Comprehensive Organometallic Chemistry IV. Volume 3: Groups 3 to 4 and the f elements - Part 1 [3] 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 3

GROUPS 3 TO 4 AND THE f ELEMENTS - PART 1 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 3 Editor Biographies

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Contributors to Volume 3

xiii

Preface 3.01

xv

Introduction to Groups 3 to 4 and the f-Elements

1

David P Mills and Stephen T Liddle

3.02

Alkyl, carbonyl and cyanide complexes of the group 3 metals and lanthanides

3

Keith Izod

3.03

Hydride, Alkyl, Aryl, Acetylide, Carbonyl, and Cyanide Complexes of the Actinides

58

Trevor W Hayton

3.04

Alkyl, Carbonyl and Cyanide Complexes of the Group 4 Metals

85

Benjamin D Ward, Owaen G Guppy, and Matthew S Shaw

3.05

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

163

Florian Jaroschik

3.06

N-Heterocyclic and Mesoionic Carbene Complexes of the Actinides

201

Stephan Hohloch and James R Pankhurst

3.07

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

227

Adrien T Normand

3.08

Alkylidene Complexes of the Group 3 Metals and Lanthanides

268

Matthew P Stevens and Fabrizio Ortu

3.09

Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

312

Erli Lu

3.10

Alkylidene Complexes of the Group 4 Transition Metals

347

Daniel J Mindiola, J Rolando Aguilar-Calderón, and Pavel Zatsepin

3.11

Alkenes and Allyl Complexes of the Group 3 Metals and Lanthanides

382

Maxime Beauvois, Yohan Champouret, Fanny Bonnet, and Marc Visseaux

3.12

Alkene, Alkyne and Allyl Complexes of the Actinides

449

Marc D Walter

3.13

Group 4 Metal Alkyne, Alkene, and Allyl Complexes

477

Hayato Tsurugi

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

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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 3 J Rolando Aguilar-Calderón University of Pennsylvania, Philadelphia, PA, United States

David P Mills Department of Chemistry, The University of Manchester, Manchester, United Kingdom

Maxime Beauvois Univ. Lille, CNRS, Univ. Artois, Centrale Lille, UMR 8181 - UCCS - Unité Catalyse et Chimie du Solide, Lille, France

Daniel J Mindiola University of Pennsylvania, Philadelphia, PA, United States

Fanny Bonnet Univ. Lille, CNRS, Univ. Artois, Centrale Lille, UMR 8181 - UCCS - Unité Catalyse et Chimie du Solide, Lille, France Yohan Champouret Univ. Lille, CNRS, Univ. Artois, Centrale Lille, UMR 8181 - UCCS - Unité Catalyse et Chimie du Solide, Lille, France Owaen G Guppy Cardiff University School of Chemistry, Cardiff, United Kingdom Trevor W Hayton Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA, United States

Adrien T Normand Institut de Chimie Moléculaire de l’Université de Bourgogne (ICMUB), Université de Bourgogne, Dijon, France Fabrizio Ortu School of Chemistry, University of Leicester, Leicester, United Kingdom James R Pankhurst École Polytechnique Fédérale de Lausanne, School of Basic Sciences, Institute of Chemical Sciences and Engineering, Sion, Switzerland Matthew S Shaw Cardiff University School of Chemistry, Cardiff, United Kingdom Matthew P Stevens School of Chemistry, University of Leicester, Leicester, United Kingdom

Stephan Hohloch University of Innsbruck, Faculty of Chemistry and Pharmacy, Institute for General, Inorganic and Theoretical Chemistry, Innsbruck, Austria

Hayato Tsurugi Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka, Japan

Keith Izod School of Chemistry, Newcastle University, Newcastle upon Tyne, United Kingdom

Marc Visseaux Univ. Lille, CNRS, Univ. Artois, Centrale Lille, UMR 8181 - UCCS - Unité Catalyse et Chimie du Solide, Lille, France

Florian Jaroschik ICGM, Univ. Montpellier, CNRS, ENSCM, Montpellier, France Stephen T Liddle Department of Chemistry, The University of Manchester, Manchester, United Kingdom Erli Lu School of Natural and Environmental Sciences, Faculty of Science, Agriculture and Engineering, Newcastle University, Newcastle upon Tyne, United Kingdom

Marc D Walter Technische Universität Braunschweig, Institut für Anorganische und Analytische Chemie, Braunschweig, Germany Benjamin D Ward Cardiff University School of Chemistry, Cardiff, United Kingdom Pavel Zatsepin University of Pennsylvania, Philadelphia, PA, United States

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

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3.01

Introduction to Groups 3 to 4 and the f-Elements

David P Mills and Stephen T Liddle, Department of Chemistry, The University of Manchester, Manchester, United Kingdom © 2022 Elsevier Ltd. All rights reserved.

Decades after its genesis, the whole field of organometallic chemistry is still burgeoning, with sustained interest across the periodic table from fundamental studies that establish the synthesis and basic properties of new organometallic complexes spanning all the way to applications in catalysis and materials science. The organometallic chemistry of groups 3 to 4 and the f-elements is certainly no exception to this opening sentiment, as evidenced by the 24 chapters within this section of Comprehensive Organometallic Chemistry-IV (COMC-IV) that we, as section editors, have had the privilege of commissioning, guiding, and editing. Since compiling COMC is a significant endeavor, and thus a periodic exercise, our brief for this section of COMC-IV was for authors to secure appropriate detail of published research from around the year 2000 onwards in order to provide comprehensive coverage of the area leading on from COMC-III. Thus, these COMC-IV chapters focus on advances in that timeframe, while making reference to prior key advances to contextualize the current contributions and to provide a retrospective link back to COMC-III. Since the choice of metals was pre-determined, the chapters in this section were organized by organometallic ligand identity, starting at C1 and working up to C9. We will now briefly highlight the astonishing range of exciting chemistry for readers to feast on. In Chapter 3.02, Izod describes advances of alkyl, carbonyl, and cyanide complexes of the group 3 and lanthanide metals. He details significant advances that include the synthesis of intrinsically reactive homoleptic alkyls, advances in carbonyl derivatives that are by definition disfavored, and lastly highlighting the still relatively under-developed nature of cyanide derivatives. In Chapter 3.03, Hayton reports on the state of alkyl, carbonyl, and cyanide actinide chemistry. He describes some of the impressive advances in isolatable s-bound organometallics in this area, now extended to transuranic elements, that have contributed to the debate on 5f-orbital participation and electronic structure of the bonding of actinides, as well as their use as precursors to many novel derivatives. In Chapter 3.04, Ward, Guppy, and Shaw review alkyl, carbonyl, and cyanide complexes of group 4 metals. This chapter emphasizes the enormous size of these areas since they often feature metallocene variants as well as ‘post-metallocene’ derivatives, together with the myriad applications in homogeneous catalysis and stoichiometric transformations. In Chapter 3.05, Jaroschik delves into N-heterocyclic and abnormal/mesoionic carbene complexes of group 3 and lanthanide metals. He amply highlights the explosive growth in this field over the past two decades, with now numerous carbene and metal variants being reported, with some now starting to find applications in small molecule activation and catalysis. In Chapter 3.06, Hohloch and Pankhurst provide an account of N-heterocyclic and mesoionic carbene complexes of the actinides. Apart from highlighting the range of complexes that have developed in an area still in its infancy, they show nicely how these ligands are already exhibiting novel bonding motifs and reactivity. In Chapter 3.07, Normand covers N-heterocyclic and mesoionic carbene complexes of the group 4 metals. This chapter shows how such complexes are far less well-developed than late metal counterparts, but it also highlights that such complexes can be exceptionally robust and active catalysts, and also provide fertile ground for stabilizing otherwise difficult to make low valent species. In Chapter 3.08, Stevens and Ortu recount alkylidene complexes of group 3 and lanthanide metals. They highlight the broad range of heteroatom-stabilized and -free alkylidenes in this emerging area, and discuss the broad range of structural diversity that such complexes exhibit along with their emerging applications in, thus far, stoichiometric substrate transformations. In Chapter 3.09, Lu catalogs alkylidene complexes of the actinides. He highlights the significant growth of the area with a range of heteroatom-stabilized derivatives and the varied electronic structure and novel reactivity patterns that such complexes are increasingly being shown to exhibit. In Chapter 3.10, Mindiola, Aguilar-Calderón, and Zatsepin chronicle alkylidene complexes of the group 4 metals. Their survey highlights the evolving methods used to prepare such complexes and the substituent groups and ancillary ligands that have diversified the area as well as the emerging stoichiometric reactivity patterns that highlight desirable opportunities for catalysis. In Chapter 3.11, Beauvois, Champouret, Bonnet, and Visseaux describe alkene, alkyne, and allyl derivatives of group 3 and lanthanide metals. Their discussion reveals the ever-broadening range of synthetic strategies to prepare such species, as well as a continued development of their applications as single-component catalysts in polymerization reactions. In Chapter 3.12, Walter describes alkene, alkyne, and allyl derivatives of the actinides. Though this area has been known for decades, this chapter shows how most of the development of this area has occurred in recent years, adding new synthetic routes and structural motifs, with interesting reactivity patterns beginning to emerge that suggest there is much to do in this area. In Chapter 3.13, Tsurugi reports on alkene, alkyne, and allyl derivatives of the group 4 metals. He shows that metallocene derivatives continue to dominate the area, but non-metallocene systems are beginning to emerge, but regardless these ligands are central to a wide range of stoichiometric and catalytic organic transformations. In volume 4, Chapter 4.01, Schädle and Anwander cover buta- and pentadienyl complexes of group 3 and lanthanide metals. They highlight the rapid growth of these areas in recent years, with new classes of ligands being introduced that exhibit novel electronic structures, a wide array of structural motifs, and applications in catalytic polymerizations.

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00113-X

1

2

Introduction to Groups 3 to 4 and the f-Elements

In Chapter 4.02, Farnaby, Chowdhury, Horsewill, and Wilson chronicle the development of buta- and pentadienyl complexes of the actinides. They cover four-carbon ligands including 2-butene-1,4-diyl, 1,3-butadiene-1,4-diyls, and cyclobutadienyls, and then five-membered metalacyclic complexes, showcasing the growing range of structural motifs and reactivity trends. In Chapter 4.03, Seth, Beretta, and Waterman provide an account of buta- and penta-dienyl complexes of the group 4 metals. They show how this relatively small area is beginning to bloom, revealing an expansion of synthetic methods to target such complexes and also their use in alkynyl couplings to produce butadienes and CdC cleavage reactions. In Chapter 4.04, Benner, Delano IV, Pugliese, and Demir discuss cyclopentadienyls and phospholyls of the group 3 and lanthanide metals. They chart the development of this now mature area, and amply show the wide range of synthetic, structural, small molecule activation, catalysis, single-molecule magnetism, and new metal oxidation state advances that have resulted. In Chapter 4.05, Gremillion and Walensky recount cyclopentadienyl and phospholyl actinide chemistry. They show how the area is burgeoning, with an ever-expanding range of derivatives spanning many types of fundamental and novel ancillary ligands that have supported new reactivity, bonding motifs, and metal oxidation states. In Chapter 4.06, Kilpatrick and Chadwick delve into cyclopentadienyl and phospholyl complexes of the group 4 metals. Reflecting the enormous size of this area after decades of endeavor, they highlight extensive synthetic efforts, numerous structural classes, and many applications in small molecule activation, dehydrocouplings, and polymerization catalysis. In Chapter 4.07, Cloke and Tsoureas review arene complexes of the group 3 and lanthanide metals. They survey the growing range of derivatives including neutral arenes, arenes appended to heteroatom-bonded chelating ligands, tripodal and anionic arenes, and inverted sandwiches featuring a wide range of co-ligands featuring novel electronic structures and emerging reactivity. In Chapter 4.08, Cryer and Liddle describe arene complexes of the actinides. This chapter reports on the synthesis and characterization of neutral and charged arene species, including inverse sandwich complexes with high charge loadings, covering a remarkable range of structural types, electronic structures, and reactivity that now even extends to transuranic elements. In Chapter 4.09, Fortier, Gomez-Torres, and Saucedo chronicle arene complexes of the group 4 metals. They reveal an expansive range of complexes, including high- and low-valent aluminates, a myriad of novel synthetic methodologies including metal vapor synthesis, bis(arene) sandwiches, tris(arene) complexes, and inverted sandwich compounds, demonstrating a burgeoning area. In Chapter 4.10, Stetsiuk, Cemortan, Simler, and Nocton report on larger aromatic complexes of the group 3 and lanthanide metals. They describe the synthesis and structures of cycloheptatrienyl, cyclooctatetraenyl, pentalenyl, and cyclononatetraenyl-based complexes, as well as surveying the magnetism and luminescence of these systems. In Chapter 4.11, Walter provides an overview of larger aromatic complexes of the actinides. He shows that while the area is still small, advances with cycloheptatrienyl, cyclooctatetraenyl, indenyl, and pentalene ligands underpin a remarkable range of electronic structure and small molecule activation studies, even extending to transuranic elements. In Chapter 4.12, last, but certainly not least, Mountford describes larger aromatic complexes of the group 4 metals. He highlights the synthetic advances that have underpinned the development of cycloheptatrienyl, cyclooctatetraene, pentalene, and indenyl derivatives, attempts to prepare cyclononatetraenyl systems, and the bonding and reactivity studies that are now emerging. It is thus self-evident that these chapters convey the continued vibrant and diverse nature of f- and early d-block organometallic chemistry. As well as serving as a major reference work, we hope that readers can draw inspiration from this formidable and exciting array of chemistry in future to realize new advances across synthesis, bonding and electronic structure, reactivity, magnetism, and materials and catalysis applications, so that organometallic chemistry continues to flourish at all levels.

3.02 Alkyl, carbonyl and cyanide complexes of the group 3 metals and lanthanides Keith Izod, School of Chemistry, Newcastle University, Newcastle upon Tyne, United Kingdom © 2022 Elsevier Ltd. All rights reserved.

3.02.1 3.02.2 3.02.3 3.02.3.1 3.02.3.1.1 3.02.3.1.2 3.02.3.1.3 3.02.3.1.4 3.02.3.2 3.02.3.2.1 3.02.3.2.2 3.02.3.2.3 3.02.3.2.4 3.02.3.3 3.02.4 3.02.5 References

3.02.1

Introduction Rare earth carbonyls Rare earth alkyls Homoleptic alkyls Neutral homoleptic Ln(III) alkyls Neutral homoleptic Ln(II) alkyls Cationic homoleptic alkyls Anionic homoleptic alkyls Heteroleptic Ln(III) alkyls Mono- and dialkyllanthanide(III) complexes with monodentate co-ligands Mono- and dialkyllanthanide(III) complexes with bidentate N- and/or O-donor co-ligands Mono- and dialkyllanthanide(III) complexes with tridentate and tetradentate N- and/or O-donor co-ligands Mono- and dialkyllanthanide(III) complexes with B- or P-donor co-ligands Heteroleptic Ln(II) alkyls Rare earth cyanides Conclusions

3 3 5 5 5 12 15 18 19 19 25 36 46 48 50 53 53

Introduction

Although well-characterized s-bonded organolanthanide compounds have been known since the 1970s, it is only in the last two decades that this area has blossomed into the vibrant research field it is today. This chapter describes recent developments in the chemistry of alkyl, carbonyl and cyanide complexes of the lanthanide and group 3 metals. Earlier work is described in the previous editions of this volume, COMC I, COMC II, and COMC III, although some seminal early discoveries are included here for context. Alkyl complexes containing cyclopentadienyl ligands (i.e. Cp2LnR, CpLnR2) are covered in the section on rare earth cyclopentadienyls and so are excluded from this section. The focus of this section is on structurally characterized, isolated compounds, however, examples of important compounds which have not been characterized in this way are also included, where the composition of the compound has been confirmed unambiguously by alternative methods. Excellent comprehensive annual reviews of developments in lanthanide organometallic chemistry up until 2017 can be found in the series of articles by Edelmann in Coordination Chemistry Reviews.1 In addition, several more specialist reviews on aspects of lanthanide alkyl, carbonyl and cyanide chemistry have been published. Mountford and Ward have reviewed the non-cyclopentadienyl organometallic chemistry of scandium.2 Zimmermann and Anwander reviewed the chemistry of homoleptic s-bonded organolanthanide complexes,3 while Trifonov reviewed the chemistry of alkyllanthanide complexes supported by guanidinate and aminopyridinate ligands,4 and also the history of bis(alkyl) rare earth(III) complexes.5 The use of lanthanide alkyls in catalytic s-bond metathesis reactions is covered in a review by Reznichenki and Hultzsch,6 while the polymerization of 1,3-conjugated dienes is described in a review by Zhang et al.,7 and Li et al. have reviewed the application of lanthanide post-metallocene complexes with chelating amido ligands in catalysis.8 The application of organolanthanide complexes in the ring-opening polymerization of cyclic esters is included in wider reviews of the use of lanthanide complexes for this application.9,10 Hou and Nishiura have reviewed the use of lanthanide dialkyls as precursors for the synthesis of polymerization catalysts and hydride clusters.11 The chemistry of cationic lanthanide organometallic compounds, including their applications in polymerization catalysis, has also been reviewed.12–14

3.02.2

Rare earth carbonyls

Due to the limited radial extension of the 4f orbitals in Ln(II) and (especially) Ln(III) cations and the consequent lack of metal valence orbitals suitable for p-back-bonding with CO, isolable rare earth carbonyls remain elusive. However, a number of transient rare earth metal carbonyls have been studied using matrix isolation or gas phase techniques. Infra-red studies on matrix isolated compounds formed by co-condensation of rare earth metals and CO have revealed Ln(CO)n species with terminal carbonyl ligands, along with Ln2(m-CO) and Ln2(m-CO)2 species (Ln ¼ Sc,15 Y, La,16–18 Gd,19 Ce,20 Tb, Ho, Dy, Er, Lu)21 containing one or two asymmetrically bridging, side-on bonded CO units, which are often activated towards CdO cleavage on warming. A further combined matrix isolation/DFT study has identified a series of monocarbonyls Ln(CO) for Ln ¼ Pr, Nd, Sm, Eu, Tb, Dy, Ho and Er.22

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00058-5

3

4

Alkyl, carbonyl and cyanide complexes of the group 3 metals and lanthanides

The cationic octacarbonyls [Ln(CO)8]+ (Ln ¼ Sc, Y, La) are 18-electron species and so are predicted to be stable. The compounds [Sc(CO)7]+ and [Y(CO)8]+ have been prepared in the gas phase by laser vaporization of the metals cooled in a pulsed nozzle with supersonic expansion of CO gas and were identified using a combination of DFT calculations and size-selective infra-red photodissociation spectroscopy.23 The Y complex was found to possess a square antiprismatic structure, while the inaccessibility of a [Sc(CO)8]+ cation was ascribed to steric overcrowding in this species. The corresponding La and Ce complexes adopt Oh geometries in the gas phase.24 Subsequently, the anionic complexes [Ln(CO)8]− (Ln ¼ Sc, Y, La, Tm, Yb, Lu)25,26 were identified by similar techniques and were shown to adopt structures with either Oh or D4h symmetry. In line with the limited radial extension of the 4f-orbitals, the bonding in these complexes is calculated to be a combination of [Ln(d)] (CO)8 s-donation and predominant [Ln(d)] ! (CO)8 p-back-donation. Reversible binding of CO by the sterically hindered divalent lanthanide complexes Cp 2Ln (Ln ¼ Sm, Eu, Yb), to give the monocarbonyls [Cp 2Ln(CO)], has been inferred from IR and NMR spectroscopy of solutions of [Cp 2Ln] in methylcyclohexane under CO at high pressures,27 although this conflicts with the observation by Evans and co-workers that CO undergoes trimerization to a ketene-carboxylate complex on reaction with [Cp 2Sm] under similar conditions.28 For ytterbium, while a monocarbonyl complex was detected at low pressures (99% selectivity in catalyzing the redistribution of a range of primary arylsilanes to secondary arylsilanes, including substrates with either electron-donating, or electron-withdrawing substituents; no redistribution reaction was observed for n-hexylsilane.

3.02.3.2.2

Mono- and dialkyllanthanide(III) complexes with bidentate N- and/or O-donor co-ligands

The problem of ligand redistribution reactions of heteroleptic alkyllanthanide complexes via Schlenk-type equilibria is frequently suppressed when bidentate co-ligands are employed. This strategy has been used for the synthesis of a wide range of heteroleptic complexes, many of which have found application as catalysts, especially for the hydroelementation of alkenes and alkynes.

26

Alkyl, carbonyl and cyanide complexes of the group 3 metals and lanthanides

The mono-amidinate complexes [[PhC{N(C6H3-2,6-iPr2)}2]Y(CH2SiMe3)2(THF)n] (116Y, n ¼ 1; 117Y, n ¼ 2) have been synthesized by the reaction between [(Me3SiCH2)3Y(THF)2] (20Y) and the corresponding amidine H[PhC{N(C6H3-2,6-iPr2)}2] in pentane.148 The difference in coordination between 116Y and 117Y is due to a difference in their isolation: the reaction mixture was evaporated to dryness during the isolation of 116Y, whereas this step was not included in the isolation of 117Y. The complex 116Y was shown to polymerize ethene with a narrow polydispersity, when activated by [PhMe2NH][B(C6F5)4]. However, the bis-THF complex 117Y was found to be inactive towards ethene polymerization under the same conditions, presumably because the extra molecule of THF blocks the necessary vacant site required for catalysis.148 A subsequent study showed that similar complexes could be prepared across the full size range of Ln(III) ions.149 The compounds [[PhC{N(C6H3-2,6-iPr2)}2]Ln(CH2SiMe3)2(THF)] (116; Ln ¼ Sc, Lu) were prepared via the reaction between [(Me3SiCH2)3Ln(THF)2] (20) and one equivalent of H[PhC {N(C6H3-2,6-iPr2)}2], whereas the compounds [[PhC{N(C6H3-2,6-iPr2)}2]Ln(CH2SiMe3)2(THF)2] (117; Ln ¼ La, Nd, Gd) were isolated by the sequential reaction of [LnX3(THF)n] (X ¼ Cl, Br) with three equivalents of Me3SiCH2Li, followed by one equivalent of H[PhC{N(C6H3-2,6-iPr2)}2].149,150 Treatment of 117 with [PhNMe2H][BPh4] in THF gave the cationic complexes [[PhC {N(C6H3-2,6-iPr2)}2]Ln(CH2SiMe3)(THF)n][BPh4] (118 Ln ¼ Sc, n ¼ 2; 119 Ln ¼ Gd, Y, Lu, n ¼ 3; 120 Ln ¼ La, Nd, n ¼ 4), which were active catalysts for the polymerization of ethene.149 A similar reaction between [(PhCH2)3La(THF)3] (34La) and H [RC{N(C6H3-2,6-iPr2)}2] (R ¼ Ph, tBu) gave the mono-amidinate complexes [[RC{N(C6H3-2,6-iPr2)}2]La(CH2Ph)2(THF)] (121).84,150 Treatment of 121 with [PhNMe2H][BPh4] in THF gave the cationic complex [[RC{N(C6H3-2,6-iPr2)}2]La(CH2Ph) (THF)3][BPh4] (122).

The closely related amidinate complex [[PhC{N(C6H3-2,6-iPr2)}2]Y(CH2C6H4-2-NMe2)2] (123Y) was prepared by the reaction of [(2-Me2NC6H4CH2)3Y] (30Y) with one equivalent of N,N0 -bis(2,6-diisopropylphenyl)benzamidine in toluene.151 Treatment of this amidinate complex with [Ph3C][B(C6F5)4] gave a precatalyst for the efficient polymerization of isoprene to give highly isotactic 3,4-poly(isoprene) when the reaction was carried out at −10  C or below. In contrast, treatment of 123Y with AlMe3 gave a precatalyst which produced largely cis-1,4-poly(isoprene) in the presence of one equivalent of the [Ph3C][B(C6F5)4] activator.151 Examination of the reaction between 123Y and five equivalents of Me3Al revealed the formation of the tetraalkylaluminate complex [[PhC{N(C6H3-2,6-iPr2)}2]Y(AlMe4)2] (124Y). While not an active catalyst itself, 124Y rapidly produces largely cis-1,4-poly(isoprene) in the presence of one equivalent of [Ph3C][B(C6F5)4]. It was suggested that the switch from 3,4-isospecific to 1,4-cis selectivity upon the addition of AlMe3 may be due to the involvement of a cationic bimetallic Y/Al species.

Alkyl, carbonyl and cyanide complexes of the group 3 metals and lanthanides

27

The reaction between the amidinate complex [[RC{N(C6H4-2,6-iPr2)}2]LnCl2] and two equivalents of 2-Me2NC6H4CH2Li gave the heteroleptic benzyl complexes [[RC{N(C6H4-2,6-iPr2)}2]Ln(CH2C6H4-2-NMe2)2] (123; R ¼ Ph, CH2C6H4-2-NMe2; Ln ¼ Sc, Y, Lu).152 These compounds undergo insertion of CO2 or PhNCO into the LndC bonds to give the corresponding carboxylate or carboxamide complexes.152 The reaction of [[PhC{N(C6H4-2,6-iPr2)}2]Ln(CH2C6H4-2-NMe2)2] (Ln ¼ Sc, Lu) with two equivalents of AlMe3 in toluene yielded the mixed methyl/methylidene complexes [[PhC{N(C6H4-2,6-iPr2)}2]3Ln3(m2-Me)3(m3-Me) (m3-CH2)] (125; Scheme 10). In contrast, treatment of [[PhC{N(C6H4-2,6-iPr2)}2]Ln(CH2C6H4-2-NMe2)2] with three equivalents of AlMe3 yielded [[PhC{N(C6H4-2,6-iPr2)}2]Ln(AlMe4)2] (124).153 Compounds 125 can also be accessed by treatment of 124 with THF.

Scheme 10 Reactions of 125 with ketones.

28

Alkyl, carbonyl and cyanide complexes of the group 3 metals and lanthanides

Treatment of 125 with cyclohexanone or acetophenone led to oxo atom transfer and the formation of [[PhC{N(C6H4-2,6iPr2)}2]3Ln3(m2-Me)3(m3-Me)(m3-O)] (126) and the corresponding alkene.153 The mechanism of this reaction has been probed using DFT calculations.154 Further treatment of 126Lu with cyclohexanone gave the methyl abstraction (enolate) product [[PhC {N(C6H4-2,6-iPr2)}2]3Ln3(m2-Me)3(m3-OC6H9)(m3-O)] (127), whereas reaction of 126Lu with acetophenone gave the insertion (alkoxide) product [[PhC{N(C6H4-2,6-iPr2)}2]3Lu3(m2-Me)3(m3-OCPhMe2)(m3-O)] (128). Unexpectedly, the Sc analogue 126Sc was unreactive towards ketones under the same conditions. Further reaction of the oxo-cluster 126 with excess PhNCS gave the sulfide cluster [[[PhC{N(C6H4-2,6-iPr2)}2]Ln(m2-Me)]3(m3-Me)(m3-S)] (129).155 The reactions of 125 with PhCN, PhC^CH, Me3SiC^CH, and CS2 have also been studied.156

The related formamidinate complexes [[HC{N(C6H3-2,6-R2)}2]2Ln(CH2SiMe3)(THF)] (130; Ln ¼ Y, Er, Dy, Sm; R ¼ Me, iPr) were isolated from the reaction between two equivalents of the corresponding formamide and [(Me3SiCH2)3Ln(THF)n] (20).157 These complexes could also be accessed by sequential reaction of LnCl3 with two equivalents of the lithium formamidinate [[HC {N(C6H3-2,6-R2)}2]Li], followed by one equivalent of Me3SiCH2Li. Compounds 130 were shown to polymerize isoprene with high 1,4-regioselectivity and narrow polydispersities in the presence of both R3Al (R ¼ Me, Et, iBu) and either [Ph3C][B(C6F5)4] or [PhNMe2H][B(C6F5)4]. Notably, the regioselectivity of the polymerization was found to depend on the sequence of addition of the catalyst, borate and trialkylaluminium; the greatest 1,4-selectivity was observed when these were added in the order 130/trialkylaluminium/borate. Metathesis reactions between [[HC{N(C6H3-2,6-iPr)}2]2LnX(THF)] (Ln ¼ Sm, La; X ¼ F, Cl) and RLi gave the corresponding terminal alkyls [[HC{N(C6H3-2,6-iPr)}2]2LnR(THF)] (131 R ¼ Me, 130 R ¼ CH2SiMe3).158

Binuclear complexes of a bis(amidinate) ligand (132) have been prepared by protonolysis of [(2-Me2NC6H4CH2)3Ln] (30; Ln ¼ Sc, Y, Lu), or by sequential salt metathesis between two equivalents of LnCl3, one equivalent of the in situ-generated dilithium complex of the bis(amidinate) ligand, and four equivalents of [2-Me2NC6H4CH2]Li.159 These complexes exhibit high activity for the living 3,4-(co-)polymerization of isoprene and myrcene when activated by [Ph3C][B(C6F5)4]. This represents the first living 3,4-polymerization of myrcene.

Alkyl, carbonyl and cyanide complexes of the group 3 metals and lanthanides

iPr iPr NMe2

H2C H2C

N

iPr

Ln NMe2 N iPr

iPr Me2N Ln

N

iPr

29

N Me N 2 iPr

CH2 CH2

iPr

132 The reaction between the chiral amidinate complexes [[tBuC{N-(S)-CH(Me)Ph}2]2LnCl]n (Ln ¼ Sc, n ¼ 1; Ln ¼ Y, Lu, n ¼ 2) and two equivalents of [(Me3Si)2CH]K gave the alkyl complexes [[tBuC{N-(S)-CH(Me)Ph}2]2Ln{CH(SiMe3)2}] (133; Ln ¼ Sc, Y, Lu), which catalyze the hydroamination/cyclization of aminoalkenes and aminoalkynes with good regioselectivity, but only poor to moderate enantioselectivity.160

The guanidinate complexes [{(Me3Si)2NC(NCy)2}2LnR] (134; Ln ¼ Y, Er, Yb; R ¼ CH2Ph, tBu) were prepared by the reaction of [{(Me3Si)2NC(NCy)2}2Ln(m-Cl)2Li(THF)2] with either PhCH2K or tBuLi.161 The complexes 134Y and 134Er (R ¼ CH2Ph) are isostructural in the solid state and feature contacts between the lanthanum ion and the carbanion center, ipso- and ortho-carbons of the benzyl ligand, while 134Yb and 134Er (R ¼ tBu) possess a five-coordinate lanthanide center with an additional agostic-type interaction with one methyl of the tert-butyl group. Compounds 134 react with elemental sulfur to give either thiolate (R ¼ PhCH2) or disulfide (R ¼ tBu) complexes.161 The related complexes [{(Me3Si)2NC(NiPr)2}2YR] (135; R ¼ CH(SiMe3)2, tBu, (m-Me)2Li(TMEDA)) were prepared from metathesis reactions between [{(Me3Si)2NC(NiPr)2}2YCl]2 and the corresponding alkyllithium.162

30

Alkyl, carbonyl and cyanide complexes of the group 3 metals and lanthanides

Protonolysis reactions were used to synthesize a series of symmetrically and unsymmetrically substituted iminophosphonamide-supported dialkyllanthanide complexes [{Ph2P(NR)2}Ln(R0 )2(THF)n] (136; Ln ¼ Sc, Y, Er, Lu, Nd, La; R ¼ tBu, Ph, o-tolyl, p-tolyl, 2-EtC6H4, 2,6-Et2C6H3, 2,6-iPr2C6H3, pyridyl; R0 ¼ CH2Ph, CH2SiMe3; n ¼ 0, 1).163–166 These compounds effect the polymerization of isoprene when activated by [Ph3C][B(C6F5)4] or [PhNMe2H][B(C6F5)4] and a triorganoaluminium compound, with regio- and stereoselectivity found to depend on the size of the lanthanide cation, the steric bulk of the iminophosphonamide ligand, and the nature of the activator.164–166 An attempt to isolate the mono-alkyl complex [{Ph2P(NtBu)2}2Sc(CH2SiMe3)(THF)n] by protonolysis of [(Me3SiCH2)3Sc(THF)2] (20Sc) failed due to the small size of the Sc(III) ion.164 However, a one-pot protocol involving the reaction of [ScCl3(THF)3] with three equivalents of the smaller alkyllithium precursor MeLi and two equivalents of iminophosphonamine cleanly gave the complex [{Ph2P(NtBu)2}2Sc(Me)(THF)] (137). In contrast, the reaction between in situ-prepared [(Me3SiCH2)3Ln(THF)3], containing the larger Nd and Sm cations, and two equivalents of iminophosphonamine gave the complexes [{Ph2P(NtBu)2}2Ln(CH2SiMe3)] (138; Ln ¼ Nd, Sm).164

The closely related complexes [[RS{N(2,6-iPr2C6H3)}2]Ln(CH2SiMe3)2(THF)n] (139; R ¼ Ph, CH2C6H4-2-NMe2; Ln ¼ Sc, Y, Lu; n ¼ 1, 2) have been synthesized by the reaction between [RS{N(2,6-iPr2C6H3)}2]Li(THF)2 and [(Me3SiCH2)2Ln(THF)n][BPh4], whereas the complexes [[RS{N(2,6-iPr2C6H3)}2]LuR0 2(THF)n (140; R, R0 ¼ CH2SiMe3, CH2C6H4-2-NMe2; n ¼ 0, 1) have been prepared by addition of R3Lu(THF)n to the sulfur diimide S{N(2,6-iPr2C6H3)2}2.165 Compounds 139 and 140 also act as precatalysts for the polymerization of isoprene, when activated by [PhNMe2H][B(C6F5)4] and iBu3Al.

A metathesis reaction between [[HC{RCN(C6H3-2,6-iPr2)}2]LnX2(THF)2] (R ¼ Me, tBu; X ¼ Cl, I) and R0 Li, R0 K, or R0 MgCl gave the b-diketiminate complexes [[HC{RCN(C6H3-2,6-iPr2)}2]LnR0 2(THF)n] (141; Ln ¼ Sc, Y; R0 ¼ Me, CH2tBu, CH2SiMe3,CH2SiMe2Ph, CH2Ph, n ¼ 0, 1).167,168 Although no solid state structure has been obtained for the complexes [[HC{RCN(C6H3-2,6iPr2)}2]YMe2], variable-temperature 1H NMR spectra indicate that the bulkier complex [[HC{tBuCN(C6H3-2,6-iPr2)}2]YMe2] is monomeric, while the less bulky complex [[HC{MeCN(C6H3-2,6-iPr2)}2]YMe2]2 exists as a dimer in solution. Thermolysis of 141 (Ln ¼ Y, R ¼ Me, R0 ¼ CH2SiMe2Ph) led to the formation of the cyclometallated species 142 due to deprotonation of one of the isopropyl methyl groups and elimination of PhSiMe3. Monitoring the thermolysis of [[HC{tBuCN(C6H3-2,6-iPr2)}2]Sc(CH2SiMe3)2] by 1H NMR spectroscopy indicated that this process is first order with respect to the Sc complex. Treatment of [[HC {tBuCN(C6H3-2,6-iPr2)}2]ScMe2] with HB(C6F5)2 yielded the contact ion pair complex [[HC{tBuCN(C6H3-2,6-iPr2)}2]ScMe {HB(Me)(C6F5)2}] (143), which underwent a similar cyclometalation and elimination of methane on warming to room temperature.169 The reaction between [[HC{tBuCN(C6H3-2,6-iPr2)}2]ScMe2] and B(C6F5)3 gave the weakly associated ion pair [HC {tBuCN(C6H3-2,6-iPr2)}2]ScMe ⋯ Me[B(C6F5)3] (144), which is stable for long periods below −30  C.170 In contrast, the reaction between [[HC{tBuCN(C6H3-2,6-iPr2)}2]ScMe2] and [CPh3][B(C6F5)4] in a variety of arene solvents gave the arene-complexed separated ion pairs [HC{tBuCN(C6H3-2,6-iPr2)}2]ScMe(Z6-ArX)][B(C6F5)4] (145; ArX ¼ PhBr, PhH, PhMe, 1,3,5-Me3C6H3).171 1H NMR studies indicate rapid exchange between free and bound arene in solution. Consistent with this, coordinated bromobenzene is readily displaced by more basic arenes, with an order of arene coordination PhBr 70  C) which favors decomposition of the alkyne to form [Cp2Ti(C4R4)] complexes. A potential use for these decaborane-carbon structures is in the synthesis of boron carbide, a hard ceramic which is used in bulletproof vests and for neutron absorption. A route to boron carbide was demonstrated by Sneddon, who polymerized compound 3.13 and then heated it to decomposition, thus furnishing boron carbide.191

122

Alkyl, Carbonyl and Cyanide Complexes of the Group 4 Metals

Scheme 52 Catalytic cycle for the hydroboration of alkenes by [Cp2Ti(CO)2]. Boron atoms within the cluster are indicated by dots.190

Group 4 carbonyl complexes containing ligands other than cyclopentadienyl or similar indenyl are uncommon within the review period, with even fewer that contain only one of these ligands. In fact, these complexes can be divided into three areas: expanded rings (i.e. arene and cycloheptatriene), 5-membered heterocycles, and non-aromatic ligands. An unusual route to dicarbonyl complexes is the reaction of an alkyl complex and carbon monoxide with mild heating and/or pressure. One such example is that by Hessen who used the carbonyls to probe the electronic and structural properties of the arene group. Two representative compounds were chosen, containing CMe2 (3.14), and CMe2CH2 (3.15) bridging groups between the cyclopentadienyl and arene. These complexes were then reacted with CO in bromobenzene (Scheme 53). Both compounds were fully consumed during the reaction, with the CMe2CH2-bridged complex taking considerably longer than the CMe2-bridged congener, however, complete conversion to the corresponding dicarbonyl complex was not observed in either case. Conducting the same reaction in the solid state provides a higher purity product over the course of several days, and as before the CMe2CH2-bridged complex was far slower to react, reaching 35% conversion after a week at room temperature. Sadly, the fate of the reactant that is not converted to the dicarbonyl species was not identified, although experiments were conducted to elucidate this observation. Upon addition of “wet” solvent to the reaction mixture an acetone by-product was formed. It was then proposed that the acetone either reacts with the dicarbonyl complex or the starting material to generate an unknown hydrolysable paramagnetic compound. Interestingly, the crystal data points to a highly distorted arene; combined with the NMR data showing less deshielded protons, it was suggested that both Z6 and Z4 resonance forms exist (A and B respectively) for this species as shown in Scheme 53.192 The IR stretching frequency of carbonyl ligands is well known for acting as a barometer for the electron density of the metal to which it is bound.9 Typically, titanium(II) species exhibit bands below 2000 cm–1, however, these cationic species show a significant deviation, with stretching frequencies typical of those observed in titanium(IV) complexes, agreeing with earlier crystal data of significant metal arene interaction resulting from partial reduction of the arene moiety. It is believed that the s-donating

Alkyl, Carbonyl and Cyanide Complexes of the Group 4 Metals

123

Scheme 53 Synthesis of linked Cp/arene titanium dicarbonyls. [B(C6F5)4]– anion omitted for clarity.192

ability of arene ligands is of greater importance than their p-backbonding, due to the significant decrease in the stretching frequency when a silyl group is attached; usually these changes are quite small (ca. 2 cm–1), but the substantially larger changes in nCO for the arene complex is explained by the arene moiety being pushed closer to the metal center (Ti(1)dArcent ¼ 2.2102(18) A˚ , R ¼ SiMe3) compared to the methyl variant, Fig. 16 (Ti(1)dArcent ¼ 2.2555(15) A˚ , R ¼ Me) due to the steric force exerted by the trimethylsilyl group. Overall, the Cp/arene ligand is capable of stabilizing low valent and electron deficient species through a blend of electron donating and accepting functionalities (Fig. 17).

Fig. 16 Alkyl precursors to mono-Cp carbonyl complexes. [B(C6F5)4]– anion omitted for clarity.

Fig. 17 Molecular structure of (Z5-cyclopentadienyl,Z6-arene) titanium dicarbonyl. H atoms omitted for clarity.192

While the carbonyl complexes for a range bis(cyclopentadienyl) and bis(indenyl) based titanium(II) species are well known, stable under a range of conditions, and are relativity easy to synthesize, troticene complexes (3.16, Scheme 54) have not enjoyed similar success in reactions with carbon monoxide. Ernsting showed that under ambient CO pressure complex 3.18 was not observed, and only by increasing the pressure of CO was any conversion detected. Variable temperature NMR spectra showed no change in resonance signals between –70 and 20  C, due to CO exchange being faster than the NMR timescale. This poor ability to form CO complexes is ascribed to the weak s-donating properties of the CO ligand and the poor p-donating ability of complex 3.17.193 Heterocycles, on the other hand, have the ability to provide a range of functionalities, with boron-substituted alkanes and aromatics being valuable tools in fine tuning catalytic activity and selectivity; subtle modulation of the electronic density can be achieved by alteration of the boron substituent.194,195 With these two features in mind, Bazan et al. attempted to combine these two ideas into the boratacyclooctatetraene (BCOT) ligand through a monocarbonyl titanium intermediate (Scheme 55).196

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Scheme 54 Synthesis of troticene carbonyl complexes.193

Scheme 55 Synthesis of [(C5H5BCH3)2Ti(CO)] (3.20).180

Addition of excess CO to compound 3.20 in C6D6 generates the dicarbonyl species, 3.21, showing broadening of the methyl-borabenzene protons, along with an additional band in the infrared spectrum corresponding to the symmetric and antisymmetric stretching of the two CO moieties at 1996 and 1949 cm–1. However, the second carbonyl group is not tightly bound to the titanium center as removal of solvent only affords complex 3.20, suggesting there is a degree of stabilization afforded by the addition of a boron substituent.196 In studying the reactivity of the 3.20 with acetylene the authors observed the formation of three products, one of which being the desired BCOT-containing complex, 3.22 with the formation of this complex favored in low concentrations of acetylene. It is believed this reaction occurs via coordination of the acetylene and loss of the CO ligand (A, Scheme 56), with a reductive elimination step (B), followed by ring expansion.

Scheme 56 Reaction of 3.18 with acetylene, along with the proposed mechanism.196

Just as boron-substituted aromatics have the capacity to alter the electronic properties of the metal center, so too, can phosphorus-substituted derivatives. The tetramethylphospholyl ligand (Scheme 57) has been shown by Tham to be a more electron-donating group than the cyclopentadienyl ligand for titanium. The same, however, is not true for the zirconium congener which is less electron-donating than the cyclopentadienyl ligand. While at first this appears contrary to the titanium result, it was found to be related to the orientation of the tetramethylphospholyl rings around the larger zirconium cation; the increased centroid–metal–centroid angle of this system indicates a reduced p-backbonding to the tetramethylphospholyl rings, and therefore a greater donation into the p orbital of the carbonyl.197 Replacement of an aromatic ligand has only been briefly investigated. Phosphinimide ligands could be a viable alternative to the Cp based structures commonly used throughout the literature, as they have comparable steric and electronic properties, and which are similarly tunable. Work by Stephan showed [Cp Ti(NPtBu)(CO)2] (Scheme 58) exhibits a carbonyl stretching frequency that lies between the values obtained for Cp2 and Cp 2 dicarbonyl complexes.198

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Scheme 57 Synthesis of a phosphametallocene.197

Scheme 58 Synthesis of a phosphinimide dicarbonyl.198

Although dicarbonyl titanocene complexes are well known, the monocarbonyl derivatives have proven difficult to isolate—even though they have been postulated to act as reactive intermediates.199,200 From the series depicted in Scheme 59 it can be observed that more sterically demanding and electron-withdrawing substituents favor formation of the monocarbonyl species. The proposed pathway for the equilibrium in Scheme 59 is via the initial dissociation of a carbonyl ligand, followed by capturing of the free carbonyl by the two cyclopentadiene rings of the titanocene complex. The resultant three-coordinate titanium(II) species should be able to react with a further ligand; with the aim of synthesizing a mixed carbonyl dinitrogen complex, 3.26-(CO) was reacted with t BuNC to produce the monocarbonyl cyanide complex. However, such a species was found to only be stable at low temperatures, decomposing to the dicarbonyl and a second unknown complex at 23  C. Just as the monocarbonyl was formed by reacting a dicarbonyl with the titanocene, so too can the mixed dinitrogen monocarbonyl species be synthesized by stoichiometrically mixing the bis(dinitrogen) and dicarbonyl complexes. Although just as with the cyanide complex, the mixed species was unstable above –20  C but was regenerated upon subsequent cooling.

Scheme 59 Synthesis and CO stretching frequencies of mono and dicarbonyls.199

Inert gas matrices can provide an effective way to observe and handle highly reactive or unusual species. Tix(CO)y clusters are one such example. Xu and co-workers were able to deposit these structures by the laser ablation of titanium atoms with carbon monoxide in an argon matrix at 7 K. Through a combination of IR spectroscopy and theoretical calculations the novel [cycloTi3[Z2(m2-CO)]3] cluster (Fig. 18, 3.27) was observed. All the clusters observed in this work possess extremely low energy CO stretching frequencies. It is believed such structures may pave the way to a new class of compounds.201

Fig. 18 Various titanium carbonyl cyclo-compounds.201

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The reaction of metal dimers with organic molecules may provide a useful pathway for the activation of typically unreactive molecules. The reaction of titanium dimers with carbon monoxide was studied by Souvi, producing both [Ti2CO] and [OTi2C]. As observed by Xu these bis(titanium) structures have incredibly weak CdO bonds owing to the increased donation from two metals into the single carbonyl p orbital. Interestingly the Ti2CO structure is converted into the OTi2C structure by irradiation with 350–400 nm light.202 The high reactivity of the Cp2Ti complex 3.29 precludes its isolation and synthetic routes that rely on this species. Recent work by Manda has explored the possibility of synthesizing this structure via UV irradiation of [Cp2Ti(CO)2] (3.28) in argon and dinitrogen matrices (Scheme 60). Irradiation within the argon matrix was not found to generate any titanocene; the monocarbonyl and dicarbonyl complexes were the only species evident by IR spectroscopy. Presumably the strong cage effects of the matrix facilitate recombination of CO and the monocarbonyl complex. Using a dinitrogen matrix however, did show increased reactivity when subjected to irradiation, forming [Cp2Ti(CO)(N2)] (3.30) and [Cp2Ti(N2)2], (3.31). Further photolysis of the 3.30 produced the mono(dinitrogen) species [Cp2Ti(N2)].203

Scheme 60 Inert gas matrix reactions.

Laser ablation of titanium, zirconium, and hafnium in an OCS atmosphere generated [M(OCS)] complexes (M ¼ Ti, Zr, Hf ) and the associated IR frequencies were recorded. Unsurprisingly the stretching frequencies for these sulfur carbonyl complexes are all far lower than that of their hexacarbonyl counterparts with stretching frequencies at nCO(sym/asym) ¼ 1878/1858, 1882/1868, and 1854/1844 cm–1 for titanium, zirconium, and hafnium respectively.204 Brathwaite and Duncan observed [Ti(CO)6]+ as the most common fragment in the vaporization of a titanium disc in a CO expansion, attesting to the stability of the hexacarbonyl complex. Species up to [Ti(CO)19] were observed, although it is likely that there are weakly-bound carbonyl ligands in the secondary coordination sphere, surrounding a strongly bound [Ti(CO)6] core. Just as Zhou et al. observed, in [M(CO)7,8]+ (M ¼ Ti, Zr, Hf ) the IR region shows two nCO bands, one from the Oh symmetric core and a second higher frequency band associated with the weakly-bound external ligand. There is no effect of the metal on the stretching frequency of the external ligand, but core ligands do show an expected decrease in the stretching frequency as the group is descended.205 The structure and bonding of a range of titanium carbonyl clusters was evaluated using infrared photodissociation. Vaporization of a titanium target generated compounds that are dominated by [TiO(CO)5]+, [Ti(CO)6]+, [Ti2(CO)9]+, and [Ti2O(CO)9]+. [Ti(CO)6]+ dissociates through the loss of a carbonyl ligand, although as the binding energy of the carbonyl ligand is far higher than the infrared photons, it is believed this is a multiphoton absorption event. As expected of an Oh symmetric structure there is a single nCO band located at 2118 cm–1. As expected, [Ti(CO)7]+ easily loses a carbonyl ligand to form the six-coordinate core and a weakly-bound CO ligand. Within the IR region there are two nCO bands, one at 2116, the other at 2168 cm–1. The first is, as with the six-coordinate structure, associated with the strongly-bound carbonyl ligands; the second corresponds to the weakly-bound outer-sphere ligand. Although the bonding energy of this outer-sphere ligand is weak (12 kcal mol–1) it is far stronger than typically observed for such ligands. However, it is worth noting that earlier work had identified the seven-coordinate structure as thermodynamically stable with respect to loss of CO. [TiO(CO)5]+ is the most abundant cation produced, showing two nCO bands at 2184 cm–1 and 2204 cm–1, with the equatorial ligands accounting for the strongest vibration seen at 2184 cm–1 and the axial ligand for the weaker at 2204 cm–1. It is anticipated that the increase in stretching frequency is due to there being a single d electron with which backbonding can occur, which is shared amongst all equatorial ligands. [TiO(CO)6]+ fragments easily, losing a CO ligand resulting in [TiO(CO)5]+ and a weakly-bound CO ligand; its IR spectrum is similar to that of [TiO(CO)5]+, with stretching frequencies found at 2170, 2182, and 2206 cm–1. [Ti2(CO)9]+ shows six bands from the terminal carbonyl ligands in the 2100–2200 cm–1 region, with a further three bands due to the bridging carbonyls at 1655, 1685, and 1942 cm–1. [Ti2O(CO)9]+ shows seven bands at 1526, 2101, 2113, 2135, 2151, 2165, and 2179 cm–1. as with [Ti2(CO)]+9, the low energy band at 1526 cm–1 is expected to correspond to a bridging carbonyl.206

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3.04.3.3

127

Zirconium and hafnium carbonyl complexes

There are significantly fewer carbonyl complexes of zirconium and hafnium compared to titanium. Arguably, the most common way to obtain zirconium dicarbonyl complexes is the in situ reduction of a Zr(IV) species with magnesium, under a carbon monoxide atmosphere (Scheme 61); sodium amalgam can also be used as a reductant but this is far less common. This method of preparing zirconium and hafnium complexes is highly dependent on the Zr(IV) species (i.e. the ligand) being suitable for this transformation, which may itself be redox active. Indeed, one of the main driving forces behind the synthesis of carbonyl complexes is to determine the electron-withdrawing or -donating nature of the supporting ligand, by measuring changes in the carbonyl stretching frequencies. Complexes that lie within the scope of this article contain exclusively cyclopentadienyl- or indenyl-based ligands.

Scheme 61 Synthesis of substituted indenyl zirconium dicarbonyls.207

Representative zirconium complexes have been reported by Chirik et.al, who prepared Zr(II) carbonyl complexes (Scheme 61, 3.32) via the magnesium-reduction of the corresponding Zr(IV) chloride congeners. By this method, a selection of symmetric and asymmetric substituted bis(indenyl) complexes were prepared (Fig. 19) and probed to evaluate the effect of various indenyl-substituents on the electron density of the zirconium center. In general, it was found that the indenyl ligand has a far stronger electron donating affect than cyclopentadienyl, although surprisingly the diisopropyl substituted indenyl was found to exhibit a similar degree of electron donation as the tetramethylcyclopentadinyl complex (average nCO ¼ 1905 cm–1).208

Fig. 19 Derivatives of zirconium carbonyl complexes 3.32.

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Interestingly, the tert-butylsilyl-substituted derivative (Fig. 20) showed an unusual orientation of the tert-butyldimethylsilyl substituents, which were rotated in such a way that the tert-butyl groups are positioned away from the zirconium metal and directed above and below the plane of their respective parent indenyl groups; the indenyl ligands were almost perfectly eclipsed with only 12.05 degree rotation between them. Along with this, the silyl groups present themselves in an unusual arrangement, being rotated to place the tert-butyl substituents away from the metal center and towards the out-of-plane positions above and below their respective indenyl groups, creating a rather open environment around the zirconium center. Typically, bent metallocene dicarbonyls display two strong bands in the carbonyl region of an IR spectrum, corresponding to the symmetric and asymmetric stretching of the two carbonyls. However, apart from isopropyl substituted indenyl derivatives, all the compounds present either four or six peaks of either equal intensity, or different sets major and minor signals. The increase in the number of signals, and the change in their relative intensity is ascribed to be the effect of various rotamers and their degree of population with intensities relative to the population of each rotamer. As such these are among the first cases of Group 4 rotamers observed by IR spectroscopy.207,208

Fig. 20 Molecular structure of tert-butyldimethylsilyl derivative of 3.32, showing the rotation of the silyl groups. H atoms omitted for clarity.207

Following this work, a further series of hafnium and zirconium complexes were synthesized (Fig. 21) to study the electron density of the metal center by analyzing the shift of the carbonyl stretching frequency in pentane. As expected of hafnium, being a third-row transition metal, the carbonyl stretching frequencies were lower in energy than those of the corresponding zirconium species. As with the earlier work on silylated substituents, a greater number of bands were observed than expected for the bent metallocene complexes, which was again postulated to be the result of rotamers, with the number and intensity of bands corresponding to population of the rotamers. Additionally, it was observed that the more highly alkylated diisopropyl indenyl ligand increases the electron density at the metal center.209 In probing higher degrees of substitution at the cyclopentadienyl-type ligand, it was found that the tetramethyl cyclopentadienyl complexes [(Z5-C5HMe4)2M(CO)2] (M ¼ Zr, Hf, Scheme 62, 3.33) display the same trend as found in the indenyl species. The hafnium center is more electron rich with two strong stretching frequencies at 1945 and 1849 cm–1, while the zirconium congener displays gives signals at 1951 and 1858 cm–1.210 As expected, the highly alkylated indenyl complex 3.34 (Scheme 63) contains a more electron rich metal center than similarly-alkylated cyclopentadienyl complexes, and has stretching frequencies of 1940 and 1844 cm–1.211

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Fig. 21 Structure and CO stretching frequencies for a series of zirconium and hafnium complexes.209

Scheme 62 Synthesis of zirconium and hafnium dicarbonyls.210

Scheme 63 Synthesis of the alkylated indenyl zirconium dicarbonyl complex.211

One of the challenges of working with low-valent Group 4 metals is their lack of stability relative to the higher oxidation state, requiring bulky ligands and/or p-backbonding groups to help stabilize them. While work with cyclopentadienyl and indenyl groups is quite expansive, the same cannot be said of the fluorenyl ligand (Scheme 64, 3.35), which has lagged behind. However, this can perhaps be attributed to the fluorenyl complexes showing a propensity to alter their hapticity to Z3 or Z1. However, the strong electron-donating properties of these ligands cannot be ignored; the work of Bazinet and Tilley examined the effect of the fluorenyl group and observed a decrease in the stretching frequency compared to cyclopentadienyl and indenyl ligands. In fact, the electron donating effects of the Flu derivative are far stronger than that of either the Cp or Ind complexes. With further work looking towards the use of these ligands in facilitating new chemical reactions.212 The reaction of the zirconium(IV) species 3.36 with carbon monoxide (Scheme 65) demonstrates its application as a source of low valent zirconium. However, it is worth noting that unlike the asymmetric ansa-metallocene reported by Chirik et al., it is not reactive towards dinitrogen, possibly due to the additional stabilization of the zirconium (IV) ground state provided by the increased electron donating ability of the Cp ligand relative to the silyl groups.213 As seen with the silyl-bridged complex 3.36, reaction with carbon monoxide results in cleavage of a ZrdC bond and formation of the reduced zirconium(II) dicarbonyl complex, Scheme 65. However, reactions with non-silyl derivatives do not necessarily only form this reduced species, nor does it have to be the major component. Work carried by Bradley demonstrated the favorable formation of the inserted acyl product, complex 3.38, resulting from a reductive elimination after insertion of carbon monoxide (Scheme 66). Therefore, such complexes can be considered to act as sequential sources of both zirconium(IV) and then

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Scheme 64 Synthesis of a zirconium fluorenyl-Cp dicarbonyl complex.212

Scheme 65 Synthesis of a dicarbonyl zirconium complex from the corresponding alkyl complex.213

Scheme 66 Synthesis of dicarbonyl and acyl zirconium complexes.211

zirconium(II).211 Later work from Chirik’s group showed that a novel zwitterionic zirconium complex 3.39 can function as a source of low valent zirconium, with the reaction of this species and carbon monoxide furnishing the expected dicarbonyl product (Scheme 67).214

Scheme 67 Synthesis of a zirconium dicarbonyl complex from a zwitterionic precursor.214

Whereas titanium carbonyl complexes have demonstrated the capacity to catalyze reactions, zirconium and hafnium derivatives have not enjoyed such success, with the vast majority of studies focused on dinitrogen activation. The addition of a highly alkylated cyclopentadienyl ligand generates an electron-rich metal center despite of the inclusion of a dimethylsilyl bridge (3.40, Scheme 68). With a greater electron donating character than the previously used [rac-(CpR)2Zr(CO)2] complexes, it was shown that these complexes are also active for dinitrogen activation.215 Hafnium carbonyl complexes are ideal candidates for nitrogen activation as they are more electron rich than their zirconium counterparts, with this in mind Chirik used the silyl-bridged ansa-cyclopentadienyl ligand with hafnium (Scheme 69). In the process of investigating dinitrogen bond cleavage in hafnocene complexes, Chirik and others observed the formation of a carbonyl species containing both terminal and bridging isocyanate ligands. After formation of the intermediate, 3.41, at 0  C, followed by warming to 23  C under 4 atmospheres of CO, quantitative formation of 3.43 was observed. It was found that the isolated intermediate 3.41 is an essential component of the reaction and not a side product formed during the reaction, as the CO

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Scheme 68 Synthesis of a dinitrogen activator.215

Scheme 69 Reaction of carbon monoxide with a dinitrogen bis(hafnium) species.216

can be either added as a reagent during the course of the reaction or a result of dissociation of CO from 3.41. Kinetic studies indicate that the rate of reaction is not perturbed to any great degree by high concentrations of CO, indicating that the terminal carbonyl ligand does not de-coordinate from the hafnium center. However, the CO ligand is labile as noted from isotopic labelling studies. Since complex 3.41 is formed as a result of reacting with three carbonyl moieties, this raised the question whether this complex could be utilized for additional CdC bond formation reactions. Reacting 3.41 with five equivalents of methyl iodide does indeed result in CdC bond formation but in a different manner to that observed in the presence of CO: producing complex 3.44, which contains a four-membered metallacycle (Scheme 70). The scope of CdC coupling potential was expanded to include tert-butyl isocyanate and CO2. As with the methyl iodide reaction, the carbonyl insertion product is trapped. It should be noted that in the absence of crystallographic data the stereochemistry of product 3.45 is unknown but is anticipated to be similar to that of 3.44. Unlike the previous two reactions which trapped the carbonyl insertion product, reaction with carbon dioxide produces a notably different product. Generating the oxo-bridged dihafnium species 3.46. Labelling studies show that one of the isocyanate ligands is derived from carbon dioxide.216 Given the recent literature on zirconium carbonyl complexes, one should not neglect the capacity for these ligands to stabilize reactive species through backbonding, generating complexes that show remarkable resilience to extreme conditions. One such example is observed in the treatment of a phosphine complex with carbon monoxide, generating the monocarbonyl complex 3.48 (Scheme 71). The reaction does not reverse to re-form 3.47, even when exposed to high temperatures and UV irradiation.217 A similar degree of stability was observed by Ernst with the sole formation of the monocarbonyl species, Fig. 22.218 An unusual application for zirconium carbonyl complexes is as a precursor to As4 (Scheme 72); As4 can be difficult to produce in known concentrations since it is often plagued by the simultaneous formation of grey arsenic. The successful formation of As4 by a zirconium carbonyl complex (Scheme 72, 3.49) was demonstrated by the reaction of a zirconium arsenic complex (3.50) with [Cp”FeBr]2, generating an octahedral complex with equatorial arsenic and axial FeCp” groups.219 In a similar fashion Scheer’s group also demonstrated that sources of [P4]2– (3.51) can be achieved via a zirconium dicarbonyl starting material (3.49). 220 In previous work by this group, they also displayed the possibility for the dicarbonyl complexes to be used to introduce Group 15 elements, such as phosphaalkynes (3.52), potentially allowing for the synthesis of targets otherwise out of the reach of conventional synthetic routes.221

3.04.3.4

Summary

In summary, the chemistry of Group 4 carbonyl complexes is much less developed than seen for alkyl complexes, with significantly fewer examples within the review period. However, it is also clear that carbonyl complexes can be used to excellent effect in

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Scheme 70 CdC bond-forming reactions with intermediate 3.41.216

Scheme 71 Reaction of carbon monoxide with a zirconium diphosphine complex.217

H3C

H3C Fig. 22 Mixed phosphine carbonyl species reported by Ernst.218

CH3

H3C

CH3

CH3

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Scheme 72 Reaction of a dicarbonyl zirconium complex with As4, P4, and tBuCP.219–222

stabilizing low-valent Group 4 complexes that exhibit interesting, and often non-conventional reactivity with unsaturated molecules. It is also of note that the fundamental science of carbonyl ligands has provided a significant degree of insight into the electron-donating nature of a range of cyclopentadienyl-type ligands, and one wonders if similar studies with non-cyclopentadienyl ligands could provide similar knowledge into the fundamental nature of many of the ligands that have been reported elsewhere in this text.

3.04.4

Cyanide and isonitrile [isocyanide] complexes

3.04.4.1

Introduction

Cyanide is a widespread ligand in coordination chemistry; it is invariably discussed in foundation year undergraduate chemistry courses as a strong-field ligand to highlight the differences between high- and low-spin transition metal complexes and is a standard addition to every coordination chemistry textbook.222 From an organometallic perspective, there are two manifestations of the “C^N” moiety acting as a ligand through coordination of the carbon: the cyanide ion MdCN (Fig. 23a) and the isonitrile molecule MdCNR (Fig 23c, more often called an isocyanide which should not be confused with the isocyanide ion MdNC, Fig. 23b, the linkage isomer of the cyanide ion).

Fig. 23 (a) Cyanide, (b) isocyanide, and (c) isonitrile ligands.

The isocyanide ion, like a nitrile ligand MdNCR, does not contain a direct MdC bond and is therefore not an organometallic ligand and is not discussed further in this article. Cyanides are s-donor / p-acceptor ligands,223 and their bonding to transition metals can be likened to carbonyl ligands, whereas isonitrile ligands are stronger s-donors and weaker p-acceptors. Cyanide ligands have a tendency to act as spectator ligands, whereas the presence of a highly polarized CdN multiple bond can give isonitriles a rich reaction chemistry when a part of an organometallic complex. In this section, Group 4 organometallic complexes of both cyanide and isonitrile ligands will be discussed, including their role as spectator ligands and their role in metal-mediated molecular transformations and catalysis.

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Cyanide complexes (MdCN)

Despite the prevalence of the cyanide ligand in transition metal coordination chemistry, there are surprisingly few cyanide complexes of the Group 4 metals. Cyanide is isoelectronic with CO and is therefore a s-donor / p-acceptor ligand. However, the negative charge on the cyanide renders it a better s-donor and worse p-acceptor than CO,223 which could make CN– more adaptable to early transition metals in high oxidation states when compared to CO. The homoleptic Ti(III) cyanide complex [Ti(CN)6]3– has been prepared but is unstable to water, giving Ti(OH)3 and HCN,224 showing that the cyanides of the Group 4 metals are unlike many of the cyanide complexes of the transition metals, which are water-stable. The comparatively small number of cyanide complexes of the Group 4 metals has been probed computationally, focusing on [Ti(CN)n]4-n, and from these studies it has emerged that the isocyanide isomer is predicted to have greater stability than the cyanide isomer for n ¼ 1–5; only for n ¼ 6 is the cyanide isomer preferred. This was attributed to the greater p-donor ability of isocyanide compared to cyanide ligands being beneficial for stabilizing the d0 complexes probed, which becomes less important as the greater s-donor ability of cyanide becomes greater for larger n.225–229 From this it may be inferred that the identity and donor ability of any co-ligands will have a profound impact on the stability of an individual cyanide complex, and also it is clear that the inherent stability of cyanide complexes for the Group 4 metals is compromised in comparison to other ligand types which are commonly found for this group of metals. Cyanide ligands rarely display reactivity at the CdN bond and act as spectator ligands, which contrasts with isonitrile ligands, which can act either as spectator ligands or else can undergo insertion chemistry, usually across the CdN bond (vide infra). Whilst there are comparatively few cyanide complexes, only a small number of these have been prepared via traditional salt metathesis (i.e. using a pre-formed cyanide anion). The examples reported within the review period are the titanium(III) complexes [CpR2Ti(CN)]3 [R ¼ H (4.1), SiMe3 (4.2), 1,3-(SiMe3)2 (4.3)] and [TpTi(CN)3]– (4.4, Tp ¼ 3,5-dimethyltrispyrazolylhydroborate). All of these complexes were prepared from the corresponding chloride complexes [CpR2TiCl] and [TpTiCl3] and cyanide salts. The molecular structures of two complexes are displayed in Fig. 24. The structure of [TpTi(CN)3] (4.4) corroborates the expected structural motif, with the cyanide being C-bound and monomeric, with a mean TidCN distance of 2.174(2) A˚ . The complex was found to have temperature-independent paramagnetism with S ¼ ½.231 The structures of [CpRTi(CN)]3 (4.1–4.3) are somewhat different in that the cyanide ligands bridge two titanium centers; the complex is thus comprised of TidCNdTi linkages, based upon the solid-state structure of the trimethylsilyl-derivative (4.2, Fig. 24, left). In this structure, the mean TidCN and TidNC bond distances are 2.131(17) A˚ and 2.161(14) A˚ respectively. EPR measurements suggest the less sterically-demanding derivatives (R ¼ H, SiMe3) form oligomeric structures in solution, with a quartet ground state clearly visible for R ¼ SiMe3, whereas the spectroscopic data for the Z5-C5H3(SiMe3)2 congener was consistent with a monomeric structure.230 A different (and more common) method by which cyanide complexes have been prepared is via the metal-mediated degradation of an organic compound, in which a (coordinated) cyanide fragment is produced. Such reactions often involve oxidation of the metal during the transformation. For example, the trimeric [Cp2Ti(CN)]3 (4.1) discussed above was previously reported by Beweries

Fig. 24 Molecular structures of the titanium cyanide complexes [CpSiMe3 Ti(CN)]3 (4.2)230 and [TpTi(CN)3]– (4.4).231 H atoms and [Et4N]+ counterion of 4.4 omitted 2 5 for clarity. Cp ¼ Z -trimethylsilylcyclopentadienyl; Tp ¼ 3,5-dimethyltrispyrazolylhydroborate.

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et al. In this case the complex was prepared by the reaction of a titanocene bis(trimethylsilyl)acetylene complex [Cp2Ti (Z2-Me3SiC2SiMe3)] (4.5) with cyanuric chloride (Scheme 73).232 The formation of complex 4.1 was thought to have proceeded via CdCl cleavage of the cyanuric chloride to yield [Cp2TiCl]2 and C3N3, which decomposed to afford the trimeric titanium(III) cyanide complex (Fig. 23). The fact that bis(trimethylsilyl)acetylene is released in the reaction indicates that the titanium species involved is effectively Cp2Ti, i.e. titanium(II), which is oxidized to titanium(III) in the product.

Scheme 73 Reactions of Cp2M(II) synthons resulting in cyanide complexes via bond cleavage of organic substrates [btmsa ¼ bis(trimethylsilyl)acetylene].232–234

Comparable reactivity was observed in the reaction of [Cp 2Zr(Z2-Me3SiC2SiMe3)] (4.6) with diphenylacetonitrile. In this case, the initial complex [Cp 2Zr(N]CHdCHPh2)(N]C]CPh2)] (4.7) was formed, effectively via the transfer of a hydrogen atom from one diphenylacetonitrile to another,234 which, upon partial oxidation, gave the zirconium(IV) cyanide complex [Cp 2Zr {k2-OC(CHPh2)NH}(CN)] (4.8), arising from CdCN cleavage of the diphenylacetonitrile (Scheme 73 and Fig. 25).233 The formation of cyanide complexes via bond scission has also been observed with isonitriles. The chemistry of isonitrile complexes will be discussed in Section 3.04.4.3, but the divide between cyanide and isonitrile complexes is less clear when an isonitrile undergoes CdN bond cleavage to afford a cyanide complex. For example, the reaction of [(rac-ebthi)Zr(Me3SiC2SiMe3)] [4.9, rac-ebthi ¼ rac-1,2-ethylene-1,10 -bis(Z5-tetrahydroindenyl)] with isonitriles initially forms a s-adduct of tert-butylisonitrile, [(rac-ebthi)Zr(Me3SiC2SiMe3)(CNtBu)] (4.10, Scheme 73); reaction of a further equivalent of isonitrile in the presence of a toluene derivative afforded complexes with Z2-iminoacyl and cyanide ligands 4.11, along with a bridging cyanide complex [(rac-ebthi)Zr(m-CN)]2 (4.12). The product complexes are thus formed via simultaneous activation of both the CdN of the isonitrile and CdH of the toluene derivative.235 Similar cyanide complex formation via isonitrile CdN bond cleavage was previously reported for the titanacyclobutane(IV) complex [Cp 2Ti{(CH2)2CHiPr}] (4.13, Scheme 74), which undergoes an initial reversible migratory insertion of tBuNC into one of the TidC bonds to afford the cyclic iminoacyl intermediate 4.14; subsequent reaction with excess tBuNC afforded the half-sandwich

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Fig. 25 Molecular structure of [Cp 2Zr{k2-OC(CHPh2)NH}(CN)] (4.8). H atoms omitted for clarity.233

complex [Cp Ti{(NtBuC)2(CH2)2CHiPr}(CN)] (4.15), which contains two isonitrile insertions into the metallacyclic moiety, along with an additional cyanide ligand coordinated to the titanium center.236

Scheme 74 Formation of a Ti(IV) cyanide complex via the cleavage of an isonitrile CdN bond.236

Cyanide complex formation via the degradation of isonitriles is not limited to cyclopentadienyl-containing complexes. Similar reactivity has been reported for the titanium(II) synthon [Ti(ketguan)(Z6-Im-DippN)]237 (4.16, Scheme 75). Complex 4.16 reacts with cyclohexylisonitrile to afford the mixed cyanide/isonitrile titanium(III) complex [Ti(ketguan)(ImDippN)(CN)(CNCy)] (4.17) which contains one cyanide ligand arising from CdN cleavage of an isonitrile, and one intact isonitrile completing the coordination sphere. The process involves a formal oxidation of the titanium, which changes from titanium(II) to titanium(III).238

Scheme 75 Synthesis of [Ti(ketguan)(ImDippN)(CN)(CNCy)] (4.17) via cyanide extraction from cyclohexylisonitrile [DIPP ¼ 2,6-diisopropylphenyl].238

As well as cyanide complexes being formed via the scission of isonitrile CdN bonds, low-valent Group 4 complexes can also undergo similar transformations with nitriles. Lamac and coworkers reported a series of Group 4 metallocene complexes bearing nitrile pendant arms.239 The nitrile groups were shown to p-coordinate to the metal center when cationic alkyl derivatives were prepared via alkyl-abstraction to stabilize the highly Lewis acidic cationic zirconium(IV) center. A similar observation was suggested to occur when the zirconium chloride complex [Cp (Z5-C5H4CH2CH2CN)ZrCl2] (4.18,Scheme 76) was reduced to zirconium(II) in [Cp (Z5:Z2-C5H4CH2CH2CN)Zr] (4.19), however in this case the p-coordinated nitrile group in 4.19 underwent CdCN bond

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scission to afford a cyanide complex and a ligated alkyl arm from the mono-substituted Cp ligand [Cp (Z5:kC1-C5H4CH2CH2)Zr(CN)] (4.20). The cyanide and alkyl ligand could be subsequently removed via protonation with HCl to afford the tert-butyl substituted zirconocene dichloride derivative 4.21.240

Scheme 76 Pendant nitrile coordination and subsequent cyanide abstraction to afford [Cp (Z5:kC1-C5H4CH2CH2)Zr(CN)] (4.20), and its subsequent reaction with HCl.240

In a different reaction motif, the hafnium dimeric and tetrameric [{Cp2Hf}2(NCO)2]n (e.g. 4.22, Scheme 77; Cp2 ¼ (Z5-C5HMe4)2, (Z5-C5H2Me3)2, Z5-C5Ht3Bu-SiMe2-Z5-C5Me4) bearing oxamidide ligands were found to undergo thermolysis to afford an oxo-bridged hafnocene dimers, along with a coordinated isocyanate and cyanide ligands in 4.23. The degradation of the oxamidide ligand is surprising since it might be expected to degrade into two NCO ligands, rather than the asymmetric scission found. The cyanide ligand was further removed by reaction either with iodotetramethylsilane to afford the corresponding iodide complex and Me3SiCN, or with MeOTf to afford the triflate complex and acetonitrile.241

Scheme 77 Synthesis of cyanide complex 4.23 via cleavage of an oxamidide ligand.241

3.04.4.3

Isonitrile complexes (MdCNR) and their reactivity

The isonitrile ligand MdCNR is more commonly referred to as isocyanide; the reader should be familiar with both terms since they are used interchangeably in the primary literature, but isonitrile will be used in this article to avoid confusion with the isocyanide ion MdNC. The chemistry of isonitrile complexes can be divided into two distinct categories. The first category contains complexes where the isonitrile ligand acts as a s-donor, and where there is no further reactivity associated with the isonitrile moiety. The second category contains complexes that may initially start with a s-coordination of the isonitrile ligand, but the isonitrile subsequently reacts, often with other ligands within the complex, to afford an entirely new ligand fragment. These reactions are commonly insertion processes, where co-ligands undergo a migratory insertion across the CdN triple bond, but other reaction motifs have also been observed. In some cases the initial s-complex can be isolated and characterized, and in other cases its existence can be inferred or is maybe more speculative. In this section both categories will be dealt with separately, starting with the description of s-complexes, followed by a discussion of their reactivity in the coordination sphere of the Group 4 metals.

3.04.4.3.1

s-complexes of isonitrile ligands

Isonitriles are isolobal and isoelectronic with CO; they are strong s-donors and comparatively weak p-acceptor ligands, when compared to CO,9 and are therefore well-suited to electron-deficient early transition metals. Isonitriles are therefore common supporting ligands in a wide variety of complexes, with several examples being reported in the review period. In many cases, the isocyanate shows no onward reactivity and acts merely as a spectator ligand, stabilizing the complex and aiding isolation. Isonitrile adducts have long been known for Group 4 metallocene complexes. Some of the early reports indicate their ability to make comparatively more stable complexes than other comparable ligands; for example, the reaction of [Cp2M(CO)2] (M ¼ Ti, Zr, Hf ) and [Cp2M(CO)(PMe3)] (M ¼ Ti, Zr, Hf ) with tBuNC displaces the carbonyl or phosphine ligands to afford [Cp2M(CO) (CNtBu)] (4.24–4.26, Scheme 78), and thereby demonstrating the superior coordinating properties of isocyanides compared to carbonyl and phosphine for the Group 4 metals.242,243 Similarly, in the pursuit of dinitrogen complexes, Chirik reported that i t [CpiPr 2 Ti(CO)] forms an isonitrile adduct [Cp Pr2Ti(CO)(CN Bu)] (4.27), but this complex was found to be unstable over minutes at room temperature, disproportionating into [CpiPr2Ti(CO)2] and unknown products.199 The above reports highlight the excellent s-donor ability of isonitriles in conjunction with the Group 4 metals. This observation was further confirmed in a recent report in which isonitriles were found to form Lewis base adducts with [Ti{N(SiMe3)2}3] (4.28) to

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Scheme 78 Synthesis of [Cp2M(CO)(CNtBu)] (4.24–4.26).242,243

afford the 5-coordinate adducts [Ti{N(SiMe3)2}3(CNR)2] [R ¼ Ad (4.29), tBu (4.30), Cy (4.31), Ph (4.32)], (Scheme 79);244 this was in stark contrast to other common s-donors such as THF, phosphines, pyridine, and arsine, which failed to coordinate, despite these being well-known donors for the Group 4 metals.

Scheme 79 Coordination of isonitriles to [Ti{N(SiMe3)2}3] (4.28).244

Isonitriles have been used as effective s-donor ligands with [Cp2M(alkyne)] complexes, which are effective M(II) synthons. For example, the Zr(II) alkyne complex [Cp2Zr{Me3SiCCB(C6F5)2}] (4.33, Scheme 80) reacts with tBuNC to afford an 18-electron adduct [Cp2Zr{Me3SiCCB(C6F5)2}(CNtBu)] (4.34), without displacing the coordinated alkyne ligand.245 However, when the alkyne is replaced by a butadiyne derivative, the corresponding M(II) complex already has an 18 valence electron count and is therefore not expected to readily accept an additional ligand. For example the complex [Cp 2Hf(Z4-Me3SiC4SiMe3)] (4.35) reacts with tBuNC to displace one of the alkyne donors; the butadiyne thereafter coordinates via a single alkyne with the remaining coordination site occupied by the isonitrile, [Cp 2Hf(Z2-Me3SiC4SiMe3)(CNtBu)] (4.36).246 Interestingly, when a conjugated enyne

Scheme 80 Reactions of Cp2M(II) synthons with isonitriles.245–247

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139

complex is used instead, as in [Cp2Zr{Me3SiC2C(Me)CH2}] (4.37), similar reactivity was observed upon addition of tBuNC, but the alkene was selectively displaced, leaving the alkyne bonded to the zirconium center in 4.38.247 Isonitrile complexes have also been probed for titanium(III) species. In the reaction of [Cp 2TiCl] (4.39) with a mixture of XylNC and lithium acetylide LiC2SiMe3, a mixture of [Cp 2Ti(CNXyl)2] (4.42) and [Cp 2Ti(C2SiMe3)2] (4.43) were formed (Scheme 81). The synthetic procedure could be rendered more selective by employing a sequential addition approach: reaction of complex 4.39 with isocyanide affords [Cp 2TiCl(CNAr)] (4.40) which can subsequently be converted to the mixed isonitrile alkynyl complex [Cp 2Ti(CNAr)(C2SiMe3)] (4.41) by halide metathesis with the appropriate lithium acetylide. However, complex 4.41 was found to readily disproportionate into the titanium(II) and titanium(IV) complexes 4.42 and 4.43 under mild conditions. The presence of an equilibrium was established by mixing samples of independently prepared 4.42 and 4.43; the mixture partially comproportionates into 4.41.248

Scheme 81 Formation of [Cp 2Ti(CNAr)(C2SiMe3)] and subsequent redox equilibria.248

As detailed in Section 3.04.2, complexes of Group 4 metallocenes can be readily converted to cationic complexes after alkyl-ligand abstraction with a strong Lewis acid. Given the propensity of isonitriles to afford highly stable adducts, it is understandable that they would find application in stabilizing such cationic species, which are often inherently unstable and prepared in situ. An example of this was reported by Green and co-workers, who prepared an ansa-zirconocene butadiene complex [{Me2C(Z5-C5H4)2Zr(Z4-C4H6)}] (4.44, Scheme 82).249 Reaction with B(C6F5)3 afforded the zwitterionic complex [{Me2C(Z5-C5H4)2Zr{Z3-C4H6B(C6F5)3}] (4.45), which was stabilized by adding a number of Lewis bases, including tBuNC, exemplified by complex 4.46, in which the weakly coordinating C6F5 group is displaced to leave a coordinated allyl ligand.

Scheme 82

t

BuNC adduct formation for cationic complexes.249,250

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Alkyl, Carbonyl and Cyanide Complexes of the Group 4 Metals

Such reactivity is not limited to classic metallocene complexes; indeed the same reactivity was observed with the cyclopentadienyl-amide constrained-geometry complex [(Z5-C5Me4SiMe2NtBu){Z3-Me(CH)2CH2B(C6F5)3}] (4.47), which reacts with tBuNC to afford the s-adduct 4.48 shown in Scheme 82; reaction of 4.47 with PMe3 did not afford the phosphine equivalent of 4.48, but instead afforded Me3PB(C6F5)3 and the parent butadiene complex [(Z5-C5H4SiMe2NtBu)Ti(Z4-CH2CHCHCHMe)] (i.e. the precursor to 4.47; reaction with phosphine effectively reverses the alkyl abstraction reaction). The fact that an adduct was formed with tBuNC once again highlights a distinction between the donor properties of isonitriles compared to other common donors such as phosphines with the Group 4 metals.250 A similar donor function was found when [Cp 2ZrMe]+ reacts with a phosphine-appended alkyne PhC2PPh2 or Me3SiC2PPh2. Rather than the phosphine acting as a donor, the methyl ligand undergoes 1,2-migratory insertion over the alkyne to give the corresponding alkenyl complexes [Cp 2Zr{C(PPh2)]C(Me)Ph}]+ (4.49) and [Cp 2Zr{C(Me)](SiMe3)PPh2}]+ (4.52), each of which contains a frustrated Lewis pair between the zirconium and phosphorus centers (Scheme 83).251,252 These complexes form adducts 4.50, 4.51, and 4.53 with alkyl isonitriles (Scheme 83); interestingly, the Zr. . .P interactions remain unaffected upon isonitrile coordination.

Scheme 83 Reactions of zirconocene frustrated Lewis pairs with isonitriles. [B(C6F5)4]– anion omitted for clarity.251,252

Whilst the chemistry of the Group 4 complexes bearing the bis(cyclopentadienyl) ligand set is extensive, there is comparatively less chemistry with complexes of cycloheptatrienyl (Cht) ligands. When studying the use of isonitriles as Lewis base adducts, much of the chemistry discussed above can be equally applied to the corresponding cycloheptatrienyl derivatives, which can be considered tri-anionic donors with the Group 4 metals. In this regard, [(Cp)(Cht)M] (M ¼ Zr, Hf ) react with isonitriles to afford the corresponding donor complexes [(Cp)(Cht)M(NCR)] [M ¼ Zr, R ¼ tBu (4.54) or Xyl (4.55); M ¼ Hf, R ¼ tBu (4.56) or Xyl (4.57), Scheme 84]; the additional valence electron contribution of the C7H3– 7 ligand means that only a single isonitrile ligand is required to afford an 18-electron complex. Interestingly the corresponding titanium complex could not be prepared, presumably due to the additional steric burden of the Cht ligand; a titanium adduct equivalent to 4.54–4.57 could only be prepared for the ansa-complexes which affords a more open coordination site for the isonitrile to coordinate. Accordingly, [(Z5-C5H4SiMe2-Z7-C7H6)Ti(CNR)] (R ¼ tBu (4.58) or Xyl (4.59)) were prepared and characterized.193,253–255 Similar chemistry was observed with the half-open analogues, in which the cyclopentadienyl ligand is replaced with a pentadienyl ligand derivative. The isonitrile adducts 4.60–4.63 were isolated and were analogous to the Cp congeners, although infrared studies indicate that the pentadienyl ligands are more effective electron donors than the analogous Cp complexes. No coupling chemistry between the coordinated isonitrile and the pentadienyl ligands were observed, even under comparatively forcing conditions.256–258 In addition to replacing the cyclopentadienyl ligand in the above sandwich complexes with an open-chain analogue, it can also be replaced entirely by the isoelectronic imidazoline-2-iminato ligand. The complex was prepared via [ChtZr(Me3SiC3H3SiMe3) (THF)]; reaction with imidazoline-2-imine affords a pogo stick zirconium complex [ChtZr(ImDIPPN)] after elimination of the allyl ligand by proton transfer. The pogo stick complex readily accepts a single additional donor ligand to afford [(Cht)Zr(ImDippN) (CNR)] (R ¼ tBu (4.64) or Xyl (4.65), Scheme 84) when reacted with isonitriles. The molecular structure of 4.64 obtained from X-ray diffraction data is given in Fig. 26; the ZrdCN bond distance is 2.3852(18) A˚ .259 In some of the above examples, isonitrile coordination is favored where other reactivity might be expected, such as in the open-chain cyclopentadienyl analogues, and in cationic complexes where phosphine reverses the alkyl abstraction process. These general observations are somewhat more complex in the case of titanium hydrazido complexes. Titanium hydrazido complexes have been shown to react with isonitriles, as will be discussed in Section 3.04.4.3.2, however in the half-sandwich titanium cyclopentadienyl-amidinate-hydrazido complex [Cp Ti(NNCPh2){MeC(NiPr)2}] (Scheme 85), despite displaying rich reaction chemistry centered about insertion reactions of the hydrazido ligand with CO2, CS2, heterocumulenes, alkynes, and silanes, reaction with isonitriles only afforded the s-adducts 4.66 and 4.67 (Scheme 85), and no insertion chemistry was observed.260

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Scheme 84 Reactions of cycloheptatrienyl complexes with isonitriles.193,253–259

Fig. 26 Molecular structure of [(Cht)Zr(ImDippN)(CNtBu)], H atoms omitted for clarity.259

A different half-sandwich complex that has found use with isonitriles is [CpTi(CO)4]–. In this case, oxidation of the Ti(0) complex using either I2 or R3SnCl (R ¼ Me, Ph) in the presence of isonitrile afforded the tetra(isonitrile) complexes [CpTi(CNR)4E] [X ¼ I (4.68), SnMe3 (4.69), SnPh3 (4.70)]. When using I2 as the oxidant, the Ti(III) complex [CpTi(CNR)2I2] was formed as a by-product.261 Whilst isonitriles usually bind via the terminal carbon in an Z1 binding mode, an unusual bonding motif was observed when an isonitrile adduct was prepared from a chloride-bridged zirconium dimer bearing fulvalene and Cp ligands, [(CpZr)2(m-Z5-C5H4-Z5-C5H4)(m-Cl)2] (4.71, Scheme 86);262 reaction with tBuNC gives a bridging isocyanide in which the terminal carbon bridges the two zirconium centers, whilst the nitrogen bonds to a single zirconium (4.72). The robustness of the isonitrile moiety (and the robustness of the binding mode) was demonstrated in that the complex can be methylated with MeMgCl to give [(CpZrMe)2(m-Z5-C5H4-Z5-C5H4)(m-CNtBu)] (4.73), whilst leaving the isonitrile unaffected. Moreover, reaction of the dimethyl complex with B(C6F5)3 affords the cationic complex [(CpZr)2(m-Z5-C5H4-Z5C5H4)(m-CH3)(m-CNtBu)][MeB(C6F5)3] (4.74), again

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Scheme 85 Reactions of half-sandwich titanium complexes with isonitriles.260,261

Scheme 86 Reactions of bimetallic zirconium fulvalene complexes. [MeB(C6F5)3]– anion omitted for clarity.262

leaving the isonitrile unaffected and adopting the same binding mode. The bonding of the isonitrile ligand can be described by two resonance forms; resonance form A involves a bridging carbon and N-lone pair donation to one zirconium center as in the parent neutral complex, whereas resonance form B involves the carbon binding to one zirconium and the CN p-bond binding to the second zirconium center. The bridging nature of the methyl ligand can be removed by the addition of a further isonitrile ligand, which binds to the non-methyl-bearing zirconium (4.75 and 4.76). Reaction of the cationic complex with water affords a m-hydroxy complex, without altering the isonitrile binding. Similarly, the bis(pentalene) dititanium complex 4.77 (Scheme 87) reacts with isonitrile to afford an adduct 4.78, in which the coordinated isonitrile adopts the same bridging mode found with the fulvalene complexes shown in Scheme 86.263 Complex 4.78 was characterized crystallographically; the proximal TidCN distance was reported as 2.016(6) A˚ , whereas the corresponding distal TidC and Ti. . .N distances were 2.326(7) and 2.147(6) A˚ respectively.

Scheme 87 Synthesis of a dititanium bis(pentalene) complex with bridging isonitrile. R ¼ SiiPr3.263

The donor properties of isonitriles can be useful in the isolation of novel complexes, but the strongly donating properties can also be used to promote reactivity that would otherwise be challenging or impossible to achieve under normal circumstances. The effective use of the strong s-donating properties of the isonitrile ligand was used to good effect in the preparation of zirconium(II)

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complexes bearing a tridentate PNP ligand. Complexes were prepared bearing an Z6-toluene ligand (4.79), which can be considered either a neutral Z6-arene (giving a formal zirconium(II) complex) or else a 1,4-diido ligand with a reduction of the aromaticity and giving a zirconium(IV) complex, with the latter case being predominant in the isolated complexes (Scheme 88). This ligand system can therefore be regarded as a masked Zr(II) synthon, whereby displacement of the neutral arene affords an arene-free Zr(II) complex, but harnessing the potential of this Zr(II) source requires a strong donor ligand to displace the arene. Isonitriles, owing to their effectiveness in forming strong s-complexes, was successful in this endeavor, affording the Zr(II) complex 4.80.159,160

Scheme 88 Zr(II) synthon containing a PNP supporting ligand.159,160

Metal-element multiple bonds can be highly reactive towards unsaturated organic substrates, and may be expected to undergo insertion reactions with isonitriles. However, this is not always the case; zirconium and hafnium complexes bearing a pincer bis(iminophosphorano)methanediide ligand, which bears a potentially reactive formal C]M bond (4.81, Scheme 89), was found to form a stable adduct 4.82 with XylNC. The authors pointed out the unexpected nature of this adduct formation since M]C complexes have a greater tendency to undergo insertion-reactivity and insertion of the isonitrile was anticipated. The formation of an adduct was attributed to the low valence electron count and strong electrophilicity of the metal center, since the adducts are 14 electron complexes.264

Scheme 89 Adduct formation with a PNP-supported zirconium complex 4.81.264

In a similar manner, the titanium silylene complex [Cp2Ti]E(THF)] [E ¼ Si4(SiMet2Bu)4 (4.83)] prepared from calcium tetrasilabicyclo[1.1.0]butane-2,4-diide and [Cp2TiCl2] was found to be unstable to prolonged storage in the solid state owing to the propensity for the THF to de-coordinate and facilitate a decomposition pathway via the resulting 16 electron intermediate. The decomposition was prevented by employing stronger s-donors such as phosphine and isocyanide, which precluded the formation of the base-free complex and afford silylene complex 4.84 (Scheme 90) that were indefinitely stable, both in the solid state and in solution.265 As with the bis(iminophosphorano)methanediide ligand, titanium imido ligands are well-known to possess a rich insertion chemistry with unsaturated substrates such as CO2, heterocumulenes, and isonitriles known to undergo insertion reactions. However, in the case of the b-diketiminato (nacnac) supported titanium imido complex 4.85 (Scheme 91) the isonitrile affords a simple s-adduct and does not show any reaction with the imido fragment. Indeed, the base-free complex was shown to react with Ph2C]C]O, effectively adding an additional alkoxide donor to the nacnac ligand, and yet this reaction was suppressed for the isonitrile-stabilized complex and demonstrated a remarkable stabilizing effect from this strong s-donor.266

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Scheme 90 Donor formation of titanium silylene complex 4.83 with isonitrile.265

Scheme 91 b-diketiminato titanium imido complex and its reaction with isonitrile. Ar ¼ Dipp.266

3.04.4.3.2

Reaction chemistry of isonitrile ligands

Whilst the complexes described in Section 3.04.4.3.1 clearly demonstrate the ability of isonitrile complexes to act as strong s-donor ligands and remain largely unaffected during any subsequent chemical reactivity, the comparatively polar carbon and nitrogen triple bond nevertheless lends itself readily to chemical reactivity in the coordination sphere of a metal. In these cases, the isonitrile ligand becomes a reagent in molecular transformations with other organic fragments around the metal. Whilst other types of reactivity also known, such as coupling reactions, the reactivity of isonitriles tends to be dominated by insertion-type chemistry in which the isonitrile undergoes migratory insertion into a metal-carbon or metal-nitrogen bond akin to the reactivity of CO. However unlike CO, de-insertion reactions are much rarer and reactions with isonitriles are more likely to involve multiple insertions, are therefore useful routes to nitrogen-containing organic compounds.267 Computational studies into the migratory insertions of isonitriles concludes that the rate-limiting step is the coordination of the isonitrile to the metal center, and that the migratory insertion is significantly more exothermic than the corresponding CO reaction, thereby offering an explanation for the rarity of de-insertion reactions of isonitriles.268,269 The examples discussed below give a comprehensive summary of the types of reactivity observed and reported in the review period. The chemistry of isonitriles has been exemplified by a study on titanium tris(ketimide) complexes [Ti(N]CtBu2)3R] [R ¼ Me (4.87), CH2Ph (4.89)]. Reaction of MesNC with the methyl congeners readily induces insertion of the isonitrile into TidCH3 to give an Z2-iminoacyl complex [Ti(N]CtBu2)3(Z2-MeC]NMes)] (4.88) shown in Fig. 27 and Scheme 92.270 Reaction with a zwitterionic complex, derived from the benzyl derivative and B(C6F5)3, [Ti(N]CtBu2)3(m-CH2Ph)B(C6F5)3] (4.90) generates the split ion-pair [Ti(N]CtBu2)3][PhCH2B(C6F5)3] (4.91) where the isonitrile acts as a Lewis base and no insertion chemistry was observed. Interestingly, the isonitrile was resistant to inserting into the TidN bonds, although this is not always the case as will be seen below. The migratory insertion of alkyl ligands into (coordinated) isonitriles is a common reaction motif and there are many examples which follow. The borole-sandwich complex [Cp{Z5-C4H3RB(C6F5)}Zr(C6F5)(OEt2)], by virtue of the dianionic borole ligand, contains one less anionic co-ligand compared to metallocene complexes. The coordinated ether in 4.92 (Scheme 93) is unsurprisingly readily displaced by adding isonitrile, although two isonitriles are coordinated to the Zr center rather than one (4.93); these complexes were found to be thermally unstable and 1,1-migratory insertion ensued, affording the corresponding Z2-iminoacyl ligand in [Cp {Z5-C4H3RB(C6F5)}Zr(CNtBu)(C6F5C]NtBu)] (4.94).271 Similarly, reaction of [Cp2ZrCl(1,4-diphenylbutenyne)PAr2] (4.95) with tBuNC readily undergoes insertion of a single isonitrile into the ZrdC(sp2) bond to afford the Z2-iminoacyl derivative (4.96). Whilst no pre-coordination of the isonitrile was detected in this study, reference to other works, such as those already alluded to, allow such an adduct to be inferred as a transient intermediate. When the chloride complex is methylated and the methyl group abstracted with B(C6F5)3 (4.97) the same reactivity is observed but with an additional isonitrile coordinating to the metal center to complete the coordination sphere in 4.98. The iminoacyl complexes were formed as atropisomers, which could interconvert via rotation about the C(sp2)dC(sp2) bond.272 In the same way, reaction of the titanacyclobutane complexes [Cp 2Ti(CH2)2CHiPr] (4.99, Scheme 93) were found to undergo isonitrile insertion into one of the TidC(sp3) bonds to afford an Z1-iminoacyl complex (4.100).236 This process was found to be reversible with tBuNC (but not with CyNC), which is comparatively rare for isonitriles. Nevertheless, the iminoacyl complex was found to be a useful synthon for further transformations, with reaction with CO leading to cyclopentenamidolates, whereas reaction with ethylene afforded [Cp 2Ti(C2H4)] and a cyclobutanimine. Attempts to induce a second isonitrile insertion into the remaining TidC site by using an excess of RNC were successful only for tBuNC, and in this case one of the cyclopentadienyl rings was removed in the process, along with cleavage of the CdNC bond of an isonitrile, affording a cyanide complex [Cp Ti(CN){(NtBu)2(CCH2)2CHiPr}].

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Fig. 27 Molecular structure of [Ti(N]CtBu2)3(Z2-MeC]NMes)] (4.88), H atoms omitted for clarity.270

Scheme 92 Reactions of titanium tris(ketamide) complexes.270

The migratory insertion of sp3 carbon centers onto isonitriles was readily achieved by reaction of tBuNC with [Cp 2ZrMe2] (4.101). The isonitrile undergoes insertion (via the initially-formed s-complex 4.102) into the ZrdMe bond to give 4.103; due to the excessive steric crowding imposed by the Cp ligands, the complex was formed as a rare example of a “N-outside” isomer; heating the sample causes isomerization to the “N-inside” isomer, in which the N-donor lies between the acyl and methyl ligands. Interestingly, the remaining methyl ligand could be removed via electrophilic cleavage either with B(C6F5)3 or with [CpCr(CO)3H].273 In the latter case, the cationic complex is stabilized by a bridging carbonyl ligand, which binds to the zirconium via a carbonyl oxygen.

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Scheme 93 Insertion reactions of isonitriles into MdC bonds of metallocene complexes. L ¼ coordinated isonitrile.236,271–273

The potential for combining multiple isonitrile molecules to construct more complex organic architectures is demonstrated by the reaction of isonitriles with the titanaallene complex [Cp 2Ti]C]CH2] (4.104, Scheme 94),274 which is itself formed via methane elimination from [Cp 2Ti(CH]CH2)Me]. Complex 4.104 reacts readily reacts with three equivalents of RNC (R ¼ Cy, Ar) via initial insertion into the Ti]C bond, affording a 5-membered metallacycle with a radialene substructure. An isomeric difference in the product was observed for CyNC compared to ArNC; for CyNC the terminal methylene group lies external to the metallacycle (4.105), whereas for ArNC the methylene lies within, giving a C]C]NAr moiety (4.106), which is presumably favored on steric grounds. Isolation and crystallographic characterization led to the conclusion that the reaction proceeds via a [2 +1] addition to afford an Z2-azaburatriene which can bind further isonitrile that can undergo the subsequent insertion steps. Titanium complexes bearing carbazolyl (Carb) ligands have been prepared bearing various alkyl ligands, which show insertion chemistry with isonitrile ligands. Formation of a monomeric complex [M(Carb)2(CH2Ph)2] (4.107) was achieved with benzyl ligands, whereas the sterically demanding CH2SiMe3 ligands afforded dinuclear species [Ti(Carb)2(m-CHSiMe3)]2 (4.109) with bridging alkylidene ligands.275 The mononuclear complex undergoes insertion of XylNC to give the expected Z2-iminoacyl complex (4.108), where both alkyl ligands have undergone insertion. Reaction with the dimeric complex was less obvious. Reaction with

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Scheme 94 Reactions of titanaallene complex with isonitriles.274

three isonitriles evokes insertion into the MdC bonds, but with the formation of bidentate XylN-CH]C(SiMe3)-C]NXyl (4.110, Scheme 95). The authors suggested a reaction pathway but acknowledged that details of the mechanism remain elusive; there is certainly CdH activation involved as evidenced by the formation of free (protonated) carbazole and the loss of a hydrogen atom from the bridging alkyldenes, but it nevertheless showcases complex coupling chemistry available with the Group 4/isonitrile combination since one ligand in 4.110 contains two isonitrile-derived xylyl groups.

