonolysis reaction with Ph3SiOH forming dimeric [Zr(CHT)(m-OSiPh3)]2 (10), the m-siloxide ligands replacing the m-Z7,Z2 COT bridging mode in 8.

Scheme 3

Whereas the protonolysis reaction of [Zr(CHT)(CH2TMS)(TMEDA)] (prepared in situ from 4) with HNImDipp formed the TMEDA adduct 9, the corresponding reaction of the CHT-allyl homologue [Zr(CHT){Z3-C3H3(TMS)2}(THF)] (5a) at 45  C for ca. 1 day cleanly afforded the “pogo stick” compound [Zr(CHT)(NImDipp)] (12, Scheme 4) along with the expected alkene side-product C3H4(TMS)2.25 The proposed intermediate in this reaction, [Zr(CHT){Z3-C3H3(TMS)2}(HNImDipp)] (11) was also isolated and structurally characterized.

Larger Aromatic Complexes of the Group 4 Metals

611

Scheme 4

The one-legged piano stool (“pogo stick”) geometry for [Zr(CHT)(NImDipp)] (12) is very unusual in group 4 chemistry, and the only analogues are Mountford’s titanium imido complexes [Ti(COTR)(NR0 )]26 described in a later section. The monomeric nature of 12, which contrasts with that of the polymeric amido counterpart 8, was confirmed by X-ray crystallography. The ZrdNIm bond distance of 1.997(2) A˚ is among the shortest reported for this ligand for zirconium and is substantially longer than in 8_THF (2.1436 (12) A˚ ).24 The corresponding ZrdNIm distances in the adducts [Zr(CHT)(NImDipp)(CNR)] (13; R ¼ Xyl (a), tBu (b)), discussed below, were identical within experimental error to that in 12. The CHTcentdZrdNim angle in 12 is essentially linear (CHTcent ¼ CHT ring centroid), whereas the angle subtended at Nim is 152.2(3)o. DFT calculations found that the ZrdNimdC bond angle was relatively “soft”, with an energy difference of 2.1 kcal mol−1 between ZrdNimdC ¼ 180o and 144o. The calculations showed that the ZrdNim linkage in 12 should be considered as a Zr^Nim triple bond (s2p4 configuration), and that 12 is isolobal with the imido complexes [Ti(COT)R(NR0 )], all possessing 16 valence electron count metals in a formal +4 oxidation state. As indicated above, compound 12 reacts with bulky isocyanides to form [Zr(CHT)(NImDipp)(CNR)] (13; R ¼ Xyl (a), tBu (b)) (Scheme 5); 13b is also obtained from [Zr(CHT)(NImDipp)(TMEDA)] (9). These adducts are stable at room temperature and at 70  C, unlike some of the corresponding isocyanide adducts of [Ti(COT)(NR)] discussed later.27 Reaction of 12 with xylylisocyanate affords the ureato(1−) complex [Zr(CHT){N(ImDipp)C(O)N(Xyl)}] (14, Scheme 5) via a [2 + 2] cycloaddition pathway. In contrast, no isolable product is obtained from the corresponding reactions with XylNCS, CO2 or CS2. Compound 12 is also able to act as an initiator for the ring-opening polymerization (ROP) of e-caprolactone, but the control of the polymer molecular weight and molecular weight distribution is relatively poor because of the slow rate of initiation relative to propagation.

Scheme 5

612

Larger Aromatic Complexes of the Group 4 Metals

4.12.2.2

Troticene, trozircene and trohafcene complexes

Mixed-ring CHT-cyclopentadienyl complexes of the type [M(CHTR)(CpR)] (CHTR and CpR ¼ general cycloheptatrienyl and cyclopentadienyl ligands) form a long-established1,6,7 class of group 4 compounds known as troticenes (M ¼ Ti), trozircenes (M ¼ Zr) and trohafcenes (M ¼ Hf ), respectively. In this section recent developments in the chemistry of this class of mixed-ring complex are described, including ansa linked derivatives ([n]troticenophanes) and compounds with additional functional groups attached to the CHTR or CpR rings, including hetero-bimetallic and -trimetallic species.

4.12.2.2.1

Complexes without additional Lewis base coordination at the C5 or C7 ring

The parent compounds [Ti(CHT)(Cp)] (15)28 and [Zr(CHT)(Cp)] (16)29 were first reported by de Liefde Meijer, and the series [M(CHT)(Cp )] (M ¼ Ti (17), Zr (18), Hf (19)) was subsequently described by Teuben30 (Kool and Rausch independently reported 1731). Green and Walker made the indenyl counterparts [M(CHT)(Ind)] (M ¼ Zr, Hf ).32 Variable photon energy PES33 and DFT calculations34 for [Ti(CHT)(Cp)] established the tetravalent, d0 nature of titanium in this compound, with three electrons being required from the metal for the CHTdTi d-bonding interactions. At the start of the period covered by this chapter, trohafcene itself had not been reported. 4.12.2.2.1.1 Synthesis of troticenes [Ti(CHT)(Cp)] (15) and [Ti(CHT)(Cp )] (17) have recently been prepared in a one-pot procedure starting from NaCp or LiCp and [TiCl4(THF)2]. This comprises the FeCl3 catalyzed reduction of in situ generated [Ti(CpR)2Cl2] by magnesium in the presence of C7H8 (Scheme 6).35,36 The yield of 17 by this route (71%) is comparable to that reported earlier by Rausch (68%) starting from [Cp TiCl3] and C7H8/Mg.31 Although 15 is obtained in only ca. 50% yield (compared to yields of up to 88% starting from [CpTiCl3] and C7H8 with Mg or a Grignard reagent as the reductant28), it is reported to be operationally more straightforward and convenient.

Scheme 6

4.12.2.2.1.2 Synthesis of new troticenes and bitroticenes by ring functionalization The principal method of introducing of functionality into the CHT or Cp rings of troticene (15) involves initial metalation, invariably using an alkyl lithium reagent. It is helpful to trace the historical and recent developments of this chemistry at this stage of the section. Under kinetic control (as recently noticed by Tamm37), the monolithiation of troticene with nBuLi occurs preferentially at the CHT ring forming [Ti(CHTLi)(Cp)] (20a; CHTLi ¼ Z7-C7H6Li).38 With a nBuLi/TMEDA protocol, 1,10 -dilithiation gives [Ti(CHTLi)(CpLi)
2TMEDA] (21_TMEDA; CpLi ¼ Z5-C5H4Li).39,40 More recently, Tamm and co-workers developed a nBuLi/ PMDETA (PMDETA ¼ pentamethyldiethylenetriamine) protocol for lithiation (forming [Ti(CHTLi)(CpLi)
PMDETA] (21_PMDETA)) and monolithiation at the Cp ring of 15 under thermodynamic control conditions giving [Ti(CHT)(CpLi)
PMDETA] (20b).37 Similar protocols afforded [Ti(CHTLi)(Cp )
PMDETA] (22) from [Ti(CHT)(Cp )] (17) and nBuLi/PMDETA in hexane.36 Switching to a lithiating mixture of tBuLi/PMDETA (ratio 17:tBuLi/PMDETA ¼ ca. 1:4:1) gives metallation of 17 at both the CHT and a methyl group on the Cp ring, forming [Ti(CHTLi)(Z5-C5Me4CH2Li)
PMDETA] (23). Reactions of [Ti(CHT)(CpLi)
PMDETA] (20b) leading to new ring-functionalized troticenes or bitroticenes are summarized in Scheme 7.41 Thus reaction of 20b with 1,2-diodoethane, ZnCl2 or SiCl2Me2 or SnClt2Bu2 form the Cp-functionalized compounds [Ti(CHT)(Z5-C5H4X)] (X ¼ I (24), ZnCl (25), SiClMe2 (26a) or SnCltBu2 (26b)). The [Pd(DPPF)Cl2]-catalyzed (DPPF ¼ 1,10 bis(diphenylphosphino)ferrocene) Negishi coupling of 25 with PhI forms [Ti(CHT)(Z5-C5H4Ph)] (27) whereas with [Ti(CHT) (Z5-C5H4I)] (24) this reaction gives the interannularly-bridged [5–5]bitroticene (28) containing a m-Z5,Z5-pentafulvalene(2-) ligand. In the solid state 28 has crystallographically imposed C2h symmetry and therefore a perfectly coplanar pentafulvalene moiety, the internal CdC bonds being slightly longer on average than in troticene itself. Bitroticene complexes linked with m-Z5,Z5-C10H8(TiCp2) (29) or m-Z5,Z5-C10H8(ER) (ER ¼ BNMe2 (30a), PPh (30b)) moieties can also be formed from 20b as also shown in Scheme 7. The geometries at Ti, B and P in these compounds are pseudo-tetrahedral, trigonal planar and pyramidal, respectively.

Larger Aromatic Complexes of the Group 4 Metals

Ti

Ti

Cl E

613

R

R 27

ER2 = SiMe2 (26a) or SntBu2 (26b)

Pd(DPPF)Cl2 PhI

ECl2R2

24

ZnCl2

Ti

Ti

Ti

Pd(DPPF)Cl2 ZnCl

Li PMDETA 20b

28

25

Ti

ECl2R2

Ti

Ti

Cp 29

Ti

Ti

E

Ti Cp

or

R ER = BNMe2 (30a) or PPh (30b)

Scheme 7

The corresponding reaction of [Ti(CHTLi)(Cp )
PMDETA] (22, metallated on the CHT ring) with ZnCl2 gives 31 (Scheme 8) which is the counterpart of the Cp-functionalized [Ti(CHT)(Z5-C5H4ZnCl)] (25).36 Compound 31 can also be coupled with PhI, forming the CHT-functionalized [Ti(Z7-C7H6Ph)(Cp )] (32). Coupling with [Ti(CHT)(Z5-C5H4I)] (24) gives the unusual interannularly-bridged pentamethyl[7–5]bitroticene (33) containing a m-Z7,Z5-sesquifulvalene ligand (Scheme 8). The X-ray

Scheme 8

614

Larger Aromatic Complexes of the Group 4 Metals

structure of 33 shows that the two linked C5 and C7 rings are twisted out of coplanarity by 22o. The UV–visible spectrum does not feature the usual intense charge-transfer band for sesquifulvalene transition metal complexes, consistent with the d0 configuration of the metal centers. Mono- or di-lithiated troticenes have also had important historic1,38–40,42 and more recent6 roles in the synthesis of Lewis donor-functionalized derivatives such as troticenophosphines. Similarly, all the recent developments in the field of troticenophanes (i.e. ansa linked systems) have exclusively been based on lithiated starting materials. These aspects are covered in the relevant sections below. 4.12.2.2.1.3 Synthesis of trozircenes and trohafcene A more versatile route to trozircenes in general was developed by Tamm et al. starting from Zr(CHT)Cl(TMEDA) (4) and MC5H4R (M ¼ Li, Na; R ¼ H, Me, TMS, 1-allyl, PPh2) or LiInd (Ind ¼ C9H7; Eq. 2).43 This method also provides improved yields and a more convenient protocol for the previously reported [Zr(CHT)(Cp)] (16),29 [Zr(CHT)(CpPPh2)]44 (CpPPh2 ¼ C5H4PPh2) and [Zr(CHT) (Ind)].32 Despite the 16 valence electron count for zirconium in [Zr(CHT)(CpC3H5)] (CpC3H5 ¼ Z5-C5H4CH2CHCH2), the pendant allylic dCH]CH2 group does not coordinate to the d0 metal center. Subsequent reports from the Tamm laboratories extended the scope of the methodology and method depicted in Eq. (2) to a range of [Zr(CHT)(CpR)] and [Zr(CHT)(IndR)] complexes with highly substituted cyclopentadienyl and indenyl ligands.45,46

ð2Þ

The previously elusive trohafcene was synthesized by reaction of [HfCp2Cl2] with magnesium and an excess of cycloheptatriene in the presence of catalytic FeCl3 (Eq. 3).18 [Hf(CHT)(Cp)] (34) was obtained in 27% yield after sublimation and structurally characterized. This synthetic method is analogous to the one-pot route to troticenes described above (Scheme 6) and differs from those first reported for its Ti and Zr congeners, and also [Hf(COT)(Cp )], which started from the respective [M(CpR)Cl3] (CpR ¼ Cp or Cp ).28–31

ð3Þ

4.12.2.2.1.4 Reactivity of trometallocenes In this part only reactions of “parent” trometallocenes involving reactions at the metal centers are discussed. Reactions at ring-bound functional groups or of more elaborate complexes are described in later sections. As described in a later section, combination of Pd(OAc)2 (OAc ¼ MeCO2) with C5- or C7-ring functionalized complexes [Ti(CHTR)(CpR)] containing an appended dPR2 group forms catalysts for the Suzuki-Miyaura coupling of aryl bromides with PhB(OH)2.47,48 As part of their mechanistic studies of this reaction, Tamm et al. examined the reactions of the parent systems, [M(CHT)(Cp)] (M ¼ Ti (15), Zr (16)) and [Ti(CHT)(Cp )] (M ¼ Ti (17), Zr (18)) with Pd(OAc)2, Ag(OAc) or Ag(OTf ) (OTf ¼ OSO2CF3), The outcomes are illustrated in Scheme 9 for 15,47 but all four compounds 15, 16, 17 and 18 react similarly forming the respective [M(CpR)(OAc)3] (CpR ¼ Cp or Cp ) and ditropyl (35).47–49

Scheme 9

Larger Aromatic Complexes of the Group 4 Metals

615

On combination of [Ti(CHT)(Cp)] (15) with Pd(OAc)2 or Ag(OAc) in the stoichiometry CHT:OAc ¼ 1:3, a reaction immediately occurs to give essentially pure [Ti(Cp)(OAc)3] (36), along with elemental Pd or Ag, and 7,70 -bi-1,3,5-cycloheptatriene (ditropyl, 35). The corresponding reaction of 15 with 4 equiv. of Ag(OTf ) (CHT:OTf ¼ 1:4) forms the tropylium salt [CpTi(OTf )4][C7H7] (37). These reactions have been described as an overall oxidation of the formal CHT3− ligand of 15, initially forming M(0) (M ¼ Ag or Pd), X− (X ¼ OAc or OTf ) and the transient radical CHT , along with (formally) [TiCp]3+. In the case of Pd(OAc)2 or Ag(OAc), [TiCp]3+ is trapped by [OAc]− giving 36, while CHT  dimerizes via CdC coupling to yield 35. In the case of Ag(OTf ) (ratio CHT:OTf ¼ 1:4), the remaining equivalent of Ag+ oxidizes CHT  to [C7H7]+, and [CpTi]3+ is therefore coordinated by four triflate anions which combine with the tropylium to give 37. Reaction of [Zr(CHT)(Cp)] (16)29 with CNtBu and CNXyl gave the structurally characterized adducts [Zr(CHT)(Cp)(CNR)] (38; R ¼ Xyl (a), tBu (b)) (Eq. 4).50 Analysis of the n(CN) IR frequencies indicates relatively weak ZrdCNR interactions, consistent with the fast ligand exchange seen in solution on the NMR timescale. DFT studies suggest that the strong, covalent ZrdCHT bonding results in a diminished capacity for 4d(p) ! C^N(p ) backbonding. Adducts between 16 and PMe3 and 1,3,4,5-tetramethylimidazolin-2-ylidene (IMe) have also been described.18,51 [Hf(CHT)(Cp)] (34) also forms a series of adducts with CNXyl, CNtBu, PMe3 and IMe.18

ð4Þ

[Ti(CHT)(Cp)] (15) and [Zr(CHT)(Cp0 )] (Cp0 ¼ Z5-C5H4Me) have been used as precursors for the atomic layer deposition (ALD) of TiO2 and ZrO2 oxide films, respectively, with O3 as the oxygen source.52 The authors note the thermal stability of 15 is higher compared to the usual ALD precursors for titanium. 4.12.2.2.1.5 Bonding and spectroscopic studies Following on from the earlier computational and PES studies of [Ti(CHT)(Cp)] (15),33,34 Kaltsoyannis and Menconi described a comprehensive DFT analysis of [M(CHT)(Cp)] for groups 4–6, including the transactinide elements for each tetrad.53 For the group 4 metals of interest in this chapter (Ti, Zr, Hf ) the study confirmed that the four electrons in the 1e2 (CHTdTi d-bonding) MOs are key to determining the character of the MdCHT bonds. The %metal character of the 1e2 level decreases down the group as the nd (n ¼ 3, 4, 5) atomic orbitals rise in energy, and more electron density is localized on the CHT. The calculated charges for CHT and Cp were consistent with CHT3− and Cp− formalisms. Interestingly, the CpMdCHT bonding energies increased from M ¼ Ti to Zr, but decreased for M ¼ Hf (and then again for M ¼ Rf ). This may be attributed to a substantial increase in Pauli repulsion which outweighs the calculated increase in the orbital and electrostatic energies as the metal’s upper core levels also increase in amplitude in the valence region. Microwave spectroscopic studies and ab initio and DFT calculations for 15 and its isotopomers were reported by Kukolich.54 The rotational constants for the Ti complex were found to be A ¼ 1720(4) MHz, B ¼ 769.269(1) MHz and C ¼ 766.131(1) MHz. The gas phase structure showed the expected features based on earlier X-ray studies. In addition, the gas-phase electronic absorption spectra of 15 (and its COT counterparts [Ti(COT)(Cp)] and [Ti(COT)(Cp )]) were measured for the first time.55 Although the gas-phase spectrum of 15 showed no clearly defined Rydberg bands, an increased absorbance in the 32,000–42,000 cm−1 region provided evidence of the e2 ! R4p Rydberg excitation.