Scheme 95 Reaction of carbazole titanium complexes with XylNC. Carb ¼ carbazolyl.275

In addition to insertion chemistry with anionic ligands, isonitrile coupling has also been observed. One such example is the titanium cluster complex [{CpTi(m-O)}3(m3-N)] (4.111) shown in Scheme 96. In this case the cluster contains a nitride and three oxide anions. Upon reaction with XylNC 4.112 is formed in which the oxides remain unaffected but the nitride couples with one isonitrile to give a coordinated carbodiimido; the titanium-bound nitrogen becomes terminal in the process.276 Mechanistically the transformation proceeds via pre-coordination of an isonitrile to one of the titanium centers, followed by a migratory insertion of the nitride ligand. In addition, two further isonitriles couple together to afford a Xyl-N]C]C]N-Xyl which bridges two titanium centers via one terminal N]C bond. Whilst the solid state structure indicates a binding mode in which the bound heterocumulene could be considered dianionic with a NdC single bond, NMR spectroscopic data suggest a degree of fluxionality which was attributed to the heterocumulene migrating from one bound N]C to the other, possibly via a linear p-bound intermediate. The formation of the heterocumulene likewise involves the pre-coordination of an isonitrile to a single titanium center, but in this case the isonitrile p-bond binds to a second titanium center, thus affording a bridging binding mode. This activates the carbon to attack by a further isonitrile to afford the heterocumulene. Further examples of the coupling of multiple isonitriles can be seen in the M(II) synthons [Cp2M(Me3SiC2SiMe3)] (Cp2 ¼ ansatetrahydroindenyl or Cp ). The titanium analogue merely forms a s-adduct, whereas the zirconium and hafnium congeners display a range of coupling reactions depending of the nature of the Cp ligand and the M : isonitrile ratio; the reactions were centered about insertion of the isocyanide into the MdC and CdSi bonds.277 Reaction with 1, 2, and 3 equivalents of isocyanide afforded adducts (4.113 and 4.116), enamine complexes (4.114 and 4.117), and an azametallacycloallene (4.115). The Cp complexes gave more limited reactivity, presumably owing to the increased steric demand, whereas the ansa-tetrahydroindenyl complexes showed greater

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Scheme 96 Isonitrile coupling with [{CpTi(m-O)}3(m3-N)].276

reactivity scope. The Cp complexes were proposed to proceed either via insertion of isonitriles into the two MdC(alkyne) bonds or else via insertion of two isonitriles into the same MdC(alkyne) bond alongside isonitrile coupling. Conversely the ansa-tetrahydroindenyl complexes proceeds via oxidative addition of a SidC bond of the alkyne; isonitriles insert into both the MdC and MdSi bonds and subsequent coupling provides a pathway to the observed products. It is interesting that despite a large number of steps to give a variety of products, each reaction with a given stoichiometry proceeds with a high level of selectivity (Scheme 97). In addition to effecting remarkable transformations of isonitriles, the complex [Cp2Ti(Me3SiC2SiMe3)] can also induce the formation of isonitriles via the decomposition of carbodiimides (Scheme 98).278 Whilst the study was primarily concerned with coordination and reactivity of the carbodiimides themselves, in some cases decomposition reactions were observed in which the carbodiimide RN]C]NR was fragmented to afford coordinated cyanide (R ¼ SiMe3 or tBu) and isonitrile (R ¼ Mes) ligands (4.119). This unusual reactivity was attributed to the radical nature of the 4-membered metallacycles expected reaction with carbodiimides (4.118). Unsaturated metallacyclic complexes have also been shown to undergo migratory insertion reactions with isonitriles. Reactions of [Cp2Zr(Z4-H2C4H2)] (4.120, Cp2 ¼ (C5H5)2, (C5Ht4Bu)2, Me2Si(C5H4)2) with tBuNC afford double-insertion products 4.121 (Scheme 99) in which the isonitrile inserts into each of the ZrdC bonds to afford 1-zircona-2,5-diazacyclopent-3-enes.279 The reaction was subject to some mechanistic uncertainty, with a number of plausible mechanisms being proposed. One of the most likely mechanisms was thought to operate via rearrangement of the precursor or complex to afford a metallacyclopropane obtained through a haptotropic shift of the pent-3-yne ligand. This highly strained ring system is expected to readily insert isonitrile into both ZrdC bonds giving a metallacyclopentene intermediate which can be arranged to afford the observed product. As with many of the insertion reactions reported in the literature, an explicit isonitrile complex was not isolated nor explicitly identified, however the existence of a s-complex as a transient intermediate in these transformations remains likely on the basis of the bulk of the literature surrounding insertion reactions of isonitrile ligands. In probing hapticity shifts in cumulene ligands, Suzuki and co-workers provided further mechanistic insight into the insertion of isonitriles into coordinated alkyne ligands. Their principal study involved proving the effects of donor ligands on the toward nation mode of [5]cumulenes (4.122, Scheme 99).280,281 The findings showed that reaction of hexapentaenes with [Cp2Zr(PMe3)2], gives isomeric complexes (different hapticity of the cumulene) depending on the presence or absence of additional s-donors. When no additional donor is present the 1-zirconacyclopent-3-yne complex was formed, whereas when PMe3 was present the Z2-p complex was favored. This parallels the work of Rosenthal, in that the present study replicates the proposed haptotropic shift that underpins isonitrile insertion.279 This comparison was verified by reaction with tBuNC, which induced the expected double-insertion into the ZrdC bonds to afford the 1-zircona-2,5-diazacyclopent-3-ene 4.123. However, in this case the reaction was sufficiently slow to allow isolation and crystallographic characterization of a complex with pre-coordinated isonitrile. It therefore seems likely that in previous studies, the proposed mechanism which proceeds via a metallic cyclopropane/Z2-alkyne is indeed correct. The haptotropic shift necessary for this transformation, whilst not observed directly in the studies of Rosenthal, is likely to be induced by the pre-coordination of an isonitrile. The isonitrile is consequently perfectly placed for the first migratory insertion on the path to the observed product. Incorporation of a carboranyl unit into zirconocene complexes offers a rich source of reactivity at the ZrdC(carboranyl) bonds. The zirconium carboranyl “ate” complex [Cp2Zr(C2B10H10)(m-Cl)][Li(OEt2)] (4.124) was prepared from Li2C2B10H10 and [Cp2ZrCl2] and can be regarded as a carboryne synthon. Complex 4.124 was found to undergo insertion chemistry with a wide variety of unsaturated organic substrates; of interest to this text is the reaction with tBuNC, which inserts into both of the ZrdC(carboranyl) bonds to afford 4.125, Scheme 100.282 Upon losing LiCl, the reaction is thought to proceed via coordination and subsequent insertion of the first isonitrile, giving a 4-membered zirconacyclobutane intermediate. This highly strained intermediate can then insert a further equivalent of isonitrile to give a zirconacyclopentane, which can thereafter ring-open to afford the product upon coordination of a third isonitrile which acts as a s-donor. Reaction of 4.124 with diethylacetylene affords the zirconacyclopentene complex 4.126. Unlike the “ate” complex precursor, complex 4.126 reacts with XylNC to afford a s-adduct 4.127; heating the complex in toluene in the presence of a second equivalent of isonitrile gave rise to a cyclopentene product (Scheme 100), in which the isonitrile has inserted into the MdC bond and then extruded the zirconium by means of a reductive elimination.283 The need for a second equivalent of isonitrile was intriguing, since it

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Scheme 97 Reactions of Cp2Zr(II) synthons with XylNC.277

Scheme 98 Formation of isonitrile from carbodiimide. L ¼ CNMes.278

is not incorporated into the product. The authors suggested that coordination of additional isonitrile facilitates dissociation of Cp2Zr(II) from the product after the reductive elimination step. By replacing the carboranyl ligand for a dicarbollide ligand (C2B9H2– 11), which is a dianionic isolobal analogue of Cp, much less sterically-demanding complexes can be prepared and the chemistry with isonitriles is different from that observed with carboranyl ligands. Xie reported that the pendant-arm derivative Me2NCH2CH2(C2B9H10) reacts with [CpZrMe3] to afford the alkyl complex [Z1:s:Z5-{(MeN(CH2)CH2CH2)C2B9H10}ZrCp] (4.128, Cp ¼ C5H3(SiMe3)2) in which one of the N-methyl groups has undergone

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Scheme 99 Insertion of isonitrile into metallacyclopentyne complexes.279–281

Scheme 100 Isonitrile reactions with zirconium-boron cluster complexes. Carbon atoms within the cluster are indicated by dots.157,282,283

CdH activation to give an organometallic ligand. In contrast to the carboranyl systems alluded to above, in this case the side-arm is the source of reactivity whereas the carbollide remains an inactive spectator ligand. The organometallic arm undergoes insertion reactions with a number of unsaturated species, including isonitriles to give a sequential double insertion of isonitrile into the Zr-C(arm), along with an additional isonitrile as a s-donor ligand (4.129, Scheme 100).157 Group 4 complexes bearing amide-appended cyclopentadienyl ligands Z5-C5Me4SiMe2NtBu (Cg), so called “constrained geometry catalysts”, have exhibited excellent performance in olefin polymerization, as described in Section 3.04.2. The chemistry

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of [CgM(2,3-dimethylbutadiene)] has been studied and offers a remarkable difference in reactivity depending on the identity of the metal under for otherwise identical complexes and reagents. All three complexes bearing titanium, zirconium, and hafnium, adopt a “supine” orientation of the dimethylbutadiene with the bonding best described as s2p, i.e. a metallacycle with the two terminal carbons bound to the metal by s-bonds and a weakly coordinated internal p-bond (4.130, Scheme 101).284 The titanium complex 4.130-Ti reacts with two equivalents of tBuNC to give a titana-aziridine and an additional coordinated isonitrile (4.132-Ti). The reaction was thought to proceed via the migratory insertion of an isonitrile into one of the TidC s bonds to give an iminoacyl complex (4.131-Ti), as has been described with other complexes above. The iminoacyl subsequently undergoes reductive elimination to afford the observed product, presumably facilitated by coordination of the second isonitrile. Reaction of 4.130-Hf with one equivalent of tBuNC gives the cyclic iminoacyl, as suggested for the titanium congener. A second equivalent of tBuNC gives the unsymmetrical bis-insertion product 4.133-Hf, which must arise from a hydride-shift. The same reaction carried out with XylNC gave a mixture of the symmetrical and unsymmetrical bis-insertion products (4.134 and 4.135 respectively), with the proportion of each dependent on the concentration of the isonitrile: adding XylNC to 4.130-Hf resulted in 92:8 unsymmetrical:symmetrical products, whereas reversing the order of addition results in >95% conversion to the symmetrical isomer. Similarly, reaction with 4.130-Zr gave both symmetrical and unsymmetrical isomers with XylNC (only intractable products were obtained with tBuNC). Addition of XylNC to 4.130-Zr gave 90:10 unsymmetrical:symmetrical; reversing the addition sequence gave 40:60 but if 4 equivalents of XylNC were used the ratio became 25:75 unsymmetrical:symmetrical.

Scheme 101 Reactions of [CgM(2,3-dimethylbutadiene)] with isonitriles.284

Similar reactivity was observed with the dimethyl complexes [CgMMe2] (4.136), in that reaction of 4.136-Ti with one equivalent of tBuNC gives the mono-insertion product with a titana-aziridine, 4.137-Ti, Scheme 102. The remaining methyl ligand undergoes a second migratory insertion with the product 4.138 stabilized by an additional isonitrile donor. Reaction of the first insertion product with XylNC gives a more surprising result in that the additional XylNC does not act as a donor ligand, but instead undergoes a second isonitrile insertion to give a 4-membered ring structure in 4.139. The hafnium congener on the other hand undergoes a single isonitrile insertion with each methyl ligand to afford a complex bearing two hafna-aziridines (4.140).

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Scheme 102 Reactions of [CgMMe2] (M ¼ Ti, Hf ) with isonitriles.284

The half-sandwich complex [(Ind)Ti(NMe2)2Me] (4.141) was found to undergo selective insertion into the MdC bond to give [(Ind)Ti(NMe2)2{C(Me)]NtBu}] (4.142); the reactivity pattern is the same as seen in other complexes described above, but in this case insertion into the MdN bond was not observed.285 DFT studies suggested that this site selectivity was due to the weaker TidC bond compared to TidN. However the MdN bond is clearly a viable reactive site, since reaction of RNC with [(Ind)M(NMe2)Cl2] (M ¼ Ti, Zr) and [(Ind)Zr(NMe2)2Cl] (i.e. no MdC bond was present to give a competitive reaction site) gave rise to single- and double-insertion reactions respectively, with each amide being converted into the carbodiimide derivative, with an exemplar being shown for the conversion of 4.143 to 4.144 in Scheme 103. The double-insertion was not observed for the titanium congener, with only single insertion being observed.

Scheme 103 Half-sandwich indenyl complexes and their reactions with tBuNC.285

Whilst most examples of reactivity with isonitriles involves reaction with a secondary substrate, there are few examples of an intramolecular process facilitated by a Group 4 metal (i.e. where the isonitrile group reacts with its substituent). One such example was reported by Arnold et al., who employed the monoanionic tripodal [N2P2] ligand tBuNSiMe2N(CH2CH2PiPr2)2. Reduction of [(N2P2)ZrCl3] (4.146) with KC8 afforded the Zr(III) complex [({(N2P2)ZrCl}2(m-Cl)2)] (4.145). Interestingly this complex reacts with the sterically demanding terphenyl isonitrile and induces disproportionation to the parent zirconium(IV) complex (4.146) and a zirconium(II) complex bearing a coordinated isonitrile ligand (4.147). When the reduction of complex 4.146 was repeated in the presence of isonitrile, a completely different reaction product was obtained. In this case, product 4.148 was obtained that contains a ring-expanded pyrrole derivative, in which the isonitrile carbon has inserted into one of the flanking mesityl rings; the resulting 7-membered ring p-coordinates to the zirconium to give a formal zirconium(IV) complex.286 Mechanistically, the reaction is thought to proceed via the formation of a transient zirconium(I) complex [(N2P2)Zr(CNAr)], facilitated by the pre-coordination of isonitrile, which lowers the reduction potential of zirconium and makes the reduction of zirconium(IV) to zirconium(I) easier. The highly reactive zirconium(I) complex thereafter facilitates insertion of the isonitrile into one of the mesityl rings; the resulting

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aromatic system is subsequently reduced by the Zr(I) (returning to Zr(IV) to give a [C7H7]3– analogue). The nitrogen is presumably protonated by adventitious water or solvent (Scheme 104).

Scheme 104 Ring-expansion of a terphenylisonitrile facilitated by a Zr(N2P2) complex.286

Scheme 105 Coupling of tBuNC by [Cp TiCl(m-Cl)].287

Reduction of [Cp TiCl3] with LiAlH4 affords the dimeric titanium(III) complex [Cp TiCl(m-Cl)]2 (4.149). Complex 4.149 dissolves in THF to afford the monomeric complex [Cp TiCl2(THF)],288 and so it is surprising that the addition of a stronger s-donor such as RNC (R ¼ tBu or Xyl) does not afford a monomeric complex, but instead increases the coordination number as one isonitrile coordinates to each titanium center to give [Cp TiCl(m-Cl)(CNR)]2 (4.150, Scheme 105). For R ¼ Xyl, the complexes were stable with no evidence of decomposition for T < 120  C, with some disproportionation observed at higher temperatures. However, for R ¼ tBu, leaving the complex in solution for 3 days at ambient temperature (or shorter times at 55  C) induces CdC coupling of two isonitrile units to afford a new species with a bridging iminoacyl ligand, in [{Cp TiCl2}2(m-Z2-Z2-tBuN]CdC]NtBu)] (4.151), which contains an unprecedented symmetric coordination mode.287 Whilst the insertion of isonitriles into alkyl ligands is well-established, less common is the migratory insertion of isonitriles into m-hydrocarbyl ligands. The study of such insertion reactions was conveniently proved using the dinuclear zirconium complex [CpZr(m-CH2)(m-Cl)(m-Z5-C5H4-Z5-C5H4)] (4.152), which bears a fulvalene ligand (Scheme 106). The complex contains a bridging methylene linker, and reaction with RNC (R ¼ tBu or Xyl) affords insertion products 4.153 and 4.154, but in this case one isonitrile inserts into each of the ZrdC bonds of the bridging methylene ligand, thus affording a bis(iminoacyl) ligand which itself bridges the two zirconium centers via two Z2 coordination modes. In addition to the migratory insertion reactions, an additional isonitrile coordinates to one of the zirconium centers, rendering the original bridging chloride coordinated to the other center.262

Scheme 106 Isonitrile coupling in bimetallic zirconocene complexes.262

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The b-diketiminato (“nacnac”) ligand has been employed with most transition metals and has repeatedly demonstrated itself to be a robust ligand framework with an ability to control and enhance reactivity of metal complexes. In the chemistry of titanium, nacnac complexes bearing alkylidene ligands [Ti(nacnac)(CHtBu)(OTf )] (4.155) have been shown to undergo insertion into the alkylidene moiety with a number of unsaturated compounds; with tBuNC, the isonitrile inserts into the alkylidene to afford an Z2-coordinated N-alkylketenimine complex [Ti(nacnac){Z2-(N,C)-(tBuN]C]CHtBu)(OTf )}] (4.156, Scheme 107).289 Conversely, reaction of tBuNC with the titanium nacnac complex bearing phosphidene (M]PR) and alkyl coligands (4.157) reacts such that the tBuNC inserts into both the M]P and MdC bonds. Insertion into the alkyl ligand affords the expected Z2-coordinated N-alkylketenimine whereas insertion at the phosphinidene affords a comparatively rare Z2-(N,C)-phosphaallene ligand (4.158). Interestingly, a further equivalent of isonitrile was seen to coordinate, with the concomitant de-coordination of one arm of the nacnac ligand, which becomes monodentate.

Scheme 107 Insertion reactions with titanium “nacnac” complexes.289,290

In addition to insertion reactions of isonitriles over metal-carbon and metal-phosphorus multiple bonds, insertions into imido ligands have also been probed. Imido ligands have a formal triple bond to transition metals, with one s and two p components. When bonded to early transition metals such as the Group 4 metals the MdN bond is highly polar and therefore reactive to a wide range of unsaturated organic compounds.291 When the Z8-cyclooctatetraene (COT) complex [Ti(NtBu)(COT)] (4.159, Scheme 108) is reacted with tBuNC, an adduct (4.160) is initially formed. On standing for 5 days at ambient temperature, NMR spectroscopic analysis indicated the formation of di(tert-butyl)carbodiimide via a formal nitrene transfer reaction, along with a paramagnetic titanium species that could not be identified. The authors proposed that the isonitrile inserted into the metal-imido bond to form 4.161, followed by the reductive elimination of the carbodiimide to afford an unidentified titanium(II) species. Such a reaction has the potential to be a titanium(II) synthon and so the reaction was performed in the presence of an excess of 1,3,5,7-cyclooctatetraene, which was successful in trapping the titanium(II) species as [Ti(COT)(Z4-C8H8)] (4.162). The insertion reaction was hindered by increasing the steric demand of either the COT (by adding silyl substituents) or the imido ligand (by adding a sterically demanding aryl group as the N-substituent).292

Scheme 108 Reactions of [Ti(NtBu)(COT)] (4.159) with tBuNC.292

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Varying reactivity with different isonitriles was also observed for the imido complexes bearing amidinate and Cp ligands, [Cp Ti {NC(C6F5)]NOtBu}{PhC(NiPr)2}] (4.163, Scheme 109). In this case reaction with tBuNC afforded a chiral-at-metal adduct 4.164 with a four-legged piano stool geometry, and which was found to be fluxional; the two enantiomeric forms interconverted on the NMR timescale giving an apparently Cs-symmetric complex, which decoalesced on measuring 1H NMR spectra at –80  C. The corresponding reaction with XylNC differs somewhat from that with tBuNC; whilst the initial reaction produces an adduct akin to complex 4.165, this species is unstable and over 16 hours transforms to [Cp Ti{PhC(NiPr)2}{NC(NOtBu)C6F4N(Xyl)C}(F)] (4.166), in which the flanking C6F5 ring has undergone CdF activation and cyclization to afford a quinazoline-type ligand. The authors proposed that the formation of complex 4.166 arises from an initial [1 +2] addition XylNC to the imido fragment, affording an Z2-carbodiimide ligand as discussed above for [Ti(NtBu)(COT)]. Subsequent to this step, nucleophilic attack of the isonitrile nitrogen on the proximal ortho CdF generates the new heterocycle and liberates a fluoride anion that subsequently coordinates to the titanium center. The reaction was slowed down significantly by replacing the C6F5 ring for a 2,6-C6H3F2 variant, thereby indicating that the electrophilicity of the aryl ring is critical in facilitating this reactivity. The transformations highlight the significant potential for Group 4 mediated transformation in the construction of complex and important molecular species; in this case, quinazoline compounds have been used in the synthesis of pharmaceuticals.293

Scheme 109 Reactions of isonitriles with titanium imido complexes293

Although structurally related to imido ligands, hydrazido ligands M]NdNR2 have shown a remarkable chemistry that is distinct from that observed with imido complexes, often owing to the additional NdN bond being involved as a reactive site. The reactivity of isonitriles with two titanium hydrazide complexes [Ti(NNPh2)(N2O)(py)2] (4.169) and [Ti(NNPh2)(NAr 2 O)(py)2] (4.167) was probed (N2O ¼ O(CH2CH2NSiMe3)2; NAr 2 O ¼ O(2-C6H4NSiMe3)2) (Scheme 110); complex 4.167 reacts with XylNC to afford a s-adduct by displacing one pyridine ligand (4.168). However, when XylNC was reacted with complex 4.169, i.e. without the aromatic arms on the tridentate supporting ligand, the hydrazido ligand was seen to cleave at the NdN bond to afford the N-coordinated N]C]NXyl and NPh2 ligands in 4.171 (a reaction step that has been reported for hydrazide complexes bearing diamido-pyridine ligands),295,296 along with insertion into one of the TidNamide bonds to afford an Z2-bonded iminoacyl ligand,

Scheme 110 Reactions of isonitriles with titanium hydrazido complexes.294

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presumably via pre-coordination of the isonitrile. In contrast, reaction of 4.169 with tBuNC leads to NdN bond cleavage of the hydrazide ligand, giving coordinated N]C]NtBu and NPh2 ligands as seen for XylNC, but without the accompanying amide insertion step (4.170).294 The transformations based upon isonitrile reactivity thus far described are invariably stoichiometric reactions. They afford novel routes to the construction of new organic fragments but nevertheless remain non-catalytic in nature. Given the nature of some of the molecular transformations described for isonitriles in the coordination sphere of a Group 4 metal, rendering these reactions catalytic would have clear advantages. One such example of a catalytic transformation catalyzed by a Group 4 metal is the three-component coupling of alkynes, amines, and isonitriles to give a,b-unsaturated b-iminoamines.297 In work by Mountford and Gade et al. the authors reported insights into the mechanism of this coupling reaction by modelling the catalytic intermediates starting from a titanium imido complex. Titanium imido complexes supported by a tridentate diamido-pyridine ligand [Ti(NtBu)(N2Npy)(py)] (4.172) (N2Npy ¼ MeC(2-C5H4N)(CH2NAr)2, Ar ¼ 3,5-C6H3Me2) have been shown to undergo [2 + 2] cycloaddition of alkynes to afford an azatitanacyclobutene, which is an intermediate in the hydroamination of alkynes. Whilst reaction of the 4-membered ring with amine completes the catalytic hydroamination reaction, reaction with isonitrile inserts into the TidC bond to afford an azatitanacyclopentene complex (Scheme 111), which was crystallographically characterized. Subsequent aminolysis liberated the a,b-unsaturated b-iminoamine and reformed the imido complex, thus completing the catalytic cycle.298

Scheme 111 Catalytic iminoamination of alkynes.298

A further example of a catalytic reaction involving isonitriles is the zirconium imido based nitrene transfer. The zirconium complex [Zr(NNN)(Cl)(CNtBu)2] (4.173) reacts with an organic azide RN3 to afford the corresponding imido complex [Zr(NNN) (NR)(Cl)(L)] (4.174), which couples with a coordinated isonitrile to form a bound carbodiimide in 4.175 which is liberated to regenerate the initial complex 4.173. The reaction is facilitated by a redox non-innocent ligand, which can engage in a 2-electron transfer and facilitate the formation of the imido complex, and the reductive elimination of the Z2-carbodiimide, which is the key component to allowing closure of the catalytic cycle (Scheme 112).299

3.04.4.4

Summary

In summary, the synthesis of Group 4 cyanide complexes has yielded only a small number of complexes within the review period, with only a handful prepared from a pre-formed cyanide source (via salt metathesis). The principal route to cyanide complexes has been via an indirect route involving the activation of organic molecules, with the cyanide moiety being produced in a bond-scission process, often accomplished using low-valent metal complexes (e.g. M2+) but has also been shown to operate with M4+ species when supported by an appropriate ligand set. Conversely, isonitriles have a rich chemistry when associated with the Group 4 metals. Their s-donating ability is profound and can be used to excellent effect in a number of synthetic strategies. For example, they can be used to aid purification and/or aid crystallization of complexes; they can be used to stabilize complexes that would otherwise be too unstable to isolate; and, they can induce reactivity that would be otherwise impossible with weaker s-donors. Even with a comparatively reactive ligand set isonitriles can act as supporting Lewis base donors without promoting undesired chemical pathways. In addition to being employed as s-donor ligands, isonitriles can be highly reactive in the coordination sphere of Group 4 metals. Migratory insertion reactions are common, but other types of reactions such as isonitrile coupling are also known. The reactivity patterns have allowed the construction of a number of complex organic fragments, which can, in some cases, be rendered catalytic. It is therefore clear that

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Scheme 112 Catalytic carbodiimide formation via azide-isonitrile coupling.299

isonitrile complexes and their reactivity have a significant role to play in synthetic chemistry beyond fundamental studies into organometallic reactivity.

3.04.5

Outlook

The organometallic chemistry of the Group 4 metals has grown in breadth as well as in depth over the last 20 years. It is clear to see that whilst there is a substantial amount of fundamental organometallic chemistry in this subject area, Group 4 complexes bearing alkyl, carbonyl, cyanide and isonitrile ligands have a rich reaction chemistry and have great capacity for inducing molecular transformations with organic substrates. The start of the review period saw an explosion in olefin polymerization with ligand environments other than bis(cyclopentadienyl), and as a consequence the chemistry of olefin polymerization has been applied in a far wider context than previously imagined. However, the scope of molecular transformations that can be enabled with the Group 4 metals is far greater, encompassing many other organic building blocks. Whilst the dominance of the +4 oxidation state renders many organometallic reaction steps, typical for later transition metals, irrelevant for the most part, this has not hindered the development of stoichiometric and catalytic processes with these metals. The application of low-valent Group 4 complexes, whether using carbonyl complexes or synthons such as Z2-alkyne complexes, has further given access to redox-based transformations that have previously been rare, and there is much exciting chemistry that has been carried out in this regard. Reaction chemistry with isonitrile ligands has also grown rapidly, and with the advent of catalytic transformations involving isonitriles, the scope of using isonitriles as novel building blocks looks promising. As the depth of understanding, and the scope of application, of the Group 4 metals is considered, one can look to the future with great anticipation of what the next 20 years will bring—and it is clear from the above text that there is much more to discover with the Group 4 metals.

Acknowledgement The authors thank the EPSRC (studentship to MSS) and Cardiff University (studentship to OGG) for financial support. We acknowledge the use of the EPSRC funded Physical Sciences Data-science Service hosted by the University of Southampton and STFC under grant number EP/S020357/1.

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3.05 N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides Florian Jaroschik, Institut Charles Gerhardt Montpellier, CNRS, ENSCM, Montpellier, France © 2022 Elsevier Ltd. All rights reserved.

3.05.1 Introduction 3.05.2 Divalent lanthanide NHC complexes 3.05.3 Trivalent lanthanide NHC complexes 3.05.3.1 Monodentate neutral NHC ligands 3.05.3.2 Bidentate monoanionic NHC ligands 3.05.3.2.1 NHC ligands with N-based tether (amido group) 3.05.3.2.2 NHC ligands with O-based tethers (alkoxy or aryloxy, enolate group) 3.05.3.2.3 NHC ligands with C-based tether 3.05.3.2.4 NHC ligands with polyatomic tether (Cp, indenyl, fluorenyl, NCO, NCN) 3.05.3.3 Tridentate NHC pincer complexes 3.05.3.3.1 Pincer ligands with one NHC unit 3.05.3.3.2 Pincer ligands with two NHC units 3.05.4 Tetravalent lanthanide NHC complexes 3.05.5 Lanthanide complexes with abnormal/mesoionic NHC ligands 3.05.6 Conclusion Acknowledgment References

3.05.1

163 163 168 168 171 171 173 185 187 189 189 192 194 196 198 198 199

Introduction

Lanthanide (Ln) N-heterocyclic carbene (NHC) complexes, including the group 3 metals Sc and Y, have received considerable attention over the last 15 years not only for their structural features concerning the intriguing lanthanide-carbene bond but also for their potential applications as readily tunable catalysts in organic chemistry and polymerization reactions. Since the previous edition (COMC-3), three reviews have appeared in this field, the most prominent being the Chemical Reviews article by P. L. Arnold in 2009.1 F. E. Kühn and colleagues reported an update on structure and applications as a book chapter in 2017,2 and in 2020, G. He and colleagues reviewed this topic with a special focus on catalysis.3 This article will be divided into four general parts depending on the oxidation state of the rare earth metals (+2, +3, +4) and the nature of the NHC ligand (normal vs meso/abnormal) to provide a full picture of the current status of lanthanide NHC research. Within each part, the complexes will be organized depending on the denticity of the NHC ligand (mono vs multi dentate ligands). Synthetic pathways and coordination chemistry of the complexes will be outlined first, followed by their reactivity and synthetic applications. NHCs are structurally versatile ligands, which have found widespread applications in organometallic chemistry and catalysis.4–7 Their electronic and steric properties can be readily modulated by introducing substituents on the nitrogen atoms or by modifying the carbon backbone (saturation, substitution, ring size).8,9 Despite their classification as soft ligands, the strong s-donor ability of NHCs allows the coordination to the highly electropositive and Lewis acidic lanthanide metals in different oxidation states. DFT calculations for trivalent lanthanide ions bound to NHC ligands have shown this bonding to be mainly ionic in the absence of any metal to carbene p-back donation.10,11 However, the valence orbital energy and hard acid/soft base mismatch as well as the influence of ancillary ligands on the lanthanide metal make these LndC bonds relatively weak compared to most LndN, LndO and LndF bonds. It has therefore proved advantageous to introduce one or more additional binding sites onto the NHC ligand via anionic donor atoms that are tethered to ring N-atoms; these further stabilize the resultant Ln-NHC complexes through increased electrostatic attraction between the ligand and the Ln cation, with additional entropic stabilization provided by chelate effects. Fig. 1 provides an overview on NHC ligands that have been applied in lanthanide chemistry so far, as well as their abbreviations that will be used throughout this article.

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164

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

Fig. 1 NHC ligands applied in lanthanide chemistry.

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

3.05.2

165

Divalent lanthanide NHC complexes

Divalent lanthanides are strong reducing agents and their complexes are often stabilized by ethereal solvents, such as tetrahydrofuran (THF), dimethoxyether (DME) or diethyl ether (Et2O). The vast majority of divalent lanthanide NHC complexes have been synthesized using an exchange reaction between THF adducts of divalent complexes and the strongly Lewis basic NHC ligand in non-polar solvents, such as toluene or hexane. This pathway has been validated for several ancillary ligands, including substituted cyclopentadienyls (Cp), amides, alkyls and the tris(pyrazole)borate (Tp) ligand (Scheme 1). Depending on the metal and/or ligand size, mono or bis(NHC) adducts were obtained. Structural and NMR parameters for these complexes are summarized in Table 1.

Scheme 1 Principal synthetic pathway to divalent Ln-NHC complexes (Cp ¼ C5Me5, Cp0 ¼ C5Me4Et, Cpt ¼ C5H4tBu, Cptt ¼ C5H3(tBu)2, Mes ¼ 2,4,6-Me3C6H2, TptBu,Me ¼ tris(3-tBu-5-Me-pyrazolyl)borate).

The seminal works by Arduengo and Schumann on divalent ytterbium and samarium metallocene complexes [Ln(NHC)(CpR)2] launched the chemistry of lanthanide NHC complexes in the mid-1990s.12–15 Complexes 1–6 shown in Scheme 1 were among the first isolated Ln-NHC complexes, and were characterized by 1H and 13C NMR spectroscopy and in certain cases by XRD analysis. These complexes show high thermal stability and remain intact even in THF or diethyl ether solutions. The varying steric bulk of the Cp and/or the NHC ligands influences the LndCcarbene bond lengths and the 13C NMR shifts of the carbene atom (Table 1). The only bis(carbene) complex, [Sm(IMe4)2(Cp )2] 6, was isolated in the case of the larger Sm metal center.13

166

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides Table 1

LndCcarbene distances and 13C NMR data on divalent lanthanide NHC complexes.

Compound name

Number

Ox state

[Yb(IMe4)(Cp )2] [Yb(IPrMe)(Cp )2] [Sm(IPrMe)(Cp )2] [Yb(IMe4)(Cp0 )2] [Yb(IPrMe)(Cp0 )2] [Yb(IMe4)(Cptt)2] [Sm(IMe4)2(Cp )2]

1-Yb 2-Yb 2-Sm 3-Yb 4-Yb 5-Yb 6

2 2 2 2 2 2 2

[Yb(IMe4)(Tp)I] [Yb(IMe4)(IMe40 )(Tp)I]

7 8

2 2

[Yb(IPrMe){N(SiMe3)2}2] [Yb(IMes){N(SiMe3)2}2] [Yb(IPr2)2{N(SiMe3)2}2]

9 10 11-Yb

2 2 2

[Sm(IPr2)2{N(SiMe3)2}2]

11-Sm

2

[Yb(IPrMe)2{N(SiMe3)2}2] [Sm(IPrMe)2{N(SiMe3)2}2]

12-Yb 12-Sm

2 2

[Yb(IMe4)2{N(SiMe3)2}2] [Yb(IMe4)3(PPh2)2]

13 14

2 2

[Yb(IBu2){C(SiHMe2)3}2] [Yb(IBu2){C(SiHMe2)3}{HB(C6F5)3}] [Sm(IBu2){C(SiHMe2)3}2] [Yb(IMes)I2(THF)3] [Eu(IMes)I2(THF)3] [Yb(PMes){N(SiMe3)2}2] [Sm(PMes){N(SiMe3)2}2]

15-Yb 16 15-Sm 17-Yb 17-Eu 20-Yb 20-Sm

2 2 2 2 2 2 2

˚) LndC (A

2.782(3) 2.552(4) 2.598(3) 2.837(7) 2.845(7) 2.641(6) 2.710(5) 2.609(5) 2.600(3) 2.637(2) 2.642(2) 2.780(2) 2.784(2) 2.803(2) 2.815(2) 2.615(3) 2.5859(19) 2.567(2) 2.591(2) 2.605(3) 2.780(3) 2.650(3) 2.749(6) 2.605(2) 2.780(2)

13

Refs.

205.0 200.7

14 14 15 12 12 14 13

C (ppm)

205.0 198.1 201.8

199.8 201.7 208.3 197.9 205.4

16 16 17 17 19 19

207.7

18,19 19

201.5 201.3

18 18

196.9 194.3

21 21 21 22 22 23 23

In 2006, Takats showed that bulky Tp ligands, such as tris(3-tBu-5-Me-pyrazolyl)borate (TptBu,Me), could also form stable divalent Ln-NHC complexes.16 Reaction of the [Yb(TptBu,Me)I] complex with IMe4 provided the expected THF/NHC exchange to yield compound 7 (Scheme 1). However, for the Yb alkyl precursor [Yb(TptBu,Me)(CH2SiMe3)], CdH activation was observed, providing the methylene bridged bis-carbene complex [Yb(IMe4)(IMe40 )(TptBu,Me)] 8, bearing one normal and one modified NHC ligand (Scheme 2). The two NHC units show different LndC bond lengths 2.710 (5) and 2.609 (5) A˚ , with the shorter belonging to the tethered ligand. A significant difference was also observed for the 13C NMR carbene resonances appearing at 208.3 and 201.7 ppm.

Scheme 2 CdH activation in IMe4 carbene ligand with [Yb(TptBu,Me)(CH2SiMe3)] precursor (TptBu,Me ¼ tris(3-tBu-5-Me-pyrazolyl)borate).

Renewed interest in divalent Ln-NHC complexes arose from the report by Cui in 2012 on the synthesis of mono(NHC) adducts of Yb amides [Yb(NHC){N(SiMe3)2}2] 9 and 10 using the bulky NHC ligands IPrMe and IMes (Scheme 1).17 The steric bulk of the NHC ligand influenced the 13C NMR shifts in these three-coordinate complexes, going from 197.9 to 205.4 ppm. These complexes were successfully employed as pre-catalysts in the dehydrogenative coupling of silanes with amines. Following this work, several

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167

divalent Yb and Sm bis(amido) bis(NHC) complexes [Ln(NHC)2{N(SiMe3)2}2] 11–13 were synthesized and structurally characterized.18,19 These four-coordinate, tetrahedral complexes show LndC bond lengths as expected from the steric bulk of the NHC ligands with IMe4 < IPr2 for Yb and IPr2 < IPrMe for Sm (see Table 1). The measured 13C NMR shift of 207 ppm for complex 12-Yb is surprisingly high, however, it was measured in THF-d8 whereas all other spectra were measured in benzene-d6. Complexes 11–13 are highly efficient pre-catalysts for hydrophosphination reactions of alkenes and alkynes. It was found that complex 13 could be recovered at the end of the hydrophosphination reaction and efficiently be re-used at least five times.18 Very recently, Roesky showed that complex 13 could also be synthesized by displacement of a pyridine-bridged bis(silylene) ligand {SiNSi} ([NC5H3{N(Et)Si[(NtBu)2CPh]}2-2,6]) from a divalent bis(amido) ytterbium complex [Yb{SiNSi}{N(SiMe3)2}2] (Scheme 3). The reaction occurred at room temperature in benzene with the IMe4 ligand, whereas no reaction was observed with the more bulky and less electron-rich IDipp ligand.20

Scheme 3 Alternative synthesis of complex 13.

The amido Ln-NHC complex 13 could be further converted to the first phosphide Ln-NHC complex [Yb(IMe4)3(PPh2)2] 14 upon reaction with diphenylphosphine (Scheme 4).18 Its formation may result from ligand rearrangement of an intermediate [Yb(IMe4)2(PPh2)2] complex, which could however not be observed. The penta-coordinate complex 14 has slightly shorter YbdC bond lengths than the amido precursor and the same 13C NMR shift for the carbene atoms. A broad peak at 10.8 ppm was observed in the 31P NMR spectrum at room temperature, whereas a sharp peak at 12.8 ppm was obtained at 80  C. This phosphide complex catalyzed the polymerization of styrene under neat conditions at room temperature to provide atactic polystyrene.

Scheme 4 Synthesis of the first Ln-NHC phosphide complex [Yb(IMe4)3(PPh2)2] 14.

The synthesis and chemistry of the first divalent alkyl Ln-NHC complexes [Ln(IBu2) {C(SiHMe2)3}3] 15 (Ln ¼ Yb, Sm) was reported by Sadow in 2016 (Scheme 1).21 The formally three-coordinate complexes with the bulky t-butyl NHC ligand show different agostic metal-hydrogen interactions depending on the Ln metal. For ytterbium these metal-hydrogen interactions are only on one alkyl ligand (15-Yb), whereas for samarium both alkyl ligands contribute to the stabilization of the metal center (15-Sm). The displacement of the NHC ligand from the Sm metal center via addition of THF was followed by UV–vis spectroscopy, revealing that at least 8 equivalents of THF were necessary to obtain the THF adduct as major complex. Both complexes 15 were active catalysts for the dehydrocoupling of primary and secondary silanes with primary and secondary amines. The reaction of 15-Yb with [B(C6F5)3] afforded the zwitterionic species [Yb(IBu2){C(SiHMe2)3}{HB(C6F5)3}] 16 via hydrogen abstraction according to NMR studies (Table 1).21 For the NHC lanthanide iodide complexes [Ln(IMes)I2(THF)3] (Ln ¼ Eu, Yb) 17 a unique reaction pathway was developed using the silver carbene complex [Ag(IMes)I] as redox-transmetallating agent in the presence of metallic ytterbium or europium (Scheme 5).22 These complexes showed YbdC and EudC bond lengths of 2.650 (3) and 2.749 (6) A˚ , respectively, according to XRD analysis. When the Eu complex 17-Eu was excited under UV light, bright yellow-green luminescence was observed in a

168

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

Scheme 5 Synthesis of divalent Ln-NHC complexes via redox-transmetalation and reaction with CO2 (Dipp ¼ 2,6-iPr2C6H3; Mes ¼ 2,4,6-Me3C6H2).

C6D6 solution and in the solid state, whereas in THF the luminescence changed to purple. Upon reaction of 17-Eu with CO2 in THF, the zwitterionic IMes-CO2 ligand with a carboxylate and an imidazolium moiety was formed without oxidation of the metal center. From this reaction the new divalent complex [{Eu(IMes-CO2)2(THF)4}I2] 18 was isolated. In contrast, with the bulkier NHC ligand-based silver complex [Ag(IDipp)I], the redox-transmetallation reaction afforded no coordination of the NHC ligand to the lanthanide metal and only [EuI2(THF)5] and non-coordinated IDipp ligand was obtained. Consequently, a one pot procedure was developed to access directly the CO2 insertion products 18 and 19 for both IMes and IDipp ligands without isolating the intermediate Ln-NHC complexes.22 Very recently, the first examples of divalent lanthanide bis(amido) complexes carrying a bulky 6-membered tetrahydropyrimidine-based NHC ligand (PMes) were reported by Trifonov and coworkers (Scheme 1).23 The complexes [Ln(PMes){N(SiMe3)2}2] 20-Yb and 20-Sm were obtained by classical THF-displacement in toluene and characterized by NMR spectroscopy and XRD analysis. These complexes show trigonal planar coordination geometry and no significant difference was observed for the YbdCcarbene bond length in 20-Yb compared to the analogous complex 10 bearing the five-membered IMes ligand (Table 1). Complexes 20 were shown to be efficient precatalysts for the hydrophosphination of 1-alkenes, cyclohexene and norbornene in combination with primary and secondary phosphines.23

3.05.3

Trivalent lanthanide NHC complexes

3.05.3.1

Monodentate neutral NHC ligands

In analogy to the synthesis of divalent Ln-NHC complexes, trivalent complexes with simple, neutral monodentate NHC ligands were mainly obtained by reaction of solvated or solvent-free Ln(L)3 or Ln(L)2(L0 ) complexes with the free carbene precursor (Scheme 6).

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

169

Scheme 6 Synthesis of trivalent lanthanide NHC complexes with neutral, monodentate NHC ligands (Cp ¼ C5Me5, Cpt ¼ C5H4tBu).

The early report on the tris(b-diketonato)lanthanide NHC complexes [Ln(IMe4){[OC(tBu)]2CH}3] 21 by Arduengo in 1994 also validated THF/carbene exchange as a synthetic approach for trivalent lanthanide complexes (Scheme 6).13 The structure of 21Eu was established by XRD analysis, whereas 13C NMR analysis of 21-Y showed the stability of the complex in solution with the first observed YdCcarbene coupling constant (1JY-C ¼ 33 Hz). Shortly afterwards, Anwander reported the synthesis of the erbium trichloride NHC complex 22 and several tris(silylamido) lanthanide NHC complexes 23–25 using the smaller IMe2 ligand.24 While complexes 22 and 23 were characterized by IR and NMR spectroscopy and elemental analysis, the Y mono and bis(carbene) complexes [Y(IMe2){N(SiHMe2)2}3] 24 and [Y(IMe2)2{N(SiHMe2)2}3] 25-Y were also structurally characterized by XRD analysis, revealing the stabilization of the metal center by YdHdSi b-agostic interactions. Later on, the La and Nd analogues 25-La and 25Nd were prepared accordingly and the structure of the Nd complex was analyzed by XRD studies.25 In contrast to the tris(alkyl) lanthanide NHC complexes discussed below, the isostructural tris(amido) bis(carbene) complexes 25-Y and 25-Nd show a slightly distorted trigonal bipyramidal coordination geometry with the two carbene ligands in the apical positions of the bipyramid.

170

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

Very few Cp-based trivalent lanthanide carbene complexes have been reported. The first examples were obtained by Baudry from the reaction of solvent-free Sm metallocene allyl or chloride precursors with the IMe4 ligand providing complexes [Sm(IMe4) (Cpt)2(C3H5)] 26 and [Sm(IMe4)(Cpt)2Cl] 27 as shown by NMR spectroscopy (Scheme 6).26 Complex 27 could also be structurally characterized by XRD analysis. In 2005, Berthet and Ephritikhine, reported on the synthesis and structural elucidation of the cerium complexes [Ce(IMe4)(Cpt)3] 28 and [Ce(IMe4)(Cp )2I] 29 (Scheme 6), and compared these with the corresponding uranium analogues.27 Longer CedI and CedC bond lengths were observed compared to the uranium analogues, in agreement with the highly ionic character of the Ce metal and the more covalent character of UdI and UdC bonds. Most interestingly, competition experiments between equimolar amounts of Ce and U precursors with one equivalent of IMe4 ligand largely favored the formation of the U-carbene complexes in a 8:2 ratio at r.t. and a 9:1 ratio at −60  C. It should further be noted that the CedNHC bonds in 27 and 28 were strong enough to resist displacement by THF or pyridine molecules.27 The most widely studied complexes with these neutral, monodentate NHC ligands are tris(alkyl) lanthanide NHC complexes. In 2007, Schumann reported on the synthesis of the erbium and lutetium mono and bis-carbene complexes [Ln(IPrMe)(CH2SiMe3)3(THF)] 30 and [Ln(IPrMe)2(CH2SiMe3)3] 31, which could be structurally characterized by XRD analysis (Scheme 6).28 For complex 30, which retains one THF molecule, the five-coordinate metal center has a distorted trigonal planar coordination geometry with the THF molecule and one alkyl group in the apical positions. For the bis(carbene) complex 31-Lu, the geometry changes to a strongly distorted square pyramid with one alkyl ligand in the apical position. Subsequently, NHC ligands bearing bulky aryl substituents (IDipp, IMes, IDmp) were employed to access the solvent-free mono-carbene bis(alkyl) lanthanide complexes 32–34 in very high yields by THF/carbene exchange in toluene at room temperature.29,30 In the case of the IMes ligand, only the Lu complex [Lu(IMes)(CH2SiMe3)3] 34 could be obtained, whereas the Y complex underwent immediate CdH activation (see Section 3.05.3.2.3).29 Complexes 32–34 show a distorted tetrahedral coordination geometry around the metal center, with only slight influences on the LndCcarbene distances by the steric bulk of the aryl groups (see Table 2). Complexes 32 and 33-Sc were applied in the homopolymerization of 1-alkenes after activation with 2 equivalents [Ph3C][B(C6F5)4] and in the copolymerization of 1-hexene with 1,5-hexadiene, showing good to excellent activity at room temperature.30 Very recently, a chiral bis(oxazoline) based NHC ligand (IBiox) was employed for the synthesis of the mono(carbene) complexes [Sc(IBiox)(CH2SiMe3)3] 35-Sc and [Ln(IBiox)(CH2SiMe3)3(THF)] 35-Y and 35-Lu (Scheme 6).31 These complexes were structurally characterized and employed in the polymerization of a-olefins after activation with 2 equivalents [Ph3C][B(C6F5)4]. Whereas the coordination geometry of 35-Sc is comparable to complexes 32–34, the ScdCcarbene bond length is significantly shorter (2.3515 (18) A˚ ) than for example in the IDipp analogue 33-Sc (2.412 (5) A˚ ). Furthermore, the carbene signal in the 13C NMR is strongly influenced by the ligand leading to an upfield shift to 163 ppm (vs 188 ppm for 33-Sc). Complexes 35-Y and 35-Lu, are distorted trigonal bipyramids with one alkyl group and the THF molecule in the apical positions, in common with complexes 30.31

Table 2

LndCcarbene distances and 13C NMR data on trivalent lanthanide NHC complexes with neutral NHC ligands. ˚) LndC (A

Compound name

Number

Ox state

[Y(IMe4){[OC(tBu)]2CH}3]

21-Y

3

[Eu(IMe4){[OC(tBu)]2CH}3] [Y(IMe2){N(SiHMe2)2}3]

21-Eu 24

3 3

[Y(IMe2)2{N(SiHMe2)2}3]

25-Y

3

[Nd(IMe2)2{N(SiHMe2)2}3]

25-Nd

3

[Sm(IPrMe)(Cpt)2Cl] [Ce(IMe4)(Cpt)3] [Ce(IMe4)(Cp )2I] [Er(IPrMe)(CH2SiMe3)3(THF)] [Lu(IPrMe)(CH2SiMe3)3(THF)] [Lu(IPrMe)2(CH2SiMe3)3]

27 28 29 30-Er 30-Lu 31-Lu

3 3 3 3 3 3

[Sc(IDmp)(CH2SiMe3)3] [Sc(IDipp)(CH2SiMe3)3] [Y(IDipp)(CH2SiMe3)3]

32 33-Sc 33-Y

3 3 3

2.663(4) 2.55(1) 2.560(9) 2.648(8) 2.671(9) 2.771(3) 2.751(3) 2.62(2) 2.797(4) 2.724(4) 2.520(6) 2.488(3) 2.557(6) 2.639(7) 2.433(9) 2.412(5) 2.555(2)

[Lu(IDipp)(CH2SiMe3)3] [Lu(IMes)(CH2SiMe3)3] [Sc(IBiox)(CH2SiMe3)3(THF)] [Y(IBiox) (CH2SiMe3)3(THF)] [Lu(IBiox) (CH2SiMe3)3(THF)]

33-Lu 34 35-Sc 35-Y 35-Lu

3 3 3 3 3

2.341(4) 2.3515(18) 2.550(2) 2.4789(2)

13

C (ppm) (1JYC (Hz))

Refs.

199.4 (33)

13

190.3 (49.6) 194.0

13 24 24 25 26 27 27 28 28 28

185.1 187.9 193.1 (37.3) 205.0 202.8 163.0 172.3 181.6

30 30 29 29 29 31 31 31

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171

Bidentate monoanionic NHC ligands

The majority of Ln-NHC complexes in the literature contain bidentate NHC ligands bearing an anionic C, O or N-based tether group as this increases the coordination stability to the metal center. Consequently, this section will be divided according to the nature of the anionic groups.

3.05.3.2.1

NHC ligands with N-based tether (amido group)

The synthesis of the first amido-tethered NHC lanthanide complexes by P. Arnold in 2003 can be considered as a milestone in NHC lanthanide chemistry, having subsequently led to a plethora of anionic bound NHC lanthanide complexes with diverse structures and reactivity.32 Reaction of the trivalent tris(amido) lanthanide precursors [Ln{N(SiMe3)2}3] with the lithium carbene amine [LiBr(HHLN)] (HLN ¼ tBuNCH2CH2[C{N(CHCH)NtBu}]) in hot toluene provided the amido-tethered lanthanide carbene complexes [Ln(HLN){N(SiMe3)2}2] 36 (Ln ¼ Sm, Y, Nd, Ce) (Scheme 7).32–34 The complexes crystallize with a pseudo tetrahedral coordination geometry and the LndCcarbene bond lengths diminish from Ce to Sm according to the lanthanide contraction (Table 3). LndSi and LndC short contacts were also observed with the SiMe3 and the amido a-CH2 groups. The yttrium complex was shown to be stable in the presence of THF, diethyl ether, PPh3 and Me3NdO, whereas N,N,N0 ,N0 -tetramethylenediamine (TMEDA) and Ph3P]O displaced the carbene ligand.32

Scheme 7 Synthesis of trivalent lanthanide NHC complexes with anionic N-tethered NHC ligands and attempted reduction to divalent complexes.

In the case of Ce, the unexpected NHC cerium bromide complex [{Ce(HLN){N(SiMe3)2}(m-Br)}2] 37 was isolated once and structurally characterized.33 The corresponding iodide derivative 38 was obtained from the reaction of 36-Ce with LiI in hot toluene. The CedCcarbene bond length is significantly longer in these dimeric complexes compared to the monomeric 36-Ce complex (e.g. 2.670 (2) A˚ for 38 vs 2.700 (3) A˚ for 36-Ce).

172

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides Table 3 LndCcarbene distances and 13C NMR data on trivalent lanthanide NHC complexes with anionic amido-tethered NHC ligands. Compound name

Number

Ox state

˚) LndC (A

13

C (ppm) (1JYC (Hz))

Refs.

[Y(HLN){N(SiMe3)2}2]

36-Y

3

2.501(5)

186.3 (54.7)

32

[Ce(HLN){N(SiMe3)2}2] [Nd(HLN){N(SiMe3)2}2] [Sm(HLN){N(SiMe3)2}2] [{Ce(HLN){N(SiMe3)2}(m-Br)}2] [{Ce(HLN){N(SiMe3)2}(m-I)}2] [{Ce(SiLN){N(SiMe3)2}(m-I)}2] [{Nd(SiLN){N(SiMe3)2}(m-I)}2] [Y(SiLN){N(SiMe3)2}2]

36-Ce 36-Nd 36-Sm 37 38 39-Ce 39-Nd 40-Y

3 3 3 3 3 3 3 3

2.670(2) 2.609(3) 2.588(2) 2.700(3) 2.699(2) 2.728(8) 2.656(5)

[Nd(SiLN){N(SiMe3)2}2] [{Sm(HLN)[N(SiMe3)2](m-OMe)}2] [{Nd(SiLN)[N(SiMe3)2](m-N3)}2] [Nd(SiLN){N(SiMe3)2}{Ga(NDippCH)2}(THF)] [Nd(SiLN){N(SiMe3)2}{FeCp(CO)2}] [Nd(SiLN){N(SiMe3)2}(NHDipp)] [Nd(SiLN){N(SiMe3)2}{NH(C6H3(C6H2iPr3)2)}]

40-Nd 41 42 43 44 45 46

3 3 3 3 3 3 3

2.648(3) 2.682(3) 2.672(3) 2.669(2) 2.606(4) 2.612(4) 2.603(6)

172.5 (55.8)

33 34 32 33 33 34 34 35 34 35 34 36 37 38 38

Importantly, it was shown that the backbone of the NHC carbene in 36-Nd and 36-Ce could be regioselectively functionalized via reaction with trimethylsilyl iodide in diethyl ether at room temperature providing the dimeric complexes [{Ln(SiLN){N (SiMe3)2}(m-I)}2] 39 (SiLN ¼ tBuNCH2CH2[C{N(CHC(SiMe3))NtBu}]) with a trimethylsilyl group on the NHC C4 position (Scheme 7).34 Mechanistically, it has been proposed that the iodide anion would initially replace one of the trimethylsilylamido groups via nucleophilic substitution. The liberated amide base could then deprotonate the NHC backbone and the resulting anion could be quenched with the trimethylsilyl cation. It should be noted that 36-Y was not transformed under these conditions, showing some steric influence in this reaction. Complexes 39 were characterized by XRD analysis revealing the electronic influence of the electropositive SiMe3 group on the NHC-Ln moieties. The resulting softer NHC ligand leads to longer LndCcarbene bond lengths as shown for example in the comparison of 38 and 39-Ce, 2.700 (3) and 2.728 (8) A˚ , respectively.34 Attempts to prepare divalent lanthanide complexes bearing this amido-tethered NHC ligand have not been successful so far (Scheme 7). Reduction of the trivalent 39-Nd with KC8 in benzene afforded the trivalent complex [Nd(SiLN){N(SiMe3)2}2] 40-Nd, probably due to some disproportionation/ligand rearrangement process.35 Furthermore, reduction of 36-Y with K(napht) also did not provide a divalent complex, but led to CdH activation on the NHC ligand (see Section 3.05.5) and the resulting anionic species could be quenched with Me3SiCl to form complex 40-Y. The most promising attempt was the reduction of 36-Sm with KC8, however, in this case ether cleavage of the DME solvent was observed, probably caused by an intermediate divalent complex, providing the trivalent methoxy-bridged dimer [{Sm(HLN)[N(SiMe3)2](m-OMe)}2] 41 (Scheme 7). Complexes 40 show again the influence of the softer trimethylsilyl-substituted NHC ligand compared to the parent ligand on the electronic properties of the complexes. In 40-Y, the 13C NMR carbene signal appears at 172.5 ppm compared to 186.3 ppm for 36-Y. Also, the NddCcarbene distance is clearly elongated from 2.609 (3) A˚ for 36-Nd to 2.648 (3) A˚ for 40-Nd.35 The reactivity of the heteroleptic neodymium complex 39-Nd in various salt metathesis reactions was then investigated (Scheme 8). Addition of one equivalent of sodium azide resulted in the formation of the azide bridged dimeric complex [{Nd (SiLN)[N(SiMe3)2](m-N3)}2] 42, which showed a slightly elongated NddCcarbene distance.34 The first complex containing a direct lanthanide-gallium bond was obtained from the reaction of 39-Nd with the gallium diyl complex [{K(TMEDA)}{Ga(NArCH)2}] in THF.36 The resulting complex [Nd(SiLN){N(SiMe3)2}{Ga(NDippCH)2}(THF)] 43 shows a NddGa bond of 3.2199 (3) A˚ and a slightly longer NddCcarbene bond length compared to the starting complex. Consequently, an unsupported LndFe bond was stabilized using 39-Nd in the reaction with K[FeCp(CO)] in THF.37 In the pseudo-tetrahedral complex [Nd(SiLN){N(SiMe3)2} {FeCp(CO)2}] 44 the NddFe distance is 2.9942 (7) A˚ and the NddCcarbene bond length is quasi-identical to the tetra-coordinate 36-Nd. Finally, the low-coordinate neodymium amide complexes 45 and 46 were obtained from the reaction of 39-Nd with sterically encumbered K[N(H)(Dipp)] and K[N(H)(terphenyl)] salts in cold THF (Scheme 8).38 Rather short NddCcarbene bond lengths were observed in these tetrahedrally arranged complexes. Further reaction of 45 or 46 with strong bases to reach terminal imido species were not successful.

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

173

Scheme 8 Transformations of trivalent neodymium NHC complex 39-Nd bearing an anionic N-tethered NHC ligand.

The application of 36-Y in rac-lactide polymerization was investigated, leading to high-molecular weight polymers with narrow polydispersity and highly regular heterotacticity.39 For the initiation step, a nucleophilic attack of the carbene on the metalcoordinated monomer was proposed. The chain growth would then occur via coordination-insertion polymerization. More recently, a theoretical study based on DFT calculations has shown that the nucleophilic attack by the Y-bound N(SiMe3)2 group on the monomer would be more favorable than attack by the carbene moiety in the initiation step.40 The electron-rich NHC ligand would serve to accelerate the carbonyl insertion and lead to improved polymerization activity. This study also proposed an explanation for the observed high stereoselectivity.

3.05.3.2.2

NHC ligands with O-based tethers (alkoxy or aryloxy, enolate group)

Various anionic O-based tether groups have been employed to stabilize trivalent Ln-NHC complexes and these will be described in the following order: unsaturated NHC ligands with alkoxide groups, saturated NHC ligands with alkoxide groups, aryloxy-tethered and enolate-functionalized NHC ligands.

3.05.3.2.2.1 Unsaturated NHC ligands with alkoxide groups The tris(alkoxycarbene) adducts [Ln(L1O)3] 47 (Ln ¼ Sc, Y, Ce; L1O ¼ OCMe2CH2[C{N(CHCH)NiPr}]) were synthesized in good yields via salt metathesis reactions from lanthanide halides and the potassium carbene precursor [K(L1O)] (Scheme 9).41–43 The isostructural Sc and Y complexes were characterized by XRD analysis showing the metal center in a pseudo-octahedral environment with the three bidentate ligands arranged in a meridional fashion. Crystals of the Sc complex contained both D-mer and L-mer enantiomers, whereas those of the Y complex contained two independent molecules of the L-mer isomers. It is interesting to note that the metal-carbene bond trans to the alkoxide group is considerably longer than the two other metal carbene bonds, for example in D-mer 47-Sc 2.452 (3) vs 2.411 (3) A˚ and 2.402 (3) A˚ . On the other side, in the same complex, the metal-oxygen distance trans to the NHC ligand is the shortest among the three, 1.989 (2) vs 2.036 (2) A˚ and 2.046 (2) A˚ . NMR studies at room temperature showed only a single set of resonances for the ligand, whereas at lower temperatures splitting of the resonances corresponding to the CH2 backbone or the isopropyl CH occurred in agreement with the meridional ligand arrangement observed by XRD analysis.43

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N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

Scheme 9 Synthetic pathways to unsaturated NHC lanthanide complexes bearing alkoxide tethers.

[LnCp3] complexes were also suitable precursors for salt metathesis reactions as shown in the synthesis of the mono and bis(carbene) cyclopentadienyl complexes [Ln(L1O)Cp2] 48 (Ln ¼ Sc, Y, Ce) and [Y(L1O)2Cp] 49 (Scheme 9).44 The Sc and Y complexes, 48-Sc and 48-Y, showed pseudo-tetrahedral coordination geometry with considerably shorter LndCcarbene distances compared to the homoleptic complexes 47 (Table 4). In the crystallized Ce complex, 48-Ce-THF, an additional THF molecule was coordinated to the metal leading to a pseudo-trigonal bipyramidal arrangement. In the mono-cyclopentadienyl yttrium complex 49, the Cp and the two alkoxy groups are in the equatorial plane of a distorted trigonal bipyramid and the carbene ligands in the axial positions. The YdCcarbene bond distances are longer than in the bis-Cp complex 48-Y and similar to the homoleptic complex 47-Y (Table 4). Another synthetic approach using protonolysis was explored starting from [Ln(CH2SiMe3)3] and the imidazolium and benzimidazolium precursors of alkoxide-appended NHC ligands L2O ¼ OCH(nBu)CH2[C{N(C6H4)NiPr}] and RL3O ¼ OCH(nBu) CH2[C{N(CHCH)NR}], giving straightforward access to the bis-alkyl complexes [Ln(L2O)(CH2SiMe3)2] 50 and [Ln(RL3O) (CH2SiMe3)2] 51 and 52 (Scheme 9).45 These bimetallic complexes form a planar Ln2O2 unit (except for 52-Y) with two metal-alkoxide bridges and a twisted trigonal bipyramidal coordination geometry around the metal centers, according to XRD analysis. Activation of 51-Sc with three equivalents [Ph3C][B(C6F5)4] yielded high activity and cis-1,4-selectivity in isoprene polymerization.45

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

175

Table 4 LndCcarbene distances and 13C NMR data on trivalent lanthanide complexes with anionic alkoxide tethered unsaturated NHC ligands. Compound name

Number

Ox state

˚) LndC (A

13

C (ppm) (1JYC (Hz))

Refs.

[Sc(L1O)3] [Sc(L1O)3-pyrrole] [Y(L1O)3]

47-Sc 47-Sc-pyrr 47-Y

3 3 3

2.436 (mean) 2.423(mean) 2.588 (mean)

195.2 194.7 197.3 (31.3)

43 44 42

[Sc(L1O)Cp2] [Y(L1O)Cp2]

48-Sc 48-Y

3 3

2.337(2) 2.489(5)

[Ce(L1O)Cp2(THF)] [Y(L1O)2Cp]

48-Ce-THF 49

3 3

[Y(L2O)(CH2SiMe3)2] [Sc(MeL3O)(CH2SiMe3)2] [Lu(MeL3O)(CH2SiMe3)2] [Y(iPrL3O)(CH2SiMe3)2] [Lu(iPrL3O)(CH2SiMe3)2] [Sc(L1O)2(L1O-CS2)] [Sc(L1O)(L1O-CS2)2]

50 51-Sc 51-Lu 52-Y 52-Lu 53 54

3 3 3 3 3 3 3

2.735(4) 2.568(3) 2.597(3) 2.557(14) 2.3846(17) 2.487(2) 2.552(7) 2.477(12) 2.411 (mean) 2.391(4)

192.1 (47.6) 194.8 (37.3)

198.0

189.9

44 44 44 44 45 45 45 45 45 43 43

The reactivity of 47-Sc toward CS2 and CO2 was investigated providing different reaction outcomes as shown in Scheme 10.43 Addition of one equivalent of CS2 in toluene provided the bis-carbene complex [Sc(L1O)2(L1O-CS2)] 53 bearing one zwitterionic thiocarboxylate-imidazolium unit, which is only bound via the alkoxide group to the metal center despite its formal negative charge on the CS2 moiety. When two or more equivalents of CS2 were added, the insertion of a maximum of two CS2 units into the ScdCcarbene bonds occurred, leading to the mono-carbene complex [Sc(L1O)(L1O-CS2)2] 54. Both 53 and 54 were characterized by XRD analysis showing shorter ScdCcarbene distances compared to 47-Sc which could be related to the lower steric bulk around the scandium ion. The monomeric 53 displays a distorted square planar pyramidal coordination geometry around the Sc center, whereas complex 54 is dimeric with an alkoxy-bridged Sc2O2 core and a distorted trigonal bipyramidal environment around the metal centers.43

Scheme 10 Reactivity of tris(alkoxycarbene) complex 47-Sc toward CO2 and CS2.

176

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

On the other hand, exposure of 47-Sc with CO2 (1 atm) at room temperature led immediately to the formation of polymeric complex 55 as a colorless precipitate, in which insertion of CO2 into all three ScdC bonds had occurred (Scheme 10). Its structure was assigned according to FTIR spectroscopy, elemental analysis and 13C and45Sc solid state NMR spectroscopy. The latter shows two resonances at 128 and 45 ppm in accordance with two [ScO6] units.43 Complexes 47–49 were further investigated in their reactions with different NdH and CdH acidic substrates and some selected examples are shown in Scheme 11.44 Initially, the reaction toward pyrrole and indole was studied. Interestingly, whereas 48-Ce did not react with pyrrole, with 48-Y an acid-base reaction occurred immediately to yield the yttrocene pyrrolide complex [Y(HL1O) Cp2(NC4H4)] 56 bearing a pendant imidazolium unit as shown by NMR spectroscopy and XRD analysis. Addition of pyrrole to 47Sc afforded the pyrrole adduct 47-Sc-pyrr, identified by XRD, in which the NH was coordinated to the oxygen atom of one alkoxide group, however, no significant changes occurred with respect to the ScdCcarbene distances. Finally, complex 49 afforded a mixture of multiple products upon reaction with pyrrole. Similar results were obtained from the reactions of indole and complexes 48 and 49.

Scheme 11 Selected examples of reactivity toward NdH and CdH acidic substrates.

Attempts to react complexes 47–49 with terminal alkynes were only successful in the case of 48-Ce, all other complexes were inert to these substrates. However, instead of an expected Ce-alkynide complex only the tris-cyclopentadienyl complex [Ce(HL1O) Cp3] 57 bearing an alkoxide bound imidazolium ligand could be isolated and characterized by XRD. This complex probably results from ligand rearrangement of an initially formed alkynide complex. Complex 57 was also cleanly obtained from the reaction of 48-Ce with freshly distilled cyclopentadiene, whereas no reaction was observed with 48-Sc or 48-Y. Finally, reaction of 48-Y with diphenylacetone yielded the zwitterionic yttrium imidazolium enolate complex 58 as shown by NMR spectroscopy and XRD analysis.44

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

177

3.05.3.2.2.2 Saturated NHC ligands with alkoxide groups All complexes in this part were synthesized starting from the bicyclic carbene alcohol adducts H(RL4O) (RL4O ¼ OCMe2CH2[C {N(CH2CH2)NR}]; R ¼ iPr, Dipp, Mes) (Scheme 12), which were obtained by deprotonation of the corresponding alcoholtethered imidazolium salts using nBuLi.46 Whereas heating these adducts in a benzene solution at 70  C did not provide any signs of a dissociated carbene-alcohol, dissolution in CDCl3 slowly led to the formation of the deuterated analogue, indicative of a possible equilibrium with the open free carbene form. Consequently, these precursors were employed in the protonolysis reaction with lanthanide amide and alkyl precursors, giving access to a large variety of new carbene complexes with interesting reactivity properties (Scheme 12).

Scheme 12 Synthesis of saturated NHC lanthanide complexes bearing alkoxide tethers via protonolysis.

The mono-carbene bis-amido lanthanide complexes [Ln(RL4O){N(SiMe3)2}2] 59–61 (Ln ¼ Y, Ce) as well as the bis-carbene amido cerium complexes [Ce(RL4O){N(SiMe3)2}2] 63 (R ¼ Dipp) and 64 (R ¼ Mes) bearing different steric bulk on the NHC ring were readily obtained at room temperature in different solvents starting from [Ln{N(SiMe3)3}] and one or two equivalents of H(RL4O).46–48 In contrast, for the formation of the bis-carbene yttrium complex 62 heating a benzene solution was necessary for the

178

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

iPr substituted ligand, whereas with Mes or Dipp substituents the reaction stopped at the mono-carbene complex 59. The tris-carbene yttrium complex [Y(iPrL4O)3] 65 could only be obtained from the HiPrL4O precursor after heating. The yttrium complexes showed characteristic 13C NMR signals for the carbene atom in the range 212–220 ppm with 1JYC coupling constants of 42 to 46 Hz for the mono-carbene complexes and significantly lower values for the bis-carbene and tris-carbene complexes, 36 and 29 Hz, respectively. This decrease could be explained with the increasingly electron-rich yttrium center due to the additional alkoxy-NHC ligands and hence less s-donation from the NHC ligand to the metal. XRD analysis of tetracoordinated complex 60Y showed a distorted tetrahedral arrangement around the metal center, with two close YdSi interactions. The YdC bond length is elongated compared to the unsaturated NHC bis-amido or bis-Cp complex 36-Y and 48-Y (2.599 (2) vs 2.501 (5) A˚ vs 2.489 (5) A˚ ). The mesityl-substituted Ce complex 61-Ce crystallized as a pyridine adduct, which can be compared to the THF adduct of the unsaturated NHC bis-Cp complex 48-Ce, indicating again a longer CedC bond length for the saturated NHC complex (2.844 (5) vs 2.735 (4) A˚ ). The five-coordinate bulky bis-carbene Ce complexes 63 (Dipp) and 64 (Mes) were structurally characterized by XRD, showing a distorted trigonal bipyramidal coordination geometry with the carbene atoms in the apical positions. Slightly longer CedC and CedO bond distances were measured in the case of the Dipp-substituted complex over the Mes-substituted complex. Protonolysis reactions of Sc and Y tris-alkyl precursors [Ln(R)3(THF)2] (R ¼ CH2SiMe3 or CH2tBu) with one or two equivalents of H(DippL4O) at low temperature provided access to mono-carbene complexes complexes [Ln(DippL4O)(CH2SiMe3)2] 66 and bis-carbene complexes [Ln(DippL4O)2(CH2SiMe3)] 67 and [Sc(DippL4O)2(CH2tBu)] 68 (Scheme 12).49 The bis-carbene complexes showed higher thermal stability than the mono-carbene complexes. Using four equivalents of LiCH2SiMe3 in the presence of LnCl3 afforded the zwitterionic Li-carbene complex 69 with a pendant tris-alkyllanthanide alkoxide moiety, which could be structurally characterized by XRD analysis. Complexes 66-Sc and 66-Y crystallized as alkoxide bridged dimers in contrast to the monomeric bis(amide) complexes 60. The five-coordinate metal centers have a distorted trigonal bipyramidal coordination geometry with the carbene atom and one alkoxy bridge in the axial positions. The YdCcarbene distance in five-coordinate 66-Y is expectedly longer than in four-coordinate 60-Y due to the additional coordination site (Table 5). The bis-carbene complex 67-Sc has a distorted trigonal bipyramidal coordination geometry with the two carbene ligands in the apical positions.49 Table 5 LndCcarbene distances and 13C NMR data on trivalent lanthanide complexes with anionic alkoxide tethered saturated NHC ligands. ˚) LndC (A

Compound name

Number

Ox state

[Y(iPrL4O){N(SiMe3)2}2]

59

3

[Y(DippL4O){N(SiMe3)2}2]

60-Y

3

[Y(MesL4O){N(SiMe3)2}2]

61-Y

3

[Ce(MesL4O){N(SiMe3)2}2(py)] [Y(iPrL4O)2{N(SiMe3)2}2]

61-Ce-py 62

3 3

2.844(5)

[Ce(DippL4O)2{N(SiMe3)2}2]

63

3

[Ce(MesL4O)2{N(SiMe3)2}2]

64

3

2.855(3) 2.813(3) 2.786(4) 2.798(4)

[Y(iPrL4O)3]

65

3

[Sc(DippL4O)(CH2SiMe3)2] [Y(DippL4O)(CH2SiMe3)2]

66-Sc 66-Y

3 3

2.4572(16) 2.625(5)

[Sc(DippL4O)2(CH2SiMe3)] [Y(DippL4O)2(CH2SiMe3)]

67-Sc 67-Y

3 3

2.442 (av)

[Sc(DippL4O)2(CH2tBu)] [Sc(DippL4O)2Cl] [Sc(DippL4O)2I] [Sc(DippL4O)2(C6F5)]

68 76 77 79

3 3 3 3

2.599(2)

13

C (ppm) (1JYC (Hz))

Refs.

212.3 (46.4) 215.5 (44) 216.3 (42)

47

216.5 (35.8)

47 47 47 48 47

220.2 (29.1)

2.416 (av) 2.4306(17) 2.422 (av)

46

215.4 (30) 217.9 (33) 236.1 215.2 215.3

47 49 49 49 49 49 49 49 49

The reactivity of the mono-carbene bis(amido) lanthanide complexes 60-Y and 60-Ce toward electrophiles was investigated. Reaction with CO2 was attempted, however, no well-defined products stemming from CO2 insertions could be isolated and structurally characterized. Furthermore, no reaction occurred with carbon monoxide.50 On the other hand, reaction with trimethylsilylhalides afforded formal addition of the SidX bond across the metal-carbene bond to yield the corresponding metal halide bis(amido) alkoxide complex 71 bearing a silylated imidazolium group (Scheme 13).48 This reactivity could be extended to azides as well as phosphinyl, stannyl and boryl reagents. Heating a benzene solution of 71 (E ¼ SiMe3) led to the elimination of

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

179

Scheme 13 Reactivity of mono-carbene bis(amide) lanthanide complexes 60 toward electrophiles (Ad ¼ adamantyl; BBN ¼ 9-Borabicyclo[3.3.1]nonane).

N(SiMe3)3 via NdSi bond formation and recovery of the Ln-carbene bond in the intermediate complex 72, which, after addition of K[N(SiMe3)2], furnished the starting complex 60. In an attempt to add adamantyl azide onto 60, no cleavage of the NdC bond was observed but the insertion product 70 was isolated. The structure was confirmed for Ce by XRD analysis showing the new covalent bond between the former carbene carbon and the azide nitrogen, as well as k2-coordination of the triazenido group to the Ce metal. This complex did not evolve further upon prolonged heating.48 The addition-elimination reactivity of electrophiles across lanthanide-carbene and also lithium carbene complexes was further exploited in CdC and C-heteroatom bond formation reactions as well as to access new organometallic complexes starting from mono- and bis-carbene alkylscandium complexes (Schemes 14 and 15).49 It was shown that an alternative route to mono-carbene bis-alkyl scandium complex 66-Sc was accessible via addition of trimethylsilylchloride or tritylchloride onto the bimetallic Li carbene complex 69. Intermediates 73 and 74 could be isolated at low temperatures and furnished the final complex 66-Sc either at room temperature (SiMe3) or after heating in toluene (CPh3) via the formation of CdSi and CdC bonds (Scheme 14).

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N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

Scheme 14 Alternative synthetic pathway to 66 based on addition-elimination of electrophiles onto 69.

The addition of halosilane, halophosphine and halostannane derivatives onto the bis(carbene) Sc alkyl complexes 67 and 68 was successful to provide C-heteroatom bond formation and the bis(carbene)Sc halide complexes [Sc(DippL4O)2X] 76 (X ¼ Cl) and 77 (X ¼ I) (Scheme 15).49 It should be noted that the reactions with XPPh2 and XPh3Sn required 5 days to go to completion whereas the reaction with XnBu3Sn was finished after 17 h. The mono-carbene Sc complex 66-Sc turned out to be less robust in these transformations. Interestingly, with C6F5I a reverse addition was observed on both the mono and bis-carbene scandium complexes to afford the new pentafluorophenyl complexes [Sc(DippL4O)2(C6F5)] 79 and [{Sc(DippL4O)(C6F5)(CH2SiMe3)}2] 81 after I-CH2SiMe3 elimination. The monomeric complexes 76, 77 and 79 were structurally characterized by XRD analysis showing a trigonal bipyramidal arrangement analogous to complex 67. A dimeric alkoxide bridged structure was found for complex 81 comparable to complex 66-Sc, however, the XRD data was not sufficiently good to discuss metrical parameters in detail. In this complex, some ScdF interactions were detected in the solid state, which was not the case in complex 79.49

3.05.3.2.2.3 Aryloxy-tethered and enolate-functionalized NHC ligands The main synthetic pathway to aryloxo-tethered NHC lanthanide complexes proceeds via protonolysis starting from the corresponding imidazolium salt. The first examples reported by Shen in 2006 showed that the ligand to metal ratio and the temperature had an important influence on the reaction outcome (Scheme 16).51,52 Reaction of the anionic ytterbium complex [LiYb {N(iPr)2}4] with imidazolium salt H2(RL1OAr)Cl (RL1OAr ¼ O-4,6-tBu2C6H2-2-CH2{C[N(CHCH)NR]}) and nBuLi in a 1:2:1 ratio in THF at 0  C provided the heteroleptic complexes [Yb(RL1ArO)2{N(SiMe3)2}] 82 and 83 in good yields. No mono-carbene complexes could be obtained by changing the reaction stoichiometry. Both complexes were analyzed by XRD studies showing a distorted trigonal-bipyramidal coordination geometry around the Yb center with the carbene atoms in the axial positions.51 Using the Y precursor, [LiY{N(iPr)2}4], and changing the molar ratio to 1:3:2 with the reaction carried out at −78  C, the first structurally characterized tris-NHC complex [Y(iPrL1ArO)3] 84 was obtained in moderate yield.52 This complex displays a distorted octahedral geometry with one oxygen atom and one carbene atom in the axial positions. The 13C NMR spectrum shows a signal at rather low-field (199.9 ppm), comparable to the tris(alkoxycarbene) complex 47-Y (197.3 ppm). Carrying out the same reaction at r.t. resulted in the isolation of the mono-NHC complex 85, which is further stabilized by a bridged bisphenoxo ligand and two imidazole rings (Scheme 16). The latter results from the cleavage of two NHC ligands, however, the mechanism has not yet been confirmed. XRD analysis of 85 showed a pseudo-octahedral geometry in which the YdCcarbene distance was shorter compared to 84 (Table 6).52

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

Scheme 15 Addition-elimination reactivity of electrophiles on mono and bis(carbene) alkyl lanthanide complexes bearing alkoxide tethers.

181

182

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

Scheme 16 Synthesis of NHC lanthanide complexes with aryloxide tethers based on ligand RL1OAr.

Table 6 LndCcarbene distances and 13C NMR data on trivalent lanthanide NHC complexes with anionic aryloxide and enolate-tethered NHC ligands. Compound name

Number

Ox state

˚) LndC (A

[Yb(MeL1ArO)2{N(SiMe3)2}]

82

3

[Yb(iPrL1ArO)2{N(SiMe3)2}]

83

3

[Y(iPrL1ArO)3]

84

3

[Y(iPrL1ArO)[(O-4,6-tBu-C6H2)2(CH2)](iPrIm)2] [Ce(iPrL2ArO)3]

85 86-Ce

3 3

[Ce(MesL2ArO)3]

88

3

[{Ce(MeL2ArO){N(SiMe3)2}(m-Br)2Li(THF)}2] [{Ce(iPrL2ArO){N(SiMe3)2}(m-Br)2Li(THF)}2] [Ce(iPrL2ArO)(SCNtBu-iPrL2ArO)2] [Y(Lenol)2{N(SiMe3)2}]

89 90 97 99-Y

3 3 3 3

[Nd(Lenol)2{N(SiMe3)2}]

99-Nd

3

[Sm(Lenol)2{N(SiMe3)2}]

99-Sm

3

[Yb(Lenol)2{N(SiMe3)2}]

99-Yb

3

2.491(4) 2.483(4) 2.543(7) 2.526(7) 2.606(3) 2.615(3) 2.641(3) 2.576(5) 2.747(6) 2.694(6) 2.785(7) 2.823(3) 2.814(3) 2.806(3) 2.639(4) 2.641(3) 2.716(4) 2.512(5) 2.513(4) 2.619(4) 2.624(4) 2.579(4) 2.585(5) 2.453(4) 2.473(4)

13

C (ppm)

Refs. 51 51

199.9

51

198.4 174.8

52 53

184.2

53

188.4 187.9

53 53 53 54 54 54 54

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

183

Homoleptic tris(aryloxycarbene) lanthanide complexes based on the NHC ligands RL2OAr (RL2OAr ¼ O-4,6-tBu2C6H2-2-[C {N(CHCH)NR}]; R ¼ iPr, tBu, Mes), having the phenoxide directly attached onto one of the NHC nitrogen atoms, were synthesized from the reaction of the imidazolium precursor H2(RL2OAr)Br (R ¼ iPr, tBu, Mes) with KN(SiMe3)2 and [LnCl3(THF)n] (Ln ¼ Ce, Sm, Eu) in DME (Scheme 17).53 The reaction afforded complexes [Ln(RL2OAr)3] 86–88 in yields varying from 15% to −76%.53 1H and 13C NMR spectroscopy studies on the paramagnetic Ce3+ complexes indicated C1 symmetry for R ¼ iPr and tBu, whereas C3 symmetry was observed for R ¼ Mes. The complexes 86-Ce and 88 were analyzed by XRD studies, showing a pseudo-octahedral coordination geometry around the metal with average CedC bond lengths of 2.742 (6) and 2.814 (3) A˚ for 86-Ce and 88, respectively. The latter are among the longest lanthanide-Ccarbene bonds, in agreement with a high degree of hemilability.

Scheme 17 Synthesis of NHC lanthanide complexes with aryloxide tethers based on ligand RL2OAr.

The heteroleptic NHC Ce bromide complexes 89 and 90 were obtained in moderate yields from the reaction of equimolar amounts of ligand H2(RL2OAr)Br, [Li(THF)Ce{N(iPr)2}4] and NBu4Br, which were characterized by XRD analysis (Scheme 17). Attempts to synthesize lanthanide NHC complexes based on the NHC ligand carrying a p-substituted phenoxy group, via salt metathesis from the corresponding sodium or potassium carbene precursor with [LnCl3(THF)n], yielded rather unstable tris(phenoxy) lanthanide complexes 91. These complexes contain pendant free carbene ligands, which did not interact with the Ln metal center, as shown by 13C NMR spectroscopy.53

184

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

The [Ce(RL2OAr)3] complexes 86-Ce, 87 and 88, underwent instant reaction with CO2 to yield the homoleptic complexes [Ce(RL2OAr-CO2)3] 92 and 93. These complexes resulted from the exclusive insertion of CO2 into the CedC bonds and without ligand redistribution, as shown by NMR spectroscopy and XRD analysis. In the case of complexes 92 with the less bulky NHC ligands (R ¼ iPr, tBu), the bound CO2 could not be removed under vacuum, whereas for the mesityl complex 93 partial reversible loss was observed (Scheme 18). Subsequently, it was shown that catalytic amounts of complex 88 could provide the formation of propylene carbonate from propylene oxide and CO2, whereas with 86-Ce no reaction was observed. This result is in agreement with the partial equilibrium concerning CO2 insertion for 88.53

Scheme 18 Reactivity of tris-NHC lanthanide complexes bearing aryloxide tethers with CO2.

Further reactivity tests of 86-Ce with other (hetero)allene substrates showed that no reaction occurred with CS2 or cyclohexylallene, whereas with mesityl isocyanate immediate insertion into all three Cedcarbene bonds was observed at room temperature providing complex 94 with full conversion (Scheme 19).53 Interestingly, with tBuNCO or tBuNCS as substrates, triple (complex 95), double (complexes 96 and 97) and mono insertion (complex 98) into the Cedcarbene bonds occurred depending on the solvent (benzene vs DME) and the temperature (r.t. vs 80  C) employed. Complex 97 could be isolated and structurally characterized by XRD analysis. The enolate-functionalized NHC lanthanide amido complexes [Ln(Lenol)2{N(SiMe3)2}] 99 were synthesized in moderate yield from the reaction of the imidazolium precursor H2(Lenol)Br (Lenol ¼ 4-OMe-C6H4COCH[C{N(CHCH)NiPr}]) with a mixture of LnCl3 and 4 NaN(SiMe3)2 (Scheme 20).54 Addition of more base afforded mainly insoluble products. The 13C NMR carbene signals for the Y complex are at rather low chemical shifts, 188.4 and 187.9 ppm. The isostructural 5-coordinate complexes show distorted trigonal bipyramidal geometries with the two enolate oxygen atoms in the axial positions, having OdLndO angles in the range 147.7–152.9 . The Y, Nd and Sm complexes showed high catalytic activity for the addition of primary and secondary cyclic amines to carbodiimides.54

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

185

Scheme 19 Reactivity of tris-NHC lanthanide complex 86-Ce toward heteroallenes.

Scheme 20 Synthesis of enolate functionalized trivalent NHC lanthanide complexes.

3.05.3.2.3

NHC ligands with C-based tether

The bulky IMes NHC ligand has been employed to access benzyl-tethered lanthanide NHC complexes via CdH activation of one of the methyl groups by metal bound alkyl groups. The first examples were reported in 2010 by Okuda from the reaction of tris(alkyl) lanthanide precursors [Ln(CH2SiMe3)3(THF)2] (Ln ¼ Y, Lu) and IMes ligand in THF at room temperature (Scheme 21).29,55 Whereas immediate CdH activation was observed in the case of Y leading to complex [Y(IMes0 )(CH2SiMe3)2(THF)2] 100-Y, the reaction took 2 h for the smaller Lu analogue to afford complex [Lu(IMes0 )(CH2SiMe3)2(THF)] 100-Lu. At shorter reaction times the simple NHC adduct was obtained for Lu (see complex 34, Section 3.05.3.1).29 The isolated complexes differ by the number of THF molecules coordinated to the metal center and both were characterized by XRD analysis and NMR spectroscopy (see Table 7). In the

186

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

Scheme 21 Synthesis of benzyl-tethered mono and bis(NHC) carbene complexes via CdH activation.

Table 7

LndCcarbene distances and 13C NMR data on trivalent benzyl-tethered lanthanide NHC complexes.

Compound name

Number

Ox state

˚) LndC (A

13 C (ppm) (1JYC (Hz))

Refs.

[Y(IMes0 )(CH2SiMe3)2]

100-Y

3

2.6420(16)

29

[Lu(IMes0 )(CH2SiMe3)2] [Y(IMes0 )2(CH2SiMe3)]

100-Lu 101-Y

3 3

2.491(2)

[Lu(IMes0 )2(CH2SiMe3)] [Y(IMes0 )(CpMe)2] [Dy(IMes0 )(CpMe)2]

101-Lu 102-Y 102-Dy

3 3 3

194.1 (37.3) 201.3 194.7 (35.6) 201.5 194.7

2.532(2) 2.534(9)

55 55 55 56 56

5-coordinate Lu complex a distorted trigonal bipyramidal coordination geometry was observed and the mesityl ring attached to the metal center is twisted by 37 from the perpendicular position compared to the imidazole ring. For comparison, in the 6-coordinate Y complex, which has a distorted octahedral geometry, this twist angle is 32 . The LudCcarbene bond length is significantly shorter than the YdCcarbene bond length, 2.491 (2) vs 2.6420 (16) A˚ , respectively, whereas less difference is observed for the M-CH2(Mes) bond lengths, which are 2.4995 (16) A˚ for 100-Y and 2.416 (3) A˚ for 100-Lu.55 Subsequently, it was shown that the reaction in toluene with an excess of IMes ligand led to the formation of the bis(carbene) complexes [Ln(IMes0 )2(CH2SiMe3)] 101 resulting from two CdH activation processes (Scheme 21).55 Once again, the reaction was considerably faster for the Y complex over the Lu complex (10 min vs 2 h). These complexes were confirmed by 1H and 13C NMR spectroscopy, showing similar characteristic signals as complexes 100. More recently, the trivalent metallocene carbene complexes [Ln(IMes0 )(CpMe)2] 102 (Ln ¼ Y, Dy) with a benzyl-tethered IMes0 ligand were synthesized in high yields from the reaction of the dimeric butyl-bridged metallocene precursors [{Ln (CpMe)2(m-butyl)}2] in benzene at room temperature (Scheme 21).56 In the tetrahedrally arranged Y complex, the YdCcarbene

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

187

and Y-benzyl bond lengths of 2.532 (2) and 2.449 (13) A˚ were shorter than in complex 100-Y, whereas a similar twist of the Mes0 ligand (30 ) with respect to the perpendicularity of the imidazole ring and nearly no difference in the 13C NMR carbene signal were observed.

3.05.3.2.4

NHC ligands with polyatomic tether (Cp, indenyl, fluorenyl, NCO, NCN)

NHC carbene ligands carrying additional Z5-Cp, indenyl or fluorenyl coordination sites have been investigated with lanthanide complexes since the early works by Cui and Danopoulos in 2007.57,58 The vast majority of complexes was obtained from a two-step procedure starting from the corresponding imidazolium salts. After formation of the free carbene with one equivalent of base, further reaction with tris(alkyl) lanthanide precursors [Ln(CH2SiMe3)3(THF)2] afforded deprotonation and coordination of the Cp-type ligand to yield the [Ln(Z5-Cp/Ind/Flu-NHC)(CH2SiMe3)2] complexes 103–107 (Scheme 22).58–63 In certain cases, the use of the lanthanide tetra(alkyl) ate complexes [LiLn(CH2SiMe3)4] as starting materials was also possible. Salt metathesis reactions starting from an anionic NHC precursor have not been successful yet. Direct protonolysis of the imidazolium salt H2(DippLInd)Br with [Y(CH2SiMe3)3(THF)2] afforded the bromide bridged dimeric complex 108.57

Scheme 22 Synthesis of Cp/indenyl/fluorenyl-tethered NHC lanthanide complexes.

The Z5/k1 coordination of the functionalized NHC ligands to the metal center in the solvent-free bis(alkyl) complexes provides a constraint-geometry configuration for 103–107. Tetrahedral coordination geometries around the metal are observed with the two alkyl ligands in cis-positions, one being endo and the other exo with respect to the NHC ring. No significant differences in the metal-carbene bond lengths were observed between cyclopentadienyl, indenyl and fluorenyl substituted complexes (Table 8). Only in the case of the bulky bis(t-butyl) substituted fluorenyl complexes 107, was an elongation of the LudC bond observed (2.482 (4) vs 2.431 (3) A˚ for simple fluorenyl complex 105-Lu).

188

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides Table 8

LndCcarbene distances and 13C NMR data on trivalent polyatom-tethered lanthanide NHC complexes.

Compound name

Number

Ox state

˚) LndC (A

[Sc(LCp)(CH2SiMe3)2] [Sc(MesLInd)(CH2SiMe3)2] [Y(MesLInd)(CH2SiMe3)2] [Ho(MesLInd)(CH2SiMe3)2] [Lu(MesLInd)(CH2SiMe3)2] [Sc(MesL1Flu)(CH2SiMe3)2] [Y(MesL1Flu)(CH2SiMe3)2]

103 104-Sc 104-Y 104-Ho 104-Lu 105-Sc 105-Y

3 3 3 3 3 3 3

2.355(5) 2.350(3) 2.501(3) 2.490(2) 2.443(3) 2.343(4)

[Dy(MesL1Flu)(CH2SiMe3)2] [Ho(MesL1Flu)(CH2SiMe3)2] [Er(MesL1Flu)(CH2SiMe3)2] [Lu(MesL1Flu)(CH2SiMe3)2] [Sc(MeL1Flu)(CH2SiMe3)2] [Lu(L2Flu)(CH2SiMe3)2] [{Y(DippLInd)(CH2SiMe3)(m-Br)}2] [Lu(iPr,PhLNN)(CH2SiMe3)2] [Y(LNO)2{N(SiMe3)2}]

105-Dy 105-Ho 105-Er 105-Lu 106 107-Lu 108 109-Lu 112-Y

3 3 3 3 3 3 3 3 3

2.502(3) 2.484(3) 2.473(11) 2.431(3) 2.339(3) 2.482(4) 2.547(5) 2.516(4) 2.640(6)

[Gd(LNO)2{N(SiMe3)2}]

112-Gd

3

[Dy(LNO)2{N(SiMe3)2}]

112-Dy

3

[Er(LNO)2{N(SiMe3)2}]

112-Er

3

2.678(10) 2.817(12) 2.645(8) 2.799(9) 2.618(9) 2.785(10)

13 C (ppm) (1JYC (Hz))

188.0 191.2 199.9 187.6 190.8 (45.8)

199.2 186.9 199.1 199.7 197.2 (27.5)

Refs.

63 58 58 59 58 59 59 61 59 61 59 62 60 57 68 70 70 70 70

Complexes 103–107 were applied as pre-catalysts in a wide range of homo and copolymerization reactions. The Cp-based complex 103 was shown to catalyze the homopolymerization of 1,6-heptadiene after activation with [Ph3C][B(C6F5)4], but was not active for the copolymerization of this diene with isoprene.63 The fluorenyl and indenyl complexes 104 and 105 could be activated with [Ph3C][B(C6F5)4]/AlR3 for the 3,4-selective polymerization of isoprene,58,59 the copolymerization of ethylene with norbornene,60 and ethylene with octane or hexene.61 No homopolymerization of styrene was possible with these NHC-based complexes, in accordance with a DFT study showing that after the first styrene insertion a stable complex is formed, in which the central metal is wrapped by the bulky ligand and phenyl ring of the inserted styrene unit, hence inhibiting further styrene insertion and consequently polymerization.64 Nevertheless, the scandium complexes 105-Sc and 106 showed the highest activity in the alternating copolymerization of ethylene and various styrene derivatives after activation with [PhMe2NH][B(C6F5)4]/AlR3.62,65,66 Interestingly, in these reactions, the smaller methyl-NHC Sc complex 106 achieved up to 10 times higher activity than the mesityl-NHC complex 105-Sc. Both complexes 105-Y and 105-Lu were also shown to exhibit high catalytic efficiency for the stereoselective methylene methylbutyrolactone polymerization.67 The amidine-tethered NHC ligands H(R1R2LNN) (R1R2LNN ¼ [2,6-(R1)2C6H3]NC(R2)NCH2CH2[C{N(CHCH)NMes}]) were obtained from the corresponding imidazolium salts via deprotonation with LiCH2SiMe3 and were employed for the synthesis of the corresponding amidino-NHC lanthanide bis(alkyl) complexes [Ln(R1R2LNN)(CH2SiMe3)2] 109–111 via protonolysis (Scheme 23).68,69 Different substituents on the amidine backbone and terminal N group were examined. The solvent-free Lu

Scheme 23 Synthesis of amidino-tethered NHC lanthanide complexes.

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

189

complex 109-Lu showed a twisted tetragonal geometry with k2-coordination to the NCN moiety and a LudCcarbene distance of 2.516 (4) A˚ . The 13C NMR signal for the carbene atom (199.7 ppm) was identical to the analogous fluorene-NHC Lu complex 105Lu. Activation of 109-Lu with [Ph3C][B(C6F5)4] led to high catalytic activity in the living polymerization of isoprene and 2-phenyl-1,3-butadiene which proceeded in a highly 3,4-selective fashion, as well as in the block copolymerization of isoprene and e-caprolactone.68,69 The lower steric bulk on the amidate-containing NHC ligand LNO (LNO ¼ OC(Ph)NCH2CH2[C{N(CHCH)NMes}]) provided access to the bis(carbene) amidate lanthanide amido complexes [Y(LNO)2{N(SiMe3)2}] 112 by reaction of the imidazolium salt with five equivalents of KN(SiMe3)2 in the presence of LnCl3 (Scheme 24).70 The seven-coordinate complexes show k2-bound amidate groups to the metal center with electron-delocalization in the NCO fragments. Two significantly different LndCcarbene bond lengths were observed in all complexes, probably caused by the steric hindrance between the neighboring carbene and NSiMe3 groups. The 1JYdC coupling was rather small (27.5 Hz). These complexes were applied as highly efficient catalysts in the hydroboration of imines (Gd) and nitriles (Er), requiring low catalyst loadings and displaying good functional group tolerance.70

Scheme 24 Synthesis of amidate-tethered NHC lanthanide complexes.

3.05.3.3

Tridentate NHC pincer complexes

The application of tridentate NHC pincer complexes provides further stability in the complexation of NHC ligands to lanthanide metals. This part will be divided according to the nature of the pincer ligand, that is containing either one or two NHC units.

3.05.3.3.1

Pincer ligands with one NHC unit

Bis(phenoxy)-functionalized saturated and unsaturated NHC ligands L1OCO-L3OCO provided a range of bimetallic lanthanide complexes incorporating alkali-metals. Depending on the combination of lanthanide precursor and base, separated ion-pairs could also be obtained and an important solvent effect was determined (Scheme 25).71–73 The reaction of the imidazolium salt H3(L1OCO)Cl (L1OCO ¼ 1,3-[O-4,6-tBu2C6H2-2-CH2]2[C{N(CHCH)N}]) with lanthanide precursors [Li(THF)Ln{N(iPr2)4}] and nBuLi in THF followed by recrystallization from DME/toluene afforded the separated ion pair complexes [{Ln(L1OCO)2}{Li(DME)3}] 113 (Ln ¼ Sm, Er, Yb).71 XRD analysis revealed a distorted octahedral coordination geometry around the lanthanide metal with two phenoxy groups in the apical positions. The bis(phenoxy) NHC ligands adopt a U-shaped conformation and the dihedral angles between two NHC rings range from 77 (Sm) to 95 (Yb). In contrast, recrystallization from THF/toluene provided the ate-complex [Sm(L1OCO)2Li(THF)] 114 showing Li-incorporation via phenoxidecoordination. The analogous sodium-diethylether complex 115 could be obtained by using [Sm{N(SiMe3)2}3]/NaN(SiMe3)2 and recrystallization from diethyl ether. In these complexes, the Sm ion has a distorted octahedral geometry with larger dihedral angles between the two NHC rings of 89 and 91 (Sm) compared to 113. The SmdCcarbene distances are only slightly influenced by the size of the alkali metal (see Table 9).71 The corresponding Li and K ate complexes 116 and 117 bearing saturated NHC ligands L2OCO (L2OCO ¼ 1,3-[O-4, 6-tBu2C6H2-2-CH2]2[C{N(CH2CH2)N}]) were synthesized accordingly, either from the combination of [Nd{N(SiMe3)2}3-m-ClLi (THF)]/KN(SiMe3)2 or [Ln{N(SiMe3)2}3]/KN(SiMe3)2.72 XRD analysis confirmed their structures to be similar to complexes 114 and 115. In addition, high frequency 13C NMR carbene resonances were observed (225.6, 220.3 ppm for La and 220.8, 214.6 ppm for Y), in agreement with carbene coordination to the metal centers. Interestingly, when the reaction was carried out with three equivalents of pro-ligand and two equivalents of [Nd{N(SiMe3)2}3], the chloride-bridged bimetallic complexes 119 were obtained, in which no deprotonation of the imidazolium rings occurred.72 The saturated six-membered NHC ligand complexes [Ln(L3OCO)2K(THF)] 118 were obtained from the pyridinium chloride precursor H3(L3OCO)Cl (L3OCO ¼ 1,3-[O-4,6-tBu2C6H2-2-CH2]2[C{N(CH2CH2CH2)N}]) and respective mixtures of [Ln {N(SiMe3)2}3]/KN(SiMe3)2. In the Nd complex, the LndCcarbene distance is slightly elongated compared to the imidazole-based complex 117-Nd and the 13C NMR signals for the carbene centers are upfield shifted for 118-Y compared to 117-Y (see Table 9).73 The bis(phenoxy)-based NHC lanthanide complexes 117 and 118 were efficient initiators for the controlled ring-opening polymerization of L-lactide and the high molecular weight polymerization of n-hexyl isocyanate.72,73

190

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

Scheme 25 Synthesis of bis(phenoxy)-functionalized NHC lanthanide complexes.

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides Table 9

191

LndCcarbene distances and 13C NMR data on trivalent lanthanide NHC pincer complexes with one NHC unit.

Compound name

Number

Ox state

˚) LndC (A

[{Sm(L1OCO)2}{Li(DME)3}]

113-Sm

3

[{Er(L1OCO)2}{Li(DME)3}]

113-Er

3

[{Yb(L1OCO)2}{Li(DME)3}]

113-Yb

3

[Sm(L1OCO)2Li(THF)]

114

3

[Sm(L1OCO)2Na(OEt2)]

115

3

[Nd(L2OCO)2Li(THF)]

116

3

[La(L2OCO)2K(THF)]

117-La

3

2.610(7) 2.606(7) 2.504(7) 2.525(7) 2.492(8) 2.515(8) 2.638(4) 2.584(4) 2.654(3) 2.601(3) 2.707(2) 2.724(2) 2.798(4) 2.776(4)

[Y(L2OCO)2K(THF)]

117-Y

3

[Nd(L2OCO)2K(THF)] [Nd(L3OCO)2K(THF)] [Y(L3OCO)2K(THF)]

117-Nd 118-Nd 118-Y

3 3 3

2.686(11) 2.759 (av)

[Nd(L1CNO)2Br] [Sm(L1CNO)2Br] [Er(L1CNO)2Br] [{Y(L1CNO)2(THF)}{Br}]

120-Nd 120-Sm 120-Er 121-Y

3 3 3 3

2.717(3) 2.685(6) 2.568(7) 2.583(7)

[{Yb(L1CNO)2(THF)}{Br}]

121-Yb

3

[{Y(L2CNO)2}{Br}]

122-Y

3

[{Lu(L2CNO)2}{Br}]

122-Lu

3

[{Er(L2CNO)2}{Br}]

122-Er

3

2.529(5) 2.536(5) 2.596(6) 2.590(6) 2.537(6) 2.538(6) 2.572(7) 2.554(7)

13

C (ppm) (1JLuC (Hz))

Refs.

71 71 71 71 71 72 225.6 220.3 220.8 214.6

211.4 212.0

196.4 196.0

72 72 72 73 73 74 74 74 75 75

187.2 186.8 190.1 (7.2)

75 75 75

Several examples of NHC-lanthanide complexes derived from salicylaldiminato-functionalized NHC ligands (LCNO) were synthesized via protonolysis starting from [LiLn{N(iPr)2}4] and the corresponding imidazolium salt (Scheme 26).74 Bis(NHC) lanthanide bromide complexes [Ln(L1CNO)2Br] 120 (L1CNO ¼ O-4,6-tBu2C6H2-2-C(H)]NCH2CH2[C{N(CHCH)NiPr}]) with varying metal size could be obtained and structurally characterized by XRD analysis. Whereas all complexes show capped octahedral coordination geometries, in the Nd and Sm complex one NHC ring is in the capping position, whereas in the Er complex the bromide is in the capping position. Also, the CdLndC angle decreases from Nd (176.5 ) to Sm (149.7 ) to Er (75.6 ), leading to the NHC rings being on opposite sides of the metal (Nd, Sm) and on the same side for the Er complex. Interestingly, in the corresponding Y and Yb complexes [{Ln(L1CNO)2(THF)}{Br}] 121, the bromide ligand was replaced by one THF molecule, leading to the cationic complexes with a non-coordinated bromide ion, which have similar coordination geometries to 120-Er.74 Reaction of the imidazolium precursor with nBuLi afforded a NHC lithium phenoxide ligand [Li(L2CNO)] (L2CNO ¼ O-4, 6-tBu2C6H2-2-CH(nBu)N(H)CH2CH2[C{N(CHCH)NiPr}]) derived from deprotonation and nucleophilic addition onto the imine moiety. This Li complex reacted by salt metathesis with LnCl3 precursors to give the new NHC ate complexes [{Ln (L2CNO)2}{Br}] 122 of the smaller lanthanides (Ln ¼ Y, Lu, Er) (Scheme 25).75 In these octahedral complexes, the NHC rings occupy the axial positions. The six-coordinate Y complex shows longer YdCcarbene bond lengths compared to the seven-coordinate complex 121-Y (see Table 9) and the carbene signal in the 13C NMR spectrum is shifted to 187 ppm compared to 196 ppm for 121-Y.

192

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

Scheme 26 Synthesis of salicylaldiminato-functionalized NHC lanthanide complexes.

3.05.3.3.2

Pincer ligands with two NHC units

Monoanionic tridentate CCC or CNC pincer ligands with two NHC units were employed for the synthesis of lanthanide dihalide or bis(amide) complexes (Schemes 27 and 28).

Scheme 27 Synthetic pathways to CCC-pincer bound bis(NHC) lanthanide dibromide complexes.

In the case of the xylene-bridged bis(imidazolium) pro-ligand H2(LCCC)Br3, (LCCC ¼ 2,6-(CH2[C{N(CHCH)NMes}])2C6H3) deprotonation and Li/Br exchange was carried out in the presence of LnCl3 salts to afford a large series of lanthanide dibromide complexes [Ln(LCCC)Br2(THF)] 123 (Scheme 27).76,77 The isostructural complexes show square-bipyramidal coordination geometry around the metal center with the two NHC and two Br ligands in trans-positions, respectively, forming the square, whereas the phenyl ring and the THF molecule are in the apical positions. The CcarbenedLndCcarbene angles diminish with increasing lanthanide size, going from 163.9 for Sc to 150.1 for Nd. An explanation for the preferred formation of the bromide compounds over the expected chloride compounds was not given. Activation of 123-Dy with an excess of Al(iBu)3 and [Ph3C][B(C6F5)4] provided a highly active catalyst system for the cis-1,4-selective isoprene polymerization, even at elevated temperature.77 The use of the diarylamido linked bis-carbene ligands (RLCNC) (RLCNC ¼ N[4-CH3-2-{C[N(CHCH)NR]}C6H3]2; R ¼ Me, iPr, Bn) for the synthesis of lanthanide bis(amido) complexes showed the influence of solvent, base, temperature, lanthanide and imidazolium precursors on the reaction outcome.78,79 The most efficient synthesis of the complexes [Ln(RLCNC){N(SiMe3)2}2] 124–126 was realized in a two-step procedure using five equivalents of NaN(SiMe3)2 and one equivalent of LnCl3 in THF at −78  C (Scheme 28). This pathway was successfully employed for methyl, isopropyl and benzyl substituted NHC rings.

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

193

Scheme 28 Synthetic pathways to CNC-pincer bound bis(NHC) lanthanide bis(amido) complexes and other derivatives.

These complexes show a distorted trigonal-bipyramidal coordination geometry around the metal center with the two silylamido and the amido groups in the equatorial positions. A comparison of the three yttrium complexes 124-Y, 125-Y, 126-Y shows the longest YdCcarbene distance for the iPr substituted NHC rings, whereas the signals in the 13C NMR spectrum are almost identical (see Table 10). The iPr substituted complexes 125 were also accessible via the reaction sequence consisting of ligand deprotonation with two equivalents of nBuLi followed by reaction with one equivalent of [Ln{N(SiMe3)2}3(m-Cl)Li(THF)3]. However, with the methyl and benzyl-substituted NHC rings, this second procedure also afforded the metal-free fused heterocyclic compounds 130, which were proposed to form via carbene CdC and CdN coupling as byproducts (Scheme 28). Furthermore, in the case of the benzyl-substitution on the NHC ring, the separated ion pairs [{Y(BnLCNC)2}{YCl[N(SiMe3)2]3}] 127 were obtained instead of the monomeric complexes. Finally, the combination of NaN(SiMe3)2 with YbCl3 afforded the separated ion pair complex 128 with the multinuclear [Na5Cl{N(SiMe3)2}5] anion. In all these zwitterionic complexes the cationic [Ln(BnLCNC)2] moiety has a distorted octahedral geometry around the metal ion. Direct reaction of [Ln{N(SiMe3)2}3(m-Cl)Li(THF)3] with proligand H3(RLNCN)X2 yielded only the fused heterocyclic compound 130 for benzyl substitution, whereas for iPr substitution a mixture of 125-Yb and the mixed Yb iodide amido complex [Yb(iPrLCNC){N(SiMe3)2}2I] 129 was isolated. The CNC-bound lanthanide bis-amide complexes 124–126 were shown to be highly efficient catalysts for the addition of terminal alkynes onto carbodiimides and of secondary phosphines onto heterocumulenes (RNCNR, RNCS, RNCO).78,79

194

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides Table 10 LndCcarbene distances and 13C NMR data on trivalent lanthanide carbene complexes with anionic pincer ligands with two NHC units.

3.05.4

Compound name

Number

Ox state

˚) LndC (A

13

C (ppm) (1JYC (Hz))

Refs.

[Sc(LCCC)Br2(THF)]

123-Sc

3

123-Y 123-Nd

3 3

193.1 193.2 192.4

76

[Y(LCCC)Br2(THF)] [Nd(LCCC)Br2(THF)] [Sm(LCCC)Br2(THF)]

123-Sm

3

[Gd(LCCC)Br2(THF)]

123-Gd

3

[Dy(LCCC)Br2(THF)]

123-Dy

3

[Lu(LCCC)Br2(THF)]

123-Lu

3

[Y(MeLCNC){N(SiMe3)2}2]

124-Y

3

[Eu(MeLCNC){N(SiMe3)2}2]

124-Eu

3

[Er(MeLCNC){N(SiMe3)2}2]

124-Er

3

[Y(iPrLCNC){N(SiMe3)2}2]

125-Y

3

[Er(iPrLCNC){N(SiMe3)2}2]

125-Er

3

[Yb(iPrLCNC){N(SiMe3)2}2]

125-Yb

3

[Er(BnLCNC){N(SiMe3)2}2]

126-Er

3

[Y(BnLCNC){N(SiMe3)2}2]

126-Y

3

[Sm(BnLCNC){N(SiMe3)2}2]

126-Sm

3

[Eu(BnLCNC){N(SiMe3)2}2]

126-Eu

3

[{Y(BnLCNC)2}{YCl[N(SiMe3)2]3}]

127-Y

3

2.397(8) 2.383(8) 2.541(3) 2.628(9) 2.640(9) 2.584(7) 2.587(7) 2.569(10) 2.579(10) 2.520(9) 2.536(9) 2.486(7) 2.482(7) 2.505(3) 2.559(3) 2.606(4) 2.552(4) 2.489(9) 2.608(9) 2.553(4) 2.542(4) 2.513(4) 2.521(4) 2.493(9) 2.495(9) 2.511(3) 2.509(3) 2.529(4) 2.521(4) 2.593(3) 2.584(3) 2.552(9) 2.575(8) 2.518(3) (av)

[{Er(BnLCNC)2}{ErCl[N(SiMe3)2]3}] [{Yb(BnLCNC)2}{YbCl[N(SiMe3)2]3}] [{Yb(BnLCNC)2}{Na5Cl[N(SiMe3)2]5}] [Yb(iPrLCNC){N(SiMe3)2}2I]

127-Er 127-Yb 128 129

3 3 3 3

2.477(3) (av) 2.461(6) (av) 2.456(4) (av) 2.420(8) 2.485(8)

77 77 76 74 74

193.1 193.2 192.0 (38)

76 79 79 79

190.3 (38)

79 79 79 78

192.0 192.5 152.5

78 78 78

194.2 194.5

78 78 78 78 79

Tetravalent lanthanide NHC complexes

A very limited number of Ce(IV)-NHC complexes have been reported, which are all related to the anionic alkoxy-tethered NHC ligands L1O and DippL4O. Three synthetic approaches have been developed starting either from trivalent or tetravalent Ce precursors as shown in Scheme 29.41,80,81 Oxidation of trivalent precursors: The homoleptic trivalent precursor [Ce(L1O)3] 47-Ce, readily obtained via salt metathesis from CeI3 and [K(L1O)] (see Section 3.05.3.2.2.1), was oxidized using benzoquinone to provide the first example of a Ce(IV)-NHC complex, [Ce(L1O)4] 131.41 The yield of the reaction could be increased upon addition of [K(L1O)] during the oxidation step. Oxidation with XeF2 or [FeCp]2[OTf] was also successful, however, providing the product in lower yield. XRD analysis of 131 showed bidentate coordination of two ligands onto the metal center with relatively short CedC bond lengths (2.693 (6), 2.652 (7) A˚ ), whereas two ligands only coordinated via the alkoxy part in a monodentate fashion. In contrast, in solution, rapid ligand exchange was observed at room temperature with only one signal for the carbene atom in the 13C NMR spectrum (213 ppm) and one set of ligands in the 1H NMR spectrum. Cooling the solution to 198 K provided a 1H NMR spectrum with three sets of ligand resonances in a 2:1:1 ratio. The presence of two free carbenes in 131 was corroborated by the addition of 9-BBN to this complex in THF, leading to the formation of the structurally characterized complex [Ce(L1O)2(L1O-BBN)2] 132. This diamagnetic compound

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

195

(A)

(B)

(C)

Scheme 29 Synthetic pathways to Ce(IV) complexes.

showed only one 13C NMR signal for the Ce-bound carbenes, whereas the boron-bound carbene was not observed. The structural features were close to those of complex 131 (Table 11).41 In a following study, the oxidation of the trivalent precursor bearing a saturated NHC ligand [Ce(DippL4O){N(SiMe3)2}2] 60-Ce (see Section 3.05.3.2.2.2) with trityl chloride in toluene gave the tetravalent chloride complex [Ce(DippL4O){N(SiMe3)2}2Cl] 133 in low isolated yield.80 This complex exhibits a significant low-field shift for the carbene center in the 13C NMR spectrum (237 ppm). The CedCcarbene bond distance in this five-coordinate complex is very close to that of complex 131 (2.692 (3) A˚ ). Computational studies on this complex and the analogous U(IV) complex showed higher ionic character and a shorter MdCcarbene bond order for the Ce complex.80

196

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides Table 11

LndCcarbene distances and 13C NMR data on tetravalent cerium NHC complexes. Number

Ox state

˚) LndC (A

13

Refs.

[Ce(L1 )4]

131

4

212.6

41

[Ce(L1O)2(L1O-9-BBN)2]

132

4

212.5

41

[Ce(DippL4O){N(SiMe3)2}2Cl]

133

4

2.693(6) 2.652(7) 2.705(2) 2.703(2) 2.692(3)

237.4

80

Compound name O

C (ppm)

Salt metathesis/oxidation approach: The reaction of [Ce(OTf )4] with 4 [K(L1O)] in hexanes yielded a mixture of the trivalent homoleptic complex 47-Ce and the expected tetravalent complex 131 (3:1 ratio).81 It was found that carrying out the reaction in diethyl ether and addition of benzoquinone provided only the tetravalent complex in good 60% yield. Furthermore it was shown that the salt metathesis approach also worked when the trivalent CeI3 precursor was used in combination with K(L1O) and benzoquinone. It should be noted that CAN or [Ce(OtBu)4] were not suitable starting compounds to access 131. Protonolysis: The mixed ligand Ce(IV)-NHC complex 133 could also be synthesized in 67% yield from the reaction of the tetravalent Ce precursor [Ce{N(SiMe3)2}3Cl] via protonolysis reaction with one equivalent of bicyclic carbene alcohol adduct H(DippL4O) (see Scheme 12) in toluene.80

3.05.5

Lanthanide complexes with abnormal/mesoionic NHC ligands

In contrast to the large variety of NHC-bound lanthanide complexes, mesoionic/abnormal NHC ligands have rarely been employed in combination with lanthanide and group 3 metals. A possible explanation may reside in the non-trivial synthesis of such complexes. The first anionic NHC-bound lanthanide complexes showing the unusual LndC4 coordination were obtained by the attempted reduction of trivalent precursors [Ln(HLN){N(SiMe3)2}2] (Ln ¼ Y, Sm) 36 (synthesis shown in Scheme 6).35 In the case of 36-Y, reaction with K(napht) provided crystals of the multi-metallic carbene complex 134-Y featuring a normal potassium carbene interaction and the abnormal LndC4 carbene formation (Scheme 30). The YdCcarbene signal in the 13C NMR spectrum is significantly upfield shifted to 167 ppm with a stronger YdC coupling of 62 Hz compared to the starting complex 36Y (186.3 ppm, 54.7 Hz). This complex also features a shorter YdC bond length (2.447 (2) A˚ ) compared to 36-Y (2.501 (5) A˚ ). It was proposed that this complex results from the reduction of the NHC ligand by K(napht) leading to the deprotonation at the C4-position, followed by rotation of the NHC ligand to bind the anionic site to the more electropositive Y metal. An alternative, high-yield synthesis of 134-Y was found in the deprotonation of the Y-bound NHC ligand using KMe in diethyl ether at low temperature, followed by addition of DME. The reduction of the trivalent 36-Sm precursor with K(Napht) yielded the expected color change for a divalent Sm metal center, however, only the trivalent 134-Sm complex could be obtained upon crystallization. Reaction of the bimetallic complex 134-Y with Me3SiCl afforded the complex 40-Y described in Scheme 6, featuring a normal YdC2 carbene interaction.35

Scheme 30 Synthetic pathways to heterobimetallic anionic dicarbene complexes featuring abnormal lanthanide coordination.

Recently, the formation of a multi-metallic Ce6Li2 cluster complex 135 (Scheme 31), featuring 6 anionic NHC-ligands bound to the Ce centers in the C2 and C4 positions, was observed as a byproduct from the reaction of the aryloxo-carbene precursor with [Li(THF)Ce(N(iPr)2)4] as described in Scheme 16.53 This complex contains two Ce centers having two normal NHC interactions,

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

197

Scheme 31 Multimetallic cerium anionic dicarbene complex featuring abnormal lanthanide coordination.

two Ce metals with one normal and one abnormal NHC interactions and two Ce centers displaying three abnormal NHCinteractions. In general, the CedC4 bond lengths were shorter (av. 2.66 A˚ ) than the CedC2 bond lengths (av. 2.72 A˚ ), except for one central CedC4 bridging interaction (2.853 A˚ ) (see Table 12). Concerning mesoionic carbene (MIC) rare earth complexes, one reaction pathway has been described starting from the N-heterocyclic olefin (NHO) CH2]C(NMeCH)2 and different [M{N(SiMe3)2}3] (M ¼ Y, La, Nd, U) precursors. The mesoionic NHC-rare earth complexes [M(aIMe3){N(SiMe3)2}3] 136, resulting from a formal 1,4-proton migration within the NHO, could be isolated in low crystalline yield (22–30%) (Scheme 32).82 The LndCcarbene distances were shorter than in normal Ln(III)-NHC complexes according to XRD analysis (see Table 12) and the MdN distances corresponded well with the Ln(III) oxidation state in these complexes. Delocalization within the MIC ligand was established from the ring bond distances which were found to be intermediate between single and double bonds. The 13C NMR shift for the carbene carbon in the Y complex was considerably up-field shifted (173 ppm) with respect to most NHC complexes and showed a large YdC coupling (1JYC ¼ 56 Hz). The electronic structure of these complexes was calculated theoretically to gain insights into the nature of the metal-carbene bond. Whereas a M] C donor-acceptor interaction was revealed in the case of U with a MICdU two electron sigma donation and a weak U-MIC one electron back donation, for the Y, La and Nd analogues only the sigma component was found due to the absence of f-electrons (Y, La) or due to energetically incompatible f-electrons (Nd). Consequently, NBO analysis showed a MdC bond order of 1.1 for U, involving some orbital U-carbene interactions, whereas for the lanthanide complexes the MdC bond order was calculated to be 0.6, with only electrostatic MIC-Ln interactions.82

Table 12 LndCcarbene distances (a ¼ abnormal, n ¼ normal) and 13C NMR data on trivalent lanthanide NHC complexes with abnormal/mesoionic NHC ligands. Compound name

Number

Ox state

˚) LndC (A

13

C (ppm) (1JYC (Hz))

Refs.

[{Y(aHLN)[N(SiMe3)2]2K(DME)}2]

134-Y

3

2.447(2)a

35

[{Sm(aHLN)[N(SiMe3)2]2K(DME)}2] [{Ce3(aMeL2OAr)3(NiPr2)5Li2(THF)2}2]

134-Sm 135

3 3

[Y(aIMe3){N(SiMe3)2}3]

136-Y

3

2.509(3)a 2.743(5)n 2.728(5)n 2.710(5)n 2.651(6)a 2.854(4)a 2.685(6)a 2.667(7)a 2.495(7)a

167.5 (62) na na

82

[La(aIMe3){N(SiMe3)2}3]

136-La

3

172.8 (56.2) na

[Nd(aIMe3){N(SiMe3)2}3]

136-Nd

3

na

82

[Y(aIMes)(CpMe)3] [Dy(aIMes)(CpMe)3]

137-Y 137-Dy

3 3

na na

56 56

2.675(14)a 2.699(5)a 2.614(12)a 2.620(11)a 2.5786(6)a 2.583(10)a

35 53

82

198

N-Heterocyclic and Abnormal/Mesoionic Carbene Complexes of the Group 3 Metals and Lanthanides

Scheme 32 Synthesis of mesoionic NHC-lanthanide complexes 136 from proton migration in NHO and their main mesomeric contributions.

Two further examples of mesoionic NHC-lanthanide complexes were obtained from the reaction of the tris(cyclopentadienyl) complexes [Ln(CpMe)3] (Ln ¼ Y, Dy) with the bulky IMes ligand in benzene at room temperature (Scheme 33).56 XRD analysis of the newly formed [Ln(aIMes)(CpMe)3] 137 complexes indicated carbene rearrangement from the normal to the mesoionic form within the coordination sphere of the lanthanide metal. In the Y complex, the YdCcarbene bond length of 2.5786 (6) A˚ was significantly longer than in the above complex 136-Y 2.495 (7) A˚ and closer to usual YdNHC complexes. This may be explained by the sterically more encumbered environment around the Y center due to the tris-Cp arrangement and also the bulky mesityl groups on the nitrogen atoms of carbene ligand. Following the reaction by NMR spectroscopy for more than 3 weeks revealed a slow rearrangement process. The Dy complex 137-Dy did not show any SMM behaviour.56

Scheme 33 Synthesis of mesoionic NHC-lanthanide complexes 137 via normal to abnormal NHC rearrangement (Mes ¼ 2,4,6-Me3C6H2).

3.05.6

Conclusion

After a relatively slow start in the mid-1990s, lanthanide-NHC chemistry has really taken off since 2005, with the introduction of functionalized anionic bi- and multi-dentate NHC ligands. Nevertheless, lanthanide NHC chemistry is still lagging far behind transition-metal NHC chemistry, which may be related to the supposed incompatibility of the hard lanthanide ions and the rather soft carbenes. However, it has now been demonstrated that the strong Lewis basicity of the NHC ligands can provide a large variety of lanthanide complexes in different oxidation states and even abnormal NHC lanthanide complexes have emerged. Various synthetic applications have been developed for divalent and trivalent NHC lanthanide complexes ranging from small molecule activation and addition-elimination reactions with electrophiles to hydrofunctionalization and polymerization reactions. Many new developments can be expected in the next decade.

Acknowledgment Financial support from the CNRS is acknowledged.

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199

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Wang, B.; Wang, D.; Cui, D.; Gao, W.; Tang, T.; Chen, X.; Jing, X. Organometallics 2007, 26, 3167–3172. 59. Wang, B.; Cui, D.; Lv, K. Macromolecules 2008, 41, 1983–1988. 60. Wang, B.; Tang, T.; Lia, Y.; Cui, D. Dalton Trans. 2009, 8963–8969. 61. Yao, C.; Wu, C.; Wang, B.; Cui, D. Organometallics 2013, 32, 2204–2209. 62. Li, S.; Liu, D.; Wang, Z.; Cui, D. ACS Catal. 2018, 8, 6086–6093. 63. Guo, F.; Nishiura, M.; Li, Y.; Hou, Z. Chem. Asian J. 2013, 8, 2471–2482. 64. Wang, X.; Lin, F.; Qu, J.; Hou, Z.; Luo, Y. Organometallics 2016, 35, 3205–3214. 65. Li, S.; Wang, M.; Cui, D. Polym. Chem. 2018, 9, 4757–4763. 66. Zhang, K.; Dou, Y.; Zhang, Z.; Jiang, Y.; Li, S.; Cui, D. Polymer 2020, 209, 123057. 67. Hu, Y.; Miyake, G. M.; Wang, B.; Cui, D.; Chen, E. Y.-X. Chem. A Eur. J. 2012, 18, 3345–3354. 68. Yao, C.; Liu, D.; Li, P.; Wu, C.; Li, S.; Liu, B.; Cui, D. Organometallics 2014, 33, 684–691.

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Yao, C.; Xie, H.; Cui, D. RSC Adv. 2015, 5, 93507–93512. Huang, Z.; Wang, S.; Zhu, X.; Yuan, Q.; Wei, Y.; Zhou, S.; Mu, X. Inorg. Chem. 2018, 57, 15069–15078. Yao, H.; Zhang, Y.; Sun, H.; Shen, Q. Eur. J. Inorg. Chem. 1920–1925, 2009. Zhang, M.; Ni, X.; Shen, Z. Organometallics 2014, 33, 6861–6867. Zhang, M.; Zhang, J.; Ni, X.; Shen, Z. RSC Adv. 2015, 5, 83295–83303. Zhang, J.; Yao, H.; Zhang, Y.; Sun, H.; Shen, Q. Organometallics 2008, 27, 2672–2675. Yao, H.; Zhang, J.; Zhang, Y.; Sun, H.; Shen, Q. Organometallics 2010, 29, 5841–5846. Lv, K.; Cui, D. Organometallics 2008, 27, 5438–5440. Lv, K.; Cui, D. Organometallics 2010, 29, 2987–2993. Gu, X.; Zhu, X.; Wei, Y.; Wang, S.; Zhou, S.; Zhang, G.; Mu, X. Organometallics 2014, 33, 2372–2379. Gu, X.; Zhang, L.; Zhu, X.; Wang, S.; Zhou, S.; Wei, Y.; Zhang, G.; Mu, X.; Huang, Z.; Hong, D.; Zhang, F. Organometallics 2015, 34, 4553–4559. Arnold, P. L.; Turner, Z. R.; Kaltsoyannis, N.; Pelekanaki, P.; Bellabarba, R. M.; Tooze, R. P. Chem. A Eur. J. 2010, 16, 9623–9629. Arnold, P. L.; Casely, I. J.; Zlatogorsky, S.; Wilson, C. Helv. Chim. Acta 2009, 92, 2291–2303. Seed, J. A.; Gregson, M.; Tuna, F.; Chilton, N. F.; Wooles, A. J.; McInnes, E. J. L.; Liddle, S. T. Angew. Chem. Int. Ed. 2017, 56, 11534–11538.

3.06

N-Heterocyclic and Mesoionic Carbene Complexes of the Actinides

Stephan Hohlocha and James R Pankhurstb, aUniversity of Innsbruck, Faculty of Chemistry and Pharmacy, Institute for General, Inorganic and Theoretical Chemistry, Innsbruck, Austria; bÉcole Polytechnique Fédérale de Lausanne, School of Basic Sciences, Institute of Chemical Sciences and Engineering, Sion, Switzerland © 2022 Elsevier Ltd. All rights reserved.

3.06.1 3.06.2 3.06.3 3.06.3.1 3.06.3.2 3.06.3.3 3.06.4 3.06.5 3.06.6 References

3.06.1

Introduction & scope Thorium–NHC complexes Uranium–NHC complexes Uranium(VI) and uranium(V) Uranium(IV) Uranium(III) Mesoionic carbenes & carbodicarbenes Conclusion & outlook Appendix

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Introduction & scope

Few classes of ligands have influenced and governed organometallic chemistry and catalysis as much as N-heterocyclic carbenes (NHCs).1–3 This unique class of ligands has contributed to a variety of important innovations in chemical technology, including the Nobel Prize for metathesis reactions4–6 in 2005; a reaction in which N-heterocyclic carbenes have proven to be superior ligands over phosphines in Grubb’s catalysts.7,8 The surge of N-heterocyclic carbene research is especially visible considering the variety of NHCs that exist today. Since the pioneering work of Wanzlick and Öfele9–11 and the isolation of free NHCs by Arduengo12 over 30 years later, a plethora of NHCs has evolved. In principle one could argue that since the early days of NHCs based on imidazole-2-ylidene, almost every nitrogencontaining 5- or 6-membered heterocycle has been converted into an N-heterocyclic carbene13–20 and even metallocene-derived mesoionic carbenes (MICs) have recently entered the field (Fig. 1).21 It should be mentioned here that this introduction will only give a brief overview of different NHC types and a short revision of their classification arguments. A more detailed discussion of the properties and applications of NHC ligands in modern organometallic chemistry will be the subject of other chapters in this book series. Currently, N-heterocyclic carbenes can be divided into three subclasses (Fig. 1),22 nNHC (normal N-heterocyclic carbenes), MIC (mesoionic carbenes) and rNHC (remote carbenes), which are classified by the following criteria:

•

nNHC: The free carbene has at least one neutral resonance form.

Typical representatives of these carbenes are Imidazol-2-ylidenes, Imidazolin-2-ylidenes, Benzimidazol-2-ylidenes, 1,2, 4-Triazol-5-ylidenes,1 Cyclic Alkyl Amino Carbenes (CAAC),14–18 Cyclic Amino Aryl Carbenes (CAArC),23 N,N-Diamidocarbenes (DAC)19 etc. (see Fig. 1, green box)

•

MIC (also known as abnormal carbenes): the free carbene must not have a resonance form that can be written without charge separation.

Typical representatives are Imidazol-4-ylidenes24,25 and 1,2,3-triazol-5-ylidenes.13,26,27 (see Fig. 1, blue box)

Comprehensive Organometallic Chemistry IV

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Fig. 1 Selected examples of N-heterocyclic carbenes (NHC) and mesoionic carbenes (MIC) explored in the literature. Ligand types that have been explored in actinide chemistry are highlighted in purple. Ligands marked with an asterisk ( ) have not been isolated in their free form yet.

•

rNHC: The carbene carbon atom must not be in an a-position with respect to any heteroatom. rNHCs can exist both as nNHCs and MICs.

Typical representatives are the 1-methyl-pyridyl-4-ylidenes and 1-methyl-pyridyl-3-ylidenes.28,29 However, this class of carbenes is only rarely explored. Despite the continuing interest of NHC ligands in the transition metal series, these ligands have barely been studied with the f-elements. This can be partly attributed to the relatively weak bonding interaction, which is an “intrinsic problem” that is also encountered with the early transition metals. Since the readily available actinides (thorium and uranium) are most commonly accessible in their highest oxidation states, these resemble very hard and strong Lewis-acidic metal ions. In contrast to this, carbene donors are, despite their electron deficiency, seen as soft donors, which causes a natural “mismatch” situation between these ligands and the metal centers. To put this into a more general context, a description of metal-NHC bonding is given below, where three main orbital interactions control the stability of the MdC bond:48

N-Heterocyclic and Mesoionic Carbene Complexes of the Actinides

s-Donation from the occupied NHC s-donor orbital to a metal acceptor orbital Despite the fact that NHCs have been described as strongly s-donating ligands, the HOMO of these ligands is quite diffuse and high in energy, leading to high ionic character in the MdC bond compared to late transition metal complexes.

Delocalization of the occupied NHC p-system into an unoccupied metal d- or f-orbital These interactions should be comparably small for heavy metals, as the coefficient of the carbene p-orbital is rather small, leading to poor overlap with the larger d- or f-orbitals.

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p-backdonation from an occupied d- or f-orbital into an empty p-orbital on the NHC This interaction can be completely neglected for actinides in their highest oxidation state as they hold no d- or f-electrons to donate. However, it should be noted that for high f-electron counts, especially in low-valent uranium complexes, this interaction becomes more and more important (vide infra).

The lack of p-backdonation from high-valent metal ions can additionally facilitate hyperconjugation or through-space interactions between the empty carbene p-orbital and the filled p-orbitals of halide ligands, which has been observed in various molecular structures.30 These hyperconjugation interactions can be described as a “pre-aggregation state” en route to reductive elimination of halo-imidazolium salts, thereby offering low-energy decomposition pathways (see Fig. 2). Although these criteria should formally render N-heterocyclic carbenes and their derivatives as unsuitable supporting ligands in actinide chemistry, the following chapter will describe the advances and surprises that have been encountered with this unique type of ligand within the actinide series so far. At the end, the reader should be convinced that actinide NHC chemistry is a versatile and incredibly interesting field, that still holds new ground to be explored and mysteries to be unraveled. It should be specified that this article will focus solely on the chemistry of N-heterocyclic carbenes. While other heteroatom-stabilized carbenes such as methandiides31–33 have also been used prominently in f-element chemistry,34–38 these examples will only be consulted for comparison with N-heterocyclic carbenes where it is useful. Additionally, those who seek a more general overview of the field of NHC ligands in f-element chemistry, including the lanthanides, are directed to two reviews written by Liddle and P. Arnold39,40 and their citing literature. We also point out that the metal-NHC bonds are drawn in the schemes and figures as generic single bonds, for clarity; in reality, M-CCarbene interactions are of a pure dative nature and these bonds could alternatively be drawn as arrows. Fig. 3 gives the scope of the NHC ligands discussed in the following sections. The discussion will be divided by the oxidation states of the elements, going from the highest to the lowest known oxidation state. A short discussion of “non-classical” NHC ligands, such as mesoionic carbenes and carbodicarbenes is then given. We close the chapter with a short summary and a possible outlook for the future of the field. Additionally, a summary of all relevant data is given for the complexes discussed (Table A1), including M-C distances and 13C NMR chemical shifts. Uranyl O]U]O IR stretching frequencies will be discussed directly in the text and are summarized in Table A2. the square bracket notation for all complexes discussed in this chapter can be found in Table A3.

Fig. 2 Hyperconjugation effects destabilizing M-NHC bonds in the presence of halogenide ligands X.

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Fig. 3 Overview of NHC ligands used in actinide chemistry so far.

3.06.2

Thorium–NHC complexes

All examples of thorium-NHC complexes focus on the use of anionic tethered NHC ligands. In fact, the chemistry of thorium NHC complexes is limited to just two NHC ligands, which are the alkoxy-NHC L1 and the bis-NHC borate ligand L7 (Fig. 3). For thorium halides, no neutral, non-anionic tethered NHC complexes have yet been isolated, unlike for their high-valent early metal halide counterparts.30,41–50 It is noteworthy that thorium is the last stable element that has been bound to an NHC ligand in the last 60 years. The field was initiated by P. Arnold and co-workers’ seminal work, describing the synthesis of the homoleptic thorium complex Th-1 using the alkoxy-tethered NHC ligand L1 (Scheme 1).51 Solution NMR spectroscopic studies revealed the thorium center in Th1 to be coordinated by two magnetically inequivalent sets of NHC ligands (arising from a slight twisting of each ligand), forming a distorted square antiprismatic coordination environment. The thorium complex was found to be eightfold coordinated by all available alkoxy and NHC donor atoms, which is indisputably attributed to the large ionic radius of the tetravalent thorium(IV) ion (1.19 A˚ ). This is noteworthy as the smaller ionic radii of uranium(IV) (1.14 A˚ )52 and the lanthanides (cf. cerium(IV) 1.11 A˚ )53 result in structural differences for the analogous uranium54 and lanthanide complexes51 (vide infra, e.g. complex U-10). While the thorium-oxygen distances in this structure are within the expected range for thorium alkoxide complexes,55 the thorium carbene distances range from 2.852(6) to 2.884(5) A˚ and are thus all significantly longer than the longest ThdC bond reported previously (2.72 A˚ ).

Scheme 1 Synthesis of the thorium(IV) NHC complex Th-1 (left) and its molecular structure (right). Hydrogen atoms have been omitted for clarity.

Even though these long bond distances might suggest very weak interactions between the thorium center and the NHC ligands, initial reactivity studies show that Th-1 does not react with protic substrates, such as pyrroles, indoles, or alkynes. NMR studies suggest that in solution, only weak hydrogen bond interactions are formed but no hydrolysis of the metal-carbene bond is observed, even under forcing conditions (Scheme 2). This is in line with other rare-earth NHC complexes, e.g., [Sc(L1)3] that are also virtually unreactive towards protic substrates.51 However, it is important to note that heteroleptic complexes, e.g., [Cp2Y(L1)], quickly

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Scheme 2 Reactivity of Th-1 towards protic substrates such as pyrrole.

hydrolyse under protic conditions to form zwitterionic alkoxy-azolium complexes.51 This implies that the stability of rare earth and f-element NHC bonds is highly dependent on the coordination environment (i.e., sterics, electronic structure and coordination number) of the complexes. In order to stabilize the mismatched bond situation between soft carbene donors and hard actinide centers, J. Arnold and co-workers investigated an approach using an NHC ligand with an anionic borate backbone, L7. The authors envisioned that due to the strong electrostatic attractions between the high-valent thorium(IV) ion and the anionic linker, the NHC ligation to the metal center would be enforced, resulting in stable and unreactive thorium-carbene bonds, while still allowing for high ligand flexibility. Based on this assumption, J. Arnold and co-workers established a rich and diverse chemistry of the bis-NHC complex Th-2, which is accessible in multigram quantities (>10 g), according to the synthetic procedure shown in Scheme 3.56

Scheme 3 Synthesis of the bis-NHC complex Th-2 and its reduction chemistry in the presence of substituted bipyridines leading to complexes Th-3a and Th-3b. (LDA, Lithium diisopropylamide [LiN(iPr)2])

For example, the authors have been able to reduce complex Th-2 in the presence of unsubstituted, neutral bipyridine as well as 6,60 -dimethyl-bipyridine, which led to complexes Th-3a and Th-3b that feature a formally direduced bpy2− (bipyridide) dianion. The metal-carbene bonds became partially elongated by up to 0.1 A˚ in Th-3a (2.656(4)–2.755(3) A˚ ) in comparison with Th-2 (2.623(6)–2.634(6) A˚ ). Nevertheless, these distances are drastically shorter than the thorium-carbene distances found in Th-1 (2.852(6)–2.844(5) A˚ ), which at first suggests a stronger metal-carbene interaction in the new complexes. However, complex Th-2 displays a coordination number of six vs. a coordination number of eight in Th-1, which plays a significant role in the shortening of the ThdC bond in Th-2. The formal assignment of a direduced bipyridine ligand was deduced from X-ray crystallographic analysis, which revealed an inter-ring CdC bond distance of 1.362(5) A˚ in the bipyridine ligand – a distance typical for bipyridine dianions.57–59 This was further supported through the observation of two reversible oxidations by cyclic voltammetry at −1.8 and −2.64 V vs. Fc+/Fc, which correspond to other bipyridine-centered oxidation processes.60 However, quantum chemical calculations on complex Th-3a revealed that the electronic structure of the complex is far more complicated than initially suggested.56 CASSCF calculations show that the ground state of complex Th-3a is a multiconfigurational open-shell singlet in which the main configuration (73%) corresponds to Th(III)-bpy1− (d1 p 1, S ¼ 0) and only a small contribution (27%) corresponds to the Th(IV)-bpy2− (d0 p 2, S ¼ 0) configuration that was initially suspected. This is also reminiscent of the results found for the [Cp 2Yb(bpy)] system of Booth et al.61 However, it is interesting that the first excited state of complex Th-3a corresponds to a pure Th(III)-bpy1− (d1 p 1, S ¼ 1) open-shell triplet state, which is only 2.7 kcal mol−1 higher in energy than the above-mentioned open-shell singlet ground state (d1 p 1, S ¼ 0). This narrow range of energies allows for thermal population of the different configurations, which is reflected in the magnetic, spectroscopic and reactivity properties of Th-3a. Notably, the closed-shell singlet state, which is the second excited state and corresponds to Th(IV)-bpy2− (d0 p 2, S ¼ 0), was found to lie another 10.2 kcal mol−1 higher in energy than the first excited state. The electronic configurations in this example are wildly different from those found in the related Cp-based thorium complexes, highlighting that the coordination environment built by the NHC ligands has a major influence on the metal electronic structure and can stabilize the thorium(III) oxidation state. For example, Zi, Walter, Maron and co-workers reported that the [Cp000 2Th(bpy)] (Cp000 ¼ 1,2,4-tri-tert-butylcyclopentadiene) complex adopts a pure closed-shell Th(IV)-bpy2− (d0 p 2, S ¼ 0) ground state, while the associated open-shell singlet and triplet states (d1 p 1, S ¼ 0 and d1 p 1, S ¼ 1) are 9.2 kcal mol−1 higher in energy (Fig. 4).62

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Fig. 4 Simplified electronic configurations of the NHC complex Th-3a (left) vs. its Cp000 analog (right). OSS, Open-Shell Singlet; OST, Open-Shell Triplet; CSS, Closed-Shell Singlet. # denotes an excited state, where the electron population is shown to indicate the spin configuration.

While the 6,60 -dimethyl-bipyridide complex, Th-3b, presents rather unselective reactivity,63 its unsubstituted bipyridide congener Th-3a shows highly diverse modes of reactivity, which reflects the unusual electron configuration of the complex. For example, as seen in Scheme 4, Th-3a reacts with ketones (giving Th-4a), aldehydes (giving Th-4b and Th-4c)56 and epoxides (giving Th-5a and Th-5b)63 akin to a classical two-electron reducing agent to yield the corresponding secondary and primary alkoxides. In all of

Scheme 4 Reactivity of complex Th-3a towards aldehydes and ketones, epoxides, azides and isonitriles.

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these transformations, the functional group (ketone, aldehyde or epoxide) is added to the 60 -position on the bpy ligand (Scheme 4). This explains why the 6,60 -dimethyl bipyridide complex shows rather unselective reactivity towards these substrates, as the 6-positions are both blocked. Similarly, complex Th-3a was found to promote a two-electron reduction in the presence of organic azides, leading to the corresponding thorium-imido complex (Th-6a, Scheme 4). The imido character was unambiguously determined by X-ray crystallography studies of Th-6a, revealing a ThdN bond distance of only 2.07(1) A˚ , which agrees well with thorium imido complexes in the literature64–67 and is about 0.35 A˚ shorter than typical thorium amido distances.64 In contrast to other previously reported actinide imido complexes prepared via this route,65,68 the bipyridine ligand in Th-6a does not dissociate and remains bound to the thorium center. This is likely attributed to the reduced steric demand of the bis-NHC-borate ligand compared to previously used tri-tert-butyl and pentamethyl-substituted cyclopentadienyl ligands.65,69 Electrochemical studies of Th-6a suggest that the bipyridine ligand can be reduced again at −2.88 V vs. Fc+/Fc. This could prompt new ventures in f-element imido chemistry, giving access to further redox-active transformations involving the imido motif. Besides acting as a “classical” two-electron donor, the thorium complex Th-3a also promotes radical cleavage of alkyl isocyanides.70 The radical pathway can be attributed to the unique open-shell configuration (d1 p 1, S ¼ 0 or S ¼ 1) of the complex and was thoroughly investigated using a “radical clock” (vide infra, Scheme 5). In contrast to previous reports of isocyanide cleavage reactions,71–73 this is a rare case where both fragments of the cleaved isocyanide are retained in the same molecule. Irrespective of the substrate, the reaction is highly regioselective and affords a product where the alkyl fragment adds to the 40 -position of the bipyridine ligand, transforming it into a pyridine-1,4-dihydropyridine-type monoanionic ligand, with localized double bonds on the substituted ring (Scheme 4). In addition to the high regioselectivity, the reaction also proceeds with moderate stereoselectivity depending on the steric bulk of the alkyl isocyanide to give the observed diastereomeric ratios of 70:30 (Th-7c) or 80:20 (Th-7a). Furthermore, the cyanide moiety of the isocyanides was found to be N-coordinated to the thorium center, displaying a Th-N distance of 2.494(5) A˚ . The N-coordination can be rationalized by a combination of density functional theory (DFT), the Pearson concept (HSAB) and X-ray crystallography studies. Although cyanide ligands often coordinate to metal ions through the “soft” C-atom, bonding is expected to occur via the “hard” nitrogen donor of the cyanide ligand in the case of the hard thorium metal center. This is also strongly suggested by DFT calculations, which show that the N-bound isocyanide complex is 5.6 kcal mol−1 more stable compared to the “classical” C-bound cyanide complex. Finally, X-ray crystallographic data also supports the isocyanide configuration, which results in well-distributed thermal ellipsoids, while an attempted cyanide refinement results in highly irregular thermal ellipsoids.

Scheme 5 Rearrangement of the cyclopropylmethyl radical to the but-3-enyl radical and the formation of Th-7e, proving the radical mechanism of the isocyanide cleavage performed by Th-3a.

The radical pathway of the alkyl isocyanide cleavage reaction can be attributed to two observations. Firstly, the qualitative reaction rates clearly correlate with the stability of carbon-centered radicals, diminishing from benzylic > 3  2 >1 alkyl isocyanides. In line with this observation, aryl isocyanides do not react at all with Th-3a, even under forcing conditions (72 h, 90  C). Secondly, when cyclopropylmethyl isocyanide was used as a substrate, the cyclopropyl fragment ring-opened and rearranged to the but-3-enyl functional group (Scheme 5). This rearrangement is a known degradation pathway of the cyclopropylmethyl-radical, further indicating that isocyanide cleavage proceeds via a radical mechanism in this case. Turning to heavier monodentate pnictide donors such as phosphorous, the diphosphido complex Th-8a, which can be obtained from the salt metathesis reaction between KPHMes and the iodo complex Th-2, was found to undergo unusual cyclometallation reactivity.74 The X-ray crystal structure of Th-8a reveals a close contact between one of the NHC mesityl groups and the phosphorous atom of one PHMes ligand (3.8 A˚ ), leading to clearly observable 13C-31P through-space coupling (JCP ¼ 4.5 Hz) in the 13C NMR spectrum of Th-8a. This interaction leads to a dynamic exchange between the hydrogen atoms on one of the mesityl-methyl groups and the phosphorous atom of the PHMes phosphide ligand. This exchange can be visualized by treating Th-8a with the deuterated phosphine PD2Mes, which results in the exchange of the deuterium atoms between the phosphide and the mesityl-methyl group in Th-8d. This most likely proceeds via the cyclometallated intermediate Th-9Mes (Scheme 6). Notably, in contrast to thorium bis-phosphido metallocene complexes,75–77 no signs of phosphinidene formation were observed even after prolonged heating of Th-8a above 60  C.78 Furthermore, Th-8a was also found to be inert towards Lewis bases, such as DMAP or pyridine, which

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Scheme 6 Synthesis of the diphosphido complex Th-8a and the proposed mechanism for deuterium exchange via a transient metallacycle Th-9Mes.

previously induced the formation of phosphinidene complexes.77 Additionally, no phosphaazaallene formation was observed with tert-butyl isocyanide78 in further contrast to the reactivity of thorium bis-phosphido metallocene complexes.79 The formation of the cyclometallated complex Th-9Mes from Th-8a highlights the lability of the phosphido ligand and has been exploited by J. Arnold and coworkers, leading to a rare example of thorium catalyzing an organic transformation (Scheme 7). Specifically, vinyl phosphines were formed catalytically from the coupling of internal alkynes and primary phosphines. Based on NMR spectroscopic investigations, Th-8a behaves as a precatalyst and forms the catalytically active species Th-9Mes after the loss of one phosphido ligand and the formation of the metallacycle as described above. After studying a series of stoichiometric reactions, the authors were able to construct a reaction mechanism. Reaction with tolane (diphenylacetylene) results in insertion of the alkyne into the PdH bond of the bound phosphide ligand, leading to the formation of the vinylphosphido thoracycle Th-10. Addition of mesityl phosphine results in the protonation of the bound vinylphosphido ligand, releasing the vinyl phosphine product (VP) and reforming Th-9Mes. Performing the reaction under a H2 atmosphere stops catalysis, and it was discovered that the vinylphosphido ligand in Th-10 is reduced by H2, leading to Th-11. When Th-10 was instead reacted with the secondary phosphine, diphenylphosphine, the diphenylphosphido complex Th-9Ph2 was isolated, which, unlike Th-9Mes, is unreactive towards tolane. This reveals that the reaction with tolane to form Th-10 occurs at the PdH bond rather than the PdTh bond. The substrate scope is therefore limited to primary phosphines and is also limited to internal alkynes. However, the pre-catalyst Th-8a does display interesting reactivity towards terminal alkynes: the reaction between Th-8a and phenylacetylene yields the thorium bis-acetylide complex Th-12.

Scheme 7 Reactivity of the diphosphido complex Th-8a towards phenylacetylene and diphenylacetylene (tolane) along with the catalytic cycle for the hydrophosphination of tolane mediated by the transient metallacycle Th-9Mes.

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The diphosphido complex Th-8a also promotes ligand-induced reductive elimination processes. Mixing Th-8a with bipyridine gave access to the PdP coupled diphosphine [PHMes]2 along with the formation of the direduced bipyridine complex Th-3a (Scheme 8).78 Overall, bipyridine functions as an oxidant in this reaction, oxidizing the phosphorous atoms from -III in Th-8a to -II in [PHMes]2 while reducing to the bpy2− dianion. This is remarkable since the reduction potentials of bipyridine are extremely negative (vide supra) and this reaction proceeds quickly at ambient temperature conditions without the addition of strong external reductants to reduce the bipyridine.56,58 Similar reactivity can be obtained when Th-8a is mixed with other oxidants, such as iodine (yielding the bis-iodo complex Th-2), p-tolyl azide, 1,10-phenanthroline or white phosphorous. However, in all of these latter cases, the fate of the thorium fragment remains unclear. Additionally, the reaction has been found to be highly dependent on the phosphide substituents, as the phenylphosphido (PHPh) complex Th-8b did not give P-P coupled products at all, while secondary thorium phosphides, such as Th-8c (R0 ¼ R00 ¼ PPh2) gave access to tetraphenyldiphosphines only after heating (Scheme 8).

Scheme 8 Ligand-induced reductive elimination of differently substituted di-phosphido complexes Th-8a,b,c with bipyridine.

In all of the reactions discussed above, the NHC-thorium bond distances change by 0.15 A˚ (see Table A1), indicating that the interactions between the carbenes and the thorium centers are based mainly on electrostatic attractions rather than formal dative bond interactions. However, the flexible coordination environment offered by the bis-NHC-borate ligand supports a wide variety of reactivities in comparison with other “innocent” supporting ligands, such as cyclopentadienes. The involvement of the mesityl NHC substituents in stabilizing metallacycle intermediates is quite unusual for an ancillary ligand but has facilitated catalytic studies involving thorium. Thus, bis-NHC-borate ligands are an ideal platform for further applications in f-element chemistry and catalysis.

3.06.3

Uranium–NHC complexes

3.06.3.1

Uranium(VI) and uranium(V)

Two decades have passed since the first uranium NHC complex was described by Costa and co-workers.80 Since then, the chemistry of uranium NHC complexes has expanded into a field of its own and is filled with many interesting examples. Thanks to the wider range of possible oxidation states that are available to uranium (most commonly U(III) to U(VI)), uranium NHC chemistry is much more prevalent than that of thorium. Nevertheless, uranium NHC chemistry is still relatively underdeveloped and many avenues are yet to be explored. Taking the uranyl motif as an example, where uranium is in its highest possible oxidation state of +VI, only three examples of NHC complexes have been reported so far. This can be only partly attributed to the notorious mismatch situation between the “soft” neutral carbene donor-atom and the “hard” metal center, but is also due to the fact that the uranyl motif is strongly oxidizing in the presence of carbon-based ligands e.g., alkyls, making uranyl-carbon bonds scarce in general.81–84 Nevertheless, in recent years many advances in the isolation of uranyl-carbon s-bonds have been made.84–90 Additionally, the uranyl motif is probably the most readily accessible source of uranium in solution for chemists, since this is the most stable naturally-occurring form of uranium (e.g., in seawater).91,92 Accordingly, the very first actinide/actinyl NHC complexes were the IMes (1,3-dimesityl-imidazol-2-ylidene) and IMesCl2 (1,3-dimesityl-4,5-dichloro-imidazol-2-ylidene) uranyl complexes U-1a and U-1b, described by Costa and co-workers (see Scheme 9).80 Despite the Lewis acidic nature of the uranyl motif, the NHC complexes U-1a and U-1b are less prone to hydrolyzation than the chemically related group VI dioxo complexes, namely those of molybdenum93 and tungsten.94 While the reasons for this are not completely explored, one possible explanation could be the occurrence of an inverse trans influence (ITI), which has been observed in uranium(IV) carbene complexes95 and rationalized by calculations for uranyl and its imido and carbene analogs.96 The uranium complexes adopt an almost perfectly octahedral coordination environment around the uranium(VI) center and the U]O bond distances fall into the same range as for previously reported uranyl complexes. The uranium carbon distances were found to be 2.626(7) (U-1a) and 2.609(4) A˚ (U-1b). Examining the IR stretching frequencies, the asymmetric O]U]O stretches appear at 938 and at 942 cm−1 for U-1a and U-1b, respectively. At the time, these belonged to the highest reported frequencies for complexes with the general formula [UO2Cl2L2] (L ¼ Ph3PO 925 cm−1, L ¼ none 940 cm−1, L ¼ DPPFO2 (diphenylphosphinoferrocene dioxide) 916 cm−1)97,98 suggesting a rather weak electron donation from the NHC ligands (vide infra).

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Scheme 9 Synthesis of the first uranium NHC complexes U-1a and U-1b (left). Molecular structure of U-1a (right), hydrogen atoms are omitted for clarity.

Interestingly, uranium-NHC complexes that feature an anionic tether are isostructural around the uranium center in comparison with the above examples. This was examined by P. Arnold and co-workers by reacting amide-tethered NHC ligands with the uranyl dication, using both salt metathesis and protonolysis protocols (Scheme 10).99 The complexes U-2a and U-2b display UdC bond distances of 2.640(5) and 2.633(7) A˚ , respectively, showing that the UdC bond distance is unaffected by the steric bulk of the amide substituents. Despite the UdC bond distances being unaffected, the anionic linkages in U-2a and U-2b did have a major influence on the metal-carbene pitch and yaw angles (Scheme 10), which were 23 /17 for U-2a (pitch/yaw) and 9 /3 for U-2b. However, one should bear in mind that such deformations in the molecular structure might not be solely related to ligand effects but also to packing effects within the crystal lattice.

Scheme 10 Synthesis of amido- and alkoxy-tethered uranyl(VI)-NHC complexes. The definition of pitch and yaw angles is shown on the right.

To compare the influence of the NHC donor and also to obviate undesired abnormal carbene (MIC, mesoionic carbene) reactivity in the NHC backbone,100 P. Arnold and co-workers also synthesized the alkoxy tethered imidazoline-2-ylidene frameworks L2 and L3.54 While the amido NHC ligands with the unsaturated backbones HL4 and HL5 can be isolated as free carbenes, isolation of the free alkoxy carbenes (L1 and L2,L3) is problematic because they form an equilibrium with their bicyclic carbene-alcohol adducts,101 which is independent of the NHC moiety used (i.e., saturated vs. unsaturated). Nevertheless, under protolytic conditions the equilibrium can be forced to the alkoxy-carbene form and the complexes U-3a and U-3b can be isolated. In the case of the saturated imidazoline-2-ylidene complexes, the UdC bond distances shorten slightly to 2.580(4) A˚ for U-3a and 2.612(2) A˚ for U-3b. Although the shortening of the UdC bond distances could be ascribed to the slightly higher donor capacity of the saturated NHC vs. unsaturated NHC ligands, it should be noted that the steric bulk of the NHC ligands L2 and L3 is lower compared to the amide ligands L4 and L5, which allows closer uranium-NHC contacts for the former. The NHC carbene 13C NMR resonances, which are observed at d 262.8 ppm for U-2a and 281.6 ppm for U-3a, are in line with the shift differences previously observed for unsaturated vs. saturated NHCs. Comparing the asymmetric stretching vibrations of the uranyl O]U]O motif by IR spectroscopy, to potentially differentiate between the NHC donor capacities of the saturated and the unsaturated NHC ligands with comparable imidazole N-substitutions, shows absorption bands at 933 cm−1 for U-2b and at 851 cm−1 for U-3a. These values (in combination with the shorter U-C distances) initially suggest that the saturated NHC ligands are much stronger donors compared to their unsaturated congeners. However, setting this into a wider context by comparing the stretching values with other uranyl complexes, such as cis-[UO2(OtBu)2(OPPh3)2] (861 cm−1) and [UO2{N(SiMe3)2}2(OPPh3)2] (901 cm−1),102 it can be seen that the

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donor atoms of anionic ligands in complexes of the general formula UO2X2L2 (X ¼ anionic donor) influences the [UO2]2+ vibrational energies to a much greater extent than the neutral donors (L). In a similar way, the role of the anionic donor atoms in tethered carbene ligands might be more significant in weakening the UO2 bonds than the type of NHC unit used. Nevertheless, the above comparison does indicate that NHC-based ligands coordinate to uranyl more strongly than phosphine oxides, which are ubiquitously used in f-element chemistry. From an electronic structure point of view, it is also interesting to observe such unexpectedly large shifts for the carbene resonances in the 13C NMR spectra of all uranyl-NHC complexes. These shifts do not only arise from electron-deficient carbene centers, but can be mostly attributed to giant spin-orbit coupling and relativistic effects that are significant for uranium.103 The coordination chemistry between NHC ligands and the uranyl motif becomes quite unusual in the absence of steric bulk or tethering groups on the NHC. Hayton and co-workers104 showed that the reaction between the BDI-supported uranyl complex U-4 (BDI ¼ b-diketiminate) and lithiated 1-methylimidazole resulted in the formation of an unusual -ate complex U-5 (Scheme 11). The 13C NMR resonance at lowest field was observed at d 329.4 ppm, which would normally be indicative of the formation of a carbene complex. However, keeping in mind that large spin-orbit couplings can cause extremely shifted carbon NMR signals,103 it is difficult to assess the nature of the UdC bond from NMR spectroscopy alone. Indeed, the X-ray crystal structure revealed unusually short UdC bond distances of 2.498(6) and 2.499(7) A˚ , which are much shorter than the shortest uranium(VI) NHC carbene distances previously reported (2.580(4) Å in complex U-3a, vide supra).54 Conversely, the UdC bond distances are much more reminiscent of those for uranyl-alkyl or metallocene complexes, for example: the UdCp centroid and UdCN distances in [UO2(C5Me5)(CN)3]2− (2.568 and 2.549 A˚ , respectively); the U-C distances in [UO2(CH2SiMe3)4] (2.497(6) and 2.481(6) A˚ )84; and in [U(CH3)6]− (2.415(5)–2.452(5) A˚ ).90 The short bond distances in U-5 therefore suggest that the heterocyclic ligands adopt a carbanionic resonance form rather than carbenes.

Scheme 11 Synthesis of a rare uranyl imidazolide complex U-5 and its transition-metal induced rearrangement to form uranyl-supported iron and cobalt carbene complexes U-6Fe and U-6Co.

The ate-complex U-5 can be used as a precursor for the preparation of heterobimetallic complexes with the transition metals, which involves a rearrangement of the coordination modes for the heterocyclic ligands. When the -ate complex is mixed with “typical divalent NHC metals” from the 3d series (e.g., iron(II) and cobalt(II)), the imidazolide unit rearranges and the heterodinuclear carbene complexes U-6Fe and U-6Co are formed. The carbene-transfer can be rationalized by the HSAB concept and has become a common synthetic procedure to synthesize late/early heterobimetallic metal complexes.101 Furthermore, the use of uranyl as a redox-active NHC-substituent might hold promise for switchable complexes in the future. The intriguing concept of redox-active side groups as redox switches for mesoionic carbenes has already been successfully demonstrated. Notably, no uranium(V) complex has been isolated with NHC-supporting ligands thus far.105–108

3.06.3.2

Uranium(IV)

While no NHC complex of the general formula [(NHC)nThCl4] (n ¼ 1 or 2) has yet been reported for thorium(IV) (vide supra), the binary NHC-halide complex U-7 was reported for uranium(IV) by Liddle and co-workers in 2009.109 Even though this was not the first uranium(IV) NHC complex reported in the literature110 (vide infra), it is a beautiful example of unsupported NHC-actinide bonding and is thus a helpful tool for describing and studying the binding of NHC ligands to tetravalent actinides. The complex U-7 can be obtained by mixing free IPr with uranium tetrachloride in THF. Notably, U-7 is the sole product of the reaction, irrespective of the number of equivalents of IPr used. Similarly to other tetravalent metal ions (e.g. in [(IPr)2TiCl4]),46,48 the uranium atom in U-7 adopts a distorted octahedral coordination environment with the NHC ligands situated trans to each other. The uranium-carbene distances were found to be 2.675(7) and 2.687(7) A˚ and are at the longer end of the range for uranium-carbene bond distances (see Table A1). For example, the U-NHC distance in U-7 is 0.07 A˚ longer than in the structurally related uranium(VI) complexes U-1a,b. Although this can mainly be attributed to the smaller ionic radius of the uranium(VI) ion compared to uranium(IV), another possible explanation is the higher steric bulk of the IPr ligand in U-7 compared with the IMes ligand in U-1. This is further supported by an earlier report by Danopoulos and co-workers,110 who synthesized the uranium(IV) carbene complex U-8, which features the pincer type bis-NHC-pyridine ligand L8. In that case, the uranium-carbene bond distances are almost 0.1 A˚ shorter (2.573(5) and 2.587(5) A˚ ) than in complex U-7. The rigid conformation of the L8 ligands enforces a non-classical coordination geometry, where the NHC ligands are no longer situated in trans positions. In U-8, the short uranium-carbene bond distances can be attributed to the pincer arrangement, which holds the steric bulk of the Dipp groups

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Fig. 5 Synthesis of the uranium(IV) NHC complexes U-7 and U-8, with the former displaying unsupported and untethered uranium(IV) NHC bonds, and their corresponding molecular structures. Hydrogen atoms have been omitted for clarity.

away from one another, and also forces the NHC groups to remain close to the metal center despite the higher coordination number of seven in U-8 (U-7 carries a coordination number of six). Even though the halide atoms in complex U-7 are not in the same plane and are slightly tilted towards the unoccupied p orbitals of the carbene carbon atoms (see Fig. 5, left), inspection of the Kohn-Sham orbitals and the Mayer bond orders between the chloride and the carbon NHC atom shows that there is essentially no interaction between the carbene and the halide ligands, which has been observed in early transition metal NHC complexes.30 In terms of anionic tethered NHC complexes of uranium(IV), only one report from P. Arnold and co-workers is present in the literature.52 Interestingly, the synthesis of the uranium(IV) NHC complexes U-9 and U-10 does not make use of a uranium(IV) starting material (Scheme 12). Rather, they are formed by the disproportionation of uranium(III) into uranium metal and uranium(IV) in the presence of the potassium salt KL1. The disproportionation of uranium(III) complexes into uranium metal and uranium(IV) is a common decomposition pathway of uranium(III) complexes.111–114 Thus, sub-stoichiometric amounts of ligand are required to achieve higher yields of the corresponding tris- and tetrakis-NHC complexes U-9 and U-10. In contrast to the homoleptic thorium complex Th-1,51 in which all possible donor atoms bind to the thorium center to form a square antiprismatic coordination environment around the thorium(IV) ion, the uranium ion in complex U-10 was found to be only sevenfold coordinated (in a pentagonal bipyramidal fashion) with one pendant, non-coordinating free NHC unit. This is a direct consequence of the smaller ionic radius of uranium(IV) compared to thorium(IV) (“the actinide contraction”) and is also reflected by the shorter

Scheme 12 Synthesis of the heteroleptic and homoleptic alkoxy-NHC complexes U-9 and U-10 and reactivity of the latter towards metals and boranes.

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An-NHC distances in U-10, which span from 2.696(3) to 2.799(3) A˚ , compared to 2.852(6) to 2.884(5) A˚ in Th-1. Notably, variable-temperature NMR spectroscopic studies suggest that a fluxional process exists between the free and uranium-coordinated NHC groups. The pendant NHC unit, as well as the weak nature of electropositive metal-NHC interactions, allows for further metalation of the complex. For example, when U-10 is reacted with either [(NBD)Mo(CO)4] (NBD ¼ norbornadiene) or [(COE)W(CO)5] (COE ¼ cyclooctene), the corresponding heterobimetallic complexes U-11 and U-12 can be obtained (Scheme 12). However, the low solubility and rapid decomposition of these complexes prevent their full characterization. When using more solubilizing NHC scavengers than group VI carbonyls, such as the reaction between U-10 and BH3SMe2, the free NHC unit in U-10 gives access to the NHC-borane complex U-13 (Scheme 12). NMR spectroscopic titration experiments show that U-10 can bind up to four equivalents of BH3SMe2, underlining the hemilabile nature of the uranium-NHC bond and suggesting that the complexes U-9 and U-10 could be prospective metalloligands in the future.

3.06.3.3

Uranium(III)

Uranium in its +III oxidation state is certainly the most deeply explored when it comes to uranium-NHC chemistry. Undoubtedly, this is attributed to the large potential uranium(III) holds in advancing several important chemical fields,115,116 including small molecule activation117–120 and catalysis.66,121–123,123a Furthermore, the magnetic properties of uranium(III) are relevant for the design of new single molecule magnets, which are highly sensitive to the coordination chemistry of the metal.124–128 Additionally, the +III oxidation state allows for direct comparisons of the chemical properties of uranium(III) complexes with non-radioactive and (mostly) redox-innocent trivalent lanthanides.129–131 Shortly after the first uranyl-NHC complexes were reported by Costa and co-workers,80 the group of Meyer extended the chemistry of N-heterocyclic carbenes, especially that of the sterically undemanding IMe4 (1,3,4,5-tetramethylimidazol-2-ylidene) ligand, to uranium(III). The group synthesized two different uranium(III) complexes. The first of these was based on the “TACN” (1,4,7-triazacyclononane) ligand, a tri-amino-tris-aryloxide multi-chelating ligand, and the second makes use of bulky hexamethyldisilylamide ([N(SiMe3)2]) ligands. The TACN complex (U-17) features a U-carbene bond length of 2.789(14) A˚ and the tris[N(SiMe3)2] complex (U-18) features a U-carbene bond length of 2.672(5) A˚ (Scheme 13, for the molecular structure of U-18 also see Fig. 6).132 In the case of U-17, the authors described that coordination of the carbene ligand to uranium(III) caused a significant movement of the uranium ion in comparison with the parent uranium(III) complex, [U(Ad,tBuArO)3TACN]. For the [U(Ad,tBuArO)3TACN] complex the uranium center is located about 0.88 A˚ below the plane defined by the three phenolate oxygen atom donors, while in U-17 the uranium atom is only 0.26 A˚ below this plane. This indicates a significant orbital interaction between the uranium center and the NHC ligand. Examining this interaction computationally by using the less complicated model complex U-18, the authors found that the SOMO-2 consists of f-type uranium orbitals as well as p-type orbitals of the carbene ligand and reflects a rare p-backbonding interaction between the NHC ligand and the electron-rich uranium(III) ion.

Scheme 13 Reactivity of the non-bulky imidazole-2-ylidene ligand IMe4 towards a variety of uranium(III) complexes.

The higher covalent character of the uranium-NHC interaction that is observed for uranium(III) holds potential for the use of NHCs and ionic liquids in waste remediation strategies. This was illustrated in the seminal work of Ephritikhine and co-workers,133 who compared the affinities of isostructural, trivalent cerium and uranium complexes towards the IMe4 ligand previously used by Meyer et al. (vide supra).132 All complexes (Ce-19, Ce-20 and U-19, U-20, Scheme 13) can be obtained in good yields from the reaction of one equivalent of IMe4 with cyclopentadienyl complexes of different steric bulk. The reactions of [Cp 2MI(L)] form the

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complexes U-19 and Ce-19, where L ¼ pyridine in the case of uranium and L ¼ none in the case of cerium. The reactions of [(Cp0 )3M] (Cp0 ¼ C5H4tBu and M ¼ Ce or U) yield the complexes U-20 and Ce-20. The uranium-NHC distances in the complexes are different (2.687(5) A˚ in U-19 vs. 2.768(5) A˚ in U-20), which is undoubtedly the result of differences in steric bulk on the cyclopentadienyl ligands rather than any electronic effects. Importantly, all uranium-NHC distances are shorter than the corresponding cerium-NHC distances, despite the larger ionic radius of uranium in comparison with cerium.134 This can be attributed to the higher covalent bond character generally observed in the early actinides, in addition to the unique p-back bonding situation for NHC ligands as described by Meyer and co-workers.132 The higher bond covalency between the NHC ligand and the uranium(III) center was also demonstrated by the preferential binding of IMe4 to uranium(III) over cerium(III). Adding one equivalent of IMe4 to an equimolar mixture of [Cp 2UI] and [Cp 2CeI] gave the corresponding complexes U-19 and Ce-19 in a ratio of 80:20 at 23  C and 90:10 at −60  C. Similar ratios of U-19:Ce-19 are observed in the NHC exchange reaction between [Cp 2UI] and Ce-19, clearly proving a strong preference of the NHC ligand for ligating uranium. Similar findings have been reported for the increased affinity of IMe4 for U-20 over Ce-20.133 Considering that a large variety of NHC ligands can be directly accessed from imidazolium-based ionic liquids,135–137 the above findings open up new avenues for the development of separation techniques and remediation strategies for radioactive nuclear waste, with the potential to help separate late actinide elements from early actinides (and fissionable uranium) and non-radioactive lanthanides.138–141 The bonding differences between U-19 and Ce-19 have also been computationally studied by the groups of Gagliardi and Cramer, who confirmed the uranium-carbene bond to be stronger than the cerium-carbene bond. Additionally, the strength of the uranium-carbene bond was found to be less dependent on the s-donor strength of the carbenes compared to cerium, which is in line with the larger nephelauxetic effect that is observed for the actinides compared to the lanthanides.142 Nevertheless, it should be noted that potential lanthanide/actinide separation strategies will be highly dependent on the specific NHC ligand used and cannot be overly generalized. This is reflected in a computational study by Maron and co-workers, who examined the binding preferences of the tridentate bis-NHC ligand L8 towards La(III) and U(III) and revealed that ligand binding is instead favored with the lanthanide in that case.143 It is important to note that despite the increased covalency found in uranium-NHC bonds (especially in the trivalent state), co-ligands can still prevent the isolation of desired NHC complexes and can in turn introduce other reaction/decomposition pathways, leading to the isolation of unexpected products. This was demonstrated by Evans and co-workers, who showed the differences between the addition of IMe4 to [UCp0 3] vs. [UCp 3] (Scheme 13). While the reaction resulted in the NHC-ligated complex U-20 for the sterically less demanding UCp0 3 complex (vide supra),133 the reaction with [UCp 3] gave only oxidized decomposition products. It can be assumed that the oxygen causing the decomposition results from residual water or oxygen in the solvents, while the NHC ligand might only have a minor influence on the observed reactivity. Nevertheless, this indicates that with the more p-donating cyclopentadienyl ligands, such as Cp , the corresponding NHC complexes could become less stable. While a full investigation of the reaction is still necessary, one particularly interesting product that was crystallized was a rare example of a NHC-coordinated terminal uranium(IV) oxo complex U-21.144 The uranium oxo distance in U-21 (1.916(6) A˚ ) is significantly longer compared to previously reported terminal mono-oxo complexes of uranium (1.76(1) A˚ –1.859(6) A˚ ),145–149 which indicates a major activation of the oxo unit. However, it remains debatable if this activation is caused by the electon-donating character of the IMe4 ligand or the Cp ligand. Transitioning to anionic-tethered NHC ligands, P. Arnold and co-workers have demonstrated the synthetic and transformative potential of these complexes.150,151 The parent uranium(III) complex U-22 (Scheme 14) is accessible via a protonolysis route using HL3.150 The uranium-NHC bond in U-22 was found to be 2.693(4) A˚ and is comparable to other uranium(III)-NHC complexes, despite the anionic tether group. As expected for a uranium(III) complex, U-22 showed remarkable potential in the activation of small molecules. For example, it cleanly reacts with trimethylsilyl iodide (Me3SiI) to give the silylated iodo-complex U-23. Hitherto, this is the only example in actinide chemistry where the NHC is not acting as a supporting ligand, but is actively participating in the reaction as a chemically non-innocent ligand. The reaction can be formally seen as the addition of Me3SiI across the uranium-NHC bond, the driving force of which is undoubtedly the higher bond stability of the uranium-iodide bond over the uranium-NHC bond. This reaction is not only limited to the actinides but is also performed by structurally related lanthanide complexes with the same NHC ligand. However, clear differences can be seen in the thermal stability of the complexes. In the case of the lanthanides, which are less prone to undergo redox reactions, thermal activation leads to the elimination of tris(trimethylsilyl)amine, N(SiMe3)3, along with the formation of heteroleptic iodo-N(SiMe3)2 lanthanide complexes.150 In contrast, thermal activation of U-23 results in the bis-iodo bis-NHC uranium(IV) complex U-24 through N(SiMe3)3 elimination and other intractable uranium products. Unlike the uranium(III) complex U-22, the uranium(IV) complex U-23 is incapable of activating another equivalent of Me3SiI (Table A3).

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Scheme 14 Reactivity of the uranium(III) complex U-22 towards the small molecules trimethylsilyl iodide, carbon dioxide, carbon monoxide and diphenyldiazomethane.

The activation of the uranium-NHC bond in U-22 seems to be unique to the reaction with Me3SiI, as, in contrast to the lanthanides,152 it is inert towards the insertion of carbon dioxide, carbon monoxide or diphenyldiazomethane (Scheme 14).151 Although P. Arnold and co-workers initially suggested the participation of the NHC ligand in the activation of carbon dioxide by U22,151 quantum chemical calculations later revealed that the activation is much more likely to occur at one of the N(SiMe3)2 ligands by forming a carbamate intermediate.153 Nevertheless, the activation product U-25 is unstable and the formulation of the NHC complex U-25 is only suggested based on IR spectroscopic data. In contrast, U-22 reacts with CO cleanly under forcing conditions (80  C for 7 days) to give the enolate complex U-26, which most likely forms via the initial C-H activation of a Me3Si -CH3 group, forming a uranium(IV) metallacycle.150 Insertion of CO into the uranium-alkyl bond and subsequent rearrangement would then form the observed enolate complex U-26. This is in line with CO insertion chemistry involving cyclometallating -SiMe3 ligands observed elsewhere.154,155 Finally, U-22 was found to activate diphenyldiazomethane to form a rare ketimido complex U-27.151 However, the mechanism of the NdN bond cleavage and the fate of the second nitrogen atom remains unclear. Beyond influencing the reactivity of uranium complexes, NHC ligands have also been shown to influence the physical properties of uranium(III) complexes, especially their magnetic properties. In a comparative study, Long’s group studied the influence of nitrogen vs. carbon donor atoms on uranium magnetism, comparing the isostructural tris-[dihydro-bis(methylpyrazolyl)borate] uranium(III) complex (U-28) with its NHC analog, tris[dihydro-bis(methylimidazol-2-ylidene)borate] uranium(III) (U-29).156 Both complexes form a trigonal prismatic coordination environment around the uranium(III) ion. Although the uranium-to-donor atom distances (i.e., N vs. C) are longer in the case of the carbene donors (2.588(4) vs. 2.662(4) A˚ in U-28 and U-29, respectively), the length of the trigonal prism around the metal is shorter for U-29, leading to an axially compressed ligand field (Scheme 15, right). This can be attributed to a more acute bite angle of the NHC ligands compared to the pyrazolyl-based ligand. Axial compression of oblate f-element ions is a key factor in enhancing their magnetic properties, in regards to raising the magnetic blocking temperature and thus the temperature range in which magnetic bistability can be observed.157–164 Thus, under an applied DC field of 750 Oe, the uranium complex U-29 was found to show distinct SMM behavior with a relaxation barrier at 33 cm−1, while the bis-pyrazolylborate complex U-28 showed no magnetic bistability under these conditions. These experiments have furthermore been investigated computationally by Gagliardi, Cramer and co-workers indicating that the consideration of the 5f3 states alone is sufficient to account for all energetically relevant spin-orbit coupling effects. This study might prove useful for the prediction and study of further uranium-based SMM complexes.165

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Scheme 15 Synthesis of the homoleptic bis(pyrazolyl)borate and bis(imidazole-2-ylidene)borate complexes U-28 and U-29 (left). Schematic view of the trigonal prismatic coordination environment (right).

3.06.4

Mesoionic carbenes & carbodicarbenes

The ligating potential of the rising class of mesoionic carbene (MIC) ligands13,22,26,166 has been studied in great depth in late transition-metal chemistry26,167–171 and main group chemistry,172–174 and these ligands are slowly finding applications even in the early transition-metal regime.175,176 However, as we move from classical N-heterocyclic carbenes to mesoionic carbene ligands, the list of thus-far studied actinide complexes shortens: only one mesoionic carbene complex has been reported for the actinide series, namely using uranium(III).177 Nevertheless, this one example (U-30) illustrates the unexpected and unusual coordination chemistry that can be discovered with uranium and certainly warrants more detailed investigations into actinide MIC chemistry in the future. The uranium-MIC complex U-30 was isolated from an unusual reaction of an N-heterocyclic olefin (NHO) with [U{N(SiMe3)2}3], where the olefin functional group in NHO was converted to a methyl group on formation of the MIC (Scheme 16). In contrast, the reaction between NHO and the tris-cyclopentadienyl thorium(III) complex resulted in N-demethylation and oxidative coupling of two NHO molecules, forming a thorium(IV) methyl complex (Th-13).178

Scheme 16 Synthesis of the first and only uranium(III) MIC complex U-30 and the same reaction with thorium that leads to to the alkyl complex Th-13, starting from the N-heterocyclic olefin precursor NHO. The important 1 and 4 positions on the NHO are labeled for clarity.

At first glance, the formation of U-30 appears to occur through a 1,4-CH shift between the NHO olefin and heterocycle groups. However, the different chemistry that takes place with thorium(III) provides an indication to alternative reaction mechanisms. Perhaps a more rational mechanism for the formation of U-30 involves the deprotonation of NHO at the 4-position by a basic N(SiMe3)2 ligand from [U{N(SiMe3)2}3], which would yield a uranium(III) bis-amido carbanion intermediate. Reprotonation of this intermediate at the basic methylene group by HN(SiMe3)2 would restore the charge neutrality on the MIC ligand as well as the uranium amide bond. The precedent for this mechanism has been established by related lanthanide complexes, namely [Ln{N(SiMe3)2}3] (Ln ¼ Y, La, Nd).177 The apparent differences in reactivity between the uranium(III) and thorium(III) examples can therefore be attributed to the choice of supporting ligands rather than the actinides themselves. Specifically, the less basic cyclopentadienyl ligands in the thorium example (Scheme 16) are likely to prevent the above-discussed mechanism from proceeding, thereby resulting in the N-demethylation product Th-13 instead of the deprotonation product, as seen for U-30.178 In U-30, the uranium-carbon bond was found to be 2.576(12)–2.598(11) A˚ (the molecular structure is strongly disordered), which is notably shorter than in the uranium(III) complex U-18 reported by Meyer et al.132 (2.672(5) A˚ , Fig. 6) and is also shorter than any other actinide(III)-NHC bond reported to date (see Table A1). This can primarily be attributed to the higher carbanionic character of the MIC carbene atom compared to a NHC carbene atom, which is generally reflected by the contraction of the M-MIC distances compared to their structurally related M-NHC congeners.22,26,166 An additional explanation for the short uranium-MIC bond distance could be related to the specific molecular orbital interactions that were identified computationally. An unprecedented p-backbonding interaction of a MIC 2p orbital and a uranium orbital consisting of 90% 5f- and 10% 6d-character hints at stronger covalent interactions between MIC ligands and the actinides.

N-Heterocyclic and Mesoionic Carbene Complexes of the Actinides

217

Fig. 6 Comparison between the molecular structures of the NHC and MIC complexes U-18 and U-30. Hydrogen atoms have been omitted for clarity.

This p back-bonding interaction is remarkable as it represents the first evidence that MIC ligands not only act as extremely strong s-donor ligands13,22,26 but also possess p-accepting properties similar to NHCs.132 It is possible that these p-accepting properties may be limited to actinides and 5f orbitals (no such interaction has yet been found with the transition metals). Since mesoionic carbenes offer generally stronger and more inert M-C bonds in transition metal complexes26,167–169 (notably, there are MIC-complexes that can survive aqua regia for several hours179) the use of MICs as innocent supporting ligands for f-elements may offer promising future applications in actinide chemistry in catalysis, small molecule activation, molecular magnetism and nuclear waste remediation. The latter is often highly acidic where the above-mentioned acid tolerance of MIC complexes might be highly fortunate and desirable. Finally, carbodicarbenes (CDCs) represent another ligand class that is closely related to N-heterocyclic carbenes and has only recently been introduced to actinide chemistry. These ligands differentiate themselves by the presence of a divalent carbon(0) donor atom stabilized by strongly donating flanking groups (mostly NHCs). Computational studies suggest CDCs are dibasic in nature, potentially acting as four-electron donors. Thus, CDCs are highly attractive ligands as they can act as both s-donors and p-donors, thereby far exceeding the donor properties of their related NHCs. Seminal work by Bart and co-workers reveals the effects of these outstanding donor ligands towards the uranyl motif.180 A reaction between the free CDC and uranyl chloride or uranyl triflate results in the clean formation of the corresponding uranium(VI) complexes U-31a,b (Scheme 17). The uranium-carbon bond in U31b was measured at 2.541(4) A˚ and is significantly shorter than in the related NHC complexes U-1a,b (Scheme 9).80 However, the uranium-carbon bonds are still drastically longer than those found in corresponding uranium-methandiide complexes (2.383 (3)–2.430(6) A˚ ),83,181 suggesting single bond character between the uranium and the CDC ligand. This was also confirmed by DFT calculations, which further suggest that the CDC ligand only acts as a two-electron donor, not a four-electron donor as calculated for uranium(IV) tetrachloride-based carbon(0) carbodiphosphorane (CDP) complexes.182 Nevertheless, when comparing the asymmetric uranyl stretching frequencies in the IR spectra of the CDC complexes U-31a,b with the related NHC complexes (U-1a,b),80 it was found that those for U-31a,b were shifted by up to 41 cm−1 to lower energies, thus indicating a significant activation of the U]O double bonds by the strongly donating CDC ligands.

Scheme 17 Synthesis of the first carbodicarbene uranyl adducts.

218

3.06.5

N-Heterocyclic and Mesoionic Carbene Complexes of the Actinides

Conclusion & outlook

Even though the first actinide-NHC complex was isolated nearly two decades ago, actinide-carbene chemistry is still in its infancy. The vast majority of actinide-carbene synthesis and reactivity studies have been performed on uranium, with the first thorium-carbene examples only being reported in 2014. Given the general surge of advances in actinide chemistry in the past decade, it is surprising that the organometallic chemistry of NHC ligands, which is ubiquitous and irreplaceable for the rest of the periodic table, remains underdeveloped for the actinides. Nevertheless, systematic studies, such as those performed by J. Arnold and co-workers, show that the chemistry, reactivity and electronic structure of NHC-supported actinide complexes can differ in impressive ways from previous, structurally-related examples and that the use of NHC ligands holds great potential for the chemistry of the actinide series. Furthermore, Meyer and Liddle have both demonstrated that the bonding situation in low-valent uranium complexes (+III) can involve unexpected p back-bonding participation for both NHCs and MICs. The case of the latter is the first and sole report of such an interaction with any metal to date. The range of subtle and unique bonding interactions that are possible between carbene ligands and the actinides makes their application in nuclear waste management a possibility, as shown by Ephritikhine and Costa, potentially allowing for the separation of lanthanide and early actinide elements in ionic liquids. Furthermore, P. Arnold and co-workers have beautifully demonstrated how f-element-NHC interactions can be used for small molecule activation and group-transfer reactivity, and have also brought to light plausible strategies towards the reduction and functionalization of CO2 and CO using this unique class of complexes. Given the limited variety of ligands that have been used in actinide chemistry so far, juxtaposed with the wide variety of different carbene ligands that are available (compare Fig. 1), we believe that the future of this topic will include significant developments in actinide organometallic chemistry. In this context, we are particularly eager to witness the development of actinide chemistry involving redox-active cyclic alkyl amino carbenes (CAACs) and further examples of mesoionic carbenes (MICs). We believe that these ligand classes will have a major impact on future reactivity and applications of the actinide elements. While this article only includes examples of thorium and uranium NHC and MIC complexes, the number of laboratories that can study highly radioactive transuranic elements is growing. No NHC or MIC complex of a transuranic element is yet known. However, these ligands offer promise in advancing the chemistry of the last bastion of the periodic table, and could potentially help to unravel the mysteries of bonding, electronic structure, and reactivity of these elements.

3.06.6 Table A1

Appendix Overview of the actinide-NHC/MIC distances and 13C NMR shifts discussed in the text, sorted by element, oxidation state and coordination number.

Complex

Ox.

CN

Carbenea

˚) M-C distanceb (A

13

C NMR (ppm)c

References

Th-1 Th-2 Th-3a Th-3b Th-4a Th-4b Th-5a Th-5b Th-6a Th-6b Th-6c Th-6e Th-7a Th-8a Th-8b Th-8c Th-9Ph2 Th-10 Th-11 Th-12 U-1a U-1b U-2a U-2b U-3a U-3b U-5

+IV +IV +IV +IV +IV +IV +IV +IV +IV +IV +IV +IV +IV +IV +IV +IV +IV +IV +IV +IV +VI +VI +VI +VI +VI +VI +VI

8 6 6 6 7 7 7 7 7 7 7 7 7 6 6 6 6 6 6 6 6 6 6 6 6 6 6

Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazolin-2-ylidene Imidazolin-2-ylidene Imidazol-2-ylidene

2.852(6)–2.884(5) 2.623(6)–2.634(6) 2.656(4)–2.755(3) 2.646(4)–2.843(4) 2.737(3)–2.883(3) 2.69(1)–2.89(1) 2.721(9)–2.862(10) – 2.690(5)–2.781(5) 2.69(1)–2.74(1) 2.664(5)–2.754(4) 2.70(1)–2.77(1) 2.70(1)–2.99(1) 2.651(2)–2.746(3) 2.637(4)–2.649(4) 2.663(3)–2.716(3) 2.584(3)–2.752(3) 2.613(4)–2.718(4) 2.691(8)–2.780(9) 2.637(4)–2.675(4) 2.626(7) 2.609(4) 2.640(5) 2.633(7) 2.580(4) 2.612(2) 2.498(6)–2.499(7)

210.3 215.8–208.2 221.9–218.5 215.7–214.0 – – 216.3–214.5 215.5–212.6 211.3–207.2 211.8–207.6 211.5–207.7 211.8–207.6 217.9–211.5 211.0–208.0 214.4–210.0 213.6–205.7 217.8–208.6 217.6–208.2 215.5–207.1 215.1–214.8 – – 262.8 – 281.6 283.6 329.4

51 56 56 63 56 56 63 70 70 70 70 56 74,78 78 78 74 74 74 74 80 80 99 99 54 54 104

N-Heterocyclic and Mesoionic Carbene Complexes of the Actinides

Table A1

219

(Continued)

Complex

Ox.

CN

Carbene

˚) M-C distance (A

U-7 U-8 U-10 U-11 U-17 U-18 U-19 U-20 U-21 U-22 U-24 U-26 U-27 U-29 U-30 U-31b

+IV +IV +IV +IV +III +III +III +III +IV +III +IV +IV +IV +III +III +VI

6 7 7 7 7 4 4 4 4 4 6 5 5 6 4 6

Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazolin-2-ylidene Imidazolin-2-ylidene Imidazolin-2-ylidene Imidazolin-2-ylidene Imidazol-2-ylidene Imidazol-4-ylidene Carbodicarbene

2.675(7)–2.687(7) 2.573(5)–2.587(5) 2.696(3)–2.799(3) 2.675(5)–2.793(5) 2.789(14) 2.672(5) 2.687(5) 2.768(5) 2.636(9) 2.693(4) 2.647(3) 2.630(6) 2.719(5) 2.661(3)–2.663(3) 2.576(12)–2.598(11)d 2.541(4)

13

C NMR (ppm)

– – – – – – – – – – – – – – – –

References 109 110 52 52 132 132 130 130 144 150 150 151 151 156 177 180

a

nNHC, normal carbene; MIC, mesoionic carbene. For multiple M-NHC distances the range between the shortest and the longest NHC bond is given. c For multiple NHC donors, the range between the highest and the lowest observed shifts is given. d The MIC is heavily disordered over three positions, which prohibits the determination of an accurate single value here. b

Table A2

Asymmetric uranyl stretching frequencies.

Complex

CN

Carbene

v in cm-1 [O]U]O]asym

U-1a U-1b U-2a U-2b U-3a U-3b U-5 U-6Fe U-6Co

6 6 6 6 6 6 6 6 7

Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazolin-2-ylidene Imidazolin-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene Imidazol-2-ylidene

938 942 929 933 851 853 886 911 911

Table A3

References 80 80 99 99 54 54 104 104 104

Square bracket notation of the complexes.

Compound

Scheme/Figure no.

Square bracket notation

L1 L2 L3 L4 L5 L6 L7 L8 Th-1 Th-2 Th-3a Th-3b Th-4a Th-4b Th-4c Th-5a Th-5b

Fig. 3 Fig. 3 Fig. 3 Fig. 3 Fig. 3 Fig. 3 Fig. 3 Fig. 3 Scheme 1 Schemes 3, 6 Schemes 3, 4, 5, 8 Scheme 3 Scheme 4 Scheme 4 Scheme 4 Scheme 4 Scheme 4

I(iPr)(CH2C(Me)2O) (SIMes)(CH2C(Me)2O) S IPr(CH2C(Me)2O) I(tBu)(CH2CH2N(tBu)) IMes(CH2CH2N(tBu)) H2B(IMe)2 H2B(IMes)2 2,6-(IPr)2-Py [Th{I(iPr)(CH2C(Me)2O-k2O,C)}4] [Th{H2B(IMes)2-k2C,C}2I2] [Th{H2B(IMes)2-k2C,C}2(2,20 -bpy)] [Th{H2B(IMes)2-k2C,C}2(6,60 -Me2-2,20 -bpy)] [Th{H2B(IMes)2-k2,C,C}2{6-H-6-C(Ph)2O-2,20 -bpy-k3,N,N,O}] [Th{H2B(IMes)2-k2,C,C}2{6-H-6-C(9-anthracenyl)(H)O-2,20 -bpy-k3,N,N,O}] [Th{H2B(IMes)2-k2,C,C}2{6-H-6-C(6-bromophenyl)(H)O-2,20 -bpy-k3,N,N,O}] [Th{H2B(IMes)2-k2,C,C}2{6-H-6-C(Me)(H)-CH2-O-2,20 -bpy-k3,N,N,O}] [Th{H2B(IMes)2-k2,C,C}2{6-H-6-(cyclohexyl-2-O)-2,20 -bpy-k3,N,N,O}] (Continued )

220

N-Heterocyclic and Mesoionic Carbene Complexes of the Actinides

Table A3

(Continued)

Compound

Scheme/Figure no.

Square bracket notation

Th-6a Th-7a Th-7b Th-7c Th-7d Th-7e Th-8a Th-8b Th-8c Th-8d Th-9Mes Th-9Ph2 Th-10 Th-11 Th-12 Th-13 U1a U1b U-2a U-2b U-3a U-3b U-4 U-5 U-6Fe U-6Co U-7 U-8 U-9 U-10 U-11 U-12 U-13 U-14 U-15 U-16 U-17 U-18 U-19 Ce-19 U-20 Ce-20 U-21 U-22 U-23 U-24 U-25 U-26 U-27 U-28 U-29 U-30 U-31a U-31b

Scheme 4 Scheme 4 Scheme 4 Scheme 4 Scheme 4 Scheme 5 Schemes 6–8 Scheme 8 Scheme 8 Scheme 6 Schemes 6, 7 Scheme 7 Scheme 7 Scheme 7 Scheme 7 Scheme 16 Scheme 9 Scheme 9 Scheme 10 Scheme 10 Scheme 10 Scheme 10 Scheme 11 Scheme 11 Scheme 11 Scheme 11 Fig. 5 Fig. 5 Scheme 12 Scheme 12 Scheme 12 Scheme 12 Scheme 12 Scheme 12 Scheme 12 Scheme 12 Scheme 13 Scheme 13 Scheme 13 Scheme 13 Scheme 13 Scheme 13 Scheme 13 Scheme 14 Scheme 14 Scheme 14 Scheme 14 Scheme 14 Scheme 14 Scheme 15 Scheme 15 Scheme 16 Scheme 17 Scheme 17

[Th{H2B(IMes)2-k2,C,C}2{2,20 -2H-bpy}(para-tolylimide)] [Th{H2B(IMes)2-k2,C,C}2{4-H-4-tBu-2,20 -bpy}(NC-k1,N)] [Th{H2B(IMes)2-k2,C,C}2{4-H-4-nBu-2,20 -bpy}(NC-k1,N)] [Th{H2B(IMes)2-k2,C,C}2{4-H-4-Bn-2,20 -bpy}(NC-k1,N)] [Th{H2B(IMes)2-k2,C,C}2{4-H-4-Bn-2,20 -bpy}(NC-k1,N)] [Th{H2B(IMes)2-k2,C,C}2{4-H-4-(but-3-ene)-2,20 -bpy}(NC-k1,N)] [Th{H2B(IMes)2-k2,C,C}2(PHMes)2] [Th{H2B(IMes)2-k2,C,C}2(PHPh)2] [Th{H2B(IMes)2-k2,C,C}2(PPh2)2] [Th{H2B(IMes)2-k2,C,C}{H2B(IMes)(d1-IMes)-k2,C,C}(PHMes)(PDMes)] [Th{H2B(IMes)2-k2,C,C}{H2B(IMes)(I-2,4-Me2-6-CH2-benzene)-k3,C,C,C}(PHMes)] [Th{H2B(IMes)2-k2,C,C}{H2B(IMes)(I-2,4-Me2-6-CH2-benzene)-k3,C,C,C}(PPH2)] [Th{H2B(IMes)2-k2,C,C}{H2B(IMes)(I-2,4-Me2-6-CH2-benzene)-k3,C,C,C}{P(Mes)(1,2-Z-diphenylethylene)}] [Th{H2B(IMes)2-k2,C,C}2{Z4-P(Mes)(1-diphenyl-2H-ethane-k4,P,C,C,C)}] [Th{H2B(IMes)2-k2,C,C}2(CCPh)2] [Th(Cp{)3Me] [UO2Cl2(IMes)2] [UO2Cl2(IMesCl2)2] [UO2{I(tBu)(CH2CH2NtBu)-k2,C,N}2] [UO2{I(Mes)(CH2CH2NtBu)-k2,C,N}2] [UO2{SIMesCH2C(Me)2O-k2,C,O}2] [UO2{SIPrCH2C(Me)2O-k2,C,O}2]] [{UO2(BDI)-m2-Cl}2] [(BDI)UO2{m-MeIm-1kC,2kN}2Li(MeIm-k1,N)] [(BDI)UO2{m-MeIm-1kN,2kC}2FeCl(MeIm-k1,N)] [(BDI)UO2{m-MeIm-1kN,2kC}2CoCl(MeIm-k1,N)] [UCl4(IPr)2] [UCl4(2,6-IPr-Py-k3,C,N,C)] [UI{I(iPr)(CH2C(Me)2O)-k2,C,O}3] [U{I(iPr)(CH2C(Me)2O)-k2,C,O}3{I(iPr)(CH2C(Me)2O)-k1,O}] [{I(iPr)(CH2C(Me)2O)-k2,C,O}2U{m-I(iPr)(CH2C(Me)2O)-1kO,2kC}2Mo(CO)4] [{I(iPr)(CH2C(Me)2O)-k2,C,O}3U{m-I(iPr)(CH2C(Me)2O)-1kO,2kC}2W(CO)5] [{I(iPr)(CH2C(Me)2O)-k2,C,O}3U{m-I(iPr)(CH2C(Me)2O)-1kO,2kC-BH3}] [{I(iPr)(CH2C(Me)2O)-k2,C,O}2U{m-I(iPr)(CH2C(Me)2O)-1kO,2kC-BH3}2] [{I(iPr)(CH2C(Me)2O)-k2,C,O}U{m-I(iPr)(CH2C(Me)2O)-1kO,2kC-BH3}3] [U{m-I(iPr)(CH2C(Me)2O)-1kO,2kC-BH3}4] [U{(Ad,tBuArO)3TACN-k6,N,N,N,O,O,O}(IMe4)] [U{N(SiMe3)2}3(IMe4)] [UI(Cp )2(IMe4)] [CeI(Cp )2(IMe4)] [U(Cp0 )3(IMe4)] [Ce(Cp0 )3(IMe4)] [UO(Cp )2(IMe4)] [U{N(SiMe3)2}2{SIPr(CH2C(Me)2O)-k2,C,O}] [UI{N(SiMe3)2}2{SIPr(CH2C(Me)2O)-2-SiMe3-k,O}] [UI2{SIPr(CH2C(Me)2O)-k2,C,O}2] [U{SIPr(CH2C(Me)2O)-k2,C,O}{N(SiMe3)2}{OSiMe3}{OCNSiMe3}] [U{SIPr(CH2C(Me)2O)-k2,C,O}{N(SiMe3)2}{N(SiMe3)Si(Me)2C(CH2)O-k2,N,O}] [U{N(SiMe3)2}2{SIPr(CH2C(Me)2O)-k2,C,O}(NCPh2)] [U{H2B(MePz)2-k2,N,N}3] [U{H2B(IMe)2-k2,N,N}3] [U{N(SiMe3)2}3(1,2,3-Me3-Im-k,C4)] [UO2Cl2(CDC)2] [UO2(OTf )2(CDC)2]

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221

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Quantum Chemical Characterization of Single Molecule Magnets Based on Uranium. J. Phys. Chem. A 2017, 121 (8), 1726–1733. https://doi.org/10.1021/acs.jpca.6b10933. 166. Vivancos, Á.; Segarra, C.; Albrecht, M. Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications. Chem. Rev. 2018, 118 (19), 9493–9586. https://doi.org/10.1021/acs.chemrev.8b00148. 167. Hohloch, S.; Su, C.-Y.; Sarkar, B. Copper(I) Complexes of Normal and Abnormal Carbenes and Their Use as Catalysts for the Huisgen [3 +2] Cycloaddition Between Azides and Alkynes. Eur. J. Inorg. Chem. 2011, 2011 (20), 3067–3075. https://doi.org/10.1002/ejic.201100363. 168. Hohloch, S.; Suntrup, L.; Sarkar, B. Arene–Ruthenium(II) and −Iridium(III) Complexes with “Click”-Based Pyridyl-Triazoles, Bis-Triazoles, and Chelating Abnormal Carbenes: Applications in Catalytic Transfer Hydrogenation of Nitrobenzene. Organometallics 2013, 32 (24), 7376–7385. https://doi.org/10.1021/om4009185. 169. Hohloch, S.; Suntrup, L.; Sarkar, B. Exploring Potential Cooperative Effects in Dicopper(I)-di-Mesoionic Carbene Complexes: Applications in Click Catalysis. Inorg. Chem. Front. 2016, 3 (1), 67–77. https://doi.org/10.1039/C5QI00163C. 170. Suntrup, L.; Hohloch, S.; Sarkar, B. Expanding the Scope of Chelating Triazolylidenes: Mesoionic Carbenes from the 1,5-“Click”-Regioisomer and Catalytic Synthesis of Secondary Amines from Nitroarenes. Chem. A Eur. J. 2016, 22 (50), 18009–18018. https://doi.org/10.1002/chem.201603901. 171. Rottschäfer, D.; Schürmann, C. J.; Lamm, J.-H.; Paesch, A. N.; Neumann, B.; Ghadwal, R. S. Abnormal-NHC Palladium(II) Complexes: Rational Synthesis, Structural Elucidation, and Catalytic Activity. Organometallics 2016, 35 (19), 3421–3429. https://doi.org/10.1021/acs.organomet.6b00662. 172. Doddi, A.; Peters, M.; Tamm, M. N-Heterocyclic Carbene Adducts of Main Group Elements and their Use as Ligands in Transition Metal Chemistry. Chem. Rev. 2019, 119 (12), 6994–7112. https://doi.org/10.1021/acs.chemrev.8b00791. 173. Nesterov, V.; Reiter, D.; Bag, P.; Frisch, P.; Holzner, R.; Porzelt, A.; Inoue, S. NHCs in Main Group Chemistry. Chem. Rev. 2018, 118 (19), 9678–9842. https://doi.org/10.1021/ acs.chemrev.8b00079. 174. Sharma, M. K.; Ebeler, F.; Glodde, T.; Neumann, B.; Stammler, H.-G.; Ghadwal, R. S. Isolation of a Ge(I) Diradicaloid and Dihydrogen Splitting. J. Am. Chem. Soc. 2021, 143 (1), 121–125. https://doi.org/10.1021/jacs.0c11828. 175. Bens, T.; Boden, P.; Di Martino-Fumo, P.; Beerhues, J.; Albold, U.; Sobottka, S.; Neuman, N. I.; Gerhards, M.; Sarkar, B. Chromium(0) and Molybdenum(0) Complexes with a Pyridyl-Mesoionic Carbene Ligand: Structural, (Spectro)Electrochemical, Photochemical, and Theoretical Investigations. Inorg. Chem. 2020, 59 (20), 15504–15513. https://doi. org/10.1021/acs.inorgchem.0c02537. 176. Baltrun, M.; Watt, F. A.; Schoch, R.; Wölper, C.; Neuba, A. G.; Hohloch, S. A New Bis-Phenolate Mesoionic Carbene Ligand for Early Transition Metal Chemistry. Dalton Trans. 2019, 48 (39), 14611–14625. https://doi.org/10.1039/c9dt03099a. 177. Seed, J. A.; Gregson, M.; Tuna, F.; Chilton, N. F.; Wooles, A. J.; McInnes, E. J. L.; Liddle, S. T. Rare-Earth- and Uranium-Mesoionic Carbenes: A New Class of f-Block Carbene Complex Derived from an N-Heterocyclic Olefin. Angew. Chem. Int. Ed. 2017, 56 (38), 11534–11538. https://doi.org/10.1002/anie.201706546.

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178. Liu, J.; Seed, J. A.; Formanuik, A.; Ortu, F.; Wooles, A. J.; Mills, D. P.; Liddle, S. T. Thorium(IV) Alkyl Synthesis From a Thorium(III) Cyclopentadienyl Complex and an N-Heterocyclic Olefin. J. Organomet. Chem. 2018, 857, 75–79. https://doi.org/10.1016/j.jorganchem.2017.08.015. 179. Vanicek, S.; Beerhues, J.; Bens, T.; Levchenko, V.; Wurst, K.; Bildstein, B.; Tilset, M.; Sarkar, B. Oxidative Access Via Aqua Regia to an Electrophilic, Mesoionic Dicobaltoceniumyltriazolylidene Gold(III) Catalyst. Organometallics 2019, 38 (22), 4383–4386. https://doi.org/10.1021/acs.organomet.9b00616. 180. Maity, A. K.; Ward, R. J.; Rupasinghe, D. M. R. Y. P.; Zeller, M.; Walensky, J. R.; Bart, S. C. Organometallic Uranyl Complexes Featuring a Carbodicarbene Ligand. Organometallics 2020, 39 (6), 783–787. https://doi.org/10.1021/acs.organomet.9b00860. 181. Tourneux, J.-C.; Berthet, J.-C.; Cantat, T.; Thuéry, P.; Mézailles, N.; Ephritikhine, M. Exploring the Uranyl Organometallic Chemistry: From Single to Double Uranium-Carbon Bonds. J. Am. Chem. Soc. 2011, 133 (16), 6162–6165. https://doi.org/10.1021/ja201276h. 182. Su, W.; Pan, S.; Sun, X.; Wang, S.; Zhao, L.; Frenking, G.; Zhu, C. Double Dative Bond Between Divalent Carbon(0) and Uranium. Nat. Commun. 2018, 9 (1), 4997. https://doi. org/10.1038/s41467-018-07377-6.

3.07

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

Adrien T Normand, Institut de Chimie Moléculaire de l’Université de Bourgogne (ICMUB), Université de Bourgogne, Dijon, France © 2022 Elsevier Ltd. All rights reserved.

3.07.1 Introduction 3.07.2 Bonding between NHCs and group 4 metals 3.07.2.1 Orbital interactions 3.07.2.2 The EDA approach 3.07.2.2.1 Principle 3.07.2.2.2 Results 3.07.2.3 Comparison with other neutral donors 3.07.3 Synthesis of group 4 metal NHC complexes 3.07.4 Complexes bearing monodentate ligands 3.07.4.1 M(IV) complexes 3.07.4.2 Lower oxidation states 3.07.5 Complexes bearing bidentate NHCs 3.07.5.1 O-functionalized ligands 3.07.5.2 N-functionalized ligands 3.07.5.3 C-functionalized and bis-NHC ligands 3.07.6 Complexes bearing tridentate NHCs 3.07.6.1 O,O functionalized ligands 3.07.6.1.1 Flexible bisaryloxy-NHC ligands 3.07.6.1.2 Rigid bisaryloxy-NHC ligands 3.07.6.2 N,O-functionalized ligands 3.07.6.3 N,N-functionalized ligands 3.07.6.4 N-C-functionalized ligands 3.07.6.5 CCC pincer NHC ligands 3.07.7 Miscellaneous 3.07.8 Conclusion Acknowledgments References Relevant Websites

228 232 232 233 233 233 234 235 236 236 239 242 242 245 246 248 248 248 251 257 258 260 260 262 263 265 265 267

Nomenclature F Ad BHT Bn Bu CAAC COD Cp DCC Dep DFT Dipp Dipp DMAD ESI Et Hex i Pr KHMDS LDA MALDI MAO

Quantum yield Adamantyl 2,6-Ditertbutyl-6-methylphenoxy Benzyl Butyl Cyclic(aminoalkyl)carbene 1,5-Cyclooctadiene Cyclopentadienide ligand Dicyclohexylcarbodiimide 2,6-Diethylphenyl Density Functional Theory 2,6-(2,6-Diisopropylphenyl)phenyl 2,6-Diisopropylphenyl Dimethyl acetylenedicarboxylate Electron spray ionization Ethyl Hexyl Isopropyl Potassium hexamethyldisilazide Lithium diisopropylamide Matrix Assisted Laser Desorption Ionization Methylaluminoxane

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00003-2

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N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

Me Mes NHC OMs PDI PLA Py t Bu TOF TON Undec Xyl

3.07.1

Methyl Mesityl N-heterocyclic carbene Methanesulfonato Polydispersity index Polylactic acid Pyridine Tertbutyl Turnover frequency Turnover number Undecyl 2,6-Dimethylphenyl

Introduction

Over the course of three decades, N-heterocyclic carbenes (NHCs) have had a profound impact on organometallic and main group chemistry.1–4 By N-heterocyclic carbenes, we mean the constantly evolving class of cyclic dicoordinate carbon compounds that contain at least one nitrogen atom within the ring. They are a tremendously useful class of ligands with a broad range of applications, especially with late transition metals.5,6 Several tens of thousands of NHC complexes of group 8–11 metals have been reported, and hundreds of them are commercially available. In this article, we explore the comparatively more contained body of literature on group 4 metal NHC complexes and their applications (mostly to catalysis). As of April 2020, there are only about 100 references dealing with Ti-, Zr- and Hf-NHC complexes, the overwhelming majority of which are based on a rather restricted

Fig. 1 Types of NHCs used with group 4 metals.