4.12.2.2.2

Heterotrozircenes

This section considers CHT complexes in which a 5 electron donor Z5- or Z6-coordinated ligand containing a heteroatom formally replaces Cp in [M(CHT)(Cp)]. To date, mixed-ring compounds of this type have been reported only for zirconium, and none prior to the period covered here. The general class of complex may be termed “heterotrozircene” and has been developed since 2011, exclusively by Tamm and co-workers.56–59 The phosphatotrozircenes [Zr(CHT)(Z5-2,5-PC4H2RR0 )] (39; R ¼ R0 ¼ H (a), Me (b); R ¼ H, R0 ¼ Ph (c)) were prepared by reaction of [Zr(CHT)Cl(TMEDA)] (4) with metallated phospholides (Scheme 10).56 Compounds 39a–b are dimeric in the solid state with intermolecular P ! Zr interactions, whereas 39c is monomeric. The relative ZrdP distances of 2.8664(7) and 2.7567 (4) A˚ for 39a and 39b are contrary to expectations on steric grounds alone. The CHT and 2-PC4H4Ph rings in 39c are effectively coplanar, whereas in 39a–b they tilt back to accommodate the P ! Zr interactions. All three complexes are apparently monomeric in solution, with variable temperature NMR studies for 39a indicating a possible monomer-dimer equilibrium at very low temperatures.

616

Larger Aromatic Complexes of the Group 4 Metals

R

Zr Cl

Me2 N N Me2

4

P

R'

R' M

Zr M = Li or K

Zr R P R'

P R

R

P

Zr

R'

39 R = R' = H (a), Me (b); R = H, R' = Ph (c)

Scheme 10

DFT calculations for monomeric 39b found a bonding situation reminiscent of that for [Zr(CHT)(Cp)] itself with a metal dz2-based LUMO and mainly CHT ligand-based d-bonding MOs which are the HOMOs. The lone-pair orbital for P is represented by HOMO-4, and it is proposed that the large energy separation of the LUMO and HOMO-4 in these systems is responsible for the relatively weak ZrdP bonding interactions. All three complexes 39a–c react with [W(CO)5(THF)], forming the adducts [Zr(CHT)(m-Z5,Z1-2,5-PC4H2RR0 )W(CO)5] (40a–c, Eq. 5), which have been crystallographically characterized. Analysis of the A1 n(CO) IR band of the adducts suggest that the P ! W donor/acceptor properties of the phospholyl ligand is comparable to that of electron-rich phosphines (e.g. PMe3). Compound 39a also reacted with [Ni(COD)2] (COD ¼ 1,4-cyclooctadiene) in a Zr:Ni stoichiometry of 4:1 to form the homoleptic compound [{Zr(CHT)(m-Z5,Z1-2,5-PC4H4)}4Ni] in which nickel has a distorted tetrahedral geometry.56

ð5Þ

Azatrozircene analogues of 39 have been prepared in a similar way using metallated bulky pyrrolyl (PyrR) or 2,4,5-tri(tert-butyl) imidazolyl ligands.57 Reaction of [Zr(CHT)Cl(TMEDA)] (4) with the respective metallated pro-ligands gave [Zr(CHT)(PyrR)] (41a–c) and [Zr(CHT)(Z5-2,4,5-N2Ct3Bu3)] (42) (Scheme 11). Compounds 41a–b and 42 were crystallographically characterized and monomeric in all cases due to the sterically demanding tert-butyl substituents adjacent to the nitrogens. A series of DFT calculations for the isosteric series [Zr(CHT)(Z5-1,3-C5Ht3Bu2)],45 [Zr(CHT)(Z5-2,5-NC4Ht2Bu2)] (41a) and [Zr(CHT) (Z5-2,4,5-N2Ct3Bu3)] (42) found that the strength of the bonding of the Z5-ring anion to [Zr(COT)]+ decreases in the order C5Ht3Bu2 > NC4Ht2Bu2 > N2Ct3Bu3.

Scheme 11

Larger Aromatic Complexes of the Group 4 Metals

617

Whereas the phospholyl, pyrrolyl and imidazolyl ligands in 39, 41 and 42 have Lewis base functional groups incorporated into the Z5-rings, boratazircenes (incorporating a BR unit into a Z6-ring) have a potentially Lewis acidic site. The new boratazircenes [Zr(CHT)(Z6-C5H5BR)] (43; R ¼ H (a), Me (b), CCTMS (c)) were synthesized as shown in Scheme 12 and crystallographically characterized for 43b–c.58 The boratabenzene ligands adopt slightly distorted Z6-coordination modes, and the CHT and C6H5BR rings are slightly bent away from coplanarity (the angle subtended at Zr between the respective ring centroids is ca. 166o). Reaction of [Zr(CHT)(Z6-C5H5BMe)] (43b) with PMe3 or DMAP gave structurally authenticated 18 valence electron adducts [Zr(CHT) (Z6-C5H5BMe)(L)] (44; L ¼ PMe3 (a) or DMAP (b)) with the Lewis base attached to zirconium and not the boron atom (Scheme 12). Compounds 44a–b are analogues of the parent trozircene adducts [Zr(CHT)(Cp)(L)] (L ¼ Lewis base) discussed earlier.18,51

Scheme 12

A 1,2-dicarbollyl analogue of trozircene and [Zr(CHT)(Z6-C5H5BR)] (43) has also been described (Scheme 13).59 Reaction of [Zr(CHT)Cl(TMEDA)] (4) with Na2C2B9H11 formed 45-Na(TMEDA) which contains the anion [Zr(CHT)(Z5-C2B9H11)]− (45−). The corresponding protonolysis reaction of [Zr(CHT){N(TMS)2}(THF)] (8_THF) with [NMe4][C2B9H12] formed the analogous tetramethyl ammonium salt [NMe4][Zr(CHT)(Z5-C2B9H11)] (45-NMe4). The crystal structure of 45-Na(TMEDA) found that one of the [Na(TMEDA)]+ units bridges two 45− anions, being bound to the carbollyl moiety of one and the CHT of another. The 45− anion itself features the expected Z7-CHT and Z5-C2B9H11 ligands. DFT calculations on the series [Zr(CHT)(X)]n– (n ¼ 0, X ¼ Cp (16), Cp (18); n ¼ 1, X ¼ C2B9H11 (45)) found that the fractional negative charges, assessed using several methodologies, on the CHT ligands increased in the order 16 < 18 < 45 showing an accumulation of negative charge on that ring. The calculations for 45− found that the LUMO was mainly based on the zirconium 4dz2 orbital and the HOMOs were the ZrdCHT d-bonding MOs, as expected from the isolobal relationship between Cp− and C2B9H2− 11. Reaction of 45-Na(TMEDA) with BaI2 in THF appears to take advantage of this charge build up on CHT by forming the inverse sandwich cation [Zr(m-Z7,Z7-C7H7){Ba(THF)5}(C2B9H11)]+ (46+) accompanied by an equivalent of 45− as the counter-anion (Scheme 13).

Scheme 13

618

Larger Aromatic Complexes of the Group 4 Metals

4.12.2.2.3

Synthesis and reactivity of troticenophanes

This section discusses advances in the synthesis and reactivity of troticene complexes in which the CHTR and CpR rings are covalently linked by one or more atoms. To date, this chemistry has not yet been extended to trozircene or trohafcene. Compounds in this class are usually either categorized as ansa bridged systems or [n]troticenophanes where n is the number of atoms in the bridge, excluding any substituents. Related compounds with transition metals in the bridging moiety, when connected by dative bond formation to functional groups (e.g. phosphines) appended to the hydrocarbyl rings are considered in a later section, along with other heterobimetallic compounds formed by C5 or C7 ring functionalization. The [1]silatroticenophane [Ti{Z7,Z5-C7H6(SiMe2)C5H4}] (47) is briefly mentioned in Green and Ng’s 1995 review as being prepared from 21_TMEDA and SiCl2Me2,1 as reported in a conference abstract.60 Since 2004, Tamm et al. have extensively developed the field of [n]troticenophane chemistry.35,36,61–65 Compound 47 and its [1]germatroticenophane congener [Ti{Z7, Z5-C7H6(GeMe2)C5H4}] (48) were prepared via 21_TMEDA and ECl2Me2 (E ¼ Si, Ge) according to Eq. (6).61,63 The compounds are isomorphous and hence present very similar molecular structures, the angle subtended at Ti between the ring centroids being ca. 161o in both. DFT and UV–visible spectroscopic studies for 47 find an increased HOMO-LUMO gap compared to that in troticene itself, but otherwise the TidC7/C5 ring bonding characteristics are very similar.

ð6Þ

Compound 47 reacts immediately and quantitatively with CNR to form [Ti{Z7,Z5-C7H6(SiMe2)C5H4}(CNR)] (49; R ¼ Xyl (a), tBu (b)) (Scheme 14), unlike troticene which does not form stable adducts with Lewis bases. This difference is attributable to the constraining ansa-SiMe2 group opening up access to the metal center. On the other hand, CO binds only weakly and reversibly to 47, as expected for a d0 compound.61 Reactions with [Pt(PEt3)3] or HX (X ¼ Cl, F) result in selective cleavage of the C7H6dSiMe2 bond forming the [2]platinasilatroticenophane 51 by net oxidative insertion of Pt(PEt3)2 into SidCipso, and [Ti(CHT)(Z5-C5H4SiMe2X)] (52; X ¼ Cl (a), F (b)) by net addition of XdH across SidCipso.62,63 Unlike the case for [Pt(PEt3)3], this latter reaction does not result in a new troticenophane.

Scheme 14

Larger Aromatic Complexes of the Group 4 Metals

619

Fig. 1 The range of [n]troticenophanes prepared from 21_PMDETA.35,64,65

The general synthetic methodology used in Eq. (6) has been extended to a range of other [n]troticenophanes starting from 21_PMDETA and the appropriate ansa bridge synthon.33,57,59 Fig. 1 illustrates the range novel compounds of this type that have been prepared with the length (n) the ansa linkage ranging from 1 to 7. As expected, structural studies show varying degrees of distortion and ring strain depending on the length and nature of the ansa linkage. Attempts to form [1]titana- or [1] zircona-troticenophane compounds, with {Cp2M} (M ¼ Zr, Hf ) moieties as the ansa link by reaction of 21_PMDETA with Cp2MCl2 were unsuccessful, giving only m-oxo hydrolysis products.41 As was found for the [1]silatroticenophane 47 (Scheme 14), the heavier congeners [Ti{Z7,Z5-C7H6(ER2)C5H4}] (ER2 ¼ GeMe2 (48), SnMes2 (53a), SntBu2 (53a)) all undergo oxidative insertion of Pt(PEt3)2 into the EdCipso bond of the C7 ring to form the corresponding expanded [2]platinatroticenophanes [Ti{Z7,Z5-C7H6(Pt(PEt3)2)(ER2)C5H4}] (59a–c) which are structurally analogous to 51 (Scheme 14). In contrast, reaction of the [2]boratroticenophane 56 with [Pt(PEt3)3] gives insertion of Pt(PEt3)2 into the BdB bond, forming the corresponding [3]troticenophane 60 (Eq. 7) containing a BdPtdB linkage.35 The geometry at the Pt centers in 51, 59a–c and 60 is distorted square planar. In all cases, structural studies showed that the insertion of Pt(PEt3)2 into the respective EdCipso or BdB bonds gave a significant reduction in ring strain as judged by decreases in the tilt angles between the CHTR and CpR rings.

ð7Þ

The [2]stanatroticenophane [Ti{Z7,Z5-C7H6(SntBu2)2C5H4}] (55) undergoes insertion reactions at the SndSn bond with elemental sulfur or selenium at elevated temperatures forming the [3]troticenophanes [Ti{Z7,Z5-C7H6(SntBu2)E(SntBu2)C5H4}] (61; E ¼ S (a), Se (b)), again with reduction in ring strain (Eq. 8).35

ð8Þ

The ability to dilithiate [Ti(CHT)(Cp )] (17) using tBuLi/PMDETA provides an additional route to troticenophanes. As summarized in Scheme 15, [Ti(CHTLi)(Z5-C5Me4CH2Li)
PMDETA] (23) reacts with PCl2R (R ¼ Ph, Mes (Mes ¼ 2,4,6-C6H2Me3)) or SiCl2Me2 to form the [2]carbaphosphatroticenophanes 62 (R ¼ Ph (a), Mes (b)) and [2]carbasilatroticenophane 63, albeit in relatively poor yields (25–33%).36

620

Larger Aromatic Complexes of the Group 4 Metals

Scheme 15

The solid state structures showed that although 62a–b and 63 are distorted compared to the non-ansa linked systems, the effect is smaller than in the [1]silatroticenophane 47 and [1]germatroticenophane 48.61,63 Compound 62b reacts with [AuCl(SMe2)] via the phosphorus lone pair to form the adduct 64. A principal focus on interest in [n]troticenophanes is their potential use via ring-opening polymerization (ROP) for preparing poly([1]troticenophane)s. Fig. 2 summarizes in general terms the ROP chemistries reported for 47, 48 and 53b, forming metallapolymers linked by CHTRdER2dCpR linkages.61–64 Differential scanning calorimetry (DSC) studies of the thermally initiated ROP for these compounds revealed polymerization onset temperatures, TROP, of 170, 130 and 233  C, respectively.63,64 These values were consistent with the trends found for the analogous [1]ferrocenophanes which have been widely studied.64 The lower TROP for 48 compared to that of 47 was attributed to GedC bonds being weaker than their SidC counterparts. The strain energies of 47 and 48 (both ca. 10.8 kcal mol−1) were slightly less than in 53b (12.4 kcal mol−1). The ROP of 47 was found to be catalyzed by the Pt(PEt3)2 insertion product 51 (3 mol%; 80  C for 64 h in C6D6) forming cyclic oligomers of 65a with m/z values between 1301 (n ¼ 5) and 5985 (n ¼ 23) according to MALDI-ToF (matrix-assisted laser desorption ionization, time of flight) mass spectroscopic analysis.62 Compound 53b underwent thermally initiated ROP at 240  C in the melt within 5 min to form low molecular weight poly([1]stannatroticenophane)s 65c (Mw ¼ 1713 g mol−1, Mn ¼ 1220 g mol−1) (Mw and Mn are weight-averaged and number-averaged molecular weights) and a polydispersity index (PDI ¼ Mw/Mn) of 1.4.64 The ROP of 53b could also be initiated by nBuLi in THF at RT, followed by quenching with TMSCl. Higher molecular weight polymers (66) were formed in this case (Mw ¼ 14,113 g mol−1, Mn ¼ 7268 g mol−1) although the PDI value of 1.9 suggested relatively poor control of the process.