Fig. 2 Monodentate NHC ligands used with group 4 metals.

Fig. 3 Bidentate NHC ligands used with group 4 metals.

Fig. 4 Tridentate NHC ligands used with group 4 metals.

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range of ligands (Figs. 1–4). Still, several reviews on this topic have appeared during the last decade, to which the reader can refer for further reading.7–10 Why are NHC complexes comparatively so rare with group 4 metals? According to some authors in the field, this is due to the ease of ligand dissociation from the metal, presumably caused by soft/hard mismatch.7,9–16 However, let us consider compound

Scheme 1 Counterintuitive (in)stability of NHC complexes in the presence of hard and soft Lewis acids.

[(F3B)(L1)] (1, R ¼ R0 ¼ Mes) (Scheme 1), obtained from the reaction of L1 (R ¼ R0 ¼ Mes) with BF3—arguably a textbook example of a hard Lewis acid. Not only is this adduct stable, but it can be recrystallised from hot acetonitrile.17 An early theoretical study by Frenking focussed on the basicity of diaminocarbenes, and compared the dissociation energies (D0) of NHC adducts with a wide range of Lewis acids. The carbene donor was modelled by the computationally less demanding acyclic diaminocarbene:C(NH2)2, and other donors were included in the study for comparison (NH3 and CO). It was found that the donor-acceptor bond was always stronger with the carbene. The D0 of the Cl4Ti C(NH2)2 and F3B C(NH2)2 adducts were 163 NMe3) has an experimental D0 and 167 kJ mol−1. For comparison, the strongest donor-acceptor bond known at the time (Cl3Al of 199 kJ mol−1.18 Therefore, the stability of compound 1 is in line with the early theoretical prediction that hard Lewis acids form strong donor-acceptor bonds with NHCs. Now, let us turn to complexes [Pd(L1)2] (2, R ¼ R0 Dipp) and [Rh(L33)(COD)][BF4] (3) (Scheme 1), which undergo ligand exchange in the presence of phosphines.19,20 These examples clearly demonstrate that (bulky) NHCs may dissociate easily from soft late transition metals.21,22 Conversely, NHC coordination to group 4 metals can be so thermodynamically favorable as to result in the dissociation of

Scheme 2 Cp dissociation upon NHC coordination to Hf.

strongly binding donors, e.g., the cyclopentadienide ligand (Cp). Such an extreme example is provided by the reaction depicted in Scheme 2.23 Arguably, complex [HfCl(Cp)2(L34)] (6) is an anionic Hf(II) species, therefore not the archetypical example of a group 4 metal complex; nevertheless, its reactivity (to give [HfCl(Cp)(L34)(L2)] (7, R ¼ Me)) should be a cautionary tale against the temptation of making sweeping statements about the weakness of the MdNHC bond in the case of group 4 metals. Thus, whilst the assertion that monodentate NHCs bind weakly to group 4 metals does contain some truth, it should be contextualized and examined critically. It is worth remembering that a few myths surrounding NHCs have been debunked over the years: for instance, that they are pure s-donors,24 or that they always form stable complexes.25 It is this author0 s opinion that the

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supposed weakness of the MdNHC bond in group 4 metal complexes is another one of these myths. This can be traced back to an oft-cited study by Arnold, who indeed reported facile NHC dissociation in rare-earth metal complexes (through competition experiments with O- and N-donors).26 However, the main issue with group 4 metal NHC complexes seems to be the electrophilicity of the metal-bound carbon atom (see Section 3.07.6).27–29 On the other hand, transmetallation of NHCs from group 4 metal complexes to late transition metals suggests a thermodynamic preference for NHCs to bind to the latter in some cases.30 In fact, Zr complexes are useful NHC transfer agents for late transition metals.31–37 Additionally, a number of studies reporting failed coordination attempts (or decomposition events) suggest some kind of mismatch between NHCs and group 4 metals in these particular instances.38–41 But as a general rule, NHCs will form relatively strong bonds compared to other neutral donors: the stronger the donor strength, the stronger the bond—even with early transition metals.42,43 As noted elsewhere,27,44–46 the notion that NHCs easily dissociate from group 4 metals is somewhat of an educated guess—but a

Fig. 5 Selected early examples of group 4 metal complexes (1994–2007).

momentous one: it has shaped the choice of ligands which quickly evolved from simple monodentate NHCs,47–50 to more sophisticated bidentate and tridentate ligands functionalized with additional O-, N- and C-donors (Fig. 5).11,31,51–53 At this point, it is worth commenting briefly on some important early examples of group 4 metal NHC complexes (Fig. 5). Herrmann first reported complexes [MCl4(L1)] (8M, R ¼ Me) in 1994, but these were only characterized by NMR spectroscopy and elemental analysis. Erker later reported structurally characterized examples of 8Zr (R ¼ Me, R’ ¼ 2-butyl, CH2Mes; R ¼ R0 ¼ iPr) and 8Hf (R ¼ R0 ¼ iPr) and found an octahedral geometry with NHCs trans to each other. Erker also studied their behavior as ethylene polymerization catalysts, and complexes 8Zr displayed moderate activity (up to 75 kg PE mol-Zr−1 h−1 bar−1 at 25  C, see the Appendix). Another interesting complex is [(TiCl3(L2))2(m-O)] (9, R ¼ iPr), which was the first crystallographically characterized example of a group 4 metal NHC complex (Fig. 5) (NB: Although they were never reported, the structures of [Cp2Ti(CO)(IMe4)] and [TiCl4(IMes)2] have been displayed on Prof. A. J. Arduengo’s personal webpage for >18 years, see Ref. 49 and list of Relevant Websites.). This complex was obtained through the intentional hydrolysis of [(TiCl4(L2)] (10, R ¼ iPr): the striking fact that the TidNHC bond

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survived these reaction conditions nicely illustrates our contention that monodentate NHCs are suitable ligands for group 4 metals (NB: Complex 10 was recently characterized by single crystal X-ray diffraction analysis, see Table 1 and Refs. 30 and 48.). In 2002, Erker reported the first examples of cationic Ti- and Zr-NHC complexes [M(Cp)2Me(L1)][X] (11M, R ¼ R0 ¼ iPr; X- ¼ BPh-4, MeB(C6F5)-3); the “in-plane” orientation of the NHC with respect of the Cp2Ti fragment suggested the absence of p interactions, but this view was later refuted by Jacobsen (see Section 3.07.2.2.2). After these initial studies, a number of research groups started reporting examples of ligands tailored specifically to bind strongly to group 4 metals. These examples served as blueprints for all the future developments in the chemistry of group 4 metal NHC complexes: they are presented here for their historical significance, and their properties will be discussed further in Sections 3.07.5 and 3.07.6. In 2003, Kawaguchi reported complex [TiCl2(L19)(THF)] (12Ti, R ¼ tBu X ¼ Cl) featuring a bisaryloxy-NHC ligand. Fryzuk reported the use of the bisamido-functionalized NHC ligand L27 to stabilize complex [ZrCl2(L27)(Py)] (13, R ¼ Tol; Py ¼ Pyridine). Gibson and Hollis developed CNC and CCC pincer-type NHC ligands and reported complexes [TiCl3(L29)] (14) and [ZrI(NMe2)2(L30)] (15, R ¼ Bu), respectively. Finally, Arnold described complex [Ti(OiPr)3(L10)] (16), in which the alkoxytethered NHC L10 binds strongly to Ti. To the best of our knowledge, the present review comprehensively references the field of group 4 metal complexes that are directly bound to NHCs, i.e., excluding NHC complexes of other metals that bind group 4 metals via a tether. In order to mirror the developments in the field, we will first examine theoretical studies on the nature and strength of the group 4 MdNHC bond (Section 3.07.2), followed by synthetic methods (Section 3.07.3) and complexes of mono-, bi- and tridentate ligands (Sections 3.07.4–3.07.6). Tables summarizing the salient features of each compound (e.g., M-CNHC distances and 13C{1H} NMR spectroscopy carbene signals) are given at the end of the sub-sections. Finally, we will highlight some more exotic examples in a dedicated section at the end of this article. Where relevant, catalytic and other applications will be discussed within the sections; a table summarizing ethylene polymerization studies is provided in the Appendix A.2. Compounds claimed in patents are included where experimental details have been provided.

3.07.2

Bonding between NHCs and group 4 metals

3.07.2.1

Orbital interactions

It is now well-established that NHCs can bind transition metals via three main orbital interactions (Fig. 6, (i): s-donation (usually via the HOMO), p-donation (via an occupied orbital resulting from the mixing of the lone pairs on nitrogens and the empty p orbital on carbon), and p-backdonation (usually via the LUMO).24,67 However, in order to interact with the metal, the NHC must reach the latter’s valence shell (d orbitals). This can be problematic with Ti, because it is a first row metal: as a consequence of the absence of nodes in 3d orbitals, they are little more extended than the core 2s,p orbitals, which therefore undergo strong Pauli repulsion with the ligand’s filled orbitals (Fig. 6, (ii). Second and third row metals (e.g., Zr, Hf ) do not have that problem because they interact with ligands through more extended 4d and 5d orbitals, respectively; therefore, they form stronger metal-ligand bonds.54,68 However, it should be noted that group 4 metals often have

Fig. 6 Orbital interactions relevant to MdNHC bonding. Reproduced from Horrer, G.; Krahfuß, M.J.; Lubitz, K.; Krummenacher, I.; Braunschweig, H.; Radius, U. Eur. J. Inorg. Chem. 2020, 2020, 281–291. (licensed from CC BY 4.0).

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empty, and hence contracted d orbitals (+IV oxidation state), which diminishes the overlap with ligand orbitals for all metals, not just Ti. Another important feature (in the particular case of NHC complexes with d0 metal chlorides) is the bending of chloride ligands that are cis to the NHC towards the latter, the consequence being a Clcis ⋯ CNHC distance that is smaller than the sum of the van der Waals radii of these atoms. This deformation is visible in the X-ray crystal structures of many complexes of group 4–5 metals.62,69,70 A proposed explanation for this phenomenon is through-space charge transfer from the chloride to the NHC (Fig. 6, (iii).62,69 However, a detailed charge displacement (CD) functional analysis conducted on [NbCl5(L)] systems (L ¼ NHC, CO, isocyanide, cyanide) more recently concluded on the absence of direct Cl ! NHC charge transfer.70 In that case, the observed Cl ⋯ NHC contact might simply originate from steric repulsion between the negatively charged chloride ligands (as observed with bromide and alkoxide ligands),71 or to a second-order Jahn-Teller distortion in the case of octahedral complexes.49,54 These aspects of group 4 metals bonding with NHCs have recently been summarized by Radius.54 Fig. 6 is reproduced from that work. Finally, what should be emphasized is that orbital interactions are not the only factor governing metal-ligand bonding: electrostatic interactions are also a key contributor to the interaction energy; in the case of group 4 metals and NHCs, the latter of these interactions can become the main one (see Section 3.07.2.2.2).24,67 Thus, fairly strong bonds can be formed despite the above comments above about diminished orbital overlap.

3.07.2.2 3.07.2.2.1

The EDA approach Principle

Several authors have used the Energy Decomposition Analysis (EDA) method in order to investigate MdNHC bonding.67,72–74 Although partitioning schemes vary, and different nomenclatures may be used, an EDA scheme considers the different contributions to the interaction energy (DEint) between two fragments A and B possessing the same geometry as in the molecule A–B.75 Thus: DEint ¼ Delstat + DEPauli + DEorb where Delstat corresponds to the electrostatic interaction, DEPauli corresponds to the Pauli repulsion, and DEorb corresponds to the orbital interaction (DEorb). Depending on the symmetry properties of the studied molecules, the latter term may be further decomposed into s and p contributions (donation and backdonation). Additionally, in order to obtain the bond dissociation energy (BDE, or De), the fragments must relax electronically and geometrically, which requires a preparation energy (DEprep), and hence:
 De ¼ − DEint + DEprep The theoretical dissociation energy D0 may be obtained by subtracting the zero-point vibrational energy (Evib(0)),76 which increases with molecular size: D0 ¼ De − Evib ð0Þ

3.07.2.2.2

Results

Jacobsen compared d0 complexes [TiCl4(L)] (10M, L ¼ L1, L5, L6) and [M(Cp)2(L1)]+ (11M, R ¼ R0 ¼ H) with a range of transition metal NHC complexes with increasing d electron count (Fig. 7).24,67 The p contribution to DEorb averaged 10% (20% for d10 metals), about two-thirds of which came from backdonation from the metal.24 With anionic d0 complexes [MCl5(L1)]- (17M,

Fig. 7 Selected examples of group 4 metal NHC complexes investigated computationally using EDA schemes.

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R ¼ R0 ¼ H), Tuononen obtained very similar results.73 Frenking and Frison/Huynh found a slightly enhanced p contribution and slightly less backdonation with complexes 10 (which span a very broad range of NHCs).43,72 Therefore, although NHCs act primarily as s-donors towards group 4 metals, p interactions are not negligible. In particular, enhanced p-backdonation can be expected for low-valent metals (see Section 3.07.4.2). However, s-donation seems to be key in determining the strength of the interaction. Indeed, a very good correlation was found between DEint and the carbene’s HOMO energy for both 10 and [AuCl(NHC)]: the higher the carbene’s HOMO energy, the stronger the M-NHC interaction.43 The electrostatic contribution to the attractive part of DEint (i.e., Delstat + DEorb) was found to oscillate around 65–75% for complexes 10, 11M and 17M.24,43,67,72,73 Jacobsen actually showed that this holds true for transition metals in general.24 On the other hand, as pointed out by Frenking, other basic neutral ligands also form mostly electrostatic bonds (e.g., NMe3, PMe3, imidazole).72,77 Jacobsen also found a considerable difference between the interaction energies of anionic (17M: DEint  −100 kJ mol−1) vs. cationic complexes (11M: DEint  −250 kJ mol−1).67 However, these systems are experimentally quite rare.44,50,56,78 For the more common neutral complexes 90 , values ranging from −166 to −283 kJ mol−1 were obtained by Frenking and Frison/Huynh (depending on the carbene ligand).43,72 Compared to other metals, these values appear to be slightly below average: most of the complexes studied by Jacobsen display DEint values ranging from −200 to −400 kJ mol−1,67 and a range of −303 to −359 kJ mol−1 was found for strongly bound [AuCl(NHC)] complexes.43,72 Therefore, while the M-NHC interaction is not the strongest for group 4 metals, it is clearly not a threat for the stability of these species. However, Frenking calculated DEprep for complexes 10 and found that relaxing the MCl4 fragment releases a copious amount of energy, thus considerably lowering the bond dissociation energy.72 Still, De values ranging from −110 to −167 kJ mol−1 were obtained, which is comparable to typical values for main group donor-acceptor adducts (e.g., H3B NH3).77,79

3.07.2.3

Comparison with other neutral donors

To the best of our knowledge, there are only two systematic studies comparing the dissociation energies of MdL bonds where M is a group 4 metal and L is a neutral donor: the theoretical study by Frenking mentioned in the introduction (which hinted at the superior donating ability of NHCs compared to NH3 and CO),18 and a series of papers by Tamm on Zr and Hf-Cht (Cht ¼ cycloheptatrienyl) complexes (Scheme 3).58–60,80,81 The separate reactions of precursors [M(Cht)X] (18M, X ¼ Cp, allyl, pentadienyl) with L readily afforded adducts [M(Cht)X(L)] (19M) in the case of the NHC and isocyanides, but less so in the case of

Scheme 3 Reaction of Zr- and Hf-cycloheptatrienyl complexes with neutral donors (the crystal structure of a Ti-cycloheptatrienyl NHC complex is shown in Ref. 80, however no experimental details were given).

PMe3 (for which ligand dissociation hampered product isolation). DFT calculations at various levels of theory consistently indicated that the enthalpy of formation of 19M was lowest for the NHC, i.e., that it is the strongest donor.57–60 Despite the numerous claims that NHCs dissociate easily from early transition metals, only a handful of reports actually deal with the stability of the MdNHC bond towards ligand exchange in the case of group 4 metals. Arnold has shown that triphenylphosphine oxide readily displaces bidentate and tridentate NHCs from oxophilic group 3 metals;26 however no reaction was observed with group 4 metal complexes (e.g., 16, Fig. 5).53,82 Similarly, Lorber has shown that the reaction of Ti imido complex [TiCl2(NAr)(NHMe2)(L1)] (20Ti, R ¼ R0 ¼ Mes, Ar ¼ Dipp or 2,6-C6H3Cl2) with an excess of pyridine displaces the dimethylamine ligand—not the NHC—to give [TiCl2(NAr)(Py)(L1)] (21).

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235

Scheme 4 Reaction of a Ti imido complex with neutral ligands.

The TiCl2NAr fragment (Ar ¼ 2,6-C6H3Cl2) is even capable of accommodating a second NHC, thus generating [TiCl2(NAr)(L1)2] (22) (Scheme 4).61 Thus, while there is little doubt that NHCs may dissociate from group 4 metals (given the generally weaker interaction energy compared to later transition metals), actual experimental evidence for processes involving NHC dissociation remains scarce, and mostly indirect.53,83 By and large, NHCs are an exceptionally versatile class of ligands for a vast range of transition metals and main group elements.

3.07.3

Synthesis of group 4 metal NHC complexes

There are two methods for preparing NHC complexes of group 4 metals: the direct reaction of a free carbene with a metal precursor (thereafter referred to as the “free carbene route”), and the reaction of an azolium salt with a metal precursor possessing basic ligands (the “alcohol/amine elimination route”). The first of these methods was used exclusively until 2004, when Abernethy reported the application of the second method to generate the Ti complex [TiCl2(NMe2)2(L1)] (23, R ¼ R0 ¼ Mes) (Scheme 5).62 Each method has its trade-offs: the free carbene route requires the use of stoichiometric amounts of NHCs, which can be

Scheme 5 Known synthetic routes to group 4 metal NHC complexes and the first example obtained by amine elimination.

unpractical; the alcohol/amine elimination route results in complexes that frequently incorporate the eliminated alcohol or amine, along with the counteranion of the azolium salt (e.g., chloride). The widely used transmetallation route using Ag-NHC species is not feasible for group 4 metals;39 in fact, Jacobsen has shown that group 4 metal NHC complexes have about the same DEint as silver NHC transfer agents [AgCl(NHC)].67 To the best of our knowledge, other methods such as the use of NHC-CO2 adducts as NHC surrogates,84 or template synthesis of an NHC at the metal,85 have not been reported.

236

3.07.4

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

Complexes bearing monodentate ligands

Until 2003 and Kawaguchi’s report of a tridentate NHC Ti complex,11 only monodentate NHC complexes were known for group 4 metals. Some of these have already been presented above (8–11 and 17–23), therefore they will not be further discussed in the first subsection. In the following subsections, we will discuss the case of low-valent group 4 metal complexes (M(III) to M(0)) separately, because of the peculiar properties of these species.

3.07.4.1

M(IV) complexes

Using the free carbene route with TiF4, Roesky described the synthesis of fluoride analogues of 8Ti [TiF4(L2)] (80 Ti, R ¼ Me, iPr) (Fig. 8). When an NHC bearing N-iPr groups was used, the resulting complex was unstable and decomposed in THF to give anionic complex [TiF5(L2)][L2H] (24, R ¼ iPr), in which the counterion is the imidazolium cation (Fig. 8). As a consequence of the anionic nature of 24, the 13C{1H} NMR signal of the carbene is strongly deshielded (207.6 ppm); the Ti-CNHC distance is slightly elongated (2.310(3) Å) compared to other Ti-NHC complexes, but not overly so (see Table 1). With N-Me substituents on the NHC, the

Fig. 8 Selected examples of monodentate NHC complexes of group 4 metals.

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

237

complex was more stable; addition of [Ti(NEt2)4] yielded the F-bridged dimer [{TiF2(NEt2)(L2)}2(m-F)2] (25). In this case the Ti-CNHC distance is much shorter at 2.2367(9) Å. The NHC donor properties were evaluated by DFT calculations: the free energy of the reaction [TiF5]− + L ! [TiF5L]- was computed and it was found that for these conditions the donor strength of NHCs fall between Cl− and F−. This was confirmed experimentally using an empirical method based on 19F NMR spectroscopy.55 Fischer also described examples of 8Ti and 80 Ti (R ¼ Dipp), along with [TiCl4(L1)] (10, R ¼ Dipp) (Fig. 8). Complexes 8Ti and 0 8 Ti both display an octahedral geometry, but the TidCNHC bond distances are considerably shorter for the latter (mean 2.262(2) Å vs. mean 2.318(3) Å). Compared to the example reported by Roesky (with L2, R ¼ iPr), those distances are also shorter (mean 2.2800(8) Å). The trigonal bipyramidal complex 10, in which only one NHC binds Ti, displays an even shorter TidCNHC bond distance (2.198(6) Å).30 Erker reported anionic complex [HfCl5(L1)][L1H] (17Hf, R ¼ R0 ¼ iPr) (Fig. 8) in 2003. This complex displays an octahedral geometry in which one of the apical coordination sites is occupied by the NHC ligand. The anionic nature of 17Hf is reflected in the long HfdCNHC bond distance (2.406(6) Å) and the relatively shielded (for Hf ) carbene signal (187.5 ppm).56 In an attempt to generate 22 (Scheme 4) by reacting [TiCl2(¼NDipp)]n with two equivalents of IMes, Lorber obtained a rare example of an anionic Ti-NHC complex [TiCl3(NDipp)(L1)][L1H] (26, R ¼ R0 ¼ Mes) in which the imido and NHC ligands are mutually cis (Fig. 8).44 As a consequence of the sterically congested coordination sphere, the TidCNHC bond distance is quite long (2.306(4) Å). Presumably, steric crowding also prevents the coordination of two IMes ligands. Using the free carbene route again, complexes [MCl2(NMe2)2(L1)] (27M, R ¼ R0 ¼ Mes or Dipp) were obtained by Abernethy, Lorber and Roesky.44,63 The TidCNHC and ZrdCNHC distances are unusually long (27Ti: R ¼ R0 ¼ Mes: 2.316(10) (Lorber), 2.313(5) (Abernethy); R ¼ R0 ¼ Dipp: 2.313 (3) Å; 27Zr: R ¼ R0 ¼ Mes: 2.453(3) Å), probably because of the presence of the strongly donating amido ligands in the same plane as the NHC. The 13C{1H} NMR signal of the carbene is rather deshielded in 27Hf (R ¼ R0 ¼ Mes: 204.0 ppm) vs. 27Ti (R ¼ R0 ¼ Mes: 194.0 ppm; R ¼ R0 ¼ Dipp: 195.2 ppm) and 27Zr (R ¼ R0 ¼ Mes: 192.4 ppm) as sometimes observed with other group 4 metal NHC complexes. Zirconium analogues of 20Ti, [ZrCl2(NAr)(NHMe2)(L1)] (20Zr, R ¼ R0 ¼ Mes, Ar ¼ Dipp or 2,6-C6H3Cl2) (Scheme 4) were also obtained. The MdCHNC bond distances in 20M fall within the normal range for Ti- and Zr-NHC complexes (Ti: 2.277(4), 2.263(5); Zr: 2.425(3) Å) and the 13C{1H} NMR signals of the carbene are strikingly similar (Ti: 191.1, 191.4; Zr: 190.8 ppm) considering the differences in MdCHNC bond distances between 20Ti and 20Zr. Considering the importance of amido- and imido-bearing group 4 metal complexes in catalytic hydroamination and multicomponent coupling reactions,106,107 complexes 20–22, 26 and 27M could be worth investigating further in catalysis. Tamm reported a series of Ti NHC complexes [TiX2(C5H4R)(L3)] 28 (R ¼ H, tBu; X ¼ Cl, Me, Fig. 8) which represent an interesting variation of the imidazole-2-ylidene template (Fig. 1A). The presence of the borate moiety in the 4-position of the imidazole ring makes the NHC anionic: thus, complexes 28 were synthesized by displacing a chloride ligand from parent [TiCl3(C5H4R)]. High catalytic activity was obtained in ethylene polymerization with one of these complexes in the presence of AliBu3/[Ph3C][B(C6F5)4] as cocatalyst (R ¼ tBu; X ¼ Cl; up to 566 kg-PE mol-Ti−1 h−1 bar−1 at 25  C); another complex in this series exhibited efficient hexene incorporation in ethylene/hexene copolymerization (R ¼ tBu; X ¼ Me; >30% hexene incorporation, up to 305 kg-polymer.mol-Ti−1 h−1 bar−1 at 25  C).64 Finally, the range of monodentate NHCs is not limited to A: Hahn and Richeson reported Ti and Zr complexes [TiCl4(L4)] (29) and [ZrX2(NMe2)2(L8)2] (30, R ¼ Tol, iPr; R0 ¼ Tol, 2-Py; X ¼ Br, OMs) respectively, which contain benzimidazol-2-ylidene (C, Fig. 2) and perimidin-2-ylidene (F) carbenes (Fig. 8). Complex 29 was obtained by the free carbene route;65 compared to complexes 8Ti and 10Ti, the 13C{1H} NMR signal is shifted downfield (196.0 vs. 180.7–186.7 ppm). At 2.221(2) Å, the TidCNHC bond Table 1

MdCNHC bond distances and 13C{1H} NMR spectroscopy carbene signal chemical shift of monodentate NHC complexes 8M-11M and 19M-30.

Nbr

M(n)

Formula

R, R0 , R00

Ar

X, X0 , X00

dMdCNHC (Å)

dCNHC (ppm)

References

7 8Ti

Hf(IV) Ti(IV)

[HfCl(Cp)2(L34)(L2)] [TiCl4(L1)]

– –

– –

2.365(5) –

197.0 180.7

23 47

8Ti

Ti(IV)

[TiCl4(L1)2]

–

–

54

Ti(IV)

[TiCl4(L1)2]

–

–

193.1

30

80 Ti 80 Ti

Ti(IV) Ti(IV)

[TiF4(L2)2] [TiF4(L2)2]

– –

– –

181.9 184.3

55 55

80 Ti

Ti(IV)

[TiF4(L1)2]

–

–

188.5

30

8Zr

Zr(IV)

[ZrCl4(L1)]

–

–

2.3358(15) 2.3343(14) 2.317(5) 2.318(5) 2.255(4) 2.2781(12) 2.2812(12) 2.261(3) 2.263(3) –

190.1

8Ti

Me Me Me Mes Mes Dipp Dipp Me i Pr

178.2

47

8Hf

Hf(IV)

[HfCl4(L1)]

–

–

–

176.2

47

Dipp Dipp Me Me Me Me

(Continued )

238

Table 1 Nbr 8Zr

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

(Continued) M(n) Zr(IV)

Formula 1

[ZrCl4(L )]

R, R0 , R00

Ar

X, X0 , X00

dMdCNHC (Å)

dCNHC (ppm)

References

i

–

–

2.432(3)

181.8

49

–

–

2.456(3)

183.5

49

–

–

2.448(3)

185.4

49

–

–

2.401(2)

189.1

49

–

–

–

48

–

–

2.194(7) 2.207(7) 2.1759(14)

48,54

– – –

– – –

– – 2.198(6)

185.2 185.3 186.7 185.5 188.8

–

BPh4

2.289(2)

178.2

50

–

Me-B(C6F5)3

–

178

50

–

–

2.406(6)

187.5

56

– – –

– – –

2.445(2) 2.453(1) 2.4381(19)

191.2 193.8 191.1

57 58 59

– 2,6-C6H3Cl2

– –

2.394(2) 2.277(4)

192.6 191.1

60 61

Dipp

–

2.263(5)

191.4

61

Dipp

–

2.425(3)

190.8

61

2,6-C6H3Cl2

–

–

192.6

61

Dipp

–

–

191.8

61

2,6-C6H3Cl2

–

192.8

61

– – –

– – –

2.315(2) 2.307(2) 2.310(3) 2.2367(9) 2.306(4)

207.6 – –

55 55 44

–

–

–

Pr Pr Me 2-butyl Me CH2Mes i Pr i Pr i Pr i Pr i Pr i

8Zr

Zr(IV)

[ZrCl4(L1)]

8Zr

Zr(IV)

[ZrCl4(L1)]

8Hf

Hf(IV)

[HfCl4(L1)]

9Ti

Ti(IV)

[(TiCl3(L2))2(m-O)]

10Ti

Ti(IV)

[(TiCl4(L2)]

10Ti 10Ti 10Ti

Ti(IV) Ti(IV) Ti(IV)

[(TiCl4(L2)] [(TiCl4(L2)] [(TiCl4(L1)]

11Ti

Ti(IV)

[Ti(Cp)2Me(L1)][X]

11Zr

Zr(IV)

[Zr(Cp)2Me(L1)][X]

17Hf

Hf(IV)

[HfCl5(L1)][L1H]

19Zr 19Zr 19Zr

Zr Zr Zr

19Hf 20Ti

Hf Ti(IV)

[Zr(Cht)(Cp)(L2)] [Zr(Cht)(2,4-C7H11)(L2)] [Zr(Cht)(C3H3(SiMe3)2) (L2)] [Hf(Cht)(Cp)(L2)] [TiCl2(NAr)(NHMe2)(L1)]

20Ti

Ti(IV)

[TiCl2(NAr)(NHMe2)(L1)]

20Zr

Zr(IV)

[ZrCl2(NAr)(NHMe2)(L1)]

21

Ti(IV)

[TiCl2(NAr)(Py)(L1)]

21

Ti(IV)

[TiCl2(NAr)(Py)(L1)]

22

Ti(IV)

[TiCl2(NAr)(L1)2]

24 25 26

Ti(IV) Ti(IV) Ti(IV)

[TiF5(L2)][L2H] [{TiF2(NEt2)(L2)}2(m-F)2] [TiCl3(NDipp)(L1)][L1H]

27Ti

Ti(IV)

[TiCl2(NMe2)2(L1)]

27Ti

Ti(IV)

[TiCl2(NMe2)2(L1)]

27Zr

Zr(IV)

[ZrCl2(NMe2)2(L1)]

27Hf

Hf

[HfCl2(NMe2)2(L1)]

28 28 28 28 29 30

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

[TiX2(C5H4R)(L3)] [TiX2(C5H4R)(L3)] [TiX2(C5H4R)(L3)] [TiX2(C5H4R)(L3)] [TiCl4(L4)] [ZrX2(NMe2)2(L8)2]

30

Zr(IV)

[ZrX2(NMe2)2(L8)2]

Me Et Dipp Dipp i Pr i Pr i Pr i Pr i Pr i Pr Me Me Me Me Mes Mes Mes Mes Mes Mes Mes Mes Mes Mes Mes Mes i Pr Me Mes Mes Mes Mes Dipp Dipp Mes Mes Mes Mes H t Bu H t Bu – Tol Tol i Pr 2-Py

48 48 30

194.0

61,62

–

2.316(10) 2.313(5) 2.313(3)

195.2

63

–

–

2.435(3)

192.4

44

–

–

–

204.0

44

– – – – – –

Cl Cl Me Me – Br

2.214(3) 2.218(10) 2.246(3) 2.230(7) 2.221(2) –

184.9 186.1 183.3 184.3 196.0 –

64 64 64 64 65 66

–

OMs

–

–

66

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

239

distance is unremarkable. Complex 30 was obtained by the amine elimination route;66 they were characterized by NMR spectroscopy only, and the carbene signal was not observed in the 13C{1H} spectra.

3.07.4.2

Lower oxidation states

Low-valent group 4 metals are interesting in the context of NHC chemistry because they can potentially backdonate much more electron density to the ligand, especially with NHCs possessing enhanced p-accepting properties such as cyclic (alkyl)(amino) carbenes (CAAC, D, Fig. 1).108 This can in turn lead to interesting properties related to the possible redox non-innocence of the NHC—in which case the oxidation state of the metal becomes indeterminate. Lorber reported the unexpected formation of a Ti(III) complex by reacting [Ti(NMe2)4] with 2 equiv. of [L1H]Cl (R ¼ R0 ¼ Mes).

Fig. 9 Low-valent NHC complexes of group 4 metals with unambiguous oxidation states.

Complex [TiCl2(X)(L1)2] (31, R ¼ R0 ¼ Mes, X ¼ NMe2) (Fig. 9) was obtained upon heating the reactants at elevated temperatures (80  C), but the authors could not explain the reduction mechanism. The EPR spectrum of a toluene solution of 31 was consistent with a d1 Ti complex (g ¼ 1.9433, no coupling to N). The TidCNHC bond distances (mean 2.295(2) Å) are unexceptional. Lorber also reported partially characterized examples of Ti(III)-NHC complexes bearing the IMes ligand, and postulated the formation of 31 (R ¼ R0 ¼ Mes, X ¼ Cl)(Fig. 9).44 Later, Roesky (R ¼ R0 ¼ Dipp) and Radius (R ¼ R0 ¼ Mes) reported the X-ray diffraction structures of 31, which were obtained using the free carbene route.54,63 Radius conducted DFT calculations on 31 (R ¼ R0 ¼ Mes, X ¼ Cl) and concluded that the unpaired spin density resides on Ti, consistent with a genuine Ti(III) complex. Compared to 31 (X ¼ NMe2), the TidCNHC bond distances in 31 (X ¼ Cl) are considerably longer (R ¼ R0 ¼ Mes: mean 2.332(3) Å; R ¼ R0 ¼ Dipp: mean 2.336(4) Å), which is counterintuitive. Interestingly, the TidCNHC bond distances in 31 (R ¼ R0 ¼ Mes, X ¼ Cl) and its Ti(IV) analogue 8Ti (R ¼ R0 ¼ Mes) are indistinguishable within 3s, therefore this parameter is not useful for discriminating both oxidation states in this particular case.54 By contrast, complex [TiCl3(L7)2] (32) displays significantly shorter TidCNHC bond lengths (mean 2.293(2) Å) compared to its Ti(IV) analogue (not shown, mean 2.330(4) Å).54,86 This is in line with the greater delocalization of unpaired electron density onto the CAAC ligand compared to IMes, because of the enhanced p-acceptor character of the former. Still, the computed electronic structure of 32 indicates a clear-cut d1 configuration.54 Deng reported the CAAC Hf(II) complex [HfX2(L7)2] (33Hf, X ¼ Cl) which was obtained by reaction of the free carbene with a 1:2 mixture of HfCl4 and KC8.87 The authors found a closed-shell singlet state (S ¼ 0) lying 57.7 kJ mol−1 lower than the lowest open-shell state (S ¼ 1) and concluded that the electronic configuration of the complex (X ¼ Cl) was d2. However, strong p-backdonation was evident in the short HfdCNHC (mean 2.169(3) Å) and long CNHCdN (mean 1.377(3) Å) bond distances.

240

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

Complex 33Hf (X ¼ Bn) was also obtained by reacting the dichloro complex 33Hf (X ¼ Cl) with 2 equiv. of KBn; in this case the HfdCNHC bond distance of 33Hf (X ¼ Bn) was longer (mean 2.197(3) Å), but the CNHCdN bonds were also slightly longer (mean 1.391(3) Å) than in the chloro complex 33Hf (X ¼ Cl). Remarkably, complex [TiX2(L7)2] (33Ti, X ¼ Cl) (Fig. 10), obtained by reduction of 32 with 1 equiv. of KC8, displays a much narrower energy gap between closed- and open-shell states (1.3–2.1 kJ mol−1 depending on the temperature), although the average TidCNHC bond distance is also quite short, at 2.109 Å. At room temperature, the triplet state (S ¼ 1) is significantly populated. The lower energy closed-shell singlet state (S ¼ 0) was found to involve considerable p-interaction between Ti and both CAAC ligands,

Fig. 10 Low-valent NHC complexes of group 4 metals with ambiguous oxidation states.

whilst the triplet state (S ¼ 1) only involves unpaired electron delocalization to one of the carbenes.86 As a result of this complex and temperature-dependent electronic structure, assigning an oxidation state to 33Ti is essentially a moot point, but can be considered as being comprised between +II and +III. In 2013, Baumgartner, Marschner and Müller reported unusual Ti-NHC/germylene complexes [Ti(Cp)2(L35)(L2)] (34, R ¼ Me, n ¼ 1, 2) (Fig. 10). The germylene ligand acts as a p-acceptor to stabilize the low-valent Ti center: the starting material for 34 is [Cp2Ti(btmsa)] (btmsa ¼ bis(trimethylsilyl)acetylene), a “masked” Ti(II) reagent. NMR spectroscopic data indicates slow rotation of the germylene ligand, therefore the double-bond character of the Ti/Ge interaction is partial and the Ti center undeniably possess some Ti(II) character. However the TidCNHC bond (for n ¼ 1) is quite long at 2.323(2) Å, probably because the germylene ligand is a better p-acceptor than the NHC.88 Braunschweig reported complex [Ti(C6H6)2(L1)] (35, R ¼ R0 ¼ Me) (Fig. 10), which was obtained by reaction of the free NHC with bis(benzene)titanium [Ti(C6H6)].109 The latter is a highly reduced Ti complex stabilized by strong d-backbonding, which can therefore be considered as a masked Ti(0) reagent.110 Not much is known about the electronic structure of 35, but the authors contended that the long TidCNHC distance of 2.323(3) Å evidences its low-valent character—which clearly contradicts the trend observed with complexes 33M. Since the benzene ligands in 35 probably accept considerable electron density from Ti by p-backdonation, it is reasonable to conclude that the TidCNHC bond distance is abnormaly long (for a low-valent Ti complex) due to the presence of a better p-acceptor. In closely related work, Beckhaus reported complexes [Ti(L36)2(L2)] (36), obtained by reaction of the free NHC with bis (Z5,Z1) pentafulvene titanium precursors.90 In spite of the interaction between Ti and the exocyclic carbon (dashed line, Fig. 10), the pentafulvene ligands in 36 display an Z4-coordination mode, indicative of the low-valent nature of Ti. Again, an elongated TidCNHC bond length (2.342(2) Å) is observed, probably due to the strong p-accepting properties of the pentafulvene ligand—

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

241

Scheme 6 Reactions of a bis(pentalene)dititanium NHC complex.

which also precludes the straightforward assignment of a meaningful oxidation state. However, the shielded carbene signals (151.7–164.8 ppm) are a clear indication of the low-valent nature of 36. Cloke reported some interesting reactivity following the coordination of an NHC to the bis(pentalene)dititanium complex [Ti2(L37)2] (37) (Scheme 6), a low-valent Ti reagent which acts as a d2 titanocene surrogate.91,92,111 It appears that the NHC enhances the reactivity of the adjacent Ti center in complex 37, resulting in CdH bond activation at the pentalene framework. When [Ti2(L37)(L38)(m-H)(L2)] (38) is reacted with H2, the Ti bis(hydride) complex [Ti2(L37)2(m-H)(H)(L2)] (39) is obtained, and deuterium labelling studies indicated that the CdH bond is formed before H2 activation takes place: this suggests a mechanism involving CdH reductive elimination, followed by oxidative addition of H2.91 Reaction of 38 with acidic reagents such as HCl or tertbutylacetylene generates complexes [Ti2(L37)2(m-H)(X)(L2)] (40). Interestingly, the use of an ammonium salt with a weakly coordinating anion enables the isolation of complex [Ti2(L37)2(m-H)(L2)][BPh4] (41), in which one of the

Table 2

MdCNHC bond distances and 13C{1H} NMR spectroscopy carbene signal chemical shift of low-valent monodentate NHC complexes 31–41.

Nbr

M(n)

Formula

R, R0 , R00

Ar

X, X0 , X00

dMdCNHC (Å)

dCNHC (ppm)

References

31

Ti(III)

[TiCl2(X)(L1)]

–

NMe2

61

Ti(III)

[TiCl2(X)(L1)]

–

Cl

2.293(3) 2.297(3) mean 2.336(4)

–

31

–

63

31

Ti(III)

[TiCl2(X)(L1)]

–

Cl

54

Ti(III)

[TiCl3(L7)2]

–

–

–

54

32

Ti(III)

[TiCl3(L7)2]

–

–

–

2.3268(40) 2.3379(37) 2.2946(33) 2.2793(42) 2.298(2) 2.301(2)

–

32

Mes Mes Dipp Dipp Mes Mes –

–

86 (Continued )

242

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

Table 2

(Continued)

Nbr

M(n)

Formula

33

Hf(II)

7

33

R, R0 , R00

Ar

X, X0 , X00

dMdCNHC (Å)

dCNHC (ppm)

References

[HfX2(L )2]

–

–

Cl

303

87

Hf(II)

[HfX2(L7)2]

–

–

Bn

304

87

33

Ti

[TiX2(L7)2]

–

–

Cl

–

86

34 34 35

Ti Ti Ti

[Ti(Cp)2(L35)(L2)] (n ¼ 1) [Ti(Cp)2(L35)(L2)] (n ¼ 2) [Ti(C6H6)2(L1)]

– – –

– – –

197.6 198.2 –

88 88 89

36 36 36 38 39 40 40 41

Ti Ti Ti Ti Ti Ti Ti Ti

[Ti(L36)2(L2)] [Ti(L36)2(L2)] [Ti(L36)2(L2)] [Ti2(L37)(L38)(m-H)(L2)] [Ti2(L37)2(m-H)(H)(L2)] [Ti2(L37)2(m-H)(X)(L2)] [Ti2(L37)2(m-H)(X)(L2)] [Ti2(L37)2(m-H)(L2)][BPh4]

Me Me Me Me Me Me Me Me Me Me Me Me

2.177(4) 2.161(3) 2.182(3) 2.211(3) 2.095(3) 2.122(2) 2.323(2) – 2.323(3)

– – – – – – – –

2-Ad Ph Tol – – Cl CCtBu –

2.342(2) – – 2.300(2) 2.291(4) – 2.267(7) 2.260(2)

164.8 151.7 152.2 197.8 198.3 195.5 196.7 185.4

90 90 90 91 91 92 92 92

Ti centers is stabilized by an agostic CdH bond.92 The TidCNHC bond distances in 38–41 are unremarkable (mean 2.280(2) Å), however protonation of 38 to give 41 has a distinct shielding effect on the carbene signal in their 13C NMR spectra (38: 197.8; 41: 185.4 ppm) (Table 2).

3.07.5

Complexes bearing bidentate NHCs

3.07.5.1

O-functionalized ligands

In 2006, Arnold reported complex [Ti(OiPr)3(L10)] (16) (Fig. 5) and used it in the ring-opening polymerization of rac-lactide. The initiator activity of 16 was relatively good for a titanium complex, producing low molecular weight polylactide (2300 Da) with narrow polydispersity (PDI 1.19) in a couple of minutes. Interestingly, chain-end analysis of the polymers by MALDI mass spectrometry showed termination by imidazolium cations, which indicates that complex 16 acts as a bifunctional catalyst. This in turn suggests that the carbene ligand could dissociate from Ti during catalysis. This hypothesis is consistent with the observation that a related Y complex, which readily undergoes carbene dissociation, is a much more active initiator than complex 16.53 However, no

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

243

Fig. 11 Examples of Ti and Zr complexes bearing O-functionalized bidentate NHCs.

further evidence (e.g., computational studies) for dissociation during catalysis was given. Reduction of complex 16 with potassium yielded a tetranuclear mixed-valence Ti(III)/Ti(IV) cluster resulting from the activation of silicone grease.112 A rare homoleptic Ti(III)-NHC complex bearing the same alkoxy-functionalized NHC as complex 16 was also reported: complex [Ti(L10)3] (42) (Fig. 11). The solid-state structure of 42 reveals three different TidCNHC bond lengths (2.263(5), 2.252(4), 2.299(4) Å).93 Longo reported a series of Zr complexes [Zr(NEt2)2(L11)2] (43) bearing alkoxy-functionalized NHCs. These complexes exist as mixtures of stereoisomers, and since they could not be characterized by X-ray diffraction, their structure remains unknown. The cyclopentanol-functionalized complex displayed low ethylene polymerization activity (up to 5.1 kg-PE mol-Zr−1 h−1 bar−1 at 50  C),94 and other complexes were even less active.95 Grubbs reported Ti and Zr complexes bearing two aryloxy-functionalized imidazolidin-2-ylidene ligands (Fig. 1B).96,97 Complexes [MCl2(L12)2] (44M, R ¼ Ad, R0 ¼ Me, R00 ¼ H, Ar ¼ Dipp) (Fig. 11) were structurally characterized and both NHCs were found trans to each other, whilst chloride ligands were cis. Both 44Zr and 44Ti were found to be weakly active in ethylene polymerization (up to 17 kg-PE mol-M−1 h−1 bar−1 at 50–80  C). Complex 44Ti was weakly active in styrene polymerization (up to 1.3 kg-PS mol-Ti−1 h−1 at 80  C), however the polymer thus obtained was highly syndiotactic and displayed a high melting temperature (Tm  250  C).97. Later, El-Batta reported that removal of the Ad group increased the catalytic efficiency of 44Ti; however, no experimental details were given.98 More recently, Le Roux also explored the effect of steric bulk reduction on 44Ti by removing the Ad and/or Me groups on the aryloxy substituent, and varying the aryl group (i.e., from Dipp to Dep and Mes). Keeping the Dipp unit on the NHC enables the

244

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

isolation of 44Ti (Ar ¼ Dipp, R ¼ R0 ¼ H, R00 ¼ H or Me). The carbene signal is more shielded in the 13C NMR spectra of 44Ti: 206.7 (R00 ¼ H) and 206.8 (R00 ¼ Me) ppm vs. 226.8 ppm for Grubbs’ complex. Additionally, the TidCNHC bond distance is shorter, consistent with the reduced steric bulk (2.2597(18) Å for 44Ti vs. 2.264(2)/2.275(2) Å for Grubbs’ complex). By contrast, switching the Dipp group for less bulky Dep or Mes substituents had a dramatic impact on the reaction of the free carbene with [TiCl4(THF)2]: instead of 44Ti, a mixture of isomers was observed in solution, which is a common occurrence in octahedral transition metal complexes.113–115 However, the reaction mixtures evolved and, astonishingly, electron-rich olefin complexes [TiCl2(L39)] (45, Ar ¼ Dep, Mes) (Fig. 11) were finally isolated (Fig. 8).83 The dimerization of the NHC ligand is evident in the more shielded olefinic carbon signal (Ar ¼ Dep: 160.7 ppm; Ar ¼ Mes: 160.8 ppm). It is not clear whether the dimerization occurred before or after coordination to Ti, but this is one of the rare cases in which NHC dissociation from a group 4 metal can reasonably be invoked. Complexes 44Ti and 45 were moderately active in ethylene polymerization (26–36 kg-PE mol-Ti−1 h−1 bar−1 at ambient temperature). Buchmeiser also reported a series of Ti complexes [TiCl3(L12)(THF)] (46, Ar ¼ Mes, Xyl, Dipp, R ¼ R0 ¼ R00 ¼ H) and [TiCl2(X) 12 (L )] (47, Ar ¼ Mes, Xyl, Dipp, R ¼ R0 ¼ R00 ¼ H, X ¼ 2,6-Dipp-phenoxy (ODipp ), 2,6-ditertbutyl-4-methylphenoxy (BHT)) using Grubb’s design. Interestingly, no NHC dimerization was observed with Ar ¼ Mes, Xyl or Dipp. The TidCNHC bond distance was found to increase from 2.211(3) Å in 46 (Ar ¼ Dipp) to 2.254(7) and 2.318(5) Å in 47 (Ar ¼ Mes, X ¼ BHT and ODipp , Table 3

MdCNHC bond distances and 13C{1H} NMR spectroscopy carbene signal chemical shift of bidentate NHC complexes 42–47.

Nbr

M(n)

Formula

R, R0 , R00

Ar

X, X0 , X00

dMdCNHC (Å)

dCNHC (ppm)

References

42

Ti(III)

[Ti(L10)3]

–

–

–

–

93

43 43 43 44

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

[Zr(NEt2)2(L11)2] [Zr(NEt2)2(L11)2] [Zr(NEt2)2(L11)2] [TiCl2(L12)2]

– – – Dipp

– – – –

197.5a 196.3a 198.6a 226.8

94 95 95 96,97

44

Zr(IV)

[ZrCl2(L12)2]

Dipp

–

2.452(4) 2.438(4)

226.8

96,97

44

Ti(IV)

[TiCl2(L12)2]

Dipp

–

–

–

98

44

Ti(IV)

[TiCl2(L12)2]

Dipp

–

2.2597(18)

206.7

83

44

Ti(IV)

[TiCl2(L12)2]

Dipp

–

–

206.8

83

45 45 46

Ti(IV) Ti(IV) Ti(IV)

[TiCl2(L39)] [TiCl2(L39)] [TiCl3(L12)(THF)]

Dep Mes Dipp

– – –

– – 2.211(3)

160.7b 160.8b 204.4

83 83 16

46

Ti(IV)

[TiCl3(L12)(THF)]

Mes

–

–

205.0

16

46

Ti(IV)

[TiCl3(L12)(THF)]

Xyl

–

–

204.8

16

47

Ti(IV)

[TiCl2(X)(L12)]

Dipp

BHT

–

211.5

16

47

Ti(IV)

[TiCl2(X)(L12)]

Mes

BHT

2.254(7)

210.6

16

47

Ti(IV)

[TiCl2(X)(L12)]

C5H8 C2H2 C5H8 Ad Me H Ad Me H H Me H H H H H H Me – – H H H H H H H H H H H H H H H H H H

2.263(5) 2.252(4) 2.299(4) – – – 2.264(2) 2.275(2)

Mes

ODipp

2.318(5)

210.8

16

Only one signal given in the experimental part. Electron-rich olefin carbon signal.

a

b

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

245

respectively). Complex 47 (Ar ¼ Dipp; X ¼ BHT) displayed high activity in ethylene polymerization (190 kg-PE mol-Ti−1 h−1 bar−1 at 60  C), and copolymerization with norbornene or cyclopentene could be achieved. Increasing the steric bulk of the aryloxy ligand (47, Ar ¼ Mes, X ¼ ODipp ) completely suppressed the catalytic activity (Table 3).16

3.07.5.2

N-functionalized ligands

Complexes [Ti(X)(X0 )(OiPr)(L13)] (48, R ¼ tBu, X ¼ Br, OiPr, X0 ¼ Br, OiPr) (Fig. 12), obtained by reacting [TiCl(OiPr)3] with Li-NHC (LiBr)n (n ¼ 0,1,2), were reported by Arnold in 2006. Interestingly, the number of bromide ligands in 48 was directly related to n, which indicates that the bromide anion binds titanium more strongly than chloride. Moreover, the carbene signal of the tris-isopropoxy complex is considerably less shielded than that of the mono-bromo bis-isopropoxy complex (205.6 vs. 187.6 ppm) (other complexes were only characterized by 1H NMR spectroscopy). The solid state structure of the latter complex displayed short

Fig. 12 Group 4 metal complexes bearing N-functionalized bidentate NHCs.

Br⋯ NHC and O⋯ NHC contacts, but DFT calculations indicated that this was due to steric interactions between the X and X0 ligands and the OiPr ligand located trans to the NHC. Reaction of 48 (X ¼ Br, X0 ¼ OiPr) with 1-tertbutylimidazole yielded cationic complex [Ti(OiPr)2(tBuIm)(L13)][Br] (49, R ¼ tBu, tBuIM ¼ 1-tertbutylimidazole) in which a bromide ligand—not the NHC—has been displaced by the incoming neutral base.71 Lavoie reported a series of adducts between a neutral imino-functionalized NHC and group 4 metals chlorides [TiCl3(L14)(THF)] (50, R ¼ Mes, R0 ¼ tBu, Ar ¼ Xyl) and [MCl4(L14)] (51M, R ¼ Mes, R0 ¼ tBu, Ar ¼ Xyl) (Fig. 12). Compared to complex 10Ti (R ¼ iPr), which bears a monodentate NHC, the TidCNHC bond distance in 51Ti is slightly shorter due to the chelating effect of the imino moiety (10Ti: 2.1759(14) Å; 51Ti: 2.167(5) Å). These complexes displayed good activity in ethylene polymerization (51Zr: 140, 51Ti: 40 kg-PE mol-M−1 h−1 bar−1 at room temperature).99 In the case of 51Ti, replacement of two chlorides by aryloxy ligands lowered the polymerization activity to negligible levels.116 A patent was granted to Slaughter and the Chevron company for a series of derivatives of 51M. The claimed Zr complexes displayed similar ethylene polymerization activity to that reported by Lavoie.105 Hydrolysis of a hafnium complex gave a m-oxo complex in which the NHC was intact. When the authors tried to make [Hf(Bn)nNHC] derivatives using [Hf(CH2Ph)4] as starting material, the Hf-coordinated benzyl ligand was found (a) to migrate to the C2-position of the imidazolium salt 52 to give

Scheme 7 Double C-N activation of an imino-functionalized NHC ligand at Hf.

compound 53 (a common decomposition pathway for NHCs bound to group 4 metals, see Section 3.07.6); or (b) to activate the imidazol-2-ylidene ring of the NHC, resulting in the extrusion of the C2 carbon to give complexes [Hf(Bn)(CBn3)(L40)] (54, R ¼ Bn, Me) (Scheme 7). Products with R ¼ Me were characterized by X-ray diffraction. Complexes 54 displayed good activity in ethylene polymerization (Bn: 174; Me: 47 kg-PE mol-Hf−1 h−1 bar−1 at 80  C).100

246

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

Another unexpected reaction was encountered by Ong when reacting L13H with [Zr(CH2Ph)]4. Depending on the substituent on the imidazole-2-ylidene ring, compounds [Zr(Bn)4(L13H)] (55, R ¼ tBu, Mes) and [Zr(Bn)3(L13)] (56, R ¼ Mes) could be isolated and characterized by single crystal X-ray diffraction analysis (Scheme 8). Interestingly, for R ¼ Mes, the ZrdCNHC bond in 56 is longer than that in 55 (2.451(2) vs. 2.420(2) Å). This is probably due to the loss of a Bn ligand, which decreases the electron density

Scheme 8 C-N activation of an amido-functionalized NHC ligand at Hf.

Table 4

MdCNHC bond distances and 13C{1H} NMR spectrocopy carbene signal chemical shift of bidentate NHC complexes 48–56.

Nbr

M(n)

Formula

R, R0 , R00

Ar

X, X0 , X00

dMdCNHC (Å)

dCNHC (ppm)

References

48

Ti(IV)

[Ti(X)(X0 )(OiPr)(L13)]

t

Bu

–

OiPr OiPr

205.6

71

48

Ti(IV)

[Ti(X)(X0 )(OiPr)(L13)]

t

Bu

–

187.6

71

48

Ti(IV)

[Ti(X)(X0 )(OiPr)(L13)]

t

Bu

–

–

–

71

49 50

Ti(IV) Ti(IV)

[Ti(OiPr)2(tBuIm)(L13)][Br] [TiCl3(L14)(THF)]

t

– Xyl

2.221(6) 2.1781(3)

– –

71 99

51Ti

Ti(IV)

[TiCl4(L14)]

Xyl

–

2.167(5)

195.3

99

51Zr

Zr(IV)

[ZrCl4(L14)]

Xyl

–

2.297(17)

197.4

99

51Hf

Hf(IV)

[HfCl4(L14)]

Xyl

–

–

201.6

99

54 54 55 55 56

Hf(IV) Hf(IV) Zr(IV) Zr(IV) Zr(IV)

[Hf(Bn)(CBn3)(L40)] [Hf(Bn)(CBn3)(L40)] [Zr(Bn)4(L13H)] [Zr(Bn)4(L13H)] [Zr(Bn)3(L13)]

Bu Mes t Bu Mes t Bu Mes t Bu Mes t Bu Me Bn t Bu Mes Mes

OiPr Br Br Br – –

2.252(3) 2.256(3) 2.241(6) –

– – – – –

– – – – –

– – 2.393(2) 2.420(2) 2.451(2)

– – 193.4 – 195.4

100 100 101 101 101

at Zr. Eventually, the thermodynamically favored aza-allyl complex [Zr(Bn)2(tBuNC2H3)(Rim)] (57, R ¼ Mes, tBu) was obtained.101 Similar to other NHC decomposition pathways for group 4 metals complexes, it seems that the reaction is driven by the formation of several MdN bonds (see Section 3.07.6) (Table 4).

3.07.5.3

C-functionalized and bis-NHC ligands

The cyclopentadienide ligand (Cp) is ubiquitous in group 4 chemistry,117 therefore the use of Cp-functionalized NHC ligands to stabilize these metals is an interesting development. Müller reported the first and (so far) only examples of piano-stool Ti and Zr complexes bearing a bidentate Cp-NHC ligand [M(NEt2)2(L15)][I] (58M) (Fig. 13). Complexes 58M were obtained by reacting one equivalent of the iodo imidazolium precursor with [M(NEt2)4]. The carbene signal of these cationic NHC complexes were quite

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

247

Fig. 13 Selected examples of Ti and Zr complexes bearing C-functionalized and Bis-NHC ligands.

shielded (58Ti: 185.3 ppm; 58Zr: 182.7 ppm). When two equivalents of imidazolium were used, neutral zirconium diiodo complex [ZrI2(NEt2)(L15)] (580 ) was obtained (Fig. 13). Complex 580 features a distorted square pyramidal geometry in which the iodo ligands are mutually cis, and the apical site is occupied by the Cp moiety. The carbene signal in the 13C NMR spectrum of 580 (187.8 ppm) is less shielded than that of 58Zr.102 Danopoulos reported a series of group 3–6 metal complexes bearing indenyl- and fluorenyl-functionalized NHC ligands. The Ti(III) complex [TiCl(NMe2)(L16)] (59) was obtained by reaction of [TiCl2(NMe2)2] with the fluorenyl-NHC potassium salt, which apparently acts as a reducing agent for the Ti(IV) precursor. The spiroimidazolium salt 60 was obtained as a by-product of the reaction.103 An indenyl-NHC Ti(III) complex [TiCl2(L17)] (61) was also obtained—this time intentionally—by reacting the parent indenyl-NHC potassium transfer agent with [TiCl3(THF)3]. The TidCNHC bond distance in 61 (2.196(5) Å) is shorter than that in 59 (2.221(2) Å), probably as a result of decreased electron density and steric crowding at Ti. Examples of d0 indenyl complexes include [TiCl(NtBu)(L17)] (62, n ¼ 1,2) and [ZrCl3(L17)] (63, n ¼ 1,2). Complexes 62 feature slightly longer TidCNHC bond distances than d1 complexes 56 and 61 (n ¼ 1: 2.227(4) Å; n ¼ 2: 2.226(2) Å). Complex 63 features a distorted square bipyramidal geometry, similar to 580 , and the ZrdCNHC bond distances of both complexes are actually equal within standard deviation limits (580 : 2.440(3) Å; 63 (n ¼ 1): 2.441(9) Å). No catalytic studies were performed in this work, although the authors acknowledged the potential catalytic utility of the reported complexes.104

Table 5

MdCNHC bond distances and 13C{1H} NMR spectroscopy carbene signal chemical shift of bidentate NHC complexes 58–64.

Nbr

M(n)

58 58 580 59 61 62 62 63 63 64

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

Formula 15

[Ti(NEt2)2(L )][I] [M(NEt2)2(L15)][I] [ZrI2(NEt2)(L15)] [TiCl(NMe2)(L16)] [TiCl2(L17)] [TiCl(NtBu)(L17)] (n ¼ 1) [TiCl(NtBu)(L17)] (n ¼ 2) [ZrCl3(L17)] (n ¼ 1) [ZrCl3(L17)] (n ¼ 2) [ZrCl4(L18)]

R, R0 , R00

Ar

X, X0 , X00

dMdCNHC (Å)

dCNHC (ppm)

References

– – – – – – – – – –

– – – – – – – – – –

– – – – – – – – – –

2.218(4) – 2.440(3) 2.221(2) 2.196(5) 2.227(4) 2.226(2) 2.441(9) – –

185.3 182.7 187.8 – – 196.7 – – – –

102 102 102 103 104 104 104 104 104 105

248

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

Finally, the aforementioned Chevron patent described the synthesis of a bis-NHC Zr complex [ZrCl4(L18)] (64) (Fig. 13), which was synthesized using the free carbene route. However, no NMR spectra or X-ray structure were reported, and the polymerization activity of 64 was not described either (Table 5).105

3.07.6

Complexes bearing tridentate NHCs

Pincer-type ligands are of especial importance in coordination chemistry: they impart great stability to transition metal centres, and compared to bidentate ligands they generally simplify the isolation of single diastereoisomers. As a consequence, they have been used thoroughly in homogeneous catalysis.118–121 Therefore, the successful application of pincer-type NHC ligand to group 4 chemistry is anything but surprising.

3.07.6.1 3.07.6.1.1

O,O functionalized ligands Flexible bisaryloxy-NHC ligands

Kawaguchi first reported the use of tridentate bisaryloxy-NHC ligands based on the imidazol-2-ylidene template (A, Fig. 1). These

Fig. 14 Bisaryloxy NHC ligands L1 originally reported by Kawaguchi.

ligands show some fluxionality in solution due to the methylene linkers, and therefore they can adopt two configurations: transoid (very common in the solid state) and cisoid (less common). In addition, since octahedral complexes are also stereochemically labile,113–115 it is often difficult to ascribe the observed fluxionality to a particular mechanism based solely on solution spectroscopic techniques. Few examples of these ligands have been reported over the years; they generally differ by the 4-position of the aryloxy ring (Fig. 14). Complex [Ti(X)2(L19)(THF)] (12Ti, R ¼ tBu, X ¼ Cl) (Fig. 5) displayed high activity in ethylene polymerization (290 kg-PE mol-Ti−1 h−1 bar−1 at 30  C).11 The carbene signal of this complex is unusually shielded (164.0 ppm). Interestingly, variable 1H NMR spectroscopy between -50  C and 25  C revealed the fluxionality of the complex, and the authors suggested that THF exchange is responsible for this phenomenon. However, the dibenzyl analogue [Ti(X)2(L19)] (65Ti, R ¼ tBu, X ¼ Bn) (Fig. 15)—which lacks a coordinated THF molecule—is also fluxional. A carbene signal at 188.3 ppm was reported in the 13C NMR spectrum of 65Ti. Complex 65Ti displays a trigonal bipyramidal geometry, whereas 12Ti is octahedral; this could explain the shorter Ti-CNHC distance in the former despite the presence of electron-releasing Bn ligands (2.187(3) vs. 2.200(9) Å). Kawaguchi also reported the aryl complexes [Zr(X)2(L19)(THF)] (12Zr, R ¼ tBu, X ¼ Cl) and [Zr(X)2(L19)] 65Zr (R ¼ tBu, X ¼ Bn, CH2SiMe3), and established that the

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

249

Fig. 15 Selected examples of Ti and Zr complexes bearing Kawaguchi’s ligands L1.

former is in slow equilibrium with an isomer in which the THF ligand is trans to the NHC. For 65Zr on the other hand, atropoisomerization (i.e., an equilibrium between transoid and cisoid forms) was invoked. As observed for Ti, trigonal bipyramidal 65Zr (X ¼ Bn) displays a shorter ZrdCNHC bond distance than octahedral 12Zr (2.309(3) vs. 2.35(1) Å). The homoleptic octahedral complex [Zr(L19)2] (66Zr, R ¼ tBu) was also obtained by reacting two equivalents of dianionic ligand precursor with ZrCl4.12 The ZrdCNHC bond distances in 66Zr are the longest reported with L19 (mean 2.382(2) Å). Zhang reported examples of structurally characterized Ti complexes in which Kawaguchi’s ligand is in cisoid conformation. Thus, reacting the [Ti(X)2(L19)(THF)] 12Ti (R ¼ tBu, X ¼ Br) with 0.5 equiv. of H2O in THF yielded the m-oxo complex [(TiBr(L19))2(m-O)] (67), while reacting 65Ti (X ¼ Bn, R ¼ tBu) with 1.0 equiv. of H2O in Et2O yielded [(Ti (L19)(m-O)2] 68 (Fig. 15). As a (likely) consequence of the large structural trans effect of the oxo ligand,122 the TidCNHC bond distance in 68 is considerably longer than that in 67 (2.250(5) vs. 2.208(5) Å). It appears that the cisoid conformation of the bisaryloxy-NHC ligand is forced by the more constrained O-Ti-O angle required by the trigonal bipyramidal geometry of 67 and 68, compared to the octahedral geometry of, e.g., 12Ti. However, it is not clear why the ligand does not coordinate to Ti in a mer fashion as in 65Ti. A homoleptic complex (66Ti, R ¼ tBu) was also reported by Zhang, with an average TidCNHC bond distance of 2.200 Å. Complexes 67 and 68 were moderately active in ethylene polymerization (67: 39; 68: 21 kg-PE mol-Ti−1 h−1 bar−1 at 25–35  C), while 12Ti (X ¼ Br) was considerably more active (164 kg-PE mol-Ti−1 h−1 bar−1 at 25–35  C).13 Zhang also reported additional examples of 65Ti (R ¼ tBu, Me; X ¼ OBn, Me), along with a Ti(III) complex [TiBr(L19)(THF)2] (69, R ¼ tBu) (Fig. 15) obtained by reacting 12Ti (R ¼ tBu, X ¼ Br) with LiBEt3H. Complex 69 displays a fairly short TidCNHC bond distance (2.18(1) Å);the high Lewis acidity, and the presence of easily exchanged THF co-ligands, in this d1 complex are likely responsible for it being fairly active in ethylene polymerization (82 kg-PE mol-Ti−1 h−1 bar−1 at 50  C). Interestingly, complex 12Ti (R ¼ tBu, X ¼ Cl) displayed lower activity compared to Kawaguchi’s report, but the authors used a different cocatalyst (MAO instead of modified MAO); this complex also showed some activity in alkyne cyclotrimerization.14 In 2010, Zhang reported the reaction of 65Zr (R ¼ tBu, X ¼ Cl) with the parent sodium imidazolide salt substituted by the same aryloxy ring, and obtained a hexameric Zr macrocycle connected by imidazolide-aryloxy ligands (not shown). The macrocyclic complex was found to be 2.5 times more active than 65Zr in ethylene polymerization, but of course it contains 6 times more Zr.123 For all the examples in Fig. 15, the complexes were synthesized using the free carbene route. In 2014, Martins reported the separate reactions of L19HBr precursors (R ¼ tBu, CMe2Ph) with [Zr(NMe2)4]. The resultant family of complexes, [ZrBr(NMe2)(L19) (THF)] were fluxional and no solid structure could be obtained by X-ray diffraction. DFT calculations indicated that the three possible isomers were all within 2 kcal mol−1 from each other. These complexes were not found to be active in alkene hydroamination.124

250

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

Fig. 16 Chiral bisaryloxy-NHC ligands L20 and their group 4 metal complexes.

Finally, Walter and Zi have reported interesting chiral bisaryloxy-NHC ligands (L20) based on the imidazolidin-2-ylidene (B, Fig. 1) template. Complexes [M(X)(NR00 2)(L20)(THF)] (70M, R ¼ R0 ¼ H, R00 ¼ Me, Et, X ¼ Cl, Br), [ZrCl(NEt2)(L20)(THF)] (71Zr, R ¼ H, R0 ¼ tBu) and [MCl(NR00 2)(L20)(THF)] (72M, R ¼ R0 ¼ tBu, R00 ¼ Me, Et) (Fig. 16, note the absolute configuration) were obtained following the amine elimination route, by separate reactions of the imidazolium bromide and chloride precursors with [M(NR00 2)4] (R00 ¼ Me, Et).

Table 6

MdCNHC bond distances and 13C{1H} NMR spectroscopy carbene signal chemical shift of tridentate NHC complexes 12M and 65M-72M.

Nbr

M(n)

Formula

12Ti 12Ti 12Ti 12Zr 65Ti 65Ti 65Ti 65Ti 65Ti 65Zr 65Zr 66Zr

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

[Ti(X)2(L19)(THF)] [Ti(X)2(L19)(THF)] [Ti(X)2(L19)(THF)] [Zr(X)2(L19)(THF)] [TiX2(L19)] [TiX2(L19)] [TiX2(L19)] [TiX2(L19)] [TiX2(L19)] [ZrX2(L19)] [ZrX2(L19)] [Zr(L19)2]

66Ti

Ti(IV)

67 68 69 70Ti

R, R0 , R00

Ar

X, X0 , X00

dMdCNHC (Å)

dCNHC (ppm)

References

Me Bu t Bu t Bu t Bu t Bu Me Me t Bu t Bu t Bu t Bu

– – – – – – – – – – – –

Cl Cl Br Cl Bn Me Me Bn OBn Bn CH2SiMe3 –

– 164 164.8 183.5 188.3 185.7 186.0 188.1 182.0 187.5 185.8 185.5

14 11 13 12 11 14 14 14 14 12 12 12

[Ti(L19)2]

t

Bu

–

–

182.4

13

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

[(TiBr(L19))2(m-O)] [(Ti (L19)(m-O)2] [TiBr(L19)(THF)2] [Ti(X)(NR00 2)(L20)(THF)]

t

– – – –

– – – Br

– 2.200(9) – 2.35(1) 2.187(3) 2.175(9) 2.179(9) 2.165(9) – 2.309(3) – 2.383(3) 2.380(3) 2.205(4) 2.196(4) 2.208(5) 2.250(5) 2.18(1) –

185.0 188.4 – 208.4

13 13 14 109

70Ti

Ti(IV)

[Ti(X)(NR00 2)(L20)(THF)]

–

Cl

–

209.4

109

70Zr

Zr(IV)

[Zr(X)(NR00 2)(L20)(THF)]

–

Br

–

209

109

70Zr

Zr(IV)

[Zr(X)(NR00 2)(L20)(THF)]

–

Br

2.416(2)

209.2

109

t

Bu Bu t Bu H H Me H H Me H H Me H H Et t

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

Table 6

(Continued)

Nbr

M(n)

Formula 20

70Zr

Zr(IV)

[Zr(X)(NR00 2)(L )(THF)]

70Zr

Zr(IV)

[Zr(X)(NR00 2)(L20)(THF)]

70Hf

Hf(IV)

[Hf(X)(NR00 2)(L20)(THF)]

70Hf

Hf(IV)

[Hf(X)(NR00 2)(L20)(THF)]

71Zr

Zr(IV)

[ZrCl(NEt2)(L20)(THF)]

72Ti

Ti(IV)

[TiCl(NR00 2)(L20)(THF)]

72Zr

Zr(IV)

[ZrCl(NR00 2)(L20)(THF)]

72Zr

Zr(IV)

[ZrCl(NR00 2)(L20)(THF)]

72Hf

Hf(IV)

[HfCl(NR00 2)(L20)(THF)]

251

R, R0 , R00

Ar

X, X0 , X00

dMdCNHC (Å)

dCNHC (ppm)

References

H H Me H H Et H H Me H H Me H t Bu Me t Bu t Bu Me t Bu t Bu Me t Bu t Bu Et t Bu t Bu Me

–

Cl

–

210.1

109

–

Cl

2.419(2)

209.3

109

–

Br

–

213.6

109

–

Cl

–

214.5

109

–

–

2.397(4)

212.9

109

–

–

2.252(2)

201.8

109

–

–

2.401(2)

206.2

109

–

–

2.410(4)

214.5

109

–

–

2.370(3)

216.9

109

Interestingly, complexes 70M-72M display stable transoid conformations, but the orientation of the aryloxy rings with respect to the phenyl rings depends upon the presence of tBu substituents on the former (Fig. 16). The MdCNHC bond distances and carbene signals of these imidazolidin-2-ylidene complexes were unremarkable (Table 6). After activation with isopropanol (1 equiv.), complexes 70M-72M (0.4 mol%, 70  C) efficiently catalyze the ring-opening polymerization of rac-lactide, yielding heterotactically enriched PLA (Pr  0.66) with rather narrow polydispersities (PDI  1.36). The Ti complexes 7Ti-72Ti were found to be less active than their Zr and Hf analogues, and they gave lower molecular weight PLA (5.9–24.6  103 g mol−1 vs. 14.9–40.4  103 g mol−1).89

3.07.6.1.2

Rigid bisaryloxy-NHC ligands

Although bisaryloxy-NHC ligands are well suited for group 4 metals, the presence of a methylene linker between the NHC and aryloxy rings (Section 3.07.6.1.1) sometimes makes the isolation of stereochemically stable complexes difficult; moreover, the corresponding free carbenes are prone to 1,2-alkyl migration.125

Fig. 17 Bisaryloxy NHC ligands without methylene linkers.

252

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

In 2005, Yagyu reported the synthesis of bisaryloxy-NHC ligands without a linker (Fig. 17, L21), and used the corresponding Pd complexes in the Heck coupling of styrene with iodobenzene.126 However, these imidazol-2-ylidene-derived ligands lack substituents next to oxygen, which could lead to aggregation problems with oxophilic metals—as observed later by Le Roux.127 In 2009, Bellemin-Laponnaz and Dagorne reported a closely related imidazolidin-2-ylidene derived ligand possessing tBu groups next to oxygen (L22).128 This ligand (R ¼ tBu) was involved in a number of studies on group 4 metal complexes.27,45,46,129–133 Further modifications to the original design by Yagyu included benzimidazol-2-ylidene (L23) and,29,134,135 more recently, mesoionic triazol-5-ylidene derived ligands (L24).136 Bellemin-Laponnaz and Dagorne reported a series of octahedral [M(X)(X0 )(L22)(THF)] (73M, R ¼ tBu, X, X0 ¼ Cl, OiPr, NMe2) and trigonal bipyramidal [M(X)(X0 )(L22)] (74M, R ¼ tBu, X, X0 ¼ Cl, OiPr, NMe2) heteroleptic group 4 metal complexes

Fig. 18 Some heteroleptic and homoleptic group 4 metals complexes bearing ligand L4.

(Fig. 18).27,45,46,129,130 Octahedral homoleptic complexes [M(L22)2] (75M) (Fig. 18) were also reported.45,130 While the heteroleptic complexes were obtained using either the free carbene or the amine/alcohol elimination route, the homoleptic complexes were obtained by reacting the carbene precursor [L22H3][Cl] with [M(Cl4)(THF)n] (M ¼ Ti, Zr, Hf; n ¼ 2, 0, 0) in the presence of NEt3. The carbene signals of 73M-75M ranged from 197.0 to 212.6 ppm, and Bn-containing complexes displayed consistently less shielded signals (204.3–212.6 ppm). The M-CNHC distances consistently increased from Ti to Hf to Zr for isostructural complexes (e.g., for 73M, X ¼ X0 ¼ Cl: 2.184(3); 2.333(6); 2.358(3) Å). The homoleptic complexes displayed elongated MdCNHC bond distances as a result of the increased steric crowding and electron density at the metal (75Ti: 2.333(2); 75Hf: 2.357(6); 75Zr 2.379(2) Å). The heteroleptic group 4 complexes have interesting catalytic properties: in particular, 73Zr (X ¼ Cl, X0 ¼ OiPr) catalyzes the ringopening polymerization of rac-lactide at room temperature with great efficiency, yielding highly heterotactic PLAs (Pr  0.82) with high molecular weight (9800  Mn  36400 Da) and narrow polydispersities (PDI  1.25).46 Complex 73Zr also catalyzes the copolymerization of rac-lactide with trimethylene carbonate, and other lactones such as b-butyrolactone and e-caprolactone may also be polymerized.138 The Ti analogue 73Ti (X ¼ Cl, X0 ¼ OiPr) is less active and gives atactic PLA.45 The homoleptic complexes, on the other hand, display interesting luminescence properties: the UV-vis spectra of complexes 75Zr and 75Hf are characterized by a strong absorption at 365 nm which, upon excitation, cause the complexes to luminesce at 485 (F ¼ 0.08) and 534 nm (F ¼ 0.12). Additionally, electrochemical oxidation of 75M occurred at the ligand, since the metals are d0; in the case of 75Zr four one-electron events—one for each aryloxy ring—were observed, three of them being reversible. These results highlight the redox non-innocent nature of L22.130 With the same ligand as Bellemin-Laponnaz and Dagorne (L22, R ¼ tBu) (Fig. 17), Le Roux reported the use of 73Ti (X ¼ Cl, 0 X ¼ OiPr; X ¼ Cl, X0 ¼ OSi(OtBu)3; X ¼ X0 ¼ N3), 73Zr (X ¼ X0 ¼ Cl; X ¼ Cl, X0 ¼ OiPr; X ¼ X0 ¼ OiPr), 73Hf (X ¼ Cl, X0 ¼ Cl; X ¼ OiPr, X0 ¼ Cl) and 74Ti (X ¼ X0 ¼ Cl; X ¼ X0 ¼ OiPr; X ¼ X0 ¼ OBn; X ¼ X0 ¼ OAc; X ¼ OiPr, X0 ¼ OSi(OtBu)3), together with a cocatalyst ([PPN][Cl]; PPN ¼ (triphenylphosphine)iminium) in the copolymerization of cyclohexene oxide with CO2.78,132,133,137 Overall, turnover numbers (275  TON  900) and frequencies (11  TOF  39) were slightly inferior to those of state-of-the-art Ti(IV) catalysts.143 Interestingly, 73Ti (X ¼ X0 N3) undergoes [2 + 2] addition with dimethyl acetylenedicarboxylate (DMAD) at the azide ligand to give triazolide-coordinated 74Ti (X ¼ X0 ¼ (k1-N1)-N3C2(CO2Me)2).133 By contrast, the catalytic activity of the anionic Hf complex [HfCl3(L22)][PPN] 76 (Fig. 18) is much higher (TON up to 1500 and TOF up to 500), and this complex can also catalyze the copolymerization of other epoxides (e.g., propylene oxide) with CO2.78 Interestingly, 76 is the product of the reaction of 73Hf (X ¼ X0 ¼ Cl) with [PPN][Cl], which are both components of the weakly active catalytic mixtures described above: therefore, it is unlikely that 76 forms in situ under the catalytic conditions—it has to be prepared separately. Both the HfdCNHC distance (2.337(4) Å) and the carbene signal (202.2 ppm) in the 13C NMR spectrum of 76 were similar to those of 73Hf. Le Roux reported [Zr(L)2] (750 Zr, L ¼ L21, R ¼ tBu; L ¼ L23, R ¼ tBu) which are analogues of the homoleptic complexes reported by Dagorne and Bellemin-Laponnaz. The ZrdCNHC bond distances are longer in the case of the benzimidazol-2-ylidene ligand (75Zr: mean 2.383(2) Å; 750 Zr(L21): mean 2.351(2) Å; 750 Zr(L23): mean 2.397(3) Å), and the carbene signals also reflect differences in ligand backbones (75Zr: 200.8 ppm; 750 Zr(L21): 186.3 ppm; 750 Zr(L23): 198.0 ppm); complexes 750 Zr were not tested in catalysis.137

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

253

Fig. 19 Ligand-dependent aggregation behavior of Ti complexes bearing ligand L22 (R ¼ H).

Le Roux also investigated the effect of removing the tBu groups in L22 (R ¼ H) (Fig. 17) on the coordination chemistry and catalytic activity of 74Ti. Unsurprisingly, trigonal bipyramidal complexes such as 74Ti (X ¼ Cl; X0 ¼ OiPr) displayed complex aggregation behavior in solution. However, it was possible to isolate well-defined dimeric (74Ti)2 (X ¼ X0 ¼ OiPr) or monomeric

Scheme 9 Synthesis of Ti complexes of bisaryloxy mesoionic carbene L6.

74Ti (X ¼ OiPr; X0 BHT) complexes, depending on the steric bulk of X and X0 (Fig. 19). As a consequence of the presence of additional bulky anionic O-donor ligands, the TidCNHC bond distance in these two complexes were quite long ((74Ti)2: mean 2.221(1) Å; 74Ti: 2.212(3) Å). Their catalytic activity in the copolymerization of cyclohexene with CO2 was found to be very low.127 Very recently, Hohloch reported a series of Ti complexes based on L24.136 The triazolium precursor of L24 can be synthesized on a scale of up to 7 g following a multistep procedure; the key step is a Cu-catalyzed alkyne azide [2 + 2]-cycloaddition, as shown in Scheme 9. Complex formation may be achieved either by the free carbene route or the alcohol elimination route. Interestingly, the reaction of [L24H][Cl] with [TiCl(OiPr)3] does not go to completion and yields zwitterionic complex [TiCl2(OiPr)(L24H)] (77) (Fig. 20) (see also Refs. 45 and 138), which may be converted to [TiCl(OiPr)(L24)] (78) by reaction with additional base (KHMDS).136 Formation of the Ti-imido complex [Ti(NtBu)(L24)(Py)] (79) may be achieved by reaction of the free carbene with Mountford’s complex [TiCl2(NtBu)(Py)3].144 Performing the deprotonation of [L24H][Cl] with triethylamine instead of LDA gave the m-oxo dimeric complex [TiCl(L24)(m-O)TiCl(L24)(THF)] (80). Traces of water present in the trimethylamine

254

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

Fig. 20 Ti complexes bearing bisaryloxy mesoionic carbene L6.

are thought to hydrolyze the Ti-imido moiety. Remarkably, complex 80 displays two different TidO bond distances (1.905(3) and 1.728 Å), which led the authors to propose a depiction using the arrow formalism (i.e., 80 is a donor-acceptor adduct). Finally, the homoleptic complex [Ti(L24)2] (81) may be obtained by reacting [L6H][Cl] with [TiCl4(THF)2] in the presence of triethylamine, this time without forming m-oxo species. Overall, compared to imidazole-2-ylidene-derived complexes, 78–81 display relatively short TidCNHC bonds (2.11–2.13 Å) and rather shielded carbene signals in their 13C NMR spectra (174.1–181.8 ppm). In the above examples, and despite the presence of potentially reactive anionic ligands (e.g., NtBu, N3), the coordinated NHC remains chemically innocent. However, in the presence of highly nucleophilic ligands (e.g., benzyl, methyl, phenyl), several authors have shown that the C2 carbon of the NHC might undergo nucleophilic attack.27–29,131,133 Thus, Bellemin-Laponnaz and Dagorne found that the reaction of [L22H][Cl] with [Zr(Bn)4] in THF yielded complex [ZrCl(L41) (THF)] 82, in which L41 adopts a k3-N,C,N mode (Scheme 10). The same reaction performed in toluene yielded the expected Zr-benzyl complex 74Zr (R ¼ tBu, X ¼ Cl, X0 ¼ Bn). However, when dissolved in toluene in the presence of THF, complex 74Zr evolved towards 82.27 Complex 82 features a strongly shielded 13C{1H} NMR signal (100.8 ppm) corresponding to the C2 atom of the NHC ligand where benzyl migration occurred. The ZrdC2 distance (2.174(3) Å) is much shorter than the mean Zr-CNHC distance (2.372(6) Å for the complexes in this review). The ZrdN distances (mean 2.309(2) Å) are within the sum of covalent radii for both atoms (2.46  0.08 Å).145 It was later found that Ti and Hf behaved similarly to Zr in the chemistry described above, although in the case of Ti benzyl migration also took place in toluene. The authors also showed that methyl and phenyl ligands

Scheme 10 Benzyl migration in Zr complexes of L4.

could migrate from Zr to the NHC.131 Mechanistic investigations (kinetics, DFT calculations) indicate that the migration is triggered by an event which increases the coordination number of 74Zr or its analogues (Zr, Hf: THF coordination; Ti: m-Cl dimer formation).131 The driving force of the reaction is likely the formation of the k3-N,C,N coordinated imidazolidinide (see Section 3.07.5.2). Le Roux also showed that reduction of 74Ti (R ¼ tBu, X ¼ X0 ¼ Cl) with LiBEt3H yielded a 2-H migration product (not shown).133 Remarkably, Despagnet-Ayoub, Labinger and Bercaw showed that benzyl migration can be reversible. Thus, reacting L5coordinated Zr complex [Zr(Bn)2(L23)] (83) with PMe3 yielded benzyl migration product [Zr(Bn)(PMe3)(L42)] (84), which contains a coordinated phosphine (Scheme 11). Complex 83 has very similar features to 74Zr (R ¼ tBu, X ¼ X0 ¼ Bn), i.e., ZrdCNHC bond distance (2.3358(17) Å) and carbene signal (199.9 ppm). On the other hand, the ZrdC2 bond distance (2.1678(17) Å), the ZrdN bond distances (mean 2.4309(11) Å) and C2 signal in the 13C NMR spectrum (159.2 ppm) of 84 differ notably from those in 82; this could be due to the presence of the bulky, electron-rich phosphine. Addition of [Ni(COD)2] to 84 removed the phosphine

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

255

Scheme 11 Reversible benzyl migration in a Zr complex bound to L5.

and restored complex 83. The authors also performed the reaction of 83 with ammonia. At low temperature (-60  C), the benzyl migration product [Zr(Bn)(NH3)(L42)] (85) is observed by 13C NMR spectroscopy as a mixture of conformers (C2 signals: 159.8/156.8 ppm). Evolution of the reaction mixture at room temperature gives complex [Zr(NH3)(m-NH2)(L42)]2 (86) following toluene elimination.29 Compared to 84, the dimeric complex 86 features an even more elongated ZrdN (mean 2.447(2) Å) and ZrdC2 bond distance (mean 2.216(3) Å); however, the C2 signal in the 13C NMR spectrum (159.8 ppm) is almost identical. The studies described above have shown that saturating the coordination sphere of the group 4 metal causes benzyl migration. Logically, removal of a ligand from already coordinatively unsaturated trigonal bipyramidal complexes yields stable cationic complexes. Bellemin-Laponnaz and Dagorne separately reacted the Zr- and Hf-dibenzyl complexes 74M (R ¼ tBu, X ¼ X0 ¼ Bn) with [HNMe2Ph][B(C6F5)4] and obtained the amine-stabilized cationic complexes [M(Bn)(L22)(NMe2Ph)][B(C6F5)4] (87M)

Fig. 21 Stable Zr- and Hf-benzyl complexes bound to L22 or L23.

256

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

(Fig. 21). Compared to the dibenzyl precursors 74M, cationic complexes 87M display slightly more shielded carbene signals in their 13 C NMR spectra (87Zr: Dd ¼ 2.8 ppm; 87Hf: Dd ¼ 5.1 ppm). Complex 89Zr was found to be a selective hexene oligomerization catalyst, yielding a 77/16/7 mixture of tri/tetra/pentamers with vinylene end groups. On the other hand, in situ cation generation using 74Zr with [Ph3C][B(C6F5)4] (presumably giving an amine-free analogue of 87Zr) gave a much less selective catalyst, and Hf complex 87Hf was almost completely inactive.129 Despagnet-Ayoub, Labinger and Bercaw reported a series of in situ-generated cationic Zr complexes [Zr(Bn)(L23)(PMe3)n] [B(C6F5)4] (n ¼ 0: 88; n ¼ 1: 89; n ¼ 2: 90) (Fig. 21) via benzyl ligand abstraction from complex 83, using the trityl salt [Ph3C [B(C6F5)4] as an abstractor. Complex 90 features a somewhat shorter ZrdCNHC bond distance (2.3235(16) Å) compared to 83, but the carbene signal in the 13C NMR spectrum of 90 (199.6 ppm) is practically identical to that of 83 (see above). The authors showed that while phosphine-free 88 is a somewhat sluggish and poorly selective hexene oligomerization catalyst, the addition of one equivalent of PMe3 to generate 89 yields a more active polymerization catalyst. Moreover, the bis-phosphine adduct 90 is completely inactive in either oligomerization or polymerization.134 A detailed experimental study followed (using kinetics, Table 7

MdCNHC bond distances and 13C{1H} NMR spectroscopy carbene signal chemical shift of tridentate NHC complexes 73M-90.

Nbr

M(n)

Formula

R, R0 , R00

Ar

X, X0 , X00

dMdCNHC (Å)

dCNHC (ppm)

References

Cl Cl Cl OiPr Cl NMe2 Cl OSi(OtBu)3 N3 N3 Cl OiPr Cl OiPr Cl Cl Cl OiPr Cl NMe2 OiPr OiPr Cl Cl Bn Cl Cl OiPr Cl Cl OiPr OiPr Cl OiPr OBn OBn OiPr OSi(OtBu)3 OAc OAc N3C2(CO2Me)2 N3C2(CO2Me)2 OiPr BHT OiPr OiPr

2.184(3)

197.4

45

2.166(3)

198.6

45

–

203.6

45

2.165(2)

198.6

133

2.183(3)

198.4

133

–

197.6

127

2.175(8)

199.6

127

2.358(3)

197.0

27

2.360(3)

200.0

46

–

202.9

129

2.398(4)

204.0

137

2.333(6)

200.5

129

–

205.2

131

2.333(3)

204.4

78

2.160(3)

200.0

45

2.212(5)

197.9

45

–

198.4

45

2.215(8)

197.9

133

2.210(2)

200.3

133

2.148(4)

197.8

133

–

197.0

133

2.212(3)

199.3

127

2.231(2) 2.210(2)

199.6

127

73Ti

Ti(IV)

[Ti(X)(X0 )(L )(THF)]

t

Bu

–

73Ti

Ti(IV)

[Ti(X)(X0 )(L22)(THF)]

t

Bu

–

73Ti

Ti(IV)

[Ti(X)(X0 )(L22)(THF)]

t

Bu

–

73Ti

Ti(IV)

[Ti(X)(X0 )(L22)(THF)]

t

Bu

–

73Ti

Ti(IV)

[Ti(X)(X0 )(L22)(THF)]

t

Bu

–

73Ti

Ti(IV)

[Ti(X)(X0 )(L22)(THF)]

H

–

73Ti

Ti(IV)

[Ti(X)(X0 )(L22)(MeCN)]

H

–

73Zr

Zr(IV)

[Zr(X)(X0 )(L22)(THF)]

t

Bu

–

73Zr

Zr(IV)

[Zr(X)(X0 )(L22)(THF)]

t

Bu

–

73Zr

Zr(IV)

[Zr(X)(X0 )(L22)(THF)]

t

Bu

–

73Zr

Zr(IV)

[Zr(X)(X0 )(L22)(THF)]

t

Bu

–

73Hf

Hf(IV)

[Hf(X)(X0 )(L22)(THF)]

t

Bu

–

73Hf

Hf(IV)

[Hf(X)(X0 )(L22)(THF)]a

t

Bu

–

73Hf

Hf(IV)

[Hf(X)(X0 )(L22)(THF)]

t

Bu

–

74Ti

Ti(IV)

[Ti(X)(X0 )(L22)]

t

Bu

–

74Ti

Ti(IV)

[Ti(X)(X0 )(L22)]

t

Bu

–

74Ti

Ti(IV)

[Ti(X)(X0 )(L22)]

t

Bu

–

74Ti

Ti(IV)

[Ti(X)(X0 )(L22)]

t

Bu

–

74Ti

Ti(IV)

[Ti(X)(X0 )(L22)]

t

Bu

–

74Ti

Ti(IV)

[Ti(X)(X0 )(L22)]

t

Bu

–

74Ti

Ti(IV)

[Ti(X)(X0 )(L22)]

t

Bu

–

74Ti

Ti(IV)

[Ti(X)(X0 )(L22)]

H

–

(74Ti)2

Ti(IV)

[Ti(X)(X0 )(L22)]2

H

–

22

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

Table 7

(Continued)

Nbr

M(n)

Formula

R, R0 , R00

Ar

X, X0 , X00

dMdCNHC (Å)

dCNHC (ppm)

References

2.333(1)

205.8

129

–

204.3

27,138

–

201.8

138

2.308(3)

212.6

129

–

209.1

131

2.233(2) 2.222(2) 2.379(2) 2.387(3) 2.352(3) 2.350(3) 2.393(3) 2.401(5) 2.357(6) 2.351(6) 2.337(4) – 2.112(5) 2.111(5) 2.130(5) – 2.174(3)

199.9

45

200.8

130

186.3

137

198.0

137

206.1

130

202.2 174.1 181.8 175.8

78 136 136 136

174.1 100.8

136 27

199.9 159.2 159.8 159.8

134 29 29 29

203.0

129

74Zr

Zr(IV)

[Zr(X)(X0 )(L )]

t

Bu

–

74Zr

Zr(IV)

[Zr(X)(X0 )(L22)]

t

Bu

–

74Zr

Zr(IV)

[Zr(X)(X0 )(L22)]

t

Bu

–

74Hf

Hf(IV)

[Hf(X)(X0 )(L22)]

t

Bu

–

74Hf

Hf(IV)

[Hf(X)(X0 )(L22)]

t

Bu

–

75Ti

Ti(IV)

[Ti(L22)2]

t

Bu

–

Bn Bn Cl Bn OiPr OiPr Bn Bn Cl Bn –

75Zr

Zr(IV)

[Zr(L22)2]

t

Bu

–

–

750 Zr

Zr(IV)

[Zr(L21)2]

t

Bu

–

–

750 Zr

Zr(IV)

[Zr(L23)2]

–

–

–

75Hf

Hf(IV)

[Hf(L22)2]

t

Bu

–

–

76 78 79 80

Hf(IV) Ti(IV) Ti(IV) Ti(IV)

[HfCl3(L22)][PPN] [TiCl(OiPr)(L24)] [Ti(NtBu)(L24)(Py)] [TiCl(L24)(m-O)TiCl(L24)(THF)]

t

Bu – – –

– – – –

– – – –

81 82

Ti(IV) Zr(IV)

[Ti(L24)2] [ZrCl(L41)(THF)]a

– –

– –

83 84 85 86

Zr(IV) Zr(IV) Zr(IV) Zr(IV)

[Zr(Bn)2(L23)] [Zr(Bn)(PMe3)(L42)]a [Zr(Bn)(NH3)(L42)]a [Zr(NH3)(m-NH2)(L42)]2a

– Bu t Bu – – – –

– – – –

– – – –

87Zr

Zr(IV)

t

Bu

–

–

87Hf

Hf(IV)

t

Bu

–

–

–

207.5

129

88 89

Zr(IV) Zr(IV)

– –

– –

– –

– –

196.0 196.4

134 134

90

Zr(IV)

[Zr(Bn)(L22)(NMe2Ph)] [B(C6F5)4] [Hf(Bn)(L22)(NMe2Ph)] [B(C6F5)4] [Zr(Bn)(L23)][B(C6F5)4] [Zr(Bn)(L23)(PMe3)] [B(C6F5)4] [Zr(Bn)(L23)(PMe3)2] [B(C6F5)4]

2.3358(17) 2.1678(17) – 2.214(4) 2.218(4) –

–

–

–

2.3235(16)

199.6

134

22

257

t

Benzyl migration product.

a

stoichiometric reactivity and chain-end analysis), which revealed that 90 can rapidly insert one unit of hexene, but that the product is strongly stabilized by the aryl group of the original Bn ligand, preventing further rapid insertion of hexene. This stabilizing effect is absent in the mono-PMe3 adduct, 89, which therefore polymerizes hexene, whilst the bis-PMe3 adduct 90 is too coordinatively saturated to insert hexene (Table 7).135

3.07.6.2

N,O-functionalized ligands

Unsymmetrical tridentate NHC ligands bearing N- and O-donor substituents are quite rare in group 4 chemistry. Attempts at coordinating N,O analogues of L19-L23 have resulted in unexpected reaction outcomes, probably because of the presence of a nucleophilic nitrogen in the vicinity of the electrophilic C2 carbon of the imidazolium/group 4 metal-coordinated NHC.28,139,140 Despagnet-Ayoub, Labinger and Bercaw reported Zr complexes [Zr(L25)2] (91) and [Zr(Bn)2(L43)] (92) (Fig. 22) in 2013.28,139 Both were obtained from the same imidazoium precursor, which is prone to rearranging into benzimidazolium. Complex 91 features the fac-coordinated rearranged ligand backbone L25, while complex 92 retains the original mer-coordinated imidazolebased backbone. However, benzyl migration occurred in this case, yielding L43. The X-ray structure of 91 only allowed the

258

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

Fig. 22 Ti and Zr complexes of N,O-functionalized NHCs.

connectivity between atoms to be established; the carbene signal was found at 200.1 ppm in the 13C NMR spectrum, as expected for a benzimidazol-2-ylidene Zr complex. Complex 92 features elongated ZrdN bond distances (mean 2.418(2) Å), and the ZrdC2 distance ( 2.617 Å) is superior to the sum of covalent radii (2.51  0.08 Å).145 Surprisingly, the signal of the C2 atom of the neutral imidazolidine ring in the 13C NMR spectrum of 92 (159.6 ppm) is very similar to those observed for 84–86 (159.2–159.8 ppm). In 2016, Zhang reported the synthesis of a series of secondary amide-functionalized imidazolium salts with an aim to generate

Table 8

MdCNHC bond distances and 13C{1H} NMR spectroscopy carbene signal chemical shift of tridentate NHC complexes 91–93.

Nbr

M(n)

Formula

R, R0 , R00

Ar

X, X0 , X00

dMdCNHC (Å)

dCNHC (ppm)

References

91 92 93

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

[Zr(L25)2] [Zr(Bn)2(L43)]a [TiCl2(L43)]

– – –

– – –

– – –

– 2.617 –

200.1 159.6 (165.4)

139 28 140

a

Benzyl migration product.

complexes of N,O-functionalized NHCs. Several rearrangements were observed, and complex [TiCl2(L26)] (93) was obtained after metalation of one of the rearranged salts. The geometry of 93 was derived from X-ray diffraction analysis of poor quality crystals, and the assignment of the carbene signal in the 13C NMR spectrum (165.4 ppm) is only tentative. Complex 93 is a moderately active ethylene polymerization catalyst (114 kg-PE mol-Ti−1 h−1 bar−1 at 50  C) (Table 8).140

3.07.6.3

N,N-functionalized ligands

Fryzuk reported a series of trigonal bipyramidal Zr and Hf complexes bearing a bisamide-functionalized NHC [M(X)2(L27)] (94M, X ¼ Cl, CH3, Bn, Et, iBu; Ar ¼ Tol, Mes) (Fig. 23). These complexes were obtained by the amine elimination route starting from [M(NR2)4] (R ¼ Me, Et); the resulting products could be converted to dichloro, then dialkyl complexes by reaction with Me3SiCl and the corresponding alkyl Grignard reagents.51,141 The carbene signals in the 13C NMR spectra of 94Zr occur around 190 ppm, whilst 94Hf displays slightly less shielded signals, around 195 ppm. The two structurally characterized examples display unexceptional MdCNHC bond distances (94Zr, Ar ¼ Tol, X ¼ CH2SiMe3: 2.415(3) Å; 94Zr, Ar ¼ Mes, X ¼ iBu: 2.385(3) Å). Complex 94Hf (Ar ¼ Mes, X ¼ Et) activates the CdH bond of one of the Mes substituents to afford complex [Hf(Et)(L28)] (95). The reaction of

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

259

Fig. 23 Zr and Hf complexes bearing a bisamido-functionalized NHC.

Table 9

MdCNHC bond distances and 13C{1H} NMR spectroscopy carbene signal chemical shift of tridentate NHC complexes 94–97.

Nbr

M(n)

Formula

R, R0 , R00

Ar

X, X0 , X00

dMdCNHC (Å)

dCNHC (ppm)

References

94Zr 94Zr 94Zr 94Zr 94Hf 94Zr 94Hf 94Zr 94Hf 94Zr 94Hf 94Hf 94Hf 95 96 97

Zr(IV) Zr(IV) Zr(IV) Zr(IV) Hf(IV) Zr(IV) Hf(IV) Zr(IV) Hf(IV) Zr(IV) Hf(IV) Hf(IV) Hf(IV) Hf(IV) Hf(IV) Hf(IV)

[Zr(X)2(L27)] [Zr(X)2(L27)] [Zr(X)2(L27)] [Zr(X)2(L27)] [Zr(X)2(L27)] [Zr(X)2(L27)] [Hf(X)2(L27)] [Zr(X)2(L27)] [Hf(X)2(L27)] [Zr(X)2(L27)] [Hf(X)2(L27)] [Hf(X)2(L27)] [Hf(X)2(L27)] [Hf(Et)(L28)] [Hf(NXylCMe)(Me)(L27)] [Hf(O(iBu)CC(iBu)O)(L27)]2

– – – – – – – – – – – – – – – –

Tol Tol Tol Mes Mes Mes Mes Mes Mes Mes Mes Mes Mes Mes Mes Mes

Cl NEt2 CH2SiMe3 NMe2 NMe2 Cl Cl Me Me Bn Bn i Bu Et – – –

– – 2.415(3) – – – – – – – – 2.385(3) – – 2.387(2) 2.387(9)

– 188.8 186.8 190.9 195.7 192.3 198.2 189.8 196.1 190.1 196.5 194.9 196.4 197.3 197.5 195.4

51 51 51 141 141 141 141 141 141 141 141 141 141 141 141 141

complex 94Hf (Ar ¼ Mes, X ¼ Me, iBu) with CO or xylyl isocyanide initially affords products corresponding to the insertion of the small molecule into the Zr-alkyl bond, which then evolve depending on the reaction conditions. Complexes 96 and 97 could be characterized by single crystal X-ray diffraction; the different crystal structures indicate that the bisamido-NHC ligand backbone is conformationnally flexible, despite the fact that the HfdCNHC bond distances almost do not vary (96: 2.387(2) Å; 97: 2.387(9) Å) (Table 9).

Fig. 24 Ti, Zr and Y complexes bearing an amido-bis NHC ligand.

260

3.07.6.4

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

N-C-functionalized ligands

Gibson reported the pyridine-bis NHC complex [TiCl3(L29)] (14) (Fig. 5) in 2004. This Ti(III) complex displays very high activity in the polymerization of ethylene (up to 791 kg-PE mol-Ti−1 h−1 bar−1 at ambient temperature), which makes it the most efficient Ti-NHC catalyst reported in the literature.52 Danopoulos later disclosed a closely related Ti(IV)-imido complex [TiCl2(NtBu)(L29)] (98) (Fig. 24) bearing the same NHC.142 The imido and pyridine ligands are trans to each other. Complex 98 features unexceptional

Table 10

MdCNHC bond distances and 13C{1H} NMR spectroscopy carbene signal chemical shift of tridentate NHC complexes 98–99.