Fig. 2 Poly([1]troticenophane)s prepared from 47, 48 and 53b.61–64

Larger Aromatic Complexes of the Group 4 Metals

621

In contrast to 47, 48 and 53b, the siloxane-bridged [n]troticenophanes 58a–c (Fig. 1) are resistant to thermally-initiated ROP. Furthermore, reaction with initiators such as potassium siloxanolate, ammonium siloxanolate or nBuLi led to only oligotroticenylsiloxanes incorporating one or more troticenyl units, ring-opened troticenes and ring-expanded [n]troticenophanes.65

4.12.2.2.4

Complexes with Lewis base functional groups attached to the C5 or C7 ring

Access to mono- or di-lithiated troticenes using the protocols described in an earlier section has allowed the development of new Lewis base functionalized troticenes (primarily phosphine groups,39,42 but also thioethers and selenoethers66) from the early 1980s.1 Fig. 3 summarizes, by way of context, the troticenophosphines known prior to 2000 and some of their adducts formed with transition metals.1,39,42 No trozircene or trohafcene congeners had been reported, and neither had analogues of 67 with functionalization at the Cp ring. Likewise, no applications in catalysis had been described.

Fig. 3 Examples of the first troticenophosphines and their coordination complexes.1

4.12.2.2.4.1 Synthesis of trometallocenophosphines and stoichiometric reactivity Routes to group 4 Cp-functionalized trometallocenophosphines based on magnesium reduction in the presence of C7H8 have been developed as shown in Scheme 16.44 Use of half-sandwich (for Ti) or sandwich (for Zr, Hf ) complexes with phosphine substituents on the cyclopentadienyl ring affords [M(CHT)(CpPR2)] (CpPR2 ¼ Z5-C5H4PR2); M ¼ Ti (71), Zr (72), Hf (73); R ¼ Ph (a), iPr (b). This is the only viable route to such complexes for 72 and 73 since attempted lithiation of one or other of the hydrocarbyl rings of the parent [M(CHT)(Cp)] (M ¼ Zr, Hf ) was unsuccessful.44

Scheme 16

622

Larger Aromatic Complexes of the Group 4 Metals

Compound 71a can be more conveniently prepared directly from [Ti(CHT)(Cp)] (15) via metallation with nBuLi/PMDETA in hexane for 4.5 h followed by addition of PClPh2, or by metallation with nBuLi in Et2O over 36 h prior to addition of PClPh2 (i.e. under thermodynamic control conditions).67 The same reaction sequence, when carried out with a 3 h metallation time afforded the CHT-functionalized isomer [M(CHTPPh2)(Cp)]42 (67a; CpPPh2 ¼ Z7-C7H6PPh2). A wide range of other troticene mono- and di-phosphines, [Ti(CHT)(CpPR2)] (71; R ¼ Cy (c), tBu (d)), Ti(CHTPR2)(CpPR2) (68; R ¼ Ph (a), iPr (b), Cy (c), tBu (d)) and [Ti(CHTPR2)(Cp )] (74; R ¼ Ph (a), Cy (c), tBu (d); R2 ¼ Cl, Ph (b)), have also been made by the nBuLi/PMDETA route (Scheme 16).36,47,48,68 The solid state structures of the new troticenophosphines feature exclusively monomeric, 16 valence electron systems as expected since the parent compounds 15 or 17 do not form adducts with Lewis bases (only the ansa bridged [1]silatroticenophane 47 forms stable adducts, as discussed earlier). A more mixed situation is found for the crystal structures of the larger metal complexes [M(CHT)(CpPR2)] (M ¼ Zr (72), Hf (73)).44 For the two systems with dPPh2 substituents (72a, 73a) the molecules form centrosymmetric dimers with two relatively long P ! M dative bonds per [72a]2 or [73a]2 moiety, reminiscent of the situation for [Zr(CHT)(2,5-PC4H3RR0 )] (39, Scheme 10). In the case of the dPiPr2 homologues, only 72b forms a dimer, with an even longer P ⋯Zr distance (2.983 A˚ ) than in [72a]2 (av. 2.889  0.04 A˚ for two different solvates), whereas 73b is monomeric with no significant intermolecular interactions. This is consistent with the increased steric bulk of dPiPr2 compared with dPPh2, and the slightly smaller covalent radius of Hf compared with Zr (due the lanthanide contraction). Many of the new trometallocenophosphines have been shown to form adducts with transition metals through P ! M dative bond formation.36,44,67–69 A selection of examples of adducts formed for various troticene systems and the zirconium congener 72b (the only heavier analogue studied so far) is given in Fig. 4. Complexes relevant to catalytic systems47,48,70 are discussed in the following section. For titanium this coordination occurs almost exclusively in the general manner found previously for 67a and 68a (Fig. 3).1 The new generation of troticenophosphines, as for the early examples, has no apparent involvement of the metal center in most of their reactions, acting as a mono- or bi-dentate donor (e.g. 75, 76 and 77a). In contrast, as anticipated from the solid state structure of 72b itself (and the affinity of trozircenes for Lewis bases in general), 72b consistently shows ambiphilic behavior in its structurally authenticated adducts.44,69 This difference between the titanium and zirconium congeners is nicely illustrated by the reaction of [M(CHT)(CpiPr)] (M ¼ Ti (71b), Zr (72b)) with [RhCl(COD)]2 forming 77 (M ¼ Ti (a), Zr (b)). In 77a the titanium is not involved in any additional interactions with the attached RhCl(COD) moiety whereas 77b features a RhdCl ! Zr interaction. The 2:1 reaction of [AuCl(THT)] (THT ¼ tetrahydrothiophene) with 72b forms 78 which represents another example of the active involvement of the Lewis acidic zirconium centers (the 1:1 reaction gives an unknown, insoluble material). The zwitterionic compound 78 formally contains a linearly coordinated d10 Au+ cation bound by a k2P bidentate [{M(CHT)(CpPiPr2)}2(m-Cl)]−. The Au ⋯ Cl distance of 3.086 A˚ is not considered to represent a covalent bond in this system but is within the sum of the van der Waals radii. [Pd{M(CHT)(CpPiPr2)}2] (79) also has a 2:1 Zr:Pd stoichiometry with a linearly coordinated d10 metal. In this case a Pd ! Zr

M(L)

Cy Cy

P Ti

P

MCl2

Ph

Ph

Ti

Ti

i

Pr i

P

P

Pr

Cy Cy

Rh

75

76

M = Pd (a), Pt (b)

M(L) = AuCl (a), Mo(CO)5 (b)

Cl

77a

i

Zr

i

Pr

Pr

P Zr

P i

Pr

Zr

Cl

i

Zr

Au

P i

Pr

Pr 78

Pr

i

Pd P

i

i

P

i

i

Rh

Pr

Pr

Zr

Cl

Pr

Pr

79

Fig. 4 Examples of complexes prepared from the new troticenophosphines and trozircenophosphines shown in Scheme 16.

77b

Larger Aromatic Complexes of the Group 4 Metals

623

dative bond is formed to one of the zirconium centers. The titanium analogues (84a–b, discussed later on) of this compound have also been structurally characterized47 and show no analogous Pd ! Ti interaction. The final example of the ambiphilic nature of 72b is shown in its reaction with elemental selenium to give [Zr(CHT) {Z5-C5H4P(Se)iPr2}] (80; Eq. 9). The solution 31P NMR spectrum showed a 1JP-Se coupling constant of 573 Hz which is significantly reduced from the corresponding value of 713 Hz in iPr3PSe.69 Together with the X-ray structure, this establishes the intramolecular Se ! Zr bond shown in Eq. (9).

ð9Þ

Despite troticene and its phosphine-functionalized derivatives showing no stable adducts with Lewis bases, several reactions have shown the metal center’s involvement in reaction outcomes. As shown in Eq. (10), addition of [PtCl2(MeCN)2] to 68c does not form the expected adduct 75b (Fig. 4) which was obtained with [PtCl2(SEt2)2], but instead gives the ketimide type compound 81 reproducibly and in high yield.68 The MeCN in this case has undergone overall insertion into a TidC bond to the CHTPCy2 ligand with concomitant reduction of Pt(+2) to Pt(0). Surprisingly, reaction of 68c with [PdCl2(MeCN)2] gave the coordination complex 75a and not an analogue of 81.

ð10Þ

Scheme 17 shows how the bulky phosphine [Ti(CHT)(CpPtBu2)] (71d) exhibits ambiphilic behavior as a metalla-FLP (FLP ¼ frustrated Lewis pair) in reactions with small molecules such as H2 and CO2. In the absence of substrate the borane B(C6F5)3 cannot form an adduct with the dPtBu2 group for steric reasons.71

Scheme 17

4.12.2.2.4.2 Catalytic applications of troticenophosphines As indicated earlier, the bulky Cp-functionalized troticenophosphines [Ti(CHT)(CpPR2)] (71; R ¼ Cy (c), tBu (d)) have been used in 2:1 M combination with Pd(OAc)2 as a catalyst system for the Suzuki-Miyaura coupling of aryl bromides with phenyl boronic acid (Eq. 11) in toluene with KOH as the external base.47,48 At 100  C with 1% catalyst loading (with respect to Pd) rapid conversions and high yields were obtained, even for bulky arylbromides, ArBr. ð11Þ

624

Larger Aromatic Complexes of the Group 4 Metals

To probe the nature of the in situ formed catalyst, the reaction of 71c–d with Pd(OAc)2 (2:1) was carried out which gave ca. 50% formation of the trimetallic Pd(0) compound [Pd{M(CHT)(CpPR2)}2] (84a–b; Eq. 12). These compounds do not show any additional Pd ! Ti interactions, in contrast to the zirconium analogue 79 (Fig. 4) which features a Pd ! Ti dative bond.

ð12Þ

The means of reduction of Pt(+2) to Pd(0) was traced through the model reactions of [M(CHT)(Cp)] (M ¼ Ti (15), Zr (16)) and [Ti(CHT)(Cp )] (M ¼ Ti (17), Zr (18)) with metal acetate complexes described in an earlier section. Adjusting the stoichiometry of the 71c–d + Pd(OAc)2 reaction to 8:3 allowed the isolation of compounds 84 in 70–80% yield. It was shown that isolated 84 were just as effective (pre)catalysts in a representative test reaction as the original 71c–d:Pd(OAc)2 dual component system. The Suzuki-Miyaura catalyst system was further developed using the CHT-functionalized troticenophosphines, [Ti(CHTPR2) (Cp)] (67; R ¼ Cy (b), tBu (c)) and [Ti(CHTPR2)(Cp )] (74; R ¼ Cy (c), tBu (d)), in combination with Pd(OAc)2 and KOH/toluene as before. Similar Pd(0)-based trimetallic systems to 84a–b were isolated in model studies. The new catalyst systems were of comparable activity to the original ones based on 71c–d. There was no difference in catalyst performance with respect to the CpR substituents (H or Me) for 67b–c and 74c–d, but the cyclohexyl-based compounds (67b and 74c) were generally superior to their tert-butyl counterparts 67b and 74c.48 The troticenophosphines [Ti(CHT)(CpPR2)] (71c–d) and [Ti(CHTPR2)(Cp)] (67b–c) have also been used to prepare the allyl palladium complexes [Pd(Z3-C3H5)(Cl){Ti(CHT)(CpPR2)}] (85a–b) and [Pd(Z3-C3H5)(Cl){Ti(CHTPR2)(Cp)}] (86a–b) (Scheme 18) and other 1- or 2-methylallyl or 1-phenylallyl homologues for use in the Buchwald-Hartwig amination reaction of N-methylaniline or morpholine with halobenzenes.70 Overall the dPtBu2 substituted systems 85b and 86b were significantly superior to their dPCy2 analogues, 85a and 86a, and 86b was the most active. Overall, however, even this was uncompetitive with other known systems that can operate under milder conditions.

Scheme 18

4.12.3

Cyclooctatetraene complexes

Cyclooctatetraene complexes of group 4, and their associated structure, bonding and reactivity studies, had reached a high level of maturity by ca. 2000. The developments in the period up to this date were covered in COMC III.2–5 Unless indicated otherwise, COTR (COTR ¼ a general cyclooctatetraene, C8R8 (R ¼ H or other)) ligands have their maximum hapticity (Z8), and in all such complexes may be assumed to have a formal 2- charge. EHMO calculations and PES studies on the complex [Zr(COTTMS)

Larger Aromatic Complexes of the Group 4 Metals

625

(Zn-COTTMS)] (87, COTTMS ¼ 1,4-C8H6(SiMe3)2) with lower hapticities (n ¼ 3 or 4) found that the Zn-COTTMS should also be treated as formally 2–, given that the ring-slippage and puckering on moving away from Z8/local D8h symmetry to local C2v did not substantially alter the frontier ring p MOs of the ligand.72 This may be a transferable result for Zn-COTR (n  4) ligands with group 4 metals in general. As in previous sections, some milestones are given here for context and orientation. By 2000, many M(+3) and M(+4) sandwich and mixed-sandwich compounds were established: [M(COT)(Z4-COT)] (M ¼ Ti (88), Zr (89), Hf (90)),73–75 [M(COTTMS) (Zn-COTTMS)] (M ¼ Zr (91), Hf (92)),72,76 [M(COT)(CpR)] (CpR ¼ Cp, M ¼ Ti (93)77; CpR ¼ Cp , M ¼ Ti (94)30,31 or Zr (95)30,78) and [Ti(COT)(Z4-C4Ph4)] (96).79 Notably, no Hf(+3) analogue of [M(COT)(CpR)] (M ¼ Ti or Zr) could be prepared.30 Mixed-sandwich monochloride and half-sandwich dichloride complexes of the following type have provided entry points to further chemistry: [M(COT)(Cp )Cl] (M ¼ Zr (97), Hf (98))78,80 [Ti(COTR)Cl(THF)n]x (COTR ¼ COT, n ¼ 1 (99)81; COT ¼ COTTMS, 2n ¼ 1 (100)82), [M(COT)Cl2(THF)n]x (n ¼ 0, M ¼ Zr (101) or Hf (102); n ¼ 1, M ¼ Zr (103) or Hf (104)),74,78 and [Zr(COTTMS)Cl(m-Cl)]2 (105).83

4.12.3.1

Complexes in oxidation state +3

The Ti(+2) complex [TiCl2(py)4] (106) reacts with an excess of COT over several hours to give [Ti(COT)Cl(py)2] (107, Eq. 13) and mer-[TiCl3(py)3]. Structurally characterized compound 107 is a rare example of a three-legged piano stool Ti(+3) COT complex.84 The sluggish nature of the reaction was attributed to a stepwise reaction mechanism in which 106 first undergoes a two-electron electron transfer reaction to form a transient Ti(+4) complex, [Ti(COT)Cl2(py)2] (not observed), which subsequently reacts with further 106 to form the final observed products.

ð13Þ

Mach and Cloke have each reported new M(+3) sandwich complexes, namely [Ti(COT)(Z5-C5Bn5)] (108, Bn ¼ CH2Ph)85 and [Zr(COTTiPs)(Cp )] (109, PnTiPS ¼ 1,4-C8H4(SiiPr3)2),86 respectively. Compound 108 was made by reaction of [Ti(COT)Cl(THF)]2 (99) with LiC5Bn5 and characterized by X-ray crystallography, EPR and UV–vis-near-IR spectroscopy. [Zr(COTTiPS)(Cp )] (109) was prepared by KC8 reduction of the Zr(+4) precursor 110a (discussed in the next section) according to Scheme 19, in a similar way to Spencer’s synthesis of [Zr(COT)(Cp )] (95) by Na/Hg reduction of [Zr(COT)(Cp )Cl].78 The X-ray structure of 109 shows the C8 and C5 rings are slightly less coplanar than in 95, attributed to increased steric effects in the former because of the bulky dSiiPr3 groups.

Scheme 19

In contrast, attempted preparation of the hafnium analogue of 109 by C8K reduction of [Hf(COTTiPS)(Cp )Cl] (110b) gave a low (18%) yield of the unusual bimetallic compound 111 which was characterized by X-ray crystallography. Compound 111 is formed by CdC coupling of two COTTiPS rings forming a new m-Z7,Z7-dicycloheptatrienyl ligand. It was proposed that each cycloheptatrienyl moiety acts as a formal 3-ligand to the hafnium to which it is bound (in the same way that CHT is formally a 3-ligand), and thus 111 is assigned as a Hf(+4) compound.86 In this context we note that the unusual formation of 111, potentially via a transient

626

Larger Aromatic Complexes of the Group 4 Metals

intermediate [Hf(COTTiPS)(Cp )], analogous to 109, may be relevant to the observation that no example of a mixed-ring Hf(+3) compound has yet been reported. Interestingly, Teuben found that while Mg or Na/Hg reduction of [Cp HfCl3] in the presence of COT in THF or toluene formed [Hf(COT)(Cp )Cl] (98) at an early stage, no [Hf(COT)(Cp )] was subsequently isolated, and a low yield of the Hf(+4) hydride [Hf(COT)(Cp )H] was obtained instead. This is in contrast to the successful formation of the titanium and zirconium congeners 94 and 95 by the same method.30 Braunschweig reported the first dilithiation of [Ti(COT)(Cp)] (93) using a nBuLi/PMDETA protocol to form [Ti(COTLi)(CpLi)
PMDETA] (112).87 Structural characterization established the presence of [Ti(COTLi)(CpLi)
PMDETA]2 dimeric units in the solid state with lithium atoms bridging C5 and C8 rings on the same titanium, and between C5 and C8 rings on neighboring atoms. Subsequent treatment of 112 with Sn2tBu4Cl2 gave the Ti(+3) ansa linked [2]stannatitanoarenophane (113) via a salt elimination reaction (Scheme 20). This represents the first example of a COT motif in [n]metalloarenophane chemistry. The overall protocol from 93 to 113 is reminiscent of the procedure used to make [Ti{Z7,Z5-C7H6(SntBu2)2C5H4}] (55, Fig. 1).35 The X-ray structure of 113 showed that the Snt2Bu4 ansa linkage only slightly perturbed the geometry compared to that of the parent 93. Compound 113 is oxidized by one-electron electron transfer using [Cp2Fe]+. The product [Ti{Z8:Z5-C8H7(SntBu2)2C5H4}]+ (113+) is isoelectronic with the CpdCHT compound 55.

Li PMDETA

t Bu Bu Sn

t

Sn2tBu4Cl2

[FeCp2] Ti

Ti Bu Sn t Bu

112

Ti Bu Sn t Bu

t

Li

t Bu Bu Sn

t

t

113

113+

Scheme 20

There have been several spectroscopic and theoretical studies of M(+3) sandwich and half-sandwich group 4 COT compounds which have focused on Rydberg transitions in [Ti(COT)(Cp )],55 computed EPR parameters for [M(COT)(Cp)] (M ¼ group 4),88 and measurement of the coherence times for [Ti(COT)(Cp)] in the context of developing organometallic systems as molecular qubits.89

4.12.3.2 4.12.3.2.1

Complexes in oxidation state +4 Half-sandwich complexes

Reaction of [Ti(NImDipp)Cl3] with K2COT gave the imidazolin-2-imininato half-sandwich (two-legged piano stool) complex [Ti(COT)(NImDipp)Cl] (114, Eq. 14).90 Compound 114 is valence isoelectronic with [Zr(CHT)(NImDipp)(CNR)] (13; R ¼ Xyl (a), tBu (b)) discussed in an earlier section (cf. Scheme 5).25 Compound 114 is also isoelectronic with the pyridine adduct27 of the COT-imido compound [Ti(COT)(NtBu)]26 discussed below.

ð14Þ

The crystal structure of 114 found the TidNim bond length (1.7905(15) A˚ ) to be somewhat longer than in the imido analogues, and that the TidNimdC angle was rather non-linear (149.5(1)o), probably due to steric crowding. DFT calculations on the model d0 neutral imido [Ti(COT)(NXyl)] and cationic imidazolin-2-imininato species [Ti(COT)(NImXyl)]+ showed that two are isolobal and that the TidN interaction in both cases should be regarded as a s2p4 Ti^N triple bond. A series of unique (in group 4 and COT chemistry) one-legged piano stool (“pogo stick”) complexes [Ti(COT)R(NR)] has been prepared (Scheme 21) by Mountford and co-workers, and their bonding and reactivity (Fig. 5 and Schemes 22 and 23) explored in collaboration with Green, Cloke and Nixon.26,27,91

Larger Aromatic Complexes of the Group 4 Metals

Scheme 21

Fig. 5 Examples of reaction products of Ti(COT)(NR) (116; R ¼ tBu (a), Dipp (b)) with various substrates.

Scheme 22

Scheme 23

627

628

Larger Aromatic Complexes of the Group 4 Metals

Reaction of [Ti(NR)Cl2(py)3] (115a–d, R ¼ tBu (a) or aryl (a–c)) with K2COT or Li2COTTMS gave monomeric [Ti(COT)(NR)] (116; R ¼ tBu (a), Dipp (b)) or [Ti(COTTMS)(NR)] (117; R ¼ tBu (a), Dipp (b, NMR tube scale only), Xyl (c), 2-C6Ht4Bu (d)) as shown in Scheme 21. Steric factors are important to prevent dimerization: whereas [Ti(COTTMS)(NXyl)] (117c) is a monomer, the less bulky COT ligand in [Ti(COT)(m-NXyl)]2 (118) allows dimer formation through m-NXyl linkages. Tert-butyl imide/arylamine exchange reactions also led to [Ti(COTR)(NAr)] (Ar ¼ Dipp, Xyl) from their respective [Ti(COTR)(NtBu)] counterparts. Reaction of [Ti(COTTMS)(NtBu)] (117a) with PhNH2 forms dimeric [Ti(COTTMS)(m-NPh)]2 (119) despite the bulky COTTMS ring. Extension of this chemistry to zirconium gave only m-imido products, for example [Zr(COTTMS)(m-NXyl)]2 (120, Eq. 15), because of the larger size of this metal.

ð15Þ

DFT and PES studies of [Ti(COT)(NR)] (R ¼ tBu (116a), Dipp (116b)) and [Ti(COTTMS)(NtBu)] (117a) were used to determine the electronic structure of these pseudo-two-coordinate complexes. The COTR ligands are bonded mainly through s- and d-interactions, whereas the NR ligands dominate the p-bonding which, together with the TidN s-bonding component, gives rise to Ti^Nimide triple bonds in these systems, consistent with the short TidNimide bond lengths and near-linear TidNimidedR bond angles found by X-ray crystallography. The COT-imido complexes react with a wide range of substrate types. A selection of products is given in Fig. 5. [Ti(COT)(NR)] (R ¼ tBu (116a), Dipp (116b)) react with organic isocyanates and CO2 and their thia-analogues via initial [2 + 2] cycloaddition. In the reactions with CO2 and tBuNCO the first-formed metallacycles undergo a rapid reverse cycloaddition to form the m-oxo dimer [Ti(COT)(m-O)]2 (121a) and the corresponding organic product RNCO or tBuNCNR. Compound 121a was also obtained from 116a and PhNO, along with cis-PhNNtBu. Compound 116a reacts with DippNCO in a similar way to tBuNCO, giving tBuNCNDipp and 121a, whereas 116b gave the isolable k2N,N0 -ureate [Ti(COT){N(Dipp)C(O)N(Dipp)}] (122). Analogous reactivity with CS2 and tBuNCS was observed in the case of [Ti(COT)(NtBu)] (116a), forming tBuNCS and tBuNCNtBu alongside [Ti(COT) (m-S)]2 (121b). [Ti(COT)(NDipp)] (116b) formed 121b and tBuNCNDipp with tBuNCS but with XylNCS the k2N,S-thiaureate [Ti(COT){N(Dipp)C(NXyl)S}] (123) was observed by NMR spectroscopy before decaying to DippNCNXyl and 116b. Reaction of 116b with CS2 formed the thermally stable k2N,S-thiocarbamate [Ti(COT){N(Dipp)C(S)S}] (124) which was isolated. These reactions of 116a and 116b with heterocumulenes are rather characteristic of group imides in general. The reactions of [Ti(COTR)(NtBu)] (COTR ¼ COT (116a), COTTMS (116a)) or [Ti(COT)(NDipp)] (116b) with tert-butylphosphaalkyne are summarized in Scheme 22 and show divergent behavior depending on the imido substituent.91 With 2 equiv. tBuCP the metallacyclic compounds [Ti(COTR){N(tBu)PC(tBu)PC(tBu)}] (125a–b) were formed, presumably via an initial [2 + 2] cycloaddition product [Ti(COT){N(tBu)PC(tBu)}] (not observed) followed by insertion of a further tBuCP into the TidC bond of the metallacycle. Thermally stable [Ti(COT){N(tBu)PC(tBu)PC(tBu)}] (125a) was studied by PES and DFT calculations which found a non-bonding, ligand-centered HOMO. In contrast, [Ti(COT)(NDipp)] (116b) reacts only slowly with t BuCP and an excess of the reagent was required to drive the reaction to completion. No metal-containing products could be identified, but the 1,2,4-azadiphosphole 126 was separated by high vacuum sublimation. DFT studies suggest that the reaction sequence to form 126 is analogous to that for 125a–b, but that in the case of 125a–b elimination of the corresponding 1,2,4-azadiphosphole is thermodynamically unfavorable because the product 1,2,4-azadiphosphole would have adjacent tert-butyl groups at the 1- and 5-positions. Reaction of [Ti(COTTMS)(NtBu)] (117a) with tBuNC formed the thermally stable adduct [Ti(COTTMS)(NtBu)(CNtBu)] (127b, Scheme 23) which was isolated and fully characterized. The homologous arylimides [Ti(COT)(NDipp)(CNR)] (R ¼ Me, Xyl, tBu) were likewise formed from 116b with the corresponding RNC, but tended to lose the Lewis base under reduced pressure except in the case of R ¼ tBu. [Ti(COT)(NtBu)] (116a) formed an isolable adduct, [Ti(COT)(NtBu)(py)] (128), with pyridine which was crystallographically characterized. The reaction of 116a with tBuNC in C6D6 formed an adduct [Ti(COT)(NtBu)(CNtBu)] (127a) analogous to 127b and 128 but lost the isocyanide under vacuum and so could not be isolated. The corresponding reaction of 116a with MeNC and XylNC gave unknown mixtures. On standing at room temperature, first-formed solutions of [Ti(COT)(NtBu)(CNtBu)] (127a) decayed over several days forming di-tert-butylcarbodiimide and unknown side-products. However, addition of COT (which does not independently react with 116a) to the mixture allowed the metal side-product to be isolated as [Ti(COT)(Z4-COT)] (88). The proposed reaction mechanism27 proceeds via an intermediate metal-bound carbodiimide species [Ti(COT)(Z2-tBuNCNtBu)] which eliminates tBuNCNtBu forming transient “Ti(COT)” which is subsequently trapped by the added COT forming 88. [Ti(COT)(NtBu)] (116a) reacts with various Brønsted acids to eliminate tert-butylamine in an analogous manner to its reactions with anilines mentioned earlier. Reaction with H2O forms the oxo dimer 121a, and pinacol forms the alkoxide 129 (Fig. 5). Surprisingly, reaction with 2 equiv. tBuSH immediately forms two homoleptic titanium products, namely [Ti(StBu)4] and [Ti(COT) (Z4-COT)] (84), apparently by redistribution of a first-formed bis(tert-butylthiolate) [Ti(COT)(StBu)2]. Reaction of 116a–b with

Larger Aromatic Complexes of the Group 4 Metals

629

AlMe3 gives the adducts [Ti(COT){N(R)AlMe2(m-Me)}] (R ¼ tBu (130a) or Dipp (130b)) which were isolated but lost AlMe3 in vacuo. DFT calculations support the proposed structures which are consistent with NMR data. Hill and Smith used [Zr(COT)Cl2(THF)] (103) for the preparation of new zirconium COT-poly(azolyl)hydroborate complexes (Scheme 24).92 Reaction of 103 with K[HB(Me2pz)3] (Me2pz ¼ N,N0 -dimethylpyrazolyl) afforded [Zr(COT){k2N,N0 -HB(Me2pz)3} Cl] (131), as confirmed by X-ray crystallography.