Nbr

M(n)

Formula

R, R0 , R00

Ar

X, X0 , X00

dMdCNHC (Å)

dCNHC (ppm)

References

98

Ti(IV)

[TiCl2(NtBu)(L29)]

–

–

–

200.7

142

99 99

Ti(IV) Zr(IV)

[TiCl3(L32)] [ZrCl3(L32)]

– –

– –

– –

2.281(6) 2.286(6) – –

– 186.2

82 82

TidCNHC bond distances (mean 2.284(4) Å); on the other hand, the carbene signal (200.7 ppm) is quite deshielded for an imidazole-2-ylidene Ti-NHC complex. In 2007, Arnold reported Ti and Zr complexes [MCl3(L32)] (99M) (Fig. 24), which bear an amido-bis NHC ligand. These complexes were obtained by the amine elimination route starting from [M(NMe2)4] and the ammonium-bisimidazolium trischloride salt. Perhaps because of the high chloride content of the ligand precursor, 99M do not incorporate dimethylamine. Complex 99Zr features an unexceptional carbene chemical sift of 186.2 ppm (the carbene signal of 99Ti was not reported). Whilst reaction of the Y(III) analogue of 99M with PPh3PO gave complex [YCl2(L32)(OPPh3)] (100), no reaction was observed with complex 99Zr which, remarkably, is also stable in neat pyridine (Table 10).

3.07.6.5

CCC pincer NHC ligands

Hollis reported a series of CCC pincer-NHC complexes of group 4 metals [Hf(X)(X0 )(X00 )(L30)] (101M: R ¼ alkyl, X, X0 , X00 ¼ Cl, Br, I, NMe2).15,31–37,146–150 Complexes 101Zr display carbene signals in their 13C NMR spectra around 190 ppm, and these values are shifted slightly more downfield for 101Hf (around 200 ppm). The structurally characterized Zr derivatives (R ¼ Bu, X ¼ Br, X0 ¼ X00 ¼ NMe2: mean 2.396(5) Å; R ¼ Bu, X ¼ I, X0 ¼ X00 ¼ NMe2: mean 2.416(2) Å) feature shorter MdCNHC bond distances than their Hf counterpart (R ¼ Bu, X ¼ X0 ¼ I, X00 ¼ NMe2: mean 2.35(2) Å). A single analogue of 101Zr bearing an imidazolidin-2Table 11

Compiled intramolecular olefin hydroamination results with CCC pincer NHC complexes 101M (R ¼ Bu).

Catalyst (M, X, X0 , X00 )

Temperature ( C)

Time (h)

Conversion (%)

References

Ti, Cl, NMe2, NMe2 Ti, I, NMe2, NMe2 Zr, Cl, Cl, NMe2 Zr, Br, Br, NMe2 Zr, I, I, NMe2 Zr, Br,NMe2, NMe2 Zr, I, NMe2, NMe2 Hf, Cl, Cl, NMe2 Hf, Br, Br, NMe2 Hf, I, I, NMe2 Zr, I, I, NMe2 Zr, Br, NMe2, NMe2 Zr, I, NMe2, NMe2

160 160 160 160 160 160 160 160 160 160 80 40 40

96 96 18 10 50 min 1.5 50 min 23 23 4.5 18 93 96

67 91 >98 >98 >98 100 100 8 64 > 98 0 23 65

15 15 148 148 146 149 149 147,148 147,148 147,148 149 149 149

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

261

ylidene CCC pincer NHC ligand [Zr(NMe2)3(L31)] (1010 Zr, R ¼ Et, not shown) was also synthesized. The carbene signal in the 13C NMR spectrum of 1010 Zr was observed at 211.8 ppm. A number of derivatives were tested in the intramolecular hydroamination of olefins,15,146–148,150 and complex 101Zr (R ¼ Bu, X ¼ I; X0 ¼ X00 ¼ NMe2) emerged as the most active catalyst (Table 11). The order of activity is Zr > Hf > Ti and I > Br > Cl. Complexes bearing two dimethylamido ligands are more active than their mono-amido counterparts. Catalyst loadings of 1 mol%

Fig. 25 Transmetallation of Zr-NHC species with late transition metals.

Table 12

MdCNHC bond distances and 13C{1H} NMR spectroscopy carbene signal chemical shift of CCC pincer NHC complexes 101–102.

Nbr

M(n)

Formula 0

00

30

R, R0 , R00

Ar

X, X0 , X00

dMdCNHC (Å)

dCNHC (ppm)

References

–

199.6

148

–

198.7

148

2.429(3) 2.402(3) 2.384(9) 2.407(4)

189.2

149

193.0

149

–

193.6

150

–

193.2

150

–

196.2

150

–

189.3

150

– 2.34(3) 2.35(3)

211.8 201

31 147

2.2024(3) 1.971(4) 1.965(4) 2.063(4) 2.061(4)

172.8 –

33 35

173.7 150.8

31

101Hf

Hf(IV)

[Hf(X)(X )(X )(L )]

Bu

–

101Hf

Hf(IV)

[Hf(X)(X0 )(X00 )(L30)]

Bu

–

101Zr

Zr(IV)

[Zr(X)(X0 )(X00 )(L30)]

Bu

–

101Zr

Zr(IV)

[Zr(X)(X0 )(X00 )(L30)]

Bu

–

101Zr

Zr(IV)

[Zr(X)(X0 )(X00 )(L30)]

Hex

–

101Zr

Zr(IV)

[Zr(X)(X0 )(X00 )(L30)]

Undec

–

101Zr

Zr(IV)

[Zr(X)(X0 )(X00 )(L30)]

Undec

–

101Zr

Zr(IV)

[Zr(X)(X0 )(X00 )(L30)]

Bu

–

1010 Zr 101Hf

Zr(IV) Hf(IV)

[Zr(NMe2)3(L30)] [Hf(X)(X0 )(X00 )(L30)]

Bu Bu

– –

102Pt 102Co

Pt(II) Co(III)

[PtCl(L30)] [CoI(acac)(L30)]

CH2SiMe3 Bu

– –

Br Br NMe2 Cl Cl NMe2 I NMe2 NMe2 Br NMe2 NMe2 I I NMe2 I I NMe2 I I I I NMe2 NMe2 – I I NMe2 – –

102Rh

Rh(III)

[RhI(m-I)(L30)]2

Et

–

–

262

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

were achieved with Zr and Hf complexes 101M by increasing reaction times,146,147 which shows that these complexes are robust—if not first-in-class as far as activity is concerned. Finally, an experimental and computational (DFT) study indicated that the active catalyst is a metal imido species, which explains why secondary amines are not cyclized.149 However, an internal alkyne could be efficiently cyclized by complexes 101Zr (R ¼ Bu, X ¼ I or Br; X0 ¼ X00 ¼ NMe2).150 Another interesting application of complexes 101Zr is their use as in situ transmetallating agents to install the CCC-pincer NHC onto late transition metals to yield NHC complexes with interesting properties (e.g., luminescence, catalysis, etc.). Zhang,33 #3235; Hollis37, 2015 #3346, #3316; Howell34, 2014 #3229; Reilly35, 2016 #3267; Denny,36 2018 #3307. This useful methodology (Fig. 25) also highlights that the weaker affinity of group 4 metals with NHCs relative to other transition metals is not as important in dissociation events as in transmetallation reactions (Table 12).

3.07.7

Miscellaneous

Fig. 26 Some unusual Ti and Zr NHC complexes.

Aside from the “classical” examples of NHC group 4 metals complexes, a handful of more “exotic” species have been reported, which are worth mentioning. Siebert reported Ti and Zr complexes with BH3-protected imidazoles as a ligand; complexes [Cp2M(L18)] (103M, R ¼ BH3) (Fig. 26) are diamagnetic and were characterized by multinuclear NMR spectroscopy and single-crystal X-ray diffraction.151 The M-CNHC distances are within the normal range for NHC complexes of group 4 metals (103Ti: mean 2.213(4) Å; 103Zr: mean 2.341 (1) Å), and so is the 13C NMR signal of the CNHC carbons in 103Zr (173.6 ppm; the corresponding signals could not be observed for 103Ti).9 However, the structure of 103Zr is desymmetrized by the presence of a Zr-(H-B) agostic interaction, and as a result the ZrdCNHC bond distances are markedly different (2.299(1) and 2.383(1) Å). In addition, a Ti(III) NHC complex [Cp2Ti(L2)] (104,

Fig. 27 N-bound Ti free NHCs and related complexes.

R ¼ BH3) was also reported by Siebert, although it could only be characterized by 11B NMR spectroscopy and ESI mass spectrometry.152 Tilley reported the separate reactions of the M(II) precursors [Ti(Cp)2(btmsa)] and [Zr(Cp)2(CO)2] with an Ir-NHC complex bearing oxygens at the C4 and C5 position of the imidazole-2-ylidene ring. The NHC thus undergoes a two-electron reduction by the low-valent metals, and the Ir(I) centers in the resulting complexes [Cp2M(L9)IrCl(COD)] (105M, COD ¼ 1,5-cyclooctadiene) exhibit significantly enriched electron density. Complexes 105M were considerably (Ti: 37 times; Zr: 20 times) more active in alcohol hydrosilylation compared to the Ir-NHC precursor complex.153

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

263

The use of [Ti(Cp)2(btmsa)] was also explored by Rosenthal and coworkers, who reported its reaction with dicyclohexylcarbodiimide (DCC). Complex [Cp2Ti(N(Cy)CN(Cy))Ti(Cp2)] 106 (Fig. 27) may be envisaged as homobimetallic Ti(III) complex of a ditopic 4-membered NHC.154 The TidCNHC bond length in 106 is 2.199(4) Å, which is within the normal range of TidNHC bond lengths.9 The intermediacy of an N-bound Ti(IV) free NHC was postulated, but the authors were themselves critical of the description of this species as an NHC. Table 13

MdCNHC bond distances and 13C{1H} NMR spectroscopy carbene signal chemical shift of NHC complexes 103–109.

Nbr

M(n)

Formula 18

R, R0 , R00

Ar

X, X0 , X00

dMdCNHC (Å)

dCNHC (ppm)

References

2.210(5) 2.215(5) 2.383(1) 2.299(1) – 2.056(4) 2.068(3) 2.199(4) – – 2.31(3) 2.31(2)

–

151

173.6

151

– 192.0 183.3 – 130.0a 162.9b –

152 153 153 154 155 155 155

103Ti

Ti(IV)

[Cp2Ti(L )]

BH3

–

–

103Zr

Zr(IV)

[Cp2Zr(L18)]

BH3

–

–

104 105Ti 105Zr 106 107 108 109

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

[Cp2Ti(L2)] [Cp2Ti(L9)IrCl(COD)] [Cp2Zr(L9)IrCl(COD)] [Cp2Ti(N(Cy)CN(Cy))Ti(Cp2)] [Cp2Ti(N(tBu)CN(tBu))] [Cp2Ti(N(tBu)C(S)N(tBu))] [Cp2Ti(N(Cy)C(N(Cy))Ti(Cp)2]

BH3 – – – – – –

– – – – – – –

– – – – – – –

Free NHC signal. C¼S signal.

a

b

However, Tonks, Goodpaster and Copéret recently reported complex [Cp2Ti(N(tBu)CN(tBu))] (107) using the same synthetic route as Rosenthal;155 various high-level computational tools (including CASSCF and QTAIM) suggest that 107 may indeed be described as a free NHC—although the biradical Ti(III)/C
resonance form is also important, since it may help in understanding the formation of 106. Additionally, the carbene signal in the 13C NMR spectrum is strongly shielded at 130.0 ppm. The reaction of 107 with elemental sulfur gave complex [Cp2Ti(N(tBu)C(S)N(tBu))] (108), thus highlighting the typical reactivity of NHCs. Interestingly, Tonks and coworkers also isolated complex [Cp2Ti(N(Cy)C(N(Cy))Ti(Cp)2] (109) from the reaction of [Ti(Cp)2(btmsa)] with DCC, which shows that controlling the reaction conditions is of paramount importance in this chemistry. Although 109 was described as a free carbene, the TidCNHC bond distances (2.310(31) and 2.307(23) Å) were (just) within the sum of covalent radii (2.33  0.08 Å) (Table 13).145

3.07.8

Conclusion

Although the first studies on NHC complexes of group 4 metals were reported soon after the isolation of the first free NHC, the field has remained remarkably contained—especially compared to late transition metals. As discussed above, the notion that NHCs are soft ligands unsuitable for hard early transition metals such as Ti, Zr and Hf obviates the undisputed fact that NHCs are very strong donors and that they form exceptionally strong bonds with most metals. Still, a critical analysis of the evidence presented in the literature allows us to draw two conclusions regarding group 4 metals and NHCs: a. due to the relatively weak group MdNHC bond compared to many other later transition metals (a notable exception being Ag), transmetallation should always be considered as a possibility. Therefore NHC complexes of group 4 metals are probably not the best candidates for bimetallic catalysis. b. the strongly polar MdNHC bond makes the NHC itself susceptible to nucleophilic attack from other ligands bound to the metal. This reactivity may be an important issue in some cases, although it may be controlled (e.g., by avoiding coordinative saturation). There are already a few examples of robust, highly active group 4 metal NHC complexes catalyzing a variety of reactions, such as ethylene,52 rac-lactide,46 or cyclohexene oxide/CO2 polymerization.78 Likewise, monodentate p-acidic NHCs (e.g., CAACs) have shown great promise for the stabilization of low-valent group 4 metals.54,86,87 It seems likely that the field will continue to grow and that many interesting and useful species will be discovered.

264

A.2 Nbr 8Zr

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

Table of ethylene polymerization catalysts. M(n) Zr(IV)

Formula 1

[ZrCl4(L )]

R, R0 , R00

Ar

X

Act.a

Co-catalyst

Temperature ( C)

References

i

–

–

75

MAO

25

49

–

–

38

MAO

25

49

–

–

46

MAO

25

49

–

–

1

MAO

25

49

– – – – – – – – – – – Dipp

Cl Cl Br – Cl Cl Me Me – – – –

96 290 164 791 265 566 58 221 5 1 1 15

MAO MMAO MAO MAO AliBu3/[Ph3C][B(C6F5)4] AliBu3/[Ph3C][B(C6F5)4] AliBu3/[Ph3C][B(C6F5)4] AliBu3/[Ph3C][B(C6F5)4] MAO MAO MAO MAO

50 30 25–35 R.T. 25 25 25 25 50 50 50 50

14 11 13 52 64 64 64 64 94 95 95 96,97

Dipp

–

17

MAO

80

96,97

Dipp

–

30

MAO

R.T.

83

Dipp

–

36

MAO

R.T.

83

Dep Mes Dipp

– – –

28 26 25

MAO MAO MAO

R.T. R.T. 50

83 83 16

Dipp

BHT

190

MAO

60

16

Mes

BHT

85

MAO

50

16

Mes

ODipp

inactive

MAO

50

16

Xyl

–

40

MAO

–

99

Xyl

–

140

MAO

–

99

– – – – – –

– – – – – –

47 174 39 21 82 114

AliBu3/sulfated alumina AliBu3/sulfated alumina MAO MAO MAO MAO

80 80 25–35 25–35 50 50

100 100 13 13 14 140

Pr Pr Me 2-butyl Me CH2Mes i Pr i Pr Me t Bu t Bu – H t Bu H t Bu C5H8 C2H2 C5H8 Ad Me H Ad Me H H H H H H Me – – H H H H H H H H H H H H Mes t Bu Mes t Bu Me Bn t Bu t Bu t Bu – i

8Zr

Zr(IV)

[ZrCl4(L1)]

8Zr

Zr(IV)

[ZrCl4(L1)]

8Hf

Hf(IV)

[HfCl4(L1)]

12Ti 12Ti 12Ti 14 28 28 28 28 43 43 43 44

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

[Ti(X)2(L19)(THF)] [Ti(X)2(L19)(THF)] [Ti(X)2(L19)(THF)] [TiCl3(L29)] [TiX2(C5H4R)(L3)] [TiX2(C5H4R)(L3)] [TiX2(C5H4R)(L3)] [TiX2(C5H4R)(L3)] [Zr(NEt2)2(L11)2] [Zr(NEt2)2(L11)2] [Zr(NEt2)2(L11)2] [TiCl2(L12)2]

44

Zr(IV)

[ZrCl2(L12)2]

44

Ti(IV)

[TiCl2(L12)2]

44

Ti(IV)

[TiCl2(L12)2]

45 45 46

Ti(IV) Ti(IV) Ti(IV)

[TiCl2(L39)] [TiCl2(L39)] [TiCl3(L12)(THF)]

47

Ti(IV)

[TiCl2(X)(L12)]

47

Ti(IV)

[TiCl2(X)(L12)]

47

Ti(IV)

[TiCl2(X)(L12)]

51Ti

Ti(IV)

[TiCl4(L14)]

51Zr

Zr(IV)

[ZrCl4(L14)]

54 54 67 68 69 93

Hf(IV) Hf(IV) Ti(IV) Ti(IV) Ti(III) Ti(IV)

[Hf(Bn)(CBn3)(L40)] [Hf(Bn)(CBn3)(L40)] [(TiBr(L19))2(m-O)] [(Ti (L19)(m-O)2] [TiBr(L19)(THF)2] [TiCl2(L43)]

a

kg PE mol-M−1 h−1 bar−1.

N-Heterocyclic and Mesoionic Carbene Complexes of the Group 4 Metals

3.07.9

Appendix

3.07.9.1

Calculation of estimated deviations (esds) for mean distances

265

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi PN 2 ð esd ð d Þ Þ i
 i¼1 We have used the following propagation formula to calculate estimated deviations (esds) of mean distances esd d ¼ , N distances obtained from crystallographic information files, and N is the number of where esd(di) are the esds of individual
bond  bond distances. Hence, the value of esd d reflects the average quality of the structures under consideration but not the dispersion of the distances of the sample itself.

Acknowledgments I would like to warmly thank Prof Paul Fleurat-Lessard and Dr Eric Daiann Sosa Carrizo for insightful discussions about Energy  values. Decomposition Analysis. I am also indebted to Dr Philippe Richard for his assistance in the calculation esd(d)

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. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51.

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Relevant Websites http://www.ajarduengo.net/Structures.html http://www.ajarduengo.net/MolSelDriver8.html—The Arduengo Research Group

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3.08

Alkylidene Complexes of the Group 3 Metals and Lanthanides

Matthew P Stevens and Fabrizio Ortu, School of Chemistry, University of Leicester, Leicester, United Kingdom © 2022 Elsevier Ltd. All rights reserved.

3.08.1 3.08.1.1 3.08.1.2 3.08.1.3 3.08.1.3.1 3.08.1.3.2 3.08.2 3.08.2.1 3.08.2.1.1 3.08.2.1.2 3.08.2.2 3.08.3 3.08.3.1 3.08.3.2 3.08.3.2.1 3.08.3.2.2 3.08.3.3 3.08.4 References

Introduction Motivation for this review Scope of this article General considerations M]CR2 chemistry RE]CR2 chemistry Methylidenes Bridging methylidenes m2-methylidenes m3-methylidenes Lewis acid-supported Alkylidenes a-Silyl-alkylidenes Phosphorano-stabilised alkylidenes Pincer-type Non-pincer type Phosphino- and phosphonio-alkylidenes Conclusions

3.08.1

Introduction

3.08.1.1

Motivation for this review

268 268 268 269 269 270 272 273 273 274 280 284 284 288 288 302 304 309 309

Compounds containing metal-ligand multiple bonding interactions are amongst the most well-studied systems in organometallic chemistry.1–3 In the case of carbon donors, these compounds play an important role in modern chemical sciences owing to their various applications in synthesis and catalysis.2,4 In the same vein as general organometallic chemistry, Rare Earth (RE) carbene chemistry is historically less developed than that of the transition metals (TM), though its origins can be traced to around a decade after Fischer’s seminal report of the first tungsten methoxymethylcarbene.5,6 There are many reasons why RE carbene chemistry has not been developed at the same pace, and these could be identified in the synthetic challenges associated with RE chemistry and the lability of RE]C interactions.7 Despite these difficulties, there has been significant progress over the last two decades, which has opened up new chemical space. Several reviews have been published on the study of RE]CR2 complexes,7–9 including one on methylidene complexes,10 and a recent comprehensive account on geminal dianions.11 Therefore, the purpose of this work is to collate the most important landmarks in RE alkylidene chemistry and its most recent developments, and to provide a perspective for future directions for this research field.

3.08.1.2

Scope of this article

This work will cover the synthesis and reactivity studies of RE complexes containing divalent carbon ligand fragments, {CR2}2− (R] H, silyl, phosphino, phosphorano) and derivatives of phosphorous ylides (Fig. 1). We will primarily focus on structurally authenticated species and their reactivity. Firstly, structural characterisation via single crystal X-ray studies is paramount for the identification of multiple bond interactions and has also proven extremely useful in the identification of challenging oligomers and clusters. Secondly, since their first report in the 1960s, TM-carbon multiple bonds have led to the discovery of unique reactivity profiles that have shaped the field of organometallic chemistry; in comparison, the reactivity scope of RE]CR2 complexes is still underdeveloped and, therefore, worthwhile highlighting in order to showcase the untapped potential of these compounds. Throughout this article we will often refer to RE]CR2 species with the term alkylidenes. This term is normally used to describe Schrock-type carbenes, with which Group 3 and Ln carbenes share analogous properties in terms of polarisation of the M]C bond and, in some cases, reactivity. To better illustrate this aspect, general concepts of carbene chemistry will be first introduced, followed by some essential considerations on RE chemistry. We will then summarise the efforts in the preparation of methylidenes (i.e. containing a {CH2}2− ligand), covering simple methylene bridged species (Section 3.08.2.1), and Lewis Acid-supported systems (Section 3.08.2.2). In Section 3.08.3, RE alkylidenes will be discussed, subdivided into a-silyl-alkylidenes (Section 3.08.3.1), phosphorano-stabilised (Section 3.08.3.2), and phosphino- and phosphonio-alkylidenes (Section 3.08.3.3) (Fig. 1). Together with a detailed discussion of the synthetic strategies, structural features and reactivity of these complexes, we will give a historical perspective of how RE alkylidene chemistry has progressed over the last four decades and which challenges and goals lie ahead for researchers working in this field.

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Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00002-0

Alkylidene Complexes of the Group 3 Metals and Lanthanides

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Fig. 1 Classes of methylidene and alkylidene complexes covered in this work. For simplicity, 2-electron dative bonds between donors and metal centers will be represented as single bonds rather than using arrows.

3.08.1.3 3.08.1.3.1

General considerations M]CR2 chemistry

Species containing a twofold metal-carbon bonding interaction are commonly referred to as carbenes, in which a formally divalent carbon fragment is trapped through interaction with a metal center, i.e. M]CR2. In 1964 Maasböl and Fischer reported the first species containing a M]CR2 fragment, identified in the tungsten complex [W{C(CH3)(OCH3)}(CO)5] (A, Fig. 3).5 TM carbene complexes are separated conventionally into two categories: Fischer and Schrock carbenes (Fig. 2). In Fischer carbenes, the ligand

Fig. 2 Schematic representation of binding modes and MO sketch of Fischer and Schrock carbenes.17

270

Alkylidene Complexes of the Group 3 Metals and Lanthanides

Fig. 3 Landmark examples of TM carbene complexes: First Fischer carbene (A),5 First Schrock alkylidene (B),12 Tebbe’s reagent (C)13,14 and the Grubbs 1st generation catalyst (D).15,16

acts as a p-acceptor and the carbon donor is electrophilic; classically, these species are obtained with metals in a low oxidation state and are stabilised with p-donor substituents. On the other hand, in Schrock carbenes the metal is usually in a high oxidation state and the ligand does not act as a p-acceptor; metal-carbon interactions are almost completely polarised towards the carbon donor, and as a result, the carbon center is nucleophilic. Therefore, in Schrock carbenes, the ligand acts formally as a 2-electron donor (e.g. X2-type or bis-alkyl) and such species are often referred to as alkylidenes, exemplified by Schrock’s seminal report of [Ta{CH(tBu)} {CH2(tBu)}] in 1974 (B, Fig. 3).12 It is noteworthy to mention that other types of carbenes are also known, in particular persistent carbenes (e.g. N-heterocyclic carbenes, cyclic alkyl-amino carbenes), which have also been employed successfully with the f-block elements;18 however, these are not classified strictly as alkylidenes and therefore are not covered in this article. Together with their fundamental importance, TM carbenes have had a huge impact in chemical synthesis and inspired the development of numerous new reactions and methodologies, such as the use of Tebbe’s Ti(IV) masked methylidene, [Ti(Cp)2(m-CH2){(m-Cl)AlMe2}] (C, Cp]{C5H5}−, Fig. 3), as a methylene transfer reagent13,14 and Grubbs’ catalysts, [Ru]] CHR (R]alkyl, aryl; D, Fig. 3), which are employed in olefin metathesis reactions.15,16 As testament of the importance of these remarkable organometallic species, the study of carbene metal complexes has delivered two Nobel Prizes in Chemistry and a total of four Nobel laureates: Fischer in 1973 (jointly awarded with Wilkinson for his work on TM organometallic chemistry),19 and Schrock, Grubbs and Chauvin in 2005.20–22

3.08.1.3.2

RE]CR2 chemistry

Carbene complexes have been obtained with metals across the whole d-block. However, analogous chemistry with Rare Earth (RE) metals has historically lagged behind.7,23,24 The reasons for this can be attributed to the nature of the metal-carbon interaction in carbene complexes. In TM complexes, the presence of a formal covalent TM]C bond imparts great stability to the complex. Conversely, the RE carbene interactions are highly polarised owing to the strong electropositive character of the metal centre and the greater HOMO/LUMO energy mismatch (Fig. 4).9 The predominant oxidation state in RE chemistry is +3; for Sc, Y, and, La this leads to a closed shell configuration where the respective 3d, 4d, and 5d valence orbitals are too high in energy to play a significant role in chemical bonding.25 In the case of Lanthanide (Ln) metals, the valence shell consists of the 4f and 5d set: the 4f orbitals have limited radial extension (“core-like”) and are less available for chemical bonding, while the 5d manifold is high in energy and therefore often inaccessible for metal-ligand interactions.7,25,26 Interestingly, Actinide (An) carbene complexes have been heavily researched over the last four decades, due to increased orbital participation in the metal-ligand interaction arising from the greater radial extension of the 5f orbitals.27 Owing to the fundamental properties of RE(III) ions, it can be expected that a theoretical RE]CR2 species would have features similar to those of Schrock-type TM alkylidenes, with high polarisation of the bond towards the carbon center and strong nucleophilic character. As a result of the large ionic radii and strong electropositive character of RE ions, the use of judicious ligand design and strict anaerobic reaction conditions are essential for the stabilisation of organometallic RE derivatives. In particular, ligand design can be used to impart kinetic stabilisation to RE complexes, achieved by saturating the metal coordination sphere to prevent undesired degradation pathways and aggregations.26 The latter aspect is especially relevant to the study of M]CR2 complexes in general, as terminal alkylidenes can often be elusive species that are hard to stabilise in the solid state, even in the case of TMs. For example, despite Tebbe’s original report of [Ti(Cp)2(m-CH2){(m-Cl)AlMe2}] dating back to 1978,13 the first structure of a terminal unsupported Ti(IV) methylidene was only reported in 2017 by Mindiola and co-workers.28

Alkylidene Complexes of the Group 3 Metals and Lanthanides

271

Fig. 4 Orbital energy diagram for Schrock-type TM]CR2 complexes (left) and RE]CR2 alkylidenes (right).8,9,25 Summerscales, O. T.; Gordon, J. C., RSC Adv. 2013, 3, 6682–6692.

In Group 3 and Ln chemistry it is common practice to design supporting ligands capable of reducing the coordination number (CN) of the metal center in order to: (1) enhance specific physicochemical properties; and (2) create a finely tuned steric pocket for further reactivity.26 While these concepts are widely applicable to the study of RE alkylidenes, a big role is also played by the design of the carbene ligand itself. In TM carbenes, the metal center can stabilise the charge of the methylidene or alkylidene dianion through a covalent interaction.11,17 However, this is difficult to achieve in RE complexes due to the electrostatic character of metalligand interactions. Therefore, carbene ligands must often be adjusted and modified to impart both additional steric protection to the metal center and electronic stability to the ligand-based dianion. The former aspect is exacerbated by the large ionic radii of the RE metals, exemplified by the tendency of RE methylidenes to oligomerise (Section 3.08.2). Additionally, owing to the high polarisation of metal-carbon bonds, carbene ligands benefit from delocalisation of the anionic charge through substituents attached to the carbon donor. This is indeed a strategy that has been applied successfully in RE chemistry, which enabled the stabilisation of terminal Group 3 and Ln alkylidenes; nevertheless, such an approach has an effect on the formal RE]C bond order (Section 3.08.3). The importance of these synthetic challenges underpinning RE chemistry is demonstrated by the variety of synthetic approaches developed over the last two decades, which have led to the structural identification of nearly 100 examples of monometallic Group 3 and Ln carbenes reported in the Chemistry Structural Database,29 though no examples of terminal and unsupported RE alkylidene complexes have been reported to date. The birth of RE alkylidene chemistry can be identified in the work of Schumann and Müller, who reacted LuCl3 and ErCl3 with Li {CH2SiMe3} to form [Li(Et2O)4][Lu(CH2SiMe3)4] and [Er(CH2SiMe3)3(THF)2] (THF ¼ tetrahydrofuran), respectively; these complexes slowly decomposed with loss of SiMe4 to yield solids formulated as [Lu{CH(SiMe3)}(CH2SiMe3)2Li] (1a), [Li(TMEDA)][Lu {CH(SiMe3)}(CH2SiMe3)2] (1b, TMEDA ¼ tetramethylethylenediamine) and [Er(CHSiMe3)(CH2SiMe3)] (2) (Scheme 1).6,30 These complexes were not structurally characterised but were analysed via NMR spectroscopy and elemental analysis. The authors also observed the occurrence of different pathways in the decomposition of [Lu(CH2SiMe3)3(THF)2] which compete with the a-H elimination route responsible for the formation of 1 and 2, especially g-H elimination of a pendant methyl group of {CH2SiMe3}− to afford [Lu{(CH2)2SiMe2}(CH2SiMe3)].30 Attempts to structurally authenticate complexes analogous to 1 and 2 have so far been unsuccessful. Nonetheless, Schumann and Müller’s work inspired the development of synthetic strategies still applied to date in the quest for RE alkylidenes (vide infra).31 Moreover, Cho and Andrews have also reported the observation of RE methylidenes via

Scheme 1 Synthesis of Ln alkylidenes 1 and 2 via a-H elimination of parent alkyl complexes.6

272

Alkylidene Complexes of the Group 3 Metals and Lanthanides

methane activation using laser ablation techniques.32 At the time of writing this work, efforts to obtain a terminal unsupported RE alkylidene have so far been unsuccessful and no structures of such species have yet been reported. In the rest of this article we will focus on the most relevant examples that have shaped this research field, dividing RE alkylidenes in two broad classes: methylidenes (Section 3.08.2) and alkylidenes (Section 3.08.3).

3.08.2

Methylidenes

Methylene, CH2, is a key intermediate in many strategic industrial processes, in particular the Fischer-Tropsch synthesis used for the conversion of syngas (CO and H2) into higher order hydrocarbons.33,34 Methylidene complexes (L)nM]CH2 have attracted a lot of interest, as they are considered models of elusive intermediates in which the alkenyl fragment is trapped through the interaction with the metal center. Complexes containing M]CH2 units are the most basic forms of alkylidene complexes, and originate from the landmark discoveries of Tebbe in the 1970s (vide supra Section 3.08.1.3.1).13,14 Since then, several research groups have embarked in the prodigious challenge of synthesising RE analogues and exploring their reactivity. Some of this chemistry has been previously reviewed,8,9 including a detailed account of RE methylidene chemistry up to 2014.10 In this section some of the key examples covered in previous reviews will be summarised, together with advances in the synthesis of RE methylidenes since then. At the time of writing, no terminal unsupported methylidene complexes have been structurally authenticated with any of the REs. Known RE methylidenes have been subdivided into bridging (Table 1; Section 3.08.2.1), and Lewis Acid-supported complexes (Table 2) (Section 3.08.2.2).

Table 1

Bridging methylidene RE complexes covered in Section 3.08.2.1, with relevant crystallographic and spectroscopic data.

Molecular formula

RE oxidation state

M–C (Å)

13

Compound

References

[{Sc(NacNacDipp)2}2(m-CH2){m-P(Dipp)}] [{Y(Cp )(THF)}3(m3-CH2)(m-Cl)3(m3-Cl)] [{La(Cp )(THF)}3(m3-CH2)(m-Cl)3(m3-Cl)] [{Sm(Cp )(THF)}3(m3-CH2)(m-Cl)3(m3-Cl)] [{Y(Cp )(THF)}3(m3-CH2)(m-Br)3(m3-Br)] [{La(Cp )(THF)}3(m3-CH2)(m-Br)3(m3-Br)] [{La(Cp )(THF)}3(m3-CH2)(m-I)3(m3-I)] [{Y(Cp0 )(THF)}3(m3-CH2)(m-Cl)3(m3-Cl)] [{La(Cp0 )(THF)}3(m3-CH2)(m-Cl)3(m3-Cl)] [{Y(Cp0 )(THF)}3(m3-CH2)(m-Br)3(m3-Br)] [{La(Cp0 )(THF)}3(m3-CH2)(m-Br)3(m3-Br)] [{La(Cp0 )(THF)}3(m3-CH2)(m-I)3(m3-I)] [{Y(Cp ´)(THF)}3(m2-Me)3(m3-Me)(m3-CH2)] [{Y{N(Dipp)(SiMe3)}3(THF)}3(m2-Me)3(m3-Me)(m3-CH2)] [{Nd{N(Dipp)(SiMe3)}3(THF)}3(m2-Me)3(m3-Me)(m3-CH2)] [{Ho{N(Dipp)(SiMe3)}3(THF)}3(m2-Me)3(m3-Me)(m3-CH2)] [{Lu{N(Dipp)(SiMe3)}3(THF)}3(m2-Me)3(m3-Me)(m3-CH2)] [{Tm(Cp ´)}3(m2-Me)3(m3-Me)(m3-CH2)] [{Tm(Cp ´)}4(m3-CH2)4] [{Lu(Cp ´)}4(m3-CH2)4] [La4Al8(C)(CH)2(CH2)2(CH3)22(C7H8)] [{Sc(NCNDipp)}3(m2-Me)3(m3-Me)(m3-CH2)] [{Y(NCNDipp)}3(m2-Me)3(m3-Me)(m3-CH2)] [{Er(NCNDipp)}3(m2-Me)3(m3-Me)(m3-CH2)] [{Dy(NCNDipp)}3(m2-Me)3(m3-Me)(m3-CH2)] [{Lu(NCNDipp)}3(m2-Me)3(m3-Me)(m3-CH2)] [{Lu(DippForm)}3(m2-Me)3(m3-Me)(m3-CH2)] [{Y(NCNDipp)}3(m2-Me)3(m3-C^CSiMe3)(m3-CH2)]

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

2.193(3) 2.424(2)–2.450(2) 2.537(3)-2.635(3) 2.464(2)–2.534(2) 2.431(3)–2.532(3) 2.553(3)–2.646(4) – – 2.572(3)–2.580(3) – – – – 2.345(5)–2.391(5) 2.475(7)–2.505(8) 2.356(10)–2.428(10) 2.310(5)–2.383(5) 2.330(5)–2.365(5) 2.324(6)–2.405(4) 2.318(11)–2.411(12) 2.549(7)–2.889(7) – – 2.357(10)–2.419(10) – 2.371(9)–2.381(8) 2.369(10)–2.381(9) 2.382(7)–2.418(8)

4 14a-Y 14a-La 14a-Sm 14b-Y 14b-La 14c 15a-Y 15a-La 15b-Y 15b-La 15c 16 20-Y 20-Nd 20-Ho 20-Lu 24 25-Tm 25-Lu 34 36-Sc 36-Y 36-Er 36-Dy 36-Lu 37 48-Y

35

[{Lu(NCNDipp)}3(m2-Me)3(m3-C^CSiMe3)(m3-CH2)] [{Y(NCNDipp)}3(m2-Me)2(m3-CH2)(m3-Z1:Z2:Z2-S2C]CH2)]

+3 +3

– –

48-Lu 49-Y

51

[{Lu(NCNDipp)}3(m2-Me)2(m3-CH2)(m3-Z1:Z2:Z2-S2C]CH2)]

+3

2.396(6)–2.459(6)

30.2 Not observed Not observed Not observed Not observed Not observed Not observed Not observed Not observed Not observed Not observed Not observed – 103.0 – – 100.2 – – Not assigned – 123.9 – – – 107.6 107.3 113.8 (q) 1 JYC ¼ 22.9 Hz 112.3 116.7 1 JYC ¼ 22.8 Hz 119.1

49-Lu

51

C (ppm)

38 38 40 39 39 39 39 39 39 39 39 39 43 44 43,44 43 45 45 45 48 49 50,51 51 52 49 53 51

51

Alkylidene Complexes of the Group 3 Metals and Lanthanides

LA-stablised methylidene RE complexes covered in Section 3.08.2.2, with relevant crystallographic and spectroscopic data.

Table 2

Molecular formula tBu,Me

[Al(Tp

273

)Me][Y(AlMe4){(m-CH2)(m-Me)AlMe2}2(AlMe2)]

[La(TptBu,Me)(m3-CH2){(m-Me)AlMe2}2] [Sc(PNPPhiPr)(m3-CH2){(m-Me)AlMe2}2] [Sc(NNfc)(m3-CH2)(AlMe2)3] [La(TMTAC){(m-Me)2AlMe2}(m3-CH2){(m-Me)AlMe2}] [{Y(TMTAC)}{RE2[(m-Me)2AlMe)]}(m6-C)(m3-CH2){(m-Me)2AlMe}{(m-Me)AlMe2}2 [{Sm(TMTAC)}{RE2[(m-Me)2AlMe)]}(m6-C)(m3-CH2){(m-Me)2AlMe}{(m-Me)AlMe2}2]

3.08.2.1 3.08.2.1.1

RE oxidation state

M–C (Å)

13

Compound

References

+3

2.344(8)2.411(9) 2.519(2) 2.317(2) 2.692(2) 2.549(2) 2.367(5) 2.408(4)

31.3, 31.0

51

59

48.9 28.8 − 4.2 −5.5 103.0 –

52 56 61 63 64-Y 64-Sm

60

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

C (ppm)

61 63 64 64 64

Bridging methylidenes m2-methylidenes

The first simple m2-methylidene RE derivative was reported in 2015 by Chen and co-workers, who prepared a scandium complex bearing both methylidene and phosphinidene ligands, [{Sc(NacNacDipp)2}2(m-CH2){m-P(Dipp)}] (4; NacNacDipp ¼ N,N-bis(2,6-diisopropylphenyl)-2,4-pentanediiminate; Dipp ¼ C6H3(iPr)2–2,6), via thermal decomposition at 60  C of a scandium methyl phosphide precursor (Scheme 2, Route A), [Sc(NacNacDipp)(Me){HP(Dipp)}] (3).35 1H NMR spectroscopy showed that in addition to 4, a second product was also formed in a 1:1 ratio, which was identified to be the bis-phosphide complex [Sc(NacNacDipp){HP(Dipp)}2] (5). The authors were able to improve their methodology for the preparation of 4 by reacting [Sc(NacNacDipp)(Me)2] (6) with one equivalent of 3, which gave the desired product in almost quantitative yield (Scheme 2, Route B). Complex 4 is the first structurally authenticated m2methylidene complex, owing to the tendency of the {CH2}2− fragment to bridge between three RE metals (vide infra Section 3.08.2.1.2 and Section 3.08.2.2).10 Crystallographic analysis of 3 revealed that the ScdC distances [2.232(3) Å] are shorter compared to RE complexes exhibiting a m3-CH2 binding motif (vide infra).35 Reaction of 4 with benzonitrile and tert-butyl isocyanide resulted in addition across the ScdC bond, yielding the insertion products [{Sc(NacNacDipp)}2{(m-N)C(Ph)CH2-k2N,C}{m-P(Dipp)}] (7) and [{Sc(NacNacDipp)}2{CH2(m-C)CN(tBu)-k3N,C,C0 } {m-P(Dipp)}] (8), respectively (Scheme 3).35 On the other hand, reaction of 4 with two equivalents of carbon dioxide led to the formation of an asymmetrically coordinated malonate anion, [{Sc(NacNacDipp)}2{(m-O)C(O)CH2CO2-k3O,O0 ,O00 }{m-P(Dipp)}] (9), through sequential addition of CO2 to the ScdC bonds. Carbon disulfide reacted in a completely different fashion with 4, inserting only one molecule of CS2 and subsequently isomerising to give [{Sc(NacNacDipp)}2{(m-S)2C]CH2-k2}{m-P(Dipp)}] (10).35 0 Levine et al. employed the pincer ligand 2,5-bis(dialkylphosphinomethyl)pyrrolide (PNPPyR ); (R0 ]Cy, tBu) to prepare a series PyR’ t of scandium and yttrium alkyl complexes of the type [RE(PNP )R2] (RE]Sc, Y; R]CH2( Bu), CH2(SiMe3); R0 ]Cy, tBu), which are capable of activating various substrates including benzene and terminal alkynes.36,37 In the case of the neopentyl derivative [Sc(PNPPyCy){CH2(tBu)}] (11-Sc), the alkyl ligand can undergo thermal activation, leading to loss of neopentane and subsequent formation of a metallacyclobutane intermediate that is converted to a transient terminal methylidene complex, [Sc(PNPPyCy)(CH2)] (12-Sc), and isobutylene (Scheme 4).37 However, the authors were not able to isolate this compound and could only identify the parent bridged methylidene dimer [{Sc(PNPPyCy)(m-CH2)}2] (13), though no solid state structural identification could be obtained; a tentative assignment of the coordination motif and the presence of bridged methylidene was formulated from 1H and DOSY NMR experiments, and by comparing 13 with the chloride-bridged analogue [{Sc(PNPPyCy)(m-Cl)}2].37 To prove the formation of

Scheme 2 Synthesis of bridged methylidene-phosphinidene complex 4.35

274

Alkylidene Complexes of the Group 3 Metals and Lanthanides

Scheme 3 Reactivity of 4 with PhCN, tBuCN, CO2 and CS2.35 Zhou, J.; Li, T.; Maron, L.; Leng, X.; Chen, Y., Organometallics 2015, 34, 470–476.

Scheme 4 Synthesis of the transient terminal methylidene 12-RE and conversion to the bridged methylene complex 13.36,37

transient 12-Sc, the authors performed an isotopic labelling experiment, which led to the formation of mixtures of d0-isobutylene and d2-isobutylene, in agreement with the reactivity pathway identified to go via the formation of a metallacycle (Scheme 4). The corresponding yttrium species 11-Y was also prepared but it decomposed at room temperature and more rapidly under vacuum, precluding characterisation beyond the simple postulation that the reaction to form 12-Y was successful.37

3.08.2.1.2

m3-methylidenes

The first m3-methylidene complexes were reported by Dietrich et al. in 2006, obtained from the cleavage of tetramethylaluminate,{AlMe4}−, in the heteroleptic aluminate precursors [{Y(Cp )}2{(m-Me)2AlMe2}2(m-Cl)2] (Cp ] {C5Me5}−) and [{La(Cp )}6{(m-Me)3AlMe}4(m3-Cl)2(m3-Cl)6].38 The addition of THF aids the cleavage of the aluminate unit (Scheme 5), inspired by Lappert’s previous observations of Lewis Base-induced methylaluminate cleavage.41 In this case, Anwander and co-workers were able to identify trimetallic species of the general formula [{RE(Cp )(THF)}3(m3-CH2)(m-Cl)3(m3-Cl)] (14a-RE; RE]Y, La); 14a-Y and 14a-La are also the first examples of structurally authenticated RE methylidene complexes with a distinct {CH2}2− unit.38 Due to the difference in ionic radii (9-coordinate ionic radii: Y 1.075 Å; La 1.216 Å),42 the YdC bond distances in 14a-Y [2.424(2)–2.450(2) Å] are shorter than the corresponding LadC interactions [2.573(3)–2.635(3) Å] in 14a-La. Anwander and co-workers subsequently reported the isostructural complex [{Sm(Cp )(THF)}3(m3-CH2)(m-Cl)3(m3-Cl)] (14a-Sm), which was obtained serendipitously from the reaction between HCp and {Sm(AlMe4)2}n in the presence of trace impurities of AlMe2Cl (Scheme 5).40 In further work, Birkelbach et al. expanded this family of complexes by employing various halide-containing aluminate starting materials, [{Y(CpR)}2{(m-Me)2AlMe2}2(m-X)2] and [{La(CpR)}6{(m-Me)3AlMe}4(m3-X)2(m3-X)6] (CpR]Cp , Cp0 ; X]Cl, Br, I), leading to the isolation of trimetallic clusters 14-RE [{RE(Cp )(THF)}3(m3-CH2)(m-X)3(m3-X)] (14a-RE: RE]Y, La; X]Cl; 14b-RE: RE]Y, La; X]Br; 14c: RE]La, X]I) and [{RE(Cp0 )(THF)}3(m3-CH2)(m-X)3(m3-X)] (15a-RE: RE]Y, La; X]Cl; 15b-RE: RE]Y, La; X]Br; 15c: RE]La, X]I).39 The methylidene/halogenido clusters 14a-Y and 14b-Y were obtained also through treatment of methyl/methylidene cluster complex [{Y(Cp0 )(THF)}3(m2-Me)3(m3-Me)(m3-CH2)](16) with SiMe3X (X]Cl, Br).39 In order to explore the utility of 14-RE and 15-RE as methylene transfer agents, these clusters were reacted with several carbonyl substrates (benzaldehyde, cyclohexanone, fluorenone and dihydrocoumarin). With the exception of dihydrocoumarin, all carbonyl substrates were converted into the corresponding exocyclic alkenes, with complexes 14a-Y, 14b-Y, 15a-Y and 15b-Y affording quantitative conversions similarly to Tebbe’s reagent.38,39 All complexes were tested also as catalysts for the ring-opening

Alkylidene Complexes of the Group 3 Metals and Lanthanides

275

Scheme 5 Synthesis of RE methylidene complexes via THF-assisted {AlMe4}− cleavage and methyl/halogen exchange (14-RE and 15-RE),38,39 together with the serendipitous preparation of the Sm analogue 14a-Sm,40 and preparation and reactivity of methyl/methylidene cluster 16.39

polymerisation of d-valerolactone, and the majority of them displayed good activity; in particular, complexes 14b-Y, 14b-La, 15b-Y and 15b-La showed high reactivity and yields >85%.38,39 Finally, 16 was reacted with neopentyl alcohol and afforded the alcoholysis product [{Y(Cp )2(OCHt2Bu)(m-OCHt2Bu)}2] (Scheme 5); it is noteworthy that selective reactivity of methyl or methylidene ligands of 16 with the substrate could not be achieved, even with stoichiometric control.39 Anwander and co-workers investigated various methods to synthesise trimetallic clusters of the type [{RE{N(Dipp) (SiMe3)}3(THF)}3(m2-Me)3(m3-Me)(m3-CH2)], (20-RE; RE]Y, Nd, Ho, Lu).43,44 Broadly, two methodologies were proposed (Scheme 6). The first was a one-step process involving the protonolysis of (LnMe3)x with the parent amine, HN(Dipp)(SiMe3) to give the intermediate [RE{N(Dipp)(SiMe3)}(Me)2(THF)2] (17-RE; RE]Y, Ho, Lu), which affords 20-RE via loss of methane. The other procedure involved treatment of Ln(EMe4)3 (E]Al, Ln]Y, Nd, Ho; E]Ga, Ln]Y) with K{N(Dipp)(SiMe3)}, followed by addition of THF to promote elimination of methane and formation of 17-RE. The intermediates [RE{N(Dipp)(SiMe3)} {(m-Me)2(AlMe2)}2(THF)2] (18-RE; RE]Y, Nd, Ho) and [Y{N(Dipp)(SiMe3)}{(m-Me)2(GaMe2)}2(THF)2] (19) were also characterised by X-ray crystallography.43,44 Complexes 20-RE also exhibited a typical Tebbe-like reactivity towards ketone substrates and provided the methylenated derivative of fluorenone in quantitative yield (Scheme 7). In the case of complex 20-Nd, the corresponding oxo-derivative [{Nd {N(Dipp)(SiMe3)}3(THF)}3(m2-Me)3(m3-Me)(m3-O)] (21) also formed and was fully characterised.44

Scheme 6 Synthesis of amido-supported m3-methylidene RE complexes 20-RE via methane elimination from respective alkyl precursors (17-RE) or methylaluminate cleavage from parent heteroleptic aluminate complexes (18-RE and 19).43,44

276

Alkylidene Complexes of the Group 3 Metals and Lanthanides

Scheme 7 Methylene transfer reaction of 20-Nd with fluorenone and formation of the oxo-derivative 21.44 Schädle, D.; Meermann-Zimmermann, M.; MaichleMössmer, C.; Schädle, C.; Törnroos, K. W.; Anwander, R., Dalton Trans. 2015, 44, 18101–18110.

In 2011 Zhang et al. isolated the first structurally authenticated RE polymethylidene complexes consisting of the aggregation of Ln(Cp0 )CH2 (Cp0 ]C5Me4SiMe3) units.45 Treatment of a precursor complex, [Ln(Cp0 )(CH2SiMe3)2(THF)] (22-Ln, Ln]Tm, Lu), with trimethylaluminium in diethyl ether yielded the hexamethyl trinuclear clusters [Ln3(Cp0 )3(m2-Me)6] (23-Ln: Ln]Tm, Lu, Scheme 8). However, while this reaction proceeded smoothly in the case of Lu, the Tm analogue presented a thermal instability and at room temperature it rapidly eliminated methane to yield the trimetallic complex [{Tm(Cp0 )}3(m2-Me)3(m3-Me)(m3-CH2)] (24). Moreover, upon heating both trinuclear clusters quantitatively formed the tetranuclear cubanes [{Ln(Cp0 )}4(m3-CH2)4] (25-Ln: Ln]Tm, Lu). Together with being the first RE polymethylidene clusters, 25-Tm and 25-Lu were also the first examples of cubanetype methylidene complexes of any metal. The assignment of the apical carbon of 24 as a methylidene was based on differences in

Scheme 8 Synthesis of cubane methylidene clusters 25-Ln via thermal decomposition of hexamethyl trimetallic cluster 23-Ln or trimetallic methylidene cluster 24.45 Adapted with permission from Zhang, W. X., et al. J. Am. Chem. Soc. 2011, 133, 5712–5715. Copyright 2011 American Chemical Society.

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277

TmdC bond lengths within the molecule and the identification of just two hydrogen atoms bound to this carbon in the Fourier difference density map in comparison to three on each of the other carbon atoms.45 Zhang et al. also investigated the reactivity of 25-Lu towards two aryl ketones, benzophenone and benzaldehyde. Both substrates underwent a Tebbe-like reaction to yield the corresponding alkenes, 1,1-diphenylethylene and styrene. The by-product of these reactions was an oxo-cluster complex, [{Lu(Cp0 )}4(m3-O)4] (25).45 The reactivity of 25-Lu was also tested with various unsaturated substrates, including CO, CO2, carbodiimides, isocyanates and isothiocyanates (Scheme 9).46 Reaction of 25-Lu with carbon monoxide yielded the ketene derivative [{Lu(Cp0 )}4(m3-CH2)2(m3-Z2-OC]CH2)2] (27). Further exposure to CO did not lead to reaction of the remaining methylidene fragments and the authors reasoned this could be due to the increased steric hindrance of the complex.46 Reaction of 25-Lu with the more sterically hindered diisopropylcarbodiimide yielded the single addition product [{Lu(Cp0 )}4(m3-CH2)3{iPrNC(]CH2)NiPr}] (28); A similar single addition product was obtained from the reaction of 25-Lu with phenylisothiocyanate, [{Lu(Cp0 )}4(m3-CH2)3{m3-Z2-PhNC(S)]CH2}] (29). On the other hand, the reactivity of 25-Lu with phenylisocyanate proceeded in a different fashion, with four molecules of substrate inserting into four LudCH2 bonds, yielding the cluster [{Lu(Cp0 )}4(m3-CH2)2{PhN]C(O)CH2(O)C]NPh}2] (30).46 This increased reactivity is likely due to the high oxophilicity of lutetium, causing more molecules of phenyl isocyanate to coordinate to the metal centers and thence react with the LudC bonds. Reaction of 25-Lu with the ammonium salt [PhMe2NH][B(C6F5)4] (Scheme 9) led to protonation of one {CH2}2− unit with the resultant formation of [{Lu(Cp0 )}4(m3-CH2)3(m3-Me)][B(C6F5)4] (31).47 There was no observed coordination of the amine by-product to the metal centers, likely due to the steric hindrance of the cyclopentadienyl ligands. Hydrogenation of 25-Lu (Scheme 9) gave a mixed tetrahydride/tetramethyl complex, [{Lu(Cp0 )}4(m2-H)2(m3-H)(m4-H)(m2-Me)4] (32).47 This is a rare example of a structurally characterised RE cluster complex with both methyl and hydride ligands. Moreover, together with the reactivity of cubane cluster 25-Lu, Hou and co-workers investigated the use of the Tm trimetallic methylidene cluster 24 with

Scheme 9 Reactivity of 25-Lu towards unsaturated substrates, small molecules and ammonium salts: ketones (26),46 carbon monoxide (27),46 diisopropylcarbodiimide (28),46 phenylisothiocyanate (29), phenylisocyanate (30),46 [PhMe2NH][B(C6F5)4] (31)47 and hydrogen (32).47 Zhang, W. X.; Wang, Z.; Nishiura, M.; Xi, Z.; Hou, Z., J. Am. Chem. Soc. 2011, 133, 5712–5715.

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benzophenone.45 In contrast with the reactivity of 25-Lu with benzophenone, in which methylene transfer occurred, the reaction of 24 with benzophenone lead to CdH activation at the 2,20 positions of benzophenone, together with insertion into one TmdCH3 bond (Scheme 10), yielding [{Tm(Cp0 )3}(m2-Me)3{(C6H4)2C(O)Me}] (33). Another high-order methylidene cluster was reported by Gerber et al., who observed that La(AlMe3)4 slowly degrades in the presence of PMe3 to give a series of polymetallic species (Scheme 11), including the dodecametallic cluster [La4Al8(C) (CH)2(CH2)2(CH3)22(C7H8)] (34).48 Complex 34 features three distinct C1 functionalities: methylidene {CH2}2−, methine {CH}3− and carbide {C}4−.48 Unlike previously observed bridging methylidenes, in this case the {CH2}2− bridges between two La metals and an Al center belonging to AlMe3; the interaction with a Lewis Acid has a stabilising effect, thus resembling the synthetic strategy employed by Tebbe for the preparation of Ti methylidenes.13 The application of this approach to RE chemistry is discussed in more detail in Section 3.08.2.2. Bidentate amidinate ligands have also been employed by Zhou and co-workers to stabilise trimetallic methylidene RE clusters of the general formula [{RE(NCNDipp)}3(m2-Me)3(m3-Me)(m3-CH2)] (Ln]Sc, Y, Er, Dy, Lu; NCNDipp]PhC({2,6-iPr2-C6H4}N)2) (36-RE).49–52 These complexes were obtained by treating heteroleptic benzyl precursors [RE(1-NMe2-2-CH2-C6H4)2(NCNDipp)] (35-RE) with trimethylaluminium (Scheme 12).49 The authors noted that by the treating 35-Sc and 35-Lu with an excess of AlMe3, the bis-aluminate complexes [RE(NCNDipp)(AlMe4)2] (38-RE; RE]Sc, Lu) formed, which could be converted to 36-RE upon dissolution in THF/toluene. Additionally, 36-Lu cleanly converts back to 38-Lu upon treatment with six equivalents of AlMe3, while no reaction with this substrate was observed for the Sc analogue 36-Sc.49 Anwander and co-workers have also reported the preparation of an analogous RE cluster using the closely related formamidinate ligands ArNCHNAr (DippForm Ar]Dipp).53 The authors reacted [LuMe3]n with the proligand DippFormH and obtained the trimetallic cluster [{Lu(DippForm)}3(m2-Me)3(m3-Me) (m3-CH2)] (37) as a minor product, likely from the degradation of the major reaction product [Lu(DippForm)2(Me)].53 The reactivity of 36-RE (RE]Sc, Y, Dy, Lu) towards ketones was also investigated (Scheme 13).49,52,54 In all permutations and with various stoichiometries, the methylidene vertex was exchanged for an oxygen atom, with 36-RE showing Tebbe-like reactivity to form [{RE(NCNDipp)}3(m2-Me)3(m3-Me)(m3-O)] (39-RE; RE]Sc, Y, Dy, Lu). However, the lutetium analogue 39-Lu underwent

Scheme 10 Reactivity of 24 with benzophenone and formation of the CdH activation/insertion product 33.45 Zhang, W. X.; Wang, Z.; Nishiura, M.; Xi, Z.; Hou, Z., J. Am. Chem. Soc. 2011, 133, 5712–5715.

Scheme 11 Degradation of La(AlMe4)3 and formation of the dodecametallic methylidene cluster 34.48 Gerber, L. C. H.; Le Roux, E.; Törnroos, K. W.; Anwander, R., Chem. - Eur. J. 2008, 14, 9555–9564.

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Scheme 12 Synthesis of the trimetallic methylidene clusters 36-RE and 37 via treatment of benzyl precursor 35-RE with AlMe3,49–52 or reaction of DippFormH with [LuMe3]n.53 Hong, J.; Zhang, L.; Yu, X.; Li, M.; Zhang, Z.; Zheng, P.; Nishiura, M.; Hou, Z.; Zhou, X., Chem. - Eur. J. 2011, 17, 2130–2137; Hong, J.; Zhang, L.; Wang, K.; Zhang, Y.; Weng, L.; Zhou, X., Chem. - Eur. J. 2013, 19 (24), 7865–7873; Hong, J.; Li, Z.; Chen, Z.; Weng, L.; Zhou, X.; Zhang, L., Dalton Trans. 2016, 45, 6641–6649; Hong, J.; Tian, H.; Zhang, L.; Zhou, X.; del Rosal, I.; Weng, L.; Maron, L., Angew. Chem., Int. Ed. 2018, 57, 1062–1067.

Scheme 13 Reactivity of 36-RE with ketones and formation of the oxo-cluster 39-RE and mixed alkoxo-oxo clusters 40 and 41.49,52 Hong, J.; Zhang, L.; Yu, X.; Li, M.; Zhang, Z.; Zheng, P.; Nishiura, M.; Hou, Z.; Zhou, X., Chem. - Eur. J. 2011, 17, 2130–2137; Hong, J.; Zhang, L.; Wang, K.; Zhang, Y.; Weng, L.; Zhou, X., Chem. - Eur. J. 2013, 19 (24), 7865–7873; Hong, J.; Li, Z.; Chen, Z.; Weng, L.; Zhou, X.; Zhang, L., Dalton Trans. 2016, 45, 6641–6649; Hong, J.; Tian, H.; Zhang, L.; Zhou, X.; del Rosal, I.; Weng, L.; Maron, L., Angew. Chem., Int. Ed. 2018, 57, 1062–1067.

further reactivity in the presence of a second equivalent of ketone, forming two different structures depending on the substrate. With cyclohexanone, simple deprotonation occurred, yielding the enolate derivative [{Lu(NCNDipp)}3(m2-Me)3(m3-O)(m3-O-C6H9)] (40), while acetophenone underwent 1,2-addition of the apical methyl group of 39-Lu to yield the tertiary alkoxide [{Lu(NCNDipp)}3(m2-Me)3(m3-O)(m3-O-CMe2Ph)] (41).49 The formation of 39-Sc was modelled by DFT analysis, which revealed that the {CH2}2−/O2− interchange occurs in a multi-stage process promoted by the synergic action of the metals in the cluster, thus differing from conventional Wittig-type reactivity.54 In further work, Hong et al. investigated the reactivity of 36-Y and 36-Lu towards amines and phenylphosphine, which led to the conversion into the corresponding imido [{RE(NCNDipp)}3(m2-Me)3(m3-Me)(m3-NR)] (42-RE: RE]Y, Lu; R]Ph, 2,6-Me2C6H3,

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Scheme 14 Reactivity of 36-RE with amines (42-RE), phenylphosphine (43-RE), diamines (44a and 44b), azobenzene and N-benzylideneaniline (45-RE).50 Hong, J.; Zhang, L.; Wang, K.; Zhang, Y.; Weng, L.; Zhou, X., Chem. - Eur. J. 2013, 19(24), 7865–7873.

p-ClC6H4, p-MeOC6H4, Me2CHCH2CH2)50 or phosphinidene [{RE(NCNDipp)}3(m2-Me)3(m3-Me)(m3-PPh)] complexes (43-RE; RE]Y, Lu),55 respectively, in high yields (Scheme 14). It is noteworthy that no reaction was observed when the more sterically demanding aniline Dipp-NH2 was employed. Similarly to what was observed in the reaction of 36-Sc with acetophenone, DFT analysis showed that the three metal centers in 42-RE act in synergy to deliver a double EdH bond activation and isomerisation.56 Furthermore, the reactions of diamines with 36-Lu led to the formation of the tethered dumbbell clusters [{[Lu(NCNDipp)]3(m2-Me)3(m3-Me)}2(m3,m3:k2-NRN0 )] (44a: R](CH2)6; 44b: R](C6H4)2), in which two trimetallic complexes are bridged by the alkyl chain of the bis-imido ligand formed upon full deprotonation of either 4,40 -diaminobiphenyl or hexamethylenediamine.50 Additionally, treatment of 36-RE (RE]Y, Lu) with half an equivalent of azobenzene yielded the imide complexes [{RE(NCNDipp)}3(m2-Me)3(m3-Me)(m-Z1:Z1:Z3-NPh)] (45-RE; RE]Y, Lu), with concomitant loss of ethylene gas (Scheme 14).50 This represents the first example of a trivalent RE-promoted N]N cleavage in azobenzene. Complexes 36-RE were also treated with N-benzylideneaniline, and 45-RE was again formed with concomitant release of styrene as a by-product (Scheme 14).50 From the mechanistic point of view, these last two reactions mimic olefin metathesis, which is a common reactivity pathway for TM carbene complexes.4 Initially, a molecule of azobenzene reacts with 36-RE to form 45-RE and N-methylideneaniline; this latter reagent is more reactive than azobenzene, so undergoes a second metathesis reaction to give ethylene and a second molecule of 45-RE.50 Zhou, Hang and co-workers also studied the reactivity of 36-Y and 36-Lu towards nitriles and alkynes. When these methylidene complexes were reacted with benzonitrile, 1-azaallyl dianionic species [{RE(NCNDipp)}3(m2-Me)3(m3-Me)(m3,Z1:Z1:Z3-CH2C(Ph) N)] (46-RE; Re]Y, Lu) were obtained (Scheme 15).51 This is in stark contrast to the reactivity of TM carbenes with nitriles, which typically form imides.1 Interestingly, 36-RE reacted with phenylacetylene and trimethylsilylacetylene in different ways. With the former substrate, the methylidene fragment underwent insertion into the terminal CalkynedH bond to yield the alkenyl dianion complexes [{RE(NCNDipp)}3(m2-Me)3(m3-Me)(m-Z1:Z3-PhC]CMe)] (47-RE; RE]Y, Lu).51 The C]C distance in 47-Lu is unusually short [1.279 (11) Å] and is intermediate between a C^C (1.21 Å) and a C]C bond (1.34 Å).51 Such a difference may be due to the dianionic nature of this ligand. Conversely, in the reaction of 36-RE with trimethylsilylacetylene protonolysis of the apical methyl group occurred, leaving the methylidene ligand intact and giving [{RE(NCNDipp)}3(m2-Me)3(m3-C^CSiMe3)(m3-CH2)] (48-RE).51 The authors explained the difference by steric considerations, owing to SiMe3 having a greater steric effect over a phenyl substituent. Finally, reactivity of 36-RE with carbon disulfide afforded the dithiolate complex [{RE(NCNDipp)}3(m2-Me)2(m3-CH2) (m3-Z1:Z2:Z2-S2C]CH2)] (49-RE; RE]Y, Lu). The unusual ethylene-1,1-dithiolate moiety was obtained by preferential attack of an equatorial methyl moiety followed by CdH activation of the same methyl group, which contrasts with the more typical insertion products in organolanthanide chemistry.51

3.08.2.2

Lewis acid-supported

This class of methylidene complexes stems from Tebbe’s original report of [Ti(Cp)2(m-CH2){(m-Cl)AlMe2}], in which the methylidene derivative ‘(Cp)2Ti]CH20 is masked by the interaction with a Lewis Acid (LA) e.g. AlMe3, AlMe2Cl (Table 2).13 As proof of this dynamic relationship, Tebbe’s reagent can be activated with the use of a Lewis Base (e.g. pyridine, THF, DMAP; DMAP ¼4-dimethylaminopyridine) and then trapped through reactions with unsaturated substrates.57 Tebbe’s original synthetic methodology has been applied successfully for other Group 4 metals and has also inspired efforts of the RE community.24

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Scheme 15 Reactivity of 36-RE with benzonitrile (46-RE), phenylacetylene (47-RE), trimethylsilylacetylene (48-RE) and carbon disulfide (49-RE).51 Hong, J.; Li, Z.; Chen, Z.; Weng, L.; Zhou, X.; Zhang, L., Dalton Trans. 2016, 45, 6641–6649.

In 2007 Piers and co-workers postulated the formation of a transient masked Sc methylidene supported by a borane, however no structural confirmation was obtained.58 A year later, the groups of Anwander and Mindiola independently completed the pursuit of a LA-stabilised Tebbe-like RE methylidene species.59–61 In the first study in 2008, Zimmermann et al. reacted RE(AlMe4)3 (RE]Y, Lu) with the proligand tris(3-t-butyl-5-methylpyrazoyl)borate (HTptBu,Me), which afforded [RE(TptBu,Me)(Me)(AlMe4)] (50-RE; RE]Y, Lu) as the main product (Scheme 16).59 However, in the case of the yttrium analogue, the supernatant also contained an ion pair formulated as [Al(TptBu,Me)Me][Y(AlMe4){(m-CH2)(m-Me)AlMe2}2(AlMe2)] (51).59 The latter contains two CH2 groups linked by an {AlMe2}+ fragment and masked by the interaction with neutral AlMe3; the remaining coordination sites on the yttrium centre are taken up by an {AlMe4}− anion. In a revised methodology, Anwander and co-workers reacted the potassium salt K(TptBu,Me) with RE(AlMe4)3 (RE]Y, La): for the yttrium reaction complex 50-Y was obtained, while employment of La(AlMe4)3 led to the formation of the methylidene complex [La(TptBu,Me)(m3-CH2){(m-Me)AlMe2}2] (52).60 Single crystal X-ray diffraction analysis of the La-methylidene interaction in 52 revealed a LadC bond distance [2.519(2) Å] shorter than those observed in methylidene clusters and other LA-stabilised RE methylidenes 51 and 34; however, because of the bonding with AlMe3 units and the resultant distribution of the dianionic charge, this interaction closely resembles LadC distances in alkyl complexes.62 At the same time of Anwander’s synthesis of 52, Mindiola and co-workers reported an analogous LA-stabilised Sc methylidene complex supported by a pincer PNP ligand, {N[2-P(CHMe2)2–4-MeC6H3]2}− (PNPPhiPr).61 In their synthetic methodology (Scheme 17), Scott et al. reacted [Sc(PNPPhiPr)(Cl)2] (53) with two equivalents of MeLi•LiBr to give the dimethyl derivative [Sc(PNPPhiPr)(Me)2] (54), which upon treatment with an excess of AlMe3 afforded the Tebbe-like methylidene complex [Sc(PNPPhiPr)(m3-CH2){(m-Me)AlMe2}2] (56).61 The X-ray crystal structure of 56 shows a Sc–CH2 distance [2.317(2) Å] that is longer than typical ScdC alkyl interactions [av. 2.236, range 2.181(4)-2.253(2) Å],29 and an asymmetric geometry of the methylidene hydrogen atoms, which indicates that an a-agostic interaction is present (ScdH 2.33 Å). It was proposed that the formation of 56 proceeds via a transient methylidene species stabilised by a single AlMe3 molecule, [Sc(PNPPhiPr)(m-CH2){(m-Me) AlMe2}] (55), based on variable temperature (VT) NMR 27Al studies. The room temperature 27Al NMR spectrum shows two broad signals (154 and 50 ppm) wherein the low field resonance corresponds to free AlMe3. Upon cooling to −30  C, these signals coalesce into one broad signal at 57 ppm, consistent with the structure of 56. This evidence points towards dissociation of 56 into 55 and AlMe3 in solution at room temperature.61 In the same report, Scott et al. also investigated the reactivity (Scheme 18) of 56 with benzophenone and 2,6-diisopropylaniline. With benzophenone, 56 underwent simple double bond metathesis to yield the corresponding oxo-complex [Sc(PNPPhiPr) (m3-O){(m-Me)AlMe2}2] (57) and Ph2C]CH2.61 Complex 56 underwent protonolysis with Dipp-NH2 to give first an imido complex, [Sc(PNPPhiPr)Me(NH-Dipp)] (58), and the addition of a second equivalent of Dipp-NH2 afforded the C2-symmetric bisimido complex [Sc(PNPPhiPr)(NH-Dipp)2] (59).61

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Scheme 16 Synthesis of LA-stabilised RE methylidene complexes 50-RE59 and 51,48 and the first Tebbe-like RE methylidene complex 52.60 Gerber, L. C. H.; Le Roux, E.; Törnroos, K. W.; Anwander, R., Chem. - Eur. J. 2008, 14, 9555–9564; Zimmermann, M.; Takats, J.; Kiel, G.; Törnroos, K. W.; Anwander, R., Chem. Commun. 2008, 8, 612–614; Litlabø, R.; Zimmermann, M.; Saliu, K.; Takats, J.; Törnroos, K. W.; Anwander, R., Angew. Chem., Int. Ed. 2008, 47, 9560–9564.

Scheme 17 Synthesis of the LA-stabilised Sc methylidene complex 56 by Mindiola and co-workers.61 Adapted with permission from Mindiola, D. J., et al., J. Am. Chem. Soc., 2008, 130, 14438. Copyright 2008 American Chemical Society.

Scheme 18 Reactivity of 56 towards benzophenone and Dipp-NH2.61 Adapted with permission from Mindiola, D. J. et al. J. Am. Chem. Soc., 2008, 130, 14438. Copyright 2008 American Chemical Society.

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Another interesting example of a Sc masked methylidene was reported by Diaconescu and co-workers in 2011.63 In their report Huang et al. described the reactivity of the heteroleptic Sc complex [Sc(NNfc){(2-Me-C4N2)-[CH]NCH]CH-N(Me)]} (60, NNfc] 1,10 -fc(NSitBuMe2)2) towards AlMe3. In their initial investigations the authors identified various Al transmetallation products, but when the reactions were carried out at high temperatures the methylidene complex [Sc(NNfc)(m3-CH2)(AlMe2)3] (61) was obtained, in which the ferrocene diamide backbone has been doubly deprotonated (Scheme 19).63 The authors were also able to obtain 61 from the direct reaction of the benzyl complex [Sc(NNfc){CH2(C6H3Me2–3,5)}(THF)] (62) with excess AlMe3. Complex 61 displays a ScdCH2 bond distance [2.69(2) Å] that is significantly longer than that observed in 56 and is more comparable to ScdCH3 distances, where the methyl acts as a bridging ligand.63 This aspect was corroborated by DFT calculations, which afforded Mayer bond orders of 0.41 for 61 and 0.83 for 56, respectively. Mitzel and co-workers reported a series of LA-stabilised RE methylidene complexes supported by the neutral ligands 1,3,5trialkyl-1,3,5-triazacyclohexane (alkyl]methyl-TMTAC, isopropyl—TiPTAC).64,65 The general synthetic methodology involves dissolution of RE aluminates and TMTAC or TiPTAC in chilled toluene solutions, which upon warming lead to the evolution of methane and the formation of CdH activation products (Scheme 20). The outcome varied significantly depending on the size of the RE ion and choice of ligand. When La(AlMe4)3 was reacted with TMTAC, upon warming to room temperature over the course of 20 h [La(TMTAC){(m-Me)2AlMe2}(m3-CH2){(m-Me)AlMe2}] (63) crystallised out of solution.64 This contains the original {AlMe4}− anion as well as the masked {CH2}2− unit, {Me3Al(CH2)AlMe3}2−, characteristic of other LA-stabilised methylidenes.

Scheme 19 Synthesis of the methylidene complex 61 via reaction of AlMe3 with aluminate complex 60 or benzyl precursor 62.63 Huang, W.; Carver, C. T.; Diaconescu, P. L., Inorg. Chem. 2011, 50, 978–984.

Scheme 20 Synthesis of LA-stabilised methylidene complexes supported by neutral ligands TMTAC and TiPTAC: monometallic derivatives (63, 65 and 66)64,65 and multimetallic methylidene/carbide clusters (64-RE).65 Venugopal, A.; Kamps, I.; Bojer, D.; Berger, R. J. F.; Mix, A.; Willner, A.; Neumann, B.; Stammler, H. G.; Mitzel, N. W., Dalton Trans. 2009, 5755–5765; Bojer, D.; Venugopal, A.; Mix, A.; Neumann, B.; Stammler, H. G.; Mitzel, N. W., Chem. - Eur. J. 2011, 17, 6248–6255.

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However, the same procedure on the yttrium and samarium analogues led to the formation of very different products, with sequential CdH activation to the point of formation of a hexacoordinate carbide ligand. The products obtained from these reactions, [{RE(TMTAC)}{RE2[(m-Me)2AlMe]}(m6-C)(m3-CH2){(m-Me)2AlMe}{(m-Me)AlMe2}2] (64-RE; RE]Y, Sm) were formed alongside the bis(trimethylaluminium) TMTAC adduct.64 In the case of samarium, the bis-CdH activation product [Sm(TMTAC) (m3-CH2)2(AlMe2){(m-Me)AlMe2}2] (65) also formed.64 Finally, the reaction between the more sterically demanding TiPTAC and Y(AlMe3)4 yields cleanly the mono-CdH activation product [Y(TiPTAC){(m-Me)AlMe3)}(m3-CH2){(m-Me)AlMe2}] (66), which is analogous to 63 (Scheme 20).65 These differences in reactivity highlight subtle changes in ionic radii between RE metals (La > Sm > Y),42 which also influence the length of the RE–CH2 interactions. The monometallic complexes 63, 65 and 66 display REdCH2 distances of 2.549(2) Å (La), 2.515(4) Å (Sm) and 2.376(3) Å (Y), respectively, in agreement with the decrease in ionic radii. These distances are close to those observed for other LA-stabilised analogues, such as the La complex 52 [2.519(2) Å]60 and the anionic Y derivative 51 [2.344(8) and 2.411(9) Å].59

3.08.3

Alkylidenes

The pursuit of a terminal unsupported RE]CH2 interaction has proven to be a formidable challenge for synthetic chemists. Together with the high polarisation of the metal-carbon interaction, the methylidene ligand does not fulfil the high steric demands of the large and electropositive RE metals, thus causing formation of dimeric species and clusters unless a LA is employed e.g. AlMe3. As a result, researchers in this field have focused their attention on another approach which involves the use of different substituents on the methylene unit with the aim of: (1) stabilising the dianionic charge via hyperconjugation; and (2) imparting kinetic stability and preventing oligomerisation. Silicon- and phosphorous-based substituents have been the most popular choices especially due the high carbanion stabilisation energies of dSiH3 and dPH2 groups.66 Schumann and Müller were the first to use this approach by identifying the formation of dianionic {CHSiMe3}2−.6 though no structural validation could be obtained and the dianionic silylcontaining fragment {(CH2)2SiMe3}2− was subsequently identified in analogous reactions.30 In this section the most important Siand P-stabilised alkylidene complexes are covered, which include a-silyl (Section 3.08.3.1), phosphorano P(V) (Section 3.08.3.2) and phosphino P(III) alkylidenes (Section 3.08.3.3), with the addition of phosphonio-alkylidenes obtained from phosphorous ylides (Section 3.08.3.3).

3.08.3.1

a-Silyl-alkylidenes

As discussed in previous sections, the first RE a-silyl-alkylidenes 1 and 2-Ln (Scheme 1) were originally reported by Schumann and Müller in 1979, though the lack of structural data did not show the state of aggregation and nuclearity of such species (Table 3).6 Almost 40 years later, Cui and co-workers succeeded in the structural authentication of a dinuclear Lu a-silyl-alkylidene, [Lu2(BODDI)(m-CHSiMe3)(CH2SiMe3)2(THF)2] (67; BODDI]{(Dipp)NC(Me)CHCOCHC(Me)N(Dipp)}2−), obtained from the reaction of the proligand BODDI-H2 with the tris-alkyl precursor [Lu(CH2SiMe3)3(THF)2] (Scheme 21).67 Treatment of 67 with one equivalent of Dipp-NH2 or benzophenone (Scheme 22) yielded the substitution products of the methylidene for an imide-, [Lu2(BODDI)(CH2SiMe3)2(THF)2(m-N-Dipp)] (68), or oxo-bridged compound, [Lu2(BODDI) (CH2SiMe3)2(THF)2(m-O)] (69).67 Interestingly, treatment of either 67 or 69 with excess benzophenone gave a product presenting one diolate and two alkoxide ligands, [Lu2(BODDI-OCPh2){OC(Ph)2(CH2SiMe3)}{m-OC(Ph)2(CH2SiMe3)}(O2CPh2-k2O,O0 ) (THF)] (70), which was proposed to form via insertion of two molecules of benzophenone into the LudCalkyl bonds, followed by a transannular addition and concomitant dissociation of the coordinating nitrogen atom, and finally insertion of Lu-m2-O into the carbonyl of a molecule of benzophenone (Scheme 22). Reaction of 67 with phenyl isocyanate gave [Lu2(BODDI){OC(NPh)CHC(NPhSiMe3)O-k3O,O0 ,N}(CH2SiMe3)2(THF)] (71), while tert-butyl-isocyanide reacted with 67 through both addition and coordination pathways to give [Lu2(BODDI){N(tBu)CCHSi(Me)2C(CH2)N(tBu)}(CH2SiMe3) (CNtBu)2] (72).67 Table 3

a-Silyl-alkylidenes RE complexes covered in Section 3.08.3.1, with relevant crystallographic and spectroscopic data.

Molecular formula

RE oxidation state

M–C (Å)

13

Compound

References

[Lu{CH(SiMe3)}(CH2SiMe3)2Li] [Li(TMEDA)][Lu{CH(SiMe3)}(CH2SiMe3)2] [Er(CHSiMe3)(CH2SiMe3)] [Lu2(BODDI)(m-CHSiMe3)(CH2SiMe3)2(THF)2] [(Lu{C(pz)3})2(m-3,5-Me2HC3N2)2(m-CHSiMe3)] [Sc(PNPPhiPr)(m-CHSiMe3){(m-Me)AlMe(CH2SiMe3)] [Li(THF)4]2[{Sc(Cl)2{m-C4(Ph)2(SiMe3)2}Sc(THF)(m-Cl)}2] [Li(THF)4]2[{Sc(Cl)2{m-C4(p-Me-C6H4)2(SiMe3)2}Sc(THF)(m-Cl)}2] [Li(THF)4]2[{Sc(Cl)2{m-C4(p-tBu-C6H4)2(SiMe3)2}Sc(THF)(m-Cl)}2]

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

– – – 2.309(6)–2.312(6) 2.0845(14) 2.549(2) 2.164(5)–2.197(3) – –

– – – 116.4 126.3 137.2 Not assigned Not assigned Not assigned

1a 1b 2 67 74 76 81a 81b 81c

6

C (ppm)

6 6 67 68 32 69 69 69

Alkylidene Complexes of the Group 3 Metals and Lanthanides

285

Scheme 21 Synthesis of the dinuclear Lu a-silyl-alkylidene 67 by Cui and co-workers.67 Li, S.; Wang, M.; Liu, B.; Li, L.; Cheng, J.; Wu, C.; Liu, D.; Lui, J.; Cui, D., Chem. - Eur. J. 2014, 20, 15493–15498.

Scheme 22 Reactivity of 67 towards aniline (68), benzophenone (69 and 70), isocyanate (71) and isocyanide (72).67 Li, S.; Wang, M.; Liu, B.; Li, L.; Cheng, J.; Wu, C.; Liu, D.; Lui, J.; Cui, D., Chem. - Eur. J. 2014, 20, 15493–15498.

A bridged alkylidene complex closely related to 67 was reported by Chen and co-workers in 2016, when studying tris(pyrazolyl) methanide, {C(pz)3}−, complexes of the small RE metals Sc, Y, Lu for the purpose of developing an isoprene polymerisation catalyst.68 The authors obtained an unexpected result in the case of lutetium: in an attempt to obtain a heteroleptic hydride derivative, the bis-alkyl complex [Lu{C(pz)3}(CH2SiMe3)2(THF)] (73) was treated with PhSiH3 (Scheme 23) to give the unexpected product [(Lu{C(pz)3})2(m-3,5-Me2HC3N2)2(m-CHSiMe3)] (74), which was characterised by 1H and 13C NMR spectroscopy and X-ray crystallography.68 NMR spectroscopic analysis revealed that PhSiH2CH2SiMe3 was formed as a by-product and it was hypothesised by the authors that an intermediate of the reaction was a lutetium hydride, which underwent ligand cleavage to yield 74.68 To date, the structural authentication of a terminal unsupported alkylidene RE complex employing the {CHSiMe3}2− ligand remains elusive. However, very recently Mindiola and co-workers have been able to isolate a LA-stabilised example, obtained through methodologies employed previously for the preparation of LA-stabilised methylidenes (vide supra Section 3.08.2.2). Zatespin et al. reacted the bis-alkyl complex [Sc(PNPPhiPr)(CH2SiMe3)2] (75) with AlMe3 (Scheme 24), leading to insertion in one of the ScdC bonds, followed by elimination of one equivalent of methane to generate the Tebbe-like complex [Sc(PNPPhiPr) (m-CHSiMe3){(m-Me)AlMe(CH2SiMe3)}] (76).31 Complex 76 exhibits a ScdCHSiMe3 distance [2.0845(14) Å] significantly shorter

286

Alkylidene Complexes of the Group 3 Metals and Lanthanides

Scheme 23 Synthesis of the bridged alkylidene complex 74.68 Li, T.; Zhang, G.; Guo, J.; Wang, S.; Leng, X.; Chen, Y. Organometalllics 2016, 35, 1565–1572.

Scheme 24 Synthesis of the LA-supported alkylidene 76.31 Scott, J.; Fan, H.; Wicker, B. F.; Fout, A. R.; Baik, M. H.; Mindiola, D. J., J. Am. Chem. Soc. 2008, 130, 14438–14439.

Scheme 25 Reactivity of 76 with DMAP to give 77 and 78, and N3Ad to afford 79.31 Scott, J.; Fan, H.; Wicker, B. F.; Fout, A. R.; Baik, M. H.; Mindiola, D. J., J. Am. Chem. Soc. 2008, 130, 14438–14439.

than that observed in the analogous Sc methylidene 56 [2.317(2) Å], in which the ScdCH2 moiety is supported by two AlMe3 fragments. Unlike Tebbe’s reagent and other LA-stabilised alkylidenes, 76 does not release an Al-containing fragment upon treatment with Lewis bases such as diazabicyclo[2.2.2]octane and PMe3.31 On the other hand, when 76 is reacted with DMAP (Scheme 25), CdH activation of the pyridyl ring occurs with concomitant formation of the adduct (DMAP)Al(Me)2(CH2SiMe3). The authors observed that without a second equivalent of DMAP the yield of the reaction is halved, suggesting that the mechanism involves formation of a transient terminal alkylidene, [Sc(PNPPhiPr)(CHSiMe3)(DMAP)] (77), which is then converted into the CdH activation product

Alkylidene Complexes of the Group 3 Metals and Lanthanides

287

[Sc(PNPPhiPr)(CH2SiMe3)(Z2-NC5H3− NMe2)] (78).31 Additionally, reaction of 76 with N3Ad (Ad ¼ adamantyl) proceeds via insertion of the azide into the ScdCHSiMe3 bond (Scheme 25), followed by abstraction of the bridging methyl group from Al to Sc to yield [Sc(PNPPhiPr)(CH2SiMe3)(Me){Z2-N3(Ad)CH(SiMe3)Al(Me)(CH2SiMe3)}] (79).31 All examples illustrated so far in this section feature a single methanediide ligand, either bridging between two metal centers or supported by a LA. Nevertheless, the library of a-silyl-alkylidenes was expanded in 2017 by Ma et al. who reported the preparation of a small family of tetrametallic Sc complexes supported by a butene-tetraanion, [Li(THF)4]2[{Sc(Cl)2{m-C4(Ar)2(SiMe3)2} Sc(THF)(m-Cl)}2] (81a, R]Ph; 81b, R]p-Me-C6H4; 81c, R]p-tBu-C6H4).69 All complexes were prepared directly via salt metathesis reactions between 1,4-dilithio-1,3-butadienyl salts [Li2{C4(Ar)2(SiMe3)2}] (80: Ar]Ph, p-Me-C6H4, p-tBu-C6H4) with ScCl3 in boiling THF (Scheme 26). The authors observed that when the reaction was carried out at −20  C the heteroleptic ate-complex [{Sc {C4(Ph)2(SiMe3)2}(THF)(m-Cl)2}2Li(THF)2] (82) was obtained, which can be readily converted into 81a by heating in THF; the concomitant formation of four equivalents of Ph-C^C-SiMe3 formed upon double reduction and CdC bond cleavage of two 1,3-butadienyl ligands.69 Owing to the presence of the butadienyl tetraanion, 81a–c can act as a two- or four-electron reducing agent. Zhang and co-workers tested the reactivity of 81a with oxidants, such as hexachloroethane, disulfides and cyclooctatetraene (Scheme 27).69 When two equivalents of hexachloroethane were employed, complex 82 was obtained together with tetrachloroethene and ScCl3; however, when four equivalents of oxidant are employed, 81a was fully converted into Ph-C^C-SiMe3, thus acting as a

Scheme 26 Synthesis of Sc bis-alkylidene complexes 81a–c.69 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.

Scheme 27 Products of two- and four-electron reductions of 81a with hexachloroethane (82), disulfides (83) and cyclooctatetraene (84).69 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.

288

Alkylidene Complexes of the Group 3 Metals and Lanthanides

four-electron reductant. Moreover, the reaction of the disulfide {SCS(NMe2)}2 with 81a yielded the disulfido complex [Sc {SC(NMe2)S-k2S,S0 }(Cl)2(THF)2] (83), while the reaction of 81a with cyclooctatetraene proceeded by a 2e− reduction to give a cyclooctatetraenyl dianion {COT}2− in the piano-stool complex [Sc(COT)(Cl)(DME)] (84, DME ¼ 1,2-dimethoxyethane).69

3.08.3.2

Phosphorano-stabilised alkylidenes

In classic TM carbene chemistry, the metal centers can impart a certain degree of stabilisation to the carbanion owing to the favorable metal-ligand frontier orbital overlap. However, such stabilisation factors are absent in RE carbene complexes owing to the highly ionic character of their metal-carbon bond interactions (Section 3.08.1). Additionally, the lack of directionality of these interactions combined with the large ionic radii of RE ions often results in oligomerisation, as shown in previous sections. Over the last 20 years, phosphorano substituents have been employed to circumvent these issues, owing to their ability to stabilise the dianionic charge of the methanediide ligand.7,11,70 Three examples of phosphorano-methanediides have been reported with the RE metals to date (Fig. 5): bis(iminophosphorano)methanediides (I, BIPM), bis(diphenylthiophosphinoyl)methanediides (II, SCS) and a-silyl-(diphenylthiophosphinoyl)methanediides (III, SCSi). The first two examples are best described as pincer-type ligands, while the latter acts as a bidentate donor (Table 4).

3.08.3.2.1

Pincer-type

The traditional Lewis structure of the methanediides {C(PPh2NR)2}2− (BIPMR; R]SiMe3, iPr, Mes) and {C(PPh2S)2}2− (SCS) (Scheme 28, A) does not give a truly accurate picture of the bonding and several resonance forms can be used to describe both BIPM and SCS dianions and their resulting complexes. This aspect has been investigated extensively over the last two decades by the groups of Mézailles, Le Floch and Liddle by combining structural data analysis and reactivity of RE methanediides with computational studies.7,11,72–74 Liddle and co-workers observed that both structural data and NBO analysis converge in characterising the RE–C interactions in yttrium BIPMR complexes intermediate between a single and a double bond interaction.71 This would necessitate abstracting charges to the methanediide carbon and the metal, hence resulting in resonance structure B (Scheme 28). However, owing to the highly ionic nature of the bonding in RE metal complexes, resonance structure C offers a more appropriate representation of the electrostatic nature of the metal-carbon interaction in these species. Lastly, there is the matter of the P]E (E]N, S) and PdC bonds. Studies by Liddle and co-workers on RE BIPM complexes showed that the PdC and PdN bonds both have bond orders consistently >1, which reflects a polarisation of the central carbon and nitrogen lone pairs towards phosphorus, although true multiple bonds are not present.75 Parallels can be drawn here with phosphorus ylides, and the R3P+–C− R2 resonance structure that is often drawn for these species. Nevertheless, an alternative and more complete picture is provided by the dipolar resonance form D, featuring two anionic amides alongside the methanediide anion, which is in good agreement with NBO analysis. Finally, there are several other resonance forms that contribute to the overall picture, including NHC-type structures E and F and carbodiphosphorane G. For simplicity, Lewis structure A will be used in all schemes and diagrams in the rest of this Section. These observations are largely mirrored in RE SCS complexes analysed by Mézailles, though the Ph2PS substituent is less efficient in delocalising the charge of the geminal carbon dianion compared to Ph2PN(SiMe3), which results in a slightly higher NBO charge on the C donor.70,73,74,76 3.08.3.2.1.1 Bis(iminophosphorano)methanediide complexes The first monometallic nucleophilic carbene complex of a RE metal was reported by Cavell and co-workers in 2000. Their synthetic approach was based on the preparation of alkylidene complexes of the Group 4 metals through protonolysis of the proligand H2C(Ph2PNSiMe3)2 (H2-BIPMTMS) with metal benzyl precursors e.g. Zr(CH2Ph)4.77 In an adapted methodology, the authors reacted H2-BIPM with [Sm(NCy2)3(THF)] (Cy ¼ cylohexyl) to afford the alkylidene [Sm(BIPMTMS)(NCy2)(THF)] (85, Scheme 29).78 Structural analysis of 85 revealed an open-book motif, with the two four-membered rings forming a dihedral angle of 37.7 (1) . The most salient feature of the structure of 85 is the SmdC distance [2.467 (4) Å], which was found to be shorter than average SmdC bonds in alkyls [ca. 2.74 Å]78 and the neutral carbene complex [Sm(Cp )2(IMe4)2] (IMe4 ¼ 1,3,4,5-tetramethyl-2-ylidene) [2.837(7) and 2.845(7)Å] reported by Arduengo et al..79 However, SmdC single bond interactions shorter than 2.45 Å have been reported since then.8,9

Fig. 5 Phosphorano-stabilised alkylidenes bis(iminophosphorano)methanediide (I), bis(thiophosphorano)methandiide (II) and a-silyl-(thiophosphorano) methandiide (III).

Alkylidene Complexes of the Group 3 Metals and Lanthanides

289

Phosphorano-stabilised alkylidene RE complexes covered in Section 3.08.3.2, with relevant crystallographic and spectroscopic data.

Table 4

RE oxidation state

M–C (Å)

13

[Sm(BIPM )(NCy2)(THF)] [Y(BIPMTMS)(CH2SiMe3)(THF)]

+3 +3

2.467(4) –

[Y(BIPMTMS)(CH2Ph)(THF)]

+3

2.357(3)

[Dy(BIPMTMS)(CH2Ph)(THF)] [Er(BIPMTMS)(CH2Ph)(THF)] [Y(BIPMTMS)(H-BIPMTMS)(THF)]

+3 +3 +3

2.364(2) 2.339(4) 2.371(4)

[La(BIPMTMS)(H-BIPMTMS)(THF)]

+3

2.512(2)

[Ce(BIPMTMS)(H-BIPMTMS)(THF)] [Pr(BIPMTMS)(H-BIPMTMS)(THF)] [Nd(BIPMTMS)(H-BIPMTMS)(THF)] [Sm(BIPMTMS)(H-BIPMTMS)(THF)] [Gd(BIPMTMS)(H-BIPMTMS)(THF)] [Tb(BIPMTMS)(H-BIPMTMS)(THF)] [Dy(BIPMTMS)(H-BIPMTMS)(THF)] [Nd(BIPMiPr)(H-BIPMiPr)] [La(BIPMMes)(H-BIPMMes)] [Ce(BIPMMes)(H-BIPMMes)] [PrBIPMMes)(H-BIPMMes)] [Gd(BIPMMes)(H-BIPMMes)] [Y(BIPMTMS)(OCPh2CH2Ph)(THF)]

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

2.472(4) 2.458(5) – 2.40(2) 2.406(2) 2.385(2) 2.364(2) 2.592(3) 2.725(5)a 2.681(11) 2.662(6)a 2.573(10)a 2.393(2)

[Y(BIPMTMS)(OCPh2CH2SiMe3)(THF)]

+3

2.422(13)

[Y(BIPMTMS){N(Ph)N(Ph)(CH2Ph)}]

+3

2.408(5)

[Y(BIPMTMS)(N]C{tBu}CH2Ph)(THF)]

+3

2.428(2)

[Y(BIPMTMS)(N3Ad-1,CH2Ph-3-k2N,N0 )(THF)]

+2

2.482(5)

[Y(BIPMTMS)(NH-Dipp)(THF)] [Y(BIPMTMS)(I)(THF)2]

+3 +3

2.426(2) 2.356(3)

[Er(BIPMTMS)(I)(THF)2] [Y{Ga[N(Dipp)CH]2}(BIPMTMS)(THF)2]

+3 +3

2.322(2) 2.348(3)

[Y(BIPMTMS){N(SiMe3)2}(THF)]

+3

–

[Y(BIPMTMS){N(SitBuMe2)2}(THF)]

+3

2.357(3)

[Ce(BIPMTMS)(I)(DME)] [Ce(BIPMTMS)(ODipp)2]

+3 +4

2.539(2) 2.441(5)

[K(18-crown-6)(THF)2][Y(BIPMTMS)2]

+3

2.455(3)–2.459(4)

– 60.1 (td) 1 JPC 131.9 Hz 1 JYC 4.9 Hz 61.8 (td) 1 JPC 207.3 Hz 1 JYC 5.0 Hz – – 66.5 (td) 1 JYC 6.9 Hz 73.8 (t) 1 JPC 171 Hz – – – – – – – – 45.1 – – – 51.0 (td) 1 JPC 134.8 Hz 1 JYC 7.0 Hz 51.3 (td) 1 JPC 133.1 Hz 1 JYC 6.5 Hz 60.0 (td) 1 JPC 207.3 Hz 1 JYC 5.0 Hz 51.4 (td) 1 JPC 152.0 Hz 1 JYC 5.0 Hz 59.8 (td) 1 JPC 185.1 Hz 1 JYC 4.0 Hz – 60.3 (td) 1 JPC 207.3 Hz 1 JYC 5.0 Hz – 61.5 (td) 1 JPC 203.3 Hz 1 JYC 5.0 Hz 57.8 (td) 1 JPC 180.0 Hz 1 JYC 5.0 Hz 63.4 (td) 1 JPC 166.2 Hz 1 JYC 8.1 Hz – 324.6 (t) 1 JPC 148.7 Hz 53.7 (td) 1 JPC 210.9 Hz 1 JYC 3.1 Hz

Molecular formula TMS

C (ppm)

Compound

References

85 86

78

87-Y

81

87-Dy 87-Er 88-Y

82

88-La

80

88-Ce 88-Pr 88-Nd 88-Sm 88-Gd 88-Tb 88-Dy 90 92-La 92-Ce 92-Pr 92-Gd 93a

80

93b

87

96

81

97

88

98

88

102 104-Y

88

104-Er 116

75

117

91

118

91

123 125

96

127-Y

82

71

80 82

80 80 80 80 83 82 85 86 86 86 86 81

75

84

96

(Continued )

290

Table 4

Alkylidene Complexes of the Group 3 Metals and Lanthanides

(Continued) RE oxidation state

M–C (Å)

13

Compound

References

[K(18-crown-6)(THF)2][Ce(BIPM )2] [K(18-crown-6)(THF)2][Pr(BIPMTMS)2] [K(18-crown-6)(THF)2][Tb(BIPMTMS)2] [K(18-crown-6)(THF)2][Dy(BIPMTMS)2] [Nd(BIPMTMS)(Cl)(THF)] [Li(THF)4][Nd(BIPMTMS)2] [Ce(BIPMTMS)2]

+3 +3 +3 +3 +3 +3 +4

2.598(3)–2.603(3) 2.573(4)–2.579(3) 2.460(5)–2.469(5) 2.431(6)–2.433(6) – 2.583(3)–2.585(3)

127-Ce 127-Pr 127-Tb 127-Dy 128 129 130

83

[Pr(BIPMTMS)2Ag] [Tb(BIPMTMS)2Ag] [{Sm(SCS)(THF)2(m-I)}2] [{Tm(SCS)(THF)2(m-I)}2] [Sc(SCS)(Cl)(py)2] [{Sc(SCS)2}{Li(THF)2}] [Sc(SCS)(H-SCS)(THF)] [Sc(SCS)(CH2Ph)(THF)2] [Sc(SCS)(Cl)(THF)2] [Sc(SCS)(Me)(THF)] [Sc(SCS){N(SiMe3)2}(THF)]

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

2.411(3) 2.347(2) 2.371(6) 2.325(5) 2.207(3) 2.212(8)–2.243(8) 2.204(4) – – – 2.247(2)

131-Pr 131-Tb 132-Sm 132-Tm 136 137 138 139 144 145 146

83

[Sc(SCS){P(SiMe3)2}(py)2] [Sc(NacNacNMe2)(SCS)] [Sc(NacNacNMe2){C(SiMe3)PPh2S-k2S,C}] [Y(NacNacNMe2){C(SiMe3)PPh2S-k2S,C}]

+3 +3 +3 +3

2.200(3)–2.225(3) 2.246(2)–2.273(3) 2.1134 (18) 2.256(3)

147 149 153-Sc 153-Y

76

[Sm(NacNacNMe2){C(SiMe3)PPh2S-k2S,C}] [Lu(NacNacNMe2){C(SiMe3)PPh2S-k2S,C}]

+3 +3

2.296(2) 2.204(3)

153-Sm 153-Lu

103,104

[Sc(NacNacNMe2){C(SiPh3)PPh2S-k2S,C}] [Y(NacNacNMe2){C(SiPh3)PPh2S-k2S,C}]

+3 +3

– 2.316(2)

154-Sc 154-Y

103,104

[La(NacNacNMe2){C(SiPh3)PPh2S-k2S,C}]

+3

2.465(2)

154-La

103,104

[Sm(NacNacNMe2){C(SiPh3)PPh2S-k2S,C}] [Lu(NacNacNMe2){C(SiPh3)PPh2S-k2S,C}]

+3 +3

2.357(4) 2.245(3)

154-Sm 154-Lu

103,104

[Sc(NacNacNiPr2){C(SiMe3)PPh2S-k2S,C}] [Sc(NacNacNiPr2){C(SiPh3)PPh2S-k2S,C}] [Y(NacNacNMe2){C(SiPh3)PPh2S}(NCPh)]

+3 +3 +3

2.125(2) 2.159(4)

155 156 164-Y

103,104

[Lu(NacNacNMe2){C(SiPh3)PPh2S}(NCPh)] [Y(NacNacNMe2){C(SiPh3)PPh2S}(CNtBu)]

+3 +3

164-Lu 166-Y

103

–

[La(NacNacNMe2){C(SiPh3)PPh2S}(CNtBu)] [Sm(NacNacNMe2){C(SiPh3)PPh2S}(CNtBu)] [Lu(NacNacNMe2){C(SiPh3)PPh2S}(CNtBu)]

+3 +3 +3

– – 2.328(2)

– – – – – – 343.5 (t) 1 JPC 170.2 Hz – – – – Not assigned Not assigned Not assigned Not assigned – – 64.4 (t) 1 JCP ¼ 71.0 Hz not assigned – 112.0 (br) 91.5 (dd) 1 JPC ¼ 17.1 Hz 1 JYC ¼ 38.6 Hz – 89.3 (d) 1 JPC ¼ 13.8 Hz 92.3 (br) 76.6 (dd) 1 JPC ¼ 12.4 Hz 1 JYC ¼ 34.0 Hz 93.3 (d) 1 JPC ¼ 46.2 Hz – 76.5 (d) 1 JPC ¼ 12.5 Hz 117.0 (br) 92.9 (br) 64.1 (d) 1 JYC ¼ 34.0 Hz 67.8 64.1 (d) 1 JPC ¼ 36.6 Hz – – 67.8

166-La 166-Sm 166-Lu

103

Molecular formula TMS

C (ppm)

83 83 82 101 101 83

83 99 100 74 76 76 76 76 76 76

103 103,104 103,104

103,104

103,104

102,103

103,104 103

103

103 103

a

All these crystal structures a statistically equivalent methanide MdCH bond distance is also present.