Cl 132

N N N N

K[HB(Me2pz)3]

K[H2B(pz)2]

H Zr

H Zr

Zr Cl

B H

Cl

THF

Cl

N N

B

N N

N N 131

103

Na[H2B(mt)2]

N Zr Cl

N

S

B

S N

133

H H

N Scheme 24

Compound 131 contains a 16 valence electron metal despite the presence of additional donors (the N of the non-coordinating Me2pz or the BdH group) on k2N-HB(Me2pz)3. This result was surprising, given that tris(pyrazolyl)hydroborate ligands are often considered to be CpR mimics and the long-known30 mixed-sandwich [Zr(COT)(Cp )Cl] (97) has an 18 valence electron zirconium and Z5-Cp . The analogous compounds [Zr(COT){k2N,N0 -H2B(pz)2}Cl] (132, pz ¼ pyrazolyl) and [Zr(COT){k2S,S0 -H2B(mt)2} Cl] (133, mt ¼ methimazolyl) also have three-legged piano stool geometries that were confirmed by X-ray crystallography. Reaction of 103 with K[H2B(Me2pz)2] did not lead to an isolable analogue of 132, and instead trace amounts of the cluster [Zr5(COT)4(m-O)4(m-Cl4)] were obtained, apparently due to trace hydrolysis. Girolimi et al. have prepared a series of mono-, di- and tri-alkyl half-sandwich complexes (Scheme 25) from [M(COT)Cl2(THF)] (M ¼ Zr (103), Hf (104)).93 The authors stress that their [M(COT)Cl2(THF)], prepared from MCl4 and K2COT (a route initially described by Spencer78) without attempted separation of the salt side-products, contains KCl, but this does not appear to impact the chemistry. Reaction of [M(COT)Cl2(THF)] with 3 equiv. LiR (R ¼ Me (Zr only), Ph or Tol (Tol ¼ 4-C6H4Me)) in the presence of TMEDA affords the “ate” complexes [Li(TMEDA)2][M(COT)R3] (M ¼ Zr (134), Hf (135); R ¼ Me (a), Ph (b), Tol (c)). In contrast, reaction with 2 equiv. of the lithiated bulky alkyl LiCH(TMS)2 afforded the neutral dialkyls [M(COT){CH(TMS)2}2] (M ¼ Zr (136), Hf (137)) provided that the products were crystallized directly from the reaction mixture. The solid state structure of 136 showed a symmetrically coordinated but distorted COT ring (bent from local D8h symmetry to C2v), due to the steric bulk and/or strong trans influence of the CH(TMS)2 ligands. Crystallization of 136 after addition of TMEDA gave the “ate” complexes [Li(TMEDA)2] [M(COT){CH(TMS)2}2Cl] (138), apparently analogous to compounds 134 and 135. Reaction of 103 with 1 equiv. LiTol and 1 equiv. TMEDA also gave an “ate” compound, namely the monotolyl derivative [Li(TMEDA)][M(COT)(Tol)Cl2] (139).The X-ray structure of 139 confirmed the presence of the [M(COT)(Tol)Cl2]− anion and the [Li(TMEDA)]+ was bound to the two ZrdCl halides forming a tetrahedral coordination environment at lithium.

Scheme 25

630

Larger Aromatic Complexes of the Group 4 Metals

4.12.3.2.2

Sandwich complexes

Reaction of the unusual bis(alkylidene) 140 with 1 equiv. COT (Eq. 16) gave [Ti(COT){Z4-1,2-C4Me2(TMS)2}] (141).94 Compound 141 is the second example of a crystallographically characterized group 4 mixed COT-butadiene compound, the other being van Oven’s 1973 [Ti(COT)(Z4-C4Ph4)] (96)79 (1,2- and 1,3-phosphabutadiene COT-supported analogues have also been structurally authenticated95).

ð16Þ

Starting from [Zr(COT)Cl2(THF)] (103), two new mixed sandwich complexes, namely [Zr(COT)(Z5-1,3-C5H3R2)Cl] (R ¼ tBu (142a), TMS (142a)) were prepared from the potassiated respective ligands. Both compounds were crystallographically characterized.96 Reaction of 142a with PhLi formed [Zr(COT)(Z5-1,3-C5Ht3Bu2)Ph], which was also structurally authenticated. Cloke et al. showed that reaction of 2 equiv. of the bulky K2COTTiPS with MCl4(THF)2 forms [M(COTTiPS)(Z4-COTTiPS)] in situ.86 Subsequent treatment with further [MCl4(THF)2] afforded the dichlorides [M(COTTiPS)Cl(m-Cl)]2 (143; M ¼ Zr (a), Hf (b)) which exist as chloride-bridged dimers. This redistribution protocol is similar to that previously reported by Floriani for other COTR ligands.83 Reaction of 143 with KCp (Scheme 26) gives the corresponding bent mixed-sandwich derivatives [M(COTTiPS) (Cp )Cl] (110; M ¼ Zr (a), Hf (b)), which have been crystallographically characterized. The reduction chemistry of 110a–b was described above and summarized in Scheme 19. The mixed-ring compound [Zr(Z8-COTTMS)(Z8-PnTMS)] (178) is described in the Pentalene section of this chapter.97

Scheme 26

An interesting ring expansion reaction was reported by Bazan in which TMSCCH or C2H2 undergo a net insertion reactions into a BdC bond of the bis(boratabenzene) complex 144 (Eq. 17).98 The so-formed Z8-C7H6B(Me)R (boratacyclooctatetraene) ligands are formally 3-moieties. Since boratabenzenes carry a formal 1-charge, compounds 145a–b are Ti(+4) 16 valence electron isoelectronic analogues of the [Ti(CHTR)(CpR)] family of mixed-sandwich cycloheptatrienyl compounds described in an earlier section. Compounds 145a and 145b have been crystallographically characterized. For the reaction of 144 with C2H2 double-insertion is possible (forming 146). The ratio 145a:146 is dependent on the alkyne pressure, with lower pressure conditions favoring single alkyne insertion (145a). The regioselectivity of the TMSCCH insertion reaction was attributed to electronic factors. The proposed mechanism involves initial substitution of the CO in 144 by RCCH and then insertion into one of the boratabenzene rings by cycloaddition and ring-expansion. For 146 two C2H2 units are oxidatively coupled at titanium prior to the coupling/insertion reaction with one of the Z6-C5H5BMe rings.

ð17Þ

Larger Aromatic Complexes of the Group 4 Metals

631

Reaction of the tris(anthracene) species [Hf(C14H10)3]2− with COT forms the dianion [Hf(Z4-C8H8)(Z3-C8H8)2]2− which was crystallographically characterized.99 As a 16 valence electron species the authors comment that it is unclear why a tris(Z4-C8H8)3 isomer is not formed instead.

4.12.4

Pentalene complexes

Unlike the chemistry of cyclooctatetraene complexes of group 4 which was very well developed by the start of 2000, that of the related pentalene ligands was still very much in its infancy. The first transition metal pentalene compounds, Li2[Fe(Z5-Pn)2] (Pn ¼ C8H6) and Li[Fe(Z5-Pn)(Z5-HPn)],100 were reported by Katz and Rosenberger in 1963, just one year after the first reported synthesis of Li2Pn by the same authors.101 However, the founding communication from Jonas et al. describing the first examples of group 4 sandwich, half-sandwich and mixed-sandwich Z8-Pn complexes, only appeared in 1997,102 with an additional short note103 and theoretical study of [Ti(Z8-Pn)2] following the next year.104 The relatively poor development in general of the organometallic chemistry of pentalene can be attributed to the synthetic challenges in working with this ligand. Recent reviews have covered the general evolution of transition metal pentalene chemistry.105–108 We begin with a summary of the important pentalene ligand developments during the period covered by this review since they are a key contributor to the chemistry of this ligand for group 4. Given the proximity in time of Jonas’s 1997 and 1998 reports102,103 to the start of the period covered by this chapter, that research is folded into the post-2000 developments described in the following sections. Although the main focus of this chapter is on complexes containing formally dianionic C8R2− 6 ligands, hydropentalenyl (C8R6H) complexes are included where contextually relevant. From a bonding analysis perspective, pentalene can be viewed conceptually as being derived either from cyclooctatetraene by transannular pinching, or from two Cp rings by the fusion of two edges. The relationship between the frontier molecular orbitals of Pn and COT, or between Pn and two fused Cp rings, have been described by Green107 and King.109

4.12.4.1

Ligand developments

Fig. 6 gives a summary of the pentalene or pentalenide(2-) ligands discussed in this chapter, along with the ring carbon numbering system110 and the definitions of some structural parameters (fold angle (f) and inter-ring torsion angle y for [M(Z8-PnR)2] sandwich compounds). Pentalene (Pn, C8H6), like its relative COT, is an 8 p-electron Hückel anti-aromatic molecule when planar. Whereas COT distorts to form its familiar “tub” shaped geometry (D2d) and is stable to dimerization or polymerization, unsubstituted Pn (C2h symmetry) rapidly dimerizes at temperatures above −196  C, and has only recently been isolated in argon matrices.111 Ring-substitution confers stability on Pn. For example, C8Ph6 (PnPh6)112 and 1,3,5-C8Ht3Bu3 (PntBu3)113,114 have been known for many decades. t

Bu

t

Bu t

H2Pn

PntBu3

Pn

Bu Pn*_iso

SiR3 2

2

2

R3Si [Pn]

6

6a

R = Me [PnTMS]2 R = iPr [PnTiPS]2

2

[Pn*]2

θ

1 φ

5

2 4

3a

3

Ring numbering

[M] Fold angle (φ)

Fig. 6 Guide to pentalenes and pentalenide dianions and definition of structural parameters.

Torsion angle (θ)

632

Larger Aromatic Complexes of the Group 4 Metals

While Pn itself is unattractive as a starting material, H2Pn (various isomers) can be obtained by flash vacuum pyrolysis (FVP) of dihydrodicyclopentadiene with concomitant C2H4 elimination, or more commonly from COT at ca. 550–650  C.101,115,116 Although H2Pn is unstable to spontaneous polymerization, it may be stored at −78  C. The dianion [Pn]2− is readily made from the pools of various isomeric dihydropentalenes H2Pn with nBuLi in heptane101 forming Li2Pn (147) or, in the presence of DME, giving the crystallographically-characterized Li2Pn
2(DME) (147_DME).117 Li2Pn has been used successfully in the synthesis of group 4 and group 5 complexes by Jonas and co-workers.102,118 Optimization of the FVP and lithiation conditions by Cloke et al. permitted the synthesis of Li2Pn
2(DME) (147_DME) in 87% yield (based on COT) on a 25 g scale.116 Quenching 147_DME with TMSCl gave a mixture of rac- and meso-1,4-C8H6(TMS)2 (H2PnTMS, 148), while reaction with TiPSOTf (TiPS ¼ SiiPr3) gave meso-1,4-C8H6(TiPS)2 (H2PnTiPS, 149). These new dihydropentalenes give access to the metallated pentalenides Li2PnTMS
2(DME) (150) or K2PnTiPS (151) in excellent yields and have allowed the development of new group 4 (and f- and d-element in general) organometallic pentalene chemistry.105–107 In a different approach to pentalene ligand development, Ashley and O’Hare reported an elegant, bottom-up synthesis of 2,3-C8HMe5(CH2) (Pn _iso, Fig. 6).119 Stepwise reaction with LS-Selectride (Li[HB{CH(Me)iPr}3]) and nBuLi/TMEDA afforded Li2Pn
2(TMEDA) (152).37 This new pentalene ligand and its derivatives have also facilitated important developments in group 4 organometallic chemistry.106–108

4.12.4.2 4.12.4.2.1

Complexes with a pentalene ligand h8-coordinated to one metal Half-sandwich titanium compounds

Reaction of K2PnTiPS (151) with [TiCl3(THF)3] afforded the mixed valence dimer [{Ti(PnTiPS)}2(m-Cl)3] (153) containing Ti(+3) and Ti(+4) centers (Eq. 18).120 The magnetic moment determined by the Evans method and SQUID magnetometry was consistent with one unpaired electron per dimer, and supported by EPR measurements. High level disorder prevented precise determination of the X-ray structure but the data clearly established the connectivity shown.

ð18Þ

Reaction of Li2Pn
x(TMEDA) (152; value of x not specified by the authors) with various Ti(+4) reagents gave undefined ligand decomposition products due to the reducing nature of Pn 2−.121 The corresponding reaction with [TiCl3(THF)3] followed by treatment with PbCl2 to oxidize the putative Ti(+3) intermediate, not observed, afforded the desired Ti(+4) half-sandwich dimer [Ti(Pn )Cl(m-Cl)]2 (154) but in only 8% yield. A softer reagent for the synthesis of 154 was the distannylated derivative cis1,4-C8Me6(SnMe3)2 (Pn SnMe3) (155). Reaction of 155 with [TiCl4(THF)2] afforded 154 in 70% yield (Eq. 19).121 The X-ray structure of 154 confirms that shown in Eq. (19) with two terminal TidCl bonds and two bridging TidCldTi units, giving each titanium a 16 valence electron count and a three-legged piano stool local geometry. Dimerization of the monomeric [Ti(Pn )Cl2] (14 valence electrons) units clearly reduces the coordinative unsaturation and electron deficiency. The fold angle of ca. 32o (see Fig. 6 for definition) was in the expected range and shows how the Pn ligand flexes at the C3adC6a bond (bridgehead carbons) to optimize interactions with the metal. The NMR spectra of 154 are consistent with fast rotation of the Pn ligand relative to the rest of the complex on the NMR timescale, even at −80  C.

ð19Þ

The related reaction of TiCl4 with 1,10 ,4,40 -C8H4(SiMe3)4 in an attempt to form [Ti(PnTMS)Cl2] or its dimer, the analogue of 154, by elimination of 2 equiv. SiClMe3 gave instead the complex 156 (Eq. 20), which contains a Z5-bound tris(trimethylsilyl)pentalene(1-) ligand.122

Larger Aromatic Complexes of the Group 4 Metals

633

ð20Þ

Two or all of the chloride ligands in [Ti(Pn )Cl(m-Cl)]2 (154) can be substituted by alkoxide or alkyl groups as summarized in Scheme 27.123,124 Reaction with 1 equiv. KOR (per Ti) forms the monoalkoxide derivatives [Ti(Pn )Cl(OR)] (157; R ¼ tBu (a), Xyl (b), 2,4-C6Ht3Bu2 (c)) in ca. 55% yield. With 2 equiv. KOR the corresponding bis(alkoxide) species 158 were formed for R ¼ tBu (a, NMR tube scale only) or Xyl (b, 62% isolated). It was noted that the tert-butyl complexes 157a and 158a are highly air-sensitive. NMR spectra and X-ray structural data were consistent with the proposed half-sandwich monomeric complexes shown in the Scheme; Pn fold angles of ca. 35-36o were reported. The high temperature 1H NMR spectra for 157b showed Pn ring rotation relative to the metal center (DG{ ca. 18.7 kcal mol−1).124

Scheme 27

Reaction of 154 with 2 equiv. methyl or benzyl Grignard per titanium gave high yields of the respective dialkyl compounds [Ti(Pn )R2] (155; R ¼ Me (a), Bn (b)).123 Both compounds were crystallographically characterized, with the structure of 155b showing a relatively small TidCH2dPh angle (ca. 102o) due to an interaction of the Ph ring p system with the 14 valence electron metal. Both compounds react with CO2 (155b being the slower of the two) by insertion into both TidC bonds to form the dicarboxylate species [Ti(Pn )(k2O-O2CR)2] (159; R ¼ Me (a), Bn (b)) in high yields. The X-ray structures for 159a–b supported the solid state IR data which indicated that the O2CR ligands in all cases are symmetrically (k2O-) bound, and confirmed the four-legged piano stool geometries illustrated in Scheme 27. Pn fold angles in the range ca. 32–36o were reported for 155a–b and 159a–b, with the smallest being for 159a–b and the largest for 155a. The coordination of the Pn ligand relative to the Ti(k2O-O2CR)2 moiety is unsymmetrical, and at lower temperatures NMR data indicate that the C2 symmetry found in the solid state is maintained in solution. Warming to 30  C gave NMR spectra consistent with average C2v symmetry due to rapid Pn ring rotation on the NMR timescale. O’Hare and co-workers have tested a number of dialkyl compounds [Ti(Pn )R2] (R ¼ Me (155a), Bn (155b), CH2EMe3 (E ¼ C or Si, mentioned in a review107)) for ethylene polymerization using borane or borate-based activators. Despite, or perhaps because of, the high electron deficiency of these compounds, only moderate activity was observed.125

634

Larger Aromatic Complexes of the Group 4 Metals

4.12.4.2.2

Half-sandwich zirconium and hafnium compounds

In their founding contribution to group 4 pentalene chemistry, Jonas et al. found that the redistribution reaction of homoleptic [Zr(Pn)2] (176a, see below) with [ZrCl4(THF)2] in THF at 20  C gave the half-sandwich species [Zr(Pn)Cl2(THF)2] (160) in 90% yield (Eq. 21).102 Structurally characterized compound 160 has a monomeric four-legged piano stool geometry with C2v symmetry; the Z8-coordinated Pn ligand has a fold angle of just under 130o.

ð21Þ

O’Hare and co-workers have reported half-sandwich zirconium and hafnium dichloride complexes with Z8-Pn ligands, namely the “ate” dimers [{M(Pn )}2(m-Z1,Z1-{LiCl2(THF)2})(m-Cl)3] (M ¼ Zr (161), Hf (162); Eq. 22) in 50% and 76% yield, respectively.121 Compounds 161 and 162 were characterized by X-ray crystallography which confirmed the incorporation of a m-Z1,Z1-{LiCl2(THF)2} moiety into the structure as shown. Attempts to remove the THF and hence promote separation of LiCl led to decomposition on heating at 130  C under high dynamic vacuum. The approximate four-legged piano stool geometries at the metal centers are analogous to that in monomeric [Zr(Pn)Cl2(THF)2] (160) and the fold angles (ca.130o) for the Pn ligands in 161 and 162 are the same within error to that in 160. The Pn ligand in 161 undergoes only slow rotation relative the rest of the complex on the NMR timescale at room temperature (moving to the fast exchange regime at ca. 42  C), whereas the Pn ligands in 162 (like those in [Ti(Pn )Cl(m-Cl)]2 (154)124) were in fast rotation on the NMR timescale down to −80  C.

ð22Þ

A number of half-sandwich pentalene zirconium and hafnium with non-cyclic hydrocarbyl co-ligands have been reported, showing rather different behavior in some cases.103,126 Jonas reported the first example of a compound in this class. Thus the bis(allyl) compound [Zr(Pn)(Z3-C3H5)2] (163; Fig. 7) was prepared from [Zr(Pn)(Cp)Cl] (185, see below) and 2 equiv. allyl lithium,

Fig. 7 Synthesis and fluxionality of compound 163.

Larger Aromatic Complexes of the Group 4 Metals

635

eliminating LiCl and LiCp.103 Compound 163 was not crystallographically characterized but its solution structure was studied in detail by NMR spectroscopy, including NOESY and 2D EXSY. Molecules of 163 have Z3-allyl ligands, and the central (meso) CdH is oriented “up” towards the Pn ligand. The overall molecular symmetry is C2, with the [Zr(Z3-C3H5)2] moiety twisted out of alignment (cf. Fig. 7) with either of the local mirror planes that bisect the bridgehead (C3a, C6a) or wingtip (C2, C5) carbons of C2v-Pn. This arrangement is reminiscent of the relationship between the Z8-bound rings in homoleptic compounds of the type [M(PnR)2] discussed later on in this section. A dynamic process exchanges the two C2 enantiomers of 163 in solution as illustrated in Fig. 7. The allyl ligands also undergo Z3–Z1–Z3 exchange, a process which may be geared with enantiomer interconversion. Compound 163, along with the dichloride [Zr(Pn)Cl2(THF)2] (160), has been cited in a patent for applications in ethylene polymerization but both show rather modest activities.127 A detailed study of the reaction of [{M(Pn )}2(m-Z1,Z1-{LiCl2(THF)2})(m-Cl)3] (M ¼ Zr (161), Hf (162)) with alkyl, allyl and phenyl Grignard reagents has been described by O’Hare and co-workers (Scheme 28).126

Scheme 28

Reaction with 2 equiv. C3H5MgCl per group 4 metal gave the bis(allyl) species [M(Pn )(Z3-C3H5)2] (M ¼ Zr (164), Hf (165)) in 60–70% yield. The X-ray structure of 164 is consistent with that proposed by Jonas103 for the homologous [Zr(Pn)(Z3-C3H5)2] (163), with exo-oriented Z3-C3H5 groups and a [Zr(Z3-C3H5)2] moiety rotated with respect to the Z8-Pn giving a compound with overall C2 symmetry. The dynamic NMR processes for 164 match those for 163. Compounds 164 and 165 react with CO2 in C6D6 on the NMR tube scale by insertion into one of the MdCH2 bonds to the allyl ligand, presumably via an intermediate with a Z1-bound allyl group. The products, [M(Pn )(k2O-O2CCH2CHCH2)2] (M ¼ Zr (166), Hf (167)) are analogous to [Ti(Pn ) (k2O-O2CR)2] (159; R ¼ Me, Bn).123 Reaction of 161 and 162 with 2 equiv. BnMgCl per metal center gave the expected [M(Pn )Bn2] (M ¼ Zr (168), Hf (169)). The X-ray structure of 168 showed very acute ZrdCH2dPh bond angles (ca. 84 and 95o) as a consequence of both phenyl groups having additional interactions with the formally 14 valence electron metal center. These interactions are more pronounced than in the titanium congener [Ti(Pn )Bn2] (155b),123 as would be expected based on the size of the metals. Further studies of the reaction in the case of 169 gave the separated ion pair [Hf(Pn )Bn3][Li(1,4-dioxane)4] (170) after addition of 1,4-dioxane. Compound 170 was crystallographically characterized. The Pn fold angle of 25 o in the [Hf(Pn )Bn3]− anion is smaller than in [Zr(Pn )Bn2] (168) because of more adverse steric and Coulombic factors in the former. The reaction of 161 and 162 with 3 equiv. PhMgCl (Scheme 28) followed by addition of 1,4-dioxane gave anionic group 4 anions, [M(Pn )Ph3]− as part of the ion pairs [M(Pn )Ph3][Li(1,4-dioxane)] (M ¼ Zr (171), Hf (172)). In contrast to ion pair-separated 170, there are close interactions in 171 in the solid state between each lithium cation and the ipso-carbons of two of the phenyl groups, forming chains of the type {[M(Pn )Ph(m-Ph)2][Li(m-1,4-dioxane)]}n. In solution the anion and cation are either separated (as depicted in Scheme 28), or the [Li(1,4-dioxane)]+ cation is rapidly interchanging between pairs of Zr-bound phenyl groups on the NMR timescale. In contrast to the methylation of [Ti(Pn )Cl(m-Cl)]2 (154) which gave thermally stable Ti(Pn )Me2 (155a, Scheme 27),123 the reactions of 161 and 162 with 2 equiv. MeMgI per metal center gave either the m-dimethyl, m-methylidene species [{Zr(Pn )}2(m-CH2)(m-Me)2] (173) or the thermally labile [Hf(Pn )Me2] (174) which decomposes within hours to unknown products, accompanied by the formation of CH4. Compound 173 was crystallographically characterized and the 13C NMR spectra showed

636

Larger Aromatic Complexes of the Group 4 Metals

resonances for the m-CH2 and m-Me groups at 127.9 and 38.9 ppm, respectively. The different outcomes for the methylation reactions of 161 and 162 was attributed to different rates of substitution vs dehydrohalogenation for zirconium and hafnium in these systems.126

4.12.4.2.3

Homoleptic sandwich and mixed-sandwich compounds

The series of homoleptic pentalene complexes [M(Pn)2] (M ¼ Ti (175a), Zr (176a), Hf (177a)) were reported by Jonas et al., prepared according to Eq. (23). The corresponding compounds [M(PnMe)2] (M ¼ Ti (175b), Zr (176b), Hf (177b)) with 2-methylpentalene (PnMe) in place of Pn were also prepared.102 It was not possible to characterize any of the compounds [M(PnR)2] crystallographically. NMR spectra for the 2-methylpentalene complexes established that the inter-ring torsion angle, y, between the two rings lies between 0 and 90o (see Fig. 6 for definition)—in other words, the two PnR rings are neither eclipsed (y ¼ 0o) nor mutually perpendicular (y ¼ 90o).

ð23Þ

At first sight, 175–177 appear to be 20 valence electron compounds. However, theoretical and PES studies have concluded that one pair of electrons is in a ligand-based non-bonding orbital.104,109,128 In addition, the DFT geometry optimization by Gleiter et al. on optimized [M(Pn)2] support the conclusions from the NMR studies that the torsion angles y in these systems (ca. 149–157o) lie between the y ¼ 0o (D2h) or 90o (D2d) extremes.128 A theoretical paper by King has advanced an explanation, based on different extents of back-bonding, for the observation that while bis(pentalene) group compounds adopt symmetric [M(PnR)2] structures, the closely related bis(cyclooctatetraene) systems adopt an unsymmetrical [M(COTR)(Z4-COTR)] arrangement. It is interesting to note that the mixed-sandwich compound [Zr(Z8-COTTMS)(Z8-PnTMS)] (178, see below) has both the pentalene and the cyclooctatetraene ligand in an octahapto arrangement.97 In 2013 Cloke reported that reaction of K2PnTiPS (151) with TiCl2 in THF resulted in the sandwich compound [Ti(PnTiPS)2] (179)129 and titanium metal in an apparent disproportionation reaction.120 More recently, the permethylated analogues [M(Pn )2] (M ¼ Zr (180), Hf (181)) were prepared according to Scheme 29.126 Surprisingly, the authors found that treatment of 161 with Li2Pn
x(TMEDA) gave no reaction, even under forcing conditions. Interestingly, reaction of 161 with an excess of LiAlH4 gave [Zr(Pn )2] (180), albeit in low yields. In contrast, reaction of [HfCl4(THF)2] with an excess of Li2Pn
x(TMEDA) in hot toluene formed [Hf(Pn )2] (181). Compounds 180 and 181 were structurally characterized. The anticipated Z8-Pn coordination was confirmed, with a torsion angle (y) of ca. 42 and 43o between the Pn ligands. These are smaller than the computed angles for unsubstituted [M(Pn)2] (M ¼ Zr, 152o; Hf, 151o) with the differences being attributed to steric factors.126

Scheme 29

Mixed-sandwich compounds of titanium and zirconium with Z8-PnR (PnR ¼ Pn or PnMe) and Z5-Cp ligands were first reported by Jonas.102 The synthetic routes are summarized in Scheme 30 for the Pn complexes. The paramagnetic Ti(+3) complex [Ti(Pn)(Cp)] (182), made by reaction of [Ti(Cp)Cl(m-Cl)]2 with Li2Pn, serves as an intermediate en route to the Ti(+4) species [Ti(Pn)(Cp)X] (183; X ¼ Cl (a), Br (b)) by oxidation with dichloro- or dibromo-ethane. The indenyl analogue of 183a, [Ti(Pn)(Ind)Cl] (184), has been claimed in the patent literature as an ethylene polymerization catalyst.127 The zirconium congener of 183a, namely 185, was prepared directly from [ZrCp2Cl2] and Li2Pn with elimination of LiCl and LiCp. Compounds 182, 183a and 185 were crystallographically characterized, confirming the Cs symmetry depicted, with 183a and 185 being isomorphous. The Pn hinge angles are 37.2, 33.7 and 32.7o, respectively, reflecting the changes in coordination number and atomic radius.

Larger Aromatic Complexes of the Group 4 Metals

637

Cl Cl 0.5

+

Ti

Ti

Ti

Li2Pn

Cl Cl 182

0.5 C2H4X2

Cl Zr

+

Ti

Li2Pn

Cl X M = Ti (183); X = Cl (a), Br (b) M = Zr, X = Cl (185) Scheme 30

O’Hare and co-workers have developed considerably the scope of pentalene complexes for olefin polymerization based on Pn complexes,130,131 following initial disclosures in the patent literature for half-sandwich and mixed-sandwich complexes by Jonas127 and Cloke132 based on Pn and PnTMS ligands, respectively. In initial studies (Eq. 24)133 it was shown that 2 equiv. NaCp (1 equiv. per metal) react with [Ti(Pn )Cl(m-Cl)]2 (154) and [{M(Pn )}2(m-Z1,Z1-{LiCl2(THF)2})(m-Cl)3] (M ¼ Zr (161), Hf (162)) in high-yielding reactions to form the mixed-sandwich derivatives [M(Pn )(Cp)Cl] (M ¼ Ti (180); Zr (181a); Hf (182a)).133 In the presence of 5 equiv. NaCp the mixed-hapticity cyclopentadienyl homologues [M(Pn )(Cp)(Z1-Cp)] (M ¼ Ti (183); Zr (184); Hf (185)) were obtained. These undergo haptotropic shifts within the Z1-Cp ring and also exchange between the Z1-Cp and Z5-Cp ligands, as the temperature is increased. For 161 and 162 the method could be extended to [M(Pn )(Cp )Cl] (M ¼ Zr (181b); Hf (182b); Eq. 24) whereas it failed for titanium.

ð24Þ

Detailed solution phase ethylene polymerization studies of [M(Pn )(CpR)X] (M ¼ Zr, Hf; CpR ¼ Cp, Cp ; X ¼ Cl, Z1-Cp) with MAO activation (MAO ¼ methylaluminoxanes) found that the sterically crowded [M(Pn )(Cp )Cl] exhibited rather poor activity, and for the other systems the best activity was found for the zirconium congeners 181a–b. Subsequently, a polymerization study by the same researchers on a wide range of compounds of the type [M(Pn )(CpR)X] and [M(Pn )(Ind)Cl] (M ¼ Ti, Zr; CpR ¼ Cp, Cp0 , C5Hn4Bu, C5Ht4Bu, 1,2,3-C5H2Me3; X ¼ Cl, Me) immobilized on various catalyst supports was reported.131 The slurry-phase polymerization activities using catalysts supported on solid MAO gave substantially improved performances compared to the equivalent solution phase systems.

638

Larger Aromatic Complexes of the Group 4 Metals

The interesting mixed eight-carbon ring compound [Zr(Z8-COTTMS)(Z8-PnTMS)] (178) mentioned earlier was prepared in 22% yield by reaction of Li2PnTMS
2(DME) (150) with [Zr(COTTMS)Cl(m-Cl)]2 (105; Eq. 25).97 The explanation for the apparent 20 valence electron count at zirconium in 178 may be analogous to that given for bis(pentalene) compounds.104,109,128

ð25Þ

Reduction of [{M(Pn )}2(m-Z1,Z1-{LiCl2(THF)2})(m-Cl)3] (161) with 4 equiv. C8K in benzene afforded the triple-decker inverted sandwich compound [{Zr(Pn )}2(m-Z6,Z6-C6H6)] (186; Eq. 26).134 X-ray crystallography confirmed the structure and showed that the Pn ligands are twisted relative to each other by ca. 45o. DFT calculations found this twisting was electronic in origin and confirmed the “twist-boat” deformation of the bridging benzene ring. A low temperature NMR spectrum found that the D2 geometry is maintained in solution. Edge energies in the zirconium K-edge XANES spectra for 186 could not differentiate between 186 and Zr(+2) and Zr(+4) reference compounds, and the DFT calculations found the bonding to be highly covalent with frontier molecular orbitals showing almost equal contributions from the bridging benzene and the ZrPn fragments.