Since Cavell’s breakthrough, protonolysis protocols have been employed very effectively across Group 3 and the f-block by using tris-alkyl precursors. Liddle et al. prepared the yttrium complex [Y(BIPMTMS)(CH2SiMe3)(THF)] (86) by reacting H2-BIPMTMS with [Y(CH2SiMe3)(THF)2] (Scheme 30).71 Complex 86 was the first RE alkylidene monometallic complex to feature both alkylidene and alkyl YdC bonds within the same coordination environment, thus allowing for a better comparison between these interactions. Remarkably, the two YdC bond lengths in 86 [Y–Calkylidene ¼ 2.406(3) Å; YdCalkyl ¼ 2.408 (3) Å] are statistically identical and within the range of YdCalkyl distances, though shorter than Y–C interactions in methanide complexes, e.g. [Y(H-BIPMTMS) (I)2(THF)2]; YdC ¼ 2.599 (2) Å.84 Liddle and co-workers extended this methodology to RE tris-benzyl reagents

Alkylidene Complexes of the Group 3 Metals and Lanthanides

291

Scheme 28 Resonance forms of bis(iminophosphorano)methanediide BIPMTMS.71 Liddle, S. T.; Mills, D. P.; Wooles, A. J., Chem. Soc. Rev. 2011, 40, 2164–2176.

Scheme 29 Synthesis of the first monometallic RE alkylidene 85 by Cavell and co-workers.78 Aparna, K.; Ferguson, M.; Cavell, R. G., J. Am. Chem. Soc. 2000, 122, 726–727.

Scheme 30 Use of RE tris-alkyl precursors in protonolysis methodologies employed by Liddle and co-workers for the preparation of heteroleptic alkylidene complexes 86,71 87-RE80,81 and 88-RE.80,82,83 Wooles, A. J.; Mills, D. P.; Lewis, W.; Blake, A. J.; Liddle, S. T., Dalton Trans. 2010, 39, 329–336; Mills, D. P.; Cooper, O. J.; McMaster, J.; Lewis, W.; Liddle, S. T., Dalton Trans. 2009, 4547–4555; Gregson, M.; Chilton, N. F.; Ariciu, A. M.; Tuna, F.; Crowe, I. F.; Lewis, W.; Blake, A. J.; Collison, D.; McInnes, E. J. L.; Winpenny, R. E. P.; Liddle, S. T., Chem. Sci. 2016, 7, 155–165; Gregson, M.; Lu, E.; Mills, D. P.; Tuna, F.; McInnes, E. J. L.; Hennig, C.; Scheinost, A. C.; McMaster, J.; Lewis, W.; Blake, A. J.; Kerridge, A.; Liddle, S. T., Nat. Commun. 2017, 8, 14137.

292

Alkylidene Complexes of the Group 3 Metals and Lanthanides

[RE(CH2Ph)3(THF)3] (RE]Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Er); in the case of the smaller metals either the methanediide-benzyl [RE(BIPMTMS)(CH2Ph)(THF)] (87-RE; RE]Y, Dy, Er)80,81 or methanediide-methanide complexes [RE(BIPMTMS)(H-BIPMTMS) (THF)] (88-RE; RE]Y, Dy, Tb) can be obtained, while for the larger RE it is not possible to control the reactivity and 88-RE (RE]La, Ce, Pr, Nd, Sm, Gd) are obtained irrespective of the stoichiometry employed (Scheme 30).80,82,83 A Nd analogue of 88-RE was also reported by Le Floch and co-workers, who employed BIPMiPr as supporting ligand,85 by first obtaining the bis-methanide complex [Nd(H-BIPMiPr)2(I)] (89) via a salt metathesis reaction between K(H-BIPMiPr) and [NdI3(THF)3.5].85 Surprisingly, the treatment of 89 with K{N(SiMe3)2} yielded the methanediide-methanide complex [Nd(BIPMiPr)(H-BIPMiPr)] (90) (Scheme 31). The original intention of the authors was to prepare an amide complex of formula [Nd(H-BIPMiPr)2{N(SiMe3)2}], however the enhanced acidity of the methanide proton led to the formation of carbene complex 90 with concomitant elimination of KI. Addition of a further equivalent of K{N(SiMe3)2} did not afford a bis-alkylidene complex. Analogous methanediide-methanide BIPMMes complexes were later reported by Liddle and co-workers (Scheme 31), in an attempt to prepare the methanediide complexes [RE(BIPMMes)(I)(THF)n] by dehydrohalogenation of [RE(H-BIPMMes)(I)2(L)x] (91a-RE: L ¼ THF, x ¼ 2, RE ¼ La, Ce, Pr, Nd, Gd; 91b-RE: L ¼ TMEDA, x ¼ 1, RE ¼ La, Gd); however, ligand scrambling occurred and led to the isolation of the heteroleptic alkylidene complexes [RE(BIPMMes)(H-BIPMMes)] (92-RE: RE ¼ La, Ce, Pr, Gd).86 Aldehydes and ketones are typical choices for preliminary reactivity tests of alkylidene complexes towards unsaturated substrates. The two alkyl-alkylidenes 86 and 87-Y were both treated with benzophenone and showed divergent reactivity (Scheme 32).81,87 For both complexes the initial step is an insertion into the Y–Calkyl bond to yield the alkoxo derivatives [Y(BIPMTMS) (OCPh2R)(THF)] (93a, R]CH2Ph; 93b, R]CH2SiMe3).81,87 The addition of a second equivalent of benzophenone to 93a under reflux conditions leads to the formation of bridged alkylidene complex [{Y(m-BIPM)(OCPh2CH2Ph)}2] (94).81 On the other hand, reaction of 93b with two further equivalents of benzophenone gave the methanide complex [Y(H-BIPMTMS)(OCPh2CH2SiMe3)O-{(CPh2)(OCPh)C6H4}] (95), which contains an isobenzofuran ligand formed through ortho-C-H aryl activation of a coordinated benzophenone and subsequent attack on the ketyl carbon of a second benzophenone molecule.87 The authors hypothesised that formation of 95 occurs via tautomerism of an oxymethylbenzophenone intermediate, which was elegantly demonstrated in the same report (vide infra). In both cases no Wittig-type reactivity was observed. The reactivity of 87-Y was also investigated with a variety of substrates by Mills et al., which include azobenzene,81 1-admantylazide, tert-butylcyanide, tert-butylisocyanates and Dipp-NH2.88 In the reaction with azobenzene, insertion occurs into the YdCalkyl bond yielding the Z2-amido derivative [Y(BIPMTMS){N(Ph)N(Ph)(CH2Ph)}] (96).88 Moreover, reactivity of 87-Y with tert-butyl cyanide and 1-adamantyl azide gave the corresponding 1,2-migratory insertion products [Y(BIPMTMS)(N]C{tBu} CH2Ph)(THF)] (97) and [Y(BIPMTMS)(N3Ad-1,CH2Ph-3-k2N,N0 )(THF)] (98), respectively, in near quantitative yield by 31P NMR spectroscopy. Addition of further equivalents of tert-butylcyanide to 97 or 1-adamantylazide to 98 did not result in reaction at the carbene centre. Furthermore, the heteroallenes N,N0 -dicyclohexylcarbodiimide, tert-butyl isocyanate and tert-butyl isothiocyanate were reacted with 87-Y, with the reactivity profile dependent upon the heteroatoms present (Scheme 33).88 The carbodiimide and isocyanate reacted with 87-Y through [2 + 2]-cycloaddition across the Y]C bond and 1,2-migratory insertion at the benzyl YdC bond, yielding [Y{C(PPh2NSiMe3)2[C(NCy)2]-k4C,N,N0 ,N00 }{C-(NCy)2(CH2Ph)-k2N,N0 }] (99) and [Y{C(PPh2NSiMe3)2[C(O) (NtBu)]-k4C,N,N0 ,O}{C-(O)(NtBu)(CH2Ph)-k2N,O}] (100), respectively. The authors could not identify the exact order of reaction, since even careful manipulation of the stoichiometric ratio led only to reduced yield. For example, if a 1:1 molar ratio was

Scheme 31 Synthesis of the methanediide-methanide complexes 9085 and 92-RE.86 Buchard, A.; Auffrant, A.; Ricard, L.; Le Goff, X. F.; Platel, R. H.; Williams, C. K.; Le Floch, P., Dalton Trans. 2009, 3, 10219–10222; Marshall, G.; Wooles, A.; Mills, D.; Lewis, W.; Blake, A.; Liddle, S. T., Inorganics 2013, 1, 46–69.

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Scheme 32 Reactivity of alkylidene-alkyl complexes 86 and 87-Y with benzophenone and formation of alkoxide derivatives 93a and 93, followed by isolation of bridging methanediide 9481 and isobenzofuran-methanide complex 95.87 Mills, D. P.; Soutar, L.; Lewis, W.; Blake, A. J.; Liddle, S. T., J. Am. Chem. Soc. 2010, 132, 14379–14381.

Scheme 33 Reactivity of 87-Y with unsaturated substrates.81,88 Mills, D. P.; Soutar, L.; Cooper, O. J.; Lewis, W.; Blake, A. J.; Liddle, S. T., Organometallics 2013, 32, 1251–1264.

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used, the yield of the double-addition product was 50% with no detectable monoaddition product. The implication behind this is that the reaction of 87-Y with one equivalent of heteroallene generates a more reactive species that binds with the second equivalent of substrate in favour of the remaining 87-Y. Conversely, the reaction of 87-Y with tBuNCS afforded [Y(H-BIPMTMS){C(S)2-(NtBu)1-kS,2-kN:m,kS0 }]2 (101).88 This reaction was proposed to initially proceed via 1,2-migratory insertion, but the resultant product then reacted further with loss of isobutylene and additional coupling of another equivalent of tert-butyl isothiocyanate to give a transient six-membered metallacycle and subsequent loss of phenylacetonitrile, resulting in an yttrium dithiocarbimidate complex, which dimerises to afford 101 (Scheme 34).88 At the time of this report, such a reactivity pathway was unprecedented for RE and TM carbenes, which had been previously found to react either via [2 + 2]-cycloaddition reactions or redox processes starting from lowvalent states.88 Finally, 87-Y was reacted separately with 2,6-diisopropylaniline and benzyl potassium. In the first instance, the YdCbenzyl bond undergoes protonolysis of one aniline proton to yield the corresponding amide [Y(BIPMTMS)(NH-Dipp)(THF)] (102) and no reaction with the Y]C bond was observed. The reaction of 87-Y with benzyl potassium yielded polymeric [Y(BIPMTMS)(m-Z1:Z6-CH2Ph)(m-Z1:Z2-CH2Ph)K]1 (103) by a 1,2-carbopotassiation reaction; interestingly, the same complex was obtained from the reaction between 86 and benzyl potassium.88 An alternative protonolysis strategy was also developed by Mills et al., which involves the use of heteroleptic benzyl-halide precursors of general formula [RE(CH2Ph)2(I)(THF)3] (RE]Y, Er) and allows for the formation of heteroleptic species which incorporate a versatile halide functionality. Reaction of [RE(CH2Ph)2(I)(THF)3] with H2-BIPMTMS afforded the alkylidene-halide complexes [RE(BIPMTMS)(I)(THF)2] (104-RE; RE]Y, Er) in excellent yields (Scheme 35).75 Alternatively, 104-Y can be prepared by deprotonation of the methanide complex [Y(H-BIPMTMS)(I)2(THF)] (105) with concomitant salt elimination upon treatment with benzyl potassium.84 104-Y and 104-Er exhibit a distorted octahedral geometry, with the RE]C bond in 104-RE positioned trans- to the halide ligand in a pseudo-axial arrangement. Remarkably, the methanediide ligands in 104-RE exhibit T-shaped geometries and planar conformations of the Y2P2C ring; the authors attributed this conformation to the increased Lewis acidity of the metal centre owing to the presence of a soft halide donor.75 The RE]C distances in 104-Y [2.356(3) Å] and 104-Er [2.322(2) Å] are comparable to those of the alkylidene-alkyl complexes 87-Y [2.357 (3) Å] and 87-Er [2.339(4) Å];80,81 on the other hand, the analogous interaction in 86 is slightly elongated [Y]C 2.406(3) Å].71 NBO analysis carried out by the authors was in good agreement with models discussed previously, featuring a strong polarisation of the metal-carbon interaction and the predominance of the resonance form comprising dianonic carbon and monoanionic nitrogen donors (D, Scheme 28). However, it is noteworthy that the frontier orbitals and atomic charges of the BIPMTMS backbone in 104-Y are very close to those of carbodiphosphorane Ph3PCPPh3, which suggests the possibility of a further resonance form in which captodative bonding is present between a carbon(0) donor and the phosphorus centers.75 Liddle and co-workers extensively investigated the reactivity of 104-Y, focusing initially on reactivity towards three aryl ketones: benzophenone, tert-butyl phenyl ketone and acetophenone (Scheme 36).87 Treatment of 104-Y with benzophenone furnished the CdH activation product [Y(H-BIPMTMS){OCPh(C6H4)-2-C(O)Ph2}(I)] (106), which upon ligand exchange with bulky the alkoxide K{OC(CH2SiMe3)Ph2} formed the isobenzofuran derivative 95.87 The identification of 106 confirmed the initial

Scheme 34 Proposed mechanism for the formation of 101.88 Mills, D. P.; Soutar, L.; Cooper, O. J.; Lewis, W.; Blake, A. J.; Liddle, S. T., Organometallics 2013, 32, 1251–1264.

Scheme 35 Preparation of the heteroleptic alkylidene-halide complexes 104-Y and 104-Er.75,84 Mills, D. P.; Wooles, A. J.; McMaster, J.; Lewis, W.; Blake, A. J.; Liddle, S. T., Organometallics 2009, 28, 6771–6776; Liddle, S. T.; Mills, D. P.; Gardner, B. M.; McMaster, J.; Jones, C.; Woodul, W. D., Inorg. Chem. 2009, 48, 3520–3522.

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Scheme 36 Reactivity of 104-Y with unsaturated subtrates.87,89 Mills, D. P.; Soutar, L.; Lewis, W.; Blake, A. J.; Liddle, S. T., J. Am. Chem. Soc. 2010, 132, 14379–14381; Mills, D. P.; Lewis, W.; Blake, A. J.; Liddle, S. T., Organometallics 2013, 32, 1239–1250.

hypothesis of Mills et al. which accounted for the initial formation of oxymethylbenzophenone in the reaction between 86 and benzophenone, followed by tautomerisation to give the isobenzofuran in 95.81 Conversely, the oxymethylbenzophenone intermediate could not be isolated when the bulkier mixed ketone Ph(tBu)C]O was employed, which only gave the corresponding substituted isobenzofuran [Y(H-BIPMTMS)(I){O(CPhtBu)(OCtBu)C6H4}] (107).81 Finally, the enolisable ketone acetophenone reacted with 104-Y by cyclotetramerization and dehydration to yield the dypnopinacol 108 (Scheme 36).87 Similarly to the detailed reactivity studies carried out on 87-Y, Mills et al. also explored the reactivity of 104-Y with various unsaturated substrates (Scheme 36).89 The separate reactivions of 104-Y with tert-butylphosphaalkyne, N,N0 dicyclohexylcarbodiimide and tert-butylisocyanate all proceeded via [2 + 2]-cycloadditions to give the propellane-type complexes [Y{C(PPh2NSiMe3)2(PCtBu)-k4C,C0 ,N,N0 }(I)] (109), [Y{C(PPh2NSiMe3)2[C(NCy)2]-k4C,N,N0 ,N00 }(I)(THF)] (110) and [Y{C(PPh2NSiMe3)2[C(O)(NtBu)]-k4C,N,N0 ,O}(I)(THF)2] (111).89 In the case of the phosphaalkyne, the reaction was proposed to take place through coordination of the phosphaalkyne to the metal in an Z2 fashion, followed by nucleophilic attack of the carbene on the phosphorus atom and subsequent conversion of the triple bond to a double, with concomitant formation of a polarised YdC bond. The other heteroallenes reacted in an analogous fashion, similarly to the [2 + 2]-cycloaddition reactions observed for 87-Y.88 However, no 1,2-migratory insertion reactions into the YdI bond were observed, which differentiates 104-Y from the alkylidenebenzyl complex 87-Y, for which reactivity with heteroallenes leads to insertion into the YdCbenzyl bond and concomitant [2 + 2]cycloaddition.88 Unexpectedly, tert-butylisothiocyanate underwent a Wittig-type reaction instead of [2 + 2]-cycloaddition to furnish the ketenimine tBuN]C]C(PPh2NSiMe3)2 (112);89 this reactivity pathway is highly unusual, particularly given the extreme oxophilicity of yttrium. Additionally, the reaction of 104-Y with 2,6-diisopropylaniline proceeded via 1,2-addition across the Y] C bond, affording the methanide-anilide complex [Y(H-BIPMTMS){HN(Dipp)}(I)(THF)] (113).89 Finally, the reaction of 104-Y with the benzopyrone coumarin afforded the ring-opened dinuclear aryloxide-enolate complex [Y2{C(PPh2NSiMe3)2(C[O]

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[CHCHC6H4O-2])-k2N,O:m,kO0 }2(I)(m-I)(THF)] (114) via ring opening of the a,b-unsaturated cyclic lactone. Initially, the ketone undergoes [2 + 2]-cycloaddition across the Y]C bond, followed by dimerization of the resultant product to yield 114.89 Complex 104-Y was also found to be capable of CdF activation with benzoyl fluoride, undergoing the usual [2 + 2]-cycloaddition pathway followed by migration of the fluoride to the yttrium centre and concomitant cleavage of the YdCmethanide bond. Rapid ligand scrambling then eliminates “YF2I(THF)n” to yield [Y{C(PPh2NSiMe3)2[C(O)(Ph)]-k2N,O}2(I)] (115).89 The presence of a halide functionality in 104-Y opened the possibility of performing salt metathesis reactions to introduce different donors. This strategy was employed successfully by Mills et al. who reacted 104-Y with the gallyl reagent [K(TMEDA)][Ga {(NDippCH)2}] to give a complex containing the first unsupported GadY bond, [Y{Ga[N(Dipp)CH]2}(BIPMTMS)(THF)2] (116), which was shown through computational studies to possess a highly polarised metal-metal bond with a very small yttrium contribution (ca. 2%). This observation is particularly significant especially when compared to the strong uranium involvement (ca. 20%) and p donation present in a GadU bond previously reported by Liddle, Jones and co-workers.84,90 Additionally, Ortu et al. reacted the yttrium methanediide 104-Y with various potassium amides with the intent of preparing the corresponding heteroleptic complexes.91 The synthetic protocol was successful when K{N(SiMe3)2} and K{N(SitBuMe2)2} were employed, affording the methanediide-amide complexes [Y(BIPMTMS){N(SiMe3)2}(THF)] (117) and [Y(BIPMTMS){N(SitBuMe2)2}(THF)] (118). Compound 118 readily undergoes CdH activation to give cyclometallated derivative [Y(H-BIPMTMS){N(SitBuMe2)(SitBuMeCH2)k2N,C}] (119). When more sterically bulky amides are employed, K{N(SiiPr3)(SitBuMe2)} and K{N(SiiPr3)2}, CdH activation is facile and no methanediide species can be isolated; the only products identified in these reactions are the methanide cyclometallated species [Y(H-BIPMTMS){N(SiiPr3)(SitBuMeCH2)-k2N,C}] (120) and [Y(H-BIPMTMS){N(SiiPr3)[SiiPr2(CHCH3CH2)]-k2N,C}] (121) (Scheme 37).91 All examples of RE and Ln carbenes discussed so far feature trivalent metal centers, largely due to the inherent stability of the +3 oxidation state for most of these metals. While the landscape of divalent RE chemistry has been significantly expanded beyond that of classic divalent Lns (Sm, Eu, Yb) over the last two decades,92,93 molecular high-oxidation state Ln chemistry is scarce and limited to few selected examples.94 Ce is the only Ln metal with a well-developed chemistry in its tetravalent state; Ce(IV) has a stable closed-shell noble gas configuration ([Xe]), can be accessed readily with mild oxidants (CeIV/III: E ¼ + 1.7 V vs SHE), and is commonly used in organic synthesis as an oxidising agent, i.e. [Ce(NO3)6][NH4]2, ceric ammonium nitrate (CAN).95 Gregson et al. utilised methanediide BIPMTMS to stabilise the first example of Ce(IV) carbene by employing a multi-step procedure (Scheme 38) starting from the methanide complex [Ce(H-BIPMTMS)(I)2(THF)] (122).96 Complex 122 was first converted into the methanediide [Ce(BIPMTMS)(I)(DME)] (123) via treatment with benzyl potassium, followed by reaction with two equivalents of K {ODipp} to yield the “ate” complex [Ce(BIPMTMS)(ODipp)2(K)(THF)]1 (124). Oxidation of 124 with silver tetraphenylborate afforded the desired product [Ce(BIPMTMS)(ODipp)2] (125). The structure was confirmed by X-ray crystallography, exhibiting a Ce]C distance [2.441(5) Å] shorter than that observed in the methanediide Ce(III) precursor 123 [2.539(2) Å], and the Ce(III)

Scheme 37 Salt metathesis reactions between 104-Y and gallyl84 and amide salts.91 Liddle, S. T.; Mills, D. P.; Gardner, B. M.; McMaster, J.; Jones, C.; Woodul, W. D., Inorg. Chem. 2009, 48, 3520–3522; Ortu, F.; Gregson, M.; Wooles, A. J.; Mills, D. P.; Liddle, S. T., Organometallics 2017, 36, 4584–4590.

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Scheme 38 Synthesis of the Ce(IV) alkylidene 125.96

methanediide-methanide complex 92-Ce [2.681(11) Å].86 Noticeably, the only other Ce methanediide-methanide complex 88-Ce exhibits a similar Ce]C distance [2.472(4) Å],80 though the authors noted the different geometries of the two carbene fragments i.e. pyramidalised in 125 vs planar in 88-Ce. The tetravalent state of 125 was further confirmed by diamagnetic behaviour observed with NMR spectroscopy and a featureless NIR absorption spectrum, indicating an absence of f ! f transitions; the assignment was further corroborated by XANES studies.72 Preliminary reactivity studies showed that 125 reacts with 9-anthracene carboxaldehyde and benzaldehyde, thus exhibiting Wittig-like chemistry.96 This is a notable experimental observation of a potential increase in covalency of the Ce]C interaction, since other RE bis(iminophosphorano)-stabilised alkylidenes classically display CdH activation chemistry when reacted with unsaturated substrates (vide supra), whereas classically more covalent U]C bonds in U(IV)BIPMTMS carbenes exhibit Wittig-type chemistry.97 Computational analysis showed that a polarised bond consisting of two electron pairs of s and p character respectively are present between the cerium(IV) centre and the methanediide carbon donor in 125.96 Further investigations by Liddle, Kerridge and co-workers also corroborated experimental evidence of the increased covalent character of the Ce(IV) alkylidene 125, by showing that U(IV) ([Rn]5f2) analogue [U(BIPMTMS)(ODipp)2] displays a similar degree of covalency in the U]C interaction.72 Quite surprisingly, the authors also found that the same interaction in closed-shell Th(IV) analogue [Th(BIPMTMS)(ODipp)2] is ionic. Collectively, these observations across three Ln/An analogues challenge common analogies made between the first members of the 4f and 5f families Ce and Th, and highlight the strong correlations between Ce(IV) and U(IV) in their metal-ligand interactions.72 Gregson et al. further developed the existing library of Ln bis(iminophosphorano)-stabilised carbenes by synthesising the bisalkylidene complexes [K(18-crown-6)(THF)2][RE(BIPMTMS)2] (127-RE; RE]Y, Ce, Pr, Tb, Dy).82,83,98 These separated ion pair species were obtained from the deprotonation of methanediide-methanide precursors 88-RE with benzyl potassium in the presence of a sequestering agent (Scheme 39). Le Floch and co-workers had previously reported similar bis-carbene species of Sc, Sm and Tm stabilised by the sulfur analogue{SCS}2−, though their synthesis was achieved using salt elimination protocols (vide infra Section 3.08.2.1.2).74,99,100 Moving across the period from Ce to Dy, the RE]C distance shortens in agreement with a contraction of the ionic radii; the longest interactions are observed in 127-Ce [2.598(3) and 2.603(3) Å], while the shortest bond lengths are detected in 127-Dy [2.431(6) and 2.434(6) Å]. It is noteworthy that the RE]C interactions in the heteroleptic precursors 88-RE are shorter than those of the bis-alkylidene species, e.g. Tb]C: 2.461(4)–2.470(4) Å (127-Tb) vs 2.385(2) Å (88-Tb).83 Additionally, the magnetic properties of 127-Dy were also investigated in a collaborative study between the research groups of Liddle and Winpenny.82 The study revealed that 127-Dy is a Single Molecule Magnet (SMM) displaying magnetic hysteresis up to 10–12 K and a barrier to the reversal of magnetisation (Ueff) of 813 K, the latter value being a record for Dy(III) monometallic SMMs at the time of the report. The remarkable magnetic properties of 127-Dy derive from the high axiality of the ligand field, comprising two dianionic methanediide carbon donors which form an almost linear C]Dy]C unit [176.6(2) ] and additional weak coordination of the imine donors about the equatorial plane.82 Recently, Cavell and co-workers prepared the neodymium methanediide complex [Nd(BIPMTMS)(Cl)(THF)] (128) by salt elimination, though no structural authentication was obtained (Scheme 40).101 In further reactivity studies, 128 was treated with CpTl to yield the heteroleptic methanide complex [Nd(H-BIPMTMS)(Cp)2] with corresponding loss of thallium chloride; the formation of the neutral heteroleptic methanediide complex [Nd(BIPMTMS)(Cp)] was not observed. The addition of a further

Scheme 39 Synthesis of bis-alkylidene complexes 127-RE.82,83,98 Gregson, M.; Chilton, N. F.; Ariciu, A. M.; Tuna, F.; Crowe, I. F.; Lewis, W.; Blake, A. J.; Collison, D.; McInnes, E. J. L.; Winpenny, R. E. P.; Liddle, S. T., Chem. Sci. 2016, 7, 155–165; Gregson, M.; Lu, E.; Mills, D. P.; Tuna, F.; McInnes, E. J. L.; Hennig, C.; Scheinost, A. C.; McMaster, J.; Lewis, W.; Blake, A. J.; Kerridge, A.; Liddle, S. T., Nat. Commun. 2017, 8, 14137. Ortu, F.; Reta, D.; Ding, Y. S.; Goodwin, C. A. P.; Gregson, M. P.; McInnes, E. J. L.; Winpenny, R. E. P.; Zheng, Y. Z.; Liddle, S. T.; Mills, D. P.; Chilton, N. F., Dalton Trans. 2019, 48, 8541–8545.

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Scheme 40 Synthesis of the Nd methanediide 128 and bis-carbene 129.101 Li, J.; Zhao, J.; Ma, G.; McDonald, R.; Cavell, R. G., J. Organomet. Chem. 2019, 895, 7–14.

equivalent of Li2(BIPMTMS) to 128 led to the formation of the bis(carbene) complex [Li(THF)4][Nd(BIPMTMS)2] (129) which, through DFT calculations, was shown to have a cumulene structure, e.g. C]Nd]C.101 Building on the previous successful synthesis of the Ce(IV) alkylidene 125, Liddle and co-workers attempted to oxidise the biscarbene derivatives 127-Ce, 127-Pr and 127-Tb to convert them into their tetravalent analogues, since both Pr and Tb have an accessible +4 oxidation state.83 Oxidation of 127-Ce with AgBPh4 produced the desired neutral bis-alkylidene [Ce(BIPMTMS)2] (130), while in the case of Pr and Tb the same reaction resulted in the insertion of silver into the bis-carbene complexes to give [Pr(BIPMTMS)2Ag] (131-Pr) and [Tb(BIPMTMS)2Ag] (131-Tb) (Scheme 41). Reportedly, the oxidation of 127-Ce to 130 was so favorable that it could be effected by traces of dry air, detectable by a color change from pale yellow to green. This color in the 4f0 complex was found to be due to LMCT transitions from the Ce]C p and s bonds to the 4f orbitals. Single crystal X-ray diffraction showed that the Ce]C distances in 130 [2.385(2) and 2.399(3) Å],83 and shorter than those of both the Ce(IV) mono-carbene 125 [2.441(5) Å]96 and the trivalent analogue 127-Ce [2.598(3) and 2.603(3) Å].72 While a shortening of the CedC bond distances should be expected because of the smaller ionic radius of Ce(IV) compared to Ce(III) (0.97 Å vs 1.196 Å),42 there was also a significant decrease of the Ce]C distances in the Ce(IV) bis-carbene derivative compared to Ce(IV) mono-carbene 125. This observation strongly suggests the presence of stabilising factors arising from the two methanediide ligands being positioned transwith respect to each other. In actinide 5f systems, the inverse-trans-influence (ITI) plays a key role and is ultimately responsible for the stability of certain species, such as the ubiquitous uranyl cation {O]U]O}2+; at the time of this report there was no evidence which suggested this phenomenon could be evoked for the 4f elements.83 Therefore, Liddle and co-workers further investigated this aspect and compared 130 to An analogues (Th and U) through reactivity and computational studies. Firstly, it was found that 130 undergoes metallo-Wittig reactivity with benzaldehyde to yield the corresponding alkene, PhC(H)]C(PPh2SiMe3)2; the same reactivity profile was also observed for the U(IV) and Th(IV) analogues, with the latter being the least reactive species.83 This suggests a more pronounced covalent character of the carbene in 130, which contrasts with the highly ionic character of other RE alkylidenes. Additionally, computational studies carried out on 130 and its An analogues showed similarities in the bonding regime which strongly suggests the ITI plays a role in both 4f and 5f species.83 3.08.3.2.1.2 Bis(thiophosphorano)methanediide Several years after Cavell’s report of the first Sm BIPMTMS-stabilised alkylidene 85, Mézailles, Le Floch and co-workers reported the RE metal complexes of bis(thiophosphorano)methanediide {SCS}2−, focussing especially on scandium, samarium and thulium.74,99,100 In their first reports, these authors employed salt metathesis methodologies which consisted of deprotonation of the proligand, (Ph2P]S)2CH2 (H2-SCS), with MeLi to give the dilithio salt Li2(SCS), which was in turn reacted with the iodide precursors [LnI3(THF)3.5] (Ln ¼ Sm, Tm) to furnish the heteroleptic iodide-bridged products [{Ln(SCS)(THF)2(m-I)}2] (132-Ln: Ln ¼ Sm, Tm; Scheme 42).99,100 By switching to a 2: 1 ligand: metal stoichiometric ratio, the bis-carbene anionic homoleptic complexes [Li(THF)4][Ln(SCS)2] (133-Ln; Ln ¼ Sm, Tm) were obtained instead. At the time of Le Floch’s report, 133-Sm was the second Sm alkylidene to be reported and the first structurally authenticated homoleptic carbene of any RE metal.99 The Sm]C

Scheme 41 Synthesis of the tetravalent cerium bis-carbene 130 and attempted oxidation of Pr and Tb analogues (131− Ln).83 Gregson, M.; Lu, E.; Mills, D. P.; Tuna, F.; McInnes, E. J. L.; Hennig, C.; Scheinost, A. C.; McMaster, J.; Lewis, W.; Blake, A. J.; Kerridge, A.; Liddle, S. T., Nat. Commun. 2017, 8, 14137.

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Scheme 42 Synthesis of the mono- and bis-carbene complexes 132-Ln and 133-Ln.99,100 Cantat, T.; Jaroschik, F.; Nief, F.; Ricard, L.; Mȩzailles, N.; Le Floch, P., Chem. Commun. 2005, 5178–5180.

interaction in 132-Sm [2.352(6) Å] was the shortest reported at the time, significantly shorter than Cavell’s seminal complex 85 [2.467(4) Å]78 and Liddle’s methanide-methanediide 88-Sm [2.41(2)].80 Le Floch and Mézailles investigated the reactivity of both 132-Ln and 133-Ln with benzophenone, which were found to react in an identical manner akin to Schrock-type carbenes, yielding the metathesis product Ph2C]C(PPh2S)2 (134);99,100 the authors postulated that polymeric Ln oxides are formed as by-products of the reaction (Scheme 43). The observation that 132-Ln and 133Ln act as Schrock-type alkylidenes is in stark contrast with the reactivity profile of 104-Y (yttrium BIPMTMS analogue of 132-Ln), for which Liddle and co-workers showed CdH activation is the preferred reaction pathway (vide supra, Scheme 36). Interestingly, the authors could also identify the intermediate “ate” complex 135-Ln, which slowly decomposes in THF to yield the alkene product 134.99,100 Fustier et al. extended the library of SCS-stabilised alkylidenes to scandium. In their synthetic methodology, Mézailles and co-workers reacted ScCl3 with Li2-SCS in a 1:1 ratio, which afforded a mixture of the heteroleptic alkylidene [Sc(SCS)(Cl)(py)2] (136, py ¼ pyridine) and a putative bis-carbene “ate” complex [Li][Sc(SCS)]2 in a 9:1 ratio, with the latter only detected via NMR spectroscopy (Scheme 44).74 When two equivalents of the ligand transfer reagent Li2-SCS was reacted with ScCl3, the homoleptic bis-carbene “ate” complex [{Sc(SCS)2}{Li(THF)2}] (137) was obtained.76 The Sc]C distances are significantly shorter in 136 [2.2072(1) Å] and 137 [2.212(8) and 2.243(8) Å] compared to the analogous distances in the LA-stabilised Sc methylidene 56 reported by Mindiola [2.317(2) Å],61 though are still in the range of ScdCalkyl bonds. The authors analysed the Sc]C interaction in 136 via DFT studies, which revealed the presence of significant s- and p-donation from the ligand into the empty scandium d-orbitals. Additionally, NBO analysis showed the ligand-metal s donation has a much higher energy (98.4 kcal/mol) than the delocalisation of the charge from the carbon dianion into the s PdC and PdS antibonding orbitals of the ligand scaffold (56.4 kcal/mol).74

Scheme 43 Reactivity of 132-Ln and 133-Ln with benzophenone.99,100 Cantat, T.; Jaroschik, F.; Nief, F.; Ricard, L.; Mȩzailles, N.; Le Floch, P., Chem. Commun. 2005, 5178–5180.

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Scheme 44 Synthesis of heteroleptic Sc alkylidene 13674 and homoleptic bis-carbene 137.76 Fustier, M.; Le Goff, X. F.; Le Floch, P.; Mézailles, N., J. Am. Chem. Soc. 2010, 132, 13108–13110; Fustier, M.; Le Goff, X. F.; Lutz, M.; Slootweg, J. C.; Mézailles, N. Organometallics 2015, 34 (1), 63–72.

Scheme 45 Synthesis of the methanide-methanediide complex 138 via protonolysis.76 Fustier, M.; Le Goff, X. F.; Le Floch, P.; Mézailles, N., J. Am. Chem. Soc. 2010, 132, 13108–13110; Fustier, M.; Le Goff, X. F.; Lutz, M.; Slootweg, J. C.; Mézailles, N. Organometallics 2015, 34 (1), 63–72.

When the solvated tribenzyl scandium precursor [Sc(CH2Ph)3(THF)3] is treated with one equivalent of proligand H2-SCS, the methanediide-methanide complex [Sc(SCS)(H-SCS)(THF)] (138) was obtained, together with trace amounts of [Sc(SCS)(CH2Ph) (THF)2] (139) which could only be observed through NMR spectroscopic analysis of the reaction mixture (Scheme 45).76 The yield for the synthesis of 138 was optimised by reacting the scandium tribenzyl precursor and H2-SCS in a 1:2 stoichiometric ratio.76 The Sc]C bond length of 138 [2.204(4) Å] is statistically identical to that of the heteroleptic alkylidene 136 [2.2072(1) Å],74 whereas, as expected, the ScdCmethanide distance is significantly longer [2.610(4) Å].76 Additionally, the molecular structure of 138 gives a clear insight into the steric properties of {SCS}2− compared to the iminophosphorano congeners {BIPMR}2−: in 138 one molecule of THF completes the metal coordination sphere, while no coordinated solvent is present in the solid state structures BIPMR-stabilised derivatives 88-RE, 90 and 92-RE, which also contain larger RE metals (vide supra Section 3.08.2.1.1). The twofold character of the ScdC interaction in 136 was further corroborated through reactivity studies. Analogously to 132-Ln and 133-Ln, 136 exhibits metallo-Wittig reactivity with benzophenone, leading to the formation of alkene 134 (Scheme 46). During the reactivity studies, tetrametallic Sc-oxo cluster [{Sc2(SCS)(m-Cl)2(m3-O)(THF)2}2] (140) was structurally authenticated; this was isolated through careful stoichiometry control and deemed as a possible intermediate of the reaction, similarly to what was observed during the formation of 135-Ln (vide supra, Scheme 43). The authors postulated that the first step of the reaction is the formation of benzophenone adduct [Sc(SCS)(Ph2CO)(Cl)(THF)2], observed via NMR spectroscopy and previously predicted in the reaction of 132-Sm with benzophenone.99,100 This species rearranges into a “ScOCl” fragment bound to alkene 134, before reacting with two equivalents of 136 to form the oxo-cluster 140. Finally, reaction of 140 with benzophenone leads to full conversion into the alkene product, thus confirming its formulation an as intermediate in the overall reaction.74 The ability of 136 to act as an alkylidene transfer reagent was also tested with TM complexes (Scheme 47).102 Fustier et al. reacted 136 with [Pd(PPh3)2Cl2], [Ru(PPh3)2Cl2], FeCl2 and CoCl2 and obtained the respective metathesis products [Pd(SCS)(PPh3)] (140), [Ru(SCS)(PPh3)2] (141), [{Fe(m-SCS)}2] (142) and [{Fe(m-SCS)}2] (143) with concomitant formation of ScCl3.102 Analogous reactions were performed using the group 4 alkylidene [{Zr(SCS)(Cl)(m-Cl)(THF)}2], for which longer reaction times were required. An illustrative example is that of cobalt: the transmetallation of SCS from the zirconium precursor required reflux in THF for 24 h, however for the scandium complex 136 the same reaction was complete after 15 min at room temperature. Additionally, reactivity of the Zr alkylidene with FeCl2 was sluggish, thus showing that for this family of reagents and conditions 136 is overall a superior metathesis reagent. The enhanced reactivity profile is likely due to the greater ionic character of the Sc]C bond relative to the Zr]C bond.102 In further reactivity studies inspired by Mindiola’s synthesis of LA-stabilised scandium methylidenes,61 Fustier et al. attempted the preparation of an analogous complex supported by the {SCS}2− scaffold.76 However, treatment of [Sc(SCS)(Cl)(THF)2] (144) with MeLi led to a mixture of products, and the presence of the target methyl complex [Sc(SCS)(Me)(THF)] (145) could only be confirmed by derivatisation from the reaction with HN(SiMe3)2 to give the amido-alkylidene product [Sc(SCS){N(SiMe3)2}(THF)]

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Scheme 46 Reactivity of 136 with benzophenone and formation of alkene 134 via oxo-cluster intermediate 139.74 Fustier, M.; Le Goff, X. F.; Le Floch, P.; Mézailles, N., J. Am. Chem. Soc. 2010, 132, 13108–13110.

Scheme 47 Transmetallation reactions of 136 with selected transition metal chloride compounds by Mézailles and co-workers.102 Fustier-Boutignon, M.; Heuclin, H.; Le Goff, X. F.; Mézailles, N., Chem. Commun. 2012, 48, 3306–3308.

(146, Scheme 48).76 Following the isolation of 146, a successful attempt was made to prepare the analogous phosphido derivative [Sc(SCS){P(SiMe3)2}(py)2] (147) by reacting 136 with the lithium phosphido salt [Li{P(SiMe3)2}(THF)2]. Structural analysis revealed that the two derivatives have distinctive geometric arrangements: the phosphido complex 147 features a planar PCP arrangement, while the amide 146 exhibits a carbene with a pyramidal geometry. This difference was probed in detail with DFT studies and the underlying reason for the different arrangements was identified in the nature of the pnictogens in trans to the carbene donor, which influence the geometry at the carbenic atom through differing orbital energies and different degrees of s- and p-donation to the scandium center. While the magnitude of the total s + p donation from C to Sc is not affected by the change in geometry at the carbon center (146: 74 kcal/mol; 147: 75 kcal/mol), a planar geometry at carbon enhances the p-component of the bond interaction with the metal center (147: s-donation 42 kcal/mol; p-donation 33 kcal/mol), compared to a pyramidal geometry which features a stronger s-component (146: s-donation 61 kcal/mol; p-donation 13 kcal/mol). Accordingly, the authors predicted that these variations in the bonding components could result in different nucleophilicities of carbene complexes.76

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Alkylidene Complexes of the Group 3 Metals and Lanthanides

Scheme 48 Synthesis of amido-alkylidene 146 and phosphido-alkylidene 147.76 Fustier, M.; Le Goff, X. F.; Lutz, M.; Slootweg, J. C.; Mézailles, N. Organometallics 2015, 34, 63–72.

One further example of an SCS-stabilised scandium alkylidene was reported by Chen and co-workers in 2018.103 As part of a larger investigation into non-pincer type alkylidenes (vide infra Section 3.08.3.2.2), Wang et al. employed the tethered diketiminate ligand {MeC(N-Dipp)CHC(Me)(NCH2CH2N(Me)2)}− (NacNacNMe2) to stabilise the Sc alkyl-chloride complex [Sc(NacNacNMe2) (Me)(Cl)] (148-Sc), which was converted to the carbene complex [Sc(NacNacNMe2)(SCS)] (149) by a salt metathesis reaction with one equivalent of Li(H-SCS) (Scheme 49).103 Two independent molecules are present in the asymmetric unit of the solid state structure of 149, with one of them comprising a slightly pyramidalised geometry of the carbene donor [Sangles ¼ 325.0(2) and 337.3(2) ].103

3.08.3.2.2

Non-pincer type

Chen and co-workers isolated the first non-pincer alkylidene complexes of scandium, supported by the tethered diketiminate ligand NacNacNR2 (NacNacNR2 ¼ {N(Dipp)C(Me)CC(Me)N(CH2CH2NR2)}−; R ¼ Me, iPr). Their synthetic approach consisted of treatment of the heteroleptic precursors 148-Sc and [Sc(NacNacNiPr2)(Cl)(Me)] (150) with the methanide salt Li{CH(SiR0 3)PPh2S} (R0 ¼ Me, Ph); this led to the elimination of LiCl and concomitant evolution of methane arising from the deprotonation of the methanide, affording a family of monomeric silyl-thiophosphinoyl-alkylidene complexes [Sc(NacNacNR2){C(SiR0 3)PPh2S-k2S,C}] (153-Sc: R ¼ Me, R0 ¼ Me; 154-Sc: R ¼ Me, R0 ¼ Ph; 155: R ¼ iPr, R0 ¼ Me; 156: R ¼ iPr, R0 ¼ Ph).104 Wang et al. extended this family by incorporating different RE metals (Y, Sm and Lu); moreover, they prepared Sm (153-Sm) and La analogues (154-La) via deprotonation of the precursors [Sm(NacNacNMe2){CH(SiMe3)PPh2S-k2S,C}(Cl)] (151) and [La(NacNacNMe2){CH(SiPh3)PPh2Sk2S,C}(Cl)] (152) with MeLi (Scheme 50).103 Complex 154-Sc features a short Sc]C distance [2.113(2) Å], as do the parent alkylidenes 155 [2.125(2) Å] and 156 [2.159(4) Å],104 which are significantly shorter than those of SCS-stabilised Sc carbenes.74,76 Remarkably, 153-RE (RE]Lu, Y, Sm) and 154-La all contain the shortest RE]C interactions reported to date for each respective metal. DFT studies were performed on 154-Sc to reveal a high degree of polarisation in the Sc]C bond and a Wiberg bond index of 0.73; the metal-carbon bonding interaction in this complex is best described as having double bond character, as no significant ylidic character could be found.104 On the other hand, DFT studies carried out on 153-Sm showed bond indexes of 0.55 and 1.23 for SmdC and PdC interactions respectively, and a HOMO delocalised across the Sm, C and P atoms. Therefore, rather than a RE]C double bond, the metal-carbon interaction in this case can be described as an allylic-type delocalisation over a three-center p system.103

Scheme 49 Synthesis of the heteroleptic carbene complex 149 by Chen and co-workers.103 Wang, C.; Mao, W.; Xiang, L.; Yang, Y.; Fang, J.; Maron, L.; Leng, X.; Chen, Y., Chem. - Eur. J. 2018, 24, 13903–13917.

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303

Scheme 50 Synthesis of the RE silyl-thiophosphinoyl-alkylidene complexes 153–156.103,104 Wang, C.; Mao, W.; Xiang, L.; Yang, Y.; Fang, J.; Maron, L.; Leng, X.; Chen, Y., Chem. - Eur. J. 2018, 24, 13903–13917; Wang, C.; Zhou, J.; Zhao, X.; Maron, L.; Leng, X.; Chen, Y., Chem. - Eur. J. 2016, 22, 1258–1261.

The reactivity of these new alkylidenes was thoroughly investigated by Chen and co-workers. Complex 155 was found to react via a simple [2 + 2]-cycloaddition reaction with N-methylbenzylideneimine (Scheme 51) to give [Sc(NacNacNiPr2){N(Me) (CPh)C(SiMe3)PPh2S–k3S,C,N}] (157) in good yield.104 Complex 155 rapidly undergoes CdH bond activation with phenylacetylene at room temperature to furnish [Sc(NacNacNiPr2){CH(SiMe3)PPh2S-k2S,C}(PhCC)] (158), also in excellent yield (Scheme 51). Interestingly, 155 reacts with elemental selenium to give the metallaselenocyclopropane complex [Sc(NacNacNiPr2){SeCH(SiMe3)PPh2S-k3S,C,Se}] (159, Scheme 51), which represents the first RE metal complex containing a RE-Se-C three-membered ring.104

Scheme 51 Reactivity of 155 with N-methylbenzylideneimine, phenylacetylene and elemental selenium.104 Wang, C.; Mao, W.; Xiang, L.; Yang, Y.; Fang, J.; Maron, L.; Leng, X.; Chen, Y., Chem. - Eur. J. 2018, 24, 13903–13917.

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Scheme 52 Reactivity of RE silyl-thiophosphinoyl-alkylidenes 153-RE, 154-RE and 155 with benzonitrile.103 Wang, C.; Mao, W.; Xiang, L.; Yang, Y.; Fang, J.; Maron, L.; Leng, X.; Chen, Y., Chem. - Eur. J. 2018, 24, 13903–13917.

Wang et al. also investigated the reactivity of the alkylidene family 153–155-RE with benzonitrile, which varies significantly depending on the RE metal involved and the substituents on the diketiminate ligand (Scheme 52).103 Complex 153-Sc reacted with benzonitrile to undergo ScdC bond cleavage and subsequent insertion across these two atoms to give [Sc(NacNacNMe2){NC(Ph)C(SiMe3)PPh2S-k2N,S}] (161), while 155 reacted in a similar fashion but also eliminated propene, turning the tertiary pendant amine of the NacNac ligand into an amide in [Sc(NacNacNiPr){NHC(Ph)C(SiMe3)PPh2S-k2N,S}] (162, NacNacNiPr ¼ {N(Dipp)C(Me)CC(Me)N(CH2CH2NiPr)}2−). When 153-Lu was reacted with benzonitrile the product first formed by Lu]C cleavage underwent reaction with a second equivalent of PhCN to yield the benzamidide derivative [Sc(NacNacNiPr){NC(Ph)NC(Ph)C(SiMe3)PPh2S-k3N,N0 ,S}] (163), whereas when the more sterically encumbered 154-RE (RE ¼ Y, Lu) were treated with benzonitrile the substrate simply coordinates to the metal forming adducts of formula [RE(NacNacNMe2){C(SiPh3)PPh2S}(NCPh)] (164-RE; RE ¼ Y, Lu).103 Similarly, the reactivity of 153-RE (RE ¼ Sc, Y, La, Sm, Lu) with tert-butyl isonitrile varied depending on the nature of the metal (Scheme 53).103 With 153-Sc, no coordination of the substrate was observed and rapid conversion to the amido-sulfido complex [Sc(NacNacNMe2){N(tBu)(CCSiMe3)}(SPPh2)] (165) was detected. However, with larger RE metals coordination and formation of the isonitrile adduct was first observed at room temperature, allowing structural authentication of the adducts [RE(NacNacNMe2){C(SiPh3)PPh2S}(CNtBu)] (166-RE; RE ¼ Y, La, Sm, Lu). Heating of 166-RE leads to the formation of similar products to 165 in which the SPPh2 fragments acts as a Z2 donor, namely [RE(NacNacNMe2){N(tBu)(CCSiMe3)}(Z2-SPPh2)] (167RE; RE ¼ Y, Sm). In the case of 166-La heating the reaction mixture led to an intractable mixture of products. Finally, Chen and co-workers tested the reactivity of RE silyl-thiophosphinoyl-alkylidenes (153–156) with phenylsilane (Scheme 54), but, with the exception of 153-Sm and 154-La, either no reaction was observed or it resulted into a complex mixture of products.103 When 153Sm and 154-La were employed as starting materials, monomeric [Sm(NacNacNiPr2){C(SiPh3)(SiHPh2)PPh2S-k2S,C}] (168) and dimeric [La2{C(SiPh3)(SiHPh2)PPh2S}2{N(CMeCHCMeNDipp)(CH2CH2NMe2)}2] (169) formed through addition of the SidH bond across the LndC bond to give a RE hydride intermediate, which subsequently activated the b-ketiminato ligand.103

3.08.3.3

Phosphino- and phosphonio-alkylidenes

Since Cavell’s report of Sm bis(iminophosphorano)methanediide 85 in 2000, the majority of research into RE alkylidenes has been focused on the use of imide- or sulfide-P(V) substituents (Table 5) (vide supra Section 3.08.3.3)78 that impart significant stability to the resulting alkylidenes by both their electron-withdrawing properties and the possibility of forming stable pincer-type or metallacycle arrangements, which can in turn temper their reactivity. In 2017 Chen and co-workers introduced a new coordination motif to the library of RE carbenes by employing a k2-phosphino-alkylidene which had been used previously with early TMs.105 Mao et al. first synthesised the methanide precursor [Sc(NacNacDipp)(Me){CH(SiMe3)(PPh2)-k2C,P}] (171) via salt metathesis between the alkyl-chloride complex [Sc(NacNacDipp)(Me)(Cl)] (170) and a lithium ligand transfer agent; CdH activation upon heating 170 in THF generated the first RE phosphino-alkylidene complex [Sc(NacNacDipp){C(SiMe3)(PPh2)-k2C,P}(THF)] (172, Scheme 55).105 Complex 172 exhibits a short Sc]C distance [2.089(3) Å], which is shorter than the ScdC bond of 171 [2.292(4)

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305

Scheme 53 Reactivity of 154-RE with tBuNC.103 Wang, C.; Mao, W.; Xiang, L.; Yang, Y.; Fang, J.; Maron, L.; Leng, X.; Chen, Y., Chem. - Eur. J. 2018, 24, 13903–13917.

Scheme 54 Reactivity of 153-Sm and 154-La with PhSiH3.103 Wang, C.; Mao, W.; Xiang, L.; Yang, Y.; Fang, J.; Maron, L.; Leng, X.; Chen, Y., Chem. - Eur. J. 2018, 24, 13903–13917.

Table 5

Phosphino and phosphonio-stabilised alkylidene RE complexes covered in Section 3.08.3.3, with relevant crystallographic and spectroscopic data.

Molecular formula

RE oxidation state

M–C (Å)

13

Compound

References

[Sc(NacNacDipp){C(SiMe3)(PPh2)-k2C,P}(THF)] [Sc(NacNacDipp){C(SiMe3)[PPh2(C6H10-O)]-k2N,O}] [Sc(NacNacDipp){C(SiMe3)[PPh2(C6H8-O)]-k2N,O}] [Sc(NacNacDipp){C(SiMe3)[PPh2(CPhNMe)]-k2N,N0 }] [Sc(NacNacDipp)(F){C(SiMe3)[PPh2(C5H3N-F)]-k2N,N0 }] [Sc(NacNacDipp){CH(PPh3)}(Me)] [Sc(NacNacDipp){CH(PPh3)}(I)] [Sc(NacNacDipp){CH(PPh3)}(OTf )] [Lu(NacNacDipp){CH(PPh3)}(CH2SiMe3)] [Sc(PNPPhiPr){CH(PPh3)}(Me)] [Sc(PNPPhiPr){m-C(PPh3)}{(m-Me)AlMe2}]

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

2.089(3) 2.148(5) – – 2.216(4) 2.057(5) 2.060(3) 2.105(2) 2.192(11) 2.121(2) 2.036(5)

Not observed Not observed Not observed Not observed – Not observed Not observed Not observed 41.6 73.99 24.92

172 182 183 184 185 187 188 189 193 196 197

106

C (ppm)

107 107 107 107 110 110 110 110 32 32

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Alkylidene Complexes of the Group 3 Metals and Lanthanides

Scheme 55 Synthesis of the phosphino-alkylidene complex 172 by Chen and co-workers.106 Mao, W.; Xiang, L.; Lamsfus, C. A.; Maron, L.; Leng, X.; Chen, Y., J. Am. Chem. Soc. 2017, 139, 1081–1084.

Å] and surpassing the previous shortest ScdC distance in the P(V)-stabilised alkylidene 154-Sc [2.113(2) Å].104 The phosphorous atom in 172 is involved in a short contact with the metal center [ScdP ¼ 2.597(1) Å] so the ligand acts effectively as a bidentate k2type donor. DFT calculations show that the HOMO is a 3-centre p-type orbital delocalised across the Sc, C and P atoms; therefore, an allylic Lewis structure better represents the bonding in this complex. Complex 172 is extremely reactive and readily CdH activates the ligand backbone at room temperature, forming [Sc{NacNac (Dipp)[C6H3-(iPr)(CH2CHCH3)]-k3N,N0 ,C}{CH(SiMe3)(PPh2)-k2C,P}] (173).106 The reactivity of 172 was further tested with pyridine, DMAP, 2-fluoropyridine and 1,3-dimethylpyrazole (Scheme 56), leading in each case to CdH activation products such as [Sc(NacNacDipp){CH(SiMe3)(PPh2)-k2C,P}{C5H2N-(R)2–1,3-k2C,N}] (174; R ¼ H, NMe2, F) and [Sc(NacNacDipp){CH(SiMe3) (PPh2)-k2C,P}{C3H2N2-(Me)(CH2)-k2C,N}] (175):106,107 in the case of pyridines, CdH activation occurs in a to the pyridyl nitrogen, whereas for pyrazoles deprotonation takes place at one of the N-methyl substituent. Moreover, 172 readily activates H2 in mild conditions, forming an unstable species which can be trapped with 1-hexene to yield the alkyl derivative [Sc(NacNacDipp)

Scheme 56 Reactivity of 172 towards hydrogen, N-, O- and B-heterocycles, alkynes and triethoxysilane.106,107 Mao, W.; Xiang, L.; Lamsfus, C. A.; Maron, L.; Leng, X.; Chen, Y., J. Am. Chem. Soc. 2017, 139, 1081–1084.

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307

(C6H13){CH(SiMe3)(PPh2)-k2C,P}] (176).106 In further studies, Mao et al. showed that 172 can perform: CdH activation/ringopening on methyl-cyclopropane in [Sc(NacNacDipp)(OCH2CHCH2){CH(SiMe3)(PPh2)-k2C,P}] (177), 1,2 addition across the Sc]C bond with dimethylisoxazole in [Sc(NacNacDipp){P(Ph)2C(SiMe3)[BH(OC(Me)2C(Me)2O]-k3P,O,O0 }] (178) and alkynes in [Sc(NacNacDipp){P(Ph)2C(SiMe3)[NC(Me)CHC(Me)O]-k4P,C,N,O}] (179), and insertion into the Sc]C bond with pinacolborane in [Sc(NacNacDipp){P(Ph)2C(SiMe3)[C(R)C(R’)]}] (180a-d; R/R0 ¼ Me, Et, tBu, Ph) and triethoxysilane in [Sc(NacNacDipp){O(Et)SiH[C(SiMe3)PPh2]O(Et)-k2O,O0 }(OEt)] (181).107 TM alkylidenes classically react with ketones in a metallo-Wittig fashion, though in the case of RE carbenes the outcome of such reactivity varies on the nature of the metal-ligand interaction. Surprisingly, when 172 is reacted with cyclohexanone and 2-cyclohexen-1-one (Scheme 57), the substrates insert into the ScdP bond forming phosphonio-alkylidene species [Sc(NacNacDipp){C(SiMe3)[PPh2(C6H10-O)]-k2N,O}] (182) and [Sc(NacNacDipp){C(SiMe3)[PPh2(C6H8-O)]-k2N,O}] (183). A similar outcome was seen for the reactions of 170 with benzylidenemethanamine and 2,6-difluoropyridine, which afforded the respective phosphonio-alkylidene complexes, [Sc(NacNacDipp){C(SiMe3)[PPh2(CPhNMe)]-k2N,N0 }] (184) and [Sc(NacNacDipp)(F){C(SiMe3)[PPh2(C5H3N-F)]-k2N,N0 }] (185).106 Phosphonio-alkylidenes are a variation of phosphonio-methylidenes {CH(PR3)}− (R ¼ alkyl, aryl), which derive from CdH activation of phosphorous ylides, R3P]CH2. Such species have been used in An chemistry since the early 1980s and are also wellestablished in TM chemistry.108,109 In 2017 Chen and co-workers developed an efficient synthetic strategy for the synthesis of RE phosphonio-methylidenes, based on the protonolysis reactions of RE alkyl precursors with phosphorous ylides (Scheme 58).110 As part of their methodology, the authors reacted [Sc(NacNacDipp)(Me)2(THF)] (186) with Ph3P]CH2 and obtained the target alkylidene complex [Sc(NacNacDipp){CH(PPh3)}(Me)] (187) with the concomitant elimination of methane. Further reactivity of 187 with Me3SiI and Me3SiOTf (Tf ¼ triflate, CF3SO−3) afforded two additional examples of Sc phosphonio-methylidenes, [Sc(NacNacDipp){CH(PPh3)}(I)] (188) and [Sc(NacNacDipp){CH(PPh3)}(OTf )] (189).110 The molecular structures of 187–189 revealed short Sc]C distances [2.105 (2) (187), 2.044 (5) (188) and 2.060 (3) Å (189)], even compared to those seen for the phosphino-alkylidene 172 [2.089 (3) Å]. DFT analysis of 187–189 showed polarised Sc]C interactions with double bond character, delocalised across the three-centre Sc-C-P p interaction, with ScdC/ScdP Wiberg bond indexes (0.70–0.84) that are similar to those observed in the three-membered ring alkylidenes 153-Sc and 172.104,106,110 187 and 189 undergo intramolecular CdH activation upon heating in toluene, yielding [Sc{NacNac(Dipp)[C6H3-(iPr)(CH2CHCH3)]-k3N,N0 ,C}{C6H4-(PPh2CH2)k2C,P}] (190) and [Sc(NacNacDipp)(OTf ){C6H4-(PPh2CH2)-k2C,C0 }] (191).110 Mao et al. attempted the preparation of a lutetium analogue of 187, but attempts to prepare a phosphonio-methylidene using the dialkyl precursor [Lu(NacNacDipp)(Me)2(THF)] were unsuccessful.110 However, when the more sterically demanding alkyl ligand {CH2SiMe3}− was employed, [Lu(NacNacDipp)(CH2SiMe3)2] (192) reacted cleanly with Ph3P]CH2 to give the phophoniomethylidene derivative [Lu(NacNacDipp){CH(PPh3)}(CH2SiMe3)] (193).110 Structural determination confirmed the formation of 193, though the data quality and large errors affecting bond metrics prevent a meaningful comparison of the Lu]C distance [2.192 (11) Å] with the previously reported silyl-thiophosphinoyl-alkylidene 153-Lu [2.204 (3) Å].103 DFT analysis shows similar polarised RE]C bonding regimes in 191 to the Sc analogues 185–187. In a similar fashion to Sc alkylidenes 185–187, complex 193 readily undergoes intramolecular CdH activation (Scheme 59) to produce the and [Lu(NacNacDipp){C6H4-(PPh2CH2)-k2C,C0 }] (194).110

Scheme 57 Conversion of 172 into phosphonio-alkylidenes 182–185.106 Mao, W.; Xiang, L.; Lamsfus, C. A.; Maron, L.; Leng, X.; Chen, Y., J. Am. Chem. Soc. 2017, 139, 1081–1084.

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Scheme 58 Synthesis of the Sc phosphonio-alkylidenes 185–187 and the intramolecular CdH activation products 190 and 191.110 Mao, W.; Xiang, L.; Maron, L.; Leng, X.; Chen, Y., J. Am. Chem. Soc. 2017, 139, 17759–17762.

Scheme 59 Synthesis of the Lu alkylidene 193 and its stability and reactivity with benzylidenemethanamine.110 Mao, W.; Xiang, L.; Maron, L.; Leng, X.; Chen, Y., J. Am. Chem. Soc. 2017, 139, 17759–17762.

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309

Scheme 60 Synthesis of the phosphonio-alkylidene 196 and the phosphonio-alkylidyne 197 by Mindiola and co-workers.31 Zatsepin, P.; Lee, E.; Gu, J.; Gau, M. R.; Carroll, P.; Baik, M.-H.; Mindiola, D. J., J. Am. Chem. Soc. 2020, 142, 10143–10152.

Additionally, reaction of 193 with benzylidenemethanamine lead to insertion into the LudC bond and CdH activation of one phenyl substituent to give [Lu(NacNacDipp){C6H4-[PPh2CHCH(Ph)NMe]-k3C,C0 ,N}] (195); this reactivity profile is analogous to that displayed by the Sc silyl-thiophosphinoyl-alkylidene 155 (vide supra scheme Scheme 51),103,104 whereas in the case of the phosphinoalkylidene 172 insertion into the REdP bond is observed (vide supra Scheme 57).106,107 Mindiola and co-workers used a similar synthetic protocol to react the dialkyl Sc complex 54 with Ph2P]CH2 (Scheme 60), affording [Sc(PNPPhiPr){CH(PPh3)}(Me)] (196).31 The same complex can also be obtained by reacting Ph2P]CH2 with the LA-stabilised a-silyl-alkylidene 76, with concomitant formation of (CH2PPh3)Al(Me)(CH2SiMe3)2. X-ray crystallography studies revealed that the ScdC distance in 196 [2.121(2) Å] is elongated with respect to Chen’s phosphonio-alkylidenes 187–189 [2.044 (5)–2.105(2) Å]110 and the phosphino-alkylidene 172 [2.089(3) Å].106 The authors reasoned that 196 could undergo an additional a-H abstraction using AlMe3, in a similar vein to the reactivity profile displayed by the bis-alkyl complex 75 in the formation of the Tebbe-like alkylidene 76. This was indeed the case and 196 was converted into the Tebbe-like alkylidyne derivative [Sc(PNPPhiPr){m-C(PPh3)}{(m-Me)AlMe2}] (197).31 Complex 197 exhibits the shortest ScdC bond reported to date [2.036(5) Å], with a computed bond order (1.31) higher than in 196 (1.02) and 75 (1.14) but lower than a formal triple-fold interaction.106 However, a participation of all three ligand-based p-orbitals was identified by the authors in their DFT analysis, with the s-component providing the major contribution to the ScdC bond and two weak interactions from the p-orbitals.31

3.08.4

Conclusions

The study of RE]CR2 chemistry is in still in its infancy compared to that of TM and An analogues. However, this gap is now rapidly being reduced and the most important advances in RE carbene chemistry are concentrated over the last two decades, demonstrating the amount of impetus the RE community is now putting into this field of research. The number of landmark complexes and the rising number of reports are testament to this renewed interest and the potential behind the use of RE alkylidenes. Nonetheless, there are still important gaps to be filled. For example, to the best of our knowledge, no alkylidene complexes have been reported to date with divalent Ln metals. Furthermore, the isolation of examples of RE]C bonds that are not supported by phosphorus substituents, as well as the first terminal unsupported methylidenes and Schrock-type alkylidenes, are highly desirable synthetic goals that have not been achieved to date. Ligand design has played a crucial role in the delivery of this remarkable library of compounds, but further innovations will be required to achieve these challenging goals, together with advances in synthetic methodologies. Nevertheless, considering the increased pace at which this field is now progressing, we predict that some of these target complexes will soon be realised. Additionally, the reactivity scope of RE alkylidenes is also rapidly expanding; so far these species have shown various reactivity profiles. In some cases these species parallel the reactivity of traditional TM alkylidenes, but many examples have been reported in which RE alkylidenes display divergent reactivity, thus opening up new possibilities for new applications in synthesis. Further to this, catalytic applications of such complexes have not been reported to date and this is undoubtedly another important challenge that researchers in this field will have to address. The significant amount of fundamental knowledge developed over the last two decades will certainly push researchers to challenge common assumptions and further contribute to the rapid expansion of this exciting research field.

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3.09

Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

Erli Lu, School of Natural and Environmental Sciences, Faculty of Science, Agriculture and Engineering, Newcastle University, Newcastle upon Tyne, United Kingdom © 2022 Elsevier Ltd. All rights reserved.

3.09.1 Terminology—Carbene or alkylidene? 3.09.2 Evidence for An]C species: Experimental and theoretical piloting works 3.09.2.1 An]C species as reaction intermediates 3.09.2.2 An]C species in an inert gas matrix 3.09.3 ‘Bottleable’ actinide nucleophilic carbene complexes: The synthetic effort 3.09.3.1 P(V)-stabilized actinide ylide carbene complexes: Synthesis, structure, reactivity 3.09.3.1.1 Actinide ylide carbene complexes [L3An(IV)(C(H)ER3)] (E: P, As): Synthesis and structure 3.09.3.1.2 Actinide ylide carbene complexes [L3An(IV)(C(H)ER3)] (E: P, As): Reactivity 3.09.3.2 P(V)-stabilized actinide pincer-type carbene complexes: Synthesis, structure, reactivity 3.09.3.2.1 U(III) pincer carbene complexes 3.09.3.2.2 U(IV) and Th(IV) pincer carbene complexes 3.09.3.2.3 U(V) pincer carbene complexes 3.09.3.2.4 U(VI) pincer carbene complexes 3.09.3.3 P(III)-stabilized actinide carbene complexes: Synthesis, structure and reactivity 3.09.3.4 U]C bonds in endohedral metallofullerenes (EMFs) 3.09.4 Conclusion and Outlook Acknowledgment References

3.09.1

312 313 313 314 314 315 315 320 322 323 324 333 335 340 343 343 344 344

Terminology—Carbene or alkylidene?

In contrast to stable carbenes in organic chemistry,1,2 in organometallic chemistry the terms ‘carbene’ and ‘alkylidene’ are both used to describe complexes containing metal-carbon double bonds (M]C),3 which play pivotal roles in modern organometallic chemistry and catalysis.4–6 Their importance has been highlighted by the 2005 Nobel Prize in Chemistry.7,8 There are two classic categories of M]C complexes: Schrock-type alkylidenes and Fischer-type carbenes, which typically differ by metal identity, metal oxidation state, substituents on the carbon center, and reactivity. For transition metal complexes, the usage of ‘alkylidene’ and ‘carbene’ are well-defined and are delineated for the Schrock-type and Fischer-type, respectively. However, for actinide metal-carbon double bond complexes, these terminologies are not entirely appropriate (Scheme 1). Schrock-type alkylidene complexes typically feature an electron-deficient, high-valent early transition metal (classically, group4/5) and a formally negatively-charged, nucleophilic [R2C2−] ligand. The M]C double bond in Schrock-type alkylidene complexes comprises one s-bond and one p-bond, which are both covalent. Hence, the usual description of the Schrock-type M]C double bond results from the formal combination of two neutral triplet fragments. The early transition metals are relatively electropositive; thus, the electron-density in the M]C covalent double bond is polarized towards the C center, i.e. [R2C2−] is strongly nucleophilic. The [R2C2−] alkylidenes bear non-heteroatomic substituents, such as hydrogen and alkyls. Thus, there is no heteroatom stabilization effect. On the other hand, Fischer-type carbene complexes feature an electron-rich, low-valent late transition metal (classically, group6-11) and a formally neutral, electrophilic, singlet [R2C:] ligand. The M]C double bond here is also comprised of one s-bond and one p-bond. However, the s-bond is a carbon-based s-donation, while the p-bond is a metal-based p-back donation, constructing a formal donor-acceptor bonding combination. The neutral [R2C:] carbene ligands bear one or two heteroatomic substituents, which can stabilize the C center via the p-lone-pair donation effect. The C center in Fischer-type carbene complexes is therefore typically electrophilic. Compared to transition metal M]C complexes, usage of the terms ‘carbene’ and ‘alkylidene’ in f-element M]C (M ¼ lanthanide, Ln; actinide, An) complexes is less well-defined. In terms of their bonding scenarios, Ln/An]C bonds are akin to a Schrock-type alkylidene (but more polarized and less covalent, due to the low electronegativity and contracted valence shell 4f/ 5f orbitals of these metals) and exhibit nucleophilic reactivity. Nonetheless, all the Ln/An]C complexes synthesized to date outside of inert gas matrix isolation or fullerene encapsulation conditions involve one or two heteroatomic substituents on the C center, which provide electronic-stabilizing effects (though by polarization mechanisms such as induction/mesomeric effects rather than lone pair donation), that are in gross terms more reminiscent of a Fischer-type carbene. Thus, Ln/An]C complexes represent their own class of ‘nucleophilic carbenes’, because metal stabilization from the conventional Schrock-alkylidene model is also supplemented by significant substituent stabilization. For didactic clarity and to avoid confusion, we will use the term ‘carbene’ to refer to An]C complexes throughout this article.

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Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00015-9

Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

313

Scheme 1 Schematic representations of the M]C bond in the classic Schrock-type alkylidene and Fischer-type carbene.

In this article, the synthesis, structure and reactivity of An]C double bond complexes with anionic nucleophilic carbene ligands, i.e. {CHER3}1− (E ¼ P, As), {C[P(]E)R2]2}2− and {CRPR0 2}2− will be covered. Neutral carbene ligands, such as N-heterocyclic carbenes (NHCs) and cyclic alkyl amino carbenes (cAACs), can form a single dative bond with electron-deficient actinide metals.1,2 Given that such complexes do not feature formal covalent M]C double bonds, they will not be covered in this article. A newly-emerging class of C(0) ligands, carbodiphosphoranes, can also form a unique double-dative bond with uranium,9 but will also not be included herein.

3.09.2

Evidence for An]C species: Experimental and theoretical piloting works

Compared to transition metal (TM) carbene/alkylidene chemistry, the development of actinide An]C complexes has lagged behind. The An]C bond, by definition, is less stable and more reactive than its transition metal counterparts. Two components underpin this inherent instability: (1) the energetic mismatch between the valence shell orbitals of the actinide metal and the carbon atom; (2) the valence shell 5f orbitals of actinide metals are spatially contracted compared to transition metal’s 3/4/5d orbitals.10 These factors render An]C bonds less covalent and less stable compared to TM]C bonds. An isolable, bona fide An]CR2 (R ¼ H, alkyls, aryls) alkylidene complex remains a highly desirable target for synthetic chemists, but has not yet been reported. However, evidence for the presence of such non-heteroatomic-stabilized An]CR2 alkylidene species as intermediates, in an inert gas matrix or in silico, can be traced back to the early 1980s.

3.09.2.1

An]C species as reaction intermediates

In 1983, as a part of their endeavors to investigate actinide applications in catalysis, Marks and co-workers reported evidence of the trapped An]C species, i.e. ‘[Cp 2An(IV)]CH2]’ (Cp ¼ [Me5C5]−; An ¼ Th, U) supported on an alumina surface.11 The alumina-supported actinide carbene species was formed by methane elimination of [Cp 2An(IV)(CH3)2] in the presence of alumina, and can be compared to the classic Tebbe’s reagent (Scheme 2).12,13 However, detailed structural data for these trapped actinide carbene species is unavailable due to the inherent difficulties of characterizing such surface-supported species.

Scheme 2 The classic Tebbe’s reagent and alumina-supported actinide carbenes.11

The methane elimination route to produce An]C bonds was investigated in silico by Maron and co-workers nearly 30 years later. In 2010, during a theoretical investigation of reactions between [Cp 2U(IV)(CH3)2] and pyridine N-oxide14 (which were experimentally reported by Kiplinger and co-workers previously, leading to CdH activation on the pyridine moiety15), the Maron group identified a ‘[Cp 2U(IV)]CH2]’ intermediate, which is highly reactive consistent with the experimentally observed CdH activation. Villiers and Ephritikhine reported additional evidence of an An]C species acting as a reaction intermediate in 2001.16 The sterically bulky ketone (tBu2C]O) was treated with [UCl4] and Li/Hg amalgam, then the reaction was quenched with water

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Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

Scheme 3 The U]C intermediate in the reaction between a ketone, UCl4 and Li-amalgam.16

(Scheme 3). Under such conditions, the alkane tBu2CH2 was observed as the major product, and there is no sign of the McMurry coupling17 product tBu2C]CtBu2. The reaction was postulated to proceed via a ‘U(IV)]CtBu2’ intermediate, which was formed by a divalent [U(II)Cl2] species, from the reduction of [UCl4] using Li/Hg amalgam. A ketyl free radical species was also hypothesized as an intermediate, but there was no reaction pathway calculated, partially due to the complexity of this reaction system.

3.09.2.2

An]C species in an inert gas matrix

Since the early 2000s, the research groups of Bursten, Andrews and Li brought laser-ablated inert gas matrices, advanced spectroscopic and modern computational technologies together, to investigate small molecules with actinide metal-carbon multiple (double and triple) bonds.18 The laser-ablated and inert gas matrix technologies allowed the preparation and trapping of extremely reactive species, such as An]C and An^C moieties. The advanced spectroscopic methods, especially infrared (IR) spectroscopy, allowed the identification and characterization of these otherwise transient species. The bonding nature of the An]C/ An^C linkages was then interrogated and interpreted by advanced computational methods. With some inherent limitations (such as the inability to directly measure types of atom and bond lengths/angles), inert gas matrix and advanced spectroscopic methods are excellent techniques to trap otherwise unstable species that are certainly not ‘bottleable’.19–23 The techniques have unique and irreplaceable values in piloting highly reactive and unstable chemical bonds/ species. For decades, such piloting works have inspired the synthetic inorganic chemistry community to replicate these unstable species in conventional, ‘bottleable’ conditions, by employing well-designed ligands. In 2001, from a reaction between laser-ablated thorium atoms and CO, a series of interesting molecules were isolated in a neon matrix below 4 K (Scheme 4).24 The geometries of the formed [:C]Th]O] and [O]Th]C]C]O] molecules were investigated by infrared spectroscopy and theoretical calculations.

Scheme 4 The neon matrix isolated CThO and OThCCO molecules with Th]C multiple bonds.24 The bond lengths and angles are based on optimized molecular structures.

After this initial success, the Bursten and Andrews groups exploited the reactions between laser-ablated metal atoms (thorium and uranium) and hydrocarbon small molecules (Th + CH4,25 Th + CH3F,26 U + CH3X,27 U + CH4,28 Th + CH2F2,29 U + CH2XY,30 U + CHX331) to produce a series of molecules featuring An]CH2 and An^CH bonds (Scheme 5). It is noteworthy that the uranium atom in the HC^UX3 molecule is in the +6 oxidation state,31 while in the other An]C molecules the actinides are in the +4 oxidation state. The parent actinide alkylidene An]CH2 molecules are volatile and reactive, and decompose at 10 K.30

3.09.3

‘Bottleable’ actinide nucleophilic carbene complexes: The synthetic effort

The inert gas matrix isolation and theoretical works outlined above proved the thermodynamic feasibility of terminal An]C bonds. The synthesis of ‘bottleable’ complexes, on the other hand, requires different and more in-depth molecular design. There are three possible strategies to stabilize an An]C bond (Scheme 6): (1) thermodynamic stabilization by introducing electronically favorable substituents, exploiting inductive/mesomeric effects; (2) kinetic stabilization by increasing steric congestion around the An]C

Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

315

Scheme 5 Reactions between laser-ablated actinide metal atoms and alkanes or haloalkanes.

Scheme 6 Three synthetic strategies to stabilize the An]C bond.

bond, or taking advantage of the chelate effect; and, (3) establishing an optimum oxidation state of the actinide metal. The synthetic community has made significant progress by employing all of these strategies, which will be covered in the following sections. By well-designed ligands and synthetic protocols, synthetic chemists have achieved several classes of bottleable An]C complexes since the 1980s. The extents of polarization and electron density delocalization of these An]C double bonds are still debated,32 but it is clear that two-fold An]C bonding interactions (usually one s-bond and one p-bond) of varying strengths have been experimentally realized and confirmed beyond reasonable doubt by theoretical investigations.33 In this section, we will summarize and categorize all reported ’bottleable’ An]C complexes, according to their metal identity, metal oxidation state and carbene ligand type. At the end of each sub-section, a table will present the essential structural parameters of the An]C bonds (bond length and calculated bond order) (Tables 1–7).

3.09.3.1 3.09.3.1.1

P(V)-stabilized actinide ylide carbene complexes: Synthesis, structure, reactivity Actinide ylide carbene complexes [L3An(IV)(C(H)ER3)] (E: P, As): Synthesis and structure

In 1978, as a part of their work investigating ylide ligands in actinide chemistry, Cramer, Gilje and co-workers discovered that the reaction between [Cp3U(IV)Cl] (Cp ¼ cyclopentadienyl) and two equivalents of the ylide lithium salt [Li(CH2)2P(C6H5)2] produced the ylide-bridging complex [{m-(CH)(CH2)PPh2}{U(C5H5)2}]2 (1) (Scheme 7).34 As a result of salt elimination and CdH

Scheme 7 Formation of a U(IV)-ylide complex 1.34

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Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

bond activation, the ylide ligand in 1 is dianionic with a formula of {(CH)(CH2)PPh2}2−. The long U(IV)-C bonds in 1 (2.52–2.66 A˚ ), longer than a typical U(IV)dC single bond in [Cp3U(IV)(n-C4H9)] (2.43(2) A˚ 35), rule out the presence of any UdC multiple bond. However, the isolation of 1 led to follow-up work from the same group, from which the first bottleable U]C multiple bond complexes were discovered. In 1981, by adjusting the stoichiometric ratio of reactions between [Cp3U(IV)Cl] and [Li(CH2)(CH2)PRPh] (R ¼ Me, Ph) from 1:2 to 1:1, the Cramer/Gilje group synthesized and characterized the first U]C multiple bond complexes [U(IV)(Cp)3(CHP{Ph} {Me}{R})] (R ¼ Me for 2; R ¼ Ph for 3) (Scheme 8).36,37 The reactions producing 2 and 3 involve an initial salt elimination, followed by CdH activation. Only 2 was structurally characterized by single crystal X-ray diffraction (SCXRD), revealing a short uranium-carbon bond (2.29(2) A˚ as initially determined by SCXRD, and 2.293(2) A˚ as refined by single crystal neutron diffraction).38 The neutron diffraction experiment also confirmed the presence of the a-CH. In conclusion, the monoanionic carbene ligands {C(H)P(Ph)(Me)(R)}1− in 2 and 3 bond to the U(IV) center via a U(IV)]C double bond. By using a proligand with a smaller substituent, an even shorter U(IV)]C bond (2.274(8) A˚ ) was achieved in 1988 in [U(Cp)3(C{H}PMe3)] (4) (Scheme 8).39

Scheme 8 Synthesis of the first U]C complexes 2–4.38,39

The U(IV)]C units in 2–4 can be represented by three resonance structures (A-C) as depicted in Scheme 9A. The experimentally observed structures of 2 and 4 are hybrids of the three resonance structures with a major contribution from the resonance form C, as depicted in Scheme 9B.40 In 1984, the first computational study of a U(IV)]C bond was conducted by Nakamura and co-worker.41 This work revealed that the U(IV)]C bonds in 2–4 are comprised of one s-bond and one p-bond, each occupied by two electrons. Overall, the U(IV)]C bond in 2–4 is best described as a 4-electron-2-center double bond. The electron-rich carbon center is stabilized by the partially positively charged P(V) center through both inductive and mesomeric effects. The bonding scenario has some resemblance to the classic Schrock type alkylidenes with a dicarbanion {CR2}2−. However, the fundamental difference in the formal ligand charge ({CHPR3}1− vs. {CR2}2−) sets the ylide carbene a unique place within the metal-carbon multiple bonding map, along with the classic Schrock alkylidene and Fischer carbene.

(A)

(A)

Scheme 9 (A) Three resonance structures (A-C) and (B) their combination for the U(IV) carbene complexes 2–4.

In 1989, instead of the tris-Cp precursor [U(Cp)3(IV)Cl] used in synthesizing 2–4, the Cramer/Gilje group reported the separate reactions between bis-Cp (Cp ¼ {C5Me5}1−) precursors [An(IV)(Cp )2Cl2] (An ¼ U, Th) and one equivalent of the ylide lithium

Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

317

Scheme 10 Reactions between {Cp 2AnCl2} and {Li(CH2)(CH2)P(R)(R0 )}.42

salts [Li(CH2)(CH2)P(R)(R0 )] (R, R0 ¼ Me, Ph; Ph, Ph; Me, Me) (Scheme 10).42 Instead of An(IV)]C carbene complexes, the reactions produced the An(IV) ylide complexes [Th(Cp )2Cl{(CH2)2P(R)(R0 )}] (5–7) and [U(Cp )2Cl{(CH2)2P(R)(R0 )}] (8–10) via salt elimination reactions. In these cases, C-H activation reactions, which led to the carbene complexes 2–4, did not occur. The formation of ylide complexes 5–10 instead of carbene complexes in these reactions may be attributed to the reduced overall steric congestion from a tris-Cp coordination environment to a bis-Cp coordination environment. Bis-Cp actinide(IV) carbene complexes remained an open quest until 2017. Nearly 30 years since the report of 5–10, Walensky, Maron and co-workers reported that the reactions between [Th(Cp )2Me2] and [CH3PPh3][X] (X ¼ Cl, Br, I) afforded not only the k2-C,C0 ylide complexes [Th(Cp )2(X){k2-C,C0 -(CH2)2PPh2}] (X ¼ Cl for 6; X ¼ Br for 13), but also the 1-C carbene complexes [Th(Cp )2(X){C(H)PPh3}] (X ¼ Br for 11; X ¼ I for 12) (Scheme 11).43 The outcomes of these reactions depend upon the halides and the reaction conditions (solvents and temperature). For X ¼ Br, changing the solvent system and temperature can alter the product from solely 11 (THF, 65  C) to a mixture of 13 and 11 (toluene/THF, 75  C). The alternative reaction mechanisms for producing the ylide complexes and the carbene complexes were probed using computational methods, which identified two viable reaction pathways. It is of note that the unexpected C6H6 identified in these reaction mixtures is postulated to be produced from a highly reactive P(V) penta-alkyl/aryl species.

Scheme 11 Reactions between {Cp 2Th(CH3)2} and {(CH3PPh3)(X)} (X ¼ Cl, Br, I).43

The Th(IV)]C bond lengths in the Th(IV) carbene complexes, 11 and 12, were determined to be 2.3137(18) and 2.299(6) A˚ , respectively. The Th]CdP angles in 11/12 are closer to linearity than those in 2/4 (ca. 160o vs. ca. 140o), which may reflect the reduced steric congestion from tris-Cp 2/3 to the bis-Cp 4/6. A natural bond orbital (NBO) analysis revealed that the Th(IV)]C bond in 11 is comprised of a strongly polarized covalent ThdC s bond (15.9% Th, 84.1% C), along with a p-type C!Th donor-acceptor bond. This bonding scenario was in accord with an earlier study41 and corroborated the Th(IV)]C double bond description.

318

Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

Compared to the reactions between [Th(Cp )2Me2] and [(CH3PPh3)(X)], the methane elimination reaction between [An(Cp )2MeX] (X ¼ Cl, Br, I) and the ylide H2C]PPh3 was found to be a more straightforward and reliable way to make bis-Cp An(IV)]C carbene complexes (Scheme 12).44 The advantages of the methane elimination protocol include higher yields and reduced likelihood of side-reactions. The products from these reactions, [An(Cp )2(CHPPh3)(X)] (An ¼ Th, X ¼ Cl for 14; An ¼ U, X ¼ Cl for 15, Br for 16, I for 17) feature short An(IV)]C bonds. The An(IV)]C bonds in the thorium carbene complexes 11, 12 and 14 (Th(IV)]C 2.299(6)–2.32351(13) A˚ ) and the uranium carbene complexes 15–17 (U(IV)]C 2.2428(2)–2.252(4) A˚ ) were the shortest uranium/thorium-carbon bonds reported at the time, and were only recently surpassed by a fullerene-encapsulated uranium-carbide45 (vide infra). For the U(IV) complexes 15–17, the Wiberg bond orders of the U(IV)]C bonds were calculated to be circa 1.17.

Scheme 12 Synthesis of the An(IV) carbene complexes 14–17 from methane eliminations between {Cp 2An(IV)MeX} and H2C]PPh3.44

Other than Cp-derived ligands, the sterically bulky amide ligand {N(SiMe3)2}1− was also used to support [An(IV)]C(H)PR3] linkages. In 2011, Walensky, Hayton and co-workers reported the synthesis of a tris-amide U(IV)]C carbene complex [U(IV){N(SiMe3)2}3(CHPPh3)] (19) from a U(III) tris-amide ylide adduct complex [U(III){N(SiMe3)2}3(CH2PPh3)] (18) (Scheme 13).46 A U(IV) tris-amide methyl complex [U(IV){N(SiMe3)2}3(CH3)] (20) and PPh3 were concomitantly produced with 19, in a 1:1:1 (19:20:PPh3) ratio. Carbene complex 19 is not stable under ambient temperature in solution and is in equilibrium with a U(IV) cyclometallated complex [U(IV){N(SiMe3)2}2{(CH2SiMe2)N(SiMe3)}] (21) and the free ylide H2C] PPh3. The reaction to produce 21 and the ylide is an intramolecular CdH activation of a methyl group on the {N(SiMe3)2}1 − ligand.

Scheme 13 Formation of a U(IV) carbene 19 from a U(III) ylide adduct 18.46

The reaction to form 19 differs from the salt/alkane elimination methods employed to make 1–17, and is postulated to involve a single-electron redox process.46 Walensky and Hayton hypothesized a process that involves an H% free radical formed by a CdH bond single-electron reductive cleavage (Scheme 13).47–49 The presence of H% was proven by isotope labelling and TEMPO trapping experiments; nonetheless, precise details of the mechanism could not be ascertained.

Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

319

The U(IV)]C bond in 19 (2.278(8) A˚ )46 is longer than those in the Cp/Cp -counterparts 2,38 439 and 15–1744 (2.23–2.25 A˚ ), which may reflect the differences in the electronic features of the supporting ligands. The amide ligand in 19 is a stronger electron donor than the Cp/Cp ligands in 2, 4 and 15–17. Thus, the U(IV) center in 19 is more electron-rich, and as a result, has a diminished interaction with the carbene ligand {CHPPh3}1−. Notwithstanding being slightly weakened, the U(IV)]C bond in 19 still features one s and one p bond, which are heavily polarized but with non-negligible U characters (12% and 8% for the s and p bond, respectively).44 The equilibrium between actinide carbene 19 and its corresponding metallacyclic complex 21 opened up a new avenue to actinide carbene complexes (Scheme 14).46 Compared to a UdC single bond, a ThdC single bond is more ionic and polarized (less metal-based f-orbital bonding interaction for Th than U); thus, is presumably more reactive.50–53 In 2017, Hayton, Hrobárik and co-workers exploited the higher reactivity of ThdC bonds and tuned the equilibrium in favor of a tris-amide Th(IV) carbene complex [Th{N(SiMe3)2}3(CHPPh3)] (23) (Scheme 15).54 The Th(IV)]C bond in 23 (2.362(2) A˚ ) is slightly longer than the Cp -supported Th(IV)]C complexes 11, 1243 and 1444 (2.299(6) and 2.3137(18) A˚ ), which is in line with the stronger electron-donating character of the {N(SiMe3)2}1− ligand. Compared to the U(IV) counterpart [U{N(SiMe3)2}3(CHPPh3)] (19), the Th(IV)]C bond in 23 has slightly reduced metal contributions in its s- and p-components. It is noteworthy that, in this work, the authors correlated the 13C NMR chemical shift of the Th(IV)]C moiety to metal-ligand covalency.54

Scheme 14 The equilibrium between the carbene 19 and the metallacyclic complexes 21.46

Scheme 15 Formation of a Th(IV)]C carbene complex 23 from a Th(IV) metallacyclic complex 22.54

In the abovementioned An]C(H)-PR3 complexes, the P(V)dC bond plays a vital role to stabilize the An]C bond. On the other hand, this stabilization diminishes the An]C double bond character. A strategy to synthesize a stronger An]C bond is by replacing the P atom with a heavier pnictogen atom E (E ¼ As, Sb, Bi). Compared to the PdC bond, heavier pnictogen EdC bonds are weaker and more ionic; thus, it is less likely to stabilize the delocalized resonance structure (Scheme 16). However, until recently, there was no reliable synthetic protocol for the arsonium ylide H2C]AsR3,55 which had impeded progression in this area.

Scheme 16 To improve the An]C double bond character by replacing P with As.

In 2020, Liddle and co-workers reported an improved and reliable synthesis of H2C]AsPh3; using this reagent, they synthesized the first metal (and thus actinide) arsonium-carbene complex [U(IV)(TrenTIPS)(CHAsPh3)] (25) (Scheme 17A).56 In this work, the authors structurally characterized H2C]AsPh3 using SCXRD for the first time. In combination with theoretical calculations, it was revealed that the As]C bond in H2C]AsPh3 is weaker than the P]C bond in H2C]PPh3. H2C]AsPh3 was subsequently treated with a U(IV) metallacyclic complex [U{N(CH2CH2NSiiPr3)2(CH2CH2SiiPr2CH(Me)CH2)}] (24) to afford the U(IV) arsonium carbene complex [U(IV)(TrenTIPS)(CHAsPh3)] (25) (Scheme 17). The phosphorus counterpart, [U(IV)(TrenTIPS)(CHPPh3)] (26), was also synthesized by a similar reaction. A side-to-side comparison between U(IV)]CdP and U(IV)]CdAs units (Scheme 17B) demonstrated that 25 features a shorter U(IV)]C bond and a substantially longer CdE bond compared to 26, even with the difference in atomic radii between P/As considered. The calculated bond orders of the U(IV)]C linkage are 1.66 (25) and 1.56 (26), respectively. The natural bonding orbital (NBO) analysis revealed that 25 has slightly higher U-character over 26 in both s- and p-components of the U(IV)]C bond (Table 1).

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Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

(A)

(B)

Scheme 17 The synthesis of a U(IV) arsonium-carbene complex 15-As and a comparison with its phosphorus counterpart 15-P.56

Table 1

Key structural parameters of An]C(H)ER3 (E: P, As) linkages in structurally characterized actinide ylide carbene complexes. 13

C chemical shift of the carbene carbon (ppm)

˚) An]C Bond length (A

An]CdE bond angle ( )

Calculated bond order

References

[U(IV)(Cp)3(CHPPhMe2)] (2) [U(IV)(Cp)3(CHPMe3)] (4) [Th(IV)(Cp )2(Br)(CHPPh3)] (11) [Th(IV)(Cp )2(I)(CHPPh3)] (12) [Th(IV)(Cp )2(Cl)(CHPPh3)] (14) [U(IV)(Cp )2(Cl)(CHPPh3)] (15) [U(IV)(Cp )2(Br)(CHPPh3)] (16) [U(IV)(Cp )2(I)(CHPPh3)] (17) [U(IV){N(SiMe3)2}3(CHPPh3)] (19) [Th(IV){N(SiMe3)2}3(CHPPh3)] (23) [U(IV)(TrenTIPS)(CHAsPh3)] (25)

– – 107.6 (d, 1JCP ¼ 20.0 Hz) (C6D6) 113.7 (d, 1JCP ¼ 20.1 Hz) (C6D6) 104.1 (d, 1JCP ¼ 19.5 Hz) (C6D6) – – – – 116.54 (d, 1JCP ¼ 22.5 Hz) (C6D6) –

2.293(2) 2.274(8) 2.3137(18) 2.299(6) 2.32351(13) 2.2428(2) 2.252(4) 2.2454(2) 2.278(8) 2.362(8) 2.272(6)

141.49(7) 143.5(5) 162.0454(16) 163.098(3) 162.1662(14) 166.0952(18) 166.1(3) 166.4890(18) 151.7(4) 148.2(1) 166.1(4)

36–38 39 43 43 44 44 44 44 46 54 56

[U(IV)(TrenTIPS)(CHPPh3)] (26)

–

2.313(3)

162.1(2)

– – – – – 1.16 (Wiberg) 1.17 (Wiberg) 1.17 (Wiberg) – – 1.56 (NalewajskiMrozek) 1.66 (NalewajskiMrozek)

3.09.3.1.2

56

Actinide ylide carbene complexes [L3An(IV)(C(H)ER3)] (E: P, As): Reactivity

Soon after their discovery,38,39 the first actinide ylide carbene complexes 3 were treated with 1 atm of CO to produce a CO insertion product [(C5H5)3U((2-OCCH)PPh2CH3)] (27) (Scheme 18).57 Though the yield of 27 was not reported, the reaction mixture was reported to change from a red color (3) into green (27). The reaction is a 1,1-migratory insertion of CO into the U(IV)]C bond, which clearly demonstrated the anionic and nucleophilic nature of the {CHPR3}1− ligand.

Scheme 18 A 1,1-migratory insertion of CO into the U]C bond of 3 to give 27.57

Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

321

Compared to CO, isocyanides RNC feature stronger p acidity and s basicity. When 3 was treated with one equivalent of cyclohexyl isocyanide (CyNC) in Et2O for one hour, the red product [(C5H5)3U{[2-N(cyclo-C6H11)CCH]PPh2CH3}] (28) was isolated in 53% yield (Scheme 19).58 Complex 28 is best described as a zwitterion with a phosphonium cationic site and negatively charged U(IV) center surrounded by three Cp−, one alkenyl carbon and one amide nitrogen. Similarly to the 1,1-migratory insertion of CO in Scheme 18, the reaction to produce 28 is a 1,1-migratory insertion of CyNC.

Scheme 19 1,1-migratory insertion of CyNC into the U]C bond of 3 to give 28.58

In contrast to the 1,1-insertions of CO and CyNC, the C^N triple bond of acetonitrile inserted into the U(IV)]C bond of 3 in a 1,2-manner (Scheme 20).59 [(C5H5)3U{1-NC(CH3)(CHPPh2CH3)}] (29) was isolated as red crystals in 50% yield from a 1:1 reaction between 3 and acetonitrile. The structure of 29 features a delocalized UdNdCdC unit, where the UdN bond can be viewed as having appreciable imido character.

Scheme 20 1,2-Migratory insertion of MeCN into the U]C bond of 3 to give 29.59

Unsaturated organic substrates with C]E double bonds were also reported to insert into U(IV)]C bonds. When complex 3 was treated with one equivalent of phenyl isocyanate (PhdN]C]O), [Cp3U{2-N,O-(NPh)(O)CCHP(CH3)(C6H5)2}] (30) was isolated in 69% yield (Scheme 21).60 The reaction was facile and completed at −78  C in toluene within 5 hours. Complex 30 features a delocalized, nearly coplanar, PdCdCdNdO unit.

Scheme 21 Insertion of PhdN]C]O into the U]C bond of 3 to give 30.60

The Brønsted basicity and monoanionic nature of the CHPR−3 carbene ligand was unequivocally demonstrated in the protonolysis reaction of complex 3 with diphenylamine (Scheme 22),61 from which a uranium amide [Cp3U(NPh2)] (31) was obtained. There was a precipitate observed during the reaction, which is presumably the ylide PhMe2P]CH2. A large excess of the diphenylamine (10 equivalents) and long reaction times (2 days) are necessary for the reaction to go to completion. In contrast, one equivalent of phenylacetylene is sufficient to convert 3 into the corresponding protonolysis product [Cp3U(CCPh)] (32) (Scheme 22).62

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Scheme 22 Protonolysis reactions of the U]C bond of 3 to give 31 and 32.61,62

Other than CO, the uranium(IV) carbene complexes 2 and 3 were also reacted with a variety of transition metal carbonyl complexes.62 In 1983, complexes 2 and 3 were reported to react with one equivalent of [CpFe(CO)2]2 in THF at ambient temperature, to produce complexes [Cp2U][{O2C2C(H)P(Ph)(Me)2}Fe(Cp)(m-CO)Fe(CO)(Cp)]2 (33) and [Cp2U][{O2C2C(H)P(Ph)2(Me)}Fe(Cp)(m-CO)Fe(CO)(Cp)]2 (34) as green crystals in ca. 45% yield, along with [Cp4U] (identified by NMR spectroscopy) and some unidentified paramagnetic products (Scheme 23).63 The oxidation states of the U and two Fe ions in 33/34 were not definitively determined by magnetometry and Mössbauer spectroscopy, but the NMR and IR spectra, as well as the SCXRD data, support a U(IV), Fe(I), Fe(I) assignment. Thus, apparently, there is no metal-based redox process during the formation of complexes 33/34. The mechanism to form 33/34 is not clear, but insertion of the coordinated CO molecules into U(IV)]C is in line with previous reports in Scheme 18.57 The bridging nature of the CO ligands in [CpFe(CO)2]2 facilitates the ensuing CO coupling and leads to the isolated products 33/34.

Scheme 23 Reactions between [CpFe(CO)2]2 and uranium carbene complexes 2 and 3, to produce complexes 33 and 34.63

By employing transition metal carbonyls with terminal coordinated CO ligands, such as [CpMn(CO)3],64 [W(CO)6],65,66 and [CpCo(CO)2],67 the separate reactions of 2 and 3 with these substrates initially gave the insertion products 35–40 in good yields (60–80%) (Scheme 24). Upon heating, complexes 35–40 underwent further reactions, such as CdH bond activation, CdO/CdP bond cleavage and migration (Scheme 24).

3.09.3.2

P(V)-stabilized actinide pincer-type carbene complexes: Synthesis, structure, reactivity

Compared to the ylide carbene ligands [HCPR3]−, dianionic alkylidene ligands CR2− 2 in the classic Schrock-type alkylidene complexes are more electron-rich. As a result, the hypothetical An]CR2 bond is more polarized and less stable than the An] C(H)PR3 bond, and thus requires extra thermodynamic/kinetic stabilization from the R groups. Ideally, a bona fide actinide alkylidene complex bears a well-developed, non-perturbed An]CR2 double bond, where the R groups are hydrocarbon (alkyl, or to a less extent, aryl) substituents. Silyl substituents, on the other hand, may introduce some

Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

323

Scheme 24 Reactions between the uranium carbene complexes 2/3 and transition-metal terminal carbonyl complexes.

extent of electronic stabilization (i.e. perturbation) to the An]C bond, via the hyperconjugation effect of silicon.1,68,69 For the all-hydrocarbon An]CR2 moiety, without any electronic stabilization (mesomeric/inductive) from the R groups, the highly polarized An]C bonds are too reactive to be isolable, thus have only been observed in inert gas matrices at extremely low temperatures (see Section 3.09.2). In the early 2000s, new families of bis-phosphorus dianionic C2− ligands were introduced to early transition metal and f-block chemistry (Scheme 25A). According to the Terminology section, the C2− center in such ligands directly links to heteroatomic groups (P(V)), so here it is described as a carbene rather than an alkylidene. These dianionic carbene ligands feature two P(V)(R2)]E (E ¼ NR0 , S, O) substituents, which form a pincer geometry upon coordination to a metal center, and provide excellent electronic and steric stabilization for the M]C bond via mesomeric and chelating effects (Scheme 25B and C). For these reasons, these pincer carbene ligand families have found widespread applications in early transition metal and f-block chemistry.70 In 2009, these pincer carbene ligands were introduced into actinide chemistry.71 Since then, An]C pincer carbene complexes have developed into one of the most vibrant research areas of actinide chemistry. In this section, we will compile the achievements in this area, according to the metal type (U and Th) and the oxidation states (U: +3, +4, +5, +6; Th: +4), covering the synthesis, structural features, and reactivity profiles. It is noteworthy that An]C bonds in the pincer carbene complexes are quite stable, due to the excellent electronic/steric protection placed by the ligand. As a result, the reactivity of the An]C bond is limited; thus, reactivity studies will be covered together with synthesis/structural discussions, rather than in dedicated separate sections. There have been several reviews in this field,72,73 which we refer interested readers to.

3.09.3.2.1

U(III) pincer carbene complexes

In the past three decades, a well-established collection of uranium-element multiple bonding (U]E/U^E, E ¼ C, O, S, Se, N, P, As) complexes have been achieved.74,75 In these U]E/U^E complexes, the uranium oxidation states are mostly +4, +5, and +6. In contrast, U(III)]E/U(III)^E complexes are scarce. There are two possible reasons for the paucity of U(III)-ligand multiple

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(A)

(B)

(C)

Scheme 25 The bis-phosphorus dianionic pincer carbene ligands C{P(E)R2}2− 2 and their mesomeric/chelating stabilizing effects on M]C double bonds.

bonded complexes: (1) from an electrostatic perspective, the U(III) ion is less electron deficient than U(IV-VI), thus is less likely to form a strong bond with E2−/E3− ligands; and, (2) strongly electron donating E2−/E3− ligands can induce U(III) disproportionation to U(0) and U(IV). In 2018, employing a bis-iminophosphorano dianionic pincer carbene ligand BIPMTMS (BIPMTMS: [C(PPh2NSiMe3)2]2−), Liddle and co-workers synthesized the first, and so far the only, U(III)]C linkages in the complexes [{U(III) (BIPMTMS)}6(m-I)3(m-6:6-arene)3] (arene ¼ C7H8 for 46; arene ¼ C6H6 for 47), by reducing a U(IV) precursor [U(IV)(BIPMTMS) (I)(m-I)]2 45 (Scheme 26) in aromatic solvents.76 Complexes 46 and 47 are arene- and iodide-bridged hexanuclear systems. It is noteworthy that the reduction of 45 in THF led to protonation of the putative U(III)]C bond, to form a complex with a U(III)dCH single bond, [{U(BIPMTMSH)(I)}2(m-6:6-C7H8)] (48), which had been reported by the Liddle group previously as a diuranium single molecule magnet (SMM).77 Complexes 46 and 47 are obtained in low but reproducible yields. A primary quest for a multimetallic complex, such as 46 and 47, is determining the oxidation states of each metal center. The single-crystal structures of 46/47 indicate localized U(III) and U(IV) centers, i.e. the complexes can be classified as Robin Day Class I systems.78,79 Among the three crystallographically independent U centers, there is one U(III)]C (2.413(8) A˚ ), one U(IV)]C (2.398(7) A˚ ) and one U(III)/U(IV) (U]C 2.30(3) and 2.47(2) A˚ ) disordered site. Superconducting quantum interference device (SQUID) magnetometry (DC measurement, wT vs. T), electron paramagnetic resonance (EPR) spectroscopy, X-ray absorption near-edge spectroscopy (XANES) and electronic absorption spectra rule out the presence of U(V) and U(VI) ions. An in-depth understanding of the electronic structure of the U(III)]C bonds in 46 and 47 was obtained via quantum chemical calculations. It should be noted that due to the multimetallic nature and the large size of the molecules, specialized computational methods, such as complete active space self-consistent field (CASSCF) and second-order perturbation theory (CASPT2), must be employed to allow a comprehensive understanding. Both the U(III)]C and U(IV)]C bonds feature polarized multiple bond character (calculated Nalewajski-Mrozek bond orders 1.15–1.16, respectively), with not only predominant C-contributions, but also non-negligible U-contributions (Table 2). The calculations also suggest a clear trend that the U-contribution in the U]C bond decreases as the oxidation states move from +4 to +3.