ð26Þ

The reduction reaction of 161 was successfully extended to other arenes: toluene, 1,2- and 1,3-xylene and C6Hi5Pr. However, the sandwich compound [Zr(Pn )2] was increasingly formed as a side-product as the steric demands of the arene increased, a process reminiscent of the reaction of K2PnTiPS (151) with TiCl2, which formed [Ti(PnTiPS)2] (179) as discussed above.120 Neither 186 nor its m-Z6,Z6-toluene homologue exchanged the bound arene with toluene or benzene solvent, respectively. Similarly, 186 did not react with a range of small molecules such as H2, CO and C2H4, indicating a high degree of kinetic stability.

4.12.4.3

Complexes with a hydropentalene ligand h5-coordinated to one metal

The 1-hydropentalene ligand (HPn ¼ 1-C8R6H; R ¼ H or other) is closely related to cyclopentadienyl. Group 4 complexes with a Z5-HPnR ligand have been made either directly from silyl- or stannyl-functionalized pentalenes and a metal halide precursor, or by ring-protonation of a metal-bound pentalene ligand. As mentioned earlier (Eq. 20), reaction of TiCl4 with 1,10 ,4,40 C8H4(SiMe3)4 gave the tris(trimethylsilyl)pentalene compound [Ti{1,3,6-C8H4(SiMe3)3}Cl3] (156) with SiClMe3 elimination instead of the desired [Ti(PnTMS)Cl2] (or its dimer).122 Turner, O’Hare et al. have used the stannylhydropentalene 1,4-C8Me6H(SnMe3) (187) as an entry point to hydropermethylpentalene complexes of group 4.135 Compound 187 itself was prepared as a 50:50 mixture of two isomers (C1 and C4 are chiral centers) by treating LiHPn with SnClMe3. The transmetallation reaction of 187 with [TiCl4(THF)2] or MCl4 (M ¼ Zr, Hf ) gave the Z5-hydropentalene complexes [Ti(HPn )Cl3] (188) and [M(HPn )Cl2(m-Cl)]2 (M ¼ Zr (189), Hf (190)) in high yield (Scheme 31). Compound 188 is a three-legged piano stool whereas the larger metals form Cl-bridged dimers to give four-legged piano stool coordination environments. The C1 methyl group is oriented anti to the metal center in all cases for steric reasons.

Scheme 31

Larger Aromatic Complexes of the Group 4 Metals

639

Compounds 188–190 have been used to prepare a series of alkoxide complexes of the type [M(HPn )Cl3-x(OR)x] (191–197; x ¼ 1–3; R ¼ tBu or aryl) by reaction with the respective KOR salts. These are summarized in Fig. 8.135 The compound [Ti(HPn ) (OXyl)3] (191a) has alternatively been prepared from [Ti(Pn )Cl(m-Cl)]2 (154) by reaction with 1 equiv. XylOH to form [M(HPn ) Cl2(OXyl)] (195) followed by addition of 2 equiv. KOXyl (Scheme 32).124 The 1-methyl group of HPn is positioned anti in 195 which may be a mechanistic consequence of the proton transfer reaction.

Fig. 8 New alkoxide complexes for the ROP of lactide.135

Cl 0.5

Ti

Cl Cl Cl 154

XylOH Ti

H OXyl

Ti Cl

Cl 1 95

2 KOXyl

H OXyl

Ti XylO

OXyl 19 1a

Scheme 32

These alkoxide compounds were evaluated for the ring-opening polymerization of rac- and L-lactide. The rate constants for the ROP were in the general order Zr > Hf > Ti. The compounds 192c and 192d had very fast rates of propagation at 100  C and gave low polydispersities at 60  C. None of the complexes gave control of the stereochemistry in the polylactides produced, and the molecular weights and MALDI-ToF data indicate more than one propagating chain per metal in some cases, as well as transesterification side-reactions. [Zr(HPn )Cl2(m-Cl)]2 (189) has also been used to prepare complexes for olefin polymerization (Scheme 33).130 Reaction of 189 with LiNP(NMe2)3 or LiFlu (Flu ¼ fluorenyl) gave the phosphinimide and mixed-sandwich complexes [Zr(HPn ){NP(NMe2)3} Cl2] (198) and [Zr(HPn )(Flu)Cl2] (199), respectively. When supported on MAO-modified layered double hydroxide supports, both complexes, as well as [Zr(HPn )Cl2(m-Cl)]2 (189), were ethylene polymerization catalysts. At both 60 and 80  C compound 199 showed the highest activity of the three compounds, with the activity at 60  C being the largest. The weight-averaged molecular weight (Mw) of the polyethylenes produced was 730,214 g mol−1 but with a very high polydispersity.

Scheme 33

640

Larger Aromatic Complexes of the Group 4 Metals

4.12.4.4 4.12.4.4.1

The chemistry of [Ti2(m-h5,h5-PnTiPS)2] Synthesis and single-bond activation reactions

Reduction of [{Ti(PnTiPS)}2(m-Cl)3] (153) with K/Hg in hexane gave the Ti(+2) dimer [Ti(m-Z5,Z5-PnTiPS)]2 (200) in 60% yield (Scheme 34).120 This diamagnetic double-sandwich compound has a TidTi bond length of 2.399(2) A˚ , and DFT studies assigned a double bond with s and p components.136 The PnTiPS rings in C2-symmetric 200 are not coplanar but bend back as illustrated in Scheme 34, with the angle a (the average of the PnTiPS ring centroid-Ti-centroid angles at each Ti) being ca. 155o. Compound 200 undergoes a one-electron oxidation with [FeCp2][B(C6F5)4] forming the mixed-valence (Ti(+2)Ti(+3)) cation 200+. The DFT computed S ¼ ½ ground state was confirmed by EPR spectroscopy and SQUID magnetometry. The crystallographically determined TidTi bond length in 200+ is 2.5091(9) A˚ with an angle a of ca. 142o, consistent with the calculations.

Scheme 34

Compound 200 reacts immediately with the dichalcogenides E2Ph2 (E ¼ S, Se, Te) at room temperature to give [Ti2(m-Z5,Z5-PnTiPS)2(EPh)2] (201; E ¼ S (a), Se (b), Te (c); Eq. 27).137 The X-ray structures showed substantially lengthened TidTi bond distances of ca. 2.65 A˚ compared to ca. 2.40 A˚ in 200, consistent with a reduction in bond order from two to one upon oxidative addition of PhEdEPh across the Ti]Ti bond. The two m-Z5,Z5-PnTiPS moieties are quite bent back from the newly formed TidEPh bonds, with a ¼ ca. 134–135o for 201a–c compared to ca. 155o in 200 itself.

ð27Þ

Addition of the N-heterocyclic carbene IMe (1,3,4,5-tetramethylimidazolin-2-ylidene) to 200 led to an intramolecular CdH bond activation of one of the pentalene SiiPr3 substituents, forming the “tuck-in” species 202 (Scheme 35).138,139 In addition to the newly-formed TidC(IMe) and TidC(CH2CH(Me)Si) bonds in 202, a bridging hydride spans the lengthened TidTi single bond (TidTi ¼ 2.5610(8) A˚ ). Compound 200 does not undergo this “tuck-in” reaction on its own, consistent with DFT calculations which found that the reaction is electronically more favorable with the added base. Addition of pyridine to 200 led to a corresponding product with py in place of IMe, but other Lewis bases such as 2,6-lutidine and PMe3 were ineffective. 2,6-dichloropyridine gave a different bond activation reaction, forming the CdCl bond cleavage product [Ti2(m-Z5,Z5-PnTiPS)2(m-Z2-NC5H3Cl)Cl] (203; Scheme 35).

Larger Aromatic Complexes of the Group 4 Metals

641

Scheme 35

Addition of H2 to 202 reverses the intramolecular bond activation and forms the dihydride [Ti2(m-Z5,Z5-PnTiPS)2(m-H)H(IMe)] (204) with a similar TidTi distance to that in 202. The reaction is partially reversible, and labelling studies using D2 formed exclusively [Ti2(m-Z5,Z5-PnTiPS)2(m-D)D(IMe)] (204-d2), showing that a reductive elimination precedes addition of H2 or D2 (i.e. the conversion of 202 to 204 or 204-d2 does not involve a s-bond metathesis reaction). Reaction of 202 with tBuCCH or HCl gave related m-hydride-acetylide or -chloride products, [Ti2(m-Z5,Z5-PnTiPS)2(m-H)X(IMe)] (X ¼ CCtBu (205), Cl (206)). Reaction of 202 with [NHEt3][BPh4] gave protonolysis of the TidC(CH2CH(Me)Si) bond leading to the agostic cation 207+.

4.12.4.4.2

Reactivity with unsaturated substrates

Compound 200 has an extensive reaction chemistry with small unsaturated molecules, as summarized in Schemes 36–39.136,137,140–142 Addition of MeNC formed the 1:1 adduct [Ti2(m-Z5,Z5-PnTiPS)2(m-Z1,Z2-MeNC)] (208; Scheme 36) which was crystallographically characterized (TidTi ¼ 2.412(2) A˚ ; a ¼ ca. 143o).137 No further addition of MeNC occurred in the presence of an excess of the ligand, attributed to steric congestion. Reaction of 200 with an excess of CO (1 atm) at −78  C gave the green-brown dicarbonyl compound [Ti2(m-Z5,Z5-PnTiPS)2(CO)2] (210), which was also obtained as one of the products of CO2 reductive cleavage by 200 (see below).140 Two n(CO) IR bands are seen for 210 (1991, 1910 cm−1) which maintains a short

Scheme 36

642

Larger Aromatic Complexes of the Group 4 Metals

R R O

Ti

C

O

R

R Ti

R

N

+ other products

Ph

R R S

212 CS2 or TolNCNTol

PhNCO

R

Ti

Ti C

or S

R R Tol N

R

213

Ti

Ti C

N Tol

214

R R

R R

R R R

O

CO2 0.5

Ti

Ti

(2 N2O for 209 only)

O R

R 209

R

200

+

Ti

Ti

0.5

R R C O

Ti

Ti

C O 210

COS (1 atm) or 2 Ph3PS

R

Ph2N2 or 2 PhN3 R

R

R = SiiPr3

S Ph N

R

R

S Ti

Ti Ph

R

R 210

N R

Ti

Ti

R

211 Scheme 37

Ti]Ti double bond (2.425(1) A˚ ). A monocarbonyl complex (209) was prepared by the addition of 1 equiv. CO −78  C. Compound 209 is structurally analogous to 208 and has a similar metal-metal bond length (2.4047(5) A˚ ).141 The IR spectrum exhibits a low n(CO) of 1655 cm−1, consistent with a bridging, side-on-bound carbonyl ligand. It was observed that, prior to warming and removal of CO, the solution formed from 200 and an excess of CO (1 atm) was orange-brown in color. Low temperature NMR and IR measurements, supported by DFT calculations, assigned the implied new compound as the tricarbonyl species [Ti2(m-Z5,Z5-PnTiPS)2(m-CO)(CO)2] (211). The bridging CO does not bond symmetrically, and a dynamic equilibrium between two non-symmetric isomers (denoted 2110 and 21100 ) accounts for the chemical equivalence of the terminal CO ligands in the 13C NMR spectrum. The reactions of 200 with heterocumulenes, together with some other reagents that effect functional group transfer reactions, are summarized in Scheme 37 and reveal a range of interesting outcomes. Additional details for the reactions with CO2 and COS are given in Scheme 38. Reaction with 2 equiv. (per dimer) of N2O, Ph3PS or PhN3 gave oxide, sulfide or imide group transfer and formation of the Ti(+4) dimers [Ti(PnTiPS)(m-E)]2 (E ¼ O (209), S (210), NPh (211)).137,140,141 In these compounds the TidTi bond has been oxidatively cleaved and two m-E moieties take the place of the m-Z5,Z5-PnTiPS ligands in linking the titanium centers. The m-oxo dimer 209 reacts with pyridine to form [Ti(Z5-PnTiPS)(m-O)(py)]2 which retains a bimetallic structure but with an Z5-coordinated pentalene ligand.140 The compounds 209, 210 and 211 are also formed in the reactions of 200 with CO2, COS and Ph2N2 by C]O, C]S and N]N bond cleavage. Reaction of 200 with 1 equiv. phenylisocyanate formed dimeric [{Ti(PnTiPS)}2(m-k2N,O,k2OdO2CNPh)] (212) which contains a bidentate phenyl-carbonimidate ligand. The two Ti(PnTiPS) moieties are rendered inequivalent by the unsymmetrically-bound bridging group.137 Each titanium in 212 has a formal Ti(+3) oxidation state and the effective magnetic moment (Evans method) was found to be 1.3 mB per metal, somewhat less than the spin-only value for a d1 ion (1.73 mB). The isolated yield of 212 (40%) and its composition shows that the experimental stoichiometry of a 1:1 ratio for 200:PhNCO is deficient in substrate. However, the reaction of 200 with 2 equiv. PhNCO gave an uncharacterizable mixture. The mechanism for forming 212 is postulated as reductive deoxygenation of PhNCO to form [{Ti(PnTiPS)}2(m-O)] (see below) and PhNC (both identified by NMR tube scale studies), followed by Ti2OdCPhNCO bond formation. In contrast to the reactions with CO2, COS and PhNCO, reaction of 200 with CS2 or TolNCNTol does not give C]S or C]N bond cleavage, and instead forms thermally stable [Ti2(m-Z5,Z5-PnTiPS)2(m-Z2,Z2-CS2)] (213) and [Ti2(m-Z5,Z5-PnTiPS)2