3.09.3.2.2

U(IV) and Th(IV) pincer carbene complexes

The +4 oxidation state dominates the field of actinide pincer carbene complexes. In 2009, Ephritikhine, Le Floch, Mézailles and co-workers reported the first members of the actinide pincer carbene family, produced from salt elimination reactions between [U(IV)(BH4)4] and Li2(SCS)
(Et2O)1.5 (SCS ¼ [C(PPh2S)]2−) (Scheme 27).71 [U(BH4)4], instead of the more conventional U(IV)

Scheme 26 Synthesis of U(III)]C pincer carbene complexes 46 and 47 from reductions of a U(IV) precursor 45.76 Table 2

Key structural parameters of U(III)]C pincer carbene complex 46.a

Complex

˚) U(III)]C Bond length (A

Calculated bond order

References

[{U(BIPMTMS)}6(m-I)3(m-6:6-C7H8)] (46)

2.413(8)

1.15 (Nalewajski-Mrozek)

76

a

The structure of 47 is very similar to 46.

Scheme 27 Synthesis and reactivity of U(IV) tris- and mono-carbene complexes 49–51 with SCS pincer carbene ligands.71

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halides, was used as starting material for its higher solubility in organic solvents. The 1:1 reaction gives a trinuclear U(IV) tris-carbene complex [U(IV)(m-SCS)3{U(IV)(BH4)3}2] (49), which can be further converted into a mononuclear carbene complex [U(IV)(SCS) (BH4)2(THF)2] (50). A 3:1 reaction between [Li2(SCS)] and [U(BH4)4] affords a U, Li, Li-trinuclear complex [U(IV)(m-SCS)3{Li(Et2O)}2] (51), which has a similar core structure to 49. The U(IV)]C bonds in the multinuclear complexes 49 and 51 (2.46 A˚ on average) are substantially longer than those in the mononuclear complex 50 (2.327(3) A˚ ), which is in line with the higher coordination numbers in the multinuclear complexes. The U(IV)]C bond in the mono-carbene complex 50 (2.327(3) A˚ ) is shorter than typical U(IV)dC single bonds in U(IV) alkyl complexes (2.4–2.5 A˚ ),80–82 and is comparable with U(IV)]C(H)PR3 bonds (2.27–2.37 A˚ ) in the U(IV) ylide carbene complexes 2–26 in the previous section (Table 1). Computational studies on truncated model complexes based on the mono-carbene complex 50 revealed that the C2− center in the [SCS]2− ligand acts as a 4e-donor to the electron-deficient U(IV) center. The U(IV)]C interaction in the SCS pincer carbene is comprised of one s-bond and one p-bond, indicated by the molecular orbital analysis, involving 5f- and 6d-orbitals of uranium. The C2− center is stabilized by not only U(IV), but also two -P(S)Ph2 substituents. Reactivity of the U(IV)]C bonds in 50 and 51 was briefly investigated. Treating 50 or 51 with 1 (for 50) or 1/3 (for 51) equivalents of benzophenone or 9-anthracenyl aldehyde afforded the carbene transfer products (Scheme 27). The facile and high-yield carbene transfer reactions clearly demonstrate the strong nucleophilicity of the C2− center. Synthesis and characterization of the U(IV)]C complexes 49–51 was a breakthrough in actinide chemistry. However, the starting material [U(BH4)4], which was employed for its solubility, is a significant drawback of this synthetic route. [U(BH4)4] is typically synthesized by a solvent-free mechanochemical method in a vacuum vibrational ball mill and purified by sublimation.83 More straightforward access to U(IV)]C pincer carbene complexes was still desirable. Shortly after the report of complexes 49–51, the same group developed a more accessible synthesis of U(IV)]C pincer carbene complexes, employing [UCl4] and [U(NEt2)4] as starting materials.84,85 [UCl4] reacts with 1, 2 and 3 equivalents of Li2SCS to yield mono-, bis- and tris-carbene complexes [U(IV)(SCS)(Cl)(THF)(m-Cl)2{(Li)(THF)2}] 52, [U(IV)(SCS)2(THF)2] 53 and 51, respectively (Scheme 28). Solvents play vital roles in these reactions. Et2O strongly favors the tris-carbene complex 51—even with lower stoichiometric ratios of Li2SCS. The mono- and bis-carbene complexes 52 and 53 have to be synthesized in a mixture of toluene and THF, instead of Et2O. The readers’ attention should also be drawn to a solvent-relevant detail: Li2SCS reacts with THF to produce LiHSCS. Thus, in the toluene/THF mixture, the THF amount must be minimized to avoid side reactions. The crucial roles of solvents in these reactions highlight the sensitivity, and the synthetic challenges, of actinide chemistry. Based on 51–53, a series of carbene-supported U(IV) complexes were derived (Scheme 28). A homoleptic U(IV) amide, [U(NEt2)4], was also used as a starting material for U(IV)]C pincer carbene complexes,85 with a series of U(IV) carbene complexes synthesized via amine (Et2NH) elimination (Scheme 29). Again, the reaction results are highly dependent on the solvent systems employed. For instance, it is not possible to prepare the bis-carbene complex 53 via a 1:2 reaction between [U(NEt2)4] and H2C(Ph2PS)2 in Et2O. Instead, a stepwise approach must be adopted, i.e. the first step to produce a carbene-methanide-amide complex [U(IV)(SCS)(SCSH)(NEt2)] (61) in Et2O, which was further converted to 53 in THF (Scheme 29). The U(IV)]C bond lengths in the structurally characterized SCS carbene complexes, 49–62, are listed in Scheme 28 and 29, ranging from 2.33 to 2.40 A˚ . A clear trend is that the lower the U(IV) center’s coordination number, the shorter the U(IV)]C bond. This trend can be explained by the increased steric congestion in higher coordination number complexes. Thus, the mono-carbene complexes (50, 52, 55–59, 60–62) have shorter U(IV)]C bonds compared to the bis- and tris-carbene complexes (49, 51, 53). In terms of the bonding scenario, a computational study85 showed that the U(IV)]C bond in the pincer carbenes are similar to that of the ylide carbene, i.e. both feature single s- and p-components, and a highly polarized U]C bond with a non-negligible metal contribution. Other than uranium, in 2011, thorium(IV) bis- and tris-carbene complexes [Th(IV)(SCS)2(DME)] (63) and [{Th(IV)(SCS)3} Li2(DME)]1 (64) were synthesized via salt eliminations between Li2SCS and [ThCl4(DME)2] in a mixture of Et2O and DME (Scheme 30).86 The Th(IV)]C bond lengths in 63 and 64 are longer than the corresponding U(IV)]C bonds in 53 and 51, respectively; which is in line with the larger ionic radius of Th(IV) cf. U(IV). Computational studies of 63 and 64 revealed that, compared to the U(IV)]C bonds in 51 and 53, the Th(IV)]C bonds in 63 and 64 exhibit a similar double-bonding scenario (one s-bond and one p-bond) but with a less metal-based orbital contribution. In conclusion, the Th(IV)]C bond in 63/64 is more polarized, more ionic and less covalent than the U(IV)]C bond in 51/53. Other than the SCS ligand, another prevalent dianionic pincer carbene ligand—bis(iminophosphorano) methandiides (BIPM, [C(PPh2NR)2]2−), was introduced to actinide chemistry in 2010 by Liddle and co-workers.87 An N-aryl BIPMMes (Mes ¼ 2,4,6-trimethylphenyl) dilithium salt was treated with uranium(III) triiodide (Scheme 31). Instead of U(III) complexes, a U(IV) bis-carbene complex [U(IV)(BIPMMes)2] (65) and U metal were obtained as products along with lithium iodide. This result can be rationalized by a salt elimination followed by a U(III) disproportionation and rearrangements, which explains the low yield of 65 (24%). The U(IV)]C bond lengths in 65 are longer than those in the corresponding SCS U(IV) bis-carbene complex 53 (2.427–2.448 vs. 2.390–2.399 A˚ ), even though the former complex has a lower coordination number (6 vs. 8). This reflects the increased steric congestion about uranium for the BIPMMes ligand compared to the SCS ligand. In 2011, using an N-trimethylsilyl BIPM ligand (BIPMTMS), Cavell and co-workers synthesized a series of U(IV) and Th(IV) mono-carbene complexes [U(IV)(BIPMTMS)(Cp)2] (66), [Th(IV)(BIPMTMS)(Cp)2] (67), [U(IV)(BIPMTMS)(Tp)(Cl)] (68) and [Th(IV) (BIPMTMS)(Tp)(Cl)] (69) with the general formula [An(IV)BIPMTMSLn] (An ¼ U or Th) (Scheme 32).88 The ancillary ligands Ln are Cp (66, 67) or a tris-pyrazolyl borate (Tp) (68, 69). The dilithium salt Li2BIPMTMS is more stable than Li2SCS in THF, thus THF was

Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

327

Scheme 28 Synthesis of a series of U(IV)]C pincer carbene complexes (51–59) with SCS ligand and their reactivity.84,85

used as the reaction solvent. Both syntheses employ a common [An(IV)BIPMTMSCl2] intermediate, which was not isolated nor characterized. The Th(IV)]C bonds in 67 (2.436(4) A˚ ) and 69 (2.469(3) A˚ ) are approximately 0.1 A˚ longer than the analogous U(IV)]C distances in 66 (2.351(2) A˚ ) and 68 (2.376(3) A˚ ), which is in line with the difference in the 6-coordinate ionic radii between Th(IV) (1.05 A˚ ) and U(IV) (0.89 A˚ ).89,90 Treatment of the [U(IV)BIPMTMSCl2] intermediate in situ with one equivalent of organic nitriles, MeCN and PhCN, led to complexes 70 and 71, respectively, which results from formal 2 + 2 cycloadditions between

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Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

Scheme 29 Synthesis of U(IV) SCS pincer carbene complexes using [U(NEt2)4] as the starting material.85

Scheme 30 The synthesis of Th(IV) SCS pincer carbene complexes.86

Scheme 31 Synthesis of BIPMMes U(IV) bis-carbene complex 65 via salt elimination and disproportionation.87

Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

329

Scheme 32 Synthesis of BIPMTMS U(IV) and Th(IV) mono-carbene complexes 66–69, and 2 + 2 cycloaddition reactions between the U(IV)]C bond and RdC^N.88

the C^N and U(IV)]C bonds. Similarly to the U(IV)/Th(IV) SCS complexes 49–64,85,86 theoretical investigations of the U(IV)/ Th(IV) BIPMTMS complexes revealed two-fold U(IV)]C and Th(IV)]C bonds, comprised of one s- and one p- bonding interaction. The bis-carbene complexes [U(IV)(SCS)2(THF)2] (53),85 [Th(IV)(SCS)2(DME)] (63)86 and [U(IV)(BIPMMes)2] (65)87 contain a metalla-allene C]An(IV)]C structure, which is comparable to the ubiquitous O]U(VI)]O in uranyl complexes. The impressively high thermodynamic stability of the uranyl unit has not only profound academic influence but also practical implications. By investigating the O]U]O moiety, researchers developed the inverse trans-influence (ITI) concept, which underpins actinide science.91–97 On the application side, the stability and water-solubility of the O]U(VI)]O moiety presents challenges in nuclear waste remediation. In this context, understanding the E]An]E bonding scenario with various actinide metals, oxidation states and ligand E atoms is of great scientific interest.98 Since the disclosure of 65, the Liddle group has made significant progress towards C]M]E (M ¼ U, Th; E ¼ C, N, O) complexes, which will be covered in the following paragraphs. For E ¼ C, in 2017, the C]M(IV)]C unit in a series of [M(IV) (BIPMTMS)2] complexes (74-M) was systematically investigated for their M]C bonding nature, where M ¼ U, Th, Ce, Tb and Pr.99 Complexes 74-U/Th/Ce were experimentally synthesized and characterized, whilst 74-Tb/Pr were investigated in silico. The synthesis of [An(IV)(BIPMTMS)2] (74-U/Th) started with a U(IV) or Th(IV) BIPMTMS ate complex [An(IV)(BIPMTMS)(Cl)(m-Cl)2Li(THF)2] (72)100 via a carbene bis-alkyl complex [An(IV)(CH2SiMe3)2(BIPMTMS)] (73) (Scheme 33).

Scheme 33 Synthesis of U(IV) and Th(IV) BIPMTMS bis-carbene complexes 74, via two-step salt elimination and alkane elimination.99

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Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

In the bis-carbene complexes 74, the Th(IV)]C bonds are circa 0.1 A˚ longer than the analogous U(IV)]C bonds, which is in line with the difference in ionic radii and the more ionic nature of the Th(IV)]C bonds. The authors discovered that the Ce(IV)]C bond in 74-Ce has a similar level of covalency to the U(IV)]C bond. This was a surprising discovery, as the lanthanide metals are typically believed not to form covalent metal-ligand bonds.10,101–103 More importantly, this work showed that the ITI operates in the polarized covalent bonded tetravalent f-element bis-carbene C]M(IV)]C (M ¼ U, Ce, Pr, Tb) system. This is a substantial extension of the ITI concept,91–95 from a niche phenomenon in high-valent actinide chemistry to a broad spectrum of the f-elements. It is instructive to compare the three U(IV) bis-carbene complexes [U(IV)(SCS)2(THF)2] (53),85 [U(IV)(BIPMMes)2] (65)87 and [U(IV)(BIPMTMS)2] (74-U)99 (Scheme 34). The U(IV)]C bond lengths in the BIPMMes/TMS complexes (65, 74-U) (2.42–2.45 A˚ ) are longer than those in the SCS complex 53 (2.39–2.40 A˚ ), which is a trend in line with the bulkier steric profile of BIPMMes/TMS ligands compared to the SCS ligand. The bulkier BIPMR ligands also prevent coordination of solvent molecules, such as the two pyridine molecules in 53, which is further translated into the lower coordination number (C.N. ¼ 6) in the BIPM complexes than the SCS complex (C.N. ¼ 8). It is noteworthy that the C]U]C angles in 53 and 65 (136.0(3) and 147.1(3) , respectively) are significantly different from that in 74-U (177.5(2) ), i.e. the C]U]C unit in 53/65 can be described as a bent geometry, whilst in 74-U, there is a linear geometry. Given that work from Liddle and co-workers suggested that the ITI operates in U(IV) chemistry,99 i.e. a linear C] U(IV)]C unit is thermodynamically favorable, it is not sure if the steric factors override the ITI and result in the bent geometry in 53 and 65. However, the structure of an actinide complex is determined by the interplay of many factors, such as actinide metal identities and their valence shell orbital characters, their oxidation states, and types of ligand atoms.

Scheme 34 A comparison of three U(IV) bis-carbene complexes 53, 65 and 74-U.

Other than the C]An(IV)]C complexes above, the Liddle group also investigated C]An(IV)]E complexes, where E is N or O. In 2016, this group synthesized two C]U(IV)]N complexes [U(IV)(BIPMTMS)(]NCPh3)(NHCPh3)(M)] (M ¼ Li for 77; M ¼ K for 78), with the coordinated alkali metal cations (Li+ and K+) (Scheme 35).104 The C]U(IV)]N units in 77 and 78 have acute angles (91.54(12)–104.4(3) , respectively). More importantly, the structural metrics are significantly influenced by the Li+ and K+ cations, depending on their capability of inducing electron density polarization. This phenomenon was rationalized by a push-pull effect along the C]U(IV)]N unit exerted by Li+/K+ cations, as illustrated by the inset of Scheme 35. As per the work described above,99 assuming that the ITI operates in the C]U(IV)]N unit, a linear, instead of bent, geometry should be expected. However, the C]U(IV)]N unit in 77 and 78 was experimentally observed as bent. It is difficult to distinguish whether a bent C]U(IV)]N geometry is thermodynamically favorable, i.e. a TI, instead of ITI, operates here; or, if the Li+/K+ cations act as an ‘anchor point’ to force the C]U(IV)]N unit to form the observed bent geometries. To address this issue, Li+/K+ free C]U(IV)]N complexes are desirable synthetic targets. Employing a different synthetic route, Liddle and co-workers obtained two alkali-metal-free C]U(IV)]N complexes, [U(IV) (BIPMTMS)(NCPh3)(BIPY)] (BIPY ¼ 2,20 -bipyridine) (79) and [U(IV)(BIPMTMS)(NCPh3)(DMAP)2] (DMAP ¼ 4-dimethyl-pyridine) (80), in cis-(79) and trans-(80) geometries, respectively (Scheme 36).105 Moreover, it was reported that, by switching the ancillary ligands DMAP or BIPY, the cis- and trans-C]U(IV)]N complexes 79 and 80 are interconvertible. The U(IV)]C and U(IV)]N bond lengths in cis-79 are shorter than those in trans-80, which may suggest a stronger bonding interaction in the cis-79, i.e. a trans-influence (TI). However, the bond length difference is too small to be conclusive. The authors thus resorted to calculations; by optimizing the C]U(IV)]N angle free of restraints in a hypothetic model complex [U(IV)(PH2NSiH3)(NCH3)], it was discovered that the most thermodynamically stable C]U(IV)]N angle is 108.18o, which is 12.3 kJ mol−1 lower in energy than the trans-geometry (180o). There is no obvious steric reason to induce the cis-C]U(IV)]N geometry. Thus, apparently, the structural preference of the C]U(IV)]E unit depends upon the E atom. In the C]U(IV)]C and C]U(IV)]N cases, the ITI and TI operate, respectively. To understand the driving force behind this diverse behavior, a joint experimental-theoretical study was necessary. A joint experimental/computational study on An pincer carbenes was done by the Liddle and Kaltsoyannis groups in 2019.106 In this work, two new cis-C]Th(IV)]N complexes were synthesized, namely [Th(IV)(BIPMTMS)(NCPh3)(NHCPh3)(K)] (83) and

Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

331

Scheme 35 Synthesis of C]U(IV)]N complexes 77 and 78 via salt eliminations followed by alkane eliminations, and (inset) the alkali metal cation induced push-pull effect.104

Scheme 36 Synthesis of the cis- and trans-C]U(IV)]N complexes 79 and 80, from alkane eliminations.105

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Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

Scheme 37 Synthesis of two cis-C]Th(IV)]N complexes 83 and 85.106

[Th(IV)(BIPMTMS)(NCPh3)(BIPY)] (85), which further expanded the library of C]An(IV)]E complexes (Scheme 37). Based on the experimentally characterized C]M(IV)]E complexes (M ¼ Th, U, Ce; E ¼ C, N), the Liddle/Kaltsoyannis groups constructed a matrix of model C]M(IV)]E complexes (M ¼ Th, U, Ce; E ¼ CMe2, NMe, O) in silico, and investigated the correlation between their C]M(IV)]E angle and the f-orbital contribution to the M(IV)]E bond (Table 3). It was discovered that the [BIPM]2− ligand induces a cis-directing electrostatic potential (ESP) upon the formation of [M(IV)BIPM]2+, i.e. from a purely electrostatic perspective, any incoming dianionic ligand E2− would preferably occupy the cis-position, which exhibits a trans-influence (TI). Only when the M]E bond has a strong f-orbital-based covalent character (high percentage of f-orbital contribution), the TI would be overridden by the trans-directing effect from the f-covalency, i.e. the inverse trans-influence (ITI) operates. The linear correlation between the f-covalency and the C]M(IV)]E angle unequivocally supports the structure-directing role of the f-orbital, which recasts a traditional perception in actinide science: FEUDAL (f’s essentially unaffected, d’s accommodate ligands).110 In 2016, the Liddle group reported the synthesis of a series of mono-carbene complexes [M(IV)(BIPMTMS)(ODipp)2] (Dipp ¼ 2,6-diisopropyl phenyl) (M ¼ U for 86; M ¼ Th for 87; M ¼ Ce for 88) (Scheme 38).107 By analyzing the M(IV)]C bond in 86–88, it was discovered that the Ce(IV)]C bond has a similar level of covalency (in the form of the metal’s orbital contribution) to the U(IV)]C bond. This discovery was surprising, as according to the traditional perception, the 4f-orbitals of a lanthanide (Ce) is less likely to participate in bonding compared to the 5f orbitals of an early actinide (U).10,101–103 However, this work demonstrated that a 4f-element could form a polarized covalent M]C bond that is comparable with an early 5f-element. Other than acting as a platform for probing the nature of metal-ligand multiple bonds, the BIPMTMS ligand has also supported a series of complexes with novel structures and reactivity profiles. In 2014, the Liddle group synthesized a Th(IV) carbene-amideketimide complex [Th(IV)(BIPMTMS)(N]CPh2)(N(SiMe3)2)] (90) (Scheme 39).108 There are three types of thorium-ligand bonds in 90: Th]C, ThdNamide, and ThdNketimide. Among them, the Th]C and ThdNamide bonds are traditionally reactive functional groups. On the other hand, the ketimide ligand [N]CPh2]− is widespread in coordination chemistry as a strongly donating inert ancillary ligand. Traditionally, the MdNketimide bond was believed to be inert. However, in 90, the ThdNketimide bond was found to be more reactive towards unsaturated organic substrates than the Th]C and ThdNamide bonds (Scheme 39). Wittig-type carbene transfer, i.e. reactions between the M]C bond in nucleophilic carbene complexes and the C]O bond in organic carbonyl compounds, is a classic reaction to demonstrate the nucleophilicity of carbenes. This is illustrated below for generic actinide BIPMTMS complexes (Scheme 40). In actinide pincer carbene chemistry, benzaldehyde, 9-anthracenecarboxaldehyde and benzophenone are the most used organic carbonyl substrates. Beyond the carbene transfer chemistry shown above, other reactivity modes involving the An(IV)]C (An ¼ U, Th) bond are not common. In most cases, BIPM or SCS ligands act as spectator ligands, instead of taking part in the reactions, likely a result of the strong stabilization and steric protection endowed by the pincer ligands. Other than the serendipitous sulfur-addition (56, Scheme 28) and [2 + 2] cycloaddition with organic nitriles (70 and 71, Scheme 32), there is only one systematic investigation of reactivity of An(IV)]C bond. In 2014, Liddle and co-workers reported the reactivity of a uranium carbene chloride LiCl-ate complex 72 towards organic carbonyl and heteroallene substrates (Scheme 41).109 The reactivity profile is diverse. Wittig-type carbene transfer was observed between 72 and one equivalent of benzophenone or

Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

Table 3

E¼O E ¼ C(CH3)2 E ¼ NCH3

333

The linear correlation between the C]M(IV)]E angles and the f-covalence in the M]E bond.106

f% in M¼E ∠C¼M¼E f% in M¼E ∠C¼M¼E f% in M¼E ∠C¼M¼E

M¼U 18.08 176.9 9.89 109.8 6.09 108.2

M ¼ Ce 17.24 165.1 11.25 128.3 5.12 116.7

M ¼ Th 6.13 116.8 3.30 111.6 2.66 108.4

Scheme 38 The mono-carbene complexes [M(IV)BIPMTMS(ODipp)2] 86–88.107

benzaldehyde. On the other hand, when a bulkier ketone, phenyl tert-butyl ketone, was used, a ketone-adduct [U(IV)(BIPMTMS) {O]C(tBu)(Ph)}(Cl)(m-Cl)]2 (93) was produced. When complex 72 was treated with an a, b-unsaturated organic carbonyl such as coumarin, a nucleophilic ring expansion product [U(IV){BIPMTMS[C(O)(CHCHC6H4O-2)]-k3-N,O,O0 }2(Cl)2(THF)] (94) was isolated in 25% yield. Reaction between complex 72 and 1 equivalent of PhCOF produced a U(VI) uranyl complex [U(VI) (O)2(Cl)2(m-Cl)2{(m-LiDME)-OC(Ph)]C(PPh2NSiMe3)(PPh2NHSiMe3)}2] (95) in a low (24%) but reproducible yield. Extracting the remaining materials from the reaction with THF and recrystallizing from pyridine furnished a U(IV) complex [U(IV)Cl2F2(pyr)4] (96). Apparently, formation of complexes 95 and 96 involves oxidation, nucleophilic attack and ligand scrambling. But the detailed mechanism is unclear, and source of the oxygen atoms in the uranyl unit of 95 could be PhCOF or adventitious water/O2. For organic heteroallene substrates, complex 72 reacted with one equivalent of N,N0 -dicyclohexylcarbodiimide (DCC) or tBuN]C]O to produce the [2 + 2] cycloaddition products [U{BIPMTMS[C(NtBu){OLi(THF)2(m-Cl)Li(THF)3}]-k4-C,N,N0 ,N00 }(Cl)3] (97) and [U{BIPMTMS[C(NCy)2]-k4-C,N,N0 ,N00 }(Cl)(m-Cl)2Li(THF)2] (98), respectively. All of these diverse reactivity profiles of 72 demonstrated the nucleophilic nature of U(IV)]C bond (Table 4).

3.09.3.2.3

U(V) pincer carbene complexes

In spite of the fact that pentavalent uranyl [O]U(V)]O]+ chemistry has been widely investigated in both aqueous and non-aqueous environments,110–114 U(V) non-uranyl coordination chemistry is less developed compared to U(III), U(IV) and

334

Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

Scheme 39 Synthesis and reactivity of a Th(IV) carbene imido ketimide complex 90.108

Scheme 40 Wittig-type carbene transfer between generic actinide BIPMTMS carbene complexes and organic carbonyl compounds.

U(VI), due to two reasons. The first is a paucity of non-uranyl U(V) starting materials. Indeed, all non-uranyl U(V) complexes were synthesized via either oxidations of U(III/IV) complexes, or reduction of U(VI) complexes.115 The second reason is that U(V) complexes tend to disproportionate to more stable U(VI) and U(IV) complexes unless they are kinetically stabilized with sterically demanding substituents. There are only three reports of U(V)]C carbene complexes, all employing pincer-type dianionic carbene ligands BIPM. Two of the three U(V)]C carbene complexes are covered in this section, while the other one, which is a U(V)]C carbene dinitrogen complex, will be covered in the following section of P(III)-Stabilized Actinide Carbene Complexes (Section 3.09.3.3).116 In 2011, the Liddle group reported a one-electron oxidation, employing 0.5 equivalent of iodine (I2) as oxidant, from a U(IV)] C complex 72 to a U(V)]C complex [U(V)(BIPMTMS)(I)2Cl] (99) (Scheme 42).100 The U centers in 72 and 99 have similar coordination geometries and compositions, which provides an ideal platform to compare the U(IV)]C and U(V)]C bonds directly. As expected, the U(V)]C bond is shorter than the U(IV)]C bond (2.268(10) vs. 2.310(4) A˚ ). Furthermore, theoretical calculations suggested that the U(V)]C bond has a larger metal contribution to both s- and the p-bonds (Scheme 42), i.e. the U(V)]C bond is more covalent than the U(IV)]C bond. In terms of reactivity, 99 undergoes a Wittig-type carbene transfer with one equivalent of 9-anthracene aldehyde. The authors attempted to oxidize U(V)]C complex 99 to a U(VI)]C complex using 0.5 equivalents of I2. However, instead of U(VI) complexes, a U(IV) complex [U(IV){(Ph2P]N(SiMe3))2C(I)}(Cl)2I] (100) was isolated as the only U-containing product in low yield. The N-substituents of the BIPM pincer carbene ligands have a substantial influence on the outcome in the pursuit of U(V)]C complexes. Compared to the BIPMTMS, the oxidations of BIPMMes/Dipp U(IV)]C complexes are much less straightforward.117

Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

335

Scheme 41 Reactivity of a U(IV) pincer carbene complex (52) toward organic carbonyl compounds.109

Oxidation of a BIPMMes U(IV)]C complex [U(IV)(BIPMMes)(Cl)2(THF)2] (102) using Me3NO led to the formation of a uranyl methanide complex [U(VI)(O)2(HBIPMMes)(Cl)(THF)] (103) (Scheme 43), which is probably a result of the insufficient steric protection from the –Mes groups. Indeed, the bulkier N-Dipp groups enabled isolation of a U(V)]C BIPMDipp complex [U(V)(BIPMDipp)(Cl)2(m-Cl)2Li(THF)2] (107) (Scheme 44),117 in which the U(V)]C bond is significantly shorter than the U(IV)]C bond in the precursor [U(IV) (BIPMDipp)(m-Cl)4(Li)2(TMEDA)] (104). As with the formation of 103, the reaction of 104 with N-oxides lead to the formation of the uranyl methanide complexes [U(VI)(O)2(HBIPMDipp)(m-Cl)]2 (105) and [U(VI)(O)2(HBIPMDipp)(m-Cl)2Li(TMEDA)] (106), instead of the desired U(V/VI) carbene complexes (Scheme 44) (Table 5).

3.09.3.2.4

U(VI) pincer carbene complexes

According to the Hard and Soft (Lewis) Acids and Bases (HSAB) theory, the hard Lewis acidic U(VI) center is not a good match with soft carbon Lewis bases, even dianionic ones; since the electron-rich and soft C1−/C2− anions may easily reduce these high oxidation state metal cations. The U(VI)-carbon linkage is often so fragile that U(VI) alkyl complexes must generally be kept at low temperature to avoid decomposition.118 However, thanks to the unique electronic and steric protective framework of pincer-type carbene ligands such as SCS and BIPM, there are a handful of U(VI)]C complexes reported since 2011, which are covered in this section. In 2011, Ephritikhine, Mézailles, Thuéry, Cantat, Berthet and co-workers employed the SCS ligand to synthesize the first U(VI)] C complex [U(VI)(SCS)(O)2(pyr)2] (pyr ¼ pyridine) (108) from a reaction between uranyl triflate, CH2(Ph2PS)2 and LiNEt2 (Scheme 45).119 In complex 108, the three strongly donating dianionic ligands (two O2−, one C2−) form a T-shape geometry

336

Table 4

Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

Key structural parameters of U(IV)]C and Th(IV)]C pincer carbene complexes.

Complex

˚) An(IV)]C Bond length (A

Calculated bond order

References

[U(IV)(m-SCS)3][U(IV)(BH4)3]2 (49) [U(IV)(SCS)(BH4)2(THF)2] (50) [U(IV)(m-SCS)3][Li(Et2O)]2 (51)

2.444(4), 2.430(4), 2.451(4) 2.327(3) 2.484(3), 2.440(4) 2.344(13) 2.399(7), 2.390(8) 2.336(4) 2.374(7) 2.359(6), 2.379(6) 2.351(8) 2.396(4) 2.395(5) 2.485(7), 2.298(7) 2.527(4), 2.552(4), 2.549(4) 2.427(8), 2.448(9) 2.351(2) 2.436(4) 2.376(3) 2.469(3) 2.337(7) 2.410(6), 2.421(6) 2.514(3), 2.516(3) 2.527(10) 2.579(3) 2.491(6) 2.500(6) 2.564(6) 2.558(3) 2.414(3) 2.508(5) 2.410(8) 2.474(8) 2.453(4) 2.463(5)

– – –

71 71 71

– – 0.86 (Wiberg) – – – – –

85 84 85 85 85 85 85 85 86 86 87 88 88 88 88 88 99 99 104 104 105 105 106 106 107 107 108 108 108 108

[U(IV)(SCS)(Cl)(THF)(m-Cl)2{Li(THF)2}] (52) [U(IV)(SCS)2(Pyr)2] (530 ) [U(IV)(SCS)(Cp)2] (55) [U(IV)(SCS){CS(Ph2PS)2}(Pyr)] (56) [Tl{U(IV)(Cp)(SCS)}2(m-Cl)3] (57) [U(IV)(COD)(SCS)(THF)] (58) [U(IV)(SCS)(Cp )2] (59) [U(IV)(SCS)(HSCS)(NEt2)] (61) [Th(IV)(SCS)2(DME)] (63) [{Th(IV)(SCS)3}Li2(DME)]n (64) [U(IV)(BIPMMes)2] (65) [U(IV)(BIPMTMS)(Cp)2] (66) [Th(IV)(BIPMTMS)(Cp)2] (67) [U(IV)(BIPMTMS)(Tp)] (68) [Th(IV)(BIPMTMS)(Tp)] (69) [{(Me3SiN]PPh2)2C}ClU(m-Cl)2UCl{NC(Me)C(Ph2P]NSiMe3)2}] (71) [U(IV)(BIPMTMS)2] (74-U) [Th(IV)(BIPMTMS)2] (74-Th) [U(IV)(BIPMTMS)(NCPh3)(NHCPh3)Li] (77) [U(IV)(BIPMTMS)(NCPh3)(NHCPh3)K] (78) [U(IV)(BIPMTMS)(NCPh3)(k2-N,N0 -BIPY)] (79) [U(IV)(BIPMTMS)(NCPh3)(DMAP)2] (80) [Th(IV)(BIPMTMS)(NHCPh3)(K)] (83) [Th(IV)(BIPMTMS)(NCPh3)(k2-N,N0 -BIPY)] (85) [U(IV)(BIPMTMS)(ODipp)2] (86) [Th(IV)(BIPMTMS)(ODipp)2] (87) [Th(IV)(BIPMTMS)(N(SiMe3)2)(m-Cl)]2 (89) [Th(IV)(BIPMTMS)(N(SiMe3)2)(NCPh2)] (90) [Th(IV)(BIPMTMS){N(SiMe3)2}{OC(H)(NCPh2)(C14H9)}] (91) [Th(IV)(BIPMTMS){N(SiMe3)2}{OC(NtBu)NCPh2}] (92)

– 1.25 (Nalewajski-Mrozek) 0.6619 (Wiberg) 0.4582 (Wiberg) – – – 1.30 (Nalewajski-Mrozek) 0.73 (Nalewajski-Mrozek) 1.04 (Nalewajski-Mrozek) 0.99 (Nalewajski-Mrozek) 1.20 (Nalewajski-Mrozek) 1.17 (Nalewajski-Mrozek) – – – – – – – –

Scheme 42 Synthesis of a U(V)]C carbene complex 99 via one-electron oxidation from a U(IV)]C complex 72, and the reactivity studies of the U(V)]C complex 99.100

Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

337

Scheme 43 The synthesis of a U(IV)]C carbene complex 102 with the BIPMMes ligand, and an attempt to oxidize 102 into a higher oxidation state.117

Scheme 44 The oxidations of a U(IV)]C carbene complex 104 to form a U(V)]C carbene complex 107 and uranyl complexes 105 and 106.117

Table 5

Key structural parameters of U(V)]C pincer carbene complexes.

Complex

U(V)]C bond ˚) length (A

Calculated bond order

References

[U(V)(BIPMTMS)(I)(Cl)2] (99)

2.268(10)

100

[U(V)(BIPMDipp)(Cl)2(m-Cl)2(Li) (THF)2] (107)

2.267(9)

[U(V)(BIPMTMS)(NAd)2(m-N2) (Li(2,2,2-cryptand))] 120

2.461(7)

1.54 (NalewajskiMrozek) 1.49 (NalewajskiMrozek) –

Note

117

116

Synthesis and structural details are described in a subsequent section in this article ‘P(III)-Stabilized Actinide Carbene Complexes’

338

Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

Scheme 45 Synthesis of a U(VI)]C carbene uranyl complex 108 from a reaction between uranyl triflate, CH2(Ph2PS)2 and LiNEt2.119

about the U(VI) center. The U(VI)]C bond length (2.430(6) A˚ ) is longer than most of the U(V)]C and U(IV)]C bonds in the previous sections, which is counterintuitive as the ionic radius of U(VI) is the smallest. The long U(VI)]C bond is likely a result of the presence of the two strongly electron-donating O2− ligands and that the uranyl dication component can be viewed as a discrete unit with secondary carbene ligation that is more electrostatic since it resides in the equatorial plane of uranyl. In accord with the long U(VI)]C bond, DFT calculations suggested that the U(VI)]C bond in 108 has a less pronounced multiple bond character, such as a relatively low Wiberg bond order of 0.91. However, the double bond character was shown by DFT calculations, as the uranium-carbon s-bond (HOMO-3) and p-bond (HOMO). In 2014, Liddle and co-workers reported a series of BIPMTMS U(VI)]C carbene complexes with several multiple bonding units [C]U(VI)(]O)(]E)] (E ¼ O, NR) (Scheme 46).120 Two U(VI) carbene oxo imido complexes, [U(VI)(BIPMTMS)(NMes) (m-O)]2 (110) and [U(VI)(BIPMTMS)(NMes)(O)(DMAP)2] (111), as well as one uranyl carbene complex [U(VI)(BIPMTMS) (O)2(DMAP)2] (112), were synthesized and characterized. Complex 111 is the first, and so far, the only complex with three formal terminal multiply bonded ligands (R2C2−, RN2−, O2−) on one metal center, where the coordinated heteroatoms derive from different element groups. Similarly to 108, the U(VI)]C bonds in 110–112 are longer than one may expect based on their U(IV)]C precursor 109 (Scheme 47). For instance, the U(VI)]C bond length in 111 (2.400(3) A˚ ) is virtually identical to the U(IV)]C bond length in 109 (2.396(10) A˚ ), considering the smaller ionic radius of U(VI) versus U(IV).89,90 The DFT calculations suggested a Nalewajski-Mrozek bond index of 1.23 for the U(VI)]C in 111, compared to 2.34 and 2.48 for the U(VI)]N and U(VI)]O bonds in the same molecule, respectively.

Scheme 46 Synthesis of U(VI) carbene-imido-oxo complexes 110 and 111 from 2-electron oxidation of a U(IV) carbene-imido complex 109; and a Wittig-type carbene transfer reaction of 111, to produce a U(VI) carbene uranyl complex 112.120

Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

339

Scheme 47 A comparison between two U(VI) carbene uranyl complexes 108119 and 112.120

A comparison between the two U(VI)]C carbene uranyl complexes 108 and 112 is intriguing (Scheme 47). The two complexes have very similar coordination geometries. In this context, the differences in the U(VI)]C bond lengths (2.430(6) A˚ of 108 vs. 2.383(3) A˚ of 112) and the O]U(VI)]O angles (171.8(2)o of 108 vs. 167.15(10)o of 112) are not negligible. The linear uranyl [O]U(VI)]O]2+ unit is thermodynamically robust, i.e. any small deviation of the O]U(VI)]O angle from 180o indicates non-trivial electronic perturbation. In 112, the P]NSiMe3 groups are better electron-donors than the P]S groups in 108, which may reasonably contribute to the more bent [O]U(VI)]O]2+ unit in 112, and resultantly, a shorter U(VI)]C bond. The O] U(VI)]O angle in 112 (167.15(10) ) is among the most bent of such [O]U(VI)]O]2+ fragments in the literature.121 From this perspective, 108 and 112 may indicate a viable avenue to tackle one of the long-standing challenges in actinide science: the synthesis of a cis-uranyl complex.122,123 In a discussion about complexes 110–112, the Liddle group attributed the relatively long U(VI)]C bond lengths to the multi-multiple bonding environments and resultant accumulated electron density surrounding the U(VI) center.120 Follow this line, it is reasonable to expect a shorter U(VI)]C bond by removing one or more of the strongly donating O2− or E2− ligands. Indeed, this was observed in a U(VI) carbene oxo dichloride complex [U(VI)(BIPMTMS)(Cl)2(O)] (113) (Scheme 48),124 where the U(VI)]C bond is ca. 0.2 A˚ shorter than those in the [C]U(VI)(O)(]E)] complexes 108/110–112 (2.183(3) A˚ in 113 vs. 2.38–2.43 A˚ in 108/110–112), along with a substantial increase of the U-contribution in both the s- and the p-bonds of the U(VI)]C linkage. However, the U(VI)]C bond in a hemilabile U(VI) carbene complex [U(VI)(2-N,C-BIPMDipp)(OtBu)3(I)] (114) (Scheme 48), where only one U]E multiple bond is present, is long (2.449(7) A˚ ).117 Though the chelating effect is less in the hemilabile complex 114 than the pincer complex 113, the significant difference in U(VI)]C bond length between 113 and 114 can be partially attributed to the inverse-trans-influence (ITI), which operates in 113 but not 114, i.e. the trans-U(VI)]O and U(VI)]C bonds in 113 enhance each other (Table 6).

Scheme 48 Synthesis of a U(VI) pincer carbene oxo dichloride complex 113,124 and a U(VI) hemi-pincer carbene complex 94.117

340

Table 6

Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

Key structural parameters of U(VI)]C pincer carbene complexes.

Complex

˚) U(VI)]C bond length (A

Calculated bond order

References

[U(VI)(SCS)(O)2] (108) [U(VI)(BIPMTMS)(NMes)(m-O)]2 (110) [U(VI)(BIPMTMS)(NMes)(O)(DMAP)2] (111) [U(VI)(BIPMTMS)(O)2(DMAP)2] (112) [U(VI)(BIPMTMS)(O)(Cl)2] (113) [U(VI)(2-N,C-BIPMDipp)(OtBu)3(I)] (114)

2.430(6) 2.408(3) 2.400(3) 2.383(3) 2.183(3) 2.449(7)

0.91 (Weiberg) – 1.23 (Nalewajski-Mrozek) – 1.50 (Nalewajski-Mrozek) 1.50 (Nalewajski-Mrozek)

119 120 120 120 124 117

3.09.3.3

P(III)-stabilized actinide carbene complexes: Synthesis, structure and reactivity

In the previous section, the two P(V)]E (E ¼ NR, S) groups in the BIPM and SCS pincer carbene ligands stabilize the An]C bond via inductive, mesomeric and chelating effects. Conversely, the P(V)]E groups perturb the An]C bond and diminish the double bond character. For pursuing better-developed An]C double bond character, novel C2− carbene ligands with less perturbing substituents are desirable. The electron-withdrawing P(V)]E substituents feature a double bond and facilitate the delocalized-type structure (Scheme 49A). On the other hand, -P(III)R2 (where R is aryl, alkyl or silyl) phosphino substituents are less likely to form delocalized structures; thus, these groups will promote the formation of a better developed An]C bond (Scheme 49B). (A)

(B)

Scheme 49 The likelihood to form delocalized electronic structures of (A) P(V)-stabilized; and (B) P(III)-stabilized carbene complexes.

The diphenylphosphino trimethylsilyl carbene ligand (Ph2PCSiMe3)2− had been used for making transition metal125 and scandium126 carbene complexes. In the scandium(III) carbene complex of this ligand, a ScdCdP 3-center unit was reported, with a weak P-–Sc interaction and a strong Sc]C bond.126 In 2018, Liddle and co-workers introduced the (Ph2PCSiMe3)2− ligand into actinide chemistry, combined with the BIPMTMS ligand, by the synthesis of three U(IV) bis-carbene complexes [U{C(SiMe3) (PPh2)}-(BIPMTMS)(m-Cl)Li(TMEDA)(m-TMEDA)0.5]2 (116), [U{C(SiMe3)(PPh2)}(BIPMTMS)(Cl)][Li(2,2,2-cryptand)] (117) and [U{C(SiMe3)(PPh2)}(BIPMTMS)(DMAP)2] (118) (Scheme 50).127 The synthesis of these complexes started from a U(IV) carbene chloride complex 72, through two steps of salt elimination and one step of intramolecular alkane elimination, to construct complex 116, which served as an entry point of this U(IV) heteroleptic bis-carbene chemistry. Complex 116 is a LiCl ate U(IV) bis-carbene complex, which has LiCl occluded in the secondary coordination sphere of the U(IV) center. To remove the LiCl, complex 116 was treated with one equivalent of 2,2,2-cryptand or two equivalents of DMAP, to produce a Li+ encapsulated separated ion pair complex 117, and a LiCl-free DMAP-adduct complex 118, respectively. In the heteroleptic bis-carbene complexes 116, 117 and 118, there are two types of U(IV)]C double bonds: one is a pincer carbene center supported by two P(V)]N(SiMe3) groups (U]CP(V)), the other is a non-pincer carbene center supported by one P(III)Ph2 group and one trimethylsilyl group (U]CP(III)). Comparison of the bond lengths and calculated bond indices between the two types of U(IV)]C bonds provided insights into the influence of P(V)/P(III) substituents on the U(IV)]C bond. In all the cases of 116–118, the U(IV)]CP(III) bonds (2.265(2)–2.296(5) A˚ ) are substantially shorter than the U(IV)]CP(V) bonds (2.405(9)–2.459(2) A˚ ). Correspondingly, the calculated Nalewajski-Mrozek bond indices of the U(IV)]CP(III) bonds (1.71–1.78) are higher than those of the U(IV)]CP(V) bonds (1.13–1.25) (Table 7). The uranium-P(III) distances in 116–118 range from 2.774(3) A˚ (116) to 2.8371(13) A˚ (118), which are at the limit of the covalent single bond radii of uranium and phosphorus (2.81 A˚ ).128 Combining these data with the acute U]CdP angle (ca. 88o), the relatively short P(III)dCcarbene bond

Actinide Metal Carbene Complexes: Synthesis, Structure and Reactivity

341

Scheme 50 Synthesis of U(IV) bis-carbene complexes 116–118 and the reactivity of 116 with PhCCPh.127

Table 7

Key structural parameters of U(IV)]CP(III) non-pincer carbene complexes.

Complex

˚) U(IV)]CP(III) bond length (A

Calculated bond order

References

[U(IV){C(SiMe3)(PPh2)}(BIPMTMS)(m-Cl)Li(TMEDA)(m-TMEDA)0.5]2 (116) [U(IV){C(SiMe3)(PPh2)}(BIPMTMS)(Cl)][Li(2,2,2-cryptand)] (117) [U(IV){C(SiMe3)(PPh2)}(BIPMTMS)(DMAP)2] (118)

2.270(10) 2.265(2) 2.296(5)

1.78 (Nalewajski-Mrozek) 1.71 (Nalewajski-Mrozek) 1.78 (Nalewajski-Mrozek)

127 127 127

(ca. 1.74 A˚ , compared to the typical PdC single bond length 1.85 A˚ 129), and the geometry and the orientation of the lone electron pair on the P atom, it could be concluded that the U]CP(III) bond in 116–118 is largely undisturbed with a small fraction of delocalization and PdC negative hyperconjugation. These data demonstrated that by replacing the two P(V)]N(SiMe3) groups with one P(III)Ph2 and one SiMe3 substituents, the multiple bond character of the U(IV)]C linkage is significantly enhanced. This finding pointed out an avenue towards stronger actinide metal-carbon multiple bonding species. Given complexes 116–118 feature two distinctively different types of U]C bonds in one molecule, it is intriguing to study the reactivity profile of these bis-carbene complexes. Carbene transfer reactions with aldehydes took place at both U]CP(V) and U] CP(III), while the C^C bond of PhCCPh selectively inserted between the weakly interacted U(IV) and P(III) atoms of 116, affording a zwitterionic bis-carbene complex [U{C(SiMe3)(Ph2PCPhCPh)}(BIPMTMS)] (119) (Scheme 50).127 The C^C bond insertion is typical for the Frustrated Lewis Pairs (FLPs), which is prevalent in main-group and transition metal chemistry130 and emerging in rare-earth metal chemistry,131 but was previously unknown in actinide chemistry. In complex 119, two U(IV)]C bond lengths are nearly the same compared to its parent complex 116. In 2019, from a reaction between complex 117 and one equivalent of adamantyl azide (AdN3), the Liddle group isolated a highly unusual U(V) end-on dinitrogen (N2) complex [U(V)(BIPMTMS)(NAd)2(m-N2)(Li(2,2,2-cryptand))] (120) (Scheme 51) in a

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Scheme 51 Reaction between a U(IV) bis-carbene complex 117 and adamantyl azide (AdN3) to produce a U(V) carbene bis-imido complex 120 with a U(V)— N2 interaction.116

moderate but reproducible yield (28% by uranium content).116 Apparently, the formation of 120 involves oxidation of uranium center from U(IV) to U(V) and ligand scrambling, but the details of this mechanism are unclear. The +5 oxidation state of the uranium center in complex 120 was confirmed by SQUID magnetometry and multiple spectroscopic methods, as well as DFT calculations. The salient structural feature of 120 is the U(V)—N2 interaction, which was characterized by X-ray crystallography, as well as IR and Raman spectra. The long U(V)—N2 distance (2.605(8) A˚ ) and short NdN distance (1.139(9) A˚ , cf. 1.0975 A˚ in free-N2132), combined with a broad n(N2) absorption centered at ca. 1940 cm−1, indicate a relatively weak interaction between the U(V) center and N2 in an end-on manner. The U(V)]C bond length in complex 120 (2.461(7) A˚ ) is much longer compared to the U(V)]C bonds in the other two U(V) carbene complexes (2.268(10) A˚ in 99,100 2.267(9) A˚ in 107117). The long U(V)]C bond length in 120 is a result of the presence of two strongly donating AdN2− imido ligands, which lead to an accumulation of electron density on the U(V) center and weaken the U(V)]C bond. Natural Bonding Orbital (NBO) analysis suggested that the U(V)]C bond in 120 is highly polarized (10% U, 90% C), corroborates the idea that the [U(V)(NAd)2]+ is the structural dictating fragment. DFT calculations reveal that the electron-deficient U(V) center of 120 donates its f1 electron to the p -orbital of a weak p-acid N2, which challenges the traditional concept of p-backdonation from an electron rich metal center in conventional d-block metal N2 activation chemistry.132–136 For uranium, as early as 1909, Haber noted that uranium was an efficient catalyst for the Haber-Bosch Process converting N2 to NH3.137 More recently, well-defined uranium-N2 molecular complexes and uranium mediated N2 activation were known for decades138–141 and received increasing interest in recent years.142–145 Nonetheless, all of these examples involved an electron rich low valent (usually +3) uranium center, which is still within the traditional metal-mediated N2 activation concept. The unusual N2 activation from a formally electron-deficient U(V) center is attributed to its electron-donating ancillary ligands, including one dianionic carbene and two dianionic imides. These strong electron-donating ligands ‘push’ the electron density from the U(V) 5f1 to the p orbital of N2. Other than organic substrates, 116 also reacts with 0.5 equivalent of [Rh(COD)(m-Cl)]2, to produce a U(IV)-Rh(I)-Rh(I) heterotrimetallic complex [U(IV)(Cl)2{C(PPh2NSiMe3)(PPh[C6H4]NSiMe3)}{Rh(COD)}{Rh(CH(SiMe3)(PPh2))}] (121), which features a very short Rh(I)dU(IV) bond (2.5835(3) A˚ ), compared to the sum of the single bond covalent radii of Rh and U (2.95 A˚ )128 (Scheme 52).146 The Rh(I)dU(IV) bond in 121 is the shortest bond reported to date between U and Rh, and the shortest intermetallic actinide-transition metal bond by the definition of formal shortness ratio (FSR).147

Scheme 52 The synthesis of a RhdU double-dative bond complex 121 from a reaction between a U(IV) heteroleptic bis-carbene complex 116 and 0.5 equivalent of [Rh(COD)(m-Cl)]2.146

The oxidation states of the metal centers in 121 were established by both experimental methods (SQUID magnetometry, UV/Vis/ IR and Raman spectra) and theoretical calculations (spin and charge populations). The short Rh(I)dU(IV) bond in 121 was

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calculated to have a Nalewajski-Mrozek bond index of 1.44, which is typical for a double bond. The nature of the Rh(I)]U(IV) bond was revealed by molecular orbital calculations, which suggest that the 4d8 Rh(I) center donates two pairs of electrons to U(IV), forming a double-dative intermetallic bond. The intermetallic double-dative bond had been postulated as a novel type of metal-metal bond shortly prior to the synthesis of complex 121,148 which is the first structurally characterized complex featuring such an intermetallic double-dative bond.

3.09.3.4

U]C bonds in endohedral metallofullerenes (EMFs)

Fullerenes provide a unique protective environment in their internal cavity, which is ideal for encapsulating and trapping highly reactive and elusive species. If the encapsulated species contains a metal, the complexes are named as endohedral metallofullerenes (EMFs). The synthesis of EMFs requires harsh conditions, such as Krätschmer-Huffman high-voltage arc discharging method, using a metal rod and graphene as starting materials.149 Though with some inherent shortfalls, such as low-yields, difficult purification (often employing HPLC/tandem HPLCs), unpredictable product structures, EMF research has developed tremendously in the past decade and provides a parallel line of enquiry (along with inert gas matrix studies) for isolating and investigating highly unstable and elusive chemical bonds. Compared to inert gas matrix technologies, the most important advantage of the EMF strategy is that it allows crystallographic characterization (both SCXRD and powder XRD) to measure bond lengths and angles directly. In actinide science, the EMF strategy has allowed the isolation of unique actinide species such as isolated metal atoms/ions and metal nitrides.150 In 2018, a C80 fullerene encapsulated diuranium carbide cluster (122) was reported.45 The core structure of 122 can be described as a U]C]U unit stabilized by strong U-arene interactions, where the arenes are subunits of the C80 fullerene (Scheme 53). The oxidation states of the two U centers in this complex are both +5. The two U(V)]C bonds (2.033(5) and 2.028 (5) A˚ ) are nearly identical and are much shorter than the U(V)]C bonds in U(V) pincer carbene complexes (2.26–2.46 A˚ ) discussed previously (Table 5). This is a result of both the constraint environment in the EMF and the removal of the P(E) R2 groups in the pincer carbene complexes. The Meyer Bond Order of the U(V)]C fragment in 121 was calculated to be 1.4. The U(V)]C]U(V) unit counterintuitively deviates from linearity, which was illustrated by the electron localization function (ELF) calculation, and was interpreted as the buildup of negative charge on the central C, giving rise to an sp/sp2-hybridized local environment at the C atom.

Scheme 53 The core structure of the EMF complex U2C@C80 (121).45

3.09.4

Conclusion and Outlook

Actinide metal-carbon multiple bond chemistry, specifically, actinide metal carbene chemistry, has developed significantly since the 1980s. Since 2010, with the introduction of U/Th pincer nucleophilic carbenes, there has been an upsurge in activity in this research area. Several milestones have been achieved to date, such as: 1. U(IV) ylide-type carbenes featuring a U(IV)]C bond (since 1981); 2. U(III)-U(VI) and Th(IV) pincer-type P(V)-stabilized carbenes featuring an An]C bond (An ¼ U(III), U(IV), U(V), U(VI), Th(IV)) (since 2009); 3. U(IV) non-pincer-type P(III)-stabilized carbenes featuring a better-developed (compared to the pincer carbenes) U(IV)]C bond (2018). However, there are still some knowledge gaps to fill in this area, such as: 1. Bona fide, non-heteroatom-stabilized An]CR2 (R ¼ H, alkyl, aryl, or silyl) alkylidene complexes have not yet been isolated; 2. Given their strategic and scientific importance, transuranic elements (Np, Pu, Am, etc.) have received an increasing research interest recently.151,152 However, their carbene complexes are unknown; 3. The reactivity scope of the reported actinide carbene complexes is still limited compared to their transition metal counterparts.

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Filling the knowledge gaps, particularly synthesizing the first An]CR2 alkylidene complexes and the first transuranic carbene complexes, will surely provide significant breakthroughs in organometallic chemistry, and create new horizons of chemical bonding and reactivity.

Acknowledgment EL thanks Newcastle University (UK) and the Newcastle University Academic Track (NUAcT) Fellowship Scheme for the generous financial support.

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123. Villiers, C.; Thuéry, P.; Ephritikhine, M. Angew. Chem. Int. Ed. 2008, 47, 5892–5893. 124. Mills, D. P.; Cooper, O. J.; Tuna, F.; McInnes, E. J. L.; Davies, E. S.; McMaster, J.; Moro, F.; Lewis, W.; Blake, A. J.; Liddle, S. T. J. Am. Chem. Soc. 2012, 134, 10047–10054. 125. Solowey, D. P.; Mane, M. V.; Kurogi, T.; Carroll, P. J.; Manor, B. C.; Baik, M.-H.; Mindiola, D. J. Nat. Chem. 2017, 9, 1126. 126. Mao, W.; Xiang, L.; Lamsfus, C. A.; Maron, L.; Leng, X.; Chen, Y. J. Am. Chem. Soc. 2017, 121, 1081. 127. Lu, E.; Boronski, J. T.; Gregson, M.; Wooles, A. J.; Liddle, S. T. Angew. Chem. Int. Ed. 2018, 57, 5506–5511. 128. Pyykkö, P. J. Phys. Chem. A 2015, 119, 2326. 129. Müller, P., Herbst-Irmer, R., Spek, A. L., Schneider, T. R., Sawaya, M. R., Eds.; In Crystal Structure Refinement; Oxford University Press, 2006. Section 3.09.12.5. 130. Jupp, A. R.; Stephan, D. W. Trend. Chem. 2019, 1, 35–48. 131. Dong, Y.; Chang, K.; Xu, X. Chin. J. Chem. 2020, 38, 559–564. 132. Fryzuk, M. D.; Johnson, S. A. Coord. Chem. Rev. 2000, 200, 379–409. 133. Nishibayashi, Y., Ed.; In Transition Metal-Dinitrogen Complexes: Preparation and Reactivity; Wiley-VCH, 2019. ISBN: 978-3-527-34425-3. 134. Gambarotta, S. J. Organomet. Chem. 1995, 500, 117–126. 135. Gambarotta, S.; Scott, J. Angew. Chem. Int. Ed. 2004, 43, 5298–5308. 136. Singh, D.; Buratto, W. R.; Tprres, J. F.; Murray, L. J. Chem. Rev. 2020, 120, 5517–5581. 137. Haber, F. Verfahren zur Herstellung von Ammoniak durch katalytische Vereinigung von Stickstoff und Wasserstoff, zweckmäßig unter hohem Druck. German patent DE 229126, 1909. 138. Roussel, P.; Scott, P. J. Am. Chem. Soc. 1998, 120, 1070–1071. 139. Cloke, F. G.; Hitchcock, P. B. J. Am. Chem. Soc. 2002, 124, 9352–9353. 140. Odom, A. L.; Arnold, P. L.; Cummins, C. C. J. Am. Chem. Soc. 1998, 120, 5836–5837. 141. Evans, W. J.; Kozimor, S. A.; Ziller, J. W. J. Am. Chem. Soc. 2003, 125, 14264–14265. 142. Falcone, M.; Chatelain, L.; Scopelliti, R.; Živkovic, I.; Mazzanti, M. Nature 2017, 547, 332–335. 143. Falcone, M.; Barluzzi, L.; Andrez, J.; Tirani, F. F.; Živkovic, I.; Fabrizio, A.; Corminboeuf, C.; Severin, K.; Mazzanti, M. Nat. Chem. 2019, 11, 154–160. 144. Barluzzi, L.; Falcone, M.; Mazzanti, M. Chem. Commun. 2019, 55, 13031–13047. 145. Xin, X.; Douair, I.; Zhao, Y.; Wang, S.; Maron, L.; Zhu, C. J. Am. Chem. Soc. 2020, 142, 15004–15011. 146. Lu, E.; Wooles, A. J.; Gregson, M. J.; Cobb, P. J.; Liddle, S. T. Angew. Chem. Int. Ed. 2018, 57, 6587–6591. 147. Cotton, F. A. In Multiple Bonds between Metal Atoms; Cotton, F. A., Murillo, C. A., Walton, R. A., Eds.; Springer Science: New York, 2005; pp 35–68. 148. Chi, C.; Wang, J.-Q.; Qu, H.; Li, W.-L.; Meng, L.; Luo, M.; Li, J.; Zhou, M. Angew. Chem. Int. Ed. 2017, 56, 6932. 149. Krätschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354–358. 150. Cai, W.; Chen, C.-H.; Chen, N.; Echegiyen, L. Acc. Chem. Res. 2019, 52, 1824–1833. 151. Kaltsoyannis, N. Chem. A Eur. J. 2018, 24, 2815–2825. 152. Magnani, N.; Caciuffo, R. Inorganics 2018, 6, 26.

3.10

Alkylidene Complexes of the Group 4 Transition Metals

Daniel J Mindiola, J Rolando Aguilar-Calderón∗, and Pavel Zatsepin∗, University of Pennsylvania, Philadelphia, PA, United States © 2022 Elsevier Ltd. All rights reserved.

3.10.1 Introduction 3.10.2 Titanium methylidenes 3.10.3 Titanium alkylidenes 3.10.4 Titanium bridging methylenes 3.10.5 Titanium bridging alkylenes 3.10.6 Titanium alkylidene clusters 3.10.7 Zirconium methylidenes 3.10.8 Terminal zirconium alkylidenes 3.10.9 Heteroatom substituted zirconium alkylidenes 3.10.10 Bridging zirconium methylenes 3.10.11 Bridging zirconium alkylenes 3.10.12 Zirconium alkylene clusters 3.10.13 Hafnium methylidenes 3.10.14 Hafnium alkylidenes 3.10.15 Hafnium bridging alkylenes 3.10.16 Hafnium alkylene clusters 3.10.17 Summary Acknowledgments References

3.10.1

347 348 350 363 364 366 368 369 369 372 373 376 377 377 377 378 379 379 379

Introduction

The unique properties of transition metals to mediate the functionalization of organic and inorganic carbon-containing molecules is the focal point of organometallic chemistry. Within this context, transition metals featuring metal-carbon double bonds such as group 4 alkylidenes, are a distinctive class of compounds implicated in a variety of fundamental transformations that include Wittig-type olefination, olefin polymerization and small molecule and CdH bond activation chemistries. Group 4 alkylidenes are commonly referred to as Schrock-type carbenes and thus are composed of a high-valent metal center bonded to one or more disubstituted carbon groups (R2C) (R ¼ hydrogen, alkyl) in a terminal fashion ([R2C]xM).1 The high electropositivity of group 4 metals drastically polarizes the alkylidene bond towards the a-carbon, providing a formal dianionic species (R2C2−), which consequently is nucleophilic; these charges are counterbalanced by the corresponding electrophilic metal cation (Mn+2). With respect to Molecular Orbital Theory, these characteristics arise from the combination of energetically high-lying and typically empty group 4 metal d-orbitals with low-lying and populated carbon orbitals which elicits an energy mismatch that renders the highest occupied molecular orbital (HOMO) with significant a-carbon character and the lowest unoccupied molecular orbitals (LUMO) more metal centric (Fig. 1, left). Nonetheless, it is also possible to envision the alkylidene bonding situation through a covalent perspective as shown in the right side of Fig. 1 (triplet carbene conceptualization). The Baik group has published an excellent review discussing the electronic intricacies of early-metal alkylidenes and late-metal carbenes as well.1 Periodic trends, such as increasing orbital size and energy anticipate heavier group 4 metal congeners to furnish alkylidenes with increased nucleophilicity. Furthermore, the choice of substitution patterns on the alkylidene fragment as well as the modularity of the ancillary ligands’ steric and electronic profiles provide limitless manifolds to tune stability and reactivity. For instance, substitution of the supporting ligand(s) with sterically encumbering moieties is an effective method which provides kinetic stabilization. This strategy has permitted the structural characterization of group 4 metal alkylidenes previously deemed too reactive for isolation. Such ample room for judicious structural modifications is also manifested in the proliferation of alkylidene functionalities substituted with groups other than hydrogens or alkyls. The structure and reactivity of these “unconventional alkylidenes” frequently diverge from classic Schrock-type carbenes into pathways that are also difficult to categorize as Fischer-type carbenes. Unarguably, these thought-provoking molecules are particularly relevant in alkylidene chemistry and warrant the reassessment of such stringent textbook standards which restrict classification to either Schrock- or Fischer-carbenes. Noteworthy, “bridging alkylidenes” of the type [(m2-CR2)xMy] are prevalent in the literature but according to IUPAC recommendations, these molecules are preferably denominated as alkylenes.2 Regardless, alkylenes have been included in this chapter given their unequivocal ties and complementary chemistry to alkylidenes. In any case, this book chapter attests that group IV metal alkylidenes (and alkylenes!) provide a unique platform for experimentalists and theoreticians to push the boundaries of fundamental and applied ∗

These authors contributed equally to the writing of this book chapter.

1

IUPAC does not recommend to call these bridging compounds “alkylidenes.” M¼CH2 is a methylidene, M-CH2-M is a methylene. Expanding to generic names alkylidenes are terminal and the bridged compounds are alkylenes.

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00026-3

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348

Alkylidene Complexes of the Group 4 Transition Metals

Fig. 1 Different molecular orbital representations of early metal alkylidene complexes.

organometallic chemistry. We have placed a marked focus on compounds that have outstanding structural elements, those of greater ligand diversity, and on more recent literature in this area that was published after the antecedent reviews.

3.10.2

Titanium methylidenes

Titanium methylidenes began their history with the notable complex m-chloro[di(cyclopenta-2,4-dien-1-yl)]dimethyl(m-methylene)titaniumaluminum [Cp2TiCH2ClAl(CH3)2] 1, commonly known as Tebbe’s reagent.3 While this molecule contains a methylene ligand bridged between titanium and aluminium centers, and thus, is not itself an alkylidene as such, early reactivity studies showed that Tebbe’s reagent behaves as a masked methylidene source since the chlorodimethylaluminium unit can be readily eliminated to produce the fleeting fragment [Cp2TiCH2].4 Later attempts by the groups of Schwartz,5 Grubbs,6 and Petasis7 to isolate this transient species [Cp2TiCH2] consisted of displacing the alane group with Lewis bases such as alkyl phosphines and 4-(dimethylamino)pyridine (DMAP) (Eq. 1). The complexes [Cp2TiCH2(PR3)] (PR3 ¼ PMe3, PEt3, PMe2Ph) are amaneble to NMR characterization but only at −40  C. At higher temperatures decomposition typically occurs resulting in bimolecular transformations.8 Such in situ approaches to assess the syntheses and reactivities of bis(cyclopentadienyl)titanium-type methylidenes dominated the literature for the greater part of the late twentieth century, and have been extensively covered elsewhere.9,10

ð1Þ

Additionally, various organic transformations in which in situ generation of titanium alkylidenes occur have been performed with the use of protocols established between the late 1970s and early 2000s. These include use of the Tebbe, Grubbs, Petasis, Takai-Lombardo, and Takeda reagents/reactions (Fig. 2). Their use has been painstakingly documented in a series of annual reviews by Herndon.11–21 Besides low-temperature conditions, the transience imposed by the reactivity of the [TiCH2] fragment can be mitigated by its coordination to a sterically demanding ligand framework as shown by the Mindiola group. With this guideline, the ligand combination bis(2-(diisopropylphosphananeyl)-4-methylphenyl)amide) (PNP−) and the bulky aryloxide 2,4,6-tritert-butylphenoxide (Mes O−) is a suitable platform to access thermally stable titanium-methylidenes.22 Specifically, [(PNP)Ti(CH3)2] 2 can be oxidized with the phenoxyl radical Mes O
23–25 to form the bisalkyl complex [(PNP)Ti(OMes ) (CH3)2] 3. The thermolysis of 3 induces the extrusion of a molecule of methane and concomitant formation of the corresponding titanium methylidene [(PNP)Ti]CH2(OMes )] 4 in high yields (Scheme 1). In the 1H NMR spectrum, the methylidene protons are particularly deshielded, resonating at 14.20 ppm. Isotopic labelling experiments unambiguously corroborate this assignment; in the isotopologue [(PNP)Ti]C13H2(OMes )] 4-13C the methylidene protons’ peak splits into a doublet (1JCH ¼ 125 Hz). Similarly, 4-13C shows a notably deshielded 13C{1H} resonance at 290.30 ppm, reminiscent of a methylidenic carbon. This peak is resolved into a doublet due to coupling to the phosphorus nuclei in the PNP ligand (2JCP ¼ 11 Hz). Interestingly, the OMes ligand imparts photostability to (PNP)Ti]C13H2(OMes ) since the analogous titanium triflate complex (PNP)Ti]CH2(OTf ) decomposes to methane, ethylene, and, other intractable products upon exposure to light.

Alkylidene Complexes of the Group 4 Transition Metals

349

Fig. 2 Various alkylenation reagents that produce titanium carbene intermediates.

Scheme 1 Synthetic route to 4.

In the same vein, oxidation of [(PN)2Ti(CH3)] 5 (PN− ¼ (N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-trimethylanilide)) with Mes O or treatment of [(PN)2Ti(CH3)(OTf )] 6 with H2CPPh3 produced the first structurally characterized titanium methylidene [(PN)2Ti]CH2] 7 (Scheme 2).26 The geometry of 7 is best described as a hybrid of square-pyramidal and trigonal bipyramidal with a t5 of 0.517, in accord with the isostructural complexes [(PN)2Ti^O] and [K(222-kryptofix)][(PN)Ti^N].27 Bond metrics are consistent with a titanium-carbon double bond (1.939(3) A˚ ) and despite the methylidene and some of PN ligand protons displaying close contacts, no agostic interactions were identified in the solid-state. The electronic structure of the [TiCH2] fragment was investigated with the aid of computational methods which show a Wiberg bond order of 1.70 and a NBO with negative partial charge of −0.74 on the on the a-carbon. A qualitative MO-diagram was constructed by mixing the [(PN)2Ti(IV)]2+ dication and methandiide [CH2]2− fragments. In this depiction, two principal bonding interactions exist: a TidC s-bond derived from the combination of the carbene sp-hybridized orbital with a metal d(y2–z2), and a dative bond between a carbon p-orbital with a d(yz) orbital along the p-plane. In agreement with Ti(IV)-d0 ion, this combination of atomic orbitals leaves the p-like orbitals empty.

Scheme 2 Two different routes to the synthesis of 7.

350

Alkylidene Complexes of the Group 4 Transition Metals

The Andrews group has employed IR spectroscopy to investigate the reactivity of laser-ablated titanium atoms with methane,28 methyl fluoride,29,30 methyl chloride,31 methyl bromide,31 difluoromethane,32 and dichloromethane to produce a variety of titanium-methylidene adducts in condensed argon matrices. The irradiation of a matrix composed of Ti/CH4/Ar with 500 nm light led to the generation of [HTidCH3] 8-H, and further photoexcitation at 290–380 nm produced methylidene [H2Ti]CH2] 9-H (Scheme 3). Similarly, it was observed that the reaction of Ti with CH3X (X ¼ F, Cl, Br) proceeded with formation of [XTidCH3] 8-X (Scheme 3), which upon irradiation with UV light of wavelengths between 240 and 380 nm would re-arrange into methylidenes [(X)(H)Ti]CH2] 9-X (Scheme 3). Interestingly, these methylidenes could revert back to their parent methyl compounds under visible light l > 420 in the case of 9-H, and nm l > 530 nm in the case of 9-X. The same conditions used to generate [(X)(H)Ti] CH2] type compounds could be applied to reactions with CH2F2 and CH2Cl2 to obtain the respective molecules [F2Ti]CH2] 10-F and [Cl2Ti]CH2] 10-Cl. For the methylidene molecules that posses a TidH bond, computed structures proposed the presence of a-agostic interactions. Isotope labelling studies with methane-d2 provided experimental evidence of structurally distorted molecules as different absorbances could be detected for four distinct isomers (Fig. 3) of [(H)(D)Ti]CHD] 9-d2.

Scheme 3 Variety of laser-generated titanium methylidenes.

Fig. 3 The four isomers of (H)(D)Ti]CHD.

Sunderlin and Armentrout studied the interactions between gaseous Ti+ ions and methane using a hot rhenium filament (ca. 2500 K) in tandem with an electron impact (EI) mass spectrometry experiment.33 They found that methane oxidative addition occurs generating the [(H)(CH3)Ti]+ 11+ ion which subsequently eliminates H2 producing the methylidene [(H2C)Ti]+ 12+. Their analyses suggested that a doublet, excited electronic state of Ti+, 2F with configuration 4s13d2, was mainly involved in the observed reactivity as opposed to the quartet, ground electronic state, 4F with configuration 4s13d2 (Table 1).

3.10.3

Titanium alkylidenes

While searching for routes to the synthesis of titanacyclobutabenzenes, the Bickelhaupt and Erker groups were able to obtain, and characterize by single-crystal XRD, early examples of phosphoniomethylidenes of titanium.34 These were obtained by heating mixtures of bis(phenyl)titanocene 13-Ph or bis(p-tolyl)titanocene 13-tol with H2CPPh3. It was proposed that elimination of an arene fragment produced the corresponding titanocene-aryne intermediate (a) which has an open coordination site to bind H2CPPh3 (b). At this stage, two competing reactions may occur: elimination of Ph3P followed by formal [2 + 2] cycloaddition between the resulting titanium-methylidene and the coordinated aryne generating titanacyclobutabenzenes 14-R as the minor products (c), and a-hydrogen migration from the bound ylide to the aryne moiety to produce aryl(triphenylphosphonio)methylidene titanocene species 15-R as the major product (d) (Scheme 4). In 15-R, the TidCa bond distance of 2.033(6) A˚ is much shorter than what is found in titanium alkyls, and markedly shorter than the TidC bond distance found in the only crystallographically characterized ylide of complex of titanium, 2.239(2) A˚ ,10 which is highly suggestive of a TidC double bond. Such alkylidenic character is further substantiated by NMR spectroscopy: the 1H NMR spectrum reveals a doublet at 8.78 ppm (1JPH ¼ 5.5 Hz) and the 13C NMR spectrum displays a resonance at 165.2 ppm (dd, 1JHC ¼ 122 Hz, 1JPC ¼ 20 Hz). It is worth noting that the methylene transfer from the ylide to titanium implied in reaction (c) is associated with the behavior of a formal Cp2Ti(II)-aryne

Table 1

Summary table for different chemical properties for titanium methylidenes.

Compound #

Formula

Oxidation state

˚) M-C bond length (A

13

References

4 7

[(PNP)Ti]CH2(OMes )] [(PN)2Ti]CH2]

IV IV

N/A 1.939(3)

290.3 291.1

22 26

C chemical shift (ppm)

Alkylidene Complexes of the Group 4 Transition Metals

351

Scheme 4 Divergent reaction pathways of ylide reaction with a titanium-aryne intermediate.

adduct, although no further studies have been performed to exclude the possibility that the PdC bond cleavage and CdC bond formation is concerted via nucleophilic attack from a Ti(IV) titanacyclopropabenzene leading to the minor products. Using half-sandwich titanium complexes featuring a bidentate, hemilabile phosphinoalkoxide ligands, van der Heijden and coworkers were able to synthesize, characterize, and study the reactivity of neopentylidene functionalities.35,36 These complexes were obtained by the addition of two equiv. of neopentyl lithium (NpLi) to the titanium dichloride complexes [Cp(R2P-CH2C(O)tBu2) TiCl2] (R ¼ Me, Ph) 16-R and [Cp(Me2P-CH2C(O)-CMe2-o-C6H4CMe2)TiCl2] 160 , yielding the respective metal alkyls [Cp(R2P-CH2C(O)tBu2)Ti(CHt2Bu)2] 17-R and [Cp(Me2P-CH2C(O)-CMe2-o-C6H4CMe2)Ti(CHt2Bu)2] 170 . At room temperature, these compounds eliminated a neopentane molecule, concomitantly forming the respective neopentylidene group (Scheme 5).The alkylidene proton and carbon resonances in the 1H and 13C{1H} NMR spectra appear at ca. 12.00 ppm and 280.00 ppm, respectively. Furthermore, the a-carbon appears as a doublet due to coupling to the phosphorus nucleus in the phosphinoalkoxide ligand. Interestingly, the 1JCH coupling constants are only approximately 95 Hz; these low values hint at agostic interactions of the CdH moieties towards the titanium center, as observed in d0 metal-alkylidene complexes. The authors were successful at obtaining a solid-state structure of [Cp(Me2P-CH2C(O)-CMe2-o-C6H4CMe2)Ti]CHtBu] 180 whose most salient feature was the short TidCa distance of 1.911(3) A˚ , suggestive of a multiple TidC bond. In addition, an acute TidCdH angle (85(3) ) and TidH distance of 2.050(5) A˚ support the presence of an agostic interaction. Neopentylidene 18 undergoes an intramolecular reaction yielding an alkyl complex cyclometallated at a phenyl group (19). 18 readily inserts a CO molecule across the titanium-alkylidene bond to form a ketene complex (20) and an ethylene molecule to form titanacyclobutane (21) by a typical [2 +2] cycloaddition step. Further heating under an ethylene atmosphere yields the respective titanocylopropane complex (22) plus a mixture of organic products derived from olefin metathesis and b-hydride elimination reactions (Scheme 5).

Scheme 5 Synthetic route to [Cp(R2P-CH2C(O)tBu2)Ti(CHt2Bu)2] (R ¼ Me, Ph) and [Cp(Me2P-CH2C(O)-CMe2-o-C6H4CMe2)Ti(CHt2Bu)2], and reaction of the latter complex with ethylene and CO.

352

Alkylidene Complexes of the Group 4 Transition Metals

The Girolami group studied the thermal decomposition pathways of tetrakis(neopentyl)titanium [Ti(CHt2Bu)4] 23, previously established as a molecular source of titanium carbide films in Chemical Vapor Deposition.37 It was found that this process occurs via a-hydrogen abstraction and neopentane elimination, generating the transient and highly reactive titanium alkylidene [Ti(CHtBu)(CHt2Bu)2] 24 (Scheme 6). The formation of this intermediate was supported by deuterium labelling, and trapping experiments with 1,2-(trimethylsilyl)ethyne. Kinetic experiments found a deuterium KIE of 5.2 between deuterated and unenriched a-positions, validating the abstraction process as an initial rate-determining step under thermal conditions. The parameters DH{ ¼ 21.5  1.4 kcal/mol and DS{ ¼ —16.6  3.8 kcal/mol for this process were obtained by an Eyring analysis. When all a-positions were deuterated, products of a g-abstraction were observable as well, indicating a second decomposition pathway estimated to be ca. 25 times slower. It was also found that the CdH bonds of benzene and cyclohexane can be activated through 1,2-addition across the Ti]CHtBu functionality. In contrast, the CdF bonds of perfluoro-n-heptane and hexafluorobenzene remained impervious to cleavage by 24 under similar conditions. As shown by the Teuben group, the implementation of a cyclopentadienyl ligand fitted with a pendant ethylene-linked amido moiety into titanium provided an entryway into isolable alkylidene complexes.38 Thermolysis of the complexes [(C5H4(CH2)2NtBu)Ti(CH2CMe2R)2] (R ¼ Me (25-Me), Ph(25-Ph)] at 80  C in the presence of trimethylphosphine (PMe3) was accompanied by loss of either neopentane (25-Me) or tert-butylbenzene (25-Ph), fashioning the alkylidene complexes (26-Me) and (26-Ph), respectively (Eq. 3). While X-ray diffraction data on these compounds was not obtained, their NMR spectra are suggestive of the proposed structural assignments since the 13C{1H} NMR signals at 251.40 ppm for 26-Me and 246.00 ppm for 26-Ph point to the presence of respective neopentylidene and neophylidene functionalities, despite the a-proton resonances at 6.39 ppm for 26-Me and 6.44 ppm for 26-Ph being somewhat shielded for typical alkylidenes moieties. These complexes were exposed to norbornene with the intent to induce ring-opening metathesis polymerization (ROMP), however, they proved to be inactive towards this transformation. In the case of ethylene as a substrate, the reaction proceeded through formation of the respective titanacyclobutanes and b-hydride elimination products rather than those of olefin metathesis.

ð3Þ

Employing the dianionic pincer ligand 2,20 -oxybis(N-isopropylanilide) (NON2−) affixed to a Ti(IV) center with two neopentyl groups (27), the Schrock group generated the neopentylidene complex (28) via trimethyl phosphine (PMe3) a-hydrogen abstraction induced at 45  C (Eq. 4).39 A remarkable feature of this complex in the solid-state is the linearity of alkylidene ligand (∠ Ti-CaCtBu ¼ 179.3(3) ) and a particularly short TidCa bond distance of 1.884(4) A˚ . While no electron density attributable to the a-proton could be found in the difference map, the significantly upfield chemical shift of 3.00 ppm in the 1H NMR spectrum and small coupling value of 1JCH ¼ 80 Hz to the a-carbon found at 230.10 ppm in the 13C spectrum are indicative of an agostic interaction. In solution, this molecule exhibits an non-localized a-hydrogen leading to C2v symmetry on the NMR timescale.

ð4Þ

Another pentacoordinate titanium-alkylidene complex was obtained by the Repo group through the low-temperature alkylation of [{(tBuN)(tBuNP)}2TiCl2] (29) with two equiv. of BnMgCl in THF.40 Under these conditions, the bis(benzyl) complex [{(tBuN) (tBuNP)}2TiBn2] 30 is obtained as the major product, and the benzylidene complex [{(tBuN)(tBuNP)}2Ti(]CHPh)(THF)] 31 as a minor product (Eq. 5). The 13C{1H} NMR chemical shift of the a-carbon in 31 was detected at 221.90 ppm, a value that is within the range of terminal alkylidenes, while in the 1H NMR spectrum, the alkylidenic proton resonates at 4.05 ppm. Such chemical shift is suggestive of an agostic interaction given that it is not significantly shifted downfield relative to the one found in the methylene group of the benzyl ligand in 30 (3.36 ppm). A 1JCH coupling constant of 162.9 Hz, however, is above the typical range of alkylidene complexes that normally exhibit agostic interactions. A single-crystal X-ray diffraction experiment revealed a TidCa bond distance of 1.901(4) A˚ which is consistent with a Ti]C double bond. An agostic interaction can be also recognized by the distinctive acute Ti-Ca-H angle of 77 and close contact between the titanium center and a-H, namely TidHa ¼1.88(7) A˚ .

Alkylidene Complexes of the Group 4 Transition Metals

353

ð5Þ

As reported by the Chirik group, addition of diazo(trimethylsilyl)methane to the Ti(II) synthon [(5-C5H3-1,3(SiMe3)2)2Ti]2(m2,1,1-N2) 32 induces N2 extrusion and intramolecular cyclometallation of a SiMe3 group in both supporting ligands producing [(5-C5H3-1-1-SiMe2CH2-3-SiMe3)2Ti] 33.41 The proposed route to this product involves the generation of an alkylidene complex which participates in an intramolecular 1,2-addition of a methyl group of the 1,3-di(trimethyl)silylcyclopentadienyl ligand. The final product could then be obtained from either generation of another alkylidene by elimination of tetramethylsilane followed by 1,2-addition, or from a s-bond metathesis reaction (Scheme 6). While no direct experimental support for this mechanism was provided, the generation of alkylidenes in this manner and cyclometallation therefrom has been reported.42–45

ð2Þ

Scheme 6 Proposed route to 33 from 32 invoking terminally bound titanium alkylidene intermediates.

Mach and coworkers showed that bis(3-butenyltetramethylcyclopentadienyl)dimethyltitanium 34 reacted under thermal conditions to form the titanacyclobutane complex [Ti{Z5-C5Me4CH2CH2CH(k-CH)}2CH2] 35, wherein the two terminal alkene groups have been stitched together by successive cyclo- and retro-cycloaddition reactions with the intermediacy of alkylidene species (Scheme 7).46

354

Alkylidene Complexes of the Group 4 Transition Metals

Scheme 7 Reaction pathway with proposed intermediates leading to the titanacyclobutane [Ti{Z5-C5Me4CH2CH2CH(k-CH)}2CH2].

The Eisch laboratory reported that addition of two equiv. of methyllithium to tetrachlorotitanium(IV) in toluene at low temperature generated dichloromethylidenetitanium(IV), [Cl2Ti]CH2] 36, through a lithium chloride adduct of the said compound (Scheme 8), rather than dichlorodimethyltitanium(IV), [Cl2TiMe2] Under these conditions, methylenation of ketones is readily observed. Reaction with diphenylacetylene followed by acidic work-up provided a mixture of both Z- and E-1,2-diphenylpropene. These products are obtained through the intemediacy of vinylcarbene intermediates which have been previously invoked in the formation of other titanium alkylidenes.47 Transient 36 is also a competent catalyst for ROMP of norbornene. The presence of LiCl is key for these transformations to occur given that THF solvation of this salt induces its dissociation from the titanium carbene thus precluding reactivity.

Scheme 8 Diagram depicting the reactivity of LiCl-stabilized [Cl2Ti]CH2] with various organic substrates.

The thermal decomposition of the dibenzyl titanium complex supported by bisphenolate-(benzene-1,3-diyl), [Ti(CH2C6H5)2 {(OC6H2-2-CMe3-4-CH3)2-C6H3}] 37 into a dimeric complex 38 featuring cyclometallation at the 2-position of the benzene1,3-diyl linker was investigated by Bercaw and his group (Scheme 9).48 Kinetics studies revealed first-order behavior with activation parameters DH{ ¼ 27.2(5) kcal/mol and DS{ ¼ −6.2(14) cal/(mol K), which are in accord with a-H abstraction at benzyl groups and subsequent elimination of toluene taking place enroute to a titanium-benzylidene intermediate 39. Isotopic-labelling studies supported this mechanism, but the presence of certain isotopologues of the products suggested that a competing s-bond metathesis reaction could be also occurring. KIE values for the a-H abstraction and s-bond metathesis mechanisms at 388 K were found to be 5.5(3) and 1.0(1), respectively.

Alkylidene Complexes of the Group 4 Transition Metals

355

Scheme 9 Competing reaction pathways from 37 to a cyclometallated dimer 38.

The same group used the related ligand bisphenoxide framework, pyridine-2,6-bis(4,6-di-tert-butylphenolate) to synthesize the dibenzyl titanium complex 40.49 This species acts as a precatalyst for the [2 +2 +2] cyclotrimerization of 2-butyne into hexamethylbenzene. The proposed mechanism involves an a-hydrogen elimination step to form a benzylidene intermediate which can undergo [2 +2] cycloaddition with 2-butyne. Insertion of a second equiv. of the alkyne and finally reductive elimination of tetramethylphenylcyclopentadiene regenerates the Ti(II) catalyst (Scheme 10).

Scheme 10 Proposed mechanism of activation of 40 towards generation of a Ti(II) catalyst for cyclotrimerization.

Mindiola and coworkers designed a novel synthetic strategy to access a variety of titanium alkylidenes.50–53 This method involves the chemical oxidation of trivalent titanium bisalkyl species to induce an alkyl-based a-hydrogen abstraction step that consequently generates the alkylidene functionality. In line with this, the first four-coordinate titanium alkylidene, featuring a b-diiminate ligand (BDI—), was conveniently synthesized in high yields.52 Accordingly, the low-temperature oxidation of [(MeBDI)Ti(CHt2Bu)2] 41 (MeBDI— ¼ [Ar]NC(Me)CHC(Me)N[Ar], Ar ¼ 2,6-(CHMe2)2C6H3) with silver trifluoromethylsulfonate (AgOTf ) resulted in the formation of the neopentylidene complex [(MeBDI)Ti(]CHtBu)(OTf )] 42, neopentane, and a silver mirror (Scheme 11). NMR spectroscopic analysis of 42 showed a 13C spectrum with the neopentylidenic carbon resonance at 271.00 ppm and a coupling constant of 1JCH of 95 Hz. In the 1H NMR spectrum, the neopentylidenic proton resonance was found at 5.23 ppm. An X-ray diffraction experiment performed on a single crystal of 42 disclosed in the solid-state a particularly short TidCa distance of 1.830(3) A˚ which is consistent with metal-ligand multiple bond character. The neopentylidenic proton could be located in the electronic density difference map and exhibited a close contact to the titanium center (TidHa ¼ 1.92(3)) A˚ , alongside a considerably large Ti-Ca-C(Me3) angle of 163.9(3) . These bond metrics in combination with the spectral features of 42 are reminiscent of the presence of an a-H agostic interaction which also is favored by the coordinative unsaturation around the metal center. Complex 42 displays Wittig-type reactivity wherein the neopentylidene fragment is readily transferred to benzophenone to produce tBu(H)C]CPh2 and the dimeric titanium-oxo complex [(MeBDI)Ti(m2-O)(m2-OTf )]2 43 (Scheme 12). While 42 is stable as a solid, in solution, the neopentylidene unit gradually undergoes intramolecular metathesis with an NAr fragment of

356

Alkylidene Complexes of the Group 4 Transition Metals

Scheme 11 Generation of (BDIMe)Ti(]CHtBu)(OTf ) and the products from its thermal decomposition.

Scheme 12 Diagram depicting selected reactions of [(MeBDI)Ti(]CHtBu)(OTf )] with different organic molecules.

Me

BDI ligand. This reaction forms the corresponding titanium imido at the expense of converting the former MeBDI ligand into an amido-diene moiety [HtBuC]C(Me)CHC(Me)N[Ar])Ti(]NAr)(k2-OTf )] 44 (Scheme 11). Additionally, under prolonged heating both methine positions in the MeBDI ligand are susceptible to intramolecular cyclometallation upon neopentane extrusion providing bisalkyl complex 45 (Scheme 11).54 In an attempt to gain insights into structure-reactivity relationships, the more sterically encumbering ligand tBuBDI (tBuBDI ¼ ([Ar]NC(tBu)CHC(tBu)N[Ar], Ar ¼2,6-(CHMe2)2C6H3) was employed.55,56 As expected for these kinetically stabilized complexes [(tBuBDI)Ti(]CHtBu)(X)] (X— ¼ OTf (46-tBu), I), both compounds remained intact even after prolonged heating at 90  C (Scheme 11).57 Further reactivity studies proved the [(MeBDI)Ti(]CHtBu)] platform to

Alkylidene Complexes of the Group 4 Transition Metals

357

be a unique avenue to access elusive functionalities and atypical small molecule activation reactivities with the alkylidene moiety. For instance, salt metathesis between 42 and LiHPR (R ¼ cyclohexyl; 2,4,6-triisopropylphenyl; 2,4,6-tritertbutylphenyl) produced the first titanium phosphinidenes [(MeBDI)Ti(]PR)(CHt2Bu)] 47 via the intermediacy of alkylidene-phosphides [(MeBDI)Ti(] CHtBu)(PHR)].58 Reactivity studies of 42 towards polar substrates have been accompanied by intriguing outcomes as well (Scheme 12).54 Cross metathesis between 42 and CO2 transiently yields tert-butyl ketene as the product of alkylidene attack to CO2 and a subsequent deoxygenation step to yield 43 (not shown). Both of these organic and metallic species combine into the bridging-oxotitanium dimer [(L1)2Ti(m2-O)(m2-OTf )][OTf] through nucleophilic attack of tert-butylketene by the g-carbon of the two b-diiminate ligands in [(MeBDI)Ti(m2-O)(m2-OTf )]2 (not shown). Reactions with tert-butylisocyanide, 2-mesitylacetonitrile, and phenyl isothiocyanate, each resulted in complete insertion in between the titanium-alkylidene bond, providing [(MeBDI)Ti(Z2-(N,C)-tBuNCCHtBu)(OTf )] 48, [(MeBDI)Ti(N[C]CHtBu(CH2Mes)])(OTf )] 49, and [(MeBDI)Ti(k2-(S,N)-SC(CHtBu)NPh)(OTf )] 50, respectively. Treatment with 1-azidoadamantane proceeds through a proposed radical mechanism where N2 is eliminated to form the imide complex [(MeBDI)Ti](NCHtBuAd)(OTf )] 51 whose adamantyl moiety is derived by migration to the former alkylidenic carbon. The reaction with diazodiphenylmethane generates the imido complex [(MeBDI)Ti(NCHPh2)(NCtBu)(OTf )] 52 whose pivalonitrile ligand is derived from a rare cleavage of the diazo N]N bond and subsequent insertion and migration steps. Reduction of 42 with potassium graphite, and of 46-OTf with tert-butyllithium resulted in cyclometallated Ti(III) complexes 53dMe and 53-tBu by deprotonation of a methyl moiety of the isopropyl groups of the ligand, ostensibly through the intermediacy of a Ti(III) alkylidene.54 This transformation is chemically reversible as re-oxidation with AgOTf restores the initial alkylidene-triflate complex (Scheme 13).

Scheme 13 Synthetic cycle of one-electron reduction of [(RBDI)Ti(]CHtBu)(OTf )] (R ¼ Me, tBu) and oxidation of the resulting cyclometallated product.

Other titanium alkylidene complexes featuring a more robust supporting pincer ligand such as [(PNP)Ti(]CHtBu)(OTf )] 54 can also be accessed employing the metal-bisalkyl one-electron oxidation protocol.59 The 13C{1H} NMR spectrum of this alkylidene shows a peak at 301.00 ppm with a coupling constant 1JCH of 102.6 Hz, which can be correlated to a proton at 8.42 ppm in the 1H NMR spectral trace. The most relevant structural features of this molecule, as revealed by X-ray crystallography, are the short TidC bond distance of 1.883(7) A˚ and the wide TidCadC(Me3) angle of 157.6(6) as well as a short TidH distance of 2.09(6) A˚ . Together, the bond metrics and NMR spectroscopic data support the presence of an alkylidene functionality with an a-H agostic interaction. Related titanium-alkylidenes [(PNP)Ti(CHR)(OTf )] (R ¼ SiMe3, iPr) have been prepared by the respective metal-bisalkyl oxidation method.50,60 An unusual discrepancy in reactivity is seen between addition of either xanthone or thioxanthone to solutions of [(PNP)Ti(] CHtBu)(CH3)] 55 (Scheme 14).61 While metallo-Wittig chemistry is observed with xanthone, generating the anticipated olefin t BuHC]CC12H8O alongside a proposed titanium oxo complex [(PNP)Ti]O(CH3)], treatment with thioxanthone yields 3,3-dimethyl-1-butene alongside [(PNP)Ti(Z2dO]CC12H8S)(OCHC12H8S)] 56 and the product of thioxanthone olefination (tBuHC]CC12H8O) as a minor component of this reaction mixture. The reaction of 55 with 2,20 -bipyridine (bipy) induced the formation of the trivalent titanium complex [(PNP)Ti(bipy)(bipyH)] 57 featuring a p-radical bipy ligand and 3,3-dimethyl-1butene. Isotopic labelling studies confirmed the intramolecular migration of the methyl group to the neopentylidene unit and formal hydride abstraction in the case of thioxanthone. For bipy, computational studies show b-hydride elimination and migratory insertion as key steps for this transformation. Further quantum chemical calculations advocate these bond cleavage and formation processes are enabled by a spin-crossover from a singlet to triplet energy surface. These events are facilitated by the adequate redox potential of thioxanthone and bipy to be reduced to a monoanionic radical species in addition to the energetic engagement between the low-lying bipy p orbitals with the frontier MOs of the metal complex. 13C and 2H isotopic labelling studies were also conducted to monitor the fate of the methylidene ligand in the dehydrocoupling reaction. Since the methyl moiety in 55 can derive from methane, this reaction constitutes a unique methane dehydrocoupling reaction with a Schrock-carbene to a terminal olefin.62

358

Alkylidene Complexes of the Group 4 Transition Metals

Scheme 14 Reactions of 55 with xanthone, thioxanthone, and 2,20 -bipyridine.

The Gade group developed the monoanionic pincer ligands 3,6-di-tert-butyl-1,8-bis((diphenylphosphaneyl)methyl)carbazolide (CbzdiphosPh) and bis((diisopropylphosphaneyl)methyl)carbazolide (CbzdiphosiPr) for the stabilization of titanium benzylidene complexes.63 Following addition of one equiv. of [Bn2Mg(THF)2] to either [(CbzdiphosiPr)TiCl3] 56 or [(CbzdiphosPh)TiI3] 57, the carbene compounds [(CbzdiphosiPr)Ti(]CHPh)(Cl)] 58 and [(CbzdiphosPh)Ti(]CHPh)(I)] 59 were conveniently synthesized (Eq. 6). The chemical shifts found in the NMR spectra of these compounds are fully consistent with other titanium alkylidenes; a 1H signal at 11.50 ppm was observed along with a 13C{1H} signal at 288.70 ppm associated with the benzylidene a-carbon of [(CbzdiphosiPr)Ti(]CHPh)(Cl)], while [(CbzdiphosPh)Ti(]CHPh)(I)] displayed 1H and 13C peaks at 10.54 ppm and 298.80 ppm respectively, with a 1JCH coupling constant of 103.5 Hz suggesting an a-H agostic interaction. A crystal structure of (CbzdiphosiPr)Ti(]CHPh)(Cl) revealed a TidCa distance of 1.918(4) A˚ and a Ti-Ca-Cipso angle of 139.5(3) . It is worth noting that this synthetic strategy was not successful in providing the Zr and Hf homologues of these alkylidenes. Instead, intramolecular cyclometallation of the methylene group linking the carbazole backbone and chelating phosphine with the metal centers was observed.

ð6Þ

Okuda and coworkers were able to promote a-H elimination from the homoleptic titanium alkyl complexes tetrakis(trimethylsilylmethyl)titanium [Ti(CH2SiMe3)4] 60 and tetrakis(dimethylphenylsilylmethyl)titanium [Ti(CH2SiPhMe2)4] 61 by treatment with the alkali amides of the macrocycle 1,4,7-trimethyl-1,4,7,10-tetraazacyclo-dodecane ((Me3TACD)H); [Li(Me3TACD)]2, [Na(Me3TACD)]3, and K(Me3TACD) (Scheme 20)64 This method produced monoanionic alkylidene complexes of the general formula [{(Me3TACD)M}Ti(¼CHSiMe2R)(CH2SiMe2R)2] whose structural features are strongly dependent on the R substituents in the silyl groups and the alkali metal cation. In the solid-state, the Na and K cations form an electrostatic interaction with the polarized methylene fragments of the -CH2SiMe3 ligands, rather than with the trimethylsilylmethylidene in compounds 62-Na and 62-K (Scheme 15). An opposite trend is observed with [{(Me3TACD)M}Ti(]CHSiMe2R)(CH2SiMe2R)2] (M+ ¼ Li, Na, K, R ¼ Ph, 63-M; M+ ¼ Li, R ¼ Me, 62-Li) where both the alkylidene and Me3TACD units sandwich the alkali cations to complete its coordination sphere. The bond metrics of these latter compounds 62-Li, 63-Li, 63-Na show TidCa bond distances ca. 1.92–1.93 A˚ with Ti-Ca-Si angles between 138 and 142 and Ti-Ca-H angles around 104–108 for the alkylidene unit. In contrast, those possessing terminal alkylidenes, 62-Na and 62-K contained shorter Ti]C bonds of 1.879(3) A˚ and 1.867(9) A˚ as well as Ti-Ca-H angles of 87.0(2) and 96.9(6) , respectively; these values suggest the presence of a-H agostic interactions for the terminal functionalities, which seem to be absent or at least significantly disrupted by alkali metal coordination.

Alkylidene Complexes of the Group 4 Transition Metals

359

Scheme 15 Reactions of homoleptic silylmethyl compounds of titanium with macrocyclic amides of lithium, sodium and potassium.

The Cavell lab developed a dilithio bis(trimethylsilyliminodiphenylphosphorano)methandiide reagent that can undergo lithium chloride (LiCl) elimination with [TiCl4(THF)2] to obtain a supported-alkylidene such as [TiCl2(k3C,N,N0 dC(Ph2P] NSiMe3)2)] 64.38 The short TidCa distance of 2.008(4) A˚ as determined by X-ray crystallography and low field chemical shift in the 13C{1H} NMR spectrum at 191.00 ppm is highly suggestive of a multiple-bond order between the Ti and Ca atoms.65 The closely related ligand bis(thiophosphinoyl)methanediide, was used by the Mézailles group to obtain the analogous titanium supported alkylidene complex [TiCl2(THF)(k3C,S,S0 dC(Ph2P]S)2] 65.66 Subsequent addition of a second equiv. of Li2(C(Ph2P]S)2) provided the homoleptic biscarbene complex [Ti(k3C,S,S0 dC(Ph2P]S)2)2] 66. While the monocarbene complex had a short TidCa distance as shown in the solid-state (1.9845(17) A˚ ), the biscarbene, for which two distinct structures were collected, displayed bond lengths of 2.108(2) and 2.069(2) A˚ in one structure, and 2.088(2) and 2.073(2) A˚ in the other. This bond elongation can be attributed to the strong trans-influence the carbenes exert on each other in 66 compared to the weaker axial donor THF ligand in complex 65. Neither the a-carbon resonance of 65 nor that of 66 could be detected by 13C{1H} NMR spectroscopy. Reactivity studies performed on 65 and 66 resemble the chemistry of terminal titanium alkylidene complexes. For instance, Metallo-Wittig reactivity is elicited with addition of benzophenone and benzaldehyde to solutions of either of the latter titanium carbene compounds (Scheme 21). Polar substrates such as N,N0 -dicyclohexylcarbodiimide and phenylisocyanate reacted with 65 in a [2 +2] fashion, followed by metathesis to obtain ketenimines and possibly titanium imides and sulfide-type species, respectively (Scheme 16).

Scheme 16 Synthesis and reactivity of bis(thiophosphinoyl)methanediide complexes of titanium with polar organic molecules.

360

Alkylidene Complexes of the Group 4 Transition Metals

Fig. 4 Drawing of [(CDPPy2)TiCl3].