Larger Aromatic Complexes of the Group 4 Metals

Scheme 38

Scheme 39

643

644

Larger Aromatic Complexes of the Group 4 Metals

(m-Z2,Z2-TolNCNTol)] (214).137,141 The substrates are bound in a m-Z2,Z2 manner to a [Ti2(m-Z5,Z5-PnTiPS)2] unit which retains its general structural integrity. The TidTi distances in 213 and 214 are 2.4432(10) and 2.4374(8) A˚ , respectively. Further details of the reductive deoxygenation or desulfurization of CO2 or COS, respectively, by 200 have been revealed through synthetic studies, NMR tube scale reactions and DFT calculations. These are summarized in Scheme 38.141 Both reactions initially form dimeric complexes with m-Z2,Z2-bound substrates, namely [Ti2(m-Z5,Z5-PnTiPS)2(m-Z2,Z2-CO2)] (215) and [Ti2(m-Z5,Z5-PnTiPS)2(m-Z2,Z2-COS)] (218). The IR spectrum of 215 at −65  C showed n(OCO) bands at 1678 (asymmetric mode) and 1236 (symmetric) cm−1. Although compounds 215 and 218 are analogues of 213 and 214 (Scheme 37), they are thermally unstable and undergo 2-electron reductive cleavage of the C]O or C]S bonds forming experimentally observed Ti(+3) intermediates [Ti2(m-Z5,Z5-Pn)2(m-O)] (216) and [Ti2(m-Z5,Z5-Pn)2(m-S)(CO)] (219). These diamagnetic species have TidTi bonds bridged by O or S ligands, with CO still bound in the case of 219. Compound 219 was prepared independently in situ from [Ti2(m-Z5,Z5-PnTiPS)2(m-S)] (220), which was itself made from 200 and 1 equiv. Ph3PS and crystallographically characterized. In the presence of an excess of COS at room temperature, 219 gives near-quantitative conversion to [Ti(PnTiPS)(m-S)]2 (210). DFT studies using Pn in place of PnTiPS of the CO2 reaction found a m-oxo CO adduct [Ti2(m-Z5,Z5-Pn)2(m-O)(CO)], the analogue of 219, as a local minimum following C]O bond cleavage from [Ti2(m-Z5,Z5-Pn)2(m-Z2,Z2-CO2)] (the DFT model for 215). However, the real compound [Ti2(m-Z5,Z5-PnTiPS)2(m-O)(CO)] was not observed experimentally. Compound 215 decomposes to the bis-m-oxo Ti(+4) dimer [Ti(PnTiPS)(m-O)]2 (209) and dicarbonyl [Ti2(m-Z5,Z5-PnTiPS)2(CO)2] (210; Scheme 36) on warming up. However, under a dynamic vacuum to remove the eliminated CO, the mono-m-oxo Ti(+3) dimer [Ti2(m-Z5,Z5-PnTiPS)2(m-O)] (216) is formed. Compound 216 can also be prepared from 200 by slow addition of 1 equiv. N2O and has been crystallographically characterized. The short TidTi bond (2.3991(7) A˚ , a ¼ ca. 140o) is consistent with the observed diamagnetism. However, 216 is thermally unstable with respect to the paramagnetic isomer [{Ti(PnTiPS)}2(m-O)] (217) formed by cleavage of the TidTi bond and change of PnTiPS bonding from bridging the two metal centers to Z8-coordination to one of each of them. The magnetic moment (Evans method) for 217 is 1.73 mB per Ti(+3) center, and the structure illustrated was confirmed by X-ray crystallography. Whereas reactions of CO and CO2 with transition metal compounds are very well developed, reactions of carbon sub-oxides such as C3O2 are quite uncommon. C3O2 is indefinitely stable at −35  C but auto-polymerizes above 0  C. The reactions of 200 with C3O2 and DFT mechanistic studies have been reported (Scheme 39).142 Reaction with 1 equiv. C3O2 while maintaining low temperatures throughout the reaction and workup gave the thermally unstable species [Ti2(m-Z5,Z5-Pn)2(m-Z2,Z2-C3O2)] (221) which was crystallographically characterized (TidTi ¼ 2.4293(14) A˚ ; a ¼ ca. 141o). The mono-m-Z2,Z2-C3O2 coordination mode in 221 is reminiscent of the first-formed reaction products of 200 with CO2, COS and CS2 (Schemes 37 and 38). Reaction of 200 with 3 equiv. C3O2, again under carefully controlled conditions, gave the tetranuclear complex [{Ti2(m-Z5,Z5-Pn)2}2(m-C9O6)] (222) in which three C3O2 units have been coupled to form a moiety containing a 4-pyrone core. Compound 222 was crystallographically characterized. Detailed DFT studies found a stepwise mechanism that proceeds via the initial formation of 221.

4.12.5

Nine-membered ring systems

4.12.5.1

Cyclononatetraenyl complexes

The cyclononatetraenide anion C9H−9 (223−, CNT− (Scheme 40)) has been known for many years and can be prepared by metallation procedures from [6.1.0]nona-2,4,6-triene precursors.143,144 As a Hückel aromatic, 10-electron anion, CNT− is isoelectronic with CHT3− and COT2−. Reaction with H2O leads to cis,cis,cis,cis-1,3,5,7-triazacyclononane (HCNT, 224) which is unstable and isomerizes to cis-4a,7a-dihydro-1H-indene (H3Ind, 225) at room temperature.145

Scheme 40

Very few organometallic CNT compounds are known.146–149 Prior to the period covered by this review, one example of a group 4 CNT compound had been reported, namely [Ti(Z7-CNT)(Cp)] (226) which was prepared from [CpTiCl3] and an excess of LiCNT.146 Compound 226 is diamagnetic and was spectroscopically characterized. By way of context, we note two non-group 4 transition metal examples, each with trihapto coordination: de Liefde Meijer’s [Cp2Nb(Z3-CNT)]147 and Murahashi’s more recent [Pd(Z3-CNT)Br(PPh3)].149 The latter is unstable in solution and converts to the 3a,7a-dihydroindenyl isomer [Pd(Z3-H2Ind) Br(PPh3)] over 12 h by an apparently analogous (but on-metal) process to the 224 ! 225 conversion shown in Scheme 40.

Larger Aromatic Complexes of the Group 4 Metals

645

Since 2000 there has been one reported attempt to prepare a group 4 complex of CNT, by Sitzmann et al. following their synthesis of barium sandwich and mixed-sandwich CNT complexes.148 Reaction of [Zr(Z5-C5Ht2Bu2R)Cl3] (227; R ¼ H, tBu) with KCNT (223_K) in an attempt to form [Zr(CNT)(Z5-C5Ht2Bu2R)Cl2] led instead to the indenyl compound [Zr(CNT)(Ind)Cl2] (228, Eq. 28).150 This process involves net loss of two H atoms as well as transannular CdC bond formation. The authors were not able to specify whether the H atoms are lost as H2 or in some other way. Although the transformations 224 ! 225 and [Pd(Z3-CNT)Br(PPh3)] ! [Pd(Z3-H2Ind)Br(PPh3)] were previously known, the combined rearrangement with dehydrogenation of a (presumed) cyclononatetraenyl ligand to form an indenyl complex in 227 ! 228 appears to be a new process.

ð28Þ

4.12.5.2

Zirconium indenyl complexes with h9-coordination

A series of papers from the Chirik group, partly in collaboration with Veiros, has disclosed a rich chemistry of bis(indenyl) and cyclopentadienyl-indenyl complexes of zirconium containing a Z9-coordinated indenyl ring. This is rather unusual since indenyl ligands usually bind to metals in an Z5-manner, with a tendency (at least with respect to their cyclopentadienyl analogues) to “ring slip” to an Z3-coordination mode. An overview of some of this chemistry was given in a review of group 4 sandwich complexes in general.151 The properties and reactivity of the compounds varies significantly with the indenyl 1,3-ring substituents in particular. For convenience, Fig. 9 summarizes the ligands studied by Chirik et al. and the abbreviations used in this chapter. IndR refers to a general indenyl ligand, C9R7 (R ¼ H or other). For the avoidance of doubt, in this section the hapticity of any indenyl ligand will always be explicitly stated, even when it is the usual pentahapto mode. Cp may always be taken as being Z5-coordinated.

4.12.5.2.1

Synthesis and structure

Initial studies in this area started with the alkylation of [Zr(Z5-IndTMS)2Cl2] (229a_Cl2) with 2 equiv. LiCHi2Pr to generate the thermally unstable alkyl-hydride, 229a_RH and isobutene (Scheme 41). Compound 229a_RH undergoes facile reductive elimination of HCMe3 to give the 18 valence electron compound [Zr(Z9-IndTMS)(Z5-IndTMS)] (229a).152 This route has been extended to a number of homoleptic (cf. Scheme 41) and mixed-ring homologues, several of which have been crystallographically

Fig. 9 Indenyl groups used and their abbreviations.

646

Larger Aromatic Complexes of the Group 4 Metals

R R Zr R

R Cl

2 LiCH2iPr

Cl

–CH2CMe2

R

R Zr

R CH2iPr

22 oC, mins

H

R

R

Zr

–HCMe3

R

R R

229_Cl 2

229_RH

229 t

R = TMS (a), SiMe2Ph (b), SiMe2 Bu (c) Scheme 41

characterized.153 In the mixed-ring species, the more electron-rich indenyl ligand adopts the higher hapticity as, for example, in [Zr(Z9-IndiPr)(Z5-IndTMS)] (229d). In the case of the electron-rich 229e the route shown in Scheme 41 is problematic because of reaction of the isobutene side-product with 229e by CdH activation forming an allyl-hydride derivative. Therefore a Na/Hg reductive route to 229e directly from 229e_Cl2 (Eq. 29) is preferred. Reductive routes from the respective dichlorides to other [Zr(Z9-IndR)(Z5-IndR)] (229) species can also be used.153 This method has been extended to the mixed-sandwich complexes [Zr(Z9-IndR)(Cp )] (230; IndR ¼ IndiPr (a), IndMe3 (b), IndMe2 (c), IndiPrMe (d), IndtBuMe (e)) from the respective [Zr(Z9-IndR)(Cp )Cl2] precursors,154 and to the benzene ring 4,7-dimethylated analogue [Zr(Z9-IndMe2iPr)(Z5-IndMe2iPr)] (231) in which the indenyl ligands are both sterically more demanding and slightly more electron-rich.155

ð29Þ

Attempted formation of [Zr(Z9-IndPh2)(L2X)] (L2X ¼ IndPh2 or Cp ) from the corresponding [Zr(Z5-IndPh2)(L2X)Cl2] with either Mg/Hg in THF or Na in benzene failed due to formation of the metal diphenylindenide salt.156 Attempts to prepare a hafnium analogue of 229a were unsuccessful, although a THF adduct [Hf(Z9-IndTMS)(Z5-IndTMS)(THF)] analogous to 231 (discussed below, Scheme 43) could be isolated.157 DFT studies of [Zr(Z9-IndiPr)(Z5-IndiPr)] (229e)153 and the model compound [Zr(Z9-Ind)(Z5-Ind)]158 were consistent with an 18 valence electron count and a HOMO based on Zr which p back-bonds to a p acceptor MO of the benzo (C6H4) moiety of the Z9-bound ligand. The X-ray structures of 229c and [Zr(Z9-IndiPr)(Z5-IndiPr)] (229e) show a folding of the C6 and C5 ring parts of the Z9-bound ring with an angle of the ca. 37–38o at the bridgehead C3adC7a bond. Lengthening of the coordinated diene-like (C4dC7) CdC bonds are consistent with the back-donation predicted by DFT studies which find that the zirconium in these compounds has a formal oxidation state between +2 and +4. Variable-temperature and EXSY (exchange correlation spectroscopy) NMR studies showed that the Z9- and Z5-rings in 229a–c and 229e are in exchange on the NMR timescale. Experimental activation parameters and DFT studies show that the barrier for this process is largest for the more electron-rich rings and that the process occurs via [Zr(Z5-IndR)2] intermediates 229_int (Scheme 42).153,158,159

Scheme 42

Larger Aromatic Complexes of the Group 4 Metals

4.12.5.2.2

647

Reactivity

Chirik et al. have reported an extensive reaction chemistry of zirconium complexes with Z9-indenyl systems.152,153,155,158,160–166 Scheme 43 summarizes a representative part of this with reference to [Zr(Z9-IndTMS)(Z5-IndTMS)] (229a). Other key reaction products of the various Z9-indenyl systems are shown in Fig. 10.

Scheme 43

Fig. 10 Products of CdH and/or CdX bond cleavage by Zr(Z9-IndTMS)(Z5-IndTMS) (229a).

Reaction of 229a with THF proceeds via an associative mechanism according to kinetic and DFT studies to form 16 valence electron [Zr(Z6-IndTMS)(Z5-IndTMS)(THF)] (231) in which one of the IndTMS rings has shifted to an Z6-coordination mode with the C1dC3 carbons of the five-membered ring no longer coordinated. DME and DMPE analogues were also made. Computational studies on the model [Zr(Z6-Ind)(Z5-Ind)(THF)] show that the negative charge on the Z6-Ind is equally distributed over both the five- and six-membered ring carbons.160 Although the related compounds 229b and 229c (with 1,3–SiMe2R substituents) also form THF adducts analogous to 231, compound 229e and mixed-ring analogues with a Z9-IndiPr group do not do so because of the stronger bonding of the dialkyl-substituted Z9-indenyl group. Whereas reaction with THF forms 231 with an Z6-bound indenyl, reaction with CO or C2R2 (R ¼ Me, Ph) give the bis(Z5-indenyl) products [Zr(Z5-IndTMS)2(CO)2] (232) and [Zr(Z5-IndTMS)2(Z2-C2R2)] (233; R ¼ Me (a), Ph (b)). The p-acceptor nature of these ligands reduces the density available at zirconium for additional back-donation to the benzo ring of an Z6-bound indenyl ligand and so, in contrast to the situation for the effectively s-only THF ligand, both indenyl ligands in these complexes have Z5-coordination. The Z2-butyne species 233a reacts with a further equiv. of alkyne forming the metallacycle 234. Reaction of

648

Larger Aromatic Complexes of the Group 4 Metals

229a with H2 afforded the dihydride [Zr(Z5-IndTMS)2H2] (235); continuing hydrogenation led to the tetrahydroindenyl derivative with a saturated six-membered ring, [Zr{Z5-1,3-C9H9(TMS)2}2H2].161 This is a relatively rare example of the indenyl ring itself becoming involved in the reactions of these systems. Another example is the reaction of [Zr(Z9-IndiPr)(Z5-IndiPr)] (229e) with cyclopentanone, which selectively inserts into the ZrdC1 and ZrdC4 bonds to the IndiPr.166 Examples of other products of CdE activation with 229a are given in Fig. 10.163 Reaction of 229a with DMAP resulted in orthoCdH activation and oxidative addition forming the Z2-dimethylaminopyridyl-hydride species 236 (Fig. 10). Rate-determining CdH addition is the first step of the CdO bond cleavage reactions of 229a with dialkyl ethers such as Et2O, MeOEt and nBu2O. This is followed by rapid b-alkoxide elimination forming alkoxy-hydride species such as [Zr(Z5-IndTMS)2(OEt)(H)] (237, from Et2O) and the corresponding olefin.162 On the other hand, CdS bond cleavage reactions with thioethers, for example with THT forming [Zr(Z5-IndTMS)2{k2C,S-S(CH2)3CH2}] (238), proceed with direct CdS addition because of the weaker CdE bond for the heavier chalcogen.

ð30Þ

Despite the access of the various [Zr(Z9-IndR)(Z5-IndR)] (229) species in solution to the Zr(+2) 16 electron intermediates [Zr(Z5-IndR)2] (229_int, Scheme 42) identified in the dynamic NMR behavior, no N2 coordination chemistry was observed for any of these systems. This is because additional binding to the C6H4 benzo moiety of the Z9-IndR ligand is more favorable than bonding to N2. To develop this chemistry further the mixed-sandwich complexes [Zr(Z9-IndR)(Cp )] (230; IndR ¼ IndiPr (a), IndMe3 (b), IndMe2 (c), IndiPrMe (d), IndtBuMe (e)) were prepared. These gave a satisfactory outcome in the cases of 230c–e,154 reversibly forming the corresponding m-dinitrogen species [{Zr(Z5-IndR)(Cp )}2(m-N2)] (239c–e, Eq. 30). The ability of the compounds with less sterically crowded IndR ligands to dimerize is the origin of the N2 stabilization in these systems, and explains why 230a–b, having the bulkier IndiPr (230a) and IndMe3 (230b) ligands, do not form m-N2 complexes.

4.12.6

Concluding remarks

Since the year 2000, two main areas with potential for further exploration have been developed, namely cycloheptatrienyl and pentalene systems. Both ligand systems have well-established synthetic entry points to their complexes, and established methods of selective ring substitution. It would be a bold reviewer who might attempt to predict specific applications of such complexes in the next decade or two before a further edition of COMC emerges. The elegant work of Chirik and co-workers on zirconium complexes of Z9-indenyl ligands seems to be quite mature now, albeit with the corresponding titanium and hafnium chemistry as yet unexplored. However, the intriguing possibility of accessing “classical” cyclononatetraenyl ligand complexes remains, provided that the apparent thermodynamic instability of C9H9 with respect to conversion to the indenyl counterparts can be kinetically controlled.

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