The Sundermeyer group reported a neutral NCN pincer, sym-bis(2-pyridyl)tetraphenylcarbodiphosphorane (CDPPy2) ligand that was coordinated to TiCl3 to produce the monomeric complex [(CDPPy2)TiCl3] 67 (Fig. 4).67 The TidCa distance found in the X-ray structure was 2.144(6) A˚ ; while this value is much longer than those reported for other chelating alkylidene compounds of tetravalent titanium, it is worth noting that Ti(III) has a larger ionic radius than Ti(IV). Floriani and coworkers described a highly-reduced m-ethene-1,2-diylidene titanium complex supported by calix[4]pyrrole ligands.68 The titanium complex ligated by the tetraanionic meso-octaethyl calix[4]pyrrole (Et8N4)4− and two THF molecules trans to each other, [(Et8N4)Ti(THF)2] 68, could be reduced with slightly more than four equiv. of lithium sand in THF under an atmosphere of ethylene. These conditions yielded the dianionic dimer [(Et8N4)Ti]C]C]Ti(N4Et8)(Li)2][Li(THF)4]2 69 (Eq. 7) which contains two Li cations chelated to the bridging C2 unit in an Z2-fashion and sandwiched between the parallel pyrrole fragments through electrostatic interactions. This complex has an average magnetic moment meff ¼ 1.80 mB per titanium center, according to the Evans’ method giving credence to an electronic description of the complex as two Ti(III) ions showing no signs of d1-d1 (anti)ferromagnetic coupling. At first glance, the magnetic data suggests that this molecule is better described as an end on and bridging acetylide titanium(III) m-ethyne-1,2-diyl complex ([Ti]dC^Cd[Ti]), but, an X-ray crystallographic analysis shows a more delocalized metal-ligand bonding scenario. In particular, the exceedingly short TidC distances of 1.809(9) and 1.757(7) A˚ alongside the virtually linear [Ti]dC^Cd[Ti] frame (178 (7) ) favors an alkylidene-type canonical structure such as [Ti]]C] C][Ti]. Further, the elongated CdC distance in the C2 unit, 1.301(11) A˚ , compared to acetylene, calcium carbide,69 or to the structurally similar dititanocene compound Cp2(Me3P)TidC^CdTi(PMe3)Cp2 (1.253(2) A˚ ),70 imply there is a higher order of bonding between the titanium centers and carbon atoms, and consequently, lower order of bonding within the C2 unit.

ð7Þ

The Mindiola group has reported a family of titanium alkyl-alkylidenes of the type [(PNP)Ti(]CHR)(CH2R’)] that are prone to intramolecular a-hydrogen abstraction. This step generates a transient, and highly reactive alkylidyne [(PNP)Ti(^CR)] (Scheme 17) which can cleave strong bonds of various small molecules that are generally considered challenging to activate. For instance, (PNP)Ti(]CHR)(CH2R’) (R ¼ tBu, R’ ¼ tBu, SiMe3) serve as precursors to [(PNP)Ti(^CR)] (R ¼ tBu, SiMe3) (where the alkylidene SiMe3 is derived from an a-H migration step) which cleaves the CdD bond of C6D6 via a 1,2-addition across the Ti^C triple bond to produce the respective titanium alkylidenes [(PNP)Ti(]CDR)(C6D5)].60 When dissolved in C6H6 and heated, an equilibrium between [(PNP)Ti(]CDR)(C6D5)] and the isotopologue [(PNP)Ti(]CHR)(C6H5)] is established. In this same study, heating of [(PNP)Ti(]CHtBu)(CHt2Bu)] 70 in neat Me4Si produced the titanium-alkylidene complex [(PNP)Ti(]CHSiMe3)(CH2SiMe3)] 71 through the activation of the CdH bonds of the methyl groups in the solvent. Furthermore, pentafluoroanisole undergoes CdO bond cleavage to generate the isolable alkylidene-alkoxide complex [(PNP)Ti{]C(tBu(C5F5R)}(OMe)] 72.71 Several fluorinated arenes (HArF) react with [(PNP)Ti(]CHtBu)(CH3)] 55 through the [Ti(^CtBu)] intermediate to regioselectively produce [(PNP)Ti(]CHtBu)(ArF)]. The linear fluoroalkane 1-fluorohexane is defluorinated to 1-hexene and [(PNP)Ti(]CHtBu)(F)] 73. The analogous alkylidene 71 performs the defluorination of 1-fluorohexane as well.71,72 Furthermore, ethers are susceptible to either dehydrogenation or dehydroalkoxylation by [(PNP)Ti(^CtBu)] to produce alkylidene-alkoxides [(PNP)Ti(]CHtBu)(OR)] 74-OR (R ¼ Me, Et, nPr, nBu, iPr, tBu).73 Complex 70 reacts with the electrophilic boron reagent B(OCH3)3 to produce the boryl substituted alkylidene complex [(PNP)Ti{(]C(tBu)(B(OCH3)2)}(OCH3)] 75.74

Alkylidene Complexes of the Group 4 Transition Metals

361

Scheme 17 Generation of transient alkylidyne species from alkyl-alkylidenes, and reactions that lead to a diverse array of new alkylidenes.

With respect to aliphatic CdH bond activation, the methyl ligand of the titanium neopentylidene 55 is readily derived from a 1,2-addition addition of methane across the [Ti(^CtBu)] linkage (Scheme 18).62,75,76 In the same vein, ethane77 as well as other linear and cyclic hydrocarbons with chain lengths of C4 to C8 are dehydrogenated to the corresponding terminal or internal olefin, followed by formation of Z2-alkene species [(PNP)Ti(CHt2Bu)(Z2-CH2CH2R)] 76-R.78 These complexes represent archetypical “masked” divalent forms of titanium that can participate in two-electron reactions with ylides.79 Accordingly, treatment of [(PNP)Ti(CHt2Bu)(Z2dH2CCH2)] 76-H with H2CPPh3 is accompanied by ethylene and neopentane extrusion and formation of a proposed titanium(II) phosphoniomethylidene fragment [(PNP)Ti(]CHPPh3)]. This intermediate undergoes oxidative addition across the PdCPh bond forming the phosphinoalkylidene complex [(PNP)Ti(]CHPPh2)(Ph)] 77 which has been characterized in the solid-state (Scheme 18). This complex selectively dehydrogenates cyclohexane (C6H12) to cyclohexene (C6H10) through a transient phosphinoalkylidyne intermediate [PNP[Ti(^CPPh2)]. Clearly, in the alkane dehydrogenation step that forms the corresponding olefin adduct 76dR the regeneration of the titanium alkyl-alkylidenes (PNP)Ti(]CHR)(CH2R’) is a tantalizing entryway to a catalytic system. In line with this, the more robust ylide P-phenyldibenzophospholium methylide H2CP(C12H8)Ph,80 which features less-vulnerable PdCPh bonds in comparison to H2CPPh3, was reacted with 70 in attempt to circumvent the formation of the thermodynamic sink 77 (vide infra).81 Indeed, this reaction conveniently produced the titanium methylneopenthylidene 55 (Scheme 19) which was used as a precatalyst in the selective dehydrogenation of linear alkanes to terminal olefins as well as cyclic alkanes to cyclic alkenes (Table 2).

Scheme 18 Reactions of titanium olefin complexes with phosphonium ylides.

362

Alkylidene Complexes of the Group 4 Transition Metals

Scheme 19 Transformation of [CpR5(tBu2C]N)TiMe(m2dMe)MeTi(N]CtBu2)CpR5] to [CpR5(tBu2C]N)Ti(m2-CH2)(m2dMe)Ti(N]CtBu2)CpR5].

Table 2

Summary table for different chemical properties of titanium alkylidenes.

Compound #

Formula

Oxidation state

˚) M-C bond length (A

13

15 18-Ph, Me 18’

[Cp2Ti(]CHPPh3)(Ph)] [Cp(R2P-CH2C(O)tBu2)Ti(CHt2Bu)2] [Cp(Me2P-CH2C(O)-CMe2-o-C6H4CMe2)Ti] CHtBu] [(C5H4(CH2)2NtBu)Ti(CH2CMe2R)2] [(NON)Ti(]CtBu)(PMe3)2] [{(tBuN)(tBuNP)}2Ti(]CHPh)(THF)] [(MeBDI)Ti(]CHtBu)(OTf )] [(tBuBDI)Ti(]CHtBu)(X)] [(PNP)Ti(]CHtBu)(OTf )] [(PNP)Ti(]CHtBu)(CH3)] [(CbzdiphosiPr)Ti(]CHPh)(Cl)] [(CbzdiphosPh)Ti(]CHPh)(I)] [{(Me3TACD)M}Ti(]CHSiMe3)(CH2SiMe3)2]

IV IV IV

2.033(6) N/A 1.911(3)

165.2 287.3, 283.5 278.1

34 35,36 36

IV IV IV IV IV IV IV IV IV IV

N/A 1.884(4) 1.901(4) 1.830(3) 1.834(2), 1.822(5) 1.883(7) 1.870(2) 1.918(4) N/A 1.929(4), 1.879(3), 1.867(9) 1.923(4), 1.921(3), N/A for 63-K 2.008(4) 1.9845(17) 2.073(2)–2.108(2) 2.144(6) 1.809(9), 1.757(7) 1.790(5) N/A 1.953(2) 1.872(2) 1.976(2) 1.922(3)

246.0, 251.4 230.1 221.9 271.0 259.6, 259.6 301.0 285.4 288.7 298.8 229.2, 224.9, 244.4

38 39 40 52 55–57 59 62,82 63 63 64

227.4, (not detected for 63-Na), 239.1 191.0 N/A N/A N/A

64

26-Ph, Me 28 31 42 46-OTf, I 54 55 58 59 62-Li, Na, K 63-Li, Na, K 64 65 66 67 69 70 71 72 73 75 77

[{(Me3TACD)M}Ti(]CHSiMe2Ph) (CH2SiMe2Ph)2] [TiCl2(k3C,N,N0 dC(Ph2P]NSiMe3)2)] [TiCl2(THF)(k3C,S,S0 dC(Ph2P]S)2] [Ti(k3C,S,S0 dC(Ph2P]S)2)2] [(CDPPy2)TiCl3] [(Et8N4)Ti]C]C]Ti(N4Et8)(Li)2][Li(THF)4]2 [(PNP)Ti(]CHtBu)(CHt2Bu)] [(PNP)Ti(]CHSiMe3)(CH2SiMe3)] [(PNP)Ti{]C(tBu(C5F5R)}(OMe)] [(PNP)Ti(]CHtBu)(F)] [(PNP)Ti{(]C(tBu)(B(OCH3)2)}(OCH3)] [(PNP)Ti(]CHPPh2)(Ph)]

IV IV IV IV III III IV IV IV IV IV IV

C chemical shift (ppm)

260.0 310.5 310.2 314.0 323.7 248.8

References

65 66 66 67 68 60 60 71 72 74 79

Alkylidene Complexes of the Group 4 Transition Metals

3.10.4

363

Titanium bridging methylenes

The Andersen laboratory reported the earliest examples of non-titanocene alkylene complexes. They obtained {[TiCHSi(Me)2Nk2dC,N]SiMe3[N(SiMe3)2]}2 78-Ti from the reduction of [((SiMe3)2N)2TiCl2] with a stoichiometric excess of Na/Hg amalgam (Eq. 8).83 While no X-ray structure was obtained for this complex, the zirconium analogue 78-Zr was unambiguously characterized through this method, and thus, by virtue of their spectral similarities as shown by NMR spectroscopy, it was surmised that both compounds share analogous connectivity. The titanium complex 78-Ti shows a noteworthy deshielded bridging methylene proton resonance at 10.50 ppm in the 1H NMR spectrum and the methylene carbon signal was located at 271.40 ppm in the 13C{1H} NMR spectrum. Although these chemical shifts are found at a lower field in comparison to the Zr analogue (7.08 ppm in 1H and 201.40 ppm in 13C{1H} NMR), they resemble those found in many terminal alkylidene complexes.

ð8Þ

Gambarotta and coworkers accessed the bridging titanium bis-methylene complex [(Cy2N)2Ti(m CH2)2Ti(NCy2)2] 79 through thermolysis of [(Cy2N)2TiMe2] (Eq. 9).84 The resonance corresponding to the protons of the m-methylene group were found as a singlet at 8.29 ppm in the 1H NMR spectrum, while the carbon signals in the 13C NMR spectrum were found downfield at 224.70 ppm. The crystal structure revealed TidC bond distances of 2.020(5) A˚ , 2.016(5) A˚ which are short in comparison with those of [(Cy2N)2TiMe2], 2.111(4) A˚ , and 2.118(3) A˚ . Lewis bases such as PMe3 or pyridine failed to induce breakage of the dimer. Thermal decomposition into ethylene and other intractable products was observed after heating toluene solutions at 70  C for 48 h. Furthermore, this complex polymerizes ethylene.

ð9Þ

The Marks and Delferro groups reported the binuclear titanium alkylene (m-Me2C-3,30 ){(Z5-Me5C5)[1-Me2Si(tBuN)][(m-CH2) Ti]}2 80, obtained from thermolysis of rac-(m-Me2C-3,30 ){(Z5Me5C5)[1-Me2Si-(tBuN)](TiMe2)}2 81. A similar cationic variant of the former complex [(m-CMe-3,30 ){(Z5Me5C5)[1-MeSi(tBuN)]}(m-CH)(m-CH)Ti][B(C5F6)4] 82 was obtained via methyl abstraction from 81 with [Ph3C]+[B(C5F6)4]− and subsequent intramolecular methyl deprotonation (Fig. 5).85

Fig. 5 Titanium bridging alkylidenes reported by the Marks and Delferro groups.

The Piers group reported dimeric titanium m-methylene compounds supported by cyclopentadienyl-type ligands: C5H5, C5Me5, C5Me4SiMe3 or CpR5 and 2,2,4,4-tetramethyl-3-pentaniminato (tBu2C]N)— (Scheme 19).86 Treatment of [CpR5(tBu2C]N)TiMe2] 83 with half an equiv. of trityl tetrakis(pentafluorophenyl)borate at −25  C generates a pair of dimeric diastereomers rac- and meso[CpR5(tBu2C]N)TiMe(m2dMe)MeTi(N]CtBu2)CpR5] rac-84 and meso-84 with a bridging methyl group. Upon warming to 25  C these compounds eliminate methane producing the methylene complexes rac- and meso-[CpR5(tBu2C]N)Ti(m2-CH2)(m2dMe)Ti(N]CtBu2)CpR5] rac-85 and meso-85 (Scheme 19). NMR spectroscopy studies on rac-[Cp (tBu2C]N)Ti(m2-CH2)(m2dMe)Ti (N]CtBu2)Cp ] identified a signal in the 1H NMR spectrum at 7.44 ppm attributable to the protons of the methylene unit, and −0.98 ppm for the m-methyl, while the 13C resonance of the m-CH2 group was found at 25.30 ppm which was further upfield than that of the m-methyl at 61.50 ppm.

364

Alkylidene Complexes of the Group 4 Transition Metals

Cuenca and coworkers studied the thermolysis of the dimer [(m-C5Me4(SiMe2O))TiMe2]2 86 which eliminated methane at 80  C to yield the complex [(m-C5Me4(SiMe2O))TiMe]2(m2-CH2) 87 (Eq. 10).87 The 1H NMR spectrum displayed a singlet at 5.90 ppm corresponding to the protons of the methylene group, and the 13C resonance for this moiety was located as a triplet at 196.90 ppm with 1JCH ¼ 118.8 Hz. The asymmetric unit of the structure in the solid state showed a TidCmethylene distance of 2.0719(13) A˚ which is slightly shorter than the TidCmethyl of 2.1215(17) A˚ in the same structure. This dititanium m-methylene complex readily engages in metallo-Wittig reactivity with benzophenone, producing the expected olefination product 1,1-diphenylethene and m-oxo complex [(m-C5Me4(SiMe2O))TiMe]2(m2-O) 88.

ð10Þ

3.10.5

Titanium bridging alkylenes

Rothwell and coworkers reported the synthesis and reactivity studies of the dimeric titanium alkylene complex [(cb)2Ti(m-CHSiMe3)2Ti(cb)2] (cb− ¼ carbazolate) 89.88 This complex is obtained via Me4Si elimination from [Ti(CH2SiMe3)4] upon addition of two equiv. of Hcb (9-H-carbazole) (Eq. 11). The Me3SiCH— 2 ligand is necessary for the formation of the alkylene unit, since the reaction between [Ti(CH2Ph)4] and two equiv. of Hcb produces the monomeric titanium bisbenzyl complex [(cb)2Ti(CH2Ph)2]. The 1H NMR spectrum of 89 contained a very low field resonance at 14.75 ppm; further downfield than typical terminal alkylidenes. The TidCa distances were 2.026(2) and 2.035(2) A˚ , shorter than that found in alkyl complexes of titanium, but in-line with other dititanacyclobutanes.

ð11Þ

The Santamaría group showed that thermolysis of [{Ti(Z5-C5Me5)(CH2SiMe3)2}2(m-O)] 90 induced elimination of two Me4Si molecules and subsequent formation of the bridging alkylene complex [Ti2(Z5-C5Me5)2(m-CH2SiMe2CH2)2(m-CHSiMe3)(m-O)] 91 (Eq. 12).89 NMR spectroscopic characterization identified the alkylenic proton at 6.68 ppm in the 1H spectrum, while the corresponding 13C signal appeared at 231.2 ppm. The distance between the bridging carbon and each titanium atom was 2.100 (8) A˚ as determined by X-ray crystallography; this is slightly longer than Rothwell’s compound with the same functionality,88 and does not deviate significantly from the length of the TidC bonds from the Ti-centers to the m-CH2SiMe2CH2 moiety (2.100 A˚ (avg)).

ð12Þ

As shown by the Wilke group, addition of four equiv. of triethylaluminium (AlEt3) to the dimer tetrakis(bicyclohexyl-1,10 -dioxy) dititanium [(biCyO2)2Ti]2 92 in toluene at −78  C provides a monomeric complex containing a m-ethan-1,1-diyl functionality between an Al and Ti center 93 - essentially the first non-titanocene Tebbe-reagent-like complex (Fig. 6).90 Two diasteriomers exist in solution; the major isomers’ 13C{1H} NMR spectroscopic signal appeared at 202.10 ppm while the minor isomer appeared at 192.20 ppm. The protons bound to the a-carbons were found at 7.71 ppm and 8.61 ppm in the 1H NMR spectrum, respectively. Single crystal X-ray diffraction studies revealed a short TidCa distance of 1.933(6) A˚ , as well as an elongated AldCa distance of 2.108(6) A˚ ; all the AldCEt bond lengths have values between 1.934 and 1.956 A˚ , indicating that the Al center is stabilizing what is otherwise a very strong TidC interaction. This complex reacts with the strained cycloalkenes such as 3,3-dimethylcyclopropene via olefin metathesis producing a new titanium adduct with a pendant m-4,4-dimethylpentan-5,5-diyl functionality. No reactivity with more equiv. of 3,3-dimethylcyclopropene was observed.

Alkylidene Complexes of the Group 4 Transition Metals

365

Fig. 6 Titanium complex 93 containing a m-ethan-1,1-diyl functionality.

Chen and coworkers investigated the thermal decomposition of the olefin polymerization catalyst Me2Si(Z5-2,3,4,6-Me4Ind) (CyN)Ti(CH3)[m-CH3Al(C6F5)3] 94. During this study, it was found that heating a benzene solution of this complex at 60  C for 12 h formed a dimeric structure containing four m3-methylene groups [Me2Si(Z5-2,3,4,6-Me4Ind)(CyN)Ti(m3-CH2)2(Al(C6F6)3) (AlMe)]2 95 (Fig. 7).91 This compound was characterized using single crystal X-ray diffraction and the structure displayed an inversion center in the symmetry of the molecule. The two TidCmethylene bond distances were 2.195(3) and 2.230(3) A˚ , hence within the range of typical TidC single bond lengths. Noteworthy characteristics of this structure are the short TidHmethylene distances (2.19(3) and 2.26(3) A˚ ) and acute TidCdH angles (77(2) and 80(2) ) found between one H atom of each methylene unit and the Ti-center. These features imply the presence of a-H agostic interactions.

Fig. 7 The dimeric titanium compound 95.

Scholz and coworkers demonstrated that titanium complexes of 1-aza-but-2-ene-1,4-diyl dianions can give way to lithium-stabilized alkylidene compounds.92 Treatment of Et2O solutions of [CpTi(Cl)(Z4-PhHCC(Me)]CHNR)] (R ¼ iPr, Cy) 96-R with methyl lithium (MeLi) in the presence of LiI at 0  C first produces the corresponding methyl complexes [CpTi(Me) (Z4-PhHCC(Me)]CHNR)] 97-R, but as the temperature increases, methane is extruded with concomitant a-H elimination from the 1-aza-but-2-ene-1,4-diyl ligand. In these titanate complexes, the Ti-center is ligated by the iodide anion and the Li cation is coordinated by the PhCC(Me)]CHNR3− fragment and an Et2O molecule. The orientation of the newly formed trianionic ligand and the location of the Li cation were dependent on the R substituent on the imide group. For instance, in [CpTi(I)(Z4-PhCC(Me)] CHNiPr)(LiOEt2)] 98-iPr (Fig. 8, left), the 1-aza-but-2-ene-1,4-diyl ligand adopts a supine geometry where the carbon atoms in the 2- and 3-position in the 1-aza-but-2-ene-1,4,4-triyl motif are tucked almost perpendicular to the Ti-Ca-N-Li plane. In this arrangement the Li cation is rendered opposite to the iodide ligand. In [CpTi(I)(Z4-PhCC(Me)]CHNCy)(LiOEt2)] 98-Cy (Fig. 8, right) the 1-aza-but-2-ene-1,4,4-triyl ligand is in a prone geometry where the alkene carbons in the 1-aza-but-2-ene-1,4,4-triyl point towards the titanium center. The Li coordination sphere is filled with highly polarized alkylidene fragment and bridging iodide ligands. The TidCa bond distances in these metallacyclic alkylidenes (R ¼ iPr, 1.958(3) A˚ ; R¼ Cy, 1.979(3) A˚ ) were shorter than those found in Ti–alkyl complexes and the distances between Ti and the 2- and 3-positions ( 2.3 A˚ ). 13C{1H} NMR spectral signals for a-carbons in CpTi(I)(Z4-PhCC(Me)]CHNCy)(LiOEt2) were found at low fields (98-iPr, d ¼244.7 ppm; 98-Cy, d ¼ 244.6 ppm).

Fig. 8 98-iPr in the supine geometry (left), and 98-Cy in the prone geometry (right).

366

Alkylidene Complexes of the Group 4 Transition Metals

The Scholz group showed a homoleptic titanium complex bearing two 1-aza-but-2-ene-1,4-diyl dianions, [(PhHCC(Me)] CHN(C6H3-2,6-iPr2))2Ti] 99 is susceptible to deprotonation by methyl lithium to the corresponding titanate complex 100 possessing a lithium-stabilized cyclic alkylidene fragment (Eq. 13).92 The TidCa bond distance observed by X-ray crystallography for the alkylidene functionality measured 1.978(3) A˚ , similar to 98-R,93 and notably shorter than the TidC distance of the second ligand which remained protonated (2.163(3) A˚ ). The alkylidene carbon atom resonance at 236.10 ppm in the 13C NMR spectrum is within the range expected of a titanium alkylidene.

ð13Þ

Throughout their examination of CdH bond activation reactions mediated by electron-rich titanium species, Stephan and coworkers found that addition of AlMe3 to [(tBu3PN)2TiMe2] produced a mixture of compounds which included the titaniumaluminum carbide [(m2-tBu3PN)Ti(m-Me)(m4-C)(AlMe2)2]2 101 and alkylene complexes [(tBu3PN)Ti(m2-tBu3PN)(m3-CH2)2(AlMe2)2(AlMe3)] 102 derived from metathesis and CdH bond cleavage reactions, respectively (Eq. 14).94 Thermolysis of 102 at 60  C for 26 h produced [Ti(m2-tBu3PN)2(m3-CH2)(m3-CH)(AlMe2)3] 103 and methane when judged by NMR spectroscopy.

ð14Þ

During the investigations of the reaction between heteroallenes, such as CS2, with the double-sandwich titanium dimer [Ti2Pn{2] (Pn{ ¼ 1,4-{SiiPr3}2C8H4) 104, the Cloke group isolated the titanium dimer [Ti2(m:Z5,Z5-Pn{)2(m:Z2,Z2-CS2)] 105.95 Although, the bridging CS2 unit might lack apparent structural features of traditional alkylidenes/alkylenes or related ligands, reactivity patterns have been suggested as a form of classification for this class of compounds.82 Complex 104 possesses two formal divalent titanium centers, which undergo a two-electron oxidation upon addition of a neutral CS2 molecule. This results in reduction of the CS2 to a dianionic CS2− 2 fragment supporting two trivalent titanium cations. Similar strategies have been employed to access other alkylidene complexes (Eq. 15).96,97

ð15Þ

3.10.6

Titanium alkylidene clusters

Santamaría and coworkers have synthesized an extensive array of titanium alkylenes of the type [{Ti(Z5-C5Me5)(m-O)}3(m-CHR) (ER’)] 106 (where R ¼ H, Me and ER’ ¼ alkoxide, iminate, OC-Mo(CO)2Cp, OC-W(CO)2Cp, and silanols)98,99 through protonation of the respective m3-alkylyne complexes [{Ti(Z5-C5Me5)(m-O)}3(m3-CR)] 107-R (R ¼ H, Me) with the corresponding alcohols as shown in Eq. (16).100

Alkylidene Complexes of the Group 4 Transition Metals

367

ð16Þ

The Yélamos laboratory found that exposure of a hexane solution of [Cp TiMe3] to an atmosphere of forming gas resulted in loss of methane, and obtention of the paramagnetic, tetrametallic, cubane cluster [(Cp Ti)4(m3-N)2(m3-CH)(m3-CH2)] 108 (Scheme 20).101 While in this complex the different bridging ligands were indistinguishable crystallographically, oxidation with either silver triflate (AgOTf ) or ferrocenium triflate (FcOTf ) provided diamagnetic [(Cp Ti)4(m3-N)2(m3-CH)(m3-CH2)][OTf] 109 (Scheme 20). In the crystal structure, a short contact between one of the methylidene group protons and one of the triflate oxygens provided a reference point for identification of this functional group. The average TidCmethylene distance was found to be 2.046(7) A˚ . Addition of chloroform-d resulted in detection of CDHCl2, [Cp TiCl3] and isolation of neutral, diamagnetic cluster [(Cp Ti)4(m3-N)2(m3-CH)2] 110 (Scheme 20).

Scheme 20 Synthesis and reactivity of cubane cluster [(Cp Ti)4(m3-N)2(m3-CH)(m3-CH2)].

A complex featuring a bidentate and chelating carbene could be generated via dilithiation of dimethyl (trimethylsilyl)phosphonate followed by addition of [TiCl(OiPr)3] in Et2O with traces of water (Fig. 9).102 This molecule 111 is a cluster comprised of two monolithiated titanium phophonate units, two equiv. of LiCl, one Li2O, lithiated dimethylphosphanate, and lithiated dimethylphosphate. The a-carbon of phosphoranoalkylidene resonance in the 13C{1H} NMR spectrum was found at 148.80 ppm. The crystal structure shows a short TidCa bond distance of 2.01 A˚ (Table 3).

Fig. 9 Cluster containing titanium phosphoranoalkylidene units.

368

Alkylidene Complexes of the Group 4 Transition Metals

Table 3

Summary table for different chemical properties of titanium alkylenes.

Compound #

Formula

Oxidation state

˚) M-C bond length (A

13

C chemical shift (ppm)

References

78-Ti 79 80

{[TiCHSi(Me)2N-k2-C,N]SiMe3[N(SiMe3)2]}2 [(Cy2N)2Ti(m CH2)2Ti(NCy2)2] (m-Me2C-3,30 ){(Z5Me5C5)[1-Me2Si(tBuN)][(m-CH2)Ti]}2

IV IV IV

271.4 224.7 227.18

83 84 85

82

[(m-CMe-3,30 ){(Z5Me5C5)[1-MeSi(tBuN)]}(m-CH)(m-CH)Ti] [B(C5F6)4] [CpR5(tBu2C]N)Ti(m2-CH2)(m2-Me)Ti(N]CtBu2)CpR5] [(m-C5Me4(SiMe2O))TiMe]2(m2-CH2) [(cb)2Ti(m-CHSiMe3)2Ti(cb)2] [Ti2(Z5-C5Me5)2(m-CH2SiMe2CH2)2(m-CHSiMe3)(m-O)] [(biCyO2)2Ti(]CHMe)(AlEt2)2] [Me2Si(Z5-2,3,4,6-Me4Ind)(CyN)Ti(m3-CH2)2(Al(C6F6)3)(AlMe)]2 [CpTi(I)(Z4-PhCC(Me)]CHNR)(LiOEt2)] [(Et2OLi)(PhHCC(Me)]CHN(C6H3–2,6-iPr2)) (PhCC(Me)] CHN(C6H3–2,6-iPr2))Ti] [(tBu3PN)Ti(m2-tBu3PN)(m3-CH2)2(AlMe2)2(AlMe3)] [Ti2(m:Z5,Z5-Pn{)2(m:Z2,Z2-CS2)] [(Cp Ti)4(m3-N)2(m3-CH)(m3-CH2)][OTf] [Ti2(C(SiMe3)P(O)(OMe)2)2(OiPr)6Li8Cl2O(P(O)(OMe)2)(PO2(OMe)2)]

IV

N/A 2.020(5), 2.016(5) 2.038(4), 2.141(4), 2.136(4) 2.08(8)–2.178(9)

227.37

85

IV IV IV IV IV IV IV IV

N/A 2.0719(13) 2.026(2), 2.035(2) 2.100(8) 1.933(6) 2.195(3), 2.230(3) 1.958(3), 1.979(3) 1.978(3)

25.30 196.9 N/A 231.2 202.1, 192.2 N/A 244.7, 244.6 236.1

86 87 88 89 90 91 92 93

IV III IV IV

2.013(7), 2.125(6) 2.27, 2.24 2.046(7) 2.01

113.3, 88.6 355.5 N/A 148.8

94 95 101

85 87 89 91 93 95 98-iPr, Cy 100 102 105 109 111

3.10.7

Zirconium methylidenes

In 1979, the Schwartz group started the quest for zirconium methylidenes complexes inspired by the successful synthesis of [Cp2Nb]CH2(CH3)], which is readily obtained via methylene transfer from H2CPPh3 to the low-valent niobium complex [Cp2NbCH3].97 In this vein, the formally divalent zirconium complex [Cp2Zr(PPh2Me)2] 112103 is a suitable reagent to mimic this transformation as it provides the two electrons needed for reduction of H2CPPh3 to PPh3 accompanied with concomitant release of the oxidized zirconium complex [Cp2Zr]CH2(PPh2Me)] 113 (Fig. 10, left). This reaction proceeds with a clear color change, but the postulated zirconium methylidene was too reactive for isolation.96 Nevertheless, evidence of its formation from NMR spectroscopy, labelling experiments, and reactivity studies are in accord with the proposed formulation, [Cp2Zr] CH2(PPh2Me)]. More than 30 years later, Mindiola and coworkers revisited the synthesis of zirconium methylidenes employing the sterically encumbering ligand framework (PNP) in combination with a bulky aryloxide moiety (OAr) [Ar ¼ {2,6-CH(CH3)2}2 C6H3] to generate a Zr complex with lower coordination number but with the appropriate kinetic stabilization.104 To this end, photolysis of [(PNP)Zr(CH3)2(OAr)] 114 induced methyl a-hydrogen abstraction and extrusion of CH4 to cleanly deliver a stable zirconium methylidene complex [(PNP)Zr(]CH2)(OAr)] 115-Zr (Fig. 10, center). The methylidene group in 115-Zr exhibits characteristic 1H and 13C{1H} NMR resonances which unambiguously correlate to the CH2 unit, as demonstrated by a 1Hd13C HSQC experiment. The solid-state structure obtained from synchrotron radiation of a small crystal shows a square pyramidal complex with equatorial PNP− and ArO− ligands and an apical methylidene moiety. Interestingly, a calculated molecular orbital diagram depicts destabilizing Zr]C p-bonding and stabilizing Zr]C p -antibonding interactions. Other zirconium methylidenes such as [H2Zr]CH2]31 116-H and [HFZr]CH2]105 116-F have been generated from laser-ablated zirconium atoms in the presence of methane and methyl fluoride, respectively, by the Andrews group (Fig. 10, right) (Table 4).

Fig. 10 Examples of zirconium methylidenes.

Alkylidene Complexes of the Group 4 Transition Metals

Table 4

369

Summary table for different chemical properties of zirconium methylidenes.

Compound #

Formula

Oxidation state

˚) M-C bond length (A

13

References

113 115-Zr

[Cp2Zr]CH2(PPh2Me)] [(PNP)Zr(]CH2)(OAr)]

IV IV

N/A 2.038(6)

248.6 230.2

103 104

3.10.8

C chemical shift (ppm)

Terminal zirconium alkylidenes

Fryzuk and coworkers reported the first stable and structurally characterized zirconium alkylidene complex which features a sterically demanding Cp-type ligand with two pendant phosphines [(P2Cp)Zr]CPhH(Cl)] (P2Cp ¼ {Z5-C5H2-1,3-(SiMe2CH2-iPPr2)2}) 117-Ph (Fig. 11).106 Complex 117-Ph is obtained from the photolyis or thermolysis of mixtures containing equimolar amounts of [(P2Cp)ZrCH2Ph(Cl)2] and [(P2Cp)Zr(CH2Ph)3]. The benzylidene fragment is identified by deshielded 1 H and 13C{1H} NMR spectroscopic resonances at 8.10 and 229.40 ppm, respectively, and the solid-state metrics indicate zirconium-carbon double bond character [Zr-Cbenzylidenic ¼ 2.024(4) A˚ ]. The same group reported a similar zirconium neopentylidene complex [(P2Cp)Zr]CHtBu(Cl)] 117-tBu, employing an analogous synthetic strategy.107

Fig. 11 First structurally characterized zirconium alkylidene complex.

The Ozerov group was able to obtain zirconium alkylidene species [(PNP)Zr(CHR)(CH2R)] (R ¼ phenyl, p-tolyl) 118-R by ahydrogen abstraction induced by alkyl elimination from the corresponding trisalkyl complex (Eq. 17).108 Variable temperature NMR spectroscopic studies provided kinetic data for this reaction consistent with a first-order rate law. Parameters taken from an Eyring plot yield △H{ ¼ 19(1) kcal/mol; △S{ ¼ −14(3) cal/(mol K); △G{298 ¼ 23(2) kcal/mol - this low barrier accounts for the ease of arene elimination. While no crystal structures were obtained for these alkylidene complexes, low-field chemical shifts in the 1H NMR spectra for the alkylidene protons (118-Ph: 7.32; 118-pTol: 7.11 ppm) as well as the respective carbon atoms in the 13C{1H} NMR spectra (118-Ph: 230.70 ppm; 118-pTol: 230.1 ppm) support the stated structural assignment.

ð17Þ

3.10.9

Heteroatom substituted zirconium alkylidenes

The Bercaw group reported the low-temperature reaction between [Cp2Zr(CO)(L)] 119-L (L ¼ CO,109 PMe3110) and [Cp 2ZrH2]111 to give the respective binuclear zirconium complexes [Cp 2(L)Zr]CHOdZr(H)Cp 2] 120-L.112 This transformation is outstanding as it is reminiscent of an archetypical Fischer-type carbene synthesis113 with early metals working in concert. Specifically, the CO ligand oxygen atom in 119-L acts as a nucleophile and attacks the zirconium center of [Cp 2ZrH2], this step is followed by hydride migration from the latter complex to the bridging CO ligand furnishing 120-L. Further reactivity studies shows that 120-CO reacts with CH3I and [Cp 2(PMe3)Zr]CHO −Zr(I)Cp 2] 120dI with CO to produce enediolate zirconocyle species [Cp2IZr −(CO] CHOZrCp 2)-k2O,O0 ] 121 (Scheme 21). Addition of pyridine to 120-CO induces intramolecular cyclometallation to the zirconium ketene complex [Cp2(py)Zr(O]C]CHOdZr(H)Cp 2]. 120-I is readily hydrolysed with excess HCl to yield [Cp2ZrCl2], [Cp 2Zr(OCH3)I], and free PMe3 (Scheme 21).

370

Alkylidene Complexes of the Group 4 Transition Metals

Scheme 21 Synthesis and reactions of [Cp 2(L)Zr]CHOdZr(H)Cp 2].

Alkylidenes have also been employed to access uncommon electron counts in zirconium complexes.65 The Cavell group reported a zirconium complex featuring a supported alkylidene ligand with iminophosphine side-arms [ZrCl2{C(Ph2P] NSiMe3)2-k3C,N,N0 }] 123 whose metal center features 12 valence electrons. This complex is readily obtained from lithium chloride elimination between (CLi2(Me2P]NSiMe3)2 and [ZrCl4(THF)2] (Scheme 22). The iminophosphine side-arms have a negligible effect on the alkylidenic carbon nucleus shielding as evidenced by 13C{1H} NMR spectroscopy. The solid-state structure of 123 is best described as two fused four-membered metallacycles whose vertex is defined by the zirconium and alkylidenic carbon atoms. The two sets of C, N, and P atoms arrange in a slightly distorted planar geometry, possibly due to steric repulsion between the bulky SiMe3 groups. The alkylidenic carbon is slightly puckered from this plane thus exhibiting a zirconium-carbon bond distance of Zr-Calkylidenic ¼ 2.190(8) A˚ . The alkylidene’s nucleophilicity in 123 is demonstrated by its protonation with 1-adamantanol (AdOH) to produce the alkoxo complex [ZrCl2{CH(Ph2P]NSiMe3)2-k3C,N,N0 }(OAd)] 124 (Scheme 22). Further experimental studies with Lewis bases, electrophiles, and cumulenes help to illustrate the nucleophilic character of 123.114

Scheme 22 Synthesis of ZrCl2{C(Ph2P]NSiMe3)2-k3C,N,N0 } and reactivity towards AdOH.

The same group reported the highly-symmetric zirconium bisalkylidene complex [Zr{C(Me2P]NSiMe3)2}2] 125 using a slightly modified synthetic protocol where two equiv. of CH2(Me2P]NSiMe3)2 were reacted with [Zr(CH2Ph)4] at high temperatures (Eq. 18).115 The most salient features of this system is the high degree of charge delocalization across both ligands as evidenced by elongated zirconium-carbon bonds obtained from the solid-state structure. This latter feature might argue against a legitimate alkylidene unit, but such lengthening can be attributed to steric congestion around the metal center as well as trans-influence of the carbenes upon each other.

ð18Þ

The So group reported an asymmetric variant of Cavell’s system, nominally [Zr{NMe2}2(CRNRS)] (RN ¼ PPh2NSiMe, RS ¼ PPh2S) 126, which is readily obtained in two steps from dimethylamine elimination between CH2(CRNRS) and [Zr(NMe2)4].116 The alkylidenic carbon exhibits unusually upshifted 13C{1H} NMR resonances (56.00 ppm) and unremarkable bond lengths in the solid-state. Interestingly, 126 reacts with water to produce the bridging oxo zirconium complex O{[ZrC{NMe2}(CRNRS)]}2 127

Alkylidene Complexes of the Group 4 Transition Metals

371

whose alkylidene remains intact. This latter compound can further insert adamantyl isocyanate across a zirconium-amide bond to deliver the ureate complex O{[Zr[OC{NMe2)NAd(CRNRS)]}2 128 (Scheme 23).

Scheme 23 Reactivity of [Zr{NMe2}2(CRNRS)].

The same group also reported the zirconium alkylidene complex [{1,3-C6H4(PhPS)2C(S)}Zr(NHMe2){C(PPh2S)2}] 129 as part of their investigation regarding the sulfur-atom-transfer properties of [{m-1,3-C6H4(PhPS)2C}Sn]2.117 The Bourissou group published a similar pincer-type zirconium alkylidene complex [Zr(NMe2)2(Ind-1,8(PPh2]NMes)2)] (Ind ¼ C9H4 Mes ¼1,3,5Me3C6H3) 130 (Eq. 19) that is synthesized in an analogous fashion to the Cavell and So methods.118 This latter compound was not stable enough to obtain a solid-state structure, however, spectral features are consistent with its proposed formulation. As expected for a Schrock-type carbene, computed molecular orbitals and atoms in molecules studies show a ligand-based HOMO with significant carbene character and a metal-centric LUMO.

ð19Þ

Le Floch and coworkers have accessed other zirconium alkylidene complexes of the type [Cp2Zr(C(Ph2PS)2)] 131 and [{C(Ph2PS)2} ZrCl(THF)]2(m2-Cl)2 132.119 The dimeric arrangement of 132 is interrupted upon treatment with excess pyridine (pyr) to produce two equiv. of the Lewis base adduct [{C(Ph2PS)2}Zr(pyr)2Cl2] 133. Both 132 and 133 exhibit reactivity patterns akin to Schrock-type carbenes since the [C(Ph2PS)2] fragment can be readily transferred to aromatic and aliphatic substituted aldehydes to produce geminal bis(diphenylthiophosphinoyl) olefins (Eq. 20). DFT studies rationalize this type of reactivity as most of the electronic density in the HOMO resides on the a-carbon.

ð20Þ

Furthermore, 132 is a versatile alkylidene transfer reagent to late-metal complexes such as CoCl2, RuCl2(PPh3)3, and PdCl2(PPh3)2 where the mono or dimeric arrangement depends on the late-metal complex in question (Scheme 24).120

Scheme 24 Alkylidene transfer properties of 132.

372

Alkylidene Complexes of the Group 4 Transition Metals

Table 5

Summary table for different chemical properties of zirconium alkylidenes.

Compound #

Formula

Oxidation state

˚) M-C bond length (A

13

117-Ph, CMe3, SiMe3

[(P2Cp)Zr]CHR(Cl)]

IV

2.024(4), (N/A for CMe3),

106,107

118-Ph, pTol 120-PMe3, CO 120-I 123 125 126 127 128 129

[(PNP)Zr(CHR)(CH2R)] [Cp 2 (L)Zr]CHOdZr(H)Cp 2] [Cp 2 (PMe3)Zr]CHOdZr(I)Cp 2] [ZrCl2{C(Ph2P]NSiMe3)2-k3C,N,N0 }] [Zr{C(Me2P]NSiMe3)2}2] [Zr{NMe2}2(CRNRS)] O{[ZrC{NMe2}(CRNRS)]}2 O{[Zr[OC{NMe2)NAd(CRNRS)]}2 [{1,3-C6H4(PhPS)2C(S)} Zr(NHMe2){C(PPh2S)2}] [Zr(NMe2)2(Ind-1,8(PPh2]NMes)2)] [Cp2Zr(C(Ph2PS)2)] [{C(Ph2PS)2}ZrCl(THF)]2(m2-Cl)2 [{C(Ph2PS)2}Zr(pyr)2Cl2]

IV IV IV IV IV IV IV IV IV

N/A N/A 2.117(7) 2.190(8) 2.288(3) 2.237(3) 2.397(2), 2.409(2) 2.231(2), 2.240(2) 2.2243(15)

229.4, 209, 213.8 230.7, 230.1 287.5, 295.0 286.3 101.7 77.8 56.0 41.27 52.8 N/A

IV IV IV IV

2.22(5) 2.251(2) 2.180(3) 2.172(2)

209 32.8 100.8 97.0

118 119 119 119

130 131 132 133

C chemical shift (ppm)

References

108 112 112 65 115 116 116 116 117

The Gessner and Kaup groups reported an exceptionally detailed theoretical comparison of many metal alkylidene complexes including Cp2Zr[C(Ph2PS)(Cy2PS)]121 where natural bond orbital analysis and Wiberg bond indices indicate electronic structures in accordance with Schrock-type carbenes (Table 5).

3.10.10

Bridging zirconium methylenes

Jordan and coworkers reported the first well-defined dinuclear zirconium methylene complex [(Cp )(C2B9H11)Zr]2(m2-CH2) 134 obtained via methane elimination from the polymer [(Cp )(C2B9H11)ZrMe]x 135.122 The Bochman group reported spectroscopic evidence for the formation of [(CpZr)2(m2-CH3)(m2-CH2)(Z5:Z5-C10H8)][B(C6F6)4] 136.123 In 2000, the Sita group published the solid-state structure of the cationic zirconium methylene dimer [{Cp ZrMe[N(tBu)C(Me)N(Et)]}2(m-CH2)(m-CH3)][B(C6F6)4]2 137 (Fig. 12), which is implicated as a relevant species in olefin polymerization reactions.124 Zirconium alkylenes have also been employed to study fundamental aspects of carbon coordination to transition metals as shown by the Bickelhaupt group who attempted to access a planar, yet four-coordinate carbon center in [(m2-CH2)(ZrCp2Me)2] 138 (Eq. 21).125 However, the geometry around the methylene ligand is best described as tetrahedral as revealed by XRD with a ZrdCH2dZr angle of 133.4 and slightly different ZrdCH2 bond lengths of Zr1dCH2 (2.225 A˚ ) and Zr2dCH2 (2.242 A˚ ). In the solid-state both methyl groups crystallize on opposite sides of the axial plane defined by the zirconium centers rendering 138 in an “antiperiplanar’ conformation. 1H NMR spectroscopy shows that this structural asymmetry is not retained in solution even at — 100  C as both methyl ligands give rise to one singlet at −0.36 ppm in toluene-d8 and at −0.46 ppm in THF-d8. The solvent polarity has an inconsequential effect in the methylenic protons’ chemical shift as shown by values of 3.68 ppm in toluene-d8 and a negligible upshifted resonance of 3.76 ppm in THF-d8. As expected for a metal-bridged methylene unit, its 13C NMR spectroscopic resonance is upshifted and located at 146.00 ppm with a small JCH coupling constant of 107 Hz.

Fig. 12 A crystallographically characterized Ziegler-Nata polymerization catalyst.

Alkylidene Complexes of the Group 4 Transition Metals

373

ð21Þ

O’Hare and coworkers employed a permethylated pentalene ligand (Pn ¼ C8Me6) to support dinuclear zirconium methylene species of the type [(Pn Zr)2(m2-CH2)(m2-Me)2] 139 (Eq. 22), for which reactivity and computational analyses have yet to be studied.126

ð22Þ

Zirconium bridging methylene units are also obtained from hydrogenolysis of metal alkyls as shown by the Mindiola group, who synthesized the dimeric zirconium complex [(PNP)Zr(CH3)]2(m2-H)2(m2-CH2) 140 from pressure controlled hydrogenation of [(PNP)Zr(CH3)3] (Eq. 23).127 An X-ray diffraction experiment reveals ZrdCH2dZr bond distances of 2.209(4) and 2.183(4) A˚ and even both m2-H ligands can be detected in the difference map exhibiting almost identical ZrdH interatomic distances of 2.09(5) and 2.000(5) A˚ . The 1H NMR spectrum of this molecule shows two signals with roughly equal intensity: a broad triplet located at 5.86 ppm and a broad resonance at 4.86 ppm. The low yield and poor solubility of this material precludes the acquisition of reliable 13 C NMR spectra (Table 6).

ð23Þ

Table 6 Compound # 134 136 137 138 139 140

3.10.11

Summary table for different chemical properties of zirconium bridging methylidenes. Formula 

[(Cp )(C2B9H11)Zr]2(m2-CH2) [(CpZr)2(m2-CH3)(m2-CH2)(Z5:Z5-C10H8)][B(C6F6)4] [{Cp ZrMe[N(tBu)C(Me)N(Et)]}2(m-CH2)(m-CH3)][B(C6F6)4]2 [(m2-CH2)(ZrCp2Me)2] [(Pn Zr)2(m2-CH2)(m2-Me)2] [(PNP)Zr(CH3)]2(m2-H)2(m2-CH2)

Oxidation state

˚) M-C bond length (A

13

References

IV IV IV IV IV IV

2.187(6) N/A 2.302(3) 2.225, 2.242 2.278(5), 2.257(5) 2.209(4), 2.183(4)

N/A 118.2 N/A 146.0 127.9 N/A

122 123 124 125 126 127

C chemical shift (ppm)

Bridging zirconium alkylenes

The first structurally characterized bridging zirconium alkylene complex was reported by the Anderson group more than 30 years ago.83 This compound was obtained from the thermolysis of neat [ZrR2(N(SiMe3)2)] 141-R (R ¼ Me, Et, N(SiMe3)2) which induced formation of free RH and a yellow oil with 1H and 13C NMR spectroscopic resonances reminiscent of then-reported alkylidene complexes, although, molecular connectivity remained ambiguous as two possible constitutional isomers could possibly be present in solution (Fig. 13A and B). The structure of this compound was conclusively established through a single-crystal X-ray diffraction experiment which disclosed two fused and planar zirconacyles formed from doubly deprotonated SiMe3 groups to yield the title compound {[ZrCHSi(Me)2N-k2-C,N]SiMe3[N(SiMe3)2]}2 142 (Fig. 13B).83

374

Alkylidene Complexes of the Group 4 Transition Metals

Fig. 13 Possible constitutional isomers of 142 formed in the thermolysis of 141-R.

The kinetics of zirconium-silylalkyl g-H deprotonation to yield alkylidenes was thoroughly investigated by the Lin and Xue groups in the formation of {[ZrCHSi(Me)2N-k2-C,N]SiMe3[NMe2]}2 143 from (Me2N)Zr[N(SiMe3)2]2(SiPht2Bu).128 Another dinuclear zirconium alkylidene complex [Zr(Cp”)Me(Z4-2,3-dimethyl-butadiene)] (Cp”¼ C5H3(SiMe3)2-1,3) 144 was obtained through intramolecular methylene deprotonation of the parent zirconium butadiene complex [Zr(CpB)Me(Z4-2,3-CH3CH] CHCH3)] by the Bochmann group.129 Schrock and coworkers reported a dimeric zirconium complex 145 whose alkylidene fragments are derived from intramolecular deprotonation of two pendant methyl groups in the supporting bis(borylamido) ligand [Mes2BNCH2CH2NBMes2]2− (Fig. 14A).130 Similar reactivity was observed by the group of Floriani to produce cylometallated zirconium alkylene species 146 (Fig. 14B).131,132

Fig. 14 Selected examples of intramolecularly cyclometallated and bridging alkylene ligands.

The group of Floriani accessed the dinuclear zirconium [{(Z8-C8H8)(Zr(OAr)}2(m2-CH2)(m2-O)] (OAr ¼ 2,6-tBu2C6H3) 147 through CO insertion and reductive activation by the bridging zirconium dihydride [{(Z8-C8H8)(Zr(OAr)}2(m2dH)2] 148 (Eq. 24).132 In the solid-state, {(Z8-C8H8)(Zr(OAr)}2(m2-CH2)(m2-O) crystallizes as a centrosymmetric molecule with elongated ZrdCH2dZr bonds (2.470 A˚ (avg)) and an angle of 115.9(6) . The CH2dO interatomic distance of 1.313(14) A˚ is suggestive of a single bond and thus the [(m2-CH2)(m2-O)] fragment is best described as formaldehyde ligand. However, the presence of a -CH2group bridged between two zirconium centers renders [{(Z8-C8H8)(Zr(OAr)}2(m2-CH2)(m2-O)] as a methylene complex. In line with this, Floriani and Gambarotta have reported a related zirconium complex [(Cp2ZrCl)2(m-CH2O)] whose “oxymethylene unit” is similarly derived from hydride to CO addition steps.133 The bond metrics of this later compound are unremarkable as shown by an X-ray crystallographic analysis. These early examples of CO reduction into CH2O assisted by zirconium-hydride complexes laid the foundation for the development of related systems that cleave the remarkably strong C^O bond, allowing further functionalization of this particularly relavant molecule.

Alkylidene Complexes of the Group 4 Transition Metals

375

ð24Þ

Kawaguchi and coworkers reported the reaction of [{[LR]Zr}2(m2-H)(m2-CHPh)] (LR2− ¼ 2,6-bis(3-tert-butyl-5-methyl-2-oxybenzyl)-4-R-anisole) 149 with dihydrogen to produce the bridging zirconium alkylene complex [{[LR]Zr}(m2-H2)(m2-CHPh)] 150.134 This latter compound is relevant to Fischer-Tropsch processes as it mediates the upgrading of CO to the hydrogenated and oxygen free organic product phenylallene (Ph2CCH2) with the aid of well-defined zirconium alkylene species. Specifically, this transformation consists of the zirconium mediated coupling of benzylene and hydride ligands to two CO molecules through subsequent insertion-deoxygenation steps with ultimate release of a dinuclear zirconium-oxo complex [{[LR]Zr}2(m2-O)] 151 and Ph2CCH2 (Scheme 25).

Scheme 25 Group transfer chemistry and synthetic cycle of {[LR]Zr}(m2-H2)(m2-CHPh) for CO upgrading.

Erker and coworkers reported the zirconium complex [(Cp2Zr)2(m2-CH−CH2B(C6F5)3})(m2-Cl)] 152 as a model to evaluate methane’s geometrical variations upon coordination to transition metals (Fig. 15).135 The solid- state structure of 152 shows a quite distorted rhombic-type core whose vertices are defined by two opposite zirconium centers and face to face chloride and methylene ligands. The ZrdCHR bond distance is particularly short (2.156(3) A˚ ) and the authors attribute this feature to donation from the highly polarized methylene ligand into the electron- deficient zirconocene center. The low-symmetry of 152 is retained in solution as revealed by 1H NMR spectroscopy. Furthermore, an enantiomeric rearrangement of the {m2-CHdCH2B(C6F5)3} fragment was determined by variable-temperature NMR spectroscopy, providing a DH{ value ca. to 12.01  0.5 kcal/mol for this process.

45

Fig. 15 Erker’s zirconium methylene complex 152.

376

Alkylidene Complexes of the Group 4 Transition Metals

The Norton group synthesized the dinuclear zirconium alkylene complexes [((CpR)2Zr)2(mCHCHNtBu)(m2-NtBu)] (R ¼ H, Me) 153-R while investigating alkyne cycloaddition reactions mediated by zirconium amide reagents.136 Specifically, compounds 153-R react with an equimolar ratio of HC^CH to produce the dimeric complexs [((CpR)2Zr)2(m2-CHCHNtBu)(m2-NtBu)] 154-R (Eq. 25). The solid-state structure of 154-H shows a particularly symmetrical Zr2(m2-C)(m2-N) core with similar Zr-NtBu bond lengths of Zr1-NtBu 2.092(4) and Zr2-NtBu 2.091(5) in addition to Zr-Calkylene interatomic distances of Zr1-C 2.267(6) and Zr2-C1 2.258(6) which are elongated in comparison to other bridging zirconium alkylenes.

ð25Þ

3.10.12

Zirconium alkylene clusters

Alkylene ligands can also arrange in cubane-type structures as shown by the Yelamos group who synthesized [{Zr(Z5-C5Me5)}3{(m3-CH)3SiMe}(m3-CSiMe3)] 155-Zr via heat-induced SiMe4 elimination from [Zr(Z5-C5Me5)(CH2SiMe3)3] (Eq. 26).137 In solution, 155-Zr is highly fluxional, producing 1H, 13C{1H} and 29Si NMR spectra with a single chemical environment for each C5Me5, (m3-CH)3SiMe and m3-CH unit. These features render 155-Zr with overall C3V symmetry on the NMR time scale. An X-ray diffraction experiment on a single crystal of 155-Zr reveals a distorted cubane-type cluster whose inner angles range from 84.5(2) to 102.9(2) . The Zr(Z5-C5Me5) units exhibit a typical three-legged piano stool geometry whose apical ligands are defined by the interstitial clusters’ CH and CSiMe3 groups.

ð26Þ

Roesky and coworkers studied the properties of the half sandwich zirconium complex [Cp ZrF3] to gain insights into the underexplored properties of group 4 fluorides towards olefin polymerization. Guided by archetypical approaches to generate polymerization catalysts, [Cp ZrF3] was treated with an equimolar amount of AlMe3. This reaction produced the dimeric zirconium complex cis-{[Cp ZrMe(m2-F)][(m2-F)2Al(CH3)2]}2 156 in a stereoselective fashion. Performing the same reaction with an excess of AlMe3 generates the polymetallic cluster [(Cp Zr)3Al6(CH3)8(CH)5(CH2)2] 157.137 The characterization of this molecule was carried on with the aid of mass spectroscopy and X-ray crystallography methods, however, it was found later by the same group that their molecular formulation was inaccurate.138 Subsequent 1H NMR spectroscopy studies proved the hapticity of the interstitial ligands in 157 to be [(Cp Zr)3Al6(CH3)8(m4-CH)4(m3-CH)(m3-CH2)2 (Fig. 16) (Table 7).

Fig. 16 Polymetallic zirconium-aluminum cluster featuring interstitial alkylene units.

Alkylidene Complexes of the Group 4 Transition Metals

Table 7 Compound #

Summary table for different chemical properties of zirconium alkylenes. Formula 2

Oxidation state

˚) M-C bond length (A

13

References

2.16(2), 2.21(2) 2.231(8)-2.252(6) 2.574(14), 2.366(14) (R]Me only) 2.335(3), 2.165(3) 2.156(3), 2.396(3) 2.0.267(6), 2.258(6)

201.4 N/A N/A (R]tBu only) 176.6

83 131 132 134

174.7 166.7

135 136

78-Zr 146 147 150

{[ZrCHSi(Me)2N-k -C,N]SiMe3[N(SiMe3)2]}2 [(COT)2Zr2(m2-Mes)(m2:k2(Cipso,CH)-C6H2-4,5-Me2(CH))] [{(Z8-C8H8)(Zr(OAr)}2(m2-CH2)(m2-O)] [{[LR]Zr}(m2-H2)(m2-CHPh)]

IV IV IV IV

152 154-H

[(Cp2Zr)2(m2-CH−CH2B(C6F5)3})(m2-Cl)] [((Cp)2Zr)2(m2-CHCHNtBu)(m2-NtBu)]

IV IV

3.10.13

377

C chemical shift (ppm)

Hafnium methylidenes

Mindiola and coworkers reported the first hafnium methylidene complex [(PNP)Hf]CH2(OAr)] 115-Hf (Section 3.10.7), obtained in analogous fashion to the zirconium congener [(PNP)Zr]CH2(OAr)] 115-Zr.104 The spectroscopic properties of 115Hf are in agreement with its formulation. Specifically, an 1Hd13C HSQC NMR experiment matches up the high-field 13C methylene resonance of 224.30 ppm with the protons of the []CH2] fragment at 8.09 ppm in the 1H NMR spectrum. Additionally, the negative DEPT-135 trace unquestionably proves the later ligand as a CH2 moiety. Andrews and coworkers generated [(H)(F)Hf]CH2] 158 from the laser ablation of hafnium atoms and methyl fluoride on solid argon matrices.139

3.10.14

Hafnium alkylidenes

The Fryzuk group reported the first structurally characterized hafnium alkylidene complex [(P2Cp)Hf]CPhH(Cl)] 159.140 Although this complex is isostructural in the solid-state to its zirconium congener 117-Ph,106 its solution behavior differs as it exists as mixture of conformational isomers due the lability of the phosphine side-arms to the harder Hf(IV) ion. Cavell and coworkers reported the metallation of (CH2(R2P]NSiMe3)2 (R ¼ Ph, Cy) with [HfCl2{N(SiMe3)2}2]141 to produce the hafnium alkylidene complexes of the type [HfCl2{C(R2P]NSiMe3)2-k3C,N,N0 }]142 160 whose structural features resemble the zirconium congeners in the solid-state.65 Lewis bases such as L (L ¼ THF and AdCN) readily bind to the metal center of 160 to produce the respective adducts [HfCl2(L){C(Ph2P]NSiMe3)2-k3C,N,N0 }] 161-L.114 Such reactivity patterns can be attributed to the metal’s electrophilicity, low-electron count, and coordinative unsaturation. Further reactions with weak acids demonstrate the nucleophilicity of the alkylidene moiety as this unit is susceptible to protonation. The asymmetrical hafnium alkylidene complex [Hf{NMe2}2(CRNRS)] 162 was reported by So and coworkers through double methylene deprotonation of CH2(CRNRS) with [Hf(NMe2)4].116 This alkylidene complex produces the bridging hafnium-oxo O[Hf(NMe2)(CRNRS)]2 163 upon treatment with degassed water. This reactivity pattern is reminiscent of the zirconium congener.116 The same group also reported a coordinatively saturated hafnium alkylidene-type complex [{1,3-C6H4(PhPS)2C(S)}Hf(NHMe2){C(PPh2S)2}] 164 (Fig. 17).117

Fig. 17 Hafnium complex 164 featuring dianionic thioketone ligands “1,3-C6H4(PhPS)2CS2−”and geminal dianions (Ph2PS)2C2−.

3.10.15

Hafnium bridging alkylenes

Teuben and coworkers reported the dihafnium complex [Cp Hf(Cl)(CHCHCH2C(Me)]C(Me)CH2-m2-CH-CH2)]2 165 whose bridging alkylene ligands are derived from acetylene insertion into the Hf-C bonds in [Cp Hf(2,3-dimethyl-1,3-butadiene)Cl] followed by subsequent tautomerization steps (Eq. 27).143 The crystal structure of 165 shows a planar and practically rectangular Hf2C2 ring with metal-alkylene bond distances of Hf-C1 ¼ 2.134 (5) A˚ and Hf-C2 ¼ 2.246(6) A˚ . These structural features are consistent with the formal reduction of the bound acetylene CdC bond order to a single bond.

378

Alkylidene Complexes of the Group 4 Transition Metals

ð27Þ

The Lin and Xue groups thermolyzed [(Me2N)Hf(N(SiMe3)2)2(SiPht2Bu)] to obtain the bridging (bis)alkylene hafnium complex {[HfCHSi(Me)2N-k2-C,N]SiMe3[NMe2]}2 166 through g-H elimination in the supporting silylamide ligands.128

3.10.16

Hafnium alkylene clusters

The cubane hafnium-alkylene complex [{Hf(Z5-C5Me5)}3{(m3-CH)3m3-SiMe}(m3-CSiMe3)] 155-Hf was obtained following the same protocol for synthesis of the parent zirconium cluster reported by the Yelamos group (Fig. 18).137 Both 155-Hf and 155-Zr exhibit the same molecular symmetry as revealed by NMR spectroscopy and similar bond metrics in the solid-state. The Roesky group reported the alkylation of [Cp HfF3] with excess equiv. of AlMe3 to produce [Cp Hf(CH3)3] in high yields in addition to trace amounts of the polynuclear alkylene cluster [(Cp Hf )3Al6(CH3)8(m3-CH2)2(m4-CH)4(m3-CH)] 167 (Eq. 28)138 (Table 8).

ð28Þ

Fig. 18 Hafnium-cubane cluster featuring bridging alkylene ligands. Table 8

Summary table for different chemical properties of hafnium complexes.

Compound #

Formula

Oxidation state

˚) M-C bond length (A

13

References

159 160 161-AdCN 162 163 164 165 166

[(P2Cp)Hf]CPhH(Cl)] [HfCl2{C(R2P]NSiMe3)2-k3C,N,N0 }] [HfCl2(NCAd){C(Ph2P]NSiMe3)2-k3C,N,N0 }] [Hf{NMe2}2(CRNRS)] O[Hf(NMe2)(CRNRS)]2 [{1,3-C6H4(PhPS)2C(S)}Hf(NHMe2){C(PPh2S)2}] [Cp Hf(Cl)(CHCHCH2C(Me)]C(Me)CH2-m2-CH-CH2)]2 {[HfCHSi(Me)2N-k2-C,N]SiMe3[NMe2]}2

IV IV IV IV IV IV IV IV

1.994(4) N/A 2.211(6) 2.218(2) 2.363(4), 2.388(5) 2.218(5), 2.407(5) 2.134(5), 2.246(6)

210 71.6 83.4 56.0 40.2 N/A 190.9

140 142 142 116 116 117 143 128

C chemical shift (ppm)

Alkylidene Complexes of the Group 4 Transition Metals

3.10.17

379

Summary

Group 4 metal constructs have enabled the generation of important alkylidene and alkylene ligands through conventional methods but also from unexpected sources and transformations. Many of these complexes possess valuable attributes for the study of well-established olefin metathesis and polymerization reactions, but also instigate the exploration of other chemistries. For instance, in recent years the recognition of greater diversity in reactivity is exemplified by the activation of CdH bonds and CO splitting among other challenging transformations. In tandem, a broadened scope of supporting ligands beyond cyclopentadienyls have been made available, providing synthetic chemists with a larger pool of options when designing and exploring new systems. Taken together, these advances lay a foundation into how to leverage the stability of group 4 metals alkylidenes and alkylenes at directing specific reactivity and obtaining new functionalities. Translating some of the stoichiometric reactions of group 4 metals alkylidenes and alkylenes such as CdH bond functionalization as well as CO upgrading and cleaving into a catalytic context stand as a key goals in this field. Additionally, the redox chemistry of group 4 metal alkylidenes and alkylenes remains to be developed.

Acknowledgments DJM thanks the Dreyfus Foundation, the Alfred P. Sloan Foundation, the Camille and Henry Dreyfus Foundation (Teacher-Scholar Award to D.J.M.), and the NSF (CHE-0348941-PECASE award, and CHE-0848248 and CHE-1152123), the JSPS, and Indiana University and the University of Pennsylvania for past and current financial support of some of the research being described.

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3.11

Alkenes and Allyl Complexes of the Group 3 Metals and Lanthanides

Maxime Beauvoisa, Yohan Champoureta, Fanny Bonnetb, and Marc Visseauxa, aUniv. Lille, CNRS, Univ. Artois, Centrale Lille, UMR 8181 - UCCS - Unité Catalyse et Chimie du Solide, Lille, France; bUniv. Lille, CNRS, INRAE, Centrale Lille, UMR 8207 - UMET - Unité Matériaux et Transformations, Lille, France © 2022 Elsevier Ltd. All rights reserved.

3.11.1 3.11.2 3.11.2.1 3.11.2.2 3.11.3 3.11.3.1 3.11.3.1.1 3.11.3.1.2 3.11.3.1.3 3.11.3.1.4 3.11.3.1.5 3.11.3.2 3.11.3.2.1 3.11.3.2.2 3.11.3.2.3 3.11.3.3 3.11.3.3.1 3.11.3.3.2 3.11.3.3.3 3.11.3.3.4 3.11.3.3.5 3.11.3.3.6 3.11.3.4 3.11.3.5 3.11.3.6 3.11.4 References

Introduction Alkene, alkyne, alkenyl, alkynyl complexes of the lanthanides Alkene and alkyne complexes Alkenyl and alkynyl complexes Allyl complexes of the lanthanides Homoleptic allyl complexes Bis-allyl complexes Tris-allyl complexes Bis-allyl cationic complexes Mono-allyl dicationic complexes Tetra-allyl anionic complexes Mono-substituted bis-allyl complexes: (Allyl)2LnX compounds (Allyl)2LnX compounds with halide ligand (Allyl)2LnX compounds with amide and related N-donor ligands (Allyl)2LnX compounds with cyclopentadienyl/indenyl ligands Bis-substituted mono-allyl complexes: (Allyl)LnX2 compounds (Allyl)LnX2 compounds with borohydride ligands (Allyl)LnX2 compounds with amide and related N-donor ligands (Allyl)LnX2 compounds with alkoxide ligands Cationic (allyl)LnX compounds with cyclopentadienyl or related ligands (Allyl)LnX2 compounds with cyclopentadienyl ligands (Allyl)LnXX0 compounds Aza-allyl lanthanide complexes Lanthanide complexes with unusual allyl coordination mode Allyl complexes of the lanthanides as intermediates Conclusion

383 383 383 393 402 402 402 403 410 412 412 413 413 413 414 419 419 422 426 426 429 436 437 439 446 446 446

Abbreviations Bd CCG CGC Cp Cp diox DFT Dipp DME Fc Flu Ip Ind MAO NMR PB PI PS ROP THF

382

1,3-Butadiene Catalyzed chain growth Constrained geometry catalyst Cyclopentadienyl Pentamethylcyclopentadienyl Dioxane Density functional theory 2,6-iPr2C6H3 Dimethoxyethane Ferrocene Fluorenyl Isoprene Indenyl Methyl aluminoxane Nuclear magnetic resonance Polybutadiene Polyisoprene Polystyrene Ring opening polymerization Tetrahydrofuran

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00055-X

Alkenes and Allyl Complexes of the Group 3 Metals and Lanthanides

TMEDA TOF TON St XRD

3.11.1

383

N,N,N0 ,N0 -Tetramethylethylenediamine Turn-over frequency Turn-over-number Styrene X-Ray diffraction

Introduction

This chapter reviews the chemistry of lanthanide and group 3 metals (which corresponds to rare earth metals and herein designed as Ln or lanthanides) alkene and allyl organometallic complexes over the period 2006–20. It follows previous developments on the chemistry of these complexes in preceding editions of COMC in 1982, 1995 and 2007.1 Alkene and alkyne complexes are compounds in which the hydrocarbon molecule behaves as L-type ligand toward the metal center. In this chapter, we will also present literature works regarding alkenyl and alkynyl (X-type ligand). As far as allyl organolanthanides are concerned, we wish to bring to the reader’s attention that this field has been addressed by Carpentier and collaborators in their well-documented review published in 2010 in C.R. Chimie.2 However, with the aim to be as exhaustive as possible, we included herein all information that seemed essential to us and was already cited earlier. All complexes comprising the “simple” [Ln](C3H5) and “substituted” [Ln](C3HnR5− n) (R ¼ alkyl, n ¼ 1–4) allyl motive will be the subject of this chapter. The complexes bearing an heteroallyl (i.e., allyl comprising a heteroatom in the carbon framework) group will be also addressed. Allyl complexes are a valuable alternative to their alkyl counterparts, which are often less stable and coordinated by an external base, for reactivity or catalysis purposes, while having appropriate activities. Regarding synthetic approaches, a convenient method to prepare [Ln]-allyl compounds is to use ionic metathesis, by reacting a lanthanide precursor ([Ln]-X, X ¼ halide, borohydride, etc.) with an anionic allyl reagent, typically a Grignard (allyl)MgX (X ¼ Cl, Br) or (allyl)nM (n ¼ 1, 2; M ¼ alkaline, alkaline-earth metal) reagent. Alternatively, the homoleptic tris(allyl)Ln(solvent)x3–6 may be used as starting material, which is reacted with a given proligand (i.e., the conjugate acid of an anionic ligand). In some rare cases, the strategy of the insertion reaction of an allene into a lanthanide hydride bond has been reported too to create the lanthanide-allyl motive7 (or the insertion of a ketenimine to produce an (azallyl)-Ln moiety). In terms of molecular structure, the allyl group is generally bonded in a trihapto 3-mode to a lanthanide metal. However, the ligand can occasionally be found under an 1-coordination arrangement. The allyl group may be fluxional in solution, which results in a typical 1H/4H or 1H/2H/2H pattern of allyl resonances by 1H NMR spectroscopy. Concerning chemical reactivity, the lanthanide allyl moiety has been the subject of numerous reactions, including insertions of small molecules, hydrogenolysis, protonation by acid reagents, hydrosilylation, substitution, photolytic activation, and reduction. All these kinds of reactivity will be addressed in this chapter. In addition, polymerization catalysis represents a whole and very specific part of the applications of lanthanide allyl complexes. In particular, it is well-established that Ln-allyl species are involved in polymerization processes of conjugated dienes when lanthanide catalysts are implemented.8–13 Advances in this field have been reviewed recently14 and they will be included here when relevant for the period covered in this chapter. Finally, complexes in which an unusual coordination mode of allyl-type is found are also reviewed. In turn, fluorenyl rare earth complexes where the ligand is in some cases coordinated in an 3-trihapto mode to the metal are not included herein. Such complexes, which have been reviewed in 2005,15 are covered in another chapter of the present edition of COMC. For ease of reading and for all practical purposes, a table containing the main structural information on the complexes described is provided at the end of each section.

3.11.2

Alkene, alkyne, alkenyl, alkynyl complexes of the lanthanides

3.11.2.1

Alkene and alkyne complexes

Isolated lanthanide-alkene species are extremely rare in the literature, even though there are some evidence of their existence via NMR studies. As a matter of facts, they are most of the time found as intermediates in various catalytic reactions such as olefin16,17 and 1,3-diene18,19 polymerization, hydrogenation,20 hydroamination,21,22 hydrophosphination23 and hydrosilylation.24 Nearly all structurally characterized lanthanide-alkene complexes involve an alkene that is tethered to another coordinating group due to the weakness and lability of the lanthanide-alkene bond. Ethylene coordination to [(C5Me5)2Eu] was highlighted via NMR paramagnetic shifts25 and dynamic NMR behavior of the yttrium alkyl-tethered alkene complex (C5Me5)2Y(1:5-CH2CH2CR2CR0 ]CH2) [1Y] (R ¼ H, Me and R0 ¼ H, Me) was extensively studied.26–34 In 1987, Andersen and Burns reported the first lanthanide-alkene structure of a divalent ytterbium-platinum (m-bridged) alkene complex, (C5Me5)2Yb(m-2:2-C2H4)Pt(PPh3)2 [2Yb].35 In 1999, the synthesis and X-ray structure of the interesting alkene ytterbium complex [Yb{1:2-C(SiMe3)2SiMe2CH]CH2}{OEt2}]2(m-I)2 [3Yb] (Fig. 1, left) was reported by the group of Smith and co-workers.36 Some years later, Evans37 and Schumann38,39 and coworkers detailed the structures of several alkene-tethered cyclopentadienyl complexes of divalent lanthanides (4Yb, 5Sm, 5Eu, 5Yb, Fig. 1, middle and right) and the closely related alkaline earths that display alkene coordination.

384

Alkenes and Allyl Complexes of the Group 3 Metals and Lanthanides

Fig. 1 Molecular structures of some alkene complexes of the lanthanides.

Fig. 2 Molecular structure of 7Sm (thermal ellipsoids are drawn at the 30% probability level). Reprinted from Berg, D. J.; Tosha, B.; Xuening, F. J. Organomet. Chem. 2010, 695, 2703–2712, with the authorization of Elsevier.

Regarding more recent work, Berg et al. reported in 2010 the synthesis, solid state structure and the solution behavior of dimeric lanthanide aryloxides bearing one or two allyl groups in the o-aryl positions: {Ln[DALP]2}2[m-DALP]2 [6Ln] (with H-DALP ¼ 2,6-diallyl-4-methylphenol and Ln ¼ La, Ce, Nd, Er, Yb, Y) and {Ln[MALP]2}2[m-MALP]2 [7Ln] (with H-MALP ¼ 2-allyl-4, 6-dimethylphenol and Ln ¼ La, Sm, Y) complexes.40 The coordination of the alkenes from the allyl groups to the Ln3+ center was observed crystallographically across the lanthanide series (Fig. 2). Variable temperature NMR studies revealed that the dimeric structures remain intact in non-coordinating solvents with rapid bridge-terminal aryloxide exchange taking place above about 275–295 K for all DALP complexes, except erbium. These complexes were the first neutral trivalent lanthanide-alkene complexes that have been structurally characterized. More recently, the group of Trifonov described the synthesis of divalent (CpBn5)2Ln [8Ln] (CpBn5 ¼ C5(CH2Ph)5, Ln ¼ Yb, Sm and Eu) by combining the potassium salt KCpBn5 with 0.5 equiv. of LnI2(THF)n (Scheme 1).41

Scheme 1 Synthesis of 8Ln complexes.

The X-ray diffraction studies revealed that the three complexes are isostructural with the Ln(II) metal being coordinated by two 5-CpBn5 rings with an additional interaction of one C-sp2 carbon in ortho position of the two pendant phenyl moiety (Fig. 3).

Alkenes and Allyl Complexes of the Group 3 Metals and Lanthanides

385

Fig. 3 Molecular structure of 8Ln complexes (Ln ¼ Yb, Sm and Eu) (thermal ellipsoids are drawn at the 30% probability level). Carbon atoms of Ph groups and hydrogen atoms are omitted for clarity. Reprinted from Selikhov, A. N.; Mahrova, T. V.; Cherkasov, A. V.; Fukin, G. K.; Larionova, J.; Long, J.; Trifonov, A. A. Organometallics 2015, 34, 1991−1999, with the authorization of the American Chemical Society.

Reaction of 8Sm with 1 equiv. of phenazine afforded the [(CpBn5)2Sm]2[m-3:3-(C12H8N2)] [80 Sm] adduct that was isolated as single crystals (Scheme 2).

Scheme 2 Synthesis of 80 Sm adduct.

X-ray diffraction analysis of 80 Sm showed that the m-bridged phenazine ligand is coordinated to both samarium centers through one nitrogen atom and two neighboring carbon atoms of the phenazine (Fig. 4). Furthermore, it was found that the Sm-C(Cp) bond distances fall in the range typical for Sm(III) compounds, indicating that oxidation of Sm(II) occurs during the reaction. The preparation of lanthanide complexes supported with a functionalized indolyl ligand was carried out by the group of Wang using [Ln(CH2SiMe3)3(THF)2] (Ln ¼ Yb, Er and Y) precursors in presence of 1 equiv. of 3-(CyN]CH)C8H5NH proligand in toluene, which yields the dinuclear lanthanide alkyl complexes [9Ln] (Scheme 3).42

386

Alkenes and Allyl Complexes of the Group 3 Metals and Lanthanides

C(44) C(43) C(45) C(41)

C(42) C(83)

C(5)

SmlD

C(82)

C(1)

N(1) C(81)

C(4) C(2) C(3)

C(86A)

C(84) C(81A)

C(85)

C(82A) C(86)

C(85A) C(84A) N(1A)

Sm(1A)

C(83A)

Fig. 4 Molecular structure of 80 Sm (thermal ellipsoids with a 30% probability level). Carbon atoms of Ph groups and hydrogen atoms are omitted for clarity. Reprinted from Selikhov, A. N.; Mahrova, T. V.; Cherkasov, A. V.; Fukin, G. K.; Larionova, J.; Long, J.; Trifonov, A. A. Organometallics 2015, 34, 1991− 1999, with the authorization of the American Chemical Society.

Scheme 3 Synthesis of complexes 9Ln.

The molecular structure of the three complexes was determined by X-ray diffraction studies and revealed, in particular, that the two metal centers are bridged by the indolyl ligand through the two carbons of the five membered ring in 2-fashion and through the nitrogen atom in an 1-mode (Fig. 5). Interestingly, the 2-coordination mode of the two carbons of the five-membered ring was not observed when the reaction was conducted in THF, due to the presence of an additional THF ligand in the coordination sphere of the metal center. In the same study, upon treatment of 3-(tBuNH-CH2)C8H5NH proligand with Ln(CH2SiMe3)3(THF)2 (Ln ¼ Y, Er and Dy) in THF, trinuclear complexes 10Ln were formed (Scheme 4, up). On the other hand, a dinuclear species 11Yb could be isolated just 1 year earlier from the reaction of the same 3-(tBuNH-CH2)C8H5NH proligand with Yb(CH2SiMe3)3(THF)2, which includes a metal with a smaller ionic radius (Scheme 4, down).43

Alkenes and Allyl Complexes of the Group 3 Metals and Lanthanides

387

Fig. 5 Representative molecular structure of complexes 9Ln showing the bridging mode as {[2:1-m-1-3-[(CyNCH(CH2SiMe3))Ind]RE(THF) (CH2SiMe3)}2 (Cy ¼ cyclohexyl, Ind ¼ indolyl, RE ¼ Ln ¼ Yb, Er, Y) lanthanide complexes (thermal ellipsoids are drawn at the 30% probability level). Hydrogen atoms are omitted for clarity. Reprinted from Zhang, G.; Deng, B.; Wang, S.; Wei, Y.; Zhou, S.; Zhu, X.; Huang, Z.; Mu, X. Dalton Trans. 2016, 45, 15445–15456, with the authorization of the Royal Society of Chemistry.

Scheme 4 Synthesis of indenyl alkyl complexes 10Ln and 11Yb showing a different result depending on the nature of the lanthanide precursor.

In the case of the trinuclear complexes 10Ln, the X-ray diffraction analysis indicates a distorted octahedral geometry around the metal center that comprises an 2-coordination mode of the carbon atoms and 1-fashion of the nitrogen atom from the indolyl moiety, one nitrogen atom from the amido group and two additional THF molecules (Fig. 6, left). Similar 2-coordination binding mode of the carbon atom of the indolyl ring was observed for the dinuclear 11Yb, which was obtained under two forms, with indolyl groups in trans (major) or cis (minor) position from each other (Fig. 6, right). In parallel, dinuclear species 12Ln were prepared from the one-pot treatment of 3-(tBuNH-CH2)C8H5NH proligand and [Ln(CH2SiMe3)3(THF)2] in the presence of [(2,6-iPr2C6H3)N]CHNH-(C6Hi3Pr2-2,6)] amidine (Ln ¼ Er, Y, Scheme 5).

388

Alkenes and Allyl Complexes of the Group 3 Metals and Lanthanides

O1

C14'

i

i C2

i N2

C9

i

i O2

i RE1

i N1

Yb1'

C20

N1

O1'

N1

N2' O2

C3

C2

RE1 C2'

N2 C20

C3'

C2

i O1

N2

N1' Yb1

Cl4

C9 O1

Fig. 6 Molecular structure of [2:1-m-1-3-(tBuNCH2)Ind]4Ln3(THF)5(CH2SiMe3) 10Ln (RE ¼ Ln ¼ Y, Er, Dy, left) and of complex trans-[{m-2:1:1-3-(tBuNCH2)Ind}Yb(THF)(CH2SiMe3)]2 11Yb (right). Thermal ellipsoids are set at the 30% probability level. Reprinted from (left) Zhang, G.; Deng, B.; Wang, S.; Wei, Y.; Zhou, S.; Zhu, X.; Huang, Z.; Mu, X. Dalton Trans. 2016, 45, 15445–15456, with the authorization of the Royal Society of Chemistry; (right) Zhang, G.; Wei, Y.; Guo, L.; Zhu, X.; Wang, S.; Zhou, S.; Mu, X. Chem. Eur. J. 2015, 21, 2519–2526, with the authorization of the Wiley-VCH Verlag GmbH.

Scheme 5 Synthesis of dinuclear complexes 12Ln.

The X-ray molecular structure revealed that in 12Ln the metal center is also coordinated to the carbon of the indolyl ring in 2-mode and the nitrogen atom in 1-fashion (Fig. 7).

Fig. 7 Molecular structure of [1-m-1:1-3-(tBuNCH2)Ind][1-m-1:3-3-(tBuNCH2)Ind]RE2(THF)[(3-2,6-iPr2C6H3)NCHN(C6Hi3Pr2-2,6)]2 [12Ln] (RE ¼ Ln ¼ Er, Y, ellipsoid are drawn at the 30% probability level). Reprinted from Zhang, G.; Deng, B.; Wang, S.; Wei, Y.; Zhou, S.; Zhu, X.; Huang, Z.; Mu, X. Dalton Trans. 2016, 45, 15445–15456, with the authorization of the Royal Society of Chemistry.

Alkenes and Allyl Complexes of the Group 3 Metals and Lanthanides

389

Fig. 8 Representative molecular structure of [trans-[{m-2:1:1-3-{tBuNCH(CH2SiMe3)}Ind}Ln(THF)(CH2SiMe3)]2] [13Ln] (RE ¼ Ln ¼ Y, Dy, Yb, ellipsoid are drawn at the 30% probability level). Reprinted from Zhang, G.; Wei, Y.; Guo, L.; Zhu, X.; Wang, S.; Zhou, S.; Mu, X. Chem. Eur. J. 2015, 21, 2519–2526, with the authorization of the Royal Society of Chemistry.

Following the same strategy as for the synthesis of complexes 9Ln (Scheme 3), but using 3-(tBuN]CH)C8H5NH as proligand, afforded the dinuclear lanthanide complexes [trans-[{m-2:1:1-3-{tBuNCH(CH2SiMe3)}Ind}Ln(THF)(CH2SiMe3)]2] [13Ln] (Ln ¼ Y, Dy, Yb), where trans represents the orientation of six membered rings of the indolyl ligands in opposite directions. According to X-ray analysis, dinuclear rare-earth metal alkyl complexes with central symmetry were isolated as dianionic species bridges with two metal alkyl units in m-2:1:1 bonding mode (Fig. 8).43 In a more recently published work by Trifonov and coworkers, divalent benzhydryl Ln(II) derivatives of the formula [(p-tBuC6H4)2CH]2Ln(L)n (14Sm, Ln ¼ Sm, L ¼ DME, n ¼ 2; 140 Ln, Ln ¼ Sm, Yb, L ¼ TMEDA, n ¼ 1) were synthesized.44 The complexes display structural peculiarities: X-ray diffraction studies showed that the benzhydryl ligands are bound to the metal center in an 2-coordination mode in 14Sm, where short contacts between Sm(II) ion and the ipso-carbon atom of one of the phenyl rings were detected, whereas 140 Yb displays 3-coordination of the benzhydryl moieties to the metal center (Fig. 9). In 140 Sm, one benzhydryl is 3-coordinated while the second one is 4-coordinated to the Sm(II) ion. Theoretical calculations were carried out in order to get more details on the nature of M− L bonding, which corroborates the presence of short interactions between the metal center and the ortho- and ipso-carbons of the benzhydryl ligands, highlighting the variety of hapticities observed.

Fig. 9 Molecular structure of a representative complex [(p-tBu-C6H4)2CH]2Ln(L)n (140 Ln, M ¼ Ln ¼ Sm, Yb). Ellipsoids are set at the 30% probability, hydrogen atoms, methyl groups of tBu-substituents, and carbon atoms of TMEDA molecules are omitted for clarity. Reprinted from Selikhov, A. N.; Plankin, G. S.; Cherkasov, A. V.; Shavyrin, A. S.; Louyriac, E.; Maron, L.; Trifonov, A. A. Inorg. Chem. 2019, 58, 5325−5334, with the authorization of the American Chemical Society.

390

Alkenes and Allyl Complexes of the Group 3 Metals and Lanthanides

Fig. 10 Molecular structure of the half-sandwich complexes 15Sm (left) and 15Yb (right). Methyl fragments of tBu groups and all hydrogen atoms except the methine hydrogen are omitted for clarity. Ellipsoids are given at 30% of the probability level. Reprinted from Selikhov, A. N.; Shavyrin, A. S.; Cherkasov, A. V.; Fukin, G. K.; Trifonov, A. A. Organometallics 2019, 38, 4615−4624, with the authorization of the American Chemical Society.

Substitution of one benzhydryl ligand by tert-butylcarbazol-9-yl (tBu4Carb) or 2,7-di-tert-butyl-fluoren-9-trimethylsilyl ( Bu2FluTMS) was also performed later by the same group.45 The molecular structure of the DME adducts of samarium and ytterbium [tBu4Carb]Ln[(p-tBu-C6H4)2CH](DME) [15Sm, 15Yb] revealed an 3-coordination mode of the benzhydryl ligand to the metal (Fig. 10, left and right, respectively) while the ytterbium complex [tBu2FluTMS]Yb[(p-tBu-C6H4)2CH](TMEDA) bearing TMEDA ligand [16Yb] shows an 1-coordination mode. Following the synthesis of [(p-tBu-C6H4)2CH]2Ln(L)n, Trifonov and coworkers prepared a series of Ln(III) (Ln ¼ La, Nd and Y) complexes supported by three benzhydryl ligands, 17Ln, from salt metathesis of [(p-tBu-C6H4)2CH]Na in presence of 1/3 equiv. of LnX3(THF)3.5 (X ¼ Cl, I) (Scheme 6).46 t

Alkenes and Allyl Complexes of the Group 3 Metals and Lanthanides

391

Scheme 6 Synthesis of complexes 17Ln.

Fig. 11 Molecular structure of complexes 17Ln (M ¼ Ln ¼ La, Nd, Y). Thermal ellipsoids are given at the 30% probability level. Methyl groups of tert-butyl substituents and hydrogen atoms except bonded with methanide carbons were omitted for clarity. Reprinted from Selikhov, A. N.; Boronin, E. N.; Cherkasov, A. V.; Fukin, G. K.; Shavyrin, A. S.; Trifonov, A. A. Adv. Synth. Catal. 2020, 362, 5432–5443, with the permission of Wiley-VCH GmbH.

Single crystals of 17Ln suitable for X-ray diffraction studies revealed that, for each lanthanide element, the benzhydryl ligand is rather coordinated in an 4-mode to the metal center with one covalent bond between Ln and the methanide carbon as well as short contacts with two ipso and one ortho-phenyl carbons of the benzhydryl part (Fig. 11). Upon reaction of the potassium salt {[2,2-(4-MeC6H3NMe2)2CH]K(THF)}2 with LnI2(THF)2(Ln ¼ Yb and Sm), the replacement of the tBu substituent on the para position of the benzhydryl ligand by NMe2 group led to the formation of the homoleptic [2,2-(4-MeC6H3NMe2)2CH]2Ln [18Ln] (Scheme 7).47

392

Alkenes and Allyl Complexes of the Group 3 Metals and Lanthanides

Scheme 7 Synthesis of complexes 18Ln.

Fig. 12 Molecular structure of 18Ln (M ¼ Yb and Sm) with a 30% thermal ellipsoid probability level. All hydrogen atoms except that of M–CH are omitted for clarity. Reprinted from Khristolyubov, D. O.; Lyubov, D. M.; Shavyrin, A. S.; Cherkasov, A. V.; Fukin, G. K.; Trifonov, A. A. Inorg. Chem. Front. 2020, 7, 2459–2477, with the permission of the Royal Society of Chemistry.

The X-ray diffraction studies revealed that, in both cases, the divalent lanthanide centers in 18Ln are coordinated by two [2,2-(4MeC6H3NMe2)2CH]− ligands through the carbon of the methanido group as well as two additional nitrogen atoms of the NMe2 substituents (Fig. 12), although, the coordination mode of both [2,2-(4-MeC6H3NMe2)2CH]− ligands was shown to be different. Upon closer examination of the distances and angles between the metals and the ligands, it was observed that one ligand coordinates in k1-N mode while the other revealed a short contact between ipso- and ortho-carbons of one phenyl substituent, resulting in an 4-CCCN coordination mode with the metal. Long et al. described the preparation of two half-sandwich complexes of the type (Cp )Dy(2,6-iPr2C6H3N −CMe]CMe − NC6Hi3Pr2 −2,6)(THF) [19Dy] and [Li(THF)3][Dy(2,6-iPr2C6H3N −CMe]CMe −NC6H3 iPr2 − 2,6)(Cp )Cl] (Cp ¼ C5Me5) [190 Dy] by treatment of DyCl3 with [K(THF)x(2,6-iPr2C6H3N −CMe]CMe − NC6Hi3Pr2 −2,6)K(THF)x] in the presence of either KCp or LiCp , respectively (Scheme 8).48

Alkenes and Allyl Complexes of the Group 3 Metals and Lanthanides

393

Scheme 8 Synthesis of 19Dy and 190 Dy starting from potassium or lithium reagent, respectively.

Fig. 13 Structure of 19Dy (left) and 190 Dy (right). Color code: orange, Dy; red, O; gray, C; green, Cl; light blue, Li. Hydrogen atoms have been omitted for clarity. Reprinted from Long, J.; Tolpygin, A. O.; Cherkasov, A. V.; Lyssenko, K. A.; Guari, Y.; Larionova, J.; Trifonov, A. A. Organometallics 2019, 38 (4), 748–752, with the permission of the American Chemical Society.

In both cases, the X-ray diffraction measurement revealed that the metal centers of both 19Dy and 190 Dy are coordinated to one dianionic NC]CN ligand, one Cp ligand and one THF or (Cl)Li(THF)3 moiety, respectively (Fig. 13). The weak interaction of the Dy(III) cation with the carbon atoms of the NC]CN part is consistent with an 2-coordination mode of the carbon double bond to the metal center (Table 1).

3.11.2.2

Alkenyl and alkynyl complexes

The reactivity of lanthanide complexes with unsaturated hydrocarbon substrates has been the subject of numerous investigations in the 1980s, but it was only in 1990 that the group of Evans succeeded in identifying the first examples of an 2-alkene lanthanide complexes [(Cp )2Sm]2(m-2:4-CH2CHPh) [20Sm] and [(Cp )2Sm]2(m-2:4-PhCHCHPh) [200 Sm] by reduction of styrene and stilbene precursors, respectively, in the presence of divalent (Cp )2Sm.9 A decade later, the same group extended the reactivity of the decamethylsamarium with isoprene and myrcene to produce the bimetallic [(Cp )2Sm]2[m-2:4-CH2CHC(Me)CH2] [21Sm] and [(Cp )2Sm]2[m-2:4-CH2CHC(CH2)CH2CH2CHCMe2] [210 Sm] complexes, respectively.49 In addition, the coordination chemistry of yttrium was also explored by reacting YCl3 with tetraphenylethylenyl M[PhCCPh2] (M ¼ Na or K) to form complexes of the type [M(THF)x][Y(Ph2CCPh2)2] [22Y].50 The extension of the chemistry of alkenyl lanthanides was at the same period pursued by Floriani and coworkers by reacting the meso-octaethylporphyrinogen (oepg) lanthanide complexes, {(oepg)Ln}Na(THF)2 (Ln ¼ Pr, Nd) with NaC10H8 in an ethylene atmosphere in the presence of 18-crown-6 in THF that yields dimeric species of formula [{(oepg)Ln}Na (THF)2(m-2:2-C2H4)]2 [23Ln] where the metals are bridged by the [C2H4]− moiety.51 According to the X-ray crystal structure of

Table 1

Structural and analytical data of alkene and alkyl complexes (in brackets for relevant complexes only, otherwise for all complexes). ˚) X-ray data typical Ln-C (alkene or alkyl) distances (A

NMR data

References

1Y

[(Cp )2Y(1:5-CH2CH2CR2CR0 ]CH2)]

–

1

26–34

2Yb 3Yb

[(Cp )2Yb(m-2:2-C2H4)Pt(PPh3)2] [[Yb{1:2-C(SiMe3)2SiMe2CH]CH2}{OEt2}]2(m-I)2]

Yb-C ¼ 2.770(3), 2.793(3) Yb-C ¼ 2.50(2)

4Yb

[Yb(C5Me4CH2CH2CH]CH2)2]

5Sm, 5Eu, 5Yb

[Ln{(C5Me4)SiMe2(CH2dCH]CH2)]2}] (Ln ¼ Sm, Eu, Yb)

1

H, 13C (5Yb)

37

6La, 6Ce, 6Nd, 6Er, 6Yb, 6Y

[{Ln[DALP]2}2[m-DALP]2] (Ln ¼ La, Ce, Nd, Er, Yb, Y)

1

H, 13C for (6Nd, 6Y)

40

7La, 7Sm, 7Y

[{Ln[MALP]2}2[m-MALP]2] (Ln ¼ La, Sm, Y)

1

H

40

8Yb, 8Sm, 8Eu

[(CpBn5)2Ln] (CpBn5 ¼ C5(CH2Ph)5, Ln ¼ Yb, Sm, Eu)

1

H, 13C (8Yb, 8Eu)

41

80 Sm 9Yb, 9Er, 9Y

[(CpBn5)2Sm]2[m-3:3-(C12H8N2)]] [{[2:1–m–1-3-(CyNCH(CH2SiMe3))Ind]Ln(THF)(CH2SiMe3)}2] (Cy ¼ cyclohexyl, Ind ¼ indolyl, Ln ¼ Yb, Er, Y)

1 1

H, 13C H (9Y), 13C (9Y)

41 42

10Y, 10Er, 10Dy

[2:1–m–1-3-(tBuNCH2)Ind]4Ln3(THF)5(CH2SiMe3) Ln ¼ Y, Er, Dy

1

H (10Y), 13C (10Y)

42

11Yb 12Er, 12Y

trans-[{m-2:1:1-3-(tBuNCH2)Ind}Yb(THF)(CH2SiMe3)]2 [[1-m-1:1-3-(tBuNCH2)Ind][1-m–1:3-3-(tBuNCH2)Ind] Ln2(THF)[(3-2,6-iPr2C6H3)NCHN(C6Hi3Pr2-2,6)]2] (Ln ¼ Er, Y) [trans-[{m-2:1:1-3-{tBuNCH(CH2SiMe3)}Ind}Ln(THF)(CH2SiMe3)]2] (Ln ¼ Y, Dy, Yb)

Yb-C ¼ 2.941(7), 3.132(7) 2.881(7), 3.032(7) Sm-C ¼ 3.249(4), 3.004(3) Eu-C ¼ 3.293(4), 3.008(3) Yb-C ¼ 3.182(3), 2.905(3) Nd-C ¼ 3.122(6), 3.024(5), 3.185(6), 3.256(5) Er-C ¼ 3.122(6), 2.928(5), 3.186(6), 3.126(5) Y-C ¼ 3.128(5), 2.970(5), 3.187(5), 3.153(5) Sm-C ¼ 3.120(4), 3.099(3), 3.149(4), 3.252(3) Y-C ¼ 3.090(3), 3.140(4), 3.123(3), 3.288(4) Yb-C ¼ 2.952(2), 3.197(2) Sm-C ¼ 2.996(2), 3.161(2) Eu-C ¼ 2.991(2), 3.163(2) Sm-C ¼ 2.869(2), 2.901(2) Yb-C ¼ 2.675(6), 2.811(7) Er-C ¼ 2.717(6), 2.825(5) Y-C ¼ 2.709(5), 2.816(5) Y-C ¼ 2.963(3), 3.044(3) Er-C ¼ 2.969(6), 3.026(5) Dy-C ¼ 2.989(5), 3.027(6) Yb-C ¼ 2.681(4), 2.787(5) Er1-C ¼ 2.794(7), Er2-C ¼ 2.733(8), 2.712(7), 3.031(6) Y1-C ¼ 2.825(6), Y2-C ¼ 2.759(7), 2.738(7), 3.022(7) Y-C ¼ 2.725(4), 2.810(4) Dy-C ¼ 2.745(3), 2.775(3) Yb-C ¼ 2.636, 2.838(3) 14Sm: Sm-C ¼ 2.863(2), 2.662(2), 2.701(2), 2.944(2), 2.963(2), 3.060(2) 140 Yb: Yb-C ¼ 2.668(2), 2.555(2), 2.617(2), 2.853(2), 2.889(2), 2.895(2) No R-xay for 140 Sm Sm-C ¼ 2.670(4), 2.904(3), 3.220(4), 3.125(4) Yb-C ¼ 2.540(4), 2.806(4), 3.084(4), 3.350(4) Yb-C ¼ 2.589(2), 2.721(2), 2.737(2) Y-Cipso ¼ 2.720(2) to 2.844(2) Y-Cortho ¼ 2.812(2), 2.814(2), 2.814(2) La-Cipso ¼ 2.841(2) to 2.915(2) La-Cortho ¼ 2.948(2), 2.958(2), 2.971(2) Nd-Cipso ¼ 2.795(3) to 2.867(3) Nd-Cortho ¼ 2.905(3), 2.918(3), 2.920(3) Yb-C ¼ 2.576(4), 2.585(4), 2.730(4), 2.742(4), 2.852(3), 2.864(4) Sm-C ¼ 2.710(2), 2.728(2), 2.779(2), 2.804(2), 2.854(2), 2.883(2) Dy-C ¼ 2.837(2), 2.845(2) Dy-C ¼ 2.764(2), 2.769(2)

H, 1H NOESY, 13C, VT 1H and 13C NMR, 1 13 H- C HMQC 1 H, 31P 1 H attempts (broad signals/ rearrangements) 1 H, 13C

– H(12Y), 13C (12Y)

43 42

1

H(13Y), 13C (13Y)

43

1

H, 13C (14Sm, 14Yb)

44

1

H (15Sm), 13C (15Sm)

45

1 1

H, 13C H (17Y, 17La), 13C (17Y, 17La)

45 46

1

H, 13C

47

13Y, 13Dy, 13Yb

14Sm, 140 Yb, 140 Sm

[[(p-tBu-C6H4)2CH]2Ln(L)n] (14Sm, Ln ¼ Sm, L ¼ DME, n ¼ 2; 140 Ln, Ln ¼ Sm, Yb, L ¼ TMEDA, n ¼ 1)

15Sm, 15Yb

[[tBu4Carb]Ln[(p-tBu-C6H4)2CH](DME)] (Ln ¼ Sm, Yb)

16Yb 17Y, 17La, 17Nd

[(tBu2FluTMS)Yb[(p-tBu-C6H4)2CH](DME)] [Ln[(p-tBu-C6H4)2CH]3] (Ln ¼ Y, La, Nd)

18Yb, 18Sm

[Ln[2,2-(4-MeC6H3NMe2)2CH]2] (Ln ¼ Yb, Sm)

19Dy 190 Dy

(Cp )Dy(2,6-iPr2C6H3NdCMe]CMedNC6Hi3Pr2 −2,6)(THF) [Li(THF)3][Dy(2,6-iPr2C6H3NdCMe]CMedNC6Hi3Pr2 −2,6)(Cp )Cl]

1

– –

35 36 38,39

48 48

Alkenes and Allyl Complexes of the Group 3 Metals and Lanthanides

Molecular formula

394

Compound number

Alkenes and Allyl Complexes of the Group 3 Metals and Lanthanides

N2

395

N4′

N3 C37′ Nd1

Nd1′

N1′

N1 C37 N3′

N4

N2′

N1C

Na1C N2

N2B

Na1 N1 C19A

C19

Pr1A Pr1 Na1A

N2C

N1A

Na1B N2A N1B

Fig. 14 Molecular structure of 23Nd (left, Na(THF)2 and counter-cationic part and meso-ethyl groups omitted for clarity) and 24Pr (right, meso-ethyl groups omitted for clarity). Reprinted from Campazzi, E.; Solari, E.; Scopelliti, R.; Floriani, C. Chem. Commun. 1999, 1617, with the permission of the Royal Society of Chemistry.

the Nd derivative 23Nd, the [C2H4]− unit is side-on bonded to the two metal atoms (Fig. 14, left). In the presence of acetylene and sodium metal, the resulting acetylido complexes were formed where the two metal centers in [{(oepg)Ln} 2 2 (Na)2(m-2:2-C2)]2 [24Ln] are bridged by the C2− 2 m- : -bonded anion as shown in the X-ray structure of 24Pr (Fig. 14, right). [(Cp00 2Sc)2-(m-2:2-C2H4)] [25Sc] was isolated in low yield